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A combined DFT/TDDFT approach has been applied for calculating the Huang-Rhys (HR) parameters along the totally symmetric normal coordinates for the 1(1)A(g)(1(1)A(1)) <--1(1)B(u) (1(1)B(2)) electronic transition in a series of oligothiophenes containing from 2 to 6 thiophene rings. The calculations required optimized molecular geometries for both the ground state and the excited molecular state. The excited state geometry optimization was carried out by means of the time-dependent density functional theory (TDDFT) based methodology implemented in the Turbomole 5.9 (1) package of programs. The results for the three smallest oligothiophenes were verified by generating the theoretical vibronic structures and comparing them with the high-resolution fluorescence spectra measured for matrix-isolated molecules. For bithiophene a comparison was also made of the theoretical results obtained for different basis sets and the most popular exchange-correlation functionals. The best results were then confronted with the HR parameters based on the molecular geometries calculated at the CASSCF level of theory. The results obtained within the DFT/TDDFT approach are in very good agreement with the available experimental data for bithiophene, terthiophene, and quaterthiophene molecules.

A model is presented that allows calculation of excitation profiles, depolarization ratios, and resonance Raman spectra of doubly degenerate (e-type) modes. It consists of a doubly degenerate excited (E-type) electronic state and a nondegenerate excited A -or B -type electronic state. The sources of Raman scattering by the e-type modes are pseudo-lahn-Teller (Herzberg-Teller) interactions between these excited states and lahn-Teller and/or Renner-Teller interaction within the E-type electronic state.Molecules of trigonal, pentagonal, and hexagonal symmetries are investigated for different coupling schemes and different energy separations between the two excited states. The results are presented in the form of three-dimensional graphs for the fundamental and the first two overtones together with the corresponding absorption spectrum. The model also allows one to follow changes in resonance Raman scattering produced by trigonal distortion of molecules possessing a triply degenerate electronic state in resonance with the incident light. The case of resonance scattering in the region of a forbidden nondegenerate state which is likely to occur in hexagonal molecules such as benzene is also discussed.

The CD spectra of (6R,15R)-(+)-6,15-dihydro-6,15-ethanonaphtho [2,3-c]pentaphene (DEP) and (7R,14R)-(+)-7,14-dihydro-7,14-ethanondibenz [a,h] anthracene (DEA) are analyzed as dimers of anthracene and naphthalene, respectively. It is shown that vibronic coupling effects are essential in understanding the vibronic structure of CD spectra in these molecules. The simple molecular dimer model is shown to explain the main vibronic features of the CD spectra, but an extension of that model is needed to achieve good quantitative agreement between experimental and theoretical CD spectra for physically reasonable exciton and vibronic coupling parameters. This extended dimer model allows for a mixing of electronic states of monomers via electronic intermonomer interaction. It is shown that the CD spectrum of DEP locates the hidden 1Lb state of the anthracene molecule about 2150 cm−1 above the origin of the 1La state, and that these two states are electronically coupled in DEP. Only these two assumptions taken together can well explain the CD and absorption spectra of DEP for the same set of parameters. We show that the CD spectra of DEA indicate strong mixing of the 1Lb and 1Bb and of the 1Bb and 1Cb states of naphthalene molecule via intermonomer interaction. The calculations presented here point out that the main reasons of previous failure to explain the vibronic structure of the CD spectra of DEP and DEA is the brakedown of the Born–Oppenheimer approximation in these dimers.