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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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Unexpected drastic decrease in the excited-state electronic communication between porphyrin chromophores covalently

Mohammed Abdelhameed1, Paul-Ludovic Karsenti, Adam Langlois

  • 1Département de Chimie, Université de Sherbrooke, Sherbrooke, PQ, J1K 2R1 (Canada).

Chemistry (Weinheim an Der Bergstrasse, Germany)
|August 16, 2014
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Summary

Researchers studied zinc(II) porphyrin dyads, finding slow singlet energy transfer. This is due to limited molecular orbital overlap and electronic shielding from the palladium iodide bridge.

Keywords:
Förster theorycarbenespalladiumporphyrinssinglet energy transfer

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Area of Science:

  • Supramolecular Chemistry
  • Photochemistry
  • Organometallic Chemistry

Background:

  • Zinc(II) porphyrin and free base porphyrin dyads with N-heterocyclic carbene (NHC) ligands were synthesized.
  • A trans-palladium(II) iodide (trans-PdI2) bridge was incorporated to link the donor and acceptor chromophores.
  • Related dimers ([Zn-Zn] and [Fb-Fb]) were also prepared for comparative studies.

Purpose of the Study:

  • To investigate the singlet energy transfer dynamics in novel porphyrin dyads.
  • To compare experimental energy transfer rates with theoretical predictions (Förster theory).
  • To elucidate the factors influencing energy transfer efficiency in these bridged systems.

Main Methods:

  • Absorption and emission spectroscopy were employed to characterize the dyads.
  • Density functional theory (DFT) computations were utilized to analyze electronic structures and orbital overlaps.
  • Kinetic analysis was performed to determine singlet energy transfer rates.

Main Results:

  • Synthesized [Zn-Fb] dyads with NHC ligands and a trans-PdI2 bridge.
  • Observed unexpectedly slow singlet energy transfer rates, deviating from Förster theory predictions.
  • Identified limited frontier molecular orbital (MO) overlap and electronic shielding by the trans-PdI2 bridge as key factors.

Conclusions:

  • The trans-PdI2 bridge hinders efficient singlet energy transfer through both double electron exchange and transition dipole interactions.
  • Molecular orbital delocalization and electronic shielding effects are critical in dictating energy transfer efficiency in such dyads.
  • Findings provide insights into the design principles for molecular systems requiring controlled energy transfer.