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. Graphical abstract Functional group dependent decomposition pathways of dihydropyrenes.

Photo-tunable linear and nonlinear optical response of cyclophanediene-dihydropyrene photoswitches

Cyclophanediene (CPD)-dihydropyrene (DHP) is a negative T-type photochrome pair having a thermodynamically stable colored form, i.e., DHP. Interconversion between cyclophanediene and dihydropyrene is associated with significant changes in dipole moment, absorption wavelength and polarizability, which can impart substantial linear and nonlinear optical response. In this study, phototunable linear and nonlinear optical response of cyclophanediene-dihdyropyrene photoswitches is described. Cyclophanedienes and dihydropyrenes are functionalized at the internal position for maximum changes in volume and polarizability. The UV-Vis spectra are calculated at ɷB97XD, which was validated through a benchmark approach. An excellent correlation is observed between theoretical and experimental absorption spectra. Several CPD-DHP pairs have been recognized for clean interconversion in UV-Vis light without formation of a photostationary state. Nonlinear optical response of dihydropyrenes is remarkably higher than that of cyclophanedienes. In general, the calculated hyperpolarizability values of dihydropyrenes are about two to three orders of magnitude higher than those for cyclophanedienes. The trends in calculated hyperpolarizabilities are rationalized through two level method. The high nonlinear optical response of dihydropyrenes stems from low excitation energies. The remarkable difference in hyperpolarizabilities of these isomeric forms paves path for the design of phototunable nonlinear optical materials.