Thermodynamics of Electron-transfer Quenching

The excited-state reduction potential, °( Cr3+/Cr2+), can be estimated using an analysis similar to Hess s law of heat summation (Fig. 8.5). Using the emission maximum (730 nm) in the luminescence spectrum and converting units yields an excited-state energy of 164 kJ mol-1 for [Cr(phen)3]3+. That means that relaxation of the 2E excited state to the ground state involves AG° = 164 kJ mol-1 or a one-electron electro- 

Description Absorption Luminescence Nonradiative decay Dynamic quenching 

Series of reactions used to estimate the excited-state reduction potential, 0( Cr3+/Cr)2+. 

The excited-state and ground-state reduction potentials indicate that a substrate (Q) having an oxidation potential (for the process Q - Q+ + e ) of E° (Q/Q+) = — 1.29 V vs NHE, for example, could be oxidized by the excited-state complex [Cr(phen)3]3+ but not by the ground state complex [Cr(phen)3]3+  

The excited-state redox reaction, equation (8.12), is thermodynamically favorable (E° 0) while ground-state reaction, equation (8.13) is not (E° 0). Therefore, a mixture of [Cr(phen)3]3+ and such a substrate will only undergo a redox reaction after the chromium complex has been excited. This is the process of photo-induced electron transfer light initiates an electron-transfer reaction. This experiment will explore how substrates such as DNA may be oxidized by the excited-state [Cr(phen)3]3+ complex. Because the electron-transfer reaction competes kinetically with luminescence, the presence of such a suitable substrate leads to a decrease in the intensity of luminescence. For this reason, the substrate is termed a quencher. 

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