J . Phys. Chem. 1988,92, 2945-2956
2945
Solvatochromic Behavior of Intramolecular Charge-Transfer Diphenylpolyenes in Homogeneous Solution and Microheterogeneous Media‘ Dong Myung Shin and David G. Whitten* Department of Chemistry, University of Rochester, Rochester, New York 14627 (Received: November 25, 1987)
A study of the solvatochromic behavior of three charge-transfer polyenes-4-(N,N-dimethylamino)-4’-nitrostilbene (NNS), 1-@-(N,N-dimethylamino)phenyl)-4-@-nitrophenyl)- 1,3-butadiene (NND), and l-@-(N,N-dimethylamino)phenyl)-6-(pnitrophenyl)-l,3,5-hexatriepe(NNT)-is reported. These three donoracceptor substituted dyes show pronounced solvatochromic behavior in homogeneous solution for both absorption and emission phenomena that may be correlated with various solvent polarity parameters such as ET(30),T * , and Py. The Taft-Kamlet treatment can be used to sort out the quantitative role of properties such as hydrogen bonding on specific solute-solvent systems. The same polyenes can be incorporated into microheterogeneous media such as aqueous micelles and phospholipid vesicles; spectroscopic evidence indicates they occupy sites in the latter assemblies that correspond to very high solvent polarities. The striking differences between absorption and fluorescence behavior of these donor-acceptor polyenes in solution and vesicles can be best accounted for in terms of a preferential solubilization in which the chromophores are partially embedded in the bilayer so that the donor (amino) end is in a relatively hydrophobic region while the acceptor (nitro) terminal is in a much more hydrophilic interface site.
Introduction Questions as to the nature and variety of solubilization sites provided by microheterogeneous media such as micelles, vesicles, and microemulsions are of fundamental interest and are central to developing any understanding of the ability of these media to modify or control reactivity.”l2 Numerous investigations have shown that the predominant solubilization sites provided by ionic micelles such as sodium dodecyl sulfate (SDS) or cetyltrimethylammonium chloride (CTAC) for aromatic hydrocarbons, moderately polar organic reagents, and inorganic ions are relatively “wet” or polar sites, perhaps best described as “interfacial”.2-8~’0*’’ Microemulsions that exist as dispersed droplets (reversed micelles and *swollen” micelles) in a continuous phase frequently show evidence of a distribution of sites ranging from nonpolar through interfacial to aqueous, depending both upon the substrate and specific m e d i ~ m . ~One , ~ of the most interesting of these media and yet in some ways least understood as a “solvent” is the bilayer structure formed by two-chain surfactants such as dioctadecyldimethylammonium cations (DODAC) and various ionic or zwitterionic phospholipids. Unlike most micellar solutions or microemulsions, several of the structures generated by suspension of these surfactants in water are thermodynamically unstable even though they may possess moderate to high kinetic (1) Photochemical Reactions in Organized Assemblies (54); part 53: Allen, M. T.; Miola, L.; Shin, D. M.; Suddaby, B. R.; Whitten, D. G. J . Membr. Sei. 1987, 33, 201. (2) Otruba, J. P.; Whitten, D. G. J . A m . Chem. Soe. 1983, 105, 6503. (3) Menger, F. M. Ace. Chem. Res. 1979, 12, 111. (4) Winkle, J. R.; Worsham, P. R.; Schanze, K. S.; Whitten, D. G. J . Am. Chem. SOC.1983, 105, 3951. (5) Schanze, K. S.; Whitten, D. G. J. A m . Chem. SOC.1983,105,6734. (6) Suddaby, B. R.; Brown, P. E.; Russell, J. C.; Whitten, D. G. J . Am. Chem. SOC.1985, 107, 5609. (7) Zachariasse, K. A.; Van Phuc, N.; Kozankiemica, B. J . Phys. Chem. 1981,85, 2676. (8) Mukerjee, P.; Cardinal, J. R.; Desai, N. R. In Micelliration, Solubilization and Microemulsions: Mittal, K. L., Ed.: Plenum: New York. 1977: Vol. I. (9) Russell, J. C.; Whitten, D. G.; Braun, A. M. J . Am. Chem. SOC.1981, 103, 3219. (10) Menger, F. M.; Doll, D. W. J . A m . Chem. SOC.1984, 106, 1109. (11) Menger, F. M.; Chow, J. F. J. A m . Chem. SOC.1983, 105, 5501. (12) Fendler, J. H. Ace. Chem. Res. 1980, 13, 7. 113) Tunuli. M. S.: Fendler. J. H. J. A m . Chem. SOC.1981. 103. 2507. (14) Lee, L. Y. C.;‘Hurst, J: K.; Politi, M.; Kurihara, K.; Fendlei, J. H. J . A m . Chem. SOC.1983, 105, 377. (15) Wong, M.; Thompson, T. E. Biochemistry 1982, 21, 4133. (16) Garber, B. P.; Sheridan, J. P. Biochim. Biophys. Acta 1982,685, 87. (17) Chang, E. L.; Garber, B. P.; Sheridan, J. P. Biophys. J . 1982,39, 197.
0022-3654/88/2092-2945$01.50/0
Thus a solution of bilayers frequently consists of different structures that do not interconvert on the time scale of many experiments. There is considerable evidence that the bilayer and vesicle structures that are formed by these amphiphiles are highly organized compared to micelles and microemulsions and that their structure may not be easily disrupted by relatively small solute molecules. Although a number of studies suggest small, moderately polar organic molecules associate with or are bound to bilayers predominantly at the water-amphiphile interface, other investigations have demonstrated solute properties much more consistent with residence in relatively nonpolar but ordered or “viscous” regions and thus suggest solubilization of the solute within the hydrocarbon interior of the Solubilization and permeation25,26of potentially reactive molecules in these latter sites are of special interest in view of the role of bilayer membranes as tunable barriers in natural biochemical processes and biomimetic systems. Although a number of bilayer structures have been well characterized by different physical techniques, major questions remain concerning the relative thicknesses of the interface and hydrocarbon regions in fluid solutions of bilayers as well as the order and effective water or electrolyte concentration as a function of distance from the edge of the bilayer.27 Several studies directed toward developing an understanding of bilayer solubilization sites have employed probe solutes that can be calibrated in terms of their behavior in homogeneous solution. Previous studies have used a variety of probes ranging from aromatic hydrocarbons such as pyrene and 1,6-diphenylhexatriene to polar organics or functionalized amphiphiles such as paranaric acid or surfactant stilbene^.^*^^-^^ Although there (18) Inoue, S.; Takemoto, H.; Yasunaga, T.; Toyoshima, Y. Mol. Cryst. Liq. Cryst. 1981, 69, 303. (19) Schanze, K. S.; Shin, D. M.; Whitten, D. G. J. Am. Chem. SOC.1985, 107, 507. (20) (a) Allen, M.-T.; Miola, L.; Suddaby, B. R.; Whitten, D. G. Tetrahedron 1987, 43, 1477. (b) Allen, M.-T.; Miola, L.; Shin, D. M.; Suddaby, B. R.; Whitten, D. G. J . Membr. Sei. 1987, 33, 201. (21) Zachariasse, K. A,; Kiihnle, W.; Weller, A. Chem. Phys. Lett. 1980, 73, 6. (22) Shinitzky, M. Isr. J . Chem. 1974, 12, 879. (23) (a) Brown, P. E.; Whitten, D. G. J . Phys. Chem. 1985,89, 1217. (b) Brown, P. E.; Mizutani, T.; Russell, J. C.; Suddaby, B. R.; Whitten, D. G. In Organic Phototransformations in Nonhomogeneous Media; ACS S y m p sium Series 278; Fox, M. A,, Ed.: American Chemical Society: Washington, DC, 1985; Chapter 11. (24) Mizutani, T.; Whitten, D. G. J . Am. Chem. SOC.1985, 107, 3621. (25) Hurst, J. K.; Thompson, D. H. P. J. Membr. Sci. 1986, 28, 3. (26) Thompson, D. H. P.; Hurst, J. K. J . Am. Chem. SOC.1987, in press. (27) Losev, A.; Mauzerall, D. Photochem. Photobiol. 1983, 38, 355.
0 1988 American Chemical Society
2946 The Journal of Physical Chemistry, Vol. 92, No. 10, 1988
Shin and Whitten
TABLE I: Absorption Maxima and Extinction Coefficients of NNB, NNS, NND, and NNT in Homogeneous Solvents‘-b NNB
NNS
h” solventc
Taftd T *
MCH cyclohexane heptane hexadecane CCI, benzene toluene dioxane THF CH,CI, acetonitrile acetone DMSO methanol ethanol 2-propanol 1 -butanol I-hexanol 1 -0ctano1
0 0 -0.08
0.294 0.588 0.535 0.553 0.576 0.802 0.713 0.683 1.000 0.586 0.540 0.505 0.503
nm
NND
Xnlm
kcal
nm
kcal
10-4c‘
356 354
80.3 80.7
3.38 3.55
420 418
68.1 68.4
2.54 3.06
366 386
78.1 74.1
2.48 3.64
426 432
67.1 66.2
2.65 2.81
372 386 394 394 392 408 392 388
76.9 74.1 72.6 72.6 71.5 70.1 72.9 73.7
2.39 3.63 4.73 3.64
430 434 440 432 434 454 428 430
66.5 65.9 65.0 66.2 65.9 62.0 66.8 66.5
2.21 2.91 2.70 2.62 2.58 2.37 2.56 2.51
3.68
3.86 3.48 3.51
NNT
h”
Amax
nm
kcal
426 428 432 438 430 450 448 440 450 454 442 447 466 440 442 442 444 446 446
67.11 66.80 66.18 65.27 66.50 63.50 63.8 65.0 63.5 63.0
64.68 64.0 61.4 65.0 64.70 64.70 64.40 64.10 64.10
10-4cp
3.27 4.26 4.13 3.70 4.43 4.31 3.64 4.19 3.41 3.04 3.55 3.46 3.35 3.83
nm
kcal
446
64.1
446 448
64.1 63.8
462 462 454 460 464 452 454 476 446 450
61.9 61.9 62.9 62.2 61.6 63.3 62.9 60.1 64.1 63.5
lO-,c‘
2.79 4.70 4.70 4.62 4.83 4.38 4.82 4.74 3.89 4.86
“p-(N,N-Dimethylamino)-p’-nitroaniline (NNB); p-(N,N-dimethylamino)-p’nitro-trans-stilbene(NNS); p-(N,N-dimethylamino)-p’-nitro-trans,trans-l,4-diphenyl-l,3-butadiene (NND); p-(N,N-dimethylamino)-p’-nitro-trans,frans,trans1,6-diphenyl-1,3,S-hexatriene ( N N T ) . Probe concenM. CMethylcyclohexane (MCH); tetrahydrofuran (THF); dimethyl sulfoxide (DMSO). dFrom ref 30. e c represents the tration is 1 X extinction coefficient. Some values may be lower than actual due to low solubility.
is agreement from several of the studies that nonpolar probe molecules incorporated in the bilayer interior experience an ordered nonpolar environment while polar or charged molecules or substances mismatched in shape with the hydrocarbon chains of the bilayer are excluded from the interior and reside at the amphiphile-water interface, relatively little information is available concerning the behavior of molecules that might be expected to span the region between the bilayer interior and the amphiphile-water interface and thus report quantitative information about its proper tie^.^^-^^ The use of molecules having intramolecular charge-transfer transitions to quantitatively assess solvent or solution properties has been developed in a number of extensive investigations over the past two decade^.^^-^^ The convenience and high precision of these spectroscopic measurements have led to widespread uses of indexes thus derived such as the Dimroth-Reichardt ET(30),37,38 Taft-Kamlet ~ * , 4 Kosower ~ , ~ ~ Z,44845 and Dong-Winnik Py39to correlate reactivity and/or a variety of physical properties. While a directly measured single-parameter index such as ET(30) is clearly attractive as an overall measure of solvent polarity, the specific sensitivity of the betaine oxide from which the ET(30) (28) Abuin, E.;Lissi, E.; Aravena, D.; Zanocco, A,; Macuer, M., private communication. (29) Kintanar, A.; Kunwar, A. C.; Oldfield, E. Eiochemisfry 1986, 25, 6517. (30) Davenport, L.;Dale, R. E.; Bisby, R. H.; Cundall, R. B. Eiochemisfry 1985, 24, 4097. (31)Chong, P. L.G.; Weber, G.Biochemistry 1983, 22, 5544. (32)Wolber. P. K.: Hudson. B. S. Eiochemistrv 1981. 20. 2800. (33) Kunitake, T.: Shimomura, M. J . A m . C h e k Soc. 1982, 104, 1757. (34)Kunitake, T.; Okahata, Y.; Shimomura, M.; Yasunami, S.; Takarabe, K. J . A m . Chem. SOC.1981, 103,5401. (35)Kunitake, T.; Nakashima, N.; Shimomura, M.; Okahata, Y.; Kano, K.; Ogawa, T. J . A m . Chem. SOC.1980, 102, 6644. (36) Kunitake, T.; Okahata, Y. J . A m . Chem. SOC.1977, 99, 3860. (37)Dimroth, K.; Reichardt, C.; Siepmann, T.; Bohlmann, F. Jusfus Liebigs Ann. Chem. 1963, 661, 1-37. (38)Reichardt, C. Angew. Chem., I n f . Ed. Engl. 1965, 4, 29. (39)Dong, D. C.;Winnik, M. A. Photochem. Photobiol. 1982, 35, 17. (40)Kamlet, M.J.; Abboud, J. L.; Taft, R. W. J . Am. Chem. SOC.1977, 99, 6027. (41)Kamlet, M. J.; Abboud, J. L.;Abraham, M. H.; Taft, R. W. J . Org. Chem. 1983, 48, 2877. (42)Grunwald, E.;Winstein, S. J . A m . Chem. SOC.1948, 70,846. (43)Reichardt, C.Soluent Effects in Organic Chemistry; Verlag Chemie: New York, 1979,and references therein. (44) Kosower, E. M. J . Am. Chem. SOC.1958, 80, 3253 (45)Kosower, E. M Acc. Chem. Res. 1982, 15, 259
scale is developed to hydrogen bond donor solvents makes this index particularly problematic for assessing solvent properties of a medium or mixture of variable protic-aprotic regions.46 Molecules such as pyrene that are less subject to specific solventsolute interactions but nonetheless quite sensitive to solvent polarity (dielectric and polarizability properties) are potentially useful in general, but pyrene’s specific utility for bilayers is somewhat restricted by the likelihood either that the bulky aromatic hydrocarbon cannot be.well solubilized in the bilayer interior or alternatively that it will disrupt the bilayer structure in the region in which it is ~ o l u b i l i z e d . ~Interesting ~ results indicating roles for both of these possibilities with pyrene have been very recently reported by Lissi and co-workers.28 A particularly attractive set of molecules for the investigation of mixed-solvent or solvent-domain properties consists of aromatic or unsaturated structures substituted with diethylamino or alkoxy donors conjugated to a nitro acceptor. Although these molecules generally contain intense charge-transfer (in accord with the Mulliken formulation) transitions that are strongly sensitive to solvent, the donor and acceptor substituents have been suggested to Kamlet, Taft, and others to be of relatively low overall sensitivity to specific solvent acidity or basicity proper tie^.^^ Incorporation of these donors and acceptors into a chromophore or structure compatible with the chains of a bilayer-forming amphiphile should lead to a molecule that can either reside in the interior of a bilayer or span the interior-interface regions and thus be sensitive simultaneously to the properties of both. Recently, we reported a study of the photophysical behavior (NMS) in phospholipid vesicles of truns-4-nitro-4’-methoxystilbene and micelles.lg Studies in homogeneous solution indicated a strong sensitivity of its triplet state to both medium polarity and viscosity. Although the photophysical behavior of N M S in micelles is consistent with solubilization at a single site of moderate polarity and viscosity, a distribution of the N M S between two sites considerably different in polarity and viscosity is indicated for DODAC, phospholipid, or dicetyl phosphate ve~icles.’~ The two sites could most consistently be interpreted as an interfacial solubilization site, characterized by moderate polarity and viscosity, and an interior or interfaceinterior spanning site, marked by relatively lower polarity but high viscosity. Here we report a study of the absorption and fluorescence behavior of a structurally related series (46)Schanze, K. S.;Mattox, T.F.; Whitten, D. G. J . Org. Chem. 1983, 2808. (47)Backer, C. A,; Whitten, D. G. J . Phys. Chem. 1987, 92, 865.
48,
The Journal of Physical Chemistry. Vol. 92, No. 10, 1988 2941
Intramolecular Charge-Transfer Diphenylpolyenes
W
Yc
m 0 CL
vr m
