Crystal growth and characterization of solvated organic charge-transfer complexes built on TTF and 9-dicyanomethylenefluorene derivatives

Article abstract of DOI:10.1039/c5ce01059d

A series of 1 : 1 organic complexes have been synthesized by reaction between TTF (tetrathiafulvalene) and three 9-dicyanomethylenefluorene derivatives: 9-dicyanomethylene-2,7-dinitrofluorene (DDF), 9-dicyanomethylene-2,4,7-trinitrofluorene (DTF) and 9-dicyanomethylene-4,5,7-trinitrofluorene-2-carboxylic acid (DC2TF). The following formulas were determined by X-ray diffraction and elemental analysis for these complexes: (TTF-DDF)·CH₃CN (1), (TTF-DDF)·0.5PhCl (2), (TTF-DTF)·CH₃CN (3), (TTF-DTF)·0.5Me₂CO (4) and (TTF-DC2TF)·H₂O (5). A sixth solvated compound was also obtained, with a different stoichiometry, (TTF)₃(DC2TF)₂·2CH₃CN (6). The degree of charge transfer in 1-5 was estimated by IR and Raman spectroscopy. The lattice solvent, acetonitrile, chlorobenzene, or acetone is slowly released from the crystals of complexes 1-4, inducing a significant decrease in charge transfer over time. These crystals converge over months towards materials close to the neutral state. Hydrate 5 is air-stable, and displays a degree of charge transfer, δ = 0.48 e^-, close to the range of semiconducting or metal-organic complexes. Finally, compound 6 is an ionic crystal, and is thus expected to be an insulating material.

Full text of DOI:10.1039/c5ce01059d

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ARTICLE TYPE
Crystal growth and characterization of solvated organic charge-transfer
complexes built on TTF and 9-dicyanomethylenefluorene derivatives
a
b
Amparo Salmerón-Valverde, Sylvain Bernès
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A series of 1:1 organic complexes has been synthesized by
reaction between TTF (tetrathiafulvalene) and three 9-
dicyanomethylenefluorene derivatives: 9-dicyanomethylene-
Of particular importance is the mode of stacking of donors and
acceptors in the crystal: segregated stacks of the D and A
components lead to quasiꢀ1D electronic systems. The metallic
5
5
6
6
7
7
8
0
5
0
5
0
5
0
conduction is then affordable, providing that the charge transfer
2
,7-dinitrofluorene
(DDF),
9-dicyanomethylene-2,4,7-
ꢊꢋ ꢊꢆ
in the complex ꢉ
ꢌ
falls within the window 0.5 < ꢍ < 0.75,
1
1
2
2
0
5
0
5
trinitrofluorene (DTF) and 9-dicyanomethylene-4,5,7-
trinitrofluorene-2-carboxylic acid (DC2TF). The following
formulas were determined by X-ray diffraction and elemental
analysis for these complexes: (TTF-DDF).CH CN (1), (TTF-
DDF).0.5PhCl
DTF).0.5Me CO (4) and (TTF-DC2TF).H O (5). A sixth
solvated compound was also obtained, with a different
stoichiometry, (TTF) (DC2TF) .2CH CN (6). The degree of
charge transfer in 1-5 was estimated by IR and Raman
spectroscopy. Lattice solvent, acetonitrile, chlorobenzene, or
acetone, is slowly released from crystals of complexes 1-4,
inducing a significant decrease of the charge transfer over
time. These crystals converge over months towards materials
close to the neutral state. Hydrate 5 is air-stable, and displays
1a
at least in the case of 1:1 complexes. However, many other
criteria must be considered when screening candidates for organic
metals, for instance the difference between redox potentials of A
and D, which should be enough to promote oxidation and
reduction of both entities, but not too high, otherwise both
3
(2),
(TTF-DTF).CH CN
(3),
(TTF-
3
2
2
3
molecules remain essentially neutral. Ionic complexes,
characterized by integral charges ꢎꢍ = 1, 2, ⋯ ꢏ, are also
undesirable in this context, since they lead to insulating materials.
Although the accumulated literature gives a number of useful
3
2
3
4
guidelines for the design of such materials, no gold rules have
been established regarding electrical properties. Surprising results
are reported from time to time, like the complex between 1,6ꢀ
diaminopyrene and pꢀbromanil, for which some polymorphs are
ꢆꢈ
ꢆꢈ
fairly conductive (ꢇ ≈ 10 ꢄS. cm at 300 K), although their
ꢆ
5
a degree of charge transfer, ꢀ = ꢁ. ꢂꢃꢄꢅ , close to the range of
crystal structures feature mixed stacks of neutral components.
semi-conducting or metallic organic complexes. Finally,
compound 6 is an ionic crystal, and is thus expected to be an
insulating material.
Also surprising was the discovery of molecular metals composed
6
of singleꢀcomponent molecules, in which ꢍ = 0 by nature.
On the other hand, some aspects of the synthesis may result
difficult to control, for example the contamination of the product
by unreacted starting materials, the variability of the
Introduction
1a
stoichiometric ratio between D and A, or the unexpected
Functionalization of the aromatic polycyclic core of 9ꢀ
dicyanomethylenefluorene with electron withdrawing groups
makes this molecule less nucleophilic. This enhanced πꢀacid
character has been used for a long time, for the synthesis of
electron acceptor molecules (A hereafter), which are useful
building blocks for the preparation of organic radical ion salts,
known as chargeꢀtransfer complexes (CTC). The combination of
these acceptors with electron donor molecules behaving as πꢀ
bases (D hereafter), like tetrathiafulvalene (TTF), results in
complexes where partial reduction and oxidation of both moieties
7
crystallization of polymorphic phases. In any case, as accurate as
3
3
4
4
0
5
0
5
possible evaluation of ꢍ is essential in the preliminary study of an
organic CTC. The semiꢀempirical method devised by Chappell et
al. in 1981 remains very popular, because it is based on common
vibrational spectroscopy, IR and Raman, without the need for
8
crystallized samples. The primary drawback to this technique is
that neutral and ion radical states of the component used as a
probe must be available. This point is never a trouble for the
neutral molecule, since it is a starting material for the synthesis of
ꢋ
ꢆ
ꢋ
ꢆ
the CTC. In contrast, authentic ionic salts like Na ꢌ or ꢉ Cl
1
allows electron transport in a more or less anisotropic way. Such
in which ꢍ = 1 are sometimes impossible to stabilize. Hopefully,
ꢍ can be computed through:
CTC systems are synthesized with the hope to obtain interesting
materials having metallic properties, despite they do not include
metals. Most of them are insulating or semiconducting materials,
but some strategies are known to orient the material toward a
metallic behaviour, characterized by a significant conductivity
2ꢎΔꢐ⁄ꢐꢑꢏ
ꢍ =ꢄ
ꢒ
1
− ꢐ ꢐꢑ
ꢎ ⁄ ꢏ
ꢈ
ꢆꢈ
85 where ꢐꢑ and ꢐ are the vibrational frequencies for neutral (ꢍ =
ꢈ
(ꢇ > 10ꢄS. cm
Bechgaard salts.
at 300 K), or even superconductivity, as in
2
0
) and ionized (ꢍ = 1) component, respectively, and ∆ꢐ =
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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ꢐꢓꢔꢓ − ꢐ is the shift observed for the same vibrational mode in
above 95% are obtained with an excess of malononitrile.
ꢑ
the CTC with respect to the neutral component used as a probe.
We have been interested in assessing the accuracy of this
method in difficult cases, for instance when comparing
50 DDF: synthesized from 2,7ꢀdinitroꢀ9ꢀfluorenone (25 mg) and
malononitrile (18.6 mg) in refluxing methanol for 9 h. and
recrystallized from acetonitrile. Yield: 98%. Bright yellow
9
10
20
5
0
5
0
polymorphic phases, complexes close to the neutral state, or
complexes with stoichiometry other than 1:1, for which some
needles, mp 306ꢀ308 °C (Lit., 298ꢀ299 °C). Found: C, 60.14; H,
2.05; N, 17.49. Calc. for C H N O : C, 60.39; H, 1.90; N,
16
6
4
4
1
1
components may not participate to the charge transfer.
During these studies, we obtained
55 17.61%.
1
2
a
set of solvated
DTF: synthesized from 2,4,7ꢀtrinitroꢀ9ꢀfluorenone (25.3 mg) and
malononitrile (8.0 mg) in refluxing methanol for 2.5 h. and
recrystallized from methanol. Yield: 98%. Yellow solid, mp 266
complexes, using TTF and three 9ꢀdicyanomethylenefluorene
derivatives (see Scheme 1) as D and A components. We realized
that the role of lattice solvent is frequently overlooked in such
1
1
2
21
°C (Lit., 266ꢀ268 °C). Found: C, 52.56; H, 1.41; N, 19.18. Calc.
compounds, despite that solvent can be proꢀactive in the 60 for C H N O : C, 52.91; H, 1.39; N, 19.28%.
16
5
5
6
mechanism of charge transfer. It seems that this issue was
DC2TF:
synthesized
from
4,5,7ꢀtrinitroꢀ9ꢀfluorenoneꢀ2ꢀ
1
3
addressed at the beginning of the TTFꢀTCNQ saga, but was
then gradually discarded, because researchers focussed on the
preparation of unsolvated species, which are crystallized in a
carboxylic acid (25 mg) and malononitrile (13.7 mg) in refluxing
methanol for 0.75 h. and recrystallized from acetonitrile or
methanol. Yield: 96%. Bright yellow solid, mp 314ꢀ316 °C
19
more controllable way.
dicyanomethylenefluorene derivatives and their complexes
characterized by Xꢀray diffraction up to 2009. For 23
compounds retrieved from the literature, five solvated complexes
A
recent review compiles 9ꢀ 65 (Lit., 307ꢀ310 °C). Found: C, 50.16; H, 1.07; N, 17.07. Calc. for
C H N O : C, 50.14; H, 1.24; N, 17.20%. For samples
1
7
5
5
8
1
4
recrystallized from MeOH, elemental analysis fits better for the
hydrated acceptor, DC2TF.H O: C, 48.51; H, 1.81; N, 16.48.
Calc. for C H N O : C, 48.01; H, 1.66; N, 16.47%.
2
15
are listed with the following solvents: chlorobenzene, 1,4ꢀ
1
7
7
5
9
16
17
18
dioxane, dichloromethane, and acetonitrile.
70
Synthesis of complexes 1-6
TTF-DDF).CH CN (1). Hot acetonitrile solutions of DDF (5.4
(
3
mg, 17 ꢁmol in 2.5 mL) and TTF (3.5 mg, 17 ꢁmol in 1 mL)
were combined. After reduction of the volume to ca. 1 mL, the
solution turned yellowish green. The solution was then cooled to
room temp., affording darkꢀbrown needles (6.6 mg, 69%), mp
7
8
8
5
0
5
1
74ꢀ176 °C. Found: C, 51.20; H, 2.09; N, 12.11; S, 22.42. Calc.
for C H N O S : C, 51.14; H, 2.32; N, 12.43; S, 22.75%. Single
24
13
5
4 4
Scheme 1 Acceptor and donor molecules used in this work
crystals suitable for Xꢀray diffraction were obtained by
evaporation over 3 d. of an acetonitrile solution (4 mL) of the raw
material.
25
The present work addresses the following question: within the
series of complexes we obtained, is the presence of lattice solvent
a factor favouring the charge transfer?
(TTF-DDF).0.5PhCl (2). The previous synthesis was repeated,
with chlorobenzene solutions of DDF (10.3 mg, 32 ꢁmol in 3
mL) and TTF (20.1 mg, 98 ꢁmol in 1 mL), affording irregular
darkꢀbrown needles (14.9 mg, 80%). mp 172 °C. Found: C,
Experimental
TTF was purchased from Sigma and purified by recrystallization
from hexane, until constant mp (120ꢀ121 °C). Acetonitrile
(
52.02; H, 2.22; N, 9.78; S, 22.11. Calc. for C25
C, 51.87; H, 2.18; N, 9.68; S, 22.15%.
H12.5Cl0.5N O S :
4 4 4
30
35
40
Merck or Baker, HPLC grade), chlorobenzene (Baker) and
acetone (Baker) were used as received. 2,7ꢀDinitroꢀ9ꢀfluorenone
Aldrich, 97%), was purified twice by chromatography over 90 were combined and the resulting solution instantaneously turned
(TTF-DTF).CH
3
CN (3). Hot acetonitrile solutions of DTF (6.2
mg, 17 ꢁmol in 2 mL) and TTF (3.5 mg, 17 ꢁmol in 1.5 mL)
(
silicagel, and then recrystallized, first from benzene and then
from ethyl acetate, until constant mp (290 °C). 2,4,7ꢀTrinitroꢀ9ꢀ
fluorenone (Matheson) was used as received. 4,5,7ꢀTrinitroꢀ9ꢀ
fluorenoneꢀ2ꢀcarboxylic acid was prepared by nitration of 9ꢀ
green. The solution was then cooled to room temp., and
evaporated, affording darkꢀbrown plates (9 mg, 90%), mp 224
°C. A typical elemental analysis gives a solvent content
corresponding to a 1:1:0.5 complex, due to acetonitrile loss over
1
9
fluorenoneꢀ2ꢀcarboxylic acid with HNO /H SO , recrystallized 95 time; found: C, 46.80; H, 1.70; N, 13.03; S, 21.32. Calc. for
3
2
4
from a mixture of acetonitrile, chloroform and hexane, and finally
heated at 95ꢀ100 °C under reduced pressure (3 mm Hg), until
C
23
H
10.5
N
5.5
O
6
S
4
: C, 46.97; H, 1.80; N, 13.10; S, 21.81%.
(TTF-DTF).0.5Me CO (4). The same procedure as for 3 was
2
1
9
constant mp (274ꢀ280 °C; lit., 267ꢀ275 °C). Malononitrile
Eastman Kodak) was purified by distillation under reduced
repeated, with acetone in place of acetonitrile, affording darkꢀ
brown plates (9.6 mg, 99%), mp 226 °C. A typical elemental
(
pressure or recrystallization from chloroform. Elemental analyses 100 analysis corresponds to the unsolvated complex; found: C, 46.68;
were performed by Desert Analytics, Tucson (AZ, USA).
H, 1.59; N, 12.22; S, 22.40. Calc. for C22
H N O S : C, 46.56; H,
9 5 6 4
1
.60; N, 12.34; S, 22.59%. Single crystals suitable for Xꢀray
45
Synthesis of acceptors
diffraction were obtained by evaporation over 4 d. of an acetone
solution (4 mL) of the raw material.
Acceptors DDF, DTF and DC2TF were prepared starting from
the corresponding 9ꢀfluorenone derivatives, using a Knoevenagel 105 (TTF-DC2TF).H O (5). Acetonitrile solutions of DC2TF
2
condensation with malononitrile, in refluxing methanol. Yields
previously recrystallized from methanol (10 mg, 25 ꢁmol in 1.9
2
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mL) and TTF (10 mg, 49 ꢁmol in 2.4 mL) were combined at
room temp., and the resulting solution instantaneously turned
dark green. The solution was slowly evaporated, over 3ꢀ4 d.,
affording brown needles (yield < 50%), mp 236ꢀ237 °C (dec.).
Found: C, 44.73; H, 1.53; N, 10.87. Calc. for C H N O S : C,
times, ca. 30 min., allowed to isolate quite pure samples of
Li DC2TF , Na DC2TF and K DC2TF .
ꢋ
ꢆ
ꢋ
ꢆ
ꢋ
ꢆ
Characterization of complexes
5
2
3
11
5
9 4
4
3.88; H, 1.76; N, 11.12%.
30 Single crystals were diffracted as soon as crystallized, in order to
minimize the impact of solvent loss. Data were measured on a
Bruker P4 diffractometer (Table 1) using the MoꢀK radiation,
α
and structures refined on the basis of absorptionꢀcorrected data,
with the SHELX package released by G. Sheldrick in 2014.
(TTF) (DC2TF) .2CH CN (6). Acetonitrile was dried over P O ,
3
2
3
2
5
2
2
2
2
3
4
and a solution of TTF was prepared (3.5 mg, 17 ꢁmol in 2 mL).
DC2TF was recrystallized from methanol, dried at 100 °C for 8 h.
and dissolved in hot acetonitrile (7 mg, 17 ꢁmol in 3.7 mL). Both
1
0
solutions were mixed in a small Büchner flask equipped with a 35 Treatment of disordered parts in 2, 3, 4 and 5 is detailed in the
CaCl2 trap, and air was evacuated with argon. After 2 d. all
solvent had evaporated, affording brown crystals (yield < 50%),
mp 234ꢀ240 °C (dec.). No elemental analysis was carried out, and
the formula was assigned from the Xꢀray refinement.
ESI, and CIF files, along with structure factors, have been
deposited with the CCDC. FTꢀIR spectra were recorded on a
ꢆꢈ
Nicolet Magna 750 spectrometer in the 3500 − 350ꢄcm range,
ꢆꢈ
1
5
with a resolution of 4ꢄcm . Samples were diluted in KBr (1/65
40
w/w) to form 3 mmꢀdiameter pellets. We took special care to
follow the same protocol for all spectra, for example by
systematically using only one single crystal for each spectrum.
Some attempts to obtain spectra using a bulk crystal with no
previous dilution were unsuccessful, since no transmittance over
Synthesis of alkaline ionic crystals
Alkaline salts of acceptors DDF, DTF and DC2TF were obtained
by reaction between the acceptor and an excess of LiI, NaI or KI,
2
0
in refluxing acetonitrile. After cooling to room temperature, 45 the sample thickness, ca. 100 ꢀm, was detectable. Full spectra
unreacted acceptor was eliminated by filtration and the solution
may be consulted in the ESI. FTꢀRaman spectra were measured
on raw crystalline materials packed in glass capillary, with a
Nicolet 800 spectrophotometer equipped with a Nd:YAG laser
(λ ≈ 1000ꢄnmꢏ operated in the 50ꢀ100 mW range.
ꢋ
ꢆ
concentrated in vacuo. In the case of DDF, impure Li DDF and
ꢋ
ꢆ
Na DDF were obtained after long reactions times, 29 h. and 72
ꢋ
ꢋ
h. respectively. With DTF, Li and Na salts were obtained after
2
5
5
0
2, and 6 h. of reaction. Finally, with DC2TF, shorter reaction
Table 1 Crystallographic data
1
2
3
4
5
6
Chemical formula
Formula mass
Space group
a/Å
C
24
H
13
N
5
O
4
S
4
C
25
H
12.5Cl0.5
578.85
ꢕ1ꢖ
N
4
O
4
S
4
C
24
H
608.64
Pbca
12
N
6
O
6
S
4
C
23.5
H
596.62
Pbca
7.1682(15)
19.902(6)
37.805(9)
90
12
N
5
O
6.5
S
4
C
23
H
629.61
C2/c
11
N
5
O
9
S
4
56 28 12 16 12
C H N O S
563.63
P2 /c
7.141(2)
17.151(4)
20.891(5)
90
1509.62
ꢕ1ꢖ
1
7.3769(11)
9.3811(9)
18.9293(19)
81.218(7)
88.805(10)
75.958(10)
1255.8(3), 2
298(2)
7.1635(16)
19.881(4)
37.705(8)
90
15.9432(14)
12.1351(9)
13.5738(11)
90
104.572(7)
90
10.0608(10)
10.3528(11)
15.612(2)
107.535(8)
100.525(9)
90.877(8)
1520.2(3), 1
298(2)
b/Å
c/Å
α
β
/°
/°
99.18(3)
90
90
90
90
90
γ
/°
3
V/Å , Z
T/K
2525.6(12), 4
298(2)
5370(2), 8
298(2)
50, 100
5393(2), 8
298(2)
45, 99.9
8389 (0.105)
3545/399/53
0.0621
2541.7(4), 4
298(2)
2
θ
max/°, complete/%
48, 99.8
50, 99.7
50.5, 99.9
50, 96.9
No. ref meas. (Rint
Data/param./restr.
[I>2σ(I)]
[all data]
)
5355 (0.066)
3950/336/0
0.0664
6313 (0.024)
4400/370/86
0.0408
6049 (0.027)
4730/417/111
0.0570
6560 (0.026)
2918/227/14
0.0547
6125 (0.047)
5172/437/1
0.0569
R
1
wR
2
0.1773
0.1098
0.1647
0.1848
0.1511
0.1567
group from the fluorene plane, of 5.3(3)° in 1 and 5.0(2)° in 2. In
both compounds, DDF is stacked with TTF molecules, forming
Results and discussion
6
7
7
5
0
5
1
:1 complexes. Mixed stacks along [100] are similar to
(
TTF-DDF).CH CN (1) and (TTF-DDF).0.5PhCl (2)
3
arrangements observed in all fluoreneꢀderivatives CTC’s obtained
so far with TTF as donor, and the geometry of the stacks is
favourable for a significant charge transfer: in 1, molecules are
With acceptor 9ꢀdicyanomethyleneꢀ2,7ꢀdinitrofluorene (DDF),
two complexes were successfully crystallized, which contain
acetonitrile (1, Fig. 1) or chlorobenzene (2, Fig. 2) as lattice
5
5
separated by 3.509 and 3.788 Å and their mean planes are almost
25
solvent. The crystal structure of neutral free DDF is known,
26
parallel, making
a dihedral angle of 1.34(8)° (Fig. 1).
showing that the ꢗ molecular symmetry is retained in the solid
ꢒ
Corresponding figures in 2 are 3.908 and 3.942 Å for DDF…TTF
separations and 2.12(4)° for the tilt angle (Fig. 2). The relative
orientation of DDF and TTF seems also to facilitate the charge
transfer, with the central π bond of the TTF placed above the
centre of gravity of the aromatic core of DDF (Fig. 1, inset), or
close to this centre (Fig. 2, inset).
state. This molecule is essentially aromatic, having the
dicyanomethylene group coplanar with the fluorene core, and the
nitro groups twisted by ca. 18° from this plane. A conformation
close to that of neutral DDF is observed in complexes 1 and 2.
The only significant modification is a slight twist of the C(CN)2
6
0
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The unexpected feature is the presence of solvent, intercalated
between TTFꢀDDF stacks. Compound 1 is a 1:1:1 complex
sharp band falls in a frequency range uncontaminated by other
vibrations of the fluorene moiety or TTF. It is thus a probe of
TTF:DDF:CH CN, with the solvent molecules placed in general
positions, and poorly interacting with the DDFꢀTTF stacks in the 35 gives a symmetrical vibration at ꢐ = 2236ꢄcm
choice for estimating ꢍ using IR spectroscopy. Neutral DDF
3
ꢆꢈ
(Fig. 3),
ꢑ
5
0
5
0
crystal. The shortest intermolecular contact involving acetonitrile
is characterized by the separation N…STTF of 3.238 Å (van der
Waals radii sum: ꢘ ꢚ ꢘ = 3.35ꢄÅ). Acetonitrile solvates of
consistent with the ꢗꢒ molecular symmetry, which makes both
cyano groups equivalent. Frequency ꢐ was estimated with
spectra of radical ionic salts Li DDF and Na DDF . These
ꢈ
ꢋ
ꢆ
ꢋ
ꢆ
ꢙ
ꢛ
complexes built with TTF derivatives are rather common and
salts, obtained by reacting DDF with LiI or NaI, are difficult to
2
7
have been Xꢀray characterized, for mixedꢀstacks as well as for 40 obtain in pure form, and all attempts we have done resulted in
2
8
ꢑ
1
1
2
segregatedꢀstacks systems. However, insertion of acetonitrile is
generally unintentional, and arises merely from the fact that
acetonitrile is a solvent of choice for the crystallization or
electrocrystallization of these compounds.
sample with residual DDF precursor, detected by a small band
ꢆꢈ
ꢆ
around 2236ꢄcm . However, the main vibration for DDF
appears at 2196 and 2194ꢄcm , for Li and Naꢋ salts,
respectively. We retained ꢐ = 2195ꢄcm for computations.
ꢆꢈ
ꢋ
ꢆꢈ
ꢈ
Less common are complexes crystallized as chlorobenzene
45
Complex 1, as analyzed just after crystallization, shows the
15
ꢆꢈ
solvates. With TTF, only two structures have been reported. For
vibration for the cyano group of DDF at ꢐ = 2221ꢄcm (Fig. 3,
compound 2, the hemisolvate is formed, TTFꢀDDF.0.5C H Cl,
left panel). The frequency shift with respect to neutral DDF is
6
5
ꢆꢈ
with the solvent molecule disordered by symmetry across
inversion centres. As in 1, solvent fills the voids between mixed
DDFꢀTTF columns, without forming strong interactions with 50 vibration, at 2254ꢄcm , accounts for the presence of acetonitrile
thus Δꢐ = 15ꢄcm , which allows to estimate the charge transfer
ꢆ
using the Chappell’s formula: ꢍ = 0.37ꢄꢝ . A less intense cyano
ꢜ
ꢆꢈ
them. However, crystal symmetry is modified, from P2 /c (1) to
as lattice solvent, since pure acetonitrile presents an intense
1
29
ꢖ
ꢕ1 (2), possibly because steric volume and shape are different for
vibration at this position. However, as time runs away, the
chlorobenzene and acetonitrile.
spectrum is modified. The measured Δꢐ shift slowly decreases,
ꢆꢈ
and stabilizes around Δꢐ = 10ꢄcm after three months. At this
ꢞ
ꢆ
5
6
6
7
5
0
5
0
point, the charge transfer dropped to ꢍ = 0.25ꢄꢝ . The
ꢜ
simultaneous disappearance of the vibration assigned to
acetonitrile, which has vanished three months after exposure of
crystals to air, shows that solvent is slowly released from the
solid. This degradation does not favour the charge transfer.
However, with these data in our hands, it cannot be determined
whether the degradation of the chargeꢀtransfer results from an
actual topochemical reaction with partial reꢀoxidation of DDF
and structural rearrangement in the solid state for DDF and TTF,
or results from the crystal lattice collapse induced by solvent loss.
Anyhow, the crystals are not stable in air over months: although
their shape and aspect seem unaltered, the crystals do no longer
diffract Xꢀray after this time.
The same behaviour is observed in complex 2 (Fig. 3, right
panel). Freshly crystallized complex displays the cyano stretching
Fig. 1 Crystal structure of 1, showing DA stacks along [100]. The inset
25
displays one stack down stacking axis.
ꢆꢈ
ꢆꢈ
band of DDF at 2220ꢄcm , giving a shift Δꢐ = 16ꢄcm
,
ꢆ
corresponding to ꢍ = 0.39ꢄꢝ . After some months, the charge
ꢟ
ꢞ
ꢆ
transfer converges to ꢍ = 0.25ꢄꢝ . We assume that almost all
ꢟ
solvent has then been extruded from the crystal, yielding, as for 1,
the stable unsolvated compound TTFꢀDDF.
Fig. 2 Crystal structure of 2, showing DA stacks along [100]. A single
orientation for the disordered chlorobenzene molecule is retained. The
inset displays one stack down stacking axis.
75
30
With 9ꢀdicyanomethylene derivatives of fluorene, the b2u
stretching mode of the cyano groups is active in IR, and this
Fig. 3 Evolution of the cyano vibrations in the IR spectra for complexes 1
(left) and 2 (right). The upper spectra (black lines) are for the neutral DDF
acceptor molecule.
4
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DOI: 10.1039/C5CE01059D
(
TTF-DTF).CH CN (3) and (TTF-DTF).0.5Me CO (4)
we limited the model to two sites, with the sum of occupancies
constrained to unity, 0.248(8) and 0.752(8). In complex 4,
acetone is also almost certainly disordered, but the crystal
structure was refined with a single position and a strongly
restrained geometry for acetone. Acetone content was estimated
by refinement of the occupancy. Compounds 3 and 4 were
expected to release solvent, and this behaviour indeed hindered
elemental analysis. For 4, analysis match a formula with no
solvent at all, while for 3, acetonitrile content is less than that
expected from Xꢀray analysis (see experimental section).
3
2
In order to check if crystal instability over long times was a
characteristic feature for solvates of CTC complexes in this
series, we repeated the work using an acceptor molecule with a
slightly higher electron affinity, namely 9ꢀdicyanomethyleneꢀ
4
4
5
5
6
6
0
5
0
5
0
5
5
2
,4,7ꢀtrinitrofluorene (DTF). Two solvates were successfully
crystallized with acetonitrile and acetone, for which formulas
were determined by Xꢀray analysis: the 1:1:1 complex (TTFꢀ
DTF).CH CN (3), and the 1:1:0.5 complex (TTFꢀDTF).0.5C H O
3
3
6
1
1
2
2
0
5
0
5
(4).
These features are fully consistent with IR data. Acetonitrile is
present in the crystal 3 just after crystallization, as reflected by
Unlike previous compounds 1 and 2, both complexes are
isomorphous, and crystallize in space group Pbca. Mixed stacks
TTFꢀDTF run in the [100] direction, with D…A separations and
relative orientation compatible with chargeꢀtransfer (See Figs. 4
and 5). For instance, the most important criterion for overlap
between π orbitals of DTF and TTF is well fitted: mean planes of
both components are nearly parallel, with tilt angles of 2.17(5)
and 1.60(8)° in 3 and 4, respectively. The nitro group at C4 in
DTF is unequally disordered over two sites, bonded to C4 and
C5, with occupancies 0.317(6)/0.683(6) in the case of 3, and
ꢆꢈ
the vibration at 2250ꢄcm . This vibration survives after one
month, but disappears after one year elapsed. The vibration of the
ꢆꢈ
cyano groups in DTF is initially found at 2214ꢄcm , shifted by
ꢆꢈ
33
1
7ꢄcm
with respect to neutral DTF. On the other hand,
parameter ꢐꢈ for fully oxidized DTF was extracted from IR
ꢋ
ꢆ
ꢋ
ꢆ
ꢆꢈ
spectra of Li DTF and Na DTF , affording ꢐ = 2184ꢄcm
.
ꢈ
The calculated charge transfer in freshly crystallized complex 3 is
ꢆ
then ꢍ = 0.37ꢄꢝ , very close to that observed in 1 and 2. After
ꢠ
one year, vibration of free acetonitrile is no longer detected, and
0
.365(10)/0.635(10) in 4. A similar disorder has been described
ꢞ
ꢆ
3
0
chargeꢀtransfer has been halved to ꢍ = 0.19ꢄꢝ (Fig. 4). For 4,
ꢠ
for neutral DTF in a low resolution study, and in other related
3
1
32
lattice acetone being a low boiling solvent, bp = 56 °C, we
assume that solvent release starts as soon as crystals are exposed
to air, precluding measurements on a 1:1:1 complex. A fresh
crystallized sample, with a stoichiometry close to 1:1:0.5 as
CTC’s with derivatives of TTF or porphyrin. It thus seems
that DTF attempts to approximate the ꢗꢒ local symmetry while
crystallizing.
observed in the Xꢀray analysis, shows the characteristic vibration
ꢆꢈ 34
of the carbonyl group for acetone, at 1712ꢄcm
.
After five
days in air, the intensity of this band decreases, and finally
vanishes after ten months. Over the same period, charge transfer
ꢞ
ꢆ
experiments a drop from ꢍ = 0.37ꢄ to ꢍ = 0.19ꢄꢝ (Fig. 5), as
ꢂ
ꢂ
observed in complex 3.
Fig. 4 Crystal structure of acetonitrile solvate 3 and evolution of the IR
cyano vibrations for this complex. For the sake of clarity, a single site for
disordered NO groups in DTF and CH CN molecules are represented.
2 3
3
0
One column of solvent is represented using spacefill model. The upper
spectrum (black line) is for the neutral DTF acceptor molecule.
Once TTFꢀDTF stacks are in place, ca. 15% of the crystal
volume is still empty, forming channels parallel to the short a
axis. These voids are filled with guest solvent, which, as in 1 and 70 Fig. 5 Crystal structure of acetone solvate 4 and evolution of the IR cyano
and carbonyl vibrations for this complex. The upper spectrum (black line)
3
5
2, poorly interact with the TTFꢀDTF host. As a consequence,
acetonitrile in 3 is disordered over a number of orientations, and
is for the neutral DTF acceptor molecule.
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(
TTF-DC2TF).H O (5) and (TTF) (DC2TF) .2CH CN (6)
For neutral DC2TF, the b ꢀcyano vibration is found at ꢐ =
2u
ꢑ
2
3
2
3
ꢆꢈ
ꢋ
ꢆ
2
234ꢄcm , while the same vibration for ionic salts Li DC2TF
We finally attempted the crystallization of solvate complexes
based on a tetraꢀsubstituted dicyanomethylenefluorene acceptor,
namely 9ꢀdicyanomethyleneꢀ4,5,7ꢀtrinitrofluoreneꢀ2ꢀcarboxylic
acid (DC2TF). First attempts of reaction between TTF and
DC2TF afforded 5, an unexpected 1:1:1 hydrate (Fig. 6).
Although the source of water is unclear, the formation of this
hydrate has been confirmed by two singleꢀcrystal refinements
from independent batch, and the presence of a broad band centred
at 3500ꢄcm
complexes (see ESI, Fig. S15).
ꢋ
ꢆ
ꢆꢈ
and Na DC2TF is shifted to ꢐ = 2188ꢄcm . The vibration
ꢈ
4
4
5
5
6
6
7
0
5
0
5
0
5
0
observed in the complex, at 2212ꢄcmꢆꢈ, corresponds to the
ꢆ
chargeꢀtransfer ꢍ = 0.48ꢄꢝ , which makes hydrate
5 a
ꢡ
5
0
5
0
5
0
prospective candidate for a semiꢀconducting or metallicꢀlike
material.
In order to assess if lattice water had an actual influence on the
chargeꢀtransfer in 5, we attempted the crystallization of the
anhydrous complex TTFꢀDC2TF, avoiding as much as possible
the presence of moisture during synthesis and crystallization
steps. However, preparation of TTFꢀDC2TF resulted very
challenging, and in spite of considerable efforts, we only obtained
an acetonitrile solvate, with different stoichiometry. Complex 6,
ꢆꢈ
1
1
2
2
3
in IR spectra, which is not observed in other
Complex (TTFꢀDC2TF).H O crystallizes in space group C2/c,
2
with the DC2TF molecule lying on the crystallographic twofold
axis. As a consequence, carboxylic and nitro groups substituting
the fluorene core at C2 and C7 are disordered by symmetry. The
TTF moiety, whose site symmetry is ꢀ1, is stacked along [001]
with the acceptor at a rather short and constant distance, 3.752 Å,
and the tilt angle between TTF and DC2TF mean planes is
(
TTF) (DC2TF) .2CH CN, was also characterized by Xꢀray
3 2 3
diffraction (Fig. 7), revealing an arrangement essentially different
from those observed in 1ꢀ5. In the asymmetric unit, one TTF and
one DC2TF molecules are in general positions, and their mean
planes are separated by 3.690 Å. The asymmetric unit is
completed by a half TTF, close to an inversion centre. This TTF
is eclipsed with respect to the first one, and located further from
the acceptor, at 3.920 Å (Fig. 7, inset). The central bond lengths
of TTF components also reflect different oxidation states. The
molecule placed on an inversion centre presents a short bond
length of 1.330(9) Å, which fits for a neutral TTF molecule
4
.06(4)° (Fig. 6). The stack arrangement allows alignment of the
centres of exocyclic double bonds of TTF with atoms C9 in
acceptors, along the stacking direction. Finally, water molecules,
with occupancy factor of ½, are disordered over inversion
centres, and are involved in hydrogen bonding with carboxylic
groups and nitro groups of DC2TF (Fig. 6, lower panel; ESI,
Table S30). Lattice solvent is thus, as in previous complexes,
filling voids between stacks, but hydrogen bonds and high boiling
point of water in comparison with acetone or acetonitrile has two
important consequences: the crystals are stable in air, and the
crystal structure is not based on an actual 1D framework, a
feature which should favour chargeꢀtransfer. This point is
confirmed by IR spectroscopy.
35
(
1.345 Å), while the TTF interacting with the acceptor has an
exocyclic bond length of 1.369(6) Å. The lengthening accounts
for oxidation of this TTF. Compound 6 is thus best described as
ꢋ
ꢆ
an ionic crystal, ꢎTTF ꢏ ꢎDC2TF ꢏ ꢎTTFꢏ. 2CH CN, and is not
ꢒ
ꢒ
ꢢ
an actual CTC. This description is confirmed by IR spectroscopy.
The cyano vibration of the acceptor in 6 is observed at
ꢆꢈ
ꢋ
ꢆ
2
185ꢄcm , close to the vibration of pure ionic salts Li DC2TF
ꢋ
ꢆ
ꢆꢈ
and Na DC2TF , at 2188ꢄcm . As a consequence, compound 6
is expected to be an insulating material, with ꢍ ≈ 1ꢄꢝ .
ꢆ
ꢣ
Fig. 6 Crystal structure of hydrate 5, omitting the disorder in DC2TF. The
inset is a part of the packing structure, as viewed down the stacking axis
[001]. One column of water molecules, each one with site occupancy of
Fig. 7 Crystal structure of ionic compound 6. The inset gives charge
assignation in the symmetric unit.
35
75
½, is represented with a spacefill style.
6
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Raman spectroscopy
complexes, which are close to the neutral state. Interestingly,
preliminary results regarding the possibility of reversible
solvationꢀdesolvation of these DA crystals appear to be positive,
at least for some solvents. In contrast, we obtained by chance a
4
4
5
0
5
0
Previous estimations for ꢍ in complexes 1ꢀ6 were based on IR
spectroscopy, but this characterization may be completed by
Raman data. The most used charge sensitive vibration is the Ag
symmetric stretching ꢐꢢ mode for the central double bond in
TTF, which we detected at 1513ꢄcm
hydrate, 5, which is an airꢀstable compound presenting a charge
5
ꢆ
ꢆꢈ 36
transfer, ꢍ
ꢡ
= 0.48ꢄꢝ , on the boundary of CTC’s exhibiting
.
The standard material
1a
metallic conductivity. We assume that hydrogen bonding plays
a role both in the stability and higher ꢍ parameter for this
compound. This is in line with other studies, pointing out that
conducting properties of some organic complexes are closely
related with interaction between D and A layers, involving
used as reference for the ionic state of the donor is TTF ꢎFeCl ꢏ,
ꢒ
ꢤ
for which the same stretching mode gives a Raman shift of
ꢆꢈ 37
1
416ꢄcm
.
As an example, Fig. 8 shows Raman spectra of
1
0
complex 1, (TTFꢀDDF).CH CN and its neutral components.
3
38
hydrogen bonds.
Acknowledgements
We acknowledge the contribution of José Francisco Padilla Rojas
(Program Estancia con un investigador, BUAP) to the first
attempts for solvationꢀdesolvation cycles.
55
Notes and references
a
Centro de Química del Instituto de Ciencias, Benemérita Universidad
Autónoma de Puebla, Ciudad Universitaria, San Manuel, 72570 Puebla,
Mexico. E-mail: asv8085@yahoo.com.mx
b
Instituto de Física, Benemérita Universidad Autónoma de Puebla, Av.
6
6
7
7
8
8
9
9
0
5
0
5
0
5
0
5
San Claudio y 18 Sur, 72570 Puebla, Mexico. E-mail:
sylvain_bernes@hotmail.com
†
Electronic Supplementary Information (ESI) available: IR spectra of
acceptor molecules DDF, DTF, and DC2TF and their alkaline salts; IR
spectra of complexes 1ꢀ6; details of crystal structure refinements of
complexes 1ꢀ6. CCDC deposition numbers for structures 1ꢀ6: 1055659ꢀ
1
055664. See DOI: 10.1039/b000000x/
3
Fig. 8 Raman spectra for TTF, DDF and complex (TTFꢀDDF).CH CN
1
(a) K. P. Goetz, D. Vermeulen, M. E. Payne, C. Kloc, L. E. McNeil
and O. D. Jurchescu, J. Mater. Chem. C, 2014, 2, 3065ꢀ3076. (b) F.
Wudl, Acc. Chem. Res., 1984, 17, 227ꢀ232.
The spectrum of 1 is poorly resolved, as a consequence of an
experimental issue common to all complexes reported here: raw
crystals packed out in a glass capillary are damaged under laser
irradiation, and this requires to measure Raman shifts with as low
as possible laser power. The ꢐꢢ vibration of TTF is nevertheless
1
2
2
3
5
0
5
0
2
(a) K. Bechgaard, K. Carneiro, F. B. Rasmussen, M. Olsen, G.
Rindorf, C. S. Jacobsen, H. J. Pedersen and J. C. Scott, J. Am. Chem.
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20.
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916ꢀ3925; (b) R. C. Wheland, J. Am. Chem. Soc., 1976, 98, 3926ꢀ
3
4
5
6
ꢆꢈ
located in the range 1490ꢀ1470 cm , corresponding to a charge
3
ꢆ
transfer in the range ꢍ = 0.24 ⋯ 0.46ꢄꢝ . Although this broad
3930.
ꢜ
(a) M. Bendikov, F. Wudl, and D. F. Perepichka, Chem. Rev., 2004,
band is not very useful for an accurate estimation of the charge
transfer, Raman data, based on the donor, are in agreement with
IR data, based on the acceptor. Since Raman spectrum of 1 was
measured on crystals few days after crystallization, some crystals
should still be unaltered solvates with a formula close to (TTFꢀ
1
04, 4891ꢀ4945; (b) J.ꢀi. Yamada, H. Akutsu, H. Nishikawa and K.
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DDF).CH CN, while partial desolvation could affect other
3
crystals. From IR spectroscopy, the degree of charge transfer
ꢆ
would thus be between 0.29 and 0.37 ꢝ (Fig. 3, IR absorptions
7
8
at 2224 and 2221 cmꢆꢈ, respectively), a range within that
determined by Raman spectroscopy.
9
1
1
0
1
A. SalmerónꢀValverde and S. Bernès, Crystals, 2015, 5, 283ꢀ293.
A. SalmerónꢀValverde and S. Bernès, C. R. Chimie, 2005, 8, 1017ꢀ
Conclusions
1
023 (in French).
For non airꢀstable solvated complexes 1-4, a similar behaviour
has been established, regardless of the nature of lattice solvent. A
gradual loss of solvent, occurring over few months, is correlated
to a decrease of the degree of chargeꢀtransfer. Such a correlation
is an evidence of the involvement of the solvent in the charge
transfer. These solvates converge slowly towards unsolvated 1:1
12 A. SalmerónꢀValverde, Ph.D. Thesis, Universidad Autónoma de
Puebla, 2008.
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3
5
1
00
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Journal Name, [year], [vol], 00–00 | 7

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6
1
1
2
2
3
3
4
4
5
5
6
6
1
1
1
7
8
9
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1
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communication to the CCDC (715820; Refcode: CMDNFL01).
Throughout this report, mean planes and centroids for D and A
molecules are calculated using all 10 nonꢀH atoms of D (TTF) and 13
C atoms of the fluorene core of A.
(a) G. Saito, H. Ikegami, Y. Yoshida, O. O. Drozdova, K. Nishimura,
S. Horiuchi, H. Yamochi, A. Otsuka, T. Hiramatsu, M. Maesato, T.
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2
2
6
7
2
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2
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J., 2014, 20, 1909ꢀ1917.
2
3
3
9
0
1
Spectral Database for Organic Compounds, SDBS N° 1218. IR
ꢀ1
vibration of the cyano group of acetonitrile is reported at 2254 cm .
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3
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(
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Page 9 of 9
CrystEngComm
View Article Online
DOI: 10.1039/C5CE01059D
A series of solvated donor-acceptor organic complexes was shown to slowly release lattice
solvent, while the degree of charge transfer decreases steadily. This behavior is not observed in
the case of a hydrate.