Fine tuning phosphorescent properties of platinum complexes via different N-heterocyclic- based CNN ligands

In this article, series of platinum(II) complexes with different [C<^>N<^>N] cyclometalated ligand scaffolds was elaborated via density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods to mainly explore how the N atomic number, the atomic number of cyclization and the position of N atom influence phosphorescent processes (radiative and nonradiative decay processes). Thereinto, the factors which determine the radiative processes, including mu(S-n) for S-0-S-n transitions, Delta E(S-n-T-1) and SOC matrix elements < T-1 | H-SOC| S-n > were calculated. In addition, according to the energy gap law, the absorption-emission Stokes shifts as well as energy gaps between T-1 and S-0 states were also computed to describe the nonradiative decay processes qualitatively. As revealed, compared with six-membered Nheterocyclic compounds, five-membered N-heterocyclic compounds exhibit obvious blue-shifted behaviors, smaller nonradiative decay rate constants and similar radiative decay rate constant, especially complex 2b with the other N atom at the 4-position of five-membered N-heterocyclic ligand, which leads to a maximum hypsochromic shift of phosphorescence band and relatively larger radiative decay rate constant and smaller nonradiative decay rate constant among these complexes. These findings successfully illustrated the structure-property relationship and provide valuable information to design highly efficient phosphorescent material. (C) 2017 Published by Elsevier B.V.