Figure 3. The luminescent properties and photooxidation stabilities of SFDBA with different aggregation states. The PL emission spectra evolution of exposed to sunlight for different time intervals: a) dilute solution, b) amorphous film and c) microcrystal film; upper: the corresponding photographs of these samples under 365 nm UV irradiation. d) PLQY of SFDBA with different aggregation states. e) The transient fluorescence decays of SFDBA with different aggregation states at 300 K. f) The FT-IR spectra of SFDBA amorphous film and microcrystal film before and after sunlight exposure for 8 h.
photos are consistent with the spectral results. As reported previously, photo-sensitive naphthylamines of SFDBA tend to be excited and interact with oxygen to form multiple excited state species and radical cation species, generating5H-spiro[dibenzo[c,h]a-crdine-7,9′-uoren]-5-one(SFDBAO) as a primary oxidation product, which is able to be further oxidized.[29]The calculated emissions of SFDBA and SFDBAO in gas phase is at 415 nm and 544 nm (Table S5), respectively, which is consistent with the experimental emissions. In Figure 3b, the initial SFDBA amorphous film exhibits a PL emission at 426 nm, however, the luminescence is quenched just after half an hour sunlight irradiation. The rapid fluorescence decline indicates the generation of radical cation species in the photooxidative process, which can act as electron acceptors to quench the singlet state even at low concentrations through an effective charge transfer.[30-32] The color of the amorphous films after 2 h sunlight irradiation varies from white to a distinct red, and the corresponding thin-layer chromatographic (TLC) analysis confirms that SFDBAO (red dots) is already formed at that point (Figure S4a). The emission spectra of microcrystal films are almost unchanged during the 8 h irradiation by sunlight, still exhibiting an obvious blue light centered at 429 nm, as displayed in Figure 3c. The appearance of the SFDBA microcrystal films keeps invariable white crystal powder without the formation of red SFDBAO, further proved by TLC analysis (Figure S4b). The corresponding XRD patterns in Figure S5 also confirm the stability of the crystalline structure of SFDBAO microcrystal films. The photoluminescence quantum efficiency (PLQY) results of SFDBA in different aggregation states in Figure 3d show that the PLQY value of microcrystal film is the highest (40.6%), which is 48% higher than SFDBA dilute solution and 327% higher than SFDBA amorphous film. The room-temperature-dependent (300 K) transient fluorescence decay curves measured in Figure 3e reveal that the three different aggregation states of SFDBA all display a fluorescent decay character with a nanosecond level (dilute solution: 4.50 ns; amorphous film: 3.06 ns; microcrystal film: 1.88 ns). The Fourier transform infrared (FT-IR) spectra of SFDBA amorphous and microcrystal films in Figure 3f indicate that the spectra are barely changed in the microcrystal film before and after sunlight exposure. However, the amorphous film after sunlight exposure shows an infrared absorption at 1640 cm-1 which can be attributed to the C=O stretching vibration on ketone moieties, while the original C-N stretching vibration and N-H bending vibration at 1099 cm-1 and 1515 cm-1 respectively are disappeared, demonstrating the SFDBA amorphous film has been oxidized. Based on the above results, a significant improvement in the photooxidation stability and luminescence of SFDBA microcrystal film compared with its amorphous phase is achieved.
To disclose the stability promotion mechanisms, the evolution of the luminescence imaging of SFDBA crystal under different sunlight exposure time intervals is recorded with a fluorescence microscope. The single crystal of SFDBA with larger size (width: 108 um) as shown in Figure 4a was adopted for clear observation. The whole SFDBA crystal after 2 h sunlight radiation exhibits a distinct blue light except the corner in the upper right, the light of which is tinted green (Figure 4b). As sunlight radiation prolonged to 4 h, the edges of the crystal exhibit a faint green glow (Figure 4c). Under sunlight irradiation for 8 h, the outline of SFDBA crystal is clearly delineated by green emission, while the main body remains blue emission (Figure 4d). The emergence and extension of green emission confirms that the oxidation process of SFDBA crystal starts at the corner and the edge. Therefore, the principle scheme in Figure