engleski

Photoinduced Nitrogen Elimination from Diazoalkanes

Photoinduced nitrogen elimination from diazoalkanes is well-known reaction and is often used in organic synthesis for in situ carbene formation. However, its mechanism is rather controversial. According to the current theory, built on the case of diazomethane [1, 2], the reaction is initiated by excitation to either S1 or S2 state, resulting with carbene in ground or first excited state respectively. In this work we studied photochemistry of three diazoalkanes - diazomethane (A), diphenyldiazomethane (B) i damantilphenyldiazomethane (C). Experimentally we found compounds B and C to absorb both VIS and UV light and afterward eliminate nitrogen, whereby the latter led to the much greater quantum yield of the reaction and to the much greater absorbance. Also, UV excitation of compound C was followed by the fluorescence, indicating anti- Kasha photochemistry of these compounds. To elucidate the observations and reveal the mechanism of N2 elimination, we simulated the nonadiabatic dynamics of all three molecules by means of surface hopping methods (FSSH and LandauZener), initiating the S0-minimum ensemble on S1 (VIS excitation) and S2/S3 (UV excitation) adiabatic surfaces. In both simulations, all three molecules underwent ultrafast relaxation to the ground electronic state through a S1/S0 conical intersection (CI). However, two types of S1/S0 CIs have been found, one of them being accessible only after UV excitation. In this work we show that only passage through the UV- accessible part of the S1/S0 CI seam leads to the efficient carbene production, which proves that N2 elimination is indeed an anti-Kasha process. In addition, we located a minimum on the S2 surface of molecule C, responsible for observed anti-Kasha fluorescence.

nitrogen photoelimination ; nonadiabatic dynamics ; diazoalkanes

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