The pressure-responsive behaviors of organic fluorophores are often irregular and highly system-dependent. Unraveling the pressure effects on them is the key to the precise design of piezochromic materials. Here, we demonstrate that the different conformational flexibilities of organic fluorophores can lead to their distinct piezochromic behaviors. By combining quantum mechanics/molecular mechanics models and thermal vibration correlation function formalism, we show that two organic fluorophores, dibenzo[b,d]thiophene 5,5-dioxide (DBTS) and carbazole (Cz), with similar chemical structures, display different conformations and conformational flexibilities, which determine the responsiveness of their piezochromic behaviors. It is found that pressurization continuously improves the planarity between the sulfonyl group and the aromatic skeleton in cross-shaped DBTS, leading to red-shifted emission. In addition, the fluorescence quantum efficiency (FQE) of DBTS increases monotonically in a wide range of 0–7 GPa and then shows a dramatic reduction upon further compression to 12 GPa. The non-monotonical FQE of DBTS is attributed to the continuously decreased radiative decay rate constant (k_r) and non-monotonically changed non-radiative decay rate constant (k_ic). The initial decrease in k_ic is due to the restriction of aromatic skeleton vibrations in the high-frequency region, and the subsequent increase in k_ic is ascribed to the acceleration of the bending and rotational vibrations of the sulfonyl group in the low-frequency region. In contrast, pressurization hardly influences the conformation of planar Cz, resulting in a slight change in emission and FQE, which is well consistent with experimental results. This study opens an efficient shortcut to the rational design of advanced piezochromic luminescent materials.

中文翻译：

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有机荧光团的构象柔性控制的压致变色行为

有机荧光团的压力响应行为通常是不规则的并且高度依赖于系统。揭示压力对它们的影响是压致变色材料精确设计的关键。在这里，我们证明了有机荧光团的不同构象灵活性可以导致它们独特的压致变色行为。通过结合量子力学/分子力学模型和热振动相关函数形式，我们证明了两个有机荧光团，二苯并 [ b , d]噻吩 5,5-二氧化物 (DBTS) 和咔唑 (Cz) 具有相似的化学结构，显示出不同的构象和构象灵活性，这决定了它们的压致变色行为的响应性。发现加压不断提高十字形 DBTS 中磺酰基和芳香骨架之间的平面性，导致红移发射。此外，DBTS 的荧光量子效率 (FQE) 在 0–7 GPa 的宽范围内单调增加，然后在进一步压缩至 12 GPa 时显着降低。DBTS 的非单调 FQE 归因于不断降低的辐射衰减率常数 ( k _r ) 和非单调变化的非辐射衰减率常数 ( k _ic). k _ic最初的减小是由于高频区芳香骨架振动的限制，随后k _ic的增加是由于磺酰基在低频区的弯曲和旋转振动的加速. 相反，加压几乎不影响平面Cz的构象，导致发射和FQE略有变化，这与实验结果非常一致。该研究为先进压致变色发光材料的合理设计开辟了一条有效的捷径。