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[(beta-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 (phi(298 K)/phi(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 ((MLCT)-M-3) 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 (MLCT)-M-3-MECP energy gap is ascribed to the ease or difficulty of the high-lying d(sigma*) orbital participating in the MECP upon thermal structural distortion. The structure dependence in energy gaps between the pi* and d(sigma*) 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).