4,5-bis(diarylmethylene)-1,3-dithiole: A novel helical electron donor exhibiting electrochromic behavior

Article abstract of DOI:10.1246/cl.2009.748

Vivid color change from intense yellow to deep purple was observed upon electrochemcal oxidation of the title donor 1 to bis(Michler's hydrol blue)-type dication 1²⁺. Since the interconversion was also accompanied by drastic geometric changes,

Full text of DOI:10.1246/cl.2009.748

748
Chemistry Letters Vol.38, No.7 (2009)
4,5-Bis(diarylmethylene)-1,3-dithiole: A Novel Helical Electron
Donor Exhibiting Electrochromic Behavior
Takanori Suzuki,ꢀ1 Shintaro Mikuni,1;y Yusuke Ishigaki,1 Hiroki Higuchi,1;yy
Hidetoshi Kawai,1;2 and Kenshu Fujiwara1
1Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810
2PRESTO, Japan Science and Technology Agency
(Received May 11, 2009; CL-090451; E-mail: tak@sci.hokudai.ac.jp)
Ar
Vivid color change from intense yellow to deep purple was
Ar
Ar
Ar
Ar
2e
2e
2a2+
S
S
S
S
S
observed upon electrochemcal oxidation of the title donor 1 to
bis(Michler’s hydrol blue)-type dication 12þ. Since the intercon-
version was also accompanied by drastic geometric changes,
configurational stability of the helical ꢀ-arrangement is modified
upon redox reactions.
Ar
3a,b
(Me2N)3P
S
Ar
Ar
Ar
S
Ar
Ar
1
S
S
2a
Ar
[ a: Ar = 4-Me2NC6H4 ; b: Ar = 4-MeOC6H4
]
Scheme 2.
ꢁ
the intermediary ion radical 1þ would be short-lived as in the
cases of redox-active 1,2-quinodimethanes.5 Negligible steady-
state concentration of the intermediate reduces the chance of side
reactions of the open-shell species, and thus favors the high re-
versibility of the electrochemical response.6 Here we report the
preparation and X-ray structure of title molecule 1 along with the
redox and electrochromic properties.
Sulfur-heterocycles1 are important components in designing
organic molecules for developing materials science. 1,3-Dithiole
and its dehydro dimer (tetrathiafulvalene, TTF2) form the most
common family. More than a thousand derivatives have been de-
scribed to date and investigated from the viewpoint of unique
solid-state properties3 (electric conduction/superconducting
behavior/magnetism) as well as in the field of supramolecular
chemistry.4 Strong electron-donating properties of TTFs are pro-
vided by the stability in cationic states thanks to the aromatic
nature of the 1,3-dithiolium unit.
We have designed and prepared here the title molecule 1,
which has very strong electron-donating units on the periphery
of the heterocycle. Those diarylmethylene units induce a strain-
ed molecular geometry with a helical arrangement of ꢀ-electron
system. Unlike TTF derivatives, the positive charges formed
upon oxidation would be located not on the dithiolium ring but
on the exocyclic auxiliaries of Michler’s hydrol blue units in
12þ (Scheme 1). Since the latter moieties are known as strong
chromophores, the present redox pair (1/12þ) would exhibit
electrochromic behavior. At the same time, one-electron oxida-
tion may cause drastic geometric change to relieve severe steric
strain between the adjacent diarylmethylene units in 1, so that
The strained donor 17 with four 4-Me2NC6H4 groups was
successfully generated upon ring-contracting desurfurization of
1,2,4-trithiane 2a by hexamethylphosphorous triamide in 72%
yield (Scheme 2), which was previously obtained by skeletal re-
arrangement of 1,3,5-trithiane 3a upon redox reaction.8 Orange
crystalline 1 can be stored infinitely under ambient conditions
despite its very strong donating properties.5d,9 According to the
voltammetric analysis, 1 undergoes reversible two-electron oxi-
dation at +0.12 V vs. SCE in MeCN (Figure S1),7 which clearly
shows that 1 is a stronger electron donor than TTF (Eox +0.31 V
under similar conditions). Upon treatment with 3 equivalents of
iodine, 1 was cleanly converted to deeply colored dication 12þ
,
which was isolated as stable triiodide salt in quantitative yield.
By reduction of 12þ(I3ꢂ)2 using Zn powder, 1 was regenerated
quantitatively. Attempted preparation of less donating 4-
MeOC6H4 derivative following the same scheme was hampered
since the skeletal rearrangement into 2b2þ does not occur
upon oxidation of 2,4-bis[di(p-anisyl)methylene]-1,3,5-trithiane
(3b).
Ar
Ar
2e
S
S
Ar
Ar
S
Ar
Ar
S
Low-temperature X-ray analysis10 of 1 has revealed that
it adopts a strained helical structure with a torsion angle of
47(1)ꢃ for the exocyclic diene unit (Figure 1). Such a large value
comes from the repulsive interaction between two inward aryl
groups (ArIN) facing each other in parallel [dihedral angle
6.2(2)ꢃ] with the shortest interatomic CꢄꢄꢄC contact of
12+
1
2e
Ar
Ar
Ar = 4-Me2NC6H4
e
S
S
S
S
˚
3.07(1) A, which is much smaller than the sum of van der Waals
S
S
S
S
˚
radii (3.40 A). This also causes a larger twisting angle of the
e
TTF
exocyclic double bond (9.9 and 11.4ꢃ) than in the case of 1,1,4,4-
tetrakis(4-dimethylaminophenyl)-1,3-butadiene (4.8ꢃ).6b
NMe2
Me2N
The 1H NMR spectrum of 1 can be rationalized by assuming
that the geometry in solution resembles the solid-state structure.
There are two pairs of resonances for the aryl groups at ꢁ 7.05,
6.63, 6.59, and 6.35 (CDCl3). The former two chemical shifts
are similar to those of related compounds with bis(4-dimethyl-
S
S
S
S
S
S
Michler's hydrol blue
1,3-dithiolium
Scheme 1.
Copyright ꢀ 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.7 (2009)
749
Figure 2. Continuous changes in the UV–vis spectra of 1 upon
constant-current electrochemical oxidation (29 mA) at 2 min in-
tervals in MeCN.
Figure 1. ORTEP drawings of (P)-1 (left: top view; right: side
view) obtained by X-ray analysis of rac-1 at 110 K.
amino)ethenyl moieties,9 thus assigned to those for aryl groups
directing outward (ArOUT). The latter two for ArIN are largely
upfield-shifted, which can be rationalized by mutual shielding
effects through ꢀ–ꢀ overlap in proximity. There are two singlets
at ꢁ 2.96 and 2.84, which are assigned to N-Me protons of ArOUT
and ArIN, respectively. We found no signs of exchange between
ArIN and ArOUT in the NMR spectra, suggesting that configura-
tion of helicity in 1 is quite stable (vide infra). On the other hand,
the spectrum indicates that 12þ adopts a C2v-symmetric struc-
ture: only two resonances for Ar groups (ꢁ 7.34 and 6.78) as well
as one singlet for N-Me protons (3.22) (CD3CN, Figure S2).7
Thus, ArOUT and ArIN in neutral 1 are rapidly interconvert-
ing or indistinguishable in 12þ, suggesting that two Michler’s
hydrol blue units are nearly perpendicular to the 1,3-dithiole ring
References and Notes
y
Present address: Faculty of Advanced Life Science, Hokkaido
University, Sapporo 060-0810
yy Present address: Institute for Materials Chemistry and Engineer-
ing, Kyushu University, Kasuga 816-8580
1
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2
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T. Mori, Chem. Rev. 2004, 104, 4947.
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in 12þ
.
Thanks to the very strong absorptions in the visible–NIR re-
gion of 12þ [UV–vis (MeCN): ꢂmax 703 nm (log " 4:75), 600
(4.69), 544 (4.74)], electrochemical interconversion of the pres-
ent pair is accompanied by vivid color change from intense yel-
low to deep purple. The presence of isosbestic points in the spec-
troelectrogram indicates clean interconversion as well as negli-
gible steady-state concentration of intermediary cation radical
species (Figure 2). This is the successful demonstration of elec-
trochromic behavior of a novel 1,3-dithiole electron donor 1.
Another interesting feature is the configurationally stable
helical geometry of 1, whereas dication 12þ is achiral due to
(time-averaged) C2v-symmetry. After many efforts, we succeed-
ed in partial optical resolution (55% ee) of helical diene 1 by us-
ing chiral HPLC (CHIRALPAK-IA, EtOAc:hexane = 1:3 with
0.5% w/w Et3N). Thanks to the helical ꢀ-system, resolved diene
1 shows very strong Cotton effects [ꢂext346 nm (ꢀ" ꢂ4:8), 335
(+25.9), 304 (ꢂ96:6), 273 (ꢂ80:6), 239 (+14.5) in MeCN; ma-
jor in the first fraction component of HPLC] (Figure S3).7 The
CD spectrum remained unchanged after several days at room
temperature, thus confirming configurational stability of helical
diene 1. In this way, it is suggested that the present pair can serve
as a chiral unit whose racemization barrier can be changed upon
redox reactions.11 Studies in this vein as well as making new ma-
terials by using 1 as a synthon are now in progress.
5
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M. Ohkita, T. Tsuji, K. Ono, M. Takenaka, Tetrahedron Lett.
2006, 47, 467.
6
a) S. Hunig, M. Kemmer, H. Wenner, I. F. Perepichka, P. Bauerle,
¨
¨
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¨
7
8
9
Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
T. Suzuki, T. Yoshino, J. Nishida, M. Ohkita, T. Tsuji, J. Org.
Chem. 2000, 65, 5514.
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1574.
ꢁ
10 Crystal data of 1: C37H42S2N4, Mr ¼ 606:89, triclinic, P1, a ¼
˚
10:253ð3Þ, b ¼ 11:225ð6Þ, c ¼ 16:472ð7Þ A, ꢃ ¼ 66:38ð2Þ,
3
ꢄ ¼ 70:378ð7Þ, ꢅ ¼ 75:929ð10Þꢃ, V ¼ 1623ð1Þ A , Z ¼ 2,
˚
Dcalcd ¼ 1:242 g cmꢂ3, independent reflection 6706 (all), 2723
(3ꢆ), R ¼ 6:3%.
11 a) J. W. Canary, Chem. Soc. Rev. 2009, 747. b) H. Goto, E.
Yashima, J. Am. Chem. Soc. 2002, 124, 7943.
Published on the web (Advance View) July 5, 2009; doi:10.1246/cl.2009.748