posted on 2022-09-16, 19:34authored byCarlos F. R. A. C. Lima, José C.
S. Costa, Artur M. S. Silva, Adélio Mendes, Luís M.
N. B. F. Santos
This work reports a comprehensive
experimental evaluation of the
solid–liquid–gas phase equilibria for five representative
phenylene-thiophene co-oligomers (3-ring aromatic compounds having
both phenyl and thienyl units). The melting temperatures and corresponding
standard molar enthalpies and entropies of fusion were measured by
differential scanning calorimetry. The equilibrium vapor pressures
of the crystalline solids as a function of temperature were measured
by a combined Knudsen/quartz-crystal effusion method, with the consequent
derivation of the standard molar enthalpies, entropies, and Gibbs
energies of sublimation. The thermodynamic properties of vaporization
were estimated from the fusion and sublimation data. The results were
analyzed together with the literature data for the corresponding phenylene
and thiophene homo-oligomers. The thermodynamic properties of fusion
and sublimation exhibited a dependence on ring identity and position
that cannot be adequately described by a simple group additivity reasoning.
The plot of the Gibbs energy of sublimation as a function of the number
of thienyl rings in the co-oligomer showed the existence of two series.
Terminal 3-thienyl rings and a linear molecular shape were found to
be consistent factors contributing to the stabilization of the crystal
phase. The higher melting temperatures and lower volatilities of crystalline
3-thienyl compounds were tentatively explained by the ability of these
rings to maximize intermolecular C–H···π
interactions independently of the sulfur position. The optical energy
gaps, as measured by UV–vis in solution, were found to lie
within the values for typical organic semiconductors (<4 eV) and
to decrease for co-oligomers containing more 2-thienyl units, following
the increased ring–ring planarity of the molecules. The surface
morphology of vapor-deposited thin films suggests a stronger tendency
of the co-oligomers, if compared to their corresponding homo-oligomers p-terphenyl and terthiophene, to form less amorphous films.