Preparation and Polymorphism of Thin Films of Unsubstituted Cobalt Phthalocyanine

Abstract

A new method for preparing thin films of unsubstituted cobalt phthalocyanine (CoPc) is reported. CoPc was dissolved in mixed media consisting of trifluoroacetic or trichloroacetic acid and trichloromethane and spread onto different aqueous subphases. In this way, thin solid films were formed on the surface that exhibit no surface pressure and break readily under mechanical stress. Films were transferred to hydrophobic glass plates by using a horizontal lifting method. Changing the subphase pH led to three different film phases (I−III) that were characterized essentially by electronic spectroscopy of monolayers transferred on solid substrates and Brewster angle microscopy of monolayers floating on the subphase as well. The narrow Q-band absorptions exhibited by phases II and III reveal increased ordering in the film structure. The strong red-shift in the Q-band for phase II reveals lateral stacking of the molecules, which would be expected for trivalent rather than for divalent metallophthalocyanines. Polarized electronic absorption spectra recorded under oblique incidence were consistent with flat-lying molecules in all cases. Heating transferred films of phase II or III produced an additional film phase, IV, that shows marked in-plane anisotropy.