Octasubstituted Metal-Free Phthalocyanine as Core of Phosphorus Dendrimers: A Probe for the Properties of the Internal Structure

Abstract

The synthesis of a new family of phosphorus dendrimers built from an octasubstituted metal-free phthalocyanine core is described up to generation 5. This core is used as a sensor and a probe for analyzing the properties of the internal structure and the influence of each structural part (core, branches, surface) upon the whole structure. UV−visible spectra show both a hyperchromic and bathochromic effect on the Q-bands with increasing generation, indicating that the chromophore is more isolated, and that the dendritic shell mimics a highly polar solvent. There is no evidence for aggregation, except for generation 0, showing again the isolation of the core. However, the dendritic shell is permeable to aqueous acids and bases, as demonstrated by the reversible splitting of the Q-band in an acidic medium (neutral form of the phthalocyanine) and the single Q-band in a basic medium (dianionic form), even for generation 4. The fluorescence quantum yield for the neutral form increases with increasing generation. The dianionic form of generation 0 is poorly fluorescent, whereas generations 3 and 4 (G₃ and G₄) exhibit better fluorescence. The cores of G₃ and G₄ are highly sensitive optical sensors for H₃O⁺ and OH^-. These experiments are carried out in THF/water mixtures, and the influence of water on the structure has been checked. The hydrodynamic radius of generation 4 is measured by NMR diffusion (pulse gradient spin−echo) experiments. R_H varies from 35.4 Å at 4 mol % of water to 32.5 Å at 64 mol % of water in THF, indicating the hydrophobic nature of these dendrimers.