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

Abstract

Tuning the thermal behavior of light driven molecular motors is fundamentally important for their future rational design. In many molecular motors thermal ratcheting steps are comprised of helicity inversions, energetically stabilizing the initial photoproducts. In this work we investigated a series of five hemithioindigo (HTI) based molecular motors to reveal the influence of steric hindrance in close proximity to the rotation axle on this process. Applying a high yielding synthetic procedure, we synthesized constitutional isomeric derivatives to distinguish between substitution effects at the aromatic and aliphatic position on the rotor fragment. The kinetics of thermal helix inversions were elucidated using low temperature 1 H NMR spectroscopy and an in situ irradiation technique. In combination with a detailed theoretical description, a comparative analysis of substituent effects on the thermal helix inversions of the rotation cycle is now possible. Such deeper understanding of the rotational cycle of HTI molecular motors is essential for speed regulation and future applications of visible light triggered nanomachines.

Designing light-driven rotary molecular motors

“…For this reason, an increase in rotation speed was observed with increasing substituent size. 136 Replacing the stereogenic methyl groups in the allylic position of first-generation HTI motor 37 with more sterically demanding ethyl groups in motor 38 was coupled with an acceleration of 10 ms. 135 A similar trend was observed in imine motors based on camphorquinone. It was found that the half-lives for the NI step decreased from 7.5 h to 10 min when moving from substitution of a cyclopropyl group in the motor to a larger t Bu group on the N atom (compare 23 and 33 ) indicating a ground state destabilisation of the metastable Z state with respect to the transition state.…”

“… 133,134 On the other hand, introducing steric hindrance in the fjord region can be used as a tool to increase the THI barriers. 135 For the first-generation HTI motor 6 the highest thermal barrier (for the E M → E S conversion) was 13.1 kcal mol −1 , equating to a half-life of 662 μs. The thermal barrier for the Z M → Z S was calculated to be 5.5 kcal mol −1 ( t 1/2 = 1.5 ns), and for this reason the Z M isomer could not be observed empirically.…”

“… 70 Similar observations were reported in a later study when comparing motors 35 and 37 with ethyl and methyl groups on the aromatic ring of the upper half, giving thermal half-lives of 36 ms and 19 ms, respectively. 135 …”

Current Reversal in a Molecular Motor

“…They are, nevertheless, sufficient to capture the steric hindrance, which has been shown to be important in other contexts, such as lightdriven machines [39]. For these light-driven molecular motors, precise control over the sterics can both increase and decrease the motor's speed [40].…”

Dibenzo[b,f][1,4,5]chalcogenadiazepine Photoswitches: Conversion of Excitation Energy into Ring Strain

“…Extending the laser excitation wavelength to 445 nm for Z ‐DBTDs also consolidate its universal applicability, especially for bioorthogonal ligation via promotion of the cycloaddition rate. The millisecond lifetime of the “metastable” E ‐DBTDs with superior photophysical features make it an ideal T‐type photoswitch element for design of light‐driven reciprocating molecular engines in which fast‐response and energy conversion are essential [52] . The photo‐switching properties of DBChDs (Ch=O, S, Se, Te), e.g., t 1/2 and ring‐strained energy, can also be widely tuned by replacing chalcogen atom on the bridge, from an ultra‐short lifetime to one‐way photo‐conversion.…”