Figure 2 Absorption, fluorescence, and phosphorescence spectra, and photographs under natural light and UV light of (a) 5-TAT-OMe, (b) 6-TAT-OMe, (c) 5-TAT-H, (d) 6-TAT-H, (e) 5-TAT-CN, and (f) 6-TAT-CN in bulk states and dichloromethane solutions.
the close molecular packing of π-planes. In addition, it is reported that the host molecules could also provide a synergistic effect to the guest at excited state via Förster resonance energy transfer (FRET) to trigger the RTP behaviors of the guest.[13,27] The host-guest hybrid films were fabricated by varying luminogen:PPh3 molar ratio via a common melt-casting method (Figure 1b and Figure S5), in which the blends of the guest and host were heated to 85℃ under ambient conditions and then rapidly cooled to reach crystalline states. As shown in Figure 3c, six melt-casting host-guest films with 0.01mol% guests all exhibit obvious visible afterglow after removal of 365 nm ultraviolet irradiation, implying the presence of RTP. As shown in Figure 3d and Figure S2, a fluorescence emission peak at 434 nm with a short lifetime of 4.15 ns, and a phosphorescence emission peak at 520 nm with a long lifetime of 83.8 ms are recorded in the photoluminescence spectra of pure PPh3 crystalline films at room temperature. The phosphorescence phenomenon of PPh3 can’t be captured by naked eyes and the photoluminescence of PPh3 crystals is weak with an overall quantum yield (ΦF+P) of 2.86%. However, after doping the luminogens into PPh3, the photoluminescence performance of the co-crystalline films is greatly enhanced and the overall quantum yields are increased by 7 to 16 times (Table 1). Especially, the RTP lifetime of host-guest hybrid films elongated to the order of seconds (Figure 3e). Note that the phosphorescence quantum yields (ΦP) of 6-TAT-OMe/PPh3, 6-TAT-H/PPh3, and 6-TAT-CN/PPh3 are 7.72%, 11.91%, and 29.35%, respectively, which are higher than that of 5-TAT-OMe/PPh3 (7.31%), 5-TAT-H/PPh3(4.66%), and 5-TAT-CN/PPh3 (12.96%), respectively. The enhancement of ΦP could be partially ascribed to the increase in Kisc (Table 1). Among six host-guest materials, 6-TAT-CN/PPh3 exhibits the largest Kisc and Krp, thus leading to the highest ΦP. Although the fast Krp brings a slight decrease in the phosphorescence lifetime of 6-TAT-CN/PPh3, it still obtains a high value of 0.76 s (Figure 3e). These results are consistent with the much brighter afterglow in 6-TAT-OMe/PPh3, 6-TAT-H/PPh3,