Near-infrared absorbing azo dyes: Synthesis and X-ray crystallographic and spectral characterization of monoazopyrroles, bisazopyrroles, and a boron-azopyrrole complex
Li, Yan; Patrick, Brian O.; Dolphin, David
Four symmetric. 2,5-bis(R,-substituted phenylazo)pyrroles [R, = 4-(dimethylamino) (I), 2-hydroxy, 2-carboxy, 4-nitro] were synthesized by a one-step coupling of the appropriate aryldiazonium salts with pyrrole under basic conditions. Four asymmetric. 2,5-bisazopyrroles were synthesized by coupling substituted phenyldiazonium salts [R''C6H4N2+Cl-] (R,, = 4-methoxy, H, 4-bromo, 4-nitro) with 2-[4-(dimethylamino)phenylazo]-1H-pyrrole under the same conditions. The reactions of I with boron trifluoride and iodomethane provided a BF2-azopyrrole complex of 1H-pyrrolo[2,1-c]-1,2,4,5-boratriazole (II) and a 2,5-bisazo-1-methylpyrrole (III). X-ray crystallography and spectral analysis showed that one compound had three crystal forms, with two of them bearing a bicyclic ring system formed via intermolecular hydrogen bonding. II 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 .ANG.) and wavelength of max. absorption (754 nm). The N=N bond distances increase in the sequence of monoazopyrrole, bisazopyrrole, bisazopyrrole with intramolecular hydrogen bonding, and II. Their max. absorptions shift bathochromically in the sequence of monoazopyrrole, bisazopyrroles, III, and II. These results are important for the design of near-IR-absorbing azo dyes and suggest an efficient path for the preparation of such dyes by effectively enhancing π-electron delocalization.