Electronic Couplings for Photoinduced Charge Transfer and Excitation Energy Transfer Based on Fragment Particle-Hole Densities.

A new scheme is proposed to calculate the electronic couplings for photoinduced charge transfer and excitation energy transfer for both singlet and triplet states. In this scheme, the locally excited and charge-transfer states are constructed from the adiabatic ones by maximally localizing the particle (i.e., electron) and hole densities in terms of predefined molecular fragments. The construction process, after which the electronic couplings are directly obtained, is highly efficient and can be combined with various kinds of preliminary electronic structure calculations as long as the adiabatic excitation energies and transition densities are available. The method also applies to the systems with multiple charge or excitation centers. Its validity is demonstrated by the applications to the 6,13-dichloropentacene dimer and tetramer and the C-Zn porphyrin dyad. The results reveal that the environment has a strong impact on the electronic couplings and can even enlarge those for long-range charge transfer by several orders of magnitude.