Quantum Chemical Study on the High-Pressure Effect for [4 + 4] Retrocycloaddition of Anthracene Cyclophane Photodimer

The [4 + 4] retrocycloaddition of bis anthracene photodimer yields anthracene cyclophane. The carbon–carbon bond cleavage in this reaction is significantly accelerated by applying high pressure. We elucidate the origin of the pressure effect using the quantum chemical method considering the high-pressure effect with the extreme pressure polarizable continuum model. The confinement by high-pressure medium significantly deforms and destabilizes the photodimer structure, and this destabilization reduces the activation energy of the bond cleavage. The pressure effect significantly depends on the shapes of the solvation cavity, indicating that the pressure effect is specific to the orientation of the molecule with respect to the surrounding matrix. This finding clearly explains the observed biexponential behavior of the reaction rate that indicates the coexistence of pressure-sensitive and pressure-insensitive components. During deformation, the bridging sp3 carbons in photodimer involve sp2 character, and it promotes the bond cleavage. From the mechanochemical point of view, a sizable mechanical force acts on atoms distanced from the reaction center, and the remote force assists the bond cleavage. This mechanism has an analogy to the principle of leverage. We also elucidate the pressure effects on ultraviolet–visible and infrared spectroscopies by quantum chemical computations for exploring the analysis methods in high-pressure chemistry