The Energetics of Electron‐transfer Reactions of Chlorophyll and Other Compounds*

Abstract

Abstract-Quite often the primary photochemical reaction of an excited state molecule is transfer of an electron to or from another molecule in its ground state. Rates of such reactions are closely dependent on differences between ground and excited state redox potentials of the reagents. The solvent also plays an important role in stabilizing ion pairs formed by the electron transfer. This Review discusses experimental data relating rates to electrochemical energy parameters in the context of a scheme which portrays the energy and electron transactions in a unified manner. Three consequences of reaction of a singlet excited state are distinguished: (Sl) quenching without detectable products, (S2) exciplex fluorescence, (S3) transient radical ion production, and energetically necessary conditions are derived for each. Similarly, four kinds of reactions involving the triplet state are distinguished, which depend on the relation between the energy of the triplet state and that of the ion pair states: (TI) rapid quenching, (T2) slow quenching, (T3) accelerated intersystem crossing and (T4) generation by reaction between radical ions 'of like spin. The last may be followed by electrochemiluminescence. Classes of compounds for which data are available include chlorophylls, porphyrins and a few other molecules of biological interest, aromatic hydrocarbons and their derivatives, heterocyclic systems, carbonyl compounds, dyes, and complexes of Ru and U. A Table compiling median or selected values of ground and excited state electrochemical potentials of chlorophylls, some porphyrins, and a few other compounds is presented.

Reversible photochemistry in a chlorophyll model system

“…One alternative is electron transfer from one molecule to another within the complex, similar to that proposed for Chl-Chl and Chl-Phe associations in this and earlier papers. However, the energy of the singlet excited state (1.84 eV) is barely sufficient to drive this reaction, according to published redox potentials for Phe (Seely 1978b). Cyclic voltammetry of our sample of Phe in acetonitrile confirmed this conclusion in that the formation ofPhe + + Phe-from (Phe)2 would require 1.90 eV.…”

A REACTION BETWEEN PHEOPHYTIN and CHLOROPHYLL ADSORBED TO POLYETHYLENE‐TETRADECANE PARTICLES*

“…3). The ion pair state is energetically accessible through the singlet excited state but probably not through the triplet excited state (Fujita et al, 1978;Seely, 1978). To account for the low quantum yield, one need only assume that most of the time ion pairs decay by reverse electron transfer to the ground states of both molecules (Eq.…”

Photoaccumulation of reduced primary electron acceptors of photosystem I of photosynthesis

“…Thus, neither the optical nor ESR evidence [4, 6,7,24] indicates that the acceptor is a Chl a dimer as earlier proposed [4,8], and in fact it seems much more likely that the acceptor is a monomer as in green bacteria [18]. The absorption difference spectrum would not exclude that the acceptor is pheophytin a [25], but this seems unlikely because of the Em of pheophytin a in vitro [26] and because of its reported absence in photosystem I particles I27].…”