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.

Introduction Azulene (C 10 H 8 ) is a non-alternant, non-benzenoid, 10 π electron aromatic hydrocarbon containing a fused seven-and fivemembered ring [1][2][3][4][5]. The electron drift from the seven-membered ring to the five-membered ring is responsible for its polarized structure which features both a 6 π electron tropylium cation and a 6 π electron cyclopentadienyl anion in the same molecule ( Figure 1). The striking feature of azulene is its permanent dipole moment (1.08 D) and blue color unlike its colorless isomer naphthalene [4]. Azulene possesses an unequal distribution of electron density between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) resulting in a relatively small electron repulsion energy in the first singlet excited state and thus, a small HOMO-LUMO (S 0 -S 1 ) gap compared to naphthalene. The large energy gap between its S 2 and S 1 states (up to 15000 cm −1 ) makes internal conversion less probable, making azulene emit from the S 2 state, violating Kasha's rule [6]. These intriguing features have encouraged researchers to use azulene derivatives as functional organic molecules in the field of optoelectronics [7][8][9][10][11][12][13][14]. Employing such stimuli-responsive, non-alternant hydrocarbon with oddmembered rings in the chemical synthesis of functional polymers is also an interesting proposition and such polymers can find promising applications in the organic electronics field such as organic field-effect transistors (OFET) and photovoltaic (PV) cells [15,16]. The synthesis of azulene-containing polymers can be envisaged through chemical and electrochemical means. Through electrochemical methods, only the five-membered rings can be incorporated onto the polymer backbone, and often the polymers produced are insoluble [17]. On the other hand, chemical synthesis provides an avenue to synthesize soluble polymers where azulene can be incorporated into the backbone through either of the rings or by involving both the rings. These substitution patterns can influence the property of the resulting polymer. However, to achieve the synthesis of such homo-and copolymers, suitably tailor-made azulene monomers with specific substitution patterns are needed, and often the synthesis of such building blocks is challenging. This could be the reason why the chemical synthesis of azulene-containing polymers is only sporadically investigated. Hence, this article is intended to provide readers an overview of the developments in the area of chemical synthesis of azulene-containing homo-and copolymers that have been achieved during the last three decades.

Review Azulene-containing homopolymers
The polyazulenes The earliest chemical synthesis of polyazulene was reported by Neoh, Kang, and Tan in 1988 [18]. Their strategy was based on the oxidative polymerization of azulene (1) by iodine or bromine to obtain polyazulene-iodine/bromine complexes (PAz-I 2 /PAz-Br 2 ) (Scheme 1).
The PAZ-I 2 complex was found to be insoluble whereas PAZ-Br 2 was sparingly soluble in most of the organic solvents. Although no structure was proposed for these polymer complexes, based on the elemental analysis, the authors provided the Scheme 1: Synthesis of polyazulene-iodine (PAz-I 2 ) and polyazulenebromine (PAz-Br 2 ) complexes. formula representation C 10 H 6 (I 2 ) 0.4 for PAZ-I 2 and C 10 H 4.9 (Br 2 ) 0.72 for PAZ-Br 2 . The gel permeation chromatography (GPC) analysis of the tetrahydrofuran (THF)-soluble fraction of PAZ-Br 2 indicated the presence of oligomeric species (average degree of polymerization of ≈7) leaving speculation about a higher degree of polymerization for the THFinsoluble component of PAZ-Br 2 . The UV-vis spectrum of the chloroform-soluble fraction of PAZ-Br 2 displayed absorption bands in the 190-330 nm region like the azulene monomer, and a long tail with a broad peak around 690 nm supporting the oligomeric nature of the soluble fraction. The thermogravimetric analysis (TG) showed that PAZ-I 2 was thermally more stable than PAZ-Br 2 . The electrical conductivity of PAZ-Br 2 (5 × 10 −3 S/cm) was far too superior compared to PAZ-I 2 (10 −6 S/cm).
In 1997, Kihara, Fukutomi, and Nakayama [19] reported the synthesis of what they described as 'true polyazulene' through a cationic polymerization reaction. Their protocol involved heating the trifluoroacetic acid (TFA) solution of azulene (1) followed by treatment with triethylamine to obtain a brown polymeric product called polyazulene (Scheme 2).
The elemental analysis of this polymer revealed that it existed as a 71:29 mixture of polymer 3 or 3' bearing a heptafulvene structure (true polyazulene) and cycloheptatrienyl trifluoroacetate 2. The temperature and reaction time determined the overall yield and the M n of the polyazulene formed. The maximum yield of 89% and the highest M n = 3600 Da was obtained when azulene was heated at 100 °C for 24 hours in TFA. However, changing the reaction medium from TFA to acetic acid or methane/trifluoromethane sulfonic acid did not facilitate the polyazulene formation. The polyazulene 3 or 3' was soluble in various organic solvents such as toluene, dichloromethane, tetrahydrofuran (THF), and N,N'-dimethylformamide (DMF). The 'true polyazulene' exhibited the conductivity of 5.38 × 10 −8 S/cm, which was increased to 8.16 × 10 −3 S/cm upon exposure to iodine atmosphere, presumably due to the formation of dehydropolyazulene via oxidative aromatization.
The polymer was partially soluble in many organic solvents such as chloroform, THF, xylenes, DMF, N-methylpyrrolidone (NMP), and also in Brønsted acids like TFA and conc. H 2 SO 4 . The molecular weight (M n ) and polydispersity (PD) of 1,3polyazulene 5 were 16,400 and 1.15, respectively, as determined by GPC in THF. The presence of resonance signals in the region δ 7.1-8.6 ppm of the 1 H NMR spectrum and the close IR spectral resemblance to azulene confirmed the integrity of the azulene units in 1,3-polyazulene 5. The robustness of the 1,3polyazulene backbone was evidenced by TG analysis where it retained over 60% of its mass even after heating to 1000 °C. The absorption spectrum recorded in chloroform revealed a considerable red shift in the absorption band of 1,3-polyazulene 5 (417 nm) compared to azulene (1, 341 nm) supporting the presence of extended conjugation prevailing in the polymer. The absorption and EPR spectral patterns of 1,3-polyazulene 5 recorded in TFA and H 2 SO 4 were contrasting to each other inferring the varied degree of protonation caused by these acids on the polymer backbone. The direct current (DC) conductivity of protonated and iodine-doped 1,3-polyazulene 5 (0.74 and 1.22 S/cm respectively) was significantly higher than its neutral form (<10 −11 S/cm). The increased conductivity of 1,3-polyazulene 5 upon protonation can be attributed to the better stabiliza-tion of cation radicals, di-, and polycations by the unique dipolar nature of azulene, and in the case of iodine doping, it was attributed to the strengthened spin-spin interaction arising due to a high radical concentration. Recently, it was also established that the water-dispersible poly(1,3-azulene)-polystyrenesulfonate (PSS) polymer can display an overall conductivity up to 0.1 S/cm under ambient conditions and an ion Seebeck coefficient value as high as 4.5 mV K −1 [21].
The majority of these polymers were soluble in common organic solvents and the GPC data obtained in chloroform showed their M n to be in the range of 2.4 to 7.2 KDa. These polymers possessed high thermal stability as well. The absorption maxima displayed by polymer 18 (409 nm) in dichloromethane solution was comparable to the above-discussed 1,3polyazulene 5 (404 nm), however, the long alkyl substituents present at the 6-position of 19 and 20 were disrupting the effective π-conjugation along the polymer backbone, resulting in the blue shift of absorption bands (λ max around 350 nm). Interestingly, the ethynyl linker in 17 was assisting the effective delocalization despite the presence of the n-alkyl group at position 6, as evidenced by its large red-shifted absorption at 456 nm compared to 19. The protonation of 17 and 18 by TFA resulted in a large red-shifted broad absorption band in the 500-900 nm region due to the formation of azulenium cations in the polymer backbone unlike the 1,3-polyazulene 5, which had no significant effect on its absorption spectrum upon protonation. The HOMO-LUMO gap for the protonated 17 (1.32 eV) was also reduced compared to its neutral form (1.49 eV). These polymers, unlike 1,3-polyazulene 5, displayed reversible acid-base chemistry. With the help of absorption spectroscopy and EPR analysis, the authors could establish the fact that effective stabilization of 6 π-electron tropylium cations can be achieved by incorporating the seven-membered rings of azulene in the polymer backbone. The enhanced stability of these 4,7-polyazulenes 17-20 in acids and their stimuli-responsible behavior make them potential materials for photonic device applications.
Poly [2,6-aminoazulene] In order to construct polyazulenes with the head-to-tail alignment of dipoles, azulene should be functionalized at the 2,6-positions, which are diagonally opposite to each other. This will lead to an alternate arrangement of seven and five-membered rings along the polymer backbone and can facilitate effective electron delocalization along the C 2 axis (C 2v symmetry) passing through 2,6-positions of azulene [23][24][25]. The first chemical synthesis of such polyazulene called poly[2,6aminoazulene] 31 was reported by Luh and co-workers [26] in 2017 and achieved by using the Buchwald-Hartwig reaction protocol (Scheme 7). The synthesis of the key building blocks 2-aminoazulene (24), 2-amino-6-bromoazulene (26), and its corresponding carbamate, tert-butyl N-(6-bromoazulen-2yl)carbamate (27), used in their synthesis is presented in Scheme 7A. The carbamate 27 was subjected to a Buchwald-Hartwig reaction by using [Pd(allyl)Cl] 2 and Jack-iePhos as a ligand to obtain the polymer 30 in 52% yield. The deprotection of the N-Boc functionality led to the formation of poly[2,6-aminoazulene] 31 in excellent yields (Scheme 7C).
The N-Boc-protected polymer 30 possessed good solubility in organic solvents and its M n and PDI were 2900 Da and 1.22, respectively. The aminoazulene dimer 29 was also synthesized (Scheme 7B) in this study as a reference compound. When  compared to the dimeric aminoazulene 29 (481 nm) and 2-aminoazulene (24, 394 nm), the polyaminoazulene 31 exhibited a far red-shifted broad absorption band spanning 400-800 nm in NMP solution with a maximum at 591 nm. Accordingly, the optical energy gap for the polymer 31 was the lowest (1.65 eV) in comparison to dimer 29 (2.15 eV) and monomer, 2-aminoazulene (24, 3.15 eV) emphasizing the advantage associated with inducing a 2,6-connectivity along the polymer backbone.
The iminium zwitterion resonance structure ( Figure 2) can contribute in extending the conjugation along the backbone of poly[2(6)-aminoazulene] 31 and this property of 31 makes it very different from the well-known polyaniline. The absorption peak positions of the protonated forms of monomer 24, dimer 29, and polyazulene 31 were further red-shifted and their respective energy gaps were also reduced compared to their neutral forms. The oxidation potential for the neutral (−0.23 V) and protonated forms of polyaminoazulene 31 (0.70 V) was significantly different unlike the case of protonated emeraldine. The Nafion membrane doped with protonated polyaminoazulene 31 displayed good proton conductivity and reduced methanol permeability, making it suitable for methanol fuel cell applications.
These polymers possessed good thermal stability (T d > 400 °C) and most of them were soluble in organic solvents such as THF, chloroform, dichloromethane, xylene, and toluene. The numberaverage molecular weights (M n ) of polymers 33-38 were in the range 16,000 to 41,000 Da (determined by GPC in THF) and the degree of polymerization was 40-60. The conjugation between the azulene-bithiophene units along the polymer backbone resulted in the red-shifted absorption bands compared to their corresponding monomers and, upon protonation, the absorption bands were even extended to the NIR region [29]. The conductivity studies performed on either iodine-doped (p-doping or oxidative doping) or protonated polymers (by using TFA) showed higher conductivities in comparison to most of the poly(thiophene-arylene) copolymers [30,31], emphasizing the role of the azulene units in stabilizing the polarons or bipolarons formed during the doping process. In particular, among the iodine-doped polymers, 38 displayed a maximum conductivity of 2.23 S/cm with 45% iodine uptake, and the protonated polymer 36 recorded a conductivity value of 1.10 S/cm. The electrochemical impedance spectroscopy (EIS) studies revealed that these polymers could be used as coatings in corrosion control applications as their conductivity was significantly enhanced (10 3 -10 4 -fold) when kept in contact with the aqueous acid solution.
Further, the coordination of polymers 37 and 39 to a multinuclear Ru cluster was investigated by the same group [32]. The organometallic complexes 40-43 were synthesized by treating polymers 37 and 39 with Ru 3 (CO) 12 in refluxing xylene (Scheme 9).
The ruthenium content in these complexes can be varied by changing the ratio of reactants during the reaction. The 1 H NMR chemical shift values of the azulene unit were used as a tool to determine the extent of ruthenium coordination to the azulene units in the polymer backbone as the coordinated azulene displays upfield-shifted resonance signals when compared to free azulenes. The absorption and electrochemical studies conducted on these complexes 40-43 revealed that their properties could be tuned by varying the ruthenium content in the polymer. A higher ruthenium content was inducing a larger blue-shift of the absorption bands and larger cathodic shift of the oxidation potential in these polymers compared to their metal-free counterparts.
The M n and PDI for polymer 45 were 5100 Da and 1.4, respectively and its thermal stability was excellent (T d = 404 °C). Polymer 45 exhibited reversible acid-base response, which was evident from its absorption and EPR studies. The optical HOMO-LUMO gap for 45 was smaller in the protonated form (1.50 eV) compared to its neutral form (1.62 eV).
In 2014, Wang, He, and co-workers [33,34]  These polymers 50, 52, 54, and 56 displayed good solubility in most of the commonly used organic solvents and GPC analysis revealed their number-average molecular weight (M n ) to be in the range of 25800 to 48600 Da with polydispersity in the range 1.29-1.9. They also exhibited excellent thermal stability as indicated by their decomposition temperature, which was over 350 °C. The polymers 50, 54, and 56 exhibited absorption bands in the 400-450 nm range due to π-π* transition. However, polymer 52, which contains a benzothiadiazole ring was an exception, as it showed a highly red-shifted band at 550 nm. The important feature of these polymers worth highlighting here is the gradual bleaching of bands due to π-π* transition and display of absorption bands in the near-IR region upon protonation with TFA. The protonated forms of these polymers displayed absorption in the region 1900-2500 nm, i.e., almost covering the entire near-IR region of the spectrum, the largest shift of 2500 nm was displayed by the polymer 56. The origin of such absorption bands in the near-IR region can be attributed to an intramolecular charge transfer (ICT) process leading to the Scheme 12: Synthesis of azulene-benzodithiophene copolymer 54 and azulene-bithiophene copolymer 56.
lowering of the HOMO-LUMO bandgap [34]. The azulene moiety plays a pivotal role here as it is polar resonance stabilized and protonation leads to a drift in its electron density from the seven-membered ring to the five-membered ring generating a stable tropylium cation making it a strong electron acceptor during the intramolecular charge transfer (ICT) process. These protonated polymers were very stable in solution for a long period of time. All these polymers exhibited good chemical and electrochemical stability and easy film-forming properties, important attributes for fabricating near-IR-based optoelectronic devices.
The final composition of the 1,3-and 4,7-regioisomers of azulene in the polymer chain was determined by 1 H NMR spectroscopy. All these polymers 61-65 were soluble in common organic solvents like THF, dichloromethane, and chloroform and had M n in the range 6000 to 21000 Da with PDI of 1.3-1.6. The polymers 61-65 displayed stimuliresponsive behavior as evidenced by shifting of their absorption bands from the UV region to the near-IR region upon protonation. This property was dependent on the composition of the azulene units and their connection pattern along the polymer backbone. For example, the protonated polymer 62 with 71% 1,3-disubstituted azulene displayed the highest absorption in the near-IR region. Also, the protonated polymer 61 showed strong fluorescence at 480 nm with a large Stokes shift of 155 nm.
In 2015, Zhang, Liu, and co-workers [36]  positions are synthesized in decent yields by using appropriately functionalized azulene and DPP monomers under Suzuki reaction conditions as delineated in Scheme 15.
All these polymers were soluble in organic solvents such as toluene, chloroform, tetrachloroethane, and the M w for 69, 71, and 72 were 41700, 38100, and 49400 Da with PDI being 3.4, 2.7, and 3.3, respectively. The polymers were thermally stable with T d above 300 °C. The absorption spectra of 69, 71, and 72 in chloroform displayed strong absorption at 667, 670, and 627 nm, and in the thin-film form they were further red-shifted. The optical energy gaps for 69, 71, and 72 were 1.33, 1.38, and 1.23 eV, respectively. Thin-films of 69 and 71, in which azulene was incorporated onto the polymer backbone in a 1,3fashion, exhibited p-type semiconducting behavior, whereas the polymer 72, in which a 4,7-connectivity pattern of azulene is present, was ambipolar with hole and electron mobilities of 0.062 and 0.021 cm 2 V −1 s −1 , respectively. The polymer 69 behaved like an electron donor for organic PV cells and the blend of thin-film 69 with PC 71 BM showed a power conversion efficiency (PCE) of 2.04%.
The solubility of all polymers was good in organic solvents like THF, dichloromethane, chloroform, and toluene, and the thermal data reflected on their high thermal stability (T d 408-434 °C). The number average molecular weight M n (GPC in THF) and polydispersity of these polymers were in the range of 6800-12500 Da and 1.29-2.21, respectively. The UV-vis absorption features of polymers 115 and 116 were altered upon protonation due to the formation of azulenium cations in the polymer backbone and they displayed a significant color change on protonation. None of these polymers were fluorescent in the neutral form, however, 115 and 116 displayed fluorescence emission at 385 nm in the protonated state. The electrochemical bandgap of these polymers was in the impressive range of 1.57-1.62 eV.
As stated above in the case of azulene-thiophene polymers 61-65, the final composition of 1,3-and 4,7-regioisomers of azulene in the polymer chain was determined by 1 H NMR spectroscopy. All these polymers were soluble in common organic solvents like THF, dichloromethane, and chloroform, and had M n in the range 9800 to 34300 Da with PDI of 1.6-2. terial to make these polymers was 2,6-dibromoazulene (125), which was synthesized from tropolone (21) in five steps (Scheme 21A). The Suzuki-Miyaura coupling reaction of 2,6-dibromoazulene (125) with 2,2'- (9,9- The two polymers 126 and 129 possessed good solubility in organic solvents and high thermal stability (T d : 418 and 432 °C for 126 and 129, respectively in a nitrogen atmosphere). The M n and PDI for 126 was 40400 Da and 2.08 and for 129 58300 Da and 1.73, respectively. The response to protonation (TFA) of these polymers was noteworthy as they exhibited rapid and reversible color changes both in solution and thin-film state. The direct current (DC) conductivity values recorded for the thin films of the protonated polymers 126 and 129 were 2.94 and 0.32 S/cm, respectively, far larger than the value noticed for its protonated 1,3-connected counterpart (10 −3 S/cm). This is presumably due to the ease of protonation of the 5-membered ring of azulene rings in the polymer as they have vacant 1,3-positions available for protonation.
Xu and Png [38], along with the above-mentioned poly(2arylazulene-alt-thiophene) 99-101, also reported the synthesis of the next generation of azulene-fluorene conjugated polymers, The polymers 136-138 were of relatively high molecular weight (M n in the range 17000-30900 Da) with PDI 1.45-2.03 and had a good thermal stability (T d > 340 °C in N 2 ). The solution-state absorption profiles (in THF solution) of these polymers displayed two bands in the region 328-349 and 392-407 nm and these peak positions were red-shifted in comparison to azulene-fluorene polymers because of the electron transfer from the azulene to benzothiadiazole units. The optical (2.19-2.38 eV) and electrochemical (2.25-2.40 eV) band gaps for the polymers 136-138 were in good agreement with each other and the gap decreased with an increase in the percentage of azulene in the polymer backbone. The thin films of these polymers exhibited electrochromism, where the color changed from yellowish green (neutral and reduced state) to greyish brown (oxidized state) with electrochromic contrasts of 17 and 13% for polymers 137 and 138, respectively, in the NIR region. In addition, the azulene-2,1,3-benzothiadiazole containing D-A-type copolymers have also been used in photovoltaic device applications [43].
These polymers exhibited M n in the range 4200-7200 Da with PDI 1.14-1.38. The oxidation potential of the benzothiadiazole-containing polymers 141-144 was low compared to all-azulenecarbazole polymer 140 due to the electron transfer from azulene to benzothiadiazole and, due to this, they exhibited better electrochromism. An electrochromic device (ECD) constructed with polymer 143 exhibited black to transmissive electrochromism with high contrast.

Azulene-methacrylate copolymers
Emrick and co-workers [45] reported the synthesis of azulenesubstituted methacrylate polymers derived from a free radical polymerization strategy, where azulenes were used as pendants.
The monomers 146 and 150 were then subjected to free radical polymerization by using azobis(isobutyronitrile) (AIBN) to obtain the polymers 151 and 152 in 73 and 82% yields, respectively (Scheme 26A and B). The M n and PDI for these polymers 151 and 152 were 13500 Da, 2.5 and 13600 Da, 2.2, respectively, and their solubility was good in organic solvents. In  The M n for the 154 and 155 series was in the range of 13900-22000 Da with a PDI range of 1.7-2.6. Likewise, for the 157 and 158 series the M n range was 18900-38900 Da with PDI 1.5-1.7. The composition of azulene in the polymers 154 and 155 was 20-40 mol % azulene, whereas the polymers 157 and 158 were containing 25-75 mol % azulene. The photophysical studies carried out on the neutral/protonated polymer series 155 and 158 were supportive of the fact that the optoelectronic property, electronic interactions, and exciton migration can be tuned by varying the density of pendant azulene units along the polymer backbone. The polymers with increasing density of pendant azulene in the backbone displayed improved device performance in comparison to the control devices featuring only poly(sulfobetainemethacrylate) interlayers. The polymer 158 containing the highest azulene density (75 mol % azulene) was found to be an effective cathode modification layer in a bulkheterojunction solar cell with a power conversion efficiency of 7.9%.

Conclusion
This review has described the chemical syntheses and key features of azulene-containing polymers reported in the last three decades. Azulene-containing homopolymers (polyazulenes) and copolymers incorporating thiophene, fluorene, benzothiadiazole, and carbazole units along the polymer backbone can be synthesized by utilizing cross-coupling strategies such as Suzuki, Sonogashira, Stille, Yamamoto, and Buchwald-Hartwig reactions. Azulene can be incorporated onto the polymer backbone by involving its five-membered rings via a 1,3-fashion, the seven-membered rings via a 4,7-fashion and, both the rings can be incorporated via 2,6-fashion, and these patterns can influence the properties of the resulting polymer. By and large, the reported azulene-containing homo-and copolymers have proven to be efficient functional materials for organic field-effect transistor (OFET), all-polymer solar cell (PSC) applications, and they can also exhibit NIR absorption and electrochromism, and electrical conductivity. However, the reports on polymers containing azulene with a 2,6-connectivity pattern are scarce in the literature. As this mode of connectivity pattern has the advantage of involving both the dipoles of azulene onto the polymer backbone, warrants further investigation. The reported 2,6-polyaminoazulene 31 behaved distinctively to polyanilines and can be a useful proton-conducting membrane material in methanol fuel cells. Also, as azulenes are known to contravene Kasha's rule, it is intriguing to explore, how their presence in the polymers can favor the energy utilization from higher excited states. This may lead to a new dimension in photoluminescent materials research. It can be concluded that the research field of azulene-containing functional polymers is still in its infancy and there is a lot of scope for chemists and material scientists to design and chemically synthesize exotic azulene-containing polymers having improved characteristics. The author hopes that this review might stimulate such research efforts in the future.