Hydrogen bonds vs. π-stacking interactions in the p-aminophenolp-cresol dimer: an experimental and theoretical study

Noncovalently bound excited-state dimers: a perspective on current time-dependent density functional theory approaches applied to aromatic excimer models

Excimers are supramolecular systems whose binding strength is influenced by many factors that are ongoing challenges for computational methods, such as charge transfer, exciton coupling, and London dispersion interactions. Treating the various intricacies of excimer binding at an adequate level is expected to be particularly challenging for time-dependent Density Functional Theory (TD-DFT) methods. In addition to well-known limitations for some TD-DFT methods in the description of charge transfer or exciton coupling, the inherent London dispersion problem from ground-state DFT translates to TD-DFT. While techniques to appropriately treat dispersion in DFT are well-developed for electronic ground states, these dispersion corrections remain largely untested for excited states. Herein, we aim to shed light on current TD-DFT methods, including some of the newest developments. The binding of four model excimers is studied across nine density functionals with and without the application of additive dispersion corrections against a wave function reference of SCS-CC2/CBS(3,4) quality, which approximates select CCSDR(3)/CBS data adequately. To our knowledge, this is the first study that presents single-reference wave function dissociation curves at the complete basis set level for the assessed model systems. It is also the first time range-separated double-hybrid density functionals are applied to excimers. In fact, those functionals turn out to be the most promising for the description of excimer binding followed by global double hybrids. Range-separated and global hybrids-particularly with large fractions of Fock exchange-are outperformed by double hybrids and yield worse dissociation energies and inter-molecular equilibrium distances. The deviation between each assessed functional and reference increases with system size, most likely due to missing dispersion interactions. Additive dispersion corrections of the DFT-D3(BJ) and DFT-D4 types reduce the average errors for TD-DFT methods but do so inconsistently and therefore do not offer a black-box solution in their ground-state parametrised form. The lack of appropriate description of dispersion effects for TD-DFT methods is likely hindering the practical application of the herein identified more efficient methods. Dispersion corrections parametrised for excited states appear to be an important next step to improve the applicability of TD-DFT methods and we hope that our work assists with the future development of such corrections.

Aromaticity effect on supramolecular aggregation. Aromatic vs. cyclic monohydroxy alcohols

In this paper, the steric hindrance effect related to the presence of either a cyclic or aromatic ring on the self-association process in the series of monohydroxy alcohols (MAs), from cyclohexanemethanol to 4-cyclohexyl-1-butanol and from benzyl alcohol to 4-phenyl-1-butanol, was studied using X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC), Fourier Transform Infrared (FTIR) spectroscopy, Broadband Dielectric Spectroscopy (BDS) and the Pendant Drop (PD) methods. Based on FTIR results, it was shown that phenyl alcohol (PhA) and cyclohexyl alcohol (CA) derivatives reveal substantial differences in the association degree, the activation energy of dissociation, and the homogeneity of supramolecular nanoassociates suggesting that the phenyl ring exerts a stronger steric impact on the self-assembling of molecules than cyclohexyl one. Additionally, XRD data revealed that phenyl moiety introduces more heterogeneity in the organization of molecules compared to the cyclic one. The changes in the self-association process of alcohols were also reflected in differences in the molecular dynamics of the H-bonded aggregates, as well as in the Kirkwood factor, defining the long-range correlation between dipoles, which were slightly higher for CAs with respect to those determined for PhAs. Unexpectedly it was also found that the surface layers of PhAs were more organized than those formed by CAs. Thus, these findings provided insight into the impact of aromaticity on the self-assembly process, H-bonding pattern, supramolecular structure, and intermolecular dynamics of the studied alcohols.

Noncovalently bound excited-state dimers: a perspective on current time-dependent density functional theory approaches applied to aromatic excimer models

Excimers are supramolecular systems whose binding strength is influenced by many factors that are ongoing challenges for computational methods, such as charge transfer, exciton coupling, and London dispersion interactions. Treating the various intricacies of excimer binding at an adequate level is expected to be particularly challenging for Time-Dependent Density Functional Theory (TD-DFT) methods. In addition to well-known limitations for some TD-DFT methods in the description of charge transfer or exciton coupling, the inherent London dispersion problem from ground-state DFT translates to TD-DFT. While techniques to appropriately treat dispersion in DFT are well-developed for electronic ground states, these dispersion corrections remain largely untested for excited states. Herein, we aim to shed light on current TD-DFT methods, including some of the newest developments. The binding of four model excimers is studied across nine density functionals with and without the application of additive dispersion corrections against a wave function reference of SCS-CC2/CBS(3,4) quality, which approximates select CCSDR(3)/CBS data adequately. To our knowledge, this is the first study that presents single-reference wave function dissociation curves at the complete basis set level for the assessed model systems. It is also the first time range-separated double-hybrid density functionals are applied to excimers. In fact, those functionals turn out to be the most promising for the description of excimer binding followed by global double hybrids. Range-separated and global hybrids-particularly with large fractions of Fock exchange-are outperformed by double hybrids and yield worse dissociation energies and inter-molecular equilibrium distances. The deviation between each assessed functional and reference increases with system size, most likely due to missing dispersion interactions. Additive dispersion corrections of the DFT-D3(BJ) and DFT-D4 types reduce the average errors for TD-DFT methods but do so inconsistently and therefore do not offer a black-box solution in their ground-state parametrised form. The lack of appropriate description of dispersion effects for TD-DFT methods is likely hindering the practical application of the herein identified more efficient methods. Dispersion corrections parametrised for excited states appear to be an important next step to improve the applicability of TD-DFT methods and we hope that our work assists with the future development of such corrections.

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.

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.

Rotamers of p‑isopropylphenol studied by hole-burning resonantly enhanced multiphoton ionization and mass analyzed threshold ionization spectroscopy

The resonance enhanced multiphoton ionization (REMPI), ultraviolet-ultraviolet (UV-UV) hole burning and mass analyzed threshold ionization (MATI) spectroscopy have been applied to investigate the vibrational features of p‑isopropylphenol in its first electronically excited state S₁ and cationic ground state D₀. Two stable conformational structures of p‑isopropylphenol are distinctly found in the supersonic molecular beam and identified as the cis and trans rotamers through REMPI and UV-UV hole burning spectroscopy. The electronic excitation energies of S₁ ← S₀ transition of two rotamers are determined to be 35,578 and 35,593 cm^-1, and the adiabatic ionization energies are 65,331 and 65,350 cm^-1, respectively. The MATI spectra recorded via different intermediate levels of S₁ state indicate the similarity in the molecular geometry between the S₁ state and the D₀ state for each rotamer of p‑isopropylphenol. Geometrical optimizations of p‑isopropylphenol have also been performed using the density functional theory (DFT) for S₀ and D₀ states, and time-dependent density functional theory (TDDFT) for S₁ state. The simulated spectra for S₁ ← S₀ and D₀ ← S₁ transitions of two rotamers are able to reproduce qualitatively the experimental spectral profile, which help us to assign the vibronic modes. Most of the observed vibrations of two rotamers in the S₁ and D₀ states are related to the in-plane ring deformation and some active modes involving isopropyl group.

Unexpected solvent effects on the UV/Vis absorption spectra of o-cresol in toluene and benzene: in contrast with non-aromatic solvents

Cresol is a prototype molecule in understanding intermolecular interactions in material and biological systems, because it offers different binding sites with various solvents and protonation states under different pH values. It is found that the UV/Vis absorption spectra of o-cresol in aromatic solvents (benzene, toluene) are characterized by a sharp peak, unlike the broad double-peaks in 11 non-aromatic solvents. Both molecular dynamics simulations and electronic structure calculations revealed the formation of intermolecular π-complexation between o-cresol and aromatic solvents. The thermal movements of solvent and solute molecules render the conformations of o-cresol changing between trans and cis isomers. The π-interaction makes the cis configuration a dominant isomer, hence leading to the single keen-edged UV/Vis absorption peak at approximately 283 nm. The free conformation changes between trans and cis in aqueous solution rationalize the broader absorption peaks in the range of 260-280 nm. The pH dependence of the UV/Vis absorption spectra in aqueous solutions is also rationalized by different protonation states of o-cresol. The explicit solvent model with long-ranged interactions is vital to describe the effects of π-complexation and electrostatic interaction on the UV/Vis absorption spectra of o-cresol in toluene and alkaline aqueous (pH > 10.3) solutions, respectively.