Silylated oligomeric poly(ether-azomethine)s from monomers containing biphenyl moieties: Synthesis and characterization

Article abstract of DOI:10.1039/c7ra10929f

In this study, four new silicon-containing poly(ether-azomethine)s with linear structures were prepared using original silicon and biphenyl moiety-containing monomers: two diamines and two dialdehydes. The oligomeric natures of the samples were established by GPC analysis, which showed chains containing 3 to 5 repetitive units. The monomers and the oligomeric samples were structurally characterized by NMR and FT-IR spectroscopy. The solubilities of the samples in common organic solvents and their thermal behavior enable improvement of their industrial and technological processability. The optical band gaps of the oligomeric samples were estimated from optical measurements (UV-vis), and their electrical behavior in films was determined using the four-point method. The surface arrangements and morphological characteristics of the films were determined via atomic force microscopy measurements. The roughness, area increase percentage and layer stiffness of the films were also measured using this technique.

Full text of DOI:10.1039/c7ra10929f

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Silylated oligomeric poly(ether-azomethine)s from
monomers containing biphenyl moieties: synthesis
and characterization
Cite this: RSC Adv., 2018, 8, 1296
Alain Tundidor-Camba,ae Carmen M. Gonzalez-Henrıquez,b Mauricio A. Sarabia-
´
´
Vallejos,c Luis H. Tagle,a Rene A. Hauyon,a Patricio A. Sobarzo,a Alexis Gonzalez,a
´
´
´
d
ae
Pablo A. Ortiz,a Eva M. Maya
and Claudio A. Terraza
*
In this study, four new silicon-containing poly(ether-azomethine)s with linear structures were prepared
using original silicon and biphenyl moiety-containing monomers: two diamines and two dialdehydes.
The oligomeric natures of the samples were established by GPC analysis, which showed chains
containing 3 to 5 repetitive units. The monomers and the oligomeric samples were structurally
characterized by NMR and FT-IR spectroscopy. The solubilities of the samples in common organic
solvents and their thermal behavior enable improvement of their industrial and technological
processability. The optical band gaps of the oligomeric samples were estimated from optical
measurements (UV-vis), and their electrical behavior in films was determined using the four-point
method. The surface arrangements and morphological characteristics of the films were determined via
atomic force microscopy measurements. The roughness, area increase percentage and layer stiffness of
the films were also measured using this technique.
Received 3rd October 2017
Accepted 14th December 2017
DOI: 10.1039/c7ra10929f
rsc.li/rsc-advances
to their optoelectronic, non-linear optic and electroluminescent
behaviors, they are excellent candidates as materials to be used
Introduction
in the fabrication of organic light-emitting diodes (OLEDs),
photovoltaic cells, and pH sensors, among other applications.3
Early studies on the synthesis of PAMs led to low molecular
weight polymers and intractable materials (both infusible and
insoluble); these characteristics hampered their potential
applications due to their low processability.4 This, in turn,
excluded the possibility of fabricating lms, thus limiting the
characterization of these materials apart from high-
temperature processability leading to the fabrication of
bers.5 The ability to form lms is desirable today for the
fabrication of devices such as OLEDs.6
In recent years, many approaches have been used to solve
this problem: namely, the introduction of side groups in the
polymer chain7 and their complexation with Brønsted acids
such as diphenyl-phosphate and di-m-cresyl phosphate and
Lewis acids such as gallium chloride to interact with the
nitrogen atom of the imine functional group.5 These strategies
enhanced the solubility of PAMs in common organic solvents
and affected the thermal behavior of the polymers, achieving
meltability without compromising the high thermal stability
that characterizes this family of materials.
Conjugated polymers have recently been used as photovoltaic
compounds in organic bulk heterojunctions due to their
intrinsic properties, such as their small bandgaps and good
hole transportation. Their ease of processing and low cost in
comparison to inorganic semiconductors, namely silicon, make
them excellent alternatives for this purpose.1 On this basis, the
synthesis of organometallic polymers with silicon atoms in their
structures may unify the features of a suitable band gap with
electron delocalization, high voltage, and low sub-bandgap
absorption, among other properties.
Conjugated poly(azomethine)s (PAMs) are a family of poly-
mers which possess an imine repeating unit, R–HC]N–R0,
where R and R0 are moieties that allow conjugation (e.g.
aromatic moieties).2 These materials have been the subject of
extensive investigation, especially in the last three decades; due
aResearch Laboratory for Organic Polymers (RLOP), Department of Organic Chemistry,
´
Ponticia Universidad Catolica de Chile, P.O. Box. 306, Post 22, Santiago, Chile.
E-mail: cterraza@uc.cl
bLaboratory of Nanotechnology and Advanced Materials (LNnMA), Chemistry
´
Department, Universidad Tecnologica Metropolitana, P.O. Box 9845, Post 21,
Santiago, Chile
Another widely used strategy to achieve better processability
of polymers is the introduction of exible –Ar–Si(R1R2)–Ar–
units, in which the R1 and R2 groups are alkyl or phenyl units. In
this sense, this strategy has already proved useful in the prep-
aration of highly thermally stable poly(ester)s and poly(amide)s
c
´
´
Departamento de Ingenierıa Estructural y Geotecnia, Escuela de Ingenierıa, Ponticia
´
Universidad Catolica de Chile, Santiago, Chile
dInstituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones
´
Cientıcas, CSIC, Spain
e
´
UC Energy Research Center, Pontica Universidad Catolica de Chile, Chile
1296 | RSC Adv., 2018, 8, 1296–1312
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with glass transition temperatures (Tg) below their thermal
decomposition temperatures (TDT) and with high solubility in
common organic solvents.8
Experimental section
Materials
The –Ar–Si(R1R2)–Ar– moiety has not only been investigated
in the improvement of the processability of polymers. More
recently, this unit has been used in the creation of materials for
phosphorescent organic light-emitting diodes (PHOLEDs),9–13
separation and capture of CO2 gas,14,15 membrane fabrication
for gas separation applications,16 fabrication of low-cost poly-
mer solar cells17,18 and synthesis of porous aromatic
frameworks.19
In the case of PAMs, Iwan and Sek3 have provided an
extensive review that considers the applications of these poly-
mers, ranging from the aerospace industry to catalyst carriers,
ion complexing agents, and anticorrosion materials as well as in
the fabrication of photovoltaic cells and OLEDs.
n-Butyllithium solution (2 M in cyclohexane), dimethyldi-
chlorosilane, diphenyldichlorosilane, bis(triphenylphosphine)
palladium(II) dichloride, anhydrous N,N-dimethylacetamide
(DMAc), anhydrous calcium chloride, p-toluenesulfonic acid, 1-
uoro-4-nitrobenzene, hydrazine monohydrate (80%) and Pd/C
(10% w/w) were obtained from Aldrich Chemical (Milwaukee,
WI, USA). 4-Hydroxyphenylboronic acid and 4-for-
mylphenylboronic acid were purchased from AK Scientic, Inc.
(San Francisco, USA). All other reagents and solvents were
purchased commercially as analytical grade (Aldrich Chemical,
WI, USA or Merck, Darmstadt, Germany). Immediately before
use, diethyl ether was reuxed with sodium under nitrogen
atmosphere and then distilled.
Considering the interesting properties of both the –Ar–
Si(R1R2)–Ar– unit and the PAMs family, it is quite worthwhile to
investigate their use in the same macromolecular material. The
Measurements
scientic literature provides very few studies devoted to this NMR spectra were acquired on a 400 MHz instrument (Bruker
subject. In fact, in studies in which the silicon atom is incor- AC-200, Germany) using CDCl3 or DMSO-d6 as solvent for the
porated into the poly(azomethine) backbone, it is not part of a – precursors and monomers, while DMAc-acetone-d6 was
Ar–Si(R1R2)–Ar– unit. In general, the silicon atom is part of employed for the oligomeric samples. In all cases, TMS was
a siloxane moiety, which implies a Si–O bond.20–25 In 2014, added as an internal standard. FT-IR spectra (KBr pellets) were
Zaltariov et al.26 reported a PAM with a central silicon atom recorded on a Perkin-Elmer (Fremont CA, USA) 1310 spectro-
bonded to four carbon atoms complexed with different metal photometer over the range of 4000 to 450 cmꢁ1. The melting
ions. However, they used a methylene unit instead of an points of the precursors and monomers were obtained using
aromatic moiety bonded to a silicon atom which, along with a Stuart Scientic SMP3 instrument. Elemental analyses were
other structural features, rendered the polymer too exible; conducted on Fisons EA 1108-CHNS-O equipment (Thermo
thus, the Tg measured for the polymer was 48 ꢀC. For a material Scientic, Waltham/MA, USA). Viscosimetric measurements
to be used for charge transport in a PHOLED device, its Tg value were performed in a Desreux–Bischof-type dilution viscosimeter
must be over 100 ꢀC to avoid deterioration of its mechanical at 25 ꢀC (c ¼ 0.5 g dLꢁ1). The weight-average (Mw) and number-
properties under high temperature conditions.9
average (Mn) molecular weights, degrees of polymerization (DP)
The aim of this work is the synthesis and chemical and and polydispersity indices (PDI) of the oligomeric samples
structural characterization of four oligomeric poly(ether- relative to polyethylene glycol oxide standards were determined
azomethine)s, designated PAM-I, PAM-II, PAM-III and PAM-IV, at 40 ꢀC using a GPC 150cv system (Waters, USA) equipped with
which contain silicon atoms and imino functions in their main a refractive index detector. For this, solutions of oligomeric
chains in addition to methyl or phenyl groups in their lateral samples in DMF-0.05 M LiBr (c ¼ 0.5 mg mLꢁ1) were ltered
segments. Thus, the structural properties of the materials were through micro-lters with pores of 2 mm; then, 100 mL of sample
studied using nuclear magnetic resonance (NMR) spectroscopy, was injected at 1 mL minꢁ1 in a MesoPore column (300 ꢂ 75
Fourier transform infrared (FT-IR) spectroscopy and elemental mm), which is suitable for samples with molecular weights up
analyses. The optical properties of the samples were measured to 25 000. Glass transition temperatures were obtained with
via UV-vis spectroscopy (optical absorption). Their solubilities a Mettler-Toledo (Gꢁre1ifensee, Switzerland) DSC 821 calorimetric
were tested in several organic solvents, and their thermal system (20 ꢀC min under N2 ow) aer the second heating
behavior was established by thermogravimetric analysis (TGA) scan. Thermogravimetric analyses were carried out on a Mettler
and differential scanning calorimetry (DSC). Finally, two (Switzerland) TA-3000 calorimetric system equipped with a TC-
different lms were prepared using the spin-coating technique 10A processor and a TG-50 thermobalance with a Mettler MT5
for deposition, using silicon wafers previously treated with microbalance. Samples of 6 to 10 mg were placed in an alumina
piranha solution (H2SO4 : H2O2) as the substrate. The samples sample holder, and the thermogravimetric measurements were
selected were PAM-I and PAM-II; both dissolved in DMAc. The carried outꢁb1etween 25 ꢀC and 800 ꢀC with a heating rate of
thicknesses of the lms were determined by ellipsometry using 20 ꢀC min under N2 ow. The absorption spectra of the
the refractive index of each layer, determined through an Abbe polymers were recorded at room temperature between 270 and
refractometer. Morphological representations of the lm 440 nm using a Lambda 35 model UV-vis spectrophotometer
surfaces were obtained by atomic force microscopy (AFM). (Perkin Elmer, USA) using solutions of DMSO (c ¼ 0.5 g Lꢁ1).
Additionally, conductivity values for the solid lms were also
A spin coater, model KW-4A from Chemat Scientic, coupled
determined through the four-point method, using a silicon with an oil-free vacuum pump (Rocker Chemker 410) was used
wafer as the calibration material.
for deposition of the polymer dissolved in DMAc over the
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pretreated silicon wafer. This instrument possesses two spin-
ning stages: (1) 500 to 2500 rpm (2 to 18 seconds) and (2) 800 to
8000 rpm (3 to 60 seconds) with a tunable acceleration rate. A
Bresser Trino Researcher II (40-1000X) trinocular microscope
coupled with a CCD color camera (5 Mp, Bresser) and a cold
light source (Optika CL-41) was used as the rst approach for
visualizing the topographies of the polymeric lms. The surface
topographies of the polymers were obtained at room tempera-
ture using an NTEGRA Prima AFM (NT-MDT Co.) in intermit-
tent contact mode at different scan ranges (25 ꢂ 25 mm2 and 50
ꢂ 50 mm2). Force spectroscopy measurements were performed
with a NaioAFM atomic force microscope (Nanosurf Inc.) using
a conical contact mode tip. These results were analyzed through
the model proposed by Hertz for tip indentation over so
surfaces according to Roa et al.27 Images were treated and
analyzed using the off-line soware Gwyddion.28 A multi-angle
laser ellipsometer, model SE 400adv, from SENTECH Instru-
ments GmbH was used to perform optical measurements with
variable incidence angles from 30ꢀ to 90ꢀ in steps of 0.5ꢀ; the
equipment possessed an attached motorized goniometer
Precursors and monomer synthesis
Bis(4-bromophenyl)di-R-silane (R ¼ methyl (1), phenyl (2)).
Under nitrogen atmosphere, n-BuLi in cyclohexane (2 M)
(42.4 mL, 84.78 mmol) was slowly added to a solution of 1,4-
dibromobenzene (20.00 g, 84.78 mmol) in 300 mL of anh.
Et2O at 0 ꢀC. Aer stirring for one hour, a solution of
dichlorodi-R-silane (42.39 mmol) in 20 mL of anh. Et2O was
added, and the mixture was stirred for 12 hours. The obtained
white solid was ltered, and the solution was washed with
water. The organic phase was dried with anh. Na2SO4 and
concentrated to obtain a yellow oil. Cold methanol was used
to obtain the dibromo derivatives as white solids; these were
ltered, washed with cold methanol and dried at 40 ꢀC under
vacuum (R ¼ methyl, 79% and R ¼ phenyl, 71%). The prod-
ucts were recrystallized from acetone with yields close to
80%.
¨
(Huber Diffraktionstechnik GmbH & Co. KG) for control inci-
dence angle variation. A stabilized He–Ne laser (l ¼ 633 nm)
˚
permitted a precision of 0.1 A in the thin lm thickness
Bis(4-bromophenyl)dimethylsilane (1). Mp ¼ 73 ꢀC to 74 ꢀC.
IR-TF (KBr, n, cmꢁ1): 3067, 3046, 3031, 3011 (C–H arom.); 2960
(C–H aliph.); 1566 (C]C); 1373 (Si–Ph); 1253 (Si–CH3); 832
measurements. This equipment was used to measure the
thicknesses of the polymeric lms (PAM-I and PAM-II). Refrac-
tive indices of these polymers were obtained using an Abbe-
¨
1
Refractometer model AR4 from A. KRUSS Optronic GmbH in
(arom. p-subst.); 500 (C–Br). H NMR (CDCl3, d, ppm): 0.44 (s,
DMAc solution at room temperature. The four-point method
was carried out using a digital multimeter from GW Instek,
model GDM-8255, which includes a 4 W test lead for conduction
measurements as an optional accessory. A DC power supply
from GW Instek, model PLR 20-18, was used to administer
a constant current to the system (6 A).
6H, H5); 7.27 (d, J ¼ 7.91 Hz, 4H, H2); 7.39 (d, J ¼ 7.86 Hz, 4H,
H3). 13C NMR (CDCl3, d, ppm): ꢁ2.42 (C5); 124.38 (C4); 131.23
(C3), 135.82 (C2), 136.55 (C1). 29Si NMR (CDCl3, d, ppm): ꢁ7.00.
Elem. anal. calcd for C14H14Br2Si; (370.17): C, 45.42%; H, 3.82%.
Found: C, 45.39%; H, 3.79%.
Bis(4-bromophenyl)diphenylsilane (2). Mp ¼ 159 ꢀC to
164 ꢀC. FT-IR (KBr, n, cmꢁ1): 3063, 3023 (C–H arom.); 1568 (C]
1
C); 1427, 729 (Si–Ph); 809 (arom. p-subst); 512 (C–Br). H NMR
Polymeric lm preparation
(CDCl3, d, ppm): 7.37 (m, 4H, H7); 7.39 (m, 4H, H2); 7.44 (m, 2H,
H8); 7.51 (m, 8H, H3, H6). 13C NMR (CDCl3, d, ppm): 123.94
(C4); 127.07 (C7); 128.97 (C8); 130.18 (C3), 131.62 (C1), 131.97
(C5), 135.18 (C6), 136.78 (C2). 29Si NMR (CDCl3, d, ppm):
ꢁ14.01. Elem. anal. calcd for C24H18Br2Si; (494.31): C, 58.31%;
H, 3.68%. Found: C, 58.26%; H, 3.61%.
Silicon wafer cleaning method. The surface nature of the
silicon wafer (Si(100)) was modied through a specic treat-
ment which improves the hydrophilicity of the surface. The
substrate was rst immersed in a solution for increased surface
density, leaving hydroxylated oxide at the top level. This pre-
treatment involves a piranha solution that removes the orig-
inal native silicon oxide lm. The silicon wafer was placed in the
piranha solution (H2SO4 : H2O2; 7 : 3 vol/vol) for 30 minutes at
Bis(4-(4-hydroxyphenyl)phenyl)di-R-silane (R ¼ methyl (3),
phenyl (4)). To a mixture of dibromo derivative (1 or 2) (13.51
mmol), 4-hydroxyphenylboronic acid (4.66 g, 33.78 mmol) and
bis(triphenylphosphine) palladium(II) dichloride (0.40 g, 0.570
mmol), a solution containing K2CO3 (12.44 g), water (45 mL)
and 1,4-dioxane (45 mL) was added; then, the mixture was
heated at 85 ꢀC for 12 hours. Aer adding 100 mL of water,
a brown solid was obtained for R ¼ methyl and a black/brown-
oil was obtained for R ¼ phenyl. The solids were recovered
through ltration and treated with acetone and chloroform,
respectively; then, the samples were ltered through celite. Aer
drying and ltering, the solutions were concentrated to produce
a brown solid for derivative 3 (94%), which was recrystallized
from abs. ethanol, and a light brown oil for 4. This oil was
dissolved in chloroform and then added to n-hexane to obtain
80 C.29,30 Aerward, the wafers were washed with HPLC water
ꢀ
and slightly sonicated in order to eliminate traces of sulfuric
acid. Then, the wafers were dried with an ultra-pure nitrogen
gas jet. This substrate treatment is important to improve the
adherence of the polymer when the lm is formed.
Thin lm deposition using spin coating. 9.5 mg of each
sample were dissolved in 500 mL of DMAc. Then, 20 mL of each
dissolved sample were deposited over a previously treated
hydrophilic silicon wafer (1 ꢂ 1 cm2). The deposition parame-
ters for the same compound were progressively varied by
changing the acceleration rate (rpm) of the spin deposition.
Aerward, the lms were subjected to strong vacuum (10ꢁ4 torr)
for a week.
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a grayish-white solid which was ltered, washed with n-hexane
and dried at 60 C under vacuum (88%).
Bis(4-(4-(4-nitrophenoxy)phenyl)phenyl)dimethylsilane (5).
Mp ¼ 98 ꢀC to 103 ꢀC. FT-IR (KBr, n, cmꢁ1): 3080, 3067, 3011 (C–
H arom.); 2954 (C–H aliph.); 1584, 1486 (C]C); 1518, 1345
(NO2); 1246 (C–O–C); 1110 (Si–Ph); 847 (Si–CH3); 819 (arom p-
subst.). 1H NMR (CDCl3, d, ppm): 0.64 (s, 6H, H13); 7.07 (d, J ¼
9.06, 4H, H10); 7.16 (d, J ¼ 8.51, 4H, H7); 7.60 (d, J ¼ 7.76, 4H,
H3); 7.65 (m, 8H, H2, H6); 8.21 (d, J ¼ 9.13, 4H, H11). 13C NMR
(CDCl3, d, ppm): ꢁ2.33 (C13); 117.27 (C10); 120.77 (C7); 125.98
(C11); 126.46 (C3); 128.96 (C6); 134.81 (C2); 137.22 (C1); 138.28
(C5); 140.77 (C4); 142.78 (C12); 154.35 (C8); 163.21 (C9). 29Si
NMR (CDCl3, d, ppm): ꢁ7.83. Elem. anal. calcd for
ꢀ
Bis(4-(4-hydroxyphenyl)phenyl)dimethylsilane (3). Mp
¼
87 ꢀC to 94 ꢀC. FT-IR (KBr, n, cmꢁ1): 3493 (O–H); 3052, 3014 (C–
H arom.); 2958 (C–H aliph.); 1520 (C]C); 1434 (Si–Ph); 1247 (Si–
CH3); 819 (arom. p-subst); 1H NMR (DMSO-d6, d, ppm): 0.56 (s,
6H, H10); 6.88 (d, J ¼ 8.54 Hz, 4H, H7); 7.50 (d, J ¼ 8.53 Hz, 4H,
H6); 7.55 (d, J ¼ 8.15 Hz, 4H, H2), 7.58 (d, J ¼ 8.19 Hz, 4H, H3),
9.59 (s, 2H, H9). 13C NMR (DMSO-d6, d, ppm): ꢁ2.57 (C10);
115.78 (C7); 125.45 (C3); 127.79 (C6); 130.73 (C5); 134.43 (C2);
135.44 (C1); 140.96 (C4); 157.34 (C8). 29Si NMR (DMSO-d6, d,
ppm): ꢁ8.64. Elem. anal. calcd for C26H24O2Si; (396.59): C,
78.74%; H, 6.11%. Found: C, 78.64%; H, 6.08%.
C
38H30N2O6Si; (638.79): C, 71.44%; H, 4.74%; N, 4.39%. Found:
C, 71.40%; H, 4.68%, N, 4.32%.
Bis(4-(4-(4-nitrophenoxy)phenyl)phenyl)diphenylsilane (6).
Mp ¼ 196 C to 197 C. FT-IR (KBr, n, cmꢁ1): 3068, 3046, 3016
(C–H arom.); 1587, 1486 (C]C); 1517, 1343 (NO2); 1246 (C–O–
C), 1111 (Si–Ph), 818 (arom. p-subst.). 1H NMR (CDCl3, d, ppm):
7.06 (d, J ¼ 9.10 Hz, 4H, H10); 7.16 (d, J ¼ 8.43 Hz, 4H, H7); 7.41
(t, J ¼ 7.40 Hz, 4H, H15); 7.47 (m, 2H, H16); 7.62 (d, J ¼ 8.10 Hz,
4H, H3); 7.64 (d, J ¼ 9.44 Hz, 4H, H14), 7.67 (d, J ¼ 9.04 Hz, 4H,
H6); 7.70 (d, J ¼ 7.31 Hz, 4H, H2), 8.21 (d, J ¼ 9.12 Hz, 4H, H11).
13C NMR (CDCl3, d, ppm): 117.29 (C10); 120.79 (C7); 125.98
(C11); 126.47 (C3); 128.03 (C15); 128.98 (C6); 129.81 (C16);
133.27 (C1); 133.94 (C13); 136.39 (C14); 137.00 (C2); 138.10 (C5);
141.13 (C4); 142.79 (C12); 154.45 (C8); 163.17 (C9). 29Si NMR
(CDCl3, d, ppm): ꢁ14.37. Elem. anal. calcd for C48H34N2O6Si;
(762.93): C, 75.56%; H, 4.50%; N, 3.67%. Found: C, 75.51%; H,
4.44%, N, 3.60%.
ꢀ
ꢀ
Bis(4-(4-hydroxyphenyl)phenyl)diphenylsilane (4). Mp
¼
159 ꢀC to 164 ꢀC. FT-IR (KBr, n, cmꢁ1): 3355 (O–H); 3066, 3045,
3017 (C–H arom.); 1519 (C]C); 1427 (Si–Ph); 815 (arom. p-
subst.). 1H NMR: (CDCl3, d, ppm): 6.89 (d, J ¼ 8.06 Hz, 4H, H7);
7.38 (d, J ¼ 6.68 Hz, 4H, H12); 7.42 (d, J ¼ 6.97 Hz, 2H, H13);
7.47 (d, J ¼ 8.22 Hz, 4H, H6); 7.54 (d, J ¼ 7.65 Hz, 4H, H3); 7.62
(d, J ¼ 7.03 Hz, 8H, H2 y H11). 5.11 (s, 2H, H9). 13C NMR (CDCl3,
d, ppm): 115.77 (C7); 126.10 (C3); 127.92 (C12); 128.38 (C6);
129.62 (C13); 132.21 (C1); 133.48 (C5); 134.35 (C10); 136.42 and
136.87 (C2 and C11); 141.78 (C4), 155.41 (C8). 29Si NMR (CDCl3,
d, ppm): ꢁ14.46. Elem. anal. calcd for C36H28O2Si; (520.73): C,
83.03%; H, 5.43%. Found: C, 82.98%; H, 5.37%.
Bis(4-(4-(4-aminophenoxy)phenyl)phenyl)di-R-silane (R
¼
methyl (7), phenyl (8)). A mixture of dinitro derivative (5 or 6)
(6.341 mmol), Pd/C 10% w/w (250 mg) and 40 mL of THF was
heated under reux. Then, 10 mL of hydrazine monohydrate
(80%) was slowly added, and the mixture was stirred for 12
hours. The remaining catalyst was removed by ltration
through celite, the mixture was washed with THF, and the ob-
tained solution was concentrated. For the purication of the
samples: 7, obtained as a white solid, was treated with water;
aer ltration, the solid was washed again with water and dried
at 100 ꢀC under vacuum. The ochre solid was puried by
column chromatography (silica/chloroform) and then recrys-
tallized from acetonitrile to obtain a light orange solid (57%). 8
was obtained as a yellow solid; it was washed with water, then
Bis(4-(4-(4-nitrophenoxy)phenyl)phenyl)di-R-silane (R
¼
methyl (5), phenyl (6)). A mixture of diphenol derivative (3 or 4)
(11.90 mmol), potassium carbonate (3.37 g, 24.40 mmol) and
15 mL of DMF was stirred and heated at 60 ꢀC. To this mixture,
1-uoro-4-nitrobenzene (3.36 g, 23.80 mmol) was added; the
mixture was stirred for 12 hours while maintaining the
temperature. The obtained solution was added to a water–
ethanol solution (1 : 1 vol/vol), and an ochre solid was formed.
The product was ltered and washed with water and then
puried through silica column chromatography. For isolation
of 3, an n-hexane–chloroform mixture (1 : 1 vol/vol)) was
employed as the mobile phase. The residue obtained aer
solvent evaporation was recrystallized from ethyl acetate to
afford a light yellow solid (88%). For isolation of 4, a petroleum
ether–chloroform mixture (1 : 1 vol/vol) was used, and the ob-
tained residue was treated with diethyl ether to produce a light
yellow solid (74%).
ꢀ
ltered and dried at 70 C under vacuum. The compound was
puried by chromatography (silica/chloroform) (69%).
Bis(4-(4-(4-aminophenoxy)phenyl)phenyl)dimethylsilane (7).
Mp ¼ 95 C to 99 C. FT-IR (KBr, n, cmꢁ1): 3342, 3448 (N–H);
3011, 3038, 3065 (C–H arom.); 2957 (C–H aliph.); 1598, 1505,
1483 (C]C); 1239 (C–O–C); 1111 (Si–Ph); 879 (Si–CH3); 811
(arom. p-subst.). 1H NMR (CDCl3, d, ppm): 0.59 (s, 6H, H13);
ꢀ
ꢀ
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3.57 (s, 4H, H17); 6.68 (d, J ¼ 8.33 Hz, 4H, H11); 6.90 (d, J ¼
7.14 Hz, 4H, H10); 6.98 (d, J ¼ 7.36 Hz, 4H, H7); 7.50 (d, J ¼
7.39 Hz, 4H, H6); 7.54 (d, J ¼ 7.41 Hz, 4H, H3); 7.59 (d, J ¼
6.91 Hz, 4H, H2). 13C NMR (CDCl3, d, ppm): ꢁ2.30 (C13); 116.24
(C11); 117.36 (C7); 121.24 (C10); 126.24 (C3); 128.22 (C6); 134.67
(C2); 134.89 (C5); 136.41 (C1); 141.34 (C4); 142.84 (C12), 148.38
(C9); 158.67 (C8). 29Si NMR (CDCl3, d, ppm): ꢁ8.09. Elem. anal.
calcd for C38H34N2O2Si; (578.83): C, 78.84%; H, 5.93%; N,
4.84%. Found: C, 78.81%; H, 5.89%, N, 4.81%.
Bꢀis(4-(4-formylphenyl)phenyl)dimethylsilane (9). Mp
¼
230 C to 233 C. FT-IR (KBr, n, cmꢁ1): 3013, 3030, 3047, 3070
(C–H arom.); 2957 (C–H aliph.); 2737, 2823 (C–H ald.); 1694 (C]
O); 1604, 1572 (C]C); 1387 (Si–CH3); 1166, 1111 (Si–Ph); 813
(arom. p-subst.). 1H NMR (DMSO-d6, d, ppm): 0.64 (s, 6H, H10);
7.70 (d, J ¼ 8.07 Hz, 4H, H2); 7.74 (d, J ¼ 8.04 Hz, 4H, H3); 7.86
(d, J ¼ 8.16 Hz, 4H, H6); 7.97 (d, J ¼ 8.23 Hz, 4H, H7); 10.08 (s,
2H, H9). 13C NMR (DMSO-d6, d, ppm): ꢁ3.48 (C10); 125.72 (C3);
126.55 (C6); 129.08 (C7); 133.83 (C2); 134.98 (C8); 137.55 (C1),
139.12 (C4); 145.14 (C5); 191.12 (C9). 29Si NMR (DMSO-d6, d,
ppm): ꢁ7.85. Elem. anal. calcd for C28H24O2Si; (420.61): C,
79.95%; H, 5.76%. Found: C, 79.91%; H, 5.70%.
ꢀ
Bis(4-(4-(4-aminophenoxy)phenyl)phenyl)diphenylsilane (8).
Mp ¼ 163 ꢀC to 164 ꢀC. FT-IR (KBr, n, cmꢁ1): 3370, 3444 (N–H);
3011, 3039, 3065 (C–H arom.); 1486, 1505, 1597 (C]C); 1235 (C–
O–C); 1111 (Si–Ph); 816 (arom. p-subst.). 1H NMR (CDCl3, d,
ppm): 3.49 (s, 4H, H17); 6.68 (d, J ¼ 8.61 Hz, 4H, H11); 6.90 (d, J
¼ 8.62 Hz, 4H, H10); 6.99 (d, J ¼ 8.56 Hz, 4H, H7), 7.39 (m, 4H,
H15); 7.44 (m, 2H, H16); 7.52 (d, J ¼ 8.58 Hz, 4H, H6); 7.56 (d, J
¼ 7.88 Hz, 4H, H3), 7.62 (d, J ¼ 6.26 Hz, 4H, H14); 7.64 (d, J ¼
7.62 Hz, 4H, H2). 13C NMR (CDCl3, d, ppm): 116.26 (C11); 117.40
(C7); 121.27 (C10); 126.24 (C3); 127.93 (C15); 128.25 (C6); 129.64
(C16); 132.42 (C1); 134.28 (C13); 134.74 (C5); 136.41 (C14);
136.87 (C2); 141.71 (C4); 142.83 (C12); 148.41 (C9); 158.78 (C8).
29Si NMR (CDCl3, d, ppm): ꢁ14.52. Elem. anal. calcd for
Bis(4-(4-formylphenyl)phenyl)diphenylsilane (10). Mp
¼
272 C to 275 C. FT-IR (KBr, n, cmꢁ1): 3015, 3049, 3068 (C–H
arom.); 2733, 2821 (C–H ald.); 1700 (C]O); 1572, 1604 (C]C);
1111, 1429 (Si–Ph); 810 (arom. p-subst.). 1H NMR (CDCl3, d,
ppm): 7.42 (d, J ¼ 7.17 Hz, 4H, H12); 7.48 (m, 2H, H13); 7.63 (d, J
¼ 7.28 Hz, 4H, H11); 7.67 (d, J ¼ 7.79 Hz, 4H, H3); 7.72 (d, J ¼
7.64 Hz, 4H, H2); 7.78 (d, J ¼ 7.88 Hz, 4H, H6); 7.96 (d, J ¼
7.85 Hz, 4H, H7); 10.06 (s, 2H, H9). 13C NMR (CDCl3, d, ppm):
126.84 (C3); 127.72 (C6); 128.09 (C12); 129.92 (C13); 130.32 (C7);
133.60 (C10); 134.50 (C1); 135.44 (C8); 136.38 (C11); 137.05 (C2);
140.86 (C4); 146.81 (C5); 191.83 (C9). 29Si NMR (CDCl3, d, ppm):
ꢁ14.36. Elem. anal. calcd for C38H28O2Si; (544.75): C, 83.78%;
H, 5.19%. Found: C, 83.72%; H, 5.13%.
ꢀ
ꢀ
C
48H38N2O2Si; (698.93): C, 82.48%; H, 4.91%; N, 4.01%. Found:
C, 82.42%; H, 4.85%, N, 3.96%.
Bis(4-(4-formylphenyl)phenyl)di-R-silane (R ¼ methyl (9),
phenyl (10)). To a mixture of dibromo compound (1 or 2) (18.50
mmol), bis(triphenylphosphine) palladium(II) dichloride
(0.55 g, 0.784 mmol) and 4-formylyphenylboronic acid (6.935 g,
46.25 mmol) a solution containing K2CO3 (16.50 g) dissolved in
126 mL of a water–1,4-dioxane mixture (1 : 1 vol/vol) was added;
then, the mixture was heated at 85 ꢀC for 12 hours. Then, water
was added, and a dark brown solid was obtained which was
ltered under vaccum. This product was treated with THF, and
the remaining solids were removed by ltration through celite.
The obtained solution was dried and concentrated to obtain
a black solid, which was puried by column chromatography
(silica/petroleum ether–chloroform, 1 : 1 vol/vol) and subse-
quent recrystallization from ethyl acetate. Following this
procedure, 9 and 10 were obtained as white solids in yields of
56% and 51%, respectively.
Silylated oligomeric poly(ether-azomethine)s synthesis
A mixture of dialdehyde (9 or 10), anhydrous calcium chloride,
p-toluenesulfonic acid (p-TsOH) and anhydrous DMAc was
placed under a nitrogen atmosphere with vigorous stirring and
heated until reux. When the dialdehyde was dissolved, a solu-
tion containing the diamine (7 or 8) in anhydrous DMAc
prepared under nitrogen ow was slowly added; the reaction
was then maintained under reux for 24 hours. Then, the
mixture was added to methanol under stirring and the obtained
solid was ltered, extensively washed with hot methanol, and
dried at 100 ꢀC under vacuum for 24 hours.
PAM-I. Yield: 79%, FT-IR (KBr, n, cmꢁ1): 3063, 3032, 3013 (C–
H arom.); 2954 (C–H aliph.); 2877 (CO–H); 1698 (C]O); 1624
(C]N); 1603, 1496, 1487 (C]C); 1244 (C–O–C); 1114 (Si–Ph);
1
872 (Si–CH3); 810 (arom. p-subst.). H NMR (DMAc, acetone-d6
internal reference (int. ref.), d, ppm): 0.47 (m, 12H, H22, H23);
7.14 (d, J ¼ 7.25 Hz, 4H, H7), 7.29 (m, 4H, H10); 7.55 (m, 8H,
H11, H20); 7.61–7.84 (m, 16H, H2, H3, H6, H19); 7.97 (m, 8H,
H15, H16); 8.80 (s, 2H, H13). 13C NMR (DMAc, acetone-d6 int.
ref., d, ppm): ꢁ3.333; ꢁ3.329 (C22, C23); 121.55 (C7); 122.82
(C10); 126.36 (C3); 126.46 (C19); 127.46, 127.56, 127.79 (C6, C11,
C16); 129.04 (C15); 132.13 (C5); 133.21; 133.22 (C2, C20); 133.61,
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133.70, 134.70, 135.63 (C1, C14, C18, C21); 139.31 (C4); 144.34 d6 int. ref., d, ppm): 0.66 (s, 6H, H26); 7.17 (m, 8H, H7, H10);
(C17); 148.94 (C12); 154.28 (C9); 156.95 (C8); 164.05 (C13). 29Si 7.40–5.56 (m, 10H, H11, H24, H25); 7.63 (m, 4H, H23); 7.68 (m,
NMR (DMAc, acetone-d6 int. ref., d, ppm): ꢁ9.82 (diamino 4H, H2); 7.74 (m, 4H, H20), 7.75–7.87 (m, 12H, H3, H6, H19);
moiety); ꢁ9.79 (dialdehyde moiety).
7.90 (m, 4H, H16), 8.10 (m, 4H, H15); 8.80 (s, 2H, H13). 13C NMR
(DMAc, acetone-d6 int. ref., d, ppm): ꢁ2.67 (C26), 119.19 (C7),
PAM-II. Yield: 86%, FT-IR (KBr, n, cmꢁ1): 3065, 3043, 3030,
3013 (C–H arom.); 2951 (C–H aliph.); 2878 (CO–H); 1700 (C]O);
1624 (C]N); 1603, 1495, 1486 (C]C); 1240 (C–O–C); 1112 (Si–
Ph); 871 (Si–CH3); 811 (arom. p-subst). 1H NMR (DMAc, acetone-
d6 int. ref., d, ppm): 0.63 (s, 6H, H26); 7.15 (m, 8H, H7, H10);
7.47 (d. J ¼ 8.11 Hz, 4H, H11); 7.53 (m, 10H, H24, H25); 7.58–
7.81 (m, 20H, H2, H3, H6, H20, H23); 7.94 (m, 8H, H16, H19);
8.13 (m, 4H, H15); 8.82 (s, 2H, H13). 13C NMR (DMAc, acetone-
d6 int. ref., d, ppm): ꢁ2.56 (C26); 119.15 (C7); 120.15 (C10);
123.28 (C11); 126.39 (C3); 127.02 (C19); 127.59 (C16); 128.65
(C24); 128.76 (C6); 129.72 (C15); 130.53 (C25); 133.95, 134.02,
134.13 (C14, C21, C22); 135.09 (C2); 135.83 (C5); 136.50 (C23);
136.99 (C1); 137.22 (C20); 141.00 (C4); 141.47 (C18); 143.09
(C17); 147.84 (C12); 155.59 (C9); 157.63 (C8); 159.62 (C13). 29Si
NMR (DMAc, acetone-d6 int. ref., d, ppm): ꢁ15.34 (dialdehyde
moiety); ꢁ9.11 (diamine moiety).
120.20 (C10); 123.29 (C11); 126.62 (C3); 126.80 (C19); 127.54
(C16); 128.61 (C24); 128.87 (C6); 129.71 (C15); 130.52 (C25);
132.88 (C1); 134.38 (C22); 135.20 (C20); 135.51 (C5); 136.49 (C23);
137.16 (C2); 138.10 (C21); 140.35 (C18); 140.88 (C14); 141.63 (C4);
143.42 (C17); 147.94 (C12); 155.54 (C9); 157.84 (C8); 159.71 (C13).
29Si NMR (DMAc, acetone-d6 int. ref., d, ppm): ꢁ15.35 (diamine
moiety); ꢁ8.33 (dialdehyde moiety).
PAM-IV. Yield: 68%, FT-IR (KBr, n, cmꢁ1): 3066, 3044, 3022,
3013 (C–H arom.); 2876, 2732 (CO–H); 1700 (C]O); 1623 (C]
N); 1603, 1496, 1486 (C]C); 1238 (C–O–C); 1110 (Si–Ph); 813
1
(arom. p-subst). H NMR (DMAc, acetona-d6 int. ref., d, ppm):
7.18 (m, 8H, H7, H10); 7.47 (m, 16H, H11, H24, H25, H28, H29);
7.65 (m, 12H, H2, H23, H27); 7.75 (m, 4H, H20); 7.82 (m, 8H,
H3, H6); 7.95 (m, 8H, H16, H19); 8.13 (d, J ¼ 6.13 Hz, 4H, H15),
8.82 (s, 2H, H13). 13C NMR (DMAc, acetone-d6 int. ref., d, ppm):
119.19 (C7); 120.19 (C10); 123.29 (C11); 126.62 (C3); 127.04
PAM-III. Yield: 65%, FT-IR (KBr, n, cmꢁ1): 3067, 3033, 3014
(C–H arom.); 2953 (C–H aliph.); 2876, 2732 (CO–H); 1700 (C]O);
1623 (C]N); 1603, 1496, 1486 (C]C); 1239 (C–O–C); 1112 (Si–
Ph); 872 (Si–CH3); 808 (arom. p-subst). 1H NMR (DMAc, acetone-
(C19); 127.60 (C16); 128.60 (C24); 128.67 (C28); 128.86 (C6);
129.74 (C15); 130.34, 130.52 (C25, C29); 132.87 (C1); 134.03
(C21); 134.35, 134.38 (C22, C26); 135.52 (C5), 136.49 (C23, C27);
137.16 (C2); 137.23 (C20); 140.96 (C18); 141.49 (C14); 141.62
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1
(C4); 143.11 (C17); 147.90 (C12); 155.56 (C9); 157.82 (C8); 159.66 stretching band observed at approximately 510 cmꢁ1. The H
(C13). 29Si NMR (DMAc, acetone-d6 int. ref., d, ppm): ꢁ15.33; and 13C NMR spectra clearly showed displacement due to the
ꢁ15.31.
high magnetic elds exerted by the silicon atoms over their
environment. Thus, the H and C nuclei of the methyl group of 1
appear at 0.44 ppm and ꢁ2.42 ppm, respectively. Also, the
aromatic carbon atoms bonded directly to the silicon atom were
observed at high eld. This effect can be observed in the spectra
of all the silylated precursors and monomers.
Results and discussion
Monomer synthesis
Both monomers use dibromo derivatives (1 or 2) as common
precursors (bis(4-bromophenyl)di-R-silanes); these were directly
obtained by means of controlled mono-lithiation of 1,4-dibro-
mobenzene and then reacted with the respective dichloro-
silanes (Fig. 1, step a). In this last step, the nucleophilic
substitution on the central silicon atom is favored because the
Si–X distances (X ¼ Cl, C) in the organo-silane compounds are
longer than the C–X bond distances. Thus, the steric hindrance
for SN2 attack was negligible, and yields of over 70% were
obtained.
Elemental analysis, FT-IR spectroscopy and NMR spectros-
copy techniques, including COSY, HMQC, and HMBC, were
employed to establish the structures of the dibromo
compounds and of all the precursors and monomers. In this
case, the main IR bands are associated with the C–Si bonds at
1373 to 1427 cmꢁ1 and 1253 cmꢁ1 according to the nature of the
carbon atom, aromatic or aliphatic, respectively, and the C–Br
The biphenyl moieties were incorporated into the monomers
and, therefore, the nal polymers by Suzuki coupling reactions.
Thus, dibromo compounds 1 or 2 were reacted with the corre-
sponding boronic acid derivatives to produce the dialdehyde
monomers 9 and 10 (Fig. 1, step e) and the diphenols 3 and 4
(Fig. 1, step b). The FT-IR spectra of the latter compounds
showed the characteristic phenolic O–H band between 3500 and
1
3400 cmꢁ1. Likewise, the H NMR spectra evidenced this func-
tion, with a narrow singlet at 9.59 ppm for 3 and a broad singlet
centered at 5.11 ppm for 4. In the latter case, the electronic
contribution of the lateral phenyl groups bonded to the silicon
atom probably promotes electronic shielding of this nucleus.
By means of arylation of the oxygen atoms using a potassium
carbonate aqueous solution as a basic medium and 1-uoro-4-
nitrobenzene, the dinitro derivatives 5 and 6 were prepared
with yields of 88% and 74%, respectively (Fig. 1, step c). For
Fig. 1 Synthesis of monomers containing silicon and biphenyl moieties. (a) (1) n-BuLi, Et2O anh., 0 ꢀC. (2) R2SiCl2 (R ¼ CH3, Ph), Et2O anh.; (b) 4-
hydroxyphenylboronic acid, Pd(PPh3)2Cl2, K2CO3 (aq.) 2 M, 1,4-dioxane, 85 ꢀC; (c) 1-fluoro-4-nitrobenzene, K2CO3, DMF, 60 ꢀC; (d) H2N–NH2
(80%), Pd/C (10%), THF, reflux; (e) 4-formylphenylboronic acid, Pd(PPh3)2Cl2, K2CO3 (aq.) 2 M, 1,4-dioxane, 85 ꢀC.
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these compounds, IR bands originating from the NO2 groups
In all cases (precursors and monomers), the 29Si NMR spectra
were observed at 1518 and 1344 cmꢁ1 and from the stretching showed two different signals. For the dimethyl series (1, 3, 5, 7
ether linkage (C–O–C) at about 1236 cmꢁ1. The aromatic zones and 9), a band could be observed between ꢁ7.00 ppm and
of the NMR spectra of both dinitro compounds showed new ꢁ8.64 ppm; meanwhile, for the diphenyl series (2, 4, 6, 8 and 10),
signals for the Ph-NO2 moieties, like an AB system, at 6.68 and this signal was observed close to ꢁ14 ppm. These values are
6.90 ppm, without evidence of the signals at about 9.6 and associated with the –Ph–Si(CH3)2–Ph– and –Ph–Si(Ph)2–Ph–
5.1 ppm attributed to the hydroxylic groups of the respective moieties, respectively, and can be explained according to the
precursors.
particular electronic effects of the methyl and phenyl groups. This
The reduction of the NO2 groups to produce diamine mono- phenomenon has been previously observed in several monomer
mers 7 and 8 was carried out following a classic procedure which and polymer families synthesized by our research group.31–33
involves the use of hydrazine and Pd/C (Fig. 1, step d). In both
cases, chromatographic techniques were employed to purify the Oligomeric poly(ether-azomethine)s synthesis and
products. In the case of diamine 7, additional recrystallization spectroscopic characterization
from acetonitrile was required. The amino group was easily
Four new silylated oligomeric poly(ether-azomethine)s were
detected in the FT-IR spectra through two stretching bands
prepared through a polycondensation procedure in DMAc
between 3340 and 3450 cmꢁ1 (Fig. 2). Also, the 1H NMR spectra
solution and under an inert atmosphere. The pH of the reaction
reported evidence of the reduction. A narrow singlet with an
was adjusted to 5.0 by adding p-TsOH, and anhydrous CaCl2 was
employed to capture water, which is the byproduct of the
dehydration of the carbonylamine intermediate (Fig. 5). The
solutions obtained aer heating at 180 ꢀC over 24 h were added
integration of 4H associated with NH2 groups was observed at
3.57 ppm for 7; meanwhile, for diamine 8, a broad and at singlet
centered at 3.49 ppm also showed an integration of 4H (Fig. 3).
Likewise, in these spectra and the 13C spectra, the aromatic zone
to methanol, precipitating light beige solids in all cases. The
was altered when NH2 replaced NO2 due to the contrasting
yields were between 65% and 86%.
electronic effects mainly exerted on the adjacent phenyl groups.
The structural characterization of the samples was performed
On the other hand, for the silicon-containing dialdehydes,
using spectroscopic techniques. KBr pellets were prepared, and
the FT-IR spectra showed the characteristic stretching band for
the IR main bands were assigned (see Experimental section).
carbonyl groups close to 1700 cmꢁ1, independently of the R
Thus, the stretching C]N of the imine group was identied at
group bonded to the silicon atom (Fig. 2). The 1H NMR spectra
1623 cmꢁ1. In all cases, a C]O band of variable intensity was
demonstrated the proton of the aldehyde group at 10 ppm,
observed close to 1700 cmꢁ1; this is not attributed to residual
while the 13C NMR spectra showed the carbonylic carbon at
monomer due to the exhaustive washing procedure of the
samples aer the isolation step. Additionally, TLC analysis using
191 ppm (Fig. 4).
CHCl3 as the mobile phase did not show a monomer signal,
indicating that the IR band is associated with terminal groups of
short chains (oligomers). In this sense, the presence of NH2 chain
terminal groups is not clearly dened in the spectra, probably
due to their lower intensity in relation to the carbonyl band.
Fig. 6 shows a representative FT-IR spectrum of the obtained
materials and the denition of the dCO and dCN distances, which
are associated with the intensity of the carbonyl and imine
bands, respectively. Using these values, it is possible to estab-
lish the aldehyde average contribution parameters inside the
chains (AC):
AC ¼ dCO/(dCO + dCN
)
Thus, the samples PAM-I to PAM-IV showed values of 0.28,
0.36, 0.52 and 0.51, respectively. According to these data, PAM-I
and PAM-II have lower proportions of CHO in their chains and
are the longest oligomers. On the other hand, PAM-III and PAM-
IV show similar aldehyde contributions and have comparable
lengths.
Initially, DMSO-d6 was used as the solvent to perform
structural characterization of the samples. The resolution of the
obtained NMR spectra was inadequate; for this reason, the
samples were analyzed in DMAc solution using a capillary with
acetone-d6 as an internal reference. In this sense, the complexity
of the aromatic zone in the NMR spectra was resolved with the
aid of DEPT-135ꢀ and 2D NMR (COSY, HMQC, and HMBC)
Fig. 2 FT-IR spectra (vertically shifted) of the silylated monomers.
Diamines 7 and 8; dialdehydes 9 and 10.
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Fig. 3 NMR spectra (CDCl3) of diamines 7 (right) and 8 (left).
analysis, enabling correct assignment of the signals. As an promoted by the diamine moiety and the corresponding exi-
example, representative 2D NMR and 29Si NMR spectra of PAM- bility of the chains did not increase the solubility of the sample.
II are shown in Fig. 7.
Among the samples, PAM-I shows the highest inherent viscosity
In all the 1H NMR spectra, a singlet at ꢃ8.8 ppm was value (Table 1); this is also associated with higher average size of
observed, while the 13C NMR analyses evidenced a signal the chains (see the GPC results) and, therefore, with lower
between 160 and 164 ppm, corroborating the formation of solubility. At the tested concentration, all the samples were
imine groups (–CH]N–).
partially soluble in DMSO and insoluble in THF and CHCl3.
In general terms, PAM-II and PAM-III, whose repetitive units
are structural isomers, showed solubility in DMAc and m-cresol;
this is probably due to the introduction of bulky phenyl groups
in one of the silicon atoms and therefore to diminished packing
forces relative to those operating in PAM-I. On the other hand,
the average molecular weights of these samples are lower than
that of PAM-I. The structural effect is more evident in PAM-IV, in
which all the lateral groups are phenyl rings. This sample, with
an average length-chain similar to that of PAM-III, was the only
one that showed solubility in DMF under the tested conditions.
It is clear that the molecular weight and the structural compo-
sition of the repetitive units of the samples strongly affected the
solubility behavior, where the possibility to establish interac-
tions between the chains was limited or favored according to
the volume of the groups bonded to the silicon atoms.
Solubility, inherent viscosity and molecular weight
The samples were tested for their ability to form solutions with
several solvents; a concentration of 5 mg mLꢁ1 at 19 ꢀC was
used, and the time to dissolve each sample with sporadic stir-
ring of similar intensity was recorded. The behavior was clas-
sied as soluble/time, partially soluble and insoluble. For these
last two classications, the samples were heated at 40 ꢀC for 24
hours. The results are shown in Table 1.
PAM-I was soluble in DMAc in >180 minutes, while in conc.
H2SO4, it dissolved in less time (40 minutes). The other samples
showed higher solubility in common polar organic solvents.
Probably, the lower volumes of the four methyl groups bonded
to the silicon atoms in PAM-I promote large packing forces
between the chains; this results in p-stacking interactions,
decreasing the solubility. In this sense, the ether linkages
Additionally, a modication to the solubility test was devel-
oped by adding LiBr (0.05 M) to dilute polymer-DMF systems
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Fig. 4 NMR spectra of the dialdehydes 9 (DMSO-d6) (right) and 10 (CDCl3) (left).
Fig. 5 Synthesis of silylated oligomeric poly(ether-azomethines).
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(0.05 mg mLꢁ1) at 40 ꢀC in order to obtain solutions. In all cases,
clear solutions were obtained and then used in the GPC
analysis.
Because no reports exist regarding the synthesis and char-
acterization of poly(ether-azomethine)s containing –PhSiR2Ph–
moieties in their chains (R ¼ methyl, phenyl), the solubility
behavior of the new materials was compared with results ob-
tained for non-silylated poly(ether-azomethine)s.34,35 These
polymers have similar structural elements to the repetitive units
of the prepared PAMs, including oxyether linkages and high
aromatic content. Likewise, the –PhSiR2Ph– moieties are
replaced by bulky and/or exible central units promoted by the
symmetric monomer moieties (Fig. 8).
In all cases, the authors report higher solubility in common
organic solvents than those observed for the silylated PAMs. The
PAMs-ref showed solubility even in THF and CHCl3 at room
temperature. These differences can be mainly associated with
the presence of oxyether bonds in the dialdehyde moiety, which
favors the solubility process. Also, the biphenyl moieties of the
silylated PAMs hinder the solubility of the chains.36 In this
sense, the respective central elements of the silylated samples
Fig. 6 The FT-IR spectrum of PAM-I with details regarding the
stretching of the C]O and C]N bands.
Fig. 7 NMR spectra (DMAc-acetone-d6) of PAM-II. (a) COSY, (b) HMQC, (c) HMBC and (d) 29Si NMR spectra.
1306 | RSC Adv., 2018, 8, 1296–1312
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Table 1 Solubility behavior of PAMs expressed by dissolution time and inherent viscositya
hinh (dL gꢁ1
)
Solubility
DMSO
DMAc
DMF
m-cresol
H2SO4 conc.
DMAc
PAM-I
Part. sol.
Part. sol.
Part. sol.
Part. sol.
Sol. in >180 min.
Sol. in 15 min.
Sol. in >180 min.
Sol. in 15 min.
Ins.
Part. sol.
Ins.
Ins.
Sol. in 40 min.
Sol. in 40 min.
Sol. in 180 min.
Sol. in >180 min.
0.32
0.20
0.14
0.10
PAM-II
PAM-III
PAM-IV
Sol. in 40 min.
Sol. in 40 min.
Sol. in 40 min.
Sol. in 15 min.
a
Ins: insoluble; Part. sol.: partially soluble; Sol.: soluble; hinh: inherent viscosity measured at 25 ꢀC (c ¼ 0.5 g dLꢁ1).
Fig. 8 General structure of the poly(ether-azomethine) used for comparison, emphasizing the natures of the central elements (R1 and R2).34,35
Table 2 Molecular weights, degrees of polymerization and poly-
and of the PAMs-ref may have similar effects on the studied
dispersity indices of the oligomers measured by GPC
properties because they are bulky elements that affect the
packing of the polymeric chains.
Mw (g molꢁ1
)
Mn (g molꢁ1
)
DPa
PDIb
Inherent viscosity measurements were used to establish an
order of the size chain in the series. Although the samples
showed solubility in concentrated sulfuric acid, this parameter
was not determined in this medium because hydrolysis of the
imine function was promoted by this reagent. According to
a report by Morgan et al.,7 although these measurements are
made quickly, it is not possible to avoid hydrolysis of the chains,
a process that is enhanced by the action of light.
PAM-I
9100
6000
4900
4900
4800
4200
4000
3600
5
4
4
3
1.90
1.42
1.23
1.36
PAM-II
PAM-III
PAM-IV
a
Approximate degree of polymerization: Mn/molecular weight of
repetitive unit. b Polydispersity index: Mw/Mn.
Thus, the inherent viscosity was recorded at 25 ꢀC in DMAc (c
¼ 0.5 g dLꢁ1); values between 0.10 and 0.32 dL gꢁ1 were ob-
tained, which are characteristic of oligomers (Table 1).
According to these data, PAM-I and PAM-II correspond to
samples with larger sizes, while PAM-III and PAM-IV are smaller
in size but are similar to each other. This observation is in
accordance with the results of the FT-IR analysis because the AC
parameter is related to the relative size of the chains; the
smaller the AC value, the larger the chain. On the other hand,
the GPC results obtained from DMF-LiBr solutions generally
ratify these observations (Table 2). The obtained oligomers have
3 to 5 repetitive units, where PAM-I shows the highest molecular
weight and PDI.
principally due to the formation of non-volatile silicon oxides
and non-oxidized organic material.30,37,38
Thermal stability studies performed on the poly(ether-
azomethines)s based on the decomposition point indicate
that they are thermally stable.39 The high aromatic content due
to the biphenyl moieties of both monomers allows inter-chain
interactions that increase the thermal stability in a similar
way to the effect discussed in the solubility section. When the
lateral groups bonded to the silicon atoms were the same (R1 ¼
R2, CH3 or phenyl group; PAM-I and PAM-IV), the samples were
more stable than those with different groups (R1 s R2, CH3 or
phenyl group; PAM-II and PAM-III). This behavior can be
attributed to the structural symmetry of the repetitive units and
the consequently higher packing forces between the chains.
These results are summarized in Table 3.
The DSC curves (Fig. 9b) show transitions between 279 ꢀC
and 204 ꢀC, which are attributed to the glass transition
temperature (Tg); however, for PAM-III, this thermal transition
was not observed. No melting temperatures were observed in
the analyses. In accordance with the results, the introduction of
the exible oxyether bond as a common structural element
lowers the rigidity of the chains, resulting in low Tg values
despite the high aromatic content of the repetitive units.
Thermal analysis
The thermal properties of the oligomers were evaluated by TGA
and DSC techniques at 20 C minꢁ1 in nitrogen atmosphere.
ꢀ
All the samples showed similar TG behaviors, with an initial
ꢀ
decomposition temperature of around 450 C (Tonset) (Fig. 9a).
The 10% weight loss (TDT10%) occured between 490 ꢀC and
ꢀ
ꢀ
510 C. The char yields at 720 C were similar in all samples,
between 55% and 60%; this is common for silylated samples,
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Fig. 9 Thermal studies: (a) TGA and (b) DSC of the samples.
Table 3 Thermal data obtained for the silylated-PAMs and PAMs-Refa
DSC
Thermogravimetric analysis (TGA)
Charge yield
at 720 ꢀC (%)
Tg
(ꢀC)
TDT10% (ꢀC)
Tonset (ꢀC)
onset
PAM-I (R1: –CH3, R2: –CH3)
PAM-II (R1: –CH3, R2: –Ph)
PAM-III (R1: –Ph, R2: –CH3)
PAM-IV (R1: –Ph, R2: –Ph)
PAM-Ref 1
279
216
n.o.
204
192
211
215
505
490
495
515
462
481
458
465
475
455
475
n.r.
420
250
53
58
57
60
n.r.
PAM-Ref 2
PAM-Ref 3
0 (at 630 ꢀC)
0 (at 630 ꢀC)
a
Tonset: temperature at which polymer decomposition starts; n.o.: not observed.; n.r.: not reported.
Inside the series, the order of the Tg values correlates to the values, in accordance with the biphenyl moieties present in
nature of the groups bonded to the silicon atoms and the sizes their repetitive units.
of the chains, which were established by GPC analysis. Thus,
PAM-I, which contains only lateral methyl groups and has the
highest average chain size, showed the highest Tg and therefore
the greatest chain rigidity; meanwhile, PAM-IV, which contains
UV/vis spectroscopy
UV/vis spectroscopy was carried out in dilute DMSO solutions
(c ¼ 0.5 mg mLꢁ1) in the wavelength range between 270 and
four lateral phenyl groups and has the lowest average size,
440 nm (Fig. 10). The PAMs showed bands within 280 to 290 nm
showed the lowest Tg. In this sense, the smaller volume of
that can be assigned to the p / p* transition of the aromatic
a methyl group compared with a phenyl group promotes the
rings of the main chain. On the other hand, a broad absorption
interaction between chains, diminishing the internal rotation.
can be found between 351 nm and 356 nm, which was assigned
to the n / p transition of the imine groups.40,41 The slight
difference between these bands can be attributed to the modi-
cation of the chain planarity. That is, the conjugation is dis-
It is clear that the decomposition temperatures and the Tg
values showed by the samples demonstrate an adequate
thermal window in view of processing of this type of aromatic
material.
rupted due to the presence of sterically hindered structures
If these thermal results are compared with those of non-
such as the phenyl rings as lateral groups in PAM-IV, showing
silylated-PAMs, similar tendencies in the Tg values can be seen
bands centered at a lower wavelength compared with the other
in selected references34,35 (Fig. 8); the bulkiest central element
samples (structural variation of the molecule rigid backbone).
These results are shown in Table 4.
From the UV-vis spectra, optical band gap values were ob-
tained using eqn (1).42,43
results in the lowest Tg. On the other hand, if PAM-I is compared
with PAM-Ref 1 or PAM-Ref 2, where the main difference is the
atom of the central unit (Si vs. C, respectively), the silylated
sample shows a higher Tg. This result is contrary to the rela-
tionship of the volume of the atoms and can also be explained
Eg ¼ 1242/lonset
(1)
by the higher number of oxyether linkages in the reference
samples, which promote higher exibility of the chains.
Regarding the TDT10%, the silylated samples show higher
where lonset is the onset wavelength, which was determined by
the intersection of the two tangents on the absorption edges
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Fig. 10 (a) UV/vis spectra for PAM-I to PAM-IV in DMSO and (b) fitted curves for PAM-II.
Table 4 Bands and optical band gap values obtained from UV/vis
Four experiments were performed by varing the rotational
speed and time (speed in rpm/time in seconds): PAM-I (1) 2000/
18 and 4000/60; PAM-II (2) 1000/18, 1500/18, 2000/18 and 4000/
60, (3) 1500/18, 2000/18 and 4000/60, (4) 2000/18 and 4000/60.
In this preparation, the homogeneity of the layers is
controlled by the physical structure of the sample. Additionally,
the viscosity of the solvent and the rotational speed used for its
deposition are factors that must be considered.
DMSO solution spectraa
lonset
1ꢀ band
2ꢀ band
(nm)
Eg (eV)
PAM-I
279.9
287.0
287.0
290.6
—
357.0
398.5
392.2
377.5
3.5
3.1
3.2
3.3
PAM-II
PAM-III
PAM-IV
356.6
354.0
351.7
Fig. 11 shows AFM images of the PAM-I (1) and PAM-II (4)
lms. For PAM-I (1), the morphological structure presents
a relatively homogeneous lm formed by large clusters with
shared boundaries, probably due to the high degree of solubility
at the moment of material deposition. Also, it can be observed
that the clusters are formed of small particles with similar sizes.
On the other hand, PAM-II (4) shows a homogeneous surface,
but with smaller clusters. Probably, the smaller size of the
chains and the replacement of the methyl group by a phenyl
group with greater volume do not promote the formation of
higher structures.
During the spin coating process, solvent evaporation reduces
the spin radius and increases the viscosity of the material; thus,
it is impossible for centrifugal force to remove the remaining
material from the wafer. Fig. 12a shows three different AFM
images of PAM-II and their respective magnications. The
variation in the topography of the sample lms is due to
changes in the spinning speed during the deposition process.
Fig. 12b shows the trends followed by the Young's modulus, the
roughness, and the area increase percentage according to the
initial spin speed used for the formation of each lm.
a
Cutoff: Lo DMSO: 330 nm and L1: 285 nm.
and was used to indicate the electronic transition of the onset
wavelength. The calculated optical band gap values are shown
in Table 4.
As can be observed, the optical band gaps are between 3.1
and 3.5 eV; these values are related to internal effects due to
morphology and carrier transport, among other factors.44 PAM-I
and PAM-IV showed the highest optical band gaps because the
two groups bonded to their silicon atoms are identical (R1 ¼ R2
¼ methyl and phenyl, respectively). This structural symmetry
around the silicon atom can strongly modulate the morpho-
logical effects on the measured parameter in a similar way to
that proposed in the thermal section. On the other hand, small
Eg values were obtained in the analysis of PAM-II and PAM-III.
This behavior can be attributed, among other factors, to the
reduction of steric hindrance and the greater delocalized p-
electron density in the aromatic segments of the chains
(conjugated polymers), which may involve electronic
delocalization.
According to these results, it can be seen that the incorpo-
ration of phenyl groups can dramatically decrease the planarity
of the system and therefore diminish the p-electron delocal-
ization along the backbone.
Atomic force microscopy and force spectroscopy
PAM-I and PAM-II were chosen for the preparation of lms
using a spin-coating technique due to their high Mn values of
4800 and 4200, respectively. DMAc solutions of the samples
were deposited over an activated silicon wafer substrate by spin-
coating with several cycles at a constant speed. Aerward, the
lms were subjected to vacuum to remove the remaining
solvent.
Fig. 11 AFM topographic images (50 ꢂ 50 mm2) of spin-coated films
obtained from DMAc solutions: (a) PAM-I (1) and (b) PAM-II (4).
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At higher initial spinning speeds, the stiffness of the surface (separation between the probes) ¼ 0.1 cm. Silicon wafer was used
increases. This may be indirectly related to the roughness of the as a calibration standard and also as a substrate. The resistivity
lm. Higher spinning speeds produce homogeneous surfaces, results obtained for the two samples are summarized in Table 5.
decreasing the roughness but maintaining the particle size. The
The refractive indices of PAM-I and PAM-II were 1.433 and
homogeneity of the lms is also indicative of the presence of 1.440, respectively; these were measured using an Abbe refrac-
interconnected microstructures inside the material, producing tometer. These values are important for measuring the thick-
network or boundary interactions that increase the Young's nesses of the lms; using an ellipsometer, thicknesses near
modulus of the surface. In the same way, by denition, the 500 nm were measured. These values were used to determine
roughness increase produces an increase in the percentage of the resistivities of the lms. The surface resistivities of the
exposed surface area; both plots follow a similar tendency.
calibration standards (silicon wafers) are certied by the
The four-point probe technique was used to measure the company (Siegert Wafer GmbH).
resistivities of the thin lms. The setup consists of four point
collinear probes, where a constant current is applied between
the two outer probes and the voltage variation is measured in
the inner probes. The surface resistivity was determined by eqn
(2), where CF corresponds to a corrector factor based on the
ratio of the probe and the substrate/lm dimensions.45,46
Table 5 Electrical studies using the four-point probe technique
Voltage
(V)
Resistivity
(U cm)
Average (U
cm)
ꢀ
ꢁ
PAM-I (1)
PAM-II (2)
PAM-II (3)
PAM-II (4)
Silicon wafer_1 (bare)
Silicon wafer_2 (bare)
3.56
32.24
35.84
31.59
15.32
17.24
2.19
19.85
22.06
19.45
9.43
2.19
20.45
V
I
s ¼
ꢂ CF
(2)
10.02
The experiments were carried out using the following
parameters: I ¼ 6 A; w (substrate thickness) ¼ 0.0525 cm, and s
10.61
Fig. 12 (a) AFM topographic images (50 ꢂ 50 mm2 and 10 ꢂ 10 mm2) of spin-coated PAM-II films obtained from DMAc solutions and (b) Young's
modulus, roughness, and area increase percentage obtained from the analysis of the images.
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Scientic reports have shown that the resistivity of a spec- through Project 1150157, and FONDEQUIP, through Project
imen increases with decreasing thickness.47 However, in this EQM120021. P. A. Sobarzo thanks CONICYT for fellowship No.
case, the thickness of the sample was not strongly affected by 21151317.
the rate of deposition, probably due to the high viscosity of the
polymer-low volatile solvent (DMAc) solution. Table 5 shows the
results obtained for PAM-I and PAM-II. The rst sample has low
resistivity (2.19 U cm), while PAM-II showed a resistivity close to
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