Discovering a rotational barrier within a charge-transfer state of a photoexcited chromophore in solution
Discovering a rotational barrier within a charge-transfer state of a photoexcited chromophore in solution
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DOI：
10.1063/1.5143441
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发表时间：
2020-03-01
影响因子：
2.8
通讯作者：
Fang, Chong
中科院分区：
文献类型：
--
作者：
Krueger, Taylor D.;Boulanger, Sean A.;Fang, Chong
甲基化发生在具有保护和调节功能的无数系统中。8-甲氧基芘-1，3，6-三磺酸盐（MPTS）是一种光酸的甲氧基衍生物，可以作为研究甲基化对激发态势能面影响的模型体系。一套光谱技术，包括瞬态吸收，波长可调谐飞秒受激拉曼光谱（FSRS），和荧光量子产率测量通过稳态电子光谱揭示MPTS的能量耗散途径的光激发。各种溶剂能够系统地表征影响随后的弛豫途径的氢键相互作用、粘度和动态溶剂化。在遇到旋转势垒之前，电荷转移态的形成发生在飞秒到皮秒的溶剂化时间尺度上。旋转弛豫与溶剂的氢键贡献强度有关，而旋转时间常数随溶剂粘度的增加而延长。时间分辨激发态FSRS，量子计算的辅助下，提供了关键的结构动力学知识，并揭示了磺酸盐基团在溶剂化过程中发挥主导作用。芘环骨架的几个突出的振动运动有助于操纵人口向更荧光的状态。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。总的来说，MPTS在水中显示出最高的荧光概率，而非质子和更粘稠的二甲基亚砜增强了非辐射途径。这些机制的见解可能适用于其他光激发的发色团，不经历激发态质子转移或保持被困在一个广泛的电子状态，也提供了设计原则，以控制分子的光学响应与特定位点的原子取代。
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.