Bifurcation of Excited-State Population Leads to Anti-Kasha Luminescence in a Disulfide-Decorated Organometallic Rhenium Photosensitizer

We report a rhenium
diimine photosensitizer equipped with a peripheral
disulfide unit on one of the bipyridine ligands, [Re(CO)3(bpy)(S–Sbpy4,4)]+ (1+, bpy = 2,2′-bipyridine, S–Sbpy4,4 = [1,2]dithiino[3,4-c:6,5-c′]dipyridine), showing anti-Kasha luminescence.
Steady-state and ultrafast time-resolved spectroscopies complemented
by nonadiabatic dynamics simulations are used to disclose its excited-state
dynamics. The calculations show that after intersystem crossing the
complex evolves to two different triplet minima: a (S–Sbpy4,4)-ligand-centered excited state (3LC)
lying at lower energy and a metal-to-(bpy)-ligand charge transfer
(3MLCT) state at higher energy, with relative yields of
90% and 10%, respectively. The 3LC state involves local
excitation of the disulfide group into the antibonding σ* orbital,
leading to significant elongation of the S–S bond. Intriguingly,
it is the higher-lying 3MLCT state, which is assigned to
display luminescence with a lifetime of 270 ns: a signature of anti-Kasha
behavior. This assignment is consistent with an energy barrier ≥
0.6 eV or negligible electronic coupling, preventing reaction toward
the 3LC state after the population is trapped in the 3MLCT state. This study represents a striking example on how
elusive excited-state dynamics of transition-metal photosensitizers
can be deciphered by synergistic experiments and state-of-the-art
calculations. Disulfide functionalization lays the foundation of a
new design strategy toward harnessing excess energy in a system for
possible bimolecular electron or energy transfer reactivity.