Chemistry Reference
In-Depth Information
(10 nm) 14 to 15 (9 nm). A plot of
0.5) provides a straight line that
can be extrapolated to 578 nm, possibly indicating the theoretical band gap of
polyheliphene.
l
max
vs 1/(N
รพ
2.3 Reactivity of [N]Phenylenes and Related Materials
Biphenylene (20) was long believed to be photoinert; however, evidence of high
internal conversion rates indicated that the picture is more complex [
44
]. Transient
absorption measurements of 20 indicate the formation of an initial, vibrationally hot
ground state S
0
* formed by internal conversion, followed by chemical reaction to
form stable products. Correlation between the transient absorbance and laser
influence data suggest that hot biphenylene must absorb a second photon to become
chemically reactive, further raising its internal energy to compete with collisional
deactivation. Several photo-products were identified including phenylacetylene,
biphenyl, naphthalene, and acenaphthylene, as well as three unknown. The authors
suggest a mechanism involving initial C-C bond cleavage to form the 2,2-biphenyl
biradical (Scheme
6
).
Work reported in the early 1990s showed that benzo[
a
]pentalene is produced as
an intermediate in the Flash Vacuum Pyrolysis (FVP) of biphenylene, further
rearrangement leading to acenaphthylene as the major product [
45
,
46
]. The driving
force of this skeletal rearrangement is believed to be the relief of strain in the four-
membered ring. More recently, however, several groups have revealed another
mechanistic pathway involved in rearrangements of larger PAHs containing the
phenylene-linkage [
47
-
49
]. Phenyl groups are able to migrate by a process in which
C-C bond cleavage occurs as described above, followed by several 1,5- or 1,6-
hydrogen transfers, and ring closure at a new position and so on (Scheme
7
).
Together, the above mechanism and ring contraction/expansion to relieve strain
can be used to explain most of the products obtained from FVP of phenylene-
containing materials. The major products obtained from FVP of benzo[
b
]bi-
phenylene (28), linear [3]phenylene (21), and angular [4]phenylene (29) are
summarized in Scheme
8
. Scott and coworkers investigated the FVP of 28 which
resulted in two major products: dibenzo[
a,e
]pentalene (30) from benzene ring
contraction and fluoranthene (31) from phenyl migration [
47
]. Vollhardt and
coworkers showed that FVP of 21 resulted in four major PAH products (the same
four products as FVP of angular [3]phenylene 22 [
50
]) as well as 1% of 22 [
48
]. The
conversion of 21 to 22 clearly occurs through the phenyl migration mechanism,
whereas the other PAH products appear to result from various benzene ring
contraction rearrangements of 22 once it has formed. Finally, 29 is converted to
the major product biphenylene dimer 32, by a mechanism similar to phenyl
migration, as well as two additional PAHs [
49
].
As mentioned previously, it has been shown that angular annulation causes
greater bond alternation and decreased diatropicity in the benzene ring at the
bend (ring C, Schemes
2
and
3
), increasing the reactivity of those double bonds.
