Light-Controlled Radical Polymerization: Mechanisms, Methods, and Applications

The use of light to mediate controlled radical polymerization has emerged as a powerful strategy for rational polymer synthesis and advanced materials fabrication. This review provides a comprehensive survey of photocontrolled, living radical polymerizations (photo-CRPs). From the perspective of mechanism, all known photo-CRPs are divided into either (1) intramolecular photochemical processes or (2) photoredox processes. Within these mechanistic regimes, a large number of methods are summarized and further classified into subcategories based on the specific reagents, catalysts, etc., involved. To provide a clear understanding of each subcategory, reaction mechanisms are discussed. In addition, applications of photo-CRP reported so far, which include surface fabrication, particle preparation, photoresponsive gel design, and continuous flow technology, are summarized. We hope this review will not only provide informative knowledge to researchers in this field but also stimulate new ideas and applications to further advance photocontrolled reactions.National Science Foundation (U.S.) (CHE-1334703

Photosynthetic Energy Conversion: Hydrogen Photoproduction by Natural and Biomimetic Means

The main function of the photosynthetic process is to capture solar energy and to store it in the form of chemical fuels. Many fuel forms such as coal, oil and gas have been intensively used and are becoming limited. Hydrogen could become an important clean fuel for the future. Among different technologies for hydrogen production, oxygenic natural and artificial photosynthesis using direct photochemistry in synthetic complexes have a great potential to produce hydrogen as both use clean and cheap sources - water and solar energy. Photosynthetic organisms capture sunlight very efficiently and convert it into organic molecules. Artificial photosynthesis is one way to produce hydrogen from water using sunlight by employing biomimetic complexes. However, splitting of water into protons and oxygen is energetically demanding and chemically difficult. In oxygenic photosynthetic microorganisms water is splitted into electrons and protons during primary photosynthetic processes. The electrons and protons are redirected through the photosynthetic electron transport chain to the hydrogen-producing enzymes-hydrogenase or nitrogenase. By these enzymes, e- and H+ recombine and form gaseous hydrogen. Biohydrogen activity of hydrogenase can be very high but it is extremely sensitive to photosynthetic O2. At the moment, the efficiency of biohydrogen production is low. However, theoretical expectations suggest that the rates of photon conversion efficiency for H2 bioproduction can be high enough (> 10%). Our review examines the main pathways of H2 photoproduction using photosynthetic organisms and biomimetic photosynthetic systems and focuses on developing new technologies based on the effective principles of photosynthesis

Latent free radical polymerization of bulk methacrylates: Organic visible-light photocatalysis and supramolecular effects

Light-activated polymerizations are important because they allow spatially and temporally controlled synthesis of polymers and polymer-based materials under ambient conditions. This capability is greatly valued in numerous applications, such as coatings, adhesives, sealants, electronics, diagnostics, dental materials, and biomaterials. Hence, the amount of precursors and synthetic routes available increases every year. However, there is limited understanding of the often intricate mechanisms via which many of these reactions work. As a consequence, this technology has not been exploited to its fullest. As a result, a need exists for the elucidation of refined mechanisms and kinetic models that aid in better understanding, predicting, and controlling such immensely valuable reactions for the production of practically relevant materials and devices. The present work delves into the refinement of the theoretical framework of free radically initiated chain growth polymerizations in solvent-free (bulk) monomer(s). This project originated to explain an unexpectedly long-lasting (>2000 s) latent polymerization observed after briefly exposing certain (meth)acrylic monomers, like 2-hydroxyethyl methacrylate, to visible-light in the presence of an organic photocatalysis composition including Methylene blue (MB+), Hünig's base and an Iodonium salt. Free radical chain growth photopolymerizations in bulk typically stop shortly (< 17 s) after irradiation is extinguished. Thus, it was clear that the available kinetic models and theories were not sufficient to account for this atypical, and potentially advantageous, phenomenon. We simultaneously monitored the photocatalyst (MB+) and monomer concentrations with UV-Vis and FT-IR spectroscopy, respectively, under several irradiation regimes. EPR spectroscopy was used to determine the nature and lifetime of the light-generated radical intermediates. Rheology confirmed that the vinyl groups consumed in the dark are in fact being polymerized. Quantum chemical calculations guided the experiments and supported the proposal of a photocatalytic mechanism via which reactive initiating free radicals can be produced long after the irradiation is extinguished. With these results, the unusually extended latent polymerization was explained by two mechanistic conclusions: 1) organic photocatalysis using MB+/Hünig's base/Iodonium salt stores energy during irradiation in the form of Leuco Methylene Blue via an e-/H+/e- transfer process instead of the typical single e- transfer; then, LMB is later used to produce radicals upon reaction with the Iodonium salts for even thousands of seconds after light cessation, and 2) hydrogen bonding exacerbates the Trommsdorff-Norrish effect via which bimolecular termination is hindered, thus resulting in the extension of the vinyl polymerization in the dark by the well-documented radical occlusion process

Ionizing Radiation-Induced Polymerization

Ionizing radiation can induce some kinds of reactions, other than polymerization, such as dimerization, oligomerization, curing, and grafting. These reactions occur through a regular radical chain causing growth of polymer by three steps, namely, initiation, propagation, and termination. To understand ionizing radiation-induced polymerization, the water radiolysis must be taken into consideration. This chapter explores the mechanism of water molecules radiolysis paying especial attention to the basic regularities of solvent radicals’ interaction with the polymer molecules for forming the crosslinked polymer. Water radiolysis is the main engine of the polymerization processes, especially the “free-radical polymerization.” The mechanisms of the free-radical polymerization and crosslinking will be discussed in detail later. Since different polymers respond differently to radiation, it is useful to quantify the response, namely in terms of crosslinking and chain scission. A parameter called the G-value is frequently used for this purpose. It represents the chemical yield of crosslinks, scissions and double bonds, etc. For the crosslinked polymer, the crosslinking density increases with increasing the radiation dose, this is reflected by the swelling degree of the polymer while being immersed in a compatible solvent. If crosslinking predominates, the crosslinking density increases and the extent of swelling decreases. If chain scission predominates, the opposite occurs. A further detailed discussion of these aspects is presented throughout this chapter

Light-induced charge-transfer dynamics in Ruthenium-polypyridine complexes

Die Arbeit beinhaltet die photophysikalische Charakterisierung verschiedener Ruthenium-polypyridyl-Komplexe mit Fokus auf dem lichtinduzierten Ladungstransfer ausgehend vom Ru-zentrum. Dabei wurden die Techniken der Absorptions-, Emissions- und Ultrakurzzeit-Pump-Probe-Spektroskopie verwendet, um die Abhängigkeit dieses Ladungstransfers von Umgebungsparametern und Substitutionen an den Polypyridyl-Liganden zu untersuchen

Taking control of charge transfer : strategic design for solar cells

The thesis is focused on the investigation of the electron transfer mechanisms leading to solar fuel production and to the identification of engineering principles that can be used to design materials able to improve charge separation. Molecular systems composed of three or more subunits arranged in a Donor-Antenna-Acceptor design are required to achieve efficient photoinduced charge separation. It is shown how structural changes in the systems design can be used to systematically optimize the energy gradients and electronic coupling between the molecular subunits, necessary to achieve controlled unidirectional charge transfer. To gain insight into the mechanisms governing the charge transfer processes within a molecular system, the process of photoinduced heterogeneous electron injection is investigated through nonadiabatic dynamics simulations. Coherent electron-nuclear vibrational effects are found to drive the electron transfer process by promoting the coherent superposition of the exciton and the charge transfer quantum state. A photoanode for solar water splitting comprising the functions of light-harvesting, charge separation and catalysis is also investigated. It is observed that, following a fast heterogeneous electron injection, the system catalytic activity is driven by a proton-coupled electron transfer mechanism in which the role of the solvent is crucial.UBL - phd migration 201

Nanoscale Polymeric Particles via Aerosol-Photopolymerization

This PhD thesis focuses on the process of aerosol-photopolymerization for the generation of various polymeric particles. Such structures are most often prepared by liquid-based methods via the well-established thermal initiation step, and aerosol-photopolymerization is presented as an alternative, aerosol-based technique which employs photoinitiated polymerization. Discussed within this thesis are the advantages and broad aspects of the process

Proton-Coupled Reduction of An Iron Nitrosyl Porphyrin in The Protic Ionic Liquid Nanodomain

The one-electron reduction of many molecules becomes much more favorable if combined with proton transfers or strong hydrogen bonding. Protic room temperature ionic liquids (RTILs), which can form nanodomains in solutions with molecular solvents (MS), can provide an efficient avenue for this process. In this work, we report on the voltammetry, UV/visible and resonance Raman spectroelectrochemistryof Fe(TPP)(NO) in the presence of aprotic/protic ammonium-based ionic liquids. While aprotic RTILs did shift the reduction to more positive potentials, similar shifts could be observed at much lower concentrations of diethylmethylammonium triflate (HAmOTf, a protic ionic liquid). Deconvolution of the rotating ring-disk electrode (RRDE) voltammetry revealed the partitioning of the reduced species into the ionic liquid nanodomains at low concentrations. The potential shift was substantially in excess of the value expected based on the pKa of the weak acid. Upon the addition of small amounts of the protic RTIL, the electrochemically or chemically generated anion, Fe(TPP)(NO)-, reacted rapidly with the HAm+ acid, forming a Fe(TPP)(HNO) complex. Further reduction to Fe(TPP)(NH2OH) could be observed on the spectroelectrochemical time scale. The outcome of this work revealed the advantageous role of protic RTIL nanodomains in accelerating the proton-coupled reductions to form more energetically favorable product

Ultrafast Vibrational Dynamics of Biomimetic Catalysts

Ultrafast two-dimensional infrared (2D-IR) spectroscopy is used in this work to study the vibrational dynamics of a series of biomimetic catalysts. We set out to investigate the vibrational dynamics of catalytic compounds in systems directly relevant to molecular reactivity, specifically reactive oxidation states, catalytically relevant self-isomerizations, dendritically-induced nano-confinement, and excitonic coherence transfer. For most of the work performed for this thesis we used diiron hexacarbonyl small-molecule mimics of the [FeFe]-hydrogenase enzyme’s active site. The vibrational dynamics of [(1,1’-bis(diphenylphosphino)ferrocene)chromium-(CO)4] (DPPFCr) in its neutral, closed-shell state were compared to the vibrational dynamics of DPPFCr as a cation radical. This comparison is possible because molecular oxidation does not significantly change the vibrational displacements of the carbonyl modes, which are studied here. Molecular oxidation induces an acceleration of the vibrational relaxation of the carbonyl modes but does not significantly affect the spectral diffusion dynamics of the carbonyl groups. We attribute this to an idiosyncrasy of the non-interacting solvent used for the experiment, CH2Cl2, which was chosen specifically for the weak nature of its solvent-solute interactions. Unexpectedly pronounced and slow spectral diffusion in the carbonyl modes of (µ-pdt)[Fe(CO)3]2 (pdt = 1,3-propanedithiolate) was observed in alkane solvents. The contribution of solvent-solute interactions in alkane solvents to spectral diffusion is expected to be minimal, and we related the spectral diffusion to fluctuations of the carbonyl potential induced by a catalytically-relevant mode of molecular fluxionality in (µ-pdt)[Fe(CO)3]2. Comparison with a different diiron hexacarbonyl compound, (µ-edt)[Fe(CO)3]2 (edt = 1,2-ethanedithiolate), effectively ruled out isomerization of the bridging organic disulfide group, and a Boltzmann distribution of states derived from electronic structure calculations supported our hypothesis by suggesting that a significant distribution of molecular conformations were present in at room temperature. Other fluxional organometallic complexes M3(CO)12 (M=Ru, Os) displayed similar spectral diffusion. This is the first use of spectral diffusion to study molecular conformational flexibility. We also observed an unexpected dependence of the rate of intracarbonyl IVR on the chain length of the alkane solvent. Nano-confinement has been reported on several occasions to favorably modulate the reactivity of diiron hexacarbonyl compounds, and dendritic assemblies with diiron hexacarbonyl cores were synthesized and the vibrational dynamics of the carbonyl groups were compared to the vibrational dynamics of carbonyls on similar diiron hexacarbonyl compounds without dendritic groups. Slower IVR and an additional timescale of spectral diffusion were observed in dendritic assemblies, which are hypothesized to reflect nano-modulation of the carbonyl group’s first solvation shell by the dendritic groups. Three diiron hexacarbonyl compounds with differing bridging disulfide groups (edt, pdt, and o-xylyldithiolate) are found to display unusual modulations of cross peak intensity which have previously been identified as spectral signatures of vibrational coherence transfer. Specific modulations of cross peak amplitude are observed in all three compounds, suggesting that certain coherence transfer events are common in diiron hexacarbonyl compounds, and an oscillatory frequency resulting from coherence transfer between bright and dark vibrational modes is identified.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145788/1/zpeckert_1.pd