Bimolecular Excited States

Case C Q is not in large excess and mutual approach of M and Q is possible during the excited-state lifetime. The bimolecular excited-state process is then diffusion-controlled. This type of quenching is called dynamic quenching (see Section 4.2.2). At high concentrations of Q, static quenching may occur in addition to dynamic quenching (see Section 4.2.4). 

Bimolecular excited state electron transfer reactions have been investigated extensively during the last decade (1-3). Electron transfer is favored thermodynamically when the excitation energy E of an initially excited molecule A exceeds the potential difference of the redox couples involved in the electron transfer process. 

The uranyl ion luminesces in fluid solutions at room temperature2 201 202 thus providing a tool for the study of bimolecular excited state processes. In several cases, however, this study is complicated by the fact that the uranyl ion forms complexes with a variety of chemical species, so that it is often difficult to distinguish between intramolecular photochemical processes involving uranyl ion-ligand complexes and intermolecular photochemical processes involving reaction between an electronically excited UO + species and the substrate2,201,202 ... 

Figure 11 Excited states created by bimolecular electron-hole recombination in single component organic solids. In contrast to photo-excitation (Fig. 10), recombination of oppositely charged, statistically independent carriers (e, h) leads to molecular and bimolecular excited states through unavoidable Coulombically correlated electron-hole pairs (e - h).

Encapsulation of a Ru atom into a caged [109, 110] or hemicaged [299] tris-bipyridine ligand extends the MLCT excited state lifetime and improves photostability relative to [Ru(bpy)3] ", while retaining the fast (diffusion controlled) bimolecular excited state electron transfer reactivity. In contrast, the [Ru(bpy)3] + in the core of a dendrimer [248] has about the same inherent lifetime as the free complex but the rate of electron transfer quenching rapidly decreases with increasing the number and size of dendrimer branches. 

Table I. Bimolecular Excited State Quenching Processes...

The bimolecular excited state quenching rate constants show some variation depending on the solvent media. Values for of 3.3 x 10 s" and... 

Bimolecular excited state rate constants for water quenching of several 9-alkylxanthyl cations and the parent xanthyl cation were determined by Stem-... 

Several singlet-excited cations have been shown to imdergo photoinduced electron transfer from aromatic donors. Fluorescence from the 9-phenylxanthyl, xanthyl, thioxanthyl, and 9-phenylthioxanthyl cations is quenched in the presence of aromatic compounds [9,10,15], Steady-state fluorescence quenching experiments gave bimolecular excited state rate constants for quenching of the cations by the aromatic donors (Table 7). The quenching rate constants increase... 

---