A new building block, bis(thiophene vinyl)-pyrimidine, for constructing excellent two-photon absorption materials: Synthesis, crystal structure and properties

Article abstract of DOI:10.1039/c2jm14766a

Two new molecules (Py1 and Py2) of D-π-A-π-D motif, where the -π-A-π- moiety is bis(thiophene vinyl)-pyrimidine as a building block, have been designed and synthesized via the Knoevenagel and Suzuki coupling reactions. The X-ray single crystal structure analysis of Py1 showed good coplanarity in the whole building block. Their two-photon absorption (2PA) properties were measured by two-photon induced fluorescence techniques with a mode-locked Ti: sapphire pulsed laser in the range of 700-940 nm. Both Py1 and Py2 exhibited large cross-section values of 1702 and 1879 GM, respectively, no matter whether the peripheral donor is weak or strong. The results demonstrate that bis(thiophene vinyl)-pyrimidine is a new building block for constructing molecules with effective 2PA.

Full text of DOI:10.1039/c2jm14766a

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PAPER
A new building block, bis(thiophene vinyl)-pyrimidine, for constructing
excellent two-photon absorption materials: synthesis, crystal structure and
properties†
*
Dugang Chen, Cheng Zhong, Xiaohu Dong, Zhihong Liu and Jingui Qin
Received 24th September 2011, Accepted 22nd November 2011
DOI: 10.1039/c2jm14766a
Two new molecules (Py1 and Py2) of D–p–A–p–D motif, where the –p–A–p– moiety is bis(thiophene
vinyl)-pyrimidine as a building block, have been designed and synthesized via the Knoevenagel and
Suzuki coupling reactions. The X-ray single crystal structure analysis of Py1 showed good coplanarity
in the whole building block. Their two-photon absorption (2PA) properties were measured by two-
photon induced fluorescence techniques with a mode-locked Ti: sapphire pulsed laser in the range of
700–940 nm. Both Py1 and Py2 exhibited large cross-section values of 1702 and 1879 GM, respectively,
no matter whether the peripheral donor is weak or strong. The results demonstrate that bis(thiophene
vinyl)-pyrimidine is a new building block for constructing molecules with effective 2PA.
dipolar analogues.21–24 In addition, quadrupolar compounds
1. Introduction
have higher fluorescence quantum yields relative to corre-
sponding dipolar compounds, most likely due to cancellation of
D–A dipoles which otherwise would tend to induce charge
transfer quenching of fluorescence.25 What is more, in quad-
rupolar systems, D–p–D and D–p–A–p–D structures are
generally more effective for 2PA than A–p–A and A–p–D–p–A
systems.26 Therefore, we focus on the design and synthesis of
molecules with D–p–A–p–D structures to gain high 2PA mate-
rials in this paper.
Our group27 and Z. Huang’s group28 have reported some
2PA molecules based on a pyrimidine core (Fig. 1a), with cross-
section (d) values of about 300–700 GM, where the conjugation
bridge is a styryl unit. On the other hand, thiophene as an
electron-rich moiety has been widely used in organic semi-
conductors, which have shown good chemical stability and
Two-photon absorption (2PA) is defined as the electronic exci-
tation of a molecule induced by a simultaneous absorption of
a pair of photons of the same or different energy.1 It was pre-
2
€
dicted theoretically by Maria Goppert-Mayer in 1931, and was
demonstrated experimentally in 1961,3 soon after the invention
of the laser. Over recent years, molecular 2PA has attracted great
interest owing to the potential applications including optical
limiting, optical data storage, up converted lasing, fluorescence
microscopy, and microfabrication,4–10 that have stimulated the
designing of materials with specific 2PA performance. In this
context, organic compounds have presented great possibilities
due to their flexibility in terms of structural engineering, allowing
the development of molecules with tunable optical nonlinearities
in accord with the target applications.11 To date, some efficient
molecular design strategies were put forward to provide guide-
lines on the development of organic conjugated molecules with
improved 2PA. Three main classes of molecules were found to be
potential 2PA chromophores: dipolar type with a donor–p
bridge–acceptor (D–p–A) motif,12–15 quadrupolar type with a D–
p–D, A–p–A, D–p–A–p–D or A–p–D–p–A motif,16–18 and
octupolar type with three-branched dipoles.19,20 Indeed,
symmetrical quadrupolar structures have experimentally and
theoretically demonstrated enhanced 2PA cross sections that are
approximately an order of magnitude greater than those of
Department of Chemistry, Wuhan University, Wuhan, 430072, China.
E-mail: jgqin@whu.edu.cn; Tel: +86-27-68752330
† Electronic supplementary information (ESI) available: The details of
the crystal data for Py1. CCDC reference number 843125. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2jm14766a
Fig. 1 Structure of pyrimidine-based chromophores: (a) reported in the
literature; (b) reported in this paper.
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charge transport ability. In addition, it is easy to modify the
thiophene unit by common chemical means to give various
derivatives. As a result, in this paper we replace the styryl unit
with a thiophene vinyl, and thus a new building block for 2PA
materials is obtained, as shown in Fig. 1b. Thiophene is a five-
membered ring, so there will be less steric hindrance and better
planarity when terminal donors are connected to the central
bis(thiophene vinyl)-pyrimidine moiety to shape conjugated
molecules, compared to the reported bis(styryl)-pyrimidine
based compounds. A hexyl group was introduced into the
pyrimidine unit in order to increase the solubility of the
molecule. Two representative molecules, Py1 and Py2, with
a weak and strong donor respectively, were synthesized and
studied here. Both of them showed much larger cross-sections
compared to the similar bis(styryl)-pyrimidine based molecules
2.3 Synthetic procedures
2.3.1 4,6-Bis((E)-2-(5-bromothiophen-2-yl)vinyl)-2-(hexyloxy)-
pyrimidine (M). A mixture of Compounds 1 (1.52 g, 9.53 mmol)
and 2 (4 g, 20.9 mmol) in 50 mL of ethanol was stirred under
argon at room temperature for 0.5 h. Hydrochloric acid (4 mL,
3.0 mol Lꢁ1) was added stepwise to the reaction mixture which
was then refluxed for 48 h. The solvent was neutralized with
0.5 mol Lꢁ1 sodium carbonate, and then the solution was fil-
trated. The residue was washed by water, ethanol and ethylether
in turn. After drying in a vacuum, the crude product (Compound
3) was used in the next reaction without further purification.
Compound 3 (1.4 g, 2.98 mmol), n-hexyl bromide (0.6 mL,
4.2 mmol) and K2CO3 (2 g, 15 mmol) were dissolved in 30 mL of
DMF, and the mixture was stirred under argon at 80 ꢀC for 24 h.
After the reaction, the solution was poured into water and
extracted with CH2Cl2 three times. The combined organic phase
was dried over Na2SO4. When the solvent was removed under
vacuum, the residue was purified by column chromatography
(silica gel, CH2Cl2/petroleum ether, 1/2) to give the product M as
when measured by
technique.
a two-photon induced fluorescence
2. Experimental
1
a yellow solid. Yield: 1.0 g, 61%. H NMR (CDCl3, 300 MHz)
2.1 Materials
d [ppm]: 7.93 (d, J ¼ 15.6 Hz, 2H), 7.02–6.97 (m, 4H), 6.75 (s,
1H), 6.67 (d, J ¼ 15.3 Hz, 2H), 4.44 (t, J ¼ 6.6 Hz, 2H), 1.89–1.82
(m, 2H), 1.52 (m, 2H), 1.37 (m, 4H), 0.91 (m, 3H).13C NMR
(CDCl3, 75 MHz) d [ppm]: 165.1, 164.1, 142.7, 130.8, 129.3,
128.7, 125.0, 114.4, 111.1, 67.4, 31.5, 28.8, 25.6, 22.5, 13.9. Anal.
Calcd for C22H22Br2N2OS2: C, 47.66; H, 4.00; N, 5.05. Found: C,
47.88; H, 3.77; N, 5.03.
THF was dried over and distilled from K–Na alloy under an
atmosphere of argon. Compounds 1,29,30 2,31 4,32 and 533 were
synthesized according to the literature. Other reagents were
commercially obtained from Sinopharm Chemical Reagent Co.
(Shanghai, China).
2.2 Instruments
2.3.2 Synthesis of Py1. Compound M (0.2 g, 0.36 mmol), 4
(0.16 g, 0.76 mmol) and Pd (PPh3)4 (18 mg, 2 mol %) were mixed
in a Schlenk tube (50 mL), which was then filled with argon under
vacuum line. 2 mL of aqueous solution of K2CO3 (2 mol Lꢁ1) and
10 mL of THF were added to the mixture by syringe. The reac-
tion was carried out at 65 ꢀC for 24 h. After the reaction cooled to
room temperature, it was poured into water, and extracted with
chloroform. The organic phase was collected and dried over
Na2SO4. When the solvent was removed under vacuum, the
residue was purified by column chromatography (silica gel,
CH2Cl2/petroleum ether, 1/2) and then recrystallized in
dichloromethane and petroleum to give the product Py1 as
a deep yellow solid. Yield: 0.16 g, 80%. 1H NMR (CDCl3,
300 MHz) d [ppm]: 8.01 (d, J ¼ 15.6 Hz, 2H), 7.26–7.24 (m, 4H),
7.14–7.13 (m, 4H), 7.06–7.05 (m, 2H), 6.77–6.72 (m, 3H), 4.45 (t,
J ¼ 7.2 Hz, 2H), 1.90–1.85 (m, 2H), 1.57 (m, 2H), 1.38 (m, 4H),
0.92 (m, 3H). 13C NMR (CDCl3, 75 MHz) d [ppm]: 165.5, 164.5,
140.3, 139.0, 137.2, 130.7, 129.5, 128.2, 125.3, 124.8, 124.5, 111.5,
67.7, 31.8, 29.2, 26.0, 22.8, 14.3. Anal. Calcd for C30H28N2OS4:
C, 64.25; H, 5.03; N, 5.00. Found: C, 64.01; H, 4.77; N, 4.84.
1H NMR and 13C NMR spectra were measured on
a
MECUYR-VX300 spectrometer. Elemental analysis of
carbon, hydrogen, and nitrogen was performed on a Vario EL
III microanalyzer. UV-Vis absorption spectra were recorded on
a Shimadzu UV-2500 recording spectrophotometer. PL spectra
were recorded on a Hitachi F-4500 fluorescence spectropho-
tometer. Thermogravimetric analysis (TGA) was undertaken
with a NETZSCH STA 449C instrument. The thermal stability
of the samples under a nitrogen atmosphere was determined by
measuring their weight loss while heating at a rate of 10 ꢀC
minꢁ1 from 30 to 500 C. The single crystal data were collected
ꢀ
using a Bruker SMART Diffractometer equipped with a CCD
detector (graphite-monochromated Mo-Ka radiation
l
¼
ꢀ
0.71073 A) at 298(2) K. Cyclic voltammetry (CV) was carried
out in nitrogen purged anhydrous CH2Cl2 solution at room
temperature with a CHI voltammetric analyzer. Tetrabuty-
lammonium hexafluorophosphate (TBAPF6) (0.1 M) was used
as the supporting electrolyte. The conventional three-electrode
configuration consisted of
a platinum working electrode,
a platinum wire auxiliary electrode, and an Ag wire pseudo-
reference electrode with ferrocenium-ferrocene (Fc+/Fc) as the
internal standard. Cyclic voltammograms were obtained at
a scan rate of 100 mV sꢁ1. Formal potentials were calculated as
the average of cyclic voltammetric anodic and cathodic peaks.
The onset potential was determined from the intersection of
two tangents drawn at the rising and background current of the
cyclic voltammogram. Two-photon excited fluorescence was
measured using a mode-locked Ti: sapphire femtosecond pulsed
laser (Chameleon Ultra II, Coherent Inc.). The pulse width and
repetition rate were 140 fs and 80 MHz, respectively.
2.3.3 Synthesis of Py2. Compound Py2 was obtained by the
same procedure as that for Py1 except that Compound 5 (0.28 g,
0.76 mmol) was used as one reactant. The product was purified
by column chromatography (silica gel, CHCl3/petroleum ether,
1/1) and then recrystallized in chloroform and ethanol to give the
product Py2 as a red solid. Yield: 0.27 g, 85%. 1H NMR (CDCl3,
300 MHz) d [ppm]: 8.01 (d, J ¼ 15.9 Hz, 2H), 7.48 (d, J ¼ 8.1 Hz,
4H), 7.31–7.26 (m, 10H), 7.17–7.06 (m, 18H), 6.78–6.73 (m, 3H),
4.46 (t, J ¼ 6.9 Hz, 2H), 1.90–1.85 (m, 2H), 1.56 (m, 2H), 1.38
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(m, 4H), 0.92 (m, 3H). 13C NMR (CDCl3, 75 MHz) d [ppm]:
164.8, 160.7, 148.1, 147.5, 146.2, 140.0, 131.2, 130.1, 129.6, 127.8,
126.9, 125.0, 124.4, 123.6, 123.4, 123.2, 111.2, 67.7, 31.9, 29.2,
26.0, 22.9, 14.3. Anal. Calcd for C58H50N4OS2: C, 78.88; H, 5.71;
N, 6.34. Found: C, 79.28; H, 6.07; N, 6.26.
3. Results and discussion
3.1 Synthesis and characterization
The synthetic routes of the new compounds are illustrated in
Scheme 1. The pyrimidine ring of Compound 1 is electron-defi-
cient, so the methyl groups connected to the ring are highly
reactive. As a result, Compound 3 was easily obtained by the
Knoevenagel reaction. The poor solubility of 3 in common
solvents makes it hard to purify. Therefore the crude product of 3
was directly used in the next step without further purification,
and an n-hexyl group was connected to the pyrimidine core by
nucleophilic substitution to give monomer M in good yield. Py1
and Py2 were obtained by the Suzuki coupling reaction with
Pd(PPh3)4 as catalyst and K2CO3 as base in the solvent of THF/
H2O at 65 ꢀC for 24 h. The X-ray single crystal structure of Py1
was determined. The new compounds were fully characterized by
1H-NMR, 13C-NMR, and elemental analysis. The thermal
properties of the two compounds were evaluated by TGA. As
shown in Fig. 2, excellent thermal stabilities were manifested in
Fig. 2 TGA plots of Py1 and Py2 with a heating rate of 10 ꢀC minꢁ1
under N2 atmosphere.
their TGA profiles showing a decomposition temperature of
ꢀ
about 385 C (5% weight loss) for both molecules.
3.2 Single crystal structure features of Py1
The X-ray single crystal structure of Py1 is shown in Fig. 3.34
Two double bonds connecting the thiophene and pyridimine unit
are both in the trans-configuration, which favors the building
Scheme 1 The synthetic routes of Py1 and Py2. Conditions (a) EtOH, HCl (3M), reflux, 12 h; (b) C6H13Br, DMF, K2CO3, 80 ꢀC, 24 h; (c) Pd(PPh3)4,
THF, K2CO3 (2M, aq), 65 ꢀC, 24 h.
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Table 1 One- and two-photon properties of the 2PA dyes
labs
lPL
Dn
l2PA
dmax
dmax
c
Compound (nm)a (nm)b (cmꢁ1
)
Fd
(nm)e (GM)f (MW)
Py1
Py2
428
459
507
579
3640
4515
0.10 810
0.50 820
1702
1879
3.47
2.31
a
The absorption maxima in the one-photon absorption spectrum. b The
emission maxima in the one-photon fluorescence spectrum. c Stokes shift
d
(Dn ¼ 1/labs ꢁ 1/lPL). Fluorescent quantum yield in chloroform.
e
f
Wavelength of maximum 2PA cross-sections. The peak of 2PA
cross-section in 10ꢁ50 cm4 s photonꢁ1 (GM).
Py2 was 31 nm red-shifted compared to that of Py1, because the
terminal triphenylamine unit in Py2 was a stronger donor than
the thiophene unit in Py1 leading to more effective ICT in the D–
p–A–p–D system. This phenomenon was also verified in their
fluorescence spectra, where the emission maximum of Py2 was
72 nm red-shifted. The fluorescent quantum yields of the two
compounds were measured in chloroform with fluorescein (in 0.1
M NaOH) as the reference molecule. The values are 0.10 for Py1
and 0.50 for Py2, so a two-photon induced fluorescence tech-
nique method can be used to study their 2PA properties.
Fig. 3 Single crystal structure plot of Py1.
block, bis(thiophene vinyl)-pyrimidine, aligning in almost the
same plane with small dihedral angles. The dihedral angles
between the terminal thiophene groups and the building block
are also small with a value of about 8ꢀ, indicating that it is
effective for the building block to be modified and produce
various planar molecules. The alkyl chain is spread out, and does
not affect the conjugation degree of the compound. As a result,
the building block shows good conjugation and coplanarity,
which can be beneficial to p-delocalization of the system and lead
to large 2PA cross-sections. It may also imply that the building
block is useful for constructing some semiconducting materials
for organic thin-film transistors (OTFTs) and polymer solar cells
(PSCs) where a coplanar –p–A–p– bridge is necessary.
3.4 Electrochemical behaviors
Cyclic voltammetry was employed to investigate the redox
behaviors of the chromophores and estimate their highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels. Fig. 5 shows the cyclic
voltammograms of the two compounds in 0.1 mol Lꢁ1 Bu4NPF6
CH2Cl2 solution. All reported potentials were calibrated against
the ferrocene/ferrocenium (Fc/Fc+) couple, which was used as the
internal standard. The HOMO energies were estimated to be
3.3 Linear absorption and single-photon-excited fluorescence
The normalized single-photon absorption and emission spectra
of the compounds in chloroform are presented in Fig. 4, and the
spectroscopic properties are summarized in Table 1. Owing to
the intramolecular charge transfer (ICT) from the peripheral
donors to the central pyrimidine acceptors, two molecules
covered a wide absorption range. The absorption maximum of
ꢁ5.20 and ꢁ5.10 eV for Py1 and Py2, respectively, according to
ox
onset
the equation HOMO ¼ ꢁ (E
+ 4.8) eV.35,36 Py2 showed
a higher HOMO energy level owing to its more electron donating
property of the terminal triphenylamine unit. The LUMO
energies were estimated from the optical band gaps and the
HOMO levels. The data are summarized in Table 2.
Fig. 4 UV-Vis absorption and single-photon fluorescence spectra of Py1
and Py2 in chloroform.
Fig. 5 The cyclic voltammograms of Py1 and Py2 in CH2Cl2 solution.
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Table 2 Energy levels of Py1 and Py2
sections of the two compounds were obtained at excitation
wavelengths of 810 and 820 nm for Py1 and Py2, respectively.
Py2 showed a value of 1879 GM, a little larger than that of Py1
with a value of 1702 GM, due to the stronger electron-donating
ability of triphenylamine. The d/MW value of Py2 was smaller
than that of Py1, because of the larger molecular weight of Py2.
As reported in our earlier papers,27 bis(styryl)-pyrimidine based
molecules, that have a similar D–p–A–p–D motif to Py1 but
with even stronger donors (diethyl amino group), showed 2PA
cross-sections of only 300–700 GM. However, Py1 with a much
weaker donor (thiophene) exhibits a much larger cross-section
(1702 GM), which is a twofold to fivefold increase. The results
indicated that our system, using bis(thiophene vinyl)-pyridimine
based chromophores, is very effective to gain a higher 2PA
performance.
ox
onset
E
Eogpt
(V)
HOMO (eV)
LUMO (eV)
(eV)a
Py1
Py2
0.4
0.3
ꢁ5.20
ꢁ5.10
ꢁ2.61
ꢁ2.72
2.59
2.38
Eogpt ¼ 1240/lonset (eV), wherein lonset is the wavelength of its onset
a
absorption edge in the longer wavelength direction.
3.5 2PA properties
The 2PA behaviour of Py1 and Py2 were measured by a two-
photon induced fluorescence technique in chloroform, which is
direct and very convenient. In order to eliminate contributions
from the excited-state absorption, a femtosecond (ꢂ140 fs)
pulsed laser was used for the measurement. A well characterized
2PA chromophore, Rhodamine B (1.0 ꢃ 10ꢁ4 M in EtOH), was
used as a reference.
4. Conclusion
We successfully synthesized two D–p–A–p–D type chromo-
phores, Py1 and Py2, constructed by a new building block,
bis(thiophene vinyl)-pyrimidine, for 2PA materials. The struc-
ture of Py1 was determined by X-ray single crystal diffraction,
and it revealed that the building block showed good coplanarity,
which can enhance the 2PA. As predicated, both compounds
showed large cross-section values, 1702 GM for Py1 and 1879
GM for Py2. The results indicate that the new building block is
very useful to construct higher 2PA materials compared to the
reported building block of bis(styryl)-pyrimidine. It may also
imply that the building block is useful for constructing some
other kinds of organic optoelectronic molecules where a strong
and coplanar –p–A–p– bridge is necessary.
The 2PA cross-section was calculated by the following equa-
tion,37 which is simplified from ref. 38 by neglecting the refractive
index correction term.
ds ¼ dr(Fs/Fr)(Fr/Fs)(cr/cs)
where the subscripts ‘s’ and ‘r’ stand for the sample and reference
molecules respectively. F is the integrated fluorescence intensity
measured at the same power as the excitation beam. F is the
fluorescence quantum yield. The number density of the molecules
in the solution was denoted c. dr is the 2PA cross-section of the
reference molecule.
Both the chromophores were stable under the test conditions,
and no obvious change was observed in the UV-vis spectra after
completion of the 2PA measurements. The two-photon excita-
tion spectra of Py1 and Py2 in CHCl3 (1 ꢃ 10ꢁ4 M) are displayed
in Fig. 6. We tested the 2PA cross-sections from 700 nm to 940
nm with an interval of 20 or 10 nm. The cross-section values
are summarized in Table 1. The 2PA cross-section per unit mass
(d/MW) is also compiled for comparison. The maximal cross-
Acknowledgements
This work was supported by the National Science Foundation of
China (No. 91022036) and the National Key Fundamental
Research Program of China (No. 2010CB630701). The authors
thank Dr Xianggao Meng of Huazhong Normal University,
Wuhan, China, for the X-ray single crystal structure
measurement.
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34 Crystal data for Py1: C30H28N2OS4, M ¼ 560.82, Monoclinic, P21/c,
ꢀ
ꢀ
ꢀ
ꢀ
a ¼ 9.9129(8) A, b ¼ 31.483(2) A, c ¼ 18.6483(14) A, a ¼ 90 , b ¼
ꢀ
ꢀ
3
ꢀ
105.132(2) , g ¼ 90 , U ¼ 5618.1(8) A , T ¼ 298(2) K, Z ¼ 8, R1 ¼
0.0734, wR2 ¼ 0.1810. CCDC number: 843125.
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4348 | J. Mater. Chem., 2012, 22, 4343–4348
This journal is ª The Royal Society of Chemistry 2012