Controlling the Conductivity of Oligomer Radical Cations by Tuning

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Controlling the Conductivity of Oligomer Radical Cations by Tuning Stacking Structures of #-Dimers Xiaoyu Chen, Li Zhang, Shenxin Yao, Gengwen Tan, and Xinping Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01554 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on August 5, 2019

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Crystal Growth & Design

Controlling the Conductivity of Oligomer Radical Cations by Tuning Stacking Structures of π-Dimers Xiaoyu Chen,† Li Zhang,# Shenxin Yao,‡ Gengwen Tan*,‡ and Xinping Wang*,‡

†

Key Laboratory of Coal Methane and Fire Control, China University of Mining and

Technology, Xuzhou 221008, China. ‡State

Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic

Materials, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, China # Center

of Materials Science and Engineering, Guangxi University of Science and Technology,

Liuzhou 545006, China KEYWORDS: Dialkoxyterphenyl, Radical cation, π-Dimer, Stacking structure, Conductivity

ABSTRACT: Salts containing radical cations of dialkoxyterphenyls (DP) including 4,4′dimethoxyterphenyl (1) and 4,4′-diethoxyterphenyl (2) have been isolated with weakly coordinating anions [Al(ORF)4]- (ORF = OC(CF3)3) or [Al(ORMe)4]- (ORMe = OC(CF3)2CH3) as the counterions. The radical cation salts have been characterized by single crystal X-ray diffraction analysis, UV-Vis absorption and EPR spectroscopy, as well as conductivity measurements. Single crystal structures indicate that the radical cations assemble into infinite stacks of π-dimer in these salts. While single crystal conductivity measurements show the

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conductivity of the salts is significantly affected by substituent groups and the interaction between cation, anion and solvent.

1 Introduction Designs and syntheses of organic conductive materials have attracted continuous interest for decades.1-3 Numerous researches reveal that the electron can migrate not only along the polymer chains, but also across the chains in form of unpaired electron interchain transport.4-5 Some organic conjugated oligomers assembled into π-dimers and π-stacks are assumed conductive if they could transfer electrons via stacks.6-9 Reports pointed out that some organic π-conjugated oligomers bearing intermolecular overlapping electron cloud feature unique electronic characters (by a mechanism for interchain charge transport).10-12 Hence π-conjugated oligomers and polymers, such as poly/oligo-phenylenes, thiophenes, phenylenevinylenes, fluorenes have been of considerable attention because of their potential applications in molecular electronics.4,13-15 However, owing to the intrinsic instability and low solubility, crystal structures of π-stacks of oxidized oligomers were rarely reported, although there was some spectroscopic evidence presented.16-21 The π-conjugated co-oligomer radical cation of 3′,4′-dibutyl-5,5′′ -diphenyl2,2′:5′,2′′-terthiophene reported in 1996 represents the first example of these π-stacked conductive materials (Scheme 1).22 Since then, no example of conductive oligomers based solely on oxidized (radical cation) π-stacks until we raised the first π-stacking conductive π-phenylene cationic oligomers23 and the following meso-helical π-stacking conductive cationic thiophene/phenylene co-oligomer (Scheme 1).24 It is presumed that the π-stacks and πinteractions are important for conduction in radical cations.25 Because of the lack of examples, there is no detailed study on the relationship between structures and conductivity of π-stacking


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radical cations. Therefore, we were interested in undertaking systemic study on conductive radical cations and controlling their conductivity by tuning π-stacking structures. In this paper, we report the syntheses, crystal structures, spectroscopic characterizations, and conductivity measurements of dialkoxyterphenyl (DP) (Scheme 2) radical cations with weakly coordinating anions [Al(ORF)4]- (ORF = OC(CF3)3) or [Al(ORMe)4]- (ORMe = OC(CF3)2CH3) (Scheme 3).25-29 Our results indicate that the formation of radical cationic π-dimers and π-stacks is important for electron migration in oligo-phenylene derivatives, and less bulky substitute group, appropriate anion-cation interaction and participation of solvent improve the conductivity.

S S

H 2N

S

NH2

S

S

Scheme 1. Conductive π-conjugated (co-)oligomer radical cations.

OR

RO DP (1, R = Me; 2, R= Et)

Scheme 2. Oligo-phenylene derivatives dialkoxylterphenyls (DP).

F3C

CF3 C

CF3 O

O

F3C

C

C

CF3 CF3

Al

F3C F3C

F3C

C

CH3 CF3 C O O

H 3C CF3

CF3

CF3 C

C

F3C

CH3 CF3

Al

F3C

O

O

F3C

CF3

O

O F3C

C

CF3 CH3

Scheme 3. Polyfuloroalkoxy aluminate anions.


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2 Experiment part 2.1 General Procedures All experiments were carried out under a nitrogen atmosphere using standard Schlenk techniques or in a nitrogen-filled glove box. NOSbF6 (Alfa Aesar) was purchased and used upon arrival. 4, 4′-dimethoxy-p-terphenyl (1), 4,4′-diethoxy-p-terphenyl (2), Li[Al(ORF)4] and Li[Al(ORMe)4] were prepared according to literatures.30-32, 25-29 Solvents were dried prior to use. EPR spectra were obtained using a Bruker EMX-10/12 spectrometer at room temperature. UVVis spectra were recorded on UV-3600 and Lambda 35 spectrometers. Element analyses were performed on an Elementar Vario EL III instrument at Shanghai Institute of Organic Chemistry, the Chinese Academy of Sciences. For conductivity measurements, single-crystal samples were affixed on glass carriers and silver paste was used to connect samples and electrodes along the crystallographic axis. I-V curves were measured by using a computer-controlled Keithley 2400 source meter. X-ray crystal structures were obtained by Bruker APEX DUO CCD detector. Single crystals were coated with Paratone-N oil and mounted using a glass fiber. Crystal data details are listed in the supporting information (Table 1, SI). Syntheses of 1•+[Al(ORF)4]-. Under anaerobic and anhydrous conditions, a mixture of 1 (0.12 g, 0.41 mmol), NOSbF6 (0.11 g, 0.41 mmol) and Li[Al(ORF)4] (0.4 g, 0.41 mmol) were dissolved in 50 mL CH2Cl2 and stirred at room temperature for 3 d (Eq.1). The resultant brown solution along with colorless precipitate (LiSbF6) was filtered. The filtrate was then concentrated and stored at ca. 5 oC for 3 d to afford X-ray-quality crystals of the radical salt 1a•+[Al(ORF)4]-


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•0.5CH2Cl2. Yield: 0.43 g, 83%; Mp: 222–224 oC; Elemental analysis (%): calcd C 33.19, H 1.47; found C 33.53, H 1.56.

DP + NOSbF6 + Li[X] DP = 1, 2

CH2Cl2

DP [X]- + LiSbF6 + NO

(1)

X = [Al(ORF)4], [Al(ORMe)4]

Syntheses of 1•+[Al(ORMe)4]-. Using the same procedure described for 1a•+[Al(ORF)4]-, 1 (0.1 g, 0.34 mmol), NOSbF6 (0.092 g, 0.036 mmol) and Li[Al(ORMe)4] led to brown crystals of 1b•+[Al(ORMe)4]-•CH2Cl2. Yield: 0.30 g, 86%; Mp: 188–192 oC (decomp.). Elemental analysis (%): calcd C 41.49, H 2.90; found C 41.30, H 3.13. Syntheses of 2•+[Al(ORF)4]-. Using the same procedure described for 1a•+[Al(ORF)4]-, 2 (0.095 g, 0.30 mmol), NOSbF6 (0.080 g, 0.03 mmol) and Li[Al(ORF)4] (0.293 g, 0.03 mmol). Yield: 0.30 g, 77%; Mp: 224–226 oC. Elemental analysis (%): calcd C 35.50, H 1.72; found C 35.68, H 1.81. 3 Results and discussion 3.1 Crystal structures 3.1.1 Crystal structure of 1a•+[Al(ORF)4]-•0.5CH2Cl2 Concentrating and cooling the CH2Cl2 solution of 1•+[Al(ORF)4]- yielded brown-yellow crystals of 1a•+[Al(ORF)4]-•0.5CH2Cl2. The crystal structure of 1a•+[Al(ORF)4]-•0.5CH2Cl2 consists of radical cations infinitely stacked along the a-axis with channels occupied by anions and CH2Cl2 solvent molecules (Crystal packing are presented in Supporting Information, Figure S1). In each


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stack, two independent radical cations stacked as a dimeric pair with essentially the same structures (Figure 1). The closest interplanar distance between the two radical cations in the dimeric pairs is 3.1941(5) Å (Figure 2), which is less than the equilibrium van der Waals separation of 3.4 Å, indicating an electronic coupling between them. Such close packing of the radical cations in dimeric pairs leads to staggered arrangement of the OCH3 groups to avoid steric crowding (Figure 1). The interdimeric – interaction is suppressed by the steric crowding but reinforced by hydrogen bonding interactions with CH2Cl2 molecules (Figure 3). Under the comprehensive effect, each dimer is partially overlapped (1/2(-C6H4C6H4-)) with two neighboring dimers and the closest interdimeric C-C distance (3.4017(5) Å) is similar to the combined van der Waals radii of a sp2 C-C interaction, implying a weak interaction between the dimers (Figure 2). In the radical cation 1a•+, the original phenyl hexagons are distorted, it can be clearly seen in the bond-length variations within the phenyl rings (Figure 4).

Figure 1. Top) Structure of the π-dimer moities in 1a•+; Bottom) The crystal structure of 1a•+ in the wireframe style (hydrogen atoms are not shown).


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b

c

3.1941(5)

a 3.4017(5)

Figure 2. Views of radical cation 1a·+, showing intermolecular interactions within and between dimers.

Figure 3. View of 1a·+ dimer with Cl…H contacts.

C18

C19 C12

O2

C13

C7

C6

O1 C17

C20 C16

C14

C11 C15

C10

C8

C5 C9

C4

C2

C1

C3

a

Figure 4. 50% Ellipsoid drawing of 1a·+. Selected bond lengths [Å]: C1–O1 1.4181(3), O1–C2 b

c

Al C H F O

1.3236(2), C2–C3 1.4118(3), C3–C4 1.3635(2), C4–C5 1.4189(3), C5–C6 1.4176(3), C6–C7


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1.3587(2), C7–C2 1.4012(3), C5–C8 1.4430(2), C8–C9 1.4161(3), C9–C10 1.3540(2), C10–C11 1.4130(3), C11–C12 1.4162(3), C12–C13 1.3640(2), C13–C8 1.4199(3), C11–C14 1.4477(2), C14–C15 1.4016(3), C15–C16 1.3689(2), C16–C17 1.3808(2), C17–C18 1.3686(3), C18–C19 1.3869(2), C19–C14 1.4008(2), C17–O2 1.3652(2) O2–C20 1.4096(3). 3.1.2 Crystal structure of 1b•+[Al(ORMe)4]-• CH2Cl2 Concentrating

the

solution

of

1•+[Al(ORMe)4]-

afforded

yellow-brown

crystals

of

1b•+[Al(ORMe)4]-•CH2Cl2. 1b•+ also has a stacking structure consisting of π-dimers, but in this stack, each dimer has a larger overlapping area (nearly entire -C6H4C6H4-) between two neighboring dimers than that in 1a•+(1/2(-C6H4C6H4-)). In 1b•+, the closest interdimeric C-C distance (3.4229(3) Å) between two dimers and interplanar distance (3.2064(3) Å) between the two radical cations are a little longer than those in 1a•+ (Figure 5), and both the chlorine atoms in one CH2Cl2 molecule exhibit the H…Cl contact with the π-dimers (Figure 6), in contrast to the one H…Cl contact in 1a•+ (Figure 6). Moreover, the H…F contact between the anion and the radical cation is weaker in 1b•+ than that in 1a•+ owing to the less amount of fluorine atoms in the anion.

a

c

3.2064(3)

b

3.4229(3)


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Figure 5. Views of radical cation 1b·+, showing intermolecular interactions within and between dimers.

Figure 6. View of 1b·+ dimer with Cl…H contacts. 3.1.3 Crystal structure of 2•+[Al(ORF)4]Crystals of 2•+[Al(ORF)4]- were obtained from its concentrated CH2Cl2 solution. Crystal structure reveals that the radical cation 2·+ stays in forms of π-dimers in the absence of solvent molecules, and the closest interplanar distance between two radical cations in a dimeric pairs is 3.1780(2) Å, similar to that of 1a·+. In contrast, the closest interdimeric distance becomes 3.9106(3) Å, indicating no van der Waals force interaction. The stacked structure of 2•+ (Figure 7) shows no overlap between two neighboring dimers, indicating steric hindrance caused by bulky ending groups decreases the interactions between neighboring dimers.


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3.1780(2) 3.9106(3)

Figure 7.

Views of radical cation 2·+ showing intermolecular interactions within but not

between dimers. 3.2 Theoretical calculations and spectroscopic properties In order to get insight into the electronic structures of the radical cation salts, we carried out theoretical calculations and UV-Vis spectroscopic studies. In terms of theoretical calculations, we took the dimeric models from the crystal structures, and the calculations were performed at the (U)M06-2X/6-31G(d) level of theory (See SI for details). The calculations revealed that the open-shell singlet state is the ground state for all the dimers, and the electron density is delocalized over the whole cations. The spectroscopic properties of DP•+ radical cations are reminiscent of those observed for previously reported sterically protected oligothiophene radical cations.24 The radical cations DP•+ in CH3CN solutions are in forms of both monomers (M) and -dimers (D), which is supported by various temperature and concentration UV-Vis spectroscopic studies. Figure 8 shows the absorption spectra of 2•+[Al(ORF)4]- in CH3CN solutions, which exhibit five main absorption bands. Similar to previous reports,29 radical cation 2•+ assembles as an equilibrium between the


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monomer (λmax: ~ 290, 500, 1100 nm) and the dimer (λmax: ~ 400, 820 nm). Warming the solution of 2•+[Al(ORF)4]- causes the intensity of the bands of the dimer to decrease while those of the monomer to increase (Figure 8a). The spectral changes are completely reversed by cooling the solutions back to room temperature. Upon diluting the solution, we could see the intensity of the bands of the dimer decreases more dramatically than those of the monomer (Figure 8b). UVVis spectra of other DP•+ salts in CH3CN solutions presented in the supporting information also indicate lower temperature (Figure S4, SI) and higher concentration (Figure S5, SI) favor dimerization. EPR spectra of 1a•+[Al(ORF)4]-•0.5CH2Cl2, 1b•+[Al(ORMe)4]-•CH2Cl2 and 2•+[Al(ORF)4]- in the solid state display a single-line signal (Figure 9). b) 3.0 M 2.5 D

2.0 1.5

3

o

10 C o 15 C o 20 C o 25 C

D

2

6.0 x 1.2 x 2.5 x 5.0 x

D

D

Absorbance

a)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


M

-5

10 M -4 10 M -4 10 M -4 10 M

M

M

1

1.0 M

0.5

0

0.0 400

600

800

1000

400

600

800

1000

1200

1400

wavelength/nm

wavelength/nm

Figure 8. a) Absorption spectra of 2×10-4 M 2•+[Al(ORF)4]- in CH3CN at a function of temperature: 10, 15, 20, 25 oC. b) Absorption spectra of 2•+[Al(ORF)4]- in CH3CN at 25 oC as a function of concentrations: 5.0×10-4, 2.5×10-4, 1.2×10-4, 6.0×10-5 M.


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c)

b)

a)

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25G 25G

25G

Figure 9. Solid-state EPR of 1a•+[Al(ORF)4]-•0.5CH2Cl2 (a, g = 2.0037), 1b•+[Al(ORMe)4]•CH2Cl2 (b, g = 2.0037) and 2•+[Al(ORF)4]- (c, g = 2.0042 ) at room temperature.

3.3 Conductivity All the DP•+ radical cation salts are quite stable in the solid state and can be handled in air for hours, but the solutions are slightly more air-sensitive with slow color fading when left in air. Two probe single crystal conductivity measurements on 1a•+[Al(ORF)4]-•0.5CH2Cl2 and 1b•+[Al(ORMe)4]-•CH2Cl2 at room temperature gave σ = 1-3×10-4 S/cm and 3-5×10-3 S/cm along the a axis (the (100) direction) and b axis (the (010) direction), respectively (Figure S6, SI). It can be expected that conductivity would be higher along the stack direction (i.e., through πstacks) because the axis of the π-stacking direction is at about 45o angle to the axises.6 It can be seen that the conductivity has a close relationship with the stacking structure. Dense stacking with interdimeric distance less than 3.4 Å enables the electron migration across the π-stacks. Larger overlapping areas of electron clouds can improve the electrical conductivity. In contrast, no electrical conductivity was observed for the crystal of 2•+[Al(ORF)4]-, which can be attributed to the long distance between the neighboring dimers.


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4 Conclusions In conclusion, we have presented the preparation, crystallization, and characterization of a series of DP radical cation salts. Conductivity, determined by the stacking structures of π-dimers, is affected by solvent, anion and substitute groups. The salts crystallized as insulators or semiconductors indicate hydrogen bonding effect caused by solvent is a reason for increasing conductivity. Replacement of anion leads to the change of interaction between the neighboring dimers, thereby affecting the conductivity. Changing the ending groups reveals that bulky substituted groups are adverse to the interdimeric interaction. These results help us to get a better understanding of conductivity of organic conductors through π-π interaction.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.xxxxx. UV-vis spectra, I-V curves and crystal data are provided. AUTHOR INFORMATION Corresponding Author [email protected] [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources We thank the National Key R&D Program of China (Grant 2016YFA0300404), the National Natural Science Foundation of China (Grants 21501195, 21525102, 21601082), the Fundamental Research Funds for the Central Universities (Grant 14380134) for financial support. The calculations were performed at the High Performance Computing Center of Nanjing University. REFERENCES (1) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J., Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH). J. Chem. Soc, Chem. Commun. 1977, 578-580. (2) Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; Gau, S. C.; MacDiarmid, A. G., Electrical Conductivity in Doped Polyacetylene. Phys. Rev. Lett. 1977, 39, 1098-1101. (3) Ratera, I.; Veciana, J., Playing with Organic Radicals as Building Blocks for Functional Molecular Materials. Chem. Soc. Rev. 2012, 41, 303-349. (4) Miller, L. L.; Mann, K. R., π-Dimers and π-Stacks in Solution and in Conducting Polymers. Acc. Chem. Res. 1996, 29, 417-423. (5) Joo, Y.; Agarkar, V.; Sung, S. H.; Savoie, B. M.; Boudouris, B. W., A Nonconjugated Radical Polymer Glass with High Electrical Conductivity. Science 2018, 359, 1391-1395. (6) Graf, D. D.; Campbell, J. P.; Miller, L. L.; Mann, K. R., Single-Crystal X-ray Structure of the Cation Radical of 3‘,4‘-Dibutyl-2,5‘‘-diphenyl-2,2‘:5‘,2‘‘-terthiophene: Definitive Evidence for π-Stacked Oxidized Oligothiophenes. J. Am. Chem. Soc. 1996, 118, 5480-5481.


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Characterization, and Crystal Structures of Benzidine Radical Cations. Chem. Eur. J. 2012, 18, 11828-11836. (24) Chen, X.; Ma, B.; Chen, S.; Li, Y.; Huang, W.; Ma, J.; Wang, X., Synthesis, Crystal Structure, and Physical Property of Sterically Unprotected Thiophene/Phenylene Co-Oligomer Radical Cations: A Conductive π–π Bonded Supermolecular meso-Helix. Chem. Asia. J. 2013, 8, 238-243. (25) Krossing, I., The Facile Preparation of Weakly Coordinating Anions: Structure and Characterisation of Silverpolyfluoroalkoxyaluminates AgAl(ORF)4, Calculation of the Alkoxide Ion Affinity. Chem. Eur. J. 2001, 7, 490-502. (26) Krossing, I.; Raabe, I., Noncoordinating Anions—Fact or Fiction? A Survey of Likely Candidates. Angew. Chem. Int. Ed. 2004, 43, 2066-2090. (27) Barbarich, T. J.; Handy, S. T.; Miller, S. M.; Anderson, O. P.; Grieco, P. A.; Strauss, S. H., LiAl(OC(Ph)(CF3)2)4: A Hydrocarbon-Soluble Catalyst for Carbon−Carbon Bond-Forming Reactions. Organometallics 1996, 15, 3776-3778. (28) Barbarich, T. J.; Miller, S. M.; Anderson, O. P.; Strauss, S. H., Coordination of the New Weakly Coordinating Anions Al(OCH(CF3)2)4−, Al(OC(CH3)(CF3)2)4−, and Al(OC(Ph)(CF3)2)4− to the Monovalent Metal Ions Li+ and Tl+. J. Mol. Catal. A 1998, 128, 289-331. (29) Ivanova, S. M.; Nolan, B. G.; Kobayashi, Y.; Miller, S. M.; Anderson, O. P.; Strauss, S. H., Relative Lewis Basicities of Six Al(ORF)4− Superweak Anions and the Structures of LiAl{OCH(CF3)2}4 and [1-Et-3-Me-1,3-C3H3N2][Li{Al{OCH(CF3)2}4}2]. Chem. Eur. J. 2001, 7, 503-510. (30) Sinclair, D. J.; Sherburn, M. S., Single and Double Suzuki−Miyaura Couplings with Symmetric Dihalobenzenes. J. Org. Chem. 2005, 70, 3730-3733.


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For Table of Contents Use Only Manuscript Title: Controlling the Conductivity of Oligomer Radical Cations by Tuning Stacking Structures of π-Dimers Author List: Xiaoyu Chen, Li Zhang, Shenxin Yao, Gengwen Tan and Xinping Wang TOC graphic: RO

OR

X

steric effect

anion effect

solvent effect

solvent effect

R = Me

R = Me

X = [Al(ORF)4]

X = [Al(ORF)4]

X = [Al(ORMe)4]

CH2Cl2 free

with CH2Cl2 semiconductor (1-3 x 10-4 S/cm2)

with CH2Cl2

R = Et

insulator

semiconductor (3-5 x 10-3 S/cm2)

Synopsis: A series of salts containing radical cations of dialkoxyterphenylenes have been isolated with weakly coordinating anions. Single crystal structures indicate the crystals are conductive because the radical cations assemble into infinite stacks of π-dimers in these salts. Single crystal conductivity measurements show the conductivity of the salts is significantly affected by the interaction of substituted groups, anion and solvent.


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Controlling the Conductivity of Oligomer Radical Cations by Tuning

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