Interpreting the Unusually Long Transient Signal in Photo-Excited Methyl Viologen in Water
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Publisher:The Ohio State University
Series/Report no.:The Ohio State University. Department of Chemistry Honors Theses; 2010
The long-lived absorption observed in femtosecond transient absorption experiments at 295 nm when methyl viologen dication (MV2+) is photo-excited in water at 266 nm has been assigned to a ground state MV2+/OH- ion pair. While a 1.68 μs lifetime component is seen in the 240 – 320 nm region, no accompanying change in IR absorption is observed. The assignment as a MV2+/OH- ion pair agrees with experiments showing that nitric acid quenches the long lived UV absorption.1 Steady-state studies have produced the MV2+/OH- ion pair and difference spectra closely match the transient absorption results. A Ketelaar plot has determined the binding constant for MV2+ and a single hydroxide molecule to be 4.8 ± 4.2 x 10-2 M-1 and the change in molar extinction coefficient at 295 nm to be 5500 ± 4900 M-1 cm-1, but further experiments should be performed to analyze and validate these values. It is proposed that the mechanism by which photo-excited MV2+ in water forms the MV2+/OH- ion pair can be described as concerted proton coupled electron transfer.