Chemistry Reference
In-Depth Information
Our recent calculations on substituted phenyl azides (S. Vyas and C. M. Hadad,
unpublished results)
149
predicted that the character of the excited states remained the
same as observed for the naphthyl azides; however, the energy gap between the S
1
and S
2
states is much larger (Fig. 2.11). The S
1
state was the (
p
p
in-plane
) state, while
,
p
) state for the phenyl moiety. The large energy gap between the
S
1
and S
2
states of phenyl azide removes the possibility of a conical intersection
between the two lowest singlet excited states, thereby increasing the lifetime of the
excited state. It is debatable whether the observed excited state of the azide was the S
1
or the S
2
state. Nevertheless, theoretical calculations on phenyl azide predict a longer
lifetime of the excited state compared to the naphthyl azide, because for phenyl
azide, a conical intersection between the S
1
and S
2
states is not available and there is
also a higher energy barrier on the S
1
surface to produce phenylnitrene.
The large energy gap between the S
1
and S
2
states for phenyl azide as compared to
the naphthyl and biphenyl azides can be attributed to the larger number of (
the S
2
state was (
p
,
p
)
states in the more conjugated aryl azides. Based on this argument, it is possible to
predict that even more extended aromatic azides will then have their lowest excited
state as the (
p
,
p
in-plane
) azide excitation will shift
higher in the excited state manifold. However, this hypothesis waits for confirmation
by future ultrafast transient absorption experiments. Our recent computational and
experimental study on nitro- and amino-substituted phenyl azide, namely, 3-nitro-4-
aminophenyl azide,
74
showed that nitrene formation takes place on the S
2
surface as
the lowest singlet excited state was localized on the nitro and aromatic units. TD-
B3LYP/TZVP calculations showed a lengthening of the N1
p
) state and conversely, the (
p
,
p
,
N2 bond length during
the relaxation on the S
2
surface. The femtosecond experiments confirmed that
exciting the molecule with low energy radiation does not generate the nitrene.
Moreover, spectroscopic signatures were correlated to intermediates by comparison
of the experimental transient absorption spectra to the computational results for the
nitrene, nitrenium ion, and other rearrangement products.
Little differences in the excited state characteristics were observed when theoret-
ical work was carried out for heteroaromatic azides. Budyka and Oshkin performed
extensive semiempirical PM3 calculations on pyridinyl azide and larger aromatic
variants.
150
Their earlier calculations suggested no differences in the singlet excited
states, anionic singlet manifold, triplet states, and the resulting photochemistry of
pyridinyl azide and phenyl azide except that the HOMO-LUMO energy gap is higher
in the case of pyridinyl azide. If one extends the
system, the azide unit is out-of-
plane due to two peri-hydrogens in azidoacridine, azidobenzacridine, and azidodi-
benzacridine; however, the geometric parameters remained similar around the azide
moiety. As the aromaticity increases from azidopyridine to dibenzacridine, the
p
p
p
(aromatic) gap decreases due to increased
-
p
-molecular orbitals, and the
p
in-plane
gap also increases. Consequently, while azidoacridine has an azide
dissociative state as S
1
, the (
p
-
p
) states become the lower-energy excited states
for the heteroaromatic azides, such as azidebenzacridine and azidodibenzacridine,
thereby rendering these larger aromatic azides to be poor photochemical sources of
nitrenes. An additional issue in heteroaromatic azides is that there is an opportunity
for protonation of the heteroatomic center, which in turn may affect the excited
p
,
