Six-branched chromophores with isolation groups: Synthesis and enhanced optical nonlinearity

Article abstract of DOI:10.1039/c2jm16406j

Three new six-branched chromophores with different isolation groups were designed and synthesized by incorporating azo chromophores onto phenyl glycerol ester via an esterification reaction. The chemical structures were confirmed by ¹H NMR, ¹³C NMR, FT-IR spectroscopy, mass spectrometry, element analysis and UV-visible absorption spectra. The six-branched dendritic chromophores can form thin films directly without adding any polymer matrix and show a quite high chromophore loading density. After electric poling, the nonlinear optical coefficients (d₃₃) of the dendrimer films were determined to be around 113, 142 and 136 pm V^-1, which are almost 3 times the maximum d₃₃ value of the azo-chromophore doped polymer films. The further improvement in optical nonlinearity was successfully realized by mixing the six-branched chromophores containing different isolation groups.

Full text of DOI:10.1039/c2jm16406j

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PAPER
Six-branched chromophores with isolation groups: synthesis and enhanced
optical nonlinearity†
*
Wenxin Lin, Yuanjing Cui, Junkuo Gao, Jiancan Yu, Ting Liang and Guodong Qian
*
Received 6th December 2011, Accepted 13th March 2012
DOI: 10.1039/c2jm16406j
Three new six-branched chromophores with different isolation groups were designed and synthesized
by incorporating azo chromophores onto phenyl glycerol ester via an esterification reaction. The
chemical structures were confirmed by 1H NMR, 13C NMR, FT-IR spectroscopy, mass spectrometry,
element analysis and UV-visible absorption spectra. The six-branched dendritic chromophores can
form thin films directly without adding any polymer matrix and show a quite high chromophore
loading density. After electric poling, the nonlinear optical coefficients (d33) of the dendrimer films were
determined to be around 113, 142 and 136 pm Vꢀ1, which are almost 3 times the maximum d33 value of
the azo-chromophore doped polymer films. The further improvement in optical nonlinearity was
successfully realized by mixing the six-branched chromophores containing different isolation groups.
macroscopic optical nonlinearity with the increase of chromo-
phore concentration.20
Introduction
Investigations of second-order nonlinear optical (NLO) mate-
rials are of current interest because of their wide applications in
all-optical switching, fast optical communications, and electro-
optic modulation.1–4 Organic NLO materials offer a number of
advantages over the traditional inorganic crystalline NLO
materials, such as large nonlinear optical coefficients, low
dielectric constants and excellent processing.5–18 For most
developmental work on NLO polymers, the guest–host system is
routinely employed because of wide selection and easy control of
the material compositions through doping chromophores in the
host matrix. However, the poled guest–host NLO materials
usually suffer from phase separation problem. Although incor-
porating the chromophores via covalent bonds onto polymer
chains can effectively alleviate this problem, it is difficult to
precisely control the compositions and properties (e.g., molec-
ular weight) of the NLO polymers from batch to batch. In
addition, how to efficiently translate the large b values of the
organic chromophores into high macroscopic NLO activities of
polymers is still a major challenge. The strong dipole–dipole
interactions among the chromophores in the guest–host polymer
systems make the chromophores tend to align in an antiparallel
fashion, diminishing the macroscopic NLO effect.19 Therefore,
the guest–host NLO polymers often display a saturated trend in
In the past few years, Dalton and Jen have pioneered the
dendritic structure to decrease the dipole–dipole interactions and
increase the poling efficiency by applying the site isolation
principle,21–23 in which the dipolar chromophores were incorpo-
rated into dendritic structures or functionalized with dendrons in
order to create a pseudospherical structure isolating the dipolar
unit from its surroundings. Compared to the small chromophore
molecules, the dendrimers display better overall mechanical
stability and film-forming properties, and offer unique charac-
teristics such as globular shape, large internal free-volume, and
an outer periphery capable of providing physical isolation of the
contents within. It has been demonstrated that the dendritic
architecture is a very promising molecular topology for the next
generation of highly efficient NLO materials.24–27
Up to now, three-branched dendritic chromophores were
mostly reported due to their convenient synthetic method.
According to the molecular theoretical studies, multi-
chromophores with more branches may exhibit larger chromo-
phore loading density and higher NLO activity.28 Our recent
work on the six-branched NLO multichromophore, prepared via
incorporating FTC-type chromophores onto phenyl glycerol
ester core, has shown excellent film-forming properties and
dramatically enhanced NLO activity.29 Considering that the
isolation groups could further benefit the optical nonlinearity of
the six-branched multichromophore, thus we introduce the
different isolation groups on to the arm of the six-branched
multichromophore and try to study the relationship between the
isolation group and properties. In this work, three six-branched
chromophores with different isolation groups (Fig. 1) were
synthesized and the films were fabricated directly using the six-
branched multichromophores without any addition of polymer
State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor
Materials and Applications, Department of Materials Science and
Engineering, Zhejiang University, Hangzhou, 310027, China. E-mail:
cuiyj@zju.edu.cn; gdqian@zju.edu.cn; Fax: +86-571-87951234; Tel:
+86-571-87952334
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm16406j
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Fig. 1 The chemical structures of six-branched chromophores S1, S2 and S3.
matrix. Then, the NLO properties of these films were discussed in
detail.
was detected by a photomultiplier monitored by a mono-
chromator to eliminate all traces of the fundamental light and
processed by a Stanford Research Systems (SRS) model SR-250
gated integrator.30–33
Experimental
Synthesis of compound 3,30,300-((ethane-1,1,1-triyltris(benzene-
4,1-diyl))tris (oxy))tris(propane-1,2-diol) (1). Compound 4,40,400-
(ethane-1,1,1-triyl)triphenol (3.00 g, 10 mmol) was dissolved in
ethanol (6 mL). A solution of NaOH (1.50 g, 37.5 mmol) in water
(5 mL) was added and the ensuing mixture was refluxed for 30
min with stirring. A solution of 3-chloropropane-1,2-diol (3.96 g,
36 mmol) in ethanol (5 mL) was added and the resulting blend
was stirred and heated at reflux for 4 h before cooled to room
temperature. Then the resulting solution was poured into water
(100 mL) and was extracted with ethyl acetate, the organic layers
were dried over MgSO4. After the solvent was evaporated under
reduced pressure, the residue was purified by silica column
chromatography using a mixture of ethanol/ethyl acetate (v : v ¼
Materials and measurements
All chemical reagents were purchased from Aldrich and Alfa
Aesar, and used without further purification. Dichloromethane
(CH2Cl2) and tetrahydrofuran (THF) were refluxed and distilled
before use, while N,N-dimethylformamide (DMF) was dried by
rotary evaporation before reaction. Other solvents, of analytical-
grade quality, were used as received.
1H and 13C NMR spectra were performed on a Bruker Avance
DMX500 spectrometer using tetramethylsilane (TMS) as an
internal standard. Fourier-transform infrared (FTIR) spectra
were recorded in the range of 400–4000 cmꢀ1 by a Thermo
Scientific Nicolet iS10 in KBr disks. Mass spectrometry was
carried out on a Bruker Daltonics esquire3000plus mass spec-
trometer. Element analysis was done on a Thermo Finnigan
Flash EA 1112 element analyzer. UV-visible absorption spectra
were studied with a Hitachi spectrometer U-4100. Thermogra-
vimetric analysis (TGA) was investigated at a heating rate of
10 ꢁC minꢀ1 under nitrogen atmosphere on a TA Instruments
SDT Q600. The thicknesses of films were determined by a Tencor
alfa-step 200 surface profiler (Lencoi). The second harmonic
generation coefficients (d33) of the poled films were studied
according to the Maker fringe technique.
1
1 : 5) as the eluent to give white solid 1. Yield: 81%. H NMR
(500 MHz, DMSO-d6, d (ppm)): 2.03 (s, 3H, CH3), 3.43 (d, 6H,
OCH2), 3.80 (m, 6H, OH), 3.95 (m, 3H, CH), 4.67(d, 3H, higher
field branch of AB quartet, CH2OH), 4.95 (d, 3H, lower field
branch of AB quartet, CH2OH), 6.82 (d, 6H, ArH), 6.91 (d, 6H,
ArH). 13C NMR (125 MHz, DMSO-d6, d (ppm)): 157.2, 141.8,
129.7, 114.1, 70.5, 69.9, 63.2, 50.6, 30.9. Elemental analysis calcd
(%) for C29H36O9: C, 65.89; H, 6.86; Found: C, 65.52, H, 7.06.
MS (ESI): exact mass calcd for C29H36O9 [M + H]+, 527.20.
Found: 527.90.
Second harmonic generation (SHG) measurement
Synthesis of 4-nitro-2-((perfluorophenyl)methoxy)aniline (2).
2-Amino-5-nitrophenol (1.54 g, 10 mmol) and 1-(bromomethyl)-
2,3,4,5,6-pentafluorobenzene (1.51 mL, 10 mmol) were dissolved
in absolute DMF, and potassium carbonate (1.66 g, 12 mmol)
The second-order optical nonlinearity of hybrid films was
determined by in situ SHG measurement. The laser source is
a Q-switched Nd:YAG pulse laser with a 1064 nm P-polarized
fundamental beam (500 mJ maximum energy, 3–5 ns pulse width,
and 10 Hz repeating rate). The generated second harmonic signal
ꢁ
was added. After it was refluxed for 12 h with stirring at 80 C,
the resulting solution was diluted with CH2Cl2 and washed with
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brine and deionized water. Then the organic phase was dried
over MgSO4 and concentrated by rotary evaporation, the residue
was purified via flash chromatography on silica gel using CH2Cl2
[M + H]+, 443.55. Found: 443.30. [M ꢀ H]ꢀ, 441.55. Found:
441.50.
1
as the eluent to yield yellow powder 2. Yield: 57.2%. H NMR
Synthesis of (E)-4-(2-(ethyl(4-((4-nitrophenyl)diazenyl) phenyl)
amino)ethoxy)-4-oxobutanoic acid (4). A mixture of chromo-
phore E1 (0.628 g, 2 mmol), succinic anhydride (0.24 g, 2.4
mmol), 4-dimethylaminopyridine (DMAP) (0.24 g, 2.4 mmol),
pyridine (0.3 mL) and anhydrous CH2Cl2 was stirred at room
temperature overnight. The resulting solution was diluted with
CH2Cl2 and washed with brine and deionized water. The organic
layer was dried over MgSO4, and the solvent was evaporated
under reduced pressure. The residue was purified by column
chromatography on silica gel using a mixture of CH2Cl2/ethyl
acetate (v : v ¼ 25 : 1) as the eluent and compound 4 was gained
(500 MHz, CDCl3, d (ppm)): 4.49 (s, 2H, OCH2), 5.24 (s, 2H,
NH2), 6.66–6.68 (d, 1H, ArH), 7.86–7.88 (d, 2H, ArH). MS
(ESI): exact mass calcd for C13H7F5N2O3 [M + Na]+, 357.20.
Found: 357.00.
Synthesis
of
2-((2-ethylhexyl)oxy)-4-nitroaniline
(3).
Compound 3 was synthesized using a similar procedure to that
1
for compound 2 as orange oil. Yield: 63.34%. H NMR (500
MHz, CDCl3, d (ppm)): 0.87–0.93 (m, 6H, CH2CH3), 1.29–1.31
(m, 4H, CH(CH2)2), 1.38–1.49 (m, 4H, CH2CH3), 1.74–1.76
(t, 1H, CH(CH2)2), 3.91–3.93 (m, 2H, OCH2), 4.74 (s, 2H, NH2),
6.62–6.64 (d, 1H, ArH), 7.61–7.62 (d, 1H, ArH), 7.75–7.77
(m, 1H, ArH). MS (ESI): exact mass calcd for C14H22N2O3 [M +
H]+, 267.34. Found: 267.00. [M ꢀ H]ꢀ, 265.34. Found: 265.10.
1
as red powder. Yield: 68.11%. H NMR (500 MHz, DMSO-d6,
d
(ppm)): 1.15–1.18 (t, 3H, CH3), 2.36–2.46 (m, 4H,
(CH2)2COOH), 3.54–3.55 (d, 2H, NCH2), 3.70–3.72 (t, 2H,
NCH2CH2), 4.23–4.25 (t, 2H, NCH2CH2), 6.92–6.93 (d, 2H,
ArH), 7.84–7.86 (d, 2H, ArH), 7.93–7.95 (d, 2H, ArH), 8.36–8.37
(d, 2H, ArH).
Synthesis of (E)-2-(ethyl(4-((4-nitrophenyl)diazenyl)phenyl)
amino)ethanol (E1). The chromophore E1 was prepared as
reported previously.34 Compound 4-nitroaniline (2.76 g,
20 mmol) was dissolved in a solution of 35% hydrochloric acid
and the mixture was stirred in an ice bath at 0–5 ꢁC. Subse-
quently, a water solution of sodium nitrite (1.38 g, 20 mmol) was
added slowly and the resulting solution was stirred in the ice bath
for 30 min, a solution of 2-(ethyl(phenyl)amino)ethanol (3.96 g,
24 mmol) in distilled ethanol was added dropwise and stirred for
1 h at 5 ꢁC. After pH of the resulting solution was adjusted to 7.0
with potassium carbonate, the reaction was stirred for another 30
min. The red solution was diluted with CH2Cl2 and washed with
brine and deionized water. The crude product was purified by
recrystallization. E1 was obtained as red powder. Yield: 72.7%.
Synthesis of (E)-4-(2-(ethyl(4-((4-nitro-2-((perfluorophenyl)
methoxy)phenyl)diazenyl)phenyl)amino)ethoxy)-4-oxobutanoic
acid (5). Compound 5 was synthesized using a similar procedure
to that for 4 as red powder. Yield: 80.33%. 1H NMR (500 MHz,
CDCl3, d (ppm)): 1.24–1.27 (t, 3H, CH3), 2.64–2.68 (m, 4H,
(CH2)2COOH), 3.53–3.54 (d, 2H, NCH2), 3.69 (s, 2H,
NCH2CH2), 4.32–4.33 (t, 2H, NCH2CH2), 5.41 (s, 2H, OCH2),
6.78–6.79 (d, 2H, ArH), 7.73–7.75 (d, 1H, ArH), 7.84–7.86 (d,
2H, ArH), 7.99 (s, 1H, ArH), 8.04–8.05 (d, 1H, ArH).
Synthesis of (E)-4-(2-(ethyl(4-((2-((2-ethylhexyl)oxy)-4-nitro-
phenyl)diazenyl)phenyl)amino)ethoxy)-4-oxobutanoic acid (6).
Compound 6 was synthesized using a similar procedure to that
for 4 as red powder. Yield: 60.92%. 1H NMR (500 MHz, CDCl3,
d (ppm)): 0.88–0.91 (t, 3H, CH3), 0.96–0.99 (t, 3H, CH3), 1.23–
1.26 (t, 3H, NCH2CH3), 1.32–1.37 (m, 8H, (CH2)2CH2,
(CH2)2CH2, CH2CH3), 1.88–1.90 (t, 1H, CH(CH2)3), 2.62–2.67
(m, 4H, (CH2)2COOH), 3.49–3.54 (m, 2H, NCH2), 3.66–3.69 (t,
2H, OCH2), 4.09–4.10 (d, 2H, NCH2CH2), 4.31–4.34 (t, 2H,
NCH2CH2), 6.77–6.79 (d, 2H, ArH), 7.65–7.67 (d, 1H, ArH),
7.85–7.89 (m, 4H, ArH).
Synthesis
of
(E)-2-(ethyl(4-((4-nitro-2-((perfluorophenyl)
methoxy)phenyl) diazenyl)phenyl)amino)ethanol (E2). The chro-
mophore E2 was synthesized using a similar procedure to that for
E1. Yield: 40.39%. 1H NMR (500 MHz, CDCl3, d (ppm)): 1.24–
1.27 (t, 3H, CH3), 3.56–3.57 (d, 2H, CH2OH), 3.61–3.64 (t, 2H,
CH2CH3), 3.89–3.90 (d, 2H, NCH2CH2), 5.40 (s, 2H, OCH2),
6.79–6.81 (d, 2H, ArH), 7.73–7.75 (d, 1H, ArH), 7.84–7.86 (d,
2H, ArH), 7.97–8.00 (t, 1H, ArH), 8.04 (d, 1H, ArH). Elemental
analysis calcd (%) for C12H19F5N4O2: C, 54.12; H, 3.75; N, 10.98.
Found: C, 54.40; H, 3.80; N, 10.54. MS (ESI): exact mass calcd
for C12H19F5N4O2 [M + H]+, 511.41. Found: 511.30. [M ꢀ H]ꢀ,
509.41. Found: 509.30.
Synthesis of six-branched chromophore S1
Compound 1 (0.52 g, 1 mmol), 4 (3.31 g, 8 mmol), EDC$HCl
(2.09 g, 11 mmol) and DMAP (0.24 g, 2 mmol) were dissolved in
mixed solvent of CH2Cl2 and THF (v : v ¼ 3 : 1). The mixture
was stirred at room temperature for 72 h. The resulting solution
was diluted with CH2Cl2 and washed with brine and water and
the organic phase was dried over MgSO4. After the solvent was
evaporated under reduced pressure, the residue was purified by
silica column chromatography on silica gel using a mixture of
CH2Cl2/THF (v : v ¼ 40 : 1) as the eluent to yield the red powder
S1. Yield: 46.21%. 1H NMR (500 MHz, CDCl3, d (ppm)): 1.21–
1.25 (m, 18H, CH2CH3), 2.01 (s, 3H, CCH3), 2.61–2.63 (m, 24H,
(OCCH2)2), 3.48–3.65 (m, 24H, NCH2), 4.04–4.05 (d, 6H,
CHCH2O), 4.28–4.44 (m, 18H, OCH2, CHCH2O), 5.35–5.37
(t, 3H, CHCH2), 6.73–6.93(m, 24H, ArH), 7.86–7.90 (m, 24H,
Synthesis of (E)-2-(ethyl(4-((2-((2-ethylhexyl)oxy)-4-nitro-
phenyl)diazenyl) phenyl)amino)ethanol (E3). The chromophore
E3 was synthesized in a similar procedure to that for E1. Yield:
69.21%. 1H NMR (500 MHz, CDCl3, d (ppm)): 0.88–0.91 (t, 3H,
CH3), 0.96–0.99 (t, 3H, CH3), 1.20–1.22 (d, 3H, NCH2CH3),
1.22–1.27 (t, 4H, CH(CH2)2), 1.33–1.38 (m, 4H, CH2CH3), 1.89
(s, 1H, CH(CH2)2), 3.55–3.59 (t, 2H, NCH2CH3), 3.61–3.63
(t, 2H, CH2OH), 3.89–3.90 (d, 2H, OCH2), 4.09–4.10 (d, 2H,
NCH2), 6.80–6.81 (d, 2H, ArH), 7.66–7.68 (d, 1H, ArH), 7.86–
7.90 (m, 4H, ArH). Elemental analysis calcd (%) for
C24H34N4O4: C, 65.14; H, 7.74; N, 12.66. Found: C, 65.11; H,
7.72; N, 12.51. MS (ESI): exact mass calcd for C24H34N4O4
9204 | J. Mater. Chem., 2012, 22, 9202–9208
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ArH), 8.28–8.29 (d, 12H, ArH). 13C NMR (125 MHz, CDCl3,
d (ppm)): 172.3, 172.2, 172.0, 171.8, 156.9, 156.4, 151.4, 147.6,
144.0, 142.5, 129.8, 126.4, 124.8, 122.8, 113.9, 111.6, 70.2, 66.1,
62.9, 61.8, 48.9, 45.8, 30.5, 29.1, 29.0, 28.9, 12.5. Elemental
analysis calcd (%) for C149H156N24O39: C, 61.56; H, 5.41; N,
11.56. Found: C, 61.31; H, 5.58; N, 11.38. MALDI-TOF MS:
exact mass calcd for C149H156N24O39 [M], 2906.97. Found:
2905.00.
Results and discussion
Synthesis and characterization
The detailed synthetic routes to the azo chromophores E1–3 and
six-branched chromophores S1–3 are shown in Scheme 1. Three
kinds of azo chromophores E1–3 with different functional
groups on the acceptor were prepared via a direct diazo coupling
reaction between 2-(ethyl(phenyl)amino)ethanol and the
different nitroaniline derivatives. The phenyl glycerol ester core 1
was produced by the Williamson ester reaction between Frechet-
type tri-phenol core 4,40,400-(ethane-1,1,1-triyl)triphenol and
3-chloropropane-1,2-diols. The esterification reaction between
azo chromophores E1–3 and succinic anhydride yielded the
compounds 4–6 with carboxylic acid. Then the six-branched
chromophores S1–3 were synthesized via esterification between
compounds 4–6 and tether core 1.
Synthesis of six-branched chromophore S2
A similar procedure to that for S1 was used to afford the six-
1
branched chromophore S2 as red powder. Yield: 48.72%. H
NMR (500 MHz, CDCl3, d (ppm)): 1.21–1.24 (m, 18H,
CH2CH3), 2.01 (s, 3H, CCH3), 2.60–2.64 (m, 24H, (OCCH2)2),
3.49–3.50 (d, 12H, NCH2), 3.64–3.66 (m, 12H, NCH2CH2), 4.04–
4.05 (d, 6H, CHCH2O), 4.27–4.29 (m, 18H, OCH2, CH2CH2O),
5.38 (d, 15H, CHCH2,CH2ArF), 6.72–6.76 (t, 18H, ArH), 6.91–
6.93 (d, 6H, ArH), 7.69–7.71 (m, 6H, ArH), 7.81–7.83 (m, 12H,
ArH), 7.94–7.96 (m, 6H, ArH), 8.01–8.02 (d, 6H, ArH). 13C
NMR (125MHz, CDCl3, d (ppm)): 172.3, 172.2, 172.0, 171.8,
156.5, 154.5, 151.5, 148.6, 148.1, 147.0, 145.0, 144.6, 142.5, 129.8,
126.6, 118.8, 118.1, 113.9, 113.2, 111.6, 109.9, 70.3, 66.1, 62.9,
61.8, 61.2, 48.9, 45.9, 29.1, 29.0, 28.9, 28.7, 12.5. Elemental
analysis calcd (%) for C191H162F30N24O45: C, 56.18; H, 4.00; N,
8.23. Found: C, 55.87; H, 4.04; N, 7.82. MALDI-TOF MS: exact
mass calcd for C191H162F30N24O45 [M], 4083.42. Found: 4082.80.
The structures of chromophores E1–3 and six-branched
chromophores S1–3 were confirmed by 1H NMR, 13C NMR, FT-
IR spectroscopy, elemental analysis and mass spectrometry. The
details of the characterizations are described in the Experimental
Section. In the 1H NMR spectra of the chromophores, the
chemical shifts were consistent with the proposed molecular
structure as demonstrated in Scheme 1. The FTIR spectra of
chromophore E2, compound 6 and six-branched chromophore
S2 were illustrated in Fig. 2 as an example. For chromophore E2,
the absorption of the hydroxy group was observed around 3500–
3200 cmꢀ1. In the spectra of compound 6, the peak at 1734 cmꢀ1
and 1708 cmꢀ1 attributed to the carboxyl group emerged obvi-
ously. In the spectra of S2, the band at 3500–3200 cmꢀ1 vanished,
while the peak at 1734 cmꢀ1 remained, indicating that the six-
branched chromophore has been successfully synthesized.
Thermal properties of azo chromophores E1–3 and six-
branched chromophores S1–3 were analyzed by thermogravi-
metric analysis (TGA) under a nitrogen atmosphere. The
decomposition temperatures (Td) of chromophore E1–3 are
determined to be 286 ꢁC, 268 ꢁC and 273 ꢁC, respectively. As
shown in Fig. 3, all the six-branched chromophores show the
higher decomposition temperatures than that of the corre-
sponding azo chromophores. The six-branched chromophoreꢁS1
displays the highest decomposition temperature (Td) of 317 C,
while S2 exhibits the lowest decomposition temperature of 283
ꢁC. The results indicate that molecular thermal stability could be
efficiently improved when chromophores were combined into
phenyl glycerol ester core to yield multichromophores, which
increased the rigidity of the chromophores. The high thermal
stability of the six-branched chromophores make them could be
widely applied in photonic device fabrications.
Synthesis of six-branched chromophore S3
The red powder chromophore S3 was synthesized using a similar
1
procedure to that for S1. Yield: 45.34%. H NMR (500 MHz,
CDCl3, d (ppm)): 0.87–0.90 (t, 18H, CH2CH3), 0.95–0.98 (t, 18H,
CH2CH3), 1.21–1.24 (m, 18H, CH2CH3), 1.31–1.59 (m, 48H,
CH2CH3, (CH2)3CH3), 1.87–1.88 (d, 6H, CH(CH2)3), 2.03 (s,
3H, CCH3), 2.62–2.66 (m, 24H, (OCCH2)2), 3.48–3.52 (m, 12H,
NCH2), 3.64–3.66 (t, 12H, OCH2), 4.05–4.09 (m, 18H,
CHCH2O, NCH2CH2), 4.28–4.30 (t, 18H, OCH2, CHCH2O),
5.37 (s, 3H, CHCH2), 6.75–6.77(m, 18H, ArH), 6.93–6.95 (d, 6H,
ArH), 7.64–7.66 (m, 6H, ArH), 7.83–7.89 (m, 24H, ArH). 13C
NMR (125MHz, CDCl3, d (ppm)): 172.3, 172.2, 172.0, 171.8,
156.4, 155.8, 151.0, 148.5, 147.3, 144.8, 142.5, 129.8, 126.4, 117.4,
116.4, 113.9, 111.6, 109.3, 72.9, 70.2, 66.1, 62.9, 61.9, 48.9, 45.9,
39.6, 30.8, 29.3, 29.1, 29.0, 28.9, 24.3, 23.2, 14.3, 12.5, 11.4.
Elemental analysis calcd (%) for C197H252N24O45: C, 64.36; H,
6.91; N, 9.14. Found: C, 63.96; H, 6.93; N, 9.04. MALDI-TOF
MS: exact mass calcd for C197H252N24O45 [M], 3676.24. Found:
3675.40.
Thin films fabrication
Linear and nonlinear optical properties
The six-branched chromophore S1, S2 and S3 were dissolved
in cyclopentanone (8% of total solid weight). After filtered by
0.22 mm Teflon membrane filters, the resulting solutions were
spin-coated on the indium-tin oxide (ITO) glass substrates ꢁto
fabricate films. The films were dried in a vacuum oven at 60 C
for 12 h to remove the residual solvent. The azo chromophore
E1, E2 and E3 doped polymer films using the poly(4-vinyl-
phenol) (PVPh) as matrix were also prepared through a similar
procedure as a reference.
The linear optical properties of the chromophores were observed
from UV-visible absorption spectra. As shown in Fig. 4, all the
chromophores exhibited strong absorption bands in the visible
region. The maximum absorption wavelengths (lmax
) of
chromophores E1–3 and S1–3 were at about 475–495 nm and
470–485 nm in CHCl3, respectively. Compared with S1, the small
red-shift of S2 and S3 (about 9–12 and 2 nm) was observed due
to the introduction of isolation substituents. The interesting
phenomena were primarily associated with the isolation groups
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Scheme 1 Synthesis of chromophores E1–3 and six-branched chromophores S1–3.
Fig. 3 TGA traces for chromophores E1, E2, E3, S1, S2 and S3.
Fig. 2 FTIR spectra of chromophore E2, compound 6 and six-branched
chromophore S2.
polymer matrix PVPh, and the largest loading densities were up
to 40 wt%. The second-order NLO properties of the thin films
were studied by SHG measurement and the poling processes
were carried out with a 5.0 kV dc voltage. Then the applied
voltage was removed after the films were cooled to room
temperature. Calculation of the SHG coefficients (d33) for the
films was based on the following equation:33
being anchored on the acceptor-end of the chromophores which
caused a decrease of electron-release ability. Meanwhile, six-
branched chromophores S1–3 blue shift about 10–20 nm in their
intrinsic absorptions compared to E1–3. This may be ascribed to
the interactions of the sub-chromophores in multichromophores
and the generation of new group (–COO–) in six-branched
chromophores S1–3. The same phenomena were also observed in
THF and DMF.
sffiffiffiffi
d33;s
d11;q
Is Lqc
Iq Ls
¼
F
(1)
To investigate the NLO properties of the six-branched chro-
mophores, the thin films (Film-S1, Film-S2, Film-S3) were
prepared, and the chromophore E1, E2 and E3 doped polymer
films were also fabricated by doping the chromophores into
where d33,s is the SHG coefficient of the poled films and d11,q is
d11 of the quartz crystals, which is equal to 0.5 pm Vꢀ1. Is and Iq
are the SHG intensities of the film and quartz, respectively. Lqc is
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Table 1 The wavelength of maximum absorption (lmax) in chloroform
and second harmonic coefficients of the films
Films
lmax (nm)
d33 (pm Vꢀ1
)
E1-doped film (40 wt%)
E2-doped film (40 wt%)
E3-doped film (40 wt%)
Film-S1
Film-S2
Film-S3
Film-M1
Film-M2
Film-M3
478
491
482
472
484
474
476
468
475
49
45
42
113
142
136
153
133
168
Fig. 4 UV-visible absorption spectra of chormophores E1, E2, E3, S1,
S2 and S3 in CHCl3.
113 pm Vꢀ1, which is almost 3 times the maximum d33 value of
the E1 doped polymer film. The above results demonstrated that
the six-branched structure can efficiently reduce the inter-
chromophore electrostatic interactions, thus improving the
active chromophore loading density and enhancing the macro-
scopic optical nonlinearities.29
the coherent length of quartz crystals, which is equal to 20.6 mm,
and Ls is the thickness of the poled films. F is the correction
factor of the apparatus and equals 1.2 when Ic [ Is.
As shown in Fig. 5, the d33 value of the chromophore E1 doped
polymer film increased with the increase of chromophore loading
density and displayed a maximum of 56 pm Vꢀ1 at 30 wt%, then
the d33 value decreased at around 40 wt% due to the strong
interchromophore electrostatic interactions. The similar behav-
iour was observed in the chromophore E2 and E3 doped polymer
films. It was reported in the literature that the chromophore
loading densities of guest–host doped NLO polymers typically
have to be kept below 25 wt%, which was quite consistent with
our results. The d33 values of the E1, E2 and E3 doped films at the
chromophore concentration of 40 wt% were calculated to be 49,
45 and 42 pm Vꢀ1, respectively. The active chromophore loading
density in the films containing six-branched chromophores (Film-
S1, Film-S2, Film-S3) were about 65, 75 and 72 wt%, respec-
tively, which was much higher than the usual loading level of
guest-host doped polymer NLO materials as shown in the
chromophore E1, E2 and E3 doped polymer films. In contrast to
the chromophore doped polymer films, the films containing
six-branched chromophores exhibited the larger d33 values. As
shown in Table 1, the d33 value of Film-S1 was determined to be
Interestingly, the Film-S2 and Film-S3 showed the higher
optical nonlinearity than that of the Film-S1. The d33 values of
Film-S2 and Film-S3 were calculated to be 142 and 136 pm Vꢀ1
,
respectively. This behaviour may be rationalized by considering
the introduction of isolation groups. In six-branched chromo-
phores S2 and S3, the isolation groups could further isolated the
chromophore branch from each other and then restrain the
interchromophore electrostatic interactions. It needs to be
mentioned that Film-S3 showed lower d33 values than Film-S2
due to the flexible substituents being too flexible to work effi-
ciently as the rigid groups. Therefore, the introduction of the
isolation groups in six-branched chromophores can further
improve the macroscopic NLO properties; furthermore, the rigid
isolation group exhibited a better effect on enhancing the d33
values than the flexible group.
To further explored the influence on NLO properties of the
isolation groups in the six-branched chromophore, we also
fabricated mixed thin films Film M1, Film-M2 and Film-M3 by
mixing the six-branched chromophores at the equally quantity.
The d33 values of the mixed films were determined to be 153 pm
Vꢀ1, 133 pm Vꢀ1 and 168 pm Vꢀ1, respectively, suggesting that the
d33 values could be further enhanced by mix the six-branched
chromophores with different isolation groups. This should be
attributed to the synergetic effect of the different isolation
groups, thus minimizing the strong intermolecular dipole–dipole
interactions. Thus, the successful examples might open up a new
avenue to achieve further good NLO polymers by mixing the
NLO dendrimers with different isolation groups.
The thermal stability of optical nonlinearity was also inves-
tigated by a depoling experiment, in which the SHG siꢁgnal was
monitored as the poled film was heated at a rate of 10 C minꢀ1
from 45 to 100 ꢁC. As shown in Fig. 6, the d33 value of Film-S1
and Film-S2 maintained half of the original signals around
70 ꢁC, indicating that the thermal stability of optical nonline-
arity were rarely influenced by the isolation groups. A further
study will aim to introduce a rigid dendritic core, which may
produce the excellent dendrimers with high thermal stability of
nonlinearity.
Fig. 5 d33 values of polymer films as a function of molecule loading
densities of chromophore E1, E2 and E3.
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Fig. 6 Decay of the normalized d33 values as a function of temperature
for Film-S1 and Film-S2.
Conclusions
In summary, three six-branched chromophores S1–3 were
designed and synthesized via incorporating azo chromophores
onto a phenyl glycerol ester core. The molecular structures were
confirmed by 1H NMR, 13C NMR, FT-IR spectroscopy,
elemental analysis and mass spectrometry. Compared with the
azo chromophores E1, E2 and E3, the six-branched chromo-
phores S1, S2 and S3 showed higher thermal decomposition
temperatures up to about 300 ꢁC. The six-branched chromo-
phores can form thin films directly without any added polymer
matrix and show a quite high chromophore loading density. The
poled films containing six-branched chromophores display 2–3
times enhancement in nonlinear optical coefficient in comparison
with the azo chromophores-doped films, in which, the rigid
isolation group exhibits better enhanced effect than the flexible
group. The further improving in optical nonlinearity are
successfully realized by mix the six-branched chromophores
containing different isolation groups, which might open up a new
avenue to achieve further good NLO polymers.
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Acknowledgements
The authors gratefully acknowledge the financial support for this
work from the National Natural Science Foundation of China
(Nos. 50802084, 50972127 and 51010002), the Zhejiang Provin-
cial Natural Science Foundation of China (No. Y4090067), and
the Fundamental Research Funds for the Central Universities.
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