Effect of Confined Hindrance in Polyphenylbenzenes

A comprehensive thermodynamic study of the whole <i>ortho</i>-polyphenylbenzenes series from biphenyl (<i>n</i> = 1) to hexaphenylbenzene (<i>n</i> = 6) is presented. Combustion calorimetry and phase equilibria measurements for 1,2,3,4-tetraphenylbenzene (<i>n</i> = 4) and pentaphenylbenzene (<i>n</i> = 5) together with literature data were used to understand and quantify the constraint effect of <i>ortho-</i>substitution on the molecular energetics and phase stability of polyaromatic compounds. All of the derived thermodynamic properties (enthalpy of sublimation, entropy of sublimation, and gas phase molecular energetics) show a marked trend shift at <i>n</i> = 4 to 5, which is related to the change of the degree of molecular flexibility after 1,2,3,4-tetraphenylbenzene (<i>n</i> = 4). The greater intramolecular constraint in the more crowded members of the series (<i>n</i> = 5 and 6) leads to a significant change in the molecular properties and cohesive energy. The trend shift in the molecular properties is related with the decrease in molecular flexibility, which leads to lower molecular entropy and destabilization of the intramolecular interaction potential due to the increased hindrance in a confined molecular space

Elucidating the Role of Aromatic Interactions in Rotational Barriers Involving Aromatic Systems

The measurement of aryl-naphthyl rotational barriers, Δ<i>G</i>^⧧, in various solvents for two substituted 1,8-diarylnaphthalenes by dynamic ¹H NMR showed that Δ<i>G</i>^⧧ trends in aromatic systems can be fully rationalized only when considering the different types of aromatic interactions that can be established in the ground and transition states, namely, intramolecular interactions involving the aromatic rings and specific solvation interactions

Effect of Self-Association on the Phase Stability of Triphenylamine Derivatives

The self-association equilibrium, i.e. formation of noncovalent dimers, in two triphenylamine derivatives, TPD (<i>N,N</i>′-bis­(3-methylphenyl)-<i>N,N</i>′-diphenylbenzidine) and mMTDAB (1,3,5-tris­[(3-methylphenyl)­phenylamino]­benzene), in solution was evaluated by ¹H NMR spectroscopy. The gas-phase energetics of the respective dimerization processes was explored by computational quantum chemistry. The results indicate that self-association is significantly more extensive in TPB than in TDAB. It is proposed that this fact helps to explain why TPB presents a stability higher than expected in the liquid phase, which is reflected in a lower melting temperature, a less volatile liquid, and possibly a higher tendency to form a glass. These results highlight the influence of self-association on the phase equilibria and thermodynamic properties of pure organic substances