Spin Effects on Decay Dynamics of Charge-Separated States Generated by Photoinduced Electron Transfer in Zinc Porphyrin−Naphthalenediimide Dyads

Abstract

Photoexcitation of zinc porphyrin−bridge−naphthalenediimide (ZP−B−NI) dyads, 1 and 2, generated the short- and long-distance charge-separated (CS) states, [ZP•+−B−NI•-], through the intramolecular electron-transfer from excited ZP to NI in solvents of various polarity. The energy level of [ZP•+−B−NI•-] was either higher (in benzene and 1,4-dioxane) or lower (in solvents of higher polarity) than that of 3ZP*−B−NI. When generated in the singlet spin state, the short-distance CS state derived from 1 rapidly (109−1010 s-1) decayed through the charge recombination (CR) leading to the ground state. On the other hand, when generated from the triplet excited state of 1, the decay of the CS state was much slower and showed magnetic field effects attributable to the level-crossing mechanism. For the long-distance CS state derived from 2, the decay dynamics and its magnetic field dependence exhibited quite different features. To examine the effects of a neighboring additional radical on the decay dynamics of these CS states, three-spin CS states [ZP•+−B−NI•-−R•] were generated from 1R• and 2R•, in which 2,2,6,6-tetramethyl-1-piperidinoxy radical (R•) was connected to the NI part in 1 and 2, respectively. The decay rate of [ZP•+−B−NI•-−R•] derived from 1R • was much faster than that of [ZP•+−B−NI•-] derived from 1. For the CS state generated from 2R•, the initial decay could be retarded compared to the CS state from 2 through the equilibration between the doublet and quartet spin states. The observed effects of R• on the decay processes of the CS states are attributed to the alteration of the energy gap between the states with different spin multiplicities and to the efficient conversion between the doublet and quartet spin states induced by the dipole−dipole interaction between NI•- and R•.