Synthesis, linear & non linear optical (NLO) properties of some indoline based chromophores

Article abstract of DOI:10.1016/j.dyepig.2010.10.010

A series of indoline-π-acceptor chromophores has been synthesized and the members first hyperpolarizabilities measured to investigate the effect of conjugation length, the use of various substituents and configurational locking on the nonlinear optical re

Full text of DOI:10.1016/j.dyepig.2010.10.010

Dyes and Pigments 89 (2011) 177e187
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, linear & non linear optical (NLO) properties of some indoline
based chromophores
,
M. Delower H. Bhuiyan a, Mohamed Ashraf a, Ayele Teshome b, Graeme J. Gainsford a, Andrew J. Kaya
*
,
Inge Asselberghs b, Koen Clays b
a Photonics, Industrial Research Limited, P. O. Box 31-310, Lower Hutt, New Zealand
b Department of Chemistry, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of indoline-p-acceptor chromophores has been synthesized and the members first hyper-
Received 15 September 2010
Received in revised form
20 October 2010
Accepted 21 October 2010
Available online 31 October 2010
polarizabilities measured to investigate the effect of conjugation length, the use of various substituents
and configurational locking on the nonlinear optical response. While increasing the length of the
conjugated interconnect enhances the optical nonlinearity, ring locking was found to have little effect,
although this is thought to be due to the electronic properties of the substituent groups, rather than
structural factors. Nonetheless, all of the compounds were found to have high molecular hyper-
polarizabilities with values of up to 1485 ꢀ 10ꢁ30 esu when measured in chloroform at 1300 nm, findings
which confirm that these chromophores are excellent candidates for further study. X-ray crystallographic
studies were performed on three of the compounds and bond length alternation values obtained; these
were found to correlate well with the observed molecular responses.
Keywords:
Nonlinear optics
Chromophore synthesis
Hyperpolarizability
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
the observed NLO response) it is often necessary to employ tech-
niques such as cross-linking in order to preserve the requisite non-
Organic nonlinear optical (NLO) chromophores containing
donor(D)e eacceptor(A) units have been widely reported in the
literature as the enabling materials for a range of photonic devices
[1e4]. The fundamental measure of a molecule’s NLO response, and
therefore potential for incorporation into NLO materials, is its
centrosymmetry [6].
p
Chromophores with highly polar (i.e., zwitterionic) ground
states often exhibit poor solubilities as well as a tendency to readily
form aggregates. This is clearly problematic when considering their
use in NLO materials as this means they cannot be incorporated into
host polymers at high loadings. Furthermore, the presence of
significant aggregation will lower the overall poling efficiency of
the final NLO material as well as increase the propensity for dele-
terious post-poling relaxation of the aligned dipoles. As a result,
further structural modifications are often required to the active
chromophores to minimise aggregation and the inclusion of bulky,
“arene-rich” substituents has been shown to be particularly effec-
tive in achieving this, thereby greatly increasing the observed
macroscopic response in NLO materials [8].
We have previously reported a synthetic methodology [13e17]
that allows entry to a number of high figure-of-merit NLO chromo-
phores with aromatisable donors, (e.g., 1,4-dihydropyridinylidene,
1,4-dihydroquinolinylidene), and containing the powerful acceptor
4,5,5-trimethyl-3-cyano-2(5H)-furanylidenepropane dinitrile (TCF)
e.g., compounds 1 and 2. While this approach allowed for ease of
synthesis and for a controlled increase in the extent of conjugation in
the molecules, the resultant “parent” merocyanines are prone to
significant amounts of aggregation [16], and this is due to their highly
figure-of-merit,
m.b, where m is the molecular dipole moment, and
b
is the first hyperpolarizability. In order to be considered a viable
candidate for further study a molecule should have a first hyper-
polarizability greater than approximately 500 ꢀ 10ꢁ30 esu and an
overall figure-of-merit greater than 5000 ꢀ 10ꢁ48 esu. Furthermore,
in order to obtain an optimal and usable electro-optic response, it is
necessary to incorporate a high weight percent of a given chro-
mophore into either a host polymer matrix, or by covalently
attaching it to a suitable polymer substrate [5e12]. In addition, for
a macroscopic NLO response to be observed it is necessary for the
molecular dipoles of the embedded chromophores to be aligned
acentrically, and this is usually achieved via either contact or corona
poling [3]. However, due to the tendency of the dipoles of the active
chromophores to relax back to an isotropic state (thereby reducing
* Corresponding author. Tel.: þ64 4 931 3210; fax: þ64 4 931 3306.
E-mail address: a.kay@irl.cri.nz (A.J. Kay).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.10.010

178
M.D. H. Bhuiyan et al. / Dyes and Pigments 89 (2011) 177e187
polar, zwitterionic ground states. Evidence for H-aggregation in these
compounds was routinely observed via the appearance of a high
energy shoulder when UVeVis absorption spectra were obtained in
solvents of low polarity such as dioxane and chloroform. Given that
the dielectric constants of these solvents resemble those found for
polymer systems, viz. 3 <10, this is clearly problematic as it suggests
these molecules will resist alignment and be prone to relaxation after
poling.
as external reference. Additional HRS measurements for compound
7 were conducted using a 800 nm fs system and using a series of
solvents with a broad range of dielectric constants, (
3); (DMSO,
3
¼ 47; DMF, ¼ 36.7; MeOH, ¼ 32.6 and CHCl3 3 ¼ 4.81). Crystal
3
3
violet dissolved in methanol was used as an octupolar external
reference (bxxx ¼ 338 ꢀ 10ꢁ30 esu) for the 800 nm measurements.
For all of the HRS measurements, a series of dilute solutions were
measured and compared to the reference concentration series. To
correct for the differences in the solvent between the chromo-
phores and the reference compound, local field correction factors
were applied [(n2 þ 2)/3]3 where n is the refractive index of the
solvent at the sodium D line, with n(DMSO) ¼ 1.48, n(DMF) ¼ 1.43,
n(MeOH) ¼ 1.32 and n(CHCl3) ¼ 1.44. The calculations of the
dynamic first hyperpolarizability were performed by taking the
ratio of the slopes of the sample and the reference compound. For
the dynamic first hyperpolarizability values obtained at 800 nm it is
necessary to consider the appropriate tensor components (dipolar
versus octupolar geometry) as the reference compound crystal
violet is octupolar whereas the chromophores under investigation
are dipolar. The static first hyperpolarizability is derived from the
simple two-level model as previously described [18,19]. The appa-
ratus and experimental procedures for the femtosecond HRS
measurements at 800 nm and 1300 nm are exactly the same as
described before [20,21].
As a continuation of this work, we describe here further
developments to our synthetic methodology, and extend the series
to include chromophores with a non-aromatisable indoline donor
group. Furthermore, we will also discuss structural modifications
that are designed to increase their NLO figure-of-merit e these
include ring locking of the
p-conjugated system along with the
2.3. Synthetic methodology
incorporation of bulky substituents, e.g., diphenyl groups, onto this
central ring system. In addition, we have substituted a halogen
atom in middle of the conjugated interconnect with a variety of
substituents in order to assess the impact from both a steric and
electronic point of view.
The methods used to synthesis the chromophores are shown in
Schemes 1 and 2. Compounds 4e6, 10, 13, 14, 20, 21 and TCF were
prepared according to the literature [15,22].
2.4. General procedure for the synthesis of chromophores 7e9 and
2. Experimental
11
2.1. Reagents and procedures
1,3,3-Trimethyl-2-methyleneindoline (3) (1.5 eq) and the
appropriate acceptor precursor (4e6 or 10) (1 eq) were dissolved in
a mixture of methanol and acetic acid (4:1 ratio). A catalytic
quantity of piperidine acetate was then added and the mixture
refluxed for 12 h. The solution was cooled to ambient temperature,
the product collected by filtration and washed with copious
quantities of hot water, followed by 2e3 small portions of cold
methanol to afford the crude product as a coloured solid. The
chromophore was obtained pure following recrystallization.
Commercially available reagents were obtained from Aldrich
and were used without additional purification. The solvents used
were of analytical grade and were also used without further puri-
fication. Column chromatography was carried out using gravity
feed column techniques on Merck silica gel type 9385 (230e400
mesh) with the stated solvent systems. Analytical thin-layer chro-
matography (TLC) analyses were performed on pre-coated plates
(Merck aluminium sheets, silica gel 60F 254, 0.2 mm). Visualisation
of compounds was achieved by illumination under ultraviolet light
(254 nm).
2.4.1. 2-{3-[2-(1,3,3-Trimethyl-1,3-dihydroindol-2-ylidene)
propenyl]-5H-furan-2-ylidene]malononitrile (7)
Black solid (55%). Recrystallization (ethyl acetate:hexanes, 1:1)
2.2. Measurements and instrumentation
afforded a dark green powder, m.p. 254e256 ꢂC. (Found: MNaþ m/z
405.1694 C24H22N4O requires MNaþ m/z 631.2048;
D
¼ 0.7 ppm). 1H
Melting points were recorded with a EZ-Melt automated melting
point apparatus and are uncorrected. 1H and 13C NMR spectra were
recorded on a Bruker AVANCE 300 MHz or 500 MHz spectrometer
and proton multiplicities are defined by the usual notations.
Accurate mass measurements were made on a Micromass Q-Tof
Premier Mass Spectrometer operating in the positive ion mode. The
UVeVisible (UVeVis) absorption spectra were recorded by a Per-
kineElmer Lambda 900 spectrometer at room temperature. Hyper-
Rayleigh scattering (HRS) measurements were conducted at
1300 nm for all of the compounds. For these measurements
a tsunami-pumped OPO (model OPAL, Spectra-Physics) was used.
The HRS measurements of compounds 7e9,17e18 and 22e25 were
performed in chloroform. Due to their limited solubility in chlo-
roform, the HRS measurements of compounds 26e28 were con-
ducted in dimethylformamide (DMF). The dipolar compound
Disperse Red 1 (DR1, bzzz ¼ 54 ꢀ 10ꢁ30 esu in chloroform) was used
NMR (500 MHz, d6-DMSO) d 8.25 (1H, app. s, CH), 7.60 (1H, m, ArH),
7.39e7.43 (1H, m, ArH), 6.30 (1H, d, J 15.0 Hz, CH), 6.13 (1H, d, J
15.0 Hz, CH), 3.69 (3H, s, NeCH3), 1.65 (6H, s, 2 ꢀ CH3), 1.58 (6H, s,
2 ꢀ CH3). 13C NMR (75 MHz, d6-DMSO)
d 177.4, 174.5, 143.7, 143.3,
142.5, 140.9. 128.4, 125.1, 122.3, 114.3, 111.3, 107.0, 102.1, 95.6, 48.9,
31.2, 26.3. lmax (CHCl3) 602 nm, log103 5.23.
2.4.2. 2-{3-Cyano-5,5-dimethyl-4-[5-(1,3,3-trimethyl-1,3-
dihydroindol-2-ylidene)penta-1,3-dienyl]-5H-furan-2-ylidene}
malononitrile (8)
Green powder (68%). Recrystallization (ethyl acetate) afforded
a green powder, m.p. 270e272 ꢂC. (Found: MNaþ m/z 431.1844
C26H24N4O requires MNaþ m/z 431.1848;
D
¼ 0.9 ppm). 1H NMR
(300 MHz, d6-DMSO)
d 8.12 (1H, t, J 12.0 Hz, CH), 7.88 (1H, t, J
12.0 Hz, CH), 7.58e7.60 (1H, m, ArH), 7.36e7.40 (2H, m, ArH),
7.21e7.27 (1H, m, ArH), 6.47 (1H, t, J 15.0 Hz, CH), 6.26 (1H, d, J

M.D. H. Bhuiyan et al. / Dyes and Pigments 89 (2011) 177e187
179
Scheme 1. Synthetic method used to prepare the parent chromophores. Reagents and Conditions. (i). MeOH:Acetic acid (4:1), piperidinium acetate, reflux 12 h.
15.0 Hz, CH), 6.04 (1H, d, J 12.0 Hz, CH), 3.59 (3H, s, NeCH3), 1.70
(6H, s, 2 ꢀ CH3), 1.64 (6H, s, 2 ꢀ CH3). 13C NMR (75 MHz, d6-DMSO)
d
176.1, 172.8, 169.2, 152.8, 150.8, 142.7, 140.9, 128.3, 125.4, 124.7,
122.2, 115.2, 114.4, 113.4, 110.9, 107.9, 103.9, 95.1, 48.7, 31.0, 26.9,
26.5. lmax (CHCl3) 698, log103 5.21.
2.4.3. 2-{3-Cyano-5,5-dimethyl-4-[7-(1,3,3-trimethyl-1,3-
dihydroindol-2-ylidene)hepta-1,3,5-trienyl]-5H-furan-2-ylidene}
malononitrile (9)
Green powder (70%). Recrystallization (dichlorometþhane)
afforded a green powder, m.p. 238e240 ꢂC. (Found: MNa m/z
457.1999C28H26N4O requires MNaþ m/z 457.2004;
D
¼ 1.1 ppm). 1H
NMR (500 MHz, d7-DMF)
d 8.01e8.21 (1H, m, ArH), 7.74e7.89 (3H,
m, ArH & CH), 7.60e7.71 (2H, m, ArH), 7.40e7.55 (1H, m, ArH),
6.72e6.80 (2H, m, CH), 6.60 (1H, d, J 15.0 Hz, CH), 6.40 (1H, d, J
12.0 Hz, CH), 3.88 (3H, s, NeCH3), 1.89 (6H, s, 2 ꢀ CH3), 1.85 (6H, s,
2 ꢀ CH3). 13C NMR (125 MHz, d7-DMF)
d 176.8, 171.7, 167.9, 154.0,
150.0, 148.1, 143.5, 141.3, 130.4, 128.9, 128.5, 126.2, 124.5, 122.3,
115.4,114.7,114.4,110.7,109.5,104.2, 95.4, 48.9, 30.8, 26.9, 26.1, 25.3.
lmax (CHCl3) 794, log103 5.12.
2.4.4. 2-{2-{2-Chloro-3-[2-(1,3,3-trimethyl-1,3-dihydroindol-2-
ylidene)ethylidene]cyclohex-1-enyl}-vinyl}-3-cyano-5,5-dimethyl-
5H-furan-2-ylidene]malononitrile (11)
Green powder (36%). Recrystallization (dichloromethane:hexþ-
anes, 2:1) afforded a green solid, m.p. 252e255 ꢂC. (Found: MNa
m/z 661.304 C41H42N4OS requires MNaþ m/z 661.294;
D
¼ 0.4 ppm).
1H NMR (300 MHz, d6-DMSO)
d 8.29 (1H, d, J 15.0 Hz, CH), 8.15 (1H,
d, J 15.0 Hz, CH), 7.60 (1H, d, J 7.5 Hz, ArH), 7.39e7.44 (2H, m, ArH),
7.24e7.30 (1H, m, ArH), 6.27 (1H, d, J 15.0 Hz, CH), 5.92 (1H, d, J
15.0 Hz, CH), 3.69 (3H, s, NeCH3), 2.67 (2H, m, CH2), 2.62 (2H, m,
CH2), 1.66 (6H, s, 2 ꢀ CH3), 1.62 (6H, s, 2 ꢀ CH3). 13C NMR (125 MHz,
d6-DMSO)
d 176.9, 167.9, 140.5, 128.5, 126.9, 126.6, 124.0, 122.3,
114.7,111.5,106.6,102.3, 95.5, 48.7, 45.5, 31.4, 27.1, 26.3. lmax (CHCl3)
802, log103 5.08.
2.5. General procedure for the synthesis of chromophores 17, 18, 24
and 25
The appropriate dialdehyde 13,14, 20 or 21 (1 eq), and TCF (1 eq)
were dissolved in iso-propyl alcohol under an argon atmosphere. A
catalytic quantity of piperidine acetate was added and the solution
was refluxed for 3 h. A solution of 1,3,3-trimethyl-2-methyl-
eneindoline (3) (1 eq) in iso-propyl alcohol was then added and the
solution refluxed for a further 3 h. The resultant solution was cooled
Scheme 2. Syntheses of the chromophores containing the either diphenylcyclohexene
or t-butyl cyclohexene interconnect. Reagents and conditions: (i) X ¼ Cl, POCl3; X ¼ Br,
PBr3, DMF, DCM, 65 ꢂC, 3 h. (ii) TCF, Piperidinium acetate, IPA, reflux 3 h. (iii) 3,
piperidinium acetate, reflux 3 h. (iv) NaH, DMF, YH.

180
M.D. H. Bhuiyan et al. / Dyes and Pigments 89 (2011) 177e187
to room temperature and the precipitated dye filtered off. The
coloured powder was washed with copious hot water and a few
small portions of cold iso-propyl alcohol to yield the crude product
as a coloured solid.
1.03 (9H, s, 3 ꢀ CH3). 13C NMR (125 MHz, d6-DMSO)
d 176.9, 168.0,
160.9, 156.6, 153.2, 148.5, 145.5, 143.7, 143.3, 142.9, 141.0, 129.3, 128.9,
128.5, 125.1, 122.3, 115.2, 114.3, 111.3, 107.0, 102.1, 95.6, 48.8, 42.1, 32.2,
31.6, 28.2, 27.2, 26.3. lmax (CHCl3) 799, log103 5.17.
2.5.1. 2-[4-(2-{2-Chloro-5,5-diphenyl-3-[2-(1,3,3-trimethyl-1,3-
dihydroindol-2-ylidene)ethylidene]cyclohex-1-enyl}-vinyl)-3-
cyano-5,5-dimethyl-5H-furan-2-ylidene]malononitrile (17)
Olive green solid (30%). Recrystallization (dichloromethane:hex-
anes, 1:1) afforded an olive green powder, m.p. 260e262 ꢂC. (Found:
MNaþ m/z 683.2559C43H37N4OCl requires MNaþ m/z 683.2554;
2.6. General procedure for the synthesis of chromophores 26e28
Under a nitrogen atmosphere and at 0 ꢂC, a slurry of NaH (1.2 eq)
(60% in mineral oil, washed with hexane) in anhydrous DMF (5 mL)
was added to a stirring solution of the appropriate phenol or thi-
ophenol derivative (1.1 eq) in anhydrous DMF (10 mL). After 30 min
the reaction was allowed to warm up to ambient temperature. This
mixture was added to a solution of 24 or 25 (1 eq) in anhydrous
DMF (10 mL). The reaction was stirred for 5 h, then quenched with
water. The solvent was evaporated under vacuum and the resulting
dye conjugate was purified by column chromatography to yield
a coloured solid.
D
¼ 0.7 ppm). 1H NMR (500 MHz, d6-DMSO)
d 8.35 (1H, d, J 14.5 Hz,
CH), 8.14 (1H, d, J 14.5 Hz, CH), 7.59 (1H, d, J 7.6 Hz, ArH), 7.41e7.43 (1H,
m, ArH), 7.30e7.35 (5H, m, ArH), 7.24e7.28 (5H, m, ArH), 7.13e7.19 (2H,
m, ArH), 6.55 (1H, d, J14.5Hz, CH), 6.36(1H, d, J14.5Hz, CH), 3.74(3H, s,
NeCH3), 3.48 (2H, br. s, CH2), 3.44 (2H, br. s, CH2),1.63 (6H, s, 2 ꢀ CH3),
1.62 (6H, s, 2 ꢀ CH3).13C NMR (125 MHz, d6-DMSO)
d178.6,168.3,147.0,
146.7, 146.6, 146.5, 144.9, 143.8, 142.8, 141.1, 128.6, 128.5, 128.1, 126.9,
126.1,126.0,125.9,125.2,125.1,122.2,114.1,111.4,106.9,102.1, 95.9, 48.8,
44.8, 36.8, 36.5, 31.8, 27.3, 26.1. lmax (CHCl3) 793, log103 5.11.
2.6.1. 2-[4-(2-{5-tert-Butyl-2-(2,6-di-tert-butyl-4-
methylphenoxy)-3-[2-(1,3,3-trimethyl-1,3-dihydroindol-2-ylidene)
ethylidene]cyclohex-1-enyl}vinyl)-3-cyano-5,5-dimethyl-5H-furan-
2-ylidene]malononitrile (26)
Green powder (55%, from 24) (2,6-di-tert-butyl-4-methylphenol
derivative). Purification by column chromatography on silica gel
(dichloromethane) afforded a green solid, m.p. 223e226 ꢂC. (Found:
MNaþ m/z 771.4566 C50H60N4O2 requires MNaþ m/z 771.4554;
2.5.2. 2-[4-(2-{2-Bromo-5,5-diphenyl-3-[2-(1,3,3-trimethyl-1,3-
dihydroindol-2-ylidene)ethylidene]cyclohex-1-enyl}-vinyl)-3-
cyano-5,5-dimethyl-5H-furan-2-ylidene]malononitrile (18)
Green solid (33%). Recrystallization (dichloromethane:hexanes,
1:1) afforded a green powder, m.p. 266e268 ꢂC. (Found: MNaþ m/z
727.2051 C43H37N4OBr requires MNaþ m/z 727.2048;
1H NMR (500 MHz, d6-DMSO)
D
¼ 0.4 ppm).
D
¼ 0.6 ppm). 1H NMR (300 MHz, d6-DMSO)
d 8.31 (1H, d, J 15.0 Hz,
d
8.29 (1H, d, J 14.5 Hz, CH), 8.17 (1H,
CH), 8.17 (1H, d, J 15.0 Hz, CH), 7.60 (1H, d, J 10.0 Hz, ArH), 7.39e7.44
(5H, m, ArH), 7.24e7.30 (1H, m, ArH), 6.27 (1H, d, J 15.0 Hz, CH), 6.08
(1H, d, J 15.0 Hz, CH), 3.68 (3H, s, NeCH3), 2.81e2.95 (2H, m, CH2),
2.30 (3H, s, CH3), 2.05e2.18 (2H, m, CH2),1.68 (18H, s, 2 o-tert-butyl),
1.64 (6H, s, 2 ꢀ CH3),1.61 (6H, s, 2 ꢀ CH3),1.41e1.49 (1H, m, CH),1.03
d, J 14.5 Hz, CH), 7.60 (1H, d, J 7.4 Hz, ArH), 7.41e7.44 (1H, m, ArH),
7.30e7.34 (5H, m, ArH), 7.24e7.28 (5H, m, ArH), 7.13e7.20 (2H, m,
ArH), 6.54 (1H, d, J 14.5 Hz, CH), 6.36 (1H, d, J 14.5 Hz, CH), 3.73 (3H,
s, NeCH3), 3.49 (2H, br. s, CH2), 3.45 (2H, br. s, CH2), 1.64 (6H, s,
2 ꢀ CH3), 1.62 (6H, s, 2 ꢀ CH3). 13C NMR (125 MHz, d6-DMSO)
(9H, s, 3 ꢀ CH3). 13C NMR (125 MHz, d6-DMSO)
d 176.7, 167.7, 161.0,
d
179.1, 168.4, 155.6, 146.4, 146.3, 146.2, 144.5, 143.4, 142.9, 141.1,
155.8,153.6,149.2,148.3,147.2,143.4,142.6,142.0,141.0,136.6,131.9,
128.5, 127.3, 125.3, 124.8,123.9, 122.5, 115.7, 114.7,111.5,106.4,102.0,
95.4, 48.5, 42.0, 33.3, 32.2, 31.2, 28.5, 27.2, 26.3, 26.0, 23.1. lmax (DMF)
896, log103 4.18.
128.6, 128.5, 128.3, 128.0, 127.7, 127.3, 126.9, 126.1, 125.0, 122.3,
114.6, 111.3, 107.2, 102.2, 95.9, 48.9, 44.8, 37.7, 37.5, 31.7, 27.2, 26.1.
lmax (CHCl3) 793, log103 5.09.
2.5.3. 2-[4-(2-{5-tert-Butyl-2-chloro-3-[2-(1,3,3-trimethyl-1,3-
dihydroindol-2-ylidene)ethylidene]cyclohex-1-enyl}-vinyl)-3-
cyano-5,5-dimethyl-5H-furan-2-ylidene]malononitrile (24)
Green solid (40%). Recrystallization (ethyl acetate:hexanes, 2:1)
afforded a green powder, m.p. 264e266 ꢂC. (Found: MNaþ m/z
2.6.2. 2-[4-(2-{5-tert-Butyl-2-phenylsulfanyl-3-[2-(1,3,3-
trimethyl-1,3-dihydroindol-2-ylidene)ethylidene]cyclohex-1-enyl}
vinyl)-3-cyano-5,5-dimethyl-5H-furan-2-ylidene]malononitrile
(27)
Green powder (62%, from 24) (thiophenol derivative). Purifica-
tion by column chromatography on silica gel (3% MeOH in
dichloromethane) afforded a green solid, m.p. 252e255 ꢂC. (Found:
MNaþ m/z 661.304 C41H42N4OS requires MNaþ m/z 661.294;
587.2564 C35H37N4OCl. requires MNaþ m/z 587.2554;
1H NMR (500 MHz, d6-DMSO)
D
¼ 1.7 ppm).
d
8.32 (1H, d, J 15.0 Hz, CH), 8.19 (1H, d,
J 15.0 Hz, CH), 7.60 (1H, d, J 10.0 Hz, ArH), 7.39e7.44 (2H, m, ArH),
7.24e7.30 (1H, m, ArH), 6.30 (1H, d, J 15.0 Hz, CH), 6.11 (1H, d, J
15.0 Hz, CH), 3.69 (3H, s, NeCH3), 2.81e2.95 (2H, m, CH2), 2.05e2.18
(2H, m, CH2),1.66 (6H, s, 2 ꢀ CH3),1.61 (6H, s, 2 ꢀ CH3),1.41e1.49 (1H,
D
¼ 0.4 ppm). 1H NMR (300 MHz, d6-DMSO)
d 8.29 (1H, d, J 15.0 Hz,
CH), 8.15 (1H, d, J 15.0 Hz, CH), 7.60 (1H, d, J 10.0 Hz, ArH), 7.39e7.44
(2H, m, ArH), 7.24e7.30 (1H, m, ArH), 7.20 (5H, m, ArH), 6.6.12 (1H,
d, J 15.0 Hz, CH), 5.92 (1H, d, J 15.0 Hz, CH), 3.69 (3H, s, NeCH3),
2.81e2.95 (2H, m, CH2), 2.05e2.18 (2H, m, CH2), 1.66 (6H, s,
2 ꢀ CH3), 1.62 (6H, s, 2 ꢀ CH3), 1.41e1.49 (1H, m, CH), 1.03 (9H, s,
m, CH), 1.03 (9H, s, 3 ꢀ CH3). 13C NMR (125 MHz, d6-DMSO)
d 176.8,
167.9, 161.1, 155.9, 153.5, 148.6, 146.9143.4, 142.9, 142.1, 141.0, 128.6,
127.4, 127.1, 125.0, 123.9, 122.3, 115.3, 114.5, 111.3, 106.6, 101.9, 95.6,
48.9, 42.0, 32.2, 31.5, 28.3, 27.2, 26.3. lmax (CHCl3) 799, log103 5.05.
3 ꢀ CH3). 13C NMR (125 MHz, d6-DMSO)
d 176.9, 167.9, 161.1, 153.6,
156.0, 148.5, 147.0, 143.5, 142.8, 142.1, 141.0, 135.7, 133.7, 132.5, 128.7,
127.8, 127.3, 125.0, 124.0, 122.6, 115.7, 114.7, 111.5, 106.6, 102.3, 95.5,
48.7, 42.1, 32.3, 31.6, 28.3, 27.1, 26.3. lmax (DMF) 907, log103 5.02.
2.5.4. 2-[4-(2-{2-Bromo-5-tert-butyl-3-[2-(1,3,3-trimethyl-1,3-
dihydroindol-2-ylidene)ethylidene]cyclohex-1-enyl}-vinyl)-3-
cyano-5,5-dimethyl-5H-furan-2-ylidene]malononitrile (25)
Dark green solid (45%). Recrystallization (ethyl acetate:hexanes,
2:1) afforded a dark greenpowder, m.p. 254e256 ꢂC.(Found:MNaþ m/
2.6.3. 2-[4-(2-{5-tert-Butyl-2-pentafluorophenylsulfanyl-3-[2-
(1,3,3-trimethyl-1,3-dihydroindol-2-ylidene)ethylidene]cyclohex-1-
enyl}-vinyl)-3-cyano-5,5-dimethyl-5H-furan-2-ylidene]
malononitrile (28)
z 631.2039 C35H37N4OBr. requires MNaþ m/z 631.2048;
1H NMR (500 MHz, d6-DMSO)
D
¼ 1.4 ppm).
d
8.25 (1H, d, J 15.0 Hz, CH), 8.20 (1H, d, J
15.0 Hz, CH), 7.60 (1H, d, J 10.0 Hz, ArH), 7.39e7.46 (2H, m, ArH),
7.24e7.31 (1H, m, ArH), 6.30 (1H, d, J 15.0 Hz, CH), 6.13 (1H, d, J 15.0 Hz,
CH), 3.69 (3H, s, NeCH3), 2.77e3.00 (2H, m, CH2), 2.05e2.25 (2H, m,
CH2), 1.68 (6H, s, 2 ꢀ CH3), 1.64 (6H, s, 2 ꢀ CH3), 1.41e1.51 (1H, m, CH),
Greenish brown solid (65%, from 24) (pentafluorothiophenol
derivative). Purification by column chromatography on silica gel
(3% MeOH in dichloromethane) afforded a greenish brown powder,
m.p. 242e246 ꢂC. (Found: MNaþ m/z 751.2562 C41H37F5N4OS

M.D. H. Bhuiyan et al. / Dyes and Pigments 89 (2011) 177e187
181
requires MNaþ m/z 751.2454;
DMSO)
D
¼ 0.2 ppm). 1H NMR (300 MHz, d6-
d
8.35 (1H, d, J 15.0 Hz, CH), 8.23 (1H, d, J 15.0 Hz, CH), 7.68
(1H, d, J 10.0 Hz, ArH), 7.40e7.46 (2H, m, ArH), 7.26e7.32 (1H, m,
ArH), 6.20 (1H, d, J 15.0 Hz, CH), 6.15 (1H, d, J 15.0 Hz, CH), 3.70 (3H,
s, NeCH3), 2.81e2.95 (2H, m, CH2), 2.05e2.18 (2H, m, CH2),1.69 (6H,
s, 2 ꢀ CH3), 1.63 (6H, s, 2 ꢀ CH3), 1.41e1.49 (1H, m, CH), 1.03 (9H, s,
3 ꢀ CH3). 13C NMR (125 MHz, d6-DMSO)
d 177.0, 167.8, 141.2, 156.2,
153.7, 149.8, 149.0, 147.2, 143.6, 143.0, 142.2, 141.1, 139.1, 136.1, 128.9,
127.5, 125.2, 124.0, 122.8, 115.9, 114.9, 111.5, 106.9, 106.6, 102.1, 95.6,
48.9, 42.5, 32.5, 31.2, 28.3, 27.4, 26.5. lmax (DMF) 906, log103 4.55.
3. Result discussion
3.1. Synthesis
The chromophores used in this study (7e9, 11, 17e18 and
24e28) were prepared as outlined in Schemes 1 and 2. The
methodology used to prepare 7e9 and 11 relies on an approach
previously described by us [14] and which utilizes the N-phenyl-
acetamide precursors 4e6 and 10. Reaction of these precursors
with 1,1,3-trimethyl-2-methyleneindoline, 3, in alcohol afforded
chromophores 7e9 and 11 as colored solids in yields of 36e87%.
Notably the crude dyes readily precipitated from solution and were
obtained essentially pure following washing with water and
methanol. The hydroxymethylene-cyclohexenecarbaldehydes 13,
14, 20 and 21 were prepared by formylation of the appropriate
cyclohexanone via the Vilsmeier reaction [22]. Condensation of
these compounds with the TCF acceptor in refluxing isopropyl
alcohol containing catalytic quantities of piperidinium acetate
yielded the cyclic enol intermediates 15, 16, 22 and 23 which were
reacted in situ with 3 to give chromophores 17, 18, 24, and 25. As
with the compounds above the final products readily precipitated
from solution. Conversion of 24 or 25 to phenylether 26 and the
thiophenylethers 27 and 28 was effected by displacement of the
vinylic halogen atoms in the precursor compounds by the appro-
priate nucleophile e presumably via a tetrahedral intermediate as
this anion would be allylic and therefore readily formed. These
substitution reactions proceeded smoothly with either chlorine or
bromine as the leaving group, although the reactions with bromide
25 were found to take place slightly faster.
Fig. 1. UveVis absorption spectra of compounds 7e9, 11, 17, and 24 in chloroform.
compounds to aggregate and in particular in environments of low
polarity such as those encountered in polymers (i.e.,
3
¼ 1e7).
Indeed it is worth noting that although a given chromophore may
show no aggregation and an excellent NLO response (viz. high
b
value) in a highly polar solvents such as DMF (
¼ 46.7), this is no indicator of how it will perform in a polymer, or
in a low polarity solvent such as chloroform ( ¼ 7.5).
¼ 4.8) or THF (
3
¼ 38) or DMSO
(
3
3
3
Consequently, in order to best replicate the response expected in
a polymer matrix the UVeVis absorption spectra and extinction
coefficients of the chromophores were measured e where possible
e in chloroform. This proved feasible for all of the compounds
except 26e28 which were found to have poor solubility in chloro-
form and their extinction coefficients were therefore obtained in
DMF; the results are presented in Figs. 1 and 2 and Table 1.
It is seen from Fig. 1 that for this series of compounds the
UVeVis absorption spectra obtained in chloroform all contain an
intense main absorption band accompanied by a blue shifted, i.e.,
high energy, shoulder. As the relative intensities of both the high
energy shoulder and the main absorption band showed no varia-
tion across a range of concentrations this confirms that the
shoulder is not due to H-aggregation. Rather as Leng et al. reported
for compounds 29 and 30 [24] and as Smith et al. demonstrated for
compounds 2 and 31 [17], the blue-shifted shoulders observed in
low polar solvents are associated with excited vibronic states. It is
It is important to note that the 1H NMR spectra of all of the
chromophores prepared indicate only one isomer to be present.
Furthermore as the coupling constants observed for the vinylic
protons of the conjugated interconnects are of the order of
12e15 Hz this confirms that they are in an all-trans configuration
[23]. This is in contrast to our previous observations for zwitter-
ionic compounds such as 1 and 2, where detailed NMR studies
showed two isomers to be present and that these were due to
rotational isomerisation about the acceptor ring and conjugated
interconnect [14]. The absence of such rotamers in the present
compounds is likely due to them being far less charge separated in
their ground states (confirmed from X-ray crystallographic studies
discussed later in the work), than, for example, 1 and 2. This, in
turn, affects bond order throughout the indoline containing
molecules and leads to more single bond character in the crucial
bond connecting the conjugated backbone and the acceptor when
compared to their zwitterionic analogues. This allows for more
freedom of rotation about this bond and therefore, presumably,
easier conversion to the thermodynamically favored transoid
rotamer.
3.2. UVevis spectroscopy
The UVevis absorption spectra of NLO chromophores in solution
can give important information about the propensity for the
Fig. 2. UveVis absorption spectra of compounds 26e28 in DMF.

182
M.D. H. Bhuiyan et al. / Dyes and Pigments 89 (2011) 177e187
therefore concluded that the high energy shoulders observed in
chloroform are also due to excited vibronic states, and not to
aggregation. As identical absorption maxima and band shapes were
observed for 17, 18, 24 and 25 in both DMF and chloroform it
implies that substituting the vinylic chlorine for bromine is inef-
fective in tuning either the ground states or excited states of this
particular suite of compounds.
twisted out of the plane of the chromophore backbone, and hence
is unable to efficiently transfer charge.
3.3. X-ray crystallography
We have managed to obtain X-ray crystallographic data for three
of the compounds e 7, 17 and 24, and the relevant results are pre-
An analysis of the linear optical data shows that shifts in the
absorption maxima are mainly related to variations in the conju-
gation length (e.g., 7 vs 8 vs 9), ring locking (c. 30 nm increase in
lmax, i.e., 9 vs 11, 17e18 and 24e25), and replacement of chlorine or
bromine with either 2,6-di-t-butyl-4-methylphenoxy (26), thio-
phenoxy (27) or pentafluorothiophenoxy (28) leads to an approx-
imate 60 nm increase in the wavelength of the absorption maxima.
The moderate increase in the absorption maximum with configu-
ration locking likely reflects improved planarity in the chromo-
phores as well as more efficient electron delocalisation. This should
also translate to improved NLO responses in these compounds. A
previous study by us demonstrated that configurational locking
across the donor-polyene unit improved the linear optical response
by up to 50% [16]. When the halogen on the cyclohexene ring is
replaced by either a 2,6-di-tert-butyl-4-methylphenoxy (26), thio-
phenoxy (27) or pentafluorothiophenoxy (28) substituent a sizable
red-shift in absorption maxima occurs, and this is to be expected as
sented in Tables 2 and 3. Table 2 contains the refinement data
whereas Table 3 contains a summary of key bond lengths in the
compounds, provides a comparison with the parent TCF and shows
the degree of bond length alternation (BLA) in the compounds. All
three crystals contain partial solvents of crystallization (Table 2) and
two of the structures (7, 24) have the molecule of interest con-
strained to a crystallographic mirror plane. For this reason, the atom
labeling is slightly different corresponding to the symmetry-related
methyl atoms in the TCF moiety (e.g., C8 & C80 in 7 & 24; C8 & C9 in
17). From the bond length data in Table 3 it is seen that the bond
order in the acceptor portion of all of the chromophores follows the
same trend as that found in the parent TCF acceptor, meaning that
these compounds should have more neutral, or non-charge-sepa-
rated, ground states (as opposed to the complete reversal of bond
order that would be expected in zwitterionic systems). However,
paradoxically, in the case of 7 the bond order at the acceptor end of
the molecule is marginally in line with what would be expected for
a charged separated, or zwitterionic, species. The bond length
alternation values shown in Table 3 reveal that in compound 7
(BLA ¼ 0.006 Å) there is near equivalence in the bond lengths of the
neighboring bonds in the polyene chain, but for both 17 and 24 the
differences are somewhat more distinct. This suggests that of the
three compounds it is 7 that will have the highest degree of charge
these auxochromes are in direct conjugation with the p-system. It
is also worth noting that the extinction coefficient of 26, which
contains two bulky substituents in close proximity to the polyene
chain, is around one order of magnitude less than the value for 27.
This would suggest that the tert-butyl groups cause some degree of
twisting in the polyene unit and/or that the phenoxy group is
Table 1
Linear and nonlinear optical properties of the chromophores used in this study. lmax, Wavelength of maximal absorption; log310, logarithm of molar extinction coefficient at
lmax bzzz, dynamic first hyperpolarizability determined at a fundamental wavelength of 800 and/or 1300 nm; bzzz0, static first hyperpolarizability derived from the two-level
model. is the fluorescence life time obtained from the demodulation fit in nanoseconds.
;
s
Compound
lmax (nm) In CHCl3
lmax (nm) In DMF
Log103
bzzz (*10ꢁ30 esu)
bzzz0 (*10ꢁ30 esu)
s
(ns)
Td (ꢂC)
,
e
7
7
7
7
7
8
9
11
17
18
24
25
26
27
28
602
602
e
599
e
5.23a
5.23
5.11
820 ꢃ 40a
1485 ꢃ 40a
1110 ꢃ 45b
90 ꢃ 5
1
254
254
254
254
254
280
249
263
270
261
278
275
150
205
225
,f
815 ꢃ 20
610 ꢃ 25
645 ꢃ 25
730 ꢃ 20
130 ꢃ 10
300 ꢃ 20
330 ꢃ 10
280 ꢃ 10
270 ꢃ 10
290 ꢃ 10
270 ꢃ 10
360 ꢃ 20
290 ꢃ 20
350 ꢃ 20
e
e
e
e
,
f
599
605c
598d
691
810
840
839
839
843
843
896
907
906
,f
e
5.20c
5.25d
5.21a
5.12a
5.08a
5.05a
5.17a
5.11a
5.09a
4.18b
5.02b
4.55b
1175 ꢃ 45c
,
f
e
1340 ꢃ 40d
,e
698
794
802
799
799
793
793
895
915
907
1230 ꢃ 120a
0.85 ꢃ 0.1
,e
,e
,e
,e
,e
,e
960 ꢃ 50a
1030 ꢃ 40a
880 ꢃ 30a
930 ꢃ 30a
950 ꢃ 50a
880 ꢃ 30a
760 ꢃ 40b
610 ꢃ 40b
720 ꢃ 40b
e
e
e
e
e
e
,
,
,
e
1.86 ꢃ 0.1
2.03 ꢃ 0.3
1.97 ꢃ 0.1
e
e
Td is taken at 5% weight loss.
a
Measured in chloroform.
Measured in dimethylformamide.
Measured in dimethyl sulfoxide.
Measured in methanol.
Measured with a 1300 nm fs operating system.
Measured with a 800 nm fs operating system.
b
c
d
e
f

M.D. H. Bhuiyan et al. / Dyes and Pigments 89 (2011) 177e187
183
Table 2
Crystallographic and structure refinement data for 7, 17 and 24.
7
17
24
Asymmetric unit
Moiety formula
MW
C24.88H23.75Cl1.75N4O
C24H22N4O.0.88CH2Cl2
456.98
C46H43ClN4O2
C43H37ClN4O.C3H6O
719.29
C35H37ClN4O1.25
C35H37ClN4O.0.25H2O
569.14
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a, Å
107 (2)
0.71073
116 (2)
0.71073
Triclinic
P-1
11.9943 (5)
12.1474 (5)
14.2267 (6)
76.315 (3)
71.363 (2)
77.448 (2)
1885.63 (14)
2
112 (2)
0.71073
Hexagonal
P63/m
27.2322 (8)
27.2322 (8)
7.3713 (3)
90
90
120
4734.1 (3)
6
1.198
Monoclinic
C2/m
24.1889 (8)
6.8064 (3)
14.8976 (6)
90
101.274 (2)
90
2405.40 (16)
4
b, Å
c, Å
a
, deg.
, deg.
b
g
, deg.
Volume, Å3
Z
r
, Mg mꢁ3
1.262
0.266
1.267
0.146
m
, mmꢁ1
0.155
Crystal size (mm)
Theta range deg.
Limiting indices
1.00 ꢀ 0.50 ꢀ 0.04
2.79e34.94
ꢁ37 ꢄ h ꢄ 35
ꢁ10 ꢄ k ꢄ 10
ꢁ22 ꢄ l ꢄ 22
43 728
0.31 ꢀ 0.21 ꢀ 0.02
2.28e26.50
ꢁ15 ꢄ h ꢄ 15
ꢁ14 ꢄ k ꢄ 15
ꢁ17 ꢄ l ꢄ 17
33 203
0.45 ꢀ 0.12 ꢀ 0.09
2.90e25.21
ꢁ32 ꢄ h ꢄ 32
ꢁ32 ꢄ k ꢄ 32
ꢁ8 ꢄ l ꢄ 8
90 267
Reflections collected
No of unique data
Rint
4913
0.0427
7793
0.0741
3074
0.1276
Absorp. Coeff. range
Restraints
0.809, 1.0
1
0.780, 1.0
1
0.645, 0.745
11
No. of parameters
Refinement method
P1, P2 coefficients of weighting schemec
Goodness-of-fit on F2
197
485
281
Full matrix least-squares on F2
0.0646, 1.8954
1.034
Full matrix least-squares on F2
0.040, 1.0761
1.005
Full matrix least-squares on F2
0.0588, 5.4309
1.029
0.051, 1943
0.157
R1 b,/data number
0.055, 3739
0.163
1.008 and ꢁ0.735
0.049, 4925
0.125
0.260 and ꢁ0.349
a
,
wR2b i(all data)
Largest diff. peak and hole (e Aꢁ3
)
a
Intensities 2.0 times their standard deviations (from counting statistics).
b
c
R1 ¼ SkFoj ꢁ jFck/SjFoj; wR2 ¼ S[[w(F2o ꢁ Fc2)2]/S[(wFo2)2]]1/2
.
Weight, w ¼ 1/[
s
2(Fo2) þ (P1 ꢀ P)2 þ P2 ꢀ P] where P ¼ (Max (Fo2, 0) þ 2 ꢀ F2c)/3.
separation across the
p
-conjugated backbone in the ground state,
over the mirror plane. The alkene C12eH12.N3(cyano) interactions
are aligned around a two fold axis of symmetry resulting in dimeric
units, with a halogen bond at one end of the molecule between N1
and the dichloromethane chlorine Cl1 atom (Table 4). The molecules
therefore lie in (010) planes with only weak interactions between the
planes provided by a C10.Cl2B (dichloromethane) contacts at an
interplane distance of 3.43 Å. The “angle of slippage” [28] is 16ꢂ;
although this is smaller than the approximated angles in related
compounds [29,30] the molecular planes in the latter crystals are not
and may even be slightly zwitterionic in nature. Nonetheless as its
BLA value is close to zero it is expected from theory to have the
lowest molecular NLO response [25e27]. This is indeed confirmed
from the HRS data which shows 7 to have a b1300 of only
820 ꢀ 10ꢁ30 esu, whereas the compound with the highest BLA value
(24, BLA ¼ 0.033) has a higher b1300 value, i.e., 950 ꢀ 10ꢁ30 esu.
3.3.1. Crystallographic features of compound 7
The asymmetric unit of 7 contains the title compound lying on
a crystallographic mirror plane (Fig. 3) and containing a disordered
and non-stoichiometric amount (0.88) of dichloromethane as solvent
of crystallization. The methyl hydrogen atoms on C21 are disordered
N2
C3
C8
O1
i
C8
Table 3
C2
Selected bond distances and magnitude of BLA (Å) for 7, 17 and 24a.
C5
C1
C6
i
Bondb
TCF
7
17
24
C22
C22
N1
C4
C2eC6
C6eC7
C7eC4
C4eC10
1.358
1.445
1.343
1.472
e
1.386 (2)
1.415 (2)
1.406 (2)
1.389 (2)
1.397 (2)
1.391 (2)
1.395 (2)
0.006
1.380 (3)
1.421 (3)
1.393 (3)
1.399 (3)
1.387 (3)
1.398 (3)
1.400 (3)
0.019
1.363 (5)
1.422 (5)
1.389 (5)
1.402 (5)
1.371 (5)
1.405 (5)
1.380 (5)
0.033
C16
C7
C11
C12
C14
C15
C17
C18
C9
C10
C13
C
C
C
10eC11
11eC12
12eC13
N3
e
e
BLA
Av. bzzz0
e
N4
C20
e
90
280
290
a
The BLA value is obtained by considering the average differences in bond length
for the polyene interconnect and acceptor only; not all relevant bond length data for
17 and 24 is shown in the table.
C19
C21
b
Using the equivalent bond labels in 7.
Fig. 3. Structure of 7 (with 50% probability ellipsoids) [31].

184
M.D. H. Bhuiyan et al. / Dyes and Pigments 89 (2011) 177e187
Table 4
ring system is disordered between half-chair & half-boat forms.
Hydrogen bond geometry (Å, deg.) for 7.
Packing within and between the mirror plane layers is provided by
bifurcated attractive CeH.N(Cyano), two other weak CeH.N and
weak halogen N2.Cl1 (3.250(6) Å), interactions (Table 6).
DeH.A
DeH
H.A
D.A
DeH.A
Symmetrya
C12eH12.N3
C32AeCl1.N1
0.95
1.784
2.53
3.160 (2)
3.462 (3)
167
173.1 (2)
1 ꢁ x, y, 1 ꢁ z
x, y, z
a
Symmetry to bring the A atom into contact.
3.4. Hyperpolarizability
present incrystallographically-imposed parallel planes. Bysymmetry
constraint, the TCF and indoline moieties are planar.
The hyper-Raleigh scattering measurements (HRS) were per-
formed at 1300 nm due to the strong absorption at 800 nm found
for those compounds with extended conjugation; the results are
presented in Table 1. In the case of compounds 7, 8, 26, 27 and 28
the HRS signal at 1300 nm was convolved with multiphoton fluo-
rescence, but the latter contribution could be extracted. By
demodulating the apparent hyperpolarizability at increasing
modulation frequencies, an accurate fluorescence-free hyper-
polarizability could be obtained [21]. The corresponding fluores-
3.3.2. Crystallographic features of compound 17
Structure 17 contains independent molecules of 2-[4-(2-{2-
chloro-5,5-diphenyl-3-[2-(1,3,3-trimethyl-1,3-dihydroindol-2-yli-
dene)ethylidene]cyclohex-1-enyl}vinyl)-3-cyano-5,5-dimethyl-5H-
furan-2-ylidene]malononitrile (Fig. 4) and one acetone molecule of
crystallization. The TCF group (atoms O1, C4eC7) and “polyene chain”
(atoms C11eC15, C31 & C32) are close to coplanar with an interplanar
twist of 1.2 (2) deg. The indol-2-ylidene group is close to planar with
the mean planes through the 5 and 6 membered rings subtending 1.8
(1)deg.. The indol-2-ylidenemeanplane is twisted with respect to the
polyene chain-TCF plane by 18.8 (1) deg.: overall the molecular
segments are twisted about the mean line passing through the
polyene atoms. The cell packing is dominated by the head to tail C
(phenyl)eH.N(cyano) H bonding link (entry 1, Table 5). This is
cence life time (s in ns) obtained from the demodulation fit is
tabulated in Table 1. The longer conjugation path present in shifting
from compound 7 to 11 resulted in a shift of the emission away
from 650 nm. The HRS signal for compounds 7e11 was constant as
a function of modulation frequency and an average over these
frequencies was obtained. The results are compiled in Table 1. The
experimental dynamic hyperpolarizability values, bzzz,1300, are
affected by resonance enhancement to varying degrees, and this
depends both on the proximity of the wavelength of the electronic
charge-transfer resonance to the fundamental wavelength and that
supplemented by CeH.Cl, CeH.p and CeCl. p interactions with
the solvent acetone providing a donor O to the C9-H9B hydrogen
bond (Table 5). The molecules pack into approximately parallel layers
(a maximum interplanar deviation of 10 deg. is noted) which are
parallel to the (ꢁ1, 2, ꢁ2) plane.
of the second-harmonic (l/2). Consequently, in order to make more
meaningful comparisons between the compounds the resonance-
free static hyperpolarizability value bzzz0,1300 is also given in Table 1.
We observe an increase in the molecular nonlinearity for all of
the compounds when the degree of the conjugation between the
donor and acceptor is increased (i.e., in moving from compounds 7
to 9). This is a well documented phenomenon and, therefore,
3.3.3. Crystallographic features of compound 24
In structure 24, the target molecule (Fig. 5) lies on a crystallo-
graphic mirror plane and the crystal contains a partial (0.25) water of
crystallization. Because of the imposed symmetry, the cyclohexene
C46
O2
C45
C44
N1
C1
C2
N2
C3
N3
C10
C7
C6
C43
O1
C36
C37
C39
C42
Cl1
C35
C8
C11
C4
C34
C5
C9
C13
C30
C12
C14
C38
C33
C32
C31
C18
C15
C16
C25
C40
N4
C41
C17
C19
C26
C27
C20
C21
C29
C28
C24
C23
C22
Fig. 4. Structure of 17 showing the asymmetric unit contents (50% probability ellipsoids) [31].

M.D. H. Bhuiyan et al. / Dyes and Pigments 89 (2011) 177e187
185
Table 5
Table 6
Hydrogen Bond geometry (Å, deg.) for 17.
DeH.A
DeH
H.A
D.A
DeH.A Symmetrya
DeH.A
DeH
H.A
D.A
DeH.A Symmetrya
C30eH30.N3
C32eH32A. N3 1.01 (6) 2.45 (6) 3.459 (8) 178 (4)
C16eH16.N1
C8eH8C.N2
0.95
2.35
3.296 (7) 176
1 ꢁ y, x ꢁ y þ 1, z
C39eH39.N2
C9eH9B.O2
C8eH8C.Cl1
0.95
0.98
0.98
2.43
2.59
2.90
3.376 (4) 171
3.505 (4) 155
3.642 (3) 133
x, y þ 1, z þ 1
x ꢁ 1, y, z
1 ꢁ y, x ꢁ y þ 1, z
y, ꢁx þ y, z þ 1/2
y, ꢁx þ y, z ꢁ 1/2
0.99 (2) 2.65 (3) 3.569 (8) 155 (4)
0.98 2.72 3.421 (5) 129
1 ꢁ x, 1 ꢁ y, ꢁz
1 ꢁ x, 2 ꢁ y, ꢁz
x þ 1, y, z
C14eCl1.Cg1b 1.749 (2) 3.6395 (13) 5.364 (3) 168
a
Symmetry to bring the A atom into contact.
C36eH36.Cg1b 0.95
2.66
3.566 (3) 159
a
Symmetry to bring the A atom into contact.
Cg1 is the centroid of the C25eC30 phenyl ring.
b
synthetic procedures used to prepare these enolealdehydes (i.e.,
via the Vilsmeier reaction on a cyclohexanone precursor) and their
replacement with electron withdrawing substituents will not be
necessarily straightforward.
entirely expected [15,16]. The inclusion of ring locking in the centre
of the conjugated -system, along with either t-butyl or diphenyl
p
In addition to the experimental results obtained for compound 7
at 1300 nm, solvent dependant HRS measurements were also
conducted using an 800 nm fs system (Table 1). A very important
observation from these measurements is that there is a modest
substitution on the central cyclohexene ring (compounds 11, 17, 18,
24, 25) has a minimal effect on the second-order nonlinear
response. This is interesting because configurational locking would
be expected to increase planarity throughout these compounds
thereby allowing for much more efficient charge transfer. One
possible reason for the negligible change may be due to the pres-
ence of the vinylic halogen atoms on the cyclohexene ring. It has
been predicted through DFT calculations that the placement of
donor groups on the even numbered methines and acceptor groups
on the odd numbered methines of a chromophore polyene back-
bone leads to a reduction in the NLO response [32]; the relevant
numbering system is shown for compound 11, Scheme 1. Thus, as
halogens are able to act as electron donor groups through reso-
nance their presence may be off-setting any gains obtained through
increase in the NLO response (from 645
ꢀ
10ꢁ30 esu to
815 ꢀ 10ꢁ30 esu) as the dielectric constant of the surrounding
medium is decreased (from
3
¼ 46.7 to 4.8). As reported by Marder
et al. [33] the reaction field model holds that the electric field
exerted on a dipolar molecule by a solvent has an effect similar to
that of donor/acceptor substitution. This solvent-mediated polari-
zation results in more stabilization of a charge separated zwitter-
ionic form in high polarity solvents (like DMSO and DMF) and in
turn leads to a reduction in the observed
polarity media such as chloroform, the solvent is very ineffective at
stabilizing charge separated species and the value is therefore
b value. Conversely, in low
b
ring locking of the
p-conjugated interconnect. This may also
increased. Therefore, the results of our solvent polarity vs NLO
response study suggest that for compound 7 the charge separated
form dominates the ground state, albeit to a moderate extent.
Lastly, there is a significant difference in the observed molecular
response for compound 7 when values obtained in chloroform at
1300 nm (b0, 90 ꢀ 10ꢁ30 esu) are compared to those obtained at
800 nm (b0, 815 ꢀ 10ꢁ30 esu). The dependence of the molecular
hyperpolarizability on wavelength is a known phenomenon [34].
The low static hyperpolarizability value found for 7 at 1300 nm
reflects the fact that the magnitude of b0 decreases the closer the
two photon absorption wavelength (viz. 650 nm for a 1300
fundamental) is to the absorption maximum of the chromophore
(602 nm). This indicates that to maximise the macroscopic
response of 7, at least, electro-optic coefficients should be
measured at wavelengths remote from c. 1300 nm. Overall, the
observed molecular hyperpolarizabilities of the compounds used in
explain the differences in the
b
values between 26e28. In the case
of compound 26 it is likely the orbitals containing the non-bonding
pairs on the vinylic oxygen atom are twisted orthogonally to the
polyene chain, while for 28 the fluorine atoms on the thiophenoxy
ring will strip electron density from the sulphur; hence in both
cases the substituents are inferior donors when compared to 27
which has a comparatively unencumbered thiophenoxy as the
donor group. As a result 26 and 28 have beta values approximately
20% higher than 27. From our previous work we found that in some
cases ring locking across the donor ring-polyene chain led to a 50%
increase in
b [16]. In the present work it is apparent that ring
locking across the centre of the polyene chain does not lead to
a corresponding increase in the NLO response, and that electronic
factors probably play an important role. While there is scope to
investigate this further it must be noted that the halogen atoms
present in the precursors 13e14 and 20e21 are artefacts of the
C8*
C25*
N2
C27
C26
C5
O1
C3
C1
Cl1
C28
C29
C24
C8
C25
C22
C2
C6
C10
C13
C7
C9
N3
C4
C14
C30
C11
C31
C23
C21
C15
N1
C12
C17
N4
C32
C16
C18
C20*
C19
C20
Fig. 5. Structure of 24 (with 50% probability ellipsoids) [31].

186
M.D. H. Bhuiyan et al. / Dyes and Pigments 89 (2011) 177e187
Fig. 6. Measured molecular hyperpolarizabilities of selected chromophores obtained at 1300 nm in chloroform. Values for 32e34 are taken from ref. [35] and values for 35 obtained
from ref. [16].
values have units of 10ꢁ30 esu.
b
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1000 ꢀ 10ꢁ30 esu and b0 ¼ 250 to 350 ꢀ 10ꢁ30 esu, and these values
compare quite favourably to other compounds for which data has
been obtained at 1300 nm. For example, the response found for 7 is
similar to that obtained for the highly extended compound 34, but
is only around 50% of the value found for 33 which has six carbon
atoms in the donoreacceptor interconnect (Fig. 6). However, the
value obtained for 11 is approximately twice that observed for the
analogous compound 35 which contains a quinolinylidene donor
and is therefore highly zwitterionic in character. Thus, given the
relative ease by which the present compounds can be synthesised
and modified, and their comparative NLO responses, it suggests
they are worthwhile candidates for further study.
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4. Summary and conclusions
A suite of chromophores containing an indoline donor and
various
p-conjugated interconnects between the powerful TCF
acceptor were prepared. The static and dynamic molecular NLO
responses of the compounds were measured and in low polarity
media values of b800 up to 1485 ꢀ 10ꢁ30 esu were obtained; the
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excellent candidates for further study. While extending the
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Acknowledgments
We would like to thank the New Zealand Foundation for
Research Science and Technology for their financial support
(Contract C08X0704) and the grants from the University of Leuven
(GOA/2006/03), Belgium.
dimethyl-2,3-dihydro-1H-indol
2-ylidene]prop-1-enyl}-3-cyano-5,5-
dimethyl-2,5-dihydrofuran-2-ylidene)-malononitrile. Acta Crystallographica
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