Chemistry Reference
In-Depth Information
experimental results. For a detailed discussion of quantum mechanical calculations
on excited states, see Chapter 2 by Hadad et al.
1.2 ARYL AZIDE EXCITED STATES AND NITRENES
1.2.1 Ultrafast UV-Vis Studies
1.2.1.1 p-Biphenylyl and o-Biphenylyl Azides. The photochemistry of aryl azides
and their corresponding nitrenes have been reviewed by Gritsan and Platz.
9
We will
take
p
-biphenylyl azide (
p
-BpN
3
) as an example to briefly illustrate the relevant
photochemical pathways (Scheme 1.1,
R
p
-biphenyl). On photolysis of
p
-BpN
3
,
the initially formed, relaxed, singlet nitrene
¼
1
p
-BpN (
1
RN, R
¼
p
-biphenyl) has
9 ns at ambient temperature.
28
At ambient temperature, the
lifetime of
1
p
-BpN is controlled by cyclization to benzazirine RAZ. At 77 K, the
singlet nitrene undergoes intersystem crossing to its lower-energy triplet spin isomer,
3
p
-BpN (
3
RN, R
l
max
¼
343 nm and
t ¼
10
6
s
1
in
3-methylpentane.
28
The benzazirines derived from most phenyl nitrenes rapidly
ring-open at ambient temperature to form 1,2,4,5-azacycloheptatetraenes, RK
(Scheme 1.1, also referred to as 1,2-didehydroazepines or cyclic ketenimines, see
Section 1.2.2 for a detailed discussion of the formation of RK).
9,10
Ultrafast laser flash photolysis (LFP) of
p
-BpN
3
(
¼
p
-biphenyl) with a rate constant
k
isc
¼
(9.3
0.4)
266 nm) in acetonitrile at
ambient temperature produces the transient spectra shown in Figure 1.1a.
26
There is a
broadly absorbing transient at 480 nm that forms within the laser pulse and decays
within the 300 fs laser pulse (Fig. 1.1a). As transient absorption decays at 480 nm, it
grows at 350 nm (Fig. 1.1a). The latter species is readily assigned to
1
p
-BpN on the
basis of nanosecond time-resolved studies.
28
The precursor of the singlet aryl nitrene
is assigned to an excited state of azide
1
p
-BpN
3
which absorbs at 480 nm.
Similarly, ultrafast LFP (
l
ex
¼
266 nm) of
ortho
-biphenylyl azide (
o
-BpN
3
)in
acetonitrile at ambient temperature produces a transient absorption at 480 nm, which
is formed within the laser pulse and decays with a time constant of 450
l
ex
¼
150 fs
(Fig. 1.2a). As this absorption decays, a new absorption at 400 nm (
1
o
-BpN) grows
with an isosbestic point at 435 nm and a time constant of 280
150 fs, the same time
constant as the decay of
1
o
-BpN
3
within experimental error.
Hackett and Hadad's calculations predict that the transition of S
0
to S
2
has a much
larger oscillator strength than the S
0
to S
1
transition in all of the aromatic azides
considered in this chapter (see Chapter 2 for details).
26
Thus, UV excitation of the
biphenylyl and naphthyl azides is predicted to promote the ground state of the azide
to the S
2
state. The S
2
state will deactivate rapidly through internal conversion to the
S
1
state of the aromatic azides. The S
1
state of the azide is predicted to undergo
nitrogen extrusion over a small barrier of
2 kcal/mol. This process is highly
exothermic (typically by
40 kcal/mol) and a vibrationally excited nitrene will
be formed. The S
1
state of the azide may also deactivate to reform the ground state,
thereby reducing the efficiency of aryl nitrene formation. Thus, the initially detected
transients in studies of the photochemistry of
p
-BpN
3
and
o
-BpN
3
are assigned to the
