Photochemistry and Photophysics of Coordination Compounds

50 N.A.P. Kane-Maguire
mination of the stereochemistry of the [Cr(sen-NH)(H2O)] 4+ photoaquation
product via chiral capillary electrophoresis analysis (CE), employing
d-tartrate as the chiral capillary additive. The primary aquation product
exhibited only a single peak, consistent with the presence of trans product –
a result anticipated by AOM theory assuming quartet excited state reactivity.
However, a control thermal aquation experiment (where isomerism is not expected)
also yielded the same data. Although an explanation was offered for
this latter result, the reviewer notes from experience [49, 73] that the CE separation
of the ∆ and Λ isomers of cis hydrolysis products is often difficult,
and a definitive assignment of a single peak to the trans isomer can be made
confidently only after several chiral additives have been tested in the capillary
buffer medium.
5.1.2
[Cr[18]aneN6] 3+
No emission was detectable from this compound in rt solution, despite
the presence of strong, long-lived 2 E → 4 A2g phosphorescence (162 µs) at
77 K [67]. A temperature dependence study of the lifetime for this transition
showed the usual low and high temperature regimes, with a single-exponential
fit to the high temperature region giving an apparent activation energy of
34 kJ mol –1 . Interestingly, however, the compound was also photoinert in ambient
solution. X-ray crystallographic data on [Cr[18]aneN6] 3+ indicated S6
point group symmetry for the complex, with no evidence for trigonal twist
strain in the [18]aneN6 ligand. The authors argue, therefore, that the Endicott
thermally activated 2 Eg relaxation model is unlikely to be operative in
this case. Instead, they propose that fast radiationless decay at rt is a consequence
of the S6 distortion from octahedral geometry, which leads to a mixing
of states with doublet and quartet character and a facilitation of 2 Eg ISC to
the ground state. In the light of data to be presented in Sect. 5.2, one could
also conjecture whether a contributing factor to the short rt lifetimes might be
a non-productive reaction pathway involving transient Cr – Nbondcleavage.
Finally, note is made of the recent communication by Sargeson and coworkers
on the remarkable photobehavior of the caged hexamine complex,
[Cr(fac-Me5-D3htricosaneN6] 3+ [74]. This photoinert compound exhibits
unique photophysical behavior for an N6 chromophoric Cr(III) species
in rt aqueous solution. In addition to displaying an exceptionally long 2 Eg
state lifetime (τ = 235 µs), the emission shows a very strong isotope effect
upon N – Hdeuteration(τ = 1.5 ms). These observations demonstrate that
2 Eg excited state decay in solution at ambient temperature is dominated by
2 Eg → 4 A2g radiationless deactivation, promoted by high frequency N – H
stretching acceptor modes. Importantly, the results also argue against thermally
activated back-intersystem crossing being a significant 2 Eg deactivation
pathway for this CrN6 system.