Dual role of lignin related compounds: Forming chromophoric dissolved organic matter (CDOM)-like spectra and quenching CDOM's fluorescence spectra.

Elucidating the molecular structure of chromophoric dissolved organic matter (CDOM) remains a central challenge in its research. Investigating the spectral changes of lignin derivatives during their transformation provides insights into the molecular basis that dictate CDOM's optical behavior. Photolysis of lignin-related compounds (LRCs), such as coniferyl aldehyde (Coni) and sinapaldehyde (Sina), yields products with spectral signatures closely resembling those of CDOM. The specific UV absorbance at 254 nm (SUVA), spectral slope (S₃₀₀-₇₀₀), and E2:E4 ratio (A/A) of the irradiated Coni and Sina were 3.98 and 3.20 L·mgC m, 0.0153 and 0.0176 nm⁻¹, and 4.38 and 7.02, respectively. All these values closely match those of Suwannee River natural organic matter (3.25 L·mgC m; 0.0152 nm; 4.58). The fluorescence quantum yields at excitation wavelengths of 375 nm (1.5-3.0 × 10) and 440 nm (1.3-2.9 × 10) were comparable to those of CDOM isolates. On the other hand, Coni and Sina quenched the fluorescence of CDOM possibly through both static and dynamic processes. Terrestrial CDOM was more readily quenched than aquatic CDOM, and the quenching efficiency was higher for untreated CDOM than for sodium borohydride-reduced CDOM. LRCs preferentially quenched fluorophores with a relatively long lifetime and caused a hypsochromic shift in the CDOM spectra. The fluorescence quenching efficiency for CDOM in water/organic solvent (isopropanol and tetrahydrofuran) mixed solutions gradually decreased with an increasing volume ratio of the organic solvent, indicating that the conformation of CDOM or the accessibility of fluorophores changes with solvent polarity.