title: Near-Infrared absorbing azo dyes: synthesis and x-ray crystallographic and spectral characterization of monoazopyrroles, bisazopyrroles, and a boron-azopyrrole complex.
abstract: 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-OCH(3); 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 horizontal lineN bond distances (1.322 and 1.300 Angstrom) and wavelength of maximum absorption (754 nm). The N horizontal lineN bond distances increase in the sequence of monoazopyrrole [1a(I): 1.253 Angstrom], bisazopyrrole (2a: 1.283 Angstrom), bisazopyrrole with intramolecular hydrogen bonding (2b: 1.293 and 1.293 Angstrom), 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 pi-electron delocalization.
authors: Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, B.C. V6T 1Z1, Canada.
authors: Yan
authors: Y
authors: Li
authors: Brian O
authors: BO
authors: Patrick
authors: David
authors: D
authors: Dolphin
mesh_terms: D001391:Azo Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D001896:Boron Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D004396:Coloring Agents / Q000138:chemical synthesis* / Q000737:chemistry; D018360:Crystallography, X-Ray; D007259:Infrared Rays; D008958:Models, Molecular; D011758:Pyrroles / Q000138:chemical synthesis* / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet
chemical_list: D001391:Azo Compounds; D001896:Boron Compounds; D004396:Coloring Agents; D011758:Pyrroles