Search

The energy-transfer properties of a regioregular silylene-spaced alternating donor–acceptor copolymer [(donor)3-SiMe2–acceptor-SiMe2]m (denoted (D3A)m) are determined with time-resolved spectroscopy; 4,4′-divinylbiphenyl serves as the donor and 4,4′-divinylstilbene as the acceptor. With excitation at wavelength 266 nm, fluorescence up-conversion curves at various detection wavelengths are measured. The observed decay of donor emission at a time constant 0.3 ps and the emission of the acceptor have a corresponding fast rise and a slower second rise at ∼2 ps. In the tail of fluorescence emission, the varied slow decay components (2–7 ps) on the donor and rise on the acceptor are observed and assigned to intrachain aggregates. With the Coulomb coupling which is beyond the multipole approximation, the Fermi golden rule rates of energy transfer are calculated between adjacent monomers and between the D2 exciton and A. In all D2A and D3A conformers calculated, we found that the coupling strengths of D–A and D–D...

By measuring the vibrational wavenumbers of their stretching modes in Raman and surface-enhanced Raman scattering (SERS) spectra, we investigated the strength of the Cr-Cr bonds in metal-string complexes Cr 5 (tpda) 4 X 2 and Cr 7 (teptra) 4 (NCS) 2 (tpda = tripyridyldiamido; teptra = tetrapyridyltriamido; X = Cl ― , NCS ― ). The bands in SERS and Raman differ insignificantly in spectral positions, indicating no major structural variation between the solid and solution forms. For SERS measurements, these complexes were bound to silver or gold nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice and to maintain the complexes in thermal equilibrium; this method is convenient to identify the stable structure. We identified both penta- and heptachromium complexes in both symmetric (s) and unsymmetric (u) forms. For pentachromium complexes, our data agree with the results obtained from structural determination of the crystalline form, but for the heptachromium complex, this experimental evidence is the first for the existence of the u-form structure. From our analysis of the vibrational normal modes, we assign the band at 280 cm ―1 to the Cr―Cr symmetric stretching mode of the s-form pentachromium complex. According to comparisons of SERS spectra obtained at either high temperatures or under oxidizing conditions, we assign 570 cm ―1 to the stretching mode of the Cr―Cr quadruple bond in the u-form for the pentachromium complex and 554/571 cm ―1 analogously for the heptachromium complex. The bands for metal-related modes in SERS spectra might be enhanced because of interaction with the metal nanoparticles. The metal-string complexes with a linear arrangement of metal ions have an increased absorption coefficient in the visible spectra and, consequently, an increased resonance Raman intensity for the metal-metal stretching modes, yielding information about the strength of chromium―chromium multiple bonding.

We recorded infrared, Raman, and surface-enhanced Raman scattering (SERS) spectra of metal−string complexes Ni5(tpda)4X2 and Co5(tpda)4X2 (tpda = tripyridyldiamido, X = Cl−, NCS−) and free ligand tripyridyldiamine (H2tpda) to determine their vibrational wavenumbers and the strength of the metal−metal bonds. For SERS measurements, these complexes were adsorbed on silver or gold nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice and to maintain the complexes in thermal equilibrium. The spectra of SERS and Raman modes show insignificant deviation in spectral features and band positions. We observe a single breathing band of pyridyl in Co5(tpda)4X2, indicating the existence of only the symmetric form, whereas split pyridyl lines are observed for Ni5(tpda)4X2 and assigned to arise because of a varied environment of coordination: square planar for the inner nickels and square pyramidal for the outer nickels in the complexes. From our analysis of the vibrational normal modes, we a...

We calculate the electron transfer (ET) rates for a series of heptacyclo[6.6.0.02,6.03,13.014,11.05,9.010,14]-tetradecane (HCTD) linked donor–acceptor molecules. The electronic coupling factor was calculated by the fragment charge difference (FCD) [19] and the generalized Mulliken–Hush (GMH) schemes [20]. We found that the FCD is less prone to problems commonly seen in the GMH scheme, especially when the coupling values are small. For a 3-state case where the charge transfer (CT) state is coupled with two different locally excited (LE) states, we tested with the 3-state approach for the GMH scheme [30], and found that it works well with the FCD scheme. A simplified direct diagonalization based on Rust's 3-state scheme was also proposed and tested. This simplified scheme does not require a manual assignment of the states, and it yields coupling values that are largely similar to those from the full Rust's approach. The overall electron transfer (ET) coupling rates were also calculated.

The energy-transfer properties of a regioregular silylene-spaced alternating donor−acceptor copolymer (donor−SiMe2−acceptor−SiMe2)m are determined with time-resolved spectroscopy; 4,4′-divinylbiphenyl serves as donor and 4,4′-divinylstilbene as acceptor. Steady-state fluorescence spectra show that the energy transfer is efficient. With excitation at 266 nm, fluorescence up-conversion curves at various detection wavelengths are measured to explore the detailed dynamics of the energy transfer. A rate ∼(0.3 ps)−1 for energy transfer between the S1 states of the donor and acceptor moieties is observed. Theoretical calculations based on fragment excitation difference provide a rate of energy transfer near the experimental value with detailed insight into the mechanism of energy transfer. The results of theoretical calculations indicate that the Coulomb interaction is dominant and the Dexter process is weak. The high-multipole interactions are important for energy transfer.

The relaxation dynamics of excited electronic states of [(1,1'-biphenyl)-4,4'-diyldi-2,1-ethenediyl]bis(dimethylsilane) dissolved in various solvents with varied polarity and viscosity have been investigated. Upon excitation at wavelength 266 nm, we measured the fluorescence curves that exhibit a rise time constant approximately 100 fs, and two decay time constants, 7-65 ps and approximately 1 ns. We attribute the former decay to upper excited states to the S(1) state, and the latter decay to geometric relaxation and the lifetime of the S(1) state. Only the tens of picosecond decay shows a dependence on the solvent viscosity, indicating that the torsional motion dominates the relaxation. Theoretical calculations were performed to obtain the optimized structures of the free [(1,1'-biphenyl)-4,4'-diyldi-2,1-ethenediyl]bis(dimethylsilane) molecule in its ground and first excited states with methods B3LYP/6-311G(d) and CIS/6-311G(d), respectively. The results of these calculations show that the dihedral angle between the two phenyl rings is approximately 34 degrees for trans and approximately 38 degrees for cis conformers in the ground state and that the first excited state has a planar structure, in agreement with the experimental results that indicate that the torsional motion of two phenyl groups elevates the relaxation of the S(1) state. Enhanced vibrational relaxation of S(1) in alcoholic solvents is observed. Rapid relaxation in methanol-OH compared with that in methanol-OD is explained by the excess energy dissipated efficiently through high-frequency vibrational mode (500 cm(-1)).

Atomic force microscopy (AFM) is employed to unveil the film morphology of 2,2‘-bistriphenylenyl (BTP), an all sp2-hybridized polyaromatic blue emitter derived from two triphenylenes (TP) in the OLED layout (organic light-emitting diode). It is generally believed that molecules with such planar cores require derivatization or steric hindrance to hamper intermolecular π−π stacking and reduce self-absorption. X-ray crystallographic results of BTP crystals show that the two triphenylenyls are coplanar, indicative of a likely tendency toward self-quenching. Surprisingly, AFM reveals grainy nanoaggregates of the BTP layer in the device, yet BTP films exhibit excellent performance for OLED applications. To correlate luminescent spectra with the molecular structure, BTP films of crystalline, nanoaggregated, and amorphous features are prepared and subjected to spectroscopic studies. These AFM and spectroscopic results in conjunction with simulation results of a small torsion barrier to rotate the TP rings suggest...

Fluorescence decays of benzosultine and naphthosultine excited at 263 nm are detected with time-correlated single-photon counting (TCSPC). Both molecules display biexponential decay with a rapid component (time constants 90 and 350 ps for benzosultine and naphthosultine, respectively) assigned to an electronically excited state S n at high energy and the slow one (constants 7.5 and 9.0 ns for benzosultine and naphthosultine, respectively) to the S 1 state. Dissociation of both molecules was investigated with nanosecond laser flash photolysis combined with transient absorption to detect the intermediates and products. With excitation at 266 nm, benzosultine yields a transient with absorption maximum at λ max = 370 nm; this transient has a short-lived component with lifetime around 1 μs and a long-lived component. Both components are insensitive to molecular oxygen. The short-lived component is tentatively assigned to the dissociation intermediate and the long-lived to be singlet o -benzoquinodimethane ( o -BQDM). Photolysis of naphthosultine yields two transient species with absorption at λ max = 420 and 520 nm; we assign the former band to triplet–triplet absorption of naphthosultine and the latter to absorption by the product singlet o -naphthoquinodimethane. Optimal geometries, energetics, and vertical transitions of benzosultine, naphthosultine, o -benzoquinodimethane, and o -naphthoquinodimethane are calculated using methods based on density-functional theory (B3LYP/6-311++G**) and time-dependent density-functional theory (TD-DFT). The results of these calculations imply that the ground electronic state of these two o -quinodimethanes ( o -QDM) is singlet with a structure of diene form. Their triplet states display a biradical structure. The energy separation between singlet and triplet states of o -benzoquinodimethane is calculated to be ca. 93.6 kJ/mol, but for o -naphthoquinodimethane, it is only ca. 27.8 kJ/mol.

Fluorescence excitation spectra for transition A 1Au–X 1Ag of trans-glyoxal in a supersonic jet were recorded with laser excitation in a wavelength region near 395 nm. With resolution 0.04 cm−1, most lines in these spectra are resolved and are assigned rotationally; of six bands assigned, three have c-type rotational structure, and another three have types a/b hybrid, a and b. Fluorescence decays with quantum beats, resulting from coherent excitation of S1 and T1 states, are observed for most rotational levels in this region. In spectra transformed to the frequency domain for these decays, the widths are larger than those obtained at low excitation energy. The widths from nonzero frequency lines, reflecting mostly the triplet character, indicate that the triplet state is dissociating. Fluorescence spectra of fragment HCO confirm that the dissociation channel correlated to the triplet surface of glyoxal involves formation of radical products. From an abrupt decrease of lifetime of the triplet state and on...

The red and orange emitters (ANA-1-3) consisting of a 4-amino-1,8-naphthalic anhydride group were synthesized. The lowest absorption band of these ANA molecules centered at approximately 450 nm is assigned to be a charge-transfer transition with emission at 514-536 nm in nonpolar solvents such as n-hexane and at approximately 590-640 nm in polar solvents such as THF and CH(2)Cl(2) and in the solid states. Emission lifetimes are measured with time-correlated single photon counting. Shorter lifetimes are observed for the ANA molecules when dissolved in polar solvents compared with those in nonpolar solvents. Strong dipole-dipole interaction of ANA molecules with solvents is indicated. At high concentrations the measured emission lifetimes, generally shortened from self-quenching, are found to remain about the same order of magnitude in ANAs. This implies that the exciton states of aggregates are formed and they exhibit a relatively long lifetime. Crystallographic data of 4-(phenyl antracen-9-yl) (ANA-2) and 4-(phenyl-2-naphthyl) amino-1,8-naphthalic anhydrides (ANA-3) show that the molecules exist as dimeric structures with antiparallel head-to-tail stacking of naphthalic anhydride planes in addition to other pi-pi stacking. The strong dipole-dipole interactions and the pi-pi stacking account for the observed red-shifted emissions of ANAs in the powders. For films prepared from vacuum sublimation, a structure similar to that in the crystal but with less crystalline order is expected based on the emission wavelength. Several electroluminescent devices based on these ANAs are reported here; they emit orange-red light at 602-628 nm with high brightness and steady external quantum efficiency.

The dynamics of photodissociation of glyoxal (HOC-COH) near the dissociation threshold on the triplet manifold are studied through measurement of distributions of nascent fragment HCO in various internal states. Three rotational levels 1(01) (*), 4(13) (*), and 3(21) (*)+3(22) (*) of vibrational state U (excitation wavelength approximately 394.4 nm, origin at 25,331.865 cm(-1)) of glyoxal in state A (1)A(u) and two other vibrational states at excitation wavelengths 390.33 and 382.65 nm are selected to produce fragment HCO. By means of fluorescence in the transition B (2)A(')-X (2)A(') of HCO, we determined the relative populations of internal states of that fragment. Rotational states of product HCO up to N=26 and K=2 are populated, and bimodal distributions of these rotational states are observed for the photolysis wavelengths used in this work. The high rotational part of the distribution with average energy near values calculated on the basis of the statistical model-phase-space theory is assigned to arise from glyoxal on its S(0) surface, and the low rotational part from the T(1) surface with an exit barrier. After photolysis near the threshold region on the triplet surface, HCO arising from the T(1) state appears to be a major component of products because these rotational levels 1(01) (*), 4(13) (*), and 3(2) (*) of U state selected are gateway states with an enhanced rate of intersystem crossing.