Structure-switching M3L2 Ir(iii) coordination cages with photo-isomerising azo-aromatic linkers

Deep-blue luminescent [{Ir(C^N)2}3(L)2]3+ coordination cages with structurally integral pyridyl-azo-phenyl groups can be reversibly photo-isomerised with no compositional change.


Introduction
Coordination cages, also referred to as metallosupramolecular cages, metallo-cages or as metal-organic polyhedra, are 3-D assemblies of metal cations and bridging ligands with well-dened discrete nano-sized structures oen related to Platonic or Archimedean solids. 1 As container molecules, they feature internal cavities in which other molecules or ions may be bound. Exploitation of their binding properties leads to a ra of potential applications as sensors, catalysts and as nano-scale reaction vessels where connement of reagents affects properties. 1 There has been recent interest in stimuli-responsive behavior of coordination cages, thus moving away from static to more complicated dynamic systems. 2 Most examples are chemo-responsive, and oen involve a degree of compositional change. 2 We were interested, however, in inducing signicant and reversible structuralbut not compositionalchange in a coordination cage. Such cages would show controllable dynamic size and shape behavior of both their external morphology and internal cavity space, the latter signicant for control of host-guest properties and concomitant functionality. Photo-irradiation is an attractively clean and efficient mechanism for structural changes. Photo-isomerisable coordination cages are rare. Clever et al. have reported the only examples of 3D coordination cage species where bridging ligands inherent to the cage structure have been photo-isomerised. 3,4 These are a [Pd 2 L 4 ] 4+ cage with a bis(3-pyridyl)dithienylethene (DTE) ligand that displays reversible ring-open 4 ring-closed photoswitching, 3 and a [Pd 3 L 6 ] 6+ 4 [Pd 24 L 48 ] 48+ cage transformation that occurs between ring-open and ring-closed ligand isomers of the bridging L which is a bis(4-pyridyl)DTE ligand. 4 One of the most widely employed photo-switching motifs is azobenzene (AZB) which exhibits reversible and robust E / Z photo-isomerisation. 5 The AZB motif has been used as a photosensitive component in a number of different types of responsive systems including liquid crystals and polymers, 6 molecular machines, 7 metal complexes, 8 and host-guest systems. 9 The use of AZB in coordination cage chemistry is limited, and photo-switching has only been shown where the AZB unit is an endohedral or exohedral decoration that is not part of the structural integrity of the cage. 10,11 Fujita et al. have reported a Pd 12 L 24 cage where the L-ligand contains pendant AZB units that occupy the inside of the cage. 10 Photo-switching of the AZB controls the internal cage hydrophobicity. Photoresponsive 2-D metallacyclic assemblies with bridging AZBtype ligands have been reported, 12,13 although photo-switching can induce both structural and compositional change. 12 Hydrogen-bonded capsule or cage assemblies have been reported with photo-responsive, though structurally peripheral, AZB groups for both hexameric calix [4]resorcinarenes cages 14 and capsule assemblies of urea-functionalised calix [4]arenes. 15 Their photo-responsive properties can be used to regulate the stability of the assembly or host-guest behaviour.
In this work, we report a series of coordination cages of type [{Ir(C^N) 2 } 3 (L) 2 ] 3+ where C^N is a cyclometallating ligand and L is a tripodal cyclotriguaiacylene (CTG) derivative functionalised with pyridyl-azo-phenyl groups. CTG is a chiral host molecule with M and P isomers and appropriate functionalisation gives classes of ligands (L) that have proved ideal for assembly of cages including M 3 L 2 (ref. [16][17][18] and M 6 L 8 (ref. 19 and 20) species. An azobenzene-appended CTG has been reported as a chemosensor for Hg(I) but its photo-responsive properties were not investigated. 21 Other classes of host molecules have also been decorated with AZB-type groups, 14,15,22 or linked together into photo-responsive dimers via an AZB. 23
The crystal structures of L2 and L3 were determined.{ There are two crystallographically distinct molecules of L1 each of opposite enantiomer. The ligands have approximate C 3symmetry and alternating enantiomers of L2 stack in an offset bowl-in-bowl fashion, forming columns running along the a axis. Adjacent columns inter-lock through p-p stacking interactions between the 3-pyridylazophenyl groups to create a crystal lattice with void channels of over 2.5 nm diameter, Fig. 1a. The solvent-accessible void space accounts for 48% of the unit cell volume, which is extremely high for a molecular crystal. L3 crystallises as a CH 3 NO 2 clathrate, and the host molecule has all three 4-pyridylazophenyl groups in different orientations, Fig. 1b.
Photo-isomerisation of each of L2-L4 can result in four possible isomers: (EEE), (EEZ), (EZZ), or (ZZZ). Changes to the 1 H NMR spectrum of L2 in CD 2 Cl 2 on irradiation with a 355 nm Nd:YAG laser indicate a mixture of isomers is formed as the diastereotopic bridging methylene CTG protons become a complex multiplet, Fig. 2a. A new doublet appears at 6.9 ppm due to an increase in shielding of the aromatic protons ortho to the azo groups on isomerisation to the Z isomer. Relative integrations gives approximately 76% conversion to Z isomers, corroborated by UV-visible spectroscopy ( Fig. S81 †) where the p / p* transition that typically occurs between 300-350 nm for E-AZB-type groups reduces in intensity on isomerisation to the Z isomer with a concomitant small increase in the n / p* transition, Fig. 3. Azobenzene itself shows 80% Z-isomer at the photo-stationary state on irradiation with 313 nm light, 5 hence the presence of three photo-isomerisable units on a single scaffold does not signicantly inhibit the isomerisation. The reverse Z / E isomerisation is induced with 450 nm light, albeit not with 100% conversion (Table S3, Fig S81 †). Solutions of E / Z switched L2 show signicant amounts of Z isomer present, even aer standing for 48 hours in the dark (Fig. S84 †), indicating a Z-rich photo-stationary state of unusually high stability. The other ligands can also be photo-switched (Table S3   standing in the dark (Fig. S85 †). This is due to the more electron-donating alkoxy substituent of L4 leads to a decreased energy barrier for the thermal isomerisation via the donation of electrons into the p* orbital. 5

Photo-switchable [{Ir(C^N) 2 } 3 (L) 2 ] 3+ coordination cages
A series of coordination cages of type [{Ir(C^N) 2 } 3 (L) 2 ]$3PF 6 were assembled in nitromethane through reaction of a 3 : 2 mixture of D,L-Ir(C^N) 2 (NCMe) 2 $PF 6 and L-type ligand Scheme 2. Cages did not form in more coordinating solvents such as dimethylsulfoxide. The cages show similar spectroscopic behaviour, hence only [{Ir(ppy) 2 } 3 (L2) 2 ]$3PF 6 (C1, ppy ¼ 2-phenylpyridinato) will be discussed in detail. Self-assembly of C1 is demonstrated by ESI-MS which shows a dominant peak at m/z 1191.3190 corresponding to [{Ir(ppy) 2 } 3 (L2) 2 ] 3+ , along with a small amount of fragmentation product, Fig. 4. This level of fragmentation is consistent with MS seen for our previously reported [{Ir(ppy) 2 } 3 (L) 2 ] 3+ cages where L are simpler 4-pyridyldecorated CTG ligands. 17 In that study, as here, M 2 L 2 species seen in MS were attributed to fragmentation in the mass spectrometer as DOSY NMR showed only a single large species in solution. 17 Despite the inertness of the d 6 Ir(III), the selfassembly of C1 is largely complete aer only one hour of equilibration, Fig. S49. † The 1 H NMR spectrum of C1 shows coordination-induced shis that are indicative of complex formation and the degree of spectral broadening is typical of coordination cages, especially where different stereoisomers may be present, Fig. 5a. Signicant shis are observed for the protons closest to the iridium centre on the ppy ligands with H 1 shied upeld from 9.12 to 8.92 ppm and H 8 shied downeld from 6.10 to 6.58 ppm. The L2 H a resonances are shied downeld from 9.20 to 9.29 ppm. Spectra were insufficiently resolved to distinguish between ppy ligands. 1 H ROESY NMR provided further evidence for cage formation with unambiguous through-space coupling between protons on the ppy and L2 ligands, which will be discussed further below in the context of the modelling studies.
Signicant shis are observed for the protons on the ppy ligands closest to the iridium centre and the proton ortho to the coordinating N on L2. 1 H DOSY NMR (Fig. 5b) shows a single large species in solution with a diffusion constant of 1.86 Â 10 À10 m 2 s À1 giving an approximate hydrodynamic radius of 17.32Å. As expected, this is larger than those reported for M 3 L 2 cages with smaller CTG-type ligands. 16,17 The diffusion constant of L2 in MeNO 2 could not be obtained due to its poor solubility. Monitoring of the 1 H NMR spectrum of C1 over 9 months indicated no degradation of the cage in solution (Fig. S50 †).
The UV-visible spectrum of C1 in CH 2 Cl 2 (Fig. S54 †) is similar to that of reported for mononuclear [Ir(ppy) 2 (N) n ] + complexes (N ¼ AZB-decorated pyridyl-donor ligand(s)), 28 with ligand-centred transitions at l max 274 nm and 293 nm localized on the ppy ligands, and a pyridylazophenyl p / p* transition in the region of 300-390 nm, which overlaps with the spinallowed mixed metal-to-ligand and ligand-to-ligand charge transfer transitions. The weak band at 400 nm is likely linked to the spin-forbidden n / p* pyridylazophenyl-centred transitions, which overlap with both the spin-allowed and spinforbidden mixed charge transfer transitions.
Despite numerous attempts, we could not obtain crystals suitable for X-ray analysis, so molecular modelling was undertaken. The 1 H NMR spectrum of C1 undergoes some sharpening on standing in CD 3 NO 2 with no concomitant changes to the ESI-MS. We have previously demonstrated that such behaviour indicates chiral self-sorting in [{Ir(ppy) 2 } 3 (L) 2 ] 3+ (ref. 17) and other cages 19 where L ¼ 4-pyridyl-appended CTG. Hence, the geometry of a chiral MM,DDD isomer of [{Ir(ppy) 2 } 3 (L1) 2 ] 3+ was optimised using a MMFF force eld. 29 The structural model is an oblate trigonal prism with a triangular arrangement of Ir(III) centres at Ir/Ir separations of 21.3, 23.6 and 25.7Å, Fig. 6. The distance from the centres of the cage to its edges averages ca. 15 A, consistent with the hydrodynamic radius of C1 measured by DOSY NMR. There is a degree of twisting of the 3-pyridylazophenyl moieties away from planarity, with average angle between aromatic azo planes of 52 . Similar twisting has been observed in coordination polymers of E-azo-bis(pyridines). 30 The energy-minimized structure is consistent with experimental observations from the 2D ROESY studies (Fig. S52 †), where through-space interactions may be observed to a separation limit of approximately 5-to-6Å. A weak ROE is observed between H a on L2 and H 8 of ppy, which are separated by 5.14Å in the model. An intense peak is observed for coupling between H a (L2) and an overlapped peak of H 1 (ppy, 2.32Å distance to H a ) and H b (L2, 4.18Å to H a ). Overall, all the observed ROEs are consistent with the structure proposed by the model and full assignments are given Table S2 (ESI †).
A family of cages with pyridyl-azophenyl-derived CTG ligands can be accessed by variation of the cyclometallating ligand, or through variation of the CTG-ligand. The analogous cages    remarkably few reported examples of other types of cage constructs. 32 Photo-isomerisation experiments on all coordination cages were performed in CH 2 Cl 2 (DCM).** All cage complexes show photo-induced changes to their absorption spectra indicative of E / Z isomerisation on irradiation at 355 nm, Fig. 7 and S87. † Control experiments with irradiation of the starting material [Ir(ppy) 2 (NCMe) 2 ]$PF 6 show no spectral changes (Fig. S86 †). For each cage, the absorbance associated with the azoaromatic p / p* decreased in intensity while the weak band associated with n / p* transition of the Z isomer increased slightly.
Genuine isosbestic points are not observed, as is expected for formation of a complicated mixture of different cage isomers that are likely produced here. Cages C1 and C4 gave the highest degree of photo-isomerisation at 39% and 40% conversion to Z isomers, compared with 35% for C1-Me and 26% for C1-F, and 16% for C2. Notably, the only cage with a 4-pyridyl donor group (C2) has the lowest conversion at only 16%, mirroring the lower photo-conversion of L3 compared with the 3-pyridyl ligands (Table S3 †). Conversions are generally signicantly higher than was observed in Fujita's Pd 12 L 24 cage with endo-pendant azobenzenes, which had 17% E/Z conversion. 10 The low degree of switching observed in Fujita's Pd 12 L 24 cage was ascribed to the dominant absorption of a Pd-Py MLCT band in the same region as the azo p / p* band, and may also reect constricted space inside the cage. 10 Similar effects are likely to be a contributing factor for the cages reported here as the azo p / p* band overlaps with Ir(III)-based mixed CT bands, but these do not appear to inhibit isomerisation to an excessive degree. The pattern of spectral changes we observe are very similar to those observed by Hecht et al. isomerisation to a photostationary state containing 46% Zisomer. 33 Mononuclear [Ir(C^N) 2 (NN) n ] + complexes with AZBdecorated ligands have been reported to have greatly reduced or even entirely suppressed E / Z photo switching due to low energy MLCT bands, which provide a pathway for relaxation of the p* azo excited state, but these studies featured very little reduction of the p / p* band on irradiation. 28 This is clearly not the case here (Fig. 7) thus there is no evidence for competing photochemical pathways substantially suppressing E / Z isomerisation of these [{Ir(C^N) 2 } 3 (L) 2 ] 3+ coordination cages. All cages could be converted back to the all-E form near quantitatively by irradiating at 450 nm for 15 minutes, Table S3 and Fig. S88-S92. † Repeated cycles of E / Z and Z / E photoisomerisations were performed for C2, Fig S93. † The cage does experience a degree of photo-chemical fatigue across eight photo-switching cycles but the degree of E / Z switching remains unchanged, while recovery of the E form decreases suggesting a small amount of decomposition with repeated cycles of photo-isomerisation.
Cage C1-F was the only cage sufficiently soluble in CD 2 Cl 2 for 1 H NMR studies. Its spectrum was recorded, re-recorded for the same sample aer irradiation at 355 nm, then recorded again aer further irradiation at 450 nm, Fig. 8. The spectrum aer 355 nm irradiation is extremely broad and cannot be interpreted, however this is as expected for a mixture of cage isomers in solution, noting there are twelve possible E,Z isomers for an intact cage. The ESI-MS of this solution, Fig. 8b, is identical to the initial, un-switched cages and dominated by the 3+ peak at m/z 1229.2631 corresponding to [{Ir(4,5,6-tFppy) 2 } 3 (L1) 2 ] 3+ . Hence, there are no compositional changes to the system. 1 H DOSY NMR spectra of the 355 nm irradiated solution could not be obtained due to thermal Z / E relaxation that occurs over the long acquisition time required for DOSY. There is near complete conversion back to the initial spectrum on irradiation at 450 nm, Fig. 8a.
To further demonstrate structural viability of the switched cage, energy-minimized molecular models were generated for    Table 1. In deaerated DCM, all cages show similar deep blue structured emissions with maxima between 410 nm and 414 nm albeit with low photoluminescence quantum yields, F PL , of approximately 1%. These emissions are signicantly blue-shied compared with those of the previously reported [{Ir(ppy) 2 } 3 (L) 2 ] 3+ cages, which showed yellow-orange emission (l PL ¼ 604 nm), or cyan emission (l PL ¼ 485 nm) in DCM. 17 DFT studies of similar mononuclear complexes indicate that the HOMOs of these coordination cages are likely located on the [Ir(C^N) 2 ] moieties, whereas the LUMOs lie on the high-energy azobenzene fragment. 28 Therefore, the presence of the AZB units attached to coordinating pyridines in C1, C1-Me, C2 and C3 implicate large HOMO-LUMO gaps and account for the deep-blue emissions exhibited by these cages. To the best of our knowledge, C1, C1-Me, C2 and C3 exhibit the bluest emissions reported for metallosupramolecular cages. On the other hand, the low F PL values of these cages is probably the result of concomitant population of emissive p* orbitals involving the azobenzene ligand, access to non-radiative higher-lying metal-centred (MC) d-d states, which are located at similar energies, and nonradiative pathways associated with the conformationally exible CTG-based ligands. 36 Spin-coated PMMA-doped lms of the four cage exhibited broad blue emissions, respectively, at l PL ¼ 452 nm, l PL ¼ 446 nm, l PL ¼ 430 nm and l PL ¼ 436 nm with low F PL values between 0.7 and 1.7%, which are similar to those observed in DCM. However, as a result of the reduction of nonradiative vibration motion in the PMMA-doped lms, the photoluminescence lifetimes, s PL , of the ester-linked cages C1 and C1-Me are signicantly longer (2922 ns for C1 and 3002 ns for C1-Me) compared to those in DCM (265 ns for C1 and 275 ns for C1-Me). This is not the case for the glycol-linked cages C2 and C3, which exhibit in both media short multi-exponential s PL in the nanosecond-time scale.

Conclusions
Ligands L2-L4 exhibits reversible photo-switching behavior, as do the family of [{Ir(C^N) 2 } 3 (L) 2 ] 3+ coordination cages. These cages represent the rst examples of coordination cages with switchable AZB-type functionality that is inherent to the structural integrity of the cage. Remarkably, despite both the caged  nature of these pyridyl-azo-phenyl groups, and overlap of the azo p / p* absorption with Ir(III)-based mixed CT bands, extensive reversible E / Z photo-isomerization is observed. This occurs without compositional change. The degree of structural change required for E 4 Z isomerization seen here is signicantly greater than for ring-open 4 ring-closed isomerization. The latter is the mechanism for the only previous example of a coordination cage exhibiting photo-isomerism of non-peripheral ligand groups. 3 The remarkable stability of the [{Ir(C^N) 2 } 3 (L) 2 ] 3+ cages to large-scale reversible structureswitching is likely to be a function of both the high degree of rotational exibility inherent in the L2-L4 ligand designs, and the chemical inertness of Ir(III) preventing dissociation of the cages. The rotational exibility, however, contributes to the low photoluminescence quantum yields of these unusual blueemitting cages. Cages form in a facile manner from [Ir(ppy) 2 ]precursors due to the trans-labilising nature of the ppy ligands. 24 Once formed they are quite inert, demonstrated by the lack of spectral changes of these cages over several months in solution, as well as by previously studied heteroleptic ligand exchange in analogous [{Ir(ppy) 2 } 3 (L) 2 ] 3+ cages occurring only over months of equilibration. 17 Given the wealth of interesting photophysical properties of many inert metal complexes, their use for coordination cage assembly is an area of considerable promise for expansion in general. The signicant conformational and subsequent size changes that these [{Ir(C^N) 2 } 3 (L) 2 ] 3+ cages undergo, coupled with their luminescent properties, points to their future development as responsive hosts or shape-changing sensors, and their molecular recognition behavior is currently under investigation.

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

Notes and references
{ CCDC 1839930 (2p$5H 2 O), 1839931 (L2), 1859217 (1p$4H 2 O), 1859218 (L3)$ 2(CH 3 NO 2 ) k Cage C3 showed higher levels of fragmentation in the mass spectra than did other cages (Fig. S76 †), and had a broader 1D 1 H NMR spectrum (Fig. S77 †), however the 1 H DOSY spectrum (Fig. S79 †) was denitive being very similar to that of cage C4 (Fig. S73 †) and clearly showing a single large species in solution, along with solvent. ** Cages are most soluble in MeNO 2 but this is not a suitable solvent for photoisomerization experiments due to its high UV cut-off and possible photodecomposition.