On the other hand, oxides (both ITO and glass) react with PPBD solutions much more slowly in acetonitrile solution. (Figure ) This reaction probably occurs analogously to the reaction of 4nitrobenzenediazonium salt, giving surface-bound aryldiazonium groups connected possibly through aryl-O bonds but more likely by strong physisorption. To coat oxide surfaces more aggressively, a solution of PPBD in deionized water can be used. (Figure ) Azo coupling between PPBD in solution and existing surface-bound aryl groups is much faster in protic solvents than in acetonitrile, accelerating the rate of PPBD bonding to the surface through N=N linkages. Additionally, in water PPBD can undergo solvolysis at an appreciable rate to form the p-hydroxybenzenediazonium cation, PHBD. At the concentration of PPBD used, PHBD can likely undergo azo coupling itself with PPBD similarly to the selfreaction of p-nitrobenzenediazonium giving an azobenzene with one phenol and two diazonium functional groups. As the solution ages, this polymer can also be incorporated into the growing surface-bound polymer film both by chemisorption and crosslinking through surface-bound diazonium groups. Chemisorption of polyphenols is known to occur on oxide surfaces and is reasonable to expect in this case. As the adsorbed film ages, it is that it will crosslink by decomposition of the bound diazonium groups, further bonding it in place. (Scheme 2) We attribute the thick, strongly yellowcolored film (Figure ) formed by PPBD decomposition in water to this combination of physisorption driven by H-bonding and cross-linking of the film by the post-physisorption decomposition of diazonium groups.