Nanometric scale investigation of the nonlinear efficiency of perhydrotriphenylene inclusion compounds

A two-photon polarized microscopy technique has been implemented to investigate the nonlinear efficiency and the molecular organization in organic inclusion compounds. Different chromophores have been included as guest molecules in perhydrotriphenylene (PHTP) channels yielding needle-shaped crystals of either millimetric or submicrometric size depending on the sample preparation. Polarization responses of second harmonic generation (SHG) and two-photon fluorescence (TPF) are shown to provide complete information on the crystalline quality of such structures. In particular, macroscopic crystals exhibit one-dimensional molecular ordering along the needle axis with occasional local orientational defects. Their zero frequency nonlinear efficiency can reach up to 340 pm/V with 4-dimethylamino-4′-nitroazobenzene as a guest, which can be ascribed to about 80% of the guest molecules setting-up the noncentrosymmetric sub-lattice. This order is furthermore preserved in submicrometric structures, which are promising systems towards SHG-active nanoparticles.