Comparing a photoinduced pericyclic ring opening and closure: differences in the excited state pathways

Area of Science:

• Photochemistry
• Molecular dynamics
• Organic chemistry

Background:

• Fulgimides exhibit photochromism, a reversible change in color upon light exposure.
• This phenomenon is attributed to the interconversion between open (E) and closed ring (C) isomers.
• The Woodward-Hoffmann rules predict photochemical interconversion between these isomers.

Purpose of the Study:

• To investigate the ultrafast dynamics of fulgimide photochromism.
• To elucidate the excited-state pathways for E --> C cyclization and C --> E cycloreversion.
• To compare experimental findings with theoretical predictions.

Main Methods:

• Femtosecond fluorescence spectroscopy
• Transient absorption spectroscopy
• Time-resolved measurements

Main Results:

• Biphasic fluorescence decay observed on the 0.1-1 ps timescale for both E --> C and C --> E reactions.
• Distinct time constants and spectral signatures for the two reaction directions (e.g., 0.06 and 0.4 ps for E --> C; 0.09 and 2.4 ps for C --> E).
• The longer decay time constants correlate with photoproduct formation times.

Conclusions:

• The excited-state pathways for E --> C cyclization and C --> E cycloreversion are not common.
• Experimental results contradict a simple interpretation of the Woodward-Hoffmann rules for fulgimide photochromism.
• Findings support recent quantum dynamic calculations on fulgimide excited-state topology.