Photoswitchable triple hydrogen-bonding motif

Abstract

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.

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The “bottom-up” organization of molecules to defined superstructures 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 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 interactions 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 photochromic reaction.⁸

Inspired by the possibility of influencing the strength of multiple hydrogen-bonding motifs by variation of π-conjugated electron-donating/accepting groups⁹ and intrigued by Irie's diarylethene photochromes,¹⁰ which allow for significant changes in π-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 in the open form is largely decoupled from the termini, whereas in the closed form the terminal donor/acceptor groups are in π-conjugation and hence should influence the association with a complementary N,N-dialkylmelamine DAD moiety.

Fig. 1 Photoswitchable triple hydrogen-bonding motif: reversible 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 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 (λ_irr = 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 (λ_irr > 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 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.‡ 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. 2 Photochromic behaviour: UV/vis absorption spectra during the course of irradiation (λ_irr = 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 × 10⁻⁵ M, 25 °C). Insets show repetitive switching cycles (λ_irr = 313 nm for ring-closure, λ_irr > 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

	Photochromism^a				Association^c
	λ max/nm	Φ ^313 nm a→b	Φ ^546 nm b→a	Conv.^b (%)	Δδmax/ppm	K a/M^−1
a In CH2Cl2 at 25 °C. b Composition of the PSS upon irradiation of the open form (1a or 2a) with UV-light (λirr = 313 nm) determined by UPLC. c Derived from NMR-titration data in CDCl3, 25 °C.^1
1a	313, 408	0.08	0.02	87	7.57 ± 0.02	132 ± 11
1b	359, 540				7.72 ± 0.06	231 ± 23
2a	321, 380	0.18	0.02	82	7.75 ± 0.05	142 ± 13
2b	317, 545				7.99 ± 0.03	318 ± 30

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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 K_a (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 K_a 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 x_{receptor 3} = 0.5.‡ 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.‡

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 °C for (a) 1a and 1b as well as (b) 2a and 2b (lines show fitted curves for the 1 ∶ 1 binding isotherms).

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 of 2b (E_p^red = −1.253 V) as compared to its ring-opened analogue 2a (E_p^red = −1.497 V). In the case of the 1a/b, the reduction potential is also reduced in the ring-closed form yet to a smaller extent (ΔE_p^red = −74 mV for 1a/b instead of ΔE_p^red = −244 mV for 2a/b, all values relative to an fc/fc⁺ standard).‡ Clearly when comparing both ring-closed isomers, the larger K_a 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 π-conjugated electron-accepting groups is best suited for achieving enhanced binding constants for the presented photoswitchable imides.

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.

Generous support by the German Research Foundation (DFGviaSFB 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.

Notes and references

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Footnotes

† This article is part of the ‘Emerging Investigators’ themed issue for ChemComm.

‡ Electronic supplementary information (ESI) available: Synthetic details and characterization data. See DOI: 10.1039/c0cc02339f

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