Differential Tuning of the Electron Transfer Parameters in 1,3,5-Triarylpyrazolines: A Rational Design Approach for Optimizing the Contrast Ratio of Fluorescent Probes

A large class of cation-responsive fluorescent sensors utilizes a donor−spacer−acceptor (D−A) molecular framework that can modulate the fluorescence emission intensity through a fast photoinduced intramolecular electron transfer (PET) process. The emission enhancement upon binding of the analyte defines the contrast ratio of the probe, a key property that is particularly relevant in fluorescence microscopy imaging applications. Due to their unusual electronic structure, 1,3,5-triarylpyrazoline fluorophores allow for the differential tuning of the excited-state energy ΔE00 and the fluorophore acceptor potential E(A/A−), both of which are critical parameters that define the electron transfer (ET) thermodynamics and thus the contrast ratio. By systematically varying the number and attachment positions of fluoro substituents on the fluorophore π-system, ΔE00 can be adjusted over a broad range (0.4 eV) without significantly altering the acceptor potential E(A/A−). Experimentally measured D−A coupling and reorg...