Theoretical study on twisted intramolecular charge transfer of 1-aminoanthraquinone in different solvents**Project supported by the Program of Outstanding Innovation Team of Hubei Normal University, China (Grant No. T201502), the Natural Science Foundation of Hubei Province, China (Grant Nos. 2014CFB349 and 2016CFC742), and the National Natural Science Foundation of China (Grant No. 11674355).

Abstract

The twisted intramolecular charge transfer and the excited state relaxation of 1-aminoanthraquinone (1-NH2-AQ) in different solvents are investigated using quantum chemical calculations in this paper. The geometries of the ground state are optimized both in gas and solvents based on the high-level ab initio calculations, the lowest excited singlet state geometry is optimized only in gas for simplicity. An intramolecular charge transfer property is substantiated by the large change of dipole moments between the S0 and S1 states. The mechanism of twisted intramolecular charge transfer is proposed by the conformational relaxation on the potential surface of the S1 state. Quantum chemical calculations present that internal conversion and intersystem crossing are important approaches to the ultrafast deactivation of the S1 state via the twisting of the amino group. The smaller energy difference between the S0 and S1 state shows that the internal conversion process is much faster in a polar solvent than in a nonpolar solvent. Energy intersections between the T2 and S1 state in cyclohexane and dioxane indicate a faster intersystem crossing process in them than in ethanol. These theoretical results agree well with the previous experimental results. Energy barriers are predicted on the potential surface of the S1 state, and they have a positive correlation to solvent viscosity, and the timescale of twisted intra-molecular charge transfer in dioxane is predicted to be longer than in cyclohexane and ethanol.