Excited State Properties of Quinoxaline-Substituted Platinum 1,2-Enedithiolates

The complexes
(dppe)M{S2C2(2-quinoxaline)(R)},
where dppe = (diphenylphosphino)ethane, M = Ni,
Pd, and Pt, and R = H and Me, have as their lowest-energy band an
intraligand charge transfer transition (ILCT).
Excitation of deaerated solutions of
(dppe)Pt{S2C2(2-quinoxaline)(R)}
lead to emissions from an 1ILCT* and an
3ILCT*. The lifetimes of these excited states
(τ) and the quantum yields for the emissions (φ) for
(dppe)Pt{S2C2(2-quinoxaline)(H)} in CH3CN are 1τ =
0.16 ns,1φ = 0.005 and 3τ = 3.3 μs,
3φ = 0.01, respectively. The
3ILCT* of
these quinoxaline-substituted complexes can undergo a diverse suite of
excited state reactions, including electron,
proton, and hydrogen atom transfers. The second order rate
constants (kq) for the quenching of the
3ILCT* emission
by acids increases with the thermodynamic driving force for the excited
state proton transfer, an observation consistent
with excited state electron and hydrogen atom transfers.
Dihydroquinone and p-methoxyphenol are
substantially
better quenching agents than excited state proton transfer would
predict and thermodynamic calculations suggest
that they quench the 3ILCT* by hydrogen atom
transfer.