A Theoretical Investigation on Intramolecular Hydrogen Bond: The ESIPT Mechanism of dmahf Sensor

The properties of intramolecular hydrogen bond of a new photochemical sensor 4'-N,N-dimethylamino-3-hydroxyflavone (dmahf) has been investigated in detail. Using Atoms-In-Molecule method, we have demonstrated that the intramolecular hydrogen bond was formed in the ground state (S-0 state). The calculated dominating bond lengths and angles involved in hydrogen bond demonstrates that the intramolecular hydrogen bond can be strengthened in the first excited state (S-1 state). In addition, the variation of hydrogen bond of dmahf has been also testified based on infrared vibrational spectra. Further, hydrogen bonding strengthening manifests the tendency of excited state intramolecular proton transfer process. According to the calculated results of potential energy curves along O-H coordinate, the potential energy barrier of about 7.49 kcal/mol is discovered in the S-0 state. However, a lower potential energy barrier of 1.61 kcal/mol has been found in the S-1 state, which demonstrates that the proton transfer process is more likely to happen in the S-1 state than the S-0 state. In other words, the proton transfer reaction can be facilitated based on the photoexcitation effectively. In turn, through the process of radiative transition, the proton-transfer form dmahf-keto regresses to the ground state with the fluorescence of 578 nm.