Switchable ambient-temperature singlet-triplet dual emission in nonconjugated donor-acceptor triarylboron-Pt complexes

Article abstract of DOI:10.1002/chem.200900641

A triarylboron compound SiBNPA (1) containing a BMes₂ acceptor and an N-(2'-pyridyl)-7-azaindolyl (NPA) donor linked by a tetrahedral silane group has been synthesised. This molecule displays unusual white singlet-triplet dual emission at 77 K,

Full text of DOI:10.1002/chem.200900641

FULL PAPER
DOI: 10.1002/chem.200900641
Switchable Ambient-Temperature Singlet–Triplet Dual Emission in
Nonconjugated Donor–Acceptor ACHTUNGTRNEUNG
Triarylboron–PtII Complexes
Zachary M. Hudson, Shu-Bin Zhao, Rui-Yao Wang, and Suning Wang*[a]
Abstract: A triarylboron compound Si-
BNPA (1) containing a BMes2 acceptor
plex [Pt(N,C-Si-BNPA)
(SMe2)Ph] (2b).
ACHTUGTNRENN(GU N,N-BNPA)Ph2] (3a) and [PtACHTUNGTRENNUNG(N,C-
This compound also displays ambient
temperature singlet–triplet dual emis-
sion, but with a much greater phos-
phorescent efficiency than 2a due to
the formation of a more stable chelate
ring. Addition of fluoride was found to
have little impact on the phosphores-
cent emission of 2a, but resulted in a
large enhancement of the phosphores-
cent emission intensity of 2b. To estab-
lish the impact of donor–acceptor ge-
ometry on this singlet–triplet dual
emission, the properties of linearly con-
jugated donor–acceptor complexes [Pt-
BNPA)Ph2] (3b) were also examined.
Consistent with 2a and 2b, the N,C-
chelate complex 3b has a much higher
phosphorescent efficiency than the
N,N-chelate 3a. Although 3a and 3b
show bright and fluoride-switchable
phosphorescence at ambient tempera-
ture, they are not dual emissive and
show only metal-to-ligand chage-trans-
fer-based phosphorescence. The non-
conjugated donor–acceptor geometry
and the overlap of the donor and ac-
ceptor singlet and triplet excitation
bands in Si-BNPA and its PtII com-
plexes may thus be the key for achiev-
ing singlet–triplet dual emission on two
separated chromophores in a single
molecule.
and
an
N-(2’-pyridyl)-7-azaindolyl
(NPA) donor linked by a tetrahedral
silane group has been synthesised. This
molecule displays unusual white sin-
glet–triplet dual emission at 77 K, with
an exceptionally long phosphorescent
decay time (2.2 s). Fluoride titration
experiments established that the singlet
and triplet emission peaks are due to
acceptor-based Mes!B charge transfer
3
and donor-based p!p* transitions, re-
spectively. This dual emission was
found to be persistent and observable
at ambient temperature in its PtII com-
plex [Pt(N,N-Si-BNPA)Ph2] (2a). Fur-
thermore, 2a was found to undergo in-
Keywords: boron · cyclometalation ·
donor–acceptor systems · fluores-
cence · platinum
ꢀ
tramolecular “roll-over” C H activa-
tion to produce the N,C-chelate com-
sors for small anions such as fluoride.[3,4] When such com-
pounds contain an electron-donor group, donor–acceptor
charge-transfer luminescence is commonly observed.[2–4] We
have shown recently that when the donor and acceptor
groups are spatially separated, the binding of fluoride ions
to the boron centre interrupts this charge transfer, activating
alternative emission pathways, such as p!p* transitions,
and enabling the use of these compounds as switch-on sen-
sors for fluoride.[4d,e]
In some triarylboron compounds, simultaneous emission
from both charge transfer and p!p* emission pathways can
be observed.[4e] Dual emissive materials are highly sought
after, due to their potential as highly sensitive ratiometric
sensors and as broad-band or white light emitters in
OLEDs.[5–6] Despite their importance and potential, dual
emissive small molecules remain rare, in part because the
molecular design requirements of such molecules are not
well understood. We have shown earlier that it is possible to
achieve dual fluorescent emission in organoboron com-
Introduction
Triarylboron compounds have recently found use in a wide
range of functional materials due to the empty pp orbital on
the boron centre.[1–4] When protected from nucleophilic
attack by bulky aryl groups such as mesityl, the electron-de-
ficient boron atom can act as a highly effective electron ac-
ceptor. Because of this, triarylboron compounds have been
used successfully as nonlinear optical materials,[1] efficient
fluorescent emitters and charge transport materials in organ-
ic light emitting diodes (OLEDs),[2] and highly selective sen-
[a] Z. M. Hudson, Dr. S.-B. Zhao, Dr. R.-Y. Wang, Prof. Dr. S. Wang
Department of Chemistry, Queenꢀs University
Kingston, Ontario, K7L 3N6 (Canada)
Fax : (+1)613-5336669
E-mail: wangs@chem.queensu.ca
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900641.
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6131

pounds by control of the donor–acceptor geometry.[4e] Fur-
thermore, we and others have recently demonstrated that
when attached to the backbone of a chelate ligand in a
metal complex, a triarylboron group can greatly facilitate
phosphorescent emission by metal-to-ligand charge trans-
fer.[7] Hence, with appropriate choice of donor and acceptor
groups and their relative geometry, it may be possible to
achieve singlet–triplet or triplet–triplet dual emission in a
metal-containing donor–acceptor organoboron compound.
Based on this principle, we have designed and synthesised a
new donor–acceptor triarylboron molecule Si-BNPA (1),
which contains a BMes2 acceptor and an N-(2’-pyridyl)-7-
azaindolyl (NPA) donor that are linked together by a SiPh4
group. This molecule was found to form PtII complexes read-
ily with switchable N,N- and N,C-chelate modes. More im-
portantly, Si-BNPA and its PtII complexes all display persis-
tent singlet–triplet dual emission at either 77 K or ambient
temperature that is also responsive to fluoride anions. The
details of our investigation are reported herein.
Results and Discussion
Scheme 2. The synthetic procedures for the PtII complexes.
Synthesis and reactivity of Si-BNPA and BNPA: Si-BNPA
was synthesised by Suzuki–Miyaura cross-coupling of
SiPh2(p-C6H4BMes2)
ACHTUNGTRENNUNG
7-azaindole by using SiPh
G
functionalisation of the NPA chelate with silane or triaryl-
boron groups has little impact on its reactivity with PtII.
Nonetheless, this facile N,N- to N,C-chelate mode switch
provides an opportunity to study the impact of chelate
mode on the emission of complexes of Si-BNPA.
al, according to Scheme 1 (see the Supporting Information
for details). BNPA was synthesised according to a procedure
reported recently by us.[7d]
Si-BNPA (1) reacts readily with [{PtPh2ACTHUNTRGNE(UNG SMe2)}n] (n=2,
3) to produce complex [Pt(N,N-Si-BNPA)Ph2] (2a), which
To establish the impact of donor–acceptor geometry on
the luminescent properties of N,N- and N,C-chelate PtII
complexes based on the NPA chromophore, we also investi-
gated PtII complexes of BNPA, which contains a BMes2 ac-
ceptor directly conjugated to an NPA donor group. The
BNPA molecule displays a similar reactivity toward the PtII
ion as Si-BNPA and NPA, and as a result, complexes [Pt-
AHCTUNTGREUN(GNN N,N-BNPA)Ph2] (3a) and [PtACHUTNRTGEG(NNUN N,C-BNPA)PhCAHTNUGTERNN(UGN SMe2)] (3b)
were obtained in the same manner as 2a and 2b
(Scheme 2).
The crystal structures of 1, 2a and 3b have been deter-
mined by single-crystal X-ray diffraction analysis[9] with 2a
and 3b shown in Figures 1 and 2, respectively. The crystal
data of 1 is provided in the Supporting Information. The
structure of 3a was reported in
ꢀ
can then be converted quantitatively to the “roll-over” C H
activation product [Pt(N,C-Si-BNPA)Ph
N
heating in the presence of a donor ligand such as SMe2
(Scheme 2). Complex 2b can also be obtained directly from
the reaction of 1 with [{PtPh2ACHTNUGRTNEUNG(SMe2)}n] at extended reaction
times at ambient temperature. The facile and quantitative
conversion of the N,N-chelate complex 2a to the N,C-che-
late complex 2b was monitored by 1H NMR experiments
(see the Supporting Information). These confirmed that the
conversion of 2a to 2b follows the same intramolecular
ꢀ
“roll-over” C H activation mechanism established for the
conversion of the NPA complex [Pt
N
ACHTUNGTRENNUNG
(N,C-NPA)(L)R] reported recently by our group.[8] Thus, the
a
preliminary
communica-
tion.[7d] Efforts to obtain single
crystals of 2b have not been
successful. It can be seen from
the structure of 3b that the {Pt-
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
NPA)} chelate to be highly
strained and puckered out-of-
plane. The poor stability of the
Scheme 1. a) i) nBuLi, THF, ꢀ788C, ii) B
ACHTUNGTRENNUNG
(BrNPA), [PdACHTUNGTRENNUNG
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Dual Emission
FULL PAPER
Figure 1. A diagram showing the molecular structure of 2a with 50%
thermal ellipsoids.
Figure 3. The fluorescent titration spectra of 1 (1.0ꢃ10ꢀ5 m) by TBAF (0–
1.8 equiv) in CH2Cl2 at 298 K (lex =300 nm). Inset: The excitation peaks
of 1 (lem =405 nm) and 1 + excess Fꢀ (lem =360 nm).
transition band on the p-biphenyl-NACHTNUTRGNEUNG(1-naph)Ph donor
group.[4e] Hence, it is tempting to assign the fluorescent
emission of Si-BNPA to NPA!B charge transfer. However,
careful comparison of the spectra of Si-BNPA with that of
SiPh2(p-C6H4-BMes2)2, (Si-BB)[4f] which lacks
a donor
group, suggests that the emission of Si-BNPA most likely
originates from Mes!B charge transfer localised on the
phenyl–BMes2 chromophore. This assignment can be made
given that the emission of Si-BNPA and Si-BB are identical
in energy, shape and response to solvent polarity (see S5.3
and S5.4 in the Supporting Information). To establish the
origin of the singlet emission of the fluoride adduct, we syn-
thesised and recorded the emission spectrum of SiPh3(p-
C6H4-NPA), lacking a BMes2 acceptor group. The emission
spectrum of this molecule matches perfectly with that of the
fluoride adduct of Si-BNPA (see S5.4 in the Supporting In-
formation), confirming that the emission of the fluoride
adduct can be assigned to an NPA-based p–p* transition.
The “switch-on” response of Si-BNPA toward fluoride can
thus be explained by the presence of distinct emission path-
ways on the NPA donor and boron acceptor chromophores.
Emission from the acceptor is turned off by Fꢀ, activating
the singlet emission from the donor group as the next lowest
energy transition in the molecule. These assignments are fur-
ther supported by TDDFT calculations at the B3LYP/6-31+
G* level of theory (see the Supporting Information).[10]
Also significant was the observation of well-separated sin-
glet and triplet emission peaks from SiBNPA in solution at
77 K, as shown in Figure 4 (lFem =363 nm, lePm =463 nm, tP =
2.2 s in THF). The singlet peak responds to Fꢀ at 77 K in
much the same manner as at ambient temperature, while
the phosphorescent peak does not change significantly with
fluoride addition (Figure 4 and Supporting Information).
Also, the shape and energy of the triplet peak are similar to
Figure 2. A diagram showing the structure of 3b with 50% thermal ellip-
soids.
six-membered non-planar N,N-chelate ring thus provides the
driving force for the facile transformation of 2a and 3a to
the more stable five-membered N,C-chelate complexes 2b
and 3b, respectively. This structural difference was also
found to have a significant impact on the photophysical
properties of the complexes. The separation distance be-
tween the B atom and the Pt atom in 2a was found to be
14.8 ꢂ.
Luminescent propertiesLigands: Both BNPA and Si-BNPA
are fluorescent with lmax =392 (F=0.25) and 405 nm (0.12),
respectively, in THF at ambient temperature. The emission
of BNPA was assigned to an NPA!B charge-transfer transi-
tion, based on its quenching response upon the addition of
fluoride (NBu4F, TBAF).[7d] To establish the origin of the Si-
BNPA emission, we also conducted fluoride titration experi-
ments in both absorption and emission modes. The 405 nm
emission peak of Si-BNPA was quenched by Fꢀ with the ap-
pearance of a new emission peak at 362 nm, as shown in
Figure 3. This “turn-on” response has some resemblance to
that of SiPh2(p-Ph-BMes2)[p-biphenyl-NACTHNUTRGNE(NUG 1-naph)Ph] report-
ed previously by us, in which Fꢀ ions quench a very weak
N!B charge transfer shoulder band and enhance a p–p*
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S. Wang et al.
ing and unusual, the 77 K temperature required is certainly
not ideal for most applications. Fortunately, this can be ad-
dressed by chelation of a PtII centre to facilitate the triplet
emission at ambient temperature.
Complexes 2a and 2b: As shown in Figure 5, both N,N- and
N,C-chelate complexes 2a and 2b display dual emission at
ambient temperature in solution, but with a distinct differ-
ence. Our data reproducibly show that the dual emission of
2a is dominated by the singlet peak (lFem =399 nm with a
shoulder at ~362 nm) with a very weak triplet emission
peak (lPem =494 nm, t=12.2(1) ms at 298 K, 65(2) ms at
77 K), while the more stable, planar N,C-chelate 2b shows a
similar singlet peak at lFmax =392 nm with a shoulder at
~348 nm and strong green phosphorescence at lPem =495 nm
(t=12.1(1) ms at 298 K, 43(2) ms at 77 K). The phosphores-
cent quantum yield of 2b (FP =0.025) is much greater than
that of 2a (0.007). Clearly the drastic decrease of the triplet
decay lifetime from 2.2 s in the free ligand to less than
100 ms in the complexes is responsible for the observation of
phosphorescence at ambient temperature. As observed in
Si-BNPA, the lmax and the excitation profiles for both singlet
and triplet emission peaks of 2b are nearly identical except
that the triplet excitation peak has a broad shoulder
(Figure 6). For 2a, the excitation bands of the singlet and
triplet emissions overlap, with the broad triplet excitation
band being shifted considerably to a longer wavelength
(Figure 7). Nonetheless, the dual emission peak profiles of
2a and 2b do not change notably with excitation energy
(300–350 nm). Thus, we are confident that the dual emission
is from a single molecule that shares similar excitation
energy.
Figure 4. The normalised full emission and excitation spectra of 1 (1.0ꢃ
10ꢀ5 m) in THF at 77 K. For comparison, the emission spectrum of 1 +
excess TBAF in THF with lex =320 nm at 77 K is also shown.
the phosphorescent peak of NPA at 77 K. Thus, the singlet
and triplet peaks of 1 may be assigned to the BMes2 and
NPA groups, respectively. The excitation profiles for both
singlet and triplet emission peaks at 77 K are essentially
identical as shown in Figure 4, supporting that the dual
emission is indeed from the same molecule. Because of this
dual emission with long-lived green phosphorescence, com-
pound 1 emits white light at 77 K under UV irradiation in
solution and the solid state, which switches to a bright green
emission when the UV light is turned off (Figure 5). In con-
trast, BNPA does not show dual emission at all under the
same conditions. These results indicate that the electronic
separation of the donor and acceptor chromophores as well
as their well-matched excitation energies are the keys to
achieving well-resolved singlet–triplet dual emission from a
single molecule. Though this dual emission is both interest-
To confirm the origin of the dual emission peaks in 2a
and 2b, we conducted fluoride titration experiments in both
absorption and emission modes. The absorption spectral
change with Fꢀ for both complexes is similar to that of 1,
Figure 5. Comparison of the full emission spectra of 1 at 77 K and its Pt
complexes at 298 K in THF. Inset: Photos of 1, 2a and 2b in the solid
state (UV-lamp, 365 nm).
Figure 6. The excitation (298 K) and emission spectra of 2b and its Fꢀ
adduct in THF at 298 and 77 K, respectively.
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Figure 8. The emission spectral change of 2b with TBAF (0 to 2 equiv) in
CH2Cl2 (1.0ꢃ10ꢀ5 m) under N2 (lex =310 nm). Inset: photos showing the
colour change of 2b before and after Fꢀ binding.
Figure 7. Excitation (298 K) and emission spectra of 2a and its Fꢀ adduct
in THF at 298 and 77 K, respectively.
sity of the triplet peak. This same phenomenon was not ob-
served in 2a, perhaps owing to the much weaker phosphor-
escence. Although the mechanistic details of this unusual
singlet–triplet dual emission require more thorough experi-
mental and theoretical work, our preliminary data have es-
tablished that the PtII ion facilitates phosphorescence
through the well-known “heavy atom effect,”[11] such that
the singlet–triplet dual emission becomes visible at ambient
temperature. Furthermore, the chelate mode has been
shown to have a significant impact on the relative ratio of
the singlet and triplet emission peaks.
and the saturation point is reached with ꢁ1.5 equivalents of
Fꢀ. Because of the extremely high sensitivity of the triplet
emission peak toward oxygen for both complexes, it is very
difficult to obtain quantitatively meaningful titration data in
emission mode. Nonetheless, the general trends revealed by
our fluoride experiments are useful in the assignment of the
emission spectra. As shown in Figures 6 and 7 (full titration
data are provided in Supporting Information), addition of
fluorides to either 2a or 2b quenches the singlet peak while
enhancing the higher-energy shoulder band in the same
manner as the free ligand Si-BNPA. Thus, the singlet emis-
sion of 2a and 2b can be assigned to mesityl!B charge
transfer, while that of their fluoride adducts may be assigned
to NPA-based 1p!p* transitions. At 77 K, the emission
spectra of 2a, 2b and their Fꢀ adducts are composed almost
entirely of phosphorescent emission, due to the large de-
crease in phosphorescent quenching at low temperature.
Furthermore, the 77 K triplet peaks have well-resolved vi-
brational features resembling those of NPA, and thus can be
assigned to an NPA-centred 3p!p* transition (Figures 6
and 7). For comparison, the dual emission spectra of 1 at
77 K and 2a and 2b at 298 K are shown together in
Figure 5.
Complexes 3a and 3b. The contrasting photophysical prop-
erties of the linearly conjugated complexes 3a and 3b fur-
ther illustrate the importance of electronically separating
the donor and acceptor chromophores to achieve dual emis-
sion in a single molecule. As shown in Figure 9, the emission
of 3a and 3b at 298 K consists of a single phosphorescent
band at lmax =543 nm, t=25.0(1) ms for 3a and lmax
=
535 nm, t=11.9(1) us for 3b. Consistent with complexes of
2, the N,C-chelate complex 3b has a much greater quantum
It is worth noting that at ambient temperature, while the
phosphorescent band of 2a does not have a significant re-
sponse to fluorides, that of 2b experiences a huge intensity
gain with the addition of ~2 equiv Fꢀ, changing the emission
colour from pale green to bright yellow-green (Figure 8).
The phosphorescent intensity gain of 2b with fluoride can
be explained as follows. First, fluoride switches the lowest
1
energy singlet transition from BMes2 to the p!p* transi-
tion of the NPA group. Since the triplet emission is NPA-
centred 3p!p* emission, the enhanced population of the
1
NPA p!p* excited state increases the population of the
Figure 9. The emission spectra of 3a (lex =360 nm) and 3b (lex =368 nm)
at 298 K in CH2Cl2 (1.0ꢃ10ꢀ5 m) with and without Fꢀ. Inset: photos show-
ing the colour change of 3b before and after Fꢀ addition.
3
NPA p!p* excited state in the metal complex promoted
by the PtII centre, thus enhancing the overall emission inten-
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S. Wang et al.
efficiency (0.032) than the N,N-chelate 3a (0.004). However,
unlike 2a and 2b, in which the triplet emission originates
from the NPA chelate, the lack of any vibrational features
in the emission band of 3a and 3b indicates that phosphor-
escence in these complexes is 3MLCT (metal-to-ligand
charge transfer) in nature. Thus, direct conjugation of the
electron-deficient BMes2 group with the NPA moiety is be-
lieved to play a key role in facilitating this charge-transfer
transition. Furthermore, addition of fluorides to either 3a or
3b quenches this emission pathway with the appearance of a
Experimental Section
All reagents were purchased from Aldrich chemical company and used
without further purification. DMF, THF, Et2O, and hexanes were purified
by using an Innovation Technology Co. solvent purification system.
CH2Cl2 was freshly distilled over P2O5 prior to use. Reactions were car-
ried out under an inert atmosphere of dry N2 unless otherwise stated.
Thin-layer and flash chromatography were performed on silica gel. 1H
and 13C NMR spectra were recorded on Bruker Avance 400, 500 or
600 MHz spectrometers. Deuterated solvents were purchased from Cam-
bridge Isotopes and used without further drying. Excitation and emission
spectra were recoded using a Photon Technologies International Quanta-
Master Model 2 spectrometer. Phosphorescence spectra and phosphores-
cent decay lifetimes were measured on a Photon Technologies Interna-
tional Phosphorimeter (Time-Master C-631F). UV/Visible spectra were
recorded on an Ocean Optics CHEMUSB4 absorbance spectrophotome-
ter. Cyclic voltammetry experiments were performed with a BAS CV-
50W analyser with a scan rate of 0.2–1.0 Vsꢀ1 using ꢁ5 mg sample in dry
DMF (3 mL). The electrochemical cell was a standard three-compart-
ment cell composed of a Pt working electrode, a Pt auxiliary electrode,
and an Ag/AgCl reference electrode. CV measurements were carried out
at room temperature with 0.1m tetrabutylammonium hexafluorophos-
phate (TBAP) as the supporting electrolyte, with ferrocene/ferrocenium
as internal standard (E8=0.55 V). High-resolution mass spectra were ob-
tained with internal calibrants on an Applied Biosystems/MDS-Sciex
QSTAR XL mass spectrometer in electrospray mode. Crystal structures
were obtained at 180 K using a Bruker AXS Apex II X-ray diffractome-
ter (50 kV, 30 mA, MoKa radiation). Photoluminescent quantum yields
were measured using the optically dilute method (Aꢁ0.1) at room tem-
perature. Fluorescent quantum yields were measured in CH2Cl2 relative
to anthracene (Fr =0.36), while phosphorescent quantum yields were
new phosphorescent band at a shorter wavelength (lmax
=
487 nm, 3a, lmax =480 nm, 3b) that resembles the corre-
sponding phosphorescent bands of 2a and 2b. These can
thus be assigned to N,N-NPA and N,C-NPA 3p–p* transi-
tions, respectively (see Supporting Information). This phos-
phorescent switching changes the emission colour of solu-
tions of 3a and 3b from yellow to green (Figure 9). The pro-
posed switching mechanisms for the conjugated 2b and non-
conjugated 3b complexes are illustrated in Scheme 3.
measured in degassed CH2Cl2 relative to fac-[IrACHTUNGRTNEUNG(ppy)3] (Fr =0.4; ppy3 =2-
phenyl pyridine anion).[12] Molecular orbital and molecular geometry cal-
culations were performed using the Gaussian 03 program suite[10] by
using crystal structures as the starting point for geometry optimisations
where possible. Calculations were performed at the B3LYP level of
theory using 6–31+G* as the basis set for all atoms except Pt, for which
LAN2LDZ was used. (N-2’-(5’-Bromopyridyl)-7-azaindole)[13] and (p-di-
mesitylboronylphenyl)(p-bromophenyl)diphenylsilane[4e] were synthesised
by our previously reported procedures. [{PtPh2ACTHNUTRGNEUNG
(SMe2)}n] (n=2 or 3)[14]
Scheme 3. The proposed emission pathways and the most likely switching
mechanisms by Fꢀ in 2b and 3b.
was prepared by methods described in the literature. Compounds BNPA
and 3a were synthesised according to previously published procedures.[7d]
The synthetic and characterisation details of Si-BNPA (1), 2a and 2b,
crystal structural data of 1, 2a and 3b, luminescence and DFT computa-
tional data are provided in Supporting Information.
Conclusions
Herein we have reported the syntheses of the first examples
of luminescent metal-containing, non-conjugated donor–ac-
ceptor triarylborane compounds. Furthermore, we have
demonstrated that persistent singlet–triplet dual emission in
organoboron compounds can be achieved in solution at am-
bient temperature. This is possible through the use of both
spatially separated chromophores that share a common exci-
tation energy and metal chelation to facilitate phosphores-
cence. In addition, we have established that N,N- to N,C-
chelate mode switching can further increase the phosphores-
cent efficiency of these complexes by the relief of ring
strain, providing a means by which the ratio of singlet-to-
triplet emission may be controlled. Lastly, we have shown
that the singlet–triplet dual emission in the non-conjugated
system can be perturbed selectively by fluoride ions through
binding to the boron receptor site, thus providing a new
strategy for the development of sensing systems based on
singlet–triplet switching.
Acknowledgements
We thank the Natural Sciences and Engineering Research Council of
Canada for financial support. Z.M.H. would like to thank NSERC for a
Canada Graduate Scholarship.
[1] a) J. C. Collings, S. Y. Poon, C. L. Droumaguet, M. Charlot, C.
Katan, L. O. Pꢄlsson, A. Beeby, J. A. Msely, H. M. Kaiser, D. Kauf-
mann, W. Y. Wong, M. Blanchard-Desce, T. B. Marder, Chem. Eur.
J. 2009, 15, 198; b) Z. Yuan, N. J. Taylor, R. Ramachandran, T. B.
Marder, Appl. Organomet. Chem. 1996, 10, 305; c) Z. Yuan, C. D.
Entwistle, J. C. Collings, D. Albesa-Jovꢅ, A. S. Batsanov, J. A. K.
Howard, H. M. Kaiser, D. E. Kaufmann, S. Y. Poon, W. Y. Wong, C.
Jardin, S. Fathallah, A. Boucekkine, J. F. Halet, N. J. Taylor, T. B.
Marder, Chem. Eur. J. 2006, 12, 2758; d) C. D. Entwistle, T. B.
Marder, Angew. Chem. 2002, 114, 3051; Angew. Chem. Int. Ed. 2002,
41, 2927; e) C. D. Entwistle, T. B. Marder, Chem. Mater. 2004, 16,
4574; f) R. Stahl, C. Lambert, C. Kaiser, R. Wortmann, R. Jakober,
Chem. Eur. J. 2006, 12, 2358; g) N. Matsumi, Y. Chujo, Polym. J.
6136
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2008, 40, 77; h) M. Lequan, R. M. Lequan, K. C. Ching, J. Mater.
[6] a) D. Gupta, M. K. Deepak, Opt. Mater. 2006, 28, 295; b) S. Tao, Y.
Zhou, C. S. Lee, S. T. Lee, D. Huang, X. Zhang, J. Mater. Chem.
2008, 18, 3981; c) Y. Liu, M. Nishiura, Y. Wang, Z. Hou, J. Am.
Chem. Soc. 2006, 128, 5592.
Chem. 1991, 1, 997; i) Z. Q. Liu, Q. Fang, D. X. Cao, D. Wang, G. B.
Xu, Org. Lett. 2004, 6, 2933; j) L. M. Tao, Y. H. Guo, X. M. Huang,
C. K. Wang, Chem. Phys. Lett. 2006, 425, 10.
[2] a) H. Doi, M. Kinoshita, K. Okumoto, Y. Shirota, Chem. Mater.
2003, 15, 1080; b) T. Noda, Y. Shirota, J. Am. Chem. Soc. 1998, 120,
9714; c) Y. Shirota, J. Mater. Chem. 2005, 15, 75; d) T. Noda, H.
Ogawa, Y. Shirota, Adv. Mater. 1999, 11, 283; e) Y. Shirota, M. Ki-
noshita, T. Noda, K. Okumoto, T. Ohara, J. Am. Chem. Soc. 2000,
122, 11021; f) W. L. Jia, D. R. Bai, T. McCormick, Q. D. Liu, M.
Motala, R. Wang, C. Seward, Y. Tao, S. Wang, Chem. Eur. J. 2004,
10, 994; g) W. L. Jia, M. J. Moran, Y. Y. Yuan, Z. H. Lu, S. Wang, J.
Mater. Chem. 2005, 15, 3326; h) W. L. Jia, X. D. Feng, D. R. Bai,
Z. H. Lu, S. Wang, G. Vamvounis, Chem. Mater. 2005, 17, 164;
i) F. H. Li, W. L. Jia, S. Wang, Y. Q. Zhao, Z. H. Lu, J. Appl. Phys.
2008, 103, 034509/1–034509/6; j) A. Wakamiya, K. Mori, S. Yama-
guchi, Angew. Chem. Int. Ed. 2007, 46, 4237; k) M. Elbing, G. C.
Bazan, Angew. Chem. 2008, 120, 846; Angew. Chem. Int. Ed. 2008,
47, 834.
[3] a) S. Yamaguchi, T. Shirasaka, S. Akiyama, K. Tamao, J. Am. Chem.
Soc. 2002, 124, 8816; b) S. Yamaguchi, S. Akiyama, K. Tamao, J.
Am. Chem. Soc. 2001, 123, 11372; c) S. Solꢅ, F. P. Gabbaꢆ, Chem.
Commun. 2004, 1284; d) M. Melaimi, F. P. Gabbaꢆ, J. Am. Chem.
Soc. 2005, 127, 9680; e) C. W. Chiu, F. P. Gabbaꢆ, J. Am. Chem. Soc.
2006, 128, 14248; f) T. W. Hudnall, M. Melaimi, F. P. Gabbaꢆ, Org.
Lett. 2006, 8, 2747; g) T. W. Hudnall, F. P. Gabbaꢆ, J. Am. Chem. Soc.
2007, 129, 11978; h) T. W. Hudnall, Y. M. Kim, M. W. P. Bebbington,
D. Bourissou, F. P. Gabbaꢆ, J. Am. Chem. Soc. 2008, 130, 10890;
i) T. W. Hudnall, C. W. Chiu, F. P. Gabbai, Acc. Chem. Res. 2009, 42,
388, and references therein; j) Y. Kim, F. P. Gabbai, J. Am. Chem.
Soc. 2009, 131, 3363.
[4] a) A. Sundararaman, K. Venkatasubbaiah, M. Victor, L. N. Zakhar-
ov, A. L. Rheingold, F. Jꢇkle, J. Am. Chem. Soc. 2006, 128, 16554;
b) K. Parab, K. Venkatasubbaiah, F. Jꢇkle, J. Am. Chem. Soc. 2006,
128, 12879; c) F. Jꢇkle, Coord. Chem. Rev. 2006, 250, 1107; d) X. Y.
Liu, D. R. Bai, S. Wang, Angew. Chem. 2006, 118, 5601; Angew.
Chem. Int. Ed. 2006, 45, 5475; e) D. R. Bai, X. Y. Liu, S. Wang,
Chem. Eur. J. 2007, 13, 5713; f) S. B. Zhao, P. Wucher, Z. M.
Hudson, T. M. McCormick, X. Y. Liu, S. Wang, X. D. Feng, Z. H.
Lu, Organometallics 2008, 27, 6446; g) G. Zhou, M. Baumgarten, K.
Mꢈllen, J. Am. Chem. Soc. 2008, 130, 12477.
[5] a) Y. M. Cheng, Y. S. Yeh, M. L. Ho, P. T. Chou, P. S. Chen, Y. Chi,
Inorg. Chem. 2005, 44, 4594; b) K. K. W. Lo, K. Y. Zhang, S. K.
Leung, M. C. Tang, Angew. Chem. 2008, 120, 2245; Angew. Chem.
Int. Ed. 2008, 47, 2213; c) R. L. Blakley, M. K. DeArmond, J. Am.
Chem. Soc. 1987, 109, 4895; d) T. Suzuki, T. Kuchiyama, S. Kishi, S.
Kaizaki, H. D. Takagi, M. Kato, Inorg. Chem. 2003, 42, 785; e) Z.
Zhao, Y. Xing, Z. Wang, P. Lu, Org. Lett. 2007, 9, 547.
[7] a) G. J. Zhou, C. L. Ho, W. Y. Wong, Q. Wang, D. G. Ma, L. X.
Wang, Z. Y. Lin, T. B. Marder, A. Beeby, Adv. Funct. Mater. 2008,
18, 499; b) Y. M. You, S. Y. Park, Adv. Mater. 2008, 20, 3820; c) Q.
Zhao, F. Y. Li, S. J. Liu, M. X. Yu, Z. Q. Liu, T. Yi, C. H. Huang,
Inorg. Chem. 2008, 47, 9256; d) S. B. Zhao, T. McCormick, S. Wang,
Inorg. Chem. 2007, 46, 10965; e) Y. Sun, N. Ross, S. B. Zhao, K. Hus-
zarik, W. L. Jia, R. Y. Wang, S. Wang, J. Am. Chem. Soc. 2007, 129,
7510; f) E. Sakuda, A. Funahashi, N. Kitamura, Inorg. Chem. 2006,
45, 10670; g) Y. Sun, S. Wang, Inorg. Chem. 2009, 48, 3755.
[8] S. B. Zhao, R. Y. Wang, S. Wang, J. Am. Chem. Soc. 2007, 129, 3092.
[9] CCDC 717963 (1), 747964 (2a), and 717965 (3b) contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[10] Gaussian 03, Revision C.02, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgom-
AHCTUNGTREGeNNUN ry, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani,
N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cio-
slowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaro-
mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wall-
ingford CT, 2004.
[11] a) D. S. McClure, J. Chem. Phys. 1949, 17, 905; b) S. P. McGlynn, R.
Sunseri, N. Christodouleas, J. Chem. Phys. 1962, 37, 1818.
[12] a) K. A. King, P. J. Spellane, R. J. Watts, J. Am. Chem. Soc. 1985,
107, 1431; b) S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-
Razzaq, R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau, M. E.
Thompson, Inorg. Chem. 2001, 40, 1704.
[13] S-B. Zhao, T. McCormick, S. Wang, Inorg. Chem. 2007, 46, 10965.
[14] D. Song, S. Wang, J. Organomet. Chem. 2002, 648, 302.
Received: March 10, 2009
Published online: May 26, 2009
Chem. Eur. J. 2009, 15, 6131 – 6137
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6137