利用低振動頻率顯微拉曼光譜儀即時監控結晶態1,1′-binaphthyl在同質異像體間變化動力學與拉曼成像分析

Abstract

除了X-光繞射分析以外，是否有其他研究晶體結構的方法？低頻率拉曼光譜提供豐富的晶體結構資訊以及分子位向。實驗室自行架設的低振動頻率拉曼光譜儀能觀測低至9.8 cm-1的拉曼訊號。此外，本系統亦能在顯微鏡下進行空間解析量測並即時分析晶體結構。在此，作者研究不同晶形的1,1’-binaphthyl以展示這套新穎的分析方法。此聯芳化合物在結晶態時因為兩個萘環間有不同的二面角而呈現兩種不同的晶形：類順式和類反式（同質異像體）。當類順式被緩慢加熱時，會透過昇華和再結晶的機制逐漸轉變成類反式。本文中，將即時監控此轉變過程並使用顯微拉曼成像法分析晶形轉變後類順式、類反式晶體的分佈情形。 類順式和類反式在指紋區的拉曼光譜非常類似，唯一的區別指標是510/532 cm-1波峰的強度比。然而，它們的光譜在低頻區截然不同。例如類順式最強的訊號在100cm-1；類反式最強的訊號在26 cm-1。是以此二低頻率特徵峰被用來區別類順式和類反式混合晶體。利用此二低頻率特徵峰建立的拉曼影像呈現了高度的選擇性與影像對比度。此外，低頻率拉曼影像更在外表看似完美的單晶中發現了分子的不規則排列。諸此結果讓我們意識到低頻率拉曼成像法，將會成為十分有用的晶體結構分析工具。

How can we understand crystal structure besides employing X-ray diffraction analysis? Low-frequency (LF) Raman spectra provide a wealth of precious information on crystal structure and orientation and can potentially be a complementary approach to the conventional X-ray diffraction method. However, observation of LF Raman spectra is very challenging due to severe interference of gigantic Rayleigh scattering. In this study, the author uses a laboratory-built LF Raman spectrometer that has the ability to observe Raman signals down to 9.8 cm-1. Because this apparatus is combined with a confocal microscope, it has become feasible to perform space-resolved measurements of crystals under the microscope for in-situ crystal structure analysis. Here, the author demonstrates this novel approach by studying crystal polymorphs of 1,1′-binaphthyl. This biaryl compound occurs as two crystal polymorphs with different dihedral angles between the two naphthyl groups: namely, the cisoid (C) and transoid (T) forms. As the C form is heated up, it gradually transforms to the T form via sublimation and subsequent recrystallization. In this thesis, the author presents tracing of this transformation dynamics and LF Raman imaging that visualizes the spatial distribution of the C and T forms after the transformation takes place. The only effective way to distinguish between the C and T forms by high-frequency Raman bands would be to calculate the intensity ratio for a pair of marker bands (510/532 cm-1). However, the two forms show much more distinct spectral patterns in the LF region. For example, the C form shows the strongest band at 100 cm-1, whereas the T form shows the most intense band at 26 cm-1. Thus, these two LF bands were chosen as markers to selectively probe different crystal polymorphs that coexist in proximity. In sharp contrast with the intensity-ratio image, the LF Raman images at 100 and 26 cm-1 show markedly high contrast and clearly discriminate one crystal form from the other. Moreover, we observed inhomogeneous distribution patterns in the LF Raman images, suggesting different crystal orientations within an apparent single crystal. Our results show that LF Raman spectroscopy and imaging should be a facile yet powerful addition to the toolbox for crystal structure analysis.