A New Highly Selective, Ratiometric and Colorimetric Fluorescence Sensor for Cu²⁺ with a Remarkable Red Shift in Absorption and Emission Spectra Based on Internal Charge Transfer

A new 1,8-diaminonaphthalene based ratiometric and highly selective colorimetric "off-on" type of fluorescent probe, receptor 2 has been designed and synthesized that senses only Cu(2+) among the other heavy and transition metal ions examined on the basis of internal charge transfer (ICT). The visual sensitivity of the receptor 2 is remarkable, showing dual color changes from colorless (receptor) to purple followed by blue and a large red shift in emission upon Cu(2+) complexation.