Near-Infrared Absorbing Azo Dyes: Synthesis and X-ray Crystallographic and Spectral Characterization of Monoazopyrroles, Bisazopyrroles, and a Boron−Azopyrrole Complex

Symmetric 2,5-bisazopyrroles 2(a−d) were synthesized by a one-step reaction of substituted phenyl diazonium salts [R′(Ph)N2+Cl−] [a, R′ = 4-N(CH3)2; b, R′ = 2-OH; c, R′ = 2-CO2H; d, R′ = 4-NO2] with pyrrole under basic conditions. Asymmetric 2,5-bisazopyrroles 3(a−d) were synthesized by reacting substituted phenyl diazonium salts [R′′(Ph)N2+Cl−] (a, R′′ = 4-OCH3; b, R′′ = H; c, R′′ = 4-Br; d, R′′ = 4-NO2) with 2-(4-dimethylaminophenylazo)-1H-pyrrole (1a) under the same conditions. The reactions of 2a with boron trifluoride and iodomethane provided a BF2-azopyrrole complex of 1H-pyrrolo[2,1-c]-1,2,4,5-boratriazole (4) and 2,5-bisazo-1-methylpyrrole 5. X-ray crystallographic and spectral analysis of 1a, 2a, 2b, and 4 showed that 1a has three crystal forms: 1a(I), 1a(II), and 1a(III), the latter two bearing a bicyclic ring system formed via intermolecular hydrogen bonding. Complex 4 was found to be the most planar due to a rigid trans-azo configuration and has the longest NN bond distances (1.322 and 1.300 Å) and wavelength of maximum absorption (754 nm). The NN bond distances increase in the sequence of monoazopyrrole [1a(I): 1.253 Å], bisazopyrrole (2a: 1.283 Å), bisazopyrrole with intramolecular hydrogen bonding (2b: 1.293 and 1.293 Å), and the BF2−azopyrrole complex. Their maximum absorptions shift bathochromically in the sequence of monoazopyrrole (1a: 443 nm), bisazopyrroles [2(a−d), 3(a−d), 5: 486−615 nm], and the BF2−azopyrrole complex. These results are important for the design of near-infrared absorbing azo dyes and suggest an efficient path for the preparation of near-infrared absorbing azo dyes by effectively enhancing π-electron delocalization.