Chameleonic Photo- and Mechanoluminescence in Pyrazolate-Bridged NHC Cyclometalated Platinum Complexes.

DFT investigations on the ground (GS) and the first triplet (T) excited state potential energy surfaces (PES) were performed on a new series of platinum-butterfly complexes, [{Pt(CC*)(μ-Rpz)}] (Rpz: pz, ; 4-Mepz, ; 3,5-dmpz, ; 3,5-dppz, ), containing a cyclometalated NHC in their wings. The geometries of two close-lying local minima corresponding to butterfly spread conformers, -, and butterfly folded ones, -, with long and short Pt-Pt separations, respectively, were optimized in the GS and T PES. A comparison of the GS and T energy profiles revealed that an opposite trend is obtained in the relative stability of folded and spread conformers, the latter being more stabilized in their GS. Small Δ (/) along with small-energy barriers in the GS support the coexistence of both kinds of conformers, which influence the photo- and mechanoluminescence of these complexes. In 5 wt % doped PMMA films in the air, these complexes exhibit intense sky-blue emissions (PLQY: 72.0-85.9%) upon excitation at λ ≤ 380 nm arising from IL/MLCT excited states, corresponding to the predominant - conformers. Upon excitation at longer wavelengths (up to 450 nm), the minor - conformers afford a blue emission as well, with PLQY still significant (40%-60%). In the solid state, the as-prepared powder of exhibits a greenish-blue emission with QY ∼ 29%, mainly due to IL/MLCT excited states of butterfly spread molecules, . Mechanical grinding resulted in an enhanced and yellowish-green emission (QY ∼ 51%) due to the MMLCT excited states of butterfly folded molecules, , in such a way that the mechanoluminescence has been associated with an intramolecular structural change induced by mechanical grinding.