Nanoscale phase analysis of molecular cooperativity and thermal transitions in dendritic nonlinear optical glasses

A broad nanoscopic study of a wide-range of dendritic organic nonlinear optical (NLO) self-assembly molecular glasses reveals an intermediate thermal phase regime responsible for both enhanced electric field poling properties and strong phase stabilization after poling. In this paper, the focus is on dendritic NLO molecular glasses involving quadrupolar, liquid crystal, and hydrogen bonding self-assembly mechanisms that, along with chromophore dipole-dipole interactions, dictate phase stability. Specifically, dendritic face-to-face interactions involving arene-perfluoroarene are contrasted to coumarin-containing liquid crystal mesogen and cinnamic ester hydrogen interactions. Both the strength of dendritic interactions and the impact of dipole fields on the relaxation behavior have been analyzed by nanoscale energetic probing and local thermal transition analysis. The presence of dendritic groups was found to fundamentally alter transition temperatures and the molecular relaxation behavior. Thermal transition analysis revealed that molecules with dendritic groups possess an incipient transition (T(1)) preceding the glass transition temperature (T(2)) that provides increased stability and a well-defined electric field poling regime (T(1) < T < T(2)), in contrast to molecular groups lacking dendrons that exhibit only single transitions. On the basis of enthalpic and entropic energetic analyses, thermally active modes below T(1) were found to be intimately connected to the dendron structure. Their corresponding activation energies, which are related to thermal stability, increased moving from cinnamic ester groups to coumarin moieties to arene-perfluoroarene interacting groups. While dendritic NLO materials were found to possess only enthalpic stabilization energies at temperatures relevant for device operation (T < T(1)), the apparent molecular binding energies above T(1) contain a substantial amount (up to ~80%) of cooperative entropic energy. The multiple interactions (from dipole-dipole interactions to local noncovalent dendritic interactions) are discussed and summarized in a model that describes the thermal transitions and phases.