Anisotropic High Electron Mobility and Photodynamics of a Self-Assembled Porphyrin Nanotube Including C₆₀ Molecules

Abstract

A cyclic porphyrin dimer (Ni2−CPDPy) linked by butadiyne moieties bearing 4-pyridyl groups includes a C60 molecule inside its cavity in solution to give a 1:1 inclusion complex (C60⊂Ni2−CPDPy). The charge-transfer (CT) band is observed at 645 nm in the UV−vis absorption spectrum of the solution of C60⊂Ni2−CPDPy. In the cyclic voltammogram of C60⊂Ni2−CPDPy, 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−CPDPy, a porphyrin nanotube is formed by the self-assembly of Ni2−CPDPy. Ni2−CPDPy 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 C60⊂Ni2−CPDPy was determined by flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements. It has an anisotropic high electron mobility (∑μ = 0.72 cm2 V−1 s−1) along the linear array of C60 (crystallographic b axis). Femtosecond laser flash photolysis of C60⊂Ni2−CPDPy in the solid state with photoexcitation at the Soret band of the porphyrin shows the formation of a triplet exciplex 3{Ni2−CPDPy···C60}*, which decays with a lifetime of 34 ps to the ground state without observation of a complete charge-separated state.