Hydrogen Bonding-Induced Unique Charge-Transfer Emission from Multichromophoric Polypyridyl Ligands: Ratiometric Probing of Methanol Impurity in Commercial Biofuels

This work is focused on utilizing fluorescent molecules for sensing the methanol content in (bio)fuels. Multichromophoric polypyridyl ligands with pyrene (1) and anthracene (2) as aryl residues have been synthesized. The twisted geometry of the probe molecules in both the solid state and aprotic solvents significantly curtails the electronic communication between the aryl moiety and pyridyl ligand (terpyridine). However, the excited-state hydrogen bonding interactions of methanol with terpyridine nitrogens result in the formation of a nearly planar conformation with larger charge separation. Thus, a broad "structureless"fluorescence spectrum was observed specifically in methanol with an exceptionally large Stokes shift (detection limit: 0.13). However, the extent of redshift was found to be attributed to the angle of twisting as well as the electronic nature of the aryl moiety. Considering its high sensitivity, probe 1 is utilized for the detection of methanol impurity in soy-based biodiesel samples, like B100, B20, and so on. Interference studies indicate that the presence of competitive byproducts, such as monoglycerides and diglycerides, or unconsumed starting materials, such as fatty acid methyl esters poses no serious threats in methanol detection. Finally, the methanol-specific fluorescence response has been utilized to design paper strips for on-location detection of methanol contamination in biofuel samples. © 2021 American Chemical Society.