Elucidating the mechanisms of acidochromic spiropyran-merocyanine interconversion

The thermal and photochemical processes associated with the acid-induced conversions of 6-nitroBIPS, SP-1, to form the protonated merocyanine (MC-OH ⁺) were investigated via UV/vis spectrophotometric studies in acetone. It was found that the mechanism of trifluoroacetic acid (TFA)-induced ring-opening of the SP and the rate of MC-OH⁺ formation follows a general acid catalysis mechanism. In accord with this mechanism, the thermal growth of the acid-induced ring-opened form (MC-OH⁺) was retarded as the concentration of TFA in the medium was increased. The N-protonated SP, i.e., SP-NH⁺, is formed in a competing side-equilibrium process as an unreactive "sink", with the nitrogen lone-pair no longer available to drive the ring-opening process and resulting in the inverse rate dependence as a linear 1/k_obs vs [HA] plot. Addition of a tertiary amine to MC-OH⁺ regenerated MC which underwent thermal ring closure to the SP, thus restoring its function as a molecular switch. NMR titration of SP samples showed a downfield shift of the N-substituent peak upon increasing the TFA concentration. However, a saturation behavior could not be observed, with SP-1 up to l M acid, unlike the model compound, N,N-dimethylaniline (N,N-DMA), which indicates a base strength order of N,N-DMA > SP-1. Further, we have demonstrated that in solvent acetone, on acidification, the normal photo- and thermochromic behavior is reversed; now MC-OH⁺ is photochemically transformed into SP-H⁺, which undergoes thermal ring-opening to MC-OH⁺.