Robust Inclusion Complexes of Crown Ether Fused Tetrathiafulvalenes with Li⁺@C₆₀ to Afford Efficient Photodriven Charge Separation

Inclusion complexes of benzo- and dithiabenzo-crown ether functionalized monopyrrolotetrathiafulvalene (MPTTF) molecules were formed with Li⁺@C₆₀ (1Li⁺@C₆₀ and 2Li⁺@C₆₀). The strong complexation has been quantified by high binding constants that exceed 10⁶M^-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 π surface of the fullerene together with the endohedral Li⁺ have a crucial role in robust complex formation. Interestingly, complexation of Li⁺@C₆₀ with crown ethers accelerates the intersystem crossing upon photoexcitation of the complex, thereby yielding ³(Li⁺@C₆₀)∗, when no charge separation by means of ¹Li⁺@C₆₀∗ occurs. Photoinduced charge separation by means of ³Li⁺@C₆₀∗ with lifetimes of 135 and 120 μs for 1Li⁺@C₆₀ and 2Li⁺@C₆₀, 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.