Cyclodextrin effects on intramolecular charge transfer of 2-biphenylcarboxylic acid: a pre-twisted molecule
Intramolecular charge transfer (ICT) of a pre-twisted 2-biphenylcarboxylic acid (2BPCA) in aqueous cyclodextrin (CD) solution has been studied by using the steady-state and time-resolved fluorescence techniques, and it has been demonstrated that the ICT interaction is accompanied by a further twist of the biphenyl moiety in order to be orthogonal in the excited state. The ICT emission of 2BPCA at 390 nm in aqueous solution is quenched upon addition of α- or β-CD, followed by an enhancement of a new emission at 330 nm. In parallel with this phenomenon, the 330 nm fluorescence decay is resolved into two decay components upon addition of CD in contrast to the single exponential decay of the ICT emission. These results, and AM1 calculations, suggest that the ICT of 2BPCA in a CD solution is inhibited by a restraint on the further twist of the biphenyl moiety of the photoexcited 2BPCA in the CD cavity. The restraint of the conformational change is due to the reduced intermolecular hydrogen bonding of 2BPCA compared with when in water, as well as to the reduced polarity of the CD cavity.