Two indigo derivatives, N-phenylpropylindigo (NPhC₃Ind) and N,N′-diphenylpropylindigo (N,N′PhC₃Ind), were synthesized using green chemistry techniques and their decay mechanisms were elucidated from ultrafast spectroscopic techniques, as well as density functional theory (DFT) and time-dependent DFT (TDDFT) studies. For NPhC₃Ind, in methylcyclohexane (MCH) and n-dodecane, a very fast (<1 ps) excited state proton transfer (ESPT) process is observed. In contrast, a bi-exponential decay is found in 2-methyltetrahydrofuran (2MeTHF), with a rising component of 9 ps and a decay of 72 ps, reminiscent of the behavior observed in indigo (IND). DFT/TDDFT calculations rationalize that the excitation of an intermolecular dimer structure, formed between hydrogen bonds involving the CO and H–N of two NPhC₃Ind units, leads to the formation of dark (S₁) and bright (S₂) states. Due to the structural distortion of the dimer, the emission is localized in one of the monomer units. Consequently, the absorption is considered to originate from a dimer while the emission (locally excited state, LE) originates from a monomer unit. In the case of N,N′PhC₃Ind, the formation of two conformers (C_IN and C_OUT) in the excited state is observed in viscous solvents, collapsing into a single conformer in 2MeTHF.

中文翻译：

---

苯丙基靛蓝衍生物的溶剂依赖性超快失活过程：了解靛蓝衰变机制的进步


采用绿色化学技术合成了两种靛蓝衍生物，N-苯丙基靛蓝 （NPhC₃Ind） 和 N，N′-二苯丙基靛蓝 （N，N′PhC₃Ind），并通过超快光谱技术以及密度泛函理论 （DFT） 和时间依赖性 DFT （TDDFT） 研究阐明了它们的衰变机制。对于 NPhC₃Ind，在甲基环己烷 （MCH） 和正十二烷中，观察到非常快 （<1 ps） 激发态质子转移 （ESPT） 过程。相比之下，在 2-甲基四氢呋喃 （2MeTHF） 中发现双指数衰变，上升分量为 9 ps，衰变为 72 ps，让人想起在靛蓝 （IND） 中观察到的行为。DFT/TDDFT 计算合理化了，在涉及两个 NPhC₃Ind 单元的 C 、O 和 H-N 的氢键之间形成的分子间二聚体结构的激发导致形成暗态 （S₁） 和亮态 （S₂）。由于二聚体的结构变形，发射局限于其中一个单体单元。因此，吸收被认为来自二聚体，而发射（局部激发态，LE）来自单体单元。在 N，N′PhC₃Ind 的情况下，在粘性溶剂中观察到两个处于激发态的构象异构体（C_IN 和 C_OUT）的形成，在 2MeTHF 中坍缩成单个构象异构体。