Alternating covalent bonding interactions in a one-dimensional chain of a phenalenyl-based singlet biradical molecule having kekule structures

Article abstract of DOI:10.1021/ja1037287

A novel naphthoquinoid singlet biradical (2a) stabilized by phenalenyl rings is prepared by a multistep procedure and is investigated in terms of covalent bonding interactions. The molecule 2a gives single crystals, in which a 1D chain is formed with a very short π-π contact at the overlapping phenalenyl rings. The unpaired electrons in 2a are involved in covalent bonding interactions not only within the molecule but also between the molecules in the 1D chain, and a linear conjugation is made of the alternating intra- and intermolecular covalent bonding interactions through conventional π-conjugation and multicenter bonding, respectively. The linear conjugation causes a lower-energy shift of the optical transition band in the crystal, but the transition energy is higher than that of the benzoquinoid singlet biradical (1a). This optical behavior and the magnetic susceptibility measurements reveal that the intermolecular covalent bonding interaction in the 1D chain of 2a is greater in strength than the intramolecular one, despite the fact that a fully conjugated Kekule structure can be drawn for 2a.

Full text of DOI:10.1021/ja1037287

Published on Web 09/27/2010
Alternating Covalent Bonding Interactions in a
One-Dimensional Chain of a Phenalenyl-Based Singlet
Biradical Molecule Having Kekule´ Structures
Akihiro Shimizu,† Takashi Kubo,*,† Mikio Uruichi,‡ Kyuya Yakushi,‡
Masayoshi Nakano,§ Daisuke Shiomi,| Kazunobu Sato,| Takeji Takui,|
Yasukazu Hirao,† Kouzou Matsumoto,† Hiroyuki Kurata,† Yasushi Morita,† and
Kazuhiro Nakasuji†
Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka,
Osaka 560-0043, Japan, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan,
Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka
UniVersity, Toyonaka, Osaka 560-8531, Japan, and Department of Chemistry and Material Science,
Graduate School of Science, Osaka City UniVersity, Sumiyoshi-ku, Osaka 558-8585, Japan
Received October 13, 2009; E-mail: kubo@chem.sci.osaka-u.ac.jp
Abstract: A novel naphthoquinoid singlet biradical (2a) stabilized by phenalenyl rings is prepared by a
multistep procedure and is investigated in terms of covalent bonding interactions. The molecule 2a gives
single crystals, in which a 1D chain is formed with a very short π-π contact at the overlapping phenalenyl
rings. The unpaired electrons in 2a are involved in covalent bonding interactions not only within the molecule
but also between the molecules in the 1D chain, and a linear conjugation is made of the alternating intra-
and intermolecular covalent bonding interactions through conventional π-conjugation and multicenter
bonding, respectively. The linear conjugation causes a lower-energy shift of the optical transition band in
the crystal, but the transition energy is higher than that of the benzoquinoid singlet biradical (1a). This
optical behavior and the magnetic susceptibility measurements reveal that the intermolecular covalent
bonding interaction in the 1D chain of 2a is greater in strength than the intramolecular one, despite the fact
that a fully conjugated Kekule´ structure can be drawn for 2a.
Chart 1
Introduction
Quinoid hydrocarbons have attracted the continuous attention
of chemists with their unusual reactivity and electronic structure
derived from a small HOMO-LUMO energy gap.1 The simplest
representatives, o- and p-quinodimethane, have been the most
extensively studied by chemical,2 spectroscopic,3 and theoreti-
cal4 methods, which illuminate the nature of a biradical with a
singlet ground state. π-Extended quinodimethanes such as
Chichibabin’s hydrocarbon,5 cyclohepta[def]fluorene,6 and poly-
acenes7 (Chart 1) and nanographenes8 have also been discussed
in terms of their biradical nature. One of the fundamental issues
of Kekule´ compounds having biradical nature would concern
the spin structure of their ground state, but it has been a
formidable task to experimentally characterize the structure as
in the case of a long history of Chichibabin’s hydrocarbon. Such
species possess the characteristic electronic structure that two
electrons on the radical centers in their biradical canonical forms
are weakly coupled within the molecule, and the case of
adequately weak coupling finds the species bearing singlet
biradical character. Because of the open-shell character, most
† Graduate School of Science, Osaka University.
‡ Institute for Molecular Science.
§ Graduate School of Engineering Science, Osaka University.
| Osaka City University.
(1) Platz, M. S. Quinodimethanes and Related Diradicals. In Diradicals;
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J. AM. CHEM. SOC. 2010, 132, 14421–14428 14421

A R T I C L E S
Shimizu et al.
Scheme 1. Resonance Structures of 1 and 2a
In this paper, we report the synthesis, crystal packing, and
covalent bonding interactions of a phenalenyl-based naphtho-
quinoid hydrocarbon 2a whose ground state is also described
in terms of the resonance of Kekule´ and biradical structures.
Our previous study of di-tert-butyl derivative 2b has demon-
strated that the intramolecular covalent bonding interaction is
weakened compared with that in 1; the reason is that the
expansion of the central π-system from benzoquinoid to
naphthoquinoid structure increases the biradical contribution in
the ground state.11 We focus our attention for 2a on the effect
of larger biradical character on the solid state properties such
as overlapping structures of phenalenyl rings, optical transition
in a crystal, magnetic properties, and electroconductivity.
Results and Discussion
a Amounts of biradical character (y) were calculated with a CASSCF(2,2)/
Theoretical Consideration of Biradical Character. The hy-
drocarbon 2 consists of 2,6-naphthoquinodimethane and two
phenalenyl moieties. The quinoid Kekule´ structure of 2 resonates
well with a biradical structure, because the formal double bond
loss in the biradical structure is compensated with aromatic
stabilization energy of the central naphthalene ring. Considering
the extent of the aromatic stabilization energy of benzene and
naphthalene, 2 owns a larger contribution of the biradical
structure in the ground state than 1.
6-31G//RB3LYP/6-31G** method.
singlet biradical species are short-lived, and this high reactivity
obscures the identification of spin structure and characteristic
functional properties. However, recent progress in stable singlet
biradicals provides the opportunity to examine them precisely
by using various analytical methods.9
In the course of our studies on stable singlet biradicals, we
have investigated benzoquinoid hydrocarbons (1a and 1b),
which are thermodynamically stabilized by exploiting the spin-
delocalizing character of phenalenyl radical, as shown in Scheme
1, showing that in the crystal, 1a and 1b form π-π one-
dimensional (1D) chains with very short π-π separation
distances between the overlapping phenalenyl rings.10 In the
1D chain, electrons of the singlet biradicals are involved in
covalent bonding interactions not only within the molecule but
also between the molecules. Furthermore, the sterically perturbed
molecule 1b gave many polymorphic crystals, in which various
separation distances (including nonoverlapping) were observed
depending on experimental conditions.10b The changes in the
separation distances varied the intermolecular covalent bonding
interaction in magnitude in the 1D chain, and consequently a
peak shift was observed in their reflection spectra. Thus, both
the intra- and intermolecular covalent bonding interactions play
a key role in the singlet-biradical-based molecular properties
or material functionalities.
As mentioned above, quinoid compounds encounter difficulty
in the experimental elucidation of their biradical character.
Instead, a theoretical study gives an obvious criterion,12 and
the amount of the biradical character is estimated by the natural
orbital occupation number (NOON).4 In a multiconfigurational
scheme, the mixing of a doubly excited configuration, 1ΦH,HfL,L
,
with a ground state configuration, 1ΦH,H, represents uncoupling
of an electron pair, and hence the occupation number of the
LUMO is a direct computational measure of the amount of
biradical character. A perfect biradical is characterized by
occupation numbers of 1.0 in HOMO and LUMO, whereas a
perfect closed-shell molecule possesses occupation numbers of
2.0 and 0.0 in HOMO and LUMO, respectively. We performed
a CASSCF(2,2)/6-31G//RB3LYP/6-31G** calculation for 1 and
2, which gave LUMO occupation numbers of 0.30 and 0.50,
and consequently the amounts of the biradical character were
estimated to be 30% and 50%, respectively. A UHF-based
broken symmetry method gives another quantitative guidance
for the biradical character.13 A broken-symmetry UB3LYP/6-
31G** calculation of 1 and 2 gave LUMO occupation numbers
of 0.37 and 0.56 with spin contaminations 〈S2〉 ) 0.65 and 0.87,
respectively. Spin densities on the phenalenyl rings are larger
for 2 than for 1 as listed in Table 1. These quantum chemical
calculations indicate more enhanced biradical character for 2
and are thoroughly consistent with the Kekule´ formula consid-
eration relevant to the biradical character described above.
The very large biradical character of 2 is associated with a
weak coupling of two unpaired electrons on the phenalenyl rings
through the naphthalene linker. With the help of a perturbation
MO analysis,14 the nonbonding singly occupied MO (SOMO)
(9) (a) Herebian, D.; Wieghardt, K. E.; Neese, F. J. Am. Chem. Soc. 2003,
125, 10997–11005. (b) Ziessel, R.; Stroh, C.; Heise, H.; Ko¨hler, F. H.;
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Chem., Int. Ed. 2004, 43, 4880–4883. (d) Porter III, W. W.; Vaid,
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J. T. L. Angew. Chem., Int. Ed. 2008, 47, 1443–1446. (g) Ueda, A.;
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1682. (h) Kamada, K.; Ohta, K.; Kubo, T.; Shimizu, A.; Morita, Y.;
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Nakano, M. Angew. Chem., Int. Ed. 2007, 46, 3544–3546. (i) Kamada,
K.; Ohta, K.; Shimizu, A.; Kubo, T.; Kishi, R.; Takahashi, H.; Botek,
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940.
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D.; Sato, K.; Takui, T.; Morita, Y.; Nakasuji, K. Org. Lett. 2007, 9,
81–84.
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Chem., Int. Ed. Engl. 1987, 26, 170–189.
(10) (a) Kubo, T.; Shimizu, A.; Sakamoto, M.; Uruichi, M.; Yakushi, K.;
Nakano, M.; Shiomi, D.; Sato, K.; Takui, T.; Morita, Y.; Nakasuji,
K. Angew. Chem., Int. Ed. 2005, 44, 6564–6568. (b) Shimizu, A.;
Uruichi, M.; Yakushi, K.; Matsuzaki, H.; Okamoto, H.; Nakano, M.;
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(14) Dewar, M. J. S.; Dougherty, R. C. The PMO Theory of Organic
Chemistry; Plenum Press: New York, 1975.
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Covalent Bonding Chain of Singlet Biradical Molecule
A R T I C L E S
Table 1. Spin Densities (F) on the Carbon Atomsa of 1 and 2
Calculated with a Broken-Symmetry UB3LYP/6-31G** Method
1
2
positions
F
positions
F
1, 3
2
-0.202
+0.100
+0.094
-0.213
+0.110
-0.218
+0.030
0.000
-0.030
+0.218
-0.110
+0.213
-0.094
+0.048
-0.096
+0.202
-0.100
-0.048
+0.096
1, 3
2
-0.229
+0.114
+0.107
-0.241
+0.124
-0.248
+0.048
-0.061
0.000
+0.061
-0.048
+0.248
-0.124
+0.241
-0.107
+0.056
-0.111
+0.229
-0.114
-0.056
+0.111
3a, 16a
4, 16
5, 15
5a, 14b
5b, 14a
6, 14
6a, 13b
6b, 13a
7, 13
8, 12
8a, 11a
8b
8c
9, 11
10
16b
16c
3a, 18a
4, 18
5, 17
5a, 16b
5b, 16a
6, 16
6a, 15a
7, 15
7a, 14b
7b, 14a
8, 14
9, 13
9a, 12a
9b
9c
10, 12
11
18b
18c
a Atom positions are shown in the following drawings:
Figure 1. (a) HOMO, (b) LUMO, and (c) spin densities of 2 and (d) SOMO
and (e) spin densities of phenalenyl radical. The calculations are performed
at RB3LYP/6-31G** (a, b) and UB3LYP/6-31G** (c, d, e) levels of theory.
Blue and green surfaces of the spin density maps represent R and ꢀ spin
densities, respectively, drawn at 0.004 e/au3 level.
Scheme 2. Synthesis of 2aa
of phenalenyl radical is weakly perturbed by the MOs of
naphthalene, because the aromatic character of naphthalene
serves a wide energy gap between the bonding and antibonding
orbitals and hence the MOs of both fragments differ consider-
ably in energy. Thus, the mixing of the phenalenyl’s SOMO
with the naphthalene’s MOs is not effective, and the HOMO
and LUMO of 2 retain the character of the SOMO of phenalenyl
radical as shown in Figure 1. For this reason, 2 has a small
energy gap as well as a large spatial overlap between the HOMO
a Reaction conditions: (a) Br2, CH2Cl2, rt, 92%; (b) CH2dCHCO2CH3,
Pd(CH3CN)2Cl2, Ph4PCl, AcONa, DMF, 140 °C; (c) Zn, AcOH, toluene,
reflux, 71% (2 steps); (d) LiI, 2,4,6-trimethylpyridine, 185 °C; (e) (COCl)2,
reflux, then AlCl3, CH2Cl2, -78 to -30 °C, 71% (2 steps); (f) NaBH4,
CH2Cl2, EtOH, rt, 100%; (g) TsOH·H2O, toluene, 90 °C, 87%; (h)
p-chloranil, benzene, reflux, 83%.
gave carboxylic acid derivatives 7. Intramolecular Friedel-Crafts
cyclization of acyl chloride of 7 with AlCl3 gave diketones 8,
which were reduced with NaBH4 and subsequently dehydrated
with TsOH to afford dihydro compounds 10. Finally 10 was
dehydrogenated with p-chloranil to give 2a. Due to the
extremely low solubility, recrystallization of the crude material
2a failed. Instead, by cooling the reaction mixture in the final
dehydrogenation step, we successfully obtained single crystals
of 2a as dark purple plates. The solid 2a was found to be stable
at room temperature in air for a couple of days.
Molecular Geometry. The X-ray crystallographic analysis of
2a at 200 K revealed effective D2h symmetry for the main core
ring. The four phenyl rings were nearly perpendicular to the
main core ring because of steric crowding (Figure 2). In the
crystal, 2a formed a 1D chain in a slipped stack arrangement
running parallel to the (0 0 1) plane (Figure 3 and Supporting
Information, Figure S1).
1
and LUMO, which are favorable for the mixing of ΦH,HfL,L
1
with ΦH,H. The small HOMO-LUMO energy gap of 2 has
been confirmed by the cyclic voltammetry measurement of 2b,
which gives ∆Eredox () Eox - Ered) of 1.04 V.11
Synthesis. The synthetic procedure for 2a is outlined in
Scheme 2. The starting material 3 was synthesized according
to the previously reported procedures15,11 and was converted
to dibromo derivatives 4 as a mixture of 3,11- and 3,12-isomers.
The individual isomers were not isolated because both com-
pounds were expected to lead to a single compound 2a. Heck
reaction of 4 with methyl acrylate and subsequent hydrogenation
of the alkenes 5 afforded ester compounds 6. Hydrolysis of 6
(15) Neudorff, W. D.; Schulte, N.; Lentz, D.; Schlu¨ter, A. D. Org. Lett.
2001, 3, 3115–3118.
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A R T I C L E S
Shimizu et al.
Figure 2. ORTEP drawings of 2a at 200 K: (a) top view and (b) side view. Displacement ellipsoids are drawn at the 50% probability level.
Figure 3. (a) Top view and (b) side view of the 1D chain of 2a at 200 K. Hydrogen atoms are omitted for clarity. In the top view, distances of the short
C-C contact in the overlapping phenalenyl rings are also shown.
The crystal structure of 2a contains two salient features: (1)
2a reproduces the superimposed phenalenyl overlap that is
almost identical to that of 1a, and (2) the phenalenyl rings of
2a retain the π-dimeric structure of the phenalenyl radical,16
not the σ-dimer, despite the fact that the spin densities on the
phenalenyl rings are larger than those of 1a. Both the π-π
separation distance (3.170 Å for 2a, 3.137 Å for 1a) and the
overlapping mode of the phenalenyl rings are similar, strongly
suggesting that the most dominant interaction controlling the
crystal packing of 2a as well as 1a is the intermolecular bonding
interaction due to the phenalenyl overlap. Recently the bonding
interaction in a π-dimer of phenalenyl radical itself has been
referred to as multicenter bonding,17 which has attracted
attention as a new type of chemical bond. A dominant attractive
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Covalent Bonding Chain of Singlet Biradical Molecule
A R T I C L E S
force in the multicenter bonding is the dispersion force, whereas
a covalent bonding interaction is also operative due to a large
SOMO-SOMO overlap, leading to the π-π overlap at a
distance less than the sum of the van der Waals radius of the
carbon atom. In the phenalenyl overlap of 2a, the distances of
the seven short-contact pairs of carbon atoms are much less
than the sum of the van der Waals radius of carbon atom (Figure
3a), and furthermore, the peripheral R-carbons,18 which bear
large spin densities, show shorter contacts than the central carbon
(C19 in Figure 2a) with no spin density. This overlapping pattern
suggests the appreciable intermolecular covalent bonding in-
teraction, as demonstrated in the recent theoretical study by
Kertesz.17d
The larger spin densities on the phenalenyl rings of 2a than
those of 1a should give a more enhanced attractive force in the
phenalenyl overlap and, naturally, would reduce the π-π
separation distance. However, the distance found in 2a was
slightly larger than that of 1a. Head-Gordon et al. performed a
CP-MRMP2 calculation to estimate the potential energy surface
of the phenalenyl radical π-dimer itself as a function of the
separation distance and found a global minimum for the
geometry at the distance of around 3.1 Å.19 The potential energy
surface is relatively shallow at the minimum point, and the
phenalenyl radical π-dimer can take various separation distances
with only small changes in total energy, whereas the potential
energy increases steeply with the distance decreasing below 3.0
Å. This theoretical finding indicates that the attractive forces
of the intermolecular covalent bonding interaction as well as
dispersion are well balanced against an interatomic repulsion
at the separation distance of ∼3.1 Å. Therefore, a stronger
intermolecular covalent bonding interaction in the 1D chain of
2a does not necessarily give a shorter separation distance.
Here, we should consider a C-C σ-bond formation, which
most organic radicals experience. Actually phenalenyl radical
has a more stabilized dimeric structure, σ-dimer, which is
supported experimentally20 and theoretically.21 Preference of
the π-dimeric structure in 2a indicates that there is still an
adequate intramolecular bonding interaction of electrons through
the naphthalene linker so as to reduce the spin densities on the
phenalenyl rings. Considering the condition-dependent π-/σ-
dimerization of a spirobiphenalenyl radical (octyl derivative),22
the actual spin densities of 2a on the phenalenyl rings can be
comparable to or less than those of the spirobiphenalenyl system,
which are almost half of those of phenalenyl radical itself.
Figure 4. Optical conductivity of 2a obtained with light on the (0 0 1)
surface (solid line) and absorption spectrum of 2b in hexane-CH2Cl2
(dashed line).
The covalent bonding interaction between the molecules
would affect the strength of the intramolecular covalent bonding
interaction. We focus on the length of the bonds denoted by a
of 2 in Scheme 1, which connect the phenalenyl rings and the
naphthalene ring, because the bond length is the most sensitive
to the contribution weight of each canonical structure in the
resonance formula (the bond a has the bond order of 1.5 in the
Kekule´ structure, whereas single bond character in the biradical
structure). The length of the bond a of 2a is longer (1.471(5)
Å) than that of 2b (1.465(7) Å),11 suggesting that the covalent
bonding interaction between the molecules weakens the in-
tramolecular one in 2a.23 Similar behavior has been observed
in 1b when the separation distance between the phenalenyl rings
is altered with various temperatures.10b On the other hand, the
bond length of 2a is shorter than the length of the corresponding
bond in 7,16-diphenylfluorantheno[8,9-k]fluoranthene (1.481(2)
Å),15 suggesting that 2a still has the adequate covalent bonding
interaction within the molecule.
Optical Property. Unlike usual intermolecular attractive forces
such as electrostatic, polarization, and dispersion, the multicenter
bonding mostly contributes to a large orbital overlap between
molecules, because the multicenter bonding involves in-phase
bonding interaction of partially occupied molecular orbitals. In
the case of 1a and 1b, strong covalent bonding interactions of
electrons through the multicenter bonding as well as conven-
tional intramolecular π-conjugation have resulted in linear
conjugation in their 1D chains, and consequently the lower-
energy shifts of HOMO-LUMO bands are observed.10
A similar shift was also observed for 2a in the solid state.
Figure 4 shows the optical conductivity of a single crystal of
2a along with the solution spectrum of 2b.24 The optical
conductivityspectrum,whichwasobtainedbytheKramers-Kronig
transformation of the reflection spectrum measured with light
(16) Goto, K.; Kubo, T.; Yamamoto, K.; Nakasuji, K.; Sato, K.; Shiomi,
D.; Takui, T.; Kubota, M.; Kobayashi, T.; Yakusi, K.; Ouyang, J.
J. Am. Chem. Soc. 1999, 121, 1619–1620. For a recent review, see:
Morita, Y.; Nishida, S. In Stable Radicals: Fundamental and Applied
Aspects of Odd-electron Compounds; Hicks, R., Ed.; Wiley-Blackwell:
New York, 2010; Chapter 3.
(17) (a) Suzuki, S.; Morita, Y.; Fukui, K.; Sato, K.; Shiomi, D.; Takui, T.;
Nakasuji, K. J. Am. Chem. Soc. 2006, 128, 2530–2531. (b) Mota, F.;
Miller, J. S.; Novoa, J. J. J. Am. Chem. Soc. 2009, 131, 7699–7707.
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132, 10648–10649.
(18) The carbon atoms of phenalenyl bearing a large coefficient in SOMO
are referred to as the R-carbon.
(23) Huang, L.; Kertesz, M. J. Am. Chem. Soc. 2007, 129, 1634–1643.
Covalent bonding interactions for 1a based on structural consideration
have been discussed in this literature. Similar discussion is also seen
in the literature. Chi, X.; Itkis, M. E.; Tham, F. S.; Oakley, R. T.;
Cordes, A. W.; Haddon, R. C. Int. J. Quantum Chem. 2003, 95, 853–
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Kochi, J. K. J. Am. Chem. Soc. 2004, 126, 13850–13858.
(20) (a) Reid, D. H. Q. ReV., Chem. Soc. 1965, 19, 274–302. (b) Gerson,
F. HelV. Chim. Acta 1966, 49, 1463–1467. (c) Zaitsev, V.; Rosokha,
S. V.; Head-Gordon, M.; Kochi, J. K. J. Org. Chem. 2006, 71, 520–
526.
(24) Due to the extremely low solubility of 2a, the monomeric properties
of 2a could not be measured. Therefore the solution absorption band
of 2a was estimated from that of 2b, which had tert-butyl groups on
the phenalenyl rings. It is noted that the tert-butyl groups effectively
prohibit the phenalenyl overlap between the neighboring molecules
even in a solid state. See ref 11.
(21) Small, D.; Rosokha, S. V.; Kochi, J. K.; Head-Gordon, M. J. Phys.
Chem. A 2005, 109, 11261–11267.
(22) Liao, P.; Itkis, M. E.; Oakley, R. T.; Tham, F. S.; Haddon, R. C. J. Am.
Chem. Soc. 2004, 126, 14297–14302.
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J. AM. CHEM. SOC. VOL. 132, NO. 41, 2010 14425

A R T I C L E S
Shimizu et al.
Figure 5. ꢁT-T plot of the solid 2a. Measured data are plotted as open
circles. Theoretical curves are drawn using the alternating Heisenberg chain
model with the parameters of Jintra ) -1200 K (fixed), Jinter ) -1300 to
-2200 K, impurity spin contamination ) 6.15%, g ) 2.00, and diamagnetic
Figure 6. Compressed pellet conductivity of 2a. The measured data are
plotted as open circles and the theoretical line is drawn in red.
susceptibility ) -562 × 10-6 emu mol-1
.
interactions; the intramolecular exchange interaction (Jintra) of
-2300 K and the intermolecular one (Jinter) of -3400 K. The
negligible spin susceptibility of 2a indicates that there is too
strong a spin-spin pairing interaction in the 1D chain to detect
changes in the susceptibility at least up to 350 K, and that the
strong interaction would be more negative than -3400 K. In
order to estimate the minimum limit of the strong spin-spin
interaction, we also used the antiferromagnetic 1D Heisenberg
chain model for the 1D chain of 2a.26 In the previous work, we
have determined the Jintra of -1900 K for 2b, which is in an
isolated monomer state (that is, Jinter ) 0) due to the bulky tert-
butyl groups. On the basis of the discussion on the bond length
of a, the Jintra for 2a in the 1D chain would be around -1200
K. Using the Jintra of -1200 K, we could deduce the Jinter of
more negative than -2000 K (Figure 5), but a more accurate
estimation of the Jinter needs further examination with the help
of theoretical calculations.
on the (0 0 1) surface of the single crystal, gave an extremely
lower-energy shifted band centered at 7822 cm-1 with respect
to the solution absorption band of 2b, indicating that the
intermolecular covalent bonding interaction affects the electronic
structure of the molecule in the 1D chain.
It should be noted that the shifted band of 2a is located at a
higher-energy region relative to that of 1a (6804 cm-1). It has
been shown that the 1D chain of 1a possesses a stronger covalent
bonding interaction between the molecules than within the
molecule.23 Extrapolation of this result allows us to presume
that the 1D chain of 2a also has a similar state but with a more
enhanced intermolecular covalent bonding interaction, because
2 has larger spin densities on the phenalenyl rings than 1 on
the basis of theoretical considerations. The higher-energy band
of 2a relative to that of 1a can be explained in that the intra-
and intermolecular covalent bonding interactions are more
unbalanced in strength in the 1D chain of 2a. The more
enhanced bonding alternation causes a greater separation in
energy between HOMO and LUMO of the 1D chain. In other
words, compared with 1a, the R-spin electron of 2a is more
localized at one phenalenyl ring while the ꢀ-spin electron is
more localized at the other (see Figure S2, Supporting Informa-
tion), and the resulting larger spin densities demand a stronger
intermolecular covalent bonding interaction, and consequently
more unbalanced covalent bonding interactions afford a larger
energy gap between the valence and conduction bands.25
Magnetic Measurements. The SQUID measurements of the
polycrystals of 2a showed basically diamagnetic behavior in
the temperature range of 30-350 K. Small positive ꢁT values
would be derived from a small amount of monoradical impuri-
ties. The ꢁT values are almost constant above 100 K (Figure
5), in high contrast to the increasing ꢁT values of 1a above 200
K.10a Huang and Kertesz have theoretically estimated the
magnetic interactions of the 1D chain of 1a using the antifer-
romagnetic 1D Heisenberg chain model.23 The theoretical study
reveals that the 1D chain of 1a has relatively balanced
Electroconductive Behavior. In the crystal, 2a forms a linear
conjugation that is made of alternating intra- and intermolecular
covalent bonding interactions, and π-electrons could delocalize
along the linear 1D chain. We measured electroconductivity of
the compressed pellet of the recrystallized 2a using a two-probe
method. The room-temperature conductivity (σrt) was 5 × 10-5
S·cm-1 with an activation energy (Ea) of 0.2 eV at 180-286
K (Figure 6). This semiconductive behavior is similar to that
of 1a (σrt ) 1 × 10-5 S cm-1, Ea ) 0.3 eV) although there is
more enhanced bonding alternation in 2a. In order to estimate
the electronic structure in the solid state, we performed a
preliminary band structure calculation using the extended Hu¨ckel
theory (EHT) for the crystal of 2a.27,28 The dispersions of
valence and conduction bands were found to be very large (0.47
and 0.42 eV, respectively) along the 1D chain (X), whereas slight
dispersions were also seen along Y (Figure 7). The smaller
activation energy for 2a than 1a would be ascribable to the two-
dimensionality of the band structure due to the slight π-π
contact between the 1D chains of 2a in contrast to the one-
(25) The direction of the transition, which gives direct information about
the relative strength of the intra- and intermolecular covalent bonding
interactions, could not be determined due to the crystal system with
a four-fold screw axis, I41/cd (See Figure S1, Supporting Information).
(26) Johnston, D. C.; Kremer, R. K.; Troyer, M.; Wang, X.; Klu¨mper, A.;
Bud’ko, S. L.; Panchula, A. F.; Canfield, P. C. Phys. ReV. B 2000,
61, 9558–9606.
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14426 J. AM. CHEM. SOC. VOL. 132, NO. 41, 2010

Covalent Bonding Chain of Singlet Biradical Molecule
A R T I C L E S
contribute to the quest for exotic genuinely organic molecular
functionalities such as purely organic magnetism involving
modification of singlet wave functions.30
Experimental Section
Materials and Methods. All experiments with moisture- or air-
sensitive compounds were performed in anhydrous solvents under
an argon atmosphere in well-dried glassware. Dichloromethane,
toluene, benzene, and DMF were dried over calcium hydride and
distilled prior to use. Ethanol was dried over magnesium ethoxide
and distilled prior to use. 2,4,6-Trimethylpyridine was obtained from
a commercial source and was used without further purification.
Column chromatography was performed with silica gel [Wako gel
C-200 (Wako)]. Infrared spectra were recorded on a JASCO FT/
IR-660M spectrometer. The electronic absorption spectrum was
measured by a Shimadzu UV-3100PC spectrometer. 1H NMR
spectra were obtained on JEOL EX-270 spectrometers. FAB mass
spectra were taken using JEOL JMS SX-102 mass spectrometers.
The polarized reflection spectrum in the infrared and visible regions
was observed using two spectrometers combined with a microscope:
FT-IR spectrometer Nicolet Magna 760 (600-12000 cm-1) and
multichannel detection system Atago Macs 320 (11000-30000
cm-1). Absolute reflectivity was determined by comparing the
reflected light from a gold mirror and silicon single crystal,
respectively. A single crystal of 2a was fixed with silicon grease
on a copper sample holder, and the crystal face was adjusted so as
to be normal to the incident light by use of goniometer head.
Temperature-dependent magnetic susceptibility was measured for
randomly oriented polycrystalline samples of 2a on a Quantum
Design SQUID magnetometer MPMS-XL with an applied field of
0.1 T in the temperature range of 30-350 K. Below 30 K, the sum
of the diamagnetic susceptibilities derived from 2a and a sample
tube is comparable to the paramagnetic susceptibility of monoradical
impurities, leading to uncertainty of the magnetic susceptibility
measured. Gradual decrease of the ꢁT values below ∼100 K would
be explained by the reasons.
1
Figure 7. Band structure of 2a along X (1/2, 0, 0) and Y (0, /2, 0) on a
(0 0 1) sheet (layer 1 in Figure S1, Supporting Information). The dotted
line represents the Fermi level.
dimensional band structure of 1a. The dispersion along Z (0, 0,
1/2) should be very small because there is no π-π contact in its
direction.
Conclusion
We have demonstrated that the spin-localizing nature on the
phenalenyl rings in the singlet biradical 2a is relevant to the
intermolecular covalent bonding interaction, which dominates
the crystal structure to afford the 1D chain. The 1D chain
possesses alternating intra- and intermolecular covalent bonding
interactions, and the linear conjugation leads to the lower-energy
shift in the optical band and to the semiconductive property.
The intermolecular covalent bonding interaction is stronger than
the intramolecular one, and the bonding alternation is more
enhanced in 2a than 1a, leading to the wider energy gap of 2a
between the valence and conduction bands.
It is a well-established concept that a covalent bond is a
chemical bond that is formed by a pairing of two electrons.29
Usually, the electron pairing is confined within a single molecule
frame to form a closed-shell ground state. However, once the
pair is weaken by forming an aromatic sextet, or more generally
by rotating a double bond or stretching a σ-bond, the partially
unpaired electron demands a second pairing and mostly
contributes to a σ-bond formation between molecules with loss
of the original pairing. Electron delocalization at the spin-bearing
reactive center leads to suppression of the σ-bond formation,
and instead, electrons are weakly paired to form a long covalent
bonding including multicenter bonding along with partial
retention of the original pairing. In this way coexistence of intra-
and intermolecular covalent bonding interactions is established
in molecular assemblages, being one of the salient features of
singlet biradical molecules. The present hydrocarbon-based
singlet biradical 2a also gives a testing ground for the occurrence
of multicenter bonding in molecular assemblages and for their
role in terms of novel chemical bonding. Alternating covalent
bonding interactions based on molecular assemblages will
7,8,15,16-Tetraphenylfluoratheno[8,9-k]fluoranthene (3). The
compound 3 was synthesized according to the previously reported
procedures.11,15
3,11-/3,12-Dibromo-7,8,15,16-tetraphenylfluoratheno[8,9-k]fluo-
ranthene (4). To a solution of 3 (2.279 g, 3.35 mmol) in
dichloromethane (2000 mL) was added bromine (1.12 g, 7.03
mmol), and the reaction mixture was stirred for 4 h at room
temperature. After the addition of aq 10% Na2SO3, the organic layer
was separated and washed with brine, dried over Na2SO4 and
filtered, and the solvent was removed in Vacuo. The dibromo
compounds 4 (2.580 g, 92%) were obtained as a vivid yellow
powder. Mp >300 °C. TLC Rf 0.76 [hexane/benzene (1:1, v/v)].
1H NMR (270 MHz, CDCl3) δ 7.69 (d, J ) 8.4 Hz, 2H), 7.26 (d,
J ) 7.9 Hz, 2H), 7.15-6.94 (m, 22H), 5.65 (d, J ) 6.9 Hz, 2H),
5.41 (d, J ) 7.9 Hz, 2H). FAB MS m/z 836 (M+). Anal. Calcd for
C54H30Br2: C, 77.34; H, 3.61. Found: C, 77.17; H, 3.61.
3,11-/3,12-Bis(2-methoxycarbonylethenyl)-7,8,15,16-tetraphe-
nylfluorantheno[8,9-k]fluoranthene (5). A mixture of 4 (2.58 g, 3.08
mmol), Pd(CH3CN)2Cl2 (31.5 mg, 0.12 mmol)), Ph4PCl (277 mg,
0.74 mmol), and AcONa (1.01 g, 12.3 mmol) in DMF (200 mL)
was degassed and purged with argon three times. Methyl acrylate
(1.66 mL, 18.5 mmol) was added to the mixture, and the mixture
was heated to 140 °C for 7.5 h. After cooling, the reaction mixture
was filtered and concentrated in Vacuo to give 5 (2.61 g) as a vivid
reddish orange powder. This material was used for the next reaction
(27) The band structure calculation was performed using the YAeHMOP
package, which is freely available on the Web at http://sourceforge.net/
projects/yaehmop/.
(28) The band structure calculation based on the EHT is a restricted method,
which may not be appropriate for singlet biradical species. Instead, a
calculation including an adequate electron-electron correlation may
give proper results.
1
without further purification. TLC Rf 0.36 (dichloromethane). H
NMR (270 MHz, CDCl3) δ 8.33 (d, J ) 16 Hz, 2H), 7.91 (d, J )
8.2 Hz, 2H), 7.44 (d, J ) 7.7 Hz, 2H), 7.25-7.02 (m, 22 H), 6.43
(29) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University: Ithaca, NY, 1960.
(30) Shiomi, D.; Sato, K.; Takui, T. J. Phys. Chem. B 2001, 105, 2932–
2938.
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J. AM. CHEM. SOC. VOL. 132, NO. 41, 2010 14427

A R T I C L E S
Shimizu et al.
(d, J ) 16 Hz, 2H), 5.74 (d, J ) 7.1 Hz, 2H), 5.70 (d, J ) 7.7 Hz,
2H), 3.79 (s, 6H). FAB MS m/z 849 ([M+H]+). IR (KBr) 1706
(m, 2H), 2.25-2.00 (m, 4H), 1.67, 1.65 (s, 2H). FAB MS m/z 792
(M+). IR (KBr) 3434 cm-1. Anal. Calcd for C60H40O2: C, 90.88;
H, 5.08. Found: C, 90.84; H, 5.17.
cm-1
.
1,10-/1,12-Dihydro-6,7,15,16-tetraphenyldicyclopenta[b,g]naph-
thaleno[1,2,3-cd;6,7,8-c′d′]diphenalene (10). To a solution of 9 (100
mg, 0.126 mmol) in toluene (50 mL) heated to 90 °C, a catalytic
amount of p-toluenesulfonic acid monohydrate was added, and the
reaction mixture was heated at 90 °C for 30 min. The mixture was
cooled in an ice-bath. The crude product was purified by column
chromatography on silica gel (toluene) to give 10 (83.0 mg, 87%)
as an air-sensitive dark yellowish brown powder. TLC Rf 0.36
[hexane/benzene (2:1, v/v)]. 1H NMR (270 MHz, CDCl3) δ
7.20-7.05 (m, 20 H), 6.95 (dt, J ) 7.6, 1.9 Hz, 2H), 6.71 (d, J )
7.3 Hz, 2H), 6.62 (dt, J ) 9.9, 2.2 Hz, 2H), 6.10 (dt, J ) 9.9, 4.0
Hz, 2H), 5.68 (d, J ) 7.6 Hz, 2H), 5.63 (d, J ) 7.3 Hz, 2H),
3.84-3.77 (m, 4H). FAB MS m/z 756 (M+).
6,7,15,16-Tetraphenyldicyclopenta[b,g]naphthaleno[1,2,3-cd;6,7,8-
c′d′]diphenalene (2a). To a solution of 10 (83.0 mg, 0.110 mmol)
in benzene (80 mL) heated at 80 °C, a solution of p-chloranil (29.7
mg, 0.121 mmol) in benzene (10 mL) was added. The reaction
mixture was slowly cooled to room temperature, and the resulting
crystals were collected and washed with acetone to give 2a (68.6
mg, 83%) as dark purple plates. Mp >300 °C (in a sealed tube).
TLC Rf 0.55 [hexane/benzene (1:1, v/v)]. FAB MS m/z 755
([M+H]+). Anal. Calcd for C60H34: C, 95.46; H, 4.54. Found: C,
95.15; H, 4.73.
3,11-/3,12-Bis(2-methoxycarbonylethyl)-7,8,15,16-tetraphenylflu-
orantheno[8,9-k]fluoranthene (6). A mixture of 5 (2.61 g, 3.08
mmol), zinc powder (10.0 g, 154 mmol), acetic acid (260 mL),
and toluene (260 mL) was refluxed for 41 h. After cooling, the
reaction mixture was filtered. The filtrate was washed with saturated
aqueous NaHCO3 and brine, dried over Na2SO4, and filtered. After
column chromatography on silica gel (dichloromethane), 6 (1.87
g, 71%, 2 steps) was obtained as a vivid yellow powder. Mp
1
273.0-285.5 °C. TLC Rf 0.38 (dichloromethane). H NMR (270
MHz, CDCl3) δ 7.71 (d, J ) 8.2 Hz, 2H), 7.23-7.03 (m, 22 H),
6.94 (d, J ) 7.4 Hz, 2H), 5.71, 5.70 (d, J ) 7.2 Hz, 2H), 5.62,
5.61 (d, J ) 7.4 Hz, 2H), 3.62 (s, 6H), 3.28 (d, J ) 7.9 Hz, 4H),
2.63 (d, J ) 7.9 Hz, 4H). FAB MS m/z 853 ([M+H]+). IR (KBr)
1739 cm-1. Anal. Calcd for C62H44O4: C, 87.30; H, 5.20. Found:
C, 86.92; H, 5.07.
3,11-/3,12-Bis(2-carboxyethyl)-7,8,15,16-tetraphenylfluoran-
theno[8,9-k]fluoranthene (7). A mixture of 6 (563 mg, 0.659 mmol),
lithium iodide (1.77 g, 13.2 mmol), and 2,4,6-trimethylpyridine (55
mL) was heated to 185 °C for 3 h. After cooling, the reaction
mixture was concentrated in Vacuo, and 2 M hydrochloric acid was
added. The solid was collected and washed with water to give 7
(551 mg) as a deep yellow powder. This material was used for the
1
next reaction without further purification. Mp >300 °C. H NMR
Computational Methods. DFT calculations were performed with
the Gaussian 03 program.31 All geometry optimizations were carried
out at the B3LYP level of density functional theory with the
6-31G** basis set. Singlet biradical character was estimated using
a CASSCF(2,2) method in the RB3LYP optimized geometry, and
using a broken-symmetry UB3LYP/6-31G** method along with
geometry optimization. A band structure calculation was performed
with an extended HMO method using the YAeHMOP package27
in the X-ray crystallographic geometry.
Crystal Data for 2b. C60H34, M ) 754.87, tetragonal, space group
I41/cd, a ) 12.4649(5) Å, c ) 47.8243(16) Å, V ) 7430.6(5) Å3,
Z ) 8, µ(Mo KR) ) 0.077 cm-1, Fcalcd ) 1.350 g cm-3, T ) 200
K, R1(wR2) ) 0.067 (0.160) for 272 parameters and 2490 unique
reflections with I > 2σ(I), GOF ) 1.06. Data collection for X-ray
crystal analysis was performed on Rigaku/Varimax diffractometer
(Mo KR, λ ) 0.710 69 Å). The structure was solved with direct
methods and refined with full-matrix least-squares.
(270 MHz, DMSO-d6) δ 7.82 (d, J ) 8.4 Hz, 2H), 7.25-7.04 (m,
22 H), 6.97 (d, J ) 7.6 Hz, 2H), 5.51, 5.50 (d, J ) 7.2 Hz, 2H),
5.41, 5.40 (d, J ) 7.6 Hz, 2H), 3.17 (t, J ) 7.2 Hz, 4H), 2.51 (t,
J ) 7.2 Hz, 4H). FAB-MS m/z 825 ([M+H]+). IR (KBr) 3414,
1704 cm-1
.
1,2,3,10,11,12-Hexahydro-1,10-/1,12-dioxo-6,7,15,16-tetraphenyl-
dicyclopenta[b,g]naphthaleno[1,2,3-cd;6,7,8-c′d′]diphenalene (8). A
mixture of 7 (551 mg, 0.659 mmol) and oxalyl chloride (30 mL)
was refluxed for 2 h. The reaction mixture was cooled and
concentrated in Vacuo. The resulting solid was dissolved in
dichloromethane (55 mL) and was cooled to -78 °C. Aluminum
chloride (464 mg, 3.48 mmol) was added, and the reaction mixture
was allowed to warm to -30 °C over 2.5 h. After the reaction
mixture was poured into ice-cold water, 2 M hydrochloric acid was
added, and the organic layer was separated. The aqueous layer was
extracted with dichloromethane. The combined organic layer was
washed with saturated aqueous NaHCO3 and brine, dried over
Na2SO4, and filtered. After column chromatography on silica gel
(dichloromethane), 8 (369 mg, 71%, 2 steps) was obtained as a
vivid orange powder. Mp >300 °C. TLC Rf 0.69 [hexane/ethyl
acetate (1:1, v/v)]. 1H NMR (270 MHz, CDCl3) δ 7.75 (d, J ) 7.6
Hz, 2H), 7.25-7.00 (m, 22 H), 5.78, 5.77 (d, J ) 7.6 Hz, 2H),
5.66, 5.65 (d, J ) 7.3 Hz, 2H), 3.30 (t, J ) 6.8 Hz, 4H), 2.88 (t,
J ) 6.8 Hz, 4H). FAB MS m/z 789 ([M+H]+). IR (KBr) 1685
cm-1. Anal. Calcd for C60H36O2: C, 91.34; H, 4.60. Found: C, 91.01;
H, 4.56.
1,2,3,10,11,12-Hexahydro-1,10-/1,12-dihydroxy-6,7,15,16-tet-
raphenyldicyclopenta[b,g]naphthaleno[1,2,3-cd;6,7,8-c′d′]diphen-
alene (9). A mixture of 8 (339 mg, 0.43 mmol), sodium boron
hydride (48.8 mg, 1.29 mmol), and dichloromethane (80 mL) and
ethanol (32 mL) was stirred for 1.5 h at room temperature. Sodium
boron hydride (32.5 mg, 0.86 mmol) was added to the reaction
mixture, and the mixture was stirred for an additional 1.5 h at room
temperature. After addition of water and 2 M HCl, the organic layer
was separated, and the aqueous layer was extracted with dichlo-
romethane. The combined organic layer was washed with saturated
aqueous NaHCO3 and brine, dried over Na2SO4, and filtered to give
9 (355 mg, 100%) as a vivid orange powder. Mp >300 °C. TLC Rf
0.58 [dichloromethane/ethyl acetate (1:1, v/v)]. 1H NMR (270 MHz,
CDCl3) δ 7.23-7.03 (m, 22 H), 6.88 (d, J ) 7.3 Hz, 2H), 5.67 (d,
J ) 7.3 Hz, 2H), 4.96 (m, 1H), 3.21-3.07 (m, 2H), 2.97-2.84
Acknowledgment. This work was supported in part by Yamada
Science Foundation (T.K.), the Grants-in-Aid for Scientific Research
on Innovative Areas (No. 2105, T.K.), Grants-in-Aid for Scientific
Research (No. 18350007, M.N.) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan, PRESTO-JST
(Y.M. and D.S.), CREST-JST (K.S. and T.T.), and the Global COE
program “Global Education and Research Center for Bio-
Environmental Chemistry” of Osaka University (A.S.). A.S.
acknowledges the JSPS Fellowship for Young Scientists.
Supporting Information Available: Crystal packing of 2a,
HOMOs of R-spin and ꢀ-spin electrons for 2 calculated with a
broken-symmetry UB3LYP method, input files for the Gaussian
calculation, details of crystallographic data collection and
structure refinement, tables of atomic coordinates, bond distances
and angles, and isotropic thermal parameters in CIF format, and
complete ref 31. This material is available free of charge via
the Internet at http://pubs.acs.org.
JA1037287
(31) Frisch, M. J.; et al. Gaussian 03, revision D.01; Gaussian, Inc.:
Wallingford, CT, 2004.
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14428 J. AM. CHEM. SOC. VOL. 132, NO. 41, 2010