The use of photoinitiated polymerization is continuously growing in industry as reflected by the large number of applications in not only conventional areas such as coatings, inks, and adhesives but ...also high-tech domains, optoelectronics, laser imaging, stereolithography, and nanotechnology. In this Perspective, the latest developments in photoinitiating systems for free radical and cationic polymerizations are presented. The potential use of photochemical methods for step-growth polymerization is also highlighted. The goal is, furthermore, to show approaches to overcome problems associated with the efficiency, wavelength flexibility, and environmental and safety issues in all photoinitiating systems for different modes of activation. Much progress has been made in the past 10 years in the preparation of complex and nano-structured macromolecules by using photoinitiated polymerizations. Thus, the new and emerging applications of photoinitiated polymerizations in the field of biomaterials, surface modification, preparation of block and graft copolymers, and nanocomposites have been addressed.
The properties of few layer (one layer (1 L) to four layer (4 L)) graphenes doped by adsorption and intercalation of Br2 and I2 vapors are investigated. The Raman spectra of the graphene G vibrations ...are observed as a function of the number of layers. There is no evidence for chemical reaction disrupting the basal plane π electron conjugation. Adsorption of bromine on 1 L graphene creates a high doped hole density, well beyond that achieved by electrical gating with an ionic polymer electrolyte. In addition, the 2D Raman band is completely quenched. The 2 L bilayer spectra indicate that the doping by adsorbed I2 and Br2 is symmetrical on the top and bottom layers. Br2 intercalates into 3 L and 4 L graphenes. The combination of both surface and interior doping with Br2 in 3 L and 4 L creates a relatively constant doping level per layer. In contrast, the G spectra of 3 L and 4 L with surface adsorbed I2 indicate that the hole doping density is larger on the surface layers than on the interior layers and that I2 does not intercalate into 3 L and 4 L. This adsorption-induced potential difference between surface and interior layers implies that a band gap opens in the bilayer type bands of 3 L and 4 L.
Due to their high sensitivity and selectivity, minimum interference with living biological systems, and ease of design and synthesis, fluorescent hybridization probes have been widely used to detect ...nucleic acids both in vivo and in vitro. Molecular beacons (MBs) and binary probes (BPs) are two very important hybridization probes that are designed based on well-established photophysical principles. These probes have shown particular applicability in a variety of studies, such as mRNA tracking, single nucleotide polymorphism (SNP) detection, polymerase chain reaction (PCR) monitoring, and microorganism identification. Molecular beacons are hairpin oligonucleotide probes that present distinctive fluorescent signatures in the presence and absence of their target. Binary probes consist of two fluorescently labeled oligonucleotide strands that can hybridize to adjacent regions of their target and generate distinctive fluorescence signals. These probes have been extensively studied and modified for different applications by modulating their structures or using various combinations of fluorophores, excimer-forming molecules, and metal complexes. This review describes the applicability and advantages of various hybridization probes that utilize novel and creative design to enhance their target detection sensitivity and specificity.
A new means for functionalizing metal oxide surfaces, specifically nanoparticles, is demostrated. This process involves the design of stable ligands that bind strongly to the surface of metal oxides ...and can undergo further chemical modification via click chemistry, with both small molecules as well as polymers, to yield metal oxide surfaces with tailored functionality. The nanoparticles are stable and easily dispersed in both polar and nonpolar solvents, a property that is controlled by the ligand. The resultant nanoparticles were characterized by TEM, TGA, FTIR, and NMR.
This Perspective presents a review and survey of the science and philosophy of my research career over the past five decades at Columbia as a physical organic chemist and photochemist. I explore the ...role of paradigms, structure, and geometric thinking in my own cognitive and intellectual development. The Perspective describes my investigations of high energy content molecules in electronically excited states and the development of electronic spin and supramolecular photochemistry chemistry. Current research dealing with the nuclear spin chemistry of H2 incarcerated in buckyballs is illustrated. In the second part of this Perspective, I recount a personal role of the philosophy and history of science and the scientific communities’ use of paradigms in their every day research and intellectual activities. Examples are given of the crucial role of geometry and structure in the rapid development of organic chemistry and physical organic chemistry over the past century.
Photoinduced electron transfer reactions of highly conjugated thiophene derivatives, 4,7-di(2,3-dihydrothieno3,4-b1,4dioxin-5-yl)benzo1,2,5thiadiazole (DTDT) and ...5,8-bis(2,3-dihydrothieno3,4-b1,4dioxin-5-yl)-2,3-di(thiophen-2-yl)quinoxaline (DTDQ), with diphenyliodonium hexafluorophosphate (Ph2I+PF6 –) and triphenylsulfonium hexafluorophosphate (Ph3S+PF6 –) were investigated by laser flash photolysis, fluorescence and phosphorescence spectroscopy, and polymerization studies. High fluorescence quantum yields, long fluorescence lifetimes (∼10 ns in aprotic solvents), and absence of detectable phosphorescence at 77 K for both compounds indicate inefficient intersystem crossing into the triplet state and dominant role of singlet excited state. Photolysis of DTDT or DTDQ in the presence of iodonium salt with visible light results in the formation of radical cations of DTDT and DTDQ as detected by laser flash photolysis. Sulfonium salts do not undergo such electron transfer reactions due to the unfavorable redox potentials. The importance of the described photoinduced electron transfer process with respect to the initiation of cationic polymerization and formation of conjugated polymers was demonstrated.
We describe the parallel, one-pot synthesis of core-photocleavable, poly(norbornene)-co-poly(ethylene glycol) (PEG) brush-arm star polymers (BASPs) via a route that combines the “graft-through” and ...“arm-first” methodologies for brush polymer and star polymer synthesis, respectively. In this method, ring-opening metathesis polymerization of a norbornene–PEG macromonomer generates small living brush initiators. Transfer of various amounts of this brush initiator to vials containing a photocleavable bis-norbornene cross-linker yielded a series of water-soluble BASPs with low polydispersities and molecular weights that increased geometrically as a function of the amount of bis-norbornene added. The BASP cores were cleaved upon exposure to UV light; the extent of photo-disassembly depended on the amount of cross-linker. EPR spectroscopy of nitroxide-labeled BASPs was used to probe differences between the BASP core and surface environments. We expect that BASPs will find applications as easy-to-synthesize, stimuli-responsive core–shell nanostructures.
The surface of highly monodisperse magnetic iron oxide (γ-Fe2O3) nanocrystals was thoroughly investigated by FTIR, NMR, and mass spectroscopy. The nanocrystals were prepared by a thermal ...decomposition method now widely used for the preparation of magnetic metal and metal oxide nanocrystals. This method takes advantage of oleic acid as a means to passivate the surface and render the particles stable with respect to aggregation or grain growth and keeps them highly dispersed in a variety of organic media. The nature of this surface in terms of ligand structure and the role of oleic acid during the synthesis remained somewhat undetermined until this report. We provide spectroscopic evidence of an oleic acid ligand structural change during γ-Fe2O3 nanocrystal synthesis.
Quantitative protein bioanalysis in complex biological fluids presents considerable challenges in biological studies and disease diagnosis. The major obstacles are the background signals from both ...the probe and the biological fluids where the proteins reside. We have molecularly engineered light-switching excimer aptamer probes for rapid and sensitive detection of a biomarker protein, platelet-derived growth factor (PDGF). Labeled with one pyrene at each end, the aptamer switches its fluorescence emission from ≈400 nm (pyrene monomer) to 485 nm (pyrene excimer) upon PDGF binding. This fluorescence wavelength change from monomer to excimer emission is a result of aptamer conformation rearrangement induced by target binding. The excimer probe is able to effectively detect picomolar PDGF in homogeneous solutions. Because the excimer has a much longer fluorescence lifetime (≈40 ns) than that of the background (≈5 ns), time-resolved measurements were used to eliminate the biological background. We thus were able to detect PDGF in a cell sample quantitatively without any sample pretreatment. This molecular engineering strategy can be used to develop other aptamer probes for protein monitoring. Combined with lifetime-based measurements and molecular engineering, light-switching excimer aptamer probes hold great potential in protein analysis for biomedical studies.
A systematic study of the zeta (ζ)-potential distribution of surfactant-wrapped individual single-walled carbon nanotubes (SWNTs) dissolved in water is presented here. The surface charge on the SWNT ...micelles, as measured by the ζ-potential, has implications for the stability of the dispersions and for the electrophoretic and dielectrophoretic assembly and alignment of SWNTs. Very highly charged SWNTs are obtained by dispersing the nanotubes in high concentrations of anionic and cationic surfactants, whereas almost neutral SWNTs are obtained by using non-ionic surfactants. The ζ-potential of the dissolved SWNTs is tuned by adjusting the surfactant concentration, the alkyl chain length of the surfactant molecule, and the solution pH in different surfactant−SWNT systems.
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