Quenching excited states

Many stabiliser species operating mainly by other means do show some quenching activity. 

Researchers at Clemson University [142-145] have shown that inclusion of 0.5-4 mole% of naphthalene dicarboxylate or biphenylene dicarboxylate in a PET synthesis results in polymers with increased UV stability, which they demonstrated was at least partly due to energy transfer processes to the copolymerised units, which could then safely dissipate the energy. Milligan [146] later suggested that such copolymers could be useful in fibres for automotive upholstery applications. 

---

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

The rhodamine B-bound complex of Ir1 (387) shows only minor alterations in the absorption spectrum of bound rhodamine B as opposed to free dye however, its fluorescence is strongly quenched.626 Fluorescence is intense when the rhodamine dye is attached to an Ir111 center. The authors conclude that the excited-state quenching mechanism is via electron transfer. 

Trigonal ML3 metal complexes exist as optically active pairs. The complexes can show enantiomeric selective binding to DNA and in excited state quenching.<34) One of the optically active enantiomers of RuLj complexes binds more strongly to chiral DNA than does the other enantiomer. In luminescence quenching of racemic mixtures of rare earth complexes, resolved ML3 complexes stereoselectively quench one of the rare earth species over the other. 35-39 Such chiral recognition promises to be a useful fundamental and practical tool in spectroscopy and biochemistry. 

Table 2 Si Lifetimes (nsec) of Selected Liquid Alkanes at Room Temperature Determined in Steady-State Excited State Quenching or in Transient Fluorescence and Absorption Studies...

TABLE 5. Excited state quenching rate constants for la in benzene"... 

This section summarizes some of the many chemical transformations thought to be significant competitors with energetically favorable excited state quenching by reverse electron exchange. 

In dealing with neutral organic molecules in a fluid solution, it has been found that the formation of the electron transfer products in an excited state quenching process may be the result of a very complicated series of events. In particular, the encounter complex may give rise to an excited state complex (exciplex), ie., a situation in which the excited state and the quencher are linked by some kind of interaction4, 38-44) (Fig. 4). Depending on its own properties and on the ex-... 

Photo-induced electron transfer between [Ru(bpy)3]2+-like centres covalently bound to positively-charged polymers (N-ethylated copolymers of vinylpyridine and [Ru(bpy)2(MVbpy)]2+) and viologens or Fe (III) has been studied using laser flash photolysis techniques. It is found that the backbone affects the rates of excited state quenching, the cage escape yield, and the back electron transfer rate because of both electrostatic and hydrophobic interactions. The effect of ionic strength on the reactions has been studied. Data on the electron transfer reactions of [Ru(bpy)3]2+ bound electrostatically or covalently to polystyrenesulphonate are also presented. 

Chromate(VI) has been reported to undergo reduction to Crv as a result of PET between its LMCT excited state and an external electron donor. In the study carried out for several aliphatic alcohols (methanol, ethanol, propan-2-ol, butan-1-ol, butan-2-ol, 2-methyl-propan-2-ol) two pathways of PET were identified one-electron transfer for intermolecular and two-electron transfer for intramolecular systems [96,97]. The intermolecular mechanism of the CrVI excited state quenching was also found for phenol or its derivatives [98], whereas in the case of an anion donor (such as oxalate) an effect of external cations was observed [99],... 

---