Synthesis of double-conjugated-segment molecules and their application as ultra-broad two-photon-absorption optical limiters

Article abstract of DOI:10.1039/b208190c

A series of novel double-conjugated-segment molecules, in which two conjugated segments are separated by an ether chain, were synthesized; these compounds provide a very broad two-photon absorption spectral range, which satisfies an urgent need in the optical limiting area.

Full text of DOI:10.1039/b208190c

Synthesis of double-conjugated-segment molecules and their application
as ultra-broad two-photon-absorption optical limiters
Junxiang Zhang,ab Yiping Cui,*a Mingliang Wangb and Juzheng Liub
a Department of Electronic Engineering, Southeast University, Nanjing 210096, P. R. China.
E-mail: ypcyz@seu.edu.cn
b Department of Chemistry, Southeast University, Nanjing 210096, P. R. China
Received (in Cambridge, UK) 21st August 2002, Accepted 13th September 2002
First published as an Advance Article on the web 30th September 2002
A series of novel double-conjugated-segment molecules, in
which two conjugated segments are separated by an ether
chain, were synthesized; these compounds provide a very
broad two-photon absorption spectral range, which satisfies
an urgent need in the optical limiting area.
contain an alkyl substituent R2 which was introduced to increase
the solubility of the resulting molecules. These molecules are
soluble in CHCl3, DMF, THF, tetrachloroethane, and so on.
While R2 was substituted by a methyl group, however, the
resulting molecules cannot dissolve in common solvents except
for NMP. The different conjugated bridges X (CNC and NNN)
were chosen to alter the charge transfer characteristics and the
molecule fundamental optical frequency. The functional groups
R1 and R3 linked to the p-conjugated systems can increase the
charge transfer ability in each conjugated chain, so as to
enhance the molecule third-order nonlinearity. Moreover, those
functional groups can also adjust the absorption wavelength. In
our targeted molecules I, there are two separated conjugated
segments. One is a donor–bridge–donor system; the other is a
donor–bridge–acceptor system. These systems efficiently pro-
vide a large third nonlinear effect.
In recent years, organic molecules exhibiting strong nonlinear-
optical behavior have received considerable interest because of
their applicability in a variety of optical devices.1–7 In
particular, optical limiters have received attention as a result of
a growing need for eye and optical sensor protection.8 Two-
photon-absorption (TPA, third-order nonlinear optical mecha-
nism) optical limiters, which offer the advantage of a high
transmission for the laser light with a frequency well below the
bandgap frequency at low incident intensity, will absorb the
light and decrease the transmission based on a two-photon-
resonance process when the incident intensity is high.9
The diiodo compounds 2 were synthesized using the
approach outlined in Scheme 2.† Methylation of hydroquinone
3 with dimethyl sulfate gave the corresponding bis-methyl ether
4 (82%). After iodination with I2 (4?5, I2, HIO3, H2SO4,
AcOH, 85%), demethylation (5?6, BBr3, 278 °C to room
temperature, 90%) was then achieved on treatment with BBr3. A
single alkyl substituent (6?7, R2Br, KOH, DMSO, 56%) was
introduced by controlling the amount of R2Br added. The diiodo
compounds 2 were synthesized by dehydration of 7 with 8
(containing one conjugated segment) under Mitsunobu reaction
conditions using Ph3P and diethyl azodicarboxylate (DEAD).
The measured linear absorption (LA) spectra of molecules I
were found to be the mixture of the absorption bands of the two
separated conjugated segments. Their absorption spectra con-
tain two peaks or one peak and one shoulder instead. Each of
these peaks corresponds to the fundamental frequency of each
conjugated segment in comparison with that of the single
conjugated molecule. For instance, the LA spectrum of If is
shown in the inset to Fig. 1. The full width of the half maximum
(FWHM) of each peak of I was selected as the parameter to
describe the absorption range (Table 1). From Table 1, one can
see that the absorption ranges are red-shifted with the increasing
electron-donor ability of R3 (H?NEt2?NPh2) or electron-
acceptor strength of R1 (SO2Me?NO2). Moreover, when the
Because of the existence of lasers with different wavelengths
in a wide spectral range, a broadband absorption nonlinear
molecule is practically needed.8 However, in traditional non-
linear absorption molecules with a single-conjugated segment,
the primary absorption coverage derived from the electron
*
transition from p orbit to p orbit is restricted to a narrow band.
One way to broaden the absorption coverage is to blend
different nonlinear molecules together. The absorptivity of the
mixture has been found to be a linear sum of those of each
compound.10 But phase-separation will be a fatal problem in the
practical application.
Recently in our laboratory, we were developing a new
synthetic strategy, which combined two TPA conjugated
segments in one molecule with a non-conjugated ether chain.
We observed the broad linear absorption band and the
broadband nonlinear absorption coverage arose from the p–p*
transition of those two conjugated segments. To the best of our
knowledge, this is the first experimental realization of broad-
band two-photon optical limiters in one compound. In this
communication, we report the synthesis of these double-
conjugated-segment molecules and their application as broad-
band two-photon-absorption optical limiters.
We have synthesized the double-conjugated-segment mole-
cules I by reaction of diiodo compounds 2 with substituted
styrene derivatives 1 in the presence of a catalytic amount of Pd
(OAc)2 and tri-o-tolylphospine (Scheme 1).† Compounds I
Scheme 2 (a) (CH3)2SO4, KOH, water, < 40 °C for 2 h, then reflux for 0.5
h, 82%; (b) I2, HIO3, H2SO4, AcOH, CCl4, 75 °C for 3 h, 85%; (c) BBr3,
CH2Cl2, 278 °C to room temperature, 5 h, 90%; (d) R2Br, KOH, DMSO,
room temperature for 20 h, 56%; (e) DEAD, Ph3P, THF, room temperature
for 48 h.
Scheme 1 (a) Pd (OAc)2, tri-(O-tolyl)phosphine, Et3N.
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Table 1 Heck reaction yield, linear absorption range, nonlinear absorption range and effective nonlinear absorption cross-section of Ia–Il, in which the alkyl
substituent R2 is C6H13
Effective nonlinear
absorption cross section/
cm4 s photon21d
Heck reaction
yield (%)
Linear absorption (LA)
range/nmb
Nonlinear absorption
(NLA) range/nmbc
Entrya
R3
R1
X
Ia
Ib
Ic
Id
Ie
If
Ig
Ih
Ii
H
H
H
H
NEt2
NEt2
NEt2
NEt2
NPh2
NPh2
NPh2
NPh2
NO2
NO2
SO2Me
SO2Me
NO2
CNC
NNN
CNC
NNN
CNC
NNN
CNC
NNN
CNC
NNN
CNC
NNN
80
85
84
88
65
67
50
58
32
39
30
44
350–485
350–525
345–450
348–502
390–470
392–518
400–454
394–498
420–487
418–530
415–458
421–500
670–970
630–970
—
—
620–950
720–1020
—
—
—
—
—
—
10247–10246
10247–10246
—
—
10247–10245
NO2
10247–10246
SO2Me
SO2Me
NO2
NO2
SO2Me
SO2Me
—
—
—
—
—
—
Ij
Ik
Il
a All compounds were characterized by MS, 1H NMR† and EA. b The absorption range here is defined as the full width of half maximum (FWHM) of each
peak. c The nonlinear absorption ranges were measured by direct nonlinear optical transmission. d The effective nonlinear absorption cross section were
deduced from T = exp(2az)/(1+ (b/a)I02(b/a)I0exp(2az)).
mino-4A-nitrostilbene (8e, 1.13 g, 3.8 mmol) and triphenylphosphine (1.50
g, 5.7 mmol) in THF (15 mL). The resulting solution was stirred overnight
and then poured into hot methanol and filtered while it was still hot. The
resulting red solid was purified by chromatography (CH2Cl2 as eluent) (2.18
g, 79%, mp 159–161 °C). 1H NMR (CDCl3, ppm): d 0.93 (m, 3H, –CH3 in
alkoxy), 1.36–1.86 (m, 8H, aliphatic protons), 3.22 (s, 3H, –NCH3), 3.88 (t,
J = 5.80 Hz, 2H, –OCH2CH2N–), 3.92 (m, 2H, –OCH2– on alkoxy side
chain), 4.14 (t, J = 5.91 Hz, 2H, –OCH2CH2N–), 6.81 (d, J = 8.12 Hz, 2H,
aromatic protons), 6.95 (d, J = 16.23 Hz, 1H, vinyl proton), 7.16 (m, 2H,
aromatic protons), 7.19 (d, J = 16.33 Hz, 1H, vinyl photon), 7.25 (d, J =
8.26 Hz, 2H, aromatic protons), 7.58 (d, J = 8.42 Hz, 2H, aromatic
protons), 8.19 (d, J = 8.64 Hz, 2H, aromatic protons). Anal. Calc. For
C
29H32N2O4I2: C, 47.93; H, 4.41; N, 3.86. Found: C, 47.97; H, 4.50; N,
3.76%.
Example of the Heck reaction (Ie): triethylamine (0.24 mL, 1.72 mmol)
was added to a solution of 2e (0.5000 g, 0.689 mmol), p-N,N-
diethylaminostyrene (0.2412 g, 1.378 mmol), Pd(OAc)2 (6.2 mg, 0.0276
mmol), and tri-o-tolylphospine (42.0 mg, 0.138 mmol) in 10 mL of DMF.
The resulting mixture was stirred at 80 °C overnight in a nitrogen
atmosphere and was then poured into methanol. The precipitated solid was
collected by filtration, then redissolved in tetrachloroethane and reprecipi-
tated in methanol. The resulting dark solid was purified by chromatography
(CHCl3+cyclohexane = 20+1 as eluent) and then recrystallized from THF–
hexane (0.367 g, 65%, mp 172–174 °C). 1H NMR (CDCl3, ppm): d 0.95 (t,
3H, –CH3 in alkoxy), 1.15–1.17 (m, 12H, –NCH2CH3), 1.40–1.89 (m, 8H,
aliphatic proton), 3.17 (s, 3H, –NCH3), 3.17–3.36 (m, 8H, –CH2CH3), 3.85
(t, J = 5.45 Hz, 2H, –OCH2CH2N–), 3.90 (m, 2H, –OCH2–), 4.28 (m, 2H,
–OCH2CH2N–), 6.70 (d, J = 8.60 Hz, 2H, aromatic protons), 6.75 (d, J =
16.38 Hz, 1H, vinyl proton), 7.10 (d, J = 16.33Hz, 1H, vinyl proton), 7.14
(d, J = 6.40 Hz, 2H, aromatic protons), 7.18 (d, J = 15.80 Hz, 2H, vinyl
protons), 7.23 (d, J = 7.82 Hz, 2H, aromatic protons), 7.26 (m, 2H, aromatic
protons), 7.30 (d, J = 16.01 Hz, 1H, vinyl proton), 7.34 (d, J = 7.92, 2H,
aromatic protons), 7.46 (d, J = 8.55 Hz, 2H, aromatic protons), 7.48 (s, J
= 7.68Hz, 2H, aromatic protons), 7.52 (d, J = 7.45 Hz, 2H, aromatic
protons), 7.56 (d, J = 16.55 Hz, vinyl proton), 7.88 (d, J = 8.00Hz, 2H,
aromatic protons). Anal. Calc. For C53H64N4O4: C, 77.56; H, 7.80; N, 6.83.
Found: C, 77.91; H, 7.51; N, 7.11%.
Fig. 1 Two-photon-absorption spectrum of If. The precision of our
apparatus is about ±15%. The inset shows the LA spectrum of If.
conjugated bridge CNC was replaced by a NNN chain the linear
absorption range was also red-shifted.
Two-photon absorption spectra of these compounds were
measured by direct nonlinear optical transmission (NLT). The
FWHM of their two-photon absorption spectra are also
presented in Table 1. Here, only data for four compounds are
shown for apparatus reasons. Extraordinarily broad spectral
ranges were provided by compounds I, i.e. the nonlinear
absorption (NLA) range of If is about 300 nm wide (Fig. 1). The
NLA ranges of I are approximately two times the LA range. The
effective nonlinear absorption cross section is of the order of
magnitude 10247–10246 cm4 s photon21
.
In summary, a new synthetic strategy was developed to
synthesize a series of double-conjugated-segment molecules,
which provided a very broadband two-photon absorption range.
The optical limiters based on these novel molecules are
promising in the near-infrared region.
This work was supported by the Excellent Young Research
Fellowship of the Education Ministry of China and National
Natural Science Foundation of China for Distinguished Young
Scholars under Grant No. 60125513.
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Notes and references
†
Example of the Mitsunobu reaction (2e): diethylazodicarboxylate
(DEAD) (1.00 g, 5.7 mmol) in THF (5 mL) was added into a solution of
compound 7 (R2NC6H13, 1.69 g, 3.8 mmol), 4-(2-hydroxyethyl)methyla-
10 J. E. Ehrlich, X. L. Wu, I.-Y. S. Lee, H. Rockel, S. R. Marder and J. W.
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