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