Sharp near-infrared (NIR) emitters, particularly those operating in the NIR-II window (1000–1700 nm), hold promise for applications in telecommunications, lasers and high-resolution in vivo imaging. Such sharp NIR emissions at a few specific wavelengths arise from shielded f→f electronic transitions in rare-earth lanthanides like Yb3+, Nd3+ and Er3+. In contrast, d→d transitions typically yield broad emission, because d-electrons strongly interact with the surrounding ligands. In this talk, I will discuss about atomic-scale structure-function link showing how Mo3+ doped in Cs2NaInCl6 double perovskites yield d→d electronic transitions emitting ultra-narrow NIR-II radiation, similar to the f→f transitions. Recently, we reported the first ambient-stable Mo3+-based ultra-narrow (FWHM < 10 meV) NIR-II emission at 1095 nm.1,2 The highly symmetric [MoCl6]3- octahedra with 4d3 electrons, result into t_2g^3 e_g^0 → t_2g^3 e_g^0 intra-configurational spin-flip (ICSF) d-d transitions. Likewise, d2 electrons in tetrahedral lattice, like Mn5+-doped into an oxide lattice also show ICSF d-d transitions with ultra-narrow emission at ~1170 nm. The ICSF d-d transitions behave similar to atomic-like f-ftransitions, with ultra-narrow spectral width, violation of Kasha’s rule, and showing upconversion photoluminescence.3