Unraveling the Influence of the Carbon Skeleton Structure and Substituent Electronic Effects on the Nontraditional Intrinsic Luminescence Properties of Nonconjugated Polyolefins

Through-space conjugation (TSC) has gradually been proven to be a vital interaction in photophysical processes, especially with the recent observation of nontraditional intrinsic luminescence (NTIL) arising from cluster/aggregation-induced molecular-scale confinement. However, although the effect of the spatial distance between molecular chains has been understood, it is not clear how the distance between adjacent conjugated structures in a single chain affects the NTIL properties of polymers, and this has become a significant research focus. Herein, four nonconjugated polyolefins with different carbon skeletons (C5/C6/C7/C8, in which the repeat units contained 5, 6, 7, and 8 carbon atoms, respectively) were synthesized, and their photophysical properties were systematically studied. The experimental and theoretical results showed that although the carbon skeleton of the C5 polymer contained only one less carbon atom than that of the C6 polymer (but possessed the same conjugated structure in the chain), the C5 polymer (QY = 23.4%) exhibited a much greater luminescence efficiency than the C6 polymer (QY = 3.3%). Moreover, the distance to the neighboring conjugated structure in the C7/C8 polymer chains was the same as that in the C5/C6 polymers, whereas the conjugated structure in the C7/C8 polymers had a longer pi conjugation region and more rigid polymer chains than the C5/C6 polymers; thus, the C7/C8 polymer had longer fluorescence emission wavelengths than the C5/C6 polymers. Moreover, simple and nonconjugated polymers of CPBB (PCPBB-C4) derivatives with electron-donating (methoxy) and electron-withdrawing (alkynyl) groups were also prepared, and the fluorescence spectra showed that both the electron-donating (increased electron density and stabilized TSC) and electron-withdrawing (increased through space electronic interactions) groups in PCPBB-C4 resulted in red-shifted luminescence and increased luminescence efficiency. Above all, these insights reveal that the carbon skeleton structure and substituent electronic effects strongly influence the NTIL properties and provide valuable strategies for optimizing the NTIL efficiency and tuning the emission colors of nonconjugated polymers.