Bright Ratiometric Fluorescent Thermometer in Spin-Crossover Molecule Enables Visualization of Spin-State Equilibria

Optically addressable molecular spins aim to tackle control and detection of weak magnetic moment when magnetic devices are scaled down to the single-molecule level. To achieve accurate readout of molecular spin states by adjacent luminescent probes, the molecular design must meet the stringent requirements of both high signal-to-noise ratio (SNR) and high sensitivity. Here, we incorporated a naphthalimide-based ratiometric fluorescent (RF) probe with a donor–acceptor (D-A) structure into a spincrossover molecule. The probe features dual emission channels arising from both a locally excited (LE) state and an intramolecular charge-transfer (ICT) state. The thermochromism associated to spin crossover spectrally overlaps with the probe’s RF bands, producing a reverse synergistic effect, achieving ratiometric fluorescent thermometer with 1.35% K⁻¹ sensitivity and 66% fluorescence contrast. More importantly, the bright fluorescence thermochromism (~50% yield) throughout the entire SCO process, enabling visualization of spin-state equilibria with high SNR (>400) and detection resolution (0.0017) even at low concentrations of 10-4 M. This tuning of the single-fluorophore RF against spin crossover thermochromism provides a new design platform for accurate read-out of spin states of molecules.