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Our results revealed that the introduction of either electron-donating or electron-withdrawing group(s) on the cyclopentadienyl rings leads to a substantial red shift of the lowest energy ligand-to-metal charge transfer (LMCT) band, generally known as 2E2g → 2E1u, relative to that of the parent ferricenium complex. The observed stabilization of the lowest unoccupied molecular orbitals (LUMO), i.e. e2g orbitals, in the electron-deficient system, was ascribed to the δ back donation from the iron atom into the antibonding molecular orbitals of the ligand rings. A series of DFT calculations reproduce the experimental trend highlighting the shift in energies of the molecular orbitals involved in the charge transfer transition. The direct relationship between the location of the LMCT band and reduction potential for specific class of substituents is also presented which allows for more informed structural modifications in the development of these complexes. 

Protein–protein interactions (PPIs) regulate signalling pathways and cell phenotypes, and the visualization of spatially resolved dynamics of PPIs would thus shed light on the activation and crosstalk of signalling networks. Here we report a method that leverages a sequential proximity ligation assay for the multiplexed profiling of PPIs with up to 47 proteins involved in multisignalling crosstalk pathways. We applied the method, followed by conventional immunofluorescence, to cell cultures and tissues of non-small-cell lung cancers with a mutated epidermal growth-factor receptor to determine the co-localization of PPIs in subcellular volumes and to reconstruct changes in the subcellular distributions of PPIs in response to perturbations by the tyrosine kinase inhibitor osimertinib. We also show that a graph convolutional network encoding spatially resolved PPIs can accurately predict the cell-treatment status of single cells. Multiplexed proximity ligation assays aided by graph-based deep learning can provide insights into the subcellular organization of PPIs towards the design of drugs for targeting the protein interactome.