Excited‐state intramolecular proton transfer of 6‐amino‐2‐(2′‐hydroxyphenyl) benzoxazole (6A‐HBO) in different solvents

Abstract

The excited‐state intramolecular proton transfer (ESIPT) process of 6‐amino‐2‐(2′‐hydroxyphenyl) benzoxazole (6A‐HBO) was investigated using density functional theory and time‐dependent density functional theory methods with B3LYP and TZVP basis sets. n‐Heptane, dichloromethane, methanol, and acetonitrile were chosen as a series of polar solvents in calculations using the IEFPCM model. To obtain a more comprehensive ESIPT mechanism, we constructed the S0 and S1 states' potential energy surfaces (PESs) by incrementally twisting the ─OH bond and increasing the distance of O–H bond. Based on the analysis of the bond lengths, the IR vibrational spectra, and the frontier molecular orbits (MOs), the intramolecular hydrogen bonding (O─H...N) is clearly strengthened, and the charge is redistributed in the S1 state. The results of calculated absorption spectrum are in accord with the experimental data. The fluorescence spectrum of 6A‐HBO‐enol showed a normal red shift, but the red shift of the 6A‐HBO‐keto is larger and increases with the solvent polarity, indicating a charge transfer. Analysis of the PESs indicates a lower potential energy barrier in S1 state for the proton transfer from the O atom to the N atom, with the excited state potential barrier slightly decreasing with the increase of the solvent polarity.

Investigation of the intramolecular hydrogen bonding interactions and excited state proton transfer mechanism for both Br-BTN and CN-BTN systems

“…It is worth mentioning that the TDDFT method has become a very useful tool to research the hydrogen bond in the excited states of the hydrogen-bond system theoretically. [21][22][23][24][25][26][27][28][29][30] Becke' three-parameter hybrid exchange functional with Lee-Yang-Parr gradient-corrected correlation (B3LYP functional) has been used in both the DFT and TDDFT methods. [43][44][45][46][47][48] The 6-311+G(d) is chosen as basis sets throughout, which has sufficient exibility to describe this system.…”

“…Experimentally, this kind of ultrafast process is in the region of femtosecond to picosecond. And the uorescence band of keto* exhibits a large Stokes shi, [21][22][23][24][25][26][27][28][29][30] which could be as large as 8000-12 000 cm À1 . In particular, the fast charge redistribution resulting from ESIPT tautomer keto*, multiple optoelectronic applications have been designed and developed, such as uorescence sensors, luminescent materials, UV absorbers, molecular logic gates, and so forth.…”

Theoretical exploration about the ESIPT mechanism and hydrogen bonding interaction for 2‐(3,5‐dichloro‐2‐hydroxy‐phenyl)‐benzoxazole‐6‐carboxylicacid

“…Since half a century ago, the ESIPT process was firstly reported by Weller and coworkers in experiment with methylsalicylate; it has become a popular research in recent years . And it is worth mentioning that the photo‐tautomer of the ESIPT reaction owns novel charge redistribution, which facilitates the applications of fluorescence sensors, biological chemosensors, molecular switches, laser dyes, UV‐light polymer stabilizers, and so forth …”

A TD‐DFT investigation of the photo‐induced excited state intramolecular proton transfer dynamics for the novel 5,5′‐(9,9‐dihexyl‐9H‐fluorene‐2,7‐diyl)bis(2‐benzo[d]thiazol‐2‐yl)phenol) system

“…In general, the proton transfer process can be regarded as a series of elementary steps, such as ion‐pair formation, hydrogen bonding rearrangement, electronic redistributions, and so forth. In view of its excited state situation, the excited state intramolecular or intermolecular proton transfer (ESIPT) process should be one of most fundamental and important research focus . It is worth mentioning that the ESIPT compounds have become extensively studied because of their wide application in serving as model systems in chemical processes and in organic light emitting diodes (OLEDs) …”