Effects of intramolecular hydrogen bonds on phosphorescence emission: A theoretical perspective

Abstract

Importing intramolecular hydrogen bond in phosphorescent transition metal complexes has been considered one of the excellent approaches to improve the electroluminescence performance of organic light‐emitting diodes in real applications. However, the relationships between such H‐bond structure and phosphorescent properties have not been theoretically revealed yet. In this study, two types of intramolecular hydrogen bonds are introduced into the two classes of traditional materials, that is, Pt(II) and Ir(III) complexes (1a and 2a) to completely elucidate their influence on the structures and properties by comparing with the original phosphors (1b and 2b) using density functional theory/time‐dependent density functional theory for the first time. A comprehensive analysis of the geometric structures, molecular orbitals, and luminescence properties (including phosphorescence emission wavelengths and radiative and nonradiative decay processes) has been carried out. Our theoretical model highlights that complexes 1a and 2a embedded with H‐bonds significantly promote the phosphorescence emission band blue‐shifted and restrict molecular deformations compared with the corresponding 1b and 2b, which can provide helpful guidance to regulate and design several aspects of highly efficient blue phosphorescent emitters.

Investigate the Relationship between Structure and Triplet Potential Energy Surface to Control the Phosphorescence Quantum Yield of Platinum(II) Complex: A Theoretical Investigation

“…In popular phosphorescent materials, for example, Pt­(II) and Ir­(III) complexes, the radiative decay rates depend on the percentage of metal-to-ligand charge transfer ( 3 MLCT) because the phosphoresce is caused by the strong spin–orbital coupling (SOC) originating from the heavy atoms (Pt­(II) and Ir­(III)). − Compared with the radiative decay rates, the nonradiative decay rates can be affected by more factors, which include the temperature-independent nonradiative decay process and the thermally activated nonradiative photo-deactivation pathway. The temperature-independent nonradiative decay process is closely related to the triplet energy, the Huang–Rhys factors ( Si ) used to elucidate the molecular rigidity, and the SOC matrix element between T 1 (lowest-lying triplet excited state) and S 0 (ground state). , In the case of the thermally activated nonradiative photo-deactivation pathway, the potential energy surface of triplet excited states should be constructed, that is, the 3 ES (emission state) → TS­( 3 ES/ 3 MC, transition state) → 3 MC (metal-centered excited state) → minimum-energy crossing point (MECP) conversion. − …”

“…The temperature-independent nonradiative decay process is closely related to the triplet energy, the Huang−Rhys factors (Si) used to elucidate the molecular rigidity, and the SOC matrix element between T 1 (lowest-lying triplet excited state) and S 0 (ground state). 6,7 In the case of the thermally activated nonradiative photo-deactivation pathway, the potential energy surface of triplet excited states should be constructed, that is, the 3 ES (emission state) → TS( 3 ES/ 3 MC, transition state) → 3 MC (metal-centered excited state) → minimum-energy crossing point (MECP) conversion. 8−11 So far, there has been abundant research focusing on the theoretical investigations of phosphorescent transition-metal complexes, such as the conjugated π extendibility of ligands, ligand stability, structural alternation of the central ligand, molecular aggregation, and triplet annihilation caused by planar molecules.…”