Tris(1-azulenylacetylene) and tris(1-azulenylethynylarylacetylene) chromophores connected to a 1,3,5-tri(1-azulenyl)benzene core have been prepared by the Pd-catalyzed alkynylations of 1-ethynylazulene with tris(3-iodo-1-azulenyl)benzene or iodoarene derivatives substituted with a 1-azulenylethynyl group with tris(3-ethynyl-1-azulenyl)benzene under Sonogashira–Hagihara cross-coupling conditions. These compounds reacted with tetracyanoethylene in formal [2+2] cycloaddition–retroelectrocyclization reactions to afford the corresponding tris(1-azulenyltetracyanobutadiene) and tris[1-azulenylbis(tetracyanobutadiene)] chromophores in excellent yields. The redox behavior of the tetracyanobutadiene (TCBD) 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.

Aryl-substituted 1,1,4,4-tetracyano-1,3-butadienes (FcTCBDs) and bis(1,1,4,4-tetracyanobutadiene)s (bis-FcTCBDs), possessing a ferrocenyl group on each terminal, were prepared by the reaction of a variety of alkynes with tetracyanoethylene (TCNE) in a [2+2] cycloaddition reaction, followed by retro-electrocyclization of the initially formed [2+2] cycloadducts (i.e., cyclobutene derivatives). The characteristic intramolecular charge transfer (ICT) between the donor (ferrocene) and acceptor (TCBD) moieties were investigated by using UV/Vis spectroscopy. The redox behaviors of FcTCBDs and bis-FcTCBDs were examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their properties of multi-electron transfer depending on the number of ferrocene and TCBD moieties. Moreover, significant color changes were observed by visible spectroscopy under the electrochemical reduction conditions.

1,3-Bis(azulenylethynyl)azulene derivatives 9–14 have been prepared by palladium-catalyzed alkynylation of 1-ethynylazulene 8 with 1,3-diiodoazulene 1 or 1,3-diethynylazulene 2 with the corresponding haloazulenes 3–7 under Sonogashira–Hagihara conditions. Bis(alkynes) 9–14 reacted with tetracyanoethylene (TCNE) in a formal [2+2] cycloaddition–retroelectrocyclization reaction to afford the corresponding new bis(tetracyanobutadiene)s (bis(TCBDs)) 15–20 in excellent yields. The redox behavior of bis(TCBD)s 15–20 was examined by using CV and differential pulse voltammetry (DPV), which revealed their reversible multistage reduction properties under the electrochemical conditions. Moreover, a significant color change of alkynes 9–14 and TCBDs 15–20 was observed by visible spectroscopy under the electrochemical reduction conditions.

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.

Ene–diyne systems possessing 1- and 2-azulenyl groups at the periphery were prepared by palladium-catalyzed cross-coupling reaction of 1- and 2-ethynylazulenes with 9-dibromomethylene-9H-fluorene and 9,10-bis(dibromomethylene)-9,10-dihydroanthracene or 2-iodoazulene with 9,10-bis(diethynylmethylene)-9,10-dihydroanthracene under Sonogashira–Hagihara conditions. The redox behavior of the ene–diyne compounds was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Moreover, a significant color change of the ene–diyne derivatives was observed with visible spectroscopy under electrochemical reduction conditions.

1-, 2-, and 6-(Ferrocenylethynyl)azulene derivatives 10–16 have been prepared by palladium-catalyzed alkynylation of ethynylferrocene with the corresponding haloazulenes under Sonogashira–Hagihara conditions. Compounds 10–16 reacted with tetracyanoethylene (TCNE) in a [2+2] cycloaddition–cycloreversion reaction to afford the corresponding 2-azulenyl-1,1,4,4,-tetracyano-3-ferrocenyl-1,3-butadiene chromophores 17–23 in excellent yields. The redox behavior of the novel azulene chromophores 17–23 was examined by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their multistep electrochemical reduction properties. Moreover, a significant color change was observed by visible spectroscopy under electrochemical reduction conditions.

An efficient synthesis of 1-heteroaryl- and 1,3-bis(heteroaryl)azulenes was established by electrophilic substitution. The reaction of azulenes and 1,1′-biazulene with the triflate of N-containing heteroarenes proceeded smoothly in the presence of excess heteroarenes to afford the corresponding 1-dihydroheteroaryl- and 1,3-bis(dihydroheteroaryl)azulene derivatives in good yields. The 1-dihydroheteroaryl- and 1,3-bis(dihydroheteroaryl)azulene derivatives were readily converted into the desired 1-heteroaryl- and 1,3-bis(heteroaryl)azulenes by treatment with tBuOK in DMSO or KOH in EtOH in excellent yields. The pyridinium salts 26⁺·I^– and 27²⁺·2I^– were also prepared by the reaction of 4a and 5a with methyl iodide. 1-Heteroaryl- and 1,3-bis(heteroaryl)azulenes exhibited a significant color change in acetic acid from that in dichloromethane due to the development of intramolecular charge-transfer (CT) absorption bands.

The ene–diyne systems 1 and 2, possessing ferrocenyl groups at the periphery, were prepared by a simple one-pot Sonogashira–Hagihara coupling reaction of ethynylferrocene with 9-dibromomethylene-9H-fluorene (4) and 9,10-bis(dibromomethylene)-9,10-dihydroanthracene (5). Ene–diyne 1 reacted with tetracyanoethylene (TCNE) in a formal [2+2] cycloaddition reaction, followed by ring opening of the initially formed [2+2] cycloadduct, cyclobutene, to afford the corresponding 1,1,4,4-tetracyanobutadiene (TCBD) derivative 6 in good yield. The redox behavior of the ene–diyne compounds 1 and 2, and the TCBD derivative 6 was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their electrochemical oxidation properties with multi-electron transfer depending on the number of ferrocene units in the molecule, in addition to the two-electron reduction of the TCBD moiety in the case of TCBD derivative 6. Moreover, a significant color change was observed by visible spectroscopy under electrochemical oxidation conditions and under reduction conditions in the case of TCBD derivative 6.

Novel bis(3-methylthio-1-azulenyl)methyl cations 3a⁺ and 3b⁺ and dications 4a²⁺ and 4b²⁺ were synthesized by thehydride abstraction reaction of the corresponding hydride derivatives 7a, 7b, 8a, and 8b, which were readily prepared by the acid-catalyzed condensation reaction of 6a and 6b with the corresponding aldehyde and dialdehyde with DDQ. The pK_R⁺ values of novel cations 3a⁺ and 3b⁺ and dications 4a²⁺ and 4b²⁺ were determined spectrophotometrically to examine the thermodynamic stability of these carbocations. The redox behavior of the mono- and dications was also examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their reversible electrochemical reduction properties, except for the case of dication 4b²⁺. Moreover, a significant color change was observed by visible spectroscopy under electrochemical reduction conditions. Dications 4a²⁺ and 4b²⁺ were also chemically reduced by Zn powder to afford thienoquinoid compounds 9a and 9b, respectively.

Ethynylated 2H-cyclohepta[b]furan-2-ones 5–15 have been prepared by Pd-catalyzed alkynylation of 3-iodo-5-isopropyl-2H-cyclohepta[b]furan-2-one (2) with the corresponding ethynylarenes or the reaction of 2-iodothiophene with 3-ethynyl-5-isopropyl-2H-cyclohepta[b]furan-2-one (4) under Sonogashira–Hagihara conditions. Compounds 5–15 reacted with tetracyanoethylene in a formal [2+2] cycloaddition reaction, followed by ring opening of the initially formed [2+2] cycloadducts, cyclobutenes, to afford the corresponding 1,1,4,4-tetracyanobutadienyl (TCBD) chromophores 16–26 in excellent yields. The intramolecular charge-transfer interactions between the 2H-cyclohepta[b]furan-2-one ring and TCBD acceptor moiety were investigated by UV/Vis spectroscopy and theoretical calculations. The redox behavior of the novel TCBD derivatives 16–26 was examined by cyclic voltammetry and differential pulse voltammetry, which revealed multistep electrochemical reduction properties, depending on the number of TCBD units in the molecule. Moreover, a significant color change was observed by UV/Vis spectroscopy under electrochemical reduction conditions.

Novel ferrocene-substituted bis(3-methylthio-1-azulenyl)methylium ions 5⁺ and 6⁺ were synthesized by the hydride abstraction reaction of the corresponding hydro derivatives 3 and 4 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Compounds 3 and 4 were readily prepared by the acid-catalyzed condensation reaction of 1-methylthioazulenes 1 and 2 with ferrocenecarbaldehyde. The redox behavior of 5⁺ and 6⁺ was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their amphoteric multistep redox properties. Moreover, these cations exhibited a significant color change under electrochemical reduction conditions, which was revealed by visible spectroscopy.