The Photoinduced Ring-Closing Reaction of Cyclopentadiene and its Methylated Derivatives studied via Ultrafast Electron Diffraction

Using ultrafast electron diffraction, the UV photoinduced structural dynamics of cyclopentadiene, and its methylated derivatives tetramethyl- and pentamethylcyclopentadiene have been captured on the femtosecond timescale. Our study uncovers the effects of the additional methyl groups in terms of the reaction dynamics and the product yields. Using a genetic algorithm, we have retrieved the time-dependent molecular structures directly from the measured diffraction signal. Upon the photoexcitation of cyclopentadiene, the strained-ring product bicyclo[2.1.0]pentene is formed along with vibrationally hot cyclopentadiene, in less than 200 fs. In tetramethylcyclopentadiene, similar reaction products are formed although on a longer timescale. The addition of a fifth methyl group prevents the ring-closing that produces the bicyclic product, and produces a different structure. The experimental results are in good agreement with the results of cyclopentadiene trajectory simulations and provide new insights into the dynamics of the methylated derivatives where full simulations are not available.