Chemically optimizing operational efficiency of molecular rotary motors

Area of Science:

• Photochemistry and Molecular Machines
• Ultrafast Spectroscopy
• Nanoscale Engineering

Background:

• Molecular rotary motors convert light energy to mechanical motion via photoinduced cis-trans isomerization.
• Focus has been on ground-state rotation frequency, with less attention on excited-state dynamics.
• Optimizing excited-state processes is crucial for enhancing motor efficiency.

Purpose of the Study:

• To investigate the excited-state dynamics of molecular motors with modified electron donor/acceptor substituents.
• To understand how substituents influence photochemical yield and motor frequency.
• To characterize the underlying excited-state relaxation pathways and their dependence on substituents and solvent.

Main Methods:

• Employed time-resolved fluorescence spectroscopy with 50 fs resolution.
• Utilized ultrafast transient absorption spectroscopy.
• Combined spectroscopic techniques to characterize excited-state dynamics and reaction pathways.

Main Results:

• Substituents modify the photochemical yield of isomerization without altering motor frequency.
• Excited-state relaxation occurs via pyramidalization at the bridging double bond, populating a substituent-dependent dark state.
• Dark state decay is influenced by solvent friction, not polarity, indicating steric factors and medium friction are key.

Conclusions:

• Chemical modification of substituents offers a method to optimize molecular motor efficiency.
• Excited-state dynamics, particularly dark state formation and decay, are critical for motor performance.
• Steric factors and medium friction, rather than intramolecular charge transfer or solvation, dictate the reaction pathway.