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Wide variety of dimensionality in phthalocyanine based molecular conductors
ELSEVIER
SyntheticMetab 86 (1997) 1799-1800
Wide variety of dimensionality
Department
of Chemistry,
in phthalocyanine
based molecular conductors
T. Inabe, K. Morimoto Faculty of Science, Hokkaido University,
Sapporo 060, Japan
Abstract We have been attempting the electrochemical oxidation of a cobalt(HI)phthalocyanine anion, [COB]‘, using various solvents, and various cations. It has been found that a rather wide variety of crystal structures can be obtained by slightly changing the counter part or solvent included. Each of the neutral radical conductors, Co(Pc)(CN)~~2CHBr3, CO(PC)(CN)~.~CHCI~, and C~(PC)(CN)~.~H~O, has completely different phthalocyanine stacking structures; the nelectron overlapping mode with systematically changes from one- to three-dimensional one. The conductivity has also been found to increase systematically increasing the dimensionality. Keywords:
Electrocrystallization,
Organic conductors based on neutral radical, Transport measurements
1. Introduction in molecular Multi-dimensional X--X overlapping conductors is important for maintaining the metallic state at We have been approaching to the low temperatures. construction of multi-dimensional I--L overlapping by using a nconjugated molecule which has projections at the center. Metal-phthalocyanines are the desirable component, since it is easy to introduce axial substituents. As shown in SCHEME, the axial substituents prevent direct overlap of the molecules; instead, the molecules need to slip a large distance to make contacts between the planar parts. This is expected to make the total overlap multi-dimensional. We have already found that electrochemical oxidation of the potassium salt of dicyanophthalocyaninatocobaIt(Il1). K[Co(Pc)(CN)2], gives K[C~(PIZ)(CN)~]~..~CH$N, in which the PC ligand is partially oxidized [ 1.21. This crystal is composed of two-dimensional sheets of Co(Pc)(CN)2 just like a stacking pattern shown in SCHEME. The cation and acetonitrile molecules are packed between the sheets. In order to study the influence of the cation and solvent on the crystal structure, we have been attempting the electrochemical oxidation of this anion using various solvents, and various 7 cations. In the course of this research, we have found that a \ rather wide variety of crystal q$$& structures can be obtained by slightly changing the counter U part or solvent included. SCHEME
short contacts with the cyan0 groups, are located between the PC chains so that the electronic interaction between them is prevented.
Fig . 1. Crystal structure of Co(Pc)(CN)2.2CHBr3. The temperature dependence of the electrical this crystal is semiconducting, and the resistivity
axis at room temperature is about I@ 62 cm. The resistivity perpendicular to the a-axis is more than two orders higher. A partially oxidized one-dimensional conductor is also the obtained by electrochemical oxidation of tetraphenylphosphonium salt [4].
3. Two-dimensional 2. One-dimensional
conductors
conductors
When the potassium salt was electrolyzed in a mixed solvent of acetonitrile and bromoform, the neutral radical crystal of Co(Pc)(CN)2.2CHBr3 is obtained [3]. In the crystal, the PC units form one-dimensional partial stacking along the (Iaxis, as shown in Fig. 1. Bromoform molecules, which have 0379-6779/97/$17.00 0 1997 Ebevier Science S.A. AlI rightsreserved PII so379-6779(%p4594-8
resistivity of along the u-
Electrochemical oxidation of the potassium salt in a mixed solvent of acetonitrile and chloroform gives another neutral radical crystal of Co(Pc)(CN)z.2CHC13 [5]. In this case, the PC units form a two-dimensional sheet parallel to the a-plane, as shown in Fig. 2. Two benzene rings in phthalocyanine overlap along the c-axis and one benzene ring overlap occurs
T. Inabe, K. Morimoto /Synthetic
1800
along the u-axis, Chloroform molecules are packed between the two-dimensional PC sheets, and form a hydrogen-bond with the cyano group.
Metals 86 (I 997) I 799- I800
Since the crystals obtained so far are small, we have not been able to determine the anisotropy of conduction from the nearly isotropic However, single-crystal measurements. conduction is suggested from the resistivity of the powder compacted sample (5 D cm); it is only slightly larger than that of the single crystal ( IO0 Q cm). Partially oxidized three-dimensional the PC units have not been obtained yet.
5. Comparison
of the neutral
conductors
radical
based
on
crystals
A systematic comparison between these three neutral radical crystals may give important information about this system. From a structural point of view, a common feature is the pattern of the molecular overlap; the existence of the axial cyano groups makes the II-JC overlap only a part of the whole
Fig.
2. Crystal structure of CO(PC)(CN)~.~CHCI~.
The electrical resistivity is, as expected from the structure, clearly anisotropic; the resistivities along the directions parallel to the two-dimensional PC sheet, -10’ Q cm. are more than two orders lower than that perpendicular to the sheet . Electrocrystallization of the same salt in acetonitrile only the aforementioned partially oxidized K[Co(Pc)gives (CN)&,XH#ZN [I ,2]. The sheet structure is essentially the same as that in C~(PC)(CN)~.~CHCI~.
4. Three-dimensional
conductors
Electrocrystallization of the potassium salt with a trace amount of water yields the third neutral radical crystal of Co(Pc)(CN)2.2H20 [2,5). The crystal structure is shown in Fig. 3. The crystal is monoclinic, space group C2/m. The water molecules in the crystal act as bridges between the PC Since units which are translationally related along the a-axis. the water molecules are aligned linearly with the axial substituents and the distance between the rings is nearly four times the thickness of aromatic rings, three other PC rings can be incorporated between those PC rings. Thus, a two benzene ring overlap exists along the _c-axis and a one benzene ring The overall IX-IC overlap along the [I 121 and [I 121 directions. interaction is completely three-dimensional.
n-conjugated system. The total n-n interaction in the crystal is, however, considerably different from each other; onedimensional in Co(Pc)(CN)2.2CHBr3, two-dimensional in Co(Pc)(CN)2.2CHCl3, and three-dimensional in Co(Pc)(CN)z2H20. Though the overlap is only partial and the distance between them is not so short (3.5-3.6 A), the conductivity data indicate that the n-n interaction is sufficient. As a matter of fact, their conductivities are much higher than that expected for neutral radical crystals. It can be noticed that the resistivity values at room temperature decrease with increasing the This fact strongly suggests that the dimensionality. conduction-limiting factor, on-site Coulomb repulsion energy, may depend on the dimensionality of the electronic system. The possibility of extrinsic conduction in these crystals The has been checked by thermoelectric power measurements. thermoelectric power has been found to be linearly correlated that the semiconduction in this crystal is to rt, indicating intrinsic in origin. Using the conductivity data, the electron to hole mobility ratio, tip+,, is obtained as nearly 1; both electrons and holes are suggested to contribute to the transport. we have found a wide variety of In conclusion, dimensionality in the PC based conductors, including highly Their resistivities are conducting neutral radical crystals. lower than those for other neutral radical solids, especially the value for Co(Pc)(CN)2.2H20, the lowest so far reported.
I@ P cm at room temperature,
is
This work was partly supported by a Grant-in-aid for Scientific Research from the Ministry of Education, Science and Culture, Japan, lzumi Science and Technology Foundation, and Tokuyama Science Foundation.
References
[II PI ]31 I41 Fig . 3. Crystal structure of Co(Pc)(CN)2.2HzO.
[51
T. lnabe and Y. Maruyama, Chem. Lett., (1989) 55. T. Inabe and Y. Maruyama, Bull. Chem. Sot. Jpn., 63 (1990) 2273. K. Morimoto and T. Inabe, Mol. Cryst. Liq. Cryst., 284 (1996) 291. H. Hasegawa, S. Takano, N. Miyajima, T. Inabe, Mol. Cryst. Liq. Cryst, in press. K. Morimoto and T. Inabe, J. Muter. Chem., 5 (1995) 1749.
SyntheticMetab 86 (1997) 1799-1800
Wide variety of dimensionality
Department
of Chemistry,
in phthalocyanine
based molecular conductors
T. Inabe, K. Morimoto Faculty of Science, Hokkaido University,
Sapporo 060, Japan
Abstract We have been attempting the electrochemical oxidation of a cobalt(HI)phthalocyanine anion, [COB]‘, using various solvents, and various cations. It has been found that a rather wide variety of crystal structures can be obtained by slightly changing the counter part or solvent included. Each of the neutral radical conductors, Co(Pc)(CN)~~2CHBr3, CO(PC)(CN)~.~CHCI~, and C~(PC)(CN)~.~H~O, has completely different phthalocyanine stacking structures; the nelectron overlapping mode with systematically changes from one- to three-dimensional one. The conductivity has also been found to increase systematically increasing the dimensionality. Keywords:
Electrocrystallization,
Organic conductors based on neutral radical, Transport measurements
1. Introduction in molecular Multi-dimensional X--X overlapping conductors is important for maintaining the metallic state at We have been approaching to the low temperatures. construction of multi-dimensional I--L overlapping by using a nconjugated molecule which has projections at the center. Metal-phthalocyanines are the desirable component, since it is easy to introduce axial substituents. As shown in SCHEME, the axial substituents prevent direct overlap of the molecules; instead, the molecules need to slip a large distance to make contacts between the planar parts. This is expected to make the total overlap multi-dimensional. We have already found that electrochemical oxidation of the potassium salt of dicyanophthalocyaninatocobaIt(Il1). K[Co(Pc)(CN)2], gives K[C~(PIZ)(CN)~]~..~CH$N, in which the PC ligand is partially oxidized [ 1.21. This crystal is composed of two-dimensional sheets of Co(Pc)(CN)2 just like a stacking pattern shown in SCHEME. The cation and acetonitrile molecules are packed between the sheets. In order to study the influence of the cation and solvent on the crystal structure, we have been attempting the electrochemical oxidation of this anion using various solvents, and various 7 cations. In the course of this research, we have found that a \ rather wide variety of crystal q$$& structures can be obtained by slightly changing the counter U part or solvent included. SCHEME
short contacts with the cyan0 groups, are located between the PC chains so that the electronic interaction between them is prevented.
Fig . 1. Crystal structure of Co(Pc)(CN)2.2CHBr3. The temperature dependence of the electrical this crystal is semiconducting, and the resistivity
axis at room temperature is about I@ 62 cm. The resistivity perpendicular to the a-axis is more than two orders higher. A partially oxidized one-dimensional conductor is also the obtained by electrochemical oxidation of tetraphenylphosphonium salt [4].
3. Two-dimensional 2. One-dimensional
conductors
conductors
When the potassium salt was electrolyzed in a mixed solvent of acetonitrile and bromoform, the neutral radical crystal of Co(Pc)(CN)2.2CHBr3 is obtained [3]. In the crystal, the PC units form one-dimensional partial stacking along the (Iaxis, as shown in Fig. 1. Bromoform molecules, which have 0379-6779/97/$17.00 0 1997 Ebevier Science S.A. AlI rightsreserved PII so379-6779(%p4594-8
resistivity of along the u-
Electrochemical oxidation of the potassium salt in a mixed solvent of acetonitrile and chloroform gives another neutral radical crystal of Co(Pc)(CN)z.2CHC13 [5]. In this case, the PC units form a two-dimensional sheet parallel to the a-plane, as shown in Fig. 2. Two benzene rings in phthalocyanine overlap along the c-axis and one benzene ring overlap occurs
T. Inabe, K. Morimoto /Synthetic
1800
along the u-axis, Chloroform molecules are packed between the two-dimensional PC sheets, and form a hydrogen-bond with the cyano group.
Metals 86 (I 997) I 799- I800
Since the crystals obtained so far are small, we have not been able to determine the anisotropy of conduction from the nearly isotropic However, single-crystal measurements. conduction is suggested from the resistivity of the powder compacted sample (5 D cm); it is only slightly larger than that of the single crystal ( IO0 Q cm). Partially oxidized three-dimensional the PC units have not been obtained yet.
5. Comparison
of the neutral
conductors
radical
based
on
crystals
A systematic comparison between these three neutral radical crystals may give important information about this system. From a structural point of view, a common feature is the pattern of the molecular overlap; the existence of the axial cyano groups makes the II-JC overlap only a part of the whole
Fig.
2. Crystal structure of CO(PC)(CN)~.~CHCI~.
The electrical resistivity is, as expected from the structure, clearly anisotropic; the resistivities along the directions parallel to the two-dimensional PC sheet, -10’ Q cm. are more than two orders lower than that perpendicular to the sheet . Electrocrystallization of the same salt in acetonitrile only the aforementioned partially oxidized K[Co(Pc)gives (CN)&,XH#ZN [I ,2]. The sheet structure is essentially the same as that in C~(PC)(CN)~.~CHCI~.
4. Three-dimensional
conductors
Electrocrystallization of the potassium salt with a trace amount of water yields the third neutral radical crystal of Co(Pc)(CN)2.2H20 [2,5). The crystal structure is shown in Fig. 3. The crystal is monoclinic, space group C2/m. The water molecules in the crystal act as bridges between the PC Since units which are translationally related along the a-axis. the water molecules are aligned linearly with the axial substituents and the distance between the rings is nearly four times the thickness of aromatic rings, three other PC rings can be incorporated between those PC rings. Thus, a two benzene ring overlap exists along the _c-axis and a one benzene ring The overall IX-IC overlap along the [I 121 and [I 121 directions. interaction is completely three-dimensional.
n-conjugated system. The total n-n interaction in the crystal is, however, considerably different from each other; onedimensional in Co(Pc)(CN)2.2CHBr3, two-dimensional in Co(Pc)(CN)2.2CHCl3, and three-dimensional in Co(Pc)(CN)z2H20. Though the overlap is only partial and the distance between them is not so short (3.5-3.6 A), the conductivity data indicate that the n-n interaction is sufficient. As a matter of fact, their conductivities are much higher than that expected for neutral radical crystals. It can be noticed that the resistivity values at room temperature decrease with increasing the This fact strongly suggests that the dimensionality. conduction-limiting factor, on-site Coulomb repulsion energy, may depend on the dimensionality of the electronic system. The possibility of extrinsic conduction in these crystals The has been checked by thermoelectric power measurements. thermoelectric power has been found to be linearly correlated that the semiconduction in this crystal is to rt, indicating intrinsic in origin. Using the conductivity data, the electron to hole mobility ratio, tip+,, is obtained as nearly 1; both electrons and holes are suggested to contribute to the transport. we have found a wide variety of In conclusion, dimensionality in the PC based conductors, including highly Their resistivities are conducting neutral radical crystals. lower than those for other neutral radical solids, especially the value for Co(Pc)(CN)2.2H20, the lowest so far reported.
I@ P cm at room temperature,
is
This work was partly supported by a Grant-in-aid for Scientific Research from the Ministry of Education, Science and Culture, Japan, lzumi Science and Technology Foundation, and Tokuyama Science Foundation.
References
[II PI ]31 I41 Fig . 3. Crystal structure of Co(Pc)(CN)2.2HzO.
[51
T. lnabe and Y. Maruyama, Chem. Lett., (1989) 55. T. Inabe and Y. Maruyama, Bull. Chem. Sot. Jpn., 63 (1990) 2273. K. Morimoto and T. Inabe, Mol. Cryst. Liq. Cryst., 284 (1996) 291. H. Hasegawa, S. Takano, N. Miyajima, T. Inabe, Mol. Cryst. Liq. Cryst, in press. K. Morimoto and T. Inabe, J. Muter. Chem., 5 (1995) 1749.