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A Tetra-Porphyrin Host Exhibiting Interannular Cooperativity.

Rhys B Murphy1,2, Duc-Truc Pham3, Martin R Johnston1

  • 1Flinders Institute for Nanoscale Science and Technology, College of Science and Engineering, Flinders University, Bedford Park, Adelaide, Australia.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|August 13, 2019
PubMed
Summary

This study reports a novel tetra-porphyrin molecular tweezer demonstrating rare interannular cooperativity with 1,4-diazabicyclo[2.2.2]octane (DABCO). Negative cooperativity was observed, suggesting DABCO may be too small for optimal binding at both sites.

Keywords:
association constantscooperativity effectsporphyrinoidsstatistical analysissupramolecular chemistry

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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.