Synthesis and electronic properties of extended, fused-ring aromatic systems containing multiple pentalene units

Article abstract of DOI:10.1021/ja1043159

A series of elongated dibenzopentalenes were synthesized via a Pd-catalyzed cyclization followed by an Fe-mediated cyclodehydrogenation. Structural geometries varying from linear to extremely bent are shown to drastically influence solubility properties as well as reduction potentials. Comparisons are made to monopentalenes and related diindenoindacenes. UV-vis absorption spectra and cyclic voltammetry measurements indicate a strong modulation of the electronic structure of these compounds mediated by the strength of interactions between pentalene centers.

Full text of DOI:10.1021/ja1043159

Published on Web 07/27/2010
Synthesis and Electronic Properties of Extended, Fused-Ring Aromatic
Systems Containing Multiple Pentalene Units
Zerubba U. Levi and T. Don Tilley*
Department of Chemistry, UniVersity of California at Berkeley, Berkeley, California 94720
Received May 19, 2010; E-mail: tdtilley@berkeley.edu
structure, reduction chemistry, and solubility properties. This class of
materials, centered around multiple electroactive pentalene units,
displays extended conjugation while benefiting from air and moisture
stability and exhibiting solubility properties that are readily modulated.
Abstract: A series of elongated dibenzopentalenes were synthe-
sized via a Pd-catalyzed cyclization followed by an Fe-mediated
cyclodehydrogenation. Structural geometries varying from linear to
extremely bent are shown to drastically influence solubility properties
as well as reduction potentials. Comparisons are made to mono-
pentalenes and related diindenoindacenes. UV-vis absorption
spectra and cyclic voltammetry measurements indicate a strong
modulation of the electronic structure of these compounds mediated
Although many extended ladder π-conjugated compounds have been
prepared,5 only one report describes elongated dibenzopentalene
derivatives with more than one pentalene moiety in a single molecule,6
obtained as a mixture of minor products upon reaction of PtCl4 with
1,2,4,5-tetrakis(phenylethynyl)benzene.
The synthesis of multipentalene based compounds 1-4 is
by the strength of interactions between pentalene centers.
outlined in Scheme 1. Bromostilbene derivatives 5, 7, and 8 were
Scheme 1. Synthesis of Extended Dibenzopentalenes 1-4a
Fused polycyclic aromatic hydrocarbons displaying extended
π-conjugation, particularly linear polyacenes, have become a primary
focus in the field of organic electronics. While pentacene derivatives
are considered to be among the most promising organic semiconduc-
tors, they are commonly susceptible to oxidative degradation and often
suffer from poor solubility. Extended pentalene derivatives1,2 and
related polyindenoindenes,3 though scarcely studied, represent an
interesting alternative to classic polyacenes. Until recently,4a-f synthetic
methodologies for accessing indenoindenes, dibenzopentalenes, and
other five-membered ring acene analogues were quite limited. This
contribution describes a series of extended dibenzopentalene derivatives
(Figure 1, 1-4) with varying structural geometries, obtained by
a Conditions: (i) N,N-diisopropylethylamine, PPh3, Pd(OAc)2, 135 °C;
(ii) FeCl3, MeNO2, CH2Cl2, CS2, RT; (iii) Fe, Br2, MeNO2, 80 °C, in the
dark, RT, 82%; (iv) N-bromosuccinimide, benzoyl peroxide, 1,2-dichlo-
robenzene, 180 °C, 49%; (v) P(OEt)3, 170 °C, 67%; (vi) (a) NaH, THF, 0
°C; (b) 4-propoxybenzaldehyde, 60 °C, 75%.
prepared with use of established Horner-Wadsworth-Emmons
(HWE) chemistry starting from commercially available para-,
meta-, and ortho-xylene, respectively; alkoxy substitutes were
Figure 1. Highly extended dibenzopentalene derivatives 1-4.
utilization of a two-step reaction sequence that allows for the controlled
cross-coupling of a bromostilbene with a diarylalkyne.4d Through
spectroscopic and electrochemical measurements, the geometric ar-
rangement of pentalene moieties is shown to influence electronic
included to aid the solubility of the final products. Under standard
Heck conditions (Pd(OAc)2, PPh3, DIPEA, DMF, 135 °C),4d
5
reacted with an excess of the appropriate diarylacetylene to afford
the s-indacenes 6a and 6b, which precipitate from solution as light
9
11012 J. AM. CHEM. SOC. 2010, 132, 11012–11014
10.1021/ja1043159  2010 American Chemical Society

C O M M U N I C A T I O N S
maroon solids. Related palladium chemistry has recently been used
to construct indacendiide ligands,7 and the formation of 6a/b likely
occurs through a similar mechanism. Oxidative cyclodehydroge-
nation of 6a/b with FeCl3 affords the linear dipentalenes 1a and
1b as dark purple solids. While 1a/b have exceedingly low
solubilities in common organic solvents, carbon disulfide (CS2) is
an excellent solvent for both chromatographic purification and
structural characterization.
Dipentalenes 2 and 3a/b were prepared by an identical two-step
reaction sequence. However, the high solubility of the corresponding
indacenes made purification of these intermediates difficult. Thus,
these indacenes were used in subsequent cyclodehydrogenations
with minimal purification. The resulting dipentalenes 2 and 3a/b
are readily soluble in common organic solvents and easily purified
by column chromatography; in the case of 3a/b, a helical twist
along the molecular framework is likely responsible for the
heightened solubility relative to 1a/b.
To extend this chemistry beyond dipentalene derivatives, the
triphenylene tribromide 12 was synthesized. Trimethyltriphenylene
was prepared according to the literature procedure8 and subjected
to electrophilic bromination conditions to afford 2,6,10-tribromo-
3,7,11-trimethyltriphenylene in high yield (82%). A 3-fold benzylic
bromination followed by an Arbuzov reaction with excess trieth-
ylphosphite afforded the triphosphonate 11. Subsequent reaction
of 11 under HWE conditions with 4-propoxybenzaldehyde furnished
the necessary tribromotrialkene 12 in good yield (75%). Treatment
of this bromoarene under standard Heck conditions with excess
diphenylacetylene followed by a 3-fold cyclodehydrogenation
afforded the dark-red tripentalene 4.
The electronic spectra (Figure 2) and electrochemical behavior
(Figure 3) of the extended pentalenes differ significantly from the
monopentalenes prepared to date.4 The lowest energy absorption
band displays a large bathochromic shift of 100 nm with a 4.8-
fold increase in the extinction coefficient upon extending the
conjugation from monopentalene A to the linear dipentalene 1b.
The linear dipentalenes also differ markedly from 2 to 4; consistent
with the linear dipentalenes having the longest effective conjugation
length, 1b also exhibits the lowest energy λmax value (Table 1) while
having extinction coefficients that are comparable to those of the
tripentalene 4. Preliminary optical characterization indicates that
these materials are nonfluorescent.
Figure 3. Cyclic voltammograms of A, 1b, and 4 in CH2Cl2: V versus
Fc/Fc+ in 0.1 M nBu4NPF6/CH2Cl2 at 100 mV/s scan rate; Fc/Fc+ ) 0.19
V; Inset: Square wave voltammogram of 1b in 0.3 M BuN4ClO4/THF.
Table 1. Comparisons of Mono-, Di-, and Tripentalenes
red
red
Compound
E
1,1/2
[V]
E
2,1/2
[V] (E1,1/2 - E2,1/2) [V] λmax (nm)
Monopentalene Aa
-1.41
-1.96
0.55
447
(-1.58) (-2.13)
Linear Dipentalene 1b
Semilinear Dipentalene 2 -1.34
Bent Dipentalene 3b
Tripentalene 4
Diindenoindacene B
-1.33
-1.65
-1.71
-1.89
-2.19
-
0.32
0.37
0.33
0.55
-
550
534
476
479
419
-1.56
-1.64
-2.75
a Literature average of single dibenzopentalenes in parantheses.4
can be observed for the dipentalenes (Figure 3, Inset). These
reductions occur at significantly more positive potentials for the
dipentalenes versus the monopentalenes (Table 1); that is, the
formation of 1b-1 occurs at a potential ca. 80 mV (240 mV) more
positive than that for A-1 (lit average for MP-1), and the formation
of 1b-2 occurs at a potential ca. 310 mV (480 mV) more positive
than that for A-2 (lit average for MP-2). Furthermore, tripentalene
4 displays three distinct reductions centered around the first
reduction potential for typical MPs (-1.4 to -1.6 V). Thus, the
absorption spectra and distinctive cyclic voltammetric behavior
suggest a strong modulation of the electronic structure of these
compounds mediated by the strength of interactions between
pentalene centers. In the case of 1, and to a lesser extent 2 and 3,
strong coupling across the π-framework results in extended
conjugation, more positive reduction potentials, and delocalized
anions. In stark contrast, the triphenylene core in 4 imparts weak
coupling across the structure resulting in three nearly independent
pentalene moieties. In addition, a noteworthy comparison can be
made to the recently prepared diindenoindacene B,9 which, lacking
a pentalene moiety, exhibits only a single reduction at -2.75 V.
This work demonstrates the versatile synthesis of elongated
dibenzopentalene derivatives containing for the first time up to three
pentalene units in a single molecule. In addition, the low reduction
potentials of the dipentalenes suggest a delocalized diradicaloid
structure for the corresponding dianions. The reduction chemistry
and enhanced optical absorption combined with air and moisture
stability make these extended materials promising candidates for
use in organic electronic devices.
Acknowledgment. The authors thank Jenna Jeffrey and Profes-
sor Richmond Sarpong for helpful discussions and gratefully
acknowledge the support of the NSF (Research Grant No.
CHE0314709) and the Director, Office of Energy Research, Office
of Basic Energy Sciences, Chemical Sciences Division of U.S. DOE
under Contract DE-AC02-05CH11231.
Figure 2. Absorption spectra of 1-4 in CH2Cl2 (10-5 M).
Supporting Information Available: Experimental details, UV-vis
absorption spectra, and cyclic voltammograms. This material is available
free of charge via the Internet at http://pubs.acs.org.
While monopentalenes (MPs) typically display one to two
reductions to form the corresponding dianion, up to four reductions
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Saito, M.; Nakamura, M.; Tajima, T. Chem.sEur. J. 2008, 14, 6062–6068.
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