Electronic levels ordering in α-sexithienyl

Abstract

The electronic level ordering in a polycrystalline thin film of α-sexithienyl (T₆) has been determined by photoluminescence, one- and two-photon excitation, and photoconductivity measurements. The lowest 1 ¹B_u Frenkel exciton is located at 17452 cm^-1. From site-selective photoluminescence we locate an extended inhomogeneous distribution of localized isolated T₆ levels (X-traps) below the 1 ¹B_u Frenkel exciton extending for 2000 cm^-1. The origin of the lowest 2 ¹A_g Frenkel exciton is located at 18350 cm^-1, i.e. 898 cm^-1 above the 1¹B_u. The photoconductivity action spectrum shows an onset at about the same energy of the 2 ¹A_g exciton band and reaches a maximum 1450 cm^-1 above the 1 ¹B_u exciton band. T₆ is proposed as a good model compound for real polythiophene. The difference in energetics and photophysical properties due to different electronic correlation effects between polyenes and polythiophene is discussed.