The intersystem crossing of a cyclic (alkyl)(amino) carbene gold (i) complex.

S Thompson,T J Penfold,J Eng

doi:10.1063/1.5032185

Abstract

The intersystem crossing (ISC) mechanism of a cyclic (alkyl)(amino) carbene gold (I) complex (CMA1) is studied using quantum dynamics. A model spin-vibronic Hamiltonian is developed, which includes 10 excited states and two important nuclear degrees of freedom. The quantum dynamics reveals that ISC from S1 → T1 occurs on the tens of picosecond time scale, consistent with recent experiments. It is driven by motion along the torsional degree of freedom of the carbazole (Cz) ligand, which causes orthogonality between the donor and acceptor groups closing the gap between the initial (S1) and final (T1) states. The role of higher triplet states through spin-vibronic interactions is also discussed. Although previous calculations, evaluated in the Condon approximation, yield large ISC rates, our present dynamical treatment, taking into account the large amplitude torsional motion, increases the calculated rate by an order of magnitude improving the agreement with experiments. The model spin-vibronic Hamiltonian developed can also be used to understand the properties of related linear metal carbene compounds, facilitating molecular design.

Highlights

Molecular triplet states represent an important outcome from excited state dynamics, and their presence can be detrimental as well as exploited
ISC rates correlated to the vibrational period of important normal modes instead of the heavy atom effect4 or thermally activated intersystem crossing pathways, which depend upon specific molecular vibrations,5 have been observed
The quantum dynamics shows that the ISC dynamics occurs on the time scale of tens of picosecond and along the torsional mode, allowing the wavepacket to reach regions of the potential where the singlet and triplet states are degenerate

Summary

Introduction

Molecular triplet states represent an important outcome from excited state dynamics, and their presence can be detrimental as well as exploited. Intersystem crossing (ISC) is driven by direct spin-orbit coupling (SOC) between two states of different multiplicity These two states are considered in isolation, and the electronic spin-orbit coupling matrix elements (SOCMEs) are treated independently from the vibrational degrees of freedom, i.e., the Condon approximation.. ISC rates correlated to the vibrational period of important normal modes instead of the heavy atom effect or thermally activated intersystem crossing pathways, which depend upon specific molecular vibrations, have been observed. These results, which signify the breakdown of the Condon approximation, place an emphasis upon explicitly understanding the coupled dynamics of the spin, electronic, and vibrational components occurring within molecular excited states

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Conclusion