Loon-Seng Tan

Multiphoton Absorbing Materials: Molecular Designs, Characterizations, and Applications

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

Covalent modification of vapour-grown carbon nanofibers via direct Friedel-Crafts acylation in polyphosphoric acid

Electrophilic functionalization of vapour-grown carbon nanofibers (VGCNF) was accomplished via Friedel–Crafts acylation with 2,4,6-trimethylphenoxybenzoic acid in polyphosphoric acid using the improved conditions that we previously described. The progress of the reaction was conveniently monitored with FT-IR spectroscopy following the growth of the keto-carbonyl band at 1664 cm−1 associated with the product. In addition to scanning electron microscopic and UV-vis spectroscopic data, the combined results from the elemental analysis and thermogravimetric analysis further suggested that there were 3 arylcarbonyl groups covalently attached to the nanotube structure for every 100 carbon sites. Because of the presence of significant hydrogen content in the starting VGCNF, the covalent attachment of the arylcarbonyl groups most probably occurred at the sp2C–H sites.

Degenerate two-photon-absorption spectral studies of highly two-photon active organic chromophores.

Degenerate two-photon absorption (TPA) spectral properties of five AFX chromophore solutions have been studied using a single and spectrally dispersed sub-picosecond white-light continuum beam. In a specially designed optical configuration, optical pathways inside the sample solution for different spectral components of the focused continuum beam were spatially separated from each other. Thus, the nondegenerate TPA processes coming from different spectral components can be eliminated, and the direct nonlinear absorption spectrum attributed to degenerate TPA processes can be readily obtained. Using this new technique, the complete TPA spectra for these five highly two-photon-active compounds (AF-380, AF-350, AF-295, AF-270, and AF-50) were obtained in the spectral range from 600 to 950 nm on an absolute scale of TPA cross section. The relationship between the molecular structures and their TPA spectral behaviors are discussed. In general the measured TPA spectra are not identical with the linear absorption spectra on the scale of absorbed photon(s) energy. Moreover, for some sample (such as AF-380), the TPA spectrum is totally different from the linear spectrum, which implies the difference of molecular transition pathways and selection rules for one- and two-photon excitation processes. At high excitation intensity levels (>or=15 GW/cm(2)), the saturation behavior of TPA transition can be observed obviously in AF-350 and AF-380 solutions that exhibit much higher nonlinear absorptivity than the other chromophores investigated.

Degenerate nonlinear absorption and optical power limiting properties of asymmetrically substituted stilbenoid chromophores

Two-photon absorption (2PA) spectra (650–1000 nm) of a series of model chromophores were measured via a newly developed nonlinear absorption spectral technique based on a single and powerful femtosecond white-light continuum beam. The experimental results suggested that when either an electron-donor or an electron-acceptor was attached to a trans-stilbene at a para-position, an enhancement in molecular two-photon absorptivity was observed in both cases, particularly in the 650–800 nm region. However, the push–pull chromophores with both the donor and acceptor groups showed larger overall two-photon absorption cross-sections within the studied spectral region as compared to their mono-substituted analogues. The combined results of the solvent effect and the 1H-NMR studies indicated that stronger acceptors produce a more efficient intramolecular charge transfer character upon excitation, leading to increased molecular two-photon responses in this model-compound set. A fairly good 2PA based optical power limiting behavior from one of the model chromophores is also demonstrated.

Large-scale self-assembly of dispersed nanodiamonds

Well-aligned rectangular-shaped nanodiamond fibers (NDFs) with a length-to-width ratio of over 1000 were formed on a large-scale via a self-assembly process by drying acid-treated nanodiamonds of a uniform size (∼4 nm) from an aqueous solution. Nanodiamond thin films (NDTFs), instead of NDFs, have also been prepared by drying the aqueous ND dispersion at a sufficiently low temperature (< 70 °C)) and/or a sufficiently low pH (< 4). Detailed conditions, including pH and temperature, for the formation of NDFs and NDTFs via the drying process, together with the structure and property of the resultant nanodiamond assemblies were investigated. The formation of NDFs and NDTFs was found to result from the delicate interplay between the ND–ND, ND–H2O, and ND–substrate interactions under the water-evaporation-induced directional convection flow. The resultant NDFs were demonstrated to show a bandgap of ∼4.5 eV.

Improved syntheses of poly(oxy-1,3-phenylenecarbonyl-1,4-phenylene) and related poly(ether-ketones) using polyphosphoric acid/P2O5 as polymerization medium

Abstract Based on the model-compound studies, the composition of polyphosphoric acid (PPA)/P 2 O 5 mixture as an effective catalytic/dehydrative medium for the preparation of poly(ether–ketones) was optimized. Thus, with the optimal weight ratio of 4:1 (PPA:P 2 O 5 ), the electrophilic substitution polycondensation of 3-phenoxybenzoic acid and related AB monomers was substantially promoted at 130 °C to yield the subject polymer abbreviated as m PEK and related poly(ether–ketones) with significantly higher molecular weights. In the cases where the polymerization systems were completely homogeneous, the ensuing polycondensation was rapid and yielded high molecular weight polymers (e.g. m PEK [η]=2.10 dl / g ) at 130 °C within 30 min., as compared to PPMA (phosphorus pentoxide/methanesulfonic acid) method which gave only moderate molecular weight polymers, e.g. m PEK ([η]=0.64 dl / g ). In some cases, where the monomers and PPA/P 2 O 5 were not fully compatible, polycondensation did proceed and reasonable molecular weight range ([η]=0.69–0.76 dl / g ) could be achieved. However, the complete incompatibility between the poly(ether–sulfone) and PPA/P 2 O 5 medium precluded the successful polymerization of 4-phenoxybenzenesulfonic acid.

Electrothermal polymer nanocomposite actuators.

Adaptive structures are desirable for a broad array of technologies, ranging from medical stents to deployable telescopes and morphing air vehicles. [ 1 ] Due to their processability and relatively high strain, [ 2 ] electroactive polymers (EAPs) have become a viable option for low-frequency actuating devices that reversibly drive a mechanically articulated substructure in a manner analogous to muscles controlling the position of an organism’s skeleton. Unfortunately, electric-fi eld driven EAPs generally require high operating voltages and exhibit low force output, challenging broader utilization. Recently, enhancements in electromechanical performance have been realized in a host of polymers through nanoparticle inclusion. [ 3– 7 ] Carbon nanotube (CNT) additives in particular, both below and above electrical percolation, have been shown to reduce the electric-fi elds required to induce actuation by up to two orders of magnitude in various electrostrictive and piezoelectric polymers. For example, Zhang et al. observed nearly a two-fold increase in electrostrictive strain (2.1%) relative to the pristine polymer at an applied fi eld of 54 MVm −1 through the addition of 0.35 vol% MWNTs to P(VDF-TrFE-CFE). [4] Likewise, Park et al. observed an electrostrictive strain of 2.6% in a LaRC-EAP fi lm containing 0.05 vol% SWNTs under an applied fi eld of 0.8 MVm −1 . [ 6 ] These enhancements have been attributed to modifi cation of the local electric-fi eld distribution within the polymer due to fi eld exclusion from the conductive particles and charge accumulation at the polymer–CNT interfaces. In general, peak electromechanical performance was reported around the electrical percolation threshold for CNT addition. The increased conductivity at these higher nanotube loadings, however, can result in a leakage current that decreases the external power effi ciency for capacitive operation of the EAP. Alternatively, if this electrical dissipation is maximized and coupled to a thermodynamically reversible dimensional transformation, effective electromechanical actuation can be envisioned via a materialderived electrothermal effect. For some materials, such as shape memory polymers [ 1 , 8,9 ] and liquid-crystal elastomers, [ 10,11 ] this can be used to replace external heat sources. Generally, with external heating, the response time is limited by the rate of heat transfer from the heater to the sample surface, and then the sample surface into the bulk. Although this can be mitigated to some extent by minimizing the sample thickness, direct incorpo-

New technique for degenerate two-photon absorption spectral measurements using femtosecond continuum generation

We present a new technique for direct measurements of degenerate two-photon absorption (TPA) spectra of two-photon absorbing materials including non-fluorescent samples. This technique is based on the use of an intense single continuum-generation beam as the coherent whitelight source with specially flattened spectral distribution. The different spectral components of the continuum beam are spatially dispersed and then passed through the sample material along different pathways so that nondegenerate TPA processes among different input spectral components can be avoided. By comparing the input and transmitted continuum spectral distributions, the TPA spectrum for a given sample can be obtained. As an example, the continuous TPA spectrum (from 550 to 1000 nm) is measured for a novel two-photon-absorbing compound (AF-389) which exhibits an extremely high TPA cross-section value of ~1x10-20 cm4/GW, or ~249 GM, around ~800-nm spectral range in femtosecond regime.

Contactless, photoinitiated snap-through in azobenzene-functionalized polymers

Significance Photomechanical effects in polymers are distinguished by the ease with which actinic light can be regulated to contactlessly trigger the magnitude and directionality of mechanical adaptivity with spatio-temporal control. The materials examined to date have not demonstrated power densities or actuation speeds necessary for applications seeking to exploit the promise of wirelessly triggered actuation. Using mechanical design, we employ two classes of azobenzene-functionalized polymers and demonstrate contactless snap-through of bistable arches realizing orders-of-magnitude enhancement in the actuation rates (∼102 mm/s) and powers (∼1 kW/m3) under moderate irradiation intensities (<<100 mW/cm2). The experimental characterization of the snap-through is supported with modeling that elucidates the effect of geometry, mechanical properties, and photogenerated strain on the actuation rate and energy output. Photomechanical effects in polymeric materials and composites transduce light into mechanical work. The ability to control the intensity, polarization, placement, and duration of light irradiation is a distinctive and potentially useful tool to tailor the location, magnitude, and directionality of photogenerated mechanical work. Unfortunately, the work generated from photoresponsive materials is often slow and yields very small power densities, which diminish their potential use in applications. Here, we investigate photoinitiated snap-through in bistable arches formed from samples composed of azobenzene-functionalized polymers (both amorphous polyimides and liquid crystal polymer networks) and report orders-of-magnitude enhancement in actuation rates (approaching 102 mm/s) and powers (as much as 1 kW/m3). The contactless, ultra-fast actuation is observed at irradiation intensities <<100 mW/cm2. Due to the bistability and symmetry of the snap-through, reversible and bidirectional actuation is demonstrated. A model is developed to elucidate the underlying mechanics of the snap-through, specifically focusing on isolating the role of sample geometry, mechanical properties of the materials, and photomechanical strain. Using light to trigger contactless, ultrafast actuation in an otherwise passive structure is a potentially versatile tool to use in mechanical design at the micro-, meso-, and millimeter scales as actuators, as well as switches that can be triggered from large standoff distances, impulse generators for microvehicles, microfluidic valves and mixers in laboratory-on-chip devices, and adaptive optical elements.

Synthesis and characterization of highly photoresponsive fullerenyl dyads with a close chromophore antenna–C60 contact and effective photodynamic potential

We report the synthesis of a new class of photoresponsive C(60)-DCE-diphenylaminofluorene nanostructures and their intramolecular photoinduced energy and electron transfer phenomena. Structural modification was made by chemical conversion of the keto group in C(60)(>DPAF-C(n)) to a stronger electron-withdrawing 1,1-dicyanoethylenyl (DCE) unit leading to C(60)(>CPAF-C(n)) with an increased electronic polarization of the molecule. The modification also led to a large bathochromic shift of the major band in visible spectrum giving measureable absorption up to 600 nm and extended the photoresponsive capability of C(60)-DCE-DPAF nanostructures to longer red wavelengths than C(60)(>DPAF-C(n)). Accordingly, C(60)(>CPAF-C(n)) may allow 2γ-PDT using a light wavelength of 1000-1200 nm for enhanced tissue penetration depth. Production efficiency of singlet oxygen by closely related C(60)(>DPAF-C(2) (M)) was found to be comparable with that of tetraphenylporphyrin photosensitizer. Remarkably, the (1)O(2) quantum yield of C(60)(>CPAF-C(2) (M)) was found to be nearly 6-fold higher than that of C(60)(>DPAF-C(2) (M)), demonstrating the large light-harvesting enhancement of the CPAF-C(2) (M) moiety and leading to more efficient triplet state generation of the C(60)> cage moiety. This led to highly effective killing of HeLa cells by C(60)(>CPAF-C(2) (M)) via photodynamic therapy (200 J cm(-2) white light). We interpret the phenomena in terms of the contributions by the extended π-conjugation and stronger electron-withdrawing capability associated with the 1,1-dicyanoethylenyl group compared to that of the keto group.