Mechanical Activation of Forbidden Photoreactivity in Oxa-di-π-methane Rearrangement

Abstract

In this work, we demonstrate that the forbidden oxirane-type photoproduct (the cyclopropyl ketone photoproduct is the allowed one) of the oxa-di-π-methane photorearrangement can be obtained by mechanochemical control of the photoreactions. This control is achieved by the application of simple force pairs rationally chosen. By analyzing in detail the effect of the applied forces on this photoreaction, it comes to light that the mechanical action affects the diverse properties of the oxa-di-π-methane rearrangement, modifying all the steps of the reaction: (i) the initial ground-state conformers’ distribution becomes affected; (ii) the new conformational population makes the triplet excitation process to be changed, responding to the magnitude of the applied force; (iii) the stability of the different intermediates along the triplet pathway also becomes affected, changing the dynamical behavior of the system and the reaction kinetics; and (iv) the intersystem crossing also becomes strongly affected, making the forbidden oxirane-type photoproduct to decay more efficiently to the ground state. All these changes provide a complex scenario where a detailed study of the effect of applied forces is necessary in order to predict its overall effect on the photoreactivity.

An Algorithm Predicting the Optimal Mechanical Response of Electronic Energy Difference

“…Of course, this algorithm overcomes the intrinsic limit of the second-order approach, permitting the application of any kind of force strength and the real effect on complex PES. This allows the description, for instance, of the potential appearance of new minima or transition states, even when considering small external forces, as could be the case of energetically accessible conformations …”

“…Considering a second-order approach in the PES, the optimal external forces and the corresponding optimal minimum energy structures can be obtained from eqs 16 and 17. Determining the optimal structures implies to solve the linear equation system: (18) where A = (2λH 0 2 − H 0 + H 1 ) −1 is a symmetric square matrix. It should be noted that, for some systems, the gradient g 1 belongs to certain symmetry groups, so that if we express the variables of the system in symmetric coordinates, eq 18 can be expressed as (19) where the superscript "sym" indicates symmetric coordinates (i.e., those totally symmetric), while "no-sym" corresponds to the non-totally symmetric coordinates.…”

“…This allows the description, for instance, of the potential appearance of new minima or transition states, even when considering small external forces, as could be the case of energetically accessible conformations. 18 3.2.1. H 2 O, a Rigid Molecular System.…”

“… 15 In this context, previous results show that mechanical control of the energy difference between electronic states is also possible by applying specific force pairs in the azobenzene photoswitch. 17 Additionally, the effect of the mechanical forces has been shown to also affect the photoreactivity, making possible the mechanical control of the photoproduct outcome, 18 or the modulation of photoisomerization and fluorescence quantum yields in molecular photoswitches. 7 , 19 Energetic parameters, like the fraction of photon energy converted to chemical energy in molecular solar thermal systems, can also be tuned with mechanical control.…”

Wandering through quantum-mechanochemistry: from concepts to reactivity and switches

“…Various techniques have been proposed to cope with this fundamental problem such as COGEF (constrained geometries simulate external force) 13 and EFEI (external forces explicitly included) 14 methods, as will be briefly summarized in Sections 2 and 3, resulting in a fair increase in quantum mechanochemical publications, sometimes also including the visualization of the distribution of the strain energy in the molecule (JEDI, Judgment of Energy Distribution). 10,15 Studies vary from the initial investigation of the mechanical strength and bond rupture in small molecules, 13 through papers on the possibility to control the stereoselectivity of pericyclic reactions including the violation of the Woodward Hoffmann rules, [16][17][18] often in synergy with experimental studies, 17,19,20 to studies on force-induced retro-click reactions, 21 mechanically induced conductance switching, 22 activation efficiency of mechanophores, 23 mechanical activation of photo-reactivity 24,25 and anthracene (4+4) cycloadducts, 26 force-induced switching of aromaticity and homo-aromaticity, 27 flex-activated mechanophores, 28 among others.…”