Thermal decomposition of syn- and anti-dihydropyrenes; functional group-dependent decomposition pathway
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 CH3- 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−1) and anti (32.10 kcal mol−1) 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.
KeywordsCyclophanediene Dihydropyrene Density functional theory Sigmatropic shift Decomposition pathways
The authors acknowledge the financial support of the Higher Education Commission of Pakistan and COMSATS University, Abbottabad Campus.
Compliance with ethical standards
The author from the King Khalid University is thankful to Deanship of Scientific Research at King Khalid University for funding the work through Research Group Project under grant number (R. G. P.1/165/40).
1. The manuscript has not been submitted to more than one journal for simultaneous consideration.
2. The manuscript has not been published previously (partly or in full).
3. A single study is not split up into several parts to increase the quantity of submissions and submitted to various journals or to one journal over time (e.g., “salami-publishing”).
4. No data have been fabricated or manipulated (including images) to support our conclusions.
5. No data, text, or theories by others are presented as if they were the author’s own (“plagiarism”). Proper acknowledgements to other’s work if quoted.
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