Reaction of Zinc Phthalocyanine Excited States with Amines in Cationic Micelles Marta E. Daraio, Axel Vo ¨lker, Pedro F. Aramendı ´a,* and Enrique San Roma ´n INQUIMAE, Departamento de Quı ´mica Inorga ´ nica, Analı ´tica y Quı ´mica Fı ´sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabello ´ n 2, Ciudad Universitaria, 1428 Buenos Aires, Argentina Received November 28, 1995. In Final Form: March 11, 1996 X Excited state deactivation of a zinc phthalocyanine (ZnPc) by amines was studied in micelles of hexadecyltrimethylammonium chloride. The singlet state deactivation was studied by fluorescence quenching. This technique allows us to determine equilibrium constants for the distribution of the amines between the aqueous and the micellar phases. It could be established that amines associate to the micelles in two different ways: with a greater affinity to a saturable number of sites, that we will term binding sites, and with a lower affinity by a partitioning mechanism. Equilibrium constants could be determined for aliphatic and aromatic amines. A kinetic scheme taking into account the simultaneous quenching of ZnPc fluorescence by the two types of micellized amines could be successfully applied to derive singlet quenching rate constants, under the assumption that micelles behave like closed compartments during the singlet deactivation. Aromatic amines are more efficient than aliphatic ones, and partitioned quenchers are more effective than bound quenchers. Aromatic amines also deactivate the triplet state of ZnPc. By flash photolysis, the absorption of the anion radical of ZnPc was detected. This species originates on singlet and triplet quenching, indicating that both proceed by electron transfer. 1. Introduction Extensive research has been carried out on reactions of excited states in micellar solutions. 1-3 From quenching measurements of excited singlet or triplet states informa- tion can be obtained on reaction mechanisms and on distribution of molecules in microheterogeneous systems. The main mechanisms for association to the micelles are partitioning and binding. Partitioning resembles the distribution of a solute between two solvents, with practically no limit to the amount of solute in either of the phases. 4 This mechanism is adequate to describe solute partitioning far from the saturation limit. On the other hand, binding 5 results in the association of solute mol- ecules to a limited number of sites in a micelle, 6 resembling an adsorption mechanism. When the occupation is much lower than the saturation, the binding distribution coincides with the partitioning prediction. The only difference can be established on molecular interaction grounds. The determination of equilibrium parameters for these distribution processes, when they exist either separated or simultaneously, is well documented in the literature. 7,8 The way of association of the solute to the micelle determines its distribution statistics, 4,6,9,10 which deeply influences the kinetics in microheterogeneous systems. The kinetics of various fluorescence quenching mech- anisms has been analyzed and analytically solved. The models include the cases where partition, 4 binding, 9 or both processes 11 describe the quencher association to the micelles. While the first two models 4,9 quite generally include the competition between excited state deactivation and quencher exchange between phases or between micelles, the last one 11 is restricted to the case of fast exchange of the quencher, i.e. the case when the equi- librium distribution of quencher between micelles is maintained during the quenching events. For molecules with short fluorescence lifetimes (of a few nanoseconds), this is not a realistic approach. Sensitizers in photodynamic therapy are believed to associate to cell membranes. Micelles are the simplest but easiest-to-realize models for biological membranes, as the competition between hydrophobic and hydrophilic interactions is responsible for the stability of both systems. Sensitizers act by two sensitization mechanisms named type I and type II, 12 depending on the way the excited state of the sensitizer is deactivated. The type I mech- anism involves the generation of radicals; electron transfer reactions are a special case of this type of photosensiti- zation. Type II is the generation of singlet molecular oxygen ( 1 Δ g ) by energy transfer from the triplet excited state of the sensitizer (generally) to dissolved ground state molecular oxygen. In an actual system both mechanisms should operate in parallel. On the other hand, electron transfer after light absorp- tion is the main path for systems converting light into chemical energy. 13 Microheterogeneous systems have been used for stabilization of species generated after charge transfer to increase the efficiency of charge * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: Orlando, FL, 1987. (2) Gra ¨ tzel, M. Heterogeneous Photochemical Electron Transfer; CRC Press: Boca Raton, FL, 1989. (3) Gehlen, M. H.; De Schryver, F. C. Chem. Rev. 1993, 93, 199. (4) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (5) The term binding might be associated with the existence of a stronger interaction of the solute with the micelle than in the case of partitioning. However, in this work, the two terms are used to distinguish between a solute which is at an occupation far away from the saturation limit, in the case of partitioning, and a solute that covers a concentration range attaining saturation, in the case of what we designate as binding. (6) Hunter, T. F. Chem. Phys. Lett. 1980, 75, 152. (7) Encinas, M. V.; Lissi, E. A. Chem. Phys. Lett. 1982, 91, 55. (8) Blatt, E.; Chatelier, R. C.; Sawyer, W. H. Chem. Phys. Lett. 1984, 108, 397. (9) Tachiya, M. J. Chem. Phys. 1982, 76, 340. (10) Daraio, M. E.; Aramendı ´a, P. F.; San Roma ´n, E. Chem. Phys. Lett. 1996, 250, 203. (11) Blatt, E.; Chatelier, R. C.; Sawyer, W. H. Biophys. J. 1986, 50, 349. (12) Jori, G., Perria, C., Eds. Photodynamic Therapy of Tumors and Other Diseases; Libreria Progetto Editore: Padova, 1985. (13) Wilkinson, F. In Photoinduced Electron Transfer; Fox. M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part A, Chapter 1.5. 2932 Langmuir 1996, 12, 2932-2938 S0743-7463(95)01082-1 CCC: $12.00 © 1996 American Chemical Society