J . Phys. Chem. 1986, 90, 5535-5539 TABLE I: Experimental and LSDA Values of Spectroscopic Constants for Cr? potential De, eV Re, au w , cm-' experiment ( I Z B t ) O 2.0 f 0.3 3.12 470b 'Et (C-") VWNc 2.4 3.27 525 ' E t (D..,,) VWNC 2.3 3.05 886 IZ' (CJ VWN" 2.6 3.12 44 1 '2' (CJ vBHe 2.8 3.12 442 Izt (c..~V B H ~ 1.8 3.21 450 'Reference 45 (see also ref 50 for a refinement to De). bEstimate from ref 44. eThis work with VWN functional (ref 24). dReference 44 with VWN functional. 'Reference 44 with von Barth-Hedin functional (ref 48). /Reference 41 with von Barth-Hedin functional (ref 48). As shown in Figure 7, the X, model fails dramatically for the F state of Cr2 This can be understood in light of the difficulties in the X, model of describing exchange-correlation consistently in a ~eries.~JThe representation of correlation as a simple scaling of the exchange overstabilizes the spin energy at the expense of overlap bonding energy. Thus, as the bond length is reduced the energy curve turns up too rapidly-a long and weak bond is predicted by the X, mode144!49for the lZ+state. These results further substantiate our recommended use of the more refined functionals (such as the VWN) which are based on derivations for exchange and correlation in the spin-polarized electron gas. Recently, Goodgame and GoddardWhave proposed a correction scheme to enhance the correlation contribution for the ionic wave function components in the generalized valence-bond method with the result that Cr2 is predicted to have a reduced bond length, in agreement with experiment. The calculation also predicts a I
(48) von Barth, U.; Hedin, L. J . Phys. C 1972, 5 , 1629. (49) Dunlap, B. I.; Phys. Reu. A 1983, 27, 2217. (50) Goodgame, M. M.; Goddard, W. A., 111 Phys. Reu. Lett. 1985, 54,
661.
5535
double-well potential curve. These results have been critically assessed by del le^,^' who deduces that the ground-state vibrational frequency corresponding to the results of ref 48 exceeds experiment by about 70%. Apparently, the theoretical description of this molecule is far from settled. V. Conclusion The local spin density theory is the most extensively utilized approach to understanding the electronic structure of solids, and the prospects are excellent that the theory will be further refined to bring it closer to the complete realization of density functional theory. Already, numerous extensions have to go beyond the local approximation, and techniques are being improved to further exploit the advantages of the inherent simplicity of the theory to construct more efficient calculational procedures. At present, the LSDA is not the preferred approach to the majority of properties of interest in small molecules-density functional theory itself is a ground-state theory and many problems need to be resolved for its use in a predictive capacity for ground-state properties. But the advantages of the conceptual simplicity of the DFT as a ground-state theory supersede questions of accuracy in isolated cases-a broadened understanding of a class of phenomena appears generally more valuable than the precise characterization of specific cases.
Acknowledgment. The author thanks Dr. F. W. Averill for his many contributions to the development of the computational techniques used in this work and his active collaboration on several stages of the work reported here. This research was sponsored by the Division of Materials Sciences, U S . Department of Energy under Contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. Registry No. Bez, 14452-60-9;Cr2, 12184-82-6. (51) Delley, B. Phys. Rev. Lett. 1985, 55, 2090. (52) Langreth, D. C.; Mehl, M. J. Phys. Reu. B 1983, 28, 1809.
On the Relatlve Contributions of Nonspecific and Specific Interactions to the Unusual Solvatochromism of a Typical Merocyanine Dye
P. Jacques Laboratoire de Photochimie Generale, UA CNRS no. 431, Ecole Nationale Superieure de Chimie de Mulhouse, 68200 Mulhouse, France (Received: July 2, 1985; In Final Form: April 15, 1986)
The wavelength A,, of a typical merocyanine dye, 4'-hydroxy- 1-methylstilbazolium betaine (M), is determined for a set of solvents covering the entire scale of solvent polarity. The unusual solvatochromism predicted by semiempirical MO methods-a bathcchromic and then an hypsochromic shift as the polarity increases-is for the first time confirmed by experimental evidence. Moreover, the solvatochromic comparison method ( T * , CY,0)recently developed by Taft and co-workers accounts very well for both shifts. Among other things, it is deduced that a = (6?/6a),* s = (6?/6n*),.Thus, depending on the nature of the solvent (i) dipole4ipole interactions and/or (ii) hydrogen bonds can contribute to the same extent to the hypsochromic shift. The delicate question of the geometry variation in this kind of compound is tentatively answered.
Introduction
since the classical work by Brmker,12 a large number of papers have been devoted to the kaleidoscopic solvatochromism of merocyanine dye^.^-'^ Most especially the stilbazolium betaine (1) Brooker, L. G. S.; Keyes, C. H.; Sprague, R. H.; Van Dyke, R. H.; Van Lare, E.; Van Zandt, G.; White, F. L.; Cressman, H. W. .I. Dent, ; S. G. J . Am. Chem. SOC.1951, 73, 5332. (2) Brooker, L. G. S.; Keyes, G. H.; Heseltlne, D. W. J . A m . Chem. SOC. 1951, 73, 5350.
(M) attracted attention because of its following unusual properties: (i) it exhibits one of the largest known solvatochromic effects, A, (3) Hiinig, S.; Rosenthal, 0. Justus Liebigs Ann. Chem. 1955, 592, 161. (4) McRae, E. G. Spectrochim. Acta 1958, 12, 192. (5) Tak, B. K.; Saxena, J. P. J . Ind. Chem. SOC.1970, 47, 791. (6) Benson, H. G.; Murrell, J. N. J. Chem. Soc., Furaday Trans. 2 1972, 68, 137. (7) Dahne, S.;Schob, F.; Nolte, K . D. 2. Chem. (Leipzig) 1973, 13, 471. (8) Gibson, H. W.; Bailey, F. C. Can. J . Chem. 1975, 53, 2162. (9) Gaines, G. L. Anal. Chem. 1976, 48, 450.
0022-36S4/86/2090-5535$01.50/00 1986 American Chemical Society
5536
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
= 620 nm in CHCI3 and 442 nm in H 2 0 ; (ii) it possesses the highest hyperpolarizability presently known” and the resulting nonlinear properties are technically important;20(iii) it contains only nitrogen and oxygen as heteroatoms and a moderate total number of atoms that allow semiempirical calculations at the CNDO level. The experimental solvatochromism of M has been characterized until now by strong negative solvatochromy with a concomittant reduction of the extinction coefficient em as solvent polarity increases. Both changes are attributed to the stabilization of the polar form M, (each benzene ring acquires an extra double bond) in polar solvents, compared to the quinonoid34 form M,, which exists in nonpolar solvents
Jacques
t
Y
18
I
-
CH,OH
i-
-
50
-%CH,CN
__
50
1M
d
Figure 1. Complex solvatochromism of M: different variations oft,,, MP
This explanation was confirmed by SCF 7~ electron calculations7 and recently by more elaborated calculations using the CNDO/S frame.21 These two studies considered explicitly the solvation energy in different manners, but both approaches employed a geometry optimization. Both lead to the same conclusions; Le., M undergoes a dramatic change in geometry as expected from M, when solvent perturbation increases. This the evolution M, theoretical information is not trivial in the sense that it implies that a “normal” solvent is able to induce such a transformation of M represented by the above-mentioned scheme. This view is at variance with all the models of solvatochromism based on the Onsager theory of dielectric^.^^ Whatever the level of sophis-
-
(10) Nolte, K. D.; Dahne, S. Ado. Mol. Relax. Processes 1977, 10, 299. (1 1) Minch, M. J.; Shah, S. S. J . Chem. Educ. 1977, 54, 709; J . Org. Chem. 1979, 44, 3252. (12) AI-Hassan, K. A,; El-Bayoumi, M. A. Cbem. Phys. Lett. 1980, 76, 121. (13) Donchi, K. F.; Robert, G. P. Ternai, B.; Derrick, P. J. Ausr. J . Chem. 1980, 33, 2199. (14) Pawelka, 2.;Sobczyk, L. J . Chem. SOC.,Faraday Trans. 1 1980, 76, 43. (15) Le Beuze, A,; Botrel, A.; Samat, A,; Appriou, P.; Guglielmetti, R. J . Chim. Phys. 1978, 75, 255, 267. (16) Simpson, W. T. J . A m . Chem. SOC.1951, 73, 5359. (17) Abdel-Mottaleb, M. S. A. 2.Naturforsch. A: Phys., Phys. Chem., Kosmosphys. 1982, 37, 1353. (18) Gruda, I.; Bolduc, F. J . Org. Cbem. 1984, 49, 3300. (19) Levine, B. F.; Bethea, C. G.; Wasserman, E.; Leenders, L. J . Chem. Phys. 1978, 68, 5042. (20) Williams, D. J. Nonlinear Optical Properties of Organic and Polymeric Materials; American Chemical Society: Washington, DC, 1983; ACS Symp. Ser. No. 233. (21) Botrel, A.; Le Beuze, A.; Jacques, P.; Strub, H. J. Chem. SOC., Faraday Trans 2 1984, 80, 1235. (22) Varma, C.; Groenen, E. Recl. Trau. Chim. Pays-Bas 1972, 91, 296. (23) Nitsche, K. S.; Suppan, P. Chimia 1982, 36, 346. (24) Reichardt, C . Solvents Effects in Organic Chemistry; Verlag Chemie: Weinheim, 1979. (25) Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W. Prog. Phys. Org. Cbem. 1981, 13, 485. (26) Kamlet, M. J. Chem. Eng. News 1985, 63(11), 20. (27) Reichardt, C.; Harbusch-Goernert, E.; Liebigs Ann. Cbem. 1983, 5, 721. (28) Kamlet, M. J.; Abboud, J. L.; Abraham, M. H.; Taft, R. W. J . Org. Chem. 1983, 48, 2877. (29) Taft, R. W.; Abboud, J. L. M.; Kamlet, M. J. J . Am. Chem. SOC. 1981, 103, 1080. ( 3 0 ) McRae, E. G. Spectrochim. Acta 1958, 12, 192.
depending on the mixture composition.
tication of these models, the solute is treated as a point dipole whose moment is assumed to be solvent independent. Moreover, the semiempirical calculations predicted a minimum in the energy of the first absorption band that is typical of some merocyanine dyes. This minimum was explained on the basis of valence-bond theory by Forster as early as 1939 (see ref 24 and 6)
R,N,i-+o
7
7
Jrthocromic
shift
R’
a polyene - Ii ke state
b polymethine-li ke state
C
-
Polyene-li ke state
increasing solvent polarity
According to his calculation, an intermediate meropolymethine b with equal contribution from both mesomeric structures a and c will have the longest wavelength absorption. Therefore, depending on the starting structure in a nonpolar solvent a merocyanine can exhibit a bathochromic or a hypsochromic shift or both shifts. Moreover, the intermediate state b will present the highest extinction coefficient emax. That is the reason the solvatochromic behavior of merocyanines is generally discussed in terms of a plot of ,,e, vs..,,E, In the case of M, low solubility does not permit the determination of emax in nonpolar or moderately polar solvents. Thus, in this paper, we will discuss separately the solvent effects on, , ,E and emax. In fact, a comprehensive study of the dependence of, , ,E on the physicochemical properties of solvents was not reported until now. (Donchi has mentioned the linear dependence oft,,, on ET(30) but for a limited number of s01vents.I~ The originality of this paper is to report reliable, , ,E values for nonpolar solvents. The values were obtained by extrapolating t,,, of mixtures (polar/nonpolar) to pure nonpolar solvent. Once a set of, ,E, values in solvents covering the entire range of polarity was available, our purpose was to test the theoretically predicted inverted solvatochromism at low polarity and moreover the most relevant theories of solvent effects on UV spectra. Experimental Part
M was synthesized according to ref 2. It was carefully dried since it is hygroscopic.” Solvents were of the highest purityJanssen or Aldrich-and care was taken to ensure that these solvents were dry. As M is extremely sensitive to traces of acids,
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5531
Solvatochromism of a Merocyanine Dye
TABLE I: Transition Energies (cm-') of the Main Band Absorption of M and Solvent Polarity Parameters
no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
solvent water trifluoroethanol ethylene glycol
formamide methanol benzyl alcohol 1-propanol 1-butanol
2-propanol nitromethane acetonitrile Me2S0 DMF acetone
nitrobenzene dichloromethane pyridine quinoline THF
dioxane chloroform benzene
carbon tetrachloride ether
triethylamine cyclohexane
cm-' 22 624 22321 21 186 20 243 20 704 19 157 19084 18904 18349 18018 17513 17483 17 182 17094 16639 16556 16 529 16393 16 393" f 40 16 287' f 40 16142" f 100 16 142" f 40 16 181" f 40 16 367" f 40 16 656' f 40 16890" f 80
ir*b
ab
1.09 0.73 0.92 0.97 0.60 0.98 0.52 0.47 0.48 0.85 0.75 1.oo 0.88 0.7 1 1.01 0.82 0.87 0.92 0.58 0.55 0.58 0.59 0.29 0.27 0.14 0
1.17 1.51 0.90 0.71 0.93 0.43 0.78 0.79 0.76 0.22 0.19 0 0 0.08 0 (0.30) 0 0 0 0 (0.44) 0 0 0 0 0
ETC 63.1 59.5 56.3 56.6 55.5 50.8 50.7 50.2 48.6 46.3 46.0 45.0 43.8 42.2 42.0 41.1 40.2 39.4 37.4 36.0 39.1 34.8 33.6 34.6 33.7 33.2
EXCL EXCL EXCL EXCL EXCL EXCL EXCL EXCL EXCL EXCL EXCL
sss sss
EXCL
AROM HA AROM AROM
sss sss
HA AROM HA
sss sss sss
"Extrapolated values (see text). *Reference 28. 'Reference 27. it was essential to maintain the solvent basic. Tetramethylammonium hydroxyde or sodium methoxide seemed to be the most convenient for this purpose. I , , , was measured in different solutions (polar/nonpolar) with the percentage of the polar solvent ranging from 100%to -30% (or less) depending on the solubility. , , ,? in the pure nonpolar solvent was estimated by graphical extrapolation and by fitting the experimental points with a second-order polynomial using regression analysis. In the case of "well-behaved" solvents, the coefficient of correlation was 0.999. Moreover, the reliability of our procedure was tested on mixtures , , , corresponding to the pure solvents can of solvents for which I be measured directly. In the case of H bonding solvents the fitting of the experimental curve involves polynomials of higher order (generally fifth) and the extrapolation to pure solvent is no longer straightforward (see the discussion concerning figure 1). The extrapolated data reported in Table I for the nonpolar solvents 19-25 were validated for each solvent by using two different non-hydroxylic cosolvents: acetonitrile and acetone. Both values were in agreement and the estimated uncertainty is mentioned for each case. However, cyclohexane is not miscible with pure acetonitrile. Thus we used as cosolvent mixtures of acetone and D M F and acetone and acetonitrile in variable proportions. This explains the larger uncertainty found in this case compared to that for other solvents. Experimental Results
A cursory glance at Table I reveals the inverted solvatochromism of M at low polarity. Moreover, the intricate solvatochromism of M is well illustrated in Figure 1, where we report the four cases of variation oft, with solvent mixture encountered. It should be borne in mind that the local polarity of a solvent mixture near a polar solute-as is the case of M-can be very different from the bulk polarity of the mixture.23 Therefore, it seems useful to give here structural interpretations for the different plots shown in Figure 1, although clearly more than one suggestion can be proposed for some cases. Case a: CH3CN/CH30H. The addition of small amounts of CH30Hin CH3CN results in a dramatic blue shift, and the curve is strongly convex. It is probable that the C H 3 0 H molecules aggregate near the C=O bond in the solute. This explains the with only a few percent alcohol. At -50% steep variation of Imax methanol composition, the hydrogen bond acceptor availability of M is fully satisfied and a further increase of the methanol concentration does not induce a noticeable change of I,,,.
Case b: THF/CH3CN. There is no specific interaction between , , , is regular, the solute and the two solvents. The variation of I the plot being quasi-linear. Nevertheless, this is fortuitous as the following case will show. Case c: N(C2Hs)3/(Cff3),CO.The variation of amaxis closely fitted by a second-order polynomial in the range 40%-100% acetone. That the curve is now concave arises from the fact that triethylamine lies on the bathochromic branch. The minimum predicted by the extrapolation is in agreement with the theoretical conclusions concerning the polymethine like state b. Case d CHC13/CH3CN. Here the variation of I , , , is more complex: it is strongly convex as in the case of CH3CN/CH30H mixtures, but it presents a maximum of , I at nearly 30% CHCl, composition. It should be noted that both solvents are similar weak HBD solvents.3s Moreover, it has been shown that the Lewis acidity of binary solvent mixtures measured with the E T parameter can go through a maximum, having the E T value higher than that of either of the pure solvent component^.^^,^^ This can explain , , , observed at 30% CHC1, comthe unexpected maximum of I position: its overall polarity is higher than that of pure acetonitrile. Moreover, M is insoluble in pure chloroform. It is likely that previous values reported in the literature refer to spectroscopic grade chloroform containing small amounts of ethanol. However, , , , in CHC13 as surprisingly low (vide we consider the value of I infra). Quantitative Treatment of the Solvatochromism
The existence of strong H bonds between M and hydroxylic solvents precluded the use of models based on the Onsager theory of dielectrics. Thus we have focused our attention on two possible correlations oft,,,: (i) an empirical parameter of polarity (we chose the E T parameter that is widely used in solvatochromism24; (ii) the most refined model available at the present time, Le., the solvatochromic comparison method (SCM).25 Correlation with the ET Parameter. Figure 2 shows that in the case of solvents for which ET > 39, t,,, is linearly correlated with E T according to the relation I , , =, 280ET 5100 (1)
+
r = 0.989
s, = 290
n = 19
(31)Dimroth, K.;Reichardt, C. Z.Anal. Chem. 1966, 225, 344. (32) Maksimovic, Z.B.; Reichardt, C.; Spiric, A. 2. Anal. Chem. 1974, 270, 100.
5538 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
Jacques
E(”() .lo-’
17 -
;/;;“.
n
16 -
.II
Figure 3. “Two branches” solvatochromism of M as a function of r*.
I \
l-
uta, x~o-3cm-1
Figure 2. t,,, as a function of ET: (0)old values; ( 0 )new values (see text).
This result is expected since it is well-known that ET can account for H-bond interaction^^^ as well as dipolarity effects. For solvents of low polarity (ET < 39), the experimental points deviate from (1) more and more as ET decreases. At first sight, , , , occurs or if the shift becomes it is not clear if a leveling of I bathochromic. However, if we consider the new values of ET proposed recentl~,~’ the scatter of the concerned points is reduced and the trend of a bathochromic shift seems to be less uncertain. Solvatochromic Comparison Method (SCM). Empirical solvent polarity scales, such as ET, proved to be useful in organizing the numerous information on solvent effects. Nevertheless, the linear correlation of a physical chemical property with a single lumped parameter, for instance ET, are less powerful for understanding the complex mechanism involved in the solute/solvent interactions. One of the aims of the S C M is to overcome this disadvantage. Recently one of the authors claimed ”I don’t think there is a property involving a solvent or a solute that we haven’t correlated to our equation”.26 As a consequence, we decided to put this assertion to the test using the complex solvatochromism of M. The methodology of SCM has been developed through a long series of papers since 1975, and a detailed summary and discussion of the current procedures of the authors have been given re~ e n t l y . ~ ~Therefore, ,~* we will only outline here the essential treatment as used here. The property X Y Z under investigation in a set of solvents can be expressed as X Y Z = XYZ,
+ S(T* + d8) + CY + b(3 + ...
(2)
It is therefore a multiparameter regression analysis in which the defined parameters are x * , solvent/solute interactions of the nonspecific kind; a,solvent hydrogen bond donor (HBD) acidities; p , solvent hydrogen bond acceptor (HBA) basicities. The 6 parameter is a “polarizability correction term” equal to 0.0 for nonchlorinated aliphatic solvents, 0.5 for polychlorinated aliphatics, and 1.O for aromatic solvents. Moreover, it is convenient to use the symbolism introduced by the authors:25 SSS, member of select solvent set; EXCL, solvent specifically excluded from select solvent set; AROM, aromatic solvent; HA, haloaliphatic solvent. From the structure of M we can logically expect that only x* and CY need be considered and we will start by considering the SSS solvents only. Inspection of Figure 3 reveals clearly the “two branches” solvatochromism, although a higher number of experimental points would be preferable. Contrary to the correlation with ET, here the bathochromic shift at very low polarity is well accounted for by x* alone (note that CC14 is not a SSS solvent). We must consider separately the set of solvents 1-20 (that is the
Fwre 4. Calculated , I from the multiple-parameterregression (eq 4) as a function of experimental t,,,.
hypsochromic branch). To include EXCL solvents for which CY # 0 a two-parameter analysis is required, corresponding to eq 3. ijcal
= 14295
r = 0.990
+ 3 3 9 3 ~ *+ 3 9 0 0 ~ ~ s, = 270
(3)
n = 16
Thus specific interactions due to hydroxylic solvents are very well accounted for by the SCM treatment. However, before accepting its reliability we have to consider also the AROM and H A solvents by determining the d parameter according to the procedure by the authors (footnote 14 of ref 29). The number of AROM solvents is low, and benzene was excluded because we were uncertain whether it should be included in the “hypsochromic” or in the bathochromic branch. From solvents 15, 17, and 18 the d parameter was evaluated as d = 0.3. The final correlations now become eq 4 and 5. ijcaI
= 14364
r = 0.992
+ 3 3 7 8 i ~ *+ 3 8 7 8 ~ ~ s, = 256
(4)
n = 19
If we add the two H A solvents the correlation becomes poorer Ical =
13 820
r = 0.976
+ 3 9 0 8 ~ *+ 3 8 9 0 ~ ~ s, = 434
(5)
n = 21
The two corresponding points deviate significantly more than the others: (i& - teXp)/bexpreaches 6.5% for the two HA solvents and
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5539
Solvatochromism of a Merocyanine Dye less than 4% for all the other solvents. It is not easy to decide whether these two points have to be excluded for the following reasons. The a values given in ref 28 are less precise than for non-HBD solvents. Moreover, the d6 term in eq 2 is not well-defined in the sense that the fixed values of 6 = 0.5 for H A and 1 for AROM solvents are not necessarily the best ones. A possible alternative consists in adapting a different d value for H A solvents. As mentioned in ref 29, this would detract for the simplicity of the solvatochromic equations. As regards CHC13, it should be remembered that it gives the lowest t,,,. Therefore, one is not sure that this solvent should obey eq 4. (Note that McRae, when studying related merocyanines, was faced with the same problem? he was not clear whether chloroform should be classified as a hydrogen bonding solvent). From Figure Id we have inferred that CHC13was acting as a H-bonding solvent even though the specific interaction with M may be increased by the high dielectric constant of CH3CN. In fact, assuming that CHC13 obeys eq 4 and that the ?r* value is reliable, we deduce a value of a! 0.1, which is quite acceptable. As regards CH2C12,the same reasoning leads 0. It should nevertheless be noted that in this to a value a! solvent we observed a vibrational structure in the absorption band (as in CHC13) that is observed only in nonpolar solvents. Therefore, it may be that the a value of both solvents are slightly too high. It is interesting that the coefficients of T * and a in eq 3 and 4 are equal. We can deduce from this important result that the dramatic hypsochromic shift is due to (i) nonspecific interaction and (ii) hydrogen bonding effects to nearly the same extent. This point underscores the power of the SCM method in resolving more fundamental information about the solute/solvent interactions than methods involving composite empirical parameters. Moreover, it is worth noting that application of eq 4 to hexafluoro-2-propanol (the most hydroxylic solvent reported in ref 28: ?r* = 0.65, a = 1.96) leads to teal = 24 130 cm-’. Although we have measured in this solvent an energy transition of 26 320 cm-I, we cannot ascertain that it corresponds to the nonprotonated M species even though added bases leave the peak unchanged. From the closeness of both values we think that in hexafluoro-2-propanol M is practically completely protonated. Recent MNDOC calc u l a t i o n ~have ~ ~ shown that the protonated compound MH’ matches the benzenoid structures M, as expected from the M O calculations including solvation. The inverted solvatochromism at low polarity can be explained only by the change in the structure of M mentioned in the Introduction. We are somewhat reluctant to draw from the preceding results significant conclusions about the share of the structural change induced by SSS solvents to the hypsochromic shift. Clearly, such a change occurs, but to what extent? We tentatively tried to address this question by studying 6(I3C) shifts. Unfortunately, due to low solute solubility, it was not possible to get decisive information.21 Intensity of the Transition. Benson and Murrel16 reported a large variation of cmax in CHC13/CH30H mixtures, which they
-
-
(33) Tavan, P.; Schulten, K. Chem. Phys. Lerr. 1984, 220, 191. (34) This form is sometimes called the “nonpolar”form. This is somewhat misleading since the dipole moment of this form amounts to nearly 26 D!22 (35) The author is grateful to one of the referees for drawing his attention to this point.
found in agreement with the theoretical suggestions of their calculations. Unfortunately, we have good reasons to think that this agreement was quite fortuitous: Although the decrease of emax they reported for the range 90%-30% CHC13 was confirmed, we were unable to reproduce the sharp increase of emax as the CHCI3 concentration tends to zero (range 30%-0% CHC1, of Figure 4 in ref 6. According to our measurements, emax is still decreasing or constant. Their assumption “there is no significant change in bandwidth over the concentration range 10-100% C H 3 0 H and hence e,,, is an advantage measure of the oscillator strength” is probably 2500 cm-I (CHCI,) and A t l j 2 invalid. Our measured 4000 cm-’ (CH30H) applied to the relation f = 4.39J-e, dv yields a nearly constant value for f. They chose the same solvents (CHCl,/CH,OH) for N M R spectroscopy and intensity studies. But use of hydroxylic solvents is not advised for comparing theoretical predictions with experimental results. Our CNDO/S calculations21indicated only a large decrease off with solvation (without predicting a minimum as did the T S C F method of Benson and Murrell). Therefore, we tried to study emax in mixtures of hydroxylic solvents. Some experimental difficulties make the determination off somewhat uncertain: (i) part of the dye may precipitate as the polarity of the mixtures decreases; (ii) the calculation o f f is not precise when performed by the trapezoidal rule adopting a step of 1 nm. We noted that a change of 0.1 nm in the position of v, results in a 5% variation off. Therefore, we are unable to draw any definitive conclusion regarding the variation of emax andfwith the solvation. Nevertheless preliminary results (for instance in THF/CH3CN mixtures) tend to indicate that the variation off(if any) is limited.
-
-
Conclusion The first experimental observation that M undergoes an inverted solvatochromism at low polarity verifies predicted shifts from semiempirical M O calculations including solvation and geometry optimization. However, the reliability of these calculations seems to be less as the dependence of the oscillator strength on the solvation varies. The solvatochromic comparison method (SCM) developed by Taft and co-workers explains the complex behavior of M. It was possible to disentangle the relative contributions of the nonspecific and specific contributions to the dramatic hypsochromic shift exhibited by M as the polarity of the solvent increases. Contrary to previous reports, both contributions can act to he same extent depending on the nature of the solvent. From these results alone it is not possible to decide to what extent the non-hydroxylic solvents really induce a change in the predicted geometry of M from the MO calculations; however, the inverted solvatochromism that we observed at low polarity implies such a change. Registry No. M, 31054-23-6; DMF, 68-12-2; THF, 109-99-9; CH,CN, 75-05-8; CH,OH, 67-56-1; Me2S0, 67-68-5; N(C2H&, 121-44-8; (CH3)2C=0, 67-64-1; CHC13, 67-66-3; water, 7732-18-5; trifluoroethanol, 75-89-8; ethylene glycol, 107-21-1; formamide, 75-12-7; benzyl alcohol, 100-51-6; 1-propanol, 71-23-8; 1-butanol, 71-36-3; 2-propanol, 67-63-0; nitromethane, 75-52-5; nitrobenzene, 98-95-3; dichloromethane, 75-09-2; pyridine, 110-86-1; quinoline, 91-22-5; dioxane, 123-91-1; cyclohexane, 110-82-7; benzene, 71-43-2; carbon tetrachloride, 56-23-5; ether, 60-29-7.
