Switchable three-state fluorescence of a nonconjugated donor-acceptor triarylborane

Article abstract of DOI:10.1021/ol102749y

A nonconjugated fluorescent molecule with a triarylboron acceptor and an alkylamine donor has been found to display bright green fluorescence due to charge transfer through space, which can be reversibly deactivated by blocking either the donor or accepto

Full text of DOI:10.1021/ol102749y

ORGANIC
LETTERS
2011
Vol. 13, No. 2
300-303
Switchable Three-State Fluorescence of
a Nonconjugated Donor-Acceptor
Triarylborane
Zachary M. Hudson, Xiang-Yang Liu, and Suning Wang*
Department of Chemistry, Queen’s UniVersity, 90 Bader Lane, Kingston, Ontario,
Canada K7L 3N6
suning.wang@chem.queensu.ca
Received November 12, 2010
ABSTRACT
A nonconjugated fluorescent molecule with a triarylboron acceptor and an alkylamine donor has been found to display bright green fluorescence
due to charge transfer through space, which can be reversibly deactivated by blocking either the donor or acceptor site. Binding of the
fluoride ion to boron switches the fluorescence color to sky blue, while protonation of the amine with acid switches the emission color to the
purple fluorescence of the acceptor chromophore.
Triarylboranes have been the subject of considerable recent
research as selective sensors for fluoride ions, as these Lewis
acidic compounds are capable of accepting electrons through
the empty pπ orbital on the boron center. When protected
by bulky aryl substituents, these materials show high
selectivity for F- binding over other anions such as Cl- and
Br-, and thus, show promise for applications such as the
detection of fluoride in drinking water with reduced interfer-
ence from other analytes. Furthermore, the electron-accepting
boron atom readily promotes intramolecular charge-transfer
(CT) luminescence, thereby facilitating rapid visual detection
of F- by visible or fluorescent color change.1
diodes (OLEDs).2 Furthermore, it has recently been dem-
onstrated that fluoride ion binding to a boron center can be
used to establish a p-n junction in LED devices.2f Since
fluoride ions can bind selectively to the boron center and
block the empty pπ orbital, a fluoride salt can thus act as a
simple probe to determine the impact of the boron moiety,
providing insight into the electronic properties of a material.
The luminescent and anion-binding properties of triarylbo-
ranes based on a 1,8-naphthyl core have been the subject of
several recent reports. When used as anion sensors, the
geometry of this linker allows for the preparation of biden-
tate Lewis acids with greatly increased binding constants
over simple triarylboranes (Scheme 1).3 Also, when donor
and acceptor chromophores are connected by this bridge, the
rigidity of the linker forces the π-systems of the chro-
Similarly, the electron-accepting ability of the boron center
has led to their development as highly efficient electron
transport materials and emitters in organic light-emitting
* To whom correspondence should be addressed. Fax: (+1)613-533-
6669.
(2) (a) Shirota, Y. J. Mater. Chem. 2005, 15, 75. (b) Li, F. H.; Jia, W. L.;
Wang, S.; Zhao, Y. Q.; Lu, Z. H. J. Appl. Phys. 2008, 103, 034509/1. (c)
Wakamiya, A.; Mori, K.; Yamaguchi, S. Angew. Chem., Int. Ed. 2007, 46,
4237. (d) Zhou, G. J.; Ho, C. L.; Wong, W. Y.; Wang, Q.; Ma, D. G.;
Wang, L. X.; Lin, Z. Y.; Marder, T. B.; Beeby, A. AdV. Funct. Mater.
2008, 18, 499. (e) Hudson, Z. M.; Sun, C.; Helander, M. G.; Amarne, H.;
Lu, Z.-H.; Wang, S. AdV. Funct. Mater. 2010, 20, 3426. (f) Hoven, C. V.;
Wang, H.; Elbing, M.; Garner, L.; Winkelhaus, D.; Bazan, G. C. Nature
Mater. 2010, 9, 249.
(1) For recent reviews, see: (a) Wade, C. R.; Broomsgrove, A. E. J.;
Aldridge, S.; Gabbaï, F. P. Chem. ReV. 2010, 110, 3958. (b) Hudnall, T. W.;
Chiu, C.-W.; Gabbaï, F. P. Acc. Chem. Res. 2009, 42, 388. (c) Hudson,
Z. M.; Wang, S. Acc. Chem. Res. 2009, 42, 1584. (d) Yamaguchi, S.;
Wakamiya, A. Pure Appl. Chem. 2006, 78, 1413. (e) Entwistle, C. D.;
Marder, T. B. Chem. Mater. 2004, 16, 4574. (f) Ja¨kle, F. Chem. ReV. 2010,
110, 3985, and references therein.
10.1021/ol102749y  2011 American Chemical Society
Published on Web 12/13/2010

in a much higher yield than more commonly used Suzuki
coupling reactions, and it is possible that the steric demands
of the substrate play a role in the efficiency of this coupling
step. The steric congestion in this molecule is apparent from
the crystal structure of 1, with torsion angles of 57.2° and
47.4° and bend angles of 118.02° and 115.22° between the
naphthyl linker and the phenyl rings of the donor and
acceptor chromophores, respectively (Figure 1). Incorporation
Scheme 1. Synthesis of Compound 1
mophores out of coplanarity with that of the linker, promot-
ing charge-transfer through-space rather than through a
directly conjugated π-system.4 The fluorescence of these
materials is switchable between dual emission pathways, as
the charge-transfer fluorescence exhibited by the pure
compound can readily be deactivated if fluoride is added.
This switches the emission color of the sample, leading to
higher-energy π-π* fluorescence from the remaining donor
chromophore.
We have herein extended this concept to a fluorescent
material switchable between triple emission pathways,
incorporating donor and acceptor fluorophores both capable
of acting as receptor sites (compound 1, Scheme 1). Using
a more basic dimethylarylamine unit, this compound can be
readily protonated to block the filled p orbital of the donor
group. In this way, blocking of either the donor group with
acid or the acceptor group with fluoride disrupts through-
space charge-transfer fluorescence, causing the material to
emit with the color of an alternate chromophore. In this way,
these simple stimuli allow for facile switching between three
emissive excited states.
Figure 1. Crystal structure of 1 with 35% thermal ellipsoids.
of a rigid linker forces the donor and acceptor atoms into
close proximity, with a B···N separation distance of 5.55 Å.
The through-space donor-acceptor charge-transfer transi-
tion may be observed as a weak low-energy band centered
at ∼386 nm in the UV-visible absorption spectrum of 1 in
CH2Cl2 (see Supporting Information, SI). This spectrum also
features an intense band at 333 nm, attributed to charge
transfer from the filled π-orbitals on the mesityl and naphthyl
rings to the boron center. These assignments have been
corroborated by theoretical absorption spectra calculated
using time-dependent density functional theory (TD-DFT,
see SI), confirming that the intense absorption band is not
in fact due to donor-acceptor charge transfer as is commonly
seen in directly conjugated systems.5 Due to the ambipolar
nature of 1, this compound undergoes both reversible
reduction and oxidation by cyclic voltammetry, at potentials
This material is best prepared by sequential Negishi
coupling of the donor and acceptor chromophores to 1,8-
diiodonaphthalene, involving one-pot metal-halogen ex-
change and formation of an organozinc intermediate, fol-
lowed by palladium-catalyzed C-C bond formation with the
appropriate aryl halide. This method was found to proceed
red
ox
0/+
of E1/2 ) -2.41 V and E1/2 ) +0.34 V vs FeCp2 in
DMF.
Compound 1 exhibits bright green charge-transfer fluo-
rescence (λmax ) 529 nm in CH2Cl2) with a quantum
efficiency of 0.43, the highest observed to date for a molecule
displaying through-space charge transfer to boron (Figure
2). The charge-transfer fluorescence is supported by the
strong solvent-dependence of the emission spectrum (e.g.,
λem ) 450 nm in hexanes, 560 nm in acetone; see SI). The
considerable increase in quantum efficiency over previously
reported nonconjugated donor-acceptor triarylboron com-
pounds is likely due to both the improved donor strength of
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Org. Lett., Vol. 13, No. 2, 2011
301

DFT calculations on compound 1 and its H+ and F-
adducts at the B3LYP/6-31G* level of theory provide insight
into the nature of fluorescence in each of these emissive
states. Calculations indicate that the donor and acceptor
chromophores make the largest contributions to the HOMO
and LUMO, respectively, consistent with through-space p(N)
r p(B) charge transfer as the lowest-energy emission
pathway. However, once the p orbital on boron is blocked
using fluoride, the lowest energy transition of [1·F]- becomes
a charge transfer from the (F)BMes2-phenyl (HOMO) to the
naphthyl (LUMO), producing blue fluorescence. If instead
the filled p orbital on nitrogen is blocked using acid, the
lowest energy transition of [1·H]+ is from the BMes2phenyl-
naph (HOMO) to the phenyl-naph (LUMO), producing
purple fluorescence with some charge transfer character. The
trend of the emission energy observed for these species
agrees with that of the calculated HOMO-LUMO gap
shown in Figure 3.
Figure 2. Normalized fluorescent emission spectra of 1 and its acid
and fluoride adducts. Inset: Photographs of the corresponding
solutions at 10-5 M in CH2Cl2, λex ) 365 nm.
the dialkylamine as well as the close donor-acceptor spacing
imposed by the naphthyl linker.
When tetrabutylammonium fluoride (TBAF) is added to
a solution of 1, the emission color is observed to rapidly
switch from green to sky blue with similar quantum
efficiency (λmax ) 469 nm, Φ ) 0.42). This is accompanied
by an increase in intensity of the strong 333 nm band in the
UV-visible spectrum of 1, likely due to a reduction in the
CT contributions of this transition and increase in π-π*
character, improving the overall overlap between participat-
ing MOs. Approximately 20 equiv of fluoride are required
to achieve complete color switching, a higher level than in
the case of most organoboranes but comparable to similar
sterically congested materials reported previously,4,6 corre-
sponding to a binding constant K of (1.7 ( 0.5) × 104 M-1.
1H and 19F NMR titrations of 1 with TBAF indicate the
quantitative conversion of 1 to [1·F]- without intermediates.
While the selectivity of dimesitylboranes for fluoride ions
over Cl-, Br-, and others is well documented, these
compounds are well-known to be responsive to cyanide ions
as well.7 Titration with tetraethylammonium cyanide (TEACN)
in CH2Cl2 thus triggers a similar fluorescent spectral change
to that induced by F- (λmax ) 466 nm, K ) (1.6 ( 0.5) ×
104 M-1). As the response to both anions is similar, further
detailed studies will focus on fluoride as a trigger for
fluorescent change here.
Figure 3.
HOMO-LUMO diagrams of 1, [1·F]-, and [1·H]+.
Remarkably, the fluorescent responses to both acid and
fluoride in this system are fully reversible by applying the
opposite trigger. Despite possessing receptor sites for both
ions, the compound exists instead as free 1 when treated with
both stimuli, and the green fluorescence is preserved.
Similarly, after treatment of 1 with excess acid or fluoride,
the CT emission can be restored by titration with the other
trigger (Figure 4b and 4d). No sample degradation is
observed in either case, and these processes can be cycled
several times in either direction without loss of fluorescent
intensity. These results are in constrast with previous studies,
in which it was shown that protonation of a proximal tertiary
amine could promote hydrolysis of an adjacent triarylborane.9
The removal of fluoride ions from triarylboranes using protic
acids including water has been reported previously.10
1H and 19F NMR titrations of [1·F]- with acid or [1·H]+
with fluoride provide insight into these processes. While
Conversely, addition of a strong acid (HBF4 or HBArF )
4
to a CH2Cl2 solution of 1 results in quantitative protonation
after addition of ∼1.5 equiv of acid, switching the emission
color of the sample to bright purple (λmax ) 398 nm). This
is accompanied by a large increase in the quantum yield of
the sample (Φ ) ∼1.0), which is consistent with other
triarylboron compounds conjugated to highly electron-rich
π-systems.6,8 As with [1·F]-, NMR data indicate clean
formation of [1·H]+ on addition of acid.
(6) Zhao, S.-B.; Wucher, P.; Hudson, Z. M.; McCormick, T. M.; Liu,
X.-Y.; Wang, S.; Feng, X.-D.; Lu, Z.-H. Organometallics 2008, 27, 6446
.
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Bresner, C.; Chislett, V.; Fallis, I. A.; Thompson, A. L.; Vidovic, D.;
Aldridge, S. Inorg. Chem. 2010, 49, 157.
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C.-W.; Gabbaï, F. P. Dalton Trans. 2008, 814. (c) Hudnall, T. W.; Gabbaï,
F. P. J. Am. Chem. Soc. 2007, 129, 11978. (d) Broomsgrove, A. E. J.; Addy,
D. A.; Bresner, C.; Fallis, I. A.; Thompson, A. L.; Aldridge, S. Chem.sEur.
J. 2008, 14, 7525.
(10) (a) Sun, Y.; Ross, N.; Zhao, S.-B.; Huszarik, K.; Jia, W.-L.; Wang,
R.-Y.; Macartney, D.; Wang, S. J. Am. Chem. Soc. 2007, 129, 7510. (b)
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302
Org. Lett., Vol. 13, No. 2, 2011

Figure 4
.
Fluorescent titration spectra for 1.0 × 10-5 M solutions of 1 in CH2Cl2 (λex ) 365 nm): (a) 1 titrated with 20 equiv of TBAF;
(b) 21 equiv of TBAF added to 1 and then titrated with 14 equiv of HBF4; (c) 1 titrated with 1.8 equiv of HBF4; (d) 1.8 equiv of HBF4
added to 1 and then titrated with 3.0 equiv of TBAF.
chelation of HF by a compound with spatially proximate
phosphine and borane groups has been recently reported,11
the 5.55 Å B···N separation distance in this case is too long
to support chelation of HF in the presence of both analytes.
Instead, it appears that fluoride abstraction from [1·F]- occurs
via a B-F···H intermediate when acid is added, as evidenced
by the appearance of a broad peak from -146 to -153 ppm
in the 19F NMR spectrum of [1·F]- on titration with HBF4.
This is similar in chemical shift to previously studied
hydrogen bond adducts of fluoride.12 Once sufficient acid is
added to completely switch the emission color back to green,
this peak disappears, indicating restoration of the material
to the original form of 1.
may not seem intuitively favorable due to their stronger
basicities in water, this is in fact consistent with the
dramatically higher basicity of fluoride in anhydrous aprotic
solvent.13
In summary, we have demonstrated a highly luminescent,
nonconjugated donor-acceptor triarylboron compound that
can rapidly and reversibly be switched between three
fluorescent states using either acid or fluoride. Moreover,
each of these stimuli can also be used to reverse the effects
of the other, making it possible to cycle compound 1 between
the three fluorescent colors using only two simple triggers.
Acknowledgment. The authors gratefully acknowledge the
Natural Sciences and Engineering Research Council of
Canada for financial support.
A similar intermediate is observed in the 19F NMR
spectrum of [1·H]+ titrated with F-, though the peak is much
sharper, appearing further upfield between -181 to -185
ppm and likely arising from a N-H···F intermediate (see
SI). Once again, after a sufficient amount of fluoride has
been added, compound 1 is restored to its original form.
Though the deprotonation of a dialkylarylamine by fluoride
Supporting Information Available: Synthetic details,
absorption spectra, TD-DFT data, 1H and 19F NMR titrations
of 1 with acid and fluoride, UV-visible and fluorescence
titration data for the reaction of 1 with CN-, and complete
X-ray crystallographic data. This material is available free
of charge via the Internet at http://pubs.acs.org.
(11) Moebs-Sanchez, S.; Saffon, N.; Bouhadir, G.; Maron, L.; Bourissou,
D. Dalton Trans. 2010, 39, 4417.
OL102749Y
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2747. (b) Shenderovich, I. G.; Smirnov, S. N.; Denisov, G. S.; Gindin, V. A.;
Golubev, N. S.; Dunger, A.; Reibke, R.; Kirpekar, S.; Malkina, O. L.;
Limbach, H.-H. Ber. Bunsenges. Phys. Chem. 1998, 102, 422.
(13) Christe, K. O.; Wilson, W. W.; Wilson, R. D.; Bau, R.; Feng, J. A.
J. Am. Chem. Soc. 1990, 112, 7619.
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