Asiri Abdullah M, Heller Harry G, Hughes David S, Hursthouse Michael B, Kendrick John, Leusen Frank Jj, Montis Riccardo
Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Jeddah, 21589 Saudi Arabia.
School of Chemistry, Cardiff University, Cardiff, Wales CF10 3AT UK.
Chem Cent J. 2014 Dec 16;8(1):70. doi: 10.1186/s13065-014-0070-3. eCollection 2014.
Derivatives of fulgides have been shown to have interesting photochromic properties. We have synthesised a number of such derivatives and have found, in some cases, that crystals can be made to change colour on crushing, a phenomenon we have termed "tribochromism". We have studied a number of derivatives by X-ray crystallography, to see if the colour is linked to molecular structure or crystal packing, or both, and our structural results have been supported by calculation of molecular and lattice energies.
A number of 5-dicyanomethylene-4-diphenylmethylene-3-disubstitutedmethylene-tetrahydrofuran-2-one compounds have been prepared and structurally characterised. The compounds are obtained as yellow or dark red crystals, or, in one case, both. In two cases where yellow crystals were obtained, we found that crushing the crystals gave a deep red powder. Structure determinations, including those of the one compound which gave both coloured forms, depending on crystallisation conditions, showed that the yellow crystals contained molecules in which the structure comprised a folded conformation at the diphenylmethylene site, whilst the red crystals contained molecules in a twisted conformation at this site. Lattice energy and molecular conformation energies were calculated for all molecules, and showed that the conformational energy of the molecule in structure IIIa (yellow) is marginally higher, and the conformation thus less stable, than that of the molecule in structure IIIb (red). However, the van der Waals energy for crystal structure IIIa, is slightly stronger than that of structure IIIb - which may be viewed as a hint of a metastable packing preference for IIIa, overcome by the contribution of a more stabilising Coulomb energy to the overall more favourable lattice energy of structure IIIb.
Our studies have shown that the crystal colour is correlated with one of two molecular conformations which are different in energy, but that the less stable conformation can be stabilised by its host crystal lattice. Graphical abstractGraphical representation of the structural and colour change in the tribochromic compound (III).
已证明俘精酸酐衍生物具有有趣的光致变色特性。我们合成了许多此类衍生物，并且在某些情况下发现，晶体在研磨时会变色，我们将这种现象称为“摩擦致色”。我们通过X射线晶体学研究了许多衍生物，以查看颜色是否与分子结构或晶体堆积有关，或者两者都有关，并且我们的结构结果得到了分子和晶格能量计算的支持。
制备并对多种5-二氰基亚甲基-4-二苯基亚甲基-3-二取代亚甲基-四氢呋喃-2-酮化合物进行了结构表征。这些化合物以黄色或深红色晶体形式获得，或者在一种情况下，两种晶体形式都有。在获得黄色晶体的两个案例中，我们发现研磨这些晶体得到深红色粉末。结构测定，包括对根据结晶条件给出两种颜色形式的一种化合物的结构测定，表明黄色晶体包含的分子在二苯基亚甲基位点具有折叠构象，而红色晶体包含的分子在此位点具有扭曲构象。计算了所有分子的晶格能量和分子构象能量，结果表明结构IIIa（黄色）中分子的构象能量略高于结构IIIb（红色）中分子的构象能量，因此构象稳定性较差。然而，晶体结构IIIa的范德华能量略强于结构IIIb的范德华能量——这可以被视为IIIa存在亚稳堆积偏好的一个迹象，但结构IIIb整体更有利的晶格能量中更稳定的库仑能量贡献克服了这种偏好。
我们的研究表明，晶体颜色与能量不同的两种分子构象之一相关，但较不稳定的构象可以通过其主体晶格得到稳定。图形摘要摩擦致色化合物（III）的结构和颜色变化的图形表示。
