Theoretical Study on the Sensing Mechanism of Fluorescence Probe LFA for Detecting Cysteine in Lysosomes

The fluorescence mechanisms of the fluorescent probes 2-(4-morpholinophenyl)-4-oxo-4 H-chromen-3-yl acrylate (LFA) for the detection of cysteine in lysosomes and 3-hydroxy-2-(4-morpholinophenyl)-4 H-chromen-4-one (LF) were investigated based on density functional theory (DFT) and time-dependent density functional theory (TDDFT). The enhancement of hydrogen bonding in the excited state of LF molecule was proved by the geometric parameters, interaction region indicator (IRI) equivalence surface and IR vibrational spectra. Due to the low barrier of the potential energy curve (PEC), the LF occurs the excited state intramolecular proton transfer (ESIPT). In addition, both LFA and LF have the characteristics of intramolecular charge transfer (ICT), as determined by the Frontier molecular orbitals (FMOs), hole electron maps and D-index analyses. Due to non-radiation transition, the LFA is quenching by the calculation of reorganization energy and transition dipole moment.