The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics

Abstract

The optical properties of dissolved organic matter influence chemical and biological processes in all aquatic ecosystems. Dissolved organic matter optical properties have been attributed to a charge-transfer model in which donor-acceptor complexes play a primary role. This model was evaluated by measuring the absorbance and fluorescence response of organic matter isolates to changes in solvent temperature, viscosity, and polarity, which affect the position and intensity of spectra for known donor-acceptor complexes of organic molecules. Absorbance and fluorescence spectral shape were largely unaffected by these changes, indicating that the distribution of absorbing and emitting species was unchanged. Overall, these results call into question the wide applicability of the charge-transfer model for explaining organic matter optical properties and suggest that future research should explore other models for dissolved organic matter photophysics.

Multiple linear regression models to predict the formation efficiency of triplet excited states of dissolved organic matter in temperate wetlands

“…A recent report using solvent and temperature adjustments to perturb DOM absorbance and fluorescence suggests that CT complexes do not form in DOM (McKay et al, ). While the hypothesis of CT complex formation has well‐founded precedent (Sharpless and Blough ), mechanisms other than formation of CT complexes may contribute to the observed trends in , including a simple screening mechanism of 3 CDOM* precursors by nontriplet forming chromophores, a disproportionate increase of nontriplet forming precursors with increasing broadband light absorption, or a complete oxidation reaction between 3 CDOM* and reduced polyphenols (McNally et al ; McKay et al ).…”

“…3 CDOM* are particularly important in biogeochemical processes because they are potent oxidants and they form secondary reactive oxidants, such as radicals and reactive oxygen species (Zepp et al 1977;Parker and Mitch 2016). 3 CDOM* are involved in the photodegradation of soluble organic pollutants (Remucal 2014), pathogen inactivation (Kohn and Nelson 2007), and the phototransformation of lignin phenols and free amino acids (McNally et al 2005;Boreen et al 2008;Waggoner et al 2017). Despite their prominent role in several photodegradation pathways, there are limited models to predict the efficiency of 3 CDOM* formation (also called the apparent quantum yield, or AQY T ) in natural systems.…”

In situ fluorescence measurements of dissolved organic matter: A review

“…Temperature DOM fluorescence intensity is inversely related to temperature (Henderson et al, 2009), due to the impact of temperature on the rate of radiationless decay mechanisms (McKay et al, 2018a). High temperature fluctuations, during in situ studies, significantly impact the measurement session outcome (Table 1).…”

Quantifying the impact of solid-phase extraction on chromophoric dissolved organic matter composition

“…There is evidence that long-wavelength absorption by CDOM may arise from charge transfer interactions between donor and acceptor molecules (Del Vecchio and Blough, 2004) although this has been recently challenged (McKay et al, 2018). In the context of the charge-transfer hypothesis, the preferential loss of, and decreased variability in, long-wavelength absorbance, may point towards a disruption of charge-transfer in DOM extracted with PPL.…”