Switches, and Actuators Masahiro Irie,*,† Tuyoshi Fukaminato,‡ Kenji Matsuda, and Seiya Kobatake †Research Center for Smart Molecules, Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501, Japan ‡Research Institute for Electronic Science, Hokkaido University, N20, W10, Kita-ku, Sapporo 001-0020, Japan Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japan

Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators

4. Survey of Novel Multiphoton Active Materials 1257 4.1. Multiphoton Absorbing Systems 1257 4.2. Organic Molecules 1257 4.3. Organic Liquids and Liquid Crystals 1259 4.4. Conjugated Polymers 1259 4.4.1. Polydiacetylenes 1261 4.4.2. Polyphenylenevinylenes (PPVs) 1261 4.4.3. Polythiophenes 1263 4.4.4. Other Conjugated Polymers 1265 4.4.5. Dendrimers 1265 4.4.6. Hyperbranched Polymers 1267 4.5. Fullerenes 1267 4.6. Coordination and Organometallic Compounds 1271 4.6.1. Metal Dithiolenes 1271 4.6.2. Pyridine-Based Multidentate Ligands 1272 4.6.3. Other Transition-Metal Complexes 1273 4.6.4. Lanthanide Complexes 1275 4.6.5. Ferrocene Derivatives 1275 4.6.6. Alkynylruthenium Complexes 1279 4.6.7. Platinum Acetylides 1279 4.7. Porphyrins and Metallophophyrins 1279 4.8. Nanoparticles 1281 4.9. Biomolecules and Derivatives 1282 5. Nonlinear Optical Characterizations of Multiphoton Active Materials 1282

Multiphoton Absorbing Materials : Molecular Designs, Characterizations, and Applications

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

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

Triplet photosensitizers (PSs) are compounds that can be efficiently excited to the triplet excited state which subsequently act as catalysts in photochemical reactions. The name is originally derived from compounds that were used to transfer the triplet energy to other compounds that have only a small intrinsic triplet state yield. Triplet PSs are not only used for triplet energy transfer, but also for photocatalytic organic reactions, photodynamic therapy (PDT), photoinduced hydrogen production from water and triplet-triplet annihilation (TTA) upconversion. A good PS should exhibit strong absorption of the excitation light, a high yield of intersystem crossing (ISC) for efficient production of the triplet state, and a long triplet lifetime to allow for the reaction with a reactant molecule. Most transition metal complexes show efficient ISC, but small molar absorption coefficients in the visible spectral region and short-lived triplet excited states, which make them unsuitable as triplet PSs. One obstacle to the development of new triplet PSs is the difficulty in predicting the ISC of chromophores, especially of organic compounds without any heavy atoms. This review article summarizes some molecular design rationales for triplet PSs, based on the molecular structural factors that facilitate ISC. The design of transition metal complexes with large molar absorption coefficients in the visible spectral region and long-lived triplet excited states is presented. A new method of using a spin converter to construct heavy atom-free organic triplet PSs is discussed, with which ISC becomes predictable, C60 being an example. To enhance the performance of triplet PSs, energy funneling based triplet PSs are proposed, which show broadband absorption in the visible region. Applications of triplet PSs in photocatalytic organic reactions, hydrogen production, triplet-triplet annihilation upconversion and luminescent oxygen sensing are briefly introduced.

Triplet photosensitizers: from molecular design to applications

Role of Structural Factors in the Nonlinear Optical Properties of Phthalocyanines and Related Compounds

OPTICALLY nonlinear organic materials show considerable potential for applications in optical signal processing and telecommunications1,2. Most materials are based on the p-nitro-aniline template, in which the optical nonlinearities are intimately associated with quasi-one-dimensional charge transfer. But there are problems associated with this conventional approach, arising from the strongly dipolar nature of the molecules2. It has recently been recognized3–5 that two- and three-dimensional stereochemistry offers new possibilities for the design and synthesis of optically nonlinear molecules, in which charge transfer is multidirectional rather than dipolar in character; octupolar nonlinearities have now been demonstrated in several molecular systems5–7. Tri-substituted ruthenium complexes6 appear particularly attractive because intense, multidirectional metal-to-ligand charge transfer leads to a significant enhancement of the optical nonlinearity, as quantified by the quadratic hyperpolarizability, β. Here we show that the choice of ligand can further increase β to values in excess of 10-27e.s.u., comparable to the best dipolar optically nonlinear molecules.

Chiral metal complexes with large octupolar optical nonlinearities

Coordination compounds are of particular interest for the design of new chromophores with large nonlinear optical (NLO) susceptibilities. They offer a wide range of metals with different oxidation states and ligands, which can give rise to tunable electronic properties. Metal ions are excellent templates to build 2D and 3D octupolar NLO-phores. Their associations with functionalized bipyridyl ligands give rise to octahedral and tetrahedral metal complexes with large first-order hyperpolarizabilities. The ability to functionalize these octupoles allows their incorporation into macromolecular architectures such as polymers and metallodendrimers, We also show in this Account two approaches to induce noncentrosymmetry at the macroscopic level, either by supramolecular organization or by the all optical poling procedure.

Molecular engineering of octupolar NLO molecules and materials based on bipyridyl metal complexes.

Recent developments in the field of metal complexes containing photochromic ligands: Modulation of linear and nonlinear optical properties

Novel photochromic dithienylethene-based platinum(II) complexes (C∧N∧N)Pt(C≡C—DTE—C6H4—D) ((C∧N∧N) = 4,4′-di(n-hexyl)-6-phenyl-2,2′-bipyridine; D = H, NMe2) were prepared and characterized. Their excellent photochromic properties allow the photoinduced switching of their second-order nonlinear optical properties in solution, as measured by the EFISH technique, due to formation of an extended π-conjugated ligand upon suitable electromagnetic radiation. Insights into the electronic structures of the complexes and the nature of their excited states have been obtained by DFT and TD-DFT calculations. These novel Pt(II) complexes were nanoorganized in polymer films which were poled, affording new materials characterized by a good second-order NLO response that can be easily switched, with an excellent NLO contrast. To the best of our knowledge, our compounds allowed designing the very first examples of switchable NLO polymer films based on metal complexes.

Second-order NLO switches from molecules to polymer films based on photochromic cyclometalated platinum(II) complexes.

Metal complexes of pyridine and bipyridine-type ligands represent an important class of nonlinear optical (NLO) materials: (i) They can incorporate a variety of functionalised ligands with a wide range of metals which can give rise to tuneable NLO properties; (ii) they are generally associated with intense low-lying charge-transfer excitations such as metal to ligand charge-transfer (MLCT) or intraligand charge-transfer (ILCT); (iii) bipyridyl ligands are also good building blocks that allow the construction of octupolar complexes with a defined geometry and symmetry. This review describes the recent developments in the design of such dipolar and octupolar chromophores for second-order nonlinear optics.

Hubert Le Bozec

Papers

Chiral metal complexes with large octupolar optical nonlinearities

Molecular engineering of octupolar NLO molecules and materials based on bipyridyl metal complexes.

Recent developments in the field of metal complexes containing photochromic ligands: Modulation of linear and nonlinear optical properties

Second-order NLO switches from molecules to polymer films based on photochromic cyclometalated platinum(II) complexes.

Dipolar and Non‐Dipolar Pyridine and Bipyridine Metal Complexes for Nonlinear Optics