A Tetra-Porphyrin Host Exhibiting Interannular Cooperativity

Area of Science:

• Supramolecular Chemistry
• Molecular Recognition
• Coordination Chemistry

Background:

• Interannular cooperativity is a rarely observed phenomenon in supramolecular chemistry.
• Molecular tweezers are sophisticated host molecules designed for guest binding.
• Porphyrin-based macrocycles offer unique electronic and structural properties for molecular recognition.

Purpose of the Study:

• To synthesize and characterize a tetra-porphyrin molecular tweezer capable of exhibiting interannular cooperativity.
• To investigate the complexation behavior of the molecular tweezer with 1,4-diazabicyclo[2.2.2]octane (DABCO).
• To determine the type and magnitude of cooperativity in the host-guest complexation.

Main Methods:

• Synthesis of a tetra-porphyrin molecular tweezer with two bis-porphyrin binding sites.
• UV/Vis and Nuclear Magnetic Resonance (NMR) titration experiments to study host-guest complexation.
• Analysis of binding constants and speciation using established models.
• Statistical analysis to quantify interannular cooperativity (γ).

Main Results:

• The tetra-porphyrin molecular tweezer successfully complexed with DABCO, exhibiting interannular cooperativity.
• UV/Vis titration data supported a complexation model involving 1:2, 2:2, and 1:4 host:guest ratios with high binding constants.
• NMR titration data indicated the formation of two sandwich species, with tetra-porphyrin·(DABCO)₂ as the major species.
• Statistical analysis revealed a negative value for interannular cooperativity (γ = 2.41×10⁻³).

Conclusions:

• The synthesized tetra-porphyrin molecular tweezer demonstrates archetypal interannular cooperativity.
• The observed negative cooperativity suggests that DABCO is too small to effectively bridge and bind optimally within both bis-porphyrin sites simultaneously.
• This study provides valuable insights into the nuanced mechanisms of cooperativity in supramolecular chemistry.