Twisting intramolecular charge transfer (TICT) is a common nonradiative relaxation pathway for a molecule with a flexible substituent, effectively reducing the fluorescence quantum yield (FQY) by swift twisting motions. In this work, we investigate coumarin 481 (C481) that contains a diethylamino group in solution by femtosecond transient absorption (fs-TA), femtosecond stimulated Raman spectroscopy (FSRS), and theoretical calculations, aided by coumarin 153 with conformational locking of the alkyl arms as a control sample. In different solvents with decreasing polarity, the transition energy barrier between the fluorescent state and TICT state increases, leading to an increase of the FQY. Correlating the fluorescence decay time constant with solvent polarity and viscosity parameters, the multivariable linear regression analysis indicates that the chromophore’s nonradiative relaxation pathway is affected by both hydrogen (H)-bond donating and accepting capabilities as well as dipolarity of the solvent. Results from the ground- and excited-state FSRS shed important light on structural dynamics of C481 undergoing prompt light-induced intramolecular charge transfer from the diethylamino group toward –C=O and –CF₃ groups, while the excited-state C=O stretch marker band tracks initial solvation and vibrational cooling dynamics in aprotic and protic solvents (regardless of polarity) as well as H-bonding dynamics in the fluorescent state for C481 in high-polarity protic solvents like methanol. The uncovered mechanistic insights into the molecular origin for the fluorogenicity of C481 as an environment-polarity sensor substantiate the generality of ultrafast TICT state formation of flexible molecules in solution, and the site-dependent substituent(s) as an effective route to modulate the fluorescence properties for such compact, engineerable, and versatile chemosensors.

中文翻译：

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超快光谱揭示二乙氨基香豆素中极性相关的扭曲分子内电荷转移

扭转分子内电荷转移 (TICT) 是具有柔性取代基的分子常见的非辐射弛豫途径，通过快速扭转运动有效地降低了荧光量子产率 (FQY)。在这项工作中，我们通过飞秒瞬态吸收 (fs-TA)、飞秒受激拉曼光谱 (FSRS) 和理论计算研究了溶液中含有二乙氨基的香豆素 481 (C481)，辅以香豆素 153 与烷基的构象锁定手臂作为对照样品。在极性降低的不同溶剂中，荧光态和TICT态之间的跃迁能垒增加，导致FQY增加。将荧光衰减时间常数与溶剂极性和粘度参数相关联，多元线性回归分析表明，发色团的非辐射弛豫途径受氢 (H) 键供体和接受能力以及溶剂的偶极性的影响。基态和激发态 FSRS 的结果为 C481 的结构动力学提供了重要信息，该结构动力学经历了快速光诱导的分子内电荷从二乙氨基转移到 -C=O 和– CF ₃基团，而激发态 C=O 拉伸标记带跟踪非质子和质子溶剂中的初始溶剂化和振动冷却动力学（无论极性如何）以及 C481 在高极性的荧光状态下的 H 键动力学质子溶剂，如甲醇。对作为环境极性传感器的 C481 荧光性的分子起源的揭示机制见解证实了溶液中柔性分子的超快 TICT 状态形成的普遍性，以及位点依赖性取代基作为调节荧光的有效途径这种紧凑、可工程化和多功能化学传感器的特性。