Designing Optical Anisotropy: Silver-Aminoalkylpyridine Nitrate Complexes with Tunable Structures

To introduce a design strategy for improving optical properties, two silver-amino alkylpyridine nitrate complexes, AgC₆H₈N₃O₃ and Ag₂C₁₄H₂₀N₆O₆, were successfully synthesized using a recrystallization method. By employing polarizable π-conjugated [NO₃^–] ions, two types of pyridine ligands, and silver cations with a high affinity for pyridine, we obtained a one-dimensional chain structure with 4-aminomethylpyridine (AgC₆H₈N₃O₃) and a zero-dimensional molecular compound by introducing a relatively flexible aliphatic chain with 4-(2-aminoethyl)pyridine (Ag₂C₁₄H₂₀N₆O₆). The compounds crystallize in the triclinic crystal system with the centrosymmetric P-1 space group, exhibiting a change in orientation between the π-conjugated system and the silver ion. Despite similar optical band gaps (3.69 eV for AgC₆H₈N₃O₃ and 3.73 eV for Ag₂C₁₄H₂₀N₆O₆), AgC₆H₈N₃O₃ shows higher absorption in the 350–600 nm range. Electronic structure calculations support the ultraviolet absorption findings, suggesting that charge transfer with π-conjugated systems influences birefringence. Ag₂C₁₄H₂₀N₆O₆ exhibits experimental birefringence (0.261@546.1 nm) surpassing that of AgC₆H₈N₃O₃ (0.212@546.1 nm), placing it among the highest recorded values within metal-pyridine incorporating nitrate complexes. The nonconventional orientation of π-conjugated [NO₃^–] ions contributes to this phenomenon, enhancing the action of free π-conjugated orbitals. This design strategy for micromodulating the alignment of the π-conjugated system promises to be an effective approach for enhancing optical properties, such as birefringence.