Tightly yet Dynamically Bound Aliphatic Guanidinium Ligands for Lead Halide Perovskite Nanocrystals

Lead halide perovskites (LHPs) have disrupted the field of visible-range emitting colloidal semiconductor nanocrystals (NCs) owing to their exceptional emissivity as classical and quantum light sources. Their appeal is countered by the stability challenges arising from inherent lattice softness and labile surface ligand bonding. Recent ligand design efforts have drastically improved the structural integrity of LHP NCs by pursuing strongly and statically bound ligands, as found with zwitterionic headgroups. Here, we venture into a new class of cationic ligands featuring guanidinium headgroups, which combine the compactness of primary ammoniums with the deprotonation resistance of quaternary ammoniums. We reasoned that strong yet dynamic ligand binding offers distinct benefits. A library of aliphatic guanidinium molecules was synthesized and surveyed as capping ligands for CsPbBr₃, FAPbBr₃, and CsPbI₃ NCs, enabling up to 95% photoluminescence quantum yields for bromide compositions. We then compare binding behavior of guanidinium ligands with those of other widely used ligands. Zwitterionic ligands phosphocholines and phosphoethanolamines bind more strongly than sulfobetaines and quaternary ammonium ligands, yet all these headgroups impart relatively static binding with exchange rates below 5 s^-1. Primary ammonium ligands are champions in dynamicity but lag behind in ligand-coverage retention and, hence, long-term stability. Guanidinium ligands strike a favorable balance: they match the binding dynamics of primary ammonium ligands while significantly enhancing binding strength, enabling rigorous purification and compatibility with relatively polar solvents such as tetrahydrofuran. We then showcase the benefits of dynamically bound ligands in photocatalytic C-C coupling reactions.