Role of TiO2 nanoparticles on the photoinduced intramolecular electron-transfer reaction within a novel synthesized donor-acceptor system

Steady-state, FT-IR and time-resolved spectroscopic measurements were carried out on the two novel synthesized pi-conjugated rigid naphthothiophene compounds, 7-methoxynaphtho[1,2-b]thiophene (7MNT) and 7-methoxynaphtho[1,2-b]thiophene-2-carboxylic acid (7MTCA) in chloroform (CHCl3) medium as well as in presence of TiO2 nanoparticles. 7MTCA multichromophoric system contains both electron donor (-OCH3) and acceptor functionalities, (-COOH) at the two different ends of the rigid naphthothiophene moiety whereas in 7MNT only the electron donor functional group, -OCH3, is present. Thus 7MNT looks like the donor part of 7MTCA where -COOH group overlaps, due to its free rotation, with the electron rich site 7MNT part. The experimental findings observed from steady-state absorption, fluorescence and time-resolved spectroscopic (fluorescence lifetime and transient absorption) measurements demonstrate that the survival duration of charge-separated species formed within 7MTCA due to intramolecular photoinduced electron-transfer (PET) reactions between the donor -OCH3 and acceptor -COOH functional groups, attached at the two different ends of this compound, could be enhanced by inhibiting the energy wasting charge recombination process through complexation of the carboxylate group (-COOH) with TiO2 nanoparticles. Thus, in the present investigation TiO2 appears not to act as an acceptor but its presence helps to survive or protect the charge-separated species formed within 7MTCA. The possibility of designing of low-cost artificial (model) solar energy conversion devices by linking the organic donor-acceptor (DA) systems like 7MTCA with TiO2 core is hinted. (c) 2007 Elsevier B.V. All rights reserved.