Variation of the Backbone Conjugation in NLO Model Compounds: Torsion‐Angle‐Restricted, Biphenyl‐Based Push‐Pull‐Systems

Abstract

Terminal piperidinyl‐ and nitro‐functionalized biphenyls, bridged between the 2 and 2′ positions by a variable number of methylene groups, are synthesized and fully characterized. These push‐pull systems with defined and restricted torsion angles between their phenyl rings are ideal model compounds to investigate the influence of the chromophore's conjugation in nonlinear optic (NLO) responses. A general synthetic route that can be implemented to access these model compounds is reported, starting from dibromo or ditriflate biphenyls. Hartwig–Buchwald cross‐coupling, a selective azacycloalkylation of diaminobiphenyls and a mild oxidation of primary amines to nitro groups in the presence of a tertiary amine summarizes the synthetic pathway towards the desired model compounds. NLO properties of the series of torsionally constrained push‐pull biphenyls are collected by electric‐field‐induced second‐harmonic generation (EFISH) experiments. The results agree qualitatively with semi‐empirical simulations based on the AM1 Hamiltonian. A linear dependence of the quadratic response on the cos2(Φ) of the inter‐aryl dihedral angle is observed, which points to oscillator strength loss as the dominant effect of increasing backbone twist.

Chemically Controlled Conductivity: Torsion‐Angle Dependence in a Single‐Molecule Biphenyldithiol Junction

“…[42] Currently we are integrating the series of biphenyl building blocks with fixed torsion angles into model compounds to study the relationship between the structure and nonlinear optical properties. [43] Furthermore, we are working on potential switching systems based on the changes of conductivity with changes in torsion angle. independent (merging r = 0.014).…”

Synthesis of Rotationally Restricted and Modular Biphenyl Building Blocks

“…Note, the Pdcatalyzed conversion to terminally amine-functionalized building blocks as precursors of model compounds with interesting nonlinear optical properties are described elsewhere. [45] The doubly chlorinated compounds 3b and 4b were further functionalized by nucleophilic aromatic substitution reactions to intermediate methylsulfanyl derivatives, which were subsequently transformed into the (acetylsulfanyl)biphenyls 3a and 4a. The model compounds were complemented by the ortho-substituted alkylbiphenyl derivatives 6-9.…”

Perturbation theory and the two-level approximation: A corollary and critique

“…Although the materials that are most effective for the utilization of optical nonlinearity in frequency conversion (especially second harmonic generation, SHG) are those whose energy level structures are significantly more complex than atoms, the two-level approximation has received wide application in such a context [5,[11][12][13][14][15][16][17][18]; it not only delivers results of a relatively simple form, it also relates well to long-established concepts of chemical structure. A wealth of synthetic studies have built on the anticipated and oft-proven connection between 'push-pull' chromophore structures [19][20][21][22] (facilitating intramolecular electron transfer) and an enhanced second harmonic response. The devising of simple structural rules, ultimately derived from the two-level model, has assisted the laboratory development of many high-efficiency nonlinear optical materials.…”