Chemistry Reference
In-Depth Information
overcrowding is enhanced due to the extension of the fjord on one side of each
double bond to a [7]helicene-like region. Related star-shaped PAHs with three-fold
symmetry are the [3]radialenes (20). The [3]radialene with three fluorenylidene
moieties (20,X
) is deep-blue in solution and forms reddish-black crystals with
a metallic luster [
105
,
106
]. The [3]radialenes with three xanthylidene (20,X
¼
¼
O)
¼
or thioxanthylidene (20,X
S) moieties form black-violet and blue crystals, respec-
tively [
106
,
107
].
X
X
X
19
20
2.5 Thermochromism, Piezochromism, Photochromism,
and Electrochromism
Thermochromism is a reversible change of the UV-vis absorption spectrum of a
compound in a certain range of temperatures [
108
]. Temperature-dependent spec-
troscopic studies of BAE solutions in high boiling point solvents showed that
thermochromism is a bona fide equilibrium A
!
kT
B between a colorless or yellow
ambient temperature form A and a blue or green high temperature form B with a
new absorption band in the visible region (at 600-740 nm) [
108
-
115
]. There is no
dissociation or association involved as the Bouguer-Lambert-Beer law is observed
[
108
,
109
,
112
]. The process is completely reversible as long as there is no
irreversible side reaction or decomposition due to excessive temperature
[
112
,
116
,
117
]. The enthalpy
kT
B is within the
range 2.7-3.9 kcal/mol for bianthrone (3)[
109
,
110
,
114
,
115
,
118
,
119
], and 4.9-
5.6 kcal/mol for dixanthylene (4)[
110
,
118
]. The effect of structural modifications
- in particular of the bridges X and Y, of substituents, and benzo-annelation - on
thermochromism has been discussed [
11
-
14
,
115
].
Several of the thermochromic BAEs show intensive colors in solid state or solid
solution under mechanical pressure [
112
,
120
]. This phenomenon is called
ʔ
H
of the transformation A
!
