Side‐Group Engineering in Benzimidazole‐Based Silver(I) Complexes: Crystal Structure, Band Composition, and Tunable Fluorescence Properties

Ligand side‐groups modulate the excitation and emission of complexes by altering electron density. In this work, three monoclinic silver(I) complexes [Ag( L 1 ) 2 ]NO 3 ( 1 ), [Ag( L 1 )( L 2 )]NO 3 ( 2 ), and [Ag( L 2 ) 2 ]NO 3 ( 3 ) (where L 1 = (1‐methyl‐1 H ‐benzimidazol‐2‐yl)methanol and L 2 = (1‐ethyl‐1 H ‐benzimidazol‐2‐yl)methanol) characterized similar coordination environments of Ag(I) centers based on single‐crystal X‐ray diffraction. However, their solid‐state fluorescence properties exhibited significant differences. Specifically, the emission peak of complex 1 was redshifted by 141 nm relative to L 1 , while the emission peak of complex 2 was redshifted by 94 nm relative to L 1 but blueshifted by 71 nm relative to L 2 . In contrast, complex 3 showed a blueshift of 93 nm compared to L 2 . This opposite shift direction arises from whether the bandgap narrows (for L 1 ) or widens (for L 2 ) upon Ag(I) coordination, as governed by the interplay of electron‐donating strength and steric hindrance of the side groups. These differences were primarily attributed to the fine‐tuning of the electronic structures, intermolecular interactions, and excited‐state charge distributions induced by the side‐group of ligand. This finding indicates that ligand side groups not only influence the emission wavelength of the free ligands but also determine the trend of photophysical property changes upon complex formation.