EVOLUTION OF THE PHOTOACTIVITY OF CHLOROALUMINUM PHTHALOCYANINE AND RELATED MATERIALS UPON HEAT-TREATMENT

Thin films and powders of chloroaluminum phthalocyanine (ClAlPc) have been heat treated at 300-degrees-C over various time periods (1-60 min). The same treatment has been applied to a highly photoactive ClAlPc, which is both structurally and chemically modified by anion uptake (Gastonguay, L.; et al. J. Electrochem. Soc. 1992,139, 337). The evolution of the photoactivity of the films (monitored photoelectrochemically) as well as their absorbance in the visible and near-infrared (NIR), their surface morphology, and their crystalline structure have been examined upon heating. Weight losses of the powders and their differential thermal calorimetry have also been measured. After heat treatments at 300-degrees-C for 60 min the following characteristics are obtained for all films: (i) a blue-shifted absorbance in the visible (the maximum of the Q band is at 630 nm); (ii) a low photoactivity of the resulting stable phase; (iii) the disappearance of the absorbance in the NIR for the structurally and chemically modified ClAlPc. This goes along with a drop of the photoactivity by at least one order of magnitude; (iv) a common film morphology (sharp-cornered parallelepipedic needles perpendicular to the substrate); (v) the same electron diffraction pattern for all films, characterized by four interplanar distances for d greater-than-or-equal-to 5 angstrom. The electron diffraction patterns may be interpreted either in terms of a slight modification of the triclinic lattice reported for ClAlPc single crystal or in terms of a cofacial stacking of the ClAlPc molecules. Heat treatments at 300-degrees-C of ClAlPc thin films also demonstrate that Pc layers sublimed at 2000 angstrom min-1 are amorphous or organized only at very short range, for more than 75% of the film. This characteristic is responsible for the easy diffusion of H+ in the disorganized phase, followed by its reaction with O2- and the uptake of anions which compensates for the loss of negative charges in highly photoactive structurally and chemically modified ClAlPc. The anions (Cl-, Br-, I3-, I-) taken up during the modification of ClAlPc are lost as hydrogen halides or I2 during the heat treatment. The maximum loss rate happens at temperatures between 130 and 150-degrees-C depending upon the anion involved.