Photoconductivity studies of the ferrocyanide ion under high pressure

A novel apparatus was developed for the investigation of photoconductivity phenomena in liquids pressurized up to 10 kbar. A variety of exciting light sources could be used. In nonpolar solvents, with an applied d.c. voltage, photo-currents as small as 10 p.a. could be resolved. In aqueous solutions, with an applied voltage oscillating at 1 khz, light-induced conductivity changes as small as 0.01% could be resolved. The photoaquation of the ferrocyanide ion was studied using the high-pressure photoconductivity apparatus and a steady-state high-pressure mercury lamp. The first-order photocurrent rise-time could be related to the relative quantum efficiency of the photoaquation process, while the dark decay of the photocurrent yielded a relative value of the bimolecular rate-constant for the reverse reaction. Kinetic measurements were carried out on dilute solutions of potassium rrocyanide in pure water, and in 20% ethanol. The photocurrent yield in aqueous solution was dependent upon secondary chemical equilibria which were sensitive to pressure in a predictable way. Inethanolic solution, the dependence of photocurrent yield on pressure followed the variation of the reciprocal solvent viscosity. In both aqueous and alcoholic solution, the photoaquation quantum efficiency decreased exponentially with pressure, as did the bimolecular rate-constant for the dark reaction in aqueous solution. The pressure dependence of the bimolecular rate-constant in the alcoholic solution indicated a diffusion-limited process. The pressure dependence of the photoaquation quantum yield, and of the bimolecular rate-constant in aqueous solution, was interpreted in terms of an activation volume model. The photoaquation data for both the aqueous and the alcoholic solutions agreed with a hypothetical mechanism whereby ligand-to-metal bond-breaking, and solvent-to-metal bond-formation, are effectively simultaneous. The results for the aqueous dark reaction strongly indicated breaking of the solvent-to-metal bond as the rate-limiting step. A concluding section discusses the wide range of photochemical and photophysical studies to which the photoconductivity apparatus described here, using high-power pulsed laser excitation, is ideally suited.U of I OnlyThesi