Intramolecular interactions in photochromic spiropyran-merocyanine polymers

Abstract

Methacrylate, acrylate, and styrene polymers with photochromic spiropyran side groups connected to the main chains through spacers of various lengths were prepared by free radical polymerization. The thermal back-reaction of photochemically formed merocyanines to spiropyrans in the polymer and monomer solutions was studied by flash photolysis. The reaction goes much slower in the polymer than in the monomer solutions. The longer the spacer, the higher the reaction rate in the polymers. Increased solvent polarity increases the rate in the polymer solutions and decreases it in the monomer ones. Whereas for polymer solutions in methyltetrahydrofuran the fading fits first-order kinetics, in benzene or toluene the fading kinetics is described by a superposition of two exponential decays. This is ascribed to the presence in the latter solvents of two types of merocyanines that also reveal different absorption spectra. It is proposed that the formation of intramolecular stacks causes these kinetic and spectroscopic effects.

Synthesis of Photoresponsive Spiropyran‐Based Hybrid Polymers and Controllable Light‐Triggered Self‐Assembly Study in Toluene

“…On the one hand, spiropyran moieties quickly transformed to the opened merocyanine form after exposure to UV light; adjacent polymer chains rapidly aggregated together, resulting in the increase of the particle diameter. On the other hand, toluene solvated poorly the polar merocyanines and forced them to form stacks with antiparallel alignment of alternate merocyanine dipoles, diminishing in this way the net dipole moment 44. The segments of the polymer chains were brought closer and closer due to the continuous formation of MC stacks, which acted as non‐covalent crosslinking points, causing the size decrease of the aggregates.…”

Light-Triggered Reversible Phase Transfer of Composite Colloids

“…Also, in toluene, in addition to main peak of λ max = 560 nm corresponding to the open MC in the trans form around the double bond, 41 two shoulder peaks with λ around 520-550 and 600-640 nm were clearly observed ( Figure 5A), which can be ascribed to the molecular H-stacks characterized by antiparallel alignment of alternate MC dipoles and the J-stacks with parallel orientation of dipoles, respectively. [41][42][43][44][45] In water, however, the absorption curve was broad, and only one main peak with λ max = 545 nm was observed. Figure S7 shows the TEM images of the PmSP/silica composite colloids suspended in the bottom water layer ( Figure 4F) and in the top toluene layer ( Figure 4H).…”

Light‐induced Effects in Photoresponsive Polymers

“…The same mechanism should also be operative in the case of the spiropyran-substituted polymers. Intra-and intermolecular stacking and aggregation among spiropyran groups and the photogenerated merocyanine forms have been proposed by Kalisky and Williams (1984), and Goldburt et al (1984). These phenomena, however, should not play a significant role in the above described solubility changes, in consideration of the small amount of spiropyran units (2 mol%) present in the polymers.…”