CYCLODEXTRIN EFFECTS ON INTRAMOLECULAR CHARGE-TRANSFER OF 2-BIPHENYLCARBOXYLIC ACID - A PRETWISTED MOLECULE

Intramolecular charge transfer (ICT) of a pre-twisted 2-biphenylcarbox ylic acid (2BPCA) in aqueous cyclodextrin (CD) solution has been studi ed 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 i n the excited state. The ICT emission of 2BPCA at 390 nm in aqueous so lution is quenched upon addition of alpha- or beta-CD,double dagger fo llowed by an enhancement of a new emission at 330 nm. In parallel with this phenomenon, the 330 nm fluorescence decay is resolved into two d ecay components upon addition of CD in contrast to the single exponent ial decay of the ICT emission. These results, and AM1 calculations, su ggest that the ICT of 2BPCA in a CD solution is inhibited by a restrai nt on the further twist of the biphenyl moiety of the photoexcited 2BP CA 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.