Redox-Switchable Bis-fused Tetrathiafulvalene Analogue

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Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Redox-Switchable Bis-fused Tetrathiafulvalene Analogue: Observation and Control of Two Different Reduction Processes from Dication to Neutral State Minami Kato,†,⊥ Yusuke Fujita,† Tomokazu Yamauchi,† Shigeki Mori,‡,§ Takashi Shirahata,†,§,¶ and Yohji Misaki*,†,§,¶

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†

Department of Applied Chemistry, Graduate School of Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan ‡ Advanced Research Support Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan § Research Unit for Development of Organic Superconductors, Ehime University, Matsuyama, Ehime 790-8577, Japan ¶ Research Unit for Power Generation and Storage Materials, Ehime University, Matsuyama, Ehime 790-8577, Japan S Supporting Information *

ABSTRACT: Derivatives of a new bis-fused donor composed of TTF and extended TTF with an anthraquinoid spacer (TTFAQ) (2) were successfully synthesized. X-ray structure analysis of the tetrakis(methylthio) derivative 2Aa and its I3− salt revealed that the TTFAQ moieties of both 2Aa and 2Aa•+ adopt the so-called saddle conformation similar to most neutral TTFAQs. The results obtained from the X-ray structure analysis and cyclic voltammetry suggest that a positive charge in 2Aa•+ is unevenly distributed on the TTF moiety, while both positive charges of 22+ are mainly located on the TTFAQ moiety. In the first two-electron redox processes, an extra cathodic wave attributed to the coexistence of a different reduction process from the oxidation process was observed for most of the derivatives.

M

olecules that exhibit redox behavior accompanied by reversible structural change are promising components for switch units or memories in single molecular devices controlled by external electric fields.1 Nonplanar organic π-electron systems often exhibit unique electrochemical properties accompanied by a large conformational change.2−4 A π-electron acceptor bianthrone is a representative example (Figure S1, Supporting Information).2 It adopts two conformations in the neutral state in different temperature regions. One is a folded structure (so-called A-form), which exists at room temperature. The other is a twisted structure (so-called B-form), which predominantly exists at high temperature. Cyclic voltammetry revealed that bianthrone is reduced from the neutral A-form to the dianionic B-form.2 The B2− species is oxidized to B at a different potential from the A/B2− redox couple. In the neutral state, bianthrone is rapidly converted from the B-form to the thermodynamically more stable A-form. As for π-electron donors, a tetrathiafulvalene (TTF) analogue with an anthraquinoid spacer (1, Figure 1; herein abbreviated as “TTFAQ”) exhibits a simultaneous two-electron oxidation accompanied by a conformational change.3 That is, (TTFAQ)2+ has a conformation with an orthogonally twisted structure between the anthracene and the two 1,3-dithiolium rings (T-form),4 while the neutral TTFAQ adopts a folded saddle-like conformation (S-form).3,4 The dication state of TTFAQ is © XXXX American Chemical Society

Figure 1. Molecular structures of 1 and 2.

thermodynamically more stable than the monocation state; in other words, the second redox potential (E2) is more negative than the first redox potential (E1).5 This phenomenon is rarely observed in most π-electron donors, whose dications are less stable than monocations because of on-site Coulomb repulsion between two positive charges. In this context, the development of analogous TTFAQ donors is of interest. We have developed various fused TTF systems as possible components for various functional materials such as molecular Received: June 25, 2018

A

DOI: 10.1021/acs.orglett.8b01985 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Redox Potentialsa of 2 and Their Related Compounds

conductors, organic rechargeable batteries, and molecular wires.6−8 We focus on fused donor systems containing one or more TTFAQ moieties as multielectron redox systems as well as candidates for functional materials.9 In the course of studies on fused TTFAQ systems, we found that a bis-fused TTF analogue containing one TTFAQ moiety (2, Figure 1) exhibits remarkable redox behavior accompanied by conformational change. We report herein the synthesis, structures, and electrochemical properties of 2. Compound 2 was synthesized according to Scheme 1. The Horner−Wadsworth−Emmons reaction of 3A,B10 with the

compd 2Aa 2Bb 2Ab 2Ba 2Bc 6 1A

E1

Emb

E2

E3

E4

+0.28

+0.39 +0.30 +0.30 +0.39 +0.60 +0.56

+0.66 +0.67 +0.67 +0.66 +0.88 +0.70

+0.03 −0.06 −0.01 −0.03 +0.03 +0.07 +0.09

Scheme 1. Synthesis of 2

V vs Fc/Fc+, scan rate 1 mV s−1 or 5 mV s−1 for Em and 50 mV s−1 for the others. bEm = (E1+E2)/2.

corresponding phosphorus ester reagents 4a,c11 in the presence of n-BuLi in THF at −78 °C gave 2Aa, 2Ba, and 2Bc in 22−56% yields. On the other hand, 2Ab and 2Bb were prepared in respective yields of 24 and 10% by P(OEt)3-mediated cross coupling between 3A,B and 5b12 in refluxing toluene. The molecular structure of 2Aa determined by X-ray structure analysis revealed that the molecule adopts a saddle-like conformation, as is observed in most TTFAQ derivatives (Figure S2).3c,4 The TTF moiety adopts a shallow tub conformation similar to most TTF derivatives. Variable-temperature (VT) NMR spectra of 2Aa indicate two conformational isomers of 2Aa exist in solution at room temperature (Figure S21). The electrochemical properties of 2 were investigated by cyclic voltammetry. Figure 2 shows deconvoluted cyclic voltammograms of 2Aa, 2Ba, and 2Ab measured in a benzonitrile solution.13 All of the derivatives exhibited three pairs of redox waves. A comparison of the peak current of each wave indicates that the first redox wave corresponds to a two-electron transfer process, while the others correspond to a one-electron transfer process. The redox potentials of 2 are summarized in Table 1. A bis-fused TTF derivative 614 exhibits four pairs of one-electron redox waves, while the TTFAQ derivative 1A15 shows one pair of simultaneous two-electron transfer waves. The E3 values of

2 are considerably sensitive to the electronic effect of the substituents on the TTF moiety. These results strongly indicate that the two positive charges formed by the first two-electron oxidation are mainly distributed over the TTFAQ moiety and that the TTF moiety contributes to the sequential two pairs of one-electron transfer processes. The UV−vis−NIR spectrum of 2Aa2+ in a benzonitrile solution generated by applying a constant voltage (+0.41 V vs Fc/Fc+) exhibited absorption bands at 387, 405, 419, 472, and 712 nm (Figure S14). The fine structure in the spectrum at 380−420 nm is characteristic of 1A2+ (382, 394, and 421 nm).4b This result strongly indicates that the two positive charges in 22+ are located on the TTFAQ moiety, as is proposed based on the result of cyclic voltammetry. Crystal growth of oxidized species of 2 was attempted to elucidate the molecular structure in the oxidation state, and single crystals of (2Aa)I3 were successfully prepared by an electrocrystallization technique. X-ray structure analysis of (2Aa)I3 revealed that two donor molecules A and B were crystallographically independent. Both molecules A and B of 2Aa•+ in (2Aa)I3 adopt a saddle-like conformation similar to the neutral 2Aa (Figure 3a,b). The dihedral angle between the two least-squares planes composed of S7−C7−C8−S8 and S9−C25−C26−S10 (θ) is 109.8(3)° for molecule A and 103.7(3)° for molecule B, respectively. The θ values of 2Aa•+ are slightly larger than that of 2Aa (97.3(2)°). The α values for molecules A (8.9(1)°) and B (7.9(1)°) are obviously smaller than that of neutral 2Aa (22.7(1)°). That is, the TTF moieties of 2Aa•+ in (2Aa)I3 show considerably higher planarity as is very often observed in cationic TTF derivatives. The I3− anions are located on the side of the TTF moiety of 2Aa•+ (Figure 3c). There are many S···I interactions shorter than the sum of the van der Waals radii

a

Figure 2. Deconvoluted cyclic voltammograms of (a) 2Aa, (b) 2Ba, and (c) 2Ab in a benzonitrile solution (3 × 10−4 M) containing nBu4N·PF6 (0.1 M) at 25 °C. Experimental conditions: Pt working and counter electrodes, scan rate 50 mV s−1. Potentials were measured against an Ag/Ag+ electrode and converted to the value vs Fc/Fc+. B

DOI: 10.1021/acs.orglett.8b01985 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

the TTFAQ one because the T-type (TTFAQ)2+ is more stable than the S-type (TTFAQ)•+. As a result, the TTF moiety returns to the neutral structure and could contribute to the following two-stage, one-electron oxidation processes to form 2T4+. The Em values of 2 are slightly affected by the electronic effect of the substituent on the TTF moiety, and those of 2Aa and 2Ab are more negative than that of 1A (see Table 1). These results suggest that the TTF moiety could participate in the first twoelectron oxidation process of 2. The first two-electron redox process of 2 except for 2Ab is irreversible; that is, an extra cathodic wave was observed at significantly negative potential ca. −0.2 V. The reversibility of this redox is enhanced when the voltammetry is carried out at lower scan rates (Figure 4) or at high temperatures (Figure S13).

Figure 3. (a) ORTEP drawing of molecule A of 2Aa in (2Aa)I3, (b) the side view, and (c) intermolecular interactions between 2Aa•+ (molecule A) and I3− in (2Aa)I3. S···I distances (in Å): d1 = 3.7313(19), d2 = 3.6787(17), d3 = 3.4938(15), d4 = 3.4888(17), d5 = 3.7407(18), d6 = 3.7098(16).

between the sulfur atoms of the TTF moiety and the iodine atoms (3.4888(17)−3.7407(18) Å for molecule A and 3.5993(19)−3.7646(17) Å for molecule B). In contrast, there is no interaction between the terminal 1,3-dithiol-2-ylideneanthraquinoid moieties and the I3− anions. The above results suggest that a positive charge in 2Aa•+ is mainly distributed on the TTF moiety and that the TTFAQ moiety adopts an almost neutral structure. The TTF moiety is elucidated to be more positive charge distribution from the viewpoint of theoretical calculation (Figure S8). The monocation state of 2Aa was found in a crystal structure, while a two-electron transfer process occurred in CV (Figure 2a). The stacked π-dimer of 2Aa•+ in the crystal packing contributes to form a monocation state in the solid state (interaction a1 and a2 in Figure S5). There are van der Waals contacts (S···S = 3.377(2), 3.325(2) Å) in the π-dimer of 2Aa•+. Unstable oxidized states in solution were often observed in solid state owing to intermolecular interactions, for example, + 0.5 valence of the TTF derivatives was found in most molecular conductors.16 A plausible oxidation process of 2 is proposed in Scheme 2. The neutral 2Aa molecule adopts the S-conformation. A positive charge formed by the first one-electron oxidation is mainly distributed over the TTF moiety and the 2Aa molecule still adopts the S-conformation, as indicated by X-ray structure analysis of (2Aa)I3. The TTFAQ moiety might contribute to the second oxidation process so that on-site Coulomb repulsion in the dication state is reduced. The resulting dicationic biradical species (2S2+) with a large y value 0.888 (see Figure S10) should have an unstable (TTFAQ)•+ structure with the S-form. Then, conformational change from the S-form to the T-form might occur associated with intramolecular charge-transfer (ICT) from the TTF unit to

Figure 4. Scan rate dependence of cyclic voltammograms of 2Ba.

These results suggest that the first two-electron redox process is associated with a conformational change. In contrast, the first two-electron redox process of 2Ab is observed as a reversible redox wave without reducing the scan rate or increasing the temperature. These results indicate that the redox process of 2 is strongly affected by the substituents and their position. The reduction process from 2T2+ to 2S is considered to be as follows. The conformational change might occur more slowly than electron transfer. When the scan rate is high enough, the conformational change cannot compete with scanning. As a result, the redox process from 2T2+ to 2T is observed in the negative voltage region. The negative shift of the reduction potential from 2T2+ to 2T compared to that of 2T2+ to 2S is attributed to the difference in stability between 2S and 2T. That is, 2T is more unstable than 2S because 2T with two orthogonally arranged DT units must have a biradical structure. In the neutral state, 2T could rapidly convert to the more stable 2S-form. At a low scan rate, 2T2+ could be reduced to 2S by way of 2S•+, because there is enough time for conformational change from 2T•+ to 2S•+ (Scheme 3). As for 2Ab, its one-electron oxidation state is more stable than those of 2Ba and 2Aa because a positive charge can be located on the dimethyl-TTF moiety with more strongly electron-donating methyl groups than methylthio groups. In that case, the conformational change from 2T+ to 2S+ associated with ICT might be accelerated, which results in observation as a reversible redox wave. The first redox wave of 2Bc does not become completely reversible even at 5 mV s−1 (Figure S12). This could be ascribed to the destabilization of 2S•+ by the

Scheme 2. Plausible Oxidation Process of 2

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DOI: 10.1021/acs.orglett.8b01985 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 3. Plausible Reduction Process of 2Aa, 2Ba, and 2Bc

Present Address

strongly electron-withdrawing effect of the methoxycarbonyl groups on the TTF moiety. In conclusion, we demonstrated a bis-fused donor system composed of TTF and TTFAQ (2) that exhibits remarkable redox behavior accompanied by conformational change. That is, two reduction processes, 2T2+ → 2S and 2T2+ → 2T, were observed, when the substituent on TTF moiety is more electronwithdrawing than the substituent on the TTFAQ moiety. The reversibility of the redox process corresponding to a redox couple 2S/2T2+ was enhanced, when the voltammetry was carried out at a sufficiently low scan rate or at high temperature. This result indicates that two different reduction processes can be controlled by the scan rate or temperature. Thus, we developed a molecular redox system that is subject to large structural change during the redox process. Conformational change of the TTFAQ unit leads to a change in the molecular length, which might be promising for the development of molecular switches. Much difference between the oxidation potential (2S → 2T2+) and the reduction potential (2T2+ → 2T) is regarded as a kind of hysteresis, which might be important for developing a molecular memory. Further investigations, in particular, chemical modification and synthesis of TTFAQ derivatives and analogues suitable for the development of the functional materials mentioned above, are in progress.

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⊥

(M.K.) Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JPSP KAKENHI Grant Nos. JP23550155, JP26410095, and JP15H00948, by MEXT KAKENHI Grant No. JP15H03798, and a Grant-in-Aid for Research Promotion, Ehime University, to The Research Unit for Power Generation and Storage Materials and to The Research Unit for Development of Organic Superconductors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01985. Experimental details, 1H and 13C NMR spectra, X-ray structure analysis, theoretical calculations, deconvoluted cyclic voltammogrammetry, and spectroelectrochemistry (PDF) Accession Codes

CCDC 1851549−1851550 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shigeki Mori: 0000-0001-6731-2357 Yohji Misaki: 0000-0002-9079-8487 D

DOI: 10.1021/acs.orglett.8b01985 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters (11) Takahashi, K.; Tanioka, H.; Fueno, H.; Misaki, Y.; Tanaka, K. Chem. Lett. 2002, 31, 1002. (12) Taniguchi, M.; Misaki, Y.; Tanaka, K.; Yamabe, T.; Mori, T. Synth. Met. 1999, 102, 1721. (13) The deconvoluted cyclic voltammograms of 2Bb and 2Bc are shown in Figure S11. (14) Misaki, Y.; Nishikawa, H.; Kawakami, K.; Koyanagi, S.; Yamabe, T.; Shiro, M. Chem. Lett. 1992, 21, 2321. (15) Batsanov, A. S.; Bryce, M. R.; Coffin, M. A.; Green, A.; Hester, R. E.; Howard, J. A. K.; Lednev, I. K.; Martín, N.; Moore, A. J.; Moore, J. N.; Ortí, E.; Sánchez, L.; Savíron, M.; Viruela, P. M.; Viruela, R.; Ye, T.Q. Chem. - Eur. J. 1998, 4, 2580. (16) Mori, T. Chem. Rev. 2004, 104, 4947.

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DOI: 10.1021/acs.orglett.8b01985 Org. Lett. XXXX, XXX, XXX−XXX