Oxidation of rubrene, and implications for device stability

Article abstract of DOI:10.1039/c7tc05775j

The rapid spontaneous photo-oxidation of rubrene to form endo-peroxide, rubrene-Ox1, was monitored via¹H NMR and UV-vis spectroscopy. The reaction is thermally reversible, which restores high mobility devices in both the crystalline thin film and single crystal. Prolonged stirring in chlorinated solvents yields a secondary, irreversible product, rubrene-Ox2, which has lost phenol, as confirmed by single crystal analysis. An acid-catalyzed rearrangement of the endo-peroxide to form rubrene-Ox2 was identified here with Density Functional Theory (DFT). Implications of the nature of these processes for the preparation of organic transistors are described.

Full text of DOI:10.1039/c7tc05775j

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B. Lin, J. Kim, H. Lee, E. K. Burnett, L. Zhang, A. Aspuru Guzik, K. N. Houk and A. Briseno, J. Mater. Chem.
C, 2018, DOI: 10.1039/C7TC05775J.
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DOI: 10.1039/C7TC05775J
In the course of studying the formation and thermally activated cycloreversion of
oxidized rubrene to pristine rubrene, we observed an irreversible, second stage oxidized
product. Such impurities present in solution processed rubrene devices will
detrimentally impact device performance and cannot be remediated through the
application of heat to reform the parent acene. Devices from pure, thermally treated
oxidized rubrene did in fact yield high carrier mobilities in both thin films and single
crystal transistors. Understanding the formation of the irreversible adduct will help one
design more chemically robust rubrene derivatives.

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Reversible and Irreversible Oxidation of Rubrene, and
Implications for Device Stability
Jack T. Ly,a Steven A. Lopez,b,c Janice B. Lin,b JaeJoon Kim,a Hyunbuk Lee,a Edmund K. Burnett,a Lei
Zhang,*a,d Alán Aspuru-Guzik,c K. N. Houk,*b and Alejandro L. Briseno*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
transport layer and leads to permanent degradation of device
performance. Here, 1H NMR and UV-Vis were utilized to monitor
the thermal cycloreversion process of the oxidized species. The
fidelity of this cycloreversion process to restore the intrinsic
properties of rubrene was demonstrated via the fabrication of high
mobility devices from previously oxidized rubrene. Prolonged
photo-oxidation in CHCl3 solution converts the endo-peroxide
irreversibly to rubrene-Ox2. It should be noted that this secondary
degradation product had been reported in 1955 23 through an acid
treatment of rubrene-Ox1, to form the same product we report
here.22–24 However, in that report, neither the mechanism of
product formation nor the crystal structure were reported. Herein,
we report the first crystal structure of rubrene-Ox2, and through
quantum mechanical computations, propose an acid catalyzed
endo-peroxide rearrangement mechanism for the generation of
rubrene-Ox2.
Dedicated to our esteemed friend and mentor, Fred Wudl
The rapid spontaneous photo-oxidation of rubrene to form
1
endo-peroxide, rubrene-Ox1, was monitored via H NMR and
UV-Vis spectroscopy. The reaction is thermally reversible,
which restores high mobility devices in both the crystalline
thin film and single crystal. Prolonged stirring in chlorinated
solvents yields a secondary, irreversible product, rubrene-
Ox2, which has lost phenol, as confirmed by single crystal
analysis. An acid-catalyzed rearrangement of the endo-peroxide
to form rubrene-Ox2 was identified here with Density Functional
Theory (DFT). Implications of the nature of these processes for the
preparation of organic transistors are described.
Organic semiconductors (OSCs) have attracted interest in large-
area devices due to their mechanical flexibility, solution
processability, and performance that rivals that of amorphous
silicon.1–5 Although these materials appear as attractive candidates
for next-generation semiconductor materials, their propensity to
degrade under ambient conditions has hindered their commercial
use. Even the benchmark organic semiconductor, rubrene, one of
the best performing materials,6–9 has been shown to readily oxidize
in both solution and amorphous thin films.10–16 The photo-oxidation
of rubrene has been well documented,16,17 involving the
photosensitized formation of singlet oxygen and subsequent
formation of the endo-peroxide Diels-Alder adduct.18 In our efforts
to monitor and clarify the photo-oxidation of rubrene11,17,19,20 and
its thermal cycloreversion,21 we observed an irreversible second
stage oxidation product, rubrene-Ox2. In addition, we describe an
acid-catalyzed process that irreversibly destroys the active
aDepartment of Polymer Science and Engineering, University of Massachusetts
Amherst, Amherst, Massachusetts 01003, USA. Email: abriseno@mail.pse.umass.edu
bDepartment of Chemistry and Biochemistry, University of California, Los Angeles, Los
Angeles, California, 02138, USA. Email: houk@chem.ucla.edu
cDepartment of Chemistry and Chemical Biology, Harvard University, Cambridge,
Massachusetts, 02138, USA.
dCollege of Energy, Beijing University of Chemical Technology, Beijing 100124, China.
Email: zhl@mail.buct.edu.cn
Figure 1. A) Molecular structure of rubrene, rubrene-Ox1, rubrene-
J. Name., 2013, 00, 1-3 | 1
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Ox2. B) 1H NMR oxidation process of rubrene and pure rubrene- displaying the efficiency of the thermolysis. Additionally, 1H NMR
Ox1. spectra (Figure 2B) were collected for the thermolysis of rubrene-
Ox1 powder at the same temperature under nitrogen
a
Rubrene-Ox1 can be easily accessed in high yield (97%) by
atmosphere; cycloreversion approached full conversion in the first
10 minutes with the disappearance of the peroxide adduct peaks
around 6.75 ppm. The bulk, white powder converted to a vibrant
red color during this heating process.
stirring a rubrene solution in chlorinated solvents under ambient
laboratory conditions. All solution concentrations for this study
were 2x10–5 M in chloroform. The formation of rubrene-Ox1 was
monitored using 1H NMR (Figure 1) through the appearance of a
new triplet and doublet peak at 6.86 and 6.75 ppm, respectively.
The oxidation of rubrene can be monitored visually as the
After 120 minutes, full conversion to rubrene-Ox1 was determined. solution color changes from red to colorless. Stirring for four days
The nmr spectrum matched that of separately purified rubrene- results in a yellow solution, indicating that a subsequent reaction in
Ox1, shown in the bottom spectrum of Figure 1B. The chlorinated solvents occurred. A previous report observed similar
characterization of rubrene-Ox1 is consistent with previously results when treating a rubrene solution with CF3COOH, yielding a
reported work.25,26
yellow product, identified as indenonaphthacene.27 That product,
however, is a different structure from the compound observed
here, and was not a product observed in our studies. The structure
of the purified rubrene-Ox2 was confirmed to be the structure
1
shown via single crystal analysis and H NMR (Figure 3). The crystal
structure of rubrene-Ox2 in Figure 3B shows that the molecular
structure is related to indenonaphthacene with the exception of a
ketone at the C-1 position in place of a phenyl substituent. This
secondary rearrangement product also lost the elements of phenol.
Prolonged photodegradation of rubrene was performed in
a
chloroform solution neutralized by Na2CO3. No observable yellow
color was produced after 4 days; however, upon the addition of
CH3SO3H, the solution immediately became yellow (see Supporting
Information). The net loss of
irreversible.
a phenol renders the process
Figure 2. A) UV-Vis spectra of thin films of rubrene-Ox1 heating at
150 oC for 30 mins (red), rubrene-Ox1 (black), and rubrene (blue),
and B) 1H NMR spectra for pristine rubrene, heat treated rubrene-
Ox1, and rubrene-Ox1.
Figure 3. Crystal structures of, A) rubrene-Ox1, B) rubrene-Ox2, and
C) 1H NMR of rubrene-Ox2.
The degradation profile of rubrene in solution was also
investigated through UV-Vis absorption to complement 1H NMR
experiments. In solution and amorphous films, pristine rubrene has
three signature absorption peaks at 529, 494, and 464 nm while
rubrene-Ox1 has no distinct peaks in the visible spectra range. UV-
Vis measurements were carried out on both solution state and thin
films of pristine rubrene and rubrene-Ox1. A rubrene half-life value
of 26 minutes in solution was determined through the careful
monitoring and depletion of the three distinct rubrene peaks under
laboratory conditions (see Supporting Information). Rubrene-Ox1
thin films were annealed to give cycloreversion to reform the
parent rubrene. Upon heating the thin films of rubrene-Ox1 at 150
˚C, distinct absorption peaks of rubrene emerged within minutes.
After 30 minutes of heating, the thin film spectrum of the thermally
treated rubrene-Ox1 sample matches that of pristine rubrene,
Figure 4 shows the proposed acid-catalyzed mechanism for
rubrene endo-peroxide rearrangement to rubrene-Ox2. Chloroform
is known to contain acidic impurities (HCl).28 The proposed
mechanism
was
evaluated
using
M06-2X/6-311+G(d,p)-
IEFPCMCHCl3//M06-2X/6-31G(d,p)-IEFPCMCHCl3 and B3LYP-D3BJ/6-
IEFPCMCHCl3calculations. Both methods predict the same
mechanism, but the following discussion is based on the computed
free energies relative to rubrene and 3O2 using M06-2X/6-
311+G(d,p)-IEFPCMCHCl3//M06-2X/6-31G(d,p)-IEFPCMCHCl3
The
.
protonation of endo-peroxide rubrene-Ox1 by model acid, H3O+,
gives cation, 2. The protonated endo-peroxide undergoes a ring
opening to afford the stabilized carbocation 3, which is 27.2 kcal
mol–1 lower in energy than 2. Attack of the cation at C-5 produces
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Wheland intermediate 4, which is higher in energy than 3 by 5.4 Finally, to demonstrate the reversibility of rubrene-Ox1 oxidation,
kcal mol–1. Subsequent deprotonation of 4 by H2O gives 5, which
we carried out physical vapor transport experiments with bulk
rearranges via a [1,2]-Ph shift and dehydration to give 6. Finally, 6
loses PhOH to afford the observed product, rubrene-Ox2.
oxidized rubrene as the source material. This experiment was
carried out at 300 oC in which two events occur: (1) the
cycloreversion process, and (2) the sublimation of the reformed
rubrene. It is interesting to note that the cycloreversion and
sublimation occur in one step to produce large, ultrathin rubrene
crystals with high purity. We fabricated single crystal transistors
from these crystals and measured carrier mobilities approaching 1.5
cm2 V-1 S-1, with Vth of 3 V, and current on/off ratios of ~106. Figure
5E shows the transfer characteristics of the single crystal rubrene
device. Output characteristics for the same device exhibits excellent
current modulation when scanned from 0 to -40 V (see supporting
information). These characteristics demonstrate that rubrene
crystals grown from a rubrene-Ox1 source can reproduce the fully
conjugated form to yield high mobility transistors. This experiment
clearly shows the reversibility of rubrene-Ox1 to rubrene to form
fully functional devices.
Figure 4. A) Proposed acid-catalyzed rearrangement mechanism. B)
Computed free energies of the proposed mechanism of rubrene-
Ox1 rearrangement to rubrene-Ox2 using M06-2X/6-311+G(d,p)-
IEFPCMCHCl3//M06-2X/6-31G(d,p)-IEFPCMCHCl3
.
Having established the structures of the rubrene oxidation
products and the means and mechanisms of interconversions, we
turned to exploration of device characteristics under different
conditions. Although rubrene-Ox2 is formed irreversibly, rubrene-
Ox1 could be a potential precursor to functional transistors. Known
examples of acene precursors often employ light and or heat-
sensitive sufinylacetamide and monoketone moieties to yield active
semiconductors.29–37 To evaluate the semiconductor properties of
thermally recovered rubrene, we fabricated thin-films via spin
casting from solution. Pristine rubrene and reduced rubrene from
the endo-peroxide are annealed to form the orthorhombic
crystals.38 As shown in Figure 5a-c, pristine rubrene and reduced
rubrene-Ox1 show interfacing crystallites with different colors
corresponding to the desired crystalline phase39–42 while rubrene-
Ox1 instead exhibits an amorphous, featureless film. Transfer
characteristics of each film are shown in Figure 5d. Pristine and
reduced rubrene-Ox1 showed hole mobility of 0.037 and 0.009 cm2
V-1 s-1, Vth of 16 and 13 V, and routine current on/off ratio of ~104
and ~103, respectively. While the mobilities are rather low, we
produced the first example of additive-free solution-processed
rubrene.43,44 The lower mobility of reduced rubrene-Ox1 may result
from the presence of a rubrene-Ox1 impurity post-thermolysis or
structural disorder at the dielectric-semiconductor interface.
Figure 5. Polarized optical images of A) pristine rubrene, B)
rubrene-Ox1, and C) reduced rubrene-Ox1 crystalline thin films.
Transfer characteristics of D) pristine rubrene, rubrene-Ox1,
reduced rubrene-Ox1 thin film transistors, and E) single crystal
rubrene crystal transistor (grown from rubrene-Ox1 source via
PVT).
Conclusions
In the course of studying the formation and thermally activated
cycloreversion of rubrene-Ox1 to pristine rubrene, we observed an
irreversible, second stage oxidized product - rubrene-Ox2. It was
found that acid impurities in chlorinated solvents lead to
a
pyramidalized ketone derivative, which is inactive in organic
transistors. Using DFT, we identify the lowest energy pathway to
the acid catalyzed formation of this irreversible product. Such
impurities present in solution processed rubrene devices will
detrimentally impact device performance and cannot be
remediated through the application of heat to reform the parent
acene. Devices from pure, thermally treated rubrene-Ox1 did in fact
yield high carrier mobilities in both thin films and single crystal
This journal is © The Royal Society of Chemistry 20xx
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ACKNOWLEDGMENT
J.L. and K.N.H. acknowledge funding from the National Science
Foundation (DMR-1508627); L.Z. thanks the Office of Naval
Research (N000147-14-1-0053) and Science Foundation of
China (21672020). J.L and A.L.B acknowledge the National
Science Foundation (DMR-1508627). S.A.L acknowledges the
DOE EERE Postdoctoral fellowship for funding. A.A.-G.
acknowledges the Department of Energy for support (DE-
SC0015959). We thank Dr. Sean Parkin at the University of
Kentucky for crystal structure analysis.
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