Ru-based
coordination compounds have important applications as
photosensitizers and catalysts. [RuII(bpy)2(bpyNO)]2+ (bpy = 2,2′-bipyridine and bpyNO = 2,2′-bipyridine-N-oxide) was reported to be extremely light-sensitive, but
its light-induced transformation pathways have not been analyzed.
Here, we elucidated a mechanism of the light-induced transformation
of [RuII(bpy)2(bpyNO)]2+ using UV–vis,
EPR, resonance Raman, and NMR spectroscopic techniques. The spectroscopic
analysis was augmented with the DFT calculations. We concluded that
upon 530–650 nm light excitation, 3[RuIII(bpyNO–•)(bpy)2]2+ is formed similarly to the 3[RuIII(bpy–•)(bpy)2]2+ light-induced
state of the well-known photosensitizer [RuII(bpy)3]2+. An electron localization on the bpyNO ligand
was confirmed by obtaining a unique EPR signal of reduced [RuII(bpy)2(bpyNO–•)]+ (g
xx
= 2.02, g
yy
= 1.99, and g
zz
= 1.87 and 14N hfs A
xx
= 12 G, A
yy
= 34 G, and A
zz
= 11 G). 3[RuIII(bpyNO–•)(bpy)2]2+ may evolve
via breaking of the Ru–O–N fragment at two different
positions resulting in [RuIVO(bpy)2(bpyout)]2+ for breakage at the O-|-N bond and [RuII(H2O)(bpy)2(bpyNOout)]2+ for breakage at the Ru-|-O bond. These pathways were found
to have comparable ΔG. A reduction of [RuIVO(bpy)2(bpyout)]2+ may result in water elimination and formation of [RuII(bpy)3]2+. The expected intermediates, [RuIII(bpy)2(bpyNO)]3+ and [RuIII(bpy)3]3+, were detected by EPR. In addition,
a new signal with g
xx
= 2.38, g
yy
= 2.10,
and g
zz
= 1.85 was observed
and tentatively assigned to a complex with the dissociated ligand,
such as [RuIII(H2O)(bpy)2(bpyNOout)]3+. The spectroscopic signatures of [RuIVO(bpy)2(bpyout)]2+ were not observed, although DFT analysis and [RuII(bpy)3]2+ formation suggest this intermediate. Thus,
[RuII(bpy)2(bpyNO)]2+ has potential
as a light-induced oxidizer.