The standard for analyzing collisional excited-state quenching is the Stern-Volmer equation. It shows that the amount of quenching has a first-order relation to the concentration of quencher (τ/τ0=1+KSV[Q]). An example of this is the oxidation of iodide by a photo-excited ruthenium complex. Here, electron transfer from iodide quenches the excited ruthenium complex and forms the reduced complex and an iodine atom (RuII* + I- -&gt; Ru+ + I●). The iodine atom further reacts with another iodide to form diiodide (I2●-). A highly charged ruthenium complex, Ru(tmam)2(deeb)6+ (tmam = 4,4’-trimethylaminomethyl-2,2’-bipyridine) however, has shown unique non-standard Stern-Volmer behavior. The addition of iodide to Ru(tmam)2(deeb)6+ yielded a second-order dependence of the excited-state quenching on iodide concentration, when the photoluminescence lifetimes were monitored (τ/τ0=1+KSV[I-]2, see Figure). Previous stopped flow literature has proposed that a second-order dependence of oxidation on iodide could indicate a concerted mechanism, where electron transfer and I-I bond formation occur together. In contrast, transient absorption spectroscopy indicated that in this system iodide was still oxidized to the iodine atom first. Proposed mechanisms that give rise to this unique second-order excited-state quenching behavior will be discussed. These results show the observation of a rare excited-state quenching behavior with a second-order dependence on the quencher. This study gives further insights into the mechanisms for the regeneration of oxidized sensitizers with iodide in dye-sensitized solar cells.

Access to a second-order excited-state quenching mechanism

251st National Meeting (2016)

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