Probing flexibility in porphyrin-based molecular wires using double electron electron resonance

Area of Science:

• Supramolecular Chemistry
• Nanotechnology
• Physical Chemistry

Background:

• Porphyrin oligomers are promising molecular wires for nanoscale applications.
• Understanding their conformational flexibility is crucial for designing advanced materials.
• Electron Paramagnetic Resonance (EPR) spectroscopy, particularly Double Electron Electron Resonance (DEER), is a powerful tool for probing distances in molecular systems.

Purpose of the Study:

• To investigate the conformational behavior of butadiyne-linked zinc porphyrin oligomers.
• To measure intramolecular end-to-end distances using DEER spectroscopy.
• To explore how ligand binding influences the shape and conformation of these molecular wires.

Main Methods:

• Synthesis of zinc porphyrin oligomers with varying numbers of porphyrin units.
• Spin-labeling of oligomer ends with TEMPO radicals.
• Utilizing Double Electron Electron Resonance (DEER) pulsed EPR spectroscopy at 50 K in solution glasses.
• Conducting molecular mechanics calculations for comparison.

Main Results:

• Porphyrin oligomers primarily adopt linear conformations in dilute solutions with monodentate ligands.
• Nonlinear conformations are less prevalent in solvents with lower glass-transition temperatures.
• Binding to star-shaped multidentate ligands induces nonlinear geometries, consistent with molecular mechanics predictions.
• DEER successfully provided structural insights into both individual oligomers and self-assembled nanostructures.

Conclusions:

• Porphyrin-based molecular wires exhibit shape persistence but are conformationally tunable via ligand interactions.
• DEER spectroscopy is effective for characterizing the structure of synthetic nanostructures.
• The findings contribute to the rational design of functional molecular materials and nanodevices.