J . Phys. Chem. 1985, 89, 491-497
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Molecular Architecture in Cyanine Dye Aggregates at the Air-Water Interface. Effect of Monolayer Composition and Organization on Fluorescent Behavior‘ S. Vaidyanathan, L. K. Patterson,* Department of Chemistry and Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556
D. Mobius, and H.-R.Gruniger Max- Planck- Institute fur Biophysikalische Chemie. Karl- Friedrich-Bonhoeffer- Institut, Abt. Molekularer Systemaufbau, D 3400 Gottingen, West Germany (Received: August 10, 1984)
The fluorescence behavior of an amphiphatic oxacyanine dye and its thiacyanine analogue has been investigated in spread monolayers at the air-water interface. J-aggregate formation as a function of area/(dye molecule) was monitored by spectral changes in pure dye monolayers and in 1:l mixtures of dye with various fatty acid coaggregates. Simultaneously, the thermodynamic behavior of these systems was characterized by the associated surface pressure-area isotherms. In all cases, J-aggregate formation may be related to a phase transition in the isotherm. The intensity of aggregate fluorescence is found to be inversely related to the work, A”, of compression of the monolayer through the transition. Inclusion of the fatty acid coaggregate was shown to facilitate J-aggregate formation in the order stearic > elaidic > oleic. Both fluorescence and thermodynamic data indicate more extensive aggregate formation in the thiacyanine systems. Aside from the paramount role played by the chromophore-chromophore interactions in determining J-aggregate phenomena, this study suggests important contributions from dispersion forces involving the long hydrocarbon moieties.
Introduction The function of a heterogeneous chemical system may be governed not only by the molecular structure of its components, but also by the extent of organization among those components. Examples of such systems are ubiquitous in biochemistry and suggest the possibility for achieving a significant measure of mechanistic control over certain classes of behavior by altering the manner in which the molecules are brought into proximity. A detailed understanding of these interdependencies can best be attained in model systems for which both chemical and physical parameters can be strictly controlled. For certain classes of molecules, the spread monolayer at the air-water interface is such a model providing (A) limited dimensionality for diffusion processes, (B) continuous control over molecular organization, and (C) the means to monitor macroscopic parameters related to molecular packing as the condition of the monolayer is altered. An attractive class of molecules with which one may illustrate these points in regard to photophysical behavior is the paraffinsubstituted cyanine dyes. Some of these have been shown to undergo transformation from monomer to extended chromophoric structures, J-aggregates, which are particularly dependent on the parameters of monolayer assembly.* While no covalent bonds between chromophores are formed on such assembly, the interactions among all components of the array undergo change and the photophysical behavior of these dyes in the aggregated state differs markedly from that of these molecules in monomeric form. Both absorption and luminescence spectra are dramatically altered, and the lifetimes of the excited states become very short indeed. The information available concerning the properties of J-aggregates in monolayers has, to date, been largely obtained from investigation of layers transferred to solid substrates on which the components of the array are bound to fixed sites.3 However, measurement of luminescence behavior at the air-water interface as a function of macroscopic thermodynamic behavior accompanying compression can provide a basis for elucidating the re(1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is document no. NDRL-2621 from the Notre Dame Radiation Laboratory. (2) Bikcher, H.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 183. (3) (a) Kuhn, H.; MBbius, D.; Btlcher, H. “Physical Methods of Chemistry”; Weissberger, A., Rocrsiter, B., Eds.; W h y : New York, 1972; Vol. 1, p 577. (b) MBbius, D.; Kuhn, H. Isr. J. Chem. 1979, 18, 375. (c) Pcnner, T. L.; Mbbius, D. J. Am. Chem. SOC.1982, 104, 7407.
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lationship between photophysical behavior and ongoing changes in monolayer organization at the molecular level. Such studies not only can provide information concerning the essential parameters of organization for aggregates at the water surface but also can facilitate optimal design of layers to be transferred. In the present study we have examined the spatial and energetic requirements of J-aggregation for two related dyes by introducing into the system a series of surfactants which differ geometrically only slightly from one another. The structures of the two dyes along with the surfactants used are the following: DYES AND CO-AGGREGATES
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