Exciplexes solution behavior

The solution behavior of exciplexes is even more complex, and accounts for the formation of radicals, triplet exciplexes and ordinary triplet states (2j0, 22). The general kinetic scheme for the photophysics of exciplexes has been summarized by Mattes and Farid (201 and is diagrammed below. Some of our results may be reconciled with the processes summarized by this scheme ... 

Bis(dimesitylboryl)-2,2 -bithiophene (BMB-2T, 242) forms a stable amorphous glass and emits pure blue color with a high fluorescence QE of 86% in THF solution [270]. However, an OLED with ITO/m-MTDATA/TPD/BMB-2T/Mg Ag emits with a broad emission due to an exciplex with TPD. The exciplex can be prevented by insertion of a thin layer of 1,3,5-tris(biphenyl-4-yl)benzene (TBB) between TPD and BMB-2T, leading to a pure blue emission. It seems that the boron complex or boron-containing compounds easily form an exciplex with common HTMs. Other similar blue emitter materials also demonstrate such behavior. 

Weller and Zachariasse thoroughly investigated exciplex formation and luminescence for donor acceptor systems in THF [18]. A particularly interesting result from their work came from an examination of the temperature dependence of radiative charge recombination between 9,10-dimethylanthracene anion (DMA") and TPTA+ in THF [19]. They found that both exciplex emission and fluorescence from DMA were observed in solution at low temperature (ca. —50°C). As the solution temperature is raised, the excimer emission decreases in relative intensity, and at room temperature the emission is nearly completely DMA fluorescence. The monomer-to-exciplex emission intensity ratio as a function of temperature follows Arrhenius kinetic behavior and yields an activation barrier that is nearly the same as the energy gap between the exciplex and the DMA states. Thus, their model consisted of reaction of the solvent-separated ions to form an intimate emissive ion pair which could dissociate to yield the singlet anthracene derivative. 

A comparison of the fluorescence behavior PBA solutions in DMAC-LiCl and in H2S0 indicates that probably the broad emission band around X = 440 nm in isotropic dilute solutions in DMAC-LiCl or H2SO11 may be attributed to exciplex formation between Li+ or in the two cases. The exciplex formation between aromatic rings and was first elucidated by Chen et al. This interpretation, of course, needs further studies and confirmation. In DMAC-LiCl solution the presence of Li - apparently hinders the stacking of phenylene chromophores of PBA chain as no difference in the fluorescence spectra of a dilute and a nematic solution could be noted. 

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