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.