Clustering and halogen effects enabled red/near-infrared room temperature phosphorescence from aliphatic cyclic imides

Pure organic room temperature phosphorescence (RTP) materials become increasingly important in advanced optoelectronic and bioelectronic applications. Current phosphors based on small aromatic molecules show emission characteristics generally limited to short wavelengths. It remains an enormous challenge to achieve red and near-infrared (NIR) RTP, particularly for those from nonaromatics. Here we demonstrate that succinimide derived cyclic imides can emit RTP in the red (665, 690 nm) and NIR (745 nm) spectral range with high efficiencies of up to 9.2%. Despite their rather limited molecular conjugations, their unique emission stems from the presence of the imide unit and heavy atoms, effective molecular clustering, and the electron delocalization of halogens. We further demonstrate that the presence of heavy atoms like halogen or chalcogen atoms in these systems is important to facilitate intersystem crossing as well as to extend through-space conjugation and to enable rigidified conformations. This universal strategy paves the way to the design of nonconventional luminophores with long wavelength emission and for emerging applications.

imides in this work?
3、The authors need to provide more references to support the statement:"The O···XC bond angles are extended from 165° to 170° (Fig. 4b), indicating gradually enhanced halogen contacts, which are beneficial to electron delocalization." 4、Some recent advances about the heavy atom effect and NIR RTP emission are suggested to be cited: Angew. Chem. Int. Ed., 2021, 60, 19735;Natl. Sci. Rev., 2021, nwab085. Reviewer #3: Remarks to the Author: Pure organic systems with room temperature phosphorescence (RTP) have attracted increasing attention in recent years. In this manuscript, the authors established a simple nonaromatic cyclic imide system with efficient red/NIR RTP, which could be ascribed to halogen effects, molecular clustering, and through-space conjugation. Furthermore, the rationality and universality of the design strategy had been proven and halogens could be replaced by other heavy atoms to result in similar long wavelength emission. These results are interesting and important to provide insights in accessing red/NIR RTP with simple building blocks. Therefore, this manuscript is recommended for publication after addressing the following minor issues: 1. The authors need to provide more references to support the statement for Fig. 4a. 2. The authors need to explain why the brominated product of 2MIP is 2BMIP. In parallel to the brominated reaction of MI, the bromination of 2MIP may also result in the product without C=C double bonds. 3. The footnote of Table 1 should be double checked and corrected. 4. Among the clustering effect, halogen effect, and pi-pi stacking, which one is the most important factor to generate the red/NIR RTP emission?
In this work, Yuan and coworkers reported a novel cyclic imide-based NIR RTP. Interestingly, despite its limited conjugation, it showed NIR emission which might originate from the presence of imide unit, heavy atoms, molecular clustering, and electron delocalization of halogens.
Although the authors' work is quite interesting, purely organic RTP is now a very well-known phenomenon. The effect of halogen atoms on RTP behavior is also very well understood in many aspects including 1) its heavy atom effect, 2) rigidification by its role in molecular interactions (such as halogen bonding), 3) its delocalization effect. In other words, this reviewer cannot find any strikingly new observation and hence, science in this work. Also, authors cannot make any advances in its applications as well. In the introduction, the authors emphasized that NIR RTP could potentially be used for optoelectronic and biological applications, but, they did not provide any of its application feasibility in this work. For those reasons, this reviewer cannot support the publication of this work in Nat. Commun.
Thanks for the reviewer's comments of our work.
Admittedly, as the reviewer implied, the effect of halogen atoms on RTP has been reported in certain aspects including 1) heavy atom effect, 2) rigidification in molecular interactions (e.g. halogen bonding), 3) electron delocalization effect. However, this work is not limited to previous understanding. And according to above understanding, one cannot directly expect the rational fabrication of NIR phosphors through halogen substitution without significant molecular conjugation. Meanwhile, other heavy atoms like sulfur can also lead to significantly red-shifted RTP in a similar way. The innovations of this work are summarized as follows: Firstly, the vast majority of previous works on purely organic RTP were focused on large conjugated structures, while little attention has been paid to that of nonaromatics. The mechanism of these nonconventional RTP compounds deserves further investigation. Moreover, most NIR emission comes from fused ring compounds or those with strong electron donating and accepting structures. Due to the highly limited conjugation, it is difficult to achieve red and NIR emission in nonaromatic compounds. Our report demonstrates the potential of achieving red and even NIR emission from the aggregates of nonaromatic compounds.
Secondly, few reports have demonstrated that the introduction of heavy atoms could cause significant redshift in the phosphorescence emission. Typically, in our work, after the introduction of Br atoms, a remarkable redshift of ˃ 200 nm for RTP is noticed, which is rare among organic luminophores. Furthermore, the introduction of sulfur also led to a bathochromically shifted RTP of ˃ 100 nm, which further verifies the feasibility of the proposal. To the best of our knowledge, this is the first reported strategy for achieving such a significant redshifted emission in nonaromatics free of large conjugation through simple molecular design and consequent clustering.
Thirdly, the effect of halogen bonding is not limited to restricting molecular motions herein. Like other noncovalent interactions among electron rich moieties, halogen and chalcogen atoms could form through-space electron delocalization with other neighboring electron rich units. In this case, heavy atoms could simultaneously lead to rigidification and electron delocalization.
As for the potential applications of these cyclic imides, we are collaborating with colleagues working on bioimaging and theranostics. As shown in Fig. C1, the cytoblasts and cytoplasts of human tendon cells could be clearly labeled using 4,6-diamidino-2-phenylindole (DAPI) and DIMI/exosome complexes, respectively. Meanwhile, we are still endeavoring to fabricate the red/NIR RTP nanoparticles for the in vivo imaging. While in this work, we mainly focus on the mechanism understanding and aim to propose a general strategy for developing nonconventional luminophores with long-wavelength emission.

Fig. C1
Confocal microscopies of human tendon cells. The cytoblasts are labeled using DAPI while the cytoplasts are marked by DIMI/exosome complexes.

QY for intersystem crossing and thus, k isc are an essential part of evaluating the full kinetics of purely organic RTP. However, in this work, this reviewer cannot find any of this process. Authors should calculate QY isc and k isc from the proper experiments.
Thanks for the reviewer's advice.
According to the previous reports (Adv. Mater. 2014, 26, 7931; Molecular Fluorescence 2012), the k isc and Փ isc (QY isc ) could be calculated by the following equations: However, fluorescence quenching occurs in DBMI, DIMI, 2BMIP, MTSI and DTSI due to their strong heavy atom effect, so that their fluorescence lifetimes (  f s) cannot be traced and corresponding k isc s also cannot be obtained. While the k isc s of the rest samples and Փ isc of all the samples are calculated and provided in Table 1, Supplementary Table 10 and 13. 2. Most PL spectra obtained from powder/crystal samples are very noisy (see Figure 2d, 5d, 6b). This should be more qualified.
Thanks for the reviewer's kind reminder.
Following the reviewer's suggestion, we have provided more qualified spectra in Fig. 2d, 5d and 6b to better demonstrate the emission.

Authors should provide low-temperature PL spectra of solution samples and should compare them with their solid-state samples. Also, authors should investigate the photophysical studies of solid solutions (phosphors-doped polymers). From those experiments, authors might have a much better in-depth understanding of the phenomena.
Thanks for the reviewer's comments and suggestion. According to the reviewer's suggestion, we have conducted additional experiments.
Firstly, we obtained the prompt (t d = 0 ms) and delayed (t d = 0.1 ms) emission spectra of the dilute solutions (10 -6 M) of DBSI, DBMI, and DIMI in tetrahydrogenfuran (THF) at 77 K (Fig.  C2), which are basically in accordance with those of pure THF (λ ex = 365 nm), indicating individual molecules of DBSI, DBMI and DIMI are virtually nonemissive even at crytotemperatures.
Furthermore, we investigated the photophysical properties of DBSI, DBMI, DIMI doped PMMA films with varying weight fractions (1, 5, 10 wt%). As shown in Fig. C3-5, apparently, the PL intensity of these films gradually increases with the increment in dopant fraction.
The DBSI/PMMA films could generate yellow emissions with maxima at 585/590 nm under 285 and 312 nm UV irradiations. Meanwhile, small shoulders are found in the range of 400~500 nm, which is decreased with increasing doping fractions and might be corresponded to the molecular fluorescence. With t d of 0.1 ms, merely peaks at ~585/590 nm are noticed for all films, which should be ascribed to the RTP emission of the DBSI aggregates with the lifetimes of 0.20~0.27 ms (Fig. C3c). Notably, for all films, with a λ ex of 365 nm UV light, faint blue emissions peaking at 443 nm are observed, which are highly consistent with that of the pristine PMMA film (Fig. C3b).
The analogous λ ex -dependent emissions are also found in DBMI/PMMA films with the orange-yellow emission (620 nm) with λ ex s of 285 and 312 nm, and faint white emission with λ ex of 365 nm. As for DIMI/PMMA films, they all display orange emission under varying UV lights with the maxima at 645 nm. The almost disappearance of the emission at 400~500 nm suggests the full quenching of fluorescence of DIMI, on account of the considerable heavy atom effect.
It is also noted that the RTP emissions of these doped films are blue-shifted with comparison to those of the corresponding crystals (Fig. C3d, C4d and C5d), which should be ascribed to their differences in molecular clustering. Obviously, better through-space conjugation is formed in crystals owing to the synergistic effect of molecular clustering, π-π stacking, and electron delocalization of halogens.   From the photophysical studies of solution samples, phosphors-doped polymers and crystals, we have gained a more in-depth understanding of the red and NIR RTP from small nonconventional luminophores, which is highly associated with molecular clustering, halogen effect, and π-π stacking.
We have placed these data into the revised Supporting Information and added corresponding discussion in the revised manuscript.
Thanks again for the reviewer's comments and suggestion.

Point-by-Point Response to the Comments and Suggestions of Reviewer #2
Recently, pure organic room temperature phosphorescence (RTP) materials have attracted extensive attention in advanced optoelectronic and bioelectronic applications. Generally, achieving long-wavelength emission, especially red and near-infrared (NIR) RTP, is still quite tough because of the difficulties in molecular designing and regulation of singlets and triplets. In this manuscript, the authors reported a series of succinimide derived cyclic imides which could emit long-wavelength RTP, even in the red and NIR spectral range with excellent efficiencies based on heavy atom effects, molecular clustering, and through-space conjugation. These results are interesting and helpful to further understand the CTE mechanism, meanwhile, they will inspire researchers a novel pathway for the construction of long-wavelength RTP. Therefore, I think the paper is suitable to be published in Nature Communications. While there are some questions that need to be addressed before publication. Compared to SI, the introduction of heavy atoms (Br, I, and S) in DBSI, DBMI, DIMI, MTSI and DTSI will remarkably promote intersystem crossing with more generation of triplet excitons, which may simultaneously enhance the radiative ( ) and nonradiative decay rate ( ) of phosphorescence (see Chem. Phys. Lett. 1980, 69, 580). The RTP lifetimes could also be calculated using the equation of <> p =1/ ( + ). Therefore, with increased values of and , the RTP lifetimes of DBSI, DBMI, DIMI, MTSI and DTSI will be much shorter when compared to that of SI.

The authors mentioned that the through-space conjugation (TSC) was formed via the interactions between heavy atoms (Br, I, S) and heteroatoms (N, O). So, what is the main difference between through-bond conjugation (TBC) in traditional polycyclic aromatic hydrocarbons (PAHs) and cyclic imides in this work?
In fact, the nature of both TSC and TBC could be all ascribed to the overlap of electron clouds and electron delocalization. However, for traditional PAHs, the conjugation is extended via explicit covalent bonds with red-shifted absorption and emission. While for cyclic imides in this work, the conjugation from covalent bonds of single molecule is limited, to achieve analogous long-wavelength emission, TSC based on the noncovalent interactions among electron rich units is proposed, which can be facilitated by halogen bonding and chalcogen bonding and is crucial for red/NIR RTP emission.
3. The authors need to provide more references to support the statement: "The O···XC bond angles are extended from 165° to 170° (Fig. 4b), indicating gradually enhanced halogen contacts, which are beneficial to electron delocalization." Thanks for the reviewer's comments and suggestion.
According to the newly added Ref. 59 (Chem. Rev. 2016, 116, 2478, the halogen bonding of O···X will be maximized when the O···XC bond angle is close to 180 o . In this work, the O···XC bond angles of DBSI, DBMI and DIMI are extended from 165° to 170°, which could well demonstrate the enhanced halogen contacts and electron delocalization.

4.
Some recent advances about the heavy atom effect and NIR RTP emission are suggested to be cited: Angew. Chem. Int. Ed., 2021, 60, 19735;Natl. Sci. Rev., 2021, nwab085. Thanks for providing the valuable references. We have carefully read these works and found them very inspiring. We have cited them as Ref. 49 and 29 in the revised manuscript to better illustrate the NIR RTP and heavy atom effect.

Point-by-Point Response to the Comments and Suggestions of Reviewer #3
Pure organic systems with room temperature phosphorescence (RTP) have attracted increasing attention in recent years. In this manuscript, the authors established a simple nonaromatic cyclic imide system with efficient red/NIR RTP, which could be ascribed to halogen effects,