The syntheses and electrooptic properties of a family of nonlinear optical chromophores are described. Typically, these species feature an ethyne-elaborated, highly polarizable porphyrinic component, and metal polypyridyl complexes that serve as integral donor and acceptor elements. The frequency dependence of the dynamic hyperpolarizability of a wide-range of these chromophores, that vary widely with respect to their electronic structure, was determined from hyper-Rayleigh light scattering (HRS) measurements carried out at fundamental incident irradiation wavelengths (λinc) of 830, 1064, and 1300 nm. These data show that: (i) Coupled oscillator photophysics and metal-mediated cross-coupling can be exploited to elaborate high βλ supermolecules that exhibit significant excited-state electronic communication between their respective pigment building blocks; (ii) High-stability metal polypyridyl compounds constitute an attractive alternative to electron releasing dialkyl- and diarylamino groups, the most commonly used donor moieties in a wide-range of established NLO dyes, and long-recognized to be the moiety that often limits the thermal stability of such compounds; (iii) This design strategy clearly enables ready elaboration of extraordinarily large βλ chromophores at telecommunication-relevant wavelengths; and (iv) Multiple charge-transfer (CT) transitions within a single chromophore can be designed to have transition dipole moments of the same or opposite sign; because the sign of the resonance enhancement factor is frequency dependent, appropriate engineering of the relative contributions of these CT states at a given wavelength provides a new means to regulate the magnitude of dynamic hyperpolarizabilities.
Electronic structural modifications of previously reported highly conjugated (polypyridyl)metal-(porphinato)zinc(II) NLO chromophores have been carried out. A primary focus of these modifications probed the role played by the porphyrin macrocycle in effecting large molecular hyperpolarizabilities; specifically, its meso-aryl substituents were replaced with electron withdrawing perfluoroalkyl groups. In doing so, we are effectively lowering HOMO and LUMO of the porphyrin fragment by 0.35eV while retaining the extensive mixing of B, Q, and CT states, and enforcing head-to-tail transition dipole alignment of the component metal-polypyridyl and porphyrin based chromophoric building blocks; this enables supermolecular structures with singly degenerate excited states polarized along the long donor-to-acceptor (D-to-A) charge transfer axis. This work will be placed in the context of ongoing electrooptic experiments and efforts aimed at fabricating new materials from these supermolecular chromophoric species.
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