Saima Bibi, Wang Yan Alexander, Hussain Riaz, Muhammad Shabbir, Ayub Khurshid
Department of Chemistry, COMSATS University, Abbottabad Campus, Abbottabad, 22060, Pakistan.
Department of Chemistry, University of British Columbia, Vancouver, V6T 1Z1, Canada.
J Mol Model. 2019 Jul 10;25(8):215. doi: 10.1007/s00894-019-4052-1.
Syn and anti dihydropyrene (DHP) are excellent thermochromes, and therefore extensively studied for their thermochromic and photochromic properties, respectively. However, they suffer from thermal decomposition due to thermal instability. In this study, we thoroughly investigated pathways for the thermal decomposition of anti- and syn- dihydropyrenes through computational methods. The decomposition pathways include sigmatropic shift and hemolytic and heterolytic (cationic and anionic) cleavages. The decomposition pathway is influenced not only by the dihydropyrene (syn- or anti-) but also by the functional groups present. For anti-dihydropyrenes, sigmatropic shift is the most plausible pathways for CN and CHO internal groups. The cascade of sigmatropic shifts is followed by elimination to deliver substituted pyrenes. For CH- and H- dihydropyrenes, hemolytic cleavage of the internal groups is the most plausible pathway for decomposition to pyrenes. The pathway is changed to heterolytic cleavage when the internal groups on the dihydropyrenes are Cl, Br, and SMe. Comparison of the activation barriers for syn (30.18 kcal mol) and anti (32.10 kcal mol) dimethyldihydropyrenes for radical pathway reveal that decomposition of syn- DHP is more facile over anti-, which is consistent with the experimental observation. The decomposition pathway for syn-dihydropyrene is also hemolytic in cleavage when the internal groups are methyl and hydrogen. Syn-dihydropyrenes (symmetrical or unsymmetrical) bearing CN group do not follow sigmatropic shift, quite contrary to the anti-dihydropyrene. The lack of tendency of the syn-dihydropyrene for sigmatropic shift is rationalized on the planarity of the scaffold. The results of the theoretical study are consistent with the experimental observations. The results here help in understanding the behavior of substituents on the dihydropyrene scaffold, which will be useful in designing new molecules with improved thermal stabilities. Graphical abstract Functional group dependent decomposition pathways of dihydropyrenes.
顺式和反式二氢芘（DHP）是优良的热致变色材料，因此分别对它们的热致变色和光致变色特性进行了广泛研究。然而，由于热不稳定性，它们会发生热分解。在本研究中，我们通过计算方法深入研究了反式和顺式二氢芘的热分解途径。分解途径包括σ迁移、均裂和异裂（阳离子和阴离子）裂解。分解途径不仅受二氢芘（顺式或反式）影响，还受存在的官能团影响。对于反式二氢芘，σ迁移是含氰基（CN）和醛基（CHO）内基团最可能的分解途径。σ迁移的级联反应之后是消除反应，生成取代芘。对于含甲基（CH）和氢（H）的二氢芘，内基团的均裂是分解生成芘最可能的途径。当二氢芘上的内基团为氯（Cl）、溴（Br）和甲硫基（SMe）时，分解途径变为异裂。自由基途径中顺式（30.18千卡/摩尔）和反式（32.10千卡/摩尔）二甲基二氢芘的活化能垒比较表明，顺式二氢芘的分解比反式更容易，这与实验观察结果一致。当内基团为甲基和氢时，顺式二氢芘的分解途径也是均裂。带有氰基的顺式二氢芘（对称或不对称）不发生σ迁移，这与反式二氢芘完全相反。顺式二氢芘缺乏σ迁移倾向可根据其骨架的平面性来解释。理论研究结果与实验观察结果一致。这里的结果有助于理解二氢芘骨架上取代基的行为，这将有助于设计具有更高热稳定性的新分子。图形摘要二氢芘的官能团依赖性分解途径。
