Search

Chiral bis - N,N,O,O -boron chelated BOPHY dyes are produced in a one-pot reaction of (1 E ,2 E)-1,2-bis((3,5-dimethyl-1 H -pyrrol-2-yl)methylene)hydrazine, boron tribromide, and an unsymmetrically substituted catechol derivative. Remarkably, in the case of methyl 2,3-dihydroxybenzoate the chiral, racemic diastereoisomeric bis -boron chelate is formed with complete stereoselectivity. In contrast, the corresponding reaction with 3,5-di- tert -butylcatechol produces a 3:1 ratio of racemic and meso (achiral) diastereoisomers. These boron chelates display two strong (ε 33–41,000 M−1 cm−1) absorption maxima between 444 and 473 nm, typical of a BOPHY-like vibronic progression. The racemic isomers were resolved by chiral HPLC and display mirror image ECD spectra. The absorption dissymmetry factor (| g abs |) for the tert -butyl catechol chelate is 5–10 fold larger than that for the ester-substituted catechol derivative, which correlates with a highly twisted core chromophore revealed by the X-ray crystal structure of the tert -butyl derivative. Both catechol chelates display BOPHY-like emission but with very low quantum yield of fluorescence (φ f ≤ 0.1%) presumably due to photoelectron transfer from the very electron-rich catechol system. • Chiral BOPHY dyes created by chelation of unsymmetrical catechols to boron. • Short diastereoselective synthesis of inherently chiral picket-fence BOPHY dyes. • Magnitude of ECD correlates with twist in chiral BOPHY core. [ABSTRACT FROM AUTHOR]

Abstract: Chiral organic fluorophores have significant promise in the development of efficient emitters of circularly polarized light. Herein we describe a helically chiral boron dipyrromethene (BODIPY) with a hitherto unreported N,N,O,C‐boron‐chelation motif, synthesised by means of a one‐pot boron metathesis, nucleophilic aromatic substitution (SNAr), Suzuki coupling, boron chelation, cascade reaction. Resolution of the racemic BODIPY (by preparative HPLC on a chiral stationary phase) allowed examination of the chiroptical properties of the resulting enantiomers (λmax(abs)=593 nm, λmax(em)=622 nm, ϵ=30 000  m−1 cm−1, φF=0.49, |glum|=3.7×10−3 (hexane)). This is the first example of circularly polarised emission from a non‐C2‐symmetric helically chiral N,N,O,C‐BODIPY and as such provides a valuable benchmark for future developments in this compound series. [ABSTRACT FROM AUTHOR]