PHOTOCHEMISTRY AND PHOTOPHYSICS OF CIS AND TRANS-DIAMMINE(1,4,8,11-TETRAAZACYCLOTETRADECANE)CHROMIUM(III) IONS IN ACIDIFIED AQUEOUS-MEDIA UNDER DIFFERENT PRESSURE AND TEMPERATURE CONDITIONS

The photochemical and photophysical behaviors of the complex ions cis- and trans-[Cr(cyclam)(NH3)2]3+, where cyclam is 1,4,8,11-tetraazacyclotetradecane, have been investigated in acidified aqueous media by pulsed-laser techniques in conjunction with conductivity and near-UV-visible optical detection methods and by steady-state photolytic procedures carried out under different pressure and temperature conditions. The lowest doublet electronically excited state for the cis complex has been characterized in terms of its absorption and emission properties and of the effects of pressure and temperature on its lifetime: DELTA-V double ending obs(tau-1) = +2.9 +/- 0.4 cm3 mol-1, and E(a)(tau-1) = 36 +/- 1 kJ mol-1. The decay of the doublet state yields an intermediate that subsequently reacts to release ammonia. The values of the ammonia quantum yield (0.15 +/- 0.05) and of the associated volume of activation (+ 0.6 +/- 0.4 cm3 mol-1) are independent of the irradiating wavelength, including that for the lowest spin-forbidden doublet band. The trans complex shows very little chemical reactivity of either a transitory or permanent nature; however, the lifetime of the lowest doublet excited state does exhibit a strong dependence on temperature, and correspondingly the observed volume of activation varies significantly with temperature: DELTA-V double ending obs(tau-1) = +1.5 +/- 0.3 and +6.6 +/- 0.6 cm3 mol-1 at 4.0 and 57.8-degrees-C, respectively. Contributions of different possible excited-state processes to the observed volumes and energies of activation are discussed.