Controlling the conformational changes in donor-acceptor [4]dendralenes through intramolecular charge-transfer processes

Article abstract of DOI:10.1002/chem.200900656

The synthesis of two [4]-dendralene compounds incorporating thiophene-(p-nitrophenyl) donor-acceptor units is presented. The dendralenes adopt two different conformers in solution and solid state and the transformation between the structures can be contro

Full text of DOI:10.1002/chem.200900656

FULL PAPER
DOI: 10.1002/chem.200900656
Controlling the Conformational Changes in Donor–Acceptor [4]-
Dendralenes through Intramolecular Charge-Transfer Processes
Alexander L. Kanibolotsky,[a] John C. Forgie,[a] Greg J. McEntee,[a]
M. Munsif A. Talpur,[a, b] Peter J. Skabara,*[a] Thomas D. J. Westgate,[c]
Joseph J. W. McDouall,[c] Michael Auinger,[d] Simon J. Coles,[e] and
Michael B. Hursthouse[e]
Abstract: The synthesis of two [4]-den-
are conjugated (b conformers), and this
process is driven by ambient light. The
structures of the two conformers of
compound 13 are confirmed by single-
crystal X-ray diffraction studies and
the structural changes in both com-
pounds have been monitored by
1H NMR spectroscopy and absorption
studies. The transformations were
found to be first-order processes with
rate constants of k=0.0027 sꢀ1 and k=
dralene compounds incorporating thio-
phene-(p-nitrophenyl) donor–acceptor
units is presented. The dendralenes
adopt two different conformers in solu-
tion and solid state and the transforma-
tion between the structures can be con-
trolled by light and heat. The electron-
donating components of the dendra-
lenes are represented by bromothienyl
(in 13) and ethylenedioxythiophene-
0.00022 sꢀ1 for 13 and 15, respectively.
Density functional theory calculations
at the B3LYP/6-31G* level give cre-
dence to the proposed mechanism for
the a!b conversion, which involves
photoinduced intramolecular charge
transfer (ICT) as the key step. The
EDOT derivative (15) can be polymer-
ised by electrochemical oxidation and a
combination of cyclic voltammetry and
UV/Vis spectroelectrochemical experi-
ments indicate that the a conformer
can be trapped and stabilised in the
solid state.
ACHTUNGTRENNUNG(EDOT)-thienyl (in 15) end-groups.
Keywords: conformational isomeri-
zation · crystallography · dendra-
lenes · electrochemistry · molecular
modeling
The most facile transformation involves
the isomerisation of donor–acceptor
conjugated systems (a conformers) into
structures in which only the thiophenes
[a] Dr. A. L. Kanibolotsky, J. C. Forgie, G. J. McEntee,
Dr. M. M. A. Talpur, Prof. P. J. Skabara
Introduction
WestCHEM, Department of Pure and Applied Chemistry
University of Strathclyde, Glasgow G1 1XL (UK)
Fax : (+44)141-548-4822
Cross-conjugated molecules have been defined as containing
three or more unsaturated groups, two of which are conju-
gated to the third but not directly to each other.[1] For exam-
ple, in benzophenone the two phenyl groups share conjuga-
tion to the carbonyl group, but are not conjugated to each
other. Similarly, in 2-methylenemalonic acid the carbonyl
groups are conjugated to the central C=C group, but not to
each other.[1] This definition is not entirely unambiguous, as
it could be said to hold for linear p-conjugated systems.[2]
The definition can be expanded, however, to structures in-
corporating two or more unsaturated groups that are all
conjugated to a central unsaturated core group, but not di-
rectly conjugated to each other.
E-mail: peter.skabara@strath.ac.uk
[b] Dr. M. M. A. Talpur
On leave from: Department of Chemistry
Shah Abdul Latif University
KhairPur (Mirꢀs), Sindh (Pakistan)
[c] Dr. T. D. J. Westgate, Dr. J. J. W. McDouall
School of Chemistry, University of Manchester
Manchester M13 9PL (UK)
[d] M. Auinger
On leave from:
Institute for Chemical Technology of Inorganic Materials
Johannes Kepler University Linz, Linz (Austria)
[e] Dr. S. J. Coles, Prof. M. B. Hursthouse
Department of Chemistry, University of Southampton
Southampton SO17 1BJ (UK)
Interest in cross-conjugated structures as candidates for
electronic components has developed because they fre-
quently provide the opportunity to investigate the funda-
mental relationships between p-electron delocalisation, con-
formation and optoelectronic properties. Scheme 1 shows
Supporting information for this article (Experimental Section: instru-
mentation, synthetic procedures and characterisation for all novel
compounds; CV, UV/Vis and computational data (Figure S1–S11;
Tables S1-S3)) is available on the WWW under http://dx.doi.org/
10.1002/chem.200900656.
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lenes are named according to the number of C=C groups in
this unit, so [3]-dendralene has n=1, [4]-dendralene has n=
2, and so on. Many of the properties described above are
common to all the cross-conjugated systems but research
into the synthetic versatility of dendralenes in cycloaddition
procedures[44–47] has taken prominence recently over interest
in their electronic and structural properties. However, the
development of analogues of tetrathiafulvalene with conju-
gated spacer groups separating two or more 1,3-dichalcole
groups has yielded a few redox-active dendralene structures,
including for example 6 and 7.[48–53]
Scheme 1. Some conjugated and cross-conjugated motifs that have been
investigated in molecular electronics: dialkynylethene 1, tetraalkynyl-
ACHTUNGTRENNUNGethene 2, oligo(p-phenylenevinylidene) 3, tetra-substituted benzene cruci-
form 4 and tetra-substituted tolane cruciform 5. R=alkyl or aryl group.
some conjugated motifs that have been well studied in this
area: dialkynylethene (1),[3–10] tetraalkynylethene (2),[11–16]
oligo(p-phenylenevinylidene) (3)[2] and cruciforms construct-
ed around a tetra-substituted benzene (4)[17–34] or tolane
core (5).[35]
ꢀ
The C C sigma bonds impart conformational flexibility
on the dendralenes where steric factors allow for it.[54,55]
Some studies on chalcogen-containing dendralenes have
highlighted the conformational variability in structures such
as 8, while others have demonstrated electrochemical con-
Several properties of these structures that are beneficial
or desirable in optical or electronic systems have been iden-
tified. Cross-conjugation allows for an extension of the con-
jugated system without the corresponding bathochromic
shift of absorption wavelength associated with linear conju-
gation. This leads to interesting candidates for optoelectron-
ic applications that rely on transparency in the visible and
near infrared regions, such as nonlinear optics.[2,9,27] The op-
posite can be achieved by appending multiple chromophores
at the cross-conjugated arms, thus engineering a broader ab-
sorption profile desirable in photovoltaic devices.[35] Multiple
conduction pathways in different dimensions are accessible
by using cross-conjugated compounds.[20–22,29,30] Electron-
donor and -acceptor moieties are frequently incorporated
into cross-conjugated structures to improve the dimensional-
ity of electronic communication.[4,5,10,22,27,36–40] Judicial place-
ment of the donor and acceptor groups allows spatial sepa-
ration of the HOMO and LUMO, as revealed by quantum-
mechanical calculations. In turn, this arrangement allows the
optical band gap to be tailored, and either of the frontier or-
bitals to be addressed independently, for example, with
host–guest interactions.[17,19,22,25,26,29–31]
trol over conformation.[56–58] Controlled molecular motion,
in conjunction with controlled intramolecular electronic
pathways, are fascinating and potentially useful features in
the design of optical switching components and molecular
machines.[59–62] In this paper, we report a novel synthetic
route to a donor–acceptor dendralene that undergoes a re-
versible conformational change driven by light and heat.
This is an unusual process, given that the two structures
appear to be “conformationally locked”. The extension of
this dendralene with ethylenedioxythiophene (EDOT) units
provides an electropolymerisable derivative. Whilst photoin-
duced trans–cis isomerism has been reported for some
Dendralenes[41–43] are a family of cross-conjugated com-
pounds that contain the motifs shown in Scheme 2. Dendra-
Scheme 2. Generic and sample structures of the dendralene family of
compounds.
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Intramolecular Charge-Transfer Processes
FULL PAPER
donor–acceptor 1,2-diethynylethenes and tetraethynyl-
Wittig reaction from 11 with p-nitrobenzyl triphenylphos-
phonium bromide (12) and potassium tert-butoxide under
reflux THF (33% yield). Coupling of dendralene 13 with 2-
trimethylstannyl-3,4-ethylenedioxythiophene (14) under
Stille conditions was achieved by using DMF as the solvent
and microwave assisted heating. Dendralene 15 was ob-
tained in 73% yield.
Compounds 21 and 22 (Scheme 4) were prepared as anal-
ogous donor–p acceptor units for comparison with the
donor–acceptor characteristics of dendralene 13. Reaction
ACHTUNGTRENNUNG
ethenes,[63] our dendralenes could be adapted in macromo-
lecular structures to give short or long conjugation lengths.
The electronic properties of such macromolecules would be
vastly different between isomeric structures and the con-
struction of these materials could be manipulated by photo-
excitation, providing a valuable and unique method for
structural and electronic control. Herein, we report the spec-
troscopic and electrochemical properties of donor–acceptor
dendralenes and relate these results to the conformational
changes observed under external stimuli.
Results and Discussion
Synthesis: The synthetic route towards [4]-dendralenes 13
and 15 is summarised in Scheme 3. Compound 11 was pre-
pared in 22% yield from 5-bromothiophene-2-carboxalde-
hyde (9) and 2,5-dimethoxytetrahydrofuran (10) by heating
at reflux in the presence of potassium acetate, acetic acid
and water.[64,65] Dendralene 13 was synthesised by a two-fold
Scheme 4. Synthesis of compounds 21 and 22: i) lithium diisopropylamide
(LDA), THF, ꢀ788C, 45–60 min, then (E)-3-(dimethylamino)acrolein
(18); ii) potassium tert-butoxide, THF, 30 min, then (p-nitrobenzyl)triphe-
nylphosphonium bromide (12), 16 h.
of 2-lithiothiophene or 2-bromo-5-lithiothiophene (gener-
ated in situ from 16 or 17) with 2-(N,N-dimethylamino)acro-
lein (18) afforded the aldehydes 19 (18% yield) or 20 (24%
yield), which were isolated in trans geometry after distilla-
tion.[66] Reaction of 19 or 20 with (p-nitrobenzyl)triphenyl-
phosphonium bromide (12) and potassium tert-butoxide
yielded 21 (19% yield) or 22 (56% yield). In both cases, a
mixture containing cis and trans geometries at the conjugat-
ed double bonds was generated. The all-trans isomer was
isolated by crystallisation from dichloromethane and petro-
leum ether; the isomers were verified by NMR and, in the
case of 22, X-ray crystallography.
X-ray crystallography: Crystalline samples of 13 were pre-
pared from the mixture of conformers by: i) Dissolution in a
minimum amount of warm, degassed dichloromethane
under nitrogen, followed by the addition of degassed hexane
to form a cloudy, yellow solution. The crystals were then al-
lowed to grow at room temperature in the dark. ii) Slow dif-
fusion of hexane into a solution of 13 in dichloromethane at
room temperature and exposed to ambient light. The two
methods gave different X-ray crystal structures, which are
shown in Figure 1. Crystals grown by method i) were found
to have the structure 13a (Figure 1, top), which consists of
two orthogonal and identical donor–p-acceptor units form-
Scheme 3. Synthesis of dendralenes 13 and 15: i) potassium acetate, acetic
acid, water, heating to reflux, 16 h; ii) potassium tert-butoxide, THF, heat-
ing to reflux, 2 days; iii) [PdACHTNUGTRNEUG(N PPh3)4], DMF, microwave, 1608C, 1 h.
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P. J. Skabara et al.
there is a notable difference of 0.108 ꢂ between the average
ꢀ
C C bond length and the average C=C bond length. In 13b,
the bond length alternation is 0.109 ꢂ. This suggests that in
both conformers, the C=C double bonds are quite localised.
The conjugated segments in both conformers provide good
coplanarity between terminal aryl units with a deviation
from planarity of 0.008(3) ꢂ for 13a (thiophene–benzene)
and 0.059(3) ꢂ for 13b (thiophene–thiophene).
Slow diffusion of hexane into a solution of 11 in chloro-
form gave suitable samples for X-ray crystallography. The
crystal structure of 11 is shown in Figure 2 (top). The pre-
Figure 1. X-ray crystal structure of conformers 13a (top) and 13b
(bottom).
Figure 2. X-ray crystal structures of compounds 11 (top) and 22 (bottom).
ferred conformation in this molecule presents two orthogo-
nal donor–acceptor half-units, which are twisted at approxi-
ing a cruciform shape. Each bromothiophene is conjugated
to a nitrobenzene unit through an all-trans butadiene bridge
(C5-C6-C7-C8 and C19-C20-C21-C22) and the torsion angle
between the two donor–p-acceptor units (C7-C6-C20-C21)
is 83.03(3)8. The single bond linking the two orthogonal
ꢀ
mately 99.06(4)8 (C5-C6-C8-C10) and joined by a C C
ꢀ
single bond (C6 C8, 1.492(4) ꢂ). This arrangement is the
same as that reported for other 1,4-diaryl-2,3-diformylbuta-
dienes where the aryl groups are alkyl- and alkoxy-substitut-
ed phenylene rings.[64] Polymorphs were not obtained by
varying the conditions of crystal growth (air and light). As a
useful comparison to conformer 13a, the crystal structure of
compound 22 was also obtained (see Figure 2, bottom). The
bond length alternation in 22 can be compared with that of
the corresponding dendralene. The difference between the
ꢀ
ꢀ
units (C6 C20) is weakly conjugated with a C C bond
length of 1.497(3) ꢂ. Crystals grown by method ii) gave the
conformer represented by 13b (Figure 1, bottom). Here, the
bromothiophene units are co-planar and conjugated through
an all-trans butadiene bridge (C5-C6-C6’-C5’) and the nitro-
benzene units are twisted out of the plane of the bromothio-
phenes with a torsion angle of 99.94(4)8 (C5-C6-C7-C8).
ꢀ
average C C and C=C bond lengths in 22 is 0.099 ꢂ, a
ꢀ
The carbon carbon bond linking the styryl units to the buta-
value very close to that in 13a. This suggests there is little
effect on the conjugation between the bromothiophene and
nitrobenzene units caused by the presence of a second
donor–acceptor arm in 13a.
ꢀ
diene fragment (C6 C7) has a length of 1.481(4) ꢂ, again
showing weak conjugation to the planar butadiene chain.
The C=C bonds between the carbons a and b to the bromo-
thiophene (C5/C6 and C19/C20 in 13a) that possessed cis
geometry in 13a have changed to a trans geometry in 13b
The different methods of crystallisation for compound 13
provide two distinct conformers. In 13a, the molecule
adopts a common dendralene motif, in which two conjugat-
ed half-units give a cruciform structure with a single non-
conjugated node. Conformer 13b, on the other hand, pro-
vides a different scenario, in which the conjugated fragment
is a 1,4-di(thiophen-2-yl)buta-1,3-diene, substituted at the
ꢀ
(C5/C6 in 13b). The C C bond length alternation gives a
measure of the extent of conjugation in each structure
above. In a highly conjugated structure, single and double
bonds will tend to become equivalent due to extensive p-de-
localisation. Within the conjugated butadiene chain of 13a,
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Intramolecular Charge-Transfer Processes
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2,3-positions by twisted nitrostyryl units and resulting in two
non-conjugated nodes within the structure. From the differ-
ent crystallisation methods applied for the generation of
each unique structure and the reproducibility of these ex-
periments, we suspected that external physical factors could
be used to enforce geometrical transformations between the
two conformers. The following sections evaluate the conver-
sion of 13a into 13b under ambient light and, on the basis
of our results, we propose a mechanism that would explain
this fascinating transformation.
Solution-state conversion between conformers 13a and 13b:
Given that two molecular conformations are accessible for
13 in the crystal phase, it is logical to assume that dendra-
lenes of this type are conformationally flexible in solution.
Indeed, it was possible to monitor the transformation be-
tween the a and b conformers by using solution-state spec-
troscopy.
1H NMR spectroscopy: The two forms of 13 can be identi-
fied by proton NMR in CDCl3 and in [D6]DMSO. Crystals
from the same batch as those that had been shown by X-ray
crystallography to have the conformation 13a were dis-
1
solved in CDCl3 in the absence of light. The H NMR spec-
trum was immediately collected from this solution. The solu-
tion was only exposed to light for a few seconds as the
NMR tube was transferred to the spectrometer magnet.
ACHTUNGTRENNUNGFigure 3a shows the spectrum obtained in this manner,
which confirms that the structure seen in 13a crystals per-
sists in solution. The solution was then exposed to ambient
light for 24 h and the proton NMR spectrum was recorded
again. This spectrum is shown in Figure 3b and can be as-
signed to the molecular conformation of 13b. The signals
for 13b can in fact be seen emerging in the spectrum of 13a
as a result of the short exposure to light that occurred be-
tween sample preparation and data acquisition. The nitro-
benzene proton signals of 13a (arising from positions 6 and
7 in Figure 3a, d=8.16 and 7.53 ppm) become shifted to
slightly lower field (d=8.21 and 7.57 ppm) in 13b. The cou-
pling constant between these two protons (J=8.8 Hz) re-
mains unchanged. The protons of the styryl C=C positions
are similarly shifted; the signal from proton 4 shifts from
d=7.29 ppm to d=7.71 ppm, and the signal from proton 5
shifts from d=6.46 ppm to d=6.71 ppm. The coupling con-
stant arising from the trans-vinyl relationship between 5 and
4 (seen in the 5 peak, obscured by solvent in the 4 peak) re-
mains unchanged at d=15.7 Hz. This downfield shift of the
signals can be attributed to the lack of conjugation from the
electron-donating bromothiophene group allowing for the
electron-withdrawing effect of the nitrobenzene group to
dominate and deshield the protons. In contrast, the proton
in position 3 loses the deshielding effect of a conjugated ni-
trobenzene group and its singlet peak shifts to higher field
(from d=7.23 ppm to d=6.75 ppm) as its position becomes
conjugated to a second electron-donating bromothiophene
group. The thiophene signals are coalesced in 13a, possibly
Figure 3. Top) 1H NMR spectrum of 13a in the region d=6.2–8.5 ppm.
Bottom) 1H NMR spectrum of the same solution after 24 h exposure to
ambient light.
as a result of the donor–acceptor sequence, but resolve into
separate peaks in 13b.
During this process of conversion, transient intermediate
signals can be seen in the NMR spectrum if it is recorded
within 24 h. Virtually, all of these signals disappear once
conversion is complete and their relative intensity is depen-
dent on the progress of the transformation. A few minor im-
purities remain in the spectrum of 13b and are likely to rep-
resent more persistent intermediates of the transformation
process. It should also be noted that if no light is allowed to
the solution at any time during the experiment then the
NMR spectra remain unchanged.
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P. J. Skabara et al.
The same effect can be seen if the solution is prepared by
using [D6]DMSO as the NMR solvent. Analogous to the
CDCl3 experiment, crystals from a batch of conformer 13a
were dissolved in [D6]DMSO and the proton NMR was re-
corded taking the same precautions to exclude light from
the solution as much as possible (see Figure S1a in the Sup-
porting Information). The solution was then exposed to am-
bient light for 24 h and the spectrum was recorded again
(see Figure S1b in the Supporting Information), allowing
identification of conformer 13b. This solution was then
transferred to a microwave reaction vial and heated to
1208C for 40 h. The cooled solution was then transferred to
an NMR tube and the proton NMR spectrum was recorded
taking care to minimise exposure to ambient light (see
ACHTUNGTRENNUNGFigure S2 in the Supporting Information). This spectrum in-
dicated that the original 13a form was now the predominant
component of the mixture, proving that the conformational
conversion is reversible. Conversion was not 100% com-
plete, however, and some intermediate signals were still
present. A repeat of the process on the same solution with
heating for an additional 6 h returned a poor quality spec-
trum, possibly as a result of decomposition during heating,
and from absorption of water by the solvent.
Figure 4. Evolution of UV/Vis absorption spectra of a solution of 13a in
CH2Cl2 over time when exposed to ambient light.
peak at l=320 nm develops. This peak is noisy due to its
low intensity, but an isosbestic point can clearly be seen in
Figure 4. If no light is allowed to the solution, the spectrum
is repeatable at any point during the process. The same
effect can be seen when chloroform is used instead of di-
chloromethane as the solvent.
1H NMR and UV/Vis analysis of 15 revealed that the two
components observed by TLC on silica are also two confor-
mational forms of the same type as 13. Because the transfor-
mation in this case was much slower, it was possible to iso-
late the components of 15 by column chromatography and
identify the pure a form, possessing donor–acceptor conju-
gation, by 1H NMR spectroscopy. It was not possible to
identify and fully characterise the b conformer as the pro-
cess of conversion was incomplete.
The rate of change of the absorbance at l=403 nm is
shown in Figure 5. The data are a good fit to the first-order
UV/Vis absorption studies: The UV/Vis absorption spectra
of compounds 13, 21 and 22 were recorded. Compounds 21
and 22 have almost identical spectra; they have a maximum
absorbance at l=395 nm and l=397 nm, respectively, and a
shoulder at approximately 320 nm (see Figure S3 in the Sup-
porting Information). The major absorbance can be assigned
to a “push-pull” intramolecular charge transfer (ICT) se-
quence from electron-donor (thiophene) to -acceptor (nitro-
benzene) units. Since the absorption maxima of 21 and 22
are so similar, it can be surmised that there is no significant
affect caused by the presence of bromine on the energy of
this transition. The molar extinction coefficient, however, is
larger (e=43000mꢀ1 cmꢀ1) for the bromothiophene deriva-
tive 22, compared to 21 (e=33000mꢀ1 cmꢀ1).
The conversion of 13a to 13b can be observed by UV/Vis
spectroscopy. Crystals of structure 13a (selected as above)
were dissolved in dry CH2Cl2 in the absence of light, then
immediately transferred to the UV/Vis spectrometer. The
spectrum of the fresh solution is very similar to the analo-
gous compounds 21 and 22. The maximum absorbance
occurs at approximately l=403 nm and is also ascribed to
ICT. If natural light is allowed to irradiate the sample, the
intensity of the longest wavelength band decreases gradually
and is slightly blue-shifted to l=397 nm (Figure 4), whilst a
Figure 5. Kinetics plot of the rate of change in the absorbance of a solu-
tion of 13 in CH2Cl2 at l=403 nm with time, upon exposure to ambient
light.
rate law, which is solved for a first order rate constant k=
0.16 minꢀ1, or 0.0027 sꢀ1. The presence of the isosbestic
point reinforces the stable first-order kinetic behaviour. The
levels of noise in the peak at l=320 nm render any exami-
nation of the rate of change in intensity of this band inaccu-
rate.
A sample of 13b was obtained by removing the CDCl3
solvent in vacuo from a solution that had been shown by
NMR to contain predominantly that conformer. The resul-
tant solid was dissolved in dichloromethane and left in am-
bient light for one hour to ensure that any 13a conformer
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Intramolecular Charge-Transfer Processes
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could convert to 13b. The UV/Vis spectrum of this solution
was then collected (see Figure S4 in the Supporting Informa-
tion). This spectrum is practically identical to the one ob-
tained after sufficient time had elapsed in the conversion ex-
periment detailed above. The lmax is seen at 395 nm, with
two higher energy peaks at l=320 nm and l=270 nm.
A similar effect can be observed in dendralene 15. A solu-
1
tion of a sample of 15 that had been identified by H NMR
as having the donor–acceptor coplanar conformation was
dissolved in dry CH2Cl2 in the absence of light. Absorption
spectra were recorded over time with exposure to ambient
light in the same fashion as for 13 detailed above. The spec-
tra are shown in Figure 6. The fresh solution before expo-
Figure 7. Top) Kinetics plot of the rate of change in the absorbance of a
solution of 15 in CH2Cl2 at l=455 nm with time, upon exposure to ambi-
ent light. Bottom) Kinetics plot of the rate of change in the absorbance
of 15 at l=368 nm with time, upon exposure to ambient light.
Figure 6. Evolution of UV/Vis absorption spectra of a solution of 15 in
CH2Cl2 over time when exposed to ambient light.
the dendralenes and, in the case of 15, polymerisation under
oxidative conditions and the properties of the resulting ma-
terial. A solution of 13a (10ꢀ4 m) and supporting electrolyte
(tetra-n-butylammonium hexafluorophosphate, 0.1m) in dry
CH2Cl2 was prepared in the absence of light, and the cyclic
voltammogram was recorded by using a glassy carbon work-
ing electrode, Pt wire counter electrode and silver wire
pseudo reference electrode. The solution was then exposed
to ambient light and the cyclic voltammogram was repeated-
ly recorded after several 15 min intervals. The cyclic voltam-
mograms are shown in Figure 8.
The oxidation of the bromothiophenes in 13a is shown by
a peak at +1025 mV, which shifts gradually to lower poten-
tials as the solution is exposed to light and finally stabilises
at +913 mV. As 13a converts to 13b the two bromothio-
phenes change from each being independently conjugated to
sure to light gives a maximum absorption at l=455 nm.
Note the bathochromic shift (52 nm) relative to 13a result-
ing from the incorporation of the EDOT group into the den-
dralene p system. Exposure to light causes the intensity of
this band to decrease, and a higher energy absorption band
at l=368 nm emerges and becomes more intense. After
300 min exposure to ambient light the spectrum ceases to
change and exhibits an absorption maximum at l=453 nm.
The kinetics of this process were compared with 13.
Figure 7 shows the rate of change of the absorbance at l=
455 nm (top), and the rate of change of the absorbance at
l=368 nm (bottom). The data are a good fit to the first-
order rate law. The value of the first order rate constant for
the diminishing band at l=455 nm (k=0.013 minꢀ1, 2.2ꢃ
10ꢀ4 sꢀ1) is equal to the rate constant of the emerging peak
centred at l=368 nm, enforcing the argument for first-order
kinetics and proving that the contribution of by-processes to
the consumption of 15 is negligible. The value of k for den-
dralene 15 is an order of magnitude lower than that for the
same process in dendralene 13 (k=0.0027 sꢀ1).
a
strongly electron-withdrawing group (nitrobenzyl) to
being conjugated to each other. The absence of a conjugated
electron-withdrawing group renders the removal of a bro-
mothiophene electron less demanding, and this is consistent
with the observed shift of the oxidation potential to lower
energy.
Cyclic voltammetry of dendralene 15 in dichloromethane
was performed in the absence of light, in the same fashion
as 13 (see Figure S5 in the Supporting Information). The
main feature is the oxidation of the EDOT group to form a
Electrochemistry: Cyclic voltammetry (CV) experiments
were conducted to probe the electrochemical properties of
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P. J. Skabara et al.
Figure 9. Electropolymerisation of 15 on glassy carbon working electrode,
by using Pt gauze counter and silver wire pseudo reference electrodes, in
CH2Cl2 (substrate concentration about 10ꢀ4 m) containing nBu4NPF6 as
supporting electrolyte (0.1m), scan rate=100 mVsꢀ1. Potentials are refer-
enced to ferrocene as internal standard.
mer backbone, at +265 mV. A cyclic voltammogram of a
film of poly(15) showing oxidation and reduction processes
is shown in Figure 10. This scan was performed in the nega-
Figure 8. Change in cyclic voltammograms of 13a over time whilst ex-
posed to ambient light. Top) Full scan. Bottom) Expansion of thiophene
oxidation peak. Glassy carbon working electrode, Pt wire counter and
silver wire pseudo reference electrodes in CH2Cl2 (substrate concentra-
tion about 10ꢀ4 m) containing nBu4NPF6 as supporting electrolyte (0.1m),
scan rate=100 mVsꢀ1. Potentials are referenced to ferrocene as internal
standard.
radical cation at +465 mV. A second oxidation process is
observed at higher potential (+1499 mV), but considering
that the two EDOT–thiophene units are not conjugated in
this dendralene structure, it is assumed that the wave at
+465 mV represents the simultaneous oxidation of both
EDOT–thiophene fragments. The cyclic voltammogram rep-
resenting the reduction of 15 is shown in Figure S6. The
first, reversible reduction wave occurs at a half-wave poten-
tial at ꢀ1403 mV and the second, irreversible reduction
occurs at ꢀ1822 mV. The final reduction, resulting in the ir-
Figure 10. Reduction and oxidation of poly(15) deposited on glassy
carbon working electrode, by using Pt counter and silver wire pseudo ref-
erence electrodes, in monomer-free acetonitrile containing nBu4NPF6 as
supporting electrolyte (0.1m), scan rate=100 mVsꢀ1. Potentials are refer-
enced to ferrocene as internal standard.
tive direction first so as to reduce interference from oxygen.
However, despite bubbling argon into the solution for
20 min, oxygen reduction was still observed as a peak at
ꢀ1293 mV. This was confirmed as an oxygen peak by run-
ning sequential differential pulse voltammetry measure-
ments on a fresh polymer film. In this more sensitive
method, the peak at ꢀ1293 mV became more intense in
each scan indicating that it is caused by oxygen. Performing
the reduction cycle first also had the effect of raising the po-
tential of the first oxidation to +548 mV, indicating that the
reductive conditions degrade the polymer. The multiple oxi-
dation peaks at +548, +787 and 1094 mV are due to the for-
mation of polarons and bipolarons in the polymer chain.
The electrochemical band gap of poly(15) was deduced
from the full-scan cyclic voltammogram by the difference
between the onset of the first reduction and oxidation
waves; HOMO/LUMO energy values were measured rela-
reversible formation of
a radical trianion, occurs at
ꢀ2273 mV.
Repeated cycling over the oxidation wave corresponding
to the EDOT–thiophene group (between ꢀ380 mV and
+670 mV, 30 cycles), caused the deposition of a film of
poly(15) on the glassy carbon working electrode. The elec-
trochemical growth of the film can be seen in Figure 9. The
development of a new peak in the range +200 to +400 mV
indicates the electroactive character of the thiophene-
EDOT-EDOT-thiophene segment within the polymer back-
bone. The cyclic voltammogram showing oxidation of the
polymer film adhered to the working electrode in monomer-
free electrolyte solution is shown in Figure S7 in the Sup-
porting Information. The main feature is the oxidation of
the thiophene-EDOT-EDOT-thiophene segment in the poly-
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Intramolecular Charge-Transfer Processes
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tive to ferrocene (see Figure S8 in the Supporting Informa-
tion). The electrochemical band gap of poly(15) was thus
calculated as 1.72 eV.
Cyclic voltammograms of a film of poly(15), recorded
over a range of scan rates, are shown in Figure S9 in the
Supporting Information, together with the corresponding
plot of scan rate versus maximum current of the oxidation
process. In Figure S9 in the Supporting Information the
peak maximum shifts to higher potentials with higher scan
rate, which is a result of the oxidation being irreversible;
this figure also shows a linear relationship of scan rate
versus peak current, concluding that the oxidation process
of the polymer is not diffusion limited.[67]
A film of poly(15) was grown on ITO-coated glass and
the UV/Vis spectrum recorded. Figure S10 in the Supporting
Information shows the spectrum of the film immediately
after growth, in the doped state. There are absorption
maxima at l=337, 424, 569 and 759 nm. The minor peak at
l=759 nm indicates a low level of doping and the transition
is due to the charge transfer of polarons and bipolarons
from one chain to another. Figure S11 in the Supporting In-
formation shows the absorption spectrum recorded after de-
doping the film by cycling between ꢀ0.1 V and +0.1 V at
100 mVsꢀ1 for one hour. The absorption maxima at l=337,
425 and 569 nm remain but the peak at l=759 nm is absent
as a result of complete de-doping. The onset of the longest
wavelength peak in Figure S11, which was found to occur at
l=690 nm, was used to estimate the optical band gap of the
polymer at 1.80 eV. This is in good agreement with the elec-
trochemically calculated band gap (1.72 eV).
UV/Vis spectroelectrochemistry was performed on a film
of de-doped poly(15) on indium tin oxide (ITO)-coated
glass. The absorption spectra were recorded at a range of
applied potentials from 0 to +2000 mV (Figure 11, top). In
the neutral state, the film appears brown, but becomes blue
when doped. At potentials above +500 mV, absorption
bands at l=569 and 759 nm intensify, the latter as a result
of the formation of polarons in the polymer chain. The
sharp and prominent peak at l=759 nm suggests that the
polarons are localised within one region of the polymer
chain, with a high probability that the bis-EDOT units are
hosts for the cation radicals. At around +1500 mV, the
bands at l=337, 425, 569 and 759 nm diminish as a result of
the generation of bipolarons and a rearrangement of the p-
conjugated character in the polymer chain. The spectra re-
corded under the range of negative applied potentials are
shown in Figure 11 (bottom). There is an increase in absorp-
tion in the wavelength range between l=500 and 900 nm at
voltages beyond ꢀ250 mV. This matches the peak seen in
the polymer reduction (seen at around ꢀ1500 mV when ref-
erenced to ferrocene), and arises from the charge transfer of
anion species between polymer chains. Due to the relatively
high oxidation potential of poly(15), compared to PEDOT
(polyethylenedioxythiophene) derivatives, together with the
value of the band gap (1.7–1.8 eV) and the sharp nature of
the spectroelectrochemical features, it is reasonable to
assume that the conjugation path of the polymer is limited
Figure 11. Top) Oxidative spectroelectrochemistry of poly(15) on ITO-
coated glass working electrode, by using Pt wire counter and silver wire
pseudo reference electrodes, in monomer-free acetonitrile containing
nBu4NPF6 as supporting electrolyte (0.1m). Bottom) Reductive spectroe-
lectrochemistry of poly(15).
to nitrostyryl-thiophene-EDOT-EDOT-thiophene-nitrostyryl
(acceptor–extended donor–acceptor) segments. This means
that the a conformer has been frozen in the electropolymeri-
sation experiment and that the conformer is persistent in
the solid state.
Computational studies: All calculations were run by using
Gaussian 03.[68] Density functional theory calculations at the
B3LYP/6-31G* level on the crystal structures of 13a and
13b showed that 13a is the lower-energy conformer, and the
energy difference between the two forms was shown to be
70.2 kJmolꢀ1. The effect of bulk solvent was considered
through the polarisable continuum model, by using a rela-
tive dielectric constant of er =4.9 for chloroform. This
showed a small increased energetic preference for 13a with
an energy difference of 73.4 kJmolꢀ1 between the two con-
formers.
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P. J. Skabara et al.
Selected molecular orbitals of 13a and 13b at isosurface
values of 0.03 are shown in Figure 12. In 13a the two highest
energy occupied orbitals, HOMOꢀ1 and HOMO are largely
localised on one or the other of the donor–acceptor “arms”,
with only a small contribution from the other arm. The
LUMO and LUMO+1 levels of 13a, on the other hand, are
evenly distributed between the two arms. Promotion of an
electron from either HOMOꢀ1 or HOMO to either of
these unoccupied molecular orbitals, therefore, involves a
component of transfer from one arm to the other. Electron
transfer by this mechanism in 13a would, therefore, occur in
the directions of the acceptor arms.
In contrast, the LUMO and LUMO+1 orbitals of 13b
are localised along the same axis. Promotion to these orbi-
tals will have the effect of transporting charge along this
axis. The HOMOꢀ2 and LUMO+2 orbitals of 13b repre-
sent p-bonding and antibonding components localised on
the nitrostyryl arms. It is noteworthy that in HOMOꢀ2 the
calculations predict a degree of conjugation between the ni-
trostyryl units, through a central node. This is an unexpected
finding given the twisted molecular geometry, marking this
conformer as a potential model for a cross-conjugated meso-
meric betaine.[69]
The eigenvalues (in eV) of the HOMOꢀ1, HOMO,
LUMO and LUMO+1 orbitals of 13a and 13b (calculated
in the gas phase and with polarisable continuum model for
chloroform) are shown in Table 1. In the gas phase and in
Table 1. Eigenvalues of HOMOꢀ1, HOMO, LUMO and LUMO+1 of
13a and 13b.
13a (gas 13a
13b (gas 13b
phase)
N
ACHTUNGTRENNUNG
HOMOꢀ1 [eV]
ꢀ5.908
ꢀ5.842
0.066
ꢀ2.794
ꢀ2.701
0.093
ꢀ5.665
ꢀ5.600
0.065
ꢀ2.703
ꢀ2.650
0.053
ꢀ6.636
ꢀ5.557
1.079
ꢀ2.547
ꢀ2.462
0.085
ꢀ6.458
ꢀ5.348
1.110
ꢀ2.493
ꢀ2.425
0.069
HOMO [eV]
DE (HOMOꢀ1/HOMO) [eV]
LUMO [eV]
LUMO+1 [eV]
DE (LUMO/LUMO+1) [eV]
the presence of chloroform there is a strong equivalence be-
tween the HOMO and HOMOꢀ1 pair, and between the
LUMO and LUMO+1 pair of 13a, as evidenced by the
small energy difference between each pair of eigenvalues.
The LUMO and LUMO+1 pair of 13b is also strongly
equivalent. These small differences in eigenvalue probably
originate from the use of the crystal structure data that do
not precisely represent the optimised gas-phase geometry. It
should be noted that there is only a small effect from incor-
porating the solvent dipole into the calculations.
The HOMO/HOMOꢀ1 pair in 13b exhibit greater differ-
ence than in 13a. The HOMO (Figure 12g) is localised on
the bis-5-bromothiophene-1,4-butadiene segment, and fol-
lows the expected double-bond pattern for the ground state.
The HOMOꢀ1 (Figure 12 f) is also located over the same
atoms but shows double-bond character between the two
central carbon atoms. This difference is reflected in the ei-
genvalue difference of around 1.1 eV. For both forms, 13a
and to a greater extent 13b, there is spatial separation be-
tween the HOMO/HOMOꢀ1 and LUMO/LUMO+1 orbi-
tal pairs. Such an arrangement is desirable for tailoring the
energy of either frontier orbital by molecular design.
Time dependant DFT calculations were performed to in-
vestigate the excited states of 13a and 13b in the gas phase
and in chloroform. Table S1 and Table S2 show the first ten
excited states of 13a and 13b, respectively. The excitation
Figure 12. Selected molecular orbitals of 13a (a–d) and 13b (e–j) at iso-
surface values of 0.03, calculated by using Gaussian 03.[68] a) HOMOꢀ1,
b) HOMO, c) LUMO, d) LUMO+1, e) HOMOꢀ2, f) HOMOꢀ1,
g) HOMO, h) LUMO, i) LUMO+1 and j) LUMO+2.
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Intramolecular Charge-Transfer Processes
FULL PAPER
energy is shown, along with the dominant transition that is
responsible for the excitation. That is, the electronic transi-
tion that has the highest wave function coefficient. The
square of the wave function coefficient is the weighting of
this transition to the overall excitation. The oscillator
strength of the excitation is also shown, which relates direct-
ly to the strength of that band in the electronic absorption
spectrum.
The excited state data can be used to interpret the UV/
Vis spectrum of 13. The first four excited states of 13a rep-
resent the longest wavelength absorptions in the spectrum.
Transitions from the HOMO or HOMOꢀ1 to the LUMO or
LUMO+1 frontier orbitals shown in Figure 12 are the larg-
est contributors to these four excitations, as evidenced by
the coefficient of the wave function. The largest oscillator
strength among these four excitations can be expected to
dominate the appearance of the spectrum and give rise to
the main bands. In the gas phase and the chloroform calcu-
lations for 13a, the largest value of the oscillator strength
occurs for the fourth excited state transition, predominantly
a HOMOꢀ1!LUMO+1 transition. The energy of this ex-
citation was calculated to be 411.91 nm in the gas phase, and
454.81 nm in chloroform. The experimental lmax of the UV/
Vis spectrum in chloroform was recorded as 404 nm.
The only other significant oscillator strength value for
transitions in 13b occurs for the transition HOMOꢀ2!
LUMO, predicted at l=324 nm (CHCl3), or l=306 nm (gas
phase). This is in good agreement with the higher energy
band seen at l=320 nm in the UV/Vis spectrum of 13b.
The Gaussian calculations predict that the HOMOꢀ2 and
the LUMO orbital of 13b are predominantly localised on
the nitrostyryl arms. Therefore, there is reasonable agree-
ment between theoretical and experimental results. The
small observed shift to higher energy of the UV/Vis absorp-
tion maximum on conversion of 13a to 13b is reflected in
the predicted largest absorption bands.
Mechanism for interconversion: A mechanism for the con-
version of 13a to 13b in solution is proposed in Scheme 5.
The conformer 13a undergoes intramolecular charge trans-
fer (ICT) by absorption of light to form a charge-separated
excited state. The proposed quinoidal p-bonding in the ex-
cited state is supported by the computational modelling re-
sults, which show that promotion of electrons from the
HOMO (Figure 12b) and HOMOꢀ1 (Figure 12a) to the
LUMO (Figure 12c) or LUMO+1 (Figure 12d) results in
ꢀ
p-bond electron density localised on the appropriate C C
bonds. The newly formed single bonds confer additional ro-
tational freedom on the molecule and the excited state
structure is able to rearrange to another excited state geom-
etry. Rotation of the bonds indicated with curved arrows
gives rise to this structure in which the bond between the
carbons a and b to the thiophenes are twisted to give a syn
relationship between the donor–acceptor pair. The excited
ACHTUNGTRENNUNG(E,E)-1,4-Di(2’-thienyl)-1,3-butadiene 23 is analogous to
the bis-bromothiophene butadiene p system in 13b. The
spectroscopic properties of 23 have
been reported elsewhere.[70] The lmax
is reported at 381 nm in CH2Cl2. This
wavelength is 14 nm shorter than the
ꢀ
absorption maximum for 13b. At first
inspection, the p–p* transition in 23
seems to be a good model for inter-
molecule is able to twist around the central C C bond to
form a planar structure. Relaxation to a neutral form is pos-
sible when the excited electrons return through the conju-
gated framework to the thiophene group, while the nitro-
benzyl groups twist out of plane to reduce steric clash with
the thiophene groups. This results in structure 13b.
pretation of the UV/Vis spectrum of 13b. A substituent
effect, which is expected to be dominated by the strongly
electron-withdrawing nitrostyryl units, could be invoked. A
comparison between 22 and 13a shows that the difference in
lmax arising from the substituent effect of one donor–accept-
or “arm” that is absent in 22 is 7 nm. Considering 23 and
13b, the dendralene absorption is affected by the presence
of two nitrostyryl groups that are absent in 23. Thus, the ob-
served substituent effect is exactly double, which is shown
by the comparison between 22 and 13a. However, the theo-
retical calculations on 13b predict that the largest value for
the oscillator strength occurs for the 3rd excited state, which
is predominantly a HOMO!LUMO+2 transition. The
energy of this excitation was calculated to be 380.35 nm in
the gas phase, and 396.16 nm in chloroform. This also coin-
cides well with the experimental lmax of the UV/Vis spec-
trum in chloroform (395 nm). The HOMO of 13b (Fig-
ure 12g) is localised on the bisbromothiophene-butadiene
unit, while the LUMO+2 orbital (Figure 12j) is localised
on the acceptor nitrobenzene units. It is surprising that this
intramolecular charge transfer process is predicted in 13b,
given the twisted geometry between these units. This pro-
cess, which is predicted only to occur in solvated conditions,
not the gas phase, must be a through-space event.
Interestingly, structure 13a, which was the predominant
conformer in the mixture that was first isolated (as identi-
fied by TLC and H NMR spectroscopy), is analogous to the
1
structure of the precursor aldehyde 11. It is reasonable to
suggest that this donor–acceptor conformation is retained
during the Wittig reaction, and kinetically stable under the
reaction conditions (heating to reflux). In the solution state,
1H NMR indicates that the aldehyde 11 has the same confor-
mation as the crystal structure. The NMR shows no change
when the solution is exposed to ambient light over several
days. This suggests that an aldehyde group is not a strong
enough acceptor for the conformational change to occur.
Conclusion
Two new [4]-dendralenes bearing thiophene–nitrostyryl
donor–acceptor units have been prepared as mixtures of
conformers. Pure samples of the conformers have been iso-
lated by recrystallisation or flash column chromatography to
allow the study of light-driven conformational transforma-
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11591

P. J. Skabara et al.
Scheme 5. Proposed mechanism for the conversion of 13a to 13b.
tions. In their application as electronic materials, the two
types of conformers represent useful monomers for semi-
conductor conjugated polymer structures (13b and 15b) or
unusual conjugated donor–acceptor architectures for second
order non-linear optics (13a and 15a). The conversion of
conformers a to b is facile and is initiated by photoexcita-
tion, giving rise to a change in double-bond character be-
tween conjugated donor–acceptor pairs through an intramo-
lecular charge transfer process. The reverse process is for-
bidden through the same mechanism since the donor–ac-
ceptor components are no longer conjugated in the b con-
formers. The single/double bonds remain localised in such
molecules and this situation prohibits the key twist in the
structure that is necessary for the regeneration of a con-
formers. Whilst through-space ICT does not allow switching
from conformer 13b to 13a, the reverse step can be partially
achieved by the application of heat. Controlling the conver-
sion between two such very different conformers in poly-
meric structures would be extremely valuable in the field of
organic electronics. From this viewpoint, these promising ex-
periments will be expanded to answer the following ques-
tions: Can the reversibility of the a!b switch be improved
by applying an alternative stimulus to heat (e.g. a redox pro-
cess)? Are the kinetics of the a!b switch size-dependent?
Can the transformation be applied to macromolecules? If
these issues can be addressed favourably, then the realisa-
tion of conjugated poly(dendralene)-based switches will pro-
vide a new and fascinating direction for plastic electronics.
Acknowledgements
A.L.K. would like to thank the EPSRC for funding (GR/T28379).
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Received: March 12, 2009
Published online: September 16, 2009
Chem. Eur. J. 2009, 15, 11581 – 11593
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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