Structures, properties, and applications of soluble polyazulene and azulene containing copolymers 4

When the azulene was inserted into the polymer backbone via 1,3-position, and after oxidation or protonation, the azulenium cation formed at the seven-membered ring and the radicals wer

radicals have been reported: (1) triarylmethyl radical, R1; (2) a per-pyridinium substituted allyl radical, R2; (3) cyclopentadienyl radical, R3; and (4)

dodecamethylcarba-closo-dodecaboranyl radical (CB11-Me12•) From analysis of these stable radical structures, we can conclude that all of these radicals are strongly stabilized thermodynamically by effective delocalization of the unpaired electron, and kinetically

by the bulky substituents.5,6

synthesis and properties of conjugated polyradicals, based on the hope that such systems can eventually be utilized as components of designable magnetic materials Numerous π-conjugated polymer substitutes with pendant radicals have been synthesized and characterized.8-11 For example, Rajca et al have created an elegant body of work on

backbone-conjugated and dendritic polyradicals, based on triarylmethyl radicals.12,13Dougherty and colleagues have also worked on the synthesis and characterization of doped plaronic conjugated materials with ferromagnetic (FM) and high spin-coupling.14,15Various researchers have synthesized conjugated polymers employing pendant polynitroxide, polynitronylnitroxide, and polyphenoxyl types of spin-bearing units.16-18 However, most of the polyradicals are designed with the radical spin sites as pendant groups It is often difficult in finding a suitable radical center that can be conjugated and linked to the polymer backbone

In Chapter 3, when we studied the electronic properties of our copolymers, we found that they formed highly stable cation radicals, either by iodine doping or TFA protonation In the solid state, a protonated copolymer film showed no significant change in the EPR signal, even after 2 weeks Furthermore, the stability was also investigated in detail using the nitrogen and oxygen permeation test The high stability was attributed to the stability

of the azulenium ion and the delocalization of the electrons along the conjugated polymer backbone

These results tell us that azulene is possibly a suitable radical center that can be linked into the conjugated polymers to form new polyradical systems This is because azulene displays many of the criteria essential for the formation of a stable cation radical For one, it has an asymmetric charge distribution It has a tendency to stabilize cations, as well as anions, due to its remarkable polarizability.19 Secondly, azulene and their

alkylated derivatives are quite unique in that the ongoing disruption of aromaticity from the neutral to the charged state is counterbalanced by the gain in resonance energy upon formation of azulenylium carbocation, a 6π-electron aromatic tropylium analogue (Figure 7-1).20 Thus, azulene appears to be an extremely novel and versatile system with regard

to radical centers

Figure 7-1 Resonance forms of azulenium carbocation

When the azulene was inserted into the polymer backbone via 1,3-position, and after

oxidation or protonation, the azulenium cation formed at the seven-membered ring and the radicals were formed on the five-membered ring that is linked to the conjugated polymer backbone Thus the un-paired electron can be mediated in a vertical by the aromatic tropylium cation; what is more, the un-paired electrons can also be delocalized along the conjugated polymer backbone This concept is illustrated clearly in Scheme 7-

of Electrons

Scheme 7-1 Illustration of the design of the stable polyradicals

Based on this analysis, we designed a conjugated polymer system containing azulene in the polymer backbone, and used a 1,3-conjugation (Figure 7-2) to demonstrate the

concept The aims of the present study were to prepare a highly stable polyradical system and study their interesting properties

R R

R R

R R

R R

R R

R R

Ox [H + ]

R = C8H17; OC8H17

Figure 7-2 The design of the high stable polyradical system based on the conjugated copolymers

containing azulene moiety

Results and Discussion

Monomer synthesis and characterization of the cation radical

To understand better of the formation of the cation radicals, the model compound, diphenylazulene, was first prepared 1,3-Diphenyl-azulene was synthesized following the general Grignard reaction (Scheme 7-1) One equiv of 1,3-dibromoazulene was reacted with 2.5 equiv of Grignard reagent of bromobenzene in anhydrous ether, yielding the green oil product, which solidified after long-term storage

The monomer 1,3-diphenylazulene was then characterized by NMR and FT-IR, then the formation of the azulenium cation radical was confirmed by respective HNMR and EPR spectrum

of TFA, an important feature noted was the formation of new peaks in the low field As shown in Figure 7-3b, two new peaks appeared at 9.08 ppm and 8.92 ppm, which were attributed to the protons 4,8 and 5,7 on the seven-membered ring of the azulenium

cation.22,23 We also found that the compound was partially protonated, as can be seen from the peaks at 8.7 ppm and 7.2 ppm that were attributed to the H4,8 and H5,7 of the neutral azulene From from the integration, about 30% protonation was found This conclusion was confirmed by further EPR and UV-vis studies Additionally, the HNMR spectrum of the protnated 1,3-diphenyl-azulene also suggests that the protonation mainly occurred on the azulene ring, as there was little change in the peaks belonging to the benzene ring (Figure 7-3b)

The UV-vis spectra of 1,3-diphenyl-azulene showed an maximum absorption wavelength (λmax) of 387 nm (in chloroform), suggesting a developed π-conjugation between the azulene and benzene rings Upon addition of TFA to the solution, two new peaks appear around 466 nm and 353 nm suggesting the formation of the azulenium cations.24 On further protonation, the new bands had a gradual increase, accompanied with a decrement

of the bands at 300 nm The color of 1,3-diphenyl-azulene solution changed from yellowish-green to brown Comparing the absorption intensity changes of the new peaks with the absorption band at 300 nm, we can conclude that the compound is partially protonated This is in agreement with what we observed in the HNMR study

0 1 2

3

TFA concentration 40%

400 600 800 0

1 2 3 4

Figure 7-4 Continuous change in UV-vis spectrum of 1,3-diphenylazulene (a), with different

TFA concentration; (b) in 10% TFA concentration with different time

Figure 7-4a also displays that the protonation can be saturated with 20% TFA solution This result may suggest that 1,3-diphenyl-azulene is easily protonated by TFA, even at low TFA concentrations At higher TFA concentrations, there is little change of the spectrum However, when we follow the UV-vis spectrum change of 1,3-diphenyl-azulene in 10% TFA concentration at different storage times, we found a kinetic behavior with the protonation process With an increase in storage time of 10% TFA/1,3-diphenyl-azulene solution, an increasing amount of 1,3-diphenyl-azulene was protonated However, the spectrum changed faster in the first 30 mins, whereas the protonation rate slowedafter 30 mins It is also interesting to note that after 2 days of storage at room temperature, there is no other change of the spectrum except in the increase of intensity

of the longest wavelength absorption This result indicates that the azulenium cation radical is very stable

The formation of the azulenium cation radical was further confirmed by EPR experiments A solution of 1,3-diphenyl-azulene in 10% TFA solution (in chloroform)

reveals a symmetrical singlet absorption with a g factor (2.0020) near to that of free

electrons, having a line width of 3.2 G The EPR signal intensity change of the compound

in 10% TFA solution with time is similar to that which we observed in the UV-vis spectra study The stability of the cation radical was measured by testing the EPR absorption intensity change after leaving the 1,3-diphenyl-azulene solution in 10% TFA, at atmosphere for 2 days The EPR spectrum showed an increase in the intensity of the EPR

signal, without obvious g value change These results are in good agreement with the

UV-vis spectra study

Polymers synthesis and characterization

The strategy for synthesis the desired polymers is outlined in Scheme 7-2 2,5-di-n-octylbenzene and 1,4-dibromo-2,5-n-octyloxybenzene were used as starting materials and synthesized according to a reference procedure.25 Here, 1,4-dibromo-2,5-di-n-octylbenzene and 1,4-dibromo-2,5-n-octyloxybenzene were reacted with excess n-butylthium at a low temperature, subsequently quenched with trimethyl borate, and then hydrolyzed with hydrochloric acid to produce 2,5-dioctyloxybenzene-1,4-bis(boronic acid), or 2,5-dioctylbenzene-1,4-bis(boronic acid).26 The purity of the di-acid was checked by HNMR spectroscopy and CNMR spectroscopy

1,4-Dibromo-R R

Br Br i

ii

R R (HO) 2 B B(OH) 2

Br Br

iii

R R

Scheme 7-2 The synthesis of PAzBzC8 and PAzBzOC8 by Suzuki coupling

Synthesis of the compolymers (Az-Bz) was carried out by a Suzuki coupling reaction, with Pd(0) catalysis.27-29 The polycondensation of equimolar quantities of the diboronic acid with 1,3-dibromoazulene was performed in a biphasic medium (toluene/aqueous sodium carbonate) solution at 800C, with catalytic amounts of tetrakis-(triphenylphosphine)palladium added The polymers were precipitated by pouring into the organic solution into methanol Purification was carried out by dissolveing the polymer in chloroform and re-precipitating in methanol

GPC analysis showed a number-average molecular weight of 6,900 and 8,100 for

PAzBzC8 and PAzBzOC8 respectively These corresponded to a chain lengths of about

32-34 aromatic rings, which is in agreement with published degrees of polymerization when using diboronic acid in the Suzuki polycondensation.30 The low molecular weight may be due to the impurities within the diboronic acids that easily condense spontaneously to boroxines to varying degrees.26

Figure 7-5 1 HNMR spectrum of polymer PAzBzOC8

These polymers were characterized by FT-IR, NMR, and elemental analyses A representative 1HNMR spectrum of the polymer PAzBzOC8 is depicted in Figure 7-5

The chemical shifts of the azulene protons were manifested at δ 8.52, 8.35, 8.10, 7.62

ppm which are associated, respectively, with the protons of H4,8, H2, and H6 on the azulene Phenyl protons appeared at δ 7.29 and 7.18 ppm The remaining resonance at δ 3.91 ppm and 0.88-1.60 ppm corresponded to the n-octyloxy pendant chains Due to the presence of both the head-to-head (HH) and head-to-tail (HT) units, the resonance peaks depicted both broad and multisignal response Similar behavior has been reported in other copolymers systems.31

The FT-IR spectra of the polymers depicted strong C-H stretching (2920 and 2850 cm-1)

of the alkyl, or alkoxyl, side group with weak stretching of azulene and benzene at 3018

cm-1 Peaks at 1570, 1200, and 736 cm-1 also confirmed the presence of the azulene

moiety and benzene ring in our copolymers Polymer PAzBzOC8, doped with iodine and

protonared with TFA, is also illustrated in Figure 7-6 The increase in absorption in the region of 1400-700 cm-1 indicated the formation of cation radicals

(c) (b) (a)

Figure 7-6 FT-IR spectrum of (a) neutral PAzBzOC8, (b), iodine doped PAzBzOC8, and (c),

TFA protonated PAzBzOC8

Thermal properties

The thermal stability of our polymers was determined using TGA in a nitrogen atmosphere The copolymers depicted very good thermal stability with an onset of degradation at about 3050C and 4120C for PAzBzC8 and PAzBzOC8, respectively The fist degradation step of PAzBzC8 occurred at 3050C,with only a 2.8% weight loss; this is possibly due to some left over oligomers The second degradation began at 4090C,

(similar to the degradation onset of PAzBzOC8), and ended at 4960C The second degradation was attributed to the cleavage of the pendant group, as calculated from the

weight change Polymer PAzBzOC8 displayed only one-step degradation that began at a

temperature of 4120C, and finished at 4740C, which was also attributed to the cleavage of the side chains From these observations, we concluded a high stability for our conjugated polymer backbones The char yield of the polymers in nitrogen atmosphere was keep at about 45-50%, even with temperature increases of up to 10000C

8 12

16

PAzBzOC8 PAzBzC8

The p-doping characteristics of our polymers were investigated by using cyclic

voltammetry The onset of the p-doping process (oxidation) of PAzBzC8 occurred at a

potential of ca.1.0 V (vs Ag/AgCl) The anodic current increased quickly with an anodic peak appearing at ca 1.2 V, and the corresponding reversible reduction peak occurred at

ca 0.98 V For polymer PAzBzOC8, the on-set of oxidation occur at ca 0.82 V (vs

Ag/AgCl) with an anodic peak observed at 1.06 V Its corresponding reversible reduction

peak appeared at ca 0.75 V, as a small peak (Figure 7-8) Concurrently, the p-doping

process of our polymers was accompanied by an obvious color change from green, in the neutral film, to dark brown in the p-doped polymeric films Comparing the

yellowish-p-doping process of PAzBzC8 and PAzBzOC8, we found a significant negative shift of the oxidation potential for PAzBzOC8 This is may be due to the electron-donating

properties of the oxygen in the alkoxy side group, which decreased the oxidation potential of the resulting conjugated polymers

PAzBzC8

PAzBzOC8 0.5 mA

Potentials (mV) vs Ag/AgCl

Figure 7-8 Cyclic voltammograms of PAzBzC8 and PAzBzOC8, measure in 0.1 M TBAHP

(tetrabutylammonium hexafluoro-phosphate) solution of acetonitrile with a scan rate of 80 mV/s

Electronic spectroscopy study

The solution-phase UV-vis absorption of our polymers was recorded at room temperature

in chloroform solution (Figure 7-9) PAzBzC8 and PAzBzOC8 exhibited the longest

wavelength absorption of up to 375 nm and 390 nm, respectively For PAzBzC8, two

main absorption peaks appeared in the UV region One strong absorption appeared at 300

nm that was attributed to the π-π* transition of the monomeric units in the polymers, with

a shoulder appearing at approximately 375 nm The latter was attributed to the π-π*

transition of the conjugated polymer backbone While for polymer PAzBzOC8, three

absorption bands were found in the UV region One main absorption band appeared at

297 nm and the other two appeared as shoulders at 340 nm and 390 nm Comparing the

longest wavelength absorption of PAzBzOC8 with that of PAzBzC8, PAzBzOC8

showed a bathochromic shift of more than 15 nm This may suggest that the donating alkoxy side group may mediate the conjugation of the polymer backbone, this was also observed in the cyclic voltammagram study

electron-300 400 500 600 0

The UV-vis measurements also suggest the formation of a stable radical cationic species

in solution As shown in Figure 7-10, when TFA was added to the polymer solution, a

new peak appeared in the visible range at 654 nm and 620 nm for PAzBzC8O and

PAzBzOC8, respectively The new absorption was attributed to the formation of

azulenium cation radicals as we previously mentioned in the monomers study.24 On further protonation, the new band in the visible region had gradual increase, accompanied

by a decrease of the absorption in UV region The color of the polymer solutions changed

to yellowish-brown and purple for PAzBzC8O and PAzBzOC8, respectively As can be

seen in Figure 7-10, we found that the polymers are sensitive to TFA concentration The polymer absorption changed greatly at low TFA concentrations, and showed little change when the TFA concentration was more than 10% Similar behavior was found in the

PAzBzC8 polymer solution

Stability of the azulenium cation radicals in solution was also investigated by UV-vis

spectrum analysis A solution of PAzBzOC8 and 10% TFA (in chloroform) was left in

atmospheric conditions for 2 days, only an increase in intensity of the longest wavelength absorption was seen The solution was further heated at 600C for 8 hours and then adjusted to its original volume; the UV-vis spectrum showed no significant changes All these results indicated that the azulenium cation radicals are stable enough in solution to endure oxygen exposure from the atmosphere

0 1

EPR spectroscopy studies

In the EPR spectrum of PAzBzOC8 in 10% TFA solution, only a single broad signal was

observed at a g-value of 2.0017, as shown in Figure 7-11a The EPR peak-to-peak line

width (∆Hpp) of the polyradical solid was 4.1 G for PAzBzOC8 and 3.7 G for PAzBzC8

In solid state, the cation radicals can be obtained by iodine doping or by TFA

protonation In fact, because of the formation of azulenium cation radicals, PAzBzOC8 and PAzBzC8 showed good solubility in TFA, despite both of them contained the long non-polar side chain Figure 7-11b shows the EPR signal of PAzBzOC8 doped with

iodine As we have previously discussed, the iodine doping induced a broad EPR signal that may have been due to the spin-spin interaction in the polyradical system An EPR

signal at 2.0011 (g-value), with a peak-to-peak width (∆Hpp) of 12.4 G, was found for

PAzBzOC8 when doped with iodine However, a relatively narrow peak was found for

the polyradical system when the polymer was protonated with TFA Figure 7-11c is the

EPR spectrum of PAzBzOC8 when protonated with TFA As we can see, it gave an EPR

signal at a g value of 2.0021, with a peak-to-peak width of 1.9 G

Figure 7-11 EPR spectra of polymer PAzBzOC8 in (a) 10% TFA solution, (b), solid state doped

with iodine, (c), solid state protonated with TFA vapor

The spin concentration of polyradicals reached a value of ca 0.45 spin/unit (based on the

azulene) upon iodine oxidation (for 4 hours) and 0.28 spin/unit upon TFA protonation (using DPPH as standard) The polyradical solid was much more stable, even under atmospheric conditions Their EPR intensities showed no significant change after storage

in atmospheric conditions for 2 months The stability of these polyradicals was also investigated by the nitrogen/oxygen permeation measurements, as we have discussed in Chapter 3 Similar results were obtained in these measurements Stability of the copolymer polyradical system, was determined with EPR measurements by heating the polyradical system at 800 C Higher stability was seen for the copoly(azulene-benzene) system, which may be due to both the stability and low reactivity of benzene moiety After heating the polyradicals, produced by protonation with TFA vapor at 800C under atmospheric conditions, there was no significant change of the EPR intensities (even after

2 days)

Conclusions

A novel and stable stable polyradical system, stabilized via the vertical delocalization and horizontal conjugation, was designed and prepared In this portion of research,

PAzBzOC8 and PAzBzC8 were synthesized to produce the polyradical systems and to

demonstrate our new concept These copolymers were synthesized via a Suzuki coupling reaction and characterized by HNMR, CNMR, and FT-IR spectrum analysis Both polymers showed high thermal stability and oxidation potentials

Upon doping with iodine or protonation with TFA, both polymers could be, as expected, converted into the stable polyradicals, which was confirmed by UV-vis, FT-IR, and EPR experiments UV-vis spectra study showed that both polymers were easily protonated to the polyradical systems at low TFA concentrations UV-vis spectra also revealed the high stability of our polyradicals in solution Polymer solution protonated with 10% TFA (in chloroform) was left in atmospheric conditions for 2 days; only an increase in the intensity of the longest absorption was seen

Further stability tests in the solid state of the polyradical systems were carried out using EPR measurements EPR intensities of the polyradical systems showed no significant change, even after storage in atmospheric conditions for 2 months

To gain insight into the formation of azulenium cation radicals, a model compound

(1,3-diphenyl-azulene) was prepared by a general Grignard coupling reaction This compound was characterized by HNMR, CNMR and FT-IR Formation of the radical cation was confirmed and investigated by HNMR, UV-vis, and EPR measurements

References

1 Coppinger, G M J Am Chem Soc 1957, 79, 501

2 Kinoshita, M.; in Handbook of Organic Conductive Molecules and Polymers, Vol 1 (Ed.:

Nalva, H S.) Wiley, New York, 1997, chapter 15, 781

3 Buchanenko, A L Russ Chem Rev 1990, 59, 307

4 Crayston, J A.; Devine, J N Walton, J C Tetrahedron 2000, 56, 7829

5 Dimagno, S.; Waterman, K.; Sperr, D.; Streitweser, A Angew Chem Int Ed Engl 1991, 30,

4679

6 Sizzmann, H.; Boese, R Angew Chem Int Ed Angl 1987, 26, 971

7 Rajca, A Chem Rev 1994, 94, 871

8 Yamaguchi, K.; Toyoda, Y.; Fueno, T Synth Met 1987, 47, 297

9 Lahti, P M.; Ichimura, A S J Org Chem 1991, 56, 3030

10 Nishide, H.; Kaneko, T.; Nii, T.; Katoh, K.; Tsuchida, E.; Yamaguchi, K J Am Chem Soc

1995, 117, 548

11 Takahashi, M.; Nakazawa, T.; Tsuchida, E.; Nishide, H Macromolecules 1999, 32, 6383

12 Rajca, A.; Utamapanya, S J Am Chem Soc 1993, 115, 2396

13 Rajca, S.; Rajca, A J Am Chem Soc 1995, 117, 9172

14 Kaisaki, D A.; Chang, W.; Dougherty, D A J Am Chem Soc 1991, 113, 2764

15 West Jr., A P.; Silverman, S K.; Dougherty, D A J Am Chem Soc 1996, 118, 1452

16 Fujii, A.; Ishida, T.; Koga, N.; Iwamura, H.; Macromolecules 1991, 24, 1077

17 Miura, Y.; Inui, K.; Markromol Chem 1992, 193, 2137

18 Abdelkader, M.; Drench, W.; Mwijer, E W Chem Mater 1991, 3, 598

19 Ito, S.; Nomura, A.; Morita, N.; Kabuto, C.; Kobayashi, H.; Maehima, S.; Fujimori, K.;

Yasunami, M J Org Chem 2002, 67, 7295

20 Oudar, J L.; Chemla, S S Chem Phys 1977, 66, 2664

21 Hendrickx, E.; Clays, K.; Dehu, C.; Brédas, J L J Am Chem Soc 1995, 117, 3547

22 Oda, M.; Kajioka, T.; Uchiyama, T.; Nagara, K.; Okujima, T.; Ito, S.; Morita, N.; Sato, T.;

Miyatake, R.; Kuroda, S Tetrahedron 1999, 55, 6081

23 Farrell, T.; Manning, A R.; Mitchell, G.; Heck, J.; Meyer-Fridrichsen, T.; Malessa, M.;

Wittenburg, C.; Prosenc, M H.; Cunningham, D.; McArdle, P Eur J Inorg Chem 2002,

1677

24 Ito, S.; Nomura, A.; Morita, N.; Kabuto, C.; Kobayashi, H.; Maejima, S.; Fujimori, K.;

Yasunami, M J Org Chem 2002, 67, 7295

25 Rehahn, M.; Schlüter, A.-D.; Feast, W J Synthesis, 1988, 386

26 Todd, M H.; Balasubramanian, S.; Abell, C Tetradron Lett 1997, 38, 6781

27 Miyaura, N.; Yanagi, T.; Suzuki, A Synth Commun 1981, 11, 513

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29 Remmers, M.; Schulze, M.; Wegner, G Macromol Rapid Commun 1996, 17, 239

30 Scherf, U.; Müllen, K Macromolecules 1992, 25, 3546

31 Yamamoto, T.; Zhou, Z H.; Kanbara, T and et al J Am Chem Soc 1996, 118, 10389

First, we designed the low band gap polymers by changing the main backbone electronic structure One successful approach to low band-gap polymers is insertion of alternation electron-donating and electron-accepting moieties along the polymers backbone.11,12 The alternating electron donor-acceptor arrangement causes an interaction of the highest occupied molecular orbital of the donor and the lowest occupied molecular orbital of the

acceptor, and hence resulting in reduction of the band gap in these materials Following this strategy, a number of low-band gap materials have been synthesized.13,14 Theoretical calculations have confirmed the band gap of polyarylenes was a strong function of molecular geometry.15 Often, the quinoid isomer gave small band gap.16 To test this idea that quinoid geometry can substantially reduce the band gap of the conjugated polymer,

Wudl and co-workers synthesized polyisothianaphthene (PITN) and showed that it has a

very small band gap (Eg = 1.13 eV) due to the preferred stabilization of the quinoid resonance form by the fused benzene ring.17 Based on the theoretical and experimental studies, led us to conclude that azulene seems to be a good candidate for designing the low band gap polymers because of its unique electronic structure First, azulene contains fused five-membered ring and seven-membered ring as a bicyclic structure and it is a nonalternant polycyclic aromatic compound with a 10-π electron system.18 A dipole moment of µ ≈ 1 D (1D = 3.33 × 10-30 cm) was observed for azulene, indicating the contribution of the dipolar structure as shown in Figure 8-1a Thus azulene can be regarded as a intra-molecule electron-donating and electron-accepting system.19,20Secondly, azulene can be easily converted to its quinoid geometry by flash vacuum pyrolysis or photooxidation.21,22 However, up to now, azulene was only scarcely employed as precursors for the synthesis of conducting polymers, and all the reported polyazulene were synthesized by linkage of the 1,3-position of azulene In the present work, the design and synthesis of the novel conjugated polymers containing azulene in the polymer backbone via coupling the 2,6-positions of the azulene will be described Effective conjugation of 1,3-azulene based and 2,6-azulene based copolymers will be compared To further understand the conjugation effect, model compounds of this series polymers were prepared and studied

by introducing steric restriction to the coplanarity of the chain rings or to the intermolecular order in the solid state This is of considerable value in design conducting polymers for various purpose.23 Aromatic substituents are particularly interesting as they present a possibility to extend its conjugation to polymer backbone For instance, 3- phenyl-thiophene is known to polymerize to a polymer having a very good conductivity and capable of undergoing facile doping-undoping process even in the cathodic region.24Whether this is due to the conjugation of the phenyl group with the oligothiophene backbone or just the steric enhancement of favorable properties is an interesting question

which has raised some controversy According to Kaeriyama et al.25 the substituent effects shown by poly(3-(4-methoxyphenyl)thiophene) support the conjugation of the

pendant aryl group with the main chain However, Yoshino et al.26 have deduced from the band gap of poly(3-phenylthiophene) that the phenyl ring is not coplanar with the thiophene rings and thus is not in conjugation with the main chain To make clear about this question, we designed and synthesized a series of model compounds with azulene moiety in the center and phenyl group substituted at different side positions as shown in

Figure 8-2b As azulene is a nonalternant compound, the conjugation influence can easily

be detected by the chemical shift of the azulene protons and the bond alternation To estimate the influence of steric and conjugation effect of the side phenyl groups, a series model compounds and related polymers with phenyl groups substituted at different positions were prepared and investigated

Molecular design

To develop low band gap conducting polymers, conjugated polymers based on the coupling of azulene were designed as shown in Figure 8-2a Two conjugated polymers with different position of the side alkyl group were designed to study the effect of steric effect on the resulting polymers’ properties Figure 8-2b is the design of the model compounds with different arrangement of side phenyl group Study of these compounds may reveal which one plays a main role in influence of the polymer conjugation, the steric or conjugation effect

2,6-CO 2 Et

CO2Et

S S

CO2Et

S S

Results and discussion

Part 1 Conjugation control by changing the main backbone conjugation type

Model compounds synthesis and characterization

To have a better understanding of electronic geometry of the molecule and the side group steric hindrance affect on the resulting conducting polymers, two model compounds were designed and synthesized as shown in Scheme 8-1

CO 2 Et

CO 2 Et

S S

C 4 H 9

CO2Et S Me

Scheme 8-1 The synthesis of model compounds for the 2,6-azulene coupling polymers Reagents

and conditions: (i), SOCl2/benzene, (ii), EtONa, ethyl cyanoacetate, (iii), Br2/CHCl3, (iv),

H 2 SO 4 /1,4-dioxane, (v), NaNO 2 /HCl, (vi), hν, thiophene, (vii), Tf 2 O/Et 3 N/CH 2 Cl 2 , (viii), 2-thenyl-tributyl stanne, (ix), LiCl/AsPh 3 , Pd(PPh 3 ) 4 , (x), 2-(4-methyl-5-phenyl)-thenyl-tributyl stanne

3-butyl-As shown in Scheme 8-1, both model compounds are prepared by Stille coupling reaction27-29 using thenylstannanes with the azulen triflates as the coupling partners We chose Stille coupling reaction because this reaction is extremely versatile, proceeds under neutral condition and can tolerate wide range of substituents on both coupling partners The synthetic intermediates azulen triflate was synthesized from tropolone via 6 steps

Azulene compoundderivative diethyl 2-aminoazulene-1,3-dicarboxylate was constructed

by Nozoe synthesis.30 Reaction of equiv amount of 2-chlorocyclohepta-2,4,6-trien-1-one ethyl cyanoacetate in ethanol, diethyl 2-aminoazulene-1,3-dicarboxylate was obtained in high yield Functionalized the 6-position was successful by bromination of diethyl 2-aminoazulene-1,3-dicarboxylate in chloroform Diazotization of diethyl 2-amino-6-bromo-1,3-dicarboxylate by sodium nitrite in dioxane-sulfuric acid afforded the diethyl 2-diazo-1,3-dicarboxylate-6-oxo-2,6-azulene, the structure of which was confirmed by NMR and IR spectrum compared with the reference.31 Photochemical reaction of thiophene and diethyl 2-diazo-1,3-dicarboxylate-6-oxo-2,6-azulene was carried out by a medium-pressure Hg lamp under an nitrogen atmosphere The photochemical reaction was first carried out between diethyl 2-diazo-1,3-dicarboxylate-6-oxo-2,6-azulene with thiophene in ethyl acetate However, the yield is low.32 We think this is may be due to the radical formation in the reaction was captured by the ethyl acetate not by thiophene as shown in the possible mechanism (Scheme 8-2).33

Scheme 8-2 Mechanism of the photochemical reaction of diethyl

2-diazo-1,3-dicarboxylate-6-oxo-2,6-azulene with thiophene

Thus we usedthiophene as solvent for the photochemical reaction, and this procedure produced the 2-thenyl substituted azulene in high yield Conversion of the 2-(diethyl-1,3-dicarboxylate-6-hydroxy-2-azulenyl)-thiophene to its triflate was successfully by treatment with triflic anhydride in the presence triethylamine as a base.34 The coupling

reaction between the azulenyl triflate with corresponding thenyl stannanes works well in dioxane at reflux with tetrakis(triphenylphosphine)palladium (0), Pd(PPh3)4, as catalyst,

in the prescence of 3 equiv of lithium chloride to give the expected products Lithium chloride is essential for the success of the reaction.35 In the absence of lithium chloride decomposition of the catalyst takes place

The structures of M26A and M26B were confirmed by 1HNMR spectroscopies as shown

in Figure 8-3 Both compounds show the characteristic signals of the azulene at the low

field For M26A, two double peaks (AB system) appeared at 9.4 ppm and 7.8 ppm with a large split constant of 11.2 Hz For compound M26B, these two peaks appeared at 9.4

ppm and 8.0 ppm The large split constant and low field shift indicate large bond alternation of the azulene ring, and a tendency to form the quinoid structure The thiophene presence was confirmed by signals in the region of 7.5-7.0 ppm As expected,

for compound M26A, one set AB and ABC systems were found in this region Regarding compound M26B, these patterns were overlapped by phenyl peaks FT-IR spectra gave

further evidence to confirm the expected structures for these model compounds

(a)

(b)

Figure 8-3 Aromatic region of the 1 HNMR spectra of models compounds in CDCl 3 : (a), M26A, (b), M26B

The absorption spectra of these model compounds were recorded in CHCl3 at room

temperature and compared with Monob that used as an example for those 1,3-coupling monomers Comparison UV-vis spectra of M26A and M26B with that of Monob (Figure 8-4), a significant red shift of the longest wavelength absorption was found for M26A and M26B Compared to the UV-vis absorption of Monob, the longest wavelength absorption red-shifted around 35 and 68 nm for M26B and M26A, respectively The

large red-shift was attributed to the conjugation type change from the 1,3-coupling of azulene to the 2,6-coupling of azulene Calculation results of various aromaticity theories indicated that azulene possesses much lower aromatic delocalization energy (4.2 kcal/mol)36 compared to benzene ( 20.0 kcal/mol), thiophene (16.1 kcal/mol), and naphthalene (30.5 kcal/mol) This strongly suggests azulene can be potential candidate to

be conjugated bridge Thus the electrons will easily delocalized along the dipolar direction (2,6-direction) of azulene because of the large dipole moment and large hyperpolarizability of azulene This is why a large red-shift of the absorption of the 2,6-coupling model compound compared to the 1,3-coupling compounds The larger red shift

of M26A compared to M26B may be due two reasons: (1) M26A has longer conjugation

length than M26B as one phenyl ring was coupled to the terminal thiophene ring; (2) the

side butyl group may induced the hindrance effect between the thiophene ring and the

azulene ring, which decrease the coplanarity of the compound, M26B

0 1

2

M26A Monob M26B

The cyclic voltammograms of M26A and M26B are shown in Figure 8-5 For M26A, the

oxidation process began at ca 1.47 V (vs Ag/AgCl) with the Epa = 2.23 V Repeat scans

found the formation of the polymer film on the electrode For M26B, because the

α-carbon of one thiophene was blocked by phenyl group, no polymerization was found

during the repeat scan However, because of the extension of the conjugation of M26B as

we observed in the UV-vis study, it displayed a lower onset and the oxidation peak during the oxidation process It began to oxidize at about 1.26 V and reach to the highest current at about 1.97 V

Compared with the CV behavior of the 1,3-coupling monomers, these 2,6-coupling compounds gave a higher oxidation potentials As we have discussed in Chapter 4, the

1,3-coupling compounds Monob display an oxidation peak at about 1.02 V with the

onset at around 0.4 V The large positive shift of the oxidation potentials was attributed to

the electron-withdraw ester group substituted at the 1,3-position of azulene These ester groups withdraw the π-electron from azulene and thus increase the oxidation potentials

M26A

M26B 0.5 mA

Structure analysis of the model compounds

Although we have studied the optical and electronic properties of these model compounds, the ultimate answer to the conformation in the solid state is given by the structure determination by X-ray diffraction Both model compounds could be obtained

as crystals of suitable dimensions for the X-ray diffraction

Molecular structure of single crystal of M26A and M26B and their atomic numbering

scheme are displayed in Figure 8-6 Their crystallographic parameters for the structure

are listed in Table 8-1 The crystals of M26A is Triclinic system belongs to P-1 space group Single crystal M26B is monoclinic system belongs to P2(1)/c space group The

major observation was the large torsion angle between the thiophene ring and azulene

ring for M26A Because of the steric hindrance of the ester group, the torsion angle

between the five-membered ring of azulene and the thiophene is about 71.60 C3) That means that the thiophene ring placed nearly vertical to the azulene ring The

(S1-C11-C2-torsion angle of the seven-membered ring of azulene and the other thiophene ring is about 85.40 (S2-C15-C8-C9), indicating the second thiophene ring is placed vertical to the azulene ring This is due to the steric effect of ester group Compared the single crystal

structure and UV-vis spectrum of the 1,3-coupling monomers (e.g Monob)and the

2,6-coupling model compounds, we can conclude that the azulene electronic structure play an important role in the resulting conjugation For the 2,6-coupling compounds, even the two thiophenes are placed nearly vertical to the azulene ring, their UV-vis spectra red -shifted about more than 30 nm This is due to the formation of quinoid structure of azulene in the the 2,6-coupling compounds

For M26B, the torsion angle between the five-membered ring of azulene and thiophene

ring is about 58.20 (S1-C11-C2-C3), and the torsion angle between the seven-membered ring of azulene and the other thiophene is only 8.60 (S2-C15-C7-C6) The small torsion angle may be due to the position of the methyl substituents Because the methyl group is far away from the seven-membered ring of azulene, which release the steric hindrance between thiophene ring and the azulene This is observation is in good agreement with what we observed in the UV-vis spectra and cyclic voltammograms study

Another Important feature is the bond length difference between the 2,6-coupling

compounds and the 1,3-coupling compounds As shown in Figure 8-7, Monob and

M26B were used as example to compare the bond length difference of two types

coupling compound First we found a larger bond alternation in the 2,6-coupling

compounds than in these 1,3-coupling compounds In the crystal structures of Monob,

the bond length difference in the seven-membered ring is about 0.008 Å, while in

2,6-coupling compound, the bond length difference was found to about 0.042 Å in M26B

And we found the longest baond on the seven-membered ring of azulene are the C6-C7

and C7-C8, the shorest bonds are the C5-C6 and C8-C9, which is characterization of the quinoid structure of azulene

Me

03

1,4 16 1.409

1.39

0 1.371

1.411 1.4 02

CO2Et

(a)

1.4 15

1.735

1.709

1.3

7 1.423 1.428

1.3 75

1.3 72

1.484

1.38

83 1.3 91 1.

39 0

1.3 8 38 5

1.708

8 1

(b)

Figure 8-7 Comparison of the bond distance in (a), M26B and (b), Monob

Figure 8-8 is a stereoscopic view of M26B crystal viewed along a axis The crystal

packing is characterized by the alternative layered structure (ABAB type) The smallest distance between the ring carbons is 3 52 Å, this is roughly twice the carbon van der Waals radius (1.70 Å) It is interesting to note that in the packing, weak C-H⋅⋅⋅π interaction was found between the proton of phenyl ring and the π-system of the seven-membered ring The angle of α was found to be 1530 and the H⋅⋅⋅Ω distance was found to

be 2.98 Å, which belongs to the weak C-H⋅⋅⋅π interaction.38 There are no π-π interaction

of the azulene ring as we observed in the 1,3-coupling compounds

Figure 8-8 Stereoscopic view of the M26B crystal viewed along a axis

Table 8-1 Crystal Data and Experimental Details

final R indices [ I>2σ(I)]

Polymers synthesis and characterization

P26A and P26B were synthesized starting from the azulen trifilate as shown in Scheme

8-3 Azulen trifilate yielded in high yield as we reported in the model compounds

synthesis The coupling position of thiophene with the azulen trifilate was controlled

from the synthesis of the thenyl stannanes.28 For P26A, the thenyl stannane was prepared

from 2-bromo-3-dodecyl-thiophene In this procedure, 2-bromo-3-dodecyl-thiophene was first converted to its Grignard reagent in THF and then react with tributyl tin chloride

afforded the 2-(3-dodecyl-thenyl) stannane For P26B, the thenyl stannane was prepared

by the reaction 3-dodecyl-thiophene with n-butyllithium at low temperature The obtained lithium reagent was then reacted with tributyltin chloride yielded the expected thenylstannane These structures and functionalized positions were confirmed by HNMR spectrum For example, 2-(3-dodecyl-thenyl)-stannane displayed one set AB system peaks in aromatic region while the 2-(4-dodecyl-thenyl)-stannane showed only two single peaks at the aromatic region Monomers were obtained following the same Stille coupling procedure as we discussed in the model compounds synthesis Polymerization was carried out using iron chloride as an oxidant in chloroform to afford the polymers

P26A and P26B The polymers were dedoped by stirring with aqueous hydrazine As

expected, these polymers can be soluble into chloroform after the dedoping process gave red-violet solution The obtained polymer was purified by Soxhlet extraction with methanol and acetone, and then re-precipitated into methanol

CO 2 Et

CO 2 Et TfO S

CO2Et

CO 2 Et

S S

2 Et

CO 2 Et

S S

C 10 H 21

n

Scheme 8-3 Synthesis of the 2,6-conjugated type polymers Reagents and conditions: (i),

LiCl/AsPh3, Pd(PPh3)4, (ii), FeCl3/CHCl3

The structures of the our polymers were confirmed by NMR spectrum Figure 8-9 is the

HNMR spectrum of aromatic resonance of P26A Because the no-symmetric structure of

the monomers and different possibility of the arrange of the 2,6-coupled azulene in the resulting polymers backbone, these peaks showed multiple pattern, FT-IR spectra gave further confirmation of the expected structure

Figure 8-9 Aromatic region of 1HNMR spectrum of P26A

The molecular weight of the polymers was determined by GPC in THF using polystyrene standard as references The GPC showed that the molecular weights of polymers were in

the moderate range For example, the number-average molecular weight of P26A is about

12,300 with a polydispersity of 2.9 and the number-average molecular weight of P26B is about 9,200 with a polydispersity of 1.65

Figure 8-10 Thermogram curve and DTA curve P26A

The thermal stability of polymers P26A and P26B were analyzed using thermogravimetry A thermogram is shown in Figure 8-10 for P26A Both polymers

exhibit an onset of degradation of at around 3500C As shown in Figure 8-10, both polymers displayed two steps degradation The fist step occurred at about 4000C which was due to the decomposition of the ester group The second step occurred at about

4700C, which was attributed to the decomposition of the alkyl side group Under nitrogen atmosphere, the polymer backbones do not decompose even when the temperature up to

10000C as can be seen from the large amount residue

peak at UV region appeared at 352 nm and the peak at visible region appeared at 500 nm

The absorption difference between P26A and P26B was attributed to the steric hindrance

of the side alkyl group For polymer P26A, the dodecyl side group was near to the

seven-membered ring, which will induce a large torsion angle between the thiophene ring and the azulene ring And this conclusion has been confirmed by the X-ray structure of the model compounds The large torsion angle will disturb the delocalization of the π-electron along the polymers backbone, and thus lead to a blue shift when compare with

the absorption of the P26B, whose side group substituents are placed far away from the

azulene ring

Compared the UV-vis spectra of P26A and P26B with that of the 1,3-coupling polymers such as Polyf, a large red shift was found for these 2,6-coupling polymers For example, compared to the UV-vis spectrum of Polyf, the longest absorption of P26A red shifted about 94 nm and P26B red shift about 116 nm The significant red shift was attributed to

the quinoid structure of azulene and the large dipole moment of azulene in the coupling polymers π-Electrons will delocalized easily along the dipolar direction of azulene

0

1

Polyf 1,3-coupling P26A P26B

Further study of the UV-vis spectrum in TFA solution revealed that no protonation

occurred in the 2,6-coupling polymers system Figure 8-12 is the UV-vis spectra of P26A

in different TFA concentration, which showed that even when the acid concentration reached up to 30%, no new peaks appeared in the spectrum and no intensity decrease of the existed absorption That may suggest that the 2,6-coupling system cannot protonated

by TFA as we observed in 1,3-coupling polymers system Analysized the structure difference between the 1,3-coupling system and the 2,6-coupling system, we can conclude that the protonation of 1,3-coupling conjugated polymers were mainly occurred

on the five-membered ring of azulene This is because both systems have similar structure and coupled units, except that all position of the five-membered ring of azulene were blocked in 2,6-coupling systems

0

1

TFA concentration 30%

Figure 8-12 UV-vis spectrum change of P26A under different TFA concentration

Part 2 Conjugation control by side aromatic substituents

Model compounds synthesis and characterization

The synthetic approach to these model compounds are shown in scheme 8-4 The starting material, 3-phenyl-thiophene, was obtained via Grignard coupling reaction of bromobenzene with 3-bromothiophene catalyzed by Ni(dppp)Cl2 The synthetic approach

to the compound DPTA1 was achieved by Grignard coupling reaction of

2-bromo-3-phenyl-thiophene and 1,3-dibromoazulene catalyzed by Ni(dppp)Cl2 in THF The

preparation of DPTA2 and DPTA3 were achieved by regio-selective reaction to control

the coupling position of thiophene.39 3-Phenyl-thiophene was regio-selectively lithiated

by LDA to give 4-phenyl-2-thienyllithium, afforded 4-phenyl-2-thienylzinc chloride in THF and then reacted with 1,3-dibromoazulene to give two product: one is the di-

substituted product DPTA2 and the other is the mono-substituted

1-bromo-3-(2-(4-phenyl-thenyl))-azulene, which was subsequently used as an intermediate to react with

the Grignard reagent of 2-bromo3-phenyl-thiophene to giveDPTA3

S S

S

Br +

S MgBr

S S

Scheme 8-4 The synthesis route for DPTA1, DPTA2, and DPTA3 Reagent and conditions: (i),

Mg/Et2O, (ii), Ni(dppp)Cl2, THF, (iii), LDA, ZnCl2, (iv), Pd(PPh3)4, THF

These compounds were characterized by NMR, FT-IR spectra Different substituted positions effects on the conjugation were studied by the HNMR, UV-vis, and cyclic voltammetry experiments

Because azulene is a nonalternant molecule, modest perturbation may induce bond alternation in azulene and thus induce large chemical shift changes of its protons.40 Thus azulene can be used as an indicator to monitor the conjugation changed induced by the

side phenyl ring For example, in compound DPTA1, where two phenyl rings are placed

at the 3-position of thiophene, protons of H4,8 of the seven-membered ring of azulene appeared at 8.19 ppm with a split constant of 9.4 Hz H2 appeared at 7.70 ppm as a single peak H5,7 on the seven-membered ring of azulene appeared at 6.96 ppm with a split

constant of 9.9 Hz While in compound DPTA2, where the side phenyl group are placed

at the 4-position of thiophene, all these peaks were shifted H4,8 in DPTA2 shifted to 8.78 ppm Compared with their chemical shifts in compound DPTA1, H4,8 shift about

0.6 ppm to the low field This indicates that the conjugation of the compound were changed when the phenyl substituted position were changed This can also be seen from others protons change on the azulene ring H2 on of azulene shifted to 8.24 ppm and H5,7

on the seven-membered ring of azulene shifted to 7.22 ppm in DPTA2