ENFOQUE CULTURAL E INTERCULTURAL
3.5. La interculturalidad de la Reforma Educativa vista por los académicos
log a = log(VR x L/a)
where
a - conductivity (Scm
R
- resistance(s)
L = thickness of crystal (cm)
2
A - area of crystal surface (cm )
lOOO/T
(K1)
(Scm“ l)
log-a3.34
-3.29
3.38
-3.39
3.44
-3.45
3.50
-3.52
3.63
-3.61
3.74
-3.71
3.85
-3.77
3.92
-3.85
3.99
-3.91
4.07
-3.97
4.16
-4.05
4.29
-4.15
4.37
-4.23
4.42
-4.37
4.48
-4.43
lOOO/T' log o ( i f ) (Scm )4.56
-4.47
4.61
-4.55
4.72
-4.66
4.82
-4-77
4.90
-4.83
4.95
-4.98
5.15
-5.09
5.29
-5.24
5.43
-5.38
5.58
-5.55
5.75
-5.73
5.92
-5.90
6.09
-6.08
6.29
-6.29
6.49
-6.49
The charge-transfer complexes of the strong tt-acceptor, 7»7»8,8-
tetracyanoquinodimethane, (TCNQ), exhibit unusual physical properties of which the most significant is their anisotropic electrical con ductivities, It was in order to more fully understand the relationship between solid state structure and electrical conductivity of such TCNQ
complexes that the crystal structures of selected complexes have been studied in the present work,
(i) Dimethyldibenzophospholium Bis-7>7,8,8-tetracyanoquinodimethanide,
This semi-conductor belongs to the pseudosoric class of TCNQ complexes. The single crystal analysis shows the TCNQ molecules to be arranged in
columns containing diadic units. The latter are held together by a network of short intermolecular contacts thereby forming two-
dimensional sheets. The resulting two-dimensionality, the lack of any disorder in the phospholium cation, and the presence of poor over lapping between adjacent TCNQ moieties lead to the low conductivity associated with the complex. The lack of disorder in the phospholium ion is attributed to its planarity and rigidity.
(ii) Dimethyldiphenylphosphonium Bis-7,7,8,8-tetracyanoquinodimethanide, (DMBP)(TCNQ), (I);
J 2
(i) 95 (Me2Ph2P)(TCNQ)2 , (ii); 179(n)
This is a good semi-conductor belonging to the homosoric class of TCNQ
complexes. The preliminary X-ray photographs indicate the presence of disorder in the lattice and this has been attributed to a statistical
disorder of the phosphonium cation. At low temperatures the TCNQ
stacks appear to be diadic in nature, but as the temperature is raised, so the cation becomes more disordered, thereby facilitating a one dimensional distortion of the TCNQ stacks which become monadic in nature. The room temperature crystal structure indicates an inter mediate monadic/diadic stacking arrangement with average inter-TCNQ
o
distances of 3.25 and 3-30A.
The related dimethyldiphenyl and methylethyldiphenylphosphonium TCNQ complexes have been examined photographically in order to relate the influence of different substituents in the phosphonium ions upon the structure adopted and thus upon the observed conductivity. The weak l'ayer-lines present in (MegEhgP) (TCNQ)g are not observed in room temperature oscillation photographs of (Et^Th^P)(TCNQ^ and this is attributed to a more homosoric stacking in the latter complex, in keeping with its higher conductivity. The relative conductivities of such phosphonium TCNQ complexes is very much dependent upon the flex ibility and capacity to disorder of the cation. As such it would be a
and, if appropriate, single crystal analyses of a series of such com plexes over a range of temperatures.
(iii) Dicyanophenyl-4,4'-bipyridylium Tetrakis-7,7,3,8-tetracyano- quinodimethanide, (DC0BP)(TCNQ)^ (ill);
(h i )
95
This is a poor semi-conductor which belongs to the pseudosoric class of TCNQ, complexes. The crystal structure shows the TCNQ molecules to be arranged in columns parallel to the b-axis and in sheets parallel to the ab plane. The TCNQ molecules in each column are arranged in tetrads but the poor overlap present in these units indicates that the stacking arrangement is better described as poorly overlapping diads. Extensive electrostatic interactions between the anion and the cation stacks increase the tri-dimensionality of the system and this has been attributed to the presence of the strongly electron-withdrawing
cyanophenyl functions. It would be of interest to examine related bipyridinium TCNQ complexes in order to more fully investigate the in fluence of both length of cation and also of the presence of electron- withdrawing groups upon the solid state structures adopted and the resulting electrical conductivities.
(iv) Diethy1-4»4^-bipyridiniumethane Tetrakis-7,7,8,8-tetracyano- quinodimethanide, (DEPA)(TCNQ)^, (IV);
2 \
CH2 ~ 0 ~ CH2_CH3
(i?)
95