J . Phys. Chem. 1989, 93, 4315-4319 entropy) for the analyzed models may be of the order of the differences between the corresponding internal energies.I9 Consequently, minimization of free energy may affect the relative stabilities of the models. In fact, the MDC model requires an important decrease in configurational entropy with respect to the HDC model, compensating the gain in stabilization associated to the decrease in electrostatic repulsion. A detailed calculation of the free energy for different Si-AI distribution models in phyllosilicates will be the subject of a forthcoming paper.
Concluding Remarks The study of the relative intensities in the 29SiN M R spectra of phyllosilicates has permitted us to deduce some characteristics (19) Herrero, C. P. Orden local de la distribucibn catibnica en redes tetraedricas bidimensionales. AnLlisis de la ordenacibn de Si y AI en filosilicatas 2:l a partir de RMN de 29Sien didos. Doctoral Thesis, Universidad Autbnoma, Madrid, 1986; 149 pp.
4315
of the Si-AI distribution in the tetrahedral sheets. In particular, from this analysis it has been possible to demonstrate that AI dispersion is intermediate between that required by Loewenstein’s rule and the maximum dispersion of charges (MDC principle). This less than maximum dispersion is mainly due to the electrostatic interaction between adjacent layers, which affects the Si-AI distribution in each layer and makes long range ordered models less favorable. As a consequence of these interactions, the most probable Si-AI distribution in phyllosilicates is one in which the tetrahedral charge is homogeneously distributed over the layers. For other aluminosilicates with different tetrahedral dimensionality (Le., pyroxenes, zeolites), the degree of AI dispersion will depend on different structural factors (tetrahedra connectivity, tcpology, size of structural rings, ...). These facts make the generalization of this study to other minerals not evident and other factors not considered here can also influence the 41 distribution in these compounds.
State-Dependent Reorientation Characteristics of Methylene Blue: The Importance of Dipolar Solvent-Solute Interactions G . J. Blanchard Bell Communications Research Inc., 331 Newman Springs Road, Red Bank, New Jersey 07701 (Received: November 7, 1988)
State-dependent rotational diffusion has been observed previously for oxazine dyes in alcohol solvents. In this paper, the rotational diffusion behavior of the monocation methylene blue, a thiazine dye, is reported for severat alcohols, nitriles, and water. In contrast to the oxazines, a state dependence is observed in both protic and aprotic solvents. Ground-state and excited-state MNDO calculations of methylene blue indicate a significant increase in its point-charge dipole moment on excitation, unlike the oxazines. Interpretation of these data in the framework of the Debye-Stokes-Einstein model shows that a solvent attachment scheme provides good agreement with experimental results.
Introduction A significant body of synthetic, physical, and analytical chemistry is performed in the liquid phase. Liquids are a relatively convenient vehicle for experimental purposes, but the choice of a specific solvent can have a profound effect on the chemistry of interest. The fundamental mechanisms of solvent-solute interactions remain, however, understood to only a limited extent. Unlike both gases and (crystalline) solids, theoretical treatments of solvent-solute interactions have proven insufficient. Because of the transient and associative nature of solvent-solute interactions, most theoretical treatments have related liquid-phase solute dynamics to solvent bulk properties. Many rotational diffusion experiments have demonstrated that polar “probe” molecules with similar volumes reorient quite differently in the same solvent. Conversely, the reorientation dynamics of a given probe molecule in different solvents with the same viscosity are usually dissimilar. These generalizations, as well as many experimental studies,’S2 underscore the fact that the Debye-Stokes-Einstein (DSE) model3 and its variants fail to provide a quantitative picture of solventsolute interactions. In order to address the limitations of the DSE model, a theory that accounts for specific interactions at the molecular level will be required. Indeed, the molecular details of solvation are becoming increasingly clear from an experimental standpoint through time-delayed fluorescence Stokes shift ( I ) Eisenthal, K. B.; Drexhage, K . H. J . Chem. Phys. 1969, 51, 5720. (2) Eisenthal, K. B. Ace. Chem. Res. 1975, 8 , 118. (3) Debye, P.Polar Molecules; Chemical Catalog Co.: New York, 1929; p 84.
0022-3654/89/2093-43 I5$01.50/0
(TDFSS)4,Sand rotational diffusion- measurements. These works have shown that simple models based on solvent bulk properties do not do an adequate job of explaining complex solvation processes. While it is obvious that the solvent and solute both contribute to the solvation process, most experimentation has focused on changes that occur with the solute on excitation and how these changes affect the solvent-solute i n t e r a ~ t i o n . ~Using - ~ TDFSS measurements, the Fleming group has demonstrated unambiguously that, following excitation, the solvent molecules surrounding the solute require a finite time to accommodate to this change and that this time varies significantly depending on the particular solvent pr~perties.~*~,~J’ Recent rotational diffusion measurements of oxazine dye molecules in alcohol solvents have shown that changes in r-electron density subsequent to excitation can cause site-specific changes in their hydrogen-bonding characteristic^.^-^ There are many interactions that can be expected to occur between the solvent and solute, including those arising from hydrogen-bonding, dipole-dipole, dipole-induced dipole, and van der Waals forces. The rotational diffusion experiments that (4) Maroncelli, M.; Fleming, G. R. J . Chem. Phys. 1987, 86, 6221. (5) Castner, E. W., Jr.; Maroncelli, M.; Fleming, G.R. J . Chem. Phys. 1987, 86, 1090. (6) Blanchard, G.J.; Cihal, C. A. J . Phys. Chem. 1988, 92, 5950. (7) Blanchard, G.J. J . Phys. Chem. 1988, 92, 6303. ( 8 ) Ben-Amotz. D.;Scott, T. W. J . Chem. Phys. 1987, 87, 3739. (9)Simon, J. D.Acc. Chem. Res. 1988, 21. 128. (10)Nagarajan, V.;Brearly. A. M.; Kang, T.-J.; Barbara, P. F. J . Chem. Phys. 1987, 86, 3183 Cl I989 American Chemical Society
4316
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989
elucidated the state dependence in the oxazine^^,^ focused on the changes in hydrogen-bonding interactions between the probe molecule and long-lived alcohol solvent networks." Other rotational diffusion experiments have demonstrated the importance of electrostatic interactions through dielectric friction m o d e l ~ ' ~ J ~ for polar noncentrosymmetric solutes in associative solvents. For nonpolar molecules in nonpolar liquids,14van der Waals forces can contribute significantly to the observed dynamics. Such systems typically follow simple DSE predictions much more closely than do more polar systems. This report focuses on ground-state (So) and excited-state (Si) rotational diffusion measurements of methylene blue, a monocationic thiazine dye, in methanol (MeOH), ethanol (EtOH), 1-propanol (PrOH), acetonitrile (MeCN), propionitrile (EtCN), butyronitrile (PrCN), and water. Methylene blue was chosen because it is similar to oxazines but is expected to differ in one important way; its excited-state dipole moment is significantly larger than its ground-state dipole moment. This difference allows, in principle, for the observation of solvation behavior attributable to dipolar as well as hydrogen-bonding interactions. A state dependence is observed in all of the alcohols as well as EtCN and PrCN, but not in MeCN or water. An explanation is offered for the character of the state dependence and, in addition, for the cases where it is not observed.
Blanchard
a>n c
y/ C H 3
i
-
*
Results and Discussion State-dependent rotational diffusion was reported recently for oxazine dyes in alcohol^.^^^ Methylene blue was chosen for this work because of its apparent similarity to oxazines. It contains a heterocyclic sulfur in place of oxygen (see Figure 1). Semiempirical MNDO calculations'* have indicated that oxazine 725, ( I I ) Carg, S . K.; Smyth, C. P. J . Phys. Chem. 1965, 69, 1294. ( 1 2 ) Philips, L. A.; Webb, S. P.; Clark, J. H. J . Chem. Phys. 1985, 83, 58 I O . ( 13) Kivelson, D.; Spears, K. G. J . Phys. Chem. 1985, 89, 1999. (14) Ben-Amotz. D.; Drake, J. M . J. Chem. Phys., submitted for publication. ( 1 5 ) Blanchard, G. J . J . Chem. Phys. 1987, 87, 6802. (16) ( a ) Bado, P.; Wilson, S . B.; Wilson, K. R. R e t . Sri. Instrum. 1982, 53, 706. (b) Andor, L.; Lorincz, A.; Siemion, J.; Smith, D. D.; Rice, S . A . Reu. Sci. Instrum. 1984, 55, 64. ( 1 7 ) (a) C R C Handbook of Chemistry and Physics, 64th ed.; Weast, R. C., Ed.: CRC Press: Boca Raton, FL, 1983. (b) Lunge's Handbook of Chemisrry, 13th ed.; McGraw-Hill Book Co.: New York, 1985.
i
F3C
CH3
b
T?-
/LA/*
R\
N/
+
I
I
R
4
Figure 1. (a) Structureof methylene blue. Only one resonance structure is shown. Geometry is consistent with the results of the MNDO calculations. (b) Structure of an oxazine. For oxazine 118 R = H; for oxazine 725, R = C I H S .
so
+,lg
i.19
i.29
\-'34
Experimental Section Laser. The laser system used here has been described fully in an earlier report.15 Briefly, a mode-locked argon ion laser (Spectra-Physics Model 171-09) was operated at 514.5nm (100-ps pulses, I-W average power, 82-MHz repetition rate) and used to pump synchronously two DCM dye lasers (Coherent Model 701-3). The pump laser was operated at 660 nm for all measurements (5-ps pulses, 130-mW average power). The probe laser was operated at 650 nm (5-ps pulses, 110-mW average power) for ground-state recovery measurements and at 700 nm (5-ps pulses, 80" average power) for excited-state stimulated gain measurements. For all measurements the probe was attenuated to be 1 R of the pump intensity. The signals generated with this system were detected by means of a radio and audio frequency triple modulation scheme.I6 The temporal resolution of this laser system is typically IO-ps fwhm. Chemicals. Methylene blue tetrafluoroborate was obtained from the Aldrich Chemical Co. at 99% purity and was used as received. The solvents were obtained from either Aldrich or EM Scientific as their highest grade available. All solvents were used as received except EtCN, which was distilled to remove residual impurities. All solutions were made in the concentration range of 18-22 pM. The sample was flowed to eliminate thermal contributions to the detected signal. The sample temperature was controlled at 27.0 0.1 OC. Solvent viscosity values were either taken from the literature directly7 or calculated from reported temperature-dependent measurement^.^^
+
-.35/
-,12 -.16
3
-.13 -.16
1
n.yJ/18 I
I
+.42
+.l9
i.19
s,
p"2D
b
+.la
\-,30
-,27
C.18
I
f.18
-.03
-,28
+.37
+.le -.30 -33
\
p-180
C.10
Figure 2. (a) Electron densities for the optimized ground state of methylene blue determined by MNDO calculations. (b) Electron densities for the optimized excited state of methylene blue, calculated in the same manner. The electron densities are reported as decimal fractions of a unit electron charge. The calculated point-charge dipole has been shown for both states.
oxazine 118, and resorufin experience an accumulation of aelectron density at the heterocyclic nitrogen on excitati~n.~.~ While such calculations should not be taken as quantitative, the trends that they predict are recognized as being correct. Recent MNDO results show that, on excitation, a-electron density is expected to increase at the heterocyclic nitrogen for many oxazines and phenoxazines, but not for phenoxaz~nes.'~MNDO calculations of both the ground state (So) and the excited state (Si) of methylene blue (see Figure 2) show that this same site-specific state-dependent change in a-electron density is predicted. These results are not surprising owing to the structural similarity of oxazines and thiazines. There are, however, differences between the two classes of molecules. One difference is that the thiazine center ring structure is distorted somewhat due to the size of the sulfur. It is interesting to note that, despite this distortion, the absorption spectra of thiazines and oxazines are quite similar. Another difference between thiazines and oxazines is that the thiazine heterocyclic sulfur carries a partial positive charge, while the oxazine heterocyclic oxygen has a slightly negative charge. Thus, on excitation, when the ring-bound nitrogen accumulates electron density, there is no significant change in the magnitude of the oxazine point-charge dipole moment, but the direction does change by 1 80°. For thiazines, however, the magnitude of the point-charge dipole moment is expected to increase substantially on excitation, with no change in direction. the point-charge dipole moment for methylene blue in the So state is calculated to be 12
-
( 1 8) Program QCPE-506, The Quantum Chemistry Program Exchange, Indiana University, Department of Chemistry, Bloomington, IN. (19) Blanchard, G. J., manuscript in preparation.
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4317
State-Dependent Rotational Diffusion of Methylene Blue
TABLE I: Ground-State and Excited-State Reorientation Times of Methylene Blue' solvent 7, cp NO) 7 , ps 0.35 f 0.02 45 f 5 0.38 f 0.03 MeOH 0.576 0.34 f 0.01 80f 1 0.34 f 0.01 EtOH I .032 0.33 f 0.01 152 f 3 0.33 f 0.01 PrOH 1.796 0.35 f 0.01 38 f 2 0.337 0.36 f 0.02 MeCN 0.34 f 0.01 48 f 2 0.36 f 0.01 EtCN 0.406 0.33 f 0.02 54f 1 0.37 f 0.01 PrCN 0.559 96 f 9 0.36 f 0.02 0.36 f 0.01 water 0.851 ~
+*,
ps
64 f 3 100 f 6 199 f 10 39 f 2 58 f 2 75 f 4 97 f 5
AT/+
0.42 0.25 0.31 0.03 0.21 0.39 0.01
f 0.18 f 0.09 f
0.09
f 0.10 f 0.08
f 0.09 f 0.15
Experimental values of reorientation times and zero-time anisotropies. Uncertainties in reported values are 95% confidence intervals for a minimum of six individual determinations. Viscosity values were taken or calculated from data in ref 17 and 18. Solvent abbreviations are as follows: MeOH, methanol; EtOH, ethanol; PrOH, 1-propanol; MeCN, acetonitrile; EtCN, propionitrile; PrCN, butyronitrile.
-
D, and for the SI state, 18 D. Due to the intrinsic limitations of M N D O calculations, these values of p and p* should be regarded as indicative of a substantial increase in 1.1 on excitation but should not be construed as quantitative. The importance of these results lies in their indication that the dipole moment is different in the two states (So and S I ) . On this basis, a state dependence in dipolar solventsolute interactions can be expected in addition to the previously reported state dependence in hydrogen-bonding interactions. If dipolar solvent-solute interactions play an important role in rotational diffususion dynamics, they should be observable in polar aprotic solvents, where hydrogenbonding interactions cannot occur. Picosecond-resolved pump-probe spectroscopy was used to measure the reorientation dynamics of methylene blue in a variety of protic and aprotic solvents. The induced orientational anisotropy function, R ( t ) ,was acquired and analyzed in the same manner as reported earlier.6*7,15For all determinations of R ( t ) no nonexponential or multiple-exponential behavior was observed; all data were fit best by a single exponential. The time constants of these decays, r (So) and r* (SI),are presented for each solvent in Table I, along with the zero-time anisotropy values, R(0) and R*(O). These data show clearly that there is a state dependence in all of the alcohols as well as in EtCN and PrCN. In each of these solvents, excited methylene blue reorients more slowly than the ground-state species. No state dependence is resolvable in MeCN or water. The rotational diffusion state dependence measured here for methylene blue differs from that seen for the oxazines. For oxazine 1 18 and oxazine 725, state-dependent rotational diffusion was observed only in protic solvent^.^^^ The data presented here show that methylene blue exhibits state-dependent reorientation characteristics in polar aprotic solvents as well. These results demonstrate that dipolar solventsolute interactions do contribute to the reorientation dynamics of methylene blue. Understanding the nature of these data offers insight into the specific types of interactions between methylene blue and its surrounding solvent cage. There are several possible explanations for state-dependent reorientation. One is that the probe molecule changes shape on excitation. Such a change would likely affect the spectroscopy of the probe by changing its measured transition polarization. The experimental observation that R(0) = R*(O) in all cases suggests that the molecular shape does not change significantly on excitation. This is supported by the MNDO calculations. Another explanation for the state dependence is that the strength of solvent-solute interaction increases on excitation. This increase would manifest itself as a longer lived interaction.20 The reorienting species would thereby appear to be larger and, consequently, its rotational diffusion time would be longer. This can be viewed, in the context of the DSE model,3 ror = T V F / k T S
(1)
as either an increase in the viscosity, 7,experienced by the solute7 or an increase in the volume, V, of the reorienting species. The terms F and S in eq 1 are modifications to the original model. F is a friction coefficient2I to account for solvent-solute boundary (20) Fleming, G. R.; Knight, A. E. W.; Morris, J. M.; Robbins, R. J.; Robinson, G.W. Chem. Phys. Lerr. 1977, 51, 399.
TABLE II: Calculated Dielectric Friction Contributions to State-Dependent Reorientation' SO~Vent Cb T ~ l PS b +df, PS +*df. PS ATdp, pS 10.4 24.5 14.1 33.7 56 MeOH 83.6 188.0 104.4 26.0 337 EtOH 21.1 430 129.0 290.0 161.0 PrOH 0.75 1.70 0.95 38.0 4.3 MeCN 1.07 28.6 4.7 2.40 1.33 EtCN PrCN 22.3 6.2 1.77 3.98 2.21 0.61 1.37 0.76 80.1 7.2c water a For this calculation, p = 12 D, p* = 18 D, and a = 5 A. values taken from ref 9. 'Taken from ref 29.
and r~
conditions, and S is a shape factor22to account for nonspherical solute shape. There are several well-recognized corrections to the DSE equation that treat various aspects of the solventsolute interaction. One way in which solvation interactions can be described is by the dielectric friction model. This model was developed originally by Nee and Z ~ a n z i gand ~ ~incorporated into descriptions of rotational diffusion by several other^.^^.^^ Dielectric friction can be thought of as the torque imposed on a solute as it rotates. This torque is the result of the polarization of the surrounding environment which was itself induced by the presence of the solute. Its effect is additive to that of the viscous resistance to solute motion and is incorporated into a description of rotational diffusion asI2 where Tdf
= p2(t - 1)rD/kTa3(2t
+
(3)
rdf is the term due to dielectric friction, where p is the solute dipole moment, t is the solvent dielectric constant, a is the dipole cavity radius, and r Dis the Debye dielectric relaxation time. Because methylene blue's ground-state dipole moment is different than that for its excited state and Tdf depends on the solute dipole, it is of interest to determine whether or not state-dependent changes in dielectric friction could explain the experimental data. This calculation is reported in terms of AT&(=r*&- rdf). The literature values of t and rDare presented in Table II?9I7 Using these values, the (MNDO) calculated values of p and p*, and a = 5 & I 2 the corrections to eq 1 were calculated for each solvent. These values are also reported in Table 11. The calculations for the nitriles predict a too-small state dependence while that predicted for the alcohols is too great. Previous studies have demonstrated that dielectric friction can contribute significantly to the reorientation dynamics of molecules having a nonsymmetric charge distribution.12 Thiazines, however, possess a C2symmetry axis coincident with the N-S axis, about which the charge is distributed symmetrically. Dielectric friction is not expected to contribute significantly to the reorientation characteristics of such molecule^.'^ From these calculations it is obvious that dielectric friction alone
( 21) Hu, C-M.; Zwanzig, R. J . Chem. Phys. 1974, 60, 4354. (22) Perrin, F. J . Phys. Radium 1934, 5, 497. (23) Nee, T.; Zwanzig, R. J . Chem. Phys. 1970, 52, 6353. (24) Hubbard, J. B.; Wolynes, P. G. J . Chem. Phys. 1978, 69, 998. (25) Madden, P.; Kivelson, D. J . Phys. Chem. 1982, 86, 4244.
4318
Blanchard
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989
TABLE 111: DSE Calculation Results Usine V As Given in Ea 4‘ solvent v ~ , , , A3 ~ ~ ~ , , 7 , ps r*, ps ATIT MeOH 36 40 47 0.18 53 71 89 0.25 EtOH PrOH 70 I24 165 0.33 MeCN 47 23 28 0.22 64 28 36 0.29 EtCN PrC\ 81 39 53 0.36 21 69 76 0.10 wuter ~~
~~~
“ F o r these calculations, F = I, S = 0.75 (oblate), and V,,
~
~~
200
-
150
-
i
.
h
a v
D
bb l o o
f
= 215
A’.
0
1 .o
0.5
cannot account for the differences between T and T*. Another explanation for the experimental data is clearly called for. The association of polar solvents with the heteroatom sites of polar solutes is well recognized. Spears and Steinmetz have shown that the end groups of resorufin can be highly associated with alcohols,26and von Jena and Lessing have reported similar results for cresyl violet in alcohols.27 Recent reports by Blanchard and Ciha16 and Blanchard’ have also shown that solvent-solute association can play a central role in state-dependent rotational diffusion behavior. On the basis of this evidence, it is worthwhile to consider whether or not a similar solvent-solute association can explain the experimental results for methylene blue. Perhaps the simplest way to model this is by using a solute volume in the DSE equation (eq I ) which for ground-state methylene blue is equal to its van der Waals (vdW) volume2*and for excited methylene blue is its van der Waals volume plus the (vdW) volume of a solvent molecule: VDSE = VMB
(4a)
1.5
2.0
solvent viscosity (cP)
loo
b. i
0.5
0.4
0.3
0.6
solvent viscosity (cP)
Figure 3. (a) Experimental reorientation times (solid circles with error bars) for both ground-state and excited-state methylene blue in MeOH, EtOH, and PrOH, plotted versus solvent viscosity (see Table I). Reorientation times (open squares) calculated from eq 1 and 4. See text for discussion. (b) Experimental reorientation times (solid circles) and calculated times (open squares) for methylene blue in MeCN, EtCN, and PrCN.
This approach is admittedly an oversimplification of the complex nature of solvation, but it is instructive in obtaining a qualitative indication of state-dependent changes in solvation. Note that this model does not imply any mechanism for the attachment. It is likely, however, that hydrogen-bonding interactions for alcohols and water, and dipolar interactions for nitriles, are largely responsible. The results of these calculations are presented in Table 0.5 I l l , using 7 values from Table I and molecular van der Waals volumcs calculated from ref 28. For the purposes of this calcu\ lation. methylene blue was assumed to be an oblate e l l i p ~ o i d , ~ ~ ~ ~ ’ ~ S = 0.75 and VMe = 215 A3.A sticking boundary condition ( F = I ) was applied. While quantitative agreement with the data I ib not achieved, the qualitative similarities between the data and 0.1 calculation are notable. The experimental data are compared with the calculated values of T and T * in Figure 3a,b. The absolute 0.01 ’ . 1 0.5 1 .o 1.5 2.0 difference between experiment and calculation, greater for the solvent viscosity (cP) nitriles than the alcohols, is likely due to the limitations of the LXE equation as well as uncertainties in estimating the actual molecular volumes. What is significant. though, is that the relatice increase in reorientation time on excitation is modeled quite well by this simple calculation. Comparison of the experimental A T / T 0.4 with the calculated A T / Tshows excellent agreement for EtOH. 0.3 PrOH. EtCN, and P r C N (shown in Figure 4a,b). Thus, by approximating the excited complex as that of the solute with one 2 0.2 solvcnt molecule “attached” and the ground-state species as the 4 solutc only. good agreement with the experimental data is achieved lor thc 1;irgcr solvents. As noted before, this calculation should be vicned a b representing a state-dependent change in the sol0.0 vent solute interaction time rather than discrete solvent attachment versus a “bare“ solute. This simple model does not do a good job. --O.? 0.3 0.4 0.5 0.6 however, of predicting the behavior of methylene blue in MeOH. solvent viscosity (cP) LlcCN. or water. Each of these cases will be discussed separately. For methylene blue in ‘MeOH. the comparatively large exFigure 4. ( a ) Experimental relative state dependence for methylene blue pcrinicntal value of A T / Targues for more than one contribution in the alcohols (solid circles with error bars) and calculated state de-
1
i
’
1
i 2 6 ) Spears. K. G.; Steinnietz. K M. J . Ph),s. Chem. 1985. 89, 3 6 2 3 . 127) von Jena. A , ; Lessing, H. E . C‘hem. Phys. Letr. 1981. 7 X . 1x7. (18) Edward. J . T J. Chetn. Educ. 1970. 4 7 . 261. ( 2 9 ) Hasted. J. B. Ayueous Die/ecrric,s: Chapman a n d Hall: [London: 1977. p 47
’
’
”
’
’
’
1
”
’
’
1
pendence (open squares). (b) Experimental relative state dependence for methylene blue in the nitriles (solid circles) and calculated values (open squares).
to the state dependence. I t is also reasonable to expect interactions similar to those i n EtOH and PrOH. From the M N D O calcu-
State-Dependent Rotational Diffusion of Methylene Blue lations as well as previous one would expect state-dependent changes in the hydrogen-bonding interactions between the solute heterocyclic nitrogen and the solvent. The increase in solute dipole moment on excitation allows also for an increased dipolar interaction. The small size of MeOH leaves space for both interactions to occur, which would produce a larger state dependence than expected from eq 4a and 4b. The greater bulk of the EtOH and PrOH aliphatic tails would preclude both of these interactions from occurring simultaneously, in agreement with the experimental data. For water, the situation should be similar to MeOH. Due to the very small volume of water, however, as well as its highly self-associative nature, state-dependent changes in hydrogenbonding or dipolar interactions would likely manifest themselves much less strongly. No state dependence is observed experimentally for methylene blue in water. MeCN does not contain any protons with which the methylene blue heterocyclic nitrogen can interact. Thus, the only statedependent change in interaction would be via a dipolar mechanism. This applies to EtCN and PrCN as well. MeCN is different from EtCN and PrCN in that it is a relatively rigid, linear molecule. Additionally, its length is approximately the same as the N-S distance in methylene blue. Because of its linear structure, the interaction of MeCN with methylene blue will not leave any solvent alkyl functionality to protrude into the surrounding solvent. The MeCN molecule interacting with the solute should thus present a minimal increase in volume, particularly for the case of rotation about the N-S axis. Thus, a state dependence in MeCN should be smaller than expected from the simple solvent attachment model. This argument does not hold for MeOH owing to its hydrogen-bonding characteristics as well as its ”bent” structure. It is perhaps surprising that the agreement between the simple solvent attachment model and the experimental data is as good as it is. It is possible to force the calculation to be in better empirical agreement with the data, but the corrections involved incorporate further speculation on solvent and solute volumes, shapes, and average interaction lifetimes. Given the limited physical reality represented by the DSE model, further assumptions beyond those already implicit in this calculation are not justified. As noted above, there are significant discrepancies between the model and experiment, but these can all be explained qualitatively in terms of solvent size and shape effects and thus
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4319 determine limiting cases for the empirical validity of this model. The value of this model lies in the simple picture which it presents; the solvent is in more intimate contact with the excited solute than it is with the ground-state species. Since the nitriles contain no acidic protons, dipolar interactions are implicated strongly as the cause of the observed behavior. While there are other potential contributions, such as dipole-induced dipole or van der Waals interactions, the uncertainty in the data and the qualitative nature of the calculations do not permit their elucidation. The model for state-dependent hydrogen-bonding interactions’ predicts behavior similar to that of the solvent attachment model for protic solvents. It is not possible to determine whether changes in hydrogen-bonding or dipolar interactions dominate the methylene blue/alcohol data. It is likely, however, that both contribute to some degree.
Conclusion The reorientation characteristics of the polar cationic thiazine dye methylene blue have been studied in several alcohols, nitriles, and water. For all alcohols as well as EtCN and PrCN a state dependence was observed. Previous reports of state-dependent reorientation have shown that protic solvents interact differently with oxazines depending on the electronic state of the solute.6*’ The probe molecule used here exhibits a significant increase in its point-charge dipole moment on excitation, in contrast to the oxazines. The observation of state-dependent reorientation of methylene blue in aprotic solvents underscores the role that dipolar solvent-solute interactions play. The difference in interaction between ground-state and excited methylene blue can be modeled qualitatively by invoking a “solvent attachment” mechanism. Actual attachment is probably not achieved, but rather, a longer solventsolute interaction time exists for the excited-state species. Further work in this area is clearly indicated to develop a quantitative understanding of this phenomenon. Acknowledgment. The author is grateful to Dr. J. P. Heritage, P. Grabbe, and S . Johnson for their assistance with the M N D O calculations and generous donation of computer time. The gift of purified methylene blue tetrafluoroborate by Dr. T. Wickersham of Aldrich Chemical Co. is greatly appreciated. Registry No. MeOH, 67-56-1; EtOH, 64-17-5; PrOH, 71-23-8; MeCN, 75-05-8; EtCN, 107-12-0; PrCN, 109-74-0; methylene blue, 61-73-4.
