New fluorescent benzo[b]thienyl amino acid derivatives based on sulfanylphenyl benzo[b]thiophenes

Fluorescent derivatives of benzo[b]thiophene were synthesized by CC and CN palladium catalysed cross-couplings of bromobenzenes with 3-(benzo[b]thienyl)boronic acid and bromo- or aminobenzenes with 7-amino or 7-bromobenzo[b]thiophene, respectively. The photophysical behaviour of the coupled compounds was investigated in acetonitrile, showing that a delicate balance between the delocalization of the electronic distribution over the entire molecule and the extent of intramolecular charge transfer interactions controls the excited-state relaxation pathway. Some of the new benzo[b]thiophene derivatives were linked to the β-carbon of alanine to obtain quasi-isosteric analogues of naturally occurring fluorescent amino acids.