Polarized Absorption and Davydov Splitting in α-sexithienyl Single Crystal at 4.2K

The polarized absorption spectra in the be face of an a-sexithienyl (T6) single crystal have been measured at 4.2K. The origin of the lowest electronic transition is at 18360 cm−1 and has been assigned to the lowest b-polarized au Davydov component of the 11Bu molecular level. The second optically allowed Davydov component (bu) 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. 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 11Bu and 21Bu have been identified at 18486 cm−1 and 18657 cm−1. The totally symmetric modes involved in the coupling are in excellent agreement with the Raman scattering data of the single crystal.