A photo-switchable molecular capsule: sequential photoinduced processes

Abstract

The metastable trilacunary heteropolyoxomolybdate [PMo₉O₃₁(py)₃]^3- - {PMo₉}; py = pyridine) and the ditopic pyridyl bearing diarylethene (DAE) (C₂₅H₁₆N₂F₆S₂) self-assemble via a facile ligand replacement methodology to yield the photo-active molecular capsule [(PMo₉O₃₁)₂(DAE)₃]^6-. The spatial arrangement and conformation of the three DAE ligands are directed by the surface chemistry of the molecular metal oxide precursor with exclusive ligation of the photo-active antiparallel rotamer to the polyoxometalate (POM) while the integrity of the assembly in solution has been verified by a suite of spectroscopic techniques. Electrocyclisation of the three DAEs occurs sequentially and has been investigated using a combination of steady-state and time-resolved spectroscopies with the discovery of a photochemical cascade whereby rapid photoinduced ring closure is followed by electron transfer from the ring-closed DAE to the POM in the latent donor-acceptor system on subsequent excitation. This interpretation is also supported by computational and detailed spectroelectrochemical analysis. Ring-closing quantum yields were also determined using a custom quantum yield determination setup (QYDS), providing insight into the impact of POM coordination on these processes.

Fig. 1. Graphical representation of the chiral molecular capsule (1(O_A)₃) with dimensions obtained from single crystal X-ray diffraction structure determination.

Fig. 2. General scheme of the rotational isomerism observed for DAEs (top) and the ditopic (DAE) used in this study (bottom left). (L1 – bottom right) was used in the transient absorption investigation as the closest non-DAE model compound to identify the intersystem crossing S₁ → T₁ spectral signature.

Fig. 3. The high resolution isotopic envelope of (1) as the trianionic {TBA₃[(PMo₉O₃₁)₂(DAE)₃]}³⁻ (exp. (top) vs. calc. (bottom)).

Fig. 4. Time-dependent UV-vis spectroscopy showing the photochemical conversion from (1(O_A)₃) to (1(C)₃) in response to 0.21 mW of 325 nm light (upper panel) in dichloromethane with the corresponding ¹H NMR spectroscopy study (bottom), where x + z = 3.

Fig. 5. (A) Femtosecond absorption data for DAE in dry acetonitrile for a laser excitation of 330 nm displayed as three separated temporal windows [0.95–1.2 ps] (top) [1.3–8 ps] (middle) and [15–2000 ps] (bottom). The absorption spectra for the PPS solution is appended (gray dashed line) for comparison (top left). (B) Femtosecond absorption data for L1 in dry acetonitrile for a laser excitation of 300 nm and temporal window [50–2000 ps]. In the lower panel, stationary absorption and emission spectra (300 nm excitation) are indicated as well. (C) Femtosecond absorption data for (1(O_A)₃) in dry acetonitrile for a laser excitation of 330 nm displayed as three separated temporal windows [0.7–1.5 ps] (top) [2–8 ps] (middle) and [12–1000 ps] (bottom). The absorption spectra for the PPS solution is appended (gray dashed line) for comparison.

Fig. 6. UV-vis-NIR spectroelectrochemistry of ((1(O_A)_x(C)_z)), (c = 1.72 × 10⁻⁴ M, 20 °C) upon electrochemical oxidation in 0.1 M n-Bu₄NPF₆ in acetonitrile/1,4-dioxane (1/1) solution at a scan rate of 20 mV^.s−1. (A) Evolution of the UV-vis-NIR spectra recorded during oxidation. Optical path length = 0.02 cm. (B) CV at a scan rate of 20 mV s⁻¹ recorded during the spectroelectrochemistry measurement. Note that the start of the recording (blue curve) is done when the equilibrium from closed to open form is reached under UV-visible illumination.

Fig. 7. CAMB3LYP/def2-SVPD orbitals (plotted with standard settings in VESTA) for the most relevant excitations in (1(O_A)₂(C)₁) as listed in (Table S6†).