Explorative computational study of the singlet fission process

Abstract

Different ab initio methods, namely multi-reference and nonorthogonal configuration interaction techniques, are explored for their applicability in studying the singlet fission problem. It has been shown for 2-methyl-1,5-hexadiene that the ¹TT state can be identified using multi-reference techniques. The geometrical and vibrational properties of the ¹TT state are such that they can be approximated with those of the ⁵TT state. A proof of principle is given for the calculation of the singlet fission pathway driven by nuclear motion: efficient singlet fission can take place if the ¹TT and S₁ states are close in energy with a large non-adiabatic coupling matrix element at the S₁ geometry, and the energy of the S₀ state is well below that of the ¹TT state at the ¹TT geometry.

The nonorthogonal configuration interaction method was used to treat a tetracene trimer. It has been shown that the first excited states can be interpreted as delocalised states; interaction with charge-transfer base states plays an important role. The ¹TT states are localised on one pair of molecules. The electronic coupling between the diabatic S[n] and ¹TT[m] states is in the meV range, confirming previous estimates. The charge-transfer base states enhance the coupling between the S[1]/S[2] and ¹TT[2] excited states.

Keywords:

• singlet fission
• ab initio calculations
• non-adiabatic coupling
• electronic coupling

Acknowledgements

Professor Dr L. D. A. Siebbeles and Dr F. C. Grozema of the Delft University of Technology are gratefully thanked for stimulating discussions. R.W.A.H. acknowledges the Netherlands Organization for Scientific Research (NWO/ECHO), grant 700.57.027 for financial support.