5946
J . Phys. Chem. 1988, 92, 5946-5950
Isotopic Dependence of Recombination Kinetics in Water A. C. Chernovitz and C. D. Jonah* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: March 14, 1988)
The decay of the hydrated electron solvated by D 2 0 molecules, eaq-(D20),and the OD radical as well as the reactions of the precursor of the hydrated electron have been investigated in D 2 0 solution by using the Argonne stroboscopic pulse radiolysis system. These processes have been compared to the analogous reactions in H 2 0 . The decay of the electron is slower in D 2 0 relative to the corresponding species in H20, while the decay of the hydroxyl radical and the efficiencies of the electron scavengers in reducing the initial yield of the hydrated electron are comparable in both solvents. The OD radical absorbance decays to 0.75 f 0.06 of its initial value from 200 ps to 3.0 ns. This parallels that for the OH radical (0.74 i 0.06). The similarity of the electron scavenging efficiencies indicate a hydrated electron precursor that exists in a distinct, localized state for both D 2 0 and H 2 0 . The slower decay of the electron in D 2 0 has been interpreted as a greater thermalization distance and a broader initial spatial distribution for the electron in D 2 0 compared to H 2 0 . The spurs of the D 2 0 and H 2 0 systems have been modeled by using a spherically symmetric electron distribution around a Gaussian hydroxyl radical core. The computer simulations agree quite favorably with the experimental results.
Introduction The nature of the initial spatial distribution of ions and radicals generated by radiolysis is of fundamental importance to the primary process involved in the deposition of energy, in spur kinetics and to the entire chemistry of the system. The interaction of ionizing particles with the solution deposits energy into the system by the excitation and ionization of the water molecules. An electron can be ejected from an electron shell of the water molecules with excess kinetic energy. The electron transfers energy as it travels through the solution, is thermalized, and is finally solvated. The initial distribution of the distance of the hydrated electron from the initial positive ion depends upon the journey of the electron during the thermalization processes. The competition between recombination of the ions and radicals created by the ionizing event(s) and the reaction of these ions and radicals with other components in the solution determine the chemical effects of ionizing radiation. The further the electron travels, the slower the recombination reactions will occur and the more effective is the reactions of these ions and radicals with other components in the solution. The importance of the competition between recombination reactions and reactions of the ions and radicals with biomolecules to biological damage has provoked a substantial theoretical effort. These have included simple in which the energy degradation spectra are used to create initial starting conditions by using prescribed diffusion kinetics4 and complete calculations that use the energy degradation spectra for electrons and calculate the kinetics by using Monte Carlo technique^.^ Two types of data have been used for the verification of such models: (1) time dependence of the transient radicals; (2) yield of final products in the presence and absence of scavengers. The latter has been the more common because data with sufficient time resolution did not exist when much of the modeling was done. Unfortunately, agreement with yield data has not guaranteed agreement with the time-dependent data. Until the present work, there existed no time-dependent data for D 2 0 and only a limited amount of yield data. For these reasons there had previously been no attempts to model the spur in D 2 0 . ( I ) (a) Mozumder, A,; Magee, J. L. Radiat. Res. 1966, 20, 203. (b) Mozumder, A.; Magee, J. L. J . Chem. Phys. 1966, 45, 3332. (c) Naleway, C. A.; Sauer, M. C.; Jonah, C. D.; Schmidt, K. H . Radiat. Res. 1979, 77,47. (d) Kuppermann, A. In Radiation Research; Silini, G., Ed.; North-Holland: Amsterdam, 1966; p 212. (2) Schwarz, H . A. J . Phys. Chem. 1969, 73, 1928. (3) Trumbore, C. N.; Short, D. R.; Fanning, J. E.; Olson, J. H . J . Phys. Chem. 1978, 82, 2762. (4) Miller, J. H.; West, M. L. J . Chem. Phys. 1977, 67, 2793. (5) (a) Zaider, M.; Brenner, D. J.; Wilson, W. E. Radiat. Res. 1983, 95, 231. (b) Turner, J. E.; Magee, J. L.; Wright, H . A,; Chatterjee, A,; Hamm, R. N.; Ritchie. R. H. Radiat. Res. (1983), 96, 437.
0022-3654/88/2092-5946$01.50/0
The dynamics of electron localization and solvation play an important role in the understanding of electron-transfer processes in condensed matter as well as in estimating the local dynamical molecular structure of the surrounding fluid in which the electron becomes solvated. The reactions and many of the characteristics of the solvated electron in various media have been extensively studied and documented.6 However, the processes involved in the travel of the electron prior to its thermalization and subsequent solvation are not well understood since their direct observation requires time-resolved subpicosecond techniques. The question has arisen as to whether there are preexisting solvation sites in the fluid into which the precursor of the solvated electron, i.e., the electron as it exists prior to solvation, becomes trapped or whether the electron induces solvent molecular reorientation during the solvation process. It has been established that electron solvation in liquid alcohols involves an intermediate trapped electron that absorbs in the near and a two-channel electron solvation mechanism has been proposed.I0 Wiesenfeld and Ippen" have measured the appearance of e,; in liquid water less than 0.3 ps following photolysis of ferrocyanide solution. Their results suggested electron capture via preexisting deep traps since the electron solvation was faster than the dielectric relaxation time of the water. Recently however, an intermediate, localized electron has been identified by Gauduel et a1.I2 using femtosecond laser photolysis of liquid water. This species absorbs light at 1250 nm, appears with a time constant of 110 fs, and relaxes in 240 fs to the hydrated state. These results suggest a two-step solvation mechanism involving a distinct localized state as the precursor of eaq-. The reactions of the precursors have been studied by measuring the initial yield of e, - as a function of scavenger concentration. Hunt and ~ o l l e a g u e ~have ~ * found '~ that the initial yield of ea; was reduced in the presence of scavengers and the fraction,f, of eaq- produced at scavenger concentration [SI may be expressed as
f = ex~(-[SlQu)
(1)
(6) (a) Matheson, M. S. In Physical Chemistry; Academic: New York, 1975; Vol. VII, Chapter IO. (b) Kestner, N . R. In Radiation Chemistry; Farhataziz, Rodgers, M. A. J., Eds.; VCH: New York, 1987; Chapter 8. ( 7 ) Baxendale, J. H.; Wardman, P. J . Chem. Soc.,Faraday Trans. I 1973, 69, 584. (8) Kenney-Wallace, G. A.; Jonah, C. D. Chem. Phys. Lett. 1976,39, 596. (9) Kenney-Wallace, G. A,; Jonah, C. D. Chem. Phys. Lett. 1977,47,362. (10) Lewis, M. A.; Jonah, C. D. J . Phys. Chem. 1986, 90, 5367. (11) Wiesenfield, J. M.; Ippen, E. P. Chem. Phys. Lett. 1980, 73, 47. (12) Migus, A.; Gauduel, Y.; Martin, J. L.; Antonetti, A. Phys. Rev. Lett. 1987, 58, 1559. (13) Wolff, R. K.; Bronskill, M. J.; Hunt, J . W. J . Chem. Phys. 1970, 53, 4211. (14) Lam, K. Y.; Hunt, J. W. Int. J . Radiat. Phys. Chem. 1975, 7, 317.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 5947
Recombination Kinetics in Water
1 .o
4
0
-m -
-
.d
C
c
0 C
.-0 L
20
0
c!
'2,
U.
0.7
I
I
I
I
I
I
I
I
I
I
I
5-
1 0.1M NaOD x 0.88
0.85
'
I
I
0.0
'
'
I
'
'
I
I
'
'
'
I
'
3.35
Time (ns)
-
d-58.0 0.0
3.35 Time (ns)
Figure 1. Four panels illustrating the effect that a deuteriated solvent has on the decay of the hydrated electron in aqueous media. T h e wavelength is 600 nm.
where Q37is the reciprocal of the concentration of scavenger that is needed to decrease the initial yield of the hydrated electron to l / e (37%) of the initial yield with no scavenger present. Thus, Q37is a measure of the efficiency of a particular solute to scavenge dry electrons. The goals of this work are to learn more about the primary processes involved in the radiolysis of liquid water. Specifically, we are interested in obtaining experimental information about the initial distribution of the primary species eaq- and OH and in interpreting these data in terms of existing mathematical models, as well as enhancing our understanding of the solvation process in general. The radiation chemistry of D 2 0 corresponds very closely to that of H20;however, there is sufficient difference between H20and D 2 0 , such as the vibrational energies and diffusion coefficients, that a solvent-dependence study can provide information about the distributions and the state of the electron that reacts or is hydrated.
Experimental Section The data were collected with a picosecond pulse radiolysis system previously d e s ~ r i b e d . ' ~The Argonne linear accelerator (linac) generated pulses of 20-22 MeV electrons 60 times per second with an approximately 25-ps pulse duration. The total dose deposited in the irradiation cell was about 7 krad. For the e,; (D20) decay and electron scavenging experiments absorption for the hydrated electron was recorded at 600 nm and room temperature (25 "C) and was between 60 and 75%. The decay of O D was determined by using an interference filter centered at 281 nm. Stock solutions of the electron scavengers were made by using D 2 0 (99.6% isotopic purity), were diluted to the appropriate concentrations, and were flowed through a 2-cm cell with suprasil windows. The deuteriated cupric and cadmium perchlorate samples were prepared by exchanging the hydrated compounds twice with DzO. Other reagents were used as obtained. Scavenger solution concentrations ranged from 0.1 to 1.2 M. The solutions were not degassed since dissolved oxygen does not affect eaqkinetics or absorption in the picosecond regime.I6 (15) Jonah, C. D. Rev. Sci. Instrum. 1975, 46, 62.
The Q3,values were obtained by alternating measurements of D 2 0 and scavenger samples. The maximum absorption of eaq(D20) was measured from the data, and the first-order decay constants were resolved from the kinetic traces by using a nonlinear least-squares fitting routine. A precalculated table, which accounts for instantaneous formation and exponential decay of the electron,I7 was used to evaluate the initial e,; (D20) yields. The OD decay curve was determined two different ways: ( 1 ) by subtracting the decay trace of the empty cell, which had a small absorption, from the decay of the O D radical decay obtained in 2 M deuteriated sulfuric acid solution; ( 2 ) by subtracting the decay of eaq-(D20) by using an interference filter centered at 643 nm from the decay trace of e,; (D20) and OD at 281 nm. Therefore, the decay of OD was measured in the presence of either D atoms or electrons. This procedure, also used to obtain the OH decay curve in this work, had been used previously to determine the OH decay curve.18 The two decay traces were comparable since the reactivity of O D with D or e,; (D20) are similar. These curves were recorded three to four times in a day to average out variations in the intensity of the linac beam. Time begins at 200 ps after the linac pulse because the decay of eaq- ( D 2 0 ) in the acidic solution occurs prior to this time. Data points were taken from a smooth curve drawn through the data.
Results Decay of eaq-( D 2 0 ) .The decay of the hydrated electron was obtained by recording the time dependence of the absorption at 600 nm in H20and D 2 0 and in 0.1 M solutions of N a O H and NaOD. The decays for eaq-were analogous to those previously measured. These traces, shown in Figure 1 , indicate that there is a large difference between the rates of decay of the electron in H,O and D20solutions. Previous studiesIg of the hydrated electron in D 2 0 with e,; (D20), D, OD, and OD, and D30+have (16) Bronskill, M. J.; Wolff, R. K.; Hunt, J. W. J . Chem. Phys. 1970, 53, 4201. (17) Jonah, C. D.; Miller, J. R.; Matheson, M. S. J . Phys. Chem. 1977, 81, 1618. (18) Jonah, C. D.; Miller, J. R. J . Phys. Chem. 1977, 81, 1974. (19) Hart, E. J.; Anbar, M. The Hydrated Electron; Wiley: New York, 1970; Chapter IV.
5948
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
Chernovitz and Jonah
TABLE I: Measured Values of O.I," '" I 10''OkQp,M-l e373
6
scaveneer acetone 2,3-butanedione CdZ+(perchlorate)d Cuz+ (perchlorate)d
1 0 -
0
9
.5
OD Decay
0 9 -
0
Cr207'NO,-
r
0
Io4-
I
u
0 8 -
I
SeOd2-
LL
0.7
I
1
1. o
0.0
I
I
2.0
3.0
I 4.0
Time (ns)
Figure 2. Experimental decay of the OD and OH radicals at 28 1 nm.
M-'
(H2Ob/D2O) 0.71 /0.93 0.89/0.85 2.63/2.78 1.11/0.80 8.33/9.09 2.38j2.17 7.14/5.00 2.38/1.85
s-I
(H20 /DzO) 0.87/0.74c 1.12/ 1.1 4.0/3.4 3.0/2.5' 8,717.7 2.0/ 1.4 1 1.0/8.9 0.36/0.17
Experimental error is approximately 10%. The measured values of
Q37are defined by eq 1. Q37and kQ,, values for H 2 0 were measured p r e v i ~ u s l y ~and ~ ~are ' ~ not corrected for time-dependent rate constants. 'These are estimated values of kQ3,obtained by an extrapolated line of a plot of k , , the pseudo-first-order rate constant, versus scavenger concentration. dUsing sulfate as the counteranion was found to be inadequate due to ion pair formation. pendent rate constants. This correction was found to be significant only for reactions with large rate constants for an analogous study of electron scavengers in H2O.I7 The Q37values in D 2 0 are generally close to those in H20; most are within 30%. The kQ3,rate constants are lower for D 2 0 than for H 2 0 , which can be attributed to the slower diffusion of the scavenger in D 2 0 compared to H 2 0 .
0.0
Time (ns)
3.9
Figure 3. Observed decay of the OD radical compared to eaq-(D20).
found that the rate constants for these reactions differ by less than 20% compared to the rate constants for the corresponding reactions in H 2 0 . Therefore the discrepancy in the rates cannot be solely due to a difference in the kinetics. The slower rates of decay in alkaline solution relative to pure solvent were discussed elsewhere." Apparently the reaction of the hydroxide ion with a proton is relatively unimportant a t early times, Le., 0-500 ps after the electron pulse. The total initial hydrated electron absorption in D 2 0 is higher than in H 2 0 due to the larger molar extinction coefficient of ea( (D20) compared to eaq.I9 Decay of OD. The decay of the O D radical compared to the decay of the OH radical is illustrated in Figure 2. The two curves drawn are averages of the individual traces. There is virtually no difference between the two decay curves. From 200 ps to 3 ns, O D decays to 0.75 f 0.06 of its initial value, which is very close to the fraction of the initial absorption for O H (0.74 f 0.06). The O H decay curve recorded here differs from the previously reported curveI8 from 200 ps to 2 ns after the electron pulse. Improvements have been made in the electronic and data acquisition system since the first O H decay curve was measured. This may account for the discrepancy between the two OH decay curves. Since the spatial distribution for the hydroxyl radical is much tighter and, therefore, more concentrated than for the hydrated electron, the OD radical was found to decay more quickly than eaq- (D20), as shown in Figure 3. Similar results were previously reported for OH.18 Q37 Values. The relative initial yields of eaq- (D20) are dependent on scavenger identity and concentration and are determined by comparing the initial absorption with and without scavengers present. The scavengers were chosen because they have a wide range of reaction radii and rate constants and have been studied in H2O.I7 The Q3, values were obtained from the slope of a semilogarithmic plot of the fraction of electron yield remaining versus the concentration of scavenger, as described by eq 1. A list of these results, along with the values of k , , , the second-order rate constants at concentration 1/Q37,is presented in Table I. The Q37measurements are not corrected for any effects of time-de(20) Jonah, C. D.; Hart, E. J.; Matheson, M. S . J . Phys. Chem. 1973,77, 1838.
Discussion Our principal findings in this study are (1) the hydrated electron decays more quickly in H20than in D20, (2) the OD radical decay is similar to that of OH, and (3) various compounds have similar reactivities in scavenging the precursor of the hydrated electron in both solvents, as determined by the Qj7 values. The slower decay for the electron in D 2 0 , which is not due to the smaller rate constants in D 2 0 , can be understood in terms of very simple theoretical considerations. If energy in excess of the excitation levels of water were lost rapidly, principally by secondary ionization and electronic excitation events, most of the electron travel would occur as a subexcitation electron. The subexcitation electron loses most of its energy to the vibrational modes of the solvent molecules. The most energetic vibrational mode of water is the 0-H (0-D) antisymmetric stretch. The energy of this mode is approximately 21i2 times greater in H 2 0 than in D20, so that 21/2more collisions are needed in D 2 0 than in H 2 0 for the energy dissipation of the electron. Thus the electron would be expected to travel a greater distance in D 2 0 compared to H20. The broader electron distribution in D 2 0 would slow the recombination reactions. The precise change in the distribution function for D 2 0 is not easily determinable since it depends critically on the amount of elastic versus inelastic scattering and the angular dependence. For instance, if the electron moves randomly while being thermalized, the increase in the width of the distribution will be proportional to "I2, where N is the number of steps. If the electron primarily travels in a straight line while being thermalized, the distance traveled will be proportional to the number of steps N . Thus one would expect that the electron distribution in D 2 0 to be between given 2'12 times more steps. 1.18 and 1.41 as large as that in H20, Since it has been assumed that most of energy is lost before the electron has traveled very far, the distribution of O H radicals would not be expected to be markedly different from that of the OD radicals. Thus the system may be simulated by assuming the electron distribution is broader while the OH distribution remains unchanged for the D 2 0 system. There have been many recent studies in which the kinetics of the recombination reactions have been examined theoretically by using a variety of techniques. The most rigorous are those in which the degradation of an ionizing particle is followed and the travel of all of the particles subsequently created is traced. This approach is excellent for determining the macroscopic spatial distribution; however, these models use various ad hoc techniques for determining the spatial distribution of the electrons. Therefore, these techniques cannot be used to directly estimate the differences between the D 2 0 and H,O systems.
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 5949
Recombination Kinetics in Water
I -D -C
c 0 7
0.0
1
1
10
OH23A OH30A
100
’000
Time(ns) 1 .o
2.0
3.0
Time(ns)
Figure 4. Comparison between experimental and theoretical data for the decay of ea; in H20and DzO.
For this work, the system has been modeled by using a simple spherically symmetric continuum diffusion kinetics procedure. The model chosen for the distribution for the initial species was that of Trumbore and co-workers3because it has provided a description of the decay of the electron and of the O H radical that is in reasonable accord with experimental data. This representation assumes that the electron is in a spherical shell surrounding the O H distribution. Figure 4 shows a comparison of the experimental and theoretical data for the electron in H 2 0 and D 2 0 . The parameters for the H 2 0 environment are precisely those of Trumbore: the radius for the electron was increased approximately 30% for D20; all the other parameters were left unchanged. There is clearly a difference between the calculated and experimental data even for H 2 0 however, the H20 parameters were not modified in any way to improve the agreement. In addition, this calculation assumes only a single-sized spur so that there are many other parameters that should also be included. Nevertheless, the general trend is satisfactory. The decrease in the decay between the H 2 0 and D 2 0 systems is quite well modeled. Also in this study, the ratio between the yield of the electron in D 2 0 and H 2 0 at long times has been calculated to be 1.10. This corresponds very well to the value of 1.1 1 determined experimentally by Fielden and Hart.21 Experimentally, the difference between the OH and O D decay is considerably smaller. This may be understood from the fact that the O H distribution was not changed in going from the H 2 0 to the D 2 0 system. Therefore, the O D reaction with itself will be unaffected because that distribution is not changed. The calculations do indeed show this effect, which is illustrated in Figure 5. There are certainly many approximations in the calculations that have been done here. Many of them have been described by Green and co-workers22in their series of papers on stochastic techniques. The distributions chosen can partially compensate for these effects. Note that the time-dependence data for the Green model and the Schwarz model2 are quite similar even though the distributions are considerably different. In addition, one would expect the errors to be similar for H 2 0 and D 2 0 and, therefore, expect the final result not to be greatly changed. Despite the above caveats, the origin of the difference between the H 2 0 and D 2 0 system appears to be well described by a wider distribution for the electron in D20. A complete quantification awaits further research. (However, the quantification of the electron decay in H 2 0 is still not complete). The similarity in the Q3,values for the presolvated electron scavengers in D 2 0 and H 2 0 , displayed in Table I, show that for the presumed increase in the travel of the presolvated electron, it does not react with a precursor scavenger as a subexcitation (21) Fielden, E. M.; Hart, E. J. Radiar. Res. 1968, 33, 426. (22) (a) Green, N. J. B.; Pilling, M. J.; Clifford, P.; Burns, W. G. J . Chem. SOC.,Farday Trans. 1 1984, 80, 1313. (b) Clifford, P.; Green, N. J. B.; Pilling, M. J. J. Phys. Chem. 1982, 86, 1318. (c) Clifford, P.; Green, N. J. B.; Pilling, M. J. J. Phys. Chem. 1982, 86, 1322.
Figure 5. Comparison of OD and OH radical theoretical decay data.
electron. If this did occur, the Q3,values would be larger in D20 than in H 2 0 because of the greater time and the greater travel in the former. The reaction then would have to be with the electron while its energy is above the ionization limit of the solvent or after the electron does not travel far, Le., while the electron is localized. Recent experimental data of Gauduel and COworkers23are consistent with the latter consideration. They have observed that the growth of the 600-nm band of the electron is faster in the presence of dry electron scavengers. This means that the reaction of the scavengers with the localized electron is in competition with solvation. The existence of several reaction channels for the electron depending on its energy has been previously proposed in alcohols. The present results in water are consistent with those proposals. One, of course, must be careful in comparing solvation in water and alcohols. The solvation processes appear to occur at the dielectric relaxation time in alcohols but are considerably faster than the dielectric relaxation time in water. Fast solvation processes also occur in glycerol and ethylene which are systems consisting of two OH bonds. Multiple channels have also been predicted from positronium annihilation experiment^.^^ Those results have shown that there is little difference between H 2 0 and D 2 0 for reaction with NO3and are similar to the results presented here. However, considerable differences were seen for Cd2+. (The reaction of cadmium was with the “damp” electron while the nitrate was with the “dry” electron.) The expected correspondence between the positronium data and the pulse radiolysis data is difficult to determine.
Conclusion The slower decay of the hydrated electron in D 2 0 relative to H 2 0 provides evidence for a broader initial spatial distribution of the electron in the spur for D20. This conclusion is supported by continuum diffusion kinetics calculations that assume a spherically symmetric distribution for ea; surrounding the O H radical core. The difference in distribution of the hydrated electron in D 2 0 is estimated to be 1.18-1.41 as large as that in H 2 0 , while the modeling calculations predict 1.30. The efficiencies of various electron scavengers have been found to be similar in both solvents. This implies that reaction or solvation of the electron does not occur concurrently with energy dissipation and that electron scavengers react with a precursor of the hydrated electron which exists in a localized state. This reasoning suggests a two-step solvation mechanism. For further elucidation of the solvation mechanism, experimental plans are in progress to investigate the hydration of the electron in viscous media in which the local structure of the water will be perturbed due to hydrogen bonding of the water molecules to the solute, In this way, the influence between the viscosity effects of the total fluid and the local structure of liquids on the solvation process may be examined. (23) Gauduel, Y., private communication. (24) Jonah, C. D., unpublished results. (25) Facetti, J.; Abbe, J. C.; Duplatre, G.; Maddock, A. G.; Haessler, A. Radiat. Phys. Chem. 1980, 15, 541. (26) Duplatre, G.; Jonah, C. D. Radiat. Phys. Chem. 1985, 24, 554.
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.
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0 1988 American Chemical Society
