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

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Abstract
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 18 360 cm−1 and has been assigned to the lowest b-polarized au Davydov component of the 1 1Bu molecular level. The second optically allowed Davydov component (bu) is polarized in the ac crystal plane and is located at 20 945 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 1 1Bu and 2 1Bu have been identified at 18 486 and 18 657 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.