Silver(I) Complexes of Diphenylpyridines: Crystal Structures, Luminescence Studies, Theoretical Insights, and Biological Activities.

A series of simple two-coordinated cationic silver(I) complexes, namely, [Ag{4-(4-R1 -phenyl)-2,6-diphenylpyridine}2 ]X (X=ClO4 - , BF4 - , or SO3 CF3 - ), with different electron-donating or -withdrawing groups (e.g., R1 =N(Me)2 , Me, H, Cl, and Br) on the phenyl ring, were successfully prepared. Extensive characterization of these complexes by various NMR spectroscopy techniques and mass spectrometry was further corroborated by single-crystal XRD analyses. Detailed photophysical investigations of [Ag{4-(4-N,N-dimethylaminophenyl)-2,6-diphenylpyridine}2 ]ClO4 (C1) displayed a strong room-temperature fluorescence in solution with an anomalously high luminescence quantum yield of 0.83. The effects of distinct substituent groups (C2-C5), π-conjugated aromatic rings (C6 and C7), and anions (C8 and C9) on the photoluminescence properties were evaluated. Furthermore, DFT and time-dependent DFT calculations were performed to discern the composition of the excited state, as well as to confirm the obtained relative emission energies upon substitution with electronically different ligands. These results indicated that the strong electron-donating substituent of N,N-dimethylamine played an important role in the unprecedented high luminescence quantum yield of C1. In addition, preliminary antimicrobial studies and confocal microscopy fluorescent imaging of HeLa cells labeled with these complexes reveal their potential applications in biological activities.