Chemical syntheses and salient features of azulene-containing homo- and copolymers

Azulene is a non-alternant, aromatic hydrocarbon with many exciting characteristics such as having a dipole moment, bright color, stimuli responsiveness, anti-Kasha photophysics, and a small HOMO-LUMO gap when compared to its isomer, naphthalene. These properties make azulene-containing polymers an intriguing entity in the field of functional polymers, especially for organic electronic applications like organic field-effect transistors (OFET) and photovoltaic (PV) cells. Since azulene has a fused five and seven-membered ring structure, it can be incorporated onto the polymer backbone through either of these rings or by involving both the rings. These azulene-connection patterns can influence the properties of the resulting polymers and the chemical synthesis in comparison to the electrochemical synthesis can be advantageous in obtaining desired patterns of substitution. Hence, this review article presents a comprehensive overview of the developments that have taken place in the last three decades in the field of chemical syntheses of azulene-containing homo- and copolymers, including brief descriptions of their key properties.

Synthesis and Halochromic Properties of 1,2,6-Tri- and 1,2,3,6-Tetra-aryl Azulenes

A series of novel 2,6-functionalized azulene molecules Azu1-3 with varied fluorene substituents at the 1- and 3-positions of azulene as well as at the 5'-position of 2-thiophene group were synthesized. Their electronic absorption and emission spectra at neutral and protonated states were examined. It was found that after functionalization with fluorenyl groups, Azu1-3 exhibited absorption maxima at 445, 451 to 468 nm, respectively. In contrast, their corresponding protonated species showed much redshifted absorption maxima at 560, 582 to 643 nm, respectively, mainly due to the extension of conjugation length and the large dipole moment along the C_2v axis of 2,6-substituted azulene molecules. Azu1-3 are non-fluorescent in their neutral forms, but became emissive in their protonated states. Analysis of absorption and emission spectra shows that substitution of the 1- or 3-position of azulene led to decrease in response to trifluoroacetic acid.

Water-dispersible conducting polyazulene and its application in thermoelectrics

We report on the facile synthesis of a water-dispersible conducting polymer, polyazulene-polystyrenesulfonate (PAZ:PSS), using PSS as a template. PAZ:PSS can achieve an overall conductivity up to 0.1 S cm^-1, and an ion Seebeck coefficient as high as 4500 μV K^-1, demonstrating that PAZ:PSS is another promising water-dispersible polymer for thermoelectrics.

Azulene in Polymers and Their Properties

Azulene, a unique isomer of naphthalene, has received much interest from researchers in different fields due to its unusual chemical structure with a negatively charged 5-membered ring fused with a positively charged 7-membered ring. In particular, incorporation of azulene into polymers has led to many interesting properties. This minireview covers functionalization methods of azulene at its various positions of 5- and 7-membered rings to form azulene derivatives including azulene monomers, and gives an overview of a wide range of azulene-containing polymers including poly(1,3-azulene), azulene-based copolymers with connectivity at 1,3-positions of the 5-membered ring, or 4,7-positions of the 7-membered ring, as well as copolymers with azulene units as side chains. Their chemical and physical properties together with applications of azulene-containing polymers have also been summarized.

Copolymers Based on 1,3-Bis(carbazol-9-yl)benzene and Three 3,4-Ethylenedioxythiophene Derivatives as Potential Anodically Coloring Copolymers in High-Contrast Electrochromic Devices

In this study, copolymers based on 1,3-bis(carbazol-9-yl)benzene (BCz) and three 3,4-ethylenedioxythiophene derivatives (3,4-ethylenedioxythiophene (EDOT), 3,4-(2,2-dimethylpropylenedioxy)thiophene (ProDOT-Me₂), and 3,4-ethylenedithiathiophene (EDTT)) were electrochemically synthesized and their electrochemical and electrochromic properties were characterized. The anodic copolymer P(BCz-co-ProDOT) with BCz/ProDOT-Me₂ = 1/1 feed molar ratio showed high optical contrast (ΔT%) and coloring efficiency (η), measured as 52.5% and 153.5 cm²∙C^-1 at 748 nm, respectively. Electrochromic devices (ECDs) based on P(BCz-co-EDOT), P(BCz-co-ProDOT), and P(BCz-co-EDTT) as anodic polymer layers, and poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) (PEDOT-PSS) as cathodic polymer layer were fabricated. P(BCz-co-ProDOT)/triple-layer PEDOT-PSS ECD showed three different colors (light yellow, yellowish-blue, and dark blue) at different applied potentials. In addition, the highest optical contrast (ΔT%) of P(BCz-co-ProDOT)/triple-layer PEDOT-PSS ECD was found to be 41% at 642 nm and the coloration efficiency was calculated to be 416.5 cm²∙C^-1 at 642 nm. All ECDs showed satisfactory optical memories and electrochemical cyclic stability.

Diketopyrrolopyrrole-Based Low-Bandgap Conjugated Polymers with Siloxane Side Chains for Electrochromic Applications

A series of conjugated copolymers P1–P3 were synthesized from 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(6-(1,1,1,3,5,5,5-heptamethyltrisiloxan-3-yl)hexyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione and 2,5-bis(trimethylstannyl)-3,4-dialkoxythiophene or its analogue via Stille coupling reactions, with molecular weights in the range of 13000–18000 g mol–1 and polydispersity indexes of 1.4–1.7. This new type of donor–acceptor polymers demonstrated reasonable switching speed, promising redox stability, together with high optical contrast and coloration efficiency. Although, at the current stage, the hybrid siloxane-terminated side chain did not significantly improve the overall performance of the resultant polymers, the unique chemical properties of the siloxane group would offer possibilities for crosslink after suitable post-polymerization treatment, and thus pave the way for future fine-tuning of the morphology of electrochromic films.