Combined Experimental and Theoretical Studies on the Rubbing-Induced Fluorescence Behavior of a Luminophore in the Solid State

It is intricate to
break and make chemical bonds in solid states
compared to their solution states, so it is imperative to ascertain
green proficient approaches by regulating the solid-state structures
and their related material properties. Here, the rubbing-induced photoluminescence
behavior of a luminophore (RIL) of the benzimidazole family in the
solid state has been accomplished. Interestingly, upon gentle rubbing
or mere scratching, solid-state fluorescence from the nonemissive
pristine RIL was observed due to the aggregation-induced emission
(AIE) phenomenon in the solid state, for which the phenolic moiety
is present in the molecule and is accountable. The structure–property
relationship of RIL and the mechanism responsible for this solid-state
fluorescence characteristics have been explained with the help of
experimental (using the single-crystal structure, powder X-ray diffraction
(PXRD), scanning electron microscopy (SEM) images, etc.) and theoretical
(by DFT and TDDFT) studies. The crystal arrangements with different
stacking interactions and the SEM images after being rubbed revealed
that the mechanical force- or pressure-induced slight deformation
in the crystal arrangement notably facilitated the strong emission
in the solid state. This rubbing-induced solid-state fluorescence
in a new luminophore (RIL) through stacking of layers restricting
the molecular motion has been developed here for the first time, and
it can be explicitly employed in steganography techniques for data
security. This present study will open up a new insight into the use
of this RIL as a solid-state smart material for data security in coding
devices in the future, and this developed approach may be helpful
to ameliorate the design of new-generation smart materials by modifying
the structure to attain other characteristics.

相较于溶液体系，固态体系中化学键的断裂与形成过程更为复杂，因此通过调控固态结构及其相关材料性能，开发绿色高效的制备方法至关重要。本研究成功实现了苯并咪唑类发光体（RIL）在固态下的摩擦诱导光致发光行为。值得注意的是，原本无发光特性的原始RIL样品，经轻微摩擦或简单刮擦后，因固态下的聚集诱导发光（aggregation-induced emission, AIE）效应而产生固态荧光，该现象与分子中含有的酚羟基官能团密切相关。本研究通过实验手段（包括单晶体结构分析、粉末X射线衍射（PXRD）、扫描电子显微镜（SEM）成像等）与理论计算（采用密度泛函理论（density functional theory, DFT）及含时密度泛函理论（time-dependent density functional theory, TDDFT）），阐明了RIL的构效关系以及该固态荧光特性的产生机制。不同堆叠相互作用的晶体排列以及摩擦后的SEM成像结果表明，机械力或压力诱导的晶体排列轻微形变，显著促进了固态下的强荧光发射。本研究首次开发了通过层堆叠限制分子运动，从而实现新型发光体（RIL）摩擦诱导固态荧光的策略，该策略可直接应用于数据安全领域的隐写技术中。本研究为未来将RIL作为固态智能材料应用于编码设备的数据安全领域提供了全新视角，同时该开发策略也可为通过结构修饰获取其他性能的新一代智能材料设计提供借鉴。