João Sotomayor

Characterization of a nematic mixture by reversed‐phase HPLC and UV spectroscopy: an application to phase behaviour studies in liquid crystal–CO2 systems

In the present work a simple but selective reversed‐phase high performance liquid chromatographic method (HPLC) was developed for the analysis of the nematic liquid crystal mixture E7 to determine precisely the composition of the liquid crystal mixture used in PDLC film preparation. Ultraviolet absorption spectrophotometry experiments were carried out to determine the HPLC detection wavelength and to characterize the absorptivity constants of E7 constituents. The technique developed is applied in the case of equilibrium solubility studies for E7 in supercritical carbon dioxide (scCO2). The results indicate unambiguously that scCO2 can fractionate the mixture towards the E7 components. The four single component peaks of the E7 mixture were distinctively separated by this method, which enabled the determination of the solubility of E7 constituents in the scCO2.

Phase transformations undergone by Triton X-100 probed by differential scanning calorimetry and dielectric relaxation spectroscopy.

The phase transformations of the surfactant Triton X-100 were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and dielectric relaxation spectroscopy (DRS). In particular, crystallization was induced at different cooling rates comprised between 13 and 0.5 K min(-1). Vitrification was detected by both DSC and DRS techniques with a glass transition temperature of ∼212 K (measured on heating by DSC) allowing classifying Triton X-100 as a glass former. A fully amorphous material was obtained by cooling at a rate ≥10 K min(-1), while crystallization was observed for lower cooling rates. The temperature of the onset of melt-crystallization was found to be dependent on the cooling scan rate, being higher the lower was the scan rate. In subsequent heating scans, the material undergoes cold-crystallization except if cooled previously at a rate ≤1 K min(-1). None of the different thermal histories led to a 100% crystalline material because always the jump typical of the glass transformation in both heat flux (DSC) and real permittivity (DRS) is observed. It was also observed that the extent/morphology of the crystalline phase depends on the degree of undercooling, with higher spherulites developing for lower undercooling degree (24 K ≤ T(m) - T(cr) ≤ 44 K) in melt-crystallization and a grain-like morphology emerging for T(m) - T(cr) ≈ 57 K either in melt- or cold-crystallization. The isothermal cold- and melt-crystallizations were monitored near above the calorimetric glass transition temperature by POM (221 K) and real-time DRS (T(cr) = 219, 220, and 221 K) to evaluate the phase transformation from an amorphous to a semicrystalline material. By DRS, the α-relaxation associated with the dynamic glass transition was followed, with the observation that it depletes upon both type of crystallizations with no significant changes either in shape or in location. Kinetic parameters were obtained from the time evolution of the normalized permittivity according to a modified Avrami model taking in account the induction time. The reason the isothermal crystallization occurs to a great extent in the vicinity of the glass transition was rationalized as the simultaneous effect of (i) a high dynamic fragile behavior and (ii) the occurrence of catastrophic nucleation/crystal growth probably enabled by a preordering tendency of the surfactant molecules. This is compatible with the estimated low Avrami exponent (1.12 ≤ n ≤ 1.6), suggesting that relative short length scale motions govern the crystal growth in Triton X-100 coherent with the observation of a grainy crystallization by POM.

Synthesis and characterization of binuclear transition metal–rhenium(VII) complexes with bridging cyanide ligands

Abstract Reacting transition metal complexes in low oxidation states, containing one or two cyanide ligands, with methyltrioxorhenium(VII) leads to bridged mixed metal compounds in good yields. The Re(VII) core is then surrounded by five or six ligands, respectively. The strength of these CN bridges and thus the stability of the newly generated bimetallic compound strongly depends on the donor strength of the ligands surrounding of the Cr/Mo/W or Fe moiety. The stability of the mixed metal molecules is reflected in the temperature dependent behavior of their 17 O-NMR spectra, in their IR (ReO) stretching frequencies and force constants, as well as several other spectroscopic data. UV–vis absorption spectra show the appearance of charge transfer bands. In the case of the mixed Mo/Re complexes the 95 Mo-NMR spectroscopy is also a helpful tool to examine the donor capability of the Mo moiety. The described compounds also show photosensitivity.

Photodecarboxylation of citrate through ion pair photochemistry: the Co(Sep)3+•citraten− (n= 1,2,3) system

Abstract The Co(Sep) 3+ complex (Sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane) forms ion pairs with citrate in aqueous solution at pH 4.0, 5.6 and 8.5, where the forms CIT − , CIT 2− and CIT 3− (CIT = citrate) predominate respectively. Irradiation in the ion pair charge transfer band for the three pH values leads to the decarboxylation of citrate with the production of CO 2 and Co(Sep) 2+ . In the presence of colloidal platinum, CO 2 and H 2 are evolved. The quantum yields for formation of CO 2 , Co(Sep) 2+ and H 2 were obtained and are discussed in terms of their dependence on pH.

Films Based on New Methacrylate Monomers: Synthesis, Characterisation and Electro-Optical Properties

A number of liquid crystal monomers based on methacrylate derivatives were synthesised by novel mild and solventless procedures under microwave irradiation. Their photo- and thermo assisted homo- and co-polymerizations with glycidyl methacrylate were investigated. The resulting polymers were characterised by NMR, GPC, DSC, and SEM to determine their structures and properties. The effects of the structures of the monomers on the electro-optical properties were correlated.

Photochemistry of the Co(Sep)3+I− system: evaluation of the quantum yield for the photochemical redox reaction of the ion pair

Abstract Spectrophotometric measurements in the region of the ion pair charge transfer band of the Co(Sepulchrate) 3+ I − system were used to assess the role played by the adducts between the complex cation and iodide anion in the photochemical behaviour of the system. No clear evidence was obtained for the presence or absence of 1:2 or higher adducts and the association constant and molar absorptivity of the 1:1 adduct (ion pair) could not be determined with a satisfactory accuracy. Nevertheless, reliable values were evaluated for the fraction of the exciting light absorbed by the ion pair in the photochemical experiments, and these values were used to correct the previously reported quantum yields, which were based on the total amount of light absorbed by the solutions. The “true” quantum yields for the ion pair are discussed in relation to the proposed reaction mechanism.

Fast synthesis employing a microwave assisted neat protocol of new monomers potentially useful for the preparation of PDLC films

AbstractIt has been reported that the length of the molecular chain and the rigidity of molecules influence the structure of the polymer network in PDLC films and hence the electro-optical properties of the composites. Herein, a series of new aromatic monomeric monomethacrylates, bismethacrylates and monovinylbenzene derivatives with a mesogenic core were successfully synthesized under microwave irradiation. The microwave assisted synthesis resulted in decreased reaction times, reduced solvent requirement, increased operational simplicity, and in most cases, improved yields and selectivity.

Photo-oxidation of carboxylic acids via photochemical redox reactions of ion pairs: evaluation of the corrected quantum yields for multi-equilibria acid—base systems

Abstract Carboxylic acid anions which form ion pairs with transition metal cations can be oxidized by electron transfer mechanisms promoted by irradiation at the ion pair charge transfer (IPCT) band. The overall quantum yield of the photochemical redox reaction of the ion pair in the presence of various carboxylic acid anions must be corrected for each species present and for the fraction of light absorbed by each reactive ion pair. These calculations require the knowledge of the acid—base equilibria of the organic substrate and the characteristic constants of the ion pair (molar absorptivities and thermodynamic association constants which provide the variation in the concentration of each ion pair over the pH range). It is shown that the variation in the overall quantum yield of the photochemical redox reaction of the ion pair as a function of pH is due to the variation in the concentration of the reactive species present in solution.

Stabilizing Unstable Amorphous Menthol through Inclusion in Mesoporous Silica Hosts

The amorphization of the readily crystallizable therapeutic ingredient and food additive, menthol, was successfully achieved by inclusion of neat menthol in mesoporous silica matrixes of 3.2 and 5.9 nm size pores. Menthol amorphization was confirmed by the calorimetric detection of a glass transition. The respective glass transition temperature, Tg = -54.3 °C, is in good agreement with the one predicted by the composition dependence of the Tg values determined for menthol:flurbiprofen therapeutic deep eutectic solvents (THEDESs). Nonisothermal crystallization was never observed for neat menthol loaded into silica hosts, which can indicate that menthol rests as a full amorphous/supercooled material inside the pores of the silica matrixes. Menthol mobility was probed by dielectric relaxation spectroscopy, which allowed to identify two relaxation processes in both pore sizes: a faster one associated with mobility of neat-like menthol molecules (α-process), and a slower, dominant one due to the hindered mobility of menthol molecules adsorbed at the inner pore walls (S-process). The fraction of molecular population governing the α-process is greater in the higher (5.9 nm) pore size matrix, although in both cases the S-process is more intense than the α-process. A dielectric glass transition temperature was estimated for each α (Tg,dielc(α)) and S (Tg,dielc(S)) molecular population from the temperature dependence of the relaxation times to 100 s. While Tg,dielc(α) agrees better with the value obtained from the linearization of the Fox equation assuming ideal behavior of the menthol:flurbiprofen THEDES, Tg,dielc(S) is close to the value determined by calorimetry for the silica composites due to a dominance of the adsorbed population inside the pores. Nevertheless, the greater fraction of more mobile bulk-like molecules in the 5.9 nm pore size matrix seems to determine the faster drug release at initial times relative to the 3.2 nm composite. However, the latter inhibits crystallization inside pores since its dimensions are inferior to menthol critical size for nucleation. This points to a suitability of these composites as drug delivery systems in which the drug release profile can be controlled by tuning the host pore size.

Improvement of permanent memory effect in PDLC films using TX-100 as an additive

ABSTRACT A number of polymer dispersed liquid crystal (PDLC) films based on dimethacrylate monomers were synthesised by polymer-induced phase separation (PIPS) using polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether (TX-100) as an additive up to total solution weight of 10% aiming to modify the initial anchorage force of the liquid crystal (LC) molecules to the polymeric matrix. The effects of the addition of this additive to the PDLC films were studied resorting to electro-optical studies. Using a fitting model, several parameters were correlated such as the permanent memory effect (PME), the voltage required to achieve 90% of maximum transmittance (E90), the average elastic constant (K) and the rotational viscosity of the director (γ). The use of TX-100 results on an increase on the PME and γ, and decrease on the E90 and K. Graphical Abstract