Structures, properties, and applications of soluble polyazulene and azulene containing copolymers 3

Chapter 4 Crystal Structures of Monomers and Oligomers Containing Azulene Unit – Model Compounds for the Corresponding Polymers Introduction In chapter 3, we have described the interes

Chapter 4

Crystal Structures of Monomers and Oligomers Containing Azulene Unit – Model Compounds for the Corresponding Polymers

Introduction

In chapter 3, we have described the interesting properties of conjugated copolymers containing azulene units in the main chain, such as stimuli-response upon protonation, stability of the radicals cations, high conductivity upon doping or protonation All these interesting properties come from their unique electronic structures However, structural studies of these copolymers have been limited by the defects of these copolymers For example, due to statistical chain length distribution and interruption of the conjugated chain by mislinkages and other defects, the obtained polymers, lack a rigidly defined structural principle and conjugation is severely disturbed.1 The structure at the chain segment level is also hard to characterize.2 These inaccessibility of the copolymer solids

is one of the reasons why model compounds, well-defined oligomeric analogues of the copolymer, have met increasing interest in the filed of conjugated organics.3

Because of the chemical purity and relatively low molecular mass, these model compounds are usually sufficiently crystalline to allow structural resolution by means of diffraction methods Although X-ray data of oligomers are scarce, X-ray data of model compounds, such as inter-ring bond lengths, torsional angles between the moieties planes, and positioning of the alkyl chains, can give valuable information on polymer properties.4-6 It also provides information to study the solid-state polymer properties.7,8

Furthermore, the physical properties of these monomers and oligomers, may be extrapolated to those of their corresponding polymers Additionally, it has been well demonstrated that oligomers themselves represent excellent candidates for electronic materials.9-12 Thus, to have a better understanding of the polymer structure and properties, we prepared the single-crystals of a series of monomers and oligomers containing azulene unit To further investigate the side chain effect on solid-state polymer properties, model compound bearing long side chains was synthesized and its single crystal structure was studied These compounds are analog of the copolymers and their solid state structures, optical and electrical properties were studied as model compounds for the corresponding polymers

Results and Discussion

Model compounds design and synthesis

Synthesis and characterization of Monoa and Monob are reported in Chapter 3

Palladium-catalyzed Stille coupling reaction13 was employed to prepare the oligomers as

described in Scheme 4-1 The common intermediate for synthesis of Oligoa and Oligob,

4’,4’’-Dibromo(1,3-(3’-methyl-thienyl))-azulene, was obtained by bromination of

Monob with NBS in chloroform and acetic acid (1:1) Because Monob is less reactive

with respect to the corresponding thiophene derivatives, bromination of Monob needed

longer reaction time 3-Methyl-2-phenyl-thiophene was prepared by Grignard coupling reaction between 2-bromo-3-methyl-thiophene and bromobenzene, it was then brominated with NBS in chloroform and acetic acid (1:1) as discussed previously The

Stille reagents for synthesis of Oligoa and Oligob were obtained in high yield by reacting

the corresponding Grignard reagent with tributylstannyl chloride in ether The obtained

Stille reagents are enough stable to store at room temperature for more than one week without significant change (detected by 1HNMR)

Stille coupling reaction of 4’,4’’-Dibromo(1,3-(3’-methyl-thienyl))-azulene with corresponding Stille reagents catalyzed by Pd(PPh3)4 in refluxing toluene, yielded Oligoa and Oligob as green and yellowish-green solid, respectively Their single crystals were

obtained by evaporating of their dilute solution in hexane and CH2Cl2 at room temperature

S S

Me Me

S S

S Me

S

Me SnBu3

S S

Me Me

S S

iii

ii

Scheme 4-1 Synthesis of Oligoa and Oligob Reagents and conditions: (i), Mg/Et2O, (ii),

Bu 3 SnCl/Et 2 O, (iii), NBS/CHCl 3 /AcOH, (iv), Bromobenzene, (v), AsPh, LiCl, Pd(Ph 3 ) 4 , dioxane

1,4-To study the long side chain effect on the solid structure of the polymers, monomers with short alkyl-substitutent (less than four carbons) had been prepared but they were found to have difficulty in crystallization We then turned to monomers with alkoxy side group,

which may crystallize easily because of the electro-negative oxygen MonoO6 was thus

designed and synthesized by Grignard coupling of 1,3-dibromo-azulene and hexyloxy-thiophene, catalyzed by Ni(dppp)Cl2 in anhydrous THF During the preparation

2-bromo-3-of MonoO6, two methods were used to prepare 3-alkoxyl-thiophene: The first attempt

was made using Ullmann synthesis with copper (I) halide catalysis.14 This method suffered from its low yields, long reaction time, and difficulty in purification In the second attempt, 3-alkoxyl-thiophene was prepared from 3-methoxyl-thiophene as shown

in Scheme 4-2.15,16 In this method, 3-bromo-thiophene was first converted to thiophene by reaction of 3-bromothiophene with MeONa in toluene 3-Hexoxyl-thiophene was then afforded by reaction of 3-methoxythiophene with 1-hexanol and NaHSO4 in toluene The second method showed advantages in high yields and simple purification Although alkoxyl-thiophene are unstable at r.t., the obtained monomers,

3-methoxy-MonoO6, however, is quite stable at r.t This is may be due to the increasing of oxidation

potentials of the product

S

Br

S OMe

S

OC6H13

S

OC6H13Br

Br

O O

iv

v

MonoO6

Scheme 4-2 Synthesis of MonoO6 Reagents and conditions: (i), MeONa/MeOH, (ii), hexanol,

toluene, (iii), NBS, CHCl3/AcOH, (iv), NBS, benzene, (v), Mg/Et2O, Ni(dppp)Cl2

Characterization

HNMR Structural characterization of the synthesized oligomers was performed using

1HNMR and 13CNMR spectra as shown in Figure 4-1a and Figure 4-2a with the

assignment of the peaks for Oligoa and Oligob Comparison of the 1HNMR of Oligoa

and Oligob, we found there is less changes of azulene protons even two more thiophenes were inserted into Oligob than in Oligoa To illustrate, H4,8, and H2 of azulene in

Oligob, negatively shifted approximately -0.02 ppm compared to that of in Oligoa In

case of H6 of azulene in Oligoa, there is about 0.04 ppm shift to the low field in Oligob

However, extension of the conjugation by inserting two more aromatic units can be seen from the β-H of thiophene For instance, two β-H on thiophene in Oligoa was found at

7.30 ppm as a single peak, and shifted to 7.14 ppm in Oligob Another two β-H on thiophene was found at 7.05 ppm in Oligob The high field shift of the β-H on thiophene ring in Oligob was attributed to the increase of conjugation

(a)

(b)

Figure 4-1 (a), HNMR spectrum of Oligoa and (b), its HMQC spectrum

For the purpose to assign the carbons in these oligomers, 2-D HMQC experiments were carried out and the results are displayed in Figure 4-1b and Figure 4-2b These results also confirmed the expected oligomers structure and revealed that the extension of conjugation has little effect on azulene’s carbons For instance, C4,8 of azulene was

found at 138 and 136 ppm in Oligoa and Oligob, respectively

(a)

(b)

Figure 4-2 (a), HNMR spectrum of Oligob and (b), its HMQC spectrum

1HMR spectrum of MonoO6 is displayed in Figure 4-3 Because of the electron-donating property of oxygen, a large shift of the protons on azulene in MonoO6 comparing to that

in the alkyl-substituted monomers such as Monob was found in its 1HMR spectrum In

Figure 4-3, H4,8 of azulene appeared at 8.58 ppm in MonoO6, while H4,8 in Monob

were found at 8.38 ppm (Chapter 3) They shifted approximately 0.2 ppm to higher filed

Similarly, H2 in MonoO6 positively shifted around 0.37 ppm when compared to that of

in Monob However, H6 and H5,7 did not show significant shifts as we observed for

H4,8 and H2, which may be due to the bond-alternation properties of azulene

Figure 4-3 HNMR spectrum of MonoO6

FT-IR FT-IR spectra showed that methyl substituted oligomers resemble those

copolymers These compounds showed characteristic frequencies at 3015 cm-1 that was assigned to the aromatic C-H stretching mode, and 2910-2850 cm-1 to the aliphatic C-H stretching There were no γα-C-H stretching at 3080 cm-1 in these oligomers as we

observed in MonoO6, which indicated that all the α position of thiophene was linked in

the conjugated oligomers Absorption at the region 1930-1659 cm-1 was attributed to characteristic absorption of the benzene ring Absorption at 1560, 1430, and 830 cm-1characterized the tri-substituted thiophene Absorption at 1591 (γC=C of azulene) and 734

cm-1(δ=C-H of azulene) characterized existence of azulene in the conjugated systems Peaks at 1170 cm-1 was attributed to the in-plane C-H deformation, 1070-1000 cm-1 was attributed to the in-plane C-H deformation, and 830-756 cm-1 was attributed to the out-plane C-H deformation

4000 3000 2000 1000 0.0

0.5 1.0 1.5

MonoO6 Oligoa Oligob

Wavenumber (cm -1 )

Figure 4-3 FT-IR spectrum of Oligoa, Oligob, and MonoO6

Structural Analysis

Structure of Monoa and Monob

Single crystal of 1,3-dithienyl-azulene (Monoa) and 1,3-di(3-methylthienyl)-azulene (Monob) were obtained and Figure 4-4 illustrates the atomic numbering scheme employed for Monoa and Monob respectively The crystallographic parameters for these

structures are listed in Table 4-1 Both crystals are orthorhombic system but a slight

difference in density is observed with Monoa at 1.406g/cm3, and Monob at 1.343g/cm3 The major observation was the large torsion angle between the thiophene ring and

azulene ring In Monoa, the thiophene ring and the five-membered ring of azulene are all

nearly planar themselves But one torsion angle between the thiophene and the membered ring of azulene was found to be 43.20 (C10-C3-C1’-S2) and the other is 34.40(C9-C1-C1’’-S1) These data are very close to the theory calculation PM3 calculation

five-has been carried out on Monoa and the calculated dihedral angle between the thiophene

ring and azulene ring are 48.50 and 520 at the lowest energy of 619.52 kJ/mol As

expected, larger steric effect of the methyl group increases the torsion angle in between

thiophene and azulene ring In Monob, one torsion angle between thiophene and the

five-membered ring of azulene ring is 44.70 (C10-C3-C1’-S2) and the other is 45.00 C1’’-S1) The large torsion angles arises from steric hindrance between the seven-membered ring of azulene and the thiophene ring, that is, the repulsion between H4, H8

(C9-C1-of azulene and the H or substituted group at 3-position (C9-C1-of the thiophene ring The large torsion angles between azulene and thiophene ring, especially the substituted one

(Monob), means that the wave functions of the HOMO for these compounds are at least

somewhat less localized than for the reported coplanar oligothiophenes.5,17

Monoa

Monob

Figure 4-4 Perspective view and atom labeling of the crystal structure of Monoa and Monob

Bonds lengths for Monoa and Monob are displayed in Figure 4-5 For example, the

bond-lengths between the azulene and the thiophene ring were found to be 1.459Å

(C3-C1’), 1.456 Å C1’’) in crystal Monoa, and to be 1.465 Å (C3-(C3-C1’), 1.461 Å C1’’) in Monob While the corresponding bond-length in the oligothiophene was found

(C1-to be 1.451 Å.18 The large steric hindrance effect on conjugation of Monob than Monoa

can also be seen from bond-length of β-β single bonds in thiophene The β-β single bond

(C2’-C3’ or C2’’-C3’’) length were found to be 1.415 – 1.416 Å in Monoa and 1.428 Å in Monob

1.423-The large torsion angle effect on the physical, optical and electrochemical properties of corresponding polymers was found in their corresponding polymers studies To illustrate

it, the conductivity of Polyb was found to decrease one-tenth of the conductivity of

Polya But to our surprise, although large torsion occurred in the thiophene-azulene

copolymer systems, these polymers still showed high conductivities in the range of 100S/cm

09

1.726

1.727

1.710 1.3

2

1.415 1.416

38 7

1.704

1.3 78 1.3

15

1.733

1.735

1.709 1.3

7

1.423 1.428

1.

37 5

1.3 72

1.484

1.38

83 1.39 1 1.

390

1.708

1.3 88 1.3

85

Monob

Figure 4-5 Bond distance in (a), Monoa; and (b), Monob

Figure 4-6a is a stereoscopic view of the Monoa crystal, viewed along a axis, which

displays molecular packing The crystal packing is characterized by the layered structure, composed of the molecular zigzags and the herringbone structure, which are also characteristic of organic crystals such as 2,6-dimethylnaphthalene21 and DMQtT (α,α’-dimethylquaterthiophene).20 The smallest distance between the carbon ring is 3.452 Å, this is roughly twice the carbon van der Waals radius (1.70 Å) The short distance demonstrates the presence of the close intact of the carbon rings because of the dipolar moment of azulene Figure 4-6b presents the overlap of the molecules from a different

view of the molecules packing Stereo views of Monob are shown in Figure 4-6c,d To our surprise, we found that Monob showed a different packing from Monoa because of

the methyl group substitution The molecule shows the herringbone structure with an

angle of ca 600 between a pair of molecular planes Figure 4-6d showed the molecular overlapping It showed that neighboring molecules are packed in opposite direction, with

a small intermolecular distance ca 3.58 Å between the C2 of azulene and neighboring

seven-membered ring of azulene It should belongs to the weak C-H⋅⋅⋅π interaction

(a)

(b)

(c)

(d)

Figure 4-6 Crystal structure of Monoa viewed from (a), a axis; (b), b axis, and Monob viewed

from (c), a axis; (d), c axis

Table 4-1 Crystal data and experimental details

empirical formula C 18 H 12 S 2 C 20 H 16 S 2

radiation wavelength, Å 0.71073 0.71073

final R indices [ I>2σ(I)]

Structure of Oligoa and Oligob

Single crystal of Oligoa and Oligob were obtained and their molecular structure and

atomic numbering are shown in Figure 4-7a The crystallographic parameters for these

structures are listed in Table 4-2 Single crystal of Oligoa is monoclinic system with

crystal of Oligoa was identified as molecular weight of 472.63 with observed density at

1.329 g/cm3, and the molecular weight of Oligob is 664.92 with density at 1.338 g/cm3

(a)

(b)

Figure 4-7 The molecular structure and atomic numbering of (a), Oligoa, and (b), Oligob

Although these oligomers showed longer conjugation than the monomers (will discuss in the following UV-vis part), large torsion angles were observed in these oligomers

between the adjacent moieties Oligoa is a systemic molecule, the torsion angle between

the thiophene and the five-membered ring of azulene was found to be 48.10 S1), and the torsion angle between the thiophene and the benzene ring was found to be 9.10 (S1-C10-C12-C17) The torsion angle between thiophene and azulene is bigger than

(C4-C5-C7-that of in the Monob However, because of the small torsion angle between the thiophene

ring and benzene ring, effective conjugation of Oligoa is longer than that of Monob For

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

ring is 48.20 (C9-C10-C11-C12) and the torsion angle between the two thiophene ring is 38.90 (C13-C14-C16-S3) To our surprise, we found that that the terminal benzene is nearly vertically positioned to the thiophene ring We think such a large torsion angle was due to the steric hindrance between the benzene ring and the methyl group on thiophene

Compared with the crystal structures of Monob, we found that methyl groups in oligomers were all arranged differently from that in Monob In both oligomers, methyl

groups are all pointed to the positions favoring the release of the steric hindrance Also

we found two adjacent thiophene rings in Oligob adopt all-anti conformation as observed

in most oligothiophenes.19,20 These arrangement may be better approach of the real arrange of the side alkyl group in the resulting copolymers Combined all the observation, such as the large torsion angle, anti-conformation of the adjacent thiophene rings, and methyl group positions, we may conclude that the alkyl side group are not arranged in a same direction Because of the large torsion angle, the long side alkyl group may arrange

a cylindrical structure with the rigid conjugated backbone in the center Such “hairy rods” structure may explain the copolymers’ properties we observed in Chapter 3, such as similar UV-vis absorption of the polymers in solution and in solid state, amorphous structures observed in XRD, and etc

(a)

(b)

Figure 4-8 ORTEP drawing of Oligoa and Oligob

It is of particular interest to compare the crystal structures of the oligomers with the

monomers Bonds lengths for Oligoa and Oligob are displayed in Figure 4-9 Because

effective conjugation extends in these oligomers, significant bond alternation should be observed in the oligomers crystals β-β Single bond in thiophene of Oligoa (C8-C9) was found to be 1.419 Å, and the α-β double bond was found to 1.361 Å (C7-C8), the difference is about 0.058 Å This difference was found to be 0.043 Å and 0.051 Å in

Monoa and Monob, respectively The bond length difference between β-β single bond and the α-β double bond in the thiophene ring was found to be 0.048 Å and 0.076 Å, respectively Such a difference may be due to the electron-donating properties of the five-membered ring of azulene and the large torsion angle between the thiophene ring and the benzene ring

1.7 27

1.3981.4 11

1.3 90 385

1.465

1.361 1.419 1

.3 58

1.7 25

1.7 28

1.420 1.

356

1.7 24

Figure 4-9 Bond distance in (a), Oligoa; and (b), Oligob

Figure 4-10a,b is a stereoscopic view of the crystal of Oligoa The overall crystalgraphic profile is characterized by a layered structure of Oligoa molecules It was noteworthy that

azulene are pointed to opposite direction at alternative layers as can be seen clearly from Figure 4-10a Such molecular packing was attributed to the large dipolar moment of azulene The smallest intermolecular distance between the seven-membered ring of azulene is 3.663 Å which is roughly twice the carbon van der Walls radius (1.70 Å).21

Stereo views of Oligob are shown in Figure 4-10c,d The molecules are packed in a layered structure consisting of slipped stacks The backbones of adjacent Oligob in the stack are laterally displaced and only slightly overlap Similar to that of Oligoa,

neighboring molecules are packed in opposite direction Interestingly, there is hydrogen bonding between the proton of seven-membered ring of azulene and the sulfur in the

thiophene near to benzene ring The C-S distance is about 3.666 Å with an angle about 144.60 This interaction may be due to the weak acidity properties of protons on the seven-membered ring of azulene and the electronegative property of sulfur atom This

interaction may arrange molecules of Oligob packed in alternative column with opposite

direction

(a)

(b)

(c)

(d)

Figure 4-10 Stereoscopic view of the oligomers: (a), crystal structure of Oligoa viewed along a

axis, (b), crystal structure of Oligoa viewed along c axis, (c), crystal structure of Oligob viewed along c axis, and (d), crystal structure of Oligob viewed along a axis

Table 4-2 Crystal Data and Experimental Details

crystal system Monoclinic Triclinic Orthorhombic

Single crystal of MonoO6 was obtained by recrystallization from dichloromethane and

hexane The obtained crystal is dark green cubic solid with density at 1.163 g/L Crystal

data for MonoO6 are given in Table 4-3 The space group of MonoO6 was determined to

be Pnaa and the crystal system was found to be a orthorhombic The numbering scheme

and the molecular structure of MonoO6 are displayed in Figure 4-11

Figure 4-11 The molecular structure and atomic numbering of MonoO6

As expected, a larger twist between the thiophene ring and the azulene ring was found The torsion angle between the thiophene and the five-membered ring of azulene is 400(C2-C3-C15-S2) and 440 (C2-C1-C11-S1) Furthermore, because of the large steric effect

of the long side group, a diminution of the angle between thiophene and azulene was found The angle between S2-C15-C3 was found to be 92.50, which is smaller than in

Monob (1180) Interestingly, because of the electron-donating property of oxygen, the bond-alternation in the seven-membered ring decreased compared with other monomers and the oligomers From Figure 4-12b, we found the bond-alternation in the seven-

membered ring of azulene is around 0.005Å While this difference in Monoa, Monob,

Oligoa, and Oligob are found to be 0.015, 0.008, 0.011, 0.007 Å respectively

(a)

S S

1.4011.4 07

1.3 84

(b)

Figure 4-12 (a), ORTEP drawing of MonoO6 and (b), bond distance in MonoO6

The packing arrangement of MonoO6 is of the “herringbone” type: the molecular axes

are parallel but the molecules in adjacent layers are inclined with respect to each other Herringbone is an usual packing of conjugated oligomers, as well as polymers.22-24 The

long axes of the molecule is parallel to the ac-plane The most noticeable feature in the

packing is the head-to-head arrangement of the molecules As can be seen in Figure 13a, all these molecules are packed in parallel and the neighboring molecules were arranged in head-to-head pattern Figure 4-13b presents a different view of the molecule

4-along c axis From this we can see that all the MonoO6 molecules are lined and form

generally zigzags along the c axis

(a)

(b)

Figure 4-13 Stereoscopic view of the oligomers: (a), crystal structure of MonoO6 viewed along

a axis, (b), crystal structure of MonoO6 viewed along c axis

UV-vis Spectra and NMR studies

We have discussed the UV-vis spectra of Monoa and Monob in Chapter 3 Compared

with these monomers, oligomers showed distinct bathochromic shift due to the extension

of the conjugation (Figure 4-14) Monob with 3 conjugated units showed a λmax of 296

nm, while Oligoa, which has 5 conjugated units in its backbone, displayed a λmax of 325

nm Oligob, which has 7 conjugated units in the backbone, was characterized by its

longest wavelength absorption at λmax of 377 nm The bathochromic shift of the longest wavelength absorption band as the number of aromatic units increase in the backbone is because addition of every aromatic units will cause hybridization of the energy level and thus yielding more and more levels This is in good agreement with the theory calculation, which showed that the energy level of oligothiophenes was shown as a function of the oligomers length.25

0 1

2

Oligob Monob Oligoa

Monob at different TFA concentration was also displayed here As we discussed in

Chapter 3, solution color of Monob gradually changed from green to light blue and to

purple with more TFA added The longest absorption at 545-555 nm was attributed to the formation of the azulene radical cations as described by the reference.26

Compared with Monob, Oligoa showed less UV-vis spectrum changes upon protonation,

as shown in Figure 4-15b When TFA wa added to the solution of Oligoa, two new bands

appeared at 571 and 467 nm, respectively However, their absorption intensities changed

insignificantly as we observed in Monob

0 1

2

TFA concentration 0%

2

TFA concentration 0%

400 600 800 1000 0

1

2

TFA concentration 0%

2

TFA concentration 0%

Upon protonation, Oligob showed two new peaks appeared at 650 and 532 nm,

respectively (Figure 4-15c) It also showed similar UV-vis spectrum changes to that of

copolymers Quite different from Oligoa, Oligob showed significant UV-vis spectrum changes during the protonation process than that of Oligoa To estimate the response

ability of the oligomers to the TFA concentration, we defined the absorption intensity

ratio between the longest absorption in neutral state (for Oligob, it appeared at 377 nm) and the newly appeared longest wavelength absorption in 40% TFA solution (for Oligob,

it appeared at 652 nm) as an index For Oligob, the index is approximately 1.05, while the index was found to be 4.63 in Oligoa

UV-vis spectrum of MonoO6 is similar to that of these monomers, except a red shift of

the longest absorption at neutral state upon protonation At neutral state, the longest absorption appears at about 313 nm, and upon protonation, the longest absorption appeared at around 607 nm

To gain insight into the mechanism of UV-vis spectrum change upon protonation,

1HNMR experiments were carried out to study these monomers and oligomers upon protonation with deuterated TFA Figure 4-16a shows 1HNMR spectrum of protonated

Monob We found that the protonation has little effect on the azulene protons As shown

in its 1HNMR spectrum, H4,8 and H2 of azulene keep intact upon protonation with deuterated TFA except some degree shift We think this is may be due to the fact that the protonation was mainly occurred at the α-position of thiophene Because of the negative

polarity of the five-membered ring of azulene,27 π-electron of the azulene will easily delocalized to the thiophene ring, which will further activate the α-position of thiophene This conclusion was also confirmed by two facts: (1) in its 1HNMR spectrum as shown in Figure 4-16a, protons of thiophene were mainly affected by TFA; (2) 1HNMR spectrum

of Monob after protonation at room temperature for two days showed that Monob had

been polymerizaed

However, protonated Oligoa showed a quite different 1HNMR spectrum from its neutral

state When deuterated TFA was added to the solution of Oligoa, all signals of Oligoa

were combined together and formed two broad peaks at around 7.5 and 8.3 ppm,

respectively This fact tells us that upon protonation, all protons in Oligoa were affected

This may be due to the formation of azulenium cation radicals (polaron) upon protonation Because the α-position of thiophene were blocked by benzene ring, polymerization cannot occur and protonation may mainly occurred on azulene ring Furthermore, because of the short conjugation length, may be only polaron can form in

Oligoa The formed polaron can hop easily along the conjugated molecule and thus the

high delocalized charge makes these peaks combine together This conclusion was also confirmed by the UV-vis-NIR spectrum and EPR study In its UV-vis-NIR spectrum upon protonation, absorption at 580 nm and 1250 nm was attributed to the formation of polaron in the conjugated system (Figure 4-15b) While in UV-vis-NIR spectrum of

Oligob, absorption at 652 nm was found to be the main absorption upon protonation,

which was attributed to the formation of bipolaron EPR gave further evidence In EPR measurement of these oligomer solutions (same concentration 10-2 M) protonated with

20% TFA, Oligoa showed stronger intensity than that of Oligb, which was attributed to the formation of EPR silent bipolaron in Oligob

Based on the above discussion, it is not difficult to understand the 1HNMR spectrum of

protonated Oligob as shown in Figure 4-16c An important feature in it is the highly

deshielded nature of the azulene hydrogen In this spectrum, we found that the H4,8 on azulene ring shifted to about 9.2 and 8.7 ppm This indicated the formation of azulenium ions.28,29 The bipolaron is relatively stable because of the short of the conjugation length, which limits the hopping of the charges

Cyclic voltammogram study

The cyclic voltammograms of Monoa and Monob, which are representative for the

oxidative process of the monomers are shown in Figure 4-17a The initiating curve (Eap =

1.02 V) clearly demonstrates the quasi-reversible cation formation The growth of the polymers film is shown in Figure 4-17c Consecutive cyclic voltammograms scans showed that the current increased with repeated scans, indicating the conducting polymers film are being formed The dark green polymer film deposited on the platinum electrode was not soluble in dichloromethane After the polymers were discharged at around -0.5 V and washed with acetonitrile, the polymers was then doped with iodine, a

high conductivities at about 7.9 and 5.1 S/cm was found for poly(Monoa) and poly(Monob) respectively The formation of the radical anion occurs at a peak potential

Ecp = -1.72 V for both monomers as shown in Figure 4-17b

1

0

Monob

Monoa 0.1 mA

Figure 4-17 Cyclic voltammograms in dichloromethane, 0.1 M TBAHFP (tetrabutylamonium

hexflouorophosphate) (a), the oxidation process of Monoa and Monob, (b), the reduction process

of Monoa and Monob, (c), the anodic electropolymerization of Monob Scan rate: 80 mV/s

The phenyl substitutions in oligomers prevent both Oligoa and Oligob from electropolymerization In Oligoa and Oligob, as shown in Figure 4-18, the oxidation

process lead to two waves in the cyclic voltammogram This fact suggests two-stage oxidation process in these oligomers which were attributed to the radical-cation and

dication formation occur in reversible one electron transfer process As expected, Oligob with longer effective conjugation display lower oxidation potentials than that of Oligoa For Oligoa, one appears at 0.82 V and the second appears at 1.20 V For Oligob, the

lower one appears at 0.80 V and the second appears at 1.02 V Both compounds showed similar reduction process as shown in Figure 4-18

800 600 400 200 0 -200

Figure 4-18 Cyclic voltammogram of (a) Oligoa (1×10 -3M) and (b) Oligob (1×10-3 M) in

CH 2 Cl 2 solution containing n-Bu 4 NPF 6 (0.1M) Scan rate 80 mV/s

Conclusions

In order to better understand the physical properties of the copolymers we obtained in Chapter 3, we designed and synthesized a series of monomers and oligomers as model compounds for the corresponding copolymers

In this chapter, models compounds such as Monoa, Monob, Oligoa, and Oligob were

designed and synthesized In order to gain insight into the side chain effect on the

polymers packing, MonoO6 was designed and synthesized These compounds were full

characterized by HNMR, CNMR, FT-IR spectra To assign the carbons in the oligomers, 2-D HMQC NMR experiments were carried out

Crystal structures of Monoa, Monob, MonoO6, Oligoa, and Oligob have been determined Both Monoa and Monob crystallize in the orthorhombic system with space group Pbca The crystal structure of Monoa is characterized by regular layered structure

composed of the molecular zigzags and the herringbone structure The shortest distance

between the ring carbons is 3.452 Å Monob showed a quite different packing from

Monoa because of the methyl group substitution The molecule showed the herringbone

structure with an angle of ca 600 between a pair of molecular planes Both compounds

showed large torsion angle between the thiophene ring and the azulene ring In Monoa,

the angle between thiophene and the five-membered ring of azulene was found to be 43.20 and 34.40 In Monob, the torsion angle between thiophene and the five-membered

ring of azulene ring was found to be 44.70 and 45.00 The large torsion angles arise from steric hindrance between the seven-membered ring of azulene and the thiophene ring

The crystal structure of Oligoa is the monoclinic system with space group C2/c while crystal Oligob is triclinic system with P-1 space group Similar to the monomers, these

oligomers also showed large torsion angle between thiophene and the azulene ring In

Oligoa, the torsion angle between thiophene and the five-membered ring of azulene was

found to be 48.10 and the torsion angle between thiophene and the benzene ring was found to be 9.10 In Oligob, the torsion angle between thiophene and the five-membered

ring of azulene ring is 48.20 and the torsion angle between the two thiophene ring is 38.90 A large torsion angle was found between the thiophene ring and the terminated

benzene ring The torsion angle between thiophene and the terminated benzene ring is about 101.60 The overall crystallographic profile is characterized by a layered structure

for Oligoa molecules The smallest intermolecular distance between the seven-membered

ring of azulene is 3.663 Å which is roughly twice the carbon van der Walls radius The

backbones of adjacent Oligob in the stack are laterally displaced and only slightly

UV-vis spectra of the monomers and oligomers showed an increase of longest wavelength absorption with the increase of the conjugated units and electron-donating substituents It is also interesting to note that all these compounds show continuous changing of the UV-vis spectra with continue addition of TFA to their chloroform solution To gain insight into the reason of the change, HNMR experiments of the protonated compounds were carried out and the formation of azulenium ion was confirmed

Electrical properties of these compounds were measured by cyclic voltammograms The experiments showed that the monomers could be electropolymerized during the oxidation

process And the phenyl substitution in the oligomers prevents both Oligoa and Oligob

from electropolymerization The monomers showed quasi-reversible cation formation and the oligomers show two oxidation waves which were attributed to the radical-cation and dication formation occurred in reversible one electron transfer process

References

1 Tour, J M Chem Rev 1996, 96, 537

2 van Hutten, P F.; Wildeman, J.; Meetsma, A.; Hadziioannou, G J Am Chem Soc 1999, 121,

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3 Electronic Materials: The Oligomer Approach; Müllen, K.; Wegner, G., Eds.; Wiley-VCH:

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5 Hotta, S.; Waragai, K Adv Mater 1993, 5, 896

6 Hotta, S.; Goto, M Adv Mater 2002, 14, 498

7 Fichou, D.; Ziegler, C.; Handbook of Oligo- and Polythiophenes (Ed: Fichou, D.) Wiley-VCH,

Weiheim, Germany, 1998

8 Antolini, L.; Horowitz, G.; Kouki, F.; Garnier, F Adv Mater 1998, 10, 382

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10 Cornil, J.; Calbert, J Ph.; Brédas, J J Am Chem Soc 2001, 123, 1250

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12 Hajlaoui, R.; Fichou, D.; Horowitz, G.; Nessakh, B.; Constant, M.; Garnier, F Adv Mater

1997, 9, 557

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14 Keegsta, M A.; Peters, T H A.; Brandsma, L Tetrahedron 1992, 48, 3633

15 Keegsta, M A.; Peters, T H A.; Brandsma, L Synth Commun 1990, 20, 213

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17 Barbarella, G.; Zambianchi, M.; Bongini, A.; Antolini, L Adv Mater 1992, 4, 282

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Polym Phys 1992, 30, 293

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Polym Phys 1986, 24, 2793

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Chapter 5

Reason for the high conductivity of the azulene containing copolymers by studying their monomer- TCNQ charge-transfer crystal structures and corresponding polymer-TCNQ charge-transfer complex Introduction

In Chapter 3, we showed the high conductivities of a series copolymers containing azulene moiety in the main chain Upon doping with iodine or protonation with TFA, these copolymers showed relatively high conductivities (1-102 S/cm) compared to those thiophene-arene copolymers (10-5–10-2 S/cm).1-3 However, single crystal structures study

of their monomers and oligomers in Chapter 4 revealed that there were large torsion angles between azulene moiety and the thiophene rings due to sulfur/substituted group and the seven-memberd ring repulsion The large torsion angles are believed to hindrance the electron moving along the polymers backbone, which would greatly decrease electrical conductivities of the resulting polymers Thus we met a question: why did the non-planar copolymer system show such high conductivity? To answer this question, charge-transfer (CT) complexes were designed and synthesized as model compounds to mimics the doping process of the polymers with the electron-acceptors In view of the role of monomer-acceptor as model system, we proceeded with a comprehensive study of

the interaction of TCNQ (tetracyano-p-quinodimethane) or TNB (trinitrobenzene) with a

variety of monomer derivatives

As we have discussed in Chapter 1, many conjugated polymers can be transformed by oxidation or reduction in the solid state to derivatives which exhibit metal-like

conductivity By the redox reaction, charge carries are generated on the polymer backbones which are counterbalanced by counterions Thus conductive polymers are salts built up by polymer chains bearing charges and counterions.4,5 The term “doping” can be understood as the redox process between the conjugated polymer backbone and electron-donating or electron-accepting reagents However, because of the insolubility and irregular structure of the polymers, it is difficult to study the conductive polymers salts Thus oligomers are considered to be ideal model compounds to investigate the doping mechanism of the corresponding conductive polymers.6-8 For example, oligothiophene materials can be doped in a manner similar to polythiophenes, using oxidating agents like iodine, nitrosyl salts NO+X-, electron-acceptor like TCNQ, or by electrochemical oxidation were investigated.9,10 In the case when the oligomers are doped with organic molecular acceptors like TCNQ, the doping produces CT complex comprising mixed column of D (donor)-A (acceptor) units.10

In fact, since the discovery of the conductive organic complex (TTF.TCNQ) by Ferraris

in 1973,11 organic charge-transfer (CT) complex are being increasing investigated, both theoretically and experimentally, as their molecular structure allows an easy preparation

of single crystal, well-characterizable compounds with high electrical conductivity.12 The formation of such CT complex is based on the interaction of an energetically high-lying HOMO (of the donor) with a low-energy (of the acceptor) Many one-dimensional 1,3,5-trinitrobenzene (TNB) as well as the donor tetrathiafulvalene (TTF) have already been investigated and superconductivity was proven even for radical ion salts.13-16 These effort have allowed the establishment of some of the electronic and structural requirement to form electrically conducting materials.17

On the other hand, the conduction of conducting polymers can be improved in the doped

state by increasing the electron mobility along the polymer backbone via π-conjugation,

as well as along the stacking direction via π-orbital overlapping.18,19 Müllen et al have

prepared a fully conjugated structure by inserting a TTF moiety into the conjugated backbone.20 However, due to its insolubility, the obtained materials could not be

characterized Yamamoto et al reported a preparation of soluble π-conjugated polymers

containing TTF units in the π-conjugated main chain.21 However, these polymers did not

form CT complex with an organic acceptor such as TCNQ Recently, Chujo et al

reported the synthesis of a new π-conjugated polymer having electron-donating dithiafulvene unit in the main chain by a cycloaddition polymerization of an aldothiokene derived from aromatic diyne monomer with its alkynethiol tautomer.22,23 The UV-vis absorption spectra showed the formation CT complex between the π-conjugated polymer and TCNQ

In this project, the charged states of the monomers were obtained through oxidation with weak acceptors such as TCNQ and TNB These charge-transfer complexes easily formed with monomers as needle crystals To further develop the charge-transfer system, charge-transfer complex between TCNQ and conjugated polymers with azulene unit as electron-donating group in the polymer backbones were prepared The specific objectives of this work are to determine: (1) the crystal structure change before and after the CT complex formation, which will give valuable inforamtion to understand the doping mechanism of the corresponding conjugated polymers, (2) the conductivities of the monomers after

“doped” with electron acceptor – TCNQ, TNB will be measured to study the CT π-orbital overlap effect on the conductivity of the CT complex, (3) the effects of different substitution on the absorbance maxima (λCT) of the CT bands of the complex of acceptors

with monomers, (4) to develop new materials based on the charge-transfer properties of azulene And at last, to answer the question: “why high conductivities was found in the large torsion angle conjugated polymers system

Results and Discussion

Synthesis of the monomers and their charge-transfer complex

Synthesis of the Monoa and Monob have been reported in detail in Chapter 3 For

Monoc, the synthesis is similar to that of Monoa, except of the preparation of the

3-methoxy-thiophene As shown in Scheme 5-1, 3-methoxy-thiophene was prepared by reaction of 3-bromothiophene with NaOMe in toluene Because the high reactive of the 3-methoxy-thiophene, bromination of 3-methoxy-thiophene was carried out in ice-water bath and the obtained product was kept at low temperature in nitrogen atmosphere

The preparation of Monoc involved the Grignard coupling reaction catalysized by

Ni(dppp)Cl2 Monoc was obtained as green solid after purification by column

chromatography Because of the high reactive of the 2-bromo-3-methoxy-thiophene, there are no mono-substituted product during the Grignard reaction if the mole ratio of Grignard reagent to the 1,3-dibromoazulene is kept 2.5 Interestingly, we found the

Monoc is quite stable at room temperature although the methoxy substituents are

believed to greatly deduce the oxidation potential of the resulting products This is may

be due to the dipolarity of azulene which share some the rich electron on the thiophene rings

Br Br

crystal of Monoa.TCNQ and Monoc.TNB are too small to do the X-ray structural analysis, only single crystal of Monoa.TNB and Monoc.TCNQ were analysized by X-

ray crystallographic measurement