Exploiting Coordination Isomerism for controlled Self-Assembly.

Herein, we exploit the inherent geometrical isomerism of a PtII complex as a new tool to control supramolecular assembly processes. UV irradiation and careful selection of solvent, temperature and concentration leads to tunable coordination isomerism, which in turn allows fully reversible switching between two distinct aggregate species (1D fibers ↔ 2D lamellae) with different photoresponsive behavior. Our findings not only broaden the scope of coordination isomerism, but also open up exciting possibilities for the development of novel stimuli-responsive nanomaterials.

Abstract: We exploited the inherent geometrical isomerism of aP t II complex as an ew tool to control supramolecular assembly processes.U Vi rradiation and careful selection of solvent, temperature,a nd concentration leads to tunable coordination isomerism, which in turn allows fully reversible switching between two distinct aggregate species (1D fibers$2D lamellae) with different photoresponsive behavior. Our findings not only broaden the scope of coordination isomerism, but also open up exciting possibilities for the development of novel stimuli-responsive nanomaterials.
The occurrence of geometrical isomerism in coordination complexes,s ometimes termed coordination isomerism, has been recognized for more than acentury,and it is acommonly observed phenomenon in the photochemistry of squareplanar complexes. [1,2] In particular, Pt II compounds have been reported to undergo geometrical isomerization upon UV irradiation, leading to photostationary states whose isomer composition primarily depends on the choice of ligands and solvent. [2,3] To date,g eometrical isomerization of Pt II complexes has been exclusively investigated at the molecular level, for instance to obtain otherwise inaccessible coordination compounds, [4] rotors, [5] and photoactivated catalysts. [6] In an attempt to broaden the scope of coordination isomerism, we reasoned that the inherently different geometry of cis and trans Pt II complexes might be exploited as an ew method to control self-assembly processes.B ased on the versatility of metal coordination in providing multiple directional interactions,t his strategy would complement the existing arsenal of tools in stimuli-responsive materials [7] and living supramolecular polymerization. [8] In order to facilitate geometrical isomerism in Pt II complexes,t he use of small and/or conformationally unrestricted coordinating ligands appears to be aprerequisite. [9,10] Otherwise,s teric repulsion between cis-coordinated ligands, along with the stronger aggregation propensity of the more preorganized trans species,w ill preferentially stabilize the trans form, which can inhibit isomerization, [11] or even induce photodecomposition. [12] While screening our library of ligands,wenoticed that the inclusion of an azobenzene moiety in the molecular design enhances the conformational freedom of the system, [13,14] which might allow ag ood balance between isomerization and aggregation. Additionally,c omplexation with Pt II inactivates the azobenzene moiety to light irradiation so it will not influence the coordination isomerism through additional isomerization possibilities. [15,16] On this basis,w ed esigned an ew Pt II L 2 Cl 2 complex (C 1 ), with Lb eing a4 -phenylazopyridyl-based ligand featuring peripheral amide groups and dodecyloxy side chains [13,14] (Scheme 1; for synthesis and characterization, see the Supporting Information). This rational choice of ligand, solubilizing groups,a nd hydrogenbonding units enables simultaneous control over coordination isomerism and self-assembly for the first time.UVirradiation and appropriate choice of solvent, temperature,a nd concentration allows fully reversible switching in the aggregate morphology (1D$2D) and represents an innovative strategy towards stimuli-responsive self-assembled materials.
Thes elf-assembly behavior of C 1 ,s ynthesized as ap ure trans form, was initially probed in methylcyclohexane (MCH) using variable-temperature (VT) UV/Vis studies at 2 10 À5 m. These experiments showed only negligible absorption changes when monomer solutions where cooled from 363 K to 293 K( Figure 1a). However,f urther cooling to 273 K causes am arked red shift in the absorption maximum from 406 nm to 418 nm along with an isosbestic point at 401 nm and ac oncurrent hyperchromism ( Figure 1a). These spectral changes,w hich are independent of the cooling rate (see Figure 1aand  : as mooth regime between 363 Ka nd around 293 K followed by as harp transition below ac ritical elongation temperature (T e % 293 K) that is characteristic of anucleated supramolecular polymerization (for thermodynamic analysis, see Figure S8 and Table S1). Thei nitial transition, which cannot be fitted to any of the existing thermodynamic models for supramolecular polymerization, suggests apre-nucleation event involving conformational changes of the azobenzene group(s), such as planarization, at higher temperatures.V T dynamic light scattering (DLS) studies at temperatures above the T e showed no significant changes in the correlation and size distribution functions ( Figure S9), thus validating our hypothesis.F urther cooling to 283 K( below the T e )d oes initiate the self-assembly of C 1 ,a sevident by the marked increase in the particle size ( Figure S9). Atomic force microscopy (AFM) on highly-oriented pyrolytic graphite (HOPG) revealed the absence of assemblies above 293 K ( Figure S10). At the T e ,short rods with auniform height of 2-3nma nd al ength of 40-70 nm are observed (Figure 1b and Figure S11), which further grow longitudinally into fibers with lengths between 60 and 700 nm (average length (l ave ) = 261 AE 73 nm) when the temperature is decreased to 273 K ( Figure 1c,a nd Figures S12, S13). Combined 1D and 2D NMR studies,b oth in CDCl 3 and MCH-d 14 (Figures S14-S17), demonstrate as lipped molecular packing stabilized by aromatic and NÀH···Cl interactions. [11b,13] This proposed arrangement is in agreement with the packing observed in the crystal state for structurally related model compound C 2 with shorter ethoxy chains,w hich exhibits NÀH···Cl, C(aromatic) À H···Cl and aromatic interactions ( Figure 1d and Figure S18).
After detailed self-assembly studies of C 1 ,w ec onfirmed that the azobenzene moieties are indeed inactive under UV irradiation when coordinated to Pt II (Figure S19-S25). [15] In ar ecent example,S hionoya and co-workers [5] elegantly showed that discrete Pt II -centred azaphosphatriptycene molecular gears efficiently undergo coordination isomerism under irradiation in appropriate solvents.P olar solvents favour efficient trans-to-cis conversion due to preferential stabilization of the dipole moment of the cis form. [3,5] On as imilar basis,w et ested whether coordination isomerism is also possible for our system (C 1 ). To our satisfaction, anew set Scheme 1. Molecular structures of C 1 and C 2 ,a nd acartoon representation of the supramolecular assembly of C 1 triggered by coordination isomerism.

Angewandte Chemie
Communications of signals corresponding to cis-C 1 are observed over time in the 1 HNMR spectra when solutions of trans-C 1 are kept under ambient conditions in moderately polar solvents such as CDCl 3 and CD 2 Cl 2 ( Figure 2). Asimilar trend is observed when aconcentrated solution of trans-C 1 in CDCl 3 (20 mm)is diluted to 1mm ( Figure S26). Forb oth time-and concentration-dependent 1 HNMR experiments in CDCl 3 ,amaximum of 33 % cis-C 1 is obtained at equilibrium. Ther atio of cis-C 1 can be further increased to 40 %inmore polar solvents such as DMSO using high temperatures ( Figure S27). However,t he strong hydrophobicity of C 1 due to the presence of long alkyl chains results in rapid precipitation even when using these harsh conditions,which precludes further analysis in polar media. Nevertheless,u sing the same experimental protocol for less hydrophobic C 2 allowed us to achieve am aximum cis ratio of 73 %( Figure S28). Decreasing the solvent polarity by using CD 2 Cl 2 leads to ar eduction in the maximum amount of formed cis-C 1 (10 %), whereas no traces of cis-C 1 were observed in nonpolar solvents such as MCH-d 14 and TCE-d 2 (c = 1 10 À3 m,F igure 2a nd Figures S29, S30). This behavior can be rationalized by comparing the relative stability of both isomers using DFT calculations ( Figure S31). Thus,w hile polar and dilute solutions stabilize cis-C 1 ,h igh concentration and solvents of low polarity favor the trans form, ap henomenon that appears to be reinforced by aggregation. Accordingly,w ee xpect that only trans-C 1 has the appropriate geometry to promote aggregation, rendering the distorted cis form as adormant species.Interestingly,the reverse cis-to-trans isomerization of C 1 can be readily achieved by UV irradiation, irrespective of the solvent polarity (CDCl 3 33 %t o1 6% and DCM-d 2 10 %t o2 %; Figure 2a nd Figure S32). Thea bsence of the free ligand in solution during this transition is indicative of at wisting mechanism as the most probable isomerization pathway. [10] Even though UV irradiation does not fully back-isomerize the system, ac omplete recovery of trans-C 1 is possible by re-dissolution of the corresponding cis-containing mixtures in an onpolar solvent (MCH) upon evaporation of the polar solvent (CDCl 3 )a thigh concentration ( Figure S33).
We envisaged that this precise control over the coordination isomerism of C 1 could represent an efficient method to tune supramolecular assembly processes.T othis end, asmall volume (30 mL) of an equilibrated mixture of trans-C 1 (67 %) and cis-C 1 (33 %) at high concentration (1 mm)inchloroform was rapidly dried using argon flow to avoid back-isomerization. Theresulting fine powder was immediately thereafter dissolved in MCH (3 mL) to afinal concentration of 1 10 À5 m (100-fold lower than in NMR experiments,i nw hich back isomerization was observed), heated to 363 Ka nd finally subjected to VT UV/Vis experiments.U pon cooling from 363 Kt o2 83 K, small fluctuations in the absorption without ac lear trend were observed, which can be attributed to aw eak coupling of the p-scaffolds ( Figure S34). Further cooling to 273 Kl eads to ab athochromic shift, which is ad istinctive spectral feature of slipped aggregate formation. However,t his spectrum differs from the one obtained for pure trans-C 1 (Figure 3a), thus indicating that adifferent selfassembly pathway occurs in the mixture of isomers.Analysis of the VT cooling curves reveals alower T e for the mixture of cis + trans-C 1 compared to the pure trans-C 1 species (Figure 3a inset, Figure S36). This delayed aggregation process (> 4hfor the mixture versus < 10 min for pure trans)i sa lso evident from kinetic UV/Vis studies ( Figure S37). VT 1 HNMR also supports the dormant nature of the cis-C 1 isomer,s ince the critical aggregation concentration of the cis + trans mixture is around 2.5 times higher (c = 2.5 10 À3 m) than that of the pure trans-C 1 in CDCl 3 .N otably,a ll proton signals from trans-C 1 in the mixture followed the same trend as for the compound in isolation (Figure 3d and Figure S38), thus suggesting asimilar molecular packing.Incontrast, most signals of the cis-C 1 species in the mixture undergo no broadening and less pronounced shifts upon cooling (Figure 3d and Figure S38). In particular, the fact that some protons,f or example,t he amide protons H' e are slightly deshielded (Figure 3d,o range signals at ca. 8ppm) suggests that these groups might add as stoppers to the active ends of the supramolecular fibers.T his attenuated growth is further supported by dispersion-corrected PM6 simulations,w hich reveal asignificantly lower stability for ahexamer containing two cis isomers instead of pure trans due to the loss of intermolecular interactions.F urther,t he simulations reveal that co-assembly with the cis isomer disrupts the alkyl chain shell around the stacked aromatic units of pure trans-C 1 (Figure 3e and Figures S39-S41). VT DLS experiments in MCH (c = 2 10 À4 m)yield considerably smaller particle sizes for the cis + trans mixture compared to the pure trans species under identical conditions (maxima at 170 nm versus 2600 nm;F igure S42). AFM measurements at 2 10 À5 m demonstrate the formation of short rigid rods (Figure 3c)a t the T e (283 K) with au niform height of 2nma nd lengths between 30 and 110 nm (l ave = 48.4 AE 11.9 nm), which is in agreement with the results obtained for pure trans-C 1 (Figures S11, S45). Interestingly,i nstead of al ongitudinal growth, further cooling to 273 Kc auses at ransformation of the short rods into 2D lamellae with similar height and length (l ave = 69.5 AE 15.6 nm) but slightly larger widths (between 20 and 60 nm), which is aproduct of bundling of the rods already observed at higher temperature (Figure 3d and Figure S44). This behavior can be rationalized by the simulations,since the alkyl chains surrounding the aromatic core of the stacks potentially offer lateral van der Waals binding sites.T his effect, together with the restriction in the degrees of freedom of the stacks containing the distorted cis isomer compared to the fibers formed by the pure trans isomer, is expected to facilitate the bundling of the rods.
Ultimately,w ev alidated the reversibility of the system through full recovery of the trans form. To this end, the lamellar aggregates from the cis + trans mixture were heated to the monomer state (363 K) and subsequently irradiated with UV light in order to back-isomerize the cis isomers present (ca. 33 %) to the trans form. Cooling the resultant hot solution led to the same aggregation pathway as the freshly prepared trans-C 1 in MCH (c = 2 10 À4 m,F igure S47). This indicates an early quantitative recovery of the trans form, as demonstrated by the observation of short fibers by AFM imaging ( Figure S48).
In conclusion, we have described anew Pt II complex (C 1 ) that undergoes both geometrical isomerism and supramolecular polymerization under controlled experimental conditions.While nonpolar media (MCH) induce the formation of thin 1D fibers of pure trans-C 1 ,the use of more polar solvents (CHCl 3 )t op repare the aggregate solution enables the formation of the distorted cis form and leads to attenuated growth into small 2D lamellae.Current work in our lab aims at optimizing the efficiency of coordination isomerism with the ultimate goal of controlling the size of supramolecular assemblies.