Carbazole-functionalized Fe(III) spin-crossover complexes exhibiting fluorescence quenching

We report the synthesis, structural characterization, magnetic properties, and photophysical behavior studies of two Fe(III) complexes derived from a carbazole-based hydrazone ligand scaffold (HL). Both complexes [Fe(L)₂]ClO₄·2CH₂Cl₂ (1) and [Fe(L)₂]BF₄·2CH₂Cl₂ (2) crystallize as mononuclear Fe(III) species and exhibit gradual, reversible spin-crossover behavior with thermal hysteresis, as confirmed by variable-temperature magnetic susceptibility measurements and an electron paramagnetic resonance study. UV-visible absorption spectra reveal ligand-centered π-π* transitions along with characteristic ligand-to-metal charge-transfer (LMCT) bands in the Fe(III) complexes. The free ligand displays intense fluorescence arising from carbazole-centered excited states, whereas coordination to Fe(III) results in pronounced quenching. This quenching is attributed to efficient nonradiative deactivation via low-lying LMCT states and enhanced spin-orbit coupling associated with the open-shell Fe(III) center. Overall, this study provides a rare example of carbazole-functionalized Fe(III) spin-crossover complexes with combined magnetic and optical characterization, highlighting the challenges and opportunities associated with coupling ligand-based photophysics to spin-state switching in paramagnetic iron systems. Electronic structure calculations have been performed to gain additional insight into the origin of the SCO process and photophysical properties.