Photoinduced intramolecular charge separation in donor/acceptor-substituted bicyclohexylidene and bicyclohexyl

Article abstract of DOI:10.1002/1521-3765(20000818)6:16<2948::AID-CHEM2948>3.0.CO;2-0

The photophysical properties of a bicyclohexylidene (1DA) and a bicyclohexyl (2DA) substituted with an anilino electron donor and a dicyanoethylene electron acceptor have been studied. Quenching of local donor emission is observed for these compounds as w

Full text of DOI:10.1002/1521-3765(20000818)6:16<2948::AID-CHEM2948>3.0.CO;2-0

FULL PAPER
Photoinduced Intramolecular Charge Separation in
Donor/Acceptor-Substituted Bicyclohexylidene and Bicyclohexyl
Frans J. Hoogesteger,[a] Cornelis A. van Walree,[a] Leonardus W. Jenneskens,*[a]
Martin R. Roest,[b] Jan W. Verhoeven,[b] Wouter Schuddeboom,[c] Jacob J. Piet,[c] and
John M. Warman[c]
Abstract: The photophysical properties
of a bicyclohexylidene (1DA) and a
bicyclohexyl (2DA) substituted with an
anilino electron donor and a dicyano-
ethylene electron acceptor have been
studied. Quenching of local donor emis-
sion is observed for these compounds as
well as quenching of the ªpseudo-localº
acceptor emission. Transient absorption
spectra show dialkylanilino-type radical-
cation and dicyanoethylene-type radi-
cal-anion absorptions. These results
show that intramolecular charge separa-
tion takes place in 1DA and 2DA. This
was corroborated by time-resolved mi-
crowave conductivity measurements
from which large excited-state dipole
moments were found for both 1DA and
2DA. Time-resolved fluorescence spec-
troscopy revealed that in the charge-
separated state in cyclohexane for 2DA,
molecular folding takes place on a nano-
second timescale. For 1DA in cyclo-
hexane, either charge separation takes
place in a (fully) folded conformation or
very rapid (subnanosecond timescale)
folding takes place subsequent to charge
separation. In addition to this difference
in conformational behavior, the pres-
ence of the exocyclic double bond be-
tween the cyclohexyl-type rings results
in efficient quenching of the anilino
donor triplet state and acceleration of
the charge recombination rate by a
factor of 20.
Keywords: electron transfer ´ laser
spectroscopy ´ synthetic methods ´
through-bond interactions
connecting saturated hydrocarbon spacers has been studied.[5]
It has been shown that no direct spatial overlap between the
donor and acceptor wavefunctions is necessary for the
occurrence of charge transfer, since the intervening bridge
has a rigid character in several of these systems. In these cases,
the interaction between D and A is a result of through-bond
orbital interactions. The concept of through-bond interactions
(TBI) was introduced in 1968 by Hoffmann et al. and
describes the intramolecular interactions between functional
groups non-conjugatively linked by orbital overlap.[6] Exper-
imental evidence for TBI has been obtained from photo-
electron and electronic spectroscopy as well as structural[7]
and chemical studies.[8]
Bi- and oligo(cyclohexylidenes)[9] are interesting candidates
for use as spacers in DSA compounds, since their alternating
s ± p ± s orbital topology allows the study of the influence of
formally nonconjugated double bonds in the bridge on
photoinduced charge-transfer phenomena. Ab initio calcula-
tions have shown that the p bonds in oligo(cyclohexylidenes)
are mutually weakly coupled by orbital interactions.[10] They
are also sufficiently easy to oxidize to render the intervening
bridge ªredox activeº.[10] Thus, the unsaturated entity can be
involved in the charge-separation process in two ways.[2, 11, 12]
Firstly, through TBI it can increase the electronic coupling
between donor and acceptor orbitals by providing relatively
Introduction
The occurrence of photoinduced electron-transfer processes
has attracted the attention of many researchers, since its study
may provide insight into the processes that play a role in the
conversion of solar energy into an electrochemical potential
in photosynthetic systems.[1] In addition, knowledge about
these phenomena is of importance for the rational develop-
ment of new optoelectronic materials that may find an
application in the fields of artificial solar-energy conversion,[2]
nonlinear optics,[3] and molecular electronics.[4] In order to
obtain a better understanding of the factors that govern
charge-transfer processes, charge transfer in donor-spacer-
acceptor (DSA) compounds that contain a variety of inter-
[a] Prof. L. W. Jenneskens, Dr. F. J. Hoogesteger, Dr. C. A. van Walree
Debye Institute, Department of Physical Organic Chemistry
Utrecht University, Padualaan 8, 3584 CH Utrecht (The Netherlands)
Fax : (‡31)302534533
E-mail: jennesk@chem.uu.nl
[b] Dr. M. R. Roest, Prof. J. W. Verhoeven
Laboratory of Organic Chemistry, University of Amsterdam
Nieuwe Achtergracht 129, 1018 WS, Amsterdam (The Netherlands)
[c] Dr. W. Schuddeboom, J. J. Piet, Dr. J. M. Warman
IRI, Delft University of Technology, Mekelweg 15
2629 JB Delft (The Netherlands)
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2948±2959
low-lying, that is, lower than the s* orbitals of the saturated
bridge, empty p* orbitals. Secondly, the unsaturated system
can function as a real anion or cation intermediate in the
electron-transfer process (sequential mechanism). By com-
parison of the photophysical properties of donor/acceptor-
substituted oligo(cyclohexylidenes) with saturated oligo-
(cyclohexyl) analogues the effect of the p bond on electron-
transfer processes can be assessed.
In addition to providing insight into the role of an isolated
double bond in charge-separation processes, functionalized
oligo(cyclohexylidenes) also offer the possibility of obtaining
well-organized optoelectronic materials. It has been shown
that the oligo(cyclohexylidene) building blockgives access to
highly ordered supramolecular structures by using the Lang-
muir± Blodgett technique, self-complementary hydrogen
bonding, and spin-coating.[13] Since this tendency to form
supramolecular structures is related to the geometry of
oligo(cyclohexylidenes), the effect of charge separation on
the conformation of these compounds is of interest. Oligo-
(cyclohexylidenes) in general possess a rodlike geometry[9a]
tigated by applying a variety of techniques, including absorp-
tion spectroscopy, steady-state and time-resolved (nanosec-
ond timescale) fluorescence spectroscopy, transient absorp-
tion (TA) spectroscopy, and time-resolved microwave con-
ductivity (TRMC) measurements. To obtain insight into their
1
conformational behaviour in the ground state, the H NMR
spectra of the synthesized compounds are also addressed. It is
shown that the presence of a double bond in the D ± A-
connecting bicyclohexylidene frameworkhas a profound
influence on both the conformational and charge-transfer
properties of the compounds investigated.
Results and Discussion
Synthesis: The synthesis of compounds 1DA and 2DA is
shown in Scheme 1. Alkylation of the dianion of carboxylic
acid 5 with N-phenylpiperidone[17] 4 afforded b-hydroxy acid
6, which was decarboxylated and dehydrated to yield acetal 7.
Hydrolysis of 7 yielded ketone 8 and subsequent condensation
that formally prevents
a
through-space overlap of the
D and A chromophores. How-
ever, conformational flexibility
of the bridge may give rise to a
more compact (folded) struc-
ture. In a folded ground-state
conformation, through-space
electron transfer may take
place upon excitation (DSA
molecules with a polymeth-
ylene bridge show this behav-
ior).[8, 14] It is also possible that
folding occurs subsequent to
charge transfer in an extended
conformation. This phenomen-
on is the result of a coulombic
attraction between the oppo-
sitely charged chromophores at
both end-positions of the mol-
Scheme 1. Synthetic scheme for 1DA and 2DA. Reagents: a) BuLi, (iPr)2NH; b) Me2NCH(OCH2CMe3)2,
CH3CN; c) H3O , THF; d) H2C(CN)2, C6H6; H2, Pd/C, THF.
‡
ecule and is operative in media that have little interaction with
the charge-separated state (gas phase or nonpolar solvents).
This so-called harpooning mechanism[15] has been observed
for certain piperidine-bridged DSA systems.[16]
In this paper the photophysical properties of a bicyclohex-
ylidene (1DA) and a bicyclohexyl (2DA) that contain
electron-donating anilino groups and electron-accepting di-
cyanoethylene groups are described. In addition, model donor
compounds (1D; 2D) and model acceptor compounds (1A;
2A) are studied. The photophysical properties were inves-
with malononitrile afforded 1DA. Saturated compound 2DA
was obtained by catalytic hydrogenation of 7, followed by
hydrolysis to ketone 10, and condensation with malononitrile.
The synthesis of donor model compound 1D is straightfor-
ward starting from 4 and cyclohexanecarboxylic acid. Cata-
lytic hydrogenation of 1D afforded 2D. Acceptor models 1A
and 2A were obtained by Knoevenagel condensation of
malononitrile and 1,1'-bicyclohexyliden-4-one[9] and 1,1'-bicy-
clohexyl-4-one,[18] respectively. All donor/acceptor com-
pounds possess high melting points and low solubilities in
common organic solvents.[19]
Ground-state conformational properties of 1DA and 2DA: In
principle, due to the possibility of ring inversion, several
conformations are accessible for compounds that contain
cyclohexyl-type rings. Unfortunately, crystals of the donor/
acceptor compounds suitable for X-ray diffraction were not
available. However, the single-crystal X-ray structure of
ketone 8 has been reported.[20] In this compound the cyclo-
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L. W. Jenneskens et al.
FULL PAPER
hexyl-type rings adopt a chair conformation and the phenyl
ring occupies an equatorial position. In the two crystallo-
graphically independent molecules in the unit cell the phenyl
ring appears to be twisted with respect to the piperidine type
ring by 40 and 508.
2A) model compounds are shown in Figure 1. Absorption
maxima and molar absorption coefficients are summarized in
Table 1.
1H NMR spectroscopy reveals that important conforma-
tional differences exist between the bicyclohexylidene (series
1) and the bicyclohexyl derivatives (series 2) in solution. The
1H NMR spectrum of 1D shows a single triplet at d ˆ 3.22 for
the four protons adjacent to the nitrogen atom. This indicates
that they are isochronous at room temperature and that their
neighboring protons are also magnetically equivalent. Indeed,
a triplet is also found for the allylic protons (d ˆ 2.46) of the
nitrogen-containing ring. This provides evidence that both
cyclohexyl-type ring inversions and nitrogen inversion are
rapid on the NMR timescale. A similar conformational
mobility behavior is found for 1A; the four protons adjacent
to the dicyanomethylene substituent and the four allylic
neighboring protons appear as triplets in the 1H NMR
spectrum. For 1DA, the aliphatic part of the 1H NMR
spectrum shows patterns for the allylic protons similar to
those of 1D and 1A. Hence, it is concluded that the
compounds containing exocyclic double bonds (1DA, 1D,
and 1A) are conformationally flexible at room temperature
on the NMR timescale.
1
The H NMR spectra of series 2 are much more complex
than those of series 1. Distinct signals are discernible for
equatorial and axial protons of the cyclohexyl-type rings. For
2DA, equatorial and axial protons adjacent to the nitrogen
atom resonate at d ˆ 3.72 and 2.62, respectively. The upfield
shift of the axial protons with respect to the equatorial ones is
a consequence of the axial orientation of the nitrogen lone
pair. This indicates that the phenyl group occupies an
equatorial position in 2DA.[21] Thus, the single bond that
connects the two cyclohexyl-type rings effectively locks the
conformation of the cyclohexyl rings and the equatorial
position of the phenyl group. This observation also applies to
2A; at room temperature it possesses a frozen conformation
on the NMR timescale.
Figure 1. UVabsorption spectra of A) 1DA, 1D and 1A and B) 2DA, 2D
and 2A. All spectra were recorded in cyclohexane.[26]
Table 1. Absorption maxima and molar extinction coefficients
e (in
parentheses) of DA compounds (1DA, 2DA), donor model compounds
(1D, 2D), and acceptor model compounds (1A, 2A) in cyclohexane at
208C.
lmax [nm] (e [103 mÀ1 cmÀ1])
The difference in conformational behavior between series 1
and 2 must find its origin in the presence of an sp2-hybridised
bridging carbon atom in the series 1. Upon chair± chair
interconversion, in the bicyclohexylidene system only the
phenyl group moves to an axial position, whereas in the
bicyclohexyl series both a phenyl and a cyclohexyl ring are
forced to adopt an axial position. The latter geometry is
energetically highly unfavorable, and the equilibrium will be
situated at the equatorial ± equatorial conformation. Another
factor that enhances the conformational mobility of the series
1 is the lowering of the barrier for chair± chair interconver-
sion by an sp2-hybridized carbon atom by about 2.5 kcalmolÀ1,
which is due to a reduction in the number of repulsive
1DA
2DA
1D
2D
1A
203 (39.8)
203 (20.6)[a]
210 (22.9)
203 (18.1)
212 (16.0)
251 (26.8)
245 (25.0)[a]
255 (15.5)
255 (13.3)
229 (16.2)
235 (14.9)
286 (5.7)
289 (2.3)[a]
289 (1.8)
289 (1.6)
274 (4.9)
2A
[a] Absorption maxima were measured in cyclohexane, while molar
Compound 2A shows a single absorption at 235 nm, which
is attributed to a p ± p* transition of the dicyanoethenyl
moiety. Compound 1A possesses an additional band at
274 nm. This transition is absent in the spectra of both
dicyanomethylenecyclohexane and 1,1'-bicyclohexylidene
and was identified previously as an intramolecular charge-
transfer (CT) absorption, which involves charge transfer from
the ethylenic double bond to the dicyanoethenyl acceptor.[23]
The absorption spectrum of the saturated donor model 2D
is similar to that of N-phenylpiperidine.[24] It consists of three
bands positioned at 289, 255 and 203 nm. The 289 nm band is
nonbonded H H interactions.[22] It should be emphasized that
À
the observed difference in conformational mobility in the
ground state may have important consequences with respect
to geometrical changes after photoexcitation (vide infra).
Absorption spectra: The UV absorption spectra of
1DA, 2DA and their donor (1D, 2D) and acceptor (1A,
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Through-Bond Interactions
2948±2959
the HOMO ± LUMO transition, that is, it is comparable to the
1Lb transition in benzene. The absorption at 255 nm has been
attributed to an intramolecular CTabsorption, which involves
an interaction between the benzene and amino moieties.[25]
The absorption spectrum of 1D is similar to the spectrum of
2D. However, in 1D the anilino-type band near 205 nm
overlaps with the p ± p* transition of the exocyclic double
bond, which is situated at 206 nm in bicyclohexylidene.[10]
Accordingly, the absorption of 1D in the 200 nm region is
more intense.
The spectra of 1DA and 2DA have a strong resemblance to
the sum spectra of 1D, 1A and 2D, 2A, respectively.[26] No
additional absorption bands are present in the spectra of the
donor/acceptor compounds. This indicates that no significant
coupling between the N-phenyl donor and the ethylenic
double bond and/or the dicyanoethylene acceptor occurs in
the ground state or Franck± Condon excited state. The molar
absorption coefficient of the long-wavelength absorption near
290 nm increases from 2300 for 2DA to about 5700mÀ1 cmÀ1
for 1DA; this is due to the additional absorption of the
double-bond/acceptor system in the latter (cf. 1A). Since in
most of the time-resolved experiments discussed below an
excitation wavelength of 308 nm was used, both the donor and
ªacceptorº chromophore will be excited in 1DA, whereas in
2DA only the dialkylanilino donor will be excited.
Figure 2. Steady-state emission spectra of A) 1DA, 1D and 1A and B)
2DA, 2D and 2A. All spectra were recorded in benzene. The insets show
the magnified spectra of 1DA and 2DA.
Steady-state emissive properties: Upon charge separation in
DSA compounds, the fluorescence of the local donor and/or
acceptor chromophores is strongly quenched. In addition,
DSA compounds generally show emission bands that are
absent in the spectra of the separate donor and acceptor
moieties.[17, 23] In general, the CT character of the emissive
state is reflected by a strong dependence of the emission
wavelength on solvent polarity.[27] The fluorescence spectra of
1DA, 2DA, and the model compounds (solvents cyclohexane
and benzene) are presented in Figure 2. Salient results are
collected in Table 2.
Table 2. Fluorescence maxima, fluorescence quantum yields Ffl, and
fluorescence lifetimes t of DA compounds (1DA, 2DA), donor model
compounds (1D, 2D), and acceptor model compound 1A in cyclohexane
and benzene (208C).[a]
cyclohexane
lem [nm] Ffl
benzene
lem [nm] Ffl
t [ns][b]
t [ns][b]
1DA 386, 521
2DA 336, 494
< 0.01, 0.02
0.01, 0.03
0.25
0.25
±
< 1, 20
< 1, 24
2
2
±
525
515
343
343
408
0.01
0.02
0.34
0.35
0.03
< 1, 2
< 1, 36
2
2
1
1D
2D
1A
336
336
±
[c]
Donor model compounds: The emissive properties of 1D and
2D are similar to those reported for N-phenylpiperidine.[24]
A
[a] Excitation wavelength used for quantum yield determination: 1D, 2D
and 1A: 290 nm; 1DA and 2DA: 300 nm. [b] Fluorescence lifetimes
determined from (multi)exponential fits at a number of wavelengths. [c] No
emission observed.
comparison of the emission spectra of 1D and 2D shows no
effect of the exocyclic double bond of the former on either the
position or quantum yield of the fluorescence. The nature of
the solvent has only a limited influence on the position of the
emission maximum, while the fluorescence is somewhat
stronger in benzene.
to rapid radiationless decay and CT fluorescence is not
observed in cyclohexane.
Acceptor model compounds: Acceptor model 2A is non-
fluorescent; this has been rationalized by invoking rapid,
radiationless relaxation accompanied by a twist around the
double bond of the dicyanoethylene chromophore in the
excited state.[28] Compound 1A shows a broad CT-type
emission at 408 nm in benzene. The charge-separated state
is a result of electron transfer from the olefinic double bond to
the dicyanoethylene acceptor.[23] The CT emission is absent in
cyclohexane. In this nonpolar solvent the charge-separated
excited state is only little stabilized, so that it lies close to the
local acceptor singlet excited state. As a consequence of
admixture with this local acceptor state the CT state is subject
Donor/acceptor compounds: The intense local donor emission
of 1D is almost completely absent in the emission spectrum of
1DA in both solvents. The emission of the acceptor/double-
bond system, which was seen to be present in the spectrum of
1A in benzene, is also completely quenched. Instead of these
ªlocalº emission bands, a new, weakand broad emission with
a maximum at 521 nm (2.38 eV; cyclohexane) or 525 nm
(2.36 eV; benzene) is discernible. Similar emission spectra are
obtained for 2DA: local emission from 2D is effectively
quenched and instead a broad emission at 494 nm (2.51 eV;
cyclohexane) or 515 nm (2.41 eV; benzene) is found. These
results point to the occurrence of charge transfer upon
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L. W. Jenneskens et al.
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photoexcitation for 1DA and 2DA. Since it is well known that
fluorescence resulting from a charge-separated state is
strongly dependent on the polarity of the medium, it is
surprising that both 1DA and 2DA have their emission
maxima at approximately the same wavelength in both
cyclohexane and benzene. It is expected that a fully stretched
CT state will be stabilized in benzene by approximately 0.4 eV
with respect to nonpolar cyclohexane.[29] This is clearly not the
case for 1DA and 2DA, which may indicate that in cyclo-
hexane and benzene emission from different excited state
conformers is observed (vide infra).
(Figure 3). The fluorescence maximum initially lies at 440 nm
and gradually shifts towards 500 nm. Single-photon-counting
experiments showed that the decay of the emission at 440 nm
is coupled to the rise of the emission at 500 nm (data not
shown). Hence, upon excitation the initially formed excited
Whether photoinduced charge separation is thermodynam-
ically feasible can be evaluated by application of the Weller
equation, which describes the dependence of the driving force
for charge separation (DGCS) on solvent and donor± acceptor
distance.[30] Values of DGCS in cyclohexane (es ˆ 2.02)[27] are
À0.52 and À0.51 eV for 1DA and 2DA, respectively. In
benzene (es ˆ 2.28),[27] DGCS values are À0.66 for 1DA and
À0.63 eV for 2DA. These data reveal that photoinduced
charge transfer is feasible for the DSA compounds under
investigation, even in the nonpolar solvent cyclohexane. They
support the experimental observation of charge separation in
the form of quenching of local emission and the occurrence of
new CT emission bands.
Figure 3. Time-resolved emission spectrum of 2DA in cyclohexane
following flash-photolysis at 308 nm.
species emits at 440 nm (2.82 eV) and converts into a second
species that emits at 500 nm (2.48 eV). If it is assumed that the
species emitting at 440 nm in cyclohexane is in a stretched
conformation, then its energy should be approximately 0.4 eV
lower in benzene. This is in good agreement with the steady
state emission maximum of 2DA in benzene (515 nm,
2.41 eV). Thus, the emission bands at 440 nm in cyclohexane
and 515 nm in benzene are attributed to a stretched CT
species, whereas the conformation of the finally formed
species in cyclohexane (emitting near 500 nm) is more
compact. Hence, for 2DA a change in molecular geometry
after charge separation is observed, involving folding from an
extended conformation to a compact conformation, which as
a consequence of the larger Coulomb stabilization emits at a
longer wavelength. The fact that in benzene no time depen-
dence of the emission is observed suggests that in this solvent
the stretched charge-separated state is stable with respect to
folding. A similar behavior has been reported for other types
of donor/acceptor compounds bearing saturated six-mem-
bered rings as spacers.[16, 32, 33]
A stretched conformation is also anticipated for the CT
state of 1DA in benzene on the basis of its structural
resemblance to 2DA and the position of its CTemission band
(525 nm, 2.36 eV). Comparison with the position of the CT
emission in cyclohexane (521 nm, 2.38 eV) indicates that in
the latter solvent a different, probably folded, conformation
must be present. However, no time dependence of the
emission wavelength was discernible for 1DA in cyclohexane,
even on the picosecond timescale. This can be rationalized in
two ways. i) Photoinduced charge separation takes place in an
already folded conformation. ii) Folding occurs on a (sub)-
picosecond timescale after charge separation in the stretched
conformation. Both interpretations are in line with the lower
barrier to ring inversion for 1DA compared with 2DA, which
followed from the NMR data. A possible folded structure of
1DA is represented in Figure 4.
Time-resolved fluorescence spectroscopy: Fluorescence life-
times (t) were measured at various wavelengths. The lifetimes
of the donor and acceptor model compounds (Table 2) are in
good agreement with values reported for similar com-
pounds.[23, 31] No significant differences are observed between
the fluorescence lifetimes of 1D and 2D (t ˆ ca. 2 ns). For
acceptor model 1A the CT emission in benzene is also
relatively short-lived (t ˆ ca. 1 ns).
A good fit to the emission decay of 1DA in cyclohexane
was obtained by assuming a biexponential decay with a short-
lived (t < 1 ns) and a long-lived component (t ˆ 20 ns). In
benzene, however, only two short-lived components are found
(t < 1 ns, 2 ns). For 2DA in cyclohexane lifetimes were similar
to those of 1DA (t < 1 ns, 24 ns), whereas in benzene a long
component of 36 ns is measured for the CT-type emission. For
both 1DA and 2DA the short-lived components are more
prominent at short wavelengths and, hence, they are attrib-
uted to residual local donor fluorescence. Thus, in benzene a
large difference between the lifetimes of the CT emission is
observed (2 and 36 ns, respectively). Apparently, deactivation
of the CT excited state in the compound with the bicyclohex-
ylidene bridge (1DA) is approximately 20 times faster than in
the compound with the bicyclohexyl bridge (2DA). In
cyclohexane, however, the lifetimes of the CT emission of
1DA and 2DA are of similar magnitude (20 and 24 ns,
respectively).
The large difference between the fluorescence lifetimes of
1DA in benzene and cyclohexane again suggests the presence
of different conformational states in these solvents. Support
for the occurrence of conformational changes upon photo-
excitation can be provided by time-dependent changes of
fluorescence spectra. Time-resolved fluorescence measure-
ments revealed that only for 2DA in cyclohexane a wave-
length shift of the emission spectrum with time is observed
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Through-Bond Interactions
2948±2959
Transient absorption (TA)
spectroscopy: So far, the occur-
rence of photoinduced charge
transfer was assumed on the
basis of quenching of local
emission and the appearance
of new emission bands. More
information on the processes
by interference of CT fluorescence near 408 nm, the spectral
region in which tetraalkyl olefinic radical cations are known to
absorb.[35] No radical anion and cation absorption could be
observed for 1A in cyclohexane, which must be a conse-
quence of the very short excited-state lifetime.
Figure 4. Possible folded struc-
ture of 1DA in the excited
state.
Donor model compounds: Surprisingly, donor model com-
pounds 1D and 2D possess pronouncedly different TA
spectra in benzene (Figure 5). For the latter, a prominent
absorption is observed at 475 nm (both in benzene and
cyclohexane) with a very long lifetime on the timescale used
(t ꢀ20 ns). This absorption band is assigned to a dialkylani-
lino-type triplet state.[36] Hence, one of the relaxation path-
ways of the S1 state of 2D involves intersystem crossing to the
triplet state. Furthermore, at short times a distinct band at
625 nm appears, the origin of which is not immediately
apparent. The TA spectra of 1D are remarkably different
from those of 2D. In cyclohexane, no absorption at all is
present; this suggests that a triplet state is either not formed or
very rapidly depopulated. In benzene, two prominent features
can be discerned: the absorption of a dialkylanilino triplet
state is completely absent and a broad, short-lived absorption
(t <5 ns) is present at 635 nm. Apparently, the presence of an
exocyclic double bond in N-phenylpiperidine derivatives
effectively quenches the dialkylanilino-type triplet state. The
absorption bands at 635 nm of 1D and at 625 nm of 2D most
probably stem from the S1 state of the dialkylanilino
chromophore. This assignment is supported by the agreement
between the TA lifetimes at 625/635 nm and the fluorescence
lifetimes (Table 2). However, it is not clear why the S1 state is
not observed in cyclohexane.
following excitation and the species formed was obtained
with transient absorption (TA) spectroscopy with an 8 ns
FWHM laser pulse. Transient absorption spectra are shown in
Figures 5 and 6, and the absorption maxima and lifetimes are
summarized in Table 3.
Donor/acceptor compounds: The TA spectra of the donor/
acceptor compounds are shown in Figure 6. Because of its low
solubility, it was not possible to record a TA spectrum of 2DA
in cyclohexane. In general, the TA spectra reveal the presence
of two absorptions that decay concurrently. No long-lived
absorptions attributable to dialkylanilino triplet states are
observed.
Figure 5. Transient absorption spectra of A) 1D and B) 2D in benzene.
For A, transients were taken at a) 0, b) 2, c) 4, and d) 20 ns. For B, transients
were taken at a) 0, b) 8, and c) 28 ns.
For 1DA, the band at about 340 nm is attributed to the
dicyanoethylene radical anion, whereas the long wavelength
absorption in the 460 ± 480 nm region is assigned to the
dialkylanilino-type radical cation. The absorption spectrum of
the dimethylaniline radical cation is well documented and
displays an absorption band at 475 nm.[37] In cyclohexane the
radical cation absorption is red-shifted by approximately
20 nm relative that observed in benzene and possesses a more
narrow and asymmetrical shape. The broadening in benzene is
either due to solute ± solvent interactions or is caused by an
increased interaction between the donor and acceptor chro-
mophores. A similar broadening of the dialkylanilino radical
cation absorption due to electronic interaction with another
chromophore has been observed for other DSA systems.[34]
Both the radical cation and radical anion decay in cyclo-
hexane with a lifetime of 21 ns. In benzene, both absorption
bands are very short-lived and a lifetime t <5 ns is observed.
These lifetimes are in agreement with those derived from
fluorescence decay measurements, which corroborates the
interpretation of the broad emission bands around about
Table 3. Transient absorption maxima and lifetimes t of DA compounds
(1DA, 2DA), donor model compounds (1D, 2D), and acceptor model
compound (1A) in cyclohexane and benzene (208C).
cyclohexane
lmax [nm]
benzene
lmax [nm]
t [ns]
t [ns]
1DA
2DA
1D
339, 480
21
±
±
339, 460
340, 490
635
< 5
35
< 5
[a]
±
[b]
±
2D
1A
475
±
ꢀ 20
475, 625
336, 410[c]
ꢀ 20
[b]
±
< 5
[a] Solubility too low. [b] No absorption observed. [c] 410 nm band due to
interference of CT emission (see text).
Acceptor model compounds: The TA spectrum of 1A in
benzene displays two bands situated at 336 nm and 410 nm.
The 336 nm band is assigned to the dicyanoethylene radical
anion.[34] The lifetime obtained from TA (<5 ns) is in
agreement with the fluorescence lifetime of 1 ns. The obser-
vation of the tetraalkyl olefinic radical cation was hampered
Chem. Eur. J. 2000, 6, No. 16
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L. W. Jenneskens et al.
FULL PAPER
overall fit curve for 1D and 2D (benzene) in Figure 7. TRMC
traces and fits for donor/acceptor compound 2DA are
depicted in Figure 8. The dipole moments calculated from
the best fits to the changes in microwave conductivity
(assuming the formation of a single transient dipolar species)
and the lifetimes used for the fitting are listed in Table 4. Since
the TRMC signal is related to the difference of the excited-
state and ground-state dipole moment, use has been made of
AM1-calculated ground-state dipole moments to obtain the
excited-state dipole moments.
Figure 6. Transient absorption spectra of A) 1DA in cyclohexane, B) 1DA
in benzene, and C) 2DA in benzene. For A, transients were taken at a) 0,
b) 14, and c) 34 ns. For B, transients were taken at a) 0, b) 4, c) 10, and
d) 16 ns. For C, transients were taken at a) 0, b) 10, c) 14, and d) 20 ns.
500 nm as originating from CT states (Table 2). The observa-
tion of both dicyanoethylene radical anion and dialkylanilino
radical cation absorption bands confirms the formation of a
charge-separated state in 1DA after photoexcitation.
Figure 7. Transient changes in the microwave conductivity (dielectric loss)
of benzene solutions of A) 1D and B) 2D (solid lines). The dashed lines
show the individual contributions from the excited singlet and triplet states
to the overall fit curve (dotted line).
For 2DA in benzene, absorption bands at 340 and 490 nm
are also present; these once again correspond to the dicyano-
ethylene radical anion and the dialkylanilino radical cation,
respectively. When compared with the TA spectrum of 1DA
in benzene, the long-wavelength absorption maximum is red-
shifted by approximately 25 nm. This may point to an excited
state interaction between the double bond and the dialkyl-
anilino group in 1DA. For 2DA both absorptions decay with a
lifetime of 35 ns, which is in good agreement with the value of
36 ns obtained from the emission spectra.
Table 4. TRMC data of 1DA, 2DA, 1D, 2D, and 1A in cyclohexane and
benzene.[a]
cyclohexane
t [ns] msph [D][b] mcyl [D][b]
benzene
t [ns] msph [D][b] mcyl [D][b]
1DA
2DA
1D
2D
1A
23
24
15
18
±
±
±
23
29
±
±
±
1.8
36
2.4
2.5
1.5
17
17
5
6
14
29
29
7
8
19
[c]
±
±
±
[c]
[d]
[a] One unit charge separated by 1 Š gives a dipole moment of 4.8 D.
[b] Assuming Fcs ˆ 1 (see text). [c] Not measured. [d] No signal observed in
cyclohexane.
Time-resolved microwave conductivity (TRMC) measure-
ments: In their excited charge-separated state, the donor/
acceptor compounds are expected to possess considerable
dipole moments. The TRMC technique is well suited for the
determination of these large dipole moments.[38, 39] TRMC
transients, reflecting the changes in microwave conductivity
upon excitation, are shown together with the individual
contributions from the excited singlet and triplet state to the
Acceptor model 1A: The acceptor model 1A has an
appreciable TRMC signal in benzene. An excited-state dipole
moment of 19 D is calculated based on a cylindrical geometry;
this corresponds to full charge separation over a distance of
3.9 Š. This is close to the spatial separation (ca. 4 Š) between
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Chem. Eur. J. 2000, 6, No. 16

Through-Bond Interactions
2948±2959
the olefinic double bond and the double bond of the
dicyanoethylene moiety. In cyclohexane no TRMC signal
whatsoever is observable for 1A. This result indicates again
that the CT state is rapidly deactivated and corroborates the
fluorescence and transient absorption data.
signal with a relatively long lifetime, assigned to the singlet
excited state, is found. This lifetime is significantly longer in
benzene (36 ns) than in cyclohexane (25 ns). Furthermore, the
contribution of the triplet state to the TRMC signal is small in
cyclohexane, and virtually absent in benzene. The lifetimes of
the excited states used for the fitting of the TRMC traces (for
both 1DA and 2AD) are in good agreement with those
obtained from both time-resolved fluorescence and TA
spectroscopy.
The absolute values of the excited-state dipole moments of
1DA and 2DA can be calculated on the basis of either a
spherical geometry or a cylindrical geometry of the charge-
separated species.[38, 39] For the folded conformation in cyclo-
hexane, the former geometry is a reasonable approximation,
whereas the latter is suitable for the calculation of the dipole
moment of the stretched conformer as found in benzene.
Thus, in cyclohexane a dipole moment of 15 D is obtained for
1DA and 18 D for 2DA. From these values it appears that the
donor and acceptor are somewhat closer together in 1DA;
this may find its origin in different conformations in the
excited states of the two compounds.
Donor model compounds: The TRMC traces of model donors
1D and 2D (both in benzene) are markedly different. The
transient of 2D displays an initial component that decays
within a few nanoseconds, followed by a component with a
lifetime of several microseconds. The fast component is
assigned to the excited singlet state of 2D, while the long-lived
component is assigned to the excited triplet state. Similar
results have been found for N,N-dimethylaniline.[40] The
TRMC trace of 1D mainly consists of a short-lived signal,
attributed to the excited singlet state, with only a minor
contribution of a long-lived triplet state. These observations
corroborate the results obtained from the TA experiments, in
which a dialkylanilino-type triplet absorption was found for
2D, whereas no evidence for triplet formation was obtained
for 1D. Singlet excited-state dipole moments of 7.3 D and
8.1 D are calculated for 1D and 2D, respectively. These dipole
moments are somewhat larger than that of N,N-dimethylani-
line (5.0 D),[40] reflecting a higher degree of charge separation
in the excited singlet state.
A value of 29 D is obtained for the excited-state dipole
moment of both 1DA and 2DA in benzene. Note that these
dipole moments are calculated under the assumption of a
quantum yield of unity for the formation of a charge-
separated species. The excited-state dipole moments of
1DA and 2DA in benzene are somewhat smaller than the
expected values of 37 and 38 D for full charge separation over
a donor± acceptor distance of 7.7 and 7.9 Š,[30] respectively.
Since the TA data revealed the formation of the both the
dicyanoethylene radical anion and the anilino radical cation,
full charge separation nevertheless must occur. It could be
that the quantum yield for charge separation is less than unity,
which might explain why the TRMC excited-state dipole
moments do not reflect full charge separation. However, the
virtually complete quenching of the local donor and acceptor
emission in the fluorescence spectra of 1DA and 2DA
strongly suggests that formation of the charge-separated state
Donor/acceptor compounds: As an illustration, TRMC
transients for donor/acceptor compound 2DA in cyclohexane
and benzene are shown in Figure 8. In both solvents, a large
.
.
‡
[D SA À]* is quantitative. The most likely explanation for the
small excited-state dipole moments left is that the geometry of
1DA and 2DA in the CT state in benzene is not a fully
stretched conformation, such as has been assumed until now.
The dipole moment of 29 D corresponds to full charge
separation over approximately 6 Š. This suggests that 1DA
and 2DA are in a bent or partially folded conformation. For
instance, in Figure 9 a structure of 1DA is depicted in which
only the piperidine ring adopts a boat conformation and
where the donor± acceptor distance is about 6 Š. Although it
is not certain that this is the correct structure for the CT state
of 1DA and 2DA in benzene, this shows that conformations
are possible in which the donor±
acceptor distance is approxi-
mately 6 Š. We note that the
actual values of the excited state
dipole moments are affected
when the geometry of the com-
pounds is not a real cylinder,
which may lead to some error in
the derived dipole moments.
Figure 8. Transient changes in the microwave conductivity (dielectric loss)
of 2DA in A) cyclohexane and B) benzene (solid lines). The dashed lines
show the individual contributions from the excited singlet and triplet states
to the overall fit curve (dotted line).
Figure 9. Partly folded struc-
ture of 1DA in the excited
state.
Chem. Eur. J. 2000, 6, No. 16
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2955

L. W. Jenneskens et al.
FULL PAPER
the presence of the double bond on the recombination
kinetics predominantly stems from the increased coupling
between the stretched CT state and the locally excited or
ground state.
Conclusion
As judged from the similar absorption spectra of 1D and 2D,
a significant ground-state interaction of the exocyclic double
bond with the anilino donor does not seem to be present. In
contrast, the absorption spectrum of acceptor model com-
pound 1A clearly shows that the double bond interacts with
the dicyanoethylene acceptor in the ground state, giving rise
to a charge-separated state upon excitation. Unfortunately,
owing to the ultrafast nature of the charge-separation process
in 1DA and 2DA, it was not possible to obtain any
information on the effect of the presence of the double bond
on this process. Nevertheless, the presence of the exocyclic
double bond in the bicyclohexylidene-type bridge system is
manifest in essentially three ways. Firstly, the double bond
renders the bridge much more flexible, facilitating the
occurrence of conformational changes in the excited state.
Thus, in cyclohexane folding takes place subsequent to charge
separation for 2DA (on an nanosecond timescale), whereas
for 1DA folding takes place either prior to charge separation,
or on a subpicosecond timescale after charge separation in the
bent or partially folded geometry. Secondly, as shown by both
the TA and TRMC data, the presence of the exocyclic double
bond results in the triplet state of the local anilino donor not
being observed. Thirdly, as indicated by the fluorescence and
TRMC lifetimes of 1DA and 2DA in benzene, the decay of
the extended CT species is about 20 times faster in presence of
the double bond. A similar effect, albeit less pronounced, has
been reported for compounds differing from 1DA and 2DA
in the donor and acceptor chromophores.[33]
The fact that an anilino triplet state is not observed in the
presence of the exocyclic double bond (compound 1D) can be
understood by considering the triplet state energies of the
anilino and tetraalkylethylene chromophores. From the
phosphorescence spectrum of N,N-dimethylaniline,[41] the
energy of the triplet state T1 of the anilino chromophore is
estimated to be 3.3 eV. The triplet energy of the double bond
in the present compounds is not exactly known, but it is
expected to be similar to or slightly lower than the triplet
energy of tetramethylethene, which is also about 3.3 eV.[42]
Therefore, we expect that once the T1 of the anilino
chromophore is populated, triplet ± triplet energy transfer to
the ethylene triplet state occurs which prevents the anilino
triplet from being detected. Subsequently, the ethylene triplet
state is rapidly deactivated by radiationless decay.[41]
In summary, we have shown that photoinduced electron
transfer occurs in donor/acceptor-substituted bicyclohexyli-
denes and bicyclohexyls, and that the presence of the
exocyclic double bond has a strong influence on the photo-
physical properties of these compounds. In this context, it is
interesting that also in longer oligo(cyclohexylidene) DSA
compounds the local emission of the dialkylanilino chromo-
phore is partly quenched; this indicates that even over a
distance of 16.1 Š (three intervening cyclohexylidene rings)
through-bond interaction between the donor and acceptor
chromophore must exist.[9b]
Experimental Section
General: All reactions were carried out under a dry nitrogen atmosphere
unless stated otherwise. Commercially available reagents were used
without purification. THF was distilled from sodium/benzophenone prior
to use. Acetonitrile was stored over 3 Š molecular sieves. For column
chromatography silica (Merckkieselgel 60, 230 ± 400 mesh ASTM) was
used. Melting points were determined by using a Mettler FP5/FP51
photoelectric melting point apparatus and are uncorrected. NMR spectra
were recorded on a Bruker AC300 spectrometer, operating at 300.13 MHz
for 1H NMR and 75.47 MHz for 13C NMR spectroscopy. Chemical shifts are
given relative to external TMS. Samples were dissolved in deuterated
chloroform unless stated otherwise. Infrared spectra of solids were
recorded on a Mattson Galaxy Series FTIR 5000 spectrophotometer with
a diffuse reflectance accessory; samples were diluted with optically pure
KBr. Elemental analyses were carried out by H. Kolbe Mikroanalytisches
Laboratorium, Mülheim a.d. Ruhr (Germany). UV spectra were measured
with a Cary 1 UV/Vis spectrophotometer in spectrophotometric grade
solvents (Janssen/Acros). Fluorescence spectra were obtained on a Spex
Fluorolog instrument.[43] Fluorescence quantum yields were determined
relative to naphthalene (Ffl ˆ 0.23)[44] (compounds 1D and 2D) or 9,10-
diphenylanthracene (Ffl ˆ 0.90)[44] (compounds 1A, 1DA, 2DA). Samples
were diluted to A1cm <0.1 at the excitation wavelength used and
deoxygenated by purging with argon for 15 minutes. Redox potentials
were determined by using cyclic voltammetry conducted with a Heka
PG287 potentiostat/galvanostat in acetonitrile (Janssen p.a. grade, freshly
distilled from calcium hydride) containing 0.1m tetrabutylammonium
hexafluorophosphate (Fluka, electrochemical grade) as supporting electro-
lyte. The scanning rate was 0.1 VsÀ1. Oxidation and reduction potentials
were determined relative to Ag/AgNO3 (0.1m in acetonitrile) and were
referenced to SCE by regularly measuring the oxidation potential of the
FeCp/FeCp‡. couple (E1/2 vs. SCE ˆ 0.31 V).[44] Both the oxidation process
of the N-phenylpiperidino donors and the reduction process of the
dicyanoethylene acceptor were irreversible. The half-wave potentials were
obtained by correction of the peakmaxima with ‡30 mV for the cathodic
wave and À30 mV for the anodic wave.
Time-resolved fluorescence measurements: Fluorescence lifetimes were
determined at single wavelengths by using a Lumonics EX700 pulsemaster
XeCl excimer laser (308 nm, 8 ns FWHM) as excitation source. The
emitted light was passed through a Carl Zeiss M4QII monochromator and
detected using an RCA1P28 photomultiplier. The signal of the detector
was fed into a TDS684A digital oscilloscope, which was triggered by a
photodiode that detects the laser light pulse. Data were analyzed by a
program based on iterative convolution which allows the calculation of
fluorescence lifetimes for mono-, bi- and tri-exponential decay functions.
Samples (A1cm(308 nm) <0.1) were degassed by purging with argon for
15 min. The equipment used for picosecond single photon-counting
measurements has been fully described elsewhere.[46] Time-resolved
fluorescence spectra (Figure 3) were obtained by using the experimental
set-up described below for transient absorption spectroscopy, but without
the xenon lamp as probe light.
The role of the olefinic double bond in the faster decay of
the CT state of 1DA relative to 2DA in benzene may be
twofold. Firstly, it is likely that it increases the coupling
between the CT state and the locally excited or ground state,
thereby favoring charge recombination. Secondly, as shown
above, through its triplet state the double bond also generates
an additional recombination pathway. The importance of this
decay channel can, however, only be small, since in cyclo-
hexane the folded CT-state lifetime of 1DA is only slightly
shorter than the folded CT-state lifetime of 2DA. Note that
for 1DA the compact CT state in cyclohexane has about the
same energy as the stretched conformation in benzene, so that
the energetic distance to the ethylenic triplet state is the same
for both conformations. It is therefore likely that the effect of
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Chem. Eur. J. 2000, 6, No. 16

Through-Bond Interactions
2948±2959
Transient absorption spectroscopy: TA spectra were obtained by using a
Lumonics EX700 XeCl excimer laser (308 nm) as the excitation source and
a 450 W high-pressure Xe-arc, pulsed with a Müller Elektronik MSP05
pulser to enhance its brightness during the observation time gate of the
detector, in right angle geometry as probe light. The probe light, after
passing through the sample cell, was collected by an optical fibre and fed
into a Jarrel-Ash monospec 27 model 1234 spectrograph in which the light
was dispersed by a grating (150 groovesmmÀ1) onto an MCP-intensified
diode array detector (EG&G 1421G, 25 mm, 1024 diodes). With this set-up
a spectral range of about 600 nm was covered with a bandwidth of 7 nm
(250 mm slit). The detector was gated at 5 ns by an EG&G 1302 pulse
generator and the start of the time window was delayed in 2 or 5 ns
increments relative to the laser pulse to obtain subsequent spectra across
the total decay time of the transients studied. The timing of the laser, the
probe light, and the optical multichannel analyser (OMA) gate pulse was
controlled by an EG&G OMAIII Model 1460 console with a 1303 pulse
generator and a digital delay generator (EG&G 9650). Spectra were
averaged over ꢁ20 pulses for each delay to improve the signal to noise
ratio. Samples were prepared in spectrophotometric grade cyclohexane and
benzene in a concentration 0.8 < A1cm(308 nm) < 1.5 and were degassed by
several freeze-pump-thaw cycles.
23.8 mmol, 71%). M.p. 166 ± 1678C; 1H NMR: d ˆ 7.25 (m, 2H), 6.92 (d,
J ˆ 7.9 Hz, 2H), 6.81 (t, J ˆ 7.3 Hz, 1H), 3.54 (s, 4H), 3.21 (t, J ˆ 5.7 Hz,
4H), 2.46 (t, J ˆ 5.7 Hz, 4H), 2.28 (m, 4H), 1.83 (m, 4H), 0.99 (s, 6H);
13C NMR: d ˆ 151.38, 129.36, 129.06, 126.07, 118.90, 115.98, 97.68, 70.12,
50.60, 33.46, 30.25, 29.11, 25.05, 22.76; IR (KBr): nÄ ˆ 3094, 3069, 3036, 3017,
2974, 2967, 2955, 2940, 2920, 2893, 2866, 2847, 2828, 1597, 1576, 1479, 1462,
1448, 1431, 1117, 752, 687 cmÀ1
.
4-(1-Phenylpiperidin-4-ylidene)cyclohexanone (8): Acetal
7
(8.12 g,
23.8 mmol) was dissolved in THF (70 mL), and 5% hydrochloric acid
(70 mL) was added. The mixture was heated to reflux for 4 h. After cooling
to room temperature, THF was removed at reduced pressure and the
resulting suspension was extracted with chloroform (2 Â 75 mL). After
washing the combined organic extracts with saturated NaHCO3 solution
and water, respectively, drying (MgSO4), and evaporation of the solvent, an
off-white solid was obtained (5.61 g, 22.0 mmol, 92%). M.p. 58 ± 608C;
1H NMR: d ˆ 7.27 (m, 2H), 6.94 (d, J ˆ 7.9 Hz, 2H), 6.84 (t, J ˆ 7.3 Hz, 1H),
3.26 (t, J ˆ 5.8 Hz, 4H), 2.60 (t, J ˆ 6.7 Hz, 4H), 2.50 (t, J ˆ 5.7 Hz, 4H),
2.43 (t, J ˆ 6.8 Hz, 4H); 13C NMR: d ˆ 212.32, 151.18, 129.13, 129.13,
125.54, 119.10, 116.00, 50.22, 40.55, 29.22, 26.54; IR (KBr): nÄ ˆ 3090, 3055,
3040, 2963, 2915, 2897, 2849, 2836, 2818, 2805, 1721, 1599, 1494, 1442, 1427,
1415, 762, 694 cmÀ1
4-(1-Phenylpiperidin-4-ylidene)cyclohexylidenepropanedinitrile (1DA):
mixture of ketone (2.35 g, 9.22 mmol), malononitrile (1.05 g,
.
Time-resolved microwave conductivity measurements: In a TRMC experi-
ment, a solution of the compound under investigation in a non-dipolar
solvent, contained in a microwave cavity, was photoexcited by a 7 ns laser
flash from a XeCl excimer laser. The formation of a dipolar excited state
lead to an increase in the high-frequency dielectric loss of the solution. This
increase wa monitored by time-resolved measurement of the change in
microwave conductivity Ds which is related to the difference in excited-
state and ground-state dipole moment. It was necessary for the calculation
of the excited state dipole moment from the TRMC transients to describe
the geometry of the charge-separated species properly. Methods are
available for spherical, cylindrical, and disklike molecular geometries. The
technique is described in detail elsewhere.[38, 39] Samples with
A1cm(308 nm) > 0.3 and preferably A1cm(308 nm) ˆ 1 were prepared in
UV spectroscopic grade cyclohexane and benzene and were deaerated by
purging with CO2 for 15 min. The solutions contained in a microwave
cavity were flash-photolysed by using a single 7 ns FWHM pulse (308 nm)
of a Lumonics HyperEX 400 excimer laser. Any transient change occurring
in the microwave conductivity (dielectric loss) of the solution was
monitored as a change in the microwave power reflected by the cavity by
using a Tektronix 7912 transient digitizer.
A
8
15.9 mmol), ammonium acetate (0.81 g), and acetic acid (2.0 mL) in
benzene (100 mL) was heated to reflux for 2 h in a Dean ± Starkapparatus.
Upon cooling a greenish precipitate formed, which was filtered off and
washed with benzene. Yield: 1.47 g (4.85 mmol, 53%) of a greenish solid.
M.p. 1708C (decomp); 1H NMR: d ˆ 7.25 (m, 2H), 6.90 (d, J ˆ 8.0 Hz, 2H),
6.85 (t, J ˆ 7.2 Hz, 1H), 3.20 (t, J ˆ 5.4 Hz, 4H), 2.78 (t, J ˆ 6.0 Hz, 4H), 2.45
(m, 8H); 13C NMR: d ˆ 184.25, 150.94, 130.32, 129.16, 124.54, 119.30,
116.07, 111.60, 83.26, 50.25, 34.42, 29.22, 28.13; IR (KBr): nÄ ˆ 3092, 3034,
2992, 2965, 2897, 2839, 2822, 2230, 1597, 1504, 1464, 1437, 754, 687 cmÀ1
;
elemental analysis calcd (%) for C20H21N3 (303.41): C 79.16, H 6.98, N
13.86; found C 79.20, H 6.90, N 13.89.
9-(1-Phenylpiperidin-4-yl)-3,3-dimethyl-1,5-dioxaspiro[5.5]undecane (9):
Acetal 7 (2.70 g, 7.92 mmol) in THF (450 mL) was stirred overnight under
an atmosphere of hydrogen (1 atm) in the presence of 10% Pd/C as a
catalyst. Filtration over Celite and evaporation of the solvent gave a white
solid in quantitative yield. M.p. 137 ± 1398C; 1H NMR: d ˆ 7.26 (m, 2H),
6.92 (d, J ˆ 8.3 Hz, 2H), 6.82 (t, J ˆ 7.3 Hz, 1H), 3.70 (brd, J ˆ 11.8 Hz,
2H), 3.53 (s, 2H), 3.48 (s, 2H), 2.63 (brt, J ˆ 11.3 Hz, 2H), 2.38 (m, 2H),
1.78 (brd, J ˆ 12.4 Hz, 2H), 1.63 (m, 2H), 150 ± 1.10 (m, 8H), 0.94 (s, 6H);
13C NMR: d ˆ 151.92, 128.99, 119.23, 116.48, 97.71, 70.07, 69.86, 50.37, 41.90,
40.65, 32.15, 30.20, 29.60, 25.73, 22.74; IR (KBr): nÄ ˆ 3096, 3069, 3040,
3025, 2951, 2904, 2864, 2830, 2812, 1599, 1460, 1440, 1111, 1099, 752,
Synthesis of donor/acceptor compounds
9-(1-Phenyl-4-hydroxy-piperidin-4-yl)-3,3-dimethyl-1,5-dioxaspiro[5.5]un-
decane-9-carboxylic acid (6): A two-necked flask was filled with THF
(150 mL) and diisopropylamine (19.05 g, 189 mmol). Butyllithium in n-
hexane (131 mL of a 1.46m solution, 191 mmol) was added to this solution
at À408C, and after stirring for 30 min 3,3-dimethyl-1,5-dioxaspiro[5.5]un-
decane-9-carboxylic acid[9] (5; 21.35 g, 93.6 mmol) in THF (25 mL) was
added at once at À408C. The reaction mixture was then stirred at 508C for
two hours and re-cooled to À408C. N-Phenyl-4-piperidone[17] (4; 16.84 g,
96.2 mmol) in THF (25 mL) was then added at once, and the reaction
mixture was stirred at 508C for another 2 h. After cooling to room
temperature the mixture was poured on ice (750 g) and diluted with diethyl
ether (500 mL). The layers were separated and the organic layer was
extracted with water (2 Â 150 mL). The combined water layers
were washed with diethyl ether (2 Â 100 mL) and subsequently acidified
to pH 1 using 3m hydrochloric acid. The resulting white precipitate was
filtered off, washed with water, and dried in vacuo over potassium
hydroxide, yielding a white solid (13.70 g, 34.0 mmol, 36%). M.p. 2228C
(decomp); 1H NMR ([D6]DMSO): d ˆ 7.81 (m, 2H), 7.52 (m, 2H), 7.47 (m,
1H), 3.68 (m, 2H), 3.38 (m, 6H), 2.40 (m, 2H), 2.19 (m, 2H), 1.92 (m, 4H),
1.60 (m, 2H), 1.21 (m, 2H), 0.87 (s, 6H); IR (KBr): nÄ ˆ 3500 ± 3000, 2976,
2957, 2949, 2903, 2868, 2900 ± 2300, 1732, 1694, 1494, 1469, 1448, 1105, 756,
687 cmÀ1
.
4-(1-Phenylpiperidin-4-yl)cyclohexanone (10): Hydrolysis of acetal
9
(2.21 g, 6.44 mmol) as described for 8 quantitatively afforded a white solid.
M.p. 1378C; 1H NMR: d ˆ 7.25 (m, 2H), 6.95 (d, J ˆ 7.9 Hz, 2H), 6.82 (t,
J ˆ 7.3 Hz, 1H), 3.70 (brd, J ˆ 13.0 Hz, 2H), 2.65 (brt, J ˆ 12.1 Hz, 2H),
2.35 (m, 4H), 2.08 (m, 2H), 1.70 ± 1.00 (m, 8H); 13C NMR: d ˆ 212.12,
151.75, 129.06, 119.47, 116.67, 50.22, 40.98, 40.93, 40.07, 29.85, 29.61; IR
(KBr): nÄ ˆ 3088, 3067, 3034, 3019, 2951, 2938, 2911, 2880, 2863, 2849, 2824,
2809, 1723, 1599, 1502, 1462, 1447, 1431, 1416, 761, 692 cmÀ1
.
4-(1-Phenylpiperidin-4-yl)cyclohexylidenepropanedinitrile (2DA): A mix-
ture of ketone 10 (1.52 g, 5.91 mmol), malononitrile (0.51 g, 7.73 mmol),
ammonium acetate (0.44 g), and acetic acid (1.0 mL) in benzene (150 mL)
was heated to reflux for 2 h in a Dean ± Starkapparatus. Upon cooling, a
precipitate formed that was filtered, washed with benzene, and dried to
give a greenish solid, which was purified by column chromatography (silica;
eluent chloroform). Yield 1.10 g (3.61 mmol, 61%) of a greenish solid. M.p.
2118C (decomp); 1H NMR: d ˆ 7.23 (m, 2H), 6.92 (d, J ˆ 8.1 Hz, 2H), 6.82
(t, J ˆ 7.2 Hz, 1H), 3.72 (dt, J ˆ 12.4, 2.3 Hz, 2H), 3.09 (brd, J ˆ 13.5 Hz,
2H), 2.62 (td, J ˆ 12.1, 2.3 Hz, 2H), 2.33 (td, J ˆ 13.5, 5.1 Hz, 2H), 2.17
(brd, J ˆ 13.1 Hz, 2H), 1.80 (brd, J ˆ 13.1 Hz, 2H), 1.60 ± 1.20 (m, 6H);
13C NMR: d ˆ 184.51, 151.65, 129.09, 119.60, 116.61, 111.64, 82.69, 50.21,
41.23, 40.07, 34.19, 30.86, 29.49; IR (KBr): nÄ ˆ 3094, 3074, 3046, 3030, 3015,
2990, 2969, 2942, 2909, 2866, 2843, 2826, 2226, 1595, 1503, 1460, 1442, 1429,
1418, 760, 688 cmÀ1; elemental analysis calcd (%) for C20H23N3 (305.43): C
78.64, H 7.60, N 13.76; found C 78.49, H 7.53, N 13.65.
691 cmÀ1
.
9-(1-Phenylpiperidin-4-ylidene)-3,3-dimethyl-1,5-dioxaspiro[5.5]undecane
(7): N,N-Dimethylformamide dineopentyl acetal (15.65 g, 67.6 mmol) was
added to
a suspension of b-hydroxy acid 6 (13.45 g, 33.4 mmol) in
acetonitrile (375 mL) and after stirring for 1 h at room temperature the
reaction mixture was heated to reflux overnight. The resulting yellow
solution was cooled to À208C, and the resulting precipitate was filtered off,
washed with cold acetonitrile, and dried to yield a white powder (8.12 g,
Chem. Eur. J. 2000, 6, No. 16
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2957

L. W. Jenneskens et al.
FULL PAPER
Synthesis of model donor compounds
[1] R. A. Marcus, N. Sutin, Biochim. Biophys. Acta 1985, 811, 265.
[2] M. R. Wasielewski, Chem. Rev. 1992, 92, 435.
1-(1-Phenyl-4-hydroxy-piperidin-4-yl)cyclohexanecarboxylic acid (11):
This compound was synthesized as described for b-hydroxy acid 6 from
cyclohexane carboxylic acid (5.25 g, 41.0 mmol) and N-phenyl-4-piperi-
done 4 (7.02 g, 40.1 mmol) by using butyllithium in n-hexane (1.46m, 58 mL,
84.7 mmol) and diisopropylamine (8.24 g, 81.6 mmol) in THF (150 mL).
The crude reaction mixture was poured on ice (150 g) and dilluted with
diethyl ether (200 mL). The water layer was separated and the organic
layer was extracted with water (2 Â 50 mL). The combined water layers
were washed with diethylether (2 Â 25 mL) and subsequently acidified to
pH 6 by using 3m hydrochloric acid. The resulting white precipitate was
filtered off, washed with water, and dried in vacuo over potassium
hydroxide. Yield 5.55 g (18.3 mmol, 45%). M.p. 2258C (decomp); IR
(KBr): nÄ ˆ 3400 ± 3200, 3067, 3056, 2982, 2968, 2932, 2854, 1687, 1654, 1494,
[3] W. Schuddeboom, B. Krijnen, J. W. Verhoeven, E. G. J. Staring,
G. L. J. A. Rikken, H. Oevering, Chem. Phys. Lett. 1991, 179, 73.
[4] For a review see: New J. Chem. 1991, 15, special issue on molecular
electronics.
[5] M. N. Paddon-Row, Acc. Chem. Res. 1994, 27, 18, and references cited
therein.
[6] R. Hoffmann, Acc. Chem. Res. 1971, 4, 1.
[7] B. Krijnen, H. B. Beverloo, J. W. Verhoeven, C. A. Reiss, K. Goubitz,
D. Heijdenrijk, J. Am. Chem. Soc. 1989, 111, 4433.
[8] W. Schuddeboom, T. Scherer, J. M. Warman, J. W. Verhoeven, J. Phys.
Chem. 1993, 97, 13092.
[9] a) F. J. Hoogesteger, R. W. A. Havenith, J. W. Zwikker, L. W. Jen-
neskens, H. Kooijman, N. Veldman, A. L. Spek, J. Org. Chem. 1995,
60, 4375; b) F. J. Hoogesteger, Ph.D. Thesis, Utrecht University,
Utrecht (The Netherlands), 1996.
[10] a) F. J. Hoogesteger, J. H. van Lenthe, L. W. Jenneskens, Chem. Phys.
Lett. 1996, 259, 178; b) R. W. A. Havenith, J. H. van Lenthe, L. W.
Jenneskens, F. J. Hoogesteger, Chem. Phys. 1997, 225, 139.
[11] M. R. Wasielewski, M. P. Niemczyk, D. G. Johnson, W. A. Svec, D. W.
Minsek, Tetrahedron 1989, 45, 4785.
[12] a) H. Heitele, M. E. Michel-Beyerle, J. Am. Chem. Soc. 1985, 107,
8286; b) H. Heitele, M. E. Michel-Beyerle, P. Finckh, Chem. Phys.
Lett. 1987, 134, 273.
1462, 1450, 1437, 1131, 761, 700 cmÀ1
.
1-Phenyl-4-cyclohexylidenepiperidine (1D): This compound was synthe-
sized from 11 (5.55 g, 18.0 mmol) and N,N-dimethylformamide dineopen-
tylacetal (9.05 g, 39.1 mmol) in acetonitrile (250 mL) as described for 7. The
crude reaction mixture was concentrated by evaporation of the solvent to
ca. 70 mL and stored at À208C. A white solid precipitated, was filtered,
washed with cold acetonitrile, and dried to yield 2.58 g of 1D. The filtrate
was diluted with chloroform (150 mL), washed with a saturated NaHCO3
solution (100 mL) and water (2 Â 100 mL), and after separation of the
layers, drying of the organic layer (MgSO4), and evaporation to dryness a
white solid was isolated. Recrystallization of the solid from methanol
afforded another 0.45 g (2.0 mmol) of 1D. Total yield 3.03 g (12.6 mmol,
70%). M.p. 818C; 1H NMR: d ˆ 7.27 (m, 2H), 6.94 (d, J ˆ 7.9 Hz, 2H), 6.82
(t, J ˆ 7.3 Hz, 1H), 3.22 (t, J ˆ 5.8 Hz, 4H), 2.46 (t, J ˆ 5.8 Hz, 4H), 2.22 (m,
4H), 1.56 (m, 6H); 13C NMR: d ˆ 151.54, 131.70, 129.10, 124.77, 118.84,
115.97, 50.73, 30.21, 29.02, 28.60, 27.20; IR (KBr): nÄ ˆ 3096, 3071, 3040, 2972,
[13] a) F. J. Hoogesteger, J. M. Kroon, L. W. Jenneskens, E. J. R. Sudhölt-
er, T. J. M. de Bruin, J. W. Zwikker, E. ten Grotenhuis, C. H. M.
Â
Maree, N. Veldman, A. L. Spek, Langmuir 1996, 12, 4760; b) F. J.
Hoogesteger, L. W. Jenneskens, H. Kooijman, N. Veldman, A. L.
Spek, Tetrahedron 1996, 52, 1773; c) E. ten Grotenhuis, J. P. van der
Eerden, F. J. Hoogesteger, L. W. Jenneskens, Chem. Phys. Lett. 1996,
250, 549; d) E. ten Grotenhuis, J. P. van der Eerden, F. J. Hoogesteger,
L. W. Jenneskens, Adv. Mater. 1996, 8, 666.
2955, 2924, 2887, 2852, 2814, 1599, 1505, 1458, 1447, 1425, 745, 682 cmÀ1
;
elemental analysis calcd (%) for C17H23N (241.38): C 84.58, H 9.61, N 5.81;
found C 84.30, H 9.59, N 5.77.
[14] F. C. de Schryver, N. Boens, J. Put, Adv. Photochem. 1977, 10,
359.
1-Phenyl-4-cyclohexylpiperidine (2D): 1-Phenyl-4-cyclohexylidenepiperi-
dine 1D (1.55 g, 6.43 mmol) in THF (250 mL) was stirred overnight under
an atmosphere of hydrogen (1 atm) using 10% Pd/C as a catalyst (500 mg).
The reaction mixture was filtered over Celite and evaporated to dryness.
The crude solid was recrystallized from methanol, yielding 1.00 g
(4.12 mmol, 64%) of a white solid. M.p. 97 ± 1008C; 1H NMR: d ˆ 7.23
(m, 2H), 6.95 (d, J ˆ 8.0 Hz, 2H), 6.83 (t, J ˆ 7.25 Hz 1H), 3.69 (brd, J ˆ
12.2 Hz, 2H), 2.65 (td, J ˆ 12.2, 2.2 Hz, 2H), 1.75 (m, 7H), 1.40 (m, 2H),
1.20 (m, 5H), 0.97 (m, 2H); 13C NMR : d ˆ 152.01, 129.00, 119.18, 116.49,
50.48, 42.60, 41.46, 30.19, 29.40, 26.78, 26.68; IR (KBr): nÄ ˆ 3092, 3069, 3032,
3019, 2920, 2870, 2849, 1599, 1503, 1460, 1447, 752, 687 cmÀ1; elemental
analysis calcd (%) for C17H25N (243.40): C 83.88, H 10.36, N 5.76; found C
83.74, H 10.30, N 5.77.
[15] T. Scherer, Ph.D. Thesis, University of Amsterdam, Amsterdam (The
Netherlands), 1994.
[16] A. M. Brouwer, R. D. Mout, P. H. Maassen van den Brink, H. J.
van Ramesdonk, J. W. Verhoeven, S. A. Jonker, J. M. Warman, Chem.
Phys. Lett. 1991, 186, 481.
[17] T. Scherer, W. Hielkema, B. Krijnen, R. M. Hermant, C. Eijckelhoff,
F. Kerkhof, A. K. F. Ng, R. Verleg, E. B. van der Tol, A. M.
Brouwer, J. W. Verhoeven, Recl. Trav. Chim. Pays-Bas 1993, 112,
535.
[18] F. J. Hoogesteger, D. M. Grove, L. W. Jenneskens, T. J. M. de Bruin,
B. A. J. Jansen, J. Chem. Soc. Perkin Trans. 2 1996, 2327.
[19] By using the described synthetic strategy, the synthesis of longer DA-
substituted oligo(cyclohexylidenes) is straightforward.[9b]
[20] H. Kooijman, A. L. Spek, F. J. Hoogesteger, L. W. Jenneskens, Acta
Crystallogr. Sect. C 1997, 53, 1156.
Synthesis of model acceptor compounds
1,1'-Bicyclohexyliden-4-ylidenepropanedinitrile (1A): A mixture of 1,1'-
bicyclohexyliden-4-one[9] (1.0 g, 5.62 mmol), malononitrile (0.38 g,
5.76 mmol), ammonium acetate (0.46 g), and acetic acid (1.0 mL) was
refluxed in benzene (50 mL) for 1.5 h. The crude reaction mixture was
washed with water (20 mL), saturated NaHCO3 solution (20 mL), and
again with water (20 mL), dried (MgSO4), and evaporated to dryness. The
crude solid was recrystallized from ethyl acetate (20 mL) to yield light
yellow needles (1.10 g, 4.87 mmol, 87%). M.p. 154 ± 1558C; 1H NMR: d ˆ
2.72 (t, J ˆ 6.5 Hz, 4H), 2.47 (t, J ˆ 6.5 Hz, 4H), 2.19 (m, 4H), 1.54 (m, 6H);
13C NMR: d ˆ 185.00, 134.93, 122.21, 111.67, 82.89. 34.74, 30.38, 28.25,
28.20, 26.77; IR (KBr): nÄ ˆ 2967, 2927, 2854, 2828, 2229, 1584, 1449, 1431,
[21] a) J. B. Lambert, S. I. Featherman, Chem. Rev. 1975, 75, 611; b) C. H.
Bushweller, S. H. Fleischman, G. L. Grady, P. McGoff, C. D. Rithner,
M. R. Whalon, J. G. Brennan, R. P. Marcantonio, R. P. Domingue, J.
Am. Chem. Soc. 1982, 104, 6224.
[22] a) J. T. Gerig, J. Am. Chem. Soc. 1968, 90, 1065; b) F. R. Jensen, B. H.
Beck, J. Am. Chem. Soc. 1968, 90, 1066.
[23] P. Pasman, F. Rob, J. W. Verhoeven, J. Am. Chem. Soc. 1982, 104, 5127.
[24] L. B. Krijnen, Ph.D. Thesis, University of Amsterdam, Amsterdam
(The Netherlands), 1990.
[25] a) J. N. Murell, J. Chem. Soc. 1956, 3779; b) K. Kimura, H. Tsubo-
mura, S. Nagakura, Bull. Chem. Soc. Jap. 1964, 37, 1336.
[26] Owing to its very limited solubility, it was not possible to quantita-
tively dissolve a known amount of 2DA in cyclohexane. Therefore, the
molar absorption coefficient of 2DA was determined in dichloro-
methane (which is a better solvent). Thus, the spectrum of 2DA in
Figure 1 was taken in cyclohexane, but the molar absorption
coefficient in dichloromethane was adopted. Although this makes a
direct comparison with the spectra of the reference chromophores
difficult, it is evident that no additional absorption peaks are present.
[27] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, 2nd
ed., VCH, Weinheim, 1988.
999 cmÀ1
.
1,1'-Bicyclohexyl-4-ylidenepropanedinitrile (2A): This compound was
obtained as described for 1A from 1,1'-bicyclohexyl-4-one (253 mg,
1.41 mmol),[18] malononitrile (103 mg, 1.56 mmol), acetic acid (0.2 mL),
and ammonium acetate (154 mg) in benzene (50 mL). The crude material
was recrystallized from methanol to yield white needles (284 mg,
1.25 mmol, 89%). M.p. 53 ± 558C; 1H NMR: d ˆ 3.01 (brd, J ˆ 13.0 Hz,
2H), 2.29 (td, J ˆ 12.9, 5.1 Hz, 2H), 2.04 (m, 2H), 1.67 (m, 6H), 1.40 ± 1.00
(m, 8H); 13C NMR: d ˆ 185.28, 111.71, 82.75, 41.88, 41.83, 34.38,
30.88, 30.22, 26.52, 26.50; IR (KBr): nÄ ˆ 2926, 2853, 2662, 2230, 1595,
1448 cmÀ1
.
[28] R. M. Weiss, A. Warshel, J. Am. Chem. Soc. 1979, 101, 6131.
2958
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Chem. Eur. J. 2000, 6, No. 16

Through-Bond Interactions
2948±2959
[29] J. M. Warman, K. J. Smit, M. P. de Haas, S. A. Jonker, M. N. Paddon-
Row, A. M. Oliver, J. Kroon, H. Oevering, J. W. Verhoeven, J. Phys.
Chem. 1991, 95, 1979.
[32] W. Schuddeboom, T. Scherer, J. M. Warman, J. W. Verhoeven, J. Phys.
Chem. 1993, 97, 13092.
[33] R. J. Willemse, Ph.D. Thesis, University of Amsterdam, Amsterdam
(The Netherlands), 1997.
[34] M. R. Roest, J. M. Lawson, M. N. Paddon-Row, J. W. Verhoeven,
Chem. Phys. Lett. 1995, 230, 536.
[35] T. Clark, M. F. Teasley, S. Nelsen, H. Wynberg, J. Am. Chem. Soc. 1987,
109, 5719.
[36] C. M. Previtali, J. Photochem. 1985, 31, 233.
[37] T. Shida, Electronic Absorption Spectra of Radical Ions, Physical
Sciences Data, Vol. 34, Elsevier, Amsterdam, 1988.
[38] M. P. de Haas, J. M. Warman, Chem. Phys. 1982, 73, 35.
[39] W. Schuddeboom, Ph.D. Thesis, Delft University of Technology, Delft
(The Netherlands), 1994.
[30] The Weller equation is given in Equation (1) below (A. Weller, Z.
Phys. Chem. 1982, 133, 93). In this equation, the first term describes
the half-wave redox potentials of the chromophores as determined in
acetonitrile (es ˆ 35.9),[27] the second term the zero ± zero excitation
energy, the third term the Coulombic energy of the ion-pair separated
by a distance RDA , and the final term the solvation energy. The
effective radii rd‡ and raÀ of the radical cation and radical anion were
obtained by using 4pr3/3 ˆ M/N1, in which N is Avogadroꢁs number, M
is the molecular weight, and 1 the density of the compound. By using
available data for N,N-dimethylaniline and tetracyanoethylene (CRC
Handbook of Chemistry and Physics, 64th ed. (Ed.: R. C. Weast),
CRC, Boca Raton, 1983), values of 3.7 Š for rd‡ and 3.4 Š for raÀ were
[40] P. C. M. Weisenborn, C. A. G. O. Varma, M. P. de Haas, J. M. Warman,
Chem. Phys. 1988, 122, 147.
[41] S. P. McGlynn, T. Azumi, M. Kinoshita, Molecular Spectroscopy of the
Triplet State, Prentice-Hall, Englewood Cliffs, 1969.
[42] N. J. Turro, Modern Molecular Spectroscopy, Benjamin/Cummings,
Menlo Park, 1978, p. 290.
[43] C. A. van Walree, M. R. Roest, W. Schuddeboom, L. W. Jenneskens,
J. W. Verhoeven, J. M. Warman, H. Kooijman, A. L. Spek, J. Am.
Chem. Soc. 1996, 118, 8395.
[44] D. F. Eaton, Pure Appl. Chem. 1988, 60, 1107.
[45] J. Dachbach, D. Blackwood, J. W. Pons, S. Pons, J. Electroanal. Chem.
1987, 237, 269.
ox
obtained. Identical values were obtained for E
of 1D and 2D
1=2
red
(0.70 V vs. SCE) and E
of 1A and 2A (À1.70 V vs. SCE, see
1=2
Experimental Section). For E00 the band edge of the lowest local
transition was taken, which is situated at 317 nm (3.91 eV, Figure 1) for
both 1DA and 2DA. The values for the donor± acceptor distance RDA
used, which were calculated with the molecular mechanics (MMX
force field as implemented in PCMODEL 4.0, Serena Software,
Bloomington, In., USA 1990) on extended conformations of the
hydrocarbon skeleton, were 7.7 Š for 1DA and 7.9 Š for 2DA.
ox
red
DGCS ˆ e(E À E † À E00
1=2
1=2
[46] S. I. van Dijk, P. G. Wiering, C. P. Groen, A. M. Brouwer, J. W.
Verhoeven, W. Schuddeboom, J. M. Warman, J. Chem. Soc. Faraday
Trans. 1995, 91, 2107.
ꢀ
ꢁꢀ
ꢁ
e2
e2
1
1
1
1
(1)
À
À
‡
À
4pe0esRDA 8pe0 rd‡ raÀ
35:9 es
[31] R. M. Hermant, Ph.D. Thesis, University of Amsterdam, Amsterdam
Received: June 29, 1999
(The Netherlands), 1990.
Revised version: November 17, 1999 [F1879]
Chem. Eur. J. 2000, 6, No. 16
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2959