Photochemistry and Photophysics of Coordination Compounds

54 N.A.P. Kane-Maguire
reaction sequence shown in Scheme 1. In this scheme, the long-lived and
short-lived Cr(III) species are labeled CrL and CrS, respectively, while kET
and k–ET are the corresponding rate constants for forward and reverse energy
transfer. Likewise, the terms kL and kS are the reciprocals of the lifetimes of
CrL and CrS, respectively, in the absence of energy transfer. Emission quenching
of CrL and CrS couldthenbefollowedbyanalyzingthedecayprofile
following pulsed excitation according to the mathematical formulation developed
by Maharaj and Winnik [95].
For the trans-dicyano and trans-diammine systems, energy transfer rate
constants at 20 ◦ C(µ = 1.0) were determined to be kET ≫ 7 × 10 6 M –1 s –1
and 9.7 × 10 6 M –1 s –1 , respectively. However, for trans-[Cr(tet a)F2] + no
energy transfer was observed, which implied that the rate constant was
≪ 3 × 10 5 M –1 s –1 . Analysis of these results using Marcus theory lead to
the important conclusion that electronic effects play a significant role in
determining the rates of energy transfer self-exchange for these series of
complexes.
7
Photoredox Behavior of [Cr(diimine)3] 3+ Systems
Arguably, the seminal report by Gafney and Adamson in 1972 [96] that the
3 MLCT excited state of [Ru(bpy)3] 2+ (where bpy is 2,2 ′ -bipyridine) could
function as an electron transfer agent was the catalytic event that led to the
extraordinary growth of transition metal photochemistry and photophysics
over the last three decades [94, 97–99]. Today, the polypyridyl compounds
of Ru(II) still hold a favored status, due to a coalescence of desirable properties
including an intense, relatively long-lived luminescence signature, and
a remarkable thermal robustness in a range of oxidation states.
The analogous polypyridyl complexes of Cr(III) are the next most investigated
[M(diimine)3] n+ systems. A few years after the Gafney and Adamson
article appeared, Bolletta et al. presented the first evidence that the ligandfield
2 Eg excited state of [Cr(bpy)3] 3+ was a strong one-electron photooxidant
[100]. This involvement of polypyridyl Cr(III) species in direct bimolecular
electron transfer reactions of the generic type represented in Eq. 1 has
since been thoroughly documented for numerous substrates, Q [101–106]:
( 2 Eg)Cr 3+ +Q→ Cr 2+ +Q + . (1)
Importantly, [Cr(diimine)3] 3+ complexes are more powerful photooxidants
than their [Ru(diimine)3] 2+ analogs. The oxidizing power of the Cr(III) 2 Eg
excited state can be assessed from the value of the 2 Eg excited state reduction
potential, E o ( ∗ Cr 3+ /Cr 2+ ). It has been shown [102, 103] that this latter quantity
can be reliably estimated from the difference between the 2 Eg → 4 A2g