Photoinitiated metal-free controlled living radical polymerization of (meth)acrylates, and vinyl monomers was investigated using the polynuclear aromatic compounds pyrene and anthracene. ...Fluorescence spectral analyses along with nuclear magnetic resonance studies were performed to determine the rate constants of initiator radical formation and to investigate the mechanisms of polymerization. The obtained polymers were analyzed by spectral and chromatographic methods. Results show that the excited state anthracene undergoes a faster electron transfer reaction with the alkyl halide initiator than the excited state of pyrene. Pyrene excimers, which are formed at higher concentrations, also react with alkyl halides to form initiator radicals. Although pyrene monomers and excimers are acting slower, polymers with higher control over the chain end functionalities and molecular weight characteristics are obtained in comparison to anthracene as sensitizer.
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 active role of phenothiazine excited states in photoinduced metal-free atom transfer radical polymerization (ATRP) was investigated by using laser flash photolysis, fluorescence, phosphorescence ...and electron spin resonance spectroscopy. Weak fluorescence emission and short lifetimes of the singlet excited state (2.01 ns) together with high intersystem crossing quantum yields (∼0.6) and long triplet lifetimes (40 μs) indicate efficient intersystem crossing into the triplet state and its dominant role in a degenerative electron transfer process in photoinduced metal free ATRP.
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•Thioxanthone-benzothiophenes absorb in the visible spectral region.•These new conjugates show high extinction coefficients and high triplet yields.•The triplet states react with ...tertiary amines to generate free radicals.•These compounds are efficient photoinitiators for free radical polymerization.
Photoinitiators for free radical polymerization based on the thioxanthone chromophore that contains benzothiophene were synthesized and characterized. Compared to thioxanthone, these compounds show a bathochromic shifted absorption up to ∼460nm. High quantum yields for intersystem crossing generate sufficient amounts of triplet states. Initiator radicals are generated by reaction of the triplet states with tertiary amines, such as diethanolamine with high rate constants (2–6×109M−1s−1) as determined by laser flash photolysis. Photoinitiated polymerization experiments of MMA showed efficient polymerization with initiator concentrations as low as 0.1mM.
Substitution of both oxygen atoms in the exocyclic carbonyl groups of the thymine chromophore by sulfur atoms results in a remarkable redshift of its absorption spectrum from an absorption maximum at ...267 nm in thymidine to 363 nm in 2,4-dithiothymine (ΔE = 9905 cm–1). A single sulfur substitution of a carbonyl group in the thymine chromophore at position 2 or 4 results in a significantly smaller redshift in the absorption maximum, which depends sensitively on the position at which the sulfur atom is substituted, varying from 275 nm in 2-thiothymine to 335 nm in 4-thiothymidine. Femtosecond transient absorption spectroscopy reveals that excitation of 2,4-dithiothymine at 335 or 360 nm leads to the ultrafast population of the triplet state, with an intersystem crossing lifetime of 180 ± 40 fsthe shortest intersystem crossing lifetime of any DNA base derivative studied so far in aqueous solution. Surprisingly, the degree and position at which the sulfur atom is substituted have important effects on the magnitude of the intersystem crossing rate constant, showing a 1.2-, 3.2-, and 4.2-fold rate increases for 2-thiothymine, 4-thiothymidine, and 2,4-dithiothymine, respectively, relative to that of thymidine, whereas the triplet yield increases 60-fold to near unity, independent of the site of sulfur atom substitution. While the natural thymine monomers owe their high degree of photostability to ultrafast internal conversion to the ground state and low triplet yields, the near-unity triplet yields in the thiothymine series account for their potent photosensitization properties. Nanosecond time-resolved luminescence spectroscopy shows that 4-thiothymidine and 2,4-dithiothymine are efficient singlet oxygen generators, with singlet oxygen quantum yields of 0.42 ± 0.02 and 0.46 ± 0.02, respectively, in O2-saturated acetonitrile solution. Taken together, these photophysical measurements strongly suggest that 2,4-dithiothymine can act as a more effective UVA chemotherapeutic agent than the currently used 4-thiothymidine, especially in deeper-tissue chemotherapeutic applications.
Can photocatalysis be performed without electron or energy transfer? To address this, organo‐photocatalysts that are based on atropisomeric thioureas and display lower excited‐state energies than the ...reactive substrates have been developed. These photocatalysts were found to be efficient in promoting the 2+2 photocycloaddition of 4‐alkenyl‐substituted coumarins, which led to the corresponding products with high enantioselectivity (77–96 % ee) at low catalyst loading (1–10 mol %). The photocatalytic cycle proceeds by energy sharing via the formation of both static and dynamic complexes (exciplex formation), which is aided by hydrogen bonding.
Organo‐photocatalysts that are based on atropisomeric thioureas and display lower excited‐state energies than the reactive substrates have been developed. These photocatalysts were found to be efficient in promoting the 2+2 photocycloaddition of 4‐alkenyl‐substituted coumarins, which led to the corresponding products with high enantioselectivity (77–96 % ee) at low catalyst loading (1–10 mol %).
Electron Delocalization in Perylene Diimide Helicenes Schuster, Nathaniel J.; Paley, Daniel W.; Jockusch, Steffen ...
Angewandte Chemie (International ed.),
October 17, 2016, Letnik:
55, Številka:
43
Journal Article
Recenzirano
Odprti dostop
We report two new helicenes derived from the double fusion of an acene with two perylene diimide (PDI) subunits. These PDI‐helicene homologs exhibit very different structural and electronic ...properties, despite differing by only a single ring in the link between the PDI units. The shorter inter‐PDI link brings the two PDI subunits closer together, and this results in the collision of their respective π‐electron clouds. This collision facilitates intramolecular through‐space electronic delocalization when the PDI‐helicene is reduced.
Two helicenes were synthesized by double fusion of an acene with two perylene diimide (PDI) subunits. These PDI‐helicene homologs show different structural and electronic properties, despite differing by only a single ring in the link between the PDI units. The shorter link brings the two PDI subunits closer together, and this results in the collision of their respective π‐electron clouds.
Supramolecular assemblies that help to preorganize reactant molecules have played an important role in the development of concepts related to the control of excited-state processes. This has led to a ...persistent search for newer supramolecular systems (hosts), and this review briefly presents our work with octa acid (OA) to a host to control excited-state processes of organic molecules. Octa acid, a water-soluble host, forms 1:1, 2:1, and 2:2 (host–guest) complexes with various organic molecules. A majority of the guest molecules are enclosed within a capsule made up of two molecules of OA whereas OA by itself remains as a monomer or aggregates. Luminescence and 1H NMR spectroscopy help to characterize the structure and dynamics of these host–guest complexes. The guest molecule as well as the host–guest complex as a whole undergoes various types of motion, suggesting that the guests possess freedom inside the confined space of the octa acid capsule. In addition, the confined guests are not isolated but are able to communicate (energy, electron, and spin) with molecules present closer to the capsule. The host–guest complexes are stable even on solid surfaces such as silica, clay, α-Zr phosphate, TiO2, and gold nanoparticles. This opens up new opportunities to explore the interaction between confined guests and active surfaces of TiO2 and gold nanoparticles. In addition, this allows the possibility of performing energy and electron transfer between organic molecules that do not adsorb on inert surfaces of silica, clay, or α-Zr phosphate. The results summarized here, in addition to providing a fundamental understanding of the behavior of molecules in a confined space provided by the host OA, are likely to have a long-range effect on the capture and release of solar energy.
Abstract
Notwithstanding the central biological role of the (6-4) photoadduct in the induction of skin cancer by sunlight, crucial mechanistic details about its formation have evaded characterization ...despite efforts spanning more than half a century. 4-Thiothymidine (4tT) has been widely used as an important model system to study its mechanism of formation, but the excited-state precursor, the intermediate species, and the time scale leading to the formation of the (6-4) photoadduct have remained elusive. Herein, steady-state and time-resolved spectroscopic techniques are combined with new and reported quantum-chemical calculations to demonstrate the excited state leading to the formation of the thietane intermediate, its rate, and the formation of the (6-4) photoadduct using the 5’-TT(4tT)T(4tT)TT-3’ DNA oligonucleotide. Efficient, sub-1 ps intersystem crossing leads to the population of a triplet minimum of the thietane intermediate in as short as 3 ps, which intersystem crosses to its ground state and rearranges to form the (6-4) photoadduct.
Photoexcitation of fluorophores commonly used for biological imaging applications generates reactive oxygen species (ROS) which can cause bleaching of the fluorophore and damage to the biological ...system under investigation. In this study, we show that singlet oxygen contributes relatively little to Cy5 and ATTO 647N photobleaching at low concentrations in aqueous solution. We also show that Cy5 generates significantly less ROS when covalently linked to the protective agents, cyclooctatetraene (COT), nitrobenzyl alcohol (NBA) or Trolox. Such fluorophores exhibit enhanced photostability both in bulk solutions and in single‐molecule fluorescence measurements. While the fluorophores ATTO 647N and ATTO 655 showed greater photostability than Cy5 and the protective–agent‐linked Cy5 derivatives investigated here, both of ATTO 647N and ATTO 655 generated singlet oxygen and hydroxyl radicals at relatively rapid rates, suggesting that they may be substantially more phototoxic than Cy5 and its derivatives.
Photoexcitation of fluorophores generates reactive oxygen species (ROS) which can cause bleaching of the fluorophore and damage to the biological system. We show that singlet oxygen contributes relatively little to Cy5 and ATTO 647N photobleaching at low concentrations in aqueous solution. We also show that Cy5 generates significantly less ROS when covalently linked to the protective agents, cyclooctatetraene (COT), nitrobenzyl alcohol (NBA) or Trolox. Such fluorophores exhibit enhanced photostability and generated less ROS than fluorophores that are commonly used, suggesting that they may be substantially less phototoxic than the common fluorophores.
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