Enhancing the photochemical stability of N,C-Chelate boryl compounds: C-C bond formation versus C=C bond cis,trans-isomerization

Article abstract of DOI:10.1021/ja906430s

N,C-Chelate boron compounds such as B(ppy)Mes₂ (ppy = 2-phenylpyridyl, Mes = mesityl) have been recently shown to undergo a facile and reversible C-C/C-B bond rearrangement upon irradiation with UV-light, quenching the emission of the sample an

Full text of DOI:10.1021/ja906430s

Published on Web 09/21/2009
Enhancing the Photochemical Stability of N,C-Chelate Boryl
Compounds: C-C Bond Formation versus CdC Bond
cis,trans-Isomerization
Chul Baik, Zachary M. Hudson, Hazem Amarne, and Suning Wang*
Department of Chemistry, Queen’s UniVersity, Kingston, Ontario, K7L 3N6, Canada
Received July 30, 2009; E-mail: wangs@chem.queensu.ca
Abstract: N,C-Chelate boron compounds such as B(ppy)Mes2 (ppy ) 2-phenylpyridyl, Mes ) mesityl)
have been recently shown to undergo a facile and reversible C-C/C-B bond rearrangement upon
irradiation with UV-light, quenching the emission of the sample and limiting their use in optoelectronic
devices. To address this problem, four molecules have been synthesized in which the π-conjugation
is extended using either vinyl or acetylene linkers. These compounds, (ph-CtC-ppy)BMes2 (B1A),
(ph-CHdCH-ppy)BMes2 (B1), [p-bis(ppy-CHdCH)benzene](BMes2)2 (B2), and [1,3,5-tris(ppy-
CHdCH)benzene](BMes2)3 (B3) have been fully characterized by NMR and single-crystal X-ray
diffraction analyses. All four compounds are light yellow and emit blue or blue-green light upon UV
irradiation. The acetylene compound B1A has been found to exhibit photochemical instability the same
as that of the parent chromophore B(ppy)Mes2. In contrast, all of the olefin-substituted compounds
are photochemically stable, instead undergoing cis-trans isomerization exclusively upon exposure to
UV light. Experimental and TD-DFT computational results establish that the presence of the olefinic
bond in B1-B3 provides an alternate energy dissipation pathway for the B(ppy)Mes2 chromophore,
stabilizing the molecule toward photochromic switching via cis-trans isomerization. Furthermore, the
incorporation of a cis-trans isomerization pathway may prove to be a useful strategy for the stabilization
of photochemically unstable chromophores in other π-systems as well.
Introduction
nomenon has been demonstrated by a number of research
groups on several classes of four-coordinate boron chelate
Conjugated organic materials are a key component of a
wide variety of optoelectonic devices. For applications in
organic light-emitting diodes (OLEDs),1 organic photovol-
taics (OPVs),2 and organic thin film transistors (OTFTs),3 it
is necessary to develop conjugated molecules with either
electron-transporting or hole-transporting properties. For this
reason, conjugated boron-containing compounds have at-
tracted much recent attention, due to their highly emissive
nature and excellent electron transport functionality. In the
case of conjugated three-coordinate triarylboron compounds,
this electron-transporting ability can largely be attributed to
the empty pπ orbital on the boron center, which stabilizes
the formation of radical anions Via conjugation with the
π-orbitals of the attached aryl groups.4,5 For four-coordinate
arylboron compounds, however, the π-conjugated backbone
is responsible for electron transport. In this case, boron
chelation acts to enhance the π-conjugation of the system,
thereby enhancing its electron-accepting ability. This phe-
chromophores, including N,O-,6a-f N,N-6g-j and N,C-chelate6k
π-systems.
Despite their potential usefulness in optoelectronic materi-
als, examples of conjugated N,C-chelate boryl compounds
remain rare. Previously known N,C-chelate ligands used for
this purpose are based on either thienylthiazole6k or 2-phe-
nylpyridine (ppy),7 which readily form highly emissive
tetrahedral boron compounds with BMes2 (Mes ) mesityl).
These N,C-chelate boron compounds are of particular interest
because they are isoelectronic with the well-known fluorene
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10.1021/ja906430s CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 14549–14559 14549

A R T I C L E S
Baik et al.
Scheme 1
chemical rearrangement under nitrogen or air. Furthermore,
we have found that cis-trans photoisomerization plays a key
role in this stabilization effect. The cis-trans isomerization
of olefins is essential for many photochemical processes, such
as vision (vitamin A)10a-c and the absorption of UV radiation
in natural and synthetic sunscreens.10d It has also been
exploited in conjugated organic and organometallic systems
as a means for switching and controlling electronic com-
munication, luminescence and molecular motion.10e-g Our
findings on the stabilization of the photochromic B(ppy)Mes2
chromophore Via π-conjugated olefinic bonds adds a new
dimension to the utility of the olefin cis-trans isomerization
phenomenon. For comparison, we have also studied the
impact of conjugation of the B(ppy)Mes2 chromophore with
a phenylacetylene substituent. The details of our investiga-
tions are presented herein.
chromophore, which is widely used in organic materials due
to its rich photophysical and electronic properties.8
However, our group has shown that B(ppy)Mes2 and its
derivatives are highly sensitive to light.7 Exposure to ambient
or ultraviolet light under an N2 atmosphere causes these
compounds to undergo a rapid structural and color change
both in solution and the solid state, with the reversible
formation and breaking of carbon-carbon bonds. Although
this photochromic behavior is unusual and may find applica-
tions in optical switching, the products of this transformation
are unstable toward oxygen. On exposure to light under air,
the newly formed C-C-bonded species reacts rapidly with
O2, eliminating the boron moiety from the chelate site
(Scheme 1). Hence, before these highly luminescent materials
can be successfully incorporated into devices such as OLEDs,
the photostability of the N,C-chelate boryl chromophore must
be addressed. Furthermore, it is necessary to understand the
various factors that influence the photochemical stability and
photochromic switching behavior of the N,C-chelate boryl
system as a point of fundamental research.
As part of our continuing efforts to understand and exploit
the unique properties of N,C-chelate boryl systems, we have
expanded our investigation to conjugated polyboryl systems
containing several B(ppy)Mes2 units and olefinic bonds.
Using olefinic bonds to build extended π-conjugated systems
and influence their optoelectronic properties has been dem-
onstrated extensively.9 In this case, however, we have found
that the olefinic bonds impart remarkable photochemical
stability to an otherwise unstable system, preventing these
mono- and polyboryl compounds from undergoing photo-
Experimental Section
General Procedure. All experiments were performed under a
nitrogen atmosphere in a Vacuum Atmospheres drybox or by
standard Schlenk technique unless otherwise noted. THF,
diethylether, and DMF were purified using the solvent purifica-
tion system (Innovation Technology, Inc.). The 1H and 13C NMR
spectra were recorded on Bruker Avance 300, 400, or 500 MHz
spectrometers. UV-vis spectra were recorded on an Ocean
Optics UV-visible spectrometer. Excitation and emission spectra
were recorded on a Photon Technologies International Quanta
Master model C-60 spectrometer. High-resolution mass spectra
(HRMS) were obtained from a Waters/Micromass GC-TOF EI-
MS spectrometer. Elemental analyses were performed at the
Canadian Microanalytical Service Ltd. Cyclic voltammetry was
performed using a BAS CV-50W analyzer with a scan rate of
200 mV/s and a typical concentration of 5 mg of the compound
in 3.0 mL of DMF using 0.10 M NBu4PF6 (TBAP) as the
supporting electrolyte and a Ag/AgCl reference electrode. The
ferrocenium/ferrocene couple was used as the internal standard
(Eo ) 0.56 V). 6-Bromo-3-pyridinecarboxaldehyde,11 1,4-
bis(diethoxyphosphorylmethyl)benzene, 1,3,5-tris(diethoxyphos-
phoryl methyl)benzene,12 2-bromo-5-iodopyridine,13 and B(p-
py)Mes27 were prepared by previously published methods in the
literature. The synthetic details for the intermediates 1, 3, 5, 6
and 8 are provided in the Supporting Information.
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14550 J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009

Photochemical Stability of N,C-Chelate Boryl Compounds
A R T I C L E S
Synthesis of (Ph-CH)CH-ppy)BMes2 (B1). 1,4-bis((E)-2-(6-bro-
mopyridin-3-yl)vinyl)benzene (2). A mixture of compound 1 (0.80
g, 3.05 mmol) and diethyl benzylphosphonate (0.70 g, 3.05
mmol) in THF (15 mL) was slowly added to a THF solution of
potassium tert-butoxide (0.40 g, 3.66 mmol). The solution was
stirred overnight, and the THF evaporated in Vacuo. After the
addition of water, the mixture was extracted with diethyl ether.
The crude product was then purified by column chromatography
(ethyl acetate:hexanes ) 1:4) to afford compound 2 in 85% yield
crude product was then purified by column chromatography
(CH2Cl2/hexanes ) 1:3) to afford compound B1A in 46% yield
1
(0.23 g). H NMR (δ, ppm, CD2Cl2): 8.70 (s, 1H), 8.14 (d, J )
7.0 Hz, 1H), 8.04 (d, J ) 8.0 Hz, 1H), 7.93 (d, J ) 8.0 Hz,
1H), 7.77 (d, J ) 7.0 Hz, 1H), 7.57 (m, 2H), 7.42 (m, 3H), 7.35
(m, 2H), 6.68 (s, 4H), 2.20 (s, 6H), 1.82 (s, 12H); 13C{1H} NMR
(δ, ppm, CD2Cl2): 158.4, 148.6, 143.3, 140.2, 134.8, 134.3,
132.0, 131.6, 131.1, 130.2, 129.6, 128.9, 125.8, 122.4, 122.1,
118.6, 117.9, 94.4, 84.6, 25.1, 20.7. Anal. Calcd for C43H48BN:
C 87.59; H 8.20; N 2.38. Found: C 86.99; H 7.62; N 2.60.
B1A′. The compound was prepared by following the general
1
(0.87 g). H NMR (δ, ppm CD2Cl2): 8.32 (d, J ) 2.0 Hz, 1H),
7.97 (dd, J ) 8.0 Hz, J ) 2.0 Hz, 1H), 7.72 (dd, J ) 8.0 Hz, J
) 1.2 Hz, 1H), 7.65 (d, J ) 8.0 Hz, 1H), 7.60 (m, 3H), 7.44
(m, 3 H), 7.32 (m, 2H), 7.30 (d, J ) 16.4 Hz, 1H), 7.20 (d, J )
16.4 Hz, 1H); 13C{1H} NMR (δ, ppm, CD2Cl2): 157.3, 148.5,
141.4, 137.1, 133.7, 132.7, 132.1, 131.9, 131.2, 130.0, 129.1,
128.6, 127.9, 127.1, 124.9, 124.8, 122.0. HRMS calcd for
C19H14BrN: m/z 335.0310; found: 335.0311.
1
procedures for NMR monitoring of the photolysis process. H
NMR (δ, ppm, C6D6): 9.17 (s, 1H), 7.71 (d, J ) 8.0 Hz, 2H),
7.58 (d, J ) 8.0 Hz, 1H), 7.25 (m, 3H), 7.22 (d, J ) 8.0 Hz,
1H), 7.10 (t, J ) 7.6 Hz, 1H), 7.05 (s, 1H), 6.99 (m, 3H), 6.86
(s, 1H), 6.01 (s, 1H), 5.53 (s, 1H), 2.98 (s, 3H), 2.25 (s, 3H),
2.20 (s, 3H), 2.05 (s, 3H), 1.81 (s, 3H), 0.61 (s, 3H).
trans-B1. To a stirred THF solution of compound 2 (0.52 g,
1.54 mmol) was added dropwise an n-BuLi solution (1.60 M in
hexanes, 1.16 mL, 1.85 mmol) at -78 °C. The resulting solution
was left to stir for 1 h at -78 °C, and BMes2F (0.80 g, 3.06
mmol) in Et2O was added. The reaction mixture was slowly
warmed up to room temperature and stirred overnight. After the
addition of water, the solution was then extracted with dichlo-
romethane. The crude product was then purified by column
chromatography (CH2Cl2/hexanes ) 1:3) to afford trans-B1 in
Synthesis of [p-bis(ppy-CHdCH)benzene](BMes2)2 (B2). Com-
pound 7. To a solution of THF (30 mL) and water (10 mL) was
added compound 6 (1.67 g, 3.22 mmol) and trifluoroacetic acid
(30 mL). The resulting reaction mixture was refluxed overnight.
The reaction was quenched with saturated aqueous Na2CO3 and
extracted with diethylether. The combined organic layers were
then dried (MgSO4), filtered, and evaporated in vacuo to yield
compound 7 as a yellow solid, which was further purified by
column chromatography (CH2Cl2/hexanes ) 1:1) (0.56 g, 40%).
1H NMR (δ, ppm, CD2Cl2): 9.97 (s, 1H), 9.01 (s, 1H), 8.47 (d,
J ) 8.4 Hz, 1H), 8.17 (d, J ) 8.4 Hz, 1H), 8.00 (d, J ) 7.6 Hz,
1H), 7.81 (d, J ) 7.6 Hz, 1H), 7.41 (t, J ) 7.6 Hz, 1H), 7.35 (t,
J ) 7.6 Hz, 1H), 6.67 (s, 4H), 2.18 (s, 6H), 1.80 (s, 12H).
13C{1H} NMR (δ, ppm, CD2Cl2): 188.5, 163.7, 149.6, 140.5,
140.2, 137.1, 134.8, 134.6, 133.1, 131.7, 130.7, 130.6, 126.5,
123.8, 119.1, 112.5, 25.5, 21.1. HRMS calcd for C30H30BNO:
m/z 431.2420; found: 431.2426.
1
81% yield (0.63 g). H NMR (δ, ppm, CD2Cl2): 8.64 (d, J )
1.8 Hz, 1H), 8.25 (dd, J ) 8.7 Hz, J ) 1.8 Hz, 1H), 8.05 (d, J
) 8.4 Hz, 1H), 7.92 (dd, J ) 6.0 Hz, J ) 2.1 Hz, 1H), 7.76 (dd,
J ) 6.3 Hz, J ) 2.1 Hz, 1H), 7.55 (d, J ) 7.2 Hz, 2H), 7.37 (m,
5H), 7.18 (d, J ) 16.2 Hz, 1H), 7.03 (d, J ) 16.2 Hz, 1H), 6.67
(s, 4H), 2.19 (s, 6H), 1.83 (s, 12H); 13C{1H} NMR (δ, ppm,
CD2Cl2): 158.4, 145.0, 140.5, 137.6, 136.8, 135.6, 134.5, 132.7,
132.5, 131.5, 131.4, 130.5, 129.5, 129.4, 127.5, 125.9, 123.7,
122.4, 118.5, 25.4, 21.0. Anal. Calcd for C37H36BN: C 87.91; H
7.18; N 2.77. Found: C 86.92; H 6.81; N 2.64.
trans,trans-B2. A mixture of 1,4-bis(diethoxyphosphorylm-
ethyl)benzene (0.13 g, 0.35 mmol) and compound7 (0.30 g, 0.70
mmol) in THF (40 mL) was slowly added to a THF solution of
potassium tert-butoxide (0.10 g, 0.89 mmol). The solution was
stirred overnight and the THF was then evaporated in Vacuo.
After addition of water, the residue was extracted with dichlo-
romethane. The crude product was then purified by column
chromatography (CH2Cl2:hexanes ) 1:1) to afford compound
cis-B1. The solution of 0.10 g of trans- B1 in benzene (20
mL) was irradiated at 365 nm for 3 h under N2. The solvent
was removed under vacuum. Cis-B1 was isolated as a pale yellow
oil in 43% yield by preparative-scale TLC (CH2Cl2:hexanes )
1
1:3). H NMR (δ, ppm, CD2Cl2): 8.47 (s, 1H), 7.87 (m, 3H),
7.71 (d, J ) 7.0 Hz, 1H), 7.30 (m, 2H), 7.24 (m, 5H), 6.84 (d,
J ) 12.0 Hz, 1H), 6.61 (s, 4H), 6.48 (d, J ) 12 Hz, 1H), 2.16
(s, 6H), 1.70 (s, 12H). 13C {1H} NMR (δ, ppm, CD2Cl2): 157.7,
146.4, 141.2, 136.1, 135.2, 134.4, 134.0, 131.9, 131.1, 131.0,
130.1, 129.2, 128.8, 128.3, 125.6, 124.9, 122.0, 117.7, 25.0, 20.8.
1
B2 in 73% yield (0.24 g). H NMR (δ, ppm, CD2Cl2): 8.66 (d,
J ) 1.5 Hz, 2H), 8.25 (dd, J ) 1.5 Hz, 8.5 Hz, 2H), 8.05 (d, J
) 8.5 Hz, 2H), 7.92 (d, J ) 7.0 Hz, 2H), 7.62 (d, J ) 7 Hz,
2H), 7.56 (s, 4H), 7.34 (m, 4H), 7.17 (d, J ) 16.0 Hz, 2H),
7.06 (d, J ) 16.0 Hz, 2H), 6.68 (s, 8H), 2.20 (s, 12H), 1.85 (s,
24H); 13C {1H} NMR (δ, ppm, CD2Cl2): 158.1, 145.8, 144.9,
138.3, 137.3, 136.8, 135.2, 134.2, 132.2, 131.6, 131.2, 131.1,
130.2, 127.7, 125.7, 124.7, 123.9, 122.1, 118.3, 25.1, 20.8. Anal.
Calcd. for C68H66B2N2•CH2Cl2: C 81.42; H 6.73; N 2.75. Found:
C 80.3; H 6.62; N 2.72.
Synthesis of (ph-C≡C-ppy)BMes2 (B1A). 2-(2-Bromophenyl)-
5-(phenylethynyl)pyridine (4). A mixture of compound3 (0.70 g,
2.71 mmol), 2-bromophenyl boronic acid (0.54 g, 2.71 mmol),
and Pd(PPh3)4 (0.15 g, 0.14 mmol) was added to the solution mixture
of degassed THF (40 mL) and degassed aqueous 1 M KOH (20 mL).
The mixture was refluxed overnight and the THF evaporated in Vacuo.
After the addition of water, the mixture was extracted with diethyl
ether. The crude product was then purified by column chromatography
(CH2Cl2/hexanes ) 1:5) to afford compound 4 in 79% yield (0.72 g).
1H NMR (δ, ppm, CDCl3): 8.87 (s, 1H), 7.90 (d, J ) 8.1 Hz, 1H),
7.70 (d, J ) 8.1 Hz, 1H), 7.65 (d, J ) 8.1 Hz, 1H), 7.59 (m, 3H),
7.44 (m, 4H), 7.28 (m, 1H). 13C{1H} NMR (δ, ppm, CD2Cl2): 157.3,
152.2, 140.9, 138.5, 135.4, 133.7, 132.1, 131.8, 130.3, 130.1, 129.2,
128.8, 127.9, 124.4, 122.9, 122.0, 119.5, 93.5, 86.4. HRMS calcd for
C19H12BrN: m/z 333.0153; found: 333.0160.
B1A. To a stirred THF solution of compound 4 (0.34 g, 1.00
mmol) was added dropwise an n-BuLi solution (1.6 M in Hx,
0.69 mL, 1.10 mmol) at -78 °C. The resulting solution was
left to stir for 1 h at -78 °C. Then a solution of dimesitylboron
fluoride (0.32 g, 1.20 mmol) in Et2O was added. The solution
was slowly warmed to room temperature and stirred overnight.
The solvent was evaporated in Vacuo, and after the addition of
water, the mixture was extracted with dichloromethane. The
trans, cis-B2. Two mg of all trans-B2 was photolyzed in a
NMR tube (C6D6, 365 nm) at ambient temperature, which led
to the formation of the trans,cis-B2 compound (∼10% conver-
1
sion). H NMR (δ, ppm, C6D6): 8.90 (s, 1H), 8.81 (s, 1H), 8.20
(d, J ) 7.5 Hz, 1H), 8.13 (d, J ) 7.5 Hz, 1H), 7.69 (d, J ) 7.5
Hz, 1H), 7.60 (d, J ) 7.5 Hz, 1H), 7.33-7.16 (m, 8H), 7.06 (d,
J ) 8.0 Hz, 2H), 7.03 (d, J ) 8.0 Hz, 2H), 6.94 (s, 4H), 6.89
(s, 4H), 6.75 (d, J ) 16.0 Hz, 1H), 6.53 (d, J ) 16.0 Hz, 1H),
6.39 (d, J ) 12.0 Hz, 1H), 5.86 (d, J ) 12.0 Hz, 1H), 2.31 (s,
6H), 2.26 (s, 12H), 2.22 (s, 6H), 2.13 (s, 12H). 13C was not
assigned for this isomer due to the low conversion.
Synthesis of [1,3,5-Tris(Ppy-CHdCH)benzene](BMes2)3 (B3).
1,3,5-Tris((E)-2-(6-(2-bromophenyl)pyridin-3-yl)vinyl)benzene (9).
A mixture of compound 8 (0.36 g, 0.58 mmol), 2-bromophenyl
boronic acid (0.35 g, 1.74 mmol), and Pd(PPh3)4 (87 mg, 0.072
mmol) were added to a degassed solution of THF (100 mL) and
aqueous 1 M KOH (50 mL). The mixture was refluxed overnight,
9
J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009 14551

A R T I C L E S
Baik et al.
and the solvent was then evaporated in Vacuo. After the addition
of water the residue was extracted with dichloromethane. The
crude product was purified by column chromatography (CH2Cl2:
ethyl acetate ) 40:1) to afford compound 9 in 38% yield (0.18
(365 nm) at room temperature, and the exposure time was
recorded. After each exposure period, the NMR spectrum was
recorded.
General Procedure for Uv-Vis Monitoring of the Photolysis
Process. A 10-5 M solution in freshly distilled toluene was
prepared and placed in a screw-cap quartz cuvette. The solution
was placed under UV (365 nm) at room temperature and the
exposure time was recorded. After each exposure period, the
UV-vis spectrum was recorded.
TD-DFT Molecular Orbital Calculations. The geometries of
compounds B1A and B1-B3 were optimized using DFT at the
B3LYP level of theory16 with 6-311G* as the basis set,17 and
these results were then used for TD-DFT calculations modeling
singlet transitions only.18 Geometric parameters from crystal
structures were used as starting points for the geometry
optimizations where possible, and all other starting points were
constructed using the GaussView software package. Calculations
were carried out using Gaussian 03,19 and were performed using
16 processors on Sunfire 25000 systems using UltraSPARC-
IV+ processors.
X-ray diffraction analysis. Single crystals of the monoboryl
and polyboryl compounds were obtained from CH2Cl2/hexanes
solution. Data were collected on a Bruker AXS Apex II single-
crystal X-ray diffractometer with graphite-monochromated Mo
KR radiation, operating at 50 kV and 30 mA at 180 K. Data
were processed on a PC with the aid of the Bruker SHELXTL
software package (version 5.10)20 and corrected for absorption
effects. All structures were solved by direct methods. The crystal
lattice of B2 contains two CH2Cl2 solvent molecules per molecule
of 2 which were modeled and refined successfully. The crystal
lattice of B1 contains disordered hexane molecules that could
not be fully modeled and were removed with the Platon
SQUEEZE program. Compound B1A cocrystallizes with 0.5
hexane per molecule, which was successfully modeled and
refined. All non-hydrogen atoms were refined anisotropically.
The positions of hydrogen atoms were calculated, and their
contributions in structural factor calculations were included. The
crystal data of the compounds are given in Table S1, Supporting
Information. Important bond lengths and angles for all com-
pounds are listed in Table S2, Supporting Information. The
complete crystal data are provided in the Supporting Information.
1
g). H NMR (δ, ppm, CD2Cl2): 8.87 (d, J ) 1.5 Hz, 3H), 8.00
(dd, J ) 8.0 Hz, 1.5 Hz, 3H), 7.73 (s, 3H), 7.70 (d, J ) 8.5 Hz,
3H), 7.67 (d, J ) 8.0 Hz, 3H), 7.60 (dd, J ) 7.5 Hz, 1.5 Hz,
3H), 7.44 (t, J ) 7.5 Hz, 3H), 7.34-7.30 (m, 9H). 13C {1H}
NMR (δ, ppm, CD2Cl2): 149.0, 143.5, 142.2, 141.1, 137.9, 135.5,
132.5, 130.9, 128.3, 126.6, 125.1, 125.0, 124.8, 123.9, 118.8.
HRMS calcd. for C45H30Br3N3: m/z 850.0062; found: 850.0061.
trans,trans,trans-B3. To a stirred THF solution of compound
9 (0.19 g, 0.23 mmol) was added dropwise an n-BuLi solution
(1.6 M in hexanes, 0.50 mL, 0.80 mmol) at -78 °C. After the
mixture was stirred for 1 h at -78 °C, a solution of BMes2F
(0.24 g, 0.88 mmol) in Et2O was added. The solution was slowly
warmed to room temperature and stirred overnight. The solvent
was then removed in Vacuo, and after the addition of water, the
mixture was extracted with dichloromethane. The crude product
was purified by column chromatography (CH2Cl2/hexanes ) 1:1)
to afford B3 in 28% yield (0.084 g). 1H NMR (δ, ppm, CD2Cl2):
8.64 (d, J ) 1.6 Hz, 3H), 8.25 (dd, J ) 8.4 Hz, 1.6 Hz, 3H),
8.04 (d, J ) 8.4 Hz, 3H), 7.90 (d, J ) 6.8 Hz, 3H), 7.74 (d, J
) 6.4 Hz, 3H), 7.59 (s, 3H), 7.31 (m, 6H), 7.18 (d, J ) 16.4
Hz, 3H), 7.10 (d, J ) 16.4 Hz, 3H), 6.66 (s, 12H), 2.21 (s, 18H),
1.86 (s, 36H). 13C{1H} NMR (δ, ppm, CD2Cl2): 158.5, 145.8,
145.1, 140.2, 137.7, 137.2, 135.1, 134.2, 131.7, 131.3, 131.1,
131.0, 130.2, 125.7, 125.6, 125.4, 124.7, 122.2, 122.1, 118.3,
25.1, 20.8, 20.7. Anal. Calcd for C99H96B3N3: C 87.41; H 7.11;
N 3.09. Found: C 85.93; H 7.48; N 2.93.
trans,trans,cis-B3. This compound was generated in situ by
photolysis of the all trans-B3 with ∼50% conversion following
the general procedures for NMR monitoring of the photolysis
1
process. H NMR (δ, ppm, C6D6): 8.94 (s, 2H), 8.71 (s, 1H),
8.19 (d, J ) 7.5 Hz, 2H), 8.06 (d, J ) 7.5 Hz, 1H), 7.70 (d, J
) 7.5 Hz, 2H), 7.58 (d, J ) 7.5 Hz, 1H), 7.45 (m, 2H), 7.43
(m, 3H), 7.39-7.17 (m, 6H), 7.11 (s, 2H), 7.02 (s, 2H), 6.92 (s,
8H), 6.77 (d, J ) 16.5 Hz, 2H), 6.76 (m, 4H), 6.57 (d, J ) 16.5
Hz, 2H), 6.45 (d, J ) 12.0 Hz, 1H), 5.96 (d, J ) 12.0 Hz, 1H),
2.27 (s, 12H), 2.25 (s, 24H), 2.21 (s, 6H), 2.03 (s, 12H).
Fluorescent Quantum Yield Measurements. The solution
photoluminescent quantum yields were measured in dilute
toluene solution (A ≈ 0.10) using the optically dilute method
relatively to 9-10-diphenylanthracene at room temperature (Φr
) 0.90). The quantum yields were calculated using previously
reported procedures.14 Solid-state quantum yields were measured
at ambient temperature using a commercial fluorimeter in
combination with an integration sphere according to literature
Results and Discussion
Syntheses. To compare the impact of an olefinic bond with
that of a CtC bond on the photochemical stability of the
B(ppy)Mes2 chromophore, compounds B1 and B1A were
synthesized according to Scheme 2. For B1, Suzuki-Miyaura
coupling of 2-bromopyridyl-5-aldehyde with o-bromophe-
nylboronic acid produced 1, which was coupled to diethyl
benzylphosphonate Via the Horner-Wadsworth-Emmons
reaction21 to produce 2. Lithiation of 2 and its subsequent
reaction with BMes2F produced B1 in 81% yield. For B1A,
2-bromo-5-iodo-pyridine was coupled to phenylacetylene Via
procedures.14b
-e
Photoisomerization Quantum Yield Measurements. The quan-
tum yields of the photoisomerization of A1 and B1A to A1′
and B1A′ were determined using ferrioxalate actinometry.15 An
Ocean Optics fiber-optic spectromphotometer connected to a
four-way temperature-controlled cuvette holder from Quantum
Northwest via 400 µm optical fibers was used to measure the
absorbance. The irradiation source was a 200 W Hg/Xe lamp
attached to a monochromator (Photon Technology International).
(16) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789. (c) Vosko, S. H.;
Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200–1211. (d) Stephens,
P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem.
1994, 11623.
General Procedures for NMR Monitoring of the Photolysis
Process. The photoisomerization was carried out by preparing a
C6D6 (dried over NaH) solution (∼0.01 M) in a NMR tube inside
an inert atmosphere drybox. The solution was placed under UV
(17) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Chem. Phys.
1980, 72, 650.
(14) (a) Demas, N. J.; Crosby, G. A. J. Am. Chem. Soc. 1970, 92, 7262.
(b) Pa˚lsson, L. O.; Monkman, A. P. AdV. Mater. 2002, 14, 757. (c)
Mello, J. C.; Wittmann, H. F.; Friend, R. H. AdV. Mater. 1997, 9,
230.
(15) (a) Parker, C. A. Proc. R. Soc. London 1953, 220, 104. (b) Abdalah,
D.; Whelan, J.; Dust, J. M.; Hoz, J. M.; Buncel, E. J. Phys. Chem. A
2009, 113, 6640.
(18) Gross, E. K. U.; Kohn, W. AdV. Quantum Chem. 1990, 21, 255.
(19) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
(20) Sheldrick, G. M. SHELXTL, Version 6.14; Bruker AXS: Madison,
WI, 2000-2003.
(21) Kim, C.; Choi, H.; Kim, S.; Baik, C.; Song, K.; Kang, M.-S.; Kang,
S. O.; Ko, J. J. Org. Chem. 2008, 73, 7072.
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Photochemical Stability of N,C-Chelate Boryl Compounds
A R T I C L E S
Scheme 2
Scheme 3
Pd-catalyzed Sonogashira coupling22 to produce compound
3, which was then coupled to o-bromophenylboronic acid
Via Suzuki-Miyaura coupling23 to produce compound 4.
Lithiation of 4, followed by the addition of BMes2F, produced
B1A in 46% yield.
To examine the impact of π-conjugated olefinic bonds on
the photochemical stability of related polyboryl compounds,
a diboryl molecule, B2, and a triboryl molecule, B3, were
synthesized as shown in Schemes 3 and 4, respectively. In
the synthesis of B2, the starting material 1 was first protected
by a neo-pentylglycol group and the protected molecule was
reacted with n-BuLi, followed by the addition of BMes2F to
produce the monoboryl 6. Deprotection of 6 produced 7,
which was reacted with a bisphosphonate compound Via a
Horner-Wadsworth-Emmons reaction to produce B2 in 73%
yield. In the synthesis of B3, the starting material 8 was
obtained first by Horner-Wadsworth-Emmons reaction of
1 with a triphosphonate molecule containing a 1,3,5-
trisubstituted benzene core, followed by its coupling to
o-bromophenylboronic acid by Suzuki-Miyaura coupling to
produce 9. The reaction of 9 with n-BuLi, followed by
addition of BMes2F, produced B3 in 28% yield. All com-
pounds were fully characterized by NMR, HRMS/elemental
analyses and single-crystal X-ray diffraction. For B1-B3, the
major product in all cases were found to contain only trans-double
bonds with only trace amounts of other isomers, which could be
readily separated by column chromatography.
(22) Grave, C.; Lentz, D.; Schafer, A.; Samor, P.; Rabe, J. P.; Franke, P.;
Schluter, A. D. J. Am. Chem. Soc. 2003, 125, 6907.
(23) Wu, J.; Watson, M. D.; Zhang, L.; Wang, Z.; Mu¨llen, K. J. Am. Chem.
Soc. 2004, 126, 177.
Crystal Structures. The structures of B1A and the trans
isomers of B1-B3 determined by single-crystal X-ray diffrac-
tion are shown in Figures 1, 2, and 3, respectively. The structure
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A R T I C L E S
Baik et al.
Scheme 4
of B2 possesses a crystallographically imposed inversion center
while the structure of B3 has approximate C3 symmetry. The
average B-C (mesityl) bond length in compounds B1A, B1,
B2, and B3 is 1.646(5) Å, which agrees well with that of
B(ppy)Mes2.7 The -CtC- bond length in B1A and the
-CHdCH- bond lengths in B1-B3 are typical, and the
vinylene-phenyl moieties in B1-B3 are approximately planar.
The vinylene-phenyl moiety does show some distortion from
planarity with the ppy chelate, however, with dihedral angles
of 29.6° (B1); 27.8° (B2); and 9.4°, 6.1°, and 30.5° (B3). The
separation distances between the boron centers are 17.78 Å in
B2, and 14.12, 14.99, 15.75 Å in B3, respectively. In the crystal
lattices of B2 and B3, all molecules stack along one direction
such that the central conjugated cores are approximately parallel.
Figure 1. Crystal structures of B1 (left) and B1A (right) with 35% thermal ellipsoids. B: green, N: blue.
Figure 2. Crystal structure of B2 with 35% thermal ellipsoids. B: green, N: blue.
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Photochemical Stability of N,C-Chelate Boryl Compounds
A R T I C L E S
Figure 3. Crystal structure of B3 with 35% thermal ellipsoids. B: green, N: blue.
Figure 4. (Left) CV diagrams of the boryl and polyboryl compounds recorded in DMF with scan rates of 100-500 mV/s. (Right) CV diagrams of B2
showing three continuous scans at 200 mV/s.
Electronic and Photophysical Properties. All four com-
pounds display reversible or quasi-reversible reduction peaks
in their CV diagrams as shown in Figure 4, with reduction
potentials 0.35-0.40 V more positive than those of B(p-
py)Mes2. The diboryl B2 displays two partially resolved and
fully reversible reduction peaks in the CV diagram, indicative
of a weak electronic communication that enhances the
electron-accepting ability of the molecule by ∼0.15 V. Two
partially resolved reduction peaks were also observed for B3,
but with a poor reversibility, suggesting that the para-
geometry of B2 is most effective in stabilizing radical anions
and promoting electronic communication Via π-conjugation,
compared to the meta-geometry. The similar reduction
potentials of B1A and B1 indicate that the CdC and CtC
linkages have a similar impact on the LUMO level of the
chromophore.
The UV-vis absorption spectra (Figure 5) show that the optical
energy gap follows the order of B2 < B3 < B1 ≈ B1A <
B(ppy)Mes2, consistent with the extent of π-conjugation in these
molecules and further supporting the effectiveness of π-conjugation
of the para-geometry in B2. All compounds emit in the blue-green
region, with substantial overlap in their absorption spectra as well.
The emission energies of these molecules follow a trend similar
to that of the absorption energies (Figure 6), although B2 has both
the highest emission energy (λmax ) 456 nm) and the smallest
Stokes’ shift. The emission spectrum of B2 displays the same well-
resolved vibronic features as that of the dibromo ligand 11 (λmax
) 417 nm), indicating that the emission in this molecule is primarily
a πfπ* transition localized on the π-backbone (see ESI). The
featureless broad emission bands in all other boron compounds
can be attributed to charge transfer transitions (mesityl f π* of
the chelate backbone), which often display large Stokes’ shifts due
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Table 1. Absorption and Luminescent Data
A, nm
(ε, M-1 cm-1
λem, nm
toluene, rt
λem, nm
solid, rt
Φ
solution/solid
cmpd
)
B(ppy)Mes2, A1
B1
B2
B3
356 (15,200)
370 (11,400)
411 (41,000)
374 (53,000)
380 (22,500)
458
487
456
500
490
n/a
15%/ n.a.
490
491
504
496
32/23 ( 2%
59/22 ( 2%
30/24 ( 2%
37/14 ( 2%
B1A
The quantum efficiencies in toluene solution were obtained under
argon using 9,10-diphenylanthracene as the standard (Φ ) 0.90). For
the olefinic compounds, the data are for the trans or all trans isomers.
mesityl group. As observed for A1′ and A2′, the transforma-
tion of B1A to B1A′ is fully thermally reversible. The
quantum efficiency15 for the phototransformation of B1A to
B1A′ was determined to be 0.33 at λ ) 365 nm, 5.0-10-4
M, which is much lower than that of the nonsubstituted
compound B(ppy)Mes2 (A1) (0.88 at λ ) 365 nm, 5.0-10-4
M). The rate constants for the conversion of A1 to A1′, and
B1A to B1A′ were determined to be 1.08-10-3 s-1 and
Figure 5. Normalized absorption and fluorescent spectra of the boryl and
polyboryl compounds recorded in toluene (1.0-10-5 M).
to interactions with solvent molecules.6 The cis-B1 molecule has
absorption and emission spectra similar to those of trans-B1 (see
ESI) but with a much lower emission efficiency, which could not
be determined accurately due to the oily nature of the cis isomer.
The solid-state quantum efficiencies of all compounds are similar,
as shown in Table 1.
1
5.44-10-5 s-1, respectively. A H NMR competition experi-
ment in which both B1A and A1 were irradiated together at
365 nm in the same NMR tube and at the same concentration
further established that A1 undergoes photoisomerization
much faster than B1A (see ESI), consistent with the low
quantum efficiency of B1A. Because B1A has a much greater
absorbance than A1 at 365 nm (Table 1), its low photoi-
somerization quantum efficiency may be attributed to the
greater π-conjugation of the ph-py chelate, compared to A1,
that stabilizes the excited state of the molecule, thus
decreasing the photoreactivity. A similar trend was also
observed between A1 and A2,7 A1 and [p-(2-py)-ph-
ppy]BMes2 where a p-(2-py)phenyl substituent is attached
to the pyridine ring of the ppy chelate.24
Photoisomerization. Although the monoboryl compounds
B1 and B1A have similar absorption spectra with B1A having
a much greater extinction coefficient, they have distinct
responses toward light. Upon prolonged exposure to UV light
(365 nm) under nitrogen, no apparent color change was
observed for B1, while the solution of B1A rapidly became
dark blue-green in color, quantitatively forming a new species
B1A′ (Figure 6) with a broad absorption band appearing at
600 nm upon irradiation (Figure 7). This is consistent with
the behavior of A1 and A2 (see Scheme 1). The fluorescent
emission intensity of B1A diminishes rapidly with irradiation,
and the new species B1A′ is not emissive (see ESI).
The characteristic chemical shifts of the pyridyl and mesityl
Surprisingly, however, no analogue of B1A′ was observed
when the olefin-substituted compound B1 was subjected to
the same irradiation conditions. Instead, the trans-isomer of
B1 was found to undergo a facile trans to cis-isomerization
at ambient temperature, reaching ∼70% conversion at equi-
1
1
protons in the H NMR spectrum of B1A′ show excellent
librium, as shown by the H NMR spectra in Figure 8. This
agreement with those of A1′ and A2′,7 confirming that B1A′
is an analogous compound in which a new C-C bond is
formed between the phenyl ring of the phenylpyridine and a
isomerization was also monitored by UV-vis and fluorescent
spectral measurements. Control experiments on a mixture of
A1 and B1 show that, upon irradiation, A1 forms exclusively
Figure 6. 1H NMR spectral change of B1A in C6D6 under N2 upon irradiation at 365 nm. The chemical shifts of the 2-pyridyl proton (Hpy) and the protons
from the mesityl groups are highlighted. Orange: B1A′, yellow: B1A.
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Photochemical Stability of N,C-Chelate Boryl Compounds
A R T I C L E S
Figure 7. Comparison of the UV-vis spectral change of B1A (left, 1s interval, ∼1.0-10-5 M) with that of A1 (right, 5s interval, 1.25-10-4 M) in
toluene upon irradiation under N2 at 365 nm and photographs of B1A and A1 solutions before and after irradiation.
Figure 8. 1H NMR spectral change of B1 in C6D6 under N2 upon irradiation at 365 nm. The chemical shifts of the 2-pyridyl proton (Hpy) and the
olefinic protons are highlighted. Yellow: trans-B1, blue: cis-B1.
Figure 9. 1H NMR spectral change of B2 in C6D6 under N2 upon irradiation at 365 nm. The chemical shifts of the 2-pyridyl proton (Hpy) and the
olefinic protons are highlighted. Yellow: all trans-B2, blue: trans,cis-B2.
A1′, while B1 simply undergoes isomerization (see ESI). This
confirms unequivocally that the olefinic bond is responsible
for the stabilization of the boron chelate chromophore toward
photolysis.
Similarly, we have found that the tetrahedral boron cores
in the diboryl compound B2 and the triboryl compound B3
(24) Rao, Y. L.; Wang, S. Unpublished results.
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A R T I C L E S
Baik et al.
Figure 10. 1H NMR spectral change of B3 in C6D6 under N2 upon irradiation at 365 nm. The chemical shifts of the 2-pyridyl proton (Hpy) and the olefinic
protons are highlighted. Yellow: all trans-B3, blue: trans,trans,cis-B3.
inherently less stable than their trans counterparts, further
irradiation preferentially reverts the existing cis double bond
to the trans form rather than forming a second cis double
bond. In the photostationary state, the conversion of tran-
s,trans-B2 to the (trans,cis) isomer is approximately 10%,
while the conversion of B3 reaches approximately 50% at
the 365 nm excitation wavelength. Changing the excitation
wavelength to 420 nm did not appreciably change the
(trans,trans) to (trans,cis) ratio for a solution of B2.
TD-DFT calculations. TD-DFT calculations were performed
for B1A and the all trans- and mono-cis isomers of B1-B3
(see ESI). As a result of the steric congestion readily apparent
from the optimized structures, these calculations indicate that
the cis isomers of compounds B1-B3 are 3.9, 4.1, and 3.9
kcal/mol higher in energy than the respective trans com-
pounds, consistent with previously reported values for cis/
trans isomers.25
The HOMO and LUMO orbitals of A1, B1A, and the trans
isomers of B1 and B2 are shown in Figure 11, with an
isocontour value of 0.02 for all surfaces. The HOMO and
LUMO diagrams for B3 can be found in the ESI. The HOMO
orbitals in all cases are dominated by both mesityl π-character
Figure 11. HOMO and LUMO orbitals of A1, B1A, trans-B1, and B2
with an isocontour value of 0.02 for all surfaces.
and the phenylpyridine C-B σ-bond. However, while the
LUMO of A1 is almost entirely composed of phenylpyridine
π* character, the vinyl or acetylene π* orbitals make large
are also stable toward UV irradiation, and no C-C bond
contributions to the LUMOs of B1A, and both isomers of
formation products were observed after prolonged UV
B1-B3. In the vinyl- and acetylene-substituted molecules,
exposure in toluene or C6D6. Instead, both B2 and B3 were
therefore, electron density is more delocalized in the excited
found to undergo trans to cis isomerization upon irradiation
state. TD-DFT calculation results for A1, B1A, trans-B1,
that causes the diminishing of the lowest absorption band
and all trans-B2 indicate that the lowest energy transition is
and the emission peak in a similar manner as observed for
almost entirely HOMO f LUMO in nature, with reasonable
B1.
oscillator strengths (>0.01); these transitions are thus likely
1
In both cases, H NMR data indicate that irradiation will
responsible for the observed fluorescence of these compounds
produce only one cis double bond at any given time, giving
(see ESI).
trans,cis-B2 or trans,trans,cis-B3 (Figures 9 and 10). Since
Several key conclusions can be drawn from the TD-DFT
data. The HOMO in all cases contains significant electron
both systems are fully conjugated and cis double bonds are
density from the phenylpyridine C-B σ-bond, consistent with
a weakening of this bond on irradiation that leads to the bond
(25) Smith, M.; March, J. AdVanced Organic Chemistry, Reactions,
Mechanisms and Structure, 5th ed.; John Wiley & Sons: New York,
2001.
cleavage observed for A1 and B1A. Also, the LUMO orbitals
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14558 J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009

Photochemical Stability of N,C-Chelate Boryl Compounds
A R T I C L E S
Scheme 5
photochemical rearrangement with an accompanying loss of
emission intensity. Cis-trans isomerization thus provides a
competing nonradiative relaxation pathway for B1-B3, and
may prove to be a useful strategy for the stabilization of a
wider variety of otherwise photochemically unstable chro-
mophores as well.
Conclusions
This work demonstrates that highly emissive and photo-
active π-conjugated systems can be achieved by incorporating
olefinic bonds into the π-backbone of N,C-chelate boryl
systems. While molecules containing the B(ppy)Mes2 chro-
mophore typically undergo photochemical rearrangement on
exposure to UV light, the isomerization of a single olefinic
bond in compounds B1-B3 completely inhibits this photo-
switching process, thereby stabilizing the chromophore
toward light. The results of comparative studies on the
monoboryl compounds A1, B1, and B1A establish unequivo-
cally that a single olefinic bond in the conjugated molecule
is sufficient to protect the B(ppy)Mes2 chromophore from
photodegradation. Furthermore, we have shown that extend-
ing the π-conjugation of the phenylpyridine chelate with an
acetylene linker does not impart this same stability to the
B(ppy)Mes2 chromophore, but does significantly reduce the
photoisomerization quantum efficiency.
for B1A and B1-B3 all contain substantial π-character from
their pendant π-bonds, which would lead to a reduction in
bond order in the excited state. In the case of B1-B3, this
creates an excited-state single bond that facilitates trans to
cis isomerization, providing an alternative pathway for energy
dissipation not available to the parent chromophore, B(p-
py)Mes2. However, this same phenomenon is not observed
for the acetylene-containing B1A, in which bond rotation
would still be restricted after a single π-bond is weakened
by photoexcitation.
A rationalization of these processes is proposed in Scheme
5. On irradiation, the singlet excited-state trans isomer may
simply fluorescence, with a radiative rate constant kr, or may
relax Via one of several nonradiative pathways, knr. Of these,
it may: (1) undergo typical vibrational deactivation to the
trans ground state (knr);26 (2) undergo isomerization to the
cis compound (knr′) or (3) undergo a photochemical rear-
rangement reaction as depicted in Scheme 1 (knr′′). (The
singlet to triplet intersystem crossing is also a possibility,26
although the role of the triplet state in our system has not
been established.) For B1-B3, cis-trans isomerization
provides a more kinetically accessible relaxation pathway
than the C-C coupling reaction, deactivating the rearrange-
ment at ambient temperatures and preserving the integrity
of the chromophore. However, with no isomerization pathway
available to B(ppy)Mes2 or B1A, these molecules undergo
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council for financial support, Dr. Rui-
Yao Wang for assistance in crystal data collection, and Fang
Liu and Professor Erwin Buncel for assisting the photoisomer-
ization quantum efficiency measurements of A1 and B1A.
Supporting Information Available: Complete crystal struc-
tural data (CIF files), synthetic and characterization details
of intermediates not included in the Experimental Section,
details of TD-DFT calculation results, reduction potentials,
complete diagrams showing the UV-vis and fluorescent
spectral change of B1A, B1, B2, and B3 upon irradiation at
365 nm, the NMR spectral data of the control photolysis
experiment of B1 in the presence of A1, the NMR competi-
tion photolysis experiment of A1 with B1A, and complete
ref 19. This material is available free of charge via the
Internet at http://pubs.acs.org.
(26) (a) Lakowicz, J. R. Principles of Fluorescent Spectroscopy, 2nd ed.;
Kluwer Academic/Plenum Publishers: New York, 1999. (b) Saltiel,
J.; D’Agostino, J.; Megarity, E. D.; Metts, L.; Neuberger, K. R.;
Wrighton, M.; Zafiriou, O. C. Org. Photochem. 1973, 3, 1.
JA906430S
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