Factors Controlling Cage Escape Yields of Closed- and Open-Shell Metal Complexes in Bimolecular Photoinduced Electron...

Area of Science:

• Photochemistry and Photophysics
• Catalysis
• Physical Chemistry

Background:

• Understanding excited-state electron transfer is crucial for developing efficient photocatalysts.
• Geminate radical pair recombination significantly impacts reaction outcomes.
• Solvent effects play a critical role in controlling charge separation and recombination dynamics.

Purpose of the Study:

• To investigate the cage escape yield of various metal photosensitizers (Fe(III), Ru(II), Ir(III)) in different solvents.
• To elucidate the influence of photosensitizer electronic structure (closed-shell vs. open-shell) and solvent dielectric properties on cage escape yields.
• To provide guidelines for optimizing photosensitizer performance in photoredox catalysis.

Main Methods:

• Experimental study of cage escape yields using six different photosensitizers (Fe(III), Ru(II), Ir(III)) and tri-p-tolylamine as an electron donor.
• Systematic variation of solvent polarity (11 solvents) to probe dielectric effects.
• Analysis of spin dynamics (intersystem crossing) and charge recombination pathways.

Main Results:

• Iridium(III) photosensitizers exhibited the highest cage escape yields (0.67-1), indicating excellent potential for photoredox catalysis.
• Ruthenium(II) and Osmium(II) photosensitizers showed moderate to good yields (around 0.65 and 0.38, respectively).
• Iron(III) photosensitizers displayed low yields (<0.1) in high dielectric constant solvents but significantly increased (up to 0.58) in low dielectric constant solvents, attributed to spin-forbidden recombination in closed-shell complexes versus spin-allowed recombination in open-shell complexes.

Conclusions:

• Cage escape yields are highly dependent on the photosensitizer's electronic configuration and the solvent environment.
• Closed-shell photosensitizers (Ir(III), Ru(II)) yield high cage escape efficiencies independent of solvent dielectric effects due to spin-forbidden recombination.
• Open-shell photosensitizers (Fe(III)) show solvent-dependent yields, offering opportunities for tuning reactivity in low dielectric media for photoredox catalysis.