EXPRESS: Absorption and Fluorescence Properties of Tracer Dyes in Complex Nonpolar Hydrocarbon Mixtures

Laser-induced Fluorescence (LIF) is widely employed in combustion research for various diagnostic applications, including the simultaneous visualisation of liquid and vapour phase as well as temperature and concentration measurements. For quantitative LIF diagnostics, liquid fuels are often doped with fluorescent tracers whose spectroscopic properties must be carefully characterised. While numerous dyes have been investigated in polar solvents like water or ethanol, data for nonpolar fluids like common combustion fuels remain scarce. However, solvent polarity is known to significantly affect solubility, their optical response, spectral shape as well as influencing shifts in absorption and emission maxima. This study provides a systematic comparison of the fluorescent dyes Coumarin 152, Fluoranthene, DCM, and Pyrromethene 567 with respect to their suitability quantitative two-colour LIF thermometry in combustion diagnostics and single-droplet evaporation experiments. Measurements are conducted in three nonpolar fuels: a fossil (B0) diesel and two synthetic alternatives, gas-to-liquid (GtL) and power-to-liquid (PtL) diesel. A systematic comparison of dye performance is presented, including solubility limits as well as temperature-dependent LIF response under 355 and 532 nm excitation. Despite its comparatively lower solubility, Pyrromethene 567 exhibits high signal intensities and a pronounced temperature-dependent response, qualifying it as suitable tracer for 2-colour LIF (2c-LIF) measurements in these fuels. The results support an informed selection of tracer-fuel combinations and contribute to improved reliability of LIF-based temperature and concentration measurements in nonpolar mixtures and real fuel application.