Academic literature on the topic 'Rehm-Weller equation'

The interaction of Zn(II) porphyrin (ZnPP) with colloidal TiO2was studied by absorption and fluorescence spectroscopy. The fluorescence emission of ZnPP was quenched by colloidal TiO2upon excitation of its absorption band. The quenching rate constant (kq) is1.24×1011 M−1 s−1. These data indicate that there is an interaction between ZnPP and colloidal TiO2nanoparticle surface. The quenching mechanism is discussed on the basis of the quenching rate constant as well as the reduction potential of the colloidal TiO2. And the mechanism of electron transfer has been confirmed by the calculation of free energy change(ΔGet)by applying Rehm-Weller equation as well as energy level diagram.

The photochemical behavior of xanthene dyes (fluorescein, erythrosine, and eosin) with colloidal SnO2nanoparticles was probed by UV-visible, steady state, and time resolved fluorescence measurements. The prepared SnO2nanoparticles were characterized by using UV-visible and powder XRD measurements. The xanthene dyes were adsorbed on the surface of colloidal SnO2nanoparticles through electrostatic interaction. Apparent association constant (Kapp) was calculated from the relevant fluorescence data. The larger value of apparent association constant indicates a strong association between xanthene dyes and SnO2nanoparticles. The fluorescence quenching is mainly attributed to electron transfer from the excited state xanthenes to the conduction band of colloidal SnO2. The electron transfer mechanism was explained based on the Rehm-Weller equation as well as the energy level diagram.

The spectroscopic and electrochemical properties of a 9-substituted fluorene ketylimine (3) were investigated and compared with those of its vinylene analogue (4) to determine the origins of the quenched fluorescence of these compounds. The predominate mode of singlet excited state deactivation of the heteroatomic fluorene was found to be internal conversion involving bond rotation. Meanwhile, its carbon counterpart was found to undergo deactivation preferentially by intersystem crossing to form its triplet, which was confirmed by laser flash photolysis. Both 3 and 4 quenched the fluorescence of fluorene with diffusion-controlled rate constants, implying that the singlet excited states of 3 and 4 are also quenched by intramolecular photoinduced electron transfer (PET). This deactivation mode was found to be exergonically favorable (–90 kJ/mol for 3 and –81 kJ/mol for 4) according to the Rehm–Weller equation. The position of the heteroatomic bond on the fluorene moiety was further found to influence the singlet excited state deactivation pathway. The 2-substituted regioisomer decayed predominately by intramolecular PET and its fluorescence can be restored by acid protonation. Conversely, the PET mechanism is a minor deactivation mode for the 9-substituted fluorene derivative and its fluorescence can be enhanced by suppressing bond rotational modes, possible at low temperature and potentially in thin films.

Chemistry<br>Ph.D.<br>Photoinduced electron transfer (PET) plays a crucial role in a wide array of biological pathways. These electron transfer reactions happen from or to the excited state of a chromophore upon absorption of light. Hence understanding the properties of excited states is necessary in elucidating the details of such pathways. The work presented in this thesis deals with PET in two systems: Fluorescent Nucleobase Analogues (FBAs) and DNA photolyase. The introductory chapter (Chapter 1) presents some background information about the two systems and sets up the stage for the reasoning behind the problems addressed in this thesis. FBAs are fluorescent analogues of naturally occurring, weakly fluorescent native nucleic acid bases. When incorporated into single stranded (ss) or double stranded (ds) DNA, the FBA fluorescence is significantly quenched. PET has been implicated to be the cause for the observed quenching. Here we have presented our attempt to correlate the quenching behavior of free FBA: nucleic acid monophosphate (NMP) pairs with the free energies associated with excited state electron transfer delta GET. Based on the delta GET values, we have tried to assign the direction of electron transfer. The quenching behavior of the FBA:NMP pairs were studied through Stern-Volmer (SV) quenching and time-resolved fluorescence studies. The above described analysis has been applied on FBAs: 4-amino-6-methyl-8-(2'-deoxy-beta-D-ribofuranosyl)-7(8H)-pteridone (6MAP), 4-amino - 2, 6 - dimethyl - 8 - (2'-deoxy-beta-d-ribofuranosyl) -7(8H) - pteridone (DMAP), 3-methyl-8-(2'-deoxy-beta-D-ribofuranosyl) isoxanthopterin (3MI) and 6-Methyl-8-(2'-deoxy-β-D-ribofuranosyl) isoxanthopterin (6MI) (Chapter 3), 2-Aminopurine (2AP) (Chapter 4), 8-Vinyl Adenosine (8VA) (Chapter 5). The final part of this thesis (Chapter 6) is on understanding the mechanistic details of a DNA repair process that is due to photoinduced electron transfer in DNA photolyase, a flavoprotein. Before the electron reaches the damaged site in the DNA, the initial electron acceptor in this repair process has been speculated to be the adenine of the flavin adenine dinucleotide (FAD). We have tested this hypothesis by measuring and comparing the various kinetic parameters associated with this process by reconstituting into apo-photolyase the natural cofactor of photolyase (FAD) and an adenine modified flavin (Etheno FAD, epsilon FAD).<br>Temple University--Theses

Une série de dimères composés de thiophène-aniline encombrée stériquement a été synthétisée. Les différents processus de désactivation de l’état singulet excité ont été étudiés par UV-visible, fluorescence, phosphorescence, photolyse par impulsion laser et calculs théoriques. Les graphiques de Stern-Volmer obtenus à partir des expériences de désactivation des états singulet et triplet ont démontré l’efficacité de l’azométhine à désactiver les fluorophores. Les calculs semi-empiriques AM1 examinant l’effet des substituants encombrés ont démontrés que les groupements tert-butyls sur l’aniline ont moins d’influence sur la barrière de rotation N-aryl que les substitutions alkyles en ont sur la rotation de thiophène-C. Les calculs Rehm-Weller basés sur les potentiels d’oxydation et de réduction ont montré que l’autodésactivation de l’état excité des azométhines se fait par transfert d’électron photoinduit menant à une éradication complète de la fluorescence. Des complexes métalliques contenant des ligands azométhines ont aussi été préparés. Le ligand est composé d’une unité hydroxyquinoline lié à un cycle thiophène. Les données photophysiques de ces complexes indiquent un déplacement bathochromique aussi bien en absorbance qu’en fluorescence. Des dispositifs de détection d’ion métallique ont été préparés et un exemple à partir d’une solution de cuivre a montré un déplacement bathochromique.<br>A series of sterically hindered thiophene-aniline azomethine dyads were prepared. The decay pathways that deactivate the singlet excited state were studied using UV-vis fluorescence and phosphorescence, laser flash photolysis and quantum calculations. Stern-Volmer relationships, derived from singlet and triplet state quenching experiments, showed that azomethines efficiently deactivate the singlet and triplet excited states of fluorophores with bimolecular kinetics. AM1 Semi-empirical quantum calculations examining the effect of bulky substituents on the bond rotational barriers demonstrate that bulky tert-butyl groups attached to the aniline moiety have less influence on the N-aryl bond rotation barrier than alkyl substitutions do on the thiophene-CH bond rotation barrier. Rehm-Weller calculations based on electrochemical potentials demonstrate that azomethines self-quench their excited states via fast and efficient intramolecular photoinduced electron transfer leading to complete fluorescence suppression. Metal complexes containing an azomethine ligand were also prepared. The ligand contains a hydroxyquinoline moiety linked with a thiophene ring. Photophysical investigations of the resulting metal complexes demonstrated significant bathochromic shifts in the absorbance and fluorescence spectra. Metal-ion sensing devices for water solutions were prepared by spin casting the ligand onto glass slides. The metal-ion sensor detected copper in water solutions through a bathochromic shift in the absorbance maximum.