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.