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.