Porphyrin bearing phenothiazine pincers as hosts for fullerene binding via concave-convex complementarity: synthesis and complexation study

In this work, we synthesized free base porphyrin hosts, m-(PTZ)(4)-H2P and p-(PTZ)(4)-H2P, that are functionalized with four phenothiazine moieties at the meso-position via a flexible ethoxy phenyl linker. The rigid and non-planar nature with the butterfly conformation of the phenothiazine moieties created a cage-like structure and hence, the porphyrins generated a new and interesting molecular topology that can be used for the size selective interaction and encapsulation with fullerenes, C-60 and C-70. The complexation of the fullerenes was mainly due to the pi-pi interactions between the phenothiazine and fullerene moieties and it was enhanced due to the pi-pi aided donor-acceptor interactions with the porphyrin core. The optical absorption studies proved the complexation of the fullerenes with these hosts. The steady-state fluorescence studies revealed the efficient diminishing of porphyrin emission with the binding constants of similar to 10(3) M-1 for C-60 and similar to 10(4) M-1 for C-70 indicating that the C-70 fits well in these hosts. The computational studies indicated that in spite of the electrostatic mechanism, dispersive forces also play a crucial role in forming strong ground state non-covalent complexes. The cyclic voltammetric studies revealed that, upon the addition of C-60 or C-70, the oxidation potentials of porphyrins were shifted to higher anodic potentials indicating that the porphyrins are harder to oxidize once complexed with fullerenes. The time-resolved fluorescence studies showed that, upon the addition of increasing amounts of fullerenes, a fluorescence decay of the phenothiazine appended porphyrins was observed proving the formation of the phenothiazine-porphyrin:fullerene host-guest complexes and revealed the photo-induced electron transfer between the fullerenes and porphyrins in these systems.