Conformationally Controlled Intramolecular Charge Transfer Complexes

Trans -1-acceptor-2-donor-substituted cyclohexanes ( 1 ), as well as their 4- (or 5-)methyl-substituted homologues ( 2 ), have been prepared and are shown to form intramolecular charge-transfer (donor−acceptor) complexes. These weak complexes are turned on and off by the chair−chair interconversion of the cyclohexane ring. The CT absorptions have been measured and the equilibrium constants for the ring reversal have been determined by UV/vis spectroscopy at 298 K, as well as by NMR spectroscopy at two temperatures: at 183 K, by direct comparison of signals due to the two chair conformations, and at 300 K, by comparison of calculated and measured widths of the α-proton signals. The Gibbs free energies assigned to the donor−acceptor interactions range between 0 and −1 kcal mol - 1 . A crystal structure of one of the complexes ( 1b ) confirms the intramolecular donor−acceptor alignment and interaction. The regioisomers of the methyl-substituted complexes were characterized by NOE interaction between the methyl and an α-proton cis to it.