Fully functionalized photorefractive polymer with infrared sensitivity based on novel chromophores

Article abstract of DOI:10.1021/ma034587o

This paper describes the synthesis and physical study of several new photorefractive (PR) polymers, which are composed of a new type of nonlinear optical (NLO) chromophore attached onto conjugated poly(p-phenylene-thiophene)s. Since the NLO chromophore is labile in many reaction conditions, the Stille coupling reaction was used to prepare these polymers. The resulting polymers exhibit high PR performances. An optical gain coefficient of 158 cm^-1 at a field of 50 V/μm and a diffraction efficiency of 68% at a field of 46 V/μm for polymer P1 were obtained, which are among the best values for fully functionalized PR polymers to date.

Full text of DOI:10.1021/ma034587o

7014
Macromolecules 2003, 36, 7014-7019
Fully Functionalized Photorefractive Polymer with Infrared Sensitivity
Based on Novel Chromophores
Wei You ,† Sh a ok u i Ca o,† Zh a n jia Hou , a n d Lu p in g Yu *
Department of Chemistry and The J ames Franck Institute, The University of Chicago,
5735 South Ellis Avenue, Chicago, Illinois 60637
Received May 7, 2003; Revised Manuscript Received August 1, 2003
ABSTRACT: This paper describes the synthesis and physical study of several new photorefractive (PR)
polymers, which are composed of a new type of nonlinear optical (NLO) chromophore attached onto
conjugated poly(p-phenylene-thiophene)s. Since the NLO chromophore is labile in many reaction
conditions, the Stille coupling reaction was used to prepare these polymers. The resulting polymers exhibit
high PR performances. An optical gain coefficient of 158 cm-1 at a field of 50 V/µm and a diffraction
efficiency of 68% at a field of 46 V/µm for polymer P 1 were obtained, which are among the best values for
fully functionalized PR polymers to date.
In tr od u ction
refractive effects in the form of monolithic materials.12,13
It is interesting to incorporate these chromophores into
polymers so that the materials possess better stability
in their amorphous state and film forming ability. To
obtain low-Tg polymers, different alkyl side chains will
be introduced. Since these polymers are multifunctional
materials made from incorporation of different func-
tional monomers which are rather sensitive to reaction
medium (especially the chromophores), many traditional
polymerization approaches are not compatible with
these monomers. It was found that the palladium-
mediated Stille coupling reaction is mild enough to
tolerate the NLO chromophore and yielded polymers
with sizable molecular weight. Excellent PR properties
were demonstrated by these polymers, and detailed
physical studies disclose insightful information on struc-
ture-property correlations.
In the past decade, organic photorefractive (PR)
materials have been investigated extensively.1-7 With
the deeper understanding of the PR mechanism, nu-
merous organic PR materials have been developed. Most
of them are composite polymeric materials2,3 in which
moieties with different functions necessary for demon-
strating the PR effect are physically mixed together with
different formulations. A very popular system utilizes
photoconducting host polymers such as poly(N-vinyl-
carbazole) (PVK) doped with guest molecules such as
nonlinear optical (NLO) chromophores and a small
amount of photosensitizer for photocharge generation.
This system is very successful in preparing high-
performance PR materials. Another class of polymeric
PR materials are fully functionalized polymeric materi-
als in which all the functional entities needed for PR
effect are covalently bonded to the PR polymers, afford-
ing improved morphological stability and good PR
properties.4-6,8-10
Resu lts a n d Discu ssion
Syn th esis of Mon om er s a n d P olym er s. The key
components for the new chromophores are the electron-
withdrawing moieties: tricyanodihydrofuran derivatives
(compounds 2 and 3 in Scheme 1),14 which can undergo
Knoevenagel condensation with substituted amino benz-
aldehyde 1 to afford the NLO chromphores/monomers
M1 and M2. As shown in Scheme 2, all the PR polymers
were synthesized with high yields by palladium-
catalyzed Stille polycondensation using Pd(PPh3)2Cl2 as
the catalyst and THF as the solvent.15 Longer alkyl
chains were used not only for the enhancement of the
solubility but also for lowering the glass transition
temperature of resulting polymers. On the basis of the
“orientational enhancement effect”,16 lowering the Tg of
the PR polymer could allow the chromophores to reori-
ent in response to the combined internal and external
fields and greatly improve the magnitude of the refrac-
tive index grating. Polymers P 1 and P 2 were synthe-
sized from the corresponding monomers M1 and M2
with di(tributyltin)yhiophene M0, respectively. Polymer
P 3 is obtained from copolymerization of M0 with M1
together with another monomer (dihexadecyldiiodoben-
zene) in the ratio of 30:70. This polymer was synthesized
in order to test our hypothesis that dilution of chromo-
In this paper, we report the synthesis and physical
studies of a new PR polymer system. This work is
motivated by our desire to prepare fully functionalized
PR polymers with high performances. Several criteria
were used in designing these polymers. First of all, the
PR polymers need to exhibit a large electrooptic coef-
ficient so that the index modulation can be optimized.
Second, it is ideal that the PR polymers possess a low
glass transition temperature so that the “orientational
effect” of dipoles due to photoinduced space charge field
can be utilized to further enhance the PR performances.
Low glass transition temperature (Tg) also allows easy
preparation of thick films for holographic studies. Third,
the polymer structures can be synthesized relatively
easily under mild conditions.
In a PR polymer, the NLO chromophores plays the
key role, and a large E-O effect in the polymer is the
necessary condition to achieve high PR performance.
Recently, our group and others have found that chromo-
phores bearing tricyanodihydrofuran derivatives as the
electron-withdrawing group exhibit promising photo-
† These two authors made equal contributions to this work.
* Corresponding author. E-mail: lupingyu@midway.uchicago.edu.
10.1021/ma034587o CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/29/2003

Macromolecules, Vol. 36, No. 19, 2003
Photorefractive Polymers 7015
Sch em e 1. Syn th esis of Mon om er s
Sch em e 2. Syn th esis of P olym er s
phores could weaken the intermolecular interaction
between adjacent chrompophores and help to shorten
the reorientation time in response to the existing field.
GPC measurement in THF with polystyrene standard
indicated the relative number-averaged molecular weight
(Mn) and the polydispersity (PDI) of all three polymers
(P 1: Mn ) 12400, PDI ) 2.30; P 2: Mn ) 15 000, PDI )
2.08; P 3: Mn ) 9000, PDI ) 1.67). Differential scanning
calorimetry (DSC) studies indicated that glass transition
temperatures for P 1, P 2, and P 3 are around 20.1, 5.6,

7016 You et al.
Macromolecules, Vol. 36, No. 19, 2003
Ta ble 1. Red ox P oten tia ls a n d En er gy Level for M1, M2 a n d P 1, P 2, P 3a
onset
ox
onset
re
Eox (V)b
Ered (V)b
E
(V)
E
(V)
EHOMO (eV)
ELUMO (eV)
Egap (eV)
M1
M2
P 1
0.54
0.55
0.55
0.79c
0.57
0.78c
0.56
-1.35
-1.30
-1.36
0.46
0.47
0.46
-1.25
-1.28
-1.16
-5.26
-5.27
-5.26
-3.55
-3.52
-3.64
1.71
1.75
1.62
P 2
P 3
-1.29
0.49
0.48
-1.18
-5.29
-5.28
-3.62
1.67
NA
NAd
NA
NA
a
b
All potentials were calibrated with the ferrocene/ferrocenium (Fc/Fc+) couple. Reported as E1/2 values taken as the average of the
anodic and cathodic peak potentials vs Fc/Fc+. c Second oxidation potential. No apparent peak observed clearly.
d
Ta ble 2. Typ ica l P h ysica l P r op er ties of th e P R P olym er s
λ
max (nm)
[CHCl3]
σa (ps/cm)
[780 nm]
compound
R (cm-1
)
n [780 nm]
Tg (°C)
µh (cm2/(V s))
b
µe (cm2/(V s))
b
P 1
P 2
P 3
595.5
593.5
592.5
24
17.5
2.8
1.688
1.679
1.611
0.072
0.065
0.025
20.1
5.6
1.3
1.6 × 10-5
9.8 × 10-5
NAc
2.6 × 10-5
1.3 × 10-4
NAc
a
b
Measured at an applied field of 33 V/µm with intensity 175 mW/cm2. Measured at an applied field of 33 V/µm with time-of-flight
method at 532 nm; for experimental details see ref 17. c No detectable transit current was observed at the condition of b.
F igu r e 1. UV-vis absorption spectra of M1, M2 and P 1, P 2,
P 3 measured in chloroform solution (1 × 10-5 M) at 25 °C
(compounds listed in order of peak height).
F igu r e 2. Cyclic voltammograms of monomer M1 and poly-
mer P 1 measured in 0.1 M Bu4NPF6 (supporting electrolyte)
in methylene chloride at 25 °C and a scan rate of 25 mV/s.
and 1.3 °C, respectively. All the glass transition tem-
peratures are lower than room temperature.
irreversible peak can be observed at -1.35 V (vs Fc/
Fc+) for M1, identical to the reversible peak for polymer
P 1 at -1.36 V (vs Fc/Fc+). The onset oxidation potential
and the reduction potential for M1, at 0.46 V and -1.25
V (vs Fc/Fc+), were used to deduce the energy levels of
M1. It was found that the HOMO and LUMO energies
for M1 are -5.26 eV and -3.55 eV (vs vacuum),
respectively. Similar analysis was applied to all the
other materials, and the results are summarized in
Table 1.
Op tica l P r op er ties. The electronic absorption spec-
tra of the monomers M1 and M2 and polymers P 1, P 2,
and P 3 are shown in Figure 1. Monomers M1 and M2
exhibit maximum absorptions at 592 and 589 nm,
respectively, which also dominate the absorptions of the
corresponding polymers, as listed in Table 2. The
refractive indexes at 780 nm for P 1, P 2, and P 3 (in
Table 2) were determined with the prism-coupling
method (Metricon model 2010 prism coupler).
P h otocon d u ctivity a n d Mobility. The photocon-
ductivity and mobility measurements provide insightful
information on structure/property relationships. It was
found that the photoconductivity measured at an in-
tensity of 17.5 mW/cm2 (780 nm) and an applied electric
field of 30 V/µm for P 1 is slightly larger than P 2 (0.072
vs 0.065 ps/cm) but much larger than P 3 (0.025 ps/cm).
This is because the concentration of the chromophores
in P 1 is higher, which also serves as the photosensitizer
and determines the amount of photogenerated charges.
Bipolar (hole and electron) carriers were observed in
these materials by the mobility measurement (time-of-
flight (TOF) experiments) as what we reported before
with a similar system.17 The bipolar transport is also
confirmed in the time-dependent diffraction efficiency
experiment (Figure 3). The cancellation and revelation
of two types of gratings were observed during the
Red ox P r op er ties. The study of the redox properties
of the monomers M1 and M2 and polymers P 1, P 2, and
P 3 were implemented by cyclic voltammetry (CV).
Under the assumption that the energy level of ferrocene/
ferrocenium is 4.8 eV below vacuum, the LUMO and
HOMO energy levels could be calculated according to
EHOMO ) -(Eonset + 4.8) eV and ELUMO ) -(Eonset + 4.8)
ox
re
eV.10 As indicated in Figure 2, compound M1 undergoes
a one-electron oxidation process, ascribable to the facile
removal of an electron from the electron-rich amino
moiety. The oxidation potential (Eox) for M1 is about
0.54 V (vs Fc/Fc+), which is almost the same as the first
oxidation potential of the polymer P 1 (0.55 V), while
the second oxidation peak of P 1 can be barely observed
at 0.79 V (vs Fc/Fc+), due to high order oxidation
potential from the electron-rich backbone. Only one

Macromolecules, Vol. 36, No. 19, 2003
Photorefractive Polymers 7017
F igu r e 3. Diffraction efficiency as a function of time of
polymer P 2 at an applied field of 31 V/µm. The writing beams
are turned on at t ) 0, and one of the writing beams is blocked
at t ) 150 s.
F igu r e 4. Optical gain coefficients of polymers P 1, P 2, and
P 3 as a function of applied field.
grating relaxation process after blocking one writing
beam (at 150 s). The detailed physical mechanisms are
discussed in ref 17.
P h otor efr a ctive P r op er ties. In our previously re-
ported PR polymeric systems and other fully function-
alized PR polymers, photosensitizers such as metal-
containing macrocycles, 2,4,7-trinitro-9-fluorenone (TNF),
or fullerene (C60) are needed for charge generation.
Polymers reported here have been shown to be photo-
conductive due to the incorporation of the novel chromo-
phores in the absence of any other photosensitizers. The
unequivocal evidence for the PR effect was provided by
the two-beam coupling (2BC) experiments where two
coherent laser beams with equal intensity were inter-
sected inside the thick polymer films. The asymmetric
energy transfer between the two beams was clearly
observed due to the phase shift between the incident
light intensity and the refractive index modulation. The
optical gain coefficient (Γ) was calculated using the
following equation:
F igu r e 5. Diffraction efficiency of polymers P 1, P 2, and P 3
as a function of applied field.
identical field18 (net gain of 160 cm-1 at a field of 50
V/µm in ref 18 for their best composite polymer).
To gain further insight of the PR nature for these
polymers, degenerate four-wave mixing (DFWM) ex-
periments were performed, in which two s-polarized
laser beams were used to write the PR grating and a
weak p-polarized, counterpropagating beam served to
read the index grating. The field dependence of the
diffraction efficiencies for P 1 and P 2 is shown in Figure
5. Polymer P 1 consistently shows higher diffraction
efficiency (η): 68% at a field of 46 V/µm, which is much
higher than the functional polymers we reported be-
fore.10 Because of the dielectric breakdown, the field
applied to the film made with P 2 could not go beyond
37 V/µm, and a diffraction efficiency of 12% was
observed at that field. For P 3, no observable diffraction
efficiency could be obtained.
1
Γ ) ln
L
γâ
1 + â - γ
(1)
(
)
where â is the intensity ratio of the two incident writing
beams, γ ) Isignal (Ipump * 0)/Isignal (Ipump ) 0) is the ratio
of intensities of the signal beam with or without the
presence of the pump beam, and L is the optical path
length of the beam with gain. As shown in Figure 4,
the gain coefficients for all three polymers (P 1, P 2, and
P 3) increase monotonically with the increment of the
applied field. While P 1 shows the highest gain coef-
ficient of 180 cm-1 at a field of 50 V/µm, P 2 exhibits a
gain coefficient of 60 cm-1 at a field of 37 V/µm before
the dielectric breakdown and P 3, 8.8 cm-1 at 69 V/µm.
Since the absorption coefficients (R) for P 1, P 2, and P 3
are 24, 17.5, and 2.8 cm-1, respectively, the net optical
gain coefficients (Γ - R) are 158, 43, and 6 cm-1 for P 1,
P 2, and P 3 at the fields mentioned. Thus, P 1 is the best
among all three polymers in terms of both gain coef-
ficient and net gain coefficient, which almost doubles
the value of the highest gain of our recently reported
polymers (83 cm-1 at a field of 60 V/µm, polymer 4 in
ref 10). The net optical gain coefficient is also compa-
rable with that of the best composite materials at the
These results can be interpreted on the basis of the
figure of merit (FOM) for low-Tg organic PR materials19
1
M
2µ2∆R
kbT
F )
9µâ +
(2)
(
)
where µ is the dipole moment, ∆R the anisotropy of the
linear polarizability, â the second-order polarizability,

7018 You et al.
Macromolecules, Vol. 36, No. 19, 2003
play a crucial role: at the same field of 31 V/µm, the
polymer P 2 with a Tg of 5.6 °C shows response time of
3.8 s, but the P 1 with much higher Tg of 20.1 °C shows
a response time of 16 s. Lower Tg means the chromo-
phores gain more free volume at room temperature to
reorient in response to the integrated fields, resulting
in a faster reorientation time.
Con clu sion
Several fully functionalized photorefractive polymers
containing conjugated backbones and novel chromophores
have been synthesized. Studies on the thermal, optical,
electrical, and photorefractive properties indicated that
these polymers exhibit low glass transition tempera-
tures and show very interesting optical properties in
relatively low external fields. High net optical gain and
diffraction efficiency can be obtained for the polymer P 1,
while faster reorientational time can be demonstrated
by polymer P 2 of lower glass transition temperature by
modifying the alkyl groups associated with the chromo-
phores. This simple polymeric PR system can be further
extended into a broader scope by introducing different
polymer backbones and incorporating various NLO
chromophores.
F igu r e 6. Response time constants of polymers P 1 and P 2
as a function of applied field.
kb the Boltzmann constant, T the temperature, and M
the molar mass. The elongation of alkyl groups inevi-
tably increases the molar mass (M in eq 2) and leads to
the decrease of the FOM. Since the optical gain coef-
ficient is a linear function of FOM, polymer P 2 exhibits
a relatively lower gain than P 1. For polymer P 3, the
low content (30%) of NLO chromophores is unfavorable
to obtain high optical gain. It is also conceivable that
the electronic transport along the chromophore is
diminished when the density of chromophores is re-
duced, confirmed with experimental results from photo-
conductivity and mobility measurements (in Table 2).
The above argument is reinforced by the observed trend
of the diffraction efficiency.
Exp er im en ta l Section
Gen er a l Meth od s. All chemicals were purchased from
commercial suppliers and used as received unless otherwise
specified. All reactions were carried out under a nitrogen
atmosphere unless otherwise noted. Tetrahydrofuran (THF)
was distilled over sodium and benzophenone. The 1H NMR
spectra at 500 MHz and 13C NMR spectra at 125 MHz were
collected on a Bru¨ker DRX-500 spectrometer. UV/vis spectra
were recorded on
a Shimadzu UV-2401PC spectrometer.
Solution UV-vis spectra were measured as 1 × 10-5
M
solutions in chloroform at 25 °C. Thermal analyses were
performed on a Shimadzu DSC-60 and TGA-50 under a
nitrogen atmosphere at a heating rate of 5 °C/min. Cyclic
voltammetry was performed on a Bioanalytical Systems CA-
50W with a three-electrode compartment cell (Bioanalytical
Systems Inc.) equipped with a platinum disk as working
electrode, a platinum wire as counter electrode, and a Ag/
AgNO3 electrode as reference electrode. The supporting elec-
trolyte used was tetrabutylammonium hexafluorophosphate
(0.1 M in methylene chloride). The scan rate was adjusted to
25 mV/s. All potentials were calibrated with ferrocene/ferro-
cenium (Fc/Fc+) couple. All reported E1/2 values are taken as
the average of the anodic and cathodic peak potentials. Mass
spectrometry was provided by a Hewlett-Packard Agilent 1100
LCMSD. Elemental analyses were performed by Atlantic
Microlab, Inc. Molecular weights and distributions were
measured with a Waters RI and UV GPC system (Waters 410
differential refractometer and Waters 486 tunable absorbance
detector) with polystyrene as the standard and THF as the
eluent. The films for two-beam coupling and four-wave mixing
experiments were prepared by sandwiching the materials
between two indium-tin oxide (ITO)-covered glass substrates.
Homogeneous film samples suitable for two-beam coupling and
degenerate four-wave mixing experiments were prepared by
sandwiching a slightly warmed material (∼30-55 °C) between
two indium-tin oxide (ITO)-coated glass substrates with
thickness controlled by a polyimide spacer, affording film
thickness of ∼130 µm. Samples so prepared were also used
for measurement of the absorption coefficients (R). Two-beam
coupling experiments were performed using a diode laser (780
nm, 50 mW) as the light source. The two split p-polarized laser
beams with equal intensity (2 × 948 mW/cm2) were intersected
in the film with an external cross-angle of 18° to generate the
refractive index grating. The film normal was tilted at an angle
of 53° with respect to the symmetric axis of the two writing
beams to provide a nonzero projection of the grating wave
However, the lower glass transition temperature for
P 2 seems to reduce the response time, measured by
using DFWM and determined by fitting numerically the
following quadratic biexponential function:20
2
-t
τ1
-t
τ2
η(t) ) η0 1 - a exp
+ (1 - a) exp
(3)
{
( )
( ) }
[
]
where η is the time-dependent diffraction efficiency, η0
is the diffraction efficiency in the steady state, τ1 and
τ2 are the fast and slow time constants, respectively,
and a is a dimensionless weighting factor.
The existence of two main processes in low-Tg organic
PR materialssthe buildup of the space charge field and
the contribution of the orientation of the chromophoress
justifies the use of the biexponential equation. The
temporal change of the space charge field could be
related to the fast time constant τ1, while the slow τ2
indicates the reorientation of the chromophore in the
combined fields. As indicated in Figure 6, the higher
the field, the faster the response time constants. At the
same electric field, both τ1 and τ2 for P 2 are smaller
than those for P 1. The fast time constant τ1 does not
vary too much for P 1 and P 2 (e.g., at external field of
31 V/µm, 2.0 s for P 1 and 1.2 s for P 2) due to structural
similarities between the two constructing chromophores
(and also the polymers). The slight difference in τ1 can
be interpreted according to the differences in the charge
carrier mobility (Table 2). The faster the mobility, the
quicker the buildup of the internal electric field which
results in the smaller τ1. However, for the slow time
constant τ2, the glass transition temperature starts to

Macromolecules, Vol. 36, No. 19, 2003
Photorefractive Polymers 7019
vector along the poling axis. Such experiment arrangement
resulted in the grating spacing is 3.66, 3.655, and 3.61 µm for
P 1, P 2, and P 3, respectively. The transmitted intensities of
the two beams were monitored by two calibrated diode
detectors. Diffraction efficiency was measured by the degener-
ate four-wave mixing (DFWM) experiment, in which two
s-polarized laser beams (780 nm) of equal intensity (2 × 948
mW/cm2) intersected in the film with an external cross-angle
of 18° and film normal was tilted at an angle of 53° with
respect to the symmetric axis of the two writing beams to write
the index grating, and a weak p-polarized beam (probe beam,
12.5 mW/cm2) counterpropagating to one of the writing beams
was used to read the index grating formed in the material.
The diffracted light intensity of the probe beam was detected
by a photodiode and subsequently amplified with a lock-in
amplifier. The diffraction efficiency η was calculated as the
ratio of the intensities of the diffracted beam to the incident
reading beam. Data were collected by a computer. The refrac-
tive indexes of the polymers were measured by using the
Metricon model 2010 prism coupler at 780 nm. Photoconduc-
tivity was measured at the wavelength 780 nm with the dc
technique and calculated as σ ) IL/VS, where I is the
photocurrent difference between total current in the presence
of light and the dark current, L is the sample thickness (25
µm in our experiment), V is the applied voltage, and S is the
sample area. The mobility was measured using time-of-flight
method as described in ref 17, calculated as µ ) d2/VtT, where
d is the sample thickness, V is the applied voltage, and tT is
the transit times for generated charges to drift across the film
thickness d.
88H, CH2), 1.62 (m, 4H, CH2), 1.90 (m, 2H, CH2), 2.08 (m, 2H,
CH2), 2.58 (t, J ) 8.0 Hz, 4H, benzyl), 3.38 (t, J ) 7.8 Hz, 4H,
NCH2), 6.65 (d, J ) 9.0 Hz, 2H, aromatic protons), 6.73 (d, J
) 15.8 Hz, 1H, trans double bond), 7.52 (d, J ) 9.0 Hz, 2H,
aromatic protons), 7.59 (d, J ) 15.8 Hz, 1H, trans double bond),
7.58 (s, 1H, aromatic proton), 7.59 (s, 1H, aromatic proton).
13C NMR (125 MHz, CDCl3, ppm): 13.95, 22.46, 22.67, 27.00,
28.95, 29.27, 29.28, 29.34, 29.37, 29.41, 29.53, 29.56, 31.36,
31.90, 39.30, 39.80, 51.33, 53.48, 96.59, 100.32, 102.36, 108.56,
111.55, 112.07, 112.15, 112.81, 121.32, 132.54, 139.25, 144.72,
144.86, 147.34, 152.15, 172.38, 177.35. MS m/z calcd from
C
78H124N4OI2 (M-H)-: 1386.67. Found: 1386.40.
P olym er iza tion . A typical polymerization procedure is
listed below: To a mixture of M0 (0.316 g, 0.477 mmol) and
M1 (0.567 g, 0.455 mmol) in 5 mL of THF was added
Pd(PPh3)2Cl2 (0.016 g, 0.02 mmol) as the catalyst. The mixture
was kept at reflux with stirring for 2 days, and then the
catalyst was removed by filtration through Celite. The polymer
was isolated through precipitation from methanol/hexane (2:1
v/v). Further purification was conducted by redissolving the
polymer in chloroform, filtering, and reprecipitating. The
polymer was a dark blue viscous semisolid after being dried
under vacuum at 40 °C overnight. Yield: 0.464 g (95%).
Ack n ow led gm en t. This work was supported by the
National Science Foundation and the Air Force Office
of Scientific Research. This work also benefited from the
support of NSF MRSEC program at The University of
Chicago.
Syn th esis of M0. Thiophene (5.25 g, 62.5 mmol) was
refluxed in 20 mL of hexane with TMEDA (21.77 g, 187.4
mmol), followed by dropwise adding n-butyllithium (187.4
mmol, 2.5 M in hexane). The mixture was kept at reflux for
2.5 h, and then tributyltin chloride (49.2 g, 151.2 mmol) was
dropwise added followed by another 0.5 h of reflux. The
mixture was then poured into water, and the organic layer
was separated and distilled under reduced pressure to yield
monomer M0 (22.84 g, 55%). 1H NMR (500 MHz, CDCl3,
ppm): δ 0.89 (t, J ) 7.3 Hz, 18H, CH3), 1.10 (t, J ) 7.0 Hz,
12H, CH2), 1.33 (m, 12H, CH2), 1.58 (m, 12H, CH2), 7.35 (s,
2H).
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M1. To a mixture of compound 1 (1.00 g, 0.938 mmol)10 and
compound 2 (0.238 g, 1.195 mm0l) in 10 mL of ethanol was
added a catalytic amount of NaOH (1.9 mg, 0.047 mmol, 15
mg/mL aqueous solution). The mixture was allowed to reflux
for 5 days. After solvent removal, the residue was purified by
flash chromatography on silica gel (hexane:ethyl acetate ) 6:1
v/v) to yield monomer M1 as a dark blue/purple solid (1.031
1
g, 88%). H NMR (500 MHz, CDCl3, ppm): δ 0.88 (t, J ) 7.1
Hz, 6H, CH3), 1.25-1.34 (m, 72H, CH2), 1.62 (m, 4H, CH2),
1.74 (s, 6H, CH3), 2.58 (t, J ) 8.1 Hz, 4H, benzyl), 3.38 (t, J )
7.9 Hz, 4H, NCH2), 6.62 (d, J ) 9.0 Hz, 2H, aromatic protons),
6.71 (d, J ) 15.8 Hz, 1H, trans double bond), 7.51 (d, J ) 9.0
Hz, 2H, aromatic protons), 7.58 (d, J ) 15.8 Hz, 1H, trans
double bond), 7.58 (s, 1H, aromatic proton), 7.59 (s, 1H,
aromatic proton). 13C NMR (125 MHz, CDCl3, ppm): 14.12,
22.68, 26.78, 27.00, 27.35, 29.29, 29.35, 29.38, 29.41, 29.49,
29.56, 29.65, 29.68, 30.18, 31.90, 39.81, 45.25, 51.32, 54.19,
93.52, 96.59, 100.33, 108.14, 111.62, 112.02, 112.82, 121.35,
132.58, 139.25, 144.73, 144.87, 148.26, 152.28, 174.06, 176.36.
MS m/z calcd from C68H104N4OI2(M-H)-: 1246.40. Found:
1246.20.
M2. Prepared in a similar way as that of M1. Silica gel,
hexane:ethyl acetate ) 25:1 v/v. Yield: 65%. 1H NMR (500
MHz, CDCl3, ppm): δ 0.83-0.88 (m, 12H, CH3), 1.23-1.34 (m,
MA034587O