Imidazole as a central π-linkage in Y-shaped push-pull chromophores

Article abstract of DOI:10.1016/j.dyepig.2010.11.004

Twelve imidazole based Y-shaped push-pull chromophores have been designed, synthesised and fully characterised featuring 4,5-bis(N,N-dimethylanilino) imidazole as a donor moiety, a systematically enlarged π-linker and nitro and cyano groups as acceptors.

Full text of DOI:10.1016/j.dyepig.2010.11.004

Dyes and Pigments 90 (2011) 48e55
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Imidazole as a central
p-linkage in Y-shaped pushepull chromophores
a
,
b
c
a
ꢀa
ꢀ
ꢀ
ꢀ
*
ꢀ
Jirí Kulhánek , Filip Bures , Tomás Mikysek , Jirí Ludvík , Oldrich Pytela
a Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, Pardubice 53210, Czech Republic
b Department of Analytical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, Pardubice 53210, Czech Republic
c
ꢀ
J. Heyrovsky Institute of Physical Chemistry, Academy of Science of the Czech Republic, v.v.i., Dolejskova 3, Prague 8 18223, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Twelve imidazole based Y-shaped pushepull chromophores have been designed, synthesised and
fully characterised featuring 4,5-bis(N,N-dimethylanilino)imidazole as a donor moiety, a systematically
Received 13 September 2010
Received in revised form
4 November 2010
Accepted 8 November 2010
Available online 18 November 2010
enlarged
p-linker and nitro and cyano groups as acceptors. The synthesised pushepull systems have
been studied by electrochemistry, UV/Vis and IR spectroscopy and nonlinear optical properties have been
calculated. Moreover, quantitative relationships between the measured and calculated properties and the
structural features have also been evaluated. Electrochemical and spectral properties have mainly been
affected by the presence of strongly conjugating acceptors, large
the chromophore.
p
-linkers and the spatial arrangement of
Keywords:
Chromophore
Imidazole
Ó 2010 Elsevier Ltd. All rights reserved.
Pushepull system
Electrochemistry
UVeVis spectroscopy
Nonlinear optics (NLO)
1. Introduction
chemical robustness, solubility and being available in reasonable
quantities (nonlinearityetransparencyesolubilityethermal stability
trade-off) [12]. Hence, various heteroaromatics have recently been
In the last several years, organic dipolar molecules that are end-
capped with electron donors and acceptors have been intensively
investigated by organic and material chemists. In contrast to inor-
ganic materials, such organic materials with readily polarisable
pushepull systems were recognized as tuneable chromophores for
nonlinear optics (NLO) and have found widespread application as
two-photon absorbing devices, opto-electronic and optical data
storage devices, organic light-emitting diodes (OLED) and organic
used for the construction of robust
p-backbones in NLO active
compounds. Among them, imidazoles [13e17] and benzimidazoles
[18e21] have proved to be sufficiently efficient and robust five-
membered aromatic systems for such purposes. The imidazole
molecule can be substituted with donor and acceptor auxiliaries at
C2 and C4/C5 (or vice versa) to form Y-shaped chromophores [22,23].
Recently, we reported the synthesis and properties of several
photovoltaic cells [1e4]. A typical organic De
consists of strong electron donors (D ¼ NR2 or OR groups) and
-conjugated
system [5e7]. Optical linear and nonlinear properties of these
molecules depend on the polarisability of the electrons localized
p
eA chromophore
De
p
eA chromophores featuring imidazole (1e2) and 2,4,5-tri-
-conjugated linkages [24e26]. Both
phenylimidazole (3) as simple
p
acceptors (A ¼ NO2 or CN groups) connected by a
p
linear and branched chromophores 1e2 consist of 4,5-dicyanoi-
midazole acceptor moieties substituted at C2 with N,N-dimethy-
lamino donor groups, separated by a systematically extended
in
isability depends mainly on its chemical structure, in particular on
the length of the -conjugated spacer and the electronic nature
of the donors and acceptors attached. The rational design of a typical
chromophore involves finding an optimal -conjugated linkage in
addition to appropriate donors and acceptors [10,11]. Successful
application and fabrication of such molecules rely on the chromo-
phore possessing large hyperpolarisability, good thermal and
p
-bonding molecular orbitals [8,9]. The chromophore polar-
p-conjugated linker. Chromophores 3, featuring various acceptors
and donors in both orientations, were readily accessible from
aldehydes and benzils. In the present work, we have combined both
approaches and report the synthesis and properties of imidazole
Y-shaped chromophores 4e9 bearing N,N-dimethylanilino (DMA)
donors and nitro and cyano acceptor groups. In contrast to their
structural analogs 1e3, the newly proposed chromophores 4e9
possess donors at C4/C5 and acceptors at C2, separated by an
p
p
additional
p-linker (Fig. 1). We report herein the synthesis, full
spectral characterisation and structureeproperty relationships of
chromophores 4e9.
* Corresponding author. Tel.: þ420 46 603 7099; fax: þ420 46 603 7068.
ꢀ
E-mail address: filip.bures@upce.cz (F. Bures).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.11.004

J. Kulhánek et al. / Dyes and Pigments 90 (2011) 48e55
49
Fig. 1. Molecular structure of the previous and new imidazole chromophores.
2. Experimental
a sealed glass pressure tube at 150 ꢀC for 24 h. The resulting mixture
was poured over ice/water, neutralised with ammonia, and the
precipitate was separated via filtration. The crude product was
purified by column chromatography (SiO2; CH2Cl2/acetone 95:5).
2.1. General
Reagents and solvents were reagent-grade and were purchased
from Penta, Aldrich, and Acros and used as received. Starting aldehydes
10a, 10b and 11a, (triphenylphosphoranylidene)acetaldehyde, 4-bro-
mobenzonitrile, 4-bromonitrobenzene, 4-formylphenylboronic acid,
vinylboronic acid pinacol ester, 4-ethynylbenzonitrile, 1-ethynyl-
4-nitrobenzene and 4-iodobenzaldehyde were commercially avail-
able. Aldehydes 11b (63%), 12a (41%) and 12b (64%) were synthesised
from 10a and 10b according to literature and obtained in the indicated
yields [27]. Aldehydes 13a (85%), 13b (89%), 14a (42%), 14b (40%), 15a
(93%) and 15b (91%) were synthesised by Suzuki-Miyaura, Heck
and Sonogashira cross-coupling reactions [28]. The condensation
reactions were carried out in a sealed glass pressure tube (Aldrich).
Column chromatography was carried out with silica gel 60 (particle
size 0.040e0.063 mm, 230e400 mesh; Merck) and commercially
available solvents. Thin-layer chromatography (TLC) was conducted on
aluminium sheets coated with silica gel 60 F254, obtained from Merck,
with visualisation by UV lamp (254 or 360 nm). Melting points (mp)
were measured on a Büchi B-540 melting-point apparatus in open
capillaries and are uncorrected.1H and 13C NMR spectra were recorded
at 400 and 100 MHz, respectively, with a Bruker AVANCE 400 instru-
ment at 25 ꢀC. Chemical shifts are reported inppm relative to the signal
2.2.1.1. 2-(4-Nitrophenyl)-4,5-bis[4-(N,N-dimethylamino)phenyl]-1H-
imidazole (4a). The title compound was synthesised from 4-nitro-
benzaldehyde 10a (302 mg) following the general procedure.
Obtained 504 mg (59%) of a red solid; mp 105e106 ꢀC (lit. [26] mp
104e106 ꢀC). NMR, IR and MS spectra were found to be identical to
those described in literature [26].
2.2.1.2. 2-(4-Cyanophenyl)-4,5-bis[4-(N,N-dimethylamino)phenyl]-1H-
imidazole (4b). The title compound was synthesised from 4-cyano-
benzaldehyde 10b (262 mg) following the general procedure.
Obtained 592 mg (73%) of a yellow solid; mp 123e125 ꢀC (lit. [26] mp
124e126 ꢀC). NMR, IR and MS spectra were found to be identical to
those described in literature [26].
2.2.1.3. (E)-2-(4-Nitrostyryl)-4,5-bis[4-(N,N-dimethylamino)phenyl]-
1H-imidazole (5a). The title compound was synthesised from (E)-
3-(4-nitrophenyl)prop-2-enal 11a (354 mg) following the general
procedure. Obtained 327 mg (36%) of a dark red solid; Rf ¼ 0.20
(SiO2; CH2Cl2/acetone 95:5); mp 163e167 ꢀC. 1H NMR (DMSO-d6,
400 MHz):
d
¼ 2.96 (br s, 12H, 2 ꢁ N(CH3)2), 6.75 (d, 4H, J ¼ 8.4 Hz,
1
of Me4Si. The residual solvent signal in the H and 13C NMR spectra
DMA), 7.32 (d, 1H, J ¼ 16.4 Hz, CH]), 7.40 (d, 4H, J ¼ 8.4 Hz, DMA),
7.51 (d,1H, J ¼ 16.4 Hz, CH]), 7.84 (d, 2H, J ¼ 8.8 Hz, Ph), 8.25 (d, 2H,
J ¼ 8.8 Hz, Ph), 12.48 (br s, 1H, NH). 13C NMR (DMSO-d6, 100 MHz):
was used as an internal reference (CDCl3 e 7.25 and 77.23 ppm;
DMSO-d6 e 2.55 and 39.51 ppm). Apparent resonance multiplicities
are described as s (singlet), br s (broad singlet), d (doublet) and m
(multiplet). Protons of the N,N-dimethylanilino group at C4/C5 were
marked as DMA. Signals of some carbons in the 13C NMR spectra were
not observed as a result of 1H-imidazole tautomerism (averaged broad
signals e br) [29]. IR spectra were recorded on a Perkin Elmer FT-IR
Spectrum BX spectrometer. Mass spectra were measured on a GC/MS
configuration comprised of an Agilent Technologies e 6890N gas
chromatograph equipped with a 5973 Network MS detector (EI 70 eV,
mass range 33e550 Da) or on an LC-MS Micromass Quattro Micro API
(Waters) instrument with a direct input (ESIþ, CH3OH, mass range
200e800 Da). Elemental analyses were performed on an EA 1108
Fisons instrument. UV/Vis spectrawere recorded on a Hewlett-Packard
8453 spectrophotometer in CH2Cl2.
d
¼ 40.41, 112.49, 122.39, 124.62, 126.28, 127.43, 128.71 (br), 143.77,
144.27, 146.40, 149.83 (br). ESI-MS: m/z ¼ 454 (M þ 1)þ, 476
(M þ 23)þ. IR (neat):
n
¼ 2795, 1611, 1587, 1513 (NO2), 1477, 1440,
1332 (NO2), 1198, 1105, 943, 815, 748 cmꢂ1. Anal. calcd. for
C27H27N5O2 (453.54): C 71.50, H 6.00, N 15.44; found: C 71.55, H
6.04, N 15.32.
2.2.1.4. (E)-2-(4-Cyanostyryl)-4,5-bis[4-(N,N-dimethylamino)phenyl]-
1H-imidazole (5b). The title compound was synthesised from (E)-
3-(4-cyanophenyl)prop-2-enal 11b (314 mg) following the general
procedure. Obtained 269 mg (31%) of a red solid; Rf ¼ 0.18 (SiO2;
CH2Cl2/acetone 95:5); mp 142e144 ꢀC. 1H NMR (DMSO-d6,
400 MHz):
d
¼ 2.96 (br s, 12H, 2 ꢁ N(CH3)2), 6.75 (d, 4H, J ¼ 8.4 Hz,
DMA), 7.27 (d, 1H, J ¼ 16.4 Hz, CH]), 7.39 (d, 4H, J ¼ 8.4 Hz, DMA),
7.46 (d, 1H, J ¼ 16.4 Hz, CH]), 7.78 (d, 2H, J ¼ 8.4 Hz, Ph), 7.86 (d, 2H,
J ¼ 8.8 Hz, Ph), 12.47 (br s, 1H, NH). 13C NMR (DMSO-d6, 100 MHz):
2.2. Synthesis
2.2.1. General procedure for the condensation of aldehydes with
d
¼ 40.00, 109.26, 112.02, 119.06, 120.85, 126.52, 126.85, 128.21 (br),
4,40-bis(N,N-dimethylamino)benzil (chromophores 4e9)
132.69, 141.55, 143.30, 149.33. ESI-MS: m/z ¼ 434 (M þ 1)þ, 456
A
mixture of 4,40-bis(N,N-dimethylamino)benzil (592 mg;
(M þ 23)þ. IR (neat):
¼ 2794, 2220 (CN),1611,1597,1519,1479,1440,
n
2.0 mmol), aldehyde 10e15 (2.0 mmol) and ammonium acetate
(1.5 g; 19.5 mmol) was heated in glacial acetic acid (30 mL) in
1349,1170, 943, 812 cmꢂ1. Anal. calcd. for C28H27N5 (433.55): C 77.57,
H 6.28, N 16.15; found: C 77.13, H 6.43, N 16.04.

50
J. Kulhánek et al. / Dyes and Pigments 90 (2011) 48e55
2.2.1.5. 2-[(1E,3E)-4-(4-Nitrophenyl)buta-1,3-dienyl]-4,5-bis[4-(N,N-
dimethylamino)phenyl]-1H-imidazole (6a). The title compound was
synthesised from (2E,4E)-5-(4-nitrophenyl)penta-2,4-dienal 12a
(406 mg) following the general procedure. Obtained 489 mg (51%)
of a dark red solid; Rf ¼ 0.19 (SiO2; CH2Cl2/acetone 95:5); mp
general procedure. Obtained 318 mg (30%) of a red solid; Rf ¼ 0.36
(SiO2; CH2Cl2/acetone 95:5); mp 165e166 ꢀC. 1H NMR (DMSO-d6,
400 MHz):
d
¼ 2.94 (s, 6H, N(CH3)2), 3.00 (s, 6H, N(CH3)2), 6.72 (d, 2H,
J ¼ 8.4 Hz, DMA), 6.82 (d, 2H, J ¼ 8.4 Hz, DMA), 7.38 (d, 2H, J ¼ 8.4 Hz,
DMA), 7.47 (d, 2H, J ¼ 8.4 Hz, DMA), 7.53 (d, 1H, J ¼ 16.4 Hz, CH]),
7.62 (d, 1H, J ¼ 16.4 Hz, CH]), 7.80 (d, 2H, J ¼ 8.4 Hz, Ph), 7.93 (d, 2H,
J ¼ 8.4 Hz, Ph), 8.14 (d, 2H, J ¼ 8.4 Hz, Ph), 8.30 (d, 2H, J ¼ 8.4 Hz, Ph),
157e160 ꢀC. 1H NMR (DMSO-d6, 400 MHz):
d
¼ 2.96 (br s, 12H,
2 ꢁ N(CH3)2), 6.64e6.86 (m, 5H, DMA þ CH]), 6.90 (d, 1H,
J ¼ 15.6 Hz, CH]), 7.25e7.50 (m, 6H, DMA þ CH]), 7.80 (d, 2H,
J ¼ 8.4 Hz, 2H, Ph), 8.24 (d, 2H, J ¼ 8.4 Hz, 2H, Ph), 12.35 (br s, 1H,
12.44 (br s, 1H, NH). 13C NMR (DMSO-d6, 100 MHz):
d
¼ 40.01,
112.05, 112.08, 118.82, 123.66, 124.10, 125.12, 126.21, 127.25, 127.49,
127.71, 129.06, 130.91, 135.44, 136.89, 143.76, 144.16, 146.12, 148.98,
149.64. ESI-MS: m/z ¼ 530 (M þ 1)þ, 552 (M þ 23)þ. IR (neat):
NH). 13C NMR (DMSO-d6, 100 MHz):
d
¼ 40.05, 122.66, 127.03,
128.21 (br), 128.66, 128.85, 129.91, 132.85, 134.25, 136.67, 143.62,
143.82, 144.16, 145.83, 149.30. ESI-MS: m/z ¼ 480 (M þ 1)þ. IR
n
¼ 2794, 2924, 1736, 1614, 1590, 1526, 1505 (NO2), 1440, 1337 (NO2),
(neat):
n
¼ 2912, 1610, 1578, 1508 (NO2), 1474, 1330 (NO2), 1224,
1186, 1108, 964, 945, 846, 816, 750, 696 cmꢂ1. Anal. calcd. for
C33H31N5O2 (529.63): C 74.84, H 5.90, N 13.22; found: C 74.93, H 6.02,
N 14.18.
1104, 984, 942, 816, 744 cmꢂ1. Anal. calcd. for C29H29N5O2 (479.57):
C 72.63, H 6.10, N 14.60; found: C 72.62, H 6.19, N 14.46.
2.2.1.6. 2-[(1E,3E)-4-(4-Cyanophenyl)buta-1,3-dienyl]-4,5-bis[4-(N,N-
dimethylamino)phenyl]-1H-imidazole (6b). The title compound was
synthesised from (2E,4E)-5-(4-cyanophenyl)penta-2,4-dienal 12b
(366 mg) following the general procedure. Obtained 291 mg (32%) of
a dark red solid; Rf ¼ 0.19 (SiO2; CH2Cl2/acetone 95:5); mp
2.2.1.10. (E)-2-[4-(4-Cyanostyryl)phenyl]-4,5-bis[4-(N,N-dimethyla-
mino)phenyl]-1H-imidazole (8b). The title compound was syn-
thesised from (E)-4-(4-cyanostyryl)benzaldehyde 14b (466 mg)
following the general procedure. Obtained 255 mg (25%) of a red
solid; Rf ¼ 0.32 (SiO2; CH2Cl2/acetone 95:5); mp 165e168 ꢀC. 1H
161e163 ꢀC. 1H NMR (DMSO-d6, 400 MHz):
d
¼ 2.94 (s, 6H, N(CH3)2),
NMR (DMSO-d6, 400 MHz):
d
¼ 2.96 (br s, 12H, 2 ꢁ N(CH3)2), 6.78
2.98 (s, 6H, N(CH3)2), 6.69e6.81 (m, 6H, DMA þ CH]), 7.23e7.43 (m,
(br s, 4H, DMA), 7.43 (br s, 4H, DMA), 7.45 (d, 1H, J ¼ 16.4 Hz, CH]),
7.56 (d, 1H, J ¼ 16.4 Hz, CH]), 7.67 (d, 2H, J ¼ 8.4 Hz, Ph), 7.84e7.90
(m, 4H, Ph), 8.13 (d, 2H, J ¼ 8.4 Hz, Ph), 12.44 (br s, 1H, NH). 13C NMR
6H, DMA þ CH]), 7.75e7.85 (m, 4H, Ph),12.30 (br s,1H, NH).13C NMR
(DMSO-d6, 100 MHz):
d
¼ 40.09 (br), 109.05, 112.00 (br), 119.09,
123.56, 126.91, 127.81, 128.56, 130.38, 132.53, 133.12, 135.96, 137.45,
(DMSO-d6, 100 MHz):
d
¼ 39.99, 109.38, 112.06, 119.09, 121.11,
141.95, 143.65, 149.01, 149.56. ESI-MS: m/z ¼ 460 (M þ 1)þ, 482
125.10, 126.66, 127.10, 127.33,128.36 (br), 131.81, 132.65, 135.56 (br),
(M þ 23)þ. IR (neat):
n
¼ 2920, 2218 (CN),1748,1591,1508,1474,1340,
141.96,143.80 (br),149.32 (br). ESI-MS: ESI-MS: m/z ¼ 510 (M þ 1)þ,
1162,1055,985,941, 815,668cmꢂ1. Anal. calcd.forC30H29N5 (459.58):
C 78.40, H 6.36, N 15.24; found: C 78.06, H 6.46, N 15.01.
532 (M þ 23)þ. IR (neat):
n
¼ 2920, 2219 (CN), 1734, 1558, 1521,
1506, 1457, 1362, 1227, 1205, 1052, 978, 817, 668 cmꢂ1. Anal. calcd.
for C34H31N5 (509.64): C 80.13, H 6.13, N 13.74; found: C 80.01, H
6.02, N 13.66.
2.2.1.7. 2-(40-Nitrobiphenyl-4-yl)-4,5-bis[4-(N,N-dimethylamino)phenyl]
-1H-imidazole (7a). The title compound was synthesised from 40-
nitrobiphenyl-4-carbaldehyde 13a (454 mg) following the general
procedure. Obtained 463 mg (46%) of a dark red solid; Rf ¼ 0.22
(SiO2; CH2Cl2/acetone 95:5); mp 159e162 ꢀC. 1H NMR (DMSO-d6,
2.2.1.11. 2-{4-[(4-Nitrophenyl)ethynyl]phenyl}-4,5-bis[4-(N,N-dime-
thylamino)phenyl]-1H-imidazole (9a). The title compound was
synthesised from 4-[(4-nitrophenyl)ethynyl]benzaldehyde 15a
(502 mg) following the general procedure. Obtained 232 mg (22%)
400 MHz):
d
¼ 2.97 (br s, 12H, 2 ꢁ N(CH3)2), 6.77 (br s, 4H, DMA),
7.43 (br s, 4H, DMA), 7.96 (d, 2H, J ¼ 8.4 Hz, Ph), 8.10 (d, 2H, J ¼ 8.4 Hz,
Ph), 8.25 (d, 2H, J ¼ 8.4 Hz, Ph), 8.37 (d, 2H, J ¼ 8.4 Hz, Ph),12.54 (br s,
of
a
red solid; Rf
¼
0.48 (SiO2; CH2Cl2/acetone 95:5); mp
¼ 2.94 (s, 6H, N
262e264 ꢀC. 1H NMR (DMSO-d6, 400 MHz):
d
1H, NH). 13C NMR (DMSO-d6, 100 MHz):
d
¼ 40.54, 112.55, 124.63,
(CH3)2), 3.00 (s, 6H, N(CH3)2), 6.72 (d, 2H, J ¼ 8.4 Hz, DMA), 6.82 (d,
2H, J ¼ 8.4 Hz, DMA), 7.37 (d, 2H, J ¼ 8.4 Hz, DMA), 7.46 (d, 2H,
J ¼ 8.4 Hz, DMA), 7.75 (d, 2H, J ¼ 8.4 Hz, Ph), 7.89 (d, 2H, J ¼ 8.4 Hz,
Ph), 8.17 (d, 2H, J ¼ 8.4 Hz, Ph), 8.34 (d, 2H, J ¼ 8.4 Hz, Ph), 12.54 (br
125.95, 127.98, 128.30 (br), 129.39 (br), 131.69, 136.89, 143.89,
146.48, 147.00. ESI-MS: m/z ¼ 504 (M þ 1)þ, 526 (M þ 23)þ. IR
(neat):
n
¼ 2920, 1609, 1591, 1505 (NO2), 1488, 1439, 1336 (NO2),
1107, 1056, 942, 815, 731 cmꢂ1. Anal. calcd. for C31H29N5O2(503.59):
C 73.93, H 5.80, N 13.91; found: C 73.87, H 5.82, N 13.78.
s, 1H, NH). 13C NMR (DMSO-d6, 100 MHz):
d
¼ 40.21, 112.07, 124.02,
124.98, 127.72, 129.07, 129.25, 129.30, 132.12, 132.16, 132.18, 132.54
(low solubility). ESI-MS: m/z ¼ 528 (M þ 1)þ, 550 (M þ 23)þ. IR
2.2.1.8. 2-(40-Cyanobiphenyl-4-yl)-4,5-bis[4-(N,N-dimethylamino)phenyl]
-1H-imidazole (7b). The title compound was synthesised from 40-
cyanobiphenyl-4-carbaldehyde 13b (414 mg) following the general
procedure. Obtained 368 mg (38%) of a yellow solid; Rf ¼ 0.28 (SiO2;
CH2Cl2/acetone 95:5); mp 170e173 ꢀC. 1H NMR (DMSO-d6,
(neat):
n
¼ 2920, 2850, 1738, 1613, 1590, 1505 (NO2), 1439, 1338
(NO2), 1226, 1166, 1105, 946, 851, 840, 812, 746, 686 cmꢂ1. Anal.
calcd. for C33H29N5O2 (527.62): C 75.12, H 5.54, N 13.27; found: C
75.29, H 5.61, N 13.12.
400 MHz):
d
¼ 2.97 (br s, 12H, 2 ꢁ N(CH3)2), 6.77 (br s, 4H, DMA),
2.2.1.12. 2-{4-[(4-Cyanophenyl)ethynyl]phenyl}-4,5-bis[4-(N,N-dime-
thylamino)phenyl]-1H-imidazole (9b). The title compound was
synthesised from 4-[(4-cyanophenyl)ethynyl]benzaldehyde 15b
(463 mg) following the general procedure. Obtained 254 mg (25%)
of a red solid; Rf ¼ 0.42 (SiO2; CH2Cl2/acetone 95:5); mp 162e165 ꢀC.
7.43 (br s, 4H, DMA), 7.91 (d, 2H, J ¼ 8.0 Hz, Ph), 7.97e8.03 (m, 4H,
Ph), 8.23 (d, 2H, J ¼ 8.4 Hz, Ph), 8.37 (d, J ¼ 8.0 Hz, Ph), 12.50 (br s,
1H, NH). 13C NMR (DMSO-d6, 100 MHz):
d
¼ 40.07, 109.88, 112.05,
118.94, 125.45, 127.27, 128.45 (br), 130.95, 132.86, 136.91, 143.50,
144.02, 149.30. ESI-MS: m/z ¼ 484 (M þ 1)þ, 506 (M þ 23)þ. IR
1H NMR (DMSO-d6, 400 MHz):
d
¼ 2.94 (s, 6H, N(CH3)2), 3.00 (s, 6H,
(neat):
n
¼ 2794, 2223 (CN), 1736, 1602, 1525, 1494, 1439,
N(CH3)2), 6.72 (d, 2H, J ¼ 8.8 Hz, DMA), 6.81 (d, 2H, J ¼ 8.8 Hz, DMA),
7.37 (d, 2H, J ¼ 8.8 Hz, DMA), 7.46 (d, 2H, J ¼ 8.8 Hz, DMA), 7.71 (d,
2H, J ¼ 8.4 Hz, Ph), 7.80 (d, 2H, J ¼ 8.4 Hz, Ph), 7.96 (d, 2H, J ¼ 8.4 Hz,
Ph), 8.17 (d, 2H, J ¼ 8.4 Hz, Ph), 12.52 (br s,1H, NH). 13C NMR (DMSO-
1350, 1225, 1124, 1058, 943, 811, 744 cmꢂ1. Anal. calcd. for
C32H29N5 (483.61): C 79.47, H 6.04, N 14.48; found: C 79.43, H 6.30,
N 14.18.
d6, 100 MHz):
d
¼ 39.98, 88.88, 93.62, 110.84, 112.01, 118.46, 120.02,
2.2.1.9. (E)-2-[4-(4-Nitrostyryl)phenyl]-4,5-bis[4-(N,N-dimethylamino)
phenyl]-1H-imidazole (8a). The title compound was synthesised from
(E)-4-(4-nitrostyryl)benzaldehyde 14a (507 mg) following the
123.46 (br), 124.93, 127.25, 127.68 (br), 129.01 (br), 131.33, 132.01,
132.06, 132.63, 143.18, 148.98, 149.67. ESI-MS: m/z ¼ 508 (M þ 1)þ,
530 (M þ 23)þ. IR (neat):
¼ 2790, 2220 (CN),1736,1611, 1597, 1524,
n

J. Kulhánek et al. / Dyes and Pigments 90 (2011) 48e55
51
1503, 1438, 1347, 1224, 1162, 1133, 1058, 942, 837, 811, 734 cmꢂ1
.
Anal. calcd. for C34H29N5 (507.63): C 80.45, H 5.76, N 13.80; found: C
80.36, H 5.87, N 13.99.
13a and 13b with central biphenyl
p
-linkers were synthesised by a
PdCl2(PPh3)2-catalysed Suzuki-Miyaura cross-coupling reaction of
4-nitro- and 4-cyanobromobenzenes with 4-formylphenylboronic
acid. Direct Heck cross-coupling of the starting 4-nitro- and 4-
cyanobromobenzenes with vinylboronic acid pinacol ester afforded
4-substituted styryldioxaborolanes 16a and 16b. These intermedi-
ates were further treated with 4-iodobenzaldehyde in a Suzuki-
Miyaura reaction to yield the desired aldehydes 14a and 14b with
the central (E)-phenylethenylphenyl
coupling of 1-ethynyl-4-nitrobenzene or 4-ethynylbenzonitrile
with 4-iodobenzaldehyde produced the final aldehydes 15a and
15b, with the central phenylethynylphenyl p-linker.
With the systematically extended aldehydes 10e15 in hand, the
aforementioned condensation of 4,40-bis(N,N-dimethylamino)
benzil afforded chromophores 4e9 as two series, a and b (Scheme
1, Table 1). Target imidazoles 4e9 were isolated in the yields of
22e73%. In some cases, the starting benzil was recovered as a main
impurity. All of the newly synthesised compounds were fully
characterised by 1H and 13C NMR, IR, EI(ESI)-MS and elemental
analyses.
2.3. Electrochemistry
Electrochemical measurements were carried out in acetonitrile
containing 0.1 M Bu4NPF6 in a three-electrode cell using dc-polar-
ography, voltammetry on Pt-rotating disk electrode (RDE) and
cyclic voltammetry (CV). The working electrode was a mercury
drop for reduction experiments and a platinum disc (2 mm in
diameter) for oxidation experiments. A saturated calomel electrode
(SCE) separated by a bridge filled with acetonitrile/Bu4NPF6 and Pt
wire was used as the reference and auxiliary electrodes. All
potentials are given vs. SCE. Voltammetric measurements were
performed using a potentiostat PGSTAT 30 (AUTOLAB, Ecochemie,
Utrecht, The Netherlands) operated via GPEs 4.8 software. All
polarographic and voltammetric data were fully consistent, and,
therefore, only anodic peak potentials (Ep,a) and cathodic peak
potentials (Ep,c) for oxidation and reduction measured by CV were
further considered.
p-linker. Sonogashira cross-
3.2. Electrochemistry
2.4. Quantum and statistical calculations
Electrochemical investigations of chromophores 4e9 were
carried out in acetonitrile by cyclic voltammetry using a platinum
stationary disk electrode (scan rates 50e1000 mV/s) and a plat-
inum rotating disk electrode (various frequencies of rotation). The
reduction studies were performed by dc-polarography (dropping
mercury electrode, DME) and cyclic voltammetry (hanging mercury
drop electrode, HMDE). The acquired data is summarized in Table 1
and representative voltammograms of compounds 4a and 4b are
shown in Fig. 2.
Initial geometries of the compounds 4e9 have been calculated
by the PM3 method (ArgusLab, [30]) and subsequently optimised
by the PM6 method (MOPAC2009, [31]). The HOMO and LUMO
energies and average second-polarisabilities
lated by employing MOPAC2009.
b were further calcu-
3. Results and discussion
3.1. Synthesis
Whereas the electrochemical oxidation processes of chromo-
phores 4e9 are most probably localized on the donor N,N-dime-
thylanilino moieties, their reductions involve the acceptor NO2 and
The retro-synthetic strategy leading to target chromophores
4e9 involves facile and one-pot condensation of 4,40-bis(N,N-
dimethylamino)benzil with extended aldehydes in the presence of
ammonium acetate in glacial acetic acid (Scheme 1) similar to that
used for the construction of chromophore 3 [26].
Whereas the starting benzil was commercially available,
extended aldehydes 11e15 needed to be synthesised (Scheme 2).
One-pot treatment of the commercially available 4-nitro and
4-cyanobenzaldehydes 10a and 10b with either 1 or 2 equivalents
of (triphenylphosphoranylidene)acetaldehyde afforded corre-
sponding cinnamaldehydes 11a and 11b or (2E,4E)-5-(4-nitro
(cyano)phenyl)penta-2,4-dienals 12a and 12b [27]. We have
recently reported a convenient synthesis for preparing donor-
CN groups in addition to the p-conjugated system. The first anodic
potentials Ep,a, measured by CV, range from 0.435 to 0.515 V for
series a (NO2) and from 0.435 to 0.506 V for series b (CN). It is
evident that the electronic nature of the appended substituents
influences the first oxidation potential only negligibly. Overall,
three anodic processes were observed. The first two steps are
reversible, diffusion controlled, one-electron oxidations and the
third process is an irreversible oxidation accompanied by electrode
inhibition (Fig. 2). With the same number of donors and acceptors,
the changes in Ep,a are clearly caused by the length and spatial
arrangement of the used
p-linker. The measured anodic peak
potential Ep,a decreases with the elongation of the
p
-linker (see
substituted
p
-conjugated linkers as building blocks for modular
chromophores 4e5e6) and increasing its planarity e e.g., compare
chromophores 4 and 5 or 7 and 8. Whereas chromophores 4 and 7
possess the 1,4-phenylene moiety connected directly to imidazole
chemistry [28]. This synthetic approach was further used for the
construction of acceptor-substituted aldehydes 13e15. Aldehydes
Scheme 1. Synthesis of chromophores 4e9.

52
J. Kulhánek et al. / Dyes and Pigments 90 (2011) 48e55
Scheme 2. Synthetic approach towards aldehydes 10e15 with a systematically extended
p-conjugated path.
C2 (a typical torsion angle for 2-phenylimidazoles is about 30ꢀ
[32]), chromophores 5 and 8 are more planar by an additional CC
double bond localized either between the imidazole C2 and the 1,4-
phenylene unit (5) or between two 1,4-phenylene units in 8.
The cathodic peak potentials Ep,c, measured by CV, range from
ꢂ0.992 to ꢂ1.104 V for series a and from ꢂ1.628 to ꢂ2.022 V for
series b. The first cathodic reductions are mostly irreversible (or
quasi-reversible) one-electron processes. The reduction of nitro-
substituted chromophores (series a) is facilitated as a result of the
reduced transfer of electron density from the DMA donors to the
Compared to series a, the electrochemical reduction of cyano-
substituted chromophores in series b occurred at more negative
potentials, approximately 0.7e0.9 V. However, the general trend in
the first reduction potentials for series b is analogous to that for
series a. Thus, elongation and planarisation of the
p-conjugated
path caused a reduction potential shift with the magnitude of
variation decreasing in the following order: 9, 6, 5, 8, 7, 4 for series
a and 6, 5, 8, 9, 7, 4 for series b. The only difference in the position of
the first reduction of chromophore 9a was discussed above.
A comparison of the measured electrochemical data obtained
for chromophore series 1 (Fig. 1, [24]) and series 4e9 seems to be
reasonable at this point. Both chromophore series differ mainly in
the orientation of acceptors and donors appended to the imidazole
ring. Whereas the first oxidation potentials of imidazole-separated
DMA groups in series 4e9 range from 0.435 to 0.515 V, the first
reduction potentials showed significant changes from ꢂ0.992 to
ꢂ2.022 V. In contrast to chromophores 4e9, chromophore series 1
nitro acceptor with increasing length and planarity of the
Among chromophores 4ae8a, chromophore 6a, featuring a long
and planar -linker, was reduced at the most positive potential
Ep,c ¼ ꢂ1.044 V. However, chromophore 9a, featuring one of the
longest but non-planar -linkers, is reduced even more easily
p-linker.
p
p
(Ep,c ¼ ꢂ0.992 V). This is caused by the presence of the electro-
negative CC triple bond, which acts as an insulator [5,25].
Table 1
Structures, yields, electrochemical data, absorption maxima (lmax), frequency of the C^N stretch, calculated energies EHOMO, ELUMO and average second-polarisabilities
chromophores 4e9.
b for
Comp.
Aa
na
Bond (—)a
Yield [%]
Ep,ab [V]
Ep,cb [V]
Ep,a ꢂ Ep,c [V]
lmax [nm (eV)]
n
(CN)c [cmꢂ1
]
EHOMO [eV]
ELUMO [eV]
b
ꢁ10ꢂ29 [esu]
4a
4b
5a
5b
6a
6b
7a
7b
8a
8b
9a
9b
NO2
CN
NO2
CN
NO2
CN
NO2
CN
NO2
CN
NO2
NO2
0
0
1
1
2
2
0
0
1
1
1
1
ꢁ
59
73
36
31
51
32
46
38
30
25
22
25
0.515
0.506
0.484
0.458
0.435
0.435
0.469
0.458
0.461
0.454
0.468
0.466
ꢂ1.104
ꢂ2.022
ꢂ1.047
ꢂ1.768
ꢂ1.044
ꢂ1.628
ꢂ1.103
ꢂ1.964
ꢂ1.067
ꢂ1.775
ꢂ0.992
ꢂ1.798
1.62
2.53
1.53
2.23
1.48
2.06
1.57
2.42
1.53
2.23
1.46
2.26
457 (2.71)
397 (3.12)
470 (2.64)
434 (2.86)
474 (2.62)
442 (2.81)
417 (2.97)
391 (3.17)
434 (2.86)
407 (3.05)
420 (2.95)
405 (3.06)
e
ꢂ7.87
ꢂ7.81
ꢂ7.79
ꢂ7.72
ꢂ7.74
ꢂ7.68
ꢂ7.75
ꢂ7.71
ꢂ7.74
ꢂ7.69
ꢂ7.77
ꢂ7.72
ꢂ1.47
ꢂ0.91
ꢂ1.44
ꢂ0.93
ꢂ1.59
ꢂ1.17
ꢂ1.41
ꢂ0.88
ꢂ1.60
ꢂ0.95
ꢂ1.63
ꢂ1.21
5.76
3.68
5.96
4.62
8.98
6.69
4.81
3.99
7.80
4.17
8.46
6.31
ꢁ
2221
e
]
]
]
]
ꢁ
2220
e
2218
e
ꢁ
2223
e
]
]
^
^
2219
e
2220
a
See Scheme 1 for the chromophore structures.
b
c
Ep,a and Ep,c e anodic and cathodic peak potentials measured by CV. All potentials are given vs. SCE.
Frequency of the C^N stretch (series b).

J. Kulhánek et al. / Dyes and Pigments 90 (2011) 48e55
53
Fig. 2. Representative CV curves of the oxidation and reduction of compounds 4a and 4b at Pt (oxidation) or HMDE (reduction) electrodes in acetonitrile.
features cyano groups at imidazole C4/C5 and N,N-dimethylamino
donor groups at imidazole C2, separated by an additional -linker.
3.3. UVevisible/IR studies and structure-hyperpolarisability
consideration of chromophores 4e9
p
The first oxidation potentials of chromophores 1 range from 0.69
to 1.38 V, while the first reduction potentials of imidazole-sepa-
rated cyano groups differ only slightly (ꢂ1.81 to ꢂ1.91 V). This
Electronic absorption spectra of imidazoles 4e9 measured in
CH2Cl2 showed intense charge-transfer (CT) absorption bands in
the UVevisible region (Fig. 3, Table 1).
implies that either donor- or acceptor-substituted p-linkers in both
series are the most electrochemically active parts of the molecule,
and that 4,5-bis(N,N-dimethylanilino)imidazole in 4e9 or 4,5-
dicyanoimidazole in 1 behave as donor or acceptor moieties,
respectively.
As a main feature, the position of the CT-band depends on the
electronic nature of the appended acceptors, taking into consider-
ation the length and planarity of the conjugated
p-backbone.
Whereas the chromophores in series a exhibited CT-bands with lmax
Fig. 3. Electronic absorption spectra of 4e9 (2 ꢁ 10ꢂ5 M solutions in dichloromethane).

54
J. Kulhánek et al. / Dyes and Pigments 90 (2011) 48e55
appearing between 417 and 474 nm, the cyano-substituted series
b showed CT-bands with lmax ranging from 397 to 442. The longest-
wavelength absorption bands of chromophores bearing NO2
acceptor groups are shifted more bathochromically than those for
chromophores with CN groups, possibly as a result of the strong
negative electronic effects of the nitro group. On the other hand, the
observed bathochromic shifts within the individual series a and
on the increasing accepting character of substituent A (e.g.,
b
(6a) ¼ 8.98 ꢁ 10ꢂ29 esu vs.
b
(6b) ¼ 6.69 ꢁ 10ꢂ29 esu), elongation of
the
p
-conjugated path (e.g., series 4a, 5a and 6a with
b
¼ 5.76, 5.96
and 8.98 ꢁ 10ꢂ29 esu) and planarity of the chromophore (e.g.,
b
(7a) ¼ 4.81 ꢁ 10ꢂ29 esu vs.
b
(8a) ¼ 7.80 ꢁ 10ꢂ29 esu).
4. Conclusion
b can be rationalised as an effect of the
p-linker between the
appended acceptors and donors. The influence of the spacer on the
efficiency of the donor-acceptor conjugation is best evaluated in
series 4e6 and 7e9, in which donor-acceptor separation is system-
atically extended. Within the series 4e6, the position of the CT-band
is shifted bathochromically with lmax of 457/397 (4a/4b), 470/434
(5a/5b) and 474/442 nm (6a/6b). When comparing chromophores 4
and 5, the change in lmax can be mainly ascribed to molecule pla-
narisation as a result of the CC double bond insertion between
imidazole C2 and the 1,4-phenylene unit. Similar to the measured
first reduction potentials (see above), the longest-wavelength tran-
sition was measured for chromophores 6a (lmax ¼ 474 nm) and 6b
Overall, twelve imidazole based Y-shaped chromophores,
featuring N,N-dimethylanilino donors at imidazole C4/C5 and nitro
and cyano acceptors at imidazole C2, were synthesised. The
acceptor substituents were further separated by an additional
systematically enlarged
p-linker. The chromophore properties
were studied by electrochemistry, UV/Vis and IR spectra, and the
experimental results were correlated with calculated properties
such as EHOMO, ELUMO and
affecting intramolecular charge-transfer were revealed. The pres-
ence of strongly conjugating acceptor, the length of the
-conjugated path and overall chromophore planarity proved to be
the most important structural factors determining electrochemical
and spectral properties of the studied pushepull De eA systems.
b. Several important structural features
a
p
(lmax ¼ 442 nm), featuring two CC double bonds and one 1,4-phe-
nylene unit in a planar arrangement. In contrast to the simplest
p
chromophores 4a and 4b, chromophores 7a (lmax ¼ 417 nm) and 7b
Considering the planarity, electrochemical behaviour, UVeVis and
IR properties, solubility and one of the highest calculated average
(
lmax ¼ 391 nm), bearing a longer twisted biphenyl
p-linker [24],
showed hypsochromically shifted CT-bands. However, partial pla-
narisation through the insertion of CC double or triple bonds, as in
chromophores 8 and 9, caused a bathochromic shift.
second-polarisabilities
b of chromophores 6a and 6b, the 4-phe-
nylbuta-1,3-dienyl -linker combined with the strong nitro or
p
cyano acceptors seems to possess one of the better balances of
performance and practicality within the studied series.
The stretching frequency of the cyano groups,
n (CN), provides
another indicator for the efficiency of the intramolecular CT. A
moderate trend, similar to the longest-wavelength absorption
maxima, can be seen in Table 1, while the lowest energy was
measured for planar chromophore 6b (2218 cmꢂ1). Employing PM3
and PM6 methods, the HOMO and the LUMO energies and average
Acknowledgements
This research was supported by the Czech Science Foundation
(203/08/0076) and the Ministry of Education, Youth and Sport of
the Czech Republic (MSM 0021627501 and 0021627502).
second-polarisabilities
b were further calculated (Table 1). The
calculated differences of EHOMO ꢂ ELUMO have been correlated
with the electrochemically measured band gap Ep,a ꢂ Ep,c. Fig. 4
shows a good linear correlation between these two quantities
(R ¼ 0.965). Moreover, the calculated average second-polarisabilities
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