Comparative studies on inter- and intramolecular electron transfer processes within 4-methoxybenzo[b]thiophene (4MBT) and p-chloroacetophenone (PCA) reacting systems by using steady-state and laser flash photolysis techniques

Methods of synthesis and characterization of the bichromophores (4MBA and 4MBAS) comprising 4-methoxybenzo[b]thiophene (4MBT) as donor and p-chloroacetophenone (PCA) as acceptor linked together by an olefinic (unsaturated in the former bichromophore and saturated in the case of the latter one) bridge have been presented in this paper. The photophysics of these bichromophores has been extensively studied both by steady-state and laser flash photolysis techniques. An attempt was made to compare the monomolecular photophysical properties of the bichromophores with the photophysics of the bimolecular systems where its constituent reacting partners (same donor and acceptor molecules as within the bichromophores) being unlinked will react by diffusion-assisted process. Both from the steady-state electronic absorption and fluorescence emission spectra it shows that the bichromophores exhibit charge transfer (CT) complex both in the ground and excited states. This observation demonstrates that mutual orientation between the donor and the acceptor moieties largely facilitates the formation of such complex. In intermolecular interactions it appears that a weak excited singlet (S I) CT complex of contact nature is formed, whereas in intramolecular systems (bichromophores 4MBA and 4MBAS) relatively stronger excited CT complexes of two types: cis (folded) and trans (extended) isomers for 4MBA and two conformers (gauche and staggered) for 4MBAS are formed, as evidenced from steady-state and time-resolved measurements. Transient absorption measurements clearly demonstrate the occurrences of photoinduced intermolecular ET reactions, whereas in intramolecular system direct excitation of ground state CT by using the third harmonic (355 nm) of a nanosecond Nd:YAG laser system produce ion-pair (D+-A(-)) state both in the singlet and triplet states. Transient absorption spectral measurements by laser flash photolysis technique reveal that energy wasting charge recombination process within the bichromophore is largely precluded on changing the nature of the spacer from unsaturated to more flexible saturated one. The necessity of incorporating two spacers (cyclophane type) instead of one to enhance the charge separation rate and to minimize the energy wasting recombination process is hinted. (C) 2002 Elsevier Science B.V. All rights reserved.