Weinstein et al.
as DNA intercalators,10 as building blocks in polynuclear
transition metal systems designed for electron/energy trans-
fer,8,11 and as sensing devices.12
reporter groups such as carboxylic acids and esters on
substituted diimine ligands have also been used24-26 to
indirectly probe charge transfer-processes using both nano-
second- and picosecond-TRIR. However, there have been
relatively few TRIR investigations of PtII diimine chromo-
phores.9,26,27
Herein, we present an investigation into the photophysics
of PtII diimines bearing perfluorinated28 thiolate ligands. We
have used UV-vis absorption spectroscopy, resonance
Raman spectroscopy, and (spectro)electrochemistry to probe
the origin of the lowest excited state in these new, highly
soluble fluorinated compounds. Our results are consistent
with previously reported data on the related non-fluorinated
Pt(diimine)(bis)thiolates.
We have introduced conjugated perfluorinated thiolate
ligands to decrease the donor capacity of the thiolate ligand
moiety and to produce a more charge-delocalized excited
state. Furthermore, the introduction of the para-CN group
into the thiolate moiety has allowed the dynamics of the
lowest excited state of Pt(bpy)(4-CN-C6F4S)2 to be moni-
tored in solution at room temperature by picosecond TRIR
spectroscopy.
The nature and dynamics of the lowest excited state is an
issue of particular importance for metal chromophores since
these possess a manifold of low-lying excited states of
different origin.13 The nature of the lowest excited state in
PtII diimine dithiolates, where diimine is a derivative of 2,2′-
bipyridine (bpy) or 1,10-phenanthroline (phen), has previ-
ously been assigned as either {charge-transfer-to-diimine}8
or ligand-ligand charge transfer (LLCT).5 The lowest excited
state for flexible bis-thiolate systems, Pt(bpy)(4-X-C6H4S)2,
has also been shown to be {charge-transfer-to-diimine} with
the highest occupied molecular orbital (HOMO) being mainly
of S(lone pair)/Pt(d) origin.7,14,15 The desired properties of
excited states such as their lifetime or degree of charge
separation can be achieved by the careful design and
modification of the ligands.
PtII diimine complexes that show emission in fluid solution
on the nanosecond or longer time scale include some
dithiolates,8,16-18 acetylides,8,9,19 and cyanides.20 However,
the majority of PtII excited states are short lived and the
elucidation of their dynamics requires fast spectroscopic
methods.
Experimental Section
Materials. All solvents were distilled over CaH2 under a dry
nitrogen atmosphere prior to use. 2,2′-Bipyridine, 1,10-phenan-
throline, ferrocene (Aldrich), potassium tert-butoxide, and hexa-
methylenetetraamine (hmt) (Sigma) were used as received. For
electrochemical experiments, anhydrous dimethylformamide (DMF)
(Fluka) was used as received. The supporting electrolyte, [NBu4]-
[BF4], was prepared from [NBu4]Cl and Na[BF4] (Aldrich) and
recrystallized twice from dichloromethane.29
A modified literature procedure was employed to synthesize Pt-
(diimine)Cl2 from K2[PtCl4] and the corresponding diimine.30 The
thiolate ligands C6F5SH and 4-CN-C6F4SH were synthesized as
described elsewhere.31
Time-resolved infrared (TRIR) spectroscopy has proved
to be a powerful technique for probing the electronic
redistribution which occurs upon formation of excited states
of coordination compounds, particularly those bearing IR
reporter ligands such as CO or CN.21-23 Peripheral IR
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Am. Chem. Soc. 1976, 98, 6159. (b) Peyratout, C. S.; Aldridge, T. K.;
Crites, D. K.; McMillin, D. R. Inorg. Chem. 1995, 34, 4484. (c) Che,
C.-M.; Yang, M. S.; Wong, K. H.; Chan, H. L.; Lam, W. Chem.-Eur.
J. 1999, 5, 3350. (d) Cusumano, M.; Di Pietro, M. L.; Giannetto, A.;
Romano, F. Inorg. Chem. 2000, 39, 50.
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Schanze, K. S. Langmuir 2000, 16, 9137. (c) Van Houten, K. A.;
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mixture of Pt(bpy)Cl2 (422 mg, 1 mmol) and hexamethylenetet-
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7078 Inorganic Chemistry, Vol. 42, No. 22, 2003