Photoinduced charge separation in cyclohexylidene-based donor - (σ-bridge) - acceptor compounds - Building blocks for materials

Article abstract of DOI:10.1002/ejoc.200200693

We describe the syntheses and properties of five semi-rigid donor - (σ-bridge) - acceptor (D-σ-A) compounds. These compounds, which are candidates for incorporation in materials, have aniline-type donors and (mixed) cyano- and alkoxycarbonyl-1,1-difunctio

Full text of DOI:10.1002/ejoc.200200693

FULL PAPER
Photoinduced Charge Separation in Cyclohexylidene-Based
Donor؊(σ-Bridge)؊Acceptor Compounds ؊ Building Blocks for Materials
Wibren D. Oosterbaan,[a] Paul C. M. van Gerven,[a] Cornelis A. van Walree,[a]
Mattijs Koeberg,[b] Jacob J. Piet,[c] Remco W. A. Havenith,[a,d] Jan W. Zwikker,[a]
Leonardus W. Jenneskens,*[a] and Rolf Gleiter[e]
Keywords: DonorϪacceptor systems / Electron transfer / Photochemistry / Solvatochromism
We describe the syntheses and properties of five semi-rigid
donor−(σ-bridge)−acceptor (D−σ−A) compounds. These com- borrows its intensity from local transitions. Upon photoexcit-
pounds, which are candidates for incorporation in materials, ation, a charge-separated state is generated in virtually
have aniline-type donors and (mixed) cyano- and alkoxycar- quantitative yield. Conformational contraction or ‘‘folding’’ of
weak intramolecular charge-transfer absorption band that
bonyl-1,1-difunctionalized vinyl acceptors that are separated
this state takes place for the two D−σ−A compounds with a
by a four-σ-bond bridge. Their electronic properties were
dicyanovinyl acceptor in apolar solvents. The absence of
studied by means of photoelectron spectroscopy in combina- folding in compounds with an alkoxycarbonyl-functionalized
tion with ab initio MO calculations, cyclic voltammetry, UV-
Vis absorption spectroscopy, (time-resolved) fluorescence
spectroscopy, and time-resolved microwave conductivity
(TRMC). We found that these D−σ−A compounds exhibit a
acceptor is attributed to a reduced lifetime of the charge-sep-
arated state.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
appreciable hyperpolarizability, which is needed for efficient
frequency doubling of laser light (second harmonic genera-
tion). Upon electronic excitation of a DϪσϪA compound,
electron transfer from the donor to the acceptor can take
place to produce a highly dipolar excited state. The fluor-
escence from these states can be tuned throughout the
whole visible region[14] and highly fluorescent DϪσϪA
compounds have also been used in electroluminescent
devices.[15]
One of our ultimate goals is the application of DϪσϪA
compounds as constituents for materials, such as mono-
layers, molecular assemblies, and polymers.[16] Therefore,
the DϪσϪA compounds should possess functionalities that
can be used as anchoring points for (supramolecular nonco-
valent) interactions or further (covalent) functionalization.
Organic materials for application in molecular electronic
and photonic devices receive considerable attention.[1] Ex-
amples are polymer-based light-emitting diodes (LEDs),[2]
materials for nonlinear optics, and electronic components,[3]
such as molecular wires, rectifiers[4Ϫ6] and switches.[7,8] In
this field of research, our interest is mainly focussed on the
role that can be played by donorϪbridgeϪacceptor com-
pounds (DϪσϪA compounds) with saturated hydrocarbon
bridges. These compounds have been proposed[9] and
successfully applied[4] as molecular rectifiers. Some of
them have been shown[10] or predicted[11Ϫ13] to possess an
[a]
Debye Institute, Department of Physical Organic Chemistry,
Utrecht University,
Padualaan 8, 3584 CH Utrecht, Netherlands
Fax: (internat.) ϩ31Ϫ(0)302534533
E-mail: jennesk@chem.uu.nl
In this article we report on the synthesis and photophys-
ical properties of a set of DϪσϪA compounds that are po-
tential candidates for incorporation in materials. The parent
compound of the set is DA1 (Figure 1), which consists of
an N,N-dimethylaniline donor and a dicyanovinyl acceptor
that are separated by a cyclohexylidene-based spacer. Re-
placement of one (DA2) or both (DA3 and DA4) cyano
groups by alkoxycarbonyl groups provides the required at-
tachment points for their incorporation in materials. In
DA2, DA3, and DA4, the methyl and dodecyl groups oc-
cupy the anchoring points. Replacement of the two methyl
[b]
Laboratory of Organic Chemistry, University of Amsterdam,
Nieuwe Achtergracht 129, 1018 WS Amsterdam, Netherlands
[c]
Interfacultary Reactor Institute, Delft University of
Technology,
Mekelweg 15, 2629 JB Delft, Netherlands
[d]
Debye Institute, Theoretical Chemistry Group, Utrecht
University,
Padualaan 8, 3584 CH Utrecht, Netherlands
[e]
Institute of Organic Chemistry, University of Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2003, 3117Ϫ3130
DOI: 10.1002/ejoc.200200693
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L. W. Jenneskens et al.
FULL PAPER
Scheme 2. Syntheses of ketone 8, D1, and 14. Conditions/reagents:
(a) THF; (b) for 5 and 12: p-toluenesulfonic acid, toluene; for 10:
CF3COOH; (c) 1 atm. H2, Pd/C, THF/EtOH; (d) HCl, H2O/THF
Figure 1. DonorϪacceptor compounds and the model donor and
model acceptor compounds studied
conditions to form 11 and 6. Catalytic hydrogenation of 11
gave D1 and hydrogenation of 6, followed by deprotection
groups of the N,N-dimethylaniline donor of DA1 by with acid, afforded ketone 8.
ϪCH2CH2OH groups allows the incorporation of the func-
The carbonyl group of 8 was converted into several di-
tional entities in step-growth polymers, such as polyesters. functionalized vinyl groups by means of Knoevenagel con-
The properties of model diacetyl ester DA5 are reported. densation reactions (Scheme 3). Reaction of 8 and 4-phe-
We show that modification of the donor (D) or acceptor nylcyclohexanone with malononitrile gave DϪσϪA com-
(A) moieties does not lead to changes of their photophys- pound DA1 and model acceptor (4-phenylcyclohexylidene)-
ical properties, i.e., their propensity to undergo photo-in- malononitrile (A1), respectively. Methyl cyanoacetate,
duced charge separation. We note that small structural dimethyl malonate, and didodecyl malonate were used to
modifications can markedly affect the excited-state proper- convert 8 into DA2, DA3, and DA4, respectively. In the case
ties of DϪσϪA compounds.[17]
Results and Discussion
Synthesis
of malononitrile, an apparent pH of 7.5Ϫ8.0 in an alcohol/
water mixture is appropriate to readily accomplish the con-
densation,[23] but in the case of methyl cyanoacetate the re-
action proceeds much slower. Dimethyl malonate and dido-
decyl malonate react only when utilizing TiCl4 and pyridine
in THF.[24]
The syntheses of the donor model compounds D1 and
D2, the acceptor model compound A1 and the DϪσϪA
compounds DA1ϪDA5 are given in Schemes 1Ϫ5. First,
the 4-bromoaniline derivative 3 was prepared by alkylation
of 4-bromoaniline (1) with two equivalents of tert-butyl 2-
chloroethyl ether (2) under conditions that minimize quat-
ernization[18] (Scheme 1). tert-Butyl ether 2 was prepared by
acid-catalysed tert-butylation of 2-chloroethanol with 2-
methylpropene.[19,20]
Scheme 3. Synthesis of DϪσϪA compounds and model acceptor
compound A1. Conditions/reagents for DA1, DA2, and A1: β-
alanine, EtOH/H2O; for DA3 and DA4: TiCl4, pyridine, THF
Scheme 1. Synthesis of 3. Conditions/reagents: K2CO3, KI, butanol
The syntheses of ketone 8 and D1 are depicted in
Scheme 2. In a broad sense, the synthetic route to 8 follows Scheme 4. Synthesis of D2 (R ϭ 4-Me2NC6H4Ϫ). Conditions/re-
the route described by Kaiho et al.[21] The Grignard reagent
agents: (a) LDA, THF; (b) Me2NCH(OCH2tBu)2, MeCN
of 4-bromo-N,N-dimethylaniline (prepared either by a dry-
stirring method[22] or by application of ultrasound[21]) was
Model donor D2 was prepared according to Scheme 4.
added to cyclohexanone (9) and ketone 4, to form 10 and The dilithium salt of 2-methylpropanoic acid was added[25]
5, respectively. These alcohols were dehydrated under acidic to ketone 8 to form β-hydroxy acid 16, which was sub-
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Photoinduced Charge Separation in Cyclohexylidene-Based Compounds
FULL PAPER
sequently converted into D2 by dehydrative decar-
boxylation.[26]
Table 1. Vertical ionization energies (Iv,j) from PES, RHF/6-31G
orbital energies (Ϫεj), and assignments for D1, D2, DA1, and DMA
Although all our attempts to prepare a Grignard reagent
from 3 failed,[27] lithiation of 3 with 2 equivalents of n-bu-
tyllithium in THF was successful. The lithiation product
was coupled to ketone 4 in the same way as the Grignard
reagent of 4-bromo-N,N-dimethylaniline (Scheme 2). A
similar reaction sequence as followed for 5 gave ketal 14.
Deprotection of 14 under aqueous acidic conditions
(Scheme 5) led to removal of both the tert-butyl groups and
the ketal functionality to give 17. Condensation of 17 with
malononitrile gave 18, which was converted into DA5 by
reaction with two equivalents of acetyl chloride.
Compound
Band (j)
Iv,j/eV
Ϫεj/eV
Assignment
D1
1
2
3
4
1
2
3
4
5
1
2
3
4
5
1
2
3
4
7.2
8.7
9.4
Ϫ
7.0
8.1
8.7
9.3
Ϫ
7.5
9.1
9.9[a]
9.9[a]
Ϫ
7.45
9.00
9.85
Ϫ
7.48
8.96
10.30
11.40
7.48
8.46
8.96
10.31
11.31
7.81
56a πar-Lp
55a πar
54a Lp-πar
53a σ
67a πar-Lp
66a π
65a πar
64a Lp-πar
63a σ
71a πar-Lp
70a πar
69a πacc
68a Lp-πar
67a σ
33a πar-Lp
32a πar
D2
DA1
9.40
10.24
10.83
12.71
7.62
9.01
10.65
12.99
DMA[32]
31a Lp-πar
30a σ
[a]
Overlap of bands.
Hence, within the resolution of the PES measurements, the
ground-state electronic interaction between the olefinic
bond and the aniline functionality is small.
Scheme 5. Synthesis of DA5. Conditions/reagents: (a) HCl, H2O/
dioxane; (b) malononitrile, β-alanine, EtOH/H2O; (c) acetyl chlo-
ride, pyridine, CH2Cl2
The PE spectrum of DA1 shows three bands of which the
two with the lowest values of Iv,j are assigned to the N,N-
dimethylaniline donor. These bands are shifted to higher
energy, relative to those in D1, by 0.3 and 0.4 eV, respec-
tively (calculated shifts: 0.33 and 0.44 eV, respectively). This
effect is attributed to an inductive shift resulting from the
introduction of the acceptor moiety.[34] The third band is
composed of two peaks, which are assigned to the πar-Lp
MO and the π[CϭC(CN)2] MO. The latter assignment is
corroborated by the reported value for Iv,j of 10.21 eV for
the π[CϭC(CN)2] MO in isopropylidenemalononitrile.[35]
These results show that, besides inductive shifts, no clear-
cut through-bond interactions are discernible between the
donor and acceptor parts of DA1.
Photoelectron Spectroscopy
To assess if ground-state electronic interactions[28Ϫ30] be-
tween the functionalities in the donorϪacceptor com-
pounds are present, He(I) photoelectron (PE) spectra of
DA1 and model donor compounds D1 and D2 were re-
corded and RHF/6-31G ab initio calculations were per-
formed. The vertical ionization energies Iv,j for D1, D2, and
DA1 are given in Table 1 together with RHF/6-31G orbital
energies εj and their assignments. The PE spectra are given
in Figure 1S (Supporting Information). The photoelectron
spectra were analysed using the following procedure. Firstly,
the PE bands of D1, D2, and DA1 were correlated with
those of model compounds. Secondly, the experimental ion-
ization potentials Iv,j were correlated with ab initio molecu-
Cyclic Voltammetry
The oxidation and reduction potentials of the
donorϪacceptor compounds and model donor and ac-
ceptor compounds were determined by cyclic voltammetry.
lar orbital energies using Koopmans’ theorem (Iv,j
ϭ
Ϫεj).[31]
The bands in the PE spectrum of D1 can be readily corre- Data are given in Table 2. The electrochemical oxidation
lated to those previously found for N,N-dimethylaniline wave of the N,N-dimethylaniline donor in D1, D2, DA1,
(DMA).[32] The first and third band of DMA are assigned DA2, DA3, and DA4 is followed by a back-reduction wave
to the two linear combinations of one of the two degenerate with a substantially smaller peak current. As this wave
π(e1g) HOMOs of benzene with the nitrogen lone pair Lp gains intensity upon increasing the scan rate (not shown),
(Lp-πar and πar-Lp). The second band is assigned to the it is evident that oxidation is followed by an irreversible
benzene π(e1g) MO, which has a nodal plane running chemical reaction.[36] Using a scan rate of 0.5 V·sϪ1, a ratio
through the nitrogen atom.[32] The PE spectrum of D2 of the cathodic peak current to the anodic peak current[37]
shows an additional band at 8.1 eV attributed to ionization (ipc/ipa) of 0.66 was obtained, allowing a reasonable estimate
from a MO centered on the olefinic bond. For isopropyl- of E°ox. It appears that the presence of an acceptor raises
idenecyclohexane, a value for Iv,j of 8.34 eV has been re- the value of E°ox of the N,N-dimethylaniline donor by 0.02
ported.[33] The presence of the olefinic bond does not have V, to 0.05 V. The data suggest that the effect is more pro-
a noticeable influence on the positions of the other bands. nounced for stronger acceptors and that it is inductive in
Eur. J. Org. Chem. 2003, 3117Ϫ3130
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L. W. Jenneskens et al.
FULL PAPER
nature (see above). In contrast to the compounds with an substantially influenced by the presence of the phenyl
N,N-dimethylaniline donor, the oxidation of the donor in group. The acceptor absorptions of DA1 (λmax ϭ 230 nm),
DA5 is quasi-reversible (ipc/ipa ϭ 0.94). Moreover, E°ox is DA2 (λmax ϭ 230 nm), and DA3 (λmax ϭ 218 nm) show up
substantially higher (by 0.14 V, relative to DA1). Thus, the clearly in the difference spectra in Figure 2(b). The values
electron donating power of the donor in DA5 is reduced of λmax are in good agreement with those for compounds
relative to that of the other DϪσϪA compounds. Presum- lacking a donor, i.e., A1, methyl cyano(cyclohexylidene)-
ably, this is due to an extra inductive effect exerted by the acetate[39] (λmax ϭ 234 nm, ε ϭ 14.7·103 Ϫ1·cmϪ1) and di-
oxygen atoms of the ester.
ethyl isopropylidenemalonate[40] (λmax ϭ 218 nm, ε ϭ
10.2·103 Ϫ1·cmϪ1 in 95% ethanol). The ‘‘peak’’ at ca.

Table 2. Redox potentials in V versus SCE as measured by cyclic
voltammetry in acetonitrile. The term E°ox denotes the half wave
potential and ∆Ep the potential difference between the peak values
of the forward and backward scans of the 0/ϩ1 electrode process
measured at a scan rate of 0.5 V·sϪ1. The values of Ep and Ep
260 nm in the difference spectra is attributed to the bor-
1
rowing of intensity by the aniline La transition from the
acceptor πǞπ* transition.[17,39]
ox
red
are the peak values of the first forward oxidation and reduction
Table 3. Absorption maxima and molar absorption coefficients of
the donorϪacceptor compounds and their model chromophores in
cyclohexane at 20 °C
scans, respectively, measured at a scan rate of 50 mV·sϪ1
Compound
E°ox (∆Εp/mV)
Ep
Ep
red
ox
Compound
λmax/nm [ε/103 Ϫ1·cmϪ1 [a]
]
D1
0.63 (150)
0.63 (130)
Ϫ
0.68 (136)
0.67 (135)
0.65 (138)
0.65 (149)
0.82 (74)[b]
0.65
0.65
Ϫ
0.71
0.69
0.69
0.69
Ϫ
Ϫ
Ϫ
D1[17]
D2[17]
A1
DA1
DA2
DA3
DA4
DA5[b]
203.5 [22.5]
204.0 [35.5]
234.0 [17.3]
203.0 [25.5]
203.5 [25.6]
204.5 [29.2]
205.0 [28.3]
253.5 [16.54]
253.0 [18.80]
302.0 [2.28]
302.5 [2.31]
D2
A1
Ϫ1.85
Ϫ1.86
Ϫ2.02
DA1
DA2
DA3
DA4
DA5
255.0 [29.3]
254.5 [28.4]
253.5 [21.9]
253.5 [20.9]
257.5 [26.2]
304.5 [2.67]
302.5 [2.64]
302.5 [2.32]
302.5 [2.18]
303.5 [2.53]
Ͻ Ϫ2.1[a]
Ͻ Ϫ2.2[a]
Ϫ1.86
[a]
[b]
Outside potential window. ∆Ep at 50 mV sϪ1 scan rate.
[a]
Error in absorption coefficients is ca. 2% for maxima near
[b]
200 nm and ca. 1% for maxima near 250 and 300 nm.
CH2Cl2.
Solvent:
The reduction waves for all donorϪacceptor compounds
are irreversible, even at scan rates of 2 V·sϪ1. In the case
of an irreversible wave, the onset can be used to monitor
differences in reduction potential.[38] For the investigated
compounds, however, the differences in the onset of the re-
duction waves were identical to differences in the peak po-
tentials. Therefore the latter are reported. The values show
that the dicyanovinyl acceptor is the strongest acceptor and
that replacement of a cyano group by an alkoxycarbonyl
group reduces the reduction potential by 0.16 V. Thus, re-
placement of both cyano groups by alkoxycarbonyl groups,
as in DA3 and DA4, is expected to reduce the reduction
potential even more. Unfortunately, the first reduction wave
of DA3 and DA4 is positioned outside the potential window
of the CV set-up that we used.
Electronic Absorption Spectra
Absorption spectra of D1, D2, A1, DA1, DA2, DA3, and
DA4 were recorded in cyclohexane. For solubility reasons,
the spectrum of DA5 was recorded in CH2Cl2. Absorption
maxima and molar absorption coefficients are given in
Table 3, while representative spectra are depicted in Fig-
ure 2. As discussed previously,[17] the N,N-dialkylaniline do-
nor in D1 and D2 exhibits three absorption bands, in the
region 195Ϫ350 nm, which are positioned near 204, 254,
and 303 nm. The acceptor chromophore in A1 displays
an absorption at 234.0 nm [Figure 2(b); ε ϭ 17.3·103
Figure 2. (a) Absorption spectra of D1 (Ϫ), DA1 (···), DA2
(Ϫ Ϫ Ϫ), and DA3 (Ϫ·Ϫ) in cyclohexane at 20 °C. The inset shows
a magnification of the region 275Ϫ375 nm. (b) Absorption spec-
trum of A1 (Ϫ) in cyclohexane at 20 °C and difference spectra
obtained upon subtraction of the absorption spectrum of D1 from
that of DA1 (···), DA2 (Ϫ Ϫ Ϫ) and DA3 (Ϫ·Ϫ). The inset shows

Ϫ1·cmϪ1, almost identical to the value reported[39] for
cyclohexylidenemalononitrile in cyclohexane (λmax
234 nm, ε ϭ 18.2·103 Ϫ1·cmϪ1)]. This result shows that the
ϭ
absorption band of the dicyanovinyl acceptor in A1 is not a magnification of the region 275Ϫ375 nm
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Photoinduced Charge Separation in Cyclohexylidene-Based Compounds
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In the spectrum of DA1, a weak tail is discernible be- cation of Equation (1) then gives V* ϭ 1200 cmϪ1. This
tween 325 and 350 nm, which is absent in the spectrum of value compares favorably with that of 1080 cmϪ1 for an-
D1 [see also the inset of Figure 2(b)] its occurrence was other four-σ-bond-bridged compound.[44] The magnitude
independent of concentration. Moreover, the intensity of this coupling denotes that, for DA1 in cyclohexane, ET
maximum at 304.5 nm is slightly red-shifted to that of D1 occurs in the adiabatic regime. Unfortunately, assessment of
(302.0 nm) and it has a significantly higher absorption coef- V is not possible, since the radiative rate constants for CT
ficient. These results reveal that a very weak absorption fluorescence in a set of solvents with different polarity are
band is present in the spectrum of DA1, which can be attri- not available.
buted to a charge-transfer (CT) transition involving elec-
The absorption spectrum of DA2 does not exhibit such a
tron transfer from the N,N-dialkylaniline donor to the dicy- clear absorption tail as does that of DA1, but the band at
anovinyl acceptor.[41] These results are in line with those 302.5 nm again has a significantly higher absorption coef-
obtained by Pasman et al.[33,39,42] and others.[43]
ficient than that of D1. In the difference spectrum of DA2
Previous work has shown[44,45] that, for DϪσϪA com- and D1 [inset of Figure 2(b)], this peak shows up as a weak
pounds with bridges of up to six σ-bonds, the CT absorp- CT absorption that is blue-shifted relative to the CT ab-
tion arises from intensity borrowing from transitions lo- sorption in DA1 by ca. 0.1 eV as judged from the onsets.
calized in the D and/or A chromophores; local excited-state This absorption finds its origin in the weaker electron-ac-
character is mixed in the CT state to a degree governed by cepting power of the (alkoxycarbonyl)cyanovinyl acceptor
the coupling energy V*. In compounds with bridges of up in DA2 as compared to the dicyanovinyl acceptor in DA1,
to six σ-bonds, V* is dominant over the coupling V of the as shown by CV (see above). For DA3 and DA4 (not
CT state with the ground state. Thus, the value of V* can shown), an even more hypsochromically shifted CT absorp-
be estimated by:[44]
tion band with an onset near 300 nm is distinguished in
the difference spectrum. This observation confirms that the
di(alkoxycarbonyl)vinyl group is a weaker acceptor than the
(1) (alkoxycarbonyl)cyanovinyl group. The absorption spec-
trum of DA4 is virtually identical to that of DA3 and the
absorption spectrum of DA5 exhibits a weak CT absorption
tail similar in appearance to that in DA1.
In this equation f and νA denote the oscillator strength
and the transition energy of the CT absorption, respectively,
while f* and ν*A represent the same quantities for the local
transition. We take the absorption of aniline at 253.5 nm as
the local excitation that donates intensity to the CT absorp-
tion. This absorption has the correct symmetry for mixing
Fluorescence Spectroscopy and Time-Resolved
Measurements
The fluorescence spectra of DA1, DA2, DA3, DA4, and
with the acceptor πϪπ* absorption[17] and, hence, also for DA5 were recorded in solvents of different polarity. For all
mixing with the CT absorption. The maximum νA (32,300 compounds the donor fluorescence, which for D1 in cyclo-
cmϪ1), absorption coefficient ε (400 cmϪ1·Ϫ1) and width hexane is located at 335 nm,[17] is strongly quenched. In-
at half height ∆ν1/2 (4000 cmϪ1) of the CT absorption of stead, a broad fluorescence band is visible of which the
DA1 in cyclohexane were estimated from the inset of Fig- maximum displays a strong bathochromic shift with in-
ure 2(b). This process gives for the oscillator strength f ϭ creasing solvent polarity (Table 4). This effect is character-
4.32·10Ϫ9 ·ε·∆ν1/2 ϭ 0.0069. The data for the aniline ab- istic for charge-transfer (CT) fluorescence.[39,42,46Ϫ48] In ad-
sorption at 253.5 nm (νA* ϭ 39450 cmϪ1) were taken from dition, dual CT fluorescence is observed for DA1 and DA5
those of 4-cyclohexyl-N,N-dimethylaniline[17] and give f* ϭ in cyclohexane (Table 4). Upon lowering the temperature
4.32·10Ϫ9 · 16540 cmϪ1·Ϫ1 · 4100 cmϪ1 ϭ 0.29. Appli- of a solution of DA1 in hexanes from 275 to 190 K the
Table 4. CT fluorescence maxima νCT in nm (quantum yield ΦCT/10Ϫ3) of DA1ϪDA5 in various solvents at 20 °C together with the
results of the solvatochromic fits
Solvent
∆f
DA1
DA2
DA3
DA4
DA5
[c]
[c]
cyclohexane
benzene[b]
0.101
0.117
0.173
0.193
0.234
0.254
0.308
491[a,b] (46)
533 (36)
411 (5.9)
531 (14)
498 (22)
509 (19)
530 (6.9)
562 (2.7)
655 (Յ 0.1)
28.0Ϯ0.8
Ϫ41.1Ϯ3.6
0.985
470[b,d](30)
508 (56)
527 (0.68)
486 (0.48)
498 (0.51)
531 (0.61)
516 (0.90)
477 (0.87)
489 (0.71)
519 (0.89)
542 (0.79)
615 (Յ 0.1)
27.2Ϯ0.5
Ϫ34.8Ϯ2.0
0.995
di-n-pentyl ether
di-n-butyl ether
diisopropyl ether
diethyl ether
THF
516[b] (52)
516[b] (48)
535 (14)
492[b] (37)
500[b] (26)
513 (29)
566 (5.0)
665 (Յ 0.1)
28.2Ϯ0.5
Ϫ41.7Ϯ2.0
0.998
556 (0.48)
529 (19)
[c]
[e]
νCT(0)/103 cmϪ1
slope/103 cmϪ1
corr. coeff.
26.1Ϯ0.4
Ϫ31.7Ϯ2.0
0.996
29.1Ϯ0.3
Ϫ40.4Ϯ1.3
0.999
[a]
[b]
[c]
A low-intensity shoulder with an estimated maximum of 420 nm is observed.
Value not used in solvatochromic fit.
No CT
[d]
[e]
fluorescence observed. A low-intensity shoulder with an estimated maximum of 400 nm is observed. Could not be measured.
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L. W. Jenneskens et al.
FULL PAPER
which is close in energy to the CT state. The decrease in the
value of ΦCT in solvents with highest polarity is attributed
to the small energy gap separating the CT and ground
states. A small gap will increase the rate of internal conver-
sion.[56]
On the assumption that the excited-state dipole moment
(µe) is large relative to that of the ground state, the
LippertϪMataga relationship[57Ϫ59] [Equation (2)] predicts
a linear relationship between the fluorescence maximum
νCT of a CT state with dipole moment µe and the solvent
polarity parameter ∆f:
Figure 3. Fluorescence of DA1 in hexanes at different temperatures.
Excitation at 300 nm
(2)
fluorescence band at 496 nm gradually disappears while the
band at 420 nm gains intensity (Figure 3). This behaviour
is rationalized by the occurrence of folding of the initially
extended CT (ECT) species to a more compact CT (CCT)
species.[49Ϫ52] Lowering the temperature suppresses this
folding process. The occurrence of the folding process in
DA1 was confirmed by time-resolved measurements (see be-
low). We note that folding has been reported for related
compounds that have an identical four-σ-bond bridge, but
with their donor and acceptor units having changed posi-
tions. These compounds have a (substituted) 1-phenylpip-
eridine donor and a (substituted) naphthyl acceptor.[52Ϫ55]
CCT emission is not observed for DA2, DA3, and DA4.
Since it is believed that, for these compounds, the Cou-
lombic attraction between the charges in the excited state is
not substantially different from that in DA1, the absence of
CCT emission is presumably related to a shorter lifetime of
the ECT state.
The overall CT fluorescence quantum yields ΦCT are low
for all DϪσϪA compounds under study (ΦCT Ͻ 0.06). As
indicated by the low intensity of the CT absorption, which
is largely dependent on the electronic coupling between
D·ϩϪσϪA·Ϫ and locally excited states, the radiative rate
constant for CT emission is expected to be small. It has
been observed for related DϪσϪA systems that the dicy-
anovinyl and the (alkoxycarbonyl)cyanovinyl acceptor
groups provide a dark decay channel that enhances internal
conversion of the CT state. This feature has been proposed
to be due to strong coupling of the CT state with non-
emissive locally excited singlet and triplet acceptor states
through torsional motion around the CϭC bond.[14,39]
Replacement of one dicyanovinyl cyano group by an al-
koxycarbonyl group, namely going from DA1 to DA2, re-
duces the value of ΦCT by a factor of 2Ϫ3. An identical
reduction in the value of ΦCT was observed by Hermant et
al. and might be ascribed to a larger rotational freedom
of the alkoxycarbonyl group, providing an additional dark
process.[14] Replacement of both cyano groups by alkoxy-
carbonyl groups (from DA1 to DA3 or DA4) reduces the
value of ΦCT by a factor of 10Ϫ110. No CT fluorescence is
detected for DA3 and DA4 in cyclohexane, although the
local donor emission is still quenched. This observation
(3)
In Equations (2) and (3), which are in CGS units, h is
Planck’s constant, c is the velocity of light, νCT(0) is the
gas-phase position of νCT, εs is the solvent dielectric con-
stant, and n is the solvent refractive index. Furthermore, ρ
denotes the effective radius of the solvent cavity enclosing
the CT species. In Figure 4, the fluorescence maxima νCT
found for DA1 to DA5 are plotted versus ∆f. A good fit to
Equation (2) is obtained for the (ECT) emission maxima
of DA2, DA3, and DA4. For DA1 and DA5, fitting of the
fluorescence maxima to Equation (2) is complicated by the
presence of two emitting species. The fluorescence of ECT
and CCT species for these compounds in cyclohexane only
partly overlaps and, hence, the maxima of the ECT and
CCT fluorescence bands can be obtained with reasonable
accuracy. In di-n-pentyl and di-n-butyl ether as solvents,
emission from CCT and ECT species is anticipated to over-
lap considerably more, making it impossible to extract pro-
per maxima without the use of time-resolved methods.
Since the driving force for folding decreases with solvent
polarity, it can be expected (and has been observed[60]) that
the CCT emission intensity decreases correspondingly.
Based upon results for similar compounds,[50,60] we believe
that the maxima in diisopropyl ether and the more polar
Figure 4. Plot of charge-transfer fluorescence maxima νCT versus
the solvent polarity parameter ∆f; ᮀ DA1; ϫ DA2; ᭞ DA3; ∆ DA4;
suggests rapid crossing to a state localized on the acceptor, Ο DA5. The fit (—) to the ECT maxima of DA1 is given as well
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Eur. J. Org. Chem. 2003, 3117Ϫ3130

Photoinduced Charge Separation in Cyclohexylidene-Based Compounds
FULL PAPER
solvents are representative for ECT fluorescence and can be cay was observed in diethyl ether, indicating that only ECT
used as such in the fitting procedure.
emission is present.
The results of linear fits of the proper (ECT) fluorescence
maxima to Equation (2) are given in Table 4. From the
2
slope, Ϫ2µe /hcρ3, the value of µe can be determined. For
Conclusion
DA1, an estimate of ρ as 0.4 times the molecular length
[14 A according to molecular mechanics (MM2 force field)
˚
Photoelectron spectroscopy, in combination with RHF/
6-31G MO calculations, do not reveal any ground-state,
through-bond interactions between the donor and acceptor
in DA1. Nevertheless, a substantial (0.3 to 0.4 eV) inductive
effect of the dicyanovinyl acceptor on the N,N-dimethylani-
line π-type MOs is present. This effect translates in cyclic
voltammetry to a 0.05 V higher oxidation potential of the
N,N-dimethylaniline donor. By means of electronic absorp-
tion spectroscopy, however, a weak charge-transfer absorp-
tion band can be distinguished for all DϪσϪA compounds,
which arises from intensity borrowing from transitions lo-
calized on the individual chromophores. An excited state
interaction between the locally excited and the CT state of
about 1200 cmϪ1 was deduced from the characteristics of
this band. This value places ET for these compounds in the
adiabatic regime.
Electron transfer occurs for all DϪσϪA compounds
upon photoexcitation, as was confirmed by the quenching
of the local donor fluorescence and the appearance of a
broad solvatochromic fluorescence band. Even for the
weakest electron acceptor, the di(alkoxycarbonyl)vinyl ac-
ceptor, charge transfer is still feasible in cyclohexane, the
most-apolar solvent studied.
Furthermore, conformational contraction or ‘‘folding’’
was found to occur upon photoexcitation of DA1 and DA5
in cyclohexane and, presumably, in slightly more polar sol-
vents as well. The absence of folding in the case of the other
DϪσϪA compounds is attributed to the short lifetime of
the intramolecular CT state of these compounds.
The results show that modification of both the electron
donor and the electron acceptor is achieved without major
changes of their characteristic photophysical properties.
This feature renders DA2ϪDA5 as suitable building blocks
for the preparation of organized DϪσϪA arrays (self-as-
sembled monolayers) as well as DϪσϪA polymers formed
by either supramolecular non-covalent interaction or
further covalent modifications.[16]
calculations] yields a value for µe of 27 D. This value corre-
˚
sponds to a unit charge separation over 5.6 A, which
˚
is in reasonable agreement with the distance of 7.1 A be-
tween the aniline nitrogen atom and the carbon atom of the
acceptor that is incorporated in the ring. For DA2 and DA5,
slopes similar to that found for DA1 were obtained, indicat-
ing that charge separation occurs over a similar distance.
The slopes for DA3 and DA4 are somewhat smaller than
those of DA1 and DA5. Fluorescence maxima for these
compounds, however, could be obtained only in a limited
range of solvent polarities. Hence, the smaller slopes for
DA3 and DA4 may point to a smaller excited-state dipole
moment and/or a larger solvent cavity, ρ.[61]
To verify the results of the steady-state measurements,
time-resolved measurements were performed on DA1. Flu-
orescence lifetimes from single-photon counting (SPC) and
excited-state dipole moments obtained by time-resolved mi-
crowave conductivity (TRMC)[62,63] of DA1 are given in
Table 5. The fluorescence lifetimes were used to fit the
TRMC transients. Two fluorescence lifetimes were found
for DA1 in cyclohexane and benzene. In both cases, the long
lifetime describes the decay of the low-energy band, while
the short lifetime describes the decay of a high-energy band
and the rise of a low-energy band. This feature shows that
the species emitting at low energy is formed out of the spec-
ies emitting at high energy. Thus, electron transfer in an
extended conformation produces an extended charge-trans-
fer (ECT) species and is followed by folding of the ECT
species to a more compact charge-transfer (CCT) species
(‘‘harpooning’’).[49Ϫ52] The TRMC excited-state dipole mo-
ment associated with the ECT species of DA1 (31 D; corre-
˚
sponding to unity charge separation over a range of 6.5 A)
is in reasonable agreement with the value obtained from
solvatochromism (27 D). The TRMC dipole moments for
the CCT species (about 5 and 11 D) represent lower limits
since they were obtained by assuming a quantum yield of
CCT formation of 1. Nevertheless, as expected, they are
smaller than those for ECT emission. Only fluorescence de-
Experimental Section
Table 5. Fluorescence lifetimes τ from SPC and excited-state dipole
moments µe of the ECT and CCT species of DA1 as obtained by
TRMC
General Remarks: All reactions and distillations were carried out
under an atmosphere of dry nitrogen unless stated otherwise. Com-
mercially available reagents were used without further purification.
4-Phenylcyclohexanone was obtained from Acros. THF and diethyl
ether were distilled from Na/benzophenone prior to use. MeCN
and pyridine were distilled from CaH2. CH2Cl2 was distilled from
ECT[a]
CCT[a]
Solvent
τ/ns
µe/D
τ/ns
µe/D
cyclohexane
benzene
diethyl ether
3.7
7
0.5
31
6.5
17
ϳ5
11
˚
P2O5. EtOH was stored on molecular sieves (3 A). Column chro-
31
matography was performed using Acros silica gel 0.035Ϫ0.070 mm,
pore diameter ca. 6 nm. Thin layer chromatography was performed
on Merck silica gel 60 F254. Spots were detected by the use of iodine
vapor and/or UV light. Melting points were determined on a home-
made melting point apparatus and are uncorrected. Gas chroma-
[b]
[a] Cylindrical and spherical geometries were used in the calculation
[b]
of the ECT and CCT dipole moments, respectively.
ured.
Not meas-
Eur. J. Org. Chem. 2003, 3117Ϫ3130
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3123

L. W. Jenneskens et al.
FULL PAPER
tography was performed on a Varian 3350 instrument equipped
with a DB-5 (ID, 0.309 mm, 25 m) capillary liquid-phase column
and an FID using nitrogen as the carrier gas. NMR spectra were
FeCp2 couple (E°ox ϭ 0.31 V versus SCE).[67] Since we obtained
typically a value E°ox ϭ 0.08 V versus Ag/AgNO3 for the FeCp2/
ϩ·
ϩ·
FeCp2 couple, we added 0.23 to our values to reference them
recorded on
a
Bruker AC 300 spectrometer operating at
to SCE. A value of ∆Ep of 74 mV at 50 mV·sϪ1 was obtained
1
300.13 MHz for H NMR and at 75.47 MHz for 13C NMR spec-
troscopy. Samples were dissolved in CDCl3 unless stated otherwise.
Chemical shifts (in ppm) are given relative to internal TMS (δ ϭ
0 ppm) in the case of 1H NMR spectra and relative to CDCl3 (δ ϭ
77.00 ppm) in the case of 13C NMR spectra. Infrared spectra were
recorded on a Mattson Galaxy Series FTIR 5000 operating with
2 cmϪ1 resolution (indicated by ‘‘FT-IR’’) or on a PerkinϪElmer
283 spectrophotometer (indicated by ‘‘IR’’); solids were measured
in KBr pellets and liquids were measured as a thin film between
NaCl plates. Peak maxima are given in cmϪ1. Peaks intensities are
designated as s (strong), m (medium), or w (weak). Elemental
analyses were performed by the Microanalytical Laboratory H.
Kolbe, Mülheim an der Ruhr, Germany. For compounds A1, D2,
and DA1, which are composed of the elements C, H, and N, the
elements C and H were determined independently. For compounds
DA2ϪDA5, which are composed of the elements C, H, N, and O,
the elements C, H, and O were determined independently.
for ferrocene.
Calculations: All quantum mechanical calculations were run on a
Silicon Graphics Power Challenge and Origin 2000 computer using
GAMESS-UK.[68] Geometries were optimized at the RHF/6-31G
level of theory [cartesian coordinates files are available upon re-
quest (L. W. J.)]. All optimized structures possess C1 symmetry. The
total RHF/6-31G energies (in hartree) were: DA1: Ϫ817.801775;
D1: Ϫ596.592471; D2: Ϫ712.459898; DMA: Ϫ363.640788.
Molecular Mechanics (MM2) calculations were performed using
the MM2 Force Field as implemented in CambridgeSoft Chem3D
Pro, version 4.0 (1997).
tert-Butyl 2-Chloroethyl Ether (2): A 500-mL pressure flask was
charged with 2-chloroethanol (84.8 g, 1.05 mol) and dry CH2Cl2
(150 mL). The magnetically stirred solution was cooled to Ϫ10 °C.
Separately condensed 2-methylpropene (150 g, 2.67 mol), cooled to
Ϫ20 °C, and concentrated H2SO4 (5 mL) were added. The flask
was stoppered and the mixture was stirred for 60 h at room tem-
perature. After cooling the mixture to Ϫ10 °C, the flask was opened
and the mixture was poured into a saturated NaHCO3 solution
(400 mL). The layers were separated and the water layer was ex-
tracted with n-pentane (2 ϫ 100 mL). The combined organic layers
were washed with water (100 mL) and dried with MgSO4. After
filtration, the solvent was carefully evaporated under reduced press-
ure. To remove most of the CH2Cl2, n-pentane (50 mL) was added
and evaporated again at the rotary evaporator. The remaining col-
orless oil (127.5 g, 89%) was essentially pure tert-butyl 2-chloro-
Photophysics: UV-Vis absorption and fluorescence spectroscopy
were performed using the equipment and procedures described in
ref.[17] For low-temperature fluorescence measurements, we used
an Oxford Instruments Optistat DN liquid nitrogen cryostat regu-
lated with an Oxford Instruments ITC 502 temperature controller.
At each temperature, the sample was thermally equilibrated for
20 min before data collection. The sample was not degassed.
Fluorescence quantum yields[64] were determined relative to a refer-
ence solution [quinine bisulfate in 1  H2SO4 (␾ ϭ 0.546)[65] or
naphthalene in cyclohexane (␾ ϭ 0.23Ϯ0.02)].[64] Spectroscopic-
grade solvents were used throughout, with the following exceptions.
Diisopropyl ether (Acros 99.5ϩ%) and di-n-butyl ether (Fluka, p.
a.; low in aromatic compounds; Ն 99.5%) were stirred with LiAlH4
for at least one day and then distilled from LiAlH4. Di-n-pentyl
ether (Fluka, 99%) was purified as follows. Five parts of the ether
were stirred with one part of sulfuric acid (Merck, p. a. 95Ϫ97%)
for 1 d. The brown mixture was poured into 2.5 parts of water and
the layers were separated. The organic layer was washed with water,
saturated NaHCO3 solution and water (similar volumes), dried
with MgSO4, and filtered. This procedure was repeated until a
brown color did not develop anymore upon stirring with sulfuric
acid. Subsequently, the ether was treated as described for diisopro-
pyl and di-n-butyl ether. Hexanes, spectrophotometric grade (Ն
93%; generally a mixture of several isomers of hexane, predomi-
nantly n-hexane, and methylcyclopentane) was obtained from
Acros.
1
ethyl ether. H NMR: δ ϭ 1.21 (s, 9 H), 3.56 and 3.61 (A2B2, J ϭ
6.3 Hz, 4 H) ppm. 13C NMR: δ ϭ 27.4, 43.6, 62.4, 73.5 ppm (in
accordance with ref.[69]). FT-IR: ν˜max ϭ 2976, 2935, 2877, 1392,
1364, 1249, 1235, 1200, 1106, 1084, 669 cmϪ1 (in accordance with
ref.[70]).
4-Bromo-N,N-bis(2-tert-butoxyethyl)aniline (3): A stirred mixture of
tert-butyl ether 2 (63.7 g, 0.466 mol), 4-bromoaniline (39.6 g, 0.230
mol), K2CO3 (64.3 g, 0.465 mol) and KI (4.16 g, 25.1 mmol) in
butanol (120 mL) was heated under reflux for 6 d. TLC (n-hexane/
acetone, 10:1 v/v) showed that most of the starting compound had
reacted. The mixture was filtered through a glass frit and the resi-
due washed with n-butanol (2 ϫ 50 mL). The filtrate was concen-
trated under reduced pressure and the remaining oil was subjected
to vacuum distillation using a 10 cm Vigreux column. Two frac-
tions were collected, one (I, 42.9 g) boiling at 50Ϫ142 °C
(0.01 Torr) and another (II, 22.3 g of a viscous colorless oil) at
142Ϫ146 °C (0.01 Torr). Fraction II proved to be pure product 3
Picosecond single-photon counting (SPC) fluorescence decay was
1
(yield: 26%). According to H NMR spectroscopy, fraction I con-
measured using the set-up and fit programs described in ref.[66]
.
sisted of 4-bromoaniline (8%), monoalkylated product (71%) and
dialkylated product 3 (21%). To this mixture were added 2 (25.6 g,
0.187 mol), K2CO3 (26.0 g, 0.188 mol), KI (6.21 g, 37.4 mmol), and
butanol (40 mL) and then the mixture was treated in the same man-
ner as the original reaction mixture. Vacuum distillation yielded,
after some first runnings (13.6 g), a colorless oil (37.1 g) collected
at 134Ϫ146 °C (0.01 Torr) which, according to 1H NMR spec-
troscopy, consisted of 3 (91%) and monoalkyl product (9%). This
The samples were degassed by purging with Ar for 10 to 15 min
and were excited at 310Ϫ320 nm.
Cyclic Voltammetry: Cyclic voltammetry (CV) measurements were
performed with an EG&G Potentiostat/Galvanostat Model 263A
in MeCN (Acros p. a. grade, freshly distilled from CaH2) contain-
ing 0.1  tetrabutylammonium hexafluorophosphate (Fluka, elec-
trochemical grade) as the supporting electrolyte at scanning rates
of 50 mV·sϪ1 to 1.5 V·sϪ1. The solute concentration was 1 to 2 m. fraction was further purified as follows. The mixture was heated
The solution was purged with N2 for at least 10 min prior to the
measurement. Redox potentials were determined relative to an Ag/
AgNO3 (0.1  in MeCN) reference electrode and were referenced to
SCE by regularly measuring the oxidation potential of the FeCp2/
with acetic acid anhydride (2.00 g, 19.6 mmol) at 120 °C for 15 min.
TLC (n-hexane/acetone, 10:1 v/v) showed that all the monoalkyl-
ated product had reacted to give the acetylated product. Column
chromatography (eluent: petroleum ether/diethyl ether, 4:1 v/v)
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Eur. J. Org. Chem. 2003, 3117Ϫ3130

Photoinduced Charge Separation in Cyclohexylidene-Based Compounds
yielded an additional amount (34.6 g, 40%) of pure 3. Total yield: through Celite and then evaporation of the solvent yielded 7 as an
FULL PAPER
66% with respect to 4-bromoaniline. 1H NMR: δ ϭ 1.16 (s, 18 H),
off-white solid (34.1 g, 98%). A small sample was purified by char-
3.47 and 3.48 (A2B2, 8 H), 6.54Ϫ6.60 (AAЈBBЈ, 2 H), 7.22Ϫ7.27 coal treatment and subsequent crystallization (60 mg mLϪ1) from
(AAЈBBЈ, 2 H) ppm. 13C NMR: δ ϭ 27.5, 52.2, 58.6, 73.1, 107.3, n-hexane/ethyl acetate (10:1 v/v) to yield 7 as colorless needles (m.p.
113.2, 131.8, 147.0 ppm. FT-IR: ν˜max ϭ 3090, 3043, 2972, 2932,
2904, 2871, 1591, 1498, 1472 (sh), 1383, 1363, 1272Ϫ1196, 1.61Ϫ1.77 (two overlapping m, 4 H), 2.34Ϫ2.39 (m, 2 H),
102Ϫ104 °C). 1H NMR: δ ϭ 0.98 (s, 6 H), 1.40Ϫ1.51 (m, 2 H),
1130Ϫ1083, 881, 806, 738 cmϪ1
.
2.42Ϫ2.53 (m, 1 H), 2.91 (s, 6 H), 3.52 (s, 2 H), 2.55 (s, 2 H),
6.71Ϫ6.73 (m, 2 H), 7.10Ϫ7.13 (m, 2 H) ppm. 13C NMR (APT):
δ ϭ 22.7, 40.7, 42.8, 112.8, 127.3 (CH and CH3); 30.1, 97.2, 134.9,
149.0 (q); 30.2, 32.5, 69.8, 70.0 (CH2) ppm. FT-IR: ν˜max ϭ 2950,
2929, 2859, 2797, 1866 (w), 1617, 1523, 1395, 1292, 1242Ϫ1209,
9-[4-(Dimethylamino)phenyl]-3,3-dimethyl-1,5-dioxaspiro[5.5]-
undecan-9-ol (5): solution of 4-bromo-N,N-dimethylaniline
A
(40.4 g, 202 mmol) in THF (200 mL) was added to magnesium
(5.16 g, 212 mmol), which had been activated by dry-stirring[22] for
2 d. The mixture began to boil and the reaction was kept under
control by occasional cooling with an ice-bath. Stirring was con-
tinued overnight at room temperature. The mixture was cooled to
1168, 1137Ϫ1103, 916, 873, 814, 797, 544, 495 cmϪ1
.
4-[4-(Dimethylamino)phenyl]cyclohexanone (8): A solution of 7
(32.9 g, 108 mmol) in 2  HCl solution (180 mL) and THF
Ϫ30 °C and a solution of 3,3-dimethyl-1,5-dioxaspiro[5.5]undecan- (180 mL) was heated under reflux for 1 h. The hot solution was
9-one (4)[71] (36.0 g, 182 mmol) in THF (120 mL) was added drop-
wise, while maintaining the temperature of the reaction mixture
below Ϫ30 °C. Stirring was continued for 1 h at Ϫ30 °C and then
overnight at room temperature. The reaction mixture was poured
into a saturated NH4Cl solution (140 mL) and extracted with
CH2Cl2 (3 ϫ 200 mL). The combined organic layers were dried
with MgSO4 and filtered. Removal of the solvent under reduced
pressure afforded a dark-green solid, which was subjected to col-
umn chromatography [SiO2 (400 g); eluent: n-hexane/ethyl acetate,
3:1 v/v, followed by ethyl acetate when the N,N-dimethylaniline im-
poured into a large Erlenmeyer flask containing solid NaHCO3
(35.4 g, 421 mmol) while stirring vigorously. When all the CO2 had
evolved, most of the THF was evaporated under reduced pressure.
The white suspension was extracted with CH2Cl2 (5 ϫ 50 mL). The
combined organic layers were washed with saturated NaHCO3
solution (25 mL), water (25 mL), and dried with MgSO4. Filtration
and then concentration of the filtrate under reduced pressure gave
7 (21.5 g, 91%) as an off-white solid, which was used in following
reactions without purification. A sample was treated with charcoal
in n-hexane. Subsequent crystallization from n-hexane (25 mg
purity had been collected]. This procedure afforded 5 (37.3 g, 64%) mLϪ1) gave analytically pure 7 as colorless crystals (m.p. 57.5Ϫ58
as an off-white solid. An analytically pure sample was prepared by
°C). 1H NMR: δ ϭ 1.83Ϫ1.97 (m, 2 H), 2.17Ϫ2.22 (m, 2 H),
2.46Ϫ2.56 (m, 4 H), 2.89Ϫ2.99 (m, 1 H), 2.92 (s, 6 H), 6.69Ϫ6.74
recrystallization from diethyl ether to yield 5 as a colorless solid
(m.p. 137Ϫ138 °C). 1H NMR: δ ϭ 0.98 (s, 6 H), 1.52 (s, OH), (AAЈBBЈ, 2 H), 7.10Ϫ7.14 (AAЈBBЈ, 2 H) ppm. 13C NMR (APT):
1.69Ϫ1.74 (m, 2 H), 1.84Ϫ1.95 (m, 2 H), 2.00Ϫ2.15 (2 ϫ m, 4 H), δ ϭ 34.2, 41.4 (CH2); 40.7, 41.7, 112.9, 127.2 (CH and CH3); 132.7,
2.92 (s, 6 H), 3.49 (s, 2 H), 3.56 (s, 2 H), 6.68Ϫ6.73 (AAЈBBЈ, 2
149.4, 211.6 (q) ppm. IR: ν˜max ϭ 2970, 2950, 2870, 2805, 1720,
H), 7.34Ϫ7.39 (AAЈBBЈ, 2 H) ppm. 13C NMR: (APT): δ ϭ 22.7, 1620, 1525, 1350, 1330, 1160, 1060, 925, 945, 825, 795, 525, 495
40.6 (CH3); 112.3, 125.4 (CH); 30.2, 72.2, 97.3, 136.4, 149.5 (q);
28.2, 35.2, 70.0 (CH2) ppm. FT-IR: ν˜max ϭ 3534 (sharp, OH), 3238
(broad, OH), 2966, 2947, 2939, 2921, 2864, 2850, 2808, 1610, 1516,
1505, 1368, 1361, 1247, 1145, 1102, 987, 937, 842, 825, 819, 573,
cmϪ1
.
1-[4-(Dimethylamino)phenyl]cyclohexanol (10): A mixture of 4-
bromo-N,N-dimethylaniline (20.3 g, 101 mmol) and Mg (2.58 g,
106 mmol) in THF (100 mL) was treated with ultrasound (bath)
for 2 h at room temperature until most of the magnesium had dis-
appeared. Caution: Occasionally the reaction proceeded so fast that
ice-cooling was necessary. A solution of cyclohexanone (9) (8.94 g,
549, 541 cmϪ1
.
9-[4-(Dimethylamino)phenyl]-3,3-dimethyl-1,5-dioxaspiro[5.5]undec-
8-ene (6): A solution of 5 (37.1 g, 116 mmol) and p-toluenesulfonic
acid monohydrate (10 mg) was heated under reflux in toluene
(400 mL) in a DeanϪStark apparatus until the theoretical amount
of water had separated (30 min). TLC (n-hexane/ethyl acetate, 3:1
v/v) showed that all starting compound had disappeared. Ethyl
˚
91.1 mmol, which had been stored on 4 A molecular sieves) in THF
(60 mL) was added dropwise over 15 min, while keeping the tem-
perature below Ϫ30 °C. The resulting suspension was stirred at
Ϫ30 °C for 1 h and then overnight at room temperature. The mix-
acetate (150 mL) was added and the catalyst was removed by flash ture was subsequently poured in a saturated NH4Cl solution
chromatography over a short silica column [6 ϫ 2 cm (cross sec-
(200 mL) and extracted with CH2Cl2 (2 ϫ 100 mL). The combined
tion)]. Evaporation of the solvent under reduced pressure afforded
extracts were dried with MgSO4 and filtered. Evaporation of the
crude 6 as an off-white solid (34.8 g, 99%). An analytically pure solvent under reduced pressure afforded a brown oil (22.1 g) which
sample of 6 was prepared by recrystallization from MeOH (80 mg
was subjected to column chromatography (eluent: n-hexane/ethyl
mLϪ1) yielding colorless needles (m.p. 113Ϫ114 °C). 1H NMR: δ ϭ
acetate, 10:1 v/v) to yield 10 (11.4 g, 57%) as an off-white solid. 1H
0.98 (s, 3 H), 1.07 (s, 3 H), 2.11Ϫ2.15 (m, 2 H), 2.52Ϫ2.57 (2 ϫ m, NMR: δ ϭ 1.22Ϫ1.36 (m, 1 H), 1.48 (s, OH), 1.55Ϫ1.89 (m, 9 H),
4 H), 2.95 (s, 6 H), 3.55 and 3.64 (AB, JAB ϭ 11.3 Hz, 2 ϫ 2 H),
2.93 (s, 6 H), 6.70Ϫ6.75 (AAЈBBЈ, 2 H), 7.35Ϫ7.40 (AAЈBBЈ, 2 H)
5.84Ϫ5.87 (m, 1 H), 6.68Ϫ6.73 (AAЈBBЈ, 2 H), 7.30Ϫ7.35 ppm. 13C NMR: δ ϭ 22.4, 25.6, 38.8, 40.6, 72.6, 112.4, 125.5,
(AAЈBBЈ, 2 H) ppm. 13C NMR (APT): δ ϭ 22.5, 22.8, 40.6 (CH3); 137.3, 149.4 ppm. IR: ν˜max ϭ 3340 (broad), 2950, 2870, 2860, 2825,
25.5, 27.9, 35.0, 70.3 (CH2); 30.2, 97.1, 129.8, 135.5, 149.5 (q);
112.3, 117.2, 125.7 (CH) ppm. FT-IR: ν˜max ϭ 2944, 2923, 2899,
2863, 2801, 1869 (w), 1611, 1524, 1395, 1246, 1229, 1203, 1166,
1625, 1530, 1320, 1200, 1140, 965, 940, 850, 820, 800, 540 cmϪ1
.
4-(Cyclohex-1-enyl)-N,N-dimethylaniline (11): Solid 10 (11.4 g,
52.0 mmol) was added to trifluoroacetic acid (65 mL) whilst stir-
ring at room temperature. After 10 min, the reaction mixture was
slowly added to a saturated NaHCO3 solution (800 mL). The re-
1113 (s), 860, 819, 798 cmϪ1
.
4-(3,3-Dimethyl-1,5-dioxaspiro[5.5]undec-9-yl)-N,N-dimethylaniline
(7): A solution of 6 (34.5 g, 114 mmol) in THF (500 mL) was sulting white suspension was extracted with diethyl ether (3 ϫ
stirred with a catalytic amount of 10% Pd on carbon under 1 atm 150 mL). The combined organic layers were dried with MgSO4 and
of H2 until, after 2 d, the uptake of hydrogen had ceased. GC filtered. Evaporation of the solvent under reduced pressure yielded
showed a conversion of over 99.9%. The solution was filtered
11 (10.2 g, 97%) as an off-white solid [m.p. 53 °C (ref.[72] 54.5Ϫ55
Eur. J. Org. Chem. 2003, 3117Ϫ3130
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3125

L. W. Jenneskens et al.
FULL PAPER
°C)], which was used in the next step without further purification. N,N-Bis(2-tert-butoxyethyl)-4-(3,3-dimethyl-1,5-dioxaspiro[5.5]-
1H NMR: δ ϭ 1.60Ϫ1.69 (m, 2 H), 1.72Ϫ1.80 (m, 2 H), 2.15Ϫ2.22
undecan-9-yl)aniline (14): A solution of 13 (36.6 g) in EtOH/THF
(m, 2 H), 2.34Ϫ2.41 (m, 2 H), 2.93 (s, 6 H), 5.99Ϫ6.02 (m, 1 H), (1:1 v/v, 220 mL) was treated as described for 7, yielding crude
6.71Ϫ6.76 (AAЈBBЈ, 2 H), 7.27Ϫ7.32 (AAЈBBЈ, 2 H) ppm. 13C 14 (36.8 g, 100%). Crystallisation from MeOH (225 mL) afforded
NMR: δ ϭ 22.3, 23.2, 25.8, 27.3, 41.0, 113.0, 121.9, 125.6, 132.0, colorless needles of pure 14 (19.2 g). The mother liquor was con-
135.9, 149.0 ppm.
centrated under reduced pressure and further concentrated by Kug-
elrohr distillation at 0.001 Torr and 130 °C, yielding a brown oil
(7.21 g), which was crystallized from MeOH (29 mL) to yield the
pure product (5.29 g). Total yield: 24.5 g of 14 (m.p. 103Ϫ104 °C).
4-Cyclohexyl-N,N-dimethylaniline (D1): A solution of 11 (10.2 g,
50.7 mmol) in THF/EtOH (1:1 v/v, 150 mL) was stirred overnight
in the presence of a catalytic amount of 10% Pd/C under 1 atm of
H2. The reaction mixture was filtered through Celite. After removal
of most of the solvent under reduced pressure, the remaining oil
was subjected to vacuum distillation using a 10 cm Vigreux column.
D1 (6.47 g, 63%) was collected as a colorless oil at 113 °C
(0.02 Torr) (ref.[73] 162Ϫ164 °C; 11 Torr). 1H NMR: δ ϭ 1.16Ϫ1.45
(m, 5 H), 1.69Ϫ1.87 (m, 5 H), 2.35Ϫ2.44 (m, 1 H), 6.67Ϫ6.72
(AAЈBBЈ, 2 H), 7.06Ϫ7.11 (AAЈBBЈ, 2 H) ppm. 13C NMR: δ ϭ
26.4, 27.0, 34.7, 40.9, 43.5, 113.0, 127.3, 136.6, 149.0 ppm.
1
This yield is 66% with respect to 12. H NMR: δ ϭ 0.98 (s, 6 H),
1.18 (s, 18 H), 1.40Ϫ1.50 (m, 2 H), 1.60Ϫ1.76 (m, 4 H), 2.34Ϫ2.38
(m, 2 H), 2.40Ϫ2.50 (m, 1 H), 3.46 and 3.49 (A2B2, J ϭ 7.2 Hz, 8
H), 3.51 (s, 2 H), 3.55 (s, 2 H), 6.61Ϫ6.64 (AAЈBBЈ, 2 H),
7.05Ϫ7.08 (AAЈBBЈ, 2 H) ppm. 13C NMR: δ ϭ 22.8, 27.5, 30.2
(q), 30.4, 32.6, 42.8, 52.1, 58.8, 69.9, 70.1, 73.0, 97.4, 111.3, 127.6,
133.8, 146.3 ppm. FT-IR: ν˜max ϭ 2969, 2948, 2934, 2910, 2863,
2856, 1618, 1521, 1391, 1383, 1361, 1202, 1187, 1132, 1114, 1097,
1080, 955, 913, 814, 796, 537 cmϪ1
.
3,3-Dimethyl-9-{4-[N,N-bis(2-tert-butoxyethyl)amino]phenyl}-1,5-
dioxaspiro[5.5]undecan-9-ol (12): n-Butyllithium in hexanes (1.59 ,
190 mL, 302 mmol) was added to a stirred solution of 3 (56.3 g,
151 mmol) in THF (150 mL), while keeping the temperature below
Ϫ70 °C. Stirring was continued for 2 h at Ϫ78 °C and then a solu-
tion of 3,3-dimethyl-1,5-dioxaspiro[5.5]undecan-9-one (4)[71]
(30.0 g, 151 mmol) in THF (100 mL) was added dropwise over
10 min, while keeping the temperature below Ϫ60 °C. The mixture
was stirred for another 2 h at Ϫ60 °C and was then warmed to
room temperature overnight. The mixture was poured into a satu-
rated NH4Cl solution (110 mL) and the organic layer was separated
from the water layer. The water layer was washed with diethyl ether
(2 ϫ 100 mL) and the combined organic layers were dried with
MgSO4. Filtration followed by removal of the solvent under re-
duced pressure afforded an oil that was subjected to column chro-
matography [SiO2 (500 g); n-hexane/ethyl acetate, 10:1 v/v]. This
procedure gave of a slightly yellow solid (46.0 g; 62% with respect
to 3) that mainly consisted of 12, which was used in the next step
without further purification. An analytically pure sample was ob-
tained by recrystallization from n-hexane (50 mg mLϪ1), which
2-{4-[4-(Dimethylamino)phenyl]-1-hydroxycyclohexyl}-2-methyl-
propanoic Acid (16): n-Butyllithium in hexanes (1.6 , 26 mL,
41.6 mmol) was added to a solution of N,N-diisopropylamine
(4.22 g, 41.7 mmol, which had been stored over KOH) in THF
(85 mL) at Ϫ40 °C. The mixture was warmed to 0 °C over 30 min
and was then recooled to Ϫ40 °C. 2-Methylpropanoic acid (1.83 g,
20.8 mmol) in THF (4 mL) was added. The mixture was heated to
50 °C and after 2 h it was cooled to Ϫ40 °C, and then 8 (3.63 g,
16.7 mmol) was added. After stirring for another 2 h, the mixture
was heated to 50 °C and kept at that temperature for 1 h. Sub-
sequently, it was poured over ice (170 g) under vigorous stirring.
The mixture was extracted with diethyl ether (4 ϫ 40 mL) and the
aqueous phase was acidified to pH 7 with 3  HCl solution
(13 mL) and then with 1  HCl to pH 3Ϫ4. The resulting white
suspension was extracted with CH2Cl2 (200 mL, followed by 3 ϫ
50 mL). The combined organic layers were dried with MgSO4 and
filtered. Evaporation of the solvent under reduced pressure gave an
off-white solid (4.34 g), which was extracted with n-pentane in a
Soxhlett apparatus overnight to remove any starting compounds.
The remaining solid (3.65 g, 72%) proved to be pure 16 (m.p.
189Ϫ191 °C, dec). 1H NMR: δ ϭ 1.29 (s, 6 H), 1.60Ϫ1.91 (m, 8 H),
2.33Ϫ2.43 (m, 1 H), 2.90 (s, 6 H), ca. 6.6 (broad, OH), 6.73Ϫ6.77
(AAЈBBЈ, 2 H), 7.10Ϫ7.15 (AAЈBBЈ, 2 H) ppm. 13C NMR: δ ϭ
21.0, 29.3, 31.8, 41.2, 42.7, 50.0, 74.0, 113.6, 127.4, 135.9, 149.0,
182.0 ppm. FT-IR: ν˜max ϭ 3541 (sharp, OH), 3439 (br., OH), 3159
(br., OH), 2998, 2970, 2955, 2942, 2929, 2879, 2857, 2617 and 2502
(br., NHϩ), 1941 (br), 1714, 1612, 1516, 1388, 1275, 1258, 1232,
1
yielded colorless needles (71%; m.p. 112Ϫ115 °C). H NMR: δ ϭ
0.90 (s, 6 H), 1.18 (s, 18 H), 1.41 (s, OH), 1.43Ϫ1.72 (m, 2 H),
1.87Ϫ1.95 (m, 2 H), 2.00Ϫ2.06 (m, 2 H), 2.10Ϫ2.15 (m, 2 H), 3.49
(s, 2 H superposed on a singlet-like A2B2 system of 8 H), 3.57 (s,
2 H), 6.64Ϫ6.68 (AAЈBBЈ, 2 H), 7.30Ϫ7.35 (AAЈBBЈ, 2 H) ppm.
13C NMR: δ ϭ 22.7, 27.5, 28.3, 30.2, 35.2, 52.1, 58.7, 70.1, 72.2,
73.0, 97.3, 111.0, 125.6, 135.3, 146.8 ppm. FT-IR: ν˜max ϭ 3486
(OH), 2974, 2922, 2867, 1614, 1521, 1390, 1364, 1197, 1111, 1065,
1157, 1139, 1048, 962, 927, 830, 818, 802, 552 cmϪ1
.
978, 814, 540 cmϪ1
.
N,N-Bis(2-tert-butoxyethyl)-4-(3,3-dimethyl-1,5-dioxaspiro[5.5]-
undec-8-en-9-yl)aniline (13): A solution of crude 12 (38.1 g) and
N,N-Dimethyl-4-[4-(1-methylethylidene)cyclohexyl]aniline
N,N-Dimethyl-1,1-bis(neopentyloxy)methanamine (6.03 g, 26.1
(D2):
a catalytic amount of p-toluenesulfonic acid monohydrate (9 mg, mmol) was added to a suspension of 16 (3.58 g, 11.7 mmol) in
0.05 mmol) in toluene (400 mL) was treated as described for 6
(using 130 mL of ethyl acetate in the workup). Crude 13 was af-
forded as an off-white solid (36.6 g). A pure sample was obtained
MeCN (140 mL). After stirring for 30 min at room temperature,
the clear solution was heated overnight under reflux. The solvent
was evaporated under reduced pressure and the resulting brown
in 100% yield from analytically pure 12 using the same procedure oil was subjected to column chromatography [silica (100 g); eluent:
1
(m.p. 77Ϫ79 °C). H NMR: δ ϭ 0.95 (s, 3 H), 1.03 (s, 3 H), 1.17 CH2Cl2]. The light-brown solid obtained (2.60 g) was treated with
(s, 18 H), 2.07Ϫ2.11 (m, 2 H), 2.49Ϫ2.53 (2 ϫ m, 4 H), 3.49 (sing-
charcoal (1 g) in n-hexane/CH2Cl2 (1:1 v/v, 60 mL) and then filtered
let-like A2B2 system, 8 H), 3.52 and 3.60 (AB, JAB ϭ 11.4 Hz, 2 ϫ through Celite. The solvent was evaporated under reduced pressure
2 H), 5.79Ϫ5.81 (m, 1 H), 6.61Ϫ6.64 (AAЈBBЈ, 2 H), 7.24Ϫ7.27 and the resulting solid was recrystallized under N2 from deoxygen-
(AAЈBBЈ, 2 H) ppm. 13C NMR: δ ϭ 22.6, 22.8, 25.5, 27.5, 28.1, ated MeOH (113 mg mLϪ1, cooled from boiling to room tempera-
30.2, 34.9, 52.1, 58.8, 70.3, 73.0, 97.1, 111.1, 116.7, 125.9, 128.7,
ture) twice. This procedure yielded colorless plate-shaped crystals
135.4, 146.8 ppm. FT-IR: ν˜max ϭ 2973, 2954, 2941, 2891, 2868, (1.03 g, 36%) of D2 (m.p. 59Ϫ60 °C). 1H NMR: δ ϭ 1.34Ϫ1.48
1608, 1522, 1393, 1380, 1364, 1359, 1200, 1116, 1089, 856, 814,
798, 790, 739, 533 cmϪ1
(m, 2 H), 1.69Ϫ1.70 (m, 6 H), 1.82Ϫ1.97 (m, 4 H), 2.53Ϫ2.64 (m,
1 H), 2.75Ϫ2.80 (m, 2 H), 2.89 (s, 6 H), 6.67Ϫ6.72 (AAЈBBЈ, 2 H),
.
3126
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Org. Chem. 2003, 3117Ϫ3130

Photoinduced Charge Separation in Cyclohexylidene-Based Compounds
FULL PAPER
7.06Ϫ7.11 (AAЈBBЈ, 2 H) ppm. 13C NMR (APT): δ ϭ 20.0, 40.9,
sequently, a solution of pyridine (2.90 g, 36.7 mmol; stored on 4 A
˚
43.8, 112.9, 127.4 (CH and CH3); 30.2, 35.5 (CH2); 120.7, 131.1, molecular sieves) in THF (20 mL) was added over ca. 10 min while
135.7, 149.1 (q) ppm. FT-IR: ν˜max ϭ 2988, 2958, 2921, 2848, 2793, maintaining the temperature between Ϫ5 and 0 °C. The resulting
1615, 1522, 1443, 1339, 1224, 1060, 978, 946, 914, 819, 798, 552, reaction mixture was stirred for 3 d at room temperature and then
531 cmϪ1. C17H25N (243.4): calcd. C 83.89, H 10.35, N 5.75; found
C 83.88, H 10.26.
saturated aqueous Na2CO3 (200 mL) and n-hexane (200 mL) were
added. The organic layer was separated and the aqueous layer was
extracted with n-hexane (2 ϫ 150 mL). The combined organic ex-
tracts were washed sequentially with saturated NaCl solution, satu-
rated Na2CO3 solution, and saturated NaCl solution. The organic
layer was dried with MgSO4 and filtered. Evaporation of the sol-
vent under reduced pressure yielded a brown oil (2.06 g), which
was purified by column chromatography (eluent: petroleum ether/
diethyl ether/chloroform, 4:1:1 v/v/v). The product was crystallized
twice from MeOH and then from EtOH, yielding colorless needles
(0.18 g, 6%; m.p. 91.0 °C). 1H NMR: δ ϭ 1.59Ϫ1.73 (m, 2 H),
2.04Ϫ2.22 (2 ϫ m, 2 ϫ 2 H), 2.68Ϫ2.78 (m, 1 H), 2.91 (s, 6 H),
3.16Ϫ3.20 (m, 2 H), 3.78 (s, 6 H), 6.67Ϫ6.72 (m, 2 H), 7.05Ϫ7.10
(m, 2 H) ppm. 13C NMR: δ ϭ 32.5, 35.3, 40.8, 42.8, 52.1, 112.9,
121.6, 127.3, 133.6, 149.3, 161.2, 166.1 ppm. FT-IR: ν˜max ϭ 2952,
2938, 2913, 2853, 2801, 1729, 1723, 1628, 1615, 1522, 1305, 1239,
1194, 1063, 810, 758, 539 cmϪ1. C19H25NO4 (331.4): calcd. C 68.86,
H 7.60, N 4.23, O 19.31; found C 68.74, H 7.68, O 19.24.
{4-[4-(Dimethylamino)phenyl]cyclohexylidene}malononitrile (DA1):
Malononitrile (1.43 g, 21.6 mmol) and β-alanine (123 mg,
1.38 mmol) were added to a solution of 8 (4.36 g, 20.1 mmol) in
85% EtOH (50 mL) stirred at room temperature. A precipitate be-
gan to form immediately. After 6 h the suspension was concen-
trated at the rotary evaporator without heating. Water (25 mL) was
added and the mixture was filtered through a glass frit. The residue
was washed with water (3 ϫ 25 mL) and dried under vacuum. This
crude product (4.64 g) was dissolved in n-hexane/ethyl acetate (1:1
v/v, 120 mL) and purified by flash chromatography over a short
silica column [6 ϫ 2 cm (cross section)]. Evaporation of the solvent
under reduced pressure gave yellow crystals (4.61 g) that were puri-
fied by three crystallizations from boiling n-hexane/ethyl acetate
(1:1 v/v, 46 mg·mLϪ1). The purity of these crystals was checked
after each crystallization by their fluorescence in diethyl ether.
1
Yield: 1.39 g (26%) of light-yellow needles (m.p. 149Ϫ151 °C). H
NMR: δ ϭ 1.65Ϫ1.79 (m, 2 H), 2.18Ϫ2.25 (m, 2 H), 2.43Ϫ2.54
(m, 2 H), 2.77Ϫ2.85 (m, 1 H), 2.94 (s, 6 H), 3.13Ϫ3.19 (m, 2 H),
6.71Ϫ6.74 (AAЈBBЈ, 2 H), 7.05Ϫ7.10 (AAЈBBЈ, 2 H) ppm. 13C
NMR (APT and CH correlation): δ ϭ 34.3, 34.5 (CH2); 40.4,
112.6, 127.0 (CH); 41.4 (CH3); 82.7, 111.5, 131.3, 149.3, 183.9 (q)
ppm. FT-IR: ν˜max ϭ 2953, 2939, 2888, 2864, 2802, 2230, 1615,
Didodecyl Malonate:
A
solution of triethylamine (15.9 g,
157 mmol) and 1-dodecanol (28.0, 150 mmol) in diethyl ether
(75 mL) was added dropwise to a stirred solution of malonoyl di-
chloride (10.5 g, 74.5 mmol) in diethyl ether (75 mL) while keeping
the temperature between Ϫ5 and 0 °C. Subsequently, the reaction
mixture was stirred overnight at room temperature. The mixture
was washed with 0.1  HCl solution (70 mL) and the aqueous layer
was extracted with diethyl ether (50 mL). The combined organic
layers were washed with water (50 mL) and dried with MgSO4.
After filtration, the solvent was evaporated under reduced pressure.
The remaining brown oil was dissolved in petroleum ether/diethyl
ether (1:1 v/v; 200 mL) and subjected to flash column chromatogra-
phy. The eluate was treated with active carbon, filtered and, after
removal of the solvent, recrystallized from EtOH and petroleum
ether to yield didodecyl malonate (11.7 g, 36%) as colorless crystals
(m.p. 33.2 °C; ref.[74] 35 °C, ref.[75] 32.5 °C). 1H NMR and 13C
1602, 1598, 1523, 1444, 1357, 1345, 1332, 946, 822, 803, 544 cmϪ1
.
C17H19N3 (265.4): calcd. C 76.95, H 7.22, N 15.84; found C 77.08,
H 7.31.
Methyl Cyano{4-[4-(dimethylamino)phenyl]cyclohexylidene}acetate
(DA2): A mixture of β-alanine (0.111 g), methyl cyanoacetate
(2.56 g, 25.8 mmol) and ketone 8 (4.34 g, 20.0 mmol) in MeOH/
water (16:3 v/v, 47.5 mL) was stirred at room temperature. A pre-
cipitate began to form after ca. 1 d. After 100 h, most of the solvent
was evaporated, using a rotary evaporator without external heating,
and then CH2Cl2 (50 mL) was added to the residue. The water layer
was separated and the organic layer was washed with water (3 ϫ
50 mL), dried with MgSO4, and filtered. The solvent was evapo-
rated under reduced pressure to afford an off-white solid. This solid
was crystallized twice from diethyl ether to give off-white crystals
(2.30 g) of DA2. The product was further purified by flash chroma-
tography (eluent: n-hexane/ethyl acetate, 1:1 v/v) followed by two
crystallizations from the same solvent mixture (115 mg·mLϪ1 and
333 mg·mLϪ1). This procedure gave DA2 (1.16 g, 19%) as colorless
crystals (m.p. 109 °C). 1H NMR: δ ϭ 1.58Ϫ1.80 (m, 2 H),
2.11Ϫ2.28 (m, 3 H), 2.41Ϫ2.52 (m, 1 H), 2.75Ϫ2.85 (m, 1 H), 2.92
(s, 6 H), 3.14Ϫ3.23 (m, 1 H), 3.84 (s, 3 H), 4.03Ϫ4.09 (m, 1 H),
6.67Ϫ6.72 (AAЈBBЈ, 2 H), 7.04Ϫ7.09 (AAЈBBЈ, 2 H) ppm. 13C
NMR (APT): δ ϭ 31.2, 35.1, 35.4, 36.6 (CH2); 40.6, 42.2, 52.5 (CH
and CH3); 102.0, 115.4, 132.4, 149.4, 162.3, 179.3 (q); 112.8, 127.2
(CH) ppm. FT-IR: ν˜max ϭ 2954, 2937, 2916, 2898, 2874, 2813,
2223, 1731, 1616, 1592, 1523, 1430, 1288, 1241, 806, 788, 778, 539
cmϪ1. C18H22N2O2 (298.4): calcd. C 72.46, H 7.43, N 9.39, O 10.72;
found C 72.41, H 7.51, O 10.66.
NMR spectra were in accordance with those reported in ref.[76]
.
Didodecyl {4-[4-(Dimethylamino)phenyl]cyclohexylidene}malonate
(DA4): A solution of TiCl4 (2.2 mL, 20 mmol) in dry CCl4 (10 mL)
was slowly added to THF (40 mL), while maintaining the tempera-
ture below 0 °C. A solution of didodecyl malonate (4.01 g,
9.10 mmol) and ketone 8 (1.97 g, 9.07 mmol) in THF (20 mL) was
added to this mixture. Subsequently, a solution of pyridine (2.89 g,
36.5 mmol) in THF (20 mL) was added over ca. 20 min while main-
taining the temperature just below 0 °C. The resulting reaction mix-
ture was stirred for 3 d at room temperature. Water (60 mL) and
diethyl ether (70 mL) were added, and then the water layer was
extracted with diethyl ether (2 ϫ 60 mL). The combined organic
layers were washed sequentially with saturated NaCl solution
(60 mL), saturated Na2CO3 solution (60 mL) and saturated NaCl
solution solution (60 mL). The combined aqueous layers were ex-
tracted again as described above and all the organic phases were
combined and dried with MgSO4. Evaporation of the solvent under
reduced pressure afforded a residue that was subjected to flash
chromatography (eluent: petroleum ether/diethyl ether, 2:1 v/v) and,
after evaporation of the solvent, the product was recrystallized
Dimethyl
{4-[4-(Dimethylamino)phenyl]cyclohexylidene}malonate
(DA3): A solution of TiCl4 (2.2 mL, 20 mmol) in dry CCl4 (20 mL;
˚
stored over 4 A molecular sieves) was slowly added to THF from MeOH, subjected to column chromatography (eluent: n-hex-
(40 mL), while maintaining the temperature below 0 °C. A solution
of dimethyl malonate (1.27 g, 9.60 mmol) and ketone 8 (1.94 g,
8.93 mmol) in THF (20 mL) was added to this mixture. Sub-
ane/ethyl acetate, 20:1 v/v) and again recrystallized from MeOH,
EtOH, and petroleum ether. This procedure yielded DA4 (0.73 g,
13%) as a colorless solid (m.p. 48.1Ϫ49.3 °C). 1H NMR: δ ϭ
Eur. J. Org. Chem. 2003, 3117Ϫ3130
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L. W. Jenneskens et al.
FULL PAPER
0.86Ϫ0.90 (m, 6 H), 1.26Ϫ1.30 (m, 40 H), 1.60Ϫ1.73 (2 ϫ m, 6 under reduced pressure gave 18 (2.41 g, 83%) as an off-white solid.
H), 2.04Ϫ2.21 (2 ϫ m, 4 H), 2.68Ϫ2.78 (m, 1 H), 2.91 (s, 6 H), Pure 18 was obtained by repeated crystallization from THF/diethyl
3.18Ϫ3.22 (m, 2 H), 4.14Ϫ4.19 (m, 4 H), 6.68Ϫ6.71 (m, 2 H),
ether (1:1 v/v, 19 mg·mLϪ1; dissolution in THF at room tempera-
7.06Ϫ7.09 (m, 2 H) ppm. 13C NMR: δ ϭ 14.1, 22.7, 26.0, 28.6, ture followed by addition of diethyl ether and crystallization at Ϫ20
29.3, 29.4, 29.6 (3 resolved signals), 29.7, 31.9, 32.4, 35.3, 40.8,
42.9, 65.1, 112.9, 122.4, 127.3, 133.7, 149.3, 160.2, 165.8 ppm. FT-
IR: ν˜max ϭ 2956 (m); 2913, 2849, 1734, 1712 (s); 1616, 1521, 1470,
°C) followed each time by flash chromatography with THF. The
fluorescence purity was monitored in diethyl ether. This procedure
gave 18 (1.54 g, 53%) as slightly yellow crystals (m.p. 137Ϫ139 °C).
1344, 1300, 1070, 941, 821, 813, 719 cmϪ1. C41H69NO4 (640.0) 1H NMR (CDCl3 with some drops of [D6]DMSO to enhance the
calcd. C 76.94, H 10.87, N 2.19, O 10.00; found C 76.87 H 10.95,
O 9.85.
solubility): δ ϭ 1.63Ϫ1.77 (m, 2 H), 2.17Ϫ2.23 (m, 2 H), 2.46Ϫ2.56
(m, 2 H), 2.75Ϫ2.85 (m, 1 H), 3.11Ϫ3.16 (m, 2 H), 3.53 (apparent
t, J ϭ 5.0 Hz, 4 H), 3.78 (apparent q, J ϭ 5.1 Hz, 4 H), 4.57
(apparent t, J ϭ 5.4 Hz, 2 ϫ OH), 6.67Ϫ6.70 (AAЈBBЈ, 2 H),
7.01Ϫ7.07 (AAЈBBЈ, 2 H) ppm. 13C NMR (CDCl3 with some drops
of [D6]DMSO): δ ϭ 34.3, 34.6, 41.4, 55.1, 60.1, 82.7, 111.4, 112.5,
131.1, 146.9, 183.8 ppm. FT-IR: ν˜max ϭ 3204 (br., s), 2951, 2943,
2922, 2882, 2865, 2230 (s, CϵN), 1612, 1598, 1522, 1358, 1187,
(4-Phenylcyclohexylidene)malononitrile (A1): Malononitrile (1.37 g,
207 mmol) and β-alanine (115 mg) were added to a stirred solution
of 4-phenylcyclohexanone (3.48 g, 200 mmol) in MeOH/water (16:3
v/v, 50 mL) at room temperature. After 10 min a white precipitate
began to form. After 5 h, most of the solvent was evaporated at
the rotary evaporator without heating. Water (25 mL) was added
and the mixture was filtered through a glass frit. The solid was
washed with water (2 ϫ 25 mL) and dried under reduced pressure
to give crude A1 (4.35 g), which was purified by flash chromatogra-
phy using n-hexane/ethyl acetate (1:1) as the eluent. Evaporation
of the solvent under reduced pressure gave a white solid that was
recrystallized from n-hexane/ethyl acetate (7:2 v/v; 87 mg·mLϪ1) to
1059, 1006, 823, 803, 656 (br., w), 561, 534, 526, 475 cmϪ1
.
[{(Acetyloxy)ethyl}{4-[4-(dicyanomethylene)cyclohexyl]phenyl}-
amino]ethyl Acetate (DA5): Compound 18 (187 mg, 0.575 mmol)
and dry pyridine (0.20 mL, 2.5 mmol) were added to an ice-cooled
stirred solution of acetyl chloride (95.6 mg, 1.22 mmol) in dry
CH2Cl2 (4.85 mL). A white precipitate formed immediately, which
give A1 (3.19 g, 72%) as colorless crystals (m.p. 104.5Ϫ105.5 °C). dissolved upon warming the mixture to room temperature. Stirring
1H NMR: δ ϭ 1.68Ϫ1.82 (m, 2 H), 2.22Ϫ2.28 (m, 2 H), 2.44Ϫ2.55
(m, 2 H), 2.83Ϫ2.93 (m, 1 H), 3.16Ϫ3.20 (m, 2 H), 7.17Ϫ7.24 (m,
was continued for 8 d at room temperature (protected from light).
The solvent was evaporated under reduced pressure and the residue
2 H), 7.25Ϫ7.27 (m, 1 H), 7.30Ϫ7.35 (m, 2 H) ppm. 13C NMR: was subjected to column chromatography (eluent: CH2Cl2/acetone,
δ ϭ 34.4 (2 overlapping signals), 42.8, 83.3, 111.5, 126.6, 126.9,
128.7, 143.5, 183.3 ppm. FT-IR: ν˜max ϭ 3086, 3063, 3032, 2963, 1.62Ϫ1.83 (m, 2 H), 2.05 (s, 6 H), 2.17Ϫ2.23 (m, 2 H), 2.43Ϫ2.54
20:1 v/v). Yield: 52 mg (43%) of a yellowish oil. 1H NMR: δ ϭ
2952, 2929, 2877, 2865, 2228 (CϵN), 1593, 1495, 1445, 1431, 1427,
1370, 1339, 1021, 1003, 986, 759, 701, 533 cmϪ1. C15H14N2 (222.3)
calcd. C 81.05, H 6.35, N 12.60; found C 80.96, H 6.35.
(m, 2 H), 2.74Ϫ2.84 (m, 1 H), 3.13Ϫ3.18 (m, 2 H), 3.60 (t, J ϭ
6.3 Hz, 4 H), 4.22 (t, J ϭ 6.3 Hz, 4 H), 6.70Ϫ6.74 (m, 2 H),
7.03Ϫ7.06 (m, 2 H) ppm. 13C NMR (APT): δ ϭ 34.5, 34.7, 49.9,
61.3 (CH2); 41.7, 112.4, 127.5 (CH); 20.8 (CH3); 83.1, 111.6, 132.1,
145.9, 170.9, 183.7 (q) ppm. FT-IR: ν˜max ϭ 2956, 2899, 2866, 2231
(CϵN), 1737, 1614, 1598, 1520, 1445, 1381, 1369, 1230, 1197, 1047,
822, 810 cmϪ1. C23H27N3O4 (409.5) calcd. C 67.46, H 6.65, N
10.26, O 15.63; found C 67.40, H 6.58, O 15.71.
4-{4-[N,N-Bis(2-hydroxyethyl)amino]phenyl}cyclohexanone (17): A
solution of 14 (28.4 g, 59.7 mmol) in dioxane/2  HCl solution (1:1
v/v, 360 mL) was heated under reflux for 3.5 h. The hot mixture
was poured over NaHCO3 (35.2 g, 419 mmol) while stirring vigor-
ously. After most of the CO2 had evolved, the mixture was ex-
tracted with CH2Cl2 (3 ϫ 100 mL). The combined organic layers
were dried with MgSO4. After filtration and removal of most of
the solvent under reduced pressure, residual solvents and 2,2-di-
methylpropane-1,3-diol were evaporated by kugelrohr distillation
at 80 °C (0.001 Torr) yielding 17 (16.4 g, 99%). The product was
further purified by column chromatography (eluent: n-hexane/
EtOH, 4:1 v/v) to yield 17 (14.8 g, 89%) as a white solid. A sample
was recrystallized from n-hexane/ethyl acetate (2:3 v/v,
22.8 mg·mLϪ1) to yield analytically pure 17 as colorless crystals
Acknowledgments
This research has been supported financially (W. D. O., J. J. P., and
M. K.) by the Council for Chemical Sciences of the Netherlands
Organization for Scientific Research (CW-NWO). We are thankful
to Dr. X. Y. Lauteslager for a helpful discussion. P. Szlachcic is
acknowledged for his contribution to the synthesis of DA2 and C.
Koper is acknowledged for her contribution to the measurements.
A. Flatow (University of Heidelberg) is acknowledged for re-
cording the PE spectra. The work in Heidelberg was supported by
the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen
Industrie, and the BASF Aktiengesellschaft in Ludwigshafen.
1
(m.p. 101 °C). H NMR: δ ϭ 1.82Ϫ1.96 (m, 2 H), 2.15Ϫ2.21 (m,
2 H), 2.46Ϫ2.56 (m, 4 H), 2.89Ϫ2.99 (m, 1 H), 3.07 (broad s, 2 H),
3.56 (t, J ϭ 4.9 Hz, 2 H), 3.85 (t, J ϭ 4.7 Hz, 2 H), 6.66Ϫ6.71
(AAЈBBЈ, 2 H), 7.08Ϫ7.13 (AAЈBBЈ, 2 H) ppm. 13C NMR: δ ϭ
34.2, 41.4, 41.6, 55.4, 60.8, 112.8, 127.4, 133.1, 146.6, 211.7 ppm.
FT-IR: ν˜max ϭ 3229, 3144 (br., s); 2944, 2923, 2898, 2866, 2850,
1698, 1617, 1521, 1461, 1425, 1348, 1239, 1225, 1187, 1161, 1111,
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