Stable photoinduced charge separation in heptacene

Article abstract of DOI:10.1039/b713059g

Heptacene, generated in inert gas matrices by photobisdecarbonylation of a bridged α-diketone precursor, undergoes ionization into radical anion and radical cation upon UV irradiation. The Royal Society of Chemistry.

Full text of DOI:10.1039/b713059g

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Stable photoinduced charge separation in heptacene{{
a
Holger F. Bettinger,* Rajib Mondal and Douglas C. Neckers
b
b
Received (in Cambridge, UK) 24th August 2007, Accepted 24th September 2007
First published as an Advance Article on the web 17th October 2007
DOI: 10.1039/b713059g
8
Heptacene, generated in inert gas matrices by photobisde-
carbonylation of a bridged a-diketone precursor, undergoes
ionization into radical anion and radical cation upon UV
irradiation.
in the infinite limit, suggests that the higher members of the acene
series may show very unusual behavior for a hydrocarbon. For
example, the low ionization potential of 1, around 6 eV based on
2
extrapolating from data up to hexacene, is similar to that of metal
9
atoms (e.g., 6.0 eV for Ga).
Acenes are important organic materials for utilization in field effect
1
transistors and in organic light emitting diodes. The potential of
This suggests unusual photochemical properties for 1, and we
here report that heptacene can undergo stable charge separation in
inert gas matices upon photoexcitation. Isolation of the a-diketone
precursor 2 in inert gas matrices is achieved by heating a sample to
acenes longer than pentacene has not yet been tapped in such
applications due to their instability. The reactivity of acenes
2
increases rapidly with size, and heptacene (1) proved to be too
3
reactive to be isolated in substance. Only in 2005 did Anthony
25
approximately 225 uC at a pressure of approximately 10 mbar.
Upon irradiation with a high-pressure mercury lamp (385 , l ,
450 nm), all the mid-IR bands of 2 decrease; prolonged photolysis
results in complete disappearance of 2. Concomitantly, a set of
new bands (Table 1, see ESI{) as well as a broad feature at
and co-workers succeed in the first synthesis of a true heptacene
4
derivative! But in spite of kinetic stabilization of the acene p
system by bulky substituents, the heptacene derivative decomposed
5
within weeks in solution. More recently, Neckers and co-workers
6
used the Strating–Zwanenburg photobisdecarbonylation of a
21
21
21
2138 cm , 2137 cm , and 2133 cm due to monomeric and
10
aggregated CO are growing in (Fig. 1). The only organic
bridged a-diketone 2 in a poly(methyl methacrylate) (PMMA)
matrix at room temperature to generate 1, which could be
characterized by its electronic absorption spectrum in the visible.
photoproduct formed in this reaction is assigned to 1 based on
comparison of the measured IR spectrum with the one computed
at the RB3LYP/6-31G* level.
Following the photochemical decomposition of 2 by UV/vis
spectroscopy at 10 K reveals six isosbestic points at 225 nm,
295 nm, 359 nm, 408 nm, 450 nm, and 504 nm (Fig. 2), confirming
that the photodecarbonylation proceeds without any detectable
intermediate. The concerted photodecarbonylation of 2 is in
agreement with earlier observations for bridged a-diketones in
6,11
12
solution and in cryogenic matrices. The electronic absorption
spectrum of green heptacene (Fig. 2) is characterized by the very
Properties of acenes related to electronic structure appear to
2,3
converge to a limit with increasing size. As the largest known
linear acene, 1 is thus of great interest for an improved
understanding of the electronic structure of its unknown higher
7
congeners, which are still receiving theoretical scrutiny. The
electronic structure, which is predicted to converge to a semimetal
a
Lehrstuhl f u¨ r Organische Chemie 2, Ruhr-Universit a¨ t Bochum,
Universit a¨ tsstrasse 150, 44780 Bochum, Germany.
E-mail: holger.bettinger@rub.de; Fax: +49 202 3214353;
Tel: +49 202 3224529
Center for Photochemical Sciences, Bowling Green State University,
Bowling Green, OH 43403, USA
Fig. 1 Mid-IR spectra (Ar, 15 K) of the photochemical decomposition
of 2. Bottom: Difference spectrum after 18 h of irradiation (385 , l ,
b
4
50 nm); bands pointing downwards decrease during irradiation, those
pointing upwards increase. Middle: IR spectrum obtained after 18 h of
irradiation. w: water, c: CO . Top: IR spectrum computed for heptacene at
the RB3LYP/6-31G* level scaled by 0.985.
{ Electronic supplementary information (ESI) available: Experimental
details and computed and experimental spectroscopic spectral data. See
DOI: 10.1039/b713059g
{
2
Contribution # 652 from the Center for Photochemical Sciences.
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Fig. 4 Mid-IR difference spectra obtained after irradiation (l , 225 nm,
1
5 K) of 1 in Ar. Bands pointing downwards decrease (h neutral hexacene,
d CH Cl ), those pointing upwards increase during irradiation. Top: Pure
Ar. Bottom: Ar doped with 1.5% CH Cl
2
2
2
2
.
Fig. 2 UV/vis spectra obtained after deposition of 2 and subsequent
irradiation (385 , l , 450 nm) after successively doubled time intervals.
Arrows pointing up and down indicate bands which increase and decrease
during irradiation. Isosbestic points are marked with small-headed arrows.
The photograph shows 1 isolated in Ar.
mid-IR and vis/near-IR bands hardly increase upon irradiation,
and disappear after long wavelength (l . 350 nm) irradiation. (ii)
Compared to the features of 1, the bands are very intense in the
mid-IR and the near-IR region, indicating that the carriers are very
polar entities. (iii) While heptacene is ESR silent at 10 K in Ar, a
highly structured transition, which could not be further analyzed
due to its complexity, occurs upon short wave irradiation. The g
factors of the two radical ions were determined to be 2.0028,
similar to those reported for the hexacene radical ions (2.0026 ¡
1
1
prominent b-band (2 B r X A ) with its most intense feature at
2u
1g
1
3
26 nm and its origin at 338 nm. The weak p-band (1 B1u r
1
X A1g) extends from 769 to 559 nm and is characterized by three
groups of signals having their strongest absorptions at 728 nm,
14
0
.0001). (iv) Doping the matrix with 1.5% dichloromethane, an
15
excellent electron trap, suppresses the set of signals ascribable to
the radical anion and results in a significantly increased yield of the
radical cation of 1. The latter shows prominent transitions at
656 nm and 601 nm. This is shifted to shorter wavelengths by
roughly 30–40 nm compared to the data obtained earlier in
5
PMMA, reflecting the lower degree of interaction between 1
4
80 nm, 971 nm, 1060 nm, 1248 nm, and a weak low-energy band
and the argon host. Hypsochromic shifts in matrices of low
polarizability and the splitting of the p-band into features with
pronounced fine structure were also observed earlier for pentacene
at 2134 nm in agreement with theory (see ESI{). The signals
at 472 nm and 1152 nm in Fig. 3 are thus due to heptacene
radical anion. As expected, the observed NIR features of
heptacene radical anion are red shifted compared to those in
13
isolated in noble gases.
It is noteworthy that the photochemistry of 2 does not show
wavelength dependence down to 254 nm. Likewise, heptacene
isolated in Ar is photostable under irradiation with light of
wavelength l ¢ 254 nm. However, treatment of 1 with the output
of a conventional low pressure Hg lamp, which includes a line at
14
hexacene anion.
In summary, we confirm that the photobisdecarbonylation of 2
is a suitable approach toward heptacene 1 and that this reaction
proceeds without any detectable intermediates even at tempera-
tures as low as 10 K. The observed photoinitiated charge transfer
of heptacene in an Ar matrix suggests that the higher acenes might
be interesting for photoelectronic applications.
1
85 nm, results in a number of new bands in the visible, near-IR
21
21
and mid-IR (1350–1400 cm and 1100–1200 cm ), while those
of heptacene only decrease slightly in intensity (Figs. 3 and 4).
The carriers of these new bands are assigned to the radical
cation and radical anion of heptacene based on the following
spectroscopic observations. (i) Once formed, the corresponding
This work was supported in Bochum by the DFG and the
Fonds der Chemischen Industrie, and in Bowling Green by the
Office of Naval Research, ONR N00014-06-1-0948 and the Ohio
Department of Development. HFB thanks Professor Wolfram
Sander for access to matrix isolation equipment and Dirk Grote
for performing the ESR measurements. RM thanks the McMaster
Endowment for a research Fellowship.
Notes and references
1
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2
2
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5
6
1
Fig. 3 Electronic absorption spectra obtained after irradiation
(l , 225 nm) of heptacene in Ar. Red trace: before irradiation.
5
210 | Chem. Commun., 2007, 5209–5211
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