Parallel-displaced Dimer Research Articles

Dynamics of benzene excimer formation from the parallel-displaced dimer.

Excimers play a key role in a variety of excited-state processes, such as exciton trapping, fluorescence quenching, and singlet-fission. The dynamics of benzene excimer formation in the first 2 ps after S1 excitation from the parallel-displaced geometry of the benzene dimer is reported here. It was simulated via nonadiabatic surface-hopping dynamics using the second-order algebraic diagrammatic construction (ADC(2)). After excitation, the benzene rings take ∼0.5-1.0 ps to approach each other in a parallel-stacked structure of the S1 minimum and stay in the excimer region for ∼0.1-0.4 ps before leaving due to excess vibrational energy. The S1-S2 gap widens considerably while the rings visit the excimer region in the potential energy surface. Our work provides detailed insight into correlations between nuclear and electronic structure in the excimer and shows that decreased ring distance goes along with enhanced charge transfer and that fast exciton transfer happens between the rings, leading to the equal probability of finding the exciton in each ring after around 1.0 ps.

Electronic structure and charge-transport properties of symmetric linear condensed bis-benzothiadiazole derivatives

The optimal geometries and electronic structures of fifteen symmetric linear condensed bis-benzothiadiazole derivatives were investigated using the density functional theory. Depending on the central core variation, the B3LYP energies of the lowest unoccupied molecular orbitals (LUMOs) varies from −2.66 eV to −4.77 eV. The interaction energies and electron drift mobilities were evaluated for model parallel-stacked and parallel-displaced dimer configurations at room temperature using Marcus theory and Einstein relation. The presented theoretical results indicate contradictory role of the pyrazine moiety fused to benzene or p-benzoquinone forming the central molecular core. These model compounds exhibit the highest electron drift mobilities, although the air stability could be lower due to the LUMO energies below −4.0 eV. The theoretical trends obtained from B3LYP calculations were compared with the results from M06-2X range-separated functional. The charge-transport properties of 5H, 11H-anthra [2,3-c:6,7-c']bis ([1,2,5]thiadiazole)-5,11-dione were also theoretically investigated for the parallel-shifted and linear dimer configurations obtained from the X-ray crystal structure.

Density-functional theoretical study of fluorination effect on the electronic structure and electron drift mobilities of symmetric pentacene derivatives

A theoretical study using density functional theory is reported to gain insights how the side-specific symmetric fluorination of pentacene can change the geometry and the electronic structure of its neutral and mono-charged states. The commercial electric p- and n-type semiconductors, pentacene and perfluorinated pentacene, were selected as reference molecules. Depending on the substitution, the energies of the lowest unoccupied molecular orbitals (LUMOs) are changed from –2.92 eV to –3.92 eV. Comparing with the pentacene, the maximal hypsochromic shift of the optical bandgap can be reached by partial outermost substitution. On the other hand, the bathochromic shifts are predicted for the fluorination of inner rings. The potential role of intermolecular hydrogen H⋯F bonds stabilizing the crystal structure was estimated by mutual comparison of interaction energies for parallel-stacked and parallel-displaced dimer configurations. The drift mobilities were evaluated for the studied dimer configurations at the room temperature using the Marcus theory and the Einstein relation. The presented theoretical results indicate contradictory role of the fluorination in preparation of electric n-semiconductors. The increasing number of fluorine atoms improves the LUMO energy level but the dimer electron drift mobilities are decreased.

Vibrational Spectroscopy and Theory of the Protonated Benzene Dimer and Trimer

Protonated benzene cluster ions, H(C(6)H(6))(2)(+) and H(C(6)H(6))(3)(+), are produced in a pulsed electrical discharge source coupled to a supersonic expansion. Mass-selected complexes are investigated with infrared photodissociation spectroscopy in the 1000-3200 cm(-1) region using the method of argon tagging. The IR spectra of H(C(6)H(6))(2)(+)-Ar and H(C(6)H(6))(3)(+)-Ar contain broad bands in the high frequency region resulting from CH-π hydrogen bonds. Sharp peaks are observed in the fingerprint region arising from the ring modes of both the C(6)H(7)(+) and C(6)H(6) moieties. M06-2X calculations have been performed to investigate the structures and vibrational spectra of energetically low-lying configurations of these complexes. H(C(6)H(6))(2)(+) is predicted to have three nearly isoenergetic conformers: the parallel displaced (PD), T-shaped (TS), and canted (C) structures [Jaeger, H. M.; Schaefer, H. F.; Hohenstein, E. G.; Sherrill, C. D. Comput. Theor. Chem. 2011, 973, 47-52]. A comparison of the experimental dimer spectrum with those predicted for the three isomers suggests an average structure between the TS and PD conformers, which is consistent with the low energy barrier predicted to separate these two structures. No evidence is found for the C dimer even though it lies only 1.2 kcal/mol above the PD dimer. Although the trimer is also computed to have many low lying isomers, the IR spectrum limits the possible species present.

Dispersion-Corrected Density Functional Theory for Aromatic Interactions in Complex Systems

Aromatic interactions play a key role in many chemical and biological systems. However, even if very simple models are chosen, the systems of interest are often too large to be handled with standard wave function theory (WFT). Although density functional theory (DFT) can easily treat systems of more than 200 atoms, standard semilocal (hybrid) density functional approximations fail to describe the London dispersion energy, a factor that is essential for accurate predictions of inter- and intramolecular noncovalent interactions. Therefore dispersion-corrected DFT provides a unique tool for the investigation and analysis of a wide range of complex aromatic systems. In this Account, we start with an analysis of the noncovalent interactions in simple model dimers of hexafluorobenzene (HFB) and benzene, with a focus on electrostatic and dispersion interactions. The minima for the parallel-displaced dimers of HFB/HFB and HFB/benzene can only be explained when taking into account all contributions to the interaction energy and not by electrostatics alone. By comparison of saturated and aromatic model complexes, we show that increased dispersion coefficients for sp(2)-hybridized carbon atoms play a major role in aromatic stacking. Modern dispersion-corrected DFT yields accurate results (about 5-10% error for the dimerization energy) for the relatively large porphyrin and coronene dimers, systems for which WFT can provide accurate reference data only with huge computational effort. In this example, it is also demonstrated that new nonlocal, density-dependent dispersion corrections and atom pairwise schemes mutually agree with each other. The dispersion energy is also important for the complex inter- and intramolecular interactions that arise in the molecular crystals of aromatic molecules. In studies of hexahelicene, dispersion-corrected DFT yields "the right answer for the right reason". By comparison, standard DFT calculations reproduce intramolecular distances quite accurately in single-molecule calculations while inter- and intramolecular distances become too large when dispersion-uncorrected solid-state calculations are carried out. Dispersion-corrected DFT can fix this problem, and these results are in excellent agreement with experimental structure and energetic (sublimation) data. Uncorrected treatments do not even yield a bound crystal state. Finally, we present calculations for the formation of a cationic, quadruply charged dimer of a porphyrin derivative, a case where dispersion is required in order to overcome strong electrostatic repulsion. A combination of dispersion-corrected DFT with an adequate continuum solvation model can accurately reproduce experimental free association enthalpies in solution. As in the previous examples, consideration of the electrostatic interactions alone does not provide a qualitatively or quantitatively correct picture of the interactions of this complex.

Benzene−Pyridine Interactions Predicted by the Effective Fragment Potential Method

The accurate representation of nitrogen-containing heterocycles is essential for modeling biological systems. In this study, the general effective fragment potential (EFP2) method is used to model dimers of benzene and pyridine, complexes for which high-level theoretical data -including large basis spin-component-scaled second-order perturbation theory (SCS-MP2), symmetry-adapted perturbation theory (SAPT), and coupled cluster with singles, doubles, and perturbative triples (CCSD(T))-are available. An extensive comparison of potential energy curves and components of the interaction energy is presented for sandwich, T-shaped, parallel displaced, and hydrogen-bonded structures of these dimers. EFP2 and CCSD(T) potential energy curves for the sandwich, T-shaped, and hydrogen-bonded dimers have an average root-mean-square deviation (RMSD) of 0.49 kcal/mol; EFP2 and SCS-MP2 curves for the parallel displaced dimers have an average RMSD of 0.52 kcal/mol. Additionally, results are presented from an EFP2 Monte Carlo/simulated annealing (MC/SA) computation to sample the potential energy surface of the benzene-pyridine and pyridine dimers.

Substituent Effects in C6F6C6H5X Stacking Interactions

Parallel displaced and sandwich configurations of hexafluorobenzene-substituted benzene dimers are studied by ab initio molecular orbital methods up to the MP2(full)/aug-cc-pVDZ level of theory to reveal how substituents influence pi-pi interactions. Two minimum energy configurations are found, one with the substituent group away from the pi-face of the hexafluorobenzene ring (2a-f) and the other with the substituent group on top of the pi-face of the hexafluorobenzene (3a-f). Higher binding energies are predicted for dimers with the substituent on the pi-face (3a-f). All sandwich dimers (4a-e) are found to be saddle points on the potential energy surfaces. A parallel-displaced minimum energy configuration is also predicted for the parent complex, C6F6-C6H6, which is in contrast to predictions based on quadrupole moments of benzene and hexafuorobenzene. The preference for the parallel displaced, rather than the sandwich configuration, is rationalized based on the smaller interplanar distance in the former. The closeness of contact in the parallel-displaced dimers leads to greater binding energies. The shape of the electron density isosurface of the monomers is suggested to provide a guide for the prediction of how arenes stack with one another. A large difference in binding energy between the C6F6 complex of aniline (3e) and N,N-dimethylaniline (3f) is calculated, and charge-transfer interactions are suggested to play a role in the latter.

Monte Carlo Studies of Isomers, Structures, and Properties in Benzene−Cyclohexane Clusters: Computation Strategy and Application to the Dimer and Trimer, (C6H6)(C6H12)n,n= 1−2

Low-temperature isomeric energies, structures, and properties of benzene-cyclohexane clusters are investigated via Monte Carlo simulations. The Monte Carlo strategy is first documented and then applied to (C(6)H(6))(C(6)H(12)) and (C(6)H(6))(C(6)H(12))(2) using four different potential energy surfaces. Results identify a single parallel-displaced dimer isomer. MP2 optimizations and frequency calculations support the Monte Carlo dimer structure and identify the van der Waals mode observed in vibronic spectra. Caloric simulations identify two temperatures where structural transitions occur and imply an experimental temperature below 10 K for dimers in cold supersonic expansions. The (C(6)H(6))(C(6)H(12))(2) studies identify eight independent trimer isomers: three form parallel-stacked (sandwich) arrangements with the two cyclohexane moieties related through a D(6)(h) transformation. The remaining five trimer isomers are trigonal, with no overall symmetry. Caloric studies indicate that the sandwich and trigonal isomeric classes coexist independently below 60 K, consistent with trimer vibronic spectra that contain two independent van der Waals progressions.

Local MP2-Based Method for Estimation of Intermolecular Interactions in Aromatic Molecules. Benzene, Naphthalene, and Pyrimidine Dimers. A Comparison with Canonical MP2 Method

Benzene, pyrimidine, and naphthalene dimers have been studied at canonical MP2, LMP2, and MP4(SDTQ) levels of theory. It has been shown that the LMP2 method is superior to canonical MP2 due to reduced BSSE. Thus, basis set limit-extrapolated LMP2 binding energies of T-shaped and parallel displaced (PD) benzene dimers are nearly the same, as has been found in higher level ab initio calculations. MP4(SDTQ) binding energies calculated at LMP2 optimized geometries are always more negative than those calculated for MP2-optimized geometries. MP4(SDTQ)/CC-pVDZ-corrected complete basis set-extrapolated LMP2 binding energies of T-shaped and PD dimers of benzene and naphthalene were found to be −2.80, −2.59 and −4.39, −6.29 kcal/mol, respectively.

A Quantum Chemistry Study of the van der Waals Dimers of Benzene, Naphthalene, and Anthracene: Crossed (D2d) and Parallel-Displaced (C2h) Dimers of Very Similar Energies in the Linear Polyacenes

A quantum chemistry study of the ground-state structures and binding energies of the van der Waals dimers of benzene, naphthalene, and anthracene has been made at the MP2/6-31G and MP2/6-31+G levels of theory. For naphthalene and anthracene, the calculations yield two low energy dimers of very similar energies: D{sub 2d} (crossed) and C{sub 2h} (parallel-displaced). Conformers, analogous to the T-shaped dimer of benzene, are less stable than the crossed and the parallel-displaced dimers. BSSE-corrected MP2/6-31+G/MP2/6-31G binding energies of the crossed and the parallel-displaced dimers are, respectively, 15.77 and 15.65 kJ/mol for naphthalene and 36.40 and 31.84 kJ/mol for anthracene. The fully optimized MP2/6-31G structures of the parallel-displaced dimer of naphthalene has a horizontal displacement of 1.5 {angstrom} along the short axes of the monomers and a vertical displacement of 3.5 {angstrom}. The corresponding horizontal and vertical separations for the parallel-displaced (PD) dimer of anthracene are 1.1 and 3.6 {angstrom}, respectively. for the crossed dimers, the vertical separation of the aromatic rings is 3.6 {angstrom} for naphthalene and 3.3 {angstrom} for anthracene. The greater binding energy and the shorter horizontal displacement of the anthracene PD dimer relative to the naphthalene PD dimer are consistent with the stronger dispersion interactions expected ofmore » a larger dimer. The greater stability and smaller vertical separation of the crossed dimer of anthracene relative to that of naphthalene can also be attributed to the disparity in the dispersion interactions.« less

Calculations of the geometry and binding energy of aromatic dimers: Benzene, toluene, and toluene–benzene

Calculations of the geometries and energies of the benzene, toluene, and toluene–benzene dimers using a combination of atom–atom and multipolar potentials are presented and discussed. An exponential-six atom–atom potential plus a molecular quadrupole–quadrupole interaction term gives the reported parallel displaced C2h benzene dimer geometry. The excited state behavior of the benzene dimer can be qualitatively understood based on this calculation. The toluene–benzene dimer is well modeled by these terms in addition to a dipole–quadrupole interaction term. Both parallel displaced and perpendicular configurations are obtained in agreement with the experimental data. Using exponential-six dipole–dipole, quadrupole–quadrupole, and dipole–quadrupole interactions the toluene dimer can be satisfactorily modeled. The calculations generally suggest that excitation of parallel displaced dimers can lead to rearrangement (and possible excimer formation) in the excited state.