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

The heat‐resistant properties towards thermal emission quenching of trans ‐bis[(β‐iminomethyl)aryloxy]platinum(II) complexes bearing 3‐iminomethyl‐2‐naphtholato‐ ( 1 ), 1‐iminomethyl‐2‐naphtholato‐ ( 2 ), 2‐iminomethyl‐1‐naphtholato‐ ( 3 ), and 2‐iminomethyl‐1‐phenolato ( 4 ) moieties, and a mechanistic rationale of these properties, are described in this report. Complex 1 a , with N , N′ ‐dipentyl groups, exhibits intense red emission in 2‐methyl‐2,3,4,5‐tetrahydrofuran (2‐MeTHF) at 298 K, whereas the analogues 2 a – 4 a 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 ( Φ 298 K / Φ 77 K ) increase in the order 1 a (0.52)> 2 a (0.09)> 3 a (0.02)>> 4 a (0.00). We investigated the emission decay and thermal‐deactivation processes using density functional theory (DFT), time‐dependent (TD) DFT, and double‐hybrid density functional theory (DHDF) calculations of N , N′ ‐diethyl forms 1 b – 4 b , 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 triplet metal–ligand charge transfer ( 3 MLCT) state and minimum‐energy crossing point between the lowest triplet state and singlet ground state (MECP) increase in the order 1 a > 2 a ,  3 a > 4 a , consistent with the experimental results for the heat‐resistant properties of these complexes. The origin of the present structure dependence of the 3 MLCT–MECP energy gap is ascribed to the ease or difficulty of the high‐lying d σ* orbital participating in the MECP upon thermal structural distortion. The structure dependence in energy gaps between the π* and d σ* orbitals, which is key for facilitating the thermal deactivation process, is rationally correlated with the extent of aromaticity on the coordination platforms ( 1 b >( 2 b ,  3 b )> 4 b ).