Effect of Aryl-, Halogen-, and Ms-Aza-Substitution on the Luminescent Properties and Photostability of Difluoroborates of 2,2′-Dipyrrometenes

Boron(III) complexes with alkyl-, phenyl-, and halogen-substituted 2,2′-dipyrromethenes (BODIPY) and meso-aza-dipyrrometenes (ms-aza-BODIPY) were synthesized. The structure relationship of the obtained coordination compounds with their luminescent characteristics is analyzed. Arylated BODIPY, in contrast to alkyl-substituted analogs, is more sensitive to interparticle interactions with a solvent, causing a decrease in the quantum yield by up to 40%. The introduction of phenyl substituents into the BODIPY molecule shifts the first absorption band bathochromic, significantly (32–37 nm) increases the Stokes shift in the emission spectrum, but reduces the probability of the S0 → S1 electronic transition as compared to alkylated complexes. Replacing the methine carbon atom with nitrogen leads to quenching of ms-aza-BODIPY fluorescence compared to BODIPY up to 5–20%. The stability of 2,2′-dipyrromethenes difluoroborates to oxidative destruction under the influence of UV irradiation in cyclohexane solutions was evaluated. It has been shown that symmetric aryl substitution in pyrrole cycles of dipyrromethene significantly increases the photostability of the corresponding compounds as compared to alkyl-substituted analogs and is an effective method of obtaining boron (III) dipyrromethenates with practically useful properties. It has been established that the replacement of the methin ms-spacer of dipyrromethene by a nitrogen atom significantly reduces the photostability of ms-aza-dipyrromethenates of boron. Halogenation of β-positions of pyrrole cycles by a factor of 5–8 reduces the photostability of difluoroborates ms-aza-dipyrromethenes in comparison with a non-halogenated analogue.