Photoswitchable tetraethynylethene-dihydroazulene chromophores

Article abstract of DOI:10.1002/1522-2675(20010418)84:4<743::AID-HLCA743>3.0.CO;2-1

The synthesis, characterization, and photophysical as well as electrochemical properties of the photochromic hybrid systems 11-16 and 18, which contain photoswitchable tetraethynylethene (TEE; 3,4-diethynylhex-3-ene-1,5-diyne) and dihydroazulene (DHA) moieties, are presented. The molecular photoswitches were synthesized by a Sonogashira cross-coupling reaction between an appropriate TEE precursor (6-10 and 17) and an iodinated DHA 1 or its vinylheptafulvene (VHF) isomer (4) (Schemes 5-7). X-Ray crystal structures of five DHA derivatives (1, trans-11a, cis-11a, 12, and 13) are discussed (Figs. 2-5). In all compounds, the cyclohexatriene moiety of the DHA chromophore adopts a clear boat conformation (Table 1). Presumably due to crystal-packing effects, the arylated TEE moieties in the hybrid systems show substantial distortions from planarity, with the dihedral angles between the planes of the central TEE core and the adjacent aryl substituents amounting to 44°. The switching properties were investigated by electronic absorption spectroscopy. Upon light absorption, DHAs 1. 12-16, and 18 underwent retro-electrocyclization in solution to give the corresponding VHFs (Figs. 6, 11, and 12). The reaction is thermally reversible, with half-lives τ_1/2 between 3.9 and 5.8 h at 25° in CH₂Cl₂ (Figs. 7 and 13 and Table 3). A comparatively slower (E) → (Z) isomerization process about the central C=C bond of the TEE moiety was also observed. The N,N-dimethylanilino-(DMA) substituted TEE-DHA hybrid systems trans-11a and cis-11a did not react to the corresponding VHFs upon irradiation (Scheme 9). Instead, only the reversible (E) → (Z) photoisomerization of the TEE core occurred (Fig. 16 and Table 4). This process was further investigated for photofatigue by electronic-emission spectroscopy (Fig. 17). After protonation of the DMA group, the usual DHA → VHF photoreaction took place. Compound 11 represents a three-way chromophoric molecular switch with three addressable sub-units (TEE core, DHA/VHF moiety, and proton sensitive DMA group) that can undergo individual, reversible switching cycles (Scheme 9). A process modeling the function of an 'AND' logic gate (Fig. 19) and three write/erase processes could be performed with this system. Cyclic and linear sweep-voltammetry studies in CH₂Cl₂ (+Bu₄NPF₆) revealed the occurrence of characteristic first-reduction steps in the TEE-DHA hybrid systems between - 1.6 and - 1.8 V vs. Fc/Fc⁺ (ferrocene/ferricinium couple) (Table 5). Oxidations occur at ca. + 1.10 V. After photoisomerization to the VHF derivatives, reduction steps at more positive and oxidation steps at more negative potentials were recorded. No DHA → VHF isomerization took place upon electrochemical oxidation or reduction (Fig. 20).

Full text of DOI:10.1002/1522-2675(20010418)84:4<743::AID-HLCA743>3.0.CO;2-1

Helvetica Chimica Acta ± Vol. 84 (2001)
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Photoswitchable Tetraethynylethene-Dihydroazulene Chromophores
by Luca Gobbi, Paul Seiler, and François Diederich*
Laboratorium für Organische Chemie, ETH-Zentrum, Universitätstrasse 16, CH-8092 Zürich
Volker Gramlich
Laboratorium für Kristallographie, ETH-Zentrum, Sonneggstrasse 5, CH-8092 Zürich
and Corinne Boudon, Jean-Paul Gisselbrecht, and Maurice Gross
Â
Laboratoire dꢀElectrochimie et de Chimie Physique du Corps Solide, Universite Louis Pasteur, C.N.R.S.:
Â
U.M.R. No. 7512, Faculte de Chimie, 4 rue Blaise Pascal, F-67000 Strasbourg
The synthesis, characterization, and photophysical as well as electrochemical properties of the photochromic
hybrid systems 11 ± 16 and 18, which contain photoswitchable tetraethynylethene (TEE; 3,4-diethynylhex-3-
ene-1,5-diyne) and dihydroazulene (DHA) moieties, are presented. The molecular photoswitches were synthesized
by a Sonogashira cross-coupling reaction between an appropriate TEE precursor (6 ± 10 and 17) and an
iodinated DHA 1 or its vinylheptafulvene (VHF) isomer (4) (Schemes 5 ± 7). X-Ray crystal structures of five
DHA derivatives (1, trans-11a, cis-11a, 12, and 13) are discussed (Figs. 2 ± 5). In all compounds, the
cyclohexatriene moiety of the DHA chromophore adopts a clear boat conformation (Table 1). Presumably due to
crystal-packing effects, the arylated TEE moieties in the hybrid systems show substantial distortions from
planarity, with the dihedral angles between the planes of the central TEE core and the adjacent aryl substituents
amounting to 448. The switching properties were investigated by electronic absorption spectroscopy. Upon light
absorption, DHAs 1, 12 ± 16, and 18 underwent retro-electrocyclization in solution to give the corresponding
VHFs (Figs. 6, 11, and 12). The reaction is thermally reversible, with half-lives t1/2 between 3.9 and 5.8 h at 258 in
CH2Cl2 (Figs. 7 and 13 and Table 3). A comparatively slower (E) ! (Z) isomerization process about the central
CˆC bond of the TEE moiety was also observed. The N,N-dimethylanilino-(DMA) substituted TEE-DHA
hybrid systems trans-11a and cis-11a did not react to the corresponding VHFs upon irradiation (Scheme 9).
Instead, only the reversible (E) ! (Z) photoisomerization of the TEE core occurred (Fig. 16 and Table 4). This
process was further investigated for photofatigue by electronic-emission spectroscopy (Fig. 17). After
protonation of the DMA group, the usual DHA ! VHF photoreaction took place. Compound 11 represents
a three-way chromophoric molecular switch with three addressable sub-units (TEE core, DHA/VHF moiety,
and proton sensitive DMA group) that can undergo individual, reversible switching cycles (Scheme 9). A
process modeling the function of an ꢁANDꢀ logic gate (Fig. 19) and three write/erase processes could be
performed with this system. Cyclic and linear sweep-voltammetry studies in CH2Cl2 (‡ Bu4NPF6) revealed the
occurrence of characteristic first-reduction steps in the TEE-DHA hybrid systems between 1.6 and 1.8 V
‡
vs. Fc/Fc (ferrocene/ferricinium couple) (Table 5). Oxidations occur at ca. ‡1.10 V. After photoisomerization
to the VHF derivatives, reduction steps at more positive and oxidation steps at more negative potentials were
recorded. No DHA ! VHF isomerization took place upon electrochemical oxidation or reduction (Fig. 20).
1. Introduction. ± Molecular switches are systems that possess at least two reversibly
interconvertible states [1]. Their study has attracted increasing interest in recent years,
owing to their potential as functional building blocks for molecular devices [2]. Possible
applications range from molecular-scale information processing and reversible high-
density data storage [3], to biomimetic engineering [4]. Photoresponsive systems [5]
have attracted particular attention, since the use of light as an external switching

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Helvetica Chimica Acta ± Vol. 84 (2001)
stimulus allows for fast and clean processes (for recent reviews, see [6]). Some of the
most promising and most widely used systems reported to date are i) the (Z)-1,2-di(3-
thienyl)ethenes developed by Irie and co-workers [7] and successfully used by other
research groups [8], ii) the ethene-based chiroptical photoswitches studied by Feringa
and co-workers [9], iii) spiropyranes [10], iv) fulgides [11], and v) azobenzenes [12]. In
1984, Daub et al. first reported the photochromic properties of dihydroazulenes (DHA;
Scheme 1) [13]. DHAs react after light irradiation in a 10-electron retro-electro-
cyclization to vinylheptafulvenes (VHF), that, in turn, undergo a thermal cyclization
back to the starting materials [14].
Scheme 1. The Dihydroazulene (DHA)-Vinylheptafulvene (VHA) Molecular Photoswitch [13][14]
With aryl-substituted tetraethynylethene (TEE; 3,4-diethynylhex-3-ene-1,5-diyne)
derivatives [15], we recently introduced a new class of molecular photoswitches that
undergo two-way photochemical interconversions, not perturbed by thermal isomer-
ization pathways [16]. Here, we describe the synthesis of a new series of photochromic
hybrid systems containing both TEE and DHA moieties as individually addressable,
photoswitchable subunits (Fig. 1). The structural and electronic characteristics of these
novel chromophores are reported, as well as detailed investigations exploring how the
switching capacity of one sub-unit is affected by the presence of the second one in the
same molecule (for a preliminary communication of parts of this work, see [17]).
R
Si(i-Pr)3
R = aryl, trialkylsilyl
(i-Pr)3Si
NC
NC
Fig. 1. Hybrid systems containing tetraethynylethene (TEE) and dihydroazulene (DHA) moieties as individually
addressable photoswitchable subunits
2. Results and Discussion. ± 2.1. Synthesis. Two routes for the synthesis of DHAs
have been reported by Daub and co-workers [13][18 ± 20]. The first one involves a [8 ‡
2] cycloaddition between 8-methoxyheptafulvene and a 1,1-dicyanoethylene (ˆ but-2-
enedinitrile) derivative, followed by elimination of MeOH to produce directly the
thermally stable dihydroazulene (DHA) chromophore [19]. An alternative protocol
starts from a tropylium salt and acetophenone derivatives, and provides first the

Helvetica Chimica Acta ± Vol. 84 (2001)
745
vinylheptafulvene (VHF) chromophore, which subsequently undergoes thermal
isomerization to the DHA form [20]. The latter route was initially used to prepare
DHA 1 bearing a 4-iodophenyl substituent for attachment to tetraethynylethene
(TEE) derivatives with a free ethynyl group by Pd0-catalyzed Sonogashira cross-
coupling [21] (Scheme 2). Commercially available 4-iodoacetophenone underwent an
acid-catalyzed nucleophilic addition to tropylium tetrafluoroborate to provide ketone
2, which produced the dicyanoethene 3 by acid-catalyzed Knoevenagel condensation
with malononitrile. Hydride abstraction with tritylium tetrafluoroborate to form the
tropylium cation and subsequent deprotonation afforded VHF 4 as a mixture with
DHA isomer 1. Upon heating this mixture to 808 in the dark, the isomeric equilibrium
was completely shifted to the thermally stable DHA form, which was isolated in 70%
yield (starting from 3). Analytically pure 4 was obtained in high yield (91%) as deep-
red crystals by irradiating a saturated solution of 1 in hexane at 366 nm.
Scheme 2. Synthesis of the 4-Iodophenyl-Substituted DHA 1 and VHF 4
O
O
a)
+
BF4
I
I
2
I
NC
CN
NC
CN
e)
f)
b)
c)
d)
NC
I
I
NC
3
4
1
a) AcOH, MeOH, 208, 6 h; 50%. b) Malononitrile, AcOH, NH4OAc, PhMe "# , 8 h; 75%. c) Ph3CBF4,
ClCH2CH2Cl, 808, 1 h. d) Et3N, PhMe, 08 ! 208, 1 h. e) 808, in the dark, 1 h; 70% (from 3). f) Hexane, hn
(366 nm), 208, 6 h; 91%.
All attempts to produce multigram quantities of 1 by the protocol described in
Scheme 2, however, failed. On the larger scale, the Knoevenagel condensation of 2 with
malononitrile worked only poorly or even not at all. At prolonged reaction times,
cleavage of the cycloheptatriene group was observed, leading to dicarbonitrile 5 in 69%
yield (Scheme 3). We assume that, in a first step, the tropylium cation is eliminated in
the reversal of the addition reaction that produced 2 (Scheme 2), and that the 4-
iodoacetophenone formed subsequently undergoes the Knoevenagel condensation
under formation of 5.
Taking these observations into account, we modified the large-scale synthesis of 4
by performing in the first step the Knoevenagel condensation between 4-iodoaceto-
phenone and malononitrile (Scheme 4). When the reaction was conducted in benzene
in the presence of AcOH and NH4OAc, dinitrile 5 was obtained in 89% yield. The use
of toluene as the solvent was less satisfactory, and almost no conversion was observed in

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Helvetica Chimica Acta ± Vol. 84 (2001)
Scheme 3. Major Side Reaction in the Large-Scale Knoevenagel Condensation of 2 with Malononitrile
O
NC
CN
a)
I
I
2
5
a) Malononitrile, AcOH, NH4OAc, PhMe, "# , 4 d; 69%.
EtOH with piperidine as the catalyst. Since the acid-catalyzed nucleophilic addition of
4-iodoacetophenone to the tropylium cation had only provided a moderate yield (50%)
of 2 (Scheme 2), we decided to explore the corresponding nucleophilic addition of 5
under basic conditions. First attempts to deprotonate 5 with DBU (1,8-diazabicy-
clo[5.4.0]undec-7-ene) in toluene at 408 or
tetrafluoroborate failed, leading only to decomposition products. At
108 in the presence of tropylium
788 in the
presence of 1.1 equiv. of DBU, the desired reaction took place and afforded a mixture 5/
3 in a 1.3 :1 ratio. A larger excess of DBU led to the formation of major, non-identified
side products, possibly initiated by deprotonation of 3. Gratifyingly, changing to the
milder base Et3N in CH2Cl2 at 788 provided the desired product in quantitative yield.
The subsequent dehydration of 3 to 4 (and the thermal conversion to 1) on the
multigram scale worked as described above (Scheme 2). This new synthetic route not
only provided sufficient quantities of 1 for the projects described in this manuscript but
also allowed the preparation of a highly efficient polymeric waveguide doped with this
molecular switch [22].
Scheme 4. Alternative Synthesis of Dinitrile 3, the Precursor to the Molecular Switch 1
NC
CN
NC
CN
O
a)
b)
I
I
I
5
3
a) Malononitrile, AcOH, NH4OAc, PhH, "# , 3 d; 89%. b) Et3N, tropylium tetrafluoroborate, CH2Cl2, 788,
1 h; 100%.
A series of TEE derivatives, with different substituents in the trans-position to the
ethynyl residue ultimately bearing the DHA/VHF moiety, was prepared by previously
introduced routes [15c]. Mono-deprotected TEE 6 [15c] was cross-coupled to various
aryl iodides (4-R-C6H4I; R ˆ Me2N, NO2, MeO, H) under Sonogashira conditions
[21][23], thereby affording the arylated TEEs 7 [15c], 8 [15c], 9, and 10 (Scheme 5).
The anisole derivative 9 turned out to be very sensitive towards light and readily
underwent rapid trans ! cis isomerization of the TEE moiety. A slower trans ! cis
isomerization was also observed for 7 and 8 upon irradiation at 366 nm or prolonged
exposure to sunlight. The Me3Si group was removed with K2CO3 in MeOH/THF, and
the mono-deprotected arylated TEEs were coupled to the iodinated VHF derivative 4,

Helvetica Chimica Acta ± Vol. 84 (2001)
747
yielding the trans-configured TEE-DHA hybrid compounds trans-11a and 12 ± 14 as
thermally stable materials (after VHF ! DHA isomerization). Similarly, cross-
coupling between 6 and 4 gave the hybrid derivative 15. To prevent photochemical
trans ! cis isomerizations, reactions were usually performed in the dark. The VHF
derivative 4 was preferentially used (instead of isomeric 1) to form the hybrid
compounds, since it was obtained in particularly high purity by recrystallization, which,
in return, benefited the yield of the cross-coupling reaction.
Scheme 5. Synthesis of the TEE-DHA Hybrid Derivatives trans-11a and 12 ± 15
R
H
Si(i-Pr)3
SiMe3
Si(i-Pr)3
SiMe3
7
8
9
R = NMe2 (68%)
R = NO2 (76%)
R = OMe (32%)
a)
10 R = H (65%)
(i-Pr)3Si
(i-Pr)3Si
6
b), c)
c)
R
Me3Si
Si(i-Pr)3
Si(i-Pr)3
trans-11a R = NMe2 (69%)
12 R = NO2 (52%)
13 R = OMe (54%)
14 R = H (78%)
15 (64%)
(i-Pr)3Si
(i-Pr)3Si
NC
NC
NC
NC
a) [PdCl2(PPh3)2], CuI, Bu4NBr, (i-Pr)2NH, THF, 4-Me2N-C6H4I ( ! 7), 4-O2N-C6H4I ( ! 8), 4-MeO-C6H4I
( ! 9), PhI ( ! 10), 208, 45 min 15 h. b) K2CO3, MeOH, THF, 208, 60 ± 90 min. c) 4, [PdCl2(PPh3)2], CuI,
Bu4NBr, (i-Pr)2NH, THF, 208, 1 ± 17 h. All steps were usually performed in the dark to prevent trans ! cis
isomerization.
However, DHA isomer 1 could also be used in the Sonogashira cross-coupling
reaction, as illustrated by the preparations of thiophene derivative 16 (obtained by the
sequence 6 ! 17 ! 16; Scheme 6) and TEE-DHA hybrid 18 bearing three (i-Pr)3Si
groups (obtained by the sequence 19 [24] ! 20 ! 18; Scheme 7). The reaction of 19
with 3.3 equiv. of (i-Pr)2NLi (LDA) in THF at 788 gave a green monometallated
lithium acetylide that was silylated with (i-Pr)3SiOSO2CF3 to give 20 in good yield
(88%). While the silylation with (i-Pr)3SiOSO2CF3 was complete after a few minutes at
788, the analogous reaction with (i-Pr)3SiCl took several hours and gave only poor
yields.
For investigations of the trans/cis photoisomerization between trans-11a and cis-
11a, a sample of cis-11a was prepared by irradiating trans-11a in CH2Cl2 at 366 nm and
separating the resulting isomeric mixture by column chromatography (Scheme 8).
2.2. X-Ray Crystal-Structure Analyses. X-Ray crystal structures of five DHA
derivatives were solved. Yellow crystals of 1 were grown at room temperature from

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Scheme 6. Synthesis of the TEE-DHA Hybrid Derivative 16 Starting from 1
Si(i-Pr)3
SiMe3
Si(i-Pr)3
S
S
a)
b), c)
6
(i-Pr)3Si
(i-Pr)3Si
16
17
NC
NC
a) [PdCl2(PPh3)2], CuI, 2-iodothiophene, (i-Pr)2NH, THF, 208, 20 min; 64%. b) K2CO3, MeOH, THF, 208, 3 h.
c) [Pd(PPh3)4], CuI, 1, (i-Pr)2NH, THF, 208, 18 h; 45% (steps b and c).
Scheme 7. Synthesis of TEE-DHA Hybrid Derivative 18
Br
Br
Si(i-Pr)3
(i-Pr)3Si
Si(i-Pr)3
(i-Pr)3Si
Si(i-Pr)3
a), b)
c), d), e)
H
H
H
(i-Pr)3Si
(i-Pr)3Si
(i-Pr)3Si
Br
Br
Br
Br
19
20
18
NC
NC
a) LDA (3.3 equiv.), THF, 788, 15 min. b) (i-Pr)3SiOSO2CF3, THF, 788, ca. 5 min, then 788 ! 08; 88%
(steps a and b). c) LDA, THF, 788, 15 min. d) Sat. aq. NH4Cl soln. e) [PdCl2(PPh3)2], CuI, 1, (i-Pr)2NH, THF,
208, 11 h; 74% (steps c, d, and e).
Scheme 8. Synthesis of cis-11a
Me2N
NMe2
Si(i-Pr)3
(i-Pr)3Si
(i-Pr)3Si
a)
(i-Pr)3Si
trans-11a
cis-11a
NC
NC
NC
NC
a) hn (366 nm), CH2Cl2, 208, 3.5 h; 39%.

Helvetica Chimica Acta ± Vol. 84 (2001)
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Et2O/hexane. The racemic compound, with a stereogenic center at C(9), crystallizes in
Å
the P1 space group, with the elemental cell occupied by two molecules. Its structure
(Fig. 2,a) closely resembles those previously reported for DHA derivatives by Daub
and co-workers [18a][25], with the seven-membered ring adopting a boat conforma-
tion (a ˆ 54.38 and b ˆ 28.08; Fig. 2,b). A clear bond-length alteration is apparent in
the linearly p-conjugated octatetraene chromophore of the DHA moiety, with C
C
bond lengths of 1.440(7) ± 1.431(7) Š and CˆC bond lengths of 1.327(7) ± 1.353(6) Š.
Fig. 2. a) ORTEP Plot of 1. Arbitrary numbering. Atomic-displacement parameters obtained at 293 K are
drawn at the 50% probability level. b) Definition of the angles a and b describing the boat conformation of the
cycloheptatriene ring in DHA derivatives [19]. Selected bond lengths [Š] and bond angles [8]: C(4) C(7)
1.463(6), C(7) C(11) 1.342(6), C(10) C(11) 1.429(6), C(10) C(16) 1.353(6), C(15) C(16) 1.431(7), C(14)
C(15) 1.327(7), C(13) C(14) 1.440(7), C(12) C(13) 1.342(7), C(9) C(12) 1.505(6), C(9) C(10) 1.514(6),
C(8) C(9) 1.564(6); C(8) C(7) C(11) 108.8(4), C(7) C(11) C(10) 113.4(4), C(11) C(10) C(16)
130.0(4), C(10) C(16) C(15) 123.8(5), C(16) C(15) C(14) 126.6(4), C(15) C(14) C(13) 125.4(5),
C(14) C(13) C(12) 125.7(5), C(13) C(12) C(9) 119.9(4), C(12) C(9) C(10) 107.1(3), C(9) C(10)
C(16) 121.6(4), C(9) C(10) C(11) 108.4(4), C(8) C(9) C(10) 103.3(3), C(7) C(8) C(9) 104.1(3),
C(17) C(8) C(18) 107.2(3). These bond lengths and angles are also characteristic for the DHA moieties in
the solid-state structures of the TEE-DHA hybrid materials.
This boat conformation is also observed for the DHA chromophores in the X-ray
crystal structures of the TEE hybrid systems (Table 1 and Figs. 3 ± 5). Appropriate
single crystals of cis-11a, 12, and 13 were grown by slow evaporation of CH2Cl2/MeCN
solutions, whereas those of trans-11a were obtained from MeOH (for a view on the X-
ray crystal structure of trans-11a, see [17]). Compounds trans-11a and 13 crystallize in
Å
the centrosymmetric P1 space group (two molecules in the unit cell), whereas 12
crystallizes in P1 (one molecule per unit cell) and cis-11a in the P2(1)/c space group (six
molecules per unit cell). As reported for other trialkylsilyl-protected TEEs, some
disorder occurs in the (i-Pr)3Si groups, which has a negative impact on the structural
accuracy [15c]. Bond angles and bond lengths in the arylated TEE cores were in the
range observed in similar X-ray crystal structures [15][16b][26]. A nearly perfect
coplanarity between the TEE core and the adjacent aryl rings was observed only for

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Helvetica Chimica Acta ± Vol. 84 (2001)
Table 1. Angles a and b Defining the Boat Conformation of the Cycloheptatriene Ring in DHA Derivatives and
Dihedral Angles between the Central TEE Core and the Adjacent Aryl Substituents A and B
Compound
a [8]
b [8]
A/TEE [8]
B/TEE [8]
1
54
28
±
17
44
6
±
ꢀ 0.1
30
26
22
a
b
b
trans-11a
cis-11a
12
)
)
)
46
47
47
24
22
24
13
21
a) From [17]. b) Not determined due to static disorder in the cycloheptatriene ring.
(i-Pr)3Si
A
B
NC
CN
TEE
Si(i-Pr)3
trans-11a [17]; in all other structures, the aryl rings show significant deviations from the
TEE plane (Table 1), probably due to packing effects.
This distortion is particularly strong for cis-11a, in which the dihedral angles
between the TEE core and the two adjacent aryl substituents amount to 30 and 448,
respectively. The five-membered ring in the DHA moiety is nearly coplanar with the
adjacent phenylene ring, which is twisted by ca. 448 out of the plane of the central TEE
core. Such geometry is undoubtedly imposed by the observed self-assembly of solid cis-
11a into supramolecular dimers (Fig. 3,b). In these dimers formed in the crystal lattice,
two neighboring cis-11a molecules arrange in such a way as to form several close
intermolecular contacts between their DHA-C6H4-TEE-DMA moieties (DMA ˆ
N,N-dimethylaniline), whereas the (i-Pr)3Si groups point away from the dimeric
assembly to interact with the same groups of neighboring dimers. The arylated DHA
moieties adopt an antiparallel arrangement in the dimer, with the aryl rings positioned
above the cycloheptatriene rings (shortest contact C(4) ´´´ C(20) ˆ 3.45 Š). The latter,
in return, interact via their C(21) C(23) bonds with the neighboring DMA rings
(shortest contact C(22) ´´´ C(31) ˆ 3.53 Š).
2.3. Photophysical Properties. A solution of yellow DHA 1 displays photochromic
properties when exposed to sunlight. The underlying process is accelerated when UV
light of 366 nm is used. The isosbestic points in the UV/VIS spectra are in accordance
with two species equilibrating. In a first study, the light-induced ring-opening of 1 to 4
was investigated in CH2Cl2 (Fig. 6). A sterilAir BLB-8 lamp, commonly employed to
view TLC plates with fluorescent coatings, was used as light source. After irradiation
(366 nm) for ca. 20 s, a complete conversion of 1 to 4 was observed under these
conditions. The isomerization was followed by UV/VIS spectroscopy, monitoring the
decrease in intensity of the characteristic absorption band of 1 at lmax of 365 nm and the
concomitant increase of a new band for 4 at lmax of 478 nm. The thermal back reaction
from 4 to 1 was investigated in CH2Cl2 at 278 (Fig. 7). The half-life t1/2 of 4 under these
conditions is 5.8 h. In the crystal, 4 is stable; its deep-red color prevails until the melting
point is reached at 1168, when an instantaneous color loss is observed. For this reason, a

Helvetica Chimica Acta ± Vol. 84 (2001)
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Fig. 3. a) ORTEP Plot of cis-11a. Arbitrary numbering. Atomic-displacement parameters obtained at 293 K are
shown at the 50% probability level. b) Aryl-DHA interactions in the supramolecular dimer formed by cis-11a in
the crystal. Bond lengths [Š] and angles [8] in the TEE core: C(2) C(3) 1.200(13), C(1) C(2) 1.429(12),
C(1) C(47) 1.442(12), C(47) C(48) 1.196(11), C(1) C(24) 1.373(10), C(24) C(25) 1.423(12), C(25) C(26)
1.180(13), C(24) C(36) 1.416(12), C(36) C(37) 1.195(11); C(1) C(2) C(3) 177.7(9), C(2) C(1) C(47)
115.8(9), C(1) C(47) C(48) 177.3(9), C(2) C(1) C(24) 121.3(9), C(47) C(1) C(24) 122.9(8), C(1)
C(24) C(25) 119.6(9), C(24) C(25) C(26) 176.6(8), C(25) C(24) C(36) 119.8(9), C(1) C(24) C(36)
120.6(8), C(24) C(36) C(37) 176.8(9).

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Fig. 4. ORTEP Plot of 12. Arbitrary numbering. Atomic-displacement parameters obtained at 295 K are shown
at the 30% probability level. Bond lengths [Š] and angles [8] in the TEE core: C(2) C(3) 1.207(8), C(1) C(2)
1.409(8), C(1) C(47) 1.434(8), C(47) C(48) 1.207(8), C(1) C(24) 1.367(8), C(24) C(25) 1.445(8), C(25)
C(26) 1.196(9), C(24) C(36) 1.421(8), C(36) C(37) 1.195(9); C(1) C(2) C(3) 175.8(6), C(2) C(1)
C(47) 118.3(5), C(1) C(47) C(48) 177.7(6), C(2) C(1) C(24) 120.1(5), C(47) C(1) C(24) 121.5(5),
C(1) C(24) C(25) 122.0(5), C(24) C(25) C(26) 174.0(7), C(25) C(24) C(36) 115.6(5), C(1) C(24)
C(36) 122.4(5), C(24) C(36) C(37) 177.5(7).
nonreactive s-trans-VHF conformation (Fig. 8) is postulated for 4 in the solid state
[18a].
All investigated DHA-TEE hybrid derivatives (trans-11a, cis-11a, 12 ± 16, and 18)
were stable crystalline yellow-to-orange-colored solids with high melting points
between 1518 and 2228. These compounds are not photochromic in the crystal. After
irradiation in solution, a dark-orange coloration was observed for all with the exception
of trans-11a and cis-11a. Fig. 9 shows the electronic-absorption spectra of all DHA
derivatives synthesized in this work. Compared to 1, the TEE-DHA hybrid derivatives
show a strong enhancement of the molar extinction coefficient and a large bath-
ochromic shift of the longest-wavelength absorption band. Correspondingly, the optical
end-absorption shifts to lower energy. The two tris-trialkylsilylated compounds 15 and
18 expectedly display nearly identical UV/VIS spectra. They both feature a shoulder
around 360 nm, which is related to a local electronic transition in the DHA moiety. The
extension of the p-system by addition of a second aryl substituent to the TEE core (12 ±
14 and 16) leads to a further increase in the molar extinction coefficient of the longest-
wavelength absorption maximum, which, together to the optical end-absorption,
appears at even lower energy. All these data are in agreement with extended p-electron
delocalization over the entire conjugated p-chromophore in these molecules.

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Fig. 5. ORTEP Plot of 13. Arbitrary numbering. Atomic-displacement parameters obtained at 213 K are shown
at the 30% probability level. Bond lengths [Š] and angles [8] in the TEE core: C(2) C(3) 1.195(11), C(1)
C(2) 1.428(12), C(1) C(35) 1.424(11), C(35) C(36) 1.216(11), C(1) C(24) 1.351(11), C(24) C(25)
1.448(13), C(25) C(26) 1.212(13), C(24) C(46) 1.452(12), C(46) C(47) 1.200(11); C(1) C(2) C(3)
179.5(8), C(2) C(1) C(24) 121.6(7), C(1) C(24) C(25) 123.5(7), C(24) C(25) C(26) 176.9(9), C(1)
C(24) C(46) 122.5(8), C(24) C(46) C(47) 176.6(10), C(46) C(24) C(25) 114.0(8), C(2) C(1) C(35)
118.4(7), C(24) C(1) C(35) 120.0(7), C(1) C(35) C(36) 177.7(8).
Efficient intramolecular donor-acceptor interactions [15c] are at the origin of the
strong bathochromic shifts measured for the longest-wavelength absorption maximum
and the optical end-absorption of the DMA derivatives trans-11a and cis-11a. The red-
shifted position of their longest-wavelength absorption band (lmax ˆ 464 nm) ± with
high intermolecular charge-transfer character ± clearly indicates a strong conjugative
interaction between the DHA acceptor and the DMA donor. This interaction is much
stronger than the one between this acceptor and the 2-thienyl donor (lmax ˆ 420 nm) in
16 or the 4-methoxyphenyl donor (lmax ˆ 419 nm) in 13 (Table 2), which both lack such
a red-shifted band. After protonation of the DMA group in trans-11a or cis-11a with
CF3CO2H, the charge-transfer absorption vanishes, and their spectra resemble those of
the other TEE-DHA conjugates (Fig. 9 and Table 2). In further support of the
proposed intermolecular charge-transfer interaction, the longest-wavelength absorp-
tion maximum of trans-11a is shifted hypsochromically when the solvent is changed
from CH2Cl2 to hexane (lmax ˆ 443 nm), an environment with a very limited capacity
for stabilizing polar states (Fig. 10).
The photochromism of the TEE-DHA hybrid derivatives 12 ± 16 and 18 (for the
special behavior of trans-11 and cis-11, see Sect. 2.4 below) was investigated in CH2Cl2
solutions. The orange color that appears after irradiation derives from a new band in
the UV/VIS spectra with a maximum at ca. 480 nm. By comparison with the spectrum
of 4 (Figs. 6 and 7), this band is assigned to the isomeric VHF form. Parallel to the
appearance of this band, the intensity of the characteristic DHA absorption around

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Fig. 6. UV/VIS Spectra recorded during irradiation of 1 at l ˆ 366 nm in CH2Cl2 (c ˆ 2.33 ´ 10 5 m). Spectra
recorded after 0, 2, 4, 6, 8, 10, 12, and 32 s.
404 ± 420 nm decreases, which is a clear indication that a photochemical switching
process is operative. This process is illustrated in Fig. 11, which depicts the UV/VIS
spectra recorded during the irradiation (366 nm, 7 min) of 14 in CH2Cl2. Since the TEE
moiety is also photoactive [16] (as mentioned in the Introduction), the DHA/VHF
isomerization could potentially also be accompanied by a trans/cis isomerization
around its central CˆC bond. However, the occurrence of clean isosbestic points shows
that only two species are at equilibrium and that the trans/cis isomerization process is
much slower than the DHA ! VHF retro-electrocyclization. The photoreaction of the
TEE group can, indeed, be observed, when samples are left for prolonged time in the
light. To prove that the changes in the absorption spectra arise from the DHA ! VHF
isomerization process, compound 18 was studied with monochromatic light (405 nm)
from a fluorimeter light source (Fig. 12). Because of three (i-Pr)3Si substituents at the
TEE core, the trans/cis isomerization is a degenerate process and leads to identical
compounds. Therefore, light-induced structural changes involve only isomerization of
the DHA moiety, leading to VHF derivative 21.
A closer look at the UV/VIS spectra revealed that the characteristic absorption
bands of the starting TEE-DHA hybrid compounds around 404 ± 420 nm never fully
faded away. An isomerization experiment with 12 monitored by thin layer chromatog-
raphy established that a quantitative conversion to the open VHF form upon

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Fig. 7. UV/VIS Spectra recorded during the thermal electrocyclization of 4 in CH2Cl2 (c ˆ 2.33 ´ 10 5 m) at 278.
Spectra recorded after 0, 52, 112, 232, and 1227 min.
I
NC
CN
NC
NC
I
s-trans-4
s-cis-4
Fig. 8. s-trans- and s-cis-Conformations of 4. The thermal electrocyclization to 1 proceeds via the s-cis-
conformation. In the crystal, 4 presumably adopts the s-trans-conformation and, hence, is thermally stable up to
the melting point temperature.
irradiation at 366 nm did not take place. The reason for this lies in the thermal VHF !
DHA back reaction. The position of the photoequilibrium between the two species
depends strongly on the experimental conditions, thus quantification was not possible.
The thermal back reaction was studied for all TEE-VHF conjugates in CH2Cl2 solution

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Fig. 9. Electronic absorption spectra of the TEE-DHA conjugates recorded at room temperature in CH2Cl2. Also
shown for comparison is the spectrum of 1.
at room temperature. Monitoring this process by UV/VIS spectroscopy is illustrated in
Fig. 13 for the conversion 21 ! 18, during which the VHF band at 481 nm faded away,
while the DHA band at 405 nm increased in intensity.
The rate constants for the thermal electrocyclization VHF ! DHA in the various
hybrid systems, k258, and the half-life times, t1/2, were calculated by monitoring the
absorption at a defined wavelength as a function of time and exponential fitting of the
resulting curves (Table 3). For all VHF-TEE systems, with the exception of the
‡
protonated DMA derivatives (cis-11aH) CF3CO2 (ˆcis-11b; cf. Scheme 9) and
‡
(trans-11aH) CF3CO2 (ˆtrans-11b; cf. Scheme 9), very similar values were obtained
in CH2Cl2, with t1/2 ranging between 4.3 and 5.0 h. Correspondingly, linearization of the
kinetic data afforded parallel lines (Fig. 14). Also, the end-absorption wavelengths of
the VHF isomers of 12 ± 16 and 18 (between 473 and 481 nm) are very similar to the
value measured for 4 (478 nm). These data, together with the similarity of the UV/VIS
spectra shown in Fig. 9, suggest that, in these conjugates, the DHA/VHF group on one
side of the central TEE core perceives only weakly the nature of the substituent on the
other. The comparison between the half-life times of 4 (5.8 h) and the TEE hybrid
derivatives shows a small increase in the rate of the thermal electrocyclization reaction.

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757
Fig. 10. Electronic absorption spectra of trans-11a in hexane (c ˆ 1.73 ´ 10 5 m) and CH2Cl2 (c ˆ 1.70 ´ 10 5 m) at
room temperature
Table 2. Maxima and Molar Extinction Coefficients of the Longest-Wavelength Absorption Bands of DHA
Derivatives in CH2Cl2
1
Compound
Substituent
lmax [nm]
e [l ´ mol 1 ´ cm
]
1
±
365
21400
trans-11a
trans-11b
4-Me2N C6H4
4-Me2HN C6H4
402/464
412
33300/44200
51600
a
‡
)
cis-11a
12
13
4-Me2N C6H4
4-O2N C6H4
4-MeO C6H4
C6H5
394/464
419
419
56600/27800
54800
56500
14
412
53200
15
Me3Si
404
46700
16
18
2-Thienyl
(i-Pr)3Si
420
406
51300
46300
a
‡
) trans-11b ˆ (trans-11aH) CF3CO2 ; see Scheme 9.

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Fig. 11. Photochromism of TEE-DHA hybrid derivative 14 in CH2Cl2 (c ˆ 1.88 ´ 10 5 m) after irradiation at
366 nm. Spectra recorded after 0, 15, 30, 45, 60, 75, 90, 105, 120, 180, and 480 s.
This increase can be readily explained with the p-electron-acceptor properties of the
TEE core [27], as previous studies by Daub et al. had shown that the VHF ! DHA
isomerization in arylated derivatives (such as 1/4) is slightly accelerated by electron-
withdrawing para-substituents on the aryl ring [18a].
The 16-times higher electrocyclization rate constant of the protonated DMA
derivative most probably finds its origin in the polar environment arising from the
addition of CF3COOH. In a control experiment, 20 ml of CF3COOH was added to a
solution of 18 in CH2Cl2 (3 ml), which represents the amount of acid used in standard
experiments to protonate trans-11a or cis-11a. The sample was irradiated at 405 nm,
and the cyclization 21 ! 18 was subsequently monitored at 481 nm (Fig. 15). A
1
remarkably enhanced rate constant k258 ˆ 2.7 ´ 10 4 s (t1/2 ˆ 43 min) was measured in
the presence of the acid. The stabilization of the postulated [28] highly polar transition
state of the VHF ! DHA electrocyclization process through the presence of
CF3COOH could account for these observations.
2.4. A Three-Way Chromophoric Molecular Switch. Among the DHA/VHF
switching experiments described in Sect. 2.3, the non-protonated conjugates trans-11a
and cis-11a are notably absent. In fact, these compounds were also included in the

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Fig. 12. Electronic absorption spectra in CH2Cl2 (c ˆ 2.12 ´ 10 5 m, Tˆ 258) recorded in 30-s intervals during the
photoisomerization of 18 to 21. Last spectrum recorded after 11 min. lirr ˆ 405 nm.
investigation, but it rapidly became clear that the switching process could not be
observed or did not take place. When we found, however, that rapid switching occurred
upon protonation of the DMA residue, we had discovered a new three-way
chromophoric molecular switch (for a report on a three-way switch consisting of a
DHA group and an acid-sensitive, redox-active anthraquinone moiety, see [29]). The
three addressable sub-units in 11, capable of undergoing individual, reversible
switching cycles are i) the TEE core, which can be reversibly photoisomerized
between its cis- and trans-forms, ii) the DHA unit, which can be transformed into a
VHF moiety upon irradiation, and iii) the proton-sensitive DMA group. With three
possible switching processes, 11 can adopt eight interconvertible states. Such a complex
system is best visualized in a three-dimensional representation by a cube with each
corner describing one distinct state (trans-11a ± d and cis-11a ± d; Scheme 9). Despite
this functional complexity, individual interconversion processes could be cleanly
addressed and six of the eight states were detected.
As a first function of this highly programmed molecular system, we investigated the
photochemical trans/cis-isomerization of the central TEE core. Irradiation at the
1
longest-wavelength absorption maximum (lmax ˆ 464 nm, e ˆ 44000 l ´ mol 1 ´ cm ) of
trans-11a induced rapid conversion to cis-11a, while irradiating into the most intense
1
absorption band of the latter (lmax ˆ 396 nm, e ˆ 55000 l ´ mol 1 ´ cm ) allowed the
reverse reaction to take place (Fig. 16). The ratio of trans- and cis-isomers at the
photostationary states and the quantum yields for the photoisomerization processes are
summarized in Table 4.

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Fig. 13. Electronic absorption spectra recorded in 30-min intervals in CH2Cl2 (c ˆ 2.12 ´ 10 5 m) at 258 during the
thermal isomerization of 21 to 18. The time-dependent decrease of the absorption at 481 nm is represented in
the top right corner. The rate constant, k258, and the half-life time, t1/2, were determined by an exponential fit to
these data.
Table 3. Half-Life Times, t1/2, and Rate Constants, k258, of the Thermal Reactions VHF ! DHA in CH2Cl2 at 258
and the Longest-Wavelength Absorption Maxima, lmax (starred), of the VHF Isomers. If the wavelength used for
the kinetic evaluation differed from lmax, both wavelengths are given.
1
DHA
t1/2 [h]
k [10 5 s
]
lmax [nm]
12
13
14
15
16
18
trans-11b
4.5
4.8
4.8
5.0
4.5
4.3
0.26
4.3
4.0
4.0
3.9
4.3
481*
500 (473*)
480*
481*
500 (474*)
481*
4.5
73.7
a
)
500*
a
‡
) trans-11b ˆ (trans-11aH) CF3CO2 ; see Scheme 9.
Both cis-11a and trans-11a displayed a strong fluorescence with a maximum at l ˆ
606 nm. This emission was monitored during several switching cycles to study the
resistance of the trans/cis isomerization process to photofatigue (Fig. 17). After 15
cycles, the intensity of the fluorescence at 606 nm decreased to about 90% of the initial
value.

Helvetica Chimica Acta ± Vol. 84 (2001)
761
Fig. 14. Linearized representation of the decrease of the VHF absorption band during the thermal VHF ! DHA
isomerization process in CH2Cl2 at 258. For the wavelength of the measurements, see Table 3.
Fig. 15. Addition of CF3COOH leads to an increase of the rate constant for the electrocyclization 21 ! 18. For
the reaction in pure CH2Cl2 a) k258 ˆ 4.5 ´ 10 5 s 1 (t1/2 ˆ 4.3 h). For the reaction in CH2Cl2 (3 ml) ‡ CF3COOH
(20 ml) b) k258 ˆ 2.7 ´ 10 4 s 1 (t1/2 ˆ 43 min). Absorbances collected at 281 nm; (c(21 ‡ 18) ˆ 2.12 ´ 10 5 m).

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Scheme 9. Three-Dimensional Switching Diagram of TEE-DHA Hybrid Derivative 11. The eight possible states
are shown as the corners of a cube. CF3CO2 is the counterion of 11b and 11c.

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Fig. 16. Electronic absorption spectrum of pure cis-11a in CH2Cl2 (----, c ˆ 2.12 ´ 10 5 m) and spectra recorded
every 10 s during the trans ! cis isomerization (lirr ˆ 464 nm), starting from pure trans-11a (Ð, c ˆ 1.70 ´ 10 5 m)
Table 4. Physical Data for the Photochemical trans/cis-Isomerization between trans-11a and cis-11a in CH2Cl2 at
258
b
b
c
c
d
la) [nm] % cis
)
% trans
)
Fc!t
)
Ft!c
)
Keq
)
ecise) [l ´ mol 1 ´ cm
]
etranse) [l ´ mol 1 ´ cm
]
1
1
396
464
35
68
65
32
0.19
0.51
0.17
0.37
1.83
2.15
55000
26000
33000
44000
a) Irradiation wavelength.
b) Percentage of cis-11a and trans-11a, respectively, in the photostationary state.
c) Partial quantum yields for the cis ! trans and trans ! cis interconversions, respectively (the quantum yield
determination is described in [17]).
d) Photoequilibrium constant, i.e., the ratio [product isomer]/[starting isomer] at the photostationary state.
e) Molar extinction coefficients of cis-11a and trans-11a at the irradiation wavelength.
This clean monitoring of the trans/cis-photoisomerization between trans-11a and
cis-11a was possible since the photoinduced retrocyclization of these hybrid derivatives
to the isomeric VHF forms cis-11d and trans-11d (Scheme 9) could not be
accomplished, as mentioned in the introduction to this Sect. However, when the
DMA moiety in 11a was protonated with CF3COOH ± leading to 11b (Scheme 9) ± the
photochemical ring opening to the isomeric VHF form 11c took place. Protonation of
1
11a to give 11b is clearly indicated by the downfield shifts of the aromatic H-NMR
signals of the DMA group (from 6.65 and 7.39 ppm to 7.51 and 7.65 ppm for the trans-
isomer, and from 6.67 and 7.38 ppm to 7.51 and 7.62 ppm for the cis-isomer). Yellow
solutions of protonated DHA derivatives cis-11b and trans-11b in CH2Cl2 both showed
absorption maxima of similar intensities at 411 nm and displayed only a weak yellow
fluorescence. The VHF-containing compounds cis-11c and trans-11c featured new
absorption band at lmax ˆ 500 nm, which renders the solution orange-brown. The

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Fig. 17. Intensity of the fluorescence of 11a in CH2Cl2 (c ˆ 2.12 ´ 10 5 m) recorded at 606 nm and 258 for 10
switching cycles starting from cis-11a. The sample was irradiated at 396 nm (cis-11a ! trans-11a) and at 464 nm
(trans-11a ! cis-11a).
conversion of trans-11b to trans-11c, upon irradiation at l ˆ 411 nm, was monitored
1
1
both by H-NMR (Fig. 18) and UV/VIS (Fig. 19) spectroscopy. Standard H-NMR
solutions (c ꢁ 10 mm) of trans-11b are too concentrated to be irradiated and
successfully switched. Diluted solutions, however, can be switched at low temperature
(
508), which must also be maintained during the recording of the 1H-NMR spectra in
order to prevent thermal VHF ! DHA back reaction during data acquisition time.
Thus, when a dilute solution (c ꢁ 0.3 mm) of trans-11b in CDCl3 in a NMR tube was
irradiated at 411 nm at 508, the DHA ! VHF conversion could be nicely observed
by 1H-NMR spectroscopy. The resonances of both DHA and VHF isomers were readily
discernable in the spectra (Fig. 18), and the absence of any non-assignable peaks
demonstrated the clean isomerization between the two forms. The VHF derivative 11c
(trans or cis) readily undergoes thermal electrocyclization back to 11b (k258 ˆ 7.37 Æ
1
0.02 ´ 10 4 s , t1/2 ˆ 15.7 min); therefore, mixtures of 11b and 11c are always observed,
even after long irradiation times at room temperature. The ratio of the DHA and VHF
isomers at equilibrium proved to be strongly dependent on the experimental
conditions; in the NMR experiment (Fig. 18) the final ratio of trans-11c to trans-11b
was ca. 90 :10.
The clean trans-11b ! trans-11c interconversion during irradiation at the specific
wavelength of 411 nm, without any isomerization to the cis-forms, was also evidenced
by several isosbestic points in the UV/VIS spectra (Fig. 19). At other wavelengths,
trans ! cis isomerization also occurred.
Treatment of 11c (a mixture of cis- and trans-isomers) with Et3N causes
deprotonation of the amino group and leads directly to the formation of the DHA-
1
containing compound 11a, as observed by UV/VIS, and H- and 13C-NMR spectro-

Helvetica Chimica Acta ± Vol. 84 (2001)
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Fig. 18. Characteristic 1H-NMR signals recorded during trans-11b ! trans-11c isomerization in CDCl3. a) DHA
Isomer trans-11b at 08 (c ˆ 1.44 ´ 10 2 m). b) After 2 h irradiation at 411 nm and 508 (c ˆ 3 ´ 10 4 m). c) After
4.5 h irradiation at 411 nm and 508 (c ˆ 3 ´ 10 4m). The latter spectrum mainly shows the resonances of the
VHF isomer with ca. 10% remaining DHA isomer. The signal around 7.10 ppm is a satellite peak from CDCl3 in
the very dilute solution.
scopy. The VHF-containing donor-acceptor system 11d could be obtained neither by
deprotonation of 11c ± even at 408 ± nor by irradiation of the DMA derivative 11a at
808. These results are tentatively attributed to a very fast thermal electrocyclization
of any intermediately formed 11d to 11a.
The addition of Et3N is not responsible for the extremely fast back reaction of 11d:
in a control experiment, a solution of 18 was irradiated, giving 21, which, after addition
of Et3N, did not show a faster thermal isomerization than in the absence of the base
(Fig. 13). Although the presence of the strongly electron-donating DMA substituent in
11 is clearly associated with the failure to observe the VHF-form 11d, its role is not

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Fig. 19. a) Electronic absorption spectra for the switching process trans-11a ! trans-11b ! trans-11c ! trans-
11a. b) in CH2Cl2 (c ˆ 1.70 ´ 10 5m) at 258. The dotted line ( ´´´´´ ) represents the starting material trans-11a. The
solid lines (ÐÐ) show the conversion trans-11b ! trans-11c, recorded at 1-min intervals during the irradiation
at l ˆ 411 nm. The dashed line (------) shows the spectrum of trans-11a, recovered after deprotonation with
Et3N.
readily explainable. Indeed, it has been reported that a simple DMA-substituted DHA
(2-(4-N,N-dimethylanilino)-8aH-azulene-1,1-dicarbonitrile) photoisomerizes in the
usual manner to give the corresponding VHF form [18a].
Beside the fast electrocyclization of 11d to 11a, photoinduced electron transfer
(PET) or electronic energy-transfer processes (EET), which have been widely used to
construct photoionically-driven fluorescent sensors and switches [30], seem also to play
a role in the failure to reach the VHF state 11d. Compounds trans-11a and cis-11a
display a very intense, solvent-dependent fluorescence, and extensive experimental and
computational analysis (to be published elsewhere [31]) indicates that these
compounds, upon photoexcitation, prefer relaxation via twisted intramolecular
charge-transfer states (TICT states) [32][33].

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When a solution of pure trans-11a in CH2Cl2 was treated with CF3COOH, then
irradiated at 411 nm, and finally Et3N was added in the dark, only trans-11a was
recovered as shown by UV/VIS (Fig. 19) and 13C-NMR spectral analysis; no trace of
diastereoisomeric cis-11a could be detected. The reversible conversion of trans-11a into
trans-11b and then into trans-11c can be described as a model process for an AND logic
gate in electronics [3]. The trans-11c state can be obtained only when both protons and
light are present; i.e., two inputs are necessary to generate one output. Nondestructive
readout of this ꢁANDꢀ logic gate is ensured by monitoring the appearance of the
absorption band of trans-11c at lmax ˆ 500 nm (Fig. 19). In contrast, the same cycle,
starting from pure cis-11a, leads to the recovery of an isomeric mixture, containing
mainly the trans-form, and thus the conversion cis-11a ! cis-11b ! cis-11c is not suited
for an ꢁANDꢀ logic gate, as cis ! trans isomerization also occurs.
In addition to the described ꢁANDꢀ logic gate, three write/erase processes are
present in system 11 that are not affected by side reactions to other states on the cubic
switching diagram when appropriate conditions are used: i) the reversible cis/trans-
photoisomerization between trans-11a and cis-11a, and ii) and iii) the reversible
protonations/deprotonations of the couples trans-11a/b and cis-11a/b. Since the
fluorescence enhancement after deprotonation of 11b amounts to a factor of ca. 300,
a very efficient nondestructive readout of information for the cis-11a/b and trans-11a/b
couples is available by using excitation light of 396 nm for the cis and of 464 nm for the
trans-isomer. The fluorescence enhancement factor is very high in comparison to
literature values. Systems with values over 1000 are known, but are very rare [34].
2.5. Electrochemistry. Cyclic voltammetry (CV) and linear-sweep voltammetry
(LSV) measurements were performed in CH2Cl2 in the presence of 0.1m Bu4NPF6
‡
(Table 5) with the ferrocene/ferricinium (Fc/Fc ) redox couple as a reference.
DHA Form 1 undergoes only irreversible reactions in both CV and LSV. The first
reduction takes place at 1.97 V and the oxidation at ‡ 1.16 V. In contrast, the VHF
isomer 4 is much more readily reduced ( 1.47 V) and oxidized (‡0.84 V). The electron-
withdrawing properties of the dicyanovinylene moiety in 4 explain the facilitated
reduction, while the formation of an aromatic tropylium ion accounts for the easier
oxidation (for previous electrochemical measurements on DHA derivatives, see [35]).
All TEE-DHA hybrid compounds, with the exception of the nitrophenyl derivative
12, undergo first a quasi-reversible or non-reversible reduction step between 1.6 and
1.8 V, that is centered on the photoactive DHA moiety. The conjugates are reduced
at ca. 300 mV more anodic potential than 1, which is readily explained by the electron-
withdrawing properties of the DHA-appended TEE group. With the exception of
DMA derivative trans-11a, all TEE-DHA conjugates undergo a first oxidation step at
ca. ‡ 1.10 V. This corresponds to a small cathodic shift of ca. 80 mV (compared to 1)
and is against expectations, considering the electron-withdrawing properties of the
TEE moiety. However, the irreversible oxidations avoid any direct comparison of redox
potentials and could therefore explain the observed potential shifts.
The first reduction step in compound 12 occurs at 1.37 V and is centered on the
nitrophenyl moiety. This potential corresponds to those of related 4-nitrophenyl
substituted TEE derivatives [27]. Similarly, the reversible, DMA-centered oxidation of
trans-11a at ‡ 0.40 V is in the range of the values measured for other DMA-substituted
TEEs [27]. The DMA-centered oxidation, therefore, is easier to perform than the one

768
Helvetica Chimica Acta ± Vol. 84 (2001)
Table 5. Cyclic Voltammetry (CV) and Linear-Sweep Voltammetry (LSV) Data Measured in CH2Cl2 ‡ 0.1m
‡
Bu4NPF6. All potentials vs. Fc/Fc . Values shown in italics correspond to redox potentials of remaining DHA
starting material in the VHF isomer obtained by photoisomerization.
Starting
material
DHA
VHF
CV
LSV
CV
LSV
b
c
d
b
c
d
E8a)
[V]
DEp
)
Ep
)
E1/2
[V]
)
Slopee) E8a)
[mV] [V]
DEp
)
Ep
)
E1/2
[V]
)
Slopee)
[mV]
[mV]
[V]
[mV]
[V]
1
1.97
2.07
1.97
74
1.47
1.48
111
‡ 1.16
‡ 1.18
1.83
1.99
‡ 0.40
‡ 1.00
1.39
135
60
65
60
75
75
60
‡ 0.87
‡ 0.84
60
trans-11a
1.80
1.94
‡ 0.40
130
55
80
12
13
14
1.37
1.55
70
70
1.38
1.65
‡ 0.84
70
80
60
1.41
2.06
‡ 0.86
90
125
60
1.57
2.04
‡ 1.08
75
180
1.76
1.92
‡ 1.05
1.58
1.42
1.77
1.92
1.56
150
‡ 1.00
75
‡ 0.85
‡ 0.95
1.69
1.71
60
95
1.71
1.89
60
60
1.37
1.85
2.28
1.69
80
1.82
2.27
‡ 1.07
‡ 1.08
‡ 1.44
70
‡ 1.37
‡ 0.82
‡ 1.06
15
1.75
1.93
‡ 1.10
73
70
80
1.34
1.71
1.84
‡ 0.82 110
93
96
80
1.38
1.76
1.93
65
75
80
60
96
1.89
‡ 1.15
‡ 0.83
‡ 1.13
‡ 1.11
a) Formal redox potential E8 ˆ (Epa ‡ Epc)/2. b) DEp ˆ Eox Ered (at n ˆ 100 mV s 1), where subscripts ox and
red refer to the conjugated oxidation and reduction steps, respectively. c) Peak potential Ep for irreversible
d) Half-wave potential E1/2
.
e) Slope of the linearized plot of E vs. log[I/
1
electron transfer at v ˆ 100 mV s
.
(Ilim I)].
of the photoactive DHA moiety. A DHA-ferrocene conjugate has previously been
reported that is also oxidized first at the ferrocene moiety rather than at the
photoactive DMA moiety. Interestingly, this compound, similar to trans-11a, does not
photoisomerize to the VHF compound [35b].
The solutions of the TEE-DHA conjugates trans-11a and 12 ± 15 were irradiated at
365 nm and the electrochemical data of the corresponding VHF forms measured. As
the conversion to the VHF isomers was not complete, weak signals corresponding to
the reduction steps of the DHA starting materials remained in some of the cyclic and
linear-sweep voltammograms (potentials shown in italics in Table 5). As expected from
the findings for 4, reversible reduction steps were observed at a more positive potential
compared to the DHA forms, while the oxidations take place at more negative

Helvetica Chimica Acta ± Vol. 84 (2001)
769
potential. Considering these observations, the DHA/VHF moiety can be described as a
photoswitchable p-electron acceptor. In the CV measured upon irradiation of the
nitrophenyl derivative 12, the first reduction potential at 1.38 V corresponds to the
overlapped signals resulting from the reversible reduction of the nitrophenyl group and
the irreversible reduction of the VHF moiety.
No data are available for trans-11b as only poorly resolved voltammograms were
obtained after addition of CF3COOH to trans-11a.
A series of spectroelectrochemical measurements on solutions of 1 and 15 was
performed in the dark to explore whether the ring opening of the DHA moiety to give
the VHF form can be forced by electrochemical oxidation or reduction. All redox steps
were irreversible in accordance with the CV measurements. The final spectra were
always different from the starting ones, but evidence for an electrochemical ring
opening of the DHA moiety could not be obtained. For compound 1, electronic
absorption spectra with isosbestic points and a new longest-wavelength absorption
band at lmax ˆ 482 nm were recorded during the oxidation process (Fig. 20). Despite
the similarity between the spectrum obtained upon oxidation and the one measured for
VHF isomer 4 (Fig. 6), the newly formed species lacks the electrochemical character-
Fig. 20. Spectroelectrochemical measurement in CH2Cl2. a) Oxidation of 1 at ‡1.55 Vand b) reduction of the so-
obtained species at 0 V. c) Oxidation of 15 at ‡ 1.6 V and d) reduction of the so-obtained species at 0 V.
Potentials given vs. Ag/AgCl.

770
Helvetica Chimica Acta ± Vol. 84 (2001)
istics of a VHF derivative. Also in the case of 15, an electrochemical DHA ! VHF ring
opening did not take place. There exists, indeed, no precedence for such a process in the
literature.
3. Conclusions. ± This paper describes the synthesis, X-ray structural character-
ization, and exploration of the photophysical and electrochemical properties of a novel
series of molecular photoswitches based on TEE-DHA hybrid systems. The DHA !
VHF photoisomerization and the corresponding thermal back reaction in these
conjugates were found to be much faster than the fully light-driven trans/cis-
isomerization of the TEE moiety. The electrochemical studies clearly demonstrated
that the DHA/VHF chromophore is a photoswitchable p-electron-acceptor moiety.
Attempts to achieve the DHA ! VHF retro-electrocyclization by electrochemical
oxidation or reduction were not successful. Most of the TEE-DHA hybrid systems
discussed in this paper (1, 12 ± 16, 18) represent simple molecular devices with the
ability to perform a basic write/erase process with convenient optical readouts. On the
other hand, DMA derivative 11 is an example of a complex, multifunctional switchable
device, capable of undergoing three individual, reversible switching cycles, namely i)
photochemical trans/cis-isomerization of the TEE core, ii) DHA ! THF photoiso-
merization and thermal back reaction, and iii) reversible protonation of the DMA
group. Of the eight possible states, six were detected and several individual
interconversion processes could be cleanly addressed, providing three write/erase
processes and one model for an electronic AND gate. The aim is now to incorporate
these systems into macroscopic devices able to perform real-life tasks. A first step has
been achieved with the reversible structuring of polymeric waveguides doped with the
simple molecular photoswitch 1 [22]. We are now further exploring the potential of
TEE derivatives and conjugates to function as fully light-driven molecular switches in
reversible information-storage devices.
Experimental Part
General. All reactions were carried out under N2. Solvents and reagents were reagent-grade and
commercially available and used without further purification unless otherwise stated. Compounds 6, 7, 8 [15c],
and 19 [24] were prepared according to literature procedures. THF and Et2O were freshly distilled from sodium
benzophenone ketyl. BuLi solns. in hexane were titrated with 2,4-dimethoxybenzyl alcohol in THF [36].
Evaporation in vacuo was conducted at H2O-aspirator pressure. Degassing of solvents was accomplished by
three freeze-thaw pump cycles. Column chromatography (CC): SiO2-60 (230 ± 400 mesh, 0.040 ± 0.063 mm) from
Fluka. M.p.: Büchi 510 or Büchi B-540, uncorrected. TLC: glass-backed Polygram SiO2-60 UV254 from
Macherey-Nagel, visualization by UV light (254 or 366 nm) or with a molybdenum-stain soln. (400 ml 10% aq.
H2SO4 soln., 20 g (NH4)6Mo7O24 ´ 6 H2O, 0.6 g Ce(SO4)2). UV/VIS: Varian Cary 5 UV/VIS/NIR spectrometer at
r.t.; lmax in nm (e in m 1 cm 1). IR Spectra (cm 1): Perkin-Elmer 1600-FT IR. NMR Spectra: Bruker AMX-500 or
AMX-400, and Varian Gemini 300 or 200 at 296 or 300 K, with solvent peak as reference. MS (m/z (%)); EI-MS:
VG TRIBRID spectrometer at 70 eV; FAB-MS: VG ZAB2-SEQ spectrometer with 3-nitrobenzyl alcohol
(NOBA) as matrix. Elemental analyses were performed by the Mikrolabor at the Laboratorium für Organische
Chemie, ETH-Zürich.
Electrochemistry. CH2Cl2 was purchased spectroscopic grade from Merck, dried over molecular sieves
(4 Š), and stored under Ar prior to use. Bu4NPF6 was purchased electrochemical grade from Fluka and used as
received.
The electrochemical experiments were carried out at 20 Æ 28 in CH2Cl2 containing 0.1m Bu4NPF6 in a
classical three-electrode cell. The working electrode was a glassy carbon disk electrode used either in motionless

Helvetica Chimica Acta ± Vol. 84 (2001)
771
1
mode for CV (10 mV s to 10 V s 1) or as rotating-disk electrode for LSV. The electrochemical cell was
connected to a computerized multipurpose electrochemical device DACFAMOV (Microtec-CNRS, Toulouse,
France) interfaced with an Apple II microcomputer. All potentials are referenced to the ferrocene/ferricinium
‡
(Fc/Fc ) couple used as an internal standard. The auxiliary electrode was a Pt wire, and an Ag wire was used as a
‡
pseudo-reference electrode. The accessible range of potentials was ‡ 1.2 to
2.2 V vs. Fc/Fc in CH2Cl2.
Spectroelectrochemical measurements were performed in a thin-layer cell (0.1 mm) through an optically
transparent thin-layer electrode (OTTLE) made of a Pt mini-grid (1000 mesh). The auxiliary electrode was a Pt
wire, and an aq. Ag/AgCl electrode was used as the reference. The OTTLE cell was placed in a Hewlett Packard
diode-array UV/VIS spectrometer model 8452A. The OTTLE cell was connected to a Bruker potentiostat
(model EI30M) for control and measurement of the electrochemical parameters.
Å
X-Ray Crystal Structure of 1. Crystal data at 293 K for (C18H11IN2) (Mr 382.20): triclinic, space group P1
(No. 2), Dc ˆ 1.679 gcm 3, Z ˆ 2, a ˆ 9.243(2), b ˆ 9.519(2), c ˆ 10.527(3) Š, a ˆ 73.89(2)8, b ˆ 65.77(2)8, g ˆ
64.35(2)8, Vˆ 755.7(3) Š3. Syntex P21 diffractometer, MoKa radiation, l ˆ 0.71073 Š. The structure was solved
by direct methods (SHELXTL PLUS) and refined with 191 parameters by full-matrix least-squares analysis
based on F2 and experimental weights; all heavy atoms were refined anisotropically and H-atoms fixed
isotropically with positions calculated from stereochemical considerations. Final R(F) ˆ 0.025 for 1304 observed
reflections with I > 2s(I) and wR(F2) ˆ 0.067 for all 1422 data. Cambridge Crystallographic Data Centre
deposition No. CCDC-151574.
X-Ray Crystal Structure of cis-11a. Crystal data at 293 K for (C54H63N3Si2) (Mr 810.29): monoclinic, space
group P2(1)/c (No. 14), Dc ˆ 1.065 gcm 3, Z ˆ 4, a ˆ 29.97(3), b ˆ 7.764(7), c ˆ 23.58(2) Š, b ˆ 113.11(6)8, Vˆ
5046(9) Š3. Picker-Stoe diffractometer, CuKa radiation, l ˆ 1.54178 Š. The structure was solved by direct
methods (SHELXTL PLUS) and refined with 547 parameters by full-matrix least-squares analysis based on F2
and experimental weights; all heavy atoms were refined anisotropically, H-atoms fixed isotropically with
positions calculated from stereochemical considerations. Final R(F) ˆ 0.079 for 2294 observed reflections with
I > 2s(I) and wR(F2) ˆ 0.215 for all 3072 data. Cambridge Crystallographic Data Centre deposition No. CCDC-
151573. For the X-ray crystal structure of trans-11a, see [17] and Cambridge Crystallographic Data Centre
deposition No. CCDC-102924.
X-Ray Crystal Structure of 12. Crystal data at 295 K for (C52H57N3O2Si2) (Mr ˆ 812.19): triclinic, space
group P1 (No. 1), Dc ˆ 1.114 gcm 3, Z ˆ 1, a ˆ 7.409(1) Š, b ˆ 13.608(1) Š, c ˆ 13.616(2) Š, a ˆ 66.27(1)8, b ˆ
75.01(1)8, g ˆ 87.74(1)8, Vˆ 1270.7(3) Š3. Nonius CAD4 diffractometer, MoKa radiation, l ˆ 0.7107 Š. Single
crystals were obtained by slow evaporation of a MeCN/CH2Cl2 soln. The structure was solved by direct methods
(SHELXS-86) [37] and refined by full-matrix least-squares analysis (SHELXL-93) [38], with an isotropic
extinction correction and w ˆ 1/[s2(Fo2 ) ‡ (0.143P)2 ‡ 0.047P], where P ˆ (Fo2 ‡ 2Fc2 )/3. All heavy atoms were
refined anisotropically (H-atoms isotropic, whereby H-positions are based on stereochemical considerations).
Final R(F) ˆ 0.055, wR(F2) ˆ 0.149 for 583 parameters, 3 restraints, and 2693 reflections with I > 2s(I) and q <
23.968. Cambridge Crystallographic Data Centre deposition No. CCDC-151798.
X-Ray Crystal Structure of 13. Crystal data at 213 K for (C53H60N2OSi2) (Mr 797.21): triclinic, space group
P1 (No. 2), Dc ˆ 1.097 g cm 3, Z ˆ 2, a ˆ 7.905(4) Š, b ˆ 16.044(7) Š, c ˆ 20.097(4) Š, a ˆ 104.96(3)8, b ˆ
Å
94.16(2)8 g ˆ 99.28(3)8, Vˆ 2413(2) Š3. Nonius CAD4 diffractometer, CuKa radiation, l ˆ 1.5418 Š. Single
crystals were obtained by slow evaporation of a MeCN/CH2Cl2 soln. The structure was solved by direct methods
(SHELXS-86) [37] and refined by full-matrix least-squares analysis (SHELXL-93) [38], with an isotropic
extinction correction and w ˆ 1/[s2(Fo2 ) ‡ (0.163P)2 ‡ 4.329P], where P ˆ (F2o ‡ 2F2c )/3. The six-membered ring
with the MeO group (C(27) to C(34)) and the (i-Pr)3Si group (Si(2) to C(45)) are disordered over two
orientations. In Fig. 5, only one orientation is shown for clarity. The disordered atoms C(27) to C(34) were
refined anisotropically with population parameters (pp) of 0.5, except that of C(34), which was set to 1.0. The
disordered atoms Si(2) to C(45) were refined isotropically with pp of 0.5. The C C bond lengths within the
disordered six-membered ring were restrained to ca. 1.39 Š, those within the disordered (i-Pr)3Si group to ca.
1.52 Š. The remaining heavy atoms were refined anisotropically (H-atoms of the ordered skeleton isotropically,
whereby H-positions are based on stereochemical considerations). Final R(F) ˆ 0.093, wR(F2) ˆ 0.273 for 597
parameters, 36 restraints, and 3538 reflections with I > 2s(I) and q < 49.978. Cambridge Crystallographic Data
Centre deposition No. CCDC-151797.
Copies of the crystallographic data (excluding structure factors) for all new structures presented here can
be obtained, free of charge, on application to the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB21EZ, UK (fax: ‡ 44(1223)336033; e-mail: deposit@ccdc.cam.ac.uk).
General Protocol for the DHA ! VHF and trans-11a/cis-11a Photoisomerizations. The isomerization
reactions in spectral-grade CH2Cl2 (typical concentrations c ꢁ 2 ´ 10 5 m) were executed at 258 in a 1-cm quartz

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Helvetica Chimica Acta ± Vol. 84 (2001)
cell and the conversions monitored with a Varian Cary 5 UV/VIS/NIR spectrometer. The cell was irradiated with
monochromatic light from a Spex Fluorolog 1680 0.22m Double Spectrometer. For irradiations at 366 nm, a
sterilAir BLB-8 UV lamp for the visualization of TLC plates was used, and the light was not monochromatic.
General Protocol for the VHF ! DHA Electrocyclizations. The quartz cells containing the irradiated solns.
were kept in the dark at 258, and the absorption spectra were recorded at regular intervals on a Varian Cary 5
UV/VIS/NIR spectrometer.
2-(4-Iodophenyl)-8aH-azulene-1,1-dicarbonitrile (1). A soln. of tritylium tetrafluoroborate (6.09 g,
18.4 mmol) in 1,2-dichlorethane (100 ml) was added via cannula to 3 (6.75 g, 17.6 mmol) in 1,2-dichlorethane
(25 ml) under N2. The mixture was stirred for 1 h at 808, then poured onto PhMe (400 ml). After cooling the red
soln. to 08, Et3N (2.69 ml, 19.3 mmol) was added dropwise and the temp. was raised over 1 h to 208. Finally, the
mixture was stirred for 1 h at 808 under exclusion of light. CC (short plug of SiO2-60; CH2Cl2/hexane 1 :1)
followed by recrystallization (hexane) afforded 1 (4.71 g, 70%). Yellow powder. M.p. 1438. UV/VIS (CH2Cl2):
277 (16200), 365 (21400). IR (KBr): 3056w, 2889w, 1484m, 1005m, 824s, 812s, 707s. 1H-NMR (200 MHz, CDCl3):
3.78 ± 3.81 (m, 1 H); 5.82 (dd, J ˆ 10.0, 3.7, 1 H); 6.27 ± 6.39 (m, 2 H); 6.46 ± 6.63 (m, 2 H); 6.91 (s, 1 H); 7.47 (d,
J ˆ 8.7, 2 H); 7.82 (d, J ˆ 8.7, 2 H). 13C-NMR (125 MHz, CDCl3): 45.02; 51.03; 96.20; 112.51; 114.90; 119.42;
‡
121.52; 127.60; 127.73; 129.91; 130.84; 131.19; 132.94; 138.32; 138.41; 139.05. EI-MS: 382.1 (78, M ), 355.0 (31,
‡
‡
‡
[M HCN] ), 255.2 (100, [M I] ), 227.1 (71), 201.1 (27), 127.1 (12, I ), 100.1 (33). Anal. calc. for C18H11N2I
(382.20): C 56.57, H 2.90, N 7.33; found: C 56.56, H 2.90, N 7.36. X-Ray, see Fig. 2.
2-(2,4,6-Cycloheptatriene-1-yl)-1-(4-iodophenyl)ethanone (2). To a suspension of tropylium tetrafluorobo-
rate (3.94 g, 22.2 mmol) in MeOH (40 ml) were added 4-iodoacetophenone (6.00 g, 24.4 mmol) and AcOH
(0.8 ml), and the resulting mixture was stirred for 6 h at 208. H2O (200 ml) was added, and the mixture was
extracted with Et2O (3 Â 150 ml). Drying of the combined org. phases (MgSO4) and CC (SiO2-60 (700 g);
CH2Cl2/hexane 2 :1) afforded 2 (3.75 g, 50%). Yellowish, needle-shaped crystals (Et2O). M.p. 768. IR (KBr):
3022w, 3002w, 2889w, 2856w, 1686s, 1577s, 1390m. 1H-NMR (200 MHz, CDCl3): 2.46 ± 2.53 (m, 1 H); 3.29 (d, J ˆ
7.5, 2 H); 5.28 (dd, J ˆ 9.5, 5.8, 2 H); 6.18 ± 6.26 (m, 2 H); 6.65 ± 6.68 (m, 2 H); 7.67 (d, J ˆ 8.5, 2 H); 7.84 (d, J ˆ
8.5, 2 H). 13C-NMR (50 MHz, CDCl3): 34.68; 41.22; 101.03; 125.06; 125.25; 129.48; 131.00; 136.17; 137.92; 197.98.
‡
‡
‡
EI-MS: 336.1 (62, M ), 231.0 (100, [IPhCO] ), 91.1 (58, [C7H7] ). Anal. calc. for C15H13OI (336.17): C 53.59, H
3.90; found: C 53.32, H 3.94.
[2-(2,4,6-Cycloheptatriene-1-yl)-1-(4-iodophenyl)ethylidene]propanedinitrile (3). From 2: In a flask
equipped with a Dean-Stark trap, 2 (1.64 g, 4.88 mmol) and malononitrile in PhMe (50 ml) were heated to
reflux for 8 h while a soln. of NH4OAc (5 g, 64.8 mmol) in AcOH (15 ml) was added portionwise. After cooling
to 208 and addition of PhMe (100 ml), the mixture was extracted with H2O (3 Â 100 ml). Drying (MgSO4) and
CC (SiO2-60 (190 g); CH2Cl2/hexane 2 :1) afforded 3 (1.42 g, 75%).
From 5: To a soln. of 5 (5.02 g, 17.1 mmol) in CH2Cl2 (170 ml) was added solid tropylium tetrafluoroborate
(3.64 g, 20.5 mmol). After sonication to form a homogeneous suspension, the mixture was cooled to 788 and
Et3N (2.62 ml, 18.8 mmol) was added dropwise. After stirring for 1 h at 788, 1m HCl was added, and the org.
phase was washed with H2O. Drying (MgSO4) afforded 3 (6.83 g, 100%). Red-brown oil. IR (neat): 3022m,
2933m, 2856m, 2222m, 1656s, 1578s, 1117s. 1H-NMR (200 MHz, CDCl3): 1.92 ± 2.07 (m, 1 H); 3.17 (d, J ˆ 8.3, 2
H); 5.16 (dd, J ˆ 9.1, 6.6, 2 H); 6.19 ± 6.27 (m, 2 H); 6.62-6.75 (m, 2 H); 7.15 (d, J ˆ 8.7, 2 H); 7.85 (d, J ˆ 8.7, 2 H).
13C-NMR (50 MHz, CDCl3): 37.60; 38.62; 86.87; 98.87; 112.17; 122.55; 126.55; 128.65; 131.16; 133.92; 138.49;
‡
‡
176.84. EI-MS: 384.0 (47, M ), 294.0 (52, [M C7H6] ), 255.1 (43), 166.1 (75), 152.1 (46), 139.0 (89), 127.0 (26,
‡
‡
‡
‡
I ), 91.0 (100, [C7H7] ). HR-EI-MS: 384.0121 (M , C18H13N2I ; calc. 384.0125).
[1-(4-Iodophenyl)-2-(2,4,6-cycloheptatriene-1-ylidene)ethylidene]propanedinitrile (4). Compound
1
(556 mg, 1.45 mmol) was dissolved in a minimum amount of boiling hexane, and the soln. was stirred and
irradiated at 366 nm at 208 for 6 h. Orange coloration and the formation of black crystals on the flask walls were
observed. The crystals were collected and washed with hexane, affording 4 (491 mg). Further irradiation
afforded more product (15 mg; total yield: 506 mg, 91%). A total of 40 mg (7%) of starting material 1 was
recovered from the soln. Black crystalline solid. M.p. 1168 (with decoloration). UV/VIS (CH2Cl2): 478 (33300).
IR (KBr): 2957w, 2922w, 2009s, 1628w, 1512s, 1466m, 1437m, 1374m, 1344s, 1256s. 1H-NMR (200 MHz, CDCl3):
5.87 ± 6.06 (m, 2 H); 6.29 ± 6.48 (m, 3 H); 6.31 (s, 1 H); 6.66 ± 6.74 (m, 1 H); 7.15 (dm, J ˆ 8.7, 2 H); 7.84 (dm, J ˆ
‡
‡
‡
8.7, 2 H). EI-MS: 382.1 (100, M ), 355.1 (36, [M HCN] ), 255.1 (94, [M I] ), 227.1 (57), 201.1 (19). Anal.
calc. for C18H11N2I (382.20): C 56.57, H 2.90, N 7.33; found: C 56.46, H 2.91, N 7.28.
2-[1-(4-Iodophenyl)ethylidene]propanedinitrile (5). In a flask equipped with a Dean-Stark trap, a soln. of p-
iodoacetophenone (5.00 g, 20.3 mmol) and malononitrile (2.68 g, 40.6 mmol) in PhH (200 ml) was heated to
reflux for 3 d. A soln. of NH4OAc (5 g, 64.8 mmol) in AcOH (15 ml) was added in portions during this time.
After cooling to 208, the soln. was diluted with Et2O (200 ml) and washed with H2O (3 Â 200 ml) and sat. aq.