研究目的
To study the effect of replacing the terminal chain of liquid crystal molecules with a more electronegative group by comparing the Raman profiles of two laterally fluorinated LC compounds with different terminal groups, specifically focusing on temperature-dependent behavior near phase transitions.
研究成果
The Raman study revealed significant differences in spectral profiles between the two compounds due to the polar NCS terminal group, affecting electron density distribution and vibrational energies. The Cr-N transition showed abrupt changes indicative of first-order behavior, while the N-I transition exhibited irregular patterns attributed to pre-transitional effects. Theoretical spectra matched well with experimental data, validating the DFT approach. Future work should explore these pre-transitional anomalies in more detail.
研究不足
The study is limited to two specific laterally fluorinated LC compounds, which may not generalize to other LC systems. The theoretical DFT calculations assume ideal conditions (e.g., vacuum), potentially leading to minor discrepancies with experimental spectra due to neglect of anharmonicity and real-world constraints. The irregular behavior near the nematic-isotropic transition suggests pre-transitional effects that require further investigation.
1：Experimental Design and Method Selection:
The study involved temperature-dependent Raman spectroscopy to analyze vibrational dynamics during phase transitions. Density functional theory (DFT) with B3LYP method and 6-31G (d,p) basis set was used for theoretical spectra generation and molecular optimization. Raman spectra were deconvoluted using Lorentzian profiles for spectral parameter extraction.
2：Sample Selection and Data Sources:
Two laterally fluorinated liquid crystal compounds, C3PP(23F)P-NCS and C3PP(23F)PC3, were obtained from the Military University of Technology, Poland. These compounds exhibit nematic phases during heating and cooling cycles, with no smectic phases or monotropic transitions observed.
3：List of Experimental Equipment and Materials:
Horiba XPlora1 Micro-Raman system with a 785 nm diode laser source, Mettler Toledo Hot stage (HS82) connected to a control unit HS1 for temperature control, liquid nitrogen dewar for cooling, and software including GaussView5.0.8, GAUSSIAN-09, and Thermocon Gramsuite for data analysis.
4：8, GAUSSIAN-09, and Thermocon Gramsuite for data analysis.
Experimental Procedures and Operational Workflow:
4. Experimental Procedures and Operational Workflow: Samples were cooled at a rate of 0.5°C/min using the hot stage. Raman spectra were recorded in the 1000–2500 cm?1 region during cooling. Spectra were analyzed by deconvoluting bands with Lorentzian fits at each temperature to extract peak positions, intensities, and linewidths.
5：5°C/min using the hot stage. Raman spectra were recorded in the 1000–2500 cm?1 region during cooling. Spectra were analyzed by deconvoluting bands with Lorentzian fits at each temperature to extract peak positions, intensities, and linewidths.
Data Analysis Methods:
5. Data Analysis Methods: Spectral parameters were analyzed using Thermocon Gramsuite software. DFT calculations were performed to generate theoretical spectra, which were compared with experimental data. Temperature-dependent changes in vibrational modes were studied to infer molecular dynamics during phase transitions.
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