Correlating Elastic Properties and Molecular Organization of an Ionic Binary Porphyrin Nanostructure

Mechanical and structural properties of a crystalline porphyrin nanostructure composed of meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) and meso-tetra(4-pyridyl)porphyrin (TPyP) will be presented.    X-ray photoelectron spectroscopy (XPS), UV-visible spectra, and elemental analysis all support a stoichiometric 1:1 TSPP to TPyP composition.  Atomic force microscopy (AFM), revealed that the porphyrin nanostructure is composed of stacked ribbons about 20 nm tall, 70 nm wide, and several microns in length.  Selected area electron diffraction (SAED) and powder x-ray diffraction patterns of TSPP:TPyP are consistent with an orthorhombic system and space group Imm2 with lattice parameters a = 26.71, Å b = 20.16 Å, and c = 8.61Å.   Crystallographic data is consistent with an arrangement of alternating face-to-face TSPP and TPyP molecules forming ordered columns along the length of the nanorods. The TSPP:TPyP are  thermally stable and do not lose their crystallinity when heated to temperatures up to 150 °C. The structural integrity of the solid is attributed to combined noncovalent interactions that include ionic, hydrogen bonding, and π-π interactions.   The values of Young's modulus obtained for the crystalline TSPP:TPyP nanorods averaged  6.5 ± 1.3 GPa.   This modulus is comparable to those reported for covalently bonded flexible polymeric systems.   The robust bonding character of the TSPP:TPyP nanostructures combined with their mechanical properties makes them excellent candidates for flexible optoelectronic devices.