Humification is a ubiquitous natural process of biomass degradation that creates multicomponent systems of nonliving organic matter, including dissolved organic matter (DOM) and humic substances (HS) in water environments, soils, and organic rocks. Despite significant differences in molecular composition, the optical properties of DOM and HS are remarkably similar, and the reason for this remains largely unknown. Here, we employed fluorescence spectroscopy with (sub)picosecond resolution to elucidate the role of electronic interactions within DOM and HS. We revealed an ultrafast decay component with a characteristic decay lifetime of 0.5–1.5 ps and spectral diffusion originating from excitation energy transfer (EET) in the system. The rate of EET was positively correlated to the fraction of aromatic species and tightness of aromatic species packing. Diminishing the number of EET donor–acceptor pairs by reduction with NaBH₄ (decrease of the acceptor number), decrease of pH (decrease of the electron-donating ability), or decrease of the average particle size by filtration (less donor–acceptor pairs within a particle) resulted in a lower impact of the ultrafast component on fluorescence decay. Our results uncover the role of electronic coupling among fluorophores in the formation of DOM and HS optical properties and provide a framework for studying photophysical processes in heterogeneous systems of natural fluorophores.

中文翻译：

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超快能量转移决定了 DOM 和腐殖质中荧光的形成

腐殖化是一种普遍存在的生物质降解的自然过程，它创造了非生物有机物质的多组分系统，包括水环境、土壤和有机岩石中的溶解有机物质 (DOM) 和腐殖质 (HS)。尽管分子组成存在显着差异，但 DOM 和 HS 的光学性质非常相似，其原因在很大程度上仍然未知。在这里，我们采用（亚）皮秒分辨率的荧光光谱来阐明 DOM 和 HS 中电子相互作用的作用。我们揭示了一种超快衰减组件，其特征衰减寿命为 0.5-1.5 ps，光谱扩散源自系统中的激发能量转移 (EET)。EET率与芳香族物质的比例和芳香族物质的堆积紧密度呈正相关。通过用 NaBH 还原来减少 EET 供体-受体对的数量₄（受体数量的减少）、pH 值的降低（给电子能力的降低）或过滤导致的平均粒径减小（颗粒内的供体-受体对减少）导致超快组分的影响降低关于荧光衰减。我们的结果揭示了荧光团之间的电子耦合在 DOM 和 HS 光学特性形成中的作用，并为研究天然荧光团异质系统中的光物理过程提供了框架。