Basic model and the rate equations

The strongly bound excitons, Sx and Tx, are intramolecular states. The interconversion process from 5ct to Sx and from Tct to Tx depends on the nature of S cT and Tct The mechanism for bimolecular interconversion is described more fully in the next section. In this section we describe the kinetics by classical rate equations. The use of classical rate equations is justified if rapid interconversion follows the ISC between Tct and S cT, as then there will be no coherence or recurrence between Tct and Sct- We also note that since interconversion is followed by rapid vibrational-relaxation (in a time of 10 s) these processes are irreversible. 

We first consider the case where ISC occurs directly via the spin-orbit cou-phng operator. This operator converts the S = 1 triplets into the singlet, 

Notice that the S — 0 component of the Tct cxciton is converted directly to the = 0 component of the Tx exciton, and cannot contribute to the singlet exciton yield. 

Solving the rate equations under the steady state conditions that 

Alternatively, we might consider ISC via a spin-randomization process, whereby the charge-transfer excitons are scattered into charge-transfer triplets with a probability of 3/4 and charge-transfer singlets with a probability of 1/4. Then the rate equations are (Wohlgenannt and Vardeny 2003), 

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