Photoisomerization and local stability in molecular and polymer-network glasses

Abstract

Although experiments have shown that photoactive molecules can actuate mechanical and optical responses in soft materials, previous computational studies of the photoresponsive molecules have largely focused on molecules in solution or vacuum. Here we use molecular dynamics simulations to study the behavior of azobenzene (AB) and disperse red one (DR1) in molecular and polymer-network glasses during and after photoactivation. In such a rigid matrix environment, the interaction of the molecule with the local environment is as important as the molecule's intrinsic electronic excitation properties, and is less understood. Simulations show that the waiting time between photoactivation and isomerization varies by orders of magnitude and depends on both intermolecular interactions and density. Specifically, we find that the distribution of waiting times follows a power-law with exponent b ≈ 1.0–1.25, where the extent of the power-law distribution grows with decreasing temperature or increasing density. The change in the wait-time distribution with density across sample composition, quench-rate and temperature is characterized, and features of the local molecular interactions are correlated with the wait time. In contrast to initial isomerization events, we find that the dynamics of photoactivated molecules and the surrounding solid after isomerization are not closely linked to the sample density or temperature.