title: Experimental and theoretical study of the rotational reorientation dynamics of 7-aminocoumarin derivatives in polar solvents: hydrogen-bonding effects.
abstract: 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.
authors.affiliation: State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, Liaoning, China.
authors.forename: Panwang
authors.initials: P
authors.lastname: Zhou
authors.forename: Peng
authors.initials: P
authors.lastname: Song
authors.forename: Jianyong
authors.initials: J
authors.lastname: Liu
authors.forename: Keli
authors.initials: K
authors.lastname: Han
authors.forename: Guozhong
authors.initials: G
authors.lastname: He