Photochemistry of organic chromophores incorporated into fatty acid monolayers

Abstract

-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...

Photochemical reactions in organized monolayer assemblies. 8. Environmental effects on photochemical reactions: contrasts in the photooxidation behavior of protoporphyrin IX in solution, monolayer films, organized monolayer assemblies, and micelles

“…Contact shifts caused by metal ions2 and by organic free radicals3 are well known, as are metal-induced pseudocontact shifts,4 but radicals are generally thought not to cause significant pseudocontact effects. 5 We now report results which appear to disprove this last belief.…”

Amphiphilic diacetylenes with pyridine and 2,2?-bipyridine headgroups ? polymerization properties in the crystalline state, in LB-multilayers, and in complexes with transition metal salts. Morphology of polymerized multilayers

“…Amphiphiles carrying pyridine or bipyridine units are of interest because of their ability to form quaternary salts and complexes with transition metal salts. For example, surfactant metal complexes are known to play an active role in photo-induced electron transfer processes in lipid bilayer membranes [12,13], and doublechain amphiphiles containing pyridine units as the head-group have been demonstrated to form synthetic vesicles [14].…”

Hydrogen Generation by Visible Light Irradiation of Aqueous Solutions of Metal Complexes. An approach to the photochemical conversion and storage of solar energy

“…Many recent studies were devoted to the interesting photochemical properties of the ruthenium (11)-tris (2,2'-bipyridyl) complex cation, Ru (bipy);+, whose excited state is thermodynamically capable of either oxidising or reducing water (see below) . A report [67] on the photochemical splitting of water into H2 and 0, using monolayers of a lipophilic derivative of Ru(bipy)$+ [68] [69] could not be reproduced by subsequent work in several laboratories 170-741. The present account describes a system generating hydrogen, via catalytic cycles, from aqueous solutions of metal complexes under continuous irradiation with visible light; it involves the photochemical production of a reducing species which thermally reduces water.…”