Theoretical Study of the Valence Isomerization of Anthracene and 9-tert-Butylanthracene to Their Dewar Forms in the Ground and Excited States

Abstract

Ab initio and semiempirical Ah41 self-consistent-field (SCF) and configuration interaction (CI) calculations have been carried out for the anthracene == 9,lO-Dewar-anthracene and 9-fert-butylanthracene * 9-tertbutyl-9,lO-Dewar anthracene valence isomerization reactions. The reaction enthalpy for the thermal rearomatization reaction in the tert-butyl-substituted compound is found to be -38 kcdmol, in good agreement with the experimental value of -41 kcallmol. The ab initio SCF optimized geometries compare favorably with experimental X-ray data. Two-dimensional energy surfaces and the reaction path connecting the isomers have been calculated. The barrier for the thermal back-reaction in the case of anthracene is theoretically found to be about 22 kcal/mol, in good agreement with kinetic data. The calculations support an approximate state correlation diagram given in the literature and offer an explanation for the photochemical stability of anthracene with respect to formation of its Dewar isomer.

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

“…Both are metastable, high energy photoisomers. Our DFT calculations place D-9TBA 184 kJ/mol above 9TBA, close to the experimental Δ H 0 = 172 kJ/mol. , PI is calculated to be only 87 kJ/mol above BA, again comparable to experimental values . 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.…”

Reaching Optimal Light‐Induced Intramolecular Spin Alignment within Photomagnetic Molecular Device Prototypes

“…1) The sign of (α–β) net electron spin contributions lying on the shared internal bonds of laterally fused rings becomes the same as that of the SD lying on vertices (rims)46 and potentially bearing substituents. 2) Some spin density arises across the bent central ring between atoms C9 An and C10 An , suggesting the onset of the formation of a bond similar to that encountered in Dewar anthracene valence isomers 44. The point is that, together with the shortening of the PB bonds showing partial double‐bond character, the central ring of anthracene displays partly quinonoid features upon saddle‐shaped distortion, in accordance with previous issues (see also Figure S II).…”

“…This saddle‐shaped (equally referred to as butterfly‐shaped) structural deformation is reminiscent of that experimentally evidenced (including by X‐ray crystallography) for 9,10‐bis(1,3‐dithiol‐2‐ylidene)‐9,10‐dihydroanthracene derivatives39 as “π‐extended TTFs”40 (TTF=tetrathiafulvalene), tetracyano‐9,10‐anthraquinodimethane species41 as “π‐extended TCNQ”42 (TCNQ=tetracyanoquinodimethane), or bis‐quinone‐anthracene,43 even though such distorted conformations are in these cases adopted in the ground state (recovering the usual axially distorted geometry upon oxidation or reduction, respectively). The computed structural deformation is also closely akin to that attached to the 9,10‐anthracene Dewar valence isomer, and can be characterized by the same set of parameters (α, φ′ and r ; see Figure 3) accordingly 44…”

“…Such decorations further contribute to the reduction of the stability of the molecule, making it more reactive. If the bulky substituent is isotropic (e.g., t Bu), structural relaxation involves the onset folding of the An backbone44, 72 that is, saddle‐shaped distortion. If the bulky group displays anisotropic features (e.g., aryl plane), conformational change (e.g., canting of the plane of the substituent) may be sufficient to relax intramolecular constraints so as to maintain the planarity of the acene skeleton, as in the case of the presently investigated PMMDs (axially distorted in the GS).…”

Photochemical formation of the elusive Dewar isomers of aromatic systems: why are substituted azaborines different?

“…13–17 Despite their high energy content, many Dewar species are predicted to be reasonably stable, as they are separated from the parent compounds by large energy barriers. 18–20 As for the possibility to pursue photochemical synthesis of Dewar species, quantum-chemical modelling has also shown that the photoisomerization reactions through which they are formed proceed in singlet excited states, 18,20 as originally suggested for Dewar benzene. 3 Thus, if such synthesis is attempted, addition of triplet sensitizers or removal of triplet quenchers is not required.…”