Novel electro-optic chromophores based on substituted benzo[1,2-b:4,5- b′]dithiophene π-conjugated bridges

Article abstract of DOI:10.1039/c4ra01767f

Two novel non-linear optical (NLO) chromophores were designed and synthesized based on the substituted benzo[1,2-b:4,5-b′]dithiophene unit (BDT), tricyanofuran (TCF) electron acceptor and two different electron donors. These new chromophores, which exhibited good thermal stability and solubility in common organic solvents, were systematically characterized by thermogravimetric analysis, UV-Vis spectra, density functional theory (DFT) calculations and measurements of the electro-optic (EO) coefficients. Compared with the dodecyl group in chromophore BDT1 that we have previously reported, the isooctane group in BDT2 could act as a suitable isolation group, and as a result, the guest-host system containing 30% of BDT2 in amorphous polycarbonate (APC) displayed the largest EO coefficient of 102 pm V^-1, which was greatly improved over BDT1 and the analogous thiophene-based chromophore with the same electron donor and acceptor. The solvatochromic analysis and DFT study demonstrated that the julolidine group in BDT3 possessed a stronger donating ability. However, the 30% BDT3/APC exhibited an EO coefficient of only 82 pm V^-1, which was probably caused by the diminishment of the hyperpolarizability value of BDT3 in the polar polymer matrix. These results indicate the potential of the isooctane-substituted benzo[1,2-b:4,5-b′]dithiophene π-conjugated bridge in chromophore design and NLO materials, and further illustrate the critical role of suitable isolation groups and fine-tuning of the donor's electron-donating strength in optimizing the non-linear properties of NLO chromophores. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra01767f

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Novel electro-optic chromophores based on
substituted benzo[1,2-b:4,5-b0]dithiophene p-
conjugated bridges†
Cite this: RSC Adv., 2014, 4, 25532
Peng Si,ab Jialei Liu,a Guowei Deng,ab Heyan Huang,ab Huajun Xu,ab Shuhui Bo,a
a
a
a
*
Ling Qiu, Zhen Zhen and Xinhou Liu
Two novel non-linear optical (NLO) chromophores were designed and synthesized based on the
substituted benzo[1,2-b:4,5-b0]dithiophene unit (BDT), tricyanofuran (TCF) electron acceptor and two
different electron donors. These new chromophores, which exhibited good thermal stability and
solubility in common organic solvents, were systematically characterized by thermogravimetric analysis,
UV-Vis spectra, density functional theory (DFT) calculations and measurements of the electro-optic (EO)
coefficients. Compared with the dodecyl group in chromophore BDT1 that we have previously reported,
the isooctane group in BDT2 could act as a suitable isolation group, and as a result, the guest–host
system containing 30% of BDT2 in amorphous polycarbonate (APC) displayed the largest EO coefficient
of 102 pm Vꢀ1
, which was greatly improved over BDT1 and the analogous thiophene-based
chromophore with the same electron donor and acceptor. The solvatochromic analysis and DFT study
demonstrated that the julolidine group in BDT3 possessed a stronger donating ability. However, the 30%
BDT3/APC exhibited an EO coefficient of only 82 pm Vꢀ1, which was probably caused by the
diminishment of the hyperpolarizability value of BDT3 in the polar polymer matrix. These results indicate
the potential of the isooctane-substituted benzo[1,2-b:4,5-b0]dithiophene p-conjugated bridge in
chromophore design and NLO materials, and further illustrate the critical role of suitable isolation groups
and fine-tuning of the donor's electron-donating strength in optimizing the non-linear properties of
NLO chromophores.
Received 28th February 2014
Accepted 11th April 2014
DOI: 10.1039/c4ra01767f
www.rsc.org/advances
key role in EO devices at both the microscopic level and
macroscopic materials level.13–15
Introduction
Organic second-order NLO chromophore molecules are the
core component of EO materials and typically consist of a p-
conjugated bridge end-capped with strong electron-donating
and -accepting groups.2,11 Among them, a p-conjugated electron
bridge helping intramolecular charge-transfer plays an essen-
tial role in the chromophore structure.11,14,16 For the purpose of
improving microscopic rst-order hyperpolarizability (b),
increasing and optimizing the p-conjugated bridge is a critical
aspect. But unfortunately, the added conjugation length oen
inuences the stability of the molecule, which will directly
decrease the practical application of NLO chromophoresc.2,11
The benzo[1,2-b:4,5-b0]dithiophene (BDT) unit containing two
thienyl rings has been widely used for photovoltaic conjugated
polymers due to its large planar conjugated structure, which
makes BDT a promising electron bridge.17–19 Previously, we rst
introduced this unit into NLO materials and synthesized a novel
chromophore BDT1 which showed promising second-order
NLO features and could meet the requirement of stability for
application.20 However, the guest–host EO materials21,22 of BDT1
doped into amorphous polycarbonate (APC) did not exhibit the
desired EO activities.
With the rocketing development of information technology,
electro-optic (EO) materials, which are widely used in telecom-
munications such as phased array radar, optical gyroscopes and
modulators, have drawn great attention in the past decades.1–10
Compared to inorganic/semi-conductor EO materials, organic
EO materials have many advantages such as large non-linear
optical (NLO) coefficient, ultra-fast response time, large band-
width, low dielectric constant and good processability.2,11,12 In
order to promote the development of EO materials, much effort
has been expended in developing NLO chromophores and EO
polymers. But to full the requirements of device fabrication
and operation, a strong need remains for improving the NLO
properties and stability of the NLO chromophores, which play a
aKey Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR
China. E-mail: xinhouliu@foxmail.com; xhliu@mail.ipc.ac.cn; Fax: +86-01-
62554670; Tel: +86-01-82543528
bUniversity of Chinese Academy of Sciences, Beijing 100043, PR China
† Electronic supplementary information (ESI) available: NMR and HRMS spectra
of resulted compounds. See DOI: 10.1039/c4ra01767f
25532 | RSC Adv., 2014, 4, 25532–25539
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Suppressing the dipolar–dipolar electrostatic interaction
RSC Advances
Experimental
and increasing the microscopic hyperpolarizability are basic
strategies to improve the EO activities from the angle of
NLO chromophores. Dipolar–dipolar electrostatic interac-
tion between chromophores, which could cause aggregation
of the chromophores, is a major obstacle hindering trans-
lation of the microscopic hyperpolarizability into macro-
scopic EO activity.5,23–30 Work from the groups of Dalton
et al. and Li et al. has demonstrated that the dipole–dipole
interactions could be effectively suppressed through the
introduction of suitable isolation groups.31–35 And the rela-
tive researches also predicted that the ideal shape for the
chromophore is spherical.2,36 On the other hand, the
hyperpolarizability is the source of EO activity, so improving
the chromophores' NLO properties is also an effective
approach to improve macroscopic activity, and the hyper-
polarizability can be tuned by optimizing the ground-state
polarization through modication of the strength of the
donor or acceptor.
To improve the macroscopic EO activities of BDT-based
NLO chromophores and further explore the potential of the
BDT p-conjugated bridge in NLO materials, we designed and
synthesized the novel chromophore BDT2 with new isolation
groups and BDT3 with a stronger electron donor as shown in
Chart 1. BDT2 adopts the shorter dendritic isooctane groups,
which we believe are more suitable for isolating and can make
the chromophore closer to a spherical shape. In BDT3, the
Materials and instruments
All reagents were purchased from commercial sources and were
used as received unless stated. DMF, THF, diethyl ether and
acetone were freshly distilled prior to use. Compound 3 was
synthesized by our group, 4,8-dihydrobenzo[1,2-b:4,5-b0]dithio-
phen-4,8-dione and the TCF acceptor were prepared according
to the literature.18,38–40 1H NMR and 13C NMR spectra were
measured using an AVANCE 400 spectrometer (Bruker) using
tetramethylsilane (TMS; d ¼ 0 ppm) as the internal standard
1
(resolution H # 0.45 Hz; 13C # 0.2 Hz). High-resolution mass
spectrometry experiments were performed using a Bruker Dal-
tonics Apex IV FTMS spectrometer (accuracy < 1–2 ppm). The
MS spectra were obtained using MALDI-TOF (Matrix Assisted
Laser Desorption/Ionization of Flight) on a GCT Premier
(Waters) (resolution 7000 FWHM; sensitivity 1 pg). Melting
points were obtained using a Beijing X4 melting point tester.
Fourier transform infrared (FT-IR) spectra were recorded using
a Varian 3100 FT-IR spectrometer (resolution 0.20 cmꢀ1; accu-
racy < 0.01 cmꢀ1). UV-Vis spectra were obtained using a Varian
Cary5000 spectrophotometer (resolution 0.05 nm; accuracy #
ꢁ0.1 nm). The TGA curve was recorded using a TA-instrument
Q50 analyzer with a heating rate of 10 ꢂC minꢀ1 under a
nitrogen atmosphere.
Syntheses
julolidine
derivative
8-hydroxy-1,1,7,7-tetramethyl-for-
myljulolidine, which has been proved to be an excellent elec-
Synthesis of compound 1. 4,8-Dihydrobenzo[1,2-b:4,5-b0]-
tron donor,13,37 is used to improve the electron-donating dithiophen-4,8-dione (4.4 g, 20 mmol), zinc powder (3.3 g, 50
ability. The thermal stability, photophysical properties, mmol), and water (60 mL) were put into a 250 mL ask, then
density functional theory (DFT) calculations and EO activities NaOH (12 g, 0.3 mol) was added into the mixture. Aer the
of these chromophores were systematically studied and mixture was stirred and reuxed for 1 h, 1-bromoisooctane
compared. The results further proved that substituted benzo (11.6 g, 60 mmol) and a catalytic amount of tetrabutylammo-
[1,2-b:4,5-b0]dithiophene is a quite promising p-conjugated nium bromide were added into the ask. The reactant was
bridge in chromophore design and that the isooctane group reuxed for 8 h and then was poured into cold water and
could be a suitable isolation group to improve the EO activities extracted by diethyl ether three times. The organic layer was
(maximum r33 value was up to 102 pm Vꢀ1), and they also dried over anhydrous MgSO4. Aer the solvent had been
illustrated the critical role of ne-tuning of donor's electron- removed, the crude product was puried by column chroma-
donating strength in optimizing the non-linear properties of tography. Compound 1 (7.1 g, yield 79%) was obtained as a pale
NLO chromophores.
yellow oil. IR (KBr), nmax cmꢀ1: 3084, 2959, 2927, 2872 (C–H),
1519 (thienyl), 1439 (benzene ring), 1198 (Ph–O–C). MS, m/z:
446.23 (M+). 1H NMR (400 MHz, CDCl3) d (ppm) 7.50 (thienyl, d,
J ¼ 5.5 Hz, 2H), 7.37 (thienyl, d, J ¼ 5.5 Hz, 2H), 4.26–4.15 (–O–
CH2–, m, 4H), 1.83 (–CH–, m, J ¼ 11.9, 5.8 Hz, 2H), 1.77–1.35
(–CH2–, m, 16H), 1.00 (–CH3, m, 12H).
Synthesis of compound 2. Compound 1 (2.23 g, 5 mmol) was
dissolved in dry THF (30 mL) in a 100 mL ask. Then n-butyl-
lithium (3.75 mL, 15 mmol, 2.5 M) was injected into the solu-
ꢂ
tion slowly at ꢀ78 C under an inert atmosphere. The solution
was stirred for 30 min at ꢀ78 ꢂC and for 30 min at ꢀ10 ꢂC. Aer
that, the solution was cooled at ꢀ78 ꢂC and excess DMF was
injected into the solution slowly. The solution was stirred at
ꢀ78 ꢂC for 30 min and then warmed to room temperature. Aer
that the reactant was quenched with water (300 mL), extracted
with diethyl ether, and dried over anhydrous MgSO4. The crude
product was puried by column chromatography. Compound 2
Chart 1 Chemical structures of the chromophores.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 25532–25539 | 25533

View Article Online
RSC Advances
Paper
(1.80 g, yield 72%) was obtained as an orange solid. Mp: 93– compound 6 according to the synthesis of compound 4.
94 ꢂC. IR (KBr),nmax cmꢀ1: 2960, 2930, 2874 (C–H), 1670 (–CHO), Compound 7 was obtained as a red oil (yield 54%). 1H NMR
1535, 1452 (aromatic ring), 1215 (Ph–O–C). MS, m/z: 502.43 (CDCl3) shows that the product is a mixture of trans and cis
1
(M+). H NMR (400 MHz, CDCl3) d (ppm) 10.14 (–CHO, s, 2H), isomers. On the basis of the integration areas of two thienyl-H
8.18 (thienyl, s, 2H), 4.28 (–O–CH2–, d, J ¼ 5.3 Hz, 4H), 1.91–1.80 signals, the trans isomer is estimated to be 82% and the cis
(–CH–, m, 2H), 1.74–1.34 (–CH2–, m, 16H), 0.98 (–CH3, m, 12H). isomer 18%. IR (KBr), nmax cmꢀ1: 2957, 2926, 2858 (C–H), 1674
Synthesis of compound 4. Compound 2 (1.10 g, 2.2 mmol), (–CHO), 1504, 1456 (aromatic ring), 1250 (Ph–O–C). MS, m/z:
compound 3 (1.01 g, 2 mmol), NaH (1.0 g, 0.04 mol), and dry 855.75 (M+). 1H NMR (400 MHz, CDCl3) d (ppm) 10.00 (–CHO, s,
diethyl ether (50 mL) were put into a 100 mL ask. The reactant 1H), 8.08 (thienyl, s, 0.18H), 8.05 (thienyl, s, 0.82H), 7.15
was stirred for 24 h at ambient temperature with the ask sealed (thienyl, phenyl and vinylic, m, 3H), 6.99 (vinylic, d, J ¼ 15.9 Hz,
up. Aer that, the mixture was poured into ice water (200 mL) 1H), 4.25–4.01 (–O–CH2–, m, 4H), 3.78–3.61 (–O–CH2–, m, 2H),
slowly. The mixture was then extracted with diethyl ether and 3.15–2.99 (–N–CH2–, m, 4H), 2.00–1.87 (–CH–, m, 1H), 1.76
dried over MgSO4. Aer removal of the diethyl ether, the crude (–CH–, m, 2H), 1.70–1.09 (–CH2– and –CH3, m, 40H), 0.94
product was puried by column chromatography. Compound 4 (–CH3, t, J ¼ 8.2 Hz, 6H), 0.89–0.84 (–CH3, m, 6H), 0.77 (–CH3, t,
1
(0.82 g, yield 58%) was obtained as a red oil. H NMR (CDCl3) J ¼ 6.9 Hz, 6H).
shows that the product is a mixture of trans and cis isomers. On
Synthesis of chromophore BDT2. Compound 4 (0.33g, 0.5
the basis of the integration areas of two –CHO signals, the trans mmol) was dissolved in chloroform (20 mL), and then a catalytic
isomer is estimated to be 90% and the cis isomer 10%. IR (KBr), account of triethylamine and TCF (0.14 g, 0.75 mmol) were
nmax cmꢀ1: 2959, 2926, 2870 (C–H), 1670 (–CHO), 1599, 1520, added. The mixture was stirred and reuxed for 5 h. Aer the
1
1449 (aromatic ring), 1265 (Ph–O–C). MS, m/z: 647.12 (M+). H solvent had been removed, the crude product was puried by
NMR (400 MHz, CDCl3) d (ppm) 10.11 (–CHO, s, 0.10H), 10.06 column chromatography. Chromophore BDT2 (0.23 g, yield
(–CHO, s, 0.90H), 8.15 (thienyl-H, s, 0.10H), 8.11 (thienyl, s, 56%) was obtained as a dark blue solid. Mp: 224–225 ꢂC. IR
0.90H), 7.39 (phenyl, d, J ¼ 8.7 Hz, 2H), 7.24 (thienyl, s, 1H), 7.09 (KBr), nmax cmꢀ1: 2960, 2928, 2870 (C–H), 2227 (C^N), 1601,
(vinylic, d, J ¼ 15.9 Hz, 1H), 6.95 (vinylic, d, J ¼ 15.9 Hz, 1H), 1539 (aromatic ring), 1285 (Ph–O–C). 1H NMR (400 MHz, CDCl3)
6.66 (phenyl, d, J ¼ 8.7 Hz, 2H), 4.36–4.09 (–O–CH2–, m, 4H), d (ppm) 7.98 (vinylic, d, J ¼ 15.8 Hz, 1H), 7.81 (thienyl, s, 1H),
3.40 (–N–CH2–, q, J ¼ 7.0 Hz, 4H), 1.88–1.76 (–CH–, m, 2H), 7.77 (phenyl, d, J ¼ 8.2 Hz, 2H), 7.68 (phenyl, d, J ¼ 8.2 Hz, 2H),
1.69–1.39 (–CH2–, m, 16H), 1.19 (–CH3, t, J ¼ 7.0 Hz, 6H), 1.03 7.46 (thienyl, s, 1H), 7.41 (vinylic, d, J ¼ 16.0 Hz, 1H), 7.03
(–CH3, t, J ¼ 7.6 Hz, 6H), 0.99–0.94 (–CH3, m, 6H).
(vinylic, d, J ¼ 16.0 Hz, 1H), 6.67 (vinylic, d, J ¼ 15.8 Hz, 1H),
Synthesis of compound 5. 8-Hydroxy-1,1,7,7-tetramethyl- 4.27 (–O–CH2–, d, J ¼ 5.6 Hz, 2H), 4.19 (–O–CH2–, d, J ¼ 5.5 Hz,
formyljulolidine (2.73 g, 0.01 mol), 1-bromoisooctane (2.90 g, 2H), 3.67 (–N–CH2–, s, 2H), 3.33 (–N–CH2–, s, 2H), 1.88–1.76
0.015 mol) and K2CO3 (3.0 g, 0.022 mol) were added into (–CH–, m, 2H), 1.81 (–CH3, s, 6H), 1.77–1.22 (–CH2– and –CH3,
anhydrous acetone (50 mL). The mixture was stirred and m, 22H), 1.04 (–CH3, t, J ¼ 7.4 Hz, 6H), 0.96 (–CH3, t, J ¼ 6.5 Hz,
reuxed for 15 h. Aer the precipitates were ltered out and the 6H). 13C NMR (101 MHz, CDCl3) d (ppm) 175.43, 172.31, 148.46,
solvent was removed, the residue was puried by column 147.07, 146.25, 143.37, 140.15, 138.50, 136.55, 133.61, 131.15,
chromatography. Compound 5 was gained as a pale yellow oil 130.80, 130.52, 128.65, 128.27, 123.04, 117.73, 116.64, 114.76,
(3.03 g, yield 81%). IR (KBr), nmax cmꢀ1: 2957, 2930, 2857 (C–H), 112.04, 111.82, 111.34, 110.85, 97.98, 97.29, 57.12, 44.58, 40.81,
1661 (–CHO), 1589, 1516 (benzene ring), 1233 (Ph–O–C). MS, m/ 30.44, 29.30, 26.47, 23.94, 23.30, 14.37, 12.85, 11.48. HRMS: for
z: 385.59 (M+). 1H NMR (400 MHz, CDCl3) d (ppm) 9.98 (–CHO, s,
1H), 7.62 (phenyl, s, 1H), 3.90–3.80 (–O–CH2–, m, 2H), 3.32–3.26
C
50H61N4O3S2, calcd: 829.41796, found: 829.41653.
Synthesis of chromophore BDT3. BDT3 was prepared from
(–N–CH2–, m, 2H), 3.25–3.19 (–N–CH2–, m, 2H), 2.00–1.89 compound 7 and TCF according to the synthesis of BDT2. BDT3
(–CH–, m, 1H), 1.76–1.65 (–NCH2–CH2–, m, 4H), 1.61–1.19 was obtained as a dark green solid (yield 48%). 1H NMR
(–CH2– and –CH3, m, 20H), 0.92 (–CH3, m, 6H).
(acetone) of the product shows two sets of similar signals,
Synthesis of compound 7. Compound 5 (3.85 g, 0.010 mol) indicating that the product is a mixture of trans and cis isomers.
was dissolved into methanol (40 mL) and stirred well, and then On the basis of the integration areas of two thienyl-H signals,
sodium borohydride (0.5 g, 0.013 mol) was added gradually at the trans isomer is estimated to be 55% and the cis isomer 45%.
0
ꢂC. Then the reactant was stirred for 24 h at ambient Mp: 103–105 C. IR (KBr), nmax cmꢀ1: 2957, 2930, 2870 (C–H),
ꢂ
temperature. Aer that, the methanol was removed and water 2228 (C^N), 1539, 1479 (aromatic ring), 1288 (Ph–O–C). 1H
was added. The aqueous solution was neutralized with dilute NMR (400 MHz, acetone) d (ppm) 8.32 (vinylic, d, J ¼ 16.1 Hz,
hydrochloric acid and extracted with dichloromethane. The 1H), 8.20 (thienyl, s, 0.45H), 8.19 (thienyl, s, 0.55H), 7.58
combined organic layer was dried over MgSO4. Dichloro- (vinylic, d, J ¼ 13.4 Hz, 0.45H), 7.51 (thienyl, s, 0.55H), 7.43
methane was removed under vacuum to get a pale yellow oil. (phenyl, s, 1H), 7.49 (thienyl, s, 0.45H), 7.37 (vinylic, d, J ¼ 16.1
The oil was dissolved in chloroform (40 mL) and triphenyl- Hz, 0.55H), 7.24 (vinylic, d, J ¼ 16.2Hz, 1H), 6.96 (vinylic, d, J ¼
phosphine hydrobromide (3.10 g, 0.009 mol) was added. The 16.0, 1H), 4.42–4.17 (–O–CH2–, m, 4H), 3.81 (–O–CH2–, dd, J ¼
reactant was reuxed for 3 h and then the chloroform was 18.6, 7.6 Hz, 2H), 3.28–3.15 (–O–CH2–, m, 4H), 1.94 (–CH3, s,
removed. The precipitates were washed several times with 6H), 1.91–1.81 (–CH–, m, 3H), 1.80–1.25 (–CH2– and –CH3, m,
diethyl ether to gain compound 6 as a white powder directly for 40H), 1.05 (–CH3, t, J ¼ 7.3 Hz, 6H), 0.97–0.93 (–CH3, m, 6H),
the next step. Compound 7 was prepared from compound 2 and
25534 | RSC Adv., 2014, 4, 25532–25539
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
0.89–0.85 (–CH3, m, 6H). HRMS: for C64H84N4O4S2, calcd:
1036.59285, found: 1036.59080.
Physical properties of the
chromophores
Thermal stability
Film preparation
Thermal stability is very important for application of the chro-
mophores in NLO devices because most processes used to
prepare NLO devices are carried out under high temperature:
the temperature should be above the glass transition tempera-
ture of the NLO polymers in the poling process; high tempera-
ture is needed to drive the solvent away in the process of lm
preparation (including cladding and coring lms); and higher
temperature would be needed in the preparation process of the
electrode. To investigate the thermal stability of the chromo-
phores, thermogravimetric analysis (TGA) was used. TGA curves
of BDT1, BDT2, and BDT3 are shown in Fig. 1 and the thermal
decomposition temperature (Td, 5% lost) values obtained from
TGA were 226, 294 and 271 ꢂC, respectively. It is easy to see that
shortening of the exible chains in the BDT unit can signi-
cantly improve the thermal stability of this kind of
For r33 measurements, guest–host polymer lms were prepared
by doping the chromophores into amorphous polycarbonate
(APC; Tg ¼ 190 ꢂC). The chromophores (20–30 mg) were mixed
with APC (100 mg) in dibromomethane (1.0–1.1 mL) as the
solvent. Stirred for 12 h, the solutions were ltered through a
0.22 mm Teon membrane lter and spin-coated onto indium
tin oxide (ITO) glass substrates at room temperature with a
spinning rate of 600 rpm to produce the lms, which were then
baked overnight at room temperature under vacuum, yielding
thin lms of optical quality and a thickness of around 3 mm. The
poling process was carried out at a temperature of about 10 ꢂC
above the Tg of the polymers.
Results and discussion
Synthesis and characterization
The synthetic approach for the new chromophores BDT2 and
BDT3 based on substituted BDT p-conjugated bridges is
depicted in Scheme 1. 4,8-Dihydrobenzo[1,2-b:4,5-b0]dithio-
phen-4,8-dione reacted with 1-bromoisooctane aer reduction
by zinc and sodium hydroxide to obtain compound 1.
Compound 2 was obtained through formylation of compound 1
with n-BuLi and DMF. 8-Hydroxy-1,1,7,7-tetramethyl-for-
myljulolidine was alkylated with 1-bromoisooctane in the
presence of potassium carbonate to obtain compound 5. Both
donors were reduced by sodium borohydride followed by a
reaction with triphenylphosphonium bromide to form Wittig
salts, which reacted with compound 2, nally followed a
Knoevenagel condensation with the TCF acceptor to obtain the
target chromophores. The structures of the chromophores were
1
conrmed by H NMR, HRMS, and UV-Vis spectroscopic anal-
ysis, and the data obtained were in full agreement with the
proposed formulations.
Fig. 1 Thermogravimetric analyses of the chromophores.
Scheme 1 Synthesis route of BDT2 and BDT3.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 25532–25539 | 25535

View Article Online
RSC Advances
Paper
chromophore to meet the higher requirements of application.
We believe that it is the overlong side chains that induce the
instability of the conjugated structure of BDT1.
Theoretical calculations
For further comparison, we have performed the quantum
chemical calculations within a framework of GAUSSIAN03 using
the split valence B3LYP 6-31G (d,p) basis set.41–43 The density
functional theory (DFT) calculations were carried out to show
the electron density of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) of
the chromophores and the b value44 of the chromophores in a
vacuum. The theoretical calculation of BDT3 was based on the
trans isomer. The data obtained from our DFT calculations are
summarized in Fig. 3 and Table 2.
In Fig. 3, it is easy to see that the electrons of the HOMO are
localized predominantly at the donor and electron-bridge, while
in the excited state those of the LUMO are localized predomi-
nantly at the acceptor and electron-bridge, which indicated that
these chromophores had good intramolecular charge-transfer
ability and that BDT was an excellent electron-bridge. The
compositions of the HOMOs and LUMOs of these chromo-
phores in the ground state were also calculated using the
Multiwfn program with Rose Schuit (SCPA) partition45 and lis-
ted in Table 2. The molecular orbital compositions of BDT1 and
BDT2 were nearly the same whereas the electron donor of BDT3
contributed a lot more to the composition of the HOMO, which
prominently showed that the julolidine moiety possesses a
stronger electron-donating ability compared to the aniline
moiety. The calculated DE (HOMO–LUMO energy gap) of the
chromophores also conrmed the results. And as we expected,
the b values of BDT1 (2079.6 ꢃ 10ꢀ30 esu) and BDT2 (2085.2 ꢃ
10ꢀ30 esu) are very close whereas that of BDT3 (2303.1 ꢃ 10ꢀ30
esu) is much larger because BDT1 and BDT2 have the same
conjugated structure while BDT3 has a stronger electron donor.
Optical properties
In order to reveal the distinctions of the electronic structures
of the chromophores, UV-Vis absorption spectra were
measured in a series of aprotic solvents with different polar-
ities so that the solvatochromic behavior of each chromo-
phore could be investigated in a wide range of dielectric
environments. The UV-Vis absorption spectra of the chro-
mophores in six different solvents are presented in Fig. 2, and
the data are summarized in Table 1.The three chromophores
in all solvents all exhibited a quite similar broad p–p* intra-
molecular charge-transfer absorption band. Also, they all
showed a bathochromic shi of its absorption maximum
initially and then reversed to a hypsochromic shi for more
polar solvents like dichloromethane, acetone and acetonitrile,
namely inverted solvatochromism. This indicated that these
three chromophores were all quite dipolar. Compared with
the spectra of BDT1 and BDT2, which were almost the same
due to their same conjugated structure, the spectra of BDT3
exhibited an obvious red-shi. The maximal absorption
wavelength (lmax) values of BDT1, BDT2, and BDT3 in chlo-
roform were 655, 659 and 672 nm, respectively. Compared to
BDT2, the lmax of BDT3 red-shied 13 nm owing to its
stronger electron donor. Dlmax (the difference in dioxane and
in chloroform) values of BDT1, BDT2 and BDT3 are 54, 58 and
83 nm, respectively. According to ref. 14, larger sol-
vatochromic effects mean a larger energy shi between
different solvents and usually implied a better electro-optic
performance.
Non-inear optical property
For application, the large hyperpolarizability has to be trans-
lated into a large EO coefficient (r33). Guest–host EO polymers
based on APC as the hosts were prepared to investigate the
macroscopic NLO property of these chromophores. The corona
poling process was carried out at a temperature about 10 ꢂC
above the Tg of the polymers. The r33 values of the poled lms
were measured using the Teng–Man simple reection tech-
nique at the wavelength of 1.31 mm.46 The r33 values were
calculated by the following equation:
3=2
3lIm
¼
4pVmIcn2
ðn2 ꢀ sin2 qÞ
1
r33
ðn2 ꢀ 2 sin qÞ sin2 q
2
where r33 is the EO coefficient of the poled polymer, l is the
optical wavelength, q is the incidence angle, Ic is the output
beam intensity, Im is the amplitude of the modulation, Vm is the
modulating voltage, and n is the refractive index of the polymer
lm.
The EO coefficients of the EO poled lms with loading
densities (chromophores into APC) of 20 and 30 wt% were
measured and are listed in Table 3. With the same loading
density, the r33 values of the poled EO lms doped with BDT2
were the largest, with BDT3 came second, and with BDT1 were
Fig. 2 UV-Vis absorption spectra of chromophores (a) BDT1, (b) BDT2
and (c) BDT3 in six different solvents (10ꢀ5 M) of varying dielectric
constants (1,4-dixoane: 2.25; toluene: 2.38; chloroform: 4.81;
dichloromethane: 8.93; acetone: 20.7; acetonitrile: 37.5) at room
temperature and (d) UV-Vis absorption spectra of chromophores in
chloroform.
25536 | RSC Adv., 2014, 4, 25532–25539
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Table 1 UV-Vis absorption and solvatochromic data of the chromophores
lmaxa/nm
lmaxb/nm
lmaxc/nm
lmaxd/nm
lmaxe/nm
lmaxf/nm
Dlmaxg/nm
BDT1
BDT2
BDT3
601
601
589
625
625
635
655
659
672
640
641
653
595
595
599
595
595
599
54
58
83
a
lmax measured in dioxane. b lmax measured in toluene. c lmax measured in chloroform. d lmax measured in dichloromethane. e lmax measured in
acetone. f lmax measured in acetonitrile. Dlmax was the difference between lmaxa and lmax
.
g
c
between the chromophores to suppress the intermolecular
dipolar–dipolar electrostatic interaction. And the shortening of
the chains on BDT probably makes the chromophore easier to
move during the poling process as well. The maximum r33 of
102 pm Vꢀ1 was obtained from 30 wt% BDT2/APC. Compared to
an analogous thiophene-based FTC-type chromophore (20–
50 pm Vꢀ1), the r33 increase was more than double and can
absolutely meet the requirement. In BDT3, the introduction of
the stronger electron donor obviously increased the hyper-
polarizability, but unexpectedly, the lms doped with BDT3
with the maximum b value did not achieve the maximum r33
value. Jen et al. once reported that the electro-optic polymer
based on poly(methyl methacrylate) and the isophorone-derived
tetraene chromophore with julolidine as the donor obtained a
low EO coefficient because the hyperpolarizability of the chro-
mophore decreased much more obviously than similar chro-
mophores based on the aniline donor in a polar medium.6 This
could also explained well why lms doped with BDT3 obtained a
lower EO coefficient, which was probably owing to the dimin-
ishment of the hyperpolarizability value of BDT3 in the polar
polymer matrix, and these results illustrate the critical role of
ne-tuning of donor's electron-donating strength in optimizing
the non-linear properties of NLO chromophores.
Fig. 3 HOMO, LUMO energy level, HOMO–LUMO energy gap (DE)
and hyperpolarizability (b) of the chromophores using DFT
calculations.
Conclusions
Table 2 Molecular orbital composition (%) in the ground state for the
chromophores
Two novel chromophores BDT2 and BDT3 were designed and
synthesized through modication of BDT1 which we reported
previously. These chromophores were all very soluble in
common solvents and were systematically characterized by MS,
NMR and UV-Vis absorption spectra. Compared with BDT1 with
overlong side chains that induce the instability of the conju-
gated structure of BDT1, the thermal stabilities of BDT2 and
BDT3 were both improved and could meet the higher require-
ments for application. Meanwhile, DFT calculations were
carried out to analyze the chromophores. Because BDT1 and
BDT2 have the same conjugated structure, their UV-Vis
absorption spectra, HOMO–LUMO gap and calculated rst-
order hyperpolarizability were nearly the same. BDT3 showed
stronger bathochromic behavior in the UV-Vis absorption
spectra, and owned a smaller energy gap and higher rst-order
hyperpolarizability in a vacuum due to its stronger electron-
donor julolidine moiety. EO polymers were prepared by the
chromophores being doped into APC and their r33 values were
BDT1
BDT2
BDT3
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
Donor
p bridge
Acceptor
56.4
39.9
3.7
2.2
50.3
47.5
56.6
39.7
3.7
2.2
50.4
47.4
65.7
31.5
2.8
2.8
50.0
47.2
Table 3 EO coefficients (pm Vꢀ1) of chromophores doped in APC with
different loading densities
Loading density
(wt%)
BDT1
BDT2
BDT3
20
30
57
66
87
102
68
82
the lowest. Compared with BDT1, the r33 of BDT2 increased over measured. The r33 values of EO polymers based on BDT2 and
50% with the alteration of the dodecane groups. That is because BDT3 were larger than the ones based on BDT1 due to the
the dendritic isooctane groups can increase the steric hindrance variation of the exible chains on the BDT unit because the
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 25532–25539 | 25537

View Article Online
RSC Advances
Paper
dendritic isooctane groups are more suitable isolation groups 13 J. Wu, S. Bo, J. Liu, T. Zhou, H. Xiao, L. Qiu, Z. Zhen and
than dodecyl groups and can suppress dipolar–dipolar electro- X. Liu, Chem. Commun., 2012, 48, 9637.
static interactions more effectively. The materials based on 14 M. He, T. M. Leslie and J. A. Sinicropi, Chem. Mater., 2002,
BDT3 with the maximum b value in a vacuum did not achieve 14, 4662–4668.
the maximum r33 value, probably due to the diminishment of 15 J. Hao, M.-J. Han, K. Guo, Y. Zhao, L. Qiu, Y. Shen and
the b value in the polar polymer matrix. The maximum r33 of X. Meng, Mater. Lett., 2008, 62, 973–976.
102 pm Vꢀ1 was obtained from 30 wt% BDT2/APC. Compared to 16 J.-M. Raimundo, P. Blanchard, P. Frere, N. Mercier,
`
an analogous thiophene-based FTC-type chromophore, the r33
increase was more than double. The substituents on the BDT
I. Ledoux-Rak, R. Hierle and J. Roncali, Tetrahedron Lett.,
2001, 42, 1507–1510.
unit signicantly inuence the properties of this type of chro- 17 Y. Liang, Y. Wu, D. Feng, S.-T. Tsai, H.-J. Son, G. Li and L. Yu,
mophore and the dendritic isooctane group is a quite suitable J. Am. Chem. Soc., 2009, 131, 56–57.
isolation group. These results indicate the importance of suit- 18 J. Hou, M.-H. Park, S. Zhang, Y. Yao, L.-M. Chen, J.-H. Li and
able isolation groups and ne-tuning of the donor's electron- Y. Yang, Macromolecules, 2008, 41, 6012–6018.
donating strength in optimizing the non-linear properties of 19 Q. Peng, X. Liu, D. Su, G. Fu, J. Xu and L. Dai, Adv. Mater.,
NLO chromophores. The BDT unit with suitable isolation 2011, 23, 4554–4558.
groups can be very helpful in chromophore design and is 20 P. Si, J. Liu, Z. Zhen, X. Liu, G. Lakshminarayana and
promising in NLO materials.
I. V. Kityk, Tetrahedron Lett., 2012, 53, 3393–3396.
21 Y. V. Pereverzev, K. N. Gunnerson, O. V. Prezhdo, P. A. Sullivan,
Y. Liao, B. C. Olbricht, A. J. P. Akelaitis, A. K. Y. Jen and
L. R. Dalton, J. Phys. Chem. C, 2008, 112, 4355–4363.
Acknowledgements
We are grateful to the Directional Program of the Chinese 22 D. M. Burland, R. D. Miller and C. A. Walsh, Chem. Rev.,
Academy of Sciences (KJCX2.YW.H02), Innovation Fund of 1994, 94, 31–75.
Chinese Academy of Sciences (CXJJ-11-M035) and National 23 T. Zhou, J. Liu, G. Deng, J. Wu, S. Bo, L. Qiu, X. Liu and
Natural Science Foundation of China (no. 11104284 and no.
61101054) for the nancial support.
Z. Zhen, Mater. Lett., 2013, 97, 117–120.
24 W. Wu, S. Xin, Z. Xu, C. Ye, J. Qin and Z. Li, Polym. Chem.,
2013, 4, 3196–3203.
25 J. Luo, H. Ma, M. Haller, A. K. Y. Jen and R. R. Barto, Chem.
Commun., 2002, 888–889.
Notes and references
1 P. A. Sullivan and L. R. Dalton, Acc. Chem. Res., 2010, 43, 10– 26 G. Deng, H. Huang, C. Peng, A. Zhang, M. Zhang, S. Bo,
18. X. Liu, Z. Zhen and L. Qiu, RSC Adv., 2014, 4, 4395–4402.
2 L. R. Dalton, P. A. Sullivan and D. H. Bale, Chem. Rev., 2010, 27 W. Wu, Z. Zhu, G. Qiu, C. Ye, J. Qin and Z. Li, J. Polym. Sci.,
110, 25–55. Part A: Polym. Chem., 2012, 50, 5124–5133.
3 L. R. Dalton, P. A. Sullivan, D. Bale, B. Olbricht, J. Davies, 28 W. Wu, C. Li, G. Yu, Y. Liu, C. Ye, J. Qin and Z. Li, Chem. –
S. Benight, I. Kosilkin, B. H. Robinson, B. E. Eichinger and
Eur. J., 2012, 18, 11019–11028.
A. K.-Y. Jen, in Organic Thin Films for Photonic Applications, 29 W. Wu, L. Huang, L. Xiao, Q. Huang, R. Tang, C. Ye, J. Qin
ed. W. N. Herman, S. R. Flom and S. H. Foulger, American
Chemical Society, 2010, vol. 1039, pp. 13–33.
4 H. Huang, G. Deng, J. Liu, J. Wu, P. Si, H. Xu, S. Bo, L. Qiu,
Z. Zhen and X. Liu, Dyes Pigm., 2013, 99, 753–758.
5 W. Wu, G. Yu, Y. Liu, C. Ye, J. Qin and Z. Li, Chem. – Eur. J.,
2013, 19, 630–641.
6 X.-H. Zhou, J. Luo, J. A. Davies, S. Huang and A. K. Y. Jen,
J. Mater. Chem., 2012, 22, 16390.
7 C. Coluccini, A. K. Sharma, M. Caricato, A. Sironi, E. Cariati,
S. Righetto, E. Tordin, C. Botta, A. Forni and D. Pasini, Phys.
Chem. Chem. Phys., 2013, 15, 1666.
and Z. Li, RSC Adv., 2012, 2, 6520–6526.
30 Z. a. Li, W. Wu, C. Ye, J. Qin and Z. Li, Polym. Chem., 2010, 1,
78–81.
31 Z. a. Li, W. Wu, Q. Li, G. Yu, L. Xiao, Y. Liu, C. Ye, J. Qin and
Z. Li, Angew. Chem., Int. Ed., 2010, 49, 2763–2767.
32 Q. Zeng, Z. a. Li, Z. Li, C. Ye, J. Qin and B. Z. Tang,
Macromolecules, 2007, 40, 5634–5637.
33 P. A. Sullivan, H. Rommel, Y. Liao, B. C. Olbricht,
A. J. P. Akelaitis, K. A. Firestone, J.-W. Kang, J. Luo,
J. A. Davies, D. H. Choi, B. E. Eichinger, P. J. Reid,
A. Chen, A. K. Y. Jen, B. H. Robinson and L. R. Dalton,
J. Am. Chem. Soc., 2007, 129, 7523–7530.
8 C. Coluccini, P. Metrangolo, M. Parachini, D. Pasini,
G. Resnati and P. Righetti, J. Mater. Sci.: Mater. Electron., 34 H. Ma, B. Chen, T. Sassa, L. R. Dalton and A. K. Y. Jen, J. Am.
2008, 46, 5202–5213. Chem. Soc., 2001, 123, 986–987.
9 I. Fuks-Janczarek, R. Miedzinski, E. Gondek, P. Szlachcic and 35 Z. a. Li, Z. Li, C. a. Di, Z. Zhu, Q. Li, Q. Zeng, K. Zhang, Y. Liu,
I. V. Kityk, J. Mater. Sci.: Mater. Electron., 2008, 19, 434–441.
C. Ye and J. Qin, Macromolecules, 2006, 39, 6951–6961.
10 I. Fuks-Janczarek, I. V. Kityk, R. Miedzinski, E. Gondek, 36 H. L. Rommel and B. H. Robinson, J. Phys. Chem. C, 2007,
´
´
J. Ebothe, L. Nzoghe-Mendome and A. Danel, J. Mater. Sci.:
Mater. Electron., 2007, 18, 519–526.
111, 18765–18777.
37 J. Wu, J. Liu, T. Zhou, S. Bo, L. Qiu, Z. Zhen and X. Liu, RSC
11 M. J. Cho, D. H. Choi, P. A. Sullivan, A. J. P. Akelaitis and
L. R. Dalton, Prog. Polym. Sci., 2008, 33, 1013–1058.
12 L. Dalton and S. Benight, Polymers, 2011, 3, 1325–1351.
Adv., 2012, 2, 1416.
38 D. W. Slocum and P. L. Gierer, J. Org. Chem., 1976, 41, 3668–
3673.
25538 | RSC Adv., 2014, 4, 25532–25539
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
39 P. Beimling and G. Koßmehl, Chem. Ber., 1986, 119, 3198–
T. Vreven, K. N. Kudin and J. C. Burant, Gaussian 03
(Revision B.03), Gaussian Inc., Pittsburgh, PA, 2003.
3203.
40 M. Q. He, T. M. Leslie and J. A. Sinicropi, Chem. Mater., 2002, 43 C. Lee and R. G. Parr, Phys. Rev. A, 1990, 42, 193–200.
14, 2393–2400.
44 K. S. Thanthiriwatte and K. M. Nalin de Silva, J. Mol. Struct.:
THEOCHEM, 2002, 617, 169–175.
45 T. Lu and F. Chen, J. Comput. Chem., 2012, 33, 580–592.
41 R. M. Dickson and A. D. Becke, J. Chem. Phys., 1993, 99,
3898–3905.
42 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, 46 C. C. Teng and H. T. Man, Appl. Phys. Lett., 1990, 56,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr,
1734.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 25532–25539 | 25539