Theoretical Investigation On The Excited-state Intramolecular Proton Transfer Of Several Typical Hydrogen-bonded Molecular Systems

In this paper,4'-methoxy-3-hydroxyl flavone?5-HB?,2-?5-carboxyphenyl?-2-hydroxyphenyl)benzothiazole?6-HB?and??Z?-4-?7-hydroxy-2,3-dihydro-1H-inden-1-ylidene?-1,2-dimethyl-1H-imidazol-5?4H?-one??7-HB?,which has five-,six-and seven-membered intramolecular hydrogen-bonded ring between proton donor and proton acceptor,respectively,were chosen to investigate the excited-state intramolecular proton transfer?ESIPT?process and photophysical properties in the gas phase by using density functional theory?DFT?and time-dependent density functional theory?TD-DFT?.A large number of calculations have revealed the enhancement mechanism of intramolecular hydrogen bonding in the excited state,and 6-HB and 7-HB are easier and faster to occur in ESIPT process than 5-HB.The larger the size of the intramolecular hydrogen-bonded ring,the easier the ESIPT process occurs.However,when the size of the intramolecular hydrogen-bonded ring is sufficiently large,the increase in the size of intramolecular hydrogen-bonded ring has little effect on the ESIPT process.In addition,the substituent effects on excited-state intramolecular proton transfer of 2-?5-carboxyphenyl?-2-hydroxyphenyl)benzothiazole?CHBT?and??Z?-4-?7-hydroxy-2,3-dihydro-1H-inden-1-ylidene?-1,2-dimethyl-imidazol-5?4H?-one??o-LHBDI?were investigated.In 4R-CHBT?R:-CH₃,-OH,-COOH,-CF₃,-CN?,through analysis of structural parameters,infrared?IR?vibrational spectra and potential energy curve,we found that the proton transfer process can not occur in the ground state,but it is easy to proceed in the excited state.The introduction of electron-withdrawing groups?R:-COOH,-CF₃,-CN?makes the ESIPT process more difficult to occur,and the electron-donating group has little effect on the ESIPT barrier of CHBT.In 4R-o-LHBDI?R:-OH,-NH₂,-CN,-CF₃,-NO₂?,by analyzing the geometric structures,infrared vibration spectra,electron density and Mulliken charges,the substitution of electron-withdrawing and electron-donating groups at the C₄ position increases ESIPT barrier.