AGGREGATION OF AN OCTASUBSTITUTED PHTHALOCYANINE IN DODECANE SOLUTION

The absorption spectra of a metal-free phthalocyanine octasubstituted with branched alkoxy chains, 2,3,9,10,16,17,23,24-octakis(3,7-dimethyloctoxy)phthalocyanine (PcH2(8,2)), in dodecane were analyzed in terms of monomer-dimer-trimer equilibria. The pure PcH2(8,2) derivative is known to have a columnar discotic mesophase at room temperature and forms a one-dimensional, self-assembled structure. In dodecane solution the same type of stacked aggregation is shown to exist with a high dimerization constant K1 = (1.50 +/- 0.04) X 10(6) M-1 and a trimerization constant K2 = (5.2 +/- 0.4) x 10(4) M-1. The absorption spectra of the monomer, dimer, and trimer in dodecane are determined. Good agreement of the assembling numbers with a modified molecular exciton theory is found. Using the aggregation constants and absorption spectra of the individual species obtained in solution, in the analysis of a spectrum of a Langmuir-Blodgett multilayer film of PcH2(8,2) a large assembling number in such a film is calculated. The origin of this strong aggregation is discussed and is probably connected with the liquid crystallinity of PcH2(8,2).