Hexamethylbenzene Crystals Research Articles

An NMR study of hexamethylbenzene (HMB) incorporated in 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine (CLPOT) framework

Molecular motion of hexamethylbenzene (HMB) incorporated in 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine (CLPOT) was investigated by 1H NMR spin-lattice relaxation time T1 measurements. In the porous space of one-dimensional (1D) CLPOT nanochannels, HMB molecules reorientate more freely compared to bulk HMB crystals. With low concentrations x < 0.5 of CLPOT-(HMB)x, the distribution of HMB seems to be quite inhomogeneous in 1D nanochannel. The activation energy of the in-plane reorientation of HMB is 5 kJ mol−1 due to the interaction with nanochannel-wall and 10 - 18 kJ mol−1 depending on the separation distance when the interaction between the adjacent HMB molecules determines the activation energy.

Sequential Epitaxial Organization of Poly(9,9-di-n-octyl-2,7-fluorene) in an Eutectic System

The phase diagram of a binary eutectic system of poly(9,9-di-n-octyl-2,7-fluorene) (PFO) and hexamethylbenzene (HMB) has been established via onset melting temperatures. Upon homogeneous mixing, the growth of PFO nematic phase in binary mixtures exhibits both lyotropic and thermotropic features. During cooling, a smectic phase of PFO developed epitaxially on the surface of prior-developed HMB crystals, establishing layer stacking via the packing of side chains. Upon the annealing at 120 °C, further epitaxial organization of PFO smectic packing on HMB crystalline substrate results in the growth of β crystalline. Both smectic and β phases of PFO yield characteristic ultraviolet (UV) absorption at 434 nm wavelength and photoluminescence at the wavelength of 441 nm. While HMB crystals were sublimated prior to the annealing treatment, α crystalline form developed instead through the transformation of smectic packing. The UV absorption and fluorescence of α form are relatively shorter wavelengths. As attributing the rise of energy bandgaps to the effect of backbone conformation, organization of less coplanar backbones within α crystalline form is proposed as a result of the lack of epitaxial effect. After the growth of smectic phase via the monotropic behavior of PFO in eutectic mixture, the following routes of phase transformation upon annealing hence unveiled the growth mechanisms of both α and β crystalline forms of PFO.

Impact of undercooling on epitaxially oriented liquid crystalline organizations of poly(3-hexylthiophene) below the eutectic temperature

For widely studied hole-transporting conjugated poly(3-hexylthiophene) (P3HT), the routes of liquid crystalline (LC) organization below the eutectic point were studied as a consequence of dissolution in molten hexamethylbenzene (HMB). Based on the variation of melting points with mixing compositions, the eutectic condition and phase diagram of these two crystallizable components were disclosed. Upon significant undercooling, the face-on LC organization of conjugated polythiophene epitaxially developed on formerly established HMB crystals. This face-on organization is featured with alternate layer stacking of organized thiophene stems with loosely aggregated regions by a nanometer scale of stacking periodicity. Differently, with a few degrees of undercooling below the eutectic point, edge-on organization preferably prevailed, which results in the anisotropic platelets extending along the chain direction. Regarding these prompt epitaxial LC organizations of conjugated polythiophene, the association of bead-like or worm-like fringe micelles in the molten state emerges as being elucidative for both structural and morphological features. After the sublimation of HMB crystals, the original face-on LC organization transformed to the crystalline packing of edge-on backbones via annealing above 160 °C. This evolution of molecular packing provides a new direction for deciphering the crystallization mechanism of conjugated polymers and the thread-like morphology.

Crystallization-Induced Orientation for Microstructures of Poly(l-lactide)-b-poly(ε-caprolactone) Diblock Copolymers

The formation of large-sized, well-oriented microdomains of poly(l-lactide)-b-poly(ε-caprolactone) (PLLA−PCL) was successfully achieved by using different crystalline substrates including benzoic acid (BA) and hexamethylbenzene (HMB), whereas spherulitic crystalline texture was obtained on amorphous substrates. Crystallization-induced orientation of PLLA−PCL microstructures is attributed to directional crystallization of eutectic type between crystalline PLLA and substrate, regardless of microphase separation. Surprisingly, a flat-on crystalline morphology (i.e., with the chain axis normal to the substrate) was obtained, as evidenced by single-crystal-like [001] zonal electron diffraction (ED). By contrast, an edge-on crystalline morphology (i.e., with the chain axis parallel to the substrate) was observed for PLLA homopolymer under the same condition. The crystallographic organization of PLLA−PCL microdomains as observed by transmission electron microscopy, ED, and scanning probe microscopy suggests that the growth direction of PLLA crystals is along the a-axis direction, which suggests that the growth direction of PLLA crystals is along the a-axis direction that corresponds well with the preferential growth direction (i.e., b-axis direction) of BA or HMB crystals. As a result, the phase-separated lamellar morphology consists of an alternation of a flat-on layer of PLLA lamellae and of an amorphous layer of PLLA and PCL components.

Pressure Effect on Phase Transitions in Hexamethylbenzene Crystals

Abstract The pressure effect on the phase transitions in hexamethylbenzene and hexa(methyl-d3)benzene crystals was studied compared with the temperature effect on the phase transitions. The transition from phase II to phase III of hexa(methyl-d3)benzene crystal takes place under lower pressure than that for hexamethylbenzene crystal. The change in the spectral structure due to the intermolecular vibrations caused by an increase in the pressure indicates that the marked interaction between the intermolecular rotational vibration and the intramolecular methyl torsional vibration of hexa(methyl-d3)benzene crystal takes place under lower pressure than that for hexamethylbenzene crystal. These facts suggest that the intramolecular methyl torsional vibration plays an important role for the II-to-III phase transition in hexamethylbenzene crystals.

Growth and lattice defects of hexamethylbenzene crystals

Single crystals of hexamethylbenzene, C 6(CH 3) 6, were grown from benzene or xylene solutions. The various crystal shapes such as plates, ribbons, whiskers and rhombohedral blocks were observed, the prism planes were (100), (010), (001) and (110). The ribbons and plates have well developed (100) or (001) planes and grew along the (010) axis. Twinned crystals were also observed; the composition plane and the twin boundary is (001). Lattice defects of the crystals were investigated by X-ray topography. Dislocations with Burgers vectors [100], [010] and [1̄10] were observed. The slip plane was (001). Dislocations with the shortest Burgers vector, [001], were not observed. The specific features of dislocations in hexamethylbenzene crystals are discussed in terms of the steric hindrance of molecules to slip.

Charge-Transfer Crystals with Ionic Triplet States: An ESR and ODMR Study of HMB-TCNB and HMBTCPA

Abstract Esr and zero-field Odmr measurements were performed for charge-transfer single crystals of hexamethylbenzene (HMB) with tetracyanobenzene (TCNB) and of HMB with tetrachlorophthalic anhydride (TCPA). Both systems exhibit very high ionic (charge-transfer) character of the triplet excitation as deduced from the small S1,–T1 separation and small fine structure parameters of |D| = |0.0285.|E| = 0.0066 cm I (HMB-TCNB) and |D| = 0.01525, |E| = 0.00342 cm−1 (HMB-TCPA). By comparison with the respective parameters of the neutrally excited acceptors a charge-transfer character is determined as e2 CT = 90% and 100%, respectively. Delocalized states (triplet excitons) are detected in HMB-TCNB in a narrow temperature range (∼ 140 K).

Exciton band of a one-molecule-per-unit-cell crystal: hexamethylbenzene first singlet

The exciton band structure has been determined experimentally, from quantitative band—band fluorescence and absorption measurements, for the first singlet excited state of the low temperature phase of the hexamethylbenzene crystal. At 2 K, the center for the band is at 35156 cm −1, its bottom (the κ = 0 state) at 35134 cm −1 and its total extent about 40 cm −1 (i.e. much larger than anticipated earlier). We derive a dispersion relation, based on the trigonal topology of the crystal, with six out-of-plane nearest neighbor pairwise interactions of −3.3 cm −1 and six in-plane ones of −0.4 cm −1. These parameters, based on the exciton band shape, are inconsistent with a transition-multiple—transition-multipole model, and in particular with an octopole—octopole one, based on the π electrons. π-σ interactions are suggested. The neat crystal data are fully consistent with a complete concentration study on isotopic mixed samples of hexamethylbenzene, at 2 K, by absorption and fluorescence spectra with a 1 cm −1 resolution. The mixed crystal results are also consistent with the energy and spectral moments given by the separated-band model and with the behaviour expected from cluster percolation and exciton percolation.

13C chemical shielding tensors in polyalkylbenzenes

High-resolution double resonance methods have been employed to study the anisotropic 13C chemical shifts in single crystals of hexamethylbenzene, hexaethylbenzene, and pentamethylbenzene. Methyl and methylene shifts have small anisotropies. Shielding of the ring carbons is a maximum when the magnetic field is normal to the ring and a minimum when it lies along the bond to the substituent. The hexaethylbenzene molecule is shown to have D3d symmetry. The orientations of the two molecules in the unit cell of pentamethylbenzene are determined.

ChemInform Abstract: PHONON SIDEBANDS OF LOCALIZED EXCITONS IN MOLECULAR CRYSTALS WITH METHYL TORSIONS, HEXAMETHYLBENZENE

Fluorescence and phosphorescence phonon sidebands of isotopic mixed hexamethylbenzene crystals at 2°K are presented. The external phonons can be observed separately from the semi‐internal (methyl torsion) ones. The nature of the electronic or vibronic state has observable but not drastic effects on the exciton‐phonon coupling function. Likewise, the exciton delocalization is of minor importance to the exciton‐phonon function in hexamethylbenzene. The coupling between external and internal vibrations is also weak in this system. The exciton‐phonon coupling appears comparable for the optical and acoustic phonons in hexamethylbenzene. The phonon sidebands give some of the phonon singularities of the low‐temperature crystal.

Phonon sidebands of localized excitons in molecular crystals with methyl torsions: Hexamethylbenzene

Fluorescence and phosphorescence phonon sidebands of isotopic mixed hexamethylbenzene crystals at 2°K are presented. The external phonons can be observed separately from the semi-internal (methyl torsion) ones. The nature of the electronic or vibronic state has observable but not drastic effects on the exciton-phonon coupling function. Likewise, the exciton delocalization is of minor importance to the exciton-phonon function in hexamethylbenzene. The coupling between external and internal vibrations is also weak in this system. The exciton-phonon coupling appears comparable for the optical and acoustic phonons in hexamethylbenzene. The phonon sidebands give some of the phonon singularities of the low-temperature crystal.

Molecular motions and λ phase transition: Raman and far-ir studies of neat and isotopic mixed hexamethylbenzene crystal

Raman and far-IR studies of hexamethylbenzene (HMB) low temperature crystalline phases (neat and isotopic mixed) are presented. The Raman phonon spectrum changes drastically during the λ-phase transition. Site splittings on two intramolecular modes of HMB- h 18 (but on only one corresponding HMB- d 18 mode) are observed in the room temperature phase (phase II) spectrum. These splittings disappear in the lower temperature phase (phase III). The methyl torsional bands are identified and a significant shift is observed for them during the phase transition. Also, while the λ-phase transition takes place at 113°K (± 2°K) for HMB- h 18 crystal, the transition temperature is 133°K (± 2°K) for HMB- d 18. Our results suggest that (for phonon interactions) the symmetry in phase Ill is close to D 3d and reduces to C i in phase II. Furthermore, the results support the mechanism of phase transition which involves a tilting of the methyl groups out of the benzene ring.

Triplet state geometry of hexachlorobenzene in different hosts at 1.6 °K

The zero-field parameters of the triplet state of hexachlorobenzene (HCB) in HCB x-traps and in hexamethylbenzene (HMB) are determined using PMDR methods. In HCB, D and E are found to be 4.66 (or 6.44) and 0.89 Ghz, respectively. The corresponding values in HMB are found to be 4.621 and 0.045 GHz. These results as well as the difference in the relative band intensities in the phosphorescence spectrum in the two hosts strongly suggest that the geometry of HCB deviates greatly from D 6h symmetry in HCB crystal but only slightly in HMB crystal.

Methyl group rotation and the low temperature transition in hexamethylbenzene. A neutron diffraction study

Neutron diffraction studies of single crystals of hexamethylbenzene at 298 K and at 130 K indicate that the molecule in phase II has approximate D3d symmetry. The amplitudes of libration of the methyl group and of rigid body motions of the molecule are consistent with earlier data, except that the barrier to methyl group rotation appears to be lower by about 0.5 kcal/mol (2100 J/mol).Consideration of intra- and inter-molecular hydrogen atom contact distances and calculated potential energy curves using a 6-exp potential function suggest that intermolecular forces are important in determining the barrier to rotation of the methyl groups and that substantial changes in the intermolecular packing must be responsible for the lambda-point transition at 116 K and the consequent profound change in the potential barrier to internal rotation which has been previously observed.

Intensities in polarized light: A1 g → B2 u transition in hexamethylbenzene

The absorption anisotropy of hexamethylbenzene crystal is investigated at room temperature for the transition occurring at 2400–2900 Å. The f-values in both directions of propagation of the light vector for faces [100] and [001] were measured. Reflectivity corrections and those due to molecular librations are discussed. The total f-value for the transition in the crystal is compared to that observed in the free molecule. The probable orientation of the transition moment induced by the crystal field in the unit cell is indicated.

Triplet—Singlet Luminescence from Methylated Benzenes in the Crystalline State and in Rigid Glass Solutions

In continuation of earlier studies, triplet—singlet emission of the following methylated benzenes is reported in their solid phases: mesitylene, durene, hexamethylbenzene. The spectra were obtained in rigid EPA solutions at 77°K, and in the crystalline state at 77° and at 4°K. Two different spectra were found for mesitylene and durene crystals, one at shorter wavelengths representing the genuine triplet—singlet emission of the substance, and the other at longer wavelengths belonging to the lowest phosphorescence of an oxidation product. Hexamethylbenzene crystal apparently exhibits only one spectrum at any temperature. No solution spectrum was obtained for it because of its poor solubility. For an interpretation of the observations, various processes which take place in these crystals upon light illumination are discussed.

Changes in the Near Ultraviolet Spectrum of Crystalline Hexamethylbenzene Accompanying a Low Temperature Phase Transition

The hexamethylbenzene crystal undergoes a phase transition at 110°K. The near ultraviolet absorption spectra of single oriented crystals of this substance have been studied in polarized light in the neighborhood of the transition temperature. A drastic and discontinuous change of the spectra is observed to accompany the phase change. The spectral change at the transition point is interpreted as indicative of a distortion of the molecule such that only twofold axes remain.

Near Ultraviolet Spectrum of Crystalline Hexamethyl Benzene and the Structure of the Hexamethyl Benzene Molecule

The absorption and fluorescence spectra of single oriented crystals of hexamethyl benzene were photographed at high resolution with polarized light at 20°K in the 2800 A region. The analysis shows that the 0–0 transition is observed with much greater intensity in light polarized perpendicular to the plane of the benzene ring than in light polarized in this plane. In the perpendicular component the vibrational structure of a weak allowed transition appears together with the 0–0 band. The in-plane spectrum is a forbidden transition, similar to the benzene 2600 A absorption. A detailed calculation shows that the perturbation by the crystal lattice cannot be the cause of the observed behavior of the 0–0 band in polarized light. It is concluded, therefore, that this behavior must be caused by the lowering of the molecular symmetry by the methyl groups. The probable symmetry of the molecule is S6. This symmetry approximately fixes the positions of the hydrogen atoms and determines the shape of the methyl rolling potential.

X-ray Investigation of the Thermal Transitions in Some Molecular Crystals.

Various phase changes which occur in crystals of hexamethylbenzene and beryllium oxyacetate have been investigated by X-ray methods.The crystal structure of hexamethylbenzene at high temperatures has been determined by single crystal oscillation method, (Cu Ka, λ=1.54 kX), in order to obtain some information about the anomalous volume change at high temperatures. It belongs to the orthorhombic system. The dimensions of the unit cell which contains four molecules of C6 (CH3) 6 are a=9.06 kX, b=15.70 kX and c=7.52 kX at 155°C. The space group was found to be D232h-Fmmm and the determined atomic parameters are shown in Table 2. Powder photographs were also taken at various temperatures, from which the change of the dimensions of the unit cell was deduced. The determined structure was discussed comparing with the structure at room temperatures which has been very accurately analyzed by Robertson and Brockway. The mechanism of the anomalous volume change was briefly discussed taking the thermal movements of the molecules into account.The crystal structures of different modifications of beryllium oxyacetate have been investigated by X-ray methods, (Cu Ka) . An attempt has been made to derive the expression for the change of the Bragg and diffuse reflections of X-rays associated with the transition about 40°C, from the viewpoint of the thermal movements of the acetate groups. It was shown that the derived intensity expression could account for the characteristic feature of the observed X-ray scattering.At 147°C the phase change of the first order occurs and when the temperature is raised above this point the substance assumes gradually the appearance of soft wax, though it melts sharply at 285°C. Powder photographs were taken at various temperatures. Most of the lines observed could be explained as due to a rhombohedral unit cell with dimensions a=9.72 kX. ar=54°20', containing one molecule in it. It will be noteworthy that this unit cell dimension is closely related to the cubic closest packing of spheres (rhombohedral lattice with ar=58°12') . This fact, combined with the observation that the X-ray intensity distribution in the powder photograph taken just below the melting point showed much resemblance to the liquid halo slightly above the melting point, suggests that the liquid state near the melting point may be considered to be cubic closest packing of molecules. Another modification of Be-oxyacetate can be obtained by sublimation, the crystal data of which are given.