Photoinduced intramolecular n-π* electron transfer in aminofullerene derivatives

The photophysical properties of two amino-C-60 derivatives, which are adducts of C-60 With N,N'-dimethyl-1,2-ethylenediamine and C-60 With piperazine, were studied systematically. The results show that there is intramolecular n (amino groups) to pi* (photoexcited fullerene cage) electron transfer in the aminofullerene molecules. The photoinduced electron transfer can be observed only in a polar solvent environment for the first compound and also in polarizable solvents for the second compound. As a result, the fluorescence quantum yields and lifetimes and the nonlinear absorptive optical Limiting responses of the amino-C-60 derivatives are strongly solvent dependent. Charge-transfer excited states are formed following electron transfer in both compounds, but only that of the second compound is emissive. There is also evidence for delayed fluorescence in polar solvents, which is probably due to the vertical excited singlet state repopulated from the charge-transfer excited state, However, the fluorescence quenchings and effects on optical limiting in polar solvents can be eliminated through protonating amino groups in the molecules by adding a small amount of trifluoroacetic acid into the sample solutions. Although the intramolecular n-pi* electron transfer in aminofullerenes shares some characteristics with the classical twisted intramolecular charge transfer in molecules represented by p-N,N-dimethylaminobenzonitrile, the amino-C-60 derivatives are in fact better classified as redox dyads. Since electron donor (amino groups) and acceptor (fullerene cage) are linked directly through only sp(3) carbons, these are the simplest donor-spacer-fullerene dyads. In more quantitative treatments, electron-transfer rate constants for the amino-C-60 derivatives under different solvent conditions were correlated with the solvent microscopic polarities, which were estimated using the strongly solvatochromic molecular probe 6-propionyl-2-(N,N-dimethylamino)naphthalene.