Gated photochromism in triarylborane-containing dithienylethenes: A new approach to a "lock-unlock" system

Article abstract of DOI:10.1021/ja208097a

Photochromic inactive dithienylethene derivatives appended with 3- or 5-dimesitylboryl-2,2′-bithiophene have been synthesized. Upon fluoride ion-binding, the photochromic reactivity is "unlocked", displaying a novel gated photochromic property.

Full text of DOI:10.1021/ja208097a

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Gated Photochromism in Triarylborane-Containing Dithienylethenes:
A New Approach to a “LockꢀUnlock” System
Chun-Ting Poon, Wai Han Lam, and Vivian Wing-Wah Yam*
Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong))
and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China
S Supporting Information
b
Tetramethyldioxaborolane is then attached to Th-DTE by
an iridium-catalyzed reaction10 (∼70% yield), followed by
ABSTRACT: Photochromic inactive dithienylethene deri-
vatives appended with 3- or 5-dimesitylboryl-2,20-bithiophene
mono-Suzuki cross-coupling reaction with the corresponding
have been synthesized. Upon fluoride ion-binding, the photo-
dibromothiophene to afford the key intermediate in 60ꢀ70%
chromic reactivity is “unlocked”, displaying a novel gated photo-
chromic property.
yield. Finally, air-stable Mes2B-substituted bis-thiophene-con-
taning dithienylethenes (∼60% yield) are obtained according to
modification of a literature method.5,6 The unsubstituted bis-
thiophene-containing dithienylethene (L1-O) has also been
prepared as a reference compound. All the compounds have
hotochromic materials, especially organic diarylethenes, have
P
received enormous attention in the past decade because of
their ability to function as potential photoswitchable molecular
devices and optical memory systems.1 Photochromic materials
with gated properties serve as an important development in
nondestructive readout and for practical application.1b In gated
photochromism, the photochromic property is suppressed or
blocked by controlling multiphoton excitation,2 intramolecular
hydrogen bonds,3a oxidation/reduction,3b,h acid/base,3c,i and
chemical reactions,3eꢀg and temperature,4 and an external sti-
mulus is required to “unlock” or reactivate the photochromic
reactivity.
1
been characterized by H and 13C{1H} NMR, mass spectro-
metry, and elemental analysis. 1H NMR spectroscopy of L1-O,
1-O, and 2-O showed the presence of only one set of 1H NMR
signals for the methyl groups on the thiophene moieties, cor-
responding to the time-averaged signals for both the antiparallel
and parallel configurations,11 indicating a fast interconversion of
the two forms12 which probably results from the lack of steric
bulk on thethiophene ring. Moreover, the 11B NMR signals of 1-O
and 2-O are found to be at δ 68.23 and 76.16 ppm, respectively,
which are typical of three-coordinated boron(III) derivatives.6b
All the compounds exhibit an irreversible oxidation wave
at ∼ +1.47 to +1.67 V vs SCE [Table S1 in the Supporting
Information (SI)]. Compounds 1-O and 2-O display two quasi-
reversible reduction waves, at ꢀ1.72, ꢀ2.06 and ꢀ1.79, ꢀ2.19 V,
respectively, while no reduction wave is observed on L1-O
within the solvent window of THF, indicative of the stabilization
of the LUMO by the extended conjugation via the boron pπ
orbital.
Recently, there has been an increasing interest in triarylbor-
ane-based materials due to their capability for optoelectronic5
and anion-sensing applications.6 The vacant pπ orbital of boron
in triarylborane allows for conjugation with an organic π system
and serves as a strong Lewis acid. After binding with Lewis base,
the trigonal planar geometry of the boron center is changed
to tetrahedral, and the electron delocalization and hence the
π-conjugation are interrupted, leading to a significant change in
photophysical properties.5,6 In addition to our continuing inter-
ests in designing various functionalized photochromic materials,7
we hypothesized that, by utilizing this unique characteristic of
boron(III),8 the photochromism of dithienylethene could be
controlled through rational design. Our design strategy is to
append the photochromic dithienylethene group on the dimesi-
tylboryl-2,20-bithiophene core, which is known to show high
fluorescence quantum yield and facile synthetic versatility.9 It is
envisaged that the photochromism could be suppressed by
spontaneous emission derived from the excited state which has
charge-transfer character from the organic π framework to the
boron pπ orbital. Addition of a Lewis base (Fꢀ) may then provide
a turn-off response in the luminescence and may cause the
turning “on” of photochromism. In order to test the hypothesis,
herein we describe the design and synthesis of a new class of
triarylborane-containing dithienylethenes and the study of their
gated photochromic behavior.
Fꢀ anion-binding properties of 1-O and 2-O have been
examined and are shown in Figure 1. Upon complexation with
Fꢀ, dramatic UVꢀvis absorption changes are obtained in both
compounds. Generally, the lowest-energy absorption bands of
1-O and 2-O, at 401 and 394 nm, respectively, tentatively assigned
as the π f π* transition with mixing of charge-transfer character
involving the boron center as the acceptor, would completely
disappear, and new bands centered at 360 and 350 nm, respec-
tively, would appear, similar to that reported previously.13 More-
over, a significant drop in emission intensity of 1-O (λmax
=
465 nm, ϕem = 0.69) and 2-O (λmax = 520 nm, ϕem = 0.73) to
almost nonemissive is observed upon addition of Fꢀ anion.
These changes can be attributed to the disruption of the
π-conjugation extended through the boron pπ orbital by the
formation of the corresponding fluoroborate. The 11B NMR
signals of 1-O and 2-O are upfield-shifted to δ 8.01 and 7.44
ppm in 1-O Fꢀ and 2-O Fꢀ, respectively, upon addition of Fꢀ
3
3
The targeted compounds are prepared as shown in Scheme 1.
Briefly, Th-DTE is synthesized by bis-Suzuki cross-coupling reaction
of 2,3-dibromothiophene and 2,5-dimethyl-3-thienylboronic acid.7h
Received: August 27, 2011
Published: November 16, 2011
r
2011 American Chemical Society
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Scheme 1
Figure 3. UVꢀVis spectral changes in (a) 1-C and (b) 2-C upon visible
irradiation at 550 nm in THF (0.05 M n-Bu4NPF6). Inset shows the plot
of absorbance change at 550 nm as a function of irradiation time with
visible light (black line) and in the dark (red line) at 298 K.
Scheme 2
Figure 1. (a) UVꢀVis and (b) emission spectral changes of 1-O (5 ꢁ
10ꢀ5 M) in THF (0.05 M n-Bu4NPF6) upon addition of various
concentrations of TBAF (from 0 to 1 ꢁ 10ꢀ4 M). (c) UVꢀVis and
(d) emission spectral changes of 2-O (5 ꢁ 10ꢀ5 M) in THF (0.05 M n-
Bu4NPF6) upon addition of various concentrations of TBAF (from 0 to
1 ꢁ 10ꢀ4 M).
photodegradation has been obtained after prolonged UV irradia-
tion (<400 nm), indicative of suppression of the photochromic
behavior upon coordination of the BMes2 group. Addition of Fꢀ
(>1.5 equiv) into a THF solution of 1-O and 2-O gives fluoro-
borate compounds, 1-O Fꢀ and 2-O Fꢀ. Both compounds
3
3
show photochromic behavior upon UV excitation, with three
absorption bands generated at ∼376, 392, and 560 nm for
1-C Fꢀ and at ∼364, 380, and 558 nm for 2-C Fꢀ, which are
Figure 2. (a) UVꢀVis spectral changes for 1-O Fꢀ upon irradiation at
3
3
3
360 nm. Inset shows the plot of absorbance change at 560 nm as a
ascribed to the absorption of the closed form (Figure 2a,c). Both
function of irradiation time. (b) UVꢀVis spectral changes of 1-C Fꢀ
1-C Fꢀ and 2-C Fꢀ display almost the same absorption spectral
3
upon addition of B(C6F5)3 after 360 nm irradiation. (c) UVꢀVis
3
3
spectral changes for 2-O Fꢀ upon irradiation at 350 nm. Inset shows
pattern as that of L1-C, with only a slight red shift in the high-
energy bands relative to L1-C. This indicates that the Fꢀ-bound
compounds have a transition similar to that of L1-C. Further-
more, abstraction of the Fꢀ ion from 1-C Fꢀ and 2-C Fꢀ can be
3
the plot of absorbance change at 558 nm as a function of irradiation time.
(d) UVꢀVis spectral changes of 2-C Fꢀ upon addition of B(C6F5)3
3
after 350 nm irradiation in THF (0.05 M n-Bu4NPF6).
3
3
done by addition of a stronger Lewis acid such as B(C6F5)3.5a,6b
The dissociation of Fꢀ can also be induced by adding an excess of
methanol, which probably would form a hydrogen bond with the
fluoride ion to compete for its binding to the boron center. After
anion, comparable to those observed in other triarylfluoroborate
systems, further supporting the binding of the Fꢀ anion to the
boron center and the change of geometry at boron from trigonal
planar to tetrahedral.6 The binding constant (log Ks) of 1-O for
Fꢀ from UVꢀvis absorption and emission studies, according to
the previously reported method,14 has been determined to
be ∼5.97 ( 0.24 and 5.96 ( 0.11, respectively, while that of 2-O
has been found to be ∼5.86 ( 0.13 and 5.89 ( 0.21, respectively.
Excitation of L1-O in THF into the π f π* transition
absorption band at ∼338 nm produces three new absorption
bands at 354, 370, and 560 nm (Figure S1), which are ascribed to
the absorptions of the closed form. On the other hand, no
photochromism of 1-O and 2-O has been observed in THF,
benzene, and 20% MeCN in benzene, except for the fact that
addition of B(C6F5)3 to 1-C Fꢀ, there is an increase in absorp-
3
tion bands at ∼400 and 600 nm, corresponding to the absorption
of 1-O and 1-C, respectively. A similar observation has been
made upon addition of B(C6F5)3 to 2-C Fꢀ, except that three
3
absorption bands at ∼302, 395, and 582 nm have grown in
intensity, with the lowest absorption band corresponding to
the absorption of 2-C (Figure 2b,d). The bathochromic shifts
observed in 1-C and 2-C, relative to those in 1-C Fꢀ and
3
2-C Fꢀ, are due to the extended conjugation of the condensed ring
3
1
π system to the boron pπ orbital. The H NMR spectral changes
of 2 after Fꢀ binding, photocyclization, and Fꢀ abstraction have
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Figure 5. Spatial plots (isovalue = 0.03) of the HOMO and LUMO for
Figure 4. Spatial plots (isovalue = 0.03) of the HOMO and LUMO for (a)
(a) 1-C Fꢀ and (b) 1-C obtained from the TDDFT/CPCM
1-O and (b) 1-O Fꢀ obtained from the TDDFT/CPCM calculations.
3
3
calculations.
been studied and depicted in Figure S2. Both 1-C and 2-C can be
converted back to their open forms upon irradiation with visible
light (>550 nm). In order to verify that the cycloreversion
involves a light-induced process, not a mere thermal-induced
reaction, the absorption decay of 1-C and 2-C in the dark has also
been studied under the same conditions. As depicted in Figure 3
inset, the absorption spectral change in the dark was less
significant than that under visible light irradiation, confirming
the occurrence of a photocycloreversion process, as depicted in
Scheme 2.
The HOMO and LUMO for 1-C Fꢀ and 2-C Fꢀ are similar to
3
3
those of L1-C (Figure 5a and S7b). Removing the Fꢀ anion from
the boron center allows the conjugation of the boron pπ orbital
into the π conjugated system (Figures 5b and S7c). A decrease in
HOMOꢀLUMO energy separation is found in the closed forms
of the three-coordinated boron compounds when compared with
that of their corresponding fluoride-bound boron compounds.
On the basis of the TDDFT/CPCM calculations, the first
singletꢀsinglet transition of the open and closed forms in each
compound, corresponding mainly to HOMO f LUMO excita-
tion (see SI), is in agreement with the trend observed in the
electronic absorption spectra. The drastic photophysical changes
upon fluoride-anion binding could be attributed to the disruption
of π-pπ conjugation and the absence of charge-transfer character
in the excited state.
The quantum yields for both photocyclization and photocy-
cloreversion processes have been determined. The ϕOfC (325 nm)
value for L1-O is found to be 0.20, while those of 1-O Fꢀ and
3
2-O Fꢀ are determined to be 0.25 and 0.18, respectively. The
3
ϕCfO (509 nm) value for L1-C is found to be 0.04, which is
improved to 0.16 and 0.08 for 1-C Fꢀ and 2-C Fꢀ, respectively.
3
3
In conclusion, a new approach for the gated photochromism
has been demonstrated. The photochromic reactivity is highly
suppressed in the three-coordinated boron compounds, due to
presence of the highly emissive π f π*/pπ(B) excited state,
while it can be restored after Fꢀ ion binding to the boron center.
This concept may provide new insights into the future design of
multiaddressable and gated photochromic materials. Further
studies on modulating the dithenylethene moiety by triarylbo-
rane and functionalization of this class of compounds as mole-
cular logic gates are now in progress.
Moreover, the conversions at the photostationary states of
1-C Fꢀ and 2-C Fꢀ are determined to be 34% and 45%,
3
3
respectively, while that of L1-C is found to be 33%. The cyclo-
reversibility of 1-O Fꢀ has also been studied, with the photo-
3
chromic property still retained after four cycles and with good
fatigue resistance (Figure S3).
Density functional theory (DFT) and time-dependent DFT
(TD-DFT) calculations have been performed to gain a deeper
insight into the molecular structures and nature of the low-
energy absorptions for the open and closed forms in this class of
compounds. Details of the optimized structures are given in the
SI (Figures S4, S5 and Tables S4, S5).
’ ASSOCIATED CONTENT
For the open forms, the HOMO and LUMO for L1-O are the
π and π* orbitals of the bithiophene core slightly mixed with
π and π* orbital of the peripheral thiophene rings, respectively
(Figure S6a). Upon incorporation of the dimesitylboryl group
into the π-conjugated bithiophene system, the LUMOs of 1-O
and 2-O show significant mixing of the pπ orbital on boron with
the π* orbital of the bithiophene system, while the HOMOs are
the π orbital localized on the dithienylbithiophene (Figures 4a
and S6b). Binding of Fꢀ to the boron center breaks the con-
jugation between the boron pπ orbital and the bithiophene
π system, leading to the increase in the HOMOꢀLUMO
separation (Table S6). As depicted in Figures 4b and S6c,
the HOMO and LUMO for 1-O Fꢀ and 2-O Fꢀ are mainly the
S
Supporting Information. Synthetic procedures, charac-
b
terization data, photophysical data, electrochemical data, cyclor-
eversibility studies, and computational details. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
wwyam@hku.hk
’ ACKNOWLEDGMENT
V.W.-W.Y. acknowledges support from The University of
Hong Kong under Distinguished Research Achievement Award
Scheme and the URC Strategic Research Theme on Molecular
Materials. This work was supported by the University Grants
3
3
respective π and π* orbitals of the bithiophene core. For the closed
form, the HOMO and LUMO of L1-C are the respective π and
π* orbitals of the ring-closed dithienylbithiophene (Figure S7a).
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COMMUNICATION
Committee Areas of Excellence Scheme (AoE/P-03/08). C.-T.P.
acknowledges the receipt of a postgraduate studentship from
The University of Hong Kong. We also thank the Computer
Center at The University of Hong Kong for providing computa-
tional resources, which were supported by a Hong Kong UGC
Special Equipment Grant (SEG HKU09).
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