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m.p. 778C. 1H NMR (400 MHz, CDCl3, 298 K): d=2.02 (s, 3H, Me),
2.05 (s, 3H, Me), 2.37 (s, 6H, Me), 2.38 (s, 3H, Me), 6.45 (s, 1H,
thienyl), 6.47 (s, 1H, thienyl), 7.07 (s, 1H, thienyl), 7.09 ppm (s, 2H,
thienyl). 13C{1H} NMR (100.6 MHz, CDCl3, 298 K): d=13.84, 14.00,
15.18, 15.21, 110.39, 120.30, 121.24, 125.68, 126.84, 126.93, 127.38,
129.95, 132.71, 133.22, 133.64, 134.04, 134.63, 135.07, 135.55,
135.73, 138.59 ppm. HRMS (MALDI-TOF) calcd for C20H17BrS4: m/z=
465.9369; found: 465.9372 [M+].
and BMes2-TTh-DTE corresponds to the p!p*/pp(B) transi-
tion, while it is predominantly the p!p* transition for 4-
BMes2-Th2-DTE. These results show that the photophysical and
photochromic properties are strongly dependent on the posi-
tion of the BMes2 substitution and the thiophene spacers.
Similar to the non-substituted compounds, the lowest-
energy absorption band for both the open and closed form of
the fluoride-bound compounds is attributed to the p–p* tran-
sition (Supporting Information, Tables S6 and S7). As binding
of Fꢀ anion to the boron center breaks the conjugation be-
tween the boron and the thiophene spacers, the lowest-
energy transition is in general found to be blue-shifted for the
fluoride-bound compounds relative to that of the fluoride-free
compounds.
(4,5-Bis(2,5-dimethylthiophen-3-yl)-thiophen-2-yl)dimesitylborane
(BMes2-Th-DTE): The reaction was performed under strictly anaero-
bic and anhydrous conditions in an inert atmosphere of nitrogen
using standard Schlenk techniques. n-Butyllithium (1.6m, 1 mL,
1.6 mmol) was added in a dropwise manner to a well-stirred solu-
tion of 2,3-bis(2,5-dimethylthiophen-3-yl)-5-iodothiophene (I-Th-
DTE) (500 mg, 1.16 mmol) in anhydrous diethyl ether (25 mL) at
ꢀ788C. The solution mixture was stirred and maintained at this
temperature for 1 hour in the dark. A solution of dimesitylboron
fluoride (430 mg, 1.6 mmol) in anhydrous diethyl ether (10 mL) was
then added to the reaction mixture in a dropwise fashion. The re-
sulting mixture was stirred in the dark for overnight and the tem-
perature allowed to rise to room temperature gradually. After-
wards, the reaction was quenched with deionized water and the
mixture was then extracted with chloroform, washed with brine,
and finally dried over anhydrous magnesium sulfate. After solvent
removal, the crude product was then purified by column chroma-
tography on silica gel (70–230 mesh). Elution with n-hexane re-
moved the side product, 2,3-bis(2,5-dimethylthiophen-3-yl)thio-
phene, as the first fraction. Then n-hexane–dichloromethane (4:1
v/v) was used to elute the desired product. Further purification
was achieved by recrystallization from hot methanol to give the
product as pale yellow crystals. Yield: 350 mg, 55%. m.p. 1648C.
1H NMR (400 MHz, CDCl3, 298 K): d=1.99 (s, 3H, -Me), 2.05 (s, 3H,
-Me), 2.17 (s, 12H, o-Me-mesityl), 2.30 (s, 6H, p-Me-mesityl), 2.33 (s,
3H, -Me), 2.34 (s, 3H, -Me), 6.38 (s, 1H, thienyl), 6.45 (s, 1H, thienyl),
6.82 (s, 4H, mesityl), 7.34 ppm (s, 1H, thienyl). 13C{1H} NMR
(100.6 MHz, CDCl3, 298 K): d=13.80, 14.22, 15.11, 15.17, 21.29,
23.58, 127.29, 127.57, 128.21, 130.87, 132.80, 133.17, 134.62,
135.23, 135.43, 136.78, 138.42, 140.84, 141.26, 143.21, 147.70 ppm.
11B NMR (160.5 MHz, CDCl3, 298 K): d=68.17 ppm. HRMS (MALDI-
TOF) calcd for C34H37BS3: m/z=522.2151; found: 552.2172 [M+]. El-
emental analysis calcd(%) for C34H37BS3: C 73.89, H 6.75; found:
C 73.65, H 6.76.
Conclusion
A series of dithienylethene-containing triarylboranes has been
successfully prepared. Electronic absorption and Lewis acid
(Fꢀ) binding studies have been performed. The photophysical
and photochromic behaviors were found to be strongly influ-
enced by the thiophene spacers (thiophene, thienothiophene,
and bithiophene) between dithienylethene and BMes2 or the
position of the BMes2 substitution in the thiophene spacers,
and the effects have been elucidated by the computational
studies. The present work has demonstrated the versatility of
these novel classes of triarylborane-containing dithienylethene
systems, which exhibit enriched photochromic and photophys-
ical properties through the rational design of the molecular
structure and the control of the chemical or structural environ-
ment at the boron(III) center.
Experimental Section
Materials: Tetrakis(triphenylphosphine)palladium(0),[15a] 2,5-dime-
thylthien-3-yl boronic acid,[15a] 2,3-bis(2,5-dimethylthiophen-3-yl)th-
iophene,[15h]
and
4,4,5,5-tetramethyl-2-(2,2’’,5,5’’-tetramethyl-
[3,2’:3’,3’’-terthiophen]-5’-yl)-1,3,2-dioxa borolane (BPin-Th-DTE)[15j]
were prepared according to reported procedures. Bis(pinacolato)di-
boron, 2,3-dibromothiophene, 2,5-dibromothiophene, dimesitylbor-
on fluoride, 4,4’-di-tert-butyl-2,2’-bipyridine, and (1,5-cyclooctadie-
ne)(methoxy)iridium(I) dimer were purchased and used as received.
All reactions were performed under strictly anaerobic conditions in
an inert atmosphere of nitrogen using standard Schlenk tech-
niques.
(4,5-Bis(2,5-dimethyl-thiophen-3-yl)thieno[3,2-b]thiophen-2-yl)dime-
sitylborane (BMes2-TTh-DTE): This was synthesized according to
a procedure similar to that of BMes2-Th-DTE except 2,3-bis(2,5-di-
methylthiophen-3-yl)-5-iodothieno[3,2-b]-thiophene
(I-TTh-DTE)
(565 mg, 1.16 mmol) was used instead of 2,3-bis(2,5-dimethylthio-
phen-3-yl)-5-iodothiophene. A pale yellow solid was obtained.
1
Yield: 410 mg, 58%. m.p. 2128C. H NMR (400 MHz, CDCl3, 298 K):
d=1.93 (s, 3H, Me), 2.05 (s, 3H, Me), 2.16 (s, 12H, o-Me-mesityl),
2.31 (s, 6H, p-Me-mesityl), 2.37 (s, 3H, -Me), 2.38 (s, 3H, Me), 6.52
(s, 1H, thienyl), 6.64 (s, 1H, thienyl), 6.83 (s, 4H, mesityl), 7.56 ppm
(s, 1H, thienyl). 13C{1H} NMR (100.6 MHz, CDCl3, 298 K): d=14.06,
15.15, 15.34, 21.24, 23.62, 125.99, 127.09, 127.31, 128.18, 131.00,
131.09, 132.55, 134.42, 135.14, 135.94, 136.12, 138.54, 138.75,
140.31, 140.93 ppm. 11B NMR (160.5 MHz, CDCl3, 298 K): d=
70.11 ppm. HRMS (MALDI-TOF) calcd for C36H37BS4: m/z=608.1872;
found: 608.1918 [M+]. Elemental analysis calcd(%) for
C36H37BS4·0.5CH3OH: C 70.17, H 6.29; found: C 70.46, H 6.23.
4’,5’-Bis(2,5-dimethylthiophen-3-yl)-4-bromo-2,2’-bithiophene (4-Br-
Th2-DTE): Aqueous cesium carbonate solution (2m, 3.03 g, 4.65 mL,
9.3 mmol) was added to a solution mixture of BPin-Th-DTE (1 g,
2.3 mmol), 2,4-dibromothiophene (1.2 g, 5 mmol), and tetrakis(tri-
phenylphosphine)palladium(0) (135 mg, 0.12 mmol) in degassed
THF (75 mL). The reaction was heated to reflux and the progress of
the reaction was monitored by TLC (about 8 h). The reaction mix-
ture 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
chromatography on silica gel (70–230 mesh) using n-hexane as the
eluent. The desired product was collected as the second fraction.
Further purification was achieved by recrystallization from hot n-
hexane to give the product as a white solid. Yield: 710 mg, 66%;
(4’,5’-Bis(2,5-dimethyl-thiophen-3-yl)-2,2’-bithiophen-4-yl)dimesityl-
borane (4-BMes2-Th2-DTE): This was synthesized according to
a procedure similar to that of BMes2-Th-DTE except 4-Br-Th2-DTE
(540 mg, 1.16 mmol) was used instead of 2,3-bis(2,5-dimethylthio-
Chem. Eur. J. 2015, 21, 2182 – 2192
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