Liquid crystalline and charge transport properties of double-decker ceriumphthalocyanine complexes{{

Fabien Nekelson,ab Hirosato Monobe,a Motoo Shiroc and Yo Shimizu*a

Received 17th November 2006, Accepted 28th February 2007

First published as an Advance Article on the web 30th March 2007

DOI: 10.1039/b616848p

A homologous series of new thioalkylated double-decker cerium phthalocyanine complexes,

[(CnH2n+1S)8Pc]2Ce, were synthesised and their mesomorphisms were clarified by DSC,

polarized optical microscopy and X-ray diffraction. Unlike other lanthanide phthalocyanines,

these double-decker cerium compounds were found by spectroscopy and X-ray crystallography

to be neutral complexes. A hexagonal columnar mesophase (Colh) was exhibited by all

homologues prior to melting to the isotropic liquid. Interestingly, the columnar mesophases

of these compounds showed strong tendencies for spontaneous homeotropic alignment on

non-treated glass substrates or ITO coated glass. The hole mobility in the Colh mesophase of

the longer chain homologue was successfully measured by a TOF technique as having a value

of 7 6 1023 cm2 V21 s21.

1. Introduction

Metallophthalocyanine complexes with rare-earth metal ions

form double-decker structures where the lanthanide ion is

sandwiched between the two phthalocyanine ligands (Fig. 1).

These materials are generally radical complexes. Since the

lanthanide ion is trivalent, it implies that one of the phthalo-

cyanine rings has a different oxidation state. Instead, one ring

is dianionic whereas the other one is radical. Single X-ray

studies revealed that the radical ligand is slightly more concave

than its dianionic counterpart.1 On the other hand, substituted

double-decker phthalocyanine complexes are known to exhibit

columnar mesophases.2–4 In the mesophase an interesting

feature of molecular dynamics is proposed. Indeed, the two

phthalocyanine rings were found to be structurally equivalent

in the mesophase due to delocalisation of the radical electron

on both ligands.5 As a result, the p–p stacking interactions

within the columns are improved on heating to the mesophase

to give high charge carrier mobility of 0.7 cm2 V21 s21, which

was evaluated by pulse-radiolysis time-resolved microwave

conductivity technique (PR-TRMC) for a thioalkylated

lutetium phthalocyanine mesogen 1 (Fig. 1).6

Unfortunately, no uniform monodomain area where the

molecules are homeotropically aligned to the substrate, which

is important for device fabrication, could be observed with

compound 1.7

Among the lanthanide phthalocyanine mesogens

reported,2,5–9 rather few substituted phthalocyanine cerium

complexes have been synthesised,10,11 though the cerium atom

in those complexes, interestingly, shows a rather peculiar

property of the valence state. The cerium ion is either tri- or

tetravalent, thus making the whole complex radical or neutral,

depending on the electronic character of the tetrapyrrole

ligands.10,12

We have recently reported the preparation of a new double-

decker cerium phthalocyanine mesogen 3f bearing thioalkyl

substituents (Scheme 1).13 In this complex, the cerium ion is

tetravalent and both phthalocyanine ligands are dianionic. As

a consequence, the mesophase stability is enhanced compared

to those of other lanthanide phthalocyanine mesogens.

Spontaneous alignment of the columnar hexagonal mesophase

on a glass substrate could also be achieved.

In this work, we prepared a homologous series of new

thioalkylated double-decker cerium phthalocyanine complexes

3a–3e (Scheme 1) in order to study their mesomorphisms

and alignment behaviour on different surfaces as well as to

measure the charge carrier transport in their mesophase by a

time of flight technique (TOF).

aNanotechnology Research Institute, National Institute of AdvancedIndustrial Science and Technology (NRI, AIST Kansai centre), 1-8-31Midorigaoka, Ikeda, Osaka 563-8577, Japan.E-mail: yo-shimizu@aist.go.jp; Fax: +81-72-751-9628;Tel: +81-72-751-9525bJSPS FellowcRigaku Corporation, Akishima-shi, Tokyo 196-8666, Japan{ Electronic supplementary information (ESI) available: 1H NMRspectrum of 3f, X-ray crystallographic file in CIF format, elementalanalysis data, XRD data and DSC thermograms. See 10.1039/b616848p{ The HTML version of this article has been enhanced with colourimages.

Fig. 1 Lanthanide phthalocyanine double-decker complexes.

PAPER www.rsc.org/materials | Journal of Materials Chemistry

This journal is � The Royal Society of Chemistry 2007 J. Mater. Chem., 2007, 17, 2607–2615 | 2607

Publ

ishe

d on

30

Mar

ch 2

007.

Dow

nloa

ded

by U

nive

rsity

of

Abe

rdee

n on

29/

08/2

013

03:1

3:44

. View Article Online / Journal Homepage / Table of Contents for this issue

2. Experimental

2.1. General measurements

IR spectra were recorded from KBr disks on a FT-IR Biorad

FTS3000 Excalibur series spectrometer. UV–visible absorption

spectra were recorded on a Shimadzu UV-2500PC spectro-

photometer. 1H NMR spectra were recorded at 500 MHz

on a JEOL JNM-A50N FT-NMR spectrometer using 5 mm

diameter tubes of 298-PP quality or better and CDCl3 as

solvent. Tetramethylsilane was used as internal standard and

the positions of the peaks are reported in ppm. Proton

decoupled 13C NMR spectra were recorded at 126 MHz on

the same spectrometer. MALDI-TOF spectra were recorded

using a-cyano-4-hydroxycinnamic acid as the matrix. Single

crystal X-ray measurements were performed using a Rigaku

RAXIS-RAPID. Near-IR spectra were obtained in spectro-

scopic grade chloroform using a HITACHI U-4100 UV/vis/

NIR spectrophotometer.

Column chromatography was performed on Silica 60

(70–230 mm ASTM mesh, Merck). Anhydrous cerium(III)

chloride was prepared by heating finely grounded crystals of

CeCl3?7H2O at 140 uC for 1 h in a vacuum oven.

2.2. Synthesis

Syntheses and purification of the products and their inter-

mediates were carried out following reported procedures with

only slight modifications. The bis(alkylsulfanyl)phthalonitrile

precursors 2a–2f were prepared following the method reported

by Ohta et al.6 However, the crude product was passed

through a short plug of silica gel before recrystallisation to

remove the impurities coming from the alkylthiols.

Bis[2,3,9,10,16,17,23,24-octakis(octylsulfanyl)phthalocyani-

nato]cerium(IV). 4,5-Bis(octylsulfanyl)phthalonitrile (0.40 g,

1 mmol) was dissolved in hot hexanol (10 mL) under an

atmosphere of nitrogen. Anhydrous cerium(III) trichloride

(0.12 g, 0.5 mmol) and DBU (0.6 mL, 4 mmol) were added to

the solution and the reaction mixture was heated at reflux for

5 h. The dark solution was cooled down to room temperature

and methanol (20 mL) was added to the flask. The dark

precipitate was collected by filtration and washed with cold

methanol. The dark solid was then chromatographed over

silica gel using a 1 : 1 mixture of hexane–dichloromethane

as eluant. Evaporation of the solvent and successive

recrystallisations from ethyl acetate and THF–methanol gave

the title compound (0.20 g, 48%) as dark crystals. Found: C,

66.28; H, 8.24; N, 6.21%. C192H288CeN16S16 requires C, 66.39;

H, 8.36; N, 6.45%; dH (500 MHz, CDCl3) 8.56 (16H, s, Harom),

3.66 (16H, m, Ar–SCHH–), 3.50 (16H, m, Ar–SCHH–), 2.08

(32H, m, Ar–SCH2–CH2–), 1.79 (32H, m, Haliphatic), 1.53

(32H, m, Haliphatic), 1.49 (32H, m, Haliphatic), 1.43 (32H, m,

Haliphatic), 1.34 (32H, m, Haliphatic), 0.90 (48H, t, J 7.0 Hz,

CH3–); dC (126 MHz, CDCl3) 141.2, 129.9, 121.8, 105.5, 34.3,

32.0, 29.5, 29.4, 29.3, 28.8, 22.7, 14.1; MALDI-MS: isotopic

clusters at 3473 D (M+, 100%); lmax (abs, log e/M21 cm21) 685

(5.2), 499 (4.6), 371 (5.0), 313 (5.2) nm (CHCl3).

2.3. Measurements of mesomorphism

The phase transition temperatures and enthalpy changes were

measured by differential scanning calorimetry (DSC, TA

instrument DSC2920) and from microscopic observations of

the optical textures (Olympus BH2 and Mettler FP90 hot

stage). The DSC measurements were carried out on 3–4 mg

samples of freshly recrystallised materials at a scanning rate of

1 or 5 uC min21. The mesophases were identified by X-ray

diffraction at temperatures corresponding to the mesophase

and the glass or crystal phase using a Rigaku RINT 2500 HF

equipped with a hand-made hot stage. Temperature-dependent

electronic spectra were measured on a Shimadzu UV2500PC

UV–vis spectrophotometer as a cast film sandwiched between

two microscope glass slides.

2.4. Measurements of charge carrier mobility

The sample cell was prepared using ITO-coated glasses as

electrodes, separated in parallel by silica bead spacers with

2 mm diameters, and fixed with silicon cement. The cell

thickness was determined by interferometry using UV–visible

spectroscopy. The effective area of the electrode was adjusted

to 0.25 cm2 by etching of the ITO thin film. The sample was

injected into the cell slit by capillary action in its isotropic

phase and the cell was cooled to room temperature very

slowly (1 uC min21) to achieve the large monodomain with the

homeotropic alignment. The positive carrier mobilities were

measured by a TOF method. The temperature conditions were

controlled within 0.2 K using a temperature controller (Chino,

SU10) and a hot stage. A N2-pulse laser was used for light

irradiation at a wavelength of 337 nm and a pulse width of

800 ps. An electric field was created in the cell using a DC

Scheme 1 Preparation of the cerium phthalocyanine complexes

2608 | J. Mater. Chem., 2007, 17, 2607–2615 This journal is � The Royal Society of Chemistry 2007

Publ

ishe

d on

30

Mar

ch 2

007.

Dow

nloa

ded

by U

nive

rsity

of

Abe

rdee

n on

29/

08/2

013

03:1

3:44

.

View Article Online

power supply (WF1941). The transient photocurrents were

detected by a digital oscilloscope (HP, infinum) with the help

of a wide band preamplifier (NF electronics instruments,

BX-31A). Hole mobilities m were calculated from the equation,

m = d2/(Vts), where d is the sample thickness, V the applied bias

and ts the transit time. The transit times of the photogenerated

carriers in the material were determined from the inflection

points on the double-logarithmic plot of the photodecay

curves.

3. Results and discussion

3.1. Synthesis

The double-decker cerium complex 3f was originally isolated

by column chromatography as a by-product during the

preparation of the thioalkylated metal-free phthalocyanine 4f

(Scheme 1) following a method reported by Ng and Lee.14 A

substantial amount of double-decker complex is known to be

produced when a large amount of cerium trichloride is used in

the reaction.14 The reaction was, therefore, carefully studied to

determine the optimum conditions leading to the selective

formation of cerium double-decker complexes 3a–3f (Table 1).

The cerium phthalocyanine compounds can be selectively

obtained by using a slight excess of cerium salt. We found that

a minimum of 0.5 equivalents with respect to the starting

material (SM) was required (entries 1, 2, and 3). However, the

use of a large excess of the cerium trichloride leads to the

formation of a red-brown side-product affecting the final yield

of product (entry 4). The amount of DBU was also found to

influence the formation of the cerium and metal-free products

(entries 3, 5 and 6). These results are reproducible with longer

chain precursors (entries 7 and 8). The best conditions were

seen when approximately 0.5 equivalents of CeCl3 and a ratio

of DBU : SM between 4 and 6 were used (entries 3 and 7).

3.2. Valence state of the cerium ion

The 1H NMR spectra of our compounds show well-defined

signals as compared with those of other reported lanthanide

double-decker phthalocyanine complexes (see ESI{).2,6 No

addition of reducing agent such as hydrazine hydrate or

NaBH4 was required.15 The NMR spectra of 3a–3f were

consistent with those of previously reported cerium double-

decker structures.10 A single signal at d 8.58 relating to the 1,4

protons of the fused-phenyl rings can be observed. The two

multiplets centered at d 3.66 and 3.50 indicate that the SCH2

protons are diastereotopic due to the two different environ-

ments inside and outside the sandwich complexes. The

molecule is therefore chiral and the two phthalocyanine rings

should be in a staggered conformation. Other signals corres-

ponding to the remaining aliphatic protons are also seen. The

signal at d 1.56 is related to the presence of a trace amount of

water in the deuterated solvent.

Lanthanide double-decker phthalocyanines generally exhibit

two broad absorption bands characteristic of a radical

phthalocyanine ligand in the 460–500 nm and 880–900 nm

regions as well as another broad band in the 1200–1600 nm

region corresponding to an intramolecular charge transfer

between the two rings.16 Therefore, near-IR spectroscopic

studies were performed on our cerium complexes. The

spectrum of one homologue 3f in CHCl3 is represented in

Fig. 2. In this spectrum, only three main absorption bands in

the UV–visible region are observed. The two absorption bands

at 685 and 371 nm were assigned to the Q-band and Soret

band, respectively. The weak shoulder attached to the Q-band

has been previously observed with lanthanide phthalocyanine

complexes and was reported to be due to the weak p–p

interactions occurring between the two ligands.17 This splitting

becomes more pronounced after oxidation with iodine whereas

it completely disappears upon reduction with LiAlH4 due to

the corresponding increase or decrease of the size of the metal

centre.10 The additional peak at 313 nm has been previously

ascribed to the N-band.18

Interestingly, the absorption band at 499 nm is rather weak

as compared to that of other substituted lanthanide double-

decker phthalocyanines.10,18 However, the intensity of this

band can also be enhanced or reduced by addition of iodine or

lithium aluminium hydride, respectively (Fig. 2). The presence

of a radical phthalocyanine ligand in the oxidised species is

confirmed by the broad absorption band at 1765 nm and the

intense one at 499 nm.

These results suggest that both ligands of our cerium

complexes are dianionic. The cerium atom is, therefore,

tetravalent and diamagnetic in nature. As a consequence,

well-defined signals can be observed in the 1H NMR spectrum

of 3f. This change of oxidation state of the cerium atom has

been explained by the hybridisation of the metal ion 4f orbital

Table 1 Influence of the DBU and CeCl3 ratios on the formation of[(RS)8Pc]2Ce complexes

Entry R SM/gDBU/eq.a

CeCl3/eq.a

[(RS)8Pc]2Ce(3) (%)b

(RS)8PcH2

(4) (%)b

1 C8H17 (3a) 0.4 4 0.125 10 402 C8H17 (3a) 0.4 4 0.25 8 243 C8H17 (3a) 0.4 4 0.5 48 04 C8H17 (3a) 0.4 4 1.0 36 05 C8H17 (3a) 0.4 2 0.5 14 146 C8H17 (3a) 0.4 10 0.5 0 307 C12H25 (3c) 0.6 6 0.5 60 08 C18H37 (3f) 0.4 4 0.5 40 0a With respect to starting material (SM). b Determined by 1H NMRof the crude product.

Fig. 2 Electronic absorption spectra of 3f in CHCl3 and after adding

iodine. The UV–vis spectrum of 3f after reduction with LiAlH4 in

CHCl3–EtOH is shown in the inset.

This journal is � The Royal Society of Chemistry 2007 J. Mater. Chem., 2007, 17, 2607–2615 | 2609

Publ

ishe

d on

30

Mar

ch 2

007.

Dow

nloa

ded

by U

nive

rsity

of

Abe

rdee

n on

29/

08/2

013

03:1

3:44

.

View Article Online

with those of the ligands. This closed shell orbital contains

only one electron which can delocalise in the p-orbitals of the

macrocycles.12 The process is facilitated by the exceptional

stability of the empty 4f0 orbital of the Ce4+ ion10 and the weak

electron donating effect of the thioalkyl groups.19

3.3. Crystal structure

The crystal structure of compound 3a (Table 2) was similar to

those of the previously described thioalkylated lanthanide

phthalocyanine derivatives.20,21 A view of the molecular

structure of 3a is depicted in Fig. 3. The cerium ion occupies

a central position in the complex and is eight-fold coordinated

to the isoindole nitrogen atoms (Niso) of the phthalocyanine

ligands. As the NMR suggested, the two phthalocyanine rings

are staggered. The staggering angle between the rings is a =

43u, which is a larger value than that of the corresponding

unsubstituted bis(phthalocyaninato)cerium complex (a =

38u).22 This could be explained by the presence of sterically

hindered sulfur atoms on the phthalocyanine rings, which

favour a staggered conformation rather than an eclipsed one.

The coordination polyhedron of cerium is a slightly distorted

antiprism. The four Niso atoms of the two phthalocyanine

rings are almost coplanar as they only deviate from their mean

planes by less than ¡0.005 A. Since their dihedral angle is only

0.77u, these two planes are practically parallel to each other.

The central cerium ion lies 1.417 A from both Niso mean

planes of the two corresponding rings. The lengths of the

Ce–Niso bonds are almost equal (Table 3). The calculated

distance between the two Niso mean planes is 2.83 A. This

value is slightly larger than that reported for other lanthanide

phthalocyanine materials (2.67–2.70 A).21,22 This can be

explained by the bigger size of the cerium ion in the lanthanide

series. Unlike the previously described lanthanide phthalocya-

nine structures,1,20 both phthalocyanine ligands of 3a are also

found to be equally distorted from planarity and, therefore,

adopt a biconcave shape. The calculated dihedral angles

between the Niso mean plane and the mean plane of each

pyrrole ring range from 4.0 to 9.3u and those of the phenyl

rings range from 2.1 to 7.4u.The crystal packing of [(C8H17S)8Pc]2Ce 3a is comparable

to those of other thioalkylated lanthanide double-decker

complexes.20,21 The molecules of 3a are organised as columns

along the a axis in a slipped-stack arrangement (Fig. 4). The

intermetallic Ce–Ce distance along the c axis is 10.970(2) A.

In contrast with other double-decker molecules,20 the shortest

Table 2 Crystallographic data and structure refinement parametersfor [(C8H17S)8Pc]2Ce 3a. CCDC reference number 627716. Forcrystallographic data in CIF format see DOI: 10.1039/b616848p

Molecular formula C192H288N16S16Ce

Molecular weight 3473.57T/K 93.1Wavelength/A 0.71075Crystal system MonoclinicSpace group P21/na/A 31.232(2)b/A 35.725(2)c/A 20.2073(10)b/u 121.196(2)V/A3 19286.3(19)Z 4Dc/g cm23 1.196mabs/mm21 0.463Independent reflections 24140 (Rint = 0.092)R1 [I . 2s(I)]a 0.0731wR2 (all data)b 0.2033a R1 = S||Fo| 2 |Fc||/S|Fo| b wR2 = [S(w(Fo2 2 Fc2)2)/Sw(Fo2)2]1/2

Fig. 3 ORTEP plot of one molecule of 3a with part of the atom

numbering scheme used and ellipsoids drawn at 50% level of

probability. The disorder model and the positions of the hydrogen

atoms are excluded.

Table 3 Selected bond lengths (A) and angles (u) for 3a

Ce–N1 2.421(3) Ce–N3 2.422(4)Ce–N5 2.421(4) Ce–N7 2.420(4)N1–Ce–N3 70.14(1) N1–Ce–N5 108.36(1)N1–Ce–N7 69.99(1) N1–Ce–N9 82.23(1)N1–Ce–N11 141.81(1) N1–Ce–N13 145.44(1)N1–Ce–N15 84.66(1) N3–Ce–N13 142.06(1)N5–Ce–N7 69.89(1) N5–Ce–N15 142.34(1)N7–Ce–N11 145.51(1) N7–Ce–N15 82.74(2)N9–Ce–N11 69.66(1) N9–Ce–N13 108.56(2)

Fig. 4 Crystal packing of 3a viewed along the a and b axes.

2610 | J. Mater. Chem., 2007, 17, 2607–2615 This journal is � The Royal Society of Chemistry 2007

Publ

ishe

d on

30

Mar

ch 2

007.

Dow

nloa

ded

by U

nive

rsity

of

Abe

rdee

n on

29/

08/2

013

03:1

3:44

.

View Article Online

intermolecular distance along the columnar axis was found,

in our case, between the conjugated rings with a value of

3.48(2) A.

3.4. Mesomorphism

All thioalkylated cerium double-decker compounds show

birefringence on heating and clearance into an isotropic liquid

at higher temperature. The phase transition temperatures

and the associated enthalpy changes determined by DSC and

polarised microscopy of our homologous series of cerium

double-decker complexes are listed in Scheme 2.

A fan texture, characteristic of a hexagonal columnar meso-

phase (Colh), was observed for all homologues upon rapid

cooling of the isotropic liquid (Fig. 5, top). An additional

mesophase–mesophase transition exhibiting a mosaic-like

texture, also distinctive of a Colh, was seen for the middle

chain length compounds 3b–3d at lower temperatures (Fig. 5,

bottom). All mesophase textures consist of domain boundaries

and some large dark areas seen under crossed polarised light.

The latter correspond to areas where the molecules are

homeotropically aligned to the substrates.13

The phase transitions were found to be reproducible by

DSC after several repetitive heating–cooling cycles (see ESI{).

Compared to other lanthanide double-decker mesogens,21 the

C8 homologue 3a shows some trace of decomposition before

clearing. A large enthalpy value (16.6 kJ mol21) was associated

to this transition. TGA experiments revealed that the decom-

position starts at around 210 uC and increases at around

250 uC. Thermal degradation of the thioalkyl chains is known

to occur at these temperatures.23 For this reason, no transition

temperatures can be detected by DSC on the cooling run for

this compound. Nevertheless, crystallisation was seen on

cooling at about 64 uC from a fresh sample of 3a heated to

its Colh mesophase. A decrease of the enthalpy values related

to the mesophase–mesophase transitions for compounds 3b–3d

was observed after the first heating–cooling cycle (Fig. 6). This

could be explained by the fact that the molecular disorder

of the supercooled sample is less important than that of the

freshly recrystallised sample.

Microscopic observations show that the middle chain length

homologues 3b and 3c are liquid crystalline materials at room

temperature. Interestingly, no crystallisation can be seen for

the long alkyl chain length compounds 3d–3f. They are, in fact,

glassy solids at room temperature as confirmed by the

persistence of their optical texture.

The mesophase stability of our cerium double-decker

materials was found to be slightly enhanced in comparison

with those of the homologous lanthanide phthalocyanine

mesogens prepared by Ohta and co-workers.6 This could be

the consequence of the structural equivalence of the two

phthalocyanine ligands favouring the p–p orbitals overlapping

over a wider temperature range.

Temperature-dependent powder X-ray diffraction analyses

were performed on the mesophase of all homologues 3a–3f to

confirm their respective symmetry lattice. Fig. 7 also shows the

temperature-dependent X-ray patterns of the C12 3c and C18 3f

homologues. Four to eight reflections with spacing ratios of

1 : 1/!3 : 1/2 : 1/!7 : 1/3 : 1/!13 : 1/4 : 1/!21 were present

in the small angle region of the X-ray patterns of the high

temperature mesophase observed for all compounds. A

hexagonal lattice symmetry was, therefore, assigned to this

columnar mesophase. The mesogenic character of this phase

Scheme 2 Mesophase transition temperatures and enthalpy changes

of the [(CnH2n+1S)8Pc]2Ce homologues.

Fig. 5 Optical textures of the Colh mesophases of 3e at 120 uC (top)

and 3c at 35 uC (bottom). Fig. 6 DSC thermogram of 3b at 1 uC min21.

This journal is � The Royal Society of Chemistry 2007 J. Mater. Chem., 2007, 17, 2607–2615 | 2611

Publ

ishe

d on

30

Mar

ch 2

007.

Dow

nloa

ded

by U

nive

rsity

of

Abe

rdee

n on

29/

08/2

013

03:1

3:44

.

View Article Online

was indicated by the broad reflections in the wide angle region

at ca. 4.3–4.6 A, which is related to the distance between the

molten thioalkyl chains. Moreover, no additional reflections

associated with the intracolumnar order c are present in this

region. As a result, this phase was described as a disordered

hexagonal columnar mesophase (Colhd).

The X-ray patterns of the lower temperature mesophase,

exhibited by the compounds 3b–3d, only differ from the high

temperature one by the apparition of two clear reflections at

ca. 7.0 and ca. 3.3–3.4 A related to the stacking periodicity

between double-decker (h) and single-decker complexes (c),

respectively. The density of the liquid crystal, which was

calculated using the value of h, is in agreement with the

ordinary Colh type mesophase of double-decker phthalocya-

nine mesogens (y1.0 g cm23).7 This mesophase was assigned

as an ordered hexagonal columnar one, Colho. Discotic liquid

crystalline materials substituted with thioalkyl groups are

known to exhibit high columnar order on cooling due to the

larger size of the sulfur atoms preventing rotational and

translational molecular dynamics within the stacks.6,24 Unlike

previously reported thioalkylated lanthanide phthalocyanine

mesogens,6 no additional reflections relating to a rectangular

lattice were observed on any of our X-ray patterns.

The glass transitions exhibited by the long chain homo-

logues 3e,3f were confirmed by the persistence of the small

angle reflections on their room temperature X-ray patterns,

which indicates that the cores of the molecules are still

arranged in a hexagonal lattice (Fig. 7). However, the

sharpening of the wide-angle reflection at ca. 4.1–4.2 A

suggests that a partial crystallisation of the alkyl chains into

the two-dimensional hexagonal packing is also occurring.25 An

intermediate broad reflection at ca. 4.3–4.4 A was seen

for compound 3d at room temperature due to incomplete

crystallisation of the alkyl chain at 23 uC. The tendency of

the molecules to keep their hexagonal columnar arrangement

at room temperature must be the consequence of the

tetravalency of the cerium atom. Indeed, we have shown that

the molecules are neutral with both phthalocyanine ligands

being structurally equivalent. Therefore, the molecular

dynamics, which has been shown in the case of other

lanthanide double-decker complexes to influence the forma-

tion of slipped columns,5 cannot occur here.

3.5. Alignment behaviour

Initial microscopic observations revealed the strong tendency

of the Colhd mesophase of 3f toward homeotropic alignment

on non-treated glass substrates.13 This phenomenon is

characterised by a dendritic growth of six-fold symmetry only

visible under uncrossed polarised light.7 However, a few small

defects, corresponding to some areas where the molecules are

slightly tilted with respect to their columnar axis, can be seen

along this growth. The Colhd mesophases of the other

homologues were also found to exhibit this tendency. The

growing rate of the dendrites is found to depend on the rate of

cooling. Large monodomains where the molecules are home-

otropically aligned can be achieved, within 2 minutes and

without apparition of additional disclinations, by keeping the

cooling rate at 1 uC min21. No birefringence can be observed

by rotating the sample stage between the crossed polarisers.

The liquid crystallinity of the compound was confirmed by the

presence of a uniform birefringent mass when the sample was

gently smeared between the glass slides.

This result is surprising because no spontaneous home-

otropic alignment could be obtained for the lutetium double-

decker complexes having the same ligands.7 Moreover, the

mesophases of our compounds show much higher viscosity,

which is known to usually lead to domain boundaries,26 than

that of their metal-free derivatives 4a–4f.

This behaviour makes our compounds ideal candidates for

the measurement of their charge carrier mobility by a TOF

technique. Therefore, the alignment behaviour of our com-

pounds on ITO-coated glass was then studied by polarised

microscopy (Fig. 8). An ITO/sample/ITO sandwiched cell of

constant thickness of 4.26 mm was prepared. Large mono-

domains of homeotropically aligned molecules can be readily

obtained by slowly cooling their isotropic liquid (Fig. 8). The

declination line observed on the picture was due to a point

defect growing along the dendritic growth. The homeotropic

columnar alignment of these compounds was also found to

persist at room temperature without any visible crystallisation.

3.6. Temperature dependence electronic spectra of a cast film

Electronic absorption spectroscopy of a thin film is a useful

technique to study the interactions between neighbouring

molecules at a given temperature. Indeed, the mesophase

transitions of an alkoxy substituted lutetium phthalocyanine

complex have been previously detected by this technique.5 The

spectral differences observed when the orientation of theFig. 7 X-Ray diffraction patterns of 3c (top) and 3f (bottom) at

different temperatures. These patterns are offset for clarity.

2612 | J. Mater. Chem., 2007, 17, 2607–2615 This journal is � The Royal Society of Chemistry 2007

Publ

ishe

d on

30

Mar

ch 2

007.

Dow

nloa

ded

by U

nive

rsity

of

Abe

rdee

n on

29/

08/2

013

03:1

3:44

.

View Article Online

molecule changes can be explained by simple molecular exciton

theory, which was developed by Kasha and co-workers.27

Temperature-dependent electronic absorption spectra of a cast

film of 3f were recorded at 5 uC intervals from 25 to 130 uC.

For clarity, only some of these UV–vis spectra are represented

in Fig. 9. However, the values of their absorbance are all

plotted against temperature in Fig. 10. Since no absorption

could be detected in the near-IR region, we limited our

analyses to the UV–visible region.

As shown in Fig. 9, no splitting of the Q-band of absorption,

indicating that the molecules are arranged in a tilted

conformation,27 are present for these phases. Therefore, the

molecules must adopt either a cofacial or a slipped arrange-

ment which is kept up to the clearing point.27 The slight

bathochromic shift observed on heating to the Colh mesophase

results from an increase of the disorder within the columnar

stacks. This result is in good agreement with our X-ray results

suggesting a columnar arrangement for the compounds.

Fig. 10 shows the sudden enhancements of the Q-band

intensities at both phase transitions. This result suggests that

no major changes in the molecular arrangement occurs, other

than the increase of the stacking distance between molecules.5

3.7. TOF measurements

The transient photocurrents detected in the hexagonal

columnar mesophase of a 5.46 mm thick sample of 3f at a

temperature of 107 uC under different applied biases are shown

in Fig. 11. The hole mobility at this temperature is kept in the

order of 7 6 1023 cm2 V21 s21. Hole transport is dominant for

the observed photocurrent. No charge carrier mobility for the

electrons could be evaluated due to the very dispersive

character of the photocurrent curves. Electron mobility is

known to be sensitive to material impurities and ambient

oxygen, which can readily trap the photogenerated charge

carriers.28 As shown in Fig. 12, the mobility is independent of

the electric field intensity in the range 104–105 V cm21.

However, mobility measurements were restricted by levels of

dark current of the sample and discharge noise of the nitrogen

laser. At an applied electric field below 104 V cm21, the decay

curve becomes too noisy to be able to observe any clear kink

whereas electric breakdown occurs above 105 V cm21.

The mobility was also found to be nearly independent of the

whole temperature range of the columnar mesophase as seen

in Fig. 13. The slight increase of mobility seen at lower

temperature can be explained by the enhancement of the

periodical stacking between the molecules within the columns.

In the isotropic phase as well as in the glass phase, the transient

photocurrents are very dispersive. Therefore, no mobility

could be determined in these phases. Small defects created by

the partial crystallisation of the alkyl chains in the glass phase

Fig. 9 Temperature-dependent electronic spectra of a cast film of 3f

in the UV–visible region.

Fig. 10 Absorbance intensities of the Q-band of a cast film of 3f at

different temperatures.

Fig. 11 Transient photocurrents in the Colhd mesophase of

[(C18H37S)8Pc]2Ce 3f at 107 uC for the positive carriers. The double

logarithmic plot of the transient photocurrents at several applied

biases is shown in the inset. The sample thickness was 5.46 mm.

Fig. 8 Microphotographs of the evolution of the homeotropic

alignment for the Colhd mesophase of 3f sandwiched between ITO-

coated glass plates.

This journal is � The Royal Society of Chemistry 2007 J. Mater. Chem., 2007, 17, 2607–2615 | 2613

Publ

ishe

d on

30

Mar

ch 2

007.

Dow

nloa

ded

by U

nive

rsity

of

Abe

rdee

n on

29/

08/2

013

03:1

3:44

.

View Article Online

and the decrease of the p–p stacking between the molecules in

the isotropic liquid might be responsible for this result.

Surprisingly, the mobility value of our cerium phthalo-

cyanine compound is much lower than that (Sm1D =

0.7 cm2 V21 s21) reported by Ohta et al. for a thioalkylated

lutetium phthalocyanine complex.6 The absence of a radical

electron on the phthalocyanine ligands, which can delocalise

to the neighbouring molecules, might be responsible for the

lower mobility of our compound. Indeed, PR-TRMC mobility

measurements performed by Warman and co-workers on

different phthalocyanine mesogens show that the carrier

transports of the free-radical lanthanide double-decker com-

plexes are much higher than those of the corresponding free-

base compounds.29

4. Conclusions

A homologous series of new double-decker cerium phthalo-

cyanine mesogens was successfully prepared; their meso-

morphism and charge transport have also been studied.

The weak electron donating effect of the thioalkyl groups

was found to have an influence on the valence state of the

cerium ion which is tetravalent with these ligands. As a result,

these double-decker complexes are neutral. Crystallographic

analyses reveal that both phthalocyanine ligands are slightly

distorted from the planarity but equivalent.

As a result, the thermal stability of their mesophase is

enhanced as compared to those of the previously reported

double-decker phthalocyanine mesogens. XRD studies show

that all homologues exhibit a Colhd mesophase before isotro-

pisation. A regioregular stacking of the molecules along the

columns was observed for the middle-chain length homologues

3b–3d at lower temperatures (Colho). XRD analyses also show

that the C10 and C12 homologues 3b,3c are liquid crystals at

room temperature whereas the longer chain complexes C16 3e

and C18 3f are glassy solids. This glass transition was, however,

not perfect as the alkyl chains were found to be partially

crystallised within the 2D hexagonal lattice of the cores.

Interestingly, microscopic observation revealed that these

compounds have a strong tendency for spontaneous home-

otropic alignment on non-treated glass substrates and ITO-

coated glass. Large monodomains of homeotropically aligned

molecules could be achieved readily and rapidly by slowly

cooling the samples from the isotropic phase. The absence of

symmetry changes within the columns throughout the whole

temperature range of the mesophase was also confirmed by

temperature-dependence electronic absorption spectroscopy.

The carrier transport of one of these materials was deter-

mined by a TOF technique. The mobility of the positive carrier

was found to be electric field and temperature independent

within the whole mesophase temperature range with a value of

7 6 1023 cm2 V21 s21. The low mobility values of these cerium

double-decker materials as compared to those of their lutetium

analogues could be explained by the absence of free-radicals,

which can delocalise within the columns.

Acknowledgements

FN acknowledges the grant-in-aid of JSPS for financial

support on the fellowship program and Dr K. Kamada

(AIST, Kansai centre) for the use of the Near-IR spectrometer.

This work was, in part, supported by a grant-in-aid for Science

Research in a Priority Area ‘‘Super-Hierarchical Structures’’

from the Ministry of Education, Culture, Sports, Science and

Technology, Japan.

References

1 A. de Cian, M. Moussavi, J. Fischer and R. Weiss, Inorg. Chem.,1985, 24, 3162–3167.

2 C. Piechocki, J. Simon, J.-J. Andre, D. Guillon, P. Petit,A. Skoulios and P. Weber, Chem. Phys. Lett., 1985, 122, 124–128.

3 K. Ohta, K. Hatsusaka, M. Sugibayashi, M. Ariyoshi, K. Ban,F. Maeda, R. Naito, K. Nishizawa, A. M. van de Craats andJ. M. Warman, Mol. Cryst. Liq. Cryst., 2003, 397, 25–45.

4 K. Binnemans and C. Gorller-Walrand, Chem. Rev., 2002, 102,2303–2345.

5 R. Naito, K. Ohta and H. Shirai, J. Porphyrins Phthalocyanines,2001, 5, 44–50.

6 K. Ban, K. Nishizawa, K. Ohta, A. M. van de Craats,J. M. Warman, I. Yamamoto and H. Shirai, J. Mater. Chem.,2001, 11, 321–331.

7 K. Hatsusaka, M. Kimura and K. Ohta, Bull. Chem. Soc. Jpn.,2003, 76, 781–787.

8 T. Komatsu, K. Ohta, T. Fujimoto and I. Yamamoto, J. Mater.Chem., 1994, 4, 533–536.

9 K. Binnemans, J. Sleven, S. De Feyter, F. C. De Shryver,B. Donnio and D. Guillon, Chem. Mater., 2003, 15, 3930–3938.

10 Y. Bian, J. Jiang, Y. Tao, M. T. M. Choi, R. Li, A. C. H. Ng,P. Zhu, N. Pan, X. Sun, D. P. Arnold, Z.-Y. Zhou, H.-W. Li,T. C. W. Mak and D. K. P. Ng, J. Am. Chem. Soc., 2003, 125,12257–12267.

Fig. 13 Temperature dependence of hole mobility for 3f under an

electric field of 2 6 104 V cm21.

Fig. 12 Electric field dependence of the hole mobility of

[(C18H37S)8Pc]2Ce at 107 uC.

2614 | J. Mater. Chem., 2007, 17, 2607–2615 This journal is � The Royal Society of Chemistry 2007

Publ

ishe

d on

30

Mar

ch 2

007.

Dow

nloa

ded

by U

nive

rsity

of

Abe

rdee

n on

29/

08/2

013

03:1

3:44

.

View Article Online

11 J. Jiang, J. Xie, D. K. P. Ng and Y. Yan, Mol. Cryst. Liq. Cryst.,1999, 337, 385–388.

12 H. Isago and M. Shimoda, Chem. Lett., 1992, 147–150.13 F. Nekelson, H. Monobe and Y. Shimizu, Chem. Commun., 2006,

3874–3876.14 C.-H. Lee and D. K. P. Ng, Tetrahedron Lett., 2002, 43, 4211–4214.15 J. Jiang, Y. Bian, F. Furuya, W. Liu, M. T. M. Choi,

N. Kobayashi, H.-W. Li, Q. Yang, T. C. W. Mak andD. K. P. Ng, Chem.–Eur. J., 2001, 7, 5059–5069.

16 D. Markovitsi, T.-H. Tran-Thi, R. Even and J. Simon, Chem.Phys. Lett., 1987, 137, 107–112.

17 A. Iwase, C. Harnoode and Y. Kameda, J. Alloys Compd., 1993,192, 280–283.

18 K. Yoshino, S. B. Lee, T. Sonoda, H. Kawagishi, R. Hidayat,K. Nakayama, M. Ozaki, K. Ban, K. Nishizawa, K. Ohta andH. Shirai, J. Appl. Phys., 2000, 88, 7137–7143.

19 L. P. Hammett, J. Am. Chem. Soc., 1937, 59, 96–103.20 A. G. Gurek, V. Ahsen, D. Luneau and J. Pecaut, Inorg. Chem.,

2001, 40, 4793–4797.

21 A. G. Gurek, T. Basova, D. Luneau, C. Lebrun, E. Kol’tsov,A. K. Hassan and V. Ahsen, Inorg. Chem., 2006, 45, 1667–1676.

22 M. S. Haghighi, C. L. Teske and H. Homborg, Z. Anorg. Allg.Chem., 1992, 608, 73–80.

23 E. Orthmann and G. Wegner, Makromol. Chem., Rapid Commun.,1986, 9, 651–657.

24 D. Adam, P. Schuhmacher, J. Simmerer, L. Haussling,K. Siemensmeyer, K. H. Etzbachi, H. Ringsdorf and D. Haarer,Nature, 1994, 371, 141–143.

25 T. Nakai, K. Ban, K. Ohta and M. Kimura, J. Mater. Chem., 2002,12, 844–850.

26 K. Hatsusaka, K. Ohta, I. Yamamoto and H. Shirai, J. Mater.Chem., 2001, 11, 423–433.

27 M. Kasha, H. R. Rawls and M. A. El-Bayoumi, Pure Appl. Chem.,1965, 11, 371–392.

28 M. Funahashi and J. Hanna, Chem. Phys. Lett., 2004, 397,319–323.

29 A. M. van de Craats, J. M. Warman, H. Hasebe, R. Naito andK. Ohta, J. Phys. Chem. B, 1997, 101, 9224–9232.

This journal is � The Royal Society of Chemistry 2007 J. Mater. Chem., 2007, 17, 2607–2615 | 2615

Publ

ishe

d on

30

Mar

ch 2

007.

Dow

nloa

ded

by U

nive

rsity

of

Abe

rdee

n on

29/

08/2

013

03:1

3:44

.

View Article Online