RESUMO
Intramolecular singlet fission and triplet-triplet annihilation (TTA) has been experimentally observed and reported. However, problems remain in theoretically accounting for the corresponding intramolecular electronic couplings and their rates. We used the fragment excitation difference (FED) scheme to calculate the coupling with states from restricted active-space spin-flip configuration interaction. We investigated three covalently linked pentacene dimers via a phenyl group in an ortho-, meta-, and para-arrangement. The singlet fission and TTA couplings were enhanced when two chromophores were covalently linked. With the Fermi golden rule, both the estimated singlet fission and TTA rates were in line with the experimental results. For systems with significant singlet-fission coupling, charge-transfer components were observed in the excited states involved, and charge-transfer states were also seen within 1 eV above the singlet excited states. Our approach allows for an analysis of through-bond versus through-space singlet fission in the full electronic wave functions. The FED scheme is useful for calculating intramolecular singlet-fission and TTA couplings.
RESUMO
The fragment excitation difference (FED) scheme is a useful method for calculating the complete diabatic couplings of various energy transfer systems. The lack of a good definition for the transformation of the transition density matrix to the off-diagonal FED matrix elements limits FED to single-excitation methods. We have developed a generalized FED scheme called the θ-optimized FED (θ-FED) scheme which does not require transforming the transition density matrices. In θ-FED, two states of interest are linearly transformed by a mixing angle θ into two mixed states. The excitation difference of each mixed state is evaluated and optimized numerically to determine the mixing angle. This approach allows for finding diabatic states and the corresponding couplings for a general set of Hamiltonians.