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Naphthalimide (NI) homo- and hetero-dimers adopting orthogonal geometry were prepared to study photo-induced symmetry-breaking charge transfer (SBCT) and charge recombination (CR)-induced intersystem crossing (ISC). The two moieties in the dimer are connected either at the 3-C or 4-C position of the NI unit. The photophysical properties of the dimers were studied with steady-state and transient absorption spectroscopic methods. Significant CT only occurs for the hetero-dimer, in which one NI unit has a 4-amino substituent and the other NI unit is without it. The CR-induced ISC is most efficient for this dimer (singlet oxygen quantum yield ?= 50.3%). For the homo-dimer, in which both NI units did not present amino substitution, SBCT was not observed. Based on the electrochemical studies, we propose that the absence of SBCT for the homo-dimer is attributed to its high oxidation potential and low reduction potential. Femtosecond transient absorption (fs TA) spectra show that there is no charge separation (CS) for the homo-dimer. Nanosecond transient absorption spectroscopy indicate the formation of a triplet state with electron delocalization for the homo dimer, with a lifetime of 72.0 ?s, while for the hetero dimer a triplet state with an intrinsic lifetime of 206.4 ?s is observed. CS (11.6 ps) and slow CR-induced ISC (>1.5 ns) were observed for the hetero-dimer. Time-resolved electron paramagnetic resonance spectra give the zero-field splitting parameters (|D| = 1894 MHz and |E| = 111 MHz) and electron spin polarization patterns (e, e, e, a, a, a) for the triplet state of the hetero-dimer, inferring that the triplet state of the hetero-dimer is confined on the amino-substituted NI moiety.

Attaching stable radicals to organic chromophores is an effective method to enhance the intersystem crossing (ISC) of the chromophores. Herein we prepared perylene-oxoverdazyl dyads either by directly connecting the two units or using an intervening phenyl spacer. We investigated the effect of the radical on the photophysical properties of perylene and observed strong fluorescence quenching due to radical enhanced ISC (REISC). Compared with a previously reported perylene-fused nitroxide radical compound (triplet lifetime, ?T=0.1 ?s), these new adducts show a longer-lived triplet excited state (?T=9.5 ?s). Based on the singlet oxygen quantum yield (??=7 %) and study of the triplet state, we propose that the radical enhanced internal conversion also plays a role in the relaxation of the excited state. Femtosecond fluorescence up-conversion indicates a fast decay of the excited state (Q1) at ~80 ps time scale. Time-resolved electron paramagnetic resonance (TREPR) spectral study confirmed the formation of the quartet sate. We observed triplet and quartet states simultaneously with weights of 0.7 and 0.3, respectively. This is attributed to two different conformations of the molecule at excited state. DFT computations showed that the interaction between the radical and the chromophore is ferromagnetic (J>0, 0.05~0.10 eV).

Although the photophysics of BODIPY compounds has been widely investigated in the last few years, their analogue N-BODIPY compounds, with nitrogen substitution at the Boron center, did not receive comparable attention. In this work we report about the systhesis and photochemical characterization of a substituted N-BODIPY compound, by means of a combined theoretical and spectroscopic approach. Compared to a standard BODIPY, the compound under investigation presents a lower fluorescence quantum yield (QY) in the visible region. The excited state relaxation dynamics of the dye was studied in different solvents, showing further fluorescence quenching in polar solvents, and a excited state decay rates strongly dependent on the environment polarity. The role of the pendant moieties and the involvement of charge transfer states in the excited state dynamics was experimentally addressed by transient absorption spectroscopy, and further analyzed with TD-DFT calculations, which allowed precise assignment of the transient signals to the correspondent electronic configuration. The complete picture of the N-BODIPY behavior shows the presence of both charge transfer and localized states, influencing the observed photophysics to different amounts, depending on the excitation conditions and the surrounding environment.

Spin-orbit charge transfer intersystem crossing (SOCT-ISC) is a promising approach to develop heavy-atom-free triplet photosensitizers. However, designing a strong visible-light harvesting heavy-atom-free triplet photosensitizer with efficient ISC ability in various solvents is still challenging. Most of the SOCT-ISC triplet photosensitizers exhibit efficient ISC only in solvent of particular polarity. To address this challenge, herein, two triads (BDP-AN-C-CZ and BDP-AN-N-CZ), composed of carbazole (CZ), anthracene (AN), and bodipy (BDP) moieties, were devised. In these triads, the distance, relative orientation, and position of CZ with respect to the AN moiety were varied to study the effect on photophysical properties, especially on SOCT-ISC efficiency. Electrochemical studies, steady-state, and time-resolved spectroscopies confirmed a sequential photoinduced electron transfer (PET) process in the triads. The fluorescence of the BDP moiety is quenched and a red-shifted CT emission band is observed in the triads, due to the enhanced PET effect, compared to the reference BDP-AN dyad. We observed that the SOCT-ISC yield can be enhanced taking advantage of sequential electron transfer. The triad BDP-AN-C-CZ, in which the CZ moiety was directly linked to the AN moiety, shows an efficient ISC ability both in low-polarity and high-polarity solvents, and unity triplet quantum yield (?T) was observed in dichloromethane. Femtosecond transient absorption spectroscopy confirmed the fast charge separation process (1.8 ps) in BDP-AN-C-CZ as compared to the other triad BDP-AN-N-CZ (4.8 ps) and the reference BDP-AN dyad (7.7 ps). The triads were used as triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion, and high upconversion quantum yield (?UC = 18%) was observed. Interestingly, long-lived (?DF = 118 ?s) and solvent-dependent color-tunable TTA delayed fluorescence was observed in the case of BDP-AN-C-CZ.

Oxoverdazyl (Vz) radical units were covalently linked to the naphthalenediimide (NDI) chromophore to study the effect of the radical on the photophysical properties, especially the radical enhanced intersystem crossing (REISC), which is a promising approach to develop heavy-atom-free triplet photosensitizers. Rigid phenyl or ethynylphenyl linkers between the two moieties were used, thus REISC and formation of doublet (D1 , total spin quantum number S=1/2) and quartet states (Q1 , S=3/2) are anticipated. The photophysical properties of the dyads were studied with steady-state and femtosecond/nanosecond transient absorption (TA) spectroscopies and DFT computations. Femtosecond transient absorption spectra show a fast electron transfer (

We prepared a series of

We prepared a series of meso-thienyl boron-dipyrromethene (Bodipy) derivatives to investigate the spin-orbit charge transfer intersystem crossing (SOCT-ISC). The photophysical properties of the compounds were studied by steady-state and femtosecond/nanosecond transient absorption spectroscopy, as well as density functional theory (DFT) computations. Different from the meso-phenyl Bodipy analogues, the meso-thienyl Bodipy are weakly fluorescent. Based on femtosecond transient absorption and DFT computations, we propose that the torsion of the thienyl group and the distortion of the Bodipy core (19.7 ps) in the S1 state lead to a conical intersection on the potential energy surface as an efficient nonradiative decay channel (408 ps), which is responsible for the observed weak fluorescence as compared to the meso-phenyl analogue. The increased fluorescence quantum yield (from 5.5 to 14.5%) in viscous solvents supports this hypothesis. With the electron donor 4'-hydroxylphenyl moiety attached to the meso-thienyl unit, the fast charge separation (CS, 15.3 ps) and charge recombination (CR, 238 ps) processes outcompete the torsion-induced nonradiative decay and induce fast ISC through the SOCT-ISC mechanism. The triplet quantum yield of the electron donor/acceptor dyad is highly dependent on solvent polarity (ΦT = 1.9-45%), which supports the SOCT-ISC mechanism, and the triplet-state lifetime is up to 247.3 μs. Using the electron donor-acceptor dyad showing SOCT-ISC as a triplet photosensitizer, efficient triplet-triplet annihilation (TTA) upconversion was observed with a quantum yield of up to 6.0%.

Donor–acceptor Stenhouse adducts (DASAs) are negative photochromes that switch with visible light and are highly promising for applications ranging from smart materials to biological systems. However, the strong solvent dependence of the photoswitching kinetics limits their application. The nature of the photoswitching mechanism in different solvents is key for addressing the solvatochromism of DASAs, but as yet has remained elusive. Here, we employ spectroscopic analyses and TD‐DFT calculations to reveal changing solvatochromic shifts and energies of the species involved in DASA photoswitching. Time‐resolved visible pump‐probe spectroscopy suggests that the primary photochemical step remains the same, irrespective of the polarity and protic nature of the solvent. Disentangling the different factors determining the solvent‐dependence of DASA photoswitching, presented here, is crucial for the rational development of applications in a wide range of different media.

Donor–acceptor Stenhouse adducts (DASAs) are a rapidly emerging class of visible light-activatable negative photochromes. They are closely related to (mero)cyanine dyes with the sole difference being a hydroxy group in the polyene chain. The presence or absence of the hydroxy group has far-reaching consequences for the photochemistry of the compound: cyanine dyes are widely used as fluorescent probes, whereas DASAs hold great promise for visible light-triggered photoswitching. Here we analyze the photophysical properties of a DASA lacking the hydroxy group. Ultrafast time-resolved pump–probe spectroscopy in both the visible and IR region show the occurrence of E–Z photoisomerization on a 20 ps time scale, similar to the photochemical behavior of DASAs, but on a slower time scale. In contrast to the parent DASA compounds, where the initial photoisomerization is constrained to a single position (next to the hydroxy group), 1H NMR in situ-irradiation studies at 213 K reveal that for nonhydroxy DASAs E–Z photoisomerization can take place at two different bonds, yielding two distinct isomers. These observations are supported by TD-DFT calculations, showing that in the excited state the hydroxy group (pre)selects the neighboring C2–C3 bond for isomerization. The TD-DFT analysis also explains the larger solvatochromic shift observed for the parent DASAs as compared to the nonhydroxy analogue, in terms of the dipole moment changes evoked upon excitation. Furthermore, computations provide helpful insights into the photoswitching energetics, indicating that without the hydroxy group the 4π-electrocyclization step is energetically forbidden. Our results establish the central role of the hydroxy group for DASA photoswitching and suggest that its introduction allows for tailoring photoisomerization pathways, presumably both through (steric) fixation via a hydrogen bond with the adjacent carbonyl group of the acceptor moiety, as well as through electronic effects on the polyene backbone. These insights are essential for the rational design of novel, improved DASA photoswitches and for a better understanding of the properties of both DASAs and cyanine dyes.

Donor-acceptor Stenhouse adducts (DASAs) are negative photochromes that hold great promise for a variety of applications. Key to optimizing their switching properties is a detailed understanding of the photoswitching mechanism, which, as yet, is absent. Here we characterize the actinic step of DASA-photoswitching and its key intermediate, which was studied using a combination of ultrafast visible and IR pump-probe spectroscopies and TD-DFT calculations. Comparison of the time-resolved IR spectra with DFT computations allowed to unambiguously identify the structure of the intermediate, confirming that light absorption induces a sequential reaction path in which a Z-E photoisomerization of C2-C3 is followed by a rotation around C3-C4 and a subsequent thermal cyclization step. First and second-generation DASAs share a common photoisomerization mechanism in chlorinated solvents with notable differences in kinetics and lifetimes of the excited states. The photogenerated intermediate of the second-generation DASA was photo-accumulated at low temperature and probed with time-resolved spectroscopy, demonstrating the photoreversibility of the isomerization process. Taken together, these results provide a detailed picture of the DASA isomerization pathway on a molecular level.

The understanding of charge carrier dynamics in hybrid materials based on colloidal semiconductor nanocrystals (or quantum dots) and organic moieties is fundamental for the design of efficient photonic and photovoltaic devices. In the present work, we investigate the interactions occurring between CdTe quantum dots, capped with a strong capping agent such as 3-mercaptopropionic acid, and a well known electron acceptor such as methylviologen molecule. The nature of the interactions and of exciton dynamics is investigated by stationary and time-resolved spectroscopies. Luminescence data recorded in presence of increasing methylviologen concentrations, indicate that the organic molecule is able to statically interact with the surface sites of CdTe quantum dots; a biphasic interaction behavior is evidenced by determining the apparent association constants. These latter are obtained through the analysis of luminescence data, and values in the range 10(3)-10(4) are determined. The nature of the interactions is characterized by nanosecond and femtosecond transient absorption spectroscopies, to clarify the dynamics and the conditions able to foster charge mobility. Nanosecond flash photolysis measurements, carried out upon quantum dots excitation, shows the absorption of methylviologen radical cation specie at 605 nm, suggesting the occurrence of electron transfer from CdTe nanocrystals to the organic acceptor; the relatively long decay time of the transient signal (10.3 mu s) indicates that back electron transfer processes are negligible. Ultrafast transient absorption measurements confirm the occurrence of an ultrafast electron transfer process; spectral and kinetic analysis of the transient data show that methylviologen radical cation is formed almost instantaneously on the ps-time scale but mainly when the samples are pumped in the energy continuum at 400 nm. This finding suggests that electron mobility from the nanocrystals to the organic units is achieved mainly when the excitonic states possess an excess of energy. The comparison of the kinetic behaviour of the signals at increasing methylviologen concentrations indicates that the electron transfer process competes with the radiative exciton recombination. In addition, the kinetic data suggests that surface trapping and Auger recombination processes might slow down the charge mobility. (C) 2017 Elsevier B.V. All rights reserved.

We prepared conceptually novel, fully rigid, spiro compact electron donor (Rhodamine B, lactam form, RB)/acceptor (naphthalimide; NI) orthogonal dyad to attain the long‐lived triplet charge‐transfer (3CT) state, based on the electron spin control using spin‐orbit charge transfer intersystem crossing (SOCT‐ISC). Transient absorption (TA) spectra indicate the first charge separation (CS) takes place within 2.5 ps, subsequent SOCT‐ISC takes 8 ns to produce the 3NI* state. Then the slow secondary CS (125 ns) gives the long‐lived 3CT state (0.94 μs in deaerated n‐hexane) with high energy level (ca. 2.12 eV). The cascade photophysical processes of the dyad upon photoexcitation are summarized as 1NI*→1CT→3NI*→3CT. With time‐resolved electron paramagnetic resonance (TREPR) spectra, an EEEAAA electron‐spin polarization pattern was observed for the naphthalimide‐localized triplet state. Our spiro compact dyad structure and the electron spin‐control approach is different to previous methods for which invoking transition‐metal coordination or chromophores with intrinsic ISC ability is mandatory., A rigid, simple, spiro compact electron donor/acceptor (D/A) dyad shows a long‐lived triplet charge transfer (3CT) state (0.94 μs) with a high energy level (ca. 2.12 eV), based on a new electron spin‐control method using spin‐orbit charge transfer intersystem crossing (SOCT‐ISC) (1NI*→1CT→3NI*→3CT; NI=naphthalimide), without heavy atoms or chromophores with intrinsic intersystem crossing (ISC) ability.

Spin-orbit charge-transfer intersystem crossing (SOCT-ISC) is useful for the preparation of heavy atom-free triplet photosensitisers (PSs). Herein, a series of perylene-Bodipy compact electron donor/acceptor dyads showing efficient SOCT-ISC is prepared. The photophysical properties of the dyads were studied with steady-state and time-resolved spectroscopies. Efficient triplet state formation (quantum yield phi(T)=60 %) was observed, with a triplet state lifetime (tau(T)=436 mu s) much longer than that accessed with the conventional heavy atom effect (tau(T)=62 mu s). The SOCT-ISC mechanism was unambiguously confirmed by direct excitation of the charge transfer (CT) absorption band by using nanosecond transient absorption spectroscopy and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The factors affecting the SOCT-ISC efficiency include the geometry, the potential energy surface of the torsion, the spin density for the atoms of the linker, solvent polarity, and the energy matching of the (CT)-C-1/(LE)-L-3 states. Remarkably, these heavy atom-free triplet PSs were demonstrated as a new type of efficient photodynamic therapy (PDT) reagents (phototoxicity, EC50=75 nm), with a negligible dark toxicity (EC50=78.1 mu m) compared with the conventional heavy atom PSs (dark toxicity, EC50=6.0 mu m, light toxicity, EC50=4.0 nm). This study provides in-depth understanding of the SOCT-ISC, unveils the design principles of triplet PSs based on SOCT-ISC, and underlines their application as a new generation of potent PDT reagents.

In order to study the relationship between the molecular structures of compact electron donor/acceptor dyads and the spin-orbit charge transfer intersystem crossing (SOCT-ISC) efficiency, we prepared a perylene (Pery)-naphthalimide (NI) dyad, in which the Pery unit is the electron donor and the NI unit is the electron acceptor, where the two units adopt an almost orthogonal geometry. The photophysical properties of the dyad were studied with steady state and femtosecond/nanosecond transient absorption (fs TA and ns TA) spectroscopies, time resolved electron paramagnetic resonance (TREPR) spectroscopy and DFT computations. A very weak charge transfer (CT) absorption band was observed, but the fluorescence of the Pery unit is strongly quenched. Upon selective excitation into the NI unit, the fast intramolecular CS process (10 ps) occurs, followed by ISC and only the 3Pery* state is observed; whereas upon selective photoexcitation into the perylene unit, an ultrafast charge separation (0.66 ps) is observed, followed by SOCT-ISC (8 ns) to populate the 3Pery* state. Moreover, the perylene radical cation is also observed on the ns timescale, presumably formed by intermolecular charge transfer. The lifetime of the 3Pery triplet state was determined to be ?T = 214 ?s with ns TA spectroscopy. The singlet oxygen quantum yield was measured as ?? = 80%, although the potential energy curve of the torsion between the donor and acceptor is shallow. The SOCT-ISC mechanism was confirmed by TREPR spectroscopy. The dyad was used as a triplet photosensitizer for the generation of delayed fluorescence (luminescence lifetime ?DF = 57.3 ?s).

5,10-dihydro-5,10-dimethylphenazine (PZ) and boron dipyrromethene (Bodipy) were linked to prepare compact electron donor/acceptor dyads and triads. The effect of the molecular geometry on the UV-Vis absorption, fluorescence and the spin-orbit charge transfer intersystem crossing (SOCT-ISC) efficiency were studied. Interestingly, a broad near-IR charge transfer (CT) absorption band (centred at 670 nm) was observed for the dyad/triad showing more coplanar geometry. Triplet state formation was observed in low polar solvents for the triad with orthogonal geometry (triplet state quantum yield is 30 % and lifetime is ?T=210 ?s). Femtosecond transient absorption spectra indicated fast charge separation (CS, 2 ps) and slow charge recombination (CR, >1.5 ns). The low-lying 3CT state of the dyads/triads in polar solvent (1.23 eV vs. the 1.65~1.69 eV of the 3LE state) was confirmed by intermolecular triplet photosensitizing experiments and the spin density surface analysis. These results are useful for design of new broadband, near-IR absorbing heavy atom-free triplet photosensitizers.

We report a combined approach of stationary and time-resolved fluorescence measurements and ultraviolet-visible (UV-vis) transient absorption spectroscopy (TAS) along with ab initio calculations, which provide an overall picture of the dynamics occurring after excitation in a push-pull molecule, namely, 4,7-bis (4,5-dibutylbenzo[1,2-b:4,3-b?]bisthiophene[1,2-b:4,3-b?]bisthiophen-2-yl)-2,1,3-benzothiadiazole. The analysis of the emission spectra in solvents of different polarities reveals the presence of three conformers whose structures differ in the orientation of the 4,5-dibutylbenzo-bisthiophene groups and in their planarity with respect to the benzothiadiazole acceptor group. The Kawski method allows us to estimate the ground- and first-excited state dipole moments (? g and ? e ) for the three conformers. We find values of ? e similar for the three conformers and higher than the relative ? g values as can be expected from a push-pull molecule undergoing a light-induced charge-transfer (CT) transition. UV-vis TAS in different solvents highlights the instantaneous (within our instrumental resolution) formation of a locally excited S 1 state (accompanied by a big change in the dipole moment with respect to S 0 ), which undergoes a rapid intramolecular CT (ICT) assisted by molecule planarization [planar ICT (PICT)]. The strong dipole-dipole interactions with the polarized solvent molecules stabilize the S 1 CT state that decays principally through fluorescence emission. Both PICT and solvation dynamics are responsible for the big Stokes' shift characterizing the molecule, particularly in polar solvents. The fluorescence lifetimes are substantially longer in polar solvents, and also fluorescence quantum yields are higher in polar solvents. We conclude that the radiative relaxation time increases when molecular planarization of the S 1 emissive state takes place, and this condition is favored in polar solvents where local dipole-dipole interactions support the structural stabilization of the CT emissive state. In the poly(methyl methacrylate) matrix, the structural and solvation dynamics are strongly inhibited, leading to reduction of nonradiative processes and to shortening of the fluorescence relaxation time

One of the challenges for achieving efficient exciton transport in solar energy conversion systems is precise structural control of the light-harvesting building blocks. Here, we create a tunable material consisting of a connected chromophore network on an ordered biological virus template. Using genetic engineering, we establish a link between the inter-chromophoric distances and emerging transport properties. The combination of spectroscopy measurements and dynamic modelling enables us to elucidate quantum coherent and classical incoherent energy transport at room temperature. Through genetic modifications, we obtain a significant enhancement of exciton diffusion length of about 68% in an intermediate quantum-classical regime.

A series of Bodipy dimers with orthogonal conformation were prepared. The photophysical properties were studied with steady-state and time-resolved transient spectroscopies. We found the triplet-state quantum yield is highly dependent on the solvent polarity in the orthogonally linked symmetric Bodipy dimers, and the intersystem crossing (ISC) is efficient in solvents with moderate polarity. The photoinduced symmetry-breaking charge transfer (SBCT) in polar solvents was confirmed by femtosecond transient absorption spectroscopy, with the charge separation (CS) kinetics on the order of a few picoseconds and the charge recombination (CR) process occurring on the nanosecond time scale in dichloromethane. These observations are supported by the calculation of the charge separated state (CSS) energy levels, which are high in nonpolar solvents, and lower in polar solvents, thus the CR-induced ISC has the largest driven force in solvents with moderate polarity. These results clarify the mechanism of SOCT-ISC in the orthogonally symmetric Bodipy dimers. The acquired information, relating molecular structure and ISC property, will be useful for devising new strategies to induce ISC in heavy atom-free organic chromophores.

Phenothiazine (PTZ)-anthracene (An) compact electron donor/acceptor dyads were synthesized. The molecular conformation was constrained by rotation restriction to achieve an orthogonal geometry between the electron donor (PTZ) and the electron acceptor (An), with the aim to enhance the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). The substitution positions on the PTZ and An moieties were varied to attain dyads with different mutual orientations of the donor/acceptor as well as different rotation-steric hindrances. The electronic coupling strengths between the electron donor and the acceptor were quantified with the matrix elements (V-DA, 0.04-0.18 eV); the smallest value was observed for the dyad with orthogonal geometry. Charge-transfer absorption and fluorescence emission bands were observed for the dyads, for which the intensity varied, manifested by the V-DA values. The fluorescence of the An moiety was significantly quenched in the dyads, efficient ISC, and the formation of the triplet state were confirmed with nanosecond transient absorption spectroscopy (Phi(Delta) = 65%, tau(T) = 209 mu s). The rotation-steric hindrance was analyzed with potential energy curves, and PTZ was found to be an ideal electron donor to attain SOCT ISC. Time-resolved electron paramagnetic resonance spectra revealed the electron-spin polarization (ESP) of the triplets of the dyads, which is drastically different from that of An, thus confirming the SOCT ISC mechanism. Moreover, we found that the ESP patterns of the dyads strongly depend on the topological features of the molecules and the structure of the electron donor, thus indicating that the relationship between the molecular conformation and the ESP parameters of the triplet state of the dyads cannot be described solely by the orthogonal geometry, as was previously observed.

The triplet excited state properties of two BODIPY phenothiazine dyads (BDP-1 and BDP-2) with different lengths of linker and orientations of the components were studied. The triplet state formation of BODIPY chromophore was achieved via photoinduced electron transfer (PET) and charge recombination (CR). BDP-1 has a longer linker between the phenothiazine and the BODIPY chromophore than BDP-2. Moreover, the two chromophores in BDP-2 assume a more orthogonal geometry both at the ground and in the first excited state (87°) than that of BDP-1 (34–40°). The fluorescence of the BODIPY moiety was significantly quenched in the dyads. The charge separation (CS) and CR dynamics of the dyads were studied with femtosecond transient absorption spectroscopy (kCS = 2.2 × 1011 s–1 and 2 × 1012 s–1 for BDP-1 and BDP-2, respectively; kCR = 4.5 × 1010 and 1.5 × 1011 s–1 for BDP-1 and BDP-2, respectively; in acetonitrile). Formation of the triplet excited state of the BODIPY moiety was observed for both dyads upon photoexci...

Dye-sensitized solar cells (DSSCs) have attracted significant interest in the last few years as effective low-cost devices for solar energy conversion. We have analyzed the excited state dynamics of several organic dyes bearing both cyanoacrylic acid and siloxane anchoring groups. The spectroscopic properties of the dyes have been studied both in solution and when adsorbed on a TiO2 film using stationary and time-resolved techniques, probing the sub-picosecond to nanosecond time interval. The comparison between the spectra registered in solution and on the solid substrate evidences different pathways for energy and electron relaxation. The transient spectra of the TiO2-adsorbed dyes show the appearance of a long wavelength excited state absorption band, attributed to the cationic dye species, which is absent in the spectra measured in solution. Furthermore, the kinetic traces of the samples adsorbed on the TiO2 film show a long decay component not present in solution which constitutes indirect evidence of electron transfer between the dye and the semiconductor. The interpretation of the experimental results has been supported by theoretical DFT calculations of the excited state energies and by the analysis of molecular orbitals of the analyzed dye molecules.

The relationships between the photophysics and structural properties of 4,7-dithien-2-yl-2,1,3-benzothiadiazole as a function of solvent polarity are investigated both experimentally and by computational methods. Stationary fluorescence measurements are consistent with a model envisaging the presence of three types of conformers in equilibrium in the ground state. They are characterized by different relative orientations of the thiophene rings. Due to a low rotational barrier, the sample in solution is characterized by a distribution of relative internal orientations. By applying the Kawski method, we evaluate the average dipole moment of ground and excited states of the three types of conformers. The ground state dipole moments are small and similar for the three types of conformers. On the contrary, dipole moments differ substantially in the excited state. X-ray diffraction of a single crystal confirms the presence of an orientational disorder of thiophene rings. Transient absorption UV-visible spectroscopy experiments allows the identification of the main mechanisms responsible for the large Stokes shift observed in this push-pull molecule. Time dependent spectra provide a picture of the relaxation processes occurring after excitation: the primary step is an internal charge transfer assisted by thiophene ring planarization which occurs on a time scale ranging from 0.88 to 1.3 picoseconds depending on solvent polarity. Moreover, time-resolved fluorescence measurements are consistent with a mechanism involving planarization accompanied by a stabilization of the charge transfer state as observed in polar solvents. In the latter, longer fluorescence lifetimes are observed along with a quantum yield decrease due to the activation of specific non-radiative relaxation channels. The photophysical behavior of 4,7-dithien-2-yl-2,1,3-benzothiadiazole in a solid matrix of polymethyl methacrylate is similar to that observed in solution, but the overall non-radiative process rate is slow with respect to that in the liquid phase. As a consequence, the radiative processes are enhanced giving rise to a fluorescence quantum yield of 90%. Such behavior is consistent with the proposed relaxation model.

The choice of a donor and an acceptor with suitable optical and self-assembly properties is essential in the design of organogel-based light harvesting systems. Organogels can provide supramolecular structures capable of enhancing energy transfer processes. In this work, we present the characterization of N -(naphthalene-1-carboxyamide)-(3 S ,4 S )-pyrrolidin-(3,4)-bisdodecyl-carbamoyldiester ( 1 ) and N -(4-nitrobenzofurazan-7-amino)-(3 S ,4 S )-pyrrolidin-(3,4)-bisdodecyl-carbamoyldiester ( 2 ) which are used as donor and acceptor moieties, respectively. The donor molecule is hardly capable to form a gelon its own but it can be assembled at reasonable concentrations with the acceptor gelator to form a two-component donor–acceptor organogels in cyclohexane. Stable organogels are formed from cyclohexane for gelator concentrations as low as ≈10 −3 M. UV–vis and steady-state fluorescence spectroscopies were used to provide a characterization of their molecular interactions. The optical changes observed during the cooling of two-component solutions of these systems are indicative of typical sol–gel transitions. The occurrence of excitation energy transfer processes in the gels is confirmed by comparison of their excitation and absorption spectra.

The irradiation of colchicone 5 led to the formation of lumicolchicone 7. The same reaction cannot be obtained by using thiocolchicone 6 as substrate. Transient absorption spectroscopy of colchicone and beta-lumicolchicone showed that probably the photoisomerization occurred on colchicone in its first excited singlet state. The spectroscopic data are in agreement with the hypothesis that lumicolchicone was generated in the ground state from the S1 state of colchicone without the presence of any intermediate. Semiempirical calculations on colchicone and thiocolchicone showed that the highest single occupied molecular orbital and the lowest unoccupied molecular orbital of the singlet excited colchicone can give a disrotatory ring closure to 7, while thiocolchicone cannot give the same type of process.

This work presents an experimental and computational study of the intramolecular electronic energy transfer process occurring in two newly synthesized bichromophoric species: N-(7-nitro-2,1,3-benzoxadiazol-4-yl)amino-bis-ethyl-2-[(4-chloro-1-naphthyl)oxy]acetate ( f-Bi ) and N-(7-nitrobenzo[c][1,2,5]oxadiazole-4-yl)-(3S, 4S)-pyrrolidin-3,4-bis-yl-2-[(4-chloro-1-naphthyl)oxy]acetate ( r-Bi ). In both f-Bi and r-Bi the donor chromophore is the [(4-chloro-1-naphthyl)oxy]acetate moiety, whereas the acceptor units belong to the family of the 4-dialkylaminonitrobenzoxadiazoles, well-known fluorescent probes. The two bichromophores differ in the structural flexibility. In f-Bi , acceptor and donors are linked by a diethanolamine moiety, whereas in r-Bi through a (3S, 4S)3,4-dihydroxypyrrolidine ring. By means of steady-state and time-resolved UV–vis spectroscopies we carried out a detailed analysis of the photo-response of donor and acceptor chromophores as individual molecules and when covalently linked in f-Bi and r-Bi . The intramolecular energy transfer process occurs very efficiently in both the bichromophores. The rate constant and the quantum efficiency of the process are k ET = (2.86 ± 0.16) × 10 11 s −1 and Q = 0.998 in f-Bi , and k ET = (1.25 ± 0.08) × 10 11 s −1 and Q = 0.996 in r-Bi . Semiempirical calculations were utilized to identify the energy and the nature of the electronic states in the isolated chromophores. Molecular mechanics calculations have been performed to identify the most stable structures of the bichromophoric compounds. The predictions of Forster theory are consistent with the experimental results and provide a suitable way to evaluate the structural differences between the two compounds.

We have analyzed the excited state dynamics of the heteroleptic [(NCS)2Ru(bpy-(COOH)2)(bpy-(C6H13)2)] Z907 solar cell sensitizer in solution and when adsorbed onto thin TiO2 films, by combining transient visible and infrared (IR) spectroscopies with ab initio Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) calculations. Upon excitation with ultra-short pulses in ethanol and dimethyl-sulphoxide solutions, the visible spectra show the appearance of a positive signal around 650 nm, within the instrumental time resolution (2) charge transfer (LML'CT). Vibrational cooling is observed in both solvents; in ethanol it is overtaken by the hydrogen bond dynamics. On the basis of DFT/TDDFT calculations, explicitly modeling the interaction of the NCS and COOH groups with solvent (ethanol) molecules, we rationalize the observed IR and visible spectral evolution as arising from the change in the hydrogen-bond network, which accompanies the transition to the lowest-energy triplet state. This interpretation provides a consistent explanation of what is also observed in the transient visible spectra. Transient IR measurements repeated for molecules adsorbed on TiO2 and ZrO2 films, allow us to identify the structural changes signaling the dye triplet excited state formation and evidence multiexponential electron injection rates into the semiconductor TiO2 film.

The valence tautomerism of low-spin Co III (Cat-N-BQ)(Cat-N-SQ) (where Cat-N-BQ is 2-(2-hydroxy-3,5-di- tert -butylphenylimino)-4,6-di- tert -butylcyclohexa-3,5-dienone and Cat-N-SQ is the dianionic radical analogue) was investigated by means of UV–vis pump–probe transient absorption spectroscopy and 1 H NMR technique in chloroform and dichloromethane. By exciting the CT transition of the complex at 480 nm, an intramolecular electron transfer process is selectively triggered. The photo-induced charge transfer is pursued by a cascade of two main molecular events characterized by the ultrafast transient absorption spectroscopy: the first gives rise to the metastable high-spin Co II (Cat-N-BQ) 2 that, secondly, reaches the chemical equilibrium with the reactant species. The rate constant of back valence tautomerization estimated by measuring the lifetime of high-spin Co II (Cat-N-BQ) 2 species and the equilibrium constant for the Co III (Cat-N-BQ)(Cat-N-SQ) ⇄ Co II (Cat-N-BQ) 2 interconversion, is significantly large (on the order of 10 9 s −1 ). It is interpreted under the point of view of the theory formulated by Jortner and Buhks et al. for non-adiabatic radiationless processes.

The valence tautomerism of low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) was investigated by means of UV-vis pump-probe transient absorption spectroscopy in chloroform. By exciting the CT transition of the complex at 480 nm, an intramolecular electron transfer process is selectively triggered. The photo-induced charge transfer is pursued by a cascade of two main molecular events characterized by the ultrafast transient absorption spectroscopy: the first gives rise to the metastable high-spin CoII(Cat-N-BQ)2 that, secondly, reaches the chemical equilibrium with the reactant species.

Transient spectra for 4‘-(N,N-dimethylamino)-3-hydroxy- (1a) and 4‘-(N,N-dimethylamino)-3-methoxyflavone (2a) in several solvents at room temperature are reported following excitation with pulses o...

Intramolecular electron transfer in the excited state of a [(bpy)ReI(CO)3(L)]+ complex (bpy = 2,2‘-bipyridine), in which L contains a pendant azacrown ether that acts as an electron donor (L = N-[4...

The exciton generation and recombination dynamics in semiconductor nanocrystals are very sensitive to small variations in dimensions, shape and surface capping. In the present work CdTe quantum dots are synthesized in water using 3-mercaptopropionic acid and 1-thioglycerol as stabilizers. Nanocrystals with an average dimension of 4.0 ± 1.0 and 3.7 ± 0.9 nm were obtained, when 3-mercaptopropionic acid or 1-thioglycerol, respectively, was used as a capping agent. The steady-state characterization shows that the two types of colloids have different luminescence behavior. In order to investigate the electronic structure and the dynamics of the exciton state, a combined study in the time domain has been carried out by using fluorescence time-correlated single photon counting and femtosecond transient absorption techniques. The electron–hole radiative recombination follows the non-exponential decay law for both colloids, which results in different average decay time values (of the order of tens of nanoseconds) for the two samples. The data demonstrate that the process is slower for 1-thioglycerol-stabilized colloids. The ultrafast transient absorption measurements are performed at two different excitation wavelengths (at the band gap and at higher energies). The spectra are dominated in both types of samples by the negative band-gap bleaching signals although transient positive absorption bands due to the electrons in the conduction band are observable. The analysis of the signals is affected by the different interactions with the defect states, due to ligand capping capacities. In particular, the data indicate that in 1-thioglycerol-stabilized colloids the non-radiative recombination processes are kinetically more competitive than the radiative recombination. Therefore the comparison of the data obtained from the two samples is interpreted in terms of the effects of the capping agents on the electronic relaxation of the colloids.

We have measured the Raman spectrum of natural and 99% isotope 35 chlorine crystal, in the lattice and in the internal mode regions, using a high resolution Raman spectrometer. We report the temperature dependence of the linewidth for both the lattice and the internal vibrational modes. The data were analyzed in terms of anharmonic interactions and contributions from the isotopic disorder. The lattice phonons are little affected by the isotopic disorder, while the internal vibrons are extremely sensitive to the presence of isotopic impurities. In the latter case the linewidths of the Ag and B3g internal vibrons are about one and two orders of magnitude, respectively, larger in the natural than in the isotopically pure sample. The Raman spectrum of natural chlorine in the internal region is correctly reproduced by an harmonic lattice dynamics calculation weighted over the statistical distribution of the isotopic species in the lattice.

In this work, we present the characterization of an enantiomeric pair of fluorescent dye organogelators and the properties of their stable gel at low concentration in organic solvents. The gels of both enantiomers and of their mixtures were analyzed by differential scanning calorimetry, circular dichroism (CD), atomic force microscopy, UV–vis absorption, and fluorescence. The acquired data were supported by molecular modeling of the helical assembly of the gelators and by the simulation of their CD spectra by means of DeVoe method, and suggested the occurrence of an enantiomeric discrimination process during the formation of the gels. Chirality, 2011. © 2011 Wiley-Liss, Inc.

A new chiroptical compound, (E)-(R(p),R(p))-1,2-bis{4-methyl-[2]paracyclo[2](5,8)quinolinophan-2-yl}ethene (trans-RPQE) has been synthesized, and its photoresponse has been investigated through steady state and time-resolved absorption and emission spectroscopies and theoretical calculations. To elucidate the relaxation mechanism of trans-RPQE after photoexcitation, the photophysics of the 2,4-dimethyl-[2]paracyclo[2](5,8)quinolinophane chromophore has also been studied. The quantum yields of the different relaxation paths for trans-RPQE have been determined. It emerges that in addition to thermal and radiative routes, trans-RPQE also photoisomerizes with a quantum yield of 8%. Trans- and cis-RPQE isomers are pseudoenantiomers exhibiting appreciably different CD spectra, whereby RPQE can be a model for the design of new promising chiroptical photoswitches.

The results of a sub-picosecond transient absorption spectroscopy study on a mononuclear and two dinuclear low-spin iron(II) complexes is reported. The dinuclear derivatives are homonuclear (i.e. Fe–Fe) and heterodinuclear (Fe–Zn) in nature. The ligands we used were 2-pyridylmethyl-ketazine and 2-pyridylmethyl-hydrazone. Irradiation was made on the metal-to-ligand CT band occurring around 500 nm. The observed pattern of the relaxation decays is consistent with the population of the metastable 5 T 2 ligand field state within the first 100 fs after the photon absorption from the three different chromophores. The suggested implication of triplet intermediate states was not detected. The ground state recovery was observed to occur with a time constant of 350 ps for the mononuclear complex and 1600–1800 ps for the two dinuclear complexes.

A dizinc complex with a polyamine macrocycle is able to selectively bind and sense uridine (U) as well as the uridine-containing ribodinucleotides U(3'-5')pU and U(3'-5')pA, thanks to an exciplex emission arising from a pi-stacked complex involving the dipyridine unit and Zn(II)-bound uridine moieties.

Experiments were performed to elucidate the excited-state behavior of 9-phenylphenalenones, which are phototoxic plant secondary metabolites involved in mechanisms of light-mediated plant defense. Using a combination of time-resolved and steady-state UV/visible spectroscopies, time-resolved IR absorption spectroscopy, time-resolved singlet oxygen phosphorescence measurements and cyclic voltammetry, we provide evidence of an intramolecular charge-transfer process in the excited singlet and the triplet states of 9-phenylphenalenones that modulates the photosensitized production of singlet oxygen.

Photochromic materials find wide use in the ophthalmic industry and it is becoming clear that future ophthalmic products will increasingly incorporate naphthopyrans (especially naphtho[2,l- b]pyrans - structure 1) as their photochromic element. The attraction of the naphthopyrans as photochromic materials stems from their broad range of intense vibrant colors combined with control over the rate of fade and resistance to fatigue. The mechanism of photochromism is shown in Scheme 1 for 3, 3-diaryl-3H-naphtho[2,l-b]pyrans. It is conventionally accepted that the trans open isomers are preferred, with the CT form preferred to the TT form. The excited state responsible for the ring scission process was proposed as the excited singlet state on the basis of nanosecond flash photolysis measurements and this is borne out by the ultrafast work of Aubard et al who report a lifetime for ring scission of 450 fs from the first excited singlet state for 1 in acetonitrile. This chapter discusses similar measurements on three naphtho[2,l-b]pyran derivatives.

The photochemical reactivity of colchicine and thiocolchicine is described. Although the irradiation of colchicine gave a well-known transposition reaction to β- and γ-lumicolchicines, thiocolchicine did not react. Femtosecond transient spectroscopy of colchicine showed a strong band with maximum at 510 nm appearing at τ = 0. It disappeared within few hundred femtoseconds, leaving a broad structureless band with a maximum around 470 nm. A second band is observed around 410 nm. The analysis in time showed that the 510-nm component appeared instantaneously and decayed following a biexponential low with time constants of 300 ′ 100 fs and 40 ps. The kinetics at 420 nm has a measurable rise time of 300 ′ 150 fs. Quantum mechanical calculations on colchicine showed that this absorption is due to a S 1 → S 1 1 transition. In thiocolchicine, the instantaneous formation of a structure with maxima out of the investigated spectral region was observed. A strong absorption around 650 nm indicated the presence of a band with a maximum at longer wavelengths (> 700 nm) and a peak around 380 nm, which partially coincides with the ground-state absorption and therefore strongly affected by its bleaching. The instantaneous formation of an absorption around 650 nm and its rapid (∼500 fs) decay was observed. At shorter wavelengths (400 nm), the decay was fitted with a biexponential curve with the first time constant of about 80 ps. The second part of the decay had a very long tail up to 500 ps. Transient spectroscopy and configuration interaction calculations are in agreement with a mechanism involving a disrotatory cyclization of colchicine in its first excited singlet state. The lack of reactivity observed in thiocolchicine was explained by considering the presence of efficient ISC to the triplet state.

Excited state intramolecular proton transfer (ESIPT) in 1-chloroacetylaminoanthraquinone (CAAQ) and 1,8-dihydroxyanthraquinone (DHAQ) in solution is investigated at room temperature by means of femtosecond transient absorption spectroscopy. While ESIPT is practically instantaneous in DHAQ, in CAAQ the same process occurs with measurable kinetics, with a time constant of 100 fs. The energy diagrams for the ground and excited states of the proton-transferred and non-proton-transferred species, compatible with these observations, are discussed and an estimate is given for the activation energy of the process.

The pigments of a fourteenth-century miniature were analyzed through the combined efforts of two powerful spectroscopic techniques, particle-induced X-ray emission (PIXE) and micro-Raman. The use of these two complementary, non-destructive techniques provided more information than either method alone, permitting unambiguous identification of many of the pigments used in the creation of the miniature. The artist used a limited range of pigments, apparently preferring to use mixtures of materials rather than introduce additional pigments. A restoration to one area of the miniature used identical pigments to the original.