Dispersion of magnetic nanoparticles in a polymorphic liquid crystal

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Dispersion of magnetic nanoparticles in a polymorphicliquid crystalMaksym F. Prodanov a , Maksym A. Kolosov a , Alexander I. Krivoshey a , Alexander P.Fedoryako a , Sergey N. Yarmolenko b , Vladimir P. Semynozhenko a , John W. Goodby c &Valery V. Vashchenko aa State Scientific Institution Institute for Single Crystals, NAS of Ukraine, Kharkov, Ukraine,Russiab North Carolina State University, Greensboro, NC, USAc Department of Chemistry, University of York, UKVersion of record first published: 14 Sep 2012.

To cite this article: Maksym F. Prodanov , Maksym A. Kolosov , Alexander I. Krivoshey , Alexander P. Fedoryako , SergeyN. Yarmolenko , Vladimir P. Semynozhenko , John W. Goodby & Valery V. Vashchenko (2012): Dispersion of magneticnanoparticles in a polymorphic liquid crystal, Liquid Crystals, 39:12, 1512-1526

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Liquid Crystals,Vol. 39, No. 12, December 2012, 1512–1526

Dispersion of magnetic nanoparticles in a polymorphic liquid crystal

Maksym F. Prodanova, Maksym A. Kolosova, Alexander I. Krivosheya, Alexander P. Fedoryakoa, Sergey N.Yarmolenkob, Vladimir P. Semynozhenkoa, John W. Goodbyc and Valery V. Vashchenkoa*aState Scientific Institution Institute for Single Crystals, NAS of Ukraine, Kharkov, Ukraine, Russia; bNorth Carolina StateUniversity, Greensboro, NC, USA; cDepartment of Chemistry, University of York, UK

(Received 12 May 2012; final version received 28 August 2012)

In order to enhance the phase stability of dispersions of magnetic nanoparticles (NPs) in a polymorphic liquidcrystal, new ligands have been designed consisting of a terphenyl-based liquid crystalline core. The most stabledispersions were obtained with 7 nm super-paramagnetic Fe3O4 NPs decorated with the new ligands in place of10 nm ferromagnetic CoFe2O4 spherical NPs.

Keywords: liquid crystals; magnetic nanoparticles; ligands; dispersion

1. Introduction

The applications of thermotropic liquid crystals(LCs) in magnetically controlled devices are oftenhindered by the very low magnetic anisotropy (�χ )of LCs, a consequence of the diamagnetic propertiesof the organic materials that normally constituteLC systems. �χ for LCs is normally of the order of10–4 cm3 mol–1 [1, 2], and the threshold value of themagnetic field is therefore required to be greater than∼0.3 T to reorientate the molecules in the LC system[3]. One of the most promising ways of increasing themagnetic susceptibility of LC materials is to disperseelongated magnetic nanoparticles (NP) in the LCmedium. This was first proposed theoretically byBrochard and de Genne [4], and later proven exper-imentally by Chen and Amer [5]. It was shown thatthe magnetic properties of a dispersion of acicularγ-Fe2O3 in NPs of about 500 nm length, coatedwith dimethyloctadecylaminopropyl-trimethoxysilylchloride in N-(4-methoxybenzylidene)-4-butylaniline(MBBA), were much greater than in MBBA on it own,and that such colloidal systems could be reoriented inan external magnetic field as low as 20 G (0.002 T),which is comparable with the Earth’s magnetic field[5]. Such high sensitivity can be employed in thevisualisation of weak magnetic fields. However, thepotential for LC-based colloids of ferromagnetic NPsin magneto-optical and magneto-electro-optical appli-cations is of considerable importance. Theoreticallythe driving magnetic field in such colloids is estimatedto be about ten times weaker than for the pure LC [6].

It was shown experimentally that filling a nematicLC host with magnetic NPs allowed the pretilt angleof the resulting material to be varied in a weak mag-netic field. No threshold voltage was observed for

*Corresponding author. Email: [email protected]

such pretilt switching [7]. Doping a nematic LC withmagnetic NPs reduced the critical magnetic field inthe magnetic Fréedericksz transition by as much as1.5 times [8, 9]. However, in spite of further investi-gations into the magnetically-controlled properties ofsuch NP/LC composite materials, known as ‘ferrone-matics’ or ‘ferrosmectics’ (see, for example, [5, 7–16]),most studies found insufficient stability of the colloidsas a function of time, aggregation of the NPs being themajor problem. Although in selected cases such aggre-gated samples might find a restricted application, forinstance in the manipulation of alignment by meansof a relatively weak magnetic field (about 0.12 T) in aLC-cell filled with NPs deposited on the glass surface[16], their aggregation/sedimentation greatly hampersthe wide application of NP colloids in LCs.

Common ligands possessing only elongated alkylor alkene chain(s) containing a polar anchoringgroup have in general been used for these systems.Furthermore, ligands possessing molecular structuresspecifically designed for compatibility with colloidalsystems, e.g. sodium dodecylbenzensulfonate (SDBS)[15] or Beycostat NB09 [17] (Scheme 1), are incapableof preventing aggregation of the magnetic NPs, even indilute dispersions.

Similarly, dispersions of semiconductor CdSenanorods grafted with triethylphosphine oxide, havealso been shown to be stable in nematic mixturesof cyanobiphenyls only over a matter of hours [18].The highest concentration for the dispersion of Au–NPs, stabilised by hexanethiol ligand in 4-cyano-4′-pentylbiphenyl (5CB), at which clustering was notobserved did not exceed 0.1 wt% [19]. Phase separa-tion was also found on dilution with common LCssuch as the so-called ‘organo–inorganic hybrid’ LCs,

ISSN 0267-8292 print/ISSN 1366-5855 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/02678292.2012.725867http://www.tandfonline.com

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Liquid Crystals 1513

S

O

O

ONaC12H25P(OH)x

O

H2n+1Cn

2-x

SDBS Beycostat NB09

O C2H4O9

Scheme 1. Non-mesogenic aryl-containing ligands applied for stabilisation of NP colloids in LC [15, 17]. Beycostat NB09 is acommercial product provided by CECA, Courbevoie, France. The structure formulae are as described by Da Cruz et al. [17].

composed of various nanoparticles (α-Fe2O3 [20],TiO2 [21]), or Au [22] coated with LC-like ligands.It has recently been reported that stable dispersionsof Au nanospheres grafted with a mixture compris-ing both common and LC-like ligands [22–27] can beachieved, but only for small NPs a few nm in diame-ter. For the dispersion of Au NPs of larger diameter(169 nm) in 5CB, reversible coalescence processes havebeen developed and studied [28, 29].

Other approaches to optically homogeneous dis-persions of NPs in nematics consist either of theaddition of an isotropic cosolvent [10], or by limita-tion of the movement of NPs in a dispersion, eitherby gelation of the LC [30] or by using side-chain LCpolymers [31].

In summary, NPs covered with ‘common’ ligandscontaining simple alkyl or alkene chain(s) do not formLC dispersions of long-term stability. Surface modifi-cation with benzenoid compounds seems to be suitableonly for very small NPs of comparable size to theLC molecules. Insufficient stability of the dispersionshas frequently been reported, almost irrespective ofthe type of the NPs deployed [5, 15–18, 22, 32–42].It is therefore not accidental that the most stablecolloids in LCs have been achieved using ligands com-prised of molecules with structures expected to sup-port mesophase formation [22, 42–46] with NPs a fewnm in size.

The influence of the type of LC host on stabil-ity of a suspension of NPs has been less studied, butit appears that suspensions in smectics are more sta-ble than in nematics [47]. However, examples of NPsdispersed in smectic phases are relatively rare [14, 15,48–50], compared with the large number of reports onferro-nematics. It is worth noting that in the case ofmagnetic NPs the dipole–dipole interaction betweenthem is an additional factor causing the instability ofa colloid. The most stable colloids in LCs are thereforesuitable for relatively weakly interactive NPs, such asthose of the noble metals [23–27]. Moreover, for mostligands used with magnetic NPs, typical anchoringgroups (carboxylic, alcoholic) do not provide reliablebinding of the ligand to the NP surface [51–54], andthis might be an additional cause for the tendency ofNPs to aggregate. We have shown recently that one ofthe ligands described here can be successfully used tostabilise a NP dispersion in the nematic host 5CB [9] in

which pronounced and reproducible enhancement ofthe sensitivity of the colloids towards a magnetic fieldhas been demonstrated.

In the present article we report (i) the synthesisof linear ligands where the molecules contain LC-likecores with effective interaction with the surround-ing LC host, and elongated alkyl chains intended toconnect the LC-like core flexibly to a suitable anchor-ing group; and (ii) the application of such ligandsin the stabilisation of dispersions of model NPs ofrelatively small size (Fe3O4 and CoFe2O4, mean diam-eter 7 and 11 nm, respectively) in polymorphic LChosts. Structures of the ligands 1a–c, and 2 (Scheme 2)were chosen since they are similar to those previouslyreported for grafting on relatively large sub-micronFe- and Ti-oxide particles [20, 21]. Furthermore, itwas expected that significant stabilisation of LC dis-persions would result for quite small (7–11 nm) NPsby comparison with sub-micron species [20, 21], whichare more than an order larger in size than the NPs usedin the current study.

Bearing in mind the smectic type of LC host cho-sen as a carrier for colloids, the 4-cyanobiphenyl unitwas not as suitable a candidate for stabilising a LC-like core as it had been in the case of stable Aucolloids in 5CB [24, 25]. Fluorinated terphenyl moi-eties were therefore preferred as basic building blocksfor the ligands (1a–c), since these units are typicallydeployed in LCs in host ferroelectric mixtures [55]. Theincorporation of fluoro-substituents in the terphenylcore is a technique used to provide much lower relativemelting points [1, 55], thereby allowing us to suppose

R

X1 X2

O (CH2)10 P(OH)2

O

X3 X4

O (CH2)11 P(OH)2

O

O

OC6H13

1a, R = C5H11, X1 = X2 = F, X3 = X4 = H

1b, R = C5H11, X1 = H, X2 = F, X3 = X4 = H

1c, R = C7H15, X1 = X2 = H, X3 = X4 = F

2

Scheme 2. Chemical structures of the ligands 1a–c, 2designed for the dispersion of magnetic NPs in the LC host.

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1514 M.F. Prodanov et al.

that a rather diffuse layer of ligand molecules formsaround the NPs as a result of the rather weak inter-molecular interactions between the ligand molecules.As an alternative we also prepared ligand 2, whichpossesses an ester moiety whose molecular structureis related to systems used in smectic LC hosts [1].The phosphonic acid group was chosen as a terminalanchoring group due to its strong affinity for the sur-face of ferric and/or cobalt oxides [51, 56], and beingthereby expected to enhance the capability of the lig-and molecules to provide reliable binding to the NPsurface.

2. Materials

The binary phenylpyrimidine eutectic mixtures [57]used as the LC host in the study possess the phasetransition temperatures shown in Scheme 3:

2.1 Syntheses of ligandsSynthetic routes to the target ligands 1a–c and 2 areshown in Schemes 4 and 5.

Compounds 1a–c (Scheme 4) were synthesised bythe following sequence of reactions: O-alkylation ofthe phenols 3a–c and the Arbuzov reaction, followedby transesterification/hydrolysis of the correspond-ing phosphonic esters. The starting phenol 3a wassynthesised by a Suzuki cross-coupling reactionin a microemulsion, similarly to that described byVashchenko et al. [58]. Phenols 3b and 3c weresynthesised using procedures in the literature (seeExperimental). Alkylation of phenols 3a–c, usinga seven-fold excess of 1,10-dibromodecane andrefluxing with K2CO3 in MEK, provided bromides4a–c. The anchoring phosphonic acid group wasintroduced (transformation of 4a–c to 1a–c) usingthe Arbuzov reaction [59, 60] followed by mildtransesterification/hydrolysis of the intermediatediethylphosphonate with Trimethylsilyl bromide(TMSBr)/THF/MeOH (aq.) [61–63]. This methodhas the advantage over that typically employed usingdialkylphosphonate hydrolysis in HBr (aq.) and/orHBr/AcOH, which uses THF in place of more polarsolvents in which the terphenyls 4a–c have poorsolubility.

Alk OH

X1 X2 X3 X4

Alk O

X1 X2X3 X4

(CH2)10

(CH2)10

Br

Alk O

X1 X2 X3 X4

P(OH)2

O

Br (CH2)10 Br i

ii, iii

3a-c

4a-c

1a-c

Scheme 4. Synthetic scheme for compounds 1a–c: (i)K2CO3, butanone, reflux; (ii) P(OEt)3, reflux; (iii) TMSBr,CHCl3, then THF, H2O, reflux.

EtO

O

OH

EtO

i

ii, iii

iv

v, vi

O

HO

O

O (CH2)11 Br

C6H13 OH

O

O

O (CH2)11 Br

C6H13

O

O

O (CH2)11 P(OH)2

C6H13

O

5

6

8

9

2

O (CH2)11 OH

7

Scheme 5. Synthesis of 2: (i) 11-bromo-undecanol, K2CO3,2-butanone, reflux; (ii) PBr3, PPh3, THF; (iii) NaOH (aq.),EtOH, ambient temperature; (iv) DCC, DMAP, DCM; (v)P(OEt)3, reflux; (vi) A. TMSBr, CHCl3, B. THF, H2O, reflux.

C8H17O

N

N

C8H17 C10H21O

N

N

C8H17

69 64 57 ~20Iso N SmA SmC Cr

Scheme 3. Chemical structures and phase transitions for the LC hosts used as a dispersion medium for magnetic NPs.

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Compound 2 (Scheme 5) was synthesised fromthe ester 5 by alkylation with 11-bromo-undecanol byrefluxing in MEK with K2CO3 to give 6, followed bynucleophilic substitution of the hydroxyl function withbromine according to the Appel reaction (CBr4/PPh3)[64, 65] and mild saponification to give carboxylic acid7. Esterification of 7 with the corresponding phenol8 was achieved using the DCC/DMAP methodologydescribed by Neises and Steglich [66] and Hassner andAlexanian [67], followed by introduction of the phos-phonic acid group by the method mentioned above forcompounds 1a–c (Scheme 4). In the case of compound2, an additional advantage of this method is that itappears to tolerate the presence of an ester function.

All the new materials gave satisfactory high-resolution mass spectra HRMS and 1H, 13C and 31PNMR spectral data. Due to the poor solubility ofthe phosphonic acids 1a–c and 2 in common sol-vents at ambient temperature, their NMR spectra wererecorded at 100◦C in DMSO-d6, and for HPLC anal-ysis the probe was dissolved in dioxane, with theaddition of a few percent of pyridine.

The new compounds 1a–c and 2 exhibited meso-morphic properties, as recently described and dis-cussed [68].

2.2 Synthesis and characterisation of nanoparticlesMagnetic NPs were obtained by pyrolysis of ironand/or cobalt acetylacetonates in diphenyl ether withconventional heating, according to known protocols[69]. In all cases oleic acid was used as ligand.In this manner spherical nanoparticles of Fe3O4 andCoFe2O4 (mean diameter 6.6 and 10.6 nm, respec-tively) covered with a hydrophobic shell were obtained(Figure 1).

Synthesis of cobalt ferrite NPs was accomplishedusing the seeding technique. Heating according tothe Sun protocol [69] first gave the NP sample(∼5.6 nm diameter). After cooling, the next por-tion of starting materials was added to the reactionmixture and the cycle of heating–cooling–addition ofacetylacetonate was repeated three times. The sizeof CoFe2O4 NPs is enlarged by between 2.5 and1.5 nm during each growth cycle, accompanied by aslight broadening of size distribution, and the NP2finally obtained had 10.6 nm mean diameter (Table 1).Particle size analysis was achieved using transmis-sion electron microscopy (TEM) at 200 kV. Elementalanalysis was performed by energy-dispersive X-rayspectroscopy (EDS) and the results are summarised inTable 2.

Table 1. Particle size analysis of NPs samples NP1 and NP2by TEM.

SampleChemicalformula No of points Size, nm

Standarddeviation

NP1 Fe3O4 770 6.6 1.6NP2 CoFe2O4 773 10.6 1.7

Table 2. EDS analysis data for Co2Fe2O4 nanoparticlesbefore and after TGA.

Before TGA After TGA

Atom % Fe Co P Fe Co P

NP2 78.3 21.7 0 79.6 20.4 0NP2–1c 74.0 20.3 5.7 74.9 20.1 5.0

20 nm 50 nm

(a) (b)

Figure 1. TEM images of the nanoparticles synthesised: (a) Fe3O4 (NP1), and (b) CoFe2O4 (NP2).

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2.3 Site exchange on nanoparticle surfaceExchange of oleic acid by the new ligands 1a–c and 2in the shell of the NPs synthesised was accomplishedby refluxing the NP with a solution of the appropriatedesigner ligand (1a–c, 2) in toluene containing a smallamount of pyridine for 10–12 h. The resulting NPswere washed thoroughly with hot toluene to removeexcess of unbound ligand, followed by decantationwhile holding the NP by a permanent magnet, dispers-ing in chloroform and filtering through a 0.45 μm fil-ter, evaporating the filtrate and drying the residue. Thecompletion of the resurfacing of the NP was confirmedqualitatively by FT–IR. Typical spectra evolution onsite exchange is shown in Figure 2 using Fe3O4 (NP1)and ligand 1a as an example.

It is clearly seen that the FT–IR spectrum ofthe free ligand 1a (Figure 2, curve c) is similar toits bound form on the NP surface (Figure 2, curveb), with almost complete disappearance of the oleicacid absorbance bands (1400–1600 cm–1). Differencesbetween the spectra within the range 1000–1100 cm–1,assigned to νP = O (Figure 2, curves b and c), may beinterpreted as the result of the conversion of the freephosphonic acid groups to bound anionic forms. Forthe other materials FT–IR spectral evolution on site-exchange of oleic acid with the new ligands was similarto those shown in Figure 2, namely a simple spectralpattern of oleic acid bound to the surface of the NPs,

transforming into a spectrum similar to that of thepure ligand (see Supplementary Information).

Since the FT–IR spectra are appropriate only forqualitative monitoring of the resurfacing procedureand are inapplicable to quantitative determination ofthe oleic acid–new ligand proportion, in order toadopt the site-exchange ratio we were guided by thefollowing considerations. It is known that carboxylicacids are quite weakly bonded to the iron/cobalt oxidesurface [51–54, 56], and they can be easily removedby washing twice with a hexane–acetone mixture [56],whereas phosphonic acids [51, 70] are much morestrongly retained and no migration from the NP sur-face into the washing solution is seen [71]. It is there-fore reasonable to assume that under specified condi-tions phosphonic acid-containing ligands are able toattach to the iron/cobalt oxide surface almost irre-versibly, and thus if they are used in the site-exchangeprocedure the oleic acid will be completely displaced.

Thermogravimetric analysis (TGA) revealed thatthe weight loss for the NP2 obtained (mainly graftedwith oleic acid) was 14%, which is in good agreementwith the reported data [72]. It is about three timeslower than that for surface-modified NP2 covered with1c ligand (53%, Figure 3).

The mass loss for the NP2–oleic acid materialsclearly corresponded to full removal of the organicconstituent, whereas in the case of NP2–1c it is

Figure 2. FT–IR spectra: (a) for Fe3O4 nanoparticles (NP1) covered with oleic acid, (b) for NP1 covered with ligand 1a, and(c) for pure compound 1a.

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0 100 200 300 400 500 60040

50

60

70

80

90

100

Perc

enta

ge m

ass (%

)

Temperature (°C)

1

2

Figure 3. TGA plots for NP2 as synthesised (curve 1) andfor NP2–1c (curve 2).

reasonable to assume that the more strongly boundphosphonic acid fragment remained on the surfaceof the inorganic core. This is due to preferable cleav-age of the P–C rather than P–O–Fe bond, as hasbeen assumed by Dai et al. [73], and is schematicallyillustrated in Figure 4.

This assumption was confirmed by EDS elemen-tal analysis of the nanoparticles before and after TGA(Table 2).

From the data in Table 2 it is also clear that theamount of phosphorus remained almost unchanged,within the limits of accuracy of TGA.

Consequently, the total contribution of organicmatter infused by the new ligands to the mass of hybridmaterial (such as NP2–1c) is higher by approximately13–16% than is indicated by TGA. The inorganic corecontent is therefore around 37–39%, rather than the47% expected from Figure 3, curve 2.

The surface density of the organic ligands was eval-uated in two ways: firstly from TGA data (similarly toYee et al. [74]), and secondly on the basis of the pro-portion of P atoms to total metal atoms (see Table 2);the calculation details are given in the SupplementaryInformation.

On the basis of the TGA data, and using thecorrected weight loss (see above), we arrived at1960–2030 molecules of 1c for each NP2, or about

5.6–5.7 molecules per 1 nm2. This is very close tothe maximum number of ligands (2000) that can bearranged on each NP surface.

An alternative evaluation of the surface ligand den-sity was based on an assumption that the majorityof P atoms after annealing (in the course of TGA)originated only from ligands which had been directlybound to the NP surface (see Figure 4). Thus, usingz = 8 and a cell volume of 0.592 nm3 for 4.8% of Pcontent, we obtained 1330 P atoms per one NP2, andapproximately 3.8 molecules of ligand per nm2. Suchan evaluation for the sample prior to TGA resulted in1527 P atoms per one NP2 and 4.3 molecules of ligandper nm2.

Our estimates of ligand surface density based onTGA and EDS data thus gave similar results and con-firmed that the organic ligands described here forma single-layer close-packed organic shell on the NPsurface.

Magnetic measurements were performed at varioustemperatures using SQUID and a vibration magne-tometer (Table 3). As expected, ferromagnetic proper-ties at room temperature were found only for cobaltferrite particles (NP2), whereas the Fe3O4 sample(NP1) was super-paramagnetic. Using NP2 and lig-and 1c as an example, we also studied the influenceof surface modification on the magnetic properties ofthe hybrid material (magnetisation curves as a func-tion of the applied field are given in SupplementaryInformation). Coercitivity and magnetisation (calcu-lated after subtraction of the mass of the organic shell)were measured both at ambient temperature and at100◦C above the SmC host mesomorphic range, andare given in Table 3.

It can be seen that the ferrite cobalt nanoparti-cles grafted with a native surfactant (oleic acid) andphosphonate ligands showed almost equal values ofmagnetisation.

3. Dispersion of the NPs in a LC host

Mixtures of NPs covered with the new LCs, 1a–c,2, were prepared by addition of an aliquot of the

NP Fe

Fe

O

O

O

O

POO

O

Heating NP Fe

Fe

O

O

O

O

POO

O O or (CH3)

Decomposedorganic materials

+

Figure 4. Scheme of desorption of phosphonate ligands from the magnetic NP surface in coarse TGA.

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Table 3. Magnetic measurements data for NPs.

Coercitivitya, Oe Mrb, emu g–1 Msb, emu g–1

at 26◦C at 100◦C at 26◦C at 100◦C at 26◦C at 100◦C

NP1 0 − − − − −NP2 478 353 25.3 16.2 78.5 72.6NP2–1c 408 387 23 14,7 66.8 71,7

aMeasured on vibration magnetometer, except NP1, studied onSQUID instrument.bMr – remanent magnetisation, Ms – saturation magnetisation.

dispersion of the NPs at a specific concentrationin chloroform to an appropriate amount of the LChost, followed by evaporation of the solvent and dry-ing of the residue in vacuo (0.2–0.5 Torr) at 120◦Cfor several hours. The concentration of NPs in theLC varied from 0.2 to 5 wt%. Fresh samples ofthe diluted dispersions of super-paramagnetic NP1appeared slightly coloured but were optically homo-geneous, both in the isotropic and LC state. The

dispersion of 5 wt% NP1 appeared to be essentiallymore viscous. In contrast, dispersions of ferromagneticNP2 in the LC were optically homogeneous only above120◦C. At lower temperatures, even in the isotropicphase (above 70◦C), dark deposits were observed inthis dispersion, presumably consisting of NPs. In orderto confirm that NP1 and NP2 remained in the LCphase we smeared the dispersions directly on a TEMgrid without solvent dilution and prepared images(Figure 5).

It can be clearly seen that the NP1–1b sample con-tained NPs distributed almost uniformly in the LChost (see Figure 5(a)), whereas CoFe2O4 NPs were col-lected in agglomerates (Figure 5(b) and 5(c)). In thecase of NP2 the total number of visible particles alsoseemed to be lower than for NP1. Therefore, in con-trast to the dispersion of NP1, the mixture of NP2 withthe LC host could be described as a coarse suspensionrather than a true colloid.

By polarising optical microscopy (POM), the tem-peratures of the phase transitions for dilute dispersions

(a)

(c)

(b)

Figure 5. TEM images of dispersion of NP in LC host: (a) NP1–1b; (b) NP2–1c; (c) NP2–2.

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Table 4. Phase transition temperatures for dispersions ofNPs covered with the new ligands, 1a–c and 2, in the SmCLC host.

Mesophases types and phase transitiontemperatures, ◦C

SampleConc.wt% Iso N SmA SmC

Pure LCa – • 68.7 • 63.8 • 57.1 •NP1–1a 0.30 • 68.2 • 62.7 • 56.8 •NP1–1b 0.21 • 68.5 • 63.6 • 57.2 •NP1–1c 0.22 • 68.3 • 63.5 • 57.0 •NP1–2 0.19 • 68.8 • 63.5 • 56.9 •

4.49 • 67.5 • 62.9 • 53.0 •NP2–1c 0.25 • 68.9 • 63.8 • 57.0 •NP2–2 0.25 • 68.8 • 63.9 • 57.2 •aIn blank experiment pure LC host was also treated as the sampleswith NPs and showed almost the same phase transition tempera-tures as before treatment.

of the NPs (0.1–0.3 wt%) remained almost unchangedfrom those of the pure LC host (Table 4).

In the case of the dispersions of NP1, the texturesobserved between untreated glass plates upon cool-ing from the isotropic state were typical of those forhomeotropic boundary conditions, whereas the purehost under similar conditions showed patterns char-acteristic of homogeneous alignment. Thus, for thedispersion of NP1–1b in LC (0.26 wt%) the nematicand SmC phases exhibited Schlieren texture, whereasthe SmA phase between crossed polarisers appearedblack (Figure 6(b) and 6(c)). Small areas of homo-geneously oriented LC were found only towards theedges of the samples. In contrast to the dispersionof NPs in the LC host, the pure host sandwichedbetween untreated glass plates typically showed focal–conic textures for the SmA phase and broken focalconic textures for the SmC phase (Figure 6(d)and 6(e)).

In fresh samples of Fe3O4 NPs in dispersions inthe host, no visible signs of NP aggregation werefound. After standing for a day, however, slightlycoloured grains and aggregations could be seen. Thesewere clearly visible both in the LC state and in theisotropic phase, just above the clearing point of thesample (see, for example, Figure 6(a)). These grainsvanished after ageing at 130–140◦C for 20–30 min, andthe samples returned to their optically homogeneousstate.

It can thus be assumed that, between untreatedglass plates at the internal surfaces of a cell, in theabsence of strong alignment of the LC, the texturesof the dispersions of NP1 are determined essentiallyby the boundary conditions at the surfaces of theNPs and the substrates. These are believed to behomeotropic due to the molecular structure of ligands

1a–c and 2 (Figure 7). It is also reasonable to assumethat, even if the NPs are uniformly distributed in thesolution, some will occur close to the inner surfacesof the cell and will become more or less bound tothe surfaces, thereby acting as homeotropic aligningmaterial.

A similar effect of LC alignment in bulk has beendescribed, for example, for gold nanodots depositedonto a glass surface, depending on the type of thegold shell [75]; the overall optical pattern for the wholecolloidal system also appears to be homeotropic.Observations of vertical instead of homogeneousalignment of NP colloids in a pure LC host have alsobeen recently described [39, 76, 77].

At higher content (5 wt%) of Fe3O4 NPs in the LChost, grains similar to those described above appearedin much greater quantities, most of them exhibitingirregular branched chains which remained visible inthe isotropic liquid throughout the whole mesomor-phic range (Figure 8).

Within the mesophase some of the chains wereseen to nucleate the disclinations around them(Figure 8). In the remaining area of the LC sam-ple homeotropic orientation occurred, similar tothat in dilute dispersions (Figures 6(c) and (6d)).Similar patterns (the formation of chains and stripes)have also been observed previously for LC dis-persions of γ-Fe2O3 NPs stabilised with commer-cial surfactant NB09 [17], and for semiconductor[41] and gold NPs [34, 48, 78].

Studying the dispersion of NP2 grafted with thenew ligands by POM showed textures similar to thepure LC host, rather than to dispersions of NP1(Figure 9). Optical nonhomogenity was found in anNP2 dispersion in LC, even in a freshly preparedsample. In addition, in contrast to the NP1 disper-sion, ageing of the NP2 colloids at temperatures wellabove the isotropic transition did not result in opti-cal clearing. This can be rationalised by taking intoaccount the reduced amount of NP2 actually dispersedin the LC host, and moreover in the material col-lected into agglomerates. In this case, the remainingNP2, in contrast to NP1, was apparently not capableof influencing the alignment of the LC.

4. Conclusions

New ligands (1a–c, and 2), molecules comprisinga three-ring liquid-crystalline unit (either fluoro-terphenyl or biphenylbenzoate), are only partiallyable to resolve the problem of the stability of mag-netic NP dispersions in a LC host, and are best atquite low concentrations of colloids. Obviously, inthe case of NPs of larger size the ability of these

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(a) (b)

(c) (d)

(e)

Figure 6. Optical textures of 0.21% dispersion of NP1–1b in LC host: (a) isotropic state, 69.4◦C; (b) SmA phase, 62.1◦C; (c)SmC phase, 54.2◦C; and for pure LC host: (d) SmA phase, 61.3◦C; (e) SmC phase, 38.5◦C.

ligands to disperse NPs in LC hosts is clearly insuf-ficient, and this will entail further research into suit-able stabilisers, already underway. In spite of theseligands not forming colloids possessing long-term sta-bility (for months or more), their dispersion ability

is nonetheless sufficient for magneto-optical assess-ments in a nematic host, in which pronounced andreproducible enhancement of the sensitivity of thecolloids obtained towards a magnetic field has beendemonstrated.

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Figure 7. Schematic representation of alignment mechanismin the dispersion of NP1–1 in LC.

5. Experimental

5.1 Materials and methodsNMR spectra were recorded on a JEOL ECX–400 NMR spectrometer (400 MHz 1H NMR spectra,100 MHz 13C NMR spectra, 162 MHz 31P NMRspectra) in CDCl3 or DMSO–d6. Mass spectra wererecorded on a Bruker microTOF mass spectrometer

coupled to an Agilent 1200 series LC system, providingthe facility for accurate mass ESI/APCI–MS, LC–MSand DEP measurements. HPLC analyses were per-formed on a Shimadzu Prominence HPLC fitted witha dual wavelength detector using a C18 5um reversed-phase column using acetonitrile–water or acetonitrile–chloroform mixtures as eluent. FT–IR spectra wererecorded on a Shimadzu Spectrum–One instrument.

Polarised optical microscopes (Olympus) with par-allel and crossed polars were used, provided witha Mettler FP900 heating stage or Instec processor/Mettler FP82 hot-stage. TGA was performed usingthe thermo-analytical system, Mettler TA 3000. TEMwas conducted on a SELMI PEM–125K instrument.EDS elemental analysis was carried out on a JEOLJSM–6390LV equipped with an X-ray microanalyticalsystem INCA–450.

Chemicals and solvents were obtained commer-cially from Aldrich and used without additional puri-fication unless otherwise noted. Starting materials in-cluded 2′-fluoro-4-hydroxy-4′′-pentyl-p-terphenyl (3b)[79], 2,3-difluoro-4′′-heptyl-4-hydroxy-p-terphenyl (3c)[80], 4′-pentyl-2,3-difluorobiphenylboronic acid [55],and 4-hexyl-4′-hydroxybiphenyl (8) [81].

(a)

(c) (d)

(b)

Figure 8. Optical textures of the 5 wt% dispersion of NP1–1b in LC host: (a) isotropic state, 73◦C; (b) nematic phase, 65.5◦C;(c) SmA phase, 54◦C; (d) SmC phase, 51◦C.

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(a)

(c) (d)

(b)

Figure 9. Optical textures of the dispersion NP2–2 in the LC host: (a) isotropic state, 71.5◦C; (b) nematic phase, 66.7◦C; (c)SmA phase, 61.3◦C; (d) SmC phase, 54.9◦C.

5.2 Scheme 45.2.1 4-Hydroxy-4′′-pentyl-2′,3′-difluoro-p-terphenyl(3a)

A mixture of a 4-bromophenol (1.55 g, 9.0 mmol), 4′-pentyl-2,3-difluorobiphenylboronic acid (3.00 g,9.9 mmol), sodium dodecylsulfate (300 mg),water (10 ml), toluene (30 ml) and hexanol (3 ml)was degassed and flushed with argon. [1,1′-Bis(diphenylphosphino)ferrocene]dichloropalladium(II)(132 mg, 0.18 mmol) was then added under argonand the degassing repeated three times. The resultantemulsion was heated under reflux with stirring anda previously degassed solution of sodium carbonate(3.82 g, 36 mmol) slowly injected by syringe. The reac-tion mixture was stirred under reflux for about 2 h,then evaporated to dryness (with isopropyl alcoholadded in portions to prevent foaming), and suspendedin CHCl3. The resultant suspension was filteredthrough a short pad of silica, washed with CHCl3and evaporated to dryness. The product obtained waspurified by crystallisation from acetonitrile and driedin vacuo (yield: 1.41 g, 45%).

1H NMR (DMSO–d6): 9.71 (s, 1N), 7.46 (dd, 2H),7.41 (dd, 2H), 7.32 (m, 2H), 7.28 (m, 2H), 6.88(t, 1H), 6.85 (t, 1H), 2.59 (t, 2H), 1.58 (p, 2H), 1.28(m, 4H), 0.84 (t, 3H).

MS (ESI, microTOF): 351.241 (M+ –H), 397.238(M+ HCOO–).

5.2.2 General procedure for alkylation of phenols with1,10-dibromodecane

A mixture of the appropriate phenol (3a–3c), 1,10-dibromodecane (7 eq. per phenol), potassium carbon-ate (3 eq. per phenol) and 2-butanone (10–15 ml per1 mmol of phenol) was stirred under reflux for about12 h. The reaction mixture was then cooled to roomtemperature, filtered and the filtrate evaporated to dry-ness. The product obtained was purified by crystallisa-tion from an appropriate solvent and dried in vacuum.

5.2.3 4-(10-Bromodecyloxy)-4′′-pentyl-2′,3′-difluoro-p-terphenyl (4a)

Quantities: 4-hydroxy-4′′-pentyl-2′,3′-difluoro-p-terphenyl (1.23 g, 3.41 mmol), 1,10-dibromodecane(7.15 g, 23.8 mmol), K2CO3 (1.41 g, 10.23 mmol) and2-butanone (40 mL). The product was recrystallisedfrom heptane (yield: 1.65 g, 85%).

1H NMR (SDCl3): 7.51 (m, 4H), 7.30 (d, 2H), 7.22(d, 2H), 7.01 (d, 2H), 4.01 (t, 2H), 3.42 (t, 2H), 2.66(t, 2H), 1.77–1.90 (m, 4H), 1.67 (m, 2H), 1.53–1.32(m, 15H), 0.92 (t, 3H).MS (ESI, TOF, EI+): 570, 572 (M+).

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5.2.4 4-(10-Bromodecyloxy)-4′′-pentyl-2′-fluoro-p-terphenyl (4b)

Quantities: 4-hydroxy-4′′-pentyl-2′-fluoro-p-terphenyl(2.16 g, 6.45 mmol), 1,10-dibromodecane (13.54 g,45.1 mmol), K2CO3 (2.23 g, 16.3 mmol) and 2-butanone (40 mL). The product was recrystallisedfrom heptane (yield: 2.95 g, 82%).

1H NMR (SDCl3): 7.53 (m, 4H), 7.35–7.47(m, 3H), 7.28 (d, 2H), 4.00 (t, 2H), 3.4 (t, 2H), 2.66(t, 2H), 1.83 (m, 4H), 1.66 (m, 2H), 1.46 (m, 4H),1.35 (m, 11H), 0.92 (t, 3H).MS (ESI, TOF, EI+): 552, 564 (M+)

5.2.5 4-(10-Bromodecyloxy)-4′′-heptyl-2,3-difluoro-p-terphenyl (4c)

Quantities: 4-hydroxy-4′′-heptyl-2,3-difluoro-p-ter-phenyl (2.00 g, 5.3 mmol), 1,10-dibromodecane(11.04 g, 36.8 mmol), K2CO3 (2.20 g, 16.0 mmol)2-butanone (50 mL). The product was recrystallisedfrom heptane (yield: 2.84 g, 89%).

1H NMR (SDCl3): 7.67 (t, 1H), 7.65 (t, 1H), 7.58(m, 1H), 7.56 (m, 2H), 7.54 (m, 1H), 7.27 (d, 2H),7.14 (m, 1H), 6.81 (m, 1H), 4.08 (t, 2H), 3.63 (q,1H), 3.41 (t, 2H), 2.65 (t, 2H), 1.81–1.89 (m, 4H),1.61–1.69 (m, 2H), 1.52–1.41 (m, 5H), 1.27–1.39 (m,15H), 0.89 (t, 3H).MS (ESI, TOF, EI+): 598, 600 (M+).

5.2.6 General procedure for Arbuzov reaction withsubsequent hydrolysis

A mixture of the appropriate bromide (2.55 mmol)and triethylphosphite (2 ml) was stirred under refluxfor 40 h. All volatile compounds were evaporated offin vacuo and the residue recrystallised from hexane.The product was then dissolved in 6 ml of anhy-drous chloroform, cooled to 0◦C and a solution oftrimethylsilylbromide (2.34 g, 15 mmol) in 3 ml chlo-roform added dropwise over 10 min. The mixture wasstirred for 30 min at 0◦C and then allowed to warmto room temperature over 3 h. The reaction mixturewas evaporated to dryness and the residue dissolvedin 25 ml THF, heated under reflux and 4 ml wateradded. After refluxing 14 h the solvent was evapo-rated off under reduced pressure and the residue wasrecrystallised from an appropriate solvent and dried invacuo.

5.2.7 4′′-Pentyl-2′,3′-difluoro-p-terphenyloxydecylphosphonic acid (1a)

Quantities: 4-(10-bromodecyloxy)-4′′-pentyl-2′,3′-di-fluoro-p-terphenyl (1.46 g, 2.55 mmol), triethylphos-

phite (2 ml, 11.49 mmol), trimethylsilylbromide(2.34 g, 15 mmol). The product was recrystallisedfrom dioxane, then from DMF (yield: 1.15 g, 78%).

1H NMR (DMSO–d6, 100◦C): 7.51 (m, 4H), 7.32(m, 4H), 7.03 (d, 2H), 4.01 (t, 2H), 2.62 (t, 2H), 1.72(t, 2H), 1.61 (t, 2H), 1.46 (m, 6H), 1.30 (m, 13H),0.87 (t, 3H).31P NMR (DMSO–d6, 100◦C): 26.467 (s).MS (ESI, microTOF): 617.283 (M+ HCOO–),571.282 (M+–H).

5.2.8 4′′-Pentyl-2′-fluoro-p-terphenyloxydecyl-phosphonic acid (1b)

Quantities: 4-(10-bromodecyloxy)-4′′-pentyl-2′-fluoro-p-terphenyl (2.75 g, 4.97 mmol), triethylphosphite(3 ml, 17.24 mmol), trimethylsilylbromide (3.94 mL,29.82 mmol). The product was recrystallised fromDMF and filtered hot, then from acetic acid (yield:1.03 g, 37%).

1H NMR (DMSO–d6, 110◦C): 7.63 (d, 2H), 7.54(d, m, 5H), 7.30 (d, 2H), 7.03 (d, 2H), 4.05 (t, 2H),2.65 (t, 2H), 1.76 (m, 2H), 1.64 (m, 2H), 1.52 (m,6H), 1.32 (m, 14H), 0.89 (t, 3H).31P NMR (DMSO–d6, 110◦C): 26.876 (s).13S NMR (DMSO–d6, 110◦C): 14.3, 22.3, 23.3,26.0, 27.5, 28.8, 29.1, 29.2, 29.4, 30.4, 30.6, 30.8,31.4, 35.3, 68.3, 115.4, 123.1, 126.9, 129.4, 130.3,131.2.MS (ESI, microTOF): 599.294 (M+ HCOO–),553.285 (M+–H).

5.2.9 4′′-Heptyl-2,3-difluoro-p-terphenyloxydecyl-phosphonic acid (1c)

Quantities: 4-(10-bromodecyloxy)-4′′-heptyl-2,3-di-fluoro-p-terphenyl (2.77 g, 4.62 mmol), triethylphos-phite (3 ml, 17.24 mmol), trimethylsilylbromide(4.00 mL, 29.9 mmol). The product was recrystallisedfrom DMF, then from acetic acid (yield: 0.98 g, 35%).

1H NMR (DMSO–d6, 110◦C): 7.70 (d, 2H), 7.57(m, 4H), 7.25 (m, 3H), 7.05 (m, 1H), 4.12 (t, 2H),2.61 (t, 2H), 1.75 (p, 2H), 1.61 (p, 2H), 1.48 (m, 6H),1.27–1.35 (m, 19H), 0.85 (t, 3H).31P NMR (DMSO–d6, 110◦C): 26.842 (s).13S NMR (DMSO–d6, 110◦C): 14.2, 22.4, 23.3,25.8, 27.5, 28.8, 28.9, 29.1, 29.3, 29.4, 30.4, 30.6,31.1, 31.7, 35.3, 70.5, 111.65, 124.5, 127.0, 127.2,129.3, 129.4, 137.5, 140.2, 142.5.MS (ESI, microTOF): 645.761 (M+HCOO–),599.731 (M+–H).

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5.3 Scheme 55.3.1 4-(11-Bromo-undecyloxy)benzoic acid (7)

A mixture of 7.07 g, 42.6 mmol of 4-hydroxybenzoicacid ethyl ester (5), 11-bromo-undecanol (10.69 g,42.6 mmol) and K2CO3 (14.72 g, 106.5 mmol) wasstirred under reflux 16 h. The reaction mixture wasthen filtered and the filtrate evaporated to dryness, giv-ing the product as a pale yellow oil, which crystallisedon standing. The product was dissolved in dry THF(200 ml) and CBr4 (17.57 g, 53 mmol) added. Theproduct was cooled to 0◦C and triphenylphosphine(17.05 g, 65 mmol) added. The mixture was stirred15 min at 0◦C and then allowed to warm to roomtemperature over 4 h. The reaction mixture was thenfiltered through a short silica pad, washed with CHCl3and evaporated to dryness. The resultant precipitatewas dissolved in DCM, giving a yellow solution, andfiltered again through a short of silica pad, result-ing in an almost colourless solution which was thenevaporated to dryness. The precipitate was dissolvedin EtOH (150 ml) and a solution of NaOH (6.72 g,168 mmol) in 50 ml water added. The mixture wasstirred at 25–30◦C for 12 h and concentrated HCl(30 ml) in 100 ml of water added. The EtOH was thenevaporated under reduced pressure and the precipitatewas filtered off and washed repeatedly with water. Theproduct was recrystallised from acetonitrile (200 ml)and dried in vacuo (yield: 10.77 g, 68%).

1H NMR (DMSO–d6): 7.88 (t, 2H), 6.99 (t, 2H),4.26 (q, 2H), 4.03 (t, 2H), 3.5 (t, 2H), 1.79 (m, 2H),1.72 (m, 2H), 1.18–1.45 (m, 14H).MS (ESI, microTOF): 369.108, 371.110 (M+).

5.3.2 4′-Hexylbiphenyl-4-yl 4-(11-bromo-undecyloxy)benzoate (9)

A mixture of 1.25 g, 4.91 mmol 4-hydroxy-4′-hexylbiphenyl (5), acid 7 (1.92 g, 5.16 mmol) and4-dimethylaminopyridine (10 mg) in dry DCM (40 ml)was cooled with stirring to 0◦C under argon. DCC(1.22 g, 5.89 mmol) was then added. The reaction mix-ture was stirred at room temperature for 19 h andfiltered through a short pad of silica. The filter waswashed several times with DCM, the filtrates collectedand evaporated to dryness. The product was recrys-tallised from acetonitrile (100 ml) and dried in vacuo(yield: 1.39 g, 47%).

1H NMR (CDCl3): 8.16 (d, 2H), 7.62 (d, 2H), 7.5(d, 2H), 7.26 (m, 5H), 6.98 (d, 2H), 4.05 (t, 2H),3.42 (t, 2H), 2.65 (t, 2H), 1.84 (m, 4H), 1.65 (m, 2H),1.31–1.52 (m, 20H), 0.90 (t, 3H).MS (ESI, TOF, EI+): 606, 608 (M+)

5.3.3 11-(4-(4′-Hexylbiphenyl-4-yl)oxycarbonyl)phenoxy-undecylphosphonic acid (2)

This compound was obtained according to gen-eral procedure for the Arbuzov reaction withsubsequent hydrolysis (see 5.2.6, above). Quantities: 4′-hexylbiphenyl-4-yl-4-(11-bromo-undecyloxy)benzoate(9, 1.28 g, 2.11 mmol), triethylphosphite (2 ml,11.49 mmol), trimethylsilylbromide (2.00 mL,15 mmol). The product was recrystallised fromdioxane, then DMF (yield: 0.70 g, 55%).

1H NMR (DMSO–d6, 100◦C): 8.05 (d, 2H), 7.68(d, 2H), 7.54 (d, 2H), 7.27 (m, 4H), 7.07 (d, 2H,),4.09 (m, 2H), 3.55 (s, 1H), 2.61 (t, 2H), 1.74 (t,2H), 1.60 (t, 2H), 1.55 (m, 6H), 1.28 (m, 16H), 0.85(t, 3H).31P NMR (DMSO–d6, 100◦C): 26.832 (s).MS (ESI, microTOF): m/z: 653.325 (M+·HCOO–),607.318 (M+–H).

Acknowledgements

Financial support is gratefully acknowledged from the RoyalSociety (International Incoming Short Visits 2008/R1), theNational Academy of Science of Ukraine (Project No0107U003550 and Research Fellowship No 288–12.10.2011),and STCU (Project No 5205). The authors are also grateful:to D. Kumar (North Carolina A&T State University) andK. Mosul (Karazin National University, Kharkov, Ukraine)for the magnetic measurements of nanoparticles; to O.Vashchenko (ISMA, NAS of Ukraine) for providing TGA;and to P. Mateichenko (ISC, NAS of Ukraine) for carryingout the EDS analysis.

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