Piotr Kaszynski

ORGANIC DERIVATIVES OF CLOSO-BORANES : A NEW CLASS OF LIQUID CRYSTAL MATERIALS

closo-Boranes and heteroboranes have been investigated as structural elements for liquid crystals. This paper reviews our progress to date in developing structure-property relationships for these boron cluster mesogens and assesses the effects of both steric and electronic influences on the liquid crystal behavior. The observation that nematic phases are favored over smectic phases is attributed principally to the steric effect of the clusters. Electronic and stereoelectronic effects can lead to materials exhibiting unusual properties. For example, compounds may exhibit high refractive indices yet low birefringence, they may be UV-transparent or, in some cases, exhibit strong charge transfer interactions with p-aromatic systems.

Photoconductivity of liquid crystalline derivatives of pyrene and carbazole

Two structurally related discogens containing either pyrene (1a) or carbazole (2a) were investigated by thermal, XRD, spectroscopic, and time-of-flight (TOF) methods. Experiments demonstrated for 1a a narrow range Colh phase, which easily forms a glass state at ambient temperature. TOF measurements showed an ambipolar charge transport for 1a with the mobilities on the order of 10−3 cm2 V−1 s−1. The carbazole 2a has two enantiotropic phases (Crcol and Colh) and behaves as a p-type semiconductor. The activation energy for positive charge mobility in 1a was found to be 0.10 ± 0.01 eV.

Photoconductive Liquid-Crystalline Derivatives of 6-Oxoverdazyl

1,3,5-Triphenyl-6-oxoverdazyl radicals 1[n], in which each phenyl group is substituted with three alkylsulfanyl groups (n = 6, 8, 10), exhibit a monotropic columnar rectangular (Col(r)) phase below 60 °C. Detailed analysis of 1[n] revealed a broad absorption band in the visible region with maxima at 540 and 610 nm and redox potentials E(1/2)(0/+1) = +0.99 V and E(1/2)(0/-1) = -0.45 V vs SCE. Photovoltaic studies of 1[8] demonstrated a hole mobility of 1.52 × 10⁻³ cm² V⁻¹ s⁻¹ in the mesophase with an activation energy of 0.06 ± 0.01 eV. Magnetization studies of 1[8] revealed nearly ideal paramagnetic behavior in either the solid or fluid phase above 200 K and weak antiferromagnetic interactions at low temperatures.

Anionic Amino Acid [closo-1-CB9H8-1-COO-10-NH3]− and Dinitrogen Acid [closo-1-CB9H8-1-COOH-10-N2] as Key Precursors to Advanced Materials: Synthesis and Reactivity†

Amino acid [closo-1-CB(9)H(8)-1-COO-10-NH(3)](-) (4) was prepared by amination of iodo acid [closo-1-CB(9)H(8)-1-COOH-10-I](-) (1) with LiHMDS in a practical and reproducible manner. The apparent dissociation constants, pK(2) = 5.6 and pK(1) > 11, were measured for 4[NMe(4)] in 50% aq. EtOH. Diazotization of 4 with NO(+)PF(6)(-) under mildly basic conditions afforded stable dinitrogen acid [closo-1-CB(9)H(8)-1-COOH-10-N(2)] (5). Activation parameters (DeltaH(++) = 33.9 +/- 1.4 kcal mol(-1) and DeltaS(++) = 10 +/- 3.5 cal mol(-1) K(-1)) for thermolysis of its methyl ester [closo-1-CB(9)H(8)-1-COOMe-10-N(2)] (11) in PhCN were established, and the heterolysis of the B-N bond is believed to be the rate-determining step. Electrochemical analysis showed a partially reversible reduction process for 11 (E(1/2)(red) = -1.03 V) and 5(-) (E(1/2)(red) = -1.21 V), which are more cathodic than reduction of [closo-1-CB(9)H(9)-1-N(2)] (17). The dinitrogen acid 5 was reacted with pyridine and N,N-dimethylthioformamide, to form pyridine acid 6 and protected mercapto acid 7, respectively, through a boronium ylide intermediate 18. Compound 7 was converted to sulfonium acid 8. The molecular and crystal structures for 5 [C(2)H(9)B(9)N(2)O(2) monoclinic, P2(1)/n, a = 7.022(2) A, b = 11.389(4) A, c = 12.815(4) A, beta = 96.212(5) degrees ; V = 1018.8(6) A(3), Z = 4,], 6 [C(7)H(14)B(9)NO(2), monoclinic, P2(1)/n, a = 14.275(4) A, b = 12.184(3) A, c = 30.538(8) A, beta = 95.377(4) degrees ; V = 5288(3) A(3), Z = 16], and 8 [C(7)H(19)B(9)O(2)S, monoclinic, P2(1)/c, a = 15.988(5) A, b = 19.377(6) A, c = 9.655(3) A, beta = 98.348(5) degrees; V = 2959.4(16) A(3), Z = 8] were determined by X-ray crystallography and compared with results of density functional theory (DFT) and MP2 calculations. Electronic structures of 5, 6, and related species were elucidated with electronic spectroscopy and assessed computationally at the B3LYP/6-31G(d,p), MP2/6-31G(d,p), and ZINDO//MP2 levels of theory.

Boron Clusters in Liquid Crystals

Abstract 3-Dimensional aromatic moiety was used for the first time in the synthesis of liquid crystals. Several of the prepared compounds, based on σ- aromatic boron clusters 1 and 2, show only nematic mesophases with relatively high Ti, good electrochemical but limited thermal stability. Absorption and emission spectra of pyridine derivative 15 are reported. Semiempirical methods were employed in evaluation of molecular properties and design of molecules with large coaxial dipole moments.

Functionalization of the [closo-1-CB9H10]- anion for the construction of new classes of liquid crystals.

The [closo-1-CB(9)H(10)](-) anion is a member of an extensive family of σ-aromatic closo-boranes that possess impressive stability and functionalization characteristics. In contrast to its bigger, more extensively studied brother, the [closo-1-CB(11)H(12)](-) anion, convenient access to the [closo-1-CB(9)H(10)](-) anion has only been recently established, and researchers have only begun to develop and understand its fundamental chemistry. The geometrical and electronic properties of the [closo-1-CB(9)H(10)](-) anion make it an attractive structural element of novel classes of either zwitterionic or ionic liquid crystals suitable for electro-optical and ion transport applications, respectively. Such materials require a 1,10-difunctionalized [closo-1-CB(9)H(10)](-) anion that permits for the formation of molecules of elongated shape. The covalent attachment of an onium fragment or the use of a counterion compensates for the negative charge. This Account highlights the progress made in the advancement and understanding of the fundamental chemistry of the [closo-1-CB(9)H(10)](-) anion. We also describe the development of 1,10-difunctionalized derivatives as key intermediates in the preparation of new classes of liquid crystalline materials. We obtained the first isomerically pure 1,10-difunctionalized derivative of the [closo-1-CB(9)H(10)](-) anion, iodo acid [closo-1-CB(9)H(8)-1-COOH-10-I](-), from decaborane through the Brellochs reaction. Functional group transformation of the C(1)-carboxyl group led to a 1-amino derivative and, subsequently, to a synthetically valuable 1-dinitrogen derivative. The latter exhibits reactivity typical for PhN(2)(+) and undergoes diazocoupling and Gomberg-Bachmann arylation reactions. The B(10)-iodine participated in Negishi alkylation and Buchwald-Hartwig amination reactions, leading to 10-hexyl and 10-amino carboxylic acids, respectively. We converted the 10-amino carboxylic acid to a 10-dinitrogen acid [closo-1-CB(9)H(8)-1-COOH-10-N(2)], which proved to be synthetically valuable in the preparation of 10-pyridinium and 10-sulfonium zwitterionic acids and their liquid crystalline esters. We investigated several intermediates using structural, spectroscopic, and kinetic methods.

Liquid crystalline behavior of tetraaryl derivatives of benzo[c]cinnoline, tetraazapyrene, phenanthrene, and pyrene: the effect of heteroatom and substitution pattern on phase stability

A series of closely related tetrasubstituted derivatives of benzo[c]cinnoline (1), tetraazapyrene (2), phenanthrene (3), and pyrene (4) were investigated for their mesogenic properties using thermal, optical, spectroscopic, and powder XRD analyses. Only three 3,4-dioctyloxyphenyl derivatives exhibited mesogenic properties. Substitution of N for CH (3 → 1 and 4 → 2 pairs) and also increase of the core element size (1 → 2 and 3 → 4 pairs) significantly increases the mesophase stability. The findings and observed trends were rationalized by analysis of conformational properties which included calculation of the planarization energy, and modeling of aliphatic chain density and fill fractions. MO calculations showed that the tetraaza derivative 2c is significantly electron deficient and suitable for electron conductive materials.

An assessment of carborane-containing liquid crystals for potential device application

Two 4-alkoxyphenyl 12-pentylcarborane-1-carboxylate nematic liquid crystals have been synthesized. The materials are found to exhibit ideal mixing of nematic phases in their binary mixtures with analogous bicyclo[2.2.2]octane derivatives and with the polar nematogen 4-(4-isothiocyanatophenyl)-1-(trans-hexyl)cyclohexane. The smectic phases for the bicyclo[2.2.2]octanes are destabilized by addition of the carborane derivative. For carborane compound 5BC5 the extrapolated dielectric anisotropy and measured optical anisotropy are 1.3 and 0.057 respectively at 20°C. The refractive indices have been correlated with the calculated electronic polarizabilities and the low birefringence measured for 5BC5 can, at least in part, be attributed to the carborane cylindrical symmetry.

The effect of carborane, bicyclo[2.2.2]octane and benzene on mesogenic and dielectric properties of laterally fluorinated three-ring mesogens

Six series of structurally similar compounds containing 12- and 10-vertex p-carborane (A and B), bicyclo[2.2.2]octane (C), and benzene (D) were prepared and their mesogenic and dielectric properties investigated. Comparative analysis showed that all carborane derivatives form significantly less stable mesophases than their carbocyclic analogs, however they exhibit a relatively high shielding ability for lateral fluorination. Depression of the clearing temperature upon fluorination of series 1, 3, and 5 is approximately constant for each series A–D and correlates with the diameter of the ring (the slope = 14.8 °C A−1 and R2 = 0.997). Compounds in series 2 (X = F) were used as low concentration additives to a nematic host, 6-CHBT. Dielectric parameters were extrapolated to pure additives and analyzed using the Maier–Meier equation. The Kirkwood factors g and apparent order parameters Sapp that are required to reproduce the extrapolated dielectric values follow the trend in the size of ring . The smallest g (0.47) and the largest Sapp (6.3) are obtained for carborane 2A, and the largest g (0.69) and the smallest Sapp (0.7) are obtained for the terphenyl derivative 2D. The increase of Sapp in the series D→A corresponds to the increasing disorder of the nematic solution with increasing size of ring .

Structural effects in three-ring mesogenic derivatives of p-carborane and their hydrocarbon analogues

Several series of structurally related three-ring esters containing benzene, cyclohexane, bicyclo[2.2.2]octane and p-carborane rings were synthesized and their mesogenic properties investigated using thermal analysis and optical microscopy. Carborane derivatives show only nematic phases, while the richest smectic polymorphism (up to three phases) was observed in the biphenyl series D. The structure–property relationships were studied by comparison of T NI between series. The ring effectiveness in stabilization of nematic phases generally follows the order carborane<benzene∼cyclohexane<bicyclo[2.2.2]octane. The results indicate that fill fraction plays a significant role in the stabilization of the mesophase. A notable positional effect of the carborane ring on T NI was also observed.