Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Chromium 41
total phosphorescence quenching is accompanied by significant but less than
total reaction quenching. Definitively establishing the precise role of the
doublet level in this quenched reaction component has proven an elusive
goal, with competing options including direct 2 Eg reaction, 2 Eg tunneling to
a ground state intermediate (GSI) surface, or “delayed” quartet excited state
reaction via thermally activated 2 Eg → 4 T2g back-intersystem crossing, BISC
(Fig. 2).
This debate has been exhaustively discussed elsewhere [4, 6, 23, 24], and
will not be a focus of this review. From the outset, it was appreciated that
the 4 T2g level of (t2g) 2 (eg) 1 orbital parentage was an attractive candidate for
substitution chemistry, based on the occupation of an eg orbital which is σ ∗
antibonding with respect to the metal–ligand (M – L) bond. At present, the
most widely employed theoretical model for rationalizing Cr(III) photosubstitution
behavior, assuming quartet reactivity, is the semi-empirical symmetry
restricted angular overlap model (AOM) developed by Vanquickenborne
and Ceulemans [25–27]. For mixed ligand systems it has had considerable
success in predicting relative ligand labilities based on identifying the plane
of labilization and assuming that the ligand with the smallest excited state
M – L bond strength is preferentially substituted. A further strength of the
model is the rationalization it provides for the stereochemical change that
is a common feature of Cr(III) photochemistry (in contrast to their corresponding
thermal behavior), especially for cases of axial ligand loss in mixed
ligand systems of D4h or C4v symmetry [4, 28]. A more recent ab initio study
of the photochemistry of Cr(III) ammine systems yielded results in good
agreement with the earlier AOM calculations [29]. Tris-polypyridyl Cr(III)
complexes may prove to be an exception to this apparent preference for quartet
excited state reactivity. Early quenching studies on [Cr(phen)3] 3+ (where
phen = 1,10-phenanthroline) revealed up to 95% reaction quenching in the
presence of doublet quenchers such as I – and NCS – [30, 31], and the data from
subsequent and more detailed investigations were most readily interpreted in
terms of a direct doublet excited state reaction pathway [32–34].
The remaining sections of this chapter are devoted to a discussion of developments
since December 1998 in a range of different focus areas of Cr(III)
photochemistry and photophysics. For convenience, the state term symbols
for Oh symmetry shown in Fig. 2 are usually employed during these discussions.
3
Ultrafast Dynamics of Cr(III) Ligand Field Excited States
Ultrafast time-resolved absorption spectroscopy constitutes one of the most
exciting and promising new frontiers in transition metal photochemistry and
photophysics. The term ultrafast is applied to photoprocesses that occur on