Dynamic Excited-State Intramolecular Proton Transfer Mechanisms of Two Novel 3-Hydroxyflavone-Based Chromophores in Two Different Surroundings

The dynamic excited-state intramolecular proton transfer (ESIPT) mechanisms of two novel 3-hydroxyflavone-based chromophores (1 and 2) in different surroundings (nonpolar cyclohexane and polar acetonitrile solvents), which are reported in the recent work (Chou et al. J. Phys. Chem. A. 2010, 114, 10412), are explored in terms of the density functional theory (DFT) and time-dependent DFT theoretical methods. The computational absorption and emission spectra for the work rendered here were in reasonable agreement with the relevant experiment. In order to present the molecular-level exposition of the ESIPT reactions for these compounds in two different solvents, we calculated the hydrogen bond (HB) parameters, corresponding infrared vibrational frequencies, frontier molecular orbitals, and maps of electron density difference between the So and Si states, and the HB strengthening tendency in S-1 states was verified, giving the probability of ESIPT reactions. In addition, to definitely expose the ESIPT mechanisms of compounds 1 and 2, we built the potential energy curves and potential energy surfaces in the S-0 and S-1, states. Calculated results exhibited that the ESIPT reaction of compound 1 in nonpolar cyclohexane solvent was more susceptible than that in polar acetonitrile solvent. For the asymmetric compound 2, only single-ESIPT processes could occur in both the solvents, and double-ESIPT processes were prohibitive due to high potential energy barriers. Moreover, the single-ESIPT processes [I (6.26 kcal/mol) and II (6.62 kcal/mol)] in cyclohexane were more susceptible than that [I' (6.91 kcal/mol) and II' (6.90 kcal/mol)] in acetonitrile. Furthermore, the single-ESIPT process I had a little advantage over the process II in cyclohexane, while the probabilities of processes I' and II' were roughly the same in acetonitrile.