Femtosecond Pump-Probe Measurements of Solvation by Hydrogen-Bonding Interactions Ehud Pines,* [a] Dina Pines, [a] Ying-Zhong Ma, [b] and Graham R. Fleming [b] 1. Introduction The study of solute-solvent interactions has long been one of the focal points of solution chemistry. [1–26] Solvation dynamics after excitation of the chromophore to a new electronic state has been the subject of many experimental studies where time-resolved fluorescence Stokes-shift measurements and ex- cited-state transient absorption [1–15] were the important tools as well as absorption line-shape analysis, [16–18] resonance Raman scattering [19–20] and coherent techniques. [21–23] Solute– solvent interactions are mainly due to the dielectric properties of the solvent. Since the range of the Coulomb interaction is large compared to the structure in the radial distribution func- tion of the solvent, such interactions are often termed “non- specific”, implying that no specific chemical bonds are formed or broken between the solvent and the solute. Another common type of solute-solvent interaction results from hydro- gen bonding. In this case, hydrogen bonds are formed be- tween specific solute and solvent atoms. [7] Both types of solva- tion interactions act to reduce the free energy of the solute. In the event of rapid excitation of the chromophore to a new electronic state, the solvent rearranges itself around the solute molecule, so as to re-minimize the free energy of the solute in its newly formed electronic state. The dynamic aspects of polar solvation have been studied extensively over the past 30 years and have resulted in a de- tailed description of polar solvation. [1–26] In contrast, relatively few studies on the dynamic aspects of solvation by specific hy- drogen-bonding interactions have been carried out. [7–8, 27–33] These experiments have been performed on a limited number of hydrogen-bonded complexes. In view of the great variety, high abundance and crucial importance of the hydrogen- bonding interaction to solution chemistry and biochemistry, it is clear that much additional experimental work is needed to comprehend the dynamic aspects of the hydrogen-bonding in- teractions. In particular, the effect of intermolecular hydrogen- bonding interactions on the electronic states of a solute has been extensively studied in steady-state solution-chemis- try. [34–36] A systematic extension of these studies to the time domain is essential for our complete understanding of such processes as solvation dynamics and proton-transfer reac- tions. [37] Another domain where understanding the role of hydrogen- bonding interactions is of great importance is in biological en- vironments, which are usually of low polarity. In low polarity environments, intra- and intermolecular hydrogen-bonding in- teractions rival the energies of nonspecific polar interactions. Indeed, hydrogen-bonding interactions often stabilize and de- termine the 3D structure of large biological structures such as proteins and DNA, and can play an important role in determin- ing the reactivity of the active site of enzymes. [38] Herein we report femtosecond time-resolved pump–probe measurements of a novel hydrogen-bonded system in solution, described below. The transient pump-probe data has been treated in terms of the time-dependent absorption correlation function, given in Equation (1): [1] CðtÞ¼ nðtÞnð1Þ nð0Þnð1Þ ð1Þ where n(t) is the first moment of the excited-state absorption spectrum at time t. In the case of the uncomplexed chromo- phore C(t) provides comparable information on solvation dy- namics to that of time-resolved fluorescence (Stokes-shift) measurements. We have found that the C(t) of the hydrogen- bonded chromophore reveals additional (over polar solvation) dynamics associated with transient hydrogen-bonding interac- [a] Dr. E. Pines, Dr. D. Pines Chemistry Department, Ben-Gurion University of the Negev P.O.B. 653, Beer-Sheva 84105, (Israel) Fax:(+ 972) 8-6472-943 E-mail: epines@bgumail.bgu.ac.il [b] Dr. Y.-Z. Ma, Prof. G. R. Fleming Department of Chemistry, University of California Berkeley Lawrence Berkeley National Laboratory, Berkeley, CA 94720–1460 (USA) An additional ultrafast blue shift in the transient absorption spec- tra of hydrogen-bonding complexes of a strong photoacid, 8-hy- droxypyrene 1,3,6-trisdimethylsulfonamide (HPTA), over the solva- tion 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  10 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 dime- thylsulfoxide (DMSO) and dioxane, respectively, and about 29% of the total C(t) of HPTA in dichloroethane (DCE) when it was hy- drogen-bonded to DMSO. We have assigned this additional dy- namic spectral shift to a transient change in the hydrogen bond (O H···O) that links HPTA to the complexing base, after the elec- tronic excitation of the photoacid. ChemPhysChem 2004, 5, 1315 – 1327 DOI: 10.1002/cphc.200301004  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1315