Structural features of meso-tetrakis(4-carboxyphenyl) porphyrin interacting with amino-terminated poly(propylene oxide)

The neutralization reaction between the meso-tetrakis(4-carboxyphenyl) porphyrin (TPPC) and O-(2-aminopropyl)-O'-(2-methoxyethyl) poly(propylene glycol) (Jeffamine M-600) in 1: 4 stoichiometric ratio affords a stable supramolecular adduct (TPPC@Jeffamine). This species has been characterized in aqueous and chloroform solution through a series of spectroscopic techniques, including H-1 and C-13 NMR spectroscopy, UV/ vis absorption, steady-state and time-resolved fluorescence emission, time-resolved fluorescence anisotropy, and resonance light scattering. All the experimental findings point to a different structural arrangement of the porphyrin chromophores within the supramolecular adduct depending on the quality of the solvent. In aqueous solution, the TPPC@Jeffamine contains mainly monomeric porphyrins while the flexible polymeric chains are folded and entangled to give larger aggregates. An insight into the structure of the basic building unit is given by SAXS measurements which, assuming a core-shell model, afford an internal core radius R-1 = 15.3 angstrom and an external radius R-2 = 21.8 angstrom, in line with the expected dimension of the single unit, based on a single porphyrin surrounded by four Jeffamine chains. In chloroform, the adduct rearranges and porphyrins oligomers are stabilized by ion-pairing with Jeffamine, which, due to its hydrophobic nature, swells in this solvent.