Molecular Orientation and Fluorescence Studies on Naphthalene Acetate Intercalated Zn2Al Layered Double Hydroxide

This paper describes a systematic study on molecular orientation and fluorescence properties for the Zn2Al layered double hydroxide (LDH) intercalated with alpha-naphthalene acetate (alpha-NAA) and beta-naphthalene acetate (beta-NAA), respectively. alpha-NAA and beta-NAA intercalated Zn2Al LDH (denoted as alpha-NAA LDH and beta-NAA LDH) were prepared by the ion-exchange method and their thin films on Si substrates were obtained by the solvent evaporation method. The powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) of alpha-NAA LDH and beta-NAA LDH confirm the intercalation of guests between sheets of LDH. The XRD and scanning electron microscope (SEM) images of alpha-NAA LDH and beta-NAA LDH thin films show that they are well c-oriented assembly with platelet-like morphology. UV-vis absorption and fluorescence spectra (emission, lifetime, and time resolution) indicate that the intercalated NAA is accommodated in its monomer form and depolarization factors exhibit no influence on its fluorescence emission process, which makes the evaluation for its interlayer molecular orientation reasonable and feasible by polarized fluorescence method. Fluorescence polarization method was applied to investigate the preferential orientation of the interlayer alpha-NAA and beta-NAA molecules, and the results show that alpha-NAA and beta-NAA are intercalated between sheets of Zn2Al LDH as monomeric units with a tilt angle Psi (defined as the angle between the transition moment of NAA molecule with respect to the normal to the LDH layer) of 60 degrees and 65 degrees, respectively. Compared with the physical mixture samples of NAA and LDH, the intercalation of NAA leads to an increase in the fluorescence lifetime. This indicates a more dilute and constrained interlayer environment for the guest, which reduces the internal mobility and the flexibility of NAA and thus improves its fluorescence lifetime.