Synthesis and liquid-crystalline properties of methacrylate monomers carrying a p-terphenyl laterally substituted with one or two cyano groups

Article abstract of DOI:10.1016/j.molliq.2017.06.042

Two methacrylate monomers, carrying a p-terphenyl laterally substituted with one or two cyano groups, were synthesized and studied from the liquid crystal and optical points of view. The synthetic route involved several reactions, including the Suzuki rea

Full text of DOI:10.1016/j.molliq.2017.06.042

Accepted Manuscript
Synthesis and liquid-crystalline properties of methacrylate
monomers carrying a p-terphenyl laterally substituted with one or
two cyano groups
Isaura Felix-Serrano, Nayely Trejo-Carbajal, Rosa Julia
Rodríguez-González, Leticia Larios-López, Ivana Moggio,
Eduardo Arias, Román Torres-Lubián, Dámaso Navarro-
Rodríguez
PII:
DOI:
Reference:
S0167-7322(17)31105-4
doi: 10.1016/j.molliq.2017.06.042
MOLLIQ 7488
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
13 March 2017
9 May 2017
9 June 2017
Please cite this article as: Isaura Felix-Serrano, Nayely Trejo-Carbajal, Rosa Julia
Rodríguez-González, Leticia Larios-López, Ivana Moggio, Eduardo Arias, Román Torres-
Lubián, Dámaso Navarro-Rodríguez , Synthesis and liquid-crystalline properties of
methacrylate monomers carrying a p-terphenyl laterally substituted with one or two cyano
groups, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.06.042
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ACCEPTED MANUSCRIPT
Synthesis and liquid-crystalline properties of methacrylate monomers carrying a p-
terphenyl laterally substituted with one or two cyano groups
Isaura Felix-Serrano, Nayely Trejo-Carbajal, Rosa Julia Rodríguez-González,
Leticia Larios-López, Ivana Moggio, Eduardo Arias, Román Torres-Lubián,
and Dámaso Navarro-Rodríguez*
Centro de Investigación en Química Aplicada (CIQA), Blvd. Enrique Reyna 140, 25294,
Saltillo, Coahuila, México
I. Felix Serrano: fesisaura@gmail.com
N. Trejo-Carbajal ntc_28@hotmail.com
R. J. Rodríguez-González: julia.rodriguez@ciqa.edu.mx
L. Larios-López: leticia.larios@ciqa.edu.mx
I. Moggio: ivana.moggio@ciqa.edu.mx
E. Arias: eduardo.arias@ciqa.edu.mx
Román Torres-Lubián roman.torres@ciqa.edu.mx
D. Navarro-Rodríguez: damaso.navarro@ciqa.edu.mx
*Corresponding author: Dámaso Navarro-Rodríguez: damaso.navarro@ciqa.edu.mx
Phone number: 52 (844) 4389846
1

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ABSTRACT
Two methacrylate monomers, carrying a p-terphenyl laterally substituted with one or two
cyano groups, were synthesized and studied from the liquid crystal and optical points of
view. The synthetic route involved several reactions, including the Suzuki reaction that
allows the coupling of aryl groups. The chemical structure of these new monomers, as well
1
as that of their corresponding precursors, was confirmed by H and 13C NMR
spectroscopies. The mesomorphic behavior of both monomers and precursors was studied
by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-
ray diffraction (XRD) analyses. Molecules bearing one cyano group melt at lower
temperature and display a mesomorphic behavior over larger temperature intervals as
compared to those bearing two cyano groups. The four compounds (two monomers and two
precursors) showed only non-ordered smectic phases. By X-ray diffraction it was
determined that molecules stack in single layers within the smectic phase, except for one of
the monomers that stacks in double layers. The electrochemical characterization revels that
these molecules behaves as p type materials, and that their electronic nature (band gap) can
be modulated by introducing CN groups. The molecules absorb and emit in the blue
spectral range, a region of high energy light. The large Stokes’ shifts indicate that the
geometry of the molecules changes from aromatic in the electronic ground state to a more
coplanar quinoid structure in the first excited state. The fluorescence quantum yield is high
(70 – 80%) suggesting that these new monomers can be used as precursors for the synthesis
of fluorescent liquid crystal polymers, which is a work in progress.
Keywords: p-terphenyl; liquid crystal; fluorescence.
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1. Introduction
In the 1970s, liquid crystals (LC) provided a technological breakthrough to the visual
display industry: the capacity to commercially produce light-weight flat-panel displays
driven by electric-field stimuli.1 Among the first known electric-field responsive liquid
crystals (nematic) for flat-panel display applications are the cyano-substituted biphenyl and
p-terphenyl derivatives.2 These two types of liquid crystals are normally combined to each
other to get a nematic order over extended temperature ranges around room temperature;3 a
condition that is virtually mandatory for LC display applications. Unlike biphenyl
derivatives, p-terphenyls possess remarkable photo-physical properties associated to their
extended -conjugation length, as for instance, a high quantum efficiency of
photoluminescence (> 0.9) and short decay times (around 1 ns).4 Thus, p-terphenyl-based
compounds have shown to be good candidates not only for use in flat-panel LC displays,
but also in their competing organic light-emitting diode (OLED) displays,5 and in many
9,10
other applications like scintillators,6 wavelength shifters,7,8 UV-laser dyes,
and so on.11
The structure – mesomorphic properties (mesophase types, transition temperatures, order
parameter, etc.) relationship for p-terphenyl liquid crystals have been studied along the
years since the 70s.2,12,13 Different chemical groups have been incorporated into the p-
terphenyl group to change its thermal and optical behavior, or to induce new properties.14,15
The cyano (or nitrile) group remains one of the most tested as it is highly polar, thermally
and chemically stable, and tolerates a wide variety of chemical groups. It induces a dipole
moment and promotes dipole – dipole interactions that may have an important effect on the
molecular stacking,16,17 as well as on the optical properties,18 depending on their number
3

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and position in the aromatic core. Short chemical substituents have proven to have a strong
influence on the mesomorphic behavior of rigid rod-like mesogenic groups.19 They could
adversely affect the mesophase, either reducing its temperature interval or inhibiting its
development.20 In this paper, the synthesis and liquid-crystalline properties of two
methacrylate monomers (and their precursors), substituted with one or two cyano groups,
are presented and discussed. Preliminary optical studies are also reported for their possible
combined luminescent and thermotropic behavior, for which the frontier orbitals HOMO
and LUMO as well as the band gap were determined by cyclic voltammetry.
2. Experimental
2.1. Materials
Bromododecane, dibromododecane, bromophenol, 5-bromo-2-hydroxy-benzonitrile, 1.6 M
butyllithium in hexanes (1.6 M BuLi), triisopropyl borate (TIPBO), hydrochloric acid,
tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4 or Pd[0]), sodium carbonate,
potassium carbonate, potassium iodide, hydroquinone (HQ), potassium hydrogen
carbonate, methacrylic acid (MA) and electrochemical grade tetrabutylammonium
hexafluorophosphate (Bu4NPF6) were purchased from Sigma-Aldrich and used as received.
Chloroform (CHCl3), deuterated chloroform (CDCl3), dimethylformamide (DMF),
methylene chloride (CH2Cl2), tetrahydrofuran (THF), triethylamine (TEA) acetone, ethyl
ether and hexanes were purchased from J. T. Baker. TEA (over KOH), THF (over a
Na/benzophenone complex) and acetonitrile (silica and CaH) were freshly distilled right
before use.
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2.2. Synthesis
The route for the synthesis of the monomers bearing a didodecyloxy-p-terphenyl group
substituted with one or two cyano groups is depicted in Scheme 1. For the sake of brevity,
monomers were labeled as mCN and m2CN. As noted in Scheme 1, the final intermediates
(here named terphenyl precursors) were labeled as tCN and t2CN. Intermediates 1a-c and
2a-c were prepared as described in a previous report for similar compounds.17 Procedures
for the synthesis of the intermediates 3a,c and 4a, as well as those for the preparation the p-
terphenyl precursors (tCN and t2CN) and monomers (mCN and m2CN), are detailed below.
Scheme 1. Synthesis route for the preparation of methacrylate monomers bearing a p-
terphenyl moiety substituted with one or two cyano groups. i) K2CO3, KI,
acetone, 48 h; ii) BuLi, –70 °C, HCl 2M; iii) Pd(PPh3)4, Na2CO3 2M, THF, 60
°C, 48 h; iv) BuLi, –40 °C, HCl 2M; v) Pd(PPh3)4, Na2CO3 2M, THF, 60 °C,
48 h; and vi) KHCO3, HQ, DMF, 20 °C, 24 h.
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4-Bromo-4’-(dodecyloxy)-1,1’-biphenyl (3a)
In a 250 mL two-neck round-bottom flask, under argon atmosphere, 1.15 g (4.90 mmol) of
1,4-dibromobenzene, 0.17 g (0.15 mmol) of Pd[0], and 80 mL of THF (freshly distilled)
were introduced, and the mixture was stirred to complete dissolution. Next, 31.8 mL of
aqueous Na2CO3 2 M and 1 g (3.26 mmol) of 2a were added and the solution was heated to
65 °C and held under stirring overnight. The reaction mixture was then allowed to reach the
room temperature to then be transferred to a separation funnel containing water and
chloroform. The organic layer was subsequently washed with water until neutral pH, dried
over MgSO4, filtered, and evaporated. The crude product was finally passed through a
silica-gel chromatographic column using hexanes:chloroform (6:4) as a mobile phase. 3a
1
was obtained as a white powder (Yield: 64 %). H NMR (CDCl3, δ = ppm): 0.91 (t, CH3,
3H, J = 6.9 Hz); 1.54-1.22 (m, CH2, 18H); 1.87-1.79 (m, CH2, 2H); 4.02 (t, CH2, 2H, J =
6.5 Hz); 6.99 (d, CH, 2H, J = 8.8 Hz); 7.44 (d, CH, 2H, J = 8.5 Hz); 7.50 (d, CH, 2H, J =
8.5 Hz); 7.55 (d, CH, 2H, J = 8.5 Hz).
4’-bromo-4-(-bromododecyl)oxy)-[1,1’-biphenyl]-3-carbonitrile (3c)
A similar procedure to that described for 3a was followed, but using 2c instead of 2a. White
1
solid (Yield: 66 %). H NMR (CDCl3, δ = ppm): 1.54-1.18 (m, CH2, 16H); 1.89-1.82 (m,
CH2, 4H); 3.43(t, CH2, 2H, J = 6.9 Hz); 4.11 (t, CH2, 2H, J = 6.6 Hz); 7.02 (d, CH, H, J =
8.8 Hz); 7.37(d, CH, 2H, J = 8.8); 7.56 (d, CH, 2H, J = 8.5 Hz); 7.68 (dd, CH, H, J = 8.8,
2.2 Hz); 7.72 (d, CH, H, J = 2.2 Hz).
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(4’-(dodecyloxy)-[1,1’-biphenyl]-4-yl)boronic acid (4a)
In a 250 mL three-neck round-bottom flask, equipped with two addition funnels (100 mL)
under argon atmosphere and stirring, 5 g (12 mmol) of 3a were dissolved in 50 mL of
freshly distilled THF. This solution was then cooled down to –40 °C (over a dry ice –
isopropanol bath) before adding 6.9 mL (30 mmol) of triisopropyl borate under vigorous
stirring. Next, 2.8 mL (30 mmol) of 1.6 M BuLi (dissolved in about 20 mL of freshly
distilled THF) were added dropwise keeping the solution at –40 °C. The reaction mixture
was stirred during 24 h allowing the temperature to rise up to room temperature. In a
following step, 12 mL of HCl 2 N were added, and 24 h later the hydrolyzed product was
transferred to a separation funnel and diethyl ether was added. The organic layer was
isolated and washed with distilled water until solution reaches neutral pH. Finally, cold
methylene chloride was added and the precipitate (4a) was recuperated by filtration to be
further dried overnight in a vacuum oven at 30 °C. 4a was obtained as a white powder
1
(Yield: 44%). H NMR (Methanol-d4, δ = ppm): 0.91 (t, CH3, 3H, J = 6.9 Hz); 1.54-1.22
(m, CH2, 18H); 1.85-1.74 (m, CH2, 2H); 4.05-3.99 (m, CH2, 2H); 6.85 (d, CH3, 6H, J = 7.9
Hz); 7.06-6.82 (m, CH2, 2H); 7.65-7.37 (m, CH2, 4H); 7.83-7.78 (m, CH2, 1H).
4-((12-bromododecyl)oxy)-4’’-(dodecyloxy)-[1,1’:4’,1’’-terphenyl]-3-carbonitrile (tCN)
In a 250 mL two-neck round-bottom flask, under argon atmosphere and magnetic stirring,
0.93 g (2.09 mmol) of 1c, 0.09 g (0.08 mmol) of Pd[0], 60 mL of THF and 17.6 mL of
aqueous solution of Na2CO3 2M, were introduced. The reaction mixture was heated to 60
°C, and then a solution (THF solvent) containing 1.04 g (2.71 mmol) of 4a was added
dropwise very slowly during 2 h. Next, the reaction mixture was held at 60 °C for 72 h
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under stirring. After this reaction period, the solution was allowed to cool down to room
temperature and 100 mL of CHCl3 were added. The solution was then transferred to a
separation funnel where the organic phase was washed with water to be further evaporated
in a rotary evaporator. The recuperated solid was introduced into a chromatographic
column (silica-gel) using a mixture of hexanes:chloroform (1:1) as eluent. tCN was
recuperated as a white powder (Yield: 62 %). 1H NMR (CDCl3, δ = ppm): 0.91 (t, CH3, 3H,
J = 6.9 Hz);1.57-1.20 (m, CH2, 34H); 2.03-1.79 (m, CH2, 6H); 3.44 (t, CH2, 2H, J = 6.9
Hz); 4.03 (t, CH2, 2H, J = 6.5 Hz); 4.14 (t, CH2, 2H, J = 6.6 Hz); 7.01 (d, CH, 2H, J = 8.8
Hz); 7.06 (d, CH, H, J = 8.8 Hz); 7.58 (dd, CH, 4H, J = 8.7, 2.0 Hz); 7.66 (d, CH, 2H, J =
8.5 Hz); 7.79 (dd, CH, H, J = 8.8, 2.4 Hz); 7.83 (d, CH, H, J = 2.2 Hz). FT-IR (KBr cm–1)
3079, 3033 (s, C-H ring stretch); 2953 (s, as, CH3); 2917 (m, as, CH2); 2879 (s, s, CH3);
2850 (m, s, CH2); 2229 (w, CN stretch); 1607, 1492, 1468 (m, C=C ring stretch); 1390 (s,
s, CH3, CH2); 1289, 1257 (m, C-O-C asym. stretch); 1179, 1129 (m, C-O-C sym. stretch);
1030 (s, C-H in plane); 815 (m, -C-H out of plane bending); 731 (s,, CH3); 722 (s, ,
13
CH2). C NMR (125 MHz, CDCl3, δ=ppm): 160.1, 158.96, 140.2, 136.9, 133.6, 132.6,
131.9, 128.0, 127.2, 126.9, 116.5, 114.9, 112.7, 102.5, 69.3, 68.2, 34.1, 32.9, 31.9, 29.7,
29.6, 29.6, 29.6, 29.5, 29.4, 29.4, 29.3, 28.9, 28.8, 28.2, 26.1, 25.9, 22.7, 14.1.
4-((12-bromododecyl)oxy)-4’’-(dodecyloxy)-[1,1’:4’,1’’-terphenyl]-3,3’’-dicarbonitrile
(t2CN)
A similar procedure to that described for tCN was followed but using 2b and 3c instead of
1c and 4a, respectively. White powder (Yield: 65 %). 1H NMR (500 MHz, CDCl3, δ=ppm):
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0.91 (t, CH3, 3H, J = 6.9 Hz);1.60-1.24 (m, CH2, 34H); 1.93-1.85 (m, CH2, 6H); 3.43 (t,
CH2, 2H, J = 6.8 Hz); 4.15 (t, CH2, 4H, J = 6.5 Hz); 7.07 (d, CH, 2H, J = 8.8 Hz); 7.61 (s,
CH, 4H); 7.78 (dd, CH, 2H, J = 8.8, 2.2 Hz); 7.83 (d, CH, 2H, J = 2.2 Hz). FT-IR (KBr cm–
1) 3072, 3046 (s, C-H ring stretch); 2950 (s, as, CH3); 2922 (m, as, CH2); 2865 (s, s,
CH3); 2850 (m, s, CH2); 2226 (w, CN stretch); 1607, 1493, 1469 (m, C=C ring stretch);
1391 (s, s, CH3, CH2); 1286, 1270 (m, C-O-C asym. stretch); 1133 (m, C-O-C asym.
stretch); 1020 (s, C-H in plane); 812 (m, -C-H out of plane bending); 727 (s,, CH3); 659
13
(s, , CH2). C NMR (125 MHz, CDCl3, δ = ppm): 160.3, 138.0, 133.2, 132.7, 132.0,
127.2, 116.4, 112.7, 102.6, 69.4, 34.0, 32.8, 31.9, 29.7, 29.6, 29.6, 29.5, 29.5, 29.4, 29.3,
29.3, 29.3, 28.9, 28.8, 28.2, 25.9, 22.7, 14.1.
12-((3-cyano-4´´-(dodecyloxy)-[1,1´:4´,1´´-terphenyl]-4-yl)oxy)dodecyl methacrylate
(mCN)
In a 250 mL round-bottom flask, under argon atmosphere and stirring, 0.21 g (2.13 mmol)
of KHCO3, 0.18 g (2.13 mmol) of methacrylic acid, and 40 mL of DMF, were introduced
and stirred to complete dissolution. Next, 7.8 mg (0.07 mmol) of hydroquinone and 0.5 g
(0.71 mmol) of tCN were added. The reaction mixture was then heated to 80 °C and held
under stirring for 24 h. Once this period elapsed, the solution was allowed to cool down to
room temperature and transferred to a separation funnel containing water (~50 mL) and
CHCl3 (~50 mL). The organic phase was subsequently washed with HCl 5 % (aqueous
solution), K2CO3 5 % (aqueous solution), and water until solution reaches neutral pH.
Finally, the organic phase was dried with sodium sulfate, filtered and evaporated. mCN was
9

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obtained as a white powder (Yield: 82 %). 1H NMR, (CDCl3, δ = ppm): 0.91 (t, CH3, 3H, J
= 6.9 Hz);1.58-1.24 (m, CH2, 34H); 1.75-1.65 (m, CH2, 2H); 1.94-1.78 (m, CH2, 4H); 1.97
(s, CH3, 3H); 4.03 (t, CH2, 2H, J = 6.6 Hz); 4.21-4.10 (m, CH2, 4H); 5.57 (s, CH2, H); 6.12
(s, CH2, H); 7.02 (d, CH, 2H, J = 8.8 Hz); 7.05 (d, CH, H, J = 8.8 Hz); 7.58 (dd, CH, 4H, J
= 8.4, 1.4 Hz); 7.66 (d, CH, 2H, J = 8.5 Hz); 7.78 (dd, CH, H, J = 8.7, 2.4 Hz); 7.83 (d, CH,
H, J = 2.4 Hz). FT-IR (KBr cm–1) 3088, 3032 (s, C-H ring stretch); 2952 (s, as, CH3); 2918
(m, as, CH2); 2869 (s, s, CH3); 2849 (m, s, CH2); 2230 (w, CN stretch); 1713 (m, C=O,
stretch); 1634 (s, C=C stretch); 1608, 1492, 1469 (m, C=C ring stretch); 1389 (s,  s, CH3,
CH2); 1290 (m, C-(C=O)-O stretch); 1254 (m, C-O-C asym. stretch); 1173 (m, O-C-C sym.
stretch); 1129 (m, C-O-C sym. stretch); 1026 (s, C-H plane bending); 915 (s, C=C-H out of
13
plane bending); 817 (m, -C-H out of plane bending); 726 (s,, CH3); 656 (s, , CH2). C
NMR (125 MHz, CDCl3, δ = ppm): 167.6, 160.1, 159.0, 140.2, 136.9, 136.6, 133.6, 132.6,
131.9, 128.0, 127.2, 126.9, 125.1, 116.5, 114.9, 112.7, 102.5, 69.3, 68.2, 64.9, 31.9, 29.7,
29.6, 29.6, 29.6, 29.5, 29.5, 29.4, 29.4, 29.3, 29.2, 28.9, 28.6, 26.1, 26.0, 25.9, 22.7, 18.3,
14.1.
12-((3,3´´-dicyano-4´´-(dodecyloxy)-[1,1´:4´,1´´-terphenyl]-4-yl)oxy)dodecyl methacrylate
(m2CN).
A procedure similar to that described for mCN was followed. White powder (Yield: 82 %).
1H NMR, (CDCl3, δ = ppm): 0.91 (t, CH3, 3H, J = 6.9 Hz); 1.58-1.23 (m, CH2, 34H); 1.75-
1.65 (m, CH2, 2H); 1.94-1.84 (m, CH2, 4H); 1.97 (s, CH3, 3H); 4.16 (m, CH2, 6H); 5.57 (s,
CH2, H); 6.12 (s, CH2, H); 7.07 (d, CH, 2H, J = 8.8 Hz); 7.61 (s, CH, 4H); 7.79 (dd, CH,
2H, J = 8.8, 2.5 Hz); 7.83 (d, CH, 2H, J = 2.5 Hz). FT-IR (KBr cm-1) 3070, 3037 (s, C–H
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ring stretch); 2947 (s, as, CH3); 2922 (m, as, CH2); 2871 (s, s, CH3); 2852 (m, s, CH2);
2225 (m, CN stretch); 1720 (m, C=O, stretch); 1636 (s, C=C stretch); 1606, 1493, 1468 (m,
C=C ring stretch); 1391 (s,  s, CH3, CH2); 1286 (m, C-(C=O)-O stretch); 1167 (m, O-C-C
asym. stretch); 1133 (m, C-O-C sym. stretch); 1011 (s, C-H plane bending); 940 (s, C=C-H
out of plane bending); 817 (m, -C-H out of plane bending); 724 (s,, CH3); 669 (s, , CH2).
13C NMR (125 MHz, CDCl3, δ = ppm): 167.6, 160.3, 138.0, 136.6, 133.1, 132.7, 132.0,
127.2, 125.1, 116.4, 112.7, 102.6, 69.4, 64.8, 31.9, 29.7, 29.6, 29.6, 29.5, 29.5, 29.4, 29.3,
29.2, 28.9, 28.6, 26.0, 25.9, 22.7, 18.3, 14.1.
2.3. Instruments
The chemical structure of intermediates, precursors and monomers was verified by proton
(1H NMR) and carbon (13C NMR) nuclear magnetic resonance performed at 25 °C in a 500
MHz NMR spectrometer (Avance III from Bruker) and using CDCl3 or methanol-d4 as
solvent. To ascertain the chemical structure of monomers, one of them was analyzed in
more detail by 2D NMR spectroscopy (COSY, HSQC, HMBC and DEPT-135) (see
Supporting information). Chemical shifts are referenced to residual non-deuterated solvent.
Chemical shifts () and coupling constants (nJH–H) are expressed in terms of ppm and hertz,
respectively. The Fourier transform infrared (FT-IR) spectra of some of the compounds
were recorded from 4000 to 400 cm–1 and 25 scans in a FT-IR spectrometer from Thermo-
Nicolet (Magna 550). The thermal stability of compounds (vacuum dried samples) was
determined in a thermogravimetric analyzer (TGA 951 from DuPont Instruments)
connected to a N2 vector gas, heating from 30 to 700 °C at 10 °C min–1. The thermal
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behavior was determined in a differential scanning calorimeter (MDSC 2920 from TA
Instruments) at heating/cooling rates of 10 °C min–1; reported data correspond to the first
cooling and second heating scans. The optical textures were obtained with samples
confined between two glass cover slides and observed through crossed polarizers in an
optical microscope (BX53 from Olympus) coupled to a hot stage (LTS420E from Linkam)
equipped with a controller (T95 LinkSys from Linkam). Textures were captured with a
digital camera (3.3 RTV from Micropublisher). The layer spacing of the smectic phases
was determined from the X-ray diffraction patterns registered at different temperatures on
cooling from the isotropic liquid, using a small/wide X-ray diffractometer (SAXSess mc2
from Anton Paar GmbH) equipped with a sealed copper tube (Cu Kα 0.154 nm), a sample
holder unit (TCS 300-C), an image plate detector, and a temperature controller (TCU50).
XRD experiments were conducted with samples encapsulated in glass capillaries having an
outer diameter and a wall thickness of 1.0 and 0.01 mm, respectively. Spectroscopic grade
THF was used for the optical characterization. UV-Vis and fluorescence spectroscopy was
carried out on a Shimadzu 2401 PC and a Perkin Elmer LS50B instruments, respectively.
The optical band gap was calculated from the intersection with the X axis (in energy units)
and the tangent to the normalized spectrum drawn at a value of absorbance of 0.1. For
fluorescence spectra, excitation wavelength was chosen at 10 nm under the absorption
lower energy peak. The quantum yield was obtained according to Williams et al.,21 and by
using quinine sulfate in 0.1 M H2SO4 as standard. Experiments were carried out at 25.0 ±
0.5 °C, with a water circulating bath. Five repetitions were realized and the average value is
reported in the Table 2. Cyclic voltammetry was performed in a C3 Stand cell from Basi
coupled to an ACM Gill AC potentiostat/galvanostat. The system consisted in a
conventional three-electrode cell: glassy carbon as a working electrode (polished with
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alumina after each run), Pt wire as the counter electrode, Ag/AgCl as reference electrode
and ferrocene/ferrocenium as internal reference (at the maximum potential in acetonitrile:
Eox = 0.590 V, Ered = 0.510 V); a value of -4.8 eV below the vacuum level was considered.
Voltammetric measurements were performed in thin films deposited on the working
electrode and using acetonitrile (deoxygenated) containing Bu4NPF6 (0.1 M) as the
supporting electrolyte at room temperature. The experiments were carried out under
nitrogen atmosphere at a scanning rate of 50 mV/s.
3. Results and Discussion
3.1. Synthesis and chemical characterization
The p-terphenyl–functionalized methacrylate monomers (mCN and m2CN) were prepared
through different reactions (Scheme 1), including the Suzuki cross-coupling reaction, which
is widely used to prepare small (oligomers) and long (polymers) -conjugated molecules.22
The Suzuki reaction proceeds under mild conditions and uses aryl boronic acids, which are
fairly stable, easy to handle, and tolerate a variety of functional groups including
cyanides.18,23 A problem of boronic acids is the relatively low temperature required for their
synthesis (typically – 40 °C and below). At such low temperature condition chemical
intermediates become hardly soluble. This was the case of intermediate 3a whose
corresponding boronic acid (4a) was obtained at low yield (44 %).
1
The H NMR spectra of tCN, t2CN, mCN, and m2CN are grouped in Figure 1. Spectral
differences between the mono- and dicyano-substituted molecules are readily seen at low
13

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fields (around 7.7 and 7.0 ppm) where some extra signals appear for tCN and mCN due to
the unsymmetrical structure of their aromatic core. On the other hand, signals appearing at
6.15, 5.5 and 4.1 ppm for mCN and m2CN confirmed the end-functionalization of
precursors (tCN and t2CN) with the methacrylate group. The first two signals correspond to
protons of the vinyl group (CH2=C), while the third one is associated to protons of the
methylene linked to the ester group (CH2OCO). In addition, the absence of a triplet at 3.5
ppm in the spectra of mCN and m2CN proves that the alkyl chain is no longer attached to
the end-bromine but to the monomer through the ester group (signal at 4.1 ppm).
Figure 1. 1H.NMR spectra of tCN, t2CN, mCN and m2CN.
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Figure 2. (a) 1H and (b) 13C NMR spectra of mCN.
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As noted, the unsymmetrical monomer (mCN) showed a complex spectrum with various
overlapped signals. In order to validate its proton assignment, a complementary
characterization was performed by COSY, HSQC, HMBC and DEPT-135. This full NMR
characterization allowed us to assign unequivocally the chemical shifts to the whole
molecule as it is shown in the 1H and 13C NMR spectra depicted in Figure 2.
1
The well-resolved H NMR spectrum of mCN (Figure 2a) allows us to distinguish six
signals in the aromatic region (low fields), which correspond to seven chemically different
protons (H5, H3, H9’, H9, H3’, H2 and H2’) out of a total of 11 aromatic protons. Such
protons, as well as those of the two methylene groups in alpha position to the oxygen atom,
(H23, H17, and H10), showed a first-order coupling pattern. The doublet centered at 7.83
4
ppm (d, JH5-H3 = 2.4 Hz) corresponds to H5, which is coupled with H3. The doublet of
doublets centered at 7.78 ppm and coupled with H2 and H5 (dd, 3JH3-H2 = 8.7, 4JH3-H5 = 2.4
Hz) was attributed to H3. The doublet centered at 7.66 ppm, and that is coupled with H9 (d,
3JH9-H9’ = 8.5 Hz), was assigned to the two equivalent H9’ hydrogen atoms. One of two
overlapped doublets at 7.58 ppm corresponds to the two equivalent hydrogens H9 (d, 3JH9-
H9’ = 8.2 Hz), and the other one to the two equivalent hydrogens H3’ (d, 3JH3’-H2’ = 8.4 Hz).
3
H9 and H3’ are coupled with H9’ and H2’, respectively. Finally, signals at 7.05 (d, JH2-H3
3
= 8.8 Hz) and 7.02 ppm (d, JH2’-H3’ = 8.9 Hz) were attributed to H2 and to the two
equivalent hydrogens H2’, respectively. H2 and H2’ are shifted towards the lowest
frequencies (around 7 ppm) of the aromatic region due to mesomeric effects by oxygen
1
atoms.24 At medium fields of the H NMR spectrum the two singlets at 5.57 y 6.12 ppm
were assigned to the two protons H26 of the vinyl group; one at the cis and the other at the
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trans position to the methyl group. Also, three triplets are distinguished at medium fields;
those centered at 4.17 ppm (t, 3JH23-H22 = 6.4 Hz) and 4.13 ppm (t, 3JH17-H18 = 6.6 Hz) were
attributed to the protons of the methylene groups alpha to the oxygen atom of the ester and
3
ether groups (H23 and H17, respectively), and that centered at 4.03 ppm (t, JH10-H11 = 6.6
Hz) was assigned to the protons of the methylene group alpha to the oxygen atom of the
ether group (H10). Finally, protons of the aliphatic chain (H11-H16 and H18-H22) and
those of the methyl group of the methyl methacrylate (H27), located in the low-frequency
region of the spectrum (between 0.9 and 2.0 ppm), were also assigned through signal
correlations.
13
The C NMR spectrum (broad band decoupling) of mCN shows 14 out of a total of 18
signals expected for carbon atoms of the aromatic cycles, meaning that four carbon-atom
pairs are chemically equivalent (Figure 2b). Such carbon atoms are C2’, C3’, C9’ and C9,
which are contiguous to the quaternary carbon atoms in the following order: C1’, C4’, C8’,
and C8. Signals at 115.1 and 112.8 ppm were assigned to C2’ and C2, respectively. Both
carbon atoms are in ortho position to a quaternary carbon atom (C1’ and C1, respectively)
linked to an oxygen atom. Signals at 132.6 and 131.9 correspond, one to one, to C3 and C5,
and that at 116.3 ppm to the carbon atom of the cyano group (C7). The two signals located
at 136.6 and 125.1 ppm were attributed to the carbon atoms of the vinyl group (C25 y C26,
respectively). The three signals shifted to the highest frequencies of the spectrum
correspond to the carbon atom (C24) of the carbonyl group and to two quaternary carbon
atoms (C1 and C1’) close to the oxygen atom. Such a high frequency shift might arise from
the inductive effect of the neighboring oxygen atoms. The assignment of all these signals
was supported by HMBC (see supporting information) and also by reported 13C NMR
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spectral data on terphenyls.25 Signals located between 63 and 70 ppm were attributed to the
carbon atoms (C17, C10 and C23) alpha to the oxygen atoms and those situated between 32
and 14 ppm to the remaining carbon atoms (C11-C16 and C18-C22) of the aliphatic chains
except for that shifted to 18 ppm, which corresponds to the carbon atom of the methyl
group (C27) of the methacrylate monomer.
3.2. Thermal behavior
Precursors (tCN, t2CN) and monomers (mCN and m2CN) were first subjected to a
thermogravimetric analysis (TGA) to determine their thermal stability. Thermograms
showed a 5 %–decomposition temperature (T5%) higher than 300 °C for all four compounds
(Figure 3). As can be noticed, mCN and m2CN are significantly more thermally stable
(around 60 °C) as compared with tCN and t2CN. This result was to certain point expected
as the dissociation energy of the C–Br bond is relatively low.26 Monomers showed no
weight lost below 300 °C, although they could have polymerized at such high temperature
condition. In order to avoid decomposition, 200 °C (well below T5%) was fixed as the
maximum temperature for their further thermotropic characterization by DSC, POM and
XRD.
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Figure 3. TGA traces of tCN, t2CN, mCN and m2CN
The liquid crystal properties of precursors and monomers were first explored by DSC. The
DSC traces (Figure 4) showed that tCN, mCN and m2CN are enantiotropic liquid crystal
whereas t2CN is monotropic. They also showed that molecules carrying one cyano group
melt at lower temperatures (near room temperature) and display a mesomorphic behavior
over larger temperature intervals as compared to those carrying two cyano groups. In a
previous work, we have found that the didodecyloxy-p-terphenyl without cyano groups
melts around 130 °C;27 since this temperature is much higher than that of the cyano-
substituted molecules, one can assume that this group perturbs the -interactions that
normally hold the aromatic cores together.
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Figure 4. DSC thermograms of precrusors (tCN, t2CN) and monomers (mCN and m2CN),
registered at heating and cooling runs (10 °C min-1).
3.3. Optical textures and mesophase structure
Once their mesomorphic range was determined, precursors and monomers were studied by
POM at variable temperature on cooling from the isotropic liquid. Both, tCN (Figure 5a)
and t2CN (Figure 5b) displayed a focal-conic fan texture coexisting with homeotropic
zones. mCN showed a focal-conical fan texture that turned into a gray schlieren texture
upon cooling, suggesting the presence of orthogonal and tilted smectic phases,
respectively.28 Figure 5c shows both textures as it was registered at the transition from one
to the other. Finally, m2CN developed batonnets that upon cooling should have merged
into the familiar focal-conic fans, but they remained separated from one another (Figure
5d). This behavior leads one to suspect that this monomer has thermally polymerized
during POM analysis.
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Figure 5. Optical textures observed on cooling from the isotropic liquid. tCN at 90 °C (a),
t2CN at 108 °C (b), mCN at 130 °C (c), and m2CN at 98 °C (d).
The molecular stacking of the smectic phases was determined from the sharp Bragg
diffraction peak (indexed as 001) appearing in the low-angle region of the XRD patterns
(Figure 6), which were captured at determined temperature on cooling from the isotropic
state. tCN displayed two equidistant Bragg reflections (d001 and d002) whereas t2CN and
mCN developed only one single peak (d001). On the contrary, m2CN showed a series of
equidistant Bragg peaks at low angles, indicating that there is a longer correlation distance
for their smectic layers as compared to that of the other studied molecules. In the wide-
angle region of the X-ray patterns, all molecules developed only one low-intense broad
diffraction peak centered around 21° that is typical of an in-plane liquid-like order of
molecules in the smectic A and C phases.
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Figure 6. X-ray diffraction patterns of tCN, mCN, t2CN and m2CN captured at 120°C
upon cooling.
Table 1. Thermal properties and structural parameters for precursors (tCN and t2CN) and
monomers (mCN and m2CN) in the mesophase.
T
d001
L
(Å)
d/L
Tilt angle
Molecule
Stacking
Mesophase
(°C) (Å)
(°)
120 38.4 45.7 0.84 Monolayer
120 37.6 45.7 0.82 Monolayer
120 41.3 49.2 0.84 Monolayer
33
35
33
37
SmC
SmC
SmC
SmAd
tCN
t2CN
mCN
m2CN
120 78.4 49.2 1.59
Bilayer
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To determine the way the molecules arrange within the lamellar structure, the experimental
smectic period (d001) was compared with the length of molecules in their most extended
conformation (L) calculated by molecular modeling software (Spartan 2010) and the
resulting d001/L ratios are gathered in Table 1. The d001/L ratios for tCN, mCN and t2CN are
close to unity, suggesting a single layer (or monolayer) stacking of molecules. The cos-1
d001/L ratios for these three molecules resulted in tilt angles of around 34° with respect to
the normal of the smectic plane. Such an angle may correspond to a tilted phase (smectic C)
as it is illustrated in Figure 7. It is to point out that for mCN two mesophases were detected
by DSC and POM, however their corresponding XRD patterns showed no significant
differences between them; the peak at low angles (001) was shifted less than 0.1° in 2 and
the broad peak at wide angles remained unchanged. For this reason the high temperature
mesophase of this compound was assigned as undefined (SmX). Finally, for m2CN the
calculated d001/L ratio was 1.6, indicating a double-layer stacking with molecules either
tilted with respect to smectic planes or orthogonal but with molecules interpenetrated from
one smectic layer to the other. The observed (by POM) batonnets surrounded by
homeotropic zones in a large temperature region is likely to be due to an orthogonal
orientation of molecules that may rather correspond to a bilayer smectic structure with
molecules interdigitated from one layer to the other (SmAd). The double layer stacking is
commonly observed in side-chain liquid crystal polymers as reported by García et al. for
polymethacrylates bearing azomesogenic moieties laterally substituted with one cyano
group.17 The double layer stacking may perhaps indicate that m2CN has polymerized
during the XRD heating/cooling runs. FTIR experiments (see Supporting information)
before and after XRD heating/cooling runs confirmed the polymerization of m2CN.
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The DSC, POM, and XRD results are summarized in the polymorphic sequences gathered
in Scheme 2.
Figure 7. Schematic representation of molecules (mCN) in a tilted smectic phase.
Scheme 2. Polymorphic behavior of monomers (mCN and m2CN) and their precursors
(tCN and t2CN).
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3.4. Optical properties in solution.
The UV-Vis absorption and emission spectra of the precursors (tCN and t2CN) and their
methacrylate monomers (mCN and m2CN) in THF are reported in Figure 8. As term of
comparison, a p-terphenyl without cyano groups but end-substituted with dodecyloxy
chains (named sCN) was also analyzed. All of the optical properties are collected in Table
2. The absorption spectra show only one single peak associated with * electronic
transition of the terphenyl moiety and with half height bandwidth (HHBW) of c.a. 50 nm,
suggesting the contribution of different conformers in the ground state. Any additional band
due to the methacrylate group was found for the monomers as the double bond absorbs in
the far UV. Moreover, as it is separated from the aromatic segment with a long aliphatic
spacer, it does not induce any electronic effect to the * electronic transition of the p-
terphenyl unit, so that the absorption maximum of the monomers is in the same position of
their corresponding p-terphenyls that is centered at around 296 nm. In general, no changes
in both, wavelength position or spectral shape were observed with respect to that of sCN,
suggesting that at least in solution, the CN groups have no relevant electronic effect in the
absorption spectra. However, all of the molecules including sCN, present a batho- and
hyperchromic trend with respect to unsubstituted p-terphenyl (max = 275 nm).29 This
behavior likely indicates that the alkoxy group, which is in the para position in the p-
terphenyl moieties, i.e. in the long molecular axis of the chromophore, has a dominant
effect with respect to p-terphenyls bearing the CN group. The optical band gap (Egopt) is of
~3.56 eV for all of the molecules, value that matches well with those electrochemically
determined (vide infra) and classifying them as semiconducting materials. In contrast to
what observed in the absorption spectra, the fluorescence maxima do shift with the number
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of substituted CN groups following the trend: without CN (sCN, 359 nm) < two CN groups
(m2CN or t2CN, ~ 363 nm) < one CN group (mCN or tCN, ~ 371 nm). The Stokes’ shift
(Δv) is in the range of values reported for substituted p-terphenyls, and is due to geometry
change from an aromatic-like geometry in the ground state to a coplanar quinoid structure
in the excited state. In this respect, it is reasonable to expect that, contrary to what
discussed for the ground state, both the alkoxy and CN groups have an electronic effect in
the excited state. In particular, the D-A-D effect expected for mCN and tCN likely gives a
larger coplanarity with respect to the symmetric D-A-A-D systems: m2CN and t2CN.
However, theoretical studies for determining the geometry in the ground and excited states
are in progress to give a better elucidation of this behavior. It is to be pointed out that sCN
has a fluorescence spectrum similar to that of p-terphenyl with an excitonic maximum at
343 nm exhibiting an intense vibronic progression at 359 nm, i.e. with a wavelength
spacing equivalent to 1295 cm-1 that corresponds to the C – C stretching vibration of the
phenyl ring. This feature was ascribed on p-terphenyl to the fact that this molecule passes
from a twisted geometry in the ground state to a more planar (quinoid) in the excited
state.30 On the contrary, the CN-substituted p-terphenyls and monomers present both broad
absorption and fluorescence spectra. A similar behavior was found by Liu for 2,2’’-
dimethyl-p-terphenyl and attributed to the fact that both ground and first excited states
present a twisted geometry.30 However, as previously mentioned, the large Stokes’ shift is
in agreement with a geometry change upon excitation. The broadness of absorption and
emission spectra of the CN substituted molecules could thus be related to the superposition
of electronic transitions arisen from different conformers. This could be consistent with the
fact that the HHBW is similar for both, the UV-Vis and fluorescence spectra;
conformational studies might elucidate this issue. As a final remark, the fluorescence
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quantum yield is high for the whole series, with the trend sCN < m2CN ~ t2CN < mCN ~
tCN and in accordance with their Stokes’ shift values. As p-terphenyl has a fluorescence
quantum yield close to 1, the deviation of the unity for the present series can thus be related
mostly to internal conversion.
Figure 8. UV-Vis (solid lines) and fluorescence (dashed lines) of p-terphenyls and
monomers in THF (see inserted structures)
Table 2. Optical properties of precursors (tCN and t2CN) and monomers (mCN and
m2CN) in THF. The reference molecule (sCN) is included.
λabs
(nm) (M-1 cm-1)
ε x 10*
HHBWabs Egopt
λem
(nm)
HHBWem
Δν
ɸ
Molecule
(nm)
(eV)
3.56
3.56
(nm)
(cm-1) (%)
296
295
7.43
7.68
49
371
363
371
49
45
6739
6350
81
69
tCN
53
t2CN
296
294
7.34
6.83
50
50
3.57
3.57
49
45
6830
6541
78
67
mCN
364
343,
359
m2CN
296
7.39
48
3.56
46
4634
61
sCN
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3.5. Electrochemical properties.
From all of the series, only m2CN and sCN compounds formed homogeneous films on the
glassy carbon electrode from THF solutions. In general, from the cyclic voltammograms
(Figure 9) we deduce that, i) both compounds are electroactive in the forward cycle scan
(from –3 to +3 V), but exhibiting irreversible redox processes, which means that the anion-
cation pairs are unstable, ii) p-terphenyls are either difficult to be oxidized (Eox > 1.8 V) or
reduced (Ered > –2.0 V), iii) however they are more susceptible for oxidation than for
reduction; behaving as a p-type material, iv) although, the electronic nature can be
modulated, e.g. sCN bearing alkoxy chains shows two oxidation peaks, assigned to the
conjugated p-terphenyl (1.86 V) and to the alkoxy groups (2.71 V), while in the reduction
process only a slight visible reduction peak is observed at 2.04 V. When two cyano groups
are introduced in the terphenyl structure (m2CN), a single oxidation peak appears at higher
potentials (1.92 V), suggesting that formed radicals are likely stabilized by the electron
withdrawing character of the cyano groups. In the cathodic region, if still a slight peak is
observed, this appears at lower potentials -1.12 V that is in agreement with the fact that the
cyano groups are easier to be reduced than to be oxidized. Since the oxidation process is
correlated with the removal of electrons (or hole injection for the development of polarized
light emitting diodes), the HOMO levels were –6.66 eV for sCN and –6.72 eV for m2CN,
while the LUMO levels were –2.76 eV for sCN and –3.68 eV for m2CN. The
electrochemical band gap (Eg) was 3.90 and 3.04 eV for sCN and m2CN, respectively.
28

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Figure 9. Cyclic voltammograms of sCN and m2CN deposited as thin films on the glassy
carbon working electrode, using Pt wire as auxiliary electrode, Ag/AgCl as
counter electrode, Bu4NPF6 as the supporting electrolyte in acetonitrile and
Fe+/Fe- as internal reference.
Conclusion
In this work, the synthesis and the thermotropic liquid crystal properties of two methacrylic
monomers and their corresponding precursors are reported. All four molecules bear a p-
terphenyl group laterally substituted with one or two cyano groups. Their chemical
1
structure was confirmed by H and 13C NMR spectroscopies, as well as by 2D NMR
techniques. By TGA was determined that their initial degradation temperature is around
29