Design of highly efficient deep-blue organic afterglow through guest sensitization and matrices rigidification

Blue/deep-blue emission is crucial for organic optoelectronics but remains a formidable challenge in organic afterglow due to the difficulties in populating and stabilizing the high-energy triplet excited states. Here, a facile strategy to realize the efficient deep-blue organic afterglow is proposed via host molecules to sensitize the triplet exciton population of guest and water implement to suppress the non-radiative decays by matrices rigidification. A series of highly luminescent deep-blue (405–428 nm) organic afterglow materials with lifetimes up to 1.67 s and quantum yields of 46.1% are developed. With these high-performance water-responsive materials, lifetime-encrypted rewritable paper has been constructed for water-jet printing of high-resolution anti-counterfeiting patterns that can retain for a long time (>1 month) and be erased by dimethyl sulfoxide vapor in 15 min with high reversibility for many write/erase cycles. These results provide a foundation for the design of high-efficient blue/deep-blue organic afterglow and stimuli-responsive materials with remarkable applications.

lifetimes (Angew. Chem. Int. Ed. 2019, 58, 15128;Mater. Horiz. 2014, 1, 46). These literatures may be helpful to the readers for better understand the development of organic phosphorescent materials and their photofunctional applications. 5. Some expressions should be carefully considered, e.g. 'ink-free rewritable OURTP paper' and 'using water as ink' in the application section.
Reviewer #3: Remarks to the Author: In this paper, the authors reported a design strategy for deep-blue afterglow materials with long lifetimes and high phosphorescence quantum yields through guest sensitization and matrices rigidification using active host and water. As authors mentioned, blue room-temperature phosphorescence from organic molecules is quite interesting, but never reported. Present mechanisms include stabilized triplet states of H aggregation or charge trapping and releasing of triplet exciton formation. These two mechanism can be hardly used to form blue phosphors, since aggregation and charge transfer can induce emission red shifts. In this paper, the authors proposed a new mechanism, doping organic phosphorescence emitters in rigid matrixes. I think it is very important that since the phosphorescence is from single molecule, it is feasible for material design with an accurate purpose for color tuning rather. The authors also proposed a rewritable paper that can be printed using water as ink to print any anti-counterfeiting patterns, even highresolution pictures. The performance reported in this paper is very high, e.g. quantum yield of 46.1% and lifetime of 1.67 s. So, I suggest this manuscript would be acceptable after a minor revision. 1. As shown in Figs.1c, 1e and 2b, the phosphorescence intensity and ratio increase significantly after water addition. The authors attributed this phenomenon to the suppression of non-radiative decays. Any other reasons can be also considered? 2. Recently, the device applications of purely organic phosphorescence materials have attracted rising attentions owing to the theoretically 100% IQE. Can CT5 used as emitters for electroluminescence as it has high PQY of 46%? 3. Can other solvents like chlorobenzene (generally used in OLED) used for erasing the anticounterfeiting patterns? How to choose the erase solvent? 4. After addition of water, the distance between CA and TMA would be elongated. However, it is showed that the energy transfer between them is not influenced. So, hydrogen bond networks would be also involved in the Dexter energy transfer. Actually, recent investigation showed that the hydrogen bond networks can be used to control the processes of carrier transport and exciton formation (Adv. Funct. Mater. 2020, 30, 1908568). The authors are suggested to add some brief explanations in this aspect. 5. Some errors or writing styles must be corrected, e.g. legend of Fig. 3e is not consistent with the figure. The abbreviation of "PQY" for phosphorescence quantum yield is quite confused. I would like to suggest "PhQY". 1
We have carefully gone through the comments of you and all the reviewers and made necessary changes to the manuscript. All these changes have been highlighted in red in the review-only version of the revised manuscript. Our responses to the comments of the reviewers are as follows.
The reply is indicated by black letters, while the comments by the reviewers are indicated by blue italic letters. We have documented the changes we have made to the original manuscript.

Reviewer: #1
In this manuscript, the authors reported a strategy to achieve efficient blue organic afterglow by designing the host-guest systems using different active hosts (e.g., trimesic acid (TMA), isophthalic acid (IPA), terephthalic acid (TPA) and phthalic acid (PA)) for triplet state sensitization, and cyanuric acid (CA) as the guest for matrices rigidification. As a result, a series of heavy-atom-free Response: Thanks for professional comments and kind recommendation of our work. We have revised the manuscript accordingly.

Comments 1. The authors confirmed the formation of H-bond among CA, TMA and water by the
solid-state 13 C-NMR and Raman spectra (Page 4). However, the description in the manuscript does not fully clarify this conclusion, please provide more detailed discussion and explanation.
Response: Thanks for the kind reminding. The description of the solid state 13 C-NMR and Raman spectra to confirm the formation of H-bond has been revised to fully clarify the related conclusion in the main text and supporting information. Detailed discussion and explanation are as follows. In the main text, "From the solid-state 13 C-NMR spectra of CT5-0, there is a splitting Response: Thanks for the kind reminding. In Fig.3i, two excitation peaks of CT5-0 located at 248 and 288 nm were observed corresponding to the excitation of CA and TMA, respectively. When CT5-0 is excited by 248 nm, CA is excited and most excitons in CA transfer to triplet state via intersystem crossing first, then to TMA through Dexter energy transfer, resulting in nearly invisible fluorescent emission band around 300 nm (blue line in Fig. 3g). Under 288 nm excitation, TMA is directly excited, leading to the more apparent fluorescent emission band before 350 nm (red line in Fig. 3g) owing to its relatively weaker intersystem crossing. Moreover, as shown in Fig. 3j, the phosphorescent intensity of CT5-20 excited by 248 nm is much higher than that excited by 288 nm. Consequently, we conclude that "triplet excitons formation of TMA by CA sensitization under 248 nm excitation are much more efficient than that under 288 nm through direct TMA excitation". We have revised the corresponding discussions to make it 4 clearer. Thanks again.

Comments 3. Does the pH of water affect the RTP performance?
Response: Thanks for the kind reminding and good suggestion. In the revised manuscript, we investigated the effects of pH on the RTP performance by adding HCl or NaOH into the TMA solution to control the pH (Supplementary Figure 10). It was found that RTP performance is dependent on the pH. The phosphorescence intensity reaches the maximum at pH = 2, and it is fully quenched when pH > 5. We have updated the related discussions in the revised manuscript.   Fig. 2d). Therefore, we can accurately measure the PQY by only integrating the emission band in the range from 365 to 620 nm which are only the phosphorescent emission without need to separate RTP from the whole emission.
We have clarified this point in the revised Supplementary Information. Many thanks.

Comments 5. The authors emphasized that blue luminescent (including fluorescent and phosphorescent) materials are indispensable for solid-state lighting and full-color display
technologies in organic optoelectronics. However, they failed to verify the importance of their 6 materials in such fields. I am wondering how to use the as-prepared materials in these fields and how much value they could have.

Response:
We totally agree with the concern of this reviewer. We think that the lifetime-encrypted anti-counterfeiting technique used in rewritable paper should be a kind of display application. For the solid-state lighting, we doped 1 wt% DPhCzT (a yellow afterglow material reported previously, Nat. Mater., 2015, 14, 685-690

Reviewer: #2
In this manuscript, the authors present an efficient strategy to design deep-blue organic afterglow materials with very long luminescence lifetime. Interestingly, both lifetime and quantum yield (PQY) of the afterglow can be enhanced simultaneously by water implement, which is contrary to the general effects of water on photo-luminance. The organic afterglow performance is impressive with PQY up to 46.1% and a water-jet printing for rewritable anti-counterfeiting paper was developed, illustrating a new approach for high-tech applications of organic afterglow materials. Thus, I suggest the acceptance of this manuscript after minor revision upon addressing the following concerns.
Response: Thanks for the kind recommendation of our work. We have carefully revised the manuscript.

Comments 1.
In mechanism section, the authors found that Dexter energy transfer is the main sensitization route through energy transfer from CA to TMA. How to support the effective Dexter energy transfer, since the doping concentration of TMA is low to 5 wt‰?
Response: Thanks for the professional comment. Indeed, reducing the doping concentration can effectively suppress the Dexter energy transfer owing to the short lifetime (several microseconds) of the triplet excitons of hosts as found in organic light-emitting diodes.
Nevertheless, phosphorescent lifetime of CA is much longer and is up to 0.36 s, which can support the much longer diffusion length of the triplet excitons of CA to sensitize the guest molecules via Dexter energy transfer. Moreover, it has been reported that H-bonds can act as the good transfer media of triplet excitons (Science 1995(Science , 269, 1409(Science -1413J. Am Chem Soc 1995, 117, 704-714;J. Phys Chem A 2008, 112, 3865-3869). Therefore, the long phosphorescent lifetime of CA and abundant H-bond networks support the efficient Dexter energy transfer from CA to TMA in a low doping concentration. We have updated related discussions in the revised manuscript.
Thanks again.
8 Comments 2. The authors claim that CA has efficient ISC process while TMA has moderate ISC. Any solid evidence can prove this?
Response: Thanks for the professional question. As shown in Fig.2a in the main text, the PL spectrum of TMA is composed mainly by fluorescence emission band with a phosphorescence tail, while the phosphorescence of CA dominates its PL spectrum, indicating that CA can form triplet excitons much easier than TMA. Further, we calculated the spin-orbital coupling (SOC) constants of CA and TMA. As shown in Supplementary Table 3, SOC constants of CA are much larger than that of TMA, which confirms the photophysical observations. Therefore, we can conclude that CA has efficient ISC process while TMA has moderate ISC. These new data have been involved in the revised manuscript. Many thanks.  Table 3. Singlet-triplet splitting energy (E S1 -E Tn ) and SOC constant from S 1 to T n of TMA and CA. The efficient intersystem crossing channels with E S1 -E Tn < 0.37 eV were highlighted in red.

Molecule
Transition E S1 -E Tn (eV) SOC (cm -1 ) Comments 5. Some expressions should be carefully considered, e.g. 'ink-free rewritable OURTP paper' and 'using water as ink' in the application section.

Response:
We are sorry for the improper expressions. We have carefully gone through the whole manuscript and corrected these expressions. Thanks for the kind reminding.

Reviewer: #3
In this paper, the authors reported a design strategy for deep-blue afterglow materials with long lifetimes and high phosphorescence quantum yields through guest sensitization and matrices rigidification using active host and water. As authors mentioned, blue room-temperature phosphorescence from organic molecules is quite interesting, but never reported. Present mechanisms include stabilized triplet states of H aggregation or charge trapping and releasing of triplet exciton formation. These two mechanism can be hardly used to form blue phosphors, since aggregation and charge transfer can induce emission red shifts. In this paper, the authors proposed a new mechanism, doping organic phosphorescence emitters in rigid matrixes. I think it is very important that since the phosphorescence is from single molecule, it is feasible for material design with an accurate purpose for color tuning rather. The authors also proposed a rewritable paper that can be printed using water as ink to print any anti-counterfeiting patterns, even high-resolution pictures. The performance reported in this paper is very high, e.g. quantum yield of 46.1% and lifetime of 1.67 s. So, I suggest this manuscript would be acceptable after a minor revision.

Response:
We appreciate the reviewer's professional and thoughtful comments and kind recommendation of our work.