David L. Uhrich

A Mössbauer temperature study of a cold nematic liquid crystal: Nematic glass–supercooled nematic

A 0.2% (by weight) solution of 1,1′‐diacetylferrocene (DAF) in n‐ (p‐methoxybenzylidene) ‐p‐butylaniline (MBBA) was prepared and studied with the Fe‐57 Mossbauer effect. The Mossbauer parameters were observed for the temperature range 110–300 K. The recoil‐free fraction (f) data showed that the MBBA crystal phase exhibited Debye behavior for the entire temperature range with a Mossbauer–Debye temperature (ϑL) of 70 K. The lower temperature data for the cold nematic also gave a linear lnf vs T plot with ϑL=47 K for the MBBA glass. The ϑL for the nematic glass was indepentent of molecular orientation. The nematic glass–supercooled nematic transition was determined to be Tg=181 K from the deviation of the lnf vs T curve from linear behavior. The nematic glass provided a nonvanishing temperature independent contribution to the Fe‐57 vibrational ellipsoil (eL). Above 180 K eL became temperature dependent. Line broadening and a reduction of the quadrupole splitting were observed for the supercooled nematic with...

A Mössbauer temperature study of two cold smectic (B and H) liquid crystals

We have used the Fe‐57 Mossbauer effect (ME) to study solutions (0.2% by weight) of 1,1′‐diacetylferrocene (DAF) in two liquid crystalline materials, 4‐n‐butoxybenzylidene‐4′‐n‐′octylaniline (BBOA or 40,8) and 4‐n‐hexoxybenzylidene‐4′‐n‐′ propylaniline (HBPA or 60,3). The ME parameters were determined for the temperature range 100–300 K in the crystalline solid phase of both systems and in the cold smectic B phase of BBOA and the cold smectic H phase of HBPA. Here ’’cold’’ means the smectic glass and the supercooled smectic phase. The lnf (recoil‐free intensity) versus T plots exhibited linear Debye behavior for the solid phases of each system over the entire temperature range. The low temperature portion of the cold smectic lnf versus T data also exhibited Debye behavior for both systems. In both instances the smectic glass gave lower Mossbauer–Debye temperatures (ϑL) than the corresponding crystalline phase. For the smectic H glass of HBPA, ϑL=46 K and is independent of molecular orientation. For the sm...

57Fe Mossbauer Study of Four Ferroene Derivatives in a Smectic B Liquid Crystalline Glass

Abstract Two monosubstituted ferroenes [ferroenyl-4′-methoxyaniline (FMA) and ferroeyl-4′-n butylaniline (FBA)] and two disubstituted ferroenes [1-1′-di-n-otanoyl ferroene (DOF) and dibenzyl l,1′-ferroene dicarboxylate (DFD)] were introduced into the smectic B liquid crystalline phase of 4-n-butoxybenzylidene-4′-n-otylaniline (BBOA, or 40.8). The resulting systems (3 solutions and one suspension) were studied by 57Fe Mossbauer spectroscopy in the smectic B glass of BBOA at 77°K. The recoil-free fraction, f, and the line asymmetry of the quadrupole split doublet (given by the area ratio of the π and the [sgrave] transitions Aπ/A[sgrave] ) were observed as functions of the angle 0 between the gamma ray direction and the preferred molecular direction as determined by a 9 kG external magnetic field. Analysis of the Aπ/A[sgrave] versus 0 measurements for solutions of DOF, FBA, and FMA in BBOA confirms a positive electric field gradient (V=) normal to the cyclopentadienyl rings and yields an intra-molecular vib...

A Mössbauer Study of a Sn-119 Bearing Solute in an Ordered Smectic Liquid Crystal, at 77°K

Abstract An ordered solute “monocrystal” was obtained by dissolving 3.5-7.0% (by weight) of triethyltin palmitate (3EtSnPalm) into 4-n-hexoxybenzylidene-4′-n-propylaniline and cooling from 90°C to room temperature through the nematic and semectic A phases to the smectic H phase (T < 60°C) in a magnetic field of 9000 Gauss. The disc-shaped sample was then removed from the magnetic field and cooled to 77°K at which temperature the Mossbauer measurements were made. From the variation of the area ratio of the two quadrupole split Mossbauer absorption peaks as a function of the angle between the orienting magnetic fields and the γ-ray beam we have determined the sign of the axial electric field gradient (Vzz) at the tin site in 3EtSnPalm to be positive. Further, the theoretical fit to the data yielded the solute order parameter, S = 0.17 and the molecular contribution to the nuclear vibrational anisotropy for the tin atom in the 3EtSnPalm molecule, ϵM = - 0.8. An independent value of ϵM = -1.50 was obtained fr...

Theory of Mössbauer Spectral Asymmetry of Quadrupole Split Lines in Liquid Crystals

Abstract We have extended the theory for the intensity asymmetry of Fe-57 quadrupole split lines to iron bearing solutes in particular liquid crystalline systems. An expression for the area ratio is obtained for solutions (or suspensions) in smectic liquid crystalline systems with complete planar alignment and also for nematic solutions (or suspensions) which have been frozen so as to preserve their liquid crystalline order. The theory will allow determination of the order parameter of the Mossbauer impurity and the asymmetry parameter of the electric field gradient at the site of the resonant nucleus.

Orientation Dependence of an Unresolved Mössbauer Quadrupole Doublet in an Aligned Liquid Crystalline Glass

Abstract In an aligned system, an unresolved Mossbauer quadrupole doublet will appear to be a broadened single line absorption with an orientation dependent line position (1.p.). Previously, we have reported an orientation dependent 1.p. for the 119Sn absorption for the solute, 4-trimethyltin-benzylidene-4′-n-butylaniline (Sn-BBA), in the smectic B glass of 4-n-butoxybenzylidene-4′-n-octylaniline (BBOA). The 1.p. depended on the angle (θ) between the preferred moleuclar direction as determined by a 9 kG magnetic field and the direction of the γ-ray beam. In this report, we present a method of obtaining the isomer shift (I.S.), the quadrupole splitting (δE Q) of the unresolved doublet, and the orientational order parameter [S = ] for the Sn-bearing molecules. The angle δ is the angle which a given molecule makes with the field direction and for the glass phase the brackets indicate a spatial average. The method consists of constructing a doublet spectrum for each θ based on initial guesses ...

A Mössbauer measurement of some lattice properties of a smectic H liquid crystal

Abstract The angular dependence of the Mossbauer recoil-free fraction of two single line Sn-absorber molecules is used to determine the tilt angle and the lattice contribution to the vibrational anisotropy (at 77°K) in a smectic H liquid crystal.

A Mössbauer Temperature Study of 1,1′-Diacetylferrocene in a Cold Smectic A Liquid Crystal

Abstract The Fe-57 Mosshauer effect (ME) was used to study a solution (0.2% by weight) of 1,1′-diacetylferrocene (DAF) in the liquid crystalline material 4-n-pentylphenyl-4′-n-heptyloxythiobenzoate (7S5). The ME parameters were determined for the temperature range 110–300K in the crystalline solid phase and 110–240K in the cold smectic C phase of 7S5. Here “cold” means the smectic glass and the supercooled smectic phase. The In recoil-free intensity (f) vs T data exhibited linear Debye behavior for the solid phase over the entire temperature range. The low temperature (T<215) portion of the cold smectic In f vs, T data also exhibited Debye behavior, and the smectic glass gave a Mossbauer-Debye temperature (θLC) about 10K lower than the crystalline phase. The smectic glass-supercooled smectic transition was determined to be Tg = 215K from the deviation of the In f vs, T data from linear behavior. Above Tg, the deviation from linear Debye behavior followed a (T-Tg)2 dependence. The intermolecular contributi...

The anomalous Mössbauer spectra of tris(diethanoldithiocarbamato)iron(III): Solvate effects

Abstract Attempts to repeat the previously reported preparation of Fe(S 2 CN(CH 2 CH 2 OH) 2 ) 3 ·3H 2 O from aqueous solution always yielded the unsolvated complex which displayed a normal two-line MOssbauer spectrum similar to those displayed by Fe(S 2 CN(CH 2 CH 2 OH) 2 ) 3 prepared in non-aqueous media. Contrary to previous reports, heating the sample did not produce an unsolvated sample, but instead, resulted in decomposition as indicated by elemental analysis, TGA and MOssbauer studies. In contrast to earlier reports, Fe(S 2 CN(CH 2 CH 2 OH) 2 ) 3 was successfully recrystallized from acetone and methanol. Each of these recrystallized complexes was, for the most part, quite ordinary in its elemental analysis, room-temperature magnetic moment (predominantly high spin) and Mossbauer spectrum. However, when Fe(S 2 CN(CH 2 CH 2 OH) 2 ) 3 prepared from ethanolic solution is recrystallized from methanol without heating during the removal of the methanol, the resulting complex yields a four-line Mossbauer spectrum at 77 K (isomer shifts: 0.726, 0.770 mm/s; quadrupole splittings: 1.18, 0.318 mm/s, respectively). The elemental analysis, room-temperature magnetic moment and EPR spectra of this complex were in accord with a predominantly high-spin, spin-crossover complex without sufficient solvate molecules to be detectable by elemental analysis. A detailed, temperature-dependent series of Mossbauer spectra are reported for this complex. These results are discussed on the basis of effects of small amounts of solvate molecules on the spin-crossover rate of these complexes and compared to reports of four-line Mossbauer spectra for Fe(S 2 CN(CH 2 ) 4 O) 3 ·CH 2 Cl 2 at 77 K.

Sn‐119 Mössbauer effect study of two Sn‐bearing liquid crystalline materials

A 119Sn Mossbauer, lattice dynamic orientational study, at 77 K, of the smectic B glass phase of the Sn‐bearing liquid crystals (LC) p‐(11‐trimethyltin)undecyloxy‐benzilidene‐p′‐n‐ butylaniline (Sn‐UBBA) and p‐(11‐trimethyltin)undecyloxy‐benzilidene‐ p′‐octylaniline) (Sn‐UBOA), and solutions of these liquid crystals in LC of similar structure which do not contain Sn as part of their molecules, is presented. The Sn‐bearing liquid crystals were designed and synthesized specifically for this study and their synthesis is described. The samples were aligned by cooling from the melt in the presence of a magnetic field. For the samples of Sn‐UBBA and Sn‐UBOA, the experimental linewidth appears to depend on the orientational angle ϑ. This is explained as due to the non‐negligible sample thickness and the rff versus ϑ variation. These ϑ dependencies were used to extract the absolute fractions for various orientations of Sn‐UBBA and then the corresponding Debye temperatures ΘD(ϑ). For the other samples the lattice ...