J . Phys. Chem. 1984, 88, 5542-5544
5542
Pyrene Solubilization In Micellar Solutions at Hlgh Pressures W. ID. Turley and H. W. Offen* Department of Chemistry and The Marine Science Institute, University of California, Santa Barbara, California 93106 (Received: May 15, 1984)
The fluorescence quenching of pyrene by 4-cyanopyridine is studied in aqueous cetyltrimethylammoniumchloride (CTAC) as a function of pressure (0-3000 bar). Quenching studies in water, in the micellar phase, and in the pressure-induced precipitation phase of CTAC confirm that pyrene remains accessible to water-soluble quenchers at all pressures. The measured activation volumes for quenching below and above the pressure-induced phase transition support the Menger model of micelles. Large discontinuities in pyrene lifetimes at CTAC and dodecane phase transitions are attributed to packing strain.
Introduction Several lines of investigations have concluded that micellesolubilized pyrene molecules are in contact with water molecules.' There is less agreement on the average site of pyrene, which could be situated at the rough surface of a hydrocarbon-like interior or within a porous cluster along water channels, according to distinct models of micellar a g g r e g a t i ~ n .The ~ ~ ~present work uses high-pressure luminescence and quenching in cetyltrimethylammonium chloride (CTAC) surfactant systems to characterize the solubilization site. Fluorescence quenching has been frequently used to study the microenvironment of the probe in micellar solutions." Waka, Hamamoto, and Mataga7 have investigated the quenching of fluorescene in sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium chloride (DoTAC) and delineated the concentration'range of surfactant and quencher Q in which simple Stern-Volmer kinetics 70/7 = 1 + kq70[Ql (1) is applicable. Here k, is the observed quenching rate constant, T the fluorescence lifetime in the presence of quencher, and T~ the lifetime in the absence of Q . The quencher 4-cyanopyridine (CNP) used in this work is reported to give exponential decays and dynamic quenching behavior, in agreement with solubility data which suggests the quencher is primarily located in the bulk aqueous phase and not adsorbed at cationic micelles. The concentrations are appropriately chosen so that [pyrene] #(H)) no isotope effect on the proton transfer yield was detected (90-350 K). N
Introduction
Since the pioneering work of Hellerl and Heller and Blattmann2s3concerping the photostability of organic compounds the intramolecular hydrogen bond (e.g., o-hydroxyphenyl group hydrogen bond acceptor in the same molecule) has been found to be the most important structural element providing photostability!~~ According to Forster’s theory6*’the acidity/basicity of
+
t Institut fuer Physikalische
Chemie.
* Institut fuer Organische Chemie.
University of Petroleum & Minerals.
thg intramolecular donor/acceptor group increases upon photoexcitation thus enabling intramolecular proton transfer in the excited state*-’* which gives rise to a large Stokes shift in the (1) Heller, H. J. Eur. Polym. J . 1969, 105-132. (2) Heller, H. J.; Blattmann, H. R. Pure Appl. Chem. 1972, 30, 145-165. (3) Heller, H. J.; Blattmann, H. R. Pure Appl. Chem. 1974, 36, 141-161. (4) Williams, D. L.; Heller, A. J . Phys. Chem. 1970, 74, 4473-4480. (5)Otterstedt, J.-E. A.J. Chem. Phys. 1973, 58, 5716-5725. (6) Forster, Th. Z . Elektrochem. Angew. Phys. Chem. 1950, 54,42-46. ( 7 ) Weller, A. In “Progress of Reaction Kinetics”; Porter, G., Ed.; Pergamon Press: London, 1961; Vol. I, pp 187-214.
0022-3654/84/2088-5544$01.50/00 1984 American Chemical Society
