Stereoselective synthesis and properties of 1,3-bis(dicyanomethylidene)indane-5-carboxylic acid acceptor fragment containing nonlinear optical chromophores

Article abstract of DOI:10.1039/c6tc00203j

A series of organic push-pull type chromophores using indane-1,3-dione 5-carboxylic acid (IDCA) and novel 1,3-bis(dicyanomethylidene)indane 5-carboxylic acid (CICA) electron acceptor fragments have been synthesized and characterized. NMR and X-ray analysis revealed that condensation reactions with the CICA fragment were stereoselective and yielded benzylidenes and azomethines with E double bond configurations. Due to the non-planar geometry these compounds are chiral and were acquired as a racemic mixture. The subsequent functionalization of the carboxylic acid group with 5,5,5-triphenylpentan-1-ol yielded solution-processable glass forming materials (6, 8, 10, 13) with glass transition temperature values of 76-134 °C. The nonlinear optical (NLO) properties of these compounds were characterized using quantum chemical calculations and second harmonic generation (SHG) measurements in corona-poled thin glassy films. The twisted geometry of the CICA based materials was shown to be beneficial to the macroscopic NLO performance due to the less pronounced solid phase stacking compared to the flat IDCA based compounds. The presence of site isolating groups at both the acceptor and donor ends of the molecule in compound 13 resulted in a considerable NLO efficiency increase. Non-centrosymmetric crystals of CICA based N,N-dimethylaminobenzylidene 7b were obtained and showed a SHG response comparable to urea.

Full text of DOI:10.1039/c6tc00203j

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2016, DOI: 10.1039/C6TC00203J.
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Stereoselective synthesis and properties of 1,3ꢀ
bis(dicyanomethylidene)indaneꢀ5ꢀcarboxylic acid acceptor fragment
containing nonlinear optical chromophores
Kaspars Traskovskisa, *, Valdis Kokarsa, Andrejs Tokmakovsb, Igors Mihailovsa,b,
Edgars Nitissb, Marina Petrovac, Sergey Belyakovc, Martins Rutkisb
a Riga Technical University, Faculty of Materials Science and Applied Chemistry, 3/7
Paula Valdena Street, Riga LVꢀ1048, Latvia
b Institute of Solid State Physics, University of Latvia, 8 Kengaraga Street, Riga LVꢀ
1063, Latvia
c Latvian Institute of Organic Synthesis, 21 Aizkraukles Street, Riga LVꢀ1006, Latvia
* Corresponding author. Tel.: +371 29148070. Eꢀmail address:
kaspars.traskovskis@rtu.lv (K. Traskovskis).
Abstract
A series of organic pushꢀpull type chromophores using indaneꢀ1,3ꢀdione 5ꢀcarboxylic
acid (IDCA) and novel 1,3ꢀbis(dicyanomethylidene)indane 5ꢀcarboxylic acid (CICA)
electron acceptor fragments have been synthesized and characterized. NMR and Xꢀ
ray analysis revealed that condensation reactions with CICA fragment were
stereoselective and yielded benzylidenes and azomethines with E double bond
configuration. Due to the nonꢀplanar geometry these compounds are chiral and were
acquired as a racemic mixture. The subsequent functionalization of the carboxylic
acid group with 5,5,5ꢀtriphenylpentanꢀ1ꢀol yielded solutionꢀprocessable glass forming
materials (6, 8, 10, 13) with glass transition temperature values 76ꢀ134° C. Nonlinear
optical (NLO) properties of these compounds were characterized by quantum
chemical calculations and second harmonic generation (SHG) measurements in
corona poled thin glassy films. The twisted geometry of CICA based materials
showed to be beneficial to macroscopic NLO performance due to less pronounced
solid phase stacking if compared to flat IDCA based compounds. The presence of site
isolating groups at both the acceptor and donor ends of the molecule in compound 13
resulted in considerable NLO efficiency increase. Nonꢀcentrosymmetric crystals of
CICA based N,Nꢀdimethylaminobenzylidene 7b were obtained and showed SHG
response comparable to urea.
1

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Introduction
In the recent decades a significant attention of the research community has
been drawn to the field of organic photonics in such areas as photovoltaics,1 organic
lightꢀemitting diodes (OLEDs),2 nonꢀlinear optical materials3 and optical sensing and
bioimaging.4 Besides improved device performance characteristics this interest is
largely driven by the costꢀefficient solution based material processing techniques, like
inkjet printing,5 which can be applied to most organic compounds. A wide group of
the compounds used for photonic applications are “donorꢀπ electron systemꢀacceptor”
(DꢀπꢀA) type molecules. The rational design can be easily applied to manage the
optical properties of the DꢀπꢀA chromophores, in particular, characteristics like
bandgap value, dipole moment and charge distribution can be modified by choosing
appropriate donor and acceptor groups or a πꢀelectron bridge. Therefore the
availability of wide variety of DꢀπꢀA building blocks is essential for the development
of new organic photonic materials.
One of the most commonly used electron acceptors is indanꢀ1,3ꢀdione.
Alongside with its structural derivative 1,3ꢀbis(dicyanomethylidene)indane and other
close structural analogues these groups have been extensively used to design materials
in different research areas: OLEDs,6,7 organic solar cells,8ꢀ10 NLO materials,11ꢀ15 twoꢀ
photon absorbing materials,16 molecular optical sensors17,18 and organic
semiconductors.19ꢀ21 However, like in the case of most other frequently used strong
electron
acceptors,
the
presence
of
indanꢀ1,3ꢀdione
or
1,3ꢀ
bis(dicyanomethylidene)indane is somewhat limiting the molecular design
possibilities of DꢀπꢀA chromophores, as these groups are no further chemically
functionalizable. Because of their large dipole moment organic chromophores often
have poor solubility and tend to be in polycrystalline solid state, making them
unsuitable for solution processing or optoelectronic applications. Structural
modifications like polymerization or introduction of solubility enhancing groups are
performed to overcome this. There are numerous areas of application, where the fineꢀ
tuning of molecular properties is also needed to maximize the performance
characteristics of the material. For instance, to achieve a good second order NLO
response in organic materials the noncentrosymmetric arrangement of NLO active
structural elements is needed. Due to large dipole moment values NLO chromophores
tend to pack centrosymmetrically and the polar order is usually achieved by an
2

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external electric field poling procedure,3 however, the strong dipolar interactions
between DꢀπꢀA molecules limit the efficiency of this process. Several molecular
design strategies can be applied to increase the attainable level of the polar order. The
so called site isolation principle22ꢀ24 is based on the introduction of sterically large
chemical substituents that increase the distance between NLO chromophores and
reduce the strength of dipolar interactions. During our recent research we showed that
pseudoꢀspherical triphenylmethyl substituents, if covalently attached to polar NLO
chromophores, can be used as efficient dipole shielding groups.25 In an attempt to
further exploit this approach we proposed a structure where a DꢀπꢀA moiety would be
encapsulated by triphenylmethyl groups at both the donor and acceptor ends of the
molecule. However, the lack of accessible, chemically easily likable and strong
electron acceptor groups was limiting our efforts. One of the promising compounds
that drew our attention was indanꢀ1,3ꢀdioneꢀ5ꢀcarboxylic acid (IDCA). This structural
fragment has an easily linkable carboxylic acid group as well as moderately increased
acceptor strength if comparted to indanꢀ1,3ꢀdione. Due to the presence of a free
carboxylic group the compound has been used to develop chromophores for dyeꢀ
sensitized solar cells,26,27 but no research effort has been made yet to evaluate the
further functionalization possibilities of compounds containing this acceptor. In this
paper we present our findings regarding the synthesis, optical, thermoplastic and NLO
properties of IDCA fragment containing materials. Additionally, we obtained much
stronger 1,3ꢀbis(dicyanomethylidene)indaneꢀ5ꢀcarboxylic acid (CICA) acceptor and
successfully performed condensation reactions to acquire NLO active benzylidenes
and azomethines. Unexpected stereospecific formation of only E configuration
condensation products was observed for this new compound providing promising
possibilities for future applications.
Experimental Section
Starting materials and general procedures
Starting materials and solvents were purchased from Acros or Alfa Aesar. Methylene
chloride (DCM) was dried over CaH2 and distilled prior to use. Compounds 2ꢀ
carbethoxyꢀ5ꢀcarboxyindaneꢀ1,3ꢀdione (2),26 β,β,βꢀtriphenylpropionic acid,28 5,5,5ꢀ
triphenylpentanꢀ1ꢀol (5),29 pꢀnitrosoꢀN,Nꢀdimethylaniline (9)30 and 4ꢀ(bis(2ꢀ
hydroxyethyl)amino)benzaldehyde (11)31 were prepared according to the known
3

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procedures. The UVꢀVis spectra were recorded with a Perkin Elmer Lambda 35
spectrometer. NMR spectra were obtained on a Bruker Avance 300 MHz or Varian 6
Unity Inova 600 MHz spectrometers using solvent residue as an internal reference.
2D NMR experiments (COSY, HSQC, HMBC) were performed to assign signals.
Mass spectra were acquired on Waters EMD 1000 MS detector (ESI+ mode, cone
voltage 30 V). The elemental analysis was carried out with a Costech Instruments
ECS 4010 CHNSꢀO Elemental Combustion System.
Synthesis
5ꢀCarboxyꢀ1,3ꢀbis(dicyanomethylidene)indane pyridinium salt (3): The solution
of compound 2 (1.84 g, 7.0 mmol) and malononitrile (1.25 g, 18.9 mmol) in pyridine
(20 mL) was stirred under reflux for 2 h. Toluene (100 mL) was then added and the
deep blue reaction mixture was allowed to cool to room temperature. The formed
precipitate was filtered off, washed with toluene and dried to yield 3 (2.34 g, 91%) as
a purple solid. The product was sufficiently pure for the use in further reactions. An
analytical sample was purified by column chromatography using ethyl acetate and
1
acetone (1:1, v/v) as the eluent. H NMR (300 MHz, DMSOꢀd6, δ, ppm): 8.89ꢀ8.87
(m, 2H), 8.52ꢀ8.45 (m, 2H), 8.04ꢀ7.94 (m, 4H), 5.79 (s, 1H). Anal. Calcd. for
C21H13N5O3 (3·H2O): C, 65.79, H, 3.41, N, 18.27. Found: C, 66.26, H, 3.06, N, 18.35.
5,5,5ꢀTriphenylpentyl 2ꢀ(4ꢀ(dimethylamino)benzylidene)ꢀ1,3ꢀdioxoꢀ2,3ꢀdihydroꢀ
1Hꢀindeneꢀ5ꢀcarboxylate (6): Compound 2 (2.0 g, 7.6 mmol) and aldehyde 4 (1.14
g, 7.6 mmol) in AcOH (5 mL) were stirred at 100 °C till the formation of gas bubbles
in the reaction mixture had stopped. The dark red precipitate was filtered off, washed
with MeOH and dried. Yield: 1.93 g (79%). The mixture of the technical
condensation product (1.5 g, 4.7 mmol), compound 5 (1.48 g, 4.7 mmol), N,N'ꢀ
dicyclohexylcarbodiimide (DCC) (1.06 g, 5.1 mmol) and 4ꢀdimethylaminopyridine
(DMAP) (0.005 g, 0.04 mmol) in DCM (20 mL) were stirred at room temperature for
12h. The resultant urea was filtered out and EtOH (50 mL) was added to the filtrate.
DCM was fractionally removed and the red precipitate was filtered off. The product
was purified by column chromatography using DCM as the eluent to afford 6 (2.09 g,
72%) as a red glass. The product is a mixture of Z:E isomers (50 : 50%), as calculated
1
by the integration of the respective proton signals. H NMR (300 MHz, CDCl3, δ,
ppm): 8.67 (m, 2H), 8.47 (d, J=1.3 Hz, 0.5H), 8.45 (d, J=1.3 Hz, 0.5H), 8.28 (dd,
4

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J=7.7 Hz, J=1.3, 0.5H), 8.25 (dd, J=7.7 Hz, J=1.3, 0.5H), 7.94 (d, J=7.7 Hz, 0.5H),
7.93 (d, J=7.7 Hz, 0.5H), 7.84 (s, 0.5H), 7.82 (s, 0.5H), 7.28ꢀ7.13 (m, 15H), 6.77 (m,
2H), 4.30 (t, J=6.6 Hz, 2H), 3.18 (s, 6H), 2.65 (m, 2H), 1.81 (quint, J=6.6 Hz, 2H),
1.27 (quint, J=6.6 Hz, 2H). 13C NMR (150 MHz, CDCl3, δ, ppm): 22.07, 29.11, 39.89,
40.11, 56.60, 65.27, 111.49, 122.00, 122.05, 122.29, 122.32, 122.64, 122.71, 123.58,
125.85, 127.82, 129.13, 135.04, 135.36, 135.39, 135.63, 138.39, 138.45, 142.04,
142.75, 147.23, 148.45, 148.52, 145.05, 154.30, 154.32, 165.25, 165.26, 188.87,
189.05, 191.01, 190.79. Anal. Calcd. for C42H37NO4: C, 81.40, H, 6.02, N, 2.26.
Found: C, 81.49, H, 6.13, N, 2.24. MS ESI+, m/z calcd. C42H38NO4: 620.3 [M+H]+,
found 620.4 [M+H]+.
1,3ꢀbis(Dicyanomethylene)ꢀ2ꢀ(4ꢀ(dimethylamino)benzylidene)ꢀ2,3ꢀdihydroꢀ1Hꢀ
indeneꢀ5ꢀcarboxylic acid (7): Compound 3 (1.5 g, 5.2 mmol) and aldehyde 4 (0.91 g,
6.2 mmol) in Ac2O (3 mL) were stirred at 100 °C for 10 min. The reaction mixture
was cooled to room temperature and MeOH (20 mL) was added. The dark blue
precipitate was filtered off and washed with MeOH to yield 7 (1.49 g, 68%). 1H NMR
(300 MHz, DMSOꢀd6, δ, ppm): 8.93 (d, J=1.1 Hz, 1H), 8.46 (s, 1H), 8.44 (d, J=8.3
Hz, 1H), 8.32 (dd, J=1.1 Hz, J=8.3 Hz, 1H), 7.67 (d, J=9.0 Hz, 2H), 6.99 (d, J=9.0 Hz,
2H), 3.29 (s, 6H). Anal. Calcd. for C25H15N5O2: C, 71.93, H, 3.62, N, 16.78. Found:
C, 71.84, H, 3.54, N, 16.61.
5,5,5ꢀTriphenylpentyl ꢀ1,3ꢀbis(dicyanomethylene)ꢀ2ꢀ(4ꢀ
(dimethylaminobenzylidene)ꢀ2,3ꢀdihydroꢀ1Hꢀindeneꢀ5ꢀcarboxylate (8):
The esterification of acid 7 with alcohol 5 was carried following the same protocol as
1
for 6 to afford 8 as a blue glass. Yield: 25 %. H NMR (300 MHz, CDCl3, δ, ppm):
9.14 (d, J=1.4 Hz, 1H), 8.76 (s, 1H), 8.62 (d, J=8.1 Hz, 1H), 8.22 (dd, J=8.1 Hz, J=1.4
Hz, 1H), 7.60 (d, J=9.1 Hz, 2H), 7.25ꢀ7.13 (m, 15H), 6.77 (d, J=9.1 Hz, 2H), 4.31 (t,
J=6.5 Hz, 2H), 3.24 (s, 6H), 2.65 (m, 2H), 1.82 (quint, J=6.5 Hz, 2H), 1.27 (quint,
J=6.5 Hz, 2H). 13C NMR (150 MHz, CDCl3, δ, ppm): 22.01, 29.06, 39.86, 40.46,
56.61, 65.69, 70.62, 70.69, 112.64, 113.96, 113.97, 114.31, 122.59, 122.85, 124.88,
125.71, 127.79, 129.14, 134.49, 134.88, 137.65, 140.71, 147.22, 147.35, 154.69,
160.09, 160.272, 164.373. Anal. Calcd. for C48H37N5O2: C, 80.54, H, 5.21, N, 9.78.
Found: C, 81.27, H, 5.75, N, 8.95. MS ESI+, m/z calcd. C48H38N5O2: 716.3 [M+H]+,
found 716.3 [M+H]+.
5

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5,5,5ꢀTriphenylpentyl 1,3ꢀbis(dicyanomethylene)ꢀ2ꢀ((4ꢀ
(dimethylamino)phenyl)imino)ꢀ2,3ꢀdihydroꢀ1Hꢀindeneꢀ5ꢀcarboxylate (10):
Compound 3 (2.6 g, 7.1 mmol) and nitrosoaniline 9 (1.2 g, 8.0 mmol) in EtOH (20
mL) were stirred under reflux for 30 min. The dark green precipitate was filtered off
and washed with MeOH. Yield: 1.6 g (53 %). The esterification of azomethine
carboxylic acid with alcohol 5 was carried following the same protocol as for 6 to
1
afford 10 as a dark green solid. Yield: 41%. H NMR (300 MHz, CDCl3, δ, ppm):
9.06 (d, J=1.4 Hz, 1H), 8.56 (d, J=8.3 Hz, 1H), 8.22 (dd, J=1.4 Hz, J=8.3 Hz, 1H),
7.39 (d, J=8.7 Hz, 2H), 7.25ꢀ7.13 (m, 15H), 6.81 (d, J=8.7 Hz, 2H), 4.30 (t, J=7.1 Hz,
2H), 3.24 (s, 6H), 2.64 (m, 2H), 1.82 (quint, J=7.1 Hz, 2H), 1.27 (quint, J=7.1 Hz,
2H). 13C NMR (150 MHz, CDCl3, δ, ppm): 21.96, 28.99, 39.79, 40.73, 56.56, 65.75,
73.75, 73.90, 112.78, 113.32, 113.65, 113.97, 125.23, 125.77, 125.97, 127.75, 129.08,
134.83, 135.16, 138.82, 141.60 147.16, 153.76, 164.08. Anal. Calcd. for C47H36N6O2:
C, 78.75, H, 5.06, N, 11.72. Found: C, 79.12, H, 5.23, N, 11.40. MS ESI+, m/z calcd.
C47H37N6O2: 717.3 [M+H]+, found 717.5 [M+H]+.
((4ꢀFormylphenyl)azanediyl)bis(ethaneꢀ2,1ꢀdiyl) bis(β,β,β ꢀtriphenylpropanoate)
(12): Compound 11 (1.82 g, 8.6 mmol) and β,β,βꢀtriphenylpropionic acid (6.54 g, 21
mmol), DCC (4.46 g, 21 mmol) and DMAP (0.01 g, 0.08 mmol) were dissolved in
DCM (50 mL). The reaction mixture was stirred for 6h. The resultant urea was
filtered out and EtOH (100 mL) was added to the filtrate. DCM was fractionally
removed and the white solid was filtered off and washed to afford 12 as a white solid
(6.08 g, 91%). 1H NMR (300 MHz, CDCl3, δ, ppm): 9.76 (s, 1H), 7.71 (d, J=8.8 Hz,
2H), 7.33ꢀ7.17 (m, 30H), 6.58 (d, J=8.8 Hz, 2H), 3.85 (t, J=6.2 Hz, 4H), 3.72 (s, 4H),
3.16 (t, J=6.2 Hz, 4H). Anal. Calcd. for C53H47NO5: C, 81.83, H, 9.09, N, 1.80.
Found: C, 81.71, H, 9.23, N, 1.72.
((4ꢀ((1,3ꢀbis(Dicyanomethylene)ꢀ5ꢀ(((5,5,5ꢀtriphenylpentyl)oxy)carbonyl)ꢀ1Hꢀ
indenꢀ2(3H)ꢀylidene)methyl)phenyl)azanediyl)bis(ethaneꢀ2,1ꢀdiyl)
bis(3,3,3ꢀ
triphenylpropanoate) (13): Aldehyde 12 (3.0 g, 3.8 mmol) was dissolved, with
warming, in acetic anhydride (20 mL). The solution was cooled to 60° C and
compound 3 (1.46 g, 4.0 mmol) was added, and the mixture was stirred for 2h. The
6

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product was precipitated with H2O and washed with EtOH. The purification by
column chromatography using ethyl acetate and toluene (1:2, v/v) as the eluent
afforded the condensation product (2.09 g, 72%) as a purple glass. The subsequent
esterification with alcohol 5 was carried following the same protocol as for 6 to afford
1
13 (16 %) as a blue glass. H NMR (300 MHz, CDCl3, δ, ppm): 9.18 (d, J=1.2 Hz,
1H), 8.69 (s, 1H), 8.66 (d, J=8.3 Hz, 1H), 8.28 (dd, J=1.2 Hz, J=8.3 Hz, 1H), 7.53 (d,
J=9.1 Hz, 2H), 7.31ꢀ7.16 (m, 45H), 6.63 (d, J=9.1 Hz, 2H), 4.35 (t, J=6.5 Hz, 2H),
3.87 (t, J=6.1 Hz, 4H), 3.73 (s, 4H), 3.19 (t, J=6.1 Hz, 4H), 2.68 (m, 2H), 1.86 (quint,
J=6.5 Hz, 2H), 1.31 (quint, J=6.5 Hz, 2H). 13C NMR (150 MHz, CDCl3, δ, ppm):
21.99, 29.04, 39.84, 45.95, 48.93, 55.82, 56.59, 60.28, 65.75, 112.50, 113.43, 113.46,
113.49, 113.79, 122.77, 124.61, 125.09, 125.79, 125.94, 126.35, 127.78, 127.85,
129.03, 129.12, 134.77, 135.21, 137.52, 140.51, 146.12, 146.31, 147.19, 152.47,
160.32, 160.37, 164.20, 170.60. Anal. Calcd. for C92H73N5O6: C, 82.18, H, 5.47, N,
5.21. Found: C, 82.09, H, 5.49, N, 5.24.
Xꢀray structural analysis
Single crystals of 7b were obtained via vapor diffusion method. Compound 7
was dissolved, by heating, in Ac2O. The saturated solution was cooled to room
temperature, filtered and transferred to a small vial. The vial was placed in a closed
chamber, which contained benzene. Blue, plateꢀshaped crystals with sufficient size
were obtained after 3 days. A good enough, cut edged crystal of 7b was used for Xꢀ
ray diffraction analysis. Diffraction data were collected at –100°C on a Brukerꢀ
Nonius KappaCCD diffractometer using graphite monochromated MoꢀKα radiation (λ
= 0.71073 Å). The crystal structure of 7b was solved by direct methods32 and refined
by fullꢀmatrix least squares.32 All nonhydrogen atoms were refined in anisotropical
approximation, all Hꢀatoms were refined by riding model. Crystal data for 7b:
monoclinic; a = 10.1268(7), b = 32.360(3), c = 8.7100(6) Å, β = 108.832(4)°; V =
2701.5(4) Å3, Z = 4, ꢁ = 0.087 mm–1, dcalc = 1.322 g/cm3; space group is Cc. A total of
8143 reflection intensities were collected up to 2θmax = 52°; for structure refinement
2643 independent reflections with I > 2σ(I) were used. The final Rꢀfactor is 0.0864.
For further details, see crystallographic data for 7b deposited with the Cambridge
Crystallographic Data Centre as Supplementary Publication Number CCDC 1436954.
Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK.
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Measurements of thermal properties
Glass transition temperatures of the compounds were determined by DSC
thermograms using Mettler Toledo DSCꢀ1/200W apparatus at a scanning rate
10°C/min. The analyzed samples underwent a cycle of a heating to 250° C, a cooling
to room temperature and a repeated heating to 250° C. The temperature corresponding
to the half vanished NLO activity (TSHI50) was evaluated from NLO activity
measurements with temperature scans at 10°C/min. Decomposition temperatures were
obtained using a Perkin Elmer STA 6000 thermal analyzer, using TG and DTA
techniques.
Quantum chemical calculations
All quantum chemical calculations were performed using Gaussian 09
package.33 For auxiliary tasks GaussView34 and Avogadro35 were used. Dipole
moment and first hyperpolarizability calculations, as well as geometry optimizations
were carried out using rangeꢀseparated hybrid density functional CAMꢀB3LYP36 and
6ꢀ31G(d,p) basis set,37 without modelling dielectric medium. Prior to QMꢀ
optimization, UFF steepest descent optimization in Avogadro was performed,
although the lowestꢀenergy conformer was selected by using chemical intuition and
the data of Xꢀray structural analysis. For compounds 6, 8 and 10 simplified structures
were employed in calculations, replacing 5,5,5ꢀtriphenylpentyl fragment with ethyl
group.
Thin film preparation
The glassy thin films used for measurements of NLO properties were prepared
by dissolving compounds in analytical grade chloroform with a typical concentration
80 mg/mL, then covering indium tin oxide (ITO) coated glass slides with the prepared
solution and spinꢀcoating them with a Laurell WSꢀ400Bꢀ6NPP/LITE spinꢀcoater
(starting speed: 0 rpm, terminal speed: 800 rpm, acceleration: 900 rpm/s, spinning
time: 30 s). The thickness of the obtained films was in the range 0.6ꢀ1.2 ꢁm. The
samples prepared for absorption measurements were 0.1ꢀ0.3 ꢁm thick and were spinꢀ
coated from 3 times more diluted solutions.
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Nonlinear optical measurements
The NLO properties of the amorphous compounds (6, 8, 10 and 13) were
characterized by means of the Maker fringe technique38 with an excitation at 1064 nm
in corona poled glassy thin films. The thickness, refractive indexes and absorption
coefficients of the samples were determined in advance. In the setup, a Qꢀswitched
DPSS Nd:YVO4 laser NL640 by EKSPLA operating at pulse energy typically at ∼16
ꢁJ, ∼10 ns pulse width, repetition rate 40 kHz was used as an excitation source. The
detailed descriptions of the experimental procedures and setup can be found in our
previous paper.25 The NLO coefficients d33, d31 and d15 were calculated by Herman
and Hayden39 approach using an xꢀcut quartz crystal as a reference (d11=0.3 pm/V)
and taking into the account the absorption of film at 532 nm. The extrapolation to the
zero frequency d33(0) according to the two level model40 was employed to benchmark
different chromophores. The second order NLO charaterization in crystals was
performed using the Kurtz powder41 and the Maker fringe methods on the same
optical setup.
Results and discussion
Synthesis and structural characterization
From the synthetic point of view the three main goals of this research were to
obtain chemically linkable IDCA and CICA based electron acceptors, to use them to
synthesize DꢀπꢀA type chromophores and to evaluate subsequent functionalization
possibilities. To be suitable for NLO studies the functionalized materials should be
highly soluble and form amorphous solid phase. We have shown previously that these
properties can be applied to a wide variety of polar molecules by incorporating in
them a triphenylmethyl group containing aliphatic carbon chains.42,43 Based on this
the esterification of the acquired chromophore molecules with 5,5,5ꢀtriphenylpentanꢀ
1ꢀol was chosen as the target molecular design in this research.
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In the first step of the synthesis, according to a known synthetic procedure,26
3ꢀhydroxyꢀ2ꢀ(ethoxycarbonyl)ꢀ1ꢀoxoꢀ1Hꢀindeneꢀ6ꢀcarboxylic acid (2) was acquired
from trimellitic anhydride (2) and ethyl acetoacetate (Scheme 1). Pyridinium salt of
1,3ꢀbis(dicyanomethylidene)indaneꢀ5ꢀcarboxylic acid (3) could be easily obtained by
heating compound 2 with malononitrile in pyridine. Condensation of 2 with 4ꢀ
(dimethylamino)benzaldehyde (4) yielded a poorly soluble DꢀπꢀA benzylidene dye.
This product was readily reacted with 5,5,5ꢀtriphenylpentanꢀ1ꢀol (5) using Steglich
esterification to yield a highly soluble and glass forming compound 6. It is worth
mentioning that indaneꢀ1,3ꢀdione precursor 2 is unstable and upon heating in polar
solvents undergoes rapid decarboxylation, presumably producing reactive indaneꢀ1,3ꢀ
dioneꢀ5ꢀcarboxylic acid. Due to this the condensation reactions involving 2 could be
performed in acetic acid as well as in most alcohols.
Slightly different reactivity was observed for compound 3. In this case the
condensation with benzaldehyde 2 or pꢀnitrosoꢀN,Nꢀdimethylaniline (9) was
performed to yield corresponding benzylidene or azomethine dyes. The reactions with
benzaldehydes gave the desired products only when Ac2O was used as the solvent,
while azomethines could also be acquired in alcohols. The following Steglich
esterification with alcohol 5 was performed to successfully acquire the final materials
8 and 10. The base free form of 3 has a strongly acidic active methylene group placed
between two dicyanomethylidene acceptor fragments and forms a blue colored anion
when dissolved in polar solvents or just by an exposure to the moisture in the air. Due
to this and the precipitation limiting high solubility, the use of pyridinium salt was
preferable for the synthetic needs, as no difference in reactivity was observed between
the two forms of compound 3. In an attempt to acquire fully triphenylmethylꢀshielded
NLO material 13 a three step synthesis was performed starting from benzaldehyde 11,
involving esterification with β,β,βꢀtriphenylpropionic acid, condensation with 3 and
subsequent esterification with alcohol 5.
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Scheme 1. Synthesis of indanꢀ1,3ꢀdioneꢀ5ꢀcarboxylic acid and 1,3ꢀ
bis(dicyanomethylidene)indaneꢀ5ꢀcarboxylic
corresponding DꢀπꢀA chromophores.
acid
acceptor
fragments
and
An unexpected stereoselective reactivity was observed during the
1
condensation reactions involving acceptor fragment 3. The obtained H (Fig. 1) and
13C NMR spectra indicated that indaneꢀ1,3ꢀdioneꢀ5ꢀcarboxylic acid containing
benzylidene 6 was acquired as a predictable 1:1 mixture of E/Z isomers. Such
reactivity is common for in benzene ring substituted indaneꢀ1,3ꢀdione derivatives.43
At the same time the molecules synthesized by employing compound 3 showed the
presence of a single stereoisomer. An accidental separation of one of the isomers was
excluded, leaving the structure specific reactivity of compound 3 as the only possible
cause for this observation. DꢀπꢀA type benzylidenes containing CICA fragment have
highly deformed and strained geometrical structures due to the sterical overcrowding.
This might result into energetically strongly preferential conformations during the
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compounds’ formation phase even if the asymmetry inducing carboxylic acid group is
located relatively far from the active reaction center. The great variance between the
chemical stability of E/Z isomers also cannot be excluded. Nevertheless, the
performed quantum chemical calculations comparing the geometrically optimized E/Z
isomers did not reveal any notable energetic or geometrical differences. At the
moment we cannot give a truly plausible explanation for the observed
stereoselectivity, and additional computational and synthetic experiments are planned
in the future to address this question in detail.
1
Fig. 1. Representative signals in H NMR spectra indicating the presence of the
mixture of E/Z isomers for 6 and a single isomer for 8, 10 and 13.
Scheme 2. Reaction of 7 with acetic anhydride.
To determine, which of the two stereoisomers was obtained during the
synthesis of the compounds containing acceptor fragment 3, a single crystal xꢀray
diffraction analysis was employed. Crystals of sufficient size and quality were
obtained in the case of benzylidene 7 from Ac2O and benzene solvent system. The
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compound initially was poorly soluble, but extended heating in Ac2O led to the
dissolution of the material. As the acquired crystal structure revealed, the acylation of
7 had happened, yielding the reaction product 7b (Scheme 2). Fig. 2 illustrates a
perspective view of the molecule 7b with thermal ellipsoids and the atomꢀnumbering
scheme given. As it can be seen, double bond C2ꢀC15 has E configuration. Based on
this we assumed the same stereochemistry in the case of compounds 8, 10 and 13. It
should be noted that molecules of 7b are chiral even though there are no asymmetric
atoms present. Due to the sterical overcrowding the aniline ring and C15 are pushed
out of the plane of indane ring resulting in a butterflyꢀtype molecular deformation.
The chirality in 7b occurs because of the presence of asymmetry inducing carboxyl
group. The crystals of 7b are racemic and has both enantiomers present, related to
each other by glide planes c. A projection of the crystal structure of 7b is shown in
Fig. 3.
It was found that there are molecules of benzene in the crystal structure. Thus,
this is benzene solvate, where ratio between molecules of benzene and molecules of
the substance is 1:1. For carbon atom C33, which is bonded with two oxygens, the
elevated electronegativity occurs as well as for neighbored methyl carbon atom C35.
It leads to the formation of a weak intermolecular hydrogen bond of CHꢂꢂꢂN type
between the methyl group and the nitrogen atom of cyano group. The length of this
bond is equal to 3.425 Å (HꢂꢂꢂN = 2.49 Å, C−HꢂꢂꢂN = 150º). The only other significant
intermolecular interaction in this crystal structure is between phenyl ring and sixꢀ
membered cycle of the indane system; the centroids of these cycles are separated by
3.89 Å, but the shortest atomꢀatom contact is C4ꢂꢂꢂC19 of 3.556 Å. All other
intermolecular contacts correspond to sums of van der Waals radii. Surprisingly, the
acquired crystals adopted the nonꢀcentrosymmetric space group Cc, unlike most
majority of the strongly polar molecules that crystalize in a headꢀtoꢀtail
centrosymmetric fashion. Usually the nonꢀcentrosymmetric packing for the
compounds with large dipole moments is achieved by strong intermolecular hydrogen
bonds44 or coulombic interactions45 that overcome the relatively weak dipoleꢀdipole
forces. In the case of 7b the main driving force for this is a combination of the
molecular geometry and πꢀπ stacking between electron poor indane ring and electron
rich aniline fragment. Because of the twisted, butterflyꢀtype structure two adjacent 7b
molecules cannot stack in a headꢀtoꢀtail conformation. Instead, the πꢀπ stacking
between two enantiomers of 7b takes place, resulting in the packing where dipole
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vectors of the interacting molecules are not parallel (Fig. S1). It is noteworthy that the
racemic nature and the presence of only E isomer is crucial for the observed nonꢀ
centrosymmetric crystal structure.
Fig. 2. Molecular structure of 7b.
Fig. 3. The unit cell of 7b viewed along the crystallographic direction [100], showing
a CHꢂꢂꢂN hydrogen bond and πꢀπ stacking interactions.
Thermal characteristics
The thermal properties of the synthesized compounds 6, 8, 10 and 13 were
examined by the means of differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA) (Fig. 4, Table 1). To be suitable for the application
in electroꢀoptic devices NLO materials must form amorphous solid phase with high
optical quality and morphological stability. DSC analysis of the compounds revealed
no signals associable with the presence of crystalline phase even during the first cycle
of the heating. Amorphous spinꢀcast film samples that had previously underwent
poling procedure and were subsequently stored for about a year revealed no visible
structural changes under examination with an optical microscope (Fig. S2). These
observations indicate that crystal formation ability of these compounds is low and
amorphous phase stability is very high despite the presence of highly polar DꢀπꢀA
chromophores. This can be mainly attributed to the incorporated alkylꢀ
triphenylmethyl moieties that create a large conformational uncertainty during the
solidꢀphase packing and partly shield dipolar interactions between the polar
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molecules. The presence of structural isomers (E/Z isomers for 6 and racemic mixture
for 8, 10 and 13) is another major factor in hindering the possible crystallization.
Regarding glass transition temperatures (Tg), a relatively large variation of this
parameter is observed for different materials. For structurally similar compounds 6, 8
and 10, where a single triphenylmethyl moiety is attached to the indane acceptor,
notably higher Tg values are observed when much CICA fragment is present (8, 10).
The same trend can be witnessed when materials 8 and 10 are compared, as more
polar compound 8 (see Table 3 for calculated dipole moment values) has increased
glass transition temperature. This is in agreement with our previous results,43 where
molecules with larger dipole moment lead to higher Tg. Assuming that only partial
dipoleꢀdipole shielding is achieved in solid phase for the synthesized materials, the
molecular motion limiting intermolecular πꢀπ interactions still take place and will be
more pronounced when the more polar chromophores are present. Unsurprisingly, a
steep drop in Tg by 37 °C is observed for compound 13 if compared to 8 despite
having the same chromophore core. More pronounced sterical shielding for 13 limits
dipolar intermolecular interactions and lowers energetic barrier for the heat induced
molecular motion. Regarding the thermal stability, the materials show moderate
decomposition temperatures (TD) in the range of 259ꢀ293 °C, with the exception of
compound 10. In the case of this azomethyne type chromophore a broad exothermic
DSC peak at 130ꢀ170 °C was detected with a simultaneous mass loss of about 2% in
TGA thermogram. The structurally analogous uncarboxylated CICA based
compounds undergo intramolecular cyclization reactions associated with an
elimination of HCN molecule upon heating in solvents.46 The observed
decomposition, most likely, can be attributed to this process. All materials showed an
excellent solubility in lowꢀpolarity solvents (DCM, toluene) but were insoluble in
alcohols.
Table 1 Thermal characteristics of the synthesized materials.
a
b
Compound
Tg , °C
TD , °C
76
134
122
97
293
267
6
8
131; 287
10
259
13
a Glass transition temperature
b Decomposition temperature (onset)
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Fig. 4. DSC thermograms of compounds 6, 8, 10 and 13 during the first heating.
Linear absorption in solutions and solid films
The acquired UVꢀVis absorption bands in different polarity solutions and
films are given in Fig. 5 but corresponding maxima values can be found in Table 2.
All compounds have a single pronounced absorption band in the visible or near
infrared wavelength range. Predictable batochromic shifts of the bands can be
observed with the increase of the acceptor strength in DꢀπꢀA system. The shortest
wavelength absorption maxima is measured for compound 6. The wellꢀdefined,
intensive and narrow peak shape for this chromophore is determined by the
structurally planar DꢀπꢀA conjugation system that is conformationally stabilized by a
CꢀH…O type intramolecular interaction.47 A stronger CICA acceptor group
containing compounds 8 and 13 show red shifted maxima values and band widths are
much broader than for 6 due to less strictly defined molecular geometry. Surprisingly
large variations by 20ꢀ47 nm between the absorption maxima values of 8 and 13 in
different polarity solvents are observed despite the presence of almost identical
chromophores. Although a minimal alterations are expected between compounds’
light absorption properties due to slightly different electronic properties of nitrogen
substituents in the aniline fragment of the molecules, the detected difference is too
large to consider only this explanation. Most likely, a solvatochromic effect is causing
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a significant band shift in compound 13 due to a dendrimer like encapsulation of the
chromophore core by large triphenylmethyl groups that cause local polarity changes.
Such effect is well known for dendrimeric, site isolated chromophores.48 Azomethine
type compound 10 shows the most batochromically shifted absorption with a wide
band in the range of 500ꢀ1000 nm. In this compound the electron withdrawing
strength of DꢀπꢀA system is even larger than in 8, as relatively to ꢀC=Cꢀ group the
electron withdrawing azomethine or imine fragment (ꢀC=Nꢀ) in the conjugation
bridge of the molecule act as an additional acceptor by itself. If compared to the
structurally identical, but uncarboxylated CICA based chromophores,49 compounds 8
and 10 have a notably redꢀshifted absorption maxima in DCM solution by 26 and 20
nm. This indicates that the electron acceptor nature of carboxyl group is strong
enough to considerably enhance the acceptor strength of this indane derivative.
A notable difference in the solvatochromic properties of the synthesized
chromophores can be observed. Compounds 6 and 13 show positive solvatochromism
that is characteristic for indane based dyes50 and indicates the presence of an aromatic
pushꢀpull type structure in the ground state that undergoes intramolecular electron
density shift upon the light excitation with a resulting increase in dipole moment. A
similar tendency can be observed for compounds 8 and 13 in low to moderately polar
solvents. However, in polar solvents (MeCN, DMSO) the wide uniform absorption
band splits in three distinctive absorption peaks. Analogous effect has been previously
reported in the case of 1,3ꢀbis(dicyanomethylidene)indane condensation products with
pyrone,51 and was attributed to the dominant zwitterionic resonance form in the
ground state of the chromophores. If compared, the split absorption peaks of 8 and 13
resemble that of the 1,3ꢀbis(dicyanomethylidene)indaneꢀ5ꢀcarboxylic acid anion 3
(Fig. 5), indicating the presence of compounds’ charge separated resonance form.
Moreover, a shift in solvatochromic behavior from positive to negative with the
increase of polarity of solvents can be observed. This is clearly evident in the case of
compound 13 where in the series of low polarity solvents (toluene, THF, DCM)
absorption bands shift batocromically but in polar solvents (MeCN, DMSO)
hypsochromically. Such behavior is different from analogous uncarboxylated 1,3ꢀ
bis(dicyanomethylidene)indane dyes52 where no absorption band splitting has been
observed and the absorption shifts in different solvents express positive
solvatochromy.
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The solid phase spectra taken in amorphous thin films show broadened and
batochromically shifted absorption bands for all compounds. This is associable with
the alterations in compounds’ electron molecular orbitals caused by the solid state
packing of polar chromophores. A slightly different pattern is observed in the case of
compound 6, where a presence of a peak shoulder can be seen, as well as a substantial
hypsochromic broadening. Compound 6 is notably less polar and has a planar
geometry in comparison with 1,3ꢀbis(dicyanomethylidene)indane fragment containing
compounds 8, 10 and 13, meaning that the solid phase packing for this material is
expected to be different. The flattened geometry is the most likely cause for the
observed absorption peak shape, as the molecules are more prone to form ordered
solid phase aggregates because of the reduced geometrical restrictions.
Fig. 5. UVꢀVis absorption spectra of the synthesized compounds in solvents with
different polarity and films.
Table 2 Light absorption characteristics of the synthesized compounds in solvents
with different polarity and films.
Compound
λmax, nm
(toluene)
489
λmax, nm
(THF)
491
λmax, nm
(DCM)
496
λmax, nm
λmax, nm
λmax, nm
(film)
510
εmax, molꢀ1·cmꢀ1 (·104)
(MeCN) (DMSO)
(DCM)
9.6
6
8
495
636
766
636
509
660
788
613
622
631
634
673
4.2
10
13
759
762
775
804
2.8
602
608
608
639
3.6
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Nonlinear optical properties
NLO properties of the synthesized amorphous phase forming materials were
characterized by the measurements of second order nonlinear coefficient d33 in
electric field poled films. The macroscopic NLO performance of a material is
proportional to molecular hyperpolarizability (β) of individual chromophores and
their number density (N) as well as average molecular acentric order parameter
〈
cos3θ .3 Since polar molecules in solid phase tend to pack in an energetically
〉
preferable centrosymmetric fashion, the attainment of sufficient polar order level is
usually the main limiting factor for most NLO materials. One of the main goals of this
research was to evaluate if a complete encapsulation of a polar chromophore with
dipolar interactions shielding nonꢀpolar triphenylmethyl groups could notably
increase the material’s macroscopic NLO efficiency. The synthesized DꢀπꢀA
molecules were initially characterized by quantum chemical calculations to determine
β and dipole moment (ꢁ) values (Table 3). The lowest β and ꢁ was predicted for
compound 6, as this molecule has a relatively weak acceptor fragment. About 50%
increase in hyperpolarizability and dipole moment was calculated for 1,3ꢀ
bis(dicyanomethylidene)indane containing compound 8. The highest NLO efficiency
was predicted for compound 10, showing an additional increase by 50%. At the same
time a notable decrease in dipole moment was calculated in comparison with
compound 8. These results are in good agreement with so called eWoD pattern
proposed by Chafin and Lindsay,53 where an increase of β and a decrease of ꢁ can be
achieved in polymethine DꢀπꢀA systems by incorporating electron acceptor
substituents at the evenꢀnumbered positions (counting from the acceptor end of the
molecule). In the case of compound 10 the electron withdrawing azomethine (ꢀC=Nꢀ)
fragment at 4 position acts as such acceptor. The experimentally determined
macroscopic NLO coefficients d33 can be found in Table 4. For compound evaluation
purposes the parameter d33(0) was taken into account to exclude the frequency
dependent enhancement. The results for structurally comparable materials 6, 8 and 10
show that the highest efficiency was achieved for compound 8. The fact that the
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observed NLO efficiencies are not consistent with calculated β values insists the
influence of acentric order diminishing dipolar interactions. This is not unexpected, as
the molecular design for the mentioned compounds involves the presence of only one
dipole shielding moiety. Despite having the lowest dipole moment, compound 6
shows the poorest NLO performance. As the previously discussed UVꢀVis spectra
indicated this can be attributed to the planar geometry of the compound that allows
more ordered solid phase packing and subsequently lowers the efficiency of external
electric field poling process. While compounds 8 and 10 are more polar, the nonꢀ
planar shape and the racemic nature leads to more random solid state packing that is
easier to orient in an external electric field. The modified version of compound 8, the
triphenylmethylꢀencapsulated material 13, shows a predictable enhancement in NLO
efficiency by approximately 30%. The performance increase is even more impressive
considering the fact that the mass fraction of the active chromophore in compound 13
is 1.8 times lower. This result once more demonstrates that the reduction of
intermolecular dipolar interactions by applying a rational molecular design can greatly
improve the measurable macroscopic nonlinearity of the material. The NLO
efficiency of the investigated molecular glasses (~20 pm/V at 1064 nm excitation) is
comparable with the values obtained for materials based on the widely used
chromophore DR1 (10ꢀ100 pm/V range).54 This performance level is also typical for
more recent molecular glass systems,55 but is notably lower than that for the stateꢀofꢀ
theꢀart monolithic molecular materials (up to 550 pm/V).56 The short length of the πꢀ
electron bridge between the donor and acceptor parts of the synthesized
chromophores is currently limiting the attainable β value and consequently the
macroscopic NLO response. An order of the magnitude enhancement of this
parameter is expected with the introduction of a longer polyenic conjugation bridge.57
The heat induced decay of polar order in poled amorphous films was
characterized by a parameter TSHI50, a temperature at which the starting second
harmonic generation intensity is lowered by 50% (Fig. 6). Like in our previous
studies, TSHI50 values for low molecular weight materials are considerably lower than
measured Tg. This can be attributed to different amorphous solid phase characteristics
between the samples in DSC and NLO experiments. Since the thin films used in
optical experiments are spin coated, less dense molecular packing with larger free
volume portion as well as solvent inclusions are the most likely causes for an
energetically less restrained molecular motion. This is particularly evident for
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compounds 8 and 10, where TSHI50 values are by 58 and 48° C lower than Tg. The less
dense solid phase packing for these compound may be facilitated by the bent shape of
1,3ꢀbis(dicyanomethylidene)indane based chromophores that promotes the formation
of cavities. The crystal structure of 7b is a good indication for this, as the compound
forms crystals with large free volume that is filled with benzene molecules. The
highest TSHI50 value of 90° C was measured for material 13, this being mainly
determined by the significantly higher molecular weight of the compound, a property
that usually can be associable with Tg increase in molecular glasses.58
Table 3 The results of quantum chemical calculations.
Compound
β(0)a, 1030 esu
ꢁa, D
45
66
91
5.3
6
8.1
8
6.8
10
a In vacuum
Table 4 Linear and nonlinear optical properties of the synthesized materials.
a
Compound
n532
n1064
d33,
pm·Vꢀ1
26.0
13.1
5.1
d33(0)
,
TSHI50
,
pm·Vꢀ1
°C
6
8
1.92
1.17
1.46
1.45
1.69
1.76
1.86
1.69
1.6
57
4.7
76
10
13
3.4
74
21.7
6.2
90
a Extrapolation to zero frequency according to twoꢀlevel model
Fig. 6. Heat induced decay of second harmonic generation in poled amorphous films.
Since compound 7b forms crystals with nonꢀcentrosymmetric space group,
they should possess a measurable NLO activity. To characterize the potential
nonlinearity of 7b crystals Kurtz powder and Maker fringe methods were employed.
In the Kurtz powder experiment the reflected second harmonic intensity as a function
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of incident beam power was collected and compared for powdered 7b, KDP and Urea
crystal samples. As evident from Fig. 7, the collected second harmonic intensity
exhibits a quadratic dependence on the incident light power. After the comparison of
the coefficients at the quadratic term of the second order polynomial fit, the nonlinear
optical efficiency of 7b was estimated to be 0.98 times that of urea and 7.8 times
higher than that of KDP. Small crystal plates (such as displayed in Fig. 8) were used
as samples for Maker fringe measurements. Crystal 7b belongs to the monoclinic
space group Cc (point group m) and has 6 independent nonlinear optical coefficients.
In these measurements the sample was excited by S and P polarized beams and the P
polarized second harmonic was then collected. According to Dou et al. only three
types of Maker fringes can be observed for monoclinic system with m point group.59
According to crystallographic measurements, the flat surface that was facing the beam
at zero incidence angle in the Maker fringe measurements was the [100] surface. Thus
the Maker fringe shape and amplitude should be determined by nonlinear optical
coefficients d13, d32 and d33 and rotation and incident light polarization axis. Due to
the poor mechanical properties of the crystals, as well as their insufficient size it was
impossible to determine the exact values of nonlinear optical coefficients. At the same
time it was expected for 7b to obtain Maker fringes, which are the superposition of
the fringes identified by Dou et al. The typically measured Maker fringe curves for
the 7b are shown in Fig. 8 and the shapes very much resemble the Maker fringes of
1st and 3rd kind. While the NLO measurements of 7b crystal revealed the NLO
activity, the material has a relatively low efficiency, as evident by Kurtz powder
measurements. This is determined by DꢀπꢀA molecule packing in the crystal, where
bulk dipole configuration of molecules are far from the most efficient for NLO
materials parallel alignment. This causes considerable cancellation of the molecular
nonlinearities and lowers the macroscopic NLO efficiency. Nevertheless, the
compound 7b is one of the few known highly polar DꢀπꢀA molecules that forms nonꢀ
centrosymmetrical crystals. Special interest can be attributed to the fact that the
packing is achieved mainly by geometry and stereochemistry of the molecule instead
of hydrogen bonding or coulombic interactions. Thus the molecular design of 7b can
be used as a promising structural base in the future crystalline NLO material
development studies.
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Fig. 7. Second harmonic intensity as a function of fundamental beam power measured
for 7b, Urea and KDP.
Fig. 8. Microscope image of a typical small 7b crystal plate used in the Maker fringe
experiments and the Maker fringes of the sample for two cases: measured P polarized
second harmonic after excitation by S and P polarized fundamental harmonic.
Conclusions
DꢀπꢀA type chromophores using indaneꢀ1,3ꢀdioneꢀ5ꢀcarboxylic acid and novel
1,3ꢀbis(dicyanomethylidene)indaneꢀ5ꢀcarboxylic acid electron acceptor fragments
have been synthesized. The reactions with the latter structural group resulted in
methine and azomethine dyes with E double bond configuration. A further
functionalization of the carboxylic acid group to acquire functional photonic materials
is possible employing simple synthetic procedures. Glass forming and solution
processable, low molecular weight compounds were acquired by synthesizing carboxy
5,5,5ꢀtriphenylpentanꢀ1ꢀol esters. A high amorphous phase stability, greatly enhanced
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by the presence of E/Z isomers or racemic mixture, was observed for these materials.
NLO activity was successfully measured in poled amorphous films and the highest
nonlinearity (d33(0) value of 6.2 pm/V) was measured for compound where
encapsulation of DꢀπꢀA molecule with dipole shielding triphenylmethyl groups at
both the donor and acceptor ends was employed. The controllable stereochemistry in
1,3ꢀbis(dicyanomethylidene)indaneꢀ5ꢀcarboxylic acid based compounds may be
potentially applied to acquire fineꢀtuned materials in NLO and other photonic fields.
The formation of nonꢀcentrosymmetric crystals in the case of compound 7b is an apt
example for this potential.
Acknowledgments
This work has been supported by the Latvian State Research Program on
Multifunctional Materials IMIS2.
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