Chemistry Reference
In-Depth Information
- H
2
ISC
3
1
A
*
A
*
C
P
D
h
h
E
or
F
B
A
Fig. 7 Generation and decay of the thermochromic and photochromic forms
piezochromism and is reversible after a certain period of time [
112
]. Piezochromism
has been studied under various conditions. Colors were observed for bianthrone (3)
dissolved in polymethyl methacrylate polymers at pressures of 40-120 kBar at
room temperature [
120
] and when pure crystals or mixtures with KBr, MgO, CaF
2
,
or BaSO
4
are strongly sheared, e.g., by grinding [
112
,
113
,
116
,
117
,
120
,
121
]. It is
generally assumed that the thermochromic and piezochromic B forms are identical
[
117
,
121
]. The B form has a smaller partial molar volume than the A form [
120
,
122
]. A recent UV-vis spectroscopic and crystallographic study of bianthrone
crystals in diamond anvil cells revealed a red coloration due to a mixture of two
phases [
60
]. The bulk of the crystal remains in a compressed A form with slightly
flattened molecules and a broadened and red-shifted UV adsorption. On the surface
of the crystals a new phase forms with an adsorption at 600-700 nm, characteristic
of the green B form. However, the diffraction data was insufficient to determine the
crystal structure of the new phase.
Hirschberg discovered photochromism in BAEs [
123
]. Solutions of bianthrone
(3) irradiated at low temperatures develop strong colors that persist after irradiation
as long as the solutions are kept at low temperatures. Upon warming, the colors fade
and disappear [
124
]. The photochromism of bistricyclic aromatic enes has been
extensively reviewed [
108
,
125
-
131
]. Photochromism in BAEs is a much more
complex phenomenon than thermochromism and piezochromism. Several photo-
chromic forms have been observed depending on the molecules studied, the irradi-
ation technique used for excitation, the temperature [
117
], and the solvent polarity
[
117
,
132
] and viscosity [
133
]. The reversible photochromic forms have been
labeled B, C, D, E, F, and P (Fig.
7
). They have been characterized by their spectra
and conditions of formation and decay [
13
,
108
,
125
,
126
]. In addition, the singlet
and triplet excited states of the normal form A and the photochromic forms have
been considered. Moreover, the situation may be complicated by various irrevers-
ible photochemical reactions such as photocyclization and oxidation.
Photochromism is observed under non-equilibrium conditions and a consider-
able or even complete conversion to the colored forms may be produced under
suitable conditions [
108
,
118
]. It is not observed in crystals or in the glass state
[
108
,
118
,
134
]. High viscosity (low temperature) reduces the quantum yield
[
128
,
135
-
138
].
