In view of the application prospects in biomedicine of (E)-1-(4-(diethyla-mino)-2-hydroxybenzylidene)-4,4-dimethylthiosemicarbazide (DAHTS), the behavior of excited-state dynamics and photophysical properties were studied using the density functional theory/time-dependent density functional theory method. A series of studies indicated that the intramolecular hydrogen-bond (IHB) intensity of DAHTS was enhanced after photoexcitation. This was conducive to promoting the excited-state intramolecular proton-transfer (ESIPT) process. Combining the analysis of the IHB and hole–electron, it revealed that the molecule underwent both the ESIPT process and the twisted charge-transfer (TICT) process. Relying on exploration of the potential energy surface, it was proposed that the different competitive mechanisms between the ESIPT and TICT processes were regulated by solvent polarity. In acetonitrile (ACN) solvent, the ESIPT process occurred first, and the TICT process occurred later. In contrast, in the CYH solvent, the molecule first underwent the TICT process and then the ESIPT process. Furthermore, we raised the possibility that the TICT behavior was the cause of weak fluorescence emission for the DAHTS in CYH and ACN solvents. By the dimer correlation analysis, the corresponding components of triple fluorescence emission were clearly assigned, corresponding to the monomer, dimer, and ESIPT isomer in turn. Our work precisely elucidated the photophysical mechanism of DAHTS and the attribution of the triple fluorescence emission components, which provided valuable guidance for the development and regulation of bioactive fluorescence probes with multiband and multicolor emission characteristics.

中文翻译：

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(E)-1-(4-(二乙基氨基)-2-羟基亚苄基)-4,4-二甲基氨基硫脲化合物的光物理性质及其三重荧光发射机制：理论视角

针对( E )-1-(4-(二乙基氨基)-2-羟基亚苯亚甲基)-4,4-二甲基氨基硫脲(DAHTS)在生物医学中的应用前景，对其激发态动力学行为和光物理性质进行了研究使用密度泛函理论/瞬态密度泛函理论方法。一系列研究表明，DAHTS 的分子内氢键（IHB）强度在光激发后增强。这有利于促进激发态分子内质子转移（ESIPT）过程。结合IHB和空穴电子的分析，揭示了该分子同时经历了ESIPT过程和扭曲电荷转移（TICT）过程。依靠对势能面的探索，提出ESIPT和TICT过程之间的不同竞争机制受到溶剂极性的调节。在乙腈（ACN）溶剂中，首先发生ESIPT过程，随后发生TICT过程。相比之下，在CYH溶剂中，分子首先经历TICT过程，然后经历ESIPT过程。此外，我们提出了 TICT 行为可能是 DAHTS 在 CYH 和 ACN 溶剂中弱荧光发射的原因。通过二聚体相关性分析，明确了三重荧光发射的相应成分，依次对应于单体、二聚体和ESIPT异构体。我们的工作精确阐明了DAHTS的光物理机制以及三重荧光发射组分的归属，为具有多波段、多色发射特性的生物活性荧光探针的开发和调控提供了有价值的指导。