Inclusion complexes of benzo-and dithiabenzocrown ether functionalized monopyrrolotetrathiafulvalene (MPTTF) molecules were formed with Li+@C-60 (1 center dot Li+@C-60 and 2 center dot Li+@C-60). The strong complexation has been quantified by high binding constants that exceed 10(6) (-1)(M) 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 ms for 1 center dot Li+@C-60 and 2 center dot 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.
Chemistry: a European Journal, 2014, Vol 20, Issue 43, p. 13976-13983