Towards photoswitchable quadruple hydrogen bonds via a reversible “photolocking” strategy for photocontrolled self-assembly

Developing new photoswitchable noncovalent interaction motifs with controllable bonding affinity is crucial for the construction of photoresponsive supramolecular systems and materials. Here we describe a unique “photolocking” strategy for realizing photoswitchable control of quadruple hydrogen-bonding interactions on the basis of modifying the ureidopyrimidinone (UPy) module with an ortho-ester substituted azobenzene unit as the “photo-lock”. Upon light irradiation, the obtained Azo-UPy motif is capable of unlocking/locking the partial H-bonding sites of the UPy unit, leading to photoswitching between homo- and heteroquadruple hydrogen-bonded dimers, which has been further applied for the fabrication of novel tunable hydrogen bonded supramolecular systems. This “photolocking” strategy appears to be broadly applicable in the rational design and construction of other H-bonding motifs with sufficiently photoswitchable noncovalent interactions.

In the past two decades, efforts have been devoted to exploring the potential of photocontrolling the dimerization bonding behavior between multiple H-bonding motifs. 63 Current strategies include (1) achieving the photo-triggered formation of multiple H-bonds by using a deprotectable "photocage", 64,65 (2) changing the H-bonding affinity by photoregulating the geometry of the bonding cavity, 66 or bonding site arrangement, 67 (3) tuning the H-bonding ability of the H-bonded motifs through light/heating conversion, 68 and (4) switching the H-bonding interactions via the reversible photoregulation of the stacking interaction, 69 or the H-bond donating-accepting ability of the multiple H-bonding motifs. 70 However, these systems are, to a considerable extent, constrained by irreversibility, 64,65 non-bidirectional photoswitchability, 66,68 or unsatisfactory photoswitching H-bonding affinities. [67][68][69][70] In addition, when employing these strategies to construct photoswitchable multiple H-bonding motifs, their structures are oen limited by strict requirements. Thus, these restrictions greatly discourage the current strategies from more innovative applications.
To break this logjam, we herein describe a reversible "photolocking" strategy, which allows for the achievement of bidirectional photoswitching of multiple H-bonding interactions by dramatically changing the binding affinity upon photoirradiation (Scheme 1). To implement this strategy, the widely used quadruple H-bonding motif, based on the ureidopyrimidinone (UPy) unit, was used and modied with a photoswitchable ortho-ester-substituted azobenzene moiety at the urea side, through which a unique photoresponsive UPy derivative of Azo-UPy was obtained (Scheme 1a). Before light irradiation, the two urea N-H hydrogens in the pristine E-Azo-UPy preferred to form intramolecular multiple H-bonds with the azo and carbonyl (C]O) groups, by which the formation of the quadruple H-bonded UPy dimer was immensely suppressed ("locked" state in Scheme 1a). Upon UV light irradiation, the E / Z photoisomerization of the azobenzene unit was triggered, which led to the breakage of the intramolecular H-bonds ("unlocked" state in Scheme 1a). The resulting Z-Azo-UPy molecule tended to form a quadruple H-bonded homodimer of (Z-Azo-UPy) 2 driven by a strengthened binding between the UPy units. Upon blue light irradiation, the reverse Z / E photoisomerization of the azo units of the (Z-Azo-UPy) 2 dimer was triggered again to dramatically weaken the dimerization, thus leading to the degradation of the (Z-Azo-UPy) 2 dimer and the regeneration of the intramolecular H-bonding "locked" E-Azo-UPy. To the best of our knowledge, this is the rst example of a quadruple H-bonding motif with bidirectionally and sufficiently photoswitchable H-bonding affinity.
We further demonstrate that the quadruple H-bonding of the non-photoactive UPy homodimer of (UPy-1) 2 was not interfered when the "locked" E-Azo-UPy was introduced (Scheme 1b). However, aer the E-Azo-UPy was irradiated with UV light in the presence of the (UPy-1) 2 dimer, the quadruple H-bonded heterodimer of (Z-Azo-UPy)$(UPy-1) could be formed (Scheme 1b). Subsequent irradiation of this heterodimer with blue light could regenerate the "locked" E-Azo-UPy to cause the degradation of the heterodimer and the reformation of the homodimer of (UPy-1) 2 . Therefore, the homo-/heterodimerization behavior of such Azo-UPy molecules could be facilely photocontrolled through the reversible photoswitching of their dimerization association constant upon irradiation with lights of different wavelengths.
To further test the potential of such unique photoswitchale Azo-UPy motifs in the construction of photocontrollable selfassembly systems, we have also prepared an Azo-UPy-appended polymer and investigated its photocontrollable aggregation behavior by photoregulating the "on/off" of the quadruple Hbonding interactions between the Azo-UPy units on the side chains. In addition, we revealed that the Azo-UPy motif could work as a photoswitchable chain capper to achieve photocontrollable supramolecular polymerization.

Results and discussion
Photoswitching behavior of the model compounds Model compounds 1a-1d (Table 1) were rst designed and prepared to test the practicability of the "photolocking" strategy. Their UV-vis absorption spectra were recorded under irradiation with light sources of different wavelengths (Fig. S1 †), according to which the optimal irradiation wavelengths could be determined when the contents of their Eor Z-isomers in the solution reached the maximum at the photostationary stationary state (PSS) ( Table  1). To further investigate reversible photoisomerization, their 1 H NMR spectra at PSS Z or PSS E were also recorded ( Fig. S2-S9 †), based on which their E/Z isomeric ratios were obtained and 1b was revealed to exhibit the best photoswitching behavior ( Table 1).
The photocontrolled "on/off" of the intramolecular Hbonding of 1b was then disclosed. Before irradiation, the amide N-H hydrogen of 1b E exhibited a downeld signal at 11.2 ppm (H E in Fig. S10a †), indicative of the formation of strong intramolecular H-bonds. Upon irradiation with UV light (l ¼ 350 nm), the E / Z photoisomerization was triggered, during which the intramolecular H-bonding was ruptured due to the fact that the geometry of 1b Z was unfavorable for the formation of such three-centered intramolecular H-bonding. In this case, the signal of the amide hydrogen of 1b Z was found to upshi to 7.3 ppm (H Z in Fig. S10b †). The intramolecular H-bonding could be formed again aer the UV-irradiated 1b was exposed to the blue light (l ¼ 460 nm), which was supported by the restoration of the downeld signal of the amide hydrogen ( Fig. S10c †). These results clearly demonstrated that the design strategy of introducing the orthoester-substituted azobenzene for photoswitching could achieve photocontrolled breakage/recovery of the intramolecular Hbonding in compound 1 (Table 1).

Photoswitchable quadruple H-bonding self-dimerization behavior of Azo-Upy
Based on the above investigation of the model compounds, we then prepared a photoresponsive UPy derivative Azo-UPy (Scheme 2). Prior to light irradiation, the formation of the intramolecular H-bonding between the urea N-H hydrogens and the azo and ester carbonyl groups, in other words, the "locking" state of Azo-UPy, was carefully evaluated through a series of NMR spectroscopic investigations. Firstly, the 1 H NMR spectrum of pure E-Azo-UPy in CDCl 3 was recorded ( Fig. 1a and S19a †), from which the proton signals (H-a E and Hb E ) of the UPy unit were found to appear at 11.8 (H-a E ) and 9.9 ppm (H-b E ), respectively. Both signals upshied as compared to those of the quadruple H-bonded UPy units, which typically appeared at around 13 and 12 ppm, respectively. [71][72][73] In particular, the chemical shi of H-a E (11.8 ppm) of E-Azo-UPy was close to that of the monomeric UPy unit (d(H-a E ) z 11.9  ppm) in CDCl 3 . 74 These results implied that E-Azo-UPy was unable to efficiently form quadruple H-bonded dimers.
Notably, the N-H hydrogen H-c E of E-Azo-UPy exhibited the maximal downeld shi (d ¼ 12.1 ppm), which indicated that H-c E might form three-centered N/H/O intramolecular Hbonding with the azo and carbonyl groups (Scheme 2). The chemical shi of H-b E (9.9 ppm) in the UPy unit of E-Azo-UPy shied signicantly downeld as compared to that of free N-H(b) (d < 8.8 ppm) in the monomeric UPy unit in CDCl 3 . 73 Meanwhile, strong NOEs were observed between H-b E and H-k E of the methoxyl group (Fig. S12 †), which also revealed the formation of intramolecular H-bonding between H-b E and the carbonyl group. All these results supported that the two urea N-H hydrogens (H-b E and H-c E ) of E-Azo-UPy formed intramolecular H-bonding with ester carbonyl groups (Scheme 2). In this way, the quadruple H-bonding sites of the UPy unit of E-Azo-UPy were partially locked, which greatly disabled the formation of intermolecular quadruple H-bonded dimers.
To further disclose the efficiency of dimerization inhibition, we recorded the 1 H NMR spectra of E-Azo-UPy of varying concentrations in CDCl 3 (Fig. S14 †), from which only one set of well-dened proton signals was observed in the concentration range of 0.25-40 mM. This might be either attributed to the fact that E-Azo-UPy was unable to form the quadruple H-bonded dimer, or the interconversion rate between the dimeric and monomeric E-Azo-UPy was faster than the NMR time scale. However, it was noteworthy that the signals of all the N-H hydrogens of E-Azo-UPy slightly shied upeld (À0.09 ppm < Dd < 0), probably owing to the weak p-p stacking of E-Azo-UPy, rather than shied downeld as the concentration increased. In addition, when competitive DMSO-d 6 (v%, 0-10%) was introduced into the solution, the spectra did not exhibit two sets of signals, which were expected to be generated by the coexisting quadruple H-bonded dimer and the uncomplexed monomer ( Fig. S15 †), respectively. These observations supported that the quadruple H-bonding dimerization of E-Azo-UPy was greatly restricted. This conclusion was further evidenced by the observation that the signals for the UPy unit of E-Azo-UPy remained unchanged aer introducing a non-photoactive quadruple H-bonded homodimer of (UPy-1) 2 into the solution of E-Azo-UPy (Fig. S16 †). In this case, if the E-Azo-UPy was capable of forming quadruple H-bonding, the signals should be changed due to the formation of the heterodimer between E-Azo-UPy and UPy-1, which was actually not observed.
Aer the evaluation of the intramolecular H-bonding of E-Azo-UPy, we then investigated its phototriggered quadruple Hbonding dimerization. Firstly, the UV-vis absorption spectra of Azo-UPy at PSS of different light sources were recorded (Fig. S17 †), from which the optimal irradiation lights were determined as l ¼ 365 nm (for E / Z) and l ¼ 460 nm (for Z / E), respectively, since the highest Z/E (or E/Z) isomeric ratios of Azo-UPy at PSS were achieved upon irradiation with these lights. Azo-UPy was found to exhibit excellent light fatigue resistance as revealed through repeated irradiation experiments (Fig. S18 †). The 19 F NMR spectra of Azo-UPy at PSS Z (365 nm) were then recorded ( Fig. S20b and S21a †), based on which the isomeric ratio was calculated as E/Z ¼ 47/53, and the content ratio of the two isomers of Z-Azo-UPy in the solution could be further determined as isomer I/isomer II ¼ 10/1, whereas the thermal stability of the obtained Z-Azo-UPy was measured as t 1/ 2 (Z / E) ¼ 64.8 h (Fig. S22 and S23 †).
The phototriggered unlocking of the intramolecular Hbonding and the quadruple H-bonding of Azo-UPy at PSS Z (365 nm) were studied using 1 H NMR (Fig. S1b and S19b †), which exhibited two new sets of signals. One signal (H-a Z , H-b Z and H-c Z ) corresponded to the UPy unit of the 4[1H]pyrimidinone dimer (isomer I in Scheme 2), and the other (black dotted signals in Fig. 1b) could be assigned to the UPy unit of the pyrimidin-4-ol tautomer of (Z-Azo-UPy) 2 (isomer II in Scheme 2). As revealed in Fig. 1b, the chemical shis of the N-H signals (H-a Z , H-b Z and H-c Z ) for isomer I of (Z-Azo-UPy) 2 appeared at 12.9, 12.5 and 12.3 ppm, respectively. For isomer II, the signals of the protons were at 12.8, 12.7 and 11.4 ppm, respectively. Compared to those of E-Azo-UPy (Fig. 1a), the N-H signals of H-a Z and H-b Z for isomer I of the (Z-Azo-UPy) 2 dimer underwent downeld shiing. Moreover, the chemical shis of the N-H signals of Z-Azo-UPy were close to those of the reported quadruple H-bonded dimers constructed by 2-Aryl UPy derivatives. 71-73 Based on these observations (Table 2), we proposed that quadruple H-bonding was formed between the UPy units of two Z-Azo-UPy molecules at PSS Z (365 nm).
To get deep insight into the phototriggered quadruple Hbonding dimerization of Azo-UPy, 2D DOSY NMR experiments were further carried out. Before irradiation, E-Azo-UPy was found to diffuse as one entity with a single diffusion coefficient of D ¼ 6.31 Â 10 À10 m 2 s À1 (Fig. S27 †). Aer irradiation with UV light (365 nm), two individually diffused entities were observed from the resulting PSS Z mixtures of Azo-UPy with the diffusion coefficients of D 1 ¼ 4.57 Â 10 À10 m 2 s À1 and D 2 ¼ 6.31 Â 10 À10 m 2 s À1 (Fig. S28 †), which could be assigned to the formed quadruple H-bonded (Z-Azo-UPy) 2 dimer and the E-Azo-UPy monomer, respectively. Based on the ratio of D 1 /D 2 ¼ 72.9%, the volume for the defusing entity formed by Z-Azo-UPy could be estimated as 2.6 times bigger than that of E-Azo-UPy, which further supported the formation of the quadruple H-bonded (Z-Azo-UPy) 2 dimer. In addition, the inuence of solvent polarity on the dimerization behavior was also revealed by the recorded 2D DOSY-NMR spectra of Azo-UPy at PSS in the strong Hbonding competitive polar solvent of DMSO-d 6 . While the pristine E-Azo-UPy in DMSO-d 6 solution revealed a single diffusion coefficient of D ¼ 1.41 Â 10 À10 m 2 s À1 (Fig. S29 †), the two components of E-Azo-UPy and Z-Azo-UPy in the PSS Z (365 nm) mixtures of the DMSO-d 6 solution of Azo-UPy were found to diffuse as species of close volume with an equal diffusion coefficient of D ¼ 1.48 Â 10 À10 m 2 s À1 (Fig. S30 †). This observation suggested that the quadruple H-bonded (Z-Azo-UPy) 2 dimer could only be formed in weakly polar solvents such as chloroform, and the quadruple H-bonds between the Z-Azo-UPy molecules were hardly formed in the solution of DMSO-d 6 , in which both the Eand Z-isomers of Azo-UPy existed as monomers.
The dimerization stability of Z-Azo-UPy was further investigated through 1 H and 19 F NMR dilution experiments of the PSS Z (365 nm) mixtures of Azo-UPy in CDCl 3 . As the concentration decreased from 20 mM to 0.25 mM, the proton (Fig. S31 †) and uorine (Fig. S33 †) signals of the monomeric Z-Azo-UPy were observed, and the dimerization constant in the formation of the (Z-Azo-UPy) 2 dimer was estimated to be K * dim ¼ 1:2 Â 10 5 M À1 : In addition, the ability of Z-Azo-UPy to form the quadruple Hbonded (Z-Azo-UPy) 2  Furthermore, when the UV-irradiated PSS Z (365 nm) solution of Azo-UPy was exposed to the blue light of l ¼ 460 nm, the reversed Z / E photoisomerization of the azo group could be triggered, during which the quadruple H-bonded (Z-Azo-UPy) 2 dimer was degenerated and the intramolecular H-bonding locked E-Azo-UPy could be regenerated, as evidenced by the observation of dramatically decreased proton signals of the Zisomer and the signicantly increased proton signals of the Eisomer of Azo-UPy at PSS E (460 nm) ( Fig. 1c and S19c †). Besides, the isomeric ratio of E/Z (92/8) could be calculated based on the recorded 19 F NMR spectrum of Azo-UPy at PSS E (460 nm) (Fig. S20 †). These results indicated that the quadruple Hbonding behavior of the Azo-UPy was bidirectionally photoswitchable.

Photoswitchable quadruple H-bonding hetero-dimerization behavior of Azo-Upy
Aer the above investigation of the reversible photocontrolled quadruple H-bonding self-dimerization of Azo-UPy, we further applied the new motif to photoregulate the quadruple Hbonding dimerization of nonphotoactive UPy-1 (Scheme 3). To this end, E-Azo-UPy was rstly mixed with UPy-1 in CDCl 3 , where E-Azo-UPy existed as the monomer with an intramolecularly H-bonding locked UPy unit, while UPy-1 existed as the quadruple H-bonded dimer of (UPy-1) 2 (Fig. 2a). Upon irradiation with UV light (365 nm), the E / Z photoisomerization of Azo-UPy in the solution was triggered to generate Z-Azo-UPy, which was found to be capable of decoupling the (UPy-1) 2 dimer through the formation of the quadruple H-bonded heterodimer of (Z-Azo-UPy)$(UPy-1), as proved by the observation of newly  Fig. 2b). The proton (Fig. S40 †) or uorine (Fig. S44 †) signals of the heterodimer of (Z-Azo-UPy)$(UPy-1) were found to be emerged and enhanced signicantly at PSS Z (365 nm) as the concentration of UPy-1 increased, while the corresponding signals for the homodimers of (UPy-1) 2 and (Z-Azo-UPy) 2 decreased accordingly. The relative association constant of the heterodimer could be obtained as K rel ¼ 3.5, based on the 1 H NMR spectra (Fig. S45 †). Notably, the formation of the heterodimer was found to be capable of improving the E / Z photoisomerization ratio of Azo-UPy, as suggested by the E/Z (42/58) value calculated from the 19 F NMR spectrum of the PSS Z (365 nm) mixtures of Azo-UPy and UPy-1 (Fig. S43 †).
When the UV-irradiated solution of the PSS Z (365 nm) mixtures of Azo-UPy and UPy-1 was further irradiated with blue Scheme 3 The schematic representation of the photocontrolled quadruple H-bonding hetero-dimerization behavior of Azo-UPy and UPy-1. light (l ¼ 460 nm), the Z / E photoisomerization of the azo group in the heterodimer of (Z-Azo-UPy)$(UPy-1) was induced, leading to the degradation of the heterodimer and the reconstruction of the homodimer, as evidenced by the observation of dramatically decreased proton signals for the (Z-Azo-UPy)$(UPy-1), as well as the enhanced proton signals for (UPy-1) 2 and E-Azo-UPy in the PSS E (460 nm) mixtures of Azo-UPy and UPy-1 (Fig. 2c). The isomeric ratio (E/Z ¼ 90/10) of Azo-UPy was obtained based on 19 F NMR of the PSS E (460 nm) mixtures of Azo-UPy and UPy-1 (Fig. S43 †). All these investigations supported that Azo-UPy not only exhibited unique photocontrollable quadruple H-bonding dimerization, but also was able to photoregulate the quadruple H-bonding dimerization behavior of the nonphotoactive UPy derivative.
Application of Azo-UPy for photocontrollable macro-/ molecular self-assembly Beneting from the dynamic nature, good directionality, versatility and high stability, multiple H-bonded motifs have been integrated into the polymeric scaffolds to fabricate H-bond cross-linked polymeric materials with tunable microstructures and high-performances. [75][76][77][78][79][80][81][82] To further apply Azo-UPy for the construction of photocontrollable macromolecular systems, a linear polymer Azo-UPy-P was then synthesized, with the side chains being modied with Azo-UPy (Fig. 3a). The photocontrolled aggregation behavior of Azo-UPy-P was then investigated by 2D DOSY-NMR spectroscopy. Before UV light irradiation, the UPy units in the side chains were locked by intramolecular H-bonding, as indicated by the observation of N-H signals ( Fig. S46 and S47a †) with chemical shis similar to those of E-Azo-UPy (Fig. 1a), and the recorded 2D DOSY-NMR spectrum of the pristine state Azo-UPy-P was found to exhibit a single diffusion coefficient of D ¼ 4.07 Â 10 À11 m 2 s À1 (Fig. 3b).
Upon irradiation with UV light (365 nm), E-Azo-UPy in the side chains of Azo-UPy-P were converted to Z-Azo-UPy, leading to the unlocking of Azo-UPy and thus enabling the macromolecular aggregation of Azo-UPy-P through the formation of intermolecular quadruple H-bonds between the Z-Azo-UPy units of different polymeric chains (Fig. S47b †), as evidenced by the observation that the PSS Z (365 nm) solution of Azo-UPy-P gave rise to a reduced diffusion coefficient of D ¼ 1.47 Â 10 À11 m 2 s À1 (Fig. 3c), which was about 36% of that of Azo-UPy-P at the pristine state. Accordingly, the volume of the UV-irradiated Azo-UPy-P was estimated as bigger as 21 times that of the nonirradiated Azo-UPy-P. Further exposing the solution of the UVirradiated Azo-UPy-P to blue light (460 nm) caused a drastic decrease of the signals of the dimerized Z-Azo-UPy motifs (Fig. S47c †), suggesting the disassembly of the polymeric aggregation. However, aer the low concentrated solution of Azo-UPy-P was irradiated with UV light, a decrease of the hydrodynamic volume of the polymer chain of Azo-UPy-P was observed (Fig. S48 †), which could be attributed to the intramolecular chain collapse of the single polymer chain of Azo-UPy-P driven by the intra chain H-bonding interactions between the Z-Azo-UPy units. 83,84 Fig. 3 (a) Chemical structure of Azo-UPy modified polymer Azo-UPy-P, and the schematic representation of its photocontrolled macromolecular self-assembly behavior. The 2D DOSY spectra (600 MHz, 40 mg mL À1 , CDCl 3 , 298 K) of polymer Azo-UPy-P under conditions of (b) before and (c) after irradiation with UV light (365 nm).
In addition to the above polymeric system, the application of the Azo-UPy motif in photoregulating the self-assembly of small molecules was also explored. To this end, Azo-UPy was employed as a photoswitchable chain capper to realize photoregulable H-bonding supramolecular polymerization. A nonphotoactive AA type monomer bifunctionalized with two terminal UPy units was thus prepared (UPy-2 in Fig. 4). The molecule was dissolved in CDCl 3 to generate a linear quadruple H-bonded supramolecular polymer (Fig. 4). 22,57 When 0.025 equivalent of the non-irradiated E-Azo-UPy was introduced into the solution, neither chemical shiing changes for these two components nor new proton signals were observed (Fig. S49ac †), indicating that the introduced E-Azo-UPy with a locked UPy unit did not obviously disturb the H-bonded supramolecular polymer based on UPy-2. However, aer UV (365 nm) irradiation, 62% of E-Azo-UPy in the mixture was converted to Z-Azo-UPy with an unlocked UPy unit. As a result, quadruple Hbonding hetero-dimerization occurred between Z-Azo-UPy and UPy-2 (Fig. 4), with the result that the degree of polymerization (DP) of the H-bonded supramolecular polymer decreased ( Fig. S49d and S50b †). In contrast, when this UV-irradiated solution of the supramolecular polymer with lower DP was further irradiated with blue light (460 nm) to reach PSS E , 82% of the capped Z-Azo-UPy was converted back to E-Azo-UPy with a locked UPy unit. Thus, the DP of the supramolecular polymer increased again (Fig. S49e and S50c †). To further visualize the photoregulable supramolecular polymerization of such a quadruple H-bonded system, the viscosity variations of the chloroform solution of the supramolecular polymer during the repeated photoswitching cycles were measured. As displayed in Fig. 5, aer introducing E-Azo-UPy into the solution of UPy-2, the decrease/increase of the specic viscosity (h sp ) of the solution could be photoswitched through irradiation with the light sources of 365 nm/460 nm alternately.

Conclusions
In summary, we have demonstrated the rational design and construction of a unique photoswitchable UPy motif (Azo-UPy) with an ortho-ester-modied azobenzene unit through the reversible E/Z photoisomerization of which the two urea N-H hydrogens of Azo-UPy could be unlocked/locked via forming intramolecular H-bonds with the carbonyl groups, thereby enabling the photocontrolled quadruple H-bonding self-/ hetero-dimerization of Azo-UPy molecules. Notably, the dimerization affinity of Azo-UPy can be dramatically changed upon alternating irradiation with UV and blue lights, and this feature has distinguished Azo-UPy-based self-assembled systems from most of the reported photoresponsive systems. Beneting from such distinctive photoswitchable quadruple H-bonding, novel photocontrolled supramolecular systems can be facilely constructed by structurally integrating the Azo-UPy motif into polymer chains or employing it as a photoresponsive chain capper to regulate the quadruple H-bonding supramolecular polymerization. In light of the wide application of multiple Hbonding interactions in supramolecular chemistry and materials science, the results presented in this work provided a fundamental design strategy, ultimately giving access to the generation of photoresponsive supramolecular self-assembled systems such as self-assembled supramolecular hosts, as well as advanced materials featuring appealing properties of photocontrolled self-healing, phase transformation, dynamic adhesion and so on. In addition, the implementation of such a "photo-locking" strategy has no special requirement for the structural skeleton of multiple H-bonding motifs, which makes it broadly applicable in the construction of other types of photoswitchable multiple H-bonding motifs.

Conflicts of interest
There are no conicts to declare.