Charged triplet excitons

AR Brown, K Pichler, NC Greenham, DDC Bradley, RH Friend, and AB Holmes, Optical spectroscopy of triplet excitons and charged excitations in poly(p-phenylenevinylene) light-emitting diodes, Chem. Phys. Lett., 210 61-66, 1993. 

The main result of the early EPR and ODMR work was a reduction of the ZFS parameters D( and when comparing monomeric 3BChl and the 3P state of the primary donor in bRCs (for a collection of data see reference 22). This has been interpreted as resulting from a delocalization of the triplet exciton in the BChl-dimer. However, due to the complexity of the electronic system, that could also involve charge transfer states, a final quantitative conclusion has been difficult. From a determination of the triplet axes in bRC single crystals104 107 it was concluded that in R. sphaeroides the triplet is indeed delocalized whereas in B. viridis it seems to be located on one monomeric BChl b half. 

Many radical ion salts such as Wurster s Blue perchlorate130 and charge transfer salts of tetracyanoquinodimethane131 are situated in their crystal lattices such that the unpaired electrons are coupled, and low-lying triplet exciton states are observable by ESR at low temperatures. 

The increased yield of the reaction products of (1) results in lower concentration of triplet excitons and, consequently, a lower number of free charge carriers formed in molecular crystals that is justified by two resonance peaks of negative polarity in the RYDMR spectrum of... 

The total concentration of holes nh is a sum of the concentration of trapped (nht) and free (nhf) carriers. However, often rihf/nht —> 0, nh nht due to a large concentration of traps. Then, the excitons are quenched by trapped carriers and the annihilation rate constant yTq is equivalent to the mobile exciton-immo-bile (trapped) charge carrier interaction rate constant yxq. Under space-charge-limited conditions, the concentration of charge is simply proportional to the applied voltage (U), nht = (3/2) o U/ed2, where d is the sample thickness, e is the electronic charge, s is the dielectric constant of the sample material, and s0 is the permittivity of free space. Thus, it may be seen that the fractional change in the triplet exciton decay rate... 

Figure 147 The relative cascade-like pattern of the increase of triplet exciton monomolecular decay rate constant (/ = t 1) as a function of charge-injecting voltage in anthracene crystal. Consecutive trap-filled limits are indicated by C/TFL (1), /TFL(2) and C/TFL (3). Dotted line indicates the averaged (linear) dependence of A/ // 0 as resulted from the standard interpretation assuming a continuous increase in the charge density proportional to the injecting voltage [334]. Adapted from Ref. 240.

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