Singlet-Triplet Splitting of Geminate Electron-Hole Pairs in Conjugated Polymers

The singlet-triplet splitting of geminate polaron pairs in a ladder-type conjugated polymer has been studied by the thermally stimulated luminescence technique. The energy gap separating the singlet and triplet states of the geminate pairs is measured to be in the range of 3–6 meV, depending on the polymer morphology. The results of correlated quantum-chemical calculations on a long ladder-type oligomer are fully consistent with the observed values of the geminate polaron pair singlet-triplet gap. Such low splitting values have important implications for the spin-dependent exciton formation in conjugated polymers.

Figure 1

Temperature dependences of the TSF and TSPh in (top) nano-MeLPPP and (bottom) mesoscopically homogeneous MeLPPP. The solid lines show the fit of the data by the hopping model of TSL [25, 27]; the following set of parameters has been used in order to fit the data on both MeLPPP polymers: Gaussian DOS width $\sigma =55\mathrm{m}\mathrm{e}\mathrm{V}$, total density of hopping sites $N=0.8\times {10}^{21}{\mathrm{c}\mathrm{m}}^{-3}$, attempt-to-jump frequency $\nu ={10}^{12}{\mathrm{s}}^{-1}$, and dielectric permittivity $ϵ=3$.

Figure 2

Electron-hole SCI wave functions of the lowest intrachain (a) and interchain (b) excitation in a model seven-ring cofacial dimer (structure shown on top).

Figure 3

Evolution with intermolecular separation, $d$, of the interchain electron-hole pair exchange energy, $\Delta$, as obtained in a model seven-ring cofacial dimer at the INDO-SCI and INDO-CCSD levels.