Interplays of excited state structures and dynamics in copper(I) diimine complexes: Implications and perspectives

• Structural dynamics on the ultrafast time scale for CuI diimine complexes.
• Key structural factors for optimal photoinduced electron transfer are identified.
• Interplays of solvent accessibility to the copper center and behaviors of the excited state are characterized.
• Structural design guidelines for CuI diimine complexes to be effective sensitizers and electron donors are outlined.
• Listed recent applications of CuI diimine complexes in solar energy conversion systems.

Although they were discovered almost four decades ago, Cu(I) diimine complexes have emerged as a group of transition metal complexes that can play important roles in solar energy conversion and utilization, and have potential to replace the quintessential ruthenium polypyridyl complexes as light sensitizers, electron donors and catalytic centers. This review includes some recent photophysical studies and transient structural studies of Cu(I) diimine complexes using ultrafast optical transient absorption and emission as well as X-ray transient absorption spectroscopy. The main focus is on identifying the key structural factors that influence the excited-state properties, such as structural reorganization, intersystem crossing and solvent quenching, with these relatively new techniques on the ultrafast time scales. Ultimately, these structural factors can be used to rationally control the energetics and dynamics of the MLCT state during the light conversion processes. This insight will serve as guidance for material design using Cu(I) diimine complexes as building blocks.

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