Aleksandra Jankowiak

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.

Anion-driven mesogenicity: a comparative study of ionic liquid crystals based on the [closo-1-CB9H10]− and [closo-1-CB11H12]−clusters

A series of structurally analogous esters derived from monocarbaborates, [closo-1-CB9H10]− (A) and [closo-1-CB11H12]− (B), containing a maximum of 4 rings in the rigid core were prepared and investigated by thermal, optical, and XRD methods. Ion pairs containing a total of 4 rings with either N-butyl-4-heptyloxypyridinium (Pyr) or cetyltrimethylammonium (Cetyl) cation form a SmA phase ranging from 90 °C to 229 °C. Ion pairs 1e[Pyr] and 2e[Pyr] containing an azo group in the anisometric anion exhibit an enantiotropic nematic phase above the SmA phase. The cesium salt 1h[Cs] did not exhibit mesogenic behavior. Partial binary phase diagrams were investigated for 1d[Pyr] and 2d[Pyr] in 2e[Pyr], and for 1f[Pyr] in non-ionic mesogens. Variable temperature powder XRD analysis of 5 ionic liquid crystals, 1e[Pyr], 1f[Pyr], 2f[Pyr], 1g[Pyr], and 2g[Pyr], demonstrated about 20% interdigitation in the SmA phase and provided thermal expansion coefficients.

Thermochromic discotic 6-oxoverdazyls

6-Oxoverdazyls 1b[n], substituted with three 3,4,5-trialkoxyphenyl groups (n = 8 and 10), exhibit a columnar hexagonal phase (Col(h)) below 130 °C. They display reversible color change from red to green at the transition to the isotropic phase. XRD and magnetization data do not support dimerization of the radicals in the mesophase.

Evaluation of carborane‐containing nematic liquid crystals for electro‐optical applications

Three new structurally similar compounds in series 1[2] and 2 containing 12‐ and 10‐vertex p‐carborane units (A and B) were prepared. Their thermal properties were investigated in the pure state and compared to those of their carbocyclic analogues. Using several higher homologues in the series 1[n], clearing temperatures for the unknown 1[4] and 1[10] were predicted. Optical, dielectric and rotational viscosity parameters for carborane derivatives 1[2] and 2 and their cyclohexane analogies were obtained by extrapolation of values measured for 10% solutions in a standard nematic host. The observed trends were rationalised using computed molecular parameters. *A visiting student from the laboratory of Professor Grzegorz Młostoń, Univeristy of Łódź, Łódź, Poland.

Copper‐Mediated CC Cross‐Coupling Reaction of Monocarba‐closo‐dodecaborate Anion for the Synthesis of Functional Molecules

a counterion for the isolation of reactive cations, and for activation of catalysts in organic synthesis. Its unique electronic and molecular structures render 1 as an attractive starting material for the construction of various functional molecules and bioactive/pharmaceutical building blocks. However, progress in the synthesis of such materials is hindered by the lack of methods for the functionalization of 1. In particular, no efficient general methods are available for introduction of aryl and sp/sp-hybridized carbon centers at the carbon vertex of C1-carborane. A cross-coupling reaction is an obvious method for the introduction of aryl groups and sp/sp-hybridized carbon centers at the carbon vertex of C1-carborane derivatives, but has not yet been successful. 10] The intrinsic difficulty appears to be associated mainly with the sterically hindered and hypervalent nature of the vertex carbon atom in 1. We report herein a general and efficient copper-mediated C C cross-coupling reaction of 1 under palladium catalysis that yields a variety of C-functionalized C1-carborane derivatives and provides a basic architecture for pharmacophores and functional materials. The reaction is demonstrated not only for the parent C1-carborane but also its B-halogenated derivatives. To demonstrate utility of the method and the resulting new class of compounds, we describe the synthesis of multivalent anions and disclose preliminary results of the biological activity and liquid crystal behavior of selected derivatives. We commenced our studies by screening for a suitable metal species at the C1-carborane carbon vertex for crosscoupling reaction with aryl iodide 2a under palladium catalysis (Table 1). Reactions of various zinc carborate derivatives 1a (i.e. Negishi-type reaction), including zincate complexes, were unsuccessful (Table 1, entry 1). The Suzuki–Miyaura-type cross-coupling with pinacol borate 1b was also examined under a variety of conditions, but failed to give the desired product, affording only halogenated C1carboranes, with most of the pinacol borate moiety remaining intact (Table 1, entry 2). These results imply that the nature of the vertex carbon atom with respect to transmetalation is quite different from that of typical aryl or alkyl carbon atoms. After extensive screening of other systems, we found that a combination of copper(I) carborate 1c and a palladium catalyst promoted the coupling reaction with 2a at room temperature to give the desired product 3a (Table 1, Scheme 1. C1-carborane (1).

How much do coulombic interactions stabilize a mesophase? Ion pair and non-ionic binary isosteric derivatives of monocarbaborates and carboranes

Replacement of the B− atom in the monocarbaborate anion, 1[10] or 1[12], and the N+ atom in the pyridinium cation [Pyr] of a liquid crystalline ion pair with C atoms leads to an isoelectronic and isosteric non-ionic binary liquid crystalline mixture of carborane (2[10] or 2[12]) and benzene ([Ph]) derivatives lacking coulombic interactions. A comparison of mesogenic properties of ion pairs, 1[10]c–[Pyr]c and 1[12]c–[Pyr]c, with their analogous non-ionic mixtures, 2[10]c–[Ph]c and 2[12]c–[Ph]c, shows a 181 K higher clearing temperature, Tc, for the ion pair. This corresponds to a DFT-calculated difference in association energy ΔΔHa = 24.5 kcal mol−1 in a typical dielectric medium (e = 2.5). Pure compounds and binary mixtures were characterized using thermal, optical, and XRD methods.

Functional Group Transformations in Derivatives of 6-Oxoverdazyl

Transformations of functional groups, such as OCH2Ph, OCOPh, NO2 and I, in 1,3,5-triphenyl-6-oxoverdazyls 1a-1e were investigated in order to expand the range of synthetic tools for incorporation of the verdazyl system into more complex molecular architectures and to increase spin delocalization. Thus, Pd-catalyzed debenzylation of the OCH2Ph group or basic hydrolysis of the OCOPh group gave the phenol functionality, which was acylated, but could not be alkylated. Orthogonal deprotection of diphenol functionality was also demonstrated in radical 1c. Pt-catalyzed reduction of the NO2 group led to the aniline derivative, which was acylated. Attempted C-C coupling reactions to iodophenyl derivatives 1e and 5e were unsuccessful. Selected verdazyl radicals were characterized by EPR and electronic absorption spectroscopy, and results were analyzed with the aid of DFT computational methods.

Anion driven ionic liquid crystals: The effect of the connecting group in [closo-1-CB9H10]- derivatives on mesogenic properties

Replacement of the connecting CH2 group in ionic liquid crystal 1a[Pyr] with an oxygen atom in 1b[Pyr] resulted in significant destabilisation of the nematic and, to a greater extent, smectic A (SmA) phases, as established from binary mixture studies in 2a[Pyr]. Density functional theory (DFT) modelling of both anions suggests that the destabilisation results from the difference in charge distribution rather than conformational changes. Binary mixture studies demonstrated that all three ionic liquid crystals are compatible with a non-ionic benzoate nematic host 3a.

[closo-B10H10]2− as a structural element for quadrupolar liquid crystals: a new class of liquid crystalline NLO chromophores

Several electrically neutral, highly quadrupolar derivatives of the [closo-B10H10]2− cluster (I) substituted in the apical positions with onium fragments derived from 4-alkoxypyridine, 4-pentylthian, 4-pentylquinuclidine, or dinitrogen were synthesized from the parent cluster in 2–5 steps, and their thermal, structural, photophysical and dielectric properties were investigated. Only derivatives 2, containing thianium and alkoxypyridinium substituents with linear alkyl chains, exhibit liquid crystalline behavior and form a nematic phase. α-Branching of the alkoxy chain leads to marked destabilization of both crystalline and nematic phases, which suggests the importance of polar interactions in phase stabilization of these compounds. Derivatives 2 and 3 with one π and one σ substituent, Q+, exhibit a directional cluster-to-Q charge transfer and increase of the net electric dipole moment to μe = 17.9 D for 2a upon photo-excitation. They are weak fluorophores (Φf = 2–7%). Detailed investigation of 4-heptyloxypyridinium derivative 2b revealed that it exhibits a nematic phase above 100 °C with Δe = +1.3 (130 °C), and a moderate negative solvatochromism. Hyper-Rayleigh scattering measurement gave βHRS = 45 ± 10 × 10−30 esu at 1064 nm. In solutions, and presumably in the melt, it exists as a mixture of interconverting trans and cis isomers at about 4 : 1 ratio with ΔG298 = 0.82 ± 0.06 kcal mol−1 and ΔG‡298 = 25.3 ± 0.6 kcal mol−1 established by VT NMR methods.

4-Substituted 1-Acyloxypyridine-2(1H)-thiones: Experimental and Computational Studies of the Substituent Effect on Electronic Absorption Spectra†

A series of eight 4-substituted 1-(adamantane-1-carbonyloxy)pyridine-2(1H)-thiones (1, X = H, OC(7)H(15), Me, CF(3), SC(3)H(7), CN, COOMe, and Cl) was prepared and characterized by UV-vis spectroscopy in MeCN and cyclohexane. The observed lowest energy transition, designated as pi(CS) --> pi*(ring), exhibits a substantial substituent effect and lambda(max) ranges from 333 (X = OC(7)H(15)) to 415 nm (X = CN). Experimental lambda(max) values for all esters except for 1b (X = OC(7)H(15)) correlate with the sigma(p)(-) parameter (rho = 0.41 +/- 0.03, r(2) = 0.95). In contrast, the energy of the absorption band at about 295 nm, designated as pi(CS) --> pi*(CS), is practically substituent independent. Both absorption bands exhibit a modest negative solvatochromic effect. The experimental absorption energies correlate better with excitation energies calculated for N-acetyloxy analogues 2 with the ZINDO//DFT than with the TD-DFT//DFT method. Calculations for a series of 12 N-acetates 2 predict the most blue-shifted pi --> pi* transition for the alkoxy substituent and most red-shifted for the NO(2) group relative to the parent 2a (X = H).