Fluoreno[4,3-c]fluorene: A closed-shell, fully conjugated hydrocarbon

Article abstract of DOI:10.1021/ol300942z

The synthesis and optoelectronic properties of 24 π-electron, formally antiaromatic 4,11-di-t-butyl-1,8-dimesitylfluoreno[4,3-c]fluorene (FF) are presented. The solid-state structure shows that the outer rings are aromatic, while the central four rings possess a bond-localized 2,6-naphthoquinone dimethide motif (in red). The biradical character of FF is assessed experimentally and computationally; the results of which implicate a closed-shell ground state.

Full text of DOI:10.1021/ol300942z

ORGANIC
LETTERS
2012
Vol. 14, No. 9
2426–2429
Fluoreno[4,3-c]fluorene: A Closed-Shell,
Fully Conjugated Hydrocarbon
Bradley D. Rose, Chris L. Vonnegut, Lev N. Zakharov, and Michael M. Haley*
Department of Chemistry and Materials Science Institute, University of Oregon,
Eugene, Oregon 97403-1253, United States
haley@uoregon.edu
Received April 11, 2012
ABSTRACT
The synthesis and optoelectronic properties of 24 π-electron, formally antiaromatic 4,11-di-t-butyl-1,8-dimesitylfluoreno[4,3-c]fluorene (FF) are
presented. The solid-state structure shows that the outer rings are aromatic, while the central four rings possess a bond-localized 2,6-naphthoquinone
dimethide motif (in red). The biradical character of FF is assessed experimentally and computationally; the results of which implicate a closed-shell
ground state.
Conjugated hydrocarbons with extended polycyclic
frameworks have fascinated chemists for over 125 years.1
Such molecules have undergone a resurgence in interest
over the last two decades because of their utilization as
materials in optical and electronic device applications,
such as organic field-effect transistors, photovoltaics,
and light emitting diodes.2 Large polycyclic aromatics
have garnered considerable attention in this area due to
π-orbital overlap of these electron-rich compounds in the
solid state, which facilitates charge transport.3
[1,2-b]fluorene5 (IF) have been reported. These acene-like
molecules show promise as electron transporting materials
due to their relatively low-lying LUMO energy levels.6
In all cases, generation of the IF skeleton utilized a
Sn(II)-mediated reductive dearomatization to furnish
the quinodimethane core. We set out to probe the limits
of this dearomatization reaction in larger arenes, such as
naphthalene.
The feasibility of dearomatizing a larger system was
first explored computationally by considering the energy
required to form the product. We employed isodesmic
reaction schemes and computationally determined the
aromatic stabilization energies (ASE) of the basic indeno-
[1,2-b]fluorene and fluoreno[4,3-c]fluorene core structures
(IF 1 and FF 2, respectively in Figure 1).7 The geometries
Recently the syntheses of derivatives of o- and p-quino-
dimethane-containing indeno[2,1-a]fluorene4 and indeno-
(1) (a) Balaban, A. T.; Banciu, M.; Ciorba, V. Annulenes, Benzo-,
Hetero-, Homo-Derivatives and their Valence Isomers; CRC Press: Boca
Raton, FL, 1987. (b) Hopf, H. Classics in Hydrocarbon Chemistry:
Syntheses, Concepts, Perspectives; Wiley-VCH: Weinheim, Germany,
2000.
(5) (a) Chase, D. T.; Rose, B. D.; McClintock, S. P.; Zakharov, L. N.;
Haley, M. M. Angew. Chem., Int. Ed. 2011, 50, 1127–1130. (b) Chase,
D. T.; Fix, A. G.; Rose, B. D.; Weber, C. D.; Nobusue, S.; Stockwell,
C. E.; Zakharov, L. N.; Lonergan, M. C.; Haley, M. M. Angew. Chem.,
Int. Ed. 2011, 50, 11103–11106. (c) Chase, D. T.; Fix, A. G.; Kang, S. J.;
Rose, B. D.; Weber, C. D.; Zhong, Y.; Zakharov, L. N.; Lonergan,
M. C.; Nuckolls, C.; Haley, M. M., submitted.
(6) Anthony, J. E.; Facchetti, A.; Heeney, M.; Marder, S. R.; Zhan,
X. Adv. Mater. 2010, 22, 3876–3892.
(7) Cramer, C. J. Essentials of Computational Chemistry: Theories
€
(2) (a) Mueller, T. J. J.; Bunz, U. H. F. Functional Organic Materials:
Syntheses, Strategies and Applications; Wiley-VCH: Weinheim, Germany,
2007. (b) M€ullen, K.; Scherf, U. Organic Light Emitting Devices: Synthe-
sis, Properties and Applications; Wiley-VCH: Weinheim, Germany, 2006.
(c) Haley, M. M.; Tykwinski, R. R. Carbon-Rich Compounds: From
Molecules to Materials; Wiley-VCH: Weinheim, Germany, 2006.
(3) Anthony, J. E. Chem. Rev. 2006, 106, 5028–5048.
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and Models; John Wiley & Sons Ltd: Chichester, U.K., 2002; pp 335ꢀ338.
r
10.1021/ol300942z
Published on Web 04/25/2012
2012 American Chemical Society

Figure 1. Calculated ASE using electronic and zero point energies from B3LYP/6-311þG(d,p)//B3LYP/6-31G(d). Compound 2 is
shown with IUPAC numbering scheme.
were optimized with DFT using the B3LYP/6-31G(d)
method8 as utilized in Gaussian 09,9 and then single point
energies and frequency calculations were performed with
the same method using the larger 6-311þG(d,p) basis set.
This method was verified against benzene, the usual stan-
dard, yielding a computed ASE value of ꢀ34.3 kcal molꢀ1
,
whereas the experimentally determined ASE is ꢀ32.2 kcal
molꢀ1 from the enthalpies of formation (ΔHf).10 The ASE
of IF 1 is surprisingly small at ꢀ13.4 kcal molꢀ1, 38% the
ASE of benzene. However, the ASE for the expanded FF 2
is ꢀ19.4 kcal molꢀ1, which is expected relative to 1 since
naphthalene has about 1.7 times the ASE of benzene.
These results indicated that the synthesis of 2 should be
possible. Although 2 is formally antiaromatic with a total
of 24 π-electrons, nucleus-independent chemical shifts
(NICS(1)zz) calculations11 (see Supporting Information)
reveal that only the peripheral benzenes are diatropic and
that the inner rings are very weakly paratropic and/or
atropic, similar to 1.
A second possibility we considered was that the fluor-
enofluorene might have an open-shell (biradical) config-
uration (e.g., 20, Figure 2). Recently a variety of extended
(8) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.;
Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter 1988, 37, 785–789.
(c) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
J. Phys. Chem. 1994, 98, 11623–11627.
(9) All calculations were performed using: Frisch, M. J. et al.
Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. See
Supporting Information for the full citation.
(10) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of
Organic Compounds, 2nd ed.; Chapman and Hall: New York, 1986.
(11) Fallah-Bagher-Shaidaei, H.; Wannere, C. S.; Corminboeuf, C.;
Puchta, R.; Schleyer, P. v R. Org. Lett. 2006, 8, 863–866.
(12) Inter alia: (a) Kubo, T.; Shimizu, A.; Uruichi, M.; Yakushi, K.;
Nakano, M.; Shiomi, D.; Sato, K.; Takui, T.; Morita, Y.; Nakasuji, K.
Org. Lett. 2007, 9, 81–84. (b) Sun, Z.; Huang, K.-W.; Wu, J. J. Am.
Chem. Soc. 2011, 133, 11896–11899. (c) Purushothaman, B.; Bruzek, M.;
Parkin, S. R.; Miller, A.-F.; Anthony, J. E. Angew. Chem., Int. Ed. 2011,
50, 7013–7017.
Figure 2. Closed-shell and OS forms of 2 and recently reported
molecules 3ꢀ5 that show significant biradical character.
polycyclic π-systems have been described that exhibit a
singlet open-shell (OS) ground state (e.g., 3) and thermally
(13) (a) Gao, X.; Hodgson, J. L.; Jiang, D.; Zhang, S. B.; Nagase, S.;
Miller, G. P.; Chen, Z. Org. Lett. 2011, 13, 3316–3319. (b) Takano, Y.;
Taniguchi, T.; Isobe, H.; Kubo, T.; Morita, Y.; Yamamoto, K.;
Nakasuji, K.; Takui, T.; Yamaguchi, K. J. Am. Chem. Soc. 2002, 124,
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77, 348–353.
Org. Lett., Vol. 14, No. 9, 2012
2427

accessible triplet states (e.g., 4, 5).12 We therefore consid-
ered an OS singlet and triplet state for 2 using symmetry
broken UB3LYP; the OS singlet energy was corrected using
the approximate spin projection method.13 Using known
2,6-quinodimethylnaphthalene 614 as a test case, the cal-
culated triplet state was estimated at 15.4 kcal molꢀ1 above
the OS singlet, which is in reasonable agreement with the
Structurally similar 6 exhibited no biradical character in
the EPR spectrum below 180 °C.15 Pure 7 is remarkably
stable;the NMR spectrum showed no decomposition
upon prolonged heating at 160 °C, and thermogravimetric
analysis revealed less than 5% decomposition by ca.
350 °C. Another sample that was kept in CDCl3 open to
the atmosphere and exposed to ambient light exhibited by
NMR only trace amounts of decomposition after 1 month.
The lowest energy transition in the absorption spectrum
of 7 (Figure 3) appears at 649 nm, compared to a λmax of
516 nm for dimesityl-IF 9.5c These values correspond to
optical energy gaps of 1.79 and 2.29 eV, respectively,17
which clearly show the effect of inclusion in the core of the
second six-membered ring and its two double bonds.
experimentally determined value of 12.6 ( 2.0 kcal molꢀ1 15
.
For 2 the closed-shell and OS singlet were calculated to
be essentially the same energy. The square of the OS
spin expectation value was miniscule (<S2> = 0.0001),
reinforcing that 2 should have a closed-shell ground state.
The calculated triplet state for 2 is 16.1 kcal molꢀ1 above the
singlet.
Of the three possible fluorenofluorene variants that
ꢀ
yield Kekule hydrocarbons, we chose to experimentally
investigate 7 since the requisite dione synthon 8 is a known
molecule that can be prepared in seven steps from com-
mercially available 2,6-dimethylnaphthalene.16 We also
felt that inclusion of mesityl groups would enhance the
kinetic and thermodynamic stability of 7. Gratifyingly,
treatment of 8 with mesityllithium followed by reduc-
tion with SnCl2 in toluene at room temperature yields
fully conjugated 7 in 86% yield (Scheme 1). Unlike the IF
derivatives, elevated temperatures for this reaction led to
Figure 3. Electronic absorption spectra of 7 (solid blue) and 9
(dashed red) in CH2Cl2.
The cyclic voltammetry (CV) data for 7 exhibit redox
amphoterism with two reversible reductions and two
oxidations;the first oxidation reversible and the second
quasi-reversible (Figure 4). The first reduction half wave
potential is ꢀ0.87 V and the second ꢀ1.29 V, whereas the
first oxidation half-wave potential is 0.82 V and the second
1.27 V.18 The electron affinity of 7 is higher than that of 9,
which has a first reduction half-wave potential of ꢀ1.12
V.5c The LUMO energy of 7 was estimated at ꢀ3.77 eV
from CV,19 an exceptionally low-lying LUMO for a poly-
cyclic hydrocarbon that does not contain electron-with-
drawing groups. This corresponds to an electrochemical
energy gap of 1.69 eV, which agrees well with the optically
determined value.
Scheme 1. Synthesis of 4,11-Di-t-butyl-1,8-dimesitylfluoreno-
[4,3-c]fluorene 7
Compound 7 cocrystallized with benzene by slow eva-
poration, yielding single crystals suitable for X-ray diffrac-
tion. The molecular structure is shown in Figure 5, and
relevant bond lengths are given in Table 1. The crystal
significant decomposition. Deep-blue solutions of 7 ex-
hibited no line broadening in the proton NMR spectrum
(i.e., no triplet character) upon heating to 160 °C (see
Supporting Information), and the EPR spectrum of the
powder and solution sample showed no triplet signal.
(17) The optical band gap was determined by taking a tangent line
from the maximum of the first derivative for the lowest energy
transition.
(18) For comparison, the potential references were changed from Fc/
Fcþ to SCE, as described in: Connelly, N. G.; Geiger, W. E. Chem. Rev.
1996, 96, 877–910.
(16) Preis, E.; Scherf, U. Macromol. Rapid Commun. 2006, 27, 1105–
1109.
(19) Reiss, H.; Heller, A. J. Phys. Chem. 1985, 89, 4207–4213.
2428
Org. Lett., Vol. 14, No. 9, 2012

Figure 4. CV data for 7. Data was recorded in a CH2Cl2 solution
of 1 mM analyte and 0.1 M Bu4NOTf using a scan rate of 50
mV sꢀ1. The working electrode was a glassy carbon electrode
with a Pt coil counter electrode and Ag wire pseudoreference.
Values reported as the half-wave potential (vs SCE) using the
Fc/Fcþ couple (0.46 V) as an internal standard; see reference 18.
structure of 7 (C6H6) establishes the closed-shell ground
3
˚
state due to the discrete, alternating short (1.356ꢀ1.378 A)
˚
and long (1.410ꢀ1.433 A) bond lengths. The B3LYP/
6-31G(d) minimized structure of 7 accurately replicates
the bond length alternation in central rings and near
homogeneity for the outer six-membered rings. To the best
of our knowledge, this is the first structural elucidation of a
neutral 2,6-naphthoquinone dimethide; the crystal struc-
ture of 5 revealed a naphthalene-like core expected for an
open-shell biradical.12a The X-ray data for the anion of
structurally similar 10 with methyltriphenylphosphonium
counterion had a 2:1 stoichiometry and possessed longer
bond lengths for the C(4)ꢀC(5) double bond (Table 1)
Figure 5. ORTEP showing top and side views of 7 (C6H6);
3
solvent molecule and hydrogens omitted for clarity. Ellipsoids
drawn at the 30% probability level.
considerable biradical character, the lack of a radical
stabilizing motif (e.g., phenalenyl unit in 5) leads to a
closed-shell ground state for 7.12a,b The redox amphoter-
ism in the CV data suggests potential as an ambipolar
electronic material, similar to IF 9,5c and the unusual
thermal and photochemical stability of 7 bode well for
such applications. We are currently exploring use
of the Sn-mediated dearomatization for the prepara-
tion of even larger, expanded π-conjugated polycyclic
hydrocarbons.
than neutral 7 (C6H6).20 These differences are attributable
3
to the negative charge distributed across the two indepen-
dent molecules of 10.
˚
Table 1. Select Bond Lengths [A]
bonda
7 (calc)b
7 (C6H6) (X-ray)
10(a)c
10(b)c
3
C(1)ꢀC(2)
C(1)ꢀC(5a)
C(2)ꢀC(3)
C(3)ꢀC(4)
C(3)ꢀC(6)
C(4)ꢀC(5)
C(5)ꢀC(5a)
1.359
1.446
1.429
1.455
1.386
1.387
1.479
1.356(6)
1.433(6)
1.410(6)
1.433(6)
1.378(6)
1.377(6)
1.459(6)
1.38
1.40
1.46
1.34
1.45
1.47
Acknowledgment. This work was supported by the
National Science Foundation (CHE-1013032). B.D.R.
acknowledges the NSF for a GK-12 fellowship (DGE-
0742540). We thank Prof. Mark Lonergan (University of
Oregon) for use of his group’s potentiostat.
d
d
ꢀ
ꢀ
1.42
1.50
1.43
1.38
1.43
1.44
a Numbering scheme for 7 (Figure 5) was applied 10(a) and 10(b).
b RB3LYP/6-31G(d) minimized structure. c See ref 20. d Not reported.
Supporting Information Available. Experimental and
computational details; NMR spectra and X-ray cif file of 7;
and complete ref 9. This material is free of charge via the
Internet at http://pubs.acs.org.
Compound 7 is an intriguing addition to the molecular
canon. In contrast to similar compounds that exhibit
(20) Sanz, F.; Daly, J. J. J. Chem. Soc., Perkin Trans. 2 1975, 1141–
1145.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 9, 2012
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