Large Molecular Hyperpolarizabilities in "Push-Pull" Porphyrins. Molecular Planarity and Auxiliary Donor-Acceptor Effects

Linear optical and SHG coefficients of a number of "push-pull" porphyrins are analyzed using the semiempirical INDO/S Hamiltonian and singles-only CI. The NLO response properties are computed at a nonresonant excitation energy of 0.1 eV using the correction vector formalism, and the calculations are carried out in both ideal planar and SYBYL force-field optimized geometries. The results strongly suggest that large NLO responses may be obtained by (1) minimizing the dihedral twist of phenyl substituents with respect to the porphyrin plane, (2) replacing the homoaromatic phenyl rings in tetraphenylporphyrins by electron excessive or deficient heteroaromatic rings such as pyrrole or tetrazine, (3) exploiting the electron-excessive and -deficient sites of the porphyrin π-electron bridge itself. Thus, functicnalizing the electron excessive β position with donor substituents and electron-deficient meso position with acceptor substituents leads to enhanced NLO response properties. The enhancement in the NLO response properties is modest in chromophores containing spacer groups which reduce the dihedral twist angles. This is attributed to the diminished inductive effect of the electron excessivity/deficiency of the porphyrin rings due to the intervening spacer and phenyl moieties.