Femtochemistry of Hydrogen Bonded Complexes after Electronic Excitation in the Liquid Phase:  The Case of Coumarin 102

Abstract

We review our results on hydrogen bonding dynamics in the electronically excited state of coumarin 102 (C102), where we deduced that a hydrogen bond cleavage occurs within 200 fs. We compare the electronic absorption and emission properties of C102 hydrogen bonded to CHCl3, phenol or 2,2-dimethyl-3-ethyl-3-pentanol. We introduce the technique of two-pulse photon echo to show that an optical coherence between the optically coupled electronic ground and excited states survives until 180−200 fs, in the case of C102−(phenol)1,2 complexes, and argue that this is an indication that an electronic state hopping occurs around this time. By comparison with the previous pump−probe and grating scattering results we deduce that this state hopping is accompanied by a hydrogen bond cleavage.

Femtosecond Pump‐Probe Measurements of Solvation by Hydrogen‐Bonding Interactions

“…[21-23, 27-33, 57-73] Elsaesser, Nibbering, and coworkers [27][28][29][30][31][32] have conducted a series of time-resolved IR absorption measurements on the hydrogen-bonding interactions of coumarin 102 (C-102). A very similar dye coumarin 153 (C-153) has been widely used in polar solvation studies.…”

“…In these hydrogen-bonding studies the C-102 molecule was found to act as a base in solution, accepting a hydrogen bond at the C=O site of the coumarin backbone. [31][32] Upon ultrafast electronic excitation the C=O···H bond was cleaved within 200 fs, which indicates that the excited coumarin molecule becomes a weaker base. Elssaeser, Nibbering, and co-workers have concluded the hydrogen-bond breakage is most probably not immediate and that hydrogen-bond dynamics are likely to occur below their time resolution (170 fs) following the electronic excitation of the hydrogen-bond acceptor.…”

“…Both DMSO and dioxane are aprotic, unassociating solvents which are known to form strong 1:1 complexes with phenol derivatives. [31,32] In the case of HPTA, a OÀH···O hydrogen bond is formed in the electronic ground state, whose existence is easily detected by IR absorption ( Figure 2) and UV absorption spectroscopy (Figures 3 and 4). The latter method also allowed the quantitative analysis of the hydrogen-bonding equilibrium constant, Figure 5.…”

Time-Dependent Density Functional Theory Investigation of the Ground and Excited States of Coumarins 102, 152, 153, and 343

“…24,[38][39][40][41][42] Excited-state proton-transfer reactions have also been widely studied using 7-aminocoumarins. [43][44][45][46][47][48] We have recently published a theoretical study of coumarins 151 and 120. 49 These coumarins have been used in a variety of applications but are small enough to admit treatment with a wide range of electronic structure techniques.…”

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

“…21 Time-resolved vibrational spectroscopic experiments on the hydrogen bonded complexes of C102 with phenol, 22,23 aniline 24 and water 25 indicated the cleavage of the hydrogen bonds in a few hundred femtoseconds following the S 0 S 1 excitation of C102. Theoretical calculations, however, indicated the initial strengthening of hydrogen bonds in these complexes after the excitation of the dye molecule.…”