Ultrafast Multisequential Photochemistry of 5-Diazo Meldrum's Acid

We disclose the light-induced dynamics and ultrafast formation of several photoproducts from the manifold of reaction pathways in the photochemistry of 5-diazo Meldrum's acid (DMA), a photoactive compound used in lithography, by femtosecond mid-infrared transient absorption spectroscopy covering several nanoseconds. After excitation of DMA dissolved in methanol to the second excited state S-2, 70% of excited molecules relax back to the S-0 ground state. In competing processes, they can undergo an intramolecular Wolff rearrangement to form ketene, which reacts with a solvent molecule to an enol intermediate and further to carboxylate ester, or they first relax to the DMA S-1 state, from where they can isomerize to a diazirine and via an intersystem crossing to a triplet carbene. For a reliable identification of the involved compounds, density functional theory calculations on the normal modes and Fourier transform infrared spectroscopy of the reactant and the photoproducts in the chemical equilibrium accompany the analysis of the transient spectra. Additional experiments in ethanol and 2-propanol lead to slight spectral shifts as well as elongated time constants due to steric hindrance in transient spectra connected with the ester formation channel, further substantiating the assignment of the occurring reaction pathways and photoproducts.