Theoretical investigation of relaxation dynamics in the Au-18(SH)(14) thiolate-protected gold nanocluster

摘要

Experimental findings of Au-18(GSH)(14) as a photosensitizer with the highest potential compared to other glutathione-protected clusters demand understanding the photophysics and relaxation dynamics of the Au-18(SR)(14) cluster. To this end, we perform ab initio real-time nonadiabatic molecular dynamics simulations on Au-18(SH)(14) to investigate its relaxation dynamics compared to the well-studied [Au-25(SR)(18)](-1) relaxation dynamics. In this work, the excitations covering up to similar to 2.6 eV in the optical absorption spectrum are analyzed to understand the electronic relaxation process of the Au-18(SH)(14) cluster. The ground state growth times of Au-18(SH)(14) are several orders of magnitude shorter than the growth times observed for the [Au-25(SH)(18)](-1) nanocluster. The S-1 (HOMO-LUMO) state gives the slowest decay time (similar to 11 ps) among all the states (S-1-S-30) considered similar to [Au-25(SH)(18)](-1). However, the S-1 state in Au-18(SH)(14) is a semiring-to-core charge transfer state, whereas S-1 in the [Au-25(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 S-2 which has the second slowest decay of 6.4 ps. The hole relaxations are faster than the electron relaxations in Au-18(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.