Triarylboron-functionalized 8-hydroxyquinolines and their aluminium(III) complexes

Article abstract of DOI:10.1039/c0cc04573j

The first examples of triarylboron-functionalized 8-hydroxyquinoline ligands and their aluminium complexes have been synthesized. These luminescent derivatives of the well-known electron transport material tris(8- hydroxyquinoline)aluminium (Alq₃) display enhanced electron-accepting ability relative to Alq₃, and can also be used as an indicator for small F^- and CN^- anions. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0cc04573j

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COMMUNICATION
Triarylboron-functionalized 8-hydroxyquinolines and their aluminium(III)
complexesw
Vladimir Zlojutro, Yi Sun, Zachary M. Hudson and Suning Wang*
Received 22nd October 2010, Accepted 26th January 2011
DOI: 10.1039/c0cc04573j
The first examples of triarylboron-functionalized 8-hydroxy-
quinoline ligands and their aluminium complexes have been
synthesized. These luminescent derivatives of the well-known
electron transport material tris(8-hydroxyquinoline)aluminium
(Alq3) display enhanced electron-accepting ability relative to
Alq3, and can also be used as an indicator for small FÀ and CNÀ
anions.
due to the susceptibility of the empty pp-orbital to attack from
small nucleophiles, giving accompanying changes in its
absorption or emission profile.9 Although Bpin-functionalized
Alq3 is known previously,5g,h triarylboron derivatives of Alq3
have not been reported in literature. The addition of a
dimesitylboron moiety to Alq3 could thus result in a multi-
functional complex capable of acting as an emissive or
electron-transport material in OLEDs, as well as an indicator
for small nucleophilic anions. To this end, we herein report the
synthesis and photophysical properties of the first examples of
8-hydroxyquinoline ligands functionalized with triarylboron
and their aluminium complexes.
The discovery of electroluminescence from thin films of
tris(8-hydroxyquinoline)aluminium (Alq3) in 1987 has since
led to a vast body of research into the design and fabrication
of organic light emitting diodes (OLEDs).1 This new technology
allows for the fabrication of efficient and durable displays with
high contrast that can be printed on wafer-thin substrates, and
is likely to bring profound changes to the display industry in
the years to come. Alq3 in particular is best known for its
electron transporting characteristics, and in this capacity has
been studied extensively for use in OLEDs.1,2 Due to a
substantial body of research and a better understanding of
the electronic characteristics of this material, many derivatives
now exist with well-controlled photophysical properties.3e,f,4,5
Nonetheless, electron transport materials (ETMs) for
OLEDs are in general less effective at transporting charge
than hole transport materials (HTMs). Electron mobilities in
organic materials are typically 1–2 orders of magnitude lower
than hole mobilities,6 often leading to reduced device efficiency
and lifetime due to poor carrier balance and an accumulation
of positive charge.7 As such, the development of improved
electron-transport materials remains a subject of considerable
research effort.
Ligands 1 and 2 were designed with rigid and flexible linkers
between the hydroxyquinoline and boron moieties, respec-
tively. Both of these linkers should promote electronic
communication, with compound 1 possessing a highly planar
acetylene
p system, and compound 2 incorporating a
hexylthiophene linker that is widely used to facilitate electron
transfer in optoelectronic materials.8a,10 The synthetic route to
each ligand is presented in Scheme 1. Free ligands were only
weakly fluorescent at room temperature in solid state and
solution (lmax = 413 nm for 1, 480 nm for 2) due to the
presence of the phenolic proton, which can quench fluorescent
emission by excited-state proton transfer.11 As this functional
group is also capable of hydrogen bonding, the response of
Triarylboron compounds have also been studied as
electron-transport materials due to the empty low-lying pp
orbital on the boron center, which allows them to act as
excellent electron acceptors. In addition, this functionality
contributes unique luminescent properties to their respective
chromophores, often promoting intense charge-transfer
luminescence.8 Furthermore, sterically protected dimesityl-
boron compounds can be used as selective chemical indicators
Scheme 1 Reagents and conditions: (i) n-BuLi, THF, À78 1C; (ii)
FBMes2, THF, À78 to 25 1C; (iii) CuI, DIPEA, Pd(PPh3)4, THF,
25 1C; (iv) piperidine, CH2Cl2, 25 1C; (v) B(pin)(OiPr), THF, À78 to
25 1C; (vi) Pd(OAc)2, SPhos, K3PO4, toluene, reflux; (vii) HCl,
CH3OH, reflux.
Department of Chemistry, Queen’s University, 90 Bader Lane,
Kingston, Ontario, Canada K7L 3N6.
E-mail: suning.wang@chem.queensu.ca
w Electronic supplementary information (ESI) available: See DOI:
10.1039/c0cc04573j
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3837–3839 3837

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these ligands to FÀ and CNÀ was also complex in emission
mode (see ESIw).
The Al(III) complexes of these ligands, Al(1)3 and Al(2)3
(Chart 1) were synthesized by reacting trimethylaluminium
with three equivalents of ligand in dry toluene. Consistent with
previous observations that mer-Alq3 is the dominating isomer
in solution with a C1 symmetry,12 both new Al(III) compounds
were found to exist as the mer-isomer, as evidenced by the
three distinct sets of peaks of the 2-, 3- and 4-positions of the
1
quinoline ring in the H NMR spectra (see ESIw).
The absorption and emission spectra of these compounds
are shown in Fig. 1, alongside those of Alq3 for comparison.
Due to the extended p conjugation and the presence of the
boron moiety, both new Al(III) complexes display considerably
stronger absorptions over a broader wavelength range than
the parent chromophore. Alq3, Al(1)3 and Al(2)3 show green,
yellow and orange fluorescence, respectively, in solid state and
solution (Fig. 1, Table 1).
Fig. 1 Absorption and normalized emission spectra of Alq3, Al(1)3
and Al(2)3. Inset: photographs of Alq3, Al(1)3 and Al(2)3 films and
CH2Cl2 solutions (10–5 M) under UV irradiation.
The red shift seen in both new complexes is likely due to
extension of the p-skeleton as well as addition of an overall
electron donating group to the C-5 position of the quinoline
moiety, which is known to cause a bathochromic shift.4a,5e In
addition, the low-lying p orbital on the boron center will lower
the LUMO level of the complex.8g This was supported by
DFT calculation results at the B3LYP/6-31G* level of theory,
which indicate that the boron centre makes a large contri-
bution to the LUMO level of both new complexes. In addition,
the HOMO and LUMO orbitals contain substantial electron
density on the phenoxide and pyridyl rings, respectively
(see ESIw), as is the case for Alq3.3 The emission quantum
efficiencies of Al(1)3 and Al(2)3 are lower than that of Alq3,
attributable to the substitution effect at the C5 position of the
quinoline.5,8 The relatively low quantum yield of Al(2)3 is
likely due to the presence of the hexyl chains that reduce the
rigidity of the compound and increase the rate of nonradiative
decay from the excited state by vibronic coupling. Compared
to Alq3, the new boron-functionalized complexes Al(1)3 and
Al(2)3 showed positive shifts in their first reduction potentials,
with values of À2.11 and À2.22 V, respectively (relative to
FeCp02/+), supporting improved electron-accepting ability.
Interestingly, both new aluminium complexes display three
distinct reduction peaks, suggesting that interactions may exist
between the ligands, making successive reductions more
difficult than the first.14 DFT calculations further suggest that
the three lowest unoccupied molecular orbitals of both Al(1)3
Fig. 2 The fluorescent titration spectra of Al(1)3 (left, lex = 420 nm)
and Al(2)3 (right, lex = 395 nm) by NEt4CN in CH2Cl2 (1.0 Â 10À5 M)
at 298 K.
and Al(2)3 are each based on the p orbital of one boron center
and the p*-orbital of the attached quinoline ring. Due to the
mer geometry of the complexes, however, the three LUMO
levels are nondegenerate. It is interesting that a similar effect is
not observed for Alq3, which displays only a single reduction
peak, suggesting that successive reduction of the complex is
not favoured in the absence of the boron centers.
The Lewis acidity of triarylboranes not only allows these
compounds to act as electron acceptors, but as receptors for
small anions as well.9 We therefore tested the ability of both
Al(1)3 and Al(2)3 to act as indicators for fluoride and cyanide,
using tetrabutylammonium fluoride (TBAF) and tetra-
ethylammonium cyanide (TEACN) as titrants (Fig. 2). In
absorption mode, both complexes show quenching of the
strong absorption band at B375 nm, that arises from a
triarylboron-based charge-transfer from the filled p-orbitals
of the mesityl groups to the boron centre as confirmed by
DFT calculations. As well, both complexes exhibit partial
quenching of the low-energy band at B430 nm, representing
the quinoline-based HOMO–LUMO charge-transfer transition
which has some contribution from the boron atom as well,
based on DFT results. In fluorescence mode, the emission of
both compounds is quenched significantly by the addition of
either anion.
Since blocking of the boron centre alone is not sufficient
cause for this loss of emission, as Alq3 itself is highly emissive
in the absence of the boron centre entirely, the emission
quenching by CNÀ for the two complexes is likely caused by
Chart 1
c
3838 Chem. Commun., 2011, 47, 3837–3839
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Table 1 Photophysical properties of Alq3 and its derivatives
Compound
l
abs/nm (log e/MÀ1 cmÀ1
)
lema/nm
F Soln.b/solidc
E1/2red d/V
HOMOe/eV
LUMO/eV
Alq3
Al(1)3
Al(2)3
260 (5.15), 393 (3.93)
266 (5.17), 370 (5.05), 427 (4.73)
270 (4.99), 337 (4.78), 373 (4.75), 430 (4.44)
507
533
570
0.12/0.14
0.10/0.06
0.01/0.02
À2.40
À2.11
À2.22
À5.24
À5.21
À5.13
À2.40
À2.68
À2.60
In CH2Cl2 at 1 Â 10À5 M. Relative to Alq3 = 0.12.13 c Measured using an integration sphere. Measured in DMF relative to FeCp2
.
a
b
d
0/+
e
Determined from the reduction potential and the optical energy gap.
the presence of highly efficient thermal relaxation pathways.
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Al(1)3 and Al(2)3 are stronger electron acceptors than Alq3,
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This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3837–3839 3839