The Conductivities of Gold-Nanoparticles Protected by Bicyclo[2.2.2]octene Annelated Oligothiophenes.
Abstract
Gold-nanoparticles (AuNPs) covered with terthiophene 7-SH and quaterthiophene 8-SH having BCO group(s) were designed and synthesized. AuNPs covered with terthiophene 9-SH having no BCO group was also synthesized as a control. The absorption spectra and TEM measurements suggested that AuNPs were successfully protected by oligothiophenes 7-SH - 9-SH.
The electronic conductances of AuNPs-8 and AuNPs-9 were moderate (~10-3 S cm-1), while the conductivity of AuNPs-7 was small (< 10-6 S cm-1) due to the inter-particle separation by sterically demanding two BCO units. After doping with iodine vapor, the conductivities of AuNPs-8 and AuNPs-9 increased 4- to 10-fold in comparison with those before doping. In contrast, the conductivity of AuNPs-7 was increased more than 10000-fold in comparison with that before doping, suggesting that this increment of the conductivity was caused by the interactions like -dimer formation between oligothiophenes protecting AuNPs.
- 46 -
3-1. Introduction.
As mentioned in Chapters 1 and 2, -dimers of oligothiophene radical cations have attracted much attention for revealing the electronic structure of p-doped polythiophenes. However, the study using DP3T showed that the conductivity of oligothiophene radical cation was decreased by forming -dimers relative to that of regular -stacks when its conductive direction was along its
-stacks direction (Figure 3-1).1 In contrast, hydrocarbon molecule DPQM having the biradical character showed conductance in neutral state due to its partially-overlapped crystal structure by the interactions like -dimers (Figure 3-2).2
(a) (b) (c)
Figure 3-1. (a) The structure of DP3T, and schematic illustrations of (b) -stacks and (c) -dimers. In this illustration, the orange arrows represent conductive directions.
(a) (b)
Figure 3-2. (a) The structure of DPQM, and schematic illustrations of (b) -dimers with slipped stacks. In this illustration, the orange arrow and dash lines represent conductive directions and intermolecular interactions like -dimers, respectively.
Thus, the conductivity of aggregate of oligothiophene dication with biradical character forming one dimensional network of partially stacked -dimer have been of interest. However, there have been no report on the issue because it required rigorous control of both stacking manner of
-dimers and biradical character. Because only the oligothiophenes having long chain length (more than 12 thiophene rings) are expected to show biradical characters,3,4 it is difficult to synthesize the
-stacks (2-3×10-3 S/cm)
-dimers (3×10-5 S/cm)
Infinite stack
- 47 -
molecule having long chain length in which the manner of -dimer is rigorously controlled. In this work, the relationship between -dimerizations and conductivities was investigated using pseudo multi-radical molecule, which is gold-nanoparticles (AuNPs) covered with a number of cationic oligothiophenes instead of using the molecules having long chain length (Figure 3-3). If it is able to control -dimerizations of these AuNPs by temperature or oxidation states, the conductivity is expected to be controlled.
(a)
(b)
Figure 3-3. The illustrations of (a) oligothiophene dication with biradical character and (b) pseudo multi-radical AuNPs. In this illustrations, black circles, dash lines, and gray circle represent bulky groups, intermolecular interactions like -dimers, and AuNPs, respectively.
AuNPs are the one of most stable metal-nanoparticles and have been extensively studied their self-assembly,5 electronic6 and optical properties,7 quantum size effect,8 and applications for chemical sensors9 and catalysts.10 In most of these studies, the AuNPs covered with organic molecules having functional groups such as thiol and amine which strongly interact with gold were used to suppress agglomeration.
The conductivities of AuNPs covered with thiol-molecules were intensively studied using various thiol-molecules. For example, the study using the AuNPs covered with various alkanethiols showed that conductivities of AuNPs depended on interparticle distance and did not depend on its particle sizes through tunneling mechanism.11 In contrast, the study on temperature-dependence of conductivity of the network of AuNPs linked by oligothiophenes having thiol groups at the both ends suggested that the conductivities of AuNPs covered with oligothiophenes were not through simple tunneling mechanism judging from temperature-dependence at extremely low temperatures.12 Wolf and co-workers reported that the conductance of AuNPs covered with oligothiophenes having diphenylphosphine increased 1000-fold by the oxidized coupling between oligothiophenes.13 Zotti and Berlin also reported that the conductance of AuNPs covered with oligothiophene thiols increased
- 48 -
100-fold by the oxidized coupling between oligothiophenes.14 These results indicated that the conductivities of AuNPs are affected by the state of their protecting molecules.15
On the other hand, the switching devices of conductance have been studied using the properties that the conductivities of AuNPs depend on linker molecules. Matsuda and co-workers reported the conductance-photoswitching devices by using of diarylethene framework.16 As the other applications of AuNPs covered with oligothiophenes, the sensors changing their conductance by volatile organic compounds were reported.14,17
Under these circumstances, in this work, the effects of electronic and chemical structures of protecting molecules of AuNPs on their conductivities will be investigated using BCO-annelated oligothiophene thiols. These AuNPs are potentially applied for the conductance-switching devices based on the redox states if the great change between before and after -dimerizations is observed.
3-2. Molecular Design.
First, thiol group, which forms a relatively stable bond with gold, was chosen as a functional group to protect AuNPs. As mentioned in Chapter 2, oligothiophenes without end-capping group shows polymerization reaction at highly reactive terminal -positions upon one-electron oxidation. Thus, in this work, methylthio groups were introduced at -position of terminal thiophene ring to stabilize the oxidized states. Methylthio group is known as a substituent that stabilizes oxidized states of oligothiophene due to conjugative electron donation by the lone pair of sulfur arom.18-20 It is thought that -dimers are formed between oligothiophenes radical cations bonded within one AuNP when the oligothiophene is without steric protection (Figure 3-4a) and the intraparticle interactions potentially decrease the conductance as observed in the radical cation salt of DP3T (see above). Thus, bulky BCO groups were introduced at -positions of thiophene rings having thiol group to bond between oligothiophenes and one AuNP (Figure 3-4b). The number of BCO group annelations were two less than the number of thiophene rings because it was considered that two thiophene rings having no-substituent at -positions were necessary to form -dimers as mentioned in Chapter 2 (5+• formed -dimer, but 6+• did not form -dimer). Based on these molecular design, relatively easily accessible quaterthiophene 7 and terthiophene 8 were synthesized, and terthiophenes having no BCO group 9 were also synthesized as a control molecule (Figure 3-5).
- 49 - (a)
(b)
Figure 3-4. The schematic illustrations of AuNPs covered with (a) unsubstituted oligothiophenes and (b) BCO-annelated oligothiophenes. In the illustration, dash lines and black circle represent intermolecular interactions like -dimers and BCO groups, respectively.
Figure 3-5. The structures of 7-9.
3-3. Synthesis.
The oligothiophene capped with methylthio groups at the both ends 7-SMe and 8-SMe were synthesized by Suzuki coupling between BCO-annelated thiophene and oligothiophene having methylthio group at one end. The oligothiophenes having thiol group (7-SH, 8-SH and 9-SH) were synthesized by thiocyanation21 of the corresponding oligothiophenes followed by decyanation with a base.
The synthetic scheme of ,-dimethylthio oligothiophenes (7-SMe and 8-SMe) was shown in Scheme 3-1. The oligothiophene 30 was synthesized by Suzuki coupling between 27, which obtained by borylation of 18,22 and 29, which was obtained by bromination of 2823 with N-bromosuccinimide (NBS). Compound 7-SMe was synthesized from 31, which obtained by bromination of 30 with NBS, and 33, which was obtained by methylthiolation of 18 with dimethyl disulfide followed by borylation. Compound 8-SMe was synthesized by Suzuki coupling between 29 and 33. The synthesis of 9-SMe was previously reported.18,24
- 50 -
Scheme 3-1. Synthetic schemes of 7-SMe and 8-SMe.
The synthetic scheme of the oligothiophene thiocyanates (R-SCN) was shown in scheme 3-2. Compound 7-SCN was synthesized by thiocyanation of 34, which was obtained from 27 and 31, using potassium thiocyanate and bromine. Compounds 8-SCN and 9-SCN were synthesized in the same way of 7-SCN using 30 and 35,24 respectively.
As shown in Scheme 3-3, oligothiophene thiols (R-SH) were quantitatively obtained by protonation using hydrochloric acid of their thiolate anions, which were reduced by lithium aluminum hydride from their respective thiocyano precursors (R-SCN).21a
- 51 -
Scheme 3-2. Synthetic schemes of 7-SCN, 8-SCN and 9-SCN.
Scheme 3-3. Synthetic schemes of thiol compounds.
3-4. Properties of Oligothiophenes Capped with Methylthio Groups.
Optical and Electronic Properties.
The electronic absorption spectra of 7-SMe – 9-SMe in dichloromethane are summarized in Figure 3-6 and Table 3-1. The longest wavelength absorptions of 7-SMe and 8-SMe were bathochromically shifted in accordance with the extension of the -systems. In contrast, in the comparison of the terthiophenes 8-SMe and 9-SMe, hypsochromic shift of the longest wavelength absorption bands by the annelation of BCO unit at the 3,4-position of thiophene rings was observed.
The distortion in the -systems caused by the steric repulsion between bridgehead protons and sulfur atoms is responsible for these phenomena, as mentioned in our previous report25 and Chapter 2-5.
The absorption spectra of their thiocyanates (R-SCN) and thiols (R-SH) were nearly identical to those of the corresponding methylthio oligothiophenes (R-SMe).
- 52 -
Figure 3-6. The absorption spectra of 7-SMe - 9-SMe.
Table 3-1. Longest Wavelength Absorption Bands (/nm) and First and Second Oxidation Potentials (E/V vs Fc/Fc+) of 7-SMe – 9-SMe in Dichloromethane.
Compd. E1/2
1 E1/2
2
7-SMe 392 0.41 (2e-)
8-SMe 380 0.35 0.46
9-SMe 388 0.89a 1.02a
aRef 18a. Measured in CH3CN and potential vs. SCE.
To examine the redox behavior of 7–SMe and 8-SMe, cyclic voltammetry was conducted in dichloromethane with tetra-n-butylammonium perchlorate as the electrolyte. The voltammograms are shown in Figure 3-7 and the values of the oxidation potentials are summarized in Table 3-1, together with those of 9-SMe.18a Both of the compounds exhibited reversible first and second oxidation waves. It was apparent that their oxidized states were stabilized by methylthio groups because methyl-capped oligothiophene (4-6) dications were instable in spite of being quaterthiophenes ( see also chapter 2-5). The one-step two-electron oxidation was observed in 7-SMe, and it was suggested that the gap between the first and the second oxidation potentials was narrowed due to elongate conjugation, as observed in mesitylthio-end-capped oligothiophenes.19,20,26 Also it was considered that the first oxidation potential was increased due to the twist in the
-system caused by the BCO groups at the -positions which suppressed effective conjugation, and that the second oxidation potential was decreased due to planarization caused by quinoidal structure
- 53 -
of oligothiophene in one-electron oxidized state which promoted effective conjugation. These predicted twisted structure of 7-SMe and planar structure of 7-SMe+• were supported by calculations at the M06-2X/6-31G(d) level (Figure 3-8).
Figure 3-7. Cyclic voltammograms of 7-SMe and 8-SMe in dichloromethane.
(a)
(b)
Figure 3-8. Top and side views of optimized structures of (a) 7-SMe and (b) 7-SMe+•. The BCO groups and hydrogens are omitted for clarity in side views.
- 54 -
-Dimerization Abilities of 7-SMe and 8-SMe.
On the basis of the reversibility of the oxidation process observed in cyclic voltammetry, radical cations of 7–SMe and 8-SMe were expected to be stable. Thus, one-electron oxidation of 7–SMe and 8-SMe were conducted in dichloromethane using nitrosonium hexafluoroantimonate as the oxidant, and immediately the electronic absorption spectra of 7-SMe+•SbF6–
and 8-SMe+•SbF6–
were measured at various temperatures. Fortunately, although the one-step two-electron oxidation wave was observed using cyclic voltammetry for 7-SMe (Figure 3-7), the both one-electron oxidation were successfully conducted although some neutral species remained unreacted. However, it was considered that some neutral species has little effect on -dimerizations. As shown in Figures 3-9 and 3-10, both of the radical cations showed two absorption bands near room temperature; the lower and higher energy bands were assigned as the HOMO–SOMO and SOMO–LUMO transitions, respectively.28 The intensity of these two bands decreased with decreases in temperature, and three new absorption bands appeared. These spectral changes were also reversible and consistent with those mentioned in Chapter 2. Thus, it was confirmed that 7-SMe+• and 8-SMe+• formed -dimers at low temperatures. The dimerization equilibrium constants were estimated from these measurements using the absorption of the monomer in the higher energy region, as described in Chapter 2. The dimerization enthalpy (Hdim) and entropy (Sdim) were then obtained by van’t Hoff plots. The results for the absorption bands of the monomer and π-dimer, Hdim and Sdim, are summarized in Tables 3-2 and 3-3.
Table 3-2. Observed and Calculated Absorption Bands (/nm) for the Radical Cations and their
-Dimers.
Radical Cation -Dimer
Compd. (exp) (calcd)
a (exp) (calcd)
a
7-SMe 712, 1334 557, 1201 600, 648, 964, 1454 469, 850, 1327 8-SMe 668, 1140 525, 949 540, 566, 882, 1148 411,423, 709, 969 9-SMe 655, 1100b 521, 957 524, 838, 1094b 413, 664, 851
aCalculated at the TD M06-2X/6-31G(d) method. bData from ref. 18a. Measured in CH3CN.
- 55 - Figure 3-9. Absorption spectra of 7-SMe+•SbF6–
(0.48 mM) at various temperatures, and van’t Hoff plots are shown in the inset. The absorption band of remaining neutral species is in the gray region.
Figure 3-10. Absorption spectra of 8-SMe+•SbF6–
(0.64 mM) at various temperatures, and van’t Hoff plots are shown in the inset. The absorption band of remaining neutral species is in the gray region.
- 56 -
Table 3-3. Observed and Calculated Dimerization Enthalpy (Hdim/kcal mol–1), Entropy (Sdim/cal K–1 mol–1) and SOMO-SOMO Interactions (ES-S/kcal mol-1).
Hdim Sdim ES-S
Compd. expa calcd expa expb calcdc
7-SMe -10.9 -17.4 -40.3 19.7 21.5
8-SMe -11.3 -15.9 -36.3 24.9 29.5
9-SMe -d -17.6 -d 26.1 33.6
aThe experimental error were within ± 1 kcal mol–1 and ± 6 cal K–1 mol–1. bCalculated from the longest absorption wavelength. cResults of TD-DFT calculations were used. dNot described in ref 18a.
(a)
(b)
(c)
Figure 3-11. Top and side views of optimized structures of (a) (7-SMe)2
2+, (b) (8-SMe)2
2+ and (c) (9-SMe)2
2+ calculated at the M06-2X/6-31G(d) level using the PCM.
3.05 Å 3.09 Å
3.06 Å
- 57 - DFT Calculations.
To consider the effects of BCO groups on -dimerization of 7-SMe+• and 8-SMe+•, DFT calculations of the -dimers were conducted at the M06-2X/6-31G(d) level,27 and banana-shaped structures were observed (Figure 3-11), as similarly shown in Chapter 2. TD-DFT calculations in dichloromethaneat the M06-2X/6-31G(d) level were also conducted for (7-SMe)2
2+ and (8-SMe)2 2+
with the most stable structures to support the assignment of the observed absorption bands. The results of the TD-DFT calculations are summarized in Table 3-2 with the corresponding experimental results. The results of calculations were in close agreement with the experimental values, although the TD-DFT calculations systematically overestimated the transition energies. The excitation of longest absorption band was dominated by the HOMO-LUMO transition, as being similar with (1)22+
- (6)22+
. The results of calculations were summarized in Table 3-3.
3-5. Properties of AuNPs Covered with Oligothiophene.
Preparation of AuNPs.
Preparation of AuNPs covered with oligothiophene 7-SH – 9-SH were conducted according to the method developed Brust et al29 (Scheme 3-4). To a stirring solution of tetra noctyl ammonium bromide in toluene at room temperature, aqueous tetrachloroauric(III) acid was added and the mixture was stirred. Then, oligothiophene thiol (R-SH) and ultrapure water were added to the solution. The residue was dispersed in methanol and collected the particles by filtration. The AuNPs covered with dodecanethiol (AuNPs-SDod) was also prepared by the same way using dodecanethiol as a control.
Scheme 3-4. Preparation of AuNPs.
Optical Properties.
The electronic absorption spectra of AuNPs-7 – AuNPs-9 and AuNPs-SDod in dichloromethane are summarized in Figures 3-12 and Tables 3-4. The absorption maxima of AuNPs-7 - AuNPs-9 were 10-20 nm red-shifted relative to those of the corresponding thiol compounds 7-SH – 9-SH. The similar bathochromic shifts were also observed in previously reported AuNPs covered with oligothiophenes.14 The surface plasmon bands30 specific to AuNPs were also
- 58 -
observed at around 520 nm. These results suggested that AuNPs covered with oligothiophenes were successfully prepared.
(a) (b)
(c)
Figure 3-12. The absorption spectra of (a)7-SH, AuNPs-7, AuNPs-SDod (b) 8-SH, AuNPs-8, AuNPs-SDod, (c) 9-SH, AuNPs-9, AuNPs-SDod.
Table 3-4. Absorption Maxima (/nm), Average Particle-Diameters (/nm) and Average Interparticle Distance (d/nm) of 7-SH – 9-SH and AuNPs Covered with them in Dichloromethane.
SH AuNPs
Compd. d
7 392 404 4.2 -
8 376 398 4.8 1.5
9 382 398 3.0 1.7
Transmission Electron Microscope (TEM) Images of AuNPs.
To examine the particle sizes and interparticle distances of prepared AuNPs, TEM
- 59 -
measurements of the drop-cast films of AuNPs were conducted (Figures 3-13), and the results were summarized in Tables 3-4. The average particle sizes of AuNPs-7, AuNPs-8 and AuNPs-9 were 4.2 nm, 4.8 nm and 3.0 nm, respectively, and was not significantly affected by the structural differences of oligothiophene thiols (Figure 3-14). The interparticle distances of AuNPs-8 and AuNPs-9 were comparable (AuNPs-8: 1.5 nm, AuNPs-9: 1.7 nm) and nearly identical to the optimized molecular length of 8-SMe (1.46 nm) and 9-SMe (1.47 nm), respectively, at M06-2X/6-31G(d) level (Figure 3-16). On the other hand, the interparticle distance of AuNPs-9 could not be estimated due to they have various interparticle distances.
(a) (b) (c)
Figure 3-13. TEM images of (a) AuNPs-7, (b) AuNPs-8 and (c) AuNPs-9.
(a) (b) (c)
Figure 3-14. Histograms of particle-diameter (a) AuNPs-7, (b) AuNPs-8 and (c) AuNPs-9.
(a) (b)
Figure 3-16. Side views of optimized structures of (a) 8-SMe and (b) 9-SMe.
1.47 nm 1.46 nm
50nm 50nm 50nm
- 60 - Conductivities of AuNPs.
The conductivities of drop-cast films prepared from dichloromethane solution of AuNPs were measured using interdigitated array (IDA) electrode. The films of AuNPs were prepared according to the following procedure: the dichloromethane solution of AuNPs was drop-casted on IDA electrode and dried, then conductivities of the films were measured, and repeated these operations until the conductivity was saturated. The saturated conductivities were used as the conductivities of the films of AuNPs. As shown in Tables 3-5, the conductivities of AuNPs-8 and AuNPs-9 were moderate and higher than the previously-reported conductivity of AuNPs capped with conjugated diphenylphosphinooligothiophenes (2×10-4 S/cm).14 Combined with the results of network structure in the films of AuNPs-8 and AuNPs-9 observed by TEM, it was considered that methylthio group, which can weakly interact with gold, and thiol group bridged to neighboring AuNPs. In contrast, the conductivity of AuNPs-7 was not detectable and less than 10-6 S/cm. The p-doped films of AuNPs were prepared by exposure to iodine vapor until the conductivity was saturated. Then, its conductivities were measured. As a result, the conductivities of doped films of AuNPs-8 and AuNPs-9 were 4-fold and 10-fold higher than those of neutral states, respectively. In sharp contrast, the conductivity of the p-doped film of AuNPs-7 was 10000-fold higher than that of neutral state.
Table 3-5. Conductivities (/S cm-1) of films of AuNPs.
neutral I2-doped
Compd.
AuNPs-7 < 10-6 3.8×10-2 AuNPs-8 1.6×10-2 5.7×10-2 AuNPs-9 5.4×10-3 5.9×10-2
3-6. Discussion.
The Effects of BCO Groups on -Dimerizations.
To consider the effects of BCO groups on -dimerization, the -dimerization capabilities of 7-SMe+• and 8-SMe+• were compared. The absolute value of enthalpy change of -dimerization of 8-SMe+• was slightly larger than that of 7-SMe+•, and it indicated that 8-SMe+• easily forms -dimer than 7-SMe+• in spite of less number of thiophene rings in 8-SMe+• than in 7-SMe+•. As mentioned in Chapter 2, the absolute value of enthalpy change of -dimerization increased with increasing chain length. On the contrary, the result that terthiophene 8-SMe+• easily form -dimer than quaterthiophene 7-SMe+• suggested that BCO groups suppressed -dimerization of 8-SMe+•. The calculated results showed that the absolute value of enthalpy change of 7-SMe+• was larger than that
- 61 -
of 8-SMe+•. However, its difference was small (only 1.7 kcal/mol against 5-10 kcal/mol observed relative to increasing one thiophene ring in Chapter 2) suggested that BCO groups suppressed
-dimerization of 7-SMe+•, again. It was considered that the suppression by the BCO groups of 7-SMe+• were caused by steric repulsion between the bridgehead protons and -protons in the neighboring thiophene rings or between the bridgehead protons themselves, where less effective overlap of the SOMOs is available, as observed in 5+• and 6+•. Judging from the calculations, it was suggested that the -dimerization energy of 8-SMe+• was smaller than that of 9-SMe+•. However, the negligible difference suggested BCO group of 8-SMe+• has few effect on -dimerization.
The Effects of differences in oligothienothiol groups on Conductivities of AuNPs.
The conductivities of AuNPs-8 showed nearly identical to that of AuNPs-9 in both neutral and iodine doped states. The calculations of dimers of 8-SMe and 9-SMe in neutral states at the M06-2X/6-31G(d) level showed that only one BCO group has few effect on interparticle distance(Figure 3-17) and the approximately same interparticle distances (AuNPs-8: 1.5 nm, AuNPs-9: 1.7 nm) indicated the approximately same conductivities due to the same oligothiophene frameworks. In contrast, the conductivity of AuNPs-7 was less than 10-6 S/cm. The reasons for the small conductivity is considered to be that two BCO groups elongated the interparticle distance of AuNPs-7 compared with AuNPs-8 and AuNPs-9, and the interaction between methylthio group and gold was prevented by steric repulsion between BCO group and methylthio group. The calculations of the dimer of 7-SMe showed that two BCO groups have great steric repulsion (Figure 3-18).
(a) (b)
Figure 3-17. Side views of optimized structures of (a) (8-SMe)2 and (b) (9-SMe)2. Hydrogens are omitted for clarity.
- 62 -
Figure 3-18. Side view of optimized structure of (7-SMe)2. Hydrogens are omitted for clarity.
Iodine doping increased the conductivities of all the AuNPs probably because their oligothiophenes were p-doped and improved their conductance. In particular, the conductivity of doped AuNPs-7 was 10000-fold higher than that of in neutral state. The possible reason is that the interparticle interaction through -dimers at thiophene rings without BCO protection in addition to the p-doping of oligothiophenes themselves. The 100- to 1000-fold increasing of the conductivities in comparison with that of before linking by the coupling reaction between oligothiophenes on AuNPs were reported previously.13,14 The increasing of the conductivity of AuNPs-7 upon iodine doping was comparable to that of AuNPs upon linking by conjugated molecules, taking the 4-10-fold increasing in AuNP-8 and AuNPs-9 by p-doping into consideration.
3-7. Conclusion.
The BCO-annelated oligothiophenes 7 and 8, and control oligothiophene 9 were synthesized and investigated effects of the BCO groups on -dimerizations. It was suggested that only one BCO group showed a small effect on -dimerizations and two BCO groups significantly suppressed -dimerizations. The AuNPs covered with 7-9 were also prepared and their conductivities were measured. The conductivity of AuNPs-7 showed the great enhancement upon iodine doping which was possibly caused by the interactions like -dimers between oligothiophenes protecting AuNPs. These result suggested AuNPs-7 would potentially applied for conductance switching device by redox stimuli.
3-8. Experimental Section.
General.
1H and 13C NMR spectra were recorded on JEOL JNM-270, LA-400, L-500, or Bruker AV500 instruments. Chemical shifts are reported in ppm with reference to tetramethylsilane, using
- 63 -
the signal of internal tetramethylsilane or the solvents. Mass spectra were recorded on a SHIMADZU GC-MS QP2020 or a Bruker MicrOTOFII-SD for EI or APCI method, respectively.
Only the more intense or structurally diagnostic mass spectral fragment ion peaks are reported.
Electronic absorption spectra were recorded on a SHIMADZU UV-Vis-NIR scanning spectrophotometer (Model UV-3101-PC). Variable-temperature measurements of electronic absorption spectra were performed using an Oxford Optistat DN liquid-nitrogen cryostat. Cyclic voltammetry (CV) was performed on a BAS-ALS620B electrochemical analyzer using a standard three-electrode cell consisting of Pt wire and Pt working electrodes, a Pt wire counter electrode, and a Ag/AgNO3 reference electrode under nitrogen atmosphere. The potentials were calibrated with ferrocene as an external standard. Transmission electron microscopy (TEM) was carried out using JEOL JEM-1010 electron microscopes. The electric conductivity was evaluated using ADCMT 6241A DC Voltage/Current Source/Monitor. Preparative gel-permeation chromatography (GPC) was performed with a JAI LC-08 chromatograph equipped with JAIGEL 1H and 2H columns. 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. bicyclo[2.2.2]octenothiophene (18),22 5-methylthio-2,2’-bithiophene (28),23 5-methylthio-2,2’;5’,2’’-terthiophene (35)24 were synthesized according to literature procedures.
Computational methods
DFT calculations were performed with the Gaussian 09 program.31 All geometry optimizations were carried out at the M06-2X/6-31G(d) basis set unless otherwise noted. For the solvation calculations, the default method (polarizable continuum model) as implemented in Gaussian 09 was used. Excitation energy was computed using time-dependent density functional theory (TD-M06-2X) with 6-31G(d) basis set in the M06-2X optimized geometry. The calculated
-dimerization enthalpy Hdim(calc) was obtained from the subtraction of twice the “sum of electronic and thermal enthalpies” of the radical cation monomer from those of the -dimer, which were given in the standard output of the vibrational analysis.
Synthesis.
Synthesis of 27.
A solution of n-butyllithium in hexane (1.67 M, 0.55 mL, 0.92 mmol) was added dropwise to a stirred solution of 3,4-bicyclo[2.2.2]octenothiophene (18) (0.137 g, 0.835 mmol) in THF (6 mL)
- 64 -
at 0 °C. After stirring at 0 °C for 1 h, triisopropyl boronic acid (0.21 mL, 0.91 mmol) was added, and the reaction mixture was stirred additionally 30 min, then, stirred 1 h at room temperature. Pinacol (0.278 g, 2.26 mml) was added and the mixture was stirred at room temperature for 20 h. The crude mixture was washed with aqueous solution of NH4Cl, the aqueous layer was extracted with ether, and the ethereal solution was dried over Na2SO4. The volatiles were removed in vacuo, and the residue was purified by column chromatography (SiO2) eluted with hexane : dichloromethane = 3:1 to afford 27 (0.212 g, 0.729 mmol, 87%) as a colorless solid. 1H-NMR (CDCl3) δ: 7.11 (1H, s), 3.58 (1H, br s), 3.09 (1H, br s), 1.85-1.69 (4H, m), 1.41-1.39 (4H, m), 1.32 (12H, s); 13C-NMR (CDCl3) δ: 156.66, 146.72, 120.77, 83.39, 31.16, 30.96, 26.53, 26.37, 24.82; MS(APCI): [M+] = 163.09; Anal.
Calcd for C16H23BO2S: C, 66.21; H, 7.99. Found: C, 66.44; H, 7.92.
Synthesis of 29.
To a solution of 28 (0.892 g, 4.20 mmol) in CHCl3 (50 mL) and acetic acid (5 mL) was added N- bromosuccinimide (NBS) (0.799 g, 4.49 mmol) in portions at 0 °C and stirred at room temperature overnight. Aqueous NaHCO3 was added to the reaction mixture, extracted with CHCl3, and dried over MgSO4. The volatiles were removed in vacuo, and the residue was purified by column chromatography (SiO2) eluted with hexane to give 29 (1.20 g, 4.14 mmol, 98%) as a pale yellow solid. 1H-NMR (CDCl3) δ: 6.97-6.93 (3H, m), 6.86 (1H, d, J = 3.8 Hz), 2.51 (3H, s).
13C-NMR (CDCl3) δ: 138.50, 138.34, 137.07, 131.63, 130.65, 124.09, 123.80, 111.17, 22.03;
HRMS(APCI) calcd for C9H7BrS3
+, 289.8888; found, 289.8887.
Synthesis of 30.
A mixture of 27 (0.525 g, 1.81 mmol), 29 (0.519 g, 1.78 mmol), Pd(PPh3)4 (0.086 g, 0.074 mmol), aqueous K2CO3 (1.0 M, 15 mL) and 30 mL of toluene under N2 was refluxed for 13 h. The reaction mixture was then allowed to cool down to room temperature, the crude mixture was washed with aqueous solution of NH4Cl, the aqueous layer was extracted with dichloromethane, and the organic layer was dried over Na2SO4. The volatiles were removed in vacuo, and the residue was
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purified by preparative GPC eluted with toluene to give 30 (0.542 g, 1.45 mmol, 81%) as a pale yellow solid: 1H-NMR (CDCl3) δ: 7.05-6.97 (4H, m), 6.79 (1H, s), 3.53 (1H, br s), 3.05 (1H, br s), 2.51 (3H, s), 1.90-1.73 (4H, m), 1.54-1.38 (4H, m); 13C-NMR (CDCl3) δ: 146.85, 141.83, 139.49, 136.16, 136.01, 135.66, 131.85, 125.66, 125.23, 124.00, 123.48, 113.14, 31.52, 30.39, 26.27, 26.19, 22.15; MS(APCI): [M+] = 374.03; Anal. Calcd for C19H18S4: C, 60.92; H, 4.84. Found: C, 60.95; H, 4.86.
Synthesis of 31.
To a solution of 30 (68 mg, 0.18 mmol) in CHCl3 (10 mL) and acetic acid (0.5 mL) was added N- bromosuccinimide (NBS) (33 mg, 0.18 mmol) in portions at 0 °C and stirred at room temperature overnight. Aqueous NaHCO3 was added to the reaction mixture, extracted with CHCl3, and dried over MgSO4. The volatiles were removed in vacuo, and the residue was purified by column chromatography (SiO2) eluted with hexane to give 31 (82 mg, 0.18 mmol, 99%) as a pale yellow solid: 1H-NMR (CDCl3) δ: 7.04-6.95 (4H, m), 3.50 (1H, br s), 3.10 (1H, br s), 2.51 (3H, s), 1.88-1.74 (4H, m), 1.51-1.41 (4H, m); 13C-NMR (CDCl3) δ: 145.83, 141.79, 139.12, 136.54, 136.20, 134.61, 131.76, 126.27, 125.63, 124.00, 123.70, 101.16, 30.78, 30.29, 25.87, 25.49, 22.10;
MS(APCI): [M+] = 451.94; Anal. Calcd for C19H17S4Br: C, 50.32; H, 3.78. Found: C, 50.00; H, 3.84.
Synthesis of 32.
A solution of n-butyllithium in hexane (1.65 M, 1.10 mL, 1.82 mmol) was added dropwise to a stirred solution of 3,4-bicyclo[2.2.2]octenothiophene (18) (0.245 g, 1.49 mmol) in THF (10 mL) at 0 °C. After stirring at 0 °C for 1 h, dimethyl disulfide (0.16 mL, 1.8 mmol) was added, and the reaction mixture was gradually warmed to room temperature and stirred at room temperature for 12 h. The crude mixture was washed with aqueous solution of NH4Cl, the aqueous layer was extracted
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with ether, and the ethereal solution was dried over MgSO4. The volatiles were removed in vacuo, and the residue was purified by column chromatography (SiO2) eluted with hexane to afford 32 (0.237 g, 1.13 mmol, 76%) as a colorless solid. 1H-NMR (CDCl3) δ: 6.86 (1H, s), 3.27 (1H, br s), 3.02 (1H, br s), 2.38 (3H, s), 1.89-1.66 (4H, m), 1.47-1.30 (4H, m); 13C-NMR (CDCl3) 145.11, 140.37, 137.41, 135.15, 132.22, 124.02, 123.26, 121.15, 31.54, 31.47, 27.50, 27.04; MS(APCI):
[M+] = 210.05; Anal. Calcd for C11H14S2: C, 62.81; H, 6.71. Found: C, 62.95; H, 6.59.
Synthesis of 33.
A solution of n-butyllithium in hexane (1.67 M, 0.56 mL, 0.92 mmol) was added dropwise to a stirred solution of 2-methylthio-3,4-bicyclo[2.2.2]octenothiophene (32) (0.176 g, 0.835 mmol) in THF (5 mL) at 0 °C. After stirring at 0 °C for 1 h, triisopropyl boronic acid (0.21 mL, 0.91 mmol) was added, and the reaction mixture was stirred additionally 30 min, then, stirred 1 h at room temperature. Pinacol (0.245 g, 2.07 mml) was added and the mixture was stirred at room temperature for 15 h. The crude mixture was washed with aqueous solution of NH4Cl, the aqueous layer was extracted with ether, and the ethereal solution was dried over Na2SO4. The volatiles were removed in vacuo, and the residue was purified by column chromatography (SiO2) eluted with hexane : dichloromethane = 1:1 to afford 33 (0.212 g, 0.633 mmol, 76%) as a colorless solid. 33 was used for other reactions without further purification. 1H-NMR (CDCl3) δ: 3.51 (1H, br s), 3.22 (1H, br s), 2.44 (3H, s), 1.86-1.69 (4H, m), 1.45-1.33 (4H, m), 1.31 (12H, s).
Synthesis of 7-SMe.
A mixture of 31 (0.161 g, 0.354 mmol), 33 (0.141 g, 0.420 mmol), Pd(PPh3)4 (0.034 g, 0.029 mmol), aqueous K2CO3 (1.0 M, 5 mL) and 10 mL of toluene under N2 was refluxed for 2 d.
The reaction mixture was then allowed to cool down to room temperature, the crude mixture was washed with aqueous solution of NH4Cl, the aqueous layer was extracted with dichloromethane, and