Singlet-triplet annihilation

One otlier common source of nonlinear response, singlet-triplet annihilation, is often tire reason for a discrepancy between fluorometric and absorjDtion kinetic measurements [27, 28 and 29]. 

Wang, X. F., Y. Koyama, H. Nagae, Y. Yamano, M. Ito, and Y. Wada. 2006. Photocurrents of solar cells sensitized by aggregate-forming polyenes Enhancement due to suppression of singlet-triplet annihilation by lowering of dye concentration or light intensity. Chem. Phys. Lett. 420 309-315. 

Figure 35 Relative fluorescence efficiency as a function of the quantal exciting intensity for a 200 pm-thick tetracene crystal excited with the 325 nm line of a He-Cd laser. The increasing segment shows the triplet—triplet fusion contribution to the fluorescence (delayed fluorescence) the decrease at high excitation levels is attributed to quenching of the singlets by singlet-triplet annihilation. Experimental data are represented by points, theoretical fits, as described in text, by the solid line. Adapted from Ref. 206.

King SM, Dai D, Rothe C, Monkman AP (2007) Exciton annihilation in a polyfluorene Low threshold for singlet-singlet annihilation and the absence of singlet-triplet annihilation. Phys Rev B 76(8)... 

There is also singlet-triplet annihilation according to the reaction scheme... 

Fig. 20 Energy level scheme and electronic transitions in 35. Upon excitation (ex) of HBC, either singlet energy transfer (EET) to the PMI periphery or intersystem crossing (ISC) to the HBC triplet state (Tj) occurs. In the case of EET, PMI fluorescence ( / ) is observed while population of Ti leads to a dark state. After ISC, singlet-triplet annihilation (STA) may quench excited singlet states (Si) of the PMI periphery through energy transfer to Ti. Subsequently, in this example, the higher excited triplet state (r ) decays non-radiatively...

In addition, it was fotmd that during the lifetime of the HBC triplet state, additional selective excitations of the PMI chromophores were quenched by singlet-triplet annihilation as sketched in Fig. 20 [38]. This result establishes a direct link to Sect. 3.2 because the energy transfer pathway in 35 has been reversed. The energy now flows from PMI to HBC. In this respect, the HBC core could be viewed as a switch for the PMI fluorescence operating via intersystem crossing. 

Kolubayev T, Geacintov N E, Paillotin G and Breton J 1985 Domain sizes in chloroplasts and chlorophyll-protein complexes probed by fluorescence yield quenching induced by singlet-triplet exciton annihilation Biochimica Biophys. Acta 808 66-76... 

According to Hercules 5> a measure of the relationship between direct excitation of the first excited singlet state by radical-ion recombination and triplet-triplet annihilation is the entropy factor FAS, estimated to be on average 0.2 eV. The enthalpy of the radical cation-radical anion recombination can be measured as the difference between the redox potentials 1/2 Ar—Ar (oxidation) and 1/2 Ar—Ar<7> (reduction). This difference has to be corrected by the entropy term. If this corrected radical-ion recombination enthalpy is equal to or larger... 

An external magnetic field was observed to have practically no effect on the intensity of the thianthrene (430 nm) emission, indicating that no triplet states are involved. The shorter-wavelength emission of the oxadiazole 102, however, is probably due to a triplet-triplet annihilation reaction of diphenyloxadiazole triplets. These are produced in the radical-ion reaction between 101 and 102, yielding thianthrene excited-singlet molecules and diphenyl-oxadiazole excited-triplet molecules ... 

At least experiments with fluoranthene and other aromatic hydrocarbons having lower triplet energies than fluoranthene resulted only in fluoranthene singlet emission- this excited singlet appears to be generated directly and not by triplet-triplet annihilation. 

Two main CL pathways are then possible If sufficient energy is available, an electron transfer reaction where the singlet excited state of A is accessible (3) otherwise an energy-deficient route whereby the energy from two triplet excited-state species are pooled, to provide sufficient energy to form the singlet excited state, in what is termed a triplet-triplet annihilation reaction, (4) and (5). 

When D and A are identical (3D + 3D —> 3D + 3D ), triplet-triplet annihilation leads to a delayed fluorescence, called P-type delayed fluorescence because it was first observed with pyrene. Part of the energy resulting from annihilation allows one of the two partners to return to the singlet state from which fluorescence is... 

In pure crystals, singlet excitons can be created by mutual annihilation of triplet excitons. The intensity of the singlet exciton fluorescence depends quadratically on the triplet exciton concentration and is therefore proportional to the square of the singlet-triplet extinction coefficient. It is interesting to compare such a delayed fluorescence excitation spectrum, observed by Avakian et cd. 52) on naphthalene, with a corresponding phosphorescence excitation spectrum (Fig. 22). 

In a suggested alternative mechanism, lower energy triplets produced by the cation-anion annihilation produce fluorescent singlets by triplet-triplet annihilation.73 This possible pathway cannot be overlooked on two accounts. First, it is known from phosphorescence emission that triplets are produced in these systems13,15,60 and second, the double potential step data analyzed by the Feldberg method seems to be in agreement with such a double annihilation mechanism.65 If triplets are initially produced in high local concentrations by the redox reaction in... 

Evidence for photoassociation in the triplet manifold is at present inconclusive. Although Hoytink et al.20 have reported excimer phosphorescence from cooled ethanolic solutions of phenanthrene and naphthalene, concentration and temperature-dependent studies of the emission characteristics must be extended in order to distinguish photoassociation of the triplet state from intersystem crossing of the singlet excimer and possible triple-triplet annihilation. Certainly the decay constant of the molecular triplet state in fluid media is relatively insensitive to solute concentration21 although this... 

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