Theoretical Studies on the Fluorescence Enhancement of Benzaldehydes by Intermolecular Hydrogen Bonding.

Density functional theory and its time-dependent extension are employed to investigate the intermolecular hydrogen-bonding- (Inter-HB-) induced fluorescence enhancement of benzaldehydes. The fluorescence quenching mechanism of benzaldehydes in chloroform is attributed to the low energy gap between the lowest ππ* and nπ* states at Franck-Condon (FC) point and conical interaction between ππ* and nπ* states. The Inter-HB between the aldehyde group of benzaldehydes and methanol can considerably increase the energy gap between ππ* and nπ* states at the FC point, and this type of hydrogen bond is strengthened in the excited ππ* state, thereby precluding the nonradiative transition from nπ* state and enhancing fluorescence. The Inter-HB between the hydroxy group of benzaldehydes and methanol is considerably stronger than that between the aldehyde group of benzaldehydes and methanol; which has a little effect on the energy gap of ππ* and nπ* states. Therefore, if Inter-HB exists between the hydroxy group of benzaldehydes and methanol, then it will play an important role in ground and excited ππ* states, and fluorescence enhancement by Inter-HB between the aldehyde group of benzaldehydes and methanol will be absent. Our proposed mechanism can explain the experimental observations and differs from that of "proximity effects".