Qian
Zhou
,
Mingxia
Feng
,
Caihong
Shi
,
Mengqiu
Qian
,
Xiurong
Ma
,
Runying
He
,
Xian
Meng
,
Yonggang
Shi
,
Qiue
Cao
and
Liyan
Zheng
*
Key Laboratory of Medicinal Chemistry for Natural Resource of Yunnan University, Ministry of Education, School of Chemical Science and Technology, Yunnan University, Kunming 650091, P. R. China. E-mail: zhengliyan@ynu.edu.cn
First published on 6th May 2025
Polymorphism is defined as the ability of a substance to exist in two or more crystalline forms, which provide a unique platform for revealing the relationship between its spatial structure and properties. However, organic crystal polymorphism can be commonly obtained by growing crystals in different solvents or at different temperatures. This study reports a compound named p-An-Br containing carbazole and anthracene chromophores with three multiple stimulus modulated crystal polymorphs with green, yellow and red fluorescence, respectively. Interestingly, switching of p-An-Br between crystal G, crystal Y and crystal R can be achieved through the uptake and release of methanol using different stimuli, which shows dynamically adjustable luminescent colors. Significantly, structure–property investigations via the in-depth analysis of molecular conformations and frameworks of the polymorphic crystals demonstrate that the diverse conformations and abundant noncovalent interactions have a predominant impact on emission behavior. Consequently, the crystals R can be used for the highly sensitive and specific sensing of methanol with a detection limit of 39.35 ppm. This study not only provides a new strategy for crystal polymorphism, but also develops an effective method for the detection of methanol.
![]() | ||
Scheme 1 (a) Previous examples of the polymorphism of crystals in the literature. (b) A brief introduction of the polymorphism of p-An-Br crystals with the color switch process in this work. |
Luminescent materials that respond to external stimuli (such as light, heat, electricity, and other chemical stimuli) have significant application potential in cutting-edge fields of optoelectronic devices,11 biochemical sensing,12 imaging,13 and anti-counterfeiting encryption,14 because of their dynamic,15 reversible,16 and adaptive conversion characteristics.17 Among various external stimuli, light is considered an ideal candidate due to its ecofriendly and noninvasive nature, spatiotemporal operation, and convenient and precise control.18 These luminescent materials can indicate the stimulus–response behavior through changes in their luminous color, intensity, and lifetimes.19 Among them, the variation of luminescent color has aroused great interest due to the unique advantages of high spatial resolution and in situ/real-time visualization.20 However, there are still large challenges in controlling the emitting color because luminescence wavelength usually depends only on the lowest energy level of the excited state and is independent of the initial excited state.21 Although several strategies, including the introduction of multiple emission centers,22 energy transfer,23 and dynamic chemical reactions,24 have been employed to regulate luminescence color, these approaches require complex molecular designs, matched host–guest energy levels, or even extra reaction reagents. Consequently, developing a fast and dynamic light-color adjustment luminescent material is of great significance.
Herein, a new compound named p-An-Br with three crystal polymorphs displaying different emission colors was reported. Tunable emission colors from green and yellow to red can be achieved through the uptake and release of methanol by illumination, heating and fumigation (Scheme 1). More significantly, the underlying mechanism of crystal polymorphism-dependent fluorescence emission was well revealed by analyzing the different molecular conformations and diverse frameworks of the polymorphic crystals. Methanol vapor can enter crystal R through the host–guest interaction to transform crystal R into crystal G, thus achieving dynamic switching of colorful luminescence between crystal G and crystal R, which can be applied in highly sensitive and selective sensing for methanol vapor.
Because compounds exhibiting an ICT effect tend to be more responsive to molecular stacking, the solid-state photophysical properties of p-An-Br were further explored. Pristine p-An-Br was a yellow crystalline powder with green fluorescence (508 nm, ΦPL = 14.5%) under UV light irradiation. Upon full grinding with a mortar and pestle, the yellow powder was converted to a brown powder and the absorption band underwent redshift (Fig. 1b). The maximum emission peak redshifted from 508 nm to 544 nm, corresponding to a fluorescence color change to yellow (ΦPL = 12.3%) (Fig. 1c). When ground powder was exposed to methanol for 3 min, it returned to its original state and could be cycled multiple times, indicating that the mechanical-chromatic behavior has good reversibility (Fig. S8†). The fluorescence lifetimes of p-An-Br before and after grinding were also measured (1.13 and 2.30 ns), indicating that the emission belonged to the fluorescence (Fig. S9†). These results indicate that molecular configurations, inter-molecular interactions and packing arrangements can be changed under an external force stimulus. Powder X-ray diffraction (PXRD) measurements were carried out to gain insight into the crystal structure change upon grinding. For the pristine powder, quantities of sharp scattering peaks were observed (Fig. S10†). After grinding, the scattering peaks vanished, indicating a grinding-induced phase transition from the crystal phase to an amorphous state.
In addition, the solid powder of p-An-Br exhibited photochromic fluorescence properties. Upon continuous ultraviolet (UV) light exposure, the fluorescence color red-shifted gradually from green (502 nm) to yellow (553 nm) (Fig. 1d). The yellow fluorescence can revert back to green fluorescence through methanol vapor treatment, and this process can be repeated cyclically, indicating good reversibility of the photochromic process (Fig. S11†). PXRD spectra were measured but no obvious change was found (Fig. S12†), indicating that the molecular structure was unchanged after UV light irradiation. Besides, the fluorescence lifetimes of p-An-Br before and after UV light irradiation were also measured (1.13 and 2.35 ns), revealing that the emission belonged to the fluorescence (Fig. S13†).
The fluorescence spectra of single crystals at different UV light irradiation times were also recorded. As shown in Fig. 2b, the emission wavelength of the single crystal shifted from 520 nm to 600 nm. Furthermore, the CIE chromaticity coordinate diagram also illustrated this redshift (Fig. 2c). Crystal G exhibited green emission (520 nm) with short lifetimes of several nanoseconds. In contrast, crystal R showed the redshifted, structureless and broadened photoluminescence (PL) spectrum with emission at 600 nm and long lifetime (τ = 34.50 ns), which is in good agreement with the characteristic of the anthracene excimer according to the previous reports (Table 1 and Fig. S16†).25 The photoluminescence quantum yields (PLQYs) of p-An-Br in different aggregation states were measured at room temperature. The rate constants kr and knr were estimated from the experimental PLQYs and the corresponding lifetimes, and the data are summarized in Table S1.† Comparatively, crystal R with long-wavelength emission showed significantly longer τ and lower knr than the other two crystal polymorphs with short-wavelength emission.
Materials | Obtained conditions | Emission wavelength (nm) | PLQY | Lifetimes (ns) |
---|---|---|---|---|
Crystal G | Liquid-phase diffusion (methanol/ethyl acetate) | 520 nm | 12.6% | 3.43 |
Crystal Y | Illuminating the crystal G with a UV lamp for 30 minutes | 550 nm | 13.8% | 3.26 |
Crystal R | Illuminating the crystal G with a UV lamp for 24 h | 600 nm | 12.9% | 34.50 |
In the crystal G, the torsion angles between the pyridine ring and anthracene were 84.93° and 85.06° (Fig. 3a). Thus, the carbazole unit exhibited a planar quasi-equatorial (QE) conformation, which was favorable for charge delocalization and strengthened the electron-donating ability and ICT effect. Additionally, the carbazole unit presented a crooked QE conformation, and the twisted angles between benzene and pyridine rings of the conformational molecule were larger (5.01° and 4.52°) than those of crystals Y and R (Fig. S17†), resulting in a weak ICT effect of the p-An-Br G crystal. Eight methanol solvent molecules are involved in the unit cell, resulting in a looser packing pattern and weaker intermolecular CT interaction. There are obvious C–H⋯O interactions to be observed between p-An-Br and solvent molecules, corresponding to the monomer emission in crystal G (Fig. S18†). As a comparison, there is no solvent molecule in crystals Y and R. Later, the adjacent molecules were connected through abundant C–H⋯π (2.734–2.884 Å) interactions and C–H⋯Br interactions (2.787–2.840 Å) (Fig. 3b). Therefore, the p-An-Br G crystal showed high excited state energy, thus resulting in a blue-shift emission compared to p-An-Br-Y and p-An-Br-R crystals.
![]() | ||
Fig. 3 The single-crystal structure, intramolecular interactions and molecular packing patterns of (a–c) crystal G, (d–f) crystal Y, and (g–i) crystal R. |
Comparatively, the carbazole unit of the p-An-Br Y crystal adopted a less crooked QE conformation (Fig. S17†). This could facilitate the conjugation between nitrogen lone pair electrons with the adjacent acceptor, leading to a more localized electronic transition upon excitation. Accordingly, the twisted angles between benzene and pyridine rings were smaller (4.43° and 4.25°) than those of crystal G. Therefore, the ICT effect of the p-An-Br Y crystal was strengthened, which led to a red shift to yellow emission. The more crooked forms (85.66° and 85.19°) of the p-An-Br moiety in crystal Y are considered the other inner reason for its red shift of emission (Fig. 3d). Furthermore, a three-dimensional (3D) supramolecular framework with the cavity units was stacked by the different conformers, which were linked via abundant intermolecular C–H⋯π (2.743 Å) and C–H⋯Br (2.759–2.825 Å) (Fig. 3e and f).
In addition, the torsion angles between the pyridine ring and anthracene in crystal R are 86.71° and 86.48° (Fig. 3g). Moreover, the twisted angles between the benzene and pyridine rings were only 3.80° and 4.02° for crystal R, respectively. Such tiny twisted angles in the acceptor moieties were beneficial for boosting the electron-withdrawing ability. These results led to the bathochromic-shift emission. Compared to crystal G and crystal Y, abundant intermolecular C–H⋯Br (2.821–2.904 Å), C–H⋯π (2.725–2.872 Å) and π⋯π (3.318 Å) interactions existed in the R-crystal (Fig. 3h). There were no π–π interactions between p-An-Br molecules in crystals G and Y due to the highly twisted p-An-Br moiety. Furthermore, more intermolecular π–π interactions in molecular packing of crystal R induce the emission red-shift (Fig. 3i).
Overall, the molecular conformations and packing modes in crystals have a significant influence on the fluorescent properties. The stacking arrangement of crystal G differs slightly from that of crystals Y and R, resulting in different fluorescence colors. As shown in Fig. 3c, f and i, the detailed stacking diagram illustrates the differences among the three polymorphs (G, Y, and R). In G crystals, methanol molecules exist in the framework voids through C–H⋯O interactions, while there are no such interactions in crystals Y and R. The stacking of the three types of crystals depends on the C–H⋯Br and C–H⋯π interactions, especially in crystal R where there is also a π⋯π interaction, resulting in a tighter stacking of crystal R and a red shift in crystal wavelength. When stimulated using pressure, the packing structures are destroyed and the transformation of the molecules from the crystalline state into an amorphous state results in the redshift of the emission. Besides, methanol molecules were found within the pores of single-crystal structures of crystal G, indicating that methanol plays a crucial role in determining the emission of single crystals. When the crystal R is placed in a methanol atmosphere, methanol molecules are absorbed into the pores of the crystal, transforming it into crystal G, thus achieving a reversible change.
To further confirm that the crystal is closely related to the fluorescent emission, PXRD patterns of the three crystals were simulated and are shown in Fig. 2d. The PXRD patterns of the three crystals are consistent with their simulated ones, indicating the high purity of the three crystals (Fig. 2d). The PXRD patterns of crystal Y and crystal R were similar. Compared with crystals Y and R, crystal G has no diffraction peaks at 5.2, 15.8, and 17.2 degrees, indicating a difference in the structure from crystals Y and R and variations in its single crystal structure. These results implied that p-An-Br possessed different aggregation states, and the molecular conformation and stacking arrangement of single crystals could provide a deep insight into the underlying luminescence mechanisms.
We further examined the possibility of excimer formation by the concentration-dependent PL spectra in the solutions. The results demonstrated that p-An-Br in high-concentration tetrahydrofuran (THF) solutions (such as 10−2 and 10−1 M) showed new emission bands which were almost the same as that of crystal R (Fig. S19†). Therefore, p-An-Br was capable of forming the excimer with the close AN dimer in solids and solutions. For the individual AN fluorophore without p-DPC, there was no emerging emission band in high-concentration THF solutions (Fig. S20†), also indicating that the molecular design was in favor of excimer formation. We tested the solution sample and the ground powder of p-An-Br, but no photochromism was observed (Fig. S21†), implying that the light-induced molecular motion may play a vital role in the crystal-state photochromism. In situ Fourier transform infrared (FT-IR) spectra were recorded but no change was found (Fig. S22†), indicating that the molecular structure was unchanged after UV lamp irradiation.
Considering the presence of methanol within crystal G, it is possible to remove methanol through heating. Upon heating, the crystal undergoes a process of turning yellow first and then red (Fig. S23†). This should be attributed to the volatilization of methanol in the lattice, leading to the crystalline transition from crystal G into crystal R. In order to verify the specific recognition mechanism of crystal R for methanol, the NMR spectra of crystal R before and after methanol fumigation were measured. As shown in Fig. S24,† additional C and H peaks of methanol appeared in the NMR spectrum after fumigation, indicating that methanol molecules entered the crystal. The thermogravimetric analysis (TGA) data indicate that compared to crystal Y and crystal R, crystal G exhibits an additional weight loss peak in the range of 96–165 °C with a thermal weight loss of 3.37% (Fig. S25†), which corresponds exactly to the relative mass of methanol molecules in the lattice, indicating that the entry of methanol molecules into the lattice of crystal R is the key factor for the recognition of methanol. We attempted to recover crystal G from crystal R by fuming crystal R with methanol vapor (Fig. S26†). As expected, after fuming crystal R for 2 min, the luminous color of the crystal R changes to yellow and then completely turns into green after continuous fuming for 10 min. The fumed crystal can be restored to its initial state by heating for 5 min. The PXRD patterns of the fumed and heated samples are almost consistent with those of crystal G and crystal R (Fig. S27†), implying the single-crystal to single-crystal structural transformations in the process of methanol adsorption. At the same time, it is shown that heating and methanol fumigation can achieve a reversible switch between crystal G and crystal R. While crystal R, which has no solvate, had voids in the crystal structure (Fig. S28†) to promote molecular motions, methanol solvent could occupy the voids through host–guest interaction. These results confirm that the recognition mechanism of methanol is probably induced by the interactions between guest methanol molecules within the host framework. The uptake and release of methanol molecules play a crucial role in the structural switching among these crystal polymorphs.
Molecular dipole moment is an indicator of charge separation, and large charge separation can cause strong CT effects, resulting in long wavelength emission. Due to the strong CT effect between ICT or TICT states, their dipole moments are greater than those of LE states. When the dipole moment of a molecule changes, its electronic and vibrational energy levels also shift accordingly, which usually leads to a shift in the emission wavelength, depending on the structure of the molecule and the external environment. During the photochromic process, rotation of the pyridine ring and anthracene led to an obvious change in the vector direction of molecular dipole moment. Orientations of the transition dipoles for the monomers of crystals G, Y and R were almost antiparallel and led to a sharp decline for the dipole moment of the dimer. Besides, the photochromic process can also affect the regular arrangement in the crystalline state. Crystallinity is equal to the proportion of the crystalline fraction in the whole, which refers to the degree of structural order of a solid material. Noticeably, each asymmetric unit in the crystal G consisted of a dimer with moderate intermolecular C–H⋯π interactions and its dipole moment was calculated to be 5.23 debye (Fig. 4d). Compared to the dipole moment of the monomer in crystal G (3.01 debye), special packing mode greatly enhanced the total dipole moment of p-An-Br in the crystalline state and promoted intense luminescence. During the process of photochromism, the production of free radicals and the strengthened π–π interaction (crystal R) led to an obvious change in the molecular dipole moment. The formed radicals reduced the molecular charge, while dimers are more tightly packed than monomers, resulting in a greater change in dipole moment. The dipole moment of crystal R drastically decreased from 3.68 D (monomer) to 2.35 D (dimer).
To quantitatively detect the concentration of methanol, crystal R was fumigated with different concentrations of methanol vapor for 10 min and then taken out for emission spectroscopy testing. As shown in Fig. 5c, the higher the methanol concentration, the more pronounced the change in the proportion (I600 nm/I520 nm) of the emission peak intensity of crystal R and crystal G. The degree of the proportion of emission peak positively correlates with methanol concentrations in the 5–70 ppm range (Fig. 5d). Based on the methanol concentration and the proportion of the emission peak of the crystal R and crystal G, the standard working curve y = −0.0206x + 1.9498 (R2 = 0.9939) can be obtained to give a limit of detection (LOD) of 39.35 ppm. Compared to other reported chemical sensors, the LOD value is one of the lowest detection limits (Table S7†), which is significantly lower than the threshold limit value (TLV) of methanol (200 ppm), where the TLV is defined as the permitted maximum concentration of a chemical during a working day for continuous exposure without producing adverse health effects.27,28 In addition, the detection sensitivity of crystal R is also far beyond that of most of the previously reported methods for detecting methanol, such as NMR, Raman and fluorescence spectroscopy, with the advantages of strong specificity, fast response, high sensitivity, and visualization.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2419510, 2419513 and 2419515. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5sc01503k |
This journal is © The Royal Society of Chemistry 2025 |