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

Experimental findings of Au(GSH) as a photosensitizer with the highest potential compared to other glutathione-protected clusters demand understanding the photophysics and relaxation dynamics of the Au(SR) cluster. To this end, we perform ab initio real-time nonadiabatic molecular dynamics simulations on Au(SH) to investigate its relaxation dynamics compared to the well-studied [Au(SR)] 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 Au(SH) cluster. The ground state growth times of Au(SH) are several orders of magnitude shorter than the growth times observed for the [Au(SH)] nanocluster. The S (HOMO-LUMO) state gives the slowest decay time (∼11 ps) among all the states (S-S) considered similar to [Au(SH)]. However, the S state in Au(SH) is a semiring-to-core charge transfer state, whereas S in the [Au(SH)] 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 S which has the second slowest decay of 6.4 ps. The hole relaxations are faster than the electron relaxations in Au(SH) 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.