DOI: 10.1002/anie.201104797
Polycycles
Electron-Accepting 6,12-Diethynylindeno[1,2-b]fluorenes: Synthesis,
Crystal Structures, and Photophysical Properties**
Daniel T. Chase, Aaron G. Fix, Bradley D. Rose, Christopher D. Weber, Shunpei Nobusue,
Chelsea E. Stockwell, Lev N. Zakharov, Mark C. Lonergan, and Michael M. Haley*
Polycyclic hydrocarbons that possess extended p conjugation
are of significant interest because of their potential use in
optical and electronic devices such as light emitting devices,
field-effect transistors, and photovoltaics.[1] While a majority
of studies have focused on acenes such as pentacene and its
derivatives (e.g., 1; Scheme 1),[2] these systems are susceptible
to oxidative and photolytic degradation;[3] thus, there is a
need for alternative, acene-like molecules. One avenue in this
search has explored compounds containing five-membered
rings, rather than the more traditional six-membered rings.
Prime examples of such molecules are dibenzopentalene (2)
and derivatives thereof, wherein the groups of Saito, Kawase,
and Tilley have recently described improved methods for
their construction.[4]
disubstitution, spirofusion).[6] Of the four fully conjugated IFs
known prior to 2011, three are rapidly oxidized by trace
amounts of oxygen[7] and the other is poorly characterized.[8]
Very recently we reported the synthesis of tetraalkyny-
lated indeno[1,2-b]fluorenes (e.g., 4).[9] The compounds
exhibited similar UV/Vis absorption profiles and slightly
larger HOMO/LUMO energy gaps compared to those of 1
while maintaining potentially superior solution stabilities;
however, the packing of 4 in the solid state resembled an
expanded herringbone pattern, a motif often found in
unsubstituted acenes. Since the steric bulk of the four
interdigitated (triisopropylsilyl)ethynyl groups was the most
likely cause for inhibiting a desirable “brick and mortar”
p stacking, we sought to examine additional IF derivatives.[10]
As a guide for experimental studies, we performed DFT
calculations (B3LYP/6-311 + G**[11] using Gaussian09)[12] on
substituted IFs to determine the effect ethynylogation of 3 has
on the HOMO (À5.53 eV) and LUMO (À3.03 eV) energy
levels, and the energy gap (2.50 eV) of the IF core (Scheme 2,
Table 1). Inclusion of the four ethynyl units in 5 significantly
lowers the LUMO by approximately 0.5 eV while the HOMO
remains unchanged, thus affording a gap energy of 1.97 eV.
Inclusion of only two acetylenes at positions 5 and 11 (e.g., 6)
slightly lowers the HOMO yet significantly raises the LUMO
compared to 5, thus affording a net gap increase of 0.41 eV. If
the two alkynes are located at positions 6 and 12, as in 7, the
HOMO (À5.51 eV) is on par with that of 5 (À5.53 eV) and
the LUMO is elevated slightly (À3.46 eV versus À3.56 eV),
thus increasing the gap by only 0.08 eV. Similar to acenes,[2]
these results illustrate that judicious positioning of the alkyne
moieties will significantly affect the electronic and photo-
physical properties. Encouraged by these initial computa-
tional studies, we targeted a number of 6,12-diethynylindeno-
[1,2-b]fluorenes for synthesis and study. We disclose herein
the preparation of IFs 8a–i along with their respective optical,
electrochemical, and computational data. We also report the
X-ray structures of 8b and 8h, thereby highlighting the effects
that substitution on the IF core has upon crystal packing.
Scheme 1. Structures 1–4. TIPS=triisopropylsilyl.
Another attractive topology is the indeno[1,2-b]fluorene
(IF) skeleton (e.g., 3), an acene analogue in which the B and
D rings each contain one fewer carbon atom, thus making the
20-p-electron molecule formally antiaromatic. While the
pentacyclic IF framework is common in the literature,
nearly all examples bear substituents on the 6- and 12-
positions, thus resulting in either cross-conjugation (e.g.,
ketones, exocyclic olefins)[5] or disrupted conjugation (e.g.,
[*] D. T. Chase, A. G. Fix, B. D. Rose, C. D. Weber, S. Nobusue,
C. E. Stockwell, Dr. L. N. Zakharov, Prof. M. C. Lonergan,
Prof. M. M. Haley
Department of Chemistry & Materials Science Institute
University of Oregon
Eugene, OR 97403-1253 (USA)
E-mail: haley@uoregon.edu
[**] The authors gratefully acknowledge the National Science Founda-
tion (CHE-1013032) for support of this research; see the Supporting
Information for the full acknowledgement.
Supporting information for this article is available on the WWW
Scheme 2. Calculated ethynylindenofluorene derivatives.
Angew. Chem. Int. Ed. 2011, 50, 11103 –11106
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11103