Polymeric ultralong organic phosphorescence with excellent humidity and temperature resistance via hydrophobic effect

Abstract

Polymeric ultralong organic phosphorescence (UOP) with persistent emission is of great importance in practical applications. However, achieving good water‐resistance for long‐term environmental stability is a formidable challenge. In this contribution, through tailoring the alkyl‐chain length of the hardeners and emitters, polymeric UOPs with varying crosslinking density and hydrophobic effect were obtained. Notably, all the polymers show no obvious decrease in UOP emission after high temperature‐humidity test (85°C/85% relative humidity for 7 days). Detailed investigations demonstrate that the rigid covalent crosslinking networks suppress the quenching of triplet excitons while the hydrophobic microenvironment affords good water/moisture‐resistance ability. Moreover, the polymers with superior processability are successfully applied as optical coatings, prepreg, and afterglow displays. With this work, we provide a new strategy to promote the long‐term stability of polymeric UOP materials in high‐temperature‐humidity conditions.

INTRODUCTION

Ultralong organic phosphorescence (UOP) materials featuring long lifetime have attracted ever-increasing attention due to their unique optical phenomenon, which can be widely applied in the fields of displays,^[] sensing,^[] information encryption,^[] and bioimaging.^[] Owing to the spin forbidden transition between the excited triplet and ground states, fast non-radiative relaxation of triplet excitons is the primary pathway for deactivation, resulting in a formidable challenge to achieve efficient phosphorescence at room temperature (RT).^[] Nevertheless, with great efforts of researchers, various rational strategies, such as crystallization,^[] host-guest doping,^[] and polymerization^[] were proposed to promote the intersystem crossing (ISC) and suppress the non-radiation.

Among these, polymeric UOPs are impressive due to good film-forming stability, cost-effectiveness, and suitability for large-area production.^[] In polymeric UOPs, the polar sites afford robust intermolecular interactions (hydrogen bond and ionic bond) between the matrix and phosphorescent emitters, which can restrict the excited-stated molecular motions and suppress non-radiation pathways.^[] However, such polar sites also exhibit strong affinity to water molecules when exposing in high humidity and temperature environment. As a result, polymeric UOPs with polar bonds will suffer from severe moisture erosion and lack long-term stability, leading to deleterious effects on the physical and optical performance.^[]

The hydrophobic effect is a phenomenon of excluding water molecules due to the aggregation of nonpolar molecules or groups, which plays a critical role in biological processes and engineering systems, such as protein folding, membrane formation, and micelle formation.^[] A common approach to enhance hydrophobicity is to introduce hydrophobic groups such as alkyl, aryl ether, and aryl ketone moieties. These groups can be further modified in their length, polarity, molecular structure, and substitution to achieve water resistance ability. Therefore, materials with robust humidity resistance can be obtained through a reasonable structural design.

With this hypothesis, polymers with different crosslinking densities were achieved by varying the length of the alkyl chains of hardeners. Meanwhile, the phosphorescent emitters with different alkyl chain lengths (methoxy and hexyloxy) were introduced into the system by doping process (Figure  and ). The polymers exhibit UOP with lifetimes of τ = 0.73–1.30 s under ambient conditions. Notably, the afterglow emissions of these polymers show robust stability (τ = 0.91–1.16 s) after immersion in water for 7 days at room temperature. Moreover, after 85°C/85% RH reliability test for 7 days, these polymeric UOPs still maintain high stability with lifetimes of τ = 0.78–1.13 s, showing excellent high humidity and temperature resistance. Detailed investigations reveal that the crosslinking density and hydrophobic effect play pivotal roles for the robust water/moisture resistance ability. The decreased density of polar sites significantly improves the water resistance of the polymers. Intriguingly, the phosphorescent emitters with different alkyl chain lengths also have an impact on the afterglow emission and water resistance of the polymers.

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RESULTS AND DISCUSSION

In this contribution, bisphenol-A (DGEBA)-based epoxy resin was selected as the polymer skeleton due to its high rigidity, low cost and good processability.^[] Based on the hydrophobic effect of alkyl chains, phosphorescent emitters such as triphenylene (TP), 2,3,6,7,10,11-Triphenylenehexol (TPO), 2,3,6,7,10,11-Hexamethoxytriphenylene (TPM), and 2,3,6,7,10,11-Hexakis(hexyloxy)triphenylene (TPH), and hardeners including 1,3-diaminopropane (A3), 1,8-octanediamine (A8), and 1,12-Diaminododecane (A12) were introduced with varying alkyl chain lengths, which can construct covalent networks with different crosslinking densities and polarity. Base on simple doping technology and curing process, a series of polymers (TP@A3, TPO@A3, TPM@A3, TPH@A3, TP@A8, TPO@A8, TPM@A8, TPH@A8, TPM@A12, and TPH@A12) with UOP features were obtained (Figure and ). The TP doped films exhibit very weak afterglow emission, while the TPO-doped polymers show poor chemical stability during the hot-curing process because of the oxidation of the phenoxy groups. In contrast, the TPM and TPH series exhibit bluish-green afterglows lasting for more than 20 s, which are easily captured by naked eyes (Figure ). The broad and structureless features of the X-ray diffraction (XRD) pattens indicate the amorphous nature of the prepared films (Figure ). Additionally, the transparent and flexible film of TPM@A3 can be easily bent and recovered (Figure ). These results demonstrate that the epoxy resins are ideal platforms for designing high-performance UOP polymers.

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As shown in Figure , the doped polymers exhibit dual-peak emission with fine vibronic features. The fluorescence emissions on the steady spectra of TPM@A3, TPM@A8, TPM@A12, TPH@A3, TPH@A8, and TPH@A12 are located at 383 nm with prompt lifetimes of 12–15 ns (Figure ), indicating that the alkyl-chain length has slightly influence on the fluorescence. The phosphorescence spectra, which are assigned to the afterglow emissions located at 488 nm, exhibit similar vibronic structures but different phosphorescence lifetimes (Figure ). The elongation of alkane chain in the hardeners results in a significant lifetime decrease in TPM series (Figure ). Meanwhile, the lifetimes in TPH series are slightly shortened when the crosslinking density decreased. Surprisingly, the doped concentration has little effect on the UOP lifetime of the system, which can be confirmed by the concentration dependent experiment (Figures ). When the concentration increases from 2‰ to 2%, the appearance of the curing films changes from transparent to non-transparent white. However, the lifetimes vary slightly with 0.86 s, 0.83 s, and 0.80 s for 2‰, 6‰, and 2%, respectively.

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To verifying the origin of the UOP feature, further experiments were performed. The UOP emission bands of the doped polymers are well overlapped with the low temperature phosphorescence spectra of the isolated molecules (∼10⁻⁵ M in dilute solutions) with maximum emission around 480 nm (Figure ). Therefore, the UOP properties of the doping polymers should be originated from the intrinsic isolated phosphorescence of TPM and TPH, due to their good dispersion in the epoxy resins. Notably, the emitter TP with polycyclic aromatic hydrocarbon feature exhibits no obvious phosphorescence emission even under cryogenic conditions. Such result demonstrates that the introduction of oxygen atoms with lone pair electrons can facilitate the intrinsic phosphorescence emission (Figures ).^[] Meanwhile, the curing reactions of the epoxy resins and hardeners generate three-dimensional covalent crosslinking networks, which provide rigid matrices to prevent the oxygen quenching.^[] Moreover, the numerous benzene rings among the epoxy resins construct a relatively rigid microenvironment, which effectively restrict the excited molecular motions to reduce vibrational energy losses.^[] In other words, the UOP emission of these polymers can be assigned to the effective intrinsic ISC process in the emitters and the rigid environment of the crosslinked polymers.

In these polymers, the crosslink density and topology are systematically altered by the introduction of hardeners with different length of alkyl-chains. The networks of these polymeric matrices become loose due to the prolonged hardeners with longer alkane chains and thus the crosslinking density decrease. These results can be demonstrated by the decreased glass transition temperature (T_g) of these doped polymers (Figure ). Intriguingly, the decreased T_g values in the TPH series indicate that the doped phosphors could affect the rigidity of the networks even with a low concentration. To reveal the relationship between polymer topology and UOP features, molecular dynamic simulations were performed to establish the crosslinking process (Figure ). Furthermore, the fraction free volume of the doped polymer increases along with the prolonged alky chain of the hardeners (Figure  and Figure ). The plenty of unoccupied space in the polymeric matrices affords structural relaxation and results in nonradiative pathways.^[] With this regard, the decreased lifetimes (Figure ) of TPM and TPH series in the polymeric matrices can be ascribed to the large free volume which afford free space for the excited molecular motions of the phosphorescent emitters. These results indicate that the UOP lifetimes in epoxy resins can be easily modulated by the alkyl chains of the hardeners.

In further set of experiment, the interactions of the UOP emitters and polymers were also calculated and shown in Figure , which are positive values of 68.73 kJ/mol, 77.12 kJ/mol, 72.46 kJ/mol, 133.39 kJ/mol, 136.22 kJ/mol, and 129.05 kJ/mol for TPM@A3, TPM@A8, TPM@A12, TPH@A3, TPH@A8, and TPH@A12, respectively. Different from previously reported negative results due to the polar hydrogen bonding,^[] the positive values in our systems can be assigned to the repulsive interactions between the alkoxyl groups of the emitters and the alkyl chains of the polymers. Such values become much larger in TPH series for the longer peripheral chains (Figure ). Due to the robust three-dimensional covalent in the polymers, the UOP emitters exhibit strong forced compatibility with the matrices. Contrast with the TPM series, the UOPs lifetimes in TPH series seem insensitive to the polymer matrices, suggesting that the repulsive interactions of the alkyl-chains also play a critical role in UOP lifetimes (Figure ).^[]

Nevertheless, the hydrophobic alkyl-chains in the polymer could decrease the water diffusion rate when exposing in the high humidity conditions, affording good water resistance ability of the polymeric UOPs. Therefore, the six polymeric UOPs films were immersed in water to evaluate the water-resistant ability. Expectedly, these films still exhibit extraordinary afterglow with a duration over 10 s, as observed by naked eyes even soaking in water for several days (Figure ). As shown in Figure , these polymers show different trends in phosphorescence lifetime when immersed in water at room temperature. TPM@A3 and TPM@A8 exhibit a tendency of decreasing first and then stabilizing, while TPM@A12 exhibits a trend of increasing and then stabilizing. TPH@A3 and TPH@A8 show a slight decrease followed by an increase and stabilization in lifetime, while TPH@A12 continues to increase and tends to stabilize after 4 days. Furthermore, the six polymers were placed at a harsh condition of 85°C/85% RH to examine the moisture-resistance ability (Figure ). The TPM series exhibit similar trend as that under the RT conditions in water (Figure ). Surprisingly, in the TPH series, the UOP lifetime of TPH@A8 increases after 7 days while other two polymers exhibit shortened emission (Figure ). The UOP features are still very bright after such trial. It should be noted that the emission intensities of the UOP are slightly decreased after 7 days but the lifetimes maintain at satisfied levels. Moreover, even after the high temperature-humidity test, there were no significant microstructure changes observed in scanning electron microscope (SEM) images of these samples. (Figure ) These results imply the excellent water/moisture-resistant ability of the polymers with alkyl chains.

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To reveal the mechanism of the robust water/moisture resistance features, the static contact angles and water absorption ratio of the six polymers were performed. As shown in Figure  and Figure , with the prolonged alkyl-chains of the hardeners, the contact angles of the TPM and TPH series increase while the water absorption ratio decrease (Figure  and Table ). These results demonstrate the enhanced hydrophobic feature in polymers constructed by the long alky chains. Meanwhile, the longer alkyl-chain polymers suggest lower crosslinking density, which indicates the lower polar environment due the less polar sites in the polymer and thus results in a lower water absorption ratio. As a result, the rigid crosslinking networks can suppress the quenching of triplet excitons while the hydrophobic microenvironment can afford good water/moisture-resistance ability. Furthermore, the increased T_g suggests the enhanced interchain interactions in the polymeric matrices with a small amount of water absorption when soaking in water for 7 days (Figure ), which further promotes the rigidity of the polymers. Benefiting from the hydrophobic microenvironment and robust covalent crosslinked networks, the triplet excitons cab be well stabilized under the harsh conditions. In general, at room temperature, the low cross-linking density polymer matrix and the doping system with long alkyl chain phosphorescent emitters exhibit better water resistance, which is achieved by the synergy of their hydrophobicity and less polar sites. However, at higher temperatures (85°C), the afterglow system with moderate cross-linking density (TPM@A8 and TPH@A8) exhibit better water resistance with prolonged lifetimes. In contrast, the lower cross-linking density in A12 systems, which have lower T_g values, promotes the movement of polymer chains and thus enhances the diffusion process of water molecules.

Considering the feasible processing, good optical performance, and adhesive ability of epoxy compounds, a set of potential applications based on the polymeric UOPs were demonstrated (–). As depicted in Figure , the TPM@A3 system was fabricated as coatings to cover different surfaces without any chemically reactive functional group. Due to the abundant polar bonds (hydroxy, ether, and C-N) of the polymer, robust adhesion can form on the glass, aluminum plate, polypropylene, and copper, which are protective and decorative. Moreover, the epoxy prepregs were pre-impregnated with UOP resins and glass fibers, after hot curing process (90°C for 4 h), the composites exhibit aquamarine emission lasting over 20 s when turning off the excitation source (UV 365 nm, Figure ). The similar process can also be applied to daily fiber such as thread. Remarkably, a prototype afterglow LED display was developed (Figure ). With frequency of electrical power changing (flash mode), the device shows a significant afterglow trace, confirming potential application for target movement trajectory display. These results certainly indicate that the polymeric UOPs based on epoxy-curing system can be applied as coatings, prepreg, and optical package materials.

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CONCLUSION

In summary, we present a novel strategy for obtaining polymeric UOPs with excellent water/moisture resistance ability via hydrophobic effect. This strategy relies on the crosslinking density and hydrophobic effect of the covalent networks. The triplet excitons are stabilized by the polymeric networks and the hydrophobic microenvironment prevents the water/moisture erosion. Strikingly, these polymeric UOPs are robust enough to maintain bright afterglow with no obvious decrease after high temperature-humidity trial (85°C/85% RH) over 1 week, exhibiting a perfect long-term stability, which is one of the most robust amorphous UOP materials. With the good processability of the epoxy resins, these polymeric UOPs are successfully applied as optical coatings, prepreg, and afterglow displays, which facilitates a myriad of possibilities for potential applications with long-term environmental stability.

ACKNOWLEDGMENTS

This work was financially supported by the Key-Area Research and Development Program of Guangdong Province (grant number: 2023B0101030002), Shenzhen R&D project (grant number: JSGGZD20220822100001002), and Guangdong Basic and Applied Basic Research Foundation (grant number: 2022A1515110927).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interests.

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