Pyrene-based aggregation-induced emission: A bridge model to regulate aggregation

Abstract

The aggregation process plays a significant role in regulating the aggregate structures from molecules toward macroscopic photophysical properties. Pyrene (Py), as the simplest dimer candidate, serves as a suitable model for studying the aggregation. Herein, a series of Py-based aggregation-induced emission (AIE) materials have been investigated by clarifying the comprehensive roles of oxygen, substituents, molecular motion, and packing during aggregation, initially realizing the aim of controlling aggregate structures. With a largely planar and conjugated conformation, Py shows anomalous AIE characteristics due to the oxygen quenching at the molecular level but turn-on fluorescence in the aggregate state because of the oxygen isolation. Although introducing substituents induces molecular motion and similarly weakened luminescence in the molecular state, the impact of substituents on the aggregate-state photophysical properties enormously differs, exhibiting from weak blue, strong cyan, and strong green to weak yellow emissions, due to variable aggregate structures. Interestingly, the natural alicycle-substituted Py-dehydroabietylamine (Py-DAA) exhibits both mechanochromism and acidichromism, which can be synergistically applied in dynamic encryption-decryption. This work not only elucidates the unique AIE property of Py for the first time but also clarifies the bridging role of aggregation between single-molecular and aggregate states, achieving preliminary control over the aggregate structures.

Graphical abstract

Public summary

• •Unique aggregation-induced emission (AIE) property of pyrene was elucidated for the first time.
• •Comprehensive effects of oxygen, substituent, molecular motion, and packing on aggregation were clarified.
• •Synergistic and dynamic encryption-decryption has been realized based on both molecular and aggregate states.

Introduction

Traditional methodology on organic solid materials is based on the “reductionistic” approach, which holds the concept that the properties of the microscopic molecular structure can determine those of the macroscopic substance. Therefore, traditional materials design usually focuses only on the properties at the molecular level.^1,2,3 However, many facts have proved that this is not always true. For example, the solid-liquid-gas phases of water and the solid-gas states of carbon dioxide all represent the same molecular structure but different macroscopic properties. For typical luminogens, perylene and hexaphenylsilole (HPS) exhibit completely different fluorescence properties in both solution and aggregate states, which are referred to as aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE) behaviors, respectively.^4,5 On this basis, it is proposed that a gap exists between molecular structure and material property, i.e., aggregate structure, which plays a bridging role in determining their different properties in different states.^6,7,8 Therefore, a specific aggregate system used as a model to elucidate the corresponding structure-property relationship between macroscopic properties and aggregate structures is extremely significant. By studying such a model system, it is possible to achieve preliminary control over macroscopic properties at a level higher than that of single molecules (Figure 1A).

Figure 1.

Schematic illustration of design approaches for the pyrene-based AIE system

(A) Schematic illustration of bridge role of mesoscopic science. ES and HB represent the electrostatic effect and hydrogen bonding, respectively.

(B) Molecular design and properties of the pyrene-based AIE system.

The aggregates include a variety of forms such as crystals,⁹ supramolecular aggregates,^10,11 nanoparticles,^12,13,14 etc. Investigating the composition and structure of different aggregates is a key focus in the study of the structure-property relationship of aggregates. Pyrene (Py) is a highly planar and conjugated molecule that can easily form dimer structures upon aggregation, resulting in significant differences in photophysical properties before and after aggregation, making it an ideal aggregate model for exploring the bridge role between molecules and aggregate properties. In addition, Py has a wide range of applications in organic light-emitting diodes (OLEDs),^15,16 bioimaging,^17,18 anti-counterfeiting,¹⁹ and other fields^20,21 due to its unique optical properties, good stability, high carrier mobility, large planar structure, and strong charge injection ability. Py’s photophysics has been studied for a long time. In the 20th century, photophysics experts like Förster and Birks et al. mainly focused on the study of the solution states.^{22,23,24,25,26,27,28} In the solution state, Py attracts much attention due to its unique photophysical properties. With the rise of aggregate science in the 21st century, researchers focused more on the study of the aggregate state of Py since materials are normally utilized as aggregates.^{18,20,21,29,30} In the aggregate state, the photophysical properties of Py can be influenced by adjusting the interactions and packings between Py molecules, such as altering the position of substituents in the isomers.³¹ By adjusting the number of substituents, the distance between Py groups can be precisely controlled to fine-tune the wavelength of the aggregate state.³² The size of the substituents and the length of alkyl chains can also be adjusted to affect the stacking and, thereby, influence the emission wavelength and quantum yield (QY) of aggregates.³³ While extensive investigations on the structure-property relationships of respective solutions and crystalline states of Py have been reported, studies on the aggregation process are still lacking. In fact, even with consistent performance in the monomer state, the aggregation process can severely affect the final properties of the aggregates. AIE research is complex, integrating monomer, aggregate, and aggregation processes.^3,6,34 Through AIE research, we can be endowed with the chance to study the bridging effect of aggregation to try to initially control the aggregate structures.

This work systematically investigates the photophysical properties of a series of Py-based AIE materials at both molecular and aggregate states to take the initial structural regulation of the aggregates. By elucidating the differences in photophysical properties before and after aggregation, the investigation reveals the combined roles of factors such as oxygen, substituents, molecular motion, and packing in the aggregation process, achieving preliminary control of the Py dimer structure toward macroscopic aggregates. Among these, Py possesses a large planar conjugated structure, exhibiting unusual AIE behavior, largely attributable to the significant influence of oxygen during the aggregation process. This work not only clarifies the unique AIE properties of Py for the first time but also highlights the important bridging role of the aggregation process between monomer and aggregate states, thus achieving preliminary regulation of aggregate-state properties (Figure 1B).

Materials and methods

Materials and reagents

Dehydroabietylamine (DAA) was obtained from the commercial disproportionated rosin amine (Guilin Songquan Forest Chemical Co., Ltd. ). Py and 1-pyrenecarboxaldehyde (Py-CHO) were obtained through ethanol recrystallization after being commercially purchased (Py: Aladdin, 95%; Py-CHO: Energy Chemical, 98%). The single crystals of Py and Py-CHO suitable for X-ray diffraction measurement could be obtained by slow evaporation of ethanol. Cyclohexylamine (CyA) (Energy Chemical, 99.5%), chloroform-d₃ (Aladdin, 99.8%), tetrahydrofuran (THF) (Aladdin, for spectroscopy, 99.5%), glycerol (Aladdin, 99.5%), trifluoroacetic acid (TFA) (Energy Chemical, 99%), and triethylamine (TEA) (Sinopharm Chemical Reagent Co., Ltd., 99.0%) were used without further purification. All organic solvents were used as received without further purification.

Characterization techniques

Melting points were determined using an OptiMelt MPA100 apparatus (SRS, USA) without applying any corrections. ¹H and ¹³C spectra were obtained using a Bruker AVANCE-III-600 spectrometer (¹H, 600 MHz; ¹³C, 150 MHz), with CDCl₃ utilized as the solvent. High-resolution mass spectroscopy (HRMS) spectra were obtained using a Q Exactive (Thermo Scientific, Germany) mass spectrometer operating in an electrospray ionization (ESI) mode. The powder X-ray diffraction (PXRD) pattern was acquired utilizing a Rigaku Ultima IV diffractometer with Cu Kα radiation. UV-visible (UV-vis) absorption spectra were collected on a Shimadzu UV2450 spectrometer. Photoluminescence (PL) spectra and solid absolute fluorescence QYs were recorded on a Horiba Fluoromax-4 spectrofluorometer. PL spectra and transient PL decay spectra of four compounds in ambient and degassed solution states, as well as other absolute fluorescence QYs were recorded on an Edinburgh FLS1000 PL spectrometer. The single-crystal data of Py were selected and mounted on an Xcalibur, Eos, Gemini diffractometer using Mo Kα radiation (λ = 0.71073 Å). The single-crystal data of Py-CHO were selected and mounted on a Bruker D8 with a PHOTON 100 detector diffractometer using synchrotron radiation (λ = 0.71073 Å). The single-crystal data of Py-CyA and Py-DAA were selected and mounted on a SuperNova, Dual, Cu at zero, AtlasS2 diffractometer using Mo Kα radiation (λ = 0.71073 Å) (Py-CyA) and Cu Kα radiation (λ = 1.54184 Å) (Py-DAA). Using Olex2, the structure was solved with the ShelXT structure solution program using intrinsic phasing and refined with the ShelXL refinement package using least squares minimization. Crystallographic Data: www.ccdc.cam.ac.uk/data_request/cif. The CCDC numbers are as follows: 2284820 (for Py), 2284852 (for Py-CHO), 2284858 (for Py-CyA), and 2335618 (for Py-DAA).

Computational details

The ground-state geometries and corresponding frontier molecular orbitals were calculated using the density functional theory (DFT) method at the B3LYP-D3BJ/6-31G(d,p) level. Analytical frequency calculations were also performed at the same level of theory to confirm the local minimum point of the optimized structures. The energy levels of singlet and triplet states were calculated based on the optimized singlet-state geometries using the time-dependent DFT method at the same level of theory. Besides, the Hirshfeld surfaces and decomposed fingerprint plots were calculated and mapped using the CrystalExplorer 17.5 package. In addition, the overlap of the dimer structure in the single-crystal analysis involves obtaining overlapping structural diagrams perpendicular to the plane perspective using Mercury, followed by separately calculating the area of overlap and the area of the Py group using Photoshop, and finally determining the proportional relationship.

Degassing details

First, Py, Py-CHO, Py-CyA, and Py-DAA solids were dissolved in THF at a concentration of 20 μM, respectively. Then, the solution was placed in a Schlenk flask, and then the solution was frozen using a Dewar of liquid nitrogen. Once the solvent was frozen, the atmosphere was evacuated under vacuum for 10 min. The Schlenk flask was removed from the Dewar, and the solution was allowed to slowly warm up and thaw while the flask was filled with nitrogen gas. The above steps were repeated three times.

Results and discussion

Molecular synthesis and characterization

For AIE systems, molecular motion is the fundamental working mechanism.³⁴ From an intuitive perspective, the size of substituents is more easily regulated to control molecular motion, making it more suitable for exploring the comprehensive influence of various factors on the aggregation process. In our previous research, we found that rosin, a natural renewable product, has a unique molecular structure that can be used to restrict the molecular motion of simple Schiff base chromophores, leading to AIE properties.³⁵ Meanwhile, it exposes a significant steric hindrance, which can regulate molecular packing, resulting in stimuli-responsive properties.^36,37 It may endow the study of the aggregation process in the Py system with unique photophysical effects. In line with this, a medium-sized cyclohexane and a smaller aldehyde group are introduced as substituents to explore the effects of variable factors during the aggregation process. As shown in Scheme S1, Py-CHO reacted with CyA and DAA via a simple Schiff base reaction to yield Py-CyA and Py-DAA. Structures were confirmed by ¹H and ¹³C nuclear magnetic resonance (NMR) and HRMS measurements (Figures S1–S6). Single crystals of Py, Py-CHO, Py-CyA, and Py-DAA were obtained via slow evaporation for X-ray diffraction measurements (please see the details in the supplemental information).

Elucidating the AIE behaviors of Py and its derivatives

The photophysical properties of all four compounds were first studied. The absorption of the four compounds is shown in Table 1. When substituents are introduced, the absorption of Py undergoes a redshift. The energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are also consistent with their absorption wavelengths (Figure S7; Tables S2–S5). They all show the typical AIE effect with weak fluorescence in a dilute solution and enhanced fluorescence at a high water fraction (f_w) (Figure 2A). Among them, the AIE effect of Py challenges our previous knowledge of the traditional ACQ phenomenon,^6,22,38,39 which always happens within planarly conjugated luminogens, driving us to find the underlying reasons for the opposite observation. As seen in the PL spectra, Py exhibits fine-structured monomer emission in the molecular state (Figure 2B). As the f_w gradually increases, the emission intensity of the monomer rises due to the polarity of the solvent (Figure S8).^40,41 When the f_w > 80%, the intensity of the monomer peak decreases and is accompanied by a new and broad peak with visible sky-blue fluorescence, which is attributed to the dimer emission generated by aggregation.⁴⁰ With the increased concentration, the dimer-emission proportion of Py also increases (Figure S9A) and can be observed by the naked eye. To further verify whether it exhibits the AIE effect, we tested the absolute QY in both the single-molecule and aggregate (f_w = 95%) states. The QY in the single-molecule state was measured as 3.3%, while upon aggregation, it increased to 27.0%, consistent with a previous report,⁴⁰ demonstrating the typical AIE effect. Still, they fail to provide a clear explanation for the following issue: as a planarly conjugated molecule, the emission of isolated Py is almost quenched. This fact suggests that the emission in the molecular state may be influenced by factors other than polarity. Either a non-radiative heat route or reactive oxygen species are always taken into consideration for the quenching of fluorescence. Obviously, the heat route is infeasible because of the rigid planar structure. Hence, the fluorescence quenching of Py can be rationally speculated to be due to the presence of oxygen in the solvent.^22,28,42,43 To verify this surmise, the PL spectra, QY, and lifetime of Py in the molecular state before and after the degassing process were compared (Figures 2C and S10; Table 1). After degassing, the PL intensity significantly increases, the QY rises from 3.3% to 54.0%, and the lifetime also increases from 15.75 to 126.07 ns. The results indicate that oxygen indeed plays an essential role in the luminescent behaviors of Py. PL spectra of Py before and after deoxygenation in dimethyl sulfoxide (DMSO) and hexane further indicate that oxygen is an important factor affecting the excited-state properties of Py. As shown in Figures S11A and S11B, after deoxygenation, the PL intensity of Py increased but decreased when air was further introduced, indicating that the excited-state excitons of Py are sensitive to oxygen. Accordingly, we also measured the PL spectra of Py in the DMSO solution upon UV light irradiation (Figure 2D). The PL intensity increases with the elongated irradiation time, suggesting the gradual removal of triplet oxygen in the DMSO solution. To the best of our knowledge, triplet oxygen can be easily transformed to singlet oxygen via the energy transfer route when faced with triplet excitons. Singlet oxygen can react with DMSO to gradually create an oxygen-free environment and enhance Py fluorescence.⁴⁴ These results indicate the production of triplet excited states in Py.^25,45 In addition, dichlorodihydrofluorescein diacetate (DCFH-DA) was used as an indicator to visualize singlet O₂ production, and the emission of the DCFH-DA/Py mixture was rapidly enhanced with irradiation time. The emission enhancement rate is significantly higher than that of DCFH-DA without Py in the DMSO solution, indicating the generation of singlet O₂ (Figures S11C and S11D). Under oxygen-containing conditions, the triplet excitons react with oxygen, resulting in non-radiative decay. However, under anaerobic conditions, the triplet excitons may undergo the triplet-triplet annihilation (TTA) process, leading to delayed fluorescence^24,25,46,47 and increased PL intensity, QY, and lifetime. To further understand the evolution of excited states, we calculated the singlet and triplet energy levels of Py. As shown in Figure 2E, the energy level of S₁ (3.450 eV) is approximately twice that of T₁ (1.719 eV), meeting the basic conditions for TTA. Besides, the molecular state of Py-CHO is also influenced by oxygen (Table 1; Figures S10B and S12A), with a lower QY than Py of only 0.5% under oxygen-containing conditions. This behavior may be related to molecular motion. After degassing, the QY of Py-CHO increased to 9.4%, along with an increase in the lifetime from 7.17 to 12.28 ns, exhibiting a similar trend to Py. From the energy levels of singlet and triplet states, Py-CHO also satisfies the possibility for TTA (Figure S13). On the contrary, Py-CyA and Py-DAA show very low QYs and lifetimes in their molecular states before and after degassing without noticeable delayed fluorescence, which may be attributed to the dominant molecular motions introduced by the largely flexible substituents (CyA and DAA), making the influence of oxygen negligible (Table 1; Figures S10C, S10D, S12B, and S12C). Based on the above results, it is proposed that the excited-state processes experienced by Py in the molecular state include fluorescence, DF of TTA, and energy transfer. The substitution of the aldehyde group could introduce molecular motions into the rigid molecular skeleton, leading to stronger non-radiative decay. When CyA and DAA are introduced, primary fluorescence and non-radiative processes occur due to the dominance of molecular motions (Figure 2F).

Table 1.

Summary of photophysical properties of Py, Py-CHO, Py-CyA, and Py-DAA in solution, aggregate, and crystal state

Comp.	λab (nm)	λem (nm)		ФF (%)		τ (ns)		λem (nm) (Aggr, fw = 95%)	ФF (%) (Aggr, fw = 95%)	λem (nm) (Cryst)	ФF (%) (Cryst)
	Solna	Solna	Solnb	Solna	Solnb	Solna	Solnb
Py	335	373	373	3.3	54.0	15.75	126.07	371, 465	27.0	465	42.4
Py-CHO	360	385	387	0.5	9.4	7.17	12.28	469	41.5	543	16.7
Py-CyA	360	391	391	0.5	0.8	NA	NA	507	35.1	495	43.5
Py-DAA	360	392	392	0.4	0.5	NA	NA	500	15.8	422	2.3

^aIn THF solution (20 μM) with air.

^bIn degassed THF solution (20 μM).

Figure 2.

AIE property of Py

(A) Fluorescence photographs of Py, Py-CHO, Py-CyA, and Py-DAA in THF/H₂O mixtures with different f_w taken under a 365 nm UV lamp. Concentration: 20 μM.

(B–D) PL spectra of Py in (B) THF/H₂O mixtures with different f_w, (C) the ambient and degassed states, and (D) DMSO solution upon UV light irradiation. Concentration: 20 μM; λ_ex = 335 nm.

(E) The calculated energy levels of Py.

(F) A simplified Jablonski diagram to illustrate the photophysical processes. Abs, absorption; F, fluorescence; NR, non-radiative relaxation; DF, delayed fluorescence; ISC, intersystem crossing; TTA, triplet-triplet annihilation.

Photophysical behavior of Py and its derivatives in the aggregate state

The detailed photophysical properties of Py under different f_w provide insights into the fundamental mechanism of weak molecular state emission and AIE properties of this conjugated planar compound. Although oxygen plays a vital role in the molecular state emission of Py, does oxygen impact the aggregate state? According to Figure 3A, after forming Py aggregates, there is an increase in dimer emission and an enhancement in monomer emission. Theoretically, the emission intensity from monomers should decrease significantly after aggregation due to the reduced proportion of monomers and increased energy transfer from monomers to dimers. The strange behavior of increased monomer emission suggests that aggregation may isolate oxygen, enabling monomers to avoid quenching and instead exhibit enhanced emission (Figure 3B).^44,48 Additionally, a different AIE trend of Py-CHO was observed compared to Py (Figure 3C). In all spectra, the monomer emission is too weak to be noticed, and a continuous redshift enhancement is observed as the f_w increases. Its QY in the aggregate state reaches 41.5% (far exceeding the molecular state of 0.5%), indicating the combined proximity effect and aggregation.⁴⁹ In addition, oxygen, unfavorable for fluorescence, is considered to be excluded after aggregation. Hence, the comprehensive causes of restriction of molecular motion, proximity effect, and TTA lead to the red-shifted AIE behavior distinguishing from that of Py. Interestingly, unlike Py, Py-CHO exhibits concentration-dependent fluorescence (Figure S9B). With increasing concentration, there is a tendency for the emission wavelength to redshift. This suggests that Py-CHO tends to form dimers with different π-π stacking arrangements at different concentrations,^50,51 further highlighting the significant role of packing in the photophysical properties of Py-CHO. Py-CyA and Py-DAA exhibit similar AIE trends to Py, wherein monomer emission is observed in the molecular state, and a new emission peak from dimer appears after aggregation (Figures 3D and 3E). The concentration and viscosity experiments also confirm their AIE properties (Figures S9C, S9D, and S14). The difference lies in the dimer emission of the latter two compounds, which is much stronger than the monomer emission. This is because the molecule-state emissions of Py-CyA and Py-DAA are extremely weak due to molecular motion. After aggregation, the restricted motions enhance fluorescence with increased QYs from 0.5%–35.1% of Py-CyA and 0.4%–15.8% of Py-DAA, respectively.

Figure 3.

Photophysical properties of Py and its derivatives in different states

(A) PL spectra of Py in THF/H₂O mixtures with f_w = 0% and 95%. Concentration: 20 μM; λ_ex = 335 nm.

(B) Schematic diagram of AIE mechanism of Py.

(C–E) PL spectra of (C) Py-CHO, (D) Py-CyA, and (E) Py-DAA in THF/H₂O mixtures with different f_w. Concentration: 20 μM; λ_ex = 360 nm.

(F–I) PL spectra of (F) Py, (G) Py-CHO, (H) Py-CyA, and (I) Py-DAA in the solution, aggregate (Py: f_w = 99%; Py-CHO: f_w = 95%; Py-CyA: f_w = 95%; and Py-DAA: f_w = 95%), and crystal states, respectively. Concentration: 20 μM. Δ represents the wavelength of the crystal minus the wavelength of the aggregate.

Interestingly, the fluorescence of the crystals and aggregates (f_w = 99% of Py and f_w = 95% of Py-CHO, Py-CyA, and Py-DAA) of the four compounds show significant differences (Figures 3F–3I). The aggregated Py exhibits emissions from both the monomer and dimer, indicating the complex packing with mixed ordered and disordered arrangements after rapid aggregation. The emission peak of the dimer is consistent with that of the crystalline state without shift (Δ = 0), suggesting that the dimer structure of Py in the aggregate state is similar to the packing in the crystalline state. However, the Py-CHO crystal is significantly redshifted compared to the aggregate state (Δ > 0), exhibiting yellow emission. On the other hand, the crystals of Py-CyA and Py-DAA exhibit varying degrees of blueshift compared to the aggregate state (Δ < 0), indicating that the dimer structures of the substituted Py derivatives in the aggregate state and crystal state differ to some extent. This difference may be attributed to the dominance of the electronic effect of the Py scaffold during the rapid formation of aggregates, favoring discrete dimers and resulting in sky-blue or green emission (wavelength range of 460–510 nm). In contrast, the substitution groups control the slow crystallization process, leading to the dark blue emission of Py-DAA (422 nm), the sky-blue emission of Py (465 nm), the green emission of Py-CyA (495 nm), and the yellow emission of Py-CHO (543 nm). The various fluorescence properties exhibited by the aggregate and crystalline states provide more insights into the critical influence of the dimer structure of the Py scaffold on the macroscopic luminescent properties. Understanding the structure-property relationship of the Py scaffold dimers is beneficial for inferring the aggregate state structure of compounds and making rational molecular designs through a “holistic” approach.

Crystal conformation and packing modes

To further understand the structure-property relationship in the aggregate state, the single-crystal structures of these four compounds were obtained (Figure 4; Table S1). As shown in Figure 4A, Py owns a planar conformation with a short distance (3.535 Å) and a large overlap (60.2%) between adjacent Py molecules, resulting in dimer emission. The stacking pattern indicates a discrete arrangement, which leads to strong sky-blue fluorescence with a QY of 42.4%. Similarly, Py-CHO possesses a relatively planar conformation (with a torsion angle of only 2.61°). Compared to Py, the distance between adjacent Py scaffolds in Py-CHO is smaller (3.475 Å), and the overlap is larger (63.8%), resulting in stronger π-π interactions. Additionally, the electronic effect brought by the aldehyde group endows Py-CHO with a head-to-tail and long-range ordered stacking, resulting in exciton diffusion and quenching.^52,53,54 Therefore, Py-CHO shows weaker yellow fluorescence with a QY of 16.7% (Figure 4B). Py-CyA and Py-DAA with larger substituents exhibit torsion angles of 21.23° and 19.42°, respectively, causing conformational distortion. The stacking of Py-CyA with a smaller steric hindrance group is similar to Py. However, with a discrete dimer alternating arrangement but a large overlap (65.3%), Py-CyA exhibits redshifted green fluorescence with a QY of 43.5% (Figure 4C). In the case of Py-DAA with the bulky rosin group, significant steric hindrance and a looser arrangement during crystallization are formed, resulting in almost no overlap of Py moieties in Py-DAA and the bluest and weakest fluorescence from its monomer (QY = 2.3%) (Figure 4D). Intermolecular interaction analysis and Hirshfeld analysis based on single-crystal structures also support their orbital overlap, different packing modes, and intermolecular interactions (Figures S15–S19). From the above results on the structure-property relationship in the crystalline state, it can be concluded that substituents can regulate the structure and properties of Py aggregates. Bulky substituents can disperse the aggregate of Py, resulting in a blueshifted and weaker fluorescence. Middle steric substituents have a minimal impact on the emission behavior of the aggregate, while substituents with both planarity and electron effects can lead to redshifted and weaker fluorescence. This explicit structure-property relationship in the aggregate state suggests that we can infer the aggregate structure of compounds and make rational molecular designs using a holistic approach.

Figure 4.

Crystal conformation and packing modes of Py and its derivatives

Molecular conformation, packing, and dimer of (A) Py (discrete dimer), (B) Py-CHO (long-range dimer), (C) Py-CyA (discrete dimer), and (D) Py-DAA (monomer).

Mechanochromic and acidochromic behaviors of Py-DAA

The above discussion highlights the vital role of the aggregate structure in macroscopic properties. Therefore, it is possible to switch macroscopic properties by changing the aggregate structure under external stimuli. Taking Py-DAA as an example, the original sample exhibits a faint dark blue fluorescence. However, after grinding, it transforms into a bright green fluorescence (Figures 5A and 5B), showing significant mechanochromic luminescence (MCL) properties. PXRD measurements were performed to examine the relationship between the solid fluorescence variations displayed by Py-DAA and its molecular morphological structure (Figure 5C). The PXRD spectra mainly exhibit distinctions at 8.0°, 12.8°, 15.4°, 16.0°, and 18.9°. After grinding, the diffraction peak at 12.8° disappeared, a new diffraction peak appeared at 18.9°, and the proportion of diffraction peaks at 8.0°, 15.4°, and 16.0° changed significantly, indicating that the change in fluorescence color is due to the alteration of the packing arrangement before and after grinding, which suggests that the Py units transition from a non-overlap state to a partial overlap state under the external force.⁴⁰ As expected, it successfully regulates macroscopic properties by modifying the aggregate structure.⁵⁵ However, fuming by volatile organic solvents or heating fails to restore it to the initial state because the strong π-π interactions between Py units could not be sufficiently disrupted. In addition, ground Py-DAA is sensitive to acid and base stimuli. When fuming with TFA, its green fluorescence changes to red. Then, when fuming with TEA, it reverts to the original fluorescence with a dark blue color. Similarly, the above color change could also be achieved by fuming the crystalline sample with TFA and TEA vapors. NMR spectra illustrate that TFA protonates the imine bonds in Py-DAA^37,56 (Figures 5D and S20). The absorption spectra show that the protonated Py-DAA exhibits an additional absorption peak of charge-transfer (CT) state within 400–500 nm (Figure 5E). Moreover, theoretical calculations based on DFT indicate that both the HOMO and the LUMO are located on the Py scaffold before the protonation of Py-DAA. After protonation, the significant CT process with a decreased energy gap is consistent with their redshifted emission (Figure 5F; Table S6). Py-CyA also exhibits the same acidochromic luminescence (ACL) property (Figures S21 and S22). Therefore, we can synergistically alter the macroscopic optical properties by separately altering the molecular and aggregate structures through external stimuli.

Figure 5.

Mechanochromic and acidochromic properties of Py-DAA

(A–C) Fluorescence photographs (A), PL spectra (B), and PXRD spectra (C) of Py-DAA under various treatments.

(D) NMR spectra of Py-DAA before and after the addition of TFA.

(E) Absorption spectra of Py-DAA solid sample before and after fuming by TFA in pure EtOH solution. Concentration: 20 μM.

(F) Frontier molecular orbitals and corresponding energy levels of Py-DAA before and after fuming by TFA.

Mechanochromic and acidochromic applications of Py-DAA

Stimuli-responsive materials have a wide range of applications in smart materials, such as anti-counterfeiting and encryption-decryption.⁵⁷ The multi-stimuli responsive properties of Py-DAA hold the potential to achieve dynamic encryption-decryption of information, thereby enhancing information security (Figure 6). We attempt to utilize this feature to design and develop unique encryption-decryption systems. On the “paper” made by mixing a pristine solid of Py-DAA with other dyes (without ACL properties), a lotus flower is drawn, thereby inputting information. The true information contained within, a “rhombus” pattern, only becomes visible after TFA fuming. However, after TEA fuming, both the lotus flower and rhombus disappear and are returned to a new paper (Figure 6A). Apart from the solid state, using Py-DAA solution as ink can also achieve stimuli-responsive effects, such as the lotus flower drawn with Py-DAA solution turning red after TFA fuming (Figure 6B). Therefore, using this feature to draw “treasure maps” can achieve encryption effects. As shown in Figure 6C, multiple paths are drawn in the treasure map, while only the correct path can be revealed by TFA fuming, with the red path indicating the safe route. A three-dimensional (3D) code consisting of solid Py-DAA in different states and dyes of the same color (without ACL properties) can be set next to the treasure chest. The 3D code containing incorrect information needs to be transformed into the correct code by TFA fuming, which contains the correct password, “rosin,” in the treasure chest, allowing access to the treasure, “natural rosin.” Thus, the only way to find the treasure is with both the correct route and 3D code (through TFA fuming). In addition to fluorescence (Figures 6B and S23), the Py-DAA solution is also expected to be used as “invisible ink” for information storage and decryption under sunlight (Figure S24). Compared to traditional single-stimulus response materials, the unique multi-stimulus response properties endow Py-DAA with better privacy and display the tremendous potential of this material in anti-counterfeiting and encryption-decryption applications. Furthermore, low-cost and readily available raw materials, along with a simple one-step synthesis, enable Py-DAA to easily reach gram-scale production, thus paving the way for further industrialization.

Figure 6.

Dynamic application of Py-DAA

(A) Application of Py-DAA based on solid state.

(B) Application of Py-DAA based on solution state.

(C) The multilevel encryption system of the "treasure, i.e. natural rosin".

Conclusion

The aggregate structures play a crucial bridging role in investigating the structure-property relationships of single-molecular structure and macroscopic photophysical properties. However, so far, the design of aggregate structures still faces challenges. In this work, Py has been adopted as an aggregate model to construct a series of derivatives by introducing substituents of -CHO, -CyA, and -DAA with different electronic and steric effects. The results indicate that factors such as oxygen, substituents, and molecular motion exert significant impacts during the aggregation process, leading to substantial differences in the optical properties of single-molecule states and aggregate states. Py possesses a large planar and conjugated structure. With a largely planar and conjugated conformation, Py exhibits anomalous AIE properties resulting from the combination of oxygen and aggregation. The highly unexpected small QY of 3.3% at the molecular level uncovers that oxygen quenches the triplet exciton, leading to efficient non-radiative decay in the monomer state. However, aggregation can isolate oxygen, leading to dimer emission and an increase in QY. Although introducing substituents induces molecular motion and weakens the emission intensity in the monomer state, their influence on the aggregate-state fluorescence properties is totally distinct. The introduction of a small and electronically conjugated substituent (-CHO) leads to long-range ordered stacking, resulting in redshifted and weakened emission. On the other hand, the larger substituent (-CyA) disrupts long-range stacking, resulting in discrete dimers with blueshifted and enhanced emissions close to those of the Py skeleton. Incorporating the bulky natural rosin structure (-DAA) with steric hindrance leads to the monomer mode in the crystalline state, resulting in intense molecular motion with the bluest and weakest emission. Furthermore, the aggregate structures can be changed by multiple external stimuli. Interestingly, Py-DAA exhibits an MCL property due to changes in the aggregate structure under external force, while the protonation of imine bonds leads to an ACL property. The synergistic effect of MCL and ACL provides excellent potential in dynamic encryption and decryption of information. This work not only elucidates the unique AIE properties of Py for the first time but also clarifies the comprehensive effects of oxygen, substituent, molecular motion, and packing on the aggregation process of Py, realizing initial control over aggregate structures. In addition, this work also provides future directions for designing organic luminescent materials with synergistic effects of aggregate structure and oxygen and expanding the biological and stimuli-response applications related to oxygen environment, such as hypoxia examination and imaging of cancer cells.

Resource availability

Materials availability

The materials generated in this study are available from the lead contact upon reasonable request.

Data and code availability

All data are available from the lead contact upon reasonable request.

Funding and acknowledgments

This article was supported by the National Natural Science Foundation of China (21601087); the Natural Science Foundation of Jiangsu Province (BK20231296); the Open Fund of Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, Guangzhou 510640, China (South China University of Technology [2023B1212060003]); the Shenzhen Key Laboratory of Functional Aggregate Materials (ZDSYS20211021111400001); the Science Technology Innovation Commission of Shenzhen Municipality (KQTD20210811090142053); and the Key Research and Development Project of Yunnan Province (202303AC100010). We also thank Jiaqin Liang from Guilin Songquan Forest Chemical Co., Ltd., for providing DAA, the AIE Institute (www.aietech.org.cn), and Guangdong Basic Research Center of Excellence for Aggregate Science for providing some AIE materials and technical assistance, as well as Professor Zhipeng Liu and Professor Xing Feng for valuable discussions.

Author contributions

X.-M.C., Z.Z., Q.Y., and B.Z.T. supervised the project. J.Z., X.-M.C., and Z.Z. conceived and designed the experiments. Y.L. and Z.T. performed the synthesis. Y.L., W.-J.W., Z.T., and X.F. conducted the measurements and analyzed the data. J.Z. performed the theoretical calculation. Y.L., J.Z., W.-J.W., X.X., X.-M.C., and Z.Z. took part in the discussion and gave important suggestions. Y.L., J.Z., F.E.K., and X.-M.C. co-wrote the paper. All authors contributed to the manuscript and approved the final version.

Declaration of interests

Y.L. and X.-M.C. are the inventors of the methodology patented by China National Intellectual Property Administration (patent no. CN202410008799.2).

Published Online: March 17, 2025

Lead contact website

https://www.x-mol.com/groups/cai_xumin