Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal.
J Phys Chem A. 2010 Feb 4;114(4):1699-708. doi: 10.1021/jp907718k.
A comprehensive investigation of the electronic spectral and photophysical properties of the oxidized form of indigo, dehydroindigo (DHI), has been carried out in solution at 293 K. It is shown that dehydroindigo readily converts into its neutral keto form, the blue indigo, in a process which depends on the solvent and water content of the medium. DHI was investigated in toluene, in benzene, and in methanol and it was found that both the oxidized and the keto indigo forms are present in solution. In marked contrast to what has been found for keto-indigo, where the internal conversion channel dominates >99% of the excited state deactivation, or with the fully reduced leuco-indigo, where fluorescence, internal conversion, and singlet-to-triplet intersystem crossing coexist, in the case of DHI in toluene and benzene, the dominant excited state deactivation channel involves the triplet state. Triplet state yields (phi(T)) of 70-80%, with negligible fluorescence (< or = 0.01%) are observed in these solvents. In methanol the phi(T) value decreases to approximately 15%, with an increase of the fluorescence quantum yield to 2%, which makes these processes competitive with the S(1) --> S(0) internal conversion deactivation process. The data are experimentally compatible with the existence of a lowest lying singlet excited state of n,pi* origin in toluene and of pi,pi* origin in methanol. A time-resolved investigation in the picosecond time domain suggests that the emission of DHI involves three interconnected species (involving rotational isomerism), with relative contributions depending on the emission wavelength. DFT calculations (B3LYP 6-31G** level) were performed in order to characterize the electronic ground (S(0)) and excited singlet (S(1)) and triplet (T(1)) states of DHI. The HOMO-LUMO transition was found to accompany an n --> pi* transition of the oxygen nonbonding orbitals to the central CC and adjacent C-N bonds. Calculations also revealed that in S(0) the two indole-like moieties deviate from planarity from ca. 20 degrees, whereas in S(1) and T(1) the predicted structure is basically planar; a gradual decrease of the carbon-carbon central bond distance is seen in the order S(0), S(1), T(1). An additional study on the blue pigment Maya Blue was made, and the comparison between the solid-state spectra of indigo, DHI, and Maya Blue suggests that, in line with recent investigations, DHI is present together with indigo in Maya Blue. These results are relevant to the discussion of the involvement of dehydroindigo in the palette of colors of the ancient Maya Blue pigment.
在 293 K 下，对氧化形式靛蓝（DHI）的电子光谱和光物理性质进行了全面研究。结果表明，DHI 很容易在依赖于溶剂和介质含水量的过程中转化为其中性酮式，即蓝色靛蓝。在甲苯、苯和甲醇中研究了 DHI，发现氧化和酮靛蓝形式都存在于溶液中。与酮靛蓝的情况形成鲜明对比，在酮靛蓝中，内部转换通道主导 >99%的激发态失活，或者对于完全还原的无色靛蓝，其中荧光、内部转换和单重态到三重态系间窜越共存，在 DHI 的甲苯和苯溶液中，主要的激发态失活通道涉及三重态。在这些溶剂中观察到的三重态产率（phi(T)）为 70-80%，荧光（<=0.01%）可忽略不计。在甲醇中，phi(T) 值降低至约 15%，荧光量子产率增加至 2%，这使得这些过程与 S(1)→S(0)内部转换失活过程具有竞争力。这些数据在实验上与甲苯中 n，pi起源的最低单线态激发态和甲醇中 pi，pi起源的存在相吻合。皮秒时间域中的时间分辨研究表明，DHI 的发射涉及三个相互关联的物种（涉及旋转异构），其相对贡献取决于发射波长。为了表征 DHI 的电子基态（S(0)）和激发单线态（S(1)）和三重态（T(1)），进行了密度泛函理论（DFT）计算（B3LYP 6-31G*水平）。发现 HOMO-LUMO 跃迁伴随着氧非键轨道到中央 CC 和相邻 C-N 键的 n→pi跃迁。计算还表明，在 S(0)中，两个吲哚样部分从约 20 度偏离平面，而在 S(1)和 T(1)中，预测的结构基本是平面的；在 S(0)、S(1)和 T(1)的顺序中，看到碳-碳中央键距离逐渐减小。还对蓝色颜料玛雅蓝进行了额外的研究，并对靛蓝、DHI 和玛雅蓝的固态光谱进行了比较，结果表明，与最近的研究一致，DHI 与玛雅蓝中的靛蓝一起存在。这些结果与讨论 DHI 在古代玛雅蓝颜料颜色中的参与有关。
