Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Chromium 51
5.2
trans-[Cr(N4)(CN)2] + (where N4 = cyclam, 1,11-C3-cyclam, and 1,4-C2-cyclam)
The Wagenknecht group has very recently studied a set of trans-dicyanochromium(III)
complexes of topologically constrained tetraazamacrocycles,
namely trans-[Cr(1,11-C3-cyclam)(CN)2] + and trans-[Cr(1,4-C2-cyclam)
(CN)2] + (Fig. 12) to determine the effect that the additional strap has on
the overall chemistry and photophysics relative to the cyclam parent complex
[69–71].
Fig. 12 Parent cyclam ligand, and the strapped derivatives 1,4-C2-cyclam and 1,11-C3cyclam
In their initial work [69, 70], differences in thermal reactivity, UV-visible absorption
spectra, and low temperature photophysics were adequately explained
on the basis of steric and symmetry arguments, and differences in numbers
of N – H oscillators in the molecule. However, marked differences in their rt
photobehavior eluded explanation. For example, the 1,11-C3-cyclam and 1,4-
C2-cyclam complexes have rt 2 Eg excited state lifetimes one and three orders of
magnitude lower, respectively, than the corresponding cyclam complex.
Furthermore, the lifetimes for complexes with the topologically constrained
ligands are strongly temperature dependent near rt in acidified aqueous solution
and the Arrhenius plots are linear [70]. Potential radiationless deactivation
pathways for the 2 Eg level in these systems are depicted in Fig. 13.
Of these possibilities, back-intersystem crossing (BISC) was considered
unlikely on energetic grounds, while net photoreaction was rejected as a significant
relaxation pathway due to the very low quantum yields for photoaquation
for all three complexes. Additionally, MM2 studies suggested that
neither solvent association nor symmetry destroying molecular “twists” are
likely causes for the data in the temperature-dependent regime [70]. In their
most recent paper [71], the authors present evidence that a photodissociation
pathway involving transient Cr-macrocyclic N-bond cleavage (followed
by rapid ring closure) was the most plausible explanation for the thermally
activated 2 Eg relaxation. This conclusion received strong support from the
observation of photodeuteration of the NH protons upon photolysis of the
cyclam complex in acidified D2O (where thermal deuteration was shown to