A study on the modification of azole rings to regulate the transition dipole moment, MLCT and T1 structural distortion of 2-pyridyl-azole copper (I) complexes for high phosphorescence performance

Abstract

Heterocyclic ligands based on 2-pyridyl-azole, have become some of the ideal ligands for metal complexes due to their flexible structure and wide wavelength tunability. In this paper, a series of Cu(N∧N) (P∧P) complexes are studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The results indicate that the combination of incorporation of N atoms and substitution of electron donor-acceptor groups can effectively control the type and the proportion of the lowest triplet states (T1) phosphorescent transition. A higher matching degree between the effective spin-orbit coupling (SOC) matrix elements and the transition dipole moments (The Sn state has both effective SOC and large transition dipole moments of Sn→S0 transition) brings about a larger transition dipole moment MTa of T1a→S0 transition, and further results in the fastest radiative decay rate (kr) for complex 2, although the main transition of which is internal-ligand charge transfer (ILCT). In addition, the non-radiative decay rate (knr) of complex 2 is lower than other complexes of series 3 (the reorganization energy produced by in-plane bending vibration of C–H bond in six-membered ring is larger than complex 2), so this complex has the highest quantum yield. Overall, the above analyses illustrate that despite the participation of Cu(I) in transition as an indispensable factor for the radiative transition, the higher component ratio does not necessarily lead to a faster kr. In addition, it is also essential to have a well-matched large transition dipole moments of the singlet states Sn→S0 transition involved in the effective SOC. Consequently, in the development of the 2-pyridyl-azole Cu(I) luminescent materials, the high quantum yield materials can be designed by adjusting the substituent at C3 position and the strength of electron donor-acceptor groups at C5 position to balance the charge transfer transition modes.