Molecular Mobility in Self-Assembled Dendritic Chromophore Glasses

Abstract

Increasing complexity in bottom-up molecular designs of amorphous structures with multiple relaxation modes demands an integrated and cognitive design approach, where chemical synthesis is guided by both analytical tools and theoretical simulations. In particular, this is apparent for novel organic second-order nonlinear optical materials of self-assembling molecular glasses involving dendritic arene stabilization moieties (phenyl, naphthyl, and anthryl) with electro-optical activities above 300 pm/V. In this study, nanoscale thermo-mechanical analyses yield direct insight into the molecular enthalpic and entropic relaxation modes. Arene-perfluoroarene interactions for coarse self-assembly are found to impose three phase relaxation regimes, with intermediate regimes of 8-15 degrees C in width and apparent activation energies between 40 and 60 kcal/mol to be the most effective for poling. Energetic analyses based on intrinsic friction microscopy (IFA) identify increasing temporal stability with increasing arene size for the low-temperature regime. Electric field poling efficiency is found to be inversely proportional to entropic cooperative contributions that can make up 80% of the overall apparent relaxation energy for the high-temperature regime. The origin for the activation energies below the incipient glass transition temperature, based on complementary molecular dynamic simulations, is tied primarily to noncovalent interactions between chromophore (dipole), dendritic (quadrupole) moieties, and combinations thereof.