Plasmon-induced indirect charge transfer and vibronic stabilization in 4-nitrobenzenethiol

Plasmon-driven reduction of 4-nitrobenzenethiol (4-NBT) has been linked experimentally to transient negative ions and nonthermal multiquantum vibrational excitation, yet the molecular nature of the electronically excited anionic state involved in this process remains unclear. Here, we present a time-dependent density functional theory (TD-DFT) study of the Au₅-4-NBT complex to examine how hot-electron injection accesses a low-lying excited anionic manifold and how these states couple to vibrational motion. Our results identify a low-lying quartet excited anionic state, denoted Q₁, that becomes energetically accessible in the Au₅-4-NBT complex under plasmon-driven vertical electron attachment (VEA) conditions. Natural transition orbital analysis shows that Q₁ has mixed local-excitation and charge-transfer character, while comparison with free 4-NBT indicates that gold coordination reorganizes and lowers the relevant excited-state manifold. Vibronically resolved one-photon absorption (OPA) calculations show that access to Q₁ is strongly influenced by the NO₂ symmetric stretching coordinate, and one-photon emission (OPE) calculations indicate that the same coordinate also plays an important role in a possible radiative decay channel from Q₁. Potential-energy profile analysis further identifies low-energy crossing regions along the NO₂ coordinate that are consistent with vibronically assisted access to the excited anionic manifold. Taken together, these results provide a molecular-level framework for interpreting the transient anionic state and nonthermal multiquantum vibrational excitation proposed experimentally for plasmon-driven 4-NBT chemistry.