Synthesis and Properties of Azulene‐Substituted Donor–Acceptor Chromophores Connected by Arylamine Cores

Abstract

1‐Ethynylazulenes connected by several arylamine cores reacted with tetracyanoethylene (TCNE) and 7,7,8,8‐tetracyanoquinodimethane (TCNQ) in a formal [2+2] cycloaddition–cycloreversion reaction to afford the corresponding tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) chromophores, respectively, in excellent yields. The intramolecular charge‐transfer (ICT) characters between the donor (azulene and arylamine cores) and acceptor (TCBD and DCNQ units) moieties were investigated by UV/Vis spectroscopy and theoretical calculations. The redox behavior of the new TCBD and DCNQ derivatives was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their multistep electrochemical reduction properties. Moreover, significant color changes were observed by visible spectroscopy under the electrochemical reduction conditions.

Synthesis, Properties, and Redox Behavior of Tris(1‐azulenyltetracyanobutadiene) and Tris[1‐azulenylbis(tetracyanobutadiene)] Chromophores Connected to a 1,3,5‐Tri(1‐azulenyl)benzene Core

“…[19] Recently, we have developed various redoxactive chromophores for the creation of stabilized electro-7 chromic materials. [20] As a part of the study, we have reported several TCBD derivatives bearing azulenyl, [10][11][12]14] ferrocenyl, [21] and 2-oxo-2H-cyclohepta[b]-3-furyl [22] substituents, and we have identified some hybrid structures of violene and cyanine with redox activities. [23] Similarly to the TCBD derivatives described previously, these new 1-AzTCBD compounds connected through a 1,3,5-tri(1-azulenyl)benzene core should exhibit a reversible color change under electrochemical reaction conditions, because these compounds showed highly reversible redox waves by CV.…”

“…Recently, we reported the redox potentials of a series of TCBD derivatives. [10][11][12]14] The study revealed that the first reduction potentials of the 1-AzTCBD compounds substituted by π-electron cores decreased as the number of substituted TCBD units increased. With respect to the number of stages for the reduction, Diederich et al reported that the compound with three DAA-substituted TCBD units connected by a 1,3,5-benzenetriyl spacer shows a six-stage reversible reduction wave within a narrow potential range.…”

“…Recently, we have also reported the synthesis and electrochemical properties of tris(1-azulenyltetracyanobutadiene) (1-AzTCBD) chromophores constructed from a 1,3,5-benzenetriyl spacer [10] and a triphenylamine core; [11] these chromophores were obtained by the formal [2+2] CA-RE reactions of the corresponding ethynylazulene derivatives with TCNE. Although these tris(1-AzTCBD) chromophores have a C 3 -symmetrical structure with respect to the central 1,3,5-benzenetriyl unit and triphenylamine core, their cyclic voltammograms displayed reversible multistage reduction waves owing to electrochemical communication between the TCBD units through the central π-electron cores.…”

“…The preparation of tris(1-azulenylacetylene) and tris(1-azulenylethynylarylacetylene) compounds connected to a 1,3,5-tri(1-azulenyl)benzene core (10)(11)(12) for the precursors of new C 3 -symmetrical molecules with multiple 1-AzTCBD units was accomplished by simple one-pot reactions involving repeated Pd-catalyzed alkynylation of the corresponding 1-ethynylazulene derivative 5 with 4 or 7 with methyl 3-[(4-iodophenyl)ethynyl]-7-isopropylazulene-1-carboxylate (8) [14] and methyl 3-[(5-iodo-2-thienyl)ethynyl]-7-isopropylazulene-1-carboxylate (9) [14] under Sonogashira-Hagihara cross-coupling conditions. [15] The cross-coupling reaction of 4 and 5 with [Pd(PPh 3 ) 4 ] as a catalyst and subsequent chromatographic purification of the reaction mixture afforded the desired product 10 in 84 % yield.…”

“…The formal [2+2] CA-RE reactions of 10-12 with TCNE to obtain the new tris(1-AzTCBD) and tris(1-AzBisTCBD) chromophores were examined according to the procedure described previously [3,4,10,11] . The reaction of 10 with TCNE yielded the tris(1-AzTCBD) 13 in 85 % yield as the sole product (Scheme 4).…”