Comprehensive structure–function correlation of photoactive ionic π-conjugated supermolecular assemblies: an experimental and computational study

Abstract

We provide a structure–function relationship study of an organic crystalline photoconductor composed of oppositely charged ionic porphyrins. Nano to millimeter size crystals with well-defined morphology composed of stoichiometric amounts of meso-tetra(N-methyl-4-pyridyl)porphyrin (TMPyP) and meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) were grown in a controlled and reproducible manner. The rod shaped TMPyP:TSPP monoclinic P2₁/c crystals have a pseudo-hexagonal cross section and their internal structure consists of highly organized molecular columns of alternating porphyrin cations and anions. Experimental characterization of the TMPyP:TSPP solid was performed using powder-XRD, AFM, SEM, DRS UV-visible, and photoconductivity measurements. For the first time the morphology of an ionic porphyrin solid is predicted. The TMPyP:TSPP crystals are non-conducting in the dark but become conductive with illumination. The n-type photoconductive response is significantly faster with excitation in the Q-band than with excitation in the Soret band. Quantum mechanical calculations were performed to determine the electronic band structure and density of states and to explain the photoconduction in TMPyP:TSPP. Based on these results we propose a model in which two types of photoconductivity occur: (1) band conduction which occurs at all excitation wavelengths and (2) hopping conductivity caused by metastable photoinduced defects that form primarily at higher energy excitations. This work combines the results from structural and theoretical studies and correlates them with electronic and optoelectronic properties thereby opening the road to the engineering of highly-organized functional materials from organic π-conjugated molecules.