Synthesis and studies on spectroscopic as well as electron donating properties of the two alkoxy benzo[b]thiophenes

Synthesis, characterization, steady state and time resolved, using time cor related single photon counting as well as laser flash photolysis techniques , spectroscopic investigations were made for two alkoxy benzo[b]thiophene m olecules: 5-methoxy benzo[b]thiophene (5MBT) and 5-methoxymethyl benzo[b]th iophene (5MMBT). In both non-polar n-heptane (NH) and polar acetonitrile (A CN) solvents and at ambient temperature the electronic absorption spectra o f these thiophenes exhibit different band systems whose assignments were ma de from the measurements of the steady state excitation polarization spectr a. Steady state fluorescence spectra of these molecules in the different po larity solvents show the presence of non-specific interactions. From the re dox properties of the benzothiophenes, measured by cyclic voltammetry, thei r electron donating properties were observed in the presence of the well-kn own electron acceptor 9cyanoanthracene (9CNA). Further, detailed studies by laser flash photolysis techniques show that ion-recombination mechanism pr edominates after the initial excitation of the acceptor moiety using the th ird harmonic of Nd:YAG laser. This recombination together with the external heavy atom effect (the donor containing 'sulphur' atom) appears to be resp onsible for the formation of the triplet of the monomeric acceptor 9CNA. Fr om the steady state experiments it is shown that both in non-polar NH and h ighly polar ACN the quenching in the fluorescence emission of 9CNA in the p resence of the benzothiophene donors is brought about primarily by the exte rnal heavy atom effect and in ACN, although the presence of the photoinduce d ET reaction is confirmed, this process seems, from the observed bimolecul ar dynamic quenching rate, k(q), to be significantly masked by the external heavy atom effect. (C) 2001 Elsevier Science B.V. All rights reserved.