Live Long and Prosper – Coordination-induced Spin State Switching in Photoactive 3d8 Complexes

• Applicants: Dr. Stephan Kupfer; Professor Dr. Dirk Schwarzer
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Inorganic Molecular Chemistry - Synthesis and Characterisation ; Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 560796377

Noble metal complexes have dominated the research field when it comes to light-driven materials and applications. These 4d and 5d metal complexes exhibit desirable photophysical properties such as strong absorption, photo-stability and long excited-state lifetimes. Such features make them suitable for a wide range of light-harvesting applications, in areas such as environmental sensors, smart pigments, information processing and storage as well as photocatalytic reactions. However, given the rarity of such metals and their associated high cost, their widespread application is unfeasible. During the last decade, there has been an increased interest in using cheaper, earth-abundant first-row transition metal complexes in light-driven applications. Unfortunately, such 3d transition metal complexes typically suffer from rather short-lived excited (spin) states in contrast to their 4d and 5d analogues. However, due to research efforts over the last decade there are now a growing number of 3d metal complexes with luminescent and photocatalytic properties that can compete with established noble metal-based compounds. One means by which the excited-state lifetime of such 3d complexes can be extended is through a process known as photo-switching. Upon interaction with light, the spin state of the system switches, prolonging the excited state and increasing its emissive behavior. Such behavior only occurs in specific complexes, where careful consideration of chemical, electronic and geometric factors has been undertaken. Our research team, which combines synthetic, spectroscopic and computational expertise, will focus on a class of square-planar 3d8, e.g. Ni(II), complexes. We will unravel the light-driven processes associated with interactive spin state switching from the low-spin to the high-spin species using a range of theoretical and experimental methods. In-depth structure-property relationships will be derived, and knowledge will be utilized to establish new design principles to allow us to further elongate the lifetime of the excited high-spin species. This will represent a crucial step toward the use of "green" and cost-effective 3d metal complexes in the fields of photophysical and photochemical applications.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2491: Interactive Spin-State Switching
• International Connection: Switzerland
• Cooperation Partner: Professor Dr. Oliver S. Wenger