Fine Tuning of the Rotational Rate of Light-Driven, Second Generation Molecular Motors by Fluorine Substitutions

TL;DR

The paper investigates how fluorine substitutions can fine-tune the THI rate in second-generation molecular motors. It employs harmonic transition state theory (HTST) to estimate thermally activated rates, with energy landscapes computed from density functional theory using B3LYP/6-31G(d,p) and transition paths via CI-NEB, validated by vibrational analyses; rates are expressed as $k_{HTST}$ and half-lives via $\tau$. The key findings show that H to F substitution at the Y site raises the transition-state energy and slows THI, while CH3 to CF3 substitution at the X site increases the metastable state's energy and speeds THI; combining both substitutions provides finer control and, in some cases, enlarged separation between absorption peaks of stable and metastable states. The calculated lifetimes agree remarkably with experimental data across multiple motors, supporting the predictive power of the approach and suggesting practical routes to tailor rotational speeds in light-driven materials. Additional insights include spectra shifts with substitutions and notes on potential N substitutions at the stator Z site; data and methods are deposited for public access.

Abstract

The relaxation time of several second generation molecular motors is analysed by calculating the minimum energy path between the metastable and stable states and evaluating the transition rate within harmonic transition state theory based on energetics obtained from density functional theory. Comparison with published experimental data shows remarkably good agreement and demonstrates the predictive capability of the theoretical approach. While previous measurements by Feringa and coworkers [Chem.\,Eur.\,J.\,(2017) 23, 6643] have shown that a replacement of the stereogenic hydrogen by a fluorine atom increases the relaxation time because of destabilization of the transition state for the thermal helix inversion, we find that a replacement of CH$_3$ by a CF$_3$ group at the same site shortens the relaxation time because of elevated energy of the metastable state without a significant shift in the transition state energy. Since these two fluorine substitutions have an opposite effect on the relaxation time, the two combined can provide a way to fine tune the rotational speed of a molecular motor.

Figures (4)

• Figure 1: Illustration of the first two steps in the rotation of a second-generation molecular motor developed by Feringa and coworkers.vicarioControllingSpeedRotation2005 After photoabsorption, the structure evolves from the stable E state to the metastable Z state, first a ca. 90$^{\circ}$ rotation in the excited electronic state and then further rotation after returning to the ground state. From there, a thermally activated transition, the thermal helix inversion (THI), can bring the molecule to the stable Z state thereby completing a ca. 180$^{\circ}$ rotation. For an efficient rotor, the transition back to the E state should have significantly lower probability than the THI. Absorption of a second photon in the stable Z state and a repeat of a THI step then completes a full circle.
• Figure 2: The molecular motors investigated in the present study to assess the effect of fluorine and nitrogen substitutions. The sites are labeled X, Y, and Z and they have a CH3, H, and CH, respectively, in the base molecules, but CF3, F, or N in the modified molecules.
• Figure 3: (a) Comparison of calculated and measured half-life of the metastable state of various second-generation molecular motors. Both the THI step and the backreaction are taken into account. Data corresponding to the base molecules (shown in figure 1) is shown in gray, while red illustrates data for molecules where H has been replaced by F at site Y. Data on additional second-generation motors is marked with blue stars. The experimental data is obtained from Refs. vicarioFineTuningRotary2006pollardRedesignLightdrivenRotary2008pollardEffectDonorAcceptor2008stackoFluorineSubstitutedMolecularMotors2017vicarioControllingSpeedRotation2005cnossenTrimerUltrafastNanomotors2009feringaControlMotionMolecular2001 (see SI). The agreement between measured and calculated values is remarkably good, the red dashed line indicating perfect agreement. (b) Calculated half-life of the THI step and its change by substitution of CH3 by CF3 at site X (blue), substitution of H by F at site Y (red), and both substitutions combined (purple). The CF3 substitution increases the rate of the THI step, whereas the F substitution decreases it, as indicated by the arrows.
• Figure 4: Effect of the H to F substitution at site Y, and the CH3 to CF3 substitution at site X of the /molecules/68 molecule. The F atoms are shown in green. (a,b,c): Change in the metastable state, transition structure, and stable state due to the H to F substitution (/molecules/16). The structures are aligned at the C=C bond. (e,f,g): Analogous change due to the CH3 to CF3 substitution (/molecules/69). The electron density rendered at 0.1 Bohr$^{-3}$ is shown for the metastable state of /molecules/16 in (d), and for the transition structure of /molecules/69 in (h). Blue: original molecule, /molecules/68. Red: Modified molecule. (i) and (j): Minimum energy path between the metastable and stable states for the base and modified molecules. The blue bars indicate the RMSD between the structure of the base and modified molecule. The zero of energy is chosen to be the energy of the state with the smallest RMSD. The H to F substitution mainly affects the transition structure and reduces the rate of THI, but the CH3 to CF3 substitution mainly affects the metastable state and increases the rate.