Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Chromium 59
and [Cr(Brphtpy)2] 3+ (where ttpy = p-tolylterpyridine and Brphtpy = pbromophenylterpyridine).
The two complexes were reported to emit strongly
in rt aqueous solution, although no emission lifetimes or spectra (except
wavelength maxima) were provided. Such emission is quite remarkable in
view of the exceedingly weak emission and very short lifetime (≈ 0.05 µs)
found for the parent terpyridine complex, [Cr(tpy)2] 3+ [103]. Based on CV
data and the reported emission spectral maxima, exceptionally high values
for E o ( ∗ Cr 3+ /Cr 2+ ) were assessed for [Cr(ttpy)2] 3+ and [Cr(Brphtpy)2] 3+
(1.65 Vand1.85 V, respectively). Consistent with these thermodynamic observations,
both complexes were demonstrated to be very powerful photooxidants.
This was especially true for [Cr(Brphtpy)2] 3+ ,whereitsemission
was quenched by all four mononucleotides (including deoxythymidine-
5 ′ -monophosphate). This statement, however, requires that the labels for
Figs. 4A and B in the paper were accidentally reversed.
8
Photoredox Involving Coordinated Ligands
Whereas Sect. 7 was concerned with examples of intermolecular electron
transfer between Cr(III) excited states and external substrates, attention
is directed in the present section to cases of intramolecular redox chemistry
involving the coordinated ligands. These studies have usually involved
photoexcitation into high-energy LMCT excited states involving
the ligand in question, which often results in the transient formation of
a Cr(II)/ligand radical pair. The subject has been reviewed by Kirk [4],
and some representative examples of molecules previously investigated are
[Cr(NH3)5Br] 2+ [121] and trans-[Cr(tfa)3] (where tfa is the anion of 1,1,1trifluoro-2,4-pentanedione)
[122]. Some of the more recent contributions in
this area are discussed in the following two sections.
8.1
Photolabilization of NO from Cr(III)-Coordinated Nitrite
It has been recently established that nitric oxide (NO) regulates a number
of mammalian biological processes, including blood pressure, neurotransmission,
and smooth muscle relaxation [123]. Additionally, tumor cells are
particularly sensitive to NO, which induces programmed cell death [124]
and limits metasis [125]. In response to these findings, Ford and coworkers
have developed a range of air-stable, water-soluble nitrito-Cr(III) macrocyclic
complexes, which display photochemically activated NO release [126–128].
The initial study involved the complex trans-[Cr(cyclam)(ONO)2] + [126],
which for convenience is labeled structure I in Fig. 19.