Mesostructured Composite Materials with Electrically Tunable Upconverting Properties

Abstract

A promising approach of designing mesostructured materials with novel physical behavior is to combine unique optical and electronic properties of solid nanoparticles with long-range ordering and facile response of soft matter to weak external stimuli. Here, orientationally ordered nematic liquid crystalline dispersions of rod-like upconversion nanoparticles are designed, practically realized, and characterized. Boundary conditions on particle surfaces, defined through surface functionalization, promote spontaneous unidirectional self-alignment of the dispersed rod-like nanoparticles, mechanically coupled to the molecular ordering direction of the thermotropic nematic liquid crystal host. As host is electrically switched at low voltages ≈ 1 V, nanorods rotate, yielding tunable upconversion and polarized luminescence properties of the composite. Spectral and polarization dependencies are characterized and explained through invoking models of electrical switching of liquid crystals and upconversion dependence on crystalline matrices of nanorods, and their potential practical uses are discussed.

Photon Upconversion Kinetic Nanosystems and Their Optical Response

“…Considering the luminescence from a collection of ions doped in solids, it can be speculated that the excitation polarization dependence represents a statistic performance of individual ions. If the transition dipoles of the doping ions are randomly oriented, as encountered in amorphous or polycrystalline structures, the excitation polarization selectivity and the emission polarization anisotropy would diminish . However, in monocrystalline lattices, where the local polyhedrons hosting the luminescence centers are well organized, the luminescence from a collection of ions could still exhibit excitation polarization selectivity, as the transition dipoles have a limited number of orientations and are oriented in relation to the principal axes of the local coordination polyhedron center of luminescence (Figure a‐b) .…”

“…Additionally, they found that the aspect ratio showed neglectable effect on the polarization anisotropy of upconversion luminescence, which is different from micropolarized spectroscopy of semiconductor systems where the aspect ratio is a critical factor which affects the polarization property of particles . In addition, it was reported that the residual polycrystallinity present in the UCNPs limited the polarization anisotropy of the individual particles . Aligned arrays of upconversion nanorods were demonstrated to exhibit polarization anisotropy, while randomly oriented nanorods would average out the anisotropy …”

“…Particularly, a proof‐of‐concept intracellular microrheometry was demonstrated where the laser driven intracellular rotation of UCNP was used to successfully determine the intracellular dynamic viscosity ( Figure ) . Similarly, the polarization anisotropy was used to determine the orientation of ordered nematic liquid crystalline dispersions of upconversion nanorods by another group …”

Modulated Visible Light Upconversion for Luminescence Patterns in Liquid Crystal Polymer Networks Loaded with Upconverting Nanoparticles

“…[30] A more detailed description for the surface functionality procedure is provided in the Supporting Information. [30] A more detailed description for the surface functionality procedure is provided in the Supporting Information.…”

“…[27,28] In their cases, UCNPs were capable of efficiently converting low-energy NIR light to UV or visible radiation, triggering the photoisomerization or photocyclization process of light-driven switches, thereby leading to the pitch change or handedness inversion of helical superstructures. However, apart from the polarized luminescence tuning of upconversion nanorod in LC media, [30] almost no attention has been focused on using LC alignment for tuning the luminescence intensity of UCNPs in LC host.In this study, for the first time, we propose a facile but effective strategy to modulate the luminescence intensity of UCNPs in the liquid crystal network (LCN) composite films and further to fabricate upconversion luminescence patterns. [29] Similarly, the doped UCNPs converted NIR light to UV radiation, triggering the trans-cis photoisomerization of the azotolane units and an alignment change of the LC molecules.…”