Indeno[2,1-b]fluorene: A 20-π-electron hydrocarbon with very low-energy light absorption

Article abstract of DOI:10.1002/anie.201302091

Smaller can be better: The first example of meta-quinodimethane embedded in an indenofluorene framework has been synthesized. 10,12-Dimesitylindeno[2,1-b] fluorene exhibits extremely low-energy light absorption, despite the small conjugation space of the molecule, which consists of only 20 πelectrons. Copyright

Full text of DOI:10.1002/anie.201302091

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DOI: 10.1002/anie.201302091
Polycyclic Hydrocarbons
Indeno[2,1-b]fluorene: A 20-p-Electron Hydrocarbon with Very Low-
Energy Light Absorption**
Akihiro Shimizu, Ryohei Kishi, Masayoshi Nakano, Daisuke Shiomi, Kazunobu Sato,
Takeji Takui, Ichiro Hisaki, Mikiji Miyata, and Yoshito Tobe*
The study of low-energy light-absorbing molecules based on
a topological alignment design for conjugated p electrons in
organic molecules has attracted much interest in connection
with functional dyes and materials for information storage.[1,2]
There are two well-established design principles for low-
energy light-absorbing hydrocarbons: 1) to extend the p-
conjugation space for alternant hydrocarbons to reduce the
energy gap between the frontier molecular orbitals (MOs), as
in relatively small acenes[3] and rylenes[4] and 2) to feature
non-alternant hydrocarbons such as azulenes[5,6] (Figure 1).
Furthermore, large singlet biradical character contributes as
well because of the small energy gap between the frontier
MOs,[7–12] as in relatively large acenes,[9] biphenalenes,[10]
anthenes,[11] and zethrenes[12] (Figure 1). As all of these
molecules generally exhibit lower-energy light absorption
with increasing p-conjugation space,[3,4,6,10–12] extension of
these p-conjugation spaces leads to lower energy light
absorption (Figure 1). In contrast, we herein report that an
indeno[2,1-b]fluorene derivative, a non-alternant hydrocar-
bon with a moderate singlet biradical character, exhibits an
extremely low-energy light absorption band whose absorption
end extends to 2000 nm, despite its small conjugation space.
We targeted indeno[2,1-b]fluorene (1a), because deriva-
tives of its structural isomers, indeno[2,1-a]fluorene (2a),[13,14]
indeno[1,2-b]fluorenes (3a),[15] and indeno[2,1-c]fluorene
(4a)[16] have been intensively studied with respect to their
unique optical properties, which are associated with their
singlet biradical characters and low-energy band gaps
(Figure 2). 1a is expected to have a larger singlet biradical
character than its isomers due to the meta-quinodimethane
subunit, in contrast to the ortho- and para-quinodimethane
structures. Indeed, theoretical calculations show that 1a has
a moderate singlet biradical character (y = 0.68 calculated by
the Yamaguchi scheme[17] using the occupation numbers of
the spin-unrestricted Hartree–Fock natural orbitals[18]), which
is larger than that of 2a (y = 0.33).[14] The spin density
distribution for 1a shows that the C10 and C12 carbons have
the largest amplitudes (Figure 3). These results indicate that
1a should be described as the resonance for the Kekulꢀ and
biradical structures (Figure 2a).
We synthesized 10,12-dimesitylindeno[2,1-b]fluorene
(1b), wherein mesityl groups were introduced to stabilize
the reactive sites through steric protection (Scheme 1). 1b
was obtained as a green solid in two steps by addition of
mesitylmagnesium bromide to the known diketone 5[19]
followed by dehydroxylation of 6 with tin(II) chloride. In
solution, 1b gradually decomposes, in contrast to 2b,[14]
indicating that 1b may have a larger singlet biradical
Figure 1. Representative hydrocarbons that exhibit low-energy light
absorptions that decrease with increasing p-conjugation space.
[*] Dr. A. Shimizu,[+] Prof. Y. Tobe
[**] This work was supported by Grants-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science and
Technology (Japan), “Quantum Cybernetics”, FIRST project on QIP,
JSPS and a Sasakawa Scientific Research Grant from the Japan
Science Society. The authors acknowledge Prof. T. Kubo of Osaka
University (Japan) for the use of facilities and valuable discussions,
Prof. S. Ito and J. Takaichi of Osaka University (Japan) for the use of
a spectrometer, and Prof. B. Champagne of Facultꢀs Universitaires
Notre-Dame de la Paix (FUNDP), Belgium for the use of computers
and the program for the theoretical calculations. The synchrotron
radiation experiments were performed at the BL38B1 of SPring-8
with the approval of the Japan Synchrotron Radiation Research
Institute (JASRI; proposal No. 2011A1341). The authors are grateful
to Dr. K. Miura, Dr. S. Baba, and Dr. N. Mizuno for crystallographic
data collection.
Division of Frontier Materials Science, Graduate School of Engi-
neering Science, Osaka University, 1-3 Machikaneyama
Toyonaka, Osaka 560-8531 (Japan)
E-mail: tobe@chem.es.osaka-u.ac.jp
Dr. R. Kishi, Prof. M. Nakano
Department of Materials Engineering Science, Graduate School of
Engineering Science, Osaka University, Osaka (Japan)
Dr. D. Shiomi, Prof. K. Sato, Prof. T. Takui
Department of Chemistry, Graduate School of Science, Osaka City
University, Osaka (Japan)
Dr. I. Hisaki, Prof. M. Miyata
Department of Material and Life Science, Graduate School of
Engineering, Osaka University, Osaka (Japan)
[+] Present address: Department of Synthetic and Biological Chemistry,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302091.
Graduate School of Engineering, Kyoto University, Kyoto (Japan)
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Figure 4. Crystal structure of 1b (measured at 113 K). a) Top view.
b) Side view. Displacement ellipsoids are drawn at the 50% probability
level.
Figure 2. Structures of indenofluorenes 1–4. a) Resonance structures
of 1. The arrows in the biradical structure represent antiparallel spins.
b) Structures of 2–4. Mes=2,4,6-trimethylphenyl.
The small singlet–triplet energy gap in 1b, which arises
from the moderate singlet biradical character (y), was
revealed experimentally. The temperature-dependent
1H NMR spectra for 1b showed no signal at + 308C because
of a thermally excited triplet species. However, after cooling
to À938C, broad signals were observed (Figure 5). Although
Figure 3. Spin density distribution of 1a calculated at the UB3LYP/
6-31G(d) level.
Figure 5. Temperature-dependent 1H NMR spectra of 1b in CD2Cl2.
The peak at 5.3 ppm is due to CDHCl2, and those at 5.55 and 5.1 ppm
are due to its sidebands. Peaks at 1.55–1.75 ppm may be due to H2O.
Scheme 1. Synthesis of 1b. Mes=2,4,6-trimethylphenyl, TFA=
trifluoroacetic acid.
assignment was not possible owing to the broadness of the
signals, they appeared at relatively high field (5.6–6.9 ppm).
This is probably due to the antiaromatic character of the
s-indacene moiety, as indicated by positive nucleus-independ-
ent chemical shift calculations (Table S8). Moreover, a triplet
species for 1a was observed by ESR spectroscopy (Fig-
ure S4a). With decreasing the temperature, the intensity of
the ESR signal decreased, which indicates a singlet ground
state in 1b (Figure S4b). The singlet-triplet energy gap was
estimated to be À2120 K (À17.6 kJmolÀ1) by temperature-
dependent magnetic susceptibility measurements (Figure S5),
which is reasonable for the DES-T for 1b, which was
theoretically estimated at À1240 K (À10.3 kJmolÀ1) at the
UB3LYP/6-31G(d) level of theory. The stabilization of the
singlet state of 1b relative to the triplet state by the
indenofluorene framework should be contrasted with
character. X-ray crystal structural analysis (Figure 4; see also
the Supporting Information, Figure S3)[25] of 1b provides
crucial information on the singlet biradical character of 1b,
because the bond lengths reflect the contribution of the
canonical structures.[20] The observed lengths for bonds
a (1.437 ꢁ) and b (1.431 ꢁ) (Figure 2a for the bond posi-
tions), which agree with the theoretical values (Table S7), lie
between the lengths of the C(sp2)-C(sp2) bond in benzene
(1.39 ꢁ) and the C(sp2)-C(sp3) bond in fluorene (1.468 ꢁ).[21]
This observation indicates that 1b has a moderate contribu-
tion of the singlet biradical canonical structure to the ground-
state electronic configuration.
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m-xylylene, the triplet ground state of which lies well below
the singlet.[22] The small energy gap between the frontier MOs
was supported by the energy difference between the first
oxidation potential and the first reduction potential (Eredox
)
measured using cyclic voltammetry. The cyclic voltammo-
gram for 1b showed two reversible and two irreversible redox
waves (E2ox,pa =+ 0.90, E1ox =+ 0.13, E1red = À1.13, and
E2red,pc = À2.03 V vs. Fc/Fc+; Figure S6). The measured Eredox
value of 1.26 eV is consistent with the HOMO–LUMO
energy gap (1.23 eV, HOMO: À4.37 eV, LUMO: À3.14 eV)
calculated at the RB3LYP/6-31G(d)//UB3LYP/6-31G(d)
level, and is less than the typical value for closed-shell
hydrocarbons.
The electronic absorption spectrum for 1b in dichloro-
methane showed a strong absorption band at 638 nm and
a weak absorption band at 850–2000 nm (Figure 6). Near-
infrared absorption other than between singlet states may be
observed because of 1) absorption contributed by impurities
such as the 1b radical cation, 2) intermolecular charge-
transfer (CT) absorption, 3) singlet-to-triplet absorption,
and 4) absorption of thermally excited triplet species.
However, the following observations rule out these
possibilities: 1) The time dependence of the electronic
absorption spectrum showed that the lowest-energy
band at 1700 nm diminished simultaneously with the
band at 638 nm (Figure S7a). 2) The lowest-energy
band with vibronic structures was independent of the
concentration of 1a (6.3 ꢂ 10À5 and 3.2 ꢂ 10À4 m; Fig-
ure S7b) and the solvent polarity (dichloromethane
and acetonitrile; Figure S7c). 3) No external heavy-
atom effect was observed in the iodoethane solution[23]
(Figure S7c). 4) The extinction coefficient for the
lowest-energy band was not dependent upon temper-
ature (Figure S7d). Moreover, TD-DFT calculations
Figure 6. UV/Vis/NIR absorption spectrum of hydrocarbon 1b. The
spectrum was recorded in CH2Cl2 at room temperature. Inset shows
a magnified view. The background absorbance at ca. 1700 nm may
À
arise from the overtone of C H vibration of the solvent.
Figure 7. Structures of biradicaloid compounds 7 and 8. The arrows in the
biradical structure represent antiparallel spins.
at the TD-UB3LYP/6-31G(d) level predict S0–S1
absorption at 1232 nm for 1b with a low oscillator
strength (f = 0.018; Table S9), which is consistent with
the experimental results. These results strongly sug-
gest that the near-infrared band with a lowest-energy
maximum at 1700 nm is from S0–S1 absorption in 1b. The
significant low-energy absorption band of 1b, despite its small
conjugation space, which consists of only 20 p electrons,
should be contrasted with those of the known systems with
similar number of p electrons, such as pentacene (22 p elec-
trons, 578 nm),[3] perylene (20 p electrons, 439 nm),[4] and the
biazulene shown in Figure 1 (20 p electrons, 633 nm),[6] as
well as structural isomers 2b (730 nm),[14] 3b (516 nm),[15b] and
4b (603 nm).[16]
To demonstrate that the moderate amplitude of the
biradical character of 1b is critical for the exceptionally low-
energy light absorption, we compared the lowest-energy
transitions of 1a (or 1b) with those of the known biphenalene
7 and the hypothetical indenofluorene congener 8, which have
larger biradical characters (0.85 for 7a and 0.79 for 8), smaller
HOMO–LUMO energy gaps (0.92 eV for 7a and 1.08 eV for
8), and larger p conjugation spaces (Figure 7). Experimen-
tally, the absorption maxima of 1b and 7b are 1700 nm and
865 nm,[10b] respectively. Moreover, theoretical transitions
estimated by TD-DFT calculations are 846 nm for 7a and
985 nm for 8 (Tables S10 and S11). These results indicate that
a molecule with a smaller p conjugation space exhibits
a lower-energy light absorption if it has a moderate biradical
character. We have revealed the theoretical background of
this behavior on the basis of the valence configuration
interaction scheme, which will be reported elsewhere.
In conclusion, the first example of meta-quinodimethane
embedded in an indenofluorene framework is presented. We
demonstrate that indeno[2,1-b]fluorene, which has a moderate
singlet biradical character (y), exhibits extremely low-energy
light absorption. We anticipate this knowledge is useful for
the design of new classes of materials based on small
molecules with low-energy light absorption.[24]
Received: March 13, 2013
Published online: April 29, 2013
Keywords: hydrocarbons · photochemistry · polycycles ·
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quinodimethanes · radicals
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