Structure-Function Correlation of Photoactive Ionic pi-Conjugated Binary Porphyrin Assemblies
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Structure-Function Correlation of Photoactive Ionic pi-Conjugated Binary Porphyrin Assemblies Morteza Adinehnia1, Bryan Borders1, Michael Ruf2, Bhaskar Chilukuri1, Ursula Mazur1 and K.W. Hipps1 1 Department of Chemistry and Materials Science and Engineering Program, Washington State University, Pullman, WA 99164-4630, USA. 2 Bruker AXS Inc., Madison, WI, 53711, USA. ABSTRACT We present the first detailed structure-function study of a photoconducting ionic porphyrin supermolecular assembly, fabricated from tetra(N-methyl-4-pyridyl)porphyrin (TMPyP) and tetra(4-sulfonatophenyl)porphyrin (TSPP) in a 1:1 stoichiometric ratio. Rod like crystals large enough for single crystal diffraction studies were grown by utilizing a nucleation and growth model described in our previous work. The unit cell of the TMPyP:TSPP crystals is monoclinic P21/c and the cell constants are a = 8.3049(11) Å, b = 16.413(2) Å, c = 29.185(3) Å, β = 92.477(9)°. These crystals have smooth well defined facets and their internal structure consists of highly organized molecular columns of alternating porphyrin cations and anions that are stacked face to face. For the first time crystal morphology (habit) of an ionic porphyrin solid is predicted by using the crystal structure data and applying attachment energy (AE) model. The predicted habit is in good agreement with the experimental structural morphology observed in AFM and SEM images of the TMPyP:TSPP crystalline solid. The TMPyP:TSPP crystals are nonconducting in the dark and are photoconducting. The photoconductive response is significantly faster with excitation in the Q-band (Red) than with excitation in the Soret band (blue). DFT calculations were performed to determine their electronic band structure and density of states. The TMPyP:TSPP crystalline system is a useful model structure that combine the elements of molecular organization and morphology along with theory and correlate them with electronic and optical electronic properties. INTRODUCTION Organic molecular semiconductors possess unique photon absorption characteristics induced by their highly polarizable π-electronic structures. These characteristics make them particularly useful for developing novel optoelectronic devices such as light-emitting diodes [1], field-effect transistors [2], photoswitches [3], sensors [4], solar cells [5], [6], [7] and memory devices [8]. Free-base porphyrins and their metal derivatives substituted with ionic groups can form nano to micro-sized supermolecular crystalline assemblies in several shapes and morphologies [9]-[16]. In these structures, strong electrostatic attraction, π-π interactions, and hydrogen bonding influence molecular ordering and influence optical and electronic properties. In the past, the molecular arrangement of self-assembled porphyrin tectons was deduced from the solution electronic absorbance spectra of the ISA product. Solid state structural studies are extremely rare [10], [17], [18] because the sizes of the crystals that can be isolated are at best suitable for powder and not fo
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