A systematic study of the structure-property relationship of a series of nonlinear optical (NLO) julolidinyl-based chromophores with a thieno[3,2-b]thiophene moiety

Article abstract of DOI:10.1039/c4tc01896f

A series of nonlinear optical (NLO) chromophores a-d bearing thieno[3,2-b]thiophene (TT) as the conjugated bridge or the lateral moiety have been synthesized and investigated. These chromophores display the same julolidinyl-based electron donor, but different electron acceptors (i.e., 2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran, TCF, or malononitrile). The solvatochromic behavior, thermal stability and electrochemical properties were evaluated to study the structure-property relationships. The solvent dependence of dipole moment (μ), static polarizability (α), hyperpolarizability (β) and bond length alternation (BLA) of all chromophores was demonstrated by density functional theory (DFT) calculations. Upon using the D-A-π-A structure, the blue-shifted phenomenon and substantially enhanced microscopic NLO properties of chromophore d were obtained. The electrooptic coefficient (r₃₃) of chromophore a (94 pm V^-1 at 1.31 μm) was four times higher than that of chromophore b (～20 pm V^-1), while the calculated hyperpolarizability (β) of chromophore b was five times larger with respect to chromophore a. All the results demonstrated that the TT unit is a highly efficient conjugated bridge, and it has some electronical and sterical effects on macroscopic electrooptic (EO) activity when it is used as the lateral moiety. Guidelines can be proposed for the design of a new series of guest-host polymers including julolidinyl-based chromophores with a TT moiety, which could be useful in organic EO device fabrication. This journal is

Full text of DOI:10.1039/c4tc01896f

Journal of
Materials Chemistry C
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PAPER
A systematic study of the structure–property
relationship of a series of nonlinear optical (NLO)
julolidinyl-based chromophores with a
thieno[3,2-b]thiophene moiety†
Cite this: J. Mater. Chem. C, 2015, 3,
370
Airui Zhang,ab Hongyan Xiao,a Shengyu Cong,ab Maolin Zhang,ab Hua Zhang,ab
a
a
a
Shuhui Bo, Qi Wang, Zhen Zhen and Xinhou Liua
*
*
A series of nonlinear optical (NLO) chromophores a–d bearing thieno[3,2-b]thiophene (TT) as the
conjugated bridge or the lateral moiety have been synthesized and investigated. These chromophores
display the same julolidinyl-based electron donor, but different electron acceptors (i.e., 2-
dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran, TCF, or malononitrile). The solvatochromic
behavior, thermal stability and electrochemical properties were evaluated to study the structure–
property relationships. The solvent dependence of dipole moment (m), static polarizability (a),
hyperpolarizability (b) and bond length alternation (BLA) of all chromophores was demonstrated by
density functional theory (DFT) calculations. Upon using the D–A–p–A structure, the blue-shifted
phenomenon and substantially enhanced microscopic NLO properties of chromophore d were obtained.
The electrooptic coefficient (r33) of chromophore a (94 pm Vꢀ1 at 1.31 mm) was four times higher than
that of chromophore b (ꢁ20 pm Vꢀ1), while the calculated hyperpolarizability (b) of chromophore b was
five times larger with respect to chromophore a. All the results demonstrated that the TT unit is a highly
efficient conjugated bridge, and it has some electronical and sterical effects on macroscopic
electrooptic (EO) activity when it is used as the lateral moiety. Guidelines can be proposed for the design
of a new series of guest–host polymers including julolidinyl-based chromophores with a TT moiety,
which could be useful in organic EO device fabrication.
Received 24th August 2014
Accepted 18th September 2014
DOI: 10.1039/c4tc01896f
www.rsc.org/MaterialsC
transfer effects14 and attaching the halogen atom24 to the
chromophores were all demonstrated to largely increase the EO
Introduction
properties of the materials. Recently, NLO chromophores with
julolidinyl groups as the stronger electron donor have been
studied and showed more excellent EO properties than chro-
mophores with the classical dialkylamine donor for EO device
applications.25–29 Among them, chromophores with a juloli-
dinyl-based donor, a thiophene-based electron bridge and a
strong acceptor (i.e., 2-dicyanomethylene-3-cyano-4-methyl-2,5-
dihydrofuran, TCF) have been developed previously by our
group which exhibit appealing NLO properties and good
stability.30,31 However, the b values of these chromophores can
still be improved. Thus, the potentially attractive microscopic
NLO properties of the julolidinyl-based chromophores are
desirable to develop. And the enhanced b values coupled with
excellent chemical and thermal stabilities are needed for prac-
tical EO applications.
Organic second-order nonlinear optical (NLO) chromophores
have been of considerable interest in ultrafast electrooptic (EO)
applications over the last few decades.1,2 The designed NLO
chromophores were required to simultaneously exhibit large
hyperpolarizability (b), strong thermal and photochemical
stability as well as good optical transparency for practical EO
device applications.3–5 To achieve better NLO properties,
signicant advances in the development of novel organic EO
materials have been made through rational molecular design6–13
to optimize the ground-state polarization of the push–pull type
chromophores by ne tuning the combination of an electron
donor (D), an electron acceptor (A) and a p-conjugated bridge
(p-bridge).14–19 The methods such as using twisted p-conjugated
systems,20–22 alkali-metal-doped effects,23 non-covalent charge
Considering the best charge transfer (CT) efficiency, a poly-
ene-type structure was introduced as the p-bridge between the
donor and the acceptor. However, chromophores with olenic
fragments always associated with poor thermal stability that
restricted their practical applications to a large extent. Thus, the
aKey Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technique Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing,
100190, PR China. E-mail: zhenzhenipc@gmail.com
bUniversity of Chinese Academy of Sciences, Beijing 100049, PR China
† Electronic supplementary information (ESI) available: Quantum mechanical
calculations, 1H and 13C NMR spectra of compounds. See DOI: 10.1039/c4tc01896f
370 | J. Mater. Chem. C, 2015, 3, 370–381
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Journal of Materials Chemistry C
replacement of a polyene-type structure by heteroaromatic
compounds with lower aromatization energies was a common
alternative to provide a good balance between NLO activity and
stability.32,33,34 Five-membered heterocyclic rings have been
successfully developed as highly efficient p-bridges for CT.32
Among them, thiophene has been widely employed in the
heterocycle-based p-bridge, and so do some more complex
structures like fused rings.35–41 The thieno[3,2-b]thiophene (TT)
unit, because of its good electron transfer ability, was mostly
introduced to provide optimized planarity and intramolecular
charge transfer (ICT) stability for organic photovoltaic applica-
tions.42,43,44 But thieno[3,2-b]thiophene (TT) as the fused ring
based p-bridge was rarely used in the NLO chromophores. And
the reported chromophores with the TT unit did not show
satised NLO properties due to the relatively weak donor and
acceptor.45,46 Moreover, thus, chromophores with the stronger
donor–acceptor and TT based bridge are expected to provide
higher NLO responses and better thermal stability.46–48
However, to the best of our knowledge, few julolidinyl-based
chromophores with a TT unit were studied, and they were
expected to have high microscopic and macroscopic NLO
properties.
According to the two-level model,49 chromophores with a
large dipole moment (m) and b value always accompanied by
long wavelength absorption, leading to the so-called nonline-
arity–transparency trade-off and resulting in a large optical loss
for EO modulator applications.26 Therefore, one crucial way to
search for the blue-shied chromophores without the loss of
nonlinearities is combining the different types of conjugated
bridges. This novel concept was proposed to overcome the
nonlinearity–transparency trade-off, but only a few studies on
this concept were reported in recent years.50,51 Consequently,
chromophore d with combined bridges was designed and
synthesized by two successive microwave methods, which were
studied by us previously. This new designed D–A–p–A structure
chromophore nely demonstrated this novel concept. In
summary, these chromophores were expected to have high NLO
properties as well as high thermal stability and to overcome the
nonlinearity–transparency trade-off.
Scheme 1 Synthesis scheme for chromophores a and b.
In this work, we focus our attention on the synthesis and
characterization of six new chromophores a–f (with a TT unit)
(see Schemes 1 and 2) bearing different p-bridges and acceptors
and a xed julolidinyl donor. The solvatochromism behaviour
and thermal stability of all chromophores were also discussed
in detail. Moreover, DFT calculations were carried out to
demonstrate the solvent dependence of the static polarizability
(a), b, m, electron density, bond length alternation (BLA) and the
energy gap (DE).26 Through the theoretical calculation, the b
value of chromophores, which contained the julolidinyl-based
donor, TCF acceptor and TT-based p-bridge, was much larger
than that of chromophores with a thiophene-based p-bridge. In
this system, chromophores a–d were designed for the rst time
to discuss the effect of the TT unit as the p-bridge or the lateral
moiety on the molecular polarization level and NLO responses.
Scheme 2 Synthesis of chromophores c, d, e and f.
amorphous polycarbonate (APC). The r33 values of poled lm a/
APC (94 pm Vꢀ1 at 1.31 mm) were almost four times larger with
respect to b/APC (ꢁ20 pm Vꢀ1 at 1.31 mm).52 The experimental
measurements and theoretical calculations described herein
helped us to recognize the electronic and geometric structures
of these chromophores.
Experimental section
General methods
Chromophores e and f were synthesized without a TT unit for All reagents were purchased from commercial sources and were
comparison. Finally, the macroscopic EO activity (r33 value) of used as received unless otherwise specied. Solvents such as
chromophores a and b was studied by doping them into the N,N-dimethylformamide (DMF) and tetrahydrofuran (THF)
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were freshly treated and distilled over calcium hydride and electrode with low reectivity and good transparency in order to
sodium, respectively, and stored under nitrogen prior to use. minimize the contribution from multiple reections.55
Phosphorus oxychloride (POCl3) was distilled before use. The
General procedure for synthesis of compound 1. Under a N2
dielectric constants of six different solvents are 1,4-dixoane: atmosphere to a solution of 8-hydroxy-1,1,7,7-tetramethyl-for-
2.25, toluene: 2.38, chloroform: 4.81, dichloromethane: 8.93, myljulolidine (2.73 g, 10 mmol) in anhydrous DMF were added
acetone: 20.7, and acetonitrile: 37.5, respectively. The thieno 1-chloroheptane (2.02 g, 15 mmol) and K2CO3 (2.08 g, 15 mmol).
[3,2-b]thiophene electron bridge and the 2-dicyanomethylen-3- The mixture was irradiated under microwaves (140 ꢂC) for 35
cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) acceptor were min and then poured into water until it was cooled to ambient
prepared according to the literature procedures,53,54 respec- temperature. The resulting solutions were ltered and then
tively. UV-Vis-NIR spectra were obtained using a HITACHI U- extracted with ethyl acetate. The combined organic extracts
2001 spectrometer. The TGA curve was recorded with a TA- were washed with brine and then dried over MgSO4 and ltered.
instrument Q50 analyzer at a heating rate of 10 ꢂC minꢀ1 under Aer the removal of the solvent, the residue was puried by
a nitrogen atmosphere. Differential scanning calorimetry (DSC) column chromatography using petroleum ether and ethyl
measurements were performed on a TA5000, 2910MDSC with a acetate (40 : 1, v/v) as an eluent to give the product as an yellow
ꢀ1
heating rate of 10 C min under the protection of nitrogen. solid (2.96 g, yield: 83%). 1H NMR (400 MHz, CDCl3, d ppm) 9.96
The MS spectra were obtained on a MALDI-TOF (Matrix Assisted (s, 1H), 7.59 (s, 1H), 3.96 (t, J ¼ 6.9 Hz, 2H), 3.29 (t, 2H), 3.23 (t,
Laser Desorption/Ionization of Flight) on a BIFLEXIII (Broker 2H), 1.93–1.82 (m, 2H), 1.76–1.66 (m, 4H), 1.46 (dd, J ¼ 8.6, 4.1
Inc.) spectrometer and on an EI (Electron Impact Source) with Hz, 2H), 1.43 (s, 6H), 1.38–1.31 (m, 4H), 1.26 (s, 6H), 0.91 (t, J ¼
an ApexIII (7.0 tesla) FTICR mass spectrometer (Bruker). Cyclic 7.0 Hz, 3H). 13C NMR (101 MHz, CDCl3, ppm) d 187.75, 162.12,
voltammetry (CV) experiments were performed on a Princeton 148.20, 126.06, 125.29, 120.71, 117.26, 78.88, 47.46, 46.82,
Applied Research Model 283 Potentiostat/Galvanostat. A three- 39.41, 35.70, 32.44, 32.04, 31.74, 30.25, 30.01, 29.73, 25.63,
electrode-system is used for the measurement and bulk elec- 22.56, 14.01. MALDI-TOF-MS (m/z): calcd: 358.0; found: 359.2.
ꢂ
trolysis, with a 3 mm glass carbon working electrode, a plat-
General procedure for synthesis of compound 2. A solution
inum sheet counter electrode, and a non-aqueous Ag/AgNO3 of thieno[3,2-b]thioꢂphene (1 g, 7.13 mmol) in dry THF (30 mL)
reference electrode for an organic solution reference electrode was cooled to ꢀ78 C and was maintained at this temperature
for aqueous solution. The electrolyte solution (0.1 M of n- under a N2 atmosphere during the dropwise addition of n-BuLi
Bu4NPF6 in CH3CN for organic solution) was degassed by in hexane (2.5 M, 14.26 mmol). Aer the mixture was stirred at
ꢂ
ꢂ
bubbling with argon for 40 min before measurement. The ꢀ78 C for 1 h, it was warmed to ꢀ10 C and held for 30 min.
working electrode was polished with 0.05 mm alumina paste and The solution was cooled back to ꢀ78 ꢂC again, and kept at this
sonicated for 15 min before use. 1H NMR and 13C NMR spectra temperature during the dropwise addition of dry DMF (10 mL).
were recorded on an AVANCE 400 (Bruker) spectrometer (400 The reaction mixture was warmed to ambient temperature, and
MHz) using tetramethylsilane (TMS; d ¼ 0 ppm) as the internal then quenched by adding 10 mL of water. The residue was
standard, respectively.
extracted with ethyl acetate (3 ꢃ 50 mL). The combined organic
The DFT quantum chemical methods using the Gaussian 09 layer was washed sequentially with brine and DI water, and then
(ref. 58) soware package in the gas phase and six different60 dried over MgSO4, ltered, concentrated and puried by ash
solvents were carried out at the B3LYP/6-31G level. Geometry chromatography on silica gel using petroleum ether and ethyl
structures of each chromophore were optimized and resulted in acetate (80 : 3, v/v) to give compound 2 as an orange solid (1.02
a three dimensional D–p–A arrangement. The calculated g, 85%). 1H NMR (400 MHz, CDCl3, d ppm) 9.96 (s, 1H), 7.93 (s,
parameters include electron density, m, a, b, BLA and DE.
1H), 7.69 (d, J ¼ 5.3 Hz, 1H), 7.32 (d, J ¼ 5.3 Hz, 1H). 13C NMR
(101 MHz, CDCl3, d ppm) 184.40, 146.56, 146.29, 140.05, 134.74,
129.92, 121.00. EI-MS (m/z): calcd: 167.9; found: 168.8.
General procedure for synthesis of compound 3. To a solu-
tion of compound 2 (1.68 g, 10 mmol) in methanol (20 mL) was
Electric eld poling and EO property measurements
To study the poling and EO activity derived from the chromo- added NaBH4 (0.46 g, 12 mmol) in batches at 0 ꢂC in an ice bath.
phores, guest–host polymers were generated by formulating Aer the temperature warmed up to room temperature and the
chromophores a (20 wt%) and b (10 wt%) into amorphous solution became clear, diluted hydrochloric acid was added to
polycarbonate (APC) using dibromomethane (CH2Br2) as the neutralize the reaction and then kept stirring for 2 h. The
solvent. The resulting solutions were ltered through a 0.2 mm solution was extracted with ethyl acetate; the combined organic
PTFE lter, and then spin-coated onto indium tin oxide (ITO) extracts were washed with brine and dried over MgSO4. Aer
glass substrates (from thin lm devices). Films of doped EO removal of the solvent, the residue was directly dried in the
polymers were baked in a vacuum oven overnight to ensure the vacuum oven for the next step without ash chromatography.
removal of the residual solvent. Corona poling was used to pole The product was a white solid (0.88 g, yield: 75%). 1H NMR (400
the lms. The poling temperature was 135 ꢂC for a/APC and MHz, acetone, d ppm) 7.36 (d, J ¼ 5.2 Hz, 1H), 7.19 (d, J ¼ 5.2 Hz,
145 ꢂC for b/APC. Then, a thin layer of aluminium (50–100 nm) 1H), 7.12 (s, 1H), 4.70 (d, J ¼ 5.9 Hz, 2H), 1.91 (dt, J ¼ 4.3, 2.2 Hz,
was sputtered onto the lms as the top electrode. The r33 values 1H). 13C NMR (101 MHz, acetone, d ppm) 149.65, 139.62, 139.40,
were measured by the Teng–Man simple reection technique at 127.49, 120.55, 117.40, 60.70. MALDI-TOF-MS (m/z): calcd:
the wavelength of 1.31 mm using a carefully selected thin ITO 170.0; found: 171.0.
372 | J. Mater. Chem. C, 2015, 3, 370–381
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General procedure for synthesis of compound 4. Into about
Journal of Materials Chemistry C
38.62, 34.80, 31.50, 30.78, 30.71, 29.15, 28.91, 28.88, 24.84,
21.67, 13.03. MALDI-TOF-MS (m/z): calcd: 521.2; found: 522.2.
Compound 7: H NMR (400 MHz, CDCl3, d ppm) 9.83 (s, 1H),
20 mL of CHCl3 was dissolved 0.5 g (2.94 mmol) of compound 3.
To this solution was added triphenylphosphine hydrobromide
1.14 g (3.32 mmol). The mixture was reuxed for 3 h. The
solvent was evaporated under reduced pressure. The product
was collected and washed with anhydrous ethyl ether, and used
without further purication. The yield was 1.4 g (97% yield). The
molecular formula without bromine was tested since the
bromine is not observed. MALDI-TOF-MS (m/z): calcd: 414.1;
found: 415.1.
1
9.51 (s, 1H), 7.81 (s, 1H), 7.57 (s, 1H), 7.29 (s, 1H), 6.95 (s, 1H),
3.86 (t, 2H), 3.10 (t, 4H), 1.81 (s, 2H), 1.62 (m, 2H), 1.46 (m, 2H),
1.33 (s, 12H), 1.16 (m, 2H), 0.82 (m, 2H), 0.70 (s, 3H). 13C NMR
(101 MHz, CDCl3, d ppm) 190.31, 182.37, 158.55, 149.38, 145.46,
144.64, 144.01, 143.66, 138.95, 128.16, 126.04, 125.80, 124.78,
120.53, 119.33, 112.41, 75.33, 46.33, 45.74, 38.43, 34.53, 31.46,
30.74, 30.68, 29.12, 28.89, 28.78, 24.81, 21.56, 13.05. MALDI-
TOF-MS (m/z): calcd: 549.2; found: 550.2.
General procedure for synthesis of compound 5. To a solu-
tion of compound 1 (1.91 g, 5.34 mmol) and compound 4 (2.64
g, 5.34 mmol) in dry THF (20 mL) was added NaH (2.40 g, 100
mmol). The solution was allowed to stir at room temperature for
24 h and then poured into water. The residue was extracted with
ethyl acetate (3 ꢃ 50 mL). The combined organic layer was
washed sequentially with brine and DI water, and then dried
over MgSO4, ltered, concentrated and puried by ash chro-
matography on silica gel using petroleum ether and ethyl
acetate (100 : 1, v/v) to give compound 5 as an orange oil (1.32 g,
50%). 1H NMR (400 MHz, CDCl3, d ppm) 7.18 (dd, J ¼ 11.9, 6.7
Hz, 1H), 7.13–7.07 (m, 1H), 7.05 (s, 1H), 7.01 (s, 1H), 6.52 (s, 1H),
3.91 (t, J ¼ 6.5 Hz, 1H), 3.86 (t, J ¼ 6.5 Hz, 2H), 3.13–3.07 (m, 2H),
3.06–2.99 (m, 2H), 1.93–1.83 (m, 2H), 1.78–1.66 (m, 4H), 1.60–
1.50 (m, 2H), 1.43 (s, 6H), 1.37 (m, 4H), 1.28 (s, 6H), 1.14 (s, 2H),
0.94–0.81 (m, 3H). 13C NMR (101 MHz, CDCl3, d ppm) 154.99,
146.19, 141.89, 138.51, 135.71, 132.72, 132.52, 127.58, 127.41,
127.35, 125.46, 125.15, 124.76, 121.48, 118.47, 116.70, 115.24,
72.32, 46.33, 45.85, 39.39, 35.83, 31.66, 31.21, 30.85, 30.42,
29.34, 29.11, 24.95, 21.59, 12.92. MALDI-TOF-MS (m/z): calcd:
493.2; found: 494.2.
General procedure for synthesis of compound 8. A solution
of compound 5 (3.95 g, 8 mmol) in dry THF (50 mL) was cooled
to ꢀ78 C and was maintained at this temperature under a N2
ꢂ
atmosphere during the dropwise addition of n-BuLi in hexane
(2.5 M, 16 mmol). Aer the mixture was stirred at ꢀ78 ꢂC for 1 h,
it was warmed to ꢀ10 ꢂC and held for 30 min. The solution was
cooled back to ꢀ78 ꢂC again, and kept at this temperature
during the dropwise addition of dry DMF (12 mL). The reaction
mixture was warmed to ambient temperature, and then
quenched by adding 10 mL of water. The residue was extracted
with ethyl acetate (3 ꢃ 70 mL). The combined organic layer was
washed sequentially with brine and DI water, and then dried
over MgSO4, ltered, concentrated and puried by ash chro-
matography on silica gel using petroleum ether and ethyl
acetate (60 : 1, v/v) to give compound 8 as an orange red solid
(2.79 g, 67%). 1H NMR (400 MHz, acetone, d ppm) 9.95 (s, 1H),
8.16 (s, 1H), 7.45 (s, 1H), 7.38 (s, 1H), 7.28 (d, J ¼ 16.1 Hz, 1H),
7.20 (d, J ¼ 16.1 Hz, 1H), 3.84 (t, J ¼ 6.6 Hz, 2H), 3.21 (t, 2H), 3.15
(t, 2H), 1.95–1.85 (m, 2H), 1.72 (t, 4H), 1.64–1.53 (m, 2H), 1.41 (s,
6H), 1.40–1.38 (m, 2H), 1.28 (s, 6H), 0.90 (s, 3H). 13C NMR (101
MHz, acetone, d ppm) 183.99, 157.63, 154.82, 147.42, 145.09,
137.61, 131.00, 129.33, 127.83, 123.78, 123.14, 118.02, 76.26,
48.05, 47.54, 41.26, 37.68, 33.56, 33.14, 32.81, 31.74, 31.07,
30.90, 26.99, 23.51, 14.53. MALDI-TOF-MS (m/z): calcd: 521.2;
found: 522.2.
General procedure for synthesis of compounds 6 and 7. A
solution of 20 mL DMF was cooled to 0 ꢂC and was maintained
at this temperature during the dropwise addition of freshly
distilled phosphorus oxychloride (POCl3, 0.64 g, 4.2 mmol). The
ꢂ
solution was kept stirring for 2 h at 0 C and this temperature
General procedure for synthesis of chromophore a. A
mixture of compound 6 (2.61 g, 5 mmol) and acceptor 9 (1.09 g,
5.5 mmol) in 20 mL of ethanol was irradiated under microwaves
(95 C) for 15 min. The resulting mixture removed the solvent
and puried through column chromatography using petroleum
was maintained during the dropwise addition of compound 5 in
10 mL DMF (1.0 g, 2 mmol). The solution was gradually warmed
to room temperature and then heated to 90 ꢂC for 3 h. The
solution of Na2CO3 (10%, 150 mL) was added to quench the
reaction which was then allowed to cool down to room
temperature. The reaction mixture was extracted with ethyl
acetate (3 ꢃ 50 mL), washed with brine, dried over MgSO4 and
ltered. Aer removal of the solvent under reduced pressure,
the crude product was puried by ash chromatography on
silica gel using petroleum ether and ethyl acetate (400 : 7, v/v) to
give compound 6 as an orange solid (0.57 g, 55%) and to give
compound 7 as an red solid (0.31 g, 28%). Compound 6: 1H
NMR (400 MHz, CDCl3 d ppm) 9.63 (s, 1H), 7.62 (s, 1H), 7.32 (d, J
¼ 5.2 Hz, 1H), 7.28 (s, 1H), 7.21 (d, J ¼ 5.1 Hz, 1H), 7.04 (s, 1H),
3.94 (t, J ¼ 6.7 Hz, 2H), 3.21–3.13 (m, 4H), 1.95–1.83 (m, 2H),
1.77–1.68 (m, 4H), 1.60–1.49 (m, 6H), 1.42 (s, 9H), 1.40–1.35 (m,
ꢂ
ether and ethyl acetate (6 : 1, v/v) as an eluent to give the
1
product as dark green solids (2.63 g, yield: 75%). H NMR (400
MHz, CDCl3, d ppm) 8.06 (d, J ¼ 15.0 Hz, 1H), 7.51 (s, 1H), 7.44
(d, J ¼ 5.2 Hz, 1H), 7.30 (d, J ¼ 5.2 Hz, 1H), 7.13 (s, 1H), 6.60 (s,
1H), 6.09 (d, J ¼ 15.0 Hz, 1H), 3.91 (t, J ¼ 6.5 Hz, 2H), 3.24 (s,
4H), 2.00–1.87 (m, 2H), 1.78–1.66 (m, 4H), 1.61 (s, 9H), 1.55–1.47
(m, 2H), 1.40 (s, 12H), 1.26 (s, 2H), 1.04–0.86 (m, 4H), 0.64 (s,
6H), 0.07 (s, 3H). 13C NMR (101 MHz, CDCl3, d ppm) 175.68,
172.06, 159.77, 153.00, 145.75, 139.43, 138.75, 138.03, 129.90,
127.82, 126.60, 126.30, 125.69, 124.96, 120.83, 119.06, 118.60,
114.79, 109.77, 95.57, 76.81, 64.54, 46.66, 46.02, 38.13, 31.44,
30.73, 30.55, 29.06, 28.52, 28.04, 25.40, 13.08. MALDI-TOF-MS
(m/z): calcd: 702.2; found: 703.2.
6H), 1.26 (s, 4H), 0.95–0.89 (m, 4H), 0.77 (s, 6H), 0.07 (s, 3H). 13
C
NMR (101 MHz, CDCl3, d ppm) 191.27, 158.30, 148.56, 144.94,
139.24, 138.21, 136.49, 132.80, 132.61, 127.43, 126.06, 125.78,
124.65, 120.43, 118.90, 118.38, 113.06, 74.86, 46.35, 45.75,
General procedure for synthesis of chromophore b. A
mixture of compound 8 (2.61 g, 5 mmol) and acceptor 9 (1.09 g,
5.5 mmol) was dissolved in ethanol (2 mL) and chloroform (18
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mL). To this solution was added 2 drops of triethylamine and 25.28, 22.67, 14.08. MALDI-TOF-MS (m/z): calcd: 689.2; found:
then was reuxed at 90 ꢂC for 3–5 h and monitored by TLC. Aer 689.8.
removal of the solvents, the residue was puried by column
General procedure for synthesis of chromophore e. A
chromatography eluting with petroleum ether and ethyl acetate mixture of compound 1 (1.79 g, 5 mmol), compound 4 (iso-
(10 : 1, v/v) to give a dark solid (2.45 g, 70%). 1H NMR (400 MHz, phorone, 0.83 g, 6 mmol), and sodium ethoxide (7 mmol, 0.17 g
CDCl3, d ppm) 7.91 (d, J ¼ 15.6 Hz, 1H), 7.56 (s, 1H), 7.30 (s, 1H), sodium dissolved in 2 mL anhydrous ethanol) in 10 mL anhy-
ꢂ
7.13 (s, 1H), 7.03 (d, J ¼ 15.9 Hz, 1H), 6.58 (d, J ¼ 15.6 Hz, 1H), drous ethanol was irradiated under microwaves (95 C) for 30
3.85 (t, J ¼ 6.7 Hz, 2H), 3.24 (t, J ¼ 5.8 Hz, 2H), 3.17 (t, 2H), 1.89 min. Then the reaction nished and cooled to room tempera-
(m, 2H), 1.77 (m, 2H), 1.76 (s, 6H), 1.57 (m, 4H), 1.44 (s, 6H), ture. Aer the solvent was evaporated, the residue and malo-
1.39 (s, 4H), 1.32 (s, 6H), 0.92 (t, J ¼ 7.0 Hz, 3H). 13C NMR (101 nonitrile (0.46 g, 7 mmol) dissolved in 10 mL anhydrous ethanol
MHz, acetone, d ppm) 182.97, 156.59, 153.78, 146.30, 144.30, were irradiated under microwaves (95 ꢂC) for 20 min. Then the
143.82, 136.63, 129.97, 128.29, 126.79, 122.74, 122.10, 117.11, reaction nished and cooled to room temperature. Aer the
116.85, 75.22, 47.01, 46.50, 40.22, 36.64, 32.52, 32.10, 31.78, solvent was evaporated, the residue was puried by column
30.71, 30.03, 29.87, 25.96, 22.48, 13.50. MALDI-TOF-MS (m/z): chromatography using petroleum ether and ethyl acetate
calcd: 702.2; found: 703.1.
(80 : 3, v/v) as an eluent to give the product as dark purple solids
1
General procedure for synthesis of chromophore c. A (1.99 g, yield: 76%). H NMR (400 MHz, CDCl3, d ppm) 7.35 (s,
mixture of compound 8 (2.61 g, 5 mmol) and malononitrile 1H), 7.29 (d, J ¼ 16.0 Hz, 1H), 6.78 (d, J ¼ 15.7 Hz, 1H), 6.77 (s,
(0.46 g, 7 mmol) was dissolved in 15 mL chloroform. To this 1H), 3.79 (t, J ¼ 6.7 Hz, 2H), 3.26 (t, 2H), 3.19 (t, 2H), 2.58 (s, 2H),
solution was added 2 drops of triethylamine and was stirred at 2.46 (s, 2H), 1.92–1.79 (m, 2H), 1.79–1.66 (m, 4H), 1.56–1.47 (m,
room temperature for 15 min. Aer removal of the solvents, the 2H), 1.42 (s, 6H), 1.39–1.34 (m, 4H), 1.31 (s, 6H), 1.06 (s, 6H),
residue was puried by column chromatography eluting with 0.92 (t, 3H). 13C NMR (101 MHz, CDCl3, d ppm) 169.07, 157.96,
petroleum ether and ethyl acetate (20 : 1, v/v) to give a dark solid 156.09, 145.02, 134.84, 127.33, 123.48, 123.05, 121.03, 114.68,
(1.28 g, 45%). 1H NMR (400 MHz, CDCl3, d ppm) 7.77 (d, J ¼ 19.1 113.97, 76.45, 47.62, 47.05, 43.09, 39.82, 39.50, 36.23, 32.36,
Hz, 2H), 7.27 (d, J ¼ 12.5 Hz, 2H), 7.12 (s, 1H), 7.02 (d, J ¼ 15.9 32.38, 32.09, 32.03, 30.87, 30.31, 30.09, 29.81, 28.21, 26.17,
Hz, 1H), 3.84 (t, J ¼ 6.6 Hz, 2H), 3.23 (t, 2H), 3.16 (t, 2H), 1.89 (m, 22.76, 14.19. MALDI-TOF-MS (m/z): calcd: 525.4; found: 526.4.
2H), 1.75 (m, 4H), 1.54 (m, 2H), 1.43 (s, 6H), 1.39 (s, 4H), 1.31 (s,
General procedure for synthesis of chromophore f. A mixture
6H), 0.91 (s, 3H). 13C NMR (101 MHz, CDCl3, d ppm) 175.56, of compound 1 (1.78 g, 5 mmol) and acceptor 9 (1.09 g, 5.5
172.08, 159.75, 152.89, 145.51, 139.52, 138.84, 138.22, 129.84, mmol) in 20 mL of ethanol was irradiated under microwaves
ꢂ
127.87, 126.58, 126.36, 125.76, 119.13, 118.61, 114.99, 111.89, (95 C) for 15 min. The resulting mixture removed the solvent
110.95, 110.88, 95.56, 76.79, 64.53, 46.73, 46.10, 38.31, 31.53, and puried through column chromatography using petroleum
30.75, 29.11, 28.62, 28.18, 25.38, 21.56, 13.02. MALDI-TOF-MS ether and ethyl acetate (10 : 1, v/v) as an eluent to give the
1
(m/z): calcd: 569.2; found: 570.1.
product as dark green solids (2.02 g, yield: 75%). H NMR (400
General procedure for synthesis of chromophore d. A MHz, CDCl3, d ppm) 7.73 (d, J ¼ 15.6 Hz, 1H), 7.42 (s, 1H), 6.73
mixture of compound 8 (2.61 g, 5 mmol), compound 4 (iso- (d, J ¼ 15.8 Hz, 1H), 3.79 (t, J ¼ 7.3 Hz, 2H), 3.42 (t, J ¼ 6.2 Hz,
phorone, 0.83 g, 6 mmol), and sodium ethoxide (7 mmol, 0.17 g 2H), 3.35 (t, 2H), 1.97–1.87 (m, 2H), 1.76 (s, 6H), 1.75–1.72 (m,
sodium dissolved in 2 mL anhydrous ethanol) in 10 mL anhy- 2H), 1.43 (s, 6H), 1.34 (dd, J ¼ 7.3, 3.4 Hz, 4H), 1.30 (s, 6H), 0.91
drous ethanol was irradiated under microwaves (95 C) for 30 (t, 3H). 13C NMR (101 MHz, CDCl3, d ppm) 174.07, 160.68,
ꢂ
min. Then the reaction was completed and cooled to room 149.17, 143.87, 130.91, 128.85, 128.32, 124.39, 122.44, 115.50,
temperature. Aer the solvent was evaporated, the residue and 113.35, 112.54, 111.90, 107.18, 96.06, 71.79, 65.55, 47.94, 47.28,
malononitrile (0.46 g, 7 mmol) dissolved in 10ꢂmL anhydrous 38.88, 35.12, 32.50, 32.09, 31.75, 29.72, 29.31, 27.12, 25.62,
ethanol were irradiated under microwaves (95 C) for 20 min. 22.57, 13.99. MALDI-TOF-MS (m/z): calcd: 538.3; found: 539.1.
Then the reaction was completed and cooled to room temper-
ature. Aer the solvent was evaporated, the residue was puried
by column chromatography using petroleum ether and ethyl
Results and discussion
acetate (10 : 1, v/v) as an eluent to give the product as black Chromophores a and b were synthesized as shown in Scheme 1.
1
solids (2.41 g, yield: 70%). H NMR (400 MHz, CDCl3, d ppm) Compound 2 with one aldehyde group was obtained through
7.27 (s, 1H), 7.25 (s, 1H), 7.21 (d, J ¼ 15.7 Hz, 1H), 7.13 (d, J ¼ two methods of the formylated reaction using n-butyllithium
15.9 Hz, 1H), 7.07 (s, 1H), 6.99 (d, J ¼ 15.9 Hz, 1H), 6.80 (s, 1H), (n-BuLi) and DMF, and the Vilsmeier reaction with phosphorus
6.72 (d, J ¼ 15.6 Hz, 1H), 6.62 (s, 1H), 3.85 (t, J ¼ 6.7 Hz, 2H), oxychloride (POCl3) and DMF. Both methods can introduce only
3.19 (t, 2H), 3.12 (t, 2H), 2.59 (s, 2H), 2.43 (s, 2H), 1.92–1.82 (m, one aldehyde group by one step approach with high yields
2H), 1.75 (m, 4H), 1.55 (s, 4H), 1.43 (s, 6H), 1.38 (m, 4H), 1.31 (s, (>90%), when n-BuLi, POCl3 and DMF were in excess.
6H), 1.08 (s, 6H), 0.91 (t, J ¼ 7.0 Hz, 3H). 13C NMR (101 MHz, Compound 2 was then reduced to alcohol 3 and transferred to
CDCl3, d ppm) 170.32, 168.65, 159.71, 156.39, 153.50, 150.58, Wittig salt 4. Aerwards, compound 1 reacted with Wittig salt 4
142.52, 141.91, 137.56, 130.90, 130.71, 128.85, 127.41, 122.85, following Wittig reaction to obtain compound 5. Through a
122.60, 120.59, 116.50, 113.17, 113.07, 112.39, 78.28, 75.34, Vilsmeier reaction, the TT unit in compound 6 was also found
47.48, 46.98, 45.69, 42.94, 42.64, 39.18, 36.70, 32.75, 32.35, as a dual-function structure and provided steric hindrance and
32.00, 31.91, 31.30, 30.28, 30.17, 29.70, 28.03, 27.81, 26.02, electronic effects.31 The formation of the unexpected compound
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Table 1 Thermal properties of chromophores a–f
7 with two aldehyde groups was resulted from the excess POCl3
and DMF. Due to the slight electron-withdrawing properties of
the aldehyde group, it was hard for compound 7 to condense
with two acceptors simultaneously with considerable yield.
Compound 8 was obtained by deprotonation of compound 5
with n-BuLi followed by reaction with DMF. Finally, compounds
6 and 8 were coupled with acceptor 9 to obtain target chromo-
phores a and b, respectively.
Compd
Tg/Tm (ꢂC)
Td (ꢂC)
a
b
c
d
e
f
205
125
217
75
166
203
238
276
174
130
296
164
To study the inuence of acceptor strength and the
combined bridge effect on the nonlinearity, chromophores c
and d were designed and shown in Scheme 2. Noticeably,
chromophore d was synthesized by two successive microwave
(MW)-assisted condensation methods introduced by us previ-
ously, which demonstrated that MW-assisted heating was a
highly efficient method in the synthesis of NLO chromophores.
All the chromophores exhibited good solubility in common
organic solvents, such as acetone, DMSO, CH2Cl2, DMF, CHCl3,
and THF. The structures of the compounds were characterized
agreement with Tg values. As a result, chromophores a, b and e
dispꢂlayed the sufficient thermal stabilities (Td is higher than
200 C) for applications in device fabrication.
UV-Vis-NIR spectroscopy
The solvatochromic behavior of chromophores is strongly
inuenced by the strength of the donor and acceptor in
combination with the p-bridge. To reveal the solvent depen-
dence effect of the p-bridge and acceptor on the electronic
structures of chromophores a–f, the UV-Vis-NIR absorption
spectra were recorded in six solvents with different dielectric
constants (Fig. 2) and the corresponding optical absorption
data are recorded in Table 2. All the chromophores showed
intense low-energy ICT absorption bands except for chromo-
phore d. Distinctively, chromophore d displayed a weak low-
energy ICT absorption band and an intense and narrow high-
energy ICT band, which was assigned to the p–p* transition of
the aromatic moiety. It was presumed that the resonant
1
unambiguously by H NMR, 13C NMR and MALDI-TOF-MS.
Thermal stability
The thermal properties of chromophores a–f were evaluated by
differential scanning calorimetry (DSC) and thermal gravimetric
analysis (TGA) in nitrogen with the results shown and tabulated
in Fig. 1 and Table 1. All chromophores except chromophore b
displayed crystalline characteristics with a melting point (Tm).
Obviously, chromophore b was obtained as a thermodynami-
cally stable amorphous glass state solid showing a glass tran-
sition temperature (Tg) at around 125 ꢂC. The amorphous state
of chromophore b was mainly attributed to both the longer p-
bridge and the stronger TCF acceptor, which disfavoured the
close packing of molecules. The strong Tm peak of chromophore
d showed that the combined bridge structure could promote
chromophore molecule crystallization.
The TGA results did not follow the same trends as DSC
results. Chromophore e showed the highest decomposition
temperature (Td), while chromophore d showed the lowest Td,
which might be due to the ‘mismatched’ combination of TT and
isophorone units.33 Chromophore b displayed higher Td than a,
suggesting that chromophores with a TT unit as the p-bridge
can increase the thermal stability compared with the reported
polyene-type chromophore (212 ꢂC).4 By comparison of chro-
mophores b and c, it can be concluded that the TCF acceptor
helped to increase the thermal stability, which was in
Fig. 1 The DSC and TGA curves of chromophores a–f with a heating Fig. 2 Solvatochromic behavior of chromophores a–f recorded in six
rate of 10 ꢂC minꢀ1 in nitrogen.
solvents (10ꢀ5 M) of varying dielectric constants at room temperature.
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Table 2 Summary of low-energy optical absorption dataa
Absorption lmax (nm)
compounds. Thus, the oxidation and reduction potentials
tested from electrochemical cyclic voltammetry (CV) measure-
ments can provide important information for the HOMO–
LUMO gap (DE). All the chromophores exhibited a single
reversible or a quasi-reversible oxidative wave in positive energy
and a single reversible reductive peak in negative energy
(Fig. S1, ESI†). Table 2 shows that the HOMOs were similar for
chromophores b and c because of the same donor part, while
the LUMOs systematically lowered with an increase of the
acceptor strength, leading to the decreased DE. Moreover, due
to the increased length of the p-bridge, chromophore b showed
the smallest DE (0.85 eV) and the chromophore f showed the
largest DE (1.44 eV). The observed electrochemical behavior was
in accord with the calculated orbital energy levels and ICT
process of HOMO and LUMO for the chromophores studied in
the following part.
As shown in Table 3, DE values of all the chromophores were
also calculated through the UV-Vis-NIR absorption spectra by
doping them into the APC polymer lms. The DE values of
spectral analysis and CV measurements followed the same
trend, and the former were larger with respect to the latter. This
can be attributed to the fact that the spectral results were tested
in polymer lms while the CV results were tested in acetonitrile
solutions.
Cmpd
DO
TL
CF
DC
AC
AN
Dlmax
a
b
c
d
e
f
628
640
546
546
525
593
668
666
560
565
532
598
700
712
591
585
562
622
703
696
583
578
558
627
688
644
557
547
549
624
693
643
555
539
547
628
75
72
45
46
37
35
a
DO: dixoane; TL: toluene; CF: CHCl3; DC: CH2Cl2; AC: acetone; AN:
acetonitrile.
structures of the combination of the TT unit and isophorone
unit disfavored the ICT from the electron donor to the
acceptor.33 In combination with the poor thermal stability, we
called this the ‘mismatched’ phenomenon. With the increase of
solvent polarity, all the chromophores red-shied initially,
reaching the maximum value in chloroform or dichloro-
methane, and then, chromophores a, e and f exhibited the
saturation behaviors. The cyanine-like characteristic (the
neutral and charge-separated limiting resonance forms
contributed equally to the ground state) band shape of them
indicated that the two limiting resonance forms have approxi-
mately the same contribution to the ground state.25 Chromo- DFT calculations
phores b and c were polarized distinctly beyond the cyanine
First- and second-order polarizability. To profoundly
limit into the zwitterionic regime in acetone and acetonitrile
solvents. While for chromophore e, a slight blue-shi was
observed. Furthermore, chromophores a and f showed the
relatively small fwhm (full width at half maximum) of the low-
energy ICT absorption band and the most intensied absor-
bance in high polar solvents due to the short polyene p-bridge
and the strong TCF acceptor.26,56
Comparison of chromophores b and c with different accep-
tors showed that the lmax blue-shied with the decrease of
acceptor strength and the lmax blue-shied from chromophores
f to e. This suggested that chromophores with a stronger
acceptor were easier to polarize. As for the chromophore d, the
lmax decreased especially in highly polar solvents, demon-
strating that chromophore d with the combination of the TT
unit and isophorone unit as the p-bridge blue-shied in
comparison with its analogue (chromophore c) with a single
type of p-bridge. The maximal wavelength difference (Dlmax) of
all chromophores in six different solvents is depicted in Table 3.
Chromophores a and b showed larger Dlmax with respect to
chromophores c and d, and chromophores e and f with shorter
p-bridges showed smaller Dlmax values than others. It was
suggested that chromophores a and b were the easiest to be
polarized among all the chromophores.
understand the microscopic NLO properties of the designed
chromophores, DFT calculations were performed at the B3LYP/
6-31G level using the Gaussian 09 program package.55,60 The
calculated average static polarizability (a) can be obtained using
eqn (1) (Table S1, ESI†). The total rst hyperpolarizability (btot
)
of the designed chromophores (Table S2, ESI†) related to the
individual tensor components can be obtained using eqn (2).
They were calculated by the nite eld method based on the
optimized molecular geometries. bi (i ¼ x, y, and z) is dened
using eqn (3)–(5) and m can be obtained using eqn (6)
a ¼ (axx + ayy + azz)/3
(1)
(2)
(3)
(4)
(5)
(6)
2
2
2 1/2
btot ¼ (bx + by + bz )
bx ¼ bxxx + bxyy + bxzz
by ¼ byyy + byzz + byxx
bx ¼ bzzz + bzxx + bzyy
2
2
2 1/2
m ¼ (mx + my + mz )
The a of chromophores a–f is shown in Fig. 3(a). It was found
that chromophores with longer or similar length of conjugated
bridges showed larger or similar a values. Moreover, the a
values of chromophores in solvents were larger than that in the
Electrochemical properties and spectroscopic analysis for DE
The electrochemical behavior is closely related to the orbital gas phase, and showed the increased tendency with the increase
structures of the highest occupied molecular orbital (HOMO) of the solvent permittivity. It was demonstrated that chromo-
and the lowest unoccupied molecular orbital (LUMO) for the phore b with the longest conjugated bridge was the most
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Table 3 Measured and calculated parameters from the electrochemical and the absorption spectroscopic experiments
Electrochemical analysis Spectroscopic analysis
DEc (ev)
lonsetd, nm
DEe (eV)
b
b
Compd
Eoxa, V
Ereda, V
EHOMO (eV)
ELUMO (eV)
a
b
c
d
e
f
0.21
0.03
0.01
0.001
0.06
0.35
ꢀ0.90
ꢀ0.82
ꢀ1.15
ꢀ1.13
ꢀ1.16
ꢀ1.09
ꢀ4.60
ꢀ4.42
ꢀ4.40
ꢀ4.39
ꢀ4.45
ꢀ4.47
ꢀ3.49
ꢀ3.57
ꢀ3.24
ꢀ3.26
ꢀ3.23
ꢀ3.30
1.11
0.85
1.16
1.13
1.22
1.44
870
948
753
766
717
715
1.42
1.31
1.65
1.62
1.73
1.73
a
b
c
Potentials vs. Ag/Ag+ in 0.1 M TBAPF6/MeCN solution. EHOMO ¼ ꢀe[4.39 V + Eox] and ELUMO ¼ ꢀe[4.39 V + Ered]. DE ¼ EHOMO ꢀ ELUMO
.
d
Estimated from the onset of the absorption at low concentration chromophores in solid lms in APC. e DE ¼ 1240/lonset
.
chromophore d that was mentioned previously. Thus, it was
demonstrated that the combination of different p-bridges could
lead to overcoming the nonlinearity–transparency trade-off.
The m of chromophores a–f is displayed in Fig. 3(c) and
tabulated (Table S3, ESI†). Chromophore b showed the largest m
due to the strong acceptor and good planarity. Chromophore a
had larger m with respect to chromophore f due to its longer p-
bridge. Though chromophore d had much larger btot than that
of chromophore c, it showed almost the same m as c. It can be
explained that the combined p-bridge could not only increase
btot, but also decrease m.
Because of the largest m and btot values, chromophore b
showed the largest mb value (Fig. 3(d), Table S4, ESI†), which
had great potential for NLO applications. Chromophore d
showed a lower mbtot value than b due to the lower m. Chro-
mophores a, e, and f showed similar mbtot values due to the
similar btot. Above all, the calculation results could provide us
useful parameters for reference in designing the novel chro-
mophores and predict their NLO properties.
Fig. 3 The solvent dependence of (a) a, (b) btot, (c) m, and (d) mb was
calculated in the gas phase and six solvents (Table S7–S10, ESI†). VC:
vacuum.
Calculated Mulliken charges. Mulliken charge analysis was
reported as a useful method to understand the ground state
electronic structure and estimate the molecular polarization.45
Therefore, the Mulliken charges on various molecular domains
for the studied chromophores were calculated in the gas phase
and are shown in Fig. 4.
vulnerable to the effects of solvent polarity of all the
chromophores.
Fig. 3(b) shows the calculated btot (Table S4, ESI†) in the gas
phase and solvents. Chromophores b and d gave a similar btot
value, which was much larger than that of other chromophores.
It was demonstrated that chromophores with longer p-bridges
and stronger acceptors offered larger btot, and were consistent
with the trend of the a value. In contrast, chromophore a
showed a lower btot value than that of chromophore c. This was
ascribed to the existence of the special lateral TT moiety that
was used as the lateral moiety and could disperse three direc-
tional bi (i ¼ x, y, and z) tensors and tended to lower the btot
value. All the chromophores showed a similar increasing trend
of btot to a certain extent as a function of increasing solvent
dielectric constant. The slopes were steep for chromophores b
and d, and relatively smooth for chromophores a, e, and f,
suggesting that chromophores with longer p-bridges showed
larger btot values and bigger changes with the increase of
dielectric constant. This followed the similar trend of the a
value. Especially, chromophore d showed the largest calculated
btot value among all six chromophores, and the blue-shied
phenomenon in UV-Vis-NIR spectra was also found for
Comparison of chromophores a and b showed that the TT
moiety both lateral moiety and p-bridge exhibited electroneg-
ativity. And the negative charge located in the TT moiety as the
lateral moiety is higher than that as the p-bridge due to the
strong electron-withdrawing stability of the TCF acceptor.
However, the charge in the TT unit for chromophore c was
electropositive, resulting in the strong TCF electron acceptor
tending to increase charge separation. In contrast, comparison
of chromophores c and d with chromophores e and f showed
that extending the conjugation length changed the electron
properties of the TT unit from electropositive to electronegative
and disfavored the charge separation. And chromophore d
showed the approximate D–A–p–A structure. These phenomena
revealed that the TT unit was regarded as an auxiliary acceptor
in d and as an auxiliary donor in c. Thus, we speculated that the
p-bridges favored to form the D–A–p–A structure, and it could
favor to overcome the nonlinearity–transparency trade-off.
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showed that the maximum b value can be achieved when the
˚
BLA values are intermediate (ꢄ0.05 ꢄ 0.01 A). Consequently, the
BLA values of the p-bridges can be regarded as a measure of
ground-state polarization and as an important parameter for
NLO response.
For the heteroaromatic ring based chromophores (a–d) and
two polyene chromophores (e and f), as shown in Fig. 5(a), the
BLA of six chromophores in the gas phase and six solvents was
calculated (Table S1–S6, ESI†) to study the solvent dependence
of BLA and the correlation between btot and BLA. For chromo-
˚
phore d with combined bridges, the BLA is between ꢀ0.04 A and
˚
ꢀ0.05 A and it also shows the largest btot, which is in consistent
with the BLA model. For chromophore b with a single TT bridge,
the absolute value of BLA shows the minimum value but the btot
is as large as that of chromophore d. We speculated that chro-
mophores using the TT unit as the p-bridge were not suitable
for this BLA model. Moreover, all the chromophores showed a
similar decaying trend in BLA with an increase of solvent
polarity. As the solvent polarity increased, the occupancy of
zwitterionic form arose to the even distribution of the two
resonance forms. The BLA of chromophore d demonstrated that
it had a larger btot value than that of other chromophores
according to the BLA model proposed by Marder et al.59
Fig. 4 Mulliken atomic charges on various molecular domains for six
studied chromophores at the B3LYP/6-31G level. The moieties in color
were considered for charge calculation.
Energy gaps (DE). The DE between HOMO and LUMO of six
chromophores was calculated in a vacuum and six different
solvents. As shown in Fig. 5(b), the DE of all the chromophores
displayed a similar decaying trend as a function of the
increasing solvent polarity. Thus, the solvent-dependent
behaviors of all the chromophores studied here suggested that
increasing the solvent dielectric constant could reduce the DE of
the chromophore molecules and tend to favor ICT. Moreover,
the DE results calculated by the DFT method were in consistent
with the experimental results using the CV method and the
spectral analysis. It also demonstrated that this calculated
method can be a good alternative to predict DE.
Frontier molecular orbitals. Frontier molecular orbitals
(FMOs) of chromophores a–f were calculated to show the rela-
tionship between the structures and properties. As shown in
Fig. 6, for all the chromophores, the HOMOs were delocalized
on the julolidinyl-based donors and p-bridges while the LUMOs
were localized in the conjugated bridges and concentrated on
the acceptors. The results indicated that increasing the length
of the p-bridges allowed an efficient ICT from the donor to the
acceptor. Moreover, for chromophore a, the optimized
Furthermore, the D–A–p–A structure chromophore could be
used as a key part in dye-sensitized solar cells (DSSCs).57
Mulliken charges on various molecular domains for above
chromophores were also calculated in six different solvents to
study the solvent dependence of the ICT. For the donor and
acceptor moieties, ICT of the six chromophores increased with
the increase of the dielectric constant of solvents. Distinctively,
the TT unit of chromophore b showed electro-negativity rst
and transferred to electro-positivity with increasing solvent
dielectric constant (Table S10–S60, ESI†). This was in relation to
the strong electron-withdrawing ability of the TCF acceptor and
the strong electron delocalization ability of the TT unit.
Furthermore, the charge difference of each unit for a given
chromophore molecule tends to decrease with the increase of
the solvent polarity, suggesting that increasing the solvent
polarity could stabilize the charge separation. Therefore, the
study of the molecular electron structure is a guideline for the
chromophore design to tune the quadratic NLO properties.
Bond-length alternation. The bond length alternation (BLA)
was reported as a key structural parameter that can be corre-
lated with the ground state geometry of the molecule and
hyperpolarizability.25 BLA was dened as the average difference
in length between adjacent carbon–carbon single and double
bonds in the molecule. Based on the two-state model,58 there is
an optimal structure about the combination of each unit of the
chromophore.25 As reported previously, the semiempirical
calculations were mainly performed on the push–pull polyene
chromophores, while few studies were performed on the het-
eroaromatic ring based chromophores. For polyene chromo-
phores, tuning the suitable combination of a donor and a
acceptor as well as the p-bridge can optimize the BLA and b. It
Fig. 5 The calculated (a) BLA and (b) energy gap values of chromo-
phores a–f in a vacuum and six different solvents.
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Journal of Materials Chemistry C
properties. Finally, the inuence of the TT unit as the lateral
moiety or p-bridge of the chromophore could be studied. Thus,
amorphous polycarbonate (APC) guest–host polymer lms
incorporating 20 wt% of chromophores a and b were prepared.
ꢂ
Both EO polymers showed a similar Tg around 140 C and the
poling temperature was about 5 ꢂC higher than Tg. Another two
critical parameters determining the r33 values were poling
ꢂ
voltage and time. The optimum condition was poled at 145 C
for 18 min under the electric eld of 11.5 kV. Aer that, the
lms were cooled to room temperature rapidly followed by the
removal of the poling voltage. A thick lm of aluminium
(around 100 nm) was sputtered on the poled lms, and then the
EO coefficients of them were measured by the Teng–Man
method at a wavelength of 1.31 mm. The maximum r33 value of
94 pm Vꢀ1 for chromophore a was achieved. However, chro-
mophore b did not show the large r33 value because of the poor
compatibility in APC lms. Then, 10 wt% of the chromophore b
was doped into APC to make the guest–host polymer lm follow
the same abovementioned procedures. The optimum condition
was poled at 155 ꢂC for 25 min under the electric eld of 11 kV.
The maximum r33 value was only around 20 pm Vꢀ1. Chromo-
phore b was calculated with the largest m and btot values,
however, these values and the good planarity also make it easy
to form the antiparallel aggregation, leading to the sharply
attenuated r33 value. As to chromophore a, the TT unit in this
molecule affected both the steric effect and the electron effect,
resulting in the decreased intermolecular dipole–dipole inter-
actions. We speculated that the steric effect could play a major
role in the macroscopic NLO properties, while the electron
effect could play a crucial role in the microscopic NLO
properties.
Photochemical stability
To investigate the photochemical stability of chromophores a
and b, they were separately dissolved in chloroform solutions (5
ꢃ 10ꢀ5 M), and then irradiated with 450 nm light for an hour. As
shown in Fig. 7(a) and (b), aer irradiation, the UV-Vis-NIR
absorption spectra of chromophore b dropped more extensively
than that of chromophore a. It also demonstrated that chro-
mophore a showed better photostability and EO properties than
those of chromophore b. This might be attributed to the lower
DE of chromophore b (0.85 eV) than that of chromophore a
Fig. 6 The HOMO and LUMO frontier molecular orbitals of chro-
mophores a–f.
perpendicular spatial conguration of the TT unit disfavored
the ICT on the main axis. It was worth highlighting that the TT
unit contributed roughly equal to both HOMOs and LUMOs of
chromophore b, which promoted the ICT. Thus, chromophores
with longer p-bridges and stronger acceptors were supposed
with lower DE, which was in accord with the experimental
results.
EO properties
According to the experimental and calculated results of chro-
mophores a–f mentioned above, chromophores a and b were
chosen to evaluate the EO activity for such reasons. Firstly, both
of them showed good thermal stability. Secondly, chromophore
b exhibited the largest btot and mbtot values according to the DFT
Fig. 7 The UV-Vis-NIR absorption spectra of the solutions were
irradiated with 450 nm light. The spectra before and after irradiation
with 450 nm light of CHCl3 solutions for chromophore a (a) and
calculations. Then, they both showed good photophysical chromophore b (b).
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(1.11 eV), which made chromophore b more sensitive to light
and oxygen than chromophore a. This was in consistent with
the experimental and calculated DE results, which showed that
chromophore b had lower DE than chromophore a. Conse-
quently, for chromophores with large microscopic NLO prop-
erties as b, how to improve the poling efficiency and solubility
and photochemical stability in host polymer lms and effi-
ciently translate the high microscopic NLO properties to
macroscopic r33 values is an urgent challenge. Such as opti-
mizing the structure by introducing the bulk groups into the
donor or acceptor part was an alternative under study.
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A series of julolidinyl-based chromophores based on different
types of p-bridges, including the TT unit and the isophorone
unit, and TCF or dicyanomethylene acceptors, have been
synthesized and investigated. The structure–property relation-
ship of the studied chromophores was explored using experi-
mental
and
calculated
data.
Electrochemical
and
solvatochromic behaviour analyses as well as DFT calculations
were also carried out to show the microscopic NLO properties of
the chromophores. Chromophore b displayed a larger mbtot
value and narrower energy gap than those of chromophore a,
whereas its r33 value was almost three times smaller with
respect to that of chromophore a. This might be attributed to
the better photochemical stability and the steric structure of the
TT unit of chromophore a, leading to the increased poling
efficiency. Chromophore d with a combination of TT unit and
isophorone unit as the p-bridge was calculated as the D–A–p–A
structure. It preferred to display the blue-shied phenomenon
of ICT absorption and high microscopic NLO properties that
helped to overcome the nonlinearity–transparency trade-off.
However, this new ‘mismatched’ combination of the p-bridge
made chromophore d display the sharply decreased thermal
stability and weak low-energy absorption because of the ineffi-
cient ICT. Thus, searching for the ‘matched’ combination of p-
bridges with good thermal stability is a big challenge. Moreover,
good thermal stability and NLO properties made chromophores
a and b have great potential for new generation organic EO
devices. Thus, the improvement of translating the large b value
into bulk EO activity of materials is a big challenge we need to
overcome in the near future.
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Acknowledgements
We are grateful to the National Natural Science Foundation of
China (nos 11104284, 21003143 and 61101054) for nancial
support. All calculations were performed on the cluster of Key
Laboratory of Theoretical and Computational Photochemistry,
Ministry of Education, China.
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