Citation

Jiang Linnan, Qian Zhenkai, Chen Yong, Yu Xiaoyong, Qiu Yugui, Xu Wen-Wen, Sun Yonghui, Xu Xiufang, Wang Lihua, Liu Yu. Double response reversible phosphorescence based on cyclodextrin supramolecular flexible elastic achieved multicolor delayed fluorescence[J]. Chin. Chem. Lett., 2025, 36(8): 110676  

Linnan Jiang^a,1, Zhenkai Qian^a,1, Yong Chen^a, Xiaoyong Yu^a, Yugui Qiu^b, Wen-Wen Xu^a, Yonghui Sun^a, Xiufang Xu^c, Lihua Wang^a, Yu Liu^a,*     

The increasing demand for high-performance functional materials [1,2] has sparked significant interest in stimuli-responsive materials [3–6] as a novel class of smart materials. Stimuli-responsive materials have the ability to change their physical or chemical properties in response to variations in external stimulus, enabling real-time monitoring of the environment and intelligent responsiveness. Compared to conventional organic fluorescence materials, purely organic room-temperature phosphorescence (RTP) materials [7–12] hold tremendous potential in various fields including information anti-counterfeiting [13], information storage [14], and bioimaging [15] due to their unprecedented photophysical properties such as long lifetimes, special radiation channels, and large Stokes shifts. However, achieving organic phosphorescence emission at room temperature remains challenging. In general, two conditions are required for achieving RTP emission: (1) Promoting intersystem crossing (ISC) from the singlet to triplet states [16]; (2) Reducing nonradiative deactivation of triplet states, ensuring they remain active long enough to emit phosphorescence [17]. Current strategies include introducing heavy atoms into chromophores [18], embedding chromophores in matrices [19], crystallization [20], polymerization [21], molecular aggregation (including H-aggregation, π–π stacking, and n–π stacking) [22], host-guest complexation [23], carbon quantum dots [24] and clustering trigger emission [25], etc. For example, Kim et al. introduced a strong non-covalent interaction between the two molecules into the system. One of them is the halogen bond between phosphors, which can not only promote the ISC process, but also inhibit the vibration of the phosphors. The other comes from the strong hydrogen bond interaction between the host matrix and phosphor groups to more effectively inhibit the diffusion/vibration of the matrix and the phosphors [26]. Fraser et al. introduced heavy iodine atoms into a dual-emission polymer system. During polymerization, they systematically varied the chain length of polylactic acid (PLA), achieving adjustment of the relative intensities of fluorescence and phosphorescence [27]. Since Turro et al. proposed that β-CD can induce RTP, macrocyclic molecules gradually entered researchers' field of view; similar to how polymer matrices provide a rigid environment, they can also restrict the vibration of guest molecules, preventing their quenching by external oxygen and moisture [28]. Liu et al. prepared highly efficient RTP materials by doping 6-bromoisoquinoline-modified β-CD into acrylamide and polymerizing them via simple in situ>the photopolymerization [29]. Zhang et al. coupled diboron diphenylmethane with PLA, obtaining single-component multi-emissive materials. This method not only significantly enhanced the fluorescence quantum yield but also yielded oxygen-sensitive green RTP and temperature-sensitive delayed fluorescence [30]. Although significant progress has been made explored it is regrettable that most RTP materials exist in the form of crystals or rigid materials, making them difficult to further process and plasticity, which significantly hinders their practical applications. Polyurethane [31–34] is an elastomer with excellent mechanical properties and easy processing, but few phosphorescent materials based on polyurethane have been reported. Therefore, combining polyurethane with cyclodextrin-based supramolecular systems may yield flexible supramolecular phosphorescent nanomaterials with both stretch and luminescent properties. This could provide an effective and environmentally sustainable alternative for optical materials and stimulate new ideas in phosphorescent material research. However, macrocyclic confinement effect and polymerization in situ cooperative expansion supramolecular elastomers RTP still face challenges. Especially, the macrocycle confinement effect [35,36], triplet-to-singlet Förster resonance energy transfer (TS-FRET) [37] achieved multicolor delayed phosphorescence (this refers to persistent luminescence) report is still rare to the best of our knowledge.

Herein, we report an approach to flexible elastomers (PU) constructed by 4-biphenylboronic acid (BOH), polyethylene glycol, 2,2-bis(hydroxymethyl)propionic acid, isophorone diamine and isophorone diisocyanate copolymer. After followed by the complexation of hydroxypropyl-β-cyclodextrin (β-CD-HP) with the biphenyl groups on the PU chain through non-covalent interactions, then the supramolecular elastomers (PU@β-CD-HP) exhibit an efficient organic RTP emission. Experimental results show that the encapsulation and hydrogen bonding interactions of β-CD-HP can effectively suppress non-radiative transitions and enhance luminescent performance. Under 254 nm UV lamp, the supramolecular PU@β-CD-HP film emits blue-green phosphorescence, with an afterglow lasting up to 18 s at room temperature and a lifetime of 1211 ms. What is important is its manifestation of distinctive reversible photo-thermal stimuli-responsive characteristics (Scheme 1). Under sustained UV irradiation, the supramolecular film gradually lighted from near imperceptibility and accompanied by visible afterglow for seconds-level by naked eye. Subsequently, the phosphorescence rapidly returned to its initial state within 3 min under 45 ℃. Notably, this photo-thermal response maintains its phosphorescence performance even after numerous cycles. Through the incorporation of suitable fluorescent dyes, such as Fluorescein isothiocyanate (FITC), Sulforhodamine 101 (SR101), Rhodamine B (RhB), into supramolecular films, the color range of the photoreactive afterglow can be expanded to encompass the green, red, and yellow regions via TS-FRET. Furthermore, benefit by excellent mechanical properties of polyurethane chain, PU@β-CD-HP film exhibits high tensile strength, and toughness characteristic which can stretch up to 3 times of its original length. Leveraging the adaptable and reversible photo-thermal responsiveness of PU@β-CD-HP film, the resultant polymer exhibits versatile utility in the fabrication of materials for diverse applications such as information encryption and photoprinting. This sophisticated supramolecular phosphorescent material not only amalgamates luminescent functionality with polyurethane attributes but also boasts scalability, thereby showing promising avenues across various domains of application.

Scheme 1

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Scheme 1. Schematic illustration of the synergistic assembly of PU@β-CD-HP and the formation of and multicolor delayed fluorescence.

The PU elastomer was synthesized in two simple steps (Scheme S1 in Supporting information). Characteristic peaks in the ¹H NMR spectrum around 7.0–7.5 ppm were attributed to the phenyl ring proton on the BOH, confirming the successful introduction of BOH (Fig. S1 in Supporting information). In the FT-IR spectrum, peaks corresponding to the bending vibration of N—H around 3350 cm⁻¹, the stretching vibration of C=O around 1633 cm⁻¹, and the bending vibration of C—H around 2944 cm⁻¹ were observed. Additionally, there was no stretching vibration peak of -N=C=O between 2050 and 2500 cm⁻¹, indicating complete reaction of the isocyanate groups. Furthermore, the stretching vibration peak of B-O near 1342 cm⁻¹ further demonstrated the successful incorporation of BOH onto the PU main chain, indicating that BOH was covalently bonded to PU, rather than in a doped form (Fig. S2 in Supporting information). The GPC testing of PU revealed a molecular weight of 44,302 for the polymer's peak, with a polydispersity index (PDI) of 1.56 (Fig. S6 in Supporting information). The thermal gravimetric analysis (TG) and differential thermogravimetric analysis (DTG) spectrum of PU showed a weight loss inflection point of the elastomer around 340 ℃ and 420 ℃, with a soft segment weight loss between 250 ℃ and 350 ℃ mainly due to the decomposition of polyols and small molecules, and a hard segment weight loss between 360 ° and 500 ℃ mainly due to the decomposition of isocyanate monomers formed by the reaction of amino formate groups with polymer polyols and chain extenders. Additionally, a significant mass loss of PU occurred between 340 ℃ and 420 ℃, attributed to the breakage of the amino formate groups connected to the main chain of PU, demonstrating its good thermal stability (Figs. S7 and S8 in Supporting information). Scanning electron microscopy (SEM) revealed the surface structure of the polyurethane film (Figs. S9 and S10 in Supporting information). As shown in the figure, after the addition of β-CD-HP, the encapsulation effect of the host-guest interaction inhibited the aggregation of BOH, resulting in a significant change in the surface morphology of the polyurethane, with a more uniform surface morphology and a transition from larger and fewer particles to smaller and more numerous ones.

In order to investigate the encapsulation behavior between β-CD-HP and BOH, we performed ¹H NMR titration experiments using BOH as the guest and β-CD-HP as the host. As shown in Fig. 1, as the equivalent amount of β-CD-HP increased from 0 to 1.5, the NMR signals of the Ha protons on BOH exhibited a significant downfield shift from 7.85 ppm to 7.91 ppm, indicating the formation of host-guest complexes between BOH and β-CD-HP. Additionally, we used the continuous variation method of equimolar titration to determine the optimal stoichiometric ratio between the host β-CD-HP and the guest BOH. The Job plot showed a maximum value at 1.0, indicating that the guest BOH was encapsulated within the cavity of β-CD-HP, forming a 1:1 type supramolecular inclusion complex (Fig. S4 in Supporting information). The stoichiometric ratio was determined to be 1:1. Through nonlinear least squares fitting analysis of the titration data, the binding constant between BOH and β-CD-HP was determined to be 3.94 × 10³ L/mol, with a stoichiometric ratio of 1:1 (Fig. S5 in Supporting information). Furthermore, two-dimensional rotating frame Overhauser enhancement spectroscopy (ROESY) NMR spectra revealed significant nuclear Overhauser effect (NOE) between the protons of β-CD-HP and BOH, further indicating that BOH is included within the cavity of β-CD-HP (Fig. S3 in Supporting information).

Figure 1

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Fig. 1. The 1H NMR spectra (400 MHz, 298 K, D2O) of BOH (1.0 mmol/L) with the addition of 0–1.5 equiv. β-CD-HP (spectra from 1 to 16).

Compared with α, β, γ-cyclodextrin [38], possessing a deeper cavity and more hydrogen bonding sites, β-CD-HP can more effectively encapsulate guest molecule with BOH, leading to effectively enhanced phosphorescence in PU@β-CD-HP film, which was prepared by thoroughly removing the solvent of the methanol-water solution of PU@β-CD-HP. In the controlled experiment, the PU film without β-CD-HP showed fluorescence emission at 332 nm and phosphorescence emission at 478 nm under the excitation of 254 nm (Figs. S13 and S16 in Supporting information). The phosphorescence lifetime of PU film was 0.647 ms, and the quantum yield was 8.08% (Figs. S11 and S19 in Supporting information). Due to the weak phosphorescence, there was almost no noticeable afterglow to the naked eye, only green fluorescence could be observed. Although the PU@β-CD-HP supramolecular elastomer remained the position of the emission peak of fluorescence and phosphorescence under the excitation of 254 nm (Fig. 2a), the phosphorescence intensity of the PU@β-CD-HP supramolecular elastomer experienced a remarkable 16.7-fold enhancement. However, after 20 s of UV (254 nm) pre-irradiation, the phosphorescence lifetime of the elastomer was effectively enhanced from 1.17 ms (Fig. S12) to 1.21 s (Fig. 2a) as compared with the one lacking of the pre-irradiation, meanwhile the quantum yield keeps to 5.04% (Fig. S18 in Supporting information), with a conspicuous long-lasting blue-green phosphorescence observable to the naked eye for several tens of seconds at room temperature (Fig. 1d and Movie S1 in Supporting information). Additionally, the phosphorescence spectrum of the PU@β-CD-HP supramolecular elastomer aligns with that of the PU film, suggesting that the phosphorescent emission source of PU@β-CD-HP originates from BOH. The extended phosphorescence lifetime of the PU@β-CD-HP implies a critical role for cyclodextrin as a matrix in safeguarding triplet excitons from quenching. To further validate the phosphorescence enhancement effect of cyclodextrin, the impact of β-CD-HP content in the polyurethane film on phosphorescence intensity was investigated through phosphorescence spectra measurements. Fig. S21 (Supporting information) illustrates a pronounced enhancement in emission near 480 nm with increasing content in the polyurethane film. These results revealed that the introduction of β-CD-HP facilitates efficient ISC from the S1 state to higher triplet states (Tn) and slow non-radiative decay rates via the macrocyclic confinement effect and hydrophobic interaction, achieving long-lived phosphorescence.

Figure 2

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Fig. 2. (a) Prompt and delayed phosphorescence spectra of PU@β-CD-HP film. Excitation by 254 nm, delayed time: 0.1 ms. (b) Lifetime decay curve of PU@β-CD-HP films collected at 480 nm. (c) Effect of light stimulation time on phosphorescent intensity of PU@β-CD-HP films. Excitation by 254 nm, delayed time: 0.1 ms. (d) The photographs of PU@β-CD-HP after UV irradiation.

On the other hand, the supramolecular elastomer PU@β-CD-HP also exhibits light-thermal response characteristics. The PU@β-CD-HP showed no visible afterglow when directly exposed to UV light in absence of pre-irradiation (Fig. 3a and Movie S2 in Supporting information). The RTP emission intensity at 480 nm increased with the time of UV irradiation and reached a maximum at 16 s (Fig. 2c), which was attributed to gradual consumption of oxygen under the continuous UV irradiation [39]. Additionally, the film could be restored to its initial state and regain its light-stimulated response properties by placing it at 45 ℃ for 3 min, resulted from allowing for triplet oxygen to re-enter the PU@β-CD-HP film, disrupting hydrogen bonding interactions and breaking the confinement effect of β-CD-HP during the heating process (heating process accelerates the re-entry of oxygen). Without heating, it takes a longer time, up to several hours or even a whole day. To confirm our hypothesis, we selected two identical PU@β-CD-HP films from the same batch and placed them separately in oxygen (high oxygen environment) and nitrogen atmospheres (low oxygen environment) after photo-activation treatment. After a certain duration, the RTP signal nearly vanished in the PU@β-CD-HP film subjected to the oxygen-rich environment. On the contrary, even after several hours, the PU@β-CD-HP film housed in the nitrogen atmosphere exhibited enduring and strong RTP emission. It is because that when under the same exposure time, the oxygen consumption in a low-oxygen environment is more noticeable because the limited oxygen is rapidly depleted, whereas in an oxygen-rich environment, the consumption is negligible, resulting in an insignificant change in phosphorescence (Movie S3 in Supporting information). Furthermore, even after undergoing repeated cycles of light stimulation and thermal deactivation dozens of times, the film still exhibited strong RTP emission (Fig. 3b). Selective stimulation of specific regions with light allowed for the creation of various patterns on the film via light printing due to this property. Moreover, the film demonstrated excellent fatigue resistance, as it could continue to perform light printing even after more than 30 cycles of light printing and thermal erasure (Fig. 3c and Movie S5 in Supporting information).

Figure 3

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Fig. 3. (a) Comparison of the same batch of PU@β-CD-HP films treated with unphotoactivated and photostimulated fluorescence in sun-light and delayed fluorescence after irradiation with ultraviolet lamp. (b) Phosphorescent intensity spectra under photostimulus/thermal reduction cycles at 480 nm. (c) Photoprint pattern and thermal erase cycle diagram.

Taking the excellent photo-thermal dual stimulation phosphorescence properties of the supramolecular elastomer, we explored its application in TS-FRET which is an effective method for transferring the T1 of long-lived organic RTP emitters to the S1 of fluorescent dyes, enabling tunable multicolor delayed fluorescence emission (Fig. 4a). To construct an efficient phosphorescence capture system, we designed a TS-FRET system using BOH@β-CD-HP as the energy donor and fluorescent dyes (such as RhB, SR101, and FITC) as the triplet-state energy acceptors to modulate the afterglow color. These fluorescent dyes have absorption spectra that overlap well with the phosphorescence spectra of PU@β-CD-HP, satisfying the requirements for energy transfer. Fig. 4b illustrates the potential mechanism of the phosphorescence capture system. Due to significant spectral overlap with FITC, SR101, and RhB, under direct excitation, the dipole oscillations of PU@β-CD-HP induce changes in the dipole distribution of FITC, RhB, and SR101. With the increase in the proportion of energy acceptor dyes, PU@β-CD-HP+FITC, PU@β-CD-HP+RhB, and PU@β-CD-HP+SR101 demonstrated a color transition from blue to green, orange, or red (Fig. 4d). Simultaneously, the phosphorescent peak intensity of the donor weakened to nearly disappear, while the delayed fluorescence peak intensity of the acceptor becomes stronger in the phosphorescence spectrum, indicating high energy transfer efficiency from the donor to the acceptor in the film (Fig. 4c). Moreover, when the weight ratio of donor to acceptor is 80:1, PU@β-CD-HP+RhB exhibited a yellow afterglow lasting for approximately 10 s after removing the excitation light; when the weight ratio of donor to acceptor is 200:1, PU@β-CD-HP+SR101 displayed a red afterglow lasting for approximately 5 s after removing the excitation light; when the weight ratio of donor to acceptor is 1000:1, PU@β-CD-HP+FITC exhibited a green afterglow lasting for approximately 5 s after removing the excitation light (Fig. 4c and Movie S6 in Supporting information), accompanied by phosphorescence lifetimes of 205.6, 647, and 455.79 ms, at their respective ratios (Figs. S22-S24 in Supporting information). At this time, the best phosphorescence intensity has been achieved. Increasing the concentration of the acceptor further will instead cause quenching. The density functional theory (DFT) calculation carried out by the Gaussian 16 program further deepens the understanding of the energy transfer mode. Selecting BOH, the phosphorescent source of PU@β-CD-HP, as the energy donor and representing RhB as the energy acceptor. After conducting conformational search, the most stable isomers of BOH and RhB are screened out and shown in Fig. S26 (Supporting information). The calculated emission phosphorescence energy range of BOH is within 2.4534–4.2460 eV. The minimum energy required for RhB to be excited is 2.9563 eV (within the energy range of the phosphorescence emitted by BOH). This indicates that this energy transfer process can take place. This result is consistent with the experimental result.

Figure 4

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Fig. 4. Luminous properties of PU@β-CD-HP +acceptor-dyes films. (a) Simplified Jablonski diagram for ultralong phosphorescence and full-color energy transfer process. (b) Normalized emission spectrum of PU@β-CD-HP (λex = 254 nm, delayed time = 0.1 ms) and absorption spectra of SR101, FITC and RhB (λex = 254 nm). (c) Delayed emission spectra of PU@β-CD-HP with different acceptor concentrations under ambient conditions (the left is PU@β-CD-HP +SR101, the middle is PU@β-CD-HP+FITC, right is PU@β-CD-HP+RhB (λex =254 nm, delayed time = 0.1 ms). (d) Full-color afterglow photographs of PU@β-CD-HP films doped with different acceptors under ambient conditions (the left is PU@β-CD-HP +SR101 films, [PU@β-CD-HP]:[SR101] is 200:1; the middle is PU@β-CD-HP+FITC film, [PU@β-CD-HP]:[FITC] is 1000:1; right is PU@β-CD-HP+RhB film, [PU@β-CD-HP]:[RhB] is 80:1).

Based on multi-color emission and time-delayed luminescence properties, supramolecular elastomer successfully applied into ink capable of writing and information encryption. Thus, a solution containing a pure acceptor fluorescent dye and PU1@β-CD-HP + acceptor was prepared and applied onto paper. As shown in Fig. 5, we wrote the digital information in different ink on the paper, which might be changed when it was exposed to different light environments, resulted from that the acceptor dye solution, due to its short lifetime, was utilized to inscribe deceptive information, while the PU1@β-CD-HP + acceptor solution was employed to conceal the true information. Besides its optical properties, PU@β-CD-HP demonstrated excellent flexibility, as displayed in Movie S4 (Supporting information). We conducted a synthesis of polyurethane films with two distinct proportions of BOH (0.1 mmol/200 mg and 0.2 mmol/200 mg) and subjected them to tensile testing to characterize their mechanical properties. Traditional RTP materials are mostly crystals or powders, which are difficult to process. Compared to it, the supramolecular elastomer is easy to shape into the required form and has better fatigue resistance. The tensile strength was approximately 3.7 MPa and the fracture elongation of PU@β-CD-HP (BOH = 0.1 mmol) was 325% (Fig. S25 in Supporting information). Intriguingly, the introduction of 1 equiv. of BOH led to a reduction in the tensile properties of the elastomer, with a corresponding decrease in fracture elongation. This could potentially be ascribed to an overabundance of added BOH, which might disrupt the polyurethane's chain structure, thereby augmenting the system's brittleness.

Figure 5

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Fig. 5. Schematic illustration of the application process for information encryption.

In summary, we constructed a novel flexible supramolecular phosphorescent elastomer, which exhibited long lifetime, high tensile strength, and reversible photo-thermal responsiveness. This supramolecular elastomer is assembled based on host-guest interactions between BOH and β-CD-HP. The supramolecular elastomer not only demonstrated excellent fluorescence and phosphorescence dual-emission, emitting blue-green phosphorescence under UV excitation with a bright afterglow lasting for over ten seconds, but also exhibited unique reversible photo-thermal stimulus-responsive characteristics, easily reverting to the initial state at 45 ℃ and demonstrating outstanding fatigue resistance through numerous cycles of photo-stimulated/thermal recovery. Furthermore, multi-color delayed fluorescence is achieved by doping organic dyes. Due to the outstanding optical properties, photo-thermal responsiveness and easy processing characteristics of this supramolecular intelligent material, it offers promising applications in information encryption, anti-counterfeiting and other fields.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Linnan Jiang: Writing – review & editing, Writing – original draft, Formal analysis, Data curation, Conceptualization. Zhenkai Qian: Writing – review & editing, Writing – original draft, Methodology, Formal analysis, Conceptualization. Yong Chen: Writing – review & editing, Supervision, Methodology, Conceptualization. Xiaoyong Yu: Writing – review & editing, Formal analysis, Data curation. Yugui Qiu: Writing – review & editing, Conceptualization, Software. Wen-Wen Xu: Writing – review & editing, Methodology, Data curation, Conceptualization. Yonghui Sun: Writing – review & editing, Supervision, Formal analysis, Conceptualization. Xiufang Xu:. Lihua Wang: Writing – review & editing, Supervision, Software, Conceptualization. Yu Liu: Writing – review & editing, Software, Resources, Project administration, Methodology, Funding acquisition, Conceptualization.

This work was financially supported by the National Natural Science Foundation of China (No. 22131008).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110676.