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

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]Cl-2) by self-assembly based on intermolecular pi-pi interactions. The crystal structure of the TTF-included PNC (PNC-TTF) was determined by X-ray crystallography. Intermolecular 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]Cl-2 occurred to give the electron-transfer state involving cation radicals of the guest molecules and one-electron reduced [H4DPP]Cl-2, {[H4DPP+center dot]Cl-2}(-), via the photoexcited singlet state of [H4DPP]Cl-2 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 x 0.23 x 0.10 mm(3)) 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 X 10(4) V cm(-1). The photocurrent showed direction dependence toward the crystallographic c axis. This indicates that the intermolecular 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 oil 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]Cl-2, 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]Cl-2 on the electrode.