Excited doublet states of electrochemically generated aromatic imide and diimide radical anions

The radical anions of aromatic diimides have been implicated recently in a wide variety of photochemical electron transfer reactions. Photoexcitation of these radical anions produces powerfully reducing species. Yet, the properties of the n excited doublet states of these organic radical anions remain obscure. The radical anions of three aromatic imides with increasingly larger n systems, N-(2,5-di-te/-?-butylphenyl)phthalimide, 1, Ar-(2,5-di-rert-butylphenyl)-l,8-naphthalimide, 2, and N-(2,5-di-/err-butylphenyl)perylene-3,4-dicarboximide, 3, as well as the three corresponding aromatic diimides, N,W-bis(2,5-di-Yer?-butylphenyl)pyromellitimide, 4a, N,N′-bis(2,5-di-/ert-butylphenyl)-naphthalene-l,8:4,5-tetracarboxydiimide, 5a, and N,N′-bis(2,5-di-tertbutylphenyl)perylene-3,4:9,10-tetracarboxydiimide, 6, were produced by electrochemical reduction of the neutral molecules in an optically transparent thin layer electrochemical cell. The radical anions of these imides and diimides all exhibit intense visible and weaker near-IR absorption bands corresponding to their D0 → D_n transitions. Excited states of the radical anions were generated by subpicosecond excitation into these absorption -bands. Excitation of 1^-- and 2^- resulted in decomposition of these radical anions, whereas excitation of 3'-6- yielded transient spectra of their D] -D_n transitions and the lifetimes of D₁. The lifetimes of the DI excited states of the radical anions of 3-6'- are all less than 600 ps and increase as the D0-D₁ energy gap increases. These results impose design constraints on the use of these excited radical anions as electron donors -in electron-transfer systems targeted toward molecular electronics and solar energy conversion.