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The primary photoprocesses of chromium (III) complexes Chen, Schoen-Nan

Abstract

Energy transfer between Reineckate ion (donor) and hexacyanochromate(III) ion (acceptor) has been studied through quenching of donor phosphorescence (lifetime and intensity) and sensitization of acceptor phosphorescence. Results from all measurements fit the expected Stern-Volmer relationship with a quenching constant kQH = 7.2 x 10⁵ M⁻¹ sec⁻¹ at -65°C. The pre-exponential factor and activation energy of kQH are 6.6 x 10¹º M⁻¹ sec⁻¹ and 4.8 Kcal/mol respectively. The constant, kQH’ is attributed entirely to an energy transfer rather than a quenching process. The electronic states directly involved are the ²Eg (and/or² T₁g) states of both donor and acceptor. Energy transfer is a diffusion-controlled (collisional) process. Hexacyanochromate(III) ion is itself quenched in the presence of Reineckate ion. The quenching constant, k'QH, which may be attributed to back energy transfer from acceptor to donor, has a pre-exponential factor of 2 x 10¹² M⁻¹ sec⁻¹ and an activation energy of 7.6 Kcal/mol. In the same system, quenching of photoaquation has also been studied at -65°C. The photoaquation quantum yield of Reineckate ion is 1.02 x 10⁻². It is reduced in the presence of hexacyanochromate(III) ion, but not as much as the phosphorescence of Reineckate ion is reduced. The limiting unquenchable part, ϕ∞chem, occurs via the ⁴T₂g state, while the quenchable part must occur through the ²Eg state as an intermediate. The actual path for the quenchable part proposed is back intersystem crossing from the ²Eg to the ⁴T₂g state, which then undergoes aquation. The primary processes of ²Eg state molecules have been investigated through the temperature dependence of phosphorescence lifetimes of some Cr(III) complexes. All the available evidence supports the idea of the thermally activated back intersystem crossing. According to this mechanism, the origins of the ⁴T₂g states of Cr (III)complexes reached by crossing are far lower in energy than has been expected. Assuming the occurrence of back intersystem crossing, the application of energy transfer to the determination of intersystem crossing quantum yield, ϕisc’ has been demonstrated. The values of ϕisc for Reineckate and hexacyanochromate (III) ions are estimated to be 0.52 and 0.35, respectively. The variation of ϕisc with temperature for these Cr(III) complexes has also been measured, which suggests that in general, internal conversion has a strong temperature dependence. From the rise of phosphorescence with time after pulse excitation, a new parameter, Tx, has been obtained which represents population of the phosphorescing state and is believed to be the lifetime of the ⁴T₂g (or less likely ²T₁g) state. Efforts have been made to confirm and identify this parameter. Studies of ϒx have been carried out as a function of temperature. Mechanisms based on different tentative assignments of ϒx are proposed and their implications examined. All primary processes, except the intrinsic radiative transitions, seem to consist of at least two components, which take different pathways and are different functions of temperature.

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