Langmuir 1993,9, 90-95
90
Phenanthrene and Triphenylene as Fluorescence Probes for Micellar Systems Kenichi Nakashima' and Inaho Tanaka Laboratory of Chemistry, College of Liberal Arts, University of Saga, 1 Hojo, Saga 840, Japan Received June 1,1992. In Find Form:October 5,1992
The solventintensificationeffect on the forbidden SI+Sotransitions of phenanthreneand triphenylene hae been studied. Both compounds show a remarkable enhancement of the origin bands on going from a nonpolar to a polar solvent. The investigation on solute-solvent interactions causing such effecta has revealed that two factors are responsible for the anomalous intensity enhancement of the origin bands bulk solventpolarity and a specificsolute-solventinteraction. The sensitivity of the origin band intensity to solvent polarity, together with the fluorescence lifetime, has been applied to the detection of critical micelle concentration (cmc) of surfactants. The results indicate the potential utility of the probes for characterizing micellar systems.
Introduction
It has been well-known that the intensity of a forbidden electronic transition of many organic molecules strongly depends on solvent properties.'-7 One typical example of this phenomenon is the Ham effect of benzene: the phenomenon that the S1 SOforbidden transition of benzene acquires a remarkable intensity in solution in carbon tetrachloride.8 A t the earliest stages of the study there were a lot of controversiesabout the origin of Ham bm1d.495*&~3 However, an answer was given to the problem by Koyanagi who attributed the Ham band to the effect of dispersion fore perturbation by solvent molecules.4 Another representative example is the solvent intensification effect on the S1 + SOspectra of pyrene, in which the 0-0 band becomes markedly strong as the polarity of solventa increasee.1 In spite of many efforts,1-3J4 however, there has never been a clear-cutexplanationfor the pyrene case. The solute-solvent interactions which contribute to the drastic enhancement of the origin band of pyrene seem to comprise both the formation of complexes with polar s 0 1 v e n t P ~and ~ usual solvent perturbations like dispersion interactions and induction interactions. Despite the difficulty of theoretical explanation of the solvent effect, pyrene has been widely applied as a fluorescence probe to the studies of micelles, vesicles, biologicalmembranes, polyelectrolyte solutionsand other
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(1)Nakajima, A. Bull. Chem. SOC.Jpn. 1971,44,3272. (2)Kalyanaauudaram, K.; Thomas, J. K. J . Am. Chem. SOC.1977,99, 2039. (3)(a)Dong,D. C.;Winnik, M. A. Photochem. Photobiol. 1982,35,17. (b)Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984,62,2580. (4)Koyanagi, M. J . Mol. Spectrosc. 1968,25,273. (5) Durocher, G.; Sandorfy, C. J. Mol. Spectrosc. 1966,20,410. (6)Baylise, N. S.;Wills-Johnson,G. Spectrochim. Acta, Part A 1968, 24,563. (7)(a) Nakaehima, K.; Uchida-Kai, K.; Koyanagi, M.; Kanda, Y. Bull. Chem. Soc. Jpn. 1982,65,416. (b) Nakashima, K.; Koyanagi, M.Bull. Chem. SOC.Jpn. 1982,56,3923. (8)Ham,J. 5.J. Chem. Phys. 196S,21,756. (9)Platt, J. R. J . Mol. Spectrosc. 1962,9,288. (10)Bayliee, N. S.;Hulme, L. A u t . J. Chem. 1963,6,257. (11)Brocklehuret, B.J. Chem. SOC.1953,3318. (12)Leach, S.;Lopez-Delgado,R.; D e b , F. J. Mol. Spectrosc. 1961, 7,304. (13)B a y h , N. S. J . Mol. Spectrosc. 1969,31,406. (14)(a) Nakajima, A. Spectrochim. Acta, Part A 1974,30,860. (b) Nakajima,A. J . MoLSpectrosc. 1976,61,467.(c)Nakz$ma,A. J . Lumin. 1976,II,429. (16)(a) Lianoa,P.;Georghiou,S.Photochem. Photobiol. 1979,30,355. (b) Liana, P.; Georghiou, S. Photochem. Photobiol. 1979,30,843. (16)Nakajima, A. Bull. Chem. SOC.Jpn. 198S,66,929. (17)N h h i m a , K.;Koyanagi, M. Photochem. Photobiol. 1986,44, 169.
0743-7463/93/2409-0$04.00/0
microheterogeneous systems.18 Since the intensity ratio of the 0-0 band to the blgvibronic band at -385 nm (the so-called 11/13ratio2p3) in the fluorescence spectrum of pyrene is quite sensitive to the polarity around the molecule, the ratio can be used as an indicator for the micropolarity of its surroundings. Thus pyrene has been employed extensively to characterize various microheterogeneous systems. Phenanthrene (molecular symmetry C d and triphenylene (Da) belong to the same category as pyrsne in the sense that they are the members of alternant hydrocarbons and their SI* SOtransitions are forbidden. In triphenylene Sl(A1') *So(Al') transitions aresymmetry forbidden because the x , y , and z vectors, respectively, belong to E', E', and A2" representations in the Da point group.lg Although the Sl(A1)*So(A1) transitionsof phenanthrene are symmetry allowed with polarization along the z-axis (in-planeshort a x i ~ ) , 6 the * ~ *observed ~~ oscillator strength for SI SOabsorption is very small (f 0.003).= For understanding the small oscillator strength of phenanthrene we might have to take into accounta confiiationinteraction ~alculation,~~ which clearly explains the forbidden character of the SI + SOtransition of pyrene.24 Such a calculation,however, has not yet been available for phenanthrene. As S1 * SOtransitions of phenanthrene and triphenylene are forbidden, one might expect that the origin band intensities of the transitions would be affected by solvents. If this proved to be the case, it would provide the experimentalist with two additional probes of micropolarity in phase-separated systems. In this study we observed SI SOabsorption and SI SOfluorescence spectra of phenanthrene and triphenylene in a wide variety of solventa. By inspectingthe correlation of the intensity of the 0-0 band with severaltypesof solvent perturbation parameters, such as dispersion interaction and induction interaction, we found that the solvent intensification effect on S1 + SOtransitions in the two compounds is governed by two factors: the dielectricfield of each solvent and a specific solute-solvent interaction.
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(18)Kalyanaeundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press, Inc.: New York, 1987. (19)Lamotta, M.; Risemberg, S.; Merle, A. M.;Jowot-Dubien, J. J. Chem. Phys. 1978,69,3639. (20)McClure, D. 5.J. Chem. Phys. 1966,25,481. (21)Azumi, T.;McGlynn, S. P. J . Chem. Phys. 1962,37,2413. (22)(a) Hochst", R. M.; Small, G. J. J. Chem. Phys. 1966,%, 2270. (b) Craig, D.P.; Small, G. J. J. Chem. Phys. 1969,60,3827.(c) Warren, J. A.; Hayes, J. M.;Small, G. J. Chem. Phys. 1986,102,323. (23)Pariser, R. J . Chem. Phys. 1966,24,250. (24)Hoijtink, G. J.;Velthorst, N. H.;Zandstra, P. J. Mol. Phy8.1960, 3, 533.
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Figure 1. The SI- &I absorption spectra of (a) phenanthrene and (b) triphenylene in various solvents at 20 OC. Solvents are hexane (-), diethyl ether (. * and acetonitrile (- - -1 for phenanthrene and hexane (-), methanol (me.), and 1,2dichloroethane (- - -) for triphenylene. a),
Furthermore, we tried to use the two molecules as fluorescence probes in micellar systems. The spectroecopic parameters examined were (i) the intensity ratio of the forbidden0band to one afthe vibronically allowed bands and (ii) the fluorescence lifetime. It turned out that the spectroscopic parameters of phenanthrene and triphenylene show significant changes assurfactantsform micelles. The results obtained in this study indicate the potential utility of phenanthrene and triphenylene as fluorescence probes. Sincethe two compoundshave different molecular sizes and different solubility in water than pyrene, we will be able to use them as supplementary probes to pyrene. Experimental Section Phenanthrene and naphthalene are zone-refinedreagents from Tokyo Kasei Kogyo Co., Ltd., and were used without further purification. Triphenylene (Tokyo Kasei Kogyo Co., Ltd., GR grade) and pyrene (Aldrich Chemical Company, Inc.) were sublimed at least twice in vacuo. The solvente employed are listed in Table I. All solvents were spectrograde and, thus, were usedas received. The surfactants, sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium chloride (HTAC), were reagent grade and these were washed with diethyl ether in a Soxhlet extractor until all fluorescent impurities were removed. An aliquot of the stock probe solution, dissolved in hexane, was transferred into a 10-mL volumetric flask, then the solvent was gently evaporated. After all the solvent was removed, 10mL of each solvent or of each aqueous surfactant solution was added to the flask and the contents were sonicated for 10 min for dissolving the probe. In the caee that water was ueed as solvent, undiseolved probe crystalswere fdtered offwith Teflon membrane (0.1 WI pore size). Absorption spectra were recorded on Jasco Ubest-50 spectrophotometer. Fluorescence spectra were observed with an Hitachi F-4000 spectrofluorometer using narrow slits (1.5 nm band widths) and were mected by conventional method with rhodamine B. Fluorescence lifetime measurements were carried out with a Horiba NAES 1100 time-resolvedspectrofluorometer which employs a time-correlated single photon counting technique.
Results and Discussion Solvent Effect on Abeorption and Fluorescence 8". In Figure 1 we present SI So absorption spectra of phenanthrene and triphenylene in various solvents at mom temperature. The assignment for the bands of phenanthrene is based on the extensive studies of Small et aI.= The band o is the origin band of SI
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Figure 2. Fluorescence spectra of (a) phenanthrene (1 X lo-' mol L-1) and (b) triphenylene (2 X 1o-Smol L-l) in various solvents at 20 "C. Solvents are hexane (-1, diethyl ether (..-), acetonitrile (- - -), and perfluorehexane (- -) for phenanthrene and hexane (-1, metahnol s), and 1,2-dichloroethane (- -) for triphenylene. The intensitiesare normalized with ref" to the bands v. (a
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SOtransition and the band v is a vibronic band which becomes allowed through a nontotally symmetrica1 mode vibration. In triphenylene the origin band of SI S,, transition partly overlaps with an adjacent e' vibronic band19 and it is difficult to resolve them in liquid phase spectra, especially in the casea of polar solvente. Thus we give the notation, 0, to the unresolved two bands in Figure lb. It is notable that the origin bands for both of the probes are strongly enhanced on going from a nonpolar to a polar solvent, whereas the shift of the bands is quite small. This feature is common to the case of pyrene. The enhancement of the bands o is not due to the solvent shift of strong, closely lying S2 So transitions becauae the intensity of the bands v remains almost constant. The enhancement should be attributed to the solvent intensification effect on SI- So forbidden transitions. Figure 2 showsthe fluorescencespectra of phenanthrene and triphenylene in polar and nonpolar solvents. The spectrum of phenanthrene in perfluorohexam ia shown for referencesince thissolvent generally gives quesi-vapor spectra25 The spectra of both probes are normalized for convenience at the bandsv, the intensityof which ie almost insensitive to solvents. The bands o are the origin bands of SI SOtransitions. The relative band inbnsitim of o to v for 20 solventa are- s ' in Table I. As in SI SOabsorption spectra, the origin bands in the fluomcence spectra of the two solutes acquire remarkable intensity as the polarity of the solvents increasee. In contrast to the intensity enhancement, slight linabroadening and red-shift are observed for polar solvente. The
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(26) Laweon, C. W.;Huayama,F.;Lip&, 8. J. Chem. Phy8.1@6@ 61,
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Nakashima and Tanaka
92 Langmuir, Vol. 9, No. 1, 1993 Table I. Solvent Dependence of the Relative Band for the Fluorescenceof Phenanthmne and Intenrities &/Iv Triphenylene
IdIv no.
solvent phena tripha perfluorohexane 1.31 0.353 1.40 0.340 2 hexane 3 methylcyclohexane 1.39 0.352 1.74 0.413 4 p-dioxane 1.42 0.343 5 cyclohexane 6 carbon tetrachloride 1.51 0.426 7 benzene 1.64 0.383 1.58 0.361 a diethyl ether 1.68 0.448 9 chloroform 1.72 0.390 10 ethyl acetate 11 dichlorometane 1.74 0.442 12 1,2-dichloroethane 1.71 0.442 1.64 0.369 13 2-propanol 1.66 0.377 14 ethanol 1.70 0.382 15 methanol 1.80 0.446 16 acetonitrile 17 NJV-dimethylformamide 1.86 0.422 1.57 0.370 ia glycerin 19 dimethyl sulfoxide 1.80 0.444 20 water 1.68 0.609 OPhen and triph stand for phenanthrene and triphenylene, respectively.
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Figure 3. Plot of the relative band intensity of fluorescence,
loss of mirror symmetry between the absorption and fluorescence spectra is intrinsically due to the vibronic coupling, 19122 although the overlap of strong S2 SO absorption on the shorter wavelength part of weak SI SO transition apparently breaks the mirror image. In order to understand the solutesolvent interaction which plays a dominant role in the enhancement of the 0-0 band of SI * SO spectra of phenanthrene and triphenylene, we attempted to correlate the relative intensity of the band o to v (I,,/&) in the fluorescence spectrawith several solvent perturbation parameters. The parameters examined are (i) dispersion interaction (a/ rS)2, where a is the mean polarizability of the solvent and r the mean intermolecular distance between the solute and the solvent, (ii) induction interaction ( P ~ / ~ Shere )~, p refers to the parmanent dipole moment of the solvent, (iii) Grunwald’s parameter (e - 1)/(2e + 11, where e is the dielectric constant of the solvent, (iv) a* scale of Kamlet and Taft,B (v) Koeower’s 2 value?’ and (vi) E ~ ( 3 0 We ).~ could not fiid any correlation between IdIv and these parameters except for Grunwald‘s parameter and the a* scale. In Figure 3 IdZVis plotted against Grunwald’s parameters. At fiit glance the correlation appears to be poor. However, if we exclude several solvents which contain an oxygen atom, we can see a considerable improvement in correlation between IdI, and Grunwald’s parameter for both probes. Figure 4 shows the plot of IdI, vs a* scale. The correlation between IdIv and a* scale is good except for water. It has been reported that some polar solventa tend to specifically interact with ~yrene.~J”l~ A similar specific interaction seems to exist between the present probes and the solvents containing an oxygen atom. We will examine the nature of such a specific interaction in future work. Here, it seems to be reaeonable to conclude that the solvent intensification effect of phenanthrene and triphenylene hae two origins: (i) a perturbation to solute electronic states from the bulk
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(28) (a) W e t , M. J.; Abboud, J. L.; TI&, R. W . J. Am. Chem. SOC. 1977, 99,8027. (b) W e t , M. J.; Abboud, J. L.; T&, R. W . J. Am. Chem. Soc. 1977,99,8326. (c)W e t , M.J.; Hall,T.N.; Boykin,J.;Taft, R. W . J. Chem. Soc., Perhin Tione.2 1979,2669. (27) Kmwer, M . K. Anhroduction to Physical Organic Chemistry; John Wiley & SOM,Inc.: New York, 1968.
ZdIv,against Grunwald’s parameter, (e - 1)/(2c + 1): (a) phenanthrene; (b) triphenylene. See Table I for the numbers of the solventa.
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dielectric field of surrounding solventa and (ii) a specific interaction between the solute and the solvent. These interferences by solventa relax the selection rule on the 51 ==SOtransitions of the solutes, resulting in the enhancement of the origin bands. It is useful and interesting to examine the correlation between 11/13 of pyrene and IdI, of phenanthrene and triphenylene. In Figure 6 we plot IdIv w 11/13for both species. Good correlation is obtained for both pyrenephenanthrene and pyrene-triphenylene paire, even if solventa with an oxygen atom are included. This suggwta that the intensity enhancement of the forbidden SI* So transitions is induced through the similar m e d ” a in
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Figure 6. Fluorescence spectra of (a) phenanthrene (-1 X 1W mol L-1) and (b) triphenylene (-2 X 10-7 mol L-1) in aqueous SDS solution. SDS concentration: 6 X mol L-1 (-), 1X W m o l L-'(- - -1 for both probes. Note that cmc of SDS in the literature (e.g. ref 18) is 8 X W3mol L-l.
water phase. The physical properties,e.g., refractiveindex, dielectricconstant, and viscosity, of the micelle are usually different from those of the bulk solvent so that the environmentcsensitive spectroscopic signals of a probe show a big difference between free and micelle-bound pyrene, phenanthrene, and triphenylene. There are some species. This is the general philosophy behind the use of notable deviations. Chlorinated solvents (9,11,12) have spectroscopic probea for micellar systems. In the pyrene a somewhat larger effect on IdI, for triphenylene than on case the value and the lifetime of fluorescence I d ISof pyrene,and the effect of water is strikinglydifferent. correspond to such spectrmcopic properties.2J8 In the Here, we will briefly look into the character of a specific determination of cmc, for example, the Illla value and the interaction between the probes and some solvente. Dong lifetime drastically change at the onset of micellization and Winnik showed that the protic ability of the solvents because pyrene is quickly solubilized into the interior of isimportantinthespecificinteractionwithpyrene.~Lianoe micelles as micelles are formed. Furthermore, pyrene has and Georghiou15 and Nakajimal6 demonstrated the exan additional advantage: ita exdmer fluorescence becomes istence of hydrogen bonded complexes between pyrene observable at the onset of micelle formation, even if the and several alcohols. We also examined the specific concentration of pyrene is so low that the excimer can not interaction between pyrene and severalpolar solventsand be detected in homogeneoussolution.18 Thisphenomenon concluded that the interaction is inherently one of complex is due to the condensation of pyrene into the micelles. We f0rmation.1~The stability constant and the stabilization expected the same effectaof micelles on the spectroscopic energy of the complex, however, were somewhat smaller properties of phenanthrene and triphenylene. thanthose anticipated for hydrogen bonding. We inferred In Figure 6 we show the fluorescence spectra of that the complex might fall into the category of a contact phenanthrene and triphenylene in the aqueous solution charge transfer comp1ex.a As for phenanthrene and of SDS below and above the cmc. As we can see, triphenylene we need further studies to characterize the triphenyleneshowsa significantchangein the fluoreecence specific interaction. At present we have an idea of some spectrum as the micelle is formed, whereas phenanthrene kind of weak complex formation as ita model, based on does not. In order to diecuss the intensity change more the aforementioned similarity to the pyrene case. quantitatively, we introduce the range of the change, R, Fluomaceme in Mieellar Syrtemr. It is well-known which is defined by that surfactante in aqueous solution form micelles above the criticalmicelle concentzation (cmc). If a small amount (1) R = [CIJI,), - CIJIv>,,,I/(IJIv>, of a hydrophobic probe is dissolved in micellar solution, the probe is preferentially incorporated into micelles where and respectively, denote the IdI, becam the interior of a micelle is less polar than the values in water and in micellar solution. The larger R value means the higher sensitivity of the probe. The R (28) Taubomura, H.; Mullikon,R.8.J. Am.Chem. SOC.1W,82,6986. values for severalalternant hydrocarbonsin SDSmicellar Figure 6. Plot of ZdZ, of (a)phenanthrene and (b)triphenylene against Z1IZs of pyrene. See Table I for the numbers of the solvents. The z1/& values are taken from ref 3.
94 Longmuir, Vol. 9, No.1, 1993
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Table 11. Intenrity Ratio between the Origin and the Allowed Vibronic Bandr of Several Aromatic Prober in Water and in SDS Micellar Solution probe molecular radiusn (A) (LJZ,.)., (Z,JZ& R 0.486 0.365 0.26 3.19 naphthalene* 1.60 0.05 3.54 1.68 phenanthrene 0.60 1.87 1.17 3.60 pyrenee 0.410 0.49 0.609 triphenylene 3.72
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