Odd–even effect on room-temperature phosphorescence for benzene-carboxylic derivatives

Abstract

In this work, a series of benzene-carboxylic derivative (BCD)-acrylamide copolymers were prepared. All the obtained copolymers exhibited blue afterglow with durations reaching up to 4.64 s. Besides, an interesting odd–even effect was observed between the number of ester groups and the phosphorescent properties.

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Liangwei Ma

Dr Liangwei Ma received his MSc in organic chemistry from South China University of Technology under the supervision of Professors Derong Cao and Sheng Wang. In 2018, he joined the research group of Professors Xiang Ma and He Tian at East China University of Technology, where he completed his PhD in applied chemistry in 2022. He continued his postdoctoral research under the supervision of Professor He Tian and joined East China University of Technology as an associate professor in the School of Chemistry and Molecular Engineering in 2024. His research focuses on the design and tuning of novel phosphorescent dyes and exploring their advanced applications in data encryption, imaging, and sensing.

In recent years, room-temperature phosphorescence (RTP) has received widespread attention owing to its remarkable large Stokes shift and long-lived delayed emission characteristics, which demonstrates application potential across a variety of fields such as information encryption,^1–4 bioimaging,^5–7 environmental monitoring^8–10 and so on.^11–14 Among RTP materials, organic ones have gained prominence over their costly and toxic inorganic counterparts due to advantages including low toxicity, ease of access, and design flexibility. However, developing pure organic RTP materials faces many challenges.¹⁵ For instance, pure organic molecules generally have weak spin–orbit coupling (SOC), leading to inefficient intersystem crossing (ISC) between singlet and triplet states. Additionally, triplet excitons are highly sensitive to the environment, such as oxygen and temperature.

Therefore, to develop efficient pure organic RTP materials, it is essential to focus on both promoting the formation of triplet excitons and suppressing the non-radiative relaxation.¹⁶ According to the El-Sayed rule,¹⁷ carbonyl substituents can effectively enhance SOC, thereby improving phosphorescence efficiency. As a result, benzene-carboxylic derivatives (BCDs) are considered ideal chromophores. In 2018, our group first achieved RTP emission in amorphous pure organic materials by employing a copolymerization strategy involving BCDs and acrylamide.¹⁸ During the copolymerization process, the dense network formed by the interaction between the polymer matrix like acrylamide and the chromophore—comprising both covalent and hydrogen bonds—provides the necessary rigidity and partially isolates the triplet excitons from oxygen. This effectively suppresses the non-radiative relaxation and oxygen-induced quenching, thus enhancing the phosphorescence efficiency. Moreover, the copolymerization approach could also offer additional advantages, including simple preparation, excellent reproducibility, and superior processability.^19,20 Up to now, numerous reports have been published on pure organic RTP materials achieved through copolymerization strategies.^21–29

To obtain copolymers with highly efficient phosphorescence, the structure of the chromophore is crucial. Among various strategies, modifying the chromophore using substituents is a common method to tune the photo-physical performance, such as wavelength and lifetime.^30–33 However, the impact of the ester substituent group number on the phosphorescence performance of BCDs has not been systematically studied yet. Herein, we prepared a series of BCDs (E1–E5) with different numbers of ester groups and copolymerized them with acrylamide using the same method. X-ray diffraction (XRD) analysis confirmed that all the obtained polymers (P1–P5) are amorphous. These copolymers exhibited blue RTP, with the longest afterglow duration of 4.64 s and phosphorescence lifetime of 550 ms. Temperature-dependent delayed spectra further verified the phosphorescent characteristics of the afterglow. Notably, the phosphorescent properties of the copolymers showed an interesting odd–even effect: compared to polymers with an odd number of substituents on the BCD chromophores, those with an even number of substituents exhibited significantly longer phosphorescence lifetimes. Furthermore, as the number of substituents increased, the odd–even effect showed a change to be less pronounced. This odd–even effect provides important insights into understanding the photophysical processes of RTP. Additionally, leveraging the distinct afterglow characteristics and excellent water solubility of copolymers, we demonstrated their potential applications in optical information encryption and anti-counterfeiting ink.

The chemical structures of E1–E5 are shown in Scheme 1a. Initially, these compounds were synthesized through esterification by reacting the corresponding carboxylic acids with an excess of 3-butene-1-ol. The structural characteristics of the BCDs were thoroughly characterized and validated using ¹H NMR, ¹³C NMR and high-resolution mass spectra (HRMS) (Fig. S1–S15, ESI†). Subsequently, based on reported methodologies,¹⁸ using 2,2′-azobis(2-methylpropionitrile) (AIBN) as a radical initiator, E1–E5 were copolymerized with acrylamide at a molar ratio of 1 : 50 to gain P1–P5. As an example, the complete synthetic route of P2 is detailed in Scheme 1b. All the resulting copolymers appeared as white powdery solids.

Scheme 1 BCD-acrylamide copolymers. (a) Chemical structures of E1–E5; (b) complete synthetic route of P2.

According to powder XRD analysis, copolymers P1–P5 exhibited diffraction patterns similar to those of polyacrylamide (PAM),¹⁸ with no distinct crystalline peaks observed, which confirmed their amorphous states (Fig. 1a). The characterizations of the polymers were determined using aqueous-phase gel permeation chromatography (GPC) (Table S1, ESI†). The results indicated that, except for P1, the polydispersity index (PDI) of the other copolymers was less than 2.0. Additionally, the number-average molecular weight (M_n) and weight-average molecular weight (M_w) of the different copolymers were on the same order of magnitude (∼10⁴), indicating minimal variation in the degree of polymerization among the samples and demonstrating good controllability during polymerization.

Fig. 1 Photophysical characterization of the polymers. (a) XRD spectra of P1–P5; (b) normalized delayed emission and excitation spectra and (c) decay curves of P2; (d) CIE 1931 coordinate chart of the delay spectra of P1–P5; (e) temperature-dependent delayed emission spectra and (f) decay curves of P2; (g) afterglow pictures of P1–P5.

Subsequently, we conducted a systematic investigation into the photophysical properties of the polymers. At room temperature (298 K), E1–E5 appeared as colorless oils and did not exhibit any noticeable phosphorescence. However, after polymerization, they all exhibited distinct blue afterglow lasting up to 4.64 s upon excitation with 254 nm light (Fig. 1g and Table S2, ESI†). A comparison of the UV-vis absorption spectra of E1–E5 in 1,4-dioxane and their corresponding copolymers revealed a strong correlation between the absorption features of the copolymers and the BCDs, except for the intrinsic PAM absorption below 240 nm^18,30 (Fig. S16, ESI†). Delayed spectra revealed that P1–P5 exhibited broad delayed emission with peaks in the range of 414 nm to 456 nm, with fitting decay lifetimes all over 100 ms, reaching up to 553 ms (Fig. 1b, c, Fig. S17, S18 and Table S3, ESI†). Furthermore, the phosphorescent quantum yields (Φ_P) peaked at a maximum of 8.9% (Table S2, ESI†). According to the CIE 1931 coordinate chart, the delayed emissions of these copolymers are primarily concentrated in the deep blue to cyan-blue region, aligning well with the observed afterglow (Fig. 1d and Table S2, ESI†). Additionally, temperature-dependent delayed spectra indicated that when the temperature was lowered to 77 K, the delayed emission intensity of P1–P5 reached its maximum, with the longest lifetimes observed (Fig. 1e, f and Fig. S19, S20, ESI†). As the temperature increased, both the emission intensity and lifetime showed a decreasing trend. Based on the above, it is clear that the afterglow observed in P1–P5 can be attributed to phosphorescence.

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) simulations were conducted by using the B3LYP method at the 6-311 g* level to form the optimized conformations of the ground and excited states of E1–E5 (Table S4, ESI†). Clearly, all of the excited-state electron clouds are mainly distributed on the benzene ring. Based on electronic conformation, the S₀ → S₁ orbital transition of E1–E5 predominantly involves π–π* transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). In contrast, the orbital transition from S₀ to excited triplet states contains more n–π* components, which could promote the occurrence of phosphorescence according to the El-Sayed rule.¹⁷ Taking E3 as an example, 99.2% of S₀ → S₁ belongs to π–π* transition, while 83.4% of S₀ → T₄ is attributed to n–π* transition (Fig. 2 and Table S4, ESI†). To investigate the possibility of ISC, further analysis on the excited singlet and triplet states have been conducted. There are multiple excited triplet states (T1, T2, T3, T4 and T5) lying below the S₁ state, and the energy difference between them and S₁ (ΔE_ST) is 1.352, 0.570, 0.566, 0.250 and 0.227 eV, respectively. Based on the optimized conformations of S₁, the SOC values between S₁ and T₄ were calculated as 0.99 cm⁻¹. This indicates that even with relatively large ΔE_ST, there are effective ISC channels between the singlet and triplet states in E3. Moreover, for the other four BCDs, the results are similar (Fig. S21 and Table S4, ESI†).

Fig. 2 Theoretical calculation results of E3, including excited-state electronic transitions, ΔE_ST and SOC values.

The odd–even effect is an interesting phenomenon in chemistry. In previous studies, the odd–even effect between the number of carbon atoms in molecular alkyl chains and photophysical properties has been investigated. It was found that materials with an even number of alkyl carbon atoms generally exhibit stronger phosphorescence or mechano-luminescence compared to those with an odd number of alkyl carbon atoms.^34,35 In this work, an intriguing odd–even effect was also discovered. Specifically, compared to polymers with an odd number of ester groups on BCDs (referred to as “odd-polymers”, P1 and P3), those with an even number of ester groups (referred to as “even-polymers”, P2 and P4) exhibited significantly longer phosphorescence lifetimes and higher quantum yields (Fig. 3a).

Fig. 3 Odd–even effect observed in BCD system. (a) Phosphorescence lifetimes and quantum yields of P1–P5; (b) phosphorescence lifetimes of P1–P5 at different temperatures; (c) ratio of lifetimes at 298 K to that at 77 K.

As the number of ester groups increased, the phosphorescence performances gradually improved, and the difference between odd-polymers and even-polymers became less obvious. For example, the phosphorescent performances of P5 were better than P1 and P3, very similar to those of P4. To further investigate this odd–even effect, the decay curves and fitted lifetimes of the polymers were obtained over a temperature range from 77 K to 277 K. At low temperature (77 K), the rigid environment minimizes non-radiative relaxation, allowing the phosphorescent properties of the polymers to closely reflect the intrinsic characteristics of the BCDs. Under these conditions, no odd–even effect was observed, and the phosphorescence lifetimes decreased with an increasing number of substituents, following the order: P1 > P2 > P3 > P5 > P4 (Fig. 3b).

As the temperature rose, differences in thermal response became apparent among the polymers (Fig. 3b, c and Table S3, ESI†). For odd-polymers, the decrease in phosphorescence lifetime was pronounced, while even-polymers exhibited a much smoother decline. Moreover, as the number of substituents increased, the rate of this decline became progressively gentler. Consequently, a pronounced odd–even effect emerged at room temperature. For example, at 77 K, the phosphorescence lifetime of P1 was as long as 2125 ms, but it rapidly decreased to only about 200 ms at room temperature, which is approximately 11.1% of its value at 77 K; similarly, the lifetime of P3 decreased from 1180 ms to 190 ms (16.1%). In contrast, for P2 and P4, the lifetime decreased from 1410 ms to 553 ms (39.2%), and from 493 ms to 327 ms (66.2%), respectively. P5, which has the largest number of substituents, also exhibited a relatively gradual decrease in phosphorescence lifetime, dropping from 551 ms to 341 ms (61.8%). Furthermore, to rule out the influence of polymer variations, water-soluble carboxylic acids A1–A5 were fabricated into films with polyvinyl alcohol (PVA) (Fig. S22a, ESI†). At room temperature, both the delayed emission spectra and lifetimes exhibited an odd–even effect similar to that observed in the polyacrylamide system (Fig. S22b–d, ESI†). This indicates that the odd–even effect is an intrinsic feature of the BCD system, rather than an artifact resulting from the polymer preparation process.

All the results suggest that this odd–even effect may be attributed to differences in the ability to stabilize excitons. This is not only related to the rigidity of the polymer itself, but also to the interactions between the BCDs and the polymer. During the process of copolymerization, the substituents on BCDs may act as anchoring groups, enhancing the crosslinking density and rigidity of the polymer network.^4,36 And that's why the odd–even effect became less obvious when the number of substituents increased. On the other hand, BCDs with even-numbered substituents may interact more effectively with the polymer matrix. And this contributes to the greater exciton stability of even-polymers, leading to their stronger phosphorescence. Ultimately, the combined effect of these factors leads to the observed odd–even effect.

The polymer powders, exhibiting similar afterglow colors but varying durations, enable the potential of time-resolved information encryption. As illustrated, different polymer powders were used to form the equation “8 + 8 = 8” (Fig. 4a). Under daylight and with 254 nm UV light on, all of the polymers stayed visible, making the equation merely display an unintelligible “8 + 8 = 8” with no meaningful information readable (Fig. 4c). However, after the removal of UV light, it momentarily displayed a misleading incorrect message “8 + 0 = 8”. Approximately 1 s later, as the short-lived afterglow of the P1 and P3 powders faded, the correct message “8 + 1 = 9” became visible.

Fig. 4 Applications of polymers in information encryption and anti-counterfeiting inks. Information encryption patterns made from (a) polymer powders and (b) polymer solvent; (c) transformation of the equation under daylight and following 254 nm UV irradiation; (d) appearance of the motto under daylight and following 254 nm UV irradiation; (e) transformation of the Chinese character from “” to “” following 254 nm UV irradiation.

Moreover, due to the excellent water solubility of polyacrylamide, the viscous liquid was formed after dissolved in water. This viscous liquid could be used as an aqueous ink of a Chinese brush, to achieve patterning and time-resolved information encryption on Xuan paper (Fig. 4b). As shown, a motto was written on the Xuan paper using inks from P2. Under daylight, no visible traces could be observed on the written paper (Fig. 4d). Even with 254 nm UV light on, the blue fluorescence of Xuan paper masks all the information. However, after turning it off, the characters displayed the intrinsic blue afterglow of P2, making the motto visible. To achieve time-resolved screening, the Chinese character “” was written by 2 types of inks made from polymers with different afterglow durations, P2 and P4. After excitation with 254 nm UV light, the paper initially displayed the Chinese character “”. After approximately 2 s, the afterglow of P4 progressively diminished, ultimately revealing the Chinese character “”, which is exclusively formed by P2 (Fig. 4e).

Conclusions

In summary, several BCD-acrylamide copolymers with efficient RTP were constructed. Besides, the relationship between the number of substituents and the phosphorescent properties of the copolymers was systematically investigated. All copolymers exhibited blue RTP, with the longest afterglow duration of 4.64 s, the longest phosphorescence lifetime of 550 ms, and the highest quantum yield reaching 8.9%. Additionally, an intriguing odd–even effect was discovered. Specifically, compared to polymers with an odd number of substituents on the BCD chromophores, those with an odd number of substituents exhibited significantly longer phosphorescence lifetimes and higher quantum yields. Additionally, as the number of substituents increased, the odd–even effect became less pronounced. Leveraging the distinct afterglow characteristics and excellent water solubility of these acrylamide copolymers, we demonstrated their potential applications in optical information encryption and anti-counterfeiting inks.

Author contributions

J. Wang, L. Zhou, L. Ma, and X. Ma designed the materials and conceived the project. J. Wang and L. Zhou conducted the molecule synthesis and characterizations. J. Wang, L. Ma, and X. Ma wrote the manuscript. All authors engaged in discussions and conducted data analysis.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (22125803, T2488302, 22020102006 and 22305080), the Science and Technology Commission of Shanghai Municipality (grant no. 24DX1400200), and the Fundamental Research Funds for the Central Universities.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc01879j

‡ These authors contributed equally to this work.

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