Electron-Rich Dipyrrolonaphthyridinediones: Synthesis and Optical Properties

This article describes the design rationale for highly electron-rich dipyrrolonaphthyridinedione (DPND) derivatives bearing substituted amino groups at the 3 and 9 positions, which exhibit absorption in the red and emission in the red/NIR region of the spectrum. These novel dyes are easily synthesized through a two-step protocol consisting of bromination of the DPND molecule followed by Buchwald-Hartwig amination. We demonstrated that the diamino-dipyrrolonaphthyridinediones have high ionization energies (∼4.7 eV) and that the spectroelectrochemical properties can be rationally tuned by altering the nature of the peripheral substituted amino groups. All amino-DPNDs exhibit solvatofluorochromism, which has not been previously reported for dyes possessing this core. Theoretical calculations reveal that in all cases, the strongest absorption is exhibited by the S₁ states which clearly correlate with the HOMO-LUMO orbital transition. As all higher states have lower oscillator strengths, it is clear that fluorescence is completely dominated by the excitation/deexcitation sequence S₀ → S₁, S₁ → S₀ and that there are no contributions to the fluorescence from excitations to higher states.