Chemistry Reference
In-Depth Information
S
n
(
< 300 fs)
τ
(
= 1.6 ps
τ
2
= 15 ps)
τ
1
S
1
270 nm
T
0
PhCON + N
2
PhNCO + N
2
S
0
PhCON
3
FIGURE 12.15.
Primary photophysical and photochemical processes followed the UV
excitation of benzoyl azide
1a
.
36a
Source
: Reprinted with permission from Ref. 36a. Copyright
2009 ACS Publications.
attributed to a mixture of the intramolecular vibrational energy redistribution (IVR),
VC, and concerted Curtius rearrangement.
36
The longer time was associated with the
decay of the thermally relaxed S
1
state of azides. Therefore, the fs TRIR experiments
unambiguously demonstrated that isocyanates arise from the lowest singlet excited
state of azides.
36
As demonstrated by the ns TRIR experiments, the intensity of the nitrene IR band
1760 cm
1
is very weak (Fig. 12.12).
17
Thus, it was difficult to analyze the
growth of this band upon femtoseconds excitation of azides
at
. Never-
theless, it was
36a
deduced that 270 nm excitation of the acyl azides must initially
populate a highly excited singlet state S
n
that decays within 300 fs to form both the
carbonyl nitrene and the IR active S
1
state. It was also proposed that a small part of
nitrenes is formed in the azide S
1
state. The following scheme of the primary
processes was proposed for the 270 ns excitation of benzoyl azide
1a
,
b,
and
7
(Fig. 12.15).
36a
Shortly after
36b
Kubicki and coworkers thoroughly investigated photochemistry of
2-naphthoyl azide
1a
using femtosecond time-resolved transient absorption and IR
spectroscopies. Experiments were performed in a series of solvents using both 270
and 350 nm excitation wavelengths.
36b
When
7
7
was excited directly to the S
1
state
(
l
ex
¼
350 nm), monoexponential decay was observed. In comparison, excitation
of
2), the S
1
state decay was
described by a bi-exponential time dependence in all solvent under study
(Table 12.5).
36b
The fast component of the bi-exponential decay was assigned to
the transformation and relaxation of the vibrationally hot S
1
state.
36b
Three processes are involved in the decay of the thermally relaxed S
1
state of
7
to the higher excited states (
l
ex
¼
270 nm, S
n
,
n
:
intersystem crossing, singlet nitrene formation, and isocyanate formation. The
lifetime of the S
1
state decreases significantly as the solvent polarity increases.
In all solvents studied, isocyanate formation correlates with the decay of the azide S
1
state. Nitrene formation correlates with the decay of the relaxed S
1
state only upon
350 nm excitation (S
0
!
7
S
1
). When S
n
(
n
2) states are populated upon excitation
(
270 nm), most nitrene formation takes place within a few picoseconds through
hot S
1
and the higher singlet excited states (S
n
)of
l
ex
¼
7
. As the solvent polarity increases,
