Construction of a femtosecond pump-probe spectroscopy system and experiments on the photochemistry of 1-diazo-indan-2-one derivates

The aim of this work was the investigation of 3-diazo-2-indolinone photoreactions as a
part of the larger project concerning 1-diazo-2-indanone derivatives1. In a recent work,
we published data on photochemistry of 3-diazo-3H-benzofuran-2-one2 and the study of
1-diazo-2-indanone is an ongoing project.
The a - carbonyl carbene was identified by ultra fast pump probe spectroscopy
experiments as the sole primary photoproduct of 3-diazo-2-indolinone in solution. It is
formed within one picosecond upon irradiation. The possibility of rapid equilibration via
intersystem crossing with its triplet state was discussed, based on the high-level ab initio
calculations. The calculations predict a triplet ground state of studied compound in the
gas phase. Ring-opening reaction of a singlet carbene is strongly favored over Wolff
rearrangement reaction or oxirane formation by the calculations.
The singlet carbene intermediate undergoes the ring opening reaction, yielding
cumulenone in aprotic solvents, k ≈ 5.9 x 1010 s-1. The cumulenone structure of the
intermediate was confirmed by step scan FTIR measurements in CD3CN. In aqueous
solutions, the primary photoproduct is intercepted by water with a rate of kH2O ≈ 7 x 109
M-1 s -1. The yield of cumulenone is thus reduced with increasing water concentration. 3-
Hydroxy-2-indolinone is the major product isolated after irradiation of 3-diazo-2-
indolinone in aqueous solutions. It is formed by two reaction pathways depending on the
water concentration in solution. They are either an O-H group insertion into the singlet
carbene or alternatively hydration of the cumulenone. The O-H insertion is obviously
favoured at higher water concentrations. We studied the mechanism of O-H insertion by
laser flash photolysis technique.
These experiments failed to detect any transient attributable to the enediol intermediate,
which would be expected to show similar absorbance at 275 nm and ketonization kinetics
as the enol of mandelic acid amide. The lack of such a transient ruled out the enol
formation pathways of singlet carbene insertion into an O-H group. The pH rate profile
has an acid-catalyzed portion kH
+ ≈ 2 x 1010 M-1s-1 followed by a complex uncatalyzed
region. This behavior is in contradiction with all known mechanisms except the ylide
formation followed by the proton transfer. This would suggest a long-lived ylide
intermediate. Such a structure has not been observed yet. Therefore, for the complete
understanding of the O-H insertion mechanism some additional experiments are
necessary.
The second goal of this work was the construction of the femtosecond pump-probe
spectroscopy setup. The constructed setup allows the direct spectroscopic observation of
the ultra short-lived intermediates in the wavelength range from 300 to 700 nm. The time
resolution achieved is around 180 fs. The repetition rate of the setup is 426 Hz and the
time resolved spectra can be measured within 1.8 ns time window.
Other projects, in which I have been involved during my PhD study, were the
investigation of photochemical reaction mechanisms of 2-nitrobenzyl compounds3, the
study of an excited state intramolecular proton transfer in 2-naphtol and the study of
energy and electron transfer in various Porphyrin - Deazaflavin systems.