Chemical stimulus-responsive tricyanopyrroline-based ICT chromophore as a potential environment-sensitive probe

Abstract

Tricyanopyrroline (TCP)-based intermolecular charge transfer (ICT) dye 1 was synthesized by incorporating phenothiazine and diphenylamine groups, and it was shown to absorb near-infrared light at 734 nm with a molar extinction coefficient (εmax) of 1.67 × 104 M−1 cm−1 in toluene, resulting in a broad absorption band ranging from 600 to 900 nm. By combining the properties of the amide (–NHCO–) group of the TCP acceptor with its ICT character, the photophysical properties of 1 were found to be environment-sensitive. Moreover, its absorption profile was significantly dependent on the nature of the solvent; for instance, in CH2Cl2, a λmax value of 781 nm was observed with an optical absorption edge (λonset) of 1018 nm, whereas the use of DMSO instead of CH2Cl2 led to a significant hypsochromic shift to a λmax value of 532 nm. Nonetheless, an increase in dye concentration caused a significant bathochromic shift in the absorption band, which was ascribable to the self-dimerization of the dye (Kd = (1.26 ± 0.84) × 104 M−1) through intermolecular hydrogen bonding between the amide moieties. The amide unit also served as a binding site for anions to generate a selective colorimetric response in the presence of AcO− and F−. Notably, 1 offers the potential to detect F− quantitatively in commercially available toothpaste. The chemical stimuli-sensitivity of the absorption properties was elucidated through several analytical techniques and theoretical calculations.