Molecules with Linear π‐Conjugated Pathways between All Substituents: Omniconjugation

Advanced Functional Materials (Impact Factor: 10.44). 02/2004; 14(3):215 - 223. DOI: 10.1002/adfm.200305003

ABSTRACT In this paper, omniconjugation is introduced as a topological phenomenon in π-conjugated systems. Omniconjugated molecules are defined by the fact that they provide direct and fully π-conjugated pathways between all substituents attached to them. Surprisingly, until now such topologies have never been explicitly recognized or investigated from this point of view. A topological design scheme is presented as a tool, which enables for the systematic construction of this novel class of π-electron molecular structures. Molecular building blocks with three or more connection points to the external moieties are proposed, which for the first time allows for the interconnection of many functional entities in a fully conjugated manner. In being truly conjugated, these pathways are expected to provide high transmission probabilities for holes and/or electrons. Omniconjugated structures may play an important role in the design of complex electronic circuitry based on organic molecules. On a larger scale, they may also give rise to special material properties. Although omniconjugation is based on a valence-bond description of the system, it is shown that our concept is in good agreement with results obtained from a molecular-orbital description of the electron probability distribution in the frontier orbitals.

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    ABSTRACT: In this work, a variety of new conjugated chromophores containing imino and hydroxyl groups are presented. Excited state intramolecular proton transfer (ESIPT) of these chromophores under one- and two-photon irradiation was surveyed. One-photon absorption spectra show the presence of internal hydrogen bond in the organic dyes containing ortho-hydroxyl group, while the corresponding dyes carrying para-hydroxyl group do not exhibit intramolecular H-bonding effect. In the most of aprotic and protic solvents, the extended chromophores bearing ortho-hydroxyl group exhibit well-separated dual emission bands with large Stokes shift (ca. 160 nm), as contrast, the molecules containing para-hydroxyl group exhibit only single band with normal Stokes shift (ca. 50 nm). Two-photon absorption (TPA) induced competitive ESIPT emission can also be observed by near-infrared (near-IR) femtosecond laser to the molecules carrying ortho-hydroxyl group. The experiments show that the molecules carrying ortho-hydroxyl group are able to undergo ESIPT under one- and two-photon excitation, while the molecules containing para-hydroxyl groups do not exhibit such properties. 2-((3′,4′-Dimethoxyl-phenylethyleneyl-phenyl-4-ylimino)methyl)phenol (C1) and 2-((3′,4′,5′- trimethoxyl-phenylethyleneyl-phenyl-4-ylimino)methyl)phenol (C3) exhibit regular and selective response to Zn2+ in DMF. The molecular modeling was further performed to analyze ESIPT occurrence theoretically.
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    ABSTRACT: We discuss the relationship between the π-conjugation pattern, molecular length, and charge transport properties of molecular wires, both from an experimental and a theoretical viewpoint. Specifically, we focus on the role of quantum interference in the conductance properties of cross-conjugated molecules. For this, we compare experiments on two series of dithiolated wires. The first set we synthesized consists of three dithiolated oligo(phenylene ethynylene) (OPE) benchmark compounds with increasing length. The second series synthesized comprises three molecules with different π-conjugation patterns, but identical lengths, i.e. an anthracene (linear conjugation), an anthraquinone (cross-conjugation), and a dihydroanthracene (broken conjugation) derivative. To benchmark reliable trends, conductance experiments on these series have been performed by various techniques. Here, we compare data obtained by conductive-probe atomic force microscopy (CP-AFM) for self-assembled monolayers (SAMs) with single-molecule break junction and multi-molecule EGaIn data from other groups. For the benchmark OPE-series, we consistently find an exponential decay of the conductance with molecular length characterized by β = 0.37 ± 0.03 Å(-1) (CP-AFM). Remarkably, for the second series, we do not only find that the linearly conjugated anthracene-containing wire is the most conductive, but also that the cross-conjugated anthraquinone-containing wire is less conductive than the broken-conjugated derivative. We attribute the low conductance values for the cross-conjugated species to quantum interference effects. Moreover, by theoretical modeling, we show that destructive quantum interference is a robust feature for cross-conjugated structures and that the energy at which complete destructive interference occurs can be tuned by the choice of side group. The latter provides an outlook for future devices in this fascinating field connecting chemistry and physics.
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