Excited State Charge Separation In Solution And In Electropolymerized Films Of Terthiophene-Fullerene Dyad And Phenothiazine-Terthiophene-Fullerene Triad

A covalently linked molecular dyad comprised of terthiophene and C-60, and a molecular triad comprised of phenothiazine, terthiophene and C-60 have been newly synthesized to probe photoexcited events in solution and at the electropolymerized film. The X-ray structure of the dyad revealed spatial disposition of the donor-acceptor entities without significant intermolecular type interactions. Optical absorbance, fluorescence, and the electrochemical studies were consistent with the structural integrity of the dyad and triad. Further, the TT-C-60 dyad (TT = terthiophene) was found to electropolymerize during multi-cycling of voltammograms. The frontier HOMO and LUMO orbitals of the phenothiazine-terthiophene-C-60 triad (PTZ-TT-C-60, PTZ = phenothiazine) were found to be located respectively on the phenothiazine-terthiophene and C-60 entities. Free-energy calculations revealed that the excited state charge transfer resulting into the formation of TT center dot+-C-60(center dot-) in the case of the dyad, and (PTZ-TT)(center dot+)-C-60(center dot-) charge separated state in the case of triad to be thermodynamically feasible. Evidence for the occurrence of excited state singlet-singlet energy transfer from (TT)-T-1* to form C-1(60)* followed by charge transfer was secured from studies involving femtosecond transient absorption studies in polar benzonitrile. Charge stabilization to some extent in the triad compared to that in the dyad was obtained. Attempts were also made to secure evidence of charge separation in the terthiophene-fullerene drop-casted and electropolymerized films on FTO surface. (C) The Author(s) 2016. Published by ECS.