Synthesis, optical properties, and acid–base indicating performance of novel ketene hydroxybenzylidene nopinone derivatives†
Three ketene nopinone derivatives (1–3) were successfully synthesized from β-pinene derivative nopinone and characterized by Fourier transform infrared spectroscope (FT-IR), nuclear magnetic resonance (NMR), mass spectrometry, and X-ray single crystal diffraction. Then, their optical properties were investigated by ultraviolet-visible and fluorescence spectroscopy. The ethanol solutions of compound 1 or 3 could change color from colorless to saffron yellow and Kelly, respectively, after adding NaOH solution. Moreover, the addition of potassium tert-butoxide could lead to a change in the UV-vis absorption and fluorescence spectra and enhance fluorescence, with compounds 1 and 3 being greatly affected by alkali. A good linear relationship between fluorescence intensity and pH values (9.01–13.38) was obtained for compound 2 (for detecting further pH value, y = 10.178x − 61.806, R2 = 0.9934). Compounds 1 and 3 were ideal acid–base indicators because of their high sensitivity, which was better than that of phenolphthalein. Therefore, these new nopinone derivatives may be candidates for future acid–base indicators.