Elucidating the Mechanisms of Acidochromic Spiropyran-Merocyanine Interconversion

Abstract

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/kobs 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 1 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+.

Diversely halogenated spiropyrans - Useful synthetic building blocks for a versatile class of molecular switches

“…[1][2][3] Molecular switches are therefore instrumental for the development of new "intelligent" materials, [4] or molecular machines. [5] Spiropyrans belong to the most important molecular switches, because of the exceptionally large variety of diverse stimuli that can be used for switching: Their isomerization between a closed spiropyran form and an open merocyanine form can be induced by light, [6] pH, [7][8][9][10] the presence of ions, [11][12][13] pressure, [14] mechanical force [1,2,[14][15][16][17][18][19][20][21] as well as electric fields. [19,22] Of those stimuli, the light induced switching provides to be easily accessible and nondestructive.…”

Switching Properties of a Spiropyran–Cucurbit[7]uril Supramolecular Assembly: Usefulness of the Anchor Approach

“…The decrease of pH yielded the disappearance of the typical absorption of the spiro form at around 350 nm and the build-up of a new band with a maximum at 310 nm, which we ascribe to the protonated spiro form 1SPH (see Figure 1). [68] The pK a shift of almost two units corroborates the close interaction between the protonated indoline nitrogen and one of the CB7 portals.…”

“…[68] However, the pH titrations of 1SP are complicated by the thermal ring opening (1SP→1MC) and therefore the experiments had to be performed under steady irradiation with visible light ( > 465 nm) which reconverts any formed merocyanine back to the spiro form. Under these conditions protonation constants of pK a = 1.4 and pK a ' = 3.2 were determined in the absence and presence of CB7, respectively (see Supporting Information).…”

Molecular machines operated by light

“…In order to trigger the self-assembly and disassembly of this pseudorotaxane by light, we identified a species (Figure 8b) that occurs in two forms, interconvertible into one another by light irradiation, exhibiting smaller and larger pK a than that of 6-H 2+ , respectively [93,94]. In the presence of an acid, the colorless spiropyran 7 is converted into the yellow protonated merocyanine form 8-H + [93].…”

“…In the presence of an acid, the colorless spiropyran 7 is converted into the yellow protonated merocyanine form 8-H + [93]. Upon irradiation with visible light, 8-H + releases a proton, isomerizing back to 7 (Figure 8b) [94].…”