Zeolites Could Speed Up Development of TADF Material
June 19, 2017
nanotechweb.org reported on a new TADF configuration that promises to increase the lifetime and efficiency of this material. The approach was developed by DR, Yuewei Zhang of State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, P. R. China. Materials with long-lived excited states, such as phosphorescence have been employed in optoelectronics, photo catalysis, molecular imaging and security applications. TADF, when excited with light form triplet excitons (electron-hole pairs) after an ‘intersystem crossing’ process from singlet excited states to triplet states,” explains co-team leader Jihong Yu. “Then, delayed fluorescence occurs by thermal activation through a reverse intersystem crossing from the triplet excited states to the singlet states to generate the final emission. The triplet excited states thus need to be stable enough for the efficient reverse intersystem crossing process to enhance the delayed fluorescent luminescence. TADFs have the advantage of being able to harvest these triplet excited states, which results in the materials having a high quantum efficiency and unique optoelectronic properties.” Until now, most TADF materials were based on metal-organic complexes and organic molecules with lifetimes limited to just several microseconds to milliseconds. More importantly, they mainly worked in oxygen-free environments because oxygen strongly quenches the triplet state emission. The triplet excitons in these materials were easily deactivated by non-radiative processes (in which energy is dissipated by molecular vibrations and rotations). Yu and colleagues are now reporting on a simple and general “dots-in-zeolites” strategy that involves embedding carbon dots (CDs) in a zeolitic matrix in a hydrothermal/solvothermal crystallization process. Zeolites are aluminosilicate and aluminophosphate materials. They are inorganic microporous crystalline structures commonly used as commercial adsorbents and catalysts and resemble highly regular sponges with networks of nanopores. In their experiments, the researchers synthesized CDs@AlPO-5 (as they are called) using triethylamine as the organic structure-directing agents in the solvent triethylene glycol at 180°C for three days. The TADFs produced have a hexagonal prism-like shape and transmission electron microscopy images show that uniform and monodispersed CDs are embedded in the AlPO-5 crystals with an average particle diameter of 3.7 nm. High-resolution images also reveal a well-resolved lattice spacing of 0.21 nm, which is close to the (100) facet of graphite. The materials have ultralong lifetimes of 350 ms and exhibit high quantum yields of up to 52.14% at ambient temperature and atmosphere. “The nanoconfined space inside the zeolites plays a key role here because it effectively stabilizes the triplet excited states of the CDs by suppressing non-radiative energy transfer processes and hinders oxygen quenching.
Figure 1: Supramolecular Structure for TADF Material
Source: Jilin University
“The design method could be applied to other luminescent nanodots and a wide range of suitable host matrices too, such as perovskite and TiO2, and so help in the development of advanced optoelectronic devices like solar cells and LEDs,” says Yu. The CDs@zeolites emit from visible blue and green to red light when excited with different wavelengths of light. They can produce strong blue emission when excited with 370-nm (UV) light, for example. This emission persists even when the UV light is switched off and it can be seen with the naked eye. “We demonstrated this feature by designing a blue rose made from our CDs@zeolite material that has a green stem and leaves made from a benzyl dye molecule (see image),” adds Yu. “After switching off the UV light, the blue petals remain illuminated while the stem and leaves disappear. This approach could have practical applications – in dual-mode security, for example.”