Spectroscopic and Theoretical Identification of Two Thermal Isomerization Pathways for Bistable Chiral Overcrowded Alkenes

Abstract

Chiroptical molecular switches play an important role in responsive materials and dynamic molecular systems. Here we present the synthesis of four chiral overcrowded alkenes and the experimental and computational study of their photochemical and thermal behavior. By irradiation with UV light, metastable diastereoisomers with opposite helicity were generated through high yielding E-Z isomerizations. Kinetic studies on metastable 1-4 using CD spectroscopy and HPLC analysis revealed two pathways at higher temperatures for the thermal isomerization, namely a thermal E-Z isomerization (TEZI) and a thermal helix inversion (THI). These processes were also studied computationally whereby a new strategy was developed for calculating the TEZI barrier for second-generation overcrowded alkenes. To demonstrate that these overcrowded alkenes can be employed as bistable switches, photochromic cycling was performed, which showed that the alkenes display good selectivity and fatigue resistance over multiple irradiation cycles. In particular, switch 3 displayed the best performance in forward and backward photoswitching, while 1 excelled in thermal stability of the photogenerated metastable form. Overall, the alkenes studied showed a remarkable and unprecedented combination of switching properties including dynamic helicity, reversibility, selectivity, fatigue resistance, and thermal stability.

Artificial molecular motors

“…From the studies on the thermal behaviour of many other molecular motors, it is evident that in general the TEZI becomes a competing pathway, only when the Δ ‡ G° for THI is sufficiently high (>110 kJ mol −1 ). 88 In this sequence of light and heat induced isomerization steps for 38, one rotor part performed a 180° clockwise rotation, while the other rotor performed a 180° anticlockwise rotation, relative to the middle stator part (Scheme 16). For an external observer, both rotor parts thus move in the same forward direction, similarly to wheels on an axle.…”

Bifunctional Molecular Photoswitches Based on Overcrowded Alkenes for Dynamic Control of Catalytic Activity in Michael Addition Reactions

“…Similar results were also obtained when dichloromethane, toluene or acetonitrile were used as solvent. Notably, the corresponding unfunctionalised molecular switches constituting the light‐responsive core of 1 and 2 have been shown to undergo efficient reversible switching with 312/365 nm and 420/450 nm light over multiple cycles with no evidence of fatigue . Moreover, the thiourea moiety was present in our previous successful examples of functionalised molecular motors employed in photo‐responsive catalytic and anion binding systems .…”

“…In our previous study, we combined a 5‐membered ring in the lower half (fluorene) with a sulfur‐containing 6‐membered ring in the upper half (5,8‐dimethylthiochromene and benzo[ f ]thiochromene). This resulted in distinctive high‐energy activation barriers for the thermal relaxation step in the rotary cycle of the second generation molecular motors and consequently, long half‐lives of the metastable species . More precisely, the THI energy barrier dramatically increased due to the high conformational constraints, resulting in an alternative and predominant thermal relaxation process, known as thermal E / Z isomerization (TEZI) [for 5,8‐dimethylthiochromene: Δ ≠ G ° THI (100 °C, 1 atm)=138 kJ mol −1 , Δ ≠ G ° TEZI (100 °C, 1 atm)=129 kJ mol −1 , t (20 °C, 1 atm)=75 years; for benzo[ f ]thiochromene: Δ ≠ G ° THI (100 °C, 1 atm)=140 kJ mol −1 , Δ ≠ G ° TEZI (100 °C, 1 atm)=129 kJ mol −1 , t (20 °C, 1 atm)=4300 years].…”

Asymmetric Synthesis of Second-Generation Light-Driven Molecular Motors

“…17 For each motor, the stable, metastable, and transition state (TS) for the THI were found by a potential energy scan (PES) using the semiempirical PM6 followed by a reoptimization with DFT. An energy diagram for 2 is given in Figure 1, and the calculated Gibbs free energies for the geometry-optimized structures for motors 1 – 4 are given in Table 1.…”