Macromol. Rapid Commun. 16, 1- 7 (1995) 1

Generation of a spin polymer through a charge-transfer complex of a ferrocenyl mesogen with tetracyanoethylene in the solid state

Hitoshi Tanaka: Kazuya Mizota

Department of Chemical Science and Technology, Faculty of Engineering, Tokushima University, 2-1 Minamijosanjima, Tokushima 770, Japan

(Received: September 12, 1994)

SUMMARY Some disubstituted ferrocenes and a polymer with flexible pendant ferrocenyl mesogens were

found to form a complex with tetracyanoethylene to give a spin molecule even in the solid state at ambient temperature.

Introduction

Metal-containing orientated polymers, such as liquid-crystalline polymers, are an interesting class of polymers. For instance, such polymers sometimes form a columnar liquid-crystal phase and the magnetic anisotropy of the polymers is systematically controlled by including different transition metals in the polymer structure3). In addition, highly orientated polymers with unpaired electrons in the structure, e. g. so- called liquid-crystalline spin polymers, are expected to represent anisotropic electronic conductivity4) and magnetism 5, as already known for small molecules. In particular, the formation of charge transfer (CT) complexes of liquid-crystalline molecules is of interest in the field of molecular devices6). There has been little known, however, on the generation and properties of unpaired electrons in such metal-containing poly- mers.

In the previous paper’), we prepared organic mesogenic spin polymers and studied their thermal properties. It was found that the phenoxy or anilino radical sites attached to the side chain of these polymers were not suitable as spin centers for the study of the spin structure because of their thermal unstability. Therefore, it was desirable to prepare more stable spin molecules. Recently, Miller et al. reported that electron- donating organic metals such as Fe(C5Me5),*)and V(C,H6),9) form a CT complex with tetracyanoethylene (TCNE) to give high-spin molecules.

In this paper we deal with the generation of unpaired electrons by the complexation of mesogenic ferrocenes 1, 2 and 3 with TCNE in the solid state.

Macromol. Rapid Commun. 16, No. 1 , January 1995

0 1995, Hiithig & Wepf Verlag, Zug CCC 1022-1336/95/$02.50

2 H. Tanaka. K . Mizota

0

R-0-C "-9 Fe

0

K-01* F'

l a (n = 5) l b (n = 9) I C ( n = 10) I d ( n = 15)

R: -1 \-c=N 1 \-O+CH,S;T-CH, Q H -Q

2a (n = 5 ) 2 c (n = 10)

2 b (n = 9)

I C-O+CH,+O--/ \--N=C--/ \-O-C II 0 H Q ''-0

I Fe

0 3 a ( n = 6 ) 3 b ( n = I I ) l

Experimental part

Compounds 2 and 1 were prepared by the condensation of either the ferrocene-1-carbonyl chloride or 1 ,l'-dicarbonyl chloride lo) with various phenol derivatives such as 4-(4-n-alkoxy- phenylisocyano)phenoIs, which were obtained by the reaction of 4-n-alkoxyaniline ') and 4-hydroxybenzaldehyde in methanol near ambient temperature. Compounds 3 were prepared by the dimethyl 2,2'-azobisisobutyrate-initiated radical polymerization of the corresponding acrylate monomers, which were synthesized similarly to the compounds 2 by using 4-(w-methacryloyl- oxy-n-alkoxyaniline a)7) instead of n-alkoxyaniline. Molecular weights of the polymers used were a, = 7700 and mw = 1 1 500 for 3a and a, = 9300 and a, = 12000 for 3b.

'H NMR (tetramethylsilane in CDCI,): l a : 6 = 0,92 (t; J =6,4 Hz, 6H), 1,33 (s; 16H), 3,98 (t; J = 6,O Hz, 4H), 4,56 (s; 4H), 5,02 (s; 4H), 6,60-7,92 (rn; 16H), 8,30 (s; 2H).

(s; 4H), 6,75-8,00 (m; 16H), 8,28 (s; 2H).

(s; 4H), 6,67-7,97 (m; 16H), 8,35 (s; 2H).

(5; 4H), 6,82-7,93 (m; 16H), 8,45 (s; 2H).

6,80-8,00 (m; 8H), 8,38 (s; 1 H).

(s; 2H), 6,78-7,98 (m; 8H), 8,32 (s; 1 H).

1 b: 6 = 0,94 (t; J = 6,4 Hz, 6H), 1,29 (s; 32H), 3,98 (t; J = 6,O Hz, 4H), 4,58 (s; 4H), 5,02

Ic: 6 = 0,92 (t; J = 6,4 Hz, 6H), 1,28 (s; 36H), 3,95 (t; J = 6,O Hz, 4H), 457 (s; 4H), 5,02

I d : 6 = 0,88 (t; J = 6,4 Hz, 6H), 1,26 (s; 56H), 3,97 (t; J = 6,O Hz, 4H), 4,64 (s; 4H), 5,09

2a: 6 = 0,95, 1,32 (s; 8H), 4,OO (t; J = 6,O Hz, 2H), 4,221 (s; SH), 4,46 (s; 2H), 4,95 (s; 2H),

2b: 6 = 0,93 (s; 3H), 1,29 (s; 16H), 4,Ol (t; J = 6,O Hz, 2H), 4,26 (s; 5H), 4,46 (s; 2H), 4,93

a) System. name: w-(4-aminophenoxy)-n-alkyl methacrylate.

Generation of a spin polymer through a charge-transfer complex . . . 3

2c: 6 = 0,94 ( s ; 3H), 1,28 ( s ; 18H), 3,99 (t; J = 6,O Hz, 2H), 4,25 ( s ; 5H), 4,46 ( s ; 2H), 4,92

3a: 6 = 1,47 (s; 8H), 1,67 (s; 2H), 1,79 (s; 3 H), 3,95 (s; 4H), 4,28 (s; 5H), 4,48 (s; 2H), 4,94

3 b : 6 = 1,33(s; 18H), 1,64(s;2H), 1,78(s;3H), 3,95(s; 4 H ) , 4 , 2 9 ( ~ ; 5 H ) , 4 , 4 9 ( ~ ; 2 H ) , 4 , 9 5

(s; 2H), 6,72-8,04 (m; 8 H), 8,40 (s; 1 H).

(s; 2H), 6,89 (s; 2H), 7,08-7,14 (m; 4H), 7,90 (s; 2H), 8,42 (s; 1H).

(s; 2H), 6,90 (s; 2H), 7,20 (s; 2H), 7,26 (s; 2H), 7,92 (s; 2H), 8,45 (s; 1 H).

C,,H,,N,O,Fe ( la: 832,8)

C,,H,,N,O,Fe (1 b: 945,O)

C,&,,N,O,Fe (IC: 973,l)

C,,H9,N206Fe (Id: 1 113,4)

C3,H3,N03Fe (2a: 509,4)

C,,H39N0,Fe (2b: 565,5)

C3,H,,N03Fe (2c: 579,6)

(C,,H,,NO,Fe), (3a: 593,5),

(C3,H,,NOSFe), (3b: 663,6),

Calc. Found

Calc. Found

Calc. Found

Calc. Found

Calc. Found

Calc. Found

Calc. Found

Calc. Found

Calc. Found

C 72,11 H 6,29 N 3,36 C 71,98 H 6,06 N 3,28

C 73,72 H 7,25 N 2,96 C 73,48 H 7,18 N 3,15

C 74,06 H 7,46 N 2,88 C 73,85 H 7,47 N 2,84

C 75,52 H 8,33 N 2,52 C 75,33 H 8,37 N 2,47

C 70,73 H 6,13 N 2,75 C 70,72 H 5,95 N 2,70

C 72,21 H 6,95 N 2,48 C 71,92 H 6,99 N 2,43

C 72,54 H 7,13 N 2,42 C 72,32 H 7,31 N 2,45

C 68,81 H 5,94 N 2,36 C 68,67 H 5,80 N 2,55

C 70,59 H 6,83 N 2,11 C 70,73 H 6,95 N 1,95

Thermal analysis was performed with a differential scanning calorimeter (Shimazu DSC-50) at a scan rate of lO"C/min under a nitrogen atmosphere. Mesogenic behavior of the materials was confirmed by optical microscopy using an Olympus polarizing microscope fitted with a Linkam TH-600PH heating stage. Specific conductivity was measured by using a YHP impedance analyzer for the disk-like sample which was prepared by mixing and pressing (300 kg/cm2) the powdered donor and acceptor molecules. The ESR spectra were recorded on a JEOL FE-2XG spectrometer equipped with an X-band microwave unit and 100 kHz field modulation. Splitting constant (a ) and g value were determined by comparison with those of Fremy's salt (a = 13,09 G and g = 2,0055) in aqueous K,CO, solution'*). Spin concentration of a complex was estimated by ESR spectroscopy standardized with 1,3,5-triphenylverdazyl and Mn2+ at ambient temperature. The 'H NMR spectra were measured on a JNM-EX400 (400 MHz) spectrometer in CDCl, at ambient temperature. Molecular weights of the polymers obtained were determined by size exclusion chromatography using a Toyosoda HLC-802A on the basis of the calibration curve established with standard polystyrene in tetrahydrofuran at 38 "C.

4 H. Tanaka, K . Mizota

Results and discussion

Fig. 1 shows the differential scanning calorimetry (DSC) charts of 1 c, 2a and 3a Two endotherms are observed in 1 c in the cooling cycle, but 2a and 3 a show only endotherm which corresponds to the melting point of these compounds. The observed endotherms in l c at 115, l and 129,l "C as well as in I d at 116,l and 128,3 "C in the cooling cycle seem to be due to the transitions from liquid crystal to crystal and from isotropic liquid to liquid crystal, respectively. All mono-substituted compounds 2a- 2c and both polymers do not show such transitions, and one broad endotherm was observed for the disubstituted compounds 1 a and 1 b. Monosubstituted ferrocenyl compounds have already been reported not to form a liquid-crystal phaseI3), which agrees with the present result. The optical micrograph between crossed polarizer confirms the liquid- crystalline nature of the opaque fluid of l c as can be seen in Fig. 2. This batonnet texture observed between two endothermic peaks in DSC may correspond to a smectic A (S,) phase. The same texture as for 1 c was also observed for 1 d, but 1 a and 1 b d o not show clearly a liquid-crystalline nature in micrography as well as in the DSC charts although the liquid-crystalline texture is sometimes observed momentarily near the melting temperature. Generation of the unpaired electrons was carried out by complexing these ferrocenyl

compounds (50 mol-Yo) with electron-accepting reagents (50 mol-%) such as TCNE. In the solid state, a mixture of equimolar amounts of TCNE and small ferrocenes such as unsubstituted ferrocene and 1 , l '-disubstituted ferrocene-carboxylic acid produced negligible amounts of unpaired electrons. Similarly, the mixture of either mono- substituted mesogenic compounds 2a-2c or polymer 3a with TCNE represented only a very small broad peak in the ESR spectrum at ambient temperature, although

heating cycle (lcl

I I I

Temperature in O C

I 100 200

Fig. 1 . and 3a

DSC curves of l c , 2a

Generation of a spin polymer through a charge-transfer complex 5

Fig. 2. Optical micrograph of l c between crossed polarizers at 120°C ( x 200)

elevating the temperature near to the melting point of their compounds accelerated the generation of unpaired electrons in some cases. Strong absorption with hyperfine structure, however, is observed in the ESR spectra of the mixtures of either disubstituted compounds 1 b -1 d or polymer 3 b with TCNE as can be typically seen in Fig. 3 (A). The observed spectrum (A) exhibits a triplet of quartet, which agrees with the computer simulated spectrum which is calculated by assuming a TCNE anion radical with the coupling constant uN (CN) = 4,48 G (g = 2,0027). The uN and g values of the TCNE anion radical which is produced by the reaction of TCNE and NaI or NaSCN in tetrahydrofuran are reported to be 1,56 G and 2,0026, respectively'4). The larger uN value obtained may be due to the rotational restriction '*) between cyano group and central C-C moiety and to the slow intermolecular electron-exchange of the TCNE anion radical in the solid state. In addition, the absorption is gradually broadened and reduced with decreasing temperatuure as can be seen in Fig. 3 probably because of preferential spin-spin relaxation, anti-ferromagnetic spin-spin interaction (exchange integral J < 0) and decrease of the concentration of the CT complex.

In solutions such as toluene, dioxane and dichloromethane such a strong absorption was not observed, and only a very small peak (broad singlet for l a - l d and 2a-2c, and multiplet for polymers) was detected in the ESR spectra at ambient temperature. At lower temperature, however, 1 c and 1 d give a strong sharp absorption with hyperfine structure in dichloromethane in contrast to other organic solvents as indicated in Fig. 4. Splitting constant and g value of this radical are estimated to be uN = 1,57 G and g = 2,0027 at - 35 "C. The ESR parameters obtained in dichloromethane resemble very closely to the reported value (a, = 1,56 G and g = 2,0026) 14) for the TCNE anion radical. These phenomena observed in dichloromethane may be due to spin- lattice relaxation, ferromagnetic spin-spin interaction ( J > 0) and increase of the concentration of the CT complex at lower temperature. The ferromagnetic interaction of the metalocene/TCNE complex observed only in dichloromethane near ambient temperature9) may give a clue to understand the interesting behavior of l c and l d .

6 H. Tanaka, K. Mizota

Fig. 3. 3 b/TCNE complex in the solid state at 23 "C (A), 0 "C (B) and - 100 "C (C)

ESR spectra of the

Fig. 4. 1 c/TCNE complex in dichloromethane at -35 "C

ESR spectrum of the

It has been known that ferrocene forms a CT complex with TCNE when they are in contact with each other in an appropriate space position, namely sandwich structure 17) , in a single-crystal state. This indicates the importance of the microstructure of each donor and acceptor, and of the entropy of the system. Therefore, bi-functional donor molecules, i. e. liquid-crystalline ferrocenes with two flexible mesogenic units (2 b-2d) or the polymer with plural flexible mesogenic ferrocenes in adjacent space position (3 b), seem particularly to be advantageous for the stacked sandwich structure to generate unpaired electrons. Furthermore, it is roughly speculated that in an entropy-controlled system the mobility-restricted media such as the solid state are sometimes more favorable to the complex formation than a fluid medium. It was also found that these complexes have a spin concentration of 10'8-10'9 spins/g and show a specific resistance of 10'o-1013 Q . cm at ambient temperature.

Generation of a spin polymer through a charge-transfer complex . . . 7

This research was supported in part by a Grant-in-Aid for Scientific Research (No. 06651030) from the Ministry of Education, Japan, to which the author (H.T.) is grateful.

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