Solution and thin-film aggregation studies of octasubstituted dendritic phthalocyanines

The synthesis and solution and thin-film characterization of eight octasubstituted dendritic phthalocyanines (Pcs) and their zinc complexes are reported. The Pc chromophore was substituted in the 2,3,9,10,16,17,23,24- positions with three generations of benzylaryl ether dendrons with either a benzyl (3a-3c) or 3,5-di-t-butylbenzyl periphery (3d-3f). Visible spectra in solution (CH₂Cl₂-EtOH mixtures, toluene, THF, dioxane, acetone, and EtOAc) indicated a varying degree of chromophore aggregation that depended on solvent, dendrimer generation, and whether the Pc was metallated. Variable-concentration visible spectroscopic studies were analyzed using a nonlinear least-squares fitting procedure giving K_d values. These values further quantitated the observations that the t-butyl-substituted dendrimers 3d-3f were all less prone to aggregation in solution than the unsubstituted dendrimers 3a-3c, with a monotonic decrease in K_d across the series 3a → 3b → 3c → 3d → 3e → 3f. Second-generation t-butyl-substituted dendrimer 3f showed little to no aggregation in all solvents studied. Thin-film studies indicated that the largest members of the two dendrimer groups, third-generation 3c and second-generation 3f, were largely monomeric as evidenced by split Q-bands, similar to that seen in dilute CH₂Cl₂ solution when deposited via spin-coating onto glass slides. The metallated zinc Pcs 4a-4f all exhibited significantly less tendency toward aggregation in both solution and thin-films than their unmetallated analogues.