This supplementary material provides detailed experimental and theoretical support for the study of interfacial charge transfer in gold nanorod (AuNR)@titanium dioxide (TiO₂) systems. It includes: 1. **Charge Transfer Efficiency from Single-Particle Spectroscopy**: The method to evaluate chemical interface damping (CID) contributions to the measured homogeneous plasmon linewidth is described, including the calculation of bulk damping, radiative damping, and CID. The CID efficiency is calculated as the ratio of CID to the measured linewidth. 2. **Charge Transfer Efficiency from IR/NIR Transient Absorption Spectroscopy**: The analysis of total charge transfer efficiencies (direct + indirect pathways) in AuNRs@TiO₂ core-shell heterostructures is detailed, comparing it with TiO₂ control samples. The assumption that the initial signal at zero pump-probe delay times is proportional to the free carrier concentration is discussed. 3. **Charge Transfer Efficiency from Visible Transient Absorption Spectroscopy**: The transient bleach recovery dynamics at the visible plasmon resonance are used to quantify the electronic temperature and interfacial charge transfer. The two-temperature model is applied to fit the data, and the electron-phonon relaxation times are extracted. 4. **Transmission Electron Microscopy of AuNRs@TiO₂**: High-resolution transmission electron microscopy images of AuNRs@TiO₂ hybrids before and after thermal annealing are provided, showing the crystallization of the TiO₂ shell and the retention of the AuNR core geometry. 5. **Atomic Force Microscopy of TiO₂**: The surface topography of TiO₂ samples is characterized, with a thickness of 500 nm and a roughness of 100 nm. 6. **Persson CID Model**: A simplified description of the Persson model is provided, including the equations for the tangential and normal components of the adsorbate-induced contribution to the SPR width. The model is used to calculate the CID for AuNRs@TiO₂, and the results are compared with experimental data. 7. **Additional Resources**: Open-source software for evaluating integrals and calculating the tangential and normal contributions to the SPR width is provided. 8. **Figures and Tables**: Additional figures and tables are included, such as high-resolution transmission electron microscopy images, absorption spectra, and fitted slopes from pump power-dependent measurements. This supplementary material supports the main findings and interpretations of the study, providing a comprehensive understanding of the interfacial charge transfer mechanisms in AuNRs@TiO₂ systems.This supplementary material provides detailed experimental and theoretical support for the study of interfacial charge transfer in gold nanorod (AuNR)@titanium dioxide (TiO₂) systems. It includes: 1. **Charge Transfer Efficiency from Single-Particle Spectroscopy**: The method to evaluate chemical interface damping (CID) contributions to the measured homogeneous plasmon linewidth is described, including the calculation of bulk damping, radiative damping, and CID. The CID efficiency is calculated as the ratio of CID to the measured linewidth. 2. **Charge Transfer Efficiency from IR/NIR Transient Absorption Spectroscopy**: The analysis of total charge transfer efficiencies (direct + indirect pathways) in AuNRs@TiO₂ core-shell heterostructures is detailed, comparing it with TiO₂ control samples. The assumption that the initial signal at zero pump-probe delay times is proportional to the free carrier concentration is discussed. 3. **Charge Transfer Efficiency from Visible Transient Absorption Spectroscopy**: The transient bleach recovery dynamics at the visible plasmon resonance are used to quantify the electronic temperature and interfacial charge transfer. The two-temperature model is applied to fit the data, and the electron-phonon relaxation times are extracted. 4. **Transmission Electron Microscopy of AuNRs@TiO₂**: High-resolution transmission electron microscopy images of AuNRs@TiO₂ hybrids before and after thermal annealing are provided, showing the crystallization of the TiO₂ shell and the retention of the AuNR core geometry. 5. **Atomic Force Microscopy of TiO₂**: The surface topography of TiO₂ samples is characterized, with a thickness of 500 nm and a roughness of 100 nm. 6. **Persson CID Model**: A simplified description of the Persson model is provided, including the equations for the tangential and normal components of the adsorbate-induced contribution to the SPR width. The model is used to calculate the CID for AuNRs@TiO₂, and the results are compared with experimental data. 7. **Additional Resources**: Open-source software for evaluating integrals and calculating the tangential and normal contributions to the SPR width is provided. 8. **Figures and Tables**: Additional figures and tables are included, such as high-resolution transmission electron microscopy images, absorption spectra, and fitted slopes from pump power-dependent measurements. This supplementary material supports the main findings and interpretations of the study, providing a comprehensive understanding of the interfacial charge transfer mechanisms in AuNRs@TiO₂ systems.