Graphical Abstract

An electron-deficient tetrathiafulvalene-conjugated bistetracene Masataka Yamashita, Hironobu Hayashi, Naoki Aratani, and Hiroko Yamada

Leave this area blank for abstract info.

1

Tetrahedron Letters

j ou rna l homepage : www.e lsev ie r .com

An electron-deficient tetrathiafulvalene-conjugated bistetracene

Masataka Yamashitaa, Hironobu Hayashia, Naoki Aratania * and Hiroko Yamadaa,b * a Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan b CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

——— * Corresponding author. Tel.: +81-743-72-6030; fax: +81-743-72-6042; e-mail: aratani@ms.naist.jp, hyamada@ms.naist.jp

Introduction

Compared to the p-type semiconductors, the line-up of n-type semiconductors is not fully developed, and the higher performances are demanded. The creation of excellent n-type materials is crucial for the fabrication of p-n junctions, bipolar transistors, and organic photovoltaic devices.1 To obtain the high electron mobility, the organic semiconductor layer should be highly ordered with strong intermolecular interactions and also have a lower LUMO energy level.

Tetrathiafulvalene (TTF) derivatives are promising candidates for semiconductors, giving high performance FETs because of their self-assembling properties leading to strong intermolecular interactions.2 However, because of the electron-rich properties, the compounds are generally weak to oxygen.3 In our previous work, we introduced fused aromatic rings to the TTF skeleton to enhance the intermolecular interaction, which consequently lead to the large charge transfer integral in the calculations based on the crystal structure (Chart 1, compound 1).4 High performance n-type organic semiconductors have recently been obtained by introducing electron-withdrawing substituents into electron-donating p-conjugated systems.5,6 Therefore, we have now introduced p-trifluoromethyl-substituent as an electron-accepting group to the peripheral aryl groups on TTF derivative 1. We report here the synthesis and physical properties of electron deficient TTF derivative 2. New TTF derivative 2 was synthesized by a phosphite coupling reaction and characterized by conventional chemical and physical methods.

S

S S

S

1CF3 CF3

CF3CF3

S

S S

S

2

Chart 1.

Synthesis and characterization

The route for the synthesis of the p-CF3-tetraphenyl TTF-conjugated bistetracene 2 starts from a Diels–Alder reaction of the quinone 47 with 1,3-bis(p-trifluoromethylphenyl)-isobenzofurane 38 to form an adduct, then subsequent dehydration provided the tetracenequinone 5 (Scheme 1). The quinone 5 was reduced with NaBH4 and SnCl2 to afford tetracene thione 6 in high yield, which was interconverted to dioxol-2-one 7 with Hg(OAc)2 in 87% yield.

A R T I C L E I N F O A B S T R A C T

Article history: Received Received in revised form Accepted Available online

An electron-deficient tetrathiafulvalene (TTF)-conjugated bistetracene 2 was synthesized as the counterpart of phenyl-substituted TTF-conjugated bistetracene 1, and was characterized in the molecular electronic structures based on the spectroscopic measurements. UV-vis absorption spectrum of 2 is slightly broader than that of 1 and fluorescence spectrum of 2 is red-shifted, reflecting the red-shifted absorption spectrum. The energy level of the highest occupied molecular orbital (HOMO) of 2 was directly determined by the atmospheric-photoelectron yield spectroscopy in powder to be –5.8 eV, which is remarkably lower than that of 1 (–5.2 eV).

2009 Elsevier Ltd. All rights reserved.

Keywords: TTF Tetracene p-Conjugation Electron deficient n-Type semiconductor

Tetrahedron 2

Scheme 1. Synthesis of p-trifluoromethylphenyl-substituted TTF-conjugated bistetracene 2.

a) b)

c)

e)

d)

f)

Figure 1. Crystal structures of a) top view and b) packing diagram of 5, c) top view and d) packing diagram of 6, and e) top view and f) packing diagram of 7. Solvent molecules are omitted for clarity. Thermal ellipsoids represent 50% probability.

The structures of 5, 6, and 7 were confirmed by their 1H-NMR and high-resolution mass spectroscopies. All those were also revealed by single crystal X-ray diffraction analysis (Figure 1).9 The structures exhibit perfectly planar skeletons unambiguously, in which the phenyl groups take perpendicular orientation. In the

solid state, all 5, 6, and 7 form a face-to-face anti-parallel dimeric structure with a stacking distance of ~3.4 Å.

Then, we tried to prepare the target compound 2 from 6 and 7 based on the phosphonate-induced coupling reaction. After several experiments, we obtained 2 in 56% yield as a purple solid. The solubility of 2 in common organic solvents is quite low, rather than that of 1. High resolution spiral MALDI-TOF-mass spectroscopy detected the parent ion peak at m/z = 1180.1184 (calcd for C66H32F12S4 = 1180.1195 [M]+). The 1H NMR spectrum of 2 in tetrachloroethane-d2 at 110°C exhibited only aromatic protons at 7.99, 7.95, 7.72, 7.69, 7.57, and 7.31 ppm. Purity of compound 2 was also confirmed by 19F and 31P NMR spectra. Sole signal at –61.37 ppm in the 19F NMR was observed, whereas no signal was found in the 31P NMR (Supporting Information).

UV-vis absorption and fluorescence spectra of 1 and 2 in toluene are shown in Figure 2. The absorption spectra of both 1 and 2 become very broad with less vibronic structure and exhibit red-shifted absorption compared to tetracene, suggesting the considerable electronic communication between two tetracenedithiole units by through-bond and/or through-space interactions.4 Compared to 1, 2 exhibits almost the same absorption peaks with slightly red-shifted absorption-edge at around 550 nm, indicating essentially the same electronic structure of 1 and 2. However, the fluorescence spectrum of 2 is red-shifted, reflecting the red-shifted absorption spectrum. The fluorescence quantum yield of 2 is 19%, which is slightly higher than that of 1 (16%).

Nor

mal

ized

abs

orba

nce

700600500400Wavelength / nm

Norm

alized fluorescence

In#toluene

S

S

R

R

S

S

R

R389,450 (sh), 480, 511 568391,450 (sh), 480, 512 586

1619

λabs / nm λem / nm Φ / %H

CF3

R

CF3 ε

2

1 2

Uv-vis Fluoresence

Figure 2. UV-vis absorption (normalized at 390 nm) and

fluorescence (excited at 390 nm) spectra of 1 (red) and 2 (blue) in toluene.

The decreased p-electron density in the p-conjugated aromatic ring leads to reduced p-electron repulsion, resulting in forming very strong aromatic interactions to be less soluble.10 The redox potentials of 2 could not be measured by cyclic voltammetry (CV) because of the insoluble properties. Instead of electrochemical measurement, the energy level of the highest occupied molecular orbital (HOMO) was directly determined by atmospheric-photoelectron yield spectroscopy in powder (Figure 3). The energy value for 2 is –5.8 eV, which is drastically lower than that for 1 (–5.2 eV). The LUMO level of 2 was estimated from the HOMO-LUMO energy gap which was obtained from the absorption onset in absorption spectrum (–2.07 eV). Introduction of CF3 groups absolutely lowers the HOMO and LUMO levels by 0.6 eV.

To understand their electronic features, MO calculations of the compounds 1 and 2 were performed at B3LYP/6-31G(d) level using Gaussian 09 package11 (Figure 4). At a glance, the HOMO and HOMO–1 of 1 and 2 are localized on the tetracene units. Interestingly, although the energy levels of the frontier orbitals

3 lower as the electron-withdrawing groups are introduced, the structures of molecular orbitals are not changed at all. Eventually we could control the energy levels with completely keeping up their frontier orbital structures.

Ioniza'on(poten'als(of(Ar#DT#TTFs

Emis

sion

yie

ld /c

ps0.

3Em

issi

on(yield(/(cps0

.3

0

5

10

15

4 5 6 7

5.2 eV

-3

2

7

12

4 5 6 7

5.8 eV

CF3

Photon energy / eV

Photon energy / eV Figure 3. Measurement of ionization potentials of 1 (upper) and 2

(lower) in powders by photoelectron spectroscopy, AC–3. Solid lines are fitting lines.

HOMO!

HOMO–1!

LUMO+1!

LUMO!

HOMO!

HOMO–1!

LUMO+1!

LUMO!

S

S

CF3

CF3

S

S

CF3

CF3

S

S

S

S

–2.15 eV

–2.19eV –2.47 eV

–2.52 eV

–4.80 eV

–4.84 eV –5.13 eV

–5.18 eV

Figure 4. Molecular orbital diagrams of 1 and 2.

In summary, the electron deficient TTF-conjugated bistetracene 2 could be synthesized and characterized in the molecular electronic structures. The precursors 5, 6, and 7 exhibit face-to-face interaction in the crystals, resulting in the 1D columnar structure as a whole. Introduction of CF3 groups was found to be very effective in lowering the HOMO and LUMO levels without any changes of orbital structures.

Acknowledgments

This work was partly supported by Grants-in-Aid for Scientific Research (Nos. 25288092, 25620061, 26288038 and 25107519 'AnApple'), for Young Scientists (A) (No. 23685030), a research grant by The Murata Science Foundation, PRESTO program by JST, the Green Photonics Project in NAIST, and the program for promoting the enhancement of research universities in NAIST supported by MEXT. The authors thank Ms. Y.

Nishikawa, Mr. S. Katao, and Mr. Asanoma in NAIST for the measurement of mass spectra, single-crystal structure analysis, and NMR measurement, respectively.

Supplementary Material

Supplementary data (synthetic detail, characterization, and X-ray diffraction analysis) associated with this article can be found, in the online version, at http://dx.doi.org/10.*/j.tetlet.2015.*.*.

References and notes

1. Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. J. Am. Chem. Soc. 2004, 126, 8138.

2. Naraso, Nishida, J.-i; Kumaki, D.; Tokito, S.; Yamashita, Y. J. Am. Chem. Soc. 2006, 128, 9598–9599.

3. Noda, B.; Katsuhara, M.; Aoyagi, I.; Mori, T.; Taguchi, T.; Kambayashi, T.; Ishikawa, K.; Takezoe, H. Chem. Lett. 2005, 34, 392.

4. Yamashita, M.; Kuzuhara, D.; Aratani, N.; Yamada, H. Chem. Eur. J. 2014, 20, 6309.

5. Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. J. Am. Chem. Soc. 2004, 126, 8138.

6. (a) Letizia, J. A.; Facchetti, A.; Stern, C. L.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 13476. (b) Ando, S.; Murakami, R.; Nishida, J.; Tada, H.; Inoue, Y.; Tokito, S.; Yamashita, Y. J. Am. Chem. Soc. 2005, 127, 14996.

7. Gautier, N.; Cariou, M.; Gorgues, A.; Hudhomme, P. Tetrahedron Lett. 2000, 41, 2091–2095.

8. Qu, H.; Chi, C. Org. Lett. 2010, 12, 3360-3363. 9. Crystallographic data for 5: C33H14F6O2S3·4MeCN, Mw =

816.8524, triclinic, space group P-1 (No. 2), a = 8.3947(7), b = 13.7360(11), c = 16.8667(14) Å, a = 87.823(2), b = 75.713(2), g = 79.223(2)°, V = 1851.4(3) Å3, rcalcd = 1.465 g/cm3, Z = 2, R1 = 0.0819 [I > 2.0s(I)], Rw = 0.2429 (all data), GOF = 1.043. Crystallographic data for 6: C33H16F6S3, Mw = 622.64, triclinic, space group P-1 (No. 2), a = 10.5893(5), b = 11.1821(6), c = 12.9433(7) Å, a = 115.5580(10), b = 102.4020(10), g = 96.8670(10)°, V = 1310.64(12) Å3, rcalcd = 1.578 g/cm3, Z = 2, R1 = 0.0576 [I > 2.0s(I)], Rw = 0.1527 (all data), GOF = 1.025. Crystallographic data for 7: C33H16F6OS3, Mw = 606.58, triclinic, space group P-1 (No. 2), a = 10.1169(3), b = 10.9830(4), c = 12.8404(4) Å, a = 73.0310(10), b = 75.4970(10), g = 81.1560(10)°, V = 1316.13(7) Å3, rcalcd = 1.531 g/cm3, Z = 2, R1 = 0.0549 [I > 2.0s(I)], Rw = 0.1558 (all data), GOF = 1.078. CCDC 1050746 (5), 1050747 (6), and 1050748 (7) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

10. Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.

11. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T. ; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. GAUSSIAN 09 Re-Vision A.2; GAUSSIAN: Wallingford, CT, 2009.