Dipole Moment Propels π-Stacking of Heterodimers of Fluorophenylacetylenes

Π-Stacking in Heterodimers of Propargylbenzene with (Fluoro)phenylacetylenes

The heterodimers of propargylbenzene (PrBz) with phenylacetylene (PHA) and monosubstituted fluorophenylacetylenes (FPHAs) were investigated using electronic and vibrational spectroscopic methods. The vibrational spectra in the acetylenic C-H stretching region show a marginal shift (0-4 cm-1) upon dimer formation, which suggests minimal perturbation of the acetylenic group. The M06-2X/aug-cc-pVDZ calculations indicate that the π-stacked structures are the most stable, followed by other structures. In general, structures incorporating aromatic C-H···π interactions are much higher in energy. The appearance of the spectra and the energy considerations clearly indicate the preference for the π-stacked structures. Furthermore, the observed trend in the stabilization energies for heterodimers with the three FPHAs is inversely proportional to the dipole moments of FPHAs. On the other hand, the absence of any clear trends in the electrostatic component of the interaction energy is attributed to the presence of the methylene group in PrBz.

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 π-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 ππ stacked dimer, which propagates incorporating C-Hπ_{Ar/Ac} and C-HF 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 ππ stacked dimers act as a nucleus for the growth of higher clusters to which other molecular units are added predominantly via the {Ar}_C-Hπ_{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 ππ stacked dimer as the core molecular unit.

Hierarchy of π-stacking determines the conformational preferences of bis-squaramates

Orientation of squaramate dimers in crystal structures is tuned by varying the degree of cooperativity between hydrogen bonding and π-stacking.