Push-pull molecules with a systematically extended π-conjugated system featuring 4,5-dicyanoimidazole

Article abstract of DOI:10.1016/j.dyepig.2009.10.004

Eighteen chromophores featuring 4,5-dicyanoimidazole as an acceptor moiety, a systematically enlarged π-conjugated spacer and methoxy and N,N-dimethylamino groups as donors were synthesised and characterised by X-ray analysis, electrochemistry, UV-Vis and fluorescence spectroscopy whilst NLO properties were calculated. Quantitative relationships between measured properties and structural features of the chromophores were also evaluated.

Full text of DOI:10.1016/j.dyepig.2009.10.004

Dyes and Pigments 85 (2010) 57e65
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Dyes and Pigments
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Push-pull molecules with a systematically extended
featuring 4,5-dicyanoimidazole
p-conjugated system
a
,
a
b
c
d
ꢀa
ꢀ
ꢁꢀ
ꢀ
ꢀ ꢂꢀ ꢀ
*
ꢀ
ꢁ
ꢁ
ꢁ
ꢁ
Jirı Kulhanek , Filip Bures , Oldrich Pytela , Tomas Mikysek , Jirı Ludvık , Ales Ruzicka
a
ꢁ
Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentska 573, Pardubice 532 10, Czech Republic
Department of Analytical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentska 573, Pardubice 532 10, Czech Republic
J. Heyrovsky Institute of Physical Chemistry, Academy of Science of the Czech Republic, Dolejskova 3, 182 23 Prague 8, Czech Republic
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentska 573, Pardubice 532 10, Czech Republic
b
c
ꢁ
ꢀ
d
ꢁ
a r t i c l e i n f o
a b s t r a c t
Article history:
Eighteen chromophores featuring 4,5-dicyanoimidazole as an acceptor moiety, a systematically enlarged
Received 20 July 2009
Received in revised form
5 October 2009
Accepted 10 October 2009
Available online 27 October 2009
p
-conjugated spacer and methoxy and N,N-dimethylamino groups as donors were synthesised and
characterised by X-ray analysis, electrochemistry, UVeVis and fluorescence spectroscopy whilst NLO
properties were calculated. Quantitative relationships between measured properties and structural
features of the chromophores were also evaluated.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Push-pull chromophores
Imidazole
Donoreacceptor systems
Electrochemistry
UVeVis spectroscopy
Nonlinear optics (NLO)
1. Introduction
[13e16]. Additionally, successful fabrication of such molecules, in
particular film deposition, relies on the chromophore possessing
Diverse, push-pull molecules with large delocalized
systems have attracted considerable attention. These materials can
be end-capped with electron donors (D) and acceptors (A) arranged
p
-electron
good thermal and chemical robustness and being available in
reasonable quantities. Hence, various heteroaromates such as
pyridine [17] or pyrazine [18] as well as five-membered rings such
as triazoles [19], thiophenes [20] or benzothiazoles [21,22] have
in either symmetrical A-p-A, D-p-D or asymmetrical A-p-D chro-
mophore orientations [1e3]. Within recent years, such readily
polarisable compounds have been studied owing to their promising
optoelectronic properties, especially second- and third-order
optical nonlinearities (NLO) [4,5]. Thus, such push-pull chromo-
phores enjoy widespread application in material chemistry as
optoelectronic [6] and optical data storage devices [7], organic
light-emitting diodes (OLED) [8], organic-inorganic hybrid mate-
rials [9], functional polymers [10] and highly functionalised den-
drimers [11,12]. A typical organic chromophore consists of strong
electron acceptors (e.g. NO2 or CN groups) and donors (e.g. NR2 or
been widely utilised as robust p-backbones for NLO active
compounds. In recent years, 1,3-diazoles e imidazoles [23e31] and
benzimidazoles [32e34] have been widely employed as suitable
and robust aromatic systems for the construction of promising
charge-transfer chromophores. Imidazole-based chromophores
usually possess donor/acceptor auxiliaries at C-2 or C-4/C-5 (and
vice versa) to form Y-shaped chromophores. The rational design of
such organic optoelectronic chromophores involves finding an
optimal
p-conjugated linkage as well as proper donors and
acceptors [35]. In general, charge-transfer chromophores that
exhibit large hyperpolarisability, good optical transparency, solu-
bility and thermal stability for fabrication processes are currently in
demand (nonlinearity-transparency-solubility-thermal stability
trade-off).
OR groups) connected by a p-conjugated system. This arrangement
ensures efficient intramolecular charge-transfer and enables
further fine-tuning of the polarisability of the chromophore
Since the first synthesis of 4,5-dicyanoimidazole was reported by
Woodward [36,37], this planar heterocyclic compound has been
utilised in the construction of the aforementioned push-pull systems
* 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 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.10.004

ꢁ
J. Kulhanek et al. / Dyes and Pigments 85 (2010) 57e65
58
with applicability in various branches of material science [38e44].
Its widespread use demonstrates that this easily accessible, soluble
and robust precursor may act as an efficient acceptor moiety.
Recently, the present authors reported the synthesis and properties
1H NMR (CDCl3, 360 MHz):
NMR (CDCl3, 90 MHz):
d
¼ 3.89 (s, 3H, CH3), 7.69 (s, 1H, Him). 13
C
d
¼ 34.13, 107.92, 111.75, 113.13, 123.09, 141.98.
EI-MS (70 eV) m/z (rel. int.): 132 (Mþ, 100), 91 (20), 77 (16), 67 (18).
Anal. calcd. for C6H4N4: C 54.54, H 3.05, N 42.41; found: C 54.60, H 3.07,
N 42.44.
of several D-p-A chromophores featuring 2,4,5-triphenylimidazole
as a basic -conjugated linkage [45]. Due to the growing interest in
p
stable heterocyclic chromophores as well as our experience in the
chemistry of imidazole and charge-transfer chromophores [13e16],
this paper concerns the synthesis and properties of imidazole-based
chromophores possessing 4,5-dicyanoimidazole as an acceptor
moiety, N,N-dimethylamino and methoxy groups as donors and
2.2.2. 2-Methoxy-1-methyl-1H-imidazole-4,5-dicarbonitrile (1b)
2-Methoxy-1H-imidazole-4,5-dicarbonitrile was synthesised
according to the literature procedure [48] in 23% yield and further
N-methylated as described by Rapoport et al. [47]. White solid,
yield 90%; mp 61e63 ꢀC (Lit. [48] mp 65e66 ꢀC). 1H NMR (CDCl3,
a systematically extended
p-conjugated path (Fig. 1).
360 MHz):
90 MHz):
d
¼ 3.53 (s, 3H, CH3), 4.10 (s, 3H, OCH3). 13C NMR (CDCl3,
d
¼ 30.79, 58.61, 108.54, 109.49, 112.12, 118.33, 154.65. EI-
MS (70 eV) m/z (rel. int.): 162 (Mþ, 100), 147 (81), 133 (21), 120 (26),
67 (73). Anal. calcd. for C7H6N4O: C 51.85, H 3.73, N 34.55; found: C
51.78, H 3.84, N 34.33.
2. Experimental
2.1. General
2.2.3. 2-(N,N-Dimethylamino)-1-methyl-1H-imidazole-4,
5-dicarbonitrile (1c)
Reagents and solvents were reagent-grade and were purchased
from Penta, Aldrich, and Acros and used as received. Boronic acids
and esters used in the SuzukieMiyaura reaction were purchased
from Aldrich or synthesised according to literature procedures [46].
2-Bromo-1-methyl-1H-imidazole-4,5-dicarbonitrile (7) was syn-
thesised from diaminomaleonitrile according to literature proce-
dures [40,47]. 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
A solution of 7 (100 mg; 0.47 mmol), dimethylamine (1 mL;
7.9 mmol; 40% aq. solution) and K2CO3 (66 mg; 0.47 mmol) in DMF
(5 mL) was heated at 110 ꢀC for 1 h and then cooled to 25 ꢀC, diluted
with H2O (30 mL) and extracted with EtOAc (2 ꢁ 30 mL). The
combined organic extracts were dried (Na2SO4) and the solvent was
evaporated to afford pure 1c as a white solid, yield 75 mg (90%); mp
82e83 ꢀC. 1H NMR (CDCl3, 360 MHz):
(s, 3H, CH3). 13C NMR (CDCl3, 90 MHz):
d
¼ 2.89 (s, 6H, N(CH3)), 3.62
d
¼ 33.40, 41.82, 109.20,
110.58, 112.42, 120.02, 156.48. EI-MS (70 eV) m/z (rel. int.): 175 (Mþ,
54), 160 (100), 146 (71), 133 (20), 119 (18), 67 (25), 42 (19). Anal.
calcd. for C8H9N5: C 54.85, H 5.18, N 39.98; found: C 54.78, H 5.23, N
39.97.
€
360 nm). Melting points (mp) were measured on a Buchi B-540
melting-point apparatus in open capillaries and are uncorrected. 1H
and 13C NMR spectra were recorded at 360/400/500 MHz and 90/
100/125 MHz, respectively, with a Bruker AMX 360 and Bruker
AVANCE 400 or 500 instruments at 25 ꢀC. Chemical shifts are
reported in ppm relative to the signal of Me4Si. The residual solvent
signal in the 1H and 13C NMR spectra was used as an internal
reference (CDCl3 e 7.25 and 77.23 ppm; DMSO-d6 e 2.55 and
39.51 ppm; acetone-d6 e 2.05 and 29.92 ppm). Apparent resonance
multiplicities are described as s (singlet), d (doublet) and m
(multiplet). Mass spectra were measured on a GC/MS configuration
comprised of an Agilent Technologies e 6890 N gas chromatograph
equipped with a 5973 Network MS detector (EI 70 eV, mass range
33e550 Da) or on a LCeMS Micromass Quattro Micro API (Waters)
2.2.4. General procedure for the synthesis of target chromophores
2e6 through SuzukieMiyaura cross-coupling
PdCl2(PPh3)2 (caution: toxic; irritant; incompatible with
oxidants, moisture; 33 mg; 0.047 mmol) and Na2CO3 (55 mg;
0.52 mmol) were added to a degassed solution of 7 (100 mg;
0.474 mmol) and boronic acid/ester (0.52 mmol) in THF/H2O
(20 mL; 4:1) under an argon atmosphere. The mixture was stirred
under Ar at 60 ꢀC for 12 h, diluted with H2O (30 mL) and extracted
with CH2Cl2 (2 ꢁ 30 mL). The combined organic extracts were dried
(Na2SO4) and concentrated in vacuo. Subsequent column chroma-
tography afforded pure products 2e6.
instrument with
200e800 Da). Elemental analyses were performed on an EA 1108
a
direct input (ESIþ, CH3OH, mass range
2.2.4.1. 1-Methyl-2-phenyl-1H-imidazole-4,5-dicarbonitrile (2a).
Obtained 48 mg (49%) of a white solid; Rf ¼ 0.33 (SiO2; Hexane/
Fisons instrument.
EtOAc 2:1); mp 134e136 ꢀC. 1H NMR (CDCl3, 360 MHz):
d
¼ 3.90 (s,
3H, CH3), 7.52e7.63 (m, 5H, Ph). 13C NMR (CDCl3, 90 MHz):
2.2. Synthesis
d
¼ 34.84, 99.46,102.20,108.58,111.91,127.11,129.25,129.43,131.48,
152.8. EI-MS (70 eV) m/z (rel. int.): 208 (Mþ, 65), 207 (100),103 (12).
Anal. calcd. for C12H8N4: C 69.22, H 3.87, N 26.91; found: C 69.18, H
3.98, N 26.87.
2.2.1. 1-Methyl-1H-imidazole-4,5-dicarbonitrile (1a)
The title compound was synthesised from diaminomaleonitrile
according to the literature procedure [47] and obtained as white solid.
The yield over 2 steps was 85%; mp 87e88 ꢀC (Lit. [47] mp 87e89 ꢀC).
2.2.4.2. 2-(4-Methoxyphenyl)-1-methyl-2-phenyl-1H-imidazole-4,5-
dicarbonitrile (2b). Obtained 64 mg (57%) as a white solid; Rf ¼ 0.25
(SiO2; Hexane/EtOAc 2:1); mp 156e158 ꢀC. 1H NMR (CDCl3,
360 MHz):
d
¼ 3.87 (s, 6H, CH3 þ OCH3), 7.03 (d, 2H, J ¼ 8.8 Hz, Ar),
7.56 (d, 2H, J ¼ 8.8 Hz, Ar). 13C NMR (CDCl3, 90 MHz):
d
¼ 34.83,
55.74, 101.81, 108.74, 112.03, 113.96, 114.85, 119.31, 122.36, 130.81,
152.78. EI-MS (70 eV) m/z (rel. int.): 238 (Mþ, 97), 237 (100), 195
(16). Anal. calcd. for C13H10N4O: C 65.54, H 4.23, N 23.52; found: C
65.52, H 4.34, N 23.52.
2.2.4.3. 2-(4-(N,N-Dimethylamino)phenyl)-1-methyl-2-phenyl-1H-
Fig. 1. Molecular structure of the proposed imidazole chromophores.Ă
imidazole-4,5-dicarbonitrile (2c). Obtained 101 mg (85%) as

ꢁ
J. Kulhanek et al. / Dyes and Pigments 85 (2010) 57e65
59
a yellowish solid; Rf ¼ 0.4 (SiO2; CH2Cl2); mp 263e265 ꢀC. 1H NMR
1H NMR (DMSO-d6, 360 MHz):
d
¼ 3.02 (s, 6H, N(CH3)2), 3.96 (s, 3H,
(DMSO-d6, 360 MHz):
d
¼ 3.05 (s, 6H, N(CH3)2), 3.89 (s, 3H, CH3),
CH3), 6.88 (d, 2H, J ¼ 8.6 Hz, Ar), 7.68 (d, 2H, J ¼ 8.3 Hz, Ar),
6.87 (d, 2H, J ¼ 8.8 Hz, Ar), 7.63 (d, 2H, J ¼ 8.8 Hz, Ar). 13C NMR
7.79e7.88 (m, 4H, Ar). 13C NMR (DMSO-d6, 125 MHz):
d
¼ 35.24,
(DMSO-d6, 90 MHz):
d
¼ 35.28, 39.02, 109.54, 111.75, 113.04, 113.75,
37.40, 109.16, 112.65, 112.72, 114.59, 120.28, 124.32, 125.66, 125.86,
127.45, 129.67, 142.58, 150.46, 152.10. EI-MS (70 eV) m/z (rel. int.):
327 (Mþ, 100), 207 (18), 163 (15). Anal. calcd. for C20H17N5: C 73.37,
H 5.23, N 21.39; found: C 73.33, H 5.25, N 21.34.
113.94, 120.30, 130.24, 151.72, 152.88. EI-MS (70 eV) m/z (rel. int.):
251 (Mþ,100), 250 (98), 234 (12), 145 (13). Anal. calcd. for C14H13N5:
C 66.92, H 5.21, N 27.87; found: C 66.92, H 5.24, N 27.87.
2.2.4.4. (E)-1-Methyl-2-styryl-1H-imidazole-4,5-dicarbonitrile (3a)
[41]. Obtained 60 mg (54%) as a white solid; Rf ¼ 0.39 (SiO2;
Hexane/EtOAc 2:1); mp 252e254 ꢀC. 1H NMR (CDCl3, 360 MHz):
2.2.4.10. (E)-1-Methyl-2-(4-styrylphenyl)-1H-imidazole-4,5-dicar-
bonitrile (5a). Obtained 72 mg (49%) as an off-white solid;
Rf ¼ 0.59 (SiO2; Hexane/EtOAc 2:1); mp 183e185 ꢀC. 1H NMR
d
¼ 3.83 (s, 3H, CH3), 6.78 (d, 1H, J ¼ 15.9 Hz, CH), 7.31e7.37 (m, 3H,
(acetone-d6, 360 MHz):
d
¼ 4.07 (s, 3H, CH3), 7.30e7.48 (m, 5H,
Ar), 7.49e7.53 (m, 2H, Ar), 7.77 (d, 1H, J ¼ 15.9 Hz, CH). 13C NMR
Ar þ CH), 7.65 (d, 2H, J ¼ 7.5 Hz, Ar), 7.78e7.83 (m, 4H, Ar þ CH).
(CDCl3, 90 MHz):
d
¼ 32.77, 108.59, 109.93, 111.90, 127.79, 128.97,
13C NMR (acetone-d6, 90 MHz):
d
¼ 35.70, 109.63, 113.23, 115.58,
129.25, 130.43, 131.11, 134.82, 141.00, 150.73. EI-MS (70 eV) m/z (rel.
int.): 234 (Mþ, 26), 233 (100), 218 (18). Anal. calcd. for C14H10N4: C
71.78, H 4.30, N 23.92; found: C 71.86, H 4.68, N 23.80.
122.13, 127.40, 127.76, 127.78, 128.25, 129.09, 129.71, 130.52, 131.88,
138.01, 140.98, 153.24. EI-MS (70 eV) m/z (rel. int.): 310 (Mþ, 100),
309 (81). Anal. calcd. for C20H14N4: C 77.40, H 4.55, N 18.05; found:
C 77.47, H 4.67, N 17.98.
2.2.4.5. (E)-2-(4-Methoxystyryl)-1-methyl-1H-imidazole-4,5-dicar-
bonitrile (3b). Obtained 71 mg (57%) as a white solid; Rf ¼ 0.31
(SiO2; Hexane/EtOAc 2:1); mp 217e219 ꢀC. 1H NMR (CDCl3,
2.2.4.11. (E)-2-[4-(4-Methoxystyry)lphenyl]-1-methyl-1H-imidazole-
4,5-dicarbonitrile (5b). Obtained 78 mg (42%) as a yellow solid;
Rf ¼ 0.42 (SiO2; Hexane/EtOAc 2:1); mp 193e195 ꢀC. 1H NMR
360 MHz):
d
¼ 3.82 (s, 3H, CH3), 3.84 (s, 3H, OCH3), 6.63 (d, 1H,
J ¼ 15.7 Hz, CH), 6.91 (d, 2H, J ¼ 8.7 Hz, Ar), 7.49 (d, 2H, J ¼ 8.7 Hz,
(acetone-d6, 360 MHz):
d
¼ 3.84 (s, 3H, OCH3), 4.06 (s, 3H, CH3),
Ar), 7.76 (d, 1H, J ¼ 15.7 Hz, CH). 13C NMR (CDCl3, 90 MHz):
6.96 (d, 2H, J ¼ 8.6 Hz, Ar), 7.19 (d, 1H, J ¼ 16.4 Hz, CH), 7.37 (d, 1H,
J ¼ 16.4 Hz, CH), 7.59 (d, 2H, J ¼ 8.6 Hz, Ar), 7.75e7.80 (m, 4H, Ar).
d
¼ 107.52, 108.80, 112.11, 112.69, 114.73, 122.61, 127.56, 129.45,
140.53,151.17,161.56. EI-MS (70 eV) m/z (rel. int.): 264 (Mþ, 46), 263
(100), 220 (12). Anal. calcd. for C15H12N4O: C 68.17, H 4.58, N 21.20;
found: C 68.15, H 4.67, N 21.09.
13C NMR (acetone-d6, 90 MHz):
d
¼ 35.71, 55.74, 109.66, 113.26,
115.71, 115.55, 122.15, 125.95, 126.93, 127.48, 129.13, 130.50, 130.66,
131.58, 114.43, 153.34, 161.01. EI-MS (70 eV) m/z (rel. int.): 340 (Mþ,
100), 281 (20), 207 (21). Anal. calcd. for C21H16N4O: C 74.10, H 4.74,
N 16.46; found: C 74.19, H 4.66, N 16.86.
2.2.4.6. (E)-2-(4-(N,N-Dimethylamino)styryl)-1-methyl-1H-imid-
azole-4,5-dicarbonitrile (3c). Obtained 89 mg (68%) as a yellow
solid; Rf ¼ 0.20 (SiO2; Hexane/EtOAc 2:1); mp 290e292 ꢀC. 1H NMR
2.2.4.12. (E)-2-{4-[4-(N,N-Dimethylamino)styryl]phenyl}-1-methyl-
1H-imidazole-4,5-dicarbonitrile (5c). Obtained 117 mg (66%) as an
orange solid; Rf ¼ 0.37 (SiO2; Hexane/EtOAc 2:1); mp 243e245 ꢀC.
(DMSO-d6, 400 MHz):
d
¼ 3.03 (s, 6H, N(CH3)2), 3.90 (s, 3H, CH3),
6.78 (d, 2H, J ¼ 8.8 Hz, Ar), 7.07 (d, 1H, J ¼ 16.0 Hz, CH), 7.60e7.68
(m, 3H, CH þ Ar). 13C NMR (CDCl3, 90 MHz):
d
¼ 32.70, 32.78,106.19,
1H NMR (acetone-d6, 360 MHz):
d
¼ 3.00 (s, 6H, N(CH3)2), 4.06 (s,
109.36, 111.81, 112.48, 112.81, 120.46, 122.55, 129.35, 139.13, 151.26,
151.70. EI-MS (70 eV) m/z (rel. int.): 277 (Mþ, 89), 276 (100), 260
(16), 137 (12). Anal. calcd. for C16H15N5: C 69.29, H 5.45, N 25.25;
found: C 69.26, H 5.59, N 25.38.
3H, CH3), 6.76 (d, 2H, J ¼ 8.7 Hz, Ar), 7.07 (d, 1H, J ¼ 16.4 Hz, CH),
7.32 (d,1H, J ¼ 16.4 Hz, CH), 7.49 (d, 2H, J ¼ 8.7 Hz, Ar), 7.71e7.79 (m,
4H, Ar). 13C NMR (acetone-d6, 90 MHz):
d
¼ 35.70, 40.46, 109.70,
113.01, 113.22, 113.23, 123.32, 125.98, 126.30, 127.10, 128.81, 128.95,
130.28, 130.45, 132.36, 142.03, 151.77. ESI-MS: m/z ¼ 376 [Mþ þ Na].
Anal. calcd. for C22H19N5: C 74.77, H 5.42, N 19.82; found: C 74.79, H
5.54, N 19.99.
2.2.4.7. 2-(Biphenyl-4-yl)-1-methyl-1H-imidazole-4,5-dicarbonitrile
(4a). Obtained 78 mg (58%) as a yellowish solid; Rf ¼ 0.42 (SiO2;
Hexane/EtOAc 2:1); mp 199e201 ꢀC. 1H NMR (CDCl3, 500 MHz):
d
¼ 3.94 (s, 3H, CH3), 7.38e7.52 (m, 3H, Ar), 7.62 (d, 2H, J ¼ 8.0 Hz,
2.2.4.13. 1-Methyl-2-[4-(phenylethynyl)phenyl]-1H-imidazole-4,5-
dicarbonitrile (6a). Obtained 61 mg (44%) as a off-white solid;
Rf ¼ 0.42 (SiO2; Hexane/EtOAc 2:1); mp 169e171 ꢀC. 1H NMR
Ar), 7.70 (d, 2H, J ¼ 8.0 Hz, Ar), 7.76 (d, 2H, J ¼ 8.3 Hz, Ar). 13C NMR
(CDCl3, 125 MHz):
d
¼ 34.93, 108.61, 111.92, 114.24, 122.56, 125.81,
127.38, 128.01, 128.57, 129.28, 129.66, 139.65, 144.32, 152.51. EI-MS
(70 eV) m/z (rel. int.): 284 (Mþ, 100), 283 (91), 179 (15), 165 (16).
Anal. calcd. for C18H12N4: C 76.04, H 4.25, N 19.71; found: C 76.02, H
4.50, N 19.67.
(CDCl3, 360 MHz):
d
¼ 3.93 (s, 3H, CH3), 7.35e7.38 (m, 3H, Ar),
7.52e7.56 (m, 2H, Ar), 7.63 (d, 2H, J ¼ 6.0 Hz, Ar), 7.68 (d, 2H,
J ¼ 6.0 Hz, Ar). 13C NMR (CDCl3, 90 MHz):
d
¼ 34.98, 88.23, 92.83,
108.51, 111.80, 114.41, 122.65, 122.68, 126.42, 126.83, 128.71, 129.15,
131.09, 131.96, 132.67, 152.03. EI-MS (70 eV) m/z (rel. int.): 308 (Mþ,
100), 307 (65), 203 (16). Anal. calcd. for C20H12N4: C 77.91, H 3.92, N
18.17; found: C 77.96, H 4.03, N 18.08.
2.2.4.8. 2-(40-Methoxybiphenyl-4-yl)-1-methyl-1H-imidazole-4,5-
dicarbonitrile (4b). Obtained 85 mg (57%) as a yellowish solid;
Rf ¼ 0.38 (SiO2; Hexane/EtOAc 2:1); mp 197e199 ꢀC. 1H NMR
(CDCl3, 360 MHz):
d
¼ 3.86 (s, 3H, CH3), 3.93 (s, 3H, OCH3), 7.00 (d,
2.2.4.14. 2-[4-(4-Methoxyphenylethynyl)phenyl]-1-methyl-1H-imid-
azole-4,5-dicarbonitrile (6b). Obtained 88 mg (56%) as a yellow
solid; Rf ¼ 0.47 (SiO2; Hexane/EtOAc 2:1); mp 199e201 ꢀC. 1H NMR
2H, J ¼ 8.6 Hz, Ar), 7.56 (d, 2H, J ¼ 8.6 Hz, Ar), 7.65e7.73 (m, 4H, Ar).
13C NMR (CDCl3, 90 MHz):
d
¼ 34.93, 55.62, 108.66, 111.98, 114.19,
114.73, 122.53, 125.13, 127.42, 128.48, 129.64, 132.02, 143.88, 152.64,
160.22. EI-MS (70 eV) m/z (rel. int.): 314 (Mþ, 100), 299 (12), 271
(10). Anal. calcd. for C19H14N4O: C 72.60, H 4.49, N 17.82; found: C
72.56, H 4.58, N 17.95.
(acetone-d6, 360 MHz):
d
¼ 3.85 (s, 3H, OCH3), 4.08 (s, 3H, CH3),
7.00 (d, 2H, J ¼ 8.7 Hz, Ar), 7.53 (d, 2H, J ¼ 8.7 Hz, Ar), 7.71 (d, 2H,
J ¼ 8.2 Hz, Ar), 7.85 (d, 2H, J ¼ 8.2 Hz, Ar), 7.92e7.95 (m, 1H, Ar). 13C
NMR (acetone-d6, 90 MHz):
d
¼ 34.54, 55.85, 87.91, 92.92, 109.56,
113.15, 115.27, 115.46, 122.19, 127.01, 128.11, 130.35, 130.84, 132.58,
133.98, 152.80, 161.40. ESI-MS: m/z ¼ 361 [Mþ þ Na]. Anal. calcd.
for C21H14N4O: C 74.54, H 4.17, N 16.56; found: C 74.66, H 4.18, N
16.66.
2.2.4.9. 2-(40-(N,N-dimethylamino)biphenyl-4-yl)-1-methyl-1H-
imidazole-4,5-dicarbonitrile (4c). Obtained 127 mg (82%) as
a yellow solid; Rf ¼ 0.38 (SiO2; Hexane/EtOAc 2:1); mp 261e263 ꢀC.

ꢁ
J. Kulhanek et al. / Dyes and Pigments 85 (2010) 57e65
60
2.2.4.15. 2-{4-[4-(N,N-Dimethylamino)phenylethynyl]phenyl-1-
methyl-1H-imidazole-4,5-dicarbonitrile (6c). Obtained 127 mg
(76%) as a yellow solid; Rf ¼ 0.35 (SiO2; Hexane/EtOAc 2:1); mp
energies and average second-polarisabilities
lated by employing MOPAC2009.
The following indicators were chosen as structural charac-
teristics of the studied chromophores: presence of the methoxy
(IndMeO; 0 or 1) or N,N-dimethylamino donor groups (IndDMA
0 or 1), presence of double (IndDB; 0 or 1) or triple bond (IndTB
b were further calcu-
242e244 ꢀC. 1H NMR (CDCl3, 500 MHz):
d
¼ 2.99 (s, 6H, N(CH3)2),
3.89 (s, 3H, CH3), 6.64 (d, 2H, J ¼ 8.8 Hz, Ar), 7.39 (d, 2H, J ¼ 8.8 Hz,
;
;
Ar), 7.56e7.61 (m, 4H, Ar). 13C NMR (CDCl3, 90 MHz):
d
¼ 34.97,
40.36, 86.69, 94.68, 108.58, 109.13, 111.86, 111.98, 114.30, 122.58,
125.40, 127.85, 129.05, 131.11, 131.95, 133.19, 150.72, 152.32. ESI-MS:
m/z ¼ 374 [Mþ þ Na]. Anal. calcd. for C22H17N5: C 75.19, H 4.88, N
19.93; found: C 75.29, H 5.07, N 19.81.
0 or 1), number of 1,4-phenylene units (nPh; 0, 1, 2), presence of
double bond appended to the imidazole 2-position (IndIm-DB
;
0 or 1) and presence of 1,4-phenylene linker appended to the
imidazole 2-position (IndIm-Ph; 0 or 1). The three of the above
structural characteristics that provided the strongest correlations
were chosen for the interpretation of the relationships between
measured or calculated quantities and structural features.
Experimental and computation data was examined by multi-
variate graphical display methods and subsequently analysed
using multiple linear regressions techniques (including t-test,
F-test, assessing multicollinearity, analysis of residuals). Resid-
uals in all of the presented regressions had Normal distribution.
The program OPstat [57] was employed for all statistical
calculations.
2.3. Crystallography
The X-ray data for crystals of 1a, 2a, 3b and 4b were obtained at
150 K using Oxford Cryostream low-temperature device on a Non-
ꢂ
ius KappaCCD diffractometer with Mo Ka radiation (
l
¼ 0.71073 A),
a graphite monochromator, and the and
f
c
scan mode. Data
reductions were performed with DENZO-SMN [49]. The absorption
was corrected by integration methods [50]. Structures were solved
by direct methods (Sir92)[51] and refined by full matrix least-
square based on F2 (SHELXL97)[52]. Hydrogen atoms were mostly
localized on a difference Fourier map, however to ensure unifor-
mity of treatment of crystal, all hydrogen were recalculated into
idealized positions (riding model) and assigned temperature
factors Hiso(H) ¼ 1.2 Ueq(pivot atom) or of 1.5 Ueq for the methyl
3. Results and discussion
3.1. Synthesis
ꢂ
ꢂ
moiety with CeH ¼ 0.96 A, 0.97, 0.98 and 0.93 A for methyl,
The 4,5-dicyanoimidazole acceptor moiety was easily prepared
by the condensation of diaminomaleonitrile with trimethyl ortho-
formate in 80% yield. Subsequent bromination and N-alkylation [47]
smoothly afforded 7 as the starting material for further cross-
coupling steps. However, exclusion of the bromination step afforded
unsubstituted 1-methyl-1H-imidazole-4,5-dicarbonitrile 1a. The
bromoimidazole 7 could be treated with an aqueous solution of
dimethylamine to effect aromatic nucleophilic substitution, which
afforded the simplest push-pull system 1d [40]. The methoxy-
substituted analogue 1c had to be prepared from diaminomaleoni-
trile using stepwise methodology described by Anderson et al. [48]
in 21% yield (Scheme 1).
methylene, methine and hydrogen atoms in aromatic ring,
ꢂ
respectively, and 0.86 A for NeH bonds.
Rint ¼ SjFo2 ꢂ Fo2,meanj/SFo2, GOF ¼ [S(w(Fo2 ꢂ F2c)2)/(Ndiffrs ꢂ Nparams)]
½
for all data, R(F) ¼ SjjFoj ꢂ jFcjj/SjFoj for observed data, wR(F2) ¼ [S(w
(Fo2 ꢂ F2c)2)/(Sw(Fo2)2)] for all data.
½
Crystallographic data for structural analysis have been depos-
ited with the Cambridge Crystallographic Data Centre, CCDC no.
739987, 739984, 739985 and 739986 for 1c, 2c, 3b and 4b,
respectively. Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road, Cambridge CB2
1EY, UK (fax: þ44 1223 336033; e mail: deposit@ccdc.cam.ac.uk or
www: http://www.ccdc.cam.ac.uk). For 2c the Flack parameter has
meaningless standard uncertainty [53,54].
We have recently reported a convenient synthesis for preparing
p
-conjugated linkers as building blocks for modular chemistry
[46]. Either unsubstituted or donor-substituted -linkers bearing
phenyl, styryl, biphenyl, phenylethenylphenyl or phenyl-
p
2.4. Electrochemistry
a
ethynylphenyl
p-conjugated backbone and pinacol acid/ester
Electrochemical measurements were carried out in acetonitrile
containing Bu4NPF6 in a three-electrode cell by cyclic voltammetry
(CV), rotating disc voltammetry (RDV) and DC polarography. The
working electrode was a platinum disc (2 mm in diameter) for CV
and RDV or dropping mercury electrode (DME). The following
conditions were applied for the polarographic measurements: drop
functionalities were synthesised in a straightforward manner.
With such boronic acids/esters in hand, we were able to carry out
PdCl2(PPh3)2-catalysed SuzukieMiyaura cross-coupling reactions
on bromoimidazole 7 to yield imidazole-4,5-dicarbonitriles 2e6
(Scheme 1, Table 1). Following this procedure, we have prepared 18
imidazole-based push-pull systems with systematically extended
time sD ¼ 1 s, scan rate
y
¼ 5 mV sꢂ1, the reference electrode was
p
-conjugated backbones. Whereas the reference series
unsubstituted, series b and c feature methoxy and N,N-dimethy-
lamino donors (A- -D chromophore) respectively. The donors
a is
a saturated calomel electrode (SCE) separated by a bridge filled
with supporting electrolyte and Pt wire, respectively. All potentials
are given vs. SCE, the Fcþ/Fc usually used as internal standard, could
not be employed due to its interaction with the analyte. Voltam-
metric measurements were performed using an electrochemical
analyser AUTOLAB (model PGSTAT 30; Ecochemie, Utrecht, The
Netherlands) operated via GPEs 4.8 software. Polarographic
experiments were accomplished with a Polarographic Analyzer PA-
p
were connected either directly to imidazole (1) or separated by
1,4-phenylene (2), (E)-phenylethenyl (3), biphenyl (4), (E)-phenyl-
ethenylphenyl (5) or phenylethynylphenyl (6)
tively. All of the newly synthesised compounds were fully
characterised by 1H and 13C NMR as well as EI(ESI)-MS and
elemental analysis.
p-linkers, respec-
ꢀ
ꢁ
ꢁ
3 (Laboratornı prıstroje, Prague, Czech Republic) connected to an
XY-recorder (ibid.).
3.2. Crystallography
2.5. Quantum and statistical calculations
Slow evaporation of CH2Cl2 or EtOAc solutions of chromophores
1c, 2c, 3b and 4b at 20 ꢀC afforded crystals suitable for X-ray
analysis. The ORTEP plots in Fig. 2 confirm the proposed molecular
structures. In contrast to the almost coplanar spatial arrangement
Initial geometries of the compounds 1e6 have been calculated
by the PM3 method (ArgusLab, [55]) and subsequently optimised
by the PM6 method (MOPAC2009, [56]). The HOMO and LUMO

ꢁ
J. Kulhanek et al. / Dyes and Pigments 85 (2010) 57e65
61
Scheme 1. Synthesis of chromophores 1e6.Ă
of the N,N-dimethylamino group in 2c, one methyl group of the
N,N-dimethylamino group in 1c is forced out of the imidazole
plane, with a dihedral angle C8N5C1N2 of about 63ꢀ as a result of
the steric repulsion with the imidazole N-methyl group. However,
the entire N,N-dimethylanilino group in 2c adopts a typical out of
imidazole plane deviation with a dihedral angle C8C5C3N1 of about
32 known for 2-phenyl-substituted imidazoles [58]. On other hand,
the 4,5-dicyanoimidazole acceptor and the 4-methoxystyryl
donor moieties in 3b are almost coplanar. Chromophore 4b, which
bears a biphenyl central spacer, adopts a typical ‘‘propeller-like”
conformation [59]. While the 4,5-dicyanoimidazole and terminal
4-methoxyphenyl ring are almost coplanar, the central 1,4-phe-
nylene unit showed substantial twist with a dihedral angle
C8C13C10N1 of about 36ꢀ.
Table 1
Yields, m.p., UVeVis (lAmax) and fluorescent (lFmax) properties as well as calculated energies EHOMO, ELUMO and average second-polarisabilities
b for chromophores 1e6.
Comp.
Yielda [%]
M.p. [ꢀC]
lAmax [nm (eV)]
lFmax [nm (eV)]
EHOMO [eV]
ELUMO [eV]
b [esu]
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
5a
5b
5c
6a
6b
6c
85b
23b
90b
49
57
85
54
57
68
58
57
82
49
42
66
44
56
76
87e88
61e63
82e83
244 (5.08)
271 (4.58)
293 (4.23)
264 (4.70)
275 (4.51)
316 (3.92)
313 (3.96)c
331 (3.75)
381 (3.25)
286 (4.34)
301 (4.12)
346 (3.58)
325 (3.82)
331 (3.75)
380 (3.26)
308 (4.03)
323 (3.84)
364 (3.41)
e
ꢂ10.42
ꢂ9.74
ꢂ9.50
ꢂ9.87
ꢂ9.37
ꢂ8.67
ꢂ9.31
ꢂ8.89
ꢂ8.35
ꢂ9.57
ꢂ9.03
ꢂ8.38
ꢂ9.07
ꢂ8.67
ꢂ8.19
ꢂ9.27
ꢂ8.84
ꢂ8.31
ꢂ1.23
ꢂ1.11
ꢂ0.99
ꢂ1.18
ꢂ1.07
ꢂ0.94
ꢂ1.33
ꢂ1.20
ꢂ1.08
ꢂ1.19
ꢂ1.13
ꢂ1.07
ꢂ1.29
ꢂ1.20
ꢂ1.12
ꢂ1.31
ꢂ1.21
ꢂ1.14
1.50 ꢁ 10ꢂ30
3.52 ꢁ 10ꢂ30
2.71 ꢁ 10ꢂ30
2.63 ꢁ 10ꢂ30
8.31 ꢁ 10ꢂ30
1.46 ꢁ 10ꢂ29
5.27 ꢁ 10ꢂ30
1.82 ꢁ 10ꢂ29
3.27 ꢁ 10ꢂ29
5.17 ꢁ 10ꢂ30
1.30 ꢁ 10ꢂ29
2.19 ꢁ 10ꢂ29
1.32 ꢁ 10ꢂ29
3.07 ꢁ 10ꢂ29
4.91 ꢁ 10ꢂ29
9.30 ꢁ 10ꢂ30
2.28 ꢁ 10ꢂ29
3.71 ꢁ 10ꢂ29
e
e
134e136
156e158
263e265
252e254
217e219
290e292
199e201
197e199
261e263
183e185
193e195
243e245
169e171
199e201
242e244
e
e
460 (2.70)
e
e
474 (2.62)
354 (3.50)
396 (3.13)
504 (2.46)
395 (3.14)
432 (2.87)
533 (2.33)
360 (3.44)
408 (3.04)
515 (2.41)
a
Yield of the final cross-coupling step.
See the Experimental section for synthesis details.
Shoulder.
b
c

ꢁ
J. Kulhanek et al. / Dyes and Pigments 85 (2010) 57e65
62
Fig. 2. ORTEP representations of the new chromophores a) 1c, b) 2c, c) 3b and d) 4b (all measured at 150 K), showing the thermal ellipsoids at 50% probability (arbitrary spheres for
H atoms).
3.3. Electrochemistry
electron processes with the reduction potentials range from ꢂ1.65 to
ꢂ1.96 V. The measured potentials are dependent on the structure of
Electrochemical investigations of chromophores 1e6 were
carried out in acetonitrile þ 0.1 M Bu4NPF6 by CV, RDV and DC
polarography. Due to the adsorption phenomena resulting in
formation of an insulating film on the electrode either during
oxidation or reduction, combination of CV, RDV and DC polarography
techniques and polishing the electrodes before each scan needed to
be applied to obtain reproducible measurements. The acquired data
is summarized inTable 2 (derived from at least two different methods
and several times repeated measurements). As a general trend, the
first oxidation occurs on the donor groups, whereas the first reduc-
the chosen p-conjugated linker, that separates the 4,5-dicyanoimi-
dazole acceptor moiety and the donors. The potentials also seemed to
depend on the nature of the appended substituent D. Along the entire
series a, b and c, the reductionpotentials were shifted to less negative
potentials, with Ered,1 ranging from ꢂ1.90 to ꢂ1.65 V (series a), from
ꢂ1.75 to ꢂ1.91 (series b) and from ꢂ1.78 to ꢂ1.96 V (series c, see
Table 2) as a result of reduced transfer of electron density from the
donor to the 4,5-dicyanoimidazole acceptor moieties with increasing
length of the p-conjugated spacer. Thus, elongation of the p-conju-
gated path caused a reduction potential shift, with of the magnitude
of variation decreasing in the following order: 3, 6, 5, 4, 2,1. It may be
the case that this can be explained by the molecular structure and
spatial arrangement of the investigated chromophores (see X-ray
structures and discussion below). Not surprisingly, CT-chromophores
possessing methoxy (series b) or N,N-dimethylamino (series c) elec-
tron-donating groups showed higher reduction potentials than those
observed for series a. Dependence of Ered,1 on the structural charac-
teristics can be described by equation (1).
tion involves the
p-conjugated core bearing the 4,5-dicyanoimida-
zole moiety. This is consistent with previous studies on analogous
donor-substituted chromophores [13,14,38]. All the first reductions
of 2-substituted 4,5-dicyanoimidazoles1e6 are reversible, one-
Table 2
Electrochemical data of chromophores 1e6 obtained by DC polarography under
conditions: sD ¼ 1 s;
y
¼ 5 mV sꢂ1 and RDV at conditions: 500 rot minꢂ1
;
y
¼ 10 mV sꢂ1 measured in acetonitrile (5 ꢁ10ꢂ4 M solutions) containing Bu4NPF6 as
supporting electrolyte.
Compound
Ered;1 ¼ ꢂð1:93 ꢃ 0:03Þ ꢂ ð4:70 ꢃ 2:03Þ10ꢂ2IndDMA
DC polarography Ered,1 [V]a
RDV Eox,1 [V]a
ꢂ2
þ ð0:133 ꢃ 0:026ÞIndImꢂDB þ ð7:22 ꢃ 0:01Þ10
n
Ph
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
5a
5b
5c
6a
6b
6c
ꢂ1.90
ꢂ1.91
ꢂ1.96
ꢂ1.86
ꢂ1.89
ꢂ1.92
ꢂ1.65
ꢂ1.75
ꢂ1.81
ꢂ1.82
ꢂ1.84
ꢂ1.85
ꢂ1.75
ꢂ1.78
ꢂ1.81
ꢂ1.74
ꢂ1.78
ꢂ1.78
e
2.18
1.38
2.20
1.76
0.93
1.72
1.33
0.69
1.88
1.50
0.79
1.55
1.25
0.58
1.89
1.47
0.72
N ¼ 18; s ¼ 3:98 ꢁ 10ꢂ2; R ¼ 0:886;
Fð3; 13Þ ¼ 15:8
ð1Þ
Although the correlation is not as strong as in other cases (see
below), it confirms the observations discussed earlier and indicates
that the lowered reduction potential is as a result of A-
-system elongation. However, influence of the imidazole-double
bond connection (IndIm-DB characteristic) implies that a planar
-conjugated spacer ensuring good communication of the donor
p-D
p
p
and acceptor moieties is crucial for the stabilisation of the reduced
form.
Except 1a, all chromophores undergo reversible one-electron
oxidation processes. The first oxidation potential Eox,1 ranges from
2.20 to 0.58 V and is shifted to less positive potentials with
a
The potentials are given vs. SCE.
increasing length of the p-conjugated path and donating character

ꢁ
J. Kulhanek et al. / Dyes and Pigments 85 (2010) 57e65
63
of the appended substituent D. The observed trend is similar to that
observed for reduction potentials. Dependence of Eox,1 on the
structural characteristics can be expressed by equation (2).
Eox;1 ¼ ð2:18 ꢃ 0:11Þ ꢂ ð0:894 ꢃ 0:094ÞIndDMA
ꢂ ð0:325 ꢃ 0:094ÞIndDB ꢂ ð0:245 ꢃ 0:065ÞnPh
N ¼ 17; s ¼ 0:184; R ¼ 0:947; Fð3; 13Þ ¼ 37:6
ð2Þ
In contrast to the equation (1), this correlation is significantly
stronger. Presence of electron-donating substituents and elonga-
tion of the p-conjugated path through a competent p-conjugating
linker reduces the oxidation potential. Dependence of Eox,1 on the
energy of the HOMO (excluding 1c as outlier) is described by
equation (3).
Eox;1 ¼ ꢂ ð7:52 ꢃ 0:46Þ ꢂ ð0:995 ꢃ 0:005ÞEHOMO
N ¼ 16; s ¼ 0:110; r ¼ 0:980; Fð1; 14Þ ¼ 33:9 ð3Þ
This strong correlation unambiguously illustrates the relation-
ship between the energy of the HOMO and the oxidation potential
Eox,1
.
3.4. UVevisible/fluorescent study and structure-hyperpolarisability
consideration of chromophores 1e6
Electronic absorption spectra of imidazoles 1e6 measured in
CH2Cl2 showed intense charge-transfer (CT) absorption bands in
the UVevisible region (Fig. 3, Table 1). The position of CT-band
depends on the electronic nature of the appended donor and length
of the conjugated
p-backbone. Whereas the chromophores in
series a exhibited CT-bands with lmax appearing between 244 and
325 nm, the donor-substituted series b and c showed CT-bands
with lmax ranging from 271 to 331 and 293e381 nm, respectively.
Compared with the unsubstituted reference series a, the longest-
wavelength absorption bands of the chromophore series b and c
are significantly bathochromically shifted as a result of the positive
mesomeric effect of the donor groups (Fig. 3, Table 1). In general,
increasing the strength of the donor or acceptor groups reduces the
energy differences between ground and excited states which
results in a bathochromically shifted CT-band [31]. This fact could
be nicely demonstrated by comparison of the chromophores in
series b and c that contained an identical p-conjugated spacer. The
CT-band of chromophores bearing stronger N,N-dimethylamino
group (series c) are shifted more bathochromically than those for
series b with weaker methoxy substituents. On the other hand, the
observed bathochromic shift within the individual series aec can
be rationalised as an effect of the p-conjugated spacer between the
appended acceptors and donors. The influence of the spacer on the
efficiency of the donoreacceptor conjugation is best evaluated in
series bec, in which donoreacceptor separation is systematically
varied. It has already been demonstrated [13,14], that if strong
donoreacceptor conjugation is achieved, the HOMO of the donor is
lowered and the LUMO of the acceptor raised. In turn, the conver-
gence of the HOMO and LUMO results in a large optical gap. On the
contrary, when the donor and acceptor are insulated by a large
p-conjugated spacer, the energy levels of HOMO and LUMO
resemble those in the free components, yielding a small optical gap.
Therefore, bathochromically shifted CT-bands are being measured.
Beginning with the simplest compound in series
c (1c,
lmax ¼ 293 nm) the longest-wavelength transition was measured
for chromophore 3c (lmax ¼ 381 nm) with (E)-styryl linker. With
respect to the performed X-ray analysis (Fig. 2), the phenyl ring
connected to C-2 in chromophores with 1,4-phenylene (2c,
lmax ¼ 316 nm) or biphenyl (4c, lmax ¼ 346 nm) spacers is sterically
forced out of the mean plane of the 4,5-dicyanoimidazole.
Fig. 3. Electronic absorption spectra of 1e6 (10ꢂ5 M solutions in dichloromethane).Ă

ꢁ
J. Kulhanek et al. / Dyes and Pigments 85 (2010) 57e65
64
Therefore, the N,N-dimethylamino donor group is engaged in less
conjugation while the bathochromic shift is not as pronounced as
for chromophores 3c featuring styryl spacer. Planarity e an
important aspect of rational design of organic chromophores e
could be further probed by comparison of chromophore 3c with
chromophore 5c, which bears an additional 1,4-phenylene unit
and, as in 2c and 4c, lacks planarity. However, lmax decreased from
381 to 380 nm. On the contrary, the olefinic linker in 5c proved to
be more ‘‘transparent” than the more electronegative acetylenic
one in 6c (lmax ¼ 364 nm). Similar conclusions could be made for
the methoxy-substituted chromophores in series b. Dependence of
1=lAmax on the bathochromic shift-inducing factors could be
statistically described by equations (4) and (5).
and methoxy groups as donors with a systematically enlarged
-conjugated spacer have been synthesised mainly by cross-coupling
p
reactions. The prepared push-pull molecules have been further
investigated by UV/Vis spectroscopy, electrochemistry, X-ray analysis
as well as by quantum-chemical calculations. From the observations
made and the performed factor analysis we can deduce the following
conclusions: (i) optical properties of polar push-pull chromophores
can be easily tailored by the modification of the appended donor or
acceptor (compare series a, b and c) and by the extension/shortening
of the
trochemical behaviour of polar push-pull chromophores depends on
the length and planarity of the -conjugated system as well as on the
p-conjugated spacer (compare chromophores 1e6); (ii) elec-
p
donating character of the appended donor; (iii) the first oxidation
potential strongly correlates with the calculated HOMO indicating
1=lAmax ¼ ð3:88 ꢃ 0:08Þ10ꢂ3 ꢂ ð4:78 ꢃ 0:76Þ10ꢂ4IndDMA
their close relationship; (iv) likewise, nonlinear optical properties (
depend on the donating character of the appended donor and on the
length and planarity of the -conjugated system.
Thus, presence of a strongly conjugating donor and the length
and planarity of the -conjugated system are the most important
structural factors determining electrochemical, spectral as well as
nonlinear optical properties of the studied push-pull A- -D
b)
ꢂ4
ꢂ ð3:87 ꢃ 0:77Þ10ꢂ4IndDB ꢂ ð2:67 ꢃ 0:78Þ10
n
Ph
p
N ¼ 18; s ¼ 1:52 ꢁ 10ꢂ4; R ¼ 0:939;
Fð3; 14Þ ¼ 35:0
p
ð4Þ
p
1=lAmax ¼ ð3:80 ꢃ 0:09Þ10ꢂ3 ꢂ ð4:79 ꢃ 0:85Þ10ꢂ4IndDMA
systems. Considering the planarity, electrochemical behaviour,
UVeVis properties, high melting point, solubility as well as one of
þð5:83 ꢃ 1:38Þ10ꢂ4IndIm Ph ꢂ ð5:97 ꢃ 0:87Þ10
n
Ph
ꢂ4
-
N ¼ 18; s ¼ 1:69 ꢁ 10ꢂ4; R ¼ 0:925;
Fð3; 14Þ ¼ 27:5
the highest calculated average second-polarisabilities
b of 3c, the
styryl -conjugated spacer combined with the strong N,N-dime-
p
ð5Þ
thylamino donor group seems to possess one of the better balances
of performance and practicality within the studied series.
Those correlations clearly demonstrate that increasing the
donating character of the substituent D (indicator IndDMA), elon-
gation of the
p-conjugated path (number of 1,4-phenylene units
Acknowledgements
nPh) and planarity of the chromophore (indicators IndDB vs. IndIm-Ph
with opposite sign) caused a bathochromic shift. In addition,
1=lAmax is highly correlated with the differences between EHOMO and
ELUMO according to equation (6).
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).
1=lAmax ¼ ꢂð2:05 ꢃ 0:09Þ10ꢂ3þð6:66 ꢃ 0:40Þ10ꢂ4
ðEHOMO ꢂ ELUMO
Þ
N ¼ 18; s ¼ 9:62 ꢁ 10ꢂ5
;
References
r ¼ 0:973; Fð1; 16Þ ¼ 28:3
ð6Þ
[1] He GS, Tan LS, Zheng Q, Prasad PN. Multiphoton absorbing materials:
molecular desings, characterizations, and applications. Chem Rev 2008;108
(4):1245e330.
Chromophores with larger
p
-conjugated systems demonstrated
fluorescent behaviour while high fluorescence was observed
mainly for the chromophores with biphenyl (4), (E)-phenyl-
ethenylphenyl (5) and phenylethynylphenyl (6) central spacers. As
a general trend, the fluorescence band is being bathochromically
[2] Kivala M, Diederich F. Acetylene-derived strong organic acceptors for
planar and nonplanar push-pull chromophores. Acc Chem Res 2009;42
(2):235e48.
ꢂ
[3] Andersson AS, Kerndrup L, Madse A[, Kilsa K, Nielsen MB, La Porta PR, et al.
Synthesis and characterization of tetrathiofulvalene-substituted di- and tet-
shifted with increasing length of the
p
-conjugated path, which is
raethenylethenes with p-nitrophenyl acceptors.
375e82.
[4] Prasad PN, Williams DJ. Nonlinear optical effects in molecules & polymers.
John Wiley Sons Inc.; 1991.
[5] Marder SR. Organic nonlinear materials: where we have been and where we
are going. Chem Commun 2006:131e4.
[6] Moonen NNP, Gist R, Boudon C, Gisselbrecht JP, Seiler P, Kawai T, et al. Donor-
substituted cyanoethenylethenes: powerful chromophores for opto-electronic
applications. Org Biomol Chem 2003;1(12):2032e4.
J Org Chem 2009;74(1):
analogous to that observed in absorption spectra (see above). A full
account of the fluorescent properties of the chromophores
described in this paper will be presented elsewhere.
In accordance with the above experimental quantities, the
calculated average second-polarisabilities
increasing donating character of substituent D (indicator IndDMA),
elongation of the -conjugated path (number of 1,4-phenylene
b (Table 1) depend on
[7] Jiang G, Michinobu T, Yuan W, Feng M, Wen Y, Du S, et al. Crystalline thin films
p
of
a donor-substituted cyanoethynylethene for nanoscale data recording
units nPh) and planarity of the chromophore (indicators IndDB), as
documented by equation (7).
through intramolecular charge-transfer interaction. Adv Mater 2005;17
(18):2170e3.
[8] Haldi A, Kimyonok A, Domercq B, Hayden LE, Jones SC, Marder SR, et al.
Optimization of orange-emitting electrophosphorescent copolymers for
organic light-emitting diodes. Adv Funct Mater 2008;18(19):3056e62.
[9] Innocenzi P, Lebeau B. Organiceinorganic hybrid materials for non-linear
optics. J Mater Chem 2005;15:3821e31.
[10] Cho MJ, Choi DH, Sullivan PA, Akelaitis AJP, Dalton LR. Recent progress in
second-order nonlinear optical polymers and dendrimers. Prog Polym Sci
2008;33(11):1013e58.
[11] Ma H, Liu S, Luo J, Suresh S, Liu L, Kang SH, et al. Highly efficient and thermally
stable electro-optical dendrimers for photonics. Adv Funct Mater 2002;12
(9):565e74.
[12] Kivala M, Boudon C, Gisselbrecht JP, Seiler P, Gross M, Diederich F. Charge-
transfer chromophores by cycloaddition-retro-electrocyclization: multi-
valent systems and cascade reactions. Angew Chem Int Ed 2007;46
(33):6357e60.
b
¼ ꢂð4:00 ꢃ 0:08Þ10ꢂ30 þ ð1:52 ꢃ 0:76Þ10ꢂ29IndDMA
ꢂ30
þ ð1:08 ꢃ 0:77Þ10ꢂ29IndDB þ ð8:65 ꢃ 2:52Þ10
n
Ph
N ¼ 18; s ¼ 7:87 ꢁ 10ꢂ30; R ¼ 0:854;
Fð3; 14Þ ¼ 12:5
ð7Þ
4. Conclusion
Eighteen chromophores 1e6 (series a,
c an d) featuring
4,5-dicyanoimidazole as an acceptor moiety and N,N-dimethylamino

ꢁ
J. Kulhanek et al. / Dyes and Pigments 85 (2010) 57e65
65
ꢀ
[13] Bures F, Schweizer WB, May JC, Boudon C, Gisselbrecht JP, Gross M, et al.
[34] Sun YF, Huang W, Lu CG, Cui YP. The synthesis, two-photon absorption and
blue upconversion fluorescent of novel, nitrogen-containing heterocyclic
chromophores. Dyes Pigm 2009;81(1):10e7.
[35] Dalton LR. Rational design of organic electro-optic materials. J Phys Condens
Matter 2003;15(20):R897e934.
[36] Woodward DW. US Patent 1950: 2534331.
[37] Webster OW, Donald DS. Synthesis of heterocycles from hydrogen cyanide
derivatives. Adv Heterocycl Chem 1987;41:1e40.
Propoerty tuning in charge-transfer chromophores by systematic modulation
of the spacer between donor and acceptor. Chem Eur J 2007;13(19):5378e87.
ꢀ
[14] Bures F, Schweizer WB, Boudon C, Gisselbrecht JP, Gross M, Diederich F. New
push-pull chromophores featuring TCAQ (11,11,12,12-tetracyano-9,10-anthra-
quinodimethane) and other dicyanovinyl acceptors. Eur J Org Chem 2008:
994e1004.
ꢀ
[15] May JC, Biaggio I, Bures F, Diederich F. Extended conjugation and
donoreacceptor substitution to improve the third-order optical nonlinearity
[38] Apen PG, Rasmussen PG. Synthesis and properties of new derivatives of 4,5-
dicyanoimidazole and 4,40,5,50-tetracyano-2,20-biimidazole. J Am Chem Soc
1991;113(16):6178e87.
of small molecules. Appl Phys Lett 2007;90: No. 251106.
ꢀ
[16] Bures F, Pytela O, Diederich F. Solvent effects on electronic absorption spectra
of donor-substituted 11,11,12,12-tetracyano-9,10-anthraquinodimethanes
(TCAQs). J Phys Org Chem 2009;22(2):155e62.
[39] Subrayan RP, Kampf JW, Rasmussen PG. Synthesis and properties of bis- and
tris(4,5-dicyano-1-methyl-2-imidazolyl)amines:
a new acidic secondary
[17] Archetti G, Abbotto A, Wortmann R. Effect of polarity and structural design on
molecular photorefractive properties of heteroaromatic-based push-pull dyes.
Chem Eur J 2006;12(27):7151e60.
[18] Hebbar N, Ramondenc Y, Ple G, Dupas G, Ple N. Push-pull structures with
a pyrazine core and hexatriene chain: synthesis and light-emitting properties.
Tetrahedron 2009;65(21):4190e200.
amine and nonbasic tertiary amines. J Org Chem 1994;59(15):4341e5.
[40] Apen PG, Rasmussen PG. Nucleophilic aromatic substitution in 4,5-dicyanoi-
midazole. Heterocycles 1989;29(7):1325e9.
[41] Densmore CG, Rasmussen PG. New acetylenic monomers and polymers from
4,5-dicyanoimidazole. Macromolecules 2004;37(16):5900e10.
[42] Shin RYC, Sonar P, Siew PS, Chen ZK, Sellinger A. Electron-accepting conju-
gated materials based on 2-vinyl-4,5-dicyanoimidazoles for application in
organic electronics. J Org Chem 2009;74(9):3293e8.
[43] Carella A, Centore R, Sirigu A, Tuzi A, Quatela A, Schutzmann S, et al. Second
order nonlinear optical performances of polymers containing imidazole
and benzimidazole chromophores. Macromol Chem Phys 2004;205(14):
1948e54.
ꢁ
ꢁ
[19] Jarowski PD, Wu YL, Schweizer WB, Diederich F. 1,2,3-Triazoles as conjugative
linkers in push-pull chromophores: importance of substitution positioning on
intramolecular charge-transfer. Org Lett 2008;10(15):3347e50.
ꢁ
[20] Oliva MM, Casado J, Raposos MMM, Fonseca AMC, Hartmann H, Henandez V,
et al. Structureeproperty relationships in push-pull amino/cyanovinyl end-
capped oligothiophenes: quantum chemical and experimental studies. J Org
Chem 2006;71(20):7509e20.
[44] Carella A, Centore R, Riccio P, Sirigu A, Quatela A, Palazzesi C, et al. Poly-
methacrylate copolymers containing 4,5-dicyanoimidazole-based chromo-
phores and their nonlinear optical behavior. Macromol Chem Phys 2005;206
(14):1399e404.
[21] Quist F, Vande Velde CML, Didier D, Teshome A, Asselberghs I, Clays K, et al.
Push-pull chromophores comprising benzothiazolium acceptor and thiophene
auxiliary donor moieties: synthesis, structure, linear and quadratic non-linear
optical properties. Dyes Pigments 2009;81(3):203e10.
ꢀ
ꢁ
ꢂꢀ ꢀ
[45] Patel A, Bures F, Ludwig M, Kulhanek J, Pytela O, Ruzicka A. Novel charge-
ꢁ
ꢁ
ꢁ
ꢁ
[22] Zajac M, Hrobarik P, Magdolen P, Foltınova P, Zahradnık P. Donor-
p-acceptor
transfer chromophores featuring imidazole as
p
-linkage. Heterocycles 2009;
-conju-
benzothiazole-derived dyes with and extended heteroaryl-containing conju-
gated systems: synthesis, DFT study and microbial activity. Tetrahedron
2008;64(46):10605e18.
78(4):999e1013.
ꢁ
ꢀ
[46] Kulhanek J, Bures F, Ludwig M. Convenients methods for preparing
p
gated linkers as building blocks for modular chemistry. Beilstein J Org Chem
2009;5(11).
[47] O'Connell JF, Parquette J, Yelle WE, Wang W, Rapoport H. Convenient synthesis
of methyl 1-methyl-2,4-dibromo-5-imidazolecarboxylate. Synthesis 1988:
767e71.
[48] Anderson WK, Bhattacharjee D, Houston DM. Design, synthesis, antineoplastic
activity, and chemical properties of bis(carbamate) derivatives of 4,5-bis
(hydroxymethyl)imidazole. J Med Chem 1989;32(1):119e27.
[49] Otwinowski Z, Minor W. Processing of X-ray diffraction data collected in
oscillation mode. Methods Enzymol 1997;276:307e26.
[23] Feng K, De Boni L, Misoguti L, Mendonxca CR, Meador M, Hsu FL, et al. Y-shaped
two-photon absorbing molecules with an imidazole-thiazole core. Chem
Commun 2004:1178e80.
[24] Ren J, Wang SM, Wu LF, Xu ZX, Dong BH. Synthesis and properties of novel Y-
shaped NLO molecules containing thiazole and imidazole chromophores. Dyes
Pigments 2008;76(2):310e4.
€
[25] Santos J, Mintz EA, Zehnder O, Bosshard C, Bu XR, Gunter P. New class of
imidazoles incorporated with thiophenevinyl conjugation pathway for robust
nonlinear optical chromophores. Tetrahedron Lett 2001;42(5):805e8.
[26] Wu W, Zhang Z, Zhang X. Design and syntheses of highly efficient and ther-
mally-stable imidazole chromophores bearing diamino groups for nonlinear
optics. J Nonlinear Opt Phys Mater 2005;14(1):61e5.
[50] Coppens P. In: Ahmed FR, Hall SR, Huber CP, editors. Crystallographic
computing. Copenhagen: Munksgaard; 1970. p. 255e70.
[51] Altomare A, Cascarano G, Giacovazzo C, Guagliardi A. Completion and
[27] Wu W, Zhang Z, Zhang X. Syntheses of NLO diamino chromophores containing
refinement of crystal structures with SIR92.
J
Appl Crystallogr 1993;26
imidazole and thiophene rings as conjugation linkage.
2004;9:617e9.
J
Chem Res
(3):343e50.
€
€
[52] Sheldrick GM. SHELXL-97. Gottingen: University of Gottingen; 1997.
[53] Flack HD, Bernardinelli G. Absolute structure and absolute configuration. Acta
Cryst 1999;A55(5):908e15.
[54] Flack HD, Bernardinelli G. Reporting and evaluating absolute-structure and
absolute-configuration determinations. J Appl Cryst 2000;33(4):1143e8.
[55] ArgusLab, Mark Thompson and Planaria Software LLC, Version 4.01, webpage:
http://www.arguslab.com.
[28] Wang S, Zhao L, Xu Z, Wu C, Cheng S. Novel nonlinearity-transparency-
thermal stability trade-off of imidazole chromophores for nonlinear optical
application. Mat Lett 2002;56(6):1035e8.
[29] Miller RD, Lee VY, Moylan CR. Substituted azole derivatives as nonlinear
optical chromophores. Chem Mater 1994;6(7):1023e32.
[30] Bu XR, Li H, Van Derveer D, Mintz EA. A novel approach to synthesis of tri-
cyanovinylthiophene for heterocyclic imidazole nonlinear optical chromo-
phores. Tetrahedron Lett 1996;37(41):7331e4.
[31] Batista RMF, Costa SPG, Belsley M, Raposo MMM. Synthesis and optical
properties of novel, thermally stable phenanthrolines bearing an arylthienyl-
imidazo conjugation pathway. Dyes Pigm 2009;80(3):329e36.
[32] CarellaA, CentoreR, FortA, PelusoA, Sirigu A, TuziA. Tuningsecond-orderoptical
nonlinearities in push-pull benzimidazoles. Eur J Org Chem 2004:2620e6.
[33] Batista RMF, Costa SPG, Belsley M, Raposo MMM. Synthesis and second-order
nonlinear optical properties of new chromophores containing benzimidazole,
thiophene, and pyrrole heterocycles. Tetrahedron 2007;63(39):9842e9.
[56] MOPAC 2009, Stewart JJP, Stewart Computational Chemistry, Version 8.315W,
webpage: http://OpenMOPAC.net.
ꢀ
[57] OPstat, Oldrich Pytela, version 6.20, webpage: http://webak.upce.cz/~koch/cz/
veda/OPgm.htm.
ꢀ
ꢁ
ꢁ
ꢀ
ꢁ
[58] Szotkowski T, Bures F, Pytela O, Kulhanek J, Travnıcek Z. Synthesis and E/Z
determination of substituted 2-phenylimidazoles. J Heterocycl Chem 2006;43:
1583e9.
[59] Corsih J, Morton-Blake DA, O'Donoghue F, Baudour JL, Beniere F, Toudic B. An
atomistic simulation investigation of the inter-ring torsion in crystalline
biphenyl. J Mol Struct (THEOCHEM) 1995;358:29e38.
ꢁ
ꢃ