Observation of spin-crossover behaviour in Co(<scp>ii</scp>)-terpyridine complexes possessing π-conjugated substations

Exploring new compounds that exhibit spin-state switching under external stimuli is crucial for advancing materials science. However, the feasibility of combining π-conjugated moieties with Co(ii)-terpy cation units <i>via</i> an ethynyl group has not yet been investigated. This approach represents a hybridization strategy wherein distinct functional moieties are incorporated into a single coordination metal complex to achieve multifunctionality. In this study, we synthesised a series of Co(ii)-terpyridine complexes [Co(R-ethynyl-terpy)₂](BF₄)₂ (R = phenyl for 1, naphthyl for 2, anthracenyl for 3, and pyrenyl for 4), resolved their crystal structures and packing arrangements, and evaluated their magnetic properties using a superconducting quantum interference device magnetometer. These Co(ii)-based complexes exhibited thermal spin-crossover behaviour in the solid state. Although complexes 1-4 were luminescent in solution, only complexes 3 and 4 emitted dual monomer and excimer fluorescence in solution due to their substituted π-conjugated moieties. Our findings are expected to serve as a platform for the synthesis of magnetically functional coordination metal compounds based on Co(ii) ions for the potential development of photophysical materials.