Supramolecular Structures and Photoelectronic Properties of the Inclusion Complex of a Cyclic Free‐Base Porphyrin Dimer and C₆₀

Abstract

A cyclic free‐base porphyrin dimer H₄‐CPD_Py (CPD=cyclic porphyrin dimer) linked by butadiyne moieties bearing 4‐pyridyl groups self‐assembles to form a novel porphyrin nanotube in the crystalline state. The cyclic molecules link together through nonclassical CH⋅⋅⋅N hydrogen bonds and π–π interactions of the pyridyl groups along the crystallographic a axis. H₄‐CPD_Py includes a C₆₀ molecule in its cavity in solution. In the crystal structure of the inclusion complex (C₆₀⊂H₄‐CPD_Py), the dimer “bites” a C₆₀ molecule by tilting the porphyrin rings with respect to each other, and there are strong π–π interactions between the porphyrin rings and C₆₀. The included C₆₀ molecules form a zigzag chain along the crystallographic b axis through van der Waals contacts with each other. Femtosecond laser flash photolysis of C₆₀⊂H₄‐CPD_Py in the solid state with photoexcitation at 420 nm shows the formation of a completely charge‐separated state {H₄‐CPD_Py^.++C₆₀^.−}, which decays with a lifetime of 470 ps to the ground state. The charge‐carrier mobility of the single crystal of C₆₀⊂H₄‐CPD_Py was determined by flash photolysis time‐resolved microwave conductivity (FP‐TRMC) measurements. C₆₀⊂H₄‐CPD_Py has an anisotropic charge mobility (Σμ=0.16 and 0.13 cm² V⁻¹ s⁻¹) along the zigzag chain of C₆₀ (which runs at 45° and parallel to the crystallographic b axis). To construct a photoelectrochemical cell, C₆₀⊂H₄‐CPD_Py was deposited onto nanostructured SnO₂ films on a transparent electrode. The solar cell exhibited photovoltaic activity with an incident photon to current conversion efficiency of 17 %.