Mesogenic properties of novel enamino ketone ligands and their copper (11) complexes

Jadwiga Szydlowska, Wiesl-aw Pyiuk, Adam Krbwczynski and Ildar Bikchantaev? Laboratory of Dielectrics and Magnetics, Department of Chemistry, University of Warsaw, Al. Zwirki i Wigury 101, 02-089 Warsaw, Poland

Several novel homologous series of ligands incorporating an enamino ketone quasi-ring and their copper complexes are synthesized and studied by thermal microscopy, DSC and EPR methods. The ligands { 1-[4'-( 4"-hexyloxyphenyloazo)phenyl]-3- alkylaminoprop-2-en-1-ones} and their complexes form enantiotropic nematic and smectic C phases with the nematic phase of some of the complexes broader than 80 "C. The EPR spectra of the paramagnetic complexes show that the chelate centre is planar in dilute solution as well as in the magnetically concentrated smectic C phase. Exchanging the hexyloxy moiety for a hexylamino or N-hexyl-N-methylamino group gives a series of ligands and complexes with lower melting and clearing temperatures. Further molecular modifications including a reversal of the substitution pattern on the enamino ketone ring are also presented.

Organometallic mesogens have been intensively studied during the last decade. Studies of paramagnetic mesogens provide a logical basis for the design and synthesis of ferromagnetic liquid crystals, which is important because of their potential applications. Also of interest are the optical properties of these complexes. As a rule, they exhibit vivid colours,' pronounced birefringence,2 dichroism3 and unusually strong optical nonlin- ear effect^.^ Their electrical properties, among them one- dimensional conductivity,' are also a promising area.

An investigation of the properties of these compounds give an opportunity for the application of several new techniques in the field of liquid crystal research. For example, Mossbauer spectroscopy (a method limited, however, to only some metals) and EXAFS, which provides information about the environ- ment of the metal centre.6 EPR spec t ros~opy~-~ and mag- netochemical methods are vital for studies of paramagnetic mesogens, which are often easily obtained by complexation of transition metal ions with appropriate ligands.

In general, liquid-crystalline phases are formed by disc-like or rod-like molecules. For geometrical reasons, it is easy to obtain discotic compounds, which are well known for several classes of 1igands.l' In contrast, calamitic paramagnetic com- plexes are restricted, in practice, to two classes of ligands,lO.'l P-diketonates and salicylaldimines. The latter seem to be more valuable materials, especially in view of their lower melting points and wider mesophase range.12

Recently, compounds incorporating quasi-ring enamino ketones, stabilized by an intermolecular hydrogen bond, were reported to be calamitic liquid-crystalline materials.13 They also appeared to be promising ligands, which enabled the synthesis of low melting, thermally stable paramagnetic nema- togens.l4.l5 In this work we present results of our studies of some novel mesomorphic enamino ketone ligands as well as their copper complexes, both designed as distinctly elongated molecules. The complexes are paramagnetic and are charac- terized by relatively good thermal stability with moderate melting temperatures.

Molecular structure

In order to obtain liquid crystalline molecules and to ensure a high length: breadth ratio for both ligands and complexes we synthesized compounds in which the enamino ketone group was substituted by a terminal alkyl chain in the 3-position and two hexyloxyazobenzene groups in the 1-position. The

7 Also: Kazan Physical-Technical Institut, Sibirsky Trakt 10/7,420029 Kazan, Russia.

resulting ligands and complexes of the main series 1 and 2, respectively were studied in detail. For comparison, some compounds of a comparative series 3 and 4, having the substituents in reversed positions, were also examined. Besides the 'reversed' azo-derivatives some 'reversed' azoxy-com- pounds, series 5 and 6, were synthesized in order to extend the mesophase temperature range. Further important modifi- cations of the parent series 1,2 were obtained by replacing the oxygen atom of the alkoxy chain by an -NH- group, series 7 and 8. Modifications in which the -NH- group was exchanged by an -N(CH3)- moiety were also synthesized and studied, series 9 and 10.

series 2 Y = H13C80 8 Y = H13CeNH

10 Y = H13C*N(CH3)

series3 x = N=N 6 X=N(O)N

series 4 x = N=N 6 X=N(O)N

J . Mater. Chem., 1996, 6( 5), 733-738 733

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by M

cMas

ter

Uni

vers

ity o

n 29

/10/

2014

14:

24:2

6.

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

The abbreviations, n-n, used to denote the structures in the text denotes the series to which the compound belongs, n, and its homologue number, n

Experimental Synthesis

All the ligands and their copper(I1) complexes were prepared in the same manner from the appropriate acetyl derivatives of azobenzene and aliphatic amines (series 1, 2 and 7-10) or aliphatic methyl ketones and derivatives of p-aminoazo- or p- aminoazoxy-benzenes (senes 3-6) The starting materials are well known compounds, and their synthesis is routine Typical synthetic procedures are described below

Preparation of 1-[ 4-( 4-hexyloxyphenylazo) phenyll-3-pro- pylaminoprop-2-en-1-one ( 1-3). 4-( 4-Hexyloxypheny1azo)- a~etophenone'~' (3 24 g, 10 mmol) and ethyl formate (1 2 g) were added to molecular sodium (0 23 g, 10 mmol) in E t20 (20 cm3) and stirred vigorously for 12 h The resulting suspen- sion of formyl ketone sodium salt was neutralized with dilute aq HC1 and the formyl ketone diethyl ether solution was separated The solution, after the addition of 1-aminopropane (06Og, ca 10mmol) in MeOH (20cm3), was kept at room temperature for 2 h Next, the diethyl ether was evaporated and the remaining solution cooled and the resulting precipitate filtered off and recrystallized from methanol Red-orange crystals of the final product 1-3 were obtained (ca 70%) (Found C, 73 2, H, 7 9, N, 10 5 Calc for C24H31N302 C, 73 24, H, 7 96, N, 10 68%) The NMR spectrum is consistent with the molecular structure, GH(CDC13) 0 80-1 95 [m, 16 H, OCH2(CH,),CH3, NCH,CH2CH3], 325 (m, 3 H, NCH,), 4 04 (t, J 6 8 Hz, 3 H, OCH,), 5 74 (d, J 7 3 , l H, H2), 6 85-7 20 (m, H', H3", H'"), 7 80-8 10 (m, 6 H, H2', H3', H", H6', H2", H6"), 10 40-10 55 (m, 1 H, NH)

Preparation of bis{ 1-[ 4-( 4-hexyloxyphenylazo)phenyl]-3- propylaminoprop-2-en-1-onato}copper (11) (2-3). A solution of Cu,(AcO), .2H20 (0 3 g) in MeOH (10 cm3) was added to a boiling solution of 1-3 (065 g, 2 mmol) in MeOH (20cm3) After 5 min at reflux, the solution was allowed to cool, and the precipitated complex (ca 90 YO) filtered off and recrystallized from hexane (Found C, 68 2, H, 7 0, N, 9 9 Calc for

Other ligands and complexes were obtained in a similar way The only difference was that ligands having terminal alkyl chains longer than decyl were recrystallized from ethanol

C48H60N604CU c , 67 93%, H, 7 14%, N, 9 goo/,)

Measurements

The identification of mesophase was based on microscopic observations of the textures and miscibility tests with reference compounds For ortho- and cono-scopy, a Zeiss Jenapol-U polarizing microscope, equipped with a Mettler FP82HT hot stage was used A typical schlieren texture was observed in both nematic (N) and smectic C (S,) phases In addition to the schlieren texture the Sc phase revealed also, a focal-conic fan or broken fan texture In the smectic A (S,) phase homeotropic or fan textures were obtained depending on the glass surface preparation The smectic F (S,) phase was recognized from its focal-conic fan texture decorated with L- shaped patches In the smectic G (S,) phase a charactenstic mosaic texture was observed l6

Calorimetric measurements were performed using a DSC7 Perkin-Elmer set-up Routine runs were performed in a dry nitrogen atmosphere at a scanning rate of 5 K min-' In the case of thermally unstable complexes the scanning rate was increased to 20 "C min-' EPR spectra at different tempera- tures were taken in the X-band on a Radiopan spectrometer equipped with a flowed-nitrogen-atmosphere heater

Results and Discussion The phase-transition temperatures and phase-transition en- thalpy changes for the compounds of series 1-10 are collected in Tables 1-4

Main series

Phase properties. Most of the compounds studied exhibited simple polymorphism For the homologues of the series 1 and 2 (Fig 1) only the nematic and smectic C phases are observed In agreement with their molecular elongation, both ligands and complexes reveal a wide enantiotropic uniaxial nematic phase with an almost 90 "C temperature range For complexes 2-9 and 2-10 the nematic phase is broader than in previously examined enamino ketone derivatives l4 The phase diagrams of the homologous series 1 and 2 are similar to those detected for other series of 1-phenyl-3-alkylaminoprop-2-en-1-one derivatives 14b For the ligands 1 the melting points monoton- ically decrease with increasing terminal alkyl chains length while for complexes 2 a shallow minimum of the melting temperatures is observed For both series 1 and 2, the long terminal alkyl chains depresses the clearing temperatures, destabilizing the nematic phase This effect is particularly strong for the complexes and suggests that the alkyl chain substituted on the nitrogen atom is slightly nonplanar with respect to the enamino ketone ring

The isotropisation entropy, which in terms of the Landau- de Gennes model reflects the magnitude of the critical contri- bution into the free energy of a system, shows a pronounced odd-even effect for the ligands 1 (Table 1) In the homologous series of organometallic compounds 2, the clearing entropy varies in a more complex way The initial, distinct entropy decrease is followed by an entropy rise with increasing terminal chain The shape of the thermal DSC isotropisation peaks for both ligands and complexes 1 and 2 is typical, showing the presence of the specific heat anomalies

( a )

150 -

'"1 0 0

L

250

150

I I

t L L Is0

5 n

15

Fig. 1 Phase diagram for (a) series 1 ligands and (b) senes 2 complexes

734 J Muter Chem , 1996, 6(5), 733-738

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by M

cMas

ter

Uni

vers

ity o

n 29

/10/

2014

14:

24:2

6.

View Article Online

Table 1 Phase transition temperatures ("C) and enthalpy changes in parentheses (kJ mol-') for ligands 1 and their copper(11) complexes 2

comp. Cry S C N Is0

1-3 1-4 1-5 1-6 1-7 1-8 1-9 1-10 1-11 1-12 1-13 1-15 1-18

2-3 2-4 2-5 2-6 2-7 2-8 2-9 2-10 2-1 1 2-12 2-13 2-15 2-18

135.8 (17.9) 141.9 (18.7) 137.2 (17.8) 132.4 (17.4) 127.5 (15.4) 122.7 (14.6) 117.9 (12.6) 112.5 (9.00) 107.0 (8.19) 102.0 (7.43 ) 97.6 (5.98) 91.2 (3.53) 97.1 (4.59)

195.3 (60.0) 160.0 (44.7) 141.8 (69.2) 125.2 (59.4) 136.4 (50.6) 121.5 (43.2) 116.6 (43.2)

115.8 (53.8) 116.2 (50.3) 122.7 (56.8) 125.5 (63.4) 118.5 (75.4)

108.0 (44.3)

-

-

- -

- -

-

121.6 (3.63) 126.0 (3.3) 128.0 (3.59) 131.7 (3.78) 133.3 (4.26) 131.9 (11.8)"

-

- - -

-

- - -

- -

-

122.6 (3.61) 130.2 (5.76)

0 182.1 (2.76) 0 165.2 (2.13)

166.9 (2.73) 0 157.3 (2.22)

157.4 (2.76) 0 151.4 (2.35)

151.8 (2.86) 148.8 (2.79)

0 148.3 (3.2) 145.0 (3.17) 145.2 (3.45)

0 140.7 (3.48) 134.8 (0.72)

256 (dec.) 0 235 (4.3)

221.5 (3.8) 213.6 (3.5)

0 214.9 (4.2) 206.6 (2.9)

0 202.7 (2.90) 0 194.1 (2.57) 0 189 (0.97)

185.9 (1.85) 0 182.5 (1.25) 0 172.2 (2.90) 0 159.2 (1.74)

"Subsequent peaks not resolved.

In both series 1 and 2 for the long homologues, besides the nematic phase the smectic C phase is observed as well. In the ligands (series 1) it appears for the terminal alkyl chain n= 10 and in the complexes (series 2) the smectic C is stabilized by the longer terminal alkyl substituents, (n 3 15).

Compounds of series 1 and 2 showed strong dichroism (yellow-orange) in the nematic and smectic phases. This points to high optical anisotropy and/or a relatively high degree of molecular orientation in the mesophases.

EPR studies. For rod-like mesogenic copper complexes, the organization of their mesophases has not yet been well eluci- dated. For complexes of the enamino ketones crystal molecular structures are not even known. However, from a comparison of the optical properties of some of the complexes with their parent ligands it was suggested that the ligand axes are not colinear. Distortion of the chelate core from planarity and pronounced intermolecular interactions have been suggested as responsible fa~t0rs . l~ ' To verify these assumptions further information needs to be obtained. In this work the EPR method was used to study the complex 2-18. Its spectra in dilute solution as well as in the condensed crystalline, smectic C, nematic and isotropic phases were analysed.

The parameters of the spectra in toluene solution (go= 2.114, A,= 1.4 mT) are close to those observed for copper complexes with a nitrogen-oxygen environment for copper ion and a planar trans configuration of the chelate core.I7 The spectrum of the solution in the glassy state is not informative because it is not sufficiently resolved. The EPR spectra for all condensed phases consisted of exchange narrowed lines and their evolution upon heating from crystal to the isotropic phase is shown in Fig. 2. The solid phase spectrum indicates the rhombicity of the magnetic parameters: g, = 2.168, g, = 2.048, gx=2.101. The isotropic phase spectrum appears to be incompletely averaged due to insufficiently fast rotational motion of the molecules.

For the mesophases, the spectrum of the Sc phase is axially symmetrical and computer simulation of the line shape (Fig. 3) provides parameters: g, = 2.056, A,, = 35 Oe; g, =gx = 2.1 1, AI = 155 Oe (A = EPR linewidthj. The high field y-line is related to

N

I

300 320 BlmT

Fig. 2 EPR spectra obtained for the polycrystalline, liquid crystalline and isotropic phases of the complex 2-18

I measured _cL

culated

3QO 320 BlmT

Fig. 3 Comparison of the experimental and simulated spectra for the Sc phase of the complex 2-18

the long molecular axis and the second line corresponds to the mean value between g-factors of the two short molecular axe~.'',~' Such a spectrum suggests an usual phase structure for the S, and S, with the long axes being parallel to each other and the short axes randomly distributed. Assuming axial symmetry of the molecular magnetic parameters, the g-tensor for the individual species can be calculated from the above presented smectic phase g data.20 For 2-18, gll'=2.164, gl'= 2.056 and the mean g-factor go'=2.092 were found. The last value points toward a planar trans configuration of the chelate core for the copper complex 2-18 not only in dilute solutions but also in magnetically concentrated mesophases.

In the nematic phase only one line was observed, coinciding with the position of the y-line for the Sc phase. This suggests a strong orientation of the long molecular axes along the external magnetic field, indicating that the anisotropy of the diamagnetic susceptibility exceeds the anisotropy of the para- magnetic susceptibility. This confirms previously reported results,21,22 that high diamagnetic anisotropy reflects the pres- ence of a large number aromatic rings in the organometallic complexes examined. It seems, that in the case of the enamino ketone copper complexes the four phenyl rings contained in the core are sufficient to orient the molecular long axes parallel to the magnetic field.23

Comparative series

In order to promote other mesophases and allow discussion of the influence of substituents and bridging groups on mesog- enity, some modifications were introduced into the structure of compounds 1 and 2. As the first modification, compounds 3 and 4 having an enamino ketone ring with reversed substitu- ents to the parent series 1 and 2 were synthesized. The resulting 'reversed core' compounds appear to have markedly higher

J . Mater. Chem., 1996, 6( 5 j, 733-738 735

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by M

cMas

ter

Uni

vers

ity o

n 29

/10/

2014

14:

24:2

6.

View Article Online

Table 2 Phase transition temperatures ("C) and enthalpy changes in parentheses (kJ mol ') for the reversed ligands 3 (azo) and 5 (azoxy) and their copper(I1) complexes, 4 and 6

comp Cry

3-3 3-5 3-1 1 4-3 4-5 4-1 1 5-11 6-1 1

161 9 (9 9) 162 5 (9 9) 136" 167" 143 3 (59 1) 156 1 (566) 922 (172)

141 5 (53 6)

S A

1948 (04) 197 3 (4 0) 1873 (114) -

-

145 5" 0

2046 (10 1) 137"

N

197 4 (2 2) - -

-

-

156" - -

2052 (1 7) 0 1998(19)

171 (dec) 180 8 (2 2) 170 4 (2 9)

0 177 8 (2 6)

Is0 -

"From microscopy

melting points It is also seen (Table2) that the phenyl substituent, if attached to the amino group, favours more ordered phases than if attached to the ketone group In both series of ligands and complexes an additional, not observed in the series 1 and 2, orthogonal smectic A phase appears In series 3, a narrow smectic A phase is stabilized by the short terminal alkyl chains In contrast, for complexes 4, the smectic A phase is stabilized for the longer homologues For the ligands 4, for the higher homologues below the smectic C phase, some tilted hexatic and crystalline smectic phases (most probably smectic F and smectic G) are also observed In addition to the differences in the mesophase sequence, the thermal stability of the mesophases and their chain length dependence is different for the alkylamino- (series 1 and 2) and the arylamino- (series 3 and 4) derivatives l5 In contrast to series 2, in series 4 the long terminal alkyl chains do not destroy the nematic phase This difference can be attributed to changes in simple geometrical factors Whether it is also connected to differences in electron density within the enamino ketone ring, cannot be decided based on our limited data

As a second modification, the azo group in some compounds of the reversed series 3, 4 was replaced by an azoxy group resulting in series 5 and 6 Such a change leads t0 an increase in the clearing temperatures, destabilization of the more ordered liquid crystalline phases and depression of the melting points More flexible moieties, such as -CH,O- were also tried as bridging groups However in this case, a significant destabilisation of mesophase thermal stability was observed As a result, only narrow enantiotropic nematic phases exist for the complexes (e g [Cu{H5C,0-CC,H, -CH20- C6H, -CO(CH)2N-C9H,9>2] has a melting point at 127 "C and an isotropisation temperature of 124 oC14a)

As a third modification, the hexyloxy group in the parent

series 1 and 2 was exchanged for a hexylamino group Not all of the resulting alkylamino- ligands of the homologous series 7 are liquid crystals (Table 3) The ligands (7) with an inter- mediate length of the terminal alkyl chain do not show mesfgenic properties, whereas for the shorter homologues the monotropic nematic phase was observed and for the longer ones only the smectic C phase appears In the complexes of the series 8 (Fig 4) the hexylamino group suppresses the clearing temperatures profoundly, and this is accompanied by a relatively smaller depression of the melting points As a result a very narrow enantiotropic nematic phase is detected However for some homologues (e g n = 3), the nematic phase, if supercooled, is observed even at 80°C, about 60°C below the clearing temperature The tendency of the long substituents

SC I I I

I 1

15 n 5

Fig. 4 Phase diagram for the series 8 complexes

Table 3 Phase transition temperatures ("C) and enthalpy changes in parentheses (in kJ mol I ) for ligands 7 and their copper(I1) complexes 8

comp Cry sc S A N Is0

7-2 7-3 7-4 7-5 7-6 7-8 7-10 7-12 7-1 8

8-2 8-3 8-4 8-5 8-6 8-8 8-10 8-12 8-18

1044 (247) 122 5 (17 2)

989 (32) 104 3 (5 7) 1000 (11 9) 974(177)

109 7 (15 9) 122 6 (30 6) 907 (422)

200" (dec ) 160" (dec)

130" 104 1 (37 2) 102 (37 0)

101 1 (39 4) 112 1 (45 5 )

99 4( 45 5)

- 0 103 (8 1)

- 0

- 0

- 0

- 0

- 0

90 6 (0 3) 0

108 l ( 6 5 ) 1048 (5 3) 101 1 (64)

106 (0 4) -

97 2 (0 4) 102 5 (0 6)

90"

228" (dec ) 145"

171 5" (dec) 149 8 (0 6) 125 3 (07) 1226 (1 1)

~~~~~ ~ ~ ~

"From microscopy

736 J Mater Chem , 1996, 6(5) , 733-738

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by M

cMas

ter

Uni

vers

ity o

n 29

/10/

2014

14:

24:2

6.

View Article Online

to destabilize the liquid crystalline phases is similar to series 2. The thermal stability of the compounds of series 8 is low and they are easily decomposed above 150°C.

It is worthwhile mentioning a phenomenon that is observed in the nematic phase for some homologues of the series 8. The conoscopic observation of a homeotropically aligned sample reveals a uniaxial nematic phase. However, when the sample is rapidly heated or cooled, the branches, visible in the conos- copic image (Fig. 5 ) separate, which is evidence of the biaxiality of the phase. The optical axes of the sample are located randomly and the angle between them depends on the rate of temperature change. In the orthoscopic observation the initially uniform dark texture becomes slightly lightened when the sample is biaxial. A similar effect can be obtained by subtle stress on a homeotropically aligned sample. The observed induced biaxiality can be plausibly explained as created by a faint horizontal flow of matter which tilts the initially vertically oriented molecules. The flow results from the stress or the temperature gradient.

Since almost all the compounds presented above have melting points close to 100°C or higher, to lower the melting temperatures compounds of series 9 and 10 were synthesized. For this purpose, in line with previous reports,24 molecules with fork-like tails were designed. The hexylamino group of the series 7 and 8 was exchanged for the N-hexyl-N-methylam- ino group. As expected, the resulting ligand series 9 and complex series 10 (Fig. 6) have distinctly lower melting points. However, the clearing temperatures were also suppressed. Unlike the amino series 7 all the compounds with N-methyl- ated ligands showed mesogenic properties, although in some cases only the monotropic nematic phase was detected (Table 4). In contrast to the series 1, 2, 4, 8 and 10, in the ligand series 9 the nematic phase is stabilized by increasing the length of the terminal alkyl chains, This reflects the comparatively smaller influence of the branching CH, group

'

75 -

50 -

Fig. 5 Conoscopic images of complex 8-3 for the homeotropically aligned (a) relaxed and ( b ) rapidly heated sample ( 10 "C min- ')

( a ) I I I I

,

Is0

v i=

125

100

75

115 n

5

Fig. 6 Phase diagram for (a) series 9 ligands and (b) series 10 complexes

Table 4 Phase transition temperatures ("C) and enthalpy changes in parentheses (kJ mol-l) for ligands 9 and their copper(11) complexes 10

comp. Cry S A N Is0

9-6 9-10 9-1 1 9-12 9-13 9-14 9-15 9-18

10-6 10-10 10-1 1 10-12 10-13 10-14 10-15 10-18

84.2 (41.4) 49.0 (13.3) 70.5 (36.3) 64.8 (34.7) 57.0 (33.3) 46.4 (37.9) 48.6 (38.4) 62.6 (52.0)

110.8 (38.1) 77.5 (53.8) 86.7 (68.7) 81.7 (50.4) 73.5 (55.5) 72.7 (52.0) 89.8 (71.1) 81.6 (72.5)

- 0 46.4 (0.5) - 0 58.9 (1.4) - 0 65.9 (2.1) - 0 65.6 (2.0) - 0 68.4 (2.0) - 0 67.7 (1.6) - 0 68.7 (3.9)

55.3 (0.8) 68.1 (2.7)

- 0 96.4 (1.1) - 0 101.9 (1.20) - 0 95.6 (1.2) - 0 94.1 (1.2) - 0 88.4 (0.97) - 0 87.6 (1.5) - 0 82.6 (2.1)

- 131.9 (1.5)

on the distortion of the molecular linearity for the longer species than for the shorter ones. For the complexes (10) only the nematic phase is observed and the mesophase stability is similar to the parent series 2.

Similarly to the main series 1 and 2, all compounds of the comparative series (3-10) showed dichroic properties (yellow- orange colour) in all liquid crystalline phases.

Conclusions We have synthesized several homologous series of elongated calamitic copper(I1) complexes and their parent ligands series is connected with subtle differences of molecular structure and electrical charge distribution.

Although the phase polymorphism depends on factors not entirely elucidated some general rules can be found. For the

J . Muter. Chem., 1996, 6(5) , 733-738 737

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by M

cMas

ter

Uni

vers

ity o

n 29

/10/

2014

14:

24:2

6.

View Article Online

ligands and their copper complexes, elongation of the alkyl chain N-substituted on the enamino ketone ring destabilizes mesophases This effect can be explained by the non-planar environment of the N-atom resulting in the non-colinear alignment of the alkyl chain with respect to the molecular core l4 In the series with reversed positions on the enamino ketone ring, where the alkyl chain is attached to the carbonyl group the influence of the terminal chain is less pronounced In this case planarity of the mesogenic core is induced by n- electron interactions between the enamino ketone and the phenyl rings l5

For the ligands and complexes, substitution on the alkyl- amino (hexylamino) group as the terminal chain significantly depresses both isotropisation temperatures and melting points The melting temperatures decrease especially strongly if a branched N-hexyl-N-methylamino chain is applied The influ- ence of this group is similar to that of the -CH(CH3)- moiety The branched fork-like tail containing the -N(CH3)- group seems to be promising for further syntheses of low- melting liquid crystals

These studies can throw light on the molecular organization of liquid-crystalline phases in organometallic compounds Previous X-ray measurements showed that the layer thickness in the smectic A phase formed by the enamino ketone Cu" complexes is significantly smaller than the molecular length (d/L-0 7-0 8) 14' This result was explained either by the non- planar structure of the chelate core or by interaction between the Cu" complexes, which form molecular clusters The present EPR studies of the Cu" complexes confirm the planarity of the enamino ketone chelate core This indicates that the mesophases of the complexes might be composed of small clusters rather than individual molecules Among possible stabilizing factors interactions of Cu 0, Cu N, Cu Cu type should be considered Copper-oxygen interactions are well known for crystals of some salicylaldymine copper com- plexes l1 25 26 Similar interactions for nickel complexes have also been reported 25 Copper-copper interactions were pre- viously assumed to be present in isotropic melts to explain EPR signals of polymeric salicylaldimine complexes 27

Significant interdigitation of alkyl chains of molecules from neighbouring layers (up to three carbon atoms) or a weak nematic orientational order parameter ( N 0 6) in the smectic layer could also be responsible for a d/L value< 1 However, high entropy loss associated with the interdigitation or con- siderably constrained in-plane translational motion of mol- ecules with strongly disordered molecular axes make both mechanisms less plausible 28

This work is a part of a research project sponsored by KBN grant 2P303 02407 Financial assistance for the EPR research part and scholarship for I Bikchantaev granted from the 12- 5O1/VII/BW-11301/33/95 is also acknowledged

References

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17 18

19

20

21

22 23

24

25

26

27

28

A Krowczynski, W Pyzuk and E Gorecka, Polish J Chem , 1994, 68,281 D W Bruce, D A Dunmurr, P M Maitlis, M M Manterfield and R Orr, J Muter Chem, 1991,1,255 D W Bruce, D A Dunmurr, S E Hunt, P M Maitlis and R Orr, J Muter Chem, 1991,1,857 C Cipparone, C Versace, D Duca, D Pucci, M Ghedini and C Umeton, Mol Cryst Liq Cryst , 1992,212,217 H Schultz, H Lehnann, M Rein and M Hanack, Struct Bonding (Berlin), 1991,74,43 G Albertini, A Guido, G Mancini, S Stizzaand and R Bartolino, Europhys Le t t , 1990,12,629 Yu G Galyametdinov, 0 N Kadkin and I V Ovtchinnikov, Izu Akad Nauk USSR, Ser Khim , 1992,12,402 (a) I V Ovtchinnikov, I G Bikchantaev, Yu G Galyametdinov and R M Galimov, in Proc 24th Ampere Congress, Poznan 1988, p 567, (b) I V Ovtchinnikov, Yu G Galyametdinov and I G Bikchantaev, Izu Akad Nauk USSR, Ser F i z , 1989,53,1870 P J Alonso, M Marcos, J I Martinez, V M Orera, M L Santjuan and J L Serrano, Liq Cryst , 1993,13,585 A M Giroud-Godquin and P M Maitlis, Angew Chem Int Ed Engl , 1991,30,375 D W Bruce, in Inorganic Materials, ed D W Bruce and D O'Hara, Wiley, England, 1992 (a) M Marcos, P Romero and J L Serrano, J Chem Soc , Chem Commun, 1989,1641, (b) J L Serrano, P Romero, M Marcos and P J Alonso, J Chem Soc, Chem Commun, 1990,859 W Pyzuk, A Krowczynski and E Gorecka, Liq Cryst , 1991, 10,593 (a) W Pyzuk, E Gorecka and A Krowczynski, Liq Cryst , 1992, 11, 797, (b) W Pyzuk, A Krowczynski, E Gorecka and J Przedmojski, Liq Cryst , 1993,14, 773 W Pyzuk, A Krowczynski and E Gorecka, Proc SPIE, 1992, 1845,277, Mol Cryst Liq Cryst , 1994,249, 17 G W Gray and J W G Goodby, in Smectic Liquid Crystal- Textures and Structures, Leonard Hill, Glasgow and London, 1984 H Yokoi, Bull Chem SOC Jpn , 1974,47,3037 R M Galimov, I G Bikchantaev and I V Ovtchinnikov, Z h Strukt Khim , 1989,30,65 R M Galimov, I G Bikchantaev, I V Ovtchinnikov and V N Konstantinov, Z h Strukt Khim , 1989,30,59 I G Bikchantaev, Yu G Galyametdinov and I V Ovtchinnikov, Z h Strukt Khim, 1987,28,61 J Barbera, A M Levelut, M Marcos, P Romero and J L Serrano, Liq Cryst , 1991,10,119 B Borchers and W Haase, Mol Cryst Liq Cryst , 1991,209,319 I G Bikchantaev, Yu G Galyametdinov, A Prosvinn, K Gnesar, E A Soto-Bustamante and W Haase, Liq Cryst , 1995,18,231 W Weissflog, G Pelzl, I Letko and S Diele, Mol Cryst Liq Cryst , 1995,260,157 B 0 West, in New Pathways in Inorganic Chemistry, ed E A V Ebsworth, A G Maddock and A G Sharpe, Cambridge University Press, 1968 H Tamura, K Ogawa, A Takeuchi and S Yamada, Chem L e t t , 1977,889, Bull Chem Soc Jpn , 1979,52,5322 M Marcos, L Oriol, J L Serrano, P J Alonso and J A Puertolas, Macromolecules, 1990,23,587 R Holyst, Phys Rev A , 1991,44,3692

738 J Muter Chem, 1996, 6(5) , 733-738

Publ

ishe

d on

01

Janu

ary

1996

. Dow

nloa

ded

by M

cMas

ter

Uni

vers

ity o

n 29

/10/

2014

14:

24:2

6.

View Article Online