Synthesis and supramolecular properties of molecular clips with anthracene sidewalls

Novel molecular clips with anthracene sidewalls (1a-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 1c and 1,2,4,5-tetracyanobenzene (TCNB) complex 14@ 1b the clips' anthracene sidewalls have to be compressed substantially during the complex formation to provide attractive pi-pi interactions between the aromatic guest molecule and the two anthracene sidewalls in the complex. Re 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 pi-pi interactions. The finding that complexes of the clips 1a-c are more stable than those of the corresponding clips 2a-c can be explained in terms of the larger van der Waals contact surfaces of the anthracene sidewalls in la-c (relative to the naphthalene sidewalls in 2a-c). Color changes resulting from charge-transfer (CT) bands are observed in complex formation by 1a-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 1b and guest 14 fluoresce from their respective excited singlet states, whilst in the complex 14@1b 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@1b is the first example of CT luminescence from a host-guest complex. The binding constant determined for the formation of the TCNB complex 14@1b from a UV/Vis titration experiment (K-a = 12400m(-1)) agrees well with the value (K-a = 12800M(-1)) obtained by H-1 NMR titration.