Photocatalytic Substrate Oxidation Catalyzed by a Ruthenium(II) Complex with a Phenazine Moiety as the Active Site Using Dioxygen as a Terminal Oxidant | Journal of the American Chemical Society

Based
on the results of fs-TAS of 1-H⁺ (see above),
the emission of 1-H⁺ at 380 nm is ascribable
to the singlet π–π* excited state of its tpphz
ligand, which is also responsible for the photocatalysis of substrate
oxidation by 1-H⁺. Normally, a higher excited
state such as the S₂ state [¹(π–π*)*]
undergoes relaxation to form a lower excited state such as the S₁ state (¹MLCT*) through thermal IC and finally
reaches the lowest triplet excited state of T₁ (³MLCT*) through intersystem crossing, which is radiatively relaxed
to the ground state ( Figure 11a), according to Kasha’s rule. (17) Not the lowest singlet excited state (S₁), i.e., ¹MLCT*, but a higher singlet excited state
(S₂), i.e., ¹(π–π*)*, of 1-H⁺ performs the photocatalytic oxidation of organic
substrates, that is, an example of anti-Kasha photocatalysis. (17) A particularly unique feature of this system
is that protonation at the vacant diimine site of the tpphz ligand
in 1-H⁺ induces emission from the S₂¹(π–π*)* state and oxidation of external
organic substrates (Figure 11b). Probably, due to the protonation at the vacant diimine
site of the tpphz ligand, spin density in the ¹(π–π*)*
excited state is biased toward the protonated diimine site, which
retards the IC to ¹MLCT*, since the spin density in the ¹MLCT* mainly localizes at the Ru^II center and the
other diimine site of the tpphz ligand bonded to the Ru^II center. (53) Insufficient orbital overlapping
in the excited electronic structures is well-known to be important
for anti-Kasha chromophores. (20a)