ELECTRONIC STARK-EFFECT STUDIES OF A PORPHYRIN-BASED PUSH-PULL CHROMOPHORE DISPLAYING A LARGE FIRST HYPERPOLARIZABILITY - STATE-SPECIFIC CONTRIBUTIONS TO BETA

Electroabsorption or stark effect spectroscopy has been applied to a p air of porphyrin-based chromophores with the aim of deriving a detaile d understanding of the Origin of the remarkable first hyperpolarizabil ity, beta, of one of the chromophores: [5-[[4'-(dimethylamino)phenyl]e thynyl]-15-[(4 henyl)-ethynyl]-10,20-diphenylporphinato]zinc(II). The measurements show that significant changes in molecular dipole moment, Delta mu, accompany excitations of x-polarized transitions of the ami nophenyl (donor)/nitrophenyl (acceptor) functionalized chromophore, bu t an absent for a related chromophore lacking the donor and acceptor g roups. For linear chromophores, changes in dipole moment are a prerequ isite to effective molecular first hyperpolarization and incident ligh t frequency doubling behavior. A more detailed consideration of the av ailable \Delta mu\ data, within the context of a conventional two-leve l model, has yielded the following: (a) specific information about the roles of each of seven electronic and vibronic excited starts in defi ning beta, (b) a semiquantitative explanation for the apparent frequen cy independence of beta found in previous hyper-Rayleigh scattering ex periments performed at 830 and 1064 nm, and (c) an explanation for the contrasting frequency dependence of the nonlinear optical response fo r the analogous Cu(II)-containing chromophore. Finally, the experiment al findings are in generally good agreement with published ZINDO calcu lations which had pointed toward the exceptional effectiveness of yne linkages in coupling donor and acceptor moieties to the highly polariz able porphyrin core assembly.