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

The polarized absorption spectra in the bc face of an alpha-sexithienyl (T- 6) Single crystal have been measured at 4.2 K. The origin of the lowest ele ctronic transition is at 18 360 cm(-1) and has been assigned to the lowest b-polarized a(u) Davydov component of the 1 B-1(u) molecular level. The sec ond optically allowed Davydov component (b(u)) is polarized in the ac cryst al plane and is located at 20 945 cm(-1). Therefore, the interchain interac tion leads to a Davydov splitting of about 2600 cm(-1) A quantum chemical m odel, which considers the total molecular wavefunctions for each transition , shows good agreement with the experimental findings for the energy and po larization of the optically allowed crystal levels. The vibronic manifold i n the absorption spectra has been interpreted in terms of the Herzberg-Tell er and Franck-Condon vibronic coupling. In particular, two false origins du e to the coupling of the molecular electronic levels 1 B-1(u) and 2 B-1(u) have been identified at 18 486 and 18 657 cm(-1). Consistently, the emissio n 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 cry stal. (C) 1998 American Institute of Physics. [S0021-9606(98)50546-6].