Two‐Phase Rubber–Plastic Matrices’ Stabilization of Organic Room‐Temperature Phosphorescence Afterglows Better than Plastic Matrix

The rigid yet polar polymers as matrices for organic‐doped room‐temperature phosphorescence (RTP) polymers are widely reported, but nonpolar rigid plastics such as polystyrene (PS) are thought to be ineffective matrix and rarely attempted. Herein, it is reported that PS–polyisoprene–PS (SIS) elastomer and rubber phase‐containing high‐impact PS (HIPS) can stabilize brighter and longer‐lived organic RTP than PS as matrix; moreover, photoactivation time for RTP production is also greatly shortened. Three N‐arylcarbazole derivatives are employed as dopants and afford the same regular results, and the afterglow lifetime of RTP elastomer is up to 1.22 s. Since the general rubbers such as polyisoprene are ineffective doping matrices and PS is not good matrix, the interface phase of polyphase polymers plays an important role in promoting and stabilizing triplet‐state emission. Based on the different confined environments in various phase regions for organic dopants, a new stabilizing RTP mechanism is discussed to understand the unique stabilizing RTP effect of multiphase matrices. This study not only develops a high‐performance RTP elastomer but also discloses a fresh strategy for enhancing RTP of organic‐doped polymers.


Introduction
[3] Their potential applications in optoelectronic devices, bioimaging, anticounterfeiting, information encryption, and glow-in-the-dark products call for not only ultralong and bright afterglows but also diverse mechanical and processing performances.10][11][12][13] To gain bright and ultralong organic RTP, enhancing triplet-state population and stabilizing triplet excitons are all indispensable.The introduction of extra heteroatoms and/or heavy halogens into conjugated organic molecules can enhance spin-orbital coupling (SOC) to facilitate intersystem crossing (ISC) for triplet-state population.[19] Therefore, like conjugated organic molecules in crystals and polymers, the RTP properties of conjugated organic molecules in different polymers are also deduced from each other because of different confined and interaction environments.
The rigid and polar polymers can usually stabilize organic RTP better, but they cannot provide the really flexible and elastic RTP materials.It is known that general rubbers such as NR (PI), SBR, NBR, and EPDM cannot stabilize the triplet state to provide RTP polymers, and nonpolar PS is rarely attempted as an effective matrix, not to mention nonpolar rubber-containing SIS (PS-PI-PS block elastomer).However, we recently present that N-(4-cyano-phenyl)carbazole (LPCN)-doped SIS can emit blue afterglow with %300 ms of RTP lifetime after 10 s photoactivation. [20]There have been a few RTP polymer reports including rubber-containing plastics as doping matrices, [21][22][23] DOI: 10.1002/sstr.202300101 The rigid yet polar polymers as matrices for organic-doped room-temperature phosphorescence (RTP) polymers are widely reported, but nonpolar rigid plastics such as polystyrene (PS) are thought to be ineffective matrix and rarely attempted.Herein, it is reported that PS-polyisoprene-PS (SIS) elastomer and rubber phase-containing high-impact PS (HIPS) can stabilize brighter and longerlived organic RTP than PS as matrix; moreover, photoactivation time for RTP production is also greatly shortened.Three N-arylcarbazole derivatives are employed as dopants and afford the same regular results, and the afterglow lifetime of RTP elastomer is up to 1.22 s.Since the general rubbers such as polyisoprene are ineffective doping matrices and PS is not good matrix, the interface phase of polyphase polymers plays an important role in promoting and stabilizing triplet-state emission.Based on the different confined environments in various phase regions for organic dopants, a new stabilizing RTP mechanism is discussed to understand the unique stabilizing RTP effect of multiphase matrices.This study not only develops a high-performance RTP elastomer but also discloses a fresh strategy for enhancing RTP of organic-doped polymers.
but there is no in-depth study and the stabilizing effect is supposedly attributed to plastic phases.
However, we now find that LPCN/PS hardly shows RTP even after 5 min UV photoactivation.Thus, we propose that rubberplastic two-phase matrix can stabilize organic RTP better than the corresponding plastic phase.To validate our hypothesis and develop ultralong RTP elastomer, in the current work, we use N-(2-cyano-5-bromophenyl)carbazole (2Q5X) and N-(2-cyano-4bromophenyl)carbazole (2Q4X) to dope PS, SIS, and HIPS.The two-in-one introduction of cyano and bromine can enhance SOC and ISC to rich triplet population and improve RTP.HIPS is a plastic-containing rubber-dispersed phase and is different from SIS with continuous rubber phase and dispersed PS phase, but they are all rubber-plastic two-phase matrices and can further test the hypothesis.It is noted that PS, SIS, and HIPS have no RTP although commercial polymers may contain trace amounts of impurities such as antioxidants.To avoid the possible effect on RTP, these polymers are purified by twice dissolutionprecipitation, and the results show that purification does not affect RTP.We now report that 2Q5X/SIS exhibits brighter (59.1 lux) and longer-lived (1.22 s) RTP afterglow than 2Q5X/ PS (22.1 lux, 0.57 s) and the maximal afterglow illuminance and RTP lifetime of 2Q5X/HIPS (34.2 lux, 0.91 s) are also remarkably superior to those of 2Q5X/PS.Moreover, the photoactivation RTP time is much shorter in rubber-containing SIS and HIPS matrices (3-5 s) than that in pure PS matrix (30-45 s).As expected, the same law is verified using 2Q4X as dopant.These results indicate that the ultralong RTP in rubber-plastic two-phase matrices is not from neither rubber phase nor plastic phase, and the interface phase has played a positive role in inhibiting triplet thermal deactivation.Interestingly, thermoplastic processing does not impair but improve RTP properties, and the doped SIS elastomer still emits RTP afterglow under arbitrary distortion and deformation.Since the improved RTP properties are not from traditional matrix polarity increase, dopant grafting, and interaction enhancement, this work discloses a fresh strategy for enhancing RTP of organic-doped polymers and represents an important RTP material and concept advance.

Results and Discussion
N-(2-cyano-5-bromophenyl)carbazole (2Q5X) and N-(2-cyano-5bromophenyl)carbazole (2Q4X) could be readily synthesized just as easily as N-(4-cyanophenyl)carbazole (LPCN) excepting that 5-bromo-2-fluorobenzonitrile and 4-bromo-2-fluorobenzonitrile were used, respectively (Figure 1a). [20]The details for synthesis, purification, and characterization are depicted in the Supporting Information.[26] However, take 2Q5X, for example; the frozen crystalline, amorphous and solution glassy states in liquid nitrogen emit strong and ultralong low-temperature phosphorescence (Figure 1b), and the quantum chemical calculations reveal the high SOC values and promising ISC channels due to the two-in-one introduction of heteroatom cyano and heavy-atom bromine (Figure 1c).Therefore, the abundant triplet-state population is validated, but crystallization itself cannot effectively inhibit the triplet thermal deactivation at room temperature.It is known that the molecular confined environments can alter RTP properties, and LPCN has shown blue RTP with afterglow lifetime of %300 ms in SIS matrix.Thus, it is reasonable that 2Q5X should exhibit better RTP afterglow properties.
We first dope 2Q5X into SIS by dissolving 0.1-1.0wt% 2Q5X (relative to SIS) and SIS into chloroform and then evaporate solvent in fume hood to obtain solution-cast 2Q5X/SIS films for the better molecular dispersion of 2Q5X in polymer matrix.Considering that densely stacked matrix is conducive to the inhibition of molecular thermal motions, we thermoplastically process the solution-cast films on 120 °C open mill and mold into 1 mm-thick plate on 160 °C plate vulcanizing machine.As shown in Figure 2a, bright and ultralong cyan RTP afterglows are observed with the duration time of about 15 s viewed by naked eyes in the dark.Notably, these afterglows are even visible to the naked eye under a fluorescent lamp (Figure 2b and Video S1, Supporting Information).Moreover, the photoactivation time for the deserved RTP production is short and only needs 3 s.Generally, the optimal doping amount is dependent on dopant and polymer properties and cannot be predicted in advance.In this case, too much doping may have caused molecular agglomeration to reduce doping efficiency, and the low doping amount can achieve brighter and longer RTP afterglows.To relatively and quantitatively compare RTP afterglow intensity, the time-dependent afterglow illuminance curves are measured under the same conditions (Figure S1, Supporting Information), and the initial (maximal) illuminance is over 50 lux in the dark for 0.1-0.5% 2Q5X/SIS (Figure 2c), which are consistent with the naked eye observation.The steady-state PL spectra, PL efficiency, and time-resolved RTP decay curves are measured.2Q5X/SIS films emit weak fluorescence and strong RTP afterglow with peak wavelengths at %400 and 500 nm (Figure 2d), respectively.Interestingly, 2Q5X/SIS films show rather low total PL efficiency (1.7-4.0%) and the calculated RTP efficiency is less than 1% (0.47-0.96%, Figure S2, Supporting Information) although the bright afterglows are observed, indicating that a large number of triplet excitons radiatively decay after excitation light ceasing.Based on the RTP decay curves (Figure 2e), the fitted RTP lifetimes are 1.11-1.22s for 0.1-1.0%2Q5X/SIS.Such bright RTP afterglows with ultralong lifetime are achieved in a nonpolar and elastic polymer matrix, which is impressive and never reported so far.
Our initial idea was to expect the rigid PS phase to stabilize RTP and the continuous PI phase to provide high-elastic properties.So 2Q5X-doped PI was prepared and no RTP was observed after UV excitation for 60 s (Figure S3, Supporting Information).Since SIS can stabilize LPCN and 2Q5X to emit RTP afterglow with lifetimes of 0.3-1.2s, it seems that PS should stabilize the same or even ultralong organic RTP.To validate this common sense idea, LPCN and 2Q5X are doped into PS, respectively.Unexpectedly, LPCN/PS hardly shows RTP even after photoactivation time is prolonged to 5 min (Figure 3a).2Q5X/PS can emit RTP (Figure 3b), but the photoactivation time required is 30-45 s, and significantly, the RTP afterglow brightness and duration time are obviously inferior to those of 2Q5X/SIS.2Q5X/PS has the similar steady-state PL spectra to 2Q5X/SIS, and the total PL efficiency and RTP efficiency are also rather low (Figure S4, Supporting Information).The measured maximal afterglow illuminance is only 6-10 lux and the RTP lifetimes are only 0.49-0.78s (Figure 3c).The overall deduced RTP afterglow properties indicate that SIS can stabilize ultralong RTP afterglows better than PS, suggesting the observed RTP for 2Q5X/SIS is not from PS phase.Alternatively, we propose that the interface phase in SIS has played an important role in promoting and stabilizing organic triplet-state emission since general rubbers are certainly effective doping matrices.
Since rubber-plastic interface phase can play an important role, let us try HIPS.Although HIPS and SIS have different polymer chain structures and phase morphologies, they are all two-phase rubber-plastic matrices.If HIPS matrix can stabilize organic RTP afterglows better than PS matrix, this will further support our viewpoint and disclose a magic enhancing RTP strategy.Figure 4a shows that RTP afterglow of 2Q5X/HIPS is indeed brighter and longer than 2Q5X/PS (0.5 wt% 2Q5X), where two samples are put together and photoactivated for 5, 30, and 60 s, respectively.It is observed that photoactivating 2Q5X/HIPS for 5 s can produce the deserved RTP afterglow, and this short time can only photoactivate 2Q5X/PS to emit inferior RTP afterglow.Figure 4b depicts the change of relative afterglow illuminance with duration time, and much the same afterglow illuminance is recorded for 2Q5X/HIPS after 5 and 30 s photoactivation (maximal 37.3 and 39.2 lux).In contrast, photoactivated 2Q5X/PS only shows the maximal afterglow illuminance of 7.5 lux.As expected, RTP lifetime of 2Q5X/HIPS is obviously longer than that of 2Q5X/PS (Figure 4c).
Both LPCN and 2Q5X give the positive effect, and 0.5% 2Q4X, an isomer of 2Q5X, is also doped into PS, HIPS, and SIS, respectively.Figure 5a shows that 2Q4X/SIS and 2Q4X/HIPS emit the   deserved afterglows after photoactivation for 5 s, and 2Q4X/PS requires 45 s photoactivation.Moreover, the RTP afterglow brightness and duration time are still 2Q4X/SIS > 2Q4X/ HIPS > 2Q4X/PS.These observations recurrence the above phenomena and again support our viewpoint.It is noted that PI (rubber) cannot stabilize organic dopants to emit RTP, and these organic dopants show weak RTP in PS matrix.We can detect the uniform and fine polyphase structure (Figure S5, Supporting Information), but the dispersion details of dopant molecules in the different phases cannot be identified at present.However, the observed bright RTP afterglows in SIS and HIPS matrices are independent of emissions in PS and PI phases, and they are dominantly from the dopants dispersed in the interface phase.Considering that rubber and plastic have very different volume shrinkage ratios (ΔV r and ΔV p ), compression modulus (E r and E p ), vibration modes (v r and v p ), cohesion energy density (CED r and CED p ), and so on, the dopant molecules dispersed in rubber, plastic, and interface phases move in different modes and degrees and are in different confined environments.Undoubtedly, polyphase polymer systems have more internal stresses, and the most concentrated area is the interface phase, where the triplet thermal deactivation should be more effectively inhibited (Figure 5b).As a result, SIS and HIPS as matrices can promote and stabilize even bright and long organic RTP afterglow.In this context, whether more multicomponent and polyphase polymers are more excellent doping matrices as well as how to design better organic dopants are worth studying, which is underway in our laboratory.
Finally, the RTP afterglows of 2Q5X/SIS are excellent, and significantly, this is a really elastic and hydrophobic RTP material and can be thermoplastic processed into self-supporting products of any shape and size without damaging RTP properties.With such flexible and elastic RTP materials in hand, we demonstrate the persistent RTP afterglow emission under arbitrary distortion and dynamic deformation (Figure 6, Video S2, Supporting Information).2Q5X/SIS strip keeps bright afterglow although afterglow duration time is slightly shortened possibly due to the deactivation effect of chain segment movements (upper).Also, a series of movements of a struggling table tennis player are showed alive via afterglow pattern (down), signifying the promising potential in flexible and dynamic RTP displaying and anticounterfeiting at least.To the best of our knowledge, this is the first RTP elastomer with so high afterglow intensity and ultralong lifetime.

Conclusion
In summary, we have demonstrated that nonopolar rigid PS as doping matrix can stabilize some N-arylcarbazole derivatives to emit RTP, but it is weak and requires a long photo-activation time.In contrast, thermoplastic elastomer SIS can stabilize even bright and long-lived organic RTP than PS, moreover the photoactivated RTP time is greatly shortened from tens of milliseconds to a few milliseconds.The promoting and stabilizing effects of two-phase polymer matrices on organic RTP are further confirmed by using both rubber-containing high impact PS (HIPS) as doping matrix and different structural N-arylcarbazole derivatives as dopants.These results reveal that the interface phase has played unique and positive roles in promoting and stabilizing organic doped RTP due to the strong stress concentration effect in these regions.Impressively, the RTP lifetime of organic doped SIS elastomer is up to 1.2 s and the bright and ultra-long afterglow is still observed under dynamically arbitrary distortion and deformation.This work has disclosed a fresh doping matrix strategy and represents an important RTP material and concept advance.The even excellent RTP elastomer and the multi-component poly-phase polymers as more effective doping matrices are being developed in our laboratory.

Figure 1 .
Figure 1.a) The synthesis and molecular structure of three N-phenylcarbazole derivatives.b) The room and low (after frozen in liquid nitrogen)temperature phosphorescence of 2Q5X in crystalline, amorphous (cooled melt), and THF solution glassy states.c) The calculated ISC probabilities (P ISC ) and spin-orbital coupling values (ζ) for possible ISC channels.

Figure 2 .
Figure 2. The room temperature PL photographs of thermoplasticized 2Q5X/SIS films before and after removing 365 nm light excitation a) in the dark and b) under fluorescent lamp.c) The relative illuminance, d) the steady-state PL spectra under 320 nm excitation, and he time-resolved RTP decay curves after 365 nm excitation for different 2Q5X doping amounts in SIS.e) The measurements are conducted after samples are activated by 365 nm light (0.3 mW cm À2 ) for 3 s.

Figure 3 .
Figure 3. a) The PL photographs of LPCN/PS after 10, 60, and 300 s photoactivation.b) The PL photographs of 2Q5X/PS with different doping amounts after 60 s photoactivation.c) The time-resolved RTP curves of 2Q5X/PS and the fit RTP lifetimes.All samples are activated by 365 nm UV light.

Figure 4 .
Figure 4. a) The PL photographs of 0.5% 2Q5X/PS and 2Q5X/HIPS films after 5, 30, and 60 s photoactivation.b) The relative illuminance for 2Q5X/HIPS and 2Q5X/PS.c) The time-resolved RTP curves of 2Q5X/HIPS and the fit long-component RTP lifetimes.

Figure 5 .
Figure 5. a) The RTP afterglow photographs of 0.5% 2Q4X-doped SIS, HIPS, and PS sheets under different photoactivation times.b) The schematic diagram of dopant molecular environments in the matrix consisting of rubber and plastic with different volume shrinkage ratio (ΔV ), compression modulus (E), vibration mode (v), cohesion energy density (CED), etc.

Figure 6 .
Figure 6.The RTP afterglow photographs of 0.5% 2Q5X/SIS elastic strip (upper) and table tennis player in motion (down) under arbitrary distortion and deformation after 365 nm light excitation.