Flat-structured boron-nitrogen-substituted heterocyclic compounds with extended conjugated π-systems such as azaborines are in high demand in the material and imaging technology markets because of their unique features such as simultaneous tunability of fluorescence color, high fluorescent quantum yields, and large Stokes Shift. Here, we have synthesized, characterized, and analyzed a series of pyrrolidinone-fused 1,2 azaborines (PFAs) as new tunable fluorescent organic materials capable to be tuned to red/NIR wavelengths. The incorporation of a phenyl group as a spacer to the left hemisphere of PFAs to extend the conjugated π-system resulted in a red-shift of the emission wavelength when compared to PFA analogues without phenyl spacers. Diverse electronic moieties (–H, –CN, –NO2,) were also incorporated on the phenyl spacer to study their photophysical effects and to help generate intramolecular charge transfer (ICT) effect. The incorporation of a –NO2 group as an electron-acceptor moiety to the phenyl spacer to create A–π–D system resulted in the most red-shifted emission with large Stokes shift of Δλ = 251 nm, or Δν = 10,468 cm-1 which is attributed to ICT state in acetonitrile. Density-functional theory and its time-dependent form were also used to gain insight into the electronic and optical properties of these novel phenyl substituted PFAs. It was concluded that the insertion of a –CN group as an electron-withdrawing moiety on the phenyl spacer, decreased the HOMO–LUMO gap when compared to the non-substituted PFA. However, the presence of a –NO2 group results in emission from partial ICT states and considerably decreases the HOMO–LUMO gap compared to the non-substituted and –CN substituted PFAs. This result supports the red-shift emission observed in the –NO2 substituted PFA since this system displayed the strongest shift between the HOMO and LUMO states.

Molecular design strategy for tuning the emission of pyrrolidinone-fused 1,2-azaborine conjugates to red/near-infrared wavelengths

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