Energy Donor Effect on the Sensing Performance for a Series of FRET-Based Two-Photon Fluorescent Hg²⁺ Probes

Nonlinear optical properties of a series of newly-synthesized molecular fluorescent probes for Hg²⁺ containing the same acceptor (rhodamine group) are analyzed by using time-dependent density functional theory in combination with analytical response theory. Special emphasis is placed on evolution of the probes' optical properties in the absence and presence of Hg²⁺. These compounds show drastic changes in their photoabsorption and photoemission properties when they react with Hg²⁺, indicating that they are excellent candidates for ratiometric and colorimetric fluorescent chemosensors. Most importantly, the energy donor moiety is found to play a dominant role in sensing performance of these probes. Two-photon absorption cross sections of the compounds are increased with the presence of Hg²⁺, which theoretically suggests the possibility of the probes to be two-photon fluorescent Hg²⁺ sensors. Moreover, analysis of molecular orbitals is presented to explore responsive mechanism of the probes, where the fluorescence resonant energy transfer process is theoretically demonstrated. Our results elucidate the available experimental measurements. This work provides guidance for designing efficient two-photon fluorescent probes that are geared towards biological and chemical applications.