Experimental and theoretical study of the rotational reorientation dynamics of 7-animocoumarin derivatives in polar solvents: hydrogen-bonding effects

The rotational reorientation dynamics of 7-aminocoumarin derivatives with different alkylation degrees in methanol, dimethylformamide, and dimethyl sulfoxide have been investigated using femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS TR SEP FD) spectroscopy. In addition to a long anisotropy decay time that accounts for the overall rotational relaxation of solutes, a short anisotropy decay time on the order of picoseconds or sub-picoseconds was also observed in hydrogen-bonding systems. Three types of hydrogen bonds involving the nitrogen lone pair, carbonyl group, and amino group of 7-aminocoumarin derivatives were denoted as types A, B, and C, respectively. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were carried out to investigate the geometric structures of isolated coumarins and hydrogen-bonded complexes in the ground and excited states, respectively. According to our results and analysis, the rapid anisotropy decays observed here in hydrogen-bonding systems may be associated with the strengthening of hydrogen bonds B or C, or both in the excited state of hydrogen-bonded coumarin–solvent complexes, and are independent of the breaking of hydrogen bond A. The strengthening of hydrogen bond C in the excited state of 7-aminocoumarin–DMF and 7-aminocoumarin–DMSO complexes has been demonstrated theoretically for the first time. Further experimental studies would be crucial to confirm this observation.