Organic fluorophore-substituted polyaza-[7]helicenes derived from 1,10-phenanthroline: Studying the chromophoric effect on fluorescence efficiency

Abstract

We report a series of fluorescent “push–pull” helicenes based on aza-[7]helicenes denoted as AZA-FLU, AZA-NPC, AZA-TPA, and AZA-2OMe for the control of the excited-state dynamics and circular dichroism properties of parent aza-[7]helicene. These symmetrical materials consist of an aza-[7]helicene central unit acting as an acceptor, which is covalently linked with different donor units, such as fluorene, N-phenyl carbazole, triphenylamine, and N,N-dimethoxy triphenylamine, and induces a strong “push–pull” character. Optoelectronic and electron distribution properties are briefly discussed in terms of electrochemical measurements and theoretical calculations. As compared to parent aza-[7]helicene (ϕ = 39% in neutral media), AZA-NPC shows a higher fluorescence quantum yield (ϕ = 71%) and fluorescence lifetime (τ₂ = 15.5 ns) due to the strong donating and intramolecular charge transport properties of the N-phenyl carbazole unit. Further, the basic nature of all aza-[7]helicenes was analyzed via acid titration with trifluoroacetic acid. Among them, AZA-NPC showed a significant change in photophysical properties, indicating a high basic nature due to the strong electron donor behavior of the N-phenyl carbazole unit.