Figure 3. A-F) PL spectra of 0.1 M PES and 0.1 M DBU in DMF at different excitation wavelengths (λ ex) and different mixing time; H) 1H NMR spectrum of PES-DBU complex (molar ratio 1:1) in CDCl3 at the different mixing time.
Then the complexation effect on CL was also systematically investigated in the other three kinds of carbonyl-based polymers, PMTC, PC, and PVAc. Figure 4 summarized the PL and 1H NMR characteristics of PMTC, PC, and PVAc upon complexation with equimolar DBU. The PL spectra of the polymer-DBU complexes were recorded at the same concentration (0.1 M) under different excitation wavelengths and mixing time. As shown in Figure 4A, PMTC also displayed a weak emission peak around 440 nm when excited at 400 nm, and no emission peak was observed at λ ex = 460 nm. Upon the addition of equimolar DBU to PMTC, a significant enhancement of fluorescence intensity occurred and the 440 nm redshifted to 490 nm under the excitation of 400 nm. Meanwhile, a new CL peak appeared around 530 nm atλ ex = 460 nm. The change of PL spectra along with the increase in standing time demonstrates the strong complexation between PMTC and DBU. Figure 4B showed the PL spectra of PC-DBU complexes. PC exhibited a typical emission peak related to the carbonyl group around 440 nm when excited at 380 nm, However, an anomalous phenomenon was observed upon the addition of equimolar DBU to PC. Under 380 nm excitation, the emission intensity of 440 nm sharply decreased after DBU addition, and then gradually recovered with time going on, surpassing the original intensity of PC after 7 days. Furthermore, a new emission peak was induced near 500 nm upon excitation at 440 nm, and its intensity kept increasing from 0 h to 7 days. These results indicated that PC also formed complexes with DBU but not as strong as PMTC.
PVAc is also one kind of polyester, which possesses carbonyl groups in the side chains. Different from PES, PVAc did not show any emission near 440 nm in acetone with c = 0.1 M. Intriguingly, when PVAc was mixed with equimolar DBU, a broad emission peak appeared in the range of 470-500 nm under 380 nm excitation, and the emission intensity continuously increased with the increase of mixing time. The emission intensity at 7 days was approximately four times higher than that at 24 hours (Figure 4C). Additionally, we also tested the PL spectra of PVAc dissolved in pure DBU, interestingly, a new long-wavelength emission peak near 550 nm was induced after 7 days of dissolution without the EDE effect (Figure S11). Meanwhile, the mixture of PVAc and pure DBU exhibited interesting apparent color change, transitioning from yellow to orange upon swelling and eventually turning blood red upon complete dissolution over time. Furthermore, 1H NMR spectra of the PMTC-DBU, PC-DBU, and PVAc-DBU complexes revealed downfield shifts of multiple protons adjacent to the tertiary amine atom in DBU as the mixing time increased (Figure 4D-F, Figure S12-14). This observation further supports the complexation between DBU and these carbonyl-based polymers, suggesting that both backbone and side-chain carbonyl groups could form the complexation with the electron-donating amine group. The strength of the complexation is expected to increase with prolonged mixing time, as evidenced by the changes observed in photographs taken under sunlight (Figure S15).