Exciplex quenching

The fact that quadricyclene and dienes quench the fluorescence of aromatic hydrocarbons despite the fact that the energetics for classical energy transfer are very unfavorable has been rationalized by the formation of an exciplex. A general mechanism is as follows  

Hammond and co-workers have suggested that the exciplex should be described in terms of the four contributing structures 

Quenching rate constants for dienes and quadricyclenes have similar sensitivities to the electronic and structural features of the excited aromatic hydrocarbon. However, during this process quadricyclene isomerizes to nor-bomadiene with a quantum yield of 0.S2, whereas dienes usually remain unchanged. Hammond has suggested that vibrational energy which is partitioned to the acceptor upon internal conversion of the exciplex can lead to isomerization ° °  

The fact that quadricyclene is isomerized and dienes are not could be a result of (a) factors that govern how the vibrational energy is partitioned and (b) the large difierence in activation energies for isomerization. 

Another point of view is that exciplex formation can in some cases involve bonding changes in the acceptor. Solomon, Steel, and Weller proposed that the quenching by quadricyclene leads to an exciplex where the quadricyclene is distorted. 

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Both the fluorescence and intersystem crossing of lt -amine exciplexes can be quenched by added amine (116). Rate constants for exciplex quenching obtained from fluorescence or t-1 isomerization quantum yield data decrease with amine a-alkylation ... 

The proposed mechanism for exciplex quenching involves formation of a triplex l(t-lT(R3N)2 ) in which the amine dimer cation radical (62) has formed a 3-electron a bond. 

The participation of the singlet states of dyes such as rose bengal (RB, tetraiodotetrachlorofluorescein, sodium salt) and MB during the sensitized oxygenation of amines was demonstrated by Davidson (40). Fluorescence quenching Stem-Volmer constants were reported for several amines, halide anions, and for 3-carotene and were consistent with charge transfer stabilization of an exciplex quenching intermediate. 

A key observation that supports the notion of ILCT character is the decreased susceptibility of these complexes to exciplex quenching. This normally occurs by attack of Lewis bases at the vacant coordination sites of the metal, and is promoted for MLCT excited states by the increased electrophilic-ity of the metal in the transient, formally oxidized + 3 state, in effect by the hole generated on the metal [85]. Contribution of ILCT character serves to delocalize the hole away from the metal and onto the ligand. 

Tears DKC, McMillin DR. Exciplex quenching of photoexcited platinum(II) terpyridines influence of the orbital parentage. Coord Chem Rev 2001 211 195-205. 

Our investigation of the mechanism of intramolecular adduct formation employed the technique of arene-amine exciplex quenching by primary amines, which had been developed in earlier investigations of exciplex quenching. These experiments provided evidence for the occurrence of adduct formation via proton transfer in the fluorescent exciplex. In the case of 39, activation parameters for exciplex formation and proton... 

A. Sharma, O. S. Wolfbeis "Fiberoptic Fluorosensor for Sulfur Dioxide Based on Energy Transfer and Exciplex Quenching, Proc. SPIE Int. Soc. Opt. Eng. 990 (1989) 116-120. 

McMillin, D. R. Kirchoff, J. R. Goodwin, K. V. Exciplex Quenching of Photoexcited Copper Complexes, Coord. Chem. Revs. 1985, 64, 83. 

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