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

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.