Excited State Dynamics in Unidirectional Photochemical Molecular Motors

Abstract

Unidirectional photochemically driven molecular motors (PMMs) convert the energy of absorbed light into continuous rotational motion. As such they are key components in the design of molecular machines. The prototypical and most widely employed class of PMMs is the overcrowded alkenes, where rotational motion is driven by successive photoisomerization and thermal helix inversion steps. The efficiency of such PMMs depends upon the speed of rotation, determined by the rate of ground state thermal helix inversion, and the quantum yield of photoisomerization, which is dependent on the excited state energy landscape. The former has been optimized by synthetic modification across three generations of overcrowded alkene PMMs. These improvements have often been at the expense of photoisomerization yield, where there remains room for improvement. In this perspective we review the application of ultrafast spectroscopy to characterize the excited state dynamics in PMMs. These measurements lead to a general mechanism for all generations of PMMs, involving subpicosecond decay of a Franck-Condon excited state to populate a dark excited state which decays within picoseconds via conical intersections with the electronic ground state. The model is discussed in the context of excited state dynamics calculations. Studies of PMM photochemical dynamics as a function of solvent suggest exploitation of intramolecular charge transfer and solvent polarity as a route to controlling photoisomerization yield. A test of these ideas for a first generation motor reveals a high degree of solvent control over isomerization yield. These results suggest a pathway to fine control over the performance of future PMMs.

Figure 1

General mechanism for the first generation PMM 1. The molecular structures and corresponding PESs are shown for the four-step cycle involving two photoisomerizations (hν) and two THI steps (Δ); the boxed step is analyzed in detail by ultrafast spectroscopy later.

Figure 2

General mechanism for operation of a second generation motor, here of the fluorene stator design, 2, showing successive photochemical and thermal steps. Adapted from ref (50). Copyright 2017 American Chemical Society.

Figure 3

Time resolved fluorescence of 2. (a) Wavelength resolved nonsingle exponential emission showing the blue-shifted contribution of the FC state and the coherences. The inset shows the Fourier transform of the residuals. (b) Time resolved emission spectra reconstructed from the data in (a). Adapted with permission from ref (47). Copyright 2012 Springer Nature.

Figure 4

Example of a trajectory calculation. (a) The excited state dynamics on the torsion–pyramidalization surface: yellow circle = FC state, green = onset of the dark state, and white = CI. (b) The subsequent formation of the product state. Adapted from ref (62). Copyright 2017 American Chemical Society.

Figure 5

Femtosecond stimulated Raman spectra of the dark excited state of molecular motor substituted forms of 2 measured at 200 fs. Asterisks represent solvent and instrumental artifacts. Red dashed arrows represent regions where substituent dependence is greatest. Adapted from ref (64). Copyright 2021 American Chemical Society.

Figure 6

A schematic one-dimensional representation of how electron withdrawing substituents enhance the yield while extending the dark state decay time of 2. Brown disks represent CIs, transmission through which will favor either the product or the original ground state, depending on substituent. The barrier between the dark state minimum and CI (largest for CN, negligible for OMe) controls the dark state lifetime. Adapted from ref (45). Copyright 2014 American Chemical Society.

Figure 7

(a) Chemical structures of 1 and 1_CNOMe. The ethylenic bond is in the plane of the page, and lighter/darker bonds indicate orientation below/above the page. (b) Absorption (solid lines) and emission (dashed lines) as a function of solvent for 1 (red) or 1_CNOMe (black). Adapted from ref (70). Copyright 2023 American Chemical Society.

Figure 8

Transient absorption for first generation substituents. (a) Ultrafast evolution of TA for 1_CNCN in cyclohexane. (b, c) EADS of 1_CNCN in cyclohexane and methanol. (d) Ultrafast evolution of TA in 1_CNOMe cyclohexane. (e, f) EADS of 1_CNOMe in cyclohexane and methanol.

Figure 9

Representation of the structure of 3 and the illustration of its ability to convert motor rotation into translational motion. Adapted with permission from ref (76). Copyright 2023 Springer Nature.

Figure 10

(a) Transient absorption spectra of 3 and (b) wavelength resolved amplitude of the transient kinetics fit to the FC → dark state→ product kinetics. Adapted with permission from ref (76). Copyright 2023 Springer Nature.

Figure 11

Evolution of the FSRS of 3 as the transformation of FC to dark state to product proceeds.

Figure 12

(a) General mechanism for PMM excited state dynamics. Initial excitation of a stable ground state populates the FC states on a repulsive part of the S₁ PES. This state decays rapidly to a dark state. As the structure evolves along the multidimensional reaction coordinate, the S₁ → S₀ transition moment decreases and the spectrum red shifts. The dark state decays to the ground state through a CI, which may be accessed by multiple pathways, which leads to nonsingle exponential dark state decay. From the CI the molecule may relax back to its original isomer or go on to the metastable isomer, which progresses further along the reaction coordinate by THI. (b) Illustration of how substituent and solvent may modify the excited state dynamics and motor efficiency. The initially excited FC state undergoes rapid (<300 fs) decay, which is weakly dependent on the substituent and solvent independent. In contrast the dark state decay is strongly influenced by the substituent and its decay is a function of viscosity, through the solvent displacement by motions along the reaction coordinate and polarity by modification of barrier heights and CI topology. (b) Adapted from ref (70). Copyright 2023 American Chemical Society.

Figure 13

Structures of oxindole, imine, and hemithioindigo motors.