Photochromic thienylpyridine-bis(alkynyl)borane complexes: Toward readily tunable fluorescence dyes and photoswitchable materials

Article abstract of DOI:10.1021/ol3004595

A series of diarylethene-containing N^{^}C chelated thienylpyridine-bis(alkynyl)borane complexes has been designed and synthesized. Their photophysical and photochromic properties have been investigated and presented. The characteristic low-energy

Full text of DOI:10.1021/ol3004595

ORGANIC
LETTERS
2012
Vol. 14, No. 7
1862–1865
Photochromic Thienylpyridineꢀ
Bis(alkynyl)borane Complexes: Toward
Readily Tunable Fluorescence Dyes and
Photoswitchable Materials
Hok-Lai Wong,†,‡ Wing-Tak Wong,‡ and Vivian Wing-Wah Yam*,†,‡
Institute of Molecular Functional Materials (Areas of Excellence Scheme,
University Grants Committee (Hong Kong)) and Department of Chemistry,
The University of Hong Kong, Pokfulam Road, Hong Kong
wwyam@hku.hk
Received February 24, 2012
ABSTRACT
A series of diarylethene-containing N∧C chelated thienylpyridineꢀbis(alkynyl)borane complexes has been designed and synthesized. Their
photophysical and photochromic properties have been investigated and presented. The characteristic low-energy absorption band of their closed
forms could be readily tuned from the visible range to the near-infrared region.
Developments of innovative boron-based fluorescence
dyes for organic light-emitting devices,1 chemosensors,2
and fluorescence probes for biological analysis2,3 have
received much attention in recent years. Tuning of their
electronic properties has been important to facilitate
charge transport with appropriate emission color in
† Institute of Molecular Functional Materials (Areas of Excellence
Scheme, University Grants Committee (Hong Kong))
(3) Yoshino, J.; Furuta, A.; Kambe, T.; Itoi, H.; Kano, N.;
Kawashima, T.; Ito, Y.; Asashima, M. Chem.;Eur. J. 2010, 16, 5026.
(4) (a) Gabe, Y.; Urano, Y.; Kikuchi, K.; Kojima, H.; Nagano, T.
J. Am. Chem. Soc. 2004, 126, 3357. (b) Zeng, L.; Miller, E. W.; Pralle, A.;
Isacoff, E. Y.; Chang, C. J. J. Am. Chem. Soc. 2006, 128, 10. (c) Peng, X.;
Du, J.; Fan, J.; Wang, J.; Wu, Y.; Zhao, J.; Sun, S.; Xu, T. J. Am. Chem.
Soc. 2007, 129, 1500. (d) Ma, G.; Muller, A. M.; Bardeen, C. J.; Cheng,
Q. Adv. Mater. 2006, 18, 55. (e) Ulrich, G.; Goze, C.; Guardigli, M.;
Roda, A.; Ziessel, R. Angew. Chem., Int. Ed. 2005, 44, 3694.
(5) (a) Rousseau, T.; Cravino, A.; Bura, T.; Ulrich, G.; Ziessel, R.;
Roncali, J. Chem. Commun. 2009, 13, 1673. (b) Erten-Ela, S.; Yilmaz,
M. D.; Icli, B.; Dede, Y.; Icli, S.; Akkaya, E. U. Org. Lett. 2008, 10, 3299.
(c) Rousseau, T.; Cravino, A.; Ripaud, E.; Leriche, P.; Rihn, S.; De
Nicola, A.; Ziessel, R.; Roncali, J. Chem. Commun. 2010, 46, 5082. (d)
Kim, B.-S.; Ma, B.; Donuru, V. R; Liu, H.; Frechet, J. M. J. Chem.
Commun. 2010, 46, 4148.
‡ Department of Chemistry, The University of Hong Kong, Pokfulam
Road, Hong Kong
(1) (a) Wang, S. Coord. Chem. Rev. 2001, 215, 79. (b) Cui, Y. D.; Liu,
Q. D.; Bai, D. R.; Jia, W. L.; Tao, Y.; Wang, S. Inorg. Chem. 2005, 44,
601. (c) Kappaun, S.; Rentenberger, S.; Pogantsch, A.; Zojer, E.;
€
Mereiter, K.; Trimmel, G.; Saf, R.; Moller, K. C.; Stelzer, F.; Slugovc,
C. Chem. Mater. 2006, 18, 3539. (d) Chen, H.-Y.; Chi, Y.; Liu, C.-S.; Yu,
J.-K.; Cheng, Y.-M.; Chen, K.-S.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H.;
Carty, A. J.; Yeh, S.-J.; Chen, C.-T. Adv. Funct. Mater. 2005, 15, 567. (e)
Wakamiya, A.; Taniguchi, T.; Yamaguchi, S. Angew. Chem., Int. Ed.
2006, 45, 3170. (f) Rao, Y.-L.; Wang, S. Inorg. Chem. 2011, 50, 12263.
(2) (a) Zhang, G.; Palmer, G. M.; Dewhirst, M. W.; Fraser, C. L. Nat.
Mater. 2009, 8, 747. (b) Kano, N.; Yoshino, J.; Kawashima, T. Org. Lett.
2005, 7, 3909. (c) Maeda, H.; Mihashi, Y.; Haketa, Y. Org. Lett. 2008,
10, 3179.
r
10.1021/ol3004595
Published on Web 03/21/2012
2012 American Chemical Society

light-emitting devices and in various applications. Among
them, the BODIPY family could be regarded as a success-
ful candidate. Notable achievementshave been made inthe
field of fluorescence sensing and labeling,4 solar
application,5 and energy transfer studies.6 Nevertheless,
functionalizations and syntheses of these dyes remain
tedious, and they generally suffer from low overall yields.
Recently, considerable efforts have been made on the
development of multidentate chelated boron-based mate-
rials, which show very promising photophysical
properties.1ꢀ3,7,8 Yet, the functionalization of this class
of materials has been mainly confined to that at the
chelating ligand. To date, there is no report on the mod-
ification of N∧C chelated boron bromide fragments. We
believe that substitution of the bromo groups with ethy-
nylaryl moieties would provide additional functionality
and fine-tuning on their photophysical properties for
various applications.
photochromic studies of a series of diarylethene-
functionalized thienylpyridine bis-alkynyl borane com-
plexes (Figure 1).
Figure 1. Structures of photochromic thienylpyridine bis-alkynyl
borane complexes.
Besides being highly fluorescent, recent reports have
shown that some four-coordinated N∧C chelated dimesityl-
boron complexes also exhibit interesting reversible photo-
chromic behaviors under inert atmosphere.9 On the
other hand, diarylethene-containing photoswitchable ma-
terials with reversible and optically addressable changes
have also gained enormous attention due to their potential
applications as optical memories or molecular switches for
modulation of the electronic properties.10 Design and
tuning of the photophysical properties of the open and
closed forms often involve tedious modification of the
diarylethene framework. This is especially important when
diarylethene molecules function as the active layer
in optoelectronic switching devices.11 With our current
interest in the study of photochromic diarylethene
frameworks,12 we anticipate that the N∧C chelated bis-
alkynyl borane complex system could serve as an ideal core
for the tuning of the photochromic and photophysical
behaviors by the systematic design of N∧C chelates
and judicious choice of alkynyls. In this communication,
we report the design, synthesis, photophysical and
The synthesis of photochromic diarylethene-containing
2-thienylpyridines has been reported by us previously.12g
Their incorporation into the boron center was accom-
plished by modification of a literature method reported
by Murakami and co-workers.8 The dibromoborane com-
plexes were subsequently reacted with the appropriate
alkynylmagnesium bromide in THF under inert atmo-
sphere and anhydrous conditions, and the target com-
pounds were obtained as yellow solids in moderate
yields. The synthetic route is summarized in Scheme 1.
Their identities have been confirmed by 1H, 11B, and 19
F
NMR, EI mass spectrometry, and satisfactory elemental
analyses. Theidentitiesof 1 and 6 have alsobeenconfirmed
by X-ray crystallography (Figure 2 and Figure S1, Sup-
porting Information (SI)). In both cases, the boron atom
adopts a distorted tetrahedral geometry, and the thienyl-
pyridine core is planarized, resulting in enhanced conjuga-
tion between them.1e The BꢀN bond distance is slightly
longer than the BꢀC(thienyl) bond distance. The average
˚
BꢀC(alkynyl) bond length in 6 is ca. 1.58 A, which is
similar to that of the tris(3,3-dimethyl-1-butynyl)borane
(6) (a) Burghart, A.; Thoresen, L. H.; Chen, J.; Burgess, K.;
˚
pyridine adduct13 and is much shorter than the BꢀBr bond
Bergstrom, F.; Johansson, L. B.-A. Chem. Commun. 2000, 22, 2203.
˚
distance in 1 (ca. 2.02 A). The bond angles at the boron
(b) Sunahara, H.; Urano, Y.; Kojima, H.; Nagano, T. J. Am. Chem. Soc.
2007, 129, 5597. (c) Harriman, A.; Izzet, G.; Ziessel, R. J. Am. Chem.
Soc. 2006, 128, 10868.
(7) (a) Kim, H.; Burghart, A.; Welch, M. B.; Reibenspies, J.; Burgess,
K. Chem. Commun. 1999, 18, 1889. (b) Kaiser, P. F.; White, J. M.;
Hutton, C. A. J. Am. Chem. Soc. 2008, 130, 16450.
(8) Ishida, N.; Moriya, T.; Goya, T.; Murakami, M. J. Org. Chem.
2010, 75, 8709.
(9) Baik, C.; Hudson, Z. M.; Amarne, H.; Wang, S. J. Am. Chem.
Soc. 2009, 131, 14549.
atom are not readily affected after the introduction of the
bis-alkynyl moiety. In addition, the diarylethene moiety
adopts an antiparallel conformation, with the two di-
methylthiophene rings pointing in opposite directions.
Details of the experimental procedures and the crystal
data have been included in the Supporting Information
(Tables S1ꢀS4).
(10) (a) Irie, M. Chem. Rev. 2000, 100, 1685. (b) Ko, C.-C.; Yam,
V. W.-W. J. Mater. Chem. 2010, 20, 2063. (c) Guerchais, V.; Ordronneau,
L.; Bozec, H. L. Coord. Chem. Rev. 2010, 254, 2533.
The electronic absorption and emission properties of the
borane complexes have been examined in various solvent
media at 298 K and are summarized in Table S5 (SI). As
shown in Figure 3a, the moderately intense absorption
band at ca. 380ꢀ450 nm is found to show strong depen-
dence on the substituents on the pyridine ring, with
electron-withdrawing groups showing lower absorption
(11) Jakobsson, F. L. E.; Marsal, P.; Braun, S.; Fahlman, M.;
Berggren, M.; Cornil, J.; Crispin, X. J. Phys. Chem. C 2009, 113, 18396.
(12) (a) Yam, V. W.-W.; Lee, J. K.-W.; Ko, C.-C.; Zhu, N. J. Am.
Chem. Soc. 2009, 131, 912. (b) Wong, H.-L.; Ko, C.-C.; Lam, W. H.;
Zhu, N.; Yam, V. W.-W. Chem.;Eur. J. 2009, 15, 10005. (c) Poon,
C.-T.; Lam, W. H.; Wong, H.-L.; Yam, V. W.-W. J. Am. Chem. Soc.
2010, 132, 13992. (d) Duan, G.; Yam, V. W.-W. Chem.;Eur. J. 2010, 16,
12642. (e) Duan, G.; Zhu, N.; Yam, V. W.-W. Chem.;Eur. J. 2010, 16,
13199. (f) Wong, H.-L; Tao, C.-H.; Zhu, N.; Yam, V. W.-W. Inorg.
Chem. 2011, 50, 471. (g) Chan, J. C.-H.; Lam, W. H.; Wong, H.-L.; Zhu,
N.; Wong, W.-T.; Yam, V. W.-W. J. Am. Chem. Soc. 2011, 133, 12690.
(13) Bayer, M. J.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem.
2002, 2069.
Org. Lett., Vol. 14, No. 7, 2012
1863

showing a bathochromic shift in emission energies. For
example, in benzene solutions, the emission energies are in
the following order: 1 (553 nm) < 2 (523 nm); 3 (496 nm)
< 6 (462 nm). By changing the bromo groups in 1 and 2 to
the alkynyl moieties in 3 and 6, respectively, the lumines-
cence quantum yields are improved [Φlum(CH2Cl2): 1
(0.05) < 3 (0.38); 2 (0.08) < 6 (0.31)]. The introduction
of the trifluoromethyl group is also found to show an
improvement in the luminescence quantum yield, which is
Scheme 1. Synthetic Route of 1ꢀ6
€
in accordance with the result reported by Jakle and co-
workers.14 Yet, the use of a more rigid alkynyl in 5 does not
show a positive influence on the luminescence quantum
yield. The electrochemical behavior has also been exam-
ined, and the data are tabulated in Table S6 (SI). The
compounds show two irreversible oxidative waves at
ca. þ1.3 and þ1.6 V vs SCE, which are almost insensitive
toward different alkynyls, and the oxidations are tenta-
tively assigned as the thiophene-centered oxidations. In
addition, an irreversible reduction wave is found at
ca. ꢀ1.55 or ꢀ1.98 V vs SCE. The potential shows a strong
dependence on the substituents on the pyridine ring, with
that bearing the electron-deficient trifluoromethyl group
being more readily reduced [3 (ꢀ1.58 V) vs 6 (ꢀ1.98 V) vs
SCE]. Thus, the first reduction is tentatively assigned as the
boron N∧C chelate core reduction. It is noteworthy that a
slight anodic shift has been observed in 4 (ꢀ1.54 V vs SCE)
when compared to 3 (ꢀ1.58 V vs SCE), which is in
agreement with the better π-accepting ability of the boron
N∧C chelate core, as supported by electronic absorption
studies. These results have demonstrated the possibility of
effective tuning of the photophysical properties in the N∧C
chelated boron bis-alkynyl complex system by the judi-
cious choice of the chelates and the alkynyls.
Upon photoexcitation, all the compounds show revers-
ible photochromism, as revealed by UVꢀvis absorption
spectral changes (Figure 4, Figure S3 (SI)) with well-
defined isosbestic points. The characteristic closed form
absorption band varies from ca. 600 nm in the visible
region in 6 to ca. 770 nm in the near-infrared region in 1
(Figure 4). They also display solvent-dependent absorp-
tion character. For example, the absorption energies of the
closed form of5 follow a trend of 701 nm (C6H6) < 665 nm
(acetone) < 659 nm (acetonitrile) (Figure S4 (SI)), which is
similar to that of the open form and is tentatively assigned
as the πꢀπ* transition of the extended thienylpyridine
borane complex core, with charge transfer character from
the condensed ring of the dithienylthiophene moiety to the
boronꢀpyridine unit. The large bathochromic shift with
respect to the open form is probably due to the increase in
the extent of π-conjugation, resulting in destabilization of
the HOMO and reduction in the HOMOꢀLUMO gap. It
is noteworthy that a simple modification on the pyridine
ring from the electron-donating methyl group to the
electron-withdrawing trifluoromethyl group is sufficient
to introduce a dramatic red shift in the lowest-energy
absorption band in the closed form, as indicated in 1 vs 2
(1820 cmꢀ1) and 3 vs 6 (1640 cmꢀ1). On the other hand, the
Figure 2. Perspective view of 1 with atomic numbering scheme.
Hydrogen atoms have been omitted for clarity. Thermal ellip-
soids are shown at the 30% probability level.
energies and longer absorption wavelengths [e.g., in ben-
zene solutions, the absorption energies are in the following
order: 1 (437 nm) < 2 (413 nm); 3 (408 nm) < 6 (387 nm)].
In addition, the electron-withdrawing alkynyl moieties
have led to a minor decrease in the absorption energy,
with the energy of 3 (408 nm in C6H6) > 4 (412 nm in
C6H6). These observations are consistent with an assign-
ment of πꢀπ* transition of the thienylpyridine borane
complex core, probably with charge transfer character
from the thiophene ring to the boron-pyridine unit due
to the sensitivity of the absorption toward various solvent
media. Upon light excitation, a solvent-dependent emis-
sion band is observed at ca. 460ꢀ580 nm (Figure S2 (SI)).
For example, the emission energies of 3 in different
solvents are in the following order: 496 nm (benzene) >
517 nm (dichloromethane) > 527 nm (acetone) > 535 nm
(acetonitrile). This is indicative of the involvement of
charge transfer excited states. The emission energy also
follows a trend (Figure 3b) similar to that of the absorption
properties, with the one bearing an electron-withdrawing
group, either on the pyridine ring or on the alkynyl group,
€
(14) Qin, Y.; Kiburu, I.; Shah, S.; Jakle, F. Org. Lett. 2006, 8, 5227.
1864
Org. Lett., Vol. 14, No. 7, 2012

modification on the alkynyl moiety provides an opportu-
nity for fine-tuning, as demonstrated by 3 vs 4 (190 cmꢀ1).
The cycloreversibility study has been performed on 6 in
degassed benzene solution as depicted in Figure S5 (SI),
which shows a reasonable fatigue resistance.
Figure 4. UVꢀvis absorption spectral changes of 1 in degassed
benzene solution upon irradiation at λ = 350 nm at 298 K.
In conclusion, a novel series of diarylethene-containing
N∧C chelated thienylpyridine bis-alkynyl borane com-
plexes has been designed and synthesized. Their photo-
physical, electrochemical, and photochromic properties
have been examined. It is envisioned that with the proper
choice of N∧C chelate and the alkynyl group, molecular
materials with desirable photophysical and photochromic
behaviors could be obtained.
Acknowledgment. V.W.-W.Y. acknowledges support
from the University Grants Committee Areas of Excel-
lence Scheme (AoE/P-03/08) and the URC Strategic
Research Theme on Molecular Materials. H.-L.W. ac-
knowledges the receipt of a University Postdoctoral Fel-
lowship. We are grateful to Dr. L. Szeto for her assis-
tance in X-ray crystal structure data collection and
determination.
Figure 3. (a) Electronic absorption and (b) normalized emis-
sion spectra of various borane complexes in benzene solution
at 298 K.
The photoisomerization also results in a reduction in
luminescence intensity (Figure S6 (SI)), probably due to
the presence of lower-lying excited states in the closed form
that quench the emission through effective energy transfer,
as is commonly observed in photochromic diarylethene
systems.12 The photochemical quantum yields have been
determined (Table S7 (SI)) and the methylpyridine-
containing compounds show a higher photocyclization
and photocycloreversion quantum yieldsthantheir trifluo-
romethylpyridine analogue (1 vs 2; 3 vs 6) with
Supporting Information Available. Detailed synthetic
procedure and characterization data for 1ꢀ6; details of
the experimental procedures and the crystal data for 1 and
6; perspective view of 6; normalized emission spectra of 4
in various solvent media; UVꢀvis spectral changes of 1
upon irradiation; UVꢀvis absorption spectra of the
closed form of 1ꢀ6 in benzene solution; UVꢀvis spectra
of the closed form of 5 in various solvent media; UVꢀvis
spectral change of 6 upon alternate photocyclization and
photocycloreversion; photophysical data for 1ꢀ6; elec-
trochemical data for 3ꢀ6; photochemical quantum yields
of 1ꢀ6. This material is available free of charge via the
Internet at http://pubs.acs.org.
Φphotocyclization(benzene):1 (0.07) < 2 (0.24) and 3 (0.10) <
6 (0.15). On the other hand, the quantum yields for
photocyclization are larger than that for photocyclorever-
sion, which is commonly observed in photochromic
diarylethene systems.10,12
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 7, 2012
1865