Photoinduced Intramolecular [2 + 2] Cycloaddition Investigated with Transient Infrared Spectroscopy

The use of visible light to drive chemical reactions has become an increasingly popular synthetic approach in organic and polymer chemistries. However, photodriven reaction dynamics and mechanisms remain difficult to elucidate. Here, we report the dynamics and kinetics of a photoinitiated intramolecular [2 + 2] cycloaddition reaction using ultrafast transient infrared spectroscopy spanning 100 fs to 1 ns after photoexcitation. Vibrational signatures of the product species, consisting of a bicyclo[3.2.0]heptane-2,4-dione core, appear within ∼300 ps following visible excitation at 430 nm. We performed global fitting analysis to a four-step sequential reaction model. The resulting kinetics are hypothesized to derive from fast vibrational relaxation within the initially excited S₃ electronic state (∼160 fs), followed by internal conversion (∼3 ps). A key intermediate state or species then forms on a ∼30 ps time scale, which is postulated to arise from intersystem crossing to the triplet manifold. The final step is the rate-limiting transformation to the product occurring with a ∼260 ps time constant.