Enhancing Near‐Infrared Absorption in Osmium(II) Complexes Under the Cumulative Influence of Terpyridine Ligand and Anions: Combined Experimental and DFT/TD‐DFT Investigation

Abstract This work presents synthesis, characterization, and thorough examination on the photophysical and electrochemical behaviours of a new class of homo‐ and heteroleptic Os(II)‐terpyridine complexes. The compounds exhibit intense singlet metal‐to‐ligand charge‐transfer ( 1 MLCT) bands in the visible region (490–500 nm) and weak and broad spin‐forbidden bands in the spectral window of 670–680 nm due to singlet ground state ( 1 GS) to 3 MLCT state transition, facilitated by spin‐orbit coupling. The complexes exhibit strong luminescence within 730–750 nm at room temperature having lifetimes in the domain of 101–158 ns, depending upon solvents. Taking advantage of active methylene protons, anion‐induced modulation of spectral and electrochemical characteristics of the complexes has been convincingly demonstrated via different optical channels and spectroscopic tools as well as by cyclic voltammetry. The augmentation in absorption spectral characteristics is quite remarkable in presence of F − and intense absorption bands spanning across the entire visible region and stretching up to NIR domain is achieved. The mode of complex‐anion interplay was elucidated via 1 H, 31 P{ 1 H} and 19 F NMR spectroscopy as well as by computational investigations using DFT and TD‐DFT methods. The complexes also act as molecular sensors and switches that can function in visible and NIR domains upon alternate addition of anion and acid.