Different types of hydrogen-bonded complexes would accelerate or delay the excited state proton transfer in 3-hydroxyflavone

Abstract

3-hydroxyflavone (3HF) has been found dual emission in polar and protic solvents for many years. The excited state proton transfer (ESPT) is the key to control the dual emissions of 3HF. We found the ESPT rate of 3HF in hexane is faster than in acetonitrile because of the lower ESPT energy barrier in hexane. Hence, it was found there are two ESPT rates in methanol, one is faster than in hexane, and the other is slower than in acetonitrile. Two solvent-solute hydrogen bonded complexes (I and II) response to the two ESPT rates. The energy barrier of ESPT was found the lowest (1.324 kcal/mol) in Complex I, corresponded to the shortest ESPT time constant (<0.09 ps). In Complex II, the ESPT time constant is the longest (21.97 ps) with the highest ESPT energy barrier (3.865 kcal/mol). Comparing the results of two complexes, we found that intermolecular hydrogen-bonds would accelerate or delay the ESPT of 3HF based on their different types.