Ground- and Excited-State Properties of Iron(II) Complexes Linked to Organic Chromophores

Two new bichromophoric complexes, [Fe(bim-ant)₂]²⁺ and [Fe(bim-pyr)₂]²⁺ ([H₂-bim]²⁺ = 1,1'-(pyridine-2,6-diyl)bis(3-methyl-1H-imidazol-3-ium); ant = 9-anthracenyl; pyr = 1-pyrenyl), are investigated to explore the possibility of tuning the excited-state behavior in photoactive iron(II) complexes to design substitutes for noble-metal compounds. The ground-state properties of both complexes are characterized thoroughly by electrochemical methods and optical absorption spectroscopy, complemented by time-dependent density functional theory calculations. The excited states are investigated by static and time-resolved luminescence and femtosecond transient absorption spectroscopy. Both complexes exhibit room temperature luminescence, which originates from singlet states dominated by the chromophore (¹Chrom). In the cationic pro-ligands and in the iron(II) complexes, the emission is shifted to red by up to 110 nm (5780 cm^-1). This offers the possibility of tuning the organic chromophore emission by metal-ion coordination. The fluorescence lifetimes of the complexes are in the nanosecond range, while triplet metal-to-ligand charge-transfer (³MLCT) lifetimes are around 14 ps. An antenna effect as in ruthenium(II) polypyridine complexes connected to an organic chromophore is found in the form of an internal conversion within 3.4 ns from the ¹Chrom to the ¹MLCT states. Because no singlet oxygen forms from triplet oxygen in the presence of the iron(II) complexes and light, efficient intersystem crossing to the triplet state of the organic chromophore (³Chrom) is not promoted in the iron(II) complexes.