Photochemical Degradation of Various Bridge-Substituted Fluorene-Based Materials

Article abstract of DOI:10.1021/acs.jpca.6b02127

Photochemical degradation is an important issue to be overcome in advancing the lifetime of fluorene-containing conjugated polymers. In order to optimize the inertness of the materials, a quantitative measure for the efficiency of degradation is needed. Here, we introduce a method to measure a relative quantum yield of the photochemical degradation by monitoring the kinetics of the process by means of UV/vis spectroscopy and liquid chromatography (LC) techniques. This method is employed to a set of differently substituted 2,7-diphenylfluorenes, serving as model compounds for polyfluorene materials. Our measurements show that the quantum yield changes by orders of magnitude upon varying the bridge substituents and that altered kinetics indicate changing degradation mechanisms.

Full text of DOI:10.1021/acs.jpca.6b02127

Article
pubs.acs.org/JPCA
Photochemical Degradation of Various Bridge-Substituted Fluorene-
Based Materials
Bjorn Kobin, Sandra Behren, Beatrice Braun-Cula, and Stefan Hecht*
̈
Department of Chemistry & IRIS Adlershof, Humboldt-Universitat zu Berlin, 12489 Berlin, Germany
̈
S
* Supporting Information
ABSTRACT: Photochemical degradation is an important
issue to be overcome in advancing the lifetime of fluorene-
containing conjugated polymers. In order to optimize the
inertness of the materials, a quantitative measure for the
efficiency of degradation is needed. Here, we introduce a
method to measure a relative quantum yield of the
photochemical degradation by monitoring the kinetics of the
process by means of UV/vis spectroscopy and liquid
chromatography (LC) techniques. This method is employed
to a set of differently substituted 2,7-diphenylfluorenes, serving
as model compounds for polyfluorene materials. Our measurements show that the quantum yield changes by orders of
magnitude upon varying the bridge substituents and that altered kinetics indicate changing degradation mechanisms.
should prove viable to the design of long-lived blue emitter
materials.
INTRODUCTION
■
Over the past years there has been a lot of research in the field
of organic electronics devoted toward exploring specific
advantages of organic materials associated with their potentially
low-cost manufacturing, light weight, and flexibility, while at the
same time trying to overcome their disadvantages related to the
efficiency and longevity of organic-based devices.1 Regarding
the aspect of enhancing lifetime, this has been a particularly
challenging topic in the context of absorbing and emitting
devices, such as organic photovoltaic cells and organic light
emitting diodes (OLEDs). In the latter case especially the
lifetime of blue emitting OLEDs is still limited.2 With regard to
conjugated polymers as blue emitting materials in OLEDs,
polyfluorenes (PFs) as well as ladder-type poly(p-phenylene)s
(LPPPs)3 have extensively been exploited,4,5 but suffer from
relatively fast (photo)degradation. In PFs the degradation
process is typically accompanied by an emerging green
emission, which has been attributed to either fluorenone
defects, hydroxyl (chain end) defects, cross-linking, or excimer
formation.6−22 Most studies propose fluorenone type defects,
which may be formed thermally23,24 as well as photochemically,
by direct excitation25,26 or via singlet oxygen mediated
oxidation,27,28 and typically occur at synthetic defect sites.
Clearly, the substitution in the bridging benzylic 9-position of
the fluorene moiety plays a crucial role in the degradation
process. This is exemplified by the observation that
introduction of aryl moieties in this position, in particular as
the spiro-bifluorene motif, renders the respective polymers
more stable toward thermal and photochemical degrada-
tion.29−37 However, no systematic study comparing the
influence of a larger variety of substituents in the 9-position
of a fluorene-type backbone has been reported thus far and yet
In this contribution, we describe the synthesis and
characterization of a variety of differently substituted fluorene
derivatives and compare their stability toward photochemical
degradation in solution in a quantitative manner. At this point
we want to emphasize that the term “stability” is somewhat
misleading as it refers to a thermodynamic property of the
ground state,38 while photodegradation is an inherently kinetic
phenomenon, in which the availability of reaction pathways
originating from an electronically excited state determines the
outcome of the photochemical reaction. Thus, we prefer to use
the term “inertness” as it refers to the lack of reactivity (and not
necessarily stability).39 This does not mean that we reject the
validity of the laws of thermodynamics, but minimization of the
ground state Gibbs free energy of the system does not
constitute the main driving force. On a more quantitative level,
the rate of a photoreaction is dependent on the intensity of
absorbed light, and hence the sensitivity toward photo-
degradation is most accurately described by a reaction quantum
yield. Since the absolute quantum yield of the unwanted
photodegradation process is typically very low and thus difficult
to measure, we chose to determine relative quantum yields,
which are normalized to the most inert, i.e., least reactive,
compound in the investigated series. In order to facilitate the
kinetic analysis of degradation, we decided to use small
molecules that carry only one fluorene unit and hence only one
methylene bridge in organic solution under ambient conditions.
To shift the optical transitions to lower energies, the fluorene
Received: February 29, 2016
Revised: June 8, 2016
© XXXX American Chemical Society
A
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core was substituted by two terminal phenyl units in 2- and 7-
positions, therefore giving rise to the investigated, differently
substituted 2,7-diphenylfluorene (DPF) derivatives (Scheme
1). The methylene bridge was symmetrically substituted with a
Scheme 1. Chemical Structures of the Investigated Fluorene
Derivatives
variety of groups, including methyl (DPF-Me2), hydrogen
(DPF),40 phenyl (DPF-Ph2), spiro-bifluorene (DPF-sp),
fluorine (DPF-F2), and oxo (DPF-O)40,41 substituents.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. The target
compounds were synthesized by Suzuki cross-coupling42 of
phenylboronic acid with the different central 2,7-dibromo-
fluorene units. The latter ones included 2,7-dibromo-9,9-
difluorofluorene, the central building block for DPF-F2, which
was synthesized according to a simplified protocol by
Katzenellenbogen,43 while 2,7-dibromo-9,9-diphenylfluorene
was prepared starting from 2,7-dibromofluorenone via a
sequence of ring opening, esterification, phenyl lithium
addition, and Friedel−Crafts alkylation according to slightly
modified literature procedures.30,44 More information about
synthesis and characterization can be found in the Supporting
Information.
Figure 1. Molecular structures in the crystalline solid of DPF-Me2,
DPF-sp, DPF-F2, and DPF-O. For DPF-F2 only one of the two
different molecular structures is shown. Atomic displacement
parameter (ADP) plots45 show atomic positions as ellipsoids drawn
at the 50% probability level.
and DPF-sp the least planar one. From the above comparison it
becomes apparent that the phenyl rings are twisted somewhat
out of plane most likely to reduce steric repulsion of the ortho
hydrogen atoms, similarly to the classical biphenyl twist. This
twist slightly reduces the overlap across the phenyl−fluorene
connection and hence limits π-conjugation in the solid.
However, due to the low barrier of rotation this effect should
be negligible in solution.
Optical Properties. The absorption and emission spectra
of all investigated compounds have been recorded in ethyl
acetate solution at room temperature (Figure 2, Table 1), and
the corresponding absorption and emission maxima as well as
emission quantum yields are summarized in Table 1. Additional
data regarding the use of other organic solvents are compiled in
the Supporting Information.
The all-hydrocarbon compounds (DPF, DPF-Me2, DPF-Ph2,
and DPF-sp) show more or less the same behavior in
absorption: There are two maxima or one maximum and one
shoulder between 310 and 330 nm. The first maximum is
slightly stronger than the second one, except for DPF-sp. Here,
the stronger absorption in the second maximum can be
attributed to the additional absorption of the spiro-bifluorene
unit. The shape of the emission spectrum is even more similar:
In the case of four derivatives we were able to obtain crystals
suitable for single crystal X-ray structural analysis, which
provided an accurate depiction of the molecular geometry in
the solid state (Figure 1). DPF-Me2 and DPF-sp crystallize in
Pna21 (orthorhombic), DPF-F2 in P21/m (monoclinic), and
DPF-O46 in Cmc21 (orthorhombic) space groups. None of the
structures incorporate solvent molecules. DPF-F2 and DPF-O
possess mirror symmetry with respect to the orientation of the
phenyl units. DPF-F2 exists in two different conformations in
the crystal structure. The twist of the planes of the two terminal
phenyl moieties with regard to the central fluorene plane
amounts to 27.08(7)° and 31.43(7)° in the case of DPF-F2 and
35.93(5)° for DPF-O and ranges between 32.22(10)° and
37.28(9)° in the case of DPF-Me2 and 40.27(6)° and 43.84(6)°
for DPF-sp, which shows that the latter two molecules are not
symmetrical with regard to the rotation of the terminal phenyl
substituents, presumably due to packing effects. Based on the
relative orientation of the aromatic planes DPF-F2 can be
considered the most planar of these structures in the solid state
B
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Figure 2. Absorption (solid line) and normalized emission (dotted) spectra of the DPFs in ethyl acetate, ≈10−5 mol L−1: DPF (red), DPF-Me2
(black), DPF-Ph2 (blue), DPF-sp (magenta), DPF-F2 (green), and DPF-O (yellow).
a low energy emission related to that transition with a
maximum around 540 nm and a ΦPL of about 0.04.
Table 1. Absorption and Emission Data of Investigated
Compounds in Solution
a
Photochemical Degradation. The experiments were
carried out in ethyl acetate as the solvent because it does not
undergo solvent-specific reactions, such as chloroform and
methylene chloride, and degradation occurs faster than in
toluene, thereby facilitating kinetic studies.28 First insight into
the photodegradation of the different derivatives was gained by
monitoring the absorption spectra upon prolonged irradiation
with UV light (300−400 nm) as illustrated for DPF and DPF-
Me2 in Figures 3a and 3b. In both cases the main absorption
band decreases while a low-energy tail and some blue-shifted
band are building up. In the case of DPF-Me2 the photo-
degradation is initially relatively slow, but then the process
clearly accelerates (Figure 3c). The respective extinction
difference (ED) plot47 (Figure 3a, inset) does not show a
linear relationship, which provides a clear indication that no
uniform photochemical reaction, converting the substrate (A)
directly into one product (B), is taking place and instead several
parallel/serial reactions seem to occur.
εmax/103,
L mol−1 cm−1
λem
/
λex/
ΦPL
compound
DPF
λmax/nm
322
(λ/nm)
nm
nm (CH2Cl2)
45 (322)
48 (325)
39 (330)
42 (309)
352,
369
322
325
330
320
0.5
0.8
DPF-Me2 325
356,
372
DPF-Ph2 313, 330
357,
374
<0.01
1.0
DPF-sp
309, 332
317
357,
374
DPF-F2
DPF-O
45 (317)
19 (322)
396
538
318
322
<0.01
0.04
286, 322,
335, 431
a
In ethyl acetate. λmax: absorption maxima. εmax(λ): absorption
coefficient and corresponding wavelength. λem: emission maxima. λex:
corresponding excitation wavelength. ΦPL: photoluminescence quan-
tum yield.
The observed acceleration effect may be related to an
autocatalytic degradation process involving singlet oxygen, as
we have recently proposed for methylated ladder-type oligo-p-
phenylenes.28 In later stages of the degradation process (not
shown here) the absorption broadens and evenly decreases
upon further irradiation. UPLC-UV/vis-absorption analysis
(Figure 3e) of degraded solutions shows a large variety of
products. This is in agreement with the ED plots,
demonstrating the large variety of involved photoreactions.
The two aryl substituted compounds DPF-Ph2 and DPF-sp also
show the same behavior (see Figures S4.3 and S4.4).
In the case of the nonmethylated derivative DPF the
conversion is much faster. The reaction starts rapidly,
decelerates over time, and stops more or less at a certain
point (Figures 3b and 3d). The ED plot (Figure 3b, inset)
clearly shows a linear relationship, and linear fitting provides
lines, whichmore or lesscross the origin. This kind of
kinetics may be related to a first-order reaction, as expected for
a uniform photoreaction (A → B). Furthermore, it was found
that the final absorption spectrum (red line in Figure 3b) is
reminiscent of the absorption spectrum of DPF-O, which was
unambiguously confirmed by UPLC analysis (Figure 3f). The
degradation of the fluorinated derivative DPF-F2 seems even
faster, also giving DPF-O as reaction product. However, the ED
plot (see Figure S4.2), which is not strictly linear, as well as the
initial increase and subsequent decrease of the band at 3 eV
There are two main peaks with nearly equal intensity between
350 and 380 nm, and two shoulders at lower energies. This
vibronic progression is related to lower flexibility of the
molecules in the excited state. Except for a slight blue shift of
DPF, the normalized PL spectra are very similar, however, the
effect of the substituent in the methylene bridge on the PL
quantum yield ΦPL in CH2Cl2 is dramatic: For DPF ΦPL is
about 0.5, methylation (DPF-Me2) increases ΦPL to 0.8, and for
the spiro-compound DPF-sp ΦPL is practically unity. But
removing the spiro linkage between the two phenyl substituents
in the 9-position leads to a drop of ΦPL below one percent for
DPF-Ph2. This may be attributed to the ability of the 9-phenyl
residues to freely rotate in the excited state. Note that the 2,7-
phenyl groups are not supposed to be freely rotating in the
excited state due to partial double bond character of the
phenyl−fluorene bond. DPF-F2 has an absorption maximum
around 320 nm and a shoulder at the low-energy side; the
emission is shifted to lower energies compared to the
hydrocarbons, having its maximum around 400 nm, and ΦPL
is negligible (<1%). The ketone DPF-O shows a behavior that
is typical for the fluorenone defect. In the region between 350
and 300 nm the absorption is decreased compared to the other
compounds, but below 300 nm it is strongly increased. Below
the “normal” absorption edge, a new, weak absorption band can
be found, which reaches far into the visible region. There is also
C
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Figure 3. Change in absorption upon photochemical degradation in ethyl acetate upon irradiation with light of 290−410 nm: (a) Changes in the
absorption spectrum of DPF-Me2, initial concentration 1.50 × 10−5 mol L−1, time interval 1 h. Inset: Extinction-difference (ED) plot, ED at 370 nm
vs ED at 325 nm (red squares, solid line: linear fit), ED at 270 nm vs ED at 325 nm (black circles, solid line: linear fit). (b) Changes in the
absorption spectrum of DPF, initial concentration 1.12 × 10−5 mol L−1, time interval 10 min. Inset: ED plot, ED at 285 nm vs ED at 325 nm (red
squares, solid line: linear fit), ED at 430 nm vs ED at 325 nm (black circles, zoomed x10, solid line: linear fit). (c) Absorbance of DPF-Me2 at 325
nm vs irradiation time. (d) Absorbance of DPF at 323 nm vs irradiation time. (e) Absorption trace (330 5 nm) of a liquid chromatography run of
pristine (black) and photodegraded (red) DPF-Me2. (f) Absorption trace (280 5 nm) of a liquid chromatography run of pristine DPF (blue),
DPF-O (red), and degraded DPF (black).
rather suggest a two-step process with some intermediate being
formed. In summary, UV irradiation of DPF and DPF-F2 leads
to rapid conversion to DPF-O without formation of significant
amounts of byproducts. The photodegradation of DPF-O itself
was also investigated, but it is much slower than all the other
derivatives, a fact that can be attributed to the lower extinction
coefficient at the irradiation wavelengths.
The strict determination of a quantum yield of the
accelerating DPF-Me2-type kinetics is somewhat difficult,
because one would need to know the exact reaction mechanism
and identify the photochemical step. Furthermore, quantum
yield values yet corresponding to different mechanisms could
not really be compared. Accordingly, we decided to treat the
different reactions as if they were of A → B type and analyzed
the initial absorption decay, associated with the first few percent
of consumed starting material. The quantum yield of that
simplified photoreaction can be determined according to eq 1
(for details see Supporting Information as well as the classic
work by Mauser48):
dE(λobs
dt
)
1
1
φ* = −
·
·
−EA(λir)
*
εA(λobs)β
∑
I0 (λir)(1 − 10
)
λir
(1)
Here, dE(λobs)/dt denotes the change of absorbance at the
observation wavelength, εA(λobs)β gives the relation of the first
term to a change in concentration, and the last term accounts
for the amount of absorbed light. The “*” in I*0 and φ* denotes
D
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that the absolute light intensity I0 is not known, but I0(λir) is at
least proportional to the actual light intensity. Hence, φ* is also
not the actual quantum yield, but a number that is proportional
to it. Finally, it will be normalized to the slowest degrading
compound to give a relative efficiency of degradation.
According to that, the slowest degrading compound will have
a φ* of 1. The larger the φ* of the other compounds, the larger
is their quantum yield for photodegradation, which was not
determined in absolute numbers. β is a factor relating a change
of concentration of the starting material to a change in
absorbance. It can be calculated as β = εA − εB/εA, when the
extinction coefficient of the product is known and the product
is stable. So for DPF β = 0.58 and for DPF-F2, β = 0.64 (see
Table 2). In the case of the other three compounds, the
conversion. Accordingly, the β-values for DPF-Me (β = 0.64),
DPF-Ph2 (β = 0.55), and DPF-sp (β = 0.59) were determined.
These values are comparable to the ones that were calculated
from the extinction coefficient of the known degradation
product.
Finally, the relative efficiencies of photodegradation for all
different DPFs (Table 2) were determined according to eq 1.
From these data one can conclude that the two aryl-substituted
compounds DPF-Ph2 and DPF-sp degrade equally the slowest
(φ* = 1). The gem-dimethyl substituted DPF-Me2 degrades
roughly three times faster. However, the unsubstituted parent
DPF degrades another 100 times faster (φ* = 290) and is
therefore much more prone to photodegradation. Surprisingly,
degradation of the fluorinated DPF-F2 is by far the fastest (φ*
= 2200), leading to a rapid and clean photoconversion to DPF-
O.
a
Table 2. Relative Efficiencies of Photodegradation
compound
β
φ*
3.3
290
1.0
1.0
2200
CONCLUSION
■
DPF-Me2
DPF
0.64
0.58
0.55
0.59
0.64
In this contribution we present a method for quantifying the
susceptibility of fluorene-containing conjugated materials
toward photodegradation. The kinetics of photodegradation
of a series of differently bridge-substituted 2,7-diphenylfluor-
enes as model compounds for polyfluorenes has been
investigated. While hydrogen or fluorine substituents in the
bridge exhibit typical decelerating first-order photokinetics
leading to the fluorenone product, in the case of aryl and alkyl
substitution degradation accelerates in the beginning and gives
a variety of different photoproducts. This points to fundamental
differences in the mechanism of photodegradation. Quantifica-
tion of the relative efficiency of the studied fluorenes shows that
aromatic substituents, such as phenyl or spiro-bifluorene, give
rise to the most robust derivatives. Replacing the aryl by alkyl
substituents leads to some loss of inertness, as indicated by the
methylated derivative degrading about three times more
efficiently. However, the unsubstituted parent derivative as
well as the fluorinated one are much more prone to
photodegradation, with the latter one degrading more than 3
orders of magnitude more efficiently than the most stable
derivatives. Taking into account both inertness toward
photodegradation and luminescence efficiency, the spiro
compound DPF-sp represents the optimal candidate. Alter-
natively, alkyl substitution still shows moderate degradation
combined with a good fluorescence quantum yield (if properly
purified). These findings should help to better understand
substituent effects in fluorene-type materials and improve their
photostability. Furthermore, we illustrate a general method to
quantify photodegradation in complex compound mixtures.
DPF-Ph2
DPF-sp
DPF-F2
a
β describes to which extent a change in concentration of the substrate
leads to a change in absorbance. φ* is the relative efficiency of
degradation.
determination of β is more demanding: While the large variety
of degradation products would still allow for the determination
of an effective εB of the product mixture, degradation does not
stop at a certain stage. However, it is possible to find an
experimental relationship between the absorbance and the
concentration of the starting material. For this purpose,
solutions of the DPF derivatives were irradiated in a
photoreactor at 300 nm and the decreasing concentration of
starting material was monitored by UPLC (integration of the
absorption peak in the chromatogram). Therefore, the
normalized absorbance of the sample can be plotted versus
the normalized concentration of the starting material (Figure
4), and for all of these three derivatives a practically linear
relationship was found. Importantly, β can be obtained from
the slope of the linear fit (Figure 4, inset), which was only
applied to the data representing the first 20% of photo-
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpca.6b02127.
Experimental synthetic details, crystal structure data,
background information on photoreaction and fluores-
cence quantum yields, and optical and NMR spectra
(PDF)
Crystallographic data for DPF-Me2 (CIF)
Crystallographic data for DPF-sp (CIF)
Crystallographic data for DPF-O (CIF)
Crystallographic data for DPF-F2 (CIF)
Figure 4. Determination of β by absorbance and UPLC. The
normalized absorbance of the product mixture is plotted versus the
normalized concentration, which was determined by UPLC (in-
tegrated absorption trace) for DPF-Me2 (red diamonds), DPF-Ph2
(black circles), and DPF-sp (blue squares). Inset: Zoom into the zone
of the linear fits.
E
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Article
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AUTHOR INFORMATION
Corresponding Author
*E-mail: sh@chemie.hu-berlin.de. Phone: +49 (0)30 2093-
7365.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Martin Herder for valuable discussions on
the evaluation of photochemical reactions by means of UV/vis
spectroscopy. Generous support by the German Research
Foundation (DFG via SFB 951) and the Helmholtz Association
(via Helmholtz Energy Alliance) is gratefully acknowledged.
BASF AG, Bayer Industry Services, and Sasol Germany are
thanked for generous donations of chemicals.
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