Photophysical Study of New Methanofullerene−TTF Dyads: An Obvious Intramolecular Charge Transfer in the Ground States

Abstract

Three isomers of a new C 6 0 -TTF dyad-C 6 o -X-TTF (X = ortho, meta, and para)-have been synthesized by changing the linking positions (ortho, meta, and para) at a phenyl group that is attached to the methano-[60]fullerene. The dyads showed clear intramolecular charge transfer (CT) absorption bands in the steady-state absorption spectra, which was indicative of an intramolecular CT interaction between the C 6 0 and TTF moieties in the ground state. The increase in intensity of the CT absorption bands followed the order C 6 0 -ortho-TTF > C 6 0 -meta-TTF >> C 6 0 -para-TTF, which can be reasonably explained by the optimized molecular structures that are calculated at the ab initio level. Extreme quenching of the fluorescence intensity from the locally excited C 6 0 moiety was observed to follow the aforementioned order, which suggests that very fast excited singlet-state dynamics are dependent on the isomers. The quenching of the absorption intensities of the triplet state of the C 6 0 moiety detected in the nanosecond region was also observed to follow the same order, which suggests that competitive paths that are more efficient than intersystem crossing are present. From subpicosecond transient absorption measurements, very short-lived transient absorption bands attributed to the overlap of S 1 -S n with the excited CT state that has strong CT character were obtained for C 6 0 -ortho-TTF (and C 6 0 -meta-TTF); appreciable charge-separated (CS) species were generated for C 6 0 -para-TTF. The lifetimes of the CT and CS states increased in the order of C 6 0 -ortho-TTF < C 6 0 -meta-TTF < C 6 0 -para-TTF. Overall, it was revealed that the ground and excited states are controlled by the difference in proximity between the C 6 0 and TTF moieties, depending on the linking positions.