Large acceleration effect of photoinduced electron transfer in porphyrin-quinone dyads with a rigid spacer involving a dihalosubstituted three-membered ring

To understand the contribution of spacer structure toward electron transfer (ET) and to regulate electronic coupling between a redox pair, porphyrin-spacer-benzoquinone molecules were prepared where spacers are trans-decalin and dihalosubstituted tricyclo[4.4.1.0]undecane including a three-membered ring. These compounds were designed to have almost the same separation distance between a redox pair, the same number of intervening bonds, and the nearly equal free energy change associated with the ET reaction. The ET rates for the charge separation process were evaluated on the basis of the fluorescence lifetimes. A quite large difference in the ET rates was observed among these compounds, and the ET rates for the compounds having the three-membered rings were ca. 50 to 60 times larger than that with trans-decalin spacer in THF. From the analysis of temperature dependence of the ET rates, it was shown that the observed rate acceleration is caused by both an increase of the electronic coupling and a decrease of the reorganization energy, Ab initio calculations of the electronic coupling elements and on molecular orbitals for the cyclopropanes predicted that the former may be due to the enhancement of the ET pathways arising from the bent geometry of the spacer or of the mixing pathway induced by a very low lying antibonding orbital in the dihalosubstituted cyclopropane.