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

Abstract

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.