Biradical Character of Linear -Conjugated Oligomer Dications Composed of

Biradical Character of Linear -Conjugated Oligomer Dications Composed of Thiophene, Pyrrole, and Methylthio End-Capping Units.

Abstract

The thiophene-pyrrole mixed hexamer to nonamer 10-13 were synthesized and their dication salts were isolated. The DFT calculations predicted that these dications have singlet biradical characters. From the results of TD-DFT calculations and UV-vis-NIR measurements in both solution and solid states, it was suggested that the ground-state electronic structures of dications of mixed octamer 12 and nonamer 13 were singlet biradical. The SQUID measurement of dication salt of 13 showed thermally excited triplet state, indicating that it has narrow singlet-triplet energy gap. This result strongly suggested that the ground-state electronic structure of 13²⁺ was dominated by a singlet biradical character, again. The conductivity of dication salt of 13 was 1000-fold higher than that of dication salt of 10, whose ground-state was suggested closed-shell singlet.

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4-1. Introduction.

Singlet biradical, which is also called as polaron pair or open-shell singlet, is a state that two radicals at different molecular orbital have antiparallel spin. Singlet biradicals and bipolarons (which is also called as closed-shell singlet) are considered to be the most likely electronic structure of long oligothiophene dications, which is the model of highly p-doped polythiophene. Janssen and co-workers reported that electronic structure of dication of duodecithiophene having dodecyl side chains have singlet biradical character.¹ It was also predicted from the DFT calculations that the electronic structures of long oligothiophene dications were singlet biradicals.^2-4 However, recent studies showed that the absorption spectrum of dication of duodecithiophene having bulky groups at

-positions was nearly identical to closed-shell singlet, and the discrepancy between the sterically unhindered and hindered oligomers may be brought about by the presence and absence of an intermolecular - interaction.⁵ In contrast, in our laboratory, it was reported that the sterically hindered sexi- and octithiophenes having bulky bicyclo[2.2.2]octene (BCO) showed absorption spectra which suggested singlet biradical characters of the sexi- and octithiophene dications.⁶ Thus, the electronic structure of long oligothiophene dications still remains unclear. This ambiguity stems partly from lack of information about solid-state properties of oligothiophene dications.

On the other hand, much attention has been focused on the functionalities of the molecule having singlet biradical character. For example, conductive and two-photon absorption properties of biradical molecule having two phenalenyl units⁷ and high performance organic field effect transistor (FET) of oligothiophene having biradical character due to quinoid-like structure by the both terminal dicyanomethyl groups⁸ have been reported. These examples suggest the relationships between functionality and biradical character.

In this work, the biradical character of the dications of thiophene-pyrrole mixed oligomers was investigated from CV, NMR, ESR, absorption spectra, and SQUID, together with the DFT calculations.

4-2. Evaluation Methods of singlet biradical character.

Spectroscopic Analysis.

Time-dependent density functional theory (TD-DFT) calculations predicted that singlet biradicals showed two absorption bands and closed-shell dications showed strong one band.^2,3 Actually, the dication of duodecithiophene reported by Janssen et al. showed two absorption bands.¹ However, the other study suggested that the difference in the number of absorption bands was caused by the intermolecular - interactions.⁵ Thus, it is considered that closed-shell singlet and singlet biradical could be distinguished by the difference in the number of absorption bands in the absence of the intermolecular interaction.

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The observation of thermally excited triplet states, which were generally not observed in closed-shell singlet molecules, were reported in various singlet biradical molecules due to their narrow singlet-triplet energy gaps.^7e,8e10 Thus, the observation of triplet state by thermal excitation with the molecule having singlet character at ground state strongly suggests that the molecule has a singlet biradical character.

Calculations.

The comparison of the calculated energies in closed-shell singlet (CS), open-shell singlet (OS) and triplet (T) states were one of general method for prediction of ground state structure of molecules which potentially have biradical characters.^2-4 In this case, CS state is calculated with the restricted method and spin multiplicity = 1, OS state is calculated with the broken-symmetry method and spin multiplicity = 1, and T state is calculated with the unrestricted methods and spin multiplicity = 3. When the ground state is CS state, the energies in CS and OS states are identical.

The OS state being more stable than CS state indicates that its ground state is OS state. As another method for prediction of biradical character, the S² values in OS state calculations are used. The S² value shows 0 suggests its ground state is CS state, and is close to 1 and 2 suggests its ground state is OS and T states, respectively.⁴ The bond length alternation (BLA)^3,4,12 and spin density distribution¹² are also the guidance for biradical character. The ideal singlet biradical of oligothiophene dication shows the BLA of the benzenoid structure at the central units with the small spin density and the quinoidal structure at the both sides with the large spin density. The most general quantification method of biradical character is biradical index, which was defined by the occupation number of LUMO in the natural orbital occupation number (NOON) analysis.¹³ The occupation number = 0 and 1 of LUMO by NOON analysis means biradical index = 0% and 100%, respectively. However, the biradical indices strongly depend on the level of theory. The comparison among the results calculated at the same level of theory is required.

4-3. Molecular Design.

The problems which are predicted to arise in the studies on long oligothiophene dications are the potential instability and poor solubility in common organic solvents. The incorporation of pyrrole in molecular structure is expected to improve the stability and solubility, because higher stability in the dication state of thiophene-pyrrole mixed pentamer DH5TP^9a than both quinquethiophene DH5T¹⁴ and thiophene-furan mixed pentamer DH5TF¹⁵ was suggested from low oxidation potential of DH5TP (Figure 4-1 and Table 4-1), and because the incorporation of long alkyl chain on nitrogen of pyrrole rings to improve the solubility is easy. Methylthio groups were

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introduced at -position of terminal thiophene ring to stabilize the oxidized states due to the protection from radical coupling in addition to the conjugative electron donation by the lone pair of sulfur arom.^16-18 Based on these molecular design, thiophene-pyrrole mixed oligomer 10-13, which was obtained by cross-coupling of easily accessible thiophene-pyrrole mixed trimer with

,-dibromooligothiophene, were designed (Figure 4-2). The dodecyl groups were chosen to improve the solubility, except for 10. In the case of 10, the hexyl groups were chosen because mixed hexamer with dodecyl side chain had too high solubility to isolate of its dication salts.¹⁹

Figure 4-1. Structures of DH5T, DH5TF and DH5TP.

Table 4-1. First and Second Oxidation Potentials (E/V vs Fc/Fc⁺) of DH5T, DH5TF and DH5TP.

Compd. E1/2 1 E1/2

2

DH5T^a,b 0.37 0.59 DH5TF^c,d 0.20 0.51 DH5TP^d,e 0.06 0.20

aRef. 14. ^bIn THF. ^cRef. 15. ^dIn dichloromethane. ^eRef. 9a.

Figure 4-2. Structures of 10-13.

4-4. DFT Calculations.

To consider the electronic structures of 10²⁺-13²⁺, the calculations of dications of 10’-13’

(Figure 4-3), whose alkyl groups on nitrogen atom of pyrrole units are methyl, as the model of 10-13 were performed at the B3LYP²⁰/6-31G(d) level.

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Figure 4-3. Structures of 10’-13’ and 10’-H – 13’-H.

Energies of Dications in Closed-Shell Singlet (CS), Open-Shell Singlet (OS) and Triplet (T) States.

The calculations of dications in CS, OS and T states were performed to consider the ground state of dications. The computational methods were as described above, and the results were summarized in Table 4-2. All of the most stable calculated electronic structures of 10’²⁺-13’²⁺ were OS states. In the case of 13’²⁺, the energy difference between CS and OS (EOS-CS; -4.2 kcal/mol) was comparable to that of 12T²⁺ which was reported to have singlet biradical character. The trend that the energies in OS and T states relative to that in CS state decreased with increasing number of heterocycles, was observed, which was similar to the results of calculations for oligothiophenes.⁴ Thus, it was suggested that 10²⁺-13²⁺ were appropriate model of p-doped polythiophene, and that their ground states were singlet biradicals.

Table 4-2. Energies (E/a.u.) and Relative Energies (E/kcal mol^-1) of 10’ ²⁺ - 13’ ²⁺ in Closed-Shell Singlet (CS), Open-Shell Singlet (OS) and Triplet (T) states. And S² values before annihilation of 10’²⁺ - 13’²⁺ in Open-Shell Singlet state.

Dication ECS EOS ET EOS-CS EOS-T ET-CS S²

10'²⁺ -3579.5420324 -3579.5447069 -3579.5405008 -1.68 -2.64 0.96 0.76 11'²⁺ -4131.3685718 -4131.3730184 -4131.3702207 -2.79 -1.76 -1.03 0.88 12'²⁺ -4683.1930923 -4683.1990169 -4683.1970699 -3.72 -1.22 -2.50 0.93 13'²⁺ -5235.0160781 -5235.0231559 -5235.0217701 -4.44 -0.87 -3.57 0.97

To examine the effects of incorporation of pyrrole units, the results of calculations for 10’-H²⁺ - 13’-H²⁺, which have hydrogen at the terminal positions (Figure 4-3), and the reported values of -linked unsubstituted oligothiophene dications 6T²⁺-9T^{2+ 4} were summarized in Table 4-3.

From the comparison between dications constructed with same number of hetero five-member rings,

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the trend that the incorporation of pyrrole stabilized the OS and T states relative to CS state was observed at all chain length. This indicated that the incorporation of pyrrole enhanced biradical character. The capping with methylthio groups also stabilized the OS and T states relative to CS state, indicated that the conjugative electron donation by methylthio groups not only stabilized the oxidized state but also enhanced the biradical character.

Table 4-3. Relative Energies (E/kcal mol^-1) of 10'-H²⁺ - 13'-H²⁺ and 6T²⁺ - 9T²⁺ in Closed-Shell Singlet (CS), Open-Shell Singlet (OS) and Triplet (T) states. And S² values before annihilation of 10'-H²⁺ - 13'-H²⁺ and 6T²⁺ - 9T²⁺ in Open-Shell Singlet state.

Dication E_OS-CS E_OS-T E_T-CS S² Dication E_OS-CS E_OS-T E_T-CS S² 6mer 10'-H²⁺ -0.45 -4.76 4.30 0.49 6T^2+a -0.2 -6.0 5.8 0.32 7mer 11'-H²⁺ -1.44 -3.08 1.64 0.73 7T^2+a -0.8 -4.0 3.1 0.60 8mer 12'-H²⁺ -2.46 -2.11 -0.31 0.85 8T^2+a -1.6 -2.7 1.1 0.77 9mer 13'-H²⁺ -3.29 -1.47 -1.82 0.91 9T^2+a -2.4 -1.9 -0.6 0.86

aRef. 4.

S² Values before Annihilation of Dications in OS States.

The S² values before annihilation calculated of dications in OS state were also summarized in Tables 4-2 and 4-3. In the case of 10’²⁺ - 13’²⁺, S² values closed to 1 with increasing five-member rings, along with oligothiophene dications.⁴ S² value of 13’²⁺ was nearly identical to that of 12T²⁺

(0.98), and this suggested that the ground state of 13²⁺ was singlet biradical, again. The incorporation of pyrrole and methylthio groups increased S² values, indicating the enhanced biradical characters.

Bond Length Alternations (BLA) of Dications in OS states.

The plots of C-C bond length of the optimized structures of dication in OS state were summarized in Figure 4-4. The plots within the same five-member ring were connected by solid lines, and the shape of line was “V” means that the ring was quinoidal structure and the shape was

“reverse V” means the ring was benzenoid structure. In the case of 10’²⁺, all of the five-member rings were quinoidal structures, and this BLA was the CS-type, whose charge delocalized in the whole molecule. In contrast, the BLAs of 12’²⁺ and 13’²⁺ showed their central rings were benzenoid and terminal rings were quinoidal structures, indicating the OS-type. The plots of the inter-ring bond lengths connected by dashed lines showed that the length between central units is longer than that near terminal units, indicating that the central rings were benzenoid structures, again.

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(c) (d)

(e)

Figure 4-4. Bond lengths plots of (a) 10’²⁺, (b) 11’²⁺, (c) 12’²⁺ and (d) 13’²⁺ with (e) bond number. In these graphs, plots connected by solid line denote the lengths within the same five-member ring, while plots connected by dashed line denote the lengths of inter-ring bond.

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Next, the BLA of 12’-H²⁺ and 8T²⁺ were shown in Figure 4-5. From the comparison with 12’²⁺ with methylthio group, the BLA of 12’-H²⁺ showed the differences that there are the quinoidal structure in the central ring and the benzenoid structure in the terminal rings, which would be caused by the lack of the conjugative electron donation with methylthio groups (Figure 4-6). The effects of incorporation of pyrrole ring were observed in the presence of the apparent quinoidal structure in the pyrrole rings which appeared to weaken the quinoidal structure in the central rings. Thus, these results indicated that the incorporation of pyrrole and methylthio groups enhanced biradical character.

(a) (b)

Figure 4-5. Bond Lengths plots of (a) 12’-H²⁺ and (b) 8T²⁺. In these graph, solid line connected plots of the same five-member ring and dashed line connected plots of inter-ring bond.

(a)

(b)

Figure 4-6. Possible resonance forms at terminal thiophene ring of (a) 12’ and (b) 12’-H.

- 82 - Spin Density Distribution of Dications in OS states.

The calculated spin density distributions of dications in OS states were shown in Figure 4-7. The antiparallel spins delocalized in the half of -systems and the separation of spins were enhanced with increasing chain length. The calculated spin density distribution of 12’-H²⁺ and 8T²⁺

in OS states were also shown in Figure 4-8. In contrast to the case of 12’-H²⁺, the spin density at sulfur of methylthio groups were observed in 12’²⁺, indicating that methylthio groups enhanced the separation of spin at the both ends. Although somewhat large spin density on pyrrole rings were observed, the effects of incorporation of pyrrole in the manner of spin distribution seems to be small.

(a)

(b)

(c)

(d)

Figure 4-7. Spin distributions of (a) 10’²⁺, (b) 11’ ²⁺, (c) 12’ ²⁺ and (d) 13’ ²⁺ in OS states.

NOON Analysis of 10’-13’.

The results of NOON analysis of 10’²⁺-13’²⁺, 12’-H²⁺ and 8T²⁺ were summarized in Table 4-4. In the case of 10’²⁺-13’²⁺, the biradical indices increased with increasing chain length. The enhancement of biradical indices by the incorporation of pyrrole and methylthio groups were observed from comparison among dications of octamer.

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(b)

Figure 4-8. Spin distributions of (a) 12’-H²⁺ and (b) 8T ²⁺ in OS states.

Table 4-4. Biradical Indexes (%) of 10’²⁺ - 13’²⁺.

Dications 10’²⁺ 11’²⁺ 12’²⁺ 13’²⁺ 12’-H²⁺ 8T²⁺

Biradical Index 48 59 66 71 55 46

Summary of DFT calculations.

Judging from the calculations of 10’²⁺-13’²⁺ as the model of 10²⁺-13²⁺, it was suggested that all of the dications were singlet biradical at ground states. Especially in 13²⁺, all of the calculated indices for singlet biradical character strongly suggested its singlet biradical character. It was suggested that the effects of incorporation of pyrrole and methylthio groups enhanced biradical character with negligible change of electronic structure of oligothiophene. In accordance with the results of these calculations, it was suggested that 10²⁺-13²⁺, especially 13²⁺, were good models of oligothiophene dication with a singlet biradical character that serves as the p-doped polythiophene.

4-5. Synthesis.

The hexamer 10 was synthesized by the oxidative coupling of 37, which was obtained by methylthiolation of 36²¹ prepared by the Paal-Knorr method. The heptamer to nonamer 11-13 were synthesized by the cross coupling reaction of 40, which was obtained by borylation of 39 prepared in the similar method for 37, with ,-dibromothiophenes.^22,23

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Scheme 4-1. Synthesis of 10-13.

4-6. Results.

Optical and electronic properties in neutral states.

The electronic absorption spectra of 10-13 in dichloromethane are summarized in Figure 4-9 and Table 4-5. The longest wavelength absorptions of 10-13 were bathochromically shifted and the extinction coefficients of 10-13 were increased in accordance with the extension of the

-systems.

Table 4-5. Longest Wavelength Absorption Bands (/nm), Extinction Coefficients (/M^-1 cm^-1) and First, Second, and Third Oxidation Potentials (E/V vs Fc/Fc⁺) of 10–13 in Dichloromethane.

Compd.   E1/2

1 (2e^-) E1/2

2 E1/2

3

10 395 39100 0.16 0.81 -

11 420 48400 0.15 0.67(pa)^a 0.54(pc)^b 0.81(pa)^a 0.54(pc)^b

12 439 57800 0.14 0.54 0.64

13 450 69300 0.12 0.51 0.67

aAnodic peak potential. ^bCathodic peak potential.

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Figure 4-9. The absorption spectra of 10-13.

(a) (b)

(c)

Figure 4-10. (a) Cyclic voltammograms of 10–13 in dichloromethane, (b) differential pulse voltammogram of 13 and liner sweep voltammogram of 13.

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To examine the redox behavior of 10–13, cyclic voltammetry, differential pulse voltammetry and liner sweep voltammetry were conducted in dichloromethane with tetra-n-butylammonium hexafluorophosphate as the electrolyte. The voltammograms are shown in Figure 4-10 and the values of the oxidation potentials are summarized in Table 4-5. Judging from cyclic voltammograms, all of the compounds exhibited reversible one-step two-electron oxidation processes, indicating that the generated dications are stable in the measurement conditions with no apparent on-site Coulombic repulsion. Differential pulse voltammetry and liner sweep voltammetry of 13 clearly showed one-step two-electron oxidation process. In the case of 12-H without the methylthio end-capping groups, the electropolymerization was observed in the same measurement conditions, whereas such electropolymerization was not observed for 10-H (Figure 4-11).¹⁹ Thus, the methylthio units play a critical role in the reversibility of the longer oligomers, and no occurrence of electropolymerization for 10-H suggests that biradical character presents only in the longer oligomer dications.

(a) (b)

(c)

Figure 4-11. Cyclic voltammograms of (a) 10-H and (b) 12-H in dichloromethane,¹⁹ and (c) structures of 10-H and 12-H.

Preparation of Dications.

Reflecting these reversible two-electron oxidations at relatively lower oxidation potentials,

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stable dark-colored precipitates were obtained when the oligomers in carbon disulfide were oxidized with ca. 2.5 equiv of pentachloroantimonate. Elemental analysis of these precipitates suggest that they are dication salts of 10²⁺(SbCl6

-)2 - 13²⁺(SbCl6

-)2 (Scheme 4-2), although they appear to be contaminated with some paramagnetic impurity as described below.

Scheme 4-2.Preparations of dication salts of 10-13.

Absorption Spectra of Dications.

As shown in Figure 4-12, the absorption spectra of 10²⁺ and 11²⁺ generated with SbCl5 in dichloromethane showed one strong absorption band at near-IR regions (10²⁺: 965 nm (log  = 5.1);

11²⁺: 1093 nm (5.1)) together with weak absorption bands at visible regions. In contrast, two broad absorption bands were observed for the longer oligomer 12²⁺ and 13²⁺ (12²⁺: 1212 nm (log  = 4.8), 648 nm (4.5); 13²⁺: 1275 nm (4.7), 692 nm (4.7)). Although one-absorption band and two-absorption bands suggested CS and OS states as mentioned above, the TD-DFT calculations of 10'²⁺-13'²⁺ in CS, OS and T states at B3LYP/6-31G(d) level were conducted to consider in more detail, and the results were summarized in Figure 4-13 and Table 4-6 with the corresponding experimental data. It was suggested that dications in CS state showed one strong absorption band, those in OS state showed two moderate absorption bands, and those in T state showed three weak absorption bands.

From the comparison of the experimental results with the corresponding results of TD-DFT calculations, 10²⁺ and 11²⁺ showing one strong absorption band suggested that their electronic structures were CS state. In sharp contrast, 13²⁺ showed two moderate absorption bands, suggesting that its electronic structure was OS state. In the case of 12²⁺, it was thought that the electronic structure was contributed of both CS and OS state because 12²⁺ showed two absorption bands having different absorbance each other. Solid-state absorption spectra of 10²⁺(SbCl6

-)2 - 13²⁺(SbCl6

-)2 in a KBr pellet were also measured (Figure 4-14), and it was found that the absorption maxima for all dications are essentially the same as those in the dilute solution. If there is a bonding intermolecular interaction in the solid state as observed in the radical cation -dimers (see Chapter 2) and phenalenyl-based singlet biradicals,⁷ the absorption maxima are expected to shift. Therefore, the

-dimer formation between dication as mentioned in previous studies⁵ seems not to take place in the solid states.

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Figure 4-12. Absorption spectra of 10²⁺-13²⁺.

(a) (b)

(c) (d)

Figure 4-13. Absorption spectra of (a) 10²⁺, (b) 11²⁺, (c) 12²⁺ and (d) 13²⁺ with the results of TD-DFT calculations in closed-shell singlet (CS),open-shell singlet (OS) and triplet (T).

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Table 4-6. Observed (10²⁺-13²⁺) and calculated (10’²⁺-13’²⁺ at the TD-B3LYP/6-31G(d) level) absorption maxima with the values of absorption coefficients () and oscillator strengths(f).

Dications 10²⁺ 11²⁺ 12²⁺ 13²⁺

Observed

max / nm ()

560 (19000) 965 (141000)

585 (25400) 1093 (138000)

648 (31500) 1212 (63900)

692(48900) 1275 (48900) Calcd. in CS

max / nm (f)

428 (0.12) 912 (2.91)

468 (0.30) 1041 (3.26)

515 (0.51) 1181 (3.53)

558 (0.80) 1337 (3.68) Calcd. in OS

max / nm (f)

622 (1.05) 996 (1.62)

612 (1.32) 1137 (1.85)

648 (1.75) 1283 (1.94)

683 (1.94) 1438 (2.04) Calcd. in T

max / nm (f)

588 (0.32) 630 (1.41) 1053 (0.76)

510 (0.42) 699 (1.52) 1201 (0.91)

558 (0.97) 756 (1.54) 1349 (1.10)

600 (1.28) 812 (1.27) 1504 (1.30)

(a) (b)

(c) (d)

Figure 4-14. Absorption spectra of (a) 10²⁺, (b) 11²⁺, (c) 12²⁺ and (d) 13²⁺ in dichloromethane (DCM) and potassium bromide (KBr).

Magnetism of Dication Salts.

The observation of thermally excited triplet were attempted using ESR measurement at various temperatures because the absorption spectra of 12²⁺ and 13²⁺ suggested that their ground

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states were singlet biradical. First, the ESR spectra of 10²⁺(SbCl6

-)2 - 13²⁺(SbCl6

-)2 at room temperature were shown in Figure 4-15. All of 10²⁺(SbCl6

-)2 - 13²⁺(SbCl6

-)2 showed the signal of ESR, and their radical concentrations were estimated 0.4%, 4%, 12% and 14% using 1,1-diphenyl-2-picrylhydrazyl (DPPH) as an external standard. It had been thought that the increment of radical concentrations with increasing chain length were corresponding to the increment of biradical characters. However, the enhancement of ESR signals according to the Curie law were observed at various temperatures (Figure 4-16). Although the ESR signal would be disappeared at low temperature if thermally excited triplet was measured at room temperature, the ESR signal was observed even at 5 K. Furthermore, the ESR signal at half-field, which was unique to triplet species, was not also observed. These results indicated that the ESR signals were derived from radical impurities, even though the purities were checked by elemental analysis.

(a) (b) (c) (d)

Figure 4-15. ESR spectra of (a) 10²⁺(SbCl6

-)2, (b) 11²⁺(SbCl6

-)2, (c) 12²⁺(SbCl6

-)2 and (d) 13²⁺(SbCl6

-)2 at room temperature.

(a) (b) (c) (d)

Figure 4-16. Intensity of ESR vs. temperature plots of (a) 10²⁺(SbCl6

-)2, (b) 11²⁺(SbCl6

-)2, (c) 12²⁺(SbCl6

-)2 and (d) 13²⁺(SbCl6

-)2.

Finally, the temperature dependence of susceptibility (M: molar susceptibility) of 13²⁺(SbCl6

-)2 measured by SQUID revealed that a MT vs. T plot (Figure 4-17) is almost flat according to the Curie law. However, when more closely examined at MT above 200 K, the values were found to gradually increase, as observed in other biradical species having singlet ground state.^7e,8e,10 The singlet-triplet model using the Bleaney-Bowers equation²⁴ gave an estimated ES-T

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value of 2000 K (4.0 kcal/mol). These results suggested that the radicals detected by ESR measurements were radical impurities, but, in the case of 13²⁺(SbCl6

-)2, a small amount of triplet biradical were included.

Figure 4-17. Thermal dependence of MT for 13²⁺(SbCl6

-)2; (○) experimental MT data; and the fitting curve (red solid lines) are drawn using the Bleaney–Bowers equation with Curie impurity term; spin contamination = 11.1 % for g = 2.00 and S = 1/2, diamagnetic susceptibility = –1.26 × 10^–3 emu mol^–1, and S–T gap = 2000 K.

Conductive Properties of Dication Salts.

To consider the effect of difference of the electronic structures on conductivity, the conductivities of 10²⁺ and 13²⁺, which were shown to be in CS and OS states, respectively, were measured. The conductivities of pellets of 10²⁺(SbCl6

-)2 and 13²⁺(SbCl6

-)2 were measured. From three times measurements, the average conductivities were 4.2×10^-6 S/cm and 6.4×10^-3 S/cm, respectively. Although the structural difference between 10 and 13 was indispensable, this results suggested that singlet biradical states improve conductivities of p-doped oligothiophenes.

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4-7. Conclusion.

Thiophene-pyrrole mixed oligomers 10-13, whose singlet biradical characters in dication states were predicted by the DFT calculations at B3LYP/6-31G(d) level, were synthesized and the properties of their dications were investigated. From the absorption spectra and corresponding TD-DFT calculations of dications suggested singlet biradical characters of 12²⁺ and 13²⁺, and dications showed almost identical spectra at the both solution and solid phase. SQUID measurements of 13²⁺(SbCl6

-)2 showed its small singlet-triplet energy gap (ES-T), and strongly suggested its singlet biradical character at ground state. The pellet of 10²⁺(SbCl6

-)2 and 13²⁺(SbCl6

-)2 showed distinctly-different conductivities, and suggested that singlet biradical states cause improvement conductivity. The solid-state properties of this type of linear oligomer dication were studied for the first time, and thus, present work provides an important information for p-doped polythiophene in the solid state.

4-8. Experimental Section.

General.

1H and ¹³C NMR spectra were recorded on JEOL JNM-270, LA-400 or L-500 instruments.

Chemical shifts are reported in ppm with reference to tetramethylsilane. Mass spectra were recorded on a SHIMADZU GC-MS QP2020 or an AXIMA-CFRD for EI or LDI-TOF method, respectively.

Only the more intense or structurally diagnostic mass spectral fragment ion peaks are reported.

Preparative gel-permeation chromatography (GPC) was performed with a JAI LC-08 chromatograph equipped with JAIGEL 1H and 2H columns. Electronic spectra were recorded on a SHIMADZU UV-Vis-NIR scanning spectrophotometer (Model UV-3101-PC). ESR spectra were recorded on JEOL JES-RE3X instruments. Magnetic measurements of 13²⁺(SbCl6–

)2 were carried out on a Quantum Design MPMS-XL SQUID magnetometer on powder samples in the temperature range from 2 K to 300 K under 5000 Oe. The molar susceptibility, M, was obtained from the raw magnetic data, subtracting a diamagnetic contribution of the sample in it. The electric conductivity was evaluated using ADVANTEST R6551 digital multimeter. Elemental analyses were performed at the microanalysis laboratory of Tokyo Metropolitan University.

Commercially available reagents were used as received. Solvents were distilled from relevant drying agents prior to use. 2,5-bis(2’-thienyl)-N-alkyl-pyrrole 36 and 38,²¹ 10-H,²¹ 5,5'-Dibromo- 2,2'-bithiophene 15²¹ and 5,5′′-Dibromo-2,2′:5′,2′′-terthiophene 16²² were prepared according to the literature procedures.

ドキュメント内 Submitted in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY (SCIENCE) (ページ 77-103)