Femtosecond pump-probe measurements of solvation by hydrogen-bonding interactions

An additional ultrafast blue shift in the transient absorption spectra of hydrogen-bonding complexes of a strong photoacid, 8 hydroxypyrene 1,3,6-trisdimethylsulfonamide (HPTA), over the solvation response of the uncomplexed HPTA and also over that of the methoxy derivative of the photoacid (MPTA) in the presence of the hydrogen-bonding base was observed on optical excitation of the photoacid. The additional 55+/-1 fs solvation response was found to be about 35% and 19% of the total C(t) of HPTA in dichloromethane (DCM) when it was hydrogen bonded to dimethylsulfoxide (DMSO) and dioxane, respectively, and about 29% of the total C(t) of HPTA in dichloroethane (DCE) when it was hydrogen-bonded to DMSO. We have assigned this additional dynamic spectral shift to a transient change in the hydrogen bond (O-H...O) that links HPTA to the complexing base, after the electronic excitation of the photoacid.