The syntheses of several surfactant analogues of tris(2,2'-bipyridine)rutheni~m(iI)~+ is described together with a study of their photochemistry in spread films, monolayer assemblies, and at a monolayer assembly-water interface. It has been found that highly purified samples of the dioctadecyl ester of (4,4'-dicarboxy-2,2'-bipyridine)bis(2,2'-bipyridine)ruthenium( 11)2+ are inactive as catalysts for the photocleavage of water in monolayer assemblies. This is contrary to our previous findings, which employed a sample of the surfactant complex which has now been shown to contain several impurities, including other surfactant ruthenium(i1) complexes. The properties of films and monolayer assemblies of these complexes are found to be quite sample sensitive even when different samples of high indicated purity are employed. The failure of the highly purified complex to serve as a catalyst appears partially due to a light-induced destruction of assemblies irradiated in contact with water, which is due at least in part to a photohydrolysis of the ester group.The photochemistry of the tris(2,2'-bipyridine)ruthenium(I1) dication (Ru(bpy)32+) and related metal complexes has been the subject of considerable recent i n v e~t i g a t i o n .~-~~ It has been found that the excited states of Ru(bpy)32+ and other complexes having relatively long excited state lifetimes can be quenched by energy transfer, complex formation, and electron transfer processes. The latter have been observed both for cases where the excited state of Ru(bpy)32+ serves as an electron d~n o r~-~ as well as for instances where it acts as an electron acceptor.8-'0 There has been intense interest in the possibility that light driven electron transfer reactions can serve as useful energy conversion and storage processes,20-2' since the initial products of electron transfer are usually highly energetic, but stable, when isolated, molecules. In solution the high-energy products are rapidly degraded through reverse electron transfer with each other to form ground-state starting material^.^.^ Recently, interest has developed in the possibility of inhibiting these back reactions by various techniques, including the initiation of competitive processes or the carrying out of the reactions in other media.11,21 I n recent investigations, we have examined the photochemical reactivity of a number of chromophores in organized monolayer a~s e m b l i e s . l .~,~~-*~ In several cases it has been found that the controlled and highly condensed environment provided by the assemblies can strongly modify photoreactivity and luminescence behavior from that observed in solution. Our interest in light driven electron transfer processes involving Ru(bpy)32+ led us to prepare surfactant complexes of ruthenium( 11) for incorporation into monolayer assemblies. In a recent communication2 we reported that the strong luminescence of a surfactant ruthenium complex could be quenched by immersion of assemblies containing the complex into water. Concomitantly, the light-induced cleavage of w...

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-The photochernical behavior of several different chromophores incorporated as surfactant molecules in organized monolayer assernblies is reported. For several systerns rather pronounced differences are observed between reacttvity in solution and in the assernblies. Many of the dtfferences can be ascrtbed to packing phenomena and to restrictions on diffusion and rnolecular motion in the condensed assemblies. Among the reactions studied which are subject to strong environmental influences in the assernblies are cis-trans photoisomerization, ligand photoejection, excimer and photodimer formation and electron transfer reactions. Photoreactions occurring at an assernblysolution interface have also been observed including the cleavage of water mediated by a layer-bound excited state of a surfactant rutheniurn complex. INTRODUtTIONIt has long been known that arnphipathic rnolecules such as fatty acids, which contain both water soluble (hydrophilic) and water insoluble (hydrophobic) groups, form rnonolayers when spread on water or other polar surfaces. Techniques for the study of such monolayer filrns were developed by Pockels (l) and Langrnuir and Blodgett (2) and in rnany cases the filrns were used to investigate rnolecular properties such as shape and size. It was found by Langrnuir as early as 1917 that rnonol ayer filrns spread on water could be transferred to solid supports (ref. 3). Although the techniques for handling these filrns have been widely used, it has been only recently that the techniques have been re:fined and applied to the construction of highly organfzed rnolecular systerns. Sirnplification and elaboration of the techniques for preparation and study of monolayer assernblies has been due largely to the investigations of Kuhn and coworkers (4-6). These investigators have also used organfzed monolayer assernblies to study a variety of phenomena rangtng from electronic energy transfer and photographic sensitization to interference and electrooptical effects (ref. 4). To a large extent the rapid progress in the use and study of monolayer assernblies has been rnade possible by the application of modern spectroscopic techniques. The facility with which absorption and luminescenca spectra can be obtained for monolayer assernblies on optically transparent supports suggested that these techniques rnight also be applied to investigate reactions and photoprocesses.One of our chief reasons for investigatfng reactions in organized rnonolayer assernblies is that the controlled regular structure of the assernbly provides a bridge between the extremes of crystalline solid and fluid solution. The intermediacy of a monolayer assernbly between liquid and solid is suggested by several observations. It is generally found that Chromophores of cornponent surfactant molecules rnafntain specffic orientatfon with respect to the layer plane (ref. 4). Studfes of the distance dependence of singlet-singlet energy transfer also fndfcate a regular structure wfth the spacing anticipated on the basis of closelypacked exten...

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