Thermochromism at room temperature in overcrowded bistricyclic aromatic enes: Closely populated twisted and folded conformations

Article abstract of DOI:10.1002/ejoc.200700414

The overcrowded thermochromic bistricyclic aromatic enes (BAEs) 10-(9′H-fluoren-9′-ylidene)-9(10H)-anthracenone (6), 10-(11′H-benzo[b]fluoren-11′-ylidene)-9(10H)-anthracenone (7), and 10-(1′,8′-diaza-9′H-fluoren-9′-ylidene)-9(10H) -anthracenone (8) were synthesized by applying Barton's two-fold extrusion diazo-thione coupling method and their crystal and molecular structures were determined. BAEs 6-8 exhibit thermochromic behavior at room temperature due to the equilibrium between the yellow anti-folded conformations and the thermochromic purple, blue, or red twisted conformations. The NMR experiments demonstrate a fast interconversion of the twisted and the anti-folded conformers of 6-8 in solution. BAE 7 readily undergoes E,Z-topomerization at room temperature with the coalescence point at 297 K and the relatively low energy barrier of ΔG_c^?(t_⊥) = 65.5 kJ/mol. B3LYP/6-311++G(d,p) calculations predict anti-folded a-6 and a-7 to be less stable than twisted t-6 and t-7 by 0.8 and 1.3 kJ/mol, respectively, whereas a-8 is more stable than t-8 by 10.7 kJ/mol. DFT calculations of 6-8, 9-(9′H-fluoren-9′-ylidene)-9H-fluorene (1), [10′-oxo- 9′(10′H)-anthracenylidene]-9(10H)-anthracenone (2) and their 1,8-diaza-substituted derivatives show that substitution in the fluorenylidene unit destabilizes the twisted conformations by 11-22 kJ/mol, while introduction of nitrogen atoms at the 1 and 8 positions of anthracenylidene unit destabilizes the anti-folded conformations by 14-18 kJ/mol. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200700414

FULL PAPER
DOI: 10.1002/ejoc.200700414
Thermochromism at Room Temperature in Overcrowded Bistricyclic Aromatic
Enes: Closely Populated Twisted and Folded Conformations
Amalia Levy,[a] Sergey Pogodin,[a] Shmuel Cohen,[a] and Israel Agranat*[a]
Keywords: Thermochromism / Strained molecules / Conformation analysis / Density functional calculations
The overcrowded thermochromic bistricyclic aromatic enes
(BAEs) 10-(9ЈH-fluoren-9Ј-ylidene)-9(10H)-anthracenone (6),
10-(11ЈH-benzo[b]fluoren-11Ј-ylidene)-9(10H)-anthracenone
the relatively low energy barrier of ∆Gc‡(tЌ) = 65.5 kJ/mol.
B3LYP/6-311++G(d,p) calculations predict anti-folded a-6
and a-7 to be less stable than twisted t-6 and t-7 by 0.8 and
(7), and 10-(1Ј,8Ј-diaza-9ЈH-fluoren-9Ј-ylidene)-9(10H)-an- 1.3 kJ/mol, respectively, whereas a-8 is more stable than t-8
thracenone (8) were synthesized by applying Barton’s two-
fold extrusion diazo-thione coupling method and their crystal
and molecular structures were determined. BAEs 6–8 exhibit
thermochromic behavior at room temperature due to the
equilibrium between the yellow anti-folded conformations
and the thermochromic purple, blue, or red twisted confor-
mations. The NMR experiments demonstrate a fast intercon-
version of the twisted and the anti-folded conformers of 6–8
in solution. BAE 7 readily undergoes E,Z-topomerization at
room temperature with the coalescence point at 297 K and
by 10.7 kJ/mol. DFT calculations of 6–8, 9-(9ЈH-fluoren-9Ј-
ylidene)-9H-fluorene (1), [10Ј-oxo-9Ј(10ЈH)-anthracenylid-
ene]-9(10H)-anthracenone (2) and their 1,8-diaza-substituted
derivatives show that substitution in the fluorenylidene unit
destabilizes the twisted conformations by 11–22 kJ/mol,
while introduction of nitrogen atoms at the 1 and 8 positions
of anthracenylidene unit destabilizes the anti-folded confor-
mations by 14–18 kJ/mol.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Johns Wort, an important remedy against depression.[10]
tetradehydrodianthracene unit, also topologically related to
2, is a beltlike pyramidalized component of the first Möbius
annulene.[11]
A
Introduction
Bistricyclic aromatic enes (BAE, Figure 1) are attractive
substrates for the study of the conformational behavior and
dynamic stereochemistry of overcrowded polycyclic aro-
matic enes and for the interplay of strain and delocalization
effects.[1] The leading member of the family, red 9-(9ЈH-flu-
oren-9Ј-ylidene)-9H-fluorene (bifluorenylidene, 1) was syn-
thesized in 1875.[2] BAEs can be classified into homomerous
bistricyclic aromatic enes (Figure 1, X = Y) and heteromer-
ous bistricyclic aromatic enes (Figure 1, X ϶ Y). The phe-
nomenon of thermochromism,[3–5] – reversible change of
color with change of temperature – was revealed in over-
crowded BAEs almost five scores years ago.[6] Meyer re-
ported that yellow solutions of 10-[10Ј-oxo-9Ј(10ЈH)-an-
thracenylidene]-9(10H)-anthracenone (bianthrone, 2) re-
versibly turn dark green upon heating.[6] This change of
color may also be triggered by pressure (piezochromism)[7]
or by UV irradiation at low temperatures (photochro-
mism).[3d,8] Thermochromic and photochromic BAEs and
related overcrowded enes serve as candidates for chiroptical
molecular switches and molecular motors.[9] Derivatives of
2 are topologically related to hypericin, widespread in St.
Figure 1. Bistricyclic aromatic enes.
Thermochromism in BAEs has been attributed to a uni-
molecular equilibrium between two distinct and intercon-
vertible conformers: a colorless or yellow ambient-tempera-
ture form A and a deep-blue or deep-green high-temperace-
ture form B:[12]
[a] Department of Organic Chemistry, The Hebrew University of
Jerusalem, Jerusalem 91904, Israel
kT
A hB
E-mail: isria@vms.huji.ac.il
Fax: +972-2-6511907
Although A absorbs in the UV region (and close to it),
the thermochromic form has a new absorption band at λ ≈
600–700 nm. In thermochromic BAEs, the enthalpy differ-
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
5198
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Eur. J. Org. Chem. 2007, 5198–5211

Thermochromism at Room Temperature
ence ∆H of the two forms is 4–30 kcal/mol[12] and is inde- conformations is to replace the fjord regions C1 and C8
pendent of the solvent.[12a] The thermochromic, photochro- atoms of one tricyclic moiety of a BAE by nitrogen atoms.
mic, and piezochromic B forms are identical.[8c,12c,12f,13] Re- Having a van der Waals radius of 150 pm, which is con-
cently the controversial nature of the thermochromic phe- siderably shorter than that of carbon, 171 pm,[16] and not
nomenon in BAEs has been resolved: the deeply colored bearing hydrogens, nitrogen atoms at the 1 and 8 positions
purple (or green) thermochromic form B was identified as would render the fjord regions of the 1,8-diaza BAE less
the twisted conformation, and its molecular structure was overcrowded. The fjord nitrogens may also affect the BAE
reported.[5] The high twist in the central double bond re- molecule in other ways. BAEs with two different tricyclic
duces the π-overlap and causes a substantial red shift. The moieties, like fluorenylidene-xanthene (5), have a poten-
yellow or colorless ambient-temperature form A was iden- tially push-pull character, in which the fluorenylidene may
tified as anti-folded or unevenly anti-folded conformations. serve as a acceptor, while the xanthylidene or the anthracen-
ylidene moieties may serve as donors.[17] The 1,8-diazofluo-
renylidene moiety may amplify this effect.[17] By contrast,
10-(9ЈH-fluoren-9Ј-ylidene)-9(10H)-anthracenone (fluoren-
ylidene-anthrone, 6) is a “pull-pull” system; its fluorenylid-
ene and anthrone moieties are polarized in the same direc-
tion.
Here, we report syntheses and molecular and crystal
structures of thermochromic BAEs fluorenylidene-anthrone
(6), 10-(11ЈH-benzo[b]fluoren-11Ј-ylidene)-9(10H)-anthra-
cenone (7) and 10-(1Ј,8Ј-diaza-9ЈH-fluoren-9Ј-ylidene)-
9(10H)-anthracenone (8), a DNMR spectroscopic study of
7, as well as a DFT study of the conformational spaces
of diaza and tetraaza substituted BAEs. The crystal and
molecular structure of 6 has previously been reported.[1a,18]
Three types of conformational behavior have been distin-
guished in BAEs.[14] The necessary conditions for the
thermochromism of the Type 1 BAEs are a global minimum
anti-folded a or unevenly anti-folded au conformation cor-
responding to the room temperature form A, a low energy
local minimum twisted conformation t corresponding to the
thermochromic form B, and an energy difference between
these conformations which is sufficiently small to allow
thermal population of t, i.e. less than 30 kJ/mol, based on
experimental data and DFT calculations.[14] Examples of
Type 1 BAEs are bianthrone (2) and 9,9Ј-bi-9H-xanthen-9-
ylidene (dixanthylene, 3). BAEs of Type 2 also adopt anti-
folded conformations as global minima, but their syn-
folded conformations s are more stable than the twisted
Results and Discussion
conformations, which cannot be populated in thermal equi-
Synthesis
BAE 6 was first synthesized in two steps by a conden-
librium. Therefore, Type 2 BAEs, for example, 9,9-bi(9H-
thioxanthen-9-ylidene) (4), do not exhibit thermochromic
behavior. BAEs of Type 3 adopt twisted conformations as
their global minima. Their thermochromic forms dominate sation between 9,9Ј-dichloro-9H-fluorene and 9-(10H)-
in the equilibrium at all temperatures and are responsible anthrone in boiling benzene to give 9Ј-chloro-9HЈ-fluor-
for the deep color of these compounds. 9-(9ЈH-fluoren-9Ј- enyl-9-anthrone followed by heating the latter in boiling
ylidene)-9H-xanthene (fluorenylidene-xanthene, 5), which nitrobenzene.[19] An attempted synthesis of 6 in one step by
has a deep purple color in solution, belongs to Type 3. The a similar condensation between 9,9Ј-dichloro-9H-fluorene
folded conformations of Type 3 BAEs, although higher in and 9-(10H)anthrone at 160 °C without solvent, according
energy than the twisted conformations, may be populated in to a literature procedure[20] afforded 6, but in unsatisfactory
equilibrium, but their color is masked by the deeply colored yield and low purity. Therefore, BAEs 6–8 were synthesized
twisted conformations. Packing effects may also favor the by applying Barton’s twofold extrusion diazo-thione coup-
folded conformations.[15]
ling method.[21] Two synthetic routes can be pursued for the
Altering the relative stabilities of twisted and folded con- preparation of 6: coupling between 9-diazofluorenone[22] (9)
formations of a BAE would result in a change of its confor- and 9-thioxoanthracenone[23] (10), or, alternatively, coup-
mational space and physical properties, including thermo- ling between 9-fluorenonethione[24] (11) and 9-diazoanthra-
chromism. One of the ways to affect the balance between cenone[23,25] (12) (Scheme 1).
Eur. J. Org. Chem. 2007, 5198–5211
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A. Levy, S. Pogodin, S. Cohen, I. Agranat
FULL PAPER
Scheme 1.
Scheme 4.
In BAEs 6–8 there is a subtle equilibrium between the
yellow anti-folded conformation and the thermochromic
deeply colored twisted conformation at room temperature.
Fluorenylidene-anthrone (6) is yellow in the solid state, but
turns purple in solution (UV/Vis spectra has a visible
absorption at 546 nm). The greenish-yellow crystals of
10-(11ЈH-benzo[b]fluoren-11Ј-yildene)-9(10H)-anthracenone
(7) give a blue solution with the visible absorption at
590 nm. 1,8-Diazafluorenylidene-anthrone (8) is yellow in
the solid state and turns dark red in solution (525 nm). The
purple, blue, and red colors of 6–8 in solution indicate that
the twisted conformations of these BAEs (reduced HOMO–
LUMO gap) are readily populated at room temperature,
while in the solid state, yellow or greenish-yellow color re-
flects the preference of anti-folded conformations.
The diazo-thione coupling between diazo 9 and thione
10 in boiling CHCl3 gave BAE 6 in 46% yield and appears
to be the preferred method for the synthesis of 6. The alter-
native coupling between diazo 12 and thione 11 in boiling
benzene afforded BAE 6 in 33% yield. The intermediate
product of Barton’s twofold extrusion, thiirane 13, can be
isolated by the reaction between diazo 9 and thione 10 in
CH2Cl2 at room temperature (Scheme 2). Heating thiirane
13 in boiling benzene with PPh3 afforded BAE 6 in 23%
yield.
Molecular and Crystal Structures
BAEs 6 and 8 crystallize in the space group P21/n. Com-
pound 7 crystallizes in the space group Pbca. Figures 2, 3
and 4 give ORTEP diagrams of one molecule of each of
BAEs 6–8, respectively, as determined by X-ray analysis
(anisotropic displacement parameters are drawn at the 50%
probability level). The crystal data of 6–8 are given in
Table 6.[28] Table 1 gives the conformations and selected
geometrical parameters of 6–8 derived from the crystal
structures and from DFT calculations (vide infra). Pure eth-
ylenic twist[1c] (ω) around C9=C9Ј is defined as the average
of the two torsion angles C9a–C9–C9Ј–C9aЈ and C8a–C9–C9Ј–
C8aЈ; folding dihedral of the tricyclic (fluorenylidene or an-
thracenylidene) moiety (A–B) and (C–D), see Figure 1, is
defined as the dihedral angle between the least-square
planes of the atoms of the benzene rings of each tricyclic
moiety and reflecting the non-planarity of the tricyclic moi-
eties; bistricyclic dihedral between the tricyclic moieties
(AEB–CFD) is defined as the dihedral angle between the
least-square planes of all the untagged and all the tagged
carbon and nitrogen atoms; pyramidalization angles[1c]
χ(C9) and χ(C9Ј) are defined as the improper torsion angles
Scheme 2.
10-(11ЈH-benzo[b]fluoren-11Ј-ylidene)-9(10H)-anthrace-
none (7), a napthologue of 6, was prepared by the diazo-
thione coupling between 9-diazobenzo[b]fluorenone[26] (14)
and thione 10 in boiling CHCl3 (Scheme 3).
Scheme 3.
1,8-Diazafluorenylidene-anthrone (8) was prepared by C9a–C9–C9Ј–C8a and C9aЈ–C9Ј–C9–C8aЈ, minus 180°. The
Barton’s diazo-thione coupling method in two steps overall conformations of the bistricyclic aromatic enes are
(Scheme 4). The coupling between 1,8-diaza-9-diazofluor- characterized by the pure twist of the central C9=C9Ј bond
enone[17,27] (15) and thione 10 in CH2Cl2 at room tempera- and by the folding dihedral of the tricyclic moieties.[1b] The
ture gave thiirane 16. Sulfur extrusion from 16 with PPh3 pyramidalization angles χ(C9), χ(C9Ј), and χ(C10) should
in boiling benzene afforded BAE 8.
also be considered.[1b]
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Eur. J. Org. Chem. 2007, 5198–5211

Thermochromism at Room Temperature
drals of 6 are 51.1° in the anthracenylidene moiety and
11.8° in the fluorenylidene moiety. Only C9 is pyramidalized
(8.5°), while C9Ј is not. The degrees of overcrowding in the
fjord regions of 6, as reflected in the intramolecular dis-
tances at the fjord regions, C1···C1Ј, C8···C8Ј, C1···H1Ј, and
C8···H8Ј are significant: 301.7, 309.1, 240.4, and 239.3 ppm.
These distances reflect 12, 10, 16, and 16% of penetration,
based on the van der Waals radii of carbon and hydrogen
which are 171 and 115 ppm.[16] Each molecule of 6 is chiral,
and the unit cell of 6 consists of two pairs of enantiomers.
The fluorenylidene units of two molecules A and B forming
a pair are parallel to each other but orientated in the oppo-
site directions. Their plane makes a dihedral angle of 14°
with the xy plane of the unit cell. The distance between the
planes of the five-membered rings of each fluorenylidene
unit is 386.0 pm, and the torsion angle C9(A)–C9Ј(A)–
C9Ј(B)–C9(B) is 180°.
Figure 2. ORTEP drawing of the molecular structure of 10-(9ЈH-
fluoren-9Ј-ylidene)-9(10H)-anthracenone (6).
BAE 7 is also chiral. The unit cell of 7 consists of four
pairs of enantiomers. The folding dihedrals of 7, as com-
pared to those of 6, are smaller in the anthracenylidene
moiety, 43.3°, and larger in the fluorenylidene moiety, 15.8°.
Both C9 (7.4°) and, to a lesser extent, C9Ј (3.0°) atoms are
pyramidalized. Compound 7 is more overcrowded in its
fjord regions than 6: the C1···C1Ј, C8···C8Ј, C1···H1Ј, and
C8···H8Ј distances are 293.2, 302.2, 236.7, and 240.1, corre-
sponding to 14, 12, 17, and 16% of penetration.
The folding dihedrals of 8 are smaller than those of 6,
47.5° in the anthracenylidene moiety and 7.5° in the fluor-
enylidene moiety. Both C9 (7.4°) and C9Ј (4.9°) atoms are
pyramidalized. The introduction of nitrogen atoms in the
fjord region of 8 eliminates short C···H distances, but 8 is
still overcrowded, its N1···C1Ј and N8···C8Ј distances are
278.1 pm (13.4% penetration) and 286.9 pm (11% penetra-
tion). The unit cell of 8 consists, as in the case of 6, of two
pairs of enantiomers. The 1Ј,8Ј-diazafluorenylidene units of
two molecules A and B forming such a pair are parallel to
each other and lie in a plane that makes a dihedral angle
of 16° with the xy plane of the unit cell. The distance be-
tween the planes of the five-membered rings of each 1Ј,8Ј-
fluorenylidene unit is 377.8.0 pm, and the torsion angle
C9(mol1)–C9Ј(mol1)–C9Ј(mol2)–C9(mol2) is 180°.
Figure 3. ORTEP drawing of the molecular structure of 10-(11ЈH-
benzo[b]fluoren-11Ј-yildene)-9(10H)-antracenone (7).
NMR Spectroscopy
1
Table 2 gives the H NMR chemical shifts of 6–8 and
related homomerous BAEs. Table 3 gives the 13C NMR
chemical shifts of 6–8 and related BAEs. It is possible to
distinguish qualitatively among the twisted conformation,
the anti-folded conformation and the syn-folded conforma-
tion of a BAE in solution, using chemical shifts of the fjord
protons H1, H8, H1Ј, and H8Ј. The fjord region protons of 6
appear at 8.33 (H1, anthracenylidene moiety) and 7.74 ppm
(H1Ј, fluorenylidene moiety). These values differ from the
Figure 4. ORTEP drawing of the molecular structure of 10-(1Ј,8Ј-
diaza-9ЈH-fluoren-9Ј-ylidene)-9(10H)-anthracenone (8).
The molecular and crystal structures of BAEs 6–8 indi- chemical shifts of twisted 1 (δ = 8.39 ppm) and of anti-
cate that these molecules adopt anti-folded conformations, folded 2 (δ = 7.06 ppm). The UV/Vis spectra shows the
which are characterized by small pure twist angles (3.2– presence of twisted conformation (absorption at 546 nm),
3.5°) and significant folding dihedrals. The folding dihe- but the observed H1 and H1Ј chemical shifts are not suffi-
Eur. J. Org. Chem. 2007, 5198–5211
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A. Levy, S. Pogodin, S. Cohen, I. Agranat
FULL PAPER
Table 1. Selected geometrical parameters of BAEs 2, 5–8 derived from their crystal structures.
ω
C9a–C9- C1–C9-
χ
χ
A–B AEB–
C1···C1Ј C8···C8Ј H1···H1Ј H8···H8Ј C1···H1Ј C8···H8Ј
twist C9Ј–C9aЈ C9Ј–C1Ј (C9) (C10) C–D CFD C9=C9Ј C10Ј–O11Ј N1···C1Ј N8···C8Ј N1···N1Ј N8···N8Ј N1···H1Ј N8···H8Ј
deg
2[a] 0.0
deg
deg
deg
deg deg[c] deg[c]
40.0
pm
pm
pm
pm
pm
pm
pm
pm
3.6
35.0
–35.0
3.6
–3.6
0.6
–1.3
0.4
–8.5
3.0
7.4
136.4
294
294
304
304
281
281
–3.6
43.3
41.4
7.2
–0.9
–2.0
8.4
5[b] 42.3
–
–
–
0.5
1.4
11.8
140.1
136.3
137.4
136.3
–
312.5
301.7
293.2
278.1
314.4
309.1
302.3
286.9
244
242
273
258
272
254
283.0
239.3
240.1
269.5
252.6
6
7
8
3.2
0.0
3.2
0.0
3.5
0.0
–19.8
27.4
27.1
–20.3
–17.2
25.9
43.4
0.0
–
255.3 234.4 288.8
240.4
250.1 232.1 236.7
277.1
7.8 51.1
15.8
5.9 43.3
7.5
6.8 47.5
122.2
–
122.9
–
–
32.6
0.0
43.8
0.0
4.7
2.2
4.9
–7.4
–
–
–
261.6
122.2
[a] Ref.[29] [b] Ref.[5] [c] See Figure 1.
1
Table 2. H NMR chemical shifts (ppm) of 6–8 and related BAEs.
H1, H8
H2, H7
H3, H6
H4, H5
Y
X
H1Ј, H8Ј
H2Ј, H7Ј
H3Ј, H6Ј
H4Ј, H5Ј
1
2
6
–
CO
CO
–
CO
–
8.386
7.062
8.331
7.735
7.211
7.148
7.448
7.003
7.332
7.400
7.497
7.273
7.709
8.093
8.235
7.642
8.275
7
CO
CO
–
–
–
–
8.442
7.825, 8.264[a]
8.607
7.751[b], 8.025
8.090
8
7.511
8.341
7.212
8.626
7.511
7.194
7.342
7.258
7.900
7.587
8.003
17[c]
9.001
[a] H10Ј. [b] H4Ј or H6Ј. [c] Ref.[17]
ciently shifted downfield as it would be expected for a (a) E,Z-isomerization or E,Z-topomerization (Figure 5).
twisted conformation. Evidently, there are fast interconver-
sions of the twisted t-6 and the anti-folded a-6 conformers
in solution. The observed chemical shifts of 6 are the
averaged shifts of its two conformers. In 7, the most down-
field fluorylidene proton appears at δ = 8.26 ppm, indicat-
ing that also in this case the twisted t-7 and the anti-folded
a-7 conformers interconvert rapidly in solution on the
NMR timescale. In the case of 8, the fjord region protons
H1 and H8 appear at δ = 8.61 ppm, which may suggest that
8 adopts only a twisted conformation in solution. The dark
red color of 8 in solution (longest wavelength absorption at
525 nm) also corresponds to a twisted conformation. How-
ever, the downfield shift of the fjord region protons H1 and
H8 can also be explained by the effect of the fjord nitrogens
at positions 1Ј and 8Ј. The H1 fjord protons in diaza-substi-
tuted 8 are shifted downfield relatively to H1 and H1Ј of the
parent 6 by 0.28 and 0.87 ppm. The latter value is charac-
teristic for twisted conformations, where the fjord protons
are bucking towards the fjord nitrogens, whereas the former
value corresponds to an anti-folded conformation.[17] We
conclude, that BAEs 6–8 demonstrate an equilibrium be-
tween a twisted conformation and a anti-folded conforma-
tion in solution at room temperature.
(b) Conformational inversion, i.e. inversion of the helicity
in twisted BAEs or inversion of the boat conformations in
the central rings of anti-folded BAEs (Figure 6).
(c) syn,anti isomerization (Figure 7).
Enantiomerization and conformational inversion may be
considered in all three processes.
In the present study, only the E,Z-topomerization of 7
has been revealed, using dynamic NMR spectroscopy. The
1H NMR spectrum of 7 at 297 K (in CD2Cl2) included one
very broad signal centered at δ = 8.37 ppm, amounting to
two protons, H1 and H8 of the anthracenylidene moiety.
However, in the 1H NMR spectrum of 7 recorded at 317 K,
a doublet at δ = 8.45 ppm replaced the broad signal,
whereas the spectrum recorded at 268 K revealed two dou-
blets at δ = 8.30 and 8.48 ppm, clearly indicating a dynamic
process. Both conformational inversion and syn, anti isom-
erization involve two parallel pathways with double barriers
of equal heights. Inversion of helicity P/M of the central
double bond through an anti-folded intermediate or syn-
folded conformer would result in two complete sets of aro-
1
matic protons appearing in the H NMR spectrum of 7 at
low temperature. Topomerization, on the other hand, is a
one step process of rotating along the central double bond,
through an almost orthogonally twisted transition state. In
this case, only the fjord region protons of the anthrone moi-
ety are expected to demonstrate a difference in their chemi-
Dynamic Stereochemistry
The BAEs undergo the following three fundamental dy- cal shifts, which agrees with the observations. The mecha-
namic processes:[1c]
nism of E,Z-topomerization of twisted BAE 7 is depicted
5202
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 5198–5211

Thermochromism at Room Temperature
Table 3. 13C NMR chemical shifts (ppm) of 6–8 and related BAEs.
C1, C8
C2, C7
C3, C6
C4, C5
C4a, C10a
C4aЈ, C4bЈ
C8a, C9a
C9
Y
X
C1Ј, C8Ј
C2Ј, C7Ј
C3Ј, C6Ј
C4Ј, C5Ј
C8aЈ, C9aЈ C9Ј
C10
1
2
6
–
CO
CO
–
CO
–
126.73
129.56
130.74
126.20
132.86
126.85
130.30
130.61
126.28
129.20
147.53
127.07
148.11
129.15
128.38
128.89
129.29
129.29
122.83
130.97
122.41
119.89
126.79
127.20
119.74
125.95
126.88
119.24
127.28
141.31
134.43
132.95
141.51
133.07
131.32
142.94
131.65
138.28
139.10
138.40
139.84
138.45
156.58
148.71
136.01
141.01
132.04
134.14
140.50
143.39
132.65
148.71
136.01
186.71
185.24
8
CO
–
–
186.47
17[a]
–
130.57
[a] Ref.[17]
Figure 5. E,Z-Isomerization of BAEs.
Figure 8. Schematic mechanism of the topomerization of 7.
DFT Study
Figure 6. Conformation inversion of twisted and anti-folded BAEs.
DFT methods are capable of generating a variety of iso-
lated molecular properties quite accurately, especially via
the hybrid functionals, and in a cost-effective way.[31] Re-
cently, the B3LYP hybrid functional was successfully em-
ployed to treat overcrowded BAEs.[14,32] Bifluorenylidene
(1), bianthrone (2), fluorenylidene-anthrone (6), and their
1,8-diaza and 1,8,1Ј,8Ј-tetraaza derivatives 9-(9ЈH-fluoren-
9Ј-ylidene)-1,8-diaza-9H-fluorene (17), 9-(1Ј,8Ј-diaza-9ЈH-
fluoren-9Ј-ylidene)-1,8-diaza-9H-fluorene (18), 10-[10Ј-oxo-
Figure 7. The syn,anti isomerization of BAEs.
in Figure 8. A dynamic NMR spectroscopic study of 7 was 9Ј(10ЈH)-anthracenylidene]-1,8-diaza-9(10H)-anthracenone
performed at low temperatures. The thermal topomeri- (19), 10-[1Ј,8Ј-diaza-10Ј-oxo-9Ј(10ЈH)-anthracenylidene]-
zation of 7 was studied by monitoring the coalescence of 1,8-diaza-9(10H)-anthracenone (20), 1Ј,8Ј-diazafluorenylid-
the two lowest field doublets of the anthracenylidene pro- ene-anthrone (8), 10-(9ЈH-fluoren-9Ј-ylidene)-1,8-diaza-
tons H1 and H8. The coalescence point was reached at 9(10H)-anthracenone (21), 10-(1Ј,8Ј-diaza-9ЈH-fluoren-9Ј-
297 K (the extrapolated frequency difference ∆νc is ylidene)-1,8-diaza-9(10H)-anthracenone (22) were chosen
51.7 Hz), which corresponds to the topomerization barrier for a systematic computational DFT study of the effect of
of 63.4 kJ/mol. The rate of passing through one of the two introducing nitrogen atoms at the fjord regions of BAEs on
alternative transition states, depicted in Figure 8, is half of their thermochromism. The relative energies of these BAEs
the effective rate for topomerization, hence the total barrier are presented in Table 4. Selected geometrical parameters of
‡
‡
height is ∆Gc (tЌ) = ∆Gc + R·T·ln2 = 65.5 kJ/mol (at BAEs under study are presented in Table 5. The following
297 K in CD2Cl2). For comparison, the E,Z-isomerization analysis is based on the DFT enthalpies ∆H298 calculated
barriers of 2,2Ј-disubstituted bianthrones lie in the range from B3LYP/6-311++G(d,p) energies and thermal correc-
83.7–90.0 kJ/mol.[30] In fluorenylidenexanthene (5), the tions to enthalpy computed at B3LYP/6-31(d) or B3LYP/6-
E,Z-isomerization barrier is 82.0 kJ/mol.[15]
311(d,p) (vide infra).
Eur. J. Org. Chem. 2007, 5198–5211
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5203

A. Levy, S. Pogodin, S. Cohen, I. Agranat
FULL PAPER
Table 4. The DFT relative energies (kJ/mol) of BAEs 1, 2, 6–8, 17–22.
B3LYP/6-31G(d)
B3LYP/6-311G(d,p)
B3LYP/6-311++G(d,p)
[a]
[a]
[b]
[b]
∆ETot
∆H298
∆G298
∆ETot
∆H298
∆ETot
∆H298
2
t
a
t
a
t
a
t
a
t
a
t
f
t
s
t
a
t
a
t
s
t
a
D2
C2h
C2
Cs
D2
C2h
C2
Cs
C1
C1
C2
Cs
C2
Cs
C2
Cs
D2
C2h
C2
Cs
M
M
M
M
M
M
M
M
M
M
M
M
M
TS
M
M
M
M
M
TS
M
TS
12.04
11.54
12.11
0.00
–2.12
0.00
–1.40
0.00
0.00
3.70
0.00
5.88
9.91
0.00
0.00
30.85
0.00
6.53
0.00
37.17
0.00
19.55
0.00
26.80
M
17.16
16.73[c]
0.00
1.09
19.11
18.67[c]
0.00
3.86
0.00
4.53
0.00
–3.02
0.00
–1.89
0.00
0.00
5.37
0.00
5.54
7.45
0.00
0.00
24.87
0.00
9.06
0.00
38.90
0.00
16.72
0.00
25.52
0.00
–3.17
0.00
–2.76
0.00
0.00
5.71
0.00
5.91
7.67
0.00
0.00
23.29
0.00
9.42
0.00
39.62
0.00
14.58
0.00
22.44
0.00
1.23
0.00
0.86
0.00
0.00
1.90
0.00
1.93
11.20
0.00
0.00
21.65
0.00
7.27
0.00
36.40
0.00
14.85
0.00
25.04
0.00
4.01
0.00
5.39
0.00
0.00
0.47
0.00
0.92
10.55
0.00
0.00
20.20
0.00
5.46
0.00
36.19
0.00
17.07
0.00
26.39
19
20
6
0.00
0.004
0.000
0.00
0.00
0.00
M
M
2.20[c]
0.00
0.78[c]
0.00
7
2.29
1.28
8
11.37[c]
0.00
10.72[c]
0.00
21
22
1
0.00
20.07
0.00
7.63
0.00
37.12
0.00
12.71
0.00
0.00
18.62
0.00
5.82
0.00
36.90
0.00
17.94
0.00
17
18
D2
C2h
21.95
23.30
[a] M = minimum, TS = transition state. [b] The thermal correction to enthalpy was computed at B3LYP/6-31G(d) unless specified
otherwise. [c] The thermal correction to enthalpy was computed at B3LYP/6-311G(d,p).
Bifluorenylidene (1), a Type 3 BAE, adopts twisted con- mation has the N1–N1Ј distance of 276.7 pm (8% penetra-
formation t-1 (pure twist 34.0°) as the global minimum. It tion), while t-18 is almost not overcrowded.
has C1···H1Ј contact distance of 259.9 pm (penetration of
Thermochromic bianthrone (2) adopts anti-folded con-
9%). Its anti-folded conformation a-1 (folding 24.3°) is less formation a-2 as its global minimum, with a folding of
stable than t-1 by 36.9 kJ/mol and significantly more over- 42.7°. It is overcrowded, with C1···C1Ј distance of 303.1 pm
crowded, with C1···C1Ј and C1···H1Ј distances of 306.0 pm (12% penetration). Its twisted conformation t-2 lies 18.7 kJ/
(penetration of 11%) and 243.8 pm (penetration of 15%), mol at B3LYP/6-311++G(d,p) and 16.7 kJ/mol at B3LYP/
respectively.
6-311G(d,p) higher than a-2. It has a pure twist of 55.4°, a
Introduction of two nitrogen atoms into 1 and 8 posi- folding of 5.0°, and is also overcrowded, with C1···C1Ј dis-
tions of 1 does not relieve 17 of overcrowding but enhances tance of 314.0 pm (penetration of 8%). The reported experi-
it: the N1···H1Ј contact distance reaches 222.5 pm (15% mental enthalpy values ∆HA–B are 11.3,[12e] 12.6Ϯ0.8,[12f]
penetration) in the twisted t-17 and 206.6 ppm (22% pene- 13.0,[12a] 13.8,[12a] 14.2,[12b,12d] 14.6Ϯ2.5,[12c] 15.5,[12g] and
tration) in the folded s-17. It is accompanied by decrease of 16.3.[12a] This newly calculated ∆HA–B value is higher than
pure twist in t-17 (28.0°) and folding in s-17 (5.7° and that reported previously [∆E = 12.0 kJ/mol at B3LYP/6-
17.4°). Unlike the anti-folded a-1, the folded conformation 31G(d)].[14]
of 17 is syn-folded (characterized by very small C8a–C9–
Introduction of two nitrogen atoms into the 1 and 8 posi-
C9Ј–C8aЈ angle, Ϯ0.4° vs. Ϯ9.6° in a-1, smaller N1–C9–C9Ј– tions of 2 lowers significantly the relative enthalpy of the
C1Ј angle than in a-1, Ϯ3.6° vs. Ϯ14.6°, and large bi- twisted conformation t-19, by 14.8 kJ/mol. In this case the
stricyclic dihedral, 30.2° vs. 0°). It is not a minimum, but a thermochromic conformation t-19 is only 3.9 kJ/mol less
transition state for the enantiomerization of t-17. No anti- stable than the anti-folded a-19 conformation. Both a-19
folded conformation was found in the conformational space and t-19 are slightly less overcrowded: the N1···C1Ј distances
of 17. The syn-folded s-17 conformation, despite its being are 289.3 pm (10% penetration) and 297.0 pm (7% penetra-
a transition state, is closer in enthalpy to the twisted t-17 tion). The relieve of steric strain is also expressed in smaller
conformation (∆H298 = 17.9 kJ/mol) than the anti-folded a- folding of the diaza-substituted tricyclic moieties of a-19
1 to the twisted t-1 (∆H298 = 36.9 kJ/mol). The folded a-18 (36.3°) and t-19 (1.2°), as well as in smaller pure twist of t-
conformation of tetraaza-substituted 18 is also a transition 19 (53.5°). Introducing two additional nitrogen atoms into
state and is higher in enthalpy than twisted t-18 by 23.3 kJ/ 19 slightly destabilizes the twisted conformation t-20, which
mol, which is more than in s-17 but less than in a-1. It is is less stable than a-20 by 4.5 kJ/mol. The twisted t-20 (pure
anti-folded (the torsion angles C8a–C9–C9Ј–C8aЈ and N1– twist 50.0°) and the anti-folded a-20 (folding 38.2°) confor-
C9–C9Ј–N1Ј are Ϯ7.3° and Ϯ12.3°, respectively) and has a mations are both moderately overcrowded.
folding of 17.7°, larger than in diaza-substituted tricyclic
Fluorenylidene-anthrone (6) is a Type 3 BAE and has a
moiety of s-17. The twisted conformation t-18 also has twisted global minimum t-6 conformation (pure twist
larger pure twist (31.0°) than t-17. The folded a-18 confor- 44.5°). However, its anti-folded a-6 conformation is only
5204
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Eur. J. Org. Chem. 2007, 5198–5211

Thermochromism at Room Temperature
Table 5. Conformations and selected geometrical parameters of 1, 2, 6–8, 17–22 derived from their crystal structures and DFT calculations
[at B3LYP/6-311++G(d,p)].
ω
C9a–C9–C9Ј–C9a C1–C9–C9Ј–C1Ј
χ
χ
A–B
C–D
deg[a]
AEB–CFD C9–C9Ј
C1···C1Ј
N1···C1Ј
pm
N1···N1Ј
N1–C9–C9Ј–N1Ј
(C9)
deg
(C10)
deg
deg
deg
deg
deg[a]
61.3
pm
pm
2
t
D2
C2h
C2
Cs
55.4
–55.4
–55.4
4.1
–4.1
–53.5
–53.5
2.6
–2.6
–50.0
–50.0
2.5
–2.5
–44.5
–44.5
5.1
–5.1
7.2
–0.9
–44.8
–44.7
5.2
–5.2
8.4
–2.0
–42.6
–42.6
1.7
–64.4
–64.4
–36.4
36.4
–61.3
–61.3
–36.2
36.2
–58.9
–58.9
–36.6
36.6
–53.4
–53.4
–25.2
25.2
–19.8
27.4
–53.3
–53.8
–25.4
25.5
–20.3
27.1
–50.3
–50.3
–20.5
20.5
0.0
0.0
5.0
142.4
314.0
–
a
t
0.0
53.5
0.0
–4.1
–4.1
0.0
0.0
1.7
3.5
0.0
0.0
–2.5
–2.5
0.0
0.0
4.1
6.2
0.4
–8.5
–0.1
0.2
–3.9
–6.5
3.0
7.4
0.0
0.0
–11.8
8.3
–8.1
42.7
0.0
59.3
6.2
136.5
142.1
136.5
140.4
135.9
139.8
136.6
136.3
140.0
136.7
137.4
139.6
137.2
136.3
139.7
137.3
138.4
136.0
137.8
137.1
138.0
138.6
136.7
136.7
303.1
297.0
289.3
–
–
–8.1
0.0
0.0
7.1
8.9
0.0
0.0
–7.9
–7.9
–
0.0
–
9.3
–
7.8
–
0.2
–
–9.3
–
5.9
–
0.0
–
8.6
–
6.8
–
0.0
–
9.0
–
0.0
–
–8.3
–
19
20
6
1.2
5.4
36.3
44.8
4.7
–
a
t
–
D2
C2h
C2
Cs
50.0
0.0
55.9
0.0
281.7
a
t
38.2
–
279.0
44.5
0.0
4.6
7.0
17.3
47.8
11.8
51.1
4.0
52.3
33.3
43.4
52.2
32.5
32.6
49.3
52.7
43.8
49.1
69.4
47.6
38.8
42.7
0.0
313.4
303.8
–
a
X
t
–
3.2
301.7
309.1
312.5
314.2
303.8
304.3
293.2
302.3
296.4
–
7
8
C1
C1
44.8
0.0
–
7.1
a
X
t
18.0
48.2
15.8
43.3
4.7
7.2
2.7
45.3
7.5
47.5
4.5
–
3.2
–
C2
Cs
42.6
0.0
–
f
280.4
–
–
–1.7
4.7
2.2
X
t
3.5
–17.2
25.9
4.9
–7.4
0.0
278.1
286.9
293.7
21
22
1
C2
Cs
41.4
0.0
–41.4
–41.4
2.8
–2.8
–40.6
–40.6
3.8
–3.8
–34.0
–34.0
9.6
–9.6
–28.0
–28.0
0.4
–48.5
–48.5
–15.9
15.9
–49.0
–49.0
–24.3
24.3
–42.0
–42.0
–14.6
14.6
–33.4
–33.4
–3.6
–
0.0
1.4
s
–19.3
13.7
0.0
0.0
–0.4
8.0
0.0
0.0
–9.6
–9.6
0.0
12.0
44.4
3.5
293.6
–
–
t
C2
Cs
40.6
0.0
284.3
279.5
6.4
a
t
13.2
41.6
2.6
–
D2
C2h
C2
Cs
34.0
0.0
321.2
306.0
297.7
284.4
–
a
t
–
24.3
17
18
28.0
0.0
–
–
–
–
–
3.6
3.2
5.7
17.4
1.9
35.0
30.2
38.4
0.0
0.0
s
12.2
–12.9
0.0
0.0
–7.3
–7.3
–0.4
–31.0
–31.0
7.3
3.6
t
D2
C2h
31.1
0.0
–38.0
–38.0
–12.3
12.3
293.7
276.7
a
–
17.7
–
–7.3
[a] See Figure 1.
0.78 kJ/mol higher than the twisted t-6. This difference be- interconversion of the twisted t-6 and the anti-folded a-6
tween the conformations is translated into ca. 42% of t-6 at diastereomers in solution (vide supra). The folding of the
equilibrium at room temperature. Therefore, BAE 6 should anthracenylidene moiety of a-6 (48.4°) is much larger than
readily demonstrate solvatochromic behavior, acquiring in the folding of the fluorenylidene moiety (17.3°). The respec-
solution a deep color, which corresponds to the twisted t-6 tive values for t-6 are 4.6° and 7.0°. The anti-folded a-6
conformation. Indeed, 6 is yellow in the solid state and pur- conformation, in spite of being close in energy to the
1
ple in solution. The H and 13C NMR spectra of 6, how- twisted t-6 conformation, is significantly overcrowded: the
ever, show only one set of signals, evidently due to the fast C1···C1Ј and C1···H1Ј distances are 303.8 pm (penetration of
Eur. J. Org. Chem. 2007, 5198–5211
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5205

A. Levy, S. Pogodin, S. Cohen, I. Agranat
FULL PAPER
ously, the DFT calculations somewhat underestimate the
stability of the twisted t-8 conformation. Note also that t-8
is more stable than its isomer t-21 by 10.4 kJ/mol. The
folded f-8 conformation cannot be unambiguously defined
as either anti-folded or syn-folded. Its fluorenylidene moiety
is nearly planar (folding 2.7°). It has small C8a–C9–C9Ј–C8aЈ
angle (Ϯ1.7), medium C1–C9–C9Ј–N1Ј angle (Ϯ20.5), large
bistricyclic dihedral (52.7°). It also has a short C1···N1Ј dis-
tance (280.4 pm, 13% penetration) and nearly normal
H1···N1Ј distance (253.7 pm, 4% penetration). The X-ray
molecular structure of 8 is in a good agreement with calcu-
lated folded f-8 conformation. The X-ray structure of 8 is
slightly more folded (7.5° vs. 2.7° for the diazafluorenylid-
ene moiety and 47.5° vs. 45.3° for the anthracenylidene moi-
ety) and more twisted (3.5°) than calculated f-8. Contrary
to f-8, the X-ray structure of 8 is clearly syn-folded. The
bistricyclic dihedral in the X-ray structure of 8 is smaller
11%) and 242.9 pm (penetration of 15%), respectively. The (43.8° vs. 52.7°), than in f-8. Both the conformations have
respective values in t-6 are 313.4 and 271.4 pm. The molec- a similar degree of overcrowding.
ular structure of 6 as determined by X-ray crystallography
Tetraaza-substituted heteromerous BAE 22 has a twisted
corresponds to the calculated anti-folded a-6 conformation. global minimum t-22 conformation (pure twist 40.6°). The
In general, there is good agreement between these two anti-folded a-22 conformation is less stable than t-22 by
structures. The X-ray molecular structure does not possess 5.8 kJ/mol. It is less folded in the anthracenylidene unit
Cs symmetry and has a small pure twist (3.2°) contrary to (41.6°) than s-21 (44.4°), but more folded in the fluorenylid-
a-6, less folded fluorenylidene moiety (11.8° vs. 17.3°) and ene unit (13.2°) than f-8 (2.7°). Both t-22 and a-22 are mod-
slightly more folded anthracenylidene moiety (51.1° vs. erately overcrowded.
48.4°). The degree of overcrowding is very similar in both
Thus, the introduction of nitrogen atoms into the fjord
structures. The only substantial difference between the ex- regions of BAEs affects dramatically the relative stabilities
perimental and the calculated structures of 6 is the bi- of their folded and twisted conformations. It is known, that
stricyclic dihedral, 43.4° vs. 33.3°. Linear benzannelation of the fluorenylidene moiety has an energetic propensity
6 does not noticeably affect the energy difference between against folding, contrary to the anthracenylidene moiety,
the anti-folded and the twisted conformations of 7, which where folding is the preferred way to relieve the steric
is 1.3 kJ/mol. The calculated geometries of t-7 and a-7 are strain.[1c] The presence of nitrogen atoms in 1 and 8 posi-
also almost identical to the respective geometries of 6 tions of fluorenylidene unit destabilizes the twisted confor-
(Table 5). The X-ray molecular structure of 7 is in the very mations, raising their energies relatively to the respective
good agreement with the calculated anti-folded a-7 confor- folded conformations by 11.5 (8), 13.6 (18), and 22.0 (17)
mation. The latter predicts slightly larger folding of both kJ/mol. On the other hand, introduction of nitrogen atoms
fluorenylidene moiety (18.0° vs. 15.8°) and anthracenylid- at the 1 and 8 positions of anthracenylidene unit destabi-
ene moiety (48.2° vs. 43.3°) than the X-ray structure.
lizes the folded conformations, raising their energies rela-
Two nitrogen atoms may be introduced in fluorenylidene- tively to the twisted conformations by 14.8 (19), 14.1 (20),
anthrone (6) either at the fluorenylidene or at the anthra- and 17.9 (21) kJ/mol. In the case of heteromerous BAE 22,
cenylidene moiety, giving BAE 8 or BAE 21, respectively. these effects work in the opposite directions, resulting in the
Fluorenylidene-1,8-diazaanthrone (21) adopts a twisted small relative destabilization of the anti-folded a-22 confor-
conformation t-21 as the global minimum, with reduced a mation by 5.0 kJ/mol.
pure twist (41.4°) and a folding (1.4°) of the diaza-substi-
The effect of introducing of nitrogen atoms into the fjord
tuted anthracenylidene moiety. The folded s-21 conforma- regions of BAEs 1, 2, and 6 upon their relative stabilities
tion is syn-folded (characterized by smaller C8a–C9–C9Ј– can also be described by a series of homodesmic reac-
C8aЈ angle, Ϯ2.8° vs. Ϯ5.2° in a-7, smaller than in a-7 N1– tions[33] (Schemes 5 and 6). Combining a 1,8-diaza-substi-
C9–C9Ј–C1Ј angle, Ϯ15.9° vs. Ϯ25.4°, and large bistricyclic tuted fluorenylidene unit with another fluorenylidene unit
dihedral, 69.4° vs. 32.5°). It is a transition state, higher in (t-17) appears to be preferable than combining it with
energy than t-21 by 18.6 kJ/mol. The s-21 conformation an anthracenylidene unit (f-8): reaction (2) is exothermic
also has very short N1···H1Ј distance, 209.2 pm (21% pene- (–16.3 kJ/mol), while reaction (1) is slightly endothermic
tration). No anti-folded conformation was found in the con- (2.4 kJ/mol). Similarly, combining a 1,8-diaza-substituted
formational space of 21. By contrast, 1Ј,8Ј-diazafluorenyli- anthracenylidene unit with a fluorenylidene unit (f-21) is
dene-anthrone (8) adopts a global minimum folded f-8 con- preferable than combining it with an anthracenylidene unit
formation, which is more stable than the twisted t-8 confor- (a-19), based on the endothermicities of the reactions (3)
mation (pure twist 42.6°) by 10.7 kJ/mol, which corre- and (4), 20.2 and 1.6 kJ/mol, respectively. Reactions (5) and
sponds to merely 1.4% of t-8 at equilibrium at 298 K. Obvi- (6) demonstrate that the twisted 1,8,1Ј,8Ј-tetraaza-substi-
5206
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Thermochromism at Room Temperature
tuted bifluorenylidene derivative t-18 is destabilized relative
to two molecules of 1,8-diaza-substituted t-17 by 34.5 kJ/
mol, whereas the folded 1,8,1Ј,8Ј-tetraaza-substituted a-20
is stabilized relative to two molecules of a-19 by 15.3 kJ/
mol. Reaction (7), which compares t-18 and a-20, indicates
a relative stabilization of folded a-20 by 14.0 kJ/mol. Thus,
tetraazasubstitution in the fjord regions promotes folding
and impairs twisting in BAEs.
Scheme 6.
the tricyclic moieties (Table S3). In the parent 6, the anthra-
cenylidene moiety bears negligibly small excess charges
(–0.003 in t-6 and 0.006 in a-6). 1,8-Diazasubstitution pulls
the electron density into the fluorenylidene moiety of 8
(–0.180 in t-8 and –0.132 in f-8) and into the anthracenylid-
ene moiety of t-21 (–0.210) and of s-21 [–0.087 at B3LYP/
6-311G(d,p); the high charge separation in s-21 at B3LYP/
6-31G++(d,p) is evidently an artefact of the diffuse func-
tion augmented basis set]. Two 1,8-diaza-substituted moie-
ties in 22 mutually neutralize their effects; the small nega-
Scheme 5.
The basis set dependence of the relative stabilities of anti- tive charges reside on the anthracenylidene half of t-22
folded and twisted conformations of the BAEs under study (–0.024) and of a-22 (–0.021). Homomerous BAEs 17 and
deserves a comment. The minimal basis set (Table S1) 19 also have excessive negative charge on their 1,8-diaza-
causes the twisted conformations to have the highest ener- substituted moieties: –0.225 in t-17 and –0.143 in s-17, and
gies relative to the corresponding anti-folded conforma- –0.245 in t-19 and –0.097 in a-19. With the exception of 22,
tions. Double split-valence basis set (Table 1) considerably the zwitterionic character is enhanced in the twisted confor-
stabilizes the twisted conformations, which become the mations due to the more planar tricyclic moieties.
global minima except in BAEs 2 and 8. The further expan-
Two somewhat bizarre reports on a thermochromism at
sion of basis set up to diffuse functions augmented triple room temperature deserve a comment. Bergmann wrote in
split-valence basis set moderately decreases the relative sta- 1948: “... dibiphenylethylene (i.e. 1), which is intensely red,
bilities of the twisted conformations. In the cases of 19 and but loses its color when chilled to low temperature, the same
20 it leads to the anti-folded conformations becoming phenomenon as in the case of dixanthylene may be assumed
global minima again.
to occur, but in a different temperature range”.[34] Ault, et
The potential push-pull character of 1,8-diaza-substi- al. reported in 1971: “Dixanthylene is a pale yellow-green
tuted 6 and related BAEs can be analyzed by summing up solid at room temperature, but becomes dark blue when
the natural atomic charges of the atoms consisting each of melted or heated in solution; at liquid nitrogen temperature,
Eur. J. Org. Chem. 2007, 5198–5211
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5207

A. Levy, S. Pogodin, S. Cohen, I. Agranat
FULL PAPER
it is completely colorless”.[35] These observations at low
temperatures have never been confirmed experimentally.
Moreover, the computational results of the present study
throw doubt on the presence of a colorless conformation of
a Type 3 BAE 1, (e.g. an anti-folded conformer) at “low
temperature” and the presence of a colorless conformer of
3, belonging to Type 1, at liquid nitrogen temperature, in
addition to the anti-folded conformer found at room tem-
perature.
range 40–60 °C) was used. X-ray data were collected on a Bruker
SMART APEX CCD diffractometer equipped with a graphite
monochromator and using MoKα radiation (λ = 0.71073 Å)
(Table 6).
10-Diazo-9(10H)-anthracenone (12): Preparation according to the
literature.[23,25] In a round-bottomed flask (250 mL) with reflux
condenser and magnetic stirrer bar, p-toluenesulfonyl chloride
(5.109 g, 26.8 mmol) was dissolved in ethanol (50 mL) under heat-
ing till everything was dissolved. Sodium azide (2.11 g, 32.5 mmol)
was dissolved in water (6 mL) and was added to the flask. The
reaction mixture was stirred at room temp. for 1–2 h. The precipi-
tate of NaCl was filtered off. Anthrone (4.42 g, 22.7 mmol) was
added and piperidine (2.5 mL) was added dropwise via syringe. The
color of the reaction mixture turned red. The reaction mixture was
stirred at room temp. for 5 h. The red precipitate was filtered off
by using a Büchner funnel. Red crystals of 12 were obtained:
4.26 g, yield 72%. Recrystallization from 1,4- dioxane gave pure
12, dec. 135–140 °C (ref.[23] dec. 150 °C). 1H NMR (CDCl3, 298 K):
Conclusions
BAEs 6–8 demonstrate a subtle equilibrium between the
twisted and anti-folded conformations at room temperature.
Their deep colors in solution indicate the presence of the
readily populated the twisted conformations, while in the
solid state these BAEs demonstrate yellowish colors of the
anti-folded conformations. The twisted and anti-folded con-
formations interconvert rapidly on the NMR timescale. In-
troduction of the nitrogen atoms into the 1 and 8 positions
of a fluorenylidene moiety affects the relative stability of
the twisted and anti-folded conformations of BAEs, thus
controlling their thermochromic behavior.
3
4
5
3
δ = 7.350 (ddd, J = 8.1, J = 1.1, J = 0.6 Hz, 2 H), 7.411 (td, J
= 8.2, J = 6.9, 4J = 1.1 Hz, 2 H), 7.702 (td, 3J = 8.1, 7.2, J = 7.2,
3
3
4J = 1.4 Hz, 2 H), 8.546 (ddd, J = 8.0, J = 1.4 Hz, J = 0.6 Hz,
2 H) ppm. 13C NMR (CDCl3, 298 K): δ = 64.66 (C=N2), 120.61
(C–H), 125.24 (C–H), 128.33 (C), 128.96 (C–H), 129.74 (C), 132.91
3
4
5
(C–H), 180.00 (C=O) ppm. IR (KBr): ν
= 1630 (C=O), 2070
˜
max
(NϵN) cm–1.
10-Thioxo-9(10H)-anthracenone (10): Preparation according to the
literature.[23] In a round-bottomed flask (25 mL) with reflux con-
denser and magnetic stirrer bar, diazo 12 (1 g, 4.5 mmol) was added
with sulfur (0.172 g, 5.4 mmol) in DMF (10 mL) (dried before on
Na2SO4 and distilled). The reaction mixture was heated in an oil
bath (15 min) at 150–157 °C and evolution of N2 was observed.
The color of the reaction mixture turned from red to dark green.
The flask was cooled down in an ice bath and the green precipitate
was filtered off by using a Büchner funnel. The green crystals were
washed with cold acetone and than dissolved in CH2Cl2, the solu-
tion was evaporated. A green powder of 10 was obtained: 0.434 g,
yield of 43%, m.p. 208–210 °C (ref.[23] 213–214 °C). 1H NMR
(CDCl3, 298 K): δ = 7.698 (td, 2 H), 7.817 (td, 2 H), 8.190 (ddd, 2
H), 8.547 (ddd, 2 H) ppm. 13C NMR (CDCl3, 298 K): δ = 126.42
(C–H), 127.91 (C), 130.07 (C–H), 133.43 (C–H), 133.48 (C–H),
138.13 (C), 185.00 (C=O), 218.59 (C=S) ppm.
Experimental Section
Melting points are uncorrected. All NMR spectra were recorded
with a Bruker DRX 400 spectrometer. H NMR spectra were re-
1
corded at 400.1 MHz using CDCl3 as solvent and as internal stan-
dard [δ(CHCl3) = 7.26 ppm]. The 1H NMR of compound 7 was
performed also at CD2Cl2 (δ = 5.31 ppm), and C2D2Cl4 (δ =
5.99 ppm) at different temperatures. 13C NMR spectra were re-
corded at 100.6 MHz using CDCl3 as solvent and as internal stan-
dard [δ(CDCl3) = 77.0 ppm]. UV/Vis spectra were measured using
an UVIKON 860 spectrometer. IR spectra were measured with a
Perkin–Elmer System 2000 FT-IR spectrometer. Complete assign-
ments were made through 2-dimentional correlation spectroscopy
(DQF-COSY, HSQC, and HMBC). Petroleum ether (PE, boiling
Table 6. Crystallographic data for BAEs 6–8.
6
7
8
Empirical formula
Temperature /K
C27H16O
295(1)
C31H18O
123(1)
C25H14N2O
295(1)
Crystal system
Space group
monoclinic
P21/n
orthorhombic
Pbca
monoclinic
P21/n
a /Å
b /Å
c /Å
α /deg
13.2593(9)
9.7518(7)
14.189(1)
90.0
8.262(5)
21.77(1)
23.00(2)
90.0
13.1607(8)
9.6404(6)
14.1537(9)
90.0
β /deg
107.568(1)
90.0
1749.1(2)
4
1.353
18977
90.0
90.0
4136(5)
8
1.305
45271
4964
108.003(1)
90.0
1707.8(2)
4
1.394
18496
γ /deg
V /Å3
Z
Density (calcd.) /Mg/m3
Reflections collected
Independent reflections
3809
3721
[R(int) = 0.0328]
R1 = 0.0424
wR2 = 0.1007
[R(int) = 0.2129]
R1 = 0.0793
wR2 = 0.1440
[R(int) = 0.0248]
R1 = 0.0421
wR2 = 0.1081
Final R indices [I Ͼ 2σI]
5208
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Thermochromism at Room Temperature
Dispiro[9H-fluorene-9,2Ј-thiirane-3Ј,9ЈЈ-(10ЈЈH)antracenone] (13):
To a stirred solution of diazo 9 (0.214 g, 1.11 mmol) in anhydrous
CH2Cl2 (20 mL) under argon atmosphere, thione 10 (0.25 g,
1.11 mmol) was added via syringe. The reaction mixture was stirred
at room temp. for 48 h. Trituration with ethanol gave a precipitate
which was filtered off on a glass frit. A yellow power of 13 was
obtained: 0.172 g in a yield 38%. 1H NMR (CDCl3, 298 K): δ =
yield 13% as a yellow green powder, m.p.172–176 °C. A single crys-
tal was obtained from sublimation in a sealed tube under vacuum
at 200 °C 1H NMR (CDCl3, 298 K): δ = 7.053 (td, 1 H), 7.312–
7.378 (m, 2 H), 7.420–7.471 (m, 3 H), 7.496–7.549 (m, 3 H), 7.751
(d, 1 H, H4Ј or H6Ј), 7.803–7.848 (m, 2 H, H1Ј, H4Ј or H6Ј), 8.025
(s, 1 H, H5Ј), 8.264 (s, 1 H, H10Ј), 8.275 (d, 2 H, H4, H5), 8.442 (br.
s, 2 H, H1, H8) ppm. 13C NMR (CDCl3, 298 K): δ = 117.71 (C–
H), 120.54 (C–H), 125.89 (C–H), 126.04 (C–H), 126.28 (C–H),
3
3
3
6.808 (dt, J = 8.3, J = 7.8 Hz, 2 H), 6.903 (d, J = 7.8 Hz, 2 H),
7.129 (dt, 3J = 8.4, 3J = 7.5 Hz, 2 H), 7.345 (dt, 3J = 8.6, 3J = 126.86 (C–H), 127.15 (C–H), 127.25 (C–H), 128.17 (C–H), 128.80
3
3
3
7.6 Hz, 2 H), 7.471 (d, J = 7.6 Hz, 2 H), 7.585 (dt, J = 8.9, J =
(C–H), 129.39 (C–H), 129.60 (C–H), 130.18, 130.55, 132.46, 132.76
(C), 132.93 (C–H), 133.98 (C), 134.12 (C), 138.29 (C), 138.54 (C),
138.68 (C), 140.38 (C), 141.37 (C), 141.49 (C), 185.18 (C10) ppm.
3
3
7.6 Hz, 2 H), 7.764 (d, J = 7.6 Hz, 2 H), 8.173 (d, J = 7.6 Hz, 2
H) ppm. 13C NMR (CDCl3, 298 K): δ = 57.38 (C–S), 58.68 (C–S),
119.73 (C–H), 124.27 (C–H), 125.87 (C–H), 127.07 (C–H), 128.07 UV/Vis (cyclohexane): c = 2.36ϫ10–5 . λmax nm (ε): 389 (11822),
1
(C–H), 128.32 (C–H), 128.37 (C–H), 130.76 (C–H), 135.93 (C), 590 (8771). H NMR (CD2Cl2, 297 K): δ = 7.057 (td, 1 H), 7.321–
140.58 (C), 141.16 (C), 141.74 (C), 185.92 (C=O) ppm.
7.379 (m, 2 H), 7.423–7.484 (m, 3 H), 7.484–7.552 (m, 3 H), 7.742
(d, 1 H), 7.826–7.857 (m, 2 H), 8.056 (s, 1 H, H5Ј), 8.199 (d, 2 H,
H4, H5), 8.259 (s, 1 H, H10Ј), 8.367 (br. s, 2 H, H1, H8) ppm. 1H
NMR (CD2Cl2, 268 K): δ = 7.046 (td, 1 H), 7.313–7.372 (m, 2 H),
7.422–7.546 (m, 6 H), 7.730 (d, J = 8.0 Hz, 1 H), 7.805–7.849 (m,
2 H), 8.052 (s, 1 H, H5Ј), 8.153 (d, J = 8.0 Hz, 1 H, H4 or H5),
8.190 (d, J = 8.0 Hz, 1 H, H4 or H5), 8.25 (s, 1 H, H10Ј), 8.296 (d,
J = 8.0 Hz, 1 H, H1 or H8), 8.478 (d, J = 7.9 Hz, 1 H, H1 or H8)
ppm. 1H NMR (C2D2Cl4, 268 K): δ = 7.083 (td, 1 H), 7.267–7.3859
(m, 2 H), 7.424–7.587 (m, 6 H), 7.702 (d, J = 8.0 Hz, 1 H), 7.778–
7.832 (m, 2 H), 8.062 (s, 1 H, H5Ј), 8.202 (d, J = 8.0 Hz, 1 H, H4
or H5), 8.233 (d, J = 8.0 Hz, 1 H), 8.245 (s, 1 H, H10Ј), 8.347 (d, J
= 7.6 Hz, 1 H, H1 or H8), 8.518 (d, J = 8.0 Hz, 1 H, H1 or H8)
ppm.
10-(9H-Fluoren-9Ј-ylidene)-9(10H)-anthracenone (6): I To a stirred
solution of diazo 12 (0.809 g, 3.76 mmol) in anhydrous benzene
(60 mL) and protected by a CaCl2 tube, thione 11 (0.757 g,
3.86 mmol) was added. The reaction mixture was refluxed for 7 h.
After cooling and standing for 48 h a precipitate was obtained
which was filtered in a sinter fritte. A yellow power of 6 was ob-
tained: 0.430 g, yield 33%, m.p. 279–281 °C (ref.[19] 280 °C dec.;[20]
283–284 °C).
II: To a stirred solution of diazo 9 (0.576 g, 2.57 mmol) in CHCl3
dried on molecular sieves (4 Å, 35 mL) and protected by a CaCl2
tube, thione 10 (0.493 g, 2.57 mmol) was added. The reaction mix-
ture was refluxed overnight. The color of the reaction mixture was
purple. The mixture was cooled to room temp., and the solvent was
removed under reduced pressure. Trituration of the crude product
in hot benzene gave a precipitate, which was filtered off. A yellow
power 0.408 g of 6 was obtained, in a yield of 46%, m.p. 280–
281 °C (ref.[19] 280 °C dec.;[20] 283–284 °C).
Dispiro[9H-1,8-diazafluorene-9,2Ј-thiirane-3Ј,9ЈЈ-(10ЈЈH)-anthrace-
none] (16): To a stirred solution of diazo[17,27] 15 (0.262 g,
1.35 mmol) dissolved in CH2Cl2 (25 mL) under argon atmosphere,
thione 10 (0.302 g, 1.35 mmol) was added via syringe. The reaction
mixture was stirred overnight at room temp. The solvent was re-
moved under reduced pressure. Trituration of the crude product in
CHCl3 gave a precipitate, which was filtered off. A grey power of
16 was obtained: 0.344 g, yield 71%, dec. 122–124 °C. 1H NMR
(CDCl3, 298 K): δ = 7.041 (td, 2 H), 7.349 (td, 2 H), 7.594 (td, 2
H), 7.719 (dd, 2 H), 7.782 (dd, 2 H), 8.089 (dd, 2 H), 8.461 (dd, 2
H) ppm. 13C NMR (CDCl3, 298 K): δ = 55.03 (C–S), 57.50 (C–S),
122.68 (C–H), 125.97 (C–H), 127.51 (C–H), 128.05 (C–H), 130.22
(C–H), 131.39 (C–H), 133.21 (C), 135.03 (C), 137.87 (C), 147.47
(C–H), 159.51 (C), 186.06 (C10) ppm.
III: To a stirred solution of thiirane 13 (0.166 g, 0.427 mmol) in
anhydrous dry benzene (15 mL) PPh3 (0.567 g, 2.16 mmol) was
added. The reaction was stirred overnight while reflux under pro-
tection of CaCl2. After cooling the solvent was evaporated till dry-
ness. A trituration in ethanol gave a precipitate which was filtered
in a sinter fritte. A yellow power of 6 was obtained, 0.035 g in a
yield 23%, m.p. 279–282 °C (ref.[19] 280 °C dec.;[20] 283–284 °C). A
single crystal was obtained from NMR tube in CDCl3.
1H NMR (CDCl3, 298 K): δ = 7.003 (td, J = 7.3 Hz, 2 H, H2Ј,
H7Ј), 7.273 (td, J = 7.3 Hz, 2 H, H3Ј, H6Ј), 7.448 (td, J = 7.7 Hz, 2
H, H2, H7), 7.497 (td, J = 7.5 Hz, 2 H, H3, H6), 7.642 (ddd, J =
7.4 Hz, 2 H, H4Ј, H5Ј), 7.735 (ddd, J = 8.0 Hz, 2 H, H1Ј, H8Ј), 8.235
(ddd, J = 7.6 Hz, 2 H, H4, H5), 8.331 (ddd, J = 7.6 Hz, 2 H, H1,
H8) ppm. 13C NMR (CDCl3, 298 K): δ = 119.74 (C4Ј, C5Ј), 126.20
(C1Ј, C8Ј), 126.28 (C2Ј, C7Ј), 127.20 (C4, C5), 128.89 (C3, C6), 129.29
(C3Ј, C6Ј), 130.61 (C2, C7), 130.74 (C1, C8), 132.95 (C4a, C10a),
134.14 (C9), 138.40 (C8a, C9a), 139.84 (C8aЈ, C9aЈ), 140.50 (C9Ј),
141.51 (C4aЈ, C4bЈ), 185.24 (C10) ppm.
10-(9H-1,8-Diazafluorene-9Ј-ylidene)-9(10H)-anthracenone (8): To a
stirred solution of thiarane 16 (0.300 g, 0.769 mmol) in anhydrous
benzene (20 mL) PPh3 (0.503 g, 1.922 mmol) was added. The reac-
tion was stirred 3 h while reflux under protection of CaCl2. After
cooling the solvent was evaporated till dryness.A trituration in eth-
anol gave a precipitate which was filtered in a sinter fritte. Purifica-
tion was performed by column chromatography on silica gel while
the compound was dissolved in CH2Cl2 and charged on the column
using CH2Cl2/Et3N (98:2) as eluent. The red fraction was collected
and evaporated; 0.090 g of 8 was obtained as yellow powder, yield
33%, m.p. 147 °C. A single crystal of 8 was obtained from CDCl3.
10-(11ЈH-Benzo[b]fluoren-11Ј-ylidene)-9(10H)-anthracenone (7): To
a stirred solution of 9-diazobenzo[b]fluorenone[26] (14) (0.278 g,
1.14 mmol) in CHCl3 (7 mL) and protected by a CaCl2 tube, thione
10 (0.246 g, 1.14 mmol) dissolved in CHCl3 (7 mL) was added via 1H NMR (CDCl3, 298 K): δ = 7.194 (td, J = 7.6, J = 4.8 Hz, 2
3
3
3
syringe. The reaction mixture was refluxed overnight. After cooling
to room temp. the reaction mixture was evaporated on silica gel
and purified by column chromatography on a dry silica gel (eluent
PE/CH2Cl2, gradient 100–70% PE). The first fraction was iden-
tified as (E)- and (Z)-11-(11ЈH-benzo[b]fluorene-11Ј-ylidene)-11H-
benzo[b]fluorene (0.040 g), the second fraction was identified as
9,10-anthraquinone (0.340 g), the third fraction was 7. A second
column chromatography at the same conditions afforded 7: 0.060 g,
H, H3Ј, H6Ј), 7.511 (m, 4 H, H3, H6, H2, H7), 7.900 (dd, J = 7.6,
3
4J = 1.5 Hz, 2 H, H4Ј, H5Ј), 8.090 (m, 2 H, H4, H5), 8.341 (ddd, J
= 4.8, 4J = 1.4 Hz, 2 H, H2Ј, H7Ј), 8.607 (m, 2 H, H1, H8) ppm.
13C NMR (CDCl3, 298 K): δ = 122.83 (C3Ј, C6Ј), 125.95 (C4, C5),
126.88 (C4Ј, C5Ј), 129.20 (C3, C6 or C2, C7), 129.29 (C2, C7 or C3,
C6), 131.32 (C4aЈ, C4bЈ), 132.65 (C9Ј), 132.86 (C1, C8), 133.07 (C4a,
C10a), 138.45 (C8a, C9a), 143.39 (C9), 147.53 (C2Ј, C7Ј), 156.58 (C8aЈ,
C9aЈ), 186.47 (C10) ppm. UV/Vis (cyclohexane): c = 2.77ϫ10–5 ,
Eur. J. Org. Chem. 2007, 5198–5211
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5209

A. Levy, S. Pogodin, S. Cohen, I. Agranat
FULL PAPER
λmax nm (ε): 370 (10288); c = 6.92ϫ10–4 , λmax nm (ε): 525 (sh,
739).
Philipp, R. Kohnen, K.-O. Hiller, K. Linde, M. Berner, Brit.
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Received: May 8, 2007
Published Online: August 30, 2007
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