Chemistry Reference
In-Depth Information
R'
R
R'
R'
R'
R
R
R'
R
R
R
R'
R'
R'
R
R'
R
R'
R'
R
R
R'
R
Fig. 10 Mechanisms for conformational inversion and
E
,
Z
-isomerization in
anti
-folded and
folded BAEs
heteromerous
BAEs
fluorenylidene-selenaxanthene
and
fluorenylidene-
telluraxanthene (1,X
Se or Te, respectively), low barriers for confor-
mational inversions (edge-passage) were observed, while the barriers for
E
,
Z
-
isomerization (orthogonally twisted transition state) were higher and only a lower
limit could be determined [
79
]. The mechanisms for conformational inversion and
E
,
Z
-isomerization in
anti
-folded and folded BAEs are illustrated in Fig.
10
[
73
,
170
].
BAEs with twisted global minima t like bifluorenylidene and fluorenylidene-
xanthene have distinct barriers for enantiomerization and
E
,
Z
-isomerization via
edge-passage and orthogonally twisted transition states, respectively [
54
,
62
].
In BAE 8 with seven-membered central rings (X,Y
¼
— and Y
¼
CH) a
syn
-folded
conformer could be isolated [
61
,
189
,
194
-
196
]. The
syn
-folded to
anti
-folded
isomerization barrier s
¼
CH
¼
G
{
¼
!
a was determined to be
ʔ
36.4 kcal/mol [
189
]. Sim-
ilar high s
a barriers were found for other BAEs with seven-membered central
rings and assigned to a seven-membered ring flip [
76
]. On the other hand, the s
!
a
barriers found for thermal decay of the photochromic E forms (
syn
-folded confor-
mation) in BAEs with central six-membered rings and X,Y
!
S (18.0 kcal/mol),
NCH
3
(15.0 kcal/mol), and CHOH (12.5 kcal/mol) (Table
3
) are much lower [
125
,
137
].
¼
2.7 Computational Studies
Theoretical methods have been applied to solve the mysteries and puzzles of BAEs
and to gain a better understanding as soon as they became available. Pariser-Parr-
Pople and H¨ckel-type calculations were used to estimate spectra, atomic charges,
and
-MO approx-
imations quantified the non-planarity and amount of steric strain in the BAE
conformations [
138
,
199
-
201
]. Semiempirical MINDO/3 and PM3 have been
used to study the interconversion of
anti
-folded and twisted conformations in
BAEs, their anions, cations, and excited state conformations [
16
,
202
,
203
]. PM3
has also been used for an extensive study systematically exploring structure-
property relationships including trends in the conformational energies, strain
π
-bond orders [
127
,
139
,
156
,
197
,
198
]. Force fields including
π
