Nellissen Anne-Charlotte, Fron Eduard, Vandenwijngaerden Jonathan B F, De Feyter Steven, Mertens Stijn F L, Van der Auweraer Mark
Laboratory for Photochemistry and Spectroscopy, KU Leuven, Chem & Tech, Celestijnenlaan 200F, 3001 Leuven, Belgium.
Department of Chemistry, Energy Lancaster and Materials Science Institute, Lancaster University, Bailrigg, LA1 4YB Lancaster, United Kingdom.
ACS Omega. 2023 Sep 20;8(39):35638-35652. doi: 10.1021/acsomega.3c02020. eCollection 2023 Oct 3.
Physisorption on hexagonal boron nitride (hBN) gained interest over the years thanks to its properties (chemically and thermally stable, insulating properties, etc.) and similarities to the well-known graphene. A recent study showed flat-on adsorption of several cationic thiacarbocyanine dyes on hBN with a tendency to form weakly coupled H- or I-type aggregates, while a zwitterionic thiacarbocyanine dye rather led to a tilted adsorption. With this in-depth time-resolved study using the TC-SPC technique, we confirm the results proven by adsorption isotherms, atomic force microscopy, and stationary state spectroscopy combined with molecular mechanics simulations and estimation of the corresponding exciton interaction. The absence of a systematic trend for the dependence of the decay times, normalized amplitudes of the decay components, and contribution of different components to the stationary emission spectra upon the emission wavelength observed for all studied dyes and coverages suggests the occurrence of a single emitting species. At low coverage levels, the non-mono-exponential character of the decays was attributed to adsorption on different sites characterized by different intramolecular rotational freedom or energy transfer to nonfluorescent traps or a combination of both. The difference between the decay rates of the four dyes reflects a different density of the nonfluorescent traps. Although the decay time of the unquenched dyes was in the order of magnitude of that of dye monomers in a rigid environment, it is also compatible with weakly coupled aggregates such as proposed earlier based on the stationary spectra. Hence, the adsorption leads to a rigid environment of the dyes, blocking internal conversion. Increasing the concentration of the dye solution from which the adsorption on hBN occurs increases not only the coverage of the hBN surface but also the extent of energy transfer to nonfluorescent traps. For TDC (5,5-dichloro-3-3'-diethyl-9-ethyl-thiacarbocyanine) and TD2 (3-3'-diethyl-9-ethyl-thiacarbocyanine), besides direct energy transfer to traps, exciton hopping between dye dimers followed by energy transfer to these traps occurs, which resulted in a decreasing decay time of the longest decaying component. For all dyes, it was also possible to analyze the fluorescence decays as a stretched exponential as would be expected for energy transfer to randomly distributed traps in a two-dimensional (2D) geometry. This analysis yielded a fluorescence decay time of the unquenched dyes similar to the longest decay time obtained by analysis of the fluorescence decays as a sum of three of four exponentials.
近年来，由于其特性（化学和热稳定、绝缘性能等）以及与著名的石墨烯相似，六方氮化硼（hBN）上的物理吸附引起了人们的关注。最近的一项研究表明，几种阳离子硫代碳菁染料在hBN上呈平躺吸附，倾向于形成弱耦合的H型或I型聚集体，而两性离子硫代碳菁染料则导致倾斜吸附。通过使用TC-SPC技术进行的这项深入的时间分辨研究，我们证实了吸附等温线、原子力显微镜以及稳态光谱结合分子力学模拟和相应激子相互作用估计所证明的结果。对于所有研究的染料和覆盖度，观察到的发射波长对衰减时间、衰减分量的归一化幅度以及不同分量对稳态发射光谱的贡献的依赖性不存在系统趋势，这表明存在单一发射物种。在低覆盖水平下，衰减的非单指数特征归因于在具有不同分子内旋转自由度的不同位点上的吸附，或向非荧光陷阱的能量转移，或两者的组合。四种染料的衰减速率差异反映了非荧光陷阱的不同密度。尽管未猝灭染料的衰减时间与刚性环境中染料单体的衰减时间处于同一数量级，但它也与基于稳态光谱先前提出的弱耦合聚集体相符。因此，吸附导致染料处于刚性环境，阻止了内部转换。增加用于在hBN上进行吸附的染料溶液的浓度，不仅会增加hBN表面的覆盖度，还会增加向非荧光陷阱的能量转移程度。对于TDC（5,5-二氯-3,3'-二乙基-9-乙基硫代碳菁）和TD2（3,3'-二乙基-9-乙基硫代碳菁），除了直接向陷阱的能量转移外，还发生染料二聚体之间的激子跳跃，随后向这些陷阱进行能量转移，这导致最长衰减分量的衰减时间缩短。对于所有染料，也可以将荧光衰减分析为拉伸指数，这与在二维（2D）几何结构中向随机分布的陷阱的能量转移预期情况相同。该分析得出的未猝灭染料的荧光衰减时间与通过将荧光衰减分析为三个或四个指数之和获得的最长衰减时间相似。
