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A detailed photophysical picture is elaborated for a structurally well-defined and symmetrical bis-tetracene dimer in solution. The molecule was designed for interrogation of the initial photophysical steps (S1 → 1TT) in intramolecular singlet fission (SF). (Triisopropylsilyl)acetylene substituents on the dimer TIPS-BT1 as well as a monomer model TIPS-Tc enable a comparison of photophysical properties, including transient absorption dynamics, as solvent polarity is varied. In nonpolar toluene solutions, TIPS-BT1 decays via radiative and nonradiative pathways to the ground state with no evidence for dynamics related to the initial stages of SF. This contrasts with the behavior of the previously reported unsubstituted dimer BT1 and is likely a consequence of energetic perturbations to the singlet excited-state manifold of TIPS-BT1 by the (trialkylsilyl)acetylene substituents. In polar benzonitrile, two key findings emerge. First, photoexcited TIPS-BT1 shows a bifurcation into both arm-localized (S1-loc) and dimer-delocalized (S1-dim) singlet exciton states. The S1-loc decays to the ground state, and weak temperature dependence of its emissive signatures suggests that once it is formed, it is isolated from S1-dim. Emissive signatures of the S1-dim state, on the other hand, are strongly temperature-dependent, and transient absorption dynamics show that S1-dim equilibrates with an intramolecular charge-transfer state in 50 ps at room temperature. This equilibrium decays to the ground state with little evidence for formation of long-lived triplets nor 1TT. These detailed studies spectrally characterize many of the key states in intramolecular SF in this class of dimers but highlight the need to tune electronic coupling and energetics for the S1 → 1TT photoreaction. [ABSTRACT FROM AUTHOR]

Quantum chemical and quantum dynamical calculations areperformedfor a bay-substituted perylene bisimide dye up to its hexameric aggregate.The aggregate structure is determined by employing the self-consistentcharge density functional tight-binding (SCC-DFTB) approach includingdispersion corrections. It is characterized by a stabilization viatwo chains of hydrogen bonds facilitated by amide functionalities.Focusing on the central embedded dimer, the Coulomb coupling for thisJ-aggregate is determined by means of the time-dependent density functionaltheory (TDDFT) to be −514 cm–1. Exciton vibrationalcoupling is treated within the shifted oscillator model from whichfive strongly coupled modes per monomer are selected for inclusioninto a minimal dynamic model. Performing wave packet propagationsfor a model employing up to 7 electronic states and 30 vibrationalmodes using the multiconfiguration time-dependent Hartree method,aggregate absorption spectra are obtained and compared to experiment. [ABSTRACT FROM AUTHOR]

A detailed investigation of the excited states accessedby UV absorptionin alternating DNA duplexes was performed by means of an extensivesampling of intra- and intermolecular degrees of freedom. The excitedstates were computed using the algebraic diagrammatic constructionmethod to second-order (ADC(2)). A realistic DNA environment was includedthrough an electrostatic embedding QM/MM coupling scheme. The resultsindicate that (i) most excited states are delocalized over at mosttwo bases, (ii) charge transfer states are located at higher energiesthan the bright states in the Franck–Condon region, but (iii)coupling between locally excited and charge transfer states may providea route to dynamical charge separation, and (iv) spectral broadeningis mainly caused by intramolecular vibrations. [ABSTRACT FROM AUTHOR]

Absorption spectra of I2 dissolved in n-heptane and CCI4 are analyzed with a quantum gas-phase model, in which spectra at four temperatures between 15° and 50 °C are least-squares fitted by bound-free spectral simulations to obtain estimates of the excited-state potential energy curves and transition moment functions for the three component bands-A ← X, B ← X, and C ← X. Compared with a phenomenological band-fitting model used previously on these spectra, the physical model (1) is better statistically, and (2) yields component bands with less variability. The results support the earlier tentative conclusion that most of the ~20% gain in intensity in solution is attributable to the C ← X transition. The T-dependent changes in the spectrum are accounted for by potential energy shifts that are linear in T and negative (giving red shifts in the spectra) and about twice as large for CCI4 as for heptane. The derived upper potentials resemble those in the gas phase, with one major exception: In the statistically best convergence mode, the A potential is much lower and steeper, with a strongly varying transition moment function. This observation leads to the realization that two markedly different potential curves can give nearly identical absorption spectra. [ABSTRACT FROM AUTHOR]

Protochlorophyllide (Pchlide) is a natural porphyrin, a precursor of chlorophyll, synthesized by plants for its photosynthetic apparatus. The pigment spontaneously forms aggregates when dissolved in neat water solution. We present here calculations of the transient absorption spectra and its comprising components (ground-state bleach, stimulated emission, and excited-state absorption) for a strongly excitonically coupled linear chain of four Pchlide chromophores, using exciton theory with phenomenological Gaussian line shapes and without energetic disorder. A refined multiexciton model that includes static disorder is applied to fit the experimental power-dependent transient absorption spectra of aqueous protochlorophyllide and the kinetics for delay times up to 20 ps after photoexcitation. We show that population up to the 4-exciton manifold is sufficient to explain the pronounced saturation of the bleaching and the shape changes in the instantaneous, t = 0.2 ps transient spectra when the pulse energy is increased from 10 to 430 nJ per pulse. The decay of the multiexciton manifold is relatively slow and is preceded by a spectroscopically distinct process. We suggest that the exciton states in the Pchlide aggregates are mixed with charge-transfer states (CTS) and that the population and repopulation of the CTS coupled to the exciton states explains the relatively slow decay of the multiexciton manifold. The relevance of our results to the optical properties and dynamics of natural photosynthetic complexes and the possible physical origin of CTS formation are discussed. [ABSTRACT FROM AUTHOR]

The photophysical properties for a series of free-base arylethynyl porphyrins and the corresponding trans-disubstituted tetraphenylporphyrin (H2TPP) derivatives lacking arylethynyl functionalities have been studied via electronic absorption and emission spectroscopy in both neutral and diacid forms. Enhanced substituent effects on porphyrin absorption spectra are observed in the arylethynyl porphyrins relative to the H2TPP derivatives, owing to the presence of the ethynyl spacer that allows for a coplanar geometry between the porphyrin macrocycle and the appended phenyl substituents. Upon protonation, both series of porphyrins exhibit substantially red shifted absorption and emission spectra and enhanced oscillator strengths, with the magnitude of the spectral shifts being more substantial in the presence of the ethynyl functionalities. Spectral features of the arylethynyl porphyrin bearing p-dimethylamino substituents closely resemble those previously classified as "hyperporphyrin spectra" and are indicative of excited-state charge-transfer character. Protonation of both series of porphyrins results in reduced fluorescence lifetimes and enhanced nonradiative decay rates, and the impact of protonation on these parameters is attenuated in the presence of the arylethynyl functionalities. Our results coupled with previous structural data showing that arylethynyl porphyrins exhibit less structural distortion upon diacid formation relative to H2TPP further substantiate the proposal that significant alteration of porphyrin photophysical properties upon diacid formation can be attributed to nonplanar structural distortions induced by protonation. [ABSTRACT FROM AUTHOR]

Electron photodetachment from the aromatic anion phenolate excited into the π--&pi* singlet excited state (S1) in aqueous solution is studied with ultrafast transient absorption spectroscopy with a time resolution of better than 50 fs. Broadband transient absorption spectra from 300 to 690 nm are recorded- The transient bands are assigned to the solvated electron, the phenoxyl radical, and the phenolate S1, excited state, and confirmation of these assignments is achieved using both KNO3 as electron quencher and time-resolved fluorescence to measure singlet excited state dynamics. The phenolate fluorescence lifetime is found to be short (∼20 ps) in water, but the fast decay is only in part due to the electron ejection channel from S1. Using global target analysis, two electron ejection channels are identified, and we propose that both vibrationally hot S1 state and the relaxed S1 state are direct precursors for the solvated electron. Therefore, electron ejection is found just to compete with picosecond time scale vibrational relaxation and electronic radiationless decay channels. This contrasts markedly with <100 fs electron detachment processes for inorganic anions. [ABSTRACT FROM AUTHOR]

The electron dynamics of gold nanorods were systematically studied by using femtosecond transient absorption experiments. Two different excitation wavelengths (400 and 800 nm) have been used as the excitation sources to selectively excite transverse and longitudinal modes. The transient absorption spectra were found to be strongly dependent on the excitation wavelength and fluence. Laser pulses of 800 nm excite the longitudinal mode directly, which cause an increase in the electronic temperatures and subsequent broadening and bleaching of both the longitudinal and transverse modes. Pulses of 400 nm excite both the transverse and longitudinal modes simultaneously. At low excitation fluences, the energy is distributed into two modes according to their steady state extinction coefficients, under which the transient spectra are similar to those under excitation at 800 nm. However, as the excitation fluence exceeds a threshold, the bleaching of the longitudinal plasmon band saturates and the input energies mainly flow to the transverse mode. As a result, the bleaching of the transverse mode increases rapidly. The electron-phonon dynamics show a strong correlation with the bleaching amplitude. We have tried to explain the results with a consistent picture: the bleaching amplitude and electron-phonon relaxation time are directly related to energy distribution into different modes, which are excitation wavelength and fluence dependent. Our studies help to clarify the seemingly inconsistent results in the previous studies by different research groups. [ABSTRACT FROM AUTHOR]

The photophysical properties of a carbonyl-containing carotenoid analogue in an s-cisconfiguration, relative to the conjugated π system, 2-(all-trans-retinylidene)-indan-1,3-dione (C20Ind), were investigated by femtosecond time-resolved spectroscopy in various solvents. The lifetime of the optically forbidden S1state of C20Ind becomes long as solvent polarity increases. This trend is completely opposite to the situation of S1-ICTdynamics of carbonyl-containing carotenoids, such as peridinin and fucoxanthin. Excitation energy dependence of the transient absorption measurements shows that the transient absorption spectra in nonpolar solvents were originated from two distinct transient species, while those in polar and protic solvents are due to a single transient species. By referring to the results of MNDO-PSDCI (modified neglect of differential overlap with partial single- and double-configuration interaction) calculations, we conclude: (1) in polar and protic solvents, the S1state is generated following excitation up to the S2state; (2) in nonpolar solvents, however, both the S1and the 1nπ* states are generated; and (3) C20Ind does not generate the S1-ICTstate, despite the fact that it has two conjugated carbonyl groups. [ABSTRACT FROM AUTHOR]

Glasses with composition 15Li2O−15K2O−xBi2O3-(65 − x)-B2O3/5V2O5(3 ≤ x≤ 15) have been prepared by the conventional melt quench technique. The electron paramagnetic resonance spectra of VO2 these glasses have been recorded in the X-band frequency (≈9.3 GHz) at room temperature. The spin Hamiltonian parameters and covalency rates were evaluated. It was found that the V4誉 exist as vanadyl (VO2) ions and are in an octahedral coordination with a tetragonal compression. The covalency rates (1 − α2) and (1 − γ2) indicate moderate covalency for the σ- and π-bonds. It was observed that the spin-Hamiltonian parameters depend slightly on the relative concentration of Bi2O3. The optical properties of this glass system are studied from the optical absorption spectra recorded in the wavelength range 200−800 nm. The fundamental absorption edge has been identified from the optical absorption spectra. The values of optical band gap for indirect allowed transitions have been determined using available theories. The direct current electrical conductivity, σ, has been measured in the temperature range 373−573 K. The conductivity decreases with the increase in Bi2O3concentration. This has been discussed in terms of the decrease in the number of mobile ions and their mobility. An attempt is made to correlate the EPR, optical, and electrical results and to find the effect of Bi2O3content on these parameters. [ABSTRACT FROM AUTHOR]

The absorption spectra of dianionic tetrocyanoethylene and dicationic tetrathiafulvalene dimers have been studied theoretically with the time-dependent density functional theory and the recently proposed Coulomb-attenuated model. The nature of the excited states was further explored by means of the two-dimensional (2D) site (transition density matrix) and three-dimensional (3D) cube (transition density and charge difference density) representations. By use of the 3D transition density and charge difference density, we visualized the orientation of transition dipole moment and also explained charge-transfer characteristics occurring in the dianionic/dicationic π-dimers system. It is found that for the dianionic/dicationic π-dimers system there exist two kinds of charge-transfer patterns for the mainly excited states, the intermolecular charge transfer and the mixture of intramolecular charge transfer coupled with intermolecular charge transfer. Meanwhile, the coupling effect of excition and the oscillation of electron−hole pairs between the monomers have been revealed with 2D site representation of transition density matrix, which also indicates the electron−hole coherence upon photon excitation. [ABSTRACT FROM AUTHOR]

Single crystal molecular structure and solution photophysical properties are reported for 1,3-diphenylisobenzofuran (1), of interest as a model compound in studies of singlet fission. For the ground state of 1and of its radical cation (1+•) and anion (1−•), we report the UV−visible absorption spectra, and for neutral 1, also the magnetic circular dichroism (MCD) and the decomposition of the absorption spectrum into purely polarized components, deduced from fluorescence polarization. These results were used to identify a series of singlet excited states. For the first excited singlet and triplet states of 1, the transient visible absorption spectra, S1→ Sxand sensitized T1→ Tx, and single exponential lifetimes, τF= ∼5.3 ns and τT= ∼200 μs, are reported. The spectra and lifetimes of S1→ S0fluorescence and sensitized T1→ Txabsorption of 1were obtained in a series of solvents, as was the fluorescence quantum yield, ΦF= 0.95−0.99. No phosphorescence has been detected. The first triplet excitation energy of solid 1(11 400 cm−1) was obtained by electron energy loss spectroscopy, in agreement with previously reported solution values. The fluorescence excitation spectrum suggests an onset of a nonradiative channel at ∼37 000 cm−1. Excitation energies and relative transition intensities are in agreement with those of ab initio (CC2) calculations after an empirical 3000 cm−1adjustment of the initial state energy to correct differentially for a better quality description of the initial relative to the terminal state of an absorption transition. The interpretation of the MCD spectrum used the semiempirical PPP method, whose results for the S0→ Sxspectrum require no empirical adjustment and are otherwise nearly identical with the CC2 results in all respects including the detailed nature of the electronic excitation. The ground state geometry of 1was also calculated by the MP2, B3LYP, and CAS methods. The calculations provided a prediction of changes of molecular geometry upon excitation or ionization and permitted an interpretation of the spectra in terms of molecular orbitals involved. Computations suggest that 1can exist as two nearly isoenergetic conformers of C2or Cssymmetry. Linear dichroism measurements in stretched polyethylene provide evidence for their existence and show that they orient to different degrees, permitting a separation of their spectra in the region of the purely polarized first absorption band. Their excitation energies are nearly identical, but the Franck−Condon envelopes of their first transition differ to a surprising degree. [ABSTRACT FROM AUTHOR]

We present a theoretical study on the temperature-dependent absorption and photoluminescence spectroscopy of rubrene multichromophores by combining the time-dependent long-range-corrected density functional theory with the exciton model. The spectra of rubrene multichromophores up to heptamers are calculated, and the effects of exciton−phonon coupling and temperature on the photophysical properties of both H- and J-aggregated oligomers are addressed. It is found that the spectral behavior of rubrene aggregates is very much dependent on aggregation details. As the temperature increases, higher excitonic states become populated, and low-energy dark states in H-aggregated oligomers become observable gradually while the peak intensities near the 0−0 transition decrease for J-aggregated oligomers. [ABSTRACT FROM AUTHOR]

Two-photon absorption (TPA) spectra of a series of highly symmetric, strong phosphorescence AuI−CuIcomplexes 1−7bearing alkynyl−phosphine ligands in solution were obtained over the biological windows (760−840 nm) by the two-photon-induced emission technique under femtosecond laser excitation. The resulting TPA cross sections are further confirmed by the open-aperture Z-scan method, which offers the advantage of reducing the excited-state reabsorption for these long-lived phosphorescence dyes. The results of these two methods are mutually in good agreement, showing that complexes 1−7exhibit moderately strong TPA cross section, for example, ∼710 GM at 760 nm for 1in CH2Cl2. Owing to the highly centrosymmetric property, the resulting trend in TPA values for 1−7can be rationalized by the unit of X-group−alkynyl−heterometallic-core−alkynyl−X-group fragment, categorized as A−π−D−π−A or D−π−A−π−D structural motif, which renders large changes in polarization upon excitation and provides significant near-IR TPA strength. [ABSTRACT FROM AUTHOR]

The lowest energy optical electronic absorption band of the three-chromophore system tris(4-bromophenyl)amine radical cation is analyzed. The lowest energy electronic transition corresponds to a p-bromophenyl orbital to nitrogen p orbital transition that places the positive charge on three equivalent p-bromophenyl chromophores. The excited electronic state is an example of excited-state mixed valence (ESMV), and the spectrum is interpreted using two ESMV models. The simplest model invokes the concept of an “effective coupling” between the three identical chromophores with an excited-state energy splitting equal to three times the coupling. A more accurate model, the “neighboring orbital model”, utilizes the coupling between the bridge’s and charge-bearing unit’s orbitals closest in energy. The three-chromophore system provides a striking illustration of the failure of an effective coupling term to account for ESMV splitting. The calculated relative energies of the diabatic and adiabatic states are different, but the calculated absorption spectra of the two models show nearly identical vibrational fine structure. Resonance Raman data and the time-dependent theory of electronic and resonance Raman spectroscopies are used to calculate the spectra. [ABSTRACT FROM AUTHOR]

Solvation dynamics of 4-aminophthalimide (4AP) in methanol is measured by broadband upconversion of the fluorescence band. The peak emission frequency ν̃(t) is determined from 100 fs onward with 85 fs time resolution. Polar solvation based on simple continuum theory, including solute polarizability, describes the temporal shape of ν̃(t) quantitatively. Extrapolation ν̃(t→0) points to an initial emission frequency which agrees with the result from stationary spectroscopy in a nonpolar solvent. The extent (4300 cm−1) of the dynamic Stokes shift is largely due (50%) to H-bonding, however. The observations imply that H-bonds with 4AP adiabatically follow the dielectric relaxation of the methanol network. The stimulated emission band is also used to measure solvation dynamics. The evolving band is monitored by transient absorption spectroscopy of supercontinuum probe pulses. But the excited-state absorption spectrum, its relative amplitude, and its evolution are needed to extract ν̃(t) from such measurements. These key data are obtained by comparison with the upconversion results. Thus calibrated photometrically, 4AP transient absorption can be used to monitor solvation dynamics in any solvent. The excited-state absorption spectrum is assigned with the help of time-dependent density-functional calculations. Fluorescence excitation and double-resonance spectroscopy of isolated 4AP, cooled in a supersonic jet, is used to determine optically active modes. An intramolecular reorganization energy is inferred which is consistent with the value in 2-methylbutane (2025 cm−1). The crystal structure is also provided. [ABSTRACT FROM AUTHOR]

Photoexcitation of a zinc phthalocyanine−perylenediimide (ZnPc−PDI) dyad and a bis(zinc phthalocyanine)−perylenediimide [(ZnPc) 2−PDI] triad results in formation of the triplet excited state of the PDI moiety without the fluorescence emission, whereas addition of Mg 2+ions to the dyad and triad results in formation of long-lived charge-separated (CS) states (ZnPc •+−PDI •−/Mg 2+and (ZnPc) 2•+−PDI •−/Mg 2+) in which PDI •−forms a complex with Mg 2+. Formation of the CS states in the presence of Mg 2+was confirmed by appearance of the absorption bands due to ZnPc •+and PDI •−/Mg 2+complex in the time-resolved transient absorption spectra of the dyad and triad. The one-electron reduction potential ( Ered) of the PDI moiety in the presence of a metal ion is shifted to a positive direction due to the binding of Mg 2+to PDI •−, whereas the one-electron oxidation potential of the ZnPc moiety remains the same. The binding of Mg 2+to PDI •−was confirmed by the ESR spectrum, which is different from that of PDI •−without Mg 2+. The energy of the CS state (ZnPc •+−PDI •−/Mg 2+) is determined to be 0.79 eV, which becomes lower that of the triplet excited state (ZnPc− 3PDI*: 1.07 eV). This is the reason why the long-lived CS states were attained in the presence of Mg 2+instead of the triplet excited state of the PDI moiety. [ABSTRACT FROM AUTHOR]

Optical spectroscopy and nanosecond flash photolysis (Nd:YAG laser, 355 nm, pulse duration 5 ns, mean energy 5 mJ/pulse) were used to study the photochemistry of Fe III(C 2O 4) 33−complex in aqueous solutions. The main photochemical process was found to be intramolecular electron transfer from the ligand to Fe(III) ion with formation of a primary radical complex [(C 2O 4) 2Fe II(C 2O 4•)] 3−. The yield of radical species (i.e., CO 2• −and C 2O 4• −) was found to be less than 6% of Fe III(C 2O 4) 33−disappeared after flash. [(C 2O 4) 2Fe II(C 2O 4•)] 3−dissociates reversibly into oxalate ion and a secondary radical complex, [(C 2O 4)Fe II(C 2O 4•)] −. The latter reacts with the initial complex and dissociates to Fe II(C 2O 4) and oxalate radical. In this framework, the absorption spectra and rate constants of the reactions of all intermediates were determined. [ABSTRACT FROM AUTHOR]

The effect of the excitation energy on the nonadiabatic photodissociation dynamics of (HI)2is explored in this work. A wave packet model is applied that simulates the photodissociation process starting from the I−HI complex left behind after dissociation of the first HI moiety within (HI)2. The probability and product fragment state distributions of the different photodissociation pathways are analyzed in a wide range of excitation energies of the I−HI absorption spectrum. It is found that the probability of electronically nonadiabatic transitions increases substantially (by a factor larger than two) in the range of excitation energies analyzed. This increase is due to an enhancement of the intensity of the spin−rotation coupling responsible for the nonadiabatic transitions with increasing excitation energy. A remarkably high fraction of bound, highly excited I2photoproducts, slowly decreasing as the excitation energy increases, is also found over the range of energies studied. The I2product state distributions show manifestations of rotational interference effects and also of rotational cooling in the case of the I2state distributions produced upon nonadiabatic transitions. Such effects become more pronounced with increasing energy. Experimental implications of these findings are discussed. [ABSTRACT FROM AUTHOR]

The photophysical properties of a novel 1,2,3,4,5,6-hexasubstituted fullerene derivative (1) are examined in this study. In addition to the ground state absorption spectrum of 1, we report its triplet-triplet absorption spectrum and molar extinction coefficient (T-T), as well as the triplet quantum yield (T), lifetime (T), and energy (ET). The saturation of a single six-member ring on the fullerene cage results in significant changes in the triplet state properties as compared to that of pristine C60. The triplet-triplet absorption spectrum shows a hypsochromic shift in long wavelength absorption, and both the triplet state lifetime and the triplet quantum yield are decreased. The triplet energy was found to be similar to that of C60. In addition, the quantum yield () of singlet oxygen generated by 1was calculated and is found to be significantly less than in the case of C60. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical study of the absorption spectra of a group of closely related pyrylium perchlorates 1−11are presented. Minor changes in the position of the substituents lead to drastic changes in the absorption spectra in this series of compounds. We have attempted to explain the observed changes using the x,y-band notation developed by Balaban and co-workers. Absorption spectra of all compounds are compared with results from time-dependent density functional theory (TDDFT) and Zerner's intermediate neglect of differential overlap (ZINDO/S) level calculations. Results of the calculations are in good agreement with experimental observations and an interesting correlation between Balaban's notations and the MO transitions are obtained for simple derivatives. It is suggested that for more complex systems such as - and -naphthyl substituted systems, the empirical method is not appropriate. [ABSTRACT FROM AUTHOR]

Electronic absorption spectra of the protonated polyacetylenic chains H2CnH(n 4, 6, 8) and the neutral H2C8H have been observed in 6 K neon matrixes after mass selection. The wavelength of the H2CnHelectronic transitions depends quasi-linearly on n, typical of carbon chains. The origin band is at 286.0, 378.6, and 467.6 nm for n 4, 6, and 8, respectively. Two ground-state vibrations of H2C4Hin the IR absorption spectrum were also detected. On the basis of the spectroscopic trends and the assignment of the vibrational frequencies in the ground and excited electronic states, it is concluded that the H2CnHspecies are C2vlinear carbon chains with one H atom on one end and two on the other. [ABSTRACT FROM AUTHOR]

Complete active space (CAS) calculations with 6-311g(3df,3pd) basis sets were performed for a large number of electronic states of the nitrate free radical (CH3N/CH3CH2N) and their positive and negative ions. All calculated states are valence states, and their characters are discussed in detail. To investigate the Jahn−Teller effect on the CH3N radical, Cssymmetry was used for both CH3N and CH3CH2N in calculations. The results (CASPT2 adiabatic excitation energies and CASSI oscillator strengths) suggest that the calculated transitions of CH3N at 32172 and 32139 cm-1are attributed to the 23A‘ ‘ → 13A‘ ‘ and 13A‘ → 13A‘ ‘, respectively, which is in accordance with the Ã3E → 3A2emission spectrum at T0 31 817 cm-1. The calculated transitions of CH3CH2N at 334 nm are attributed to the 13A‘ ‘ → 23A‘ ‘ and 13A‘ ‘ → 13A‘, respectively, which is in accordance with the UV absorption spectrum of a series of 11 bands beginning at 335 nm. The vertical and adiabatic ionization energies were obtained to compare with the PES data. These results are in agreement with previous experimental data, which is discussed in detail. [ABSTRACT FROM AUTHOR]

The present work deals with singlet excitation energy transfer (EET) occurring in contact ion pairs (CIPs) of several anionic oxonol analogues (acting as EE donors) and cationic cyanines (acting as acceptors) characterized by off resonance individual transitions. Combining conductometric and spectroscopic measurements with decreasing solvent polarity, we were able to observe a progressive ion pairing leading first to solvent-separated ion pairs (SSIPs) and then to CIPs. Analysis of the absorption spectra of three selected salts (A2,C1, A2,C2, and A1,C4) in chloroform−toluene mixtures showed that the transformation of SSIP into CIP involves the appearance of a certain exciton coupling, the extent of which decreases regularly with increasing gap between the local excitation energies. Fluorescence excitation spectra showed that EET occurs in CIP, and EET efficiencies were evaluated with a procedure expressly devised for weakly emitting donors. These were between 0.2 and 0.65 for the examined ion pairs involving anions A1 and A2. The spectroscopic study was complemented by a theoretical investigation aimed at establishing the dynamic regime of the observed EET. From classical MD simulations and local full geometry optimizations, A2,C1 and A2,C2 were found to form rather stable sandwich-type CIP structures with interchromophore distances (R) of about 0.45−0.50 nm. The donor−acceptor electronic coupling was calculated in terms of Coulombic interactions between atomic transition charges. For CIP, the electronic coupling was decidedly beyond the limit of the weak coupling required for an incoherent Förster-type mechanism. Thus, we tried to arrange the EET dynamics within the theory developed by Kimura, Kakitani, and Yamato (J. Phys. Chem. B 2000, 104, 9276) for the intermediate coupling case, which provides analytical expressions of time-dependent occupation probability, EET rate, and coherency in terms of two basic quantities:  the electronic coupling and a correlation time related to the Franck−Condon factor. The latter was shown to be primarily modulated by Förster''s spectral overlap integral (related in turn to the excitation energy gap). Calculations were carried out for the three sample systems using three values of the electronic coupling roughly corresponding to CIP, 1.0, and 2.0 nm interchromophore distances. At the CIP distance, EET in both A2,C1 and A2,C2 was predicted to occur with a partial exciton mechanism, very short transfer times (about 10 fs), and high degree of coherence. In A1,C4 (having the largest energy gap), EET was found to occur with a hot-transfer mechanism. More or less hot-transfer dynamics appeared to be retained by all three systems at R = 1.0 nm. Fully incoherent EET appeared to become operative only at distances larger than 2.0 nm. [ABSTRACT FROM AUTHOR]

The high resolution, single site emission and absorption spectra of magnesium isobacteriochlorin (MgiBC) in n-octane matrixes at 7 K are reported. Its emission and Q and Soret band absorption regions have been investigated. The vibrational frequencies of the ground and the lowest energy ππ* singlet excited states were determined from luminescence and excitation spectra, respectively. The emission from MgiBC at 7 K seems to be entirely fluorescence. The luminescence and Qy region spectra of the complex are similar, having intense, narrow origin bands followed by relatively weak, but orderly vibrational structure. The Qx region does not have a clear origin and exhibits complex vibrational structure that increases in intensity going to higher energy. In the Soret region the individual ππ* origins are clearly identifiable and some vibrational structure was also observed. [ABSTRACT FROM AUTHOR]

The six multichromophoric species 1−6, containing the potentially luminescent Ru(II) polypyridine subunits and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene fluorophores (dipyrromethene−Mi2 dyes, herein after called bodipy), have been prepared and their absorption spectra, luminescence properties (both at room temperature in fluid solution and at 77 K in rigid matrix), and redox properties have been investigated (for the structural formulas of all the compounds, see Figure 1). For comparison purposes, also the same properties of the bodipy-based free ligands have been examined. Three of the multichromophoric species (1−3) are based on the Ru(bpy)3-type metal subunit, whereas 4−6 are based on the Ru(terpy)2-type metal subunit. Transient absorption spectroscopy at room temperature of all the compounds has also been performed. The absorption spectra of all the metal complexes show features that can be assigned to the Ru(II) polypyridine subunits and to the bodipy centers. In particular, the lowest energy spin-allowed π−π* transition of the bodipy groups dominates the visible region, peaking at about 530 nm. All the new complexes exhibit a rich redox behavior, with reversible processes attributed to specific sites, indicating a small perturbation of each redox center and therefore highlighting the supramolecular nature of the multichromophoric assemblies. Despite the good luminescence properties of the separated components, 1−6 do not exhibit any luminescence at room temperature; however, transient absorption spectroscopy evidences that for all of them a long-lived (microsecond time scale) excited state is formed, which is identified as the bodipy-based triplet state. Pump−probe transient absorption spectroscopy suggests that such a triplet state is formed from the promptly prepared bodipy-based 1π−π* state in most cases by the intervention of a charge-separated level. At 77 K, all the complexes except complex 1 exhibit the bodipy-based fluorescence, although with a slightly shortened lifetime compared to the corresponding free ligand(s), and 4−6 also exhibit a phosphorescence assigned to the bodipy subunits. Phosphorescence of bodipy species had never been reported in the literature to the best of our knowledge:  in the present cases we propose that it is an effective decay process thanks to the presence of the ruthenium heavy atom and of the closely lying 3MLCT state of the Ru(terpy)2-type subunits. [ABSTRACT FROM AUTHOR]

In this work, we describe the spectroscopic properties of a series of platinum complexes containing one acetylide ligand per platinum, having the chemical formula trans-Pt(PBu3)2((C≡CC6H4)n—H)Cl, n = 1−3 (designated as half-PEn-Pt) and compare their spectroscopic behavior with the well-characterized series trans-Pt(PBu3)2((C≡CC6H4)n—H)2, n = 1−3 (designated as PEn-Pt). This comparison aims to determine if the triplet state of PEn-Pt is confined to one ligand or delocalized across the central platinum atom. We measured ground-state absorption spectra, fluorescence spectra, phosphorescence spectra, and triplet-state absorption spectra. The ground-state absorption spectra and fluorescence spectra both showed a blue shift when comparing half-PEn-Pt with PEn-Pt, showing the S1 state is delocalized across the platinum. In contrast, the phosphorescence spectra of the two types of compounds had the same 0−0 band energy, showing the T1 state was confined to one ligand in PEn-Pt. The triplet state absorption spectra blue shifted when comparing half-PEn-Pt with PEn-Pt, showing the Tn state was delocalized across the central platinum. This comparison supports recently published work that suggested this confinement effect (Rogers, J. E et al. J. Chem. Phys. 2005, 122, 214701). [ABSTRACT FROM AUTHOR]

The absorption spectrum of the propargyl radical in the region from 180 to 400 nm is investigated in detail by means of theory. Vertical excitation energies and potential energy surfaces are determined by highly accurate multireference configuration interaction (MRCI) calculations. The vibrational dynamics of several electronic states is studied, and line intensities and positions are calculated with respect to the electronic and vibrational ground state. Four electronic states absorb in the region of interest:  1 2B2, 2 2B1, 2 2B2, and 3 2B1. However, electronic excitations into the 2B2 states are dipole-forbidden from the X̃ 2B1 ground state and corresponding vibronically allowed transitions are shown to be weak. The spectrum is dominated by the strong 2 2B1 ← 1 2B1 band which is computed in overall good agreement with available experiments. A strong absorption at 242 nm, which has been assigned to propargyl, is not confirmed by the calculations, and only very weak absorptions are found at wavelengths shorter than 280 nm. The present results strongly suggest that the 242 nm feature must be due to a different species. [ABSTRACT FROM AUTHOR]

The spectroscopic properties of the self-associated complexes formed between the anionic surface docking site of spinach plastocyanin and the cationic metalloporphyrins, in which the tyrosine 83 (Y83) moiety is placed just below the docking site, tetrakis(N-methyl-4-pyridyl)porphyrin (Pd(II)TMPyP4+ and Zn(II)TMPyP4+), have been studied and reported herein. The fluorescence quenching phenomenon of the self-assembled complex of Zn(II)TMPyP4+/plastocyanin has also been discovered. The observed red-shifting of the Soret and Q-bands of the UV−visible spectra, ca. 9 nm for Pd(II)TMPyP4+/plastocyanin and ca. 6 nm for the Zn(II)TMPyP4+/plastocyanin complexes, was explained in terms of exciton theory coupled with the Gouterman model. Thus, the hydroxyphenyl terminus of the Y83 residue of the self-associated plastocyanin/cationic porphyrin complexes was implicated in the charge-transfer ligation with the central metal atoms of these metalloporphyrins. Moreover, ground-state spectrometric-binding studies between Pd(II)TMPyP4+ and the Y83 mutant plastocyanin (Y83F-PC) system proved that Y83 moiety of plastocyanin played a critical role in the formation of such ion-pair complexes. Difference absorption spectra and the Job''s plots showed that the electrostatic attractions between the cationic porphyrins and the anionic patch of plastocyanin, bearing the nearby Y83 residue, led to the predominant formation of a self-associated 1:1 complex in the ground-state with significantly high binding constants (K = (8.0 ± 1.1) × 105 M-1 and (2.7 ± 0.8) × 106 M-1 for Pd(II)TMPyP4+ and zinc variant, respectively) in low ionic strength buffer, 1 mM KCl and 1 mM phosphate buffer (pH 7.4). Molecular modeling calculations supported the formation of a 1:1 self-associated complex between the porphyrin and plastocyanin with an average distance of ca. 9 Å between the centers of mass of the porphyrin and Y83 positioned just behind the anionic surface docking site on the protein surface. The photoexcited singlet state of Zn(II)TMPyP4+ was quenched by the Y83 residue of the self-associated plastocyanin in a static mechanism as evidenced by steady-state and time-resolved fluorescence experiments. Even when all the porphyrin was complexed (more than 97%), significant residual fluorescence from the complex was observed such that the amplitude of quenching of the singlet state of uncomplexed species was enormously obscured. [ABSTRACT FROM AUTHOR]

The nature of the linear dichroism in the near-edge X-ray absorption fine structure (NEXAFS) spectra of linear n-alkanes is a matter of long-standing controversy. Linear dichroism in the carbon 1s → σ*C-C transition has been interpreted within a building block model and a molecular orbital model, leading to two different descriptions for the angular dependence of this feature. When used for measurement of molecular orientation, the application of these two different models will lead to different results. We have explored the linear dichroism in the carbon 1s NEXAFS spectra of single crystals of the linear n-alkane hexacontane (n. 60H122). An analysis of the angular dependence in this spectrum shows that the transition dipole moment associated with the carbon 1s → σ*C-C transition is oriented along the macromolecular chain axis, contradicting the predictions of the building block model. However, other transitions are observed in the σ*C-H and the σ*C-C bands that are orthogonal to the dominant transitions for each band. We also observe that radiation damage can be manifest in the form of molecular reorientation in highly ordered organic thin films. [ABSTRACT FROM AUTHOR]

Transient UV absorption spectra and kinetics of the CH2I radical in the gas phase have been investigated at 313 K. Following laser photolysis of 1−3 mbar CH2I2 at 308 nm, transient spectra in the wavelength range 330−390 nm were measured at delay times between 60 ns and a few microseconds. The change of the absorption spectra at early times was attributed to vibrational cooling of highly excited CH2I radicals by collisional energy transfer to CH2I2 molecules. From transient absorption decays measured at specific wavelengths, time-dependent concentrations of vibrationally “hot” and “cold” CH2I and CH2I2 were extracted by kinetic modeling. In addition, the transient absorption spectrum of CH2I* radicals between 330 and 400 nm was reconstructed from the simulated concentration−time profiles. The evolution of the absorption spectra of CH2I* radicals and CH2I2* due to collisional energy transfer was simulated in the framework of a modified Sulzer−Wieland model. Additional master equation simulations for the collisional deactivation of CH2I* by CH2I2 yield 〈ΔE〉 values in reasonable agreement with earlier direct studies on the collisional relaxation of other systems. In addition, the simulations show that the shape of the vibrational population distribution of the hot CH2I* radicals has no influence on the measured UV absorption signals. The implications of our results with respect to spectral assignments in recent ultrafast spectrokinetic studies of the photolysis of CH2I2 in dense fluids are discussed. [ABSTRACT FROM AUTHOR]

Excited-state absorption spectroscopy and ionization threshold measurements for coumarin 314 (C314) adsorbed to the surface of NaCl aerosols characterize the chemical environment of the particle surface as a function of relative humidity (RH). An atmospheric pressure flow of aerosol passes through an ionization cell where two-photon laser ionization of the adsorbed molecules produces a net charge on the particle. Monitoring this charge as a function of the laser wavelength produces either the absorption spectrum of the S1 ← S0 transition or the ionization threshold. The wavelength of maximum absorption for the S1 excited-state shifts from 448 nm for RH < 5% to 441 nm for RH = 60%, indicating that adsorbed water decreases the polarity of the surface. Similarly, the ionization threshold increases from 5.10 to 5.27 eV over a similar range of RH. The decrease in polarity is attributable to changes in the local electric field experienced by C314, which is on the order of 1 × 107 V/cm, and is correlated with changes in the surface topography. Using a continuum model, we estimate the contributions to the measured thresholds of the polarization response of the surface ions and the electric field and calculate an effective dielectric constant for the adsorbed water film. For a multilayer water coverage (RH = 65%), the effective dielectric constant is approximately 2.4. These results demonstrate that the changes in surface topography with adsorbed water are as important as direct water−solute interactions in determining the solvent character of the surface. [ABSTRACT FROM AUTHOR]

FTIR absorption spectra of water-containing ethene:Ar matrices, with compositions of ethene up to 1:10 ethene:Ar, have been recorded. Systematically increasing the concentration of ethene reveals features in the spectra consistent with the known 1:1 ethene:water complex, which subsequently disappear on further increase in ethene concentration. At high concentrations of ethene, new features are observed at 3669 and 3585 cm-1, which are red-shifted with respect to matrix-isolated &ngr;3 and &ngr;1 O−H stretching modes of water and the 1:1 ethene:water complex. These shifts are consistent with a &pgr;···H interaction of a 2:1 ethene:water complex of the form (C2H4···H−O−H···C2H4). The analogous (C2D4)2H2O complex shows little shifting from positions associated with (C2H4)2H2O, while the (C2H4)2D2O isotopomer shows large shifts to 2722.3 and 2617.2 cm-1, having identical &ngr;3(H2O)/&ngr;3(D2O) and &ngr;1(H2O)/&ngr;1(D2O) values when compared with monomeric water isotopomers. Features at 3626.1 and 2666.2 cm-1 are also observed and are attributed to (C2H4)2HDO. DFT calculations at the B3LYP/6-311+G(d,p) level for each isotopomer are presented, and the predicted vibrational frequencies are directly compared with experimental values. The interaction energy for the formation of the 2:1 ethene:water complex from the 1:1 ethene:water complex is also presented. [ABSTRACT FROM AUTHOR]

The propargyl radical has twelve fundamental vibrational modes, Γvib(HCCCH2) = 5a1 ⊕ 3b1 ⊕ 4b2, and nine have been detected in a cryogenic matrix. Ab initio coupled-cluster anharmonic force field calculations were used to help guide some of the assignments. The experimental HC=C‒̈CH2 matrix frequencies (cm-1) and polarizations are a1 modes—3308.5 ± 0.5, 3028.3 ± 0.6, 1935.4 ± 0.4, 1440.4 ± 0.5, 1061.6 ± 0.8; b1 modes—686.6 ± 0.4, 483.6 ± 0.5; b2 modes—1016.7 ± 0.4, 620 ± 2. We recommend a complete set of gas-phase vibrational frequencies for the propargyl radical, HC=C‒̈CH2 X̃ 2B1. From an analysis of the vibrational spectra, the small electric dipole moment, μD(HCCCH2) = 0.150 D, and the large resonance energy (HCCCH2), roughly 11 kcal mol-1, we conclude that propargyl is a completely delocalized hydrocarbon radical and is best written as HC=C‒̈CH2. [ABSTRACT FROM AUTHOR]

The 5f3 → 5f 26d1 absorption spectra of the U3+ ions incorporated in SrCl2 single crystals were recorded at 4.2 K in the 15 000−50 000 cm-1 spectral range. From an analysis of the vibronic structure, 32 zero-phonon lines corresponding to transitions from the 4I9/2 ground multiplet of the 5f 3 configuration to the 5f 26d(eg)1 excited levels were assigned. A theoretical model proposed by Reid et al. (Reid, H. F.; van Pieterson, L.; Wegh, R. T.; Meijerink, A. Phys. Rev. B 2000, 62, 14744) that extends the established model for energy-level calculations of nf N states has been applied for analysis of the spectrum. The Fk(ff) (k = 2, 4), ζ5f (ff), [inline equation] (ff), %@mt;sys@%%@ital@%B%@rsf@%%@sx@%0%@be@%6%@sxx@%%@mx@% (ff), Fk(fd) (k = 2, 4), and Gj(fd) (j = 1, 3) Hamiltonian parameters were determined by a least-squares fitting of the calculated energies to the experimental data. A good overall agreement between the calculated and experimentally observed energy levels has been achieved, with the root-mean-square (rms) deviation equal to 95 cm-1 for 32 fitted levels and 9 varied parameters. Adjusted values of Fk(ff) and ζ5f (ff) parameters for the 5f 2 core electrons are closer to the values characteristic of the 5f 2 (U4+) configuration than to those of the 5f 3 (U3+) configuration. For the U3+ ion, the f−d Coulomb interaction parameters are significantly more reduced from the values calculated using Cowan''s computer code than they are for lanthanide ions. Moreover, because of weaker f−d Coulomb interactions for the U3+ ion than for the isoelectronic Nd3+ lanthanide ion, the very simple model assuming the coupling of crystal-field levels of the 6d1 electron with the lattice and the multiplet structure of the 5f 2 configuration may be employed for the qualitative description of the general structure of the U3+ ion f−d spectrum. [ABSTRACT FROM AUTHOR]

Transient absorption spectra and decay profiles of HO2 have been measured using cw near-IR two-tone frequency modulation absorption spectroscopy at 297 K and 50 Torr in diluent of N2 in the presence of water. From the depletion of the HO2 absorption peak area following the addition of water, the equilibrium constant of the reaction HO2 + H2O ↔ HO2−H2O was determined to be K2 = (5.2 ± 3.2) × 10-19 cm3 molecule-1 at 297 K. Substituting K2 into the water dependence of the HO2 decay rate, the rate coefficient of the reaction HO2 + HO2−H2O was estimated to be (1.5 ± 0.1) × 10-11 cm3 molecule-1 s-1 at 297 K and 50 Torr with N2 as the diluent. This reaction is much faster than the HO2 self-reaction without water. It is suggested that the apparent rate of the HO2 self-reaction is enhanced by the formation of the HO2−H2O complex and its subsequent reaction. Results are discussed with respect to the kinetics and atmospheric chemistry of the HO2−H2O complex. At 297 K and 50% humidity, the concentration ratio of [HO2−H2O]/[HO2] was estimated from the value of K2 to be 0.19 ± 0.11. [ABSTRACT FROM AUTHOR]

Resonance Raman spectra were obtained for nitrobenzene in cyclohexane solution with excitation wavelengths in resonance with the charge-transfer (CT) band absorption spectrum. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mainly along the nominal NO2 symmetric stretch mode (ν11), the nominal benzene ring stretch mode (ν7), accompanied by a moderate degree of motion along the nominal ONO symmetry bend/benzene ring stretch mode (ν23), the nominal C-N stretch/benzene ring breathing mode (ν16), the nominal CCC bending mode (ν20) and the nominal CCH in-plane bending mode (ν14). A preliminary resonance Raman intensity analysis was done and the results for nitrobenzene were compared to previously reported results for several nitroalkanes. [ABSTRACT FROM AUTHOR]

The 193-nm photochemistry of allene (H2C=C=CH2), propyne (H3C-C≡CH), and 2-butyne (H3C-C≡C-CH3) has been examined, and the UV spectral region between 220 and 350 nm has been surveyed for UV-absorption detection of transient species generated from the photolysis of these molecules. Time-resolved UV-absorption spectroscopy was used for detection of transient absorption. Gas chromatographic/mass spectroscopic (GC/MS) analysis of the photolyzed samples were employed for identification of the final photodissociation products. An emphasis of the study has been on the examination of possibilities of formation of different C3H3 isomeric radicals, that is, propargyl (H2CCCH) or propynyl (H3CCC), from the 193-nm photolysis of these molecules. Survey of the UV spectral region, following the 193-nm photolysis of dilute mixtures of allene/He resulted in detection of a strong absorption band around 230 nm and a weaker band in the 320-nm region with a relative intensity of about 8:1. The time-resolved absorption traces after the photolysis event show an instantaneous rise, followed by a simple decay. The spectral features, observed in this work, following 193-nm photolysis of allene are in good agreement with the previously reported spectrum of H2CCCH radical in the 240- and 320-nm regions and are believed to originate primarily from propargyl radicals. In comparison, the spectra obtained from the 193-nm photolysis of dilute mixtures of HCCCH3/He and CH3CCCH3/He were nearly identical, consisting of two relatively broad bands centered at about 240- and 320-nm regions with a relative intensity of about 2:1, respectively. In addition, the time-resolved absorption traces after photolysis of propyne and 2-butyne samples, both in the 240 and 320 nm regions, indicated an instant rise followed by an additional slower absorption rise. The distinct differences between the results of allene with those of propyne and 2-butyne suggest the observed absorption features following 193-nm photolysis of these molecules are likely to be composite with contributions from a number of transient species other than propargyl radicals. Propyne and 2-butyne are structurally similar. The methyl (CH3) and propynyl (CH3C≡C) radicals are likely to be among the photodissociation products of 2-butyne, and similarly, propynyl is likely to be a photodissociation product of propyne. GC/MS product analysis of photolyzed 2-butyne/He mixtures indicates the formation of C2H6 (formed from the combination of CH3 radicals), and a number of C6H6 and C4H6 isomers formed from self- and cross reactions of C3H3 and CH3 radicals, including 1,5-hexadiyne and 2,4-hexadyine, that are potential products of combination reactions of propargyl as well as propynyl radicals. [ABSTRACT FROM AUTHOR]

The effects of 4-cyano and 3-cyano substituents on the spectroscopic properties and photoacidity of 3- and 4-hydroxystilbene have been investigated. In nonpolar solvents, the 3-hydroxycyanostilbenes have much longer singlet lifetimes and larger fluorescence quantum yields than do the 4-hydroxycyanostilbenes. The longer lifetimes of 3-hydroxystilbene and its cyano derivatives are attributed to a “meta effect” on the stilbene torsional barrier, similar to that previously observed for the aminostilbenes. The cyano substituent causes a marked increase in both ground state and excited-state acidity of the hydroxystilbenes in aqueous solution. The dynamics of excited-state proton transfer in methanol-water solution have been investigated by means of femtosecond time-resolved transient absorption spectroscopy. Assignment of the transient absorption spectra is facilitated by comparison to the spectra of the corresponding potassium salts of the conjugate bases and the methyl ethers, which do not undergo excited-state proton transfer. The 4-cyanohydroxystilbenes undergo excited-state proton transfer with rate constants of 5 × 1011 s-1. These rate constants are comparable to the fastest that have been reported to date for a hydroxyaromatic photoacid and approach the theoretical limit for water-mediated proton transfer. The isotope effect for proton transfer in deuterated methanol-water is 1.3 ± 0.2, similar to the isotope effect for the dielectric response of water. The barrier for excited state double bond torsion of the conjugate bases is small for 4-cyano-4-hydroxystilbene but large for 4-cyano-3-hydroxystilbene. Thus the “meta effect” is observed for the singlet states of both the neutral and conjugate base. [ABSTRACT FROM AUTHOR]

The UV absorption spectrum of the permanganate anion is a prototype transition-metal complex spectrum. Despite this being a simple d0 Td system, for which a beautiful spectrum with detailed vibrational structure has been available since 1967, the assignment of the second and third bands is still very controversial. The issue can be resolved only by an elucidation of the intricate vibronic structure of the spectrum. We investigate the vibronic coupling by means of linear-response time-dependent density functional calculations. By means of a diabatizing scheme that employs the transition densities obtained in the TDDFT calculations in many geometries around Re, we construct a Taylor series expansion in the normal coordinates of a diabatic potential energy matrix, coupling 24 excited states. The simulated vibronic structure is in good agreement with the experimental absorption spectrum after the adjustment of some of the calculated vertical excitation energies. The peculiar blurred vibronic structure of the second band, which is a very distinctive feature of the experimental spectrum, is fully reproduced in the calculations. It is caused by the double-well shape of the adiabatic energy surface along the Jahn−Teller active e mode of the allowed 1E state arising from the second 1T2 state, which exhibits a Jahn−Teller splitting into 1B2 and 1E states. We trace the double-well shape to an avoided crossing between two diabatic states with different orbital-excitation character. The crossing can be explained at the molecular orbital level from the Jahn−Teller splitting of the set of 7t2{3dxy, 3dxz, 3dyz} orbitals (the LUMO + 1), to which the excitations characterizing the diabatic states take place. In contrast to its character in the two well regions, at Re the 21T2 state is not predominantly an excitation to the LUMO + 1, but has more HOMO − 1 → LUMO (2e = {3dx2-y2, 3dz2}) character. The changing character of the 21T2 − 1E state along the e mode implies that the assignment of the experimental bands to single orbital transitions is too simplistic intrinsically. This spectrum, and notably the blurring of the vibronic structure in the second band, can be understood only from the extensive configurational mixing and vibronic coupling between the excited states. This solves the long-standing assignment problem of these bands. [ABSTRACT FROM AUTHOR]

The pulse radiolysis of N2O saturated aqueous solutions of KSCN was studied under neutral pH conditions. The observed optical absorption spectrum of the SCN• radical in solution is more complex than previously reported, but it is in good agreement with that measured in the gas phase. Kinetic traces at 330 and 472 nm corresponding to SCN• and (SCN)2•-, respectively, were fit using a Monte Carlo simulation kinetic model. The rate coefficient for the oxidation of SCN- ions by OH radicals, an important reaction used in competition kinetics measurements, was found to be (1.4 ± 0.1) × 1010 M-1 s-1, about 30% higher than the normally accepted value. A detailed discussion of the reaction mechanism is presented. [ABSTRACT FROM AUTHOR]

A correlation function formalism is applied to compute the two-photon absorption spectrum of benzene. Using harmonic Hamiltonians for the ground and excited electronic states, we find that the theory agrees qualitatively with the experimentally observed sparsity of the thermal two-photon absorption spectrum as compared with the single-photon absorption spectrum. An expression for the average vibrational energy in the excited state is derived. We find that cooling of the nascent vibrational energy in the electronically excited state is not as extensive in the two-photon absorption process as compared to the single-photon case. [ABSTRACT FROM AUTHOR]

MR-CISD and MR-CISD+Q calculations have been carried out to investigate geometries, energies, and electronic absorption spectra of formamide and its O- and N-protonated forms. The vertical excitation energies for formamide are in good agreement with available experimental data and with the results of best calculations reported so far. Analysis of the calculated electronic absorption spectra reveals that the lowest excited state in the parent molecule and in its N-protonated form corresponds to the n-π* valence excited state, whereas the O-protonated form shows the π-π* excited valence state as the lowest one. The second excited valence states in the neutral molecule and the N-protonated ion are the π-π* state, whereas it is the n-π* state for the O-protonated formamide. Adiabatic excitation energies are reported for the first excited valence state of all three species with structures optimized at the MR-CISD level. All structures exhibit strong deviation from planarity characterized by pyramidalization of the C and the N atoms and rotation of the NH2 with respect to the plane of the COH group. It appears that oxygen is the most basic site of formamide both in the ground state and in the first singlet excited state. Its calculated gas-phase basicity (GB) and proton affinity (PA) in the latter are smaller than in the ground state by 2.1 kcal mol-1 and 2.4 kcal mol-1, respectively. The difference in basicity between oxygen and nitrogen positions drops from 16 kcal mol-1 (ground state) to 6 kcal mol-1 (first singlet excited state). These results are analyzed within the Förster thermodynamic cycle. [ABSTRACT FROM AUTHOR]

The functional dependence of excited-state geometries and normal modes calculated with time-dependent density functional theory (TDDFT) is investigated on the basis of vibronic structure calculations of the absorption spectra of large molecules. For a set of molecules covering a wide range of different structures including organic dyes, biological chromophores, and molecules of importance in material science, quantum mechanical simulations of the vibronic structure are performed. In total over 40 singlet-singlet transitions of neutral closed-shell compounds and doublet-doublet transitions of neutral radicals, radical cations, and anions are considered. Calculations with different standard density functionals show that the predicted vibronic structure critically depends on the fraction of the “exact” Hartree-Fock exchange (EEX) included in hybrid functionals. The effect can been traced back to a large influence of EEX on the geometrical displacement upon excitation. On the contrary, the dependence of the results on the choice of the local exchange-correlation functional is found to be rather small. On the basis of detailed comparisons with experimental spectra conclusions are drawn concerning the optimum amount of EEX mixing for a proper description of the excited-state properties. The relationship of the quality of the simulated spectra with the errors for 0-0 transition energies is discussed. For the investigated singlet-singlet π → π* transitions and the first strongly dipole-allowed transitions of PAH radical cations some rules of thumb concerning the optimum portion of EEX are derived. However, in general no universal amount of EEX seems to exist that gives a uniformly good description for all systems and states. Nevertheless an inclusion of about 30-40% of EEX in the functional is found empirically to yield in most cases simulated spectra that compare very well with those from experiment and thus seems to be necessary for an accurate description of the excited-state geometry. Pure density functionals that are computationally more efficient provide less accurate spectra in most cases and their application is recommended solely for comparison purposes to obtain estimates for the reliability of the theoretical predictions. [ABSTRACT FROM AUTHOR]

Absorption spectra, resonance Raman spectra and depolarization ratios, fluorescence spectra and emission polarizations, and simulations of the resonance Raman excitation profiles and absorption spectra are reported for a donor-acceptor substituted distyrylbenzene (DADSB) and two covalent “dimers” formed by joining two DADSB chains at their center phenyl ring through a paracyclophane moiety. Semiempirical and density-functional theory calculations of the ground-state geometries and ZINDO calculations of the electronic excitations are also reported. Both the spectroscopic results and the electronic structure calculations agree that the electronic states of these “dimers” are not adequately described by excitonic coupling between the nominally degenerate electronic transitions localized on each DADSB moiety. The resonance Raman spectra of both dimers are essentially identical but show additional lines and intensity differences relative to the monomer. The excitation profiles of all three molecules exhibit interference effects between the Raman amplitudes for the first two strongly allowed electronic transitions. All three molecules exhibit Raman depolarization ratios of 1/3 throughout the lowest-energy absorption band, but the fluorescence of both dimers immobilized in polymer matrixes is considerably depolarized relative to that of the monomer. This suggests that the electronic excitation, initially delocalized over both conjugated chains of the dimers, may become localized on a single chain as geometric relaxation and solvent reorganization occur. [ABSTRACT FROM AUTHOR]

Using the output of a frequency-tripled, pulsed dye laser, we have extended the ionization-detected one-photon absorption spectrum of HCO to include higher excited vibronic levels of the 3pπ 2Π Rydberg state in the interval from 47 900 to 50 500 cm-1. Included in this spectrum are band systems associated with bending overtones from (030) through (060), as well as cold-band transitions to bend-CO-stretch combinations from (011) through (031). The use of a separate, synchronized, fixed-frequency visible ionization pulse, in a technique that we term assisted REMPI, reveals a number of weak features. We find sequence-band transitions to confirmed states as well as to the higher excited levels, (041) and (070), which fall out of range as cold bands. We find a very weak structure that we assign to the (100)-(000) transition terminating on the CH-stretch fundamental. This band appears in the 3pπ state at a frequency very close to the value found for ω1 in the HCO+ cation. Continuing a trend established in earlier observations, we find that the Renner-Teller coupling of the electronic orbital and vibrational angular momentum in the 3pπ 2Π state of HCO varies with bending quantum number, decreasing linearly with v2 in both fundamental as well as bend-CO-stretch combination states. [ABSTRACT FROM AUTHOR]

X-ray absorption spectra (XAS) at the Cl and S K edge and Mo L edge have been calculated at the TDDFT level for a series of dioxomolybdenum complexes LMoO2X (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate, X = Cl, SCH2Ph, OPh), which play an important role in modeling the catalytic cycle of the sulfite oxidase enzyme. Also, the XAS spectra of model molecules of the Mo complexes have been simulated and interpreted in terms of the Mo 4d orbital splitting, in order to find possible correlations with the spectral pattern of the complexes. Comparison with the available experimental data allows us to assess the performances of the present computational scheme to describe the core excitations in large bioinorganic systems. The theoretical interpretation of the spectral features of both the metal and ligand core excitations in terms of the oscillator strength distribution provides important insight into the covalency of the metal-ligand bond. [ABSTRACT FROM AUTHOR]

When a high-frequency discharged CS2:O2:Ar sample was condensed at 4 K, new absorptions at 1003.1 and 567.7 cm-1 were produced. Isotopic substitution (13CS2/O2, C34S2/O2, CS2/18O2, and mixtures) shows that these new absorptions are due to S-O and S-S stretching vibrations, which involve two inequivalent S atoms and one O atom. The photosensitive behavior and the agreement with frequencies and isotopic frequency ratios from quantum chemical calculations substantiate assignment of these absorptions to the SSO- anion, which was predicted to have a bent structure with elongated S-S and S-O bond lengths relative to those of S2O. [ABSTRACT FROM AUTHOR]

5fN → 5fN-16d1 absorption spectra of U4+- and U3+-doped Cs2NaYCl6, Cs2LiYCl6, Cs2NaYBr6, CsCdBr3, and Cs3Lu2Cl9 single crystals were recorded at 4.2 K in the 14 000-50 000 cm-1 spectral range. The 5f2 → 5f16d1 absorption bands of U4+ ions were observed in the 30 000-37 000 cm-1 energy range, and have been assigned as transitions from the 3H4 ground multiplet of the 5f2 configuration to crystal-field levels of the 5f16d(t2g)1 manifold. The energies of the lowest levels of the 5f16d(t2g)1 configuration have been for the first time determined for U4+ ions doped in a chloride or bromide host crystal. They are equal to 26 360 and 28 590 cm-1 for U4+:CsCdBr3 and U4+:Cs2NaYBr6 respectively and range from 30 360 to 31 010 cm-1 for U4+ doped in the chloride hosts. Highly structured vibronic bands of 5f3 → 5f26d(t2g)1 absorption transitions of the U3+ ions appeared at an energy as low as 14 158 cm-1, and have been assigned to transitions from the 4I9/2 ground multiplet of the 5f3 configuration to crystal-field levels arising from the configurations 5f2(3H4)6d(t2g)1(Γ8g) (barycenter at ~16 000 cm-1) and 5f2(3H4)6d(t2g)1(Γ7g) (barycenter at ~19 000 cm-1). Besides, some low intensity bands observed at ~18 600 and above 21 000 cm-1 have been assigned as transitions to the 5f2(3F2)6d(t2g)1(Γ8g) and 5f2(3F2)6d(t2g)1(Γ7g) levels, respectively. Transitions to the 5f26d(eg)1 levels of U3+ ions have been observed as broad and unstructured bands at wavenumbers higher than 38 000 cm-1. The crystal field splitting (10Dq) of the 5f26d1 configuration for U3+ doped in Cs2NaYCl6 single crystals amounts to ~23 000 cm-1, and the separation between Γ8g and Γ7g components of the t2g state, resulting from spin-orbit interaction for the 6d electron, can be evaluated as 3350 cm-1. The splitting of the 5f2(2S+1LJ)6d(γ)1(Γig) (γ = t2g or eg; i = 7 or 8) configurations, resulting from the coupling of the 5f2 core electrons with the 6d electron is somewhat smaller and amounts to ~3000 cm-1. The main feature of the crystal level structure observed in the 5f3-5f26d1 absorption spectra of U3+ ions reflects the dominating influence of the crystal field splitting and spin-orbit coupling of the 6d electron. [ABSTRACT FROM AUTHOR]

Oscillator strengths for C 1s, N 1s, and O 1s excitation spectra of gaseous glycine and the dipeptide, glycyl-glycine, have been derived from inner-shell electron energy-loss spectroscopy recorded under scattering conditions where electric dipole transitions dominate (2.5 keV residual energy, θ ≉ 2°). X-ray absorption spectra of solid glycine, glycyl-glycine, glycyl-glycyl-glycine, and a large protein, fibrinogen, were recorded in a scanning transmission X-ray microscope. The experimental spectra are assigned through interspecies comparisons and by comparison to ab initio computed spectra of various conformations of glycine and glycyl-glycine. Inner-shell excitation spectral features characteristic of the peptide bond are readily identified by comparison of the spectra of gas-phase glycine and glycyl-glycine. They include a clear broadening and a ~0.3 eV shift of the C 1s → π*C=O peak and introduction of a new pre-edge feature in the N 1s spectrum. These effects are due to 1s → π*amide transitions introduced with formation of the peptide bond. Similar changes occur in the spectra of the solids. The computational results support the interpretation of the experimental inner-shell spectra and provide insight into electron density distributions in the core excited states. Possible conformational dependence of the inner-shell excitation spectra was explored by computing the spectra of neutral glycine in its four most common conformations, and of glycyl-glycine in planar and two twisted conformations. A strong dependence of the computed C 1s, N 1s, and O 1s spectra of glycyl-glycine on the conformation about the amide linkage was confirmed by additional ab initio calculations of the conformational dependence of the spectra of formamide. [ABSTRACT FROM AUTHOR]