Insight into the isoelectronic character of azomethines and vinylenes using representative models: A spectroscopic and electrochemical study

Article abstract of DOI:10.1021/jo401497z

A series of azomethine and vinylene dyad and triad analogues were prepared. Their absorbance, fluorescence, and redox properties were examined experimentally and theoretically using density functional theory (DFT) calculations. These measurements were done to determine the effect of the heteroatom of the azomethine relative to its all-carbon counterpart and to assess the isoelectronic character of the two bonds. The orientation of the azomethine was found to have little effect on the absorbance, fluorescence, and electrochemical properties. In contrast, the spectral and electrochemical properties were highly contingent on the electronic groups and degree of conjugation. The spectral properties could be tuned 200 nm across the visible region. More importantly, the heteroatom in the conjugated bond was found to give rise to only a 20 nm bathochromic shift in the absorbance and fluorescence spectra. The fluorescence quantum yield (Φ_Fl) of the vinylene derivatives varied between 5% and 20% with fluorescence quenching occurring by photoisomerization from the E to Z isomers. In contrast, the fluorescence of the analogous azomethine derivatives was completely quenched. The collective spectroscopic and electrochemical ab initio DFT data additionally confirmed that the azomethine and its analogous vinylene are isoelectronic. It was also found that a conjugated thiophene vinylene dyad with primary amines in the α,α′-positions could be prepared and isolated. The compound was stable under aerobic conditions providing electron withdrawing (either ester or nitrile) groups were located in the adjacent positions.

Full text of DOI:10.1021/jo401497z

Article
pubs.acs.org/joc
Insight into the Isoelectronic Character of Azomethines and
Vinylenes Using Representative Models: A Spectroscopic and
Electrochemical Study
Andreanne Bolduc,† Abelaziz Al Ouahabi,†,‡ Charlotte Mallet,†,§ and W. G. Skene*,†
́
†Laboratoire de caracter
́
isation photophysique des mater
Centre-ville Montreal, QC H3C 3J7, Canada
‡Institut Charles Sadron (UPR22-CNRS) 23 rue du Loess, BP 84047, 67034 Strasbourg Cedex 2, France
́ ́ ́ ́ ́
iaux conjugues, Departement de Chimie, Universite de Montreal, CP 6128,
§Department of Polymer Science and Engineering, University of Massachusetts - Amherst, Conte center for polymer research, 120
Governors Drive, Amherst, Massachusetts 01003, United States
S
* Supporting Information
ABSTRACT: A series of azomethine and vinylene dyad and triad
analogues were prepared. Their absorbance, fluorescence, and redox
properties were examined experimentally and theoretically using
density functional theory (DFT) calculations. These measurements
were done to determine the effect of the heteroatom of the
azomethine relative to its all-carbon counterpart and to assess the
isoelectronic character of the two bonds. The orientation of the azomethine was found to have little effect on the absorbance,
fluorescence, and electrochemical properties. In contrast, the spectral and electrochemical properties were highly contingent on
the electronic groups and degree of conjugation. The spectral properties could be tuned 200 nm across the visible region. More
importantly, the heteroatom in the conjugated bond was found to give rise to only a 20 nm bathochromic shift in the absorbance
and fluorescence spectra. The fluorescence quantum yield (ΦFl) of the vinylene derivatives varied between 5% and 20% with
fluorescence quenching occurring by photoisomerization from the E to Z isomers. In contrast, the fluorescence of the analogous
azomethine derivatives was completely quenched. The collective spectroscopic and electrochemical ab initio DFT data
additionally confirmed that the azomethine and its analogous vinylene are isoelectronic. It was also found that a conjugated
thiophene vinylene dyad with primary amines in the α,α′-positions could be prepared and isolated. The compound was stable
under aerobic conditions providing electron withdrawing (either ester or nitrile) groups were located in the adjacent positions.
room temperature with catalytic mineral acids.11,12 The
products can further be purified by straightforward precip-
INTRODUCTION
■
Conjugated materials have received much attention, owing to
itation. Azomethines are additionally interesting as functional
their optoelectronic properties that are suitable for plastic
materials because of the inherent electron-withdrawing
electronic applications.1−4 Thiophenes have been the choice
property of the heteroatomic bond. When coupled to electron
building block for preparing such materials. This is in part due
to their low oxidation potentials that are ideal for both chemical
rich heterocycles, this results in highly conjugated π-donor−
acceptor materials. These compounds can have optoelectronic
and electrochemical synthesis of highly conjugated compounds.
properties that are of interest for use in device applications and
Despite this advantage, the chemical synthesis of conjugated
thiophenes involving carbon−carbon coupling requires strin-
that are comparable to their all-carbon counterparts.
gent reactions such as Suzuki coupling,5 Stille coupling6,7 and
Azomethines are commonly accepted as being isoelectronic
to their all-carbon counterparts.13 There are nonetheless
Heck coupling,8 to name but a few. The synthesis of donor−
acceptor conjugated materials is additionally challenging
because of reactivity incompatibilities between reagents and
catalysts that lead to low yields and unselective product
differences in their properties, notably their photostability,
fluorescence, and redox potentials.14 Extensive structure−
property studies have subsequently been undertaken for
optimizing the optoelectronic properties of azomethines.11
formation. Therefore, methods for preparing donor−acceptor
Despite these studies, there is still little known about the
collective effect of the azomethine bond and various electronic
groups on the optoelectronic properties. This is in part owing
to the limited number of stable aminothiophenes available for
conjugated materials that have desired optoelectronic proper-
ties are of interest.9,10
We recently addressed this challenge by using azomethines
(−NCH−) as functional materials. The advantage of the
preparing conjugated azomethines. As a result, the stable 2,5-
heteroatomic materials over their vinylene counterparts is the
ease of preparation. Azomethines are prepared by the
straightforward condensation of complementary amines and
aldehydes. The reaction can be done using mild conditions at
Received: July 10, 2013
Published: August 15, 2013
© 2013 American Chemical Society
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diamine (1; Chart 1) has predominately been used for
preparing conjugated azomethines that are stable under
use of vinyl derivatives in plastic electronics such as organic
photovoltaics15,16 and organic light emitting diodes,17,18 would
suggest that azomethines would also be suitable for such
applications. It is therefore important to understand the effect
of the various electronic groups on the optoelectronic
properties for designing and preparing azomethines with
enhanced properties relative to their analogous vinyl deriva-
tives. Most importantly, the effect of the nitrogen atom in the
azomethine and stabilizing esters are of importance for
comparing the properties among similar compounds. For this
reason, we prepared the previously unreported and air-stable 2-
aminothiophene vinylene 4, whose synthesis, spectroscopy, and
electrochemistry are herein presented. The effect of the esters
and the azomethine nitrogen on the spectroscopic and
electrochemical properties is further examined by comparing
to the analogous compounds in Chart 1. The spectroscopic
properties and energy levels of 2-aminothiophene derivatives
that could not be synthesized were additionally calculated
theoretically. The collective experimental and theoretical data
were compared for understanding the structural modifications
and electronic effects on the spectroscopic and electrochemical
properties of conjugated oligomers.
Chart 1. Vinylene and Azomethine Analogues Investigated
RESULTS/DISCUSSION
■
Synthesis. The ester-free derivative (7) would be an ideal
model compound for direct comparison of the physical
properties to both the vinylene analogue (2) and the
azomethines in Chart 1. However, it could not be isolated
and it spontaneously decomposed. For this reason, 4 was
targeted. It was surmised that the electron withdrawing ester in
the 3-position would stabilize the targeted product, similar to its
unsubstituted counterpart 2-aminothiophene.19−21
ambient aerobic conditions. The stability of 1 and its
azomethine products are courtesy of the stabilizing effect of
the esters in the 3 and 4 positions.
Their synthetic and purification advantages concomitant with
their isoelectronic character make azomethines interesting
replacements for their all-carbon counterparts. The successful
The synthesis of 4 was done according to the synthetic
scheme outlined in Scheme 1. This involved preparing the
activated ester 4a according to known means in high yields.22 It
was then reacted with 1,4-dithiane-2,5-diol via the Gewald
reaction under basic conditions to afford the air-stable 2-
Scheme 1. Synthetic Scheme for the Preparation of 4: (i) 80 °C, 16 h; (ii) 45 °C, DMF; (iii) rt 16 h, CH2Cl2; (iv) 60 °C, 16 h,
Dioxane; (v) TiCl4/ZnCl2, THF
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Scheme 2. Synthetic Scheme for the Preparation of 8: (i) POCl3, 60 °C, DMF; (ii) TiCl4/ZnCl2, THF
aminothiophene 4b.23 Although the product is inert, the amine
group is still reactive, especially under Vilsmeier−Haack and
McMurry reaction conditions. 4b was subsequently protected
as an amide with trifluoroacetic anhydride, which was
butylthiophene-2-carboxaldehyde starting material. The yields
are nonetheless consistent with those of 4 and 8. The low
overall yields for the aminothiophene vinylenes are a result of
the stringent reactions, multistep preparation, and mixture of E
and Z isomers.
1
confirmed by both H and 19F NMR. The protection was
t
The vinylene model compound (2) was prepared according
to standard McMurry coupling procedures.26 No extensive
purification was required for the compound. The desired E
isomer was isolated by recrystallization in carbon tetrachloride,
extremely sluggish, unlike that for protecting 4b with BOC.
The latter was originally chosen, but the protecting group did
not withstand the McMurry coupling conditions. The desired
aldehyde was incorporated into 4c via Vilsmeier−Haack
formylation. The isolated 4d was then subjected to McMurry
coupling. Formation of the desired vinylene was found to occur
only when zinc was added to a TiCl4 solution in
tetrahydrofuran (THF) and allowing it to stir for 30 min
before adding 4d. The resulting product was found to be
unstable. It was subsequently deprotected with potassium
carbonate without isolating it beforehand. The reactions were
additionally done in the dark to prevent photoisomerization of
the vinylene products. Although the E isomer of 4 was
predominately formed, it could be isolated from the undesired
Z isomer by column chromatography. The targeted 4 was
isolated as a yellow solid. Although the product could be
isolated and handled under ambient conditions, it was
photochemically unstable, requiring protection against ambient
light when stored.
1
as confirmed by H NMR.
X-ray Structure. Monocrystals of 8 suitable for X-ray
diffraction (XRD) analyses were grown from the slow
evaporation of acetone. These crystals were grown to see
whether the resulting compound was coplanar similar to
analogous azomethines.27 XRD further provides unequivocal
confirmation that the E isomer of the unsaturated bond was
obtained, as was confirmed by 1H NMR. As shown in Figure 1,
Compound 8 was also targeted. This was because the
azomethine formed from the dimethylformamidine (DMF)
protecting group has a limited degree of conjugation relative to
conjugated azomethines such as 11, 13, and 21. Therefore, the
effect of the azomethine bond on the photophysical and
electrochemical properties relative to conjugated azomethines
can be examined. The targeted 8 was prepared similarly to 4
but starting from 8a (Scheme 2). 8a was formylated directly,
and it was protected in situ by DMF. The protecting group was
expected to be removed with conditions similar to those of 4.
In contrast, only the protected product was obtained. Multiple
attempts to deprotect 8 were unsuccessful with known
methods, including hydrochloric acid and trifluoroacetic acid.
The electron withdrawing differences of the nitrile and ester
groups are most likely responsible for the different stability of
the protecting group.
Figure 1. Face (top) and edge (bottom) views of the resolved XRD
crystal structure of 8.
The syntheses of 10, 21, and 23 were done according to
previously described methods.24,25 The azomethine derivatives
15, 17, and 19 were prepared from the corresponding amine
and aldehyde in a 1:1 ratio in ethanol along with a catalytic
amount of trifluoroacetic acid. The reaction mixture was then
stirred overnight at room temperature. No inert atmosphere or
anhydrous solvents were required. In the case of 15, the
resulting dimer was insoluble in ethanol, and the product
precipitated overnight. The desired product was obtained pure
in a considerable yield by filtering the solution and washing it
with cold ethanol. In contrast, 17 and 19 required additional
purifications to isolate them from the undesired trimers that
were formed as byproducts. Purification by flash column
chromatography was thus performed and the compounds were
obtained in moderate yield. Differences in yields were observed
for 17 (20%) and 19 (75%) owing to the instability of the 5-
8 is highly coplanar with a mean plane torsion angle between
the two thiophene rings of 1.96(1)°. The high degree of
coplanarity adopted by the thiophenes with respect to the
vinylene is evident in the face and edge views of Figure 1. The
configuration adopted is similar to that of corresponding
azomethines.27
8 organized itself in a zigzag packing in the crystal lattice
(Figure 2) and it crystallized in the orthorhombic Pbca space
group. Its organization is driven mostly by hydrogen bonding
between the cyano group and the adjacent hydrogens as shown
in top of Figure 3. Two different hydrogen bonding motifs were
found in the crystal packing. Bonding between the cyano
groups and hydrogen from the methyl terminal group was
found at a distance of 2.647(1) Å. The short distance is
common for H-bonding.28 This is assumed to lead to the zigzag
configuration, because the molecules organize themselves at
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behavior was observed in azomethines, where the thiophene
centroid had π interactions with the azomethine between
thiophenes.30
Photophysical Properties. The photophysical properties
of the azomethines and vinylene analogues from Chart 1 were
investigated. Notably, the absorbance, fluorescence, and
fluorescence quantum yields of these compounds were
measured to assess the structural and electronic effects on the
spectroscopic properties. These effects can be assigned by
comparing the spectroscopic properties of the different
compounds. For example, the impact of the degree of
conjugation can be seen by comparing 2, 4, 8, and 10 as well
as azomethine derivatives 15, 17, 18, 21, and 23. The effects of
the azomethine bond, amine electron donating, and ester
withdrawing groups can be assigned by comparing both 21 and
23 to 10. The electronic effects can similarly be evaluated from
comparing 2, 4, and 8. Meanwhile, the electron donating and
withdrawing effects can be evaluated by comparing the spectral
properties of 2, 4, 15, 17, and 19.
As can be seen by the collective absorbance spectra in Figure
4 and the data in Table 1, the absorbance can be tailored
Figure 2. Crystal lattice packing of 8.
□
●
Figure 4. Normalized absorbance of 2 (black ), 4 (red ), 8 (blue
□
●
▲
▽
▲
), 10 (red ), 15 (purple ), 17 (orange ), 19 (green ), 21
▽
●
(brown ), and 23 (green ) measured in dichloromethane.
approximately 200 nm across the visible region with subtle
structural modifications and with electronic groups. For
example, an 87 nm red shift in the absorbance occurs as a
Table 1. Photophysical Properties of the Compounds
Measured in Deaerated Dichloromethane
a
b
c
Figure 3. Intramolecular hydrogen bonding (top) and π-stacking
(bottom) found in the crystal lattice of 8.
compound λabs (nm) λem (nm) Φfl (%)
ΔE (eV)
Eg spectro (eV)
2
353
382
440
445
382
406
408
427
493
412
500
497
498
468
500
497
459
594
5
7
3.2
2.8
2.6
2.7
3.0
2.8
2.8
2.5
2.2
3.1
2.7
2.5
2.6
2.7
2.6
2.6
2.2
2.1
4
68.6(1)° from each other. The molecules also organize
themselves on the same plane via cyano−hydrogen interactions.
The cyano groups also hydrogen bond at 2.575(1) Å with the
adjacent thiophene.
The molecular organization in the lattice is also driven by
various intermolecular π-stacking interactions. One occurs
between the thiophene’s centroid and the vinyl bond and
another takes place between the remaining thiophene’s centroid
and the azomethine bond of the dimethylformamide protecting
group. These interactions occur respectively at distances of
3.637(1) and 3.340(1) Å and they are within the acceptable
range for π−π stacking.29 Interestingly, a similar packing
8
20
10
15
17
19
21
15
≈0
≈0
≈0
≈0
≈ 0
e
23e
a
Absolute quantum yield measured with an integrating sphere.
b
Absolute energy gap taken from the intercept of the normalized
c
absorbance and fluorescence spectra. Spectroscopically derived
energy-gap. From the literature.25 From the literature.12
d
e
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result of increasing the degree of conjugation from 2 (353 nm)
to 10 (445 nm). A similar red shift of 45 nm is also observed
between 15 and 21 as a result of the increased degree of
conjugation. However, the observed red shift cannot be
assigned uniquely to the increase in conjugation, given the
different electron withdrawing groups of 15 and 21. The
stronger electron withdrawing character of the cyano group
affects the optoelectronic properties (vide supra). The
additional thiophene in 15−21 is also in part responsible of
the observed red shift. Meanwhile, the 66 nm bathochromic
shift of 23 relative to that of 21 is owing to the electron
donating effect of the terminal amines. These two compounds
can be directly compared because the orientation of the
azomethine does not effect the HOMO and LUMO levels (see
Figure 9 and Table 3). The effect of the terminal amines is
further seen when comparing 2 with 4, whose absorbance
difference is 30 nm. The shift between 2 (353 nm) and 4 (382
nm) is less than that for 21 (427 nm) and 23 (493 nm) owing
to the different number of ester withdrawing groups. The
electronic push−pull effect resulting from the amine/
azomethine is further seen by the 58 nm bathochromic shift
of 8 relative to 4.27 This effect is highly noticeable from the
absorbance maximum that is red-shifted by 90 nm relative to
that of 2. Of particular interest is the effect of the heteroatom in
the azomethine on the spectroscopic properties. Although the
unique effect of the azomethine on the spectral properties
cannot unequivocally be assessed relative to its all-carbon
counterpart, 2 and 15 are the best representative models for
such a comparison. The weak donating effect of the 2,2′-methyl
groups of 2 accounts for a red shift of 2 nm, according to the
spectral shifts observed for 17 (406 nm) and 19 (408 nm). The
azomethine therefore gives rise to a 27 nm bathochromic shift.
Although large spectral differences were not observed between
the all-carbon bond and azomethine counterparts, the
heteroatomic bond nonetheless results in red-shifted spectra.
Enhanced spectral shifts are possible by taking advantage of the
electronic effect of the azomethine by conjugating it with
electron rich and/or electron donating groups. The resulting
electronic push−pull systems absorb in the visible such as 21
and 23.11,27
The same trend in absorbance shifts as a function of structure
was also observed for the fluorescence of the compounds
examined. The exceptions to the trend were 4 (382 nm) and 21
(427 nm) that were red and blue shifted, respectively, from
what was expected. Although the exact reason for these
exceptions is not known, there was a significant difference in
the fluorescence quantum yields (Φfl) for the compounds. As
seen in Table 1, the azomethines essentially do not fluoresce.
To some extent, this is not surprising because the heteroatomic
bond is known to rapidly deactivate the singlet excited state by
photoinduced electron transfer.31 The excited state is addition-
ally known to be efficiently deactivated by internal conversion
involving bond rotation.32 In the case of thiophene azomethine
derivatives, intersystem crossing to the triplet state is also an
efficient deactivation mode.33 The collective deactivation
modes ensure the quenched fluorescence of azomethines such
that the Φfl is below what can be accurately measured by an
integrating sphere (<2%).
cross (vide supra).34−36 The triplet formed by this process can
be spectroscopically detected by laser flash photolysis. This
technique was subsequently used to determine the presence of
triplets. No triplet transient was detected either for 2 or 4. The
intense laser pulse at 355 nm led exclusively to a photoproduct
having a much longer lifetime than the resolution of the
instrument (>100 μs). The long lifetime precludes a triplet
transient given they typically have unimolecular lifetimes <100
μs.37 To further identify the photoproduct, 2 was irradiated at
350 nm under steady-state conditions. As seen in Figure 5, the
Figure 5. Change in absorbance spectra of 2 when irradiated at 350
nm between 0 (black) and 170 min (orange) in deaerated and
deuterated chloroform.
original absorbance spectra 2 (λmax = 350 nm) are converted
into a spectrum whose absorbance is shifted to 295 nm. The
blue shift in the photoproduct absorbance suggests it has a
decreased degree of conjugation relative to 2. Meanwhile, the
isosbestic point at 310 nm confirms the presence of only two
species that are interdependent. The photoproduct was
1
subsequently analyzed by H NMR and it was found to be
the cis isomer. The low Φfl observed for 2 therefore is from
trans → cis photoisomerization and not singlet excited state
quenching by intersystem crossing. The vinylene compounds
are therefore photounstable. This is in contrast to their
azomethine counterparts that are photostable and do not
photoisomerize even at prolonged irradiated times.38
Electrochemical Properties. The electrochemical proper-
ties of the compounds were also examined by cyclic
voltammetry to determine the oxidation and reduction
potentials of the compounds. This was done to accurately
assess the structure−property relationships and correlate the
electrochemical properties with the spectroscopic properties.
The reduction process was irreversible for all the compounds
investigated. In contrast, the oxidation process was reversible
for all compounds except for 15. Electrochemical reversibility
was quantitatively confirmed from the equal peak current of the
forward and reverse scans.39 The oxidation process was found
to be a one-electron, according to known means.40 Meanwhile,
the formal oxidation potential (Eo) was calculated from the
forward and reverse oxidation potentials.
In contrast to the azomethines, the vinylene derivatives
fluoresced in appreciable amounts (5−20%), and these could
be measured accurately with an integrating sphere. The
measured values were well below unity, which is not surprising
because oligothiophenes are known to efficiently intersystem
Cyclic voltammetric measurements were done at various scan
rates to further confirm quantitatively the oxidation reversi-
bility, type of process (reversible, quasi-reversible, and pseudo-
reversible), and number of electrons transferred in the
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oxidation. The number of electrons (n) involved in the
oxidation was quantified by comparing the peak currents of
ferrocene to the given sample for the forward scans, according
to the Randles−Sevcik equation:41
unequivocaly assigned, the LUMO energy levels cannot be
accurately evaluated electrochemically. Nonetheless, the
collective electrochemical data for the compounds from Chart
1 are summarized in Table 2 and representative anodic cyclic
voltammograms are found in Figure 7.
i = (2.69 × 105)n3/2AD1/2Cv1/2
where i is the anodic peak current, n the number of electrons
transferred, A the working electrode area, D the diffusion
coefficient, C the concentration, and v the scan rate. The
equation simplifies to
Table 2. Electrochemical Properties of Compounds from
Chart 1 Measured in Deaerated Dichloromethane
a
b
compound Eforward (E0) (V) HOMO (eV) LUMO (eV) Eg electro (eV)
2
1.09 (1.02)
0.54 (0.51)
0.65 (0.63)
0.82 (0.80)
1.65 (−)
0.88 (0.85)
0.89 (0.86)
0.86 (0.83)
0.87 (0.82)
−5.4
−4.8
−5.0
−5.1
−6.0
−5.2
−5.2
−5.2
−5.2
−3.2
−3.0
−2.9
−3.4
−2.9
−2.9
−2.9
−3.0
−3.1
2.2
1.8
2.1
1.7
3.1
2.3
2.3
2.2
2.1
i = n3/2 1/2
× constant
v
4
8
when equimolar ferrocene is used as the internal reference. The
desired n can be determined by plotting the anodic peak
current as a function of scan rate (Figure 6). Because the
10
15
17
19
21
23
c
d
a
b
Measured against Ag/Ag+. Eforward refers to the oxidation potential
for the forward scan. Value in parentheses refers to formal potential
c
determined from the reversible oxidation couple. From the
d
literature.25 From the literature.12
■
Figure 6. Current as a function of scan rate for 10 ( ) and ferrocene
●
( ). Inset: anodic cyclic voltammograms of 10 measured at 10
(black), 25 (red), 50 (green), 75 (blue), 100 (light blue), 200 (pink),
300 (yellow), 400 (tan), 500 (gray), 600 (purple), 700 (brown), 800
(dark green), 900 (teal), and 100 (dark blue) mV s−1 with 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) in anhydrous
and deaerated dichloromethane.
oxidation of ferrocene is a one-electron process, the ratio of the
slopes (Figure 6) gives the desired n.42 According to this
means, all the compounds, except 15, were found to reversibly
undergo a one-electron oxidation, resulting in the radical cation.
The reversible oxidation observed confirms that the resulting
radical cation is stable and that it does not couple according to
standard anodic means.43
Figure 7. Anodic cyclic voltammograms of 2 (black), 4 (red), 8 (blue),
10 (green), 15 (purple), 17 (orange), 19 (gray), 21 (brown), and 23
(pink) vs Ag/Ag+ with 0.1 M TBAPF6 in anhydrous and deaerated
dichloromethane with ferrocene as an internal reference. Ferrocene
was not added to 4 and 8 because of overlapping oxidation potentials.
The forward scan (Eforward) oxidation potentials for the
various compounds were used to examine the effect of structure
and electronic effects on the potentials. The typically used Epa
could not be applied for comparing the potentials of the
different compounds because of the irreversible oxidation of 15.
Eforward is independent of the oxidation reversibility making for
accurate assessment of the oxidation potential contingent on
structure. The oxidation onset was additionally used to
calculate the HOMO energy level. This was done according
to the commonly accepted approximation: HOMO = −e-
(Epaonset + 4.72), where the potentials are measured against Ag/
Ag+,44−47 and by taking the E′ of the internal reference
It is evident from the electrochemical data that the vinyl
derivatives have lower oxidation potentials than their
azomethine analogues. This is obvious upon comparing 2
whose oxidation potential is 560 mV less positive than that of
15. Similarly, the oxidation potentials of 17 and 19 are 360 mV
more positive than the analogous 4. The more positive
potentials observed for the azomethines relative to the all-
carbon counterparts can be ascribed to the electron with-
drawing effect of the heteroatomic bond. Interestingly, 4 (540
mV) had the lowest oxidation potential of the compounds
examined, owing to the electron donating amines. The
measured value of 4 was more positive than the reduction
potential of oxygen, confirming its observed stability under
ambient conditions. The effect of degree of conjugation and
electronic effects on the oxidation potential are consistent with
the spectroscopic data. For example, Epa of 10 is 20 mV lower
ferrocene as 0.35 V vs Ag/AgCl.48 Similarly, the LUMO energy
onset
level was calculated according to LUMO = −e(Epc
+ 4.72).
The approximations are valid if the redox processes form radical
ions. Given the reduction process of the compounds cannot be
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than that of 2, owing to the increased degree of conjugation.
Meanwhile, the Epa of 4 is 10 mV lower than that of 8 as a result
of the amine electron donating effect as opposed to the
withdrawing azomethine character. While 10 (820 mV), 17
(880 mV), 21 (860 mV), and 23 (870 mV) are structurally
different, they have similar Epa. This is owing to the collective
electronic effects and degree of conjugation.
for the calculated LUMO energy levels being less negative than
the experimentally measured values. However, it was shown
that the B3LYP basis set is adequate to calculate the energy
levels of azomethine derivatives.51 The similar trend of HOMO
and LUMO energy levels contingent on structure for both
experimental and calculated values confirms that the chosen
theoretical method is suitable for predicting the properties of
the compounds from Chart 1 that otherwise could not be
prepared. The effect of the azomethine bond with the electron
withdrawing groups (2 versus 3, 4 versus 5, etc.), orientation of
the heteroatomic bond (11 versus 12), and symmetry of the
azomethines (11 versus 13) can also be accurately evaluated
empirically.
The HOMO and LUMO energy levels of the compounds
that could not be synthesized were subsequently calculated.
The energy gap was additionally calculated from the difference
between the HOMO and LUMO energy levels. The maximum
absorbance was also calculated using time-dependent density
functional theory of the first excited state. The properties were
both analyzed in a vacuum and dichloromethane to assign
solvent effects. This is particularly important because the
energy levels, and hence the absorbance spectra of amine
containing compounds such as 16−19, 22, and 23 are highly
dependent on solvent polarity.52,53 The combined calculated
values are found in Table 3. The effect of the azomethine on
the properties is evident from the collective theoretical
calculations. For example, comparing 2 with its counterpart 3,
the HOMO and LUMO energy levels in both a vacuum and
dichloromethane are lower for the azomethine. This leads to a
bathochromic shift in the theoretical absorbance maximum.
This is consistent with the experimental data. The HOMO and
LUMO levels are further lower for 5 versus 4, 15 versus 14, 17
versus 16, 19 versus 18, 11, 12, and 13 versus 10, 21 versus 20,
and 23 vs 22. The HOMO energy level of 8 (−5.0 eV) is
additionally lower than that of 9 (−4.8 eV). However, it has a
0.3 eV higher LUMO energy level owing to the electron
withdrawing character of the azomethine. The contribution of
the heteroatomic bond in 9 causes a blue shift (417 nm) in the
absorbance spectra relative to its all-carbon counterpart (426
nm).
Theoretical Calculations. Given the synthetic challenges
in isolating aminothiophene vinylene derivatives such as 11 and
13 for analyzing their spectroscopic and electrochemical
properties, the desired properties were empirically calculated.
This method was chosen to accurately evaluate the effect of the
azomethine on the optoelectronic properties without the
electron withdrawing ester/nitrile group, which is otherwise
required for synthetically preparing the compounds. The
properties were calculated using DFT and 6-31G* basis sets.
The geometries were first optimized semi-empirically using
Austin model 1 (AM1) followed by single point energy
calculations with DFT. This approach was selected because of
the shorter AM1 geometry optimization computation times
without sacrificing accuracy. The latter was confirmed by
comparing the bond lengths and energies of optimized
geometries for a model 8 calculated semi-empirically and by
DFT. Both methods gave similar results. Moreover, the
calculated bond angles and lengths for the AM1 calculations
correlated better with X-ray diffraction data. The properties of
the synthesized compounds from Chart 1 were first calculated.
These were used as a benchmark to verify the empirically
calculated values against the experimentally measured proper-
ties. Although absolute values cannot be precisely calculated
empirically, relative values, however, can be accurately
measured within a given series. Figure 8 shows a comparison
of the experimental and theoretical data. The calculated and
measured HOMO energy levels are similar. This is not
surprising because the B3LYP 6-31G* basis set is known to
accurately estimate the HOMO energy levels of conjugated
materials.49,50 This is in contrast to the LUMO energy values
that cannot be as accurately calculated. This, in part, accounts
The effect of the withdrawing and donating groups on the
energy levels can be seen by comparing the calculated values of
4, 5, 6, and 7. The electron withdrawing azomethine bond
decreases the HOMO and LUMO energy values for 5 versus 4
and 7 versus 6. This gives rise to a bathochromic shift for all the
absorbance of the azomethine derivatives compared to their all-
carbon counterparts. The electron withdrawing ester also
stabilizes the HOMO and LUMO levels, resulting in a small
bathochromic shift in the absorbance maximum. This also was
observed when we compare 20 to 10. The effect of the terminal
amine on the properties can also be seen when we compare 2
versus 6 and 3 versus 7. The electron donating group increases
the HOMO and LUMO energy levels for both series. This
leads to a bathochromic shift in the absorbance for the amine-
terminated compounds, owing to the electronic push−push
effect of the conjugated amine−azomethine groups.
The effect of the azomethine orientation can also be assessed
by examining the calculated energy levels of 11, 12, and 13.
The data shows that the energy difference between the HOMO
and LUMO energy values is consistent, regardless of the
azomethine orientation. The orientation of the dipole moment
created by the azomethine perturbs the HOMO energy levels
by 0.2 eV. Similarly, the LUMO energy levels are perturbed by
Figure 8. Comparison of experimental (open symbols) and calculated
●
▲
(filled symbols) HOMO ( ) and LUMO ( ) in dichloromethane.
The HOMO energy levels for 8, 17, and 23 were not included because
the calculated and experimental values are identical.
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Table 3. HOMO and LUMO Energy Levels, Energy Gaps, and Absorbance Maximum Theoretically Calculated by DFT B3LYP
6-31G* in Both a Vacuum and Dichloromethane
vacuum
LUMO (eV)
dichloromethane
LUMO (eV)
compound
HOMO (eV)
Eg (eV)
λabs (nm)
HOMO (eV)
Eg (eV)
λabs (nm)
2
−4.9
−5.2
−4.6
−4.8
−4.2
−4.4
−5.0
−4.5
−4.6
−5.1
−4.9
−5.0
−5.4
−5.8
−4.9
−5.2
−4.9
−5.1
−5.0
−5.2
−4.8
−5.1
−1.3
−1.7
−1.2
−1.5
−0.9
−1.3
−2.1
−1.5
−1.7
−2.3
−2.0
−2.1
−1.9
−2.4
−1.4
−1.8
−1.3
−1.7
−1.7
−2.0
−1.9
−2.3
3.6
3.5
3.4
3.3
3.3
3.1
2.9
3.0
2.9
2.8
2.9
2.9
3.5
3.4
3.5
3.4
3.6
3.4
3.3
3.2
2.9
2.8
331
338
355
370
352
364
426
417
415
423
416
429
344
347
352
361
343
356
396
416
434
469
−5.0
−5.3
−4.6
−4.9
−4.3
−4.6
−5.0
−4.8
−4.7
−5.2
−5.1
−5.2
−5.3
−5.8
−4.9
−5.2
−5.0
−5.1
−5.1
−5.4
−4.8
−5.2
−1.4
−1.8
−1.3
−1.7
−1.1
−1.5
−2.1
−1.8
−1.8
−2.4
−2.2
−2.3
−2.0
−2.4
−1.5
−1.8
−1.4
−1.8
−1.8
−2.3
−1.9
−2.5
3.6
3.5
3.3
3.2
3.2
3.1
2.9
3.0
2.9
2.8
2.9
2.9
3.3
3.4
3.4
3.4
3.6
3.3
3.3
3.1
2.9
2.7
332
340
364
380
359
372
426
417
416
424
414
421
359
347
356
367
350
361
400
422
443
487
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Figure 9. Calculated HOMO and LUMO energy levels for 10, 11, 12, 13, and 21 relative to vacuum.
0.3 eV. Lower energy values were found when the heteroatom
was adjacent to the external thiophene (11). The calculated
absorbance maxima were also consistent with the calculated
energy gaps. The differences in the energy level are seen in
Figure 9. This figure also clearly shows the effect of adding the
azomethine linkages on the calculated HOMO energy level.
Although the HOMO energy levels are lower, the energy gap is
consistent because both the HOMO and LUMO energy levels
are similarly perturbed.
The frontier orbitals were also investigated. Figure 10 shows
an example for 4 and its analog 5. It can be observed that, in
both cases, the HOMO frontier orbitals are evenly distributed
across the conjugated network. The same trend was also
observed for the triads. In contrast, LUMO frontier orbitals are
localized on the vinyl and azomethine bonds. This trend was
similarly observed for all the dyads (see the Supporting
Information). However, the LUMO was extended over the
central thiophene and the conjugated bonds for the triads. This
leads to intramolecular charge transfer of density over the entire
molecule. This was previously observed with conjugated
azomethine derivatives.32 The vinyl and azomethine also
show the same trend in terms of electron delocalization.
Therefore, the DFT calculation clearly supports the fact that
the azomethine and vinyl linkages are isoelectronic.
CONCLUSION
■
The photophysical and electrochemical properties of azome-
thines and their analogous all-carbon counterparts were
analyzed. It was shown that it is possible to adjust the
optoelectronic properties by incorporating different functional
groups in the molecules. The vinyl compounds showed
reversible oxidation in comparison to the azomethine,
indicating that they are more stable under harsh conditions
than their analogs. The collective spectral and electrochemical
data revealed that the heteroatom had little effect on the
properties of the conjugated dyads and triads. Only a 20 nm
spectral red shift was observed between all-carbon and
azomethine analogues. The only effect of the heteroatom was
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Figure 10. Calculated HOMO (top) and LUMO (bottom) frontier orbitals of 4 (left) and 5 (right) in dichloromethane.
for the fluorescence quantum yields. The azomethine
fluorescence was not quenched by photoisomerization between
the E → Z isomers, unlike its vinylene counterparts. The similar
properties between the isoelectronic bonds were further
confirmed by DFT studies. Taking into consideration the
photochemical stability, greater ease of synthesis, and similar
optoelectronic properties, azomethines are good alternatives to
their all-carbon counterparts. Azomethines having potentially
suitable properties for use in plastic devices, can therefore be
designed on the basis of their all-carbon counterparts that have
been successfully used as functional materials in working
devices.
Table 4. Details of the Crystal Structure Determination of 8
formula
C18H18N6S2
mol wt (g/mol); F(000)
cryst color and form
cryst size (mm)
T (K); dcalcd (g/cm3)
cryst syst
382.50 g/mol; 1600
orange platelet
0.18 × 0.06 × 0.04
150 (2); 1.363
orthorhombic
Pbca
space group
unit cell: a (Å)
16.0473 (6)
8.2489 (3)
b (Å)
c (Å)
28.1693 (11)
90.000
α (deg)
β (deg)
90.000
EXPERIMENTAL SECTION
■
γ (deg)
V (Å3); Z
90.000
General Procedures. All chemicals and reagents were obtained
from commercial sources unless otherwise stated. Anhydrous and
deaerated solvents were obtained from an activated alumina solvent
purification system. CDCl3 was repeatedly passed over a plug of
activated basic alumina to remove undesired acid contaminants.
Spectroscopy. Absorbance measurements were done on a
commercial UV−visible−NIR absorbance spectrophotometer and
the fluorescence measurements were performed on UV−visible
combined time-resolved and steady-state fluorometer after deaerating
the samples with nitrogen for 20 min. The absolute quantum yields
were measured using an integrating sphere.
3728.8 (2); 8
3.14−72.54; 0.982
49300/3680; 0.053
2.705 semiempirical
0.0541; 0.1413
0.0651; 0.1467
1.009
θ range (deg); completeness
collected/independent reflections; Rint
μ (mm−1) abs. corr.
R1(F); wR (F2) [I > 2σ(I)]
R1(F); wR (F2) (all data)
GOF(F2)
max. residual e− density
1.398 e−·Å−3
Electrochemistry. Cyclic voltammetry measurements were made
on a multichannel potentiostat. Compounds were dissolved in
deaerated dichloromethane at 10−4 M with 0.5 M NBu4PF6. A
platinum electrode was used as the working electrode with a platinum
wire as the auxiliary electrode. The reference electrode was a silver
wire. Ferrocene was added to the solution as an internal reference (Epa
= 0.435 V versus saturated calomel electrode (SCE)).54
first optimized semi-empirically using AM1. These geometries were
compared to the crystal structures of known compounds. Single point
energies were then calculated from the optimized geometries using the
DFT ab initio method with the B3LYP 6-31 G* basis set.
Synthesis. The synthesis of 10, 21, and 23 were done according to
known methods.24,25 All high-resolution mass spectrometry (HRMS)
measurements were done in the electrospray ionization (ESI) mode
with a time-of-flight (TOF) analyzer.
(E)-1,2-Bis(5-methylthiophen-2-yl)ethene (2). In a two-necked
round-bottom flask, 5-methylthiophene-2-carbaldehyde (0.50 g, 1.4
mmol) was dissolved in anhydrous THF (8 mL). Titanium(IV)
chloride (0.52 mL, 4.7 mmol) was then added dropwise at −18 °C,
and the solution was stirred for 30 min. Zinc powder (0.62 g, 9.5
mmol) was added over a period of 30 min, and the reaction mixture
was stirred for another 30 min at −18 °C. The reaction mixture was
then refluxed for 3.5 h. After, the mixture was then poured into iced
water. The organic layer was extracted with ethyl acetate. The product
Crystal Structure Determination. Suitable X-ray monocrystals of
8 were obtained by the slow evaporation of acetone (Table 4). X-ray
diffraction measurements were performed on a diffractometer using
graphite-monochromatized Cu Kα radiation with 1.541 78 Å. The
structures were solved by direct methods (SHELXS97). All non-
2
hydrogen atoms were refined based on FFobs (SHELXS97), whereas
hydrogen atoms were refined on the calculated positions with fixed
isotropic U, using riding model techniques.
Theoretical Calculations. Theoretical calculations were done
with a commercially available software package.55 The geometries were
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was recrystallized in carbon tetrachloride, and it was washed with cold
(E)-Dibutyl 5,5′-(Ethene-1,2-diyl)bis(2-aminothiophene-3-carbox-
ylate) (4). In a two-necked flask, 2d (0.20 g, 0.6 mmol) was dissolved
in anhydrous THF (14 mL). Titanium(IV) chloride (0.32 mL, 2.4
mmol) was added dropwise at −18 °C, and the reaction mixture was
stirred for 30 min. Zinc powder (0.44 g, 4.7 mmol) was added over a
period of 30 min, and the slurry was stirred at the same temperature
for another 30 min. The reaction mixture was then refluxed for 3.5 h.
Afterward, the resulting solution was poured into ice water and the
organic layer was extracted with ethyl acetate. The crude mixture was
then poured into a solution of K2CO3 (0.5 g in 20 mL methanol and 4
mL water) and it was stirred for 3 h under nitrogen. The solvent was
evaporated, and the crude product was extracted with ethyl acetate and
concentrated under a vacuum. Purification of the crude mixture over
silica gel flash chromatography (70/30, hexanes/ethyl acetate) gave 4
as a yellow powder (97 mg, 77%). 1H NMR (acetone-d6): δ = 7.17 (s,
1.6H), 6.87 (s, 1H), 6.52 (s, 1H), 4.2 (m, 2H), 1.70 (m, 2H), 1.47 (m,
2H), 0.97 (m, 3H). 13C NMR (acetone-d6): δ = 165.1, 163.3, 124.6,
123.7, 119.0, 105.7, 63.3, 31.2, 19.4, 13.5. HRMS (ESI+): calcd for
C20H27N2O4S2 [M + H+], 423.14068; found, 423.1395.
1
ethanol to afford the product as a yellow powder (453 mg, 52%). H
NMR (CDCl3): δ = 6.83 (s, 1H), 6.78 (dJ = 3.6 Hz, 1H), 6.62 (m,
1H), 2.47 (dJ = 0.8 Hz, 3H). 13C NMR (CDCl3): δ = 141.0, 139.3,
126.3, 126.2, 121.1, 16.1. HRMS (ESI+): calcd for C12H13S2 [M + H+],
221.0453; found, 221.0450.
Butyl 2-Cyanoacetate (4a). Cyanoacetic acid (10 g, 117.5 mmol)
was dissolved in n-butanol (10.9 mL, 117.5 mmol). Zinc perchlorate
hexahydrate (21.9 g, 58.7 mmol) and magnesium sulfate (14.14 g,
117.5 mmol) were then added to the solution. The resulting slurry was
stirred at 80 °C overnight. The reaction mixture was cooled to room
temperature. The resulting precipitate was filtered, and it was washed
with ethyl acetate. The organic extracts were concentrated, the residue
was taken up into dichloromethane, and the solid was filtered. The
filtrate was then washed with water and then with saturated NaHCO3.
After removing the solvent under a vacuum, we purified the crude
product by silica gel flash column chromatography using hexane/ethyl
acetate (90/10) as the eluent. The product was isolated as a colorless
oil (12.73 g, 77%). 1H NMR (acetone-d6): δ = 4.18 (tJ = 6.4 Hz, 2H),
3.79 (s, 2H), 1.67−1.60 (m, 2H), 1.45−1.35 (m, 2H), 0.94−0.90 (tJ =
7.6 Hz, 3H). 13C NMR (acetone-d6): δ = 165.7, 115.8, 67.6, 32.1, 25.8,
20.5, 20.5, 14.8. HRMS (ESI+): calcd for C7H11NO2 [M + Ag+],
247.9835; found, 247.9837.
(E)-N′-(3-Cyano-5-formylthiophen-2-yl)-N,N-dimethylformimida-
mide (8b). In an oven-dried two-necked flask, phosphorus oxychloride
(12 mL, 128 mmol) was added dropwise at 0 °C in anhydrous
dimethylformamide (40 mL, 516 mmol). The solution was stirred for
20 min, followed by the addition of 8a (4 g, 32 mmol). The reaction
mixture was stirred at room temperature for 40 min. The mixture was
then heated at 50 °C overnight, and afterward the reaction mixture was
poured into ice water and the crude product was extracted with ethyl
acetate. The organic layer was further washed with water. The organic
layers were combined, dried with magnesium sulfate, filtered, and then
Butyl 2-Aminothiophene-3-carboxylate (4b). In a two-necked
round-bottom flask, 4-dithiane-2,5-diol (0.54 g, 3.54 mmol), and 4a (1
g, 7.08 mmol) were added. Anhydrous dimethylformamide (0.5 mL)
was added followed by triethylamine (0.5 mL, 3.54 mmol) at 0 °C.
The reaction mixture was then heated to 45 °C for 1.5 h. The reaction
mixture was poured into ice water. The organic layer was extracted
with dichloromethane, and it was then dried with magnesium sulfate.
The salts were removed, and the organic layers were concentrated.
The product was purified by silica gel flash chromatography with
hexanes/ethyl acetate (70/30) to give the title compound as a yellow
oil (0.41 g, 58%). 1H NMR (acetone-d6): δ = 6.93−6.91 (dJ = 6.4 Hz,
1H), 6.87 (s, 1.4H), 4.21−4.18 (tJ = 6.4 Hz, 2H), 1.70−1.63 (m, 2H),
1.47−1.38 (m, 2H), 0.96−0.92 (tJ = 7.6 Hz, 3H).13C NMR (acetone-
d6): δ = 166.7, 165.6, 127.2, 107.9, 107.3, 64.7, 32.6, 20.9, 15.0. HRMS
(ESI+): calcd for C9H13NO2S [M + Ag+], 305.9712; found, 305.9699.
Butyl 2-(((Trifluoromethoxy)carbonyl)amino)thiophene-3-car-
boxylate (4c). In an oven-dried round-bottom flask, 4b (1.15 g, 5.81
mmol) was dissolved in dichloromethane (5 mL). Trifluoroacetic
anhydride (TFAA; 0.96 mL, 6.97 mmol) was added at 0 °C, followed
by the dropwise addition of triethylamine (1 mL). The reaction
mixture was stirred at room temperature overnight. It was then diluted
in dichloromethane and washed with brine and water. The organic
layer was dried over sodium sulfate, filtered, concentrated, and then
purified by silica gel flash chromatography with hexanes/ethyl acetate
(90/10). The title compound was isolated as a white solid (895 mg,
1
concentrated to give a yellow solid (3.5 g, 53%). H NMR (acetone-
d6): δ = 9.78 (s, 1H), 8.20 (s, 1H), 7.94 (s, 1H), 3.32 (s, 1H), 3.20 (s,
1H). 13C NMR (acetone-d6): δ = 183.2, 175.6, 159.1, 140.6, 132.5,
116.4, 100.3, 42.2, 36.5. HRMS (ESI+): calcd for [C9H9N3OS + H] +,
208.0539; found, 208.0534.
(1E,1′E)-N′,N″-(5,5′-((E)-Ethene-1,2-diyl)bis(3-cyanothiophene-
5,2-diyl))bis(N,N-dimethylformimidamide) (8). In a two-necked
round-bottom flask 8b (0.50 g, 2.4 mmol) was dissolved in anhydrous
THF (8 mL). Titanium(IV) chloride (0.82 mL, 8.1 mmol) was added
dropwise at −18 °C, and the reaction mixture was stirred for 30 min.
Zinc powder (1.1 g, 16.3 mmol) was added over a period of 30 min,
and the slurry was stirred at the same temperature for another 30 min.
The reaction mixture was then refluxed for 3.5 h. It was then poured
into ice water and the organic layer was extracted with ethyl acetate.
The product was purified by silica gel chromatography using ethyl
acetate/hexane (20/80) to give the product as a yellow powder (0.53
1
g, 57%). H NMR (CDCl3): δ = 9.66 (s, 1H), 8.16 (s, 1H), 7.92 (s,
1H), 3.31 (s, 3H), 3.18 (s, 3H). 13C NMR (acetone-d6): δ = 165.8,
155.9, 130.4, 125.9, 120.6, 115.7, 97.5, 73.3, 72.3, 40.3. 34.6. HRMS
(ESI+): calcd for C18H19N6S2 [M + H+], 383.1107; found, 383.1090.
(E)-2-((5-Methylthiophen-2-yl)methyleneamino)thiophene-3-car-
bonitrile (15). In a round-bottom flask, 2-aminothiophene-3-carbon-
itrile (100 mg, 0.8 mmol) and 5-methylthiophene-2-carbaldehyde (67
mg, 0.53 mmol) were dissolved in anhydrous ethanol. A catalytic
amount of trifluoroacetic acid (TFA) was added and the mixture was
stirred overnight at room temperature. The resulting precipitate was
filtered and it was washed with cold ethanol to give the title compound
as a yellow powder (0.11 g, 88%). 1H NMR (acetone-d6): δ = 8.78 (s,
1H), 7.65 (dJ = 3.6 Hz, 1H), 7.40 (dJ = 5.6 Hz, 1H), 7.27 (dJ = 5.6 Hz,
1H), 6.98 (dJ = 2.8 Hz, 1H), 2.60 (s, 1H). 13C NMR (acetone-d6): δ =
165.2, 156.6, 151.1, 141.2, 138.4, 129.5, 129.3, 123.5, 116.1, 106.8,
17.0. HRMS (ESI+): calcd for C11H9SN2S [M + H+], 233.0202;
found, 233.0198.
1
50%). H NMR (acetone-d6): δ = 12.00 (s, 1H), 7.31 (dJ = 5.6 Hz,
1H), 7.19 (dJ = 5.6 Hz, 1H), 4.38−4.35 (tJ = 6.4 Hz, 2H), 1.79−1.76
(m, 2H), 1.51−1.47 (m, 2H), 1.01−0.98 (tJ = 7.6 Hz, 3H). 13C NMR
(acetone-d6): δ = 167.0, 155.0, 146.9, 126.0, 120.6, 118.1, 66.9, 32.3,
20.8, 15.0. HRMS (ESI+): calcd for [C11H12F3NO3S + H]+, 296.0569;
found, 296.0555.
Butyl 2-(((Trifluoromethoxy)carbonyl)amino)formylthiophene-3-
carboxylate (4d). In a dry two-necked flask, phosphorus oxychloride
(0.13 mL, 1.44 mmol) was added to dimethylformamide (0.11 mL,
1.44 mmol) at 0 °C. 4c (0.150 g, 0.48 mmol) diluted in
tetrahydrofuran (5 mL) was then added. The reaction mixture was
heated to 60 °C overnight, and then it was poured into ice water. The
crude product was extracted with dichloromethane, dried with
magnesium sulfate, filtered, and then concentrated. The product was
purified by silica gel flash chromatography with hexanes/ethyl acetate
(80/20) to afford the title compound as a white powder (116.6 mg,
72%). 1H NMR (acetone-d6): δ = 9.67 (s, 1H), 8.01 (s, 1H), 7.97, (s,
1H), 4.20−4.16 (tJ = 6.4 Hz, 2H), 3.24 (s, 3H), 3.13 (s, 3H), 1.72−
1.65 (m, 2H), 1.51−1.42 (m, 2H), 0.97−0.93 (tJ = 7.2 Hz, 3H). 13C
NMR (acetone-d6): δ = 183.7, 173.7, 164.3, 158.1, 142.4, 131.3, 119.8,
65.3, 41.7, 36.1, 32.6, 20.9, 15.0.
(E)-Diethyl 2-Amino-5-((5-butylthiophen-2-yl)methyleneamino)-
thiophene-3,4-dicarboxylate (17). Both 2, 5-diaminothiophene-3,4-
dicarboxylic acid (77 mg, 0.3 mmol) and 5-butylthiophene-2-
carboxaldehyde (50 mg, 0.3 mmol) were dissolved in ethanol (10
mL) before adding a catalytic amount of TFA (5 μL). The mixture was
stirred at room temperature overnight and the organic layer was
washed with water, and extracted with dichloromethane. After the
solvent was removed under reduced pressure, the product was purified
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by silica gel chromatography using dichloromethane and triethylamine
(10%) as an eluent. The product was isolated as a yellow powder (0.02
g, 20%). H NMR (acetone-d6): δ = 8.17 (s, 1H), 7.45 (s, 2H), 7.34
enedioxy thiophene)( 3,4-ethylenedioxythiophene) electron donor
cell. CN 102504208, June 20, 2012.
(8) Shin, R. Y. C.; Sonar, P.; Siew, P. S.; Chen, Z.-K.; Sellinger, A. J.
1
(d, 1H, J = 3.6 Hz), 6.88 (d, 1H, J = 3.6 Hz), 4.33 (dd, 2H, J = 7.2
Hz), 4.21 (dd, 2H, J = 7.2 Hz), 2.87 (t, 6H, J = 8.8 Hz), 1.72 (m, 2H),
1.40−1.26 (m, 4H), 0.95 (t, 3H, J = 7.6 Hz). 13C NMR (acetone-d6): δ
= 165.1, 164.4, 160.9, 151.7, 146.4, 140.8, 133.3, 132.5, 129.8, 126.0,
61.0, 59.9, 33.9, 22.3, 14.3, 14.1, 13.5. HRMS (ESI+): calcd for
C19H24N2O4S2 [M + H+], 409.1250; found 409.1249.
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̈
(CDCl3): δ = 7.93 (s, 1H), 7.03 (s, 1H), 6.25 (s, 2H), 4.39 (dd, 2H, J
= 7.2 Hz), 4.22 (dd, 2H, J = 7.2 Hz), 2.35 (s, 3H),2.10 (s, 3H), 1.42 (t,
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ASSOCIATED CONTENT
* Supporting Information
■
S
1H and 13C NMR spectra of all compounds, UV−visible and
fluorescence spectra, cyclic voltammograms, calculated HOMO
and LUMO frontier orbitals, calculated atom coordinates, and
absolute energies. A simplified chart of synthesized and
synthetically unavailable compounds is also presented. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*W. G. Skene. E-mail: w.skene@umontreal.ca.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Natural Sciences and Engineering
Research Council of Canada is acknowledged for Discovery,
Strategic Research, and Research Tools and Instrument grants.
The Canadian Foundation for Innovation is also acknowledged
for additional equipment and infrastructures. The Center for
Self-Assembled Chemical Structures is also acknowledged. A.B.
thanks NSERC, Fondation Queb
Nature et Technologies and the Universite
graduate scholarships. Amelie Guerin is acknowledged for her
́
ec
́
oise pour la Recherche en
̧ nita, A. L.; Elisei, F. J.
́
de Montreal for
́
́
́
(37) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular
Photochemistry Of Organic Molecules; University Science Books:
Sausalito, CA, 2010.
help with preliminary cyclic voltammetry measurements.
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