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(L₁)₂]NO₃ (1), [Ag(L₁)(L₂)]NO₃ (2), and [Ag(L₂)₂]NO₃ (3) (where L₁ = (1-methyl-1H-benzimidazol-2-yl)methanol and L₂ = (1-ethyl-1H-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 L₁, while the emission peak of complex 2 was redshifted by 94 nm relative to L₁ but blueshifted by 71 nm relative to L₂. In contrast, complex 3 showed a blueshift of 93 nm compared to L₂. This opposite shift direction arises from whether the bandgap narrows (for L₁) or widens (for L₂) 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.