Beyond Spin Crossover: Optical and Electronic Horizons of 2,6-Bis(pyrazol-1-yl)pyridine Ligands and Complexes.

The 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligand family is widely recognized for its versatile coordination abilities and broad functionalization potential. This review examines bpp and its modifications at the pyridine ring's 4-position, focusing on their influence on magnetic, optical, and electronic properties. Key applications discussed include spin crossover (SCO), single-ion and single-molecule magnetism (SIM and SMM), luminescence, redox flow batteries (RFBs), and photonic devices. We provide a comprehensive overview of ligand modifications involving carboxylates, extended aromatic systems, radicals, and redox-active units such as tetrathiafulvalene (TTF), alongside supramolecular architectures. The review highlights fundamental design principles, particularly the role of substituents in tuning the SCO behavior, photophysical properties, and self-assembly into functional nanostructures. Notable advancements include SCO-driven conductivity modulation, reversible luminescent switching, and amphiphilic bpp-based vesicles for multicolor emission. By analyzing the interplay between ligand structure and magnetic, optical, and electronic functions, we provide insights into the potential of bpp derivatives for advanced materials design. This review presents recent experimental and theoretical developments, offering a foundation for future exploration of bpp-based compounds in multifunctional devices.