| Summary: | 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. === Science, Faculty of === Chemistry, Department of === Graduate
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