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Synthesis and photophysical properties of 3-azinyl-7-aminocoumarins: tuning solvatochromism, HLCT excited state, and static fluorescence quenching by picric acid
A series of new coumarin-azine fluorophores based on 1,2,4-triazinyl and pyridinyl substituted derivatives of 7-aminocoumarin was synthesized via a nucleophilic substitution of hydrogen (SNH) reaction followed by an inverse electron-demand Diels–Alder transformation. The primary photophysical properties of 3-triazinyl coumarins and their pyridine analogs was investigated in toluene and acetonitrile solutions. Thus, the emission of the fluorophores in acetonitrile solution covers the range from blue to green-yellow with emission maxima from 470 to 547 nm with absolute quantum yields (QYs) of up to 85%. The emission in toluene solution covers only blue region and QYs values decreased to 60%. A study of the solvent effect using the Lippert-Mataga equation showed that two derivatives of 3-triazinyl coumarin are solvatochromic and demonstrate hybrid local and charge transfer exited state, while 3-pyridyl coumarins turned out to be non-solvatochromic. Six fluorophores were tested as sensors for detecting nitroaromatic compounds. Turned out that they’re selective sensors for picric acid (PA) with a very insignificant inner filter effect that can be ignored. Fluorescence is quenched by PA via the true static photoinduced electron transfer (PET) mechanism confirmed by density functional theory calculations and fluorescent titration data. The best sensory properties in this coumarin-azine dyads showed compound 5g which has Stern-Volmer constant of 2.10 × 106 M−1, limit of detection of 0.366 ppm (1.60 μM) and quenching efficiency of 86%.