Vibronic fine structure in the absorption spectrum of oligothiophene thin films.

A multimode Holstein Hamiltonian is used to describe optical excitations in quaterthiophene pinwheel aggregates. The Hamiltonian includes the coupling of excitons originating from the 1Ag→1Bu 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 A1 band in the low-energy region of the absorption spectrum, ranging from the band origin to ≈1500 cm-1 beyond. The calculated A1 band of quaterthiophene aggregates agrees favorably with that observed from thin films. [ABSTRACT FROM AUTHOR]