Triplet exciplex emission

ECL emission has been also observed in the mixed ECL systems involving PAHs with reaction partners like aromatic amines or ketones forming radical cations D + or radical anions A-, respectively.114127 Such approach solves the problems caused by the instability of ECL reactants but lowers distinctly the free energy available for the formation of an excited state. Usually, the energy released in electron transfer between A- + D + ions is insufficient to populate emissive 11A or D states directly and the annihilation of the radical ions usually generates only nonemissive3 A or 3 D triplets that produce light via triplet-triplet annihilation. Consequently ECL efficiencies in the mixed ECL systems are usually very low. Only in some cases, when radiative electron transfer between A + D+ species is operative, relatively intense [A D + ] exciplex emission can be observed. 

Shim and coworkers have published several examples of the 2+2 between aryl-substituted alkynes and alkenes. Their data suggests that the singlet and triplet pathways give rise to the observed cyclobutenes. Exciplex emission was detected for the alkyne-alkene pairs. 

Aromatic alkynes, such as 1-naphthylbuta-1,3-diyne (120) in the presence of 2,3-dimethylbutene, undergo a [2 + 2] photocycloaddition reaction to form a cyclobutene derivative 121 in 77% chemical yield (Scheme 6.52). This particular addition occurs exclusively on the C1-C2 triple bond. Quenching, fluorescence and exciplex emission studies suggested that this reaction proceeds from both singlet and triplet excited states.717... 

Inorganic anions (Br and I ) are efficient fluorescence quenchers for azastilbenes mainly in acidic medium. Charge-transfer complexes and enhanced intersystem crossing to the triplet state account for the quenching mechanism [205,282], Fluorescence quenching by aliphatic amines is associated with exciplex emission [282, 303, 304] (Section V). Exciplex forma-... 

In nearly all radical anion recombination reactions with TPTA exciplex emission is observed - the exciplex being formed directly from the radical ions. Mixed triplet-triplet annihilation appears to be impossible as the triplet lifetime of TPTA is very short [37]. 

Parker and Joyce125 have also observed a new band in the delayed emission spectrum of solutions of anthracene (A) and 9,10-diphenylanthracene (B) in ethanol at — 75°C, which they attribute to the exciplex of these species formed in the process of mixed triplet-triplet annihilation... 

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