Ambient-temperature metal-to-ligand charge-transfer phosphorescence facilitated by triarylboron: Bnpa and its metal complexes

Article abstract of DOI:10.1021/ic7021215

A Cu(I) complex, 1, and a Pt(II) complex, 2a, of a triarylboron ligand, Bnpa, with bright ambient-temperature phosphorescence have been obtained. The phosphorescence of these complexes is highly sensitive toward molecular oxygen and has a distinct response to fluoride ions. For 1, the fluoride ion causes phosphorescent quenching and Bnpa dissociation, and for 2a, it switches phosphorescent color from yellow to green. The Cu(I) complex has an exceptionally high emission quantum yield (0.88) in the solid state.

Full text of DOI:10.1021/ic7021215

Inorg. Chem. 2007, 46, 10965−10967
Ambient-Temperature Metal-to-Ligand Charge-Transfer Phosphorescence
Facilitated by Triarylboron: Bnpa and Its Metal Complexes
Shu-Bin Zhao, Theresa McCormick, and Suning Wang*
Department of Chemistry, Queen’s UniVersity, Kingston, Ontario K7L 3N6, Canada
Received October 26, 2007
A Cu(I) complex, 1, and a Pt(II) complex, 2a, of a triarylboron
ligand, Bnpa, with bright ambient-temperature phosphorescence
have been obtained. The phosphorescence of these complexes
is highly sensitive toward molecular oxygen and has a distinct
response to fluoride ions. For 1, the fluoride ion causes
phosphorescent quenching and Bnpa dissociation, and for 2a, it
switches phosphorescent color from yellow to green. The Cu(I)
complex has an exceptionally high emission quantum yield (0.88)
in the solid state.
known previously, their emissions are all dominated by
fluorescence at ambient temperature.1,2 Phosphorescent ma-
terials have several key features distinguishing them from
fluorescent materials, including high device efficiency of
OLEDs if used as emitters,4,5 high sensitivity to triplet-state
quenchers such as oxygen thus enabling them as potential
oxygen sensors,6 and high color tunability via metal ions.
Phosphorescent triarylboron-containing metal complexes are
especially attractive because we and others have demon-
strated recently that metal chelation/binding to a conjugate
triarylboron ligand can greatly enhance the electron-accepting
ability of the boron center,7 which, when combined with
phosphorescence, enables their potential use as bifunctional
electron transport-phosphorescent emitters in OLEDs.9c
Furthermore, phosphorescent triarylboron metal complexes
offer the possibility for direct probing of the impact of anion
binding at the boron site and ancillary ligands on photo-
physical properties of the metal complexes, which can, in
turn, lead to the development of better phosphorescent
emitters, more sensitive and versatile sensing systems.
Despite the great potentials and the increasing number of
Luminescent triarylborons have versatile applications in
materials sciences. For example, they can function as
effective and highly selective sensors toward anions such as
fluoride or cyanide ions,1 as emitters or electron transport
materials in organic light-emitting devices (OLEDs),2 or as
nonlinear optical materials in photonic devices.3 Although
many examples (small molecules, oligomers, and polymers)
of luminescent three-coordinate organoboron compounds are
* To whom correspondence should be addressed. E-mail: wangs@
chem.queensu.ca.
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10.1021/ic7021215 CCC: $37.00
Published on Web 12/01/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 26, 2007 10965

COMMUNICATION
positive (0.16 V for 2a and 0.32 V for 2b) relative to the
free ligand, albeit much less dramatically than B2bipy
complexes. The fact that 2b has a more positive reduction
potential than 2a supports the presence of electronic com-
munication between the metal and the boron center and is
consistent with the chloride ligand being a weaker electron
donor relative to phenyl. The metal-to-ligand charge-transfer
(MLCT) absorption band of 2a appears at a lower energy
than that of 2b, further supporting such electronic com-
munication.
Figure 1. Crystal structure of 2a: top view (left) and side view (right).
Scheme 1. Structures of Bnpa and Complexes 1, 2a, and 2b
When irradiated by UV light, Bnpa is fluorescent in the
violet-blue region, with the emission energy shifting toward
a longer wavelength with increasing solvent polarity (see
Figure S4.1 of the Supporting Information), consistent with
charge-transfer emission between the boron center and the
npa portion. In contrast, the emission of complexes 1 and
2a is dominated by a yellow and highly visible phospho-
rescence (see Figure S4.2 of the Supporting Information) at
ambient temperature with λmax ) 540 nm, τ ) 8.0 µs, and
Φ ) 0.04 for 1 and λmax ) 543 nm, τ ) 25.0 µs, and Φ )
0.004 for 2a in CH2Cl2. The phosphorescent nature of the
emission spectra of 1 and 2a is confirmed by their high
sensitivity toward oxygen, which quenches the emission of
the complexes. The Pt complex 2a is much more sensitive
toward oxygen than the Cu complex 1 because ∼2% O2
causes >90% quenching of the phosphorescent peak of 2a
(at 4.0 × 10-5 M) while more than 20% O2 is needed to
achieve the same effect for 1 (see Figure S8 of the Supporting
Information). This difference is most likely caused by the
much longer phosphorescent decay lifetime of the Pt
complex, compared to the Cu complex. Complexes 1 and
2a are stable in solution under air, and their phosphorescent
response toward oxygen is fully reversible by using nitrogen
or argon, thus making them potentially useful as oxygen-
sensing reagents.
Both 1 and 2a also display bright-yellow-green phospho-
rescence in the solid state at ambient temperature with
emission wavelengths at 527 nm (τ ) 7.5 µs) and 519 nm
(τ ) 10.9 µs), respectively. Most impressive is compound
1, which has an unusually high emission quantum efficiency
of 0.88 in the solid state (see Figure S4 of the Supporting
Information; Φ ) 0.10 for 2a), determined by using an
integration sphere fluorimeter. Although a number of phos-
phorescent Cu(I) complexes with coordination environments
similar to that of 1 and N,N-chelate ligands such as 1,10-
phenanthroline10a (e.g., [Cu(9,10-phenanthroline)(PPh3)2]-
[BF4]) and 2-(2-pyridyl)benzimidazole10b,c have been dem-
onstrated successfully for use as triplet emitters in OLEDs,
their emission quantum efficiencies are all much lower than
that of 1. The highest solid-state emission quantum efficiency
at ambient temperature for previously known Cu(I) com-
plexes is 0.69 for [Cu(dnbp)(DPEphos)][BF4] (dnbp ) 2,9-
di-n-butyl-1,10-phenanthroline; DPEphos ) bis[2-(diphen-
ylphosphino)phenyl] ether) in a poly(methyl methacrylate)
matrix, reported by Wang and co-workers, which was shown
to be an excellent emitter in OLEDs.10a The exceptionally
examples of boron-containing metal complexes in recent
literature,1j,7-9 ambient-temperature phosphorescent metal
complexes that contain a triarylboron group remain elusive.9
In search of bright phosphorescent emitters with a triaryl-
boron functional group, we have developed a new ligand
system, Bnpa (Scheme 1). We have found that Cu(I) and
Pt(II) complexes of Bnpa with appropriate ancillary ligands
display bright room-temperature phosphorescence in the
solution and solid state that is very sensitive toward oxygen
and anions such as fluoride ions. The preliminary results are
reported herein.
The synthesis of Bnpa was achieved by Ullmann conden-
sation of 7-azaindole with 2,5-dibromopyridine to produce
N-(2-Brpy)-7-azaindole (Brnpa), followed by the substitution
of the bromo group with BMes2. A Cu(I) complex, [Cu-
(Bnpa)(PPh3)2][BF4] (1), and two Pt(II) complexes, Pt(Bnpa)-
Ph2 (2a) and Pt(Bnpa)Cl2 (2b), were obtained by the reaction
of Bnpa with [Cu(MeCN)2(PPh3)2][BF4], [PtPh2(µ-SMe2)]n
(n ) 2 and 3), and K2PtCl4, respectively. The crystal
structures of Bnpa and complexes 2a and 2b were determined
by single-crystal X-ray diffraction analyses, and the structure
of 2a is shown in Figure 1. The Pt(II) center in 2a and 2b
has a typical square-planar geometry. Compared to the free
Bnpa ligand, where the py and azaindolyl rings are nearly
coplanar, the Bnpa ligand in both complexes has considerable
distortion due to the formation of the six-membered chelate
ring, as evidenced by a large torsion angle between the py
and azaindolyl rings (25.01° for 2a and 36.65° for 2b).
Although the structure of 1 was not determined because of
the lack of suitable crystals, the coordination environment
around the Cu center is believed to be similar to that in [Cu-
(npa)(PPh3)2][BF4] [3; npa ) N-(2-py)-7-azaindolyl], syn-
thesized by us, where the Cu(I) center has a distorted
tetrahedral geometry [N-Cu-N ) 92.17(15)°] and the py
and azaindolyl rings have a very large torsion angle of 38.67°
(see Figure S12 of the Supporting Information).
Bnpa displays a reversible reduction peak at E
red
1/2
)
-2.22 V vs FeCp20/+ in the cyclic voltammetry (CV) diagram
recorded in N,N-dimethylformamide, corresponding to the
reduction of the boron center. Consistent with the previously
reported B2bipy complexes,7a the binding of metal ions to
the Bnpa ligand shifts the first reduction potential to more
(10) (a) Zhang, Q.; Zhou, Q.; Cheng, Y.; Wang, L. X.; Ma, D.; Jing, X.;
Wang, F. AdV. Mater. 2004, 16, 432. (b) McCormick, T.; Jia, W. L.;
Wang, S. Inorg. Chem. 2006, 45, 147. (c) Jia, W. L.; McCormick, T.;
Tao, Y.; Lu, J. P.; Wang, S. Inorg. Chem. 2005, 44, 5706.
10966 Inorganic Chemistry, Vol. 46, No. 26, 2007

COMMUNICATION
(4). The phosphorescent color switch of 2a can therefore be
explained as fluoride-induced color switching from MLCT
to ligand-centered emission due to the impact of fluoride
ion binding on the energy level of the MLCT state. The much
brighter emission intensity of 2a‚F-, compared to that of 4,
may be attributed to the bulky BMes2 group that can
effectively reduce intermolecular interactions, hence triplet-
triplet annihilation of 2a‚F-. Although the green emission
of 2a‚F- is readily quenched by air, this adduct is stable
toward air.
The behavior of the complex 1 toward fluoride ions is
quite different. The addition of <0.8 equiv of fluoride ions
under nitrogen results in quenching of the phosphorescent
peak and a rise in the Bnpa fluorescent peak, which is
subsequently quenched with additional fluoride ions in the
same manner as the free Bnpa does, resulting in a nonemis-
sive solution, as shown by the photograph in Figure 2. The
instability of 1 toward fluoride ions was further confirmed
by UV-vis and NMR spectral data (see Figure S11 of the
Supporting Information), which can be attributed to the
known poor N,N-chelating ability of the npa moiety11 and
the steric congestion caused by the two PPh3 ligands. This
is further supported by the NMR titration experiment of 3,
which was found to undergo decomposition upon the addition
of fluoride ions. The consequence of the instability of 1
toward fluoride ions is that it has the potential to serve as a
turn-on sensor for fluoride ions at [F-]/[1] < 0.8 ([1] )
∼10-5 M) under air because the phosphorescent emission
of 1 is mostly suppressed by oxygen and the fluorescence
of Bnpa is switched on in this concentration regime, as
confirmed by the fluorescent titration experiment performed
under air.
In summary, we have demonstrated the first examples of
bright ambient-temperature phosphorescent transition-metal
complexes with a triarylboron group. We have shown that
the boron group in these complexes plays a key role in
achieving the bright phosphorescence. We have also il-
lustrated that the Pt(II) and Cu(I) complexes have a distinct
phosphorescent response toward fluoride ions at ambient
temperature due to a fluoride-triggered energy level change
in 2a and ligand dissociation in 1. Last we have shown that
the Cu(I) complex 1 is an exceptionally bright phosphores-
cent emitter in the solid state and thus a very promising
candidate for use as a phosphorescent emitter or a bifunc-
tional electron-transport phosphorescent emitter in OLEDs.
Figure 2. Phosphorescent spectral change of 2a (λex ) 360 nm) with the
addition of Bu4NF (TBAF; [2a] ) 4.0 × 10-5 M in CH2Cl2). Inset:
photograph of the solutions of 1 and 2a (4.0 × 10-5 M in CH2Cl2) with
and without the addition of excess TBAF under UV light. The solutions in
this experiment were prepared in a drybox under nitrogen.
high emission quantum efficiency and the incorporation of
an electron-transporting boron functionality make 1 a very
promising candidate as an efficient bifunctional phospho-
rescent emitter for OLEDs, which is being investigated in
our laboratory. 2b, a chloride analogue of 2a, does not show
any phosphorescence at ambient temperature in solution or
in the solid state. The significant impact by ancillary ligands
on the reduction potential, the MLCT absorption energy, and
phosphorescence leads us to suggest that the phosphorescence
in 2a is most likely from a MLCT state.
To establish the role of the boron center in the phospho-
rescence of 1 and 2a, we examined the photophysical
properties of their non-boron-containing analogues [Cu(npa)-
(PPh3)2][BF4] (3) and Pt(npa)Ph2 (4). 3 has no detectable
phosphorescence, and 4 only displays very weak and hardly
visible greenish phosphorescence (λmax ) 487 nm; τ ) 18.5
µs) at ambient temperature (see Figure S4.2 of the Supporting
Information). The contrasting properties of 1 and 2a versus
3 and 4 illustrate that the BMes2 group in complexes 1 and
2a is critical for achieving the bright ambient-temperature
phosphorescence in solution. The role of the BMes2 group
is most likely to lower the energy level of the MLCT state
below that of the ligand-centered excited state, because of
its electron-accepting nature, thus facilitating the MLCT
emission.
The impact of anion binding at the boron center on
phosphorescence was investigated by examining the phos-
phorescent spectral change of 1 and 2a in the presence of
various amounts of fluoride ions. In contrast to the free ligand
Bnpa, which experiences a typical fluorescent quenching
upon the addition of fluoride ions (see Figure S6 of the
Supporting Information), the addition of fluoride ions to the
solution of 2a causes a phosphorescent color switch from
yellow to green, as shown by Figure 2 (see also Figure S10
of the Supporting Information). The yellow phosphorescent
band at 543 nm diminishes, while a new oxygen-sensitive
green phosphorescent emission band at λmax ) 497 nm (τ )
10.9 µs, determined in the presence of excess fluoride ions)
appears and increases in intensity with the amount of fluoride
ions, which can be attributed to the adduct 2a‚F-. Fluoride
ion binding to the boron in 2a was also confirmed by 19F
NMR and UV-vis titration experiments (see Figures S5 and
S11 of the Supporting Information). The green emission band
of 2a‚F- has the distinct vibrational structure of npa and
resembles the weak phosphorescent spectrum of Pt(npa)Ph2
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for financial
support. We thank Dr. Jianping Lu at the National Research
Council of Canada for assisting in the measurement of the
solid-state emission quantum efficiency.
Supporting Information Available: Experimental section, CV
diagrams, UV-vis spectra, emission spectra, UV-vis titration
spectra, fluorescent titration spectra, phosphorescent titration spectra,
NMR titration, and crystal structural data. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC7021215
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3092. (b) Wu, Q.; Lavigne, J. A.; Tao, Y.; D’Iorio, M.; Wang, S.
Inorg. Chem. 2000, 39, 5248.
Inorganic Chemistry, Vol. 46, No. 26, 2007 10967