Spectral shape of intense exciton absorption in oligothiophene crystals

The peculiar shape of $c$-polarized absorption spectra of crystalline oligothiophenes is attributed to the coupling between the intense upper Davydov component originating from the lowest molecular excitation and the continuum of unbound exciton-phonon states deriving from excitons off the center of the exciton Brillouin zone. This formulation is the energy-domain description of the first step of intraband exciton relaxation and, in analogy to intramolecular radiationless transitions, is couched in terms of the configuration interaction between the discrete state and the one-phonon continuum, using an exact theory originally developed by Fano. The experimental absorption shapes for individual oligothiophenes and the spectral changes along the series are very well reproduced within a unified treatment, providing a plausible explanation in terms of the interplay between the trends imposed by the changes of the exciton band width and of the effective vibronic coupling constant, with the latter parameter obtained from ab initio quantum chemistry calculations.

Figure 1

Comparison between the experimental $c$-polarized absorption spectra of quaterthiophene (4T), quinquethiophene (5T), and sexithiophene (6T) (Ref. 12). The spectra are offset (in the abscissa) and scaled (in the ordinate) in such a way that their intense high-energy peaks coincide.

Figure 2

Quaterthiophene, quinquethiophene, and sexithiophene molecules.

Figure 3

Unit cell of the low-temperature phase of the quaterthiophene crystal.

Figure 4

Calculated (broken line) vs experimental (Ref. 12) (solid line) absorption spectra of oligothiophenes.