Chemistry Reference
In-Depth Information
15
0.2 ps
0.4 ps
1 ps
2 ps
10 ps
10
5
10
10
0
350
400
450
500
550
600
Wavelength (nm)
FIGURE 1.4.
Transient absorption spectra produced by 266 nm photolysis of 2-naphthyl
azide in acetonitrile at ambient temperature with a time window of 0.2-10 ps.
Source
:
Reprinted with permission from Ref. 54.
study as the absorption maximum is outside the spectral probe range of 350-620 nm.
As in the case of
1
o
-BpN, evidence of vibrational cooling of
1
1-NpN is not present.
Thus, either
1
1-NpN is formed thermally relaxed or it is formed vibrationally excited
and isomerizes to naphthazirine (1-NpAZ) at the same or a faster rate that it
undergoes vibrational relaxation. Deuteration of the solvent has no discernable
influence on the observed dynamics (
15 ps).
The spectral analysis of 2-NpN
3
is much more complicated than that of 1-NpN
3
.
Ultrafast LFP (
l
ex
¼
270 nm,
t
1
¼
0.8 ps,
t
2
¼
266 nm) of 2-naphthyl azide (2-NpN
3
) in acetonitrile produces
the transient spectrum of Figure 1.4.
54
Transient absorption bands centered at 350
and 420 nm are formed within the time resolution of the spectrometer (300 fs). At
longer delay times (
l
ex
¼
1 ps), only transient absorption at 420 nm is observed. The
carrier of the 350 nm band has a shorter lifetime than that of the 420 nm peak, thus we
must be detecting the transient absorption of at least two distinct species whose
spectra overlap severely. The lifetime of the species absorbing at 420 nm is 1.8 ps (the
long-lived component in Fig. 1.5). The lifetime of the carrier of the 350 nm transient
absorption is within the instrument response (300 fs, the short-lived component in
Fig. 1.5). Similar time constants were obtained in methanol. In this case, theory
cannot confidently assign the transient spectra.
53
Tsao and Platz have extensively studied
ortho,ortho
0
-disubstituted phenyl nitrenes
and found that the rate of cyclization of a di-
ortho
-substituted singlet aryl nitrene
can be retarded by a steric effect.
55
A similar effect was also observed for singlet
3,5-dichloro-
ortho
-biphenylyl nitrene.
40
These
ortho
modifications of aryl nitrenes
do not dramatically alter the spectral position of the absorption band.
40,55
For reasons
of synthetic convenience, 1-chloro-2-naphthyl azide (Cl-2-NpN
3
) was prepared and
studied by ultrafast time-resolved spectroscopic techniques (Scheme 1.4) in order to
assign the spectral band of
1
2-NpN.
54
Ultrafast photolysis (
>
308 nm) of 1-chloro-2-naphthyl azide (Cl-2-NpN
3
)in
methanol produces the transient spectrum shown in Figure 1.6. There is a small weak
l
ex
¼
