Visible-Light-Induced Heptacene Generation under Ambient Conditions: Utilization of Single-crystal Interior as an Isolated Reaction Site

Article abstract of DOI:10.1002/chem.202002155

The photo-induced generation of unstable molecules generally requires stringent conditions to prevent oxidation and the concomitant decomposition of the products. The visible-light-induced conversion of two heptacene precursors to heptacene was studied. Single crystals of bis- and mono-α-diketone-type heptacene precursors (7-DK2 and 7-DK1, respectively), were prepared to investigate the effect of precursor structure on reactivity. The photoirradiation of a 7-DK2 single crystal cleaved only one α-diketone group, forming an intermediate bearing a pentacene subunit, while that of a 7-DK1 single crystal gave rise to characteristic absorption peaks of heptacene and their increase in intensity with photoirradiation time, indicating the generation of heptacene without decomposition. Heptacene production was not observed when the precursors were photoirradiated in solution, implying that the single crystal interior provided isolation from the external environment, thus preventing heptacene oxidation.

Full text of DOI:10.1002/chem.202002155

Accepted Article
Accepted Article
Title: Visible-light-induced Heptacene Generation under Ambient
Conditions: Utilization of Single-crystal Interior as an Isolated
Reaction Site
Authors: Hironobu Hayashi, Nao Hieda, Mitsuaki Yamauchi, Yee Seng
Chan, Naoki Aratani, Sadahiro Masuo, and Hiroko Yamada
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202002155
Link to VoR: https://doi.org/10.1002/chem.202002155
01/2020

10.1002/chem.202002155
Chemistry - A European Journal
COMMUNICATION
Visible-light-induced Heptacene Generation under Ambient
Conditions: Utilization of Single-crystal Interior as an Isolated
Reaction Site
Hironobu Hayashi,*[a] Nao Hieda,[a] Mitsuaki Yamauchi,[b] Yee Seng Chan,[a] Naoki Aratani,[a] Sadahiro
Masuo,*[b] Hiroko Yamada*[a]
[a]
Dr. H. Hayashi, N. Hieda, Y. S. Chan, Prof. Dr. N. Aratani, Prof. Dr. H. Yamada
Division of Materials Science
Nara Institute of Science and Technology (NAIST)
8916-5 Takayama-cho, Ikoma 630-0192 (Japan)
E-mail: hhayashi@ms.naist.jp, hyamada@ms.naist.jp
Dr. M. Yamauchi, Prof. Dr. S. Masuo
[b]
Department of Applied Chemistry for Environment
KwanseiGakuin University
2-1 Gakuen, Sanda, Hyogo 669-1337, Japan
E-mail: masuo@kwansei.ac.jp
Supporting information for this article is given via a link at the end of the document.
Abstract: The photo-induced generation of unstable molecules
generally requires stringent conditions to prevent oxidation and the
concomitant decomposition of the products. The visible-light-induced
conversion of two heptacene precursors to heptacene was studied.
Single crystals of bis- and mono-a-diketone-type heptacene
precursors (7-DK2 and 7-DK1, respectively), were prepared to
investigate the effect of precursor structure on reactivity. The
photoirradiation of a 7-DK2 single crystal cleaved only one a-diketone
group, forming an intermediate bearing a pentacene subunit, while
that of a 7-DK1 single crystal gave rise to characteristic absorption
peaks of heptacene and their increase in intensity with
photoirradiation time, indicating the generation of heptacene without
decomposition. Heptacene production was not observed when the
precursors were photoirradiated in solution, implying that the single
crystal interior provided isolation from the external environment, thus
preventing heptacene oxidation.
graphene nanostructures.22 At the same time, however, the
zigzag-edged structure of polyacenes results in ultra-narrow
highest occupied molecular orbital (HOMO)-lowest unoccupied
molecular orbital (LUMO) gaps with increasing acene length,
which causes a decrease in stability. Furthermore, polyacenes
are poorly soluble, due to their uncomplicated structure, which
lacks functional groups. A successful strategy for the synthesis of
polyacenes is the Strating-Zwanenburg reaction,23 whereby a-
diketone
groups
undergo
visible-light-induced
photodecarbonylation at the final stage to generate polyacenes.
Quantitative conversion from precursors to polyacenes can be
achieved by simple photoirradiation with only gaseous byproducts.
A number of polyacenes have been synthesized via the Strating-
Zwanenburg reaction and characterized by matrix-isolation
techniques at low temperatures, or via state-of-the-art on-surface
synthesis under ultra-high vacuum conditions, enabling
experimental investigation of their electronic properties.22,24–28
However, the generation of polyacenes via visible-light-induced
photodecarbonylation of a-diketone groups is in stark contrast
with the fact that polyacenes are unstable against photoirradiation
because of their high HOMO levels. Heptacene generation by
thermally-induced cycloreversion from diheptacenes under an
argon stream29 and by thermal decarbonylation of monoketone
heptacene precursors,30 have been reported, thereby establishing
that heptacene can persist in the solid state for an extended
period of time. However, such conversions necessitate anaerobic
conditions. Consequently, the generation of polyacenes via
visible-light irradiation and ambient conditions is considerably
challenging, while its success would provide a facile access to the
fundamental properties of polyacenes.
Performing chemical reactions within single crystals or molecular
cages is challenging but significant, because their specific
packing and void spaces enable highly selective reactions.
Indeed, a number of efficient reactions inside porous materials
have been achieved.1–7 Moreover, single-crystal-to-single-crystal
transformations can be realized by controlling external conditions
such as light, temperature, and chemical reactants, as
demonstrated by the creation of photoswitchable materials8,9 and
two-dimensional networks.10–12 In the above-mentioned studies,
generally the whole crystal is exposed to external stimuli to
achieve efficient reactions. In this study, we focused on another
potential use of the single crystal interior: the possibility of
isolating a reaction site from the external environment, thereby
allowing an unstable product to remain intact.13–21
A polyacene was chosen as the unstable compound for the
study. Polyacenes, composed of multiple linearly fused benzene
rings, have drawn considerable attention because of their
potential utility as organic semiconductor materials. Their intrinsic
optical and electronic properties, stemming from their zigzag-
edge, render them suitable candidates for exploring magnetism in
The isolation of generated polyacenes from the external
environment prevents their oxidation. In this context, we have
successfully demonstrated the photoconversion of a crystalline
phase a-diketone-type pentacene precursor into pentacene, with
the use of a 488 nm CW laser.31,32 Importantly, the CW laser
selectively irradiates the interior of a single crystal, suggesting its
potential utility as an isolated site for unstable compounds. This
1
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effective technique was applied herein for the generation of
heptacene under ambient conditions.
This result implies that the n–p* transition of the intermediate
could not be efficiently excited further, as the generated
pentacene moiety displays an intense absorption in the same
region as the n–p* transition, which prevents the intermediates
from further photoconversion to heptacene. By contrast, the
photoirradiation of 7-DK1 (470±10 nm, 200 mW cm–2) afforded
orange precipitates, accompanied by a small increase in
absorption at 481 and 518 nm. The characterization of these
precipitates was not pursued at present, however, analysis of their
absorption spectra suggests that they are likely to be a mixture of
diheptacene, heptacenequinone and related oxidized species.29
The presence of oxygen molecules in the system is unavoidable
even with prolonged N2 bubbling. Thus, the generated
heptacenes immediately undergo dimerization and/or oxidation
reactions, demonstrating the difficulty in heptacene isolation in
solution even under anaerobic conditions.
Two heptacene precursors, bearing two or one a-diketone
group, 7-DK222 and 7-DK125 (Figure 1), respectively, were
prepared according to literature procedures, in order to
investigate the structural effect on the conversion, when it is
accompanied by a large structural change in the crystal. Upon
photoirradiation, 7-DK2 can be converted to heptacene via an
intermediate bearing a pentacene subunit, while 7-DK1 can be
converted to heptacene without generating an intermediate.
Figure 1. a-Diketone-type heptacene precursors and their expected
photoconversion to heptacene.
The absorption spectra of 7-DK2 and 7-DK1 in toluene
exhibited broad peaks at approximately 430–500 nm, which were
assigned to the n–p* transition of the a-diketone moiety (Figure
2). Considering that the anthracene moiety does not absorb
beyond approximately 400 nm, photoirradiation at 450–500 nm
can selectively excite the n–p* transition in the precursors.
Figure 3. UV-vis absorption spectra changes during photoirradiation of (a) 7-
DK2 and (b) 7-DK1 in toluene under N2 atmosphere. Conditions for
photoirradiation: LED lamp with 470±10 nm (200 mW cm–2). Images show the
color of 7-DK2 and 7-DK1 solutions before and after photoirradiation.
For photoconversion in the solid state, single crystals of 7-
DK2 and 7-DK1 were prepared by the diffusion of hexane vapor
into a dichloromethane solution of the precursors. Block crystals34
were obtained from the 7-DK2 solution, while needle crystals35
were grown from the 7-DK1 solution (Figure 4a,d). In both cases,
the dihedral angle between the two aromatic regions bridged by
the a-diketone-containing moiety was 108º (Figure 4b,e). In the
crystals, the 7-DK2 molecules are densely packed through p–p
interactions, CH–p interactions, and hydrogen bonding. The
benzene moieties are stacked in the crystals with a p–p distance
of 3.88 Å (Figure 4c). Similarly, 7-DK1 molecules are packed
through CH–p interactions and hydrogen bonding. Importantly, 7-
DK1 molecules on their own cannot provide a sufficiently dense
space packing, thus the inclusion of dichloromethane molecules
occurs in the packing (Figure 4f). Therefore, the intermolecular
interactions between 7-DK2 molecules are expected to be
stronger than those of 7-DK1, suggesting that the significant
structural change accompanying the photoconversion is more
favorable for 7-DK1 compared with 7-DK2.
Finally, photoconversion in the crystals was investigated
(see experimental details in Supporting Information). Briefly, in
this experiment, the coverslip on which the single crystal was
placed onto the stage of an inverted microscope. The light source
from a 488 nm CW laser was introduced through an objective lens.
For the observation of the single crystal absorption spectra, a
halogen lamp with a Köhler illumination system was used as the
incident light. Note that the absorption spectra in the regions of
450–700 nm and 650–1000 nm were measured separately, as a
different spectral measurement system was employed.
Figure 2. UV-vis absorption spectra of 7-DK1 (blue) and 7-DK2 (red) in toluene.
First, photoconversion of the precursors in toluene under N2
atmosphere was investigated to confirm the instability of
heptacene and, hence, the significance of the solid state
conversion. A blue LED (470±10 nm, 200 mW cm–2) was used
as a light source for the photoconversion. In the case of 7-DK2,
as the photoirradiation progressed, new peaks at 490, 546, and
593 nm appeared and grew in intensity (Figure 3). The color of
the solution changed from yellow to purple, suggesting the
conversion of 7-DK2 to the pentacene-containing intermediate.31
However, further photoirradiation, and/or photoirradiation at a
higher power (400 mW cm–2), could not convert the intermediate
to heptacene, affording precipitates of the intermediates instead.
2
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7-DK1 crystal during photoirradiation could be assigned to
heptacene, strongly suggesting the generation of heptacene in
the crystal. This result is in contrast with that observed in solution,
where the generated heptacene was immediately decomposed
and/or dimerized. Indeed, no significant changes in the spectrum
were noted during the measurement. The broad peak at
approximately 900 nm is attributable to heptacene aggregation
via p–p stacking in the crystal, although the degree of aggregation
appears to be low.32
Figure 4. Optical images of (a) 7-DK2 and (d) 7-DK1 single crystals. Single-
crystal X-ray structures of (b) 7-DK2 and (e) 7-DK1. Thermal ellipsoids
represent 50% probability. Solvent molecules are omitted for clarity (7-DK1).
Packing structure of (c) 7-DK2 and (f) 7-DK1 single crystals with space filling
model. Hydrogen atoms are omitted for clarity.
In the transmission image of the 7-DK2 crystal (Figure 5a), a red
emission was observed during photoirradiation, and the focal
point became dark-orange after photoirradiation. Spectroscopic
analysis of the 7-DK2 crystal indicated increasing absorption at
565 and 607 nm as the photoirradiation progressed (Figure 5c).
Considering the results obtained for the photoirradiation in
solution, these observed peaks were confidently assigned to the
intermediate containing a pentacene subunit. The observed red
emission additionally supports the formation of a pentacene
moiety.31 Notably, absorptions corresponding to the aggregation
of the intermediate, which should appear at 625 nm and 663 nm,
were not observed, unlike in the case of pristine pentacene.31 This
is likely as a result of the remaining bulky a-diketone groups
preventing pentacene stacking. Further photoirradiation, and/or
irradiation at a higher energy (200 µW) exhibited the same trend
as seen in solution. Specifically, no significant peaks were
observed in the region of 700–1000 nm (Figure 5d), indicating the
cleavage of a single a-diketone moiety in 7-DK2, without the
formation of heptacene. On the contrary, photoirradiation of the 7-
DK1 crystal yielded differing results (Figure 5b). Upon
photoirradiation, a broad peak appeared in the range of 550–700
nm (Figure 5e). More interestingly, characteristic peaks with
vibronic structures were observed at 652, 721, 791, and 820
Figure 5. Transmission images of (a) 7-DK2 and (b) 7-DK1 crystals before (left),
during (middle), and after (right) photoirradiation. UV-vis-NIR absorption spectra
recorded during photoirradiation of (c, d) 7-DK2 and (e, f) 7-DK1 crystals under
an ambient atmosphere. Conditions for photoirradiation: 488 nm CW laser with
50 µW.
It should be mentioned that recent studies have reported
good thermal stability of heptacene up to 420 ºC. In addition, it
has been reported that no significant changes in the spectrum of
heptacene were observed after it was stored in a glove box for six
weeks. One matter for concern is the dimerization of heptacene
in the solid state. However, the probability for the dimerization
event is strongly dependent on the packing structure.29,30
Specifically, heptacene gradually undergoes cyclodimerization to
diheptacene in the solid state, in the case of heptacene generated
by the thermal decomposition of diheptacene.29 This is likely due
to the face-to-face packing of heptacenes, resulting from minor
structural rearrangements in the solid state during the conversion.
On the other hand, considering that only minor structural
rearrangements in the solid state are possible during the
conversion process, the packing structure of 7-DK1 would impede
the dimerization process. These factors contribute to the increase
in heptacene absorption intensity with photoirradiation duration.
Additionally, conversion from the α-diketone precursor to
heptacene requires considerable structural modifications, from
the bent to the planar structure. Thus, the structural and optical
(shoulder) nm, along with
a broad absorption peak at
approximately 900 nm (Figure 5f). According to a previous report,
heptacene films deposited on a sapphire at 16 K exhibited a broad
absorption in the range of 700–800 nm with the maximum
absorption at roughly 800 nm,29 while the absorption spectrum of
heptacene, obtained by heating a solution of diheptacene in 1-
methylnaphthalene, displayed characteristic peaks at 623, 682,
753, and 792 (shoulder) nm.29 Thus, the peaks observed for the
3
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generation of photo-unstable heptacenes presents as being an
“unreasonable” strategy. However, herein, the intensity of
heptacene absorption had unquestionably increased with
increasing photoirradiation time, even under ambient conditions.
This result strongly indicated that the generated heptacene
molecules in the crystal were protected from oxidation, while the
possibility of heptacene decomposition cannot be entirely
excluded. The favorable thermal stability of heptacene following
the conversion would provide for further applications, such as
organic field-effect transistors and organic solar cells. Notably,
comparable results were obtained for three individual crystals of
7-DK1, when subjected to the same conditions, guaranteeing the
reliability of this approach.
In summary, we have demonstrated the visible-light-
induced generation of heptacene under ambient conditions.
Pivotal to the success of this method was the selective irradiation
of the crystal interior, with the generated heptacenes being
shielded by heptacene precursors on the outer regions of the
crystal. This strategy provides efficient access to otherwise
inaccessible compounds. We believe that this approach is of
general relevance for the synthesis and characterization of
unstable molecules.
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Acknowledgements
[34] Crystallographic data for 7-DK2: C34H18O4, MW = 490.48, Monoclinic,
space group C2/c, a = 21.829(5), b = 8.367(2), c = 13.012(3) Å, b =
98.919(5)°, V = 2347.8(10) Å3, T = 90 K, Z = 4, reflections measured
5298, 1743 unique. The final R1 was 0.0559 (I > 2s(I)), and the final wR
on F2 was 0.1841 (all data), GOF = 0.998.
This work was partly supported by CREST JST (No.
JPMJCR15F1) and Grants-in-Aid for Scientific Research (Nos.
JP17H03042, JP19H04584,
JP18K14195, and JP20H02816).
JP18K14190,
JP18H01958,
[35] Crystallographic data for 7-DK1: C32.5H19O2Cl, MW = 476.93, Monoclinic,
space group P21/c, a = 18.385(15), b = 6.379(5), c = 24.22(2) Å, b =
127.879(10)°, V = 2242(3) Å3, T = 90 K, Z = 4, reflections measured 9756,
3212 unique. The final R1 was 0.0978 (I > 2s(I)), and the final wR on F2
was 0.2408 (all data), GOF = 1.022.
Keywords: heptacene • single crystal • photoconversion •
isolated reaction site • precursor approach
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4
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Entry for the Table of Contents
The single-crystal interior provided a reaction site isolated from the external environment thus preventing oxidation of unstable
products. Visible-light-irradiation of the inner region of a mono-a-diketone-type heptacene precursor crystal resulted in characteristic
heptacene absorption peaks, suggesting successful heptacene generation under ambient conditions.
5
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