Cyclohexadiene Revisited: A Time-Resolved Photoelectron Spectroscopy and ab Initio Study

Area of Science:

• Chemical Dynamics
• Photochemistry
• Spectroscopy

Background:

• Cyclohexa-1,3-diene (CHD) is a key molecule for studying photochemical reactions.
• Understanding its excited state dynamics is crucial for predicting reaction pathways.

Purpose of the Study:

• To elucidate the excited state dynamics of CHD after excitation to the lowest lying ππ* (1B) state.
• To combine experimental and theoretical approaches for a comprehensive analysis.

Main Methods:

• Time-resolved photoelectron spectroscopy was employed for experimental investigation.
• Fewest switches surface hopping molecular dynamics based on linear response time-dependent density functional theory was used for theoretical modeling.

Main Results:

• Dynamics were found to progress on a single excited state surface.
• An incubation time of 35 ± 10 fs was observed, followed by a 60 ± 10 fs progression to the ground state via ring-opening or back-reaction.
• Ring-opening is favored with positive velocity in the bond alternation coordinate during nonadiabatic coupling.
• A revival of the photoelectron signal after 100 fs confirmed parallel reaction channels with distinct time constants.

Conclusions:

• The study confirms a model of two parallel reaction channels for CHD excited state dynamics.
• The formation of hexatriene rotamers, including trans-Z-trans, precedes the attainment of thermodynamic equilibrium.