Figure 5. Schematic representation of the mechanism of the through-space n ‧‧‧π complex.
Based on our previous studies on the CL properties of carbonyl-based aliphatic polymers, the 440 nm emission in these CLgens arises from the (n , π*) transition of the carbonyl groups.[25, 36-37] The emission intensity of the 440 nm CL is influenced by the electronic structure and conformational flexibility of the subunits. Generally, a relatively rigid structure favors the (n , π*) transition of carbonyl groups. In terms of flexibility, PES exhibits the highest flexibility, followed by PMTC, and finally PBLG and PC, aligning with the observed emission intensities of 440 nm CL. Meanwhile, the flexible backbone of PVAc makes it difficult to stabilize the (n , π*) transition of side-chain carbonyl groups, preventing effective clusterization between adjacent carbonyl groups. However, the complexation between carbonyl groups and DBU widely exists in main-chain and side-chain polymers. Then, we propose the through-space n ‧‧‧π complex mechanism for the complexation, as illustrated in Figure 5. Upon the addition of the electron-rich organic base (DBU) to the carbonyl-based polymers, the electron-withdrawing carbonyl group can accept electrons from the electron-rich nitrogen in DBU, leading to the formation of a complex between carbonyl groups and the Lewis base DBU, thereby modulating the photophysical properties of the CLgens.
It needs to be emphasized that not only DBU, but other nitrogen-containing organic bases could also form complexes with carbonyl groups, for example, triethylamine (TEA). However, according to our studies, the complexation between TEA and carbonyl groups is relatively weak, as reflected by the PL changes. Overall, this work presents an effective approach to induce through-space n ‧‧‧π complexes by introducing the nitrogen-containing organic base for clusterization. Further exploration of the through-space n ‧‧‧π complex theory, such as achieving n ‧‧‧π complexes in a broader range of carbonyl-containing molecules, especially for small molecules with distinct structures, is currently underway in our research group, aiming to establish a more comprehensive theoretical framework.
Conclusions
In conclusion, we have discovered a general complexation phenomenon between carbonyl-based polymers and the nitrogen-containing organic base of DBU, which is demonstrated by the PL and 1H NMR characterization. The complexation could not only enhance the intrinsic 440 nm emission of carbonyl groups, arising from its (n , π*) transition, but also induce a new long-wavelength CL corresponding to the complexes. By understanding the complexation strategies employed in nature and adopting an engineering approach, researchers can overcome the challenges to achieve efficient clusteroluminescence. The study of emerging clusteroluminescence from the complexes between carbonyl-based polymers and organic bases presents exciting opportunities for advancing the field of optoelectronics and photonics. Further investigations are needed to elucidate the underlying mechanisms, optimize the cluster structures, and explore new materials for complexation. These advancements will pave the way for the creation of novel luminescent materials with enhanced performance and expanded applications in various fields.
Supporting Information
The supporting information for this article is available on the WWW under https://doi.org/10.1002/cjoc.2023xxxxx.
Acknowledgement
H. Zhang thanks for the financial support from the National Science Foundation of China (22205197) and the Youth Talent Excellence Program of ZJU-Hangzhou Global Scientific and Technological Innovation Center.
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