Cation modulation of carbonyldipyrrinone (CDP) fluorescence: emission-ratiometric sensing of calcium

Optically dilute solutions of 2,3,7,8-tetraethyl-N,N′-carbonyldipyrrinone displays an absorbance peak at 401 nm and an emission peak at 493 nm in acetonitrile. The two carbonyl groups in the structure seems to be optimally placed for cation coordination, and when calcium ions are added in the form of a perchlorate salt, a remarkable bathochromic change in the absorption and emission spectrum takes place: the absorption peak shifts by 24 nm and the emission peak shifts by 49 nm towards the red end of the visible spectrum. The shift in the emission spectrum allows wavelength ratiometric assessment of calcium concentration. In addition, the response is highly selective with respect to the cations added.