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

A cyclic porphyrin dimer (Ni-2-CPDpy) linked by butadiyne moieties bearing 4-pyridyl groups includes a C-60 molecule inside its cavity in solution to give a 1:1 inclusion complex (C-60 subset of Ni-2-CPDPy). The charge-transfer (CT) band is observed at 645 nm in the UV-vis absorption spectrum of the solution of C-60 subset of Ni-2-CPDPPy. In the cyclic voltammogram of C-60 subset of Ni-2-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 C-60. In the crystal structure of C-60 subset of Ni-2-CPDPy a porphyrin nanotube is formed by the self-assembly of Ni-2-CPDPy . Ni-2-CPDPy molecules link together through nonclassical C-H center dot center dot center dot N hydrogen bonds and pi-pi interactions of the pyridyl groups along the crystallographic b axis. The included C-60 molecules are linearly arranged in the nanotube to afford a supramolecular peapod. The charge-carrier mobility of the single crystal of C-60 subset of Ni-2-CPDPy was determined by flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements. It has an anisotropic high electron mobility (Sigma mu = 0.72 cm(2) V-1 s(-1)) along the linear army of C-60 (crystallographic b axis). Femtosecond laser flash photolysis of C-60 subset of Ni2CPDPy in the solid state with photoexcitation at the Soret band of the porphyrin shows the formation of a triplet exciplex (3){Ni-x-CPDPy center dot center dot center dot C-60}*, which decays with a lifetime of 34 ps to the ground state without observation of a complete charge-separated state.