Experimental demonstration of the relationship between the second- and third-order polarizabilities of conjugated donor-acceptor molecules

The dependence of the second- and third-order polarizabilities ((beta) and (gamma) ) on ground-state polarization was measured for a series of donor-acceptor polyenes using electric field induced second harmonic generation and third harmonic generation, respectively. The changes in ground-state polarization, associated with the donor/acceptor strength or solvent polarity, were probed by x-ray crystallography, 1H-NMR, electronic absorption, and Raman spectroscopies. The observed behavior of (beta) and (gamma) as a function of ground- state polarization agrees well with theoretical predictions. In particular, positive and negative peaks, as well as sign changes, were observed for both (beta) and (gamma) . The dependences for (beta) and (gamma) are consistent with a derivative relationship between them. In addition, the third-order polarizability of a series of molecules possessing zero bond length alternation was found to be negative, in agreement with predictions based on the relationship between the polarizabilities and ground-state geometry

Direct observation of reduced bond-length alternation in donor/acceptor polyenes

There has been tremendous interest in asymmetric cyanine and merocyanine compounds because of their applications as photographic sensitizers, membrane potential probe, and photochromic dyes for all-optical memory. On the basis of UV-visible spectroscopic data, Brooker suggested that merocyanine molecules could be described by a superposition of neutral and charge-separated canonical resonance forms and that, by changing the basicity of the endgroups and/or the solvent polarity, one could tune the molecular structure from neutral and polyene-like through polar and cyanine-like (with equal contributions from neutral and charge-separated resonance forms) to highly polar, charge-separated polyene-like

Direct observation of reduced bond-length alternation in donor/acceptor polyenes

There has been tremendous interest in asymmetric cyanine and merocyanine compounds because of their applications as photographic sensitizers, membrane potential probe, and photochromic dyes for all-optical memory. On the basis of UV-visible spectroscopic data, Brooker suggested that merocyanine molecules could be described by a superposition of neutral and charge-separated canonical resonance forms and that, by changing the basicity of the endgroups and/or the solvent polarity, one could tune the molecular structure from neutral and polyene-like through polar and cyanine-like (with equal contributions from neutral and charge-separated resonance forms) to highly polar, charge-separated polyene-like

Experimental Demonstration of the Dependence of the First Hyperpolarizability of Donor-Acceptor-Substituted Polyenes on the Ground-State Polarization and Bond Length Alternation

It has been suggested that optimizing the first hyperpolarizability, β, of donor-acceptor compounds requires a specific donor-acceptor strength for a given conjugated bridge. For donor-acceptor polyenes, β can be maximized when an optimal degree of mixing between neutral and charge-separated canonical resonance forms This degree of mixing is related to the donor-acceptor strength and a molecular parameter, bond length alternation (BLA), defined as the difference between the average carbon-carbon single and double bond lengths in the polymethine backbone. The degree of BLA arises from the linear combination, or mixing, of the two-limiting charge-transfer resonance forms of the molecule (Figure 1).4 For unsubstituted polyenes or chromophores with weak donors-acceptors, the neutral canonical form is the dominant contributor to the ground state, resulting in large positive BLA. As the donor-acceptor strength increases, the charge-separated resonance structure contributes more to the ground state, resulting in smaller BLA, until both resonance forms contribute equally and the ground-state structure possesses essentially zero BLA, analogous to a symmetrical cyanine. Increasing the ground-state polarization further results in the charge-separated canonical form dominating the ground-state structure, leading to negative BLA

Experimental Demonstration of the Dependence of the First Hyperpolarizability of Donor-Acceptor-Substituted Polyenes on the Ground-State Polarization and Bond Length Alternation

It has been suggested that optimizing the first hyperpolarizability, β, of donor-acceptor compounds requires a specific donor-acceptor strength for a given conjugated bridge. For donor-acceptor polyenes, β can be maximized when an optimal degree of mixing between neutral and charge-separated canonical resonance forms This degree of mixing is related to the donor-acceptor strength and a molecular parameter, bond length alternation (BLA), defined as the difference between the average carbon-carbon single and double bond lengths in the polymethine backbone. The degree of BLA arises from the linear combination, or mixing, of the two-limiting charge-transfer resonance forms of the molecule (Figure 1).4 For unsubstituted polyenes or chromophores with weak donors-acceptors, the neutral canonical form is the dominant contributor to the ground state, resulting in large positive BLA. As the donor-acceptor strength increases, the charge-separated resonance structure contributes more to the ground state, resulting in smaller BLA, until both resonance forms contribute equally and the ground-state structure possesses essentially zero BLA, analogous to a symmetrical cyanine. Increasing the ground-state polarization further results in the charge-separated canonical form dominating the ground-state structure, leading to negative BLA

Structures of Pb(II) porphyrins: 5,10,15,20-tetrakis-triisopropylsilylethynylporphinato lead(II) and 5,15-bis-(3,5-bis-tert-butylphenyl)-10,20-bis-triisopropylsilylethynylp orphinato lead(II)

The crystal structures of two lead containing porphyrins are described. Structure 1 contains unprecedented stacks of three metalloporphyrins. Each structure shows disorder of the Pb atoms which are situated out of the mean plane of the porphyrin ring

A Unified Description of Linear and Nonlinear Polarization in Organic Polymethine Dyes

An internal or external electric field F can drive the chemical structure, bond order alternation, and electronic structure of linear polymethine dyes from a neutral, bond-alternated, polyene-like structure, through a cyanine-like structure, and ultimately to a zwitterionic (charge-separated) bond-alternated structure. As the structure evolves under the influence of F, the linear polarizability α, the first hyperpolarizability β, and the second hyperpolarizability γ are seen to be derivatives, with respect to F, of their next lower order polarization (for α) or polarizability (for β and γ). These derivative relations provide a unified picture of the dependence of the polarizability and hyperpolarizabilities on the structure in linear polymethine dyes. In addition, they allow for predictions of structure-property relations of higher order hyperpolarizabilities