Background and Originality Content
All organic room temperature phosphorescence (RTP) luminogens have drawn
extensive attentions and shown intelligent potentials in the field of
optoelectronic devices,[1-3]sensor,[4,5] information
storage,[6-10] biological
imaging,[11,12] and
anti-counterfeiting.[13-15] Generally, organic
luminogens in a single component seldom exhibit persistent
phosphorescence at ambient conditions due to their intrinsic
spin-forbidden intersystem crossing (ISC) from the lowest single state
(S1) to the triplet state (Tn) as well
as their ultrafast triplet exciton deactivation. Tremendous endeavors,
including the introduction of heteroatoms,[16,17]heavy halogen atoms,[3,18] sulfonyl
groups,[13] keto
groups,[19-21] and
multimers,[22-24] have been made to enhance the
ISC efficiency and populate triplet excitons, successfully turn on the
persistent phosphorescence of pure organic luminogens. Nevertheless,
most of them turn on RTP relying on highly ordered crystalline
structures. The cultivation of crystals is hard to control and repeat,
and the resulting crystal phosphorescent materials are also difficult to
process, thus limiting the actual application
scenarios.[21,25,26]
Recently, a host-guest system via a simple melt-casting method is
promising to solve the above problems.[1,13,27-29]Dispersing the guest (organic luminogens) into the host matrix could
provide a rigid environment for guest to suppress the possible
non-radiative transitions. Besides, the host molecules could participate
in the formation of charge-transfer states under photo-excitation or
provide a synergistic effect to the guest at excited state via Förster
resonance energy transfer (FRET) to trigger
RTP.[1,13,27] For the guest materials, most
reported organic luminogens tend to employ a twisted molecular
configuration to avoid the possible quenching effect by π-π stacking of
large aromatic rings.[3,30-32] However, there are
still a few guests with large conjugated backbones showing bright
emissions in aggregated states considering that the rigid conjugated
structures could restrict molecular vibrations and
rotations.[22,33] Therefore, the molecular packing
of π-planes in aggregated states is important.[26]Beneficial from the intrinsic advantages of rigid host materials, guest
with large conjugated π-planes is likely to realize strong emissions as
well as persistent phosphorescence in host-guest hybrid films. In
addition, more examples of host-guest systems employing the guest with
large conjugated π-planes are also in high demand to further understand
the complex relationships among chemical structure, molecular packing,
and RTP performance.
Herein, six triazatruxene-based organic luminogens with different
peripheral substituents are designed and synthesized, as shown in Figure
1a. These luminogens in aggregated states demonstrate fluorescence
quantum yields ranging from 15.73% to 37.58% but no RTP. Dispersing
these individual luminogen as guest into the host (PPh3)
via a simple melt-casting method could turn on the persistent RTP. Among
six luminogens, 6-TAT-CN/PPh3 films obtain the highest
phosphorescence yield (Фp) of 29.35% with a
phosphorescence lifetime of 0.76 s, and 5-TAT-H/PPh3 films exhibit the
longest phosphorescence lifetime of 0.99 s with a Фpvalue of 4.66%. The experimental results and theoretical simulation
indicate that PPh3 acts as not only a rigid matrix to
suppress the non-radiative transitions of the guest, but also provides
energy transfer channels to the guest. Furthermore, based on the
different afterglow duration of these host-guest systems, multi-level
dynamic data encryption and anti-counterfeiting are realized. This work
deepens the insight for low cost, halogen free, and facile fabrication
of all-organic persistent RTP materials, with demonstrated promising
applications in data protection.
Results and Discussion
As a strong electron-donating unit, triazatruxene (TAT) has proven to be
a promising platform for optoelectronic materials in multitudinous
fields, i.e. organic solar cells, field-effect transistors,
organic light-emitting diodes, etc. [34-36]TAT possesses an inherently rigid and planar backbone, allowing
electrons to delocalize along conjugated π-systems, facilitating long
excited-state lifetimes and suppressing the possible non-radiative
transitions induced by molecular vibrations and
rotations.[33,37] In addition, theC3h symmetric structures endow TAT with multiple
reactive sites. Herein, 5-TAT-OMe and its isomer (6-TAT-OMe, Figure 1a)
were designed and synthesized via structural modification to
systematically investigate the effect of stereochemical configurations
on luminescence. To further tune molecular configurations and energy
levels, 5-TAT-H, 5-TAT-CN, and their corresponding isomers with rational
selected substituents were constructed as well.