Four styryl phenanthroline derivatives as excellent acidochromic probes

Four newly-synthetized styryl-phenanthroline derivatives were found to exhibit great acidochromism starting from neutral pH values down to negative Ho. The aqueous solution of all of the four molecules features a yellow color at nearly neutral pH, but when the acidity of the solution increases, the color changes and, depending on the compound, an orange, pink, purple or colorless hue is observed. Spectrophotometric and fluorimetric titration were performed to describe the changes in the spectral properties of the molecules with pH. The huge acidochromic shifts shown by these molecules come from the protonation of the nitrogen atoms of the central phenanthroline rings or those of the dimethylamino or diphenylamino side substituents. Thanks to H-1 NMR titration, quantum-mechanical calculations and femtosecond-resolved excited state absorption, a full characterization of the differently protonated forms in terms of protonation sites, spectral and photophysical properties, is reported. These features thus make the molecules' chromophores suitable candidates for applications as acidochromic probes.