Porphyrin and Tetrabenzoporphyrin Dendrimers: Tunable Membrane-Impermeable Fluorescent pH Nanosensors

The pH dependencies of the UV−vis and fluorescent spectra of new water-soluble dendritic porphyrins and tetrabenzoporphyrins were studied. Because of extended π-conjugation and nonplanar distortion, the absorption and the emission bands of tetraaryltetrabenzoporphyrins (Ar4TBP) are red-shifted and do not overlap with those of regular tetraarylporphyrins (Ar4P). When encapsulated inside dendrimers with hydrophilic outer layers, Ar4Ps and Ar4TBPs become water soluble and can serve as pH indicators, with pK's adjustable by the peripheral charges on the dendrimers. Two new dendritic porphyrins, Gen 4 polyglutamic porphyrin dendrimer H2P-Glu4OH (1) with 64 peripheral carboxylates and Gen 1 poly(ester amide) Newkome-type tetrabenzoporphyrin dendrimer H2TBP-Nw1OH (2) with 36 peripheral carboxylates, were synthesized and characterized. The pK's of the encapsulated porphyrins (pKH2P-Glu4OH = 6.2 and pKH2TBP-Nw1OH = 6.3) were found to be strongly influenced by the dendrimers, revealing significant electrostatic shi...