Time-dependent configuration-interaction simulations for exciton-exciton fusion in perylene dimers after laser pulse excitation

We present laser driven many electron dynamics within the time-dependent configuration interaction (TD-CI) approach for the simulation of exciton-exciton fusion in perylene dimers. We use a selected active orbital window and a small distortion of the ground state geometry to construct a reasonably sized model system based on ab initio quantum chemical wave functions, which shows a large exciton-exciton fusion probability. For the TD-CI simulations, we adopt a CI variant, i.e., the time-dependent double configuration interaction singles (TD-dCIS), which allows a direct mapping of excited monomer and charge transfer states to single and double exciton states in aggregates. We use semicanonical orbitals, which are localized on one of the perylene molecules in the dimer and are very similar to canonical Hartree-Fock orbitals of the isolated perylene molecule. We compare the TD-dCIS results to few-state and many-state molecular exciton Hamiltonian (MEH) models employing the point dipole approximation for exactly the same systems. We find that the usual few-state MEH models perform quite well compared with TD-dCIS, while including many states in MEH models leads to large deviations if the usual point dipole approximation is used. Further, we prove that even in strongly coupled dimers the double exciton state can be efficiently excited using ultrashort laser pulses.