Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Chromium 49
cal effect. It was also argued that 2 Eg back-intersystem crossing (BISC) was
not likely to be an important component of 2 Eg excited state deactivation
for Cr(III) complexes with N6 or N4C2 chromophores [64]. This latter conclusion
has been questioned by Kirk [4, 68], and in Sect. 5.1 the subject is
discussed further. In Sect. 5.2 some very recent work by the Wagenknecht
group employing a new series of sterically constrained N4-macrocycles is featured
[69–71].
5.1
[Cr(sen)] 3+ and [Cr[18]aneN6] 3+
5.1.1
[Cr(sen)3] 3+
In a recent report, Kirk and coworkers have reinvestigated the photobehavior
of [Cr(sen)] 3+ , and compared it with that for the macrocyclic complex,
[Cr[18]aneN6] 3+ [68].
The data obtained for [Cr(sen)] 3+ supported the earlier report of a very
short doublet lifetime in rt aqueous solution, and the photoaquation quantum
yield of 0.10 determined upon 4 A2g → 4 T2g excitation was in excellent
agreement with that recorded earlier.
However, based on a more detailed investigation of Cr(sen)] 3+ photoaquation,
it was proposed that this process occurs via the 4 T2g excited state after
back-intersystem crossing (BISC). The more convincing argument presented
was that direct irradiation into the 2 Eg state yielded a photoaquation quantum
yield 22% lower than that for 4 T2g excitation excitation. However, as
noted by the authors, direct spin-forbidden doublet excitation experiments
are fraught with difficulties. Their second argument was based on a deter-
Fig. 11 Macrocyclic-N6 ligand, [18]aneN6