Effect of stereoregularity on excitation‐dependent fluorescence and room‐temperature phosphorescence of poly(2‐vinylpyridine)

Preparation of non‐conjugated luminescent polymers (NCLPs) with excellent cluster luminescence (CL) performance is of great significance for scientific and industrious applications, and yet improving the performance of NCLPs through proper structural design is still a huge challenge. Herein, we report a non‐conjugated ionized polymeric system consisting of (−)‐camphorsulfonic acid ((−)‐CSA) and poly(2‐vinylpyridine) (P2VP). These acid‐base complexes exhibit typical excitation‐dependent fluorescence and room‐temperature phosphorescence (RTP) with a lifetime up to 364 ms. We discover that changing the stereoregularity from atactic to isotactic significantly improves the CL performance of the complex. It (1) broadens the fluorescence emission spectra to cover the entire visible region, (2) enhances the fluorescence emission intensity at long wavelength beyond 500 nm, (3) enhances the phosphorescence intensity, and (4) extends the phosphorescence lifetime. Systematical experimental characterization and molecular dynamics simulation unravel the key role of stereoregularity in determining the formation of different pyridine aggregates that strongly influence the CL performance. Moreover, the different luminescence shows great potential in excitation divided information display and time‐resolved encrypted display. This work not only points to a new direction for developing NCLPs with excellent performance, but also broadens the applications of NCLPs materials.

Second, the entangled long chains of the NCLPs can prevent triplet excitons quenching by oxygen. [36][37][38] For example, Huang and An et al. reported the ionized derivatives of poly(4-vinylpyridine) (P4VP) exhibit both fluorescence and ultralong RTP with multicolor emission at room temperature, while ionized pyridine monomers only have phosphorescence emission at 77 K, and they prove that ionic bonding played a critical role in suppressing nonradiative transitions for RTP enhancement. [33] However, there remains a number of challenges in this field. For example, most of the emission of NCLPs is limited to 400-500 nm and it is difficult to prolong the lifetime of RTP. [39] The macromolecular structures are invariably closely linked to luminescent properties, i.e., the β-barrel structure plays an important role in the fluorescence emission of green fluorescent proteins. [40,41] In NCLPs, regulating the structure to affect the chromophores clusters is a concise and effective strategy to manipulate the CL performance. [26] Regulating the polymeric properties, such as molar mass, [42,43] architecture, [44][45][46][47][48] sequences, [49] and hierarchical structures [50][51][52] can effectively improve the CL performance of NCLPs. Most of these strategies involved adjustment of appropriate conformations of the clustered chromophores to form a large or condensed conjugated structure. In addition to the above characteristics, different stereoregularities also give rise to distinct arrangements of the chromophores. For example, changing the stereoregularity of polymeric chains in polystyrene from isotactic to atactic has been found to induce the red-shifted fluorescence emission. [53] This research also implies that increasing the main chain stereoregularity can affect the conformations of clustered chromophores through the rigid polymeric environment. Hence, improving the CL performance through proper stereoregularity design is highly demanded for mechanistic investigation and practical application. However, the relationship between stereoregularity and the CL performance in NCLPs is still unclear due to the difficulty in the synthesis of the highly isostatic main chain polymers.
In this work, we demonstrate that stereoregularity has a great effect on CL performance. We select macromolecular acid-base complex as the model, which is consisted of (−)-CSA and atactic poly(2-vinylpyridine) (aP2VP) or isotactic poly(2-vinylpyridine) (iP2VP). The addition of (−)-CSA to P2VP can form ionic bonds, which can suppress the nonradiative transitions and restrict the chromophore motions, easily leading to the long-lived RTP. [30,34] By changing the stereoregularity from atactic to isotactic, the emission spectrum is broadened to cover the entire visible region, and the emission intensity over 500 nm is strongly enhanced. The RTP lifetimes of iP2VP are prolonged, which are up to ∼3 times as long as that of aP2VP. Moreover, the other complexes with various sulfonic acids can further tune the CL performance. Systematic experimental results and molecular dynamics simulations show that iP2VP complexes are more favorable to form more condensed clusters with larger conjugation structure than aP2VP complexes. The stereoregularitydependent luminescence would expand applications of P2VP in high-level security protection, including excitationdependent displaying and time-resolved information encryption.

RESULTS AND DISCUSSIONS
iP2VP with a high isotacticity of mmmm > 99% was synthesized via coordination polymerization [54] and aP2VP with the similar M n was commercial available (Table S1 and Figure S2). The complexes of iP2VP-(−)-CSA and aP2VP-(−)-CSA were synthesized by mixing (−)-CSA with iP2VP or aP2VP in equal equivalent in chloroform (Scheme S1). The chemical structures of iP2VP and aP2VP complexes were characterized by Fourier transform infrared spectroscopy (FTIR), 1 H-Nuclear magnetic resonance spectra ( 1 H-NMR), and powder X-ray diffraction (PXRD). After the addition of (−)-CSA, the stretching vibrations of the C═N bond at 1590 cm −1 disappeared; meanwhile, a new band attributed to the stretching vibrations of the C═NH + bond appeared at 1625 cm −1 (Figures S3 and S4). In addition, the maximum changes of the chemical shift of the ortho-position of pyridine are 0.3 ppm ( Figures S5-S7). The above results confirmed the complete protonation of P2VP by (−)-CSA. [55] Moreover, the absence of any diffraction peaks in the highangle region in the PXRD pattern of P2VPs revealed its amorphous state in the solid state ( Figure S8). [33] To verify the formation of clustered chromophores in the complexes, we measured the photophysical properties of iP2VP-(−)-CSA and aP2VP-(−)-CSA in dichloromethane solution. The originally weak fluorescence emission is continuously enhanced with increasing the concentration of the complex, exhibiting a typical concentration-enhanced emission effect ( Figure 1A and Figures S9 and S10). Meanwhile, with increasing the concentration of complexes the onset wavelength of absorption gradually extends over 300 nm ( Figure 1B and Figure S11), indicating the generation of multiple chromophore aggregates in the complex. Moreover, for the concentrated solutions (c = 100 mg mL −1 ), the maximum emission wavelength is red-shifted with an increase of λ ex ( Figure 1C and Figure S12). Such an excitation-dependent fluorescence (EDF) strongly suggests the presence of multiple emissive species, which is further confirmed by the various lifetimes (<τ>) ( Figure 1D and Figure S12, and Tables S2 and S3). All of the above results strongly confirm that both iP2VP-(−)-CSA and aP2VP-(−)-CSA complexes exhibit typical characteristics of NCLPs. [9] Although it is obvious that both iP2VP-(−)-CSA and aP2VP-(−)-CSA exhibit EDF characteristics in concentrated solutions, their emission intensities are quite different. For iP2VP-(−)-CSA, the relative emission intensity located beyond ∼500 nm is much stronger than that of aP2VP-(−)-CSA. ( Figure 1C and Figure S12a). To further verify the effect of the stereoregularity on CL performance, we examined the EDF emission of iP2VP-(−)-CSA and aP2VP-(−)-CSA in powder state. As shown in Figure 2A, when the λ ex gradually changes from 340 to 560 nm, the maximum emission wavelength of iP2VP-(−)-CSA exhibits a bathochromic shift from 438 to 590 nm. However, for the aP2VP-(−)-CSA, the maximum emission wavelength varies from 437 to 570 nm and the emission intensity at larger wavelength than 500 nm is much weaker than that of iP2VP-(−)-CSA under the identical conditions ( Figure 2B).
We summarize the maximal emission wavelengths and normalized PL intensities at different λ ex for both complexes in Figure 2C. The maximal emission wavelength of The EDF spectral measurements are also confirmed by the luminescence photos under different λ ex : when the λ ex changes from 340 to 520 nm, the emission colors of iP2VP-(−)-CSA complex vary from sky blue to yellow and finally to red, covering the entire visible region ( Figure 2C, upper panel). However, the red emission of the aP2VP-(−)-CSA complex is barely distinguishable under the same conditions ( Figure 2C, lower panel). The CIE coordinates of the complexes are also consistent with the different EDF emissions caused by the different stereoregularity of the P2VP chains ( Figure 2D). The above results suggest that the iP2VP-(−)-CSA complex formed more condensed clusters with larger conjugation than that of the aP2VP-(−)-CSA complex, which can also be verified in their excitation and absorption spectra ( Figures S13 and S14). [35] The P2VP-(−)-CSA complex also exhibits long-lived RTP emission because the ionic bond between pyridine pendants and sulfonic acid restricts the motions of the chromophores. [33] We further studied the effect of the stereoregularity on the RTP performance of P2VP-(−)-CSA complexes. As shown in Figure 3A, when irradiated with a 365 nm UV lamp, the iP2VP-(−)-CSA complex shows blue emission in solid state at room temperature. After turning off the UV lamp, yellow-green ultralong phosphorescence is observed by the naked eye, and can last for several seconds under ambient conditions. However, the phosphorescence emission of aP2VP-(−)-CSA complex is hard to be distinguished by naked eyes after the UV lamp was turned off for 0.5 s. Moreover, it can be seen from the photos in Figure 3A that the afterglow of the iP2VP-(−)-CSA is brighter than that of aP2VP-(−)-CSA under the same conditions, which is consistent with the phosphorescence quantum yield: iP2VP-(−)-CSA is 7.8% while aP2VP-(−)-CSA is 4.2%. The different RTP performance is also evident in emission spectra (Figures S16-S20). Moreover, the RTP lifetime of iP2VP-(−)-CSA is 364.74 ms, which is ∼3 times as long as that of aP2VP-(−)-CSA ( Figure 3B and Figure  S15, and Tables S4 and S5). Such a huge difference in RTP performance results from the distinct stereoregularity will inevitably affect the aggregation of monomers in the complex. A previous study reported that the RTP lifetime of poly(styrene sulfonic acid) (PSS) was independent on stereoregularity, [31] which is markedly different from our results probably due to the lower stereoregularity of iPSS (mm = 90%) than that of iP2VP (mmmm > 99%). The highly isostatic polymeric chain is critical to prolong the RTP lifetime. To the best of our knowledge, we provide the first example that the stereoregularity remarkably affects the RTP performance of NCLPs.
According to previous studies, water vapor can destroy the ionic bonding, thus weakening the RTP intensity of NCLPs. [34] Based on this feature, we explore the reversible RTP in the iP2VP-(−)-CSA complex. As can be seen in Figure 3C,E after the addition of water vapor, both the RTP emission intensity and the lifetime significantly decreased, which originate from the breakage of ionic bonds between (−)-CSA and iP2VP ( Figure S21). After drying at 80 • C in vacuum for 6 h, the RTP emission is recovered due to repair of the ionic bonds. The reversibility of the RTP is examined by alternately adding water vapor and drying in cycles ( Figure 3D). Benefiting from the excellent reversibility, the iP2VP-(−)-CSA complex can be used as information encryption and reversible storage materials. As shown in Figure 3E, the letters "PKU" are depicted on a piece of nonfluorescent paper with iP2VP-(−)-CSA powder. In the dry state, the letters are sky blue under UV light and rapidly turn into yellow-green when the UV light is turned off. After absorbing water vapor, the fluorescence emission color is almost unchanged under UV light ( Figure 3E, left column). However, it becomes virtually invisible after the removal of UV light ( Figure 3E, right column). The reversible display of "PKU" information can be achieved between the two states with distinct RTP emission. In brief, one can write, read, or erase information efficiently in such a bi-stable system.
To discover the underlying mechanism for the stereoregularity effect, we simulated the aggregation behavior of pyridine chromophores in P2VP-(−)-CSA complexes by molecular dynamics simulation. The simulated complex contains 100 pyridine units that are completely protonated by (−)-CSA. In iP2VP-(−)-CSA, we can observe multiple aggregates produced by pyridine π-π stacking, such as Aggregate II, Aggregate III, Aggregate IV, and other larger aggregates ( Figure 4A). While in aP2VP-(−)-CSA, one can only observe Aggregate II and Aggregate III, without larger aggregates ( Figure 4B). The difference in aggregation modes between iP2VP and aP2VP is further confirmed by the number distribution of aggregates formed by different amounts of pyridine chromophores ( Figure 4C). For all aggregation modes, the numbers of aggregates in iP2VP-(−)-CSA are larger than aP2VP-(−)-CSA. The nor-malized 1D scattering profiles in the q z direction of the 2D Gi-WAXS pattern of iP2VP and aP2VP complexes also prove the existence of multiple aggregates ( Figure S22). [33] Additionally, we examine the luminescence performance of 2VPy-(−)-CSA monomer to further understand the different aggregation modes between iP2VP-(−)-CSA and aP2VP-(−)-CSA. 2VPy-(−)-CSA has fluorescence emission but no RTP emission, and the fluorescence emission of 2VPy-(−)-CSA under different λ ex are highly similar to that of aP2VP-(−)-CSA ( Figure S23). This result indicates that the aggregation modes in aP2VP(−)-CSA is similar to the monomeric complex, which is unfavorable for the stacking of pyridine chromophores to form a variety of large aggregates.
Based on the above results, we propose a possible mechanism to rationalize the stereoregularity-dependent luminescence ( Figure 4D). The multicolor EDF emission originates from multiple aggregate states in the P2VP complexes. In comparison with aP2VP-(−)-CSA, the highly isotactic configuration of iP2VP-(−)-CSA is more conducive to ππ stacking among pyridines, resulting in more condensed aggregates with a larger conjugation structure. Consequently, iP2VP-(−)-CSA shows a comprehensive CL improvement, including a broader EDF spectrum covering the entire visible region, stronger emission intensity at long wavelengths above 500 nm, stronger RTP intensity, and a longer RTP lifetime.
To further demonstrate this stereoregularity improved CL performance, we test the photophysical properties of acidbase complexes with different sulfonic structures. First, we replace the rigid (−)-CSA with ethanesulfonic acid (ESA) that contains a flexible alkyl chain ( Figure 5A). Both iP2VP-ESA and aP2VP-ESA exhibit similar EDF emission just as the (−)-CSA complex in powder state. For iP2VP-ESA, the EDF spectral range is 10 nm wider than that of aP2VP-ESA. Meanwhile, the strongest emission wavelength changes from 438 to 530 nm with an increasing λ ex , but the strongest emission band of aP2VP-ESA has been fixed at 438 nm. The relative emission intensity located over 500 nm of iP2VP-ESA is much stronger than that of aP2VP-ESA ( Figure 5B,C and Figure S24). Moreover, the RTP lifetime of iP2VP-ESA is longer than aP2VP-ESA complex, showing the similar trend to the case of (−)-CSA complexes. However, the RTP lifetime of the complexes with ESA is shorter than that with (−)-CSA ( Figure 5D), which may be due to the ethyl group in ESA can freely rotate and enhance the nonradiative decay in comparison to the rigid structure of (−)-CSA.
Next, we replace the sulfonate anion with dodecylbenzenesulfonic acid (DBSA) to form iP2VP-DBSA and aP2VP-DBSA complexes ( Figure 5A). Both the iP2VP-DBSA and aP2VP-DBSA complexes exhibit typical EDF features ( Figures S25 and S26). However, the EDF spectral range of iP2VP-DBSA and aP2VP-DBSA complexes are larger than the complexes with (−)-CSA or ESA. This broaden emission spectra may be attributed to the phenyl group in DBSA, which can also participate in the aggregates of pyridines and lead to larger conjugated structure than (−)-CSA or ESA ( Figure 5F,G and Figure S24). In addition, the RTP lifetime of iP2VP-DBSA is twice as long as the case of aP2VP-DBSA ( Figure 5H and Figures S27-S29 and Tables S6-S9), consistent with the previous results in (−)-CSA and ESA complexes. However, the RTP lifetime of the complexes with DBSA is much shorter than that of the complexes with the other two sulfonic acids. According to previous report, the anion can strongly affect the RTP lifetime of PSS derivatives. [34] The short RTP lifetime of the complexes with DBSA may result from the long alkyl chain in DBSA anions. Moreover, we tested the photophysical properties of pure iP2VP and aP2VP, they showed similar variation trend as the P2VP complex ( Figures S30 and S31). The above results not only confirm the improved CL performance based on increasing stereoregularity of P2VP main chain, but also reveal the feasibility of tunable CL properties of NCLPs upon varying anion structures.
The huge difference in CL performance between iP2VP-(−)-CSA and aP2VP-(−)-CSA can be applied to the anti-counterfeiting and encryption display. As shown in Figure 6A, iP2VP-(−)-CSA is spread on the center region of the substrate, and the surrounding is covered by aP2VP-(−)-CSA powder. Due to their different fluorescence intensity under different λ ex s (Figure 2A,B), selective emission patterns can be realized by tuning λ ex from 420 to 520 nm. When the excitation wavelength is 420 nm, the sample is filled up by green emission. However, when the excitation wavelength is switched to 520 nm, the emission glow around the sides is fainted, leaving only the red emission in the center of the sample. These excitation wavelength-controlled responses can be exploited for anti-counterfeiting.
In addition, by taking advantage of the difference in the RTP lifetime of iP2VP-(−)-CSA and aP2VP-(−)-CSA, we can hide the encrypted information at a certain time node to achieve high-level information encryption and security protection. For example, real information "34" is hidden in a fake message "88" according to the tunable RTP lifetime of iP2VP-(−)-CSA and aP2VP-(−)-CSA complex. The encrypted information "34" can be revealed at about 1.5 s after the UV light is turned off ( Figure 6B).

CONCLUSION
NCLPs with tunable excitation-dependent fluorescence and long-lived room-temperature phosphorescence are achieved in the P2VP complexes. We discover that the CL performance of iP2VP complexes and aP2VP complexes are quite different. The iP2VP complexes have broader emission regions, stronger emission intensities at long wavelengths above 500 nm, and longer RTP lifetime in comparison with aP2VP complexes. Systematic experimental characterization and molecular dynamics simulation revealed that the stereoregularity of polymeric chains plays a key role in CL performance. Moreover, the universality of this stereoregularity improvement strategy is also proved with different sulfonic acids: P2VP-ESA and P2VP-DBSA. On the basis of this stereoregularity-dependent CL, high-level information encryption display can be achieved in our work. This finding not only contributes to the study of the structure-property relationships of these new luminescent materials but also provides a new strategy for the development of advanced information security and encryption materials.

A C K N O W L E D G M E N T S
The financial support from the National Natural Science Foundation of China (Nos. 51833001 and 51921002) is greatly appreciated. The authors also thank Mr. Mingxing Chen from the Analytical Instrumentation Center of Peking University for his generous help in photoluminescence tests.

C O N F L I C T O F I N T E R E S T
The authors declare no conflict of interest.