A triarylamine-triarylborane dyad with a photochromic dithienylethene bridge

Article abstract of DOI:10.1021/jo301083a

A molecular triad composed of a triarylamine donor, a triarylborane acceptor, and a photoisomerizable dithienylethene bridge has been synthesized and explored by cyclic voltammetry, UV-vis, and luminescence spectroscopy. The effects of irradiation with UV light and fluoride addition on the electrochemical and optical spectroscopic properties of the donor-bridge- acceptor molecule were investigated. Photoisomerization of the dithienylethene bridge affects the triarylboron reduction potential, but not the triarylamine oxidation potential. UV-vis experiments reveal that the association constant for fluoride binding at the triarylborane site is independent of the isomerization state of the bridge. Irradiation of a THF solution of our donor-bridge-acceptor molecule with UV light, followed by F^- addition, leads to a different color of the sample than UV irradiation alone or F^- addition alone.

Full text of DOI:10.1021/jo301083a

Article
pubs.acs.org/joc
A Triarylamine−Triarylborane Dyad with a Photochromic
Dithienylethene Bridge
Andreas K. C. Mengel, Bice He, and Oliver S. Wenger*
Georg-August-Universitat Gottingen, Institut fur Anorganische Chemie, Tammannstrasse 4, D-37077 Gottingen, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: A molecular triad composed of a triarylamine
donor, a triarylborane acceptor, and a photoisomerizable
dithienylethene bridge has been synthesized and explored by
cyclic voltammetry, UV−vis, and luminescence spectroscopy.
The effects of irradiation with UV light and fluoride addition
on the electrochemical and optical spectroscopic properties of
the donor−bridge−acceptor molecule were investigated.
Photoisomerization of the dithienylethene bridge affects the
triarylboron reduction potential, but not the triarylamine
oxidation potential. UV−vis experiments reveal that the
association constant for fluoride binding at the triarylborane site is independent of the isomerization state of the bridge.
Irradiation of a THF solution of our donor−bridge−acceptor molecule with UV light, followed by F− addition, leads to a
different color of the sample than UV irradiation alone or F− addition alone.
visible light, it is possible to switch between class I, class II, and
class III mixed-valence behavior,54 that is, from complete charge
localization to either partial or full charge delocalization. In the
present study, we synthesized and investigated the chemically
related triarylamine−dithienylethene−triarylborane molecule
shown in the right half of Scheme 1 (molecule 1), and we
explored the effects of photoisomerization and fluoride addition
on the electrochemical, optical absorption, and luminescence
properties of this compound.
The design principle of the investigated molecule is simple.
The triarylamine with two p-anisyl substituents is a popular
electron donor because of its comparatively high chemical
stability and relatively low oxidation potential;55,56 dimesityl-
borane is a suitable acceptor because the electron-deficient
boron atom is sterically protected from its environment and,
therefore, relatively stable, yet, at the same time, is a reasonably
good electron acceptor.1 The photoisomerization reactions of
dithienylethenes are highly reversible in many systems; hence,
this particular photochromic unit appeared as an attractive
choice,57−63 also in light of a recent study of fluoride and
mercuric(II) cation sensing by a photochromic organoboron
compound.36
INTRODUCTION
■
Triarylboranes have received much attention in the context of
fluoride sensing in recent years,1−12 but also their possible
application as light-emitting materials or as electron-deficient π-
conjugated units of conducting polymers has stimulated much
work.13−22 One branch of research in this multifaceted area
deals with intramolecular charge transfer between electron-rich
triarylamine groups and electron-poor triarylborane moi-
eties.23−29 In systems with π-conjugated bridges, amine-to-
borane charge transfer commonly manifests as an absorption
band in the electronic (UV−vis) spectrum.1,24−26,28−34 There
have been recent studies of triarylborane-containing dithieny-
lethenes,35,36 but we are unaware of prior investigations of
triarylamine−triarylborane donor−acceptor systems with pho-
tochromic bridges.
In this work, we aimed to explore to what extent the
isomerization state of a photochromic bridge affects intra-
molecular charge transfer between amine and borane groups.
The issue of controlling π-conjugation pathways for electron
transfer in photochromic systems is of long-standing
interest.37−44 However, it is not trivial to investigate photo-
induced electron transfer in photochromic donor−bridge−
acceptor molecules because the light energy that is used to
trigger electron transfer in many cases also induces photo-
isomerization.38,40,42,44 Recent studies on photoswitchable
mixed-valence systems demonstrated that the extent of charge
delocalization between two redox-active centers can be
controlled by light when the two redox moieties are connected
covalently via a photoisomerizable unit.45−52 In our own work,
we had explored the monocationic forms of the dithienyl-
ethene-bridged bis(triarylamine) molecules shown on the left
of Scheme 1 (n = 0, 1),53 and we found that, by using UV and
RESULTS AND DISCUSSION
■
Synthesis. The photochromic backbone of donor−bridge−
acceptor molecule 1 is built into the overall system using
dithienylperfluorocyclopentene building block 2, which has
been previously described in the literature (Scheme 2).37,53
Trimethylsilyl−halogen exchange on molecule 2 yields diiodo
Received: June 1, 2012
Published: July 16, 2012
© 2012 American Chemical Society
6545
dx.doi.org/10.1021/jo301083a | J. Org. Chem. 2012, 77, 6545−6552

The Journal of Organic Chemistry
Article
Scheme 1. (a) Open and Photocyclized Forms of Two Dithienylethene-Bridged Bis(triarylamines) (n = 0, 1; R = C6H4OCH3),
the One-Electron Oxidized Forms of Which Were Previously Investigated in the Context of Photoswitchable Organic Mixed
Valence.45,53 (b) Open and Closed Forms of the Donor−Bridge−Acceptor Molecule of Central Interest in This Study
a
Scheme 2. Synthetic Steps Leading to the Donor−Bridge−Acceptor Molecule 1
a
(a) ICl, CH2Cl2, −5 °C; (b) bis(pinacolato)diboron, Pd(PPh3)Cl2, NaOAc, PEG600, 90 °C; (c) Pd(PPh3)4, THF, Na2CO3 (aq), 80 °C.
compound 3, which can subsequently be used for Suzuki-type
C−C couplings with suitable reagents. The first coupling
partner was molecule 5, which is a boronic acid pinacol ester of
the triarylamine unit, prepared in one step from di-p-anisyl-p-
bromophenylamine (4).64 The resulting coupling product is
molecule 6. A boronic acid pinacol ester of the triarylborane
unit (8), prepared from previously known (4-bromophenyl)-
dimesitylborane (7),65,66 was subsequently coupled to molecule
6, yielding the target molecule 1. Detailed synthetic protocols
and product characterization data of all new compounds are
given in the Experimental Section. In addition, selected NMR
and ESI-TOF-HRMS spectra are shown in the Supporting
Information.
Electrochemistry. Figure 1 shows cyclic voltammograms of
the donor−bridge−acceptor molecule 1. We have found that
triarylamine oxidation is best monitored in CH2Cl2 or CH3CN,
whereas triarylborane reduction is much more cleanly
detectable in dry THF solution. The upper half of Figure 1
shows oxidative voltage sweeps recorded from a 5·10−4
M
solution of molecule 1 in dry and deoxygenated CH2Cl2 in the
presence of 0.1 M tetrabutylammonium hexafluorophosphate
(TBAPF6), while the lower half of Figure 1 shows reductive
6546
dx.doi.org/10.1021/jo301083a | J. Org. Chem. 2012, 77, 6545−6552

The Journal of Organic Chemistry
Article
85% closed isomer.68 The voltammograms in the upper half of
Figure 1 are virtually identical to each other. Triarylamine
oxidation occurs in a reversible fashion at 0.27 V vs Fc+/Fc in
both the open and the closed isomers of 1, in agreement with
previously reported oxidation potentials for this particular
redox-active unit.69,70 Thiophene oxidations in dithienylcyclo-
pentenes are known to occur at significantly higher potentials
and are outside the potential range considered here;44,53,71,72
electrocyclization is not observed.71−74 The reductive sweeps in
the lower half of Figure 1 are significantly different from each
other. While the weak currents between −1.5 and −2.0 V vs
Fc+/Fc are probably caused by impurities, the strong
irreversible waves below −2.5 V vs Fc+/Fc are attributed to
triarylborane reduction, in agreement with literature values for
comparable systems.75−77 It is obvious from Figure 1 that
triarylborane reduction occurs at somewhat less negative
potentials for the closed form of 1 than for its open isomer:
The peak currents are at −2.6 V vs Fc+/Fc for the closed form
and at −2.7 V vs Fc+/Fc for the open isomer. Addition of
TBAF (as a fluoride source) has led to very low quality
voltammograms, which do not permit any meaningful
conclusions except that triarylborane reduction cannot be
detected any more (data not shown).
Reverse photoisomerization from the closed to the open
form is possible when using a 610 nm cutoff filter in front of an
incandescent lamp as an irradiation source. However, this
process is extremely slow for solutions with millimolar
concentrations (>hours), presumably due to the relatively low
photon flux and the low quantum yield for closed-to-open
isomerization, which is not an uncommon phenomenon for
dithienylethenes.41,42 What is more, the sample suffers from
photodegradation, and therefore, it has not been possible to
demonstrate that the photoinduced changes in the voltammo-
gram of Figure 1b are indeed reversible.
Figure 1. Cyclic voltammograms of the open (blue traces) and closed
(red traces) forms of 1. (a) Oxidative sweeps with 100 mV/s in dry
CH2Cl2. (b) Reductive sweeps with 100 mV/s in dry THF. 0.1 M
TBAPF6 was used as an electrolyte in both cases; the waves at −0.51 V
vs Fc+/Fc are due to the decamethylferrocene internal reference. The
waves marked by the asterisks are assumed to be due to
electrochemical decomposition products.67
voltage sweeps obtained from a 10−3 M solution of molecule 1
in dry and deoxygenated THF in the presence of 0.1 M
TBAPF6 electrolyte. Trace amounts of decamethylferrocene
were added for internal voltage calibration, and this causes the
reversible redox waves at −0.51 V vs Fc+/Fc in all four
voltammograms (dashed vertical line).
The blue traces in Figure 1 are voltammograms obtained
from the open form of molecule 1, whereas the red traces are
voltammograms of the photocyclized (closed) form. For
photoisomerization, the samples were irradiated with a hand-
UV−vis Spectroscopy. The blue trace in Figure 2a is the
optical absorption spectrum of a 3.2·10−5 M solution of the
open form of molecule 1 in THF. This sample is essentially
colorless because there are no absorptions in the visible spectral
range. Upon irradiation with UV light, the sample turns blue
1
held 8 W UV lamp. According to H NMR experiments, this
leads to a photostationary state composed of 15% open and
Figure 2. UV−vis spectra of 1 in THF. (a) Photoisomerization of the open form (blue trace) to the closed form (red trace) upon UV irradiation. (b)
Effect of fluoride addition to the closed form. (c) Titration of the open form (blue trace) with TBAF. (d) Photoisomerization of the open fluoride
adduct (orange trace) to the closed fluoride adduct (green trace). Note that the green traces in (b) and (d) are identical.
6547
dx.doi.org/10.1021/jo301083a | J. Org. Chem. 2012, 77, 6545−6552

The Journal of Organic Chemistry
Article
because of increasing absorptions at 643, 452, and 398 nm
(dotted black traces and upward arrows in Figure 2a); at the
same time, the absorbance at 352 nm is decreasing. Ultimately,
a photostationary state (with 85% closed and 15% open form;
see above) is reached (red trace). Figure 2b illustrates the effect
of fluoride addition on the optical absorption spectrum of the
photocyclized solution; the red trace in Figure 2b is the same
spectrum as the red trace in Figure 2a. When adding
tetrabutylammonium fluoride (TBAF), the maximum of the
longest-wavelength absorption shifts from 643 to 620 nm,
resulting in a color change from blue to blue-green. In addition,
there are spectral changes between 500 and 250 nm with clean
isosbestic points at 514, 465, and 373 nm. After 1 equiv of F−
has been added, no further spectral changes are observed and
the green trace in Figure 2b is obtained.
Figure 4. Absorbance changes at (a) 353 and (b) 335 nm in the
course of TBAF addition to 3.2·10−5 M solutions of the open (a) and
closed (b) forms of 1 in THF. The solid lines are least-squares fits to
the experimental data with eq 1.12,78
When reversing the sequence of light irradiation and F−
addition, one ultimately arrives at exactly the same spectrum, a
fact that is illustrated by the lower half of Figure 2: The blue
trace in Figure 2c is the same spectrum as the blue trace in
Figure 2a, that is, the spectral signature of the open form of
molecule 1 in THF. When adding F− to this solution, the
absorbance at 353 nm decreases, whereas a band at 302 nm
gains intensity. After addition of 1 equiv of TBAF, the spectrum
represented by the orange trace in Figure 2c is obtained. The
spectrum of this sample is shown again in Figure 2d (orange
trace), and when irradiated subsequently with UV light, it
undergoes the spectral changes shown in Figure 2d. The green
trace in Figure 2d is the spectrum of the final photostationary
state and corresponds precisely to the green spectrum of Figure
2b, that is, the solution that has been irradiated prior to fluoride
addition. We note that, in Figure 2c, the band of the closed
form at 642 nm is already detectable because part of the sample
unavoidably photoisomerizes in the absorption spectrometer.
The photograph in Figure 3 illustrates the color changes that
are associated with UV irradiation and fluoride addition to a
function of the total (free and bound) F− concentration. An
analogous fluoride titration curve monitoring the absorbance at
335 nm of an equally dilute THF solution of the closed form of
1 is shown in Figure 4b. The solid lines in Figure 4 are the
results of least-squares fits to the experimental data with eq 1,
which is appropriate for determination of binding constants
(Ks) of 1:1 adducts.12,78
A = A0 + [(Alim − A0)/2·c0]·[c0 + cF + Ks−1
[(c0 + cF + Ks−1)2 − 4·c0·cF]1/2
−
]
(1)
In eq 1, A0 is the absorbance of the sample at a selected
wavelength in the absence of any titrant, A is the absorbance at
the same wavelength in the presence of titrant, and Alim is the
limiting absorbance value obtained once the solution has been
saturated with titrant. c0 is the concentration of molecule 1
(3.2·10−5 M), and cF is the concentration of added F− (see x
axes in Figure 4). Fits to the two data sets in Figure 4 yield Ks
values of (1.6 0.8)·107 M−1 (Figure 4a) and (2.3 1.0)·107
M−1 (Figure 4b). Thus, the fluoride binding constants of the
open and closed forms of molecule 1 are identical within
experimental accuracy. The magnitude of our Ks values (107
M−1) is similar to that reported previously for comparable
triarylborane systems.1 We note that fluoride binding constants
may be strongly susceptible to trace water impurities in the
solvent.
19F and 11B NMR Spectroscopy. The 19F NMR spectrum
of the closed form of molecule 1 in CHCl3 exhibits resonances
at −110 and −131 ppm due to the fluorine atoms on the
perfluorocyclopentene backbone (see the Supporting Informa-
tion). Upon addition of TBAF, an additional resonance shows
up at −172 ppm, which is typical for F− bound to a triarylamine
group.1,29 In the 11B NMR spectrum of the same sample, there
is a broad resonance at −74 ppm, which is typical for
triarylborane (see the Supporting Information).1,29 Upon
fluoride addition, this resonance shifts to 4 ppm, in agreement
with previously reported chemical shifts for fluorinated
dimesitylboryl groups.1,29 No attempts to determine fluoride
binding constants from NMR experiments were made; in light
of the high Ks values found above, this did not appear to be
meaningful.
Discussion of Charge-Transfer Properties. We now turn
our attention back to the UV−vis data in Figure 2. The fluoride
titration experiment in Figure 2c suggests that the absorbance
at 353 nm of the open isomer is caused, at least in part, by an
electronic transition involving the boron center. Indeed, in
aromatic nitrogen−boron systems, there are frequently N → B
charge-transfer transitions in this spectral range.1,25,26,28 The
Figure 3. Photograph showing (a) the open form of 1, (b) the open
fluoride adduct of 1, (c) the closed form of 1, and (d) the closed
fluoride adduct of 1.
THF solution of molecule 1: Adding F− to the open isomer has
essentially no influence on the color of the solution; the light
yellowish appearance of the solution (Figure 3b) is mostly due
to TBAF itself. Photoisomerization alone will produce a blue
solution (Figure 3c); photoisomerization combined with
fluoride addition leads to a blue-green color (Figure 3d).
From the F− titrations in Figure 2b,c, it is possible to
determine fluoride binding constants for the closed and open
forms of molecule 1. Figure 4a plots the absorbance of a
3.2·10−5 M THF solution of the open isomer at 353 nm as a
6548
dx.doi.org/10.1021/jo301083a | J. Org. Chem. 2012, 77, 6545−6552

The Journal of Organic Chemistry
Article
difference in triarylamine oxidation and triarylborane reduction
potentials found above amounts to ∼3 V. On this basis, one
might expect the N → B charge-transfer transition at ∼24 000
cm−1 or ∼410 nm, but this represents obviously a crude
estimate at best. Nevertheless, it appears reasonable to conclude
that the absorption band at 353 nm (open isomer) has
significant N → B charge-transfer character.79
photocyclization. The closed form of 1 is nonemissive because
of energetically low-lying π−π* transitions on the dithienylper-
fluorocyclopentene bridge (Figure 2a).41 At long irradiation
times, a photostationary state is reached (see above), and the
remaining percentage (∼15%) of open isomer accounts for the
residual emission intensity. Compared to the initial lumines-
cence intensity detected from a freshly prepared solution of the
open form of 1, the emission intensity decreases only by a
factor of ∼3, but it should be kept in mind that our samples are
highly sensitive to UV light and photoisomerize to a significant
extent already in the course of recording the very first
luminescence spectrum. This difficulty precludes verification
of the F− binding constants from above by complementary
luminescence titration experiments. Samples to which F− has
been added are nonemissive, and it appears plausible to assign
the luminescence of the open (and unfluorinated) form of
molecule 1 to a charge-transfer transition involving the
triarylamine and triarylborane moieties.
As outlined in the Introduction, we anticipated that
photocyclization of the dithienylethene bridge would affect N
→ B charge transfer. However, it is not straightforward to
identify the respective electronic transition in the closed isomer.
One would expect N → B charge transfer to be suppressed
when F− is added, and consequently, we are searching for
spectral regions in the UV−vis data of Figure 2b in which the
absorbance is decreasing upon fluoride addition. This turns out
to be the case between 514 and 466 nm, as well as between 372
and 273 nm; the spectral changes beyond 514 nm are not
considered here because the longest-wavelength absorption is
quite clearly caused by dithienylperfluorocyclopentene-localized
π−π* transitions that may be perturbed by F− complex-
ation.40,44,57,61,62,71,72 From the electrochemical data in Figure
1, we learn that triarylborane reduction is ∼0.1 V easier in the
closed form of molecule 1 than in its open isomer. One might
thus expect a red shift of the N → B charge-transfer transition
by ∼800 cm−1 following photoisomerization, but this cannot be
reconciled with the UV−vis data in Figure 2b. The strongly F−-
sensitive bands between 372 and 273 nm are at higher energy
than the N → B transition in the open form, whereas the
weakly F−-sensitive absorbance between 514 and 466 nm seems
too weak and too red shifted in order to be assigned to the N
→ B transition of the closed isomer. Given the more extensive
π-conjugation of the photocyclized bridge, we would have
expected the oscillator strength of the N → B transition to be
even higher in the closed form than in the open isomer. Thus,
contrary to what we originally hoped, it appears that in-depth
computational studies are necessary to gain insight into N → B
charge transfer in the two isomers of molecule 1, but this is
beyond the scope of our experimental investigations.
SUMMARY AND CONCLUSIONS
■
The N → B charge-transfer transition can be easily identified in
the open form of molecule 1, but for its closed isomer, this
seems impossible with experimental means alone. Thus, we
conclude that photoisomerizable organic mixed-valence sys-
tems, such as those shown on the left of Scheme 1,45,53
represent more attractive models for investigating charge
transfer across photoswitchable bridges than our triaryl-
amine−triarylborane donor−acceptor system. In molecule 1,
there is a small influence of photoisomerization on the
triarylborane reduction potential, but virtually none on the
triarylamine oxidation. We anticipate that this influence would
be significantly greater in a molecule in which the N and B
atoms of the respective redox-active groups would be attached
directly to the two different thiophene units of the photo-
isomerizable bridge. However, in our hands, such a molecule
turned out to be synthetically much less easily accessible than
molecule 1, despite a prior report of an analogous compound
with an organoboron unit attached directly to a thiophene of a
dithienylethene bridge.36
Luminescence Spectroscopy. The open form of molecule
1 is emissive when irradiated with UV light. Figure 5 shows a
series of luminescence spectra that were obtained from a THF
solution of the open form of 1. With increasing irradiation time,
the emission intensity decreases, which is a manifestation of
An interesting finding from our study is that addition of F−
to a solution of molecule 1 induces no significant color change,
whereas photoisomerization alone produces a blue solution.
Only the combined input of UV light and F− ions leads to a
blue-green color.
EXPERIMENTAL SECTION
■
Dithienylperfluorocyclopentene building block 2 was synthesized
following previously published protocols.37,53 Briefly, 2-methylthio-
phene was brominated,80 and the resulting 3,5-dibromo-2-methyl-
thiophene molecule was reacted with n-butyllithium and trimethylsi-
lane to produce 3-bromo-2-methyl-5-trimethylsilylthiophene.81 Sub-
sequent treatment with n-butyllithium and perfluorocyclopentene
yielded molecule 2 (4.93 g/9.6 mmol, 17% yield; starting from 13.9 g/
56.1 mmol of 3-bromo-2-methyl-5-trimethylsilylthiophene and 4.51
mL/33.6 mmol of perfluorocyclopentene).37 1H NMR (300 MHz,
CDCl3): δ (ppm) = 0.37 (s, 18H), 2.49 (s, 6H), 7.11 (s, 2H).
Deprotection of the trimethylsilyl groups occurred with iodine
monochloride following standard protocols,82,838586 and this gave
molecule 3 in 72% yield (650 mg/1.05 mmol, starting from 750 mg/
1.46 mmol of compound 2).53,82,84 1H NMR (300 MHz, CDCl3): δ
(ppm) = 1.91 (s, 6H), 7.20 (s, 2H).
Figure 5. Luminescence of molecule 1 in THF solution measured after
different time intervals following irradiation at 355 nm in the
fluorimeter. The initial spectrum (solid trace) is from a solution
containing mostly the open isomer. Subsequent spectra (dotted
traces) contain an increasing proportion of the nonemissive closed
isomer. Intensities are normalized arbitrarily to the first spectrum.
For the synthesis of molecule 4, 4-bromoaniline (5.00 g, 29.2
mmol) and 4-iodoanisole (14.4 g, 61.3 mmol) were reacted in toluene
(30 mL) in the presence of CuI (0.28 g, 1.46 mmol), KOH (12.8 g,
6549
dx.doi.org/10.1021/jo301083a | J. Org. Chem. 2012, 77, 6545−6552

The Journal of Organic Chemistry
Article
228 mmol), and 1,10-phenanthroline (0.26 g, 1.46 mmol). After
heating to 90 °C for 3 days, the reaction mixture was cooled to room
temperature, diluted with CH2Cl2 (200 mL), and washed four times
with 150 mL portions of water. The combined organic phases were
dried over anhydrous MgSO4, and the solvent was evaporated
subsequently. Purification of the raw product occurred by column
chromatography on silica gel using an eluent mixture composed of
CH2Cl2 and pentane (1:1). This procedure afforded molecule 4 in
57% yield (6.35 g, 16.6 mmol).64 1H NMR (300 MHz, CDCl3): δ
(ppm) = 3.82 (s, 6H), 6.83 (m, 2H), 6.85 (m, 4H), 7.06 (m, 4H), 7.26
(m, 2H).
Compound 4 (1.77 g, 4.60 mmol) was dissolved in PEG600 (23
mL) along with bis(pinacolato)diboron (1.75 g, 6.90 mmol),
bis(triphenylphosphine)palladium(II) chloride (0.16 g, 0.23 mmol),
and sodium acetate (1.77 g, 18.4 mmol). After stirring the reaction
mixture at 85 °C overnight, water was added (50 mL), and the cooled
solution was extracted with diethyl ether (50 mL). The resulting
organic phase was washed three times with 75 mL portions of water,
and the aqueous phases were re-extracted subsequently with diethyl
ether (3 × 100 mL). The combined ether phases were dried over
anhydrous MgSO4, and the solvent was removed on a rotary
evaporator. The raw product was purified by column chromatography
on silica gel using a 1:1 (v:v) mixture of CH2Cl2 and pentane as an
eluent. Subsequent washing of the dry product with pentane gave pure
compound 5 in 32% yield (0.63 g, 1.46 mmol).66 1H NMR (300 MHz,
CDCl3): δ (ppm) = 1.34 (s, 12H), 3.82 (s, 6H), 6.85 (d, J = 8.9 Hz,
4H), 6.89 (d, J = 8.6 Hz, 2H), 7.09 (d, J = 8.9 Hz, 4H), 7.62 (d, J = 8.6
Hz, 2H).
Dithienylethene compound 3 (893 mg, 1.44 mmol) and triaryl-
amine unit 5 (388 mg, 0.90 mmol) were dissolved in a mixture of THF
(6 mL) and 1 M aqueous sodium carbonate solution (10 mL). After
bubbling with N2 during 20 min, tetrakis(triphenylphosphine)-
palladium(0) (160 mg, 0.14 mmol) was added, and the reaction
mixture was heated to 80 °C under N2 overnight. Diethyl ether (25
mL) was added after cooling to room temperature, and the reaction
mixture was washed with water (3 × 25 mL). The organic phase was
dried over anhydrous MgSO4 prior to evaporating the solvent under
reduced pressure. Purification of the raw product occurred using
column chromatography on silica gel using a 9:1 (v:v) mixture of
pentane and diethyl ether. Molecule 6 was obtained in 32% yield (370
mg, 0.46 mmol) as an amorphous solid. 1H NMR (300 MHz, CDCl3):
δ (ppm) = 1.90 (s, 3H), 1.92 (s, 3H), 3.80 (s, 6H), 6.84 (d, J = 9.0 Hz,
3H), 6.90 (d, J = 8.7 Hz, 2H), 7.06 (d, J = 9.0 Hz, 4H), 7.09 (s, 1H),
7.22 (s, 1H), 7.31 (d, J = 8.7 Hz, 2H). 13C NMR (300 MHz, CDCl3):
δ (ppm) = 14.3, 14.5, 29.7, 55.5, 114.8, 119.5, 120.3, 120.6, 126.8,
128.5, 128.7, 133.6, 133.9, 136.2, 140.5, 148.7, 156.1. HRMS (ESI-
TOF) m/z [M+] Calcd for C35H26NO2F6IS2: 797.0354. Found:
797.0347.
For the synthesis of triarylborane unit 7, p-dibromobenzene (1.75 g,
7.57 mmol) was dissolved in dry diethyl ether (20 mL). After cooling
to −78 °C, a 1.6 M solution of n-butyllithium in hexane was added
dropwise (3.4 mL, 6.6 mmol), and the reaction mixture was stirred for
3 h at this temperature. Trimesitylboron fluoride was then added
slowly (1.56 g, 5.5 mmol). After stirring the mixture at room
temperature overnight, it was washed with aqueous NH4Cl solution
(100 mL) and water (100 mL). The organic phase was dried over
anhydrous MgSO4, and the solvent was removed on a rotary
evaporator. After recrystallization from pentane, molecule 7 was
obtained in 67% yield (2.05 g, 5.1 mmol).65 1H NMR (300 MHz,
CDCl3): δ (ppm) = 2.00 (s, 12H), 2.31 (s, 6H), 6.83 (s, 4H), 7.37 (d,
J = 8.2 Hz, 2H), 7.49 (d, J = 8.2 Hz, 2H).
diethyl ether afforded pure product 8 in 90% yield (1.36 g, 3.0
mmol).66 1H NMR (300 MHz, CDCl3): δ (ppm) = 1.36 (s, 12H),
1.98 (s, 12H), 2.31 (s, 6H), 6.82 (s, 4H), 7.50 (d, J = 8.0 Hz, 2H), 7.77
(d, J = 8.0 Hz, 2H).
Compound 8 (259 mg, 0.57 mmol) and dithienylethene unit 6 (370
mg, 0.46 mmol) were dissolved together in THF (4 mL) and 1 M
aqueous Na2CO3 solution (3 mL). Prior to adding tetrakis-
(triphenylphosphine)palladium(0) (120 mg, 0.10 mmol), the reaction
mixture was bubbled with N2 during 20 min. After heating to 80 °C
under N2 overnight, the mixture was cooled to room temperature and
diethyl ether (50 mL) was added. The mixture was then washed with
water (3 × 50 mL), and the combined aqueous phases were re-
extracted with diethyl ether (2 × 50 mL). The combined organic
phases were dried over anhydrous MgSO4 before evaporating the
solvent under reduced pressure. Column chromatography on silica gel
using a 9:1 (v:v) mixture of pentane and diethyl ether afforded the
open form of molecule 1 in 57% yield (262 mg, 0.26 mmol). 1H NMR
(300 MHz, CDCl3): δ (ppm) = 1.96 (s, 3H), 1.97 (s, 3H), 2.00 (s,
12H), 2.34 (s, 6H), 3.82 (s, 6H), 6.81−6.91 (m, 8H), 6.93 (d, J = 8.7
Hz, 2H), 7.09 (d, J = 8.9 Hz, 2H), 7.16 (s, 1H), 7.35 (d, J = 8.7 Hz,
2H), 7.41 (s, 1H), 7.54 (s, 4H). 11B NMR (160 MHz, CDCl3): δ
(ppm) = 73.9 (s, 1B). 19F NMR (282 MHz, CDCl3): δ (ppm) =
−110.0 (m, 4F), −131.9 (m, 2F). HRMS (ESI-TOF-MS) m/z [M+]
Calcd for C59H52NO2BF6S2: 995.3437. Found: 995.3451. Anal. Calcd
for C59H52NO2BF6S2·0.2 C4H8O (%): C, 71.07; H, 5.37; N, 1.38; S,
1
6.32. Found: C, 71.27; H, 5.67; N, 1.21; S, 6.58. Closed isomer: H
NMR (300 MHz, CDCl3): δ (ppm) = 2.03 (s, 12H), 2.15 (s, 6H),
2.33 (s, 6H), 3.83 (s, 6H), 6.74−6.93 (m, 12H), 7.11 (d, J = 8.7 Hz,
4H), 7.36 (d, J = 8.9 Hz, 2H), 7.50−7.58 (m, 4H). 13C NMR (300
MHz, CDCl3): δ (ppm) = 14.2, 15.8, 19.8, 20.4, 21.1, 23.5, 24.9, 55.5,
60.4, 115.0, 118.2, 122.8, 127.4, 127.7, 128.1, 128.3, 129.1, 129.3,
136.4, 139.0, 140.8, 141.5, 149.9, 156.9. Melting point: (78 4) °C.
Cyclic voltammetry was performed using a glassy carbon working
electrode and a silver counter electrode, and a silver wire served as a
quasi-reference electrode. The supporting electrolyte was 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6); decamethylfer-
rocene was added in small quantities for internal voltage calibration.
Nitrogen was bubbled through the dried solvents before initiating
voltage sweeps at 100 mV/s. UV−vis spectra were recorded on a diode
array spectrophotometer; luminescence was measured on a
commercial fluorimeter equipped with a double monochromator.
Quartz cuvettes were used for all optical spectroscopic experiments.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of all new compounds, 11B and 19F
NMR spectra of molecule 1, and ESI-TOF-MS spectra of the
new compounds 1 and 6. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: oliver.wenger@chemie.uni-goettingen.de.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Funding from the Deutsche Forschungsgemeinschaft (DFG)
through grant number WE4815/3-1 is acknowledged.
■
Molecule 7 (1.35 g, 3.3 mmol) was dissolved in PEG600 (60 mL)
along with bis(pinacolato)diboron (1.26 g, 4.95 mmol), bis-
(triphenylphosphine)palladium(II) chloride (0.20 g, 0.28 mmol),
and sodium acetate (10.8 g, 130 mmol). After heating to 90 °C for 4 h,
the reaction mixture was cooled to room temperature, diluted with
water (300 mL), and extracted with diethyl ether (3 × 200 mL). The
combined organic phases were dried over anhydrous MgSO4, and the
solvent was removed subsequently on a rotary evaporator. Column
chromatography on silica gel using a 9:1 (v:v) mixture of pentane and
REFERENCES
■
(1) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge, S.; Gabbaï, F. P.
Chem. Rev. 2010, 110, 3958−3984.
(2) Dusemund, C.; Sandanayake, K.; Shinkai, S. J. Chem. Soc., Chem.
Commun. 1995, 333−334.
(3) Aldridge, S.; Bresner, C.; Fallis, I. A.; Coles, S. J.; Hursthouse, M.
B. Chem. Commun. 2002, 740−741.
6550
dx.doi.org/10.1021/jo301083a | J. Org. Chem. 2012, 77, 6545−6552

The Journal of Organic Chemistry
Article
(4) Bresner, C.; Day, J. K.; Coombs, N. D.; Fallis, I. A.; Aldridge, S.;
Coles, S. J.; Hursthouse, M. B. Dalton Trans. 2006, 3660−3667.
(5) Lee, M. H.; Agou, T.; Kobayashi, J.; Kawashima, T.; Gabbaï, F. P.
Chem. Commun. 2007, 1133−1135.
(39) Liddell, P. A.; Kodis, G.; Moore, A. L.; Moore, T. A.; Gust, D. J.
Am. Chem. Soc. 2002, 124, 7668−7669.
(40) Jukes, R. T. F.; Adamo, V.; Hartl, F.; Belser, P.; De Cola, L.
Inorg. Chem. 2004, 43, 2779−2792.
(41) Wenger, O. S.; Henling, L. M.; Day, M. W.; Winkler, J. R.; Gray,
H. B. Polyhedron 2004, 23, 2955−2958.
(6) Hudnall, T. W.; Gabbaï, F. P. J. Am. Chem. Soc. 2007, 129,
11978−11986.
(42) Jukes, R. T. F.; Adamo, V.; Hartl, F.; Belser, P.; De Cola, L.
Coord. Chem. Rev. 2005, 249, 1327−1335.
(7) Hudnall, T. W.; Kim, Y. M.; Bebbington, M. W. P.; Bourissou, D.;
Gabbaï, F. P. J. Am. Chem. Soc. 2008, 130, 10890−10891.
(8) Broomsgrove, A. E. J.; Addy, D. A.; Bresner, C.; Fallis, I. A.;
Thompson, A. L.; Aldridge, S. Chem.Eur. J. 2008, 14, 7525−7529.
(9) You, Y. M.; Park, S. Y. Adv. Mater. 2008, 20, 3820−3826.
(10) Kim, Y.; Gabbai, F. P. J. Am. Chem. Soc. 2009, 131, 3363−3369.
(11) Hudnall, T. W.; Chiu, C. W.; Gabbaï, F. P. Acc. Chem. Res. 2009,
42, 388−397.
(43) Roberts, M. N.; Nagle, J. K.; Finden, J. G.; Branda, N. R.; Wolf,
M. O. Inorg. Chem. 2009, 48, 19−21.
(44) He, B.; Wenger, O. S. Inorg. Chem. 2012, 51, 4335−4342.
(45) Wenger, O. S. Chem. Soc. Rev. 2012, 41, 3772−3779.
(46) Fraysse, S.; Coudret, C.; Launay, J.-P. Eur. J. Inorg. Chem. 2000,
1581−1590.
(47) Kurihara, M.; Matsuda, T.; Hirooka, A.; Yutaka, T.; Nishihara,
H. J. Am. Chem. Soc. 2000, 122, 12373−12374.
(48) Motoyama, K.; Koike, T.; Akita, M. Chem. Commun. 2008,
5812−5814.
(49) Motoyama, K.; Li, H. F.; Koike, T.; Hatakeyama, M.; Yokojima,
S.; Nakamura, S.; Akita, M. Dalton Trans. 2011, 40, 10643−10657.
(50) Tanaka, Y.; Inagaki, A.; Akita, M. Chem. Commun. 2007, 1169−
1171.
(51) Tanaka, Y.; Ishisaka, T.; Inagaki, A.; Koike, T.; Lapinte, C.;
Akita, M. Chem.Eur. J. 2010, 16, 4762−4776.
(52) Sakamoto, R.; Murata, M.; Nishihara, H. Angew. Chem., Int. Ed.
2006, 45, 4793−4795.
(53) He, B.; Wenger, O. S. J. Am. Chem. Soc. 2011, 133, 17027−
17036.
(12) Lam, S. T.; Zhu, N. A. Y.; Yam, V. W.-W. Inorg. Chem. 2009, 48,
9664−9670.
(13) Kinoshita, M.; Kita, H.; Shirota, Y. Adv. Funct. Mater. 2002, 12,
780−786.
(14) Entwistle, C. D.; Marder, T. B. Angew. Chem., Int. Ed. 2002, 41,
2927−2931.
(15) Entwistle, C. D.; Marder, T. B. Chem. Mater. 2004, 16, 4574−
4585.
(16) Yamaguchi, S.; Wakamiya, A. Pure Appl. Chem. 2006, 78, 1413−
1424.
(17) Sundararaman, A.; Venkatasubbaiah, K.; Victor, M.; Zakharov,
L. N.; Rheingold, A. L.; Jakle, F. J. Am. Chem. Soc. 2006, 128, 16554−
̈
16565.
(18) Zhao, C. H.; Wakamiya, A.; Inukai, Y.; Yamaguchi, S. J. Am.
(54) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 10,
Chem. Soc. 2006, 128, 15934−15935.
247−422.
(19) Parab, K.; Venkatasubbaiah, K.; Jakle, F. J. Am. Chem. Soc. 2006,
̈
(55) Heckmann, A.; Lambert, C. Angew. Chem., Int. Ed. 2012, 51,
326−392.
128, 12879−12885.
(20) Li, H. Y.; Sundararaman, A.; Venkatasubbaiah, K.; Jakle, F. J.
Am. Chem. Soc. 2007, 129, 5792−5793.
(21) Elbing, M.; Bazan, G. C. Angew. Chem., Int. Ed. 2008, 47, 834−
838.
̈
(56) Hankache, J.; Wenger, O. S. Chem. Rev. 2011, 111, 5138−5178.
(57) Feringa, B. L. Molecular Switches; Wiley-VCH: Weinheim,
Germany, 2001.
(58) Irie, M. Chem. Rev. 2000, 100, 1685−1716.
(59) Raymo, F. M.; Tomasulo, M. J. Phys. Chem. A 2005, 109, 7343−
7352.
(22) Lorbach, A.; Bolte, M.; Li, H. Y.; Lerner, H. W.; Holthausen, M.
C.; Jakle, F.; Wagner, M. Angew. Chem., Int. Ed. 2009, 48, 4584−4588.
̈
(23) Liu, X. Y.; Bai, D. R.; Wang, S. N. Angew. Chem., Int. Ed. 2006,
45, 5475−5478.
(60) Murguly, E.; Norsten, T. B.; Branda, N. R. Angew. Chem., Int. Ed.
2001, 40, 1752−1755.
(61) Peters, A.; McDonald, R.; Branda, N. R. Chem. Commun. 2002,
2274−2275.
(62) Belser, P.; De Cola, L.; Hartl, F.; Adamo, V.; Bozic, B.; Chriqui,
Y.; Iyer, V. M.; Jukes, R. T. F.; Kuhni, J.; Querol, M.; Roma, S.; Salluce,
N. Adv. Funct. Mater. 2006, 16, 195−208.
(24) Stahl, R.; Lambert, C.; Kaiser, C.; Wortmann, R.; Jakober, R.
Chem.Eur. J. 2006, 12, 2358−2370.
(25) Yuan, M. S.; Liu, Z. Q.; Fang, Q. J. Org. Chem. 2007, 72, 7915−
7922.
(26) Zhou, G.; Baumgarten, M.; Mullen, K. J. Am. Chem. Soc. 2008,
̈
130, 12477−12484.
(63) Areephong, J.; Hurenkamp, J. H.; Milder, M. T. W.; Meetsma,
A.; Herek, J. L.; Browne, W. R.; Feringa, B. L. Org. Lett. 2009, 11,
721−724.
(27) Sun, Y.; Wang, S. N. Inorg. Chem. 2009, 48, 3755−3767.
(28) Pron, A.; Zhou, G.; Norouzi-Arasi, H.; Baumgarten, M.; Mullen,
̈
K. Org. Lett. 2009, 11, 3550−3553.
(64) Bushby, R. J.; McGill, D. R.; Ng, K. M.; Taylor, N. J. Chem. Soc.,
Perkin Trans. 2 1997, 1405−1414.
(29) Schmidt, H. C.; Reuter, L. G.; Hamacek, J.; Wenger, O. S. J. Org.
Chem. 2011, 76, 9081−9085.
(65) Mutaguchi, D.; Okumoto, K.; Ohsedo, Y.; Moriwaki, K.; Shirota,
Y. Org. Electron. 2003, 4, 49−59.
(30) Liu, Z. Q.; Shi, M.; Li, F. Y.; Fang, Q.; Chen, Z. H.; Yi, T.;
Huang, C. H. Org. Lett. 2005, 7, 5481−5484.
(31) Zhou, Z. G.; Li, F. Y.; Yi, T.; Huang, C. H. Tetrahedron Lett.
2007, 48, 6633−6636.
(32) Cao, D. X.; Liu, Z. Q.; Li, G. Z. Sens. Actuators, B 2008, 133,
489−492.
(33) Hudson, Z. M.; Liu, X. Y.; Wang, S. N. Org. Lett. 2011, 13, 300−
303.
(66) Lu, J.; Guan, Z. Z.; Gao, J. W.; Zhang, Z. H. Appl. Organomet.
Chem. 2011, 25, 537−541.
(67) We note that the respective waves become increasingly
important when performing a series of repeated voltage sweeps on
the same solution; moreover, a new wave appears at −0.85 V vs Fc+/
Fc.
(68) Particularly diagnostic for distinguishing between the two forms
is the resonance from the methyl groups in the ortho position to the
C−B bond because the respective signal does not overlap with any
other resonances.
(34) Li, H. Y.; Jakle, F. Macromol. Rapid Commun. 2010, 31, 915−
̈
920.
(35) Poon, C. T.; Lam, W. H.; Yam, V. W. W. J. Am. Chem. Soc. 2011,
133, 19622−19625.
(69) Sreenath, K.; Suneesh, C. V.; Gopidas, K. R.; Flowers, R. A. J.
(36) Zhou, Z. G.; Yang, H.; Shi, M.; Xiao, S. Z.; Li, F. Y.; Yi, T.;
Huang, C. H. ChemPhysChem 2007, 8, 1289−1292.
(37) Gilat, S. L.; Kawai, S. H.; Lehn, J. M. Chem.Eur. J. 1995, 1,
275−284.
Phys. Chem. A 2009, 113, 6477−6483.
(70) Sreenath, K.; Thomas, T. G.; Gopidas, K. R. Org. Lett. 2011, 13,
1134−1137.
(71) Browne, W. R.; de Jong, J. J. D.; Kudernac, T.; Walko, M.;
Lucas, L. N.; Uchida, K.; van Esch, J. H.; Feringa, B. L. Chem.Eur. J.
2005, 11, 6414−6429.
(38) Endtner, J. M.; Effenberger, F.; Hartschuh, A.; Port, H. J. Am.
Chem. Soc. 2000, 122, 3037−3046.
6551
dx.doi.org/10.1021/jo301083a | J. Org. Chem. 2012, 77, 6545−6552

The Journal of Organic Chemistry
Article
(72) Browne, W. R.; de Jong, J. J. D.; Kudernac, T.; Walko, M.;
Lucas, L. N.; Uchida, K.; van Esch, J. H.; Feringa, B. L. Chem.Eur. J.
2005, 11, 6430−6441.
(73) Peters, A.; Branda, N. R. Chem. Commun. 2003, 954−955.
(74) Peters, A.; Branda, N. R. J. Am. Chem. Soc. 2003, 125, 3404−
3405.
(75) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000,
122, 6335−6336.
(76) Kaim, W.; Schulz, A. Angew. Chem., Int. Ed. 1984, 23, 615−616.
́
(77) Zalis, S.; Kaim, W. Main Group Chem. 2006, 5, 267−276.
(78) Bourson, J.; Pouget, J.; Valeur, B. J. Phys. Chem. 1993, 97, 4552−
4557.
(79) In an attempt to test our hypothesis, we tried to suppress the N
→ B charge-transfer transition by adding Bronsted acids to our
̈
solutions; but unfortunately, this seemed to induce sample
decomposition.
(80) Pu, S. Z.; Liu, G.; Li, G. Z.; Wang, R. J.; Yang, T. S. J. Mol. Struct.
2007, 833, 23−29.
(81) de Meijere, A.; Zhao, L. G.; Belov, V. N.; Bossi, M.; Noltemeyer,
M.; Hell, S. W. Chem.Eur. J. 2007, 13, 2503−2516.
(82) Hensel, V.; Schluter, A. D. Liebigs Ann. 1997, 303−309.
̈
(83) Hanss, D.; Wenger, O. S. Eur. J. Inorg. Chem. 2009, 3778−3790.
(84) Hensel, V.; Lutzow, K.; Jacob, J.; Gessler, K.; Saenger, W.;
Schluter, A. D. Angew. Chem., Int. Ed. 1997, 36, 2654−2656.
̈
(85) Walther, M. E.; Wenger, O. S. Dalton Trans. 2008, 6311−6318.
(86) Hanss, D.; Walther, M. E.; Wenger, O. S. Coord. Chem. Rev.
2010, 254, 2584−2592.
6552
dx.doi.org/10.1021/jo301083a | J. Org. Chem. 2012, 77, 6545−6552