I. Polyzos

Benzothiazoles with Tunable Electron-Withdrawing Strength and Reverse Polarity: A Route to Triphenylamine-Based Chromophores with Enhanced Two-Photon Absorption

A series of dipolar and octupolar triphenylamine-derived dyes containing a benzothiazole positioned in the matched or mismatched fashion have been designed and synthesized via palladium-catalyzed Sonogashira cross-coupling reactions. Linear and nonlinear optical properties of the designed molecules were tuned by an additional electron-withdrawing group (EWG) and by changing the relative positions of the donor and acceptor substituents on the heterocyclic ring. This allowed us to examine the effect of positional isomerism and extend the structure-property relationships useful in the engineering of novel heteroaromatic-based systems with enhanced two-photon absorption (TPA). The TPA cross-sections (δ(TPA)) in the target compounds dramatically increased with the branching of the triphenylamine core and with the strength of the auxiliary acceptor. In addition, a change from the commonly used polarity in push-pull benzothiazoles to a reverse one has been revealed as a particularly useful strategy (regioisomeric control) for enhancing TPA cross-sections and shifting the absorption and emission maxima to longer wavelengths. The maximum TPA cross-sections of the star-shaped three-branched triphenylamines are ∼500-2300 GM in the near-infrared region (740-810 nm); thereby the molecular weight normalized δ(TPA)/MW values of the best performing dyes within the series (2.0-2.4 GM·g(-1)·mol) are comparable to those of the most efficient TPA chromophores reported to date. The large TPA cross-sections combined with high emission quantum yields and large Stokes shifts make these compounds excellent candidates for various TPA applications, including two-photon fluorescence microscopy.

Two-photon absorption properties of novel organic materials for three-dimensional optical memories

Abstract The two-photon absorption (TPA) properties of a novel group of pyrylium-based compounds were studied. The molecules of this group were synthesized by systematically changing the chemical structure of a specific substituent in an initial chromophore. TPA cross-sections as large as 1.8×10 −47 cm 4 s photon −1 and high quantum yields were obtained. The results combined with the low cost and ease of synthesis of these compounds make them candidates for TPA applications. Particularly, three-dimensional data storage in polymer matrices of the synthesized compounds was realized through the photobleaching process. The high efficiency of recording and the submicron resolution reveal the great potential of this group of molecules as memory materials.

Intensity dependent nonlinear absorption of pyrylium chromophores

Abstract The intensity dependence of nonlinear absorption (NLA) of a series of pyrylium chromophores was studied with femtosecond pulses in their two-photon absorption spectral region to assess their potential as optical limiters. The open z-scan technique using different excitation intensities was employed. Fifth-order effects appear above a critical value of the irradiance and consequently of the first excited singlet-state population. Reverse saturable absorption (RSA) based on two-photon (TPA) and excited-state absorption (ESA) was dominant in most cases. The third- and fifth-order NLA coefficients and high ESA cross-sections of the order of 10 −15 cm 2 are presented at 760, 790 and 840 nm.

Stress transfer mechanisms at the submicron level for graphene/polymer systems.

The stress transfer mechanism from a polymer substrate to a nanoinclusion, such as a graphene flake, is of extreme interest for the production of effective nanocomposites. Previous work conducted mainly at the micron scale has shown that the intrinsic mechanism of stress transfer is shear at the interface. However, since the interfacial shear takes its maximum value at the very edge of the nanoinclusion it is of extreme interest to assess the effect of edge integrity upon axial stress transfer at the submicron scale. Here, we conduct a detailed Raman line mapping near the edges of a monolayer graphene flake that is simply supported onto an epoxy-based photoresist (SU8)/poly(methyl methacrylate) matrix at steps as small as 100 nm. We show for the first time that the distribution of axial strain (stress) along the flake deviates somewhat from the classical shear-lag prediction for a region of ∼2 μm from the edge. This behavior is mainly attributed to the presence of residual stresses, unintentional doping, and/or edge effects (deviation from the equilibrium values of bond lengths and angles, as well as different edge chiralities). By considering a simple balance of shear-to-normal stresses at the interface we are able to directly convert the strain (stress) gradient to values of interfacial shear stress for all the applied tensile levels without assuming classical shear-lag behavior. For large flakes a maximum value of interfacial shear stress of 0.4 MPa is obtained prior to flake slipping.

Three-photon induced photobleaching in a three-dimensional memory material

The photobleaching properties of a 3D memory material based on pyrylium salts are studied. It is shown that under ultrashort pulsed irradiation photobleaching occurs through two diverse mechanisms. One is major and causes fast bleaching, whereas the other is minor and causes slow bleaching. Furthermore, it is proved that both mechanisms are three- or more-photon processes. This fact gives the capability of significantly increasing data storage density.

Z-scan analysis for near-Gaussian beams through Hermite-Gaussian decomposition

Gaussian decomposition is used as a theoretical infrastructure with which Z-scan experiments are analyzed. This procedure is extended here to the interesting, from a practical point of view, case in which the laser beam used is not perfectly Gaussian. We follow a perturbative approach to obtain the far-field pattern of the beam after the beam passes through a nonlinear sample. The procedure is based on the decomposition of the electric field at the exit plane of the sample to a linear combination of Hermite–Gaussian functions. To a first-order approximation, each mode of the incident beam is decomposed to a linear combination of different-order modes that do not exceed the order of the original mode. Finally, the effects of the simultaneous presence of first and higher-order refractive nonlinearities or first-order refractive nonlinearity and nonlinear absorption are studied.

Modulation of (non)linear optical properties in tripodal molecules by variation of the peripheral cyano acceptor moieties and the π-spacer

A series of twelve tripodal push-pull molecules with a central triphenylamine donor and peripheral cyano substituted acceptors has been prepared. These molecules possess systematically altered π-linkers as well as cyano acceptors. Based on the experimental properties measured by differential scanning calorimetry, electrochemistry, one and two photon absorption/emission spectroscopy, supported by the DFT calculations, thorough structure–property relationships were elucidated.

Strong two photon absorption and photophysical properties of symmetrical chromophores with electron accepting edge substituents.

Two photon absorption (TPA) and photophysical properties of three new symmetrical chromophores with electron accepting phthalimide edge substituents have been studied. The three chromophores contain fluorene, alcoxy-substituted divinyl benzene, and carbazole moieties as central cores, respectively. The femtosecond time-resolved fluorescence upconversion spectroscopy and two photon excited fluorescence technique have been carried out. The effect of solvent polarity on TPA and on photophysics has also been determined. Ultrafast fluorescence dynamics, with decay times ranging from 1 to 13 ps, are revealed in polar solvents. This is attributed to the relaxation of the chromophores to the intramolecular charge transfer state. The chromophore bearing fluorene central core, being of the type A-pi-A, is the most efficient concerning TPA. Strong TPA, with a cross section value as high as 2100 GM at an excitation wavelength of 770 nm is found in acetophenone which is a solvent of intermediate polarity. The TPA spectra were also reproduced using a sum over states three-state model. A study of the TPA induced photobleaching of the fluorene molecule, doped in a solid poly(methyl-methacrylate) film, has shown that this material is very promising for efficient TPA optical data storage.

Molecular Modeling Combined with Advanced Chemistry for the Rational Design of Efficient Graphene Dispersing Agents

Pyrene-functional PMMAs were prepared via ATRP-controlled polymerization and click reaction, as efficient dispersing agents for the exfoliation of few-layered graphene sheets (GS) in easily processable low boiling point chloroform. In parallel, detailed atomistic simulations showed fine dispersion of the GS/polymer hybrids in good agreement with the experiment. Moreover, the molecular dynamics simulations revealed interesting conformations (bridges, loops, dangling ends, free chains) of GS/polymer hybrids and allowed us to monitor their time evolution both in solution and in the polymer nanocomposite where the solvent molecules were replaced with PMMA chains. Microscopic information about these structures is very important for optimizing mechanical performance. It seems that the combination of atomistic simulation with advanced chemistry constitutes a powerful tool for the design of effective graphene dispersing agents that could be used for the production of graphene-based nanocomposites with tailor-made...

Two- and three-photon absorption of organic ionic pyrylium based materials

Pyrylium dyes having the same basic chemical structure, differing only to a specific substituent, have been used as novel materials for multiphoton three-dimensional data storage. Electronic absorption spectra, two-photon and three-photon absorption properties of this class of pyrylium dyes, have been studied theoretically and compared to experimental results. The effects of the counteranion, the surrounding solvent, and electron releasing and electron withdrawing groups in specific positions of the basic structure have been explored in detail. It is argued that on grounds of the quality of experimental spectroscopic agreement, the computed two- and three-photon data may be used in pulse propagation simulations of three-dimensional recording in optical memories.