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Emission and absorption studies of octahedral chromium (III) complexes Witzke, Horst

Abstract

I.The luminescence spectra of a number of molecular Cr(III) complexes have been investigated using standard low-temperature spectroscopic techniques. The observed transitions have been assigned, assuming octahedral micro-symmetry, as emission from either the lowest lying doublet state, ²Eg, or the lowest lying quartet state, ⁴T₂g. An empirical classification is proposed which allows one to predict whether a given complex will show phosphorescence or fluorescence at low temperatures. In general, complexes with relatively high ligand field strengths show only phosphorescence because the ²Eg state lies far below the zeroth level of the ⁴T₂g. state. Compounds with relatively weak ligand field strengths have the ⁴T₂g state as their lowest excited state and therefore show only fluorescence. Complex ions with intermediate ligand fields may emit from both levels since the latter lie within a few hundred wavenumbers of each other. The phosphorescence spectra are sharp and show vibrational structure. A line coincident in energy in absorption and emission identifies the origin of the ²Eg ↔ ⁴A₂g transitions. As these spin-forbidden transitions are intra-configurational (in the strong field limit), the vibration structure in emission and absorption is very similar, although a true mirror image relationship is not realized. In many cases the vibrational spacings have been correlated with known infra-red data. The fluorescence emission is broad and shows no vibrational structure. The ⁴T₂g ↔ ⁴A₂g transitions are inter-configurational and the emission maximum is Stokes-shifted by a few thousand wavenumbers from the absorption maximum, as expected. II. The methods that have been used in the past to derive the electron repulsion parameters, B and C, from the spectra of octahedral Cr(III) complexes have been reinvestigated. A number of Cr(III) complexes were chosen for which the spin-forbidden transitions to the three lowest doublet states are known. The complete strong field and weak field matrices have been used to derive the parameters B and C from the known spectral data. A convenient graphical method is given which makes it a simple matter to choose the "best" values of B and C. The assumption that C=4B was discarded because it is incompatible with the nephelauxetic effect. The three parameter (Δ, B, C) crystal field theory reproduces the band positions of the d-d transitions of most Cr(III) complexes to within a few hundred wavenumbers. Because the free ion theory of Condon and Shortley involves about the same uncertainty, it is felt that the results obtained here represent the best that can be obtained from the three parameter crystal field theory.

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