Dipole Moment Propels π-Stacking of Heterodimers of Fluorophenylacetylenes

Abstract

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.

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

“…It must be pointed out that "π-stacking" in the heterodimer structures is loosely defined based on the geometrical criterion. 14,21 The stacking of heterodimers is further investigated with the energy decomposition analysis using the SAPT0 method, which calculates the contribution of various energy compo- nents such as the electrostatic (E Elec ), induction (E Ind ), dispersion (E Disp ), and exchange (E Exch ) to the total interaction energy (E SAPT0 ). The energy components from SAPT0 analysis for all calculated structures of the heterodimers of PrBz are listed in Tables S1−S4 (see the Supporting Information).…”

“…Similar observations were made in the case of homodimers of PrBz, 19 PHA, 28 2FPHA, 3FPHA, and 4FPHA 20 and heterodimers of PHA with 2FPHA, 3FPHA, and 4FPHA. 21 In an effort to assign the structures responsible for the observed spectra, DFT calculations were carried out for all four sets of heterodimers. A structural search followed by geometry optimization using the M06-2X/aug-cc-pVDZ level of theory (see the Methods section) resulted in 13, 19, 23, and 14 structures for the heterodimers of PrBz with PHA (Figure S2, Tables S1), 2FPA (Figure S3, Table S2), 3FPHA (Figure S4, Table S3), and 4FPHA (Figure S5, Table S4), respectively.…”

“…The electronic spectra of all the monomers (PrBz, PHA, 2FPHA, 3FPHA, and 4FPHA) have been reported earlier and show sharp bands corresponding to the 0 0 0 (band-origin) excitation of the S 0 →S 1 transition (Figure S1, see the Supporting Information). 19,21 The electronic spectra of the heterodimers of PrBz with PHA, 2FPHA, 3FPHA, and 4FPHA, recorded using the one color resonant two-photon ionization (1C-R2PI) method by monitoring the parent mass signal, are depicted in Figure 2. The electronic spectra of the PrBz heterodimers with PHA, 2FPHA, 3FPHA, and 4FPHA are very generally red-shifted and show broad bands, similar to the homodimers of PHA and FPHAs, and can be attributed to several reasons which include (i) excitonic coupling, (ii) presence of multiple isomers, (iii) distributed Franck−Condon factors, and combinations thereof.…”

“…Further, the broadening of the electronic spectra (see Figure 1) in combination with marginal red shifts in the acetylenic C−H stretching vibrations (see Figure 2) have earlier been assigned to (π-)stacked structures of homo-and heterodimers of PHA and FPHAs along with the homodimer of PrBz. [19][20][21]28 Therefore, based on the observed spectra, electronic structure calculations, and comparison with similar systems, the present set of heterodimers of PrBz with PHA and FPHAs are assigned to a set of π-stacked structures within an energy band of 1.5 kJ mol −1 (the mean error for π-stacking interaction energy at the M06-2X level) from the global minima. It must be pointed out that "π-stacking" in the heterodimer structures is loosely defined based on the geometrical criterion.…”

“…The justification for the theoretical methods used in the present work is that the M06-2X/aug-cc-pVDZ method is adequate to reliably calculate the geometries and relative energies of various types of intermolecular interactions including π-stacking interaction 20 and to further compare the present set of results with our earlier work on heterodimers of PHA with FPHA. 21 The analysis of the interaction energies of various structures of heterodimers was carried out using the symmetry-adapted perturbation theory (SAPT). 40 The simplest of the SAPT approaches, SAPT0, was performed using the cc-pVTZ basis set along with the cc-pVTZ-JKFIT basis for the Hartree−Fock and cc-pVTZ-RI basis for the SAPT procedure.…”