/-[Pt(CN)(C≡CR)(CNBu)] Isomers: Synthesis, Photophysical, DFT Studies, and Chemosensory Behavior.

/ Isomerism can be a crucial factor for photophysical properties. Here, we report the synthesis and optical properties of a series of - and -alkynyl/isocyanide cycloplatinated compounds [Pt(CN)(C≡CR)(CNBu)] [R = CH-4-OMe , 3-CHS ; CN = 2-(2,4-difluorophenyl)pyridine (dfppy) (), 4-(2-pyridyl)benzaldehyde (ppy-CHO) ()]. The -forms do not isomerize thermally in MeCN solution to the forms, but upon photochemical irradiation in this medium at 298 K, a variable isomerization to the forms was observed. This behavior is in good agreement with the theoretically calculated energy values. The / configuration, the identity of the cyclometalated, and the alkynyl ligand influence on the absorption and emission properties of the complexes in solution, polystyrene (PS) films, and solid state are reported. All complexes are efficient triplet emitters in all media (except for - and - in CHCl solution at 298 K), with emission wavelengths depending mainly on the cyclometalated ligand in the region 473-490 nm (dfppy), 510-550 (ppy-CHO), and quantum yields (ϕ) ranging from 18.5 to 40.7% in PS films. The combined photophysical data and time-dependent density functional theory calculations (TD-DFT) at the excited-state T geometry reveal triplet excited states of L'LCT (C≡CR → CN)/IL (CN) character with minor MLCT contribution. The dfppy () complexes show a greater tendency to aggregate in rigid media than the ppy-CHO () and the with respect to the , showing red-shifted structureless bands of MMLCT and/or excimer-like nature. Interestingly, and undergo significant changes in the ultraviolet (UV) and emission spectra with Hg ions enabling their use for sensing of Hg ions in solution. This is clearly shown by the hypsochromic shift and substantial decrease of the low-energy absorption band and an increase of the intensity of the emission in the MeCN solution upon the addition of a solution of Hg(ClO) (1:5 molar ratio). Job's plot analysis estimated a 1:1 stoichiometry in the complexation mode of Hg by . The binding constant (log ) calculated for this system from absorption titration data resulted to be 2.56, and the limit of the detection (LOD) was 6.54 × 10 M.