J. Phys. Chem. 1988, 92, 6303-6307
6303
A Study of the State-Dependent Reorientation Dynamics of Oxazine 725 in Primary Normal Aliphatic Alcohols G . J. Blanchard Bell Communications Research Inc., Red Bank, New Jersey 07701 (Received: March 17, 1988; In Final Form: May 23, 1988)
Certain oxazine laser dyes have been shown to exhibit state-dependent rotational diffusion behavior before. In this work the state-dependent dynamical behavior of oxazine 725 is examined in the primary n-aliphatic alcohols methanol (C,) through 1-decanol (Clo). The data demonstrate that the excited probe molecule reorients more slowly than the ground-state probe in all of the alcohols. This state dependence is shown to vary with solvent aliphatic chain length in a regular manner. An explanation is offered for this regularity in terms of a state-dependent change in the interaction energy between the solute and the solvent surrounding it owing to a site-specific change in the Lewis basicity of the solute molecule.
Introduction
primary n-aliphatic alcohols, represent a series of solvents with regularly varying alkyl and polar characteristics. Many early The study of the fundamental nature of solvation has attracted studies of the liquid phase were performed using dielectric dismuch attention in the recent A significant amount of persion techniques. Primary n-aliphatic alcohols displayed three this work has been focused on the dynamics of polar organic characteristic motions detectable by these dispersion measure“probe” molecules in polar organic solvents due to their widespread ments. Among these motions was one which was characteristic use and the availability of experimental systems capable of of the formation and existence of hydrogen-bonded net~orks.’~J’ studying the~n.l-~ One of the more elegant approaches to the study The lifetimes of these networks were found to be rather long of solvation has been through the time-resolved measurement of (between 100 ps for methanol and 2 ns for 1-decanol), suggesting the fluoroescence Stokes shift exhibited by polar dyes in various that the local environments formed by these solvents are stable polar This work showed that electronic excitation of on the time scale of a probe molecule’s rotational motion. The the probe molecule can induce subsequent changes in the solvent influence of these long-lived solvent networks on probe molecular cage surrounding it. Understanding the dynamics of this process motion remains at present, however, an open question. required treatment at the molecular level rather than by a conThe present study was undertaken to understand more comtinuum approach. pletely the nature of the state-dependent rotational diffusion In addition to the aforementioned technique, rotational diffusion behavior reported earlier.15 Oxazine 725 (see Figure 1) was measurements are also sensitive to the solvent environment surstudied in the series of primary n-aliphatic alcohols methanol rounding the reorienting molecule. Most of the previous work through 1-decanol, hereafter referred to as primary n-alcohols. in this area has related rotational diffusion behavior to the bulk viscosity and the polarity of the solvents being s t ~ d i e d . ~ - ’ ~ The ~ ~ ~rotational diffusion dynamics of oxazine 725 were measured for both the ground state (So) and first excited singlet state (SI), Interpretations in terms of bulk solvent properties, however, have and a state dependence was resolved. Oxazine 725 was chosen not provided a quantitative molecular level picture of solvation. because of its similarity to oxazine 118 as well as previous work Recently, it has been shown that certain polar organic dye relating to its dynamical b e h a ~ i o r .This ~ state dependence was molecules exhibit state-dependent rotational diffusion characfound to correlate with the solvent primary n-alcohol chain length. teristics in the series of butanols.15 In that study both the hyIt is the purpose of this paper to report these results and offer an drogen-bonding characteristics of the solvents and the changes explanation for the observed behavior. produced in the probe molecules on excitation were shown to be important to understanding the observed behavior. Thus it is Experimental Section becoming increasingly clear that the process of solvation needs a. Laser. The picosecond pumpprobe laser spectrometer used to be addressed at the microscopic rather than the macroscopic in these experiments has been described in detail elsewhere.l8 level in order to gain insight into its fundamental nature. Briefly, a mode-locked argon ion laser (Spectra-Physics Model Alcohols have been the subject of a number of studies of polar 17 1-09) was used to produce 1 W of average power at 5 14.5 nm solvation.5-’5~18~20 This is, in part, because alcohols, particularly with 100-ps pulses at 82-MHz repetition rate. This laser was used to pump synchronously two dye lasers (Coherent Model 701-3). Triple frequency modulation techniques were used for signal ( I ) Castner, E. W. Jr.; Maroncelli, M.; Fleming, G. R. J . Chem. Phys. encoding with sum frequency synchronous demodulation detection. 1987, 86, 1090. The pump dye laser was operated at 640 nm, producing 5-ps pulses (2) Nagarajan, V.;Brearly, A. M.; Kang, T-J.; Barbara, P. F. J . Chem. and 130-mW average power. The probe dye laser was operated Phys. 1987, 86, 3183. ( 3 ) Maroncelli, M.; Fleming, G. R. J . Chem. Phys. 1987, 86, 6221. at 635 nm (5-ps pulses, 120-mW average power) for ground-state (4) Ben-Amotz, D.; Scott, T. W. J . Chem. Phys. 1987,87, 3739. recovery measurements and at 690 nm (5-ps pulses, 90-mW av( 5 ) Chuang, T. J.; Eisenthal, K. B. Chem. Phys. Leu. 1976, 11, 368. erage power) for excited-state stimulated gain measurements. (6) von Jena, A,; Lessing, H. E. Chem. Phys. 1979, 40, 245. Both dye lasers were operated with DCM dye (Exciton). The (7) Fleming, G.R.; Morris, J. M.; Robinson, G. W. Chem. Phys. 1976, 17, 91. time resolution of this system is determined by the cross-correlation (8) Spears, K. G.; Cramer, L. E. Chem. Phys. 1978, 30, 1 . of the two dye lasers, which is typically 10 ps fwhm.I8 (9) Waldeck, D.; Cross, A. J. Jr.; McDonald, D. B.; Fleming, G. R. J.
Chem. Phys. 1981, 74, 3381.
(10) Waldeck, D. H.; Fleming, G. R. J . Phys. Chem. 1981, 85, 2614. ( I I ) Gudgin Templeton, E. F.; Quitevis, E. L.; Kenney-Wallace, G. A. J . Phys. Chem. 1985, 89, 3238. (12) Gudgin Templeton, E. F.; Kenney-Wallace, G. A. J . Phys. Chem. 1986, 90, 2896. (13) von Jena, A.; Lessing, H. E. Chem. Phys. Leu. 1981, 78, 187. (14) Spears, K. G.; Steinmetz, K. M. J . Phys. Chem. 1985, 89, 3623.
(15) Blanchard, G. J.; Cihal, C. A. J . Phys. Chem. 1988, 92, 5950. (16) Garg, S. K.; Smyth, C. P. J . Phys. Chem. 1965, 69, 1294. (17) Fellner-Feldegg, H. J . Phys. Chem. 1969, 73, 616. (18) Blanchard, G. J. J . Chem. Phys. 1987, 87, 6802. ( 1 9) Perrin, F. J . Phys. Radium 1934, 5 , 497. (20) Millar, D. P.; Shah, R.; Zewail, A. H. Chem. Phys. Lett. 1979, 66, 435.
0022-3654/88/2092-6303~0l.50/0 0 1988 American Chemical Society
6304 The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
Blanchard
TABLE I: Solvent Viscosities and Ground- and Excited-State Zero-Time Anisotropies and Reorientation Times
MeOH EtOH 1 -PrOH
1-BuOH I-PeOH 1-HxOH 1-HpOH 1-0cOH I-NOOH I-DeOH DMSO
0.576 f 0.012 1.032 f 0.023 1.796 f 0.031 2.377 f 0.042 3.160 f 0.055 4.146 f 0.089 5.427 f 0.104 6.878 f 0.145 8.685 f 0.153 10.524 f 0.219 1.Ib
0.34 f 0.02 0.34 f 0.01 0.34 f 0.01 0.33 f 0.01 0.34 f 0.01 0.33 f 0.01 0.35 f 0.01 0.34 f 0.01 0.34 f 0.01 0.33 f 0.01 0.37 f 0.01
0.33 f 0.01 0.34 f 0.01 0.35 f 0.01 0.34 f 0.01 0.33 f 0.01 0.34 f 0.01 0.32 f 0.01 0.36 f 0.01 0.35 f 0.01 0.34 f 0.01 0.38 f 0.01
72 f 5 112 f 5 193 f 10 281 f 1 1 385 f 22 509 f 13 685 f 17 892 f 17 1045 f 22 1210 f 36 266 f 20
82 f 3 129 f 5 229 f 13 337 f 7 456 f 16 607 f 14 773 f 11 1017 f 21 1190 f 21 1409 f 31 279 f 14
“The solvent abbreviations are as follows: MeOH = methanol, EtOH = ethanol, 1-PrOH = 1-propanol, 1-BuOH = I-butanol, 1-PeOH = I-pentanol, 1-HxOH = I-hexanol, 1-HpOH = I-heptanol, I-OcOH = I-octanol, 1-NoOH = 1-nonanol, 1-DeOH = 1-decanol,DMSO = dimethyl sulfoxide. *From the Merck Index, 10th ed.; Merck & Co.: Rahway, NJ, 1983.
000
L
200
F
n v)
Q
Figure 1. The structure of oxazine 725. Only one of the possible resonance structures is presented here. The counterion (not shown) is perchlorate. The structure of oxazine 118 is the same except that it has protons on the end nitrogens instead of ethyl groups.
b. Absorption Spectroscopy. All absorption spectra reported here were measured on a Varian Model 134 scanning absorption spectrometer, operating with 0.2-nm resolution. The spectra were digitized by using a Jandel Scientific digitizing tablet. c. Chemicals. Oxazine 725 perchlorate was purchased from Exciton Chemical Co. and was used as obtained. All solvents except ethanol were purchased from either Aldrich or E M Scientific as their highest grade available and used without further purification. Absolute ethanol (anhydrous) was obtained from US1 Chemical Co. and also used as purchased. For the rotational diffusion experiments the sample was flowed through a 1 mm path length flow cell to minimize thermal contributions to the signal and to allow for temperature control. Sample temperature was maintained at 27.0 f 0.1 “C with a Neslab Model EX100-DD temperature bath. All sample solutions were measured by absorption spectroscopy to be in the concentration range of 19-22 PM.
d . Computation. Semiempirical MNDO calculations were performed for both the ground state and first excited singlet state of oxazine 725 using the AMPAC (QCPE-506) software package. All calculations were performed on a Digital Equipment Corp. (DEC) Vaxstation 2000 computer system. e. Viscosity Measurements. The viscosity of each solvent used here was measured at 27.0 f 0.1 “C by using a falling ball type viscometer immersed in a Tamson Model 45 temperature bath.
Results and Discussion Picosecond pump-probe spectroscopy was used to measure the rotational diffusion of oxazine 725 in this work. The experimental functions Zil(t)and I L ( t ) were obtained independently of one another. The induced orientational anisotropy function, R(t),from which orientational relaxation times were calculated, was generated according to
Both ground-state and excited-state rotational diffusion times of oxazine 725 were measured in each of the primary n-alcohols methanol through I-decanol as well as for dimethyl sulfoxide (DMSO). These results are presented in Table I along with the solvent viscosity. Ail of the experimental decay times reported here represent the best-fit value of a regression of the data [In
v
b
O
’
L-
0 1
I
2
3
4
5
6 7
8
u
9 101112
Solvent Viscosity ( c p ) Figure 2. Ground and excited state reorientation times of oxazine 725 in the primary n-alcohols methanol through I-decanol plotted vs solvent viscosity. T indicates ground state and T* indicates excited state. The solid line, labeled DSE, is calculated from eq 2 for comparison to the experimental data.
R ( t ) vs t ] . For all measurements no nonexponential or multiple-exponential behavior could be resolved. The reported zerptime anisotropy values are from the regression of the data for delay times >30 ps.I8 The uncertainties reported for each quantity are 95% confidence intervals for six or more individual determinations. The data demonstrate clearly that there is a state dependence in the rotational diffusion times for each of the alcohols, the excited probe molecule always reorienting more slowly than the ground-state probe. These data are presented in Figure 2 and show that the difference between ground- and excited-state rotational diffusion times increases with increasing solvent viscosity. In an earlier report,15 oxazine 118 in the series of butanols was also shown to exhibit state-dependent rotational diffusion behavior. This was interpreted in terms of a change in the Lewis base character of the probe molecule ring-bound nitrogen causing a change in its hydrogen-bonding characteristics. In a polar aprotic solvent, 2-butanone, oxazine 118 did not exhibit the state dependence. The state-dependent change in the Lewis base character of oxazine 11 8’s ring-bound nitrogen was indicated by MNDO calculations of the electron densities for both the ground state (So) and first excited singlet state (SI). While MNDO calculations are recognized to be semiquantitative at best, especially for excited states and large molecules, the trends which they predict are accepted as being ~ o r r e c t .MNDO ~ calculations were performed for both the So and SI states of oxazine 725 in order to determine whether or not the same site-specific change in electron density was indicated. The results of these calculations are presented in Figure 3a for the ground state and Figure 3b for the excited state. They are quite similar to those for oxazine 118, which is not
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6305
Reorientation Dynamics of Oxazine 725 in Alcohols
a
300
250 200
1
I
t.141
+.om
b
c
0
m., . . 1
,,I
.
,
I . .
,
-.
.. . .. . I
,,
..
,
. , , .. . . , . . . a
I
6 7 8 9 1 0 1 1 1 2 Solvent Viscosity (cp) Figure 4. The difference between the experimental ground and excitedstate reorientation times, AT ( = T * - T ) , plotted versus solvent viscosity. The slope of the best fit line through the data is 17.5 ps/cP. 0
+.l41
1
2
3
4
5
+.ooz
Figure 3. (a) Electron densities for the optimized ground state of oxazine 725 determined by MNDO calculations. (b) Electron densities for the optimized excited state of oxazine 725, calculated in the same manner. The electron densities are reported as decimal fractions of a unit electron charge. The proton charges have not been shown. The orientations of the end ethyl groups approximate those obtained from the calculations.
surprising because of the structural similarity of oxazine 725 and oxazine 1 18. Thus the state-dependent reorientation behavior exhibited by oxazine 725 in primary n-alcohols can be related to the site-specific change in Lewis basicity of the probe’s ring-bound nitrogen. As before, in a polar aprotic solvent, state-dependent reorientation behavior should not be observed. Oxazine 725 exhibits no state dependence, within the experimental uncertainty, in the solvent DMSO. It is expected that if the state dependence were owing to an increased excited-state dipole-dipole interaction it would manifest itself in DMSO due to that solvent’s large permanent dipole. The rotational diffusion of many dyes in polar solvents can be approximated reasonably well by using the modified DebyeStokes-Einstein equation.9~’3+’4~20*23
= qVF/kTS
lor
(2)
where rotis the probe molecule orientational relaxation time, q is the solvent bulk viscosity, Vis the probe molecule hydrodynamic volume, calculated by the method of van der Waals increments2] to be 294 A3,S is a shape factor to account for nonspherical shape, calculated from Perrin’s equationsI9 to be 0.759, k is the Boltzmann constant, and T is the temperature. F is a friction coefficient, equal to 1 for the “sticking” boundary condition and less than one for the “slipping” boundary condition, its exact value depending on the shape of the probe molecule.22 It should be noted that the calculation of S requires the assumption of an effective rotor shape. In a previous paper9 oxazine 725 was modeled as a prolate ellipsoid. Based on the data presented here as well as recent work relating the number of decays in R ( t ) and the transition dipole orientation to an effective rotor shape,’* an oblate ellipsoidal model shape was chosen instead. However, since oxazine 725 does not resemble a regular ellipsoidal shape very closely, the assignment of one particular rotor shape over another ~~~
~
~
(21) Edward, J. T. J . Chem. Educ. 1970, 847, 261. (22) Hu, C-M; Zwanzig, R. J . Chem. Phys. 1974, 60, 4354. (23) Debye, P. Polar Molecules, Chemical Catalog Co.: New York, 1929; D 84. (24) Moore, W. J. Physical Chemistry, 3rd ed.;Prentice-Hall: Englewood Cliffs, NJ, 1963; p 724. (25) C R C Handbook of Chemistry and Physics, 64th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1983; pp C691-C695. (26) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; Freeman: San Francisco, 1960; p 213. (27) von Jena, A.; Lessing, H. E. Ber. Bunsen-Ges. Phys. Chem. 1979.83, 121 .-..
(28) Blanchard, G . J.; Wirth, M. J. J . Phys. Chem. 1986, 90, 2521. (29) Ben-Amotz, D.;Harris, C. B. J . Chem. Phys. 1987, 86, 4856.
should not be given too much emphasis. The modified DSE model assumes a hard ellipsoid reorienting in a continuum medium and is likely not physically realistic. It fails to take into account the complex nature of the solvent network surrounding the probe molecule by assuming that the solvent bulk viscosity is the same as the “microscopic viscosity” experienced by the probe. There is no mechanism in this model to account for either multipleexponential decays of R(t)I8or state-dependent reorientation.15 In spite of these limitations, however, it serves as a useful model for approximating rotational diffusion times for a wide variety of solvent-solute systems. While nonpolar probe molecules in solvents of low polarity behave, in the framework of the DSE model, close to the slip polar and/or charged probe molecules in polar solvents are approximated more closely by use of the stick boundary condition. In certain cases, reorientation times exceeding those predicted by the stick limit have been o b s e r ~ e d . ~ - ~ ~Similar ’ ~ ~ ’ ~to~ ~ the’ report for oxazine 118 and resorufin,l5 the experimental reorientation times for oxazine 725 are longer than predicted by the (stick) DSE model. This behavior has been taken in the past to be indicative of strong solvent-solute interaction.’**J4 The magnitude of the reorientation state dependence reported here correlates with the solvent bulk viscosity. This correlation is presented in Figure 4. The simplest interpretation, in terms of the DSE model, is that the probe molecule is increasing in volume on excitation. From the slope of the best-fit regression line for these data, and assuming no change in probe shape, the volume change is calculated to be 55 A3. It should be noted that the calculation of this value is highly dependent on the particular assumptions made. It is interesting to note that this change is very similar, perhaps fortuitously, to the 75-A3 change in polarizability anisotropy of oxazine 725 measured earlier by Waldeck et aL9 Such a change would represent an approximate 20% increase in molecular volume on excitation. If the probe molecule actually increased in volume on excitation, however, it should also be observable in DMSO, which it is not. The observed state dependence is therefore attributable to some change in interaction between the solvent and solute rather than to a change in solute volume. It is appropriate at this point to examine in more detail the nature of the state-dependent reorientation behavior of oxazine 725 in the primary n-alcohols. Steady-state absorption spectroscopy can serve as an indicator of the probe molecule’s local environment. The absorption spectra of the oxazine class of dyes are known to be sensitive to their local solvent environment.28 Cresyl violet, for example, exhibits a blue-shifted absorption spectrum (S, So) in strong hydrogen-bonding solvents such as methanol, which moves to the red in higher primary n-alcohols. The SI So absorption spectra of oxazine 725 in the primary n-alcohols methanol through 1-decanol are presented in Figure 5a. The absorption maximum shifts to the red with increasing alcohol chain length up to C , and remains approximately constant through Clo. The absorption maxima are plotted against primary
-
-
6306 The Journal of Physical Chemistry, Vol, 92, No. 22, 1988
Blanchard
TABLE 11: Microscopic Viscosities As Determined from Reorientation Data, (Aq,,,/q,) Values, E, (=Eria), and AEma 95% confidence
Eb,b
solvent MeOH EtOH I-PrOH 1 -BuOH 1 -PeOH 1 -HxOH 1-HpOH I -0cOH 1-NOOH 1-DeOH DMSO
'Im. cp
'Irn*,cp
1.34 f 0.09 2.08 f 0.09 3.58 f 0.19 5.21 f 0.21 7.14 f 0.41 9.44 f 0.24 12.7 f 0.30 16.5 f 0.36 19.4 f 0.40 22.4 f 0.70 2.84 i 0.21
1.52 f 0.06 2.39 f 0.10 4.25 f 0.24 6.25 f 0.13 8.46 f 0.30 11.3 f 0.26 14.3 f 0.25 18.9 f 0.36 22.1 f 0.37 26.1 f 0.60 2.98 f 0.15
A'Irn/'lb
0.31 f 0.26 0.30 f 0.18 0.37 f 0.24 0.44 f 0.14 0.42 f 0.22 0.44 f 0.12 0.29 f 0.1 1 0.35 f 0.10 0.31 f 0.09 0.35 f 0.13 0.13 f 0.33
kcal/mol
kcal/mol
interval
2.48 3.37 4.30 4.60 4.95 5.36 5.82 6.00 6.30 6.53 3.62
1.78 2.65 3.71 4.11 4.43 4.87 5.08 5.37 5.60 5.90 2.40
+0.36, -1.1 +0.28, -0.54 +0.30, -0.63 +0.17, -0.23 +0.25, -0.44 +0.14, -0.19 +0.19, -0.28 +0.15, -0.20 +0.15, -0.20 +0.19, -0.27 +0.76, -2.7
'See text for discussion of these terms. Solvent abbreviationsare the same as for Table 1. b E bvalues taken or estimated from data in ref 29 and 25
I
...
aI 7 :
P
i
p
7
8
P
i
0.00
L' '
635
'
'
640
'
'
.
'
'
'
" '
'
'
" '
'
'
" '
'
'
"
650 655 660 Wavelength (nm)
645
665
P
A
I 670
-
--
9
lo
Solvent aliphatic chain length 655
( n u m b e r o f carbons)
7
Figure 6. AErncalculated from eq 7 plotted vs solvent alcohol aliphatic chain length. Uncertainties shown for each of the points have been propagated through the calculations from the input quantities determined experimentally.
I I I
i
i 640 D
where Evisis the viscous flow energy, A may be thought of as a collisional frequency factor, R and T have the usual meanings. The microscopic viscosity being probed by the oxazine 725 molecule is the interaction between itself and the solvent. The DSE equation could be modified to account for this = v,V/kTS (4)
'
~
1
2
'
3
'
'
'
"
4 5 6 7 8 Number of Carbons
"
'
9 1 0 1 1 1 2
Figure 5. (a) Absorption profiles for oxazine 725 in the primary nalcohols methanol through 1-decanol. The relative intensity of each profile has been normalized to unity. (b) Position of the absorption maximum of oxazine 725 in the alcohols, presented as a function of
solvent aliphatic chain length. n-alcohol chain length in Figure 5b, indicating a decreasingly polar environment with increasing solvent alkyl character up to C,, In DMSO, the absorption maximum of oxazine 725 is centered at 656 nm, indicating that changes in solvent hydrogen-bonding strength are at least partially responsible for the spectral shifts observed in the alcohols. Thus, oxazine 725 is presented with a less hydrogen-bonding environment in 1-decanol than in methanol. As was mentioned earlier, alcohols are known to form complex, long-lived (on the time scale of the rotational diffusion measurements) hydrogen-bonded network^.'^.'^ Due to the relative lifetimes of these networks, it is possible that the probe is presented with a relatively "constant" environment during its reorientation. If this is the case, any change in the probe on excitation should manifest itself as an approximately constant change in rotational diffusion behavior, once solvent self-interactions are accounted for. It is instructive to consider the nature of the solvent "viscosity" which the probe molecule senses. The measured bulk viscosity of a liquid is a manifestation of the interaction between solvent molecules as they move amongst one another. The energy required for a liquid to "flow" is termed "viscous flow energy", and is related to the measured bulk viscosity by 7 = A exP(Evis/RT)
(3)
where vm is the microscopic viscosity experienced by the probe molecule. A similar treatment can be envisaged for the excited probe molecule assuming that its shape is not state dependent. The experimental observation that R(0) = R*(O)in each solvent supports this assumption (see Table I). The term vm implicitly incorporates whichever boundary condition may be applicable. The microscopic viscosity term can be approximated from the experimental reorientation data for both the ground and excited states and is presented in Table 11. The difference between the microscopic 1and v* values increases with increasing solvent bulk viscosity. Since solvent-solvent as well as solute-solvent interactions are important to a description of rotational diffusion,'* it is necessary to account for solvent-solvent interactions as well. A convenient representation of these interactions is the solvent bulk viscosity. When the difference between the ground- and excited-state microscopic viscosities is corrected for solvent self-interactions in this manner, a relatively constant fractional change is observed. This fractional change is presented in the form of (Avm/qb) in Table 11, where the subscripts m and b indicate microscopic and bulk, respectively. Using eq 3 it can be shown that ( & , , / v b )is related to the difference in solvent-solute interaction energy between the two probe staLes exp(AEm/RT) = (Avrn/vb) exp(Eb/RT) (5) assuming that the microscopic and bulk preexponential factors are equal. This is a reasonable approximation for the solute concentrations used here. The values of Eb (Evisin eq 3) have been determined previously29 or can be estimated from AH, data.24,25E, is presented in Table I 1 for each primary n-alcohol
Reorientation Dynamics of Oxazine 725 in Alcohols along with AE, calculated by using eq 5. There are several noteworthy points regarding these data. A& for each solvent is plotted against alcohol aliphatic chain length in Figure 6 and vary little with alcohol alkyl character. Further, this energy is on the order of 2-6 kcal/mol, comparable to that for an alcoholic hydrogen bond (typically about 3-6 kcal/mo1).26 Due to the relatively small energies involved and the fact that they are microscopic in nature, their accurate determination presents a difficult task. The difference in solvent-solute interaction energies between the ground and excited states of oxazine 725 is similar in each solvent, suggesting that it is some property of the probe molecule rather than of the solvent which is dominant. It is clear, however, from these data and those presented in Figure 5 that solvent properties do to some degree affect the observed state dependence. The results of the M N D O calculations are consistent with the observation that the change in interaction energy is on the order of a hydrogen bond (see Figure 3). The excitation-dependent increase in electron density at the oxazine 725 ring-bound nitrogen site causes a stronger excited-state hydrogen-bonding interaction to occur between the solvent and the solute. The observed change in interaction energy between the ground and excited states is of the correct magnitude and direction to be consistent with the MNDO calculations as well. Thus, on excitation, oxazine 725 interacts more strongly with the solvent surrounding it owing to the increased Lewis base character of its ring bound nitrogen. This stronger interaction is observed as an increase in the energy required for the solvent to “flow” about the probe molecule. From this argument it is expected that if the solvent does not contain an acidic proton with which the probe can interact, state-dependent reorientation would not be observed. This prediction is verified experimentally through the rotational diffusion measurements of oxazine 725 in DMSO. The state-dependent rotational diffusion behavior presented here and in a previous report15 concerns changes in solvation at the ring-bound nitrogen site of oxazine 725 and oxazine 118. While this particular heteroatom site plays an important role in the observed reorientation dynamics of these molecules, there are other sites where solvent interactions might be important as well. In order to understand the solvation behavior at other heteroatom sites within these probe molecules, it is useful to compare their absorption and rotational diffusion characteristics. The electronic structure of these two dyes is quite similar. On that basis it is expected that their absorption spectra will not differ much. Oxazine 118 in 1-butanol exhibits an absorption maximum at 594 nm15 while the absorption spectrum of oxazine 725 in 1-butanol is centered at 647 nm (see Figure 5). As discussed previously, solvent hydrogen-bonding interactions are known to blue shift the absorption spectra of oxazine dye molecules. The position of the
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6307 oxazine 118 absorption maximum can be attributed to strong solvent interactions with the acidic amino protons on the end group nitrogens. For oxazine 725, the substitution of the acidic protons with ethyl groups precludes such end group interactions and causes the absorption spectrum to be red-shifted with respect to oxazine 118. The rotational diffusion characteristics of the two dyes in 1-butanol also provide insight into interactions with their end groups. Oxazine 725 is approximately 1/3 greater in volume than oxazine 118, owing to its additional end ethyl groups. Based on the DSE model, oxazine 118 would be expected to reorient more rapidly than oxazine 725 in 1-butanol. The experimental observation that oxazine 118 reorients more slowly than oxazine 725, by a factor of almost 2 in either state, also indicates strong solvent interaction with the oxazine 118 end amino groups. Despite the importance of these interactions in determining the dynamical behavior of these two dyes, they are not expected to contribute to the state-dependent reorientation characteristics of either dye based on the results of the MNDO calculations.
Conclusion The rotational diffusion behavior of oxazine 725 in the primary n-alcohols methanol through 1-decanol as well as in DMSO was measured for both the ground and first excited singlet states. It was found that the rotational diffusion times exhibited a state dependence in the alcohols but not in DMSO. MNDO calculations show that on excitation the oxazine 725 ring-bound nitrogen becomes a stronger Lewis base, allowing for stronger interactions with solvent acidic protons. The state dependence was observed to correlate with the alcohol bulk viscosity, suggesting that the probe molecule as well as the solvent was responsible for the observed behavior. By use of the rotational diffusion times determined experimentally to infer the “microviscosity” being experienced by the probe molecule, a state-dependent change in the energy of interaction between the solvent and solute was calculated. This energy varied little between alcohols and was on the other of 2-6 kcal/mol, qualitatively consistent with the predictions of the MNDO calculations. More work in this area is clearly needed to understand on a quantitative basis the nature of the solvent cage surrounding the probe molecule and to be able to model it theoretically.
Acknowledgment. The author is indebted to J. P. Heritage and P. Grabbe for their assistance with the MNDO calculations and for their generous donation of computer time. The author is also grateful to D. Ben-Amotz for stimulating discussions of this work, and to C. J. Weschler and B. T. Reagor for valuable critical readings of the manuscript. Registry No. Oxazine 725, 24796-94-9.
