Fluorescence and absorption spectra of oligophenylenevinylenes: Vibronic coupling, band shapes, and solvatochromism

Abstract

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.

Structure, photophysics, and the order-disorder transition to theβphase in poly(9,9-(di-n,n-octyl)fluorene)

“…More likely it represents a superposition of modes with the strongest Huang-Rhys coupling parameters. To represent these "two" sub-bands we follow the nomenclature of Gierschner et al 49 and specify the two peaks, with approximate offsets of 160 meV and 200 meV, as a 1 and b 1 modes respectively.…”

Wavelength and Temperature Dependence of the Femtosecond Pump−Probe Anisotropies in the Conjugated Polymer MEH-PPV: Implications for Energy-Transfer Dynamics

“…The actual absorption line shape of MEH-PPV probably involves additional vibronic progressions, 45 a more complicated inhomogeneous distribution where different frequency components have different line shapes, 46 and may also involve vibrational frequency changes in the excited state. 47 Nevertheless, this simple model manages to capture the essence of the dispersive transport, as we show below. Figure 7 shows an example of two different fluorescence spectra obtained by excitation at 17 500 and 19 000 cm -1 respectively.…”

Picosecond Structural Relaxation of Abietic Acid Based Amine End Capped Para‐Phenylenevinylene Trimers in Solution

“…Given the weakly polar nature of solutes and solvent, the large Stokes shift suggests conformational differences between the ground-and the excited state, [16] with more planar species in the relaxed excited state, absorbing and emitting at lower energy. Table 2 shows that increasing the van der Waals volume of the AECPV3 end-groups and/or side chains, the shortest decay time t 3 increases.…”