Engineering high-brightness and long-lived organic room-temperature phosphorescence via systematic molecular design

Abstract

We report a systematic molecular design in BF₂bdk-based afterglow emitters with photoluminescence quantum yields up to 46.3% and lifetimes around 1 s. Suitable excited-state types, diverse excited state species, relatively small singlet–triplet energy gaps and strong dipole–dipole interactions are critical in determining the afterglow properties.

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Organic photofunctional materials exhibit wide application in diverse fields such as optical analysis, light harvesting, cosmetics, textile printing and dyeing.¹ Among them, organic room-temperature phosphorescence (RTP) and afterglow materials, characterized by their long-lived emission post-excitation source removal, hold significant promise for advanced anti-counterfeiting, bioimaging, and oxygen/microenvironment probing applications.² Among these, high-brightness organic afterglow materials, particularly those with phosphorescence lifetimes exceeding 100 ms, are especially noteworthy due to their easily detectable afterglow phenomena, even by the naked eye or low-cost equipment.³ This makes them ideal for high-contrast biological monitoring, high-sensitivity analytical detection, information encryption and authentication, and organic optoelectronic devices.⁴ The afterglow brightness L(t) of organic afterglow materials at time t can be described by the equation L(t) ∼ ε*Φ* exp(−t/τ), where ε is the molar absorption coefficients, Φ is the quantum efficiency, and τ is afterglow lifetime.⁵ Hence, achieving high-brightness organic afterglow materials necessitates not only a large ε but also a high Φ and a long τ. However, in organic systems, triplet excited states are difficult to form due to spin–forbidden transitions and weak spin–orbit coupling, and they are easily deactivated at room temperature or body temperature, making the attainment of bright and long-lived organic room-temperature afterglow materials challenging.

To circumvent these limitations, researchers have employed strategies such as promoting intersystem crossing (ISC) through n–π* transitions⁶ and heavy atom effects (HAE),⁷ protecting triplets in crystalline or glassy environments,^7,8 and other molecular design and material engineering strategies,⁹ leading to the development of over a thousand examples of organic afterglow materials (Fig. S1, ESI†). Fig. S1 (ESI†) summarizes the performance metrics of organic afterglow materials in recent years, highlighting that efficient and long-lived organic afterglow materials with quantum efficiencies above 40% and lifetimes over 1 s still remain scarce (below 1% of the total organic afterglow materials). This scarcity stems from the fact that the introduction of HAE or n–π* transitions in conventional molecular designs, while increasing the phosphorescence quantum yield (Φ_P), also tends to increase both the intersystem crossing rate (k_ISC) and the phosphorescence rate (k_P), resulting in shorter phosphorescence lifetimes (τ_P). Thus, selectively enhancing k_ISC to elevate Φ_P, while maintaining a low k_P, and simultaneously suppressing non-radiative (k_nr) and quenching (k_q) rates, is paramount for achieving high-efficiency long-lived phosphorescence.

We envision that by designing and synthesizing organic luminescent molecules with high ε, high k_ISC and low k_P, and subsequently doping them into suitable organic matrices, it is possible to achieve bright and long-lived organic afterglow materials. This material design strategy, on the one hand, can take advantage of the rigid and protective environment provided by the organic matrix to suppress k_nr and k_q of the luminescent molecules. Simultaneously, the dipole–dipole interactions between the organic matrix and the luminescent molecules can facilitate ISC process, leading to a high k_ISC, thereby realizing materials with enhanced efficiency and longevity.

Difluoroboron β-diketonate (BF₂bdk) molecules, due to their large ε and high Φ, have been proven to be an excellent precursor for high-performance organic afterglow in previous studies by our group and others.¹⁰ Moreover, BF₂bdk molecules possess significant dipole moments in their S₁ excited states, enabling strong dipole–dipole interactions with an organic matrix having a large ground-state dipole moment, which can promote their ISC and obtain a large k_ISC. Besides, phenyl benzoate (PhB) has been proved to be an ideal organic matrix for BF₂bdk molecules, due to its large ground-state dipole moment, as well as its rigid and protective environment. Therefore, in this work, we designed and synthesized a series of BF₂bdk molecules as efficient luminescent components, and doped them into PhB to prepare bright and long-lived two-component afterglow materials. By modulating the structure of the BF₂bdk molecules and manipulating their afterglow performance, bright and long-lived organic room-temperature afterglow materials with high quantum efficiencies of up to 46.3% and afterglow lifetimes of 976 ms have been achieved successfully. Further investigations reveal that suitable types of excited state transitions, diverse excited state species, relatively small singlet–triplet energy gaps (ΔE_ST ∼ 0.3–0.4 eV), and strong dipole–dipole interactions all favour ISC and the emission of bright persistent luminescence, providing new insights and directions for the development of high-brightness, long-lived organic afterglow systems. These bright and long-lived organic afterglow materials also demonstrate promising applications in advanced anti-counterfeiting and information encryption.

Scheme 1 illustrates the chemical structures of the BF₂bdk molecules and the PhB matrix employed in this work, with the synthesis of the BF₂bdk molecules achieved via the cascade reaction developed by our group.¹⁰ Comprehensive structural characterization has been performed on the synthesized BF₂bdk compounds, and their high purity can be confirmed through HPLC measurements following multiple recrystallizations using spectroscopic-grade dichloromethane/hexane (Fig. S2, ESI†). Table S1 (ESI†) summarizes the photophysical properties of these BF₂bdk compounds in different solvents, showing their large ε, which attest to their superior photophysical properties and potential for intense luminescence (Fig. S3, ESI†). For example, compound 4 in dichloromethane solution exhibits a strong UV-vis absorption maximum at 380 nm (ε = 4.07 × 10⁴ M⁻¹ cm⁻¹) and an intense fluorescence emission at 462 nm with PLQY of 48.6%. The absorption band at 325–425 nm coincides well with the S₀–S₁ ICT transition as revealed by TD-DFT calculation and can be assigned to be ICT transition from the the aromatic donor to the dioxaborine acceptor (Fig. S4, ESI†). Besides, compounds 2 to 4 show positive solvatochromicity (Fig. S3, ESI†), which is consistent with the intramolecular charge transfer (ICT) nature in their S₁ states (Fig. S4–S6, ESI†). Compound 1 of localized excitation (LE) character in its S₁ state show similar fluorescence maxima in different solvents (Fig. S3 and S7, ESI†).

Scheme 1 Chemical structures of BF₂bdk and PhB matrix.

No noticeable afterglow can be observed from either solution-state or solid-powder-state BF₂bdk compounds at room temperature (Fig. S8, ESI†). In the powder and crystal states of BF₂bdk, molecular aggregation of BF₂bdk can cause afterglow quenching in the organic systems. PhB matrix can disperse luminescent dopants and prevent afterglow quenching caused by the electronic coupling of BF₂bdk aggregates. On the other hand, the dipole–dipole interaction between ground-state PhB and BF₂bdk's S₁ state can enhance ISC of the organic system. Therefore, PhB matrix is introduced to obtain BF₂bdk-PhB materials with room-temperature afterglow property (Text S1, ESI†). When compound 1 was doped into PhB at a concentration of 0.01 wt%, the resultant 0.01%-1-PhB materials exhibit a weak green afterglow lasting approximately 5 s at room temperature (Fig. 1A), with a maximum emission peak at 485 nm and afterglow lifetime of 412 ms (Fig. 1C and E). This afterglow emission, which has already been confirmed to originate from the T₁ emission of compound 1 in our group's previous work,^10c is an RTP-type afterglow emission. The steady-state emission spectrum of 0.01%-1-PhB afterglow materials exhibits a maximum emission peak at 390 nm, from which S₁ level can be estimated to be 3.18 eV (Fig. 1C). At 77 K, the phosphorescence peak of 0.01%-1-PhB afterglow materials is at 495 nm, closely matching the room-temperature value, from which we can estimate the T₁ energy level to be 2.51 eV (Fig. S9, ESI†). The room-temperature photoluminescence quantum yield (PLQY) value of 0.01%-1-PhB was measured to be as low as 0.43%, consistent with the low PLQY value of its solution. Unlike the BF₂bdk-matrix afterglow materials in our previous work where the BF₂bdk's S₁ states have ICT character,¹⁰ because a single benzene ring has limited electron-donating strength, compound 1 shows prominent LE character in its S₁ state as revealed by TD-DFT calculation (Fig. 1F). It is known that LE system usually has large singlet–triplet splitting energy (ΔE_ST); here 1-PhB sample has ΔE_ST value of 0.67 eV as estimated from its fluorescence and phosphorescence maxima (Fig. 1C). Because of the large ΔE_ST, 1-PhB system has limited ISC and thus weak RTP (Fig. 1A).

Fig. 1 (A) and (B) Photographs of 1-PhB (A) and 2-PhB (B) materials. (C) and (D) Steady-state and delayed emission spectra of 1-PhB (C) and 2-PhB (D) excited at 365 nm. (E) Delayed emission profiles of 1-PhB and 2-PhB monitored at 485 nm and 530 nm. (F) TD-DFT calculation of 1's and 2's S₁ states.

To enhance luminescence efficiency and afterglow brightness of BF₂bdk-PhB materials, a naphthyl substituent group, known for its greater ISC capability than phenyl, is further introduced to obtain compound 2.^10b The resultant 0.01%-2-PhB materials display bright blue fluorescence at room temperature, with a significantly increased PLQY of 26.1% (Fig. 1B). In a dark room, 2-PhB materials can exhibit a bright yellow-green afterglow (Fig. 1B), which has been previously established to be RTP-type afterglow originating from 2's T₁ emission.^10g The steady-state and delayed emission spectra of the 2-PhB materials reveal a fluorescence maximum at 442 nm (S₁ = 2.81 eV), a phosphorescence peak at 529 nm (T₁ = 2.34 eV) with phosphorescence lifetime of 829 ms (Fig. 1D and E). These results indicate a marked improvement in both afterglow brightness and lifetime of 2-PhB compared to 1-PhB. This enhancement aligns with theoretical calculated results. TD-DFT calculation demonstrates that 2's S₁ states exhibit a pronounced ICT nature, distinctly different from the LE characteristics of 1 (Fig. 1F). Moreover, ΔE_ST of 2-PhB materials can be estimated to be 0.47 eV, which would facilitate ISC to a certain extent and favor efficient phosphorescence emission.

Further enhancement of the PLQY is achieved by introducing an electron-donating methoxy group into compound 2, resulting in 0.01%-3-PhB materials with PLQY of 39.11%. At room temperature, 3-PhB materials display a bright blue fluorescence, and a yellow RTP-type afterglow (Text S2 and S3, ESI†) lasting for 7 s (Fig. 2A). The steady-state emission spectrum of 3-PhB materials shows a fluorescence band at 455 nm (Fig. 2B, S₁ = 2.73 eV), while their delayed emission spectrum shows a phosphorescence peak at 550 nm (T₁ = 2.25 eV) with lifetime of 929 ms (Fig. 2C and E). TD-DFT calculation shows that, the sum of the SOCMEs between the triplet excited states (T_n, n = 1–3) and S₁ in 3, denoted as SOCME(sum) = SOCME(T₁–S₁) + SOCME(T₂–S₁) + SOCME(T₃–S₁), increases significantly from 0.88 to 1.16 cm⁻¹, suggesting a notable improvement in ISC ability (Fig. S5 and S6, ESI†). The enhancement in SOCME contribute to the strengthened afterglow brightness of 3-PhB compared to 2-PhB. The estimated ΔE_ST of 3-PhB materials is 0.48 eV, similar to that of 2-PhB materials, indicating that the introduction of the methoxy group has little effect on ΔE_ST in this system.

Fig. 2 (A) and (B) Photographs of 3-PhB (A) and 4-PhB (B) materials. (C) and (D) Steady-state and delayed emission spectra of 3-PhB (C) and 4-PhB (D) excited at 365 nm. (E) and (F) Emission decay profiles of 3-PhB (E) and 4-PhB (F) monitored at 550 nm and 534 nm.

Twisted donor units, as demonstrated in our and others' previous work, can reduce ΔE_ST, promoting ISC and efficient afterglow emission.^10a,d,f Inspired by this, a twisted polycyclic aromatic unit is further introduced into 3 to obtain 4. The resultant 0.01%-4-PhB materials still exhibit a bright blue fluorescence at room temperature with PLQY as high as 46.3% (Fig. 2B), and their steady-state emission spectrum features a fluorescence peak at 467 nm (Fig. 2D, S₁ = 2.66 eV). In a dark room, 4-PhB materials can exhibit a very bright yellow-green RTP-type afterglow (Text S2 and S3, ESI†) lasting approximately 9 s (Fig. 2B), with a phosphorescence band at 534 nm (Fig. 2D, T₁ = 2.32 eV) in their delayed emission spectrum and lifetime of 976 ms (Fig. 2F). 4-PhB materials can also exhibit significant afterglow at 77 K with a similar phosphorescence peak at 550 nm (Fig. S10, ESI†). The estimated ΔE_ST of 4-PhB afterglow materials is 0.34 eV, notably lower than that of other BF₂bdk-PhB materials. Furthermore, TD-DFT calculation indicates that, besides S₁-to-T₁ ISC channel, S₁-to-T_n (n = 2 or 3) channels are responsible for the population of T₁ states in BF₂bdk systems; the S₁–T_n energy gap is smaller than that of S₁–T₁. Besides, compared to 2 and 3, 4 exhibits more distinct differences in the excited states between T_n (n = 1–3) and S₁ states, along with larger SOCME_Tn–S1 values, particularly SOCME_T1–S1 = 0.46 cm⁻¹ (Fig. S4, ESI†). According to the energy gap law and the EI-Sayed rule, such differences in excited states and larger SOCME_Tn–S1 values can significantly facilitate the formation of multiple ISC pathways between T_n and S₁ states, promoting ISC process and efficient phosphorescence.

Similarly, TD-DFT calculation also reveals that the S₁ dipole moments of the BF₂bdk molecules follow the order 4 > 3 > 2 > 1 (Table S2, ESI†). PhB matrix, which has a ground-state dipole moment of 1.94 D, can interact with BF₂bdk's S₁ excited state via dipole–dipole interaction (with less influence on BF₂bdk's T₁ energy levels), lower BF₂bdk's S₁ energy levels and thus narrow ΔE_ST to enhance intersystem crossing.^10e–h Thus, a larger S₁ dipole moment of the luminescent molecules leads to stronger dipole–dipole interactions with the matrix, favoring the enhancement of ISC process as well as the afterglow performance of BF₂bdk-PhB materials.^10e,g Besides, we select cyclo olefin polymer (COP), poly(methyl methacrylate) (PMMA), PhB and 4-methoxybenzophenone (MeOBP) to serve as organic matrix to prepare BF₂bdk-matrix samples. Corresponding steady-state and delayed emission spectra, as well as fluorescence decay profiles, have been collected (Fig. S11–S14 and Table S3 and S4, ESI†). The emission studies show that BF₂bdk's S₁ energy levels decrease with the dipole moments of organic matrices while BF₂bdk's T₁ energy levels have insignificant change. Overall, BF₂bdk molecules in organic matrices with large dipole moments have small ΔE_ST and strong tendency of intersystem crossing. The fluorescence lifetimes of BF₂bdk's S₁ states have been found first increase with dipole moments from COP through PMMA to PhB and then decrease from PhB to MeOBP. The lifetime increase is attributed to the decrease of BF₂bdk's fluorescence decay rates in polar environment as supported by the reported studies, while the significant increase of intersystem crossing from PhB to MeOBP should be responsible for the lifetime decrease. These results reveal the effect of dipole–dipole interaction between ground-state matrix and S₁-state BF₂bdk for the enhancement of intersystem crossing of the BF₂bdk-matrix system, which has also been supported by our previous study and the reported studies.^10e,g

In summary, appropriate types of excited state transitions, a rich diversity of excited states, relatively small ΔE_ST (0.3–0.4 eV), and strong dipole–dipole interactions are the reasons behind the superior afterglow brightness and PLQY of 4-PhB materials compared to 3-PhB > 2-PhB > 1-PhB.

Notably, the afterglow colors of these BF₂bdk-PhB materials were distinctly red-shifted relative to their fluorescence colors, with the afterglow peaks being red-shifted relative to the fluorescence peaks. This suggests that their afterglow emissions belong to organic room-temperature phosphorescence. The phosphorescence emissions of 1-PhB and 2-PhB materials were previously attributed to the T₁ emission of the BF₂bdk, and the same should hold true for the persistent emissions of 3-PhB and 4-PhB materials.

In the field of information encryption and anti-counterfeiting technologies, fluorescence inks have gained widespread use due to their excellent printability and high-resolution features.^4e,f However, their ease of replication limits their application in scenarios requiring higher security levels. Organic room-temperature afterglow materials, with their unique long-lived excited state properties enabling time-resolved multiplexed information encryption, show vast potential for advanced anti-counterfeiting and information encryption applications. PhB matrix, with its low melting point (70 °C), endows BF₂bdk-PhB afterglow materials with outstanding processability, allowing them to be fabricated into afterglow plate, as well as various persistent afterglow patterns (Fig. 3A and B). These BF₂bdk-PhB afterglow plates appear as inconspicuous white glass plates under daylight but exhibit intense blue fluorescence under a 365 nm lamp (Fig. 3A). After removing the lamp, they can display bright yellow-green afterglow. Moreover, when using different pre-designed masks, these afterglow plates can exhibit yellow-green afterglow with different patterns after the excitation source is removed, demonstrating potential applications in the display sector (Fig. 3B).

Fig. 3 (A) Photographs of 4-PhB afterglow plates (20 cm × 20 cm) at different conditions. (B) Photographs of 4-PhB afterglow plates behind different pre-designed masks.

In conclusion, through systematic molecular design and strategic incorporation into protective organic matrices, we have successfully developed a series of high-brightness and long-lived organic room-temperature afterglow materials. By modulating the structure of the BF₂bdk molecules and manipulating their afterglow performance, achieving afterglow materials with PLQY > 40% and afterglow lifetime around 1 s. Our findings underscore the significance of suitable excited state transitions, diverse excited state species, small singlet–triplet energy gaps, and strong dipole–dipole interactions in the realization of high-performance afterglow materials. These materials not only advance the fundamental understanding of organic room-temperature afterglow materials but also open up new avenues for their practical applications in advanced anti-counterfeiting, information encryption, and display technologies.

Data availability

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

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the financial supports from National Natural Science Foundation of China (22175194), Hundred Talents Program from Shanghai Institute of Organic Chemistry (Y121078), Pioneer Hundred Talents Program of Chinese Academy of Sciences (E320021), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB0610000), and Ningbo Natural Science Foundation (2023J243). We thank the staff at BL17B1 beamline of the National Facility for Protein Science in Shanghai (NFPS), Shanghai Advanced Research Institute, CAS, for providing technical support in X-ray diffraction data collection and analysis.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental procedures, spectroscopic data. CCDC 2226200. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cp02927e

‡ Equal contribution.

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