Cation-triggered photoinduced intramolecular charge transfer and fluorescence red-shift in fluorescence probes

We report the synthesis of two new nonpolar fluorescence cation probes (DDS-crown, 5 and DDB-crown, 6) designed from stilbene and 1,4-diphenyl 1,3-butadiene, respectively, by substitution at the two ends with two electron-donor groups (D) (dimethylamino and monoaza-15-crown-5), one of which is able to chelate a cation. The absorption and fluorescence spectra in several solvents of different polarity and the picosecond transient absorption spectra give an estimate of the Intramolecular Charge Transfer (ICT) strength in the excited state. When a cation (Ca2+) is chelated by the macrocycle, the ICT process is increased and the fluorescence is red-shifted. These spectroscopic effects of cation-chelation are enhanced in a third probe (DDCS-crown, 7) derived from DDS-crown by inserting an electron-acceptor (A) group (CN) at the ortho position of the macrocycle in order to increase the ICT.