Prussian blue (PB) represents a class of metal-organic coordinated compounds with fascinating electrochromic properties. Although the electronic structure has been studied intensively, its electrochromic mechanism remains unresolved due to the lack of electronic-transition analysis. Herein, we investigate the electrochromism of Prussian blue (PB) and its derivatives by combining optical characterization and density functional theory (DFT) calculations. We unambiguously determine the optical gaps of PB-related derivatives and construct a smart window exhibiting excellent electrochromic performance and temperature control. DFT calculations demonstrate that the coloring of Prussian yellow (PY) is critically governed by two absorption bands centered at ca. 2.4 eV and 3.0 eV respectively. The former is weak and is generated by the charge-transfer transitions from the Fe(I)-t_2g (Fe ions connected with C) band to the Fe(II)-t_2g (Fe ions connected with N) band. The latter is strong and is induced by the electronic excitations from the Fe(I)-t_2g band to the antibonding Fe(II)-e_g band. The color change from yellow to blue is induced by the reduction of Fe(I) ions, which redshifts and enhances the former transitions but suppresses the latter transitions. In contrast, the color change from blue to transparency (Prussian white) is induced by the reduction of Fe(II) ions, which enlarges the bandgap and permits transmittance of visible light. The band-edge transition oscillator strengths are critically governed by the state weight of C-p, which provides opportunities for optical modulation via anionic doping. The insights obtained in this work are fundamental for understanding and controlling the opto-electronic properties of PB-related compounds for intelligent applications.

普鲁士蓝（PB）代表一类具有令人着迷的电致变色特性的金属有机配位化合物。尽管已经对电子结构进行了深入研究，但由于缺乏电子跃迁分析，其电致变色机理仍未解决。在这里，我们通过结合光学表征和密度泛函理论（DFT）计算来研究普鲁士蓝（PB）及其衍生物的电致变色现象。我们毫不含糊地确定PB相关衍生物的光学间隙，并构建一个展示出色电致变色性能和温度控制的智能窗口。DFT计算表明，普鲁士黄（PY）的颜色主要受两个以ca为中心的吸收带支配。分别为2.4 eV和3.0 eV。前者是弱的，是由从Fe（I）-t _2g（与C连接的Fe离子）带到Fe（II）-t _2g（与N连接的Fe离子）的电荷转移跃迁产生的。后者很强，是由Fe（I）-t _2g谱带到反键Fe（II）-e _g的电子激发引起的带。Fe（I）离子的还原会引起从黄色到蓝色的颜色变化，Fe（I）离子会发生红移并增强前者的跃迁，但抑制后者的跃迁。相比之下，Fe（II）离子的还原会导致颜色从蓝色变为透明（普鲁士白色），这会增大带隙并允许可见光透射。带边缘跃迁振荡器的强度由Cp的状态权重严格控制，这为通过阴离子掺杂进行光调制提供了机会。在这项工作中获得的见解对于理解和控制PB相关化合物在智能应用中的光电特性至关重要。