DFT/TD-DFT study to decipher the fluoride induced ring opening process of spiropyran

DFT/TD-DFT methods were used to determine the fluoride anion sensing mechanism of 3',6'-Bis(tert-butyldimethylsilyloxy)spiro[benzo[f]chromene3,9'ffuorene], abbreviated as SP. The description of ring opening in the ground state of SP molecule and its isomerization in open form is presented. It was revealed from the study that in the ground state, SP is the most stable form in contrast with the isomer obtained in the open form. To initiate the ring opening, at first, the fluoride ion attacks as a nucleophile to de-silylate the SP molecule. This attack of fluoride ion may induce Cspiro-O bond cleavage leading to the formation of two anionic species, i.e., MC-D1 and MC-D2 respectively (MC is merocyanin). The mono-de-silylation process was endogenic, which was followed by the ring opening process. Furthermore, the orthogonal geometry of probe SP does not show ICT character, whereas, MC-D1 and MC-D2 displayed ICT character owing to the formation of planar geometry along with an increase in conjugation. The fluorescence property of SP, and most stable isomers of open form (CT, MC-D1, and MC-D2) were predicted theoretically. The calculated emission spectra uncovered that SP may show fluorescence, which could be quenched in presence of fluoride anion.