Location of a 1A g state in bithiophene

Abstract

The 0–0 excitation energy of a ¹A_g state of 2,2’‐bithiophene has been determined by measuring the two‐photon fluorescence excitation spectrum of a dilute solution of this molecule in crystalline n‐hexane at 77 K. Because the 0–0 energy is what would have been predicted by extrapolating previously measured α,ω‐dithienylpolyene 2 ¹A_g 0–0 energies to zero polyene chain length, it is assigned to the 2 ¹A_g state. The 0–0 band is centered at 36 173 cm⁻¹, approximately 6570 cm⁻¹ above the 0–0 of the 1 ¹A_g to 1 ¹B_u transition. This order of bithiophene excited singlet states is opposite to that of the linear polyene with the same number of double bond but may reverse for chains longer than six repeat units.