Exploiting substituent effects for the synthesis of a photooxidatively resistant heptacene derivative

Full text of DOI:10.1021/ja808881x

Published on Web 02/25/2009
Exploiting Substituent Effects for the Synthesis of a Photooxidatively
Resistant Heptacene Derivative
Irvinder Kaur, Nathan N. Stein, Ryan P. Kopreski, and Glen P. Miller*
Department of Chemistry and Materials Science Program, UniVersity of New Hampshire,
Durham, New Hampshire 03824-3598
Received November 12, 2008; E-mail: glen.miller@unh.edu
Polycyclic aromatic hydrocarbons composed of linearly annelated
benzene rings are called acenes,1 anthracene representing the
smallest member of the series. Tetracene and pentacene, the next
two largest, are organic semiconductor compounds that have been
utilized extensively in both organic field-effect transistor2 and
organic light-emitting diode3 applications. Both compounds pho-
tooxidize under ambient conditions,4 a serious problem for acenes
that becomes increasingly problematic with increasing length.
Likewise, hexacene and heptacene have eluded extensive experi-
mental study because of the ease with which they photooxidize.
Recently however, Neckers and co-workers5 demonstrated a
photochemical route for the synthesis of hexacene and heptacene
embedded in rigid polymeric media, where the molecules show
lifetimes of ∼12 and 4 h, respectively. Octacene, nonacene, and
larger acenes have never been isolated. They are predicted to have
open-shell singlet ground states.6 Larger acenes are expected to
exhibit singlet polyradical character in their ground states.7
While acenes larger than pentacene degrade under ambient condi-
tions, their resistance to photooxidation can be altered through chemical
functionalization. Anthony and co-workers8 prepared silylethynyl
derivatives of hexacene and heptacene, both of which exhibited
enhanced stability relative to their parent hydrocarbons. Recently, Wudl
and co-workers9 prepared a similar silylethynyl derivative of heptacene
bearing additional phenyl substituents at positions 5, 9, 14, and 18.
Aside from Anthony’s and Wudl’s silylethynyl derivatives, no other
functionalized hexacene or heptacene compounds have been reported,
despite the fact that large acenes promise reduced band gaps, greater
charge carrier mobilities and lower reorganization energies in molecular
electronic applications.10
Here we report the successful synthesis of four heptacene derivatives
with varying levels of photooxidative resistance (1 < 2 < 3 < 4).
Heptacene derivatives 1 and 3 bear p-(t-butyl)phenyl and p-(t-
butyl)thiophenyl substituents, respectively, at positions 7 and 16,
while 2 and 4 possess additional o-dimethylphenyl substituents at
positions 5, 9, 14, and 18. As predicted, derivative 4 is an especially
persistent species. It possesses a small optical HOMO-LUMO gap
(1.37 eV), rivaled only by Wudl’s recent silylethynylated heptacene
derivative (1.35 eV).9
Recently, we completed a substituent-effect study for a large
series of pentacene derivatives11 in which it was revealed that steric
effects, electronic effects, and the positional location of substituents
are all important factors for determining photooxidative resistances
and HOMO-LUMO gaps. For example, phenyl substituents alone
do little to enhance photooxidative resistance on pentacene, but
those with o-alkyl groups provide steric resistance to photooxidation.
The phenyl groups reside in time-averaged orthogonal orientations
relative to the acene backbone, constraining the o-alkyl groups to
lie directly over and under the acene π system.The o-alkyl groups
effectively shield otherwise reactive π electrons leading to longer-
lived species. The most impressive substituent effects, however,
were observed with thioalkyl and thioaryl substituents, as each
dramatically enhanced photooxidative resistance through special
electronic effects that render type-II photooxidations12 less viable.
In fact, alkylthio and arylthio substituents are considerably better
than silylethynyl substituents at enhancing pentacene photooxidative
resistance,11 and this fact alone suggests new opportunities for
preparing large, persistent acenes. We reasoned that by combining
arylthio and o-dimethylphenyl substituent effects, we could produce
a heptacene derivative with enhanced photooxidative resistance.
In view of the ease with which simple aryl-substituted pentacenes
photooxidize,4,11 heptacene derivative 1 was expected to degrade
rapidly in solution, and it did not disappoint. Derivative 2, on the
other hand, showed modestly improved photooxidative resistance
due to the presence of the sterically demanding o-dimethylphenyl
substituents at the 5, 9, 14, and 18 positions. With p-(t-butyl)th-
iophenyl substituents located at the most reactive carbons, C7 and
C16, heptacene derivative 3 is even longer lived, corroborating the
impressive substituent effects afforded pentacenes by alkylthio and
arylthio substituents.11 The combination of p-(t-butyl)thiophenyl
substituents at positions 7 and 16 (i.e., arylthio substituents attached
to the most reactive ring) and o-dimethylphenyl substituents at
positions 5, 9, 14, and 18 (i.e., steric resistance on neighboring
rings) make heptacene derivative 4 an especially persistent species
(Figure 1a) that endures for weeks as a solid, 1-2 days in solution
if shielded from light, and several hours in solution when directly
exposed to both light and air. Heptacene 4 is soluble in a variety
1
of solvents and has been fully characterized in solution using H
NMR, 13C NMR, UV-vis, and fluorescence spectroscopies.13 The
solution-phase stability of 4 makes it amenable to thin-film
formation using simple, high-rate methods such as blade coating
and spin coating.
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2009 American Chemical Society

C O M M U N I C A T I O N S
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Attempts to characterize 1-3 by H NMR spectroscopy were
complicated by their relatively rapid degradation in solution, but
useful spectra could nonetheless be obtained by working quickly.13
As solids stored in the dark, 1-3 have significant lifetimes, enabling
mass spectral characterization. Laser desorption ionization (LDI)
mass spectra of 1 revealed a strong molecular ion at m/z 642.6
along with smaller signals at (M+ + 32) and (M+ + 16)
corresponding to a 1O2 adduct of 1 and its fragment, respectively.13
LDI mass spectra of 2 and 3 showed molecular ions at m/z 1058.6
13
1
(2) and 706.3 (3) with no sign of O2 adducts. High-resolution
mass spectra (FAB+) of 1-3 indicated molecular ions at m/z
642.3257 (1, M+, calcd 642.3287, error ) -4.5 ppm), 1058.5770
(2, M+, calcd 1058.5791, error ) -1.9 ppm) and 707.2792 (3,
MH+, calcd 707.2806, error ) -2.0 ppm).
The UV-vis and fluorescence spectra for 4 (Figure 1b) are
consistent with a highly conjugated structure possessing a small
HOMO-LUMO gap. The longest-wavelength absorption for 4 in
CH2Cl2 is centered at 865 nm.14 On the basis of the onset of this
absorption, we determined the optical HOMO-LUMO gap for 4
to be 1.37 eV. For comparison purposes, the optical HOMO-LUMO
gaps for pentacene and TIPS-pentacene, benchmark organic semi-
conductor compounds, are 2.08 and 1.81 eV, respectively.11 LDI
mass spectra of 4 showed a strong molecular ion at m/z 1122.2
13
1
with no sign of O2 adduct formation. HR-MS (FAB) indicated
an ion at m/z 1123.5298 (MH+, calcd 1123.5310, error ) -1.1
ppm).
Figure 1. (top) Normalized absorbances of CH2Cl2 solutions of heptacene
derivatives 1-4 as a function of time exposed to light and air (1 × 10-4
M
initial concentration) at 25 °C. (bottom) Normalized UV-vis (blue line)
and fluorescence (green line) spectra of heptacene derivative 4 that shows
unusual photo-oxidative resistance.
In summary, we have demonstrated that a combination of arylthio
and o-dialkylphenyl substituents can be utilized to produce an
unusually persistent heptacene derivative. Conventional wisdom
suggesting that “the central ring [of heptacene] must be function-
alized with alkylsilylethynyl groups in order to provide sufficient
stability for isolation and characterization”9 is now recognized to
be overly restrictive. The synthetic-materials chemist interested in
preparing large, persistent acenes has additional, attractive options.
Scheme 1. Synthesis of Heptacene Derivatives 1-4
Acknowledgment. The authors acknowledge the Nanoscale
Science & Engineering Center for High-Rate Nanomanufacturing
(NSF-0425826) for financial support.
Supporting Information Available: 1H and 13C NMR spectra,
UV-vis spectra, fluorescence spectra, LDI mass spectra, digital
photographs, and synthetic details for 1-4 and selected precursors. This
material is available free of charge via the Internet at http://pubs.acs.org.
References
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(13) See the Supporting Information.
(14) As illustrated in Figure 1, a weaker band of longer wavelength was also
observed at 958 nm.
The syntheses of heptacene derivatives 1-4 are outlined in
Scheme 1. All four compounds pass through 7,16-disubstituted
heptacene-5,9,14,18-tetraones prepared via Diels-Alder reactions
involving 1,4-naphthaquinone and appropriately substituted 1,2,4,5-
tetramethylenebenzene precursors. For 1 and 3, the tetraones were
reduced to tetraols using borohydride and then reductively aroma-
tized using SnCl2. For 2 and 4, the tetraones were reacted with
o-dimethylphenyllithium, prepared in situ, to produce a unique set
of tetraols that were then reductively aromatized using SnCl2. All
four derivatives precipitated as dark-green solids, were isolated by
vacuum filtration, washed with water and methanol, and finally dried
under vacuum.13
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