Structure-function correlation of photoactive ionic π-conjugated porphyrin structures

Morteza Adinehnia

Crystallization of a binary porphyrin nanostructure (BPN) of TSPP (meso-tetra(4-sulfonatophenyl)porphyrin) and TMPyP (meso-tetra(N-methyl-4-pyridyl)porphyrin) was studied. The morphology and crystallinity of the BPN was investigated using TEM and AFM. The composition of the BPN was analyzed using XPS, elemental analysis and UV-Visible spectroscopy. In this work, we have developed a model to simulate the growth of these organic monocrystalline materials for the first time. The model is then validated by comparing the simulation results with experimental crystallization histograms. This investigation will help in better understanding of crystallization and size control in this class of photoactive organic materials. As a major achievement of this work, we provide a structure-function relationship study of organic crystalline photoconductors composed of oppositely charged ionic porphyrins. Nano to millimeter size crystals with well-defined morphology. Our model system, the rod shaped TMPyP:TSPP monoclinic P21/c crystals have pseudo-hexagonal cross sectional and their internal structure consists of highly organized molecular columns of alternating porphyrin cations and anions. 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 and (2) hopping conductivity. To provide further information, we measured mechanical properties of seven different BPNs and correlated the obtained values with their structural properties. The electrostatic charge of the tectons, measured densities and π- π interactions of these porphyrin solids follow the same trend as their elastics moduli. In a broader perspective, this study highlights the extraordinary mechanical behavior of porphyrin assemblies bound together by ionic and π-bonding. This work combines the results from structural and theoretical studies and correlates them with mechanical, electronic and optoelectronic properties thereby opening the road to the engineering of highly-organized functional materials from organic π-conjugated molecules.

Structure-function correlation of photoactive ionic π-conjugated porphyrin structures

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