Theoretical study of the mechanism of electron transfer at photosynthetic reaction centers. I. Singlet excited states of free base porphin

Free base porphin is a key unit in the electron transfer reaction at photos ynthetic reaction centers. For the electron transfer reaction, the transfer integral of the rate constant depends strongly on the quality of the wave functions of porphin-based chromophores. Therefore, we need a stable method for calculating the wave functions of optically allowed excited states of the porphin. We developed such a method and verified its stability by calcu lating the wave functions for an ethylene molecule. We confirmed that the o ptically allowed excited states required the entire amount of valence molec ular orbitals for the active space to adequately describe the wave function with molecular in-out polarization. We applied our calculation method to i nvestigate the wave function of free base porphin. Our ab initio calculatio n used 4-31 G plus d polarization functions for the carbon and nitrogen ato ms, and Rydberg 2d basis functions on the center of each pyrrol ring simult aneously. We also proposed an assignment for the controversial B and N band s. This assignment is reasonably consistent with experimental data within 0 .36 eV for Q and B bands, and 0.75 eV for N bands. The reproduction accurac y of the lower excitation spectra up to B band confirms the reliability of the transfer integral part to calculate the rate constants of the electron transfer reaction at photosynthetic reaction centers. (C) 1999 American Ins titute of Physics. [S0021-9606(99)30906-5].