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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