Photon-, Electron-, and Proton-Induced Isomerization Behavior of Ferrocenylazobenzenes

3-, 4-, and 2-ferrocenylazobenzenes, 1, 2, and 3, respectively, and several derivatives of 1 were synthesized, and
their photoisomerization behaviors were examined. The molecular structures of 1 and its derivatives, 2-chloro-5-ferrocenylazobenzene (5) and 3-ferrocenyl-4‘-hydroxylazobenzene (11), were determined by X-ray diffraction analysis.
3-Ferrocenyl compound 1 undergoes reversible trans-to-cis isomerization with a single green light source and the
FeIII/FeII redox change. 4- and 2-Ferrocenyl compounds, 2 and 3, also respond to green light in addition to UV
light, exciting the π−π* transition, but the cis molar ratio in the photostationary state (PSS) is lower than that of
1. The response to green light in 2 and 3 is caused by the MLCT (from Fe d orbital to azo π* orbital) band
excitation, while the character of the MLCT band, as estimated by time-dependent density functional theory
calculations, differs between 1 and 2. The oxidized form of 2 undergoes facile cis-to-trans thermal isomerization.
Both 1 and 2 undergo facile protonation and show proton-catalyzed cis-to-trans isomerization. Among the derivatives
of 1, 2-chloro-5-ferrocenylazobenzene (5) exhibits the highest cis molar ratio (47%) in the PSS of green light
irradiation.

分别编号为1、2、3的3-二茂铁基偶氮苯、4-二茂铁基偶氮苯与2-二茂铁基偶氮苯，以及1的数种衍生物均被合成，并对其光异构化行为开展了研究。化合物1及其衍生物2-氯-5-二茂铁基偶氮苯（编号5）、3-二茂铁基-4'-羟基偶氮苯（编号11）的分子结构经X射线衍射（X-ray diffraction）分析得以确定。3-二茂铁基偶氮苯（即化合物1）可在单一绿色光源与FeIII/FeII氧化还原变化的协同作用下发生可逆的反式-顺式异构化反应。4-二茂铁基与2-二茂铁基偶氮苯（即化合物2和3）除可响应紫外光外，亦可通过激发π-π*跃迁响应绿色光，但二者在光稳态（photostationary state, PSS）中的顺式摩尔占比均低于化合物1。化合物2与3对绿色光的响应由金属到配体电荷转移（Metal-to-Ligand Charge Transfer, MLCT）带激发所引发，该电荷转移过程为从Fe的d轨道至偶氮基团的π*轨道；而通过含时密度泛函理论（time-dependent density functional theory）计算所表征的MLCT带特征，在化合物1与2之间存在显著差异。化合物2的氧化态可快速发生顺式-反式热异构化反应。化合物1与2均可发生快速质子化反应，并表现出质子催化的顺式-反式异构化行为。在化合物1的衍生物中，2-氯-5-二茂铁基偶氮苯（5）在绿色光辐照下的光稳态中顺式摩尔占比最高，达47%。