Bisquinolones as chiral fluorophores – A combined experimental and computational study of absorption and emission characteristics

Biscarbostyrils (4,4′-bisquinolones) can be synthesized from 4-chloro-2-quinolinones using a Pd-catalyzed one-pot borylation/Suzuki cross-coupling protocol or via Ni(0)-mediated reductive homocoupling. The electronic spectra of biscarbostyrils 4b–8 exhibit unusual properties in comparison to the corresponding carbostyrils 1–3. Similar absorption spectra are accompanied by red-shifted emission maxima up to 520 nm. Unsubstituted biscarbostyril 4b displays the unusual property of a 2500 cm−1 fluorescence redshift in water as compared to dimethylsulfoxide instead of an expected 800 cm−1 blueshift. In order to further improve redshifts and fluorescence quantum yields, varying substitution patterns were created. In bisquinolone 7, an additional diphenylphosphinoxide substitution in position 3 and 3′ (15c) increased the quantum yield to 20% and the epsilon value to 25,000. A crown ether linkage from position 6 to 6′ in biscarbostyrils improved the emission maximum from 470 to 500 nm, but the fluorescence quantum yield was raised only from 3% to 6%. Time-dependent density functional calculations of absorption and emission spectra of selected derivatives show good agreement with the corresponding experimental data. Especially, the unusual large Stoke’s shift observed for biscarbostyrils as well as their rather low fluorescence quantum yields can be rationalized on the basis of these calculations. Like 1,1′ binaphthalenes the biscarbostyril structures are axially chiral and can be functionalized in position 3 and 3′ with diphenylphosphine. Most of the racemates were baseline HPLC separated on the Pirkle type ULMO column, with separation factors of up to 2.4 for BINAP type intermediate 15a.