Synthesis, EPR study and photophysical properties of a mononuclear Fe(III) Schiff base complex functionalized by 3,6-di-tert-butyl-carbazole moieties

An azomethine iron(III) complex containing 3,6-di-tert-butyl-carbazole fragments on periphery with a PF6ˉ counter-ion was synthesized and characterized by a wide range of experimental techniques. According to the X-ray powder diffraction data, the unit cell contains four molecules, which adopt a facial isomeric form. EPR spectroscopy revealed the presence of only high-spin (HS) iron(III) centers in the temperature range between 3.8 and 300 K. At low temperatures, the majority of complexes form the supramolecular dimers, wherein the two HS iron(III) centers are weakly antiferromagnetically coupled, but an increase in temperature causes the cleavage of intermolecular hydrogen bonds and results in the formation of a monomeric packing structure. The thermal stability and phase behavior were studied by thermogravimetric analysis and differential scanning calorimetry. The UV/Vis absorption properties were studied in dichloromethane and rationalized by time-dependent density functional theory calculations. Upon excitation at 345 nm, the complex exhibits intense fluorescence centered at 449 nm. A single emission band was assigned to a πcarb–π* intraligand charge-transfer excited state. In addition to a significant Stokes shift (104 nm), a high fluorescence quantum yield (54%) was observed.