Excited State Structure Correlates with Efficient Photoconversion in Unidirectional Motors.

Area of Science:

• Photochemistry
• Molecular machines
• Ultrafast spectroscopy

Background:

• Unidirectional photomolecular motors rely on ultrafast photochemical isomerization.
• An intermediate dark excited state mediates this reaction through a conical intersection (CI) with the ground state.
• The relationship between molecular structure and photoisomerization efficiency remains unclear.

Purpose of the Study:

• To investigate the correlation between molecular structure and photoisomerization efficiency in molecular motors.
• To capture vibrational spectra of the dark state using femtosecond stimulated Raman spectroscopy.
• To establish guidelines for designing more efficient molecular motor derivatives.

Main Methods:

• Femtosecond stimulated Raman spectroscopy (FSRS) was employed.
• Vibrational spectra of the dark state were recorded for molecular motors with varying substituents.
• Photoisomerization quantum yield and dark state lifetime were measured.

Main Results:

• A direct correlation was found between isomerization quantum yield, dark state lifetime, and excited state vibrational spectrum.
• Electron-withdrawing substituents induced activity in lower frequency modes, linked to pyramidalization distortion at the ethylenic axle within 100 fs.
• Electron-donating substituents did not promote this distortion, maintaining double bond character at the axle.

Conclusions:

• The dark state structure significantly influences photoconversion performance.
• Substituent effects on molecular structure, particularly ethylenic axle distortion, are critical for motor efficiency.
• These findings provide a basis for rational design of enhanced photomolecular motors.