Optoelectronic property tailoring of conjugated heterocyclic azomethines - The effect of pyrrole, thiophene and furans

Article abstract of DOI:10.1002/poc.1894

A series of de novo symmetric heterocyclic azomethine dyads and triads consisting uniquely of furans, thiophenes, pyrroles and methyl-pyrroles were prepared. These were prepared to investigate the effect of various heterocycles and the degree of conjugati

Full text of DOI:10.1002/poc.1894

Research Article
Received: 25 February 2011,
Revised: 6 May 2011,
Accepted: 10 May 2011,
Published online in Wiley Online Library: 19 July 2011
(wileyonlinelibrary.com) DOI 10.1002/poc.1894
Optoelectronic property tailoring of
conjugated heterocyclic azomethines – the
effect of pyrrole, thiophene and furans
Stéphane Dufresnea and W. G. Skenea*
A series of de novo symmetric heterocyclic azomethine dyads and triads consisting uniquely of furans, thiophenes,
pyrroles and methyl‐pyrroles were prepared. These were prepared to investigate the effect of various heterocycles
and the degree of conjugation on the spectroscopic and electrochemical properties. The crystallographic structures
of the symmetric azomethines were also compared with their unsymmetric and heterocyclic and homoaryl
counterparts. It was found that for a similar series, bathochromic absorbance and fluorescence shifts occurred
when progressing in the order of furan < thiophene < pyrrole ≈ methyl‐pyrrole. The spectroscopic properties of the
heterocyclic azomethines were also bathochromically shifted relative to their homoaryl analogues as a result of
increased degree of conjugation and electronic effects. The former was in part confirmed by crystallographic studies
showing the heterocycles adopted co‐planar and antiparallel arrangements. Although all the compounds studied
showed weak fluorescence at room temperature, their fluorescence could be restored at low temperatures, implying
deactivation of the singlet excited manifold by bond rotation. Meanwhile, irreversible oxidation was observed for
all the azomethines studied and their oxidation potentials were contingent on the heterocycle and number of
azomethines, ranging between 0.8 and 1.4 V versus Ag+. The irreversible oxidation was due to radical cation
cross‐coupling resulting in products with higher degrees of conjugation. Copyright © 2011 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: fluorescence; imines; physical properties; synthesis
of particular importance given that azomethines are generally
accepted to be hydrolytically, oxidatively and reductively
INTRODUCTION
unstable in addition to being acid sensitive, precluding their
use as functional materials in working devices.[15–17] Although
some property improvements are possible with homoaryl
azomethines,[11,18] their optoelectronic properties are not
comparable to their vinylene analogues. This is a result of
inherent twisting around the aryl–azomethine bonds that limits
their degree of conjugation.[10,19–27] Enhanced azomethine
optoelectronic properties comparable to their vinylene analogues
are however possible with thiophene derivatives.[6,7,28,29] This is in
part a result of coplanarization of the thiophene–azomethine moie-
ties. This leads to higher degrees of conjugation relative to the
homoaryl azomethines, resulting in delocalization of the heterocy-
clic electronic effects across the entire conjugated framework.
Although enhanced properties are possible with azomethines
prepared exclusively with thiophenes, azomethines derived
from other heterocycles are unknown and their optoelectronic
properties remain unexplored.[30] In fact, to the best of our
knowledge, there are no previous reports examining the
Conjugated materials have found uses in organic electronic
devices including light emitting diodes, photovoltaics and field
effect transistors.[1–3] This is partly because of their optoelectronic
properties that are compatible with such devices. Although
these materials are prepared from aryl–aryl coupling protocols,
property enhancements resulting in increased device efficiencies
are possible by incorporating vinylene linkages.[4,5] These unsat-
urated connecting units are understood to prevent rotation
around the aryl–aryl bond leading to increased planarity. Despite
the enhanced optoelectronic properties that are possible with
vinylene derivatives, their preparation requires stringent reaction
conditions.[6–8] Their preparation additionally produces substan-
tial amounts of unwanted by‐products relative to the desired
products, requiring extensive purification for achieving optimal
properties and device performance.
Azomethines (−N = CH–) are interesting alternatives to vinylene
linkages in part owing to their straightforward preparation
that does not require stringent reaction conditions.[9] Moreover,
they are isoelectronic to their carbon counterparts.[10] Given
the synthetic advantages of azomethines and their expected
similar properties to their vinyl cousins, much work is still
required for assessing their suitability for replacing functional
materials currently used in organic electronic devices. Additional
azomethine examples are further required to demonstrate that
functional materials‐like properties are possible with these
heteroatomic conjugated systems, in contrast to the limited proper-
ties exhibited by previously investigated compounds.[11–14] This is
* Correspondence to: W. G. Skene, Laboratoire de caractérisation photophysique
des matériaux conjugués, Département de Chimie, Université de Montréal, CP
6128, Centre‐ville, Montreal, QC, Canada.
E‐mail: w.skene@umontreal.ca
a S. Dufresne, W. G. Skene
Laboratoire de Caractérisation Photophysique des Matériaux Conjugués,
Département de Chimie, Université de Montréal, CP 6128, Centre‐ville,
Montreal, QC, Canada
J. Phys. Org. Chem. 2012, 25 211–221
Copyright © 2011 John Wiley & Sons, Ltd.

S. DUFRESNE AND W. G. SKENE
preparation and structure/property studies of azomethines
consisting uniquely of heterocycles. We therefore were interested
in investigating the effect of different heterocycles and symmetric
and unsymmetric azomethines consisting entirely of heterocycles
on the photophysical and electrochemical properties. This is of
particular interest for gaining an important understanding of
azomethine structure/property relationships and for the design
and preparation of new functional materials with enhanced
properties for potential use in devices. To this means, dyad and
triad model compounds (Chart 1) consisting of one and two
azomethines, respectively, were prepared by condensing
furans, pyrroles and thiophenes monoaldehydes with amino‐
thiophenes (1 and 2). De novo symmetric azomethines were
prepared and their optoelectronic properties were compared
with their unsymmetric derivatives in addition to their
corresponding homoaryl counterparts for examining the
possibility of optoelectronic property tailoring. In particular, the
absorbance, fluorescence and fluorescence quantum yields
contingent on azomethine structure were examined. The
preparation, photophysical and electrochemical properties of
these conjugated heterocyclic azomethines in addition to their
crystallographic data are herein presented to illustrate the
enhanced properties possible with these conjugated heteroatom
and all‐heterocyclic compounds.
EXPERIMENTAL SECTION
Materials and general methods
All chemicals were commercially available from Aldrich (Oakville,
ON, Canada) including 3–6 and were used as received unless oth-
erwise stated. Anhydrous and deaerated solvents were obtained
with a Glass Contour (Irvine, CA, USA) solvent purification system.
NMR spectra were recorded on a Bruker (Milton, ON, Canada)
400 MHz spectrometer with the appropriate deuterated solvents.
Spectroscopic methods
The absorption measurements were done on a Varian Cary‐
500 spectrometer (Dorval, QC, Canada) while the fluorescence
studies were performed on an Edinburgh Instruments FLS‐920
fluorimeter after deaerating the samples thoroughly with nitrogen
for 20 min with NSG Precision Cells (Farmingdale, NY, USA) cu-
vettes. Fluorescence quantum yields were measured with samples
of low sample concentration (10–5 M) in dichloromethane excited
close to their maximum absorption wavelength relative to
bithiophene (ϕf = 0.013). Phosphorescence measurements were
done on a Varian Cary Eclipse (Dorval, QC, Canada) at 77 K in a
4:1 ethanol/methanol solvent. The same matrix was used for
low temperature fluorescence measurements. The absolute
energy difference (ΔE) was taken from the intercept of the
normalized absorbance and fluorescence spectra for the
corresponding compound. Meanwhile, the spectroscopic energy
gap (Egspec) was calculated from the onset of absorbance.
Electrochemical methods
Cyclic voltammetry measurements were performed on a Bio
Analytical Systems (West Lafayette, IN) EC Epsilon potentiostat.
Compounds were dissolved in anhydrous and deaerated
dichloromethane at 10–4 M with NBu4PF6 ranging between 10–2
and 10–3 M concentration to achieve conductive solutions. A
platinum electrode was used as auxiliary and working electrodes,
while a saturated Ag/AgCl (aq.) electrode was used as the refer-
ence electrode. To ensure consistent redox measurements be-
tween the various azomethines analysed, ferrocene was added
at the end of each measurement and used as an internal refer-
ence.[31] The highest occupied molecular orbital (HOMO) values
were calculated from the corresponding oxidation onset
(Epaonset) according to Epaonset + 4.4 eV. Similarly, the lowest unoc-
cupied molecular orbital (LUMO) values were calculated from
the corresponding reduction onset (Epconset) according to
Epconset + 4.4 eV.
Crystal structure determination
Diffraction data for 20 were collected on a Bruker FR591
diffractometer using graphite‐monochromatized Cu Kα radiation
with 1.54178 Å. The structures were solved by direct methods
(SHELXS97). All nonhydrogen atoms were refined based on
Fobs2 (SHELXS97), while hydrogen atoms were refined on
calculated positions with fixed isotropic U, using riding
model techniques.
Chart 1. Heterocyclic azomethines, representative homoaryl analogues
and anodically coupled products prepared and investigated
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Copyright © 2011 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2012, 25 211–221

HETEROCYCLIC EFFECT ON AZOMETHINE OPTOELECTRONIC PROPERTIES
Experimental for anodic coupled products
chromatography (SiO2) eluted with hexanes/ethyl acetate (90/10)
up to hexanes/ethyl acetate (50/50). M.p.: 77–79 °C. 1H‐NMR
(acetone‐d6): δ = 11.35 (s, NH), 8.45 (s, 1H), 7.29 (s, 1H, J = 1.8 Hz),
7.26 (d, 1H, J = 5.7 Hz), 7.19 (d, 1H, J = 5.7 Hz), 6.96 (t, 1H,
J = 1.8 Hz), 6.37 (m, 1H, J = 1.8 Hz). 13 C‐NMR (acetone‐d6):
δ = 165.4, 151.2, 130.2, 127.8, 127.0, 120.6, 120.4, 115.1, 111.6,
103.5. HRMS(+) calculated for [C10H7N3S + H]+: 202.04334 found
202.04380.
Single‐sided indium tin oxide (ITO) coated plates were purchased
from Delta Technologies (Loveland, CO, USA.) (RS = 5–10 Ω). These
were cut into rectangular shapes and were used as the working
electrode. The substrates were used after cleaning with standard
washing protocols with water, acetone and dichloromethane for
20 min followed by ultrasonication. The cross‐coupled products
were electrochemically produced by 50 repeated anodic scans
ranging from 0 to 1.4 V at 100 mV/s in dichloromethane. Alterna-
tively, the anodic coupling was done by applying an anodic po-
tential 100 mV greater than the corresponding radical cation and
held at the potential for 10 min in dichloromethane. In both cases,
the cross‐coupled products were physisorbed onto the ITO elec-
trode. The transparent electrodes were removed from the
reagent/electrolyte solution and rinsed thoroughly with dichloro-
methane to remove unreacted reagent and the supporting elec-
trolyte. The electrode was then submerged in a dichloromethane
solution of NBu4PF6 and a final potential of −100 mV was applied
for 2 min to ensure the resulting products were in the neutral
form.
Sufficient material of the anodically coupled product for stan-
dard characterization methods was obtained according to the
experimental procedures described above with the exception of
the working electrode. The button platinum working electrode
was substituted with a large area platinum mesh gauze. Several
milligrams of 19 were dissolved in anhydrous dichloromethane
(75 mL) and a potential 100 mV greater than its first Epa (i.e.,
1.2 V) was applied for 20 min under a blanket of nitrogen. The
resulting products were neutralized for 2 min by applying a
potential of −100 mV. The solvent was removed under reduced
pressure and the resulting product was dissolved in a mixture
of ethyl acetate/diethyl ether (50:50) and the residual NBu4PF6
was removed by filtration.
2‐[(Thiophen‐2‐ylmethylene)‐amino]‐thiophene‐3‐carbonitrile (9)
2 (50 mg, 0.40 mmol) and 5 (54 mg, 0.48 mmol) were mixed in
anhydrous isopropanol (20 mL) with a catalytic amount of TFA
and then refluxed for 20 h. The reaction mixture was concen-
trated and then purified by flash chromatography eluted with
hexanes/ethyl acetate (90/10 v/v) up to hexanes/ethyl acetate
(60/40 v/v) to afford the product as an orange solid (61 mg, 70
%). M.p.: 58–60 °C. 1H‐NMR (acetone‐d6): δ = 8.49 (s, 1H), 7.90
(d, 1H, J = 5.0 Hz), 7.82 (dd, 1H, 3 J = 3.7 Hz and 1.0 Hz), 7.42
(d, 1H, J = 5.7 Hz), 7.26 (d, 1H, J = 5.6 Hz), 7.26 (dd, 1H, J = 4.8 Hz
and J = 3.5 Hz). 13 C‐NMR (acetone‐d6): δ = 163.4, 155.4, 141.8,
136.2, 133.9, 129.0, 128.1, 122.5, 114.6, 105.8. HRMS(+) calculated
for [C10H6N2S2 + H]+: 219.00452, found: 219.00514.
2‐((Furan‐2‐yl)methyleneamino)thiophene‐3‐carbonitrile (10)
6 (54 mg, 0.56 mmol) and 2 (70 mg, 0.56 mmol) were refluxed in
anhydrous isopropanol (20 mL) in a 50‐mL flask where the solu-
tion turned orange after 12 h of stirring under nitrogen. The
solution was then concentrated under vacuum to near dryness.
The crude product was loaded onto a silica column and eluted
with hexanes/ethyl acetate (90/10) up to hexanes/ethyl acetate
(75/25) to give the product as a yellow–orange solid (95 mg, 83 %).
M.p.: 86–88 °C. 1H‐NMR (acetone‐d6): δ = 8.55 (s, 1H), 7.95 (d, 1H,
J = 1.5 Hz), 7.45 (d, 1H, J = 5.7 Hz), 7.35 (d, 1H, J = 3.4 Hz), 7.30
(d, 1H, J = 5.7 Hz), 6.77 (dd, 1H, 3 J = 3.5 Hz and 1.7 Hz). 13 C‐NMR
(acetone‐d6): δ = 163.8, 151.7, 149.1, 148.4, 128.2, 122.6, 120.5,
114.6, 113.5, 106.0. HRMS(+) calculated for [C10H6N2OS + Na]+:
225.00930 found 225.00885.
Synthesis
The preparation of 1–2, 13–14, 17 and 19–21 were done
according to previously reported methods.[9,29,30]
Diethyl 2‐((1‐methyl‐1H‐pyrrol‐2‐yl)methyleneamino)‐5‐aminothiophene‐
3,4‐dicarboxylate (11)
2‐((1‐Methyl‐1H‐pyrrol‐2‐yl)methyleneamino)thiophene‐3‐carbonitrile (7)
3 (53 mg, 0.48 mmol) and 2 (60 mg, 0.48 mmol) were mixed in
anhydrous isopropanol (20 mL) with a catalytic amount of
trifluoroacetic acid (TFA) and then refluxed for 12 h. The reaction
mixture was concentrated in vacuo and the crude mixture was
purified by flash chromatography eluted with hexanes/ethyl
acetate (90/10) up to hexanes/ethyl acetate (60/40) to afford
the product as a yellow solid (30 mg, 60%). M.p.: 116–118 °C.
1H‐NMR (acetone‐d6): δ = 8.47 (s, 1H), 7.28 (d, 1H, 3 J = 5.7 Hz),
7.21 (d, 1H, 3 J = 5.7 Hz), 7.19 (dd, 1H, 3 J = 2.0 Hz and 5 J = 1.8 Hz),
6.95 (dd, 1H, 3 J = 4.0 Hz and 5 J = 1.7), 6.27 (dd, 1H, 3 J = 4.0 Hz
and 3 J = 2.1 Hz), 4.12 (s, 3 Hz). 13 C‐NMR (acetone‐d6): δ= 165.7,
151.6, 133.1, 129.5, 127.5, 122.9, 120.5, 115.1, 110.2, 104.0, 37.0.
HRMS(+)calculated for [C11H9N3S + H]+: 216.05955 found 216.05889.
3 (32 mg, 0.29 mmol) and 1 (75 mg, 0.29 mmol) were mixed in
anhydrous isopropanol (20 mL) with a catalytic amount of TFA
and then refluxed for 12 h. The reaction mixture was concen-
trated and then purified by flash chromatography eluted with
hexanes/ethyl acetate (50/50) to afford the product as a yellow
solid (81 mg, 80%). M.p.: 130–132 °C. 1H‐NMR (acetone‐d6):
δ = 7.94 (s, 1H), 7.35 (s, NH2), 6.96 (dd, 1H, 3 J = 4.1 Hz and
5 J = 2.0 Hz), 6.64 (dd, 1H, 3 J = 3.9 Hz and 5 J = 1.8 Hz), 6.14
(dd, 1H, 3 J = 3.8 Hz and 2.5 Hz), 4.29 (q, 2H, J = 7.1 Hz), 4.21
(q, 2H, J = 7.1 Hz), 3.95 (s, 3H), 1.34 (t, 3H, J = 7.1 Hz), 1.28 (t, 3H,
J = 7.1 Hz). 13 C‐NMR (acetone‐d6): δ = 165.5, 164.4, 160.1, 144.3,
134.6, 130.2, 130.0, 128.0, 119.0, 109.1, 101.7, 61.0, 59.8, 36.7, 14.1,
14.1. HRMS(+) calculated for [C16H19N3O4S + H]+: 350.11690 found
350.11781.
2‐((1H‐Pyrrol‐2‐yl)methyleneamino)thiophene‐3‐carbonitrile (8)
Diethyl 2‐((1H‐pyrrol‐2‐yl)methyleneamino)‐5‐aminothiophene‐3,
4‐dicarboxylate (12)
In a 50‐mL round bottom flask, 2 (60 mg, 0.48 mmol) was dis-
solved in isopropanol (20 mL). To this was added, with vigorous
stirring, 4 (46 mg, 0.48 mmol) and a catalytic amount of TFA
followed by refluxing for 12 h. The title compound was
isolated as a yellow powder (68%) after purification by flash
4 (28 mg, 0.29 mmol) and 1 (75 mg, 0.29 mmol) were mixed
in anhydrous isopropanol (20 mL) with a catalytic amount of
TFA and followed by refluxing for 12 h. The reaction was
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S. DUFRESNE AND W. G. SKENE
concentrated and then purified by flash chromatography
eluted with hexanes/ether (50/50) up to diethyl ether (100%) to
afford the product as an orange powder (74mg, 76%). M.p.:
N1,N4‐Bis((1‐methyl‐1H‐pyrrol‐2‐yl)methylene)benzene‐1,4‐diamine
(22)
In a 50‐mL round bottom flask, 1,4‐phenylenediamine (70 mg,
0.65 mmol) was dissolved in isopropanol (20 mL). To this was
added 3 (139 μL, 1.36 mmol) and a catalytic amount of TFA with
vigorous stirring followed by refluxing for 12 h. The title com-
pound was isolated as a yellow solid (120 mg, 64%) by precipita-
tion in ethyl acetate/hexanes (5/95). M.p.: 127–129 °C. 1H‐NMR
(acetone‐d6): δ = 8.46 (s, 2H), 7.24 (s, 4H), 6.98 (t, 2H, J = 1.9 Hz),
6.73 (dd, 2H, 3 J = 3.8 Hz and 5 J =1.7 Hz), 6.18 (dd, 2H, J = 3.8 Hz
and 2.6 Hz). 13 C‐NMR (acetone‐d6): δ = 150.4, 150.3, 130.9,
129.6, 121.9, 118.6, 108.9, 36.4. HRMS(+) calculated for
[C18H18N4 + H]+: 291.16042 found 291.16030.
1
161–163 °C. H‐NMR (acetone‐d6): δ = 10.68 (s, NH), 7.92 (s, 1H),
7.34 (s, NH2), 7.04 (dd, 1H, 3 J = 3.6 and 5 J = 2.4 Hz), 6.65 (dd, 1H,
3 J = 3.6 and 5 J = 2.2 Hz), 6.23 (dd, 1H, 3 J = 3.6 and 2.4 Hz).
13 C‐NMR (acetone‐d6): δ = 165.3, 164.4, 160.0, 143.8, 134.5,
130.7, 127.6, 124.1, 116.3, 110.5, 101.8, 60.9, 59.8, 14.1, 14.0.
HRMS(+) calculated for [C15H17N3O4S + H]+: 336.10125 found
336.10208.
Diethyl 2,5‐bis((1‐methyl‐1H‐pyrrol‐2‐yl)methyleneamino)thiophene‐
3,4‐dicarboxylate (15)
3 (200 mg, 1.83 mmol) and DABCO (200 mg, 1.83 mmol) were dis-
solved in anhydrous toluene (20 mL) at 0 °C followed by the
slow addition of TiCl4 (1.0 M, 1.83 mL, 1.83 mmol). 1 (90 mg,
0.35 mmol) was subsequently added and the reaction mixture
was refluxed for 3–4 h. The solvent was evaporated and the
product isolated as a red solid after purification by flash chroma-
tography with 1% triethylamine in hexanes up to hexanes/
ethyl acetate (50/50) (57 mg, 37%). M.p.: 134–136 °C. 1H‐NMR
(acetone‐d6): δ = 8.33 (s, 2H), 7.11 (d, 2H, J = 2.0 Hz), 6.85
(dd, 2H, 3 J = 4.0 and 5 J = 1.8 Hz), 6.24 (dd, 2H, J = 4.0 Hz and
2.5 Hz), 4.29 (q, 4H, J = 7.1 Hz), 4.07 (s, 6H), 1.33 (t, 6H,
J = 7.1 Hz). 13 C‐NMR (acetone‐d6): δ = 163.5, 149.5, 149.3, 132.2,
129.9, 126.1, 121.8, 109.9, 60.9, 36.9, 14.1. HRMS(+) calculated
for [C22H24N4O4S + H]+: 441.15910 found 441.15811.
N1,N4‐Bis((1H‐pyrrol‐2‐yl)methylene)benzene‐1,4‐diamine (23)
In a 50‐mL round bottom flask, 1,4‐phenylenediamine (70 mg,
0.65 mmol) was dissolved in isopropanol (20 mL). To this was
added 4 (129 mg, 1.36 mmol) with vigorous stirring along with
a catalytic amount of TFA followed by refluxing for 12 h. The title
compound was isolated as a yellow powder (147 mg, 86%) by
precipitation in ethyl acetate/hexanes (5/95). M.p.: 205–207 °C.
1H‐NMR (acetone‐d6): δ = 10.93 (s, NH), 8.40 (s, 2H), 7.23 (s, 4H),
7.10 (s, 2H), 6.71 (dd, 2H, 3 J = 3.5 Hz and 5 J =1.3 Hz), 6.27 (dd,
2H, J = 3.4 Hz and 2.7 Hz). 13 C‐NMR (acetone‐d6): δ = 150.0,
149.2, 131.7, 123.5, 122.0, 116.3, 110.1. HRMS(+) calculated for
[C16H14N4 + H]+: 263.12912 found 263.12930.
N1,N4‐Bis((thiophen‐2‐yl)methylene)benzene‐1,4‐diamine (24)
Diethyl 2,5‐bis((1H‐pyrrol‐2‐yl)methyleneamino)thiophene‐3,
4‐dicarboxylate (16)
In a 50‐mL round bottom flask, 1,4‐phenylenediamine (70 mg,
0.65 mmol) was dissolved in isopropanol (20 mL). To this was
added 5 (124 μL, 1.36 mmol) with vigorous stirring and a cata-
lytic amount of TFA followed by refluxing for 12 h. The title
compound was isolated as a yellow solid (163 mg, 85%) by
precipitation in ethyl acetate/hexanes (5/95). M.p.: 171–173 °C.
1H‐NMR (acetone‐d6): δ = 8.84 (s, 2H), 7.74 (d, 2H, J = 5.0 Hz ),
7.67 (d, 2H, J = 3.5 Hz), 7.35 (s, 4H), 7.23 (dd, 2H, J = 4.5 Hz and
3.8 Hz). 13 C‐NMR (acetone‐d6): δ = 153.0, 149.8, 143.7, 133.2,
130.8, 128.3, 122.4. HRMS(+) calculated for [C16H12N2S2 + H]+:
297.05147 found 297.05132.
4 (173 mg, 1.74 mmol) and DABCO (195 mg, 1.74 mmol) were
dissolved in anhydrous toluene (20 mL) at 0 °C followed by the
slow addition of TiCl4 (1.0 M, 1.74 mL, 1.74 mmol). 1 (90 mg,
0.35 mmol) was then added and the mixture was refluxed for
30 min. The title compound was isolated as a red solid (64 mg,
44%) after flash chromatography with hexanes/ether (34/66)
with 1% triethylamine and up to diethyl ether (100%). M.p.:
1
185–187 °C. H‐NMR (acetone‐d6): δ = 10.98 (s, NH), 8.31 (s, 2H),
7.20 (d, 2H, J = 0.4 Hz), 6.87 (d, 2H, J = 1.2 and 2.0 Hz), 6.33
(d, 2H, J = 2.2 Hz), 4.26 (q, 4H, J = 7.1 Hz), 1.30 (t, 6H, J = 7.1 Hz).
13 C‐NMR (acetone‐d6): δ = 163.4, 149.7, 149.4, 130.6, 126.0,
125.2, 119.0, 111.2, 60.9, 14.0. HRMS(+) calculated for
[C20H20N4O4S + H]+: 413.12780 found 413.12778.
N1,N4‐Bis((furan‐2‐yl)methylene)benzene‐1,4‐diamine (25)
In a 50‐mL round bottom flask, 1,4‐phenylenediamine (70 mg,
0.65 mmol) was dissolved in isopropanol (20 mL). To this, was
added 6 (113 μL, 1.36 mmol) with vigorous stirring and a cata-
lytic amount of TFA followed by refluxing for 12 h. The title com-
pound was isolated as a yellow powder (115 mg, 67%) by
precipitation in ethyl acetate/hexanes (5/95). M.p.: 144–146 °C.
1H‐NMR (acetone‐d6): δ = 8.49 (s, 2H), 7.83 (d, 2H, J = 1.4 Hz),
7.34 (s, 2H), 7.12 (d, 2H, J = 3.4 Hz), 6.69 (dd, 2H, J = 3.4 Hz and
1.8 Hz). 13 C‐NMR (acetone‐d6): δ = 153.8, 150.7, 148.2, 146.8,
122.8, 116.5, 113.1. HRMS(+) calculated for [C16H12N2O2 + H]+:
265.09715 found 265.09747.
Diethyl 2,5‐bis((furan‐2‐yl)methyleneamino)thiophene‐3,4‐
dicarboxylate (18)
In a 50‐mL round bottom flask was added 6 (112 mg, 1.16 mmol)
dissolved in anhydrous toluene (25 mL) to which was subse-
quently added DABCO (434 mg, 3.87 mmol) and TiCl4 (1.0 M,
1.55 mL, 1.55 mmol) at 0 °C. 1 (100 mg, 0.39 mmol) was then
added and the mixture was refluxed for 2 h after which the
solvent was then evaporated. Purification by flash chromatog-
raphy (SiO2) with hexanes and increased up to hexanes/ethyl
acetate (50/50) yielded the title product as a red solid (30 mg,
19%). M.p.: 145–147 °C. 1H‐NMR (acetone‐d6): δ = 8.40 (s, 2H),
7.91 (d, 2H, J = 1.2 Hz), 7.26 (d, 2H, J = 3.5 Hz), 6.74 (dd, 2H,
J = 3.5 Hz and 1.8 Hz), 4.31 (q, 4H, J = 7.1 Hz), 1.34 (t, 6H,
J = 7.1 Hz). 13 C‐NMR (acetone‐d6): δ = 162.9, 152.2, 149.8, 147.9,
147.8, 127.5, 119.0, 113.4, 61.2, 14.0. HRMS(+) calculated for
[C20H18N2O6S + H]+: 415.09583 found 415.09530.
RESULTS AND DISCUSSION
Spectroscopy
The series of compounds reported in Chart 1 were prepared to
investigate the effect of the heterocycles in addition to the de-
gree of conjugation on the electrochemical and photophysical
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Copyright © 2011 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2012, 25 211–221

HETEROCYCLIC EFFECT ON AZOMETHINE OPTOELECTRONIC PROPERTIES
properties. In particular, the fluorescence, fluorescence quantum
yields and phosphorescence of the various azomethines were
investigated. Upon comparing spectroscopic properties of the
similar heterocyclic compounds for example, 7, 11 and 15 in
Table 1 and Fig. 1, the effect of the degree of conjugation on
the spectroscopic properties can be elucidated. In all cases, both
the absorbance and fluorescence maxima were bathochromically
shifted by about 70 nm upon progressing from the dyads to the
triads. Meanwhile, the homoaryl derivatives 22–25 exhibited
absorbance and fluorescence maxima comparable to their
corresponding dyads. This is in contrast to the homoaryl benchmark
26, whose absorbance and fluorescence maxima that are
hypsochromically shifted relative to the triads. The spectroscopic
data confirm that extended degrees of conjugation are
possible with the heterocyclic derivatives relative to their
homoaryl azomethines.
properties. The spectroscopic data further imply that the
homoaryl 26 is a suitable reference for benchmarking the
spectroscopic properties. Comparing the data of the various
compounds, it is evident that the spectroscopic properties
can discretely be tailored as a function of structure and
heterocycle. The significant bathochromic shifts observed for
the heterocyclic azomethines relative to the homoaryl analogues
imply that the heterocyclic derivatives have enhanced
delocalization. This is most likely due in part to the high degree
of coplanarity (vide infra) of the heterocyclic azomethines
combined with the electronic effects of the π‐rich heterocycles.
Also, the steady‐state spectroscopic data suggest that the
degree of conjugation for the homoaryl derivatives 22–25 is
confined to the heterocycle–azomethine moiety and that
delocalization across the entire molecule is reduced. This is in
part based on their hyposchromic shifts relative to their
respective analogues 15–18.
Although an unsubstituted amino derivative of 1 would be
advantageous for direct comparison of the properties with 26,
this is not possible owing to the instability of both such a
precursor and its corresponding azomethines. 1 was used in lieu
of preparing the targeted compounds because both 1 and its
azomethine derivatives are stable under ambient conditions.
Although an ester derivative of 2 is preferred for direct property
comparison for compounds prepared from 1 and 2, the ester
analogues of 7–10 were unstable, precluding their characteriza-
tion. Despite this, the azomethines derived from 1 and 2 can be
compared owing to the similar electron withdrawing effect of
the cyano and ester groups.[32,33] Moreover, given that 1 and 2
were used for preparing all the heterocyclic azomethines, the
effect of their electron withdrawing group on the observed
properties is consistent for all the azomethine derivatives.
The general trend of the various heterocycles’ impact on
the spectroscopic properties is represented in Fig. 1. It is
evident that the absorbance is bathochromically shifted by
about 30 nm for each heterocycle upon progressing in the
order of furan < thiophene < pyrrole < methyl pyrrole for the
symmetric azomethines. The same trend was observed for
all the azomethines studied and is consistent with the
increasing electron richness of the heterocycle.[38] In the case
of the unsymmetric azomethines, the spectroscopic properties
resemble those of the corresponding symmetric azomethines
with the greatest electron‐rich heterocycle.
Although the azomethine bond is electron withdrawing, its
electronic effect is insufficient to offset any gain achieved by
increasing the degree of conjugation. This is corroborated by
the spectral bathochromic shifts observed for 15–18 relative to
their dyad analogues 7–10. The limited spectroscopic property
perturbation from the electronic groups is further evidenced
from the dyads 7–14. This is apparent from the absorption
spectra of 11–14 that are shifted by only 10 nm relative to
7–10, while the fluorescence spectra are relatively similar,
despite the strong electron donating group in the terminal
position of 11–14. The terminal amine’s donating effect is most
likely further offset by the adjacent withdrawing ester. The
collective spectroscopic data confirm that the spectroscopic
properties are governed more by the degree of conjugation than
the electronic groups along the conjugated framework, which
affect predominately the redox potentials (vide infra). Direct
property comparison of the 3‐monocyano substituted
azomethines (7–10) to their 3,4‐disubstituted counterparts
(11–18) is therefore possible because the additional cyano
group is not expected to significantly affect the spectroscopic
All the compounds studied exhibited weak fluorescence
quantum yields (Φf) on the order of < 10–3. This is in contrast
to their vinylene analogues that fluoresced with Φf ranging
between 0.68 and 0.99.[39,40] Although many assumptions are
made for deriving quantum yields relative to standards such as
bithiophene (Φf = 0.013), the fluorescence for all the compounds
is nonetheless extremely low. We verified the accuracy of the
fluorescence measurements using an integrating sphere. This
provides absolute fluorescence yields regardless of the
excitation and emission wavelengths and emission yields.[41]
The fluorescence yields for the azomethines were all below the
detection limit (Φf < 0.01), confirming the quenched fluorescence
regardless of the degree of conjugation and type of heterocycle.
Low temperature fluorescence measurements were
subsequently undertaken to gain insight into the origins of the
fluorescence quenching and deactivation modes of the
azomethines. At 77 K, internal conversion deactivation modes
by either simple bond rotation or intramolecular processes
requiring geometric alignment and bond rotations are sup-
pressed. Increased fluorescence should therefore be observed
if nonradiative internal conversion deactivation by either of
these rotation modes is an efficient deactivation manifold.
Homogeneous glass matrices suitable for low temperature
measurements were done with a 4:1 ethanol/methanol matrix.
As seen in Table 1, the low temperature fluorescence is several
orders of magnitude greater than that at room temperature. The
measured values suggest that the fluorescence can be restored
to near quantitative values at low temperature. The azomethines
therefore have temperature sensor‐like properties such that the
fluorescence can be turned‐on and turned‐off with temperature.
Although the temperature dependent fluorescence confirms
that the singlet excited state of the azomethines is deactivated
by internal conversion, the quantum yield of this process cannot
be accurately measured. This is a result of the large error associ-
ated in comparing the intense fluorescence signal at 77 K to the
extremely noisy and weak room temperature signal (inset Fig. 1).
The temperature‐dependent fluorescence nonetheless confirms
that the azomethine fluorescence is temperature dependent. It
further confirms that deactivation by internal conversion involv-
ing bond rotation/alignment is a major deactivation mode of
azomethine singlet excited state deactivation. Meanwhile, deac-
tivation of the singlet manifold is consistent for all the azo-
methines and is independent of the type of heterocycle and
degree of conjugation. It is noteworthy that weak fluorescence
was observed regardless of solvent polarity and proticity
J. Phys. Org. Chem. 2012, 25 211–221
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S. DUFRESNE AND W. G. SKENE
Table 1. Spectroscopica and electrochemicalb data for the various symmetric and unsymmetricc azomethines
absITO
(nm)l
f
g
spec
(ITO)
(ITO)
Compound Abs Fluo
(nm)d (nm)e
Φfl (Φ77 K)
ΔE Eg
Epa Epc Eg HOMO LUMO Epa
(V)l
Epc
(V)l
(eV)h (eV)i (V) (V) (eV)j (−eV)k (−eV)k
1
2
7
8
9
305
279
382
380
370
363
390
391
400
390
457
452
440
430
457
452
420
367
365
366
360
354
335 2.5 × 10–3 (8.9 × 10–3) 3.7
304 3 × 10–6 (7.7 × 10–6) 4.2
3.0 0.6
3.9 1.3
—
—
—
—
4.9
5.6
5.7
5.7
6.0
6.0
5.1
5.1
5.3
5.3
5.1
5.3
5.2
5.3
5.3
5.4
5.4
5.3
5.3
5.7
5.6
—
1.2
1.5
2.8
2.8
3.0
3.0
2.5
2.5
3.0
3.1
3.0
3.0
3.1
3.2
3.1
3.1
3.2
2.2
2.2
2.6
2.6
—
—
—
—
—
—
—
Dyads
473
472
439
429
472
474
480
469
538
504
534
494
531
531
519
419
404
416
422
410
9.4 × 10–5 (0.76)
1.1 × 10–4 (0.42)
3.9 × 10–3 (0.79)
2.3 × 10–4 (0.56)
4.3 × 10–4 (0.59)
2.6 × 10–4 (0.48)
2.9 × 10–3 (0.97)
7.7 × 10–4 (0.88)
5.1 × 10–3 (0.82)
2.5 × 10–3 (0.88)
2.8 × 10–3 (0.74)
9.6 × 10–4 (0.81)
3.8 × 10–4 (0.39)
1.2 × 10–3 (0.43)
6.5 × 10–4 (0.21)
1.2 × 10–3 (0.71)
3.3 × 10–3 (0.69)
3.7 × 10–4 (0.63)
4.6 × 10–4 (0.59)
0.68–0.99
2.9
3.0
3.2
3.1
2.9
2.9
2.9
2.9
2.5
2.6
2.6
2.8
2.4
2.5
2.6
3.2
3.1
3.1
3.2
—
2.7 1.4 −1.6 2.9
2.7 1.4 −1.6 2.9
2.7 1.7 −1.4 3.0
2.8 1.7 −1.4 3.0
2.7 0.8 −1.9 2.6
2.7 0.8 −1.9 2.6
2.6 1.0 −1.4 2.3
2.7 1.0 −1.3 2.2
2.3 0.8 −1.4 2.1
2.3 1.0 −1.4 2.3
2.3 0.9 −1.3 2.1
2.4 1.0 −1.2 2.1
2.2 1.0 −1.3 2.2
2.3 1.1 −1.3 2.3
2.3 1.1 −1.2 2.2
2.8 1.0 −2.2 3.1
2.7 1.0 −2.2 3.1
2.9 1.4 −1.8 3.1
2.9 1.3 −1.8 3.0
1.3
1.3
1.3
1.4
0.6
0.6
0.7
0.7
0.8
0.8
0.7
0.8
0.7
1.0
0.8
0.8
0.8
1.3
1.0
—
−1.1
−1.1
−1.2
−1.1
−1.3
−1.4
−1.2
−1.4
−1.1
−1.2
439
433
433
412
424
430
434
431
486
483
485
480
483
477
455
404
397
420
448
—
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26m
Triads
−1.3
−1.2
−1.1
−1.3
−1.3
−1.2
−1.2
−1.4
—
Analogues
2.7
—
—
—
aIn anhydrous acetonitrile.
bValues reported against Ag/AgCl (sat’d) in CH2Cl2.
cLiterature values.[30]
dAbsorbance maximum.
eEmission maximum.
fFluorescence quantum yields at λex = 303 nm, relative to bithiophene.[34]
gValues in parentheses are absolute fluorescence quantum yields at 77 K relative to room temperature.
hAbsolute HOMO–LUMO spectral difference.
iSpectroscopic energy gap.
jElectrochemical energy gap.
kHOMO and LUMO values were determined from the oxidation and reduction onsets, respectively, relative to vacuum.
lData for anodically coupled products immobilized on ITO electrodes.
mLiterature values.[35–37]
implying that simple polar effects and perturbation of the
1.00
singlet–triplet manifolds are not the reason for the quenched
fluorescence. It is further noteworthy that the emission wave-
length does not shift with temperature (inset Fig. 1). This implies
that the compounds adopt a similar extended degree of
conjugation arrangement both in the solid matrix and at room
temperature. The temperature induced fluorescence turn‐on
makes the azomethines interesting fluorimetric temperature
sensors.
Laser flash photolysis (LFP) was used to further investigate
the excited state deactivation modes of the azomethines.
Short‐lived transients on the order of µs can be spectroscopically
detected and assigned by this time resolved method. No
transient was detected for any of the azomethines, implying that
intramolecular deactivation involving either radical or radical ion
transients does not occur. Although no signal was measured by
LFP, triplet formation by intersystem crossing (ISC) cannot be
unequivocally dismissed. This is because the shortest transient
0.75
0.50
0.25
0.00
Wavelength (nm)
400
450
500
550
Wavelength (nm)
Figure 1. Effect of different heterocycles on the absorbance spectra:
15 , 16 , 17 (●), 18 . Inset: fluorescence of 20 at room
temperature magnified 400 times (■) and 77 K
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HETEROCYCLIC EFFECT ON AZOMETHINE OPTOELECTRONIC PROPERTIES
lifetime detectable by the system is ≈100 ns and triplet quench-
ing by intramolecular energy transfer would occur on a shorter
timescale. Phosphorescence measurements were subsequently
done in a glass matrix at 77 K. At this low temperature, collisional
deactivation processes that would otherwise thermally deacti-
vate the triplet are eliminated. Weak phosphorescence should
subsequently be seen if the triplet state is formed. Even though
phosphorescence (see Supporting Information) was observed
for all the azomethines studied, only qualitative information
is unsymmetric and that it consists of a central thiophene sand-
wiched between a furan and pyrrole. The figure also shows that
the heterocycles adopt an antiparallel arrangement. It is further
evident that the two connecting azomethine bonds both adopt
the E isomer and that the compound is linear. Most importantly,
the mean plane angles between the terminal heterocycles and
the central thiophene are about 18o.
The crystallographic data for 11–21[6,30,42–46] were compiled
to examine the effect of the various heterocycles on the molec-
ular arrangement and for further understanding the spectro-
scopic data. The data are collectively summarized in Table 2. It
can be seen that the mean plane angles between the terminal
heterocycles and the central thiophene range between 2.5–34o.
This is in contrast to their homoaryl derivative 23 whose analo-
gous mean planes are twisted between 32.9 and 56.4o.[47] The
consistent large aryl–azomethine twisting angle of the homoaryl
regarding the triplet state can be obtained given that Φphos
<
ΦISC. Azomethine phosphorescence confirms that the triplet is
produced at 77 K and that it is efficiently deactivated at room
temperature.
In light of the high fluorescence yields measured at low
temperatures, deactivation by ISC is not a major deactivation
mode. The collective steady‐state and time‐resolved spectro-
scopic data confirm that the suppressed fluorescence is
predominately a result of deactivation of the singlet excited state
by internal conversion involving bond rotation. The fluorescence
can therefore be restored at low temperatures by suppressing
these energy wasting modes. Meanwhile, the steady‐state
spectroscopic data confirm that both the absorbance and
fluorescence are contingent not only on the nature of the
heterocycle, but also on the number of azomethine bonds. Also,
significant absorbance and fluorescence bathochromic shifts are
possible with the heterocyclic azomethines relative to their
homoaryl containing counterparts. It is evident from the data
that the color and fluorescence emission can be discretely tuned
to nearly any wavelength, contingent of heterocycle, degree of
conjugation and electronic effects. Heterocyclic azomethines
therefore exhibit enhanced functional materials‐like properties
relative to their homoaryl analogues and similar properties to
those of their vinylene counterparts.
derivatives imply
a reduced degree of conjugation and
reduced delocalization of the phenylene azomethines relative
to their heterocycle counterparts. Meanwhile, the antiparallel
and coplanar arrangements of 11–21 contribute to the en-
hanced degree of conjugation of the heterocyclic derivatives.
The azomethine and azomethine–aryl bonds were found to be
shorter than the corresponding CH–aryl bonds. This suggests
an increased degree of conjugation of the azomethines relative
to their homologues, resulting in delocalization of the electron‐
rich heterocycles across the entire molecule. This is responsible
for the strong visible absorbance and emission properties of
the heterocyclic azomethines. Also, given that no temperature‐
dependent emission shifts were observed, the configuration ob-
served in the crystal state is most likely comparable to that
adopted in solution.
According to Fig. 3, the crystal packing shows several
interactions between the pyrrole–furan moieties. These are
consistent with donor–acceptor type interactions between the
two heterocycles. These interactions can be assumed from the
centroid‐to‐centroid distance (3.808 Å) and the interatom dis-
tances (3.3–3.6 Å) that are below the sum of the van der Waals
radii (3.4–3.5 Å). The donor–acceptor interactions lead to regio‐
regular crystal packing involving alternating pyrrole–furan
repeating motifs. Although hydrogen bonding is nonexistent
Crystallographic data
Single crystal X‐ray diffraction analysis of 20 was investigated to
determine the twisting between the aryl–azomethine planes.
The X‐ray crystallography data shown in Fig. 2 confirm that 20
Figure 2. ORTEP representation of 20 with the ellipsoids drawn at 30% probability
J. Phys. Org. Chem. 2012, 25 211–221
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S. DUFRESNE AND W. G. SKENE
for 20, unlike other azomethines such as 11–14, the heterocyclic
donor–acceptor interactions are sufficiently strong as evidenced
by the fact that X‐ray quality crystals of this unsymmetric com-
pound could be easily obtained. Nonetheless, the collective co-
planarity of the aryl–azomethine segments and the short bond
distances are responsible for the significantly different spectro-
scopic properties between the homo‐ and heterocyclic
azomethines.
Electrochemistry
Cyclic voltammetry was undertaken to further investigate the
contribution of the various heterocycles and degree of conjuga-
tion on the oxidation (Epa) and reduction potentials (Epc). A typi-
cal voltammogram is seen in Fig. 4. The effect of the different
heterocycles is evident from the Epa for 7–10. As seen in Table 1,
the Epa for the furan and thiophene derivatives are between
100 and 300 mV more positive than their corresponding pyrrole
analogues. Conversely, the Epc of the furan and thiophene
derivatives are less positive than their pyrrole counterparts.
These trends are independent of the degree of conjugation
and are consistent with the π‐richness of the heterocycle.[38]
Figure 3. Extended view along the ac axis of the crystal lattice of 20
25
20
15
10
5
0
-5
-10
-2
-1
1
2
Potential (V) versus Ag/AgCl (Sat'd)
Figure 4. Cyclic voltammogram of 18 measured in anhydrous and dea-
erated dichloromethane at a scan rate of 100 mV/s
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Copyright © 2011 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2012, 25 211–221

HETEROCYCLIC EFFECT ON AZOMETHINE OPTOELECTRONIC PROPERTIES
The electronic effect on the oxidation potential is evident
upon comparing the Epa of 11–14 to their corresponding dyads.
The electron donating terminal amine reduces the Epa by about
600 mV. Meanwhile, the same reduction in Epa occurs with
the additional azomethine for the triads relative to the
corresponding dyads. The reduced Epa is a result of the increased
degree of conjugation and confirms the extended delocalization
of the azomethines. The higher Epa and energy gap (Eg) of the
homoaryl derivatives (22–25) relative to their all‐heterocyclic
counterparts corroborate the limited degree of conjugation that
was spectroscopically observed. The collective electrochemical
data confirm that both the anodic and cathodic potentials can
be tailored by incorporating different heterocycles into the
azomethines and via the number of azomethine bonds.
0.2
0.0
Anodic coupling
0.6
-0.2
As seen in Fig. 5, both the oxidation and reduction processes
of 18 are irreversible. This behaviour is consistent for all the
azomethines reported in Chart 1. It can be argued that the irre-
versible processes are a result of azomethine decomposition
and reduction of the heteroconjugated bond. The latter can be
dismissed because the cathodic process was found to be a
one‐electron transfer, determined relative to ferrocene. Con-
versely, azomethine reduction is a two‐electron process. The
anodic process is similarly a one‐electron transfer process
resulting in the radical cation. The irreversible oxidation is a
result of radical cation cross‐coupling according to standard
means.[48,49] This was previously determined with terminally
substituted derivatives such as 27 that were found to yield
persistent radical cations.[6,7,50,51]
0.4
0.2
-0.4
0.0
400
500
600
-0.6
Wavelength (nm)
0.4
0.6
0.8
1.0
Potential (V) versus SCE
Figure 5. Anodic coupling of 15 at 100 mV/s in dichloromethane. Inset:
absorbance of 15 deposited on ITO an electrode after applying Epa = 1 V
for 0 (black), 1 (red) and 5 min (blue)
In light of the unsubstituted 2,2′‐terminal positions of the
heterocyclic azomethines, we investigated whether the radical
cation could undergo standard cross‐coupling to afford coupled
products with increased degrees of conjugation. This was done
both by cyclic oxidation scans and chronometrically by applying
a potential slightly more positive than the Epa of the azomethine
of study. ITO‐coated glass electrodes were used for spectroscop-
ically and electrochemically characterizing the anodically
coupled products physisorbed to the transparent electrode. A
typical voltammogram of the anodic coupling is represented in
Fig. 5 for 15. The anodic wave shifts to less positive potentials
with cycling between 1.1 and 0.4 V concomitant with a shift of
the cathodic wave to more positive potentials. The former
implies that the anodically coupled product is more conjugated
than its corresponding monomer. Moreover, the resulting
product physisorbed to the electrode exhibited a one‐electron
reversible oxidation. The increased degree of conjugation is
further confirmed by a 30‐nm bathochromic shift in the
absorbance relative to the original 15. To ensure that the
measured properties were that of the neutral product and not
700
650
600
15
550
17
500
450
400
350
300
11/13
1
0.00
0.05
0.10
0.15
1/ # atoms
Figure 6. Evolution of spectroscopic Eg as a function of the number of
atoms along the conjugated framework for the methyl‐pyrrole and thio-
phene containing dyads and triads
Figure 7. Mass spectrum of P19 in positive ESI mode
J. Phys. Org. Chem. 2012, 25 211–221
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S. DUFRESNE AND W. G. SKENE
its corresponding oxidized intermediate, a potential of 0 V was
applied for 2 min prior to measuring the Epa of the coupled
products.
heterocycle. The azomethines investigated illustrate that
heterocyclic azomethines are robust and that functional
materials‐like properties are possible. Future generation of
materials for a given application can therefore be prepared
with the knowledge gained from the discovered fluorescence
turn‐on and turn‐off, oxidation potentials and property tailoring
contingent on structure.
Less positive Epa and bathochromic absorbance shifts
were observed for the anodically coupled products of all the
azomethines. Given that the anodic coupling of 11–14 can only
yield dimers, their similar absorbance shifts to the other anodi-
cally coupled products in Chart 1 suggest that the anodically
coupled products P15–P25 are low molecular weight oligomers.
This was spectroscopically investigated with a calibration curve
of absorbance as a function of the reciprocal number of atoms
along the conjugated framework (Fig. 6). The estimated degree
of oligomerization determined by this method for 15 is two.
Both the oxidation and absorbance shifts nonetheless provide
the first reported evidence for anodically coupled products
where increased degrees of conjugation relative to their
corresponding monomers are formed.
Absolute evidence for coupled product formation was further
sought to corroborate the qualitative spectroscopic data
obtained from the anodically coupled products deposited on
the ITO electrodes. 19 was therefore anodically coupled using
a large surface area platinum mesh gauze for obtaining sufficient
material for standard characterization. However, only minute
amounts of soluble product for mass spectrometry analyses were
isolated. The resulting Electrospray Ionisation (ESI) mass spectrum
is shown in Fig. 7. The major molecular ion peaks (m/z = 791
and 700) confirm that anodically coupled products are formed.
The weaker molecular ion peaks (m/z = 457 and 545) suggest
some azomethine hydrolysis, which most likely occurs during
analysis/sample preparation. Although higher order oligomers
or polymers cannot be confirmed because of their reduced
solubility, the mass spectrometry (MS) data nevertheless provide
information relating to the lower molecular weight limit of the
coupled products that are produced. It is noteworthy that
azomethine anodic product coupling has not previously been
confirmed. The collective MS and spectroscopic data provide
unprecedented supporting evidence that anodic coupling
occurs and that products with increased degree of conjugations
relative to 19 are formed. The data further suggest that anodi-
cally produced polyazomethine is possible, providing its oligo-
meric precursors do not precipitate during anodic coupling.
This would be possible with 3,4‐dialkylated monomer derivatives
of 15–21.
Acknowledgements
Natural Sciences and Engineering Council Canada (NSERC)
Canada is thanked for Discover and Research Tools and Instru-
ments grants allowing this work to be performed in addition to
Canada Foundation for Innovation for additional equipment
funding. S.D. also thanks NSERC for a graduate scholarship.
W.G.S. also acknowledges both the Alexander von Humboldt
Foundation and the Royal Society of Chemistry for a JWT
Jones Travelling fellowship, allowing the completion of this
manuscript.
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Conjugated azomethines consisting uniquely of heterocycles
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