Etelvina de Matos Gomes

Synthesis, properties and photochromism of novel charge transfer compounds with Keggin anions and protonated 2,2′-biquinoline

Abstract Several novel compounds with protonated 2,2′-biquinoline (biqui) and Keggin polyoxoanions (α-isomers), having the general formula (Hbiqui) m [XM 12 O 40 ]· n solv, X=P ( m =3), Si ( m =4); M=Mo, W; n =0, 3; solv=H 2 O, N , N -dimethylformamide (dmf), were synthesized and characterized by analytical, spectroscopic and X-ray diffraction techniques. Electronic spectroscopy (visible/UV) indicated the presence of intermolecular charge transfer between the organic and inorganic moieties in the solid state. Evidence for the existence of intermolecular electronic interaction in solution was found for compounds with the [SiW 12 O 40 ] 4− anion, a fact quite uncommon for charge transfer compounds based on Keggin anions. A single crystal X-ray diffraction study could be performed on (Hbiqui) 4 [SiW 12 O 40 ]·3dmf crystals, but the refinements revealed the presence of highly disordered 2,2′-biquinoline molecules and α-[SiW 12 O 40 ] 4− anions. Photosensitivity to sunlight and to W-lamp visible light was assessed for all compounds. Photochromic properties were found for solids with [XMo 12 O 40 ] m − Keggin anions. Reduction of these anions upon irradiation was observed by diffuse reflectance and EPR spectroscopy. After a minimum of 5 h under sunlight, the extent of anion reduction followed the order (Hbiqui) 3 [PMo 12 O 40 ]·3dmf>(Hbiqui) 4 [SiMo 12 O 40 ]·3dmf>(Hbiqui) 3 [PMo 12 O 40 ]>(Hbiqui) 4 [SiMo 12 O 40 ]·4H 2 O. The Kurtz powder test was used to evaluate the second-order non-linear optical properties of the prepared compounds. (Hbiqui) 4 [SiW 12 O 40 ]·3H 2 O originated a second harmonic generation signal with intensity about 15% that of urea for 1064 nm radiation.

Oriented single-crystal-like molecular arrangement of optically nonlinear 2- methyl-4-nitroaniline in electrospun nanofibers

In-plane aligned nanofibers of organic 2-methyl-4-nitroaniline (MNA) were produced by the electrospinning technique using a 1:1 weight ratio with poly(l-lactic acid). The fibers are capable of enormous efficient optical second harmonic generation as strong as pure MNA crystals in powder form. Structural, spectroscopic, and second harmonic generation polarimetry studies show that the MNA crystallizes within the fibers in an orientation in which the aromatic rings of MNA are predominantly orientated edge-on with respect to the plane of the fiber array and with their dipole moments aligned with the fiber axis. The results show that the electrospinning technique is an effective method to fabricate all-organic molecular functional devices based on polymer nanofibers with guest molecules possessing strong nonlinear optical and/or polar properties.

Ferroelectric characterization of aligned barium titanate nanofibres

We report the synthesis, structural and ferroelectric characterization of continuous well-aligned nanofibres of barium titanate produced by the electrospinning technique. The fibres with average diameter of 150–400 nm consist of connected nanoparticles of BaTiO3 stacked together to form the shape of a long filament. The tetragonal phase in the obtained nanofibres was revealed by the x-ray diffraction and Raman spectroscopy and has been also confirmed by the second harmonic generation (SHG) and piezoresponse force microscopy (PFM). The temperature dependence of the SHG in the vicinity of the paraelectric–ferroelectric phase transition suggests that barium titanate nanofibres are indeed ferroelectric with an apparent glass-like state caused by metastable polar nanoregions. The existence of domain structure and local switching studied by PFM present clear evidence of the polar phase at room temperature.

Strong enhancement of second harmonic generation in 2-methyl-4-nitroaniline nanofibers

An effective control of the second harmonic generation (SHG) efficiency in electrospun nanofibers of nonlinear optically active 2-methyl-4-nitroaniline and carrier polymer poly(l-lactic acid) (MNA-PLLA) is presented. The SHG efficiency of the MNA-PLLA fibers strongly depends on the diameter of the nanofibers and can be increased up to an order of magnitude by controlling the electrospinning processing parameters. For optimal electrospinning process conditions, MNA-PLLA nanofibers with an effective nonlinear optical coefficient that is two orders of magnitude greater than the counterpart bulk powder MNA may be obtained. The work can be used as a guideline for the manufacture of nanophotonic devices.

Intense optical second harmonic generation from centrosymmetric nanocrystalline para-nitroaniline

We demonstrate an approach that uses normally centrosymmetric, but highly polarizable organic molecules of para-nitroaniline (p-NA) as very efficient second harmonic generator (SHG) for near infra-red light. The approach is based on an effective manipulation of the nanocrystalline size and their assembly into a highly orientated mesocrystalline structures. The resulting mesocrystalline form of p-NA consists of multiple nanocrystals with common orientation, providing a set of surfaces with highly aligned molecules forming a head-to-tail polar arrangement. The second harmonic generation efficiency from this arrangement of mesocrystalline nanostructures is comparable to that from the conventional non-centrosymmetric organic crystals. The experimental results indicate that a very strong second-order nonlinear response can be obtained from small D-π-A organic molecules with elevated molecular hyperpolarizabilities even if these molecules tend to crystallize in centrosymmetric structures.

Probing ferroelectric behaviour in charge-transfer organic meta-nitroaniline

Potential ferroelectricity in charge-transfer organic materials is often masked by the intrinsic conductivity. Here, we report the compelling evidence of ferroelectricity in organic π-conjugated meta-nitroaniline (m-NA) crystals as shown by the local electromechanical measurements using the piezoresponse force microscopy (PFM) technique. m-NA is a charge-transfer molecular material with the exceptional optical non-linearity and perceptible conductivity along the crystallographic polar axis. While standard Sawyer-Tower measurements revealed an apparently lossy-dielectric hysteresis, The PFM switching spectroscopy indicated clear ferroelectric behaviour in this technologically important multifunctional material. Further study of the pyroelectric properties in m-NA crystals confirmed their high spontaneous polarization of 18 μC/cm2 at room temperature, comparable to the best known organic ferroelectrics.

Synthesis of polymer-based triglycine sulfate nanofibres by electrospinning

In this work we present the synthesis and characterization of polyethylene oxide (PEO) based triglycine sulfate (NH2(CH2OOH)3H2S04, TGS) nanofibres obtained by electrospinning. The fibres, with typical diameters of about 190?750?nm and above several hundred micrometres in length, present the nanocrystals of TGS embedded in a polymer matrix. The obtained nanofibres were characterized by FT-IR spectroscopy and the domain structure was examined by piezoforce microscopy. Dielectric permittivity measurements on the TGS?PEO nanofibres exhibit the characteristic ferroelectric?paraelectric phase transition at around 50??C.

Crystal structure and second harmonic generation in cesium hydrogen malate hydrate

Cesium hydrogen L-malate monohydrate, CsH(C4H4O5).H2O, is a new non-linear optical semi-organic crystalline material with a second harmonic generation efficiency roughly 2.5 times greater than KDP. Its crystal structure, space group P21, shows that the malate anions, are interconnected through directional O-H•••O hydrogen bonding, in a head-to-tail arrangement, creating extended anionic layers. The water molecules provide a cross-link, through hydrogen bonding, between adjacent layers. Especially noteworthy is that the Cesium cations and the COO- group from the malate anions, form a sequence of nearly perfectly aligned dipoles oriented along the b crystallographic axis giving a permanent dipole moment of 38 Debye per unit cell. As the crystals are non hygroscopic and easy to grow, they are potential new material for nonlinear optical and pyroelectric applications.

L-alaninium perrhenate: crystal structure and non-linear optical properties

AbstractThe crystal structure and non-linear optical properties of L-alaninium perrhenate, C3H8NO2+ ReO4−, are reported. The protonated amino acid and the perrhenate anion have their usual geometries. The three-dimensional hydrogen-bonded network can be seen as a stacking of layers parallel to the (100) planes. Each layer is formed by chains of alternating positive and negative ions along the b and c axes. Hydrogen bonding of adjacent layers forms alternating chains along the a axis. A high damage threshold and a second-harmonic generation efficiency three times that of KDP make this new material potentially useful in non-linear optics.

The influence of nanocrystal size on optical second harmonic generation by para-nitroanaline embedded in electro-spun polymeric fibers

Poly(methyl methacrylate) electro-spun fibers with embedded nanocrystals of the paradigmatic donor–acceptor nonlinear chromophore para-nitroaniline have been recently demonstrated to be efficient generators of second harmonic light. To understand the influence of the size and local strain experienced by the embedded para-nitroaniline nanocrystals, a Williamson−Hall analysis was carried out on the X-ray diffraction intensity. Both the mean crystal size and strain can be tuned by simple changes in the deposition parameters of flow rate and applied voltage. The observed second harmonic signal is well correlated with the ratio of the fiber diameter to the mean para-nitroaniline crystal size suggesting that surface effects are the main source of the strong nonlinear optical response. Adjusting the electro-spinning deposition parameters when producing polymeric fibers doped with strong nonlinear organic chromophores with high dipole moments has the potential to provide a versatile and efficient method for developing second-order nonlinear optical materials.