Estimation of Electronic Coupling for Singlet Excitation Energy Transfer

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 (mu(A) and mu(B)) of the individual chromophores: V = (E-i - E-j)([(MiMj)(mu(2)(A) - mu(2)(B)) - (mu(A)mu(B))(M-i(2) - M-j(2))]/[(M-i(2) - M-j(2))(2) + 4(MiMj)(2)]}. 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, pi-stacked chromophores. For a system of two identical chromophores, the coupling is given by V = (E-i - E-j)[(EiFj - EjFi)/(EiFj + EjFi)] [1/(2 cos theta)] where F-i and F-j are the corresponding oscillator strengths and cos theta 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 pi-stacks of pyrimidine nucleobases 5'-TT-3' and 5'-CT-3' showing quite different EET properties.