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

Abstract

The present investigation was made to study the photophysical properties of a newly synthesized benzothiophene compound “4-methoxy-benzo[b] thiophene (4MBT)” in different environments both at the room and at 77 K temperatures. The studies were mainly made by electrochemical and spectroscopic (both by steady-state and time resolved) techniques. 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. In the presence of the electron acceptors 2-nitrofluorene (2NF) and 9-fluorenone (9FL) the room temperature fluorescence spectra of 4MBT are found to be quenched significantly. In the presence of the latter quencher, a long-wavelength band was found to develop at around 508 nm region with a concomitant reduction in the 4MBT fluorescence band. Fluorescence lifetime data reveal that this long-wavelength band might be due to the formation of acceptor fluorescence by Förster's long-range singlet–singlet energy transfer process. The observed highly negative Δ G 0 values of the present donor-acceptor systems coupled with large overlapping of fluorescence spectra of the donor and electronic absorption spectra of the acceptor are explained by competing energy transfer and photoinduced electron transfer (ET) reactions. In the presence of the acceptors, presently studied, excited donor 4MBT deactivates non-radiatively through both the static and the dynamic quenching processes in which the former process appears to be dominant. Positive deviation from linearity of Stern–Volmer (SV) plot along with the observation of unaffected fluorescence lifetime of 4MBT in the presence of different concentrations of the quenchers (acceptors) demonstrates this fact. In dynamic quenching, Förster's energy transfer process appears to play a major role. However, when acceptors are excited to their first singlet excited states in the presence of unexcited (ground state) donor 4MBT, only dynamic processes (ET associated with some other non-radiative processes) seem to be operative. From the present investigations it was inferred that benzothiophene is a stronger electron donor than the analogous compound 4MI, though the structures of these two donors are similar except for the presence of a heavy atom S in the 1-position of 4MBT. At 77 K as the value of R 0, Förster critical energy transfer distance, is high (∼20 Å), the quenching of the donor fluorescence by the acceptors seems to proceed mainly through the Förster's energy transfer mechanism. The reduction of phosphorescence intensity of this donor in the presence of an acceptor seems to be due to the involvement of a non-radiative pathway through which singlet – singlet ( S D 1→ S A 1) energy transfer occurs efficiently followed by intersystem crossing to populate the triplet level of the acceptor at the expense of the population in donor triplet. At this temperature the exact mechanism of the observed fluorescence quenching effect of the acceptor (2NF and 9FL) in the presence of the donor 4MBT containing S atom is not very clear at the present stage of investigation. Two possible non-radiative mechanisms, one by external heavy-atom effect and the other by photoinduced ET process, seem to have minor role at 77 K as sulfur is known to be weak heavy-atom perturber and ET is thermally activated process.