Electronic stark effect studies of a porphyrin-based push-pull chromophore displaying a large first hyperpolarizability: State-specific contributions to β

Electroabsorption or Stark effect spectroscopy has been applied to a pair of porphyrin-based chromophores with the aim of deriving a detailed understanding of the origin of the remarkable first hyperpolarizability, β, of one of the chromophores: [5-[[4'-(dimethylamino)phenyl]ethynyl]-15-[(4_- nitrophenyl)-ethynyl]-10,20-diphenylporphinato]zinc(II). The measurements show that significant changes in molecular dipole moment, Δμ, accompany excitations of x-polarized transitions of the aminophenyl (donor)/nitrophenyl (acceptor) functionalized chromophore, but are absent for a related chromophore lacking the donor and acceptor groups. For linear chromophores, changes in dipole moment are a prerequisite to effective molecular first hyperpolarization and incident light frequency doubling behavior. A more detailed consideration of the available |Δμ| data, within the context of a conventional two-level model, has yielded the following: (a) specific information about the roles of each of seven electronic and vibronic excited states in defining β, (b) a semiquantitative explanation for the apparent frequency independence of β found in previous hyper-Rayleigh scattering experiments performed at 830 and 1064 nm, and (c) an explanation for the contrasting frequency dependence of the nonlinear optical response for the analogous Cu(II)-containing chromophore. Finally, the experimental findings are in generally good agreement with published ZINDO calculations which had pointed toward the exceptional effectiveness of yne linkages in coupling donor and acceptor moieties to the highly polarizable porphyrin core assembly.