Probing Structural Confinement in Organic Nanoparticles by Intertwining Photochromism and Nanomechanics

Functional organic nanoparticles, largely praised for optoelectronics and bioimaging applications, are usually manufactured through the mere self-assembly of molecular dyes. Undesirable dissociation due to noncovalent interactions between the molecular constituents can be prevented by further photo-cross-linking. In order to study the reactivity within nanoparticles and the impact of photo-cross-linking on their mechanical properties of nanoparticles, we elaborate nanoparticles and films based on a photoactivatable network derived from reacting bis-acrylate-functionalized azo photochromic units and bis(dithiolane) cross-linkers of variable flexibility. Photochromic studies, recording azo relaxation dynamics after photoisomerization, are combined with force spectroscopy investigations using atomic force microscopy, following a nanocompression method, to give access to the elastic modulus at the single nanoparticle level. We find that the average thermal relaxation of the azo units in cross-linked nanoparticles is largely enhanced in comparison to that in cross-linked films, whereas similar values are obtained for both the more flexible and stiffer networks. While the elastic modulus slightly increases for thin films incorporating stiffer cross-linkers, the difference is amplified 10-fold in nanoparticles. These results first demonstrate that high structural confinement in nanoparticles upon nanoprecipitation favors large reactivity and thereby a large incorporation of the cross-linkers in the final three-dimensional networks. Second, they point out that nanoparticle elasticity is mainly ruled by the stiffness of the cross-linkers. The enhanced discrepancy in elasticity at the nanoparticle level is thus tentatively interpreted as nanoconfinement effects, impacting the free volume distribution and chain mobility.