Molecular clips with extended aromatic sidewalls as receptors for electron-acceptor molecules. Synthesis and NMR, photophysical, and electrochemical properties

Article abstract of DOI:10.1021/jo8007513

(Figure Presented) We have synthesized molecular clips 1 comprising (i) two benzo[k]fluoranthene sidewalk and (ii) a dimethylene-connected benzene bridge that carries two acetoxy (1a), hydroxy (1b), or methoxy (1c) substituents in the para position. Their NMR spectra, single-crystal structures, and photophysical (fluorescence intensity, lifetime, depolarization) and electrochemical properties are discussed. For the purpose of comparison, similar compounds (2 and 3) containing only one benzo[k]fluoranthene unit have been prepared and studied. The strongly fluorescent clips 1 form stable complexes with electron-acceptor guests because of a highly negative electrostatic potential on the inner van der Waals surface of their cavity. The complexation constants in chloroform solution for a variety of guests, determined by NMR and fluorescence titration, are much larger than those of the corresponding anthracene and naphthalene clips (4 and 5), particularly in the case of extended aromatic guests. The effect of the substituents in the para position of the benzene spacer unit of clips 1 is discussed on the basis of the host-guest complex structures obtained by X-ray analysis and molecular mechanics simulations. In the case of 9-dicyanomethylene-2,4,7-trinitrofluorene (TNF) guest, complex formation with clip 1a causes dramatic changes in the photophysical and electrochemical properties: (i) a new charge-transfer band at 600 nm arises, (ii) a very efficient quenching of the strong benzo[k]fluoranthene fluorescence takes place, (iii) shifts of both the first oxidation (clip-centered) and reduction (TNF-centered) potentials are observed, and (iv) reversible disassembling of the complex can be obtained by electrochemical stimulation.

Full text of DOI:10.1021/jo8007513

Molecular Clips with Extended Aromatic Sidewalls as Receptors for
Electron-Acceptor Molecules. Synthesis and NMR, Photophysical,
and Electrochemical Properties
†
†
,†
‡
Barbara Branchi, Vincenzo Balzani, Paola Ceroni,* Mireia Campa n˜ a´ Kuchenbrandt,
,
‡
‡
§
Frank-Gerrit Kl a¨ rner,* Dieter Bl a¨ ser, and Roland Boese
Dipartimento di Chimica “G. Ciamician”, UniVersit a` di Bologna, Via Selmi 2, I-40126 Bologna, Italy, and
Institut f u¨ r Organische Chemie and Institut f u¨ r Anorganische Chemie der UniVersit a¨ t Duisburg-Essen,
Campus Essen, UniVersit a¨ tsstr. 5, 45117 Essen, Germany
paola.ceroni@unibo.it; frank.klaerner@uni-duisburg-essen.de
ReceiVed April 8, 2008
We have synthesized molecular clips 1 comprising (i) two benzo[k]fluoranthene sidewalls and (ii) a dimethylene-
connected benzene bridge that carries two acetoxy (1a), hydroxy (1b), or methoxy (1c) substituents in the
para position. Their NMR spectra, single-crystal structures, and photophysical (fluorescence intensity, lifetime,
depolarization) and electrochemical properties are discussed. For the purpose of comparison, similar compounds
(2 and 3) containing only one benzo[k]fluoranthene unit have been prepared and studied. The strongly fluorescent
clips 1 form stable complexes with electron-acceptor guests because of a highly negative electrostatic potential
on the inner van der Waals surface of their cavity. The complexation constants in chloroform solution for a
variety of guests, determined by NMR and fluorescence titration, are much larger than those of the corresponding
anthracene and naphthalene clips (4 and 5), particularly in the case of extended aromatic guests. The effect of
the substituents in the para position of the benzene spacer unit of clips 1 is discussed on the basis of the
host-guest complex structures obtained by X-ray analysis and molecular mechanics simulations. In the case
of 9-dicyanomethylene-2,4,7-trinitrofluorene (TNF) guest, complex formation with clip 1a causes dramatic
changes in the photophysical and electrochemical properties: (i) a new charge-transfer band at 600 nm arises,
(ii) a very efficient quenching of the strong benzo[k]fluoranthene fluorescence takes place, (iii) shifts of both
the first oxidation (clip-centered) and reduction (TNF-centered) potentials are observed, and (iv) reversible
disassembling of the complex can be obtained by electrochemical stimulation.
1
The design of efficient synthetic receptors may be useful
not only to selectively and efficiently recognize molecules or
ions but also to better understand the structure and functionality
species since they have a well-organized yet relatively flexible
shape that enables interactions with guests of different size. The
recognition process can be based on electron donor-acceptor
2–4
5
6
of natural receptors. Molecular clips are very interesting host
interactions, hydrogen bonding, hydrophobic effects in aque-
7
8
9
ous media, ion pairing, and arene-arene interactions.
†
Universit a` di Bologna.
Institut f u¨ r Organische Chemie der Universit a¨ t Duisburg-Essen.
Institut f u¨ r Anorganische Chemie der Universit a¨ t Duisburg-Essen.
‡
(2) (a) Kl a¨ rner, F.-G.; Kahlert, B. Acc. Chem. Res. 2003, 36, 919. (b) Kl a¨ rner,
F. G.; Campa o` a Kuchenbrandt, M. Synthesis of molecular tweezers and clips
by the use of a molecular Lego set and their supramolecular functionsIn Strategies
and Tactics in Organic Synthesis; Harmata, M., Ed.; Academic Press-Elsevier:
Amsterdam, 2008; Vol. 7, Chapter 4. (c) Rowan, A. E.; Elemans, J.; Nolte, R. J.
Acc. Chem. Res. 1999, 32, 995.
§
(
1) For some recent reviews, see:(a) Schneider, H. J.; Yatsimirsky, A. K.
Chem. Soc. ReV. 2008, 37, 263. (b) Biros, S. M.; Rebek, J. Chem. Soc. ReV.
007, 36, 93. (c) Baldini, L.; Casnati, A.; Sansone, F.; Ungaro, R. Chem. Soc.
ReV. 200736, 254.
2
1
0.1021/jo8007513 CCC: $40.75  2008 American Chemical Society
J. Org. Chem. 2008, 73, 5839–5851 5839
Published on Web 07/09/2008

Branchi et al.
SCHEME 1. Structures of Benzo[k]fluoranthene (BkF),
Molecular Clips 1, 2, 4, and 5, and Half-Clip 3
molecular clips 4 and 5, which contain anthracene and naph-
thalene sidewalls, respectively.
The benzo[k]fluoranthene moieties have been selected because
11
they are strongly fluorescent polycyclic aromatic hydrocarbons
1
2
with good electron-donor ability. The resulting clips are
soluble in organic solvents and present a highly negative
electrostatic potential on the inner van der Waals surface of the
cavity that forms a recognition site for electron-deficient guest
molecules. Besides a thorough investigation of the novel
compounds, we report the host-guest complex formation of
clips 1-3 with different guest molecules substituted with
electron-acceptor groups. The highly stable host-guest complex
of clip 1a with 9-dicyanomethylene-2,4,7-trinitrofluorene (TNF)
has been studied in detail.
Results and Discussion
Synthesis of Molecular Clips 1a-c, 2a, and 3a. Starting
5a
from the known tetrabromonaphthalene clip 5d, the molecular
clip 1c with expanded benzo[k]fluoranthene sidewalls was
prepared by a sequence of palladium-catalyzed Suzuki-Heck-
type couplings with 1-naphthaleneboronic acid analogously to
the coupling between 1,2-dibromobenzene and 1-naphthale-
SCHEME 2. Synthesis of Molecular Clips 1a-c
1
3
neboronic acid leading to fluoranthene. The dimethoxy-
substituted clip 1c was converted into the hydroquinone clip
1b by reaction with boron tribromide and 1b into the diacetoxy-
substituted derivative 1a by esterification of the hydroquinone
1
b with acetic anhydride.
Because the yield of the Suzuki-Heck-type coupling is only
moderate (20%), we tried to synthesize clip 1a with an
independent route, namely, by repetitive Diels-Alder reactions
of bisdienophile 8a with two molecules of the o-quinodimethane
derivative 7 (generated in situ by debromination of bis(dibro-
momethyl)fluoranthene 6) followed by dehydrobromination of
the Diels-Alder adduct analogously to the successful synthesis
1
4
2
of molecular clips containing naphthalene sidewalls.
Molecular tweezers and clips containing aromatic sidewalls
are able to form stable complexes with electron-deficient
aromatic and aliphatic molecules, as well as inorganic and
Bis(dibromomethyl)fluoranthene 6 was prepared in three steps
as shown in Scheme 3: (1) Diels-Alder cycloaddition of
acenaphthylene to 2,3-dimethyl-1,3-butadiene, (2) oxidative
dehydrogenation of the Diels-Alder adduct by the use of
chloranil, and (3) photochemically induced N-bromosuccinimide
+
organic cations, including NAD (the important cofactor of
1
0
many redox enzymes). The driving force for the formation of
these adducts is based on charge-transfer interactions, coupled
to CH-π and π-π interactions and, in some cases, electrostatic
forces.
(4) Most recently, molecular clips have been used for the programmed
synthesis of polycyclic nano objects:(a) Ghosh, S.; Mukherjee, P. S Organo-
metallics 2008, 27, 316. (b) Ghosh, S.; Mukherjee, P. S. Dalton Trans. 2007,
2542. (c) Yang, H.-B.; Ghosh, K.; Das, N.; Stang, P. J. Org. Lett. 2006, 3991.
We have synthesized three molecular clips 1 (Schemes 1 and
) containing extended aromatic sidewalls (namely, benzo[k-
fluoranthene units). The two benzo[k]fluoranthene sidewalls are
connected by a dimethylene-bridged spacer containing a benzene
ring that carries two acetoxy (1a), hydroxy (1b), or methoxy
1c) substituents in the para position. Their NMR spectra, single-
crystal structures, photophysical and electrochemical behavior,
and host properties toward electron-deficient guest molecules
have been investigated.
To elucidate the contribution of the expanded aromatic
sidewalls to the binding of guest molecules, we also prepared
molecular clip 2a and half-clip 3a, which contain only one
benzo[k]fluoranthene sidewall, and we have compared the
properties of the new compounds 1-3 with those of the
2
]
(5) (a) Kl a¨ rner; F G.; Kahlert, B.; Boese, R.; Bl a¨ ser, D.; Juris, A.; Marchioni,
F. Chem. Eur. J. 2005, 11, 3363. (b) Marchioni, F.; Juris, A.; Lobert, M.;
Seelbach, U. P.; Kahlert, B.; Kl a¨ rner, F.-G. New J. Chem. 2005, 29, 780.
(6) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. Angew. Chem., Int. Ed.
2
001, 40, 2383.
(
(
7) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int. Ed.
2
003, 42, 1210.
(8) Gallivan, J. P.; Dougherty, D. A. J. Am. Chem. Soc. 2000, 122, 870.
(9) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J. Chem. Soc.,
Perkin Trans. 2 2001, 651.
(10) (a) Fokkens, M.; Jasper, C.; Schrader, T.; Koziol, F.; Ochsenfeld, C.;
Polkowska, J.; Lobert, M.; Kahlert, B.; Kl a¨ rner, F.-G. Chem. Eur. J. 2005, 11,
4
77–494. (b) Jasper, C.; Schrader, T.; Panitzky, J.; Kl a¨ rner, F.-G. Angew. Chem.,
Int. Ed. 2002, 41, 1355–1358. (c) Kl a¨ rner, F.-G.; Burkert, U.; Kamieth, M.;
Boese, R. J. Phys. Org. Chem. 2000, 13, 604.
(11) Rivera-Figueroa, A. M.; Ramazan, K. A.; Finlayson-Pitts, B. J. J. Chem.
Educ. 2004, 81, 242.
(12) Uibopuu, H.; Vodzinskii, Y V.; Tikhova, N. Y.; Kirso, U.; Jacquignon,
(
3) (a) Holder, S. J.; Elemans, J. A. A. W.; Donners, J. J. J. M.; Boerakker,
P. C. Polynuclear Heterocyclic Hydrocarbons; Plenum Publishing Corporation:
New York, 1987; p 1233.
M. J.; de Gelder, R.; Barber a` , J.; Rowan, A. E.; Nolte, R. J. M. J. Org. Chem.
2001, 66, 391. (b) Shimizu, K. D.; Dewey, T. M.; Rebeck, J. J. Am. Chem. Soc.
1994, 116, 5145. (c) Zimmerman, S. C.; Wu, W.; Zeng, Z. J. Am. Chem. Soc.
1991, 113, 196. (d) Nedar, K. M.; Whitlock, H. W. J. Am. Chem. Soc. 1990,
112, 7269.
(13) Wegner, H. A.; Scott, L. T.; De Meijere, A. J. Org. Chem. 2003, 68,
883–887.
(14) Kl a¨ rner, F.-G.; Panitzky, J.; Blaser, D.; Boese, R. Tetrahedron 2001,
57, 3573–3687.
5
840 J. Org. Chem. Vol. 73, No. 15, 2008

Molecular Clips with Extended Aromatic Sidewalls
SCHEME 3. Synthesis of Clip 2a (Containing One
Benzo[k]fluoranthene and One Naphthalene Sidewall) and
“
Half-Clip” 3a (Containing Only One Benzo[k]fluoranthene
Sidewall)
FIGURE 1. Two crystallographically independent molecules of
a·2CHCl displayed as capped sticks without the CHCl molecules,
hydrogen atoms, and disorder.
1
3
3
1
The concentration dependence of H NMR spectrum of 1a
in chloroform (CDCl3), especially the concentration-dependent
shift of the signal assigned to the central naphthalene protons
(Figure S2 in the Supporting Information), indicates the forma-
tion of a weak self-assembled dimer (1a)2 having an intertwined
structure similar to that found in the crystal. With the assumption
that the central naphthalene protons in (1a)2 experience an
1
upfield H NMR shift similar to that (∆δmax ) 2.3 ppm) found
for the corresponding protons in the self-assembled dimer of
the methanephosphonate-substituted clip having anthracene
sidewalls, the binding constant can be estimated for the
(
NBS) bromination produced the compound 6. However, the
1
6
reaction of 6 with the bisdienophile 8a led only to the (1:1)
adduct 9a, even with a large excess of diene precursor 6. Steric
hindrance presumably prevents the anellation of a second
expanded aromatic sidewall in 9a, but this unexpected result
allows us to isolate 9a and to prepare the unsymmetrical clip
formation of (1a)2 from the observed shift ∆δobs ) 0.05 ppm
2
-1
to be KDim ) [(1a)2]/[1a] e 10 M . The missing concentration
1
dependence of H NMR spectra of the clips 1c and 2a indicate
that these compounds exist monomolecular-disperse in solution
2
a by the reaction of 9a with o-bis(dibromomethyl)benzene,
(Figures S3 and S4 in the Supporting Information).
which proceeds smoothly contrary to corresponding reaction
Host-Guest Complex Formation. NMR Properties. The
1
with 6. The “half clip” 3a was prepared by the reaction of
magnetic anisotropy of the clip arene units makes H NMR
1
5
diacetoxybenzonorbornadiene 10a with bis(dibromometh-
yl)fluoranthene 6 in the presence of sodium iodide and calcium
carbonate analogously to 9a. The structures of all new com-
pounds were unambiguously characterized by their spectral data
spectroscopy a very sensitive probe for uncovering the binding
of a guest molecule inside the clip cavity. The host-guest
complex formation can be easily detected by pronounced upfield
1
shifts of the H NMR signals of the guest protons after addition
(
see Experimental Section and Supporting Information). In the
of the clip as host molecule. In all binding processes reported
here the host-guest association and dissociation are fast
processes with respect to the NMR time scale. Thus, the
case of the diacetoxy-substituted clip 1a, crystals could be grown
that were suitable for a single-crystal structure analysis. Their
analysis independently confirms the bowl-shaped structure of
the clip molecule 1a. In the crystal lattice of compound
1
maximum complexation-induced H NMR shifts, ∆δmax, of the
guest signals (∆δmax ) δ0 _- δC; where δ0, δC are the chemical
1
1
a · 2CHCl3, two pairwise arranged clips are interpenetrating
each other in an almost perpendicular fashion as shown in Figure
. The main planes through the molecules form an interplanar
H NMR shifts of the protons of the free and complexed guest
molecule, respectively) the association constants, Ka, and the
1
1
free enthalpies of association, ∆G, could be determined by H
angle of 78° [plane(C13, C51, C52, C37)/(C113, C151, C152,
C137) ] (see Experimental Section for the numbering of the
carbon atoms) with the CHCl3 groups filling gaps without
obvious and specific interactions. The maximum distances of
the sidewalls are similar (C13 · · ·C37 6.85 Å and C113 · · ·C137
NMR titration. In these experiments the dependence of the
1
complexation-induced H NMR shifts, ∆δobs (∆δobs ) δ0 -
1
δobs, where δobs is the chemical H NMR shift of the guest proton
observed in the presence of the host molecule and, hence, the
weighted average between δ0 and δC), of the guest signals from
the host concentration, [H]0, at constant guest concentration,
[G]0 ) constant, is measured, as described in the Experimental
Section. Scheme 4 shows the guest molecules studied in this
work.
6
.97 Å), which is in the range of π stacked aromatic compounds.
This highly ordered arrangement of intertwined clip molecules
is evidently stabilized by intermolecular π-π stacking of the
benzo[k]fluoranthene sidewalls.
(
15) (a) Vaughan, W. R.; Yoshimine, M. J. Org. Chem. 1957, 22, 7–12. (b)
(16) Kl a¨ rner, F.-G.; Kahlert, B.; Nellesen, A.; Zienau, J.; Ochsenfeld, C.;
Schrader, T. J. Am. Chem. Soc. 2006, 128, 4831–4841.
Meinwald, J.; Wiley, G. A. J. Am. Chem. Soc. 1958, 80, 3667–3671.
J. Org. Chem. Vol. 73, No. 15, 2008 5841

Branchi et al.
SCHEME 4. Structures of Guest Molecules That Form
Host-Guest Complexes with Molecular Clips
TABLE 1. Association Constants (Ka) Determined for the
a
a
Formation of Host-Guest Complexes with 1:1 Stoichiometry
Ka (M-1)
guest
TCNB
TCNQ
KS
NDCA
TNF
AQ
host 1a (R ) OAC)
host 1c (R ) OMe)
2
2
b
2
2
9.2 × 10
4.4 × 10 ; 1.6 × 10
2
2
b
5.8 × 10
2.3 × 10 ; 1.2 × 10
5.0 × 10
2
4.8 × 10
4
c
4
d
4
3
c
3.5 × 10 ; 6.3 × 10
5
e
1.2 × 10
6.9 × 10
5
d
2.5 × 10 ; 3.3 × 10
4.4 × 10
3
3
c
c
3
3
c
c
1
1
,8-DNAQ
,5-DNAQ
4.7 × 10
5.0 × 10
2.3 × 10
2.7 × 10
a
Unless otherwise noted, determined by 1H NMR titration
b
experiments in chloroform at 298 K. Association constant for the 2:1
host:guest complex.
Determined by spectrofluorimetric titration (see below), Determined
by spectrophotometric titration (vide infra).
c
Determined by solid-liquid extraction.
d
e
found for all other complexes by this kind of analysis (Table
1
). A single-crystal structure analysis of the crystalline complex
a
1
,2,4,5-tetracyanobenzene (TCNB), tetracyanoquinodimethane (TCNQ),
between the dimethoxy clip 1c and TCNB shows a (2:1)
host-guest stoichiometry supporting the results in solution (vide
infra).
Kosower salt (KS), 1,4,5,8-naphthalene-1,4,5,8-tetracarboxylic acid-dian-
hydride (NDCA), 9-dicyanomethylene-2,4,7-trinitrofluorene (TNF), anthra-
,10-quinone (AQ), 1,8-dinitroanthra-9,10-quinone (1,8-DNAQ), 1,5-
9
dinitroanthra-9,10-quinone (1,5-DNAQ).
Stability of Host-Guest Complexes. The data summarized
for the host-guest complex formation of the benzo[k]fluoran-
thene clips 1a,c in Table 1 allow the following conclusions: (i)
all complexes of the diacetoxy-substituted clip 1a are more
stable than the corresponding ones of the methoxy-substituted
clip 1c; (ii) the complexes of the benzo[k]fluoranthene clips 1a,c
are more stable than the corresponding complexes of the
anthracene or naphthalene clips 4a,c or 5a,c.
In the case of naphthalene-1,4,5,8-tetracarboxylic acid-
dianhydride (NDCA), and 1,8- and 1,5-dinitroanthra-9,10-
quinone (1,8- and 1,5-DNAQ), which are only slightly soluble
in chloroform, it was not possible to determine the host-guest
complex formation by conventional NMR titration experiments.
Since the solubility of these molecules substantially increases
in the presence of the molecular clip 1a or 1c by forming soluble
host-guest complexes, the association constant, Ka, and the
The effect of the substituents at the central spacer unit has
5,14
previously been observed for clips 4a,c or 5a,c.
It has been
∆
δmax values of the guest signals could be determined by
explained by their different steric size and conformations with
respect to the clip cavity. In the case of the dimethoxy-
substituted systems, the syn,syn conformation (in which both
OMe groups point toward the clip cavity) is calculated by force
field to be the preferred one. In this conformation the sterically
relatively large OMe substituents shield the clip cavity and,
hence, disfavor the complex formation. In the case of the
diacetoxy-substituted system 5b the anti,anti conformation was
calculated to be the most stable one. In this conformation there
is no steric hindrance to complex formation. In the single-crystal
structures of some complexes of the diacetoxy-substituted
naphthalene clip 5a a syn,anti conformation is observed in which
the carbonyl oxygen atom (pointing toward the cavity) obviously
forms an additional attractive interaction with the guest molecule.
1
solid-liquid extraction by means of H NMR spectroscopy, as
described in the Experimental Section. The host-guest complex
between TNF and the diacetoxy-substituted clip 1a is too stable
for the determination of the Ka value by H NMR titration. (The
δobs values of the TNF protons are almost independent from
the concentration of 1a and close to the ∆δmax value). In this
case the binding constant, Ka, could be determined in more dilute
solution either by spectrophotometric and spectrofluorimetric
titration (vide infra).
Stoichiometry of Host-Guest Complexes. According to the
analysis of the NMR titration experiments by the use of the
1
∆
1
7
HOSTEST program, TCNB and TCNQ form host-guest
complexes of mixed (1:1) and (2:1) stoichiometry with the
dimethoxy-substituted clip 1c, whereas a (1:1) stoichiometry is
The higher stability of the host-guest complexes of the
benzo[k]fluoranthene clips 1a,c with respect to the corresponding
complexes of the anthracene or naphthalene clips 4a,c or 5a,c
can be rationalized with the larger van der Waals contact surface
of the benzo[k]fluoranthene sidewalls in the molecular clips of
type 1 compared to the anthracene or naphthalene sidewalls in
the clips of type 4 or 5. This stabilizing effect is relatively small
in the case of the monocyclic guest molecules such as TCNB,
TCNQ, and Kosower salt (KS, Scheme 4). The complexes of
(
(
17) HOSTEST V5.60; University of Pittsburgh: Pittsburgh, PA.
18) The electrostatic potential surfaces (EPS) shown in Figure 2 were
calculated by the use of computer program SPARTAN 04 Version 1.0.0
Wavefunction Inc.) as described by Kamieth, M.; Kl a¨ rner, F.-G.; Diederich, F.
(
Angew. Chem., Int. Ed. 1998, 37, 3303–3306. Kl a¨ rner, F. G.; Panitzky, J.; Preda,
D.; Scott, L. J. Mol. Model. 2000, 6, 318–327.
(19) (a) Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Lipton,
M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput. Chem.
990, 11, 440–467. (b) Macromodel, V. 6.5; Schr o¨ dinger: Portland, OR.
(
1
20) The dynamic processes, association-dissociation and guest rotation
inside the host cavity, have observed by the use of NMR spectroscopy at low
temperatures. Lobert, M.; Bandmann, H.; Burkert, U.; Buchele, U. P.; Podsad-
lowski, V.; Kl a¨ rner, F.-G. Chem.-Eur. J. 2006, 12, 1629–1641.
1
a with these guest molecules are only more stable by a factor
of 1.3-4.5 or 3.4-19.3 than the corresponding complexes of
the anthracene clip 4a or the naphthalene clip 5a.
A dramatic increase (by a factor g500) in the stability of the
TNF complex with 1a compared to that with the anthracene
clip 4a (Table 2) was found. The comparison of the binding
constant of complex 1a⊃TNF with that of 2a⊃TNF or 3a⊃TNF
(
(
21) Mukherjee, T. K.; Levasseur, L. A. J. Org. Chem. 1965, 30, 644.
22) (a) Hellrung, B.; Balli, H. HelV. Chim. Acta 1990, 73, 81. (b) Loufty,
R. O.; Hsiao, C. K.; Ong, B. S.; Keoshkerian, B. Can. J. Chem. 1984, 62, 1877.
(23) (a) Binstead, R. A. Specfit; Spectrum Software Associates: Chapell Hill,
NC, 1996. (b) Gampp, H.; Maeder, M.; Meyer, C. J.; Zuberbulher, A. Talanta
1
985, 32, 257.
24) F o¨ rster, Th. Discuss. Faraday Trans 1959, 27, 7.
(
5
842 J. Org. Chem. Vol. 73, No. 15, 2008

Molecular Clips with Extended Aromatic Sidewalls
1
TABLE 2. Maximum Complexation-Induced H NMR Shifts of TNF Protons, ∆δmax, and Association Constants, Ka, for the Formation of the
a
TNF complexes of 1a, 2a, 3a, and 4a
host
guest TNF
1a
2a
3a
4a
5
b
2
2
2
Ka
2.9×10
6.1×10
1.2×10
5.7×10
∆δmax Ha
∆δmax Hb
∆δmax Hc
∆δmax Hd
∆δmax He
1.6
c
1.6
2.0
1.1
1.0
1.9
2.4
1.6
1.2
1.4
1.3
1.8
0.6
2.7
2.8
1.2
2.1
c
1.9
a
1
b
Deteremined by H NMR titration experiments in CDCl3 at 298 K. Mean value obtained from spectrofluorimetric and spectrophotometric titration.
Not detectable.
c
∆
δmax, of the guest protons in the complexes of the molecular
clips 1a,c (Table 2 and Table S1 in the Supporting Information)
are of comparable size to those observed in corresponding
complexes of the clips 4a,c and 5a,c containing the smaller
anthracene or naphthalene sidewalls. This finding indicates that
in all these complexes with 1a,c, 4a,c, and 5a,c the guest
molecules are positioned inside the clip cavity similarly. The
complex structures (calculated by force field using a Monte-
Carlo conformer search, Figure 3) agree well with the observed
large ∆δmax values which are affected by the magnetic anisot-
ropy of the surrounding clip arene-units (Figure 4).
The calculated structures show that in the case of the
monocyclic guest molecules such as TCNB or TCNQ only the
upper naphthalene units of the benzo[k]fluoranthene sidewalls
are needed for the binding of the guest molecule, whereas in
the case of the larger polycyclic guest molecules such as NDCA,
TNF, etc. the complete benzo[k]fluoranthene sidewalls are used
for the binding of the guest molecule. This certainly explains
why the increase in the stability of the complexes with 1a,c is
much larger with the polycyclic guest molecules than that found
for the smaller monocyclic guest molecules. According to the
calculations (Figure 3), the chemically equivalent protons in
the free guest molecules are expected to be nonequivalent in
FIGURE 2. Electrostatic potential surfaces (EPSs) of the diacetoxy-
substituted benzo[k]fluoranthene clip 1a (left) and guest molecules
(
(
right) calculated by AM1. The color code spans from -25 kcal/mol
red) to +25 kcal/mol (blue). The listed molecular electrostatic
potentials (MEP in kcal/mol) were calculated at the marked positions
by AM1 and in the case of 1a additionally by B3LYP/6-31G**//AM1.
25
Both methods led to highly negative MEP values.
demonstrates that both benzo[k]fluoranthene sidewalls in 1a are
essential for the formation of this extraordinarily stable host-guest
complex. Finally, the finding that the complexes of the dinitro-
substituted anthra-9,10-quinone derivatives (1,8- and 1,5-
DNAQ) with molecular clip 1a are substantially more stable
than that of the parent anthra-9,10-quinone (AQ, Table 1) allows
the conclusion that electron donor-acceptor interactions provide
an important contribution to the host-guest binding in these
systems despite their large van der Waals contact surfaces. The
electrostatic potential surfaces (EPS calculated by quantum
chemical methods) show the complementary nature of the
negatively polarized host cavity and the positively polarized
guest molecules substituted by electron-withdrawing nitro or
cyano groups leading to the tight binding of 1,5- DNAQ, 1,8-
DNAQ, and TNF by molecular clip 1a, whereas the EPS of the
benzene rings of anthra-9,10-quinone (AQ) is still slightly
negative though this molecule contains the electron-withdrawing
carbonyl functions (Figure 2). The negatively polarized AQ is
only weakly bound to 1a.
1
the host-guest complexes each showing two separated H NMR
1
signals. The observation of only one H NMR signal for the
bound guest molecule such as TCNB, TCNQ, or NDCA
providesevidencethatthemutualcomplexdissociation-association,
as well as the rotation of the guest molecule inside the clip
cavity, are fast processes with respect to the NMR time scale.
Thus, only one signal as average of the signals of all possible
2
0
conformers is observed. This result confirms the assumption
1
which has been made for the analysis of the H NMR titration
experiments. The finding of similar ∆δmax values for the TNF
protons Ha, Hc, and He in the highly stable complex 1a⊃TNF
(
Table 2) indicates that this complex exists as a dynamic
equilibrium between both conformers shown in Figure 3
contrary to complex 4a⊃TNF, which seems to exist mainly in
the higher energy conformer according to the larger ∆δmax
5
a
values found for the TNF protons Hc and Hd.
Complex Structures Calculated by Molecular Mechanics.
1
Single Crystal Structure of a Host-Guest Complex. The
single-crystal structure determined for the (2:1) complex
between the dimethoxy-substituted clip 1c and TCNB as guest
molecule displays a different position of the guest molecule. In
The large complexation-induced shifts of the H NMR signals,
(
25) Debad, J. D.; Morris, J. C.; Magnus, P.; Bard, A. J. J. Org. Chem. 1997,
6
2, 530.
J. Org. Chem. Vol. 73, No. 15, 2008 5843

Branchi et al.
FIGURE 5. Absorption spectra of compounds 1a, BkF, and TNF in
CHCl solution.
3
clips, the walls are bent outward. Thus, they can adopt the
TCNB between one sidewall of each clip. The high flexibility
of the clips is apparent due to the expansion by 4.7 Å (distance
C4· · ·C28 11.36 Å). Two of the CHCl3 molecules are situated
inside the clips where they employ C-H· · ·Cl and C-H· · ·π
contacts (2.82 and 2.61 Å). The CHCl3 molecules outside the
clips are linked via C-H· · ·N contacts (2.53 Å) (Figure 5). The
two methoxy groups attached to the central spacer unit of 1c
point toward the clip cavity and evidently shield the upper part
of the cavity from binding the TCNB molecule in this part of
the clip cavity. This observation may explain why 1c also forms
(
2:1) complexes with the small guest molecules TCNB and
TCNQ in solution.
FIGURE 3. Lowest energy structures of the host-guest complexes
of the diacetoxy-substituted clip 1a with various guest molecules
calculated by a Monte-Carlo conformer search (MacroModel 6.5, force
field: AMBER*, 5000 structures): (a) front view and (b) side view
of the energy minimum structure of complex1a⊃TNF; (c) side view
of a structure calculated to be higher in energy by 3.7 kcal/mol.
Photophysical Properties. The benzo[k]fluoranthene chro-
mophore presents two structured ππ* bands with maximum at
309 and 404 nm in CHCl3 solution at room temperature (Figure
1
9
1
0
5
).
The absorption spectra of the two clips 1a,c are similar to
that of model compound BkF with slightly broader and red-
shifted bands. The molar absorption coefficients of clips 1a and
1
c (Table 3) are lower than expected, taking into account that
each molecule contains two chromophores. This effect may be
due to a perturbation of the chromophore properties upon
functionalization with the bridge, while a through-space interac-
tion of the two chromophores is not likely because of the
distance and quite low flexibility of the bridge. Compounds 1a,c
and BkF are highly fluorescent (Figure 6 and Table 3) with a
structured band with πmax ≈ 415 nm. In the case of the two
clips, the emission band is slightly red-shifted and corresponds
to a smaller quantum yield compared to BkF, while the lifetime
of the emitting excited-state is the same for the three compounds.
These results suggest slight differences in the radiative and
nonradiative rate constants of the fluorescent excited-state in
going from BkF to clips 1a and 1c. At 77 K in CH2Cl2/CHCl3
1
:1 (v/v) rigid matrix, the emission spectra of the three
FIGURE 4. Clips in 1c · 1/2TCNB (C10
H
2
N
4
) · 2CHCl
3
displayed as
compounds show a slightly blue-shifted fluorescent band and a
structured and much weaker phosphorescence with πmax ) 568
nm and lifetime π ) 0.4 s (Table 3, Figure 6).
No evidence of concentration dependence of either absorption
or emission spectra of 1a and 1c confirms that in dilute solutions
this molecular clip exists as a monomer (vide supra).
TNF shows two absorption bands at 310 and 350 nm, a tail
extending up to 450 nm (green line in Figure 5), and a very
capped sticks with blue lines between the side walls indicating the
π-stacking (C32 · · ·C74 3.37 Å) and the C-H· · ·π interactions from
the CHCl
.53 Å).
3
molecules (H71· · ·C45 2.61 Å) and to the TCNB (H60 · · ·N2
2
this structure (formally consisting of 1c · 1/2TCNB (C10H2-
N4)·2CHCl3) where TCNB is additionally complexed between
the sidewalls of two pairwise and centrosymmetric arranged
5
844 J. Org. Chem. Vol. 73, No. 15, 2008

Molecular Clips with Extended Aromatic Sidewalls
TABLE 3. Photophysical Data in Air-Equilibrated CHCl3 Solution, unless Otherwise Noted
a
2
98 K
77 K
absorption
fluorescence
fluorescence
phosphorescence
λmax (nm) τ (s)
4
-1
λmax (nm)
ε (10 M cm-1
)
λmax (nm)
Φem
τ (ns)
λmax (nm)
τ (ns)
1
1
BkF
TNF
a
c
405
405
402
364
2.3
1.8
1.3
1.8
415
415
412
530
0.5
0.5
7.0
7.0
7.0
0.8
412
412
408
7.0
7.0
7.0
568
568
568
0.4
0.4
0.4
0.7
10-
3
a
Emission spectra performed in CH2Cl2/CHCl3 1:1 (v/v).
FIGURE 6. Fluorescence (solid line, CHCl
3
solution, 298 K) and
phosphorescence (dashed line, CH Cl /CHCl 1:1 v/v, 77 K) spectra
2 2 3
of compounds 1a. λex ) 380 nm.
FIGURE 8. Fluorescence spectral changes upon titration of a 4.4 ×
-
6
1
0
3
M solution of TNF with 1a in CHCl solution. Blue and pink
lines correspond to the addition of 0 and 3 equiv of TNF per clip,
respectively. The inset shows the emission intensity changes at 440
2
2
nm and the fitting curve obtained by Specfit.
cm- at 600 nm is formed. The fitting of the emission spectra
1
with the same procedure yields a similar value within the
5
-1
experimental error: Ka ) 2.5 × 10 M . As previously
discussed, the formation of the 1a⊃TNF adduct is due to
electron donor-acceptor interactions between the strong electron-
21,22
acceptor guest
(see also electrochemical experiments below)
and the electron-donating cavity of the clip. The formation of
a complex with charge transfer (CT) character is consistent with
both the presence of a low-energy CT absorption band with
maximum at 600 nm and the quenching of the fluorescent
excited-state of the benzo[k]fluoranthene moiety of the clip. The
CT band resulting from the complex formation is responsible
for the color change from yellow to green (Figures 5 and 7) of
the solution containing clip 1a upon addition of TNF. A color
change is also observed upon titration of clip 1a with other guest
molecules, for example, with NDCA the color changes from
yellow to red.
-6
FIGURE 7. Absorption spectral changes upon titration of a 7.2 × 10
M solution of 1a with TNF in CHCl solution (cuvette optical path )
cm). Blue and pink lines correspond to the addition of 0 and 3 equiv
of TNF per clip, respectively. The inset shows the absorbance changes
3
5
2
7
at 600 nm with the fitting curve obtained by Specfit.
No change in the photophysical properties of BkF has been
observed upon addition of up to 10 equiv of TNF in CHCl3
solution, demonstrating that the supramolecular structure of clip
weak fluorescence band at 530 nm (Table 3). Upon titration of
-6
-3
a 7.2 × 10 M CHCl3 solution of 1a with a 2 × 10 M CHCl3
solution of TNF, a new absorption band at 600 nm increases
1
a is necessary for the formation of a stable adduct with high
(
Figure 7), and concomitantly the fluorescence intensity of the
association constant.
Fluorescence Anisotropy. The fluorescence anisotropy is
defined as
clip, upon excitation at the 402 nm isosbestic point, decreases
(
Figure 8).
Global fitting of the changes in the absorption spectra show
5
-1
-1
that a 1:1 adduct with Ka ) 3.2 × 10 M and π ) 1000 M
r ) I -I ⁄ I +2I
)
(1)
(
||
⊥
) ( ||
⊥
(
26) Kaifer, A. E.; G o´ mez-Kaifer, M. Supramolecular Electrochemistry;
Wiley-VCH: Weinheim, 1999; Chapter 9.
27) The nonlinear regression analyses of the NMR titration experiments were
where I| and I⊥ are the emission intensities registered when the
emission and excitation polarizers are oriented parallelly or
perpendicularly, respectively. For a single molecular species in
a rigid matrix, the fluorescence anisotropy corresponds to an
(
performed by means of the computer program TableCurVe 2D 5.01; SYSTAT
Software, Inc.: Richmond, CA.
J. Org. Chem. Vol. 73, No. 15, 2008 5845

Branchi et al.
TABLE 4. Steady-State Fluorescence Anisotropy Values (rss) and
the compound. The rss values obtained in rigid matrix reflect
the intrinsic anisotropy r0 for BkF but not for clips 1a and 1c.
Indeed, the r0 value is the same for the benzo[k]fluoranthene
chromophore isolated or inside the clip structure, and the lower
value of rss in rigid matrix for 1a and 1c is due to fluorescence
depolarization by energy migration. On the other hand, in fluid
solution the situation is opposite: the values of rss are higher
for clips 1a and 1c compared to BkF. This result can be
rationalized considering that, under these experimental condi-
tions, the main mechanism of depolarization is rotation for all
of the investigated compounds and that rotation is slower for
the clips because of their higher hydrodynamic volume. This
hypothesis is also confirmed by the fact that, upon decreasing
the solvent viscosity, i.e., in going from CHCl3/propylene glycol
a
Rotational Relaxation Times (θ)
CHCl3/propylene glycol
rss
θ (ns)
1
a
1
1
1
:1
:1
:99
0.02
(0.2)
0.08
b
9
8
2
1
c
1
1
1
:1
:1
:99
0.02
(0.2)
0.08
b
BkF
1
1
1
:1
:1
:99
0.01
b
(0.3)
0.05
a
In air-equilibrated CHCl3/propylene glycol mixtures at 298 K, unless
b
1
:99 to 1:1 (v/v) mixtures, a decrease of ca. 4 times in the
otherwise noted. Steady-state fluorescence anisotropy in CHCl3/CH2Cl2
:1 (v/v) at 77 K.
1
steady-state anisotropy has been observed for both BkF and the
clips and that complete depolarization takes place in pure CHCl3
solution. As to the fluorescence anisotropy decay, the experi-
mental data can be fitted considering an apparent r0 value of
0.20 for the clips and an r0 value of 0.30 for BkF. Indeed,
depolarization by energy migration takes place on a much
shorter time-scale that cannot be investigated with our experi-
mental setup. Energy migration is a very fast process of
intrinsic value r0 that depends only on the different orientation
of the absorption and emission transition moments. In the case
of a fluid solution, the value of the anisotropy decreases if the
molecule undergoes a change in orientation during the excited-
state lifetime. In particular, in the case of a spherical rotor, the
decay of r can be fitted by a monoexponential model (eq 2
2
4
depolarization since, based on the F o¨ rster theory: (i) ken )
0
-t⁄θ
r t
( )
) r e
(2)
11 -1
1
0
s
for a distance of 1 nm between the two fluorophores
where θ is the rotational relaxation time, which can be related
to the hydrodynamic volume Vh by the Stokes-Einstein-Debye
equation:
of the clips, compared to an intrinsic deactivation rate constant
8
-1
k ) 1/τ ) 1.1 × 10 s (τ ) 9 ns in CHCl3/propylene glycol
1:99 v/v); (ii) the distance value where the rate of energy transfer
and that of intrinsic deactivation are equal (R0) is 3 nm.
Therefore, fluorescence anisotropy decay observed in the ns time
scale for compounds 1a,c and BkF at 298 K is due to
depolarization by rotation and can thus be correlated with the
corresponding hydrodynamic volumes.
Vhη
θ) kBT
(3)
where η is the viscosity of the solvent, kB is the Boltzmann
constant, and T is the absolute temperature.
For species containing only a fluorophoric unit as BkF, the
value of the rotational relaxation time (θ) in fluid solution yields
information on the hydrodynamic volume (Vh) of the molecule.
On the other hand, for clips 1a and 1c, which contain two
identical fluorophoric units, fluorescence depolarization can
occur also by energy migration from the originally excited unit
to the other differently oriented one. The single-crystal structure
of clip 1a shows that the two benzo[k]fluoranthene moieties of
the clips are not randomly oriented because of the constraint
imposed by the central spacer unit. However, the symmetrical
geometry of the clips does not imply a collinear orientation of
the emission transition moments of the two fluorophores, so
that energy migration may, in principle, cause fluorescence
depolarization.
Electrochemical Properties. The oxidation of BkF is known
to be associated with a dimerization reaction. In particular, the
(7,12-diphenyl)benzo[k]fluoranthene undergoes a dimerization
process upon oxidation: a dimer is formed upon removal of two
2
5
electrons and two hydrogen atoms.
Cyclic voltammograms performed on benzo[k]fluoranthene
and 1a in CH2Cl2/TBAPF6 solutions at 298 K show no reduction
process in the available negative potential window and oxidation
processes (Figure 9a and Table 5) with partial chemical
irreversibility.
In the case of BkF, chemical reversibility for the one-electron
oxidation process is achieved at 1 V/s, although the correspond-
ing E1/2 value reported in Table 5 is partially affected by
adsorption phenomena upon the reverse cathodic scan. The clip
The measurements in fluid solution were performed in CHCl3/
propylene glycol mixtures, because in the low-viscosity CHCl3
solvent, complete depolarization takes place during the excited-
state lifetime for compounds 1a,c and BkF, while in pure
propylene glycol the investigated compounds are not soluble.
Experimental data reported in Table 4 show that (i) no significant
difference has been observed for 1a and 1c, as expected; (ii) rss
and θ values are higher for the clips compared to BkF in fluid
solution; (iii) the steady-state fluorescence anisotropy measured
in a rigid matrix of CH2Cl2/CHCl3 1:1 (v/v) at 77 K is lower
for the clips compared to BkF. These results suggest that in
fluid solution fluorescence depolarization of the clip takes place
by two channels, namely, rotation and energy migration, whereas
in the case of BkF only the former channel is present, but with
a higher efficiency due to the lower hydrodynamic volume of
1
a presents three oxidation processes: the first two processes
are one-electron transfers associated to chemical reactions. They
likely correspond to the first oxidation of each benzo[k]fluo-
ranthene in the clip occurring at distinct potential values because
of electrostatic and/or electronic interactions between the two
moieties: the first is oxidized at less positive potential and the
second at approximately the same value as that observed for
benzo[k]fluoranthene. Upon increasing the scan rate, the height
of the reverse peaks at ca. 1.2 V (vs SCE) in Figure 9a increases
in intensity and that of the peak at 1.05 V (vs SCE) decreases,
indicating that the last process corresponds to the rereduction
of a product formed by the chemical reaction coupled to the
first two electron transfer processes. Complete chemical revers-
ibility is not observed even at scan rates of 20 V/s. The third
5
846 J. Org. Chem. Vol. 73, No. 15, 2008

Molecular Clips with Extended Aromatic Sidewalls
FIGURE 10. Cyclic voltammograms of TNF (c ) 1.2 mM) upon
addition of from 0 (green line) up to 2.0 (pink line) equiv of clip 1a in
2 2 6
CH Cl /TBAPF solution. Working electrode, glassy carbon; scan rate,
0
.05 V/s. Ferrocene (Fc) reversible oxidation is also reported.
SCHEME 5. Square Scheme Representing Formation of the
a⊃TNF Complex and the Reduction Processes
1
line in Figure 9b), compared to the cyclic voltammograms
of the two separated components. The fact that the benzo-
[
k]fluoranthene moiety is more difficult to oxidize and TNF
is more difficult to reduce when they are associated with one
another demonstrates the charge transfer character of this
complex. Moreover, lower current intensities are registered
for 1a⊃TNF because of the lower diffusion coefficient of
the complex compared to the separated components. In-
deed, the first oxidation process of 1a⊃TNF is superimposed
to the second oxidation process of the clip, but the current
intensity is not doubled (Figure 9a). On the other hand,
successive reduction and oxidation processes occur at the
same potentials of the isolated components because upon one-
electron oxidation or one-electron reduction, the complex is
destroyed.
FIGURE 9. Cyclic voltammograms of compounds 1a (blue line) and
TNF (green line) and an equimolar solution of 1a and TNF (pink line)
with concentration of 1.3 mM in CH Cl /TBAPF solution. Working
2 2 6
electrode, glassy carbon; scan rate, 0.05 V/s. In panel a, ferrocene (Fc)
reversible oxidation is also reported.
TABLE 5. Electrochemical Data in CH2Cl2/TBAPF6 Solution at
2
98 K
IIIred
IIred
Ired
Iox
IIox
IIIox
a
a
1
BkF
TNF
a
+1.28
+1.33
+1.55
b
+1.33
-1.19
-1.19
-0.62
-0.62
-0.02
-0.12
c
1
a⊃TNF
+1.33
+1.55
A more careful investigation of the cathodic region and,
in particular, of the first process has been performed because
of its complete chemical reversibility. Under the experimental
conditions used in the experiments reported in Figure 9, the
percentage of association is above 90%, estimated on the
basis of the association constant measured in chloroform
solution by photophysical data. By successive addition (from
0.5 to 3 equiv) of clip 1a to a 1.2 mM solution of TNF, a
progressive shift of the first reduction process is observed
(Figure 10): the maximum shift of the half-wave potential
compared to the free TNF is 95 mV, registered upon addition
of 2 equiv of clip per guest molecule. Moreover, upon
addition of clip 1a to a TNF solution, a broadening and
intensity decrease is observed (Figure 10), demonstrating that
the diffusion coefficient (vide supra) and the rate of
heterogeneous electron transfer is lower in the complex since
the electroactive guest is encapsulated inside the cavity of
the host.
a
b
Chemically irreversible process; Epa value at 0.2 V/s. Adsorption
on the working electrode partially affects the reverse scan. Bielectronic
process.
c
electron transfer process is broader and chemical irreversible,
thus precluding a careful estimation of the number of electrons
exchanged.
Under the same experimental conditions, TNF shows three
reversible one-electron reduction processes (green line in Figure
9
b, Table 5): the first process occurs at -0.02 V (vs SCE)
demonstrating the strong electron-acceptor character of this
compound. No oxidation process is observed in the available
potential window.
2
2
An equimolar solution of 1a and TNF (c ) 1.3 mM) in
CH2Cl2/TBAPF6 shows cyclic voltammograms in which the
first oxidation process is positively shifted (pink line in Figure
9
a) and the first reduction process is negatively shifted (pink
J. Org. Chem. Vol. 73, No. 15, 2008 5847

Branchi et al.
The observed electrochemical behavior can be rationalized
on the basis of the square scheme reported in Scheme 5, in
which the association constant Ka of 1a and TNF is much larger
than that of 1a and TNF• (Ka′). The two constants, Ka and Ka′,
Therefore, the complex 1a⊃TNF can be disassembled by
electrochemical stimuli.
-
Experimental Section
2
6
are linked by the following equation:
Synthesis. Dimethoxy-benzo[k]fluoranthene Clip (1c). A Schlenk
0
0
2 3
flask was equipped with a magnetic stirring bar, Pd (dba) (0.33 g,
Ka
F
(
E - E C)
0
.37 mmol, dba ) dibenzylidene acetone, Ph-CHdCH-C(dO)-
ln
)
(4)
(
)
Ka′
RT
3
CHdCH-Ph), P(Cy) (0.55 g, 1.95 mmol, tricyclohexylphosphine,
6 11 3
P(c-C H ) ), (0.87 g, 1.11 mmol) 5d, 1-naphthaleneboronic acid
(0.5 g, 2.91 mmol), and DBU (2.5 mL, 1,8-diazabicyclo[5.4.0]undec-
7-ene) in DMF (33 mL). The resulting mixture was stirred at 155
0
0
where E and E C are the standard reduction potentials for TNF
and 1a⊃TNF, respectively. A good estimation of these values
are the E1/2 values reported in Table 5, from which the ratio
Ka/Ka′ results are of the order of 40.
°
C for 48 h. After dilution with 50 mL of CH
washed three times with 50 mL of HCl (5%) and once with 50 mL
of saturated NaHCO and 50 mL of H O. Drying the organic phase
over Na SO and evaporation of the solvent gave the crude product.
2 2
Cl , the mixture was
3
2
Upon variation of the scan rate in the range 0.05-20 V/s, no
change in the shape of the cyclic voltammogram is observed,
indicating that both the forward and backward scan of the first
reduction process are due to the redox couple 1a⊃TNF/
2
4
Column chromatography on silica gel, eluting with EtOAc/
cylohexane (1:3) yielded 1c as a yellow solid (150 mg, 0.21 mmol,
1
3
19%): mp >240 °C (dec); H NMR (CDCl , 500 MHz) δ [ppm]
•
-
2
1
a⊃TNF . The heterogeneous electron transfer rates are, in
2.49 (br dt, 2H, J(27i-H, 27a-H) ) 7.15 Hz, 27a-H, 28a-H), 2.59
(br dt, 2H, 27i-H, 28i-H), 3.91 (s, 6H, -CH ), 4.61 (br t, 4H, 9-H,
11-H, 22-H, 24-H), 7.54 (dd, 4H, J(1-H, 2-H))7 Hz, J(2-H, 3-H)
8.05 Hz, 2-H, 5-H, 15-H, 18-H), 7.65 (s, 4H, 8-H, 12-H, 21-H,
5-H), 7.72 (d, 4H, 3-H, 4-H, 16-H, 17-H), 7.83 (d, 4H, 1-H, 6-H,
any case, higher than the association and dissociation rate
3
•-
3
3
constants of the complex between 1a and TNF or 1a and TNF .
)
2
1
3
Conclusions
14-H, 19-H), 8.03 (s, 4H, 7-H, 13-H, 20-H, 26-H); C NMR
(
(
CDCl
-OCH
3
, 125 MHz) δ [ppm] 47.8 (C-9, C-11, C-22, C-24), 61.6
3
Molecular clips (1) containing two extended aromatic side-
walls, namely, two benzo[k]fluoranthene units, linked by a
dimethylene-bridged spacer unit containing a benzene ring
substituted with two acetoxy (1a), hydroxy (1b), or methoxy
), 64.5 (C-27, C-28), 119.0 (C-1, C-6, C-14, C-19), 120.3
(C-7, C-13, C-20, C-26), 120.6 (C-8, C-12, C-21, C-25), 126.0 (C-
3, C-4, C-16, C-17), 128.1 (C-2, C-5, C-15, C-18), 130.4 (C-3a,
C-16a), 132.2 (C-7a, C-12a, C-20a, C-25a), 134.8 (C-3c, C-16c),
1
37.1 (C-6a, C-13b, C-19a, C-26b), 137.4 (C-6b, C-13a, C-19b,
(
1c) groups in the para position, have been synthesized and fully
C-26a), 139.9 (C-9a, C-10a, C-22a, C-23a), 145.7 (C-10, C-23),
characterized (NMR spectra, single-crystal structures, photo-
physical and electrochemical properties). These clips are strongly
fluorescent, and anisotropy measurements demonstrate that
fluorescence depolarization takes place by a very fast (subns)
energy migration between the two identical benzo[k]fluoranthene
units and, in fluid solution, by rotation of the clip. For the
purpose of comparison, similar compounds (2 and 3), containing
only one benzo[k]fluoranthene unit, have been prepared and
studied.
-1
1
47.8 (C-8a, C-11a, C-21a, C-24a); IR (KBr) ν˜ [cm ] 3044 (C-H),
992 (C-H), 2960 (C-H), 2926 (C-H), 2852 (C-H), 1477
2
(
3
3
CdC), 1287 (C-O); UV-vis (CHCl ) λmax [nm] (log ε) 246 (4.77),
12 (4.75), 362 (3.87), 382 (4.07), 405 (4.09); MS (70 eV) m/z
+
+
(
7
%) 714 (100) [M ], 699 (25) [M - CH
14.2604; 714.2559 calcd for C54
Hydroquinone Benzo[k]fluoranthene Clip (1b). Under argon,
3
]; HR-MS (70 eV) found
34 2
H O .
a stirred solution of 1c (198 mg, 0.28 mmol) in dichloromethane
(25 mL) was cooled down to -78 °C and treated with boron
tribromide (700 µL, 7.27 mmol). The solution was allowed to warm
up slowly to room temperature over 24 h. Methanol (5 mL) was
slowly added to the solution, which was cooled again with ice/
water, to quench the excess of boron tribromide. After removal of
the solvents the red-brown crude product was suspended in
methanol, filtered and washed with 1 mL of acetonitrile. 1b (159
The highly negative electrostatic potential on the inner cavity
of these clips constitutes the recognition site for electron-
deficient guest molecules. The effect on the association constants
of (i) the presence of one or two identical aromatic sidewalls,
(
ii) different substituents on the bridging unit, and (iii) the nature
of the aromatic sidewalls have been investigated with a variety
of electron-acceptor molecules. In the case of polycyclic guests,
clips 1 evidence hosting ability stronger than the corresponding
anthracene and naphthalene clips (4 and 5). This finding can
be rationalized with the larger van der Waals contact surface
of the benzo[k]fluoranthene compared to the anthracene or
naphthalene sidewalls.
mg, 0.23 mmol, 84%) was isolated as a brown-orange solid: mp >
1
300 °C; H NMR (DMSO-d
6
, 500 MHz) δ [ppm] 2.32 (br dt, 2H,
2
J(27i-H, 27a-H) ) 8.40 Hz, 27a-H, 28a-H), 2.40 (br dt, 2H, 27i-
H, 28i-H), 4.60 (br t, 4H, 9-H, 11-H, 22-H, 24-H), 7.62 (dd, 4H,
J(1-H, 2-H)) 6.90 Hz, J(2-H, 3-H)) 8.15 Hz, 2-H, 5-H, 15-H,
8-H), 7.69 (s, 4H, 8-H, 12-H, 21-H, 25-H), 7.82 (d, 4H, 3-H, 4-H,
6-H, 17-H), 8.00 (d, 4H, 1-H, 6-H, 14-H, 19-H), 8.23 (s, 4H, 7-H,
6
3-H, 20-H, 26-H), 8.64 (2, 2H, -OH); C NMR (DMSO-d , 125.7
3
3
1
1
1
13
Among the investigated systems, the most stable complex is
MHz) δ [ppm] 46.4 (C-9, C-11, C-22, C-24), 64.2 (C-27, C-28),
119.5 (C-1, C-6, C-14, C-19), 120.0 (C-8, C-12, C-21, C-25), 120.4
(C-7, C-13, C-20, C-26), 125.9 (C-3, C-4, C-16, C-17), 128.3 (C-
2, C-5, C-15, C-18), 129.9 (C-3a, C-16a), 131.5 (C-7a, C-12a,
that of clip 1a with 9-dicyanomethylene-2,4,7-trinitrofluorene
5
-1
guest. The association constant (Ka ≈ 10 M , in chloroform
solution) has been estimated by spectrophotometric and spec-
trofluorimetric titrations. Indeed, profound changes of the
photophysical properties are brought about by the formation of
the complex 1a⊃TNF: the presence of a new charge-transfer
state is demonstrated by the presence of a broad absorption band
at 600 nm and the quenching of the strong benzo[k]fluoranthene
emission. Moreover, the electrochemical investigation shows a
positive shift of the clip oxidation process and a concomitant
negative shift of the TNF reduction process in the complex,
while further reduction and oxidation processes are unaffected.
1
1
C-20a, C-25a), 133.7 (C-3a , C-16a ), 133.9 (C-9a, C-10a, C-22a,
C-23a), 136.1 (C-6b, C-13a, C-19b, C-26a), 136.3 (C-6a, C-13b,
C-19a, C-26b), 139.1 (C-10, C-23), 148.2 (C-8a, C-11a, C-21a,
-1
C-24a); IR (KBr) ν˜ [cm ] 3338 (O-H), 3042 (C-H), 2991 (C-H),
936 (C-H), 2862 (C-H), 1485 (CdC), 1277 (C-O); UV-vis
CHCl ) λmax [nm] (log ε) 252 (4.74), 309 (4.80), 362 (4.02), 382
4.22), 405 (4.27); MS (70 eV) m/z (%) 686 (45) [M ]; HR-MS
ESI+) m/z 709.2083 found; 709.2138 calcd for C52 Na.
Diacetoxy Benzo[k]fluoranthene Clip (1a). Freshly distilled
acetic anhydride (1.9 mL, 20.09 mmol) was added under argon to
2
(
(
(
3
+
30 2
H O
5
848 J. Org. Chem. Vol. 73, No. 15, 2008

Molecular Clips with Extended Aromatic Sidewalls
a stirred solution of 1b (120 mg, 0.17 mmol) in pyridine (23 mL).
The solution was stirred for 24 h at room temperature and then
poured into ice-water (100 mL). The precipitate was filtered,
concentrated in vacuo in a rotary evaporator. Purification by column
chromatography on silica gel with a mixture of EtOAc/cyclohexane
(1:3) as eluent gave 9a (460 mg, 0.84 mmol, 28%) as a light yellow
1
washed with water, and dried over P
chromatography on silica gel with a mixture of EtOAc/cylohexane
1:3) as eluent gave 1a (107 mg, 0.139 mmol, 82%) as a yellow
2
O
5
. Purification by column
solid: mp 212 °C; H NMR (CDCl
3
, 500 MHz) δ [ppm] 2.20 (dt,
2
1H, J(20i-H, 20a-H) ) 6.90 Hz, 20i-H), 2.26 (dt, 1H, 20a-H),
2
(
3
2.44 (s, 6H, -CH ), 2.51 (dt, 1H, J(19i-H, 19a-H) ) 8.10 Hz, 19i-
1
solid: mp > 300 °C; H NMR (CDCl
3
, 500 MHz) δ [ppm] 2.51
H), 2.69 (dt, 1H, 19a-H), 3.81 (m, 2H, 11-H, 14-H), 4.35 (br t, 2H,
2
3
(
6
br dt, 2H, J(27i-H, 27a-H) ) 7.95 Hz, 27i-H, 28i-H), 2.56 (s,
9-H, 16-H), 6.65 (m, 2H, 12-H, 13-H), 7.63 (dd, 2H, J(1-H, 2-H)
) 6.9 Hz, J(2-H, 3-H) ) 8.15 Hz, 2-H, 5-H), 7.70 (s, 2H, 8-H,
17-H), 7.80 (d, 2H, 3-H, 4-H), 7.93 (d, 2H, 1-H, 6-H), 8.16 (s, 2H,
3
H, -CH
3
), 2.72 (br dt, 2H, 27a-H, 28a-H), 4.36 (br t, 4H, 9-H,
1-H, 22-H, 24-H), 7.52 (dd, 4H, J(1-H, 2-H) ) 6.90 Hz, J(2-
3
3
1
1
3
H,3-H) ) 8.10 Hz, 2-H, 5-H, 15-H, 18-H), 7.61 (s, 4H, 8-H, 12-H,
3 3
7-H, 18-H); C NMR (CDCl , 125.7 MHz) δ [ppm] 21.0 (-CH ),
2
1
1-H, 25-H), 7.70 (d, 4H, 3-H, 4-H, 16-H, 17-H), 7.78 (d, 4H,
48.0 (C-9, C-16), 48.3 (C-11, C-14), 65.3 (C-19), 69.7 (C-20), 119.1
(C-1, C-6), 120.5 (C-7, C-18), 121.1 (C-8, C-17), 126.1 (C-3, C-4),
128.2 (C-2, C-5), 130.6 (C-3a), 132.4 (C-7a, C-17a), 135.0 (C-
1
3
-H, 6-H, 14-H, 19-H), 7.95 (s, 4H, 7-H, 13-H, 20-H, 26-H);
, 125.7 MHz) δ [ppm] 21.1 (-CH ), 48.4 (C-9, C-11,
C-22, C-24), 65.1 (C-27, C-28), 118.9 (C-1, C-6, C-14, C-19), 120.2
C-7, C-13, C-20, C-26), 121.2 (C-8, C-12, C-21, C-25), 125.9 (C-
, C-4, C-16, C-17), 128.1 (C-2, C-5, C-15, C-18), 130.4 (C-3a,
C
NMR (CDCl
3
3
1
3a ), 137.4 (C-6a, C-18b), 137.5 (C-6b, C-18a), 137.6 (C-10a,
(
3
C-14a), 140.2 (C-9a, C-15a), 142.8 (C-12, C-13), 143.2 (C-10,
C-23), 146.9 (C-8a, C-16a), 169.0 (CdO); IR (KBr) ν˜ [cm- ] 2989
(C-H), 2936 (C-H), 2867 (C-H), 1762 (CdO), 1203 (C-O);
1
1
1
C-16a), 132.3 (C-7a, C-12a, C-20a, C-25a), 134.8 (C-3a , C-16a ),
37.1 (C-6a, C-13b, C-19a, C-26b), 137.3 (C-6b, C-13a, C-19b,
C-26a), 137.5 (C-10, C-23), 141.0 (C-9a, C-10a, C-22a, C-23a),
46.3 (C-8a, C-11a, C-21a, C-24a), 168.8 (CdO); IR (KBr) ν˜
1
UV-vis (CHCl
3
) λmax [nm] (log ε) 245 (4.66), 309 (4.67), 359
+
(3.96), 379 (4.03), 403 (3.99); MS (70 eV) m/z (%): 546 (45) [M ],
+
+
1
504 (12) [M -CH
2
CO], 462 (27) [M - 2CH CO]; HR-MS (ESI+)
2
-1
[cm ] 3044 (C-H), 2989 (C-H), 2936 (C-H), 2857 (C-H), 1763
(CdO), 1205 (C-O); UV-vis (CHCl ) λmax [nm] (log ε) 247
(5.09), 314 (5.10), 362 (4.10), 384 (4.22), 405 (4.24); MS (70 eV)
found 569.1728; 569.1723 calcd for C38H26NaO .
4
3
Diacetoxy Clip with One Benzo[k]fluoranthene and One
Naphthalene Sidewall (2a). A mixture of 9a (250 mg, 0.46 mmol),
o-bis(dibromomethyl)benzene (1.16 g, 2.75 mmol), anhydrous NaI
+
+
CO], 686 (65) [M+
2
m/z (%) 770 (100) [M ], 728 (38) [M - CH
2·CH CO]; HR-MS (ESI+) found 793.2351; 793.2349 calcd
Na.
,9-Bis(dibromomethyl)fluoranthene (6). A reaction vessel
quartz glass) equipped with a photolysis lamp (150 W mercury
high-pressure lamp) was charged with 8,9-dimethylfluoranthene
4.48 g, 19.45 mmol), N-bromosuccinimide (15.20 g, 85.40 mmol),
and 200 mL of CCl . The reaction started by heating the stirred
-
for C56
2
(3.5 g, 23.35 mmol), anhydrous CaCO (0.79 g, 7.89 mmol), and
3
H
34
O
4
anhydrous DMF (20 mL) was stirred under argon for 30 min at
8
room temperature and then heated to 55 °C under vacuum (100
mbar) for 5 h. The reaction mixture was poured into ice (100 g),
the brown mixture was, after decolorization by addition of aqueous
sodium hydrogen sulfite, extracted with dichloromethane (3 × 75
mL), and the combined organic layers were filtered, washed with
saturated aqueous sodium hydrogen carbonate (2 × 75 mL) and
water (2 × 75 mL), dried over Na SO , and concentrated in vacuo
(
(
4
mixture to reflux under irradiation for 36 h. The reaction mixture
was cooled down to room temperature, and the precipitate consisting
of insoluble succinimide and of product only slightly soluble in
2
4
in a rotary evaporator. Purification by column chromatography on
silica gel with a mixture of EtOAc/cyclohexane (1:3) as eluent gave
2a (150 mg, 0.23 mmol, 51%) as a light yellow solid: mp > 300
4
CCl was filtered off. The soluble portion of the product was in
the filtrate. The succinimide was dissolved in water by extraction
of the filtered solid. The remaining solid insoluble in water
contained the major portion of the product, which was dissolved
in chloroform. The filtrate and the chloroform solution were worked
up separately. To remove the excess of bromine, the organic phases
1
2
°C; H NMR (CDCl , 500 MHz) δ [ppm] 2.44, 2.49 (2dt, 2H, J(23i-
3
H, 23a-H) ) 7.95 Hz, 23a-H, 24a-H), 2.51 (s, 6H, -CH ), 2.68,
2.71 (m, 2H, 23i-H, 24i-H), 4.32 (br t, 2H, 11-H, 18-H or 9-H,
3
20-H), 4.33 (br t, 2H, 11-H, 18-H or 9-H, 20-H), 7.21 (m, 2H,
14-H, 15-H), 7.53 (s, 2H, 12-H, 17-H), 7.55 (m, 2H, 13-H, 16-H),
were washed with saturated aqueous NaHSO
washing with water several times the organic phases were dried
over MgSO and evaporation of the solvent gave product 6 (10.36
g, 18.98 mol, 98%) as a light yellow solid: mp > 170 °C (dec); H
NMR (CDCl , 500 MHz) δ [ppm] 7.30 (br s, 2H, 11-H, 12-H),
.71 (dd, 2H, J(2-H, 1-H) ) 6.95 Hz, J(2-H, 3-H) ) 8.20 Hz, 2-H,
-H), 7.94 (d, 2H, 3-H, 4-H), 8.05 (d, 2H, 1-H, 6-H), 8.21 (br s,
3
solution. After
3
3
7.59 (dd, 2H, J(2-H, 3-H) ) 8.10 Hz, J(2-H, 1-H) ) 6.90 Hz,
4
2-H, 5-H), 7.64 (s, 2H, 8-H, 21-H), 7.76 (d, 2H, 3-H, 4-H), 7.88
(d, 2H, 1-H, 6-H), 8.06 (s, 2H, 7-H, 22-H); C NMR (CDCl , 125.7
1
13
3
3
MHz) δ [ppm] 21.1 (-CH ), 48.2, 48.4 (C-9, C-11, C-18, C-20),
3
7
5
2
1
1
3
65.2 (C-23, C-24), 119.0 (C-1, C-6), 120.2 (C-12, C-17), 120.4
(C-7, C-22), 121.2 (C-8, C-21), 125.4 (C-14, C-15), 126.0 (C-3,
C-4), 127.8 (C-13, C-16), 128.2 (C-2, C-5), 130.5 (C-3a). 132.28
1
3
H, 7-H, 10-H); C NMR (CDCl
1, C-12), 121.8 (C-1, C-6), 128.2 (C-3, C-4), 128.3 (C-2, C-5),
3
, 125.7 MHz) δ [ppm] 37.3 (C-
1
(C-12a, C-16a), 132.33 (C-7a, C-21a), 134.9 (C-3a ), 137.29 (C-
1
-1
30.2 (C-3a), 133.4 (C-3a ), 135.1 (C-6a, C-10b); IR (KBr) ν˜ [cm ]
045 (C-H), 2997 (C-H), 1460 (CdC), 665 (C-Br); UV-vis
6a, C-22b), 137.40 (C-6b, C-22a), 137.41 (C-10, C-19), 140.9 (C-
9a, C-10a, C-18a, C-19a), 145.9 (C-11a, C-17a), 146.4 (C-8a,
C-20a), 168.8 (CdO); IR (KBr) ν˜ [cm- ] 2963 (C-H), 2931
3
1
(
3
CHCl ) λmax [nm] (log ε) 246 (4.52), 262 (4.35), 296 (4.45), 332
+
(3.87), 365 (4.09), 373 (4.11); MS (70 eV) m/z (%) ) 546 (3) [M ],
(C-H), 2858 (C-H), 1762 (CdO), 1198 (C-O); UV-vis (CHCl )
+
+
+
4
2
5
66 (15) [M - Br], 386 (20) [M - 2·Br], 307 (30) [M - 3·Br],
26 (100) [M - 4·Br], isotopic pattern: m/z (%) 541.7517 (15),
λ
max
[nm] (log ε) 243 (4.82), 315 (4.77), 362 (3.91), 382 (4.05),
+
+
+
405 (4.05); MS (70 eV) m/z (%) 646 (100) [M ], 604 (60) [M -
+
43.7497 (67), 545.7477 (100), 547.7469 (70), 549.7442 (15); HR-
CH CO], 562 (100) [M - 2·CH CO]; HR-MS (ESI+) found
669.2086; 669.2036 calcd for C H NaO .
46 30 4
2
2
MS (70 eV) found 545.7477; 545.7476 calcd for C18
Half-Clip” (9a). A mixture of bisdienophile 8a (0.99 g, 2.97
mmol), 8,9-bis(dibromomethyl)fluoranthene 6 (5.48 g, 10.0 mmol),
anhydrous NaI (9.41 g, 62.78 mmol), anhydrous CaCO (2.05 g,
0.32 mmol), and anhydrous DMF (72 mL) was stirred under argon
H10Br
4
.
“
Diacetoxy Benzo[k]fluoranthene “Half-Clip” (3a). A mixture
of dienophile 10a (214 mg, 0.83 mmol), 8,9-bis(dibromometh-
yl)fluoranthene 6 (1.53 g, 2.8 mmol), anhydrous NaI (2.62 g, 17.48
3
2
3
mmol), anhydrous CaCO (0.57 g, 5.69 mmol), and anhydrous DMF
for 30 min at room temperature and then heated to 55 °C under
vacuum (100 mbar) for 5 h. The reaction mixture was poured into
ice (300 g), the brown mixture was, after decolorization by addition
of aqueous sodium hydrogen sulfite, extracted with dichloromethane
(20 mL) was stirred under argon for 30 min at room temperature
and then heated to 55 °C under vacuum (100 mbar) for 5 h. The
reaction mixture was poured into ice (120 g), the brown mixture
was, after decolorization by addition of aqueous sodium hydrogen
sulfite, extracted with dichloromethane (3 × 100 mL), and the
combined organic layers were filtered, washed with saturated
aqueous sodium hydrogen carbonate (2 × 100 mL) and water (2
(
3 × 200 mL), and the combined organic layers were filtered,
washed with saturated aqueous sodium hydrogen carbonate (2 ×
50 mL) and water (2 × 150 mL), dried over Na SO , and
1
2
4
J. Org. Chem. Vol. 73, No. 15, 2008 5849

Branchi et al.
×
100 mL), dried over Na
rotary evaporator. Purification by column chromatography on silica
2
SO
4
, and concentrated in vacuo in a
after: 0.0753 /0.0552, max/min transmission 1.00/0.89; structure
solution with direct methods (SHELXS) and refinement on F
2
gel with a mixture of EtOAc/cyclohexane (1:3) as eluent gave 3a
(SHELXTL 6.12) (641 parameters), the hydrogen atom positions
were calculated and refined as riding models on idealized geometries
with the 1.2-fold (1.5-fold for methyl groups) isotropic displacement
parameters of the equivalent Uij of the corresponding carbon atoms.
The ADPs of the atoms C12, C13, C15, N1, N2, c71 and C72
indicate some disorder which was not resolved. R1 ) 0.0818, wR2
1
(
150 mg, 0.31 mmol, 38%) as a light yellow solid: mp 201 °C; H
3 3
NMR (CDCl , 500 MHz) δ [ppm] 2.42 (s, 6H, -CH ), 2.53(br dt,1H,
2
J(17i-H, 17a-H)) 8.00 Hz, 17i-H), 2.68 (br dt, 1H, 17a-H), 4.45
3
(br t, 2H, 9-H, 14-H), 6.72 (s, 2H, 11-H, 12-H), 7.64 (dd, 2H, J(1-
3
H, 2-H)) 7 Hz, J(2-H, 3-H)) 8 Hz, 2-H, 5-H), 7.79(s, 2H, 8-H,
1
7
4
1
2
5-H), 7.81(d, 2H, 3-H, 4-H), 7.95(d, 2H, 1-H, 6-H), 8.2(s, 2H,
(all data) ) 0.2122, maximum residual electron density 0.818 e
1
3
-3
-H, 16-H); C NMR (CDCl
3
, 125.7 MHz): π [ppm] 21.1 (-CH
3
),
Å
.
8.9 (C-9, C-14), 64.3 (C-17), 119.2 (C-1, C-6), 120.3 (C-11, C-12),
20.5 (C-7, C-16), 121.7 (C-8, C-15), 126.1 (C-3, C-4), 128.3 (C-
1
, C-5), 130.6 (C-3a), 132.4 (C-7a, C-15a), 135.0 (C-3a ), 137.3
C-6a, C-16b), 137.6 (C-6b, C-16a), 142.5 (C-10, C-13), 142.8 (C-
1
]
CdO), 1187 (C-O); UV-vis (CHCl
3
2
H
22NaO
4
.
3
Crystal Structure Determinations. 1a ·2CHCl
3
. C56H
32 ·2CHCl ,
3
9.656(2)°, V ) 4724.2(4) Å , triclinic crystal system, Z ) 4, dcalcd
-
3
-1
1.424 g cm , µ ) 0.414 mm , space group P1, data collection
Determination of K by Solid-liquid Extraction Experiments.
a
See Supporting Information.
program multiscan. Rmerg before/after: 0.0853/0.0503, max/min
1
a
Determination of K by H NMR Titration Method. See ref
transmission 0.97/0.85; structure solution with direct methods
2
3a.
(
SHELXS) and refinement on F (SHELXTL 6.12) (1288 param-
The results of all titration experiments are given in the Supporting
Information.
eters), the hydrogen atom positions were calculated and refined as
riding models on idealized geometries with the 1.2-fold (1.5-fold
for methyl groups) isotropic displacement parameters of the
equivalent Uij of the corresponding carbon atoms. The trichlo-
romethane atoms Cl(1) to Cl(3) and Cl(10) to Cl(12) are disordered
over two sites with SOF 0.8 and 0.2. Further disorder was detected
at the oxygen atom O6 at two sites with SOF 0.6 and 0.4. The
poor R-values and residual electron density resulted from nonre-
solved disorder and poor crystal quality. Therefore, the results are
taken for a confirmation of the overall molecular geometry and
The estimated experimental error on the association constant is
1
5%. The errors result from standard deviation of the nonlinear
24
regression.
Photophysics. The experiments were carried out in air-
equilibrated CHCl solution at 298 K. Global fitting of absorption
and emission spectra has been performed by SPECFIT software.
3
2
3
The fluorescent decay curves were collected under 0° and 90°
polarization angles. Global analysis on fluorescence anisotropy
decay curves has been performed by FAST software (version 1.8.1).
The estimated experimental errors are 2 nm on the band maximum,
packing mode. R1 ) 0.0915, wR2 (all data) ) 0.2205, maximum
residual electron density 0.954 e Å-
3
.
5
% on the molar absorption coefficient, fluorescence lifetime,
steady-state fluorescence anisotropy at 298 K and logK values, 10%
on the fluorescence quantum yield, and 15% on rotational correla-
tion times, and steady-state fluorescence anisotropy at 77 K.
Electrochemistry. The electrochemical experiments were carried
2 2
out in argon-purged CH Cl solution at 298 K. In the cyclic
voltammetry (CV) the working electrode was a glassy carbon
2
electrode (0.08 cm ), the counter electrode was a Pt spiral, and a
silver wire was employed as a quasi-reference electrode (AgQRE).
The potentials reported are referred to SCE by measuring the
AgQRE potential with respect to ferrocene (E1/2 ) 0.46 V vs SCE
+
for Fc /Fc). The concentration of the compounds examined was
-
3
of the order of 1 × 10 M; 0.1 M tetrabuthylammonium
hexafluorophosphate (TBAPF ) was added as supporting electro-
lytes. Cyclic voltammograms were obtained with scan rates in the
6
-1
range 0.05-20 V s . The estimated experimental error on the E1/2
value is (10 mV.
1
c·1/2C10
H
2
N
4
·2CHCl
3
· C54
H
34 ·1/2 C10
H
2
N
4
3
·2CHCl , crystal
dimensions 0.17 × 0.14 × 0.11 mm, crystal color, orange. T )
Acknowledgment. This work has been supported in Italy by
MIUR (PRIN 2006034123_003) and in Germany by the
“Deutsche Forschungsgemeinschaft” (DFG, KL 299/13-1). We
thank Heinz Bandmann and Dr. Torsten Schaller for NMR
measurements, Willi Sicking for assistance with the HOSTEST
calculations, and Professor Craig Wilcox for providing us access
to the HOSTEST program.
2
2
03(2) K. Cell dimensions a ) 34.167(3), b ) 14.8708(15), c )
3
2.173(2) Å, ꢀ ) 117.776(2), V ) 9967.8(17) Å , monoclinic
crystal system, Z ) 8, dcalcd ) 1.390 g cm- , µ ) 0. 393 mm
3
-1
,
space group C2/c, data collection of 64258 intensities, 12403
independent (Rmerg ) 0.0865, 1.66° ) Θ ) 28.35°), 4948 observed
o
[F g 4σ(F)], Datareduction and absorption correction with Bruker
AXS APEX2 Vers. 10-27 2005 program multiscan. Rmerg before/
5
850 J. Org. Chem. Vol. 73, No. 15, 2008

Molecular Clips with Extended Aromatic Sidewalls
Supporting Information Available: General experimental
details. H NMR spectra of the new compounds. Pdb files
These data can be obtained free of charge at www.ccdc.ca-
m.ac.uk/conts/retrieving.html [or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB2
1
containing atom coordinates for the calculated structures.
Experimental data for the determination of association
1EZ,UK.Fax:+44-1223-336-033.E-mail:deposit@ccdc.cam.ac.uk].
1
constants Ka and complexation-induced chemical H NMR
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
shifts, ∆δmax, by H NMR titrations. CCDC-682029 (for
1
a·2CHCl3) and CCDC-682030 (for 1c·1/2C10H2N4 ·2CHCl3)
contain the supplementary crystallographic data for this paper.
JO8007513
J. Org. Chem. Vol. 73, No. 15, 2008 5851