Plasmon-Driven Charge Transfer Aids Selective Nonthermal Vibrational Excitation in Nitrobenzenethiol on Silver Nanoparticles: Insights from Embedded Correlated Wavefunction Calculations

Abstract

In plasmonic photocatalysis, a commonly invoked reaction enhancement mechanism is the excitation of a metal-to-molecule charge transfer state, resulting in transient-ion formation. Once accessed, a transient-ion state leads to changes in the molecular geometry that can leave the molecule in a highly vibrationally excited state upon relaxation to the electronic ground state. In this work, we investigate this phenomenon as it applies to 4-nitrobenzenethiol (4-NBT) on a silver (Ag) surface, which has been observed to form dimercaptoazobisbenzene upon plasmonic excitation. At the nascent stage of the reaction under illumination, it is hypothesized that 4-NBT may acquire an electron that modifies its chemistry. Here, we use density functional embedding theory and embedded correlated wavefunction theory to test this hypothesis by investigating a manifold of electronically excited states for adsorbed 4-NBT. We map out the potential energy curve along 4-NBT’s NO2 symmetric stretch and scissor bend vibrational modes to gauge how electronic excitation and associated molecular distortions may give rise to selective ground-state vibrational excitation. With embedded state-averaged density-matrix renormalization group self-consistent field theory and subsequent embedded N-electron valence state second order perturbation theory, we obtain the manifold of electronic excited states along the vibrational normal mode vectors and determine whether any of the excited states are in resonance with the plasmon peak frequency. Our calculations show that a metal-to-molecule charge-transfer state exists at an energy within the plasmon resonance for motion along 4-NBT’s NO2 symmetric stretch mode but not along the scissor bend mode. The charge-transfer state, involving transfer from the Ag surface 5s-states at the Fermi level to a π* orbital of 4-NBT, leads to distortions that lengthen the N–O bonds, resulting in a higher vibrational quantum state upon relaxation to the electronic ground state. This suggests that 4-NBT’s NO2 symmetric stretch can be nonthermally excited (enhanced via plasmon resonance) and potentially contribute to its release of dioxygen and subsequent dimerization under illumination.