Singlet formation

Reaction leading to the triplet diradical is exothermic by 1.0 kcal mol-1 but singlet formation would be exothermic by 27.4 kcal mol-1. 

The experimentally obtained values show larger divergence based on alkane fluorescence measurement. Walter et al. [148] and Luthjens et al. [65,128] published 0.8-0.9. Later the latter authors modified their value to 0.65 [149]. In solutions of isooctane, cyclohexane, or -hexane with naphthalene, Sauer and Jonah established this value as 0.5 0.1 this singlet formation probability is approximately constant during the decay of charged species up to 70 nsec [150]. In final product experiments/ = 0.34 [151], 0.53, and 0.47 [84] was estimated for cyclohexane, cix-decalin, and trans-6eca m, respectively. The low value for cyclohexane is due to the ionic reactions before recombination. 

Figure 1. Illustration of the copper-oxygen singlet formation on the square lattice. The additional doped hole cannot go to the copper site due to strong on-site Coulomb repulsion and is distributed among four oxygen sites forming together with the copper spin a Zhang-Rice singlet.

The index pd corresponds to a Zhang-Rice singlet formation with one hole placed on the copper site whereas the second hole is distributed on the neighboring oxygen sites [12], Namely, T/ " " = -2= is the copper-oxygen creation operator in terms of copper (X) and oxygen... 

It is evident from the above discussion that the threshold voltage, current density, power efficiency, luminous efficiency and, to some extent, device lifetime of OLEDs using organic low-molar-mass compounds, oligomers and polymers depends on intrinsic molecular properties, such as HOMO and LUMO energy levels, efficiency of hole and electron injection and subsequent transport, efficiency of singlet formation and fluorescence efficiency. The... 

The above calculation suggests that a singlet formation due to strong correlations with a triplet excited state should be found in appropriate molecules. The effect requires an even total number of valence electrons. In order to detect it one should search e.g. for molecules containing Ce, which are diamagnetic, but which show a f-electron count close to 1, when photoemission experiments are performed. 

Localized fluorescence that results from cross-electron-transfer reactions can occur by direct singlet formation or by reaction to yield triplets that annihilate to... 

The reaction mechanism of excited singlet formation in rubrene ECL seems to be a superposition of two parallel processes. It is produced directly in the electron transfer reaction and/or in the efficient triplet-triplet annihilation within the geminate triplet pair. Thus, in the rubrene ECL system 0eci is the sum of efficiencies for the separate reaction pathways and it can be described as follows ... 

The rates of direct excited singlet formation, occurring in the normal Marcus region for both the ECL systems discussed with AG] 0, can be expressed as follows ... 

Typically, is near to one for Ce systems with the bare f-level well below Ep. Note that the Anderson width A is much smaller than that conduction-band width, so that N (Ep) is indeed large when compared with the bare conduction-band DOS. On the other hand, it should be clear that singlet formation via an indirect exchange coupling cannot transfer much spectral weight from the bare local level up to Ep. Thus, a narrow resonance near the Fermi level of maximum height (jrAY and width k Tp < A arises. It should be emphasized that the local DOS considered here is not the quasiparticle DOS that is used in Fermi-liquid theory to express thermodynamic quantities via free-particle for-... 

Fukuroi K, Takahashi K, Mochida T, Sakurai T, Ohta H, Yamamoto T, Einaga Y, Mori H (2014) Synergistic spin transition between spin crossover and spin-Peierls-like singlet formation in the halogen-bonded molecular hybrid system [Fe(Iqsal)2][Ni(dmit)2]-CH3CN H20. Angew Chem Int Ed 53 1983-1986... 

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