*Recent Advancements in Schiff Base Fluorescence Chemosensors for the Detection of Heavy… DOI: http://dx.doi.org/10.5772/intechopen.109022*

(CHEQ ), and ring-opening mechanisms, have been proposed for the interaction between metal ions and various chemosensors. In this chapter, we also briefly address the mechanisms underlying the chemosensory applications of Schiff base derivatives. (i) Electron transfer (ET): Out of all of these traditional processes, ET-primarily via photo-induced electron transfer (PET)—has been the one used the most frequently in fluorescent chemosensors. A fluorophore's fluorescence is typically quenched by the PET process; however, it may be restored if guest molecules can prevent the PET process. (ii) Charge transfer (CT): ICT (intramolecular charge transfer), MLCT (metalligand charge transfer), and twisted ICT are examples of CT processes (TICT). A ratiometric signal is produced by an ICT-based chemosensor when an ICT process is enhanced or suppressed. This ratiometric signal can remove the majority of ambiguities by self-calibrating two emission bands and enable quantitative determination in more advanced applications, such as imaging in living cells and tissues. On the other hand, transition metal complexes like those of ruthenium, rhenium, and iridium, etc., frequently exhibit MLCT, in which CT occurs from a ligand to a transition metal cation. Through the influence of MLCT energy level by analytes, it can also be employed for chemosensor design. Additionally, TICT is a potent intramolecular CT that takes place in the excited state and requires solvent relaxation surrounding the molecule to produce an ongoing rotation of the electron donor and acceptor until it is twisted roughly 90 degrees [30, 31]. Since intramolecular rotation and charge separation in the TICT state depend on polar solvent relaxation, the fluorescence behavior is extremely sensitive to micropolarity and/or steric barrier for molecular rotation [29, 32, 33]. (iii) Energy transfer (ET): Depending on the interaction distance between the energy donor and energy acceptor, ET can be divided into two categories: electronic energy transfer (EET) and fluorescence resonance energy transfer (FRET). While FRET requires a specific amount of spectral overlap between the emission spectrum of the donor and the acceptor, EET, also known as Dexter electron transfer, requires a distance between the donor and acceptor of less than 10 to be effective. For effective FRET to happen, the distance between the donor and acceptor needs to be between 10 and 100. As a result, the chemosensors based on the energy transfer mechanism are highly dependent on distance. (iv) Excimer/exciplex: An excimer is a complex that is created when a fluorophore interacts with another fluorophore of the same structure in the ground state while it is stimulated. The resulting complex is known as an exciplex if the fluorophore in the excited state differs from the fluorophore in the ground state. A dual emission from the monomer and excimer/exciplex is frequently recorded at the same time because the emission spectra of an excimer/exciplex are red-shifted in comparison to that of the monomer. The excimer/exciplex band can therefore be monitored to detect excimer/exciplex production or deformation in response to interaction with a guest species. However, there are currently just a few examples of chemosensors that work by forming exciplexes [32, 34, 35].
