Redox-active, near-infrared dyes based on 'Nindigo' (indigo-N,N′-diarylimine) boron chelate complexesElectronic supplementary information (ESI) available: Characterization details for new compounds. CCDC 895624-895629. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2sc21307a

Reactions of indigo- N , N ′-diarylimine ('Nindigo') derivatives with BF 3 ·Et 2 O give mono- or bis-BF 2 chelate complexes 2 or 3 respectively. The product distribution between 2 and 3 is sensitive to the auxiliary base and solvent. Although the bis-BF 2 complexes 3 are isolable, they gradually decompose in solution to the corresponding mono-BF 2 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 ( ∼ 3 × 10 4 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. Nindigo derivatives can be converted into their corresponding mono- and bis-BF 2 complexes, both of which exhibit intense near-infrared absorbance, weak emission, and redox activity.