Structure and physical properties of a hydrogen-bonded self-assembled material composed of a carbamoylmethyl substituted TTF derivative

Article abstract of DOI:10.1039/a800509e

Crystal structures of the carbamoylmethyl substituted TTF derivative AMET are characterized by polymeric hydrogen bonding between amide groups. As a result, the TTF moieties stack in parallel even in the neutral crystal. The ν_NH absorbtions of

Full text of DOI:10.1039/a800509e

J O U R N A L O F
Structure and physical properties of a hydrogen-bonded self-assembled
material composed of a carbamoylmethyl substituted TTF derivative
Go Ono,a Akira Izuoka,a Tadashi Sugawara*a and Yoko Sugawarab
aDepartment of Basic Science, Graduate School of Arts and Sciences, the University of T okyo,
Komaba, Meguro, T okyo 153, Japan
C H E M I S T R Y
bSchool of Science, Kitasato University, Sagamihara, Kanagawa 288, Japan
Crystal structures of the carbamoylmethyl substituted TTF derivative AMET are characterized by polymeric hydrogen bonding
between amide groups. As a result, the TTF moieties stack in parallel even in the neutral crystal. The n absorbtions of neutral
NH
AMET at 3426 and 3184 cm−1 show shifts to lower wavenumber, Dk, of 37 and 58 cm−1, respectively, at 4.1 GPa. Therefore the
iodine-doped samples at doping ratios of less than 45% was exactly the same as that for a neutral sample, suggesting that the
hydrogen bonding pattern is not affected significantly upon doping.
˚
shrinkage of the NMH,O distance is estimated to be ca. 0.04 A at this pressure. The pressure dependence of the IR spectra of
Although crystalline AMET is an insulator in the neutral state (s =ca. 10−8 S cm−1), the conductivity is enhanced by a factor
external pressure, and the s of a 5% iodine-doped sample increased three-fold at 1.0 GPa. The enhanced conductivity of iodine-
rt
of 107 upon iodine doping of 45 mol% (s =1.2×10−1 S cm−1). Furthermore, the conductivity increases as a function of the
doped samples may be ascribed to the increase in the overlap between the donor moieties based on the shrinkage of the hydrogen
bond of the carbamoylmethyl group.
rt
rt
Construction of molecular self-assemblies has become a current
topic of interest in materials chemistry.1 Although various
sophisticated molecular architectures are now known, the
number of molecular assemblies which exhibit prominent
functionalities is still limited. In this respect, it may be interes-
ting to design a novel organic donor as a building block for
the self-assembly, the functionalities of which can be controlled
by means of external stimuli.2
It is well-known that organic donors, e.g. perylene, TTF,
BEDT-TTF and TMTSF, prefer to align in a herringbone
structure in the neutral state in order to avoid electronic
Fig. 1 Crystal structure of benzamide viewed along the c axis
repulsion between p-electron rich donor molecules.3 On the
other hand, when the donors are partially oxidized, they tend
to stack one above the other, with intermolecular contacts
between heavy atoms, e.g. sulfur or selenium. The heteroatom
contacts, then, construct conduction paths by themselves, as
long as the HOMO of the donor has reasonably large
coefficients on these heteroatoms.4 If a stacking structure can
be obtained even for the neutral donors by means of specific
intermolecular forces, the molecular assembly is expected to
exhibit controllable transportational phenomena by hole-
doping, application of an external pressure, etc.
In this respect, we were intrigued by the hydrogen-bonded
crystal structures of primary amides.5,6 The crystal structure
of benzamide is characterized by a one-dimensional chain
composed of a doubly hydrogen-bonded dimers; the dimer is
formed via one of the amide hydrogens, while the other amide
hydrogen forms a polymeric hydrogen-bonded chain6 (Fig. 1).
Moreover, the one-dimensional arrangement of the phenyl ring
of benzamide in the crystal is entirely different from a typical
herring-bone type packing, as observed in crystals of aromatic
hydrocarbons. Hydrogen-bonded molecular assemblies con-
structed using amide groups, therefore, are likely to be effective
for regulating the structure of molecular self-assemblies com-
posed of organic donors.
the direction of the hydrogen bond is restricted to the hori-
zontal direction with reference to the donor plane. Thus the
packing pattern is not significantly influenced by the hydrogen
bond of the thioamide group.
Recently, we have prepared BEDT-TTF derivative AMET
1 carrying a carbamoylmethyl group (CH CONH ). The
2
2
difference in the conformations of the hydrogen-bonding
groups between thioamide TTF and AMET may be explained
by the presence of a methylene unit between the amide group
and the donor moiety; the hydrogen bond in AMET is expected
to be formed perpendicular to the donor plane due to the
flexibility of the carbamoylmethyl group.
In the present paper, the hydrogen-bonding scheme of
neutral crystals of AMET is investigated by X-ray crystallog-
raphy and IR spectroscopy. The conducting behavior of iodine-
doped AMET is measured as a function of the doping ratio.
The change in the hydrogen-bonding scheme of the iodine-
doped sample is also examined by IR spectroscopy. Finally
the effect of external pressure on the conductivity and the
hydrogen bonding scheme of AMET is investigated with
reference to the possible pressure control of the transportation
properties of hydrogen-bonded organic conducting materials.
Bryce and co-workers reported a TTF derivative substituted
with a thioamide group.7 Although a polymeric hydrogen-
bonded chain is constructed by the thioamide groups, the
packing of the donor moieties is of a herringbone type. Since
the thioamide group is directly attached to the donor moiety,
Experimental
Materials
Syntheses of zinc chelate 8,8,9 4,5-ethylenedithio-1,3-dithiol-2-
one 28 and 4-acetylthio-5-methylthio-1,3-dithiole-2-thione 510
J. Mater. Chem., 1998, 8(8), 1703–1709
1703

were performed according to published procedures.
Tetrahydrofuran (THF) was distilled over sodium metal in the
presence of benzophenone prior to use under a nitrogen
atmosphere. Methanol was distilled under a nitrogen atmos-
phere after an addition of sodium metal. Dichloromethane and
1,1,2-trichloroethane were treated with sulfuric acid, then
washed with 100 ml saturated aqueous NaHCO and 100 ml
3
of water twice. The solution was dried over anhydrous MgSO
4
and filtered, and the solvent was evaporated under reduced
pressure. The product was purified by column chromatography
with silica gel using chloroform–hexane (151) as eluent.
Evaporation of the solvent gave 3 as a yellow oil (4.00 g,
13.5 mmol, 95%); d (CDCl ) 1.29 (t, J=7.3 Hz, 3H), 2.48 (s,
washed with saturated aqueous NaHCO , water and brine,
successively. The solvents were distilled after being dried over
3
H
3
3H), 2.69 (t, J=7.3 Hz, 2H), 3.10 (t, J=7.3 Hz, 2H), 4.18 (q,
J=7.3 Hz, 2H); n (KBr)/cm−1 2982 and 2924 (CH ), 1733
CaCl for several hours at room temperature. Acetone, 2-
bromoethanol and 1,3-dibromopropane were treated with
K CO and distilled. Triethyl phosphite was dried over sodium
2
max
2
(CNO, ester), 1669 (CNO, 1,3-dithiol-2-one), 1425 (CNC),
1186 and 1022 (CMO), 741 (CMSMC).
2
3
metal and distilled under a nitrogen atmosphere. All other
reagents were used as purchased without further purification.
Gel permeation chromatography was performed using an
LC-08 instrument (Japan Analytical Industry Co., Ltd.)
equipped with JAIGEL-1H and -2H columns. Chloroform was
used as eluent.
4,5-Ethylenedithio-4∞-(2-ethoxycarbonylethylthio)-5∞-
methylthiotetrathiafulvalene 4. In a 100 ml round-bottomed,
short-necked flask fitted with an air condenser, 2 (2.81 g,
13.5 mmol) and 3 (4.00 g, 13.5 mmol) were dissolved in 20 ml
of triethyl phosphite. The mixture was heated in an oil bath
at 110 °C with stirring for 3 h under nitrogen. The solvent and
triethyl phosphate were then evaporated under reduced press-
ure at 100 °C. The resulting red residue was extracted with
chloroform and filtered off with suction to remove BEDT-
TTF. The product 4 was separated from homo-coupled TTF
derivatives by silica gel column chromatography using
dichloromethane–hexane (357) as eluent. Evaporation of the
solvent gave 4 as red oil (1.76 g, 3.72 mmol, 23%); d (CDCl )
Cyclic voltammetry
Cyclic voltammograms (CVs) were measured in dichloro-
methane in the presence of tetrabutylammonium perchlorate
(0.1 mol l−1) as electrolyte with a platinum working electrode
using
a potentiostat/galvanostat HAB 151 (HOKUTO
DENKO Ltd.). An Ag/AgCl electrode was used as the reference
electrode. The scanning rate was 200 mV s−1.
Spectral measurements
H
3
1.27 (t, J=7.3 Hz, 3H), 2.47 (s, 3H), 2.65 (t, J=7.3 Hz, 2H),
3.04 (t, J=7.3 Hz, 2H), 3.30 (s, 4H), 4.16 (q, J=7.3 Hz, 2H);
n
(KBr)/cm−1 2969 and 2921 (CH ), 1731 (CNO), 1417
max
2
1H NMR spectra were measured using a JEOL GSX-270
spectrometer; chemical shifts in CDCl solution are reported
in d units relative to tetramethylsilane as internal standard.
(CNC), 1183 and 1022 (CMO), 773 (CMSMC).
3
4,5-Ethylenedithio-4∞-carbamoylmethylthio-5∞-
UV–VIS–NIR spectra were measured using a Nihon Bunko
V-570 series UV–VIS–NIR spectrometer using KBr pellets.
Infrared spectra were recorded using a Perkin-Elmer 1400
series infrared spectrometer using KBr pellets. The pressure
dependence of the IR spectra was measured by pressing a KBr
pellet using a diamond anvil cell.
methylthiotetrathiafulvalene 1. In a 50 ml round-bottomed,
short-necked flask fitted with a dropping funnel, 4 (781 mg,
1.65 mmol) and 2-bromoacetamide (1.13 g, 8.25 mmol) were
dissolved in 50 ml of methanol–dichloromethane (951), the
flask being cooled with an ice bath. To this solution was added
dropwise sodium methoxide (357 mg, 6.60 mmol) in 5 ml of
methanol with stirring under nitrogen. The resulting red solu-
tion was stirred for 12 h at room temperature. The mixture
was filtered off with suction and the residue was dissolved in
chloroform to remove sodium bromide. The orange solution
Preparation
4-(2-Ethoxycarbonylethylthio)-5-methylthio-1,3-dithiole-2-
thione 6. In a 200 ml round-bottomed, short-necked flask fitted
with a dropping funnel, 4-acetylthio-5-methylthio-1,3-dithiole-
2-thione 5 (3.60 g, 15.1 mmol) was dissolved in 100 ml of
methanol, and the flask was cooled with an ice bath. To this
solution was added dropwise sodium methoxide (820 mg,
15.1 mmol) in 20 ml of methanol with stirring under nitrogen.
The resulting red solution was stirred for 1 h at room tempera-
ture. The solution was cooled with an ice bath again, and ethyl
3-bromopropionate (2.73 g, 15.1 mmol) in 20 ml of methanol
was added under nitrogen. The mixture was stirred for 12 h at
room temperature. The solvent was evaporated and the residue
was extracted with dichloromethane to remove sodium bro-
mide. The orange solution was washed with 150 ml of saturated
was washed with 50 ml of saturated aqueous NaHCO and
50 ml of water twice. The mixture was dried over anhydrous
3
MgSO and filtered, and then the solvent was evaporated. The
crude product was recrystallized from chloroform to give 1 as
4
orange needles (673 mg, 1.57 mmol, 98%); mp 139 °C;
d (CDCl ) 2.48 (s, 3H), 3.31 (s, 4H), 3.50 (s, 2H), 6.85 (s, 2H);
H
max
3
n
(KBr)/cm−1 3426, 3285, 3257 and 3184 (NH ), 2976 and
2
2921 (CH ), 1664 (CNO), 1601 (NH ), 1483 (CNC, center),
2
2
1407 (CNC, peripheral), 1370 (CMN), 774 (CMSMC), 587
(NMCNO) (Calc. for C H ONS : C, 30.75; H, 2.58; N, 3.26;
S, 59.69. Found C, 30.57; H, 2.57; N, 3.49; S, 60.00%).
11 11
8
aqueous NaHCO and 150 ml of water twice. The mixture was
dried over anhydrous MgSO and filtered, and then the solvent
3
General procedure for preparing iodine-doped samples of
AMET
4
was evaporated. The product, 4-(2-ethoxycarbonylethylthio)-
5-methylthio-1,3-dithiole-2-thione 6, was obtained as a yellow
oil (4.45 g, 14.2 mmol, 93%); d (CDCl ) 1.28 (t, J=7.3 Hz,
Doping of the crystals of AMET with iodine was carried out
using the following procedure. The crystals of AMET were
ground in a mortar with a pestle of agate. Then, the obtained
crystallites were added to carbon tetrachloride containing a
given amount of iodine and the suspension was stirred for 2 h
at room temperature. The purple color of the suspension faded
completely, indicating that the dissolved iodine had been
homogeneously absorbed by the crystallites of AMET. The
color of the crystallites of AMET changed from orange to
black upon iodine doping. Doping of iodine into crystals of
BEDT-TTF also carried out according to the same procedure.
The doping ratio was evaluated via the increased weight of the
doped sample.
H
3
3H), 2.53 (s, 3H), 2.70 (t, J=7.3 Hz, 2H), 3.11 (t, J=7.3 Hz,
2H), 4.19 (q, J=7.3 Hz, 2H); n (KBr)/cm−1 2982, 2928
max
(CH ), 1733 (CNO, ester), 1423 (CNC), 1029 (CNS), 744
2
(CMSMC).
4-(2-Ethoxycarbonylethylthio)-5-methylthio-1,3-dithiole-2-
one 3. In a 200 ml round-bottomed, short-necked flask, (4.45 g,
14.2 mmol) and mercury() acetate (13.60 g, 42.7 mmol) were
dissolved in 20 ml of acetic acid and 60 ml of chloroform. The
mixture was stirred for 3 h at room temperature. The mixture
was filtered to remove mercury salts, and the filtrate was
1704
J. Mater. Chem., 1998, 8(8), 1703–1709

X-Ray crystallographic analysis
samples of AMET was measured by pressing a pellet of
crystallites of AMET using a Be–Cu cell: DEMNUM S-20
(Daikin Industries Co, Ltd.) was used as a pressure transmit-
ting medium.
A single crystal of AMET was mounted on a Rigaku AFC-5
four-circle diffractometer using graphite mono-chromatized
˚
Mo-Ka radiation (l=0.7107A). The crystal system is ortho-
rhombic, space group Pca2 (#29), and the cell constants
1
Results and Discussion
refined with 20 reflections (4<2h<25°) are a=15.193(2), b=
˚
˚
4.7566(8), c=23.472(3) A, V =1696.2(4) A3. Intensity data
were measured at ambient temperature; v scan (4<2h∏45°)
and v–2h scan (45<2h<55°), scan speed 4 degrees min−1,
0<h<19, 0<k<30, 0<l<6. Three standard reflections (8 0
0, 0 14 0 and 0 4 2) were measured for every 100 reflections,
with no significant variations throughout the data collection;
among 2417 reflections measured (4<2h<55°), 2312 were
Preparation of AMET
The preparative route to AMET 1 is shown in Scheme 1. The
key reaction is the hetero-coupling between ethylenedithio
ketone 2 and unsymmetrically substituted ketone 3 carrying
an ethoxycarbonylethylthio group. After the coupling, the
protecting group is efficiently removed by sodium methoxide
via a retro-Michael addition mechanism (Scheme 2). The
ethoxycarbonylethyl group is, therefore, considered to be an
excellent protecting group for a thiolate in the coupling
procedure.13 The thiolate is reacted with 2-bromoacetamide to
afford carbamoylmethylthio-substituted TTF derivative 1. The
yield of each step is reasonably high, and the total yield
starting from [Zn(dmit) ]Ω(Et N) 8 is 4–7%. Single crystals
unique (R =0.010).
int
The structure was solved by direct methods using SIR9211
and expanded using Fourier techniques.12 The non-hydrogen
atoms were refined anisotropically. The hydrogen atom coordi-
nates were refined, their isotropic B values being fixed. The
refinement of the structure was performed by a full-matrix
least-squares method based on 1607 observed reflections
[I>3.00s(I)]. Final refinement with anisotropic thermal fac-
2
4
2
of neutral AMET were obtained by slow evaporation of a
tors for all atoms; 223 parameters, R=0.032 and R =0.032;
chloroform solution.
The donating ability of AMET was estimated via redox
potentials determined by cyclic voltammetry. Two oxidation
w
w=1/s2(F ); S=0.03, max. (shift/e.s.d)=0.09, max. and min.
o
˚
heights in final difference Fourier map 0.27, −0.25 e A−3. The
atomic scattering factors used throughout the analysis were
obtained from International Tables for X-ray Crystallography
(1974). All calculations were performed using the teXsan
crystallographic software package from the Molecular
Structure Corporation.
waves were measured: E1/2 =0.51 V (reversible) and
OX1
E1/2 =0.84 V (irreversible). These values were practically the
OX2
same as those of BEDT-TTF [E1/2 =0.51 V (reversible) and
OX1
E1/2 =0.85 V (irreversible)]. This result suggests that intro-
OX2
duction of a carbamoylmethyl group does not affect the
Full crystallographic details, excluding structure factors,
have been deposited at the Cambridge Crystallographic Data
Centre (CCDC). See Information for Authors, J. Mater. Chem.,
1998, Issue 1. Any request to the CCDC for this material
should quote the full literature citation and the reference
number 1145/100.
donating ability of BEDT-TTF.
Crystal structure of AMET
The crystal structure of AMET is shown in Fig. 2. The long
molecular axes of the AMET molecules are aligned in parallel
along the a axis, being inclined by ca. 40°, and they are flipped
alternately along the c axis. The conformation (t , t , t , see
Measurements of electric conductivity
1
2
3
Electric conductivities were measured by a four or two probe
method. Gold wires (25 mm w) were attached to the sample
with gold paste. The sample was fixed in the chamber of a
homemade cryostat and cooled slowly. The temperature was
measured using a (0.03atom% Fe)-gold-Chromel thermo-
couple. The activation energies of the semiconducting samples
were estimated by the slope of the straight line, obtained via a
semilogarithmic plot of the conductivity vs. temperature. The
pressure dependence of the conductivity of iodine-doped
O
O
–H+
–
CH2CH2
R S
CH2CH2
S
C
R
C
OEt
OEt
R S–
CH2=CH-COOEt
Scheme 2 Generation of a thiolate anion via retro-Michael addition
SCOPh
SCOPh
S
S
S
S
S
S
S
S
S
S
S
S
S
S
i
ii
iii
Zn
S
S
S
O
(NEt4)2
85%
75%
91%
8
7
2
SAc
S
S
S
S
COOEt
S
S
v
iv
32%
COOEt
iii
S
S
O
93%
95%
S
S
SMe
SMe
SMe
5
6
3
S
S
S
SCH2CONH2
SMe
S
S
S
S
S
COOEt
S
S
S
S
vi
23%
vii
2 + 3
98%
S
SMe
4
AMET 1
Scheme 1 Reagents and conditions: i, BzCl, acetone, room temp., 3 h; ii, NaOMe, MeOH, 0 °C, 1 h, then BrCH CH Br, MeOH, room temp., 12
2
2
h; iii, Hg(OAc) , CHCl , room temp., 3 h; iv, AcCl, acetone, −5 °C, 2 h, then MeI, acetone, room temp., 1 h; v, NaOMe, MeOH, 0 °C, 1 h, then
2
3
BrCH CH COOEt, MeOH, room temp, 8 h; vi, P(OEt) , 110 °C, 3 h; vii, NaOMe, MeOH, 0 °C, 1 h, then BrCH CONH , MeOH, room temp.,
4 h
2
2
3
2
2
J. Mater. Chem., 1998, 8(8), 1703–1709
1705

Fig. 2 Crystal structure of AMET viewed along the b axis
˚
Fig. 5 The NMH,O hydrogen bond (2.80 A; dotted line) and
˚
NMH,S intermolecular interaction (3.52 A; broken line) of AMET
Fig. 3) of the carbamoylmethyl group of AMET is (−sc)-(+sc)-
(+sc). The donor moiety has a concave shape, and the dihedral
angles between the tetrathioethylene moieties of the central
part and of the two edge parts are 22.3 and 13.1°, respectively
(Fig. 3). No disorder was observed at the ethylenedithio group.
short in spite of the electronic repulsion between the p-electrons
of the donor moieties. This structure is in sharp contrast to
the herringbone structure of a neutral crystal of BEDT-TTF
molecules.3
The carbamoylmethyl group forms
hydrogen-bonded chain along the b axis (Fig. 4). The hydrogen
a one-dimensional
Primary amides, in general, form a double hydrogen-bonded
chain utilizing the two hydrogen atoms of the amide group
(Fig. 1).5,6 In the case of the hydrogen-bonded crystal of
AMET, however, only one of the hydrogen atoms of the amide
group participates in a polymeric hydrogen bond. The other
amide hydrogen is found in close proximity to the sulfur atom
of a methylthio group belonging to an AMET molecule in an
adjacent column. A weak NMH,S hydrogen bond is, thus,
˚
bond distance of NMH,O is 2.80 A; this value is approxi-
mately an average distance for hydrogen bonds of amide
groups.14 As a result of these hydrogen bonds, the AMET
moieties form columnar stacks along the b axis. The shortest
˚
S,S contact in this column is 3.76 A. The distance is fairly
˚
considered to be formed, with an N,S distance of 3.52 A
(Fig. 5). It should be noted that the hydrogen-bonded chain
runs perpendicular to the stacking of the donors. Since the
carbamoylmethyl group of AMET has conformational flexi-
bility due to the methylene unit between the amide group and
the donor moiety, the amide group can bend to give a (−sc)-
(+sc)-(+sc) conformation.
Although the donor AMET moieties are stacked in parallel,
the conductivity of the neutral crystal is not appreciably high,
especially compared with the neutral crystal of tetrakis(decyl-
thio)-TTF.15 In the case of tetrakis(decylthio)-TTF, a one-
dimensional stack of TTF moieties is achieved due to the
dispersion force of the alkyl groups. This effect is called a
‘fastener effect’ in order to stress the importance of the disper-
sion force of the long alkyl chains. The S,S distance within
a column of tetrakis(decylthio)-TTF molecules can be as short
Fig. 3 Conformation of the carbamoylmethyl group of AMET
˚
as 3.5 A, and an exceptionally low resistivity (2.7×105 V cm)
was recorded for a crystal of the neutral donor. Compared
with tetrakis(decylthio)-TTF, the resistivity of neutral AMET
is much higher (ca. 108 V cm). This may be explained by the
slightly longer S,S contacts in neutral AMET and the inclined
stacking of donor moieties.
Pressure dependence of IR spectra of undoped and iodine-doped
AMET
In order to obtain information on the hydrogen-bonding
pattern of the carbamoylmethyl group of AMET, especially
under an external pressure, IR spectra of KBr pellets of neutral
and iodine-doped AMET were recorded. The stretching modes
of the amide group, n and n
mation mode, d
purpose.
(Amide I), and the defor-
, were chosen as key bands for this
NH
C=O
N–C–O
The four NMH stretching bands, presumably resulting from
site-splitting, were observed at 3426, 3285, 3257 and 3184 cm−1
(Fig. 6). Based on the assignments of acetamide,16 the
absorptions at 1664 and 587 cm−1 were assigned to the n
C=O
stretching (Amide I) and the NMCNO deformation (d
respectively.
),
Fig. 4 Hydrogen-bonding scheme of AMET. The NMH,O distance
is 2.80 A
N–C–O
˚
1706
J. Mater. Chem., 1998, 8(8), 1703–1709

Fig. 7 UV–VIS spectra of AMET and iodine-doped samples; (a) neutral
sample, (b) 12% I -doped sample, (c) 32% I -doped sample
2
2
Fig. 6 Pressure dependence of IR spectrum of neutral AMET (n
region)
NH
UV–VIS spectra of undoped and iodine-doped AMET
The UV–VIS spectra of iodine-doped AMET were recorded
in order to examine the electronic structure of the doped
sample (Fig. 7). While the spectrum of neutral AMET showed
absorption maxima at 400 and 490 nm, a broad absorption
band at 930 nm was observed for the doped samples. This
characteristic broad band can be assigned to the charge
transfer band.
When an external pressure was applied to the neutral sample,
at 3426 and 3184 cm−1 showed small shifts to lower
n
NH
wavenumber, Dk, of 37 and 58 cm−1, respectively, at 4.1 GPa
(Fig. 6), whereas the peaks at 3258 and 3257 cm−1 were
relatively unaffected. On the other hand, n
at 1664 cm−1
C=O
Besides the CT band, two absorption bands at 285 and
380 nm were recognized. The former band may be assigned to
showed a shift to higher wavenumber of 5 cm−1. The d
N–C–O
at 578 cm−1 was also shifted to higher wavenumber by
60 cm−1.
the absorption of I− and the latter to I −, respectively, judging
3
from reference spectra for these species.19 Thus, the doped
These results suggest that the N,O hydrogen bond distance
becomes shorter when an external pressure was applied, and
that the NMH hydrogen is shifted towards the oxygen atom,
resulting in weakening of the NMH band. On the other hand,
iodine is converted to the counter ions I− and I −, which are
3
considered to be in equilibrium with each other.
Effect of doping on the conductivities of neutral AMET and
BEDT-TTF
the d
(in plane) deformation peak showed a shift to higher
N–C–O
wavenumber. This is because the force constant of the bending
mode of the NMCNO group becomes larger due to the shorter
hydrogen bond. Nakamoto et al. proposed a relation between
a shift to lower wavenumber of the n stretching (Dk/cm−1)
The conductivities of pellets of neutral and iodine-doped
AMET were measured as a function of the doping ratio
(mol%). Although the conductivity of neutral AMET is as low
NH
˚
and a shortening of the hydrogen-bonded distance (Dd/A),
as s =ca. 10−8 S cm−1, it is enhanced remarkably as the
rt
Dd=7.7×10−4×Dk.17 According to the above equation, the
doping ratio increases. The conductivity of the 5% doped
shrinkage in the NMH,O distance of AMET is estimated to
be 0.04 A at 4.1 GPa.
sample is 2×10−4 S cm−1, 104 times larger than that of neutral
AMET. The maximum value of the conductivity (1.2×10−1
S cm−1) was obtained at a doping ratio of 45% (Fig. 8).
Although the maximum value of the conductivity of iodine-
˚
The pressure dependence of n and d
NH N–C–O
ined for the 5% iodine-doped sample. The n
was also exam-
and d
NH
N–C–O
absorptions showed shifts of −24 and +66 cm−1 respectively
doped BEDT-TTF (s =4×10−3 S cm−1) is also obtained at
maximum value for iodine-doped AMET.
rt
under 4.0 GPa. The shifts in n
and d
are found to
a doping ratio of 45%, the value is 102 times smaller than the
NH
N–C–O
exhibit the same tendencies as those of the neutral sample.
This result strongly suggests that the hydrogen-bonded scheme
is not perturbed significantly, at least up to a doping ratio of
5%. For doping ratios higher than 45% a new absorption
Some poly(vinyl-TTF) polymers have been synthesised in
order to obtain conducting polymers carrying donor units.20
Although these polymers were oxidized with bromine or
TCNQ, the doped polymers exhibited poor conductivities
peak, assigned to n
of a more weakly hydrogen-bonded
C=O
amide group, appeared at 1685 cm−1. This may be explained
by the partial disruption of the uniform polymeric hydrogen-
bonded chain due to heavy doping. In other words, the
hydrogen bonded chain supports the columnar stacking of
donors as long as the doping ratio does not exceed ca. 45%.
It is well-documented that, in the IR spectrum of crystalline
N-methylacetamide, the intensity of the small peak near amide
II (1525 cm−1) increases pronouncedly at lower temperatures.18
This side-band peak is possibly assigned to the resonance line
between the amide II and the lattice phonons, the intensity of
which is enhanced due to the polaron along the hydrogen-
bonded chain, although the origin of the side-band peak is
still disputed. In crystals of AMET, such coupling has not
been detected so far. If a coupling between polaronic migration
and the transportational phenomena is observed, a new physi-
cal property might be explored, leading to a novel functionality
characteristic of the hydrogen-bonded conductive materials.
(s <10−8 S cm−1). The failure to obtain high conducting
rt
polymers may be ascribed to the difficulty of maintaining a
face-to-face stack of donors along the backbone of the polymer.
On the other hand, the donor units of AMET are arranged in
a face-to-face stack along the hydrogen-bonded chain formed
by the amide units, and the stacking structure is reasonably
unperturbed by doping or the application of high pressures,
as judged from the IR spectra of the doped samples.
Pressure dependence of the conductivities of iodine-doped
AMET
The conductivities of 5 and 37% iodine-doped AMET samples
under atmospheric pressure showed semiconducting behavior
in the temperature range of 300–150 K. The activation energies
of the 5 and 37% doped samples under atmospheric pressure
were evaluated to be E =0.2 and 0.09 eV, respectively.
a
J. Mater. Chem., 1998, 8(8), 1703–1709
1707

Fig. 10 External pressure dependence of activation energy of the ($)
5 and (#) 37% iodine-doped sample of AMET
salts.21 Since the conductivity was measured using pellets, the
possibility of a change in the contact resistance between
crystallites cannot be ruled out.
Fig. 8 Conductivity of the iodine-doped sample of ($) AMET and
(#) BEDT-TTF
Conclusion
In the case of the 5% doped sample, the external pressure
dependence of the conductivity was almost linear, and the
conductivity under 1.0 GPa was about 3.5 times larger than
that under atmospheric pressure (Fig. 9) on the left scale).
However, the increase in conductivity of the 37% doped
sample reached saturation at around 0.3 GPa (Fig. 9, on the
right scale). The external pressure dependence of the activation
energy of the iodine-doped sample was also examined (Fig. 10).
The activation energy of the 5% doped sample decreased to
According to the IR spectroscopic analysis of the hydrogen
bond formed by the carbamoylmethyl group of AMET, this
group appears to be effective in maintaining the hydrogen-
bonded chain even in the doped sample under high pressure.
Furthermore, the hydrogen bond distance is sensitive to the
applied pressure. Such an effect may be derived from the
polymeric hydrogen bond network, which is formed parallel
to the p-stacking of the donor moieties.
It is to be stressed that the donor stacking is realized by
simply introducing a carbamoylmethyl group into the donor
moiety. This hydrogen-bonded molecular assembly composed
of AMET is, therefore, appropriate for manifesting pressure-
sensitive conducting properties under hole-doped conditions.
E =0.1 eV under 0.7 GPa, showing a minimum value at this
pressure. On the other hand, the activation energy of the 37%
a
doped sample was relatively unaffected by the application of
external pressure.
The increase of the conductivity of iodine-doped AMET
may be mainly rationalized by a decrease in the inter-donor
distance within the stack, coupled with the shortening of the
hydrogen bond distance of the amide groups. The degree of
This work was partly supported by CREST of JST. The
authors are indebted to Professor Tokura of the University
of Tokyo, JRCAT, and Professor Moritomo of Nagoya
University for the measurement of the IR spectra of AMET
under high pressures. The authors also thank Dr Matsushita,
Tokyo Metropolitan University, for discussions concerning the
preparation of AMET.
increase of the s value is of about the same order as those of
the TTF–TCNQ, (BEDT-TTF) Cu(NCS) and (TMTSF) PF
rt
2
2
2
6
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1709