Bay Functionalized Perylenediimide with Pyridine Positional Isomers: NIR Absorption and Selective Colorimetric/Fluorescent Sensing of Fe 3+ and Al 3+ Ions.

Bay functionalized perylene diimide substituted with pyridine isomers, (2-pyridine (2HMP-PDI), 3-pyridine (3-HMP-PDI) and 4-pyridine (4-HMP-PDI)) have been synthesized and explored for selective coloro/fluorimetric sensing of heavy transition metal ions. HMP-PDIs showed strong NIR absorption (760-765 nm) in DMF. The absorption and fluorescence of HMP-PDIs have been tuned by make use of pyridine isomers. Reddish-orange color was observed for 2-HMP-PDI (λmax  = 437, 551, 765 nm) whereas 4-HMP-PDI exhibited light green (λmax  = 432, 522, 765 nm). 3-HMP-PDI showed orange-yellow (λmax  = 431, 524, 762 nm). The fluorescence spectra of 2-, 3- and 4-HMP-PDI showed λmax at 585, 538, 546 nm, respectively. Interestingly, HMP-PDI dyes showed selective color change (intense pink color) and fluorescence quenching for Fe3+ and Al3+ metal ions in DMF. Absorbance spectra revealed complete disappearance of NIR absorption and intensification/appearance of new peak at lower wavelength. The concentration dependent studies suggest that 4-HMP-PDI can detect up to 36.52 ppb of Fe3+ and 43.12 ppb of Al3+ colorimetrically. The interference studies in presence of other metal ions confirmed the good selectivity for Fe3+ and Al3+ . The mechanistic studies indicate that Lewis acidic character of Fe3+ and Al3+ ions were responsible for selective color change and fluorescence quenching.