Single boryl isomerization in silyl-bridged photochromic diboryl dyes

Article abstract of DOI:10.1021/ol102319t

Silyl-bridged dimers of a ppy-BMes₂ (ppy = 2-phenylpyridine, Mes = mesityl) photochrome were found to undergo photochromic switching involving a single boryl unit only. A through-space intramolecular energy transfer was found to be responsible

Full text of DOI:10.1021/ol102319t

ORGANIC
LETTERS
2010
Vol. 12, No. 22
5266-5269
Single Boryl Isomerization in
Silyl-Bridged Photochromic Diboryl Dyes
Stephen K. Murphy, Chul Baik, Jia-sheng Lu, and Suning Wang*
Department of Chemistry, Queen’s UniVersity, 90 Bader Lane, K7L 3N6, Kingston,
Ontario, Canada
wangs@chem.queensu.ca
Received September 26, 2010
ABSTRACT
Silyl-bridged dimers of a ppy-BMes2 (ppy ) 2-phenylpyridine, Mes ) mesityl) photochrome were found to undergo photochromic switching
involving a single boryl unit only. A through-space intramolecular energy transfer was found to be responsible for the single-chromophore
isomerization phenomenon and fluorescence quenching. Steric congestion in the diboryl molecules was found to have an impact on
photoisomerization quantum efficiency.
Thermally reversible (t-type) photochromic dyes such as
spiropyrans, spirooxazines, and naphthopyrans have attracted
Scheme 1. Photoisomerization and Oxidation of B(ppy)Mes2
great attention due to their applications in smart windows
and ophthalmic glasses.1a-e While modification of these
existing photochromes allows fine-tuning of their photo-
chromic properties, the development of novel t-type photo-
chromic dyes with different switching kinetics, thermal
stabilities, and coloration is important for the development
of new switching devices and applications. We have recently
reported that four-coordinate organoboron compounds based
on a ppy-BMes2 (ppy ) 2-phenylpyridine, Mes ) mesityl)
chromophore can undergo a thermally reversible photoi-
somerization process accompanied by a distinct color change
from either colorless or light yellow to dark blue or green
(Scheme 1).2
matic color changes can be achieved even at low conversion;
and (3) the photoisomerization is accompanied by reversible
fluorescence quenching. One challenge is to understand the
various factors that influence the quantum efficiency of the
switching process in this new photochromic system. Re-
cently, we have shown that in π-conjugated polyboryl
molecules that contain more than one switchable B(ppy)Mes2
unit and acetylene linkers only one chromophore undergoes
photoisomerization.2d Furthermore, we have observed that
The B(ppy)Mes2 chromophore is attractive for use in
photochromic applications for several reasons: (1) it can be
quantitatively switched between the two isomers; (2) dra-
(1) (a) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. ReV. 2000, 100,
1741. (b) Kobatake, S.; Irie, M. Annu. Rep. Prog. Chem., Sect. C 2003, 99,
277. (c) Crano, J. C.; Guglielmetti, R. J. Organic Photochromic and
Thermochromic Compounds; Plenum Press: New York, 1999. (d) Ko, C.-
C.; Yam, V. W. W. J. Mater. Chem. 2010, 20, 2063, and references therein.
(e) Corns, S. N.; Partington, S. M.; Towns, A. D. Color. Technol. 2009,
125, 249. (f) Feringa, B. L. Molecular Switches; Wiley-VCH: Weinheim,
Germany, 2001.
(2) (a) Rao, Y. L.; Amarne, H.; Zhao, S. B.; McCormick, T. M.; Martic´,
S.; Sun, Y.; Wang, R. Y.; Wang, S. J. Am. Chem. Soc. 2008, 130, 12898.
(b) Baik, C.; Hudson, Z. M.; Amarne, H.; Wang, S. J. Am. Chem. Soc.
2009, 131, 14549. (c) Amarne, H.; Baik, C.; Murphy, S. K.; Wang, S.
Chem.sEur. J. 2010, 16, 4750. (d) Baik, C.; Murphy, S. K.; Wang, S.
Angew. Chem., Int. Ed. 2010, ASAP (DOI: 10.1002/anie.201003144).
10.1021/ol102319t  2010 American Chemical Society
Published on Web 10/14/2010

the single boryl isomerization in the polyboryl system appears
to be more efficient than the corresponding monoboryl
compound due to the antenna effect.3 To understand the role
of π-conjugation on the unusual single-boryl switching
phenomenon in polyboryl systems, we decided to study
diboryl compounds that contain a nonconjugated linker such
as a SiMe2 group.
Two new silyl-bridged diboryl compounds SiB1 and SiB2
have thus been synthesized (Figure 1). We have found that
Figure 2. Crystal structure of SiB1. Blue, N; green, B; red, Si.
atoms of the boryl units. While the SiB2 structure was not
determined by X-ray diffraction, molecular modeling shows
that SiB2 is much less crowded with the two B centers being
17.6 Å apart (see Supporting Information).
Two well-resolved reduction peaks were observed in the
CV diagram of SiB1, which may be attributed to the
sequential reduction of the two boryl units, by comparing to
that of B1 (Table 1 and Supporting Information), thus
supporting some degree of interaction between the two B
units. In contrast, the SiB2 molecule has only a single
reduction peak. The behavior of SiB2 is similar to that of
Me2Si(-t -CpFeCp)2 where no interactions between the
two Fe(II) centers were observed.8 Both compounds display
a low-energy shoulder in the absorption spectra which is
attributable to charge transfer from the mesityl groups
(HOMO) to the N,C-chelate backbone (LUMO) based on
previous work.2b The energy of this transition is lower for
B2 and SiB2 due to the greater π-conjugation in those
molecules. Comparison of the optical energy gaps of the
dimers to the corresponding monomers shows that dimer-
ization has a greater impact on SiB1 than SiB2 since SiB1
has a much smaller optical energy gap than B1, while SiB2
and B2 are similar in that respect. This can also be attributed
to the closer proximity of the two boryl chromophores in
SiB1 compared to SiB2.
Figure 1. Silyl-bridged dimers and corresponding monomers.
these molecules also display a single-chromophore isomer-
ization phenomenon, similar to that observed in previously
reported conjugated molecules.2d In addition, we have shown
that the second boryl unit can be isomerized by introduction
of an oxidant. The details are presented herein.
SiB1 was synthesized via monolithiation of 2,5-dibro-
mopyridine and silylation with Si(CH3)2Cl2, followed by
Suzuki-Miyaura coupling4 with 2 equiv of 2-Br-PhB(OH)2
and finally dilithiation and electrophilic quenching with
BMes2F. SiB2 was prepared via monolithiation of 1,4-
diiodobenzene and silylation with Si(CH3)2Cl2, followed by
Sonogashira coupling5 with H-Ct C-ppy-BMes2.2d Full
synthetic details can be found in the Supporting Information.
SiB1 crystallizes in the chiral space group P212121, and its
structure6 determined by single-crystal X-ray diffraction
analysis is shown in Figure 2. The bond lengths and angles
around the B center are similar to previously known N,C-
chelate BMes2 compounds.2,7 SiB1 is a very congested
molecule with a separation of 8.64 Å between the two B
Upon irradiation with 365 nm light, both compounds
underwent quantitative transformations to the dark colored
isomers SiB1a and SiB2a, respectively, which were moni-
Table 1. Photophysical and Electrochemical Properties
λmax (ε)a
optical energy gap
λEm
(nm)a
458
ΦF
(%)b
10
E1/2
kthermal × 106
thermal reversal t1/2
red
d
e
compound
(nm, M-1 cm-1
)
(eV)
3.10
(V)c
Φphotoiso.
(s-1
)
(h)
52f
B1
320 (15200)
357 (3207)
327 (21705)
366 (9160)
313 (24 900)
377 (11 700)
322 (63476)
384 (27561)
-2.30
0.85
3.7
8.2
SiB1
B2
2.85
2.88
2.72
479
490
484
36
37
48
-2.23, -2.41
-2.03
0.60
0.33
0.58
23f
6
30.9
24.8
SiB2
-2.06
8
a At 10-5 in toluene. b With 9,10-diphenylanthracene (ΦF ) 90%) as the standard in toluene. c Relative to FeCp20/+ in DMF. d At 25 °C in toluene, using
ferrioxalate actinometry at 360 nm excitation, the value is the average of four measurements. e At 35 °C in d6-benzene. f Calculated from the first-order rate
constants.
Org. Lett., Vol. 12, No. 22, 2010
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tored by UV-vis, fluorescence, and NMR spectroscopy. The
UV-vis spectra (Figure 3) of both compounds showed the
Figure 4.
1H NMR spectral change of SiB1 during irradiation.
1
Similar H NMR spectral changes were observed for SiB2
(see Supporting Information). Solids of SiB1a and SiB2a
may be isolated from a pure oxygen-free hexane solvent.
Because of the lack of π-conjugation between the two
boryl units in the silyl diboryl molecules, this single-
chromophore isomerization phenomenon can be attributed
to a through-space intramolecular energy transfer from the
nonisomerized chromophore to the low-energy absorption
band of the isomerized chromophore based on Kasha’s
principle,9 which dissipates the excited state energy, thus
preventing isomerization of the second chromophore. Com-
plete fluorescence quenching was also observed as a con-
sequence of the energy transfer in SiB1. However, the
isomerized product of SiB2 has a weak emission peak despite
the reaction being quantitative, indicating that the larger
separation of the two boryl units results in weaker through-
space interactions.
The single-chromophore photoisomerization phenomenon
of the silyl-bridged boryl systems is in contrast to the
extensively studied dithienylethene (DTE) photochromic
system, where simultaneous photoisomerization of multiple
chromophores in a single molecule is frequently observed.10
For example, a silyl-bridged DTE dimer was found to
undergo multichromophore switching, reaching a photosta-
tionary state consisting of the nonisomerized, singly isomer-
ized, and doubly isomerized compounds, despite the presence
of a low-energy absorption band in the isomerized state of
the DTE system.11 This difference may be caused by the
much slower photoisomerization rate of the N,C-chelate boryl
compounds that cannot compete kinetically with the energy
transfer process. Efforts are being taken by us to determine
the rates of the various competing excited state processes of
the N,C-chelate boryl systems.
Figure 3. Changes in the absorption spectra (left) and emission
spectra (right) of SiB1 (top) and SiB2 (bottom) during irradiation
with 365 nm light. Insets: Photographs showing color changes of
the compounds before and after irradiation.
appearance and rapid growth of a peak at λmax ) 624 nm
for SiB1 and λmax ) 653 nm for SiB2 upon irradiation, which
is characteristic of the isomerized unit shown in Scheme 1.
However, the intensity of this peak relative to the largest
peak at the completion of isomerization was smaller for the
diboryl compounds: 32% for B1, 18% for SiB1, 20% for
B2, and 8% for SiB2. Thus, the spectral change of the diboryl
compounds is approximately one-half that of the correspond-
ing monoboryl compounds, indicating that only one of the
ppy-BMes2 groups likely undergoes isomerization in the
silyl-bridged dimers.
This was in fact confirmed by 1H NMR spectra. As shown
in Figure 4, the 1H NMR spectrum of SiB1 upon irradiation
shows the complete disappearance of the original pyridyl
proton signal, H1, at 8.83 ppm, and the growth of two new
peaks H1′ and H1′′, at 8.79 ppm and 8.67 ppm, respectively,
that retain a 1:1 ratio throughout the entire photolysis process.
The appearance of these peaks is accompanied by peaks H3
and H4 that integrate to one proton each and are characteristic
of the isomerized species. The peak ratios and integration
values unambiguously confirmed that only one boryl unit
undergoes isomerization, forming SiB1a quantitatively.
(3) Kawa, M. Top. Curr. Chem. 2003, 228, 193, and references therein.
(4) (a) Suzuki, A. Acc. Chem. Res. 1982, 15, 178. (b) Miyaura, N.;
Suzuki, A. Chem. ReV. 1995, 95, 2457.
(9) (a) Kasha, M. Discuss. Faraday Soc. 1950, 9, 14. (b) Kaieda, T.;
Kobatake, S.; Miyasaka, H.; Murakami, M.; Iwai, N.; Nagata, Y.; Itaya,
A.; Irie, M. J. Am. Chem. Soc. 2002, 124, 2015.
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P.; Schluter, A. D. J. Am. Chem. Soc. 2003, 125, 6907.
(6) The crystal data of SiB1 have been deposited at the Cambridge
Crystallographic Data Centre (CCDC 789494).
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V.; Ishow, E.; Hoang-Thi, K.; Ledoux, I.; Nakatani, K.; Le Bozec, H. Angew.
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2007, 5, 1170.
5268
Org. Lett., Vol. 12, No. 22, 2010

The photoisomerization quantum efficiencies of the silyl-
bridged diboryl compounds were measured in toluene using
ferrioxalate actinometry at 360 nm excitation.12 The diboryl
compound SiB1 was found to have a lower quantum
efficiency (0.60) than the corresponding monoboryl B1
(0.85), while SiB2 was found to nearly double the quantum
efficiency (0.58), compared to that (0.33) of B2. The greatly
enhanced photoisomerization quantum efficiency of SiB2
compared to B2 may be attributed to the antenna effect3
where one boryl center facilitates the other boryl center to
undergo isomerization via fast intramolecular energy transfer.
The decreased quantum efficiency of SiB1 relative to B1
may be attributed to the greater steric congestion in the
diboryl molecule that offsets the antenna effect. Thus, we
have illustrated that linking two photoswitchable boryl
centers together via a silyl group in a nonsterically congested
molecule may be an effective stragegy in enhancing a single
boryl photoisomerization quantum efficiency of polyboryl
compounds.
The rate of the reverse thermochromic reaction was
measured at 35 °C in C6D6 (see Figure S8 in Supporting
Information). All compounds undergo a first-order reaction
to restore the starting material at this temperature. The half-
lives of the isomerized species decreased in the order B1 >
SiB1 > SiB2 ≈ B2 (Table 1), indicating that increased
π-conjugation and steric congestion enhance the rate of the
thermal reversal, as previously observed.2d
Because the C-C coupled dark isomer is known to react
with oxygen to form a colorless C-C coupled product B1b
as shown in Scheme 1, oxygen may be used to eliminate
the low-energy absorption band of the isomerized species
and trigger isomerization of the second boryl in the diboryl
molecules. This is demonstrated by the reaction of the dark
isomer SiB1a with oxygen that resulted in a quantitative
formation of SiB1b, accompanied by the reappearance of
an emission peak that is slightly blue-shifted (λmax ) 470
nm) compared to SiB1, and the disappearance of the low-
energy absorption band at 624 nm. This further supports that
only one of the boryl units had undergone isomerization and
subsequent oxidation, leaving the second boryl intact. The
hypsochromic shift of the emission maximum of SiB1b
relative to that of SiB1 (Figure 5) is also consistent with the
interactions between the two boryl units in SiB1. Irradiating
the solution of SiB1b under N2 at 365 nm resulted in once
Figure 5. Normalized fluoresence spectra of SiB1 and its C-C
coupled products.
again fluorescence quenching and the appearance of the low-
energy absorption band of the dark isomer SiB1c at 614 nm
(see Supporting Information). Exposure of SiB1c to O2
quenched the low-energy absorption band and led to the
appearance of a very weak emission band at 440 nm,
attributed to the double C-C coupled product SiB1d, a much
weaker emitter than SiB1 (see Supporting Information). The
stepwise deborylation, C-C coupling, and color change of
diboryl compounds with oxygen is certainly not desirable
for photochromic switches but may find use in other
applications.
In summary, we have demonstrated that the single-boryl
isomerization phenomenon observed previously is not limited
to π-conjugated systems. Further, we have shown that linking
two photoswitchable boryl units together by a silyl group
may greatly enhance the photoisomerization quantum ef-
ficiency in a sterically noncongested system. Intramolecular
energy transfer and steric factors play an important role in
the photoisomerization process of the diboryl compounds.
Acknowledgment. This work was supported by the
Natural Sciences and Engineering Research Council of
Canada.
Supporting Information Available: Synthesis and char-
acterization of SiB1 and SiB2, cif file of SiB1, calculated
structure of SiB2, electrochemical, photophysical, photolysis,
and thermolysis data. This material is available free of charge
via the Internet at http://pubs.acs.org.
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D.; Whelan, J.; Dust, J. M.; Hoz, J. M; Buncel, E. J. Phys. Chem. A 2009,
113, 6640.
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