ELECTRON-TRANSFER PHOTOFRAGMENTATION REACTIONS IN MONOLAYER FILMS AT THE AIR WATER INTERFACE/

A series of photoinduced electron-transfer fragmentation reactions hav e been studied in compressed monolayer films at the air/water interfac e. The reactions investigated involve amphiphilic and polymeric deriva tives of fragmentable amino alcohol, 1,2-diamine, and pinacol donors a nd light-absorbing accepters, which are reactive in solution-phase stu dies from their triplet states. For intralayer studies a surfactant an thraquinone derivative was the light-absorbing acceptor. For comparabl e ''interfacial'' studies, the water soluble cation tris(2,2'-bipyridi ne)ruthenium(II)(2+) (Ru(bpy)(3)(2+)) was the photoactive acceptor fro m the subphase. The fragmentation reactions all involve oxidative clea vage of a relatively strong C-C bond in the donor. Reaction was follow ed in each case by monitoring changes in surface pressure that occur w hen the compressed film is irradiated and maintained at a constant are a. Reaction was readily observed in most cases where the donor and lig ht-absorbing substrate are present; however the consequences were foun d to be quite dependent upon the specific donor substrate. Thus for si mple single-chain amphiphiles containing either amino alcohol or 1,2-d iamine donor sites, both intralayer and interfacial reactions result i n rapid decrease in surface pressure, consistent with destruction of t he film as the more hydrophilic redox products are solubilized into th e subphase. For a polymeric diamine, much more complex behavior is obs erved, consistent with a situation where single fragmentation events d o not lead to removal of material from the film but multiple fragmenta tion reactions culminate in film solubilization. Finally, a doublechai n amphiphilic pinacol was found to undergo interfacial fragmentation w ith Ru(bpy)(3)(2+) in the subphase with a concurrent increase in surfa ce pressure to form stable films that do not ''dissolve'' into the sub phase. The isotherms observed following irradiation, decompression, an d recompression are consistent with an expansion that occurs as the tw o-chain amphiphile undergoes redox fragmentation to produce two equiva lents of a single-chain amphiphile.