Photoconductivity of Porphyrin Nanochannels Composed of Diprotonated Porphyrin Dications with Saddle Distortion and Electron Donors

Abstract

Supramolecular architecture named as porphyrin nanochannels (PNCs), including tetrathiafulvalene (TTF) and p-aminophenol as electron-donating guests in the inner space, was prepared with the hydrochloride salt of dodecaphenylporphyrin ([H4DPP]Cl2) by self-assembly based on intermolecular π−π interactions. The crystal structure of the TTF-included PNC (PNC−TTF) was determined by X-ray crystallography. Intermolecular π−π interaction was recognized among peripheral phenyl groups of the porphyrin, mainly in the direction of the crystallographic c axis to form a column structure. Photoinduced electron transfer from the guest molecules to [H4DPP]Cl2 occurred to give the electron-transfer state involving cation radicals of the guest molecules and one-electron reduced [H4DPP]Cl2, {[H4DPP+•]Cl2}−, via the photoexcited singlet state of [H4DPP]Cl2 in PNC. The reactions were examined by solid-state femtosecond laser flash photolysis and ESR measurements to determine the rate constants of electron transfer and electronic structures of the cation radicals included in the cavity, respectively. A single crystal (0.87 × 0.23 × 0.10 mm3) of PNC−TTF exhibited photoconductivity upon photoirradiation at 633 nm with a He−Ne laser (5 mW), and the photocurrent was 0.7 nA at electrical field strength of 3.5 × 104 V cm−1. The photocurrent showed direction dependence toward the crystallographic c axis. This indicates that the intermolecular π−π interaction is the main conduction pathway. Various PNC supramolecules including TTF and other electron-donating guest molecules were also employed to construct photoelectrochemical cells with use of SnO2 transparent electrodes. Short-circuit photocurrent measurements were made on the cells with the OTE/SnO2/PNC−guest photoanodes, and they exhibited clear photoresponse upon photoirradiation. The photocurrents increase with increasing the rate constants of the photoinduced electron transfer from the guest molecules to [H4DPP]Cl2, exhibiting saturation behavior. The performance of the cell with the OTE/SnO2/PNC−TTF electrode exhibited the maximum IPCE (incident photon-to-current efficiency) value of 10.1% at 460 nm, which corresponded to the absorption maximum of the Soret band of [H4DPP]Cl2 on the electrode.