As an important organic photofunctional material, spirooxazine usually does not show photochromism in the solid state because the π-π stacking between molecules hinders photoisomerization. The development of solid-state photochromic spirooxazines is crucial for practical applications but remains challenging.
In this work, Nankai University Associate Professor Han Jie and Jilin University Professor Yang Yingwei et al. designed and synthesized a series of spirooxazine derivatives (SO1-SO4). They carry bulky aromatic substituents at positions 4 and 7 of the skeleton, providing them with bulk for solid-state photochromism under mild conditions. All compounds SO1-SO4 have adjustable solid-state photochromism , excellent fatigue resistance and high thermal stability . It is worth noting that it only takes 15 s for SO4 to reach absorption intensity saturation, which is the fastest solid-state photoresponse of spirooxazine to the authors' knowledge. The X-ray crystal structure of the intermediate compound SO0 and the product SO1-SO2 as well as computational studies show that large aromatic groups can lead to a subtractive effect, thereby achieving photochromism of solid SO. The ideal photochromic properties of these spirooxazines open new avenues for their applications in UV printing, quick response (QR) coding and related fields. This work was published in "Advanced Materials" under the title "Tunable Photochromism of Spirooxazine in the Solid State: A New Design Strategy Based on the Hypochromic Effect".
[Photochromic Properties]
Figure 1. Structure and properties of SO4 molecules
The author took SO4 powder sample as an example to demonstrate the reversible photoswitching process, as shown in Figure 1. The solid sample immediately changes from white to blue (within a few seconds), with the blue gradually darkening as the irradiation time continues to increase to 15 seconds and reaches maximum intensity. Other samples SO1-SO3 also showed similar phenomena. The observed color changes are in good agreement with the UV-visible spectral results of SO0-SO4 powder (Fig. 2a). All SO1-SO4 have almost no background color, which is probably due to the obvious subtractive color effect caused by the introduction of a large number of substituents . Therefore, the bulky substituents on the SO skeleton may promote the photochromism of the solid, which preliminarily shows that the author's design strategy is feasible.
Figure 2. UV-visible spectrum absorption study
In order to further understand the influence of aromatic groups on photochromic properties, the author conducted a comparative study on the photoresponse behavior of SO1-SO4 powders. Figure 2c shows the absorption as a function of time at the absorption maximum. Under photostable steady state (PSS), SO1 takes approximately 80 s to reach the maximum absorption (maximum abs) value, while the SO2-SO4 samples reach the maximum abs value at 30 s, 25 s, and 15 s respectively at PSS. The time for SO1-SO4 to reach PSS gradually decreased from 80 s to 15 s, indicating that larger substituents can provide larger free volume for photoisomerization of molecules and promote solid photocolorization. Similar to the photochromic behavior in the powder state, compounds SO1-SO4 in thin poly(methyl acrylate) (PMMA) films also exhibit reversible color changes under alternating UV and visible light or heat treatment. After 80 s of UV light irradiation, new strong absorption spectra centered at 599, 612, 611, and 610 nm appeared in the SO1-SO4 doped film (Figure 2b), and the color of the transparent film simultaneously changed to purple or blue. Subsequently, when the PSS was reached by irradiation with 365 nm light at 25°C, the thermal fading behavior of the photochromic film was continuously monitored after stopping the irradiation. After stopping the irradiation, the absorption band decreased rapidly and completed fading within 1.2 h at room temperature (Figure 2d), accompanied by the color change from purple or blue to colorless. At the same time, after heating at 60°C for 1 min, the film exposed to ultraviolet light can return to its original state.
[Subtractive color effect analysis]
Figure 3. Subtractive color effect analysis
The author studied the color of the compound in the solid state (Figure 3a). The powders of SO and SO0 appear orange-red and dark yellow respectively. In contrast, with the introduction of spatial aromatic groups on the SO skeleton, the color of the SO1-SO4 powder changes from light yellow to white, which provides direct evidence for the subtractive color effect. Through theoretical calculations, the author reveals the relationship between the structure of SO1~SO4 and the subtractive color effect.The large dihedral angle between the aromatic group and the SO group will significantly hinder the conjugation between the two, further hindering the overlap of π electron clouds, which can also partially explain the reason why the background color of these compounds becomes lighter. Furthermore, as shown in Figure 3c, some of the isosurfaces were separated, indicating that the introduced aromatic substituents were not coupled to the SO moiety, resulting in a subtractive effect.
[Morphological changes of SO0~SO4 under solid-state conditions]
Figure 4. Morphological analysis
As shown in Figures 4a-4d, the author used scanning electron microscopy (SEM) to study the surface morphology of SO1-SO4, revealing the relationship between its photochromic properties and material structure. The peaks in the PXRD of SO2-SO4 are greatly broadened, indicating that they are amorphous or amorphous. The difference in peak shape between PXRD of SO2-SO4 is caused by different spatial aromatic groups. Disordered packing also weakens intermolecular packing, leading to an increase in the free volume of solid-state photochromism, which is consistent with experimental and computational results.
[Material Application]
Figure 5. Summary of Material Application
, the author of designed and synthesized a series of new spirooxazines SO1-SO4 for the first time based on the new strategy of subtractive effect, showing excellent photochromic properties, such as fast light response speed, high fatigue resistance and thermal stability. SO1-SO4 has been successfully used in UV printing, QR codes, photochromic fingerprint technology, etc.
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Source: Frontiers of Polymer Science
