Detecting Coordination-Induced Spin-State Switching in Coumarin-Tagged Nickel(II) Complexes <i>via</i> the Fluorescence Quenching Method

In this study, coumarin-functionalized Ni(II) complex <b>3</b> has been demonstrated to act as a fluorescent probe for the quantification of coordination-induced spin-state switching (CISSS) behavior <i>via</i> fluorescence quenching. Upon coordination with pyridine, the Ni(II) center undergoes a spin-state transition from a diamagnetic to a paramagnetic form, leading to the observable changes in fluorescence intensity. Deconvolution of quenching contributions based on three distinct models─CISSS quenching, sphere of action, and dynamic (collisional) quenching─reveals that static quenching mechanisms (involving CISSS quenching and sphere of action) dominate throughout the titration process, with CISSS quenching contributing up to 28% maximally. Theoretical analysis further confirms that the elevation of the highest occupied molecular orbital (HOMO) energy level in the ligand upon coordination with nickel(II) ions facilitates the photoinduced electron transfer (PET) process. Nickel(II) complexes in diamagnetic and paramagnetic states exhibit distinct PET rates (<i>k</i>_PET-dia = 0.37(1) ns^-1 for diamagnetic species; <i>k</i>_PET-para = 0.48(2) ns^-1 for paramagnetic species), which, in turn, induces detectable CISSS-dependent fluorescence quenching. This work demonstrates the feasibility of using fluorescence quenching as a visual tool to monitor CISSS behavior in coordination complexes, offering new insights into spin-state-responsive molecular sensing.