Bright Ratiometric Fluorescent Thermometer in a Spin Crossover Molecule toward 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 spin crossover (SCO) molecule. The probe features dual emission channels arising from a locally excited (LE) state and an intramolecular charge-transfer (ICT) state. The thermochromism associated with SCO spectrally overlaps with the probe’s RF bands, producing a reverse synergistic effect, achieving ratiometric fluorescent thermometers (RFTs) with 1.35% K–1 sensitivity and 66% fluorescence contrast. More importantly, the bright fluorescence thermochromism (∼50% quantum yield) is maintained throughout the entire SCO process, enabling direct 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 SCO thermochromism provides a new design platform for the accurate readout of spin states of molecules.