5950
J . Phys. Chem. 1988, 92, 5950-5954
Acknowledgment. We sincerely thank Dr. David Bartels for his helpful and enlightening discussions. We are grateful to Donald Ficht, George Cox, and Edwin Kemereit for their superb operation of the linac. Work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE
under Contract No. W-31-109-ENG-38. Registry No. D,O, 7789-20-0; OD, 13587-54-7;Cd(C10J2, 1376037-7; C ~ ( C ~ O ,13770-18-8; )~, Cr20,2-, 13907-47-6;NO,-, 14797-55-8; IO^, 15056-35-6;SeO>-, 14124-68-6;acetone, 67-64-1; 2,3-butanedione, 430-03-8.
Orientational Relaxation Dynamics of Oxazine 118 and Resorufin in the Butanols. Valence- and State-Dependent Solvation Effects G . J. Blanchard* and C. A. Cihal' Bell Communications Research Inc.. 331 Newman Springs Road, Red Bank, New Jersey 07701 (Received: November 13, 1987; In Final Form: April 20, 1988)
The picosecond-resolved reorientation behavior of the monocation oxazine 1 18 and the monoanion resorufin were examined in the series of butanols (1-butanol, 2-butanol, 2-methyl-l-propanol, and 2-methyl-2-propanol). These measurements revealed strong state-dependent reorientation characteristics for oxazine 1 18 and a much more subtle state dependence for resorufin. Semiempirical MNDO molecular orbital calculations indicate that, on excitation, the a-electron density increases significantly at the ring-bound nitrogen in both molecules. The observed state-dependent reorientation of both molecules is interpreted in terms of excitation-dependent changes in Lewis basicity at their heteroatom sites.
Introduction
Measurement of the orientational relaxation properties of molecules in low-viscosity solvents has been used extensively to elucidate the fundamental nature of solventsolute interactions.'+ Since the development of mode-locked laser technology, studies of ground- and excited-state rotational diffusion behavior with several-picosecond time resolution have become p ~ s s i b l e . ~A~ ~ large number of these studies have been directed toward molecules that absorb light in the visible region of the spectrum, due to the availability of mode-locked lasers that operate in this wavelength range. By use of molecules such as laser dyes for these studies, much has been learned about the dynamics of ~ o l v a t i o n . ~This .~ class of molecules, however, typically suffers from two limitations. First, many are of very low or nonexistent point group symmetry, making their motion difficult to model and limiting the experimental determination of their transition dipole orientation. The second added complexity is based on the fact that many of these molecules contain polar chromophores, allowing for strong solvent interaction, or even a t t a ~ h m e n t .In~ dilute solution, some of these molecules even exist as charged species. The use of less polar and/or more symmetric probe molecules would thus be of great benefit but would require that experiments be performed at wavelengths where it is difficult to generate picosecond laser pulses. The limitations inherent to polar dye molecules have therefore been tolerated. A variety of studies have been made in the past to examine the implications of these limitations on rotational diffusion measurements. One area of interest has been the effect of ionic charge on dynamical b e h a v i ~ r . ~ .In ' ~ those studies, the interpretation of the results was complicated somewhat by the fact that the structures of the oppositely charged probe molecules were not identical. Two polar probe molecules that appear to be particularly well suited to a study of ionic charge do exist, however. They are the monocation oxazine 1 18 and the monoanion resorufin. Their structures are presented in Figure I . These two molecules are both of effective C , symmetry in dilute solution, are structurally very similar, and are isoelectronic, but have opposite ionic charge. In addition, their absorption spectra lie in a region easily accessible to mode-locked lasers. *Author to whom correspondence should be addressed. 'Summer Student. Present address: Department of Chemistry, Iowa State University, Arnes, IA 50011.
0022-3654/88/2092-5950$0l.50/0
The rotational diffusion behavior of these two dye molecules was examined in the series of butanols (1-butanol, 2-butanol, 2-methyl- 1-propanol, and 2-methyl-2-propanol). These alcohols were chosen for this study due to the wide range of bulk properties and molecular shapes available in an isomeric series. Both ground-state and excited-state rotational diffusion times were measured in this work in order to determine whether or not any state dependence was resolvable. Examination of the orientational relaxation behavior of these two dyes revealed that, in fact, a state dependence does exist, but only in certain solvents. It is the purpose of this paper to report this anomalous reorientation behavior and offer an explanation for its existence.
Experimental Section Laser. The picosecond pump-probe spectrometer used in this work has been described in detail elsewhere." Briefly, an argon ion laser (Spectra-Physics Model 17 1-06) mode-locked at 4 1.06 MHz was used to pump synchronously two dye lasers (Coherent Model 701-3). Triple frequency modulation was employed for signal encoding, with sum frequency synchronous demodulation detection. The pump dye laser was operated with Rhodamine 6G (Exciton) at 570 nm for measurements of resorufin and at 590 nrn for measurements of oxazine 118. For ground-state-recovery measurements the probe laser was operated using R6G, at 570 nm for resorufin and at 585 nm for oxazine 118. For excited-state stimulated-gain measurements the probe dye laser was operated with DCM dye (Exciton), at 630 nm for resorufin and 640 nm for oxazine 118. The probe wavelengths were chosen so that the measurements were not affected by the overlap of the absorption ( 1 ) Spears, K. G.; Steinmetz, K. M . J . Phys. Chem. 1985, 89, 3623. (2) Sanders, M. J.; Wirth, M. J. Chem. Phys. Let?. 1983, 101, 361. (3) Gudgin Templeton, E. F.: Quitevis, E. L.; Kenney-Wallace, G. A. J . Phys. Chem. 1985,89, 3238. (4) Von Jena, A,; Lessing, H. E. Chem. Phys. 1979, 40, 245. ( 5 ) Eisenthal, K. B. Acc. Chem. Res. 1975, 8, 118. (6) Fleming, G. R.; Morris, J. M.; Robinson, G.W. Chem. Phys. 1976, 17 , -91- .
(7) Shank, C. V.; Ippen, E. P. Appl. Phys. Lett. 1975, 26, 62. (8) Millar, D. P.: Shah, R.; Zewail, A. H. Chem. Phys. Let?. 1979,66, 435. (9) Gudgin Templeton, E. F.: Kenney-Wallace, G. A. J . Phys. Chem. 1986, 90, 2896. (10) Von Jena, A,; Lessing, H. E. Ber. Bunsen-Ges. Phys. Chem. 1979,83, 1 0 1
101.
( 1 1 ) Blanchard, G. J. J . Chem. Phys. 1987, 87, 6802
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 5951
Relaxation Dynamics of Oxazine 118 and Resorufin
TABLE I: Experimental Zero Time Anisotropy Values and Anisotropy Decay Constants for Oxazine 118 in the Alcohols as Well as in 2-Butanone'
R(O) f solvent 1-butanol 2-methyl-1propanol 2-butanol 2-methyl-2propanol 2-butanone Figure 1. Molecular structures of oxazine 118 (a) and resorufin (b). Only one resonance structure is presented for each molecule. The counterion for resorufin is sodium and for oxazine 118 is chloride.
E 6E4
E
24 4
s -b S
4
2E4
o
450
500
550 600 650 Wavolongth (nm)
700
6E4 1
750
I
t I
- -450
500
550 600 650 Wavolongth (nm)
700
750
Figure 2. (a) Absorption and emission spectra of oxazine 118 in I-butanol. e = 58 000 L mol-l cm-l at 594 nm. (b) Absorption and emission spectra of resorufin in 1-butanol. e = 53 350 L mol-' cm-' at 579 nm. The emission intensities are in arbitrary units.
and emission spectra (see Figure 2) since the detection scheme used here responds identically to both ground- and excited-state signals. For all measurements the pump power was maintained at 20-25 mW at the sample and the probe at 150 pW. The experimental time resolution of this system is determined by the cross-correlation of the pump and probe lasers and is typically 10 ps fwhm. Steady-State Spectroscopy. The absorption spectra of all solutions measured were taken on a Varian Model 634 UV-visible absorption spectrometer. The spectra presented in this work were acquired with 1.O-nm resolution. The emission spectral profiles were recorded with approximately 1-nm resolution using a Princeton Applied Research Model 1453 optical multichannel analyzer (OMA). Calculations. Semiempirical M N D O calculations were performed on both probe molecules using the M N D O software package developed by Thiel et a1.'* A Digital Equipment Corp.
-
(12) Program QCPE-353, written by W. Thiel, revised by K. E. Gilbert and J. J. Gajewski. The Quantum Chemistry Program Exchange, Department of Chemistry, Indiana University, Bloomington, IN 47405.
T*
f
95% cl, ps 667 f 13 776 f 22
0.31 f 0.01 0.32 f 0.01
701 f 22 1165 f 37
0.31 f 0.01 0.33 f 0.01
847 f 13 1456 f 46
0.36 i 0.02
51 i 5
0.36 f 0.02
53 f 4
"The asterisk indicates a measurement of the excited state. All measurements were made at 27.0 f 0.1 OC. TABLE 11: Experimental Zero Time Anisotropy Values and Anisotropy Decay Constants for Resorufin in the Alcohols" solvent
\
E
R*(O) f
95% cl, ps 95% cl 518 f 16 0.30 f 0.02 710 f 42 0.29 f 0.01
R(O) f 7f R*(O) f 95% cl 95% cl, ps 95% cl 1-butanol 0.34 f 0.01 466 f 48 0.37 f 0.01 2-methyl-1- 0.34 f 0.01 524 & 24 0.35 f 0.02 propanol 2-butanol 0.38 i 0.03 513 f 35 0.35 f 0.01 2-methyl-2- 0.36 f 0.02 982 f 98 0.34 f 0.03 propanol
-I
z
T f
95% cl 0.30 f 0.01 0.31 f 0.01
T*
f
95% cl, ps 467 f 18 599 f 21 523 f 24 893 f 69
"The asterisk indicates a measurement of the excited state. The data for 2-methyl-2-propanol have a relatively large uncertainty due to partial protonation of the carbonyl moieties (see text for discussion). All measurements were made at 27.0 f 0.1 "C.
(DEC) Vaxstation 2000 computer using the VMS operating system was used throughout. These calculations were done for both the ground state (So) and the first excited singlet state (SI) for each probe molecule. (Using this system, approximately 40 CPU days were required for completion of all calculations.) Chemicals. All alcohols used were either EM Omni-Solv grade or Aldrich Gold Label grade and were used without further purification. Rotational diffusion times were identical for each dye molecule in either brand of a given solvent. Solvent 2-butanone (98%) was purchased from E M and distilled prior to its use in order to remove any water. Resorufin (sodium salt) was purchased from Aldrich and used without further purification. A sample of oxazine 118 (chloride) was obtained from the Exciton Chemical Co. and used as received. The molar absorptivities of both resorufin and oxazine 118 were determined in order to establish the appropriate concentrations to use in the pumpprobe experiments. The molar absorptivity of resorufin in 1-butanol was measured to be 53 350 L mol-' cm-' at 579 nm and that of oxazine 118 in 1-butanol as 58 000 L mol-I cm-' at 594 nm. The solutions used for reorientation measurements were all prepared in the range of 19-22 pM and were changed daily. For all measurements the sample was flowed through a 1-mm path length flow cell to minimize thermal contributions to the signal. The actual path length of the measurement is on the order of 50 pm.13 The temperature of the sample was controlled at 27.0 f 0.1 OC.
Results and Discussion The rotational diffusion times of oxazine 118 and resorufin were measured using picosecond resolved pump-probe spectroscopy. Z,l(f) and Z l ( t ) were measured independently in order to obtain the zero-time induced orientational anisotropy, R(0). The anisotropy function used in characterizing the probe molecule dynamical behavior is given by R(t) = I q t ) - ~
+
L ( ~ ) I / ~ q21,(t)l ~ )
(1)
The time constants of the anisotropy decays were measured by regression of the function (ln R ( t ) }vs time. All of the decays measured here were best fit by a single exponential. Nonexponential or multiple-exponential behavior could not be resolved. (13) Blanchard, G. J.; Wirth, M. J. Anal. Chem. 1986, 58, 532.
5952 The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 TABLE 111: Calculated DSE Times for Resorufin and Oxazine 118 in the Solvents Used in These Exwriments“ resorufin oxazine 118
solvent 1-butanol 2-methyl-1-propanol 2-butanol 2-methyl-2-propanol 2-butanone
ll, c p 2.38 f 0.04 3.19 f 0.04 2.81 f 0.04 3.96 f 0.04 0.42
ps 124 166 146 205 22
Tilick.
Tdip
51 68 60 85 9
ps
ps 137 184 162 228 24
Tstickr
Tslip,
ps
59 79 70 98 10
“Viscosities of the alcohols were measured experimentally at 27.0 f 0.1 OC. The viscosity of 2-butanone was taken from ref 25. i ( t i & and rSlip are calculated according to eq 2, using values and dimensions detailed in the text.
The values of the ground-state and excited-state anisotropy decay constants for oxazine 118 are presented in Table I and for resorufin in Table 11, along with the zero-time anisotropy values. R(0) and R*(O)were obtained by regression of the experimental data for times >30-ps delay in order to avoid pulse overlap interference effects. There are several notable points regarding these data. The first is that resorufin reorients, within the experimental uncertainty, identically in both its ground and excited states in I-butanol, 2-butanol, and 2-methyl-2-propanol. In 2-methyl- 1propanol, however, it exhibits a slight state dependence. For oxazine 1 18, there is a dramatic state dependence in relaxation times for all of the alcohols. Before we discuss in detail the state-dependent dynamical behavior of these two molecules, it is in order to compare the experimental results to established models for rotational diffusion.14,15This will be of value in estimating the degree of solvent interaction being experienced by these dyes. The modified Debye-Stokes-Einstein (DSE) model of rotational diffusion is given by
= oVF/kTS
lor
(2)
where 7 is the solvent bulk viscosity, V is the probe molecule hydrodynamic volume, k is the Boltzmann constant, T is the temperature, and F is a “friction coefficient”, equal to 1 for the “sticking” bounding condition and < I for the “slipping” boundary condition, its exact value dependent on probe molecular shape. S is a shape factor, calculated from Perrin’s equations.16 In the past resorufin has been modeled as a prolate ellipsoid,l and its transition dipole has been assumed to lie along its long axis3 Recent results on cresyl violet” combined with the data presented here suggest that an effective oblate rotor shape may be more appropriate for both dye molecules. The assignment of a specific rotor shape, however, should not be given too much emphasis due to the limitations inherent to the DSE model. The modified DSE model assumes an ellipsoidal solute reorienting in a continuum solvent and allows for a variable frictional drag between the solvent and solute. Although it is perhaps an oversimplification of the highly complex nature of solvation, this model has been shown to be useful in the past for the description of some dye molecules reorienting in low-viscosity polar solvent^.^*^^ Values of io, were calculated for both resorufin and oxazine 1 18, using values for solvent viscosity at 27 “C determined experimentally. The hydrodynamic volume of resorufin was calculated by the method of van der Waals increments’* to be 165 A3,in agreement with an earlier report by Spears and Steinmetz.’ On the basis of molecular dimensions consistent with this volume (12.7 8, X 5.0 8, X 5.0 8,)the shape factor, S, is calculated to be 0.768. Similarly, oxazine 118 is calculated to have a hydrodynamic volume of 180 A3 and a shape factor of 0.754 for dimensions (13.3 8, X 5.1 A X 5.1 8,). Using these quantities and the appropriate values of F from Hu and Zwanzig,IS the calculated values of T,, (14) Debye, P. Polar Molecules; Chemical Catalog Company: New York, 1929; p 84. (15) Hu, C.-M.; Zwanzig, R. J. J . Chem. Phys. 1974, 60, 4354. (16) Perrin, F. J . Phys. Radium 1934, 5, 497. (17) Chuang, T. J.; Eisenthal, K. B. Chem. Phys. Lett. 1971, 11, 368. (18) Edward, J. T. J . Chem. Educ. 1970, 47,261.
Blanchard and Cihal are presented in Table 111. In all cases the calculated values of T,, track qualitatively with the experimental data but are substantially faster, even in the stick limit. Similar behavior has been reported previously for other dye molecules in alcohol solvents and has been interpreted as indicative of strong solvent-solute intera~tion.~~’~ At this point we will turn to a discussion of the observed state-dependent reorientation behavior of these probe molecules. Since this state dependence is exhibited much more strongly for oxazine 118 than for resorufin, it is appropriate to consider the two dyes separately. Oxazine 118. The observed state-dependent reorientation of oxazine 118 manifests itself as a significant increase in the time constant of the anisotropy decay function on excitation. In earlier work” large wavelength-dependent changes in the value of T were observed for cresyl violet. That wavelength dependence was accounted for quantitatively by changes in the relative orientation of the pumped and probed transition dipoles. Their relative orientation was observable directly through the measurement of R(t) at early times. Measurement of this quantity required the acquisition of the experimental signals Z,,(r) and Z L ( t ) independent of one another. By comparison, in this report, for a given solvent R(0)changes negligibly within the experimental uncertainty for the two probe wavelengths used (see Tables I and 11). Thus the observed state-dependent changes in T are not due to a wavelength-dependent change in the angle between the pumped and probed transition dipoles. The T values measured for the different states are directly comparable. This applies to oxazine 118 as well as resorufin. There are several reasons why one might expect an increase in reorientation time on excitation. One possibility is that the probe molecule changes shape or volume on excitation. Such a change would, however, likely have an observable effect on the measured value of R(0). Another possibility is that it is not the shape or volume, but rather the a-electron distribution within the probe molecule that changes on excitation. Such a change could conceivably allow for a modification of solvation behavior without affecting the molecular shape significantly. Since oxazine 118 is a polar molecule containing four heteroatoms, investigation of this possibility is worthwhile. Oxazine 118 is different from resorufin not only in terms of ionic charge, but also with respect to its polar end groups. Since oxazine 118 contains amino protons which could conceivably become more “acidic” on excitation, they could be the site of a change in solvation. It is also possible that one or both of the ring-bound heteroatoms could be responsible. In an attempt to elucidate the reasons for the observed state dependence experimentally, the rotational diffusion times of oxazine 118 in both the ground and excited states were measured in 2-butanone. The motivation for this experiment was to present oxazine 118 with an aprotic solvent molecule which is structurally similar to one of the butanols. The measured value of T at 27 O C was 51 f 5 ps and T* was 53 f 4 ps. [R(O)= R*(O) = 0.36 f 0.02 for oxazine 118 in 2-butanone (see Table I).] Thus no state dependence was resolvable, suggesting that the alcohol proton does play a role in the observed anomalous state-dependent reorientation. In an effort to understand this behavior more completely, semiempirical MNDO calculations21*22 were performed for both the ground state and the first excited singlet state of oxazine 118. It is recognized that the ground-state results are in reasonable agreement with experimental measurements, even for rather complex molecules.21 Excited-state calculations are recognized to be less accurate than those for the ground state since the parameterization employed in these calculations depends on ground-state information. At best, these calculated results can be considered only semiquantitative. It is accepted, however, that the qualitative trends predicted by such calculations are correct. (19) Spears, K. G.; Cramer, L. E. Chem. Phys. 1978, 30, 1. (20) Blanchard, G. J.; Wirth, M. J. J . Chem. Phys. 1985, 82, 39. (21) Dewar, M. J. S.; Thiel, W. J . Am. Chem. SOC.1977, 99,4899,4907. (22) Maroncelli, M.; Fleming, G. R. J . Chem. Phys. 1987, 86, 6221.
-
Relaxation Dynamics of Oxazine 118 and Resorufin
7211
HP-
+.28
+.20 -.21
+.21
712
+.29
-.21
-.28
"2
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 5953
L v
- 0.75 O
rn
i7i
0.50
c
.-
c
p 0.25 0
n 4 0.00
400 -.IS
Figure 3. (a) r-electron densities for the optimized ground state of oxazine 118 determined by MNDO calculations. (b) n-electron densities for the optimized first excited singlet state of oxazine 118, calculated in the same manner. A discussion of the results is presented in the text. Electron densities are reported here in decimal fractions of a unit electron charge.
Calculated changes in quantities such as electron density could possibly provide some insight into the anomalous state-dependent orientational relaxation measurements. The ground- and excited-state r-electron densities calculated for oxazine 118 are presented in Figure 3. More significant than examining the results for the individual states is to note the difference between the two states. Notably, the amino chromophores change little on excitation, as with the ring-bound oxygen. Likewise, the electron densities at the ring carbons change little except for those adjacent to the ring-bound nitrogen. The ring-bound nitrogen experiences a substantial increase in electron density and the carbons adjacent to it a corresponding decrease. The experimental observation that the solvent alcohol proton plays a role in the reorientation-state dependence is consistent with the calculated result. The oxazine 118 ring-bound nitrogen becomes a stronger Lewis base on excitation, allowing for an increased interaction between itself and the solvent alcohol proton. It has been recognized by Fleming et a1.26that the lifetime of such an interaction is proportional to its strength. If the lifetime of the solvent-solute interaction becomes closer to the solute reorientation time constant, the reorienting entity will appear, on average, larger. Thus, a stronger solventsolute interaction in the excited state should manifest itself as an increase in the observed orientational relaxation time. The calculation results suggest that in an aprotic solvent, such as 2-butanone, state-dependent orientational relaxation should not be observed, in agreement with the experimental finding. The results of the calculation thus provide an explanation for the observed anomaly; a state-dependent change in the n-electron density, specifically at the ring-bound nitrogen site, causes a change in the strength of (protic) solvent interaction at that site. Resorufin. The state-dependent dynamical behavior observed for resorufin is much less pronounced than it is for oxazine 118. Perhaps one reason for this lies in the fundamentally different chemistry of carbonyl and amino moieties. We observed that in primary alcohols the visible absorption spectrum of resorufin displayed a single maximum at 579 nm. In 2-butanol, a secondary alcohol, the absorption at 579 nm decreased and was accompanied by an increase in a very broad absorption band centered near 480 nm. In 2-methyl-2-propano1, a tertiary alcohol, the absorption band at 579 nm decreased further and the 480-nm band increased correspondingly. This progression is presented in Figure 4 and can be understood in terms of protonation of one of the resorufin carbonyl moieties. Protonation removes the added stabilization afforded by resonance delocalization. The more dissociative the (23) Blanchard, G. J.; Wirth, M. J. J . Phys. Chem. 1986, 90, 2521. (24) Drexhage, K. H. in Dye Lasers; Schafer, F. P., Ed.; Topics in Applied Physics, Vol. 1; Springer: Berlin, 1973. (25) Lange's Handbook of Chemistry, 12th ed.; Dean, J. A,, Ed.; McGraw-Hill: New York, 1979. (26) Fleming, G. R.; Knight, A. E. W.; Morris, J. M.; Robbins, R. J.i Robinson, G. W. Chem. Phys. Lett. 1977, 51, 399.
450
550 600 Wavelength (nm)
500
650
700
Figure 4. Solvent dependence of the absorption spectrum of resorufin in the alcohols. (1) is for 2-methyl-1-propanoland 1-butanol,(2) represents 2-butano1, and (3) represents 2-methyl-2-propanol.
Figure 5. (a) n-electron densities for the optimized ground state of resorufin determined by MNDO calculations. (b) *-electron densities for the optimized first excited singlet state of resorufin, calculated in the same manner. Electron densities are presented as in Figure 3, as decimal fractions of a unit electron charge.
alcohol proton, the more pronounced this effect will be. This experimental observation provides evidence for strong solvent interaction at the end groups. No similar observation was made for oxazine 118. By setting the pump wavelength to 570 nm, only the deprotonated form of resorufin was selected spectroscopically. Thus, for a given formal concentration, a greater relative fraction of resorufin is observable in primary alcohols than in secondary alcohols, which is greater than tertiary alcohols. The resultant uncertainties in the values of T and T* measured experimentally are greatest for 2-methyl-Zpropano1, followed by 2-butanol, and are least for 1-butanol and 2-methyl-1-propanol. Therefore, if a weak state dependence exists for resorufin in 2-butanol or 2methyl-2-propanol, it is possible that it was not resolved experimentally. In order to understand the more subtle nature of the statedependent reorientation behavior of resorufin, MNDO calculations were performed for both the ground and first excited singlet states. The results are presented in Figure 5. For these calculations it is important to examine both the individual results and the difference between the ground and excited states. The calculated changes in r-electron density on excitation are very similar to those for oxazine 118; the ring-bound nitrogen changes most significantly with the other heteroatoms changing only slightly. Examination of the individual calculation results, however, reveals the presence of a relatively high electron density on the carbonyl end group oxygens in both states. Further, these oxygens have free lone electron pairs with which to interact readily with a Lewis acid. The calculations for resorufin indicate that a change in electron density occurs at the ring-bound nitrogen on excitation. This increased electron density has to compete with the strong Lewis base character of the carbonyl moieties for solvation by the Lewis acid solvent. The excitation-dependent change in the ring-bound nitrogen electron density is thus less significant relatively for resorufin than for oxazine 118. The calculations are consistent
J. Phys. Chem. 1988, 92, 5954-5958
5954
Figure 6. The structure of cresyl violet.
with the less pronounced state-dependent reorientation dynamics observed experimentally. It is interesting to note that cresyl violet, a molecule structurally similar to oxazine 118, does not exhibit state-dependent reorientation properties in a wide variety of protic s o l ~ e n t s . ~ Cresyl ~-~2~ violet differs from oxazine 118 by containing an extra fused ring (see Figure 6). The absorption spectrum of cresyl violet has a maximum very close to that of oxazine 118. This added ring contributes little to the position of the absorption spectrum but does make it broader and more featureless. These effects have been attributed to steric interference of cresyl violet's end amino group by the additional ring structure.24 It is likely that steric blocking of the ring-bound nitrogen by this same ring accounts for the lack of state dependence reported in reorientation measurements.
Conclusions The rotational diffusion behavior of oxazine 118 in the series of butanols is observed to be highly state dependent. This state dependence is not resolved in the solvent 2-butanone, suggesting that the alcohol proton plays a role. The state-dependent reorientation of resorufin in the butanols is much more subtle than
for oxazine 118. The absorption spectrum of resorufin is observed to be solvent dependent, indicating a strong interaction of the solvent alcohol protons with its carbonyl end groups. No analogous solvent dependence of the absorption spectrum is seen for oxazine 118. Semiempirical MNDO calculations of the ground state and the first excited singlet state for both molecules show a significant increase in a-electron density at their ring-bound nitrogen. For oxazine 118 the ring-bound nitrogen becomes a stronger Lewis base on excitation. For resorufin this also occurs, but the carbonyl moieties are relatively strong Lewis bases in both states. Thus, on excitation, the increase in electron density at the ring-bound nitrogen is of less significance for resorufin than it is for oxazine 118. In conclusion, it is not possible to examine the role that ionic charge plays in orientational relaxation measurements in as straightforward a way as was intended originally. This is due primarily to the fundamentally different chemistry of the (C=O) and the (C-NH2) moieties. This work does report, however, unambiguous state-dependent rotational diffusion times for both resorufin and oxazine 118 in certain solvents. It also demonstrates that changes in electron density at a specific atom within the probe molecule can be responsible for significant changes in its local environment. Acknowledgment. We are indebted to A. S. Gozdz for his assistance with the emission measurements as well as with the MNDO calculations. The efforts of J. P. Heritage and P. Grabbe are greatly appreciated for their assistance with the MNDO calculations and for their generous donation of computer time. We are also grateful to R. Steppel of the Exciton Chemical Company for providing the sample of oxazine 1 18 chloride. Registry No. Resorufin sodium salt, 34994-50-8;oxazine 118 chloride, 53669-98-0.
Characterization of a Pulsed Supersonic Beam of Ammonia Monomer and Clusters Using the Hexapole Electric Field Kazuhiko Ohashi, Toshio Kasai, and Keiji Kuwata* Department of Chemistry, Faculty of Science, Osaka University, Toyonaka 560, Japan (Received: December 29, 1987)
An intense pulsed supersonic beam of ammonia monomer is focused and state selected by a hexapole electric field. The intensity of the focused beam is estimated to be 10'' molecules sr-' s-'. The state-selected monomer beam is characterized by comparison of its intensity with the numerical calculation of trajectories according to the Stark effect for two nearby levels, and the rotational state distribution is obtained. Ammonia clusters with n members ( n = 2-10) are found to be unfocused by the hexapole field. Thus, neither a symmetric-top structure nor a very large electric dipole moment is expected for ammonia dimer and larger clusters. The analysis on the nozzle-stagnation pressure dependence of the focusing behavior shows a method to estimate the fraction of clusters in the beam by the hexapole technique. N
1. Introduction So far, the rotational state distribution of ammonia molecules in a supersonic beam or a free jet has been determined by spectroscopic methods such as IR absorption,14 coherent anti-Stokes Raman scattering (CARS),Smultiphoton ionization (MPI)? and
bolometric detection.' Due to small number density in molecular beam experiments, those methods often have a common difficulty in practical application. The electrostatic state selection may be an alternative technique to determine the rotational distribution for such a polar symmetric-top m ~ l e c u l e . * ~ ~
(1) Snavely, D. L.; Colson, S. D.; Wiberg, K. B. J . Chem. Phys. 1981, 74, 6975. (2) Baidacchini, G.; Marchetti, S.;Montelatici, V. Chem. Phys. Left. 1982, 91, 423. (3) Mizugai, Y . ;Kuze, H.; Jones, H.; Takami, M. Appl. Phys. B 1983, B32, 43. (4) Veeken, K.; Reuss, J. Appl. Phys. E 1984, 834, 149.
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0022-3654/88/2092-5954$01 SO10
0 1988 American Chemical Society
