(263f) Molecular Mobility in Self-Assembled Dendritic Chromophore Glasses

Organic second-order nonlinear optical (NLO) materials are being actively pursued for applications in photonic devices such as high-speed electro-optic (EO) modulators, optical switches, and frequency converters. For practical applications, NLO materials must have both high macroscopic EO activity and thermal stability within the operating temperature range. High macroscopic EO activity can be achieved by acentrically ordering a system containing a high density of high dipole chromophores via electric field poling at elevated temperatures. Thermal stability requires the system to have internal constraints to prevent collapse of the acentric order at operating temperatures. Recent efforts for achieving both requirements have focused on dendrons capable of self-assembly through arene-perfluoroarene (Ar^H-Ar^F) quadrupolar interactions within self-assembled glassy chromophore (SAMGC) systems, which provided excellent EO activity above 300pm/V and good thermal stability.

In this study, we use experimental (intrinsic friction microscopy, shear modulation force microscopy) and theoretical techniques (MD simulation) to determine the energetics of the underlying molecular mobility within SAMGCs incorporating phenyl-, naphthyl- or anthryl-pentafluorophenyl Ar^H-Ar^F interactions. We further tie the energetics to their thermal relaxation behavior, wherein two thermal transitions (T₁ and T₂) have been observed. Below the apparent onset of the glass transition (T1

), the activation energy for mobility, E_a,1, increases with increasing size in the aromatic group within the dendritic moiety, indicating that a tailored choice in the arene moieties can affect temporal and thermal stability. The source of E_a,1 is rooted in non-covalent interactions, and the presence of these interactions correlates well with theoretically modeled glass transition behavior. Various interactions were apparent from MD simulation, including the expected aromatic-pentafluorophenyl face-to-face quadrupole interactions

Above the lower transition temperature, T₁, molecular mobilities become increasingly cooperative. Sufficient mobility exists in the region of T₁2 to allow for chromophore acentric electric field alignment, as non-covalent interactions associated with stabilization of the system below T₁ are in competition with melt-like effects. Further, the energy related to the entropic molecular cooperativity increases with increasing arene size, and accounts for approximately 80% of the observed apparent activation energy above T₂. Although beneficial to temporal stability with increased operating temperatures, cooperativity was found to lower the poling efficiency. This study provides an important stepping stone towards cognitive molecular engineering based on nanoscopic investigation of the thermomechanical phase behavior, and computer simulation.