Role of Intramolecular and Intermolecular Hydrogen Bonding in Both Singlet and Triplet Excited States of Aminofluorenones on Internal Conversion, Intersystem Crossing, and Twisted Intramolecular Charge Transfer

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Abstract

Time-dependent density functional theory method was performed to investigate the intramolecular and intermolecular hydrogen bonding in both the singlet and triplet electronic excited states of aminofluorenones AF, MAF, and DMAF in alcoholic solutions as well as their important roles on the excited-state photophysical processes of these aminofluorenones, such as internal conversion, intersystem crossing (ISC), twisted intramolecular charge transfer (TICT), and so forth. The intramolecular hydrogen bond C=O...H-N can be formed between the carbonyl group and amino group for the isolated AF and MAF. However, no intramolecular hydrogen bond for DMAF can be formed. At the same time, the most stable conformation of DMAF is out-of-plane structure, where the two dihedral angles formed between dimethyl groups and fluorenone plane are 163.1 degrees and 41.74 degrees, respectively. The formation of intramolecular hydrogen bond for AF and MAF is tightly associated with the intersystem crossing of these aminofluorenones. Furthermore, the ISC process can be dominantly determined by the change of intramolecular hydrogen bond between S(1) and T(1) states of aminofluorenones. Since the change of hydrogen bond between S(1) and T(1) states of AF is stronger than that of MAF, the rate of ISC process for AF is faster than that for MAF. Moreover, the rate constant of the ISC process of DMAF is nearly close to zero because of the absence of intramolecular hydrogen bond. On the other hand, the intermolecular hydrogen bond C=O...H-O can be also formed between all aminofluorenones and alcoholic solvents. The internal conversion process from S(1) to S(0) state of these aminofluorenones is facilitated by the intermolecular hydrogen bond strengthening in the electronic excited state of aminofluorenones because of the decrease of energy gap between S(1) and S(0) states. At the same time, the change of intermolecular hydrogen bond between S(1) and T(1) states for AF is much stronger than that for MAF, which may also contribute to the faster ISC process for AF than that for MAF in the same solvents. The TICT process plays an important role in the deactivation of the photoexcited DMAF, since the TICT process along the twisted dihedral angle is nearly barrierless in the S(1) state of DMAF. However, the TICT cannot take place for AF and MAF because of the presence of the intramolecular hydrogen bond.

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