β,β-Directly Linked Porphyrin Rings: Synthesis, Photophysical Properties, and Fullerene Binding.

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Materials Science

Background:

• Cyclic porphyrin oligomers serve as models for photosynthetic light-harvesting systems and supramolecular receptors.
• Direct β,β-linkages in cyclic porphyrins are synthetically challenging and underexplored.

Purpose of the Study:

• To synthesize unprecedented β,β-directly linked cyclic zinc porphyrin trimers (CP3) and tetramers (CP4).
• To investigate the three-dimensional structures, photophysical properties, and supramolecular receptor capabilities of these novel oligomers.

Main Methods:

• Yamamoto coupling of a 2,3-dibromoporphyrin precursor.
• Structural characterization using NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction.
• Computational modeling using density functional theory (DFT) for geometry optimization.
• Photophysical and electrochemical property measurements.
• Fullerene binding studies using NMR titration and X-ray diffraction.

Main Results:

• Successful synthesis of CP3 and CP4 cyclic zinc porphyrin oligomers.
• Determination of propeller (CP3) and saddle (CP4) 3D structures.
• CP3 exhibits stronger π-conjugation and red-shifted absorption compared to CP4 due to smaller dihedral angles.
• CP3 central ring is partially aromatic (HOMA 0.52), while CP4 central ring is non-aromatic (HOMA -0.02).
• CP4 demonstrates ditopic fullerene receptor activity with significant binding affinities for C70 and C60.

Conclusions:

• The β,β-direct linkage enables the formation of novel cyclic porphyrin architectures with tunable properties.
• Geometric differences significantly impact electronic structure, π-conjugation, and aromaticity.
• The saddle-shaped CP4 is an effective ditopic receptor for fullerenes, highlighting potential applications in molecular recognition and self-assembly.