Synthesis and Optical Properties of Acidochromic Amine-Substituted Benzo[a]phenazines

A new series of alkylamine- or arylamine-substituted benzo[a]phenazines have been synthesized from 1,2-naphthoquinones by employing simple sequential Michael-type addition with a variety of primary and secondary amines and the condensation reaction of the resulting amine-substituted 1,2-naphthoquinones with o-phenylenediamine. They exhibited absorption peaks originating from the charge transfer transition between the amine and pyrazine segments and benzo[a]phenazine localized π–π* transitions. Although the absorption spectra of the dyes were not significantly influenced by the nature of the solvents, addition of TFA led to a prominent red-shift in the absorption spectra owing to the protonation at the quinoxaline segment which enhanced the electron-accepting ability. The qualitative trends observed in the optical properties and acidochromism were supported by density functional theoretical computations. The dyes displayed positive solvatochromism in the emission spectra suggestive of a more polar excited state. The dyes were also characterized by a quasi-reversible reduction couple originating from the pyrazine segment which underwent shifts corresponding to electron-donating strength of the amine segment.本研究以1,2-萘醌（1,2-naphthoquinones）为起始原料，依次与多种伯胺、仲胺进行迈克尔加成（Michael-type addition）反应，再将所得胺取代的1,2-萘醌与邻苯二胺（o-phenylenediamine）进行缩合反应，合成了一系列烷基胺或芳基胺取代的苯并[a]吩嗪（benzo[a]phenazines）类化合物。该系列化合物的吸收峰源于胺片段与吡嗪（pyrazine）片段间的电荷转移跃迁，以及苯并[a]吩嗪骨架的局域π–π*跃迁。尽管该类染料的吸收光谱基本不受溶剂性质的影响，但加入三氟乙酸（TFA，trifluoroacetic acid）后，由于喹喔啉（quinoxaline）片段发生质子化作用、提升了其电子接受能力，吸收光谱出现显著红移。通过密度泛函理论（density functional theory, DFT）计算，验证了该类染料光学性质与酸致变色（acidochromism）行为的定性变化趋势。该类染料的发射光谱呈现正溶剂致变色（solvatochromism）效应，表明其激发态极性更强。此外，该系列染料还表现出源于吡嗪片段的准可逆还原电对，其还原电位随胺片段的给电子能力强弱发生相应偏移。