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).