Theoretical investigation of relaxation dynamics in the Au18(SH)14 thiolate-protected gold nanocluster.

Abstract

Experimental findings of Au18(GSH)14 as a photosensitizer with the highest potential compared to other glutathione-protected clusters demand understanding the photophysics and relaxation dynamics of the Au18(SR)14 cluster. To this end, we perform ab initio real-time nonadiabatic molecular dynamics simulations on Au18(SH)14 to investigate its relaxation dynamics compared to the well-studied [Au25(SR)18]-1 relaxation dynamics. In this work, the excitations covering up to ∼2.6 eV in the optical absorption spectrum are analyzed to understand the electronic relaxation process of the Au18(SH)14 cluster. The ground state growth times of Au18(SH)14 are several orders of magnitude shorter than the growth times observed for the [Au25(SH)18]-1 nanocluster. The S1 (HOMO-LUMO) state gives the slowest decay time (∼11 ps) among all the states (S1-S30) considered similar to [Au25(SH)18]-1. However, the S1 state in Au18(SH)14 is a semiring-to-core charge transfer state, whereas S1 in the [Au25(SH)18]-1 cluster is a core-to-core transition. The remaining higher excited states have very short decay time constants less than 1.4 ps except for S2 which has the second slowest decay of 6.4 ps. The hole relaxations are faster than the electron relaxations in Au18(SH)14 due to the closely packed HOMOs in the electronic structure. Radiative relaxations are also examined using the time-dependent density functional theory method, and the excited state emission energy and lifetime are found to be in good agreement with experiment.