Estimation of Electronic Coupling for Singlet Excitation Energy Transfer

Abstract

Electronic coupling is a key parameter that controls the efficiency of excitation energy transfer (EET) and exciton delocalization. A new approach to estimate electronic coupling is introduced. Within a two-state model, the EET coupling V of two chromophores is expressed via the vertical excitation energies (E i and E j ), transition dipole moments (M i and M j ) of the system and transition moments (μ A and μ B ) of the individual chromophores:}. These quantities are directly available from quantum mechanical calculations. As the estimated coupling accounts for both short-range and long-range interactions, this approach allows for the treatment of systems with short intermolecular distances, in particular, π-stacked chromophores. For a system of two identical chromophores, the coupling is given bywhere F i and F j are the corresponding oscillator strengths and cos θ is determined by the relative position of the chromophores in the dimer. Thus, the coupling can be derived from purely experimental data. The developed approach is used to calculate the EET coupling and exciton delocalization in two π-stacks of pyrimidine nucleobases 5′-TT-3′ and 5′-CT-3′ showing quite different EET properties. ■ INTRODUCTIONTheoretical and computational methods have been successfully applied to study excitation energy transfer (EET) in various molecular systems. Recent developments in this field were considered in detail in several reviews. 1−4 The efficiency of EET between two chromophores A and B, A*−B → A−B*, is controlled by electronic coupling of two diabatic states φ A and φ B . In the reference states, the exciton is localized on the corresponding molecules. Commonly, φ A and φ B are defined as excited states of the separated chromophores. The electronic coupling of the reference excited states is a key parameter that controls delocalization of excited states and the probability of energy transfer between the chromophores. 1−4The coupling of singlet excited states V includes both a shortrange term, which depends on the orbital overlap between the excited states, and a long-range Coulomb contribution. The Coulomb coupling of two molecules A and B can be estimated as the interaction of their transition dipole moments μ A and μ Bwhere R AB is the intermolecular distance. This widely used dipole−dipole scheme suggested by Forster 60 year ago 5 provides good estimates when the spatial extension of the transition densities in the molecules is much smaller than the distance R AB . More accurate values can be computed using transition atomic charges 6In eq 2, i runs over all atoms of the molecule A, and j runs over all atoms of B; q i and q j are transition charges derived from quantum mechanical (QM) calculations of excited states φ A and φ B of the individual molecules. At large distances between the chromophores, V dd ≈ V tc . Both eq 1 and eq 2 account only for the long-range interaction of the excited states, whereas the orbital and exchange terms are neglected. The performance of the transition charge (TC) model was discussed...

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