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

Abstract

DFT investigations on the ground (GS) and the first triplet (T 1 ) 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; 3,5dppz, 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 T 1 PES. A comparison of the GS and T 1 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 smallenergy 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 3 IL/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 3 IL/ 3 MLCT excited states of butterfly spread molecules, 4s. Mechanical grinding resulted in an enhanced and yellowish-green emission (QY ∼ 51%) due to the 3 MMLCT 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.

Computational Protocol to Calculate the Phosphorescence Energy of Pt(II) Complexes: Is the Lowest Triplet Excited State Always Involved in Emission? A Comprehensive Benchmark Study

“…However, one cannot disregard that this is always the case for any given complex as, e.g., a significant activation barrier between different triplet excited states might prevent the population of the lowest adiabatic triplet state at a given temperature. This situation has been described on related platinum–butterfly complexes …”

“…This situation has been described on related platinum−butterfly complexes. 82 After reassigning the actual triplet excited state involved in the emission for complexes 6, 7, 10, 11, and 12, i.e., the T em state, we are now ready to discuss the statistical errors (i.e., mean absolute deviation (MAD), the mean signed deviation (MSD), and the root-mean-square deviation (RMSD) values) of the different DLPNO-CCSD(T) approaches for the phosphorescence energies of complexes 1−12. The statistical errors are shown in Figure 5.…”

Phosphorescent Tris-cyclometalated Pt(IV) Complexes with Mesoionic N-Heterocyclic Carbene and 2-Arylpyridine Ligands

“…Bidentate cyclometalated aryl-substituted NHC ligands (aryl-NHCs, C∧C*) have been extraordinarily successful with the Ir­(III) − and Pt­(II) − ions as a replacement of cyclometalated 2-arylpyridines (C∧N), enabling better photostabilities, wider color tunability, and higher emission efficiencies. These enhancements are brought about by the larger ligand-field splitting induced by the NHC moiety with respect to the pyridine and, consequently, the reduced thermal accessibility of MC states from the triplet, mixed ligand-centered/metal-to-ligand charge-transfer ( 3 LC/MLCT) emissive state of Ir­(III) and Pt­(II) complexes.…”

Metal–Metal-to-Ligand Charge Transfer in Pt(II) Dimers Bridged by Pyridyl and Quinoline Thiols

“…The energetic splitting between the dσ and dσ* orbitals is determined in part by the ground-state Pt–Pt distance, so the energy of the MMLCT transitions can be coarsely modulated by tuning this orbital overlap. This concept was first exploited by Thompson and co-workers in pyrazolate-bridged C ∧ N terminated Pt­(II) complexes, illustrating that variation in the pyrazolate 3- and 5-substituents effectively controlled the Pt–Pt separation and the nature of the electronic transitions observed. , Our research group expanded upon this effort using distinct cyclometalating ligands in concert with pyrazolate bridging ligands leading to the conclusion that judicious selection of both ligand types influences the resultant triplet character in the excited state. − The Ma group has more recently developed new classes of Pt­(II) dimer chromophores featuring strikingly bright phosphorescence. − In parallel, work from Sicilia and co-workers has generated numerous examples of interesting behavior, − particularly noteworthy was the recent discovery of chameleonic photo- and mechanoluminescence response from NHC cyclometalated Pt­(II) dimers …”

“…44−47 In parallel, work from Sicilia and co-workers has generated numerous examples of interesting behavior, 48−52 particularly noteworthy was the recent discovery of chameleonic photo-and mechanoluminescence response from NHC cyclometalated Pt(II) dimers. 53 The structural effects leading to MMLCT excited-state characters in platinum dimers deserve further consideration as these design features are critical toward developing molecules featuring long lifetimes that can be systematically tuned through subtle geometry changes. Pyrazolate bridging ligands nominally represent symmetric 3−5-substitiuted N ∧ N-type ligands valuable for constructing Pt(II) dimers with MMLCT excited states, whereas asymmetric pyridine derivatives including 6-substituted-2-hydroxypyridines (N ∧ O), 54 2-mercaptopyridines (N ∧ S), 37,38,55−57 and 2-mercaptobenzothiazoles have also realized success.…”