Location of a 1A g state in bithiophene

The 0–0 excitation energy of a 1Ag 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 1Ag 0–0 energies to zero polyene chain length, it is assigned to the 2 1Ag state. The 0–0 band is centered at 36 173 cm−1, approximately 6570 cm−1 above the 0–0 of the 1 1Ag to 1 1Bu 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.