image: (a) Design strategy for the through-space charge transfer based TADF in the host-guest MOF Tpl@NKU-111 and the corresponding chemical structures (M: metal center; A: acceptor; D: donor); (b) Jablonski diagram summarizing all possible fundamental photophysical processes of Tpl@NKU-111. view more
Credit: ©Science China Press
This study is led by the team of Prof. Xian-He Bu (academician of CAS) and Prof. Ze Chang from School of Materials Science and Engineering, Nankai University. They have previously developed a host-guest MOF platform for the rational construction of crystalline D-A materials and D-A heterostructure crystals (Adv. Mater. 2018, 30, 1804715 and Angew. Chem. Int. Ed. 2019, 58, 13890-6). Based on the construction principles of host-guest MOFs based on D-A moieties, this class of MOF are promising platforms for through-space CT to obtain TADF materials. First, the confined space of the host MOF framework and relatively strong coordination bonding can fix the position and orientation of D and A molecules. Spatially separated D and A molecules can then be obtained, which leads to a small overlap of HOMO and LUMO and small ΔEST. Proper spatial CT interactions can also be obtained, through which the electron clouds of D and A moieties can interact with each other, which will enhance radiative transition processes and increase PLQY. Second, the relatively rigid framework of MOF can suppress non-radiative relaxation induced by rotational and vibrational motion of components to further increase PLQY. In addition, the crystalline nature of MOFs allows for the straightforward structural modulation of D/A components, which is beneficial for the investigation of structure-property relationships.
By introducing electron-rich triphenylene (Tpl) molecules into the cages of NKU-111, face-to-face π-π stacking of Tpl and the electron-deficient Tpt ligand was achieved. The resulting Tpl@NKU-111 displayed enhanced emission intensity upon increasing the temperature, which fits the characteristics of TADF. Detailed photophysical characterization of Tpl@NKU-111 confirmed the presence of TADF, which was further supported by first-principles calculations. The results show that Tpl@NKU-111 had a separated LUMO and HOMO, which led to a small ΔEST (0.11 eV), making it an ideal platform for triplet excitation state harvesting (Scheme 1). In addition to achieving TADF, the PLQY and PL intensity of Tpl@NKU-111 could be optimized by tuning the Tpl loading in the host framework. A maximum PLQY of 57.36% was achieved under an air atmosphere and room temperature when the ratio of Tpl loaded in the cages was 24.0%. These results suggest that utilizing crystalline host-guest MOFs is an effective and flexible strategy for through-space CT to achieve TADF with modular PLQY and PL intensity.
See the article:
Manipulating Spatial Alignment of Donor and Acceptor in Host-Guest MOF for TADF
https://doi.org/10.1093/nsr/nwab222
Journal
National Science Review