Supramolecular structures and photoelectronic properties of the inclusion complex of a cyclic free-base porphyrin dimer and C₆₀

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 C ₆₀ 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 C ₆₀⊂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⊂H ₄-CPD_Py was determined by flash photolysis time-resolved microwave conductivity (FP-TRMC) measurements. C₆₀cH ₄-CPD_Py has an anisotropic charge mobility (σμ = 0.16 and 0.13cm ²V^-1s^-1) along the zigzag chain of C₆₀ (which runs at 45° and parallel to the crystallographic b axis). To construct a photoelectrochemical cell, C₆₀⊂H ₄-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 %.