Study On Multi-mode Liquid Crystal Photonic Materials Based On Fluorescent Photoswitches Of Sulfone-substituted Diarylethenes

Cholesteric liquid crystals（CLCs）with long-range ordering helical structures are considered to be ideal liquid crystal（LC）photonic materials due to their unique optical properties and stimulus-responsive mechanisms.Introducing fluorescent photoswitches into CLC systems can generate phototunable circularly polarized luminescence（CPL）.CPL can carry abundant optical information（i.e.color,intensity,and polarization）and afford a high-level visual perception,showing great superiority in the field of 3D displaying,CP-OLED,information encryption,asymmetric catalysis,and chiral sensing.Luminescent CLCs can be regarded as excellent CPL-active materials because the highly-ordered molecular arrangement can induce ultra-dissymmetric CPL with large luminescent dissymmetry factor(g_lum).In addition,based on the responsive nature of CLCs under external fields,various CPL-active materials in response to photo,temperature,and electric field has been developed.Among them,photo-responsive CPL is expected to apply for advanced photonic devices and becoming a rapidly evolving research topic because of the irreplaceable merits like local irradiation,precise tailoring,and remote operation.Currently,phototunable CPL materials still meet following challenges:1)enriching the regulation mode and being not limited to the intensity and handedness of CPL;2)realizing the complete reversibility and overcoming the intermediate status;3)improving the processing property and satisfying the practical application.To address the issues 1)and 2),the novel fluorescent photoswitches based on sulfone-substituted diarylethenes（DAE）have been designed and synthesized in this thesis,featuring the fully reversible photocyclization/cycloreversion.Their open-form isomers show no emission while closed-form isomers emit bright fluorescence,and the interconversion between these two isomers shows good reversibility and fatigue resistance.CLCs constructed by sulfone derivatives of DAEs exhibit instantaneously activated CPL upon excited light,which achieves the qualitative change of CPL signal from“0”to“1”.To solve the problem 3),we constantly optimize the material systems.From small molecular CLC system to liquid crystal photonic capsules（LCPCs）,and then to cholesteric liquid crystal elastomer（CLCE）,the photonic device has completed the transformation from the upper and lower substrate to the lower substrate,and then to the flexible self-supporting film without the substrates,realizing the phototunable CPL of solid materials.The applications of LC photonic materials with phototunable CPL in the field of circularly polarized display,multi-level information encryption,and anti-counterfeiting are fully discussed and the main research contents are listed as follows:1.Chiral fluorescent photoswitch（S,S）-switch 6 was synthesized by covalent linking axial chiral group binaphthyl to sulfone-substituted DAE scaffold.（S,S）-switch 6 can undergo the completely reversible photocyclization/cycloreversion upon irradiation with UV and visible light,and exhibit the absolute“off-on-off”responsive CPL in solution.（S,S）-switch 6 can effectively induce the nematic LC to form the helical structure of CLC,and CLC generates intense CPL in a“Turn-On”manner,of which the g_lum value is amplified by 2 orders of magnitude.This turn-on type CPL material has great potential in the field of anti-counterfeiting and information encryption.By constructing the double-layer CPL system consisting of a CPL layer and a polymer cholesteric reflective layer,the g_lum value of CPL signal is further amplified to 1.43 and the visualization of circularly polarized luminescent patterns can be successfully achieved;2.Two chiral blue fluorescent molecules（S）-CNB and（R）-CNB were synthesized as donor,and sulfone-substituted DAE-based fluorescent photoswitches with green and red colors DG and DR were synthesized as acceptors.The F?rster resonance energy transfer（FRET）platforms was constructed in LCPCs.These LCPCs exhibit photoswitchable CPL from initial blue emission to RGB trichromatic signals under UV irradiation due to the synergistic effect of energy and chirality transfer and show strong time dependence because of the different FRET efficiencies at each time node.By mixing different ratio of tricolor LCPCs into PVA matrix,photoswitchable CPL films from blue to full-color were prepared.Based on these phototunable CPL,full-color emission,and time response characteristics,the concept of multilevel data encryption by using LCPC films is demonstrated;3.Fluorescent photoswitch BSD was designed and synthesized by introducing pyridine moiety into sulfone-substituted DAE scaffold.BSD shows distinctive fluorochromic behavior,with open-isomer emitting weak blue fluorescence,while closed-isomer exhibiting intense green emission.BSD can spontaneously form hydrogen-bonded complexes with donor AHBA by intermolecular electrostatic interactions,and covalently incorporated into the network of CLCE,resulting in a good mechanical performance.CLCE shows photo-force dual-responsive CPL and can control the dissymmetry degree and“on/off”state of CPL.CLCE film integrates mechanochromism,fluorochromism,and photo-force dual-responsive CPL characteristics,and can apply for high-end anti-counterfeiting application as multi-level photonic labels.In summary,many kinds of sulfone-DAE-based fluorescent photoswitches are synthesized,and their photophysical properties are systematically discussed in this thesis.The LC photonic materials with multiple phototunable CPL behaviors are developed by using sulfone-DAEs as photosensitive units.Such LC photonic materials show tremendous potential in the field of information encryption and anti-counterfeiting,and have great significance for the development and application of smart materials.