Fluorescence-Detected Wavepacket Interferometry Reveals Time-Varying Exciton Relaxation Pathways in Single...

Area of Science:

• Photosynthesis research
• Biophysics of light-harvesting complexes
• Quantum dynamics in biological systems

Background:

• Photosynthesis requires efficient energy relaxation in pigment-protein complexes.
• Protein flexibility causes energetic and structural disorder, leading to complex, fluctuating excited-state energy levels.
• The specific exciton states and vibrational modes driving relaxation remain debated.

Purpose of the Study:

• To investigate the role of environmental fluctuations in energy relaxation pathways.
• To identify the exciton states and vibrational modes involved in relaxation dynamics.
• To understand how disorder impacts energy transfer in single pigment-protein complexes.

Main Methods:

• Utilized two-pulse ultrashort laser excitation on single pigment-protein complexes from purple bacteria.
• Generated interfering exciton wave packets to probe excited-state dynamics.
• Analyzed emission intensity modulations as a function of pulse delay time (femtosecond timescale).

Main Results:

• Observed femtosecond-timescale decay of interference due to environmental fluctuations.
• Detected slower variations (tens of seconds) in interference patterns for individual complexes.
• These slower variations indicate fluctuating energy relaxation pathways toward the lowest exciton states.

Conclusions:

• Energy relaxation pathways in photosynthetic complexes are not static but fluctuate over time.
• Temporal variations in coupling between electronic excitations and low-frequency vibrational modes drive this relaxation.
• Environmental disorder significantly influences exciton dynamics and energy transfer efficiency in photosynthesis.