Chemistry Reference
In-Depth Information
FIGURE 1.2.
Transient absorption spectra produced by 266 nm photolysis of
o
-biphenylyl
azide in acetonitrile at ambient temperature with time windows of (a) 0.3-1.3 ps and (b) 2-
500 ps.
Source
: Reprinted with permission from Ref. 26.
As expected, relaxed singlet nitrene
1
p
-BpN does not exhibit any significant
population decay on the 100 ps timescale (
9 ns).
28
However, the transient
absorption spectrum of the initially formed singlet nitrene (
1
p
-BpN) undergoes
vibrational cooling (VC)
36-39
(Fig. 1.1b), which manifests as a decay on the red edge
(380 nm) and a rise on the blue edge (345 nm) of the absorption band with the same
time constant (
t
t ΒΌ
11 ps). The time-dependent band narrowing is characteristic of
vibrational cooling of species initially formed with excess vibrational energy. A
derivative of
1
o
-BpN, 3,5-dichloro-
ortho
-biphenyl nitrene also undergoes vibra-
tional cooling in cyclohexane with a time constant of 11 ps.
40
Furthermore, in a
related ultrafast study of 2-fluorenyl azide, a similar spectral evolution was also
observed.
41
This rules out the possibility that the spectral changes are due to rotation
around the C
C bond of the biphenyl moiety.
1
o
-BpN decays with a time constant of 16
3 ps (Fig. 1.2b). The 16 ps time
constant represents the population decay of the singlet nitrene (
1
o
-BpN) by
isomerization to both isocarbazole and a benzazirine (and subsequently the benza-
zirine ring expands to form a 1,2-didehydroazepine, Scheme 1.2). The spectrum of
1
o
-BpN does not undergo reshaping characteristic of vibrational cooling, even
though the decay of
1
o
-BpN takes place on the timescale of vibrational cooling.
