The reactions of long-chain diazo ketones and azides in adsorbed monolayers on inorganic solids were investigated and conpared with the corresponding behaviour in monolayers at the air-water interface. The isolated products indicate that reaction of photochemically generated intermediates occurs both with co-adsorbed water and with hydrolCy1 groups on the solid surface. In the case of l-diazo-2oxoheptadecane, for example, the products of these two reactions are heptadecanoic acid and a surface-grafted long-chain ester. The latter can be removed as methyl heptadecanoate by ester exchange using mthanolic HCl. Pronounced differences were observed between alumina, silica gel and fumed silica as supports, and were attributed to differences in the amount of adsorbed Hz0 and in the density of surface hydroxyl groups.
Recently there has been considerable interest in
the photochemistry of compounds adsorbed on the polar surfaces of inorganic materials such as silica gel and alumina.'-' For the most part, however, such research is concerned with possible modification of the mechanisms or product distributions of photoreactfons as a result of adsorption. Much less attention has been devoted to photoreactions between adsorbed molecules and functional groups on the underlying surface. This publication describes the development of photochemical reactions of adsorbed monolayers as a technique for modifying solid surfaces. This surface grafting technique differs from the well-established silylation of hydrophilic surfaces6-g in that it is a two-step procedure, as illustrated conceptually in Fig. 1. In the first step chemisorption of a long-chain compound stable to hydrolysis occurs from hydrocarbon solution to give monolayer coverage of the surface." In the second step the grafting reaction is carried out by UV irradiation or heating. Many classes of photosensitive compounds have been investigated in such organized systems as liposomes or monolayers at the air-water interface. These include diazonium salts," azobenzene derivatives", azenesA3 , azides and diazoketones14-16. For the present study of photochemistry in monolayers at the solid-liquid interface, the long-chain azides and diazoketones shown in Chart I were chosen, both for their ease of synthesis and because they have conpact hydrophilic groups similar in polarity to the carboxylic acid functional group. rl CHa(CH,),CCH=N, n = 14, 16 s CH,K%),sO-Ch P C'h(cHz)~d% Chart I