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Discovering a rotational barrier within a charge-transfer state of a photoexcited chromophore in solution
Structural Dynamics ( IF 3.670 ) Pub Date : 2020-03-04 , DOI: 10.1063/1.5143441
Taylor D. Krueger 1 , Sean A. Boulanger 1 , Liangdong Zhu 1 , Longteng Tang 1 , Chong Fang 1
Affiliation  

Methylation occurs in a myriad of systems with protective and regulatory functions. 8-methoxypyrene-1,3,6-trisulfonate (MPTS), a methoxy derivative of a photoacid, serves as a model system to study effects of methylation on the excited state potential energy landscape. A suite of spectroscopic techniques including transient absorption, wavelength-tunable femtosecond stimulated Raman spectroscopy (FSRS), and fluorescence quantum yield measurements via steady-state electronic spectroscopy reveal the energy dissipation pathways of MPTS following photoexcitation. Various solvents enable a systematic characterization of the H-bonding interaction, viscosity, and dynamic solvation that influence the ensuing relaxation pathways. The formation of a charge-transfer state out of the Franck–Condon region occurs on the femtosecond-to-picosecond solvation timescale before encountering a rotational barrier. The rotational relaxation correlates with the H-bond donating strength of solvent, while the rotational time constant lengthens as solvent viscosity increases. Time-resolved excited-state FSRS, aided by quantum calculations, provides crucial structural dynamics knowledge and reveals the sulfonate groups playing a dominant role during solvation. Several prominent vibrational motions of the pyrene ring backbone help maneuver the population toward the more fluorescent state. These ultrafast correlated electronic and nuclear motions ultimately govern the fate of the photoexcited chromophore in solution. Overall, MPTS in water displays the highest probability to fluoresce, while the aprotic and more viscous dimethyl sulfoxide enhances the nonradiative pathways. These mechanistic insights may apply robustly to other photoexcited chromophores that do not undergo excited-state proton transfer or remain trapped in a broad electronic state and also provide design principles to control molecular optical responses with site-specific atomic substitution.

中文翻译:

在溶液中光激发发色团的电荷转移状态下发现旋转势垒

甲基化发生在无数具有保护和调节功能的系统中。光酸的甲氧基衍生物8-甲氧基6--1,3,6-三磺酸盐(MPTS)可作为模型系统研究甲基化对激发态势能态的影响。包括瞬态吸收,波长可调飞秒激发拉曼光谱(FSRS)以及通过稳态电子光谱进行的荧光量子产率测量在内的一系列光谱技术揭示了光激发后MPTS的能量耗散途径。各种溶剂可以对影响随后的松弛途径的H键相互作用,粘度和动态溶剂化进行系统表征。弗兰克-康登区外电荷转移状态的形成发生在飞秒到皮秒的溶剂化时间尺度上,然后遇到旋转势垒。旋转弛豫与溶剂的氢键给体强度有关,而旋转时间常数随着溶剂粘度的增加而延长。时间分辨的激发态FSRS,借助量子计算,提供了至关重要的结构动力学知识,并揭示了在溶剂化过程中起主导作用的磺酸盐基团。ring环骨架的几个突出的振动运动有助于使种群趋向更荧光的状态。这些超快相关的电子和核运动最终决定了溶液中光激发发色团的命运。总体而言,水中的MPTS发出荧光的可能性最高,非质子且粘度更高的二甲基亚砜增强了非辐射途径。这些机制的见解可以强有力地应用于不经历激发态质子转移或保持陷于宽电子状态的其他光激发发色团,并且还提供设计原理来控制具有特定位置原子取代的分子光学响应。
更新日期:2020-03-04
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