Albert Fratini

Crystal structures, phase transitions and energy calculations of poly(p-phenylene) oligomers

The room temperature crystal structures, unit cell dimensions at 110K and phase transitions of three poly(p-phenylene) oligomers are reported. The structures of p-quinquephenyl (PQP), C30H22,p-sexiphenyl (PSP), C36H26, and p-septiphenyl (PSeptiP), C42H30, each belonging to space group P21/c, are similar to those of shorter oligomers. The unit cell dimensions are a = 22.056 A, b = 5.581 A, c = 8.070 A and β = 97.91° for PQP, a = 26.241 A, b = 5.568 A, c = 8.091 A and β = 98.17 ° for PSP, and a = 30.577 A, b = 5.547 A, c = 8.034 A and β = 100.52 ° for PSeptiP. The a axis increases with molecular length. The molecules are linear and planar in all three structures. The herringbone nature of the packing is similar for PQP and PSeptiP, while a considerably greater tilt occurs in PSP. At 110K, the unit cell parameters b and c are approximately doubled while a remains nearly the same as in the room temperature cell. A time-dependent solid state transition is observed for PQP, PSP and PSeptiP when crystals are cooled to 110K. At elevated temperatures, thermal measurements indicate the oligomers to be thermotropic liquid crystals. The crystal-smectic transition temperatures are reported for PQP, PSP, PSeptiP and p-octiphenyl (POP), C48H34. The results of a molecular mechanics study on the conformation and packing of PSP are also presented. The competition between intramolecular forces (such as ortho hydrogen repulsions) and intermolecular crystal packing forces was examined in particular. Molecular mechanics calculations predict non-planar conformations in isolated polyphenyls, implying that conjugation between phenyl rings is insufficient to overcome ortho hydrogen repulsions. In a crystalline environment, however, intermolecular forces tend to force a planar conformation. Calculations on arrays of PSP molecules show that changing the phenyl-phenyl torsion angles from the coplanar value increases the total energy of the structure. The most favourable intermolecular interactions between oligomers are achieved for conformations having the phenyl rings coplanar.

Evidence for the presence of hydrogen-bonded ion-ion interactions in the molten salt precursor, 1-methyl-3-ethylimidazolium chloride

Abstract The crystal structure analysis and IR study of the molten salt precursor, 1-methyl-3-ethylimidazolium chloride (MEICl) has been undertaken as part of an investigation of the ion-ion interactions in room temperature melts, where the mole fraction of AlCl 3 is less than 0.5. Hygroscopic crystals of MEICl have been grown in acetonitrile and sealed under helium gas in a capillary tube. The orthorhombic space group is P 2 1 2 1 2 1 , with a = 10.087(1), b = 11.179(1), c = 28.733(4) A, V = 3240.0 A 3 , mol. wt. = 146.62 and D calc = 1.204 g cm −3 for Z = 16. The asymmetric unit contains four MEI + ⋯Cl − ion pairs. The MEI + ions cluster in four distinct layers perpendicular to the c axis. Similarly, the arrangement of Cl − ions is a layered one. Each Cl − interacts with three MEI + ions and each MEI + is associated with three nearest Cl − ions. The distance of Cl − from a ring carbon atom averages 3.55 A. Cl − ions are situated in hydrogen-bonded positions rather than at random, characteristic of a CH⋯Cl − hydrogen-bond interaction. Evidence for the presence of hydrogen bonding of Cl − at the three ring CH bonds in basic MEICl/AlCl 3 melts is presented.

Refinement of the structure of PEEK fibre in an orthorhombic unit cell

Abstract The crystalline structure of oriented fibres of poly(ether-ether-ketone) (PEEK) has been analysed by X-ray diffraction and refined by the ‘linked-atom least-squares’ procedure. The conformation of chains, consisting of a six-aryl ring unit, is approximated by the refinement of a two-aryl ring unit within the orthorhombic unit subcell with dimensions: a=7.83±0.02 A , b=5.94±0.01 A and c=9.86±0.04 A . Certain constraints imposed by Pbcn space group symmetry are relaxed during refinement. The results of the two-ring refinement indicate that a single torsion angle can be used to describe the conformation of the six-ring unit. The torsion angle corresponds to the average tilt of the phenylene rings out of the (100) face, and the best fit is obtained with an angle of 37°. The simulated powder diffraction pattern based on the atomic coordinates of the six-ring unit matches very closely previously reported patterns for a variety of PEEK specimens. The analysis supports and extends to oriented fibres the previously reported finding that space group Pbcn is a valid representation for the structure of PEEK.

Molecular Packing and Crystalline Order in Polybenzobisoxazole and Polybenzobisthiazole Fibers

The structures of poly(p-phenylenebenzobisthiazole) (PBZT) and poly(p-phenylenebenzobisoxazole) (PBO) fibers have been investigated by fiber diffraction techniques. d-spacings were obtained from equatorial and meridional scans recorded on a four-circle diffractometer. Intensity data were derived from x-ray rotation patterns taken on Weissenberg and vacuum cylindrical cameras. Unit cells were found to be monoclinic and non-primitive, each containing two chains per cell of dimensions a = 11.79(2), b = 3.539(5), c = 12.514(9) A, γ = 94.0(2) o for PBZT; and = 11.20(1), b = 3.540(2), c = 12.050(3) A, γ = 101.3(1) for PBO. The fiber axes correspond to c. The conformational torsion angle between the bisthiazole and phenylene units and the orientation of chains within the unit cells were obtained from a ‘linked-atom least-squares’ (LALS)refinement procedure. A packing model is proposed for each polymer in which two independent molecular chains are displaced longitudinally by discrete rather than random increments. Primitive unit cells (Z = 1), besides requiring perfect axial registry of molecular chains, suffer from the occurrence of short intermolecular contacts and are rejected from further consideration.

Photophysical properties of a series of electron-donating and -withdrawing platinum acetylide two-photon chromophores.

To explore spectroscopic structure-property relationships in platinum acetylides, we synthesized a series of complexes having the molecular formula trans-bis(tributylphosphine)-bis(4-((9,9-diethyl-7-ethynyl-9H-fluoren-2-yl)ethynyl)-R)-platinum. The substituent, R = NH(2), OCH(3), N(phenyl)(2), t-butyl, CH(3), H, F, benzothiazole, CF(3), CN, and NO(2), was chosen for a systematic variation in electron-donating and -withdrawing properties as described by the Hammett parameter σ(p). UV/vis, fluorescence, and phosphorescence spectra, transient absorption spectra on the fs-ps time scale, and longer time scale flash photolysis on the ns time scale were collected. DFT and TDDFT calculations of the T(1) and S(1) energies were performed. The E(S) and E(T) values measured from linear spectra correlate well with the calculated results, giving evidence for the delocalized MLCT character of the S(1) state and confinement of the T(1) exciton on one ligand. The calculated T(1) state dipole moment ranges from 0.5 to 14 D, showing the polar, charge-transfer character of the T(1) state. The ultrafast absorption spectra have broad absorption bands from 575 to 675 nm and long wavelength contribution, which is shown from flash photolysis measurements to be from the T(1) state. The T(1) energy obtained from phosphorescence, the T(1)-T(n) transition energy obtained from flash photolysis measurements, and the triplet-state radiative rate constant are functions of the calculated spin density distribution on the ligand. The calculations show that the triplet exciton of chromophores with electron-withdrawing substituents is localized away from the central platinum atom, red-shifting the spectra and increasing the triplet-state lifetime. Electron-donating substituents have the opposite effect on the location of the triplet exciton, the spectra, and the triplet-state lifetime. The relation between the intersystem crossing rate constant and the S(1)-T(1) energy gap shows a Marcus relationship with a reorganization energy of 0.83 eV. The calculations show that intersystem crossing occurs by conversion from a nonpolar, delocalized S(1) state to a polar, charge-transfer T(1) state confined to one ligand, accompanied by conformation changes and charge transfer, supporting the experimental evidence for Marcus behavior.

The structure of poly-2,5-benzoxazole (ABPBO) and poly-2,6-benzothiazole (ABPBT) fibers by X-ray diffraction

Abstract The determination of the crystalline structure of oriented fibers of poly-2,5-benzoxazole (ABPBO) and poly-2,6-ben-zothiazole (ABPBT) is described. Both unit cells are metrically orthorhombic, with the parameters: a = 6.061 (17), b = 3.384 (13), c (fiber axis) = 11.575 (6) A for ABPBO; and a = 6.044 (6), b = 3.417 (7), c (fiber axis) = 12.194 (18) A for ABPBT. The fiber repeat consists in each structure of two fused ring groups arranged in a planar, zigzag conformation. The conformational torsion angle and orientation of chains within the unit cells are derived from a linked-atom least squares refinement technique. Polymer chains pack laterally through van der Waals interactions. A plausible disorder model which involves defects in chain direction is presented. Refinement of a static disorder model for ABPBO in which 50% of the chains have their chain directions reversed leads to a lower R residual and sum of constraints.

Spectroscopic structure-property relationships of a series of polyaromatic platinum acetylides.

To develop a structure-spectroscopic property relationship in platinum acetylides having poly(aromatic hydrocarbon) ligands, we synthesized a series of chromophores with systematic variation in the number of fused aromatic rings (nFAR) and ligand topology (polyacene (L), polyphenanthrene (Z), or compact(C)). We measured ground-state absorption, fluorescence, and phosphorescence spectra. We also performed nanosecond and femtosecond transient absorption experiments. To extend the range of compounds in the structure-property relationship, we did DFT calculations on an expanded series of chromophores. Both the DFT results and experiments show that the S(1) and T(1) state energies are a function of both nFAR and the ligand topology. In the L chromophores, the S(1) and T(1) state energies decrease linearly with nFAR. In contrast, the S(1) and T(1) state energies of the Z chromophores oscillate around a fixed value with increasing nFAR. The C chromophores have behavior intermediate between the L and Z chromophores. A parallel series of calculations on the ligands shows the same behavior. The S(1)-S(n) energy obtained from ultrafast time-resolved spectra has a linear variation in nFAR. The rate constant for nonradiative decay, k(nr), was calculated from the S(1) state lifetime and decreases with an increasing number of π electrons in the aromatic ring. The result is consistent with the spin-orbit coupling caused by the central platinum heavy atom decreasing with larger nFAR. The present work shows that the framework developed for the analysis of poly(aromatic hydrocarbon) properties is useful for the understanding of the corresponding platinum acetylide complexes.

Crystal structures of poly-paraphenylene oligomers containing pendant phenyl groups

Abstract The room temperature crystal structures of 1,2,4-triphenylbenzene (TPB), C24H18; 22,45-diphenyl-p-quinquephenyl (DPQ), C42H30; and 22,65-diphenyl-p-septiphenyl (DPS), C54H38, have been investigated as part of a research programme in rigid-rod polymers, materials which are of great interest for aerospace and electro-optical applications. The molecules are non-planar, in contrast to the planar structures found at room temperature for the unsubstituted polyphenyls. The oligomer axis does not align with any of the crystallographic axes. The pendant-oligomer bond, however, does align with the longest crystallographic axis. The pendant torsion angle is greater than 45° and increases with increasing chain length. Knowledge of molecular structure and crystal packing of oligomeric model compounds will be useful in further calculations of mechanical, optical, and electro-optical properties for the corresponding rigid-rod polymer structures.

Syn and Anti Isomers of [2.2]Paracyclonaphthane

The structures of the anti (1) and syn (2) isomers of [2.2]paracyclonaphthane, C 24 H 20 , can be understood in terms of molecules composed of four segments, two naphthalene rings and two ethylene groups bridging the naphthalene rings. Distortions from ideal geometries are described and attributed primarily to intramolecular effects. Several observations have been made: the bridged portion of the naphthalene ring is deformed into a boat shape in both (1) and (2); much of the π-π repulsion is taken up by bending angles α and β; the naphthalene rings are parallel to each other in (1), but are nonparallel in (2); one naphthalene ring in (2) is twisted relative to the other, indicating a mode of strain reduction which is unique to this molecule, and the C-C distances and valency angles in the ethylene bridges are larger than normal values

1-(4-Nitrophenyl)-4-piperidinol

The title compound, C 11 H 14 N 2 O 3 , is a non-linear optical chromophore. The piperidinol ring is in a chair conformation. The C-N-C fragment of the piperidinol moiety is nearly coplanar with the nitrophenyl ring system. The molecular stacking allows hydrogen bonding between the piperidinol hydroxy group and the nitro group.