Photoconductive behavior of binary porphyrin crystalline assemblies

The mechanism of photoconductivity in a crystalline photoconductor synthesized from 1: 1 ratio of meso-tetra(4-pyridyl) porphyrin (TPyP) and meso-tetra(4-sulfonatophenyl) porphyrin (TSPP) ionic tectons was examined. The rod-like crystals of TPyP: TSPP insulate in the dark but become photoconducting on illumination and a portion of the photoinduced current persists after the laser light is turned off. This persistent photoconductivity (PPC) is investigated as a function of laser illumination wavelength, laser power, and sample temperature. The primary charge carriers in the TPyP: TSPP upon photoexcitation are electrons and the charge recombination mechanism follows monomolecular kinetics. The number of electrons contributing to the photocurrent is directly proportional to the number of photons absorbed thus, the mechanisms of the photoconductivity resulting from excitations within the Soret band and the Q-band are the same. The PPC is interpreted to be the result of the formation of photoinduced metastable defects that allow for Miller-Abrahams-like hopping conductivity. The TPyP: TSPP has an incommensurately modulated crystal lattice and its proposed model structure is based on both ionic and neutral porphyrin tectons. The thermogravimetric analysis shows that the porphyrin crystals undergo dehydration on heating (similar to 50 degrees C) by losing water molecules located in the crystalline channels. Temperature dependent XRD indicates that dehydration causes irreversible changes to the crystal structure. The loss of crystallinity observed with heating the TPyP: TSPP crystals above 90 degrees C causes approximately 25% loss in photoconductivity but has little effect on the lifetime associated with the persistent photoconductivity.