Intramolecular electronic interactions between nonconjugated arene and quinone chromophores

Article abstract of DOI:10.1021/ja910362j

The novel surprisingly colorful dark blue and orange-red molecular clips 1 and 2 containing a central p-benzoquinone spacer-unit and anthracene or napththalene sidewalls were synthesized by DDQ oxidation of the corresponding colorless hydroquinone clips 7 and 8. The colors of the quinone clips result from broad absorption bands in the visible range (1, λ_max = 537 nm and 2, λ_max = 423 and λ_shoulder =515 nm) showing bathochromic shifts of 112 and 90 nm, respectively, compared to the similarly tetraalkyl-substituted duroquinone 31, even though the clips 1 and 2 only contain insulated π systems as chromophores, a central tetraalkyl-substituted p-benzoquinone spacer-unit and two anthracene or two naphthalene sidewalls. To elucidate the electronic properties of these clips, we prepared the compound 3, the anti-configured isomer of clip 2, and the benzene-, naphthalene-, and anthracene-substituted quinones 4, 5, and 6, the so-called half-clips . The half-clips 6 and 5 show a similar color change and the same trend in the UV/vis absorption spectra as the anthracene and naphthalene clip 1 and 2. This finding already rules out that the color of these systems is a result of through-space π-π interactions between the aromatic sidewalls in the molecular clips 1 and 2. Quantum chemical ab initio calculations provide good evidence that the bathochromic shift of the absorption band at the longest wavelength observed in the UV/vis spectra of the clip quinones 2, 3, and 1 and the half- clip quinones 4, 5, and 6 with an increasing number of rings in the anellated aromatic unit (from benzene to anthracene) is the result of an increasing configuration interaction between a n → π* excitation of the quinoid component and a π → π* excitation with intramolecular charge transfer (CT) character. The initial π orbitals involved here and in higher lying transitions mainly stem from through-space interactions between π orbitals of the aromatic sidewalls and π orbitals of the quinone moiety with varying degree of mixing. The configuration interaction in the excited states can be considered to be a homoconjugation, that is, the relevant charge transfer states are formed across an allegedly insulating aliphatic bridge. The UV/vis spectra of the molecular clips 1-3, the half-clips 4-6, and the quinones 32 and 33 simulated by means of quantum chemical ab initio calculations agree well with the experimental spectra.

Full text of DOI:10.1021/ja910362j

Published on Web 06/03/2010
Intramolecular Electronic Interactions between Nonconjugated
Arene and Quinone Chromophores
Georg Jansen,* Bj o¨ rn Kahlert, Frank-Gerrit Kl a¨ rner,* Roland Boese, and
Dieter Bl a¨ ser
Institut f u¨ r Organische Chemie and Institut f u¨ r Anorganische Chemie, UniVersit a¨ t
Duisburg-Essen, 45117 Essen, Germany
Received December 14, 2009; E-mail: georg.jansen@uni-duisburg-essen.de;
frank.klaerner@uni-duisburg-essen.de
Abstract: The novel surprisingly colorful dark blue and orange-red molecular clips 1 and 2 containing a
central p-benzoquinone spacer-unit and anthracene or napththalene sidewalls were synthesized by DDQ
oxidation of the corresponding colorless hydroquinone clips 7 and 8. The colors of the quinone clips result
from broad absorption bands in the visible range (1, λmax ) 537 nm and 2, λmax ) 423 and λshoulder )515
nm) showing bathochromic shifts of 112 and 90 nm, respectively, compared to the similarly tetraalkyl-
substituted duroquinone 31, even though the clips 1 and 2 only contain insulated π systems as
chromophores, a central tetraalkyl-substituted p-benzoquinone spacer-unit and two anthracene or two
naphthalene sidewalls. To elucidate the electronic properties of these clips, we prepared the compound 3,
the anti-configured isomer of clip 2, and the benzene-, naphthalene-, and anthracene-substituted quinones
4, 5, and 6, the so-called “half-clips”. The “half-clips” 6 and 5 show a similar color change and the same
trend in the UV/vis absorption spectra as the anthracene and naphthalene clip 1 and 2. This finding already
rules out that the color of these systems is a result of “through-space” π-π interactions between the aromatic
sidewalls in the molecular clips 1 and 2. Quantum chemical ab initio calculations provide good evidence
that the bathochromic shift of the absorption band at the longest wavelength observed in the UV/vis spectra
of the clip quinones 2, 3, and 1 and the “half-clip” quinones 4, 5, and 6 with an increasing number of rings
in the anellated aromatic unit (from benzene to anthracene) is the result of an increasing configuration
interaction between a n f π* excitation of the quinoid component and a π f π* excitation with intramolecular
charge transfer (CT) character. The initial π orbitals involved here and in higher lying transitions mainly
stem from through-space interactions between π orbitals of the aromatic sidewalls and π orbitals of the
quinone moiety with varying degree of mixing. The configuration interaction in the excited states can be
considered to be a homoconjugation, that is, the relevant charge transfer states are formed across an
allegedly insulating aliphatic bridge. The UV/vis spectra of the molecular clips 1-3, the “half-clips” 4-6,
and the quinones 32 and 33 simulated by means of quantum chemical ab initio calculations agree well
with the experimental spectra.
Introduction
properties (e.g., photoelectron, UV/vis absorption, and emission
spectra), reactivity (for example, redox reactions), and electric
conductivity of these systems. For several decades, it is known
In organic molecules such as polycyclic aromatic-aliphatic
systems and biomolecules like proteins and DNA, through-space
and through-bond interactions have been proposed to understand
the electronic interactions between π systems that are insulated
by one or more saturated carbon atoms from each other. These
interactions are important for the understanding of spectroscopic
2
that rapid long-range electron and hole transfer processes
through saturated carbon bridges in donor-bridge-acceptor
systems can take place with high efficiencies even for long
1
3
bridges, highlighting the role of through-bond interactions. A
recent, surprising finding is that bridges containing π systems
which are not conjugated with the donor and acceptor moieties
(
1) “Through-space”, “through-bond” interactions: (a) Hoffmann, R.;
Heilbronner, E.; Gleiter, R. J. Am. Chem. Soc. 1970, 92, 706–707.
(
b) Hoffmann, R. Acc. Chem. Res. 1971, 4, 1–9. (c) Filipescu, N.;
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M. M.; Barcina, J. O.; Martinez, A. G.; del Rosario Torres Salvador,
M. Org. Lett. 2007, 9, 2943–2946. (b) Bell, T. D. M.; Stefan, A.;
Lemaur, V.; Bernhardt, S.; Muellen, K.; Cornil, J.; Beljonne, D.;
Hofkens, J.; Van der Auweraer, M.; De Schryver, F. C. Photochem.
Photobiol. Sci. 2007, 6, 406–415. (c) Kobayashi, T.; Kobayashi, S.
Eur. J. Org. Chem. 2002, 2066–2073.
Minn, F. L.; Thomas, A. V. J. Chem. Soc., Perkin Trans. 2 1972,
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K. N.; Paddon-Row, M. N. HelV. Chim. Acta 1982, 65, 1743–1751.
e) Jordan, K. D.; Paddon-Row, M. N. Chem. ReV. 1992, 92, 395–
10. Electron transfer through DNA and peptides: (f) Giese, B. Bioorg.
1
(
4
Med. Chem. 2006, 4, 6139–6143. (g) Breeger, S.; von Meltzer, M.;
Hennecke, U.; Carell, T. Chem.sEur. J. 2006, 12, 6469–6477. (h)
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3638–3645. (b) Oevering, H.; Paddon-Row, M. N.; Heppener, M.;
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Res. 1994, 27, 18–25.
4
7, 9067–9070. (j) Ghosh, A.; Joy, A.; Schuster, G. B.; Douki, T.;
Cadet, J. Org. Biomol. Chem. 2008, 6, 916–928.
10.1021/ja910362j  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 8581–8592 9 8581

A R T I C L E S
Jansen et al.
do not lead to increased photoinduced charge transfer rates
4
compared to geometrically similar fully saturated bridges. The
charge transport through the σ system was even observed to be
insensitive to changes in hybridization of the bridge carbon
atoms. Hyperconjugation between neighboring π and σ systems
recently was found to be nearly half as strong as conjugation
5
between adjacent π systems. π,σ hyperconjugations are also
known to contribute to the interactions between the two
dimethylene-bridged double bonds of norbonadiene, which are
mainly attributed to direct through-space homoconjugation.
Figure 1. Synthesis of the quinone clips 1 and 2.
6
analogously to 1 and 2 starting in this case by the hydrolysis of
1
1
They were also proposed to explain the strong donor-acceptor
the known diacetoxy-substituted compound 9 and subsequent
DDQ oxidation of the hydroquinone 10 resulting from the
hydrolysis.
7
coupling in an intramolecular triptycene quinhydrone system.
Here, we report the synthesis and spectroscopic properties
of the dimethylene-bridged molecular clips 1 and 2. Even though
each system only contains insulated π systems (a central
p-benzoquinone spacer-unit and two aromatic sidewalls) and,
therefore, is expected to be yellow resulting from the quinone
For the synthesis of the compounds 4-6 (Figures 3-5), new
routes were developed. Diels-Alder cycloaddition of 1,4-
1
2
dichloro-1,3-butadiene 11 to dimethoxybenzonorbornadiene
1
3
1
2
followed by HCl elimination with potassium tertiary-
8
moiety as chromophore, the anthracene clip 1 is dark blue and
butoxide without isolation of the primary Diels-Alder adduct
leads in one step to the dibenzonorbornadiene derivative 13.
The methoxy groups in 13 were cleaved by BBr and the
3
resulting hydroquinone was again oxidized with DDQ producing
the desired benzo-substituted quinone 4. The synthesis of the
naphthalene- and anthracene-substituted quinones 5 and 6 started
the naphthalene clip 2 is orange-red. To elucidate the origin of
the surprising color especially of clip 1, we synthesized
compound 3, the anti-configured isomer of 2, and the benzene-,
naphthalene-, and anthracene-substituted quinones 4- 6 and
calculated the electronic spectra of 1- 6 by ab initio methods.
9
1
3,14
The anti configuration of the two arene moieties in 3 and the
both from diacetoxybenzonorbornadiene 19
using the
lack the of the second arene sidewall in 4- 6 prevent a through-
space interaction between the arene moieties of the sidewalls
which may be possible in the molecular clips 1 and 2.
methodology for the anellation of naphthalene units to CC
1
5
double bonds developed by Cava et al. and later refined by
1
6
2
Paddon-Row et al. 1,4 Br -elimination of tetrabromo- or
hexabromo-o-xylene 15 or 16 with sodium iodide led to the
o-quinodimethane 17 or 18 as reactive intermediate which was
trapped by Diels-Alder reaction with 19. The primary
Diels-Alder adduct was not stable under the conditions of
reaction and eliminated two molecules of HBr leading to the
benzonaphthonorbornadiene 22 and 23, respectively. The ac-
etoxy functions in 22 were hydrolyzed and the resulting
hydroquinone 24 was again oxidized by DDQ producing the
desired naphthalene-substituted quinone 5.
For the preparation of the anthracene-substituted quinone 6,
it was necessary to convert the diacetoxybenzo-dibromonaph-
thalene-substituted compound 23 to the corresponding dimethoxy
derivative 25. This conversion was performed in one step by
basic hydrolysis of the acetoxy groups and subsequent methy-
Results and Discussion
The quinone clips 1 and 2 (Figure 1) were synthesized by
oxidation of the known hydroquinones 7 and 8 with 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Compound 3
1
0
11
lation of the resulting phenolate groups with methyl iodide. Br
2
elimination in 25 with n-butyllithium led to the corresponding
aryne as reactive intermediate which was trapped with furan
leading to a mixture of syn-26 and anti-26 in a ratio of (2:3)
which was isolated but not separated. Deoxygenation of the
mixture of syn-26 and anti-26 with titanium tetrachloride and
zinc powder gave the anthracene-substituted dimethoxyben-
zonorbornadiene 27. The methoxy groups in 27 were again
(
Figure 2), the anti-configured isomer of 2, was prepared
(
(
(
4) Goldsmith, R. H.; Vura-Weis, J.; Scott, A. M.; Borkar, S.; Sen, A.;
Ratner, M. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2008, 130,
7
659–7669.
5) (a) Cappel, D.; T u¨ llmann, S.; Krapp, A.; Frenking, G. Angew. Chem.,
Int. Ed. 2005, 44, 3617–3620. (b) Fernandez, I.; Frenking, G.
Chem.sEur. J. 2006, 12, 3617–3629.
6) (a) Heilbronner, E.; Schmelzer, A. HelV. Chim. Acta 1975, 58, 936–
cleaved with BBr
oxidized with DDQ to give the desired anthracene-substituted
quinone 6. If a larger excess of BBr (40 instead of 20
molequivalents, containing a small amount of Br ) was used
for the reaction with 27, the 9,10-dibromoanthracene derivative
9 was isolated after DDQ oxidation of the primary product.
3
and the resulting hydroquinone 28 was
9
2
67. (b) Martin, H.-D.; Mayer, B. Angew. Chem., Int. Ed. 1983, 22,
83–314. (c) Houk, K. N.; Rondan, N. G.; Paddon-Row, M. N.;
3
Jefford, C. W.; Thanh Huy, P.; Burrow, P. D.; Jordan, K. D. J. Am.
Chem. Soc. 1983, 105, 5563–5568.
2
(
(
(
7) Iwamura, H.; Makino, K. J. Chem. Soc., Chem. Commun. 1978, 720–
7
21.
2
8) (a) Vischer, E. B. J. Chem. Soc. 1953, 815–820. (b) Flaig, W.; Salfeld,
J.-C. Justus Liebigs Ann. Chem. 1958, 618, 117–139.
9) Attemps to synthesize the anti-configured anthracene clip for a
comparison with clip 1 failed presumably because of the low solubility
of the tetrabromoderivative of 9 which did not react with n-butyllithium
and furan. Kahlert, B. Dissertation, University of Duisburg-Essen,
(12) Bartlett, P. D.; Wallbillich, G. E. H. J. Am. Chem. Soc. 1969, 91,
409–414.
(13) Meinwald, J.; Wiley, G. A. J. Am. Chem. Soc. 1958, 80, 3667–3671.
(14) Vaughan, W. R.; Yoshimine, M. J. Org. Chem. 1957, 22, 7–12.
(15) (a) Cava, M. P.; Napier, D. R. J. Am. Chem. Soc. 1957, 79, 1701–
1709. (b) Cava, M. P.; Shirley, R. L. J. Am. Chem. Soc. 1960, 82,
654–656.
2
005.
(
10) Kl a¨ rner, F.-G.; Kahlert, B.; Boese, R.; Blaeser, D.; Juris, A.; Marchioni,
F. Chem.sEur. J. 2005, 11, 3363–3374.
(
11) Kl a¨ rner, F.-G.; Panitzky, J.; Bl a¨ ser, D.; Boese, R. Tetrahedron 2001,
(16) Paddon-Row, M. N.; Patney, H. K.; Harish, K. Synthesis 1986, 328–
330.
3
673–3687.
8
582 J. AM. CHEM. SOC. 9 VOL. 132, NO. 25, 2010

Intramolecular Electronic Interactions
A R T I C L E S
Figure 2. Synthesis of the quinone 3.
Figure 3. Synthesis of the quinone 4.
Figure 4. Synthesis of the quinone 5.
The structures of all new compounds were assigned by the
spectral data as described in the Experimental Details in
Supporting Information. In the case of compounds 22, 23, and
in the visible region in addition to the R, ꢀ, and p bands which
are characteristic for the aromatic anthracene or naphthalene
system. The absorption band of 1 at the longest wavelength
(λmax ) 537 nm, log ε ) 317) shows a bathochromic shift
compared to that of 2 (λmax ) 423 nm, 515 nm (shoulder), log
ε ) 2.98, 2.58) and 3 (λmax ) 425 nm, log ε ) 3.19). The UV/
vis spectra of the isomeric naphthalene quinone 2 and 3 are
very similar to each other with exception of the shoulder at λ
) 515 nm only observed in the spectra of 2 (Figure 8).
The position and high intensity of these absorption bands are
unusual for nonconjugated p-benzoquinone derivatives. For
example, p-benzoquinone (30) itself and duroquinone 31 as well
as the norbornadiene-anellated quinones 32 and 33 are yellow
27, the precursors of the naphthalene- and anthracene-substituted
quinones 5 and 6, an independent structural proof came from
single-crystal structure analysis (Figure 6). In all three systems,
the naphthalene or anthracene CC bond fused to the norborna-
diene moiety is remarkably longer than the other naphthalene
or anthracene bonds C15-C18 d [Å] ) 1.440(4), C15-C18
1
.425(5), C17-C22 1.442(2) for 22, 23 and 27, respectively.
The bond lengths in the diacetoxy- or dimethoxy-substituted
benzene ring in 22, 23, or 27 are analogously but less strongly
influenced by the fused norbornadiene unit. These findings
indicate that the norbornadiene unit causes a partial localization
of the π bonds in the fused arene moieties comparable to that
(17) (a) B u¨ rgi, H.-B.; Baldridge, K. K.; Hardcastle, K.; Frank, N. L.;
Gantzel, P.; Siegel, J. S.; Ziller, J. Angew. Chem., Int. Ed. Engl. 1995,
34, 1454–1456. (b) Frank, N. L.; Baldridge, K. K.; Siegel, J. S. J. Am.
Chem. Soc. 1995, 117, 2102–2103.
1
7
observed in tris(bicyclo[2.1.1]hexeno)benzene
(
and tris-
1
8
bicyclo[2.2.1]hepteno)benzene derivatives.
As already pointed out in the introduction, the anthracene
(18) (a) Frank, N. L.; Baldridge, K. K.; Gantzel, P.; Siegel, J. S. Tetrahedron
Lett. 1995, 36, 4389–4392. (b) Rathore, R.; Lindemann, S. V.; Kumar,
A. S.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 6012–6018. (c)
Cardullo, F.; Giuffrida, D.; Kohnke, F. H.; Raymo, F. M.; Stoddart,
J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 339–
341.
and naphthalene quinone clips 1 and 2 are colorful substances.
Crystals of 1 are dark blue almost black and those of 2 and the
anti-isomer 3 are orange-red or orange (Figure 7). Each of the
UV/vis spectra of 1, 2, and 3 exhibits a broad absorption band
J. AM. CHEM. SOC. 9 VOL. 132, NO. 25, 2010 8583

A R T I C L E S
Jansen et al.
Figure 5. Synthesis of the quinones 6 and 29.
Figure 6. Single-crystal structures of the quinone precursors 22, 23, and 26.
band at the longest wavelength in the UV/vis spectra of the
quinones 4, 5, and 6 shows a bathochromic shift with the
transition of the benzene-substituted norbornadienoquinone 4
to the naphthalene- and finally anthracene-substituted system 5
and 6, respectively (Figure 13). Bromosubstitution of the
anthracene moiety in 29 leads to a hypsochromic shift compared
to 6 (Figure 14).
The finding that the monosubstituted quinones 6 and 5 show
a similar color change and the same trend in the UV/vis
spectrum as the corresponding naphthalene and anthracene clip
2
and 1, respectively, already rules out that the color of these
systems is a result of a through-space π-π interaction between
the aromatic sidewalls in the molecular clips. To elucidate the
mechanism of the bathochromic shift observed here, the UV/
vis spectra of benzene (34); naphthalene (35); anthracene (36);
p-benzoquinone (30); and the benzene-, naphthalene-, and
anthracene- substituted quinones 4, 5, and 6; the clips 1, 2, and
38 as well as the anti-isomers 3 and 37, 39, and 40 (Figure 15
were calculated by the use of quantum chemical methods, that
Figure 7. Structures and colors of the solids of the quinones 1-3.
8
to orange (Figure 9) showing only weak absorption bands in
the visible region, however, at shorter wavelengths than the
quinone clips 1 and 2, which were assigned to the forbidden n
f π* transitions (Figures 10, 11).
The color of arene-substituted norbornadieno-quinones 4, 5,
6
, and 29 changes from yellow over blue-gray to dark violet
1
9
comparable to the color change observed for naphthalene- and
is, by coupled-cluster theory in the CC2 approximation. The
anthracene-substituted clips (Figures 7 and 12). The absorption
underlying Hartree-Fock (HF) molecular orbitals (MOs) were
8
584 J. AM. CHEM. SOC. 9 VOL. 132, NO. 25, 2010

Intramolecular Electronic Interactions
A R T I C L E S
Figure 10. UV/vis spectra of the p-benzoquinone 30 (solid line) and
duroquinone 31 (dashed line) in CDCl at 25 °C.
3
Figure 8. UV/vis spectra of the quinones 1 (solid line), 2 (dashed line),
and 3 (dotted line): (a) experimental in CDCl at 25 °C, (b) simulated (see
3
text).
Figure 9. Colors of the solids of the quinones 30-33.
expanded in a correlation-consistent Gaussian basis set of
2
0
triple-ꢁ valence quality (cc-pVTZ). For the CC2 calculations,
Figure 11. UV/vis spectra of the quinones 32 (solid line) and 33 (dashed
line): (a) experimental in CDCl at 25 °C, (b) simulated (see text).
the resolution-of-the-identity (RI) approximation was employed
3
(
further computational details are found in the Supporting
starting in antisymmetric and symmetric linear combinations
of the lone-pair oxygen orbitals, respectively. Note that the σ
orbitals of the carbon backbone also contribute to the n orbitals
and that they are not the highest occupied HF MOs. The third
and fourth excited states result from π f π* transitions starting
from the HOMO and HOMO-1, respectively. Only the fourth
transition is symmetry-allowed and displays a large oscillator
strength. The complete active space second-order perturbation
2
1
Information).
Transition energies and corresponding oscillator strengths
obtained from the “mixed” velocity-gradient length-velocity
formula for p-benzoquinone (30) are presented in Table 1.
Graphical representations of the HF MOs involved in the
electronic transitions of 30 may be found in Figure 16. All low
energy transitions for 30 shown in Table 1 correspond to
transitions into the low-lying π* LUMO. The first two excited
states result from symmetry-forbidden n f π* transitions
22
theory (CASPT2) calculations of Weber et al. yield this state
as the seventh excited state. According to CASPT2, the singlet-
excited states number four to six are essentially characterized
by double excitations. Thus, they cannot be correctly described
(19) Christiansen, O.; Koch, H.; Jørgensen, P. Chem. Phys. Lett. 1995, 243,
4
09–418.
(
(
20) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007–1023.
21) (a) H a¨ ttig, C.; Weigend, F. J. Chem. Phys. 2000, 113, 5154–5161.
(22) Weber, J.; Malsch, K.; Hohlneicher, G. Chem. Phys. Lett. 2001, 264,
(
b) H a¨ ttig, C.; K o¨ hn, A. J. Chem. Phys. 2002, 117, 6939–6951.
275–318.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 25, 2010 8585

A R T I C L E S
Jansen et al.
Figure 12. Colors of the solid quinones 4, 5, 6, and 29. 5 shows a reversible thermochromic color change from yellow to red at 205 °C.
Figure 13. UV/vis spectra of the quinones 4 (dotted line), 5 (dashed line), and 6 (solid line): (a) experimental in CDCl at 25 °C, and (b) simulated (see
3
text).
by CC2. Two of the corresponding transitions are symmetry-
forbidden; the remaining one was found to have a small
oscillator strength of 0.00041. The value of the so-called D1
diagnostics indicating multireference character of the ground
state of 30 is found to be 0.08 and thus considerably larger
than the largest value of 0.05 recommended for “safe” applica-
tion of CC2. Comparison with the experimental results given
in Table 1 reveals that the calculated excitation energies are
overestimated by up to 0.3 eV, which we deem an acceptable
error for the series of molecules considered here. The D1
diagnostics for the free arene molecules are below 0.04 and the
largest D1 diagnostics of 0.09 for all investigated molecules
was found for the syn- and anti-anthracene clips 1 and 40,
respectively.
In norbornadienoquinone 32, the symmetry is lowered from
D
2h to Cs. Thus, all electronic transitions become symmetry-
8
586 J. AM. CHEM. SOC. 9 VOL. 132, NO. 25, 2010

Intramolecular Electronic Interactions
A R T I C L E S
Figure 14. UV/vis spectra of the quinone 29 in CDCl at 25 °C.
3
Figure 16. Selected molecular (HF/cc-pVTZ) orbitals of p-benzoquinone
3
0. Orbital energies in electronvolts (eV).
Table 2. Calculated (CC2/cc-pVTZ) Excited States for 32 (cf.
Figure 17)
state
E
(calc)
λ
oscil. str.
composition
1
1
A′
2.751
2.875
3.493
4.229
5.297
451
431
355
293
234
0.00010
0.00006
0.00991
0.00389
0.28955
0.944 (18a′′ f 19a′′)
0.966 (24a′ f 19a′′)
0.954 (27a′ f 19a′′)
0.937 (26a′ f 19a′′)
0.929 (25a′ f 19a′′)
1
1
1
1
1
2
3
4
A′′
A′′
A′′
A′′
Figure 15. Additional structures of benzene 34, naphthalene 35, anthracene
6, and the quinones 37-40 used for quantum chemical calculations.
3
Table 1. Calculated (CC2/cc-pVTZ) Excited States for
a
p-Benzoquinone 30 (cf. Figure 16)
state
E
(exp)
E
(calc)
λ
oscil. str.
composition
1 1
B
A
2.7
2.7
4.4-4.6
5.1-5.4
2.810
2.946
4.645
5.603
441
421
267
221
-
-
-
0.956 (4b1g f 2b3g
0.972 (5b3u f 2b3g
0.989 (1b2g f 2b3g
0.961 (2b1u f 2b3g
)
)
)
)
2g
1
1
1
1
1
1
u
B
B
1g
2u
0.41821
a
Excitation energies in eV, corresponding wavelengths in
nanometers. Only contributions with a coefficient larger than 0.03 are
shown.
allowed. The data collected in Table 2 demonstrate, however,
that the first two transitions corresponding to n f π* transitions
within the quinone moiety have low oscillator strengths. They
display a bathochromic shift of 10 nm with respect to p-
benzoquinone 30. Note that the orbitals 24a′, 18a′′, and 19a′′
involved in the transition are essentially unaltered n and π*
orbitals of p-benzoquinone, respectively, as can be seen from
Figure 17. This is not true for the occupied orbitals involved in
the remaining three transitions: the orbital 25a′ is a linear
combination of the two occupied π orbitals of the quinone
Figure 17. Selected molecular (HF/cc-pVTZ) orbitals of 32. Orbital
energies in electronvolts (eV).
+
moiety, denoted as π in the following. The orbitals 26a′ and
2
7a′ mainly stem from through-space interactions between π
nm, respectively (cf. Figure 11). The remaining fifth transition
may be considered as a symmetry-perturbed π f π* excitation
within the quinone moiety. It displays a hypochromic and
bathochromic shift of 13 nm with respect to the allowed π f
π* excitation of 30. Figure 11b shows a simulation of the UV/
vis spectra of 30 and 32 based on the results of Tables 1 and 2.
All transition energies were down-shifted by 0.25 eV and
inhomogeneous broadening of each of the lines was represented
through a Gaussian distribution with a width of 0.5 eV (fwhm).
The reasonable agreement of the band maxima and of the
general shape of the spectrum with experiment (Figure 11a)
confirms our calculations and the resulting assignment.
orbitals: they are formed by the remaining linear combination
-
π
of the quinone π orbitals and the π orbital of the
norbornadiene rest. Note that orbital 26a′ looks very similar to
the orbital denoted as 10a in norbornadiene, and 27a′ resembles
of norbonadiene, that is, orbitals which to some extent are
1
6b
2
6
influenced by π,σ hyperconjugations. The third and fourth
transitions (27a′ f 19a′′ and 26a′ f 19a′′) may be considered
as mixtures between a quinone π f π* excitation and a charge
transfer (CT) excitation from an ethene moiety fused to
p-benzoquinone. These excitations are responsible for the
experimentally observed bands with maxima at 386 and 330
J. AM. CHEM. SOC. 9 VOL. 132, NO. 25, 2010 8587

A R T I C L E S
Jansen et al.
Table 3. Calculated (CC2/cc-pVTZ) Excited States for 4 (cf. Figure
18)
state
E
(calc)
λ
oscil. str.
composition
1 1
A′ 2.734 454 0.00032 0.932 (23a′′ f 25a′′)
1
1
2
3
2
A′′ 2.875 431 0.00006 0.964 (31a′ f 25a′′)
1
A′′ 3.367 368 0.00607 0.918 (34a′ f 25a′′) +0.318 (33a′ f 25a′′)
A′′ 4.077 304 0.00737 0.895 (33a′ f 25a′′) -0.300 (34a′ f 25a′′)
A′ 4.248 292 0.01152 0.943 (24a′′ f 25a′′)
1
1
1
3
A′ 5.106 243 0.01150 0.716 (34a′ f 35a′) -0.524 (24a′′ f 26a′′)
A′′ 5.269 235 0.26311 0.927 (32a′ f 25a′′)
1
4
Table 4. Calculated (CC2/cc-pVTZ) Excited States for 5 (cf. Figure
S1)
state
E
(calc)
λ
oscil. str.
composition
1 1
A′ 2.710 458 0.00189 0.897 (28a′′ f 31a′′)
1
1
2
2
A′′ 2.878 431 0.00007 0.955 (37a′ f 31a′′)
1
A′ 3.368 368 0.02539 0.921 (30a′′ f 31a′′)
1
A′′ 3.389 366 0.00606 0.875 (41a′ f 31a′′) +0.420 (40a′ f 31a′′)
A′′ 4.052 306 0.00731 0.842 (40a′ f 31a′′) -0.380 (41a′ f 31a′′)
A′ 4.334 286 0.00722 0.703 (41a′ f 42a′) -0.610 (30a′′ f 32a′′)
A′′ 4.823 257 0.05621 0.958 (30a′′ f 42a′)
1
3
3
4
5
1
1
1
A′′ 5.263 236 0.27007 0.925 (39a′ f 31a′′)
Table 5. Calculated (CC2/cc-pVTZ) Excited States for 6 (cf. Figure
S2)
state
E
(calc)
λ
oscil. str.
composition
1
1
1
2
2
3
3
4
A′ 2.627 472 0.01734 0.695 (33a′′ f 37a′′) -0.625 (36a′′ f 37a′′)
A′′ 2.878 431 0.00007 0.962 (44a′ f 37a′′)
1
1
1
1
1
1
A′ 2.917 425 0.02362 0.718 (36a′′ f37a′′) +0.625 (33a′′ f 37a′′)
A′′ 3.415 363 0.00702 0.829 (48a′ f 37a′′) +0.510 (47a′ f 37a′′)
A′′ 3.647 340 0.06732 0.980 (36a′′ f 49a′)
A′ 3.797 327 0.00798 0.704 (48a′ f 49a′) -0.635 (36a′′ f 38a′′)
A′′ 4.047 306 0.00915 0.752 (47a′ f 37a′′) -0.436 (48a′ f 37a′′)
Figure 18. Selected molecular (HF/cc-pVTZ) orbitals of 4. Orbital energies
in electronvolts (eV).
Table 6. Calculated (CC2/cc-pVTZ) Excited States for Benzene 34
a
+0.309 (46a′ f 37a′′) -0.309 (45a′ f 37a′′)
(cf. Figure S3)
1
1
1
4
5
5
A′ 4.660 266 0.00001 0.901 (35a′ f 37a′′)
A′′ 5.058 245 0.00078 0.756 (36a′′ f 50a′) +0.611 (35a′′ f 49a′)
A′ 5.067 245 1.74606 0.714 (36a′′ f 38a′′) +0.600 (48a′ f 49a′)
state
E(exp)
E(calc)
λ
oscil. str.
composition
1 1
1
B
3u ( L
b
)
5.06 5.303 234
6.44 6.609 188
-
u
0.700 (1b3g f 1a )
-
0.700 (1b2g f 2b1u
0.699 (1b3g f 2b1u
0.699 (1b2g f 1a
)
1 1
B
2u ( L
1
)
-
)
u
)
a
+
Figure 13b shows a simulation of the UV/vis spectra of the
quinones 4-6, the “half-clips”, generated in an identical manner
using the calculated transition energies and oscillator strengths
of Tables 3-5 (for graphical representations of the orbitals, see
Figure 18 and Figures S1 and S2 in the Supporting Information).
Clearly, the calculations correctly reproduce the observed strong
batho- and hyperchromic shift of the first absorption maximum
with increasing size of the arene moiety. Much of what has
been discussed above also applies to the half-clips. For example,
the first, the second, and the seventh excitation in 4 correspond
a
Point group D2h (rather than D6h) was used for technical reasons.
smaller degree of mixing (cf. Figure 18). The through-space
π-π interactions between the quinone and the benzene moieties
now rather become visible at the level of configuration interac-
tion. Note that mixing between two occupied orbitals and
configuration mixing become equivalent for single excitations
into the same virtual orbital, that is, for the case at hand. A
state of essentially pure CT character is observed as the fifth
+
to slightly perturbed n f π* and π f π* excitations within
1
state, labeled as 2 A′. The initial orbital of the corresponding
the quinone moiety. Their transition energies are nearly identical
to those of the corresponding excitations in 32. The same holds
true for the first, the second, and the eighth transition in 5, and
also for the second excitation in 6. Note that the small oscillator
strength of this second n f π* excitation is practically constant
along the entire series of molecule 32 and 4-6. The much larger
transition, the HOMO-1, looks like one component of the
degenerate HOMO of benzene 34 (cf. Figure S3). For symmetry
reasons, it cannot mix with the occupied π orbitals of the
quinone moiety. Finally, the sixth excited state of 4 predomi-
nantly results from the lowest π f π* excitation within the
benzene moiety with small admixtures of CT character. It shows
+
oscillator strength of the π f π* excitation within the quinone
a bathochromic shift of 9 nm with respect to the calculated
symmetry-forbidden transition into the L
in 34 (cf. Table 6).
The transitions observed in 5 may be characterized in a similar
moiety also hardly changes for 32, 4, and 5. However, it is
smaller than the oscillator strength of the corresponding
transition in 30, which is easily explained by the fact that the
1
b
state (the R band)
+
π orbital, loosely speaking, represents only half of that of the
1
manner. The main differences to 4 are that the 2 A′ state of
π orbitals of 30 from which excitation into π* is allowed.
predominant CT character is already found as the third excited
state (cf. Table 4). It corresponds to the HOMO f LUMO
excitation. Note that the HOMO of 5 is practically identical to
the HOMO of naphthalene 35 and that, as in 4, for symmetry
reasons it cannot mix with the occupied π orbitals of the quinone
As in 32, the third and fourth electronic states in 4 are found
to be mixtures between CT and quinone moiety π f π*
excitations (cf. Table 3). Compared to 32, the occupied orbitals
33a′ and 34a′ participating in these transitions show a somewhat
8
588 J. AM. CHEM. SOC. 9 VOL. 132, NO. 25, 2010

Intramolecular Electronic Interactions
A R T I C L E S
Table 7. Calculated (CC2/cc-pVTZ) Excited States for
Naphthalene 35 (cf. Figure S4)
Table 8. Calculated (CC2/cc-pVTZ) Excited States for 36 (cf.
Figure S5)
state
E
(exp)
E
(calc)
λ
oscil. str.
composition
f 2b2g
f 2b3g
f 2b2g
state
E
(exp)
E
(calc)
λ
oscil. str.
composition
1 1
B
3u ( L
1
)
4.13 4.438 279 0.00003 0.699 (2b1
0.699 (1a
4.66 4.834 256 0.08117 0.968 (1a
)
)
1
1
3.60 3.625 342 0.07740 0.982 (2b3g f 3b1u
3.64 3.865 321 0.00034 0.703 (2b2g f 3b1u
)
)
u
)
b
u
a
1 B2u ( L )
1
1
-
u
b
1 B3u ( L )
1 1
B
2u ( L
1
)
)
-0.690 (2b3g f 2a
0.831 (1a f 3b1u
0.525 (2b3g f 3b2g
0.828 (2b3g f 3b2g
0.522 (1a f 3b1u
5.417 229 2.25389 0.703 (2b3g f 2a
0.684 (2b2gf 3b1u
a
u
1 1
2 1
2 1
B
B
B
5.108 243
5.304 234
-
-
)
1g
1g
3u
u
+
)
)
-
u
)
u
)
+
)
Table 9. Calculated (CC2/cc-pVTZ) Excited States for 33 (cf.
Figure 20)
state
E
(calc)
λ
oscil. str.
composition
1
1
B
A
A
1
2.696
2.799
2.991
3.904
3.977
4.802
460
443
415
318
312
258
0.00048
-
-
0.01664
-
0.20048
0.933 (10a
0.938 (15b
0.957 (17b
0.950 (21a
0.941 (16b
0.906 (20a
2 2
f 15b )
1
1
1
1
1
1
2
1
3
2
2
2
1
f 15b
f 15b
2
)
1
2
)
)
B
A
B
2
1
f 15b
f 15b
f 15b
2
2
1
2
)
)
2
1
2
second n f π* excitation, on the other hand, is still found at
the same transition energy as in the other half-clips. It is slightly
lower than the excitation energy of the remaining linear
1
combination 2 A′ of quinone moiety n f π* and CT
1
excitations. The transition into 2 A′ also has a noticeable
oscillator strength and is the main contributor to the low-energy
band in the spectrum of 6 (cf. Figure 13). Figure 19 demonstrates
1
that the next state, 2 A′′, has a fairly constant energy for all of
the half-clips. As mentioned above, this state is a linear
combination of a CT and a quinone moiety π f π* excitation.
The other linearly independent combination of these two
1
excitations gives rise to the 3 A′′ state in 32, 4, and 5, while
1
the corresponding state in 6 is dubbed 4 A′′. Again, its energy
1
is fairly constant for all of the half-clips. In 6, the 2 A′′ and 4
1
A′′ states are separated by two states which predominantly stem
from π f π* excitations within the anthracene moiety. The
corresponding excitations in anthracene 36 give rise to the nearly
degenerate L and L states, respectively (cf. Table 8). With
a b
Figure 19. Calculated levels of the half-clip quinones. Solid lines denote
states of predominant CT character, dashed lines quinone moiety excitations,
dashed-dotted lines states with partial CT character, and dotted lines arene
moiety excitations.
1
1
respect to these, one observes only a slight bathochromic shift
of a few nanometers in 6. There is one essentially “pure” CT
1
moiety (cf. Figures S1 and S4). Furthermore, there are two states
of essentially pure naphthalene moiety π f π* character with
energies below that of the intense quinone moiety π f π*
state (4 A’) in 6, at a fairly high energy of 4.66 eV. We stopped
1
our calculations at the first state (5 A′) with an oscillator
strength larger than 0.2. It is of predominant π f π* anthracene
1
1
transition. While the lower one (3 A′) of these shows a
character, like its nearly degenerate companion 5 A′′. Note that
bathochromic shift of 7 nm with respect to excitation into the
the latter is the only state in any of the half-clips which shows
a down-shift of more than 0.1 eV when diffuse basis functions
are added (cf. Table S1), indicating that Rydberg mixing plays
a role here (as it also does for the higher excited states of 36
not given in Table S1).
1
1
L
b
state in 35, the wavelength of the second one (4 A′′) is
1
practically the same as that of excitation into the L
a
state giving
rise to the p band (cf. Table 7).
The findings for norbornadienoquinone 32, and its benzo- or
naphtho-substituted derivatives 4 and 5, respectively, discussed
above are collected in Figure 19. As can be seen from the figure,
For the quinone 33 and the clips 38, 2, and 1, only the first
few transitions were considered, as for the corresponding anti-
configured isomers 37, 39, 3, and 40 which, for convenience,
will be dubbed “anti-clips” in the following. Concentrating first
on 33 and 37, Tables 9 and 10 and Figures 20 and 21,
respectively, demonstrate that the first two transitions in each
case correspond to n f π* excitations within the quinone
moiety. All of the remaining transitions should best be qualified
as mixtures between π f π* excitations within the quinone
moiety and CT excitations from appropriate symmetric and
antisymmetric linear combinations of the π orbitals on the
norbonadiene rests.
there is a slight lowering of the energy of the first excited state
1
(
1 A′) along this series. A slight, yet increasing amount of
mixing with CT excitations is responsible for this effect. This
becomes obvious for the lowest transition in the anthraceno-
substituted system 6 which displays a marked bathochromic shift
of 31 nm with respect to parent p-benzoquinone 30 and, in
particular, a strong increase of its oscillatory strength (cf. Table
5
). Here, the CT excitation out of the HOMO, which looks like
the HOMO of anthracene 36 (cf. Figures S2 and S5), mixes
with the first n f π* excitation of the quinone moiety. The
J. AM. CHEM. SOC. 9 VOL. 132, NO. 25, 2010 8589

A R T I C L E S
Jansen et al.
Table 10. Calculated (CC2/cc-pVTZ) Excited States for 37 (cf.
Figure 21)
Table 11. Calculated (CC2/cc-pVTZ) Excited States for 38 (cf.
Figure S6)
state
E
(calc)
λ
oscil. str.
composition
state
E
(calc)
λ
oscil. str.
composition
1 1
A
A
2.695
2.810
2.999
3.737
4.235
4.712
460
441
413
332
293
263
-
0.933 (11b
f 12b
)
)
)
1 B
1
2.655
2.789
2.915
467
445
425
0.00111
-
-
0.906 (15a
0.901 (22b
0.877 (24b
g
g
u
g
1
2 2
f 21b )
1
1
1
1
1
1
1
1
2
2
3
u
0.00015
-
0.00690
-
0.20699
0.959 (16b
0.970 (20a
0.933 (18b
0.950 (19a
0.902 (17b
f 12b
f 12b
f 12b
g
1 A
2
1
f 21b
f 21b
2
)
)
1
B
A
B
A
g
g
g
2 A
2
1
2
u
u
g
)
)
+0.347 (23b1 f 21b
0.976 (28a f 21b
0.881 (23b f 21b
2
2
)
)
1
g
g
f 12b
f 12b
g
1 B
2
3.545
3.786
350
327
0.00609
-
1
2
)
1
u
u
g
)
3 A
2
1
2
)
-
0.368 (24b1 f 21b
1
2
1
B
1
4.113
4.123
301
301
0.00644
0.01404
0.936 (16a
2
f 21b
f 21b
2
)
1
A
1
0.972 (20b
2
2
)
Table 12. Calculated (CC2/cc-pVTZ) Excited States for 39 (cf.
Figure S7)
state
E
(calc)
λ
oscil. str.
composition
1
1
1
1
A
A
g
2.659
2.809
2.935
466
441
423
-
0.908 (16b
0.954 (23b
0.893 (27a
g
f 18b
f 18b
f 18b
0.427 (26a f 18b
g
)
1
1
u
0.00013
-
u
g
)
B
g
g
g
)
+
g
g
g
)
)
1
1
2
2
A
B
u
3.484
3.955
356
313
0.00381
-
0.964 (25b
u
f 18b
f 18b
0.397 (27a
g
)
)
g
0.883 (26a
g
g
-
g
f 18b
1
1
2
1
A
B
g
4.154
4.169
298
297
-
0.937 (17b
g
f 18b
g
)
)
u
0.02261
0.971 (19a
u
f 18b
g
Table 13. Calculated (CC2/cc-pVTZ) Excited States for 2 (cf.
Figure S8)
state
E
(calc)
λ
oscil. str.
composition
Figure 20. Selected molecular (HF/cc-pVTZ) orbitals of 33. Orbital
energies in electronvolts (eV).
1 1
B
2.590
479
0.00466
0.819 (20a
f 27b
)
1
2
2
2 2
+0.428 (22a f 27b )
1
1
1
2
A
A
2
2.790
2.929
444
423
-
-
0.882 (28b
0.823 (31b
1
f 27b
f 27b
2
)
)
2
1
2
+
0.455 (30b
0.974 (26b f 27b
0.871 (22a f 27b
0.417 (20a
1 2
f 27b )
1
1
1
2
A
1
3.157
3.222
393
385
0.02867
0.01281
2
2
)
)
B
1
2
2
-
2
f 27b
2
)
Table 14. Calculated (CC2/cc-pVTZ) Excited States for 3 (cf.
Figure S9)
state
E
(calc)
λ
oscil. str.
composition
1 1
A
g
2.608
475
-
0.838 (21b
f 24b
g
g
)
+
g g
0.387 (23b f 24b )
1
1
1
1
A
u
2.812
2.954
441
420
0.00013
-
0.933 (29b
u
f 24b
f 24b
g
g
)
)
B
g
u
0.825 (34a
g
+
0.548 (33a
0.972 (25a f 24b
0.889 (23b f 24b
g
0.377 (21b f 24b )
g g
f 24b )
1
1
1
2
B
A
3.221
3.274
385
379
0.05487
-
u
g
)
g
g
g
)
-
g
second is symmetry-forbidden, in 37, it is the other way round.
This should lead to a hypsochromic shift of about 20 nm for
the first band between 33 and 37, despite the virtually identical
energies of the first two electronically excited states. Note,
however, that vibronic coupling, which could not be taken into
account in this study, will influence the transition intensities.
This may explain the seeming absence of the experimentally
observed low-energy maximum at 426 nm in the simulated
spectrum of 33 shown in Figure 11b: without vibronic coupling,
the oscillator strength of the first transition is calculated too
low.
Figure 21. Selected molecular (HF/cc-pVTZ) orbitals of 37. Orbital
energies in electronvolts (eV).
The mixing here becomes visible in the mixing of the orbitals,
with a direct participation of quinone π orbitals (rather than of
+
-
π and π orbitals as for the half-clips). For both 33 and 37,
inspection of the orbitals suggests that for the fourth state the
CT character somewhat prevails while it is the quinone moiety
π f π* character for the sixth state. The most important
difference between 33 and 37 is found for the oscillator
strengths: in the C2V point group of 33, all transitions become
symmetry-allowed, except for transitions into states of A
symmetry, while for 37 with its C2h point group, only transitions
into states of A and B symmetry are allowed. In consequence,
while for 33 the first electronic transition is allowed and the
2
The results for the clips 38, 2, and 1 and the anti-clips 39, 3,
and 40 are collected in Tables 10-16, and corresponding level
schemes including a qualitative characterization of the states
are presented in Figures 22 and 23, respectively.
u
u
8
590 J. AM. CHEM. SOC. 9 VOL. 132, NO. 25, 2010

Intramolecular Electronic Interactions
A R T I C L E S
Table 15. Calculated (CC2/cc-pVTZ) Excited States for 1 (cf.
Figure S10)
state
E
(calc)
λ
oscil. str.
composition
1 1
B
1
2.400
517
0.01784
0.790 (28a
f 33b
2
2
)
+
0.507 (25a
2
f 33b
2
)
)
1 1
A
A
2.581
2.791
2.833
480
444
438
0.04296
-
0.00551
0.971 (32b
f 33b
f 33b
)
)
1
2
2
1
1
2
2
0.897 (35b
0.753 (25a
1
2
1
B
1
2
f 33b
0.553 (28a f 33b
f 33b
f 33b
2
)
-
2
2
2 1
A
2.941
422
-
0.775 (38b
2
1
2
)
+
0.537 (37b
1
2
)
Table 16. Calculated (CC2/cc-pVTZ) Excited States for 40 (cf.
Figure S11)
state
E
(calc)
λ
oscil. str.
composition
1 1
A
g
2.466
503
-
0.728 (29b
f 30b
)
g
g
+
0.579 (26b
g
f 30b
g
)
)
1 1
B
2.663
2.815
2.862
466
440
433
0.08481
0.00013
-
0.969 (31a
f 30b
u
u
g
)
1
1
2
A
A
u
0.952 (36b
0.700 (26b
u
g
f 30b
f 30b
g
)
)
1
g
g
-
0.628 (29b
g
f 30b
f 30b
f 30b
g
1 1
B
2.976
417
-
0.763 (41a
g
g
g
)
+
0.633 (40a
g
g
)
Figure 23. Calculated levels of the clip quinones. Solid lines denote states
of predominant CT character, dashed lines quinone moiety excitations, and
dashed-dotted lines states with partial CT character.
interaction level (cf. Tables 12-15). The other state of quinone
moiety n f π* character does not suffer from mixing at the
orbital or configuration interaction level. It thus has a virtually
constant transition energy for all of the clips and anti-clips. In
1
and 40, this leads to the situation that it becomes the third
excited state, after a state of predominant CT character as the
second excited state. As already discussed above for 38 and
3
9, electronic transitions into the lowest excited state are
symmetry-allowed for the clips, while they are forbidden for
the anti-clips. This explains the “shoulder” at 515 nm in the
experimental spectrum of 2, which is absent in the spectrum of
the corresponding anti-configured isomer 3 (cf. Figure 8), as
can be seen from the comparison with the simulated low-energy
part of the spectra shown in Figure 8b (based on the data in
Tables 12-14, employing a down-shift of 0.25 eV and Gaussian
broadening with width 0.5 eV). Note that most of the intensity
of the low-energy band in 1 is due to the transition into the
second excited state, that is, the lowest state of essentially pure
CT character. The transition into the first excited state is not
visible as a separate band but rather helps to broaden the low-
energy band.
Figure 22. Calculated levels of the clip quinones. Solid lines denote states
of predominant CT character, dashed lines quinone moiety excitations, and
dashed-dotted lines states with partial CT character.
Conclusion
The main differences of the clips and anti-clips with respect
to the half-clips are the higher symmetry and the larger number
of arene moiety π orbitals participating in the electronic
transitions. These form symmetric and antisymmetric linear
combinations, respectively, which in turn may mix with the two
quinone π orbitals. In 38 and in 39, respectively, this leads to
The novel surprisingly colorful dark blue and orange-red
molecular clips 1 and 2 were synthesized by DDQ oxidation of
the corresponding colorless hydroquinone clips 7 and 9. The
colors of the quinone clips result from broad absorption bands
in the visible range (1, λmax ) 537 nm, and 2, λmax ) 423 and
λ
shoulder ) 515 nm) showing bathochromic shifts of 112 and 90
a low-lying state of mixed CT and quinone moiety π f π*
nm, respectively, compared to the similarly tetraalkyl-substituted
duroquinone 31, even though the clips 1 and 2 only contain
insulated π systems as chromophores, a central tetraalkyl-
substituted p-benzoquinone spacer-unit, and two anthracene or
two naphthalene sidewalls. To elucidate the electronic properties
of these clips, we prepared the compound 3, the anti-configured
isomer of clip 2, and the benzene-, naphthalene-, and anthracene-
substituted quinones 4, 5, and 6, the so-called “half-clips”. The
1
character as the third excited state. Corresponding 2 A
2
and 1
1
g
B states, respectively, are also found for the remaining clips
and anti-clips. Transitions into these states are symmetry-
forbidden. The first two excited states in 38 and 39, respectively,
essentially are only slightly perturbed quinone moiety n f π*
transitions. For the lowest state of 1, 2, 3,, and 40, however,
mixing in of CT character becomes evident at the configuration
J. AM. CHEM. SOC. 9 VOL. 132, NO. 25, 2010 8591

A R T I C L E S
Jansen et al.
“
half-clips” 6 and 5 show a similar color change and the same
bridge. The increasing CT character of the absorption band at
the longest wavelength in UV/vis spectra of the “half-clip”
quinones 4, 5, and 6 is consistent with the decreasing redox
potential of benzene, naphthalene, and anthracene toward
p-benzoquinone, which is also reflected by the absorption band
in the UV/vis spectra of the intermolecular CT complexes
between p-benzoquinone and naphthalene (λmax ) 373 nm) or
anthracene (λmax ) 450 nm). The bathochromic shift of
absorption band at the longest wavelength in the UV/vis spectra
of the naphthalene clips 2 and 3 to the anthracene clip 1 can be
explained analogously to that observed in the spectra of the
corresponding “half-clips” 5 and 6.
trend in the UV/vis absorption spectra as the anthracene and
naphthalene clip 1 and 2. This finding already rules out that
the color of these systems is a result of “through-space” π-π
interactions between the aromatic sidewalls in the molecular
clips 1 and 2. The UV/vis spectra of the molecular clips 1-3
(
3
Figure 8), the “half-clips” 4-6 (Figure 13), and the quinones
2 and 33 (Figure 11) simulated by means of quantum chemical
ab initio calculations agree well with the experimental spectra.
The quantum chemical calculations allow the following conclu-
sions: The bathochromic shift of the absorption band at the
longest wavelength in the “half-clip” quinones 4, 5, and 6 with
the increasing number of rings in the anellated aromatic unit
Acknowledgment. We gratefully acknowledge financial support
by the Deutsche Forschungsgemeinschaft.
(
from benzene to anthracene) is the result of an increasing
configuration interaction between a n f π* excitation of the
quinoid component and a π f π* excitation with intramolecular
charge transfer (CT) character. The initial π orbitals involved
in the various transitions mainly stem from through-space
interactions between π orbitals of the aromatic sidewalls and π
orbitals of the quinone moiety with varying degree of mixing.
The configuration interaction in the excited states can be
considered to be a homoconjugation, that is, the relevant charge
transfer states are formed across an allegedly insulating aliphatic
Supporting Information Available: Details on the quantum
chemical calculations including plots of relevant molecular
1
orbitals, experimental details, H NMR spectra of the new
compounds, and supplementary crystallographic data (cif files).
This material is available free of charge via the Internet at http://
pubs.acs.org.
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