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

摘要

Symmetric 2,5-bisazopyrroles 2(a-d) were synthesized by a one-step reaction of substituted phenyldiazonium salts [R'(Ph)N_2~+Cl~-] [a, R' = 4-N(CH_3)_2; b, 2-OH; c, R' = CO_2H; d,R'= NO_2]with pyrrole under basic conditions. Asymmetric 2,5-bisazopyrroles 3(a-d) were synthesized byreacting substituted phenyl diazonium salts [R"(Ph)N_2±Cl~-] (a, R" = 4-OCH_3; b, R" = H; c, 4-Br; d, R" = 4-NO_2) with 2-(4-dimethylaminophenylazo)-1H-pyrrole(la)under the same condi-tions. The reactions of 2a with boron trifluoride and iodomethane provided a BF2-azopyrrolecomplex of 1H-pyrrolo[2,1-c]-1,2,4,5-boratriazole (4) and 2,5-bisazo- 1-methylpyrrole 5. X-raycrystallographic and spectral analysis of 1a, 2a, 2b, and 4 showed that 1a has three crystal forms:I), 1a(II),and III),the latter two bearing a bicyclic ring system formed via intermolecularhydrogen bonding. Complex 4 was found to be the most planar due to a rigid trans-azo configurationand has the longest N=N bond distances (1.322 and 1.300 A) and wavelength of maximumabsorption (754 nm). The N=N bond distances increase in the sequence of monoazopyrrole [1a(I):1.253 A], bisazopyrrole (2a: 1.283 A), bisazopyrrole with intramolecular hydrogen bonding (2b: 1.293and 1.293 A), and the BF_2-azopyrrole complex. Their maximum absorptions shift bathochromicallyin the sequence of monoazopyrrole (la: 443 nm), bisazopyrroles [2(a-d), 3(a-d), 5: 486-615 nm],and the BF_2-azopyrrole complex. These results are important for the design of near-infraredabsorbing azo dyes and suggest an efficient path for the preparation of near-infrared absorbing azodyes by effectively enhancing Jr-electron delocalization.