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
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