Photoswitchable triple hydrogen-bonding motif

Article abstract of DOI:10.1039/c0cc02339f

Photochromic bis(thiazol-4-yl)maleimides, displaying enhanced binding affinity to complementary melamine receptors in their ring-closed switching state, have been developed and could pave the way to light-responsive supramolecular assemblies.

Full text of DOI:10.1039/c0cc02339f

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Photoswitchable triple hydrogen-bonding motifwz
¨
Martin Herder, Michael Patzel, Lutz Grubert and Stefan Hecht*
Received 2nd July 2010, Accepted 10th September 2010
DOI: 10.1039/c0cc02339f
Photochromic bis(thiazol-4-yl)maleimides, displaying enhanced
binding affinity to complementary melamine receptors in their
ring-closed switching state, have been developed and could pave
the way to light-responsive supramolecular assemblies.
The ‘‘bottom-up’’ organization of molecules to defined super-
structures and resulting materials requires the use of weak and
reversible non-covalent interactions between the molecular
building blocks as they enable the generation of defect free
structures. As structure formation relies on the primary event
of molecular recognition, hydrogen-bonds have been extensively
exploited as they offer directionality as well as complementarity
Fig. 1 Photoswitchable triple hydrogen-bonding motif: reversible
and can be adjusted in their strength.1,2 Another advantage of
using the supramolecular assembly approach relies on its
ability to respond to external stimuli giving rise to ‘‘smart’’
materials.3 For this purpose, hydrogen-bonding interactions
can be controlled by changes in temperature, solvent, and
pH.1,2 The formation of multiple hydrogen bonding inter-
actions can also be tuned by oxidation/reduction, altering the
intrinsic ability of one partner to act as a hydrogen-bond
acceptor and/or donor.4 In contrast to the above mentioned
stimuli, light represents a truly non-invasive stimulus combined
with unprecedented spatio-temporal resolution that, in principle,
constitutes an exquisite tool to remotely control hydrogen-
bonding interactions.2a,c Thus far such photochemical control
has been achieved primarily by manipulating the geometry and
flexibility of the key building blocks involved in the formation
of hydrogen-bonding networks5 and their higher aggregates.6
Another example relies on the photomodulation of secondary
interactions and their impact on the formation of a discrete
hydrogen-bonding complex.7 Here, we present an alternative
approach to photochemically modulate the inherent binding
strength of a triple hydrogen-bonding motif by means of
changing its electronic nature during the course of a photo-
chromic reaction.8
photochemical ring-closure (opening) leads to an enhanced (diminished)
binding of the central ADA imide moiety in 1b/2b (1a/2a) to a
complementary DAD melamine receptor 3.
in the open form is largely decoupled from the termini,
whereas in the closed form the terminal donor/acceptor groups
are in p-conjugation and hence should influence the association
with a complementary N,N-dialkylmelamine DAD moiety.
In order to readily vary the substitution pattern, a modular
synthesis of compounds 1a and 2a was devised, involving
Suzuki cross-coupling between the dibromomaleimide core
and suitable 5-methyl-2-phenylthiazol-4-ylboronic esters,
carrying either electron-donating ether or electron-accepting
ester groups in the para-position of the 2-phenyl moiety.
Thiazole termini were chosen as they are readily available
and offer great performance, in particular with regard to their
thermal and photochemical stability.12 To facilitate solubility,
n-hexyl chains were attached to both termini in each case (1a, 2a)
and furthermore incorporated into the complementary melamine
moiety 3. The respective ring-closed isomers 1b and 2b were
isolated after preparative irradiations of their ring-open
derivatives and subsequent column chromatography.
Compounds 1a/b and 2a/b display excellent photochromic
behaviour (Fig. 2). Irradiation with UV-light (lirr = 313 nm)
of a solution of 1a or 2a in methylene chloride leads to rapid
change in colour from yellow to deep-red, reflected in new
bands arising at 540 nm and 545 nm, respectively. Isosbestic
points at 329 nm in the case of 1a/b and at 256, 336, 352, 379,
and 408 nm in the case of 2a/b indicate clean two-component
processes. The thus reached photostationary states (PSSs)
contain 87% of 1b and 82% of 2b, respectively (Table 1).
Subsequent irradiation with visible light (lirr > 500 nm) effects
decolorization and the original UV-spectra are completely
restored, i.e. the ring-closed isomers are quantitatively
converted into their ring-opened forms. Repetitive switching
cycles revealed slight decomposition of 1a/b13 while 2a/b
showed excellent fatigue resistance (Fig. 2, insets). Furthermore,
both ring-closed isomers were found to be thermally stable
Inspired by the possibility of influencing the strength of
multiple hydrogen-bonding motifs by variation of p-conjugated
electron-donating/accepting groups9 and intrigued by Irie’s
diarylethene photochromes,10 which allow for significant
changes in p-conjugation when switching from the open to
the closed form, we chose to merge both aspects in our design
(Fig. 1).11 The central maleimide core is acting as a triple
hydrogen-bonding acceptor–donor–acceptor (ADA) site, which
Department of Chemistry, Humboldt-Universitat zu Berlin,
¨
Brook-Taylor-Str. 2, 12489 Berlin, Germany.
E-mail: sh@chemie.hu-berlin.de; Fax: +49 30 2093-6940;
Tel: +49 30 2093-7365
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
z Electronic supplementary information (ESI) available: Synthetic
details and characterization data. See DOI: 10.1039/c0cc02339f
c
460 Chem. Commun., 2011, 47, 460–462
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To investigate the association behaviour of each photochromic
ADA maleimide pair (1a/b and 2a/b), NMR titrations with the
complementary DAD receptor (3) were carried out in CDCl3.
Changing chemical shifts of the central imide NH-proton were
monitored with increasing amount of receptor and show
characteristic binding isotherms (Fig. 3). In both cases, the
binding curve of ring-closed isomer (1b or 2b) displays a
steeper slope that is indicative of a stronger association.
Evaluation of the data following established procedures15
allowed us to determine the respective association constants
Ka (Table 1), which are in the expected range of related
imide–melamine complexes.9d,15c Indeed, the association of
each of the ring-closed isomers to the melamine receptor is
stronger as compared to the respective ring-opened isomers.
The Ka values approximately double upon switching to the
ring-closed forms and a slightly stronger enhancement of
binding was observed in the case of the ester derivatives
2a/b. The 1 : 1 binding stoichiometry was verified by Job plots
showing maxima at molar fractions xreceptor 3 = 0.5.z Most
importantly, photochemical switching behaviour remains
unaffected by the presence of melamine 3, as essentially the
same quantum yields are observed even if the majority of the
switch is complexed.z
The observed general enhancement in the association of the
ring-closed isomers with the receptor can primarily be related
to the reduced electron-density of the maleimide core.16 This is
nicely reflected in the significantly reduced reduction potential
Fig. 2 Photochromic behaviour: UV/vis absorption spectra during
the course of irradiation (lirr = 313 nm) until reaching the PSS of (a)
1a (time intervals: t = 0, 60, 120, 180, and 500 s) and (b) 2a (time
intervals: t = 0, 10, 30, 60, and 120 s) in CH2Cl2 (c = 2 ꢀ 10ꢁ5 M,
25 1C). Insets show repetitive switching cycles (lirr = 313 nm for
ring-closure, lirr > 500 nm for ring-opening) between (a) 1a and 1b as
well as (b) 2a and 2b.
Table 1 Photochromic and association properties of both
switch pairs
Photochromisma
Associationc
Conv.b
(%)
313 nm
a-b
546 nm
b-a
lmax/nm
F
F
Ddmax/ppm Ka/Mꢁ1
1a 313, 408 0.08
1b 359, 540
2a 321, 380 0.18
2b 317, 545
0.02
0.02
87
82
7.57 ꢂ 0.02 132 ꢂ 11
7.72 ꢂ 0.06 231 ꢂ 23
7.75 ꢂ 0.05 142 ꢂ 13
7.99 ꢂ 0.03 318 ꢂ 30
a
b
In CH2Cl2 at 25 1C. Composition of the PSS upon irradiation of
the open form (1a or 2a) with UV-light (lirr = 313 nm) determined by
c
UPLC. Derived from NMR-titration data in CDCl3, 25 1C.w
over extended periods of time (days and weeks). Interestingly,
the photochromic performance of 1a/b is strongly dependent
on solvent polarity since in acetonitrile prolonged irradiation
times are necessary to reach the PSS, which is composed of
only 44% of 1b. This finding is further supported by comparing
the quantum yields for ring closure, which show a marked
decrease when going from methylene chloride to acetonitrile.z
This effect can be attributed to the generation of a twisted
intramolecular charge transfer (TICT) state upon excitation of
1a as suggested previously to explain solvent-dependent
photochromism of related donor–acceptor dithienylethenes.14
Fig. 3 Association behaviour: NMR-titration of ring-open isomers
(blue circles) and ring-closed isomers (red diamonds) with receptor 3 in
CDCl3 at 25 1C for (a) 1a and 1b as well as (b) 2a and 2b (lines show
fitted curves for the 1 : 1 binding isotherms).
c
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Chem. Commun., 2011, 47, 460–462 461

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red
of 2b (Ep
red
analogue 2a (Ep = ꢁ1.497 V). In the case of the 1a/b, the
= ꢁ1.253 V) as compared to its ring-opened
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reduction potential is also reduced in the ring-closed form yet
red
to a smaller extent (DEp = ꢁ74 mV for 1a/b instead of
red
DEp = ꢁ244 mV for 2a/b, all values relative to an fc/fc+
standard).z Clearly when comparing both ring-closed isomers,
the larger Ka value for 2b is due to the presence of the coupled
electron-accepting ester groups, which further diminish
electron density in the maleimide core. The overall reduction
of electron density upon ring-closure is associated with an
increased acidity of the central imide N–H group, whose
interaction with the melamine’s endocyclic N-atom seems to
dominate the association event. This finding is in line with
the geometry of the five-membered maleimide ring that
leads to elongated and hence weaker interactions of both
terminal imide carbonyl O-atoms with the two exocyclic
N–H groups of the melamine receptor. From our experiments
it seems that ring-closure in combination with p-conjugated
electron-accepting groups is best suited for achieving
enhanced binding constants for the presented photoswitchable
imides.
7 A. Goodman, E. Breinlinger, M. Ober and V. M. Rotello, J. Am.
Chem. Soc., 2001, 123, 6213–6214.
8 An impressive, conceptually related example of a photoswitchable
fulgimide has been described in T. Okuyama, Y. Yokoyama and
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The first encouraging example of a photoswitchable triple
hydrogen-bonding motif, based on an electronic modulation
of the participating basicity/acidity and therefore binding
strength, has been developed. Ongoing efforts in our laboratories
are concerned with the design of improved systems, which
provide larger differences in association strength (ON/OFF
ratios) as well as inherently stronger association. The latter
could be accomplished by introducing more hydrogen-bonding
sites, for example Meijer’s quadruple hydrogen-bonding
ureidopyrimidines,2b or simply by working in less polar
media.15c In principle, such photoswitchable supramolecular
‘‘glue’’ should allow for the construction of light-responsive
soft materials, for example by utilizing multivalent constructs
as photochromic monomers or crosslinkers. Furthermore, our
approach of exploiting the bridge of diarylethenes as a tunable
functionality,16,17 which is electronically modulated by coupling
terminal donors/acceptors appears to be a general strategy to
photomodulate various functions and work along these lines
will be reported in due course.
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base pairing with thymidine have recently been reported in:
¨
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bleaching of the absorption band is observed, yet no by-product
could be detected.
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Generous support by the German Research Foundation
(DFG via SFB 658) and the Fonds der Chemischen Industrie
is gratefully acknowledged. Wacker Chemie AG, BASF AG,
Bayer Industry Services, and Sasol Germany are thanked for
generous donations of chemicals.
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Notes and references
16 This effect is somewhat related to photoswitchable changes in
Lewis-acidity observed by Branda and coworkers in dioxaborolane
bridged dithienylethenes, which interconvert between a (hetero)-
aromatic open and a cross-conjugated closed isomer: V. Lemieux,
M. D. Spantulescu, K. K. Baldridge and N. R. Branda, Angew.
Chem., Int. Ed., 2008, 47, 5034–5037.
17 For elegant examples exploiting chemical reactivity differences of
the bridge functionality, see: (a) D. Sud, T. J. Wigglesworth and
N. R. Branda, Angew. Chem., Int. Ed., 2007, 46, 8017–8019;
(b) V. Lemieux, S. Gauthier and N. R. Branda, Angew. Chem.,
Int. Ed., 2006, 45, 6820–6824; (c) V. Lemieux and N. R. Branda,
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1 G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford
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3 For recent overviews, in particular dealing with photoresponsive
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c
462 Chem. Commun., 2011, 47, 460–462
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