Tuning the Ground and Excited State Dynamics of Hemithioindigo Molecular Motors by Changing Substituents

Abstract

Efficiency and performance of light triggered molecular motors are crucial features that need to be mechanistically understood to improve the performance and enable conscious property tailoringf or specific applications. In this work, three differenth emithioindigo-based molecular motors are investigated and all four steps in their complete unidirectionalr otation are unraveled fully quantitatively. Transient absorption spectroscopy across twelveo rders of magnitude in time is used to probe the fs nuclear motions up to the ms thermal kinetics, covering the timeframe of the whole motor rotation. The newly knownf ull mechanisms allow simulation of the motor systems to scrutinize their performance at realistic illumination conditions. This highlights the importance of photoisomerization quantum yields for the rotationspeed. The substitution pattern in close proximity to the rotation axle influences the excited and ground state properties. Reduction of electron donation and concomitant increaseo fs teric hindrance leads to faster photoisomerization reactions with quasi-ballistic behavior,b ut also to as light decrease in the quantume fficiency. The expected decelerating effects of increased stericsa re primarily manifested in the ground state. Ap romising approach for nextgeneration hemithioindigom otors is to elevate electron donation at the rotor fragment followed by an increaseo f steric hindrance.

Steric Effects on the Thermal Processes of Hemithioindigo Based Molecular Motor Rotation

“…Due to very fast thermal relaxation the corresponding ΔG � B-C values for thermal B-1 to C-1 and B-2 to C-2 conversions were obtained by time resolved absorption spectroscopy at 22°C in CH 2 Cl 2 solution using a multiscale broadband setup covering 12 orders of magnitude in time, as previously reported. [12] The experimental data obtained for derivatives 3 and 4 evidence unidirectional motor behavior analogous to the previously studied HTI motor derivatives 1, 2, and 5. The photoreactions of pure isomers C-3 and C-4 at À 80°C were followed by recording a series of 1 H NMR spectra during 470 nm illumination.…”

“…Kinetic analysis confirmed the helix inversions as first order reactions allowing to determine the Δ G ǂ values for these processes. Due to very fast thermal relaxation the corresponding Δ G ǂ B ‐ C values for thermal B ‐ 1 to C ‐ 1 and B ‐ 2 to C ‐ 2 conversions were obtained by time resolved absorption spectroscopy at 22 °C in CH 2 Cl 2 solution using a multiscale broadband setup covering 12 orders of magnitude in time, as previously reported [12] …”

“…The resulting higher ground state barriers for thermal helix inversions enabled a direct experimental observation of the sequential population of all four ground state intermediates and thus of the unidirectionality. Later studies with ultrafast spectroscopy enabled also a comprehensive mechanistic picture of the excited state processes in the original HTI motor [2f] and in a series of three HTI motors with increased steric hindrance at one position of the rotor fragment [12] . In the present work we used our previously developed synthetic methodology [2g] to synthesize two new and constitutional isomeric HTI motor derivatives in order to investigate the influence of sterically demanding substituents at different sides of the rotor fragment.…”

“…Later studies with ultrafast spectroscopy enabled also a comprehensive mechanistic picture of the excited state processes in the original HTI motor [2f] and in a series of three HTI motors with increased steric hindrance at one position of the rotor fragment. [12] In the present work we used our previously developed synthetic methodology [2g] to synthesize two new and constitutional isomeric HTI motor derivatives in order to investigate the influence of sterically demanding substituents at different sides of the rotor fragment. With these new derivatives in hand a series of five HTI motors 1 – 5 with different steric hindrance in close proximity to the photoisomerizable double bond could now quantitatively be compared with respect to their thermal ratcheting processes.…”

Designing light-driven rotary molecular motors

“…10%) when the methoxy groups on the upper half were exchanged for methyl or iso-propyl groups, however only small differences in the QYs for the E / Z and triplet state pathways were observed. 126 Extensive computational research into the quantum chemical processes of light-driven rotary molecular motors in recent years has proven to be a powerful tool in studying the elusive photoisomerisation step. 49,65,66,69,124 These calculations have led to the development of several theoretical motor scaffolds (Fig.…”