Vibronic fine structure in the absorption spectrum of oligothiophene thin films

Abstract

A multimode Holstein Hamiltonian is used to describe optical excitations in quaterthiophene pinwheel aggregates. The Hamiltonian includes the coupling of excitons originating from the 1A(g)-->1B(u) electronic transition to phonons originating from the five intramolecular vibrational modes known from oligothiophene solution absorption/emission spectroscopy. The resulting eigenstates with lowest energy are best described as hybrid polaron phonons. The polarons are formed by coupling excitons with the higher frequency (688, 1235, and 1551 cm(-1)) vibrational modes, while the (optical) phonons arise from the lower frequency (161 and 333 cm(-1)) modes. The polaron phonons are responsible for the fine structure defining the A(1) band in the low-energy region of the absorption spectrum, ranging from the band origin to approximately 1500 cm(-1) beyond. The calculated A(1) band of quaterthiophene aggregates agrees favorably with that observed from thin films.

Beyond the strong vibronic coupling approximation – sexithiophene single crystal

“…In the calculations of the consecutive spectra the cut-off range of the resonance interaction is increased; the corresponding lattice sums are consistently calculated with the respective cut-off range. In the simplest case (short-dash line) only the firstand second-nearest neighbor interactions are included, as it was often done in the literature [2][3][4][5]. While the lowenergy vibronic bands look qualitatively reasonable, their intensity in c polarization is much too large; more importantly, the energy gap between the b-polarized absorption origin and the intense peak in c polarization is dramatically underestimated with respect to experiment.…”

“…The transition is also coupled to several intramolecular vibrational modes, which give rise to a complicated vibronic structure at the low-energy absorption onset. Interpretation of this structure, starting from a simplistic weak-coupling approach [1], has recently developed into a full-fledged theory [2][3][4][5] addressing the issue in considerable detail and yielding impressive agreement with experiment.…”

“…[5] where all relevant modes are explicitly included). The pertinent mode is identified with the effective 0.18 eV stretching vibration of the rings; there is ample evidence showing that its (linear) coupling to the electronic excitation is intermediate, but rather on the strong side [5]. This favors a basis set rooted in the strong-vibronic-coupling approximation, as offering good prospects of rapid convergence.…”

“…It takes into account one Frenkel excitation per molecule, and, for the sake of simplicity, only one vibrational mode per molecule (in contrast to the sophisticated treatment of Ref. [5] where all relevant modes are explicitly included). The pertinent mode is identified with the effective 0.18 eV stretching vibration of the rings; there is ample evidence showing that its (linear) coupling to the electronic excitation is intermediate, but rather on the strong side [5].…”

The influence of geometry on the vibronic spectra of quantum aggregates

“…Then the Hamiltonian and G(E) again are diagonalised in this expanded basis. In view of the explosion in the number of vibronic basis states however, this strategy is only possible for very small aggregates and only a few vibrational modes [31,[36][37][38][39][40][41][42]. Alternatively one can seek approximations.…”