Synthesis and Photoactivity of Chemically Asymmetric Polymeric Porphyrin Films Made by Interfacial Polymerization

Thin polymeric porphyrin films have been prepared by interfacial polymerization of a pair of reactive comonomers. one or both of which are derivatized tetraphenylporphyrins. Combinations that have successfully yielded polymeric films include the following; tetrakis(4-aminophenyl)porphyrin (TAPP) in DMSO with tetrakis [4-(chlorocarbonyl)phenyl] porphyrin (TCCPP) in ethyl acetate, various aliphatic diamines in water with TCCPP in chloroform, various aliphatic diacyl chlorides in hexane with TAPP in DMSO, tetrakis(4-hydroxyphenyl)porphyrin (THPP) in aqueous base with TCCPP in chloroform, and ZnTHPP in aqueous base with CuTCCPP in chloroform. Typical film thicknesses are in the range 0.01-10 μm, depending on the particular monomers, reaction time, and reaction conditions. The films display a unique chemical asymmetry, in the sense that opposite surfaces of the films show distinctive differences in the concentration and type of functional groups that are present, demonstrated by X-ray photoelectron spectroscopy (XPS) and by contact angle studies. This asymmetry is a consequence of the interfacial polymerization method; for example, the surface formed in contact with TAPP solution has an excess of unreacted amine groups, while the surface formed in contact with TCCPP solution has an excess of unreacted acid chloride groups, which become carboxylic acid groups after the usual aqueous workup. When placed between identical semitransparent electrodes and irradiated with either broad-band light or a pulsed laser, these films develop directional photopotentials, whereby the film surface that was prepared in contact with the TCCPP solution (the acid surface) develops a more negative potential than the opposite (amine or hydroxyl) surface. Maximum photopotentials are currently about 25 mV. Films that are optically dense at the irradiation wavelength display photopotentials from a competing hole-injection mechanism; these effects are smaller, but occasionally they prevent the observation of the inherent photopotential asymmetry, which is seen consistently in thinner films. We consider the directionality of the photopotentials to be a manifestation of the chemical asymmetry of these interfacial films—that is, photoinduced charge separation involves electron transfer toward the acid surface of the film, which corresponds to the predicted trends of oxidation and reduction potentials of the various porphyrins within the polymer film.