Preparation And Properties Of Flexible Organic Fluorescent Crystal Composites

In recent years,flexible organic optoelectronic materials have found widespread applications in fields such as organic photovoltaics(OPVs),organic light-emitting diodes(OLEDs),and organic field-effect transistors(OFETs),offering new possibilities for flexible electronics.Organic crystals with high refractive index and optical transparency are gaining attention as prospective materials for next-generation optoelectronic devices.However,unlike viscoelastic polymers with flexible chains,single crystals composed of densely packed anisotropic organic molecules have not been considered viable functional materials due to their mechanical rigidity and brittleness.Recently,breakthroughs in the development of flexible organic crystal materials have emerged in the field of organic solids,introducing a unique class of pliable or elastic engineered materials.These materials hold the potential to revolutionize the concept of organic crystal electronics.The low fatigue resistance and poor environmental adaptability of single crystals typically lead to poor cycling performance,which poses challenges for processing and integration into sensor devices,ultimately limiting their application range.Addressing this challenge,this dissertation proposes an original academic concept of combining functional polymeric soft materials with flexible molecular crystals.We have established a universal strategy for assembling polymer films on the surface of molecular crystals,successfully developing a series of organic molecular crystal composites with unique functions.This method leverages the excellent properties of polymers and the mechanical compliance of organic crystals,while preserving the crystals’ superior optical and electrical properties,enabling broader applications such as solvent resistance and sensing actuation.In Chapter 2,we applied layer-by-layer assembly technology to coat the surface of organic crystals with polymer composites,achieving solvent resistance for the first time while maintaining the crystals’ photophysical properties.This method is versatile and efficient,suitable for various organic molecular crystals,and allows long-term preservation in most organic solvents.Leveraging the optical waveguiding properties and solvent resistance of these crystals,we achieved optical waveguides in solvent environments.This research not only advances the application of organic crystals under extreme conditions but also provides new insights for integrating organic crystals with polymer materials.In Chapter 3,we utilized the humidity sensitivity of polyvinyl alcohol(PVA)and the mechanical flexibility of needle-like elastic crystals to propose a novel method for converting elastic molecular crystals into actuating elements.This approach offers a simple route for non-contact and controllable mechanical deformation of molecular crystals.By assembling polymer layers at different positions on the crystal,precise control of the crystal’s shape change is possible.The hybrid crystals exhibit linear responses over a wide range of air humidity,demonstrating excellent humidity sensing capabilities.Finally,by adjusting the external humidity,precise spatial control of light signal output was achieved.In Chapter 4,we combined organic crystals with low-temperature mechanical flexibility and thermoresponsive polymer materials to fabricate flexible composite crystal materials capable of rapid,reversible deformation at low temperatures.This deformation results from strain differences caused by the varying thermal expansion coefficients of the crystals and polymers.The hybrid materials exhibit excellent twostage linear responses within the temperature range of 15 to-120°C.Even after 2000 cycles of temperature variation,the hybrid crystals maintain high sensitivity,demonstrating good stability and fatigue resistance.By integrating optical waveguiding and low-temperature responsiveness,we created hybrid optical elements with temperature-dependent signal output positions,suitable for temperature monitoring in low-temperature environments.Additionally,these materials showed potential as microrobots capable of autonomous movement and object manipulation on surfaces.In Chapter 5,we demonstrated a method for "welding" damaged organic crystals using the viscosity and electrostatic interactions of polyelectrolytes through layer-bylayer assembly.The polymer fills the gaps between crystal fragments,ensuring firm contact and acting as an optical transmission medium,allowing hybrid crystals to retain certain waveguiding capabilities.This method also enables the combination of different crystals to form heterogeneous structured hybrid crystals,producing variable intensity and wavelength when excited at different positions,thus achieving organic integrated optical circuits based on crystals.By integrating organic crystal bundles with complementary fluorescence spectra,white light mixed signal waveguides can be achieved.This strategy offers a way to restore the optical and mechanical properties of damaged crystals and is expected to stimulate interest in the processing and integration of lightweight organic photonics.These innovative and cost-effective universal strategies optimize and improve the performance of lightweight sensing,electronic,or optical actuating elements,providing new opportunities for the development of flexible electronic technologies.