Effects of solvent and cyclodextrin on the photophysical properties of 4-acetylbiphenyl: Intramolecular charge transfer associated with hydrogen-bonding effect

The photophysical properties of 4-acetylbiphenyl (ACBP) in various solvents and in aqueous α-cyclodextrin (α-CD) solution have been investigated using steady-state and time-resolved fluorescence spectroscopy. In non-polar solvents, the fluorescence spectrum with an emission maximum at 310 nm exhibits a vibrational structure with a non-mirror image to the absorption spectrum, reflecting the change in excited-state geometry toward the coplanarity of the biphenyl moiety. As the solvent polarity increases, the fluorescence spectrum denotes the structureless normal emission and an additional large Stokes' shift band around 380 nm. This behavior indicates the formation of an intramolecular charge transfer (ICT) state through relaxation from the normal excited state where the excited-state geometry change is hindered. Especially in water, the ICT emission is further red shifted to 400 nm with the normal emission band at 310 nm, and the relative intensities between 310 and 400 nm emission bands are affected by the excitation wavelength. However, this excitation wavelength dependence is not so large in organic polar solvents and aqueous α-CD solutions. The 400 nm emission in water exhibits a single exponential decay with a very short decay time (34 ps) while it shows a bi-exponential or triple-exponential decay in organic solvents and aqueous α-CD solutions. These results suggest that the ICT state in water is stabilized through exciplex formation by the hydrogen-bonding interaction between the acetyl group and water.