Reversible Valence Photoisomerization between Closed-Shell Quinoidal and Open-Shell Biradical Forms.

Area of Science:

• Photochemistry
• Chemical Kinetics
• Supramolecular Chemistry

Background:

• Photochromic materials exhibit reversible color changes upon light exposure.
• Radical complexes can undergo valence isomerization, transitioning between different electronic states.
• Understanding the kinetics and thermodynamics of these transitions is crucial for material design.

Purpose of the Study:

• To kinetically study the thermal equilibrium between biradical and quinoidal forms.
• To determine the thermodynamic parameters of this valence isomerization process.
• To investigate photoisomerization pathways induced by visible light.

Main Methods:

• Utilized nanosecond laser flash photolysis to observe microsecond timescale processes.
• Initiated the study from the singlet biradical form.
• Employed UV and visible light irradiation to induce and probe photoisomerization.

Main Results:

• Observed and characterized the thermal equilibrium between singlet biradical and quinoidal forms.
• Determined thermodynamic parameters for the biradical-to-quinoidal valence isomerization for the first time.
• Demonstrated visible-light-induced photoisomerization from quinoidal to biradical form.

Conclusions:

• The study provides the first kinetic and thermodynamic data for the biradical-quinoidal equilibrium.
• The photochromic phenoxyl-imidazolyl radical complex exhibits reversible valence isomerization.
• This work advances the understanding of photo-switchable radical systems.