Figure 2. A-F) PL spectra of 0.1 M PBLG and 0.1 M DBU in DMF at different excitation wavelengths (λ ex) and different mixing time; H) 1H NMR spectrum of PBLG-DBU complex (molar ratio 1:1) in CDCl3 at the different mixing time.
Besides polyamide, polyester of PES was also investigated upon complexation with equimolar DBU through excitation-dependent and time-dependent PL spectra. As shown in Figure 3A, PES exhibited a quite weak emission around 440 nm when it was dissolved in DMF with cof 0.1 M. When equimolar DBU was added, the emission was intensified, meanwhile, an obvious EDE effect was observed (Figure 3B). However, unlike PBLG, the maximum CL wavelength redshifted to 470 nm under 380 nm excitation at the initial mixing. After standing for 1 day, an extra CL peak at 510 nm was observed, and its intensity increased with time going to 2 days. Interestingly, after 1 day, two distinct peaks (λ em = 450 nm and 510 nm) are excitation-wavelength independent (Figure 3C-F). Moreover, with the increase in standing time, the intensity ratio of 510 nm / 450 nm increased and the complexation-induced emission became dominant. These results indicated that the complexation between PES and DBU is stronger than polyamide and the formed complexes are more stable. Time-dependent1H NMR spectra revealed downfield chemical shifts of the protons 1,5, and 8 in DBU and upfield chemical shifts of the ester group protons in PES as the mixing time increased, which further supports the complexation between PES and DBU (Figure 3H, Figure S10).