Chemistry Reference
In-Depth Information
-electron system of BAEs leads to a bathochromic
shift of the long-wavelength UV-vis absorptions as compared to a hypothetical
planar conformation, in particular in the case where the central double bond is
affected [
108
,
127
,
139
-
141
]. In a long and controversial debate on the nature of the
colored form, which was instrumental in forming today's understanding of the
interrelation of color and structure in organic compounds, zwitterionic valence
isomers [
142
] and betain-type mesomeric structures [
143
-
145
], biradicals [
146
-
154
], thermally populated triplets [
109
-
111
,
155
,
156
], cyclized forms [
127
,
157
,
158
] including Woodward and Wasserman's electrocyclized isomers [
159
], planar
[
13
,
160
-
165
], twisted [
108
,
112
-
114
,
122
,
125
,
132
,
134
,
155
,
158
,
166
-
168
], and
double chair [
116
,
129
,
169
] conformations have been discussed as the colored
B form. Today the puzzle is resolved [
63
] and it is generally accepted that the room
temperature form A is the
anti
-folded conformation (a)[
108
,
112
,
115
,
134
,
146
,
166
,
170
-
173
]. The deeply colored B form is the twisted conformation (t)[
112
-
114
,
122
,
125
,
134
,
158
,
166
-
168
,
173
]. Each of the two tricyclic moieties are
nearly planar; however, the molecules are twisted about the central double bond by
ca. 50
-60
[
138
,
167
]. Thermochromism, piezochromism, and the photochromic
B form have been attributed to this conformational change [
63
,
113
,
122
,
141
,
166
].
A criterion for thermochromism has been derived from ab initio DFT calcula-
tions of the
anti
-folded,
syn
-folded, and twisted conformations of thermochromic
and non-thermochromic overcrowded bistricyclic aromatic enes (BAEs). The nec-
essary condition is a small energy difference,
The non-planarity of the
π
7 kcal/mol, between the global
minimum
anti
-folded and the twisted conformations [
141
]. Even a small population
of the twisted conformation at elevated temperatures leads to a striking change of
the visible color from yellow to green or blue-green. While easily seen with the
naked eye and clearly detected by UV-vis spectroscopy, the concentrations remain
too low for X-ray crystallography [
116
,
155
]. Therefore, the molecular structures of
dixanthylene (4) in the yellow
<
ʲ
ʱ
-form are identical within the
limits of experimental error [
70
,
155
]. A recent state-of-the-art crystallographic
study of the solid-state thermochromism in bianthrone (3) and another BAE-related
compound also showed no significant change of conformation with temperature
that could explain the color change [
59
]. Instead, the increased absorption in the
UV-vis spectral range at 400-500 K was assigned to dynamic large amplitude
vibrations increasing with temperature. It should be noted that the reported spectra
[
59
] do not
- and blue-green
show the absorption at 600-740 nm characteristic for
the
thermochromic B form.
Fluorenylidene-anthrone (21), its benzo[
b
]fluorenylidene derivative 22, and
1,8-diazafluorenylidene derivative 23 have folded conformations in their yellow
or greenish-yellow crystals. However, these compounds give deeply colored purple,
blue, or red solutions at room temperature, indicating the presence of the twisted
thermochromic B form [
78
]. This has been termed “thermochromism at room
temperature.” Indeed, DFT calculations showed that the energy differences
between twisted and folded conformations are very small, favoring the twisted
