We simulate subpicosecond charge separation in two donor–acceptor molecular dyads. Charge separation dynamics is described using a quantum master equation, with parameters of the dyad Hamiltonian obtained from density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations and the rate of energy dissipation estimated from Ehrenfest-TDDFT molecular dynamics simulations. We find that higher-energy charge transfer states must be included in the dyad Hamiltonian in order to obtain agreement of charge separation rates with the experimental values. Our results show that efficient and irreversible charge separation involves both coherent electron transfer from the donor excited state to higher-energy unoccupied states on the acceptor and incoherent energy dissipation that relaxes the dyad to the lowest energy charge transfer state. The role of coherence depends on the initial excited state, with electron delocalization within Hamiltonian eigenstates found to be more important than coherence between eigenstates. We conclude that ultrafast charge separation is most likely to occur in donor–acceptor dyads possessing dense manifolds of charge transfer states at energies close to those of Frenkel excitons on the donor, with strong couplings to these states enabling partial delocalization of eigenstates over acceptor and donor.

We study the dynamics of charge separation in bulk heterojunction organic photovoltaic systems in light of recent experimental observations that this process is characterized by multiple time scales in the range of 10 fs to 100 ps. Coherent evolution of the excitonic state has been suggested to dominate the early stages of the charge separation process and diffusion of localized excitons to be dominant at longer times. Both of these processes obviously depend on the system morphology, in particular on the grain sizes of the donor and acceptor phases. Here we analyze these mechanisms and their characteristic time scales, aiming to verify the consistency of the proposed mechanisms with the experimentally observed time scales of charge separation. We suggest that the coherent mechanism that dominates the early stage of charge separation involves delocalized excitons. These excitons are formed by optical excitation of clusters of strongly interacting donor sites, and the charge separation rate is determined by the probability that such sites lie at the donor–acceptor interface. The (relatively) slow diffusive rate is estimated from the mean first passage time for a diffusing exciton to reach the donor grain surface. Our estimates, based on available exciton diffusion rates and morphology data, are consistent with experimental observations.

We present time-dependent density functional theory (TDDFT) calculations for single and dimerized Coumarin-343 molecules to investigate the quantum mechanical effects of chromophore aggregation in extended systems designed to function as a new generation of sensors and light-harvesting devices. Using the single-chromophore results, we describe the construction of effective Hamiltonians to predict the excitonic properties of aggregate systems. We compare the electronic coupling properties predicted by such effective Hamiltonians to those obtained from TDDFT calculations of dimers and to the coupling predicted by the transition density cube (TDC) method. We determine the accuracy of the dipole–dipole approximation and TDC with respect to the separation distance and orientation of the dimers. In particular, we investigate the effects of including Coulomb coupling terms ignored in the typical tight-binding effective Hamiltonian. We also examine effects of orbital relaxation which cannot be captured by either of these models.

We present a theoretical analysis of the electronic absorption spectra of tetracene in 4He droplets based on many-body quantum simulations. Using the path integral ground state approach, we calculate one- and two-body reduced density matrices of the most strongly localized He atoms near the molecule surface and use these to investigate the helium ground-state quantum coherence and correlations when tetracene is in its electronic ground and excited states. We identify a trio of quasi-one-dimensional, strongly localized atoms adsorbed along the long axis of the molecule that show some quantum coherence among themselves but far less with the remaining solvating helium. We evaluate the single-particle natural orbitals of the localized He atoms by diagonalization of the one-body density matrix and use these to construct single- and many-particle solvating helium basis states with which the zero-phonon spectral features of the tetracene–4HeN absorption spectrum are then calculated. The absorption spectrum resulting from the three-body density matrix for the strongly bound trio of helium atoms is in very good agreement with the experimental data, accounting quantitatively for the anomalous splitting of the zero-phonon line [Hartmann, M.; Lindinger, A.; Toennies, J. P.; Vilesov, A. F. Chem. Phys. 1998, 239, 139; Krasnokutski, S.; Rouillé, G.; Huisken, F. Chem. Phys. Lett. 2005, 406, 386]. Our results indicate that the combination of strong localization and the quasi-one-dimensional nature of trios of helium atoms adsorbed along the long axis of tetracene leads to a quantum coherent, yet highly correlated ground state for the helium density closest to the molecule. The spectroscopic analysis shows that this feature accounts quantitatively for the anomalous splittings and hitherto unexplained fine structure observed in the absorption spectra of tetracene and suggests that it may be responsible for the corresponding zero-phonon splittings in other quasi-one-dimensional planar aromatic molecules.