58
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012) BCSJ AWARD ARTICLE
¹
¹
radicals, I3 , I2, I , and chlorobenzene. Taking into account that
the ratio of the molecules of 1a and the anionic charges based on
formation of stable ³-dimers. To the best of our knowledge,
this is the first report of NMR spectra of a diamagnetic TTF
³-dimer. Our results provide detailed information about the
interactions in the MV and ³-dimer states of TTF derivatives
and will be useful in the design of supramolecular structures
and functional nanostructures.
¹
¹
I3 and I is 4:3, cation radical 4 possesses an average charge
per molecule of 1a of +0.75. The two cation radicals (1a2
1.5+
)
adopt a syn-conformation (Figure 10a), reflecting the stability
of the face-to-face stacking structure in the cation radical and
dication states. Interestingly, these dimeric TTFs are aligned in
an edge-to-face mode to form a two-dimensional sheet structure
(Figure 10b). The dihedral angles between the naphthalene and
TTF rings were determined to be 52°-56°. Although one
TTF ring adopts a planar structure with a maximum deviation
of 0.1 ¡ from the least-squares plane, the other three TTF
rings have bent 1,3-dithiole rings. The syn arrangement of
the TTF moieties causes the 1,3-dithiole rings to form envelope-
like structures, and the intramolecular distances between
S(1a)£(7a), S(2a)£(8a), S(1b)£(7b), and S(2b)£(8b) are shorter
than the sum of the van der Waals radii (3.70 ¡). Although there
is only one short S£I contact in 4 (I2£S(9b) 3.53 ¡), there are
a number of short intermolecular S£S contacts, as shown in
Figure 10b, and these S£S contacts result in the formation of
a sheet structure. In spite of the ¬-like structure,29 electric
conductivity of 4 was not very high (5.0 © 10¹5 S cm¹1), which
is thought to be due to inhibition of the b axis conductivity
pathway by the naphthalene moiety. Moreover, 4 in DMSO
underwent a charge recombination, and in the UV-vis-NIR
spectrum of 4 in DMSO, absorptions at 836 (SOMO¹1 ¼
SOMO) and 1800 nm (CR band) were observed, similar to those
in the spectrum of 1a•+ shown in Figure 4.27 Finally, X-ray
analysis of 4 experimentally confirmed that the syn-conforma-
tion in the cation radical and dication states of 1a was
much more stable than the corresponding anti-conformation,
although neutral 1a existed as a mixture of two isomers in
solution in an anti-syn ratio of 93:7 (Table 1) and adopted the
anti-form in the solid state (Figure 1) in order to avoid steric
crowding.
Experimental
1,8-Bis[(4,5-ethylenedithio)tetrathiafulvalenyl]naphtha-
lene (1a). To a solution of 2a (770 mg, 2.6 mmol) in THF
(40 mL) was added n-C4H9Li (1.65 mL, 2.6 mmol) dropwise
at ¹78 °C under N2 atmosphere. The mixture was stirred for
90 min. Then, a suspension of anhydrous ZnCl2 (442 mg,
3.2 mmol) in THF (3 mL) was added to the reaction mixture at
¹65 °C. The mixture was stirred for 1 h at the same temperature
and warmed up to ¹20 °C. A solution of 1,8-diiodonaphthalene
(355 mg, 0.9 mmol) in THF (2 mL) and [Pd(PPh3)4] (216 mg,
0.19 mmol) was added. The mixture was stirred for 1 h at
¹20 °C and warmed up to 45 °C. After stirring for 18 h,
aqueous NH4Cl solution was added to the mixture and
extracted with CS2. The combined organic layer was washed
with saturated brine and dried over MgSO4. The solution was
filtered, and solvent was removed in vacuo. The residue was
purified by column chromatography on silica gel (activity V)
with CS2 as the eluent. Recrystallization from CS2-diisopropyl
ether gave red crystals of 1a (513 mg, 78%). 1a: mp 199.3-
200.4 °C; LDI-TOF-MS m/z = 712 (M+); 1H NMR (CDCl3:
CS2 = 1:1, 500 MHz): ¤ 7.85 (dd, J = 7.5 and 1.2 Hz, 2H),
7.56 (dd, J = 7.5 and 1.2 Hz, 2H), 7.46 (t, J = 7.5 Hz, 2H),
6.03 (s, 2H), 3.29 (s, 8H); 13C NMR (CDCl3:CS2 = 1:1,
125 MHz): ¤ 135.2, 133.2, 131.0, 130.4, 129.5, 125.4, 118.4,
114.8, 114.2 (2C), 105.4 (2C), 30.5; UV-vis-NIR (4.2 ©
10¹5 M, CH2Cl2): -max (¾) 312 (30000), 338sh (22000) nm; IR
(KBr): 2957, 2923, 2853 cm¹1; Anal. Calcd for C26H16S12: C,
43.79; H, 2.26%. Found: C, 43.63; H, 2.25%.
1,8-Bis[4,5-bis(methylthio)tetrathiafulvalenyl]naphtha-
lene (1b). The synthesis of 1b was carried out from 2b and
1,8-diiodonaphthalene in 59% yield in a similar manner as
described for 1a. 1b: a red powder; mp 86.9-87.8 °C; MALDI-
TOF-MS m/z = 716 (M+); 1H NMR (CDCl3, 500 MHz):
¤ 7.88 (dd, J = 7.5 and 1.2 Hz, 2H), 7.58 (dd, J = 7.5 and
1.2 Hz, 2H), 7.48 (t, J = 7.5 Hz, 2H), 6.07 (s, 2H), 2.47
(s, 6H), 2.44 (s, 6H); 13C NMR (CDCl3, 125 MHz): ¤ 135.1,
133.5, 130.9, 130.3, 129.4, 127.9, 126.7, 125.4, 117.8, 115.4,
106.8 (2C), 19.3 (2C); UV-vis-NIR (2.5 © 10¹5 M, CH2Cl2):
Conclusion
We studied in detail the interactions between two TTF units
by using 1,8-bis(tetrathiafulvalenyl)naphthalenes 1a-1c as a
model. From X-ray analyses and molecular structural calcu-
lations, the anti-isomer of neutral 1a-1c is more stable than
the syn-isomer due to steric effect, although 1a-1c exist as a
mixture of anti- and syn-isomers in solution. CV measurements
revealed that 1a-1c underwent two one-electron and one two-
electron redox processes. Large Kc values were obtained due to
a stable mixed-valence (MV) state in 1a-1c, and the thioalkyl
groups at the 4 and 5 positions of the TTF moieties stabilize the
MV state. Cationic species 1a•+, 1a2+, and 1a4+ were prepared
by chemical and electrochemical oxidation methods, and their
electronic structures were elucidated from absorption spectra.
In the case of 1a•+, a strong charge resonance (CR) band was
observed arising from the MV state due to the face-to-face
arrangement of the TTF moieties. For 1a2+, the lowest energy
absorption band was split, indicating a Davydov blue shift due
to the formation of a ³-dimer. The experimentally observed
transitions could be reproduced by TD-DFT calculations. From
an ESR spectrum of 1a•+, a hyperfine coupling constant (hfcc)
consistent with that for an MV state was obtained. The
1H NMR spectra of 1a2+ and 1a4+ were consistent with the
-
(¾) 308 (33000), 390sh (5800) nm; IR (KBr): 2918,
max
2853 cm¹1; Anal. Calcd for C26H20S12: C, 43.54; H, 2.81%.
Found: C, 43.34; H, 3.05%.
1,8-Bis(tetrathiafulvalenyl)naphthalene (1c). The syn-
thesis of 1c was carried out from tetrathiafulvalene and 1,8-
diiodonaphthalene in 85% yield in a similar manner as
described for 1a. 1c: an orange powder; mp 145.6-146.7 °C;
1
LDI-TOF-MS m/z = 532 (M+); H NMR (CDCl3, 400 MHz):
¤ 7.86 (dd, J = 8.0 and 1.0 Hz, 2H), 7.60 (dd, J = 8.0 and
1.0 Hz, 2H), 7.46 (t, J = 8.0 Hz, 2H), 6.30 (d, J = 6.8 Hz, 2H),
6.28 (d, J = 6.8 Hz, 2H), 6.05 (s, 2H); 13C NMR (CDCl3,
100 MHz): ¤ 135.2, 133.5, 130.9, 130.3, 129.8, 125.5, 119.2
(2C), 118.9, 118.1, 111.1, 110.7; UV-vis-NIR (4.2 © 10¹5 M,
CH2Cl2): -max (¾) 310 (30000), 400 (3100) nm; IR (KBr):