Ionic modulation of the charge transfer transitions in host–guest complexes of carbon nanorings and fullerenes

The hoop-shaped π-conjugated cycloparaphenylenes (CPPs), which constitute powerful hosts for complexation with fullerenes, were investigated by time-dependent density functional theory calculations and high-level second-order algebraic diagrammatic construction methods. For certain questions of open-shell character, complete active space self-consistent field and multireference ab initio calculations (strongly contracted N-electron valence state perturbation theory) have been performed as well. Host-guest complexes composed of [n]CPPs (n = 9, 10, 11) and pristine fullerenes (C60 and C70) were considered. Charge transfer (CT) processes between the fullerenes and the CPPs in ground state and excited states were the focus of this work. Our calculations clearly demonstrate the advantages of endohedral insertion of metal cations (Li+, Be2+, Na+, and Mg2+) into the fullerenes for tuning the CT states. The analysis of the pristine complexes shows the occurrence of local excitonic states on the CPP units as well as on the fullerenes. The latter represent the lowest excited states. Into these series on nonpolar states, CT states are embedded. Moving to the complexes with endohedral fullerene encapsulation leads to dramatic changes in the spectral composition. CT states are strongly stabilized and form extended bands of CT states that form the energetically lower end of the spectrum. Ionic modulation of CT states in these fullerene-CPP complexes offers a new insight for constructing photoactive supramolecular systems with efficient charge transfer between the donor and acceptor parts.