Combining high electron affinity and intramolecular charge transfer in 1,3-dithiole-nitrofluorene push-pull diads

Article abstract of DOI:10.1002/chem.200701459

Attaching electron-rich 1,3-dithiol-2-ylidene moieties to polynitrofluorene electron acceptors leads to the formation of highly conjugated compounds 6 to 11, which combine high electron affinity with a pronounced intramolecular charge transfer (ICT) that is manifested as an intense absorption band in their visible spectra. Such a rare combination of optical and electronic properties is beneficial for several applications in optoelectronics. Thus, incorporation of fluorene-dithiole derivative 6 a into photoconductive films affords photothermoplastic storage media with dramatically increased photosensitivity in the ICT region. A wide structural variation of the dithiole and fluorene parts of the molecules reveals excellent correlation between the ICT energy and the reduction potential with the Hammett's parameters for the substituents. Although only a small solvatochromism of the ICT band was observed, heating the solution led to a pronounced blueshift, which was probably as a result of increased twisting around the C9=C14 bond that links the fluorene and dithiole moieties. X-ray crystallography analysis of 7 a, 8 a, 10 a, 11a and 13 a confirms an ICT interaction in the ground state of the molecules. The C9=C14 double bond between the donor and acceptor is substantially elongated and its length increases as the donor character of the dithiole moiety is enhanced.

Full text of DOI:10.1002/chem.200701459

FULL PAPER
DOI: 10.1002/chem.200701459
Combining High Electron Affinity and Intramolecular Charge Transfer in
1,3-Dithiole–Nitrofluorene Push–Pull Diads**
[
a]
[b, c]
[a]
Dmitrii F. Perepichka,* Igor F. Perepichka,*
Oleksandr Ivasenko,
[d]
[
c, f]
[
[c]
Adrian J. Moore,
Martin R. Bryce,* LyudmilaG. Kuzꢀmina,
Andrei S. Batsanov, and Nikolai I. Sokolov
c]
[e]
Abstract: Attaching electron-rich 1,3-
dithiol-2-ylidene moieties to polynitro-
fluorene electron acceptors leads to the
formation of highly conjugated com-
pounds 6 to 11, which combine high
electron affinity with a pronounced in-
tramolecular charge transfer (ICT)that
is manifested as an intense absorption
band in their visible spectra. Such a
rare combination of optical and elec-
tronic properties is beneficial for sever-
al applications in optoelectronics. Thus,
incorporation of fluorene–dithiole de-
rivative 6a into photoconductive films
affords photothermoplastic storage
media with dramatically increased pho-
tosensitivity in the ICT region. A wide
structural variation of the dithiole and
fluorene parts of the molecules reveals
excellent correlation between the ICT
energy and the reduction potential
with the Hammettꢀs parameters for the
substituents. Although only a small sol-
vatochromism of the ICT band was ob-
served, heating the solution led to a
pronounced blueshift, which was prob-
ably as a result of increased twisting
around the C9=C14 bond that links the
fluorene and dithiole moieties. X-ray
crystallographic analysis of 7a, 8a, 10a,
11a and 13a confirms an ICT interac-
tion in the ground state of the mole-
cules. The C9=C14 double bond be-
tween the donor and acceptor is sub-
stantially elongated and its length in-
creases as the donor character of the
dithiole moiety is enhanced.
Keywords: charge transfer · donor–
acceptor systems
· electron-defi-
cient compounds · photoconductive
materials · redox chemistry
[
a] Prof. D. F. Perepichka, O. Ivasenko
[e] N. I. Sokolov
Department of Chemistry, McGill University
Laboratory of Holography, Faculty of Natural Sciences
8
01 Sherbrooke Street West, Montreal H3A 2K6, QC (Canada)
University “Kievo-Mogilyanskaya Academy”
Kiev 04145 (Ukraine)
Fax : (+1)514-398-3797
E-mail: dmitrii.perepichka@mcgill.ca
[
f] Dr. A. J. Moore
[
b] Dr. I. F. Perepichka
University of Sunderland, Fleming Building
Wharncliffe Street, Sunderland SR1 3SD (UK)
Centre for Materials Science
Faculty of Science and Technology
University of Central Lancashire
Preston PR1 2HE (UK)
Fax : (+44)1772-89-4981
E-mail: iperepichka@uclan.ac.uk
[
**] Electron Acceptors of the Fluorene Series, Part 15; for Part 14 see:
D. F. Perepichka, M. R. Bryce, I. F. Perepichka, S. B. Lyubchik, N.
Godbert, C. A. Christensen, A. S. Batsanov, E. Levillain, E. J. L.
McInnes, J. P. Zhao, J. Am. Chem. Soc. 2002, 124, 14227–14238
Supporting Information for this article is available on the WWW
under http://www.chemistry.org from the authors. It contains the ther-
mochromic behaviour of 11a; additional correlation plots; orbital
[
c] Dr. I. F. Perepichka, Dr. A. J. Moore, Prof. M. R. Bryce,
Dr. A. S. Batsanov
Department of Chemistry, Durham University
South Road, Durham DH1 3LE (UK)
Fax : (+44)191-384-4737
energy diagrams for compounds 8 and 11; colour versions of Figures 3,
1
4
, and 7; H NMR of 9a; the results of elemental and mass spectral
analyses for compounds 6 to 11; and their DFT optimised geometries
XYZ coordinates).
E-mail: m.r.bryce@durham.ac.uk
(
[
d] Dr. L. G. Kuzꢀmina
Institute of General and Inorganic Chemistry
Russian Academy of Sciences
Moscow 119991 (Russian Federation)
Chem. Eur. J. 2008, 14, 2757 – 2770
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2757

Introduction
sible reduction waves in cyclic voltammetry (CV)experi-
ments, which reveals their extraordinary electron-acceptor
[18]
Conjugated donor–p-acceptor molecules (D–p-A)with
strong electronic coupling between the D and A moieties
have been of general interest for a number of electronic and
optoelectronic applications. Large second-order polarisabili-
ty and electro-optic effects render them the most promising
non-linear optical and electro-optical materials for telecom-
capacity. The 1,3-dithiole heterocyclic ring, a structural el-
ement of the famous p-electron donor tetrathiafulvalene
(TTF), is also a promising building block for different types
of supramolecular systems and a terminal donor moiety in
[19]
D–p-A non-linear optical chromophores.
We suggested that combining these two structural blocks
(dithiole and nitrofluorene)would yield D– p-A molecules
with interesting electronic properties. An important feature
of such a combination is that full intramolecular charge
[
1–4]
munications.
usually incorporating several D–p-A moieties)have been
explored for their large two- and three-photon absorption
More elaborate structures in this family
(
[5–7]
+
ꢀ
cross-sections,
which are important properties for several
transfer, which results in a zwitterionic state D –p-A ,
would cause aromatisation of both donor and acceptor moi-
eties (6 and 14e systems, respectively; resonance structure
1b). This aromatisation would stabilise the charge-separated
excited state, which is of paramount importance for optoe-
lectronic applications. The electronic properties of such
diads could be tuned across a wide range by the synthetical-
ly convenient introduction of electron-donating and elec-
tron-withdrawing groups (EDG and EWG, respectively).
applications, such as optical limiting and fluorescent imag-
ing. Recently, D–p-A derivatives have also been extensively
studied in molecular electronics, particularly for use as un-
[
8,9]
imolecular rectifiers.
Donor–acceptor (HOMO–LUMO)interactions in such
molecules usually result in intramolecular charge-transfer
ICT)absorption bands in the visible or near-IR region of
their electronic spectra. The strong sensitivity of the ICT
band to variations in the molecular structure and media
makes D–p-A compounds convenient model systems in
physical organic chemistry. Thus, solvatochromism in one
such molecule, the dye 4-hydroxyphenylpyridinium, was
(
used to establish the solvent polarity scale E . At the same
T
time, a strong HOMO–LUMO interaction in the majority of
such molecules makes them neither strong electron donors
nor acceptors. It is, however, desirable for several applica-
tions to combine a pronounced electron affinity and strong
light absorption in the same compound. A possible ap-
proach to achieve this combination would be to use a p-ex-
tended electron-acceptor moiety with numerous electron-
withdrawing substituents, which could maintain an electron-
deficient character in the presence of the ICT interaction.
Electron acceptors in the fluorene series are known for
their unique ability to sensitise photoconductivity of poly-N-
vinylcarbazole-type (PVK)semiconductive polymers and re-
Previously we reported some representatives of the gener-
al structure 1 and showed their ability to sensitise the photo-
[
20,21]
conductivity of polymers that contain carbazole.
The
present work describes the synthesis and detailed studies of
the structural effects in both fluorene and dithiole rings on
the spectral and electrochemical properties of these new
donor–acceptor diads, as well as on their supramolecular or-
ganisation in the solid state. Electrochemical and spectro-
scopic data have been supported by theoretical DFT calcu-
lations. We also demonstrate the remarkable stability and
great potential of these materials in optoelectronic applica-
tions, that is, very high efficiency of these compounds as sen-
sitisers of the conductivity of poly[N-(2,3-epoxypropyl)car-
bazole] (PEPK)in optical data storage processes (e.g., pho-
[10–13]
lated amorphous hole-transporting materials.
cess is based on the formation of a charge-transfer complex
CTC)between the fluorene acceptor and a carbazole
The pro-
(
donor, which is highly efficient because of the great structur-
al similarity between these components. However, the rela-
tively weak intensity of the charge-transfer band (e~1000–
ꢀ
1
ꢀ1[14]
2
000m cm
)affects the efficiency of the photosensitisa-
[11,22]
tion. Some improvements in photosensitisation were found
when a nitrofluorene was covalently linked to a fullerene ac-
tothermoplastic hologram recording
).
[15]
ceptor with an intrinsic absorption in the visible region.
CTCs of dendrimers that contain nitrofluorene and carba-
zole were used to design columnar liquid crystals with im-
Results and Discussion
[16]
proved charge-transport characteristics.
Very recently, a
Synthesis: Nitrosubstituted fluorenes 4a–g have been pre-
trinitrofluorenone acceptor moiety was used together with a
hexabenzocoronene donor core to build supramolecular
pared by nitration of fluorene or its 4-substituted derivatives
with nitric acid, or a mixture of nitric acid with acetic or sul-
[17]
[20,23–25]
nanotubes with substantial axial photoconductivity.
The
furic acids, as previously described.
As strong CꢀH
[26]
electron affinity of the strongest fluorene acceptor, 9-dicya-
nomethylene-2,4,5,7-tetranitrofluorene, is close to that of
acids, derivatives 4a–g readily participate in condensation
reactions with various electrophiles, which include aldehy-
[4b,27]
[26]
[20,21,28]
7,7,8,8-tetracyano-p-quinodimethane (TCNQ); moreover, 9-
des,
amides,
and dithiolium salts.
Thus, treat-
dicyanomethylene–polynitrofluorenes have up to four rever-
ment of nitrosubstituted fluorenes 4a–g with dithiolium salts
2758
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ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 2757 – 2770

Push–Pull Diads
FULL PAPER
3
A–F, which were obtained by methylation of the corre-
sponding 1,3-dithiol-2-thiones 2A–F with methyl trifluoro-
[29]
sulphonate, results in the formation of 9-(1,3-dithiol-2-yli-
dene)fluorenes 6–11 (Scheme 1). The reaction proceeds
Scheme 2. Protonation of 1,3-dithiol-2-ylidene-fluorene conjugates 6 to
1
1 in strong acids.
ble those of the corresponding 9H-fluorenes (Figure 1). Re-
ducing the acidity of the solution shifts the equilibrium to-
wards non-protonated forms 6 to 11 (Scheme 2), which leads
Scheme 1. Synthesis of 9-(1,3-dithiol-2-ylidene)fluorenes 6 to 11 by con-
densation of nitrofluorenes 4 with dithiolium salts 3.
Figure 1. Electronic absorption spectra of 7a in mixtures of H
acid with various concentrations of H SO , a)94, b)62, c)45, d)35 and
e)29% H SO . The absorbance is corrected for the dilution.
2 4
SO /acetic
2
4
2
4
smoothly in N,N-dimethylformamide (DMF)with no addi-
tional catalyst (in contrast with common base-catalysed re-
actions of activated methylene compounds)and its rate posi-
tively correlates with the electron-withdrawing strength of
substituents in the fluorene nucleus. Thus, for 4a the reac-
tion is completed within 1 h at room temperature, whereas
to a gradual restoration of the ICT bands upon dilution of
the solution in H SO with acetic acid (Figure 1). The basici-
2
4
ty of diads 6 to 11 decreased with the increasing electron-
withdrawing ability of substituents X and Y, which destabil-
ises the dithiolium cation. For dicarbomethoxy-substituted
11a only a partial protonation was observed in concentrated
H SO , as revealed by a residual ICT absorption in the UV/
4
g typically requires over 24 h at 65 to 708C. Compounds 6
to 11 are very intensely coloured (red to black), air- and
light-stable solids. No signs of decomposition were observed
after storage for several years on the bench or after heating
to the melting points (typically, above 250–3008C).
2
4
Vis spectra. The protonation can be followed by a downfield
shift of the proton signals in the NMR spectra. For partially
protonated compound 11a, two sets of signals that corre-
spond to the neutral and protonated forms, were observed
(see the NMR spectroscopy data given in Table 8).
Compounds 7 to 11 have a limited solubility in most or-
ganic solvents (moderate for derivatives that contain carbo-
methoxy substituents). Thus, long-chain-substituted deriva-
tive 6a was specifically synthesised to improve the processa-
bility for device applications. Compounds 7 to 11, however,
all dissolve in concentrated sulfuric and trifluoromethanesul-
fonic acids. This results in a reversible protonation of the
molecules at the C9 position of the fluorene nucleus to form
cations 6’ to 11’, which have a positive charge delocalised
Electrochemistry: The electrochemical behaviour of com-
pounds 6 to 11 was studied by CV in either dichloromethane
or N,N-dimethylacetamide (DMA)with 0.1 m Bu NPF as
4
6
the supporting electrolyte. The presence of both electron-ac-
ceptor (nitrofluorene)and electron-donor (dithiole)frag-
ments gives compounds 6 to 11 highly amphoteric multire-
dox behaviour, with up to four single-electron reduction
waves (the first two or three reduction processes are reversi-
ble)and up to two single-electron oxidations (Figure 2,
Table 1). It is quite unusual for organic molecules to show
[30]
[20]
over an aromatic dithiolium ring
(Scheme 2).
Similar
behaviour was demonstrated by other 1,3-dithiole deriva-
[31]
tives, notably TTFs. Such protonation eliminates the pre-
requisite conditions for ICT (donor character of the dithiole
moiety, conjugation path), which affords 6’ to 11’ as almost
colourless salts with UV/Vis absorption spectra that resem-
Chem. Eur. J. 2008, 14, 2757 – 2770
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2759

D. F. Perepichka, I. F. Perepichka, M. R. Bryce et al.
Figure 2. Cyclic voltammograms for tetranitro derivative 6a and trinitro-
carbomethoxy derivative 7c in CH Cl that contains 0.1m Bu NPF (scan
2
2
4
6
ꢀ
1
rate 100 mVs ).
red1
red2
Figure 3. Dependence of the E1=2 (filled shapes)and E1=2 (open shapes)
reduction potentials of series 7 (circles), 8 (squares)and 11 (triangles)
versus the sum of the s constants in the fluorene moiety. Lines show the
p
five to seven distinct redox states (neutral, radical anion,
dianion, radical trianion, tetraanion, radical cation and di-
cation), although other examples (fullerenes, D–s-A diads)
linear fits of the experimental data from Table 1 (for correlation parame-
ters see Table 2).
[32]
are known.
red1
red2
The observed first and second reduction potentials are in
good linear correlation with the sum of the Hammett con-
The potentials E1=2 and E1=2 both showed good linear cor-
A
relations (correlation coefficient r=0.980–0.993)on ꢀsp
A
stants of the substituents in the benzene rings of the fluo-
with sensitivity parameters of 1 ꢁ0.35 for red1 and ꢁ0.30
A
p
A
rene moiety (s ) as shown by Equation (1), (Figure 3).
for red2 processes (Table 2). The 1 values were essentially
independent of the substituents in the dithiole ring (cf.
series 7, 8 and 11; Table 2), which means that there is no
cross-correlation between the variations of the fluorene and
the dithiole structures.
In the reduced (anionic)states, the negative charge
should be localised almost exclusively on the fluorene frag-
ment, which is also in agreement with LUMO orbital locali-
sation on the fluorene moiety (see the Theoretical studies
X
0
1=2
A
A
p
E1=2 ¼ E þ1
s
ð1Þ
0
1=2
in which E1/2 is the observed half-wave redox potentials, E
is the E1/2 value of the reference compound (unsubstituted
benzene rings in the fluorene nuclei, ꢀs =0), and 1 is a
sensitivity parameter of the redox potential to substituents
effects.
A
p
A
[
a]
2 2
Table 1. CV data for solutions of compounds 6 to 11 in DMA and CH Cl .
ox1
=2
red1
1=2
(A!AC )
red2
red3
1=2
red4
pa
[b]
[b]
4[c]
Solvent
E1
E
E
E
E
HOMO
[eV]
LUMO
[eV]
K
dispr ”10
]
1=2
+
ꢀ
ꢀ
2ꢀ
(A2ꢀ!AC3ꢀ
(AC3ꢀ!A4ꢀ
ꢀ1
A
C
H
T
R
E
U
N
G
(D!DC )
0.86, 1.22 ir
A
H
R
U
G
A
H
R
U
G
)
A
H
R
U
G
)
A
H
R
U
G
)
A
H
R
U
G
[
d]
6
6
7
7
7
7
7
7
7
8
8
8
8
8
9
1
1
1
1
1
a
a
a
b
c
c
d
e
f
a
b
c
d
e
a
CH
2
Cl
2
2
ꢀ0.91
ꢀ0.80
ꢀ0.83
ꢀ0.94
ꢀ1.11
ꢀ0.98
ꢀ1.12
ꢀ1.18
ꢀ1.28
ꢀ0.78
ꢀ0.90
ꢀ0.94
ꢀ1.08
ꢀ1.14
ꢀ0.76
ꢀ0.77
ꢀ0.76
ꢀ0.87
ꢀ1.05
ꢀ1.11
ꢀ1.10
ꢀ1.04
ꢀ1.07
ꢀ1.15
ꢀ1.21
ꢀ1.22
ꢀ1.35
ꢀ1.37
ꢀ1.43
ꢀ1.02
ꢀ1.11
ꢀ1.18
ꢀ1.29
ꢀ1.32
ꢀ1.01
ꢀ1.02
ꢀ0.98
ꢀ1.06
ꢀ1.25
ꢀ1.31
ꢀ1.91
ꢀ2.04
ꢀ2.01
ꢀ2.04
ꢀ1.89
ꢀ2.05
ꢀ2.02
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
ꢀ5.9
–
ꢀ4.1
ꢀ4.2
ꢀ4.2
ꢀ4.1
ꢀ3.9
ꢀ4.0
ꢀ3.9
ꢀ3.8
ꢀ3.7
ꢀ4.2
ꢀ4.1
ꢀ4.1
ꢀ3.9
ꢀ3.9
ꢀ4.2
ꢀ4.2
ꢀ4.2
ꢀ4.1
ꢀ3.9
ꢀ3.9
6.0
0.9
0.9
[
e]
DMA
DMA
DMA
–
0.85 ir
0.84 ir
0.82, 1.35 ir
0.79 ir
0.76 ir
0.73 ir
–
0.79 ir
0.77 ir
0.75 ir
0.73 ir
0.72 ir
ꢀ5.8
ꢀ5.8
ꢀ5.8
ꢀ5.8
ꢀ5.8
ꢀ5.7
–
ꢀ5.8
ꢀ5.8
ꢀ5.8
ꢀ5.7
ꢀ5.7
–
2.8
CH
2
Cl
ꢁ200.0
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
0.9
1.3
6.0
29.0
0.9
2.8
1.9
2.8
8.9
0.6
5.8
1.9
6.0
4.1
4.1
–
ꢀ2.00
ꢀ2.03
ꢀ2.05
ꢀ2.00
ꢀ1.53 ir
ꢀ1.99
ꢀ2.00
ꢀ1.79
ꢀ1.81
ꢀ1.83
ꢀ
0a
1a
1b
1d
1e
1.18 ir
ꢀ6.2
[
[
[
e]
e]
e]
–
–
–
–
ꢀ2.08
ꢀ2.10
ꢀ2.03
–
–
–
–
–
[
e]
ꢀ1.88
ꢀ
1
+
[
a] 0.1m Bu
4
NPF
6
was used as the supporting electrolyte (scan rate 100 mVs ). Potentials are given versus Fc/Fc . [b] Calculated from MO=ꢀ5.0ꢀE1/
+
[59]
ꢀ
2ꢀ
C ÐA+A , which was estimated from the equation for the single-elec-
2
(vs. Fc/Fc ). [c] Kdispr is the disproportionation constant in the equilibrium 2A
redð1ꢀ2Þ
red1
red2
1=2
[34]
+
2+
tron redox process, DE1=2
=E1=2 ꢀE =ꢀ0.059 log Kdispr
.
[d] ir: irreversible, values correspond to Epa. The second value is for the DC !D process
(
oxidation to dication). [e] The estimated uncertainties for these values are Æ0.05 V.
2760
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Chem. Eur. J. 2008, 14, 2757 – 2770

Push–Pull Diads
FULL PAPER
Table 2. Correlation parameters for the reduction potentials of com-
pounds 7, 8 and 11 from Equation (1).
state (a radical ion copper salt of 2,4,5,7-tetranitro-9-dicya-
nomethylenefluorene was isolated and analysed by X-ray
crystallography), all attempts to isolate or analyse the rad-
[
a]
0
1A [V]
[37]
Redox
process
red1
E
[eV]
r
s
0
n
1=2
ical cation salts of 7 and 8 by chemical oxidation with
7
7
8
8
1
1
a–f
a–f
a–e
a–e
1a,b,d,e
1a,b,d,e
E1
ꢀ1.998Æ0.072 0.366Æ0.027 0.989 0.028
ꢀ2.059Æ0.053 0.308Æ0.020 0.991 0.021
ꢀ1.923Æ0.112 0.354Æ0.041 0.980 0.033
ꢀ2.012Æ0.093 0.307Æ0.029 0.982 0.027
ꢀ1.877Æ0.122 0.348Æ0.046 0.984 0.036
ꢀ2.055Æ0.076 0.339Æ0.028 0.993 0.022
6
6
5
5
4
4
=2
red2
NOBF have failed. Nevertheless, the above-mentioned rad-
4
E1
=2
ical cations are reasonably stable on the CV timescale in sol-
vents such as CH Cl (Figure 2), and even the second (irre-
red1
E1
=2
red2
E1
2
2
=2
red1
E1
versible)oxidation peak was observed. Unfortunately, owing
to the low solubility of the other derivatives, the use of
highly nucleophilic DMA as the solvent was unavoidable,
and only irreversible oxidation peaks have been observed in
this solvent (Table 1).
=2
red2
E1
=2
[
(
a] The following Hammett constants were used, s
CN), 0.45 (CO
2
p
=0.78 (NO
Me), 0 (SMe) and ꢀ0.17 (Me); r is the correlation co-
0
is the standard deviation and n is the number of points used
2
), 0.66
[
38]
efficient, s
in the correlation.
Intramolecular charge transfer: electron-absorption spec-
troscopy studies: The intense colour of compounds 6 to 11 is
the result of intramolecular charge transfer (ICT)from the
donor dithiole moiety onto the acceptor nitrofluorene nu-
cleus, which can be visualised by resonance structure 1b.
The ICT manifests itself in a long-wavelength absorbance
band (ICT band)with maxima in the range of 470 to 610 nm
section). Therefore, the sensitivity of the reduction poten-
tials to substituents in the fluorene ring (1 )is five to ten
A
times higher than the corresponding sensitivity to substitu-
D
ents in the dithiole moiety (1 =0.04–0.07 V for both reduc-
[33]
ꢀ1
ꢀ1
tion waves).
and a high extinction coefficient (~10000m cm ). Not un-
The thermodynamic stability of the radical anion versus
the corresponding dianion is described by the equilibrium
expectedly, increasing the electron-withdrawing strength of
substituents R and R in the acceptor fluorene fragment
1
2
ꢀ
0
2ꢀ
2
AC ÐA +A with a disproportionation constant Kdispr. It
can be determined from CV experiments as E and Kdispr
(H<CO Me<CN<NO )and the electron-releasing ability
2
2
of substituents X and Y (CO Me<I<SMe<Me)in the
1
/2
2
[34]
are related through Equation (2):
donor dithiole fragment results in a pronounced bathochro-
mic shift of the ICT band. Remarkably, however, the energy
of the ICT band maxima (hnICT, eV)is in good linear de-
pendence with the sum of the Hammett parameters of the
red1
=2
red2
E1 ꢀE1=2 ¼ ꢀ0:059log Kdispr
ð2Þ
[38]
substituents (ꢀsp) in either acceptor [Eq. (3), see Table 3
for correlation parameters in series 7, 8 and 11] or donor
ꢀ1
For systems with Kdispr values greater than 10 , two redox
processes coalesce into a single two-electron redox wave in
CV experiments. We have previously observed a large varia-
tion in the thermodynamic stability of radical anions of fluo-
D
moieties [Eq. (4), sensitivity parameter 1 =0.226Æ0.030,
correlation coefficient r=0.967).
[
14]
X
rene-based acceptor molecules
as well as of radical cat-
0
ICT
A
A
p
hnICT ¼ hn þ1
s
s
ð3Þ
ð4Þ
[35]
ions of 1,3-dithiole-based donor molecules.
In the most
X
extreme case, the radical cation of a p-extended dithiole de-
0
D
D
p
hnICT ¼ hn þ1
ICT
rivative is completely destabilised, which gives rise to a truly
[35,36]
two-electron (inverted potentials)oxidation process.
In
A
D
dithiole–fluorene diads, a complete coalescence of the first
and second reduction waves has never been observed, al-
The sensitivity parameters, 1 and 1 , for an acceptor and a
donor series, respectively, have an opposite sign, but are of
almost equal absolute value within experimental error. Thus
the cross-correlation term is insignificant and the ICT
ꢀ
though the thermodynamic stability of species AC has a
slight tendency to decrease with a reduction in the electron-
withdrawing character of substituents in the fluorene moiety
p
A
p
D
energy correlates well with the ꢀs ꢀꢀs term [Eq. 5,
ꢀ
4
ꢀ4
(
cf. Kdisp =0.9”10 for 7a and 29”10 for 7 f, Table 1). A
Figure 4]. The correlation on Figure 4 incorporates the pre-
viously reported fluorene–di-
relative stabilisation of the radical anion was achieved in
polar solvents; the disproportionation constant is decreased
by over two orders of magnitude when low polarity CH Cl
[20]
thioles series 12a–d. Such re-
markably linear dependence of
the hnICT on the substituents
throughout almost the entire
visible range of the spectrum
(lmax =470–610 nm)allows fine
tuning of the absorption band,
which is important for tailoring
the sensitivity of the photocon-
ductive materials based on
these compounds (see below).
2
2
ꢀ
4
is replaced by highly polar DMA (e.g., K =0.9”10 and
00”10 for compound 7c in DMA and CH Cl , respec-
disp
ꢀ
4
2
2 2
tively).
A single-electron electrochemical oxidation of compounds
6
to 11 produces radical cation species, in which the positive
charge is expected to mainly localise on the dithiole frag-
ment (as the aromatic dithiolium cation)with the unpaired
electron residing on the fluorene moiety. Although this type
of radical was shown to be very stable in the radical anion
Chem. Eur. J. 2008, 14, 2757 – 2770
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2761

D. F. Perepichka, I. F. Perepichka, M. R. Bryce et al.
Table 3. Maxima of the ICT bands of compounds 6–11 in different sol-
vents and correlation parameters for the series 7, 8 and 11b from Equa-
tion (3).
(ꢁ0.05 eV)in different solvents (Table 3.) As for other
push–pull fluorene derivatives, the highest wavelength ab-
sorption was observed in highly polarisable (aromatic and
chlorinated)solvents, whereas the polarity of the media did
A
C
H
T
R
E
U
N
G
(CH
2
Cl)
2
DMF
Acetone
l
ICT [nm]
l
ICT [nm]
lICT [nm]
[27d]
ꢀ
1
ꢀ1
not have a major effect.
This suggests that there is a sig-
A
H
R
N
A
H
R
U
G
nificant ICT in the ground state (i.e., the contribution of res-
onance form 1b is high), with relatively small changes in the
dipole moment of the molecules during photoexcitation. An
interesting media effect is, however, a pronounced thermo-
chromic behaviour. Solutions of compounds 6 to 11 demon-
strate a hypsochromic shift of their ICT bands with increas-
ing temperature (Figure 5). The ICT band energy is inverse-
[
a]
6
7
7
7
7
7
7
8
8
8
8
8
8
9
1
1
1
1
1
1
1
a
a
590
611
589
580
554
540
–
–
[
b]
609.5
588
582
546.5
542
532
589
567
563
531.5
523.5
495
572.5
560
546
527.5
523
595 (8600)
[
b]
b
–
–
–
–
[
b]
c
[
b]
b]
b]
d
[
e
[
f
535.5
592
572
568
538
529
498
558
557
544
528.5
520
496
488
472
–
[
c]
a
b
578 (9700)
[
c]
–
–
–
–
–
–
–
[
c]
c
[
c]
c]
c]
d
[
e
g
a
0a
1a
1b
1c
1d
[
[
d]
535 (9800)
[
d]
–
–
–
–
–
[
d]
[
d]
d]
d]
495.5
–
474.5
[
1e
1g
[
[
2 2
a] Recorded in CH Cl . [b] Correlation parameters determined by
0
Eq. (3), in which R is the multiple correlation coefficient, hn =(2.83Æ
ICT
0
(
(
.05)((CH
2
Cl)
2
), (2.86Æ0.05)eV (DMF);
1
A
=(ꢀ0.249Æ0.020)
(CH Cl)
2
2
)
, ꢀ(0.259Æ0.018)eV (DMF); R=0.987 ((CH
2 2
Cl) ), 0.991
0
ICT
Figure 5. The effect of the temperature on the position of ICT bands in
solutions of 6a in 1,2-dichlorobenzene was observed (no correction for
the solution thermal expansion was made, which led to a minor decrease
in the observed absorbance with increasing temperature). The inset
shows photographs of solutions in chlorobenzene at 1008C (left)and
DMF). [c] hn =(2.88Æ0.03), ((CH
2
Cl)
2
), (2.91Æ0.04)eV (DMF);
1
A
=(ꢀ0.245Æ0.013)((CH
2
Cl)
2
) , ꢀ( 0.248Æ0.017)eV (DMF); R=0.994
0
ICT
(
0
(
(CH
2
Cl)
2
), 0.990 (DMF). [d] hn =(2.99Æ0.05)((CH
2
Cl)
2
), (2.95Æ
.07)eV (DMF);
1
A
=(ꢀ0.221Æ0.021)((CH
2
Cl)
2
) , ꢀ( 0.208Æ0.025)eV
2 2
DMF); R=0.981 ((CH Cl) ), 0.978 (DMF).
1
08C (right).
ly proportional to the temperature [Eq. (6)], and the tem-
perature sensitivity parameters a (ꢀ71 for 6a and ꢀ82 eVK
for 11a in chlorobenzene, see the Supporting Information)
are very similar to those we have previously observed for
[20]
other fluorene–dichalcogenole diads.
hnICT ¼ hn TI CðT1Þþa=T
ð6Þ
Similar behaviour is documented for conjugated oligo- and
polythiophenes, for which it is explained by torsional distor-
tions of the conjugated chain (rotation around CꢀC inter-
[39]
unit bonds)at elevated temperatures.
It is likely that the
Figure 4. Cross-correlation of the first electronic transition (hnICT)in
compounds 9–12 with varying the substituents in both the fluorene and
the dithiole moieties (&: series 7, ~: series 8, *: compound 9a, +: com-
pound 10a, ^: series 11, ~: series 12 (from ref. [24]). The solid line shows
the linear fit according to Equation (5)(total number of points n=24,
correlation coefficient r=0.993).
increased bond length of the C9=C14 bond between the flu-
orene and dithiole parts of the molecule (see the section on
X-ray crystallography below)facilitates the twist around this
bond at elevated temperature, which, in turn, suppresses
conjugation to cause a blue shift of the ICT band.
X
X
X-ray crystallography: To assess the structural variations in
fluorene–dithiolylidene diads, we performed X-ray crystallo-
graphic studies on derivatives 7a, 8a (briefly reported earli-
er ), 10a, 11a and 13a (Figure 6)with the same (2,4,5,7-
tetranitro-)substitution in the fluorene acceptor moiety and
varied 4,5-substituents in the dithiole donor moiety. These
hnICT ¼ ð2:81 Æ 0:02Þþðꢀ0:22 Æ 0:01Þð
s ꢀ
A
s Þ
D
ð5Þ
[21]
Solvato- and thermochromism: The compounds studied
reveal a relatively weak solvatochromic effect of their ICT
bands, in which the lICT varies by less than 20 nm
2762
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Chem. Eur. J. 2008, 14, 2757 – 2770

Push–Pull Diads
FULL PAPER
Figure 6. X-ray molecular structures of 7a, 8a, 10a, 11a and 13a. Thermal ellipsoids are drawn at the 50% probability level. The DMSO molecule (10a),
disorder (11a)and the second independent molecule of 13a are omitted.
variations play two different roles, firstly, their electronic in-
fluence affects the distribution of the electronic density
ity of the dithiole fragment, that is, from 11a with two elec-
tron-withdrawing CO Me substituents to 7a with two elec-
2
(
ICT)in the molecule. The C9 ꢀC14 bond, which must be a
tron-releasing methyl substituents, is manifested by a slight
increase in the length of the C9=C14 bond from 1.376 to
1.398 Š. Therefore, the observed bond lengthening is clearly
attributable to ICT, as visualised by resonance forms
1a$1b. Nevertheless, the length of this bond is still closer
to a typical double bond, which suggests the neutral (non-
zwitterionic)ground state of the molecules.
single bond in 1b, is indeed longer in compounds 7 to 13
than in 14 (1.367(3)Š) in which the fluorene moiety has
[35]
little electron-withdrawing ability
(see Table 4). The
lengthening (as well as the twist around this bond)is stron-
ger when the dithiole moiety carries electron-releasing sub-
stituents (7a, 8a)and weaker when it has electron-with-
drawing CO Me substituents (10a, 11a), although the effect
2
only marginally exceeds experimental error, and the C15=
C16 bond distance (which must lengthen with the increase
in ICT)is even less sensitive. Thus, the increased donor abil-
Table 4. Selected bond lengths [Š] from single crystal X-ray diffraction
and DFT B3LYP/6-31G(d)calculations.
C9ꢀC14 bond
DFT
C15ꢀC16 bond
DFT
X-ray
X-ray
7
7
7
7
7
7
7
8
1
1
1
a
b
c
d
e
f
g
a
1.398(2)1.3832
1.341(2)1.3462
–
–
–
–
–
–
1.3826
1.3808
1.3807
1.3791
1.3785
1.3762
1.3808
–
–
–
–
–
–
1.3462
1.3459
1.3459
1.3460
1.3455
1.3458
1.3530
In each case the fluorene moiety is warped as a result of
steric repulsion between the 4- and 5-nitro substituents,
which (together with the C4 and C5 atoms)tilt to opposite
sides of the fluorene plane. The puckering is not strong
enough to disrupt the aromatic system. Molecule 11a is dis-
ordered between two oppositely warped conformations, of
which the major one (72% occupancy)is shown. Compound
[
21]
[21]
1.395(5)
1.352(5)
0a
1a
3a
1.384(2)1.3800
1.376(3)1.3801
1.391(4)1.3810
1.337(2)1.3444
1.339(3)1.3508
1.396(4)1.3982
Chem. Eur. J. 2008, 14, 2757 – 2770
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2763

D. F. Perepichka, I. F. Perepichka, M. R. Bryce et al.
1
0a crystallised as a 1:1 solvate with DMSO, which is disor-
dered, but forms a relatively strong secondary bond S1··O
.910(2)Š with 10a. The asymmetric unit of 13a comprises
rounding dipoles. Thus, compounds 7a and 8a with the
strongest ICT have the longest C9=C14 bonds, which are
further elongated in the solid state (Table 4).
2
two independent molecules of similar geometry, in which
the dithiole ring (planar in other molecules)is folded along
the S···S vector by 7.4 or 2.88. Molecule 8a is twisted by 208
around the C9=C14 bond; this twist amounts to 68 in 7a and
Throughout the range of substituents in the donor and ac-
ceptor moieties, the LUMO is almost exclusively localised
on the acceptor nitrofluorene moiety (Figure 7). The highest
1
to 38 in other compounds. Thus the donor and acceptor
moieties are almost coplanar and such a conformation fa-
vours a partial ICT, as represented by resonance forms
1
a$1b.
All five crystal structures contain continuous stacks of
molecules related through inversion centres (and thus are
rigorously parallel)or, in 11a, related through a glide plane
and roughly parallel (within 108) . In7a and 8a the stacking
is mainly between acceptor moieties (A), whereas the donor
groups (D)are separated as widely as possible within the
stack. In 10a and 11a the stacking is of D–A type, in 13a
both D–A and A–A overlaps are present simultaneously.
The mean interplanar separations are fairly uniform in 7a
(
3.48–3.50 Š), 8a (3.36–3.46 Š)and
whereas in 10a these separations alternate between 3.39 and
.79 Š, that is, the stack effectively consists of dimers. The
13a (3.48–3.60 Š),
3
inter-stack motif is herringbone, except in the triclinic struc-
ture of 8a in which all of the molecules are parallel. Thus,
changing the substituents affects the crystal packing, through
a change of molecular dimensions and intermolecular inter-
actions S···O in 7a, 8a and 11a (3.19 to 3.25 Š), O···N in
Figure 7. B3LYP/6-31G(d)frontier orbital coefficients in compounds 7a
and 11g.
13a (2.93 Š)and S···S in 8a (3.54 Š), although these are
rather weak. More detailed analysis of the crystal packing in
fluorene–dithiole push–pull compounds, which includes
dyads 7a, 8a, 10a, 11a and 13a, will be published else-
LUMO density is seen on the electron-withdrawing substitu-
ents and on the C11ꢀC12 bond within this unit. Thus, the re-
duction potentials of 7 to 11 are predicted to depend strong-
ly on the substituents in the fluorene moiety and be less sen-
sitive to the structure of the dithiole moiety, precisely as was
observed in the electrochemical experiments (see above).
Conversely, the HOMO is much more delocalised, and is ex-
pected to depend strongly on substituents in both the ac-
ceptor and donor parts of the molecule. Although the irre-
versible character of the oxidation process does not allow
accurate correlations to be made, a positive shift of 80 mV
of the oxidation potential of the 4,5-dimethyl-1,3-dithiole
derivatives upon introducing the third nitrogroup in the flu-
orene moiety (7d!7a, Table 1)is indicative of this behav-
iour. The calculated frontier orbital energies (Figure 8)pre-
dict that changing the fluorene substitution pattern from
[40]
where.
Theoretical studies: The geometry and electronic structures
of the fluorene–dithiole diads were calculated by using the
hybrid B3LYP functional of density functional theory
[41]
(
DFT). This level of theory with a medium basis set (6-
31G(d)) is now routinely used to study medium-sized organ-
ic molecules and has been shown to accurately predict the
structure and energy of other related p-functional materi-
[
42]
als.
The calculated gas-phase geometries of 7 to 11 very close-
ly resemble those found in the solid by X-ray crystallogra-
phy (see above). The difference between the calculated and
observed bond lengths of most bonds does not exceed 0.005
to 0.008 Š. The exceptions to this are the CꢀS bonds in the
two (7g)to four ( 7a)nitro groups ( Dꢀs =1.56)raises the
p
LUMO by ~0.8 eV and HOMO by ~0.6 eV, that is, substitu-
ents in the fluorene moiety have a slightly larger effect on
the LUMOs than on the HOMOs (for series 8a–g and 11a–
g see the Supporting Information). A similar change in the
substituents in the donor dithiole moiety (from 7a to 11a,
dithiole ring (D=0.025–0.030 Š)and the exocyclic C9 =C14
bond (D=0.004–0.018 Š, Table 4). Whereas the small dis-
crepancies in the CꢀS bond length are possibly owing to in-
trinsic limitations of the method, the deviations for the C9=
C14 bond are likely to be as a result of intermolecular inter-
actions in the solid state. As noted above, the order and the
length of this bond is sensitive to the extent of the ICT in
the molecules (1a$1b). For diads with significant ICT, the
resonance input of structure 1b is substantial, and the parti-
ally zwitterionic ground state is highly stabilised by the sur-
Dꢀs =1.24)decreases the LUMO and HOMO energies by
p
0.15 and 0.29 eV, respectively (see the Supporting Informa-
tion). Thus, substituents in the dithiole moiety have a stron-
ger effect on HOMO orbital energies. The calculated orbital
energies are within 0.6 eV of values determined by electro-
chemistry, but surprisingly, even larger differences are found
2764
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Chem. Eur. J. 2008, 14, 2757 – 2770

Push–Pull Diads
FULL PAPER
ed values varies significantly with substituents, and the diads
consisting of strong acceptor and donor fragments have an
ICT energy that is lower than the calculated value by up to
0
.15 eV. This behaviour is consistent with the solvent effects,
which should be stronger for compounds with pronounced
charge-transfer character.
Materials applications: Carbazole-containing polymers are
widely used hole-transporting materials with pronounced
photoconductivity in the ultraviolet region of the spec-
[12,13]
trum.
2,4,7-Trinitrofluorenone (TNF)and 2,4,7-trinitro-
9
-dicyanomethylenefluorene (DTF)are known as efficient
fluorene acceptors, commercially used as sensitisers of elec-
trophotographic and photothermoplastic materials based on
[11,43,44]
carbazole-containing photoconductive polymers.
Thus,
PVK/TNF blend was the first commercially used organic
material to replace selenium in the electrographic process
[45]
(
IBM Copier I Series, 1970). The broad absorption of the
Figure 8. B3LYP/6-31G(d)orbital energy diagram and the HOMO–
LUMO gap (Eg)for compounds 7a–g.
PVK/TNF CTC extends the photosensitivity throughout the
visible region of the spectrum. However, a low absorptivity
of the intermolecular CTC limits the photosensitisation effi-
ciency and requires a high concentration of acceptor units.
Furthermore, the spectral zone of the photosensitivity of
such materials cannot be easily expanded into the long-
wavelength visible and near-IR regions (as required for
some applications, for example, holographic interferometry
by using He/Ne or solid state NIR lasers). A number of dye
molecules dispersed in a polymer matrix (e.g., in PVK or
poly(N-epoxypropylcarbazole)(PEPK)have been success-
fully used as photoreceptors in the electrophotographic pro-
cess to increase the sensitivity in a certain region of the
for the electrochemical and computed HOMO–LUMO gaps
(
>1 eV, cf. Table 1 and Figure 8). In part, this is owing to
the irreversible character of the oxidation process, which
makes it impossible to accurately determine the thermody-
namic redox potentials. A much better correlation was ob-
served between the first optical transition (optical gap,
hnICT)and the calculated HOMO–LUMO gaps (see the Sup-
porting Information). According to time-dependent (TD)
DFT calculations, the lowest energy absorption of com-
pounds 7 to 11 is always a simple HOMO–LUMO transi-
tion. The calculated transition energies are in good agree-
ment with the observed hnICT values (Figure 9). Remarkably,
however, the offset between the experimental and calculat-
[46]
spectrum (at the absorption maximum of the dye). How-
ever, they are generally not suitable for photothermoplastic
processes owing to poor rheological properties of the films
and inefficient thermodeformation during the development
of the latent image. On the other hand, the photothermo-
plastic process has some important advantages over electro-
photography (real-time information recording, reversibility
of the image formation–erase process, high sensitivity, suita-
bility for hologram recording), and consequently, wider
areas of applications (e.g., holographic interferometry). Pho-
tothermoplastic storage media (PTSM)based on polymer
donor–molecular acceptor CTCs are widely studied in this
[11]
instance. Combining strong absorption and electron affini-
ty in fluorene acceptors with ICT, we hoped to increase the
photoresponse of PTSM in the ICT region of the acceptor.
Thus, we have prepared thin films ((1.2Æ0.1) mm)of the
hole-transporting material PEPK blended in different ratios
Figure 9. Relationships between the hn calculated from TD-DFT B3LYP/
6
-31G(d)in the gas phase and experimental hnICT energies from electron
absorption spectra of compounds 7a–f, 8a–e,g, 9a, 10a and 11a–e,g in
,2-dichloroethane (*)and DMF ( &). The dashed line corresponds to a
slope of 458 of equal energies, the solid line shows the linear fit for data
in 1,2-dichloroethane (hnICT =ꢀ(1.37Æ0.31)+(1.57Æ0.13)hnTD-DFT
.05, r=0.941, n=20).
1
;
0
s =
0
Chem. Eur. J. 2008, 14, 2757 – 2770
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2765

D. F. Perepichka, I. F. Perepichka, M. R. Bryce et al.
with fluorene acceptor 6a and used them as photothermo-
plastic storage media. Analogous films have been prepared
with TNF, which has an electron affinity very close to that
of 6a, but lacks ICT. Figure 10 shows the spectral distribu-
of PEPK or PVK with TNF or DTF at concentrations great-
er than 10 wt% of the acceptors. This leads to a high diffrac-
tion efficiency (hmax =25%)that is not far from the theoreti-
[47]
cal limit for plane light wave holograms (33.9%) and al-
lowed the films to attain an extremely high level of holo-
2
ꢀ1
graphic sensitivity (S =250–300 m J , at h=1%). Remark-
h
ably, only one to two mol% of 6a is sufficient to reach the
maximum sensitivity of the PTSM. In addition, PTSM with
6
a as an acceptor shows a higher dynamic range of holo-
gram recording compared with TNF, which allows better
registration of low-contrast objects. The significant (more
than an order of magnitude)increase in the holographic sen-
sitivity and the fact that the spectral response of this sensi-
tivity is linked to a position of the ICT band (and thus can
be tuned by substituents)makes this type of acceptor very
promising as a sensitiser for recording holograms on PTSM
materials.
Figure 10. Spectral distribution of the electrophotographic response (SDV
of PEPK films sensitised by 5 wt% of acceptors 6a (~)and TNF (
).
)
&
Experimental Section
General: Acetone, DMF and DMA were purified as described previous-
[
48]
tion of the electrophotographic response (SDV)of PEPK
ly.
acid, washed with water, dried over MgSO
from CaO. Methyl trifluoromethylsulfonate (methyl triflate, ꢂ98%, Al-
drich)and Bu NPF (Fluka, puriss)were used as received. UV/Vis spec-
tra were acquired by using a Varian Cary 5000 UV/Vis-NIR spectrome-
Chlorobenzene and 1,2-dichloroethane were stirred with sulfuric
4
, distilled from P and then
2 5
O
films sensitised by acceptor 6a in comparison with TNF.
Whereas the spectral response of PEPK/TNF films in the
electrophotographic regime decreases above approximately
4
6
[
23]
[20]
560 nm, PTSM with acceptor 6a demonstrates an increased
ter. Nitrosubstituted fluorenes 4b,e and 4c,f were prepared by nitra-
tion of fluorene or its 4-substituted derivatives with nitric acid or its mix-
ture with acetic or sulfuric acid, as described previously.
response in the region of 500 to 700 nm with a maximum at
around 600 nm, which corresponds well to the ICT energy of
the acceptor (Table 3). The long-chain C H groups in 6a
Cyclic voltammetry: Electrochemical experiments were carried out by
using a BAS Epsilon electrochemical analyser. CV was performed in a
three-electrode cell equipped with a platinum disk (f=1.6 mm)as the
working electrode, platinum wire as a counter electrode and a non-aque-
1
8
37
substantially increase the solubility of the acceptor and en-
hance the rheological properties of PTSM, which makes
them suitable for hologram recording by using a simple He/
Ne laser (l=632.9 nm)even without the additional plasticis-
er used in other acceptors (20–30 wt%), such as TNF, to
achieve appropriate thermodeformation properties of the
films, but which decrease the photoresponse of the material
owing to a dilution effect.
+
ous Ag/Ag reference electrode (0.01m AgNO3 and 0.1m Bu NPF in
4
6
MeCN). Cyclic voltammograms were recorded at room temperature in
dry CH Cl or DMA, deoxygenated by bubbling with argon gas, with
.1m Bu NPF as supporting electrolyte. The potentials are given versus
ferrocene (Fc), which was used as an internal standard and showed the
following potentials against the reference electrode: +0.20 V (vs. Ag/
2
2
0
4
6
+
+
2 2
Ag in CH Cl ), and +0.10 V (vs. Ag/Ag in DMA).
In contrast to TNF, for which SDV decreases with increas-
ing wavelength in the visible region, compound 6a displays
increased sensitivity in its ICT region. At the He/Ne laser
wavelength, PEPK/6a PTSM
X-Ray crystallography: X-ray diffraction experiments were carried out
by using a SMART 3-circle diffractometer with a 1 K CCD area detector
and graphite-monochromated MoKa radiation (l=0.71073 Š)and
a
Cryostream (Oxford Cryosystems)open-flow N
2
cryostat. The structures
achieves an electrophotographic
Table 5. Results of PTSM tests based on PEPK sensitised by fluorene acceptors.
sensitivity approximately five
times higher than that of the
PEPK/TNF material, even at
much lower concentrations of
the acceptor (Table 5). The
long-chain substituents in 6a
l=632.9 nm
[
a]
[b]
[c]
[d]
2
[f]
Acceptor
TNF
Concentration
[wt%] (mol%)
V
0
[V]
DV/V
0
[%]
S
DV
S
h
h
max
[e]
DR
2
ꢀ1
ꢀ1
A
H
R
U
G
A
H
R
U
G
]
A
H
R
U
G
]
[%]
1000
ꢂ25
ꢂ25
5.0 (3.5)210
2.0 (0.44)170
5.0 (1.1)160
10 (2.2)155
14
10
10
10–15
20–25
1.9
8
10
12–16
10–12
9
210
250
15
6
6
a
a
ꢂ3000
ꢂ3000
ꢂ2000
have a pronounced plasticising 6a
effect on the PTSM and im- 6a
250–300
200–250
25
20
20 (4.4)135–140
prove its rheological properties. [a] V is the surface potential of the film, charged in the dark. [b] DV/V is the dark decay of the surface poten-
0
0
Even at high concentrations of tial for 30 s. [c] SDV is the electrophotographic response of the film estimated by the latent electrostatic image
as 20% decay of the surface potential under illumination. [d] S is the photoresponse estimated by visualised
h
6
a (20 wt%), no phase segrega-
image at the level of diffraction efficiency of the hologram h=1%. [e] hmax is the maximal diffraction efficien-
cy of the film for plane wave hologram recording. [f] DR is dynamic range of hologram recording, defined as
the maximal difference between the intensity of the reference and object laser beams to achieve the diffraction
tion or crystallisation of the ac-
ceptor from the blend were ob-
served, which occurs for films efficiency h=1%.
2766
www.chemeurj.org
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 2757 – 2770

Push–Pull Diads
FULL PAPER
were solved by direct methods and refined by full-matrix least squares
dry chloroform (10–30 mL), which caused an exothermic reaction. The
reaction mixture was stirred at 458C for 1 h, then evaporated in vacuum.
The product was used in the condensation reactions with the fluorenes
without further purification. Triflates 3A–F can be stored at ꢀ208C
under argon for several weeks without appreciable decomposition.
2
[49]
against F of all reflections by using SHELXTL software
(Table 6).
CCDC 114934 (8a SADWUP), 656461 (7a), 656462 (10a), 656463 11a
and 656464 (13a)contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[
24]
2,4,5,7-Tetranitrofluorene 4a:
Fuming nitric acid (60 mL, d=
ꢀ
3
1.50 gcm )was cooled to 0 8C and fluorene (16.6 g, 0.1 mol)was added
with intense stirring whilst the temper-
ature was kept below 108C. Additional
[
a]
Table 6. Crystal data for compounds 7a, 10a, 11a, and 13a.
fuming HNO
(40 mL)was then
3
added, the mixture was stirred at 608C
for 1.5 h and the product was left to
crystallise. The product was filtered,
Compound
7a10a
11a13a
formula
C
18
H
10
N
4
O
8
S
2
C
582.53
105
18
H
8
N
4
O
9
S
2
·C
2
H
6
OS
C
562.44
100
20
H
10
N
4
O
12
S
2
20 8 4 8 2
C H N O S
496.42
120
M
r
474.42
120
10.994(2)13.4384(14)
9.831(2)8.0904(8)
17.003(3)21.379(2)
91.84(1)99.77(2)
1836.7(5)2290.7(4)
4
0.35
washed with nitric acid (10 mL, d=
T [K]
a [Š]
b [Š]
c [Š]
b [8]
ꢀ3
1
.35 gcm ), then with water until the
10.8860(1 71 )8 .406(2)
27.790(4)25.936(3)
7.2462(12)8.067(1)
96.18(1) 100.76(4)
2179.4(6) 3783.3(8)
washings were neutral and then dried
to give 4a (22.05 g, 64%)as a light
yellow powder (m.p. 275–2768C
(
decomp)) suitable for further synthe-
ses. It can be additionally recrystal-
lised from minimal amount of
fuming nitric acid. M.p. 278–2808C
decomp); lit. m.p. 275–276 8C (de-
3
V [Š ]
Z
4
0.40
27536
6118, 3.8
3.6, 9.3
4
8
a
-
1
m [mm ]
0.33
19407
4812, 5.1
4.5, 10.8
0.35
27264
8727, 7.3
4.8, 11.0
reflns collected
reflns unique, Rint [%]
22279
4865, 3.5
3.0, 8.1
(
[24b]
comp);
dioxane 10:1). H NMR (400 MHz,
[D ]acetone): d=9.07 (2H, d, J=
f 2
R =0.30 (SiO , m-xylene/
R
1
, wR
2
[%]
1
[
a] All compounds are monoclinic with space group P2
1
/c (no. 14). R
1
=ꢀjjF
o
jꢀjF
c
jj/ꢀjF
o
j on the data with
6
2
2
2
2
2
1/2
I>2s(I). wR
2
={ꢀ[w
A
C
H
T
R
E
U
N
G
(F
o
ꢀF
c
) /ꢀ[w
A
T
E
N
(F
o
) ]}
.
2.0 Hz; H-1,8), 8.89 (2H, d, J=2.0 Hz;
H-3,6), 3.58 ppm (2H, s; H-9);
1
3
6
C NMR (100 MHz, [D ]acetone): d=
Computational procedures: DFT computations of the geometries of com-
151.1, 147.2, 140.2, 138.1, 126.8 (CH),
[
50]
pounds 7 to 12 were carried out with the Gaussian 03 package of pro-
grams by using Popleꢀs 6-31G split valence basis set supplemented by d-
polarisation functions for heavy atoms. Beckeꢀs three-parameter hybrid
2
123.5 (CH), 67.7 ppm (CH ).
[25]
2
,4,7-Trinitrofluorene 4d:
Fuming nitric acid (150 mL, d=1.50)was
cooled to 08C and fluorene (20 g, 0.12 mol)was added with intense stir-
ring over approximately 30 min while the temperature was kept below
+58C. The mixture was stirred at 0 to +58C for 30 min and then poured
onto ice (1 kg). The solid was filtered, washed with water until the wash-
ings were neutral and then dried to give crude 4d (33 g, 91%). The crude
product was recrystallised twice from acetic acid to give 4d as yellow
[
51]
exchange functional
with Lee–Yang–Parr gradient-corrected correla-
[
52]
tion functional (B3LYP)
were employed. Thus, the geometries were
optimised at the B3LYP/6-31G(d)level of theory and the electronic
structures were calculated at the same level of theory. The lowest energy
absorptions were obtained from TD-DFT calculations (at the same
B3LYP/6-31G(d)level)for optimised geometries. Contours of HOMO
and LUMO orbitals were visualised at a cut-off of 0.05 by using the Mo-
[
25]
crystals (21.5 g, 59%). M.p. 210–2118C; lit. m.p. 210–2118C; R =0.70
f
1
(SiO ; benzene/acetone 30:1); H NMR (400 MHz, [D ]acetone): d=8.91
2
6
[
53]
lekel v.4.3 program.
Photophysical measurements of sensitised PEPK films:
prepared in the following way: PEPK (0.5 g)obtained by anionic poly-
(1H, d, J=2.0 Hz; H-1), 8.86 (1H, d, J=2.0 Hz; H-3), 8.65 (1H, d, J=
2.0 Hz; H-8), 8.42 (1H, dd, J=8.8, 2.0 Hz; H-6), 8.28 (1H, d, J=8.8 Hz;
[
11]
PTSM were
[
54]
merisation of N-(2,3-epoxypropyl)carbazole
amount of 6a were dissolved separately in methyl ethyl ketone (both in
mL), and then the solutions were combined and filtered. The resulting
and a corresponding
Table 7. Conditions for the preparation of compounds 6–11.
[
a]
[b]
[c]
5
t
[h]
T [8C]
Y1 [%]
Y2 [%]
M.p. [8C]
solution was supported on an ITO-coated glass base. The final thickness
of the photoconductive films was (1.2Æ0.1) mm. The surface of the film
was charged by a positive corona discharge grid; the final potentials were
6
7
7
7
7
7
7 f
8a
8b
8c
8d
8e
8g
9a
10a
11a
11b
11c
11d
11e
a
a
b
c
d
e
2
1
3
15
20
22
25
1
3
3.5
25
25
1
2
1
1
3
6
30
30
20
20
30
30
40
50
70
20
25
20
60
70
110
20
20
20
20
25
50
65
75
95
95
92
95
84
55
56
100
71
55
21
43
–
90
85
90
74
33
28
50
90
90
86
91
81
51
40
88
57
50
–
11
75
–
74
76
49
30
17
131–135
>360
>360
>300 (decomp)
>360
>360
>300 (decomp)
321–322.5
305–310
240–242
248–250
296–297.5
289–292
>360
350 (decomp)
282–284
278–282
258–263
248–252
283–287
ꢀ
1
V
0
ꢁ120–160 Vmm (measured by the dynamic sonde method). The rela-
tive dark decay of the surface potential (DV”100/V )was estimated for a
time of 30 s (DV= V , in which V is the charge potential of the sur-
ꢀV
face in the dark after t=30 s)and lies in the range of 10 to 15%. The
0
0
t
t
2
ꢀ1
electrophotographic sensitivity (SDV [m J ])was estimated at a level of
2
4
0% decay of the initial potential under illumination with wavelengths of
ꢀ
1
00 to 900 nm and an intensity of 0.1 mWcm . Real holographic sensitiv-
2
ꢀ1
ity (S
h
[m J ])was estimated at the level of 1% diffraction efficiency
(
h=1%)of the visualised image by recording holograms of the planar
ꢀ
1
light wave at the spatial frequency of n=450 mm with irradiation of a
He/Ne laser (l=632.9 nm). The maximal diffraction efficiency, which
was achieved without amplification of the hologram recording (hmax [%]),
was found to be the ratio of the beam intensity diffracted into the first
order of a diffraction to the intensity of the beam grazed on the holo-
gram when the ratio of the intensities of the integrated beams was 1:3.
The dynamic range (DR)of the hologram recording was estimated to be
a maximal ratio of the reference and object beams for achieving h=1%.
2
-Methylthio-1,3-dithiolium triflates 3A–F: Methyl triflate (3 mmol)was
[a] Reaction time. [b] Crude yield. [c] Purified yield after recrystallisa-
tion.
[
55]
[56]
[57]
[58]
added to a solution of 2A, 2B, 2C, 2D, 2E or 2F (2 mmol)in
Chem. Eur. J. 2008, 14, 2757 – 2770
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2767

D. F. Perepichka, I. F. Perepichka, M. R. Bryce et al.
1
[a]
Table 8. H NMR data for compounds 6 to 11.
1
-H (J1.3)8-H (
J
6.8)3-H
6-H
5-H (
J
5.6)Other signals
3
.11 (t, J=7.2 Hz; 2SCH
1.56–1.44 (m; 2CH ), 1.20–1.40 (m; 14CH
.89 (t, J=6.8 Hz; 2CH
2 2
), 1.82–1.71 (m; 2CH )
[
[
b,c]
c]
6
a
a
9.12 (d, J=1.6 H– z)8.78 (d)
2
2
)
0
3
)
7
7
7
7
7
7
8
8
8
8
8
8
9
1
1
1
9.20–(brs)8.70 (brs)
2.49 (s; 2C
H
H
3
)
)
[
d]
b
9.09
A
H
R
U
G
J=2.0 Hz)–
2.44 (s; 2C
3.86 (s; CO
J=8.0 Hz)2.41 (s; 2C
J=8.0 Hz)2.41 (s, 2C
J=9.0 Hz)4.09 (s; CO
3
[
e]
c
9.04br
8.60 (brs)8.41 (brs)–
2
CH
H )
3
H )
3
CH
3
3
3
), 2.41 (s; 2CH
3
)
)
[
e]
e]
e]
d
8.98 (d, J=2.0 Hz)8.82 (d,
8.98 (s)8.82 (s )888. .7.36973 (((dds, )
8.90 (s)8.77 (s )888. .5.24317 (((dds, )
J=2.0 Hz)8.63 (d) 8.37 (dd) 8.79 (d,
[
e
f
[
2
), 2.38 (s; 2CH
3
[
f]
a’
b
c
– 9.31 (s)9.06 (s)
2.92 (s; 2SC
H
H
3
)
)
8.96 (brs)
8.80 (brs)8.71 (brs)8. 68 0. 3– (8 b (r bs r) s)
8.79 (d, J=2.0 Hz)8.62 (d,
8.84 (d)8.69 (d)8. 748 574 (d ,d) )
8.80 (brs)8.71 (brs)–
3.85 (s; CO
2.70 (s; 2SC
3
2
CH
3
), 2.67 (s; 2SCH )
d
e
J=2.0 Hz)8.74 (d) 8.28 (dd) 8.07 (d,
J=8.8 Hz)2.68 (s; 2SC
J=8.4 Hz)2.67 (s, 2SC
J=8.4 Hz)2.99 (s; 2SC
–
H
3
)
H
H
3
)
[
f]
g
a
8.40 (d, J=2.1 H 8z .) 28 7. 1 (8 d (, dd)
3
)
8.82 (brs)8.68 (d,
8.78 (brs)8.63 (d,
9.85–(brs)8.70 (brs)
J=1.6 Hz)–
J=1.8 Hz)–
[
g]
[
0a
1a
1a’
8.48 (s; dithiole-
H), 3.99 (s; CO
CH
4.71 (s), 4.32 (s; 11a)
2
CH
3
)
3.93 (s, 2CO
2
3
)
f]
9.33 (brs), 9.31 (d, J=1.6 Hz; 1, 1a)9.17 (brs)
.09 (d, J=1.6 Hz; 11a)
–
9
11b
11c
11d
8.83 (d, J=1.6 Hz)8.77 (d,
8.80 (d, J=1.6 Hz)8.69 (d,
8.70 (d, J=2.0 Hz)8.34 (d,
J=1.6 H8 .z7 )6 (d)8.72 (d)–
J=1.6 H8 .z6 )5 (d)8.42 (d)–
J=2.0 Hz)8.56
3.95 (s; 2CO
3.95 (s, 2CO
2 3
CH )
2
CH
CH
3
), 3.85 (s, CO
)
2
CH
3
)
8.23 (dd) 7.96 (d,
J=9.0 Hz)3.96 (s, 2CO
2
3
[
a] Spectra were recorded in [D
CDCl . [c] Recorded at 508C. [d] Recorded at 908C. [e] Recorded at 1208C. [f] Recorded in D
the sample.
6
]DMSO at 300 MHz by using tetramethylsilane as an internal standard at a temperature of 208C. [b] Recorded in
3
2
SO at 500 MHz. [g] One drop of CF CO D was added to
4
3
2
1
3
H-5), 4.53 ppm (2H, s; H-9); C NMR (100 MHz, [D
6
]acetone): d=
51.1, 149.4, 149.0, 148.0, 146.0, 142.0, 137.7, 127.0 (CH), 125.2 (CH),
23.7 (CH), 121.3 (CH), 120.0 (CH), 38.4 ppm (CH ).
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Blanchard-Desce, Org. Lett. 2004, 6, 47–50.
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1
1
2
7
67–770.
General method for the preparation of 6 to 11: Dithiolium triflate 3A–F
0.42–0.45 mmol)was added to a solution of fluorene 4a–g (0.4 mmol)in
DMF (2 mL)and the reaction mixture was stirred at 20 to 70 8C for 1 to
0 h. The precipitate that formed was filtered, washed successively with
[
8] R. M. Metzger, Chem. Rev. 2003, 103, 3803–3834.
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(
3
DMF, acetone and 2-propanol, and dried in vacuo to give compounds 6
to 11, which were purified by recrystallisation from DMF. The reaction
temperature, time and yields are collected in Table 7. The H NMR spec-
[10] W. D. Gill in Photoconductivity and Related Phenomena; (Eds.: J.
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1
[11] a)I. F. Perepichka in NATO Science Series 3 High Technology,
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Acknowledgements
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Young Professor award. I.F.P. thanks The Royal Society of Chemistry for
the Journals Grant for International Authors supporting his visit to
McGill University. The work in Durham was funded by the EPSRC.
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ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 2757 – 2770

Push–Pull Diads
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Chem. Eur. J. 2008, 14, 2757 – 2770
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2769

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Published online: January 31, 2008
2770
www.chemeurj.org
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 2757 – 2770