Theoretical Study on the Design of Fluorescent Probe Molecules for Bio-Imaging

Abstract

Recently development of fluorescent molecular probes attracts much interest for in-vivo medicinal analysis. One of the typical examples is the fluoroscein derivates called Tokyo Green (TG). In this work, excited states and electronic spectra of protonated and deprotonated forms of TG were theoretically investigated by using the time-dependent density functional theory (TDDFT). Here we focus on revealing structure-property relationship on chemical modification of the benzene moiety. It is shown that the TDDFT method successfully reproduces the experimentally observed excitation energies. The calculation reveals that there are two important excited states assigned as ?-?* and charge-transfer (CT) excited states. Depending on the functional group introduced in the benzene moiety, the excitation energy to the CT excited state changes. As a result, the CT excited state becomes lower than the ?-?* excited state in some molecules from which fluorescence is not observed. Since the CT excitation is electric-dipole forbidden, we predict that the energy lowering is the origin of quenching of fluorescence. The solvent effect of the excitation energies was also studied, indicating that it can also be a factor controlling the emission property of this series of compounds.