Photoconductive behavior of binary porphyrin crystalline assemblies

Abstract

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 (˜50 ∘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 ∘C causes approximately 25% loss in photoconductivity but has little effect on the lifetime associated with the persistent photoconductivity.

Mechanical behavior of crystalline ionic porphyrins

“…We note that heating the BP solids up to 130 °C did not modify their elemental charges or composition, based on X-ray photoelectron spectroscopy, (XPS) measurements [18,20,28,29,36]. Thermogravimetric analysis (TGA) combined with XPS showed that the crystals experienced reversible loss of water from within their crystalline channels at temperatures below 110 °C and became amorphous with annealing above 125 °C [29,30]. Representative morphologies of BP crystals obtained from optical and SEM imaging are depicted in Fig.…”

“…SEM images of TPyP:TSPP ( Fig. 2d) crystals synthesized in a pH 3 solution show that the crystals have rectangular cross sections and squared off ends; the larger structures are hollow [30]. Eskelsen [35].…”

“…Detailed procedures for the ISA crystal preparation and isolation are described elsewhere [30,35,36]. The different porphyrin crystals were all prepared at 65 °C in neutral or acidic (HCl) aqueous media from positive and negative tecton ion combinations in 1:1 ratios.…”

“…For single crystal XRD evaluation, a Bruker D8 VENTURE single crystal diffractometer equipped with a Ga METALJET X-ray source (1.341398 Å) and Photon 100 detector was used for data acquisition from single crystal BP samples. The details of data collection and structural refinement for the free-base and metallated crystals are presented elsewhere [28][29][30].…”

“…Here, we present a quantitative elastic properties and structure correlation study of a series of ionically self-assembled (ISA) [17][18][19][20] crystalline p-conjugated solids fabricated from oppositely charged porphyrin tectons (binary porphyrins, BP). These materials are known to serve as active components in many optoelectronic devices such as solar cells [21][22][23][24], photovoltaics [1,17,25], and photosensors [26][27][28][29][30]. Until recently, reports of crystal structures of BPs were rare because synthesized crystals were not suitable for single crystal X-ray diffraction (XRD) [31][32][33][34].…”