Cyclodextrin effects on excited-state geometry change and intramolecular charge transfer of 4-biphenylcarboxylic acid

The cyclodextrin (CDx) effects on the spectroscopic properties of a pretwisted molecule, 4-biphenylcarboxylic acid (4-BPCA), in aqueous solution have been examined. It is observed that the intramolecular charge transfer (ICT) emission of 4-BPCA is blue-shifted upon addition of α-CDx and β-CDx, manifesting that the excited-state ICT of 4-BPCA is partially inhibited by forming the 1:1 CDx inclusion complex. Particularly in the presence of α-CDx, two isoemissive points and dual fluorescence decays are observed, being consistent with the consecutive formation of 2:1 inclusion complex between α-CDx and 4-BPCA. Temperature effects on the fluorescence spectrum and decay kinetics of the inclusion complex suggest that the excited-state geometry change toward coplanarity of the biphenyl moiety becomes slightly accessible with suppression of the ICT in the 2:1 complex, revealing the internal rotational barrier energy of 6 ± 1 kJ/mol. This can be attributed to the inhibition of the interaction of carboxyl group with the polar and hydrogen-bonding water as well as the reduced polarity inside the CDx cavity.