Chemistry Reference
In-Depth Information
R
R
hv
1
N
3
N
3
*
1
RN
3
*
RN
3
-N
2
R
H
+
R
1
N
R
N
NH
+
1
RN
RNH
+
RAZ
R
R
3
N
N
3
RN
RK
R
N
N
R
SCHEME 1.1.
General reaction pathways for aryl azides.
independent photochemistry.
3
However, exceptions to Kasha's rule are known. A
very well-known exception is provided by azulene, whose fluorescence originates
from the S
2
state and whose corresponding absorption and emission spectra are not
mirror images.
4
Also, numerous photochemical wavelength-dependent reactions are
known. For example, the photolysis of diazirines using different excitation wave-
lengths produces different quantum yields and even different photoproducts.
5,6
Therefore, it is important to understand the nature of the excited state of a
photochemical reaction precursor and the competition between all of the pathways
by which it can decay.
The photolysis of aromatic azides promotes nitrogen extrusion and the release of
singlet nitrenes (Scheme 1.1).
7,8
The chemistry of aryl nitrenes has been extensively
studied by chemical, physical, and computational methods.
9,10
The quantum yields
of light-induced decomposition of the naphthyl azides are close to unity and that of
simple phenyl azides fall in the range 0.1-0.7 and depend on the concentration of the
azide.
11-18
To the best of our knowledge, simple phenyl, biphenylyl, and naphthyl
azides lack observable fluorescence, which is consistent with their large quantum
yields for extrusion of molecular nitrogen. Otherwise, essentially nothing was known
of the details by which aryl azide excited states decompose to form singlet nitrenes at
the outset of this project. The development of ultrafast spectroscopic techniques and
modern quantum chemical computational methods provides tools with which to
begin to understand how the excited state surfaces of aryl azides connect to the
ground state surfaces of the nitrenes.
