Reversible piezofluorochromic nature and mechanism of aggregation-induced emission-active compounds based on simple modification

Abstract

Simple modification of triphenylamine generates four luminophores with aggregation-induced emission characteristics. Three of them can be smartly switched in fluorescence emissions by various external stimuli such as grinding, annealing and solvent-fuming, which may be attributed to the transformation from the crystalline state to the amorphous state and vice versa.

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Stimuli-responsive luminophores have been considered as one of the most important organic materials due to their potential applications in sensors or optical information storage,¹ which were frequently constructed by varying the substituent groups. However, the disadvantage of irreversibility makes it difficult to alter fluorescence simply by chemical modification in the solid state. One promising approach to provide the materials with reversible color transition is to control the mode of molecular packing by force. However, the fluorescence efficiency of organic luminescent materials often becomes weak at high concentrations or in the solid state as a result of the ACQ (aggregation-caused quenching) effect.

Several anti-ACQ materials were reported by Tang et al. and Park et al.² in 2001 and 2002, respectively, and these substances are termed aggregation-induced emission (AIE) and aggregation-induced emission enhancement (AIEE) materials. Some AIE or AIEE materials among these functional materials not only exhibit strong emission in the solid state but also possess stimuli-responsive switching properties. Hence, taking advantage of AIE-active materials as promising candidates to develop stimuli-responsive materials has a promising future. Although a lot of research has been carried out in this area, the piezofluorochromic mechanism is still under debate. The structure–property relationships of these compounds need to be further studied to help the design and synthesis of more novel stimulus-response materials.

During our research for new AIE fluorophores based on triphenylamine,³ we accidentally found that the reactant 4-diphenylamino-benzaldehyde is not only AIE-active but also PIE(piezofluorochromism-induced emission)-active, which conformed with the concept of PAIE (piezofluorochromic aggregation-induced emission).⁴ With this in mind, we made some simple modifications to triphenylamine to further study the structure–property relationship of PAIE materials. It was found that the emission of those AIE materials could be switched between two different fluorescent colors by modulating their molecular packing pattern.⁵

The target compounds (Scheme 1) were prepared according to the synthetic routes shown in Scheme S1.† We make a full description of the synthetic approach and the characterization of the corresponding compounds in the ESI.†

Scheme 1 Chemical structure of compounds 1–4.

Fig. 1a and b show the PL (photoluminescence) spectra and UV-vis absorption spectra of 2 in solution mixtures with different water fractions. Compound 2 exhibited a very weak PL in pure DMF (dimethyl formamide). However, when the water fraction (f_w) is increased to 80%, the PL intensity was significantly enhanced. The enhancement of intensity from 91 to 282 a.u. was observed as the water fraction is increased from 80% to 90%. Since water is a non-solvent of 2, the molecules may aggregate and the free rotations were restricted, and thus, the fluorescence emission was induced.

Fig. 1 (a) PL spectra of 2 in the DMF–H₂O mixture with different water fractions. Concentration of 2: 10 μM; excitation wavelength: 365 nm. The top inset shows the changes in the PL peak intensity. The bottom inset shows the emission photos of compound 2 in pure DMF as well as solvent mixtures with 10%, 80% and 90% water fractions taken under 365 nm UV illumination; (b) UV-vis absorption spectra of 2 in DMF–H₂O mixtures with different water fractions; (c) normalized PL spectra of 2 powder upon repeated fuming and grinding; (d) maximum emission wavelength change versus repeating cycles (auxiliary photos: luminescence of solids for 2 taken at room temperature under UV light after grinding and after initializing by fuming process).

As seen in Fig. 1b, almost no change appears in the absorption curves when the water fraction is increased from 0% to 90%. However, as the water fraction is increased to 99%, a red shift of the absorption curve with a level-off tail can be obviously observed, which provided us with further evidence of nanoscale aggregation formations in the aqueous mixtures with higher water fractions. The results above indicate that the emission is induced by nano-aggregation, and thus, 2 is AIE-active. Similar phenomena can also be observed for the other compounds (Fig. S10 and Fig. S11†). Herein, 1 exhibited strong yellowish green photoluminescence when its pure solution is photoexcited; however, when the water fraction (f_w) is increased, a significant decrease in the emission intensity can be observed. This phenomenon may be attributed to an increase in the solvent polarity and energy loss in the excited state caused by the enhanced ICT (intramolecular charge transfer) process.⁶ From pure DMF to mixture of 10% f_w, the solvation and ICT process are dominant, which may contribute to the non-radiative excitation caused by the free rotation of the σ bonds, and thus, no light emission can be observed. However, the PL intensity was swiftly promoted with a water fraction of 80%. Although water was further added, solvent polarity may no longer have a considerable influence on the optical properties. A positive solvatochromism of 1 can be observed in Fig. S12† and the maximum emission wavelength in polar solvents' red shifts as compared to those in apolar solvents. Indeed, according to the molecular simulation, the electron distribution in the HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) levels are located around the triphenylamine and aldehyde/malononitrile groups (Fig. S13†), respectively, which is indicative of the ICT process and demonstrates our previous inference.

The as-synthesized compound 2 exhibited strong yellow luminescence (581 nm) in the solid state. After grinding, the sample showed a strong orange emission (602 nm) with a red shift of 21 nm (Fig. 1c and Fig. 1d, others in Fig. S15 and Fig. S16†), which is a typical mechanochromic phenomenon. Upon further fuming with DCM (dichloromethane) vapor for 5 minutes or annealing at 80 °C for 10 minutes, the original color can be restored. The conversion between yellow and orange emission colors can be repeated many times without fatigue.

Fig. 2 shows the real-time pictures of the evaporation procedure of drops of solutions 1–4 in dichloromethane placed on a filter paper. During the early development, the wet spots could scarcely be visualized under irradiation at 365 nm by a UV lamp (Fig. 2a). As the solvent evaporated, all the spots except 4 changed to a brighter color (Fig. 2b) and finally reached full-size brightness (Fig. 2c) after the solvent completely volatilized, which further indicates that compounds 1–3 are AIE-active and have potential applications as security inks. Herein, no obvious change in the fluorescence emission intensity of 4 can be observed, which corresponds with the weak AIE properties, as shown in Fig. S10.†

Fig. 2 Photographs of several drops of solutions 1–4 in CH₂Cl₂ taken on a filter paper under irradiation at 365 nm (time: a = 2 s, b = 5 s, c = 9 s).

The as-synthesized sample of 2 was outspread on filter papers and labeled as “AHU” with a metal spatula in contrast to the yellow surrounding regions. Under a handheld UV lamp, the letter “AHU” emits orange light, which can be erased by either vapor fuming or thermal annealing treatment (Fig. 3a). Thus, various kinds of patterns can be recorded simply by handwriting and be erased by annealing or fuming, which makes 1–3 as promising candidates for optical recording and pressure-sensing systems.

Fig. 3 (a) Photographs of 2 in filter paper upon piezowriting–erasing by fuming, annealing and grinding under UV light (365 nm); (b) SEM of the as-synthesized sample; (c) the ground sample of 2; (d) WAXD of 2: (A) as-synthesized sample, (B) ground sample, (C) fuming B in CH₂Cl₂ vapor for 5 minutes, (D) annealing B at 80 °C for 10 minutes.

Fig. 3b and c show the scanning electron microscope (SEM) images of the as-synthesized and ground sample of 2, respectively. As shown, the as-synthesized sample shows a rectangular shape with different lengths. After it was ground, the crystal particles were destroyed and broke up into amorphous pieces, resulting in a crude surface. This process leads to the red shift in the emission of 2 from yellow to orange. To gain further insight into the mechanism of the piezofluorochromic effect, we also carried out the wide-angle X-ray diffraction (WAXD) measurement. According to the WAXD data of the as-prepared 2 solid (Fig. 3d-A), the diffraction pattern exhibited multiple strong and sharp diffraction peaks, indicating its crystalline structure. In contrast, no noticeable diffraction peak can be observed in the profiles of the ground sample (Fig. 3d-B), which implies that most of the crystalline structures were destroyed and transformed to the amorphous state. However, the WAXD curve diffraction peaks reemerged after simple fuming or annealing treatments (Fig. 3d-C and D), which realized the reversible conversion of piezofluorochromism. This interesting phenomenon can also be observed for other compounds except for 4. With regard to 4, no diffraction peaks can be observed in the profiles of either the as-synthesized sample or the ground sample. Meanwhile, no obvious change can be observed in the change of the maximum emission wavelength (the fifth figure, as shown in Fig. S15†) and SEM images (Fig. S17†) when as-synthesized or ground, indicating that 4 is amorphous even when as-synthesized.⁷ These results demonstrate that the change of molecules from the crystalline state to the amorphous state can indeed induce significant PIE properties.⁸

The single crystals of compounds 2 and 3 (Fig. 4, single crystal of 1 in Fig. S19†) are easy to be obtained by recrystallization with ethanol. The highly twisted conformations, which support for the active intramolecular rotations in good solutions, can effectively consume the energies of the excitation state and subsequently make 2 molecules non-luminescent. However, when the molecules aggregate as nanoparticles, such rotations can be restricted. The crystalline structure shows that the molecules of 2 are packed in antiparallel patterns with weaker interactions (π⋯π, 3.484 Å) and hydrogen bonds (C–H⋯N, 2.608 Å), which can help to rigidify the conformations of the molecules, thereby yielding enhanced emissions.⁹ The highly twisted benzene rings in these crystal molecules can also prevent the formation of exciplex or excimer, which could further favor the boosted emissions. Fortunately, such twists were released when the conformation was planarized upon facile mechanical stimuli, which could also be restored upon thermal annealing or solvent fuming. The highly twisted stacking architectures could result in shorter effective conjugation lengths and bluer emissions, while the planar conformation could result in increased effective lengths and redder emissions.¹⁰ Similar twisted conformations can also be observed in the single crystals of 1 and 3. Obviously, the aromatic rings of one molecule in crystals of 3 are all restricted by the weak interactions or hydrogen bonds, which provide more twisted and compressible space than that of 2. Thus, a redder shift of the emission wavelength (nearly 50 nm) can be observed in contrast to 2 (nearly 20 nm) after grinding.

Fig. 4 (a) Molecular packing of 2 in single crystals with π⋯π (3.484 Å), C–H⋯N (2.608 Å) hydrogen bonds indicated; (b) molecular packing of 3 in single crystals with O⋯C (3.200 Å), C–H⋯O (2.523 Å) and C–H⋯O (2.661 Å) hydrogen bonds. The left insets show the photographs of the single crystal of 2 (yellow) and 3 (blue) irradiated under UV light (365 nm).

In summary, we have easily synthesized and fully characterized a class of triphenylamine derivatives. In spite of very simple molecular structures, all the compounds except 4 show excellent AIE activities. Besides, compounds 1–3 exhibit reversible off/on fluorescent switching feature to organic solvents and showed significant piezofluorochromic properties, which can be switched via external stimulation including grinding, annealing and fuming with organic vapors. Morphological changes from the crystalline phase to the amorphous phase may account for this phenomenon. Obviously, taking advantage of triphenylamine as a core to develop PAIE materials may be feasible and effective. Meanwhile, further modification by enlarging the substituent is also underway.

Acknowledgements

This work was supported by the Program for New Century Excellent Talents in University (China), the Doctoral Program Foundation of the Ministry of Education of China (20113401110004), the National Natural Science Foundation of China (21271003, 21271004 and 51372003), the Ministry of Education, the Natural Science Foundation of Education Committee of Anhui Province (KJ2012A024), the 211 Project of Anhui University, Higher Education Revitalization Plan Talent Project of (2013).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterization of compounds 1–4; HOMO and LUMO energy levels; change in the PL spectrum of 1–3 by repeated grinding–heating cycles, XRD profiles of 1–3 in different states; change in the PL spectrum, TEM images and XRD profile of as-synthesized sample and ground sample of 4. CCDC 994493, 990297 and 990298. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra03042g

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