a KrF = ФFF; bKiscpF;c The absolute phosphorescence efficiencies were calculated according to the following equation: Фp = Sp/SPL × ФF+P, where SPL and SP stands for the integral area of the total photoluminescence and phosphorescence spectra, respectively, and ФF+P refers to the overall absolute photoluminescence quantum yield of the host-guest crystalline film;d KrP = ФPP; eKnrP = (1-Фp)/ τP.
and 6-TAT-CN/PPh3 films (Figure 3c). In addition, these host-guest films present different afterglow lifetime by naked eyes (Figure 3c). For example, a strong green afterglow with a duration of around 10 s is observed in the 6-TAT-H/PPh3 film, while the afterglow of 5-TAT-OMe/PPh3 is much shorter. This visual difference stems from the different phosphoresce intensity and lifetime between the 6-TAT-H/PPh3 and 5-TAT-OMe/PPh3 films. Such different visible afterglow duration makes these host-guest materials have potential applications in high secure-level information encryption and anti-counterfeiting.
Time-dependent density functional theory (TD-DFT) calculations are carried out to gain a deep insight into the RTP phenomena of the host-guest systems.[39-45] The singlet and triplet energy levels of PPh3 and six luminogens are listed as Figure S7. The energy of the lowest singlet state (S1) for PPh3 is 4.55 eV. According to the Franck-Condon principle, the energy transfer could occur from the S1state of PPh3 to the Sn states of guests with energy within ES1 ± 0.3 eV of PPh3. There exist more energy transfer channels (marked as blue) for 6-TAT-OMe, 6-TAT-H, and 6-TAT-CN in contrast with their isomers, respectively, promoting the energy transfer process in their host-guest co-crystal systems (Figure 4b). In addition, compared with the isomers, 6-TAT-OMe, 6-TAT-H, and 6-TAT-CN show better absorption spectral overlaps with the emission spectrum of PPh3(Figure 4a), in which 6-TAT-CN among six luminogens exhibits the best absorption capacity and largest overlaps, supporting the efficient energy transfer process. And the good crystallinity of PPh3 provides the guest molecules an appropriate rigid environment to restrict molecular rotations and vibrations, inhibiting the non-radiative transitions. These results may contribute to the high phosphorescence quantum yield of 6-TAT-CN/PPh3co-crystalline films and understand the persistent RTP via host-guest co-crystalline strategy.
Since these host-guest systems exhibit different afterglow duration, it is promising to achieve high secure level data encryption and anti-counterfeiting applications. As shown in Figure 5a, we use three kinds of host-guest systems (6-TAT-H/PPh3, 6-TAT-CN/PPh3, and 5-TAT-OMe/PPh3) to fabricate a simple anti-counterfeiting pattern “Fortune cat” via a mask. Under a 365 nm UV-lamp irradiation, “Fortune cat” emits violet blue and greenish fluorescence. After stopping UV light irradiation, “Fortune cat” with yellow-green