Robust Inclusion Complexes of Crown Ether Fused Tetrathiafulvalenes with Li~+@C_(60) to Afford Efficient Photodriven Charge Separation

摘要

Inclusion complexes of benzo- and dithiabenzocrown ether functionalized monopyrrolotetrathiafulvalene (MPTTF) molecules were formed with Li~+@C_(60) (1·Li~+@C_(60) and 2·Li~+@C_(60)). The strong complexation has been quantified by high binding constants that exceed 10~6m~(-1) obtained by UV/Vis titrations in benzonitrile (PhCN) at room temperature. On the basis of DFT studies at the B3LYP/6-311G(d,p) level, the orbital interactions between the crown ether moieties and the p surface of the fullerene together with the endohedral Li~+ have a crucial role in robust complex formation. Interestingly, complexation of Li~+@C_(60) with crown ethers accelerates the intersystem crossing upon photoexcitation of the complex, thereby yielding ~3(Li~+@C_(60))*, when no charge separation by means of ~1Li~+@C_(60)* occurs. Photoinduced charge separation by means of ~3Li~+@C_(60)* with lifetimes of 135 and 120 μs for 1·Li~+@C_(60) and 2·Li~+@C_(60), respectively, and quantum yields of 0.82 in PhCN have been observed by utilizing time-resolved transient absorption spectroscopy and then confirmed by electron paramagnetic resonance measurements at 4 K. The difference in crown ether structures affects the binding constant and the rates of photoinduced electron-transfer events in the corresponding complex.