A versatile photochromic dithienylethene-containing β-diketonate ligand: Near-infrared photochromic behavior and photoswitchable luminescence properties upon incorporation of a boron(III) center

Article abstract of DOI:10.1021/ja105537j

A versatile dithienylethene-containing β-diketonate ligand and its boron(III) compounds have been successfully synthesized. Upon photocyclization, the ligand shows a new absorption band at 630 nm with good fatigue resistance and high thermal stability. Incorporation of the boron center has been demonstrated to shift the photochromic behavior to the NIR region.

Full text of DOI:10.1021/ja105537j

Published on Web 09/21/2010
A Versatile Photochromic Dithienylethene-Containing ꢀ-Diketonate Ligand:
Near-Infrared Photochromic Behavior and Photoswitchable Luminescence
Properties upon Incorporation of a Boron(III) Center
Chun-Ting Poon, Wai Han Lam, Hok-Lai Wong, and Vivian Wing-Wah Yam*
Institute of Molecular Functional Materials and Department of Chemistry, The UniVersity of Hong Kong,
Pokfulam Road, Hong Kong, P. R. China
Received June 24, 2010; E-mail: wwyam@hku.hk
Scheme 1
Abstract: A versatile dithienylethene-containing ꢀ-diketonate ligand
and its boron(III) compounds have been successfully synthesized.
Upon photocyclization, the ligand shows a new absorption band at
630 nm with good fatigue resistance and high thermal stability.
Incorporation of the boron center has been demonstrated to shift
the photochromic behavior to the NIR region.
B(C6F5)2F·OEt2, and BPh3 afforded the air-stable boron(III) com-
pounds 3ThacacBF2, 3ThacacB(C6F5)2, and 3ThacacBPh2, respec-
In the past decade, photochromic materials have received much
attention because of their ability to function as potential photoswit-
chable molecular devices and optical memory storage systems.1 Among
all kinds of such materials, organic diarylethene derivatives have been
extensively studied because of their excellent fatigue resistance and
thermal irreversibility.2 Recently, the coordination of photochromic
derivatives to metal centers to enhance the stability of the photochromic
system and modulate the photochromic reactivity has drawn much
attention.3,4 While ꢀ-diketonate is well-known as O,O-donor ligands
for transition-metal centers and lanthanides,5 there has been no report
on the functionalization of this class of ligands as diarylethene-
functionalized ligands. With our successful demonstration of the design
of various classes of versatile diarylethene-containing ligands,4c-e we
hypothesized that incorporation of the diarylethene moiety into the
ꢀ-diketonate ligand would open up new avenues and opportunities
for the development of new classes of photochromic materials. In
particular, boron(III) diketonates are known to exhibit rich photo-
physical properties.6 In spite of the increasing interest in boron-based
materials for optoelectronic,7 sensing,8 and various other applications,6e,f
the development of boron-based materials with photochromic functions
is extremely rare and underdeveloped.9 In view of the electron-
accepting ability of boron(III)7a and the high luminescence quantum
yield of boron diketonate,6 inclusion of a diarylethene-containing
ꢀ-diketonate ligand at the boron(III) center may readily give rise to
an interesting class of near-infrared (NIR) photochromic materials
that can enhance the semiconductor diode laser susceptibility for
applications in optical memory storage,10 without the need for the
tedious modification of the diarylethene framework,11 and may lead
to a new class of photoswitchable luminescence materials.12 Herein,
we report the syntheses and photochromic properties of a versatile
dithienylethene-containing ꢀ-diketonate ligand and its boron(III)
coordination compounds; their NIR photochromic behavior and
photoswitchable luminescence properties upon boron(III) coordination
are also described.
tively, in 40-75% yield (Scheme 1). Two isomeric forms, namely
1
the keto and enol forms, were observed. The H NMR spectrum of
3ThacacH showed that the enol form predominates in both CD2Cl2
and C6D6, with a population of ca. 95% in the enol form, which is
stabilized by an intramolecular hydrogen bond. Moreover, the 11B
NMR signals of 3ThacacBF2, 3ThacacB(C6F5)2, and 3ThacacBPh2
were found to be at 3.82, 8.32, and 11.49 ppm, respectively, which
are typical of these boron(III) derivatives.6c,d All of the compounds
exhibited an irreversible oxidation wave at ca. +1.6 V vs SCE [Table
S1 in the Supporting Information (SI)], while the irreversible reduction
wave of 3ThacacH became quasi-reversible and was shifted anodically
from -1.7 V to ca. -1.1 to -1.3 V upon coordination to boron(III)
(Figure S1 in the SI). 3ThacacH in its open form was found to dissolve
in benzene to give a pale-yellow solution with an intense absorption
band at ca. 368 nm, corresponding to the intraligand (IL) π f π*
transition of the 1-(2-thiophenyl)-1,3-butanedione moiety, with mixing
of the π f π* transitions of the dimethylthiophene moieties. Upon
excitation, blue luminescence at 461 nm with φem ) 0.009 was
observed. Upon coordination to the boron(III) center, the IL absorption
band was red-shifted from 368 nm to 415-434 nm (Table S2). The
red shift is attributed to the strong electron-accepting ability of boron,
which decreases both the HOMO and LUMO energy levels, with the
LUMO being lowered by a greater amount because it is more localized
on the ꢀ-diketone core; the result is a decrease in the transition energy
(see the SI). Upon excitation at λ e 420 nm, 3ThacacBF2 (502 nm,
φem ) 0.167) and 3ThacacB(C6F5)2 (517 nm, φem ) 0.160) displayed
an intense green emission. In contrast, 3ThacacBPh2 exhibited a weak
emission at 484 nm with φem ) 0.007 (Table S3). Similar observations
have been reported for a related system.6c
UV excitation of 3ThacacH produced a new absorption band at
ca. 630 nm (Figure S2), which is ascribed to the absorption of the
closed form. In addition, the emission was quenched with φem
)
0.004 in the photostationary state (Figure S3). Both 3ThacacBF2
and 3ThacacB(C6F5)2 showed NIR photochromic behavior upon
excitation at λ e 420 nm (Scheme 2), with four absorption bands
generated at ca. 298, 355, 506, and 758 nm in 3ThacacBF2 and
three absorption bands at ca. 297, 543, and 810 nm in
3ThacacB(C6F5)2 (Figure 1), while the absorptions of the closed
forms of the “boron-free” organic diarylethene derivatives were
Reaction of methyl 4,5-dibromothiophene-2-carboxylate and 2,5-
dimethyl-3-thienylboronic acid via a bis-Suzuki cross-coupling reaction
to give 3ThCOOMe4c-e followed by Claisen condensation of
3ThCOOMe with acetone in the presence of a strong base afforded
the dithienylethene-containing ꢀ-diketone 3ThacacH in moderate yield
(ca. 55%). Subsequent reactions of 3ThacacH with BF3 ·OEt2,
9
13992 J. AM. CHEM. SOC. 2010, 132, 13992–13993
10.1021/ja105537j  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
absorption for the open forms can be assigned as the π f π*
transition from the π orbital localized in the dithienylthiophene unit
to the π* orbital localized in the thienyl-containing ꢀ-diketone
(enol)/ꢀ-diketonate core, while that for the closed forms is the π
f π* transition from the π orbital localized on the condensed ring
of the dithienylthiophene unit to the π* orbital delocalized over
the whole 3ThacacH/3Thacac moiety.
In conclusion, a versatile dithienylethene-containing ꢀ-diketonate
ligand and its boron(III) compounds have been synthesized, and their
photophysical and photochromic properties have been studied. The
incorporation of boron(III) has been demonstrated to shift the photo-
chromic behavior to the NIR region, with a pronounced red shift in
the absorption maxima upon photocyclization. This facile modulation
may provide new insights into the future design of NIR photochromic
materials. Further investigations of the inclusion of this ligand at other
transition-metal and lanthanide centers are in progress.
observed at λ e 750 nm.11 These results suggest that a pronounced
perturbation of the dithienylethene ligand is achieved by incorpora-
tion of the boron(III) center. Moreover, the luminescence quantum
yields of 3ThacacBF2 and 3ThacacB(C6F5)2 were found to be
reduced by over 90% to 0.012 and 0.008, respectively. On the other
hand, 3ThacacBPh2 showed no photochromic behavior, with only
photodegradation after prolonged UV irradiation.
Acknowledgment. V.W.-W.Y. acknowledges support from The
University of Hong Kong under the Distinguished Research
Achievement Award Scheme and the URC Strategic Research
Theme on Molecular Materials. This work was supported by the
University Grants Committee Areas of Excellence Scheme (AoE/
P-03/08). C.-T.P. acknowledges the receipt of a postgraduate
studentship from The University of Hong Kong. We also thank
the Computer Center at The University of Hong Kong for providing
computational resources, which were supported by a Hong Kong
UGC Special Equipment Grant (SEG HKU09).
Figure 1. (a) UV-vis absorption and (b) normalized emission spectral
changes of 3ThacacBF2, and (c) UV-vis absorption and (d) normalized
emission spectral changes of 3ThacacB(C6F5)2 in benzene (5 × 10-5 M)
upon UV excitation at 298 K.
Supporting Information Available: Synthetic procedures, charac-
terization data, photophysical data, electrochemical data, cycloreversibility
and thermal stability studies, and computational details. This material is
available free of charge via the Internet at http://pubs.acs.org.
The quantum yields for both the photocyclization and photocyclo-
reversion processes were determined (Table S4). The φOfC(390) value
for 3ThacacH was found to be 0.2, while after incorporation of boron,
φOfC(390) improved to ca. 0.4. The φCfO(509) value for 3ThacacH was
found to be 0.0019, whereas the φCfO(509) values for the boron(III)
compounds were found to be 0.0004-0.0005. Furthermore, both
3ThacacH (Figure S4) and 3ThacacBF2 (Figure S5) showed good
cycloreversibility, as they did not lose their photochromic properties
over five repeating cycles. Photocycloreversion could also be achieved
upon irradiation at 650 nm (Figure S6). In addition, the closed forms
of 3ThacacH and 3ThacacBF2 were found to undergo slow thermal
backward reactions. The half-life for the closed form of 3ThacacH,
which was estimated from the absorption decay, was found to be 1376
min at 328 K and 65525 min at 298 K, while that of 3ThacacBF2
was found to be 598 min at 328 K and 8455 min at 298 K. A slightly
lower activation energy for thermal cycloreversion of 3ThacacBF2
(Ea ) 103.3 kJ mol-1) than of 3ThacacH (Ea ) 106.7 kJ mol-1) was
observed (Figures S7-S10).
Density functional theory (DFT) and time-dependent DFT (TD-
DFT) calculations were performed to gain deeper insight into the
structural geometries and nature of the low-energy absorptions for
the open and closed forms of this class of compounds. Although
the closed form of 3ThacacBPh2 cannot be obtained from experi-
ment, calculations were also performed to predict its properties (see
the SI). The first singlet-singlet transitions for the open and closed
forms, respectively, were computed to be at 371 and 691 nm for
3ThacacH (the most stable enol tautomer), 421 and 809 nm for
3ThacacBF2, 436 and 858 nm for 3ThacacB(C6F5)2, and 414 and
788 nm for 3ThacacBPh2, and these mainly correspond to
excitation from the HOMO to the LUMO (see Tables S8 and S9).
It is noted that the HOMO and LUMO of the two forms in the free
ligand are very similar to those of the boron compounds. On the
basis of the topologies of the HOMO and LUMO, the lowest-energy
References
(1) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim, Germany,
1990.
(2) (a) Irie, M. Chem. ReV. 2000, 100, 1685. (b) Tian, H.; Yang, S. Chem.
Soc. ReV. 2004, 33, 85.
(3) (a) Ko, C.-C.; Yam, V. W.-W. J. Mater. Chem. 2010, 20, 2063. (b) Hasegawa,
Y.; Nakagawa, T.; Kawai, T. Coord. Chem. ReV. 2010, 254, 2643.
(4) (a) Ferna´ndez-Acebes, A.; Lehn, J.-M. AdV. Mater. 1998, 10, 1519. (b)
Murguly, E.; Norsten, T. B.; Branda, N. R. Angew. Chem., Int. Ed. 2001,
40, 1752. (c) Yam, V. W.-W.; Ko, C.-C.; Zhu, N. J. Am. Chem. Soc. 2004,
126, 12734. (d) Lee, P. H.-M.; Ko, C.-C.; Zhu, N.; Yam, V. W.-W. J. Am.
Chem. Soc. 2007, 129, 6058. (e) Yam, V. W.-W.; Lee, J. K.-W.; Ko, C.-
C.; Zhu, N. J. Am. Chem. Soc. 2009, 131, 912.
(5) Vigato, P. A.; Peruzzo, V.; Tamburini, S. Coord. Chem. ReV. 2009, 253, 1099.
(6) (a) Cogne´-Laage, E.; Allemand, J.-F.; Ruel, O.; Baudin, J.-B.; Croquette,
V.; Blanchard-Desce, M.; Jullien, L. Chem.sEur. J. 2004, 10, 1445. (b)
Zhang, G.; Chen, J.; Payne, S. J.; Kooi, S. E.; Demas, J. N.; Fraser, C. L.
J. Am. Chem. Soc. 2007, 129, 8942. (c) Nagai, A.; Kokado, K.; Nagata,
Y.; Arita, M.; Chujo, Y. J. Org. Chem. 2008, 73, 8605. (d) Macedo, F. P.;
Gwengo, C.; Lindeman, S. V.; Smith, M. D.; Gardinier, J. R. Eur. J. Inorg.
Chem. 2008, 3200. (e) Zhang, G.; Palmer, G. M.; Dewhirst, M. W.; Fraser,
C. L. Nat. Mater. 2009, 8, 747. (f) Zhang, G.; Lu, J.; Sabat, M.; Fraser,
C. L. J. Am. Chem. Soc. 2010, 132, 2160.
(7) (a) Yamaguchi, S.; Wakamiya, A. Pure Appl. Chem. 2006, 78, 1413. (b)
Wakamiya, A.; Taniguchi, T.; Yamaguchi, S. Angew. Chem., Int. Ed. 2008,
47, 834.
(8) (a) Ja¨kle, F. Coord. Chem. ReV. 2006, 250, 1107. (b) Wase, C. R.;
Broomsgrove, A. E. J.; Aldridge, S.; Gabba¨ı, F. P. Chem. ReV. 2010, 110,
3958.
(9) (a) Zhou, Z.; Xiao, S.; Xu, J.; Liu, Z.; Shi, M.; Li, F.; Yi, T.; Huang, C.
Org. Lett. 2006, 8, 3911. (b) Lemieux, V.; Spantulescu, M. D.; Baldridge,
K. K.; Branda, N. R. Angew. Chem., Int. Ed. 2008, 47, 5034. (c) 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.
(10) Tsujioka, T.; Kume, M.; Irie, M. Jpn. J. Appl. Phys. 1996, 35, 4353.
(11) (a) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Chem.sEur. J. 1995, 1, 275.
(b) Chen, B. Z.; Wang, M. Z.; Wu, Y. Q.; Tian, H. Chem. Commun. 2002,
1060. (c) Tian, H.; Chen, B. Z.; Tu, H.; Mu¨llen, K. AdV. Mater. 2002, 14,
918. (d) Odo, Y.; Matsuda, K.; Irie, M. Chem.sEur. J. 2006, 12, 4283.
(12) Yun, C.; You, J.; Kim, J.; Huh, J.; Kim, E. J. Photochem. Photobiol., C
2009, 10, 111.
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