A supramolecular photoswitch constructed by intermolecular hydrogen bond between BTEPy and TTF-COOH

Article abstract of DOI:10.1016/j.cplett.2008.02.091

A novel supramolecular photoswitch containing bisthienylethene-pyridine (BTEPy) and carboxyl attached tetrathiafluvalene (TTF-COOH) was constructed via intermolecular hydrogen bond. FT-IR spectra, XPS characterizations, ¹H NMR and theoretical calculation were carried out to verify the formation of the intermolecular hydrogen bond. The supramolecular self-assemblies BTEPy · 2TTF show good photochromic properties. A molecular switch with UV/vis light as inputs and electrochemical signals as outputs was obtained.

Full text of DOI:10.1016/j.cplett.2008.02.091

Chemical Physics Letters 455 (2008) 256–260
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
A supramolecular photoswitch constructed by intermolecular hydrogen bond
between BTEPy and TTF–COOH
Yanli Feng a, Qiong Zhang a,b, Wenjuan Tan a, Deqing Zhang c, Yaoquan Tu b, Hans Ågren b, He Tian a,*
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, PR China
b Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, SE-10691 Stockholm, Sweden
c Key Laboratory of Organic Solid States, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
novel supramolecular photoswitch containing bisthienylethene-pyridine (BTEPy) and carboxyl
Received 7 December 2007
In final form 26 February 2008
Available online 2 March 2008
attached tetrathiafluvalene (TTF–COOH) was constructed via intermolecular hydrogen bond. FT-IR spec-
tra, XPS characterizations, 1H NMR and theoretical calculation were carried out to verify the formation of
the intermolecular hydrogen bond. The supramolecular self-assemblies BTEPy Á 2TTF show good photo-
chromic properties. A molecular switch with UV/vis light as inputs and electrochemical signals as outputs
was obtained.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
Here, we have designed a self-assembled molecule (BTE-
Py Á 2TTF) constructed by intermolecular hydrogen-bonding inter-
action between bisthienylethene-pyridine and carboxyl attached
tetrathiafluvalene. Pyridine groups are widely used for interaction
with carboxylic acids via intermolecular hydrogen bonds [21–29].
The self-assembled molecule BTEPy Á 2TTF (Scheme 1) shows good
photochromic properties which could be triggered between the
colorless ring-open and colored ring-closed forms by alternating
irradiation with UV and visible light. Electrochemical properties
of the self-assembled molecule BTEPy Á 2TTF were also discussed
both experimentally and theoretically. The oxidation potential of
the self-assembled complex BTEPy Á 2TTF could be reversibly mod-
ulated by alternating UV and visible light irradiation. And by apply-
ing different potentials (vs Ag/AgCl), absorption spectra showed
distinct absorption bands for the three states of TTF, TTFÅ+ and
The self-assembly of functional molecules into supramolecules
has attracted much interest, since it is one of the most powerful
approaches towards development of new materials and devices.
Among various types of photochromic compounds, bisthienyleth-
ene derivatives (BTEs) are one of the most promising materials be-
cause of their excellent fatigue resistance and thermal stability in
both isomeric forms, picosecond switching times, and fairly high
photocyclization quantum yields [1–4]. The open and closed iso-
mers of BTEs differ from each other not only in their absorption
but also in various physical and chemical properties, such as lumi-
nescence, refractive indices, oxidation/reduction potentials, chiral
properties and magnetic interactions [5–8]. Reversible light-
responsive supramolecules are most desirable since only light is
used as the input stimulus without other additives needed [9,10].
Therefore, lots of supramolecular self-assembled systems employ-
ing a photochromic bisthienylethene moiety as a photoreversible
switch have been reported [11–13].
TTF2+
.
2. Experimental
Tetrathiafulvalene (TTF) [14] is a reversible and stable two-elec-
tron donor which has been intensively studied for more than two
decades, mainly with the aim of developing organic superconduc-
tors. TTF and its derivatives are strong electron donors, which can
be reversibly transformed into the corresponding radical cation
TTFÅ+ and dication TTF2+ through either chemical or electrochemi-
cal oxidations at easily accessible potentials [15]. Because of this
feature, functional molecules and supramolecules containing TTF
units were used for molecular switches, logic gates, sensors, and
machines [16–20].
Reagents and starting materials were used as received. Solvents
were distilled and dried before use. The precursors BTEPy [30] and
TTF–COOH [31] were synthesized according to published proce-
dures, and well characterized by 1H NMR and MS. The self-assem-
bled composite was yielded by dissolving and stirring a mixture of
BTEPy and TTF–COOH with molar ratio of 1:2 in THF and the sol-
vent was removed in vacuo.
Absorption spectra were measured on a Varian Cary500 UV-Vis
spectrophotometer. The optical switch experiments were carried
out in a photochemical reaction apparatus with a 200 W Hg
lamp. Cyclic voltammetry measurements were carried out at a
platinum electrode using millimolar solutions in CH2Cl2 (distilled
over CaH2 prior to use) containing 0.1 M of the support electrolyte,
* Corresponding author. Fax: +86 21 64252288.
E-mail address: tianhe@ecust.edu.cn (H. Tian).
0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.02.091

Y. Feng et al. / Chemical Physics Letters 455 (2008) 256–260
257
Scheme 1. Photochromic process in BTEPy Á 2TTF.
tetrabutylammonium hexafluorophosphate. And it was in a three
electrode cell (Pt working and counter electrodes and Ag/AgCl ref-
erence electrode) and potentiostat assembly was by VersaStarII
electrochemical analyzer. Electrochemical oxidation and reduction
were carried out at graphite plate as working electrode. FT-IR spec-
tra were performed on Nicolet 380 with KBr pellets. X-ray photo-
electron spectra (XPS) were measured on a Perkin Elmer PHI
5000C ESCA System. All theoretical calculations were carried out
within the DFT approach in gas phase using the GAUSSIAN 03 pro-
gram [32]. DFT calculations were performed using hybrid B3LYP
exchange-correlation functional and the 6-31GÃ basis set.
Throughout the calculations, the long hexylthio side chains on
TTF units have been replaced by methylthio groups in order to
cut the computational cost with no significant change in the
electronic properties of TTF [33].
in BTEPy was 400.4 eV (Fig. 1b). A higher binding energy feature
(400.8 eV) was observed for the self-assembled complex BTE-
Py Á 2TTF, larger by 0.4 eV than that in BTEPy. In principle, the for-
mation of hydrogen bond or protonated nitrogen leads to an
upshift of the binding energy of N 1 s due to the decrease in elec-
tron density in nitrogen atoms [22,24,26]. The binding energy
change of 0.4 eV in the present BTEPy Á 2TTF complex relative to
BTEPy may be ascribed a hydrogen bond interaction [27]. And it
should not be ionic interaction, because the change was supposed
to be more than 2.0 eV for the ionic interaction in a protonated en-
tity [22,24,26,27]. The binding energy of mixture of BTEPy and TTF
powder (BTEPy (open form): TTF = 1:2) did not have obvious up-
shift compared with that in BTEPy. These results were consistent
with the IR spectral data.
1H NMR is a useful tool to study supramolecular interactions. In
Fig. 1c, BTEPy (open form) in DMSO-d6 shows an array of reso-
nances corresponding to the aromatic protons in the molecule.
The signal of carboxylic acid of TTF–COOH was observed at about
d = 12.700 in dried DMSO-d6. The formation of hydrogen bonds
could affect the protons at the a-position of pyridine (H1). As
shown in Fig. 1c, the H1 protons exhibited double peaks at 8.476.
With the addition of 2 equiv of TTF–COOH, the peaks moved to
8.532 and there was a downfield shift for this pair of peaks. The
hydrogen bond formation also influenced the protons at the b-po-
sition (H2) of pyridine, the change of chemical shift Dd was 0.042
when the ratio of BTEPy (open form): TTF–COOH = 1:2. A single
peak at d = 7.554 was for thienyl protons (H3), which also exhibited
a small downfield shift after the addition of TTF–COOH. These
changes arose from the fact that the weak acid TTF–COOH, acting
as a proton donor, could reduce the electron density on the aryl
ring of BTEPy. When TTF–COOH was increased to 3 equiv, 1H
NMR spectrum had no obvious changes, compared with that of
2 equiv of TTF–COOH.
3. Results and discussion
To verify the interaction mode between pyridine and carboxylic
acid during the self-assembly, FT-IR spectra, XPS and 1H NMR spec-
tra were measured (The measurements were performed on the
ring-open form of the BTEPy and BTEPy Á 2TTF) and theoretical cal-
culation was carried out. Room temperature FT-IR spectra of BTE-
Py, TTF–COOH, self-assembled BTEPy Á 2TTF and mixture of BTEPy
and TTF powder (BTEPy (open form):TTF = 1:2) were showed in
Fig. 1a. Carboxylic acid dimers (H-bonded carbonyl) could be cre-
ated in TTF–COOH solution. The band at 1686 cmÀ1 in TTF–COOH
was ascribed to the stretching vibration of carboxylic acid dimers
(H-bonded carbonyl) [21,25,29]. This band diminished and a new
band centered at 1695 cmÀ1 was observed in the self-assembled
BTEPy Á 2TTF, which was attributed to the complex formation be-
tween the pyridine unit of BTEPy and the carboxylic acid units of
TTF–COOH [23,25,28,29]. Upon formation of the composite, the
characteristic modes of the pyridine group at 1596, 1498 and
1462 cmÀ1 shifted to 1603, 1501 and 1466 cmÀ1, respectively.
The vibrations of the pyridine group at 1539 and 1411 cmÀ1 dimin-
ished completely in the complex. The most striking feature was the
presence of a broad band at 1922 cmÀ1 in BTEPy Á 2TTF, indicative
of strong hydrogen bonds between the carboxylic acid and pyri-
dine moieties [21,24,25]. No apparent new absorption band around
1620 cmÀ1, corresponding to a characteristic band of protonated
pyridine, was observed in assembly, suggesting that the complex
is formed through hydrogen bond interaction rather than ionic
interaction [23]. Obviously, the intermolecular hydrogen bond
interaction did act as a driving force for the formation of self-
assembled composite, where the carboxylic acid served as the do-
nor and pyridine group with a lone electron pair as the acceptor.
From FT-IR spectra of mixture of BTEPy and TTF powder (BTEPy
(open form): TTF = 1:2) in Fig. 1a, it could be get that just having
a powder mixture of the two precursors might not have the same
effect in creating intermolecular hydrogen bond.
The B3LYP/6-31G* optimized geometries of ring-open and ring-
closed isomers for BTEPy Á 2TTF were shown in Fig. S3 (Supporting
Information). Single point energy calculations were carried out for
BTEPy, TTF–COOH and BTEPy Á 2TTF at the B3LYP/6-31+G* level
with the corresponding B3LYP/6-31G* optimized geometries.
Hydrogen bond energy was calculated and shown in Table S1 (Sup-
porting Information). The hydrogen bond energy in the BTE-
Py Á 2TTF ring-open form (EH-bond–O
) was calculated to be
0.4810 eV, and that in the ring-closed form (EH-bond–C
0.4785 eV.
) was
Absorption spectra of the BTEPy, TTF–COOH and BTEPy Á 2TTF
were investigated and shown in Fig. 2. Upon irradiation with light
of 254 nm for 20 min, the colorless open-ring form BTEPy became
purple (k = 558 nm), which was due to the transformation of BTEPy
by photocyclization into its closed form. BTEPy could be photo-
chemically regenerated from its closed form by irradiation with
light of 570 nm. Similarly, upon UV light irradiation at 254 nm
for 20 min, BTEPy Á 2TTF was also photoswitched to its closed form
with the appearance of its characteristic absorption band at
k1max = 374 nm, k2max = 557 nm. From Fig. 2b, no obvious differ-
ence in the spectra of the closed ring form between BTEPy and BTE-
Py Á 2TTF was got. The absorption spectrum of BTEPy Á 2TTF (ring
open form) was approximately the sum of the absorption spectra
Since the binding energy of N 1 s is very sensitive to the envi-
ronmental perturbation, XPS characterization was used to verify
the hydrogen bond formation [10]. The realistic error margin on
the N 1 s binding energy value was about 0.1 eV in the XPS mea-
surements. The N 1s binding energy value of the pyridine group

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Y. Feng et al. / Chemical Physics Letters 455 (2008) 256–260
Fig. 1. Verification of the intermolecular hydrogen bond: (a) Room temperature FT-IR spectra of mixture (a mixture of powder BTEPy (open form): TTF = 1:2), self-assembled
BTEPy Á 2TTF (open form), TTF–COOH and BTEPy (open form); (b) N 1 s spectra (XPS) of BTEPy (open form), self-assembled BTEPy Á 2TTF (open form) and mixture (a mixture of
powder BTEPy (open form): TTF = 1:2); (c) Partial 1H NMR spectra (in DMSO-d6): line 1 and a are partial 1H NMR spectra of BTEPy (open form): TTF–COOH = 1:3; line 2 and b
are partial 1H NMR spectra of BTEPy (open form): TTF–COOH = 1:2; line 3 and c are partial 1H NMR spectra of BTEPy (open form); line 4 and d are partial 1H NMR spectra of
TTF–COOH.
of compounds BTEPy (ring-open form) and TTF–COOH (in Fig. 2a).
The absorption position of the ring closed form was almost similar
(in Fig. 2b). On the basis of the above results, it could be concluded
that no strong electronic interaction existed between the TTF unit
and the BTEPy unit in BTEPy Á 2TTF.
by ca. 27 mV. The small oxidation wave at 1.3 V was shifted to
1.22 V, accompanied by the appearance of a small oxidation wave
at 0.79 V.
To elucidate the electrochemical properties, the molecular orbi-
tal for the BTEPy Á 2TTF both in its ring-open form and ring-closed
form was analyzed for the B3LYP/6-31GÃ optimized geometries.
The HOMO (the Highest Occupied Molecular Orbital) and SOMO
(the Second-highest Occupied Molecular Orbital) of the BTE-
Py Á 2TTF (open form) were found to be mainly localized on the
two TTF units with close energy of À4.907 eV and À4.914 eV (See
Supporting information Table S3 and Fig. S4). This helped to ex-
plain the first couple at 0.65 V covered two unresolved oxidation
processes, corresponding to two electrons almost simultaneously
removed from the two TTF groups to form two TTFÅ+. The other
two molecular orbitals on these two TTF units were also with the
The electrochemical studies of BTEPy Á 2TTF were performed on
VersaStar II electrochemical analyzer. The cyclic voltammograms
of BTEPy Á 2TTF in dichloromethane containing tetrabutylammo-
nium hexafluorophosphate (0.1 M) before and after UV irradiation
were shown in Fig. 3. Two conspicuous quasi-reversible redox
waves for the open-ring isomer were observed at Eox1 = 0.65 V
and Eox2 = 0.99 V (vs Ag/AgCl), and a small oxidation wave was at
1.3 V. After the solution of BTEPy Á 2TTF was exposed to UV light
(254 nm) for 30 min, the anodic potential Eox1 was shifted to the
positive region by ca. 11 mV while the Eox2 was positively shifted

Y. Feng et al. / Chemical Physics Letters 455 (2008) 256–260
259
Fig. 2. Absorption spectra of TTF–COOH (2.0 Â 10À5 mol LÀ1), open form (O) of BTEPy (1.0 Â 10À5 mol LÀ1), open form (O) of BTEPy Á 2TTF (1.0 Â 10À5 mol LÀ1) in THF, Sum of
absorption spectra of TTF–COOH (2.0 Â 10À5 mol LÀ1) and open form (O) of BTEPy (1.0 Â 10À5 mol LÀ1), in (a). Absorption spectra of TTF–COOH (2.0 Â 10À5 mol LÀ1), photo-
stationary state (PSS) of BTEPy (1.0 Â 10À5 mol LÀ1), photo-stationary state (PSS) of BTEPy Á 2TTF (1.0 Â 10À5 mol LÀ1) in THF, Sum of absorption spectra of TTF–COOH
(2.0 Â 10À5 mol LÀ1) and photo-stationary state (PSS) of BTEPy (1.0 Â 10À5 mol LÀ1), in (b).
SOMO: À6.145 eV). As a result, oxidation waves of the BTEPy in
the closed form were at lower potentials compared with those of
BTEPy in the open form, leading to the appearance at 0.79 V of
the originally covered oxidation wave (at 1.0 V for open form) of
BTEPy. Interestingly, after irradiation with the light (>540 nm),
the initial cyclic voltammograms were almost restored. This can
be employed to construct a molecular switch with UV/visible light
as inputs and electrochemical signal as outputs.
TTF exhibits two reversible one-electron oxidation processes,
corresponding to TTF/TTFÅ+ and TTFÅ+/TTF2+ redox reactions, respec-
tively. Thus, the transformation of TTF into TTFÅ+ and TTF2+ can be
achieved by electrochemical oxidations. TTF units in the molecule
could be transformed into TTFÅ+ by application of an oxidation po-
tential at +0.75 V (vs Ag/AgCl) and the formation of TTF2+ could be
obtained by sequential oxidation at +1.1 V (vs Ag/AgCl). Further-
more, both TTFÅ+ and TTF2+ could be reduced to the neutral TTF
after applying a reduction potential at À0.1 V (vs Ag/AgCl). The
absorption spectra of BTEPy Á 2TTF (5 Â 10À5 M in dry CH2Cl2)
Fig. 3. Cyclic voltammograms of BTEPy Á 2TTF in CH2Cl2 (5.0 Â 10À4 mol LÀ1) with a
and those after electrochemical redox were shown in Fig. 4. After
scan rate 50 mV sÀ1
.
applying an oxidation potential at +0.75 V (vs Ag/AgCl) for 1 h,
similar energy of À6.191 eV and À6.196 eV (See Supporting infor-
mation Table S3 and Fig. S5), which indicated that two more elec-
trons were again almost simultaneously removed from the two
TTFÅ+ at a higher potential (Eox2 = 0.99 V) to form two TTF2+. Two
conspicuous quasi-reversible redox waves for the closed-ring iso-
mer were also ascribed to the formation of the two TTFÅ+ and two
TTF2+. Our calculation results indicated that HOMOs of both the
ring-open and ring-closed isomers of BTEPy Á 2TTF were mainly lo-
cated on the TTF units, and the HOMO energy of the ring-open iso-
mer (EHOMO = À4.907 eV) was found to be higher than that of the
ring-closed isomer (EHOMO = À4.915 eV). The higher was the HOMO
energy of the ring-open isomer, the lower its Eox. This is consistent
with our experimental results as discussed above.
The cyclic voltammetry of BTEPy (open form) showed two oxi-
dation waves at 1.0 V and 1.3 V (See Supporting information
Fig. S2). Therefore, in the cyclic voltammetry of BTEPy Á 2TTF
(ring-open form) the oxidation wave at 1.0 V belonging to the BTE-
Py unit was superposed with the second redox wave (TTFÅ+/TTF2+
)
at Eox2 = 0.99 V (vs Ag/AgCl). The HOMO and SOMO energies of
BTEPy in closed form (HOMO: À4.859 eV and SOMO: À6.083 eV)
were higher than those in the open form (HOMO: À5.709 eV and
Fig. 4. Absorption spectra of BTEPy Á 2TTF (open form) in CH2Cl2 (5.0 Â 10À5 mol -
LÀ1) after applying different potentials.

260
Y. Feng et al. / Chemical Physics Letters 455 (2008) 256–260
new absorption bands at 456 nm and 851 nm appeared, and the
absorption at 321 nm (due to neutral TTF) decreased. The absorp-
tion band at 456 nm was attributed to TTFÅ+ and that at 851 nm
was ascribed to p-dimer (TTF22+) [34]. Sequential application of
an oxidation potential at +1.1 V (vs Ag/AgCl) for another 1 h, the
absorption bands at 456 nm and 851 nm almost disappeared and
the absorption band at 362 nm appeared which was attributed to
TTF2+. Then, by the application of a reduction potential at À0.1 V
(vs Ag/AgCl) for 2 h, the absorption band at 362 nm was decreased,
indicating the reduction of TTF2+ to the neutral TTF.
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In summary, we designed a novel supramolecular photoswitch
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This work was supported by National Foundation of China
(90401026 and 50673025), National Basic Research 973 Program
(2006CB806200) and Scientific Committee of Shanghai. We thank
Prof. Fuyou Li (Fudan University, Shanghai) for his assistant to
measure the XPS.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2008.02.091.