Photochromic dithienylethene-containing triarylborane derivatives: Facile approach to modulate photochromic properties with multi-addressable functions

Article abstract of DOI:10.1002/chem.201404784

A series of dithienylethene-containing triarylboranes has been designed, synthesized, and characterized. The electrochemistry, photophysics, and photochromic behavior have also been studied. The photophysical and photochromic properties could be facilely

Full text of DOI:10.1002/chem.201404784

DOI: 10.1002/chem.201404784
Full Paper
&
Photochromic Systems
Photochromic Dithienylethene-Containing Triarylborane
Derivatives: Facile Approach to Modulate Photochromic
Properties with Multi-addressable Functions
Chun-Ting Poon, Wai Han Lam, Hok-Lai Wong, and Vivian Wing-Wah Yam*[a]
Abstract: A series of dithienylethene-containing triarylbor-
anes has been designed, synthesized, and characterized. The
electrochemistry, photophysics, and photochromic behavior
have also been studied. The photophysical and photochro-
mic properties could be facilely tuned in this system by vary-
ing the thiophene spacers (thiophene, thienothiophene, and
bithiophene) between the dithienylethene and the dimesi-
tylboron (BMes2) or the position of the BMes2 substitution in
the thiophene spacers. The absorption of closed form has
been found to be more sensitive towards the structural
modification upon incorporation of the BMes2 unit. More-
over, multi-addressable photochromic reactivity is obtained
upon addition of Lewis base (Fꢀ), which is due to the forma-
tion of boron–Lewis base adduct. The dependence of the
photophysical and photochromic properties on the thio-
phene spacers and the position of the BMes2 substitution
has been further supported by computational studies.
intensity,[13] or even the photochromic reactivity,[14] could be
modulated by external stimuli, such as light-triggered, chemi-
cal-induced, and thermally responsive, which may allow high-
density storage, non-destructive read-out and data and signal
processing. With our continuing interest in the design and
study of various functionalized photochromic materials,[15] we
believe that the incorporation of triarylborane into photochro-
mic dithienylethene through different p-conjugate formation
would open up new avenues and opportunities for the devel-
opment of new classes of photochromic materials, in which
the photochromic properties could be facilely modulated, and
multi-addressable photochromic properties could also been
achieved upon addition of Lewis base. Herein, we describe the
design and synthesis of a series of dithienylethene-containing
triarylboranes. The dependence of the thiophene spacers and
the position of the BMes2 substitution on the photophysical
and photochromic properties have been demonstrated, sug-
gestive of the high versatility of this class of compounds. Com-
putational studies have been performed to provide a deeper
Introduction
In recent decades, there has been increasing interest in triaryl-
borane-based materials owing to their potential for optoelec-
tronic[1] and anion-sensing applications.[2] The vacant pp orbital
of boron in triarylborane allows for conjugation with organic p
system,[3] in which the photophysical properties could be readi-
ly tuned by attaching substituents at different positions and
the introduction of p-conjugated linkers.[4] Furthermore, it
serves as a strong Lewis acid, which could bind strongly with
Lewis base because of the strong electron-accepting proper-
ties of the boron center that could lead to the disruption of
the p-conjugation, giving rise to a significant change in the
photophysical properties.[1,2,5]
On the other hand, photochromic dithienylethene com-
pounds are amongst one of the most important and well-stud-
ied photochromic materials; the excellent thermal reversibility,
fatigue resistance, and distinguishable absorption spectra of
the open form and the closed form have led to their potential
use in photoswitchable devices and optical memory storage
systems.[6] Furthermore, the utilization of photochromic dithie-
nylethene moiety would provide an additional photo-controlla-
ble switching functionality that could modulate the lumines-
cence,[7] magnetic[8] and non-linear optical properties,[9] chirali-
ty,[10] and conductivity.[11] Besides that, by introducing addition-
al reversible responses, the absorption profile[12] and emission
[a] Dr. C.-T. Poon, Dr. W. H. Lam, Dr. H.-L. Wong, Prof. Dr. V. W.-W. Yam
Institute of Molecular Functional Materials [Area of Excellence Scheme, Uni-
versity Grants Committee (Hong Kong)] and Department of Chemistry,
The University of Hong Kong, Pokfulam Road, Hong Kong (P.R. China)
E-mail: wwyam@hku.hk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404784.
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understanding on the effect of the thiophene spacers and the
position of the BMes2 substitution in the thiophene spacers on
the photophysical and photochromic properties, which would
Table 1. Electrochemical data for non-substituted and BMes2-substituted
compounds[a]
Compound
Oxidation
pa/V vs SCE[b]
Reduction
E1/2/V vs SCE[c] (Epc/V vs SCE)[d]
allow
a better design of multifunctional photochromic
E
materials.
[e]
Th-DTE
TTh-DTE
Th2-DTE
BMes2-Th-DTE
BMes2-TTh-DTE
5-BMes2-Th2-DTE[f]
4-BMes2-Th2-DTE
3-BMes2-Th2-DTE[f]
+1.76
+1.57
+1.47
+1.85
+1.76
+1.67
+1.68
+1.59
–
–
–
[e]
[e]
Results and Discussion
ꢀ1.92
ꢀ1.81, ꢀ2.24
ꢀ1.72, ꢀ2.06
(ꢀ2.12)
Synthesis and characterization
2,3-Bis(2,5-dimethylthiophen-3-yl)thiophene (Th-DTE), 2,3-bis-
(2,5-dimethylthiophen-3-yl)thieno[3,2-b]thiophene (TTh-DTE),
2,3-bis(2,5-dimethylthiophen-3-yl)-5-iodothiophene (I-Th-DTE),
and 2,3-bis(2,5-dimethyl-thiophen-3-yl)-5-iodothieno[3,2-b]thio-
phene (I-TTh-DTE) were prepared as previously reported.[15h]
The dithienyl-substituted derivatives were synthesized by the
initial preparation of BPin-Th-DTE from the reaction between
tetramethyldioxaborolane and 2,3-bis(2,5-dimethylthiophen-3-
yl)thiophene using the iridium-catalyzed reaction (ca. 70%
yield). Then mono-Suzuki cross-coupling reaction between
BPin-Th-DTE and 2,4-dibromothiophene afforded 4-Br-Th2-
DTE with ca. 66% yield (Supporting Information, Scheme S1).
Finally, air-stable Mes2B-substituted dithienylethenes (ca. 60%
yield) were obtained by lithiation of the bromo or iodo group
with 1.2 equivalents of nBuLi, followed by addition of Mes2BF
solution (Supporting Information, Scheme S2). The air-stable
compounds were obtained and isolated in their pure forms by
standard workup procedures and column chromatography.
The identities of the targeted compounds have been charac-
ꢀ1.79, ꢀ2.19
[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] Epc refers
to the cathodic peak potential for irreversible reduction waves. [e] Not
observed within the solvent window of THF. [f] From Ref. [15j].
es, the oxidation is tentatively assigned as oxidation of the
thiophene spacers with some mixing of the dithienylethene
unit. No reduction wave was found for Th-DTE, TTh-DTE, and
Th2-DTE within the electrochemical window of the solvent,
while one or two quasi-reversible reduction waves were ob-
served at ꢀ1.72 to ꢀ2.24 V (vs SCE) upon incorporation of the
BMes2 moiety, indicative of the stabilization of the LUMO
(lowest unoccupied molecular orbital) upon extended p-conju-
gation via the boron pp orbital. With the exception of 4-
BMes2-Th2-DTE, in general, the first reduction wave was shifted
to less negative potential upon going from BMes2-Th-DTE to
BMes2-TTh-DTE to BMes2-Th2-DTE. The first reduction wave
can be assigned as the reduction of the thiophene spacers
with mixing of boron-center reduction character. For 4-BMes2-
Th2-DTE, the first reduction process was found to occur at the
most negative potential among the three BMes2-Th2-DTE com-
pounds. Interestingly, the potential for the first reduction
couple of 4-BMes2-Th2-DTE was found to fall in a similar po-
tential region as that of other related trimesitylboranes,[1c,4c,5g]
suggesting the reduction is localized on the triarylborane unit
(see below).
1
terized by H and 11B NMR spectroscopy and FAB-MS and gave
1
satisfactory elemental analyses. In the H NMR studies, BMes2-
Th-DTE and BMes2-TTh-DTE showed two singlet signals at d=
6.2–6.8 ppm, corresponding to the two thienyl protons on the
dimethylthiophene rings, with the exception of 4-BMes2-Th2-
DTE, which showed a singlet. This suggests that the dime-
thylthiophene rings were freely rotating, so that the parallel
and antiparallel configurations could interconvert between
each other on the NMR timescale at room temperature to give
1
only one signal. Moreover, the H NMR signals from the mesityl
groups were almost the same in all of the Mes2B-substituted
compounds, which showed that there was little influence on
the mesityl groups by changing the p-conjugated linker. Fur-
thermore, the 11B NMR spectra of the Mes2B-substituted com-
pounds were found to be at d=68–72 ppm, which indicated
that the boron centers were three-coordinated adopting
a trigonal planar geometry.[1,2]
Electronic absorption and emission studies
Th-DTE, TTh-DTE, and Th2-DTE were found to dissolve in ben-
zene to give colorless solutions with intense absorption bands
at ca. 290, 300, and 340 nm, respectively. The photophysical
data are summarized in Table 2 and the electronic absorption
and normalized emission spectra of BMes2-substituted com-
pounds in benzene are shown in Figure 1 and 2, respectively.
The absorption band is red-shifted upon going from Th-DTE
to TTh-DTE to Th2-DTE and is attributed to the p!p* transi-
tion of the thiophene spacers, with mixing of the p!p* transi-
tion of the dimethylthiophene. Upon incorporation of the
BMes2 moiety, the low-energy absorption band was found to
be red-shifted by 55–80 nm, (4095–7065 cmꢀ1) with the excep-
tion of 4-BMes2-Th2-DTE and can be assigned as p!p*/pp(B)
Electrochemical studies
All of the compounds exhibited an irreversible oxidation wave
at ca. +1.47 to +1.85 V vs. SCE. The electrochemical data for
the complexes are given in Table 1. The shift of the oxidation
wave to a less positive potential upon going from Th-DTE to
TTh-DTE to Th2-DTE was observed. A similar trend was found
for the corresponding BMes2-substituted compounds. As a less
positive potential of the oxidation wave was observed when
the number of double bonds in the thiophene spacers increas-
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Table 2. Photophysical data of non-substituted and BMes2-substituted compounds in the open form at 293 K.
Compound
Solvent
Absorption
labs/nm (e/dm3 molꢀ1 cmꢀ1
Emission
lem/nm (to/ns)
[a]
f
lum
)
[b]
[b]
[b]
Th-DTE
TTh-DTE
Th2-DTE
benzene
benzene
benzene
THF
benzene
THF
benzene
THF
benzene
THF[c]
290 (6300)
300 (15400)
285 (8200), 340 (14000)
278 sh (14500), 338 (17700)
295 sh (5100), 330 sh (8600), 365 (12500)
328 sh (12800), 365 (19000)
295 sh (6400), 345 sh (14100), 380 (25500)
295 sh (8800), 343 sh (17100), 380 (32800)
285 sh (7600), 330 (11100), 405 (30000)
326 (11100), 401 (31000)
–
–
–
–
[b]
424 (<0.5)
425 (<0.5)
433 (<0.5)
440 (<0.5)
440 (<0.5)
446 (<0.5)
465 (0.97)
475 (1.42)
454 (0.85)
465 (1.05)
517 (6.58)
520 (8.18)
0.036
0.021
0.014
0.020
0.026
0.029
0.720
0.690
0.084
0.032
0.890
0.730
BMes2-Th-DTE
BMes2-TTh-DTE
5-BMes2-Th2-DTE
4-BMes2-Th2-DTE
3-BMes2-Th2-DTE
benzene
THF
benzene
THF[c]
290 sh (14200), 330 (24300)
262 (25900), 328 (24800)
310 (16200), 395 (5900)
309 (15700), 394 (5400)
[a] Luminescence quantum yield determined by using quinine sulfate in 1.0n sulfuric acid (flum =0.546, lex =365 nm) as reference. [b] Non-emissive.
[c] From Ref. [15j].
transition. The red-shift is due to the conjugation of the boron
pp orbital to the p system of the thiophene spacers, leading
to a decrease in the LUMO energy and hence a decrease in the
transition energy. No significant solvatochromic effect was ob-
served upon changing the solvent from benzene to THF, which
is typical of the triarylborane systems with dimesityl
substituents.[1–5]
Upon photoexcitation, Th2-DTE displayed blue emission in
benzene at ca. 424 nm, while Th-DTE and TTh-DTE were
found to be non-emissive. The rather weak emission of Th2-
DTE is probably derived from excited states arising from the
p!p* transition of dithienyl bithiophene. BMes2-Th-DTE,
BMes2-TTh-DTE and 4-BMes2-Th2-DTE also showed weak blue
emission at ca. 440, 446 and 465 nm, respectively, with fluores-
cence quantum yields of less than 0.1 in benzene. A slight
Figure 1. Electronic absorption spectra of the open form of BMes2-substitut-
bathochromic shift in the fluorescence maxima of all of the
compounds was observed by changing the solvent from ben-
zene to THF since the emissive state has partial charge transfer
character, which is similar to those of the previous reports.[1] It
is surprising to note that a significant difference in both the
absorption and emission properties between 4-BMes2-Th2-DTE
and its related compounds, 5-BMes2-Th2-DTE and 3-BMes2-
Th2-DTE, was found in which a lower-energy absorption band
and high luminescence quantum yield were observed in the
latter two compounds.[15j] A possible explanation is due to the
fact that the BMes2 group is poorly conjugated with the bithio-
phene at the 4-position, which will be discussed in more detail
in the computational study below.
ed compounds in benzene.
Fꢀ binding studies
The Fꢀ anion-binding properties of these compounds in their
open form were explored using UV/Vis absorption and NMR
spectroscopy. The binding constants of the BMes2-substituted
compounds are summarized in Table 3. For BMes2-Th-DTE, the
low-energy absorption showed a drop in absorbance and van-
ished completely upon an increase in Fꢀ concentration, with
Figure 2. Normalized corrected emission spectra of the open form of BMes2-
substituted compounds in degassed benzene.
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tonitrile–benzene mixture, which is similar to that of triarylbor-
ane compounds.[2] Table 3 also lists the data from 19F NMR and
11B NMR spectroscopy for the BMes2-substituted compounds
upon addition of TBAF solution. For 19F NMR studies, upfield
shifts of about 40–50 ppm from ca. d=ꢀ117 to ꢀ170 ppm
were obtained in the boron-bound fluoride signal. Moreover,
the 11B NMR resonances of the Mes2B-substituted compounds
were found to be at ca. d=68.17 to 76.16 ppm and showed
upfield shifts to ca. d=7.44 to 9.10 ppm upon Fꢀ ion-binding.
This further supported that the coordination number of the
boron center was changed from three to four and the geome-
try from trigonal planar to tetrahedral.[2,5]
Table 3. Binding constants (log Ks) and 19F NMR and 11B NMR chemical
shifts of the open forms of BMes2-substituted compounds upon addition
of Fꢀ ions.
Compound
Log Ks
19F NMR[a]
11B NMR[a]
Free Bound
d/ppm
Bound
UV/Vis
Free
BMes2-Th-DTE
BMes2-TTh-DTE
5-BMes2-Th2-DTE
7.17ꢁ0.31[b]
6.27ꢁ0.12[b]
6.70ꢁ0.17[b]
5.97ꢁ0.24[d]
4.47ꢁ0.03[b]
6.18ꢁ0.16[b]
5.86ꢁ0.13[d]
ꢀ116.3[c]
ꢀ117.0
ꢀ165.3
ꢀ164.7
ꢀ164.5
68.2
70.1
68.2
8.4
7.8
8.0
ꢀ116.3[c]
4-BMes2-Th2-DTE
3-BMes2-Th2-DTE
ꢀ116.3[c]
ꢀ116.6
ꢀ170.9
ꢀ174.5
71.6
76.2
9.1
7.4
[a] Measured in 20% CD3CN–C6D6. [b] Measured in 20% acetonitrile–ben-
zene (0.05m nBu4NPF6). [c] No signals due to free Fꢀ were observed in
the presence of excess TBAF. A 19F NMR chemical shift of free TBAF in
20% CD3CN–C6D6 of d=ꢀ116.3 ppm was used for comparison. [d] From
Ref. [15j], measured in THF (0.05m nBu4NPF6).
Photochromic studies
Upon excitation into the p!p* transition absorption band of
Th-DTE, TTh-DTE, and Th2-DTE in degassed benzene, typical
photochromic behavior was observed. The solution was
changed from colorless to reddish purple, with the growth of
an intense absorption band at about 330–355 nm and a broad
absorption at about 546–565 nm in the UV/Vis absorption
spectra. There was a progressive red-shift from Th-DTE to TTh-
DTE and to Th2-DTE, which is in line with the trend of absorp-
tion observed in the corresponding open forms. After incorpo-
ration of the BMes2 group, BMes2-Th-DTE also displayed pho-
tochromic reactivity upon UV irradiation, and the solution was
changed from colorless to green. Four absorption bands were
generated at ca. 290, 390, 445, and 660 nm upon UV excitation,
as shown in Figure 4. A significant red-shift of ca. 3163 cmꢀ1
a growth in the higher energy absorption shoulder. BMes2-
TTh-DTE showed a similar spectral change. A well-defined iso-
sbestic point was obtained at ca. 308 and 336 nm upon addi-
tion of Fꢀ ions to BMes2-Th-DTE and BMes2-TTh-DTE, respec-
tively, and saturation was reached when the concentration of
Fꢀ ions was about 5ꢁ10ꢀ5 m. These could be attributed to the
destruction of the p-conjugation between the empty pp orbi-
tal on boron and the p* orbital of the thiophene spacers such
that the p!p*/pp(B) transition would disappear, leading to
the appearance of only the high-energy p!p* absorption.
The UV/Vis spectral changes upon addition of TBAF to BMes2-
Th-DTE, BMes2-TTh-DTE and 4-BMes2-Th2-DTE are shown in
Figure 3 and Figure S1 and S2 in Supporting Information, re-
Figure 4. Electronic absorption spectral changes of BMes2-Th-DTE (ca.
5ꢁ10ꢀ5 m) upon excitation at 365 nm in degassed 20% acetonitrile–benzene
(0.05m nBu4NPF6). Inset: UV/Vis absorption spectral changes of BMes2-Th-
DTE (ca. 1.5ꢁ10ꢀ4 m) at 650 nm on alternate excitation at 365 and 480 nm
over five cycles.
Figure 3. UV/Vis spectral changes of BMes2-Th-DTE (ca. 5ꢁ10ꢀ5 m) in its
open form upon addition of various concentrations of TBAF in 20% acetoni-
trile–benzene. Inset: A plot of absorbance at 366 nm as a function of the
concentration of Fꢀ with theoretical fits for 1:1 binding.
was observed in its closed form when compared with Th-DTE.
The red-shift is ascribed to the increase in conjugation by the
effective overlap of the empty pp orbital on the boron center
and the p* orbital of the condensed ring system of the closed
form. BMes2-TTh-DTE showed photochromic behavior as well
upon UV excitation. Similarly, four absorption bands were gen-
erated at ca. 295, 400, 435 and 610 nm, as shown in Figure S3
spectively, and the corresponding theoretical fits to the equa-
tion for the formation of a 1:1 adduct were also plotted and
shown in the inset. The binding constants, log Ks, for the Fꢀ
anions were found to be in the range of 6.18–7.17 in 20% ace-
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in Supporting Information, while relatively less significant red-
shift of ca. 1722 cmꢀ1 was observed when compared with the
closed form of TTh-DTE. It was surprising to note that a blue-
shift of the lower-energy absorption band was observed upon
going from BMes2-Th-DTE to BMes2-TTh-DTE. Furthermore,
a polymethylmetharylate (PMMA) film of BMes2-Th-DTE was
spin-coated on a quartz plate to demonstrate the photochro-
mic properties in rigid medium. Upon light exposure, the color
of the film was changed from colorless to green and the color
is stable in the dark for a period of time (Supporting Informa-
tion, Figure S4).
When the BMes2 moiety was attached at different positions
of the bithiophene, a dramatic difference in the photochromic
properties was found. 4-BMes2-Th2-DTE was the only bithio-
phene analogues that displayed photochromic property, while
5-BMes2-Th2-DTE and 3-BMes2-Th2-DTE displayed gated pho-
tochromic reactivity, in which the photochromic properties
were only found in the presence of Fꢀ anion.[15j] Three absorp-
tion bands appeared at ca. 355, 375, and 570 nm upon photo-
excitation of 4-BMes2-Th2-DTE (Supporting Information, Fig-
ure S5). In contrast to BMes2-Th-DTE and BMes2-TTh-DTE, the
closed form absorption spectrum of 4-BMes2-Th2-DTE showed
a slight red-shift of ca. 5 nm when compared with Th2-DTE,[15j]
indicative of the similar excited state character for Th2-DTE
and 4-BMes2-Th2-DTE, which would be discussed in more
detail in the DFT study.
Figure 5. UV/Vis spectral changes of BMes2-Th-DTE (ca. 2.5ꢁ10ꢀ4 m) in its
closed form upon addition of various concentrations of TBAF in 20% aceto-
nitrile–benzene.
Moreover, there was a reduction in luminescence intensity in
BMes2-Th-DTE and BMes2-TTh-DTE upon photo-irradiation at
their isosbestic wavelength, as shown in Figure S6 and S7 in
Supporting Information, respectively. The decrease in the emis-
sion intensity upon conversion to the closed forms of BMes2-
Th-DTE and BMes2-TTh-DTE might be attributed to the
quenching of the emissive excited state by the lower-lying ex-
cited state of the closed form moiety. Furthermore, this could
be due to the fact that the absorption shoulder at ca. 440 nm
was found in the closed form of BMes2-Th-DTE and BMes2-
TTh-DTE, which matched well with the open form emission
spectra, leading to an excitation energy transfer from the open
form to the closed form. As a result, the emission intensity
gradually decreased upon photocyclization.
Figure 6. Electronic absorption spectral changes of BMes2-Th-DTE·Fꢀ (ca.
5ꢁ10ꢀ5 m) upon excitation at 310 nm in degassed 20% acetonitrile–benzene
(0.05m nBu4NPF6). Inset: UV/Vis absorption spectral changes of BMes2-Th-
DTE·Fꢀ (ca. 8.5ꢁ10ꢀ5 m) at 550 nm on alternate excitation at 310 and
550 nm over five cycles.
the spectral changes in BMes2-Th-DTE in its closed form upon
addition of various concentrations of TBAF. Furthermore, the
cycloreversibility has been demonstrated in BMes2-Th-DTE and
BMes2-Th-DTE·Fꢀ with at least five repeating cycles, as shown
in the inset in Figure 4 and 6, which indicated that these
chemical processes are all inter-related and reversible in each
direction. Representations of the summarized results are
shown in Scheme 1.
Photochromic studies via Fꢀ binding
The photochromic properties of BMes2-containing dithienyle-
thene compounds with their Fꢀ ion-bound compounds were
studied. Besides the binding of Fꢀ ion to the open form, UV/
Vis spectral changes were also observed upon addition of Fꢀ
ion to the closed form. Upon addition of TBAF, the green solu-
tion of the closed form of BMes2-Th-DTE was gradually
changed to a purple solution and the lowest energy absorp-
tion disappeared with an increase in the higher energy absorp-
tion, similar to that of the open form, as shown in Figure 5.
Moreover, the UV/Vis spectral change was also observed for
the open form Fꢀ ion-bound complex, BMes2-Th-DTE·Fꢀ, with
the appearance of two absorption bands at ca. 326 and
548 nm, as shown in Figure 6, in which the lowest energy ab-
sorption band was in good agreement with the observation of
Similar multi-addressable photochromic reactivity could be
found in both BMes2-TTh-DTE and BMes2-TTh-DTE·Fꢀ (Sup-
porting Information, Figure S8), where typical photochromic
activities were observed. The closed form absorption of
BMes2-TTh-DTE·Fꢀ also resembled the UV/Vis spectral changes
of BMes2-TTh-DTE in its closed form upon addition of TBAF
(Supporting Information, Figure S9). It is interesting to note
that the absorption maximum of the closed form BMes2-TTh-
DTE·Fꢀ (554 nm) was found to be slightly red-shifted when
compared to that of BMes2-Th-DTE·Fꢀ (548 nm), which is in
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hand, the quantum yield of photocycloreversion of BMes2-Th-
DTE and BMes2-TTh-DTE was found to be significantly in-
creased from 0.002 to 0.026 and from 0.008 to 0.03 upon addi-
tion of Fꢀ, respectively. Furthermore, the quantum yield of
photocycloreversion of Fꢀ-bound compounds was found to be
almost the same as the boron-free compounds. This suggested
that the BMes2 group would suppress the photocycloreversion
by at least one order of magnitude when compared to boron-
free compounds.
Computational studies
Density functional theory (DFT) and time-dependent DFT
(TDDFT) calculations have been performed on the dithienyle-
thene-containing triarylboranes (see computational details
below). For the bithiophene compounds, both the s-cis and s-
trans conformations were considered in our study. The molecu-
lar orbitals and the electronic transitions for the bithiophene
compounds shown were based on the study of lower-energy
conformation (Supporting Information, Tables S1 and S2).
Scheme 1. Representation of the multi-addressable photochromic reactivity
upon addition of Fꢀ ion to BMes2-Th-DTE in 20% acetonitrile–benzene
(0.05m nBu4NPF6).
contrast with the trend observed in the corresponding fluo-
ride-free closed forms. The close resemblance of the photo-
chromic activities of the Mes2B-substituted compound to that
of the free ligands upon the addition of Fꢀ ions is likely due to
the destruction of pp–p conjugation between the boron
center and the thiophene spacer so that the fluoroborate
would act like a sp3 substituent with a minor effect on the ex-
cited state properties, and hence the origin of the transition
would resemble that of a p!p* transition of the dithienyl
thiophene/thienothiophene.
The HOMO (highest occupied molecular orbital) for the
open forms of the non-substituted and BMes2-substituted
compounds is the p orbital of the thiophene spacers, mixed
with the p orbitals of the peripheral thiophenes (Figure 7; Sup-
porting Information, Figures S11,S12). The HOMO energy level
increases as the number of the C=C bonds increases in the
thiophene spacers (Supporting Information, Table S3). The
LUMO of the non-substituted compounds corresponds to the
p* orbital of the thiophene spacers with some mixing of the
p* orbitals of the peripheral thiophenes (Supporting Informa-
tion, Figure S11). On the other hand, the LUMO for the BMes2-
substituted compounds with the exception of 4-BMes2-Th2-
DTE show substantial mixing of the pp orbital on boron with
the p* orbital of the thiophene spacers (Figure 7a; Supporting
Information, Figure S12). Unlike the other BMes2-substituted
compounds, the LUMO in 4-BMes2-Th2-DTE is localized on the
triarylborane (Figure 7b). In general, a significant decrease in
the LUMO energy is found upon incorporation of BMes2 unit.
However, the LUMO level in 4-BMes2-Th2-DTE is higher than
On the other hand, only slight UV/Vis spectral changes of 4-
BMes2-Th2-DTE in its closed form was observed upon addition
of TBAF (Supporting Information, Figure S10), resembling the
results of the open form. Such difference is attributed to the
poor conjugation between the BMes2 group the bithiophene
at the 4-position.
Moreover, the quantum yields for both the photocyclization
and photocycloreversion processes have been determined for
Fꢀ-bound or Fꢀ-free compounds and are summarized in
Table 4. The quantum yield of photocyclization was found to
be insensitive towards the addition of Fꢀ ion. On the other
Table 4. Electronic absorption data in the closed form and photochemical quantum yields for non-substituted and BMes2-substituted compounds and
their fluoride-bound compounds.
Compound
Absorption
labs/nm (e/dm3 molꢀ1 cmꢀ1
fO!C
fC!O
)
Th-DTE[a]
328 (9700), 546 (5700)
338 (21300), 552 (7800)
355 sh (30500), 565 (12300)
290 (7100), 390 (9200), 445 (5500), 660 (4600)
326 (20800), 548 (5600)
295 (8500), 400 (20500), 435 (14800), 610 (7,100)
354 (41400), 554 (7500)
376 (34400), 392 (36300), 560 (7500)
355 (25800), 375 (27300), 560 (7200)
364 (33100), 380 (33400), 558 (7300)
0.39[d]
0.42[d]
0.20[d]
0.41[e]
0.48[e]
0.32[e]
0.37[e]
0.25
0.046[f]
0.039[f]
0.041[f]
0.002[g]
0.026[f]
0.008[g]
0.030[f]
0.164
TTh-DTE[a]
Th2-DTE[a]
BMes2-Th-DTE[b]
BMes2-Th-DTE·Fꢀ[b]
BMes2-TTh-DTE[b]
BMes2-TTh-DTE·Fꢀ[b]
5-BMes2-Th2-DTE·Fꢀ[c]
4-BMes2-Th2-DTE[b]
3-BMes2-Th2-DTE·Fꢀ[c]
0.17[e]
0.18
0.054[f]
0.077
[a] Measured in degassed benzene. [b] Measured in degassed 20% acetonitrile–benzene. [c] From Ref. [15j], measured in THF (0.05m nBu4NPF6). [d] Data
obtained using 310 nm as excitation source. [e] Data obtained using 325 nm as excitation source. [f] Data obtained using 510 nm as excitation source.
[g] Data obtained using 480 nm as excitation source.
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Th2-DTE (607 nm). It is noted that the magnitude of the red-
shift for the lower-energy absorption band from the open
form to the closed form is decreased in the order of Th-DTE>
TTh-DTE>Th2-DTE. This probably arises from the fact that the
HOMO and LUMO of the closed form is more localized on the
condensed ring of the dithienylthiophene, and thus a larger
shift in the HOMO and LUMO energy would be expected in
the closed form if the HOMO and LUMO of the open form
have a larger contribution from the dithienylthiophene.
As mentioned above, three BMes2-substituted compounds
displayed photochromic behavior in the absence of Fꢀ anion,
which are BMes2-Th-DTE, BMes2-TTh-DTE, and 4-BMes2-Th2-
DTE. The HOMO for the closed forms of the BMes2-substituted
compounds is similar to its non-substituted compounds
(Figure 8; Supporting Information, Figure S14). The mixing of
Figure 7. Spatial plots (isovalue=0.03) of the HOMO (left) and LUMO (right)
for the open form in (a) BMes2-TTh-DTE and (b) 4-BMes2-Th2-DTE.
that in 3-BMes2-Th2-DTE and 5-BMes2-Th2-DTE. The different
stabilizing effects can be explained on the basis of the contri-
bution in the LUMO at the site of substitution in the bithio-
phene. The LUMO has a larger contribution on the carbon
atoms at the 3- and 5-positions but least at the 4-position
(Supporting Information, Figure S11c). The effect of the BMes2
at the 4-position is much less, leading to the least stabilization
of the LUMO level. This is in agreement with the most positive
potential for oxidation found in 4-BMes2-Th2-DTE when com-
pared to that of 3-BMes2-Th2-DTE and 5-BMes2-Th2-DTE.
On the basis of the TDDFT calculations, the lowest-energy
absorption band for the non-substituted compounds is attrib-
uted to the p–p* transition of the dithienylethene-containing
thiophene spacers (Supporting Information, Tables S4). The cal-
culated transition is found to be red-shifted in energy in the
order of Th-DTE (290 nm)>TTh-DTE (309 nm)>Th2-DTE
(347 nm), which is in line with the increase in extended p-con-
jugation. With the exception of 4-BMes2-Th2-DTE, the lowest-
energy transition for the BMes2-substituted compounds contri-
buting to the HOMO!LUMO excitation is significantly red-
shifted, relative to the corresponding non-substituted com-
pounds. The result support the assignment of the p!p*/pp(B)
transition. For 4-BMes2-Th2-DTE, the S0!S1 transition is com-
puted to be relatively less intense, as it mainly corresponds to
the charge transfer transition from the dithienyl bithiophene
to the triarylborane. The more intense S0!S2 transition com-
puted at 344 nm for 4-BMes2-Th2-DTE is mainly the p–p* tran-
sition of the dithienyl bithiophene. As shown in Table 2, the
peak maximum of the low-energy absorption band of the 4-
BMes2-Th2-DTE is only slightly perturbed when compared to
that of Th2-DTE, since the intense absorption band is mainly
due to p–p* transition.
Figure 8. Spatial plots (isovalue=0.03) of the HOMO (left) and LUMO (right)
for the closed form in (a) BMes2-TTh-DTE and (b) 4-BMes2-Th2-DTE.
the boron pp orbital in the LUMO can also be found in BMes2-
Th-DTE and BMes2-TTh-DTE, but not in 4-BMes2-Th2-DTE. It is
also due to the fact that the LUMO of the bithiophene shows
no contribution from the carbon atom at the 4-position (Sup-
porting Information, Figure S13c), leading to poor conjugation
of the BMes2 group with the bithiophene. The S0!S1 transition
of 4-BMes2-Th2-DTE is computed at 608 nm, which is similar to
the non-substituted compounds. On the other hands, a signifi-
cant red-shift was found for the S0!S1 transition of BMes2-Th-
DTE and BMes2-TTh-DTE, when compared to that of Th-DTE
and TTh-DTE, due to the pp–p conjugation. The transition is
blue-shifted upon going from BMes2-Th-DTE to BMes2-TTh-
DTE, which is in agreement with the experimental observation.
Upon incorporation of the BMes2 group, a larger stabilization
of the LUMO for the closed form in BMes2-Th-DTE than that in
BMes2-TTh-DTE is found (Supporting Information, Table S3).
This can be rationalized by the different contribution at the
carbon where the BMes2 group is attached, which is found to
be larger in Th-DTE than that in TTh-DTE, which allows
a better overlap between the boron pp orbital and the p* or-
bital of the thiophene spacer and thus a larger stabilization of
the LUMO, leading to a red-shift of the transition. The low-
energy absorption band for the closed form of BMes2-Th-DTE
The HOMO and LUMO of the closed forms for the non-sub-
stituted compounds are the respective p and p* orbitals being
more localized on the fused dithienylthiophene unit (Support-
ing Information, Figure S13). As shown in Table S5, the lower-
energy absorption is mainly contributed by the HOMO!
LUMO excitation. The transition is computed to be red-shifted
in the energy order of Th-DTE (565 nm)>TTh-DTE (572 nm)>
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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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phen-3-yl)-5-iodothiophene. A pale yellow solid was obtained.
Yield: 380 mg, 52%. m.p. 1088C. H NMR (400 MHz, CDCl3, 298 K):
with the open form. A 2 mL aliquot of that solution was transferred
into a 1 cm path-length quartz cuvette equipped with a screw cap,
which had been pre-flushed with argon for at least 30 min, by a sy-
ringe needle. The titration was then performed in the dark at 208C
as soon as possible to eliminate complications that are due to pho-
tocycloreversion and to minimize the occurrence of the thermal
backward reaction.
1
d=2.00 (s, 3H, Me), 2.04 (s, 3H, Me), 2.09 (s, 12H, o-Me-mesityl),
2.31 (s, 6H, p-Me-mesityl), 2.36 (s, 3H, Me), 2.37 (s, 3H, Me), 6.45 (s,
2H, thienyl), 6.82 (s, 4H, mesityl), 7.09 (s, 1H, thienyl), 7.21 (s, 1H,
thienyl), 7.53 ppm (s, 1H, thienyl). 13C{1H} NMR (100.6 MHz, CDCl3,
298 K): d=13.78, 13.97, 15.13, 15.17, 21.22, 23.30, 126.44, 126.96,
127.39, 128.22, 129.37, 133.05, 134.89, 134.94, 135.39, 135.51,
137.89, 138.60, 139.56, 140.69 ppm. 11B NMR (160.5 MHz, CDCl3,
298 K): d=71.60 ppm. HRMS (MALDI-TOF) calcd for C38H39BS4: m/
z=635.2107; found: 635.2057 [M+]. Elemental analysis calcd(%) for
C38H39BS4: C 71.90, H 6.19; found: C 71.76, H 6.21.
The binding constants for 1:1 complexation were determined by
a nonlinear least-squares fit of the absorbance versus the concen-
tration of ion added according to Equation (1):[16]
Physical measurements and instrumentation: 1H, 13C{1H}, and
19F{1H} NMR spectra were recorded on a Bruker AV400 at 298 K
with chemical shifts (d, ppm) relative to tetramethylsilane (Me4Si)
and trichlorofluoromethane (CFCl3), respectively. 11B NMR spectra
were recorded on a Bruker DRX 500 at 298 K with chemical shifts
(d, ppm) relative to boron trifluoride diethyl etherate (BF3·OEt2). El-
emental analyses of the new compounds were performed on
a Carlo Erba 1106 elemental analyzer at the Institute of Chemistry,
Chinese Academy of Sciences, Beijing. UV/Vis absorption spectra
for photochromic studies were measured by a Hewlett–Packard
8452A diode-array spectrophotometer and the photoirradiation
was carried out using 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 absorption spectra
for the closed form were recorded on a Varian Cary 50 spectropho-
tometer equipped with a Xenon flash lamp. All measurements
were studied at room temperature unless specified otherwise.
Steady-state emission spectra at room temperature and 77 K were
recorded using a Spex Fluorolog-3 Model FL3-211 spectrofluorome-
ter equipped with a R2658P PMT detector. Time-resolved emission
studies were performed by using 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 by using a CH Instru-
ments, Inc. model CHI 620 Electrochemical Analyzer. All measure-
ments were conducted in CH2Cl2 solutions with 0.1m nBu4NPF6 as
supporting electrolyte at room temperature. A three-electrode
system was employed, with Ag/AgNO3 (0.1m in acetonitrile), glassy
carbon (CH Instrument), and platinum wire as reference electrode,
working electrode, and counter electrode, respectively, in a glass
cell with compartments separating the working electrode and the
reference electrode from the counter electrode.
(
)
1
hꢀ
ꢁ
i
=
2
Alim ꢀ Ao
2 Co
2
1
1
A ¼ Ao þ
Co þ CA þ =
ꢀ
Co þ CA þ =
ꢀ4 CoCA
KS
KS
ð1Þ
where Ao and A are the absorbance of the complex at a selected
wavelength in the absence of and in the presence of the ion, re-
spectively, Co and CA is the total concentration of the complex and
the concentration of the ion, respectively, Alim is the limiting value
of absorbance in the presence of excess ion, and Ks is the stability
constant or binding constant.
Photochemical quantum yield measurements: Chemical actino-
metry was employed for the photochemical quantum yield deter-
mination.[17] Incident light intensities were taken from the average
values measured just before and after each photolysis experiment.
In the determination of the photochemical quantum yield, the
sample solutions were prepared at concentrations with absorbance
slightly greater than 2.0 at the excitation wavelength. The solution
of the open form sample with known concentration (ca. 10ꢀ4 m)
was placed in a degassing cell with a Pyrex round-bottomed flask
connected by a sidearm to a 1 cm quartz fluorescence cuvette and
was sealed from the atmosphere by a Rotaflo HP6/6 quick-release
Teflon stopper. The solution sample was degassed with at least
four freeze–pump–thaw cycles. The quantum yield was determined
at a small percentage of conversion by monitoring the initial rate
of change of absorbance (DA/Dt) in the absorption maximum of
the closed forms in the visible region. To minimize the error, the
measurement was carried in the dark.
Computational details: Calculations were carried out using Gaussi-
an09 software package.[18] The ground-state geometries of the
open and closed forms (if it is photochromic-active) for the non-
substituted and BMes2-substituted compounds as well as the corre-
sponding the fluoride-bound compounds were fully optimized in
benzene by using density functional theory (DFT) calculations with
the hybrid Perdew, Burke, and Ernzerhof functional (PBE0)[19] in
conjunction with the conductor-like polarizable continuum model
(CPCM).[20] On the basis of the ground-state-optimized geometries,
time-dependent density functional theory (TDDFT) calculations[21]
at the same level associated with the CPCM (benzene) was em-
ployed to compute the singlet–singlet transitions in the electronic
absorption spectra of the two forms. Vibrational frequency calcula-
tions were performed for all stationary points to verify that each
was a minimum (NIMAG=0) on the potential-energy surface. For
all the calculations, the 6-31G(d,p) basis set was employed to de-
scribe the S, C, B and H atoms while 6-31+G(d) basis set was used
for the F atom.[22] The DFT and TDDFT calculations were performed
with a pruned (99,590) grid.
Fluoride (Fꢀ) binding measurements: The electronic absorption
spectral titration was performed on a Varian Cary 50 UV/Vis spec-
trophotometer at room temperature and its emission titration was
recorded on a Spex Fluorolog-3 Model FL3-211 spectrofluorometer.
The measurements were performed in 20% acetonitrile in benzene
or THF solution with 0.05m nBu4NPF6 as supporting electrolyte to
maintain a constant ionic strength of the sample solution. The
open-form Fꢀ binding was simply conducted under ambient at-
mosphere without any difficulties. Owing to the instability of the
closed form in air, the closed-form Fꢀ binding was conducted
under an argon atmosphere. A solution of the open form sample
with known concentration was placed in a degassing cell with
a Pyrex round-bottomed flask connected by a sidearm to a 1 cm
quartz fluorescence cuvette and was sealed from the atmosphere
by a Rotaflo HP6/6 quick-release Teflon stopper. The solution
sample was degassed with at least four freeze–pump–thaw cycles.
The solution was then irradiated with a 300 W Oriel Corporation
Model 66011 Xe (ozone free) lamp to give the closed form, mixed
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Mukherjee, P. Thilagar, Chem. Commun. 2013, 49, 993; l) K. Sarkar, P. Thi-
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Acknowledgements
V.W.-W.Y. acknowledges support from The University of Hong
Kong and the URC Strategic Research Theme on New Materi-
als. This work was supported by the University Grants Commit-
tee Areas of Excellence Scheme (AoE/P-03/08), and the General
Research Fund (GRF) Grant (HKU 7060/12P) from the Research
Grants Council of Hong Kong Special Administrative Region,
China. We are grateful to the Information Technology Services
of The University of Hong Kong for providing computational
resources.
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Keywords: boron
·
density functional calculations
·
dithienylethene · multi-addressable functions · photochromic
systems
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