Mapping the Excited-State Potential Energy Surface of a Photomolecular Motor

Area of Science:

• Photochemistry
• Molecular Machines
• Spectroscopy

Background:

• Unidirectional photomolecular rotary motors enable molecular nanomachines.
• Their function relies on light-induced conformational changes (1a to 1b).
• The excited-state surface of 1a is known, but 1b remains less understood.

Purpose of the Study:

• To experimentally characterize the metastable 1b excited state of photomolecular rotary motors.
• To investigate the excited-state dynamics and reaction pathways.
• To compare findings with previous theoretical calculations.

Main Methods:

• Ultrafast transient absorption spectroscopy
• Femtosecond stimulated Raman spectroscopy
• Excited-state spectroscopy

Main Results:

• The "dark" excited-state intermediate between 1a and 1b was probed.
• The lifetime and structure of this intermediate vary based on the initial excited ground-state conformation.
• This indicates distinct reaction coordinates for forward and reverse photochemical processes.

Conclusions:

• The excited-state dynamics of photomolecular rotary motors are conformation-dependent.
• Understanding these differences is vital for optimizing motor operation.
• Findings challenge previous computational models and provide new experimental insights.