Spacer controlled photo-induced intramolecular electron transfer in a series of phenothiazine-boron dipyrromethene donor-acceptor dyads

A series of phenothiazine (PTZ)-boron dipyrromethene (BODIPY) based electron donor-acceptor dyads, PB1, PB2, and PB3, containing PTZ as an electron donor and BODIPY as an electron acceptor, tethered together with spacers of varied sizes i.e., directly connected (absence of the spacer), phenyl bridge, and ethoxy phenyl spacer, respectively, are reported. Optical absorption spectra of the dyads showed marginal ground state interactions between the chromophores and, these interactions are found more pronounced in PB1 and PB2. Electrochemical studies on the dyads revealed minor shifts in the first oxidation of PTZ moiety and the first reduction of BODIPY moiety, compared to their control compounds. In the newly synthesized dyads, the free-energy calculations performed by employing the redox and singlet excited state energies, in both polar and non-polar solvents, suggested the possibility of electron transfer from the ground state of PTZ to the singlet excited BODIPY moiety (PTZ -> (BODIPY)-B-1*) to produce PTZ(center dot+)-BODIPY center dot-. Accordingly, steady state emission studies revealed the efficient fluorescence quenching of the singlet excited BODIPY in the dyads and the efficiency of quenching for all the dyads is higher in polar solvents. More importantly, these emission studies also revealed that, in a given solvent, as the length of the spacer between the chromophores is increased, the efficiency of fluorescence quenching is decreased. The degree of electronic communication between the chromophores w.r.t. the spacer type and size was supported by density functional theory (DFT) and time dependent-DFT (TD-DFT) calculations. The electron transfer rates (k(ET)) of these dyads were found to be solvent dependent and were in the range 0.06 x 10(8) to 1.96 x 10(10) s(-1). (C) 2015 Elsevier B.V. All rights reserved.