This supplementary text provides detailed analysis of charge transfer efficiencies in plasmonic systems, focusing on the role of plasmon in interfacial charge transfer. The study evaluates the contributions of chemical interface damping (CID) to the measured homogeneous plasmon linewidth, $\Gamma$, using the equation $\Gamma = \Gamma_{Bulk} + \Gamma_{rad} + \Gamma_{CID}$. The bulk damping contribution is calculated based on the energy-dependent bulk dielectric function, while the radiative damping is estimated using a proportionality constant derived from previous studies. The CID efficiency, $\eta_{CID}$, is calculated as the ratio of $\Gamma_{CID}$ to $\Gamma$, representing the direct charge transfer efficiency. The study also considers the resonance energy dependence of $\Gamma_{rad}$, which may affect charge transfer efficiency, but this effect is deemed minor compared to experimental errors. Charge transfer efficiencies are further analyzed using IR/NIR and visible transient absorption spectroscopy. The total charge transfer efficiencies (direct + indirect pathways) are determined by comparing the signal amplitude of gold nanorod@TiO₂ core-shell heterostructures with direct bandgap excitation of TiO₂ control samples. The analysis assumes that the initial signal at zero pump-probe delay times is proportional to the free carrier concentration and scales linearly with the absorbed photon density. The study also notes that different excitation wavelengths may lead to different initial electron energies, affecting absorption cross sections, but the excellent agreement between wavelength-dependent charge transfer efficiencies in the IR/NIR and visible regions suggests this effect is minor for the system studied. The visible plasmon resonance dynamics are proportional to the electronic temperature, which is lowered by interfacial charge transfer in AuNR@TiO₂ heterostructures. The charge transfer efficiency is determined by the ratio of the slopes of fluence-dependent measurements of the bleach recovery dynamics for AuNRs@TiO₂ compared to AuNRs. The study uses a two-temperature model to quantify the plasmon bleach dynamics, considering electron-phonon interactions and thermalization processes. The Persson model is used to quantify the influence of the chemical environment on the surface plasmon resonance (SPR) of metal particles. The model calculates the width of the SPR based on the Fermi velocity, particle radius, and contributions from diffusive scattering of electrons at the particle surface. The study applies the model to AuNRs@TiO₂, considering the effective mean free path of the AuNR core and the number of TiO₂ resonance states per unit surface area. The total width due to CID is calculated as the sum of the tangential and normal components, resulting in a CID width of 19 ± 1 meV. The study also considers hypothetical systems where broadening arises from isolated defect sites at the surface, but the results show that the broadening is due to states inherent to the interface itself rather than isolated defect sites. Open-source software isThis supplementary text provides detailed analysis of charge transfer efficiencies in plasmonic systems, focusing on the role of plasmon in interfacial charge transfer. The study evaluates the contributions of chemical interface damping (CID) to the measured homogeneous plasmon linewidth, $\Gamma$, using the equation $\Gamma = \Gamma_{Bulk} + \Gamma_{rad} + \Gamma_{CID}$. The bulk damping contribution is calculated based on the energy-dependent bulk dielectric function, while the radiative damping is estimated using a proportionality constant derived from previous studies. The CID efficiency, $\eta_{CID}$, is calculated as the ratio of $\Gamma_{CID}$ to $\Gamma$, representing the direct charge transfer efficiency. The study also considers the resonance energy dependence of $\Gamma_{rad}$, which may affect charge transfer efficiency, but this effect is deemed minor compared to experimental errors. Charge transfer efficiencies are further analyzed using IR/NIR and visible transient absorption spectroscopy. The total charge transfer efficiencies (direct + indirect pathways) are determined by comparing the signal amplitude of gold nanorod@TiO₂ core-shell heterostructures with direct bandgap excitation of TiO₂ control samples. The analysis assumes that the initial signal at zero pump-probe delay times is proportional to the free carrier concentration and scales linearly with the absorbed photon density. The study also notes that different excitation wavelengths may lead to different initial electron energies, affecting absorption cross sections, but the excellent agreement between wavelength-dependent charge transfer efficiencies in the IR/NIR and visible regions suggests this effect is minor for the system studied. The visible plasmon resonance dynamics are proportional to the electronic temperature, which is lowered by interfacial charge transfer in AuNR@TiO₂ heterostructures. The charge transfer efficiency is determined by the ratio of the slopes of fluence-dependent measurements of the bleach recovery dynamics for AuNRs@TiO₂ compared to AuNRs. The study uses a two-temperature model to quantify the plasmon bleach dynamics, considering electron-phonon interactions and thermalization processes. The Persson model is used to quantify the influence of the chemical environment on the surface plasmon resonance (SPR) of metal particles. The model calculates the width of the SPR based on the Fermi velocity, particle radius, and contributions from diffusive scattering of electrons at the particle surface. The study applies the model to AuNRs@TiO₂, considering the effective mean free path of the AuNR core and the number of TiO₂ resonance states per unit surface area. The total width due to CID is calculated as the sum of the tangential and normal components, resulting in a CID width of 19 ± 1 meV. The study also considers hypothetical systems where broadening arises from isolated defect sites at the surface, but the results show that the broadening is due to states inherent to the interface itself rather than isolated defect sites. Open-source software is