The role of charge-transfer states in the spectral tuning of antenna complexes of purple bacteria

In this paper a novel approach to study the formation and relaxation of excited states in solution is presented within the integral equation formalism version of the polarizable continuum model. Such an approach uses the excited state relaxed density matrix to correct the time dependent density functional theory excitation energies and it introduces a state-specific solvent response, which can be further generalized within a time dependent formalism. This generalization is based on the use of a complex dielectric permittivity as a function of the frequency, epsilonomega. The approach is here presented in its theoretical formulation and applied to the various steps involved in the formation and relaxation of electronic excited states in solvated molecules. In particular, vertical excitations (and emissions), as well as time dependent Stokes shift and complete relaxation from vertical excited states back to ground state, can be obtained as different applications of the same theory. Numerical results on two molecular systems are reported to better illustrate the features of the model.

The design of optimal light-harvesting (supra)molecular systems and materials is one of the most challenging frontiers of science. Theoretical methods and computational models play a fundamental role in this difficult task, as they allow the establishment of structural blueprints inspired by natural photosynthetic organisms that can be applied to the design of novel artificial light-harvesting devices. Among theoretical strategies, the application of quantum chemical tools represents an important reality that has already reached an evident degree of maturity, although it still has to show its real potentials. This Review presents an overview of the state of the art of this strategy, showing the actual fields of applicability but also indicating its current limitations, which need to be solved in future developments.

We present a combined quantum mechanics and molecular mechanics (QM/MM) method to study electronic energy transfer (EET) in condensed phases. The method introduces a quantum mechanically based linear response (LR) scheme to describe both chromophore electronic excitations and electronic couplings, while the environment is described through a classical polarizable force field. Explicit treatment of the solvent electronic polarization is a key aspect of the model, as this allows account of solvent screening effects in the coupling. The method is tested on a model perylene diimide (PDI) dimer in water solution. We find an excellent agreement between the QM/MM method and "exact" supermolecule calculations in which the complete solute-solvent system is described at the QM level. In addition, the estimation of the electronic coupling is shown to be very sensitive to the quality of the parameters used to describe solvent polarization. Finally, we compare ensemble-averaged QM/MM results to the predictions of the PCM-LR method, which is based on a continuum dielectric description of the solvent. We find that both continuum and atomistic solvent models behave similarly in homogeneous media such as water. Our findings demonstrate the potential of the method to investigate the role of complex heterogeneous environments, e.g. proteins or nanostructured host materials, on EET.