Five alkyl-substituted difluoroboron dipyrrin (BODIPY) dyes have been synthesized: three with a methyl, isopropyl or tert-butyl group at the meso-position of the BODIPY core and two substituted with one or two tert-butyl functions at the 3/5-positions. X-Ray structural analysis, UV-vis absorption spectroscopy and fluorescence (steady-state and time-resolved) techniques have been used to study the structures and the spectroscopic/photophysical properties of these dyes. All but one of these BODIPYs are highly fluorescent in all the solvents tested, the exception being meso-tert-butylBODIPY (2). Derivative 2 differs from the other alkylated boron dipyrrins as it exhibits a broad and red-shifted fluorescence band with a large Stokes shift. In addition, very low fluorescence quantum yields and short fluorescence lifetimes characterize 2. Quantum chemical calculations indicate that 2 has a distorted, nonplanar geometry in the S₁ excited state due to the rotation of 8-tert-butyl group. Our results lead us to the conclusion that the torsional rotation about the bond connecting the meso-C and the quarternary C of the tert-butyl group of 2 plays a crucial role in the fast radiationless deactivation of this isomer.

Carbon monoxide is a key C₁ feedstock for the industrial production of hydrocarbons, where it is used to make millions of tonnes of chemicals, fuels, and solvents per annum. Many transition metal complexes can coordinate CO, but the formation of new C−C bonds in well-defined compounds from the scission and subsequent coupling of two or more CO moieties at a transition metal centre remains a challenge. Herein, we report the use of low-coordinate iron(II) complexes for the selective scission and homologation of CO affording unusual squaraines and iron carboxylates at ambient temperature and pressure. A modification of the ligand framework allows for the isolation and structural characterisation of a proposed metallacyclic Fe(II) carbene intermediate. These results indicate that, with the appropriate choice of supporting ligands, it is possible to cleave and homologate carbon monoxide under mild conditions using an abundant and environmentally benign low-coordinate, first row transition metal.