Metal Dipyrrin and Aryl-Linked Bis-Porphyrin Hosts for Fullerenes and the Supramolecular Assembly of Multi-Chromophore Porphyrin – Fullerene Systems

Abstract

Photoactive systems can be assembled using the spontaneous π-π attraction between porphyrins and fullerenes. We prepared several bis-porphyrin hosts for fullerenes that demonstrate fullerene binding in solution. An aryl-alkyne linked bis-porphyrin host was prepared by a Sonogashira coupling. Aryl-linked hosts with 3,5-pyridyl or isophthaloyl head groups were prepared by amide coupling reactions using acid chloride or activated ester methods. Computational modelling demonstrated that the relatively rigid linkers of these hosts arranged the porphyrins close to parallel at distances of approximately 12 Å, suitable for the inclusion of a fullerene guest between the two porphyrins. UV-Visible spectrophotometric titrations showed that association constants of these hosts were between 1500 - 3100 M⁻¹ for C₆₀ in toluene, and were influenced by the number of close contacts available with porphyrin meso-position aryl substituents. Association constants for C₇₀ were approximately an order of magnitude larger, and association constants were greatly increased in 1:1 toluene:acetonitrile. The 3,5-pyridyl-linked bis-porphyrin hosts were coordinated to ruthenium porphyrins as an axial ligand, allowing the assembly in solution of ruthenium porphyrin - free base porphyrin - fullerene triads, and tetrads with a ferrocene-appended ruthenium porphyrin. Coordination to a ruthenium porphyrin results in a small increase in the fullerene binding constant of the pyridyl-linked bis-porphyrin. Three porphyrins were linked to a cobalt dipyrrin complex, forming tris-porphyrin hosts for fullerenes where the linker is also a photoactive chromophore. The binding constants of these hosts for C₆₀ are 1500-1800 M⁻¹ in toluene. These multichromophore systems are suitable for investigation of their photophysical properties by transient absorption spectroscopy. The presence of secondary donors is expected to result in these multichromophore systems exhibiting a longer-lived excited state due to sequential electron transfer causing greater spatial separation of the radical ion pair.