Photochemistry that can be driven at low incident photon flux on optically excited plasmonic nanoparticles is attracting increasing research interest because plasmon-driven reactions offer new pathways for efficient conversion of the abundant solar energy into fuel. The confinement of the electromagnetic field by plasmonic metal nanoparticle and consequent field enhancement gives a dual advantage of ultrasensitive in situ and operando spectroscopic methods to monitor enhanced field-driven photochemical transformation. Charge carrier driven either via direct or indirect excitation mechanism are well-established pathways in plasmon-driven reactions to date. However, the excitation mechanism via plasmon-pumped electron transition from an occupied molecular orbital (HOMO) to an unoccupied molecular orbital (LUMO) of the adsorbate is not well understood and hence not reported yet. Here, we address for the first-time excitation wavelength-dependent plasmon-pumped electronic excitation mechanism to initiate surface photochemistry based on the N-demethylation of methylene blue (MB) on gold nanostructures. At excitation wavelength that overlaps with the resonances of MB and the localized surface plasmon resonance of gold nanoparticles, we enhanced selectively conversion of MB to thionine (TH) in the presence of oxygen in the atmosphere and water in the surface–molecule complex. In this mechanism, the intense electric field pushes the ground state of the adsorbed MB and increase the population of the excited state that increases the quantum yield of singlet oxygen generated via energy transfer from MB triplet excited state to oxygen in triplet ground state. The chemical changes are monitored by detecting the vibrational signatures of the reactant and product species in situ using surface-enhanced Raman scattering (SERS) spectroscopy.
It is important to note that enhanced surface chemistry and ultrasensitive spectroscopic methods depend strongly on enhanced adsorbate absorption. However, accurate determination of the adsorbate on the surface of metal nanoparticle is often ignored because of the technical difficulties for direct absorption measurements. Here, we show that molecule-plasmon weak excitation coupling can be used for determining the electronic absorption band of resonant adsorbates with sensitivity down to sub-monolayer surface coverage. By comparing the absorbance of gold nanoisland (AuNI) with and without adsorbate of resonant molecules (methylene blue (MB) and thionine (TH)), induced transparency is observed at the absorption band of the adsorbates. Apart from significant spectral broadening and red shifting of peak wavelength, the inverted transparency spectrum has a surprising similarity to the absorption spectrum of the corresponding dye in solution. Interestingly, the adsorption isotherm determined based on the integral area under the inverted transparency spectra is linearly correlated to the corresponding isotherm determined based on adsorbate induced plasmon resonance red shift. The results presented in this work demonstrate a simple, sensitive and reliable approach for determining adsorbate absorption spectrum from molecule-plasmon excitation coupling.
Photochemistry, Plasmon-pumped excitation, charge transfer excitation, singlet oxygen generation, methylene blue, N-demethylation, resonant excitation, absorption band of adsorbate
Level of Degree
Department of Chemistry and Chemical Biology
First Committee Member (Chair)
Terefe G. Habteyes
Second Committee Member
Third Committee Member
Fourth Committee Member
Tesema, Tefera Entele. "Charge Transfer and Plasmon-Pumped Molecular Excitation Mechanisms in Surface Enhanced Photochemistry." (2020). https://digitalrepository.unm.edu/chem_etds/173