Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Chromium 47
As shown in Fig. 8, the quenchable and non-quenchable dissociation pathways
are both thought to proceed through lower-lying porphyrin π–Cr dπ
charge transfer (CT) states [48]. For example, an increase in the electron
density in a Cr dπ orbital with a z-axis component should lead to a weakening
in the Cr-axial ligand bond. The rich photobehavior of these systems suggests
that they would make good candidates for future ultrafast spectroscopic
studies.
5
Thermally Activated 2 Eg Excited State Relaxation Studies
of Sterically Constrained Systems
Early studies on Cr(III) complexes of the type [Cr(N4)X2] n+ where N4 is
the macrocyclic ligand cyclam or tet a (see Fig. 9) and X = Cl – [53, 54],
CN – [55, 56], NH3 [56–58], and F – [59] revealed a marked difference in the
photobehavior of the geometric isomers. All the trans isomers are photoinert
while the cis species are photoreactive. The cyano, ammine, and fluoro systems
drew particular attention because of their strong emission in rt solution,
with accompanying lifetimes almost identical to those reported at 77 K. These
observations were attributed to the steric rigidity of the macrocyclic ring restricting
access to the thermally activated photochemical relaxation channels
available to their non-macrocylic analogs.
Fig. 9 Macrocyclic-N4 ligands, cyclam and tet a
An extensive literature now exists on the effects of ligand steric constraint
on 2 Eg excited state relaxation [4, 60–71], with studies by Endicott and coworkers
being especially noteworthy. Hexaam(m)ine Cr(III) systems have been
one key area of study of Endicott’s group, where a range of complexes were
synthesized containing ligands that would be trigonally strained if coordinated
octahedrally to Cr(III). Their studies provided convincing evidence that
the more trigonally strained ligand systems underwent more rapid 2 Eg deactivation.
In an illustrative example [63], the photobehavior of [Cr(en)3] 3+ was