17,17-Dialkyltetrabenzo[a,c,g,i]fluorenes with extremely high solid-state fluorescent quantum yields: relationship between crystal structure and fluorescent properties

Six 17,17-dialkyltetrabenzoa,c,g,i]fluorenes (dialkylTBFs) 2–7 (methyl to n-hexyl) were prepared by treatment of 17H-tetrabenzoa,c,g,i]fluorene (TBF) 1 with n-alkyl halides under basic conditions in moderate yields upon column chromatography and recrystallization. All the compounds show intense blue fluorescence in solutions. On the other hand, in the solid state, the fluorescent spectra and the quantum yields (Φ_p) vary with their alkyl chains. In particular, the Φ_p values of 4–7 bearing n-propyl to n-hexyl groups are close to unity, while the fluorescence of 1 is almost quenched. Crystallographic analyses of 2–7 revealed that the TBF moieties have helicene-like chiral structures in the crystals, and that the alkyl groups basically take an all-trans planar conformation. Because the longer alkyl substituents can serve as a spacer to prevent TBF nuclei from aggregation, π–π interactions of the TBF moieties probably play an important role in quenching the solid-state fluorescence. The relationship between the optical properties and molecular arrangements in the crystals are discussed.