New 1,3-dithiol-2-ylidene donor-π-acceptor chromophores with intramolecular charge-transfer properties, and related donor-π-donor molecules: Synthesis, electrochemistry, X-ray crystal structures, non-linear optical properties and theoretical calculations

Article abstract of DOI:10.1039/a708078f

New donor-π-acceptor chromophores 5a-c, 7a-c, 8a-c, 13a-c and 14b-c have been synthesised: substituted 1,3-dithiole derivatives span a range of donor abilities [viz. 4,5-dimethyl > 4,5-bis(methylsulfanyl) > 4,5-bis(methoxycarbonyl)], a conjugated ethylenic or oligoethylenic linker varies the central π-electron unit (one, two and four conjugated double bonds) and dicyanomethylene and N-cyanoimine groups are the acceptor units. Extended tetrathiafulvalene derivatives 15a-c have also been synthesised. The electronic absorption spectra of compounds 5,7,8 and 14 reveal a broad low-energy intramolecular charge-transfer band [λ_max(MeCN) 354-533 nm] which shifts bathochromically with increasing donor strength of the dithiole ring, and with increasing length of the conjugative pathway. The solution redox properties of 5,7,8 and 14, studied by cyclic voltammetry, reveal a reversible one-electron oxidation wave, attributed to the formation of the radical cation of the 1,3-dithiol-2-ylidene moiety, and an irreversible one-electron reduction to form the radical anion located on the dicyanomethylene or N-cyanoimine groups. For the bis(dithiole) donors 15a-c a single two-electron redox wave is observed. The non-linear optical (NLO) properties of 7a-c and 14b have been determined using the EFISH technique: moderate NLO properties are observed for compounds 7a-c [μ.β(0) 85-112 × 10^-48esu] whereas for the more extensively conjugated compound 14b, the value is increased to μ.β(0) 1170 × 10^-48 esu. The molecular structure and electronic properties of the unsubstituted (R = H) compounds 7, 8, 14 and 15 have been calculated within the density functional theory approach. The minimum energy conformation corresponds to the trans orientation of the side chain for 7, and the cis orientation for 8, in agreement with X-ray crystal structures and solution NMR data. The redox properties and electronic spectra are discussed on the basis of molecular orbital energies and topologies. The X-ray crystal structures of compounds 7b, 8b and 15b are reported. In 7b the 1,3-dithiol-2-ylidene and dicyanopropene systems are planar and form an angle of 3° between each other; the 'butadiene' unit has a trans configuration. Molecule 8b is planar (with the exception of the methyl groups) and the imine nitrogen atom is in a syn orientation towards the dithiole group, forming a short intramolecular S ... N contact [2.719(6) A]. For both 7b and 8b π-delocalisation is observed within the central conjugated spacer unit. Molecule 15b has a predominantly planar structure, with the central tetraene unit in an all-trans configuration.

Full text of DOI:10.1039/a708078f

J O U R N A L O F
New 1,3-dithiol-2-ylidene donor–p–acceptor chromophores with
intramolecular charge-transfer properties, and related donor–p–donor
molecules: synthesis, electrochemistry, X-ray crystal structures,
non-linear optical properties and theoretical calculations
C H E M I S T R Y
Adrian J. Moore,a Martin R. Bryce,*,a Andrei S. Batsanov,a Andrew Green,a
Judith A. K. Howard,a M. Anthony McKervey,b Peter McGuigan,b Isabelle Ledoux,c
Enrique Ort´ı,d Rafael Viruela,d Pedro M. Viruelad and Brian Tarbite,†
aDepartment of Chemistry, University of Durham, Durham, UK DH1 3L E
bSchool of Chemistry, T he Queen’s University, Belfast, N. Ireland, UK BT 9 5AG
cCNET -France T e´le´com, 196, Avenue Henri Ravera BP 107, 92225 Bagneux, France
dDepartamento de Quımica Fısica, Universidad de Valencia, E-46100 Burjassot, Valencia, Spain
´
´
eGreat L akes Chemical (Europe) L td, AycliVe Industrial Estate, Newton AycliVe, Co. Durham,
UK DL 5 5HA
New donor–p–acceptor chromophores 5a–c, 7a–c, 8a–c, 13a–c and 14b–c have been synthesised: substituted 1,3-dithiole
derivatives span a range of donor abilities [viz. 4,5-dimethyl>4,5-bis(methylsulfanyl)>4,5-bis(methoxycarbonyl)], a conjugated
ethylenic or oligoethylenic linker varies the central p-electron unit (one, two and four conjugated double bonds) and
dicyanomethylene and N-cyanoimine groups are the acceptor units. Extended tetrathiafulvalene derivatives 15a–c have also been
synthesised. The electronic absorption spectra of compounds 5, 7, 8 and 14 reveal a broad low-energy intramolecular charge-
transfer band [l (MeCN) 354–533 nm] which shifts bathochromically with increasing donor strength of the dithiole ring, and
with increasing length of the conjugative pathway. The solution redox properties of 5, 7, 8 and 14, studied by cyclic voltammetry,
max
reveal a reversible one-electron oxidation wave, attributed to the formation of the radical cation of the 1,3-dithiol-2-ylidene moiety,
and an irreversible one-electron reduction to form the radical anion located on the dicyanomethylene or N-cyanoimine groups.
For the bis(dithiole) donors 15a–c a single two-electron redox wave is observed. The non-linear optical (NLO) properties of 7a–c
and 14b have been determined using the EFISH technique: moderate NLO properties are observed for compounds 7a–c [m.b(0)
85–112×10−48 esu] whereas for the more extensively conjugated compound 14b, the value is increased to m.b(0) 1170×10−48 esu.
The molecular structure and electronic properties of the unsubstituted (R=H) compounds 7, 8, 14 and 15 have been calculated
within the density functional theory approach. The minimum energy conformation corresponds to the trans orientation of the side
chain for 7, and the cis orientation for 8, in agreement with X-ray crystal structures and solution NMR data. The redox properties
and electronic spectra are discussed on the basis of molecular orbital energies and topologies. The X-ray crystal structures of
compounds 7b, 8b and 15b are reported. In 7b the 1,3-dithiol-2-ylidene and dicyanopropene systems are planar and form an angle
of 3° between each other; the ‘butadiene’ unit has a trans configuration. Molecule 8b is planar (with the exception of the methyl
groups) and the imine nitrogen atom is in a syn orientation towards the dithiole group, forming a short intramolecular S · · · N
˚
contact [2.719(6) A]. For both 7b and 8b p-delocalisation is observed within the central conjugated spacer unit. Molecule 15b has
a predominantly planar structure, with the central tetraene unit in an all-trans configuration.
Asymmetric conjugated molecules containing electron donor
and electron acceptor substituents (D–p–A) have been thor-
oughly explored in the design of materials having highly
efficient second-order non-linear optical (NLO) chromo-
phores.1 A prototypical example is 4-nitroaniline. Molecular
non-linearity (bm) of such compounds is dependent on the
effective length of p-conjugation and the strength of the donor
and acceptor substituents. Whilst a wide variety of acceptor
groups have been examined for their role in influencing the
molecular NLO properties, relatively few electron donor moiet-
ies have continued to receive the most attention (e.g. dialkyl-
amino and alkoxy groups). In the search for other electron
donating groups we focus in this paper on derivatives of the
1,3-dithiole heterocycle. This ring system has, to date, received
limited attention as a donor component in D–p–A materials.
This is surprising since it has been the centre of studies within
the field of organic molecular metals, as it is a constituent part
of the well known p-donor tetrathiafulvalene (TTF) 1, and the
synthesis and electrochemistry of 1,3-dithiole derivatives is well
developed and understood.2 The use of the 1,3-dithiol-2-ylidene
group as an electron donor has been reported previously by
Gompper et al.,3 and by Katz et al.4 in their investigations of
benzenoid type non-linear optical chromophores. Lehn and
co-workers5 have also employed the 1,3-benzodithiol-2-ylidene
group in their studies on push–pull carotenoids. Jen et al.6
have discussed the role of a number of 1,3-dithiol-2-ylidene
derivatives in thiophene derived chromophores. Baudy-Floc’h
and co-workers7 have studied the NLO properties of some 2-
imino-4-amino-1,3-dithioles and Wegner and co-workers8 have
S
S
S
S
S
S
CN
CN
*E-mail m.r.bryce@durham.ac.uk
†Present address: Seal Sands Chemicals Ltd., Seal Sands Road, Seal
Sands, Middlesbrough, UK TS2 1UB.
2
1
J. Mater. Chem., 1998, 8(5), 1173–1184
1173

incorporated a substituted benzo-1,3-dithiol-2-ylidenedicyano-
methylene unit into the backbone of a main-chain polymer.
During the preparation of this manuscript, other groups have
reported new 1,3-dithiole–p–A systems.9
the reaction of aldehydes 6a–c with N,N-bis(trimethylsilyl)-
carbodiimide and titanium tetrachloride (according to the
conditions developed by Aumuller and Hunig19) in dichloro-
¨
¨
methane at room temperature resulted in the formation of the
corresponding N-cyanoimine derivatives 8a–c (78–85%).
Malealdehyde 9, which can be prepared in a high state of
purity by the oxidation of furan with distilled dimethyldi-
oxirane (DMD) in acetone,20 has been efficiently intercepted
in Wittig reactions.21 In particular, in situ reaction with
one equivalent of triphenylphosphoranylideneacetaldehyde
afforded, after work-up and isomerisation (induced either
thermally or by treatment with iodine), (E,E)-mucondialdehyde
1022 (62%) (Scheme 3). Compounds 14b and 14c were con-
veniently prepared in a two-step procedure from compound
10. Compound 10 was first condensed with ylides 12a–c
(generated in situ from phosphonium salts 11a,17 11b18 and
11c23 in tetrahydrofuran by addition of triethylamine) to afford
compounds 13a–c (54–79% yields). Compounds 13a and 13b
were the only isolated products, even when an excess of ylides
12a and 12b was used in this reaction, whereas the extended
bis(1,3-dithiole) donor 15c (14%) was isolated along with
compound 13c (54%) when one equivalent of ylide 12c was
employed (see below). Subsequent reaction of compounds 13b
and 13c with Lehnert’s reagent (as described above) afforded
compounds 14b and 14c (43 and 87% yield, respectively).
Surprisingly, compound 14a could not be obtained by this
procedure. Attempts at reacting compounds 13a–c with N,N-
bis(trimethylsilyl)carbodiimide and titanium tetrachloride in
dichloromethane (as above for compounds 6a–c) gave only
unchanged starting aldehydes.
To date, very little attention has been given to the potentially
important interplay that functionality on the 1,3-dithiole ring
may have in modulating the degree of intramolecular charge-
transfer in D–p–A materials. With this in mind, we have
synthesised a range of novel D–p–A structures, varying the D,
p and A fragments: substituted 1,3-dithiol-2-ylidene deriva-
tives span a range of donor abilities [viz. 4,5-dimethyl>4,5-
bis(methylsulfanyl)>4,5-bis(methoxycarbonyl)], a conjugated
ethylenic or oligoethylenic linker varies the central p-electron
unit (one, two and four conjugated double bonds) and the
dicyanomethylene and N-cyanoimine groups have provided
the acceptor functionality.10
Results and Discussion
Synthesis of 5a–c, 7a–c, 8a–c, 13a–c and 14b–c
The first, and one of the simplest, D–p–A compounds utilising
a 1,3-dithiol-2-ylidene donor, compound 2,11 was prepared by
Mayer et al. during early pioneering investigations of the
chemistry of the 1,3-dithiolium cation.12 Using a similar pro-
cedure, we have now synthesised derivatives 5a–c (Scheme 1).
Thus, methylation of 1,3-dithiole-2-thiones 3a,13 3b14 and 3c15
with methyl trifluoromethanesulfonate afforded 1,3-dithiolium
cations 4a–c (93–97%), which on reaction with malononitrile
using pyridine as base gave, after spontaneous elimination of
methanethiol, compounds 5a–c (61–67% yields). We next
targeted compounds 7a–c with an additional double bond in
the spacer unit. These were readily prepared in 64–71% yields
by reaction of Lehnert’s reagent (malononitrile, titanium tetra-
chloride and pyridine)16 with the known aldehydes 6a,17 6b18
and 6c17 in refluxing dichloromethane (Scheme 2). Similarly,
Synthesis of bis(1,3-dithiol-2-ylidene)hexa-2,4-diene derivatives
15a–c
The incorporation of conjugated linking groups between
the 1,3-dithiole rings of TTF has been widely explored
as a structural modification of the p-donor unit. In parti-
R
R
R
R
R
R
S
S
S
S
S
S
CN
CN
(i)
(ii)
S
SMe
O
O
(i)
(ii), (iii)
CF3SO3
O
O
O
9
R = Me
5a
10
R = Me
R = Me
3a
4a
4b R = MeS
4c
5b R = MeS
3b R = MeS
5c
R = CO2Me
3c R = CO
2Me
R = CO2Me
Scheme 1 Reagents and conditions: (i) CF SO Me, CH Cl , 20 °C, 2 h;
R
R
3
3
2
2
R
R
S
S
PR'3
S
S
(iv)
(ii) H C(CN) , pyridine, PriOH, 20 °C, 3 h
2
2
PR'3
H
–
BF4
R
S
S
HA
(i)
CN
CN
R = Me, R' = Bu
11a
12a
12b
12c
R = Me, R' = Bu
R = MeS, R' = Ph
R = CO2Me, R' = Bu
11b R = MeS, R' = Ph
R
11c
R = CO2Me, R' = Bu
HB
7a
7b
7c
R = Me
R
R
S
S
R = MeS
R = CO2Me
O
R
R
R
R
6a
6b
6c
R = Me
HA
HA
S
S
S
R
R
R = MeS
(v)
S
HA
HC
(ii)
HC
R = CO2Me
S
HE
HE
S
HB
HB
HB
CN
CN
N
CN
HD
O
HD
HF
HF
13a
13b
13c
R = Me
14b
14c
R = MeS
R = CO2Me
R = MeS
R = Me
8a
R = CO2Me
8b R = MeS
8c
R = CO2Me
Scheme 3 Reagents and conditions: (i) DMD, acetone, 0 °C, 0.5 h;
(ii) Ph PCHCHO, CH Cl , 020 °C, 3 h; (iii) I , CH Cl , 20 °C,
Scheme 2 Reagents and conditions: (i) CH (CN) , TiCl , pyridine,
CH Cl , reflux, 24 h; (ii) Me SiNCNSiMe , TiCl , pyridine, CH Cl ,
20 °C, 48 h
2
2
4
3
2
2
2
2
2
0.5 h; (iv) 10, NEt , THF, 20 °C, 24 h; (v) CH (CN) , TiCl , pyridine,
2
2
3
3
4
2
2
3
CH Cl , reflux, 24 h
2
2
4
2
2
1174
J. Mater. Chem., 1998, 8(5), 1173–1184

R
R
R
R
R
R
S
S
S
S
S
S
PBu3
(i)
H
R
R
O
S
S
BF4
R = Me
R = MeS
R = CO2Me
16a R = Me
11a R = Me
11c R = CO2Me
11d R = MeS
15a
15b
15c
16b R = CO2Me
16c R-R = (CH=CH)2
16d R-R = SCH2CH2S
Scheme 4 Reagents and conditions: (i) (a) For compound 15a: LDA,
13a, THF, −7820 °C, 16 h; (b) for compounds 15b and 15c: 10,
NEt , THF, 20 °C, 24 h
3
for an (ABC) spin system were simulated using LAOCOON
software and good agreement with the experimental data was
2
cular, olefinic,24 quinonoid25 and heterocyclic26 spacers have
attracted considerable attention and extensively conjugated
derivatives are of interest in the context of organic molecular
wires.27 The rationale behind the design of TTF derivatives
with extended conjugation is that the oxidised states respon-
sible for conduction in charge-transfer complexes and radical
cation salts should be stabilised by decreased intramolecular
Coulombic repulsion. As a continuation of our interest in
extended p-donors24d,f,j,25a and having a ready supply of
dialdehyde 10, we were attracted to the target compounds
15a–c. As noted above, reaction of equimolar amounts of the
ylide derived from phosphonium salt 11c and dialdehyde 10
afforded both the desired aldehyde 13c and the bis(1,3-dithiole)
derivative 15c (54 and 14% yields, respectively). Reaction of
2.5 equivalents of phosphorane 12c with 10 increased the yield
of 15c (66%, aldehyde 13c was also isolated in 14% yield).
Similar reactions of dialdehyde 10 with up to 4 equivalents of
phosphoranes 12a and 12b gave only aldehydes 13a and 13b
as the isolated products (45 and 70% yields, respectively).
Using the more reactive ylide derived from tributylphosphon-
ium salt 11d28 (2.5 equivalents) in this reaction gave extended
donor derivative 15b in 68% yield (along with aldehyde 13b,
18%). The tetramethyl analogue 15a was obtained (40%) by
the deprotonation of Wittig reagent 11a in THF at −78 °C
using LDA as base, following the known literature pro-
cedure,17,24a in the presence of aldehyde 13a (Scheme 4).
found.32 Definitive confirmation of the all trans stereochemistry
of compound 15b was provided by X-ray analysis (see below).
X-Ray molecular structures of compounds 7b, 8b and 15b
The single crystal structures of compounds 7b, 8b and 15b
have been determined by X-ray diffraction (Table 1). In mol-
ecule 7b (Fig. 1) the dithiol-2-ylidene and dicyanopropene
systems are planar and form an angle of 3° between each
other. The torsion angles around the C(2)–S(3) and C(3)–S(4)
bonds are 7 and 86°, respectively. Thus all the non-hydrogen
atoms, except C(10), are approximately coplanar and all the
multiple bonds and sulfur atoms [except S(4)] can participate
in p-conjugation. The ‘butadiene’ moiety has a trans-configur-
˚
ation; the formally single C(4)–C(5) bond is only 0.016 A (4s)
longer than the formally double bonds. Thus, the system is
much closer to a cyanine (bond-equivalent) structure, than to
genuine all-trans polyenes with the mean alternation (D)
˚
between single and double bond lengths of 0.10–0.12 A.33 Such
Table 1 Crystal data
compound
formula
M
7b
C H N S
284.42
8b
C H N S
260.40
15b
C H S
466.78
10
8
2 4
8
8
2 4
16 18 8
symmetry
˚
triclinic
5.062(1)
9.144(3)
14.051(5)
85.75(3)
83.24(3)
87.92(2)
643.8(3)
150
Mo-Ka
0.71073
:
P1 (No. 2)
2
orthorhombic
15.791(1)
8.140(1)
8.878(1)
90
monoclinic
9.808(2)
7.575(2)
14.090(3)
90
93.92(2)
90
1044.4(4)
150
a/A
˚
b/A
˚
c/A
1H NMR Spectra of compounds 7, 8, 14 and 15
a (°)
b (°)
c (°)
90
90
Previous structural studies on 1,3-dithiole derivatives,29 1,2-
dithiole derivatives30 and trithiapentalene analogues31 bearing
substituents capable of strong intramolecular S · · · X (X=O, S)
interactions have shown they exist in a favoured s-cis confor-
mation. Comparison of the 1H NMR spectra for compounds
7a–c and 8a–c suggested that, in solution, compounds 7a–c
adopt very different conformations from compounds 8a–c. In
˚
U/A3
1141.2(2)
150
T/K
radiation
Mo-Ka
0.71073
Pca2 (No. 29)
Cu-Ka
1.54184
P2 /c (No. 14)
˚
l/A
Space group
Z
1
1
4
7.9
2
m/cm−1
crystal size/mm
7.1
1.47
78.9
1.48
the former series, the presence of large 3J
(12.5–12.8 Hz) is indicative of an s-trans conformation; in
coupling
D /g cm−3
scan mode
1.52
AB
c
0.1×0.4×0.4 0.35×0.16×0.015 0.05×0.3×0.5
v/2h
60
3753
3185
contrast, compounds 8a–c exhibited a significantly smaller 3J
coupling (4.8–4.9 Hz) suggesting an s-cis conformation which
v
v
AB
2h
(°)
data total
51.2
5636
1855
150
2132
1888
max
could be stabilised by a close S · · · N intramolecular contact
(for the solid-state structures of 7b and 8b determined by X-
ray analysis, see below). In solution, the ethylenic chain in
compounds 13a–c and 14a–c retains an all s-trans confor-
mation; this was established by analysis of the coupling
data unique
data observed,
I>2s(I)
2336
0.039
1491
0.074
1459
0.019
R
int
absorption
constants 3J , 3J =13.9–15.1 Hz and 3J , 3J , 3J
=
correction
transmission
min5max
empiricala
0.9451.00
177
numerical
0.69050.988
160
empiricala
0.3251.00
113
BC DE AB CD EF
8.0–11.6 Hz. COSEY (1H , 1H) spectra for compounds 13a–c
and 14a–c allowed definitive identification of all the ethylenic
protons. After completion of our work, a similar synthesis of
analogues 16a–d was described by Gorgues and co-workers,24k
for which comparable 1H NMR data were obtained, and they
too concluded an all trans configuration in solution.
Confirmation of the all s-trans configuration was obtained by
X-ray analysis of compound 16a.24k Evidence for the retention
of the all trans stereochemistry of dialdehyde 10 in the product
15b obtained therefrom was provided by 1H NMR simulation.
The absolute values of proton shifts were determined by
(1H, 13C) HETCOR and, using literature J values, the spectra
no. of refined
variables
weighting scheme,
A, Bb
wR(F2), all data
goodness-of-fit
R(F), obs. data
0.046, 0.75
0.073
1.05
0.038
0.55
0, 2.66
0.149
1.36
0.056
0.44
0.108, 3.21
0.191
1.11
0.072
0.89
˚
˚
Dr /e A−3
max
Dr /e A−3
−0.54
−0.37
−0.52
min
a72 y-scans of 2 reflections, TEXSAN software.43 bw−1=s2(F2)+
(AP)2+BP, where P=(F 2+2F 2)/3
o
c
J. Mater. Chem., 1998, 8(5), 1173–1184
1175

Fig. 1 Molecular structure of 7b showing overlap of adjacent molecules
˚
in a stack. Bond distances (A): C(1)–C(4) 1.387, C(4)–C(5) 1.402,
C(5)–C(6) 1.384, C(1)–S(1) 1.743, C(1)–S(2) 1.727, S(1)–C(2) 1.757,
S(2)–C(3) 1.753, C(2)–C(3) 1.343, mean C–S(Me) 1.813, C(6)–C(N)
1.430, C–N 1.149 (e.s.d. 0.004, for C–S 0.003).
˚
a degree of p-delocalisation (D∏0.02 A) is usual for polyene
Fig. 3 (a) Molecular structure of 15b (primed atoms are inversion-
˚
related). Bond distances (A): C(1)–C(6) 1.348(9), C(6)–C(7) 1.430(10),
C(7)–C(8) 1.369(9), C(8)–C(8∞) 1.417(13), C(1)–S(1) 1.749(7),
C(1)–S(2) 1.769(6), S(1)–C(2) 1.770(6), S(2)–C(3) 1.771(6),
C(2)–C(3) 1.332(8). (b) Molecular structure of 15b, side-view,
showing planarity.
chains with an electron-withdrawing dicyanomethylene group
of one end and an electron-releasing (e.g. amino) substituent
at the other end (e.g. compounds 17 and 18).34 1,2-Dithiole
and 1,3-dithiole heterocycles have been shown to produce a
similar effect, e.g. compounds 19 (D=0.02 A)35 and 20,36 the
latter having the same conjugated path and the same acceptor
˚
˚
group as 7b, with the same D value (0.016 A). Molecules in the
crystal of 7b, related via an x translation, form a stack with
˚
an interplanar separation of ca. 3.4 A and a lateral shift of
˚
3.5 A, such that the oppositely charged dithiole and dicyano-
methylene moieties (arising as a result of intramolecular elec-
tron transfer) of adjacent molecules are nearly overlapping
(Fig. 1).
In molecule 8b (Fig. 2) all non-hydrogen atoms lie in one
plane within 0.03 A, except the methyl atoms C(4) and C(5)
˚
which are situated at 1.24 and 1.67 A, respectively, from this
˚
plane (on the same side). The N(1) atom, which is in a syn-
orientation towards the dithiole group, forms a short intramol-
˚
ecular contact of 2.719(6) A with S(1) which is substantially
O
O
N
less than the sum of their respective van der Waals radii
˚
N
(3.35 A).37 This fact, together with p-delocalisation along the
O
O
C(1)C(6)C(7)N(1) chain, suggests some degree of intramolecu-
lar sulfur · · · nitrogen bonding interaction. The only noteworthy
intermolecular contact N(2) · · · S(2) (x−1/2, 2−y, z) which is
nearly coplanar with the ring and trans to the S(2)–C(3) bond,
CN
CN
17
18
NC
NC
˚
is not particularly short (3.28 A).
The central tetraene moiety of compound 15b adopts an all-
trans configuration [Fig. 3(a)]. It is notable that the alternation
CN
NC
Ph
Me
S
S
˚
of bond lengths is smaller (D=0.07 A) than in non-substituted
CN
CN
˚
S
octatetraene (D=0.12 A).33a Molecule 15b lies on a crystallo-
S
S
graphic inversion centre and has a predominantly planar
structure [Fig. 3(b)], distorted by: (i) small twists (5°) around
the C(6)–C(7) bond; (ii) folding of the dithiole rings along the
S(1) · · · S(2) vectors by 8°; and (iii) out-of-plane conformation
of the methyl groups, i.e. twists around the C(2)–S(3) and
C(3)–S(4) bonds by 18 and 64°, respectively. Molecules in the
crystal contact in a perpendicular edge-to-face fashion and
thus no stacks exist (cf. extended donor 21 which stacks in
uniform fashion along the x-axis24j).
S
Ph
20
19
S
R
S
R
S
S
S
S
S
R
S
R
21 R = MeS
22 R = H
23
SMe
SMe
MeS
MeS
S
S
S
S
˚
Fig. 2 Molecular structure of 8b. Bond distances (A): S(1)–C(1)
1.739(7), S(1)–C(2) 1.751(7), S(1) · · · N(1) 2.719(6), S(2)–C(1)
1.741(7), S(2)–C(3) 1.743(8), N(1)–C(8) 1.340(9), C(1)–C(6)
1.376(10), C(6)–C(7) 1.398(10), N(1)–C(7) 1.313(9), C(8)–N(2)
1.148(9); angles (°): C(2)S(1)N(1) 171.4(2), C(7)N(1)C(8) 119.8(6).
24
1176
J. Mater. Chem., 1998, 8(5), 1173–1184

Fig. 5 Cyclic voltammograms of compounds 7b (- - - ) and 14b (CC)
· · ·
an irreversible one-electron reduction wave, to form the radical
anion located on the dicyanomethylene or N-cyanoimine
group. Within each series of D–p–A systems studied (e.g.
compounds 7a–c) there is a small cathodic shift of both the
oxidation and the reduction peaks with increasing donor
strength. Similarly, when series 5, 7 and 14 are compared, there
is a significant cathodic shift in the oxidation wave and an
anodic shift in the reduction wave on increasing the conju-
gation length (Fig. 5). The UV and CV data are entirely
consistent with an increase in electron delocalisation with
increasing conjugated chain length. Furthermore, they confirm
that the relative donor and acceptor strenths follow the series:
4,5-dimethyl > 4,5-bis(methylsufanyl) > 4,5-bis(methoxy-
carbonyl) and dicyanomethylene>N-cyanoimine.
Fig. 4 UV–Visible absorption spectra for compound 14b in various
solvents, at 20 °C, demonstrating solvatochromism. Solvents:
CF CH OH (CC) l
556 nm, DMSO ( ) l
······
553 nm, CH Cl (-
3
2
max
555 nm, MeCN (-----) l
max
526 nm.
max
2
2
max
- ) l
·· ··
533 nm, Et O (C C C) l
max
2
528 nm, hexane (- - - - ) l
· · · ·
max
Electronic absorption spectra of compounds 5, 7, 8 and 14
The electronic absorption spectra of compounds 5, 7, 8 and
14 are dominated by a broad low-energy band [l (MeCN)
354–533 nm, dependent on substituents and conjugative length
max
The solution redox properties of the bis(1,3-dithiole) donors
15a–c are collated in Table 3, along with selected model
compounds 21–24 for comparison. For all the new extended
donors, only one redox couple involving a two-electron transfer
is observed [this redox couple could not be resolved into two
separate couples by: (i) lowering the polarity of the solvent;
(ii) reducing the scan rate; (iii) lowering the temperature to
−78 °C; or (iv) differential pulse voltammetry]. It is unclear
whether this wave is the result of two inseparable one-electron
oxidations, or a concomitant two-electron transfer. In either
case, these data are entirely consistent with previous work
with vinylogous TTF systems17,24 which has established the
following general trends in the redox properties with increasing
conjugative length. There is: (i) a lowering of the first oxidation
(Table 2)]. Following precedents for other D–p–A systems,
this band is attributed to a photoinduced intramolecular
electron transfer from the 1,3-dithiol-2-ylidene moiety to the
acceptor fragment. For each series of compounds 5, 7, 8 and
14, the variation of donor substitution has a small, but
significant, effect on the position of the charge-transfer band.
In all cases, the absorption maxima in the 1,3-dithiole series
shift bathochromically with increased donor strength [e.g.
l
son of absorption spectra for compounds 5b, 7b and 14b shows
(MeCN) 7a 489; 7b 475; 7c 439 nm]. Similarly, a compari-
max
that the maxima shift bathochromically with increasing number
of conjugated double bonds [l (MeCN) 5b 364; 7b 475; 14b
max
533 nm]. The absorption spectra of compounds 14b and 14c
exhibit marked solvatochromism (exemplified in Fig. 4 for
compound 14b).
potential (E ) due to the increased electron delocalisation; and,
1
(ii) a smaller difference (DE) between E 1/2 and E 1/2 (tending
1
2
to zero as the conjugation length increases), indicative of
increased stabilisation of the dicationic state as a result of
increased charge separation (and thus reduced on-site
Coulombic repulsion). These observations on the influence of
conjugative chain length are shown clearly in a comparison
of the cyclic voltammograms of compounds 156, 21 and 24
(Fig. 6). Predictably, compound 15a is the best donor in the
series due to the presence of the electron releasing methyl
substituents on the 1,3-dithiole ring.
Cyclic voltammetry of compounds 5, 7, 8, 14 and 15
The solution redox properties of D–p–A compounds 5, 7, 8
and 14 have been studied by cyclic voltammetry and the results
are collated in Table 2. All the new compounds display a
reversible one-electron oxidation wave, attributed to the forma-
tion of the radical cation of the 1,3-dithiole donor moiety, and
Table 2 Cyclic voltammetric dataa and electronic absorption maximab
for D–p–A compounds 5a–c, 7a–c, 8a–c and 14b–c
Table 3 Cyclic voltammetric dataa for extended TTF vinylogues 15a–c,
with the data for compounds 1 and 21–24 for comparison
E
/V
E
/V
l
/nm
ox
red
max
5a
5b
5c
7a
7b
7c
8a
8b
8c
14b
14c
1.70
1.76
2.15
1.09
1.14
1.49
1.25
1.17
1.65
0.73
0.97
−1.67
−1.52
−1.24
−1.18
−1.06
−1.01
−1.37
−1.22
−1.14
−0.84
−0.81
387
E 1/2/V
E 1/2/V
E 1/2−E 1/2/V
1
2
2
1
364
354
489
475
439
450
448
426
533
492
TTF 1
2217
2317
24
2118,24j
15a
15b
0.34
0.20
0.217
0.53
0.50
0.18b
0.36b
0.56b
0.71
0.36
0.223
0.78
0.63
—
0.37
0.16
0.006
0.25
0.12
—
—
—
—
—
15c
a10−5  compound in dry MeCN under argon vs. Ag/AgCl, Pt
working and counter electrodes, 20 °C, 100 mV s−1 scan rate. bTwo
a10−5  compound in dry MeCN under argon vs. Ag/AgCl, Pt
working and counter electrodes, 20 °C, 100 mV s−1 scan rate. bIn
MeCN solution, 20 °C.
electron redox couple with peaks observed as
a single wave,
E
−E =56 mV, indicative of two non-interacting redox centres
ox red
J. Mater. Chem., 1998, 8(5), 1173–1184
1177

Theoretical calculations
To gain a deeper understanding of the experimental trends
reported above, the molecular structure and electronic proper-
ties of the unsubstituted (R=H) compounds 7, 8, 14 and 15
were calculated within the density functional theory (DFT)
approach.40 Compared to Hartree–Fock (HF) methods, DFT-
based calculations have the advantage of including electron
correlation effects and have been shown to provide more
accurate optimised geometries and energetic data.41 All calcu-
lations were performed using the B3P86 density functional and
the polarised 6-31G* basis set.
Molecular structure of 7, 8, 14 and 15 (R=H). The molecular
structures of 7 and 8 were optimised both in cis and trans
conformations. The minimum energy conformation corre-
sponds to the trans orientation for 7 and to the cis structure
for 8 in agreement with NMR and X-ray observations. The
optimised bond lengths and bond angles calculated for these
structures are shown in Fig. 7. In order to compare the
optimised structures with those obtained experimentally, the
average deviations between the experimental X-ray data
reported for 7b and 8b and the theoretical structures obtained
for unsubstituted 7 and 8 were calculated for the bond lengths,
Fig. 6 Cyclic voltammograms of compounds 15b (CC), 21 (-----) and
24 (- - )
· ·
Table 4 Dipole moment and non-linear optical dataa
b(0)/
10−30 esu
m.b(0)/
10−48 esu
compound
l /nm
m/D
b/10−30 esu
31
0
7a
7b
7c
14b
489
475
439
556
8.8
6.9
6.8
—
13
15
12
—
112
105
85
37
24
m.b=4554
1170
:
:
d(R), and the bond angles, d(a). The deviations were found to
˚
aMeasured in CHCl . Relative errors: m<3%; b<10%; b(0)<10%.
:
:
3
be very small both for 7 [d(R)=0.012 A, d(a)=0.7°] and 8
[d(R)=0.009 A, d(a)=0.5°], demonstrating the agreement
between theory and experiment.
˚
:
:
Non-linear optical properties
The cis conformer of 7 was found to be destabilised by
7.91 kcal mol−1 with respect to the trans structure due to the
short S(2) · · · C(7) and S(2) · · · N(1) contacts resulting from the
rotation around C(4)–C(5) [see Fig. 7(a)]. To alleviate these
contacts, the S(2)–C(1)–C(4), C(1)–C(4)–C(5), C(4)–C(5)–
C(6) and C(5)–C(6)–C(7) bond angles widen to 129.0, 134.9,
136.9 and 128.5°, respectively. These values are very much
larger than those found for the trans conformer [see Fig. 7(b)]
indicating the strain present in the cis conformer.
The non-linear optical properties of compounds 7a–c and 14b
were determined using the EFISH technique performed at
1.34 mm and the data are presented in Table 4. From experimen-
tal b values at 1.34 mm it is possible to infer ‘static’ b(0) values
using a two-level dispersion model,38 which can be considered
as valid in the present case of unidimensional electronic charge-
transfer molecules:39
3B2
W 4
b=
f Dm
2mW 3 (W 2−B2v2)(W 2−4B2v2)
In contrast to 7, the cis conformer of 8 is calculated to be
more stable than the trans orientation by 3.85 kcal mol−1.
The optimised distance between S(1) and N(1) in the cis
W 4
=
b(0)
(1)
(2)
(W 2−B2v2)(W 2−4B2v2)
˚
orientation is 2.722 A, in very good agreement with the
˚
observed X-ray value of 2.719(6) A. This short intramolecular
where
contact is stabilised by the opposite charges borne by S(1)
and N(1) (see Fig. 8), which give rise to a bonding electrostatic-
type interaction. For the trans conformer, both sulfur atoms
present net atomic charges of +0.34e. For the preferred cis
conformer, the S(1) atom augments its charge to +0.43e and
3B2 f Dm
b(0)=b =
0
2m W 3
W being the energy of the charge transfer electronic transition,
f the corresponding oscillator strength, Dm=m −m (where m
is the dipole moment of the first excited state).
1
0
1
Compounds 7a–c exhibit moderate non-linear optical
properties [cf. the relevant application parameter is m.b(0),
the standard value for the ‘classical’ NLO dye DR1 is
ca. 500×10−48 esu] (Table 4). The dependence of b(0) upon
the nature of R substitution is weak, m.b(0) being slightly
smaller for the weakly electron donating CO Me substituent.
The b(0) values are comparable to those of p-nitroaniline
2
derivatives. The quadratic hyperpolarisability dramatically
increases when adding two double bonds, leading to m.b(0)
values that are double those of the standard dye DR1. Although
the dipole moment of 14b could not be measured (due to a
moderate solubility in non-polar solvents), m was estimated to
be ca. 7 D. This enhancement of m.b(0) is strongly correlated
with the increase in electron delocalisation with increasing the
conjugated length. The strong red-shift of the maximum
absorption wavelength l
corresponds to a significant
max
decrease of W. The large solvatochromism of compound 14b
indicates a high Dm value, and the strong hyperchromic effect
observed when increasing the number of double bonds corre-
sponds to a high oscillator strength f. All these factors result
in the observed enhancement of m.b(0) (by a factor of two) for
compound 14b.
Fig. 7 B3P86/6-31G*-optimised minimum energy structures of
˚
(trans) and 8 (cis). (a) Bond lengths in A. (b) Bond angles in degrees
7
1178
J. Mater. Chem., 1998, 8(5), 1173–1184

consequence of the magnitude of the electron transfer between
the donor and the acceptor units. Mulliken population analysis
based on B3P86/6-31G* calculations predicts that the net
charge of the dithiole ring decreases in passing from 2 (+0.28e)
to 7 (+0.17e) and to 14 (+0.11e). The charge transfer thus
decreases as the length of the conjugated spacer increases,
justifying the increase of the C(1)–S bond lengths and of the
single–double bond alternation along the series 2<7<14.
However, it should be noted that although the charge transfer
decreases along this series, the dipole moment increases due
to the separation of the donor and the acceptor moieties. The
dipole moments calculated at the B3P86/6-31G* level are
7.9 D (2)<10.2 D (7)<13.7 D (14). Although these values
overestimate the experimental data for compounds 7 (see
Table 4), they can be used as a good estimate of the variation
of the dipole moment as the length of the conjugated spacer
increases.
Fig. 8 B3P86/6-31G* net atomic charges (in e) for cis and trans
conformations of 8 (top) and for trans-7 (bottom)
˚
Compound 15 presents mean C(1)–S bond lengths of 1.771 A
˚
and a mean alternation in the octatetraene spacer of 0.067 A.
These values are in good agreement with the experimental X-
˚
ray values (1.759 and 0.07 A, respectively) and are larger than
˚
those theoretically obtained for 14 (1.760 and 0.047 A). Despite
the absence of charge transfer, the alternation of the conjugated
spacer in 15 is significantly shorter than that calculated for the
˚
octatetraene molecule (0.101 A) using the same theoretical
approach. The terminal C(1)–C(6) and C(6)–C(7) bonds of
15 present the largest differences with the equivalent bonds of
octatetraene (see Fig. 9).
Electronic structure. We turn now to discuss the redox
properties on the basis of molecular orbital energies and
topologies. Table 5 summarises the energies of the highest
occupied molecular orbital (HOMO) and the lowest unoccu-
pied molecular orbital (LUMO) of 7, 8, 14 and 15 and of
selected compounds TTF (1), 2, 22 and 23 for comparison.
Table 5 B3P86/6-31G* molecular orbital energies e
e
/
e
/
e
/
e
/
HOMO
eV
LUMO
eV
HOMO
eV
LUMO
eV
compound
compound
2
7
7a
7c
8
−7.07
−6.63
−6.37
−6.92
−6.60
−6.11
−2.94
−3.37
−3.20
−3.53
−2.95
−3.60
TTF 1
22
23
−5.09
−4.99
−4.93
−4.89
−1.54
−1.35
−1.82
−2.12
15
14
˚
Fig. 9 B3P86/6-31G*-optimised bond lengths (in A) of 2, 14, 15 and
octatetraene. 2 exhibits C symmetry. 15 and octatetraene exhibit
C
2v
symmetry.
2h
faces the negatively charged N(1) atom (−0.51e). The adoption
of a cis orientation by compounds 8 thus results not only in
an attractive coulombic interaction, but also determines a
more effective charge transfer between the donor dithiole ring
and the acceptor cyanoimine unit. It should be also stressed
that, for 8, the adoption of the cis conformation causes no
special distortion of the bond angles [see Fig. 7(b)] as was the
case for cis-7. Fig. 9 shows the bond lengths calculated for
unsubstituted 14 and 15 in their most stable all-trans confor-
mation; the values obtained for 2 and for octatetraene are
included for comparison. As the length of the conjugated linker
along the series 2, 7, 14 increases (see Fig. 7 and 9), the mean
length of the C(1)–S bonds becomes longer passing from
˚
˚
˚
1.743 A (2) to 1.754 A (7) and to 1.760 A (14). This evolution
is accompanied by an increase of the mean alternation between
single and double bonds in the spacer unit which varies from
˚
˚
0.039 A for 7 to 0.047 A for 14. Both geometrical trends are a
Fig. 10 Electronic density contours of the HOMO and LUMO of 7
J. Mater. Chem., 1998, 8(5), 1173–1184 1179

The topology of these orbitals is exemplified in Fig. 10 for
compound 7. The HOMO resides mainly on the 1,3-dithiol-2-
ylidene group while the LUMO spreads over the
Conclusions
A series of novel highly-conjugated donor–p–acceptor chromo-
phores 5a–c, 7a–c, 8a–c, 13a–c and 14b–c have been syn-
thesised, in which the donor units (substituted 1,3-dithioles)
are separated from dicyanomethylene and N-cyanoimine
acceptor groups by an ethylenic or oligoethylenic linker (one,
two and four conjugated double bonds). Extended tetrathiaful-
valene derivatives 15a–c have also been synthesised. The
electronic spectra, redox properties, non-linear optical proper-
ties and X-ray crystal structures of representative derivatives
have been studied. These experimental data are supported by
theoretical calculations. A broad low-energy intramolecular
charge-transfer band observed for compounds 5, 7, 8 and 14
shifts bathochromically with increasing donor strength of the
dithiole ring, and with increasing length of conjugation between
the donor and acceptor moieties. Cyclic voltammetric data
establish that compounds 5, 7, 8 and 14 undergo a reversible
one-electron oxidation, and an irreversible one-electron
reduction, to form the radical cation and radical anion species,
respectively. For the bis(dithiole) donors 15a–c a single two-
electron redox wave is observed. Moderate NLO properties
are observed for compounds 7a–c [m.b(0) 85–112×10−48 esu]
whereas for the more extensively conjugated compound 14b,
the value is increased to m.b(0) 1170×10−48 esu. The minimum
energy conformations predicted for 7 and 8 are in agreement
with X-ray crystal structures and solution NMR data. These
results should stimulate further studies on the use of 1,3-
dithiol-2-ylidene units as electron donor components in D–p–A
materials, which by virtue of intramolecular charge-transfer
will possess interesting electronic, optical, non-linear optical
and structural properties.
CNC–CNC(CN) unit. Similar atomic orbital compositions
are found for compounds 2, 8 and 14.
2
From a MO standpoint, oxidation implies the extraction of
an electron from the HOMO, and reduction implies the
introduction of an electron into the LUMO. More positive
oxidation potentials are therefore to be expected for com-
pounds with lower energy HOMOs and more negative
reduction potentials for compounds with higher energy
LUMOs. This trend is observed when comparing the calculated
MO energies (Table 5) with the redox potentials obtained
experimentally by CV (Tables 2 and 3). For the D–p–A
compounds, the HOMO increases in energy and the LUMO
decreases in energy as the length of the conjugated p spacer
increases along the series 2, 7, 14. The destabilisation of the
HOMO accounts for the cathodic shift of the oxidation
potential and the stabilisation of the LUMO justifies the
anodic shift of the reduction potential along that series. The
effect of substituents has been studied for compound 7. As
the donor strength of the substituent increases [7c (R=
CO Me)<7 (R=H)<7a (R=Me)], both the energy of the
2
HOMO [−6.92 eV<−6.63 eV<−6.37 eV] and the LUMO
[−3.53 eV<−3.37 eV<−3.20 eV] increase accounting for
the cathodic shift of the oxidation and reduction potentials
along this series. The destabilisation effect is more important
for the HOMO in agreement with the larger cathodic shifts
observed for the oxidation potential (see Table 2).
The relative energies of the LUMO of 7 (−3.37 eV) and 8
(−2.95 eV) confirm the higher acceptor capability of the di-
cyanomethylene group in accord with CV data. The almost
identical energies obtained for the HOMO of 7 (−6.63 eV)
and 8 (−6.60 eV) suggest, however, that similar donor proper-
ties should be expected for both compounds. This prediction
contrasts with the lower oxidation potentials measured for
compounds 7. The additional factor that should be taken into
account to explain the experimental trend is the different
conformations displayed by compounds 7 and 8. As mentioned
above, the cis conformation adopted by 8 allows for a more
effective charge transfer due to the S(1) · · · N(1) bonding
interaction. The dithiole ring exhibits a net charge of +0.23e
in 8 compared to the charge of +0.17e calculated for 7. The
greater charge defect accumulated by that ring in 8 makes it
harder to extract an electron upon oxidation, since it is mainly
removed from the dithiole moiety, thus leading to a higher
oxidation potential.
The atomic orbital composition of the HOMO and the
LUMO of D–p–A compounds suggests that the lowest-energy
HOMOLUMO electronic transition implies an electron
transfer from the dithiole environment to the acceptor part of
the molecule. This supports the intramolecular electron transfer
character of the first absorption band observed in the electronic
spectra. The HOMO–LUMO energy gap decreases with the
length of the conjugated spacer (2>7>14) and with the
donor strength of the substituents (7c>7>7a) (see Table 5).
These trends justify the bathochromic shifts observed exper-
imentally along those series (see Table 2). The smaller values
Experimental
General methods
1H NMR Spectra were obtained on a Bruker AC 250 spec-
trometer operating at 250.134 MHz. 13C NMR Spectra were
obtained on
a Varian 400 spectrometer operating at
100.581 MHz. Mass spectra were recorded on a VG7070E
spectrometer operating at 70 eV. IR Spectra were recorded on
a Perkin-Elmer 1615 FTIR spectrometer operated from a
Grams Analyst 1600. UV–VIS Spectra were obtained on a
Kontron Uvicon 930 spectrophotometer using quartz cells;
extinction coefficients (e) are quoted in −1 cm−1. Melting
points were obtained on a Kofler hot-stage microscope appar-
atus and are uncorrected. Cyclic voltammetric data were
obtained on a BAS 50W electrochemical analyser (1×10−5 
solution of donor in acetonitrile under argon, 1×10−1 
Bu NClO supporting electrolyte, platinum working and coun-
4
4
ter electrodes, Ag/AgCl reference electrode, 20 °C). Column
chromatography was performed on Merck silica gel (70–230
mesh), unless otherwise stated, and solvents were distilled prior
to use. All reagents were of commercial quality and used as
supplied unless otherwise stated; solvents were dried where
necessary using standard procedures.
of l
measured for compounds 8 with respect to 7 are due
max
to the higher energy of the LUMO of 8 which enlarges the
HOMO–LUMO gap.
General procedure for compounds 4a–c
To a stirred solution of thiones 3a,13 3b14 or 3c15 (30 mmol)
in dry dichloromethane (100 ml) was added methyl trifluoro-
methanesulfonate (3.5 ml, 30 mmol). The resultant mixture
was stirred under an argon atmosphere for 2 h at 20 °C
whereupon addition of anhydrous diethyl ether (100 ml) pre-
cipitated a solid, which was filtered, washed with anhydrous
diethyl ether and dried to afford the products. The products
are moisture sensitive, but may be stored under argon for at
least 3 months. The following were obtained.
The HOMO of the bis(dithiole) compounds 22, 23 and 15
spreads over the whole molecule involving both dithiole moiet-
ies and the central polyenic chain. This determines that its
energy slightly increases with the length of the chain as is the
case for polyenes. As a consequence, the first oxidation poten-
tial lowers with increasing the conjugate length (see Table 3
and Fig. 6). The LUMO decreases in energy more rapidly
along the series TTF, 22, 23, 15 as it is localised primarily on
the conjugated chain.
1180
J. Mater. Chem., 1998, 8(5), 1173–1184

4,5-Dimethyl-2-methylsulfanyl-1,3-dithiolium triflate 4a.
(9.65 g, 96%) White solid, mp 71 °C (Analysis found: C, 26.0;
H, 2.9; C H F O S requires: C, 25.8; H, 2.8%); d [(CD ) CO]
m/z (CI) 221 (M++1, 25%), 238 (M++NH , 100%);
(3H, s), 2.13 (3H, s); d (CDCl ) 167.3, 151.0, 127.5, 127.2, 116.0,
4
d (CDCl ) 7.35 (H , d, J 12.8), 6.54 (H , d, J 12.8), 2.15
H
3
B
AB
A
AB
7
9 3 3 4
H
3 2
C
3
3.27 (3H, s), 2.72 (6H, s).
113.9, 106.1, 68.5, 13.7, 13.3; n (KBr)/cm−1 2208 and 2200
max
(both CON), 1540 (CNC); l (MeCN)/nm (e) 489 (4.2×104),
max
467 (3.5×104), 249 (7.4×103).
2,4,5-Tris(methylsulfanyl)-1,3-dithiolium triflate 4b. (11.68 g,
97%) Yellow solid, mp 75 °C (Analysis found: C, 21.5; H, 2.4;
C H F O S requires: C, 21.5; H, 2.3%); d [(CD ) CO] 3.34
1-[4,5-Bis(methylsulfanyl)-1,3-dithiol-2-ylidene]-3,3-
dicyanoprop-2-ene 7b. (504 mg, 71%) Red solid, mp 149–150 °C
(Analysis found: C, 42.2; H, 2.8; N, 9.8; C H N S requires:
7
9 3 3 6
H
3 2
(3H, s), 2.83 (6H, s).
10
8 2 4
C, 42.2; H, 2.8; N, 9.9%); m/z (CI) 285 (M++1, 100%), 302
d, J 12.7), 2.47 (s, 3H), 2.46 (s, 3H); d (CDCl ) 164.2, 150.8,
4,5-Bis(methoxycarbonyl)-2-methylsulfanyl-1,3-dithiolium
triflate 4c. (11.88 g, 93%) White solid, mp 97 °C (Analysis
found: C, 26.0; H, 2.2; C H F O S requires: C, 26.1; H, 2.2%);
(M++NH , 12%); d (CDCl ) 7.30 (H , d, J 12.7), 6.65 (H ,
4
H
3
B
AB
A
AB
C
3
9
9 3 7 4
130.8, 130.3, 115.1, 113.1, 108.0, 71.7, 19.2, 19.1; n (KBr)/cm−1
d [(CD ) CO] 4.04 (6H, s), 3.44 (3H, s).
max
2210 (CON), 2198 (CON), 1554 (CNC); l (MeCN)/nm (e)
max
H
3 2
475 (5.3×104), 318 (1.0×104), 244 (1.0×104). A crystal suitable
General procedure for compounds 5a–c
for X-ray analysis was grown by slow evaporation of its
CH Cl solution.
To a stirred suspension of salt 4 (15 mmol) in dry propan-2-
ol (25 ml) at 20 °C under argon was first added malononitrile
(1.1 g, 16.8 mmol) followed by dry pyridine (3 ml, excess). After
stirring for 3 h, water (150 ml) was added and the mixture
extracted into CH Cl (3×75 ml) and the organic phase
2
2
1-[4,5-Bis(methoxycarbonyl)-1,3-dithiol-2-ylidene]-3,3-
dicyanoprop-2-ene 7c. (492 mg, 64%) Orange solid, mp
202–203 °C (sublimes from ca. 150 °C) (Analysis found: C, 46.9;
H, 2.7; N, 9.0; C H N O S requires: C, 46.8; H, 2.6; N,
2
2
washed with water (2×200 ml). After drying (MgSO ) and
4
12
8 2 4 2
evaporation in vacuo, column chromatography eluting with
CH Cl , and, if necessary, recrystallisation from CHCl the
following were obtained.
9.1%); m/z (CI) 309 (M++1, 80%), 326 (M++NH , 100%);
4
d (CDCl ) 7.32 (H , d, J 12.5), 6.60 (H , d, J 12.5), 3.92
2
2
3
H
3
B
AB
A
AB
(6H, s); n (KBr)/cm−1 2218 (CON), 2208 (CON), 1707
max
[C(NO)–O–];
(5.8×103).
l
(MeCN)/nm (e) 439 (2.5×104), 241
max
(4,5-Dimethyl-1,3-dithiol-2-ylidene)dicyanomethane
5a.
(2.04 g, 67%) Tan needles, mp 228–229 °C (sublimes from ca.
150 °C) (Analysis found: C, 50.2; H, 2.9; N, 14.2; C H N S
General procedure for compounds 8a–c
8
6 2 2
requires: C, 49.5; H, 3.1; N, 14.4%); m/z (EI) 194 (M+, 100%);
d (CDCl ) 2.21 (6H, s); d [(CDCl ) ] 180.9, 129.8, 114.4, 60.7,
To a stirred solution of aldehyde 6a17, 6b18 or 6c17 (3 mmol)
in dry CH Cl (25 ml) under argon at 20 °C were added
sequentially (i) titanium tetrachloride (3.3 ml, 3.3 mmol, 1  in
m/z (CI) 195 (M++1, 25%), 212 (M++NH , 100%);
4
2
2
H
3
max
C
2 2
14.0; n (KBr)/cm−1 2205 (CON); l (MeCN)/nm (e) 387
max
CH Cl ), (ii) N,N-bis(trimethylsilyl)carbodiimide (0.75 ml,
3.3 mmol) and (iii) dry pyridine (1 ml, excess) and the mixture
was stirred for a further 48 h. After dilution with CH Cl
(1.3×104), 370 (1.5×404), 233 (4.8×104).
2
2
[4,5-Bis(methylsufanyl)-1,3-dithiol-2-ylidene]dicyano-
methane 5b. (2.36 g, 61%) Pale yellow solid, mp 100–101 °C
(Analysis found: C, 37.4; H, 2.4; N, 11.0; C H N S requires:
2
2
(250 ml) the mixture was washed with water (3×100 ml). After
drying (MgSO ), the solvent was evaporated in vacuo and the
4
8
6 2 4
residue was chromatographed eluting with ethyl acetate to
afford the products. The following were thus obtained.
C, 37.2; H, 2.3; N, 10.8%); m/z (EI) 258 (M+, 100%); m/z (CI)
259 (M++1, 50%), 276 (M++NH , 100%); d (CDCl ) 2.52
(MeCN)/nm (e) 364
4
H
3
(6H, s);
n
(KBr)/cm−1 2207;
l
max
max
1-(N-Cyanoimino)-2-(4,5-dimethyl-1,3-dithiol-2-
(1.3×104), 353 (1.6×104), 214 (9.2×103).
ylidene)ethane 8a. (459 mg, 78%) Orange solid, mp 181–182 °C
(sublimes from ca. 130 °C) (Analysis found: C, 48.8; H, 3.8; N,
14.3; C H N S requires: C, 49.0; H, 4.1; N, 14.3%); m/z (CI)
[4,5-Bis(methoxycarbonyl)-1,3-dithiol-2-ylidene]dicyano-
methane 5c. (2.34 g, 65%) Cream solid, mp 104–105 °C
(Analysis found: C, 42.7; H, 2.1; N, 10.0; C H N O S requires:
8
8 2 2
197 (M++1, 100%); d (CDCl ) 8.24 (H , d, J 4.9), 6.39
H
3
B
AB
10
6 2 4 2
(H , d, J 4.9), 2.23 (3H, s), 2.22 ((3H, s); d (CDCl ) 167.6,
C, 42.5; H, 2.1; N, 9.9%); m/z (EI) 282 (M+, 100%); m/z (CI)
A
AB
C
3
166.3, 131.9, 126.9, 118.8, 103.4, 13.5, 13.0; n (KBr)/cm−1
283 (M++1, 40%); d (CDCl ) 3.96 (6H, s); d [(CD ) CO]
max
2155 (CON), 1542 (CNC), 1457 (CNN); l (MeCN)/nm (e)
max
H
3
C
3 2
182.4, 161.6, 138.2, 116.7, 67.4, 59.4, 57.3; n (KBr)/cm−1 2207
max
450 (5.0×104), 432 (3.7×104), 236 (9.7×103).
(CON), 1710 [C(NO)–O–];
(2.3×104), 214 (1.5×104).
l
(MeCN)/nm (e) 354
max
1-(N-Cyanoimino)-2-[4,5-bis(methylsufanyl)-1,3-dithiol-2-
ylidene]ethane 8b. (660 mg, 85%) Orange solid, mp 127–128 °C
(Analysis found: C, 37.0; H, 2.9; N, 10.9; C H N S requires:
General procedure for compounds 7a–c
8
8 2 4
To a stirred solution of aldehyde 6a,17 6b18 or 6c17 (2.5 mmol)
in dry CH Cl (50 ml) under argon at 20 °C were added
sequentially: (i) malononitrile (248 mg, 3.75 mmol), (ii) tit-
C, 36.9; H, 3.1; N, 10.7%); m/z (CI) 261 (M++1, 100%);
(3H, s), 2.51 (3H, s); d (CDCl ) 167.3, 166.1, 134.9, 129.5, 117.9,
d (CDCl ) 8.32 (H , d, J 4.8), 6.48 (H , d, J 4.8), 2.56
2
2
H
3
B
AB
A
AB
C
3
anium tetrachloride (3.75 ml, 3.75 mol, 1  in CH Cl ) and
105.4, 19.2, 19.0; n (KBr)/cm−1 2161 (CON), 1540 (CNC),
2
2
max
max
(iii) dry pyridine (1 ml, excess) and the mixture was then
refluxed for 24 h. After cooling, the mixture was diluted with
CH Cl (250 cm3) and washed with water (3×100 ml). After
1455 (CNN);
l
(MeCN)/nm (e) 448 (1.3×104), 307
(2.5×103), 201 (8.1×103). A crystal suitable for X-ray analysis
was grown by slow evaporation of its CH Cl solution.
2
2
2
2
drying (MgSO ), the solvent was evaporated in vacuo and the
4
residue was chromatographed eluting with CH Cl to afford
the products. The following were obtained.
1-(N-Cyanoimino)-2-[4,5-Bis(methoxycarbonyl)-1,3-dithiol-
2-ylidene]ethane 8c. (681 mg, 80%) Bright yellow solid, mp
156–157 °C (Analysis found: C, 42.0; H, 2.7; N, 10.2;
2
2
1-(4,5-Dimethyl-1,3-dithiol-2-ylidene)-3,3-dicyanoprop-2-ene
7a. (358 mg, 65%) Red solid, mp 222–223 °C (sublimes from
ca. 185 °C) (Analysis found: C, 54.2; H, 3.6; N, 12.9; C H N S
requires: C, 54.5; H, 3.7; N, 12.7%); m/z (EI) 220 (M+, 100%);
C H N O S requires: C, 42.2; H, 2.8; N, 9.9%); m/z (CI) 261
10
8 2 4 2
(M++1, 100%); d (CDCl ) 8.45 (H , d, J 4.9), 6.59 (H , d,
H
3
B
AB
max
A
J
4.9), 3.95 (3H, s), 3.93 (3H, s); n (KBr)/cm−1 2189
10
8
2 2
AB
(CON), 2169 (CON), 1733 and 1710 [both C(NO)–O], 1575
J. Mater. Chem., 1998, 8(5), 1173–1184
1181

and 1559 (both CNC), 1471 (CNN); l (MeCN)/nm (e) 426
14.1), 6.65 (H , dd, J 14.1, J 11.9), 6.55 (H , dd, J 11.6,
ED EF BA
J 13.9), 6.24 (H , d, J 11.6), 6.18 (H , dd, J 13.9, J
max
E
B
(3.5×104), 414 (3.6×104), 227 (1.3×104).
BC
A
AB
C
CB
CD
11.5), 2.45 (3H, s), 2.43 (3H, s); n (KBr)/cm−1 2215 (CON),
max
General procedure for compounds 13a–c
1581 (CNC);
l
(MeCN)/nm (e) 533 (2.3×104), 326
max
(6.8×103), 264 (6.2×103).
To a stirred solution of Wittig reagent 11a,17 11b18 or 11c23
(1.5 mmol) and dialdehyde 1021 (1.5 mmol) in dry THF (50 ml)
under argon at 20 °C was added dry triethylamine (1 ml,
excess) and stirring was continued for 16 h. After evaporation
of the solvent in vacuo, the residue was extracted with CH Cl
(E,E)-1-[4,5-Bis(methoxycarbonyl)-1,3-dithiol-2-ylidene]-
7,7-dicyanohepta-2,4,6-triene 14c. Eluting with CH Cl
(219 mg, 87%) as a deep red solid, mp 213–214 °C (Analysis
2
2
2
2
found: C, 53.4; H, 3.5; N, 7.6; C
H N O S requires: C, 53.3;
(250 ml) and washed with water (3×100 ml). The organic
layer was dried (MgSO ) and the solvent evaporated in vacuo.
16 12
2 4 2
H, 3.3; N, 7.7%); m/z (CI) 361 (M++1, 100%); d (CDCl )
H
3
4
7.39 (H , d, J 12.0), 6.92 (H , dd, J 11.6, J 14.1), 6.67
Purification of the products was achieved by column chroma-
tography. The following were thus obtained.
F
EF
D
DC
DE
(H , dd, J 14.1, J 12.0), 6.50 (H , dd, J 11.5, J 13.9),
ED EF BA BC
6.22 (H , d, J 11.5), 6.21 (H , dd, J 13.9, J 11.6), 3.87
AB CB CD
(6H, s); d (CDCl ) 159.1, 159.0, 158.8, 147.9, 142.8, 138.8, 133.0,
E
B
A
C
(E,E)-6-(4,5-Dimethyl-1,3-dithiol-2-ylidene)hexa-2,4-dienal
13a. Elution with CH Cl (150 mg, 45%) as a red solid, mp
C
3
131.1, 127.3, 125.1, 114.0, 112.1, 109.7, 79.8, 53.8, 53.7;
n
2
2
(KBr)/cm−1 2217 (CON), 1717 and 1700 [both
108–110 °C (Analysis found: C, 59.0; H, 5.4; C H OS
max
11 12
2
C(NO)–O], 1566 (CNC); l (MeCN)/nm (e) 492 (7.5×104),
requires: C, 58.8; H, 5.4%); m/z (DCI) 225 (M++1, 100%);
15.0), 6.57 (H , dd, J 12.0, J 11.2)., 6.21–6.02 (H , H ,
max
302 (1.0×104), 252 (1.0×104).
d (CDCl ), 9.50 (H , d, J 8.1), 7.14 (H , dd, J 11.5, J
H
3
F
EF
D
DC
DE
C
B
BA
BC
A
(E,E)-1,6-Bis(4,5-dimethyl-1,3-dithiol-2-ylidene)hexa-2,4-
diene 15a
H , m); n (KBr)/cm−1 1667 (CNO) 1603, 1567, 1501 (all
E
max
max
CNC); l (MeCN)/nm (e) 451 (1.5×104), 280 (9.9×103), 229
(6.4×104).
To a solution of Wittig reagent 11a17 (410 mg, 0.9 mmol) and
aldehyde 13a (50 mg, 0.23 mmol) in dry THF (50 ml) at
−78 °C was added LDA (1  in cyclohexane, 0.6 ml, 0.9 mmol)
and the solution allowed to attain room temperature over
16 h. The solvent was evaporated and the residue extracted
into toluene (100 ml), washed with water (100 ml) and the
(E,E)-[4,5-Bis(methylsulfanyl)-1,3-dithiol-2-ylidene]hexa-
2,4-dienal 13b. Elution with CH Cl (341 mg, 79%) as a deep
2
2
red solid, mp 95–96 °C (Analysis found: C, 45.8; H, 4.4;
C
100%); d (CDCl ) 9.52 (H , d, J 8.1), 7.12 (H , dd, J
H OS requires: C, 45.8; H, 4.2%); m/z (CI) 289 (M++1,
11 12
4
H
organic phase dried (MgSO ). After evaporation of the solvent,
the residue was chromatographed (neutral alumina, 70–230
3
F
EF
D
DC
4
11.1, J 15.0), 6.52 (H , dd, J 11.6, J 14.1), 6.20 (H , dd,
DE BA BC
14.1, J 11.1), 6.13 (H , d, J 11.6), 6.09 (H , dd, J
CD AB ED
15.0, J 8.1), 2.41 (3H, s), 2.40 (3H, s); d (CDCl ) 193.2, 152.2,
EF
143.3, 137.5, 129.6, 127.8, 126.7, 125.4, 113.2, 19.0, 18.9;
B
C
J
mesh) eluting with hexane–toluene (651 v/v) to afford com-
pound 15a as a black solid (30 mg, 40%); mp 170–173 °C
CB
A
E
C
3
(Analysis found: C, 57.0; H, 5.3; C
H S requires: C, 56.8; H,
14 18 4
n
l
(KBr)/cm−1 1670 (CNO), 1595 and 1573 (both CNC);
5.3%); m/z (DCI) 339 (M++1, 100%); d (CDCl ) 6.00–5.98
max
max
H
3
(MeCN)/nm (e) 438 (2.1×104), 293 (7.6×103), 229
(6H, m), 1.92 (6H, s) 1.89 (6H, s); l (MeCN)/nm (e) 476
max
(4.4×103).
(2.9×104), 452 (2.0×104), 270 (9.9×103), 215 (6.4×103).
(E,E)-6-[4,5-Bis(methoxycarbonyl)-1,3-dithiol-2-ylidene]-
hexa-2,4-dienal 13c. Initial elution with CH Cl afforded com-
pound 15c (108 mg, 14%); continued elution with
General procedure for compounds 15b–c
2
2
To a stirred solution of Wittig reagent 11c23 or 11d28 (2.0 mmol)
and dialdehyde 1021 (0.9 mmol) in dry THF (50 ml) under
argon at 20 °C was added dry triethylamine (1.5 ml, excess)
and stirring was continued for 16 h. After evaporation of the
solvent in vacuo the residue was chromatographed to afford
the products. The following were thus obtained.
CH Cl –acetone mixture (1051 v/v) afforded the product 13c
2
2
(252 mg, 54%) as an orange solid, mp 147–148 °C (Analysis
found: C, 51.3; H, 3.7; C H O S requires: C, 50.0; H, 3.9%);
13 12 5 2
m/z (CI) 313 (M++1, 100%); d (CDCl ) 9.55 (H , d, J 8.0),
14.3), 6.25 (H , dd, J 14.3, J 11.1), 6.16 (H , d, J 11.2),
H
3
F
EF
7.11 (H , dd, J 15.1, J 11.1), 6.47 (H , dd, J 11.2, J
D
DE
DC
B
BA
BC
C
CB
CD
A
AB
6.10 (H , dd, J 15.1, J 8.0), 3.86 (6H, s); n (KBr)/cm−1
(E,E)-1,6-Bis[4,5-bis(methylsulfanyl)-1,3-dithiol-2-
ylidene]hexa-2,4-diene 15b. Elution with CH Cl –hexane (151
v/v) (285 mg, 68%) as a dark orange solid, mp 165–167 °C
E
ED
EF
max
1730 and 1702 [both C(NO)–O], 1664 (CNO), 1603 (CNC),
2
2
1581 (CNC);
l
(MeCN)/nm (e) 410 (2.9×104), 264
max
(8.2×103), 229 (7.8×103).
(Analysis found: C, 41.0; H, 4.0; C
H S requires: C, 41.2; H,
16 18 8
3.9%); m/z (CI) 467 (M++1, 100%); d (CDCl ) 6.05–5.85
129.0, 127.8, 126.6, 125.7, 125.5, 124.4, 124.3, 115.3, 115.1, 111.1,
H
3
General procedure for compounds 14b–c
(6H, m), 2.34 (6H, s), 2.32 (6H, s); d (CDCl ) 132.1, 129.5,
C
3
To a stirred solution of aldehyde 13b–c (0.7 mmol) in dry
CH Cl (50 ml) under argon at 20 °C were added sequentially:
(i) malononitrile (66 mg, 1 mmol), (ii) titanium tetrachloride
18.9, 18.8; l (MeCN)/nm (e) 456 (2.6×104), 430 (2.2×104),
max
2
2
273 (8.5×103), 213 (8.6×103). A crystal suitable for X-ray
analysis was grown by slow evaporation of its MeCN solution.
(1.0 ml, 0.75 mol, 1  in CH Cl ) and (iii) dry pyridine (1 ml,
excess) and the mixture was then refluxed for 24 h. After
cooling, the mixture was diluted with CH Cl (250 ml) and
2
2
Continued elution with CH Cl afforded compound 13b
(47 mg, 18%).
2
2
2
2
washed with water (3×100 ml). After drying (MgSO ), the
solvent was evaporated in vacuo and the residue was chromato-
4
(E,E)-1,6-Bis[4,5-bis(methoxycarbonyl)-1,3-dithiol-2-
ylidene]hexa-2,4-diene 15c. Elution with CH Cl (310 mg,
graphed to afford the products. The following were thus
obtained.
2
2
66%) as a dark orange solid, mp 186–188 °C (Analysis found:
C, 46.5; H, 3.6; C O S requires: C, 46.7; H, 3.5%); m/z
H
20 18 8 4
(E,E)-1-[4,5-Bis(methylsulfanyl)-1,3-dithiol-2-ylidene]-7,7-
dicyanohepta-2,4,6-triene 14b. Eluting with CH Cl –hexane
(CI) 515 (M++1, 100%); d (CDCl ) 6.05–5.97 (6H, m), 3.83
H
3
(6H, s) 3.81 (6H, s); n (KBr)/cm−1 1749, 1730 and 1702 [all
2
2
max
(251 v/v) (101 mg, 43%) as a deep red solid, mp 160–162 °C
(Analysis found: C, 50.1; H, 3.7; N, 8.2; C N S requires:
C(NO)–O], 1598 and 1563 (both CNC); l (MeCN)/nm (e)
max
H
433 (9.6×104), 410 (8.5×104), 388 (5.0×104), 239 (1.5×104).
14 12 2 4
C, 50.0; H, 3.6; N, 8.3%); m/z (CI) 337 (M++1, 100%;
Continued elution with CH Cl –acetone (1051 v/v) afforded
compound 13c (40 mg, 14%).
2
2
d (CDCl ) 7.42 (H , d, J 11.9), 6.95 (H , dd, J 11.5, J
H
3
F
EF
D
DC
DE
1182
J. Mater. Chem., 1998, 8(5), 1173–1184

Chem., 1995; 5, 365; (e) T. J. Marks and M. A. Ratner, Angew.
Chem., Int. Ed. Engl., 1995, 34, 155.
X-Ray crystallography
Single-crystal X-ray diffraction experiments were carried out
on a Rigaku AFC6S 4-circle diffractometer (7b, 15b) or a
Siemens 3-circle SMART diffractometer with a CCD area
detector (8b), using graphite monochromators and Oxford
2
Reviews: (a) M. Narita and C. V. Pittman, Synthesis, 1976, 489;
(b) A. Krief, T etrahedron, 1986, 42, 1237; (c) G. Schukat, A. M.
Richter and E. Fanghanel, Sulfur Rep., 1987, 7, 155; (d) G. Schukat
¨
J. Becher, Adv. Mater., 1993, 5, 288.
¨
and E. Fanghanel, Sulfur Rep., 1993, 13, 254; (e) T. K. Hansen and
Cryosystems open-flow N gas cooling devices. The structures
were solved by direct methods and refined by full-matrix least
3
4
5
R. Gompper, H.-U. Wagner and E. Kutter, Chem. Ber., 1968, 101,
4123.
2
squares against F2 on all data, using SHELXTL software.42
Non-H atoms were refined anisotropically; all H atoms in 7b
and 15b were refined isotropically; in 8b methyl groups were
refined as rigid bodies, other H atoms were treated as ‘riding’.
Highest peaks of the residual electron density in all structures
occur near sulfur atoms. Crystal data and experimental details
are listed in Table 1; atomic coordinates and thermal param-
eters, bond distances and angles 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 1145/88.
H. E. Katz, K. D. Singer, J. E. Sohn, C. W. Dirk, L. A. King and
H. M. Gordon, J. Am. Chem. Soc., 1987, 109, 6561.
(a) M. Blanchard-Desce, I. Ledoux, J.-M. Lehn, J. Malthete
and J. Zyss, J. Chem. Soc., Chem. Commun., 1988, 737;
(b) M. Barzoukas, M. Blanchard-Desce, D. Josse, J.-M. Lehn and
J. Zyss, Chem. Phys., 1989, 133, 323.
A. K.-Y. Jen, V. P. Rao, K. J. Drost, K. Y. Wong and M. P. Cava,
J. Chem. Soc., Chem. Commun., 1994, 2057.
M. Guillemet, J. M. Raoul, F. Pelle´, A. Robert and
M. Baudy-Floc’h, J. Mater. Chem., 1995, 5, 35.
C.-S. Kang, C. Heldmann, H.-J. Winkelhahn, M. Schulze,
6
7
8
D. Neher, G. Wegner, R. Wortmann, C. Glania and P. Kramer,
¨
Macromolecules, 1994, 27, 6156.
9
(a) A. I. de Lucas, N. Mart´ın, L. Sanchez, C. Seoane, J. Gar´ın,
J. Orduna, R. Alcala´ and B. Villacampa, T etrahedron L ett., 1997,
38, 6107; (b) S. Inoue, Y. Aso and T. Otsubo, Chem. Commun.,
1997, 1105; (c) K. Kondo, T. Fujitani and N. Ohnishi, J. Mater.
Chem., 1997, 7, 429.
Non-linear optical measurements
b Measurements were obtained in chloroform, the fundamental
wavelength was 1.34 mm, and the source was a Nd3+ YAG Q-
switched, mode-locked laser, emitting 90 ns duration pulse
trains, the duration of each pulse being 160 ps. To complete
these measurements the dipole moments were separately deter-
mined in chloroform solution with a WTW dipole meter
coupled with an Abbe refractometer, in order to measure the
relative permittivity and the refractive index of the solutions.
From these experimental data the dipole moment was deduced
using the Guggenheim model.44 Static b(0) values were deduced
from the experimental values using a two-level dispersion
model [eqn. (3)]
10 For reviews of dicyanomethylene and N-cyanoimine groups in
materials chemistry see: (a) S. Hunig, J. Mater. Chem., 1995, 5,
¨
1469; (b) N. Mart´ın, J. L. Segura and C. Seoane, J. Mater. Chem.,
1997, 7, 1661.
11 R. Mayer and B. Gebhardt, Chem. Ber., 1964, 97, 1298.
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b(0)=b(l)[1−(2l /2l)2][1−(l /l)2]
(3)
0
(l being the fundamental wavelength and l the first maximum
0
0
absorption wavelength).
Computational details
18 A. J. Moore and M. R. Bryce, T etrahedron L ett., 1992, 33, 1373.
19 A. Aumuller and S. Hunig, Angew. Chem., Int. Ed. Engl., 1984,
¨
¨
The calculations were performed using the GAUSSIAN 9445
system of programs on IBM RS/6000 workstations and on a
SGI Power Challenge L R8000 computer at the Department
of Qu´ımica F´ısica of the University of Valencia. All the
calculations were carried out at the DFT level using the hybrid
gradient-corrected B3P86 density functional46 and the 6-31G*
basis set,47 which includes polarisation d functions on sulfur,
nitrogen and carbon atoms. The Berny analytical gradient
method48 was used in all the geometry optimisations. The
requested convergence on the density matrix was 10−8 and the
threshold values for the maximum force and the maximum
displacement were 0.00045 and 0.0018 atomic units, respect-
ively. All the molecules were restricted to be planar during
geometry optimisation.
23, 447.
20 B. J. Adger, C. Barrett, J. Brennan, M. A. McKervey and
R. W. Murray, J. Chem. Soc., Chem. Commun., 1991, 1553.
21 B. J. Adger, C. Barrett, J. Brennan, P. McGuigan, M. A. McKervey
and B. Tarbit, J. Chem. Soc., Chem. Commun., 1993, 1220.
22 B. Golding, G. Kennedy and W. P. Watson, T etrahedron L ett.,
1988, 29, 5991.
23 N. C. Gonella and M. P. Cava, J. Org. Chem., 1979, 44, 930.
24 (a) Z. Yoshida, T. Kawase, H. Awaji, I. Sugimoto, T. Sugimoto and
S. Yoneda, T etrahedron L ett., 1983, 24, 3469; (b) Z. Yoshida,
H. Awaji and T. Sugimoto, T etrahedron L ett., 1984, 25, 4227;
(c) T. K. Hansen, M. V. Lakshmikantham, M. P. Cava, R. M.
Metzger and J. Becher, J. Org. Chem., 1991, 56, 2720; (d) A. J.
Moore, M. R. Bryce, D. J. Ando and M. B. Hursthouse, J. Chem.
Soc., Chem. Commun., 1991, 329; (e) T. K. Hansen, M. V.
Lakshmikantham, M. P. Cava and J. Becher, J. Chem. Soc., Perkin
T rans. 1, 1991, 2873; ( f ) M. R. Bryce, M. A. Coffin and W. Clegg,
J. Org. Chem., 1992, 57, 1696; (g) D. Lorcy, R. Carlier, A. Robert,
A. Tallec, P. Le Magueres and L. Ouahab, J. Org. Chem., 1995, 60,
2443; (h) Y. Misaki, T. Ohta, N. Higuchi, H. Fujiwara, T. Yamabe,
T. Mori, H. Mori and S. Tanaka, J. Mater. Chem., 1995, 5, 1571;
(i) Review; M. R. Bryce, J. Mater. Chem., 1995, 5, 1481; ( j) M. R.
Bryce, A. J. Moore, B. K. Tanner, R. Whitehead, W. Clegg,
F. Gerson and A. Lamprecht, Chem. Mater., 1996, 8, 1182; (k)T.-T.
Nguyen, Y. Gouriou, M. Salle´, P. Fre`re, M. Jubault, A. Gorgues,
L. Youpet and A. Riou, Bull. Soc. Chim. Fr., 1996, 133, 301.
25 (a) M. R. Bryce, A. J. Moore, M. Hasan, G. J. Ashwell, A. T. Fraser,
W. Clegg, M. B. Hursthouse and A. I. Karaulov, Angew. Chem.,
Int. Ed. Engl., 1990, 29, 1450; (b) J. Dong, K. Yakushi and
Y. Yamashita, J. Mater. Chem., 1995, 5, 1735.
We thank the EPSRC and Great Lakes Chemicals (Europe)
for funding the work in Durham and Belfast. The work in
Valencia was supported by the CICYT Grant PB95-0428-C02-
02. We are grateful to the Leverhulme Trust for a scholarship
(to A.S.B.) and the Royal Society for Leverhulme Senior
Research Fellowship (to J.A.K.H.).
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Paper 7/08078F; Received 10th November, 1997
1184
J. Mater. Chem., 1998, 8(5), 1173–1184