Mariacristina Rumi

Two-photon absorption: an overview of measurements and principles

The range of organic compounds whose degenerate two-photon absorption (2PA) spectrum has been reported has increased rapidly in recent years, in parallel with the growing interest in applications based on the 2PA process. The comparison of results from different techniques is not always straightforward, and experimental conditions employed may vary significantly. We overview the concepts underlying 2PA measurements and the common assumptions and approximations used in the data analysis for various techniques. The importance of selecting appropriate excitation regimes under which measurements should be performed and of avoiding contributions from absorption mechanisms in addition to 2PA will be emphasized.

Two-Photon Absorbing Materials and Two-Photon-Induced Chemistry

Two-photon absorption, the process by which an excited molecule or material is produced by the simultaneousabsorption of two photons, has been studied extensively in recent years, from both fundamental and applicationpoints of view. On one side, the field has been expanded with the measurement of two-photon absorptionspectra and cross sections for a wide range of conjugated molecules. In this contribution, we willreview the two-photon properties of some of these classes of materials, and we will discuss structure/propertyrelationships that have been developed from these investigations. On the other side, two-photon absorptionhas been exploited as a means to activate a variety of chemical and physical processes with sub-diffraction–limitedresolution, because three-dimensional spatial confinement of the excitation volume in a material irradiatedby a tightly focused laser beam can be achieved via two-photon or, more generally, multi-photon absorptionprocesses. This characteristic has led to applications in a number of technological fields, such asmicrofabrication and laser scanning fluorescence microscopy. Here, we will survey material systems thathave been developed to activate radical or cationic polymerization reactions, deprotection of functionalgroups, and singlet oxygen generation via two-photon excitation of one of the components in the system.

Structure-processing-property correlations in solution-processed, small-molecule, organic solar cells

Alkyl chains are often attached to the periphery of semiconductor molecules to impart solubility and they represent a pervasive structural element in solution processable, organic photovoltaics (OPV). It is important to understand the effects of such substitutions on the morphology and performance of organic solar cells. This investigation focuses on determining structure–property correlations in OPV devices constructed with small-molecule, solution processable electron donors based on benzothiadiazole–dithienopyrrole, mixed with the electron acceptor PCBM. Two donor molecules with the same opto-electronic molecular properties but differing alkyl substituents – without (BD) or with (BD6) hexyl side chains – are studied. The resulting device data for fabricated solar cells, across a range of processing conditions, is compared to thin-film morphology, spectroscopy, thermal analysis, and molecular dynamics simulations. Two device states of higher and lower performance, depending on the casting solvent, are obtained for the molecule without the side chains (BD); both states have amorphous mesoscale structure, but show subtle differences in the nanoscale phase separation. In contrast, for the molecule with side chains (BD6) devices have highly variable reproducibility and middling efficiency and photocurrent. The BD6 donor exhibits lower miscibility with PCBM, which correlates with the formation of a donor-enriched layer on the surface of the solar cell.

Theoretical Design of Organic Chromophores with Large Two-Photon Absorption Cross-Sections

Design strategies and structure-property relationships for two-photon absorption in conjugated molecules are described on the basis of correlated quantum-chemical calculations. We first focus on stilbene derivatives with centrosymmetric structures. We found that derivatization of the conjugated molecule with electroactive groups in a quadrupolarlike arrangement leads to a large increase in the two-photon absorption cross section, δ. Quantum-chemical description provides rich insight into the mechanisms for the two-photon absorption phenomenon.

Two-photon absorption in cross-shaped chromophores with phenylene-vinylene backbones

While early investigations in two-photon absorption (TPA) mainly focused on developing structure/property relationships for organic molecules with a conjugated path extending in one dimension, more recent research efforts have also included molecules containing multiple TPA-active units or with multi-dimensional conjugated backbones. However, a complete understanding of how the number and structure of branches in chromophores affect TPA properties has not yet emerged. Here, we describe chromophores consisting of four donor-substituted branches linked to an aromatic core (benzene or pyrazine) and compare them with linear analogues with only two branches (donor-Π-donor distryrylbenzenes). It is shown that this design does not lead to large enhancement of the TPA cross sections, with only a modest increase being observed going from compounds with two to compounds with four branches. It is also shown that a molecular exciton model can describe semi-quantitatively both the one-photon and the two-photon spectra of this type of multi-branched compounds. In contrast to the one-photon case, the model shows that pure additivity of the TPA cross section should not, in general, be expected when two monomer units are coupled, and that the TPA cross section of the composite molecule depends on the relative orientation of the constituent units and on the strength and sign of the coupling interaction.

Two-photon polymerization initiators for efficient three-dimensional optical data storage and microfabrication

Summary form only given. Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has enabled the development of 3D fluorescence imaging, 3D optical data storage, and 3D lithographic microfabrication. Each of these applications takes advantage of the fact that the two-photon absorption probability depends quadratically on intensity, and therefore under tight-focusing conditions, the absorption is confined at the focus to a volume of order /spl lambda//sup 3/, where /spl lambda/ is the laser wavelength. Any subsequent process, such as fluorescence or a photo-induced chemical reaction, is also localized in this small volume. For instance, two-photon excitation can initiate conventional reactions such as side-group deprotection, radical generation, and polymerization, through energy transfer or electron transfer. However, the efficiency of such processes depends critically on the strength of the chromophores two-photon absorptivity. We have developed a wide array of chromophores which hold great promise for 3D optical data storage and 3D microfabrication. These materials are based on donor-/spl pi/-donor, donor-acceptor-donor, or acceptor-donor-acceptor structural motifs.

Two photon absorbing chromophores for broadband optical limiting

Summary form only given. Optical limiters are devices that strongly attenuate intense optical beam while exhibiting high transmittance for low intensity ambient light levels. General requirements for a material to be a good optical limiter are high pulse energy suppression, broad bandwidth, wide temporal response, operation in fast optics, and high ambient transmission. Molecules with large two-photon absorption cross sections that form strongly absorbing excited states have potential for good optical limiting response. We discuss our recent results on obtaining relatively broadband optical limiting response through the use of such two-photon absorbing chromophores.

Polymer stabilization of cholesteric liquid crystals in the oblique helicoidal state

Electrical control of the pitch has been reported in a variant of the cholesteric liquid crystal phase composed of chiral dopants and liquid crystal dimers with a bent conformation, such as CB7CB. For a finite range of applied electric field, the dimeric mesogens assume an oblique helicoidal structure, in which the helical axis is aligned along the electric field and the local director is tilted towards the helical axis (rather than being perpendicular to it). An electric field can directly regulate the periodicity (pitch), allowing reconfiguration of the optical response from a scattering or transparent state to a reflective state. Here, we employ po stabilization to retain the oblique helicoidal state absent an applied field. The polymer stabilized oblique helicoidal structures were investigated under various conditions and material compositions. With polymer stabilization, the magnitude of the selective reflection is found to be dependent on the strength of the applied field. Comparison of the electro-optical response of samples with and without a polymer network elucidates the relative role of boundary conditions, anchoring strength, and elastic energy on the stability of the oblique helicoidal state.

New derivatives of cyclohexanone and piperidone compounds for bioluminous sensing

We present the results of our investigation of new derivatives of cyclohexanone and piperidone compounds that have exhibited in the past anticancer effect due to biochemical destruction of cancer cells. In this study we focus on using these compounds as markers for malignant cells because of their strong two-photon excited fluorescence. Their molecular cross-section of two-photon absorption can be as high as 3000x10-50cm4s/photon and compares well with Rhodamine B, well-known fluorescent molecular probe. This provides an option for monitoring the biochemical destruction of cancer cells by means of two-photon excited fluorescence spectroscopy. Initially we studied the two-photon fluorescence of the solutions of pure compounds. Then we mixed the compounds with amino acids (Glycine and Alanine), the major building blocks of proteins in cells, hoping that the fluorescence will give some insight in the interaction between the compounds and bio substances. We discuss the solubility issues, the cross-section of twophoton absorption of the compounds, and also the features of the spectrum of the two-photon excited fluorescence. Compounds combining both properties (cytotoxicity and two-photon excited fluorescence), which are now carried by different chemical agents, are expected to improve the efficiency of cancer treatment and lower the cost.

Water-soluble 1,4-bis(4-aminostyryl)benzene derivatives for biological two-photon applications

1,4-Bis(4-aminostyryl)benzene derivatives have been shown to exhibit large two-photon cross-sections at ca. 730-745 nm and are typically highly fluorescent in organic solvents. For biological imaging applications, we have been interested in identifying water-soluble dyes with high two-photon cross-sections and fluorescence quantum yields. Four new bis(aminostyryl)benzene chromophores have been synthesized; these chromophores are functionalized with phosphate, sulfonate, and sulfate hydrophilic groups. In some cases, this substitution pattern renders the chromophores water-soluble and, for the sulfonate and sulfate cases, moderate fluorescence quantum yields are retained in water.