Excitonic Coupling in Polythiophenes: Comparison of Different Calculation Methods

On an ultra-short time-scale several measurements, e.g. anisotropy decay in polythiophenes [Gra03], suggest significant deviations of the excitation transfer process from the Förster like hopping between the spectroscopic units. Therefore, the model for the excitonic coupling has to be revised, particularly the validity of the point-dipole approximation.

Distance dependency of the excitonic coupling V₁₂ of two skew sedecimthiophenes turned to each other by an angle of 45º (see figure below) is calculated for several models:

• Point dipole approximation
  
  

• Line dipole approximation, where one distributes the transition dipole moments along the two oligomers (of length L) and integrates over the interactions between the infinitesimal dipoles.
  
  

• Coulomb interaction between the transition densities ρ_TD1(r) and ρ_TD2(r) of the two oligothiophenes.
  
  

• ZINDO calculation to obtain excitonic energy splits, which, however, can not be assigned to V₁₂ unequivocally for R₁₂ < 4Å because of a third transition near by.

For the more sophisticated models the resulting effective excitonic coupling V₁₂ (see figure below) is always smaller than that obtained in the point-dipole approximation.

Thus, for polythiophene in solution the line-dipole approximation gave more reliable results [Gra01]. However, excitonic coupling can not be responsible for the experimentally found ultra-short components in the anisotropy decay in polythiophene films [Gra03]. To explain these components one has to suppose a different mechanism.

References

• W. J. D. Beenken, T. Pullerits, Excitonic coupling in polythiophenes: Comparison of different calculation methods, J. Chem. Phys. 120 (5), 2490-2495 (2004) [Details]
• M. M. L. Grage, Y. Zaushitsyn, A. Yartsev, M. Chachisvilis, V. Sundström, T. Pullerits, Ultrafast excitation transfer and trapping in a thin polymer film, Phys. Rev. B 67 (20), 205207 (2003) [Details]
• M. M. L. Grage, P. W. Wood, A. Ruseckas, T. Pullerits, W. Mitchell, P. L. Burn, I. D. W. Samuel, V. Sundström, Conformational disorder and energy migration in MEH-PPV with partially broken conjugation, J. Chem. Phys. 118 (16), 7644-7650 (2003) [Details]