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J. Phys. Chem. 1982, 86, 4642-4648
The concentrations of amphiphiles required to attain three-phase behavior with protosurfactants are comparable to those required with surfactants. The lower tendency to be adsorbed on minerals and the higher solubilities of salts of divalent cations of low-equivalent-weight salts would be important practical advantages in oil-recovery applications.20 Exploration of only a minute fraction of the possible variables which could increase the oil-mobilization properties of these systems has been possible here. In view of the sharp differences obtained by variations in the alcohol variable alone: it does not seem unlikely that conditions could be found for favorable interfacial tensions between all three phases. In any case, many of the properties which have been
attributed to the complex aggregation characteristics of surfactants seem to be also observed with protosurfactants, and some with systems in which alcohols are the only am~hiphile.~~
Acknowledgment. I am very much indebted to Dr. J. S. Johnson, Jr., of the Oak Ridge National Laboratory Chemistry Division for many helpful discussions. This research was sponsored by the Office of Oil, Gas, and Shale, U S . Department of Energy, under contract W7405-eng-26 with Union Carbide Corp. (33) Knickerbocker, B. M.; Pesheck, C. V.; Scriven, L. E.; Davis, H. T. J . Phys. Chem. 1979,83, 1984.
Liquid Crystalline Solvents as Mechanistic Probes. 9. Dynamics of Intramolecular Fluorescence Quenching Processes of N,N-Dimethyl-4-[ 3-( I-pyrenyl)propyl]aniline in the Liquid Crystalline and Isotropic Phases of a Cholesteric Solvent' V. C. Anderson, B. B. Craig,$ and R. 0. Weiss' Depaflment of chemistry, Georgetown University, Washington, D.C. 20057 (Received: May 10, 1982; In Final Form: Ju/y 28, 1982)
The dynamics of the intramolecular quenching of pyrene singlet ('P) fluorescence by an aromatic amino group in N,N-dimethyl-4-[3-(l-pyrenyl)propyl]aniline (A) have been investigated in a nonpolar, cholesteric liquid crystalline solvent [59.5/15.6/24.9 (wf w/w) cholesteryl oleate/cholesterylnonanoate/cholesterylchloride]. The disappearance of the 'P fluorescence is shown to correlate kinetically with the formation of a fluorescent exciplex state. The monoexponential lP emission decay disfavors the existence of energetically distinct ground-state conformers of A. Employing 1-ethylpyrene as a model compound allows determination of rate constants and activation parameters for the intramolecular 'P quenching process. The small differences in the activation parameters [E,'(cholesteric) = 9.7 f 1.2 kcal mol-l and E,'(isotropic) = 7.8 f 0.3 kcal mol-'; AS*(cholesteric) = 3 f 3 eu and AS'(isotropic) = -4 f 1 eu] suggest that cholesteric order has little effect on the quenching efficiency. A mechanistic explanation is advanced.
Introduction Recently, we have investigated several photochemical and photophysical processes in liquid crystalline medias2" The steric constraints imposed by the liquid crystalline order on the movement of solute molecules have been exploited to elucidate the spatial requirements and conformational geometries of transition states. A process of particular interest has been the quenching of aromatic hydrocarbon fluorescence by amines.2 In a study of the intermolecular quenching of pyrene singlet emission by an aliphatic tertiary amine, 5a-cholestan-3@-yldimethylamine, in the cholesteric and isotropic phases of a 59.5f 15.6/24.9 (w/w/w) mixture of cholesteryl oleate/cholesteryl nonanoate/cholesteryl chloride (CM), it was concluded that the preferred quenching geometry is highly specific and resembles closely the commonly accepted exciplex configuration, i.e., an orientation which places the nitrogen lone pair of the amine orthogonal to the plane of the pyrene T ~ystem.~ Here, we use similar techniques to investigate the influence of liquid crystalline order on the intramolecular quenching of pyrenyl singlet ('P) fluorescence by an aromatic amino group (D) in N,N-dimethyl-4-[3-(l-pyreny1)propylIaniline (A).
* Naval Research Laboratory, Washington, DC
20375.
0022-3654/82/2086-4642$0 1.25/0
A
B
Fluorescence spectra of A in various isotropic solvents exhibit the emission characteristics of a 'P monomer as well as the structureless, red-shifted band of an exciplex ~ t a t e . ~The , ~ exciplex band shows a remarkable bathochromic shift with increase of the solvent polarity (indicative of charge-transfer character), while the wavelength maxima of the 'P monomer emission are insensitive to s ~ l v e n t Appearance .~ of the exciplex state has been shown (1) Part 8: Anderson, V. C.;Craig, B. B.; Weiss, R. G. J. Am. Chem. SOC.1982, 104, 2972. (2) Anderson, V. C.;Craig, B. B.; Weiss, R. G. J . Am. Chem. SOC.1981, 10.1 _ " _7, 1 ~ 9
(3) See for instance: Nerbonne, J. M.; Weiss, R. G . J. Am. Chem. SOC. 1978, 100, 2571. 1979,101, 402. (4) Gordon, M.; Ware, W. R., Eds. "The Exciplex";Academic Press: New York, 1975. Mataga, N.; Ottolenghi,M. In 'Molecular Association"; Foster, R., Ed.; Academic Press: London, 1979; Val. 2, Chapter 1. Taylor, G. N.; Chandross, E. A.; Schiebel, A. H. J.Am. Chem. SOC.1974,96,2693. (5) Mataga, N.; Okada, T.; Masuhara. H.: Nakashima. N. J. Lumin. 1976, 12/13, 159.
0 1982 American Chemical Society
Liquid Crystalline Solvents as Mechanistic Probes
to correlate kinetically with the quenching of 1P.617The accepted geometry for this intramolecular exciplex is a sandwich-typestructure which results in maximum overlap of the 'P and D 7 systems. Picosecond laser studies6t9indicate that in polar solvents a nonstereospecific charge-transfer state precedes the sandwich-type ~ t r u c t u r e . ~ However, ?~ in nonpolar media it is thought that exciplex formation may occur only after attainment of a conformation in which the two moieties overlap in a sandwich-like arrangement. The solvent system CM represents a "nonpol"' solvent in this context.2 There is no evidence for a ground-state interaction between P and D, hence it is likely that the lowest energy ground-state conformations of A in isotropic solvents extend P and D away from one another. Consequently, exciplex formation involves rotation about the carboncarbon bonds of the linking methylene groups. Indeed, minimization of steric repulsions within the hydrocarbon chain of A will play an important role in the energetics of exciplex production: for solvents of similar dielectric constant, the rate of formation is governed then largely by the dynamics of the propyl chain and the medium viscosity. For example, the rise of the exciplex emission has been characterized with a time constant of 4.5 and 7.5 ns in n-hexane (7 = 0.313 cP,'O t = 1.89") and decalin (7 = 3.38 cP,12 e = 2.20i3), respectively.6 The high viscosity of the isotropic and cholesteric phases of CM should strongly influence the efficiency of 'P-D quenching in A. The solvent order in the cholesteric phase may also affect the quenching process. On the basis of space-filling CPK molecular models, A is expected to exist preferentially in an extended conformation and reside in or between the parallel solvent "layers" of a cholesteric phase. If quenching of lP in A proceeds via a specific sandwich-typegeometry, its efficiency should be decreased by a cholesteric phase as a result of the greater bulk viscosity and of the solvent reorganization involved in folding of the hydrocarbon chain. In this paper, we report dynamic quenching experiments with A in the cholesteric and isotropic phases of CM. The results are interpreted with the aid of the data from a model compound, 1-ethylpyrene (B).
Experimental Section Corrected melting points and transition temperatures were obtained on a Kofler micro hot-stage microscope with polarizing lenses. 1-Ethylpyrene, mp 95-95.5 "C (lit. mp 94-95 "CI4),and N,N-dimethyl-4-[3-(l-pyrenyl)propyl]aniline, mp 91.5-93 "C (lit. mp 96-97 "C15),were obtained from Molecular Probes and used without further purification. Analysis at 254 nm with a Waters high-performance liquid chromatograph using a Waters Rad-Pak B silica column and 1 / 2 (vol/vol) chloroform/n-hexane as eluant showed A and B to have greater than 97 and 99% (6)Okada, T.; Saito, T.; Mataga, N.; Sakata, Y.; Misumi, S. Bull. Chem. SOC.Jpn. 1977,50,331. (7)Migita, M.; Okada, T.; Mataga, N.; Nakashima, N.; Yoshihara, K.; Sakata, Y.; Misumi, S. Chem. Phys. Lett. 1980,72,229. (8) Migita, M.; Kawai, M.; Mataga, N.; Sakata, Y.; Misumi, S. Chem. Phys. Lett. 1978,53,67. (9)Okada, T.; Migita, M.; Mataga, N.; Sakata, Y.; Misumi, S. J. Am. Chem. SOC.1981,103,4715. (10)Dunstan, A. E.; Thole, F. B. J. Chem. SOC.1913,127. (11)Williams, J. W.; Ogg, E. F. J. Am. Chem. SOC.1928,50,94. (12)Seyer, W. F.;Leslie, J. D. J. Am. Chem. SOC.1942,64, 1912. (13)Staudhammer, P.; Seyer, W. F. J. Am. Chem. SOC.1968,80,6491. (14)Josephy, E.; Radt, F., Eds. "Elsevier's Encyclopedia of Organic Chemistry"; Elsevier: New York, 1940; Series 111, Vol. 14,p 379. (15)Katusin-Razem, B.;Wong, M.; Thomas, J. K. J. Am. Chem. SOC. 1978,100,1679.
The Journal of Physical Chemistry, Vol. 86, No. 23, 1982 4643
purity, respectively. Cholesteryl oleate, cholesteryl nonanoate, and cholesteryl chloride were purified as described previously.2 Dynamic Measurements. Samples were mixed and stirred in the isotropic phase of the liquid crystal until homogeneous as determined visually and with a polarizing microscope. Aliquots were used to fill 0.4-mm width flat Pyrex capillary tubes which were flame sealed after several heat (to isotropic temperatures)-pump-freeze cycles at ca. 0.1 torr. Decay constants were measured with a singlephoton counter described previously.2 The excitation and !P emission wavelengths were filtered as before.2 A Schott OG-515 glass filter was used to monitor the 'P-D exciplex emission above 500 nm. The excitation light ( r l I 2N 2.5 ns) was focused onto the sample cell positioned at 45" to the excitation beam. Photons emitted from the front surface of the sample were then focused onto the photomultiplier tube placed at right angles to the excitation source. An emission counting efficiency of
