Photoinduced electron transfer in Langmuir–Blodgett films between the donor [5,10,15,20-tetra(pentyl-oxy-biphenyl-oxy- m-phenyl)-porphyrinato]zinc(II) and quinone acceptors, either separated or linked by a cyclohexylene-bridge

Abstract

Complexes of zinc porphyrins with mesogenic substituents and quinones, as well as the corresponding mixtures of unlinked porphyrins and quinones, have been investigated in Langmuir–Blodgett (LB) mono- and multilayer films by means of steady-state as well as time-resolved fluorescence spectroscopy. The films were deposited onto quartz substrates. They consist of one or more layers of cadmium behenate (cadmium salt of docosanoic acid) containing either the donor (a zinc porphyrin), the acceptor (a quinone), or both. In the latter case, a covalently linked zinc porphyrin–quinone and mixtures of a zinc porphyrin and a quinone have been investigated. Steady-state S 2-fluorescence of zinc porphyrins was utilized as a reference to determine the rates of photoinduced electron transfer (PET), in order to characterize the intramolecular and intermolecular PET. This approach was found to be particularly useful for monolayer films. For PET of covalently linked zinc porphyrin–quinone compounds in monofilms, the same PET rates as in ethanol were observed (5.3×10 10 s −1). In the case of unlinked zinc-porphyrin/quinone mixtures, the PET rates depend on the concentration of quinones in the acceptor layers. Furthermore, the competition of energy transfer (between zinc porphyrins) and PET (between zinc porphyrins and quinones) in the cadmium behenate matrix is discussed.