Nonlinear optical properties of diaromatic stilbene, butadiene and thiophene derivatives

Article abstract of DOI:10.1039/d1nj00456e

Series of highly polar stilbene (1a-e), diphenylbutadiene (2a-c) and phenylethenylthiophene (3a-c) derivatives were preparedviaHorner-Wadsworth-Emmons method with a view to produce new and efficient materials for second harmonic generation (SHG) in the so

Full text of DOI:10.1039/d1nj00456e

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Nonlinear optical properties of diaromatic
stilbene, butadiene and thiophene derivatives†
Cite this: New J. Chem., 2021,
45, 6640
a
a
b
a
Esa Kukkonen,
Elmeri Lahtinen,
Pasi Myllyperkio,
Matti Haukka
and
¨
a
Jari Konu
*
Series of highly polar stilbene (1a–e), diphenylbutadiene (2a–c) and phenylethenylthiophene (3a–c) derivatives
were prepared via Horner–Wadsworth–Emmons method with a view to produce new and efficient materials
for second harmonic generation (SHG) in the solid-state. The single-crystal X-ray structures of compounds
1–3 reveal extensive polymorphism and a peculiar photodimerization of the 2-chloro-3,4-dimethoxy-40-
nitrostilbene derivative 1a to afford two polymorphs of tetra-aryl cyclobutane 4. The stilbene congeners
2-chloro-3,4-dimethoxy-40-nitrostilbene (1aꢀnon-centro), 5-bromo-2-hydroxy-3-nitro-40-nitrostilbene
(1b) and 4-dimethylamino-40-nitrostilbene (1e), as well as 40-fluoro-400-nitro-1,4-diphenyl-1,3-butadiene
(2a) present the ideal, non-centrosymmetric arrangement of the chromophores for nonlinear optical
(NLO) activity. Compounds 1b and 2a exhibit only relatively low intensity for second harmonic generation
(0.04 and 0.18 times that of urea reference, respectively), while the stilbene polymorph 1aꢀnon-centro
shows NLO activity of over 32 times that of urea. In addition, the conjugated diaromatic compounds 1–3
display fluorescence behaviour in CH2Cl2 solutions with the exception of stilbene derivative 1b.
Received 28th January 2021,
Accepted 19th March 2021
DOI: 10.1039/d1nj00456e
rsc.li/njc
NLO active material crystallizes in a non-centrosymmetric space
group (lack of inversion symmetry) to avoid the effective cancela-
Introduction
The study of nonlinear optical (NLO) materials has gained tion of hyperpolarizability, even though centrosymmetric crystals
increasing interest especially owing to their significance in have also been reported to display SHG through processes
laser technology and optoelectronics. Ultrafast electro-optical involving intermolecular interactions.10,11
switches, optical signal processing and computing, data storage,
Although most of the commercially available NLO crystals
and photonic technologies are just some of the various applica- are based on inorganic compounds such as b-barium borate
tions where these materials have become indispensable.1–8 One (BBO) and potassium titanyl phosphate (KTP), primarily owing
of the most sought-after nonlinear properties for these applica- to their high chemical and physical stability,12–14 the academic
tions is the second harmonic generation (SHG), a phenomenon research on new NLO active materials has largely focused on
in which the material combines two photons into one, creating organic ‘‘push–pull’’ molecules for which high SHG activities
radiation with twice the frequency of the original. The funda- have been observed frequently.15–18 A group of compounds
mental requirement for a material to have SHG activity is to consistently showing remarkable SHG efficiencies are diary-
possess non-zero second-order nonlinear susceptibility w(2), which lethenes, commonly known as stilbenes (compound 1,
necessitates the compound to have non-zero first-order hyperpo- Scheme 1). They have been studied extensively by both theore-
larizability b at the molecular level.9 It is highly beneficial if the tical and experimental methods,19,20 and, for example,
3-methyl-4-methoxy-40-nitrostilbene (MMONS, Scheme 1), the
most efficient second order NLO material reported to date with
a
the SHG efficiency of up to 1250 times of urea,21 belongs to this
group of conjugated systems. Additionally, a variety of hetero-
aromatic, diarylethene-based systems where one or both aryl
moieties have been replaced with thiophene, thiazole, or pyr-
¨
¨
Department of Chemistry, University of Jyvaskyla, P. O. Box 35, FI-40014
¨
¨
Jyvaskyla, Finland. E-mail: jari.a.konu@jyu.fi; Tel: +358-40-805-4406
b
¨
¨
¨
¨
Nanoscience Center, University of Jyvaskyla, P. O. Box 35, FI-40014 Jyvaskyla,
Finland
† Electronic supplementary information (ESI) available: A pdf file containing
tables of crystallographic data (Tables S1–S3) and pertinent bond parameters role rings have also been investigated.22–25 The primary aim of
(Tables S4–S16, together with molecular figures for atomic numbering scheme) as
these studies was to increase the hyperpolarizability (b) of the
chromophores by altering the charge-transfer properties. While
the electric field induced second harmonic generation (EFISHG)
measurements of especially thiophene containing diarylethenes
in solution have shown to improve the NLO properties at molecular
well as 1H NMR (Fig. S1–S11) and IR spectra (Fig. S12–S22) for all compounds.
This material is available free of charge. CCDC 2058507–2058519 for compounds
1aꢀcentro, 1aꢀnon-centro, 1b, 1c, 1d, 1e, 4ꢀPcell, 4ꢀCcell, 2a, 3aꢀmono, 3aꢀortho, 3b,
and 3c. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/d1nj00456e
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Experimental methods
Reagents and general procedures
The reactions were carried out in air. All the starting materials and
solvents were purchased from commercial sources. Solvents were
dried over 3 Å molecular sieves and the remaining reagents
were used without further purification: diethyl(4-nitrobenzyl)-
phosphonate (TCI Chemicals, 497.0%), 4-methoxy-3-methylbenzal-
dehyde (Sigma-Aldrich, 99%), 2-chloro-3,4-dimethoxybenzaldehyde
(TCI Chemicals, 498.0%), 5-bromo-3-nitrosalicylaldehyde (TCI
Chemicals, 497.0%), 4-bromothiophene-2-carboxaldehyde (TCI
Chemicals, 498.0%), 5-methylthiophene-2-carboxaldehyde
(TCI Chemicals, 497.0%), 4-fluorocinnamaldehyde (TCI Chemicals,
495.0%), sodium hydroxide (VWR Chemicals, 99%), 4-anisal-
dehyde (Merck, 99%), 3,4-dimethoxybenzaldehyde (Aldrich,
99%), 5-(methylthio)thiophenecarboxaldehyde (TCI Chemicals,
498.0%), 4-methoxycinnamaldehyde (Sigma-Aldrich, Z98%),
4-dimethylaminocinnamaldehyde (TCI Chemicals, 498.0%),
CH2Cl2 (VWR Chemicals, 99%), chloroform (Fisher Chemical,
analytical reagent grade), acetone (Mallinckrodt, 499.5%), Ethanol
(Altia, 499.5%). Elemental analyses were performed by analytical
¨
¨
services at the Department of Chemistry, University of Jyvaskyla.
Spectroscopic methods
The 1H NMR spectra were obtained in CD2Cl2 at 30 1C on
Bruker Avance III 300 spectrometer operating at 300.15 MHz.
1H NMR spectra are referenced to the solvent signal and the
chemical shifts are reported relative to (CH3)4Si. The IR spectra
were measured with Bruker Alpha FTIR spectrometer. The
extinction coefficients (molar absorptivity) were determined
with PerkinElmer Lambda 650 spectrophotometer by measuring
the absorbances of 1 mM and 10 mM solutions in CH2Cl2 (800 to
250 nm). The fluorescence spectra of the compounds were
recorded on a Varian Cary Eclipse spectrophotometer from
0.1–10 mM solutions in CH2Cl2.
Scheme 1 Synthesis of the stilbene (1), diphenylbutadiene (2), and phe-
nylethenylthiophene (3) derivatives.
level,22–25 the information on SHG activity of these systems in
the solid-state, requiring the most advantageous non-centro-
symmetric molecular packing in the crystal lattice, is much more
scarce.
We recently reported a novel method for producing NLO
active lenses by stereolithographic (SLA) 3D printing tech-
nique.26 This new approach avoids the often tedious and
Nonlinear optical (NLO) measurements
time-consuming process of growing large single crystals by The nonlinear optical properties of compounds 1a–e, 2a–c, and
utilizing microcrystalline powders of NLO active components 3a–c were measured with Kurtz–Perry powder method27 by
similarly to the Kurtz–Perry powder method for determination using femtosecond laser radiation at 1030 nm. In addition,
of relative SHG intensities.27 At the same time, the protective the second harmonic generation (SHG) was double-checked by
layer of photopolymer resin used in the SLA 3D printing could using spectrally resolving detection scheme using Kurtz–Perry
enable the use of labile organic chromophores as NLO active sample geometry. For these experiments, both femtosecond
units.26 In this context and with the above considerations in and nanosecond laser pulses were used. Femtosecond experi-
mind, we have now investigated the conjugated, –NO2 sub- ments were performed with the setup consisting an amplified
stituted stilbene-based systems with the goal of producing new femtosecond laser source at 1030 nm (Pharos, Light Conversion
NLO active materials in the solid-state by the means of (1) Ltd) running at 600 kHz repetition rate. Generated SHG signal
changing the functional groups at the electron-donating end of was detected by using a high-resolution spectrometer equipped
stilbene, (2) extending the chain length between the aromatic with 300 mm spectrograph (Acton SpectraPro 300i) with spectro-
groups to afford diarylbutadiene derivatives, and (3) changing scopy CCD detector (Newton DU971N-BV, Andor). Nano-second
one of the phenyl groups to thiophene. Consequently, we report laser pulses at 100 Hz repetition rate and at 1030 nm and
here the synthesis, spectroscopic (1H NMR, IR, UV-Visible and 1060 nm were taken from the ns-OPO (Ekspla Ltd.). Spectrum
fluorescence) and structural characterization (single-crystal XRD), of the SHG signal was recorded by using time-gated ICCD
and the SHG properties (Kurtz–Perry powder method)27 of stilbene detector (ISTAR, Andor) with 150 mm spectrograph. (Acton
(1a–e), diphenylbutadiene (2a–c), and phenylethenylthiophene SpectraPro 150i). The crystalline samples, apart from 1b, were
(3a–c) derivatives (Scheme 1).
sieved to a particle size of o125 mm and placed into capillary
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tubes for the determination of relative SHG intensity of the The product was then extracted with CH2Cl2/H2O-mixture followed
compounds. A capillary tube with urea sieved to the same particle by evaporation of the organic phase to give an orange powder of
size was used as a reference.
0.023 g (6% yield) as the spectroscopically pure product.
X-Ray crystallography. Crystallographic data for compounds
Anal. calcd (%): C, 46.05; H, 2.48; N, 7.67 found: C, 44.98; H,
1aꢀcentro, 1aꢀnon-centro, 1b, 1c, 1d, 1e, 4ꢀPcell, 4ꢀCcell, 2a, 3aꢀ 2.64; N, 7.86%. 1H NMR (CD3CN, 30 1C): d 11.10 [s, 1H, –OH], d
mono, 3aꢀortho, 3b, and 3c are summarized in Tables S1–S3 8.24 [m, 3H, –C6H4NO2 (2H) and –C6H2 (1H)], d 8.04 [m, 1H,
(cf. ESI,† also for molecular figures with atomic numbering –C6H2], d 7.72 [m, 2H, –C6H4NO2], d 7.58 [d, 1H, –CHQCH–,
schemes and the tables of pertinent bond parameters). Crystals 3J (1H,1H) = 16 Hz], d 7.32 [d, 1H, –CHQCH–, 3J (1H,1H) = 16 Hz].
were coated with FomblinsY oil and mounted on a MiTeGen X-Ray quality crystals of compound 1b were obtained by slow
loop. Diffraction data were collected on Rikagu-Oxford Super- evaporation of ethanol/chloroform solution at 23 1C.
Nova Single or Dual Source diffractometers equipped with Atlas
Preparation of4-methoxy-40-nitrostilbene (1c). 0.133 g (1.00 mmol)
CCD area-detector using graphite monochromatized CuKa of 4-anisaldehyde was dissolved in 10 mL of ethanol and 0.22 mL
radiation (l = 1.54184 Å; 1aꢀcentro, 1aꢀnon-centro, 1b, 1c, 1e, (1.00 mmol) of diethyl(4-nitrobenzyl)phosphonate was added at 23 1C.
4ꢀPcell, 4ꢀCcell, 2a, and 3b) or MoKa radiation (l = 0.71073 Å; 3 mL of 1.5 M solution of sodium hydroxide in ethanol was then
1d, 3aꢀmono, 3aꢀortho, and 3c) at ꢁ120 or ꢁ150 1C. The data added and the reaction mixture was stirred overnight. The
were processed primarily by performing analytical numeric absorp- resulting precipitate was then filtered and washed with 5 mL
tion correction using a multifaceted crystal with CrysAlisPro of cold ethanol. The reaction afforded 0.170 g of yellow powder
program.28 All structures were solved by direct methods with (66% yield).
SHELXS or SHELXT and refined by using SHELXL implemented
Anal. calcd (%): C, 70.57; H, 5.13; N, 5.49 found: C, 69.83; H,
in the Olex2 program package.29,30 After full-matrix least-squares 4.82; N, 5.52%. 1H NMR (CD2Cl2, 30 1C): d 8.19 [m, 2H,
refinement of the non-hydrogen atoms with anisotropic thermal –C6H4NO2], d 7.63 [m, 2H, –C6H4NO2], d 7.52 [m, 2H,
parameters, the hydrogen atoms were placed in calculated posi- –C6H4OMe], d 7.27 [d, 1H, –CHQCH–, 3J (1H,1H) = 16 Hz], d
tions (C–H = 0.93 Å for –CH and 0.98 Å for –CH3 hydrogen atoms, 7.05 [d, 1H, –CHQCH–, 3J (1H,1H) = 16 Hz], d 6.94 [m, 2H,
and O–H = 0.82 Å for –OH hydrogen atoms). The isotropic thermal –C6H4OMe], d 3.84 [s, 3H, –OCH3]. X-Ray quality crystals of
parameters of the calculated hydrogen atoms were fixed at 1.2 compound 1c were obtained by slow evaporation of methanol
(–CH) or 1.5 (–CH3) times that of the corresponding carbon or solution at 23 1C.
oxygen. In the final refinement, the calculated hydrogen atoms
were riding on their respective carbon or oxygen atoms.
Preparation of 3,4-dimethoxy-40-nitrostilbene (1d). 0.165 g
(1.00 mmol) of solid 3,4-dimethoxybenzaldehyde was dissolved
Preparation of 2-chloro-3,4-dimethoxy-40-nitrostilbene (1a). in 10 mL of ethanol at 23 1C. 0.22 mL (1.00 mmol) of diethyl-
0.202 g (1.00 mmol) of 2-chloro-3,4-dimethoxybenzaldehyde (4-nitrobenzyl)phosphonate was added to the solution. 3 mL of
was dissolved in 10 mL of ethanol and 0.22 mL (1.00 mmol) 1.5 M solution of sodium hydroxide in ethanol was then added
of diethyl(4-nitrobenzyl)phosphonate was added to the solution and the reaction mixture was stirred overnight. The resulting
at 23 1C. 3 mL of 1.5 M of sodium hydroxide solution in ethanol precipitate was then filtered and washed with 5 mL of cold
was then added and the reaction mixture was stirred overnight. ethanol. The reaction afforded 0.219 g of yellow powder (77%
The resulting precipitate was then filtered and washed with yield).
5 mL of cold ethanol. The reaction afforded 0.255 g of yellow
powder (80% yield).
Anal. calcd (%): C, 67.36; H, 5.30; N, 4.91 found: C, 66.46; H,
5.08; N, 4.81%. 1H NMR (CD2Cl2, 30 1C): d 8.20 [m, 2H,
Anal. calcd (%): C, 60.10; H, 4.41; N, 4.38 found: C, 59.54; H, –C6H4NO2],
d 7.64 [m, 2H, –C6H4NO2], d 7.26 [d, 1H,
3
4.19; N, 4.40%. 1H NMR (CD2Cl2, 30 1C): d 8.21 [m, 2H, –C6H4NO2], –CHQCH–, J (1H,1H) = 16 Hz], d 7.12 [m, 2H, –C6H3], d 7.05
3
d 7.68 [m, 2H, –C6H4NO2], d 7.65 [d, 1H, –CHQCH–, J (1H,1H) = [d, 1H, –CHQCH–, 3J (1H,1H) = 16 Hz], d 6.90 [m, 2H, –C6H3O], d
16 Hz], d 7.49 [m, 1H, –C6H2], d 7.05 [d, 1H, –CHQCH–, 3J (1H,1H) = 3.91 [s, 3H, –OCH3] d 3.87 [s, 3H, –OCH3]. X-Ray quality crystals
17 Hz], d 6.93 [m, 1H, –C6H2], d 3.91 [s, 3H, –OCH3], d 3.85 [s, 3H, of compound 1d were obtained by slow evaporation of metha-
–OCH3]. X-Ray quality crystals of the two polymorphs of compound nol solution at 23 1C.
1a (1aꢀcentro and 1aꢀnon-centro) as well as two polymorphs of the
Preparation of 4-dimethylamino-40-nitrostilbene (1e). 0.148 g
subsequent dimerization products (4ꢀPcell and 4ꢀCcell, cf. main text) (1.00 mmol) of 4-dimethylaminobenzaldehyde was dissolved in
were obtained by slow evaporation of ethanol/chloroform and 10 mL of ethanol at 23 1C. 0.22 mL (1.00 mmol) of diethyl-
ethanol/dichloromethane solutions at 23 1C.
(4-nitrobenzyl)phosphonate was added to the solution. 3 mL of
Preparation of 5-bromo-2-hydroxy-3-nitro-40-nitrostilbene 1.5 M solution of sodium hydroxide in ethanol was then added
(1b). 0.247 g (1.00 mmol) of solid 5-bromo-3-nitrosalicylaldehyde and the reaction mixture was stirred overnight. The resulting
was dissolved in 15 mL of ethanol at 23 1C. 0.22 mL (1.00 mmol) of precipitate was then filtered and washed with 5 mL of cold
diethyl(4-nitrobenzyl)phosphonate was added to the solution. The ethanol. The reaction afforded 0.182 g of red powder (68% yield).
flask was gently heated until all aldehyde had dissolved. 3 mL
Anal. calcd (%): C, 71.62; H, 6.01; N, 10.44 found: C, 70.25;
of 1.5 M solution of sodium hydroxide in ethanol was then H, 5.81; N, 10.32%. 1H NMR (CD2Cl2, 30 1C): d 8.17 [m, 2H,
added and the reaction mixture was stirred overnight. The –C6H4NO2], d 7.59 [m, 2H, –C6H4NO2], d 7.45 [m, 2H,
resulting precipitate was then filtered and washed with 5 mL –C6H4NMe2], d 7.24 [d, 1H, –CHQCH–, 3J (1H,1H) = 16 Hz],
of cold ethanol, affording 0.200 g of red-brown product (55% yield). d 6.96 [d, 1H, –CHQCH–, J (1H,1H) = 16 Hz], d 6.72 [m, 2H,
3
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–C6H4NMe2], d 3.01 [s, 6H, –N(CH3)2]. X-Ray quality crystals of was dissolved in 10 mL of ethanol at 23 1C. 0.22 mL (1.00 mmol) of
the compound 1e were obtained by slow evaporation of metha- diethyl(4-nitrobenzyl)phosphonate was added to the solution. 3 mL
nol solution at 23 1C.
of 1.5 M solution of sodium hydroxide in ethanol was then added
Preparation of 40-fluoro-400-nitro-1,4-diphenyl-1,3-butadiene and the reaction mixture was stirred overnight. The resulting
(2a). 0.151 g (1.00 mmol) of solid 4-fluorocinnamaldehyde was precipitate was then filtered and washed with 5 mL of cold ethanol.
dissolved in 10 mL of ethanol at 23 1C. 0.22 mL (1.00 mmol) of The reaction afforded 0.186 g of yellow powder (60% yield).
diethyl(4-nitrobenzyl)phosphonate was added to the solution.
Anal. calcd (%): C, 46.47; H, 2.60; N, 4.52 found: C, 45.91; H,
3 mL of 1.5 M solution of sodium hydroxide in ethanol was 2.62; N, 4.53%. 1H NMR (CD2Cl2, 30 1C): d 8.20 [m, 2H, –C6H4NO2], d
then added and the reaction mixture was stirred overnight. The 7.62 [m, 2H, –C6H4NO2], d 7.34 [d, 1H, –CHQCH–, J (1H,1H) = 16
3
resulting precipitate was then filtered and washed with 5 mL of Hz], d 7.23 [m, 1H, –C4H2S], d 7.12 [m, 1H, –C4H2S], d 7.01 [d, 1H,
cold ethanol. The reaction afforded 0.093 g of yellow powder –CHQCH–, 3J (1H,1H) = 16 Hz]. X-Ray quality crystals of compound
(35% yield).
3aꢀmono were obtained by slow evaporation of ethanol/chloroform
Anal. calcd (%): C, 71.36; H, 4.49; N, 5.20 found: C, 70.05; H, solution at 23 1C, while the crystals of 3aꢀortho were obtained by slow
4.33; N, 5.18%. 1H NMR (CD2Cl2, 30 1C): d 8.18 [m, 2H, evaporation of toluene solution at 23 1C.
–C6H4NO2], d 7.58 [m, 2H, –C6H4NO2], d 7.47 [m, 2H,
Preparation of 2-[(4-nitrophenyl)ethenyl]-5-methylthiophene
–C6H4OMe], d 7.18–6.71 [m, 6H, –C6H4OMe & –CHQCH–]. (3b). 0.11 mL (1.00 mmol) of liquid 5-methylthiophene-2-
X-Ray quality crystals of compound 2a were obtained by slow carboxaldehyde was dissolved in 10 mL of ethanol at 23 1C.
evaporation of acetone/water solution at 23 1C.
0.22 mL (1.00 mmol) of diethyl(4-nitrobenzyl)phosphonate was
Preparation of 40-methoxy-400-nitro-1,4-diphenyl-1,3-butadiene added to the solution. 3 mL of 1.5 M solution of sodium
(2b). 0.162 g (1.00 mmol) of solid 4-methoxycinnamaldehyde was hydroxide in ethanol was then added and the reaction mixture
dissolved in 10 mL of ethanol at 23 1C. 0.22 mL (1.00 mmol) of was stirred overnight. The resulting precipitate was then fil-
diethyl(4-nitrobenzyl)phosphonate was added to the solution. tered and washed with 5 mL of cold ethanol. The reaction
3 mL of 1.5 M solution of sodium hydroxide in ethanol was then afforded 0.191 g of yellow powder (78% yield).
added and the reaction mixture was stirred overnight. The
Anal. calcd (%): C, 63.65; H, 4.52; N, 5.71 found: C, 62.70; H,
resulting precipitate was then filtered and washed with 5 mL 4.40; N, 5.59%. 1H NMR (CD2Cl2, 30 1C): d 8.18 [m, 2H, –C6H4NO2],
of cold ethanol. The reaction afforded 0.175 g of orange powder d 7.58 [m, 2H, –C6H4NO2], d 7.35 [d, 1H, –CHQCH–, J (1H,1H) =
(62% yield).
3
16 Hz], d 6.99 [m, 1H, –C4H2S], d 6.84 [d, 1H, –CHQCH–, 3J (1H,1H) =
Anal. calcd (%): C, 72.58; H, 5.38; N, 4.98 found: C, 71.36; H, 16 Hz], d 6.72 [m, 1H, –C4H2S], d 2.51 [s, 3H, –CH3]. X-Ray quality
5.27; N, 5.01%. 1H NMR (CD2Cl2, 30 1C): d 8.17 [m, 2H, –C6H4NO2], crystals of compound 3b were obtained by slow evaporation of
d 7.57 [m, 2H, –C6H4NO2], d 7.43 [m, 2H, –C6H4OMe], d 7.14 [q, 1H, ethanol/chloroform solution at 23 1C.
–CHQCH–], d 6.90 [m, 3H, –C6H4OMe & –CHQCH–], d 6.78 [d, 1H,
Preparation of 2-[(4-nitrophenyl)ethenyl]-5-(methylthio)thiophe
–CHQCH–, 3J (1H,1H) = 15 Hz], d 6.69 [d, 1H, –CHQCH–, (3c). 0.152 g (1.00 mmol) of liquid (5-methylthio)thiophene-
3J (1H,1H) = 16 Hz], d 3.82 [s, 3H, –OCH3]. X-Ray quality crystals 2-carboxaldehyde was dissolved in 10 mL of ethanol at 23 1C.
of compound 2b were obtained by slow evaporation of dichlor- 0.22 mL (1.00 mmol) of diethyl(4-nitrobenzyl)phosphonate was
omethane solution at 23 1C and the structure was confirmed to be added to the solution. 3 mL of 1.5 M of sodium hydroxide in
analogous to that reported previously31 by unit cell measurement. ethanol was then added and the reaction mixture was stirred
Preparation of 40-dimethylamino-400-nitro-1,4-diphenyl-1,3- overnight. The precipitate was then filtered and washed with
butadiene (2c). 0.175 g (1.00 mmol) of solid 4-dimethylaminocin- 5 mL of cold ethanol. The reaction afforded 0.224 g of orange
namaldehyde was dissolved in 15 mL of ethanol at 23 1C. 0.22 mL powder (81% yield).
(1.00 mmol) of diethyl(4-nitrobenzyl)phosphonate was added to
Anal. calcd (%): C, 56.29; H, 4.00; N, 5.05 found: C, 55.53; H,
the solution. 3 mL of 1.5 M solution of sodium hydroxide in 3.97; N, 5.05%. 1H NMR (CD2Cl2, 30 1C): d 8.19 [m, 2H,
ethanol was then added and the reaction mixture was stirred –C6H4NO2], d 7.59 [m, 2H, –C6H4NO2], d 7.33 [d, 1H, –CHQCH–,
overnight. The resulting precipitate was then filtered and washed 3J (1H,1H) = 16 Hz], d 7.04 [m, 1H, –C4H2S], d 6.96 [m, 1H, –C4H2S],
with 5 mL of cold ethanol. The reaction afforded 0.1451 g of d 6.87 [d, 1H, –CHQCH–, 3J (1H,1H) = 16 Hz], d 2.55 [s, 3H, –CH3].
copper-brown product (49% yield).
X-Ray quality crystals of compound 3c were obtained by slow
Anal. calcd (%): C, 73.44; H, 6.16; N, 9.52 found: C, 72.83; H, evaporation of dichloromethane solution at 23 1C.
6.07; N, 9.47%. 1H NMR (CD2Cl2, 30 1C): d 8.15 [m, 2H, –C6H4NO2],
d 7.54 [m, 2H, –C6H4NO2], d 7.36 [m, 2H, –C6H4NMe2], d 7.14 [q,
1H, –CHQCH–], d 6.87–6.68 [m, 4H, –C6H4NMe2 & –CHQCH–], d
Results and discussion
Synthesis and crystal structures of compounds 1–4
6.62 [d, 1H, –CHQCH–, 3J (1H,1H) = 16 Hz], d 2.99 [s, 6H,
–N(CH3)2]. X-Ray quality crystals of compound 2c were obtained
by slow evaporation of dichloromethane solution at 23 1C and Horner–Wadsworth–Emmons synthetic method33 was used to
the structure was confirmed to be analogous to that reported synthesize the compounds 1–3 (Scheme 1). Equimolar amounts of
previously32 by unit cell measurement.
aromatic aldehyde and phosphonate were reacted in basic condi-
Preparation of 2-[(4-nitrophenyl)ethenyl]-4-bromothiophene (3a). tions to yield spectroscopically and analytically pure products in
0.191 g (1.00 mmol) of solid 4-bromothiophene-2-carboxaldehyde good yields (cf. ESI† for the 1H NMR and IR spectra). The reactions
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resulted in the precipitation of pure products with the exception of been previously observed with other stilbenes and diaromatic
compound 1b which was extracted with a mixture of CH2Cl2 and olefins where a distance of ca. 3.5–4.2 Å between the central CQC
H2O followed by evaporation of the organic phase. All the com- bonds of adjacent molecules facilitates the solid state photo-
pounds 1–3 possess –NO2 group at the electron withdrawing end dimerization process.36–38 Suitable alignment of molecules and
of the push–pull molecules. Various substituents at the electron a distance of ca. 3.7 Å between the double bonds can be found in
donating end were selected with a view to have an effect on the 1aꢀcentro whereas no such arrangement is observable in 1aꢀnon-
charge-transfer properties and therefore the molecular hyperpolari- centro therefore suggesting that the former polymorph is the
zability as well as to increase the possibility for the ideal, non- primary source for the formation of tetra-aryl cyclobutane dimer 4.
centrosymmetric packing of the chromophores in the crystal lattice
(Scheme 1).
The cyclobutane dimer 4 also shows polymorphism with
crystallization in two monoclinic space groups, P21/n (4ꢀPcell)
and C2/c (4ꢀCcell). The difference in crystal packing can also be
seen in a slightly different orientation of the phenyl groups
Single-crystal X-ray structures of stilbene derivatives
Similarly to many reported stilbene derivatives,20,21,34,35 the 2- around the central cyclobutane unit (Fig. 2). The dimerization
chloro-3,4-dimethoxy-40-nitrostilbene compound 1a shows poly- results in the expected lengthening of the double bond in the
morphism by crystallization in both centrosymmetric (P21/c, 1aꢀ ethenyl part of the original monomers from 1.333(2)–1.333(4) Å
centro) and non-centrosymmetric (P21, 1aꢀnon-centro) space in 1aꢀnon-centro and 1aꢀcentro to 1.546(2)–1.558(2) Å in 4ꢀPcell
groups (Fig. 1, cf. ESI† for molecular figures of all the structures and 4ꢀCcell. In addition, the C–C bonds between the two
with atomic numbering schemes and the tables of pertinent stilbene units in cyclobutane rings in 4ꢀPcell and 4ꢀCcell are
bond parameters). Analogously to 3-methyl-4-methoxy-40-nitro- slightly longer than within each stilbene unit at 1.583(2)–
stilbene (MMONS),34 the discrete molecules in the centrosym- 1.600(2) Å vs. 1.546(2)–1.558(2) Å, respectively (cf. ESI†).
metric form 1aꢀcentro are nearly planar with ca. 3.81 angle
Stilbene derivative 1b with –OH, –Br and two –NO2 substi-
between the aryl rings. By contrast, more pronounced twisting tuents crystallizes in a non-centrosymmetric space group Cc.
is notable in the non-centrosymmetric polymorph 1aꢀnon-centro The phenyl rings within each molecule are strongly twisted
as evidenced by the corresponding angle of ca. 19.21 in the compared to both polymorphs of 1a with an angle of 29.71
molecule. The primary driving force of the crystal packing in between the aryl groups (Fig. 3a). Similarly to 1a, the crystal
both polymorphs of 1a seems to be p–p interactions, and while packing in 1b can be primarily contributed to p–p interactions
in 1aꢀcentro this leads to a herringbone-like linear arrangement even though weak Brꢀ ꢀ ꢀONO halogen contacts of ca. 3.51 and
of molecular chains (Fig. 1a), in 1aꢀnon-centro half of the 3.37 Å, analogous to the Clꢀ ꢀ ꢀONO contacts in 1aꢀnon-centro,
molecules are rotated by ca. 901 (Fig. 1b). Consequently, only also exist. The two orientations of independent molecules in 1b
1aꢀnon-centro displays intermolecular Clꢀ ꢀ ꢀONO interactions of (Fig. 3b) and those observed in 1aꢀnon-centro (Fig. 1b) also bear
3.27 Å whereas the remaining close contacts (e.g. MeOꢀ ꢀ ꢀH, a close resemblance.
NO2ꢀ ꢀ ꢀH) are virtually insignificant in both polymorphs.
Stilbene 1c with –NO2 and –MeO substituents displays three
Stilbene 1a was also noted to go through, presumably, a independent molecules in the crystal lattice arranged in a
%
photodimerization process of the ethenyl linkage to afford tetra- rather random manner in triclinic space group P1 (Fig. 4a).
aryl cyclobutane compound 4 (Fig. 2). Analogous behaviour has In contrast to the structures of 1a and 1b, there is an apparent
lack of p–p interactions in 1c. Instead, one of the three indepen-
dent molecules forms centrosymmetric dimers through weak
Hꢀ ꢀ ꢀOMe hydrogen bonds between the MeO group and one of
the hydrogens of the phenyl rings (Fig. 4b).
Fig. 1 Molecular packing in (a) 1aꢀcentro (along c-axis) and (b) 1aꢀnon-
centro (along a-axis).
Fig. 2 The difference in phenyl ring and substituent orientations between
P-cell (red) and C-cell (blue) tetra-aryl cyclobutanes 4.
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Fig. 5 (a) Rotation of the phenyl rings, and (b) the dimeric unit in 1d.
Fig. 3 (a) Crystal packing in compound 1b showing the orientation of the
molecules within each layer (along a-axis) and (b) side view of the layer
revealing the molecular distortion.
Stilbene derivative 1d with one –NO2 and two –MeO groups
crystallizes in a centrosymmetric space group P21/c. The aromatic
rings within each molecule are heavily twisted with an angle of
43.71 between the rings (Fig. 5a). The molecules in 1d form dimeric
units through MeOꢀꢀꢀH3CO interactions (Fig. 5b) while the overall
arrangement of molecules in the crystal lattice is rather arbitrary.
Stilbene congener 1e with –NO2 and –NMe2 substituents
crystallizes in a non-centrosymmetric space group P21. The
structure shows disorder in which the molecules are organized
in two orientations in the crystal lattice in 60 : 40 ratio (cf. ESI†
for a molecular figure of disorders in 1e). The slightly twisted
stilbene molecules form planes along a-axis, and chains
through weak NO2ꢀ ꢀ ꢀ(H3C)N hydrogen bonding along c-axis
with alternating chain directions in the planes (Fig. 6).
Fig. 6 Molecular packing of stilbene derivative 1e viewed along a-axis (H
atoms removed for clarity, disorder indicated with lighter colours).
Single-crystal X-ray structures of diphenylbutadiene derivatives
The diphenylbutadiene 2a crystallizes in non-centrosymmetric
space group Pc. Despite multiple crystallizations and several
data collections, the crystal structure consistently shows orien-
tational disorder where adjacent molecules are placed opposite
Fig. 7 Crystal structure of 2a with herringbone-style layers viewed along
a-axis (H atoms removed for clarity, disorder indicated with lighter colours).
directions in head-to-tail fashion (cf. ESI† for a molecular figure
of disorder in 2a).39 The slightly twisted diphenylbutadienes in
2a form planes along a-axis with alternating orientation of the
molecules within each plane, and a herringbone style layering
between the planes (Fig. 7). The crystal structures of diphenyl-
butadienes 2b and 2c have been reported previously (CSD
entries YIQJEQ and EDUSAA).31,32 The former crystallizes in
%
centrosymmetric space group P1 with head-to-tail organization
of the molecules. Despite the increase in chain length com-
pared to stilbenes derivatives 1a–e, the conjugated molecules in
2b are notably planar (Fig. 8) forming a sheet-like structure with
p–p interactions leading to a relatively short distance of 3.70 Å
between centroids of the phenyl rings. The diphenylbutadiene
2c crystallizes in centrosymmetric space group P21/c and,
analogously to 2b, with markedly planar geometry within the
molecules. In contrast to 2b, however, there is some angling of the
molecules between the layers (Fig. 8c), and therefore no p–p
interactions can be observed. Despite the extensive crystallization
Fig. 4 (a) Orientation of stilbene molecules in the crystal lattice, and
(b) close contacts in 1c.
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Fig. 8 (a) Side view and (b) end view of molecular arrangement in 2b (CSD
entry ‘‘YIQJEQ’’),31 and (c) molecular arrangement in 2c (CSD entry
‘‘EDUSAA’’).32 Disorder in 2c has been removed for clarity.
efforts, no polymorphism and discovery of new, non-centro-
symmetric crystal structures of 2b and 2c were observed.
Fig. 9 (a) Crystal structure of 3aꢀmono viewed along b-axis, and (b) side
view of 3aꢀortho.
Single-crystal X-ray structures of phenylethenylthiophene
derivatives
Nonlinear optical properties of compounds 1a–e, 2a–c and 3a–c
Similarly to stilbene 1a, the phenylethenylthiophene 3a displays
polymorphism by crystallizing in two centrosymmetric space
groups, monoclinic P21/c (3aꢀmono) and orthorhombic Pbca
(3aꢀortho). Most notably, the phenyl and thiophene rings in 3aꢀ
mono are significantly twisted (by 12.41 and 17.51 in two
independent molecules, respectively) while the analogous twisting
in 3aꢀortho is less pronounced (4.71). Both polymorphs exhibit
significant halogen bonding through NO2ꢀꢀꢀBr contacts of ca. 3.06
and 3.00 Å in 3aꢀmono and 3aꢀortho, respectively. Overall, the
molecules form layered structure in 3aꢀmono (Fig. 9a) whereas in
3aꢀortho the distortion between layers is notable (Fig. 9b).
Phenylethenylthiophene derivative 3b crystallizes in a centro-
symmetric space group P21/n (Fig. 10a). The crystal structure
displays two independent molecules in the unit cell with only
slight deviation from planarity (ca. 6.81 and 5.71 angles between
the aromatic rings, respectively). One of the two molecules in
asymmetric unit exhibits significant rotation of the –NO2 sub-
stituent with respect to the phenyl ring (3.61 vs. 27.81 in the two
distinct molecules). This behaviour is similar to 3aꢀmono, in
which the –NO2 groups show various degrees of rotation
(3.21 and 13.71 degrees), whereas in 3aꢀortho all the substituents
are virtually planar with respect to the phenyl and thiophene
rings. The weak NO2ꢀ ꢀ ꢀH3C interactions result in chain forma-
tion with alternating directions and V-shaped geometry along
the chains (Fig. 10b).
The nonlinear optical properties of compounds 1a–e, 2a–c, and
3a–c were initially measured with Kurtz–Perry powder method27
by using femtosecond laser radiation at 1030 nm. In these
experiments the SHG efficiency of MMONS (Scheme 1) dis-
played intensity of 61 times compared to that of urea (Table 1),
far from the reported literatures values of ca. 750–1250 times
urea.21 In view of the possible competing processes, such as
two-photon fluorescence, the SHG intensities of MMONS, urea
and the most promising new stilbene derivative, 1aꢀnon-centro,
were also measured by using nanosecond laser radiation at
both 1030 and 1060 nm to closely reproduce the literature experi-
ments (Table 1 and Fig. 12).21 While the switch from femto-
second to nanosecond laser source did increase the SHG intensity
of MMONS from 61 to ca. 200 times to that of urea, the change of
the wavelength from 1030 to 1060 nm did not show significant
effect on the SHG intensity consistently with the solid-state
absorption maxima of MMONS reported at lower wavelength
(346 and 474 nm)21 than either of the SHG signals at 515 and
Phenylethenylthiophene 3c crystallizes in centrosymmetric
space group P21/n. The molecular chains form herringbone-
style planes with altering direction of the chains within each
layer (Fig. 11). The only intermolecular interactions are the
weak NO2ꢀ ꢀ ꢀH3CS contacts between the two substituents.
Fig. 10 (a) Crystal structure of compound 3b viewed along b-axis, and
(b) side view of the layers showing the V-shaped arrangement.
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Fig. 11 Herringbone style layers of 3c viewed along a-axis.
Table 1 Areas of SHG signals of 1aꢀnon-centro, 1b and 2a compared to
urea and MMONSa
Femtosecond laser radiation at 1030 nm (laser power: b1.3 mW, c6.2 mW)
Signal
Samplea areab (a.u.) (times urea) Sampleb areac (a.u.) (times urea)
Relative areab
Signal
Relative areac
Urea
31 000
1
Urea
1b
2a
397 800
15 600
71 300
1
0.04
0.18
MMONS 1 890 000 61.0
1aꢀnon-
169 700
5.5
centro
e
Nanosecond laser radiation at d1030 and 1060 nm
Signal Relative area Signal
Relative areae
Samplea aread (a.u.) (times urea)d Sampled areae (a.u.) (times urea)
Urea
MMONS 977 100
4760
1
205
7.7
Urea
7120
1
MMONS 1 498 300 210
1aꢀnon- 230 650 32.4
centro
1aꢀnon- 36 640
centro
Fig. 12 SHG signals of 1aꢀnon-centro, MMONS and urea generated at (a)
a
Crystalline samples sieved to particle size of o125 mm.
1030 nm and (b) 1060 nm with nanosecond laser source.
530 nm. However, the SHG intensity of MMONS still remained evident to be two-photon induced process as the intensity is not
notably lower than the literature values (200 vs. 750–1250 times of clearly dependent on the duration of laser pulse, suggesting
urea) possibly owing to the fragmentation of fragile MMONS that the generated SHG signal is to some extent absorbed in the
samples into too small microcrystalline material in the sieving sample.
process to reduce the SHG intensity,21 or because of partial
The stilbene derivatives 1b and 1e also display non-centro-
contamination of the samples with inactive polymorphs of symmetric crystal packing in the solid state with the space
MMONS.34 The latter, however, was rendered unlikely by space groups Cc and P21, respectively. However, only negligible SHG
group determinations of several individual crystals and by X-ray activity is observed for compound 1b (0.04 times that of urea),
powder measurements.
and virtually no SHG activity is detected for 1e with femto-
Hand-picked crystalline sample of the non-centrosymmetric second laser experiments at 1030 nm. The stilbene derivative 1c has
polymorph 1aꢀnon-centro displays SHG intensity of ca. 5.5 times also been reported to give rise to SHG signal, and polymorphism by
to that of urea in the femtosecond laser experiments at 1030 nm. crystallization in both centrosymmetric and non-centrosymmetric
The SHG intensity of 1aꢀnon-centro increases to 7.7 and, signifi- triclinic space groups.20,40,41 However, our investigation revealed
42
%
cantly, 32.4 times to that of urea when nanosecond laser source only centrosymmetric space group (P1) and no SHG activity for 1c.
is used at 1030 and 1060 nm wavelength, respectively. In all
While the stilbene derivatives 1a–e expectedly display varying
measurements the microcrystalline sample of 1aꢀnon-centro degree of SHG activity in the solid state depending on the
shows detectable broad fluorescence indicating that some SHG efficiency of crystal packing, the alterations in the chain length
efficiency is lost due to electronic excitation of the material, between aryl groups to give diarylbutadienes 2 and changing
namely two-photon absorption of the laser radiation or absorp- one of the aryl groups to thiophene in compounds 3 resulted
tion of the generated SH signal. The absorbance at 515 nm is primarily in centrosymmetric arrangements of the chromo-
virtually non-existing in CH2Cl2 solution (see below), but in the phores despite the extensive crystallization efforts to obtain
microcrystalline sample signal faces multiple interactions non-centrosymmetric polymorphs. Consequently, among all
increasing absorption significantly. This may explain the the derivatives of 2 and 3, only the diarylbutadiene compound
observed difference in the intensity of SHG signal between 2a shows SHG intensity (0.18 times of urea) consistently with the
two fundamental wavelengths. In addition, fluorescence is not non-centrosymmetric space group Pc.
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It has been suggested that the highest recorded SHG activity
Paper
of 3-methyl-4-methoxy-40-nitrostilbene (MMONS) arises partly
from the strong p–p interactions that enhance the intermole-
cular charge-transfer and, consequently, the second order
polarizability.21 In accordance, MMONS displays distances as
short as ca. 3.56 Å between the phenyl ring centroids despite
the two distinct orientations of the molecules. The same
distances in stilbene derivatives 1aꢀnon-centro and 1b are ca.
4.22 and 3.82 Å, respectively, even though it is the former
chromophore that displays higher SHG intensity. This is likely
a result of the better arrangement of polar centres in the crystal
lattice of 1aꢀnon-centro and the presence of electron withdrawing
–NO2 group also in the ‘‘push’’ part of the chromophore 1b.
In contrast to the aforementioned compounds, the non-centro-
symmetric stilbene derivative 1e does not show significant p–p
interactions, which is possibly contributing to the virtual absence
of SHG activity.
Optical spectroscopy of compounds 1–3
The absorption spectra of compounds 1–3 were measured in
CH2Cl2 at room temperature (Table 2 and Fig. 13). All com-
pounds display one high and one low energy absorption maximum
in solution, albeit the former is observable only as a small shoulder
with the solvent signal in the case of stilbene derivatives 1a and 1c.
Additionally, stilbene congener 1b, the only compound with –NO2
group at both ends of the molecule, displays a distinct shoulder
also at 410 nm. The high energy (low wavelength) absorption
maxima show relatively narrow range of 278–303 nm for stilbene
derivatives 1, 289–322 nm for diphenylbutadiene compounds 2,
Table 2 Absorption and fluorescence spectroscopic properties of com-
pounds 1–3
Absorption bandsa
Fig. 13 Visible spectra of compounds (a) 1a–e, (b) 2a–c and (c) 3a–c (1 mM
and 10 mM sol. in CH2Cl2 at 23 1C).
b
b
lAbs (nm)
ec (Mꢁ1 cmꢁ1
)
lAbs (nm)
ec (Mꢁ1 cmꢁ1
)
1a
1b
1c
1d
1e
365
17 200
2a
2b
2c
3a
3b
3c
381
289
401
301
452
322
358
272
390
278
397
291
23 350
10 700
36 450
16 350
35 350
18 450
10 600
13 750
16 300
10 800
27 350
8850
336
278
376
23 800
13 700
23 100
and 272–291 nm for phenylethenylthiophene species 3, while the
corresponding low energy (high wavelength) bands exhibit signifi-
cantly broader range of 336–439 nm for 1a–e, 381–452 nm for 2a–c
and 358–397 nm for 3a–c. Expectedly, all the extinction coefficient
values of ca. 9–37 ꢂ 103 Mꢁ1 cmꢁ1 are in the range typically
observed for p - p* transition. Given the wider wavelength range
observed for the low energy absorption maxima, these bands are
likely reflecting the modifications made by substituent alterations
at the electron donating part of compounds 1–3 whereas the high
energy bands in the absorption spectra are contributed to the
electron withdrawing, –NO2 substituted end of the conjugated
push–pull systems where no alterations were made within each
series. Consistently, both diphenylbutadiene (2) and phenyl-
ethenylthiophene (3) derivatives exhibit a steady red shift in the
low energy absorption maximum following the approximate
order of increasing electron donating power of the substituents:
F (2a, 381 nm) o OCH3 (2b, 401 nm) o NMe2 (2c, 452 nm), and
Br (3a, 358 nm) o CH3 (3b, 390 nm) o SCH3 (3c, 397 nm),
respectively. Similarly, stilbene derivative 1e with the strongest
385
280
439
303
24 350
13 000
23 850
12 750
Fluorescence bandsd
lEx (nm) lEm (nm) Dng (cmꢁ1
)
lEx (nm) lEm (nm) Dng (cmꢁ1
)
e
f
e
f
1a 380
1c 380
1d 390
1e 450
560
585
605
730
9540
9502
9445
9080
2a 380
2b 400
2c 460
3a 380
3b 395
3c 400
550
615
780
520
570
635
8065
8677
9303
8702
8097
9441
a
b
1 mM and 10 mM sol. in CH2Cl2 at 23 1C. Absorption maximum.
Extinction coefficient. 0.1–10 mM solutions in CH2Cl2 at 23 1C.
Excitation maximum. Emission maximum. Stokes shift in wave-
c
e
d
f
g
numbers calculated by the equation (107/lAbs) ꢁ (107/lEm).
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together with the increasing electron donating power of the sub-
stituent(s) within each series. Accordingly, the –NMe2 containing
derivatives 1e and 2c give rise to the highest wavelength excitation
and emission maxima. The large Stokes red shift values expectedly
indicate high dipole moment in the excited state for all the com-
pounds 1–3 in CH2Cl2 as has been previously demonstrated, for
example, for compound 1e.43
Conclusions
We report here the preparation and characterization of conju-
gated stilbene, diphenylbutadiene and phenylethenylthiophene
based diaromatic compounds and their SHG activities measured
from the crystalline samples by the Kurtz–Perry powder method.
The most significant activity was observed for the stilbene 1a,
which produces SHG signal with intensity ca. 32 times stronger
than that of the urea reference when nanosecond laser radiation
at 1060 nm is applied. The stilbene derivative 1b and diphenyl-
butadiene 2a also display SHG activity, albeit with significantly
lower intensity compared to 1a. The often high SHG intensities
reported for stilbene derivatives have been contributed, in part,
to the strong p–p interactions that enhance the intermolecular
charge-transfer and, consequently, the second order polariz-
ability.21 Consistently, the stilbene derivative 1a exhibits somewhat
weaker p–p interactions compared to 3-methyl-4-methoxy-40-nitro-
stilbene (MMONS, Scheme 1) with significantly higher SHG
intensity.
Our efforts to further investigate the effect of weak interactions
to the NLO properties by changing the functional groups at the
electron-donating end of stilbene, the extended chain length
between the aromatic groups in diarylbutadienes and with thio-
phene derivatives were hampered by the extensive polymorphism
that resulted in mainly centrosymmetric, NLO inactive chromo-
phores despite the comprehensive crystallization efforts. Even the
stilbene 1a with notable SHG activity also crystallizes in centro-
symmetric, inactive space group and goes through, presumably, a
photodimerization process to form two polymorphs of the tetra-
aryl cyclobutane 4. Nevertheless, the high SHG intensities of
especially 3-methyl-4-methoxy-40-nitrostilbene (MMONS) and con-
generic derivative 1a make them attractive chromophores for
future studies to produce NLO active lenses by stereolithographic
3D printing technique.26 Furthermore, the current study illustrates
the versatility of the structural, chemical and optical properties in
these diaromatic, conjugated systems as also evidenced by their
fluorescent nature in solution.
Fig. 14 Fluorescence spectra of compounds (a) 1a, 1c–1e, (b) 2a–c and
(c) 3a–c (emission spectra: solid line, excitation spectra: dotted line, 0.1–10 mM
sol. in CH2Cl2 at 23 1C).
electron donating group (NMe2) gives the lowest energy absorp-
tion maximum of the series at 439 nm.
The fluorescent nature of stilbene and its derivatives is well
established and various aspects of their optical, photophysical
and photochemical properties have been exhaustively studied.43–47
Consistently with the high optical activity, the stilbene (1), diphenyl-
butadiene (2) and phenylethenylthiophene (3) derivatives in this
investigation all show fluorescent properties with the exception of Conflicts of interest
compound 1b (Table 2 and Fig. 14). In contrast to the two
absorption bands, only single maximum with poorly resolved
There are no conflicts to declare.
shoulders is observed in the excitation spectra of all the com-
pounds 1–3 in CH2Cl2 at room temperature. However, the wave-
length of the excitation maximum of each derivative displays close
Acknowledgements
correlation with the corresponding low energy absorption band. The authors gratefully acknowledge financial support from
Similarly to the absorption spectra, both the excitation and emis- the Finnish Cultural Foundation and the Magnus Ehrnrooth
sion spectra of compounds 1–3 exhibit more pronounced red shift Foundation.
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¨
26 E. Kukkonen, E. Lahtinen, P. Myllyperkio, J. Konu and
M. Haukka, ACS Omega, 2018, 3, 11558–11561.
27 S. K. Kurtz and T. T. Perry, J. Appl. Phys., 1968, 39, 3798–3813.
28 CrysAlis PRO, Agilent Technologies Ltd, Yarnton, Oxford-
shire, England.
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