Interchain interaction in a prototypical conjugated oligomer from polarized absorption at 4.2 K: α-sexithienyl single crystal

The polarized absorption spectra in the bc face of an α-sexithienyl (T₆) single crystal have been measured at 4.2 K. The origin of the lowest electronic transition is at 18360 cm^-1 and has been assigned to the lowest b-polarized a_u Davydov component of the 1¹B_u molecular level. The second optically allowed Davydov component (b_u) is polarized in the ac crystal plane and is located at 20945 cm^-1. Therefore, the interchain interaction leads to a Davydov splitting of about 2600 cm^-1. A quantum chemical model, which considers the total molecular wavefunctions for each transition, shows good agreement with the experimental findings for the energy and polarization of the optically allowed crystal levels. The vibronic manifold in the absorption spectra has been interpreted in terms of the Herzberg-Teller and Franck-Condon vibronic coupling. In particular, two false origins due to the coupling of the molecular electronic levels 1¹B_u and 2¹B_u have been identified at 18486 and 18657 cm^-1. Consistently, the emission counterpart of the two false orgins has been identified in the polarized fluorescence spectra. The totally symmetric modes involved in the coupling are in excellent agreement with the Raman scattering data of the single crystal.