Theoretical Analysis of Solvent Effect on NAPBr Dye's Two-photon Absorption Ability and Non-Radiative Transition in Lipid Droplets Detection

Two-photon fluorescence imaging has shown a promising application in biomedical imaging due to its outstanding advantages such as large penetration depth, low photo-damage, and photo-bleaching, etc. Among them, the two-photon fluorescent dye NAPBr, which can effectively select and monitor lipid droplets in living cells and biological tissues, has attracted extensive attention because of its excellent fluorescent properties. However, the research on the fluorescent abilities of two-photon fluorescent dyes in solvent environment is not sufficient. In our work, theoretical analysis reveals the internal mechanism of the solvent effect on geometric structure and photophysical properties of two-photon fluorescent dyes, especially non-radiative transition process, and holes-electrons distribution and transfer. This can provide a reference for the development of efficient two-photon absorption (TPA) molecules with aggregation-induced emission (AIE) characteristics. Related data also showed good regularity. Moreover, dye in four solvents have excellent photophysical properties: high fluorescence quantum efficiency (up to 66.60%), large Stokes shift (up to 108696 cm-1), and two-photon absorption cross section (up to 3658 GM). The medium dielectric constant solution environment can achieve a balance between two-photon absorption and fluorescence emission capabilities better, which lays a solid foundation for the study of TPA molecules with AIE functions in terms of solvent effects.