Photoinduced Electron Transfer in Pophyrin- and Phthalocyanine-Fullerene Dyads in Non-Coordinating and Halide-Coordinating Environments

Abstract

Photoinduced electron transfer (ET) in dyads consisting of zinc porphyrins or zinc phthalocyanines as the electron donors and fullerene C60 as the electron acceptor was studied in different environments. Time-resolved absorption and emission spectroscopy studies were performed to monitor the populations of the transient states involved in the ET reaction. In porphyrin-fullerene dyads the reaction proceeds via an intramolecular exiplex intermediate. The precursor of this state can be observed as a broad absorption band, also known as charge transfer (CT) band, in the far red part of the spectrum. The molar absorption of the CT band was strong enough to populate the intramolecular exciplex directly from the ground state by exciting the dyad at its CT band. By using this strategy the number of intermediate states in the excitation relaxation process was reduced to two, and the quantitative analysis of the ET reactions involving the exciplex and the complete charge-separated (CS) state was carried out by constructing a kinetic model based on the Marcus theory of ET and Boltzmann distribution law. The model was successfully applied to a zinc porphyrin-fullerene dyad and it allowed to evaluate the ET parameters, e.g. the free and reorganization energies, the electronic couplings for the exciplex and the complete CS state, and the charge separation degree in the exciplex. The developed model can be applied for such dyads at any value of the dielectric constant of the solvent.

Chloride binding to the central zinc of a zinc porphyrin-fullerene dyad results in drastic changes in the energies and lifetimes of the locally excited singlet state of porphyrin, the exciplex, and the complete CS state. The effects of the chloride coordination on the energies and lifetimes of the ET intermediates were examined and explained by the electric field effect on the donor chromophore induced by the negative charge of the chloride ion ligated to the central metal of porphyrin. The ligation results also in an increase of the oscillator strength of the CT absorption band.

Compared to porphyrin-fullerene dyads the chloride binding effect was found to be much weaker in zinc phthalocyanine-fullerene dyads. The forward ET reaction in the zinc phthalocyaninefullerene dyads was found to occur in the Marcus inverted region. The latter conclusion was made because the chloride ligation to the zinc phthalocyanine moiety of the dyad was found to result in (i) an increase in the driving force of the ET reaction and (ii) a decrease in the ET rate constant.