Using stable radicals to protect pentacene derivatives from photodegradation

Article abstract of DOI:10.1002/anie.201300595

A radical solution: The photochemical instability and insolubility in organic solvents of pentacene derivatives prevent their use in molecular electronics. These issues were solved by using pentacene derivatives with stable radicals (Y=radical moiety, 1 a

Full text of DOI:10.1002/anie.201300595

Angewandte
Chemie
DOI: 10.1002/anie.201300595
Conducting Materials
Using Stable Radicals To Protect Pentacene Derivatives from
Photodegradation**
Yusuke Kawanaka, Akihiro Shimizu, Tetsuro Shinada, Rika Tanaka, and Yoshio Teki*
Organic transistors and organic devices have a variety of
applications in molecular electronics[1–3] and in the up-and-
coming area of organic molecular spintronics.[4] The use of
inexpensive and facile ink-jet methods[5] is possible for the
fabrication of organic semiconductors. Pentacene and its
derivatives[6,7] have attracted increasing interest as promising
electronic materials owing to their high hole mobility.[8]
Pentacene is the most promising candidate for organic field-
effect transistor (OFET) applications.[3,8] However, its chem-
ical instability in the presence of light and air[9] prevents
practical applications. Efforts have been made to improve the
stability by the addition of substituents.[6,7] The most notable
example is 6,13-bis(triisopropylsilylethynyl)pentacene,[7,10] in
which the 6- and 13-positions of pentacene are protected by
triisopropylsilylethynyl groups. For this pentacene derivative,
extremely high hole mobility of 1.8 cm2 Vꢀ1 sꢀ1 was reported
for the thin film.[8] However, the addition of substituents to
photoactive carbon sites prevents further functional modifi-
cations because both the 6- and 13-positions are blocked by
the substituents and also the characteristic nature of the
pentacene moiety is changed.
Pen–Ph–NN (2a) and their precursors (1b and 2b) were
synthesized (Scheme 1; Pen, Ph, OV, and NN denote penta-
cene, phenyl, oxo-verdazyl radical, and nitronyl nitroxide
radical moieties, respectively). We demonstrate the remark-
able photochemical stability induced by the attachment of
Herein, we report a new method that utilizes a stable
radical to protect pentacene derivatives from photodegrada-
tion. During the course of our systematic studies of p-
conjugated spin systems with high-spin photo-excited states
for functional materials,[11] we have discovered that a combi-
nation of two unstable species (photoreactive pentacene and
a radical) leads to remarkable protection from photodegra-
dation and an enhancement in solubility in common organic
solvents. These effects are advantageous for practical appli-
cations of acene derivatives in molecular electronic devices.
Radicals are well-known energy scavengers of the photo-
excited state. We have utilized this characteristic of radicals to
scavenge the energy of the photoexcited state of pentacene.
Two novel radical pentacene hybrids, Pen–Ph–OV (1a) and
Scheme 1. Pentacene, 1a, 2a, and their precursors 1b and 2b,
respectively.
a radical moiety to pentacene. The electrochemical properties
of pentacene required for applications in molecular electron-
ics were conserved and photochemical instability and sol-
ubility were considerably improved by this approach.
1a and 2a were synthesized from 3 in five steps
(Scheme 2). The ESR spectrum of solutions of 1a and 2a
are shown in Figure 1a and b together with simulated spectra.
Their g values and the hyperfine splitting (see Figure 1
caption) are coincident with those of the phenylverdazyl[12]
and phenyl nitronyl nitroxide radical,[13] showing the unpaired
spin localized in the radical moiety, thus, indicating that the
characteristic electronic properties of the pentacene moiety
do not change by the attachment of a radical moiety. The X-
ray crystal structure of 1a is shown in the Supporting
Information. The molecular packing was modified compared
with pentacene because the phenyl group is twisted relative to
the pentacene plane. This will lead to a decrease in the
electron or hole mobility in the crystals. However, the
molecular packing could be improved easily by replacing
the phenyl linker with another linker such as an acetylene.
The decay profiles of the UV/Vis absorption spectra of
THF solutions of 1a, 1b, 2a, and 2b under ambient light (the
light of a fluorescent lamp in the laboratory) in the saturated
air conditions are depicted in Figure 2. In the precursors 1b
[*] Y. Kawanaka, A. Shimizu, Prof. Dr. T. Shinada, Prof. Dr. Y. Teki
Division of Molecular Material Science
Graduate School of Science, Osaka City University
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan)
E-mail: teki@sci.osaka-cu.ac.jp
Dr. R. Tanaka
X-ray Crystal Structure Analysis Laboratory
Graduate School of Engineering, Osaka City University
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan)
[**] This work was supported by the Grant-in-Aid for Scientific Research
(B) (No. 24350076) from Japan Society for the Promotion of Science
(JSPS)). We acknowledge Profs. Daisuke Kosumi and Hideki
Hashimoto (Osaka City University) for their helpful discussion.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300595.
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which is commonly used as a solvent in organic syntheses,
because of the polarity of the radical moiety. Thus, issues with
the solubility of pentacene for device fabrication were
simultaneously solved by the attachment of the radical
moiety. Similar protection from photodegradation (see Fig-
ure 2c and Figure 2d) and improvement in solubility were
observed for 2a. We also confirmed these characteristic
phenomena for CH2Cl2 solutions (see the Supporting Infor-
mation, Figures S4 and S5). Figure 3 shows the decay profiles
of the low-energy absorption band of l = ca. 590 nm of 1a, 1b,
2a, and 2b in the air-saturated THF solution (see the
Supporting Information, Figure S5 for the air-saturated
CH2Cl2 solution). The accurate wavelengths of the low-
energy lmax are listed in Table 1 as well as the decay time (t)
obtained from the least square fitting by using the following
equations;
AðtÞ=Að0Þ ¼ expðꢀt=tÞ or
ð1Þ
ð2Þ
AðtÞ=Að0Þ ¼ aexpðꢀt=t1Þ þ ð1ꢀaÞ ꢁ expðꢀt=t2Þ,
where A(t) is the absorbance of the low-energy lmax at time
t and a is the purity of the radical and the contribution of
(1ꢀa) comes probably from the precursor, because the
lifetime, t2, is close to that of the precursor. The lifetime of
1a is approximately 202 times (ca. 235 times for t1) longer
than that of 1b in THF and approximately 384 times (ca. 480
times for t1) longer in CH2Cl2, thus showing the remarkable
protection provided by the radical component. No notable
photostabilization was observed for 6-phenylpentacene (see
the Supporting Information). These results show clearly that
the present photo-stabilization does not come from the steric
buffering. Similarly, the lifetime of 2a was approximately 380
times (ca. 400 times for t1) longer than that of 2b in THF. The
photochemical stability of 2a was 500 times better than that of
pentacene in THF solution. 2a was more stable in CH2Cl2 (the
lifetime determined from the least-square fitting of the decay
is 2077 min for 2a). The corresponding data for a CH2Cl2
solution of pentacene could not be obtained because penta-
cene is not soluble in CH2Cl2.
Scheme 2. Synthesis routes for 1a, 1b, 2a, and 2b.
To investigate the electrochemical properties, cyclic
voltammetry (CV) measurements were carried out (see the
Supporting Information, Figure S6). The redox potentials of
1a, 1b, 2a, and 2b are listed in Table 1, and show that the
electrochemical properties of the pentacene moieties in these
compounds are similar to those of pentacene. The first
oxidation potential of the verdazyl radical overlaps with that
of the pentacene moiety (1a, 294 mV).[12] In contrast, the first
oxidation potential of the nitronyl nitroxide radical is split
from that of the pentacene moiety[13,14] (see 2a, 298 mV and
408 mV; see the Supporting Information, Figure S6c). A
similar situation also occurs for the first reduction potentials.
It is difficult to measure the redox potentials of pentacene
because of its insolubility. Therefore, in Table 1, we cited the
electrochemical potentials of 6,13-diphenylpentacene
reported in the literature (compound 9 in Ref. [7]) instead
of pentacene itself. Here, half-potentials (E1/2) were reported
to be E1/2[Ox] =+ 682 mV and E1/2[red] = ꢀ1396 mV vs. Ag/
Ag+, which correspond to E1/2[Ox] =+ 170 mVand E1/2[red] =
ꢀ1910 mV vs. Fc/Fc+. As the redox waves at ca. + 290 mV,
Figure 1. Observed and simulated X-band ESR spectra at room tem-
perature in toluene solution. a) Spectrum of 1a. The microwave
frequency is 9.09575 GHz. The g value and hyperfine coupling con-
stants (A) of the nitrogen nuclei and protons of the methyl groups
determined by the spectral simulation are g=2.0040, A1N(2)=0.65 mT,
A
2N(2)=0.52 mT, and AH(6)=0.52 mT, where the numbers in paren-
theses denote the number of equivalent nuclei. b) Spectrum of 2a. The
microwave frequency is 9.09108 GHz. The g value and hyperfine
coupling constants (A) determined by the spectral simulation are
g=2.0065 and AN(2)=0.750 mT.
and 2b, the absorption band at 460–610 nm, which is
characteristic of the pentacene moiety, decayed rapidly
within a few minutes because of photochemical instability
similar to that of pentacene. In contrast, almost no change was
observed for 1a under the same conditions, showing the
remarkable protection from photodegradation provided by
the stable radical. In addition, 1a was easily dissolved in THF,
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that of pentacene. Thus, only the photochemical property
was remarkably improved without loss of the electron-
donor ability.
Figure 4 shows two possible mechanisms to explain
this remarkable protection from photodegradation. Rad-
ical species are known as energy scavengers of the excited
state because of their strong non-irradiative transition to
the electronic ground state. Therefore, an effective
intramolecular energy transfer from the S1 state of the
pentacene moiety to the D1 state of the radical moiety
(Pen*(S1)–Ph–R(D0)!Pen(S0)–Ph–R*(D1)) is expected,
with subsequent rapid energy relaxation to the electronic
ground state (Pen(S0)–Ph–R*(D1)!Pen(S0)–Ph–R(D0))
by the strong non-irradiative transition, that is, the
internal conversion (IC) process of the radical moiety.
As shown in Figure 4, another possible mechanism is
intersystem crossing (ISC) enhanced by the radical
moiety; this enhanced ISC mechanism has been discussed
for the p-conjugated spin systems in our previous
work.[11a] During the energy transfer or “intersystem”
crossing from Pen*(S1)–Ph–R(D0) to the D1 state of
Pen*(T1)–Ph–R(D0), the whole spin state is unchanged.
Thus, spin-allowed transitions are expected, leading to
the effective quench of the fluorescence of the pentacene
moiety.
Figure 2. Typical UV/Vis spectra obtained at a variety of different time at
room temperature under saturated air conditions in THF solutions: a) 1a
(0.944ꢀ10ꢀ4 m); b) 1b (0.945ꢀ10ꢀ4 m); c) 2a (1.02ꢀ10ꢀ4 m); d) 2b
(0.983ꢀ10ꢀ4 m). A typical UV/Vis spectrum for pentacene is shown in
Figure S3 of the Supporting Information.
To clarify the mechanism, we measured fluorescence
spectra (see Figure S7 in the Supporting Information).
The fluorescence, resulting from the pentacene moiety,
observed for 1b and 2b was completely quenched in the
spectrum of 1a and 2a. The Fçrster mechanism[15] and other
type of the energy transfer mechanisms (Dexter mecha-
nism[16] and quantum-mechanical coherent energy-transfer
mechanism[17] through p-conjugation) are expected to occur
as well as the enhanced ISC (see the Supporting Information
for further discussion). The energy transfer or enhanced ISC
and subsequent strong non-adiabatic energy relaxation is
a plausible mechanism for the protection of these systems
from photodegradation. The lifetime of the S1-excited-state of
ꢀ1400 mV, and ꢀ1850 mV appeared in the cyclic voltammo-
grams of 1a, 1b, 2a, and 2b, we assigned them to the
pentacene moiety. The first oxidation and reduction poten-
tials (the half-potentials) of the pentacene moiety of 2a, were
+ 298 mV (310 mV) and ꢀ1372 mV (ꢀ1450 mV) vs. Fc/Fc+
(see CV curve of 2a in the Supporting Information). The
oxidation potential of 9 is expected to be lower than that of
pentacene, because of the phenyl substituents, and the
reduction potential to be higher. Therefore, the oxidation
and reduction potentials for our compounds may be more
close to those of pentacene itself. In contrast, the lifetimes of
1a and 2a are much longer (ca. 1090 times in CH2Cl2) than
Table 1: Electrochemical (oxidation and reduction potentials, E[Ox] and E[red]) in CH2Cl2 and optical properties (low-energy lmax, lifetime (t)) of
pentacene and pentacene derivatives in organic solvents (THF and CH2Cl2).
Redox potentials
vs. Fc/Fc+
THF solution
CH2Cl2 solution
E[Ox]
E[red]
lmax
t
a
t1
t2
lmax
t
a
t1
t2
[mV]
[mV]
[nm] [min]
[min]
[min]
[nm] [min]
[min]
[min]
Pentacene
(9)[c]
1a
–
ꢀ–
575 1.5ꢂ0.03
–
–
–
–
–
–
insoluble
(+170)[b]
+294,[a]
+908[b]
+246,
+838[b]
+298,
+408,
+861[b]
+289,
+850[b]
(ꢀ1910)[b]
ꢀ1380,[a]
ꢀ1881
ꢀ1382,
ꢀ1856
ꢀ1372,
ꢀ1580,[b]
ꢀ1828
ꢀ1349,
ꢀ1863
–
–
–
–
–
–
–
587 404.9ꢂ12.0 0.978 470.4ꢂ6.4 4.6ꢂ0.4
586 2.0ꢂ0.1
588 728.1ꢂ12.1 0.994 762.6ꢂ9.2 2.2ꢂ0.5
589 768.7ꢂ32.7 0.982 962.9ꢂ25.3 4.5ꢂ0.4
1b
2a
–
–
–
589 2.0ꢂ0.1
–
–
–
–
589 1760.5ꢂ31.3 0.993 2076.7ꢂ51.1
2b
587 1.9 ꢂ0.1
–
–
–
589 1.9ꢂ0.1
–
–
–
[a] The oxidation and reduction potentials of the phenyl verdazyl radical are +270 mV and ꢀ1280 mV,[12] which are close to the the potentials of 1a
(+294 mV and ꢀ1380 mV), and overlap with those of the pentacene moiety. The relative height of the oxidation wave of 1a compared to the reduction
potential is bigger than that of the precursor 1b. [b] E1/2 value. [c] These data were calculated from the potentials vs. Ag/Ag+ reported in Ref. [7].
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Thus there is potential for magnetic control of hole and/or
electron transport. A variety of potential applications of
radical species, for example in molecular spin batteries,[18] in
a high performance OFET,[19] as a photoconductive liquid-
crystalline verdazyl radical,[20] and applications derived from
the large negative magnetoresistance,[4] and the Kondo
effect[21] were recently reported, thus opening the new
research area of organic molecular spintronics. Our present
method will open a promising way toward organic molecular
spintronics and organic molecular electronics.
Experimental Section
Syntheses: The synthesis routes to the stable radicals, 1a and 2a, and
their precursors, 1b and 2b are depicted in Scheme 2. Radicals 1a and
2a were characterized by spectroscopic analysis. Full experimental
details for the synthesis are given in the Supporting Information.
Measurements: UV/Vis spectra were measured on a HITACHI
U-3500 at room temperature. Cyclic voltammetry (CV) was carried
out in a solution of CH2Cl2 (BuCN, 0.1m TBAPF6) by using
a multipurpose electrochemical apparatus (Hokuto Dennko HSV-
100) under Ar in the glove box. Measurements were made using
a standard three-electrode configuration comprising a glassy-carbon-
disk electrode as the working electrode, a Pt wire as the counter
electrode and an Ag/Ag+ electrode as the reference. Ferrocene was
used as internal standard. The fluorescence spectra were taken at
room temperature by a HITACHI F-4500T spectrometer. A conven-
tional X-band ESR spectrometer (JEOLTE300) was used for the cw-
ESR experiment. Samples used in the ESR measurements were
degassed by repeated freeze–pump–thaw cycles. X-ray diffraction
data were collected on a Rigaku AFC-7/Mercury CCD area-detector
diffractometer with graphite monochromated Mo-Ka radiation (l =
0.7107 ꢀ). The details are described in the Supporting Information.
Ab initio molecular orbital calculations were performed using the
Gaussian 09W package.[22] Ubecke 3 LYP DFT theory and 6-31G(d,p)
basis sets were applied.
Figure 3. Decay profiles of the THF solutions for the characteristic
~
*
;
absorption band of pentacene ( ; l=575 nm). 1a (blue
&
*
l=587 nm), 1b (red ; l=586 nm), 2a (violet ; l=588 nm), and
2b (green &; l=587 nm). Similar data for CH2Cl2 solutions are given
in the Supporting Information.
Received: January 23, 2013
Published online: May 21, 2013
Keywords: conducting materials · energy transfer ·
.
Figure 4. A possible mechanism for the remarkable protection from
photodegradation provided by stable radicals. Efficient energy transfer
occurs from Pen*(S1)–Ph–R(D0) to Pen*(S0)–Ph–R*(D1) (from S1
excited state of pentacene moiety to D1 excited state of the radical
moiety) followed by strong energy relaxation to the ground state by an
effective non-irradiative transition. Enhanced ISC is also a possible
mechanism.
pentacene derivatives · radicals · spintronics
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