Porphyrin molecules are highly conjugated macrocyclic organic molecules. Due to their unique electronic structure these molecules can function as photocatalysts, aiding in the break-down of air pollutants. Further, porphyrins such as tetrakis(4-carboxyphenyl) (TCPP) have been shown to  self-assemble into nanostructures with enhanced photocatalytic properties compared to their monomer form. One example is TCPP nanowires, formed through titration with hydrochloric acid (HCl). The nanowires form through electrostatic interactions of the protonated TCPP core and chloride ions, thus the nanostructures can be controlled under different pH conditions. Nanowires are formed at pH values below 2, but nanoplatelets are observed at a pH of 3. The mechanism driving the transition between assembled TCPP nanostructures, however, is not  known. Therefore, the goal of this work is to explore the structural details of TCPP assemblies under systematically changed pH conditions. Atomic Force Microscopy (AFM) was utilized to monitor the morphological changes of TCPP nanostructures. The nanowires measured 0.6-1.5 μm in length with diameters of 100-300 nm. Both global and local changes in nanowire structure were measured by either immersing the entire sample in specific pH solutions for set times or using Dip-pen nanolithography. For Dip-pen nanolithography, the AFM tip was used to induce local structural changes utilizing humidity to alter the pH where the AFM tip is in contact with the sample. Preliminary results suggest that in pH 3 solution the wires disassemble primarily in the longitudinal direction at a growth rate of 4 nm/min. When placed in pH 1 the wires assemble at 9 nm/min.

Structural analysis of TCPP nanostructures

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.