Kasha or state selective behavior in the photochemistry of ortho-nitrobenzaldehyde?

Abstract

The photochemistry of ortho nitrobenzaldehyde dissolved in tetrahydrofuran was studied by means of femtosecond UV/Vis and IR spectroscopy. Comparison was made of the spectral and temporal signatures for ∼400 nm and ∼260 nm excitation. The 400 nm excitation promotes NBA to its lowest excited singlet state of nπ* character whereas for 260 nm an upper excited state of ππ* character is addressed. On the picosecond time scale, the molecule undergoes hydrogen transfer, yielding a ketene intermediate, internal conversion recovering the starting material, and intersystem crossing. Time constants and yields of these processes are virtually not affected by the excitation wavelength. For 400 nm excitation a ∼100 fs decay component seen in the 260 nm experiment is absent, indicating that this component is due to a ππ* → nπ* internal conversion. In contrast to its formation, the decay of the ketene intermediate is influenced by the excitation wavelength. This can be attributed to different amounts of vibrational excitation.

Mechanism of Ultrafast Intersystem Crossing in 2‐Nitronaphthalene

“…In contrast, ISC was expected to be considerably slower when the SOCs are very small, as is the case in organic molecules composed solely of light atoms of the first period. For certain classes of organic molecules, such as nitro polycyclic aromatic hydrocarbons (NPAHs)3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 and the closely related nitrobenzene derivatives,15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 ISC has been measured to occur in an ultrafast sub‐picosecond timescale, challenging this paradigm.…”

Nonadiabatic Dynamics Simulation Predict Intersystem Crossing in Nitroaromatic Molecules on a Picosecond Time Scale

“…Despite this general rule of thumb, a number of organic molecules have been reported to display ultrafast ISC on a sub‐picosecond time scale after photoexcitation, despite possessing small SOCs. Of these molecules, a large part belong to the class of nitro polycyclic aromatic hydrocarbons (NPAHs) or the closely related nitrobenzene derivatives . The assumptions about the processes occurring after excitation and the existence of ultrafast ISC are derived mainly from the interpretation of time‐resolved spectroscopic experiments.…”

The photoformation of a phthalide: a ketene intermediate traced by FSRS

“…The model was first developed for the photochemistry of ortho-nitrobenzaldehyde (oNBA), a compound on which we conducted a series of femtosecond spectroscopy measurements. [7][8][9][10][11] In oNBA, photoexcitation triggers the transfer of a hydrogen atom from the aldehyde substituent to the nitro group. The transfer occurs within a few 100 fs and results in the formation of a ketene intermediate 12 in its electronic ground state.…”