Unusual features of the quaterthiophene electro-absorption spectrum

A previously used model describing the coupling between Frenkel and charge-transfer (CT) excited states in a crystal with molecules arranged in close-packed planes of herringbone pattern, is generalized to include some CT states engaging the molecules from the different planes. The corresponding Hamiltonian is parametrized for quaterthiophene and numerically diagonalized, to yield ultimately the electro-absorption (EA) spectrum Contrary to our recent expectations, the contribution from the newly introduced CT states is only minor. It turns out I that the high-energy EA signal of quaterthiophene is dominated by the contributions from the CT states with the electron transferred along the longer lattice period in the close-packed plane, in contrast to the more familiar case of sexithiophene, where these states barely contribute. The reasons of this difference are pinpointed and discussed. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim