Dipole Moment Propels π-Stacking of Heterodimers of Fluorophenylacetylenes

Electronic and vibrational spectroscopic investigations in combination with quantum chemical calculations were carried out to probe the formation of four sets of heterodimers of phenylacetylene with 2-fluorohenylacetylene, 3-fluorophenylacetylene, 4-fluorophenylacetylene, and 2,6-difluorophenylacetylene. The interaction of phenylacetylene with fluorophenylacetylenes leads to marginal (2-9 cm^-1) red-shifts in the acetylenic C-H stretching frequencies of fluorophenylacetylenes, which suggests that constituent monomers are minimally perturbed in the heterodimer. On the other hand, the density-functional-theory-based calculations indicate that π-stacked structures outweigh other structures incorporating C-H···π and C-H···F interactions by about 8 kJ mol^-1 or more. The IR spectra in the acetylenic C-H stretching region were interpreted based on the perturbed dipole model, which suggests formation of predominantly antiparallel π-stacked structures, propelled by dipole moment. However, the energy decomposition analysis suggests that among stabilizing components dispersion dominates, while electrostatics plays a pivotal role in the formation of the π-stacked structures. Interestingly, the ability of 2-fluorophenylacetylene and 2,6-difluorophenylacetylene to π-stack differs significantly, even though both of them have almost identical dipole moments and the dipole moment propels the formation of π-stack structures. These results suggest π-stacking transcends the classical electrostatic description in terms of dipole moment.