Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Chromium 61
Fig. 20 Mechanism of NO release following light absorption by a pendant aromatic antennae
strongly absorbing pendant aromatic chromophores were attached to the
macrocyclic ring [128–130]. Two of these second-generation Cr(III)nitrito
systems are shown in Fig. 19, where molecules II and III have anthracenyl
and pyrenyl pendant arms, respectively. The normally strong fluorescence
of these tethered aromatics was largely quenched upon Cr(III) coordination,
consistent with fast intramolecular energy transfer to the lower lying
Cr(III) ligand field excited states followed by NO gas release according to
Scheme 2 [128]. The overall NO-generating process for these promising lightgathering
antennae systems is depicted in Fig. 20 for the pyrenyl-pendant
complex (molecule III).
8.2
Photogeneration of Nitrido Complexes from Cr(III) Coordinated Azide
The complex [Cr(NH3)5N3] 2+ containing the azido ligand, N3 – ,wasthesubject
of several photochemical studies during the 1970s [131–134]. The results
from irradiations in the LMCT region (λ ≤ 330 nm)identifiedthepresenceof
two competing processes, involving the formation of azide radical, N3 · and
nitrene, N – , intermediates (Eqs. 3 and 4, respectively) [132–134]:
[CrIII (NH3)5N3] 2+ + hν → [CrII (NH3)5(N3·)] 2+ → 1.5N2
[CrIII (NH3)5N3] 2+ + hν → [CrIII (NH3)5N] 2+ +N2
Product analysis was complicated by the thermal reactions of the radical
species generated. However, based in part on the differences expected in the
N2 gas yields for the two processes, Katz and Gafney [132, 134] concluded that
initial nitrene formation was the dominant reaction pathway. Although the fi-
(3)
(4)