Heat-resistant Properties in the Phosphorescence of trans-Bis[β-(iminomethyl)aryloxy]platinum(II) Complexes: Effect of Aromaticity on d-π Conjugation Platforms.

Heat resistant properties towards thermal emission quenching of trans-bis[(β-iminomethyl)aryloxy]platinum(II) complexes 1-4, and a mechanistic rationale of these properties, are described in this report. Complex 1a exhibits intense red emission in 2-MeTHF at 298 K, whereas the analogues 2a-4a are less or non-emissive under the same measurement conditions. All four complexes are highly emissive at 77 K. The heat resistant properties toward thermal emission quenching (Φ298K/Φ77K) increase in the order 1a (0.52) > 2a (0.09) > 3a (0.02) > 4a (0.00). We investigated the emission decay and thermal deactivation processes using DFT, TD-DFT, and DHDF calculations of N,N'-diethyl forms 1b-4b, and discuss the results with a focus on the energy levels, molecular structures, and electronic configurations in the triplet excited states. The energy differences between the 3MLCT state and MECP increase in the order 1a > 2a, 3a > 4a, consistent with the experimental results for the heat resistant properties of these complexes. The origin of the present structure dependence of the 3MLCT-MECP energy gap is ascribed to the ease and difficulty of the high-lying dσ* orbital participating in the MECP upon thermal structural distortion. The structure-dependence in energy gaps between π* and dσ* orbitals, which is key for facilitating the thermal deactivation process, is rationally correlated with the extent of aromaticity on the coordination platforms (1b > (2b, 3b) > 4b).