ELECTRONIC COUPLING MATRIX ELEMENT FOR ELECTRONIC EXCITATION-ENERGY TRANSFER - S-1-1-]S-2+S-0 EXCITED-STATE ANNIHILATION(S)

For the singlet-singlet annihilation process S-1+S-1-->S-2+S-0, an exp ression is derived for the electronic coupling matrix element involved in electronic excitation energy transfer (EET) between two equivalent chromophores. Allowance is made for vibronic coupling to occur betwee n the S-1 and S-2 states of the chromophores, for each of which the S- 1<-- S-0 and S-2<-- S-0 excitations are, respectively, electric-dipole forbidden and allowed. Attention is also given to aspects of the theo ry for singlet-singlet (pi(D)pi(D))(n(A)n(A))-->(pi(D)pi(D))(n(A)pi(A )) EET with vibronic coupling. The theory for S-1+S-1-->S-2+S-0 is il lustrated via the results of some model calculations for two C=S chrom ophores. For each chromophore, the calculations treat explicitly the v alence-shell p pi atomic orbitals and a sulphur lone-pair atomic orbit al. The results of the calculations highlight the nature of the primar y contributors to the vibronic coupling component of the electronic EE T matrix element, namely Coulombic-type terms. The theory also shows t hat a Coulombic term is the primary contributor to the vibronic coupli ng component for singlet-singlet (pi(D)pi(D))(n(A)n(A)) -->(pi(D)pi(D ))(n(A)pi(A)) EET. (C) 1998 American Institute of Physics.