Correlation between molecular conformation, packing, and dynamics in oligofluorenes: a theoretical/experimental study

Fluorene-based systems have shown great potential as components in organic electronics and optoelectronics (organic photovoltaics, OPVs, organic light emitting diodes, OLEDs, and organic transistors, OTFTs). These systems have drawn attention primarily because they exhibit strong blue emission associated with relatively good thermal stability. It is well-known that the electronic properties of polymers are directly related to the molecular conformations and chain packing of polymers. Here, we used three oligofluorenes (trimer, pentamer, and heptamer) as model systems to theoretically investigate the conformational properties of fluorene molecules, starting with the identification of preferred conformations. The hybrid exchange-correlation functional, OPBE, and ZINDO/S-CI showed that each oligomer exhibits a tendency to adopt a specific chain arrangement, which could be distinguished by comparing their UV/vis electronic absorption and (13)C NMR spectra. This feature was used to identify the preferred conformation of the oligomer chains in chloroform-cast films by comparing experimental and theoretical UV/vis and (13)C NMR spectra. Moreover, the oligomer chain packing and dynamics in the films were studied by DSC and several solid-state NMR techniques, which indicated that the phase behavior of the films may be influenced by the tendency that each oligomeric chain has to adopt a given conformation.     

Crystal and molecular structures of two sulfonium ylides: Influence of secondary S...O interactions on the molecular conformation and packing

Results are reported of an x-ray structure study of two sulfonium ylides formed by reaction of dimethylacetylenedicarboxylate with DMSO. It is demonstrated that one of these forms inter- and the other intramolecular secondary S...O interactions. In the first case they affect the molecular packing in the crystal; in the second, the molecular conformation.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 81–87, January, 1991.The authors thank V. A. Palyulin for calculating the geometry of the cyclic fragment using the Kramer-Pople method.     

Structure and conformation in molecular peroxides

Molecular structures and energies have been calculated in the MINDO approximation for fifteen neutral and anionic peroxides: fully optimized torsional potential functions have been calculated for twelve of these, and torsional potential functions, subject to constrained optimizations, for a further two peroxides. Bond dissociation energies D(R₁O—OR₂) were also calculated. Equilibrium structures and energies were calculated for the polyoxo species H₂O_n, HO_nF, F₂O_n, HO_n and FO_n (n ? 4), and a complete set of bond dissociation energies derived for H₂O_n, HO_nF, and F₂O_n.     

Structure and packing arrangement of molecular compounds

The calculation of electronic structures of TCNQ and several electron-donor molecules has been performed by the Iterative Extended Hückel Method, taking into account all valence electrons of various atoms and retaining the overlap integrals. Calculations of intermolecular overlaps involving the lowest vacant molecular orbital of TCNQ and one or more highest occupied orbitals of each of the donors have been correlated with the observed donor-TCNQ geometrical configurations. The results of this simple approach, which is based on Mulliken's “Overlap and Orientation Principle”, suggest that the charge-transfer interaction plays an important role in the particular observed packing arrangement in the solid state.     

The interlayer swelling and molecular packing in organoclays

Understanding the interlayer swelling and molecular packing in organoclays is important to the formation and design of polymer nanocomposites. This paper presents recent experimental and molecular simulation studies on a variety of organoclays that show a linear relationship between the increase of d-spacing and the mass ratio between organic and clay. A denser molecular packing is observed in organoclays containing surfactants with hydroxyl-ethyl units. Moreover, our simulation results show that the head (nitrogen) groups are essentially tethered to the clay surface while the long hydrocarbon chains tend to adopt a layering structure with disordered conformation, which contrasts with the previous assumptions of either the chains lying parallel to the clay surface or being tilted at rather precise angles.     

Synthesis and characterization of monodisperse oligofluorenes

An efficient synthesis of 9,9-bis(2-ethylhexyl)fluorene oligomers up to the heptamer is reported, with repetitive Suzuki and Yamamoto coupling reactions employed in the synthesis. The key steps for preparation of the essential intermediates include Pd-catalyzed transformation of aryl bromides to aryl boronic esters (Miyaura reaction) and the application of the much higher reactivity of aryl boronic esters over aryl bromides in the Pd-catalyzed cross-coupling reaction with aryl diazonium salts. Variation of the UV/Vis absorption and photoluminescence characteristics with chain length is reported. Moreover, glass transition and liquid-crystal characteristics of the oligomers are described and compared with those of the polymer.     

Synthesis of oligofluorenes by endcapping

An efficient one-step synthesis of 9,9-di(2-ethylhexyl)-2,7-fluorene oligomers via an endcapping reaction is reported. Controlled endcapping demands a full conversion of functional groups and thus the Yamamoto reaction was chosen as the aryl-aryl coupling method. SEC analysis showed that the endcapping is complete. The molecular weights were adjusted in the range from 1300 to 3800 g mol^-1 by different amounts of endcapper. The mixtures exhibit broad mesophases and the transition temperatures strongly increase with molecular weight. In the series of oligomers reported here, clearing temperatures between 57°C and over 360°C could be realized.     

Synthesis and energy-transfer properties of hydrogen-bonded oligofluorenes

A set of fluorene oligomers has been synthesized by stepwise palladium-catalyzed (Suzuki) couplings of fluorene monomers. Ureidopyrimidinones (UPy), functional groups that can dimerize via quadruple hydrogen bonds, were attached to both ends of the oligofluorenes. The resulting bis-UPy-terminated oligomers self-assemble into supramolecular chain polymers. For comparison, oligofluorenes of the same oligomer lengths but without terminal hydrogen-bonding groups were synthesized. Chains of hydrogen-bonded fluorenes can be simply endcapped by a variety of chain stoppers, molecules that have one UPy group. In this manner, we have endcapped the hydrogen-bonded fluorene chains with either oligo(p-phenylenevinylene) or perylene bisimide. Energy-transfer experiments in solution and the solid state demonstrate that oligofluorenes can donate energy to a variety of energy acceptors, but that this energy transfer occurs most effectively when the donor fluorene is hydrogen-bonded to the acceptor.     

Infrared dichroism and molecular conformation of crystalline tetramethyldisiloxanediol

Vibrational spectra of tetramethyldisiloxanediol (TMDD) have been studied in solution and also in the solid state. Polarized IR spectra of oriented crystalline films have been recorded in the frequency range 4000–200 cm^?1. The structure of the molecules in solution has been found to be of C_2v symmetry. On the basis of the IR dichroitic data obtained for the oriented crystalline films, the molecules should have no symmetry in crystals (C₁ site symmetry) and the structure of OC₂SiOSiC₂O skeletal fragment may be described with an angle of torsion of about 70° around the (Si)O-Si bond.     

The molecular structure and conformation of 1,2-cyclononadiene

The molecular structure and conformation of 1,2-cyclononadiene has been studied by the gas electron diffraction method. The experimental data were interpreted in terms of an unsymmetrical (C₁) molecular model and there were no indications of other conformers being present.     

The molecular structure and conformation of 1,2,6-cyclononatriene

The molecular structure and conformation of 1,2,6-cyclononatriene were studied by the gas electron diffraction method. Two different models, differing primarily in the degree of distortion from planarity of the bonds at the isolated CC double bond, give satisfactory agreement with the experimental data.     

Variety of the molecular conformation in Peptide nanorings and nanotubes

Possible molecular conformations in peptide nanorings and nanotubes were theoretically investigated by a mathematical conformation analysis as well as ab initio Hartree-Fock calculations. The mathematical analysis predicts not only the conventional nanorings having an extended-type (E-type) backbone (trans zigzag) but also the novel ones having bound-type (B-type) backbones with a smaller internal diameter. Ab initio calculations for the amino acid substitution reveal that all 20 encoded residues can form both types of the above nanorings as a local minimum. However, the energetically stable type is determined in accordance with the kind of the replaced side chains. Moreover, the present work theoretically reveals that both types of nanorings stack to form nanotubes through inter-ring hydrogen bonds, i.e., larger E-type nanotubes and smaller B-type nanotubes. Electronically, the HOMO and LUMO states of the nanoring and nanotube backbones are formed by the in-plane pi state. The replacement by the appropriate residues is furthermore predicted to intrude additional levels in the energy gap and to form the frontier states localized at the side chains.     

Structural disturbance and lattice relaxation in molecular crystal packing calculations

Lattice calculations on impurity-containing crystals are reported, made with the help of standard intermolecular potentials. A lattice containing a substitutional impurity, or a vacancy, relaxes in its vicinity, the equilibrium structure being little changed in either orientation or translation. The small changes do however enable the strain energy to be greatly reduced. The energies are sensitive to the assumed intermolecular potentials but the optimised structures are rather insensitive. An impurity larger than the host (tetracene in anthracene) adopts closely the host orientation. This applies also to a smaller impurity (e.g. napthalene in anthracene) but in this case the impurity may move away from the lattice site to one of two inversion related displaced positions. The cage surrounding a vacant site relaxes little, leaving the “hole” more or less intact. Lattice relaxations found in this way seem too small to accommodate guest—host photochemical reactions in those lattices which, in the pure crystal, are photochemically stable.     

Understanding glycine conformation through molecular orbitals

The four most stable C(s) conformers of glycine have been investigated using a variety of quantum-mechanical methods based on Hartree-Fock theory, density-functional theory (B3LYP and statistical average of orbital potential), and electron propagation (OVGF) treatments. Information obtained from these models were analyzed in coordinate and momentum spaces using dual space analysis to provide insight based on orbitals into the bonding mechanisms of glycine conformers, which are generated by rotation of C-O(H) (II), C-C (III), and C-N (IV) bonds from the global minimum structure (I). Wave functions generated from the B3LYP/TZVP model revealed that each rotation produced a unique set of fingerprint orbitals that correspond to a specific group of outer valence orbitals, generally of a' symmetry. Orbitals 14a', 13a', 12a', and 11a' are identified as the fingerprint orbitals for the C-O(H) (II) rotation, whereas fingerprint orbitals for the C-C (III) bond rotation are located as 16a' [highest occupied molecular orbital (HOMO)], 15a' [next highest molecular occupied molecular orbital (NHOMO)], 14a', and 12a' orbitals. Fingerprint orbitals for IV generated by the combined rotations around the C-C, C-O(H), and C-N bonds are found as 16a', 15a', 14a', 13a', and 11a', as well as in orbitals 2a" and 1a". Orbital 14a' is identified as the fingerprint orbital for all three conformational processes, as it is the only orbital in the outer valence region which is significantly affected by the conformational processes regardless rotation of which bond. Binding energies, molecular geometries, and other molecular properties such as dipole moments calculated based on the specified treatments agree well with available experimental measurements and with previous theoretical calculation.     

Effect of molecular packing on corannulene-based materials electroluminescence

The present investigation reports for the first time a detailed theoretical analysis of the optical absorption spectra of corannulene-based materials using state-of-the-art first-principles many-body GW-BSE theory. The study specifically addresses the nature of optical excitations for predictions regarding suitability for device fabrication. The well-defined structure-correlation relationship in functionalized corannulenes is used in a focused investigation of the predicted optoelectronic properties in both the isolated state and bulk crystals. The findings suggest that the excitonic properties are strongly dependent on the specific substituent group as well as the crystalline arrangement. Arylethynyl-substituted corannulene derivatives are shown to be the most suitable for device purposes.     

Polarons, bipolarons, and side-by-side polarons in reduction of oligofluorenes

The nature of charge carriers in conjugated polymers was elucidated through optical spectroscopy following single- and multielectron reduction of 2,7-(9,9-dihexylfluorene) oligomers, F(n), n = 1-10, yielding spectra with the two bands typical of polarons upon single reduction. For short oligomers addition of a second electron gave a single band demonstrating the classic polaron-bipolaron transition. However, for long oligomers double reductions yielded spectra with two bands, better described as two polarons, possibly residing side-by-side in the F(n) chains. The singly reduced anions do not appear to delocalize over the entire length of the longer conjugated systems; instead they are polarons occupying approximately four fluorene repeat units. The polarons of F(3) and F(4) display sharp absorption bands, but for longer oligomers the bands broaden, possibly due to fluctuations of the lengths of these unconfined polarons. DFT calculations with long-range-corrected functionals were fully consistent with the experiments describing polarons in anions, bipolarons in dianions of short oligomers, and side-by-side polarons in dianions of long oligomers, while results from standard functionals were not compatible with the experimental results. The computations found F(10)(2-), for example, to be an open-shell singlet ( ≈ 1), with electrons in two side-by-side orbitals, while dianions of shorter oligomers experienced a gradual transition to bipolarons with states of intermediate character at intermediate lengths. The energies and extinction coefficients of each anionic species were measured by ultraviolet-visible-near-infrared absorption spectroscopy with chemical reduction and pulse radiolysis. Reduction potentials determined from equilibria mirrored oxidation potentials reported by Chi and Wegner. Anions of oligomers four or more units in length contained vestigial neutral (VN) absorption bands that arise from neutral parts of the chain. Energies of the VN bands correspond to those of oligomers shorter by four units.