THE EFFECT OF STRUCTURAL MODIFICATIONS ON CHARGE MIGRATION IN MESOMORPHIC PHTHALOCYANINES

The conductive properties of metal-free phthalocyanines, peripherally octa-substituted with n-alkoxy, n-alkoxymethyl, n-alkyl, and branched alkoxy hydrocarbon chains have been studied using the pulse radiolysis time-resolved microwave conductivity technique within the temperature range of -100 to +200 degrees C. For n-alkoxy-Pcs the sum of the charge carrier mobilities, Sigma mu(s), within organized columnar domains is estimated to be 8.5 x 10(-6) m(2)/Vs at room temperature in the crystalline solid (K phase). This corresponds to a Pc-to-Pc intracolumnar jump time for one-dimensional charge migration of 0.14 ps. Sigma mu(s), increases slightly with temperature in the K phase (activation energy ca. 0.03 eV) up to the K --> D (discotic mesophase) transition at which it decreases abruptly by a factor of ca. 5 despite the change from tilted to horizontal stacking of the Pc macrocycles. In the D phase of the n-alkoxy-Pcs, Sigma mu(s), is almost independent of temperature. The charge mobility in the tilted columnar mesophase of a n-alkoxymethyl-Pc is close to that found in the horizontally stacked mesophase of the n-alkoxy-Pcs. At the D --> I (isotropic liquid phase) transition of a triply-branched alkoxy derivative Sigma mu(s) decreases by a factor of ca. 7 to 0.2 X 10(-6) m(2)/Vs which is still much higher than expected for transport via molecular ion displacement.