Towards the engineering of a photon-only two-stroke rotary molecular motor

Abstract

The rational engineering of photoresponsive materials, e.g., light-driven molecular motors, is a challenging task. Here, we use structure-related design rules to prepare a prototype molecular rotary motor capable of completing an entire revolution using, exclusively, the sequential absorption of two photons; i.e., a photon-only two-stroke motor. The mechanism of rotation is then characterised using a combination of non-adiabatic dynamics simulations and transient absorption spectroscopy measurements. The results show that the rotor moiety rotates axially relative to the stator and produces, within a few picoseconds at ambient T, an intermediate with the same helicity as the starting structure. We discuss how such properties, that include a 0.25 quantum efficiency, can help overcome the operational limitations of the classical overcrowded alkene designs.

Fig. 1. Chemical formulae of the classic light-driven rotary motor (LDRM) and the LDRMs studied here.

The red arrow shows the direction, in which rotation of the upper part occurs with respect to the lower part of the motor.

Fig. 2. Schematic representation of the reaction path for the first half of LDRM working cycle starting from an EP configuration.

A Classic unidirectional EP $\stackrel{h\nu }{\to }$ ZM $\stackrel{T^{\circ }}{\to }$ ZP half-cycle with the possibility of loss of EP $\stackrel{h\nu }{\to }$ ZP directionality. The labels marking the system potential limitations (see text) are given in purple. B Target unidirectional, efficient, and ultrafast EP $\stackrel{h\nu }{\to }$ ZP half-cycle. The acronyms EP, ZP, etc. stand for the structures with a specific helicity (P or M) in a specific configuration (Z or E). CI stands for conical intersection. The gray dots represent the initial structures, the red dots represent the productive trajectories, and the blue dots the unproductive trajectories.

Fig. 3. Rotary cycle (also working cycle or photochemical cycle).

A Rotary cycle of a classic 4-stroke LDRM of Fig. 1. B Hypothetical rotary cycle of a 2-stroke LDRM proposed here. The two sequentially absorbed photons are, in general, of different wavelengths.

Fig. 4. Simulated gas-phase photoisomerisation of MTDP.

A Projection of the adiabatic minimum energy path (MEP) EP → ZP of the MTDP motor onto θ (see Fig. 1 for the definition of θ) calculated with the SSR method (triangles). The structures below the plot show the geometries of the reactant (EP), the conical intersection (CI${}_{S_1/S_0}$) and the product (ZP). The arrows indicate a CCW motion. The dashed energy profiles correspond to 3-root state-average XMS-CASPT2 energies calculated with a 2 electrons in 2 π-orbitals complete active space. The dotted energy profiles show the corresponding 5-root state-average with a 10 electrons in 10 π-orbitals complete active space. The insets display the relationship between the S₁ and S₂ states along the framed region. An avoided crossing between a charge transfer and locally excited state is supported by plotting the charge residing on the pyrrolidinone (also called oxindole) ring in the two states. The S₁ → S₀ nonadiabatic relaxation occurs near the geometry of CI${}_{S_1/S_0}$, which is shown by the red filled triangle. B The same for the ZP → EP step. C θ propagation during the quantum-classical population dynamics starting from EP. The propagation along the S₁ PES (the black lines) is connected with the productive (the red lines) and unproductive (the blue lines) propagation along the S₀ PES by the corresponding hop points (the red and blue circles) “imaging” a segment of the CI${}_{S_1/S_0}$ seam. D The same for the ZP → EP step. Source data are provided as a Source Data file.

Fig. 5. Synthesis of MTDP.

Reagents and conditions: (i) LiHMDS, BF₃ ⋅ Et₂O, THF dry, −78 °C; (ii) TFA, DCM, 0 °C to room temperature.

Fig. 6. Transient absorption (TA) spectroscopy of a methanol solution of MTDP upon 290 nm light excitation.

A False-color representation of the pump-induced absorption change (ΔA, in mOD) as a function of probe wavelength (nm) and pump-probe delay (in ps). A selection of TA spectra (ΔA) at late, intermediate, and early pump-probe time delays is displayed in B–D, respectively. E Decay-Associated Spectra (DAS) obtained from the global fit of the entire TA dataset displayed in A by a multiexponential kinetics involving 4 time constants (see also Supplementary Fig. 31). Source data are provided as a Source Data file.

Fig. 7. Illustration of the factors contributing to absence of the stable M conformations in MTDP.

A Z-2 structure highlighting the strained unit (blue) and the clashing fjord region (red). B Pictorial illustration of the equatorial to axial relaxation imposed by the strain in a model of the ECPE unit. In the equatorial position the Me in position 1 is almost parallel/aligned with the methyl substituent in position 3. The large dihedral angle (in red) in the axial conformer is consistent with removal of the strain. C Geometrical parameters (planar and dihedral angles in red) justifying the reduced steric repulsion in Z-2 vs. Z-3. The energy difference (kcal/mol) between the M and P conformers of Z-3 is also given.