Dipole moment enhanced π-π stacking in fluorophenylacetylenes is carried over from gas-phase dimers to crystal structures propagated through liquid like clusters

The aggregates of monofluorinated phenylacetylenes in the gas-phase, investigated using the IR-UV double resonance spectroscopic method in combination with extensive structural search and electronic structure calculations, reveal the formation of liquid-like clusters with a pi-stacked dimeric core. The structural assignment based on the IR spectra in the acetylenic and aromatic C-H stretching regions suggests that, unlike the parent non-fluorinated phenylacetylene, the substitution of a F atom on the phenyl ring increases the dipole moment, leading to robustness in the formation of a pi MIDLINE HORIZONTAL ELLIPSIS pi stacked dimer, which propagates incorporating C-HMIDLINE HORIZONTAL ELLIPSIS pi_{Ar/Ac} and C-HMIDLINE HORIZONTAL ELLIPSISF interactions involving both acetylenic and aromatic C-H groups. The structural evolution of fluorophenylacetylene aggregates in the gas phase shows marginal effects due to fluorine atom position on the phenyl ring, with substitution in the para-position tending towards phenylacetylene. The present study signifies that the pi MIDLINE HORIZONTAL ELLIPSIS pi stacked dimers act as a nucleus for the growth of higher clusters to which other molecular units are added predominantly via the {Ar}_C-HMIDLINE HORIZONTAL ELLIPSIS pi_{Ar} type of interaction and the dominant interactions present in the crystal structures gradually emerge with increasing cluster size. Based on these features, gas-phase clusters of fluorophenylacetylene are hypothesized as "liquid-like clusters" acting as intermediates in the generation of various polymorphic forms starting from a pi MIDLINE HORIZONTAL ELLIPSIS pi stacked dimer as the core molecular unit.