Structural Transformation of a Multicompartment Micelle Induced by Photo-Switchable Spiropyran–Merocyanine Transition: Dissipative Particle Dynamics Simulation Approach

This work highlights the structural transformation of a multicompartment micelle consisting of an amphiphilic A-b-(C-spiropyran) diblock copolymer in water, induced by the photo-switched transition between hydrophobic nonionic spiropyran and hydrophilic zwitterionic merocyanine, using a set of computational methods such as density functional theory (DFT), molecular dynamics (MD) simulation, and dissipated particle dynamics (DPD) simulation. By employing a new computational framework for the Flory–Huggins χ-parameter, which is improved by considering solvation free energy, it is demonstrated that the χ-parameter for a merocyanine–water pair is significantly smaller than that for a spiropyran–water pair, indicating that hydrophobic spiropyran becomes hydrophilic merocyanine through the photo-switched transition. Our computational procedure of χ-parameter calculation is further validated by investigating the deprotonation of syndiotactic and isotatic acrylic acid in water. It is revealed that a reasonable χ-parameter of −0.717 and −1.23 can be calculated from a singly deprotonated syndiotactic and isotatic trimer acrylic acid in water, respectively, which is consistent with the weak acidity of acrylic acid. Finally, using a set of calculated χ-parameters, DPD simulations are performed employing two different block copolymers with two different block lengths: A10-b-(C6-Spiro12) (Micelle I) and A6-b-(C10-Spiro12) (Micelle II), demonstrating that both micelles undergo distinctive inside-out transformations as a function of the block length ratio. Through the structure analysis, it turns out that Micelle I keeps a well-defined core–shell structural feature before and after the transformation, whereas Micelle II loses such structural feature because short Block A is not capable of forming the core of micelle.