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

DFT investigations on the ground (GS) and the first triplet (T₁) excited state potential energy surfaces (PES) were performed on a new series of platinum-butterfly complexes, [{Pt(C^∧C*)(μ-Rpz)}₂] (Rpz: pz, 1; 4-Mepz, 2; 3,5-dmpz, 3; 3,5-dppz, 4), containing a cyclometalated NHC in their wings. The geometries of two close-lying local minima corresponding to butterfly spread conformers, 1s-4s, and butterfly folded ones, 1f-4f, with long and short Pt-Pt separations, respectively, were optimized in the GS and T₁ PES. A comparison of the GS and T₁ 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 ΔG (s/f) 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 ³IL/MLCT excited states, corresponding to the predominant 1s-4s conformers. Upon excitation at longer wavelengths (up to 450 nm), the minor 1f-4f conformers afford a blue emission as well, with PLQY still significant (40%-60%). In the solid state, the as-prepared powder of 4 exhibits a greenish-blue emission with QY ∼ 29%, mainly due to ³IL/³MLCT excited states of butterfly spread molecules, 4s. Mechanical grinding resulted in an enhanced and yellowish-green emission (QY ∼ 51%) due to the ³MMLCT excited states of butterfly folded molecules, 4f, in such a way that the mechanoluminescence has been associated with an intramolecular structural change induced by mechanical grinding.