A thiosemicarbazone based chemo and fluorogenic sensor for Zn2+ with CHEF and ESIPT behaviour: computational studies and cell imaging application

We report herein the development of a novel, diformyl-p-cresol (DFC)-thiosemicarbazide (TS) based sensor (DFC-TS) that selectively and sensitively recognizes Zn2+ by both UV-Vis and fluorescence methods. The gradual addition of Zn2+ to a solution of the ligand developed a new absorption band at 430 nm, while the bands at 370 and 316 nm gradually decrease generating one well defined isosbestic point at 390 nm exhibiting similar to 17 fold turn-on fluorescent enhancement (FE). When we plot absorbance (at 430 nm) vs. [Zn2+] there is a gradual increase in absorption with [Zn2+], becoming saturated at similar to 1 equivalent of Zn2+ and then again it increases with the increase in [Zn2+] and ultimately becomes saturated at similar to 2 equivalents of added Zn2+. This clearly demonstrates that the Zn2+ binding event to the ligand occurs in two steps, one at a time. Non-linear least-squares computer-fitting of these data gives the parameters: K-f1' = (9.70 +/- 5.51) x 10(5) M-1, n = (1.28 +/- 0.05) for the first step and K-f2' = (1.11 +/- 0.65) x 10(5) M-1 and n = (1.01 +/- 0.06) for the second step. So far, this study provides the opportunity where we have successfully, for the first time, determined the stepwise formation constants; though they have values of the same order of magnitude. The ground state geometries of DFC-TS, both enol and keto forms and [Zn(DFC-TS)(OAc)], [Zn(DFC-TS)(OAc)](-), and [Zn-2(DFC-TS)(OAc)(2)] were optimized using the Gaussian-03 suit program and bond distances of all species are in reasonable agreement with the reported values.