Thermal Reversion Mechanism of N-Functionalized Merocyanines to Spiropyrans:  A Solvatochromic, Solvatokinetic, and Semiempirical Study

Abstract

In continuing studies of the effect of solvent and molecular structure on the behavior of photochromic and thermochromic dye molecules, especially spiropyran (SP)-merocyanine (MC) interconversions, we have examined a series of 6′-nitrobenzoindolinospiropyrans (6-nitro-BIPS) with varying N-functionalities (R ) CH 3 , CH 2 CH 2 COOH, CH 2 CH 2 CH 2 SO 3 -, CH 2 CH 2 COO-Cholesteryl). The solvent effect was assessed by following the thermal decay of the photochemically ring-opened merocyanine to the spiropyran (MC H SP) via UV/vis spectroscopy at the λ max of the MC form. It was found that while modification of the N-moiety produced no perturbations in the solvatochromic behavior of these dyes, there was a marked effect on the solvatokinetic behavior. In nonpolar solvents, where the MCs possess predominantly quinoid character (unit central bond order), a constant thermal reversion rate was observed for the MCs with electron-rich N-ligands. This was attributed to electronic and steric interactions between the ligands and the phenoxide moiety. However, in polar solvents the increased zwitterionic character of the MCs (central bond order ∼2) leads to inhibition of the thermal reversion rate for the MCs in this study, independent of N-functionality. The MC H SP interconversion has also been examined by means of semiempirical calculations. These reveal the lowest energy pathway for conversion of the trans-MC to a cis-MC form via sequential bond rotation of the three central dihedral angles (R, β, and γ). The calculations support the observed solvatokinetic behavior, leading to the assignment of the trans/cis thermal isomerization as the rate-determining step in the overall process.

Physicochemical study of spiropyran–terthiophene derivatives: photochemistry and thermodynamics

“…As is well established in the literature, merocyanines exhibit negative solvatochromism as a result of both a decrease in the dipole moment on electronic excitation and an increase in the dipolar nature of the ground state. 43 It has also been shown that this tends to be independent of the nature of the N-substituent, and this is indeed the case for MC-2 and MC-3, which show near identical solvatochromic behaviour.…”

“…This suggests that the MC-2 adopts a ground state conformation somewhat closer to the transition state leading to the spiropyran (BSP-2) than MC-3 does on isomerising to BSP-3. Given that the formation of spiropyran from merocyanine typically requires rotation about the three bonds linking the indoline and phenolate components of the merocyanine, as discussed extensively by Wojtyk et al, 43 a p-p interaction between the terthiophene and the phenolate component of MC-2 that brought the phenolate oxygen (in polar solvents) or quinone oxygen (in non-polar solvents) closer to the indoline nitrogen should lower DH z . Such an MC conformation could be determined by p-p interactions during the ring opening of the spiropyran to form the merocyanine initially.…”

Structural Analysis of Spiropyran Polimers using ATR Spectroscopy

“…The same figure shows a 10 cm -1 frequency shift between the spyro in powder and spyro plus methylene spectra. This shift is due to the solvent effect on the spyro [7][8][9]. As we can observe in figure 4, some peaks of spyropiran molecule are covered by solvent peaks, for example, peaks for frequencies lower than 750 cm -1 .…”

“…Infrared 1,[3][4][5][6] and Raman 4,6 spectrometries studies have been used to observe the conformational and intermolecular interactions between the polymeric base and the spyropiran. Others works analized the solvent polarity effects into spyropiran molecule, using only the absorption spectrum of the structure [7][8][9] . In this work we used infrared spectroscopy in attenuated total reflection (ATR) mode to analyze the interactions between the polymeric base, solvent and SP.…”

Synthesis of spiropyran-containing random copolymer by atom transfer radical polymerization and its complexation with metal ions

“…In polar media, the ground state of the bipolar colored form is more stabilized than the excited state (μ ground >μ excited ). [45] This will give rise to a greater energy gap between the two MC states and a hypsochromic shift of the absorption band. [46] In addition, an obvious color change is also observed because of the 'negative' solvatochromism of the photochromic species at different solvents as presented in Figure 5 inset.…”