Chemistry Reference
In-Depth Information
Table 2 Photophysical properties of [N]phenylenes in THF [
34
]
Compound
Stokes shift (cm
1
)
E(S
1
) (eV)
f
F
f
ISC
10
4
20
2.98
4,750
2.6
0.01
<
10
4
21
2.11
0.01
<
<
22
2.85
750
0.07
0.30
23
2.66
70
0.12
0.25
24
2.57
65
0.21
0.45
25
2.81
150
0.15
0.03
Table 3 Lowest energy
l
Compound
Lowest E
l
max
(nm)
E
g
(eV)
max
for some [4]- and [5]
phenylenes [
27
-
29
,
42
,
43
]
2a
486
2.55
26
488
2.54
5a
505
2.46
10b
523
2.37
27
530
2.34
TMS
TMS
TMS
TMS
TMS
TMS
26
27
Fig. 5 Structures of [N]phenylenes 26 and 27 [
42
,
43
]
The larger [N]phenylenes 23-25 have very small Stokes shifts (70-150 cm
1
)as
compared to both 22 (750 cm
1
) and 20 (4,750 cm
1
), indicating a more rigid
structure in which the geometry does not change much in going from S
0
to S
1
[
34
].
This data together with increased fluorescence quantum yields indicate slower rates
of internal conversion (IC) for these species. As indicated in Table
2
, the zigzag [N]
phenylenes 23 and 24 undergo intersystem crossing (ISC) fairly efficiently with ISC
quantum yields ranging from 25% to 30%. Therefore this process is a likely
contributor to the deactivation of the S
1
state. However, this is not the case for 20
and 21 for which energy is presumably lost via fast internal conversion (IC).
An interesting pattern also emerges in examination of the band gap (estimated
from the lowest energy absorption) of the bent [N]phenylene derivatives discussed
in the previous section (Table
3
). The lowest energy absorptions of linear 2,3-bis
(trimethylsilyl)-[4]phenylene [
42
] 26) and 2,3,9,10-tetrakis(trimethylsilyl)-[5]
phenylene [
43
](27, Fig.
5
) are also included in Table
3
for comparison. Bent [4]
phenylene 2a, has almost the same lowest energy
l
max
as its linear counterpart [
27
],
as does singly bent [5]phenylene 10b [
29
]. The
l
max
of doublebent [5]phenylene 5a,
on the other hand, lies between that of the linear 27 and the higher energy
absorption of other known isomers including zigzag (see above) [
28
]. From exami-
nation of this data, it appears that the first angular ring annulation has a stronger
effect on the band gap than does a second.
The lowest energy
l
max
of the heliphenes also increases with length as expected
[
30
,
31
]. The bathochromic shift decreases in going from 12 to 13 (12 nm), 13 to 14
