UMD Theses and Dissertations

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Subject: search.filters.subject.photoinduced electron transfer

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    LIGHT CONTROL OF CHEMICAL SYSTEMS: PHOTOCHEMICAL ELECTRON TRANSFER METHODS FOR RELEASING CALCIUM IONS AND THE PHOTOISOMERIZATION OF ALKENES TO MODULATE RHEOLOGICAL CHANGES.
    (2021) Heymann Loor, Romina R; Falvey, Daniel E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Our research combines organic photochemistry with the engineering principles of rheology through the study of photorheological fluids (PR). The two photochemical systems researched show changes in the rheological properties brought about by the addition of light. The investigated systems are the photoisomerization of cinnamic acid derivatives in the surfactant, Cetyltrimethylammonium bromide (CTAB), and calcium release through degradation ethylenediaminetetraacetic acid (EDTA) caused by an electron transfer mechanism. The CTAB system shows how a change in molecular conformation can cause significant changes in the bulk property of a solution. The calcium EDTA system employs targeted electron transfer to cause calcium release, which gels the biopolymer alginate with inexpensive, readily available materials. Chapter 2 details how the orientational binding, intermolecular interactions, and molecular geometry of cinnamic acid derivatives contribute to the rheological changes in CTAB. 1H NMR titration studies in CTAB identified binding patterns of the additives in CTAB. From those studies orientational binding models were developed for trans-ortho-methoxycinnamic acid (tOMCA), cis-ortho-methoxycinnamic acid (cOMCA), meta-methoxycinnamic acid (mMCA), para-methoxycinnamic (pMCA), ortho-hydroxycinnamic acid (oCoum), meta-hydroxycinnamic acid (mCoum), and para-hydroxycinnamic acid (pCoum). 1H-1H 2D NOESY spectra identified through space intermolecular interactions occurring within the micelle. Preliminary data into possible π-anion interaction between tOMCA molecules within the micelle is presented. Photolysis confirmed the creation of cis isomers for all additives but also identified coumarin by-products for oCoum. B3LYP calculations indicated out-of-plane geometry for all the cis isomers and possible intramolecular hydrogen bonding of oCoum. Finally, a model of binding interactions that lead to changes in the packing parameter of the surfactant and, therefore, a change into wormlike micelles for tOMCA versus cOMCA is introduced. In chapters three and four, we investigated calcium release using sensitizers that promote photoinduced electron transfer. Anthraquinones derivatives were shown in Chapter 3 to release calcium in stoichiometry amounts with UV light irradiation. In Chapter 4, flavins produced 1000-fold calcium release to sensitizer concentration in the visible light spectrum. In both chapters, there are detailed calcium release studies, degradation studies, and alginate experiments. We present calcium release studies at acidic and neutral pH, quantum yields, degradation of EDTA, sensitizer reoxidation studies, sensitizer degradation data, fluorescence, and transient spectra. While enough calcium was released to produce alginate gels, none were made in vitro at neutral and acidic pH.
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    Visible Light Photorelease of Carboxylate Anions by Mediated Photoinduced Electron Transfer to Pyridinium-based Protecting Groups
    (2009) Borak, John Brian; Falvey, Daniel E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The use of sensitized photoinduced electron transfer (PET) to trigger release of redox-active photoremovable protecting groups (PRPGs) allows a broad range of chromophores to be selected that absorb in difference wavelength ranges. Mediated electron transfer (MET) is particularly advantageous as sub-stoichiometric amounts of the often costly sensitizer (relative to the amount of protected substrate) can be combined with an excess amount of an inexpensive electron donor. Thus, the sensitizer acts as an electron shuttle between the donor and the protecting group to initiate release. The development of improved MET release systems using visible light as the trigger is the focus of the current work. The N-alkylpicolinium (NAP) group has demonstrated its utility as an aqueous-compatible PET-based PRPG, releasing protected substrates upon one electron reduction. Adaptation of MET PRPG release to visible light absorbing mediators began with employing ketocoumarin dyes that primarily form excited triplet states. These chromophores demonstrated high rates of release of NAP-protected carboxylates using sub-stoichiometric concentrations of mediator. Subsequently, nanomolar concentrations of gold nanoparticles were used to mediate electron transfer to NAP-protected compounds. This system exhibited rapid deprotection with very high release quantum efficiencies. In an effort to use highly stable visible-light-absorbing metal-centered dyes with modest redox properties, the NAP group has been synthetically modified to adjust its reduction potential to more positive values. Photolysis of solutions containing the protected substrate, a large excess of an electron donor, and substoichiometric amounts of the dye tris(bipyridyl)ruthenium(II) released the free carboxylates in high yields while photodegradation of the chromophore was minimal. To demonstrate the utility of the NAP group, a quasi-reversible photorheological fluid has been developed based on the formation and disruption of aqueous micelles. In solutions containing the surfactant cetyltrimethylammonium bromide, visible light photorelease of a carboxylate additive from the NAP-ester derivative induces a 105 increase in solution viscosity due to the formation of an interpenetrating micelle network. Subsequent irradiation of the viscoelastic fluid with UV light induces a cis-trans isomerization within the released carboxylate thereby disrupting the micelle network and decreasing solution viscosity by 102.5.
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    Photoinduced Electron Transfer in Ionic Media
    (2008-12-08) Vieira, Rebecca; Falvey, Daniel E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The goal of this research was to explore the use of room temperature ionic liquids (RTILs) and ionic liquids crystals (ILCs) as reaction media for photoinduced electron transfer (PET) processes. Photoinduced electron transfer in two room-temperature ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) and 1-octyl-3-methylimidazolium hexafluorophosphate (OMIM-PF6) has been investigated using steady-state fluorescence quenching of 9,10-dicyanoanthracene (DCA) with a series of single electron donors. From these fluorescence quenching rates, reorganization energy values and kdiff values can be derived from a Rehm-Weller analysis. In many cases, these fluorescence quenching reactions occur at rates larger than what would be expected based on the Smoluchowski equation. In addition, reorganization energy values of 10.1 kcal/mol and 16.3 kcal/mol for BMIM-PF6 and OMIM-PF6, respectively, have been determined. The dynamics of electron transfer reactions in butyl pyridinium bis(trifluoromethanesulfonyl)imide (BuPyr-NTf2) and other solvents have been explored using laser flash photolysis. In these experiments, benzophenone (BP), duroquinone (DQ), and 9-cyanoanthracene (9CA) were used as excited-state acceptors, 1,4-diazabicyclo[2.2.2]octane (DABCO) and hexamethylbenzene (HMB) were used as ground-state donors, and methyl viologen (MV2+) was used as a probe molecule. Analysis of kinetic and spectroscopic data from these experiments shows that electron transfer from photoreduced acceptors to the probe occurs via one or more solvent ions in cases where the acceptor anion radical has a reduction potential that is more negative than the solvent ions (BP- and 9CA- in BuPyr-NTf2). Mediated electron transfer was demonstrated to significantly enhance quantum efficiencies of photoinduced electron transfer in cases where back electron transfer would otherwise predominate. In addition to RTILs, a pyridinium and imidazolium ILC were used as reaction media in LFP experiments. In these experiments, BP and a pyrromethene dye were used as excited-state acceptors, DABCO and DMB were used as ground-state donors, and MV2+ was used as a probe molecule. In these experiments it was determined that the pyridinium ILC was able to mediate electron transfer with both BP and PM such that quantum efficiencies (Φ) using both acceptors are improved significantly. Additionally, a rate enhancement for PET is observed using the pyridinium ILC as opposed to the imidazolium ILC for the BP system.