Design, synthesis, and properties of nonlinear optical chromophores based on a verbenone bridge with a novel dendritic acceptor

Article abstract of DOI:10.1039/c8tc00083b

Two novel second order nonlinear optical (NLO) chromophores based on N,N-diethylaniline as a donor, verbenone based tetraene as a bridge, and tricyanofuran (TCF) or tricyanofuran derivatives with a dendritic moiety as an acceptor have been synthesized in good overall yields and systematically characterized. Besides, a facile applicable synthetic approach for a NLO dendritic acceptor was developed. Compared with C7, after introducing dendritic derivative steric hindrance groups into the acceptor, chromophore C8 had good thermal stabilities with high thermal decomposition temperatures which were 33 °C higher than that of chromophore C7. At the same time, cyclic voltammetry (CV) experiments were performed to determine the different redox properties. The conjugated verbenone tetraene segments in two chromophores could significantly improve the glass-forming ability and molecular polarization of chromophores as revealed by UV-vis-NIR absorption measurements. The bulky dendritic moiety linked by a short C-C bond is closer to the TCF acceptor, which is the most polar part in the chromophore, compared to conventional isolation groups. The results obtained from electro-optic (EO) coefficients indicate that this TCF acceptor with a unique dendritic structure can prevent antiparallel packing between chromophores, improving the poling efficiency and enhancing the EO performance. These properties, together with the good solubility, suggest the potential use of these new chromophores as materials for advanced photonic devices.

Full text of DOI:10.1039/c8tc00083b

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Materials Chemistry C
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Design, synthesis, and properties of nonlinear
optical chromophores based on a verbenone
bridge with a novel dendritic acceptor
Cite this:DOI: 10.1039/c8tc00083b
ab
bc
de
Hejing Sun,
Jingdong Luo
Zhong’an Li,
*
Jieyun Wu,
Zhenhua Jiang,a
and Alex K.-Y. Jen*bd
bd
Two novel second order nonlinear optical (NLO) chromophores based on N,N-diethylaniline as a donor,
verbenone based tetraene as a bridge, and tricyanofuran (TCF) or tricyanofuran derivatives with a
dendritic moiety as an acceptor have been synthesized in good overall yields and systematically
characterized. Besides, a facile applicable synthetic approach for a NLO dendritic acceptor was
developed. Compared with C7, after introducing dendritic derivative steric hindrance groups into the
acceptor, chromophore C8 had good thermal stabilities with high thermal decomposition temperatures
which were 33 1C higher than that of chromophore C7. At the same time, cyclic voltammetry (CV)
experiments were performed to determine the different redox properties. The conjugated verbenone
tetraene segments in two chromophores could significantly improve the glass-forming ability and
molecular polarization of chromophores as revealed by UV-vis-NIR absorption measurements. The bulky
dendritic moiety linked by a short C–C bond is closer to the TCF acceptor, which is the most polar part
in the chromophore, compared to conventional isolation groups. The results obtained from electro-optic
(EO) coefficients indicate that this TCF acceptor with a unique dendritic structure can prevent antiparallel
packing between chromophores, improving the poling efficiency and enhancing the EO performance.
These properties, together with the good solubility, suggest the potential use of these new
chromophores as materials for advanced photonic devices.
Received 6th January 2018,
Accepted 16th February 2018
DOI: 10.1039/c8tc00083b
rsc.li/materials-c
organic chromophores into high macroscopic NLO activities,
since the strong intermolecular dipole–dipole interactions
Introduction
Organic and polymeric second-order nonlinear optical (NLO) among the chromophore moieties in the polymeric system would
materials are promising enabling material systems for broad- impede the poling induced noncentrosymmetric alignment.9–12
band optical modulation and terahertz photonics, due to their
Based on the site-isolation principle, dendritic NLO materials,
large electro-optic (EO) activity, relatively low dielectric constants, have been intensively explored in the NLO field, and considered
ultrafast response time, and exceptional processibility for large- to be a very promising molecular topology for the next generation
scale integrating photonics.1–8 In this area, one of the most of highly efficient NLO materials, owing to their tree-like mono-
critical challenges is to efficiently translate the large mb values disperse structures and attractive features.13–25 In 1997, through
(where m is the dipole moment, and b is the first polarizability) of theoretical analysis and extensive experiments Dalton and
co-workers proposed that intermolecular electrostatic interaction
a The Key Laboratory for High Performance Polymer of the Ministry Education of
China, College of Chemistry, Jilin University, Changchun 130012,
People’s Republic of China
b Department of Materials Science and Engineering, University of Washington,
Seattle, Washington 98195, USA. E-mail: ajen@uw.edu; Fax: +1 206 543 3100;
Tel: +1 206 543 2626
c School of Chemistry and Chemical Engineering, Huazhong University of Science &
Technology, Wuhan, Hubei 430074, People’s Republic of China
d Department of Chemistry, City University of Hong Kong, Tat Chee Avenue,
Kowloon, Hong Kong SAR, People’s Republic of China.
could be minimized through modification of the shape of
chromophores.3,7,14 From 2011 until now, various cross-linkable
NLO dendrimers have been designed, which exhibit excellent
thermal stability and large optical nonlinearity, due to spatial
isolation from the dendrimer shell.9,26,27 In 2002, taking advantage
of the properties of fluorinated materials, such as high thermal
stability, chemical inertness, and low dielectric constants, Jen’s
group developed a novel 3-D shaped chromophore shielded
by highly-fluorinated dendrons, which exhibits a significantly
improved three times higher EO coefficient compared to its
pristine analog.28 They also developed a generally applicable
E-mail: jingdluo@cityu.edu.hk, alexjen@cityu.edu.hk
e University of Electronic Science and Technology of China, School of Optoelectronic
Science and Engineering, Chengdu 611731, People’s Republic of China
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method for the post-functionalization of side-chain dendronized acceptor unit for comparison. Both the chromophores contain
NLO polymers with high poling efficiency.29 In 2006, the N,N-diethylaniline as a donor and the same verbenone based
influence of binding mode and attachment flexibility of dendritic tetraene as a bridge, which has high polarization and glass-
NLO chromophores on their EO performance was investigated.30 forming ability. The three-dimensional verbenone-based ring
Then, inspired by arene–perfluoroarene (ArH–ArF) interaction, structures around the planar tetraene chain can increase steric
Jen’s group developed a novel class of dendritic NLO chromo- hindrance compared to the widely used isophorone-based
phores substituted by phenyl/pentafluorophenyl dendrons which tetraene (CLD) bridge, and can prevent close packing of molecules
can reversibly self-assemble to build an extended supramolecular and hence crystallization.40 The solvatochromism behavior,
structure, in order to improve poling efficiency and good thermal thermal stability, electrochemical and nonlinear optical properties
stability.22,31 Meanwhile, they also studied the relationship of the two chromophores were thoroughly investigated to under-
between the molecular mobility energetics of dendritic chromo- stand their structure–property relationships.
phores in the condensed phases and their thermal transition
(relaxation) behaviour, and found that subtle variations in the
dendritic molecular structure influence the critical parameters
Experimental
Materials and measurements
significantly.32 Recently, many novel NLO materials have employed
dendritic structures to modify NLO chromophores and polymers,
All chemicals were purchased from Aldrich and used as received.
and the role of the site-isolation effect in determining the material
properties was systematically investigated.31,33–36 However, the
Solvents such as tetrahydrofuran (THF), ethanol, and dibromo-
methane (DBM) were treated and distilled prior to use according to
research of the dendritic chromophore mostly focuses on the
common purification procedures. 1H NMR spectra were recorded
modification of donor and bridge groups with flexible binding
on a Bruker AV300 spectrometer using CDCl3 as a solvent in all
by post-modification. The dendritic acceptor, the analogue of TCF
cases. High resolution mass spectrometry (HRMS) was performed
moieties, has seldom been noticed, especially for chromophores
on a Bruker APEX III 47e Fourier transform mass spectrometer,
linked with dendritic groups by short rigid binding, possibly
and ESI-MS spectra were recorded on a Bruker Daltonics Esquire
due to the challenging and complicated synthesis of dendritic
ion trap mass spectrometer. Cyclic voltammetric data were
acceptors. Most importantly, compared to the electron donor
measured on a BAS CV-50W voltammetric analyzer using a
and bridge, the electron acceptor as the most polar moiety in
conventional three-electrode cell, a glassy carbon working electrode,
chromophores can have a pronounced effect on the molecular
a Pt wire counter-electrode, and a Ag/AgCl reference electrode at a
hyperpolarizability. The tricyano-dihydrofuran (TCF) acceptor,
scan rate of 100 mV sÀ1. 0.1 M tetrabutyl ammonium hexafluoro-
which has a unique dihydrofuran ring structure, has been
phosphate (TBAPF) in CH2Cl2 was used as the electrolyte. UV-vis-
increasingly used in the new generation chromophores. The
NIR spectra were recorded on a UV Varian Cary 5000 UV-vis-NIR
dihydrofuran ring with three cyano groups is highly planar and
Spectrophotometer. Microwave irradiation was carried out on a
strongly dipolar, because of the cumulative effect of rigidly
CEM Discovery focused microwave system. Thermogravimetric
aligned cyano groups. When such an acceptor is used to construct
analysis (TGA) was performed on a Pyris 1 thermogravimetric
analyzer.
chromophores, the chromophores tend to pack in antiparallel
alignment because of the strong dipole–dipole interactions. Thus,
a dendritic acceptor with short rigid binding is more suitable for
preventing the chromophores from antiparallel packing and
Poling and r33 measurements
improving the poling process.37–39 This rigid short linkage brings For studying the EO properties derived from the chromophores,
the dendritic group closer to the acceptor than the traditional TCF guest–host polymers were generated by formulating chromo-
acceptor, and may encapsulate the chromophore to help minimize phores C7 and C8 in poly(methyl methacrylate) (PMMA) using
dipole–dipole interactions and prevent aggregation. In previous DBM as the solvent. The resulting solutions were filtered through
research, we found that it is possible to improve material stability a 0.2 mm PTFE filter and spin-coated onto thin-film device (TFD)
without increasing the cooperativity (that also means no sacrifice indium tin oxide (ITO) glass substrates. Films of doped polymers
of efficiency) through controlling the structural rigidity of the were baked in a vacuum oven at 60 1C overnight to ensure the
materials.32 In this regard, we have developed a novel synthetic removal of the residual solvent. Then, a thin layer of gold was
route to control the structural rigidity of the materials. The sputtered onto the films as a top electrode for contact poling. The
acceptor with a dendritic moiety not only provides different r33 values were measured using the Teng–Man simple reflection
dendritic structures to modify NLO chromophores, but is also technique at a wavelength of 1310 nm.
expected to facilitate the preparation procedure of dendritic
chromophores.
Compound 1. To a solution of 4-(diethylamino)benzaldehyde
(2.00 g, 11.28 mmol) and (1S)-(À)-verbenone (2.04 g, 13.58 mmol)
To deepen the exploration of the design parameters of novel in anhydrous ethanol was added a freshly prepared solution of
dendritic chromophores, in this work we focus our attention on sodium ethoxide (14.35 mmol) using a syringe. The reaction
the synthesis and characterization of a new chromophore C8 mixture was stirred at 70 1C for 24 h and then was poured into
with a dendritic moiety on the acceptor via a rigid C–C bond, a saturated solution of ammonium chloride and extracted with
through the focused microwave assisted synthesis. Chromo- ethyl acetate; the combined organic extracts were washed with
phore C7 was synthesized with tricyanovinyldihydrofuran as an brine and dried over Na2SO4. After the removal of the solvent,
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the residue was purified by column chromatography using give a white solid product. (3.49 g, yield: 80%) 1H NMR (300 MHz,
hexane/ethyl acetate as an eluent to give the product as red CDCl3) d 7.59–7.28 (m, 10H, Ar–H), 7.20 (s, 2H, Ar–H), 6.81 (d,
solids (2.66 g, yield: 76%). 1H NMR (CDCl3, 300 MHz, ppm): J = 1.4 Hz, 1H, Ar–H), 5.07 (s, 4H, CH2), 2.56 (s, 3H, CH3). HRMS
d 7.40 (d, J = 7.8 Hz, 2H, Ar–H), 6.83 (dd, J = 36.6, 16.0 Hz, 2H, (ESI) (M+, C22H20O3): calcd: 332.1412; found: 333.1499.
CH), 6.66 (d, J = 7.8 Hz, 2H, Ar–H), 5.85 (s, 1H, CH), 3.42 (d,
Compound 5. To a solution of ethyl vinyl ether (15 mmol,
J = 6.5 Hz, 4H, NCH2), 3.15 (s, 1H, CH), 2.91 (d, J = 4.4 Hz, 1H, 1.08 g) in THF was added 5.9 ml of t-BuLi (1.7 M, 10 mmol) in
CH2), 2.72 (s, 1H, CH), 2.13 (d, J = 9.2 Hz, 1H, CH2), 1.59 (s, 3H, pentane dropwise at À78 1C, under nitrogen for 2 h. The
CH3), 1.20 (t, J = 13.8 Hz, 6H, CH3), 1.04 (s, 3H, CH3). MS: calcd solution was warmed on an ice bath and stirred for 1 h. And
for C21H27NO: 309.2; found: 310.1.
then cooled to À78 1C. A solution of compound 4 (5 mmol,
Compound 2. Under a N2 atmosphere to the mixture of NaH 1.66 g) in THF was added to the lithiated ether solution
(0.133 g, 5.54 mmol) in dry THF was added diethyl cyano- dropwise at À78 1C. The resulting mixture was stirred for 1 h
methylphosphonate (0.98 g, 5.52 mmol) dropwise using a at À78 1C, allowed to warm up to room temperature slowly for
syringe at 0 1C in an ice bath. After the solution became clear, 2 h. The reaction was then quenched with 30 ml of NH4Cl
compound 1 (0.85 g, 2.75 mmol) in dry THF was added. The aqueous, and the mixture was extracted with CH2Cl2. The
mixture was refluxed for 3 h and then poured into a saturated combined organic layer was washed with water and dried over
solution of ammonium chloride and extracted with ethyl acetate; magnesium sulfate. After the solvent was evaporated, the
the combined organic extracts were washed with brine and dried residue was dissolved in 20 ml methanol and HCl(aq) (1 M, 6 mmol)
over Na2SO4. After the removal of the solvent, the residue was was added dropwise at room temperature. The exothermal reaction
purified by column chromatography using hexane/ethyl acetate was cooled using a water bath and monitored by thin layer
as the eluent to give the product as an orange-red solid (0.78 g, chromatography (TLC). After stirring for 2 h, the resulting mixture
yield: 85%). The ratio of the Z :E isomers is 60 : 40% calculated by was neutralized with NaHCO3, concentrated via rotary evaporation,
the integration of respective protons. 1H NMR (CDCl3, 300 MHz, and extracted with CH2Cl2. After the solvent was evaporated, the
ppm): d 7.35 (s, 2H, Ar–H), d 6.74 (s, 1H, CH), 6.66 (d, J = 8.0 Hz, product was purified by chromatography to give a white solid
1
2H, Ar–H), 6.50 (s, 0.6H, CH), 6.07 (s, 0.4H, CH), 4.94 (s, 0.4H, (1.22 g, yield: 65%). H NMR (CDCl3, 300 MHz, ppm): d 7.52–7.30
CH), 4.82 (s, 0.6H, CH), 3.41 (d, J = 5.9 Hz, 4H, N CH2), 3.01 (s, (m, 10H, Ar–H), 6.67 (s, 2H, Ar–H), 6.57 (s, 1H, Ar–H), 5.18–4.92
1H,CH), 2.72 (s, 2H, CH2), 1.54 (d, J = 15.4 Hz, 3H, CH3), 1.25 (m, 4H, CH2), 4.56 (s, 1H, OH), 2.00 (d, J = 34.0 Hz, 3H, CH3),
(d, J = 22.6 Hz, 6H, CH3), 0.87 (s, 3H, CH3). MS: calcd for C23H28N2 1.73 (s, 3H, CH3) HRMS (ESI) (M+, C24H24O4): calcd: 376.1675;
332.48; found: 333.1.
found: 377.1714.
Compound 3. The solution of compound 2 (0.5 g, 1.50 mmol)
Compound 6. Compound 5 (1 mmol, 376 mg), malonitrile
in dry toluene was cooled to À78 1C and the solution of (2 mmol, 132 mg) and NaOEt (0.15 mmol) in ethanol were
diisobutyl aluminum hydride (DIBAL) in hexane (1.0 M, 2.6 mL, heated under 30 W microwave reflux for 30 min. The solvent
2.6 mmol) was added dropwise using a syringe. After keeping at was removed by rotary evaporation and the residue was purified
À78 1C for 2 h, wet silica gel was added to quench the reaction and via flash chromatography on silica gel with a gradient eluent
the mixture was stirred at 0 1C for 2 h. The product mixture was of CH2Cl2 to 2% EtOH in CH2Cl2 to afford a solid product.
1
filtered off, and the resulting precipitates were washed with ethyl (293 mg, yield: 62%). H NMR (CDCl3, 300 MHz, ppm): d 7.40
acetate. Evaporation of the solvent and purification by column (d, J = 3.3 Hz, 10H, Ar–H), 6.65 (d, J = 13.3 Hz, 1H, Ar–H), 6.31
chromatography using hexane/ethyl acetate as the eluent gave the (d, J = 2.1 Hz, 2H, Ar–H), 5.04 (d, J = 7.9 Hz, 4H, CH2), 2.22–1.98
product as dark red oil (0.39 g, yield: 78%). The ratio of the Z : E (m, 3H, CH3), 1.87 (d, J = 17.3 Hz, 3H, CH3). HRMS (ESI)
isomers is 40 : 60% calculated by the integration of respective (M+, C30H23N3O3): calcd: 473.1739; found: 474.1835.
protons. 1H NMR (300 MHz, CDCl3) d 10.14 (d, J = 8.2 Hz, 0.4H,
Compound 7. Compound 3 (101 mg, 0.3 mmol) and the TCF
CHO), 9.89 (d, J = 8.5 Hz, 0.6H, CHO), 7.44–7.27 (m, 2.4H, Ar–H), acceptor (59 mg, 0.3 mmol) were dissolved in the solvent
6.93 (s, 0.6H, CH), 6.71 (t, J = 9.6 Hz, 2.4H, Ar–H), 6.63 (d, J = mixture of anhydrous ethanol and CH2Cl2. The reaction mixture
8.7 Hz, 2H, CH), 6.09 (s, 1H, CH), 5.84 (d, J = 8.5 Hz, 0.6H, CH), was allowed to stir at 65 1C for 4 h and monitored by TLC. After the
5.62 (d, J = 8.1 Hz, 0.4H, CH), 3.39 (dd, J = 14.0, 7.0 Hz, 4H, removal of the solvents, the residue was purified by column
N CH2), 3.03 (d, J = 4.9 Hz, 1H, CH), 2.87–2.57 (m, 1H, CH), chromatography eluting with hexane/ethyl acetate. Further purifi-
1.89–1.66 (m, 2H, CH2), 1.66–1.45 (m, 5H, CH3), 1.20 (dd, J = cation of the product by reprecipitation from methanol/dichloro-
17.9, 11.0 Hz, 6H, CH3), 0.89 (d, J = 1.7 Hz, 4H, CH3). MS: calcd methane afforded the desired chromophore as a dark solid
for C23H29NO 335.5; found: 336.2.
(124 mg, yield: 80%). The ratio of the Z : E isomers is 30 : 70%
Compound 4. A mixture of benzyl bromide (4.47 g, 26.3 mmol), calculated by the integration of respective protons. 1H NMR
3,5-dihydroxyacetophenone (2.0 g, 13.15 mmol), potassium (CDCl3, 300 MHz, ppm): d 8.27 (t, J = 13.4 Hz, 0.3H, CH), 8.02
carbonate (3.63 g, 26.3 mmol), and KI (0.33 g, 1.97 mmol) (t, J = 13.7 Hz, 0.7H, CH), 7.40 (d, J = 7.9 Hz, 2H, Ar–H), 7.29
was dissolved in dry DMF. The reaction mixture was heated at (s, 3H), 6.92–6.73 (m, 2H, CH), 6.72–6.56 (m, 2H, Ar–H), 6.22
85 1C for 20 h. After cooling to room temperature, the reaction (dd, J = 27.8, 16.5 Hz, 2H, CH), 6.04 (d, J = 12.8 Hz, 0.3H, CH),
mixture was poured into water. The mixture was extracted with 3.43 (d, J = 6.7 Hz, 4H, CH2), 3.30 (s, 0.7H, CH), 3.13 (s, 1.3H,
CH2Cl2 and dried. After the removal of the solvents under reduced CH), 2.82 (d, J = 20.7 Hz, 1.7H, CH2), 1.79 (d, J = 8.9 Hz, 1.3H,
pressure, the residue was purified by column chromatography to CH3), 1.69 (d, J = 12.8 Hz, 6H, CH3), 1.59 (d, J = 11.3 Hz, 6H,
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CH3), 1.22 (t, J = 6.7 Hz, 6H, CH3), 0.88 (s, 3H, CH3). HRMS(ESI) same electron donor N,N-diethylaniline group and the (1S)-(À)-
(M+, C34H36N4O): calcd: 516.2889; found: 517.2975.
verbenone-based tetraene bridge, but different side groups on the
´
Compound 8. Compound 3 (101 mg, 0.3 mmol) and com- acceptor. The novel functional acceptor with Frechet-type benzyl
pound 6 (142 mg, 0.3 mmol) were dissolved in the solvent ether dendrons of generation1 was synthesized in three steps.
mixture of anhydrous ethanol and CH2Cl2. The reaction mix- The precursor dendritic ketones were prepared according to
ture was allowed to stir at 65 1C for 12 h and monitored by TLC. the literature.41,42 Then, ethyl vinyl ether was reacted with
After the removal of the solvents, the residue was purified by t-butyllithium at À78 1C and the vinyl anion formed. The
column chromatography eluting with hexane/ethyl acetate. dendritic ketones were added to the vinyllithium solution, with
Further purification of the product by reprecipitation from the anion reacting with the pro-chiral ketone. After acid hydrolysis,
methanol/dichloromethane afforded the desired chromophore racemic a-ketol was obtained as shown in Scheme 1. Through the
as a dark solid (137 mg, yield: 58%). The ratio of the Z : E focused microwave assisted synthesis,43 the a-ketol with a
isomers is 20 : 80% calculated by the integration of respective dendritic substituent was condensed with malononitrile under
protons. 1H NMR (CDCl3, 300 MHz, ppm): d 7.91 (m, 0.2H, CH), basic conditions to form a 2,5-dihydrofuranderived electron
7.73 (m, 0.3H, CH), 7.48–7.27 (m, 12H, Ar–H), 6.85–6.70 (m, 1H, acceptor (C6). Starting from the amine donor, N-ethyl-N-(2-hydroxy-
Ar–H), 6.63 (d, J = 8.7 Hz, 2H, Ar–H), 6.55 (d, J = 8.7 Hz, 0.5H, ethyl) aminobenzaldehyde, a novel verbenone based bridge was
Ar–H), 6.49 (s, 1H, CH), 6.45 (s, 1H, CH), 6.32 (d, J = 8.1 Hz, 1H, synthesized in good overall yields through simple 3-step reactions.
Ar–H), 6.13 (t, J = 13.9 Hz, 2H, CH), 5.87 (d, J = 12.2 Hz, 0.5H, In the presence of sodium ethoxide as the base, (1S)-(À)-verbenone
CH), 5.05 (d, J = 9.9 Hz, 4H, CH2), 3.40 (d, J = 6.7 Hz, 4H, CH2), reacted directly with the amine donor via the Knoevenagel
3.05 (s, 1H, CH), 2.89 (s, 0.3H, CH2), 2.70 (s, 1H, CH), 2.10 condensation to obtain the corresponding aminophenyldienone
(s, 0.3H, CH2), 2.04–1.82 (m, 3H, CH3), 1.68 (d, J = 9.1 Hz, 1H, derivatives. The extension of the conjugated p-bridge to yield the
CH2), 1.53 (d, J = 10.6 Hz, 4H, CH3), 1.45 (s, 2H, CH3), 1.21 key aminophenyltrienal intermediates was accomplished in
(dd, J = 16.6, 9.8 Hz, 6H, CH3), 0.81 (d, J = 6.5 Hz, 2H, CH3), 0.67 good overall yields by condensing the dienones with diethyl-
(s, 1H, CH3). HRMS (ESI) (M+, C53H50N4O3): calcd: 790.3883; (cyanomethyl)phosphonate using the Wittig–Horner reaction,
found: 791.3973.
followed by reducing the resultant trienenitriles with DIBAL and
subsequent hydrolysis. Finally, the novel acceptor with a dendritic
substituent and a TCF type acceptor were condensed with a
verbenone-based tetraene bridge to afford the desired phenyl-
tetraene-based chromophores C8 and C7, respectively. All these
chromophores possess good solubility in common organic
solvents, such as dichloromethane, THF and acetone. All the
chromophores were completely characterized by H NMR, MS,
and UV-vis spectroscopic analysis and the data obtained were in
full agreement with the proposed formulations.
Results and discussion
Synthesis and characterization of chromophores
Scheme 1 shows the chemical structures and the synthetic
approach for this series of ring-locked tetraene chromophores
C7 and C8. These chromophores were designed to have the
1
Thermal stability
The NLO chromophores must be thermally stable in order to
withstand the poling process and subsequent processing for
use. To investigate the thermal stability of the chromophore,
thermogravimetric analysis (TGA) was performed with a heat-
ing rate of 10 1C minÀ1 under a nitrogen atmosphere. The
TGA curve of the chromophores is shown in Fig. 1. The two
chromophores with a similar verbenone structure on the p-electron
bridge exhibited good thermal stabilities with the decomposition
temperatures (Td) higher than 220 1C. Chromophore C8 had the
highest decomposition temperature (Td = 273 1C), followed
by C7 (Td = 240 1C). While chromophore C8 had a much
higher decomposition temperature than chromophore C7 with
no steric hindrance on the acceptor. The enhanced thermal
stability of chromophore C8 over chromophore C7 is due to the
introduction of dendric group derivative steric hindrance
groups into the acceptor. These data indicate that the novel
TCF acceptor with dendric steric hindrance groups can help
to increase the thermal stability of the chromophore. The
excellent thermal stability of these chromophores makes
them suitable for practical device fabrication and EO device
preparation.
Scheme 1 Chemical structures and synthetic scheme for chromophores
C7 and C8.
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tetrabutylammonium hexafluorophosphate (TBAPF) as the sup-
porting electrolyte. The relative data and voltammograms of
1 Â 10À4 mol LÀ1 chromophores C7 and C8 were recorded.
As shown in Fig. 2, both chromophores C7 and C8 exhibited
one quasi reversible oxidative wave with a half-wave potential,
E1/2 = 0.5(Eox + Ered), versus ferrocene/ferrocenium at about 0.142
and 0.14 V, respectively. Meanwhile, these chromophores had
an irreversible reduction wave corresponding to the acceptor
moieties at Ered = À1.213 V and À1.114 V. This may suggest that
the strength of the electron acceptor could be increased by
replacing the methyl group with the dendritic moiety substituent.
This may result from the fact that the dendritic moiety has a
stronger electron-withdrawing ability than the methyl group. The
energy gaps between the HOMO and LUMO energy for chromo-
phores C7 and C8 were 1.382 eV and 1.364 eV, respectively, while
the chromophore FTC exhibited the energy gap (DE) values of
1.529 eV.30 This comparison demonstrated that the new verbenone
based bridge narrowed the energy gap indicating the excellent
charge-transfer of verbenone based bridge chromophores.
Fig. 1 TGA curves of chromophores C7 and C8 with a heating rate of
10 1C minÀ1 in a nitrogen atmosphere.
Electrochemical properties
The HOMO and LUMO levels of the two chromophores were
calculated from their corresponding oxidation and reduction
potentials. The HOMO levels of C7 and C8 were estimated to be
À4.89 eV and À4.85 eV, respectively. And, the corresponding
LUMO levels of C7 and C8 were estimated to be À3.51 eV, À3.49 eV,
respectively.
To determine the redox properties of chromophores, cyclic
voltammetry (CV) measurements were conducted in degassed
anhydrous dichloromethane solutions containing 0.1
M
Optical properties
To explore the different charge-transfer (CT) absorption properties
of each chromophore, UV-vis absorption spectra of the two
chromophores were measured in series solvents with different
dielectric constants. As shown in Fig. 3, chromophores C7 and
C8 exhibited the maximum absorption (lmax) at 723 nm and
731 nm in chloroform, respectively. Compared to chromophore
C7, chromophore C8 showed a blue shift (D = 8 nm), which is
attributed to the different acceptors: the dendritic group would
enhance the electron-withdrawing ability of the acceptor. It may
somehow enhance the strength of the acceptor. The spectrum
data are summarized in Table 1.
Besides, the solvatochromic behavior was also an important
aspect to investigate the polarity of chromophores. When
increasing the solvent dielectric constant, the two chromophores
showed very large bathochromic shifts of 74 nm and 72 nm from
Fig. 2 Cyclic voltammograms of chromophores C7 and C8 recorded in
CH2Cl2 solutions containing a 0.1 M TBAPF supporting electrolyte at a scan
rate of 100 mV sÀ1
.
Fig. 3 UV-Vis absorption spectra of chromophores C7 and C8 in seven kinds of aprotic solvents with varying dielectric constants.
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Table 1 Spectral data and the absorption maxima of chromophores C7 and C8 in seven kinds of solvents with varying dielectric constants
Absorption maximum (nm)
Compd
Dioxane
Toluene
Dichloromethane
Tetrahydrofuran
Chloroform
Acetone
Acetonitrile
Dla
C7
C8
649
659
671
685
714
729
673
689
723
731
678
690
678
699
74
72
a
Dl = lmax(chloroform) À lmax(dioxane)
.
Table 2 Physical properties and EO coefficients of chromophores C7 and
C8
dioxane to chloroform, respectively. C7 and C8 display larger
solvatochromism than those of CLD (60 nm)44 and a FTC
(61 nm)45 chromophore. It indicated that chromophores C7
and C8 are more polarizable than the traditional CLD and FTC
chromophores. With a further increase of solvent polarity,
saturation behavior was found for both chromophores in more
polar solvents, such as acetone and acetonitrile. For example, C7
and C8 showed hypsochromic shifts of 45 nm and 41 nm,
respectively, from chloroform to acetone. Such a phenomenon
was reported by Davies46 and was ascribed to the back-electron
transfer from the acceptor side to the donor side in polar solvents
causing a blue-shift in the absorption spectrum.47
Compounds
wt%
r33 (pm VÀ1 a
)
Nb
r33/Nc
C7
10
25
35
23
39
47
1.164
2.911
4.075
19.75
13.40
11.53
C8
10
25
35
13
34
68
0.762
1.905
2.677
17.06
17.84
25.40
a
b
Experimental value from simple reflection at 1310 nm. Chromophore
number density; in units of Â1020 molecules per cc. r33 normalized by
c
chromophore number density; in units of Â10À20 pm cc per (V molecules).
Electro-optic performance
increase of the chromophore density, the intermolecular dipole–
dipole interactions would become stronger and stronger, and
lead to the unfavorable antiparallel packing of chromophores. It
could not translate the microscopic hyperpolarizability into macro-
scopic EO response effectively. The graph becomes nonlinear.
Films of C8 were prepared at 10, 25, and 35 total chromo-
phore wt%, corresponding to 0.762, 1.905, and 2.677 Â
1020 molecules per cc, respectively. When the concentration
of the chromophore in PMMA is low, the C7/PMMA film
displayed a larger r33 value than the C8/PMMA film. As the
chromophore loading increases, this trend is reversed. Meanwhile,
as compared to C7, N-normalized r33 (r33/N) of C8 maintained high
values and did not decrease with increasing chromophore content.
Fig. 5 shows schematic illustration of chromophore C8 poling
processes. At the close chromophore number (N) density (25 wt%
for C7 and 35 wt% for C8), the contribution of every chromophore
to the r33 value (r33/N) of C8 (25.40) is nearly two times that of C7
(13.40). And as the chromophore loading density increased, the
r33/N of C8 increased, rather than decreased as for C7. Due to
optimized configurations of C8 the rigid C–C bond linked
dendritic moiety on the acceptor encapsulates the chromo-
phores effectively to prevent the formation of an antiparallel
dimer, and provides more space for efficient orientation of the
chromophores; it could help to keep chromophore–chromo-
phore electrostatic interactions to a low level, and improve the
poling process. It means that this unique dendritic structure
might be desirable in reducing dipole–dipole interactions and
maintaining a parallel pattern, resulting in translation of the
microscopic hyperpolarizability into a macroscopic EO response
more effectively. Besides, solvatochromic behavior indicated
that chromophores C7 and C8 are more easily polarizable than
chromophores CLD and FTC. Therefore, in the poling process,
chromophores C7 and C8 are orientated more easily in contrast
to chromophores CLD and FTC.
To test the effect of the novel dendritic acceptor on the EO
activities, the r33 values of C7/PMMA and C8/PMMA films were
measured in different loading densities, as shown in Fig. 4. C7
and C8 displayed excellent compatibility with the polymer
matrix when guest–host polymers were prepared. For chromo-
phore C7, the r33 values gradually improved from 23 pm VÀ1
(10 wt%) to 47 pm VÀ1 (35 wt%). However, as shown in Table 2,
N-normalized r33 (r33/N), which effectively normalizes the r33
value for the relative chromophore content, decreases with
increasing number density. This can be explained by the fact
that, when in a low-density range, the intermolecular dipolar
interactions are relatively weak, the performance of the materials
was more related to the intrinsic molecular properties. With the
Fig. 4 Comparison of the EO coefficients of NLO chromophores C7 and
C8 containing different loading densities and substituent modifications.
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based tetraene p-conjugation as the bridges have been synthe-
sized and systematically characterized by NMR, MS and UV-vis
absorption spectroscopy. Thermogravimetric analysis showed
that chromophore C8 had excellent thermal stability with a
decomposition temperature of 273 1C, and this decomposition
temperature was 33 1C higher after introducing the dendritic
substituent steric hindrance group into the acceptor when
compared with chromophore C7. The effects of bathochromic
and solvatochromic behaviors on UV-vis absorption were also
investigated to compare different electron withdrawing abilities
and polarizabilities. Both the chromophores showed good
solubility and comparability with the polymer, which were
crucial for making high quality EO devices. Incorporation of
chromophores C7 and C8 into PMMA provided large electro-
optic coefficients of 47 and 68 pm VÀ1 at 1310 nm with a high
loading of 35 wt%. Moreover, dendritic substituents can reduce
dipole–dipole interactions for highly efficient poling at a high
chromophore loading density. The number of aligned chromo-
phores (translated microscopic hyperpolarizability into macro-
scopic electro-optic coefficients) of C8 was nearly two times that
of C7. It also suggests that intelligently designed dendritic EO
chromophores are promising candidates for further development.
Fig. 5 The normalized poling current of chromophore alignment as a
function of temperature in PMMA.
Conflicts of interest
Fig. 6 Proposed scheme of poling processes to generate the macro-
scopic polar order of chromophore C8 in a polymer matrix.
There are no conflicts to declare.
In terms of temporal stability of electro-optic coefficients,
widely adaptable strategies include the self-assembled dendrimers,
simple guest–host doping with high glass transition temperature.
For C7 and C8 in host PMMA with the relatively close chromo-
phore number density (25 wt% for C7 and 35 wt% for C8), despite
of the different molecular structures, their films showed very
similar highest poling temperatures, which are dictated by their
glass transition temperatures during poling, respectively (shown in
Fig. 5). In such typical guest-host systems with PMMA as the host,
their temporal stability of electro-optic coefficients at room tem-
perature is around 90% after initial fast decay and then tend to be
stabilized. Such temporal stability is routine and it indicated that
their stability is related to the glass transition temperature. We
expect that good temporal stability at elevated temperatures can be
achievable by using polymers with higher glass transition
temperature, which will be reported in due course.
Acknowledgements
This work was supported by Air Force Office of Scientific
Research (FA8650-14-C-5005), New Faculty Start-up Grant of City
University of Hong Kong (7200550), and the State-Sponsored
Scholarship for Graduate Students from China Scholarship
Council (201506170098).
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