Mapping charge-transfer excitations in Bacteriochlorophyll dimers from first principles

Abstract

Photoinduced charge-transfer excitations are key to understand the primary processes of natural photosynthesis and for designing photovoltaic and photocatalytic devices. In this paper, we use Bacteriochlorophyll dimers extracted from the light harvesting apparatus and reaction center of a photosynthetic purple bacterium as model systems to study such excitations using first-principles numerical simulation methods. We distinguish four different regimes of intermolecular coupling, ranging from very weakly coupled to strongly coupled, and identify the factors that determine the energy and character of charge-transfer excitations in each case. We also construct an artificial dimer to systematically study the effects of intermolecular distance and orientation on charge-transfer excitations, as well as the impact of molecular vibrations on these excitations. Our results provide design rules for tailoring charge-transfer excitations in Bacteriochloropylls and related photoactive molecules, and highlight the importance of including charge-transfer excitations in accurate models of the excited-state structure and dynamics of Bacteriochlorophyll aggregates.

Quantum–Quantum and Quantum–Quantum-Classical Schemes for Near-Gap Excitations with Projection-Based-Embedded GW-Bethe–Salpeter Equation

“…Many-body Green’s functions Theory employing the GW approximation and the Bethe–Salpeter equation (BSE) has been a widely established method for the determination of electronic excitations in solid-state physics. Over the past decade, it has gradually found more and more application in traditionally molecular quantum chemistry settings. − It was shown that GW -BSE provides an effective single- and two-particle picture with accurate energies for charged and neutral excitations of different character, e.g., photoionization and localized vs charge-transfer type excitations, without the need for any adaptations. ,, Even though its scaling (dependent on details of the implementation) is favorable compared to wave function based methods such as ADC(2) or CC2, the direct application of GW -BSE to many complex molecular systems remains computationally challenging. Examples of such molecular systems are polymers with complex internal architecture, either solvated or pure or mixed blends, more general solvent–solute systems with nonequilibrium relaxation dynamics, or molecular aggregates as in organic semiconductor films …”

“…Over the past decade, it has gradually found more and more application in traditionally molecular quantum chemistry settings. 2 − 10 It was shown that GW -BSE provides an effective single- and two-particle picture with accurate energies for charged and neutral excitations of different character, e.g., photoionization and localized vs charge-transfer type excitations, without the need for any adaptations. 4 , 11 , 12 Even though its scaling (dependent on details of the implementation) is favorable compared to wave function based methods such as ADC(2) 13 or CC2, 14 the direct application of GW -BSE to many complex molecular systems remains computationally challenging.…”