Enhanced Electron-Transfer Properties of Cofacial Porphyrin Dimers through pi-pi Interactions

The radical cation of zinc tetrapentylporphyrin is dimerized with an excess of the neutral counterpart to form the dimer radical cation in which the unpaired electron is delocalized over both porphyrin rings. The dimeric radical cation exhibits an NIR absorption spectrum characteristic of weak pi-bond formation between the porphyrin rings. When cofacial porphyrin dimers, linked by different spacers, are oxidized such pi-bond formation between the porphyrin rings is also recognized in cyclic voltammetry, and Vis/NIR and ESR spectroscopic measurements. The dynamics of photoinduced electron transfer from the triplet excited states of cofacial porphyrin dimers to a series of electron acceptors were investigated by using laser flash photolysis measurements and compared with the porphyrin monomer. The rates of photoinduced electron-transfer reactions of cofacial porphyrin dimers are prominently accelerated compared with the reference monomer. The driving-force dependence of the rate constants of photoinduced electron-transfer reactions was analyzed in light of the Marcus theory of electron transfer to afford the reorganization energies of electron transfer (lambda). The lambda values of cofacial porphyrin dimers are significantly smaller than those of the porphyrin monomer when compared at the same driving force of the photoinduced electron transfer. The lambda values increase linearly with an increase in the driving force of the photoinduced electron transfer. This is accompanied by an increase in the distance between electron donor and acceptor molecules, where the electron transfer occurs. The enhanced electron-transfer properties of cofacial porphyrin dimers, in relation with the important role of the special pair in the photosynthetic reaction center, result from the smaller reorganization energy (lambda) together with the larger driving force of the photoinduced electron transfer due to the pi-electron delocalization in the dimer radical cations.