Fluorescence spectroscopy has been developed as one of the most common tools in a wide range of fields such as sensing and biomedical diagnostics. Fluorescence sensing has a lot of advantages including high sensitivity, excellent selectivity, availability of versatile fluorophores, etc. Fluorescence sensing requires a change in the spectral response to the analyte, for example, changes in the intensity, lifetime, anisotropy, and excitation or emission spectrum. Charge transfer typically takes place when the electronic wavefunctions of the donor are coupled to the acceptor. The charge transfer can be categorized into electron exchange, intramolecular photoinduced electron transfer (PET), and interfacial electron transfer. In addition to the Dexter interaction, PET is another type of electron transfer. In PET, a complex is usually formed between the electron donor and the acceptor. The excited complex can return to the ground state from the excited state without the emission of a photon, and the extra electron is eventually returned to the electron donor.