Photodynamics of the Molecular Ruby [Cr(ddpd)₂]³

Area of Science:

• Inorganic Chemistry
• Photochemistry
• Computational Chemistry

Background:

• Strong-field ligands are crucial for achieving luminescence in first-row transition-metal complexes.
• Earth-abundant near-infrared emitters can be developed using early 3d metals.
• The molecular ruby [Cr(ddpd)2]3+ exhibits high phosphorescence quantum yields in aqueous solution at room temperature.

Purpose of the Study:

• To understand the photodynamics of the molecular ruby [Cr(ddpd)2]3+ in water.
• To elucidate the electronic processes responsible for its luminescence properties.
• To correlate molecular structure and dynamics with observed emission characteristics.

Main Methods:

• Photodynamics simulations in water using trajectory surface hopping.
• Parametrization of linear vibronic coupling potentials from multiconfigurational CASSCF/CASPT2 calculations.
• Analysis of electronic state relaxation pathways and geometric changes.

Main Results:

• Excitation to the second absorption band initiates a relaxation cascade through metal-centered states.
• The complex undergoes intersystem crossing and relaxation through quartet states to low-lying doublet states.
• Elongation of Cr-ligand bonds and twisting of pyridine units drive these electronic processes.
• Low-lying doublet states are reached within 1-2 ps, close to their minima, explaining high quantum yields.

Conclusions:

• The simulated photodynamics accurately explain the high phosphorescence quantum yield of [Cr(ddpd)2]3+.
• Metal-centered state relaxation pathways are key to the observed luminescence.
• Computational methods provide valuable insights into the photophysical behavior of transition-metal complexes.