Redox and photochemical behaviour of a porphyrin monolayer on an indium-tin oxide electrode

In order to investigate photoluminescence behaviour of an ordered molecular porphyrin monolayer and its quenching properties by oxygen gas, a porphyrin with long alkyl chains, 5,10,15,20-tetrakis[4-(11-carboxylundecane-1-oxy)phenyl]porphyrin (4), was synthesized and adsorbed onto an indium-tin oxide (ITO) substrate by a chemical dipping method. Cyclic voltammetry was used to analyze the ITO electrode coated with 4. The peak current of the first oxidation was proportional to the sweep rate, and the surface coverage was estimated to be 2.3-2.5 × 10−10 mol cm−2. The UV-vis spectrum of the monolayer showed a broadened Soret band, which shifted to longer wavelength. These features suggest that the porphyrin moieties of 4 are packed to form a J-type structure. The oxygen quenching ratio of the porphyrin 4 monolayer on the ITO electrode, I0/I100, was estimated to be 1.25, where I0 and I100 are, respectively, luminescence intensity values in 100% argon and 100% oxygen. On repeated step cycling between 100% argon and 100% oxygen atmospheres, the response times of luminescence quenching were 10 s (argon to oxygen) and 23 s (oxygen to argon). These findings suggest that a monolayer of sensing dye is applicable for oxygen sensing system without deterioration of size-accuracy of models.