Synthesis and Relative Thermal Stabilities of Diphenylamino- vs Piperidinyl-Substituted Bithiophene Chromophores for Nonlinear Optical Materials

Electrooptic poled polymers can be used in a variety of photonic applications involving the switching or modulation of light. These polymers contain second-order nonlinear optical chromophores that have been aligned by electric-field poling near the glass transition temperature of the polymer host. Loss of poling-induced alignment of the chromophores results in a decay of the macroscopic nonlinearity and presents an important issue that must be addressed if these materials are to have a commercial impact. Accordingly, researchers have attempted to incorporate chromophores into polymers with glass transition temperatures far in excess of the anticipated operating temperatures, such that over the lifetime of the devices the decay of chromophore alignment is minimized. This has created a need for chromophores that not only are very nonlinear but also have adequate thermal stability to survive the poling process which may occur at temperatures exceeding 200 °C. Accordingly, as part of our ongoing effort to develop chromophores for nonlinear optical applications we synthesized a series bithiophene-based chromophores (Figure 1) with the hope that they may have both high optical nonlinearity and good thermal stability. The donor−acceptor bithiophene chromophores have the advantage of having moderate aromatic stabilization in the ground state, making them more oxidatively stable than the analogous polyene chromophores and more polarizable than the analogous biphenyl chromophores. In addition, several reports in the literature demonstrated that protons α to the amine donor attenuate the thermal stability of other nonlinear optical chromophores. Therefore we examined the relative stability of compounds substituted with either piperidinyl or diphenylamino donors.