Spectroscopic and TDDFT investigation on highly selective fluorogenic chemosensor and construction of molecular logic gates.

1,5-Bis(2-fluorene)thiocarbohydrazone (FBTC) was designed and synthesized for selective sensing of fluoride and copper ions. The binding constants of FBTC towards fluoride and copper ions have been calculated using the Benesi-Hildebrand equation, and FBTC has more binding affinity towards copper ion than fluoride ion. The H NMR and C NMR titration studies strongly support the deprotonation was taken from the N-H protons followed by the formation of hydrogen bond via N-HF. To understand the fluoride ion sensing mechanism, theoretical investigation had been carried out using the density functional theory and time-dependent density functional theory. The theoretical data well reproduced the experimental results. The deprotonation process has a moderate transition barrier (481.55kcal/mol). The calculated ΔE and ΔG values (-253.92 and -192.41kcal/mol respectively) suggest the feasibility of sensing process. The potential energy curves give the optimized structures of FBTC-F complex in the ground state and excited state, which states the proton transition occurs at the excited state. The excited state proton transition mechanism was further confirmed with natural bond orbital analysis. The reversibility of the sensor was monitored by the alternate addition of F and Cu ions, which was explained with "Read-Erase-Write-Read" behaviour. The multi-ion detection of sensor used to construct the molecular logic gate, such as AND, OR, NOR and INHIBITION logic gates.