Indigo is a blue dye molecule that has been used since antiquity, although it is better known today for its use in blue jeans. Indigo has previously been shown to exhibit remarkable photostability due to fast excited state dynamics mediated by an excited state intramolecular proton transfer. Study of this process is complicated by the fact that the photophysics of indigo is very sensitive to the environment. In order to disentangle the intrinsic photodynamics of indigo from the effects contributed by the environment, we studied indigo in a molecular beam using resonance enhanced multiphoton ionization. We obtained excited state lifetimes of individual vibronic bands with pump-probe spectroscopy ranging from 60 ps to 24 ns. We have mapped a barrier to relaxation at about 700 cm^-1, beyond which fast excited state dynamics dominate. Below this barrier two decay processes compete and mode-specific relaxation occurs with certain vibronic bands near the origin relaxing faster than others, or exhibiting different partitioning between the two relaxation channels. Computational studies at the ADC(2)/MP2/cc-pVDZ level indicate that two low-barrier reaction paths exist in the S₁ state of indigo, one corresponding to proton transfer, the other to hydrogen transfer. In both cases the charge distribution changes drastically upon de-excitation. These data provide a sensitive probe of the potential energy landscape, responsible for the response to absorption of light. The results may help in understanding the photostability that preserves the blue color of indigo dyes.

靛蓝是自上古以来就已使用的一种蓝色染料分子，尽管今天它在蓝色牛仔裤中的使用已广为人知。先前已显示出靛蓝由于由激发态分子内质子转移介导的快速激发态动力学而显示出显着的光稳定性。靛蓝的光物理对环境非常敏感的事实使对该过程的研究变得复杂。为了使靛蓝的固有光动力学与环境所造成的影响脱钩，我们使用共振增强型多光子电离技术在分子束中研究了靛蓝。我们用泵浦探针光谱法获得了60 ps至24 ns范围内的各个振动带的激发态寿命。我们在700 cm ^-1处绘制了放松的障碍，除此之外，快速激发态动力学也占主导地位。在此障碍之下，两个衰减过程相互竞争，并且与原始位置附近的某些振动带相比，其他区域更快地放松，或者在两个放松通道之间表现出不同的划分，从而发生了模式特定的放松。在ADC（2）/ MP2 / cc-pVDZ级别的计算研究表明，靛蓝的S ₁状态存在两条低势垒反应路径，一条对应于质子转移，另一条对应于氢转移。在这两种情况下，去激励后电荷分布都会急剧变化。这些数据提供了对势能态势的灵敏探测，负责对光吸收的响应。结果可能有助于理解保留靛蓝染料蓝色的光稳定性。