Singlet Exciton Energy Transfer

Excitons in conjugated polymers can move by transfer from their current location (the donor site) to an acceptor location on the same polymer chain or on another nearby polymer backbone. This motion of energy plays important roles in the photophysics of the material, and is critical to the function of optoelectronic devices such as solar cells. Singlet motion is usually considered in terms of Forster 

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Singlet excitonic energy transfer between chls is most commonly discussed in terms of the two limiting cases of very strong and very weak electronic coupling (J) between donor (D) and acceptor (A) transition dipoles [159-161]. J(cm" ) may be calculated by the expression given by Pearlstein [162]... 

Ohkita et al. [64] studied the influence of this excess energy with respect to the relaxed charge transfer state on the yield of spatially separated charge carriers with a series of polythiophenes vdth differing singlet exciton energy levels (Figure 4.14). 

The existence of differential reactivity for various sites suggests the possibility that energy absorbed at one site on the chain may be transferred down the chain until it localizes in a site with an unusually high cross section for reaction. Shulman, Gueron, and Eisinger154 claim that energy absorbed in poly dAT at the excited singlet level is transferred to a common excimer between A and T, whence it crosses to a triplet triplet excitons have been observed in poly A with a jump time of 10"8 to 10 10 sec. 

A theoretical study by the same author suggests that RDX forms charge transfer complexes upon crystn which are unique because their charge transfer exciton band is of lower energy than the singlet exciton band of their molecular crysts. Static reactivity indices were used to predict the likely primary dissociative products obtainable from each excited state of secondary nitramines theory predicts that the axial and equatorial nitramine groups of the polynitra-mines RDX, alpha- and beta-HMX, may possess quite novel selective decompn paths and hence give different primary dissociative products... 

Strong coupling would not be expected for many fluid solvents but weak coupling could lead to energy transfer from solvent to solvent molecule at rates in excess of diffusion rates. This could occur as a result of singlet or even triplet exciton migration. 

The exchange term is usually dominant at close approach of M d and Ma, and allows triplet exciton transfer to occur when the donor and acceptor transitions are spin-forbidden. Therefore, four energy transfer processes from a singlet (1M [) excited molecule and triplet (3Mp excited molecule to an unexcited molecule in the singlet state (1Ma) are possible ... 

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