Redox-active, near-infrared dyes based on ‘Nindigo’ (indigo-N,N′-diarylimine) boron chelate complexes

Reactions of indigo-N,N′-diarylimine (‘Nindigo’) derivatives with BF3·Et2O give mono- or bis-BF2 chelate complexes 2 or 3 respectively. The product distribution between 2 and 3 is sensitive to the auxiliary base and solvent. Although the bis-BF2 complexes 3 are isolable, they gradually decompose in solution to the corresponding mono-BF2 species 2; this process is accelerated by water. The instability of 3 is believed to be due to ring stain effects based on structural analyses of 2. Electrochemical studies of 2 reveal one quasi-reversible oxidation process and two irreversible reductions, whereas derivatives of 3 possess a reversible oxidation and two sequential reversible reductions. The electronic spectra of 2 and 3 contain intense (e ∼ 3 × 104 M−1 cm−1) long-wavelength absorptions near 650 nm and 750 nm respectively. Both series of compounds are weakly emissive in the near-infrared. Time-dependent DFT calculations reveal the electronic transitions to be π–π* in nature.