Ultrafast Spectroscopic Study of the Photochemistry and Photophysics of Arylhalodiazirines:  Direct Observation of Carbene and Zwitterion Formation

Abstract

Ultrafast photolysis (lambdaex = 270, 350, or 360 nm) of bromophenyl, chlorophenyl, fluorophenyl, and fluoro-para-trifluoromethylphenyl diazirines produces transient species which absorb broadly in the UV and visible regions. Transient decay can be fit to either mono- or biexponential functions (tau1 approximately 0.3-10 ps, tau2 approximately 10-350 ps; dependent on solvent and halogen). Fluoro- and chlorophenylcarbene are formed within the time resolution of the spectrometer (300 fs, 270 nm excitation). Bromophenyl diazirine decay (270 nm excitation) correlates with the growth of bromophenylcarbene. Solvent and substituent effects on the slower decays of the transient absorptions are consistent with assigning the carriers of transient absorption in the visible region to ring-opened zwitterionic species.

Theoretical study of non-Hammettvs.Hammett behaviour in the thermolysis and photolysis of arylchlorodiazirines

“…17,18 They yield carbenes photochemically under mild conditions. 19,20 Ultrafast time-resolved laser flash photolysis of aryl diazo compounds [21][22][23] and aryldiazirines [24][25][26] has provided valuable information on the dynamics of the precursor excited states and the generated singlet arylcarbenes. Using ultrafast infrared spectroscopy, the singlet aryldiazomethane and arylcarbene intermediates were detected in the photolysis of phenyldiazirine, phenylchlorodiazirine and p-methoxy-3-phenyl-3-methyldiazirine in less than 1 ps upon excitation.…”

Channels to Singlet and Triplet Phenylcarbenes in Phenyldiazomethane: A CASSCF and MRCI Study

“…A photon excites C 6 H 5 C(H)N 2 to the S n (n > 1) state followed by the decay to the S 1 state by internal conversion or the S 1 state is directly populated with low excitation energies as conducted in many available experiments. [11,12,[14][15][16][17][18][19]21] On arrival at the S 1 state, two considerable routes exist: 1) the S 1 direct dissociation to the excited singlet C 6 H 5 CH and 2) internal conversion to the S 0 state via the minimum-energy crossing points S1-S0 and S1-S0-ISO. In route 1, C 6 H 5 CH decays to the lowest singlet state as a result of the minimum-energy crossing point between the excited and lowest singlet states, S1-C-S2-C.…”

“…Since most of the available experiments observing the ultrafast formation of singlet arylcarbene were conducted with low excitation energy, it is rational to expect that the S 1 state can be directly populated. [11,12,[14][15][16][17][18][19]21] The channels from S1-MIN are close to those from S1-MIN-ISO, and thus we only focus on those from S1-MIN-ISO in the following discussion. Section 3 is divided into several subsections.…”

“…Since most of the available experiments observing ultrafast formation of singlet arylcarbene were conducted around this wavelength, one could expect that this ultrafast event should mainly interrelate with the S 1 or S 0 state. [11,12,[14][15][16][17][18][19]21] Additionally, although the initially populated singlet state may vary due to different excitation wavelengths, the CN bond fission should predominantly occur in the S 1 state as a result of the S n !S 1 (n > 1) internal conversion.…”

“…Combined with ultrafast laser flash technique, this UV-VIS and IR spectroscopy becomes a powerful tool to probe the decay dynamics of the excited states of azide, diazo, and diazirine, and the formation and decay of nitrene or carbene reactive intermediates. [9,[11][12][13][14][15][16][17][18][19] For example, the singlet excited states of aryldiazo compounds such as CH 3 OC 6 H 4 C(CH 3 )N 2 and C 6 H 5 C(H)N 2 , and their excited singlet carbenes have recently been detected by Platz and coworkers with ultrafast time-resolved UV-VIS and IR spectroscopy. [15,[17][18][19][20][21] The excited states of these aryldiazo compounds typically decay within 300 fs to form singlet arylcarbenes, which subsequently undergo various intra-and intermolecular reactions.…”