Figure 4 Optical properties of EDTA-CDs and UPy-CDs. (a) UV-vis
spectra of EDTA-CDs, UPy-NCO and UPy-CDs in DMF. (b) Photoluminescence
(PL) spectra of EDTA-CDs, UPy-NCO and UPy-CDs in DMF. (c)
Photoluminescence (PL) spectra of EDTA-CDs and UPy-CDs in solid state.
(d) Time-resolved fluorescence spectra of EDTA-CDs and UPy-CDs in solid
state.
Subsequently, the optical properties of EDTA-CDs and UPy-CDs were
investigated. EDTA-CDs exhibit a strong absorption centered at 276 and
283 nm, which could be attributed to the π-π* transition of C=C and n-π*
transition of C=O domains [28]. These absorption
bands are retained for UPy-CDs, indicating that the structure of carbon
core is less affected. Besides, a shoulder peak at around 293 nm is
present for UPy-CDs. This broad absorption should be derived from the
UPy moieties [42,43]. Upon excitation at 365 nm,
EDTA-CDs
and UPy-CDs show blue fluorescence in DMF (50 mg mL-1)
with the identical emission
maxima at 415 nm. Considering that UPy is almost non-fluorescent, the
fluorescence of UPy-CDs and EDTA-CDs in DMF should be of the same
origin, that is, the o -PD derived carbon core. In contrast to the
blue PL in solution, the emission of EDTA-CDs and UPy-CDs show a
remarkable red-shift in solid state, with an emission maximum of 460 and
487 nm, respectively. Concomitantly, the emission band shape is also
distinct, with structured and broad band present for the solution-state
and solid-state emission, respectively. These phenomena should be
ascribed to the aggregation induced emission spectral shift. Notably,
the emission of UPy-CDs is more red-shifted than EDTA-CDs, suggesting
that the inter-particle interaction of UPy-CDs is stronger than that of
EDTA-CDs. According to the
time-resolved luminescence decay traces, the emission lifetime of
EDTA-CDs and UPy-CDs are 0.81 and 4.42 ns, both fall in the range of
nanoseconds, indicating their emission of singlet parentage (Fig. 4d).
The lifetime of UPy-CDs is 33.4 ms times longer than that of EDTA-CDs,
indicative of the stabilization of singlet sates via strengthen of
interparticle association.
To our gratifyingly interest, the
introduction of UPy not only stabilize the singlet excited state, but
also result in the efficient population of triplet state. An afterglow
emission can be easily observed by the naked eye from UPy-CDs powder
under ambient conditions. The yellow RTP could lasting for over 6 s
after ceasing the UV lamp (365 nm) irradiation. Transient analysis of
the phosphorescence signal at 534 nm shows an average emission lifetime
of 33.6 ms. Note that no afterglow can be detected from EDTA-CDs or
UPy-NCO under the same conditions. This implies that the RTP emission of
the UPy-CDs should primarily attributed to the stabilization of triplet
excited state by rigidifying effect, which could be further traced back
to the UPy induced multiple hydrogen bonding (Scheme 1). In other
words, the UPy recognition motifs
on the surface could serves as the “rigid matrix” by incorporation of
strong inter-CDs interaction [44,45].
Hence, matrix-free RTP could be
induced in the solid state, which has rarely reported for CDs for the
best of our knowledge. Moreover, we envisage that besides the
rigidifying effect, the surface UPy units could also act as partition to
separate the adjacent phosphors and thus mitigate the aggregation caused
quenching.