Fluorescence emission and excitation spectra of para-phenylene vinylenes nPV with nϭ1 -4 styryl units are investigated experimentally and theoretically as a function of the temperature and the polarizability of the solvent. At low temperatures, the vibronic structures of the S 0 ↔S 1 emission and excitation bands are mirror symmetrical with negligible 0-0 energy gaps. The frequencies of the prominent vibrational modes are assigned to the second longitudinal acoustic phonon modes of the entire molecules and to localized carbon-carbon stretching vibrations. The complete vibronic structures of the spectra are calculated at the ab initio Hartree-Fock (HF/6-311G*) and restricted configuration interaction singles (RCIS/6-311G*) levels of theory assuming planar C 2h molecular symmetry. The theoretically predicted spectra are in good agreement with the experiments. At room temperature, a 0-0 energy gap between the first band maxima opens, and the mirror symmetry between absorption and emission is lost. The vibronic band shapes and 0-0 band gaps are successfully simulated with a combination of Gaussian and exponential broadening of the low temperature spectra. The exponential term reflects the differences in thermal population of the phenyl-vinyl torsional modes in the S 0 and S 1 electronic states. Spectral shifts upon changes in temperature and solvents are quantitatively explained by changes in the refractive index of the environment. From extrapolation of the experimental data the vertical and adiabatic transition energies of the oligomers in vacuo are obtained and compared to RCIS and semiempirical quantum chemical calculations, respectively.

The present Perspective critically re-examines the photophysics of paradistyrylbenzene (DSB) as a prototype of herringbone-arranged H-aggregates to resolve the apparent contradiction of the frequently reported "aggregation-induced emission quenching" in H-aggregates on one side and highly emissive DSB crystals on the other and discusses the signatures and fate of excitons in single-and polycrystalline samples, including size and polarization effects.

Solid state luminescence enhancement (SLE) of conjugated organic materials has had a great impact in materials science, but a deep understanding has been rather limited to date. Here, we investigate a prototype example of SLE materials, cyano-substituted distyrylbenzene (DCS), by varying systematically and subtly the substitution pattern (inter alia of the position of the cyano-substituent) to give largely different photoresponse in fluid and solid solution as well in the crystalline state. The combination of quantitative (ultra)­fast optical spectroscopic techniques, appropriate quantum-chemical methods, and structural (X-ray) data allows us to elucidate and rationalize all details of the SLE process, including steric versus electronic factors, radiative versus nonradiative decay channels, and intra- versus intermolecular contributions, providing a first holistic picture of SLE.

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The influence of environmental factors on the degradation process of P3HT film has been investigated quantitatively. The decay kinetics of the polymer absorption during variation of intensity and spectral distribution of the incident light, oxygen concentration, humidity level as well as temperature are monitored using infrared and UV/vis absorption spectroscopy. Additionally, the oxygen diffusion into the polymer film has been investigated using fluorescence spectroscopy under the same experimental conditions. The degradation process is light initiated with a strong increase of the effectiveness toward the ultraviolet region of the spectrum. The observed photo oxidation is not oxygen diffusion limited although an activation energy of 26 kJmol -1 was observed for both degradation and oxygen diffusion. The observed kinetics, especially its dependence on wavelength of the incident light, point to a radical-based degradation process in the solid state rather than a singlet oxygen-based mechanism as it is observed in the liquid phase. Furthermore the presence of humidity strongly affects the degradation process although water itself does not decompose the polymer. Changing the structure of the polymer from regioregular to regiorandom significantly accelerates the degradation, probably due to the higher triplet yield of the regiorandom polymer.

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We investigated the transient bleaching and absorption of the asymmetric core/shell CdSe/CdS nanorods using the pump-probe technique. We observed ultrafast carrier relaxation and identified hole localization dynamics with 650 +/- 80 fs time constant. Upon pumping the CdSe core, we found an intense bleaching signal in the CdS spectral region, which we assigned to the delocalization of the electronic wave function on the basis of envelope-function theoretical calculations.