Copper Octacarboxyphthalocyanine Aggregation in Buffered Solutions

Results of spectral UV-Vis absorption investigations on association of copper octa-carboxyphthalocyanine, CuPcOC, are reported. Influence of pH on association is discussed. In pH < 5.5 only ag gre gates are present. Monomer appears above pH = 5.5 and at pH Greater-than or equal to 6.2 only monomeric Pc forms exist. CuPcOC exhibits higher tendency to wards association in phosphate buffer solutions than in redistilled water. In pH 5.9–6.0 monomers, dimers and trimers coexist in equilibrium in 1.0×10–6 – 1.2×10–5 mol/dm3 concentration range at 25°C. The average assembling number n = 3 was determined for pH 5.6, 5.9 and 6.0 solutions. The spectrum of aggregate was calculated with two maxima at gammamax approx. 600 nm and 630 nm attributed to trimer and dimer band, respectively. Decrease of pH causes an in crease of cumulated association constant Kn. Kn of or der of 109 (dm3/mol)2 was calculated for pH 6.0 and 1010 (dm3/mol)2 for pH 5.9. For pH 5.6 considerably higher Kn was evaluated. A strong influence of pH on CuPcOC spectrum and equilibrium constants suggests that be sides pi-pi inter actions hydrogen bonding between Pc molecules can also occur.