Odd Number Of Methylene Units Research Articles

Shape-Directed Patterning and Surface Reaction of Tetra-diacetylene Monolayers: Formation of Linear and Two-Dimensional Grid Polydiacetylene Alternating Copolymers.

Side chains containing two diacetylene units spaced by an odd number of methylene units exhibit pronounced “bumps” composed of 0.3 nm steps, in opposite directions, at odd and even side-chain positions. In densely packed self-assembled monolayers, the bis-diacetylene bumps stack into each other, similar to the stacking of paper cups. Bis-diacetylene side chain structure and associated packing constraints can be tailored by altering the bump width, direction, side-chain location, and overall side-chain length as a means to direct the identities and alignments of adjacent molecules within monolayers. Scanning tunneling microscopy (STM) at the solution–HOPG interface confirms the high selectivity and fidelity with which bis-diacetylene bump stacking directs the packing of shape-complementary side chains within one-component monolayers and within two-component, 1-D self-patterned monolayers. Drop cast or moderately annealed monolayers of anthracenes bearing two bis-diacetylene side chains assemble single domains as large as 10(5) nm2. Light-induced cross-linking of two-component, 1-D patterned monolayers generates linear polydiacetylene alternating copolymers (A-B-)x and 2-D grid polydiacetylene alternating copolymers (A(-B-)(-B-)A(-B-)(-B-))x that covalently lock in monolayer structure and patterns.

The relationship between molecular structure and the incidence of the NTB phase

In this work, we present the first part of a study into the relationship between molecular structure and the occurrence of the ‘twist-bend nematic phase’ (NTB). Given the large amount of chemical space that might reasonably be expected to give rise to the NTB phase, this paper is only concerned with methylene-linked bimesogens bearing polar terminal groups based on the initial work of George Gray on cyanobiphenyls. As with other studies, we find that the NTB phase is observed only for materials that contain an odd number of methylene units in the spacer chain. It also appears that, in a given series of materials, there is a weak negative correlation between the dipole moment of the individual mesogenic units and the thermal stability of the NTB phase. Furthermore, we find that increasing the length–breadth ratio of the individual mesogenic units also provides a significant increase in the thermal stability of the NTB phase. The electrooptic behaviour of two materials, one with a terminal nitrile unit and one with an isothiocyanate group, was investigated. The NTB phase of the NCS-terminated material can be switched with a large applied voltage (20 V μm−1); however, the analogous nitrile-terminated material showed no electrooptic response under these conditions. Either the threshold voltage to switching is simply lower for isothiocyanate materials than nitriles or that there is more than one phase currently identified as the twist-bend nematic.

Mechanistic DFT studies – helicate-type complexes with different alcylic spacers

Metal-controlled self-assembly of complexes is of high interest in the field of Supramolecular Chemistry [1,2]. In the current study, we synthesized binuclear complexes with different spacers and study the influence of chain length on their relative energy. The considered complexes prefer the zigzag conformation. Thus a bridge with an odd number of methylene units forms a meso-Helicate (ΔΛ or ΛΔ) and one with an even number leads to a Helicate (ΔΔ or ΛΛ) (figure ​(figure1)1) [3,4]. Figure 1 Geometrically optimized complexes with Ti(IV); left: energies of the complexes with different alcylic spacers; right: ΔΔ and ΛΔ form of the complex with a propylene spacer. Comparison of the calculated transition energies for the non-dissociative interconversions of the diastereomeres with experimental results provides inside into the isomerization process. Moreover, insertion of different cations (templates) into the cavities of the binuclear complexes and corresponding calculations allow prediction of their influence on the isomerization. Enlargement of the studied system results in binuclear complexes with imino-bridged ligands. The obtained computational results provide a possible explanation for the experimentally observed high diastereoselectivity. As the DFT functionals like B3LYP do not describe long-range interactions properly, we chose the coulomb-attenuating method CAM-B3LYP [5] which corrects the exchange interaction at long range. The complexes with Ti(IV) in their helical or meso form have been geometrically optimized at the CAM-B3LYP level of theory with the TZVP basis set and MDF10 as ECP for Ti(IV) as implemented in the program package Gaussian09 [6].

Structural characterization of self-assembled monolayers of pyridine-terminated thiolates on gold

Self-assembled monolayers (SAMs) fabricated on Au(111) substrates from a homologous series of pyridine-terminated organothiols have been investigated using ultra high vacuum infrared reflection adsorption spectroscopy (UHV-IRRAS), X-ray photoelectron spectroscopy (XPS), scanning tunnelling microscopy (STM) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. A total of 4 different pyridine-based organothiols have been investigated, consisting of a pyridine unit, one or two phenyl units, a spacer of between one and three methylene units and, finally, a thiol unit. For all pyridine-terminated thiols the immersion of Au-substrates in the corresponding ethanolic solutions was found to result in the formation of highly ordered and densely packed SAMs. For an even number of the methylene spacers between the SH group and the aromatic moieties, the SAM unit-cell is rather large, (5sq.rt(3) x 3)rect, whereas in case of an odd number of methylene units a smaller unit cell is adopted, (2sq.rt(3) x sq.rt(3))R30 degrees. The tilt angle of the molecules amounts to 15 degrees . In contrast to expectation, the pyridine-terminated organic surfaces exposed by the corresponding SAMs showed a surprisingly strong resistance with regard to protonation.

Liquid crystalline properties and dependence of transition temperatures on the length of the flexible alkylene spacer of symmetric dimers composed of bent‐core units

Three new series of symmetric dimers composed of bent‐core molecules connected by a flexible alkylene spacer have been synthesized and their mesomorphic properties studied. The effect of varying the length of the terminal chains for fixed odd and even spacers, as well as varying the spacer length for a fixed terminal chain length, on the mesomorphic properties has been investigated. Remarkably, all the dimers investigated show the same type of mesophase, which has been characterized as a ColobPF phase, on the basis of X‐ray diffraction and electro‐optical measurements. It is found that for a fixed spacer length, the thermal range of the mesophase increases with increase in the number of methylene units in the terminal chain. On the other hand, interestingly an odd–even effect is observed when the length of the flexible spacer is varied; dimers having an odd number of methylene units possess higher clearing points.

Effects of chromophore orientation and molecule conformation on surface-enhanced Raman scattering studied with alkanoic acids and colloidal silver nanoparticles

Experimental studies have been carried out to gain a better understanding of the effects of chromophore orientation and molecular conformation on surface-enhanced Raman scattering (SERS) based on metal nanostructures. A series of alkanoic acids that contain a phenyl ring separated by methylene groups from the carboxylic acid, including phenylacetic acid, 3-phenylpropionic acid, 4-phenylbutyric acid, 5-phenylvaleric acid, and 6-phenylhexanoic acid, was investigated as model molecules with colloidal silver nanoparticles as SERS substrates. As the number of methylene groups increases, the molecules display an interesting zigzag intensity pattern of the phenyl ring bending mode around 1000 cm(-1) as well as a trend of appearance and disappearance of either the degenerate ring breathing mode or C[Double Bond]O vibrational mode near 1585 and 1630 cm(-1), respectively. Molecules containing an odd number of methylene units display a higher ring bending intensity and degenerate ring breathing mode and are suggested to have a trans conformation on the particle surface. Molecules with an even number of methylene units show a C[Double Bond]O vibrational mode and weaker ring bending in their SERS spectra and are suggested to have a gauche conformation on the silver nanoparticle surface. The different conformation is attributed to the varying interactions of the carboxylic group or the phenyl ring pi electrons with the silver surface. The SERS intensity was found to change little as the length between the phenyl ring and the carboxylic group was increased by adding CH(2) spacers. This is possibly because the effective distance between the phenyl ring and the silver surface does not change much with increasing number of CH(2) spacers as a result of changes in molecular conformation and variations in the phenyl ring orientation with CH(2) addition. The insight gained from this study is important for understanding SERS of complex molecules for which chromophore orientation and molecular conformation must be taken into careful consideration.

Structures and phase transformations of odd-numbered asymmetric main-chain liquid crystalline polyesters

Two new asymmetric odd-numbered main-chain liquid crystalline (LC) polyesters (BPE-Cn) were synthesized through the condensation polymerization of A–B type asymmetric α,ω-carboxylic acid–hydroxyl terminated monomers with odd-numbers of methylene units in the spacers (BPCA-Cn-PmOH, where n=7 and 9). The phase behaviors and structures of these odd-numbered BPE-Cn samples were characterized using the combined techniques of differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), selected area electron diffraction (SAED) and polarized light microscopy (PLM). One-dimensional (1D) powder WAXD results at different temperatures combined with DSC results revealed that during cooling these two polyesters enter a low-ordered LC phase before developing a crystalline phase at lower temperatures. Based on 2D WAXD of oriented fiber patterns, the low ordered LC phase of odd-numbered BPE-Cn was identified to be a smectic A (SmA) phase, which was constructed by each chemical repeating unit along the fiber drawn direction. The crystalline structures were determined to be monoclinic unit cells (KM). However, BPE-C7 possessed a γ=87°, while BPE-C9 had a β=100°. These crystalline structure identifications were also confirmed by SAED from single crystals. Two-chain packing models of the KM phases with four chemical repeating units were proposed on the basis of the experimental diffraction patterns. The different structures were attributed to the interplay between the mesogenic group ordering propensity and the chain constraints dictated by the methylene spacer and the meta-linkage at the end of mesogen. In the ordered phases of these two odd-numbered BPE-Cn polyesters, aromatic mesogenic groups are more or less parallel to each other along the chain with small angles of deviation from the c-axis of the crystal lattice, different from those large zigzag conformation structures in even-numbered BPE-Cn crystals. Crystallographic calculations of these two unit cell structures indicated that the neighboring chains in the crystals are translated along the c-axis (in the case of BPE-C9) or twisted away from the b-axis (in the case of BPE-C7) in order to incorporate both of the bend attributed to the odd-numbered methylene spacers and of the configurational meta-linkage at the end of mesogens. The simulated diffractions based on these calculations qualitatively agreed with the experimental observations. The phase identifications were also supported by the observed texture changes in PLM.

Photoresponsive vitrifiable chiral dimesogens: photo-thermal modulation of microscopic disordering in helical superstructure and glass-forming properties

Thermal, photooptical as well as glass-forming properties of a series of photoresponsive chiral dimesogenic compounds containing cholesterol–azobenzene moieties linked to varying alkyl chain lengths (6–14) are described. All compounds except the dimesogen containing 7 methylene spacers show a cholesteric phase. Selective reflections observed in the cholesteric liquid crystals were found to be blue shifted upon increasing the temperature due to variations in smectic fluctuations. X-Ray diffraction studies of these compounds in the cholesteric phase show a low intensity reflection at small angles, the half widths of which decrease on decreasing the temperature, confirming the presence of smectic cybotactic groups. These materials also exhibit a strong odd–even effect in their reflection wavelength as a function of spacer length. Compounds with even numbers of methylene units show a drastic change in their reflection wavelength (102–225 nm) on temperature change compared to those with odd numbers of methylene units (9–14 nm). UV photolysis, using light of 366 nm wavelength, in the cholesteric super-cooled state exhibits a hypsochromic shift in the reflection wavelength. This can be explained by the formation of the cis isomer destroying the smectic clusters in the helical superstructure due to its bent geometry, which in turn causes a decrease in the reflected wavelength. A maximum wavelength shift of 215 nm (703–488 nm) has been observed on photolysis at 80 °C (cooling cycle) for the dimesogen having 12-methylene spacers. By rapid cooling of the compounds from the cholesteric temperature to 0 °C glassy liquid crystalline films maintaining helical molecular order were obtained. The optical properties of these dimesogens, such as cholesteric reflections, controlled by light and temperature combined with their glass forming properties, can be utilized for the construction of information storage materials.

Crystallization kinetics of poly(trimethylene terephthalate)

AbstractThe bulk isothermal crystallization kinetics of poly(trimethylene terephthalate) (PTT) was studied using a differential scanning calorimeter. Avrami's theory was used to analyze the data. Based on crystallinity growth rate, Avrami rate constant, K, and crystallization half‐time, PTT's crystallization rate is between those of poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) when compared at the same degree of undercooling. PBT has the highest crystallization rate with K in the order of 10−2 to 10−1 min−n. It is about an order of magnitude faster than PTT at 10−3 to 10−2 min−n, which in turn is an order of magnitude faster than PET with K of 10−4 to 10−2 min−n. Contrary to previous reports (PTT was not included in the study) that aromatic polyesters with odd numbers of methylene units were more difficult to crystallize than the even‐numbered polyesters, PTT did not fit in the prediction and did not follow the odd‐even effect.

Phase Identification in a Series of Liquid Crystalline TPP Polyethers and Copolyethers Having Highly Ordered Mesophase Structures. 8. Phase and Structural Evolution in a Series of Copolyethers Containing Odd-Numbered Methylene Units in Both Comonomers

A series of liquid crystalline copolyethers have been synthesized from 1-(4-hydroxy-4‘-biphenyl)-2-(4-hydroxyphenyl)propane with 1,7-dibromoheptane and 1,11-dibromoundecane (coTPPs-7/11). This represents the copolyethers containing only odd numbers of methylene units in both comonomers. The molar ratio between these two comonomers in this series ranges from 1/9 to 9/1. The coTPPs-7/11 exhibit multiple phase transitions during cooling and heating in differential scanning calorimetry experiments. In each of these thermal transitions, only a small undercooling and superheating dependence is observed upon cooling and heating at different rates. All of the coTPPs-7/11 possess similar phase behavior based on the structural analyses using wide-angle X-ray diffraction (WAXD) on both powder and fiber samples and electron diffraction experiments on thin-film samples in transmission electron microscopy. The low-temperature phase is identified as a tilted hexagonal columnar (ΦTH) phase with multiple tilt angles between molecular axes and the hexagonal lattice normal. However, the hexagonal lateral dimensions in a ΦTH phase with various tilt angles are identical. This ΦTH phase transforms to another columnar (Φ‘) phase during heating at approximately 140 °C, as evidenced by the temperature-resolved WAXD fiber patterns. In this transition, the laterally ordered reflections from the (110) and (200) planes in the quadrants move toward the equator, corresponding to the decreased tilt angles. The discontinuous change in both the lateral d spacings and tilt angles represents a first-order transition. The columnar−columnar phase transition may be associated with the conformational disorder of the methylene units according to the infrared spectroscopic observations. Further heating of the samples leads to transitions from the Φ‘ to a nematic (N) phase and from the N phase to the isotropic melt. Combining the copolymer phase behaviors observed with the corresponding homopolymers TPPs(n = 7 and 11), a phase diagram describing transition temperatures with respect to the composition can be constructed. Their transition properties are also discussed.

Synthesis and characterization of segmented polyurethanes containing aromatic diol chain extenders

A series of segmented polyurethanes (SPUs), based on hexamethylene diisocyanate (HDI) and polytetramethylene glycol (PTMG) as soft segments and then extended with alkylene di(4-hydroxybenzoate), HB as hard segments, were synthesized in this study. The effect of the number of methylene units in HB (from 2 to 6) on the microphase separation of the SPUs was evaluated by means of a differential scanning calorimeter (DSC) and a polarized optical microscope (POM). The kinetic reaction between HB and HDI was determined by Fourier transform infrared spectroscopy. The rate constants of the second-order kinetics showed that the reactions of HDI and HB with odd numbers of methylene units appeared to be faster than those with even numbers of methylene units. The results from POM showed that the SPUs with even numbers of methylene units in the HB have a higher degree of phase separation between the hard and soft segments than those with odd numbers of methylene units in HB.

U-Shaped dimeric liquid crystals derived from phthalic acid

Details of the transitional properties of four homologous series of U-shaped liquid crystal materials, the benzene-1,2-di-(4-carboxyalkoxybenzylidene-4-n-alkylanilines), are presented. The spacers incorporate 3 to 6 methylene units, whilst terminal aliphatic chain lengths are varied from 1 to 12 units. Members of the two homologous series with an odd number of methylene units in the spacer form nematic and smectic phases as a function of terminal chain length. Materials with an even number of methylene units in the spacer are purely smectogenic, and in both series the first two homologues form only smectic B phases. For all four series the higher homologues show the phase sequence SF/I-SC-SA-I. X-ray diffraction studies have shown that these smectic phases are composed of molecules arranged in bilayers.

Odd-even effects in the thermotropic and optical properties of three series of chiral twin liquid crystals

Abstract Three series of novel chiral twin liquid crystals consisting of a cholesteryl and a 4′-cyanobiphenyl-4-yloxy group (III) or a 4-cyanophenoxy group (IV) and of a dihydro-cholesteryl and a 4′-cyanobiphenyl-4-yloxy group (V) connected by an alkanoate spacer with a varying number (1–7 and 10) of methylene units were synthesized and their mesogenic properties investigated. Strong odd-even effects were observed as a function of spacer length for the phase transition temperatures, the corresponding entropy changes and the selective reflection wavelengths associated with the chiral nematic phase. The compounds with an even number of methylene units have a smaller pitch than the compounds with an odd number of methylene units. Replacement of the cholesteryl group by a dihydrocholesteryl group results in an larger pitch, whereas replacement of the 4′-cyanobiphenyl-4-yloxy group by a 4-cyanophenoxy group gives a smaller pitch.

Crystallization behavior of polyethers containing odd numbers of methylene spacers from the isotropic and liquid crystalline states

A series of thermotropic polyethers synthesized from 1-(4-hydroxyphenyl) - 2 - (2 - methyl - 4 - hydroxyphenyl) - ethane and α,ω-dibromo-n-alkanes with odd numbers of methylene units (MBPE-n = odd) shows monotropic mesophase behavior. In isothermal differential scanning calorimetry (DSC) experiments, two—sometimes even three—exothermic transition processes can be observed when the crystallization temperature is below the mesophase transition temperature, while only one exothermic process is present above the mesophase transition temperature. The melting behavior of the crystals grown from the mesophase and from the isotropic melt states is different. The crystals grown from the mesophase state exhibit a larger overall heat of transition and a higher transition temperature compared with those grown from the isotropic melt. This may be attributed to the molecular interfacial connections between the crystal and amorphous regions when MBPEs crystallize from the mesophase state. The difference in morphology between the crystals grown from the different states has also been studied with polarized light microscopy (PLM) and transmission electronic microscopy (TEM). The structures of the crystals grown from the different states are, however, the same, as evidenced through wide-angle X-ray diffraction (WAXD) measurements. From the banded morphology of MBPE samples observed from PLM, the defect textures observed through TEM and the results of WAXD experiments, this mesophase can be identified as a nematic liquid crystal state.

Influence of molecular structure on the nematic-nematic transition in polyethers based on 1-(4-hydroxyphenyl)-2-(2-R-4-hydroxyphenyl)ethane where R=CH3 and Cl, and flexible spacers with an odd number of methylene units

In a previous paper (reference 1b) we reported that flexible main chain liquid crystalline copolymers based on various molar ratios between the flexible mesogenic groups 1-(4-hydroxyphenyl)-2-(2-R-4-hydroxyphenyl)ethane (RBPE) where R=F, Cl, CH3 and several different molar ratios between flexible spacers containing odd numbers of methylenic units, i.e. 5, 7, and 9 display two enantiotropic uniaxial nematic mesophases. The homopolymers RBPE-X (where R=-CH3 and-Cl) and X=number of methylenic units in the flexible spacer, i.e. X=5, 7, 9, 11, and 13) display also two nematic (n1, n2) mesophases which are however, monotropic or virtual. This paper will discuss the dependence of the phase transition temperatures between these two nematic mesophases (Tn1n2), the isotropization temperature (Tin1), and of their associated thermodynamic parameters of the MBPE-X (M=CH3) and C1BPE-X homopolyethers as a function of spacer length X. These results will be compared with the predictions of the only available theory which predicts the existence of two uniaxial nematic mesophases in flexible main chain liquid crystalline polymers (reference 6).

Mesogen conformation changes in thermotropic polyesters at phase transitions

AbstractMesogen conformations in thermotropic polyesters with alternating mesogens and poly(methylene) spacers were investigated by IR spectroscopy with polarized and unpolarized light. The selective deuteration of terephthaloyl‐bis(p‐oxybenzoate) mesogens gave a possibility of assigning part of the IR bands to terephthalic or to oxybenzoic fragments. It is shown that in the liquid crystalline melt the mesogen conformation is planar, but in the semicrystalline state it is mainly nonplanar. Differences in mesogen conformations of polymers containing an even or an odd number of methylene units in the spacer are detected. Thus, LC ordering seems to affect not only the conformation of the spacer but also the conformation of the mesogen.

Odd‐even effects in the optical properties of chiral twin liquid‐crystalline cholesteryl ω‐(cyanobiphenylyloxy) alkanoates

AbstractA series of novel cholesteryl w‐(cyanobiphenylyloxy)alkanoate esters with a varying number (2–7 and 10) of methylene units in the alkanoate spacer was synthesized and their mesogenic properties were investigated by polarization microscopy and differential scanning calorimetry. All compounds show a broad range of enantiotropic liquid‐crystalline behavior. Both the cholesteric‐isotropic and the smectic‐cholesteric transition temperatures show a pronounced odd‐even effect as a function of spacer length. The selective reflection wavelength of the cholesteric phases has been determined as a function of spacer length and temperature. For the first time, we have shown that the reflection wavelength also shows a pronounced odd‐even effect. The compounds with an even number of methylene units reflect at much lower wavelengths than the compounds with an odd number of methylene units. The reflection wavelength of the latter compounds decreases strongly with temperature, whereas the reflection wavelength of the compounds with the even spacers is almost independent of the temperature. The odd‐even effect in the optical properties attenuates with increasing spacer length.

Liquid crystalline polyethers based on conformational isomerism. 20. Nematic-nematic transition in polyethers and copolyethers based on 1-(4-hydroxyphenyl)2-(2-R-4-hydroxyphenyl)ethane with R = fluoro, chloro and methyl and flexible spacers containing an odd number of methylene units

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTLiquid crystalline polyethers based on conformational isomerism. 20. Nematic-nematic transition in polyethers and copolyethers based on 1-(4-hydroxyphenyl)2-(2-R-4-hydroxyphenyl)ethane with R = fluoro, chloro and methyl and flexible spacers containing an odd number of methylene unitsG. Ungar, V. Percec, and M. ZuberCite this: Macromolecules 1992, 25, 1, 75–80Publication Date (Print):January 1, 1992Publication History Published online1 May 2002Published inissue 1 January 1992https://doi.org/10.1021/ma00027a013RIGHTS & PERMISSIONSArticle Views218Altmetric-Citations66LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts

Synthesis and Characterization of Thermotropic Polyethers and Copolyethers Based on 4,4′-dihydroxy-α-methylstilbene and Flexible Spacers Containing Odd Numbers of Methylene Units

Abstract This paper describes the synthesis and characterization of thermotropic polyethers and copolyethers based on 4,4′-dihydroxy-α-methylstilbene (HMS) and flexible spacers containing odd number of methylene units [i.e., HMS-X/Y(A/B), where X is the number of methylene units in one of the spacers, Y is the number of methylene units in the other spacer, and A/B refers to the molar ratio of the two spacers]. The polyethers HMS-X are binary copolymers containing structural units derived from the two constitutional isomers of HMS and the flexible spacer. The binary copolymers HMS-X/Y(A/B) contain four structural units derived from combinations of the two constitutional isomers of HMS and the two flexible spacers. The following polymers and copolymers were synthesized and characterized: HMS-5, HMS-7, HMS-9, HMS-11, HMS-13(A/B), HMS-5/7(A/B), HMS-5/9(A/B), HMS-5/ll(A/B), HMS-7/9(A/B), HMS-9/ll(A/B), and HMS-9/13(A/B). All polymers had number average molecular weights higher than 15,000 and exhibited an enan...

Nematic-isotropic pretransitional behaviour in dimers with odd and even spacer lengths

Abstract Magnetic field induced optical birefringence measurements in the isotropic phase are reported for two members of a series of phenyl benzoate dimers: one having an odd number of methylene units in the spacer and one having an even number. The discontinuity at the nematic-isotropic transition, as determined by the difference between the first order phase transition temperature TNI and the supercooling limit of the isotropic phase T∗, is approximately 8 ± 3 times larger for the even species than for the odd. In addition, the product of the Cotton-Mouton coefficient dΔn/dH2 and T–T∗ is the same for both materials. In terms of both steric and molecular field theories, the results indicate that the orientations of the terminal mesogenic groups are essentially uncorrected in the isotropic phase.