Solvatochromic Probes Displaying Unprecedented Organic Liquids Discriminating Characteristics

Four highly fluorescent derivatives of bis(phenyl-ethynyl-)-2-naphthyl (BPEN) with push-pull structures were designed and synthesized, of which azetidine was adopted as an electron-donating unit. For the electron withdrawing moiety, it varies from hydrogen, to formyl, then to the 2-ethoxyethyl derivative of dicyanovinyl, and finally to dicyanovinyl itself, and the corresponding fluorophores are denoted as A1, A2, A3, and A4, respectively. To enhance the solubility of the compounds, two n-hexadecyl residues were grafted onto the side positions of BPEN. Interestingly, introduction of azetidine not only improves the fluorescence quantum yield and enlarges the Stoke's shift of the parent compound but also endows them, in particular A2 and A4, exceptional capability to distinguish structurally relevant organic liquids, such as ethylbenzene and its isomers (o-xylenes, m-xylenes, and p-xylenes), monoalkyl-substituted benzene derivatives, gasolines of different grades, and other organic liquids. Theoretical calculation and Lippert-Mataga equation-based tests revealed the intramolecular charge transfer (ICT) nature of the solvatochromic properties of the compounds. Further quantitative analysis of the data obtained from studies of the probes/n-hexane-dioxane systems revealed the big differences in the dipole moments between the excited and ground states of A1, A2, A3, and A4, which are about 23, 29, 43, and 38 D, respectively. Moreover, the four novel fluorophores possess exceptional photochemical stability as demonstrated by the fact that more than 2 h of UV light illumination did not result in detectable reduction in the fluorescence emission of the fluorophores. It is the long wavelength absorption (>380, ≈400, >410, and >430 nm), large molar absorption coefficient (>59 000, >52 000, >39 000, and >34 000 cm^-1 M^-1), great color change (400-620 nm), and good solubility in common organic liquids that makes the as developed compounds, in particular A2 and A4, very competitive solvatochromic probes.