Nonadiabatic dynamics simulation method for transition metal complexes: Theory, developments, and applications

Abstract

Excited-state related properties of transition metal complexes have received much attention in recent years due to their potential applications in the field of energy, material and biology. To fully understand the underlying mechanism, we always need to resort to ultrafast time-resolved spectroscopies as well as reliable ab initio nonadiabatic dynamics simulations. However, nonadiabatic dynamics simulation methods that are suitable for investigating excited-state dynamics of transition metal complexes still demand further developments to improve simulation efficiency and accuracy. In this short review, we have shortly present some recent developments of non-adiabatic dynamics methods that are suitable for investigating excited state properties of transition metal complexes, mainly including Multi-Configuration Time Dependent Hartree (MCTDH) and mixed quantum-classical surface hopping methods. In addition, we have introduced our recently developed TD-DFT-based generalized trajectory surface hopping method in which an efficient algorithm of computing nonadiabatic couplings is realized. In combination with the classical path approximation, this newly developed method can effectively simulate internal conversion and intersystem crossing processes of transition metal complexes on an equal footing. With this newly developed method, we have systematically investigated the early-time ultrafast excited-state dynamics of three Ir(III) complexes bearing distinct ligands. The ISC rates estimated by our simulations are 65, 81 and 140 fs, which agree quite well with experimentally measured ca. 80, 80 and 110 fs. These Ir(III) complexes show similar macroscopic phenomena, i.e., ultrafast IC and ISC, their microscopic excited-state relaxation dynamics are individually different, for example, electron and hole transfer dynamics. These new insights for excited-state dynamics of Ir(III) complexes could be helpful for rationally designing Ir-containing compounds with excellent photoluminescence performance. In addition, we have also investigated the ultrafast intersystem crossing process of Au(I) naphthalene derivatives. We have found that (1) ultrafast and sub-picosecond intersystem crossing processes are mainly caused by small energy gaps and large spin-orbit couplings between S1 and T n . (2) Adding the second gold(I)-phosphine group does not increase spin-orbit couplings between S1 and T n but decrease their values remarkably, which implies that heavy-atom effects are state-specific, not-universal. (3) The position at which the second gold(I)-phosphine group is attached has remarkable influence on electronic structures of S1 and T n and their relative energies, which affect energy gaps and spin-orbit couplings between S1 and T n and eventually modulate intersystem crossing rates from S1 to T n . This work exemplifies that different isomers of a compound could have distinct excited-state relaxation dynamics. Above simulation results not only agree pretty well with experiments, but also provide valuable insights into excited-state relaxation dynamics for these transition-metal-containing complexes. In one word, our present developments have established an efficient and accurate simulation tool for investigating excited-state dynamics of transition metal complexes.