Pressure Catalyzed Bond Dissociation in an Anthracene Cyclophane Photodimer

Abstract

The anthracene cyclophane bis-anthracene (BA) can undergo a [4 + 4] photocycloaddition reaction that results in a photodimer with two cyclobutane rings. We find that the subsequent dissociation of the dimer, which involves the rupture of two carbon-carbon bonds, is strongly accelerated by the application of mild pressures. The reaction kinetics of the dimer dissociation in a Zeonex (polycycloolefin) polymer matrix were measured at various pressures and temperatures. Biexponential reaction kinetics were observed for all pressures, consistent with the presence of two different isomers of bis(anthracene). One of the rates showed a strong dependence on pressure, yielding a negative activation volume for the dissociation reaction of ΔV(++) = -16 Å(3). The 93 kJ/mol activation energy for the dissociation reaction at ambient pressure is lowered by more than an order of magnitude from 93 to 7 kJ/mol with the application of modest pressure (0.9 GPa). Both observations are consistent with a transition state that is stabilized at higher pressures, and a mechanism for this is proposed in terms of a two-step process where a flattening of the anthracene rings precedes rupture of the cyclobutane rings. The ability to catalyze covalent bond breakage in isolated small molecules using compressive forces may present opportunities for the development of materials that can be activated by acoustic shock or stress.

Pressure Dependence of the Forward and Backward Rates of 9-tert-Butylanthracene Dewar Isomerization

“…[34][35] PI is calculated to be only 87 kJ/mol above BA, again comparable to experimental values. 20 The higher energy of D-9TBA is likely due to the formation of two adjacent cylcobutene rings with higher strain than the two separate cyclobutane rings in PI. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 12 differences and volume changes, we would expect the high-energy, high-volume D-9TBA to be more sensitive to pressure than PI.…”

“…Despite their enthalpic differences, the PI BA and D-9TBA 9TBA reactions have very similar activation energies of 93-96 kJ/mol at room temperature and pressure 20,[24][25]. Thus from an energetic standpoint, there is no reason to suspect that PI would become so reactive at high pressure relative to D-9TBA.…”

Photo-induced dynamic association of coumarin pendants within amphiphilic random copolymer micelles

“…where PD(t) corresponded to the time-dependent photo-dimerization degree, k 1 and k 2 were rate constants, and X 1 and X 2 were the relative weight fractions [31,32]. To be emphasized, the sum of X 1 and X 2 determined the maximum photodimerization extent (PD max ) that the coumarin pendants could reach after the long enough irradiation time [33].…”

Coarse grain model for coupled thermo-mechano-chemical processes and its application to pressure-induced endothermic chemical reactions

“…Such endothermic chemistry with volume reduction could provide the desired capability of shockwave energy dissipation. One possible system with such characteristics is the anthracene molecule; Jezowski et al [21] have found that the application of mechanical force reduces the rection rate involved in chemical bond formation, while Slepetz et al [22] have calculated a possible intermediate pathway occurring between reactants and products, which involves molecular volume reduction. With the potential energy fully determined in terms of the system variables, we can write the system Hamiltonian from which we will derive equations of motion:…”