Measurements of the Solute Dependence of Solvation Dynamics in 1

Solvent Dependence of the Spectra and Kinetics of Excited-State Charge ... Does PRODAN Possess a Planar or Twisted Charge-Transfer Excited State?...
0 downloads 0 Views 1MB Size
Download PDF
J. Phys. Chem. 1995, 99, 4811-4819

4811

Measurements of the Solute Dependence of Solvation Dynamics in l-Propanol: The Role of Specific Hydrogen-Bonding Interactions C. F. Chapman, R. S. Fee, and M. Maroncelli* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 Received: November 22, 1994; In Final Form: January 10, 1995@

Time-resolved Stokes shift measurements (-20 ps effective resolution) are used to measure the solvation times of 16 different probe solutes in a single solvent, l-propanol (253 K). The solutes were chosen so as to provide a wide range of chemical structures and physical properties consistent with the requirements of strong fluorescence and high solvent sensitivity. Rather than exhibiting a continuous range of solvation times, the dynamics observed with these solutes indicated a division into two distinct classes. The majority (1 1) of the solutes were observed to fall into a “normal” category. The solvation times of these solutes span a relatively narrow range, 0.14-0.22 ns, times consistent with expectations based on nonspecific theories of solvation dynamics. The remaining five solutes, all simple aromatic amines, form a distinct group whose solvation dynamics is at least 2-fold faster than those of the “normal” solutes. The difference between these two classes of solutes appears to reflect differences in the nature of their hydrogen-bonding interactions with the solvent.

I. Introduction A large number of recent studies have used time-dependent fluorescence spectroscopy to measure the dynamics of solvation in polar The observable in these experiments is the time-dependent Stokes shift of the fluorescence spectrum of some solvatochromic probe as a function of time after excitation. The time evolution of the spectrum reflects the solvation energy relaxation subsequent to the perturbation by the electronic transition. Studies of this sort have mainly focused on the solvent dependence of the relaxation. Typically, one or two probe solutes have been used to study how the dynamics varies in a range of solvents. In cases where more than one solute has been employed, differences in the dynamical response measured with different solutes have been found to be much smaller than variations observed when a single probe is examined in different solvents. Thus, the dynamics being observed can be thought of as being largely a property of the solvent. Indeed, the least sophisticated of theoretical models for this dynamics, those that describe the solvent as a dielectric continuum, predict dynamics that is virtually independent of the identity of the probe solute or the nature of the charge redistribution involved in the electronic transition. However, theories that consider the molecular nature of solute-solvent interactions do predict a dependence on solute. The relative solute/solventsize ratio and the nature of the charge perturbation, i.e., whether the perturbation involves a change in solute charge, dipole moment, etc., are predicted to have important effects on the dynamics. 1,4,5 Molecular dynamics simulations also show significant variations with solute and with the nature of the perturbation i n ~ o l v e d . ~ These -~ latter results, along with the theoretical predictions, suggest that it would be of interest to try to elucidate the nature of the solute dependence of solvation dynamics in experimental systems. Although variations in the dynamics as a function of solute are expected to be smaller than the now well-characterizedsolvent dependence, these more subtle variations are likely to be more sensitive to how the molecular-level details of solvation influences the dynamics. In spite of the fact that a wide variety of experimental solvation probes have now been used to measure solvation @Abstractpublished in Advance ACS Abstracts, March 1, 1995.

0022-365419512099-4811$09.0010

dynami~s,l-~ only two previous studies have examined the solute dependence in any detail. Su and SimonlO compared solvation dynamics of the TICT state in (dimethylamino)- and (diethy1amino)benzonitrile in propanol at several temperatures. They observed that solvation of the dimethyl compound was consistently -15% faster than that of the larger, diethyl compound, in contrast to the size dependence originally anticipated on the basis of existing theory.11.12In their extensive studies of solvent dependence Barbara and co-workers2J3 also compared solvation times of two similar probes, coumarin 152 and 153 (see Figure 1). In a wide range of protic and aprotic solvents, the latter authors observed solvation of coumarin 152 to be uniformly 20-40% faster than that of coumarin 153. In both of these studies the two probes (at least superficially) differed only in size by virtue of the addition of alkyl substituents. In both cases the smaller of the two probes exhibited the faster solvation dynamics, in direct contradiction to the original prediction of size effects made by molecular theories. 1,4,5,11,12 In order to explore the solute dependence of solvation dynamics more fully, we have measured time-resolved fluorescence spectra of 16 probe solutes in a single reference solvent, l-propanol at 253 K. This particular solvent and temperature were chosen in order to obtain solvation response times that could be readily measured with the -20 ps effective time resolution available with our time-correlated single photon counting apparatus. (We also attempted to perform parallel studies in a polar aprotic solvent, propylene carbonate, but found the dynamics to be too fast to measure accurately, even at reduced temperatures.) The solutes examined are illustrated in Figure 1. Rather than choosing or designing solutes to test preconceived ideas of what features should most affect the dynamics, we merely chose solutes to represent as wide a range of chemical and physical properties as possible, consistent with the proviso that they possess good fluorescence properties and substantial (’500 cm-l) Stokes shifts in propanol. As can be seen from Figure 1, many of the solutes are chemically similar. Virtually all of them possess an amino group which (presumably) donates some electron density to the remainder of the molecule upon excitation and gives rise to strong solvato-

0 1995 American Chemical Society

4812 J. Phys. Chem., Vol. 99, No. 13, 1995

Chapman et al.

.

Aniline: R=H DMA: R=CH3

v

v

W

2-AA

1-AN

1-APY R

X I

A C1: XSH3; R=C~HS C120: XSH3; R=H C152: X=CF,; R S H 3 N R :-*/ \

0 4-AP: R=H 4-AMP R=CH3

U

H

c339

q

C102: R=CH3 C153: R S F 3 N’‘

R

indole: R=H 1-MI: R S H 3

DMABN

Prodan

Figure 1. Structures of the solutes examined in this work. chromism. In most of these molecules we assume that electronic excitation populates a single excited state and that the observed dynamics reflect merely solvent relaxation in the electrostatic field generated by this excited state. Some of the molecules have amino groups that are free to rotate, and these might be expected to form twisted intramolecular charge transfer (TICT) states.14 We include one probe, DMABN, which is in fact the archetypal TICT molecule. However, in propanol this electron transfer process is much faster than s o l ~ a t i o n , ~and ~ Jit~ appears that one can monitor solvation dynamics of the TICT state with little interference from the reaction step. This molecule is included mainly to show its similarity to the other, presumably simpler solutes studied. The results obtained from this study were not those anticipated. Instead of finding a continuous distribution of solvation times correlated to solute properties such as size or charge distribution, we observed a highly bimodal distribution of times, which suggests a separation of the solutes into two different classes. The distinction between these two solute groups appears to be associated with whether or not specific hydrogen-bonding interactions with the solvent play an important role in the dynamics. In the majority of solutes studied it does not. The dynamics of all of these “normal” probes are all similar to one another and in keeping with expectations based on dielectric theories of solvation dynamics. The other group of solutes, which are all aromatic amines, undergo much faster solvation. We conjecture that the dynamics observed with these solutes has mainly to do with a hydrogen-bonding rearrangement between the probe and a single solvent molecule rather than being the result of nonspecific solvation of the sort usually considered.

11. Materials and Methods Schematic structures of the probes used in this study are shown in Figure 1. Table 1 explains the solute nomenclature and provides some characterization of the ground-state properties of these molecules. The latter information was obtained from semiempirical quantum chemical calculations with the MNDO Hamiltonian.16 Calculations were performed at the RHF level with full geometry optimization using the AMPAC program package.l7

TABLE 1: Solute Designations and Ground-State Propertie* group volume dipole effective probe solute abbrev label (A3) moment (D) moment aniline aniline A 93 1.46 2.0 1-aminonaphthalene 1-AN A 136 1.45 2.1 2-aminoanthracene 2-AA A 179 1.46 3.2 1-aminopyrene 1-APY A 195 1.47 2.5 dimethylaniline DMA A 128 0.64 2.2 coumarin 120 c120 C 152 5.08 1.9 coumarin 1 c1 c 221 4.09 1.6 coumarin 339 c339 C 192 5.44 1.8 coumarin 102 c102 C 233 5.38 1.7 coumarin 153 C153 C 246 5.46 1.6 4-aminophthalimide 4-AP P 133 4.35 2.1 4-amino-N-methyl- 4-AMP P 151 3.98 2.1 phthalimide indole indole I 109 1.61 1.9 1-methylindole 1-MI I 126 1.91 1.7 “prodan” prodan D 216 2.71 2.4 (dimethylamino)- DMABN D 146 3.88 1.9 benzonitrile “Molecular volumes were calculated from van der Waals increments?* Dipole moments were calculated from semiempirical quantum chemical calculations with the MNDO Hamiltonian using the AMPAC package (see Figures 4 and 5 ) . Effective moments (eqs 4 and 5 ) were determined from charges generated by an electrostatic potential fit of the MNDO wave function. All of the coumarin probes used in this study were laser grade materials from Eastman Kodak. Aniline (ACS reagent), indole (99+%), 1-methylindole (97+%), and 1-aminopyrene (97%) were obtained from Aldrich, and prodan was from Molecular Probes Inc. These probes showed no signs of fluorescence impurities and were used as received. Other probes required recrystallization prior to use. 4-Aminophthalimide (practical grade; Eastman Kodak), I-aminonaphthalene (practical grade; Sigma), and 4-amino-N-methylphthalimide(97%, Aldrich) were recrystallized from methanol. DMABN (98%, Aldrich) was recrystallized from methylcyclohexane, and 2-aminoanthracene (95%, Pfaltz and Bauer) was recrystallized from benzene. The solvent 1-propanol (99.5%) was dried over molecular sieves before use. Other solvents used in solvatochromic measurements were typically Aldrich “HPLC” grade. Steady-state measurements were made using a Perkin-Elmer Lambda-6 UV-visible spectrometer and a Spex Fluorolog

Solvation Dynamics in 1-Propanol Model F2 121fluorimeter. Fluorescence spectra were corrected for instrumental response. All of the fluorescence samples were bubbled with nitrogen to remove dissolved oxygen. Quantum yields were measured relative to a quinine sulfate/l .O N sulfuric acid standard whose yield was taken to be 0.546 at 360 nm excitation. Uncertainties in the reported frequencies and quantum yields are expected to be f l O O cm-l and f20%, respectively. Time-resolved fluorescence decays were measured with a time-correlated single photon counting instrument previously described.lg Briefly, the apparatus consists of a Nd:YAG pumped synchronous dye laser, l/4 m monochromator, and 6 p m microchannel plate detector combination that typically produces 50-60 ps instrument response functions (fwhm). Solutes were excited as near to their lowest energy absorption maxima as possible. A series of temporal decays were collected with 4 or 8 nm band-pass for 10-16 wavelengths spanning the emission band. These decays were first individually fit to a multiexponentialform using a convolute and compare algorithm and then relatively normalized according to the steady-state emission spectrum. Finally, these fitted and normalized decays were used to construct the time-resolved emission spectra employed in further analysis. (See refs 19 and 20 for details.) To compare the dynamics observed with different probes, we focus on the normalized spectral response function S,(t) defined by

where v(t)is some characteristic frequency of the fluorescence spectrum at time t. In order to determine this response, we first fit the spectrum at a given time to a log-normal line shape function20 so as to obtain the best estimates of the peak and average (first moment) frequencies. The time evolution of each of these two frequency measures was then fit to a multiexponential form, from which apparent ~ ( 0 and ) Y(=) values and a normalized S,(t) function were obtained. In most cases, the functions derived from the peak and average frequency measures are comparable, and some combination of these two functions was used to provide the best overall characterization of the spectral response function. The functions so obtained are in general not simple exponential functions. For comparing time scales obtained with different solutes, we use both the l/e times, tl,, and the average times,

On the basis of repeated measurements of the same probe, we expect the uncertainties in the values of the observed shifts ~ ( 0 ) - Y(=) and the time constants t l , and (Z)obs to be on the order of &20%. The method for characterizing the solvation response just described, in particular determination of ~ ( 0 )from the timeresolved data alone, does not take into account the effects of finite time resolution on the measured response. If spectral dynamics are occumng on a time scale faster than -20 ps (our expected time resolution after deconvolution), these dynamics would not be included in the &(t) functions so obtained, and the times determined from them would therefore be too large. In order to overcome this deficiency,we have recently developed a method for independently evaluating the position of the emission spectrum before any solvent relaxation has occurred. The method involves comparing steady-state absorption and emission spectra in the solvent of interest and in a nonpolar

J. Phys. Chem., Vol. 99, No. 13, 1995 4813 reference solvent. It is described and tested in some detail in ref 21. In the present work we use the independent estimate of the time-zero frequency and thus the dependent shift ~ ( 0 ) Y(-) obtained in this way to compute “corrected” solvation times,

(3) This time is really a lower bound to the actual solvation time, but it is a good approximation if the unobserved components of the solvation dynamics are much faster than (Z)obs. In the present case our time resolution should allow for observation of all solvation components slower than -20 ps. Since values of ( t ) & s are in the range 100-300 ps, (t)com computed from eq 3 should provide a good estimate of the average solvation time in nearly all instances. We therefore use this estimate in attempts to correlate solvation times with probe properties. 111. Results and Discussion

A. Solute Characterization. In order to understand the differences observed in the solvation dynamics of different solutes, we have considered a number of properties that might be of importance. Before reporting on the observed dynamics, we will first summarize these properties. Three molecular characteristics obtained from semiempiricalcalculation are listed in Table 1. The first of these is the solute van der Waals volume.22 This property is of interest since molecular theories predict that solvation times should vary with the probe/solvent size ratio.11J2 Over the range of solutes considered here the molecular volumes vary by a factor of about 2.5. For the solvent 1-propanol these volumes translate into solute/solvent (linear) size ratios of between 1.1 and 1.5. This variation in linear dimensions is rather modest, and it would be difficult to span a much wider range using polyatomic solutes. However, we note that the volume itself might be of direct relevance if solute rotational diffusion plays some role in the solvation dynamics. Although only one theoretical description has considered this aspect of the solute’s influence on the dynamics,23simulations have shown that for solutes the size of benzene the effect can be significant.6 The second property listed in Table 1 is the (ground state) dipole moment of the solute. The dipole moment is not expected to directly influence the solvation dynamics, but it provides some indication of the likely strength of the interaction between the solute and solvent (proportional to p2/V for a continuum solvent). The dipole moments of these molecules vary appreciably over the different solute familities considered. The final property in Table 1 is the “effective moment” (m,ff) of the solute’s ground-state charge distribution. This characteristic, which relates to how the charge distribution of the solute is sensed by the solvent is defined in terms of the electrostatic energy of interaction (Uel)between an arbitrary charge distribution {qi} inside of a cavity of radius a and a continuum solvent. This energy can be written in terms of a sum over multipole moments Qm,

and where r k and rl are distances from the center of the solute, e k l is the angle between these two radius vectors, and P,(x) denotes the Legendre polynomial of rank m.24 The effective moment is the solvation energy-weighted average:

4814 J. Phys. Chem., Vol. 99, No. 13, 1995

cmQm/a2m+’ m=O

m=O

Thus, values of m,ff = 0, 1, and 2 denote that the distribution of charge within the solute is such as to appear to the solvent as predominantly that of a charge, a dipole, or a quadrupole, respectively. The effective moment is of interest because recent simulations of solvation dynamics of polyatomic solutes have shown that the value of meff associated with the change in the solute’s charge distribution between the ground and excited states is directly linked to the speed of the solvation response.6 (For the reasons discussed below, we have not calculated meff associated with the change in the SO SIcharge distribution, but rather meff associated with SOalone.) From Table 1 one finds that the values of this effective moment, with only one 0.5. Thus, there is exception, fall within the range 2.0 (unfortunately) little differentiation among the various probes with respect to this characteristic. However, the fact that these values are all significantly greater than unity and clustered around the value two is an interesting result in its own right. It indicates that, with respect to interactions with a solvent, the charge distributions of typical solvatochromic probes are not best viewed in terms of point dipole models, an assumption almost universally employed in theories of solvation dynamics.25 Rather, a better first-order description of the ground-state charge distribution of these molecules would be in terms of a point quadrupole. The solute properties considered above are all features of the solute’s ground electronic state. The solvation dynamics measured in experiment are more closely related to properties of the SI state and to differences between SOand SI. Whereas we expect that the properties listed in Table 1 for the ground state can be reliably calculated with semiempirical methods, it is not at all clear that the same is true for the properties of the S1 states of these molecules. Many of them have more than one low-lying excited singlet state whose energies may be solvent dependent, and this feature renders the correct interpretation of gas phase calculations difficult. Rather than rely on calculation, we therefore choose to characterize the excited states and SO S1 changes only in terms of spectroscopically measured quantities. This characterization is camed out in Table 2, where we list a number of observables obtained from steady-state spectra of the probes in propanol and other solvents at room temperature. In addition to listing several characteristics of the absorption and emission spectra of all of the probes in 1-propanol, Table 2 contains three sets of data related to the changes in solvation brought about by electronic excitation. The first of these involves the solvatochromic shifts observed in polar aprotic solvents. For each probe we measured characteristic frequencies of the absorption and emission spectra as a function of solvent polarity in a series of “selected” (in the sense of ref 26) polar aprotic solvents. We characterize the polarity of these solvents using the “x*” solvatochromic polarity scale.26 This scale is constructed such that the n* value of a given solvent is meant to reflect only its polarity/polarizability independent of the (presumed additive) effects of its hydrogen bond donating or accepting abilities. For all of the probes examined here the spectral frequencies in selected polar aprotic solvents were found to be reasonably linear functions of n*. Examples of such data for two of the probes, 1-AN and C153, are shown in Figure 2. The values Sabs and ,S, listed in Table 2 are the slopes of the

-

*

-

Chapman et al. best-fit lines through the selected solvent series (open symnols in Figure 2). AS = Sem - Sabs provides a measure of the magnitude of the change in the probe’s polarity (‘‘W’) between the ground and first excited states of the probe. Since hydrogenbonding solvents (filled symbols in Figure 2) have been excluded from the fits, these values provide a relative measure of the change in the solute’s electronic structure that is related to nonspecific interactions with a polar solvent. In all probes studied the value of AS is positive, which indicates that the solutes all become more “polar” as a result of the SO SI transition. The magnitudes observed for this quantity span a range of more than a factor of 5 . The columns labeled “c-hex-propanol” in Table 2 serve to characterizethe probes’ interactions with the solvent 1-propanol, in a manner similar that just described in the case of polar aprotic solvents. Here we compare the absorption and emission shifts, hvabsand Av,,, in 1-propanolrelative to the nonpolar reference solvent cyclohexane (i.e., Av = v(chex) - v(proh)). These shifts reflect the sensitivity of the various probes to the effects of all electrostatic interactions with propanol, including the effects due to specific hydrogen bond formation. The quantity AAv = Av,, - Avabsrelates to the differential sensitivity of the ground versus the excited state with respect to such interactions. It also provides one measure of the magnitude of the time-dependent emission shift expected in propanol. Estimates of the influence of solute-solvent hydrogen bonding on the propanol spectral shifts are provided in the two columns labeled “HB effect” in Table 2. The values listed are the differences between the observed frequencies in 1-propanol and the frequencies predicted from the n* regressions made using polar aprotic solvents. (Refemng to Figure 2, AvaH measures the vertical displacement of the absorption regression line from the respective 1-propanol point, and similarly for AyeH.) Under the assumption that the n* scale has properly calibrated the polarity/polarizability of propanol due to interactions other than hydrogen bonding, these displacements quantify the importance of specific hydrogen-bonding interactions to the spectral shifts. As will be discussed shortly, such interactions appear to hold the key to understanding the bimodal distribution of solvation times observed in propanol. The final column in Table 2 lists our best estimates for the magnitudes of the time-dependent shifts expected in 1-propanol (253 K). These values result from “time-zero” analyses of steady-state spectra using the procedure detailed in ref 21. This analysis is similar in spirit to the cyclohexane/propanol comparisons (AAv) using room temperature spectra. The full analysis takes into account the effects of inhomogeneous broadening of the polar spectra and results in values that are higher than the former values by an average of -25%. On the basis of the tests provided in refs 21 and 27, we anticipate that in favorable cases (for example C153, C102, and 4-AP) these predicted shifts should be accurate to within f 2 0 0 cm-’. In cases where the lowest energy absorption band consists of overlapping transitions, as in indole or 1-APY, the method is less reliable. (These values are shown in parentheses in Table 2.) However, even here we expect the predictions to be good to about f 5 0 0 cm-’. Since this uncertainty is only 3~25%of the average shifts expected for the set of probes examined, these estimates and the corrected solvation times calculated from them are useful even in these less favorable cases. B. Time-Resolved Spectra and Solvation Dynamics. Typical time-resolved emission spectra are provided in Figure 3. The connected points in this figure are the instantaneous spectra at a series of times (0- 1 ns), and the continuous curves display the corresponding absorption, estimated time-zero

-

J. Phys. Chem., Vol. 99, No. 13, I995 4815

Solvation Dynamics in 1-Propanol TABLE 2: Steady-State Spectral Characteristics (293 K) propanol SS" solvatochromismb probe solute (v),bS (I&, #em N -&bs -Sem aniline 35.08 29.82 0.18 4 0.88 2.20 8 1.57 3.12 1-AN 31.61 22.96 0.84 2-AA 24.93 19.23 0.52 9 1.01 2.18 1-APY 27.07 22.70 0.64 4 (1.4) 1.74 DMA 33.4 28.39 8 0.88 1.85 5 1.82 2.40 c120 28.31 22.98 0.82 c1 27.46 22.13 0.49 4 1.53 3.00 c339 27.11 22.13 0.81 4 1.78 3.0 c102 25.96 20.95 0.79 9 1.49 3.03 C153 23.75 18.40 0.36 9 2.14 3.73 4-AP 26.89 18.75 0.28 8 2.32 3.60 4-AMP 26.32 18.33 0.23 6 2.19 3.28 indole 36.92 30.16 0.45 8 0.09 3.01 1-MI 36.28 29.98 0.64 5 0.42 2.75 prodan 27.57 20.17 0.83 5 1.50 3.79 DMABN 34.14 a

c-hex-propanol'

HB effect!

AS

AYabs

Avem

AAv

AvaH

AyeH

1.32 1.55 1.18 (0.3) 0.97 0.58 1.47 1.22 1.53 1.59 1.27 1.09 2.93 2.32 2.29

0.30

1.68 3.33 2.22 1.30 1.17 3.22 3.31 3.55 3.46 3.59 5.06 4.81 2.09 1.29 4.44

1.38 2.90 1.64 0.63 0.75 1.13 1.72 1.40 1.81 1.95 2.45 2.31 1.83 1.20 2.81

-0.6 -0.7 -0.7 -0.4 -0.3 1.0 0.7 1.1

0.6 1.4 0.8 0.4 0.1 1.6 1.5 1.5 1.6 1.3 3.1 2.9 1.2 0.6 2.7

0.44

0.58 0.67 0.41 2.10 1.59 2.15 1.65 1.65 2.61 2.51 0.26 0.09 1.62

0.5

0.7 1.3 1.4 0.0

-0.1 0.7

time P ~ ( 0-) Y(-) 1.7 3.3 1.8 (0.8) 1.1 1.3 1.8 1.4 2.1 2.2 3.2 2.7 (1.8) ( 1.7) (3.2)

Average absorption and emission frequencies and emission quantum yields in 1-propanol. For absorption, the average frequencies reported are

+

(vi v-)/2 where v i are the frequencies of the upper and lower half intensity points of the lowest absorption band. For emission the frequencies

are the Fist moment of the spectrum. All frequencies here and in subsequent tables are given in lo3 cm-I. Characterization of the polarity dependence of the absorption and emission spectra using a series of selected polar aprotic solvents and the n* scale of solvent polarity.26 N is the number of solvents measured, and Sabs and Semare the slopes of the linear regression of frequency versus n* (see text and Figure 2). Frequency shifts between 1-propanol and cyclohexane. Absorption frequencies were measured as the frequency of the half intensity point on the low frequency edge of the band (v-) and emission frequencies were similarly measured using the high-frequency half point (v+). AAv = Avem- AVabsprovides one measure of the expected magnitude of the time-dependent emission shift in 1-propanol.*Effect of hydrogen bonding on absorption and emission frequencies as measured by the difference between the frequencies predicted for propanol based on the n* regressions using polar aprotic solvents and the actual frequencies observed. Expected magnitude of the time-dependent shift in 1-propanol estimated from "time-zero" analysis of steadystate spectra as described in ref 21.

n

0.

7

E0

c)

-2.

0 3

--4. d . a i t L

0.

\

4-AP

1

;-2. Gi

-4.

0

.5

1.

7T*

Indole

~

Figure 2. Relative absorption (circles) and steady-state emission

(squares) frequencies of 1-AN and C153 as a function of solvent polarity. The frequencies plotted are the first spectral moments of the respective bands, shifted for clarity. (The frequencies are defined to be 0.0 in emission and 1.0 in absorption in cyclohexane solvent (n*= 0.) The solvent polarity is measured with the empirical JC* scale of solvent polarities.26 The "selected" polar aprotic solvents (open symbols) represented here are hexane (n*= -0.1 1). cyclohexane (0), diethyl ether (0.24), butyl acetate (0.46), tetrahydrofuran (0.55), acetonitrile (0.66), dimethylfonnamide(0.88), and dimethyl sulfoxide (1). The solid lines are the fits to these data sets. The solid symbols represent the alcohols: 1-butanol (n*= 0.47), 2-propanol (0.48). 1-propanol (0.52). ethanol (0.54), and methanol (0.60). emission, and the steady-state spectra (from high to low frequency). Before discussing the dynamics in detail, it is useful to make a few points concerning the suitability of the various probes for measuring solvation. First, to within the resolution with which the time-resolved spectra were recorded (-10 nm), all of the spectra observed (excluding DMABNZ8)appear to be simple, single-component spectra, well fit by the log-normal line shape used in the time-dependent analysis. As a function

Figure 3. Time-resolved,steady-state, and time-zero spectra of three solutes in 1-propanol (253 K). The continuous curves are, from low

to high frequency, the steady-state emission, the estimated time-zero spectrum (see text), and the absorption spectrum. The connected points are the time-resolved spectra at times of 0, 50, 100, 200, and lo00 ps from high to low frequency. of time, the spectra are observed to shift without large concomitant changes in shape or width. Typically, tripleexponential fits were needed to accurately reproduce the emission decays on the blue edges of the spectrum, although in some cases, decays across the emission spectrum could be adequately fit using only biexponential functions. However, in such cases the time constants were found to vary systematically with wavelength. All of these observations point to the spectral dynamics being the result of a continuous solvation process rather than one involving transitions between just a few discrete states. Furthermore, a number of probes such as C102, C153,and 4-AP have been studied in sufficient detail (and with

4816 J. Phys. Chem., Vol. 99,No. 13, 1995

Chapman et al.

TABLE 3: Time-Resolved Data in 1-Propanol (253 K) observed time dependenceb correctedc lex,“ rem tie (4 % (t) probe solute (nm) (ns) v(0)- v(-) (ns) (ns) missed (ns) aniline 291, 286 3.2 0.39 0.09 0.10 77 0.023 1-AN 2-AA 1-APY DMA C120 c1 C339 C102 C153 4-AP

295 18.7 354 23.8 360, 368 4.9 315 2.6 360 4.0 370 3.9 370 4.0 360 4.9 390,405 4.9 291 17.6 4-AMP 307 9.3 indole 287 4.7 1-MI 289 5.7 prodan 360, 364 3.6 DMABN 292 2.7

1.29 0.97 0.43 0.61 0.91 1.21 1.06 1.69 1.62 2.43 2.20 1.40 1.03 2.68 2.50

0.13 0.17

0.08 0.11 0.11 0.14 0.10 0.14

0.19 0.21 0.20 0.20 0.16 0.23 0.18

0.21 0.29 0.27 0.21 0.20

0.21 0.22 0.20 0.26 0.22 0.23 0.29 0.27 0.24 0.23

60 46 (43) 43 32 33 26 24 25 23 19 (23) (39) (17)

I

.

.

’

I

0.067

0.060 (0.081) 0.079 0.14 0.14 0.16 0.15 0.19 0.17 0.18 (0.22) (0.17) (0.18)

Multiple wavelengths indicate that time-resolved spectra were recorded multiple times. The shifts and time constants listed in these cases are averages over the multiple runs. re, is the decay time of the emission determined at times long compared to the spectral shift. v(0) - v(-) is the magnitude, and tl, and (r) are the lle and integral average times of the observed shift of the emission spectrum. Percent of the time-dependent shift missed due to finite time resolution and “corrected”average solvation time (eq 3) determined using the estimates of the time-resolved shift listed in the last column of Table 2. sufficient time resolution) that it is clear that the spectral dynamics being observed results only from solvation dynamics free from the interference of other excited-state processes.2,13,21,27 Nevertheless, one should bear in mind that the excited-state behavior of many of probes employed here is far from completely understood, and there may be complicating features with some solutes. For example, in the cases of the indoles, prodan, and 1-APY the absorption spectra exhibit structure which is indicative of overlapping absorption bands (see indole spectra in Figure 3). As already mentioned, this renders accurate determination of their time-zero spectra difficult. A more important concern is that in the case of the indoles the unusual high-frequency tail seen in their steadystate emission (Figure 3) may indicate that emission also arises from more than a single electronic transition. The existence of such unusual dual emission has been conclusively demonstrated in the case of tryptophan, where ‘L$’Lb equilibration has been found to occur on a 1 ps time scale in water.29 It may be that similar electronic dynamics are occurring in the indoles and possibly in some of the other probes in propanol. However, on the basis of the remarkable similarities between these probes and some of the better characterized probes (Table 3), it must be that, just as in the case of DMABN, these electronic processes occur on a time scale much faster than that which we observe. We therefore will henceforth assume that the spectral dynamics measured in these experiments are indeed a reflection of only the process of excited-state solvation. Spectral response functions (Sv(t),eq 1) constructed from the data of Figure 3 are shown in Figure 4. A summary of the time constants and related quantities measured in the timeresolved experiments is also provided in Table 3. The &(t) functions plotted in Figure 4a are those obtained directly from the time-resolved spectra, assuming that all relaxation has been observed. The l/e times (tie) and average ((&bs) times listed in columns 5 and 6 in Table 3 are the times associated with such “uncorrected” data. Several features of these data are noteworthy. First, the decays observed are typically not exponential functions of time. However, the values of tle and (7)obs differ only slight (-10%) in most cases. From the values

Time (ns) Figure 4. Spectral response functions of several probes in 1-propanol at 253 K. The number of short dashes (1-6) designates the probes according to the following scheme: 1, aniline; 2, 2-AA; 3, DMA; 4, C153; 5 , indole; 6, 4-AP. Panel a shows uncorrected S&) functions obtained from eq 1 using the observed value of ~ ( 0 ) .Panel b shows the same data “corrected”by using the value of v(0)obtained from the estimated time-zero spectrum. The two solid curves shown in panel b are the predictions of the simple continuum (SC) and dynamical MSA (MSA) models (see text). of (&bs listed in Table 3, it seems that the observed times cover the entire range between 100 and 300 ps. Nevertheless, as suggested by Figure 4, the data set divides naturally into two groups having similar dynamics, the “amines” (“A”) and the group formed by all remaining probes, with the former set being significantly faster than the latter. However, it is dangerous to base comparisons of the dynamics of different probes on the observed dynamics alone. As illustrated by Figure 3, all of the observed time-zero spectra (higher-frequency dashed spectra) are red-shifted from the position of the spectra expected prior to solvent relaxation. In all cases the time-zero predictions indicate that some of the solvation dynamics has been missed due to the finite time resolution of our instrumentation. As can be seen from Table 3, for the majority of the probes the fraction missed is between 20 and 35%. In the case of the “A” solvents, an even larger fraction appears to be too fast for us to detect. In the worst case (aniline, Figure 3), our time-zero estimates indicate that we only actually observe one-fourth of the spectral relaxation in the present experiments. Clearly, it is important to account for this missed portion of the response when making comparisons to theory or when trying to understand differences between solutes.30 We therefore now turn attention to the “corrected” spectral response functions shown in Figure 4b and the corrected average times eq 3) provided in the last column of Table 3. With the exception of aniline, whose average time is shifted considerably away from the remaining “A’ solvents, correcting for the unobserved portion of the response only enhances the apparent bimodal distribution of solvation times. The data divide cleanly into the set of simple aromatic amines “A”, which all have solvation times of ‘80 ps, and a set consisting of all of the remaining solutes, which are all slower than the “A” set by a factor of 2. For convenience we will refer to the latter group as the “ N ’ group, where N denotes “normal”. Excluding DMABN, for which we cannot estimate the time-zero spectrum, the 10 “N’ solutes have average solvation times that span a relatively narrow range of 0.17 & 0.04 ns. We term these the “normal” group in the sense that the solvation times observed are consistent with expectations based on theoretical and

Solvation Dynamics in 1-Propanol

3.1

’

ri

’

A A AA

0

>

2

’

3. 2.1 A

’

J. Phys. Chem., Vol. 99, No. 13, 1995 4817 ’

’

1

’

’

L

PP

D

I

‘A A i+

P

3l.+ .

< r > (ns) Figure 5. Plots illustrating the lack of any obvious correlationbetween the (corrected) solvation times and three solute properties: the solute

volume (top), its effective moment (middle), and its sensitivity to nonspecific polar interactions (bottom). See text for details. simulation studies of solvation dynamics in the linear regime as well as experimental results on several well-studied solutes in a wide range of solvents.’S2 Simulations show that solvation of molecular solutes generally proceeds at a rate comparable to but slower than the rate predicted by simple continuum models of In Figure 4b we compare the predictions of the simple continuum (“SC”) theory to the observed response functions. We also show here the predictions of a slightly more sophisticated theory of solvation, the dynamical MSA theory (for ionic solutes).”.’* Experiments with much higher time resolution using the solute C15327show that this latter theory provides predictions of solvation times that are within a factor of 2 of those observed in virtually all polar solvents examined. From Figure 4b one observes that the tle times of the “N’solutes lie clustered around the value of 0.16 ns predicted by the DMSA theory. The average solvation times of these solutes (0.140.22 ns) are bracketed by the average times of 0.13 and 0.28 ns predicted by the SC and DMSA theories, respectively. Thus, the “N’ solutes display dynamics that apparently reflects nonspecific solvation of the sort usually considered in discussions of polar solvation dynamics. In contrast to this “typical” behavior of the “ N ’ solvents, the solvation times observed with the “A” solutes are much shorter than even the time predicted by the simple continuum theory (usually considered a lower limit for the solvation time). This observation suggests that perhaps a qualitatively different sort of dynamics is being observed with this latter group. In order to understand what this dynamics might be and what it is that distinguishes the two groups of solutes, we now return to the various solute characteristics quantified in the last section. Before it became clear that the solutes could be usefully categorized into two groups, we made extensive attempts to correlate observed solvation times with a wide range of solute proper tie^.^' Figure 5 illustrates typical results of such efforts, using as examples the three solute properties originally thought to be most closely related to the solvation time. These are the solute volume, effective moment, and its sensitivity to nonspecific interactions with polar solvents as measured by AS (see section A). As can be seen from Figure 5, none of these three solute attributes appear to be closely correlated to its solvation time. In order to hopefully see beyond the scatter in plots such

as these, we also performed correlation analyses using the set of all solute properties listed in Tables 1-3 as well as a number of other related quantities (such as radiative and nonradiative rates, et^.).^^ No particularly useful correlations emerged from such analyses. Given the uncertainties inherent in the data, it appears we can only say with confidence that the solutes do divide up into the two groupings already described. Except for the case of aniline, which appears to be much faster than the other “A” solutes, the spread in solvation times within each grouping is insufficient for further analysis. Given these limitations, we can at least attempt to discem what solute attributes distinguish between the “A” and “N’ groupings and lead to their different solvation dynamics. In looking over all of the collected data on these solutes, we have noted only two points that separate the “A” and “N’solutes and which might therefore account for their distinct dynamics. Both relate to how the solutes hydrogen bond to a solvent such as propanol. The first point of distinction is that in the “A” group there is a single site, the amino nitrogen lone pair, that should interact strongly with a hydrogen bond donating solvent such as propanol. All of the other solutes either have no particularly favorable accepting sites (i.e., the indoles) or have multiple sites for such interactions. The second distinguishing characteristic of the “A’ solutes is more subtle. It is displayed in the columns labeled “HB effect” in Table 2 and illustrated graphically in Figure 2. We observe that in essentially all “N’ solutes the hydrogen bond donating ability of alcohol solvents causes an extra red shift in both the absorption and emission spectra beyond what would be expected in a similarly polar solvent incapable of hydrogen bonding. In the “A” solutes, while hydrogen bonding also causes a red shift of the emission, it causes a blue shift of the absorption spectrum. Clearly, different patterns of hydrogen bonding is the SO and SI states in the two groups of solutes must be responsible for this spectral behavior. Unfortunately, at present we can only speculate as to what the difference might be. It is reasonable (but not necessary) to interpret the spectroscopy of the “A’ solutes as reflecting a destabilization of the initially formed FranckCondon excited state (the state reached before any nuclear motion occurs) by hydrogen bonding. In these solutes one might imagine that the hydrogen-bonding structure optimized for ground-state interaction with the N atom is energetically unfavorable in the S1 state, perhaps due to rehybridization occurring upon excitation.32 Reaching equilibrium in the excited state might therefore mainly be a matter of restructuring (breaking and re-forming?) a single hydrogen bond at the amino nitrogen atom. In the remaining solutes, the spectral evidence indicates that less dramatic reorganization occurs. The solutesolvent hydrogen bonding in the presence of ground state “N’ solutes may already be favorable for solvation of their excited states such that only modest changes in the hydrogen-bonding structure are required for equilibration. Either of the two differences just noted can be used to rationalize the differences in the solvation dynamics of the two solute classes. As a first step, we note that in all solutes a substantial fraction of the solvation response is in some way connected to changes in solute-solvent hydrogen bonding between the ground and excited states. Using the data of Table 2, the fraction of the solvation response due to hydrogen bonding can be crudely estimated from the ratio

7..

A v , ~- A v . ~

[ ~ ( o-) ~(w)lpred

(6)

For the different solute groupings we find values of F ( A ) =

Chapman et al.

4818 J. Phys. Chem., Vol. 99, No. 13, 1995 68%, Y(C) = 39%, fH(P) = 57%, y(Z)= 54%, and fH(0) = 64%, and for the collection of all “N’ solutes, y(N)= 48%.

One must be careful about taking these percentages too literally. They depend on the z* value of propanol for providing the correct partitioning of the “polarity” of propanol into nonspecific and hydrogen-bonding components, and the accuracy of this partitioning is unknown. Nevertheless, these values seem to indicate that specific solute-solvent hydrogen bonding contributes significantly to the dynamics of all of the solutes in propanol. It appears to be most important in the “A” solutes and least important in the coumarins (and especially C15320,27). Given the apparent dominance of specific hydrogen bonding in the solvation of the “A” solutes, it is easy to understand why the dynamics observed are not consistent with the predictions of either the SC or the MSA theories. The dynamical content of these theories is based solely on the dielectric response of the pure solvent. While this response implicitly contains information about the dynamics of solvent-solvent hydrogen bonding, it carries no information at all about solvent-solute interactions. These theories assume that the solute causes no important perturbations to the solvent’s structure or dynamics33 and that the dynamics sensed by the solute are the same electrical fluctuationsthat occur in its absence. If, as is probably the case in the “A’ solutes, the solvation change in response to the SO SItransition involves mainly the reorganization of a single, critical solute-solvent hydrogen bond, such theories cannot be expected to apply. This assertion is even more true if, as is suggested by the observed absorption blue shift, the perturbation caused by excitation in these solutes is large enough to “drive” the dynamics in a nonlinear fashion. Simulations of solvation in methanol show that, especially in the solvation of negative charges, the hydrogen-bonding rearrangements that accompany electronic excitation are often quite different from the dynamics predicted by linear solvation theories such as the SC or MSA t h e ~ r i e s .Thus, ~ the “anomalous” behavior of the “A” solutes is readily explained. What is perhaps harder to understand is why the “N’ solutes behave “normally”, that is, why they should appear to follow the predictions of nonspecific solvation theories. We have just noted that in many solutes besides those in the “A” group over half of the solvation response is estimated to be due to hydrogenbonding effects. In these cases one might anticipate that simple theories should similarly fail and that the dynamics observed should be quite variable given the variability of the specific solute-solvent bonding present in this collection of probes. Yet, what is observed is that the solvation dynamics varies little among the “N’ solutes. In addition, this dynamics can indeed be predicted from simple theories to just the same accuracy that the dynamics in polar aprotic solvents can be predicted.*’ This apparent paradox can be rationalized on the basis of simulations of polyatomic solutes in Such simulations show that when the solvation change upon excitation is spread around several interaction sites, and when the overall perturbation is in the linear regime, the solvation response is not particularly sensitive to the details of the individual hydrogen-bonding interactions taking place. This is true in spite of the fact that collectively these hydrogen-bonding interactions may be responsible for a large fraction of the solvation energy change. The time scale of the response is found to be mainly determined by the overall moment (m,ff, in the sense of section A) of the charge redistribution. Furthermore, the response that one finds is indeed reasonably close to the predictions of nonspecific solvation theories such as the SC and DMSA theories.6 Thus, the fact that the hydrogen-bonding contributions to the total solvation change are sizable in many of the “ N ’ solutes is not

-

inconsistent with their displaying “normal” behavior. The key features of the “A’ solutes that we propose makes them different are (i) that the hydrogen-bonding effect is localized to a single interaction and (ii) that partly as a result of this localization the perturbation caused by the SO SItransition makes the response “driven” in a nonlinear fashion.

-

IV. Summary and Conclusions In this study we have used the time-resolved Stokes shift method to measure the solvation times of 16 probe solutes in 1-propanol (253 K). Contrary to initial expectations that solute variations would provide a continuous range of solvation times, the dynamics observed indicate a division between two distinct types of solute. The majority (1 1) of the solutes studied fall into the “normal” category. The solvation times of these solutes span a relatively narrow range of 0.17 f 0.04 ns, times consistent with expectations based on nonspecific theories of solvation dynamics. The remaining five solutes, all simple aromatic amines “A”, form a separate group whose solvation dynamics is at least 2-fold faster than those of the “normal” solutes. We rationalized the difference between these two groups of solutes in terms of differences in the nature of their hydrogenbonding interactions with the solvent. We hypothesize that in the “A’ solutes a large fraction of the solvation response to the SO SI transition involves a localized hydrogen-bonding interaction with a single solvent molecule. The dynamics observed with these solutes is not well modeled by available theories of solvation, since these completely ignore such interactions. In the remaining solutes, although hydrogen bonding to the solvent is still important energetically, we hypothesize that the effect is not localized to a specific site and is not tied to the dynamics of any particular interaction. In this case it appears that the dynamics is fairly insensitive to the details of the solute structure or electronic distribution, and these dynamics are reasonably modeled by the same types of theories used to treat solvation in aprotic solvents. We conclude by considering the above results in the context of previous measurements of solvation dynamics in alcohol solvents. In keeping with the preponderance of solutes displaying “normal” dynamics here, the vast majority of previous experiments have found solvation times roughly consistent with predictions of simple solvation t h e o r i e ~ . l - ~ In % ~contrast, ~ observation of “deviant” behavior of the sort measured in the “A” solvents is rare. One clear example appears to be the solvation of the benzophenone anion studied by Lin and Jonah.35 Using electron attachment and transient absorption spectroscopy, these authors were able to measure the time-dependent solvation of this anion subsequent to its formation. They observed solvation times in several alcohols that were -5 times faster than times observed with “normal” probes in these solvents.35 This behavior is similar to what is reported here in the “A’ solutes, and it is reasonable to assume that in the benzophenone anion, as in the amines, the majority of the solvent relaxation involves the specific association of an alcohol molecule with the localized charge present in the anion. In another study, using time-resolved fluorescence measurements, Declemy and cow o r k e r ~observed ~~ spectral evidence for both specific and nonspecific components in the solvation dynamics of the probe MPQB in alcohol solvents. (MPQB has a structure similar to C153, but with an N substitution opposite the ring 0.) In contrast to the situation observed here in the “A’ solutes, in 1-propanol at 250 K the spectral changes that were associated with specific hydrogen bond formation occurred more slowly than the continuous spectral shift. The latter dynamics was

-

J. Phys. Chem., Vol. 99, No. 13, 1995 4819

Solvation Dynamics in 1-Propanol observed to take place in roughly the same time that we observe for “normal” solutes. A final study of relevance to the present work is due to Berg and c o - ~ o r k e r s ,who ~ ~ recently used a transient absorption technique to measure hydrogen-bonding dynamics to the solute resorufin. It turns out that the spectral shifts of this probe molecule due to nonspecific interactions are minimal, enabling these authors to observe the dynamics of specific hydrogen bonding to the solute without interference from Stokes shift dynamics. In ethanol and two diols, Berg and co-workers found that the time required for hydrogen-bond equilibrationto resorufin was similar to the nonspecific solvation times previously measured in these solvents.37 This similarity is interesting in that, if it is not merely coincidental, it might offer another explanation for why many of the probes studied here, which do form hydrogen bonds to the solute, nevertheless exhibit dynamics that seem unaffected by such interactions. Further experimental studies, especially in dilute alcohol solutions, would be helpful in addressing this possibility. It would also be of interest to examine some of the same probes employed in the present work in a polar aprotic solvent in order to both test whether or not the dynamics of the “A” solutes remains distinct when hydrogen-bonding interactions are absent. Our attempts along these lines in low-temperaturepropylene carbonate31 show that such measurements will require much higher time resolution than employed here. Finally, it is clear that a great deal more could be learned about the specifichonspecific aspects of solvation dynamics in alcohols through detailed computer simulations of the sort performed by Fonseca and Ladanyi9 on small solutes. The “only” impediment to connecting such simulations to experiments of the sort described here is the need for realistic descriptions of the excited states of such complex molecules.

Acknowledgment. The authors thank Vijaya Kumar for perfoming the semiempirical calculations reported here and Joseph Gardecki for help with some of the experimental measurements and analysis. This work was supported by the Office of Basic Energy Sciences of the US. Department of Energy. References and Notes (1) Maroncelli, M. J . Mol. Liq. 1993, 57, 1. (2) Barbara, P. F.; Jarzeba, W. Adv. Photochem. 1990, 15, 1. (3) Simon, J. D. Acc. Chem. Res. 1988, 21, 128. (4) Bagchi, B.; Chandra, A. Adv. Chem. Phys. 1991, 80, 1. (5) Raineri, F. 0.;Zhou, Y.; Friedman, H. L. Chem. Phys. 1991,152, 201. (6) Kumar, P. V.; Maroncelli, M. Simulation Studies of the Dynamics of Solvation of Polyatomic Solutes. Submitted to J . Chem. Phys. (7) Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. Nature 1994, 369, 471. (8) Rosenthal, S. J.; Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. J . Mol. Liq. 1994, 60, 25. (9) Fonseca, T.; Ladanyi, B. M. J . Mol. Liq. 1994, 60, 1. (10) Su, S.-G.; Simon, J. D. J. Phys. Chem. 1989, 93, 753. (1 1) (a) Rips, I.; Klafter, J.; Jortner, J. J . Chem. Phys. 1988, 88, 3246. (b) Rips, I.; Klafter, J.; Jortner, J. J. Chem. Phys. 1988, 89, 4288. (12) Nichols In, A. L.; Calef, D. F. J . Chem. Phys. 1988, 89, 3783. (13) Jarzeba, W.; Walker, G. C.; Johnson, A. E.; Barbara, P. F. Chem. Phys. 1991, 152, 57. (14) Lippert, E.; Rettig, W. Adv. Chem. Phys. 1987,68, 1. (15) Su, S.-G.; Simon, J. D. J. Chem. Phys. 1988, 89, 908. (16) See for example: Sadlej, J. Semi-Empirical Methods of Quantum Chemistry; Ellis-Honvood: Chichester, 1985. (17) AMPAC 4.5, 1993 Semichem, 12715 W. 66th Terrace, Shawnee, KS 66216.

(18) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. (19) Chapman, C. F.; Fee, R. S.; Maroncelli, M. J. Phys. Chem. 1990, 94, 4929. (20) (a) Maroncelli, M.; Fleming, G. R.J . Chem. Phys. 1987,86,6221. (b) Maroncelli, M.; Fleming, G. R. J. Chem. Phys. 1990, 92, 3251. (21) Fee, R. S.; Maroncelli, M. Chem. Phys. 1994, 183, 235. (22) Edwards, J. T. J . Chem. Educ. 1970, 47, 261. (23) Bagchi, B.; Oxtoby, D. W.; Fleming, G. R. Chem. Phys. 1984.86, 257. (24) Beveridge, D. L.; Schnuelle, G. W. J . Phys. Chem. 1975, 79,2562. The value computed for meff depends upon the value chosen for the solute radius a. The values listed in Table 1 result from a choosing a to be twice the radius obtained from the van der Waals volume of the solute. For nearly planar molecules of the type examined here, this choice results in radii sufficient to just exclude the continuum solvent from the region of the atomic sites r,. (25) Recent theoretical studies by Ranieri, Freidman, and co-workers have attempted to employ more realistic models for the solute’s charge distribution. See for example: Raineri, F. 0.;Resat, H.; Pemg, B. C.; Hirata, F.; Friedman, H. L. J. Chem. Phys. 1994, 100, 1477. (26) (a) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J . Org. Chem. 1983, 48, 2877. (b) Laurence, C.; Nicolet, P.; Dalati, M. T.; Abbound, J. L. M.; Notario, R. J. Phys. Chem. 1994, 98, 5807. (27) Homg, M. L.; Gardecki, J.; Papazyan, A.; Maroncelli, M. SubPicosecond Measurements of Polar Solvation Dynamics: Coumarin 153 Revisited. To be submitted to J. Phys. Chem. (28) As already mentioned, DMABN undergoes a well-known TICT reaction, and its emission spectrum clearly reflects this reaction. It is composed of two emission bands, a blue emission due to the “locally excited” or “LE’ state formed upon excitation and a red emission due to the product “CT” state. In measuring solvation dynamics using DMABN, we have ignored the nearly unresolvable dynamics of the LE band and focused on the time-dependent spectral shift of the CT band, which occurs at later times. (29) Ruggiero, A. J.; Todd, D. C.; Fleming, G. R. J . Am. Chem. SOC. 1990, 112, 1003. (30) In a preliminary discussion of some of this work in ref 1, we noted an apparent correlation between the average solvation times and the magnitudes of the spectral shifts v(0) - v(-) observed in a number of probes. This correlation turns out to be an uninteresting result of the fact that a strong connection exists between the speed of the observed response and how much of the overall dynamics is too fast to resolve. That is, the solutes with the smallest (&bs values also are the solutes for which the greatest fraction of the response is not observed. Thus, these solutes have the smallest apparent shifts. Total or corrected shifts, estimated using the time-zero analysis, do not show any clear correlation with either the observed or the corrected solvation times. (31) Chapman, C. F. Time-Resolved Fluorescence Studies of Solvation Dynamics and Chemical Reaction. Ph.D. Thesis, The Pennsylvania State University, 1992. (32) We note that the excited-state calculations reported in ref 8 on 1-AN indicated only a very modest change in charge at the N atom as a result of the SO S1 transition. The calculations showed instead the charge redistribution to be localized mainly to the ring carbon atoms. However, these excited-state calculations were not expected to be accurate. The present experimental results would point to the existence of a much more substantial charge donation from the amino group to the aromatic ring. (33) The DMSA does acknowledge the fact that the solute excludes volume and that this exclusion does slightly change the translational ordering of solvent molecules in the vicinity of the solute. However, this solute effect is completely unrelated to the specific interactions of importance here. (34) Many of these results have been summarized in ref 1 and in: Fee, R. S. Time-Resolved Fluorescence Studies of Spectral Dynamics and Complexation Phenomena. Ph.D. Thesis, The Pennsylvania State University, 1994. Other recent studies are: Zhang, X.; Kozik, M.; Sutin, N.; Winkler, J. R. J . Am. Chem. SOC.1991,16,247. Khundkar, L. R.; Bartlett, J. T.; Biswas, M. Dynamic Fluorescence Stokes’ Shift of an AcetyleneBridged Door-Acceptor Compound in Alcohols at Low Temperatures. J . Phys. Chem., in press. (35) (a) Lin, Y.; Jonah, C. D. In Ultrafast Dynamics of Chemical Systems; Simon, J. D., Ed.; Kluwer Academic Publishers: Dordrecht, 1994; pp 137-162. (b) Lin, Y.; Jonah, C . D. J . Phys. Chem. 1993, 97,295. Lin, Y.; Jonah, C. D. J. Phys. Chem. 1992, 96, 10119. (36) Declemy, A.; Rulliere, C.; Kottis, P. Laser Chem. 1990, 10, 413. (37) Yu, J.; Berg, M. Chem. Phys. Lett. 1993, 208, 315.

-

JP943 1 189