Molecular Aggregation Dynamics via a Liquid-like Cluster Intermediate during Heterogeneous Evaporation as Revealed by Hyperspectral Camera Fluorescence Imaging

A model for inversion of hyperspectral characteristics of phosphate content in mural plaster based on fractional-order differential algorithm

The Dunhuang murals, a significant part of Chinese heritage, have suffered deterioration primarily due to environmental and chemical factors, notably salt damage. This study proposes a sophisticated method that synergizes Fractional Order Differentiation (FOD) and Partial Least Squares Regression (PLSR) to accurately invert the phosphate content in the mural plaster of the Dunhuang paintings. The focal points of the research include: (1) To address the issue of information loss and reduced modeling precision caused by integer-order differentiation algorithms, the FOD method is employed for preprocessing spectral data. This approach ensures the fine spectral differences in the phosphate content of the mural plaster are precisely captured, (2) Utilizing PLSR, the study models the spectral bands identified at a significance level of 0.01 with measured conductivity values, thereby enabling the precise prediction of the phosphate content in the murals. The research outcomes reveal: (1) The FOD method can elucidate the nonlinear characteristics and variation patterns of the mural samples in the spectral data. As the order increases from zero to two, the number of spectral bands meeting the 0.01 significance test initially decreases and then increases. The highest absolute value of the positive correlation coefficient is observed at the 1.9 order, corresponding to the 2077 nm band, (2) For predicting the phosphate content in the murals, the model at the 1.9 order is most suitable for inversion. This model, after cross-validation, achieves a maximum R2 value of 0.861. This study created an efficient FOD-based model for estimating phosphate in mural plaster.

Photoluminescence variations in organic fluorescent crystals by changing the surface energy of the substrate

Organic fluorescent crystals were obtained using single-benzene-based diethyl 2,5-dihydroxyterephthalate (DDT) molecules through crystallization from a droplet of the DDT solution on an Au substrate. To control the size of the DDT crystals, the surface energy of the Au substrate was modified with air plasma treatment, producing a hydrophilic surface and a hydrophobic self-assembled monolayer (SAM) coating. The size of DDT crystals increased as the surface energy of the substrate decreased. The averaged cross-section area of the DDT crystals on the Au substrates increased in the order of the air-plasma-treated substrate (∼23.43 μm2) < pristine substrate (∼225.6 μm2) < hydrophobic SAM-coated substrate (∼2240 μm2). On the other hand, the main emission of the DDT crystals redshifted from blue to green as the crystal size increased, which is related to the aggregation of the DDT crystals. Moreover, the coffee-ring effect during the DDT crystallization was hindered by controlling the solvent evaporation conditions. As examples of the application of the proposed technique, patterned DDT crystals were obtained using selectively patterned hydrophobic and hydrophilic substrates.

Flapping motion as a fluorescent probe for assembly process involving highly viscous liquid-like cluster intermediates during evaporative crystallization

Fluorescence probes are widely used to assess the molecular environment based on their photo-physical properties. Specifically, flexible and aromatic photo-functional system (FLAP) is unique viscosity probe owing to the excited-state planarization of anthracene wings. We have previously applied fluorescence spectroscopy to monitor the evaporative crystallization of solvents. The fluorescence color and spectral changes, which depend on the aggregation form, enable direct fluorescence visualization during evaporative crystallization. The fluorescence visualization of the liquid-like cluster intermediate proposed in the two-step nucleation model for the nucleation process has been achieved. However, the physical properties of these clusters, especially the viscosity, molecular motion, and intermolecular interactions, are still unclear. In this study, FLAPs are used as probes for local-viscosity changes and space limitations of the liquid-like cluster state during evaporative crystallization by observing the fluorescence-spectral changes and using hyperspectral-camera (HSC) imaging. Green emission originates from the monomer in the solution owing to the free-flapping motion. The fluorescence color turns blue with increasing viscosity under crowding conditions. If the survival time of the liquid-like cluster state is sufficient, crystalline phase (R-phase) formation proceeds via a 2-fold π-stacked array of the V-shaped molecules. It is difficult to form the V-shaped stacked columnar structures in the liquid-like cluster state region, resulting in the deposition of head-to-tail dimer structures, such as the yellow-emissive phase (Y-phase). In the case of the FLAP, the stacking intermediate does not form during solvent evaporation in the liquid-like cluster; rather, it is deposited in an amorphous form that exhibits blue emission (B-phase). These findings suggest that it is important to the maintenance of the survival time of the liquid-like cluster states to organize and rearrange the stacking forms. We have achieved the fluorescence probing of viscosity changes at local molecular motion with solvent depletion during solvent evaporation for the first time.