Large molecular third-order optical nonlinearities in polarized carotenoids

Aplicaciones de los métodos computacionales al estudio de la estructura y propiedades de polímeros

En este trabajo se revisan las técnicas de simulación molecular más habituales y potentes para la descripción de los polímeros a escala atómica y molecular, las cuales se han clasificado en cuánticas o clásicas dependiendo de cómo se describen las interacciones entre las partículas. Se presentan asimismo diversas aplicaciones de dichas metodologías, realizadas en nuestro laboratorio, en el contexto del estudio de la estructura y propiedades de polímeros. En particular, se muestran aplicaciones de las técnicas clásicas a la determinación de estructuras cristalinas, a estudio del plegamiento lamelar de los nylons, a la estabilidad de las estructuras supramoleculares observadas en algunos complejos tensioactivo·polielectrolito y a la difusión de gases en matrices poliméricas, mientras que el uso de técnicas cuánticas se ha ilustrado presentando estudios dedicados a la predicción de efectos cooperativos, interacciones específicas y parámetros espectroscópicos.

High third-order nonlinear optical susceptibility in new fluorinated poly(p-phenylenevinylene) copolymers measured with the Z-scan technique

The third-order nonlinear optical properties of a series of copoly(2, 3, 5, 6-tetrafluoro-1, 4-phenylenevinylene-2, 5-dioctyloxy-1, 4-phenylenevinylene) that contain variable ratios of two differently substituted monomers have been studied in chloroform solutions at l=1064 nm by the picosecond Z-scan technique. Nonlinear refractive index n(2) of the samples investigated has been found to be negative, and a strong dependence of its magnitude on the copolymer's composition has been observed. The highest third-order nonlinear optical susceptibility, |x((3))|=(6 +/- 2)x 10(-10) esu, was measured for a copolymer obtained by reaction of equimolar quantitites of the parent monomers.

Multiple relaxation pathways in push-pull polyenes

Subpicosecond absorption and gain spectroscopy are used to investigate the excited-state behavior of push-pull polyenes made of a diethylthiobarbituric acid electron-acceptor group and a dibutylaniline electron-donor group linked by a pi-conjugated chain. Four polyenes of increasing length, ranging from n = 2 to 5 double bonds, are compared. The relaxation path and relaxation kinetics are studied in dioxane and in cyclohexane, a polar and a nonpolar solvent, respectively. In dioxane, the results provide evidence for the formation of an emissive transient state on an ultrashort time scale (2-3 ps) attributed to a charge transfer (CT) state. The regular shift of the gain peak of this transient state with increase in the chain length (ca. 100 nm per added double bond) indicates that its structure is similar to that of a cyanine, i.e. with a fully conjugated polyenic chain. Its lifetime ranges from a few tens to a few hundreds of picoseconds depending on the chain length. When the number of double bonds increases from n = 2 to 3, the lifetime increases, then decreases continuously for longer chains. In cyclohexane, where the transient CT state is not formed, the decay of the initial excited state follows the same trend when the chain length increases but the lifetimes are shorter than that of the CT state in dioxane. In both solvents, the characterization of long-lived photoproducts by synchronizing two low repetition-rate subpicosecond laser systems demonstrates a change in the relaxation route as the chain length increases. Isomerization occurs for n = 2, whereas intersystem crossing to the triplet state occurs for n = 4. The change in the relaxation channel is observed for n = 3 in both solvents with however a solvent-dependent behavior. In dioxane, relaxation to the triplet state is already observed for n = 3, while an intermediate regime with a relaxation directly to the ground state is observed in cyclohexane. The photophysics of the studied push-pull polyenes is tentatively compared to that of polymethine cyanines and substituted carotenoids.

A comparative study of third-order nonlinear optical properties of silver phenylacetylide and related compounds via ultrafast optical Kerr effect measurements

A comparative study of the third-order nonlinear optical properties, via the newly developed heterodyned optical Kerr effect (OHD-OKE) measurements, of silver phenylacetylide and related compounds is reported. [AgC[triple bond]CC(6)H(5)](n) (1) was found to exhibit efficient third-order nonlinear optical susceptibility chi((3)) of 2.4 x 10(-14) esu, and second hyperpolarizability gamma of 9.07 x 10(-32) esu. These results are compared with those of two related silver phenylacetylide compounds, namely, a double salt, (silver phenylacetylide).(silver tert-butylthiolate) [AgC[triple bond]CC(6)H(5).AgS(t-C(4)H(9))](n) complex (2), and a cluster, triphenylphosphine silver phenylacetylide tetramer, [(C(6)H(5))(3)PAgC[triple bond]CC(6)H(5)](4) (3), as well as that of the related organic polymer polyphenylacetylene (4). These four compounds represent different types of phenylacetylide derivatives: 1 is an organometallic polymer, 2 a polymeric double salt, 3 a discrete metal cluster, and 4 an organic polymer. It was found that the third-order optical nonlinear response was enhanced by the incorporation of silver d electrons into the delocalized conjugated organic pi system, and its magnitude is highly dependent upon the extent of the pi delocalization. Specifically, the relative magnitudes of chi((3)) and gamma follow the order silver phenylacetylide polymer (1) > (silver phenylacetylide).(silver tert-butylthiolate) double salt (2) > polyphenylacetylene polymer (4) > tetrameric (triphenylphosphine silver phenylacetylide)(4) cluster (3). The observed trend may be attributed to the decreasing length of pi conjugation. It is interesting to note that the incorporation of Ag(I) into the polymeric framework of polyphenylacetylene enhances the chi((3)) by 25-fold for the same degree of polymerization (n = 7). The signs of chi((3)) and gamma, which are related to the response mechanisms, were found to be solvent dependent.

Negative differential resistance behavior in conjugated molecular wires incorporating spacers: a quantum-chemical description

Recent experimental studies have demonstrated that single molecules or a small number of self-assembled molecules can perform the basic functions of traditional electronic components, such as wires and diodes. In particular, molecular wires inserted into nanopores can be used as active elements for the fabrication of resonant tunneling diodes (RTDs), whose I/V characteristics reveal a Negative Differential Resistance (NDR) behavior (i.e., a negative slope in the I/V curve). Here, quantum-chemical calculations are used to describe on a qualitative basis the mechanism leading to NDR in polyphenylene-based molecular wires incorporating saturated spacers. This description is based on the characterization of the evolution of the wire electronic structure as a function of a static electric field applied along the molecular axis, which simulates the driving voltage between the two electrodes in the RTD devices. We illustrate that the main parameters controlling the NDR behavior can be modulated through molecular engineering of the wires.

Origin of giant optical nonlinearity in charge-transfer-mott insulators: a new paradigm for nonlinear optics

Neither pure Mott insulators nor pure charge-transfer insulators have ever been considered as a possible candidate for nonlinear optical (NLO) materials since individually neither the strong correlation (U) nor the large charge transfer (Delta) is favorable to the NLO response. However, in their composites, charge-transfer-Mott insulators, jointly Delta and U can enhance the hyperpolarizability (gamma) by guiding the ground states into the antiferromagnetic phase and the excited states into the charge-transfer phase. These Delta and U that maximize gamma form a unique golden Delta-U line, on which the recently observed giant nonlinear optical effect is just a single point, whose physical origin is that the system is driven into a phase-separated region for the ground and excited states. This novel mechanism may suggest a conceptually new paradigm to explore an even larger optical nonlinearity.