Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 7819
molecular interactions in the solid state.34 In the present case,
the bulky [n-Bu4N]þ cation prevents close contacts of the
[Pt(4,6-dFppy)(CN)2]- anions in the crystal packing. This
leads to a close similarity of the emission spectra of the neat
powder and of a dilute fluid solution. On the other hand,
concerning the emission decay times and the emission quan-
tum yields, distinct differences are observed. The origin of
these differences is ascribed to the crystal structure and a
stabilization of quenching metal-centered dd* states in solu-
tion via molecular distortions and/or bond length elonga-
tions. Further insight into the nature of the emitting T1 state
is gainedfrom decay time measurements infrozen THF down
to 1.2 K and from a highly resolved low-temperature single-
crystal emission spectrum.
Figure 1. Structure of [n-Bu4N][Pt(4,6-dFppy)(CN)2].
ligands with high ligand field strengths, such as carbenes.20
Further, it has been reported that in Pt(II) compounds with
terdentate dipyridylbenzene N∧C∧N ligands, the dd* states
are shifted to higher energies when compared to compounds
with bidentate phenylpyridine-based N∧C ligands because of
shorter Pt-C bond lengths in the terdentate complexes.21-23
Another possibility to shift these quenching states out of
reach at ambient temperature is given by the use of strong-
field ancillary ligands such as CO and CN-.24-26
In this contribution, we report on the synthesis and crystal
structure of [n-Bu4N][Pt(4,6-dFppy)(CN)2] with 4,6-dFppy =
(40,60-difluorophenyl)pyridinate and n-Bu = n-butyl (Figure 1)
as well as on detailed photophysical studies of the compound
as neat powder and dissolved in tetrahydrofuran (THF),
respectively. The applied strategy of attaching electron with-
drawing fluorine atoms at the 4- and 6-positions of the phenyl
moiety has proven to be successful in stabilizing the highest
occupied molecular orbital (HOMO). This leads to a blue-shift
of the emission with respect to non-fluorinated compounds.27-29
In general, solid state samples of Pt(II) compounds often
show photophysical properties which are remarkably differ-
ent from those of dilute fluid solutions, since the (pseudo)
square-planar geometry facilitates interactions of adjacent
monomers via Pt-5dz2 and Pt-6s, 6pz-overlap or π-stacking
effects.30-34 As a consequence, low-energy emission bands
stemming from molecular aggregates30,31,35 or excimers21,22,36
occur. However, bulky counterions can suppress such inter-
2. Experimental Section
Absorption spectra were recorded with a Varian Cary 300
double beam spectrometer. Emission spectra at 300 and at
77 K were measured with a steady-state fluorescence spectro-
meter (Jobin Yvon Fluorolog 3). Luminescence quantum
yields were determined with an integrating sphere (Labsphere,
4P-GPS-033-SL) with highly reflective Spectralon inside
coating. The estimated relative error is about (0.10. Fluid
solutions were degassed by at least three pump-freeze-thaw
cycles with a final vapor pressure at 77 K of ≈10-5 mbar.
Experiments at lower temperatures were carried out in a He
cryostat (Cryovac Konti Cryostat IT) in which the He gas
flow, He pressure, and heating were controlled. Highly re-
solved emission spectra were detected with a cooled photo-
multiplier (RCA C7164R) attached to a triple spectrograph
(S&I Trivista TR 555). A pulsed diode laser (PicoQuant PDL
800-B) with a pulse width of about 500 ps and an excitation
wavelength of 372 nm or a Nd:YAG laser (IB Laser Inc.,
DiNY pQ 02) with a pulse width of about 7 ns, using the third
harmonic at 355 nm, were applied as excitation sources for
decay time measurements. The Nd:YAG laser was also used
as excitation source for low-temperature emission spectra.
Decay times were registered using a FAST Comtec multi-
channel scaler PCI card with a time resolution of 250 ps.
Elemental analyses were carried out by the Center for Che-
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Thompson, M. E.; Holmes, R. J.; Forrest, S. R. Inorg. Chem. 2005, 44, 7992.
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€
mical Analysis of the Faculty of Chemistry at the Universitat
Regensburg. NMR spectra were measured with a Bruker
Avance 300 instrument (T=300 K). Chemical shifts are
reported in δ [ppm] relative to external standards (solvent
residualpeak, 85%H3PO4). TheNMRspectrawereanalyzed
byfirstorder. Characterizationofthesignals:s= singlet, d=
doublet, t =triplet, q= quartet, m= multiplet, dd =double
doublet, dt = double triplet, ddd = double double doublet.
Mass spectra were recorded on a Finnigan MAT TSQ 7000
(ESI) instrument.
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€
Kalyanasundaram, K.; Bolink, H. J.; Gratzel, M.; Nazeeruddin, M. K.
Materials and Synthesis. K2PtCl4, [n-Bu4N]Cl, KCN, 4,6-
difluorophenylboronic acid, 2-bromopyridine, and 2-ethoxy-
ethanol are commercially available and were used without
further purification. H-4,6-dFppy is prepared via the Suzuki
coupling reactions of 4,6-dFppy boronic acid with 2-bromopyr-
idine. [Pt(4,6-dFppy)(H-4,6-dFppy)Cl] is prepared according to
a literature method from H-4,6-dFppy and K2PtCl4.37 [n-Bu4N]-
[Pt(4,6-dFppy)(CN)2] is synthesized starting from a solution of
[Pt(4,6-dFppy)(H-4,6-dFppy)Cl] (100 mg, 0.16 mmol) in CH2Cl2
(20 mL) and adding an excess of [n-Bu4N]Cl (200 mg, 0.72
mmol) and KCN (50 mg, 0.77 mmol). After 2 h stirring, the
solvent is removed under reduced pressure. The residue is washed
with water and extracted with CH2Cl2. Yellow crystals are ob-
tained by recrystallization from CH2Cl2/Et2O. Yield: 98 mg,
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