Abstract
Novel molecular clips with anthracene sidewalls (1 a–c) were synthesized; they form stable host–guest complexes with a variety of electron‐deficient aromatic and quinoid molecules. According to single‐crystal structure analyses of clip 1 c and 1,2,4,5‐tetracyanobenzene (TCNB) complex 14@1 b, the clips’ anthracene sidewalls have to be compressed substantially during the complex formation to provide attractive π–π interactions between the aromatic guest molecule and the two anthracene sidewalls in the complex. The compression and expansion of aromatic sidewalls are calculated by molecular mechanics to be low‐energy processes, so the energy required for compression of the anthracene sidewalls during complex formation is apparently overcompensated by the gain in energy resulting from the attractive π–π interactions. The finding that complexes of the clips 1 a–c are more stable than those of the corresponding clips 2 a–c can be explained in terms of the larger van der Waals contact surfaces of the anthracene sidewalls in 1 a–c (relative to the naphthalene sidewalls in 2 a–c). Color changes resulting from charge‐transfer (CT) bands are observed in complex formation by 1 a–c: from colorless to red or purple with TCNB (14), and from yellow to green with 2,4,7‐trinitro‐9‐fluorenone TNF (17). Independently, the host 1 b and guest 14 fluoresce from their respective excited singlet states, whilst in the complex 14@1 b the charge‐transfer state quenches the higher‐energy singlet states of the two components, and as a result luminescence is only observed from this new CT state. To the best of our knowledge, complex 14@1 b is the first example of CT luminescence from a host–guest complex. The binding constant determined for the formation of the TCNB complex 14@1 b from a UV/Vis titration experiment (K~a~ = 12 400 m^−1^) agrees well with the value (K~a~ = 12 800 m^−1^) obtained by ^1^H NMR titration.