Early Time Hydrogen-Bonding Dynamics of Photoexcited Coumarin 102 in Hydrogen-Donating Solvents:  Theoretical Study

Abstract

To study the early time hydrogen-bonding dynamics of chromophore in hydrogen-donating solvents upon photoexcitation, the infrared spectra of the hydrogen-bonded solute-solvent complexes in electronically excited states have been calculated using the time-dependent density functional theory (TDDFT) method. The hydrogen-bonding dynamics in electronically excited states can be widely monitored by the spectral shifts of some characteristic vibrational modes involved in the formation of hydrogen bonds. In this study, we have demonstrated that the intermolecular hydrogen bonds between coumarin 102 (C102) and hydrogen-donating solvents are strengthened in the early time of photoexcitation to the electronically excited state by theoretically monitoring the stretching modes of C=O and H-O groups. This is significantly contrasted with the ultrafast hydrogen bond cleavage taking place within a 200-fs time scale upon electronic excitation, proposed in many femtosecond time-resolved vibrational spectroscopy experiments. The transient hydrogen bond strengthening behaviors in excited states of chromophores in hydrogen-donating solvents, which we have demonstrated here for the first time, may take place widely in many other systems in solution and are very important to explain the fluorescence-quenching phenomena associated with some radiationless deactivation processes, for example, the ultrafast solute-solvent intermolecular electron transfer and the internal conversion process from the fluorescent state to the ground state.

Validation and Divergence of the Activation Energy Barrier Crossing Transition at the AOT/Lecithin Reverse Micellar Interface

“…It should be noted that hydrogenbonding interactions have been proved to play a vital role in the electronically excited states of other fluorophores including coumarin dye (e.g. Coumarin 102 33 ) in recent literature. [34][35][36][37] In a recent spectroscopic study 32 on the hydrogen-bonding properties of C-500, it has been demonstrated that the quantum yield of the dye C-500 in hexane (devoid of hydrogen bonding, Φ ) 0.76) is comparable to those in CHCl 3 (hydrogen-bond donor, Φ ) 0.81) and THF (hydrogen-bond acceptor, Φ ) 0.81).…”

Wagging motion of hydrogen-bonded wire in the excited-state multiple proton transfer process of 7-hydroxyquinoline·(NH3)3 cluster

“…However, little is known about electronic excited-state hydrogen bond, because the structure and dynamics of which are difficult to analyze for both theoretical and experimental studies. Recently, Zhao and Han [12][13][14][15][16][17][18][19] have determined theoretically that intermolecular hydrogen bonds between solute and alcoholic molecules can be significantly strengthened in the electronic excited state upon photoexcitation. In previous works [20][21][22][23], we have demonstrated that, the excited-state hydrogen bonding behavior would play an important role in many photochemical reactions such as fluorescence quenching [20], excited-state proton transfer [21], tuning effects on photochemistry [22].…”

Solvation and Protonation of Coumarin 102 in Aqueous Media: A Fluorescence Spectroscopic and Theoretical Study

“…Theoretical calculations, however, indicated the initial strengthening of hydrogen bonds in these complexes after the excitation of the dye molecule. [26][27][28][29] Excited state proton transfer of C102 was observed in the acidic aqueous 30 and ethanol 31 solutions of the dye. It was manifested as a stretch of the rising edge of the fluorescence decay curves measured in the emission wavelength range of the protonated species.…”