4-Methoxybenzo[b]thiophene

4-Methoxybenzo[b]thiophene (4MBT) is an electron donor. The presence of the heteroatom sulfur (S) in 4MBT seems to be responsible for the observed significant  changes in the transition energies of this molecule compared to that of analogous compound 4-methylindole (4MI) in spite of the structural similarities of these two compounds[1].

[1] Maiti M, et al. Photophysics of 4-methoxy-benzo[b]thiophene in different environments. Its role in non-radiative transitions both as an electron and as an energy donor. Journal of Luminescence, 1999; 82: 259-276.