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Photolabile protecting groups offer synthetic routes with facile deprotection by photolysis, providing higher yields with less workup. Protecting groups producing only gaseous byproducts upon removal are particularly desirable as sustainable reagents. Cheletropic reactions provide just this combination of photocleavable groups with gaseous byproducts. However, only few protecting groups for cheletropic reactions have been described and little is known about their photochemistry and associated stereochemistry. Here we show that the sulfolene protecting group, used for diene protection and functionalization, can be photochemically removed. First, using TD‐DFT, we conclude that the photochemical ring‐opening occurs conrotatorily in a concerted process, consistent with the Woodward‐Hoffmann rules. Experimentally, we demonstrate that this process can be photoinitiated by radiation between 180 and 300 nm and confirm the proposed stereochemistry in solution. Using time‐resolved photoelectron spectroscopy, we determine that the first steps in the photochemical ring‐opening of sulfolene occur on ultrafast timescales (≤84 fs), consistent with a fully concerted process. Our findings expand applications of the sulfolene protecting group and promise an easy strategy for control of cis‐trans isomerization. [ABSTRACT FROM AUTHOR]

To investigate the influence of amine structure on the internal conversion from S 2 to S 1 , three aliphatic amines containing the same number of degrees of freedom, but with different degree of N -substitution, were investigated with femtosecond time-resolved mass spectrometry. As N -substituents lower the excitation energies, and the excitation in all cases is by a 200 nm photon, the S 1 density-of-states in the Franck–Condon region will be high for the more N -alkylated amine. This, according to standard models, should lead to faster internal conversion. The experimental results are in contrast to this, indicating non-ergodic behavior and a coupling of the states by very few molecular vibrations. This was further investigated by substituting differing numbers of hydrogen atoms with deuterium in two amines, one primary and one tertiary, thus freezing the degrees of freedom while altering the N -substituent mass. In both case, deuteration resulted in a significantly longer lifetime of the initially excited state, confirming the importance of the N- inversion motion in the description of the dynamics of internal conversion. [ABSTRACT FROM AUTHOR]

Internal conversion: The energy difference between the Franck–Condon and equilibrium geometries and the vibrational frequency of one or a few modes determine the relative importance of adiabatic and nonadiabatic dynamics and thus the rate of electronic energy dissipation (see picture). In the cycloketones, variations in these quantities lead to a difference in the timescale for the S2→S1 transition. [ABSTRACT FROM AUTHOR]

Femtosecond time‐resolved spectroscopy can open the window to fascinating new perspectives of internal conversion processes. In their Communication on page 2247 ff., T. I. Sølling, K. B. Møller, and co‐workers show that the S2 to S1 transition of a series of cyclic ketones is mediated by very few degrees of freedom. This knowledge makes it possible to predict and influence the transition dynamics through modification of the molecular structure. [ABSTRACT FROM AUTHOR]