Fluorescence quenching rate constants, kq, ranging from 106 to 2 × 1010 M−1 sec−1, of more than 60 typical electron donor-acceptor systems have been measured in de-oxygenated acetonitrile and are shown to be correlated with the free enthalpy change, ΔG23, involved in the actual electron transfer process

in the encounter complex and varying between + 5 and −60 kcal/mole. The correlation which is based on the mechanism of adiabatic outer-sphere electron transfer requires ΔG≠23, the activation free enthalpy of this process to be a monotonous function of ΔG23 and allows the calculation of rate constants of electron transfer quenching from spectroscopic and electrochemical data.

A detailed study of some systems where the calculated quenching constants differ from the experimental ones by several orders of magnitude revealed that the quenching mechanism operative in these cases was hydrogen-atom rather than electron transfer.

The conditions under which these different mechanisms apply and their consequences are discussed.

Kinetics of Fluorescence Quenching by Electron and H‐Atom Transfer

Die Fluoreszenz aromatischer Kohlenwasserstoffe und Azaverbindungen A, wird durch typische Elektrondonoren D (amino- und methoxy-substituierte Benzole) in polaren Losungsmitteln dynamisch geloscht. Ein Vergleich der in (sauerstoff-freiem) Acetonitril gemessenen Geschwindigkeitskonstanten kq der Fluoreszenzloschung mit der aus spektroskopischen und elektrochemischen (Redoxpotentialen) Daten berechneten freien Enthalpieanderung Δ G23 fur die Elektronubertragung im Begegnungskomplex:










zeigt, das die Loschreaktion diffusionsbestimmt verlauft (kq = 1,6 · 1010 M−1 sec−1), wenn Δ G23 5 kcal/mol kq proportional zu exp (– Δ G23/RT) abnimmt. Die kinetische Analyse der Mesergebnisse liefert die Beziehung:










wobei Δ G, die zu k23 gehorende freie Aktivierungsenthalpie eine monotone Funktion von Δ G23 ist und daraus berechnet werden kann. Damit ist es moglich, die Geschwindigkeitskonstanten von Elektronubertragungsreaktionen auf Grund thermodynamischer Daten zu berechnen.



Ein uber die intermediare Bildung angeregter EDA-Komplexe verlaufender Loschmechanismus kann ausgeschlossen werden.



Rate constants, kq, of fluorescence quenching by typical electron donors (D) of aromatic hydrocarbons and aza-compounds (A), measured in de-oxygenated acetonitrile, are shown to be related to the enthalpy change, Δ G23, involved in the electron transfer process:










in the encounter complex. Δ G23 can be calculated from spectroscopic data and polarographic redox-potentials of the molecules involved. kq reaches the diffusion-controlled limit of 1.6 · 1010 M−1 sec−1 for Δ G23 5 kcal/mol. A kinetic analysis yields the expression:










where Δ G, the free enthalpie of activation of the electron transfer process (k23), is a monotonic function of Δ G23 from which it can be calculated. This allows rate constants of electron transfer reactions to be calculated from thermodynamic data.



The results cannot be interpreted by a quenching mechanism involving intermediate formation of excited charge-transfer complexes.

Kinetik und Mechanismus der Elektronübertragung bei der Fluoreszenzlöschung in Acetonitril

Specific removal of o-methoxybenzyl protection by DDQ oxidation.

Metal 1,2-Dithiolene and Related Complexes

On the selectivity of deprotection of benzyl, mpm (4-methoxybenzyl) and dmpm (3,4-dimethoxybenzyl) protecting groups for hydroxy functions

Mechanism of the Formation of Di-t-butylnitroxide from t-Nitrobutane and Sodium Metal

Free Radicals in Amine Solutions of Elemental Sulfur

Cleavage Reactions of Tertiary Nitro Anion Radicals

REACTIONS OF THE t-NITROBUTYL ANION RADICAL

π-System Properties of Pyromellitonitrile

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