The photochemistry and photophysics of cis- and trans-[Cr(cyclam)(NCS)2]+ ions in aqueous and nonaqueous solvents
Date
1999-04-01
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Degree Level
Doctoral
Abstract
The external parameters of solvent, pressure, and temperature were used to study the effects of these agents on the photochemical and photophysical properties of cis-[Cr(cyclam)(NCS)2]ClO4. This study was performed in number of solvents: water, dimethylformamide, acetonitrile and nitromethane. The quantum yield for the release of the thiocyanato ligand was found to be 0.0702 ± 0.005, 0.0635 ± 0.003, 0.0108 ± 0.001 and 0.00537 ± 0.001 for water, dimethy1formamide, acetonitrile and nitromethane at room temperature and pressure. The solvent was found affect the radiative lifetime ([tau]) (2Eg [right arrow] 4A2g, Oh approximation) of the complex. The values were determined to be 11.0, 2.24, 4.09, 7.49, 3.15 and 4.1 microseconds in acidified (pH 3.0) water, dimethylformamide, acetonitrile, nitromethane, hexamethylphosphoramide and acetone respectively. The effect of temperature on the observed radiative lifetime ([tau]) for doublet decay (2Eg) was determined. The temperature behavior is model by the Arrhenius equation with only slight deviations from linearity. Thus for the temperatures studied here, most of the data corresponds to the high temperature (strongly coupled region) of nonradiative decay (Engleman and Jortner). The apparent activation energies were determine to span a short range (i.e.41.2 ± 1.7 to 49.8 ± 1.3 kJ mol-1 for water to nitromethane respectively). The values of the apparent activation energies were compared with the energy spacing between the excited quartet and excited doublet state, as determined by the 5% rule of Fleischauer et al. The values of the estimated energy spacing between the excited states were found to be larger than the apparent activation energies. The effect of temperature on the photochemical quantum yield of thiocyanate, release was determined in 0.001 M HClO4. The temperature has only a marginal effect on the quantum yield. An Arrhenius plot of the data revealed a slope of 3 ± 8 kJ mol-1, which is zero within the error. The effect of pressure on the radiative lifetime and photochemical quantum yield in 0.001 M HClO4, dimethyfformamide, acetonitrile and nitromethane, was determined. The volumes of activation [Delta]V* ([tau]-1 ) are +1.9 ± 0.3, +3.9 ± 0.5, +4.2 ± 0.5 and +4.6 ± 0.6 cm3 mol-1 for 0.001 M HClO4, dimethy1formarnide, acetonitrile and nitromethane, respectively. The volume behavior indicates that the complex undergoes a shift in dominant decay modes as temperature is reduced. The effect of pressure on the photochemical quantum yield was determined and are -2.6 ± 0.7, -8.6 ± 0.7 and 3.2 ± 1.0 cm3 mol-1for 0.001 M HClO4, acetonitrile and dimethylformamide respectively. The sensitivity of excited state lifetime ([tau]) of the cis- and trans-isomers of [Cr(cyclam)(NCS)2]+ towards dioxygen was investigated. Th sensitivity is due to efficient energy transfer from the excited complex to molecular oxygen to form singlet delta (1[Delta]g O2) oxygen. The measurements resulted in intersystem-crossing vields which approached a limiting value of 0.5 suggesting that a statistically limited spin-exchange mechanism may be responsible for the energy transfer in these systems.
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Keywords
chemistry, CR (III) complexes, photochemistry, chromium
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Chemistry
Program
Chemistry