a KrF =
ФF/τF; bKisc =Фp/τF;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 =
ФP/τP; 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