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

Abstract

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, C₆₀ and C₇₀. The complexation of the fullerenes was mainly due to the π–π interactions between the phenothiazine and fullerene moieties and it was enhanced due to the π–π 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 ∼10³ M⁻¹ for C₆₀ and ∼10⁴ M⁻¹ for C₇₀ indicating that the C₇₀ 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 C₆₀ or C₇₀, 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.