Figure 1. Left: chemical structures of carbonyl-based polymers and DBU; right: photos of the complexes between carbonyl-based polymers and DBU in DMF (PBLG, PES, PMTC, PC) or acetone (PVAc) at different mixing time taken under 365 nm UV lamp, c = 0.1 M, control is the pure polymer solution without DBU.
Results and Discussion
As shown in Figure 1, five different kinds of carbonyl-based polymers, poly(benzyl-L-glutamate) (PBLG),[36] Polyester (PES),[37] poly(monothiocarbonate) (PMTC), polycarbonate (PC), and polyvinyl acetate (PVAc), were dissolved in N,N-dimethylformamide (DMF) or acetone, and then DBU was added to the solution (Figure S1-S6). With the increase in standing time, from 0 h to 7 days, the emission intensity was intensified. To exclude the influence of solvents and DBU on the enhanced emission, the photoluminescence (PL) spectra of DBU in DMF and acetone were recorded under different standing time and excitation wavelength, respectively (Figure S7 and S8). The results suggested that DBU was almost non-emissive and its PL spectra were unchanged from 0 h to 7 days. Then, the PL spectra of PBLG in DMF with concentration (c ) of 0.1 M were measured at various excitation wavelengths ranging from 320 to 500 nm (Figure 2). According to our previous report, PLGA exhibited two kinds of emission peaks, the presence of benzene rings in the PBLG side chains resulted in an emission peak around 300 nm when excited below 320 nm, which can be attributed to the isolated benzene ring.[36]Furthermore, PBLG exhibited a weak blue emission around 440 nm when the excitation wavelength (λ ex) was above 320 nm and the emission intensity reached the maximum at λ ex= 360 nm (Figure 2A). Afterward, same molar concentration of DBU (0.1 M) was added into the PBLG solution. As shown in Figure 2B, upon the initial addition of DBU (0 h), no significant redshift in the emission peak was observed. However, the emission intensity increased by approximately 1.5 times. Figures 2C-F depict the time-dependent PL spectra of PBLG with equimolar DBU at different standing time, 1 day, 2 days, 4 days, and 7 days. With increasing time, a noticeable enhancement in the emission intensity and a significant redshift in the CL peak was observed. Apparently, a new peak around 520 nm emerged atλ ex = 460 nm, and this peak also showed excitation-dependent emission (EDE) effect,[26]the maximum emission wavelength (λ em) shifted to 550 nm at λ ex = 500 nm, which is unprecedented in unmodified peptides.
To further investigate the nature of the PBLG-DBU interaction, the1H NMR spectra of the PBLG-DBU mixture were measured at the corresponding mixing time (Figure 2H, Figure S9). Because the signals of PBLG are too complicated to analyze, the proton signals of DBU were monitored. With the increase in mixing time, the proton signals in DBU experienced downfield shifts, indicating a change in the resonance frequency due to electron transfer from the electron-rich in DBU to PBLG. It is noteworthy that protons 1, 8, and 5 showed a larger downfield shift than the other protons. So, it is proposed that the complexation of PBLG and DBU should occur between carbonyl groups of PBLG and the tertiary amine of DBU. Meanwhile, the absence of new peaks in the NMR spectrum confirms that the interaction between PBLG and DBU is a complexation process without any chemical reactions.