Mesogenic, Luminescence, and Nonlinear Optical Properties of New Bipyrimidine-Based Multifunctional Octupoles

A series of new bipyrimidine-based chromophores have been prepared presenting alkoxystyryl donor groups carrying aliphatic chains in the 3,4, 3,5 or 3,4,5 positions, connected to electron-accepting 2,2'-bipyrimidine cores. Their linear and nonlinear optical properties were investigated as well as their mesomorphic properties by various techniques (light-transmission measurements, polarized-light optical microscopy, and differential scanning calorimetry measurements). Only two derivatives, BPM-3,4-C12 and BPM-3,4-C16, were found to exhibit liquid-crystalline behavior with the formation of lamella-columnar phases and/or hexagonal columnar phases over large temperature ranges. Small-angle X-ray scattering analysis allowed proposing a stacking model inside the mesophase in which the molecules are interdigitated alternatively along their long axis and their short axis to form columns. Dielectric measurements were performed as a function of the temperature, showing the centrosymmetric nature of the mesophases. Large quadratic hyperpolarizabilities have been measured for the individual mesogens in solution by using hyper-Rayleigh scattering. These chromophores exhibit also cubic nonlinear optical properties, revealing relatively large two-photon absorption cross sections. The nonlinear optical properties in the liquid crystalline state of compounds BPM-3,4-C12 and BPM-3,4-C16 have been studied by wide-field second-harmonic generation and two-photon fluorescence microscopy, confirming centrosymmetry for these achiral mesogens and the excellent third-order nonlinearity for multiphoton imaging.