Intermolecular and Intramolecular Electron Transfer Processes from Excited Naphthalene Diimide Radical Anions

Excited radical ions are interesting reactive intermediates owing to powerful redox reactivities, which are applicable to various reactions. Although their reactivities have been examined for many years, their dynamics are not well-defined. In this study, we examined intermolecular and intramolecular electron transfer (ET) processes from excited radical anions of naphthalene-1,4,5,8-tetracarboxydiimide (NDI(•-)*). Intermolecular ET processes between NDI(•-)* and various electron acceptors were confirmed by transient absorption measurements during laser flash photolysis of NDI(•-) generated by pulse radiolysis. Although three different imide compounds were employed as acceptors for NDI(•-)*, the bimolecular ET rate constants were similar in each acceptor, indicating that ET is not the rate-determining step. Intramolecular ET processes were examined by applying femtosecond laser flash photolysis to two series of dyad compounds, where NDI was selectively reduced chemically. The distance dependence of the ET rate constants was described by a β value of 0.3 Å(-1), which is similar or slightly smaller than the reported values for donor-acceptor dyads with phenylene spacers. Furthermore, by applying the Marcus theory to the driving force dependence of the ET rate constants, the electronic coupling for the present ET processes was determined.