Supramolecular Assembly of Helical Porphyrin Nanostructures on Single Stranded DNA Via Directional Hydrogen Bonds

Chiral nanomaterials and metamaterials have great potential for applications in plasmonics, biological sensing, labelling, chiral memory, data storage, and optical communication. Porphyrins’ modular photophysical and structural properties, high stability, and accessible synthetic routes make them one of the most commonly utilized functional building blocks in molecular engineering. Single stranded DNA (ssDNA) represents a versatile supramolecular template, since its nucleotide sequence and structure can be easily copied into newly formed periodically arranged helical nanostructures. This lecture will present the synthesis of porphyrin-purine conjugates and their assembly into helical supramolecular porphyrin and metalloporphyrin nanostructures formed on a single stranded oligothymidine template via directional hydrogen bonding (see Figure).^[1,2] We will describe how the helicity of ssDNA-templated porphyrin and metalloporphyrin nanoarrays can be controlled and switched using external stimuli to preferentially prepare left-handed and right-handed nanoassemblies. We will also present external stimuli induced, reversible switching of helical twist of ssDNA-templated porphyrin nanoassemblies. Time-dependent density functional theory (TD DFT) simulations of UV-vis and circular dichroism (CD) spectra for model porphyrin nanostacks have been used to confirm the origin of observed chiroptical properties and to assign the handedness of porphyrin nanoassemblies. We will also discuss successful use of dialysis to separate porphyrin nanoassemblies from porphyrin monomers. The chiral DNA-templated multiporphyrin nanostructures exhibited high thermal and acid/base stability: the helicity was fully preserved up to +85 °C and pH between 3 and 12. High resolution transition electron microscopy (HRTEM) has showed formation of metalloporphyrin nanoassemblies and their assembly into helical fibrils with micrometer lengths.^[2]

1. G. Sargsyan, B. M. Leonard, J. Kubelka, M. Balaz, ‘Supramolecular ssDNA templated porphyrin and metalloporphyrin nanoassemblies with tunable helicity.’ Chem. Eur. J., 2014, 20, 1878-1892 (VIP)

2. G. Sargsyan, A. A. Schatz, J. Kubelka, M. Balaz, ‘Formation and helicity control of ssDNA templated porphyrin nanoassemblies.’ Chem. Commun., 2013, 49, 1020-1022