Paper EM-WeM1
Nanoscale Guided Molecular Engineering of Organic Electro-Optical Materials

Wednesday, November 11, 2009, 8:00 am, Room B1

Increasing complexity in bottom-up molecular designs of amorphous structures with multiple relaxation modes demand an integrated and cognitive design approach, where chemical synthesis is guided by both analytical tools and theoretical simulations. This is true of organic second-order nonlinear optical (NLO) materials, which are being actively pursued for applications in photonic devices. For practical applications, NLO materials must have both high macroscopic EO activity and thermal stability. High 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 systems, which provided excellent EO activity above 300pm/V and good thermal stability.

In this study, nanoscale scanning probe based thermo-mechanical analyses, intrinsic friction microscopy (IFM) and shear-modulation force microscopy yield direct insight into the molecular enthalpic and entropic relaxation modes of these materials. Ar^H-Ar^F interactions of dendritic moieties for coarse self assembly are found to impose three phase relaxation regimes with two transition temperatures, T₁ and T₂. Energetic analyses based on IFM 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. Based on a molecular dynamic simulation, activation energies are tied primarily to interactions between chromophore (dipole), dendritic (quadrupole) moieties and combinations thereof below the incipient glass transition temperature. Above T₁, molecular mobility becomes increasingly cooperative. Sufficient mobility exists in the region of T₁<T<T₂ 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, 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. Future synthesis efforts therefore must balance cooperativity to obtain satisfactory results in both stability and efficiency.