Samir Farid

Bio: Samir Farid is an academic researcher from University of Rochester. The author has contributed to research in topics: Electron transfer & Radical ion. The author has an hindex of 42, co-authored 146 publications receiving 5879 citations. Previous affiliations of Samir Farid include Columbia University & Northwest University (United States).

Efficiencies of photoinduced electron-transfer reactions: role of the Marcus inverted region in return electron transfer within geminate radical-ion pairs

Abstract: In photoinduced electron-transfer processes the primary step is conversion of the electronic energy of an excited state into chemical energy retained in the form of a redox (geminate radical-ion) pair (A + D A'-/D'+). In polar solvents, separation of the geminate pair occurs with formation of free radical ions in solution. The quantum yields of product formation, from reactions of either the free ions, or of the geminate pair, are often low, however, due to the return electron transfer reaction (A'-/D'+ - A + D), an energy-wasting step that competes with the useful reactions of the ion pair. The present study was undertaken to investigate the parameters controlling the rates of these return electron transfer reactions. Quantum yields of free radical ion formation were measured for ion pairs formed upon electron-transfer quenching of the first excited singlet states of cyanoanthracenes by simple aromatic hydrocarbon donors in aceonitrile at room temperature. The free-ion yields are determined by the competition between the rates of separation and return electron transfer. By assuming a constant rate of separation, the rates of the return electron transfer process are obtained. These highly exothermic return electron transfer reactions (-AG,, = 2-3 eV) were found to be strongly dependent on the reaction exothermicity. The electron-transfer rates showed a marked decrease (ea. 2 orders of magnitude in this AG, range) with increasing exothermicity. This effect represents a clear example of the Marcus "inverted region". Semiquantum mechanical electron-transfer theories were used to analyze the data quantitatively. The electron-transfer rates were found also to depend upon the degree of charge delocalization within the ions of the pair, which is attributed to variations in the solvent reorganization energy and electronic coupling matrix element. Accordingly, mostly on the basis of redox potentials, one can vary the quantum yield of free-ion formation from a few percent to values approaching unity. Use of a strong donor with a strong acceptor to induce reactions based on electron transfer is likely to be inefficient because of the fast return electron transfer in the resulting low-energy ion pair. A system with the smallest possible driving force for the initial charge-separation reaction results in a high-energy, and therefore long-lived ion pair, which allows the desired processes to occur more efficiently. The use of an indirect path based on secondary electron transfer, a concept called "cosensitization", results in efficient radical-ion formation even when the direct path results in a very low quantum yield.

Radiative and nonradiative electron transfer in contact radical-ion pairs

Abstract: The relationship between radiative and nonradiative electron transfer is explored for return electron transfer processes in the contact radical-ion pairs formed by excitation of ground state CT complexes. Using a conventional nonadiabatic theory of electron transfer, absolute rate constants for nonradiative return electron transfer, varying over more than two orders of magnitude, can be predicted from information obtained from analyses of the corresponding radiative processes. The effects of solvent polarity, driving force and molecular dimension on the rates of nonradiative return electron transfer are studied.

Dynamics of Bimolecular Photoinduced Electron-Transfer Reactions

Abstract: From the accumulated results of several research groups over the last 25 years, it is clear that photoinduced electron-transfer reactions have significantly broadened the scope of organic photochemistry.1 The fundamental mechanistic principle is that when quenching of an excited state via electron transfer is sufficiently exothermic, the reaction occurs at or close to the diffusion-controlled limit (kdiff). In polar solvents, where most reactions are carried out, the primary intermediate is a geminate radical-ion pair, A•-/D•+ (eq 1).3 Return electron transfer within the

Contact and solvent-separated geminate radical ion pairs in electron-transfer photochemistry

Abstract: The two primary intermediates that play a major role in determining the efficiencies of bimolecular photoinduced electron-transfer reactions are the contact (A{sup {sm bullet}{minus}}D{sup {sm bullet}+}) and the solvent-separated (A{sup {sm bullet}{minus}}(S)D{sup {sm bullet}+}) radical ion pairs, CRIP and SSRIP, respectively. These two species are distinguished by differences in electronic coupling, which is much smaller for the SSRIP compared to the CRIP, and solvation, which is much larger for the SSRIP compared to the CRIP. The present work addresses the quantitative aspects of these and other factors that influence the rates of energy-wasting return electron transfer within the ion-pair intermediates. The electron acceptor tetracyanoanthracene (TCA) forms ground-state charge-transfer complexes with alkyl-substituted benzene donors. By a change of the excitation wavelength and/or donor concentration, either the free TCA or the CT complex can be excited. Quenching of free {sup 1}TCA{sup *} by the alkylbenzene donors that have low oxidation potentials, such as pentamethylbenzene and hexamethylbenzene, in acetonitrile solution leads to the direct formation of geminate SSRIP. Excitation of the corresponding charge-transfer complexes leads to the formation of geminate CRIP. Rates of return electron transfer within the two types of ion pair are determined from quantum yields for formation of free radicalmore » ions pairs depend upon the reaction exothermicity in a manner consistent with the Marcus inverted region.« less

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.

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

Abstract: 6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

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...

Luminescent and Redox-Active Polynuclear Transition Metal Complexes.

Abstract: Great attention is currently paid to the synthesis of polynuclear transition metal complexes and the study of their photochemical, photophysical, and electrochemical properties. This interest is stimulated, in particular, by attempts to design and construct multicomponent systems (often called supramolecular species) capable of performing useful lightand/or redox-induced functions.1-16 A great deal of investigations on mononuclear transition metal complexes had previously shown that several families of these compounds are very interesting from the electrochemical, photochemical, and photophysical viewpoints.17-22 The metalligand interaction, in fact, is often (i) weak enough to allow the manifestation of intrinsic properties of metal and ligands (e.g., ligand-centered and metalcentered absorption bands and redox waves) and, at the same time, (ii) strong enough to cause the appearance of new properties, characteristic of the whole compound (e.g., metal-to-ligand or ligand-tometal charge-transfer bands). On passing from mononuclear to polynuclear transition metal complexes, the situation becomes even more interesting because in the latter (supramolecular) compounds one can find, besides properties related to each metal-based component, properties related to the structure and composition of the whole array. A suitable choice of the mononuclear building blocks and bridging ligands and an appropriate design of the (supramolecular) structure can in fact allow the occurrence of very interesting and potentially useful processes such as energy transfer along predetermined pathways, photoinduced charge separation, multielectron exchange at a predetermined potential, etc. The knowledge on the luminescence and redox properties of polynuclear transition metal complexes is rapidly accumulating, but it is disperse in a great number of journals. We have made an attempt to collect the available results, and we present them together with some fundamental introductory concepts and a few comments. One of the main problems, of course, was to delimit the field of this review. Using personal criteria which are related to our own research interests, we decided to consider only polynuclear transition metal complexes that can be defined as supramolecular species (section 2.2) and that are reported to exhibit luminescence. For such compounds only, the electrochemical properties have also been reviewed. Furthermore, we decided to include only classical (Werner-type) polynuclear transition metal compounds where the number of metal-based units is exactly known and the connection between the metal centers is provided only by bridging ligands. Thus, a number of interesting systems such as polymer-appended metal † In memoriam of Mauro Ciano. 759 Chem. Rev. 1996, 96, 759−833