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

The radical anions of aromatic diimides have been implicated recently ill a wide variety of photochemical electron transfer reactions. Photoexcitation of these radical anions produces powerfully reducing species. Yet, the properties of the pi* excited doublet states of these organic radical anions remain obscure. The radical anions of three aromatic imides with increasingly larger pi systems, N-(2,5-di-tert-butylphenyl)phthalimide, 2, N-(2,5-di-tert-butylphenyl)-1,8-naphthalimide, 2, and N-(2,5-di-tert-butylphenyl)perylene-3,4-dicarboximide, 3, as well as the three corresponding aromatic diimides, N,N'-bis(3,5-di-tert-butylphenyl)pyromellitimide, 4a, N,N'-bis(2,5-di-tert-butylphenyl)-naphthalene-1,8:4,5-tetracarboxydiimide, 5a, and N,N'-bis(2,5-di-tert-butylphenyl)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 D-0 --> 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-1 --> D-n transitions and the lifetimes of D-1. The lifetimes of the D-1 excited states of the radical anions of 3(.-)-6(.-) are all less than 600 ps and increase as the Do-DI 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.