Tuning the Excited-State Properties of Platinum(II) Diimine Dithiolate Complexes

Two series of Pt(diimine)(dithiolate)
complexes have been prepared in order to investigate the effects
of
molecular design on the excited-state properties of this chromophore.
The first series comprises
Pt(dbbpy)(dithiolate)
complexes where dbbpy = 4,4‘-di-tert-butyl-2,2‘-bipyridine
and the dithiolates are 1-(tert-butylcarboxy)-1-cyanoethylene-2,2-dithiolate (tbcda),
1-diethylphosphonate-1-cyanoethylene-2,2-dithiolate (cpdt),
6,7-dimethyl-quinoxaline-2,3-dithiolate (dmqdt), maleonitriledithiolate (mnt), and
toluene-3,4-dithiolate (tdt). The second series
comprises Pt(diimine)(tdt) complexes where the diimines are
3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen),
4,4‘-di-tert-butyl-2,2‘-bipyridine (dbbpy),
4,4‘-dimethyl-2,2‘-bipyridine (dmbpy), 2,2‘-bipyridine (bpy),
1,10-phenanthroline
(phen), 5-chloro-1,10-phenanthroline (Cl-phen),
4,4‘-dichloro-2,2‘-bipyridine (Cl2bpy), and
4,4‘-bis(ethoxycarbonyl)-2,2‘-bipyridine (EC-bpy). All of the compounds display
solvatochromic absorption bands and solution luminescence,
which are attributed to a common charge-transfer-to-diimine excited
state. The excited-state energies can be tuned
by approximately 1 eV through ligand variation. Solution lifetimes
range from 1 ns to over 1000 ns and Φem
range
from 6.4 × 10-3 to less than 10-5 in
CH2Cl2. Based on these data, the
nonradiative and radiative decay rate constants
have been calculated. For the Pt(diimine)(tdt) series,
the nonradiative decay rate constants increase
exponentially
with decreasing energy, in agreement with the Energy Gap Law, while
those for the Pt(dbbpy)(dithiolate) complexes
do not exhibit a similar correlation. Excited-state redox
potentials have been estimated for all of the complexes
from spectroscopic and electrochemical data. The ability to tune
the driving force for bimolecular excited-state
electron-transfer reactions has been demonstrated for eight of the
complexes using reductive and oxidative quenching
experiments.