Steric and electronic influence on photochromic switching of N,C-chelate four-coordinate organoboron compounds

Article abstract of DOI:10.1002/chem.200903582

A four-coordinate organoboron compound B(ppy)Mes₂ (1, ppy = 2-phenylpyridyl, Mes=mesityl) was previously found to undergo reversible photochromic switching through the formation/breaking of a C-C bond, accompanied by a dramatic color change fro

Full text of DOI:10.1002/chem.200903582

DOI: 10.1002/chem.200903582
Steric and Electronic Influence on Photochromic Switching of N,C-Chelate
Four-Coordinate Organoboron Compounds
Hazem Amarne, Chul Baik, Stephen K. Murphy, and Suning Wang*[a]
4750
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Chem. Eur. J. 2010, 16, 4750 – 4761

FULL PAPER
Abstract: A four-coordinate organo-
boron compound B(ppy)Mes2 (1, ppy=
2-phenylpyridyl, Mes=mesityl) was
previously found to undergo reversible
photochromic switching through the
also been synthesized and their boron
compounds were investigated. The
crystal structures of most of the new
compounds were determined by X-ray
diffraction analysis. UV/Vis, NMR, and
electrochemical methods were used to
monitor the photoisomerization pro-
cess. DFT calculations were performed
for all compounds to understand the
photophysical and electronic properties
of this class of molecules. The results
of our study showed that the bulky me-
sityl group is necessary for photochro-
mic switching. Electron-donating and
electron-withdrawing groups on the
ppy chelate have a distinct impact on
the photoisomerization rate and the
photochemical stability of the mole-
cule. Furthermore, we have found that
the non-ppy-based N,C-chelate ligands
such as benzo[b]thiophenepyridyl can
also promote photoisomerization of the
boron chromophore in the same
manner as the ppy chelate, but the
product is thermally unstable.
ACHTUNGTRENNUNG
ꢀ
formation/breaking of a C C bond, ac-
companied by a dramatic color change
from colorless to dark blue. To under-
stand this unusual phenomenon,
a
series of new four-coordinate boron
compounds based on the ppy-chelate
ligand and its derivatives have been
synthesized. In addition, new N,C-che-
late ligands based on benzo[b]thiophe-
nylpyridine and indolylpyridine have
Keywords: boron · N ligands · pho-
tochemistry · reactivity · structure
elucidation
Introduction
phore, such as an olefin bond in the N,C-chelate ligand, can
inhibit the photoisomerization process by dissipating energy
through the alternative trans–cis isomerization pathway,[4b]
hence enhancing the stability of the four-coordinate boron
chromophore. In view of the importance and potential appli-
cations of organic photochromic systems[5] and the rarity of
photochromic systems based on four-coordinate organobor-
on compounds, we have conducted a comprehensive study
on the ppy-based organoboron compounds. To fully under-
stand the factors influencing the photochromic properties of
N,C-chelate four-coordinate boron compounds, we have syn-
thesized a series of analogues with the general formula of
Four-coordinate organoboron compounds have been exten-
sively investigated in the past decade as emissive materials
for use in organic light-emitting diodes (OLEDs), because
of their high thermal and chemical stability.[1,2] In contrast to
three-coordinate boron compounds, which can function as a
Lewis acid or an electron-transport material in optoelectron-
ic devices through the empty p orbital of the boron center,[3]
four-coordinate organoboron compounds can also function
as electron-transport materials by means of the boron-stabi-
lized p* orbital of the conjugated chelate ligands.[1,2] We
have recently discovered that certain four-coordinate boron
BACTHUNTRGNE(NUG N,C-L)Ar2 (1–10) and various N,C-L ligands and Ar
compounds such as B
(ppy)Mes2 (1, ppy=2-phenylpyridyl,
groups. We have found that both steric and electronic fac-
tors have a significant impact on the photochromic behavior
and photochemical stability of this class of molecules. The
details are presented herein.
Mes=mesityl) are not only highly emissive, but also readily
undergo an unusual and reversible photochromic switching
upon exposure to light (Scheme 1).[4a] We have further dis-
covered that incorporating a second photosensitive chromo-
Results and Discussion
Syntheses: The organoboron compounds used in this study
with the general formula of BACHTNUTRGNENG(U N,C-L)Ar2 can be classified
into two groups depending on the nature of L: group A with
a ppy or its derivative as the chelate ligand (compounds 1–
8) and group B with other N,C-chelate ligands (9 and 10).
Compounds in group A can be further divided into two
groups based on whether or not the Ar group is a mesityl
(1–6) or a non-mesityl group (7 and 8). The syntheses of
compounds 1, 3, and 5 have been described previously.[4]
Compound 2 was prepared according to Scheme 2. Suzuki–
Miyaura coupling[6] of 2-bromopyridine with 2,3,4,5-tetra-
fluorophenyl boronic acid produced the ligand 2’, which,
upon lithiation and the subsequent addition of BMes2F pro-
duced 2 in 39% yield.
Scheme 1.
[a] H. Amarne, Dr. C. Baik, S. K. Murphy, Prof. Dr. S. Wang
Department of Chemistry
Queenꢁs University, Kingston
Ontario, K7L 3N6 (Canada)
Fax : (+1)613-5336669
The syntheses of compounds 4 and 6 follow a similar pro-
cedure, namely the synthesis of the 2-(2-bromophenyl)pyri-
dine derivative and its subsequent reaction with nBuLi fol-
E-mail: wangs@chem.queensu.ca
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903582.
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S. Wang et al.
mopyridine and benzo[b]thio-
phen-2-yl boronic acid. Its sub-
sequent reaction with nBuLi
and BMes2F produced
9 in
62% yield. The 2-indolylpyri-
dine ligand, which was prepared
by using an Ullmann coupling
reaction similar to that for the
synthesis of 7-azaindolylpyri-
dine,[7] was treated with LDA
followed by addition of BMes2F
to produce 10 in 72% yield. All
compounds were fully charac-
terized by NMR spectroscopy,
HRMS, and/or elemental analy-
ses.
Crystal structures: The crystal
structural data of compounds 1
Scheme 2.
lowed by addition of BMes2F, as shown in Scheme 2 (yield
81-83%). The ligand 4’ was obtained by Suzuki–Miyaura
coupling of 2-bromo-5-(trimethylsilyl)pyridine with 2-bro-
mophenyl boronic acid. In the same manner, the ligand 6’
was obtained by Suzuki–Miyaura coupling of 2-bromo-5-[2-
(trimethylsilyl)ethynyl]pyridine with 2-bromophenyl boronic
acid. Compounds 7 and 8 were prepared by lithiation of 2-
(2-bromophenyl)pyridine[4a] and the subsequent addition of
BPh2Cl and (C6F5)2BF, respectively, as shown in Scheme 3.
Compounds
9 and 10 were prepared according to
Scheme 4. The 2-benzo[b]thienylpyridine ligand was pre-
pared using a Suzuki–Miyaura coupling reaction of 2-bro-
Scheme 4.
and 5 were reported previously.[4a,b] The crystal structures of
2, 4, and 6–9, determined by single-crystal X-ray diffraction
analysis in this work, are shown in Figure 1–3, respectively.
The important bond angles and bond lengths of 1, 2, 4–9 are
shown in Table 1 for comparison.
Scheme 3.
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Organoboron Compounds
FULL PAPER
Figure 2. The crystal structures of compound 6 (top, showing the two sets
of the disordered SiMe3 group) and 7 (bottom) with 50% ellipsoids and
labelling schemes.
Table 1. Comparison of bond lengths and angles of compounds 1, 2, 4–9.
ꢀ
ꢀ
ꢀ
B N
[ꢂ]
B Cchelate B CAr
N-B-Cchelate
[8]
CAr-B-CAr
[8]
[ꢂ]
[ꢂ]
Figure 1. The crystal structures of compound 2 (top) and 4 (bottom) with
50% ellipsoids and labelling schemes.
1.644(2)
1.654(2)
1.629(5)
1.636(5)
1.648(5)
1.649(6)
1.633(6)
1.661(5)
1.650(4)
1.650(4)
1.632(5)
1.643(5)
1.611(3)
1.616(3)
1.635(2)
1.639(2)
1.632(7)
1.633(7)
1
2
1.6531(19) 1.625(2)
95.13(10)
94.5(2)
114.91(11)
1.661(4)
1.638(5)
1.643(6)
1.642(3)
1.667(4)
1.618(3)
1.612(2)
1.659(6)
1.628(5)
1.639(6)
1.641(6)
1.631(4)
1.621(4)
1.628(3)
1.610(2)
1.617(6)
116.1(3)
115.9(3)
116.3(3)
115.3(2)
115.6(2)
113.48(15)
114.34(12)
116.2(4)
Group A: The compounds in group A (1–8) all contain the
ppy or functionalized ppy as the chelate. There are, howev-
er, significant variations in B N and B C bond lengths
among this group of compounds. Comparison of structural
data of compound 1 with that of 7 clearly shows that the
bulky mesityl groups in 1 destabilize the compound, as evi-
95.2(3)
4
ꢀ
ꢀ
95.1(3)
5
6
7
8
9
95.44(18)
95.2(4)
ꢀ
ꢀ
denced by the much longer B N (1.653(2) ꢂ) and B CAr
bonds (1.649(2) ꢂ average)[4a] of
1
versus those of
7
95.58(13)
97.63(11)
94.8(4)
(1.618(3) and 1.613(3) ꢂ, respectively, average) while the C-
B-N chelating angle is essentially the same in both com-
pounds. The C6F5 group in 8 also destabilizes the molecule
relative to the phenyl group in 7 by weakening the B CAr
bond (1.637(2) ꢂ average). This effect can be attributed to
the electron-withdrawing nature of the C6F5 group. The elec-
ꢀ
tron-donating group SiMe3 in 4, the phenyl
G
tron-withdrawing fluorinated phenyl ring of the ppy chelate
in 5,[4b] and the SiMe3-acetylene group in 6 of the ppy che-
in
2 shortens the B CMes bond lengths considerably
ꢀ
late do not have significant impacts on the B CMes and the
(1.632(5) ꢂ average), while having no significant impact on
ꢀ
ꢀ
B Cchelate bond lengths compared to 1. In contrast, the elec-
the B Cchelate bond.
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S. Wang et al.
Photophysical and electrochemical properties: UV/Vis ab-
sorption spectra and fluorescence spectra for compounds 1–
10 are shown in Figure 4. The lmax of the lowest energy ab-
sorption band, the absorption edge (optical energy gap), and
the lmax of the emission spectra along with fluorescence
quantum efficiencies are provided in Table 2. All com-
pounds have intense absorption bands at 290–400 nm. For
the mesityl compounds 1–6, 9, and 10, there is a low-energy
shoulder in the 350–470 nm region that can be attributed to
Figure 3. The crystal structures of compound 8 (top) and 9 (bottom) with
50% ellipsoids and labelling schemes.
Group B: The structure of compound 9 provides an impor-
tant comparison of non-ppy-based N,C-chelate ligands with
those of ppy and derivatives. Comparison of the structural
Figure 4. Top: UV/Vis spectra recorded in toluene (ꢁ10ꢀ5 m) . Bottom:
fluorescence spectra recorded in toluene (ꢁ10ꢀ5 m).
ꢀ
data of 9 and 1 shows that the B CMes bonds in 9
ꢀ
ꢀ
(1.632(7) ꢂ average) are shorter, while the B N and B
Cchelate bond lengths are similar for both compounds. None-
theless, the overall structural features of 9 resemble those of
1. The crystal structure of 10 was not determined due to the
lack of adequate crystals.
Table 2. Photophysical and electrochemical properties of 1-10 in toluene
(ꢁ10ꢀ5 m).
red
Absorption lmax [nm] Optical energy lem
F in
E
1=2
The crystal structural data indicates that despite the bulky
mesityl groups in compounds 1–6 and 9 and 10, the boron
center retains a four-coordinate environment similar to that
of the phenyl and pentafluorophenyl compounds 7 and 8 in
the solid state. In solution, these compounds also retain the
four-coordinate geometry, as confirmed by their 11B chemi-
cal shift.[8] In addition, unlike three-coordinate triarylboron
compounds that readily react with fluoride ions,[3] com-
pounds 1–10 do not show any reactivity with fluoride ions,
as established by an NMR study (see Supporting Informa-
tion), further supporting their four-coordinate geometry in
solution.
(e
N
gap [eV]
[nm] toluene[a] [V][b]
1
2
3
4
5
6
7
8
9
357 (3207)
349 (2397)
390 (3780)
355 (4672)
377 (11726)
371 (8161)
334 (6503)
341 (6164)
386 (8923)
3.10
3.13
2.70
3.08
2.90
2.93
3.51
3.48
2.89
2.81
458
470
527
418
490
490
413
360
455
461
0.10
0.05
0.15
0.12
0.37
0.28
0.31
0.22
0.30
0.024
ꢀ2.30
ꢀ2.09
ꢀ1.68
ꢀ2.32
ꢀ2.03
ꢀ2.04
ꢀ2.28
ꢀ2.09
ꢀ2.19
ꢀ2.23
10 362 (6264)
[a] Determined using 9,10-diphenylanthracene as the standard. [b] From
+/0
CV diagrams recorded in DMF, relative to the potential of FeCp2
.
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Organoboron Compounds
FULL PAPER
a charge-transfer transition from the mesityl to the ppy che-
late based on the results of our previous study of com-
pounds 1 and 5.[4] The fact that this low-energy shoulder is
absent in the absorption spectra of the non-mesityl com-
pounds 7 and 8 is consistent with this assignment. The
phenyl and pentafluorophenyl compounds 7 and 8, respec-
tively, have the largest optical energy gap. All compounds
except 10 are either modest or bright fluorescent emitters
with lmax ranging from 360 to 527 nm (Figure 4). The emis-
sion energy of compounds 1–4 follows the order of 4>1>
2>3, which is also in agreement with a charge-transfer tran-
sition from the mesityl p to the ppy-chelate p* level, since
an electron-donating SiMe3 group in 4 raises the p* level of
the ppy, while an electron-withdrawing group, such as the
carbonyl in 3, stabilizes it. Extended p-conjugation of the
ppy chelate with an acetylene group also significantly de-
creases the energy of the charge-transfer transition due to
the stabilization of the p* level, as evidenced by the much
longer emission wavelength of compounds 5 and 6, relative
to that of 1. The benzo[b]thienylpyridine compound 9 is a
bright emitter at a wavelength similar to that of 1, while the
indolylpyridine compound 10 emits in the same region, but
much more weakly.
observations are all consistent with the LUMO level being
the p* orbital of the ppy chelate. The diphenyl compound 7
has a similar reduction potential as that of 1, despite the
weaker N,C-chelating bonds in 1 that should destabilize the
p* level somewhat. The pentafluoro compound 8 has a
much more positive reduction potential than that of 1. This
can be attributed to the greater s donation of the ppy ligand
ꢀ
to the boron center, as evidenced by the much shorter B N
ꢀ
and B Cchelate bond lengths in 8 (Table 1), which causes sta-
bilization of the p* level. This supports the fact that the
nature of the aryl group has an impact on the p* level of
the chelate moiety due to their influence on the electron
density of the B center. The non-ppy-chelate compound 9
has a more positive reduction potential than 1, which is at-
tributable to the greater p conjugation. Compound 10, how-
ever, has a potential similar to that of 1, despite the much
bigger indolylpyridine chelate ligand. It is worth noting that
compounds 2 and 10 are unstable toward electrochemical re-
duction even in the absence of light, as evidenced by the ap-
pearance of new species in the CV diagrams after reduction
(see Supporting Information), while compounds 1, 3, 4, and
6 have the best stability toward reduction. The reduction po-
tentials for all compounds are provided in Table 2.
In the cyclic voltammetry diagrams, all compounds display
a reduction peak that can be attributed to the reduction of
the p* orbital of the N,C-chelate ligand (Figure 5). Com-
pound 1 has a reduction peak at ꢀ2.30 V (vs. FeCp2+/0),
which is much more negative than those of the triarylboron
isomers[9] of 1, 2-(4-BMes2-C6H4)pyridine (ꢀ2.16 V) and 5-
BMes2-2-C6H5-pyridine (ꢀ2.12 V), indicating that triarylbor-
ons are better electron acceptors. Electron-withdrawing
groups such as fluoro and carbonyl groups (2 and 3) greatly
shift the reduction potential to more positive values, while
the electron-donating group SiMe3 (4) shifts the potential to
a more negative value. The p conjugation of the acetylene
group in the ppy ligand (5, 6) also shifts the reduction po-
tential to more positive values, relative to that of 1. These
DFT calculations for compounds 1–10: Our previous TD-
DFT computations[4] on compounds 1 and 5 established that
the HOMO–LUMO (mesityl to ppy) transition is responsi-
ble for the lowest energy transition observed in the absorp-
tion and emission spectra for these two compounds. It is
therefore reasonable to assume that the same is true for the
new dimesitylboron compounds. Hence, only non-time-de-
pendent DFT calculations were performed for the new com-
pounds to validate the general trend of the experimentally
observed photophysical properties of this class of molecules.
The calculated HOMO–LUMO energies and gaps along
with the experimental values determined from CV and ab-
sorption data are provided in Figure 6. The data show that
Figure 5. The CV diagrams showing the reduction peak of compounds 1–
Figure 6. The calculated and experimental HOMO–LUMO energy levels
of compounds 1–10.
10, recorded in DMF with scan rates 200 or 300 mVsꢀ1
.
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S. Wang et al.
the general trend of the calculated HOMO and LUMO en-
ergies agree with the experimentally observed trend. The
HOMO and LUMO diagrams of all new compounds are
provided in the Supporting Information. For all molecules,
the HOMO level is mainly concentrated on one of the aryl
groups, while the LUMO level exclusively nearly involves
the p* orbital of the N,C-chelate group. Thus, the lowest
energy electronic transition in all molecules can be consid-
ered a charge-transfer transition from the aryl group to the
N,C-chelate unit. For compounds 7 and 8, this transition
most likely overlaps with the p!p* transition localized on
the ppy chelate, leading to the absence of a low-energy
shoulder in the absorption spectra. The large HOMO–
LUMO gaps in 7 and 8 are clearly due to the deep HOMO
levels caused by the phenyl and the pentafluorophenyl
groups, respectively, relative to that of mesityl. Some of the
large discrepancies between calculated and experimental
values can be attributed to the limitation of the ground state
DFT calculations.
Photochromic properties: Compounds 1–10 display distinct
responses toward irradiation by light. For example, com-
pounds 3, 4, 6, and 9 undergo photochromic switching from
either colorless or light yellow to dark blue or dark green
upon irradiation by UV light (350 or 365 nm) under nitro-
gen (Figure 7). This phenomenon is reminiscent of the pho-
tochromism of compounds 1 and 5, which have been shown
previously to change color from colorless or light yellow to
dark blue upon irradiation by UV light through the forma-
tion of the species 1a and its analogue 5a, respectively
1
(Scheme 1).[4] Indeed, UV/Vis (Figure 8) and H NMR spec-
tral data (see Supporting Information). confirmed that com-
pounds 3, 4, 6, and 9 form the same type of species as 1a
Figure 8. The UV/Vis spectral change of compound 3, 4, 6, and 9 in tolu-
ene after being exposed to UV light (365 nm). The spectra were recorded
at 5 seconds intervals for 3, 4, and 9 and 3 seconds for 6 of UV exposure.
Figure 7. Photographs of compounds 1–10 in toluene showing their colors
(middle), fluorescent colors (top), the color change after being irradiated
by a UV lamp (365 nm) for ꢁ5 min for 1, 3, 4, and 6 and ꢁ20 min for all
other compounds (bottom).
and 5a upon irradiation. In contrast, the diphenyl compound
7 and the pentafluorophenyl compound 8 are stable toward
UV irradiation at either 350 or 310 nm and no photochromic
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Organoboron Compounds
FULL PAPER
ꢀ
ꢀ
switching was observed, while compounds 2 and 10 undergo
decomposition upon irradiation at 350 nm.
addition, the B N (1.660(5) ꢂ) and B Cchelate (1.650(6) ꢂ)
bond lengths in B[5-(7-azaindolyl)-2-Br-py]Mes2 are also
among the longest. Due to the similarity of the azaindolyl-
pyridyl and indolylpyridyl chelate ligands, it is reasonable to
assume that compound 10 likely possesses the same structur-
al characteristics. Hence, the poor photochemical stability of
10 is likely due to the poor stability of the indolylpyridyl
chelate.
Steric factor: The key difference between compound 7 and 1
is that the former has two phenyl groups, while the latter
has two mesityl groups attached to the boron center. Crystal
ꢀ
structural data indicated that the B mesityl bonds in 1 are
ꢀ
much longer than those of the B C6H5 bonds in 7 owing to
the fact that mesityl groups are much bulkier than phenyl
ꢀ
groups. The weaker B mesityl bonds and the electron-do-
Electronic factor: To investigate the impact of electron-do-
nating and electron-withdrawing groups on photoisomeriza-
tion of the ppy-chelate compounds, we compared the photo-
isomerization rate of 3 and 4 relative to that of 1. For com-
pound 3, the photoisomerization rate is somewhat smaller
than that of 1 (k(1)/k(3)ꢁ1.2), as determined by the com-
petitive photolysis NMR experiments in C6D6 However,
complete conversion of 3 to 3a could not be achieved due
to the decomposition of 3 after extended exposure to UV ir-
radiation. For 4, the isomerization rate is faster than 1 with
k(1)/k(4)ꢁ0.80 and the full conversion to 4a can be ach-
ieved readily. As shown in Figure 6, the electron-donating
group SiMe3 destabilizes the p* level, hence destabilizing
the excited state, which appears to accelerate the photoiso-
merization process. Compounds 5 and 6 have an acetylene
group attached to the ppy and both compounds can be fully
converted to 5a and 6a, respectively. The rate of conversion
of compound 6 was found to be similar to that of 4 (k(1)/
k(6)ꢁ0.80), while the rate for 5 is much slower than that of
1 (k(1)/k(5)ꢁ20). The extended p conjugation of the che-
late ligand with the phenyl group is believed to be responsi-
ble for the slow photochromic response of 5, as is the case
for compound 9. The similar photoisomerization rates of 4
nating methyl groups are clearly responsible for the low-
energy charge-transfer transition from the mesityl!ppy in 1
with respect to the phenyl!ppy transition in 7. Nonetheless,
the lack of any photochromic switching of 7, even under ir-
radiation at 310 nm that is near the lmax of its low-energy ab-
sorption band, suggests that the steric congestion in 1 is
likely the key factor responsible for the photochromic prop-
erties of 1. The non-photoresponsive behavior of 7 and 8 is
also in sharp contrast to that of BPh4 and BPh
ꢀ
N
ꢀ
which are known to produce C C coupled products with the
formation of a BC2 ring upon irradiation at 254 nm.[10] The
greater photochemical stability of 7 and 8 versus that of
ꢀ
BPh4 or BPh
chelate ligand.
3ACHTUNGTRENNUNG
(biphenyl)ꢀ can only be attributed to the ppy-
Chelate ligands: Compounds 9 and 10 are the only members
that do not have ppy or its derivative as the chelate ligand.
The N,C-chelate units in both molecules involve a pyridyl
and a five-membered thiophene or pyrrole ring. Compound
9 undergoes a photochromic switching process similar to
ꢀ
that of 1. In the presence of oxygen, the C C coupled prod-
uct 9b—an analogue of 1b shown in Scheme 1—was isolat-
ꢀ
ꢀ
ed and fully characterized by NMR and MS analyses. This
and 6 suggest that the electron donating effect by the CC
SiMe3 group in 6 may be dominant. The ratio of the photo-
ꢀ
demonstrates that the photochromic switching and C C
coupling phenomenon is not limited to the ppy-chelate chro-
mophore. Using 1 as the internal standard, a competitive
photolysis experiment at 365 nm monitored by NMR spec-
troscopy established that the rate of photoisomerization of 9
to 9a is much slower than that of 1 and the ratio of the rela-
tive rate constants of k(1)/k(9) was determined to be ꢁ15
(see Supporting Information), despite the much greater ab-
sorbance of 9 at 365 nm. The slower conversion of 9 to 9a
relative to 1 to 1a can be attributed to the greater p conju-
gation of the benzo[b]thiophenepyridyl chelate, which stabil-
izes the excited state and enhances the radiative decay path-
way. This explanation is supported by the significantly
higher quantum efficiency of 9 compared to that of 1, as
shown in Table 2.
ACHTUNGTRENiNUNG somerization rate constants of 1/1a and 5/5a were previous-
ly determined to be about 20[4b] by UV/Vis spectroscopy,
which agrees well with the value determined here by the
NMR experiments. Using the absorbance data and the rela-
tive rate constants,[12] the relative photoisomerization quan-
tum efficiency of 1 versus that of 3–6 and 9 was estimated to
be approximately 2.5, 1.2, 74, 2.2, and 36, respectively, under
NMR photolysis conditions. Thus, compound 1, the simplest
in this group of compounds, is the most efficient in the pho-
toisomerization process.
ꢀ
The C C coupled products 3b–6b—analogues of 1b—
were obtained by either exposing 3a–6a to air, or by irradi-
ating solutions of 3 or 4 in benzene or toluene in the pres-
ence of O2, and have been identified and characterized by
MS analyses. This further supports that these compounds
undergo photoisomerization in the same manner as 1 does.
In addition, the photoisomerizations of 4–6 are also fully re-
versible and thermally slow at ambient temperature and fast
upon heating to 708C, as is the case for compound 1 (see
Supporting Information). Compound 9a does not reverse
back to 9 at ambient temperature. Heating the solution of
9a in benzene or toluene at 708C led to unidentified decom-
position products (see Supporting Information), an indica-
In contrast to compound 9, the indolylpyridyl chelate
compound 10 undergoes decomposition upon irradiation in-
ꢀ
stead of photoisomerization. The C C coupled product 10b
was not observed when the photolysis was conducted under
air. Although we were not able to obtain a single-crystal
structure for 10, the structure of a closely related compound
B[5-(7-azaindolyl)-2-Br-py]Mes2 was determined (see Sup-
porting Information) which has a Mes-B-Mes angle of
120.2(3)8—the largest among this group of compounds. In
Chem. Eur. J. 2010, 16, 4750 – 4761
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4757

S. Wang et al.
tion that compound 9a may be thermally unstable. Attempts
to reverse the photoisomerization process by using visible
light have not been successful. The fluoro-substituted com-
pound 2 was found to be unstable toward photolysis and an
insoluble orange solid was observed after the solution of 2
in benzene or toluene was irradiated by UV light under the
same conditions as for 1, and 3–6. The fluoro substituents
on the phenyl ring are clearly responsible for the photo-
chemical instability of the molecule, although the exact
cause is not known. Our attempts to synthesize mono- and
difluoro-substituted analogues to better understand this phe-
nomenon were unsuccessful due to difficulties in controlling
the lithiation site on the fluoro-substituted phenyl ring.
grows in intensity with exposure time. Because this peak is
absent when the UV lamp is off, it can be assigned to the
species of 1a, 3a, 4a, and 6a, which are generated by photo-
isomerization. For 1, 4, and 6, we were able to obtain the
CV diagrams representing the pure species 1a, 4a, and 6a,
because of the fast photoisomerization. The reduction peaks
of 1a, 4a, and 6a appear at a similar potential as those of 1,
4, and 6, indicating that the LUMO energy level does not
change significantly upon isomerization, which is consistent
with the TD-DFT results obtained for 1 and 1a.[4a] The low
oxidation potentials of 1a, 3a, 4a, and 6a indicate that the
dark species have a high HOMO level (ꢀ4.26, ꢀ4.30, ꢀ4.28,
and ꢀ4.31 eV, respectively), thus explaining their high sus-
ceptibility toward oxidation by oxygen. The electrochemical
energy gaps of 1a, 3a, 4a, and 6a, obtained from the reduc-
tion and the oxidation potentials, follow the same trend as
that of the optical energy gaps obtained from the absorption
edge as shown in Table 3. For compounds 5 and 9, it was not
possible to detect the dark species conclusively in the CV di-
agram due to the very slow conversion of 5 and 9 to 5a and
9a.
Photophysical and electrochemical properties of the dark
species: The dark colored species of 1a, 3a–6a, and 9a are
characterized by one intense and broad absorption band in
the visible region, as shown in Figure 8. Compound 3a has
the longest absorption wavelength (728 nm), while com-
pounds 1a and 4a have the shortest wavelength (605 nm).
Previous TD-DFT calculation results[4a] for 1a showed that
its HOMO is dominated by the “bent bonds”[11] of the BC2
ring between the B and the C atoms and the p orbitals of
the cyclohexadienyl ring, while its LUMO is mostly the p*
of the ppy chelate. In addition, TD-DFT results confirmed
that the dark color of 1a arises predominately from the
HOMO to LUMO transition. The observed trend of the ab-
sorption band of 1a, 3a–6a, and 9a is consistent with this
transition assignment.
Table 3. Photophysical and electrochemical data of 1a, 3a–6a, and 9a.
ox
1=2
[V][b]
red
1=2
[V][b]
Absorption[a] Optical energy
E
E
Electrochemical
energy gap [eV]
lmax [nm]
gap [nmeVꢀ1
]
1a 605
770/1.61
ꢁ930/ꢁ1.33
770/1.61
820/1.51
825/1.50
ꢀ0.54 ꢀ2.30 1.76
ꢀ0.50 ꢀ1.68 1.18
ꢀ0.52 ꢀ2.32 1.80
ꢀ0.51 ꢀ2.03 1.52
3a 728
4a 605
5a 640
6a 650
9a 628
The dark colored species of 1a, 3a, 4a, and 6a can also
be detected readily by electrochemical analysis. As shown in
Figure 9, a distinct oxidation peak appears at about ꢀ0.50 V
(vs. FeCp2+/0) in the CV diagram during irradiation that
800/1.55
[a] Recorded in toluene (ꢁ1.0ꢀ5 m). [b] Recorded in DMF, relative to the
+/0
potential of FeCp2
.
Conclusion
Based on the results of this study, we suggest the following.
1) The photoisomerization of the N,C-chelate four-coordi-
nate boron compounds is most likely initiated by a pho-
toinduced charge transfer from the HOMO localized on
the aryl group to the LUMO localized on the chelate
unit.
2) The bulky mesityl groups are required to observe the
photoisomerization phenomenon of the ppy-based four-
coordinate boron compounds, which can be attributed to
the destabilization of the molecule and the HOMO level
by the bulky mesityl. Other bulky aryl groups most likely
have the same impact on the photoisomerization of N,C-
chelate boron chromophores, which should be a topic of
future work.
3) Electron-donating groups such as SiMe3 on the ppy che-
late appear to accelerate the photoisomerization rate by
destabilizing the excited state.
4) Extended p conjugation on the ppy chelate appears to
retard the photoisomerization process by stabilizing the
excited state.
Figure 9. CV diagrams of 1, 3, 4, and 6 before and after UV irradiation
(365 nm) in DMF with NBu4PF6 as the electrolyte and 200 mVsꢀ1 scan
rate. Inset: Photographs showing the color change of the solution of com-
pound 6 in the electrochemical cell with and without exposure to UV
light.
4758
www.chemeurj.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 4750 – 4761

Organoboron Compounds
FULL PAPER
procedure. Yellow crystals of 2 were obtained (0.07 g, 39%). 1H NMR
(CD2Cl2): d=8.72 (d, J=6.0 Hz, 1H), 8.29 (d, J=8.0 Hz, 1H), 8.12 (t, J=
7.8 Hz, 1H), 7.41 (t, J=6.6 Hz, 1H), 6.69 (s, 4H), 2.19 (s, 6H), 1.80 ppm
(s, 12H); 13C NMR (CD2Cl2): d=154.2, 148.4–148.3 (m), 146.3, 146.0–
146.1 (m), 143.9–143.8 (m), 141.9, 140.6, 137.9–137.8 (m), 134.9, 130.4,
123.4, 122.6 (d, JC,F =12.6 Hz), 24.6, 20.6 ppm; 19F NMR (CD2Cl2): d=
ꢀ129.9 (t, JF =20.9 Hz, 1F), ꢀ144.4 (td, JF =20.5, 4.9 Hz, 1F), ꢀ152.2 to
ꢀ152.4 (m, 1F), ꢀ161.6 ppm (t, JF =19.4 Hz, 1F); 11B NMR (CD2Cl2):
d=8.08 ppm; HRMS: m/z calcd for C29H26BF4N: 475.2028 [M]+; found:
475.2093; elemental analysis calcd (%) for C29H26BF4N: C 73.28, H 5.51,
N 2.95; found: C 73.08, H 5.61, N 2.91.
5) Electron-withdrawing groups, such as fluoro or carbonyl
groups on the ppy chelate, appear to either slow down
the photoisomerization process and/or cause irreversible
photodecomposition of the molecule, and thus these sub-
stitution patterns may not be desirable for achieving
stable and reversible photoswitching systems.
6) The photoisomerization is not limited to the ppy-chelate
system, as demonstrated by compound 9. However, the
boron compounds based on the non-ppy-chelate ligands
benzo[b]thiophenepyridyl or indolylpyridyl have either a
poor thermal reversibility for the photoisomerization
process or decompose upon photolysis, thus making
them unsuitable for use in photochromic systems.
Compound 4: 2-(2-Bromophenyl)-5-(trimethylsilyl)pyridine (0.64 g,
2.10 mmol) in Et2O, nBuLi (1.60m in hexane, 1.43 mL, 2.30 mmol), and
at BMes2F (0.71 g, 2.64 mmol) were treated as described in the general
procedure. Colorless crystals of 4 were obtained (0.81 g, 81%). 1H NMR
(CD2Cl2): d=8.68 (s, 1H), 8.09 (d, J=8.0 Hz, 1H), 8.00 (d, J=8.0 Hz,
1H), 7.91 (d, J=6.8 Hz, 1H), 7.74 (d, J=6.8 Hz, 1H), 7.31–7.27 (m, 2H),
6.65 (s, 4H), 2.17 (s, 6H), 1.79 (s, 12H), 0.28 ppm (s, 9H); 13C NMR
(CDCl3): d=159.2, 150.0, 145.6, 140.0, 135.4,133.9, 133.7, 131.2, 130.1,
125.5, 122.0, 117.6, 24.9, 20.7, ꢀ1.67 ppm; 11B NMR (CD2Cl2): d=
7.21 ppm; HRMS: m/z calcd for C32H38BNSi: 475.2867 [M]+; found:
475.2867; elemental analysis calcd (%) for C32H38BNSi: C 80.82, H 8.05,
N 2.95; found: C 80.70, H 7.99, N 2.97.
Although it is not clear yet whether or not other unex-
plored N,C-chelate ligands will perform better, the ppy-
based compounds are most promising as reversible photo-
thermal switching materials. Compound 1, the simplest
member of the ppy group, is the most efficient in the photo-
ACHTUNGTRENNUNG
Compound 6: 2-(2-Bromophenyl)-5-[(trimethylsilyl)ethynyl]pyridine
(0.25 g, 0.76 mmol), nBuLi (1.60m in hexane, 0.57 mL, 0.91 mmol), and
BMes2F (0.27 g, 1 mmol) were treated as described in the general proce-
dure. Yellow crystals of
6
were obtained (0.31 g, 83%). 1H NMR
(CDCl3): d=8.69 (s, 1H), 7.98 (d, J=8.0 Hz, 1H), 7.90 (d, J=8.0 Hz,
1H), 7.81 (d, J=7.2 Hz, 1H), 7.79 (d, J=7.2 Hz, 1H), 7.32 (d, J=7.2 Hz,
1H), 7.27 (d, J=7.2 Hz, 1H), 6.68 (s, 4H), 2.21 (s, 6H), 1.79 (s, 12H),
0.27 ppm (s, 9H); 13C NMR (CDCl3): d=166.7, 158.7, 149.4, 145.5, 143.5,
140.6, 134.8, 134.4, 132.2, 131.6, 130.3, 125.9, 122.3, 118.5, 117.6, 100.9,
100.1, 25.4, 21.2, 0.1 ppm; 11B NMR (CD2Cl2): d=7.93 ppm; HRMS: m/z
calcd for C34H38BNSi: 499.2867 [M]+; found: 499.2891; elemental analysis
calcd (%) for C34H38BNSi: C 81.74, H 7.67, N 2.80; found: C 82.91, H
7.81, N 2.83.
Experimental Section
General: Starting materials were purchased from Aldrich Chemical Co.
and were used with out further purification. Solvents were dried using an
activated alumina column system, purchased from Innovative Technology
Inc., while C6D6 was dried over CaH2 in a glove box. All reactions were
carried out under an atmosphere of dry nitrogen using standard Schlenk
techniques. Purifications using column chromatography were performed
using ultra pure silica gel (70–230 Mesh) 60 ꢂ, purchased from Silicycle.
NMR spectra (1H,13C, 11B, and 19F) were recorded at room temperature
on a Bruker Avance 400 or 500 MHz. Proton and 13C chemical shifts
were reported with respect to solvent peaks as internal standard, while
BF3.Et2O and CFCl3 were used as references for 11B and 19F NMR sig-
nals, respectively. Excitation and emission spectra were recorded on a
Photon Technologies International QuantaMaster Model C-60 spectrom-
eter. UV/VIS spectra were recorded on an Ocean Optics Model CHEM-
USB4-UV/VIS. Elemental analyses were performed by Canadian Micro-
analytical Services Ltd. Delta, BC. Cyclic voltammetry was performed
using a BAS CV-50W analyzer with a scan rate of 200 mVsꢀ1 and sample
concentrations of 5 mg/3.0 mL DMF, using 0.10m NBu4PF6 (TBAP) as
supporting electrolyte and Ag/AgCl reference electrode. The ferroceni-
um/ferrocene couple was used as the internal standard (Eo =0.56 V). 2-
(2-Bromophenyl)pyridine,[4a] 2-bromo-5-(trimethylsilyl)pyridine,[13] 6-bro-
Compound 7: 2-(2-Bromophenyl)pyridine (1.17 g, 5.0 mmol) in THF
(50 mL), nBuLi (1.6m in hexane, 3.13 mL, 5.0 mmol), and BPh2Cl (1.1 g,
5.0 mmol) were treated by the general procedure. Colorless crystals of 7
were obtained (0.52 g, 33%). 1H NMR (CD2Cl2): d=8.52 (dt, J=5.6 Hz,
J=1.2 Hz, 1H), 8.20–8.05 (m, 2H), 7.96 (d, J=8.0 Hz, 1H), 7.71 (d, J=
7.2 Hz, 1H), 7.55–7.42 (m, 2H), 7.38 (td, J=7.4 Hz, J=1.2 Hz, 1H),
7.30–7.10 ppm (m, 10H); 13C NMR (CD2Cl2): d=158.6, 144.3, 141.2,
136.4, 133.2, 131.3, 130.7, 127.6, 126.3, 125.9, 122.5, 122.1, 118.7 ppm;
11B NMR (CD2Cl2): d=6.48 ppm; HRMS: m/z calcd for C23H18BN:
319.1532 [M]+; found: 319.1530; elemental analysis calcd (%) for
C23H18BN: C 86.54, H 5.68, N 4.39; found: C 86.46, H 5.81, N 4.42.
Compound 8: 2-(2-Bromophenyl)pyridine (0.58 g, 2.5 mmol) in THF
(50 mL), nBuLi (1.6m in hexane, 1.56 mL, 2.5 mmol), and BACHTUNGTRENNUNG(C6F5)2F
(2.5 mmol based on 100% yield) were treated as described in the general
procedure. Colorless crystals of 8 were obtained (0.20 g, 16%). 1H NMR
(CD2Cl2): d=8.61 (d, J=5.6 Hz, 1H), 8.21 (td, J=7.8 Hz, J=1.2 Hz,
1H), 8.12 (d, J=8.4 Hz, 1H), 7.94 (d, J=6.8 Hz, 1H), 7.75 (d, J=7.2 Hz,
1H), 7.40–7.53 ppm (m, 3H); 13C NMR (CD2Cl2): d=159.2, 149.2–149.3
(m), 147.0–147.1 (m), 144.9, 142.9, 141.0–141.1 (m), 138.7–138.8 (m),
136.2–136.3 (m), 135.7, 132.1, 130.4, 127.5, 123.0, 122.2, 118.9 ppm;
19F NMR (CD2Cl2): d=ꢀ133.41 (dd, JF =25.2 Hz, JF =9.2 Hz, 4F),
ꢀ159.87 (t, JF =20.1 Hz, 2F), ꢀ165.16 to ꢀ165.29 ppm (m, 4F); 11B NMR
(CD2Cl2): d=0.77; HRMS: m/z calcd for C23H8BF10N, 499.0590 [M]+;
found: 499.0584; elemental analysis calcd (%) for C23H8BF10N: C 55.35,
H 1.62, N 2.81; found: C 55.51, H 1.90, N 2.76.
monicotinaldehyde,[14]
BACHTUNGTRENNUNG
(C6F5)2F,[15] BPh2Cl,[16] compounds[4] 1, 3, and 5
were synthesized by previously reported procedures. Synthetic proce-
dures for all N,C-chelate ligands are provided in the supporting informa-
tion.
General procedure for preparation of the 2-[2-(dimesitylboryl)phenyl]-
pyridines: nBuLi (1.6m in hexane, ca. 1.1 equiv for all compounds except
10 for which LDA was used) was added slowly to a solution of phenyl-
pyridine ligands (1 equiv) in THF (or Et2O), at ꢀ788C; the resulting so-
lution was stirred for 1 h at ꢀ788. Then, diaryl boron halide (1.3 equiv)
was added quickly under nitrogen and the solution was stirred at the
same temperature for 2 h and then stirred overnight at ambient tempera-
ture. After the solution was extracted with H2O/diethyl ether, the crude
product was purified by column chromatography using hexane/CH2Cl2 to
afford the pure product.
Compound 9: 2-(Benzo[b]thiophen-2-yl)pyridine (0.42 g, 2.0 mmol) in
THF (50 mL), nBuLi (1.6m in hexane, 1.25 mL, 2.0 mmol), and BMes2F
(0.6 g, 2.0 mmol) were treated as described in the general procedure.
1
Yellow crystals of 9 were obtained (0.56 g, 61%). H NMR (CD2Cl2): d=
Compound 2: 2-(2,3,4,5-Tetrafluorophenyl)pyridine (0.087 g, 0.38 mmol)
in THF (30 mL), nBuLi (1.6m in hexane, 0.24 mL, 0.38 mmol), and
BMes2F (0.114 g, 0.38 mmol) were treated as described in the general
8.65 (d, J=5.6 Hz, 1H), 8.04 (d, J=7.6 Hz, 1H), 7.98–7.89 (m, 2H), 7.66
(d, J=8.0 Hz, 1H), 7.35–7.27 (m, 2H), 7.18 (t, J=6.6 Hz, 1H), 6.68 (s,
4H), 2.18 (s, 6H), 1.90 ppm (s, 12H); 13C NMR (CD2Cl2): d=155.6,
Chem. Eur. J. 2010, 16, 4750 – 4761
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4759

S. Wang et al.
146.1, 146.0, 141.4, 141.2, 140.3 134.5, 134.1, 130.4, 126.9, 126.2, 125.8,
125.4, 125.0, 123.8, 120.4, 118.5, 24.9, 20.7 ppm; 11B NMR (CD2Cl2): d=
7.96 ppm; HRMS: m/z calcd for C31H30BNS: 459.2192 [M]+; found:
459.2198; elemental analysis calcd (%) for C31H30BNS: C 81.04, H 6.58,
N 3.05; found: C 81.68, H 6.81, N 2.99.
Acknowledgements
We thank the Natural Science and Engineering Research Council of
Canada for financial support. We are in debt to Dr. Rui-Yao Wang for
his assistance in crystal data collections. This work is financially support-
ed by the Natural Sciences and Engineering Research Council of
Canada.
Compound 10: 1-(Pyridin-2-yl)-1H-indole (0.58 g, 3.0 mmol) in Et2O
(50 mL), LDA (1.8m in heptane/THF/ethylbenzene, 1.8 mL, 3.2 mmol),
and BMes2F (0.89 g, 3.0 mmol) were reacted by the general procedure.
1
Yellow powder of 10 was obtained (1.10 g, 83%). H NMR (CD2Cl2): d=
8.23 (d, J=6.0 Hz, 1H), 8.12 (td, J=8.0 Hz, J=1.6 Hz, 1H), 7.87 (d, J=
8.8 Hz, 1H), 7.78 (d, J=8.0 Hz, 1H), 7.49 (dd, J=6.6 Hz, J=1.8 Hz,
1H), 7.28–7.18 (m, 2H), 7.11 (t, J=6.6 Hz, 1H), 6.67 (s, 4H), 6.54 (s,
1H), 2.19 (s, 6H), 1.89 ppm (s, 12H); 13C NMR (CD2Cl2): d=151.2,
146.1, 143.6, 140.2, 136.8, 134.5, 132.5, 129.9, 122.9, 121.6, 120.8, 117.6,
111.4, 109.8, 105.6, 24.4, 20.7 ppm; 11B NMR (CD2Cl2): d=3.38 ppm;
HRMS: m/z calcd for C31H31BN2: 442.2580 [M]+; found: 442.2577; ele-
mental analysis calcd (%) for C31H31BN2: C 84.16, H 7.06, N 6.33; found:
C 83.21, H 7.22, N 5.65.
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spectroscopy: Samples were dissolved in dry C6D6 in an NMR tube
under inert atmosphere. To remove any traces of oxygen that might be
present in the NMR tube, three freeze–pump–thaw cycles were per-
formed using liquid N2. The photolysis for all compounds except 7 was
then performed with either a Rayonet photochemical reactor with irradi-
ation at 350 nm at room temperature or a hand-held UV lamp at 365 nm,
followed by recording the 1H NMR spectra after different time periods.
For compound 7, the photolysis was carried out in C6D6 with a Spectro-
line UV lamp at 312 nm.
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cuvette (ꢁ10ꢀ5 m), under inert atmosphere in a glove box. After wrap-
ping the cuvette with aluminum foil, it was transferred out and photolysis
was then performed using a UVP UVGL-25 Compact UV lamp (365 nm)
at room temperature. The UV/Vis spectra were recorded after each ex-
posure time and the exposure times were added up.
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calculations, which were carried out at the B3LYP level of theory with 6–
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geometric parameters were used as starting points for the geometry opti-
mizations, while the Gaussview software package was used to input start-
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X-ray diffraction analysis: Single crystals of compounds 2, 4, 6–9, and
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CCDC-759906 (2), -759907 (4), -759908 (6), -759909 (7), -759910 (8),
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Chem. Eur. J. 2010, 16, 4750 – 4761

Organoboron Compounds
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Received: December 30, 2009
Published online: April 8, 2010
Chem. Eur. J. 2010, 16, 4750 – 4761
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