Henry Linschitz

Bio: Henry Linschitz is an academic researcher from Brandeis University. The author has contributed to research in topics: Flash photolysis & Porphyrin. The author has an hindex of 36, co-authored 73 publications receiving 3988 citations. Previous affiliations of Henry Linschitz include Hungarian Academy of Sciences & Carnegie Mellon University.

Hydrogen-Bonding and Protonation Effects in Electrochemistry of Quinones in Aprotic Solvents

Abstract: Hydrogen-bonding and protonation are fundamental factors controlling potentials and mechanisms in the reduction of quinones. These are studied systematically in benzonitrile, acetonitrile, and dimethylsulfoxide solutions by cyclic voltammetry of a series of quinones of increasing basicity (chloranil to duroquinone), in the presence of hydroxylic additives of increasing hydrogen-bonding power (tert-butyl alcohol to hexafluoro-2-propanol) or acidity (trifluoroacetic acid). Electrochemical effects are demonstrated over the complete interaction range, from hydrogen bonding of reduced dianions to protonation of unreduced quinones. With increasing concentrations of additives, three clearly different types of electrochemical behavior are observed for weakly (I), moderately (II), and strongly (III) interacting quinone-additive pairs, as follows: (I) Two well-separated reduction waves, corresponding to formation of quinone mono- and dianions, shift positively, with no loss of reversibility. The second wave is sma...

Studies on Metastable States of Porphyrins. II. Spectra and Decay Kinetics of Tetraphenylporphine, Zinc Tetraphenylporphine and Bacteriochlorophyll1

Abstract: The flash technique used earlier to study the metastable states of chlorophyll was applied to tetraphenylporphine, - zinc tetraphenylporphine, and bacteriochlorophyll in toluene and pyridine solution. The spectra all show a main band just below the Soret peak, as well as other regularities. The decay law is again --dC*/dt = k1C* + k2(C*)2 + The respective rate constants for all the compounds are almost the same, except for a much faster first-order decay in bacteriochlorophyll. Finally, little or no flash-bleaching is observed with the Cu2+, Co2+, and Ni2+ complexes of tetraphenylporphine.

Quenching Processes in Hydrogen-Bonded Pairs: Interactions of Excited Fluorenone with Alcohols and Phenols

Abstract: In order to clarify mechanisms of excited state interactions in hydrogen-bonded pairs, we have studied the kinetics of dynamic quenching of singlet and triplet fluorenone by a series of alcohols, phenols, and related compounds, in which hydrogen-bonding power, redox potential, and acidity are systematically varied. In addition, effects of solvent basicity or polarity and deuteration help identify the role of hydrogen-bonding in physical or chemical quenching processes. Alcohols and weak acids, with high oxidation potentials, do not quench the triplet, but quench the singlet at rates which parallel hydrogen-bonding power. This is attributed to a physical mechanism, involving vibronic coupling to the ground state via the hydrogen bond. This is much stronger in the excited state than in the ground state, and provides efficient energy dissipation in the radiationless transition. Phenols, with hydrogen-bonding power comparable to that of the alcohols but with much lower oxidation potentials, quench both single...

Electron transfers in chemistry and biology

Abstract: Electron-transfer reactions between ions and molecules in solution have been the subject of considerable experimental study during the past three decades. Experimental results have also been obtained on related phenomena, such as reactions between ions or molecules and electrodes, charge-transfer spectra, photoelectric emission spectra of ionic solutions, chemiluminescent electron transfers, electron transfer through frozen media, and electron transfer through thin hydrocarbon-like films on electrodes.

Organic Photoredox Catalysis

Abstract: In this review, we highlight the use of organic photoredox catalysts in a myriad of synthetic transformations with a range of applications. This overview is arranged by catalyst class where the photophysics and electrochemical characteristics of each is discussed to underscore the differences and advantages to each type of single electron redox agent. We highlight both net reductive and oxidative as well as redox neutral transformations that can be accomplished using purely organic photoredox-active catalysts. An overview of the basic photophysics and electron transfer theory is presented in order to provide a comprehensive guide for employing this class of catalysts in photoredox manifolds.

Proton-Coupled Electron Transfer

Abstract: ▪ Abstract Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several are...

Spectra of porphyrins

Abstract: The review opens by presenting the absorption spectra for three series of porphyrins derived from the basic skeleton: (a) compounds obtained by simple substitution; (b) compounds obtained by reduction of one or more pyrrole rings; and (c) compounds obtained from fusion of aromatic rings onto the basic skeleton. The spectra are discussed in terms of a four orbital model—that is intensity changes and energy shifts are related to the properties of two top filled and two lowest empty pi orbitals. Emission spectra of metal porphyrins are then discussed, three metal series being distinguished: (1) In closed shell metals, the continuous enhancement of phosphorescence at the expense of fluorescence is attributed to spin-orbit coupling. (2) In paramagnetic metals, observed effects are attributed to the existence of a state at the same energy as the usual triplet but with multiplicity the same as the ground state; its intensity is ascribed to exchange interactions. (3) In diamagnetic metals with unfilled d shells, peculiar emission properties are attributed to enhanced spin orbit coupling due to low lying metal triplets. The review closes by discussing n-π transitions and triplet-triplet spectra.

Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

Abstract: Many, if not most, redox reactions are coupled to proton transfers. This includes most common sources of chemical potential energy, from the bioenergetic processes that power cells to the fossil fuel combustion that powers cars. These proton-coupled electron transfer or PCET processes may involve multiple electrons and multiple protons, as in the 4 e–, 4 H+ reduction of dioxygen (O2) to water (eq 1), or can involve one electron and one proton such as the formation of tyrosyl radicals from tyrosine residues (TyrOH) in enzymatic catalytic cycles (eq 2). In addition, many multi-electron, multi-proton processes proceed in one-electron and one-proton steps. Organic reactions that proceed in one-electron steps involve radical intermediates, which play critical roles in a wide range of chemical, biological, and industrial processes. This broad and diverse class of PCET reactions are central to a great many chemical and biochemical processes, from biological catalysis and energy transduction, to bulk industrial chemical processes, to new approaches to solar energy conversion. PCET is therefore of broad and increasing interest, as illustrated by this issue and a number of other recent reviews.