J. Phys. Chem. 1988,92, 7039-7041
7039
Femtosecond Microscopic Solvation Dynamics of Aqueous Solutions Wlodzimierz Jarzqba,+Gilbert C. Walker, Alan E. Johnson, Michael A. Kahlow, and Paul F. Barbara*gt Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: September 26, 1988)
Solvation dynamics around a solute dissolved in water have been measured for the first time on the femtosecond time scale. The measurements were made by the fluorescence time-dependent Stokes shift technique, which employs polar fluorescent probes and ultrafast fluorescence spectroscopy to characterize the solvation dynamics of electronically excited molecules. The probe used in this study is the 7-(dimethylamino)coumarin-4-acetateion. The observed microscopic solvation dynamics are well modeled by a biexponential decay with lifetimes (7,,72) and amplitudes ( A l ,A z ) as follows: 71 = 0.16 ps ( A , = 0.33) and 72 = 1.2 ps (Az = 0.67). The experimental results have been compared to predictions from the dielectric continuum theory, the linearized MSA theory, and recently published molecular dynamics simulations of water solvation.
Introduction
The measurement of the dynamics of solvation by ultrafast spectroscopy has been the subject of numerous recent experiand studies. Typically, solvation relaxation has been studied by measuring the time-resolved emission spectrum of a fluorescing probe molecule that has a much smaller dipole moment in the ground state So than the excited state S,. Optical excitation of such a probe prepares the solute/solvent system in a nonequilibrium configuration. Relaxation of the solvent polarization about the excited-state dipole leads to a time-dependent red shift of the fluorescence spectrum, which is usually quantified by the correlation function e(?). P(t) '(') = p(0)
- v(m) - p(m)
pered by the need for extraordinarily fast time resolution to study this liquid and the absence of good water soluble solvation probes. We have recently been able to overcome these limitations,16 and in this paper we report the first solvation measurement on water by the fluorescence technique. Experimental Section
7-(Dimethylamino)coumarin-4-acetic acid was obtained from Molecular Probes, Inc., and was used without further purification. The sodium salt of 7-(dimethylamino)coumarin-4-aceticacid was prepared prior to the experiment by mixing a solution of the acid in water with the stoichiometric amount of NaOH solution.
Here e(t) is the frequency of the fluorescence spectrum maximum (1) Nagarajan, V.; Brearley, A. M.; Kang, T. J.; Barbara, P. F. J . Chem. at time t . Phys. 1987, 86, 3183. (2) Kahlow, M. A.; Kang, T. J.; Barbara, P. F. J. Chem. Phys. 1988,88, Early papers on transient solvation measurements by this 2372. technique were restricted to slowly relaxing glassy and associated (3) Kahlow, M. A,; Jarzpba, W.; Kang, T. J.; Barbara, P. F.Femtosecond solvents due to the limited time resolution of the technology of Resolved Solvation Dynamics in Polar Solvents. J . Chem. Phys., in press. transient emission measurements. Very recently, with the at(4) Maroncelli, M.; Fleming, G. R. J . Chem. Phys. 1987, 86, 6221. tainment of femtosecond-resolvedemission data in the ultraviolet16 Maroncelli, M.; Fleming, G. R. J . Chem. Phys. 1988, 89, 785. it has been possible to study the transient solvation dynamics of (5) Castner, Jr., E. W.; Maroncelli, M.; Fleming, G. R. J . Chem. Phys. 1987, 86, 1090. Castner, Jr., E. W.; Bagchi, B.; Maroncelli, M.; Webb, S. a broad range of quickly relaxing solvent^.^ The ultraviolet region P.; Ruggiero, A. J.; Fleming, G. R. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, of the spectrum is particularly advantageous, as opposed to the 363. more easily obtained visible region, because the optimal probe (6) Su,S. G.; Simon, J. D. J . Phys. Chcm. 1987, 91, 2693. molecules for transient fluorescence absorb in the ~ l t r a v i o l e t . ' ~ ~ ~ ~ (7) Meech, S.R.; OConnor, D.V.;Phillips, D. J . Chcm. Soc., Faraday The dynamics of polar solvation are interesting from many Trans. 2 1983, 79, 1563. perspectives, such as the study of the coupling of solvent motion (8) Declimy, A,; Rullite, C. Chcm. Phys. Left. 1988, 146, 1. and chemical reaction rates, i.e., the so-called dynamic solvent (9) Kinoshita, S.; Nishi, N.; Kushida, T. Chcm. Phys. Lett. 1987,134,605. (10) Mazurenko, Y.T.; Bakshiev, N. G. Opr. Spcctrosc. 1970, 28, 490. effe~t.'"'~ Also, solvation dynamics offer a means to evaluate (11) Bagchi, B.; Oxtoby, D. W.; Fleming, G. R. Chcm. Phys. 1984, 86, the theoretical models for the structure and dynamics of liquids. 257. For example, there is a rough correlation between experimental (12) van der Zwan, G.; Hynes, J. T. J . Phys. Chcm. 1985, 89, 4181. data on solvation dynamics and theoretical prediction from a (13) Wolynes, P. G. J . Chem. Phys. 1987,86, 5133. dielectric continuum model of solvation, but in many cases the (14) Rips, I.; Klafter, J.; Jortner, J. J. Chcm. Phys. 1988,88, 3246. Rips, quantitative agreement is p00r.l-l~ Interestingly, the linearized I.; Klafter, J.; Jortner, J. J . Chem. Phys. 1988, 89, 4288. mean spherical approximation (MSA) theory, a new model for (15) Loring, R. F.; Mukamel, S. J . Chem. Phys. 1987, 87, 1272. solvation dynamics that incorporates a structured solvent, seems (16) Kahlow, M. A,; Jarqba, W.; DuBruil, T.; Barbara, P. F. Rev.Sci. Instrum. 1988, 59, 1098. able to rationalize certain failures of the continuum mode1.3p4J3J4 (17) Kahlow, M. A.; Kang, T. J.; Barbara, P. F. J. Phys. Chem. 1987,91, The solvent water is noticeably missing from the list of solvents 6452. studied to date by the transient fluorescence method of measuring (18) Kang, T. J.; Kahlow, M. A,; Gisser, D.; Swallen, S.;Nagarajan, V.; solvation dynamics. From a practical and fundamental standpoint Jarzpba, W.; Barbara, P. F. Dynamic Solvent Effect in the Electron Transfer it is arguably the most important solvent. Only for water have Kinetics Bianthryls. J . Phys. Chem., in press. the dielectric properties and solvation dynamics been theoretically (19) For recent reviews and well-referenced articles on experiments on dynamic solvent effects see: (a) Barbara, P.F.; Jarzpba, W. Acc. Chem. Res. modeled in detail by molecular dynamics. Experimentally, the 1988, 21, 195. (b) Barbara, P. F.; Walker, G. C. Reu. Chem. Infermed. 1988, dielectric dispersion of bulk water is uniquely well characterized.20 10, 1. (c) Simon, J. D. Acc. Chem. Res. 1988,21, 128. (d) Kosower, E. M.; Despite the excellent opportunity that water offers to compare Huppert, D. Annu. Rev. Phys. Chem. 1986, 37, 127; (e) McManis, G. E.; theory and experiment, solvation measurements have been hamMishna, A. K.; Weaver, M. J. J . Chem. Phys. 1987,86, 5550. ?On leave from the Faculty of Chemistry, Jagiellonian University, 3 Karasia, 30-060 Krakbw, Poland. *Alfred P. Sloan Fellow and Presidential Young Investigator.
(20) For recent summary of the dielectric data on water see: Mason, P. R.; Hasted, J. B.; Moore, L. Adu. Mol. Relaxation Processes 1974, 6, 217. Hasted, J. B.; Husain, S. K.; Frescura, F. A. M.; Birch, J. R. Chem. Phys. Left. 1985, 118, 622.
0022-365418812092-7039$0 1.5010 0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 25, 1988
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17.0
Energy ( x 1 0 3 c m - ' )
26.0
Figure 2. Reconstructed fluorescence spectra of 7-(dimethylamino)coumarin-4-acetate ion 0.1 and 1 ps after excitation. The solid line represents the best fit of the log normal distribution function to the data.
0.0
time ( p s )
5.0
Figure 3. The upper section of the figure is a superposition of an ex-1.0
time (ps)
4.0
Figure 1. Fluorescence transients of 7-(dimethylamino)coumarin-4-
acetate ion in water recorded at 445 nm (upper), 483 nm (middle), and 509 nm (lower). The solid line through the points is a fit of the data to a multiexponentialdecay. The peak near zero time in the upper panel is the instrument response function (280 fs fwhm). Femtosecond time-resolved emission data were recorded with a recently constructed ultraviolet, fluorescence apparatus that uses the upconversion technique for time resolution. The apparatus is described in detail elsewhere.I6 The laser source is a femtosecond dye laser (styryl 8 dye, HITCI saturable absorber, 792 nm, 70 fs fwhm) that is synchronously pumped by the second harmonic of an actively mode-locked Nd:YAG laser (Quantronix Model 416). The dye laser is amplified at an 8.2-kHz repetition rate by seven passes through a thick dye jet that is optically pumped by a copper vapor laser. The sample is excited by the second harmonic of the amplified laser at 396 nm ( ~ 1 3 fs 0 fwhm) which travels through a variable optical delay. Time resolution is achieved by mixing the fluorescence with the 792-nm light in a KDP crystal to generate light at the sum frequency (upconversion). The apparatus has a 280-fs width (fwhm) instrument response function. Time-resolved emission data in the nanosecond range were collected by the time-correlated single-photon-counting technique. Results and Discussion Coumarin dyes have been shown to be excellent fluorescence probes for solvation dynamics, because they exhibit large dynamic fluorescence shifts and are less susceptible than many polar fluorescent molecules to nonideal probe effects such as excited-state intramolecular charge transfer.24 Unfortunately, ordinary coumarin dyes are not water soluble. In this work we employ a water-soluble coumarin, the sodium salt of 7-(dimethylamino)coumarin-4-acetic acid.
hH,COO- Na' Preliminary results suggest that this compound is an excellent dynamic solvation probe. The emission spectrum of 74dimethylamino)coumarin-4-aceticacid in alcohols analogous to that of other coumarins exhibits a shift toward longer wavelength when solvents of increasing polarity are used.4 The radiative rate constant of SIis not a strong function of the solvent, suggesting
perimentally determined C(t) function for the fluorescing probe 7-(dimethylamino)coumarin-4-acetateion in water and a biexponential fit of C(t) with the parameter given in the front row of Table I. The lower section in the figure portrays the difference between the experimental C(r) and the biexponential fit on a scale that is expanded by a factor of 3 in the Y direction.
that low-lying excited charge-transfer states are not a complication for this probe. The photophysical properties are sensitive, however, to whether the ion or the nondissociated acid is studied. In water we observe photophysical behavior of 7-(dimethylamino)coumarin-4-acetate anion (in solution of salt or low concentrated acid) with a different fluorescence lifetime (1.2 ns) than the nondissociated acid in alcohols (3.0-3.9 ns). We have measured the C(t) function for water by the spectral reconstruction method. Briefly, the fluorescence transients at the various emission wavelength regions are fit to a convolution of the instrument response function and a multiexponential model for the emission Figure 1 shows examples of transients at different wavelengths measured at 298 K. The deconvoluted emission transients for different wavelengths were calibrated to the static spectrum and used to reconstruct emission spectra at different times after excitation. The instantaneous fluorescence frequency maximum ~ ( tis) obtained from a fit of the reconstructed spectrum to a log normal distribution function! Figure 2 shows examples of the reconstructed spectra at different times after excitation with fits to a log normal distribution function. The C(t) function is calculated by using the p ( t ) values from the fitting. Figure 3 shows the C ( t ) function for water in the range 0.04-5 ps obtained by using the procedure described above. The observed C ( t ) function for water decays nonmonoexponentially. However, it can be very well fit by a biexponential function with time constants 0.16 and 1.2 ps and average time 0.86 ps, see Table I. In any dynamic solvation study it is possible that the apparent C(t)may be in error due to nonsolvation effects such as vibrational relaxation of the probe, intramolecular charge transfer of the probe, hydrogen-bonding dynamics between the probe and solvent, and even proton transfer between the probe and solvent. Due to the rapid solvation dynamics of water and its strong solvent interactions, nonideal effects are particularly challenging in this solvent. Our data on the photophysical properties of the probe used in this paper strongly suggest that nonideal effects are not a factor for this probe, but further investigation is necessary to completely establish this important point. It is interesting to compare the experimentally observed solvation dynamics with theoretical predictions (Table I). The first theory
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The Journal of Physical Chemistry, Vol. 92, No. 25, 1988 7041
TABLE I: Comparison of Experimental and Theoretical Values for Solvation Times in Aqueous Solutions 71,
method experimental DCM one relaxation time DCMb Cole-Cole distribution MSA/ionC M D simulations (MCY water) (ST2 water) (TIP4P water)
PS 0.16 (33%) 0.59" ( 100%) 0.48 (96%) 0.59 (82%) 10.1 0.025d (-50%) -
729
(7),
PS 0.86
reference this work
-
0.59
20
1.40 (4%) 3.20 (18%)
0.52
20
1.05
14, 20
1.2 (67%)
1.0 0.3-1.0d (-50%)
-
ps
-
23 0.09-0.33d 24, 25
0.4-0.7
26
"This value is commonly called 7 , the longitudinal relaxation time which is given by 71 = s&e, cc)/(2c0 + cc). For this calculation cc was assumed to be equal to unity. The other parameters, 7D = 8.72, to = 79.2, and em = 4.84, are from Hasted and co-workers." bThe dielectric continuum model was solved by using a numerical method from the literature and a Cole-Cole distribution function for the frequency-dependent dielectric response of water with the parameters of Hasted and co-workers,20 Le., so = 8.7 ps, c, = 4.2, and cr = 0.013. The C(t) calculated by this procedure was well fit by a biexponential form with the lifetimes and amplitudes that are listed in the table. eThe amplitudes and lifetimes for the MSA ion entry in the table were calculated by fitting the simulated c(r)to a biexponential function. The simulations were made using the method of Rips, Klafter, and Jortner" and a computer program from Mark Maroncelli. The dielectric data in note a of this table were used as parameters for the MSA ion simulations, where the solute was considered to have a radius of 4.0 A and the solvent 1.4 A. dThe theoretical solvent relaxation functions are nonexponential and vary depending on the size and charge of the solute. The average time constant was evaluated by integrating the decay from 0.0 to 1.0 ps.
+
is the simple dielectric continuum model (DCM).'b12 This model treats the solvent as a uniform dielectric continuum that is characterized by its bulk frequency dependent dielectric constant e(@). The solute is represented by a point dipole moment in a spherical unpolarizable cavity. In the main dispersion region the complex dielectric constant of water has been modeled with a single dispersion region (Debye relaxation time T~ = 8.7 PS),~Oand alternatively by a C o l d o l e distribution functionB The predicted solvation dynamical parameters from the DCM with the two different models for t(u) are similar to each other and noticeably different than the experimental results as shown in Table I. The experimental results are more dramatically nonmonoexponential, perhaps due to the importance of molecular interactions in the r e l a ~ a t i o n . ' ~The J ~ average relaxation time ( T ) predicted by the DCM is significantly shorter than experiment. Indeed, the dynamical MSA m0de1'~J~ for solvation, which involves a structured solvent, predicts that the solvation of an ion in water occurs on a range of time scales, which is well fit by a biexponential relaxation, see Table I. The MSA calculations a r e
more obviously nonmonoexponential (like the experiment) than the DCM calculations. The MSA relaxation times, however, differ significantly from experiment. In fact, the deviation between the both of these theories and experiment is in the range observed for other l i q ~ i d s . ~ It is important to note that both the DCM and MSA calculations presented here use dielectric data for bulk water from the main dispersion frequency region only. It may be that the higher frequency region of the dielectric response of water (which is ignored in our treatment of water) may play a larger role in the relaxation than the amplitudes of these components for bulk water would indicate; see NeumannSz1 Recently, water relaxation around a dissolved solute molecule has been studied by using the molecular dynamics method.22-26 Engstrom et al.23used molecular dynamics computer simulations to study quadrupole relaxation mechanisms for Li+, Na+, and C1ions in dilute aqueous solution. They found that the N M R relaxation rate for these ions was determined by the relaxation of water molecules in the first shell. Relaxation dynamics in this case can be expressed by biexponential decay with time constants I0.1 ps and T~ 1 ps for all three ions. Maroncelli and Fleming24,25studied the time dependence of solvation of monoatomic ions immersed in large spherical clusters of ST2 water. Calculations for solutes of different size and charge predict nonexponential hydration dynamics with a very short component (10-20 fs) due to librational motions and a longer nonexponential component with an average time constant of a few hundred femtoseconds. Karim et aLZ6studied hydration dynamics using the TIP4P model of water in molecular dynamics simulations. Calculations were performed for changes in the dipole moment of a spherical solute. In this case nonexponential hydration dynamics were also observed with an average time constant for the first solvation shell in the range of 0.4-0.7 ps. Generally, the presented molecular dynamics simultations are in qualitative agreement with experiment, predicting nonexponential hydration dynamics with an average time constant below 1 ps. Quantitative agreement with experimental data is, however, not achieved. Future experiments from our group on water will explore the probe and temperature dependence of C(t) in an attempt to obtain a clear picture of the molecular aspects of the solvation dynamics of water. Acknowledgment. Acknowledgment is made to the National Science Foundation (Grant No. CHE-8251158), to the National Institutes of Health (shared instrument grant No. RR01439), to the University of Minnesota Microelectronics and Information Science Center, and to Unisys Co. (21) Neumann, M. J . Chem. Phys. 1986,85, 1567. (22) Rao, M.; Berne, B. J. J . Phys. Chem. 1981,85, 1498. (23) Engstrom, S.;Jonsson, B. J . Chem. Phys. 1984,80, 5483. (24) Maroncelli, M.; Castner, Jr., E. W.; Bagchi, B.; Fleming, G. R. Faraday Discuss. Chem. SOC.1988,85, 1. (25) Maroncelli, M.; Fleming, G. R. Computer Simulation of the Dynamics of Aqueous Solvation. J . Chem. Phvs.. in Dress. (26) Karim, 0. A.; Haymet, A. D. J.; Banet,'M.j.; Simon, J. D. J . Phys. Chem. 1988, 92, 3391.
