Anisotropic high electron mobility and photodynamics of a self-assembled porphyrin nanotube including C₆₀ molecules

A cyclic porphyrin dimer (Ni₂-CPD_Py) linked by butadiyne moieties bearing 4-pyridyl groups includes a C₆₀ molecule inside its cavity in solution to give a 1:1 inclusion complex (C ₆₀⊂Ni₂-CPD_Py). The charge-transfer (CT) band is observed at 645 nm in the UV-vis absorption spectrum of the solution of C₆₀⊂Ni₂-CPD_Py. In the cyclic voltammogram of C₆₀⊂Ni₂-CPD_Py, a small anodic shift of the porphyrin oxidation potential and a small cathodic shift of the fullerene reduction potential compared with their original redox potentials are indicative of CT interaction from the porphyrin to C₆₀. In the crystal structure of C₆₀⊂Ni₂-CPD_Py, a porphyrin nanotube is formed by the self-assembly of Ni₂-CPD_Py. Ni₂-CPD_Py molecules link together through nonclassical C-H· · ·N hydrogen bonds and π-π interactions of the pyridyl groups along the crystallographic b axis. The included C₆₀ molecules are linearly arranged in the nanotube to afford a supramolecular peapod. The charge-carrier mobility of the single crystal of C ₆₀⊂Ni₂-CPD_Py was determined by flash-photolysis timeresolved microwave conductivity (FP-TRMC) measurements. It has an anisotropic high electron mobility (Σμ ) 0.72 cm² V^-1 s^-1) along the linear array of C₆₀ (crystallographic b axis). Femtosecond laser flash photolysis of C ₆₀⊂Ni₂-CPD_Py in the solid state with photoexcitation at the Soret band of the porphyrin shows the formation of a triplet exciplex ³{Ni₂-CPD_Py · · ·C₆₀}*, which decays with a lifetime of 34 ps to the ground state without observation of a complete charge-separated state.