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'' porph yrins are analyzed using the semiempirical INDO/S Hamiltonian and sing les-only CI. The NLO response properties are computed at a nonresonant excitation energy of 0.1 eV using the correction vector formalism, an d the calculations are carried out in both ideal planar and SYBYL forc e-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 ex cessive or deficient heteroaromatic rings such as pyrrole or tetrazine , (3) exploiting the electron-excessive and -deficient sites of the po rphyrin pi-electron bridge itself. Thus, functionalizing the electron excessive beta position with donor substituents and electron-deficient meso position with acceptor substituents leads to enhanced NLO respon se properties. The enhancement in the NLO response properties is modes t in chromophores containing spacer groups which reduce the dihedral t wist 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.