Synthesis, photochromic, and computational studies of dithienylethene- containing β-diketonate derivatives and their near-infrared photochromic behavior upon coordination of a boron(III) center

Article abstract of DOI:10.1002/chem.201203105

A series of dithienylethene-containing 1-thienyl-3-aryl-propane-1,3-diones (aryl=phenyl (Ph), thienyl (Th), and 4,5-bis(2,5-dimethylthiophen-3-yl)thiophen- 2-yl (DTE-Th)) and the corresponding boron(III) diketonates, (O^O)BR₂ (R=F, C₆F₅, and Ph), have been designed and synthesized. Their photophysical, electrochemical, and photochromic properties have been studied. Upon coordination of a boron(III) center, the closed forms of the dithienylethene-containing β-diketonates show near-infrared response and the photochromic behavior was also found to be affected by the aryl substituents at the 3-position of the β-diketonates. Moreover, computational studies have been performed that help to provide an understanding of the effect of substituents on the photophysical and photochromic properties. NIR photoswitch: A near-infrared photochromic, responsive, dithienylethene-containing material (see scheme) can be easily prepared and the photochromic properties can be tuned by attaching different boryl groups to various dithienylethene-containing β-diketones ligands. Copyright

Full text of DOI:10.1002/chem.201203105

FULL PAPER
DOI: 10.1002/chem.201203105
Synthesis, Photochromic, and Computational Studies of Dithienylethene-
Containing b-Diketonate Derivatives and Their Near-Infrared Photochromic
Behavior Upon Coordination of a BoronACTHNUTRGNEU(GN III) Center
Chun-Ting Poon, Wai Han Lam,* and Vivian Wing-Wah Yam*[a]
Abstract: A series of dithienylethene-
containing 1-thienyl-3-aryl-propane-
1,3-diones (aryl=phenyl (Ph), thienyl
(Th), and 4,5-bis(2,5-dimethylthiophen-
3-yl)thiophen-2-yl (DTE-Th)) and the
photochromic properties have been
studied. Upon coordination of a boron-
ACHTUNGTRENN(UNG III) center, the closed forms of the di-
show near-infrared response and the
photochromic behavior was also found
to be affected by the aryl substituents
at the 3-position of the b-diketonates.
Moreover, computational studies have
been performed that help to provide
an understanding of the effect of sub-
stituents on the photophysical and pho-
tochromic properties.
thienylethene-containing b-diketonates
corresponding boronACTHNUTRGNEUGN(III) diketonates,
Keywords: boron · sulfur heterocy-
cles · luminescence · near-infrared ·
photochromism
(O^O)BR2 (R=F, C6F5, and Ph), have
been designed and synthesized. Their
photophysical, electrochemical, and
Introduction
through metal-coordination-assisted planarization of the ex-
tended p-conjugated system in dithienylethene-containing 2-
(2-pyridyl)imidazoles and their rhenium(I) complexes, such
that the tedious and nontrivial synthesis could be replaced
by the judicious design and suitable choice of ligands and
metal centers.[5] Since then, several NIR-responsive photo-
chromic dithienylethenes consisting of metal complex sys-
tems have been reported.[6,7] In addition to metal complexa-
tion, Yam and co-workers have recently communicated a
new class of dithienylethene-containing 1-thienyl-butane-
1,3-diones with which NIR-responsive photochromic behav-
Photochromic materials have received much attention due
to their capacity to function as potential photoswitchable
molecular devices and optical memory storage systems.[1]
Among the various types of photochromic materials, organic
diarylethene-type compounds, which undergo reversible cy-
ACHTUNGTRENNUNGclization and cycloreversion reactions between open and
closed forms upon irradiation with appropriate wavelengths
of light, have been extensively studied because of their ex-
cellent fatigue resistance and thermal irreversibility.[2] To en-
hance the semiconductor diode laser susceptibility for appli-
cation in optical memory storage,[2a,3] a number of attempts
have been made to design near-infrared (NIR)-responsive
photochromic materials by extending the p conjugation of
the thiophene moiety to shift the absorption maxima to
lower energy.[4] In 2007, Yam and co-workers reported an in-
novative way to achieve NIR photochromic behavior
ior could be achieved upon coordination of a boronACHTUNGTRENNUNG(III)
center.[8] To extend the structural diversity and to study the
structure–property relationships in this system, in which the
photophysical properties of this class of compound has been
found to be sensitive towards peripheral modification,[9]
herein we describe the design and synthesis of a series of 1-
thienyl-3-aryl-propane-1,3-diones (aryl=phenyl (Ph), thien-
yl (Th), and 4,5-bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl
(DTE-Th)) and the corresponding boronACHTNUTRGNEUNG(III) compounds,
[a] Dr. C.-T. Poon, Dr. W. H. Lam, Prof. V. W.-W. Yam
Institute of Molecular Functional Materials
and Department of Chemistry
The University of Hong Kong
Pokfulam Road
(O^O)BR2 (R=F, C6F5, and Ph), and their photophysical,
electrochemical, and photochromic behavior. Density func-
tional theory (DFT) and time-dependent density functional
theory (TDDFT) calculations have been performed to pro-
vide further insight into the effect of the substituents on the
photophysical and photochromic properties.
Hong Kong (P.R. China)
E-mail: wwyam@hku.hk
chsue@hku.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203105. It contains TDDFT/
CPCM singlet–singlet transitions for the open and closed forms; elec-
Results and Discussion
tronic absorption spectral changes of L1–L3, 1-BF2–3-BF2, 1-B
and 2-B (C6F5)2–3-B(C6F5)2
(C6F5)2; 1H NMR spectral changes of 1-B
upon photo-irradiation; corrected emission spectral changes of 1-
BF2–3-BF2, 1-B(C6F5)2, and 2-B(C6F5)2; decay trace of the closed
ACHTNUGTERN(NGNU C6F5)2,
A
R
ACHTUNGTRENNUNG
Synthesis and characterization: In the previous communica-
tion,[8] the dithienylethene-containing 1-thienyl-butane-1,3-
dione was prepared by Claisen condensation of methyl 4,5-
bis(2,5-dimethylthiophen-3-yl)thiophen-2-carboxylate with
acetone in the presence of NaNH2 as base. However, only a
A
ACHTUNGTRENNUNG
form of 2-BF2 at 800 nm; Arrhenius plot for the thermal backward re-
action of 2-BF2; PBE0 optimized geometries; spatial plots of the
HOMO and LUMO and Cartesian coordinates for the optimized ge-
ometries.
Chem. Eur. J. 2013, 19, 3467 – 3476
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3467

Scheme 1. Synthetic route to dithienylethene-containing b-diketone ligands and the corresponding boronACHTNUTRGNEUNG(III) compounds.
few derivatives could be prepared because the availability of
the acetyl precursors was limited. Therefore, in the present
work, a different synthetic route has been developed, as
shown in Scheme 1. 1-(4,5-Dibromothiophen-2-yl)ethanone
was used as the starting material to give 1-[4,5-bis(2,5-
benzene to give yellow solutions with an intense absorption
band at around 392–425 nm, corresponding to the intralig-
and (IL) p!p* transition of the 1-thienyl-3-aryl-propane-
1,3-dione, with mixing of the p!p* transitions of the di-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHUTNGERNmNUG ethHCATUNGTRENNyUGN lthiophene moieties. When compared with the previ-
dimeth
G
(DTE-Th-
ous studies,[8] a significant redshift of approximately 24–
57 nm (1664–3645 cmꢀ1) was observed when the methyl
group was changed to an aryl group at the 3-position of the
b-diketone, which is indicative of the increase in the extent
COMe) in moderate yield (65%) through a twofold Suzuki
cross-coupling reaction, followed by reaction with methyl
benzoate, methyl thiophene-2-carboxylate, and methyl 4,5-
bis(2,5-dimethylthiophen-3-yl)thiophen-2-carboxylate[8]
to
of p conjugation. Upon coordination to the boronACHTUNGTRENNUNG(III)
give the corresponding dithienylethene-containing 1-thienyl-
3-aryl-propane-1,3-dione, for which aryl=Ph (L1), Th (L2),
and DTE-Th (L3). Subsequent reactions of dithienylethene-
containing 1-thienyl-3-aryl-propane-1,3-dione derivatives
center, a further redshift of the IL absorption band was ob-
served, corresponding to the strong electron-accepting abili-
ꢀ
ty of the BR2 moiety that decreases both the HOMO and
LUMO energy levels, but the LUMO would be lowered to a
larger extent because it would be more localized on the b-
diketone core and hence would lower the transition energy
(see below).
with BF3·OEt2, B
ACHTUNGTRENNUNG(C6F5)2F·OEt2, and BPh3 afforded air-
stable boron(III) compounds with approximately 45–75%
ACHTUNGTRENNUNG
yield. The identities of all compounds were confirmed by
1H NMR spectroscopic analysis and by 19F NMR spectro-
scopic analysis for the fluorine-containing compounds, in ad-
dition to fast atom bombardment (FAB) mass spectroscopy
and satisfactory elemental analyses. Only signals of the enol
Upon excitation into the IL p!p* absorption band, L1,
L2, and L3 displayed blue luminescence with a lifetime of
less than 0.5 ns, with the emission maxima found at around
463, 463, and 471 nm, respectively. Slight bathochromic
shifts in emission maxima and increase in the fluorescence
quantum yield were observed across the series, which is con-
sistent with the trend of the absorption spectra due to the
increase in the p conjugation. A pronounced redshift with
significant emission enhancement was observed after coordi-
1
form were observed in the H NMR spectra of L1, L2, and
L3, which is consistent with stabilization of the enol form by
the aromatic system.[10] Moreover, all the b-diketone and b-
diketonate derivatives showed two singlet signals at around
d=6.43–6.49 ppm, corresponding to the two thienyl protons
on the dimethylthiophene rings. This suggested that the di-
methylthiophene rings are free to rotate so that no parallel
or antiparallel configurations within the NMR time-scale
ꢀ
ꢀ
BACHTUNGTRENNUNG
nation of the BF2 or
ꢀ
ꢀ
BACHTUNGTRENNUNG
BF2 and
bright-yellow emission, with the emission maxima found at
around 526–557 nm. The emission lifetime of approximately
2.04–3.07 ns indicated that the emission origin was of singlet
character. The fluorescence quantum yield was also found
to increase when compared with those reported previously,[8]
which is indicative of a strong influence of the aryl group at
1
were recorded.[2a] In addition, a rather simple H NMR spec-
trum was recorded for samples with L3, which indicated
that the two 4,5-bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl
units are symmetrically identical. Upon coordination of the
boron
diketonate core was always observed, which is ascribed to
the electron-withdrawing ability of the boron(III) center
and the increase in the electron delocalization of the b-di-
ketonate core upon coordination to the boron(III) center.
ACHTUNGTRENNUNG(III) center, a downfield shift of the proton at the b-
ꢀ
the 3-position. However, after inclusion of BPh2, it was
E
found to be weakly emissive and a relatively weak influence
on the emission maxima was observed when compared with
the former two cases. A similar observation has been report-
ed but the exact origin of this was not well-understood.[11]
The photophysical data are summarized in Table 1 and the
electronic absorption and normalized emission spectra of
AHCTUNGTRENNUNG
Electronic absorption and emission studies: Compounds L1,
L2, and L3 in their open forms were found to dissolve in
3468
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 3467 – 3476

b-Diketonate Derivatives
FULL PAPER
Table 1. Photophysical data of dithienylethene-containing b-diACTHNUTRGNEkNUG etone compounds and
Electrochemical studies: All the compounds exhibit
an irreversible oxidation wave at around +1.51 to
+1.59 V versus SCE. The electrochemical data are
summarized in Table 2. No significant shift was ob-
served at this oxidation wave by increasing the p
conjugation through the use of different aryl groups
attached at the 3-position of the b-diketone and b-
diketonate core of the boron compounds. This im-
plied that the HOMO would be mainly localized on
the corresponding boronACHTUNGTRENNUNG
(III) compounds in the open form.[a]
[b]
Compound
Absorption labs
Emission lem
fem
[nm]
A
[nm]
ACHTUNGTERN(NUNG to [ns])
(eꢁ10ꢀ3 [dm3 molꢀ1 cmꢀ1])
L1
L2
L3
293 (6.0), 392 (20.3)
290 (8.5), 400 (25)
296 (10), 425 (36)
298 (8.8), 450 (20)
463 (<0.5)
463 (<0.5)
471 (<0.5)
513 (<0.5)
511 (<0.5)
513 (<0.5)
526 (3.07)
529 (2.94)
545 (1.63)
545 (3.19)
554 (3.70)
557 (2.04)
0.05
0.10
0.22
0.02
0.03
0.06
0.39
0.40
0.30
0.32
0.36
0.34
1-BPh2
2-BPh2
3-BPh2
1-BF2
2-BF2
3-BF2
1-B
2-B
3-B
300 (9.0), 451 (23)
316 (8.8), 460 sh (29.2), 490 (38.2)
303 (6.8), 360 (9.9), 448 (29)
305 (5.8), 372 sh (7.8), 460 (28.8)
316 (6.5), 380 sh (6.4), 492 (44.2)
300 (8.9), 383 (7.4), 466 (28.3)
310 (9.9), 391 (9.3), 475 (42.3)
318 (6.4), 405 sh (5.8), 510 (40.7)
the dithiACTHNUTRGNEUGeN nylthiophene core, which is rather insensi-
tive to peripheral modifications. This oxidation
wave was probably due to oxidation of the dithie-
nylthiophene core.
A quasi-reversible reduction couple of L1–L3
was observed from ꢀ1.41 to ꢀ1.46 V versus SCE in
CH2Cl2 and this reduction couple was found to be
anodically shifted when the aryl group was used in
place of the methyl group at the 3-position of the b-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[a] In benzene at 293 K. [b] Luminescence quantum yield determined by using quinine
sulfate in 1.0 N sulfuric acid (flum =0.546, lex =365 nm) as reference.
Table 2. Electrochemical data for the dithienylethene-containing b-dike-
ꢀ
ꢀ
the BF2- and
BACHTUNGTRENNUNG(C6F5)2-appended dithienylethene-contain-
tone compounds and the corresponding boron
ACHTUNGTREN(NUNG III) compounds in the
ing b-diketonate compounds are shown in Figure 1 and
open form.[a]
Figure 2, respectively.
Compound
Oxidation
Reduction
Epa [V vs. SCE][b]
E1/2 [V vs SCE][c] (DEp [mV])[d]
L1
L2
L3
+1.53
+1.53
+1.52, +1.86
+1.54
+1.56
+1.52
+1.53
+1.51
+1.52
ꢀ1.46 (156)
ꢀ1.43 (152)
ꢀ1.41 (128)
ꢀ1.02 (123)
ꢀ1.03 (89)
ꢀ1.01 (77)
ꢀ0.88 (93)
ꢀ0.85 (86)
ꢀ0.86 (77)
ꢀ0.90 (140)
ꢀ0.89 (82)
ꢀ0.90 (74)
1-BPh2
2-BPh2
3-BPh2
1-BF2
2-BF2
3-BF2
1-B
2-B
3-B
G
+1.59
+1.52
+1.52
R
A
[a] In dichloromethane with 0.1m nBu4NPF4 as supporting electrolyte at
room temperature; scan rate 200 mVsꢀ1. [b] Epa refers to the anodic peak
potential for irreversible oxidation waves. [c] E1/2 =(Epa+Epc)/2; Epa and
Epc are peak anodic and peak cathodic potentials, respectively. [d] DEp =
(EpaꢀEpc).
Figure 1. Electronic absorption and normalized emission spectra of the
open form of 1-BF2–3-BF2.
di
ACHTUNGTRENNUNG
served upon coordination to the boronAHCUTNGTRENNUNG
findings indicated that the LUMO was sensitive towards pe-
ripheral modifications. The reduction couple for L1–L3 and
the corresponding boron compounds arise from b-diketone-
based and b-diketonate-based reductions, respectively,
which have also been supported by DFT calculations.
Photochromic studies: Upon excitation into the intraACHTUNGTRENNUNGliACHTUNGTRENNUNGgand
p!p* transition band in degassed benzene, L1–L3 under-
went photocyclization; the color of the solution changed
from yellow to green, with the growth of an absorption
shoulder at around 455–480 nm and a broad absorption at
around 655–658 nm in the UV/Vis spectra, as shown in Fig-
ures S1–S3 in the Supporting Information. There was a red-
shift in the closed form absorption upon going from L1 to
L3. The bathochromic shift could be attributed to the in-
Figure 2. Electronic absorption and normalized emission spectra of the
open form of 1-BACHTUNGTRENNUNG(C6F5)2–3-BHCAUTNGTREN(NUGN C6F5)2.
Chem. Eur. J. 2013, 19, 3467 – 3476
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3469

W. H. Lam, V. W.-W. Yam and C.-T. Poon
crease in p-electron conjugation, similar to the trend ob-
double photocyclized product is probably due to the effec-
tive energy transfer from DTE-Th (open-form moiety) to
8a,8b-dimethyl-1,8-dithia-as-indacene (closed form moiety)
after the first cyclization, which quenched the second cycli-
zation. Such lack of activity of the second cyclization pro-
ꢀ
served in the open form absorption. After inclusion of the
BF2 group, a pronounced redshift in absorption maxima was
observed upon photocyclization. The lowest energy absorp-
tion maximum of the closed form of 1-BF2, 2-BF2, and 3-
BF2 was found to be at around 800, 800, and 807 nm, respec-
tively, as shown in Figures S4–S6 in the Supporting Informa-
tion, and they behaved as NIR-responsive photochromic
materials, which is indicative of a pronounced perturbation
AHCTUNGTREGcNNNU ess in molecules containing two dithienylethene cores has
also been reported.[13] On the other hand, no photochromic
ꢀ
behavior was observed for the BPh2 derivatives, with only
photodegradation being observed after prolonged UV irra-
diation. For all other compounds, a significant reduction in
the luminescence intensity of both the ligands and the
upon coordination to the boronACTHNUGRTNEUNG(III) center. A further red-
ꢀ
shift was observed when the
absorption maximum of the closed form of 1-B
B
G
U
boronACHTUGNTERN(UNG III) compounds was observed upon photoirradiation
A
E
at their isosbestic wavelength. It was also found that the
closed form of all the compounds was nonemissive in the
NIR region (up to 1000 nm). The emission spectral changes
and 865 nm, as shown in Figures S7 and S8 in the Supporting
Information and Figure 3, respectively, suggesting that the
upon photoexcitation of 3-BACHTUNRTGNEUNG(C6F5)2 are shown in Figure 4 as
Figure 3. Electronic absorption spectral changes of 3-BACHTUNRGTNEUNG(C6F5)2 upon exci-
tation at 510 nm in degassed benzene.
Figure 4. Corrected emission spectral changes of 3-BACHTUNRGTNEUNG(C6F5)2 upon excita-
tion at 463 nm in degassed benzene.
photochromic behavior could be easily tuned by adjusting
the periphery of the boron(III) center. Representative
1H NMR spectral changes of 1-B
(C6F5)2, 2-B(C6F5)2, and 3-
(C6F5)2 after photocyclization are been depicted in Figur-
es S9–S11. In accordance with the predictions from the
Woodward–Hoffmann rule,[12] only the antiparallel forms
are photoactive, undergoing conrotatory cyclization upon
light excitation that leads to a trans arrangement of the
methyl groups in the closed form.
a representative example; the others are provided in Fig-
ACHTUNGTRENuNUGN res S12–S16 in the Supporting Information. Such a large
difference in emission intensity may suggest that this class of
compounds can potentially act as a new class of photo-
switchable luminescence materials.[14]
The quantum yields for both the photocyclization and
photocycloreversion processes have been determined and
are summarized in Table 3. The fO!C values of the dithi-
G
E
G
BACHTUNGTRENNUNG
ACHUTNGERNeNUG nylHCATUNGTRENNeUGN thene-containing b-diketones were found to be in the
Moreover, it is interesting to note that well-defined isos-
ACHTUNGTRENNUNGbestic points were found in L3, 3-BF2, and 3-BACHTUNTGREN(NUGN C6F5)2 such
that no double photocyclized product was observed even
after prolonged irradiation (Scheme 2). The absence of
range 0.09–0.13, whereas the fC!O values were determined
to be 0.033–0.080, which are similar to those reported for
photochromic dithienylethene materials.[2a] Upon incorpora-
ꢀ
ꢀ
tion of BF2 and
BACHTUGNRETNNU(G C6F5)2 groups, the fO!C was found to
Scheme 2. Photochromic reaction of 3-BF2.
3470
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Chem. Eur. J. 2013, 19, 3467 – 3476

b-Diketonate Derivatives
FULL PAPER
Table 3. Electronic absorption data of dithienylethene-containing b-diketone com-
were considered in our studies for the open forms
of L1–L3, the keto and cis-enol forms. The cis-enol
forms of the ligands were calculated to be 3.9–
5.1 kcalmolꢀ1 lower in energy than that for the keto
forms. The higher stability of the enol forms is asso-
pounds and the corresponding boron
ACHTUNGTRENUN(NG III) compounds in the closed form and their
photochemical quantum yields.[a]
Compound
Absorption labs [nm]
(eꢁ10ꢀ3 [dm3 molꢀ1 cmꢀ1])
fO!C
fC!O
ACHTUNGTRENNUNG
L1
L2
L3
290 (12.1), 386 (21.4), 460 sh (11.2), 655 (7.4)
291 (11.1), 403 (21.5), 480 sh (7.3), 655 (6.2)
296 (13.7), 429 (27.8), 455 (27.5), 658 (9.9)
0.09[c]
0.11[c]
0.13[c]
0.042[d]
0.080[d]
0.033[d]
ꢀ
ciated with the presence of an intramolecular O
H···O hydrogen bond in the six-membered cyclic
structure (Figure S19 in the Supporting Informa-
tion).
[b]
[b]
[b]
1-BPh2
2-BPh2
3-BPh2
1-BF2
2-BF2
3-BF2
1-B
2-B
3-B
–
–
–
–
–
–
–
[b]
[b]
[b]
–
[b]
[b]
[b]
–
For all the open-form boron compounds, the cen-
tral boron atom has a distorted tetrahedral geome-
303 (10.7), 438 (24.3), 546 (4.2), 800 (5.3)
302 (10.5), 436 (23.4), 550 sh (6.3), 800 (7.9)
306 (17.3), 473 sh (23.2), 522 (41.2), 807 (18.7)
306 (17.9), 445 (23.0), 568 sh (7.9), 853 (10.2)
308 (16.0), 465 (31.6), 577 sh (6.1), 863 (8.6)
307 (16.7), 492 sh (20.0), 538 (31.7), 865 (13.7)
0.12[c]
0.12[c]
0.05[c]
0.15[c]
0.14[c]
0.03[c]
0.004[d]
0.002[d]
0.003[d]
ꢀ
try (Figure S19). The average B O bond distance
G
0.004[d] of 1-BPh2–3-BPh2 (1.535–1.538 ꢂ) is calculated to
E
0.002[d]
be slightly longer than 1-BF2–3-BF2 (1.512–1.514 ꢂ)
G
0.004[d]
and 1-B
G
E
A
[a] In benzene at 293 K. [b] Could not be determined due to the lack of photochromic
property. [c] Data obtained using 400 nm as excitation source. [d] Data obtained using
600 nm as excitation source.
longer average B O distance was also observed in
the X-ray crystal structure of the diphenylboron di-
ketonate, relative to the BACHTNUTRGNE(UNG C6F5)2 diketonate ana-
logues,[11b] which could be explained by the stronger
ꢀ
electron-withdrawing effect of the C6F5 groups rel-
ꢀ
be 0.03–0.15, in which a reduction in the fO!C was obtained
with an increase in the p conjugation at the b-diketonate
group when compared with previous reports on compounds
with a methyl substituent.[8] One possible explanation could
be that the fluorescence quantum yields of the compounds
were found to be enhanced by almost three times, relative
to the previous report,[8] such that the increase in the radia-
tive rate would suppress the rate of photocyclization be-
cause both processes were in competition with each other.[15]
A further reduction in fO!C was observed in the bis-dithien-
yl-ethene-containing compounds. This might be due to intra-
molecular energy transfer between the two dithienylethene
ative to Ph, making the boron center more electron defi-
ꢀ
cient and hence strengthen the B O interactions.
For the closed forms of the enol tautomer of L1–L3 and
boron compounds [1-BF2–3-BF2, and 1-B
ACHTNUGERTN(NGNU C6F5)2–3-B-
AHCTUNGTRENNUNG
compounds do not vary by much compared to their open
forms, with the exception of the thiophene core and the con-
densed ring units (Figure S20 in the Supporting Informa-
tion).
The first singlet–singlet transitions of the open and closed
forms are listed in the Supporting Information, in Table S1
and Table S2, respectively. The S1 excited state arises from
one-electron excitation from the HOMO to the LUMO. The
spatial plots of the HOMO and LUMO for the open forms
of selected compounds are shown in Figure 5 and Figure S21
in the Supporting Information.
rings. The fC!O of the boronACTHNUTRGENUGN(III) compounds were found to
be in the range 0.002–0.004, which showed a tenfold reduc-
tion in values relative to that of the ligand themselves.
A study of the thermal stability of the closed form has
been performed on 2-BF2 and the plots of ln
N
For the open form, the HOMO and LUMO of ligand L1–
L3 are the p and p* orbitals localized on the dithienylthio-
phene unit(s) and the diaryl b-diketone enol core, respec-
tively. The HOMO and LUMO in the boron compounds are
time for the absorbance decay of 2-BF2 at 800 nm at various
temperatures are shown in Figure S17 in the Supporting In-
formation. The half-life for the closed form of 2-BF2 was
found to be 2360 and 27 min at 303 and 343 K, respectively.
The activation energy of the thermal cycloreversion of 2-
BF2, determined by plotting ln(k) versus Tꢀ1 (Figure S18 in
the Supporting Information) using the Arrhenius equation,
was found to be 94.1ꢁ7.6 kJmolꢀ1, which is smaller than
those reported in the previous study,[8] suggesting that intro-
duction of the aryl group decreases the thermal stability of
the closed form.
Computational studies: Density functional theory (DFT)
and time-dependent density functional theory (TDDFT) cal-
culations have been performed to gain a deeper insight into
the nature of the low-energy absorptions and the photochro-
mic reactivity of this class of compounds. The optimized
structures of the open and closed forms for the selected
compounds are shown in the Supporting Information, in Fig-
ures S19 and S20, respectively. Two possible structural forms
Figure 5. Spatial plots (isovalue=0.03) of the HOMO and LUMO for the
open forms in a) L1 and b) 1-BF2 obtained from the TDDFT/CPCM cal-
culations.
Chem. Eur. J. 2013, 19, 3467 – 3476
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3471

W. H. Lam, V. W.-W. Yam and C.-T. Poon
closed forms of the ligand and
the boron compounds relative
to the open forms.
The S0!S1 transition energy
for the open form is computed
to be redshifted in the order of
L1–L3 (392–420 nm)>1-BPh2–
3-BPh2 (436–464 nm)>1-BF2–
3-BF2
A
(441–469 nm)>1-B-
ACHTNUGERTN(NGNU C6F5)2 (459–
486 nm), in line with the
HOMO–LUMO energy separa-
tion. For the closed forms, the
energy of the S0!S1 transitions
is computed to be redshifted
from the enol tautomer of L1–
L3 (717–728 nm) to 1-BF2–3-
BF2 (842–852 nm) and to 1-B-
A
E
(898–
the lowest energy absorption
for the open and closed forms
Figure 6. Orbital energy diagram of selected molecular orbitals (H=HOMO and L=LUMO).
very similar to the corresponding ligands. Figure 6 depicts
the energy diagram of selected molecular orbitals for all the
compounds. The HOMO–LUMO energy gap decreases in
the order Ar=Ph>Th >DTE-Th for both diketones and
boron diketonates, which is consistent with the increase in
the extent of p-conjugation of the Ar groups. Upon ligand
coordination to the boron atom, both the HOMO and
LUMO energies decrease, indicating that the boron center
is highly electrophilic. Because the LUMO is more localized
on the b-diketone core, the energy of the LUMO level is de-
creased to a larger extent than that of the HOMO level in
each of the boron compounds, leading to a smaller HOMO–
LUMO energy gap in the boron compounds. It should be
Figure 7. Spatial plots (isovalue=0.03) of the HOMO and LUMO for the
closed forms in a) L1 and b) 1-BF2 obtained from the TDDFT/CPCM
calculations.
noted that the HOMO–LUMO energy gap decreases upon
going from 1-BPh2–3-BPh2 to 1-B(C6F5)2–3-B(C6F5)2. Given
the stronger electron-withdrawing ability of C6F5 compared
with Ph, the decrease in the LUMO level for 1-B(C6F5)2–3-
(C6F5)2 is larger than that with 1-BPh2–3-BPh2. When com-
pared with 1-BF2–3-BF2, a larger decrease in LUMO energy
in 1-B(C6F5)2–3-B(C6F5)2 can also be found, leading to a
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
B
A
for the b-diketone and the corresponding boron compounds
can be assigned as the p!p* transition of the dithienyl-
A
U
ACHTUNGTRENeNUNG thene-containing b-diketone (enolic form)/b-diketonate
smaller HOMO–LUMO separation. This is probably due to
the fact that the stronger B–F interaction weakens the B–O
interaction, in turn enhancing the C–O interaction and
giving rise to a higher p* orbital energy.
For the closed forms of L1–L3 as well as the boron com-
pounds (1-BF2–3-BF2 and 1-BACTHNUTGRENN(UG C6F5)2–3-BHCAUTNGTREN(NUGN C6F5)2), the
HOMO is the p orbital localized on the condensed ring of
the dithienylthiophene unit, whereas the LUMO is the p*
orbital delocalized over the entire dithienylethene-contain-
ing b-diketone (enolic form)/b-diketonate core (see Figure 7
and Figure S22 in the Supporting Information). A significant
decrease in the HOMO–LUMO energy gap is found in the
core. In general, the calculated wavelengths of the p!p*
transitions are in agreement with the observed trend in the
low-energy absorption band.
It is interesting to note that fluorine substituents on the
aryl ring not only have an influence on the photophysical
properties, but also on the photochromic behavior. It can be
seen that the luminescent quantum yield is low when R=
Ph. In addition, no photochromic behavior was observed for
ꢀ
the BPh2 derivatives, with only photodegradation being
found after prolonged exposure to UV irradiation. On the
ꢀ
other hand, the
chromic active and show luminescence.
B
G
3472
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 3467 – 3476

b-Diketonate Derivatives
FULL PAPER
The homolytic bond dissociation energy/enthalpy for the
Experimental Section
C
C
reactions, 1-BR2!1-BR +R (R=Ph and C6F5), have been
Materials: Tetrakis(triphenylphosphane)palladium(0),[18] 2,5-dimethyl-
thien-3-yl boronic acid,[19] 1-(4,5-dibromothiophen-2-yl)ethanone,[20] and
ꢀ
computed. The B C bond dissociation energy (DE)/enthalpy
(DH) for R=C6F5 is 18.1/18.3 kcalmolꢀ1 higher than that of
AHCTUNGTRENNUNG
fluorobis(pentafluorophenyl)borane diethyl etherate[21] were prepared ac-
cording to literature procedures. Triphenylborane, boron trifluoride di-
ethyl etherate, and sodium amide were purchased and used as received.
All reactions were performed under strictly anaerobic conditions in an
inert atmosphere of nitrogen using standard Schlenk techniques.
ꢀ
R=Ph, indicating that the B C6F5 bond is stronger than the
ꢀ
B C6H5 bond. It is possible that the photoinstability of the
ꢀ
BPh2 analogues is a result of the effective generation of
ꢀ
C
C
the radical pair [ BPh +Ph ], which is followed by subse-
quent chemical reactions. Indeed, the cleavage of one of the
Physical measurements and instrumentation: 1H and 19F NMR spectra
were recorded with a Bruker AV400 at 298 K with chemical shifts (d,
ppm) relative to tetramethylsilane (Me4Si) and trichlorofluoromethane
(CFCl3), respectively. 11B NMR spectra were recorded with a Bruker
DRX 500 at 298 K with chemical shifts (d, ppm) relative to boron tri-
fluoride diethyl etherate (BF3·OEt2). Positive-ion FAB mass spectra were
recorded with a Finnigan MAT 95 mass spectrometer. Elemental analyses
of the new compounds were performed with a Carlo Erba 1106 elemental
analyzer at the Institute of Chemistry, Chinese Academy of Sciences,
Beijing. UV/Vis absorption spectra for photochromic studies were meas-
ured with a Hewlett–Packard 8452A diode array spectrophotometer and
the photoirradiation was carried out with a 300 W Oriel Corporation
Model 66011 Xe (ozone free) lamp with Applied Photophysics F 3.4
monochromator to control the wavelength-selection. UV/Vis-NIR ab-
sorption spectra for the closed form were recorded with a Varian Cary 50
spectrophotometer equipped with a Xenon flash lamp. All measurements
were studied at RT unless specified otherwise. Steady-state emission
spectra at RT and 77 K were recorded with a Spex Fluorolog-3 Model
FL3–211 spectrofluorometer equipped with a R2658P PMT detector.
Time-resolved emission studies were performed with a Horiba Jobin
Yvon FluoroCube based on a time-correlated single photon counting
method, using a nanoLED with peak wavelength and pulse duration
equal to 371 nm and <200 ps, respectively, as the excitation source.
Cyclic voltammetric measurements were performed with a CH Instru-
ments model CHI 620 Electrochemical Analyzer. All measurements
were conducted in CH2Cl2 solutions with 0.1m nBu4NPF6 as supporting
electrolyte at RT. A three-electrode system was employed, with Ag/
AgNO3 (0.1m in acetonitrile), glassy carbon (CH Instrument), and plati-
num wire as reference electrode, working electrode, and counter elec-
trode, respectively, in a glass cell with compartments separating the work-
ing electrode and the reference electrode from the counter electrode.
ꢀ
B C bonds was proposed to be the primary photoreaction
in the photoalkylation of dialkylboron diketonate com-
pounds.[16] Because photocleavage of the B C6H5 bond
would compete with the photocyclization and fluorescence
processes, this might lead to low quantum yields for both
processes. Based on the calculation, the presence of fluori-
nated substituents would enhance the B C bond strength
and hence increase the photostability of the
ꢀ
ꢀ
ꢀ
B
N
ꢀ
logues when compared with the BPh2 counterparts. Inter-
estingly, tetra
G
N
todegradation, which is much less pronounced in the corre-
sponding fluorinated analogue.[17]
Conclusion
A series of photochromic dithienylethene-containing b-di-
ACHTUNGTRENNUNGketones and the corresponding boronCAHTUNGTRNE(NUGN III) compounds,
(O^O)BR2 (R=F, C6F5, and Ph), have been successfully
prepared. Electronic absorption and emission studies have
been performed. An intense absorption band at around
400 nm was observed in the dithienylethene-containing b-di-
ketone derivatives, and this lowest-energy absorption band,
originated from p!p* transition, was found to be redshifted
upon incorporation of a boronACTHNUTRGNEUGN(III) center. Moreover, the
1-[4,5-Bis(2,5-dimethylthiophen-3-yl)thiophene-2-yl]ethanone (DTE-Th-
COMe): Aqueous sodium carbonate solution (2m, 1.7 g, 8 mL, 16 mmol)
electrochemical study confirmed that the LUMO was sensi-
tive towards an increase in p conjugation at the 3-position
of b-diketone/b-diketonate and also suggested that the
LUMO energy level was lowered after incorporation of a
was added to
a solution of 1-(4,5-dibromothiophen-2-yl)ethanone
(568 mg, 2 mmol), 2,5-dimethylthien-3-yl boronic acid (1.87 g, 12 mmol),
and tetrakis(triphenylphosphane)palladium(0) (321 mg, 0.36 mmol) in
THF (200 mL), and the reaction mixture was stirred and heated at reflux
in the dark. The progress of the reaction was monitored by TLC. The re-
action mixture was then extracted with chloroform, washed with brine,
and finally dried over anhydrous magnesium sulfate. After filtration and
removal of the solvent, the crude product was purified by column chro-
matography on silica gel (70–230 mesh; n-hexane/dichloromethane, 4:1 v/
v). Further purification was achieved by recrystallization from hot
hexane to give the product as a very pale-yellow solid. Yield: 450 mg
boron
ACHTUNGTRENNUNG(III) center. Upon photoexcitation, the dithienyl-
ACHTUNGTRENNUNG
photochromic behavior and the absorption maximum of the
closed form was found to be approximately 655–658 nm.
Upon incorporation of BF2 and
frared responsive photochromic behavior was observed in
which the absorption of the closed form was found at
around 800–865 nm, suggesting that a pronounced perturba-
tion of the dithienylethene-containing b-diketonate ligand
ꢀ
ꢀ
BACHTNGUTERNNU(G C6F5)2 groups, near-in-
1
(65%); H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): d=2.01 (s,
3H; Me), 2.08 (s, 3H; Me), 2.36 (s, 3H; Me), 2.38 (s, 3H; Me), 2.55 (s,
3H; COMe), 6.43 (s, 1H; thienyl), 6.45 (s, 1H; thienyl), 7.60 ppm (s, 1H;
thienyl); FAB-MS (+ve): m/z: 346 [M]+; elemental analysis calcd (%) for
C18H18OS3: C 62.39, H 5.24; found: C 62.25, H 5.17.
was achieved upon coordination to the boronACTHNURTGNEU(GN III) center.
Computational studies have been performed to elucidate
the substituent effect on the photophysical and photochro-
mic properties and are generally in agreement with the ex-
perimental results. A significant emission quenching was
also observed upon photocyclization, suggesting their poten-
tial application as photoswitchable luminescence material.
1-[4,5-Bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl]-3-phenylpropane-1,3-
dione (L1): Sodium amide (156 mg, 4 mmol) was added to a solution
DTE-Th-COMe (277 mg, 0.8 mmol) and methyl benzoate (0.25 mL,
2 mmol) in THF (20 mL) and the reaction mixture was stirred and
heated at reflux overnight in the dark. The reaction was quenched by ad-
dition of HCl (1m), extracted with chloroform, washed with brine, and fi-
nally dried over anhydrous magnesium sulfate. After filtration and re-
moval of the solvent, the crude product was purified by column chroma-
tography on silica gel (70–230 mesh; n-hexane/chloroform, 4:1 v/v). Fur-
ther purification was achieved by recrystallization from hot methanol to
Chem. Eur. J. 2013, 19, 3467 – 3476
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3473

W. H. Lam, V. W.-W. Yam and C.-T. Poon
give the product as a yellow solid. Yield: 210 mg (58%); 1H NMR
(400 MHz, CDCl3, 298 K, relative to Me4Si): d=2.03 (s, 3H; Me), 2.10 (s,
3H; Me), 2.37 (s, 3H; Me), 2.39 (s, 3H; Me), 6.47 (s, 1H; thienyl), 6.48
(s, 1H; thienyl), 6.67 (s, 1H; enol-H), 7.46–7.56 (m, 3H; phenyl), 7.72 (s,
2.37 (s, 6H; Me), 2.40 (s, 6H; Me), 6.45 (s, 2H; thienyl), 6.46 (s, 2H;
thienyl), 6.69 (s, 1H; diketonate-H), 7.92 ppm (s, 1H; thienyl); 19F NMR
(376.4 MHz, CDCl3, 298 K, relative to CFCl3): d=ꢀ141.98 ppm (s);
11B NMR (160.5 MHz, CDCl3, 298 K, relative to BF3·OEt2): d=3.95 ppm
(s); FAB-MS (+ve): m/z: 705 [MꢀF]+; elemental analysis calcd (%) for
C35H31BF2O2S6·0.5H2O: C 57.29, H 4.40; found: C 57.46, H 4.38.
1H; thienyl), 7.94 (d, JACHTUNGTRENNUNG(H,H)=7.1 Hz, 2H; phenyl), 16.30 ppm (br, 1H;
enol-OH); FAB-MS (+ve): m/z: 450 [M]+; elemental analysis calcd (%)
for C25H22O2S3·0.5 H2O: C 65.33, H 5.04; found: C 65.24, H 4.89.
Bis(pentafluorophenyl)boryl{1-
ACHTUNGTRENNUNG
1-[4,5-Bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl]-3-(thiophen-2-yl)pro-
pane-1,3-dione (L2): Synthesized according to a procedure similar to that
used with L1 except DTE-Th-COMe (277 mg, 0.8 mmol) was used as the
starting reagent to react with methyl thiophene-2-carboxylate. Yield:
175 mg (48%); yellow solid; 1H NMR (400 MHz, CDCl3, 298 K, relative
to Me4Si): d=2.02 (s, 3H; Me), 2.09 (s, 3H; Me), 2.37 (s, 3H; Me), 2.39
(s, 3H; Me), 6.46 (s, 1H; thienyl), 6.48 (s, 1H; thienyl), 6.51 (s, 1H; enol-
phen-2-yl]-3-phenylpropane-1,3-dionate} [1-BAHCTUNGTRENNUNG
fluorophenyl)borane diethyl etherate (350 mg, 0.8 mmol) was added to a
stirred solution of L1 (58 mg, 0.128 mmol) in anhydrous dichloromethane
(20 mL) at RT and the reaction mixture was stirred overnight. The reac-
tion was quenched by the addition of water, extracted with chloroform,
washed with brine, and dried over anhydrous magnesium sulfate. After
filtration and removal of the solvent, the crude product was purified by
column chromatography on silica gel (70–230 mesh; n-hexane/chloro-
form, 4:1 v/v). Further purification was achieved by recrystallization
from hot hexane to give the product as an orange solid. Yield: 66 mg
(65%); 1H NMR (400 MHz, CD2Cl2, 298 K, relative to Me4Si): d=2.06
(s, 3H; Me), 2.11 (s, 3H; Me), 2.39 (s, 3H; Me), 2.40 (s, 3H; Me), 6.50 (s,
1H; thienyl), 6.56 (s, 1H; thienyl), 6.98 (s, 1H; diketonate-H), 7.60 (t, J-
H), 7.16 (t, J
ACHTUNGTRENNUNG(H,H)=4.4 Hz, 1H; thienyl), 7.60 (d, JACHTUNGTRENNUNG
thienyl), 7.72 (s, 1H; thienyl), 7.76 (d, JACHTUNGTRENNNUG
16.20 ppm (br, 1H; enol-OH); FAB-MS (+ve): m/z: 456 [M]+; elemental
analysis calcd (%) for C23H20O2S4: C 60.49, H 4.41; found: C 60.75, H
4.49.
1,3-BisACHTUNGTRENNUNG[4,5-bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl]propane-1,3-
A
ACHTUNGTNER(NUNG H,H)=7.5 Hz, 1H; phenyl), 8.06
dione (L3): Synthesized according to a procedure similar to that used
with L1 except DTE-Th-COMe (277 mg, 0.8 mmol) was used as the start-
ing reagent to react with methyl 4,5-bis(2,5-dimethylthiophen-3-yl)thio-
phen-2-carboxylate. Yield: 205 mg (38%); deep-yellow solid; 1H NMR
(400 MHz, CDCl3, 298 K, relative to Me4Si): d=2.02 (s, 6H; Me), 2.09 (s,
6H; Me), 2.37 (s, 6H; Me), 2.39 (s, 6H; Me), 6.46 (s, 2H; thienyl), 6.48
(s, 2H; thienyl), 6.49 (s, 1H; enol-H), 7.67 (s, 2H; thienyl), 16.21 ppm
(br, 1H; enol-OH); FAB-MS (+ve): m/z: 677 [M]+; elemental analysis
calcd (%) for C35H32O2S6·CH3OH: C 60.98, H 5.12; found: C 60.86, H
4.83.
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
22.5 Hz; C6F5), ꢀ157.83 (t,
J
G
ꢀ164.76 ppm (m, C6F5); 11B NMR (160.5 MHz, CDCl3, 298 K, relative to
BF3·OEt2): d=8.65 ppm (s); FAB-MS (+ve): m/z: 628 [MꢀC6F5]+; ele-
mental analysis calcd (%) for C37H21BF10O2S3·0.5H2O: C 55.57, H 2.86;
found: C 55.68, H 3.04.
Bis(pentafluorophenyl)boryl{1-
phen-2-yl]-3-(thiophen-2-yl)propane-1,3-dionate} [2-B
sized according to a procedure similar to that of 1-BAHCTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Difluoroboryl{1-ACHTUNGTRENNUNG[4,5-bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl]-3-
(58 mg, 0.128 mmol) was used as the starting reagent. An orange solid
was obtained. Yield: 64 mg (62%); 1H NMR (400 MHz, CD2Cl2, 298 K,
relative to Me4Si): d=2.06 (s, 3H; Me), 2.11 (s, 3H; Me), 2.39 (s, 3H;
Me), 2.40 (s, 3H; Me), 6.49 (s, 1H; thienyl), 6.55 (s, 1H; thienyl), 6.77 (s,
phenyl-propane-1,3-dionate} (1-BF2): Boron trifluoride diethyl etherate
(0.03 mL, 0.243 mmol) was added to a stirred solution of L1 (58 mg,
0.128 mmol) in anhydrous dichloromethane (20 mL) at RT and the reac-
tion mixture was stirred for 3 h. The reaction was quenched by addition
of water, extracted with chloroform, washed with brine, and finally dried
over anhydrous magnesium sulfate. After filtration and removal of the
solvent, the crude product was purified by column chromatography on
silica gel (70–230 mesh; n-hexane/chloroform, 4:1 v/v). Further purifica-
tion was achieved by recrystallization from hot hexane to give the prod-
uct as an orange solid. Yield: 48 mg (75%). 1H NMR (400 MHz, CDCl3,
298 K, relative to Me4Si): d=2.03 (s, 3H; Me), 2.15 (s, 3H; Me), 2.38 (s,
3H; Me), 2.40 (s, 3H; Me), 6.46 (s, 1H; thienyl), 6.47 (s, 1H; thienyl),
1H; diketonate-H), 7.31 (t, J
(H,H)=4.9, 1.0 Hz, 1H; thienyl), 7.97 (s, 1H; thienyl), 8.08 ppm (dd, J-
(H,H)=4.0, 1.0 Hz, 1H; thienyl); 19F NMR (376.4 MHz, CDCl3, 298 K,
relative to CFCl3): d=ꢀ135.95 (d, J(F,F)=15.0 Hz, C6F5), ꢀ157.87 (t, J-
ACHTUNGTRENN(UNG H,H)=4.4 Hz, 1H; thienyl), 7.93 (dd, J-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A
(160.5 MHz, CDCl3, 298 K, relative to BF3·OEt2): d=8.37 ppm (s). FAB-
MS (+ve): m/z: 633 [MꢀC6F5]+; elemental analysis calcd (%) for
C35H19BF10O2S4·0.5H2O: C 51.92, H 2.49; found: C 52.11, H 2.61.
6.94 (s, 1H; diketonate-H), 7.55 (t, J
ACHTUNGTRNE(NUNG H,H)=7.7 Hz, 1H; phenyl), 7.67 (t,
Bis(pentafluorophenyl)boryl
thiophen-2-yl]propane-1,3-dionate} [3-B
to procedure similar to that of 1-B
0.128 mmol) was used as the starting reagent. A red solid was obtained.
Yield: 89 mg (68%); 1H NMR (400 MHz, CD2Cl2, 298 K, relative to
Me4Si): d=2.03 (s, 6H; Me), 2.09 (s, 6H; Me), 2.38 (s, 6H; Me), 2.40 (s,
6H; Me), 6.46 (s, 2H; thienyl), 6.50 (s, 2H; thienyl), 6.60 (s, 1H; diketo-
nate-H), 7.91 ppm (s, 2H; thienyl); 19F NMR (376.4 MHz, CDCl3, 298 K,
ACHUTGTNRNEUG{N 1,3-bisACHTGNUTRENNUNG
JACHTUNGTRENNUNG
(H,H)=7.3 Hz, 1H; phenyl), 8.01 (s, 1H; thienyl), 8.11 ppm (d, J-
G
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
(H,H)=7.4 Hz, 1H; phenyl); 19F NMR (376.4 MHz, CDCl3, 298 K, rela-
a
ACHTUNGTRENNUNG
tive to CFCl3): d=ꢀ140.90 (75%) (s), ꢀ140.84 ppm (25%) (s); 11B NMR
(160.5 MHz, CDCl3, 298 K, relative to BF3·OEt2): d=4.18 ppm (s). FAB-
MS (+ve): m/z: 479 [MꢀF]+; elemental analysis calcd (%) for
C25H21BF2O2S3·0.5 H2O: C 59.17, H 4.37; found: C 58.93, H 4.25.
Difluoroboryl{1-ACHTUNGTRENNUNG[4,5-bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl]-3-(thio-
relative to CFCl3): d=ꢀ135.45 (d, J(F,F)=22.6 Hz, C6F5), ꢀ157.02 (t, J-
G
phen-2-yl)-propane-1,3-dionate} (2-BF2): Synthesized according to a pro-
cedure similar to that used with 1-BF2 except L2 (58 mg, 0.128 mmol)
was used as the starting reagent. Yield: 45 mg (70%); orange solid;
1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): d=2.02 (s, 3H;
Me), 2.13 (s, 3H; Me), 2.35 (s, 3H; Me), 2.37 (s, 3H; Me), 6.46 (s, 1H;
thienyl), 6.47 (s, 1H; thienyl), 6.81 (s, 1H; diketonate-H), 7.26–7.30 (m,
E
(160.5 MHz, CDCl3, 298 K, relative to BF3·OEt2): d=9.04 ppm (s); FAB-
MS (+ve): m/z: 853 [MꢀC6F5]+; elemental analysis calcd (%) for
C47H31BF10O2S6·H2O: C 54.33, H 3.20; found: C 54.51, H 3.36.
Diphenylboryl{1-
phenyl-propane-1,3-dionate} (1-BPh2): Triphenylborane (0.25m in THF,
2 mL, 0.5 mmol) was added to stirred solution of L1 (58 mg,
ACHTUNGTREN[NUNG 4,5-bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl]-3-
1H; thienyl), 7.88 (dd, JACHTGNUTRENNUNG
thienyl), 8.03 ppm (dd,
JACHTUNGTRENNUNG
a
(376.4 MHz, CDCl3, 298 K, relative to CFCl3): d=ꢀ141.84 ppm (s).
11B NMR (160.5 MHz, CDCl3, 298 K, relative to BF3·OEt2): d=3.91ppm
(s); FAB-MS (+ve): m/z: 504 [M]+; elemental analysis calcd (%) for
C23H19BF2O2S4: C 54.76, H 3.80; found: C 54.63, H 3.86.
0.128 mmol) in anhydrous toluene (20 mL) and the reaction mixture was
stirred and heated at reflux overnight. The solvent was then evaporated
under vacuum and the crude product was purified by column chromatog-
raphy on silica gel (70–230 mesh; n-hexane/chloroform, 4:1 v/v). Further
purification was achieved by recrystallization from hot hexane to give the
product as an orange solid. Yield: 35 mg (45%); 1H NMR (400 MHz,
CD2Cl2, 298 K, relative to Me4Si): d=2.05 (s, 3H; Me), 2.14 (s, 3H; Me),
2.39 (s, 3H; Me), 2.40 (s, 3H; Me), 6.51 (s, 1H; thienyl), 6.56 (s, 1H;
thienyl), 6.89 (s, 1H; diketonate-H), 7.16–7.21 (m, 2H; phenyl), 7.24–7.28
DifluoroborylACHTUNGTRENNUNG{1,3-bisACHTUNGTRENNUNG[4,5-bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl]-
propane-1,3-dionate} (3-BF2): Synthesized according to a procedure simi-
lar to that used with 1-BF2 except L3 (87 mg, 0.128 mmol) was used as
the starting reagent. Yield: 66 mg (72%); red solid; 1H NMR (400 MHz,
CDCl3, 298 K, relative to Me4Si): d=2.02 (s, 6H; Me), 2.12 (s, 6H; Me),
3474
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Chem. Eur. J. 2013, 19, 3467 – 3476

b-Diketonate Derivatives
FULL PAPER
(m, 4H; phenyl), 7.57–7.61 (m, 6H; phenyl), 7.70 (t, J
(H,H)=7.4 Hz,
transitions in the electronic absorption spectra of the two forms in the
ligand (the enol tautomer) and boron compounds. For all the calcula-
tions, the 6–31G
B, and H atoms.
1H; phenyl), 8.00 (s, 1H; thienyl), 8.20 ppm (d, JACTHUNRTGNEUNG(H,H)=7.2 Hz, 2H;
phenyl); 11B NMR (160.5 MHz, CDCl3, 298 K, relative to BF3·OEt2): d=
11.69 ppm (s). FAB-MS (+ve): m/z: 537 [MꢀPh]+; elemental analysis
calcd (%) for C37H31BO2S3·0.5CHCl3: C 67.79, H 4.71; found: C 67.80, H
4.77.
(d,p) basis set[26] was employed to describe the S, F, O, C,
ACHTUNGTRENNUNG
Diphenylboryl{1-ACHTUNGTRENNUNG[4,5-bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl]-3-
(thiophen-2-yl)propane-1,3-dionate} (2-BPh2): Synthesized according to a
procedure similar to that of 1-BPh2, except L2 (58 mg, 0.128 mmol) was
used as the starting reagent. An orange solid was obtained. Yield: 40 mg
(52%); 1H NMR (400 MHz, CD2Cl2, 298 K, relative to Me4Si): d=2.05
(s, 3H; Me), 2.14 (s, 3H; Me), 2.39 (s, 3H; Me), 2.40 (s, 3H; Me), 6.51 (s,
1H; thienyl), 6.55 (s, 1H; thienyl), 6.72 (s, 1H; diketonate-H), 7.16–7.30
(m, 7H; phenyl and thienyl), 7.57–7.59 (m, 4H; phenyl), 7.86 (dd, J-
Acknowledgements
V. W.-W. Y. acknowledges support from the University of Hong Kong
and the URC Strategic Research Theme on Molecular Materials. This
work was supported by the University Grants Committee Areas of Excel-
lence Scheme (AoE/P-03/08). C.-T. P. acknowledges the receipt of a post-
graduate studentship from the University of Hong Kong. We also thank
the Computer Center of The University of Hong Kong for providing
computational resources.
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
relative to BF3·OEt2): d=12.05 ppm (s); FAB-MS (+ve): m/z: 543
[MꢀPh]+; elemental analysis calcd (%) for C35H29BO2S4·0.5C6H14: C
68.76, H 5.47; found: C 68.89, H 5.40.
DiphenylborylACHTUNGTRENNUNG{1,3-bisACHTUNGTRENNUNG[4,5-bis(2,5-dimethylthien-3-yl)thiophen-2-yl]pro-
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to that of 1-BPh2, except L3 (87 mg, 0.128 mmol) was used as the starting
reagent. An orange solid was obtained. Yield: 73 mg (68%); 1H NMR
(400 MHz, CD2Cl2, 298 K, relative to Me4Si): d=2.04 (s, 6H; Me), 2.13
(s, 6H; Me), 2.38 (s, 6H; Me), 2.39 (s, 6H; Me), 6.50 (s, 2H; thienyl),
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BF3·OEt2): d=9.20 ppm (s); FAB-MS (+ve): m/z: 763 [MꢀPh]+; ele-
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C 65.53, H 4.92.
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Received: September 2, 2012
Revised: October 24, 2012
Published online: January 24, 2013
3476
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Chem. Eur. J. 2013, 19, 3467 – 3476