Ultralong organic phosphorescence from isolated molecules with repulsive interactions for multifunctional applications

Intermolecular interactions, including attractive and repulsive interactions, play a vital role in manipulating functionalization of the materials from micro to macro dimensions. Despite great success in generation of ultralong organic phosphorescence (UOP) by suppressing non-radiative transitions through attractive interactions recently, there is still no consideration of repulsive interactions on UOP. Herein, we proposed a feasible approach by introducing carboxyl groups into organic phosphors, enabling formation of the intense repulsive interactions between the isolated molecules and the matrix in rigid environment. Our experimental results show a phosphor with a record lifetime and quantum efficiency up to 3.16 s and 50.0% simultaneously in film under ambient conditions. Considering the multiple functions of the flexible films, the potential applications in anti-counterfeiting, afterglow display and visual frequency indicators were demonstrated. This finding not only outlines a fundamental principle to achieve bright organic phosphorescence in film, but also expands the potential applications of UOP materials.

Intermolecular interactions, including attractive and repulsive interactions, play a vital role in manipulating functionalization of the materials from micro to macro dimensions. Despite great success in generation of ultralong organic phosphorescence (UOP) by suppressing non-radiative transitions through attractive interactions recently, there is still no consideration of repulsive interactions on UOP. Herein, we proposed a feasible approach by introducing carboxyl groups into organic phosphors, enabling formation of the intense repulsive interactions between the isolated molecules and the matrix in rigid environment. Our experimental results show a phosphor with a record lifetime and quantum efficiency up to 3.16 s and 50.0% simultaneously in film under ambient conditions. Considering the multiple functions of the flexible films, the potential applications in anti-counterfeiting, afterglow display and visual frequency indicators were demonstrated. This finding not only outlines a fundamental principle to achieve bright organic phosphorescence in film, but also expands the potential applications of UOP materials.
Repulsive interactions widely exist in material science ranging from macroscopic matters to microscopic molecules [44][45][46] . For instance, a composite hydrogel with anisotropic mechanical properties developed, which is dominated by electrostatic repulsive force between negatively charged nanosheets embedded within it 46 . In macromolecules, the repulsive interactions play a vital role in stabilizing molecule conformations for functionalization, such as crystalline superstructures formed with soft clusters 47 , peptide folding between phosphorylated amino acids and tryptophan 48 , and the crystallization behavior of DNA-functionalized nanoparticles 49 . Despite great success in materials science, repulsive interactions have rarely been mentioned to improve phosphorescence properties of organic materials. It is worth noting that repulsion and attraction usually coexist in molecular systems 50 . Therefore, we reason that introduction of the carboxylic group and a short alkane may shorten the distance between the emitter and PVA polymer chain through hydrogen bonding, enabling the formation of the intense repulsive interactions between the aromatic hydrogens and the PVA polymer chains (Fig. 1b). Consequently, a rigid molecular environment was constructed to enhance phosphorescence at room temperature.

Results
Photophysical properties for CzA doped film  Table 1). After a delay time of 8 ms, a structured spectrum emerges with main peaks at 417 and 444 nm. Impressively, the emission lifetimes were over 2.7 s. It is worth noting that the profile of the phosphorescence spectrum in film is consistent with that in dilute solution at 77 K, indicating the emitter CzA is isolated with the PVA matrix in single-molecule state (Supplementary Fig. 30). With the variation of the excitation wavelengths from 250 to 350 nm, there is no change to the profiles of the phosphorescence spectra (Fig. 2b). The corresponding Commission International de l'Eclairage (CIE) coordinates are at (0.15, 0.08), which are in the deep blue region (Fig. 2c).
In a further set of experiments, we investigated the influence of the ratio of CzA emitters doping into polymer matrix and atmospheres on phosphorescence performance at room temperature. With the concentration of CzA molecule increasing from 0.1 to 5.0 wt%, both phosphorescence lifetimes and efficiencies firstly increase and then decline in film (Fig. 2d, e, Supplementary Figs. 32 and 33 and Supplementary Table 3). Remarkably, the phosphorescence lifetime of CzA emitter reaches 2.76 s when the ratio of CzA is 0.7 wt.%. The highest phosphorescent quantum efficiency of CzA/PVA is 20.1%. Notably, the phosphorescence decay curves of films show multi-exponential decay characteristic, which was ascribed to the relaxation of the PVA polymer chains at room temperature 51,52 . Because we found that the phosphorescence decay curve of the CzA film becomes single exponential at 77 K (Supplementary Fig. 34 Fig. 35). However, when the humidity reaches 68%, the phosphorescence intensity of CzA/PVA films dramatically decreased to 50% within 4 min ( Supplementary Fig. 36). However, there is no change in the profiles of phosphorescent spectra.

Investigation for UOP mechanism of the molecules in film
To gain deeper insight into high performance of phosphorescence for CzA/PVA, a set of control experiments on the change of the end groups and the alkane lengthen of the CzA molecule and matrix were conducted. As shown in Fig. 3a, b, the profiles of the phosphorescence spectra have little change, but the phosphorescence lifetimes and quantum efficiency exhibited a sharp fall. After concentration optimization, the control model of 9-ethyl-9H-carbazole (EtCz) without the carboxylic group showed an ultralong lifetime of 1.75 s and phosphorescence quantum efficiency of 15.0%. At 77 K, the profile of the phosphorescence spectrum for EtCz in dilute m-THF solution is consistent with CzA ( Fig. 3a bottom), indicating there is no influence of the carboxylic group on triplet energy levels. Notably, the phosphorescence properties of CzA/PVA and EtCz/PVA based on synthesized carbazole are consistent with that by commercial carbazoles (Supplementary Fig. 41). For the model of 8-(9H-carbazol-9-yl)octanoic acid (CzOA) molecule, the phosphorescence lifetime becomes shorter than that of CzA ( Supplementary Fig. 42). From PXRD analysis, it is found that there is no change of crystallinity for the PVA film after doping with CzA ( Supplementary Fig. 43). The XRD diffraction peaks for the CzA/PVA and neat PVA films with specific values (19.59°) can be clearly identified. There is no change in the FT-IR spectra and the losing weight curves for the doped PVA films (Supplementary Figs. 44 and 45), indicating that the trace doping of the CzA molecules has little effect on the physical properties of the PVA matrix. Taken these results together, we speculated that the molecular interactions between CzA and PVA matrix were vital for the phosphorescence improvement of the isolated CzA molecules in PVA film. Subsequently, we studied the supramolecular interactions between the emitters and polymer matrix by NMR spectrometer (Fig. 3c, d, Supplementary Figs. 46-55 the aromatic hydrogens of CzA were shielded with the hydroxyl hydrogen of PVA by the repulsive interaction [53][54][55][56] (Fig. 3e). Inversely, there is no change of the signals of the aromatic protons in EtCz and CzOA molecules (Fig. 3c and Supplementary Fig. 53). Meanwhile, the correlation of the protons was further validated by a nuclear overhauser effect spectroscopy (NOESY) NMR spectrum of CzA/PVA (Fig. 3d). There indeed exist intense correlations between the hydroxyl of PVA and the H a in CzA, as well as moderate correlations between the matrix and the H b , H c , and H d in CzA, suggesting that aromatic hydrogens of CzA was efficiently restricted into the PVA matrix by repulsive interactions. In contrast, the 1 H and NOESY NMR spectra of EtCz/PVA showed the absence of interaction between PVA and EtCz ( Fig. 3c and Supplementary Fig. 55). Therefore, we concluded that the intense repulsive interactions between CzA and the PVA matrix play a critical role in phosphorescence enhancement. Apart from the investigation of the influence of the end groups for the CzA molecule on phosphorescence, we further conducted a series of control experiments on different matrices, such as polyacrylic acid (PAA) and polyacrylamide (PAM). Like the molecules in PVA matrix, there is no change in the profiles of phosphorescence spectra for films with CzA and EtCz dopants (Supplementary Fig. 56a and b). For the phosphorescence lifetimes, the CzA-based films were longer, which was ascribed to the existence of the repulsive interaction between aromatic units and polymer matrices ( Supplementary Fig. 56c-f and Supplementary Table 6). Notably, in comparison with the molecules in PAA matrix, there existed significant increase for phosphorescence lifetimes in the PAM matrix owing to intensive repulsive interactions ( Supplementary Figs. 56e and 57). This finding further confirms the importance of the repulsive interactions for phosphorescence enhancement.
Given the above results, we proposed a conceivable mechanism for UOP of the CzA molecules in the PVA matrix, as shown in Fig. 3e. The introduction of the carboxylic group enables the formation of the intense repulsive interactions between carbazole units and the PVA matrix. After dispersed into the PVA matrix, the CzA molecules are isolated through repulsive interactions between carbazole groups and hydroxyl groups in PVA, further benefiting the suppression of nonradiative transitions. Thus, ultralong phosphorescence from the isolated molecules was obtained in CzA/PVA film under ambient conditions after photoexcitation (Supplementary Fig. 58).

Extended experiments on the universality of the design principle
To prove the universality of our approach, a series of new molecules with carboxyl (FA, PXZA, and NTIA) were also mixed with the PVA polymers matrix, which were named FA/PVA, PXZA/PVA, and NTIA/ PVA, respectively. Like CzA, we found that there also existed strong repulsive interactions between the aromatic hydrogen of the emitters and the hydroxyl hydrogen of the PVA matrix, which was verified by 1 H NMR spectra ( Supplementary Fig. 59a-c). As anticipated, all films showed intense afterglow emission after ceasing of the irradiation (Supplementary Movie 1). As shown in Fig. 4a, the emission peaks of FA/PVA, PXZA/PVA, and NTIA/PVA in steady-state PL spectra are at 305, 395, and 384 nm, of which the corresponding emission lifetimes are 4.9, 6.2, and 2.4 ns, respectively (Supplementary Fig. 29c-e). Specifically, FA/PVA and NTIA/PVA displayed structured phosphorescence spectra with maximum emission peaks at 458 and 544 nm, respectively, whereas PXZA/PVA showed broad phosphorescence emission with a main and at around 477 nm. It is worth noting that the profiles of the phosphorescence spectra in film are consistent with those in dilute solution at 77 K, indicating the emitters of FA, PXZA and NTIA are isolated with PVA matrix in single-molecule state ( Supplementary Fig. 30c-e). Impressively, with conjugation variation of the molecules from FA, PXZA to NTIA, the afterglow emission of the films changed from sky blue, green to yellow, spanning a large emission color gamut (Fig. 4b). After optimization, FA/PVA, PXZA/PVA and NTIA/PVA showed the longest phosphorescence lifetimes of 3.21 s (0.1 wt%), 876.89 ms (1.0 wt%), and 362.61 ms (0.5 wt%) and the highest quantum efficiency of 50.0% (0.3 wt%), 2.8% (1.0 wt%), and 8.0% (0.7 wt%), respectively (Fig. 4c and Supplementary Figs. 67-72 and Supplementary  Tables 7-9). To the best of our knowledge, it is the best phosphorescence material (FA/PVA at 0.3 wt%) considering both ultralong lifetimes and high quantum efficiency of phosphorescence (Supplementary Fig. 73). With the variation of the excitation wavelengths, there is no change for the profiles of the phosphorescence spectra (Fig. 4d).

Applications of flexible UOP films
Considering the feature of the long-lived afterglow of the emitters and the processability, flexibility and self-healing ability of PVA film, a set of potential applications were demonstrated. We firstly prepared pieces of transparent films based on different emitters in single or multiple components. With Chinese paper-cutting technology, a transparent pattern of art craft with the mouse was tailored, which showed intense blue afterglow (Fig. 5a and Supplementary Movie 2). Similarly, another Chinese paper-cut pattern based on two-component (CzA and NTIA) afterglow emitters showed a half of blue and a half of orange UOP after switching off the lamp (Fig. 5b). Regarding the standout self-healability of the PVA composite materials, an emissive 3D cube with blue afterglow was fabricated with CzA/PVA film (Fig. 5c, d

and Supplementary
Movie 3). Moreover, several ribbons with permutation and combination of PVA films doped with different afterglow molecules were prepared by utilization of self-healing ability. With variation of excitation (302 or 365 nm), the afterglow of ribbons varies with time ( Fig. 5e), which is promising in potential applications of anticounterfeiting and encryption.
Remarkably, the molecule doping PVA films were used as lighting panels for afterglow displays. As shown in Fig. 5f and Supplementary Movie 4, a demo of four electroluminescent devices was illustrated. After switching off the power source, the integrated devices displayed colorful afterglow with time. From Fig. 5g and Supplementary Movie 5, it is found that different tracks displayed with colorful afterglow from blue to white and then to yellow were clearly captured by the naked eye when the PVA film with mixed afterglow molecules of CzA and NTIA. Impressively, with frequency of electrical power change from 5.0, 1.0 to 0.2 Hz, the device showed reversible cycle of colorful afterglow (Fig. 5h, Supplementary Fig. 90 and Supplementary Movie 6), demonstrating the potential application for visual frequency indicators. For instance, the afterglow cycles between yellow and white (I-II-III) when the current frequency is 1.0 Hz.

Discussion
In conclusion, we have reported a molecular design strategy to improve phosphorescence performance of the isolated molecules in film under ambient conditions. The introduction of the carboxylic group enables the formation of the intense repulsive interactions between the molecules and the PVA matrix, constructing a rigid molecular environment. Remarkably, the films display a phosphorescence lifetime of 3.16 s and a highly phosphorescence quantum efficiency up to 50% simultaneously. With tailoring the molecular structures of the emitters, the UOP colors of the films were regulated from deep blue to yellow. In view of self-healing capability, flexibility and tailorability, the films with UOP feature were applied into anticounterfeiting, afterglow display and visual frequency indicators. These results not only provide a view to gain deeper insight into the phosphorescence of molecules in film, but also expand the scope of the potential applications of the UOP materials.

Data availability
All relevant data are included in this article and its Supplementary Information files. Source data are provided with this paper.