Hexaarylbutadiene: A Versatile Scaffold with Tunable Redox Properties towards Organic Near‐Infrared Electrochromic Material

Abstract

When the 1,1,4,4‐tetraanilinobutadiene skeleton is attached with two halogenated aryl units at the 2,3‐positions, they undergo facile two‐electron oxidation to give stable dicationic dyes which exhibit a near‐infrared (NIR) absorption whereas the neutral dienes show only pale color. Therefore, a distinct electrochromic response with an absorption change in the NIR region is achieved, which is attracting considerable recent attention from the viewpoint of bioimaging. Herein, we demonstrate that the redox potentials of the 1,1,4,4‐tetraanilinobutadiene can be precisely controlled by the donating properties of the amino group on the aniline unit as well as the number of halogen atoms on the aryl units at 2,3‐positions on the butadiene. In contrast, the NIR absorption bands mainly depend on the number of halogen atoms irrespective to the donating properties of aniline unit. Thus, the hexaarylbutadiene skeleton is proven to be a versatile scaffold to develop less‐explored organic NIR electrochromic materials, whose redox and spectroscopic properties can be finely tuned by modifying/attaching the proper substituents.

Redox‐Active Hydrocarbons: Isolation and Structural Determination of Cationic States toward Advanced Response Systems

“…Indeed, aromatic hydrocarbons are known to exhibit unique electrochemical and spectroscopic properties based on their π-conjugated systems, and several systems capable of redox interconversion between neutral and cationic (anionic) states have been reported by effectively stabilizing the charged state by embedding heteroatoms or introducing heteroatom-substituents into the π-skeleton. [21][22][23][24][25][26][27][28][29][30][31][32] On the other hand, since it is difficult to stabilize such charged species without heteroatoms, the development of pure hydrocarbonbased redox systems is challenging from the viewpoint of the stability of cationic (anionic) species. In general, even though the incorporation of heteroatoms is an effective method for stabilizing charged species and modulating the electronic properties of organic molecules, the potential instability of bonds between carbon and heteroatoms in the neutral state may reduce fatigue resistance in organic electronic applications.…”

“…In addition, delocalization of a charge over an extended π‐conjugation is effective for the synthesis and isolation of charged species as stable entities with carbon‐centered ions. Indeed, aromatic hydrocarbons are known to exhibit unique electrochemical and spectroscopic properties based on their π‐conjugated systems, and several systems capable of redox interconversion between neutral and cationic (anionic) states have been reported by effectively stabilizing the charged state by embedding heteroatoms or introducing heteroatom‐substituents into the π‐skeleton [21–32] . On the other hand, since it is difficult to stabilize such charged species without heteroatoms, the development of pure hydrocarbon‐based redox systems is challenging from the viewpoint of the stability of cationic (anionic) species.…”

Near‐Infrared Electrochromic Behavior of Dibenzothiepin Derivatives Attached with Two Michler's Hydrol Blue Units

“…Only a limited number of such NIR‐EM have been reported so far, [7] and the absence of heavy metals is favorable for their use in biological applications. For this purpose, the hexaarylbutadiene framework of I / I 2+ can serve as a very versatile platform because the redox potentials for a color change as well as the NIR‐absorption wavelength can be modified systematically by attaching the proper substituents [5] . Finally, the stability of I 2+ under aqueous/physiological conditions enabled us to make use of these EM to create a NIR‐bioimaging protocol by targeting the redox species in biological systems (e.g., hydrogen sulfide as a reductant and hydroxy radical as an oxidant) [8]…”

Generation of hydroxyl radical-activatable ratiometric near-infrared bimodal probes for early monitoring of tumor response to therapy

“…1 ). This nanoprobe is designed on the basis of our works on synthesis and engineering of organic π-electron electrochromic materials (EMs), which distinctly switch color upon electron transfer (i.e., a redox process), to build activatable optical probes for in vivo imaging 56 – 58 . To achieve fast reaction kinetics toward •OH, three EM 1 (e.g., 1-F-Me, 1-Br-Me and 1-Br-Et) sharing the same diene scaffold are synthesized and screened (Fig.…”