Tunable spectroscopic and electrochemical properties of conjugated push-push, push-pull and pull-pull thiopheno azomethines

Article abstract of DOI:10.1039/b616379c

Novel azomethines consisting uniquely of thiophene units were examined. The highly conjugated compounds were prepared by condensing air stable aminothiophenes with 2-thiophene aldehydes, which were substituted with various electronic groups. The resulting azomethines are highly conjugated and are both reductively and hydrolytically resistant. Various electron donating and accepting groups placed in the 2-position of 5-thiophene carboxaldehyde lead to electronically delocalized push-push, pull-pull, and push-pull azomethines. These electronic groups affect both the HOMO and the LUMO levels, which influence the absorption and emission spectra. Colors spanning the entire visible spectrum ranging from yellow to blue are possible with these nitrogen containing conjugated compounds. Excited state deactivation of the singlet excited state occurs predominately by internal conversion while only a small amount of energy is dissipated by intersystem crossing to the triplet state and by fluorescence. The ensuing fluorescence and phosphorescence of the thiopheno azomethines are similar to those of their thiophene analogues currently used in functional devices, but with the advantage of a low triplet state and tunable HOMO-LUMO energy levels extending from 3.0 to 1.9 eV. Quasi-reversible electrochemical radical cation formation is possible while the oxidation potential is dependent on the nature of the electronic group appended to the thiophene. The crystallographic data of the electronic push-push system show the azomethine bonds are planar and linear and they adopt the E isomer. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b616379c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Tunable spectroscopic and electrochemical properties of conjugated
push–push, push–pull and pull–pull thiopheno azomethines{
St e´ phane Dufresne, Marie Bourgeaux and W. G. Skene*
Received 9th November 2006, Accepted 22nd December 2006
First published as an Advance Article on the web 23rd January 2007
DOI: 10.1039/b616379c
Novel azomethines consisting uniquely of thiophene units were examined. The highly conjugated
compounds were prepared by condensing air stable aminothiophenes with 2-thiophene aldehydes,
which were substituted with various electronic groups. The resulting azomethines are highly
conjugated and are both reductively and hydrolytically resistant. Various electron donating and
accepting groups placed in the 2-position of 5-thiophene carboxaldehyde lead to electronically
delocalized push–push, pull–pull, and push–pull azomethines. These electronic groups affect both
the HOMO and the LUMO levels, which influence the absorption and emission spectra. Colors
spanning the entire visible spectrum ranging from yellow to blue are possible with these nitrogen
containing conjugated compounds. Excited state deactivation of the singlet excited state occurs
predominately by internal conversion while only a small amount of energy is dissipated by
intersystem crossing to the triplet state and by fluorescence. The ensuing fluorescence and
phosphorescence of the thiopheno azomethines are similar to those of their thiophene analogues
currently used in functional devices, but with the advantage of a low triplet state and tunable
HOMO–LUMO energy levels extending from 3.0 to 1.9 eV. Quasi-reversible electrochemical
radical cation formation is possible while the oxidation potential is dependent on the nature of the
electronic group appended to the thiophene. The crystallographic data of the electronic push–push
system show the azomethine bonds are planar and linear and they adopt the E isomer.
be readily obtained by these normal coupling means. Standard
Introduction
synthetic means furthermore do not easily promote the
Conjugated materials have received much attention as they
selective one-pot addition of monomer units in high yields to
offer many new possibilities for devices combining unique
1
optical, electrical, and mechanical properties. Particular
afford unsymmetric electronic p-conjugated push–pull systems.
Such conjugated compounds along with their electronic
interest is found with thiophene based materials because of
2
their low oxidation potentials and their equally low band-gap
pull–pull and push–push analogues offer a viable means for
spectroscopic property tuning and they provide a new realm of
3
properties, which make them useful for many electronic
easily synthesized functional materials for enhanced emitting
9
devices. Moreover, these compounds show such versatility
devices or non-linear optical materials.
because of their semiconductor like properties obtained with
Azomethines (–NLCH–) are appealing alternatives for
chemical doping that further make them suitable for commer-
conjugated materials because of the mild reaction conditions
cial applications. As a result of the diverse emissive and
conductive properties and various synthetic means for their
formation, conjugated thiophene materials have found many
applications including uses as hole injection layers in OLED/
PLEDs, in flexible light displays, solar cells, flat panel displays,
field effect transistors, non-linear optical materials, sensors,
required for their synthesis. They have the advantage of
increased yields and selective addition through the condensa-
tion reaction between an amine and an aldehyde, requiring less
difficult purifications. The result of the easy reaction between
the two complementary groups is a robust azomethine
connection that is conjugated and exhibits a high stability
towards hydrolysis and reduction when aryl precursors are
4
,5
and/or low power consumption products, to name but a few.
1
0–13
As attractive as these conjugated materials are for their
physical properties, their synthesis is however challenging.
employed.
Research progress regarding functional
azomethine materials has unfortunately not been as prolific
as their carbon analogues because of the limited number of
available stable aryl diamino precursors. Such precursors often
undergo undesired oxidative decomposition while the resulting
azomethine products do not have suitable properties for
functional applications and suffer from problematic irrever-
6
Typical synthetic methods such as Suzuki and Wittig
7
8
strategies or electropolymerization used to obtain conjugated
thiophenes are often quite stringent. Alternating p-conjugated
units with various electron donor and acceptor groups cannot
1
4,15
D e´ partement de Chimie, Pavillon JA Bombardier, Universit e´ de
Montr e´ al, CP 6128, succ. Centre-ville, Montr e´ al, Qu e´ bec, Canada
H3C 3J7. E-mail: w.skene@umontreal.ca
sible oxidation.
Only recently have efforts focused on
taking advantage of these robust bonds to end cap oligomers
16
using thiophene aldehyde precursors. The recent introduc-
{
Electronic supplementary information (ESI) available: experimental
tion of stable aminothiophenes (1 and 3) have further
section; absorption and emission spectra; cyclic voltammetry; reduc-
tion analyses. See DOI: 10.1039/b616379c
helped to develop azomethine research leading to conjugated
1
166 | J. Mater. Chem., 2007, 17, 1166–1177
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Chart 1 Conjugated azomethines examined, their precursors, and some representative carbon analogues.
1
7,18
azomethines consisting uniquely of thiophene units.
These
the equilibrium in favor of the products without the need
of stringent conditions, unlike standard carbon coupling
methods. The versatile reaction supports a wide variety of
common organic solvents with selective product formation
(Scheme 1) through the judicious choice of solvent, tempera-
ture and reagent stoichiometry. This selectivity is courtesy of
the different reactivity of the amino groups of 1 in its pristine
form relative to its monoazomethine form. Selective mono-
addition to yield the amino monoazomethines is done with one
equivalent of the corresponding aldehyde by condensing it
with 1 at room temperature or by refluxing in ethanol. Both
instances lead exclusively to the monoadducts, owing to the
deactivation by the electron withdrawing ester group. The
deactivating group is further responsible for the increased
stability of 1 relative to its unsubstituted 2,5-diaminothiophene
analogue, which spontaneously undergoes oxidative decom-
position under ambient conditions and cannot be isolated. The
electron withdrawing group is also responsible for decreasing
the amine nucleophilicity. As a result, only mild reaction
conditions are required for the condensation of monoazo-
methines. The electron withdrawing effect of the azomethine
bond in the monoazomethine further reduces the amine
reactivity. More stringent reaction conditions are therefore
required to push the second condensation to form the
bisazomethine. This provides a means to tailor product
formation by changing either the reagent stoichiometry or
the solvents used. The resulting azomethine bond is sufficiently
robust that it resists reduction with common reducing
agents such as NaBH4 and resists even strong reductants
extremely stable compounds do not suffer from the limitations
1
of previous azomethines. Furthermore, they have the added
9
2
0
advantage of being isoelectronic to their carbon analogues,
making them suitable replacements for conventional thiophene
materials.
Recently, we developed a route to synthesize oligothiopheno
2
1,22
azomethines by selective reagent addition.
The potential
advantages of azomethine-based materials concomitant with
1
3
our previous success with such compounds have prompted us
to develop new symmetric and unsymmetric thiopheno
azomethines. The main objective is to examine the physical
properties, among which are the spectroscopic and photo-
physical properties including the cyclic voltammetry, of these
new compounds. These are of particular interest giving the
limited number of the photophysical and electrochemical
studies regarding azomethines especially their thiophene
derivatives. Herein, we present the preparation and charac-
terization of conjugated thiopheno azomethines containing
terminal electron donating and accepting groups. The sequen-
tial assembly of these electronic push–push, pull–pull and push–
pull p-conjugated compounds is presented. The steady-state
and time-resolved photophysics, electrochemical properties,
HOMO–LUMO energy gaps, and crystal structures of these
unique thiopheno azomethines are also investigated.
Results and discussion
Synthesis
including DIBAL and H /Pd using standard protocols. Only
2
The reaction conditions to afford the azomethines involve
shifting the equilibrium by simple dehydration via hydroscopic
solvents (i.e., absolute ethanol and anhydrous toluene and
THF) or mild drying agents (i.e., molecular sieves and alkali
sulfates). These parameters concomitant with the formation of
the thermodynamically favorable conjugated products displace
in the case of refluxing with DIBAL for 10 hours did reduction
of the azomethine occur. Azomethine hydrolysis occurs only
with prolonged heating with concentrated acid in water
saturated organic solvents. No product decomposition was
observed even after eighteen months in solution under normal
conditions.
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J. Mater. Chem., 2007, 17, 1166–1177 | 1167

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Scheme 1 Conjugated thiophene assembly by complementary precursors leading to tunable properties. Experimental conditions: i) EtOH, D,
4
ii) i-PrOH, D, iii) TiCl , DABCO, toluene, D, iv) n-BuOH, D.
The monoazomethines 5–8 are formed exclusively in ethanol
regardless of the aldehyde used and the stoichiometry
employed. By simply changing the solvent polarity and using
different boiling point solvents, such as isopropanol or
n-butanol, the reaction can be pushed to afford the corre-
sponding bisazomethines in most cases (vide infra).
Condensation can be done sequentially in a one-pot fashion
directly from 1 with two equivalents of the corresponding
aldehyde. Alternatively, an additional equivalent of aldehyde
can be added to 5 to afford 14. In all cases in which controlled
addition was possible, the condensation reaction was optimally
catalyzed with ca. 5 mol% of trifluoroacetic acid. The
controlled addition methodology is amenable to any aldehyde
combination with 1 and provides a useful method for
preparing new conjugated thiophenes. The reduced amine
reactivity arising from the electron withdrawing ethyl ester
group is evident with monomer 1 in which 14 can be obtained
only by using high boiling point solvents, such as n-butanol.
Unlike 1, selective product formation was not observed with
the monoamine precursor 3. The addition of one equivalent of
addition. The unreacted amount of 3 therefore condenses
preferentially with 13 immediately upon its formation resulting
in the observed abundance of 20 relative to 13.
Synthesis of the substituted bisazomethines is not straight-
forward owing to the different reactivity of the amino group of
1 towards the various aldehydes. The bisazomethines are
obtained with mild dehydration methods only with prolonged
reaction times that result in significant aldehyde decomposition.
The strong electron donating character of the diethylamino
group of 2 deactivates the aldehyde such that nucleophilic
addition of 1 cannot be done under mild conditions. Alterna-
tively, its reactivity can be increased with a Lewis acid such as
2
4
TiCl4. This route significantly decreases the reaction time
while improving the yields of the coupling reaction to afford
conjugated monoazomethines (7 and 11) and bisazomethines
(16 and 18). Conversely, the electron withdrawing nitro
group of 5-nitrothiophene-2-carboxaldehyde allowed for easy
azomethine formation by simple heating with an acid catalyst
in isopropanol only with the amino precursors 1 and 3. The
strong electron withdrawing nature of the nitro group, the
high degree of conjugation, concomitant with the ester with-
drawing capacity of 6, further reduces the amine’s nucleo-
philic character and prevents bisazomethine formation. This
explains the difficulty in forming 15, which could be obtained
only by the Lewis acid method from 1 or 6. Since these com-
pounds are highly conjugated, the electronic withdrawing/
donating groups exert their influence throughout the entire
molecule. The reactivity of the terminal amine in the mono-
azomethines is therefore influenced by these electronic effects.
The synthesis of the unsymmetric compounds was further
challenging because of their dynamic covalent character
3
to 2,5-thiophene dicarboxaldehyde afforded a mixture of
monoadduct 13 with a predominate amount of bisazomethine
0. Product control with 3 was not possible even in ethanol at
2
room temperature, which otherwise led to product control
with 1. Unlike previous examples that exploited solubility
1
3,23
differences to afford selective product formation,
such
selectivity was not observed with 3. The lack of selectivity is
attributed to the electron withdrawing cyano group that
deactivates the amine and results in reduced reactivity
towards the aldehyde. The cyano electron withdrawing effect
of 13 increases the aldehyde reactivity towards nucleophilic
1
168 | J. Mater. Chem., 2007, 17, 1166–1177
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Scheme 2 Possible Lewis acid promoted dynamic component exchange of different monomers.
resulting in Lewis acid induced component exchange illu-
excited electronic p–p* levels owing to the stabilization from
increasing the degree of conjugation. The molar absorptivity
coefficients are substantially high for all the azomethines
because of the highly allowed p–p* transitions. The linear
bathochromic shift with the reciprocal number of unsub-
stituted thiophene units observed (Fig. 1 inset) further supports
the conjugated nature of the azomethine bond. The batho-
chromic shift is consistent with a planar rigid p-conjugation of
the compounds. Moreover, extrapolation of the curve gives the
potential absorption maximum for an azomethine polymer
consisting entirely of thiophene units, being ca. 500 nm.
The combined spectroscopic results for the various thio-
pheno azomethines are reported in Table 1. The absorption
and emission spectra intercept provides the relative energy
2
5,26
strated in Scheme 2.
Under our coupling conditions,
attempts to obtain the unsymmetric 17 from 5 resulted
quantitatively in 6. Instead of undergoing the second con-
densation to form a bisazomethine, the terminal thiophene was
exchanged with 2-nitrothiophene carboxaldehyde, according
to Scheme 2. The resulting product is the thermodynamically
stable 6 and its formation is consistent with previous
2
6,27
examples of Lewis acid catalyzed component exchange.
Consequently, the synthesis of 17 was achieved by exploiting
the dynamic component reaction to exchange the thiophenes
under stoichiometric conditions from 14. The 2-diethylamino
thiophene can also sustain a dynamic component exchange
and be replaced either by thiophene carboxaldehyde or by the
2
-nitrothiophene carboxaldehyde component in the conju-
gated bisazomethines. Thus, as predicted by the thermody-
namic stability and the reactivity of each compound, the
nitrothiophene preferentially exchanges with the thiophene
compound which, in turn, can displace the diethylamino
thiophene unit according to Scheme 2. The advantage of this
dynamic behavior is a tunable synthesis for each compound.
The observed dynamic behavior assures formation of the most
thermodynamically stable product, further supporting the
conjugated character of the azomethine compounds. A diverse
library of electronic push–push, pull–pull and push–pull
conjugated compounds is therefore possible via the step-wise
pathways and opens a new synthetic means of tunable
materials.
Spectroscopic properties
The extent of oligomerization can visually be followed by the
intense color change from yellow (1) to orange (5) and to deep
red (14) with the unsubstituted compounds. Their pronounced
bathochromic change (Fig. 1 and Table 1) is indicative of the
conjugation degree arising from azomethine bond formation.
The observed transitions are dominated by lowering of the
Fig. 1 Influence of terminal electronic groups on the absorption
spectra of 14 (closed diamonds), 15 (closed squares), 16 (open circles),
1
7 (open squares), 18 (closed triangles). Inset: reciprocal number of
thiophene units versus the change in absorption maximum for the
unsubstituted thiophenes.
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J. Mater. Chem., 2007, 17, 1166–1177 | 1169

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Table 1 Spectroscopic values for the various reagents and azomethines measured in anhydrous acetonitrile
a
b
Em. /
nm
i
j
Phos. /
nm
Abs. /
Module nm
e
M
lmax
/
cm
k
ns
nr
/
21
2
1
21
c
23
d
e
f
g
/eV
h
r
21
/ms
W
fl (610
)
Wfl (77 K)
t
fl /ns DE /eV
E
g
k
Reagents
1
2
3
4
5
6
7
8
9
1
1
1
1
305
371
279
295
400
490
452
404
370
400
473
379
385
440
481
555
470
588
427
451
277
312
423
376
351
37 000
32 000
15 000
10 000
19 000
29 000
42 000
16 000
14 000
36 000
49 000
17 000
22 000
64 000
37 000
34 000
30 000
45 000
46 000
65 000
24 000
27 000
—
335
408
304
323
480
637
542
516
439
454
545
451
426
534
589
659
614
660
559
500
306
344
—
2.5
0.5
3 6 10
0.2
2.9
1.2
2.7
2.3
3.9
2.6
8.7
1.3
1.5
2.8
1.6
1.9
5.6
2.1
1.3
1.4
0.2
0.6
0.0089
0.0058
7.7 6 10
0.00058
0.97
0.89
0.96
0.94
0.79
0.68
0.82
0.68
0.72
0.71
0.15
0.95
0.70
0.51
0.90
0.56
0.76
0.50
—
13.5
0.3
3.9
4.6
14.0
16.7
13.5
1.0
5.4
5.8
11.1
0.5
1.5
13.2
11.9
16.6
14.1
9.3
3.7
3.2
4.2
4.0
2.9
2.3
2.5
2.7
3.2
2.9
2.4
2.9
3.0
2.6
2.4
2.0
2.3
1.9
2.5
2.6
4.2
3.7
2.5
—
3.0
3.0
3.9
3.3
2.6
2.0
2.3
2.3
2.7
2.5
2.3
2.7
2.6
2.3
2.1
2.0
2.1
1.6
2.2
2.3
3.8
3.5
2.4
—
0.19
1.67
8 6 10
0.04
0.21
0.07
0.93
2.30
0.72
0.45
0.78
2.60
1.00
0.21
0.13
0.11
0.40
0.23
0.68
0.67
0.05
0.38
0.08 479
3.34 543
0.27 493
0.20 500
0.07 525
0.06 698
0.07 549
1.00 530
0.18 479
0.17 536
0.09 578
2.00 520
0.67 481
0.07 560
0.08 600
0.06 751
0.07 695
0.11 686
0.52 598
0.48 575
0.25 481
0.63 490
2
3
26
24
Aminomonoazomethine
Monoazomethine
0
1
2
3
Bisazomethine
14
1
1
1
1
1
2
5
6
7
8
9
0
1.9
2.1
4.3
1.6
Model compounds
21
22
23
24
25
b
k
40
1.8
22
—
280
0.53
—
4.72
—
—
—
l
l
28 400
24 000
—
422
—
56
—
—
—
0.2
m
n
m
n
n
o
o
3.2
3.1
a
c
30 d
Absorption maximum. Emission maximum. Fluorescence quantum yields at lex = 303 nm, relative to bithiophene.
Fluorescence
e
quantum yields at 77 K relative to room temperature. Monoexponential fluorescence lifetime measured at lem max. Absolute HOMO–
f
g
LUMO spectral difference. Spectroscopic band-gap.
h
i
j
k
r
= Wfl/tfl
From Gawinecki and Muzalewski, and Coville and Neuse.
.
k
nr = k
r
(1 2 Wfl)/Wfl
.
Phosphorescence measured in 1 : 4 methanol–
From Seixas de
k
ethanol matrix at 77 K. From Fr e` re and coworkers.
57
l
58 59 m
3
0–32,35 n
57 o
60
Melo and coworkers.
From Roncali and coworkers.
From Marks and coworkers.
2
8
difference between the ground and the excited states. From
the combined normalized absorption–emission spectra, the
relative HOMO–LUMO transition can be calculated from the
spectral overlap. The absorption method further provides an
estimate for the band-gap that can be derived from the
absorption onset in the red region of the spectrum. This
method applies to our compounds since only a slight
bathochromic shift of ca. 20 nm between solution and thin
film was observed. The measured values by this method show
the azomethines possess lower band-gaps relative to their
carbon analogues such as 23 and 25. The similarity of the
azomethines to structurally comparable nitrogen free thio-
phenes is evident from the photophysical data presented in
Table 1. The electronic groups chosen are known to influence
both the HOMO and LUMO levels, which is evident from the
spectroscopic data. In general, the electron donating and
the electron withdrawing groups are known to decrease the
HOMO and increase the LUMO levels, respectively. Such
perturbations translate into apparent energy gap differences
and intense absorption and emission spectral variations.
The monoazomethines 9–13 serve as reference compounds
to illustrate the effect of the electronic groups. The amine
group greatly affects both the absorption and the emission
compared to the nitro group. This is supported by the
pronounced spectral shifts of 100 nm for the donor group
and only slight shifts of 30 nm for the acceptor group, relative
to the reference compound 9. The similar spectroscopic
properties of the methylated analogue 9 and 12 confirm the
pronounced spectroscopic change is a result of the electronic
groups. The same tendency is observed also with the amino
monoazomethines (5–8) whose terminal –NH2 further
influences the properties via its donating character. The
electronic terminal groups can collectively influence the
resulting properties courtesy of the highly conjugated
azomethine bond. The withdrawing –NO
2
and donating
–NH2 of 6 jointly stabilize the ground and excited states,
respectively, leading to an electronic push–pull system. This
results in pronounced bathochromic absorption and emission
shifts relative to 5. Only slight bathochromic shifts are
observed with 7 because of the two amino groups that work
in opposition leading to an electronic push–push system.
The bisazomethines exhibit the same spectroscopic trend as
the monoazomethines. Both the absorption and the emission
spectra of the bisazomethine undergo large bathochromic
shifts relative to the monoazomethines because of the
increased conjugation. The pronounced stabilization implies
a high degree of delocalization and planarity throughout the
azomethines. The electronic group cooperativity is more
pronounced for the bisazomethines than for the mono-
azomethines, supported by the spectral differences observed
for 15–18 relative to the unsubstituted 14. The push–push
compounds 15 and 17 exhibit slight spectral shifts of 40 nm
while the corresponding amino substituted pull–pull compound
undergoes a large bathochromic shift around 70 nm, relative to
14. The electronic push–pull system (18), combining both a
donor and an acceptor group, exhibits the most pronounced
absorption and emission bathochromic shifts. The push–pull
effect greatly perturbs the electronic transitions and reduces
the HOMO–LUMO energy gap between the ground and
excited states. The net outcome is an intense visible blue color
1
170 | J. Mater. Chem., 2007, 17, 1166–1177
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for 18 in addition to an extremely low HOMO–LUMO energy
gap of 1.9 eV. From these observations, it can be concluded
that the donating group has a greater effect on the electronic
transitions than the nitro group, supported by the pronounced
bathochromic shifts. This is concomitant with the ester groups
on the thiophene central ring that also influence the spectro-
scopic properties through their withdrawing character. Thus,
depending on the substituent chosen, it is possible to tailor the
electronic properties of a given bisazomethine by perturbing
either the HOMO or LUMO energy levels.
The influence of the nitrogen placement within the
azomethine bond upon the resulting spectroscopic properties
was examined with model compounds 14 and 20. The
spectroscopic properties, including HOMO–LUMO energy
gap and molar absorption coefficients, of these two com-
pounds are very similar, which confirms the comparable
withdrawing effect of the cyano and the ester groups upon the
electronic transitions. This is further supported by the similar
spectroscopic results for the analogues 9 versus 12 and 14
versus 19. The placement of the nitrogen within the azomethine
bond does not perturb the electronic transitions given the
comparable spectroscopic results among the studied com-
pounds. Conversely, the nature and the number of substituents
in the 5 and 59 positions (Fig. 2) greatly affect the electronic
transitions which, in turn, influence the absorption and emis-
sion spectra in addition to the HOMO–LUMO energy gap.
Steady-state and time-resolved fluorescence measurements
further provide valuable insight into the excited state deactiva-
tion pathways of these compounds. Both the similar ns
fluorescence lifetimes and the calculated radiative rate
Fig. 3 Normalized absorption (open squares) and fluorescence
(
closed squares) spectra of 16 in acetonitrile. Phosphorescence (closed
circles) spectrum measured in 4 : 1 ethanol–methanol matrix at 77 K.
Inset: phosphorescence decay monitored at 760 nm.
relative to the fluorescence and a monoexponential emission
decay (Fig. 3). The phosphorescence quantum yields at this
low temperature can be calculated by relative actinometry with
3
6
fluorenone. The phosphorescence quantum yields measured
for all the studied compounds are approximately the same,
Wphos ¡ 0.1. The triplet energies can be calculated from the
E(0,0) transition from the phosphorescence spectra and these
3
3,37
values are comparable to those of 23 and 25.
Further evidence of the triplet state is provided by ns-laser
flash photolysis (LFP) from the weakly absorbing transients
observed at 280 and 370 nm for the monoazomethine and
bisazomethine, respectively (Fig. 4). The transients produced
by the intense laser pulse are assigned to the triplets owing to
their monoexponential decay kinetics that are similar to benzo-
phenone (Fig. 4 inset) in addition to being quenched with
standard triplet quenchers such as oxygen, 1,4-cyclohexadiene,
constants (k ) for all the azomethines imply similar deactiva-
f
tion modes of the singlet excited state for all the compounds.
The monoexponential lifetimes suggest unimolecular deactiva-
tion which is further supported by the dilute experimental
conditions, which preclude bimolecular self-quenching. The
low fluorescence quantum yields are indicative of nonradiative
quenching deactivation processes such as intersystem crossing
(
ISC) to the triplet state or by internal conversion (IC).
Photoisomerization between the E and the Z isomers is also a
2
9
possible deactivation mode.
However, no such photo-
products were observed. The low fluorescence quantum yield
for the thiopheno azomethines is however not surprising since
thiophenes are known to weakly fluoresce because of efficient
ISC to the triplet state. This is understood to occur by heavy
3
0–35
atom induced spin–orbit coupling by the sulfur atoms.
This behavior is expected to be enhanced further by the
azomethine nitrogen that contributes to the heavy atom effect
concomitant with the increased degree of conjugation, which
favors ISC by narrowing the singlet–triplet gap. Population of
the triplet manifold is supported by the phosphorescence
measurements at 77 K that show a bathochromic emission
Fig. 4 Transient absorption spectra of benzophenone (open squares),
6
(open circles), and 17 (closed squares) recorded 100 ms after the laser
pulse at 266 nm in deaerated acetonitrile. Inset: the decay of triplet
benzophenone recorded at 525 nm.
Fig. 2 Numbering scheme of thiopheno azomethines.
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3
8
b-carotene, and methylnaphthalene. Also apparent in the
transient absorption spectra are negative absorptions resulting
from the strong ground state bleaching of the azomethines.
These are mirror images of the ground state absorption spectra
and confirm the transient absorption is much weaker than the
ground state absorption. Even though there is direct evidence
for the triplet state by LFP, this manifold is not preferentially
formed and therefore it is not responsible for deactivating the
singlet state. This is derived from the surprisingly weak
azomethine transient signal. Large triplet extinction coeffi-
intense one at 77 K. Nonetheless, the observed fluorescence
signal decrease at room temperature is from efficient
nonradiative deactivation of the singlet excited state by IC
according to W (77 K) 2 W (RT) # W . This is in contrast
fl
fl
IC
to conventional oligothiophenes, such as 25, that efficiently
3
0–32
populate their triplet state by ISC with WT ¢ 0.8.
Even
though the major deactivation pathway is by IC, a certain
amount of the excited state energy remains unaccounted
for according to the following energy conservation equation:
W + W
fl
+ WIC # 1. The residual excited state energy must
ISC
2
1
21
cm
cients (e
azomethines because of their high degree of conjugation.
Owing to the intense e , a transient signal of equal intensity to
T
)
¢10 000
M
are assumed for the
be dissipated by ISC to the triplet. Since only a weak signal is
seen by LFP, the imine bond must therefore be a good triplet
deactivator leading to rapid and efficient intramolecular self-
quenching of this excited state.
3
9
T
2
1
21
that of benzophenone (e
T
= 8 400 M
T
cm , W = 1) is
expected if the azomethine triplet is produced in significant
amounts. Since both WT and eT contribute to the LFP signal,
the weak azomethine signal obtained under identical experi-
ment conditions as benzophenone (Fig. 4) confirms the
azomethine triplet formation is inefficient. An upper limit of
WT # 0.05 is derived from the LFP signal relative to
benzophenone and is in agreement with the phosphorescence
quantum yields. Evidence for the low triplet efficiency of the
azomethines is further supported by the calculated nonradia-
tive rate constants (knr) that are greater than 25, which is
Cyclic voltammetry
Thiophene azomethines undergo two detectable step-wise
oxidations by cyclic voltammetry. The first oxidation corre-
sponds to a one electron process from the thiophene to form
a radical cation. This is followed by removal of an additional
electron to form a dication. Even through the specific thio-
phene unit that undergoes oxidation cannot be unambiguously
assigned, the first oxidation process consistently occurs
between 0.4 and 1.6 V and is greatly affected by the electronic
groups. Furthermore, the first oxidation step is quasi-
reversible (Fig. 6) unlike homologous aryl azomethines that
exhibit irreversible behavior regardless of the scan rate and
experimental conditions.
3
1
known to efficiently populate its triplet state. The faster knr
values taken together with the weak LFP signal implies that
deactivation of the singlet state occurs predominately by
nonradiative IC and not by a manifold shift to the triplet state
as with conventional thiophenes.
In addition to an oxidation process, the azomethine bond
can potentially be reduced. This process normally involves two
rapid and undistinguishable sequential steps each involving
Evidence for the IC deactivation mode is confirmed by the
relative fluorescence at 77 K that is two and half orders of
magnitude greater than that at room temperature (Fig. 5). All
modes of excited state deactivation involving bond rotation
and diffusion controlled quenching processes are suppressed at
this low temperature leading to Wfl (77 K) ranging between
4
0,41
one electron transfer.
However, no such processes were
observed for any of the studied azomethines either in dichloro-
methane or at large negative potentials in DMF. This is
4
2,43
in accordance with other reductive imine studies
and
0
.53 and 0.97 for the compounds studied. Accurate values for
the low temperature fluorescence yields cannot be derived
because of experimental errors associated with calculating the
weak fluorescence signal at room temperature relative to the
supports the observed robustness of the conjugated
azomethine bond towards chemical reductants. A reversible
six electron reduction was however observed for 6, 10, 15, 17,
and 18, which is assigned to the reduction of the nitro group.
Fig. 5 Fluorescence of 15 at 77 K (closed squares) relative to room
Fig. 6 Typical cyclic voltammogram of 16 in anhydrous dichloro-
2
1
temperature (open squares, magnified 50 times).
4 6
methane at 100 mV s with 0.1 M NBu PF as an electrolyte.
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Conversely, an irreversible reduction was observed at low
azomethine electrochemical reduction was observed under our
experimental conditions, the electrochemical oxidation onset
and the spectroscopically measured HOMO–LUMO energy
gaps can be combined to calculate the LUMO energy level
potentials only for 13 and 20. Chemical reduction of these two
compounds was also possible with NaBH
4
. The irreversible
process is most likely from cyanide ion formation followed by
4
4
complete thiophene decomposition. NMR product studies
support this in which no thiophene or azomethine is detectable
according to LUMO = HOMO 2 E . The resulting values are
g
reported in Table 2. The oxidation potential taken together
after the chemical reduction of 13 and 20 with NaBH
4
in
with the spectroscopic E data provide an accurate representa-
g
methanol and at room temperature.
tion of the HOMO–LUMO energy levels. The resulting energy
The ability of the electronic groups to modify the HOMO/
LUMO levels is apparent from the measured oxidation
potentials. The donating amino group increases the HOMO
level, resulting in a decrease in the oxidation potential. On
average, a 500 mV reduction in the oxidation potential was
observed for the amino containing compounds relative to their
unsubstituted analogues. A pronounced reduction in the
oxidation potential is apparent with the push–push system 16
where the two amino groups combine to increase the HOMO
level. Unfortunately, the ability of the withdrawing group to
decrease the LUMO level is not apparent since the reduction
potentials cannot be measured. This notwithstanding, an
increase in the oxidation potential was observed for the nitro
containing thiophenes and suggests a destabilization of the
HOMO level. The measured HOMO/LUMO values and how
they are influenced by the electronic groups are consistent with
the spectroscopic measurements.
diagram shown in Fig. 7 illustrates the perturbation effect of
the various electronic groups upon the HOMO and LUMO
energy levels. Since the oxidation potential for quasi-irrever-
sible processes is dependent upon the scan rate and hence the
calculated HOMO energy level, the cyclic voltammetry
measurements of all the compounds were performed under
identical scan rates. In general, the azomethine oxidation
potentials and their HOMO–LUMO energy gaps can be
modulated such that they are similar to their carbon analogues
4
6
(23 and 25) and lower than their aryl azomethine homo-
logues (24), making them compatible for use in functional
devices. Moreover, the HOMO level can be tailored courtesy
of the electronic groups such that the energy levels are
compatible with that of ITO in OLEDs, thus making these
compounds potential hole injection layers in working devices.
Compounds 21 and 22 were synthesized as models to
systematically assign the effect of the simple azomethine and
the electronic groups on the oxidation potentials. These model
compounds also help to deconvolute the azomethine oxidation
processes, which have only been partially examined to
The oxidation onset provides the ionization potential (IP)
according to IP = Eox (SCE) + 4.4, where Eox (SCE) is the
onset
onset
4
5
oxidation potential onset in volts versus the SCE electrode.
1
0,43,47
The ionization potentials are obtained from the measured
oxidation potentials by applying a 0.39 V correction factor.
Similarly, the LUMO values are calculated from the electron
date.
The oxidation potentials of 21 and 22 are
mutually identical and they are similar to the other thiopheno
azomethines. This implies the simple azomethine bond exerts
2,3,5,48
affinity (EA) given by the reduction onset potential (Ered
)
little effect on the oxidation potentials of the thiophenes.
onset
according to EA = E or end set(SCE) + 4.4. Since no apparent
Rather, the oxidation potentials are influenced uniquely by the
a
Table 2 Cyclic voltammetry values for the various thiopheno azomethines measured in anhydrous acetonitrile
1
2
1
2
Module
E
pa /V
E
pa /V
E
pc /V
E
pc /V
E
pa(poly)/V
HOMO/2eV
LUMO/2eV
Reagents
1
2
3
4
5
6
7
8
9
0.6
1.0
1.3
1.2
1.0
1.1
0.4
0.8
0.9
1.3
0.7
1.2
1.2
1.0
1.1
0.4
1.0
0.7
0.9
1.2
1.0
1.0
0.9
1.5
1.1
1.1
—
—
1.7
1.4
—
1.0
—
—
—
1.7
—
—
—
—
20.7
—
20.7
—
—
—
—
—
—
—
1.6
—
—
1.6
—
—
—
1.7
—
—
—
—
0.7
—
—
—
2.1
—
—
1.4
—
—
—
—
4.9
5.3
5.7
5.6
5.4
5.5
4.7
5.1
5.2
5.6
5.0
5.5
5.5
5.1
5.4
4.7
5.3
5.0
5.2
5.5
5.3
5.3
—
1.2
2.1
1.5
1.6
2.5
3.2
2.2
2.4
2.0
2.7
2.6
2.6
2.5
2.5
3.0
2.7
3.0
3.1
3.0
2.9
1.1
1.6
—
Aminomonoazomethine
Monoazomethine
—
21.1
—
—
—
—
10
11
12
13
20.6
—
—
21.0
—
20.5
—
20.6
20.7
—
20.9
—
—
—
—
20.9
—
—
21.3
—
21.0
—
21.2
21.1
—
21.2
—
—
—
—
—
Bisazomethine
14
1.3
1.6
1.1
1.4
1.4
1.1
—
—
—
—
—
15
16
17
18
19
20
Model compounds
21
22
23
24
25
b
c
—
—
—
—
d
—
—
—
a
21
b
Values reported are against a Ag/AgCl (sat’d) reference electrode done in CH
57 c
coworkers.
2
Cl
2
and a scan rate of 100 mV s
.
From Fr e` re and
41 d
57
From Rao et al.
From Roncali and coworkers.
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J. Mater. Chem., 2007, 17, 1166–1177 | 1173

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Fig. 7 Schematic representation of the relative HOMO–LUMO energy gaps influenced by different terminal electronic groups.
electronic groups and the degree of conjugation. Unlike with
other thiopheno azomethines, 21 undergoes irreversible
oxidation resulting from radical cation coupling to form a
dimer. The product eventually is deposited as a thin film on
the working electrode upon prolonged oxidation times.
Conversely, the one electron oxidation to generate the radical
cation is quasi-reversible for 22 and represents one of only a
few examples of azomethine quasi-reversible oxidation. The
reversible-like behavior is a result of substitution in the 5 and
difference was observed for the oxidation potentials of 8,
12, and 19 relative to their unalkylated analogues 5, 9, and
14, respectively. This suggests that simple alkylation does
not affect the oxidation potential. The absence of significant
differences in the oxidation potentials validates the
irreversible process for azomethine oxidation and confirms
that the coupling of intermediates occurs preferentially by a–a
coupling via the 5,59 positions. This is further supported by
the lack of coupling products with monoazomethines 10–13
along with the previous electrochemical polymerization of an
59 positions (Fig. 2) that prevents coupling between radical
2
2
cations. This suggests that product formation occurs predomi-
nately via these two positions. The quasi-reversible radical
cation formation is in contrast to other azomethines that
undergo irreversible oxidation regardless of substitution, scan
speeds, and experimental conditions because of the extremely
alkylated derivative of 14.
Crystal structure
Similar to the carbon analogues, two geometric azomethine
isomers are possible; E and Z. Unlike their carbon analogues,
differentiation between the two azomethine isomers is difficult
4
9
reactive radical cation.
coupling was observed only for the compounds that possess
Dimer formation by oxidative
1
2
,50
by H-NMR. The NMR spectra of all the azomethines studied
one substitution in either the 5 or 59 positions.
Conversely,
showed exclusively one imine peak in the 8–9 ppm region
confirming the exclusive formation of one isomer, which is
assumed to be the more thermodynamically stable E isomer.
electrochemically induced polymerization was possible only
2
with 14 owing to its unsubstituted 5,59 positions.
2
To further verify the observed irreversible oxidation was a
result of radical cation coupling via the a,a positions,
compounds 8, 12 and 19 were examined. The terminal methyl
groups of these model compounds suppress a–a oxidative
coupling via the 5,59 positions, but do not prevent any b–b
coupling defects via the 3, 39, 4, and 49 positions. No
electrochemically induced oxidative product coupling was
observed for these compounds, while they exhibit reversible
one electron oxidation. The lack of coupling products for 8
and 12 suggest coupling of the radical cations occurs
exclusively on the thiophene Ring B (Fig. 2) and not on
Ring A, to which the amine is directly bound. This may be
from the electron withdrawing ester that increases the
oxidation potential of thiophene A such that the oxidation
occurs preferentially on Ring B. Furthermore, no significant
5
1
52
The crystal structures obtained for 5, 14 and 16 are all
consistent and confirm that both azomethine bonds in these
structures adopt the E configuration. To confirm the E isomer
did not preferentially crystallize, NMR spectra of the crystals
used for X-ray diffraction and the mother liquor were both
1
measured and they yielded identical H-NMR spectra. This is
further corroborated by repeated crystallization of various
compounds from Chart 1 that consistently yielded the same
crystal structure. Accurate correlation between the solid state
and solution structures can be assumed because similar
absorption spectra were observed for samples both in solution
and in thin film. The latter are known to increase the planarity
of twisted molecules leading to increased conjugation resulting
in bathochromic shifts in the absorption spectra.
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174 | J. Mater. Chem., 2007, 17, 1166–1177
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ortho hydrogen on the homoaryl ring and the azomethine
1
A high
3,55
˚
hydrogen, which are separated only by 2.16 A.
degree of intramolecular hydrogen bonding and intermole-
cular p-stacking further contribute to the observed planar
structures. This is evident from the four contacts present for 16
…
…
…
…
involving H5 S2, H10 S2, H27A S3, and H17A S1.
…
There exists also a CH O intermolecular hydrogen bond
…
between C17–H17 O1 [i = (2x, 2y, 2z)]. The geometric
…
˚
…
parameters of this arrangement are H O 2.34 A, C O
˚
…
3
.198(12) A, C–H O 141u and the pairs are linked by an
inversion center to form dimers. The crystallographic data
furthermore show the azomethine bond distances to be shorter
than those of its carbon analogue (Table 3), which are in part
responsible for the increased planarity and high degree of
5
6
conjugation.
Conclusion
Fig. 8 Schematic representations of the crystal structure of 16, top
view (top) and seen along the a crystal lattice parallel to the thiophene
units (bottom).
The first examples of electronic push–push, pull–pull and
push–pull conjugated thiophene azomethines were presented.
The formation of the highly conjugated azomethine bond is
responsible for displacing the equilibrium in favor of
reductively and hydrolytically resistant new materials.
Through the selective synthesis, different substituents and
electronic groups can be incorporated into the conjugated
framework leading to symmetric and unsymmetric com-
pounds. These electronic groups provide a means to control
the physical properties such as HOMO–LUMO energy gap,
absorption, and color emission because of the inherent
conjugation of the azomethine bond that allows perturbation
of the HOMO and LUMO energy levels. The spectroscopic
and electrochemical properties can be tailored by varying the
number of thiophene units and the nature of the electronic
groups substituted on the thiophenes. Reversible-like oxida-
tion behavior and low potentials make these azomethines
suitable for p-type doping while their linear and planar
conformations impart a high degree of conjugation. Such
properties are particularly useful because they permit fine-
tuning of the various properties of the products formed. This,
in turn, allows for a myriad of possibilities in the application of
these compounds to meet the needs of the electronics industry,
which is not easily achieved using conventional materials.
The crystal structure of 16,{ represented in Fig. 8, clearly
shows the heteroatom units adopt an anti-parallel arrange-
ment. The same anti-parallel arrangement of the sulfur atoms
was also observed with other monoazomethines and bisazo-
1
7,51,53
methines.
The added advantage of the anti-parallel
conformation and the resulting linearity confers a high degree
of conjugation and validates the spectroscopic results. This
arrangement is also ideal for providing high conductivities that
1
7
are expected with doping. The average mean planes of the
terminal thiophenes were found to be twisted 5.8u and 10.6u
from the mean plane of the central thiophene and its two
azomethine bonds. The low twisting angle provides a planar
and linear configuration to the molecule. The measured mean
plane angles for 14 and 16 are in contrast to homoaryl
azomethines that have twisted mean planes varying between
1
2,14,54
5
5u and 65u from the central thiophene.
A 45u angle
between the two thiophene mean planes is also apparent with
azomethines derived from thiophene dicarboxaldehyde such as
1
3,23,55
2
4.
The deviation from planarity is required for the
homoaryl azomethines to avoid steric hindrance between the
¯
, M = 588.79, triclinic, P1, a =
{
1
9
Crystal data: C28
H
30
N
4
O
4
S
3
0.5863(4), b = 10.9741(4), c = 14.5969(5) s, a = 96.217(2), b =
Acknowledgements
3
7.195(2), c = 111.441(2)u, U = 1544.06(10) s , T = 100(2) K, Z = 2,
2
1
m = 2.508 mm , 18808 reflections measured, 2540 unique (Rint
0
=
.056), R1 = 0.1054, wR2 = 0.2912 (I . 2s(I)). CCDC reference
The authors acknowledge financial support from the Natural
Sciences and Engineering Research Council Canada, Fonds de
Recherche sur la Nature et les Technologies, the Centre for
number 626758. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b616379c
Table 3 Selected crystallographic data of thiophene azomethines and a carbon analogue
b
4
c
23
1
16
a
5
Side A
Side B
Side A
Side B
Side A
Side B
d
Plane angle
7.3u
1.283 A
9.1u
1.277 A
25.3u
1.286 A
10.6u
1.296 A
5.8u
1.282 A
0u
1.278 A
6u
1.439 A
e
˚
˚
˚
˚
˚
˚
˚
–
CLN–
e
˚
˚
˚
˚
˚
˚
˚
LN–Aryl–
1.351 A
1.381 A
1.393 A
1.363 A
1.374 A
1.585 A
1.312 A
˚
1.426 A
˚
1.443 A
52
˚
1.435 A
56 d
˚
1.432 A
˚
1.418 A
˚
1.614 A
˚
1.430 A
LCH–Aryl–
From Skene et al.
terminal thiophene units. For 23, the value refers to the –CLC– bond distance.
a
51
b
c
From Dufresne et al.
From Zobel.
Refers to the mean plane angle between the central thiophene and the
e
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36, 2712–2720; (e) N. Giuseppone and J.-M. Lehn, Polym. Mater.:
Sci. Eng., 2004, 90, 725.
6 N. Giuseppone, J.-L. Schmitt, E. Schwartz and J.-M. Lehn, J. Am.
Chem. Soc., 2005, 127, 5528–5539.
27 (a) N. Giuseppone and J.-M. Lehn, Chem.–Eur. J., 2006, 12,
Self-Assembled Chemical Structures, and Canada Foundation
for Innovation. Gratitude is extended to Dr M. Simard for
assistance with the crystal structure analyses and to Prof.
D. Zargarian for helpful suggestions. M.B. thanks the
Universit e´ de Montr e´ al for a graduate scholarship.
2
1
2
723–1735; (b) N. Giuseppone and J.-M. Lehn, Chem.–Eur. J.,
006, 12, 1715–1722.
2
8 J. F. A. Van Der Looy, G. J. H. Thys, P. E. M. Dieltiens,
D. De Schrijver, C. Van Alsenoy and H. J. Geise, Tetrahedron,
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