Ultrafast photofragment ion spectroscopy of the Wolff rearrangement in 5-diazo Meldrum's acid

Abstract

We investigate the gas-phase photochemistry of 5-diazo Meldrum's acid (DMA), a photoactive compound used in lithography, by femtosecond photofragment ion spectroscopy. Transient-absorption studies in solution had revealed an ultrafast intramolecular Wolff rearrangement to a ketene after UV excitation, followed by reactions which also involve the solvent. Due to the absence of solvent molecules in this gas-phase study, we are able to focus purely on the photochemistry of the Wolff rearrangement and subsequent reaction steps. The observation of the time-resolved photofragment ion signals allows us to discriminate the dynamics of ketene and carbene products. By identification of the different possible molecular origins for a certain fragment ion signal, the time scale of the Wolff rearrangement and the lifetime of the ketene product are inferred. We further identified experimental signatures of a second Wolff rearrangement emanating from the carbene product, as had been conjectured indirectly for this molecule from pyrolysis studies.

Early events in the photochemistry of 5-diazo Meldrum's acid: formation of a product manifold in C–N bound and pre-dissociated intersection seam regions

“…17 Moreover, they suggest that excitation at 266 nm or 267 nm in time-resolved UV-vis spectroscopic experiments populates the S 2 state of DMA. 20,22,23 Our calculations are also in good agreement with previous TD-DFT calculations, which give an S 1 energy of 3.4-3.5 eV and an energy of 5.2-5.3 eV for the bright state. 17,20 These calculations give a number of additional states with near zero oscillator strength between 4.3 and 5.3 eV that are probably Rydberg states not considered in our calculations.…”

“…22 Recently, femtosecond time-resolved photofragment ion spectroscopy was employed to investigate the gas-phase photochemistry of DMA, where solvent and vibrational energy loss effects are avoided, simplifying comparisons to theoretical studies. 23 These results show that DMA rearranges to form a ketene within a time scale of 27 fs after 267 nm excitation, which provides strong evidence for the concerted WR mechanism. 23 In addition, the formation of the carbene II product was observed with a time constant of 358 fs.…”

“…23 These results show that DMA rearranges to form a ketene within a time scale of 27 fs after 267 nm excitation, which provides strong evidence for the concerted WR mechanism. 23 In addition, the formation of the carbene II product was observed with a time constant of 358 fs.…”

“…Initial decay from DMA-S 0 Excitation at 266 nm or 267 nm (approximately 4.65 eV) 20,22,23 populates the pp* state, which is S 2 at DMA-S 0 . The relaxation path is initially dominated by C 5 -N 8 bond stretching followed by bending of the quasi-linear C 5 -N 8 -N 9 moiety until a CI point between the pp* state and pp* NN states is reached, (S 2 /S 1 ) X (see Fig.…”

“…Experimental studies have investigated its spectroscopic properties, 16 thermochemical decomposition, 3,[17][18][19] and photolysis under various conditions. 3,15,17,18,[20][21][22][23] The proposed primary processes of DMA upon UV photoexcitation are illustrated in Scheme 1.…”