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Novelhigh birefringent isothiocyanates based onquaterphenyland phenylethynyltolane m olecular
cores
a

a

a

a

Jakub Herm an , Jerzy Dziaduszek , Rom an Dbrow ski , Jerzy Kdzierski , Krystianabb

Kow iorski , Venkata SaiDasari , Surajit Dhara & Przem ysaw Kulaaa

Departm ent of Advanced Technologies and Chem istry , M ilitary University of Technology ,W arsaw , PolandbSchoolof Physics , University of Hyderabad , Hyderabad , India
Published online: 18 Jun 2013.

To cite this article: Jakub Herm an , Jerzy Dziaduszek , Rom an Dbrow ski, Jerzy Kdzierski, Krystian Kow iorski, Venkata
SaiDasari, Surajit Dhara & Przem ysaw Kula (2013) Novelhigh birefringent isothiocyanates based on quaterphenyland
phenylethynyltolane m olecular cores, Liquid Crystals, 40:9, 1174-1182, DOI: 10.1080/02678292.2013.808768
To link to this article: http://dx.doi.org/10.1080/02678292.2013.808768

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Liquid Crystals, 2013
Vol. 40, No. 9, 1174–1182, http://dx.doi.org/10.1080/02678292.2013.808768

Novel high birefringent isothiocyanates based on quaterphenyl and phenylethynyltolane molecularcoresaJakub Hermana*, Jerzy Dziaduszeka, Roman Dabrowski
˛, Jerzy K˛edzierskia, Krystian Kowiorskia,bbVenkata Sai Dasari , Surajit Dhara and Przemysaw Kulaaa
Department of Advanced Technologies and Chemistry, Military University of Technology, Warsaw, Poland; b School of Physics,
University of Hyderabad, Hyderabad, India

Downloaded by [University of Hyderabad] at 05:38 17 August 2013

(Received 10 August 2012; final version received 22 May 2013)
We report the synthesis of new laterally alkyl substituted high birefringent quaterphenyl and phenylethynyltolane
liquid crystals with isothiocyanate terminal group. The thermal and optical properties such as birefringence aremeasured and discussed based on their molecular structures. The compounds exhibit birefringence (1n) in therange of 0.44–0.63, and are expected to be appropriate components of high birefringent nematic mixtures forvarious applications.
Keywords: high birefringence; nematic; isothiocyanate; tolane derivative

1. Introduction
High birefringence (1n) liquid crystals (LCs) play animportant role not only among conventional display
devices but also in laser beam steering [1] and tunablefocus lenses [2]. Despite a great number of visible
range applications [3–8], liquid crystal birefringenceis also saturated in the IR region as reported by Wu[9]. Finally, compounds with increased birefringence
gained a huge importance for radio frequency applications in the GHz and THz part of the spectrum [10,11].
Studies of LCs in the millimetre and terahertz ranges
have been demonstrated and followed by different
liquid crystal-based devices [12–16].
The birefringence of a liquid crystal is mainly
determined by the p-electron conjugation of themolecular core, thus the more linearly conjugated liquid crystal would exhibit the larger optical anisotropy[17]. High 1n values can be obtained by increasing the molecular conjugation length and designing
the proper molecular shape. Representative functionalgroups that contribute to the molecular conjugation
length include unsaturated rings (benzene, naphthyl,
thiophene) and unsaturated bonds. Nevertheless highmelting temperature, increased viscosity and degradedphoto stability are the major concerns for high 1nliquid crystals. Reduced UV stability is caused byincreased UV absorption as well as intrinsic reactivity of the triple bond of compounds with elongatedp-electron system. That forces a special protection ofthese materials form direct UV exposure. Despite thefact that application of high birefringent liquid crystals can be limited in visible and ultraviolet regions,they can be successfully utilised for long-wavelength*Corresponding author. Email: jherman@wat.edu.pl© 2013 Taylor & Francis

radio frequency applications (GHz/THz frequencies)[12–16].
High melting temperatures, enthalpies, andincreased viscosity is often a consequence of the elongated molecular structures of the high birefringent
compounds. While the conjugation length increase
leads to high melting points, high 1n liquid crystals
show a poor solubility, when eutectic mixtures areto be formed. If melting temperatures as well as
enthalpies can be successfully improved (lowered)by introducing into molecule various lateral alkyl
substitution, that viscosity may still reach highvalues. An efficient way to reduce it is the elevatedtemperature operation applied for these compounds.
Introduction of a number of lateral substituentsacross the long axis of the liquid crystalline molecule
is a conventional method for the design of physicochemical properties. The lateral fluorine substitution
has an ability to reduce viscosity and increase thedielectric anisotropy [18]. But the most effective way
for lowering melting point is the insertion of thelateral alkyl group, to widen the molecular width [19].
Continuing our interest in the design of new
medium to high birefringent LCs [20,21], we havebeen focused in this paper, on the synthesis of theisothiocyanate quaterphenyl and phenylethynyltolane
species, as the promising components of high birefringent liquid crystal applicable mixtures, with thecommon formulas given in Figure 1.
Compounds possessing four phenyl ring system(quaterphenyl) laterally substituted by one or few substituents in chosen rings were the object of low attention so far, mainly due to the extremely high melting

Liquid Crystals

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Figure 1. General formulas for compounds of series 1 and
2 wherein: X = methyl, ethyl, fluorine, chlorine and n = 3.5.

temperatures of known compounds. Such a molecular core together with an appropriate substitution
can affect 1n values up to 0.43–0.45 [22]. From theother side, both non-polar and polar phenylethynyltolane species gained a bigger interest as the promisinghigh birefringent materials [18,23–27]. Especially polar
phenylethynyltolane derivatives seem to be a desirable for many potential applications. Polar compounds
reported so far consist either of fluorine atom or cyanogroup as a terminal groups [19]; Liao et al reported
also some derivatives with isothiocyanate polar group[27]. These compounds exhibit not only satisfactory1" values, but also very high birefringence (1n  0.5)
in the visible spectral region.
Cyano (CN) and isothiocyanate (NCS) groups are
two commonly employed polar groups used for elongating the molecular conjugation [28]. Many investigations conducted recently and in the past by ourand other laboratories show that liquid crystalline
isothiocyanates are attractive LCs from the application point of view [29], followed by proposalsof high 1n mixtures [30]. Comparing the physicochemical properties of analogous isothiocyanate compounds with those consisting of cyano group, thelatter are more viscous and possess a larger dipole
moments, caused by the very strong dipole momentof the carbon–nitrogen triple bond. Therefore, theisothiocyanates exhibit lower dielectric anisotropy
than the cyano compounds, but their birefringence
1n is much higher [31]. Based on our experience andreasons mentioned above, we synthesised some isothiocyanate compounds with several lateral substitution.
Moreover the discussion about optimal number of lateral substituents, their optimal position in molecularcore, their shape and length is presented.
2. Experimental
The phase transition temperatures and enthalpy data
were determined by polarising optical microscopy(POM) with an OLYMPUS BX51 equipped witha Linkam hot stage THMS-600 and differential

1175

scanning calorimeter (SETARAM DSC 141) during heating/cooling cycles (rate, 2 degrees/min).
The purity of synthesised compounds were determined by thin layer chromatography (TLC),SHIMADZU GCMS-QP2010s series gas chromatograph equipped with quadrupole mass analyser(MS), high performance liquid chromatography
HPLC-PDA-MS (APCI-ESI-dual source) Schimadzu
LCMS 2010EV equipped with polychromatic UV-VIS
detector and MS (ESI/APCI) analyser. Proton (1 H)
nuclear magnetic resonance (NMR) spectra in CDCl3
were collected using a Varian 500 spectrometer.
General synthetic methods of investigated compounds of series 1 and 2 are given in Schemes 1
and 2. For the series with quaterphenyl unit, 4-nalkylphenylboronic acids were used as a starting
materials. Suzuki–Miyaura hetero cross-coupling
reaction was the major incorporated during synthesisof quaterphenyl rigid core, while Sonogashira–Hagihara coupling was predominant for thesynthesis of phenylethynyltolane unit. Here 4-nalkylphenylacetylene and 4-iodo-3-alkylaniline wereused as starting reagents. Introduction of the NCS
polar group into molecules in all cases was carriedout at the last step with thiophosgene as a reactant.
Experimental procedures are given below.

2.1 Synthetic procedures
2.1.1 40 -pentyl-3-methylbiphenyl (2) – Suzuki–
Miyaura procedure
To the mixture of 4-pentylphenylboronic acid (85 g;
0.442 mol), 3-iodotoluene (80.3 g; 0.368 mol), K2 CO3(153 g; 1.1 mol) in 500 ml of acetone and 200 ml ofwater, all flushed with nitrogen, a 0.2 g (0.3% mol) of
catalyst PdCl2 was added. The reaction mixture washeated under reflux for 3 h. After reaction completion mixture was poured into 5% HCl solution. Crude
product was extracted with hexane, washed with water,
dried over MgSO4 . The hexane was evaporated andresidue distilled at reduced pressure. Yield, 74.2 g(87%); b.p., 130 C (0.4 mmHg); MS(EI): 238 (M+ ),181, 166.

2.1.2 40 -pentyl-4-iodo-3-methylbiphenyl (4)The mixture consisted of 40 -pentyl-3-methylbiphenyl(2) (73.6 g; 0.31 mol), I2 (31.4 g; 0.124 mol), HIO3
(11 g; 62 mmol), glacial acetic acid (350 ml), water(80 ml) and concentrated sulphuric acid (31 ml)
was stirred for 3 h under reflux. The mixture waspoured into Na2 SO3 water solution. The product was
extracted to dichloromethane, washed with water,
dried over MgSO4 . Solvent was evaporated and the
product was purified by column chromatography

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1176

Scheme 1. Synthetic route to quaterphenyl liquid crystals (series 1). Synthetic conditions: a – Pd(OAc)2 , K2 CO3 , Acetone,
H2 O; b – I2 , HIO3 , H2 SO4 , CH3 COOH, H2 O; c – (1) sec-BuLi, THF, –75 C, (2) B(OC3 H7 )3 , –75 C, (3) H3 O+ , H2 O; d – CSCl2 ,CHCl3 , H2 O.

(silica gel) using hexane as eluent to give a deep
yellow liquid; yield, 90.5 g (80%); MS(EI): 364 (M+ ),307, 165.
2.1.3 3-chloro-20 -methyl-400 -pentyl-[1,10 :40 ,100 ]terphenyl (6)Compound 6 was prepared using a procedure similar
to that described for synthesis of compound 2. Product6 was purified by column chromatography (silica gel)using hexane/methylene chloride as eluents to give
yellow oil. Yield, 23.7 g (70%); MS(EI): 348 (M+ ), 291,
239, 165.

2.1.4 2-(3-chloro-20 -methyl-400 -pentyl-[1,10 :40 ,100 ]terphenyl-4-yl)-1,3,2-dioxaborinane (8)A solution of compound 6 (23.7 g; 68 mmol) inanhydrous Tetrahydrofuran (200 ml) was stirred andcooled to –75 C on a dry ice/acetone bath, all flushedwith nitrogen. The sec-Butyllithium in cyclohexanehexane solution (68 ml; 1.3 m) was added dropwise
and stirred for 3 h. After that, the tripropyl borate(17.5 ml; 80 mmol) was added dropwise and temperature kept at –75 C. Stirring was continued while
mixture was allowed to reach room temperature. Thensolvents were evaporated and residue was poured

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Liquid Crystals

1177

Scheme 2. Synthetic route to phenylethynyltolane liquid crystals (series 2). Synthetic conditions: a – PdCl2 (PPh3 )2 , CuI, TEA,
DBU, THF; b – HCl, dioxane, NaNO2 , KI; c – NaH, toluene; d –CSCl2 , CaCO3 , H2 O, CHCl3 .

into water and acidified with 10% HCl to pH  2–3.
Purification of boronic acid was continued by carryingout an esterification to cyclic boronic ester derivative(8) with subsequent amount of 1,3-propanediol(4.9 ml; 68 mmol) in toluene solution. Crude productwas purified by column chromatography (silica gel,methylene chloride as eluent) to give yellow low
melting crystals. Yield, 27.8 g (70% GC-MS purity);MS(EI): 432 (M+ ), 375, 317, 239.
2.1.5 20 -chloro-4-isothiocyanato-200 -methyl-4000 pentyl-[1,10 :40 ,100 :400 ,1000 ]-quaterphenyl (12)Compound 10 was synthesised using standard
Suzuki–Miyaura cross-coupling protocol. The solidhydrochloride of 10 amine was filtered off, washedwith cold acetone and then dissolved in chloroform(30 ml). The solution was dropped slowly to the
mixture consisted of CHCl3 (30 ml), water (30 ml)

and thiophosgene (0.8 ml; 10.6 mmol) at 0–5 C. The
mixture was stirred for 5 h at room temperature. The
organic layer was separated, dried over MgSO4 , thesolvent was evaporated and the solid residue waspurified by column chromatography (silica gel) using
boiling heptane as eluent to give white solid crystal,
which was recrystallised from acetone. Yield, 1.7 g(50%); m.p., 107 C; MS(EI): 481 (M+ ), 424, 392,
212. 1 H NMR (200MHz, CDCl3 )  7.53–7.51 (d, 2H,
Ar–H); 7.48–7.46 (d, 2H, Ar–H); 7.37–7.32 (m, 5H,
Ar–H); 7.29–7.26 (m, 5H, Ar–H); 2.67–2.62 (t, 2H,
Ar–CH2 –); 2.39 (s, 3H, Ar–CH3 ); 1.75–1.59 (m, 2H,–CH2 –); 1.39–1.26 (m, 4H, –CH2 CH2 -); 0.95–0.88(t, 3H, –CH3 ).
2.1.6 2-methyl-4-[(4-pentylphenyl)ethynyl] aniline(19) – Sonogashira–Hagihara protocol
To the mixture of 4-iodo-2-methylaniline (100 g;
0.43 mol), triethylamine (71 ml; 0.52 mol), 1,

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8-Diazabicyclo[5.4.0]undec-7-ene (64 ml; 0.43 mol),anhydrous Tetrahydrofuran (500 ml), PdCl2 (PPh3 )2(5 g; 3% mol), CuI (2.5 g; 3% mol), a solution of
77.6 g (0.45 mol) 4-pentyl-1-ethynylbenzene in 200 mlof anhydrous Tetrahydrofuran was slowly added over
30 min at room temperature. A consequent exothermic reaction raised the temperature up to 50 C.
The mixture was refluxed for 3 h, then cooled andpoured into water. Crude product was extracted withtoluene. Organic layer was washed with water, driedover MgSO4 and filtrated. Toluene was evaporated
under reduced pressure conditions and the residue wasrecrystallised form a hexane solution to give a brown
crystals. Yield, 95 g (80%), m.p., 71.7–72.2 C; MS(EI):
277 (M+ ), 220, 204.
2.1.7 1-iodo2-methyl-4-[(4-pentylphenyl)ethynyl]
benzene (21)To a solution of compound 19 in dioxane (250 ml),
100 mL of 30% HCl solution was added dropwise at
0–5 C. After that, aqueous solution of sodium nitrite(32.4 g; 0.47 mol) was added slowly and the mixture
was stirred for 1 h, maintaining the temperature at thesame level of 0–5 C. Finally the reaction mixture wastreated with aqueous solution of 68.6 g of NaI. Afterthe temperature of the mixture rose to room temperature, it was poured into a water solution of Na2 SO3 ,then the product was extracted with toluene, washedwith water, brine and dried over MgSO4 . Organic
solvent was evaporated and the crude product was
purified by column chromatography (silica gel, hexane
as eluent) to give a deep brown liquid. MS(EI): 388(M+ ), 331, 261, 202.
2.1.8 1-ethynyl-2-methyl-4-[(4-pentylphenyl)
ethynyl] benzene (23)Compound 23 was obtained with standard
Sonogashira protocol and then by the deprotection of carbon–carbon triple bond with catalytic
amount of sodium hydride in anhydrous toluene.
Product 23 does not crystallise and stay in form of
dark oil. Yield, 15 g (72%); MS(EI): 286 (M+ ), 229,
202.

2.1.9 4-[2-methyl-4-[(4-pentylphenyl)ethynyl]
phenyl]ethynyl aniline (25)Compound 25 was prepared using a procedure similar
to that described for the synthesis of 19. Product 25
was purified by recrystallisation from hexane to give
deep brown crystals.

2.1.10 1-[(4-isothiocyanatophenyl)ethynyl]-2methyl-4-[(4-pentylphenyl)ethynyl]
benzene (27)
To the mixture of compound 4-[2-methyl-4-[(4pentylphenyl)ethynyl]phenyl]ethynyl aniline (25) (6 g;
16 mmol), CaCO3 (4 g; 40 mmol) in CHCl3 (30 ml)
and water (30 ml), the straight thiophosgene (1.9 ml;
24 mmol) was added slowly while stirring and maintaining temperature at 0–5 C. Then the mixture wasstirred for 5 h at room temperature. The organic
layer was separated, dried over MgSO4 and the solvent was evaporated at lower pressure. Crude productwas purified by column chromatography (silica gel,hexane as eluent) and recrystallised from acetone togive white crystals. Yield, 2.8 g (42%); m.p., 100 C;
MS(EI): 419 (M+ ), 362, 330, 302. 1 H NMR (200MHz,
CDCl3 )  7.56–7.51 (m, 2H, Ar–H); 7.49–7.45 (m, 2H,
Ar–H); 7.43–7.41 (m, 2H, Ar–H); 7.35–7.30 (m, 1H,
Ar–H); 7.19–7.14 (m, 4H, Ar–H); 2.65–2.56 (t, 2H,
Ar–CH2 -); 2.50 (s, 3H, Ar–CH3 ); 1.69–1.58 (m, 2H,–CH2 –); 1.35–1.28 (m, 4H, –CH2 CH2 –); 0.93–0.86 (t,3H, –CH3 ).

3. Results and discussion
The mesomorphic properties of prepared compounds
are listed in Table 1. Final liquid crystals are whitesolids, show a simple mesophase sequence, and possess only nematic phases in broad temperature ranges.
For the comparison of properties also other neverreported so far fluorinated analogues (15–18, 33–35),
synthesised in our lab too, are matched in Table 1.
Compounds of series 1 (see Table 1) exhibit low
melting temperatures, which are decreasing with thelengthening of alkyl terminal chain. It seems that substitution of quaterphenyl core with methyl group andchlorine/fluorine atoms simultaneously lower melting temperatures successfully. Additionally increasing number of fluorine atoms in quaterphenyl coredecreases the clearing point of such compounds. Such
substitution makes these mesogens easily supercooled
to room temperatures. Introducing carbon–carbon
triple bond inside quaterphenyl core (see compound
16, Table 1) extends extra molecular conjugation leading to increased birefringence (1n) without deterioration of liquid crystal properties. Until now only few
mesogens with methyl lateral substitution based onphenylethynyltolane core were reported earlier [27].
Our results confirm that data. The interesting situation is that phase transition enthalpies are much lower
than those of fluorine substituted homologues (seecompounds 33–35, Table 1). Alkyl lateral substitution
widens molecular separation considerably comparingLiquid Crystals

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Table 1. Phase transition temperatures and enthalpies (kJ/mol) of compounds measured by DSC method(onset point) in a heating cycle at 2 C/min.

No.

n

X1

X2

Phase transition temperatures [ C](enthalpy [kJ/mol])

11
12
13
14

3
5
5
5

H
H
F
F

H
H
H
F

Cr 148.6 (42.6) N 206.1 Iso
Cr 106.6 (28.0) N 161.9 Iso
Cr 86.3 (24.9) N 142.0 Iso
Cr 95.2 (26.5) N 117.0 Iso

15

Cr 77.3 (28.7) N 265.0 Iso

16

Cr 83.5 (24.9) N 211.9 Iso

17

Cr 114.7 (32.8) N >300 Iso18

Cr 95.9 (33.6) N 280.0 Iso

27
28
29
30
31
32
33
34
35

n

R1

R2

X1

X2

5
5
5
5
5
5
3
3
4

CH3
CH3
CH3
C2 H5
C2 H5
C2 H5
F
F
H

H
H
H
H
H
H
F
F
H

H
F
F
H
F
F
H
F
F

H
H
F
H
H
F
H
H
F

to other substituents leading to improvement of melting properties.
To measure the 1n values, a set of 5–20% wt.
nematic guest–host systems were prepared. Mixtures
were composed of investigated compounds and hostnematic 1816 (1n = 0.220 measured at 25o C for
sodium line D) prepared in our lab (method a).
As a second host in method b, known single
compound nematic was applied.1 The 1n values
for investigated isothiocyanates can be extrapolated
from guest–host mixtures results by comparison withhost nematic system. For the birefringence measurements of guest–host mixtures, the method based on
exploiting interference in a wedge cells of small apex

Cr 101.1 (24.2) N 234.1 Iso
Cr 107.7(27.1) N 220.4 Iso
Cr 99.5 (23.7) N 195.4 Iso
Cr 76.6 (24.6) N 157.3 Iso
Cr 73.0 (29.3) N 144.8 Iso
Cr 115.9 (38.9) N 126.1 Iso
Cr 147.3 (41.8) N >300.0 Iso
Cr 156.3 (41.9) N 253.2 Iso
Cr 165.0 (40.9) N 243.0 Iso

angle was used [32,33], which is a modification of
method described in [34]. Birefringence values, determined with this method are marked as method a, possess the systematic error of about 2% which appears
due to low accuracy of determining the wedge geometry. The errors involved in the temperature determination are of about 0.3 K. Method b applied here
uses the phase modulation technique to measure thebirefringence of the sample [35]. The setup consists of
two crossed Glan–Thompson (GT) polarisers, a photoelastic modulator (PEM), a He–Ne laser source anda detector. The plane polarised light after first GT
polariser passes through PEM and sample where bothare kept at 45 with respect to the first polariser. The

1180

J. Herman et al.

light passes through the second GT polariser and thento a detector. The detector output is connected to alock-in amplifier for the detection of the first and thesecond harmonics of the ac signal.
The birefringence of the sample is given by

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1n =


I1 f
J2 (A0 )arctan2 d
I2 f
J1 (A0 )where  is the wavelength of laser light, d is the cell
thickness, I 1f and I 2f are the first and second harmonic
signals measured by the lock-in amplifier, Ao is theretardation amplitude of the PEM and J 1 and J 2
are the first and second order Bessel functions. Thismethod is very sensitive and capable of measuring
retardation much less than a nanometer. The measurements were made as a function of temperature using
an Instec temperature controller with an accuracy of
20 mK. The whole experiment was controlled by acomputer.
Results of birefringence measurements for chosen
structures are given in Table 2. The lowest values ofbirefringence are found for compounds with quaterphenyl core (compounds 13 and 15). Four phenyl
rings connected together and combined with theisothiocyante group generate 1n values in the range
0.44–0.45. Introduction of one triple carbon–carbon
bond inside this core (compounds 16 and 17) raised theTable 2. Birefringence values of chosen structures extrapolated from guest–host systems (measured at 25 C) using
wedge cells @589 nma and phase modulation technique@633 nmb .
1na

1nb

13

0.44

0.43

15

0.45

16

0.53

17

0.55

28

0.63

30

0.63

31

0.62

No.

Structure

0.52

birefringence above 0.5 level. Whereas phenylethynyltolane (bistolane) unit in connection with high polarisable polar NCS group show the highest values of 1n(compounds 28, 30 and 31). Results show strong conjugation effect of p-electron system especially insidebistolane unit, for which experimental results exceed
the most the theoretical assumptions. This kind of
structural sequence appears as a great advantage overquaterphenyl unit. The effect of carbon–carbon triple
bond on birefringence increase can be noticed comparing values of quaterphenyl structures 13 and 15 withresults obtained for 16 and 17. Such bond helps toelongate p-electron system and increases birefringencevalues of about 0.09.

4. Conclusions
Two homologous serie of compounds exhibiting
nematic phase in broad temperature range, fromquaterphenyl and phenylethynyltolane families havebeen synthesised and characterised. Synthesised compounds are attractive to formulation of new highbirefringent mixtures with broad temperature rangeof nematic phase. Careful selection of lateral substituent, their position and sequence among rigid
cores of molecules have the biggest influence on the
optimisation of mesogenic properties of liquid crystalline molecules. In our case, proper connection ofthese substituents in lateral positions allowed to reachreasonable levels both in melting temperatures andenthalpies, what is very important in mixture formulation. Further experiments confirmed valuable optical properties of investigated structures. New materials are characterised with the birefringence values from 0.44 for quaterphenyl core up to 0.63 forphenylethynyltolane core. Further work is now underway to further investigate the viscoelastic properties ofprepared compounds.

Acknowledgement
This work was supported by the Military University ofTechnology Grant 08-714.

Note
1.
0.62

1,4-bis[(4-butylphenyl)ethynyl]-2-ethylbenzene, TCr–N =29.5 C, TN-I = 121.0 C, 1n = 0.37 (633 nm, 25 C).

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