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

Abstract

Cyclotriguaiacylene has been functionalised with 3- or 4-pyridyl-azo-phenyl groups to form a series of molecular hosts with three azobenzene-type groups that exhibit reversible photo-isomerisation. Reaction of the host molecules with [Ir(C^N)₂(NCMe)₂]⁺ where C^N is the cyclometallating 2-phenylpyridinato, 2-(4-methylphenyl)pyridinato or 2-(4,5,6-trifluorophenyl)pyridinato results in the self-assembly of a family of five different [{Ir(C^N)₂}₃(L)₂]³⁺ coordination cages. Photo-irradiation of each of the cages with a high energy laser results in E → Z photo-isomerisation of the pyridyl-azo-phenyl groups with up to 40% of groups isomerising. Isomerisation can be reversed by exposure to blue light. Thus, the cages show reversible structure-switching while maintaining their compositional integrity. This represents the largest photo-induced structural change yet reported for a structurally-integral component of a coordination cage. Energy minimised molecular models indicate a switched cage has a smaller internal space than the initial all-E isomer. The [Ir(C^N)₂(NCMe)₂]⁺ cages are weakly emissive, each with a deep blue luminescence at ca. 450 nm.

Scheme 1. Synthesis of (a) ester-linked ligands L1 and L2; (b) glycol-linked ligands L3 and L4.

Fig. 1. From the crystal structures of (a) L2 (some disorder not shown, see Fig. S3†); (b) L3·2(MeNO₂).

Fig. 2. ¹H NMR spectra (300 MHz) in CD₂Cl₂. (a) Ligand L2 and (b) ligand L4 showing in each case (i) initial spectrum, (ii) after irradiation with 355 nm laser to Z-rich form; (iii) after irradiation with 450 nm light to return to E state. Red asterix indicates CTG–CH₂– proton and blue circle indicates aromatic proton shielded in Z-isomer.

Fig. 3. UV-visible spectra of L2 in CHCl₃ (30 μM), blue trace initial spectrum, red trace after irradiation with 355 nm laser to photo-stationary state.

Scheme 2. Synthesis of [{Ir(C^N)₂}₃(L)₂]·3PF₆ coordination cages.

Fig. 4. ESI-MS of [{Ir(ppy)₂}₃(L2)₂]·3PF₆C1 after 24 h equilibration. Inset shows (a) experimental and (b) calculated isotope pattern for [{Ir(ppy)₂}₃(L2)₂]³⁺.

Fig. 5. Interpreted ¹H NMR of C1 in CD₃NO₂ with numbering scheme. (a) ¹H spectra (300 MHz) of (i) ligand L2; (ii) precursor [Ir(ppy)₂(NCMe)₂]·2PF₆; (iii) [{Ir(ppy)₂}₃(L2)₂]·3PF₆C1 after overnight equilibration. (b) Section of ¹H DOSY spectra (600 MHz) of C1.

Fig. 6. Energy-minimized structure of [{Ir(ppy)₂}₃(L2)₂]³⁺.

Fig. 7. UV-visible spectra showing E → Z photo-isomerization of cages in DCM solution indicated by reduction in intensity of π → π* absorption on step-wise irradiation with 355 nm Nd:YAG laser, and insets showing expansion of n → π* transition increasing in intensity with isomerisation. Each sample was irradiated in steps to a photo-stationary state where further irradiation did not produce more spectral changes. (a) Cage C1 with total irradiation time 255 s; (b) cage C1-Me with total 967 s irradiation; (c) cage C1-F irradiated for total 767 s; (d) cage 3 irradiated for a total of 499 s. Black line is initial spectrum, grey lines spectra on subsequent irradiations.

Fig. 8. (a) ¹H NMR (300 MHZ, CD₂Cl₂) of (i) initial C1-F; (ii) after irradiation with 355 nm laser for 200 s; (iii) after re-irradiation with 450 nm Xe lamp for 40 min. (b) ESI-MS of solution taken at conditions (ii).

Fig. 9. Energy-minimised models of [{Ir(ppy)₂}₃(L2)₂]³⁺ with differing numbers of Z-isomer ligand arms. Hydrogens excluded for clarity.

Fig. 10. Normalized photoluminescence spectra of coordination cages in deaerated DCM solution. Inset shows darkroom images of emission of C1-Me in DCM (upper) and in PMMA matrix (lower).