Design principles to tune the optical properties of 1,3,4-oxadiazole- containing molecules

Article abstract of DOI:10.1039/b616996a

We have synthesized a series of oxadiazole compounds and ethynylene analogs. Our data reveal that the ring is both optically transparent in the visible range and fully conjugating while we have also discovered the presence of a non-radiative mechanism active in molecules containing common para-dialkoxy substituents adjacent to the oxadiazole ring(s). This structure leads to a greatly reduced quantum yield, in our example dropping from 95.0% to 48.0%. Through our thorough study we have revealed evidence that this is the result of a repulsive interaction between the oxadiazole and the adjacent alkoxy oxygen atom, which we believe prevents excited-state planarity. This quantum yield reduction is preventable through the design principles presented here. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b616996a

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www.rsc.org/materials | Journal of Materials Chemistry
Design principles to tune the optical properties of 1,3,4-oxadiazole-
containing molecules{
Onas Boltona and Jinsang Kim*ab
Received 21st November 2006, Accepted 11th January 2007
First published as an Advance Article on the web 30th January 2007
DOI: 10.1039/b616996a
We have synthesized a series of oxadiazole compounds and ethynylene analogs. Our data reveal
that the ring is both optically transparent in the visible range and fully conjugating while we have
also discovered the presence of a non-radiative mechanism active in molecules containing
common para-dialkoxy substituents adjacent to the oxadiazole ring(s). This structure leads to a
greatly reduced quantum yield, in our example dropping from 95.0% to 48.0%. Through our
thorough study we have revealed evidence that this is the result of a repulsive interaction between
the oxadiazole and the adjacent alkoxy oxygen atom, which we believe prevents excited-state
planarity. This quantum yield reduction is preventable through the design principles presented
here.
functional detriment to adding such sidechains to 1,3,4-
Introduction
oxadizole compounds.
The future of organic opto-electronics depends on the
In this work we present an in-depth investigation into this
improvement of device efficiency and longevity to levels
phenomenon and the specific effect 1,3,4-oxadiazole moieties
competitive with established inorganic alternatives. The overall
have on the optical properties of their compounds by studying
device efficiency can be greatly improved by the balance of
a complete array of oxadiazole oligomers and a small
charge mobilities in the device’s active materials. While most
complement of oligo(phenylene ethynylenes).
organic semiconducting compounds exhibit good hole mobility
Conjugated molecules containing either the 1,3,4-oxadiazole
and poor electron mobility, a handful of electron-transporting
or, in their place, ethynyl moieties have been synthesized and
organic moieties are known and have been employed to
compared systematically. The structures of each can be seen in
improve electron transport in optoelectronic materials. Among
Fig. 1 and again in Fig. 2–7. Alkoxy or alkyl substituents are
this group there have been none more popular than the 1,3,4-
varied in presence and position amongst the molecules at the end
oxadiazole moiety, which is shown in Scheme 1a.1,2
and central phenyl rings. These chains are added to mimic and
The exploration of conjugated 1,3,4-oxadiazole-containing
study those common to poly(phenylene oxadiazole) polymers.
polymers and small molecules has led to a host of new materials
favorable for opto-electronics by offering not only improved
balance of charge mobility but enhanced thermal and photo-
stability as well. Having robust functionality and many well-
documented synthetic routes, 1,3,4-oxadiazole has been recently
incorporated into polymer and small molecule mixtures,3–7
sidechains,8–13 and backbones14–19 to increase external quantum
efficiencies and overall device performance.1–7,9–13,15,16,18
The known drawbacks to 1,3,4-oxadiazole-containing poly-
mers are 1) the poor solubility they impart and 2) the lack of
thorough understanding as to the specific contribution
oxadiazole makes to the optical and electronic characteristics
of their compounds. Practically, the issue of solubility has been
addressed by the addition of flexible sidechains, often alkoxy
structures of various lengths, to the rigid oxadiazole back-
bone.20,21 While this is not thought to greatly alter the optical
functionality of these polymers, our findings suggest a notable
aDepartment of Materials Science and Engineering, University of
Michigan, Ann Arbor, MI, USA
bDepartments of Chemical Engineering, Macromolecular Engineering,
and Biomedical Engineering, University of Michigan, Ann Arbor, MI,
USA. E-mail: jinsang@umich.edu; Fax: +1-734-763-4788;
Tel: +1-734-936-4681
{ This paper is part of a Journal of Materials Chemistry issue
highlighting the work of emerging investigators in materials chemistry.
Scheme 1 The 1,3,4-oxadiazole moiety (a), synthetic route to
oxadiazole compounds O1–O7 (b), synthetic route to ethynylene
compounds A1 and A2 (c).
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Fig. 3 Compounds A2 and O2 with UV absorption (a) and PL
excitation/emission (b) shown for 1 ppm chloroform solutions.
Fig. 4 Compounds O2 and O3 with UV absorption (a) and PL
excitation/emission (b) shown for 1 ppm chloroform solutions.
Fig. 1 Complete list of all compounds synthesized.
Fig. 5 Compounds O1 and O4 with UV absorption (a) and PL
excitation/emission (b) shown for 1 ppm chloroform solutions.
Technologies International Inc. Quantamaster fluorometer
1
equipped with an Integrating Sphere. H NMR spectra were
Fig. 2 Compounds A1 and O1 with UV absorption (a) and PL
collected from a Varian Mercury 300 (300 MHz) and a Varian
Inova 500 (500 MHz) as noted.
excitation/emission (b) shown for 1 ppm chloroform solutions.
Experimental
Synthesis
General
The synthetic route to the conjugated molecules is depicted in
Scheme 1b and c. All compounds were found to be soluble in
All chemicals used in syntheses were of reagent grade
purchased from Aldrich. Solvents for 1H NMR were
purchased from Cambridge Isotope Labs, Inc. of Andover,
MA, USA. UV absorption data were collected on a Varian
Cary 50 Bio. PL excitation and emission spectra as well as
1
chloroform and structurally verified by H NMR.
Preparation of compounds 1. For O1, O4, and O7,
terephthaloyl dichloride was purchased from Aldrich and used
without further purification. O3 was obtained from Kangwon
quantum efficiency data were collected on
a Photon
1982 | J. Mater. Chem., 2007, 17, 1981–1988
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saturated sodium chloride–water solution respectively. The
ethyl acetate layer is then dried over magnesium sulfate and
the solvent is removed under reduced pressure. The resulting
solids are recrystallized from ethanol to produce a white
powder (1.50 g, 63.0%).
Hexadecoxy substituents are added to terephthalyl precur-
sors to compounds 1 for the synthesis of compounds O2 and
O6. The specific method used in the synthesis of compound O2
is presented here. A 100 ml 2-neck round-bottomed flask is
charged with diethyl 2,5-(dihydroxy)terephthalate (0.508 g,
2 mmol, 1 equiv), 1-bromohexadecane (1.466 g, 4.8 mmol,
1.2 equiv), potassium carbonate (0.829 g, 6 mmol, 3 equiv),
and dimethyl formamide (20 mL). The flask is sealed under
argon, heated to 80 uC, and stirred. After 24 hours the inhomo-
geneous solution is gravity filtered to remove insoluble pota-
ssium carbonate. The filtrate is then extracted with dichlor-
omethane and deionized water. The dichloromethane layer is
then dried over magnesium sulfate and the solvent is removed
under reduced pressure. The resulting solids are recrystallized
from ethanol to yield a white powder (0.79 g, 55.6%).
Ethyl-protected precursors to compounds 1 are de-protected
by potassium hydroxide. Again, the specific method used in
the synthesis of compound O2 is presented here. A 100 ml
2-neck round-bottomed flask is charged with diethyl 2,5-
(hexadecyloxy)terephthalate (0.40 g, 0.56 mmol, 1 equiv),
potassium hydroxide–water solution (2 g KOH, 18 mL
deionized water), and ethanol (5 ml). The solution is heated
to 80 uC and stirred for 24 hours as some white precipitate
forms. The solution is then cooled and neutralized by the
dropwise addition of hydrochloric acid–water solution (37%)
producing further white precipitate. The precipitates are
collected and recrystallized from dimethyl sulfoxide to yield
a white power (0.28 g, 76.7%).
Fig. 6 Compounds O1 and O5 with UV absorption (a) and PL
excitation/emission (b) shown for 1 ppm chloroform solutions.
Fig. 7 Compounds O6 and O7 with UV absorption (a) and PL
excitation/emission (b) shown for 1 ppm chloroform solutions.
Lee. For O5, 2,5-dihexadecylterephthaloyl dichloride was
synthesized following exactly the procedure described by
Rehahn et al.22 The following describes the procedure used
to synthesize compounds 1 used for O2 and O6.
Terephthalyl precursors to compounds 1 are chlorinated by
thionyl chloride. A 100 ml 2-neck round-bottomed flask is
charged with 2,5-(hexadecoxy)terephthalate (0.20 g, 0.30 mmol,
1 equiv) and vacuum flushed with argon three times. Thionyl
chloride (10 ml, excess) is then added to the solution and
refluxed at 80 uC for 24 hours. The remaining thionyl chloride
is then removed under reduced pressure and the solid product
is used immediately in the next step of the synthesis to prevent
deactivation of terephthaloyl chlorides.
For O6 hydroxyterephthalate was prepared from dimethyl-
(amino)-terephthalate by the Sandmeyer reaction.23 A 100 ml
2-neck round-bottomed flask is charged with dimethyl(amino)
terephthalate (10.5 g, 50 mmol, 1 equiv) and a solution of
sulfuric acid (15 mL, 168.9 mmol, 16.8 equiv), deionized water
(10 mL), and deionized crushed ice (41.5 g). The solution is
cooled to 0 uC and a solution of sodium nitrite (3.65 g,
50 mmol, 1 equiv) in 10 ml of deionized water is added
dropwise over 30 minutes. The mixture is stirred at 0 uC for
30 minutes and then added slowly to a solution of sulfuric acid
(46 mL) and deionized water (34 mL) preheated to 120 uC.
This solution is stirred at 120 uC for 20 minutes, cooled to 0 uC,
and filtered to collect precipitates. These solids are recrystal-
lized from ethanol–water (200 ml EtOH, 400 ml H2O) to yield
a white powder (6.05 g, 66.5%). Note, methyl protection is
removed as a side reaction, which was confirmed by the
Preparation of compounds 2. 2-Hydroxybenzoic acid and
3-hydroxybenzoic acid were purchased from Aldrich and
alkoxy-substituted by the same method described earlier for
compounds 1. Compounds 2 are synthesized from ethyl (2 or
3)-hexadecyloxybenzoate. A 100 ml 2-neck round-bottomed
flask is charged with ethyl 3-hexadecyloxybenzoate (0.39 g,
1.0 mmol, 1 equiv), hydrazine monohydrate (1.5 g, 30 mmol,
30 equiv), and methanol (15 mL). The solution is heated to
60 uC and stirred as an insoluble white precipitate forms. After
24 hours the suspension is cooled. Precipitates are collected by
filtration and recrystallized from ethanol to yield a white
powder (0.21 g, 56%).
1
absence of methoxy peaks in H NMR.
Hydroxyterephthalate is ethyl protected prior to alkoxyla-
tion. A 100 ml 2-neck round-bottomed flask is charged with
hydroxyterephthalate (1.82 g, 1 mmol, 1 equiv), sulfuric acid
(0.5 mL, catalytic), and ethanol (15 mL). The flask is then
stirred and refluxed at 80 uC for 48 h. The solvent is then
removed under reduced pressure and the resulting solids are
dissolved in ethyl acetate and extracted with 10 wt% sodium
bicarbonate–water solution, pure deionized water, and
Preparation of compounds 3. A 100 ml 2-neck round-
bottomed flask is charged with 2,5-(hexadecyloxy)
terephthaloyl dichloride (1, 0.20 g, 0.30 mmol, 1 equiv),
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3-(hexadecyloxy)benzohydrazide (2, 0.226 g, 0.60 mmol,
1 equiv), and chloroform (10 mL) and stirred at 25 uC for
2 hours. Precipitates form and are filtered off before being
recrystallized from dimethyl sulfoxide to yield a white powder
(0.20 g, 48.9%).
(t 4H), 1.91–1.86 (m 4H), 1.81–1.77 (m 4H), 1.62–1.47 (m 8H),
1.41–1.20 (m 96H), 0.88–0.85 (t 12H).
Preparation of compounds 4. For A1 1,4-diiodobenzene was
purchased from Aldrich and used without further purification.
Compound 4 used as a precursor to compound A2 was
synthesized from 2,5-diiodobenzene-1,4-diol using a method
identical to alkoxylations used for compounds 1.
Preparation of compounds O1–O7. A 100 ml 2-neck round-
bottomed flask is charged with 2,5-(hexadecyloxy)-N91,N94-(3-
(hexadecyloxy)benzoyl)terephthalohydrazide (3, 0.20 g,
0.15 mmol, 1 equiv) and phosphorus oxychloride (7 mL,
excess). The solution is heated to 100 uC and stirred under
reflux for 2 hours. The solution is poured into deionized water
at 0 uC creating a precipitate, which is collected by filtration
and recrystallized from chloroform to yield a white power
(0.10 g, 50.2%).
Preparation of compounds 5. Sonogashira–Hagihawa reac-
tions were conducted to produce compounds 5. A 100 ml
3-neck round-bottomed flask was charged with 1,4-bis(hex-
adecyloxy)-2,5-diiodobenzene (4, 0.60 g, 0.74 mmol, 1 equiv),
trimethylsilyl acetylene (purchased from GFS Chemicals)
(0.22 g, 2.22 mmol, 1.5 equiv), copper(I) iodide (10 mg,
52.5 mmol, 0.07 equiv), dichlorobis(triphenylphosphine)
palladium(II) (25 mg, 35.5 mmol, 0.048 equiv), and a solvent
mixture of diisopropylamine (30 ml), triethylamine (5 ml), and
tetrahydrofuran (5 ml). The solution is sealed under argon and
stirred at 60 uC for 96 hours followed by removal of solvent by
reduced pressure. The resulting solids are purified by column
chromatography (10% ethyl acetate, 90% hexanes) to yield
pure compounds 5 (0.55 g, 98%).
O1. Recrystallization from ethanol yields an off-white
1
powder (83.4%). H NMR (500 MHz, CDCl3): d 8.32 (s 4H),
7.73–7.69 (m 4H), 7.46–7.43 (t 2H), 7.12–7.10 (d 2H), 4.08–
4.06 (t 4H), 1.85–1.81 (m 4H), 1.52–1.50 (m 4H), 1.39–1.27 (m
48H), 0.89–0.86 (t 6H).
O2. Recrystallization from chloroform yields
a white
1
powder (50.2%). H NMR (500 MHz, CDCl3): d 7.84 (s 2H),
7.71–7.68 (m 4H), 7.44–7.41 (t 2H), 7.10–7.08 (d 2H), 4.20–
4.18 (t 4H), 4.07–4.04 (t 4H), 1.96–1.91 (m 4H), 1.86–1.80 (m
4H), 1.58–1.52 (m 4H), 1.52–1.46 (m 4H), 1.37–1.23 (m 96H),
0.89–0.86 (t 12H).
Preparation of compounds 6. Trimethylsilane is removed by
basic deprotection. A 100 ml 3-neck round bottomed flask was
charged with (2,5-bis(hexadecyloxy)-1,4-phenylene)bis(ethyne-
2,1-diyl)bis(trimethylsilane) (5, 0.5 g, 0.67 mmol, 1 equiv),
tetrahydrofuran (10 ml), and methanol (10 ml). As the mixture
is stirred a 1 M solution of potassium hydroxide in deionized
water (2 ml) is added dropwise. After 60 minutes the solvent is
removed under reduced pressure. The resulting solids are
extracted in dichloromethane and deionized water. The
organic layer is collected and dried over magnesium sulfate
followed by removal of the solvent under reduced pressure to
yield a dark brown powder (0.374 g, 93.5%).
O3. Donated by Kangwon Lee. 1H NMR (300 MHz,
CDCl3): d 8.05–8.03 (d 4H), 7.86 (s 2H), 7.71–7.69 (d 4H),
4.12–4.06 (m 4H), 1.87–1.84 (m 2H), 1.67–1.62 (m 4H), 1.59–
1.48 (m 4H), 1.36–1.28 (m 8H), 0.98–0.95 (t 6H), 0.90–0.87
(t 6H).
O4. Recrystallization from toluene yields a white powder
(58.6%). 1H NMR (500 MHz, CDCl3): d 8.29 (s 4H), 8.11–8.10
(d 2H), 7.54–7.50 (t 2H), 7.11–7.06 (m 4H), 4.16–4.13 (t 4H),
1.95–1.89 (m 4H), 1.57–1.53 (m 4H), 1.39–1.19 (m 48H), 0.88–
0.85 (t 6H).
Preparation of compounds 7. Compounds 7 are purchased
from Sigma-Aldrich and used without any further purification.
Preparation of compounds A1 and A2. Compounds A1 and
A2 are prepared by a method similar to the preparation of
compounds 5. Sonogashira–Hagihawa reactions are carried
out as described between 1,4-diethynyl-2,5-bis(hexadecyloxy)-
benzene and 1-iodo-3-methoxybenzene with copper iodide and
palladium(II) catalysts. Compounds are purified by column
chromatography.
O5. Column chromatography (5% ethyl acetate, 95% hexane)
yields a pale orange powder (51.5%). 1H NMR (500 MHz,
CDCl3): d 7.97, 7.62 (s 2H), 7.70–7.68 (m 4H), 7.47–7.45 (t 2H),
7.27–7.11 (d 2H), 4.08–4.05 (t 4H), 3.13–3.10, 2.91–2.88 (t 4H),
1.86–1.82 (m 4H), 1.75–1.72 (m 4H), 1.68–1.63 (m 4H), 1.52–
1.45 (m 4H), 1.44–1.25 (m 100H), 0.90–0.88 (t 12H).
O6. Column chromatography (20% ethyl acetate, 75%
hexane, 5% chloroform) yields an off-white powder (58.6%).
1H NMR (300 MHz, CDCl3): d 8.29–8.27 (d 1H), 7.86 (s 1H),
7.81–7.78 (d 1H), 7.74–7.67 (m 4H), 7.45–7.41 (m 2H), 7.12–
7.09 (t 2H), 4.28–4.26 (t 2H), 4.08–4.04 (t 4H), 1.99–1.98 (m
2H), 1.85–1.83 (m 4H), 1.60–1.58 (m 2H), 1.52–1.47 (m 4H),
1.38–1.24 (m 72H), 0.89–0.87 (t 9H).
A1. Column chromatography (5% ethyl acetate, 95%
hexanes) yields a bright yellow solid (33.3%). 1H NMR
(500 MHz, CDCl3): d 7.51 (s 4H), 7.28–7.25 (m 2H), 7.15–
7.13 (d 2H), 7.06 (s 2H), 6.92–6.90 (d 2H), 3.84 (s 6H).
A2. Column chromatography (5% ethyl acetate, 95%
hexanes) yields a yellow–orange solid (88.2%). 1H NMR
(300 MHz, CDCl3): d 7.28–7.23 (t 2H), 7.15–7.12 (d 2H), 7.07
(s 2H), 7.02 (s 2H), 6.91–6.87 (d 2H), 4.05–3.99 (t 4H), 3.82
(s 6H), 1.89–1.80 (m 4H), 1.59–1.54 (m 4H), 1.47–1.24 (m 48H),
0.90–0.86 (t 6H).
O7. Recrystallization from acetone yields a pale yellow
powder (42.2%). 1H NMR (300 MHz, CDCl3): d 8.34–8.24 (m
4H), 7.63 (s 2H), 7.09–6.95 (m 4H), 4.10–4.04 (t 4H), 4.01–3.94
1984 | J. Mater. Chem., 2007, 17, 1981–1988
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O6 has only a single central alkoxy chain. Depicted in Fig. 6,
O5 shows only one peak at the lowest wavelength of any
compound, 300 nm, implying that the feature of double
absorption peaks of O2 originates from the dialkoxy substitu-
tion on the central phenyl ring. Interestingly, O6, which has
only a single alkoxy side chain on the central phenyl ring,
shows in Fig. 7 what appears to be the beginnings of a single
peak splitting into two in the UV-abs and PL excitation
spectra. This supports our hypothesis that the dialkoxy
substitution is responsible for the feature of double absorp-
tion/excitation peaks as it can be seen in lower proportion
when there is only one alkoxy chain.
Results and discussion
Optical activity of 1,3,4-oxadiazole
There is no current consensus regarding the role of 1,3,4-
oxadiazole in molecular conjugation. The moiety is frequently
considered to be an independent chromophore24 while some
also believe that 1,3,4-oxadiazole does not provide full
conjugation.25,26 To verify these hypotheses we prepared a
series of 1,3,4-oxadiazole-containing conjugated molecules.
Fig. 2 and 3 show the structures, UV absorption (UV-abs), and
photoluminescent (PL) excitation and emission of compounds
A1, A2, O1, and O2 observed in 1 ppm chloroform solution.
By comparing ethynyl compounds A1 and A2 to their
oxadiazole analogs, O1 and O2, we can begin to discern the
optical functionality of 1,3,4-oxadiazole. Firstly, striking
similarity exists between the UV-abs and PL spectra of A1
and A2, and O1 and O2, respectively. This agreement coupled
with the knowledge that the acetylene moiety absorbs below
220 nm proves that the 1,3,4-oxadiazole ring is itself
transparent in the visible and near-UV range, with a large
bandgap similar to the ethynylene unit.27 We can also infer
that the oxadiazole ring is fully conjugating,21,28,29 as is the
ethynyl moiety, which should lay to rest any suspicion that
1,3,4-oxadiazole disrupts molecular conjugation.
We also synthesized O7, which has para-dialkoxy chains on
its terminal phenyl rings, to see the effect of para-dialkoxy
substitution at the conjugative end of the molecule. O7 shows,
also in Fig. 7, a broad single peak with a small splitting in the
middle exactly like O6 instead of two discrete peaks observed
in O2 and O3 indicating that the para-dialkoxy phenyl group is
not a separate chromophore, but para-dialkoxy phenylene is.
We have concluded based on the results that the 360–370 nm
peak seen in the UV-abs and PL excitation spectra of
compounds A2, O2, and O3 is from the chromophore created
by the fully conjugated main chain. The observed red-shift in
the absorption lmax of this chromophore relative to all
compounds without dialkoxy substitution on the central ring
is likely due to the increased electron density (lower bandgap
due to an elevated highest occupied molecular orbital
(HOMO) level) from the central dialkoxy substituents.30 The
other peak (310–320 nm) seen in the UV-abs and PL excitation
spectra of A2, O2, and O3 is then taken to be a new
chromophore created by the central para-alkoxy chains as
illustrated in Fig. 8. In each case, though, both of these active
absorbing chromophores proceed to the same radiant decay
mechanism as their PL emission spectra are identical regard-
less of which chromophore is excited.
An interesting finding about the optical properties of
compounds A2 and O2 is the double peak seen in their UV
absorption and PL excitation spectra. These compounds vary
from A1 and O1 only in the presence of the dialkoxy
substituents on their central phenyl rings, which represent a
commonly used design principle to provide better solubility.
The pronounced double peaks mean that this substitution has
a large influence over the optical activities of compounds A2
and O2. It is surprising that the alkoxy chains exhibit a greater
effect on the compounds’ optical properties than the oxadia-
zole moiety does, as both A2 and O2 have very similar optical
spectra that vary greatly from compounds A1 and O1. Given
the moiety’s aromatic ring structure and position in the
conjugated molecular backbone, it is surprising that the
oxadiazole ring does not more drastically affect the optical
spectra. Following this, we systematically investigated the role
of side chain substitution on the optical properties of other
oxadiazole-containing conjugated molecules. We began by
exploring the importance of alkoxy chain structure and
position.
These findings are significant for the molecular design of
1,3,4-oxadiazole-containing conjugated compounds and pro-
vide a more solid understanding. Knowing that this oxadiazole
structure is visibly transparent, conjugating, optically passive,
and similar to other well-studied conjugation units such as the
ethynylene moiety will simplify design and broaden the scope
of its application into conjugated organic molecules and
polymers for organic optoelectronic applications.
The effect of 1,3,4-oxadiazole structure on quantum efficiency
Variations to the substitution of the terminal phenyl rings
show that these alkoxy chains hold little influence over the
compounds’ optical properties. Comparison between com-
pounds O2 and O3, as can be seen in Fig. 4, reveals that the
alkoxy chains at the compounds’ ends have little effect on their
optical characteristics as compound O3 has only bromine
groups on its ends yet exhibits an optical spectrum identical to
that of O2. Comparison between compounds O1 and O4,
shown in Fig. 5, proves that the location of alkoxy chains on
the backbone-end phenyl ring is also inconsequential as
compound O4 has its terminal alkoxy chains placed ortho to
the oxadiazole ring and its molecular conjugation.
Investigating further into the optical properties of 1,3,4-
oxadiazole we examined the quantum yields (QYs) of each
To investigate the effects of substitution on the central
phenyl ring we prepared compounds O5 and O6. O5 is an
analog to O2 with alkyl rather that alkoxy center chains and
Fig. 8 Two chromophores present in compounds A2 and O2.
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this theory by placing para-alkoxy substituents adjacent to a
Table 1 Quantum yields of all compounds in chloroform solution
single 1,3,4-oxadiazole ring, and its data support our hypoth-
esis. Its UV-abs and PL excitation are most similar to O1, O4,
and O6, but its PL emission is much redder, more like those of
A2, O2 and O3. The QY of O7 is 69.1–69.3%, indicating that
oxadiazole-adjacent para-dialkoxy substituents promote a
lowered quantum yield as well as a red shift (relative to O1
and O4). This red shift indicates that the para-alkoxy chains
alter the excited state activity of the compounds.
ID
Quantum efficiency (%)
lEx/nm
Conc. (ppm)
A1
A2
80.6 ¡ 0.1
76.7 ¡ 0.4
76.4 ¡ 0.6
95.0 ¡ 0.7
48.7 ¡ 1.6
48.0 ¡ 0.3
47.1 ¡ 0.6
43.1 ¡ 0.6
40.8 ¡ 0.5
46.3 ¡ 1.4
43.6 ¡ 1.1
98.1 ¡ 1.3
79.0 ¡ 1.2
85.9 ¡ 1.4
69.2 ¡ 0.1
327
318
368
330
313
368
368
368
368
313
370
336
310
342
350
1
1
1
1
O1
O2
1
1
10
100
1000
1
Compound O5 includes alkyl substituents in place of O2’s
alkoxy chains. This compound exhibits a QY of 77.8–80.2%,
much larger than the QY of O2 and O3, implying that the
oxygen atoms of the alkoxy chains in O2 and O3 are
interacting with the oxadiazole rings to create non-radiative
decay pathways. This also indicates that the observed low QY
is the result of an intramolecular excited-state phenomenon
and not the result of any agglomerative or intermolecular
event as O5 is substituted equally to O2 and O3 and will be no
more or less likely to agglomerate in solution.
O3
1
1
1
1
O4
O5
O6
O7
1
compound in chloroform solution, which are shown in Table 1.
Oxadiazole compound O1 exhibited a very high QY of 94.3%–
95.7% in 1 ppm chloroform solution while O2 recorded much
lower values, 47.1%–50.3%, in the same conditions. In
contrast, ethynylene compounds A1 and A2 both exhibited
similar QYs of 80.5–80.7% and 75.8–77.0%, respectively. To
ensure that this was not due to some unexpectedly poor
solubility{ we tested the QY of O2 in higher concentrations
and found a slight drop from 47.7%–48.3% at 1 ppm to 40.3%–
41.3% at 1000 ppm. This concentration-dependent drop
indicates that some agglomeration does occur yet it is not
significant enough to account for the large discrepancy
between the QYs of O1 and O2. We hypothesized that the
lowered QY is likely the result of an interaction between the
dialkoxy substituents and the adjacent 1,3,4-oxadiazole rings.
The remaining oxadiazole compounds were designed so as
to explore this hypothesis. O3, structurally very similar to O2
but with ethylhexoxyl substituents that are slightly more bulky
near the oxadiazole, behaved identically to O2 with low QY of
42.5–47.7% regardless of which excitable chromophore is
excited. O4 was synthesized to probe the specific effect of
alkoxy substituents placed ortho to the oxadiazole, in this case
on the terminal phenyl rings. Surprisingly, this compound
exhibited a very high QY of 96.8–99.4%, indicating that the
low QY of O2 and O3 is not the result of simple interaction
between an adjacent alkoxy and oxadiazole group but
resultant from interactions between para-substituted dialkoxy
side chains and adjacent oxadiazole rings within the molecular
conjugation.
Further exploring this hypothesis we tested the PL activity
and QY of 1 ppm chloroform solutions of O1 and O2 in
increasing concentrations of trifluoroacetic acid (TFA). As the
TFA concentration is increased protonation of the oxadiazole
nitrogen atoms alters their electronic activity. As can be seen in
Fig. 9, increasing protonation of O1 leads only to greatly
reduced PL emission and QY. O2, on the other hand, exhibits
not a quenching but a 38% increase in QY followed by a long,
sustained QY value still 10% higher than the initial value. It
should be noted also that increasing acid concentration
reduced UV-abs for both compounds, which must be
considered in order to find agreement between data in Fig. 9a
and c. Also, as the oxadiazole rings of O2 are protonated there
is a red shift in the PL emission indicating an evolution of the
chromophore. O1, in contrast, exhibits a rapid quenching with
no appreciable color shift.
We conclude that the reason for the low QY in compounds
O2 and O3 results from a repulsive interaction between
chromophoric dialkoxy substituents and adjacent oxadiazole
rings, which hinders excited state planarity. The bonding
nature of excited state 1,3,4-oxadiazole creates a p-bond
between the oxadiazole and phenyl rings, which is hindered in
O2 and O3 by this effect leading to non-radiative decay.
Comparing the PL emission spectra of A1 and O1 (Fig. 2) we
can see that the redder peaks are dominant in the case of O1,
indicating greater excited state molecular planarity. It is
unlikely that these peaks arise from agglomerates as O1 has
such a high QY, substantially higher than A1. This is much less
pronounced in comparison between A2 and O2, where the PL
emission is almost identical (Fig. 3). We hypothesize that the
lone-pair electrons of the oxadiazole ring are repelled by the
lone-pair electrons of oxygen atoms of the para dialkoxy
substituents, inhibiting molecular and excited state planarity.
This hypothesis is illustrated in Fig. 10. Our TFA study
supports this as protonation of the oxadiazole rings creates
hydrogen bonding between the oxadiazole and alkoxy groups.
This results in both enhanced QY as well as the red shift seen
from the PL emission of O2 under increasingly acidic
conditions. We believe that this hydrogen bonding lessens
the quenching effect of the charged oxadiazole ring, which
Compound O6 is identical to O2 with the removal of one
central alkoxy chain and was prepared to investigate this
hypothesis. This compound behaved somewhat like O2 and O3
in its UV absorption (a bi-modal spectrum, though much less
profound) but much like O1 and O4 in PL excitation and
emission, and showed a higher QY of 84.5–87.3%. Clearly,
removing one of the alkoxy chains significantly hinders the
inefficient, non-radiative mechanism at play in O2 and O3,
which can to this point only be the result of para-alkoxy
substituents adjacent to 1,3,4-oxadiazole. Compound O7 tests
{ Unexpected, because the extra alkoxy substituents on O2 are
expected to enhance its solubility.
1986 | J. Mater. Chem., 2007, 17, 1981–1988
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View Article Online
near-UV range and that this ring is fully conjugating and can
be incorporated into conjugated polymers in a capacity nearly
identical to the well-known and widely used ethynylene
moiety. Our thorough study shows that para-dialkoxy
substituents on the central phenyl ring create a separate
absorbing chromophore and, when placed adjacent to one or
more 1,3,4-oxadiazole rings, hinder excited state planarity
enabling a non-radiative decay mechanism and causing a
sizeable drop in the quantum efficiency of the compounds
(a 49.5% loss, in the case presented). Our study also
demonstrates that replacing para-alkoxy substituents with
single alkoxys or para-alkyls provides an alternate means to
enhance oxadiazole compound solubility without dramatically
lowering quantum efficiency. With much research being done
on the incorporation of 1,3,4-oxadiazole into conjugated
molecules for organic opto-electronics, these findings give
valuable insight into the characteristics of the moiety and a
design principle for improving the QY of oxadiazole com-
pounds, enhancing their efficiency and applicability.
Acknowledgements
This work was supported by the University of Michigan
College of Engineering start-up fund and partly supported by
the National Science Foundation (NSF BES 0428010). We
thank Mr Kangwon Lee for contribution of one compound,
O3.
References
Fig. 9 Compounds O1 and O2 in 1 ppm chloroform solution.
Quantum yields as TFA concentration is increased (a), PL emission
of O1 as TFA concentration is increased (b), and PL emission of O2 as
TFA concentration is increased (c).
1 A. P. Kulkarni, C. J. Tonzola, A. Babel and S. A. Jenekhe, Chem.
Mater., 2004, 16, 4556.
2 G. Hughes and M. R. Bryce, J. Mater. Chem., 2005, 15, 94.
3 C. Adachi, T. Tsutsui and S. Saito, Appl. Phys. Lett., 1990, 56, 799.
4 A. R. Brown, D. D. C. Bradley, J. H. Burroughes, R. H. Friend,
N. C. Greenham, P. L. Burn and A. Kraft, Appl. Phys. Lett., 1992,
61, 2793.
would explain why O2 remains fluorescent as O1 is rapidly
quenched in the same acidic conditions.
5 M. Strukelj, F. Papadimitrakopoulos, T. M. Miller and
L. J. Rothberg, Science, 1995, 267, 1969.
6 C. Zhang, S. Ho¨ger, K. Pakbaz, F. Wudl and A. J. Heeger,
J. Electron. Mater., 1994, 23, 453.
Conclusions
7 J. H. Ahn, C. Wang, C. Pearson, M. R. Bryce and M. C. Petty,
Appl. Phys. Lett., 2004, 85, 1283.
8 X.-C. Li, F. Cacialli, M. Giles, J. Gru¨ner, R. H. Friend,
A. B. Holmes, S. C. Moratti and T. M. Yong, Adv. Mater.,
1995, 7, 898.
In this study we have established a strong understanding of the
optical properties of the 1,3,4-oxadiazole moiety and present
an important design principle for conjugated molecules
containing it. Firstly, we have demonstrated that the 1,3,4-
oxadiazole structure is optically transparent in the visible and
9 S.-J. Chung, K.-Y. Kwon, S.-W. Lee, J.-I. Jin, C. H. Lee, C. E. Lee
and Y. Park, Adv. Mater., 1998, 10, 1112.
10 Z. Bao, Z. Peng, M. E. Galvin and E. A. Chandross, Chem. Mater.,
1998, 10, 1201.
11 Z.-K. Chen, H. Meng, Y.-H. Lai and W. Huang, Macromolecules,
1999, 32, 4351.
12 D. W. Lee, K.-Y. Kwon, J.-I. Jin, Y. Park, Y.-R. Kim and
I.-W. Hwang, Chem. Mater., 2001, 13, 565.
13 Y. Z. Lee, X. Chen, S. A. Chen, P. K. Wei and W. S. Fann, J. Am.
Chem. Soc., 2001, 123, 2296.
14 X.-C. Li, A. B. Holmes, A. Kraft, S. C. Moratti, G. C. W. Spencer,
F. Cacialli, J. Gru¨ner and R. H. Friend, J. Chem. Soc., Chem.
Commun., 1995, 2211.
Fig. 10 Dialkoxy/1,3,4-oxadiazole electron repulsion inhibits
planarity in compounds O2 and O3. This electronic repulsion is
believed to lead to non-radiative decay and quantum yields lower than
those of similar, non-substituted compounds (a). Hydrogen-bonding
scheme present in acidic conditions that induces molecular planarity, a
red-shift in photoluminescent emission, and, we believe, increased
quantum yield (b).
15 Q. Pei and Y. Yang, Chem. Mater., 1995, 7, 1568.
16 Z. Peng, Z. Bao and M. E. Galvin, Chem. Mater., 1998, 10, 2086.
17 X. Jiang, R. A. Register, K. A. Killeen, M. E. Thompson,
F. Pschenitzka and J. C. Sturm, Chem. Mater., 2000, 12, 2542.
18 J. A. Mikroyannidis, I. K. Spiliopoulos, T. S. Kasimis,
A. P. Kulkarni and S. A. Jenekhe, Macromolecules, 2003, 36, 9295.
19 Z. Zhang, Y. Hu, H. Li, L. Wang, X. Jing, F. Wang and D. Ma,
J. Mater. Chem., 2003, 13, 773.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 1981–1988 | 1987

View Article Online
20 S. Janietz and S. Anlauf, Macromol. Chem. Phys., 2002, 203, 427.
21 W. Huang, H. Meng, W.-L. Yu, J. Pei, Z.-K. Chen and Y.-H. Lai,
Macromolecules, 1999, 32, 118.
22 M. Rehahn, A.-D. Schlu¨ter and W. J. Feast, Synthesis, 1988, 5,
386.
23 K. Weinges, U. Reinel, W. Maurer and N. Gaessler, Liebigs Ann.
Chem., 1987, 10, 833.
24 J. A. Mikroyannidis, H. Hlı´dkova´, D. Vy´prachticky´ and
V. Cimrova´, J. Polym. Sci., Part A: Polym. Chem., 2005, 43,
3079.
25 N. C. Yang, S. M. Lee, Y. M. Yoo, J. K. Kim and D. H. Suh,
J. Polym. Sci., Part A: Polym. Chem., 2004, 42, 1058.
26 Y. Zou, S. Tan, Z. Yuan and Z. Yu, J. Mater. Sci., 2005, 40, 3561.
27 A. J. Fahr, Mol. Spectrosc., 2003, 217, 249.
28 S. W. Cha, S.-H. Choi, K. Kim and J.-I. Jin, J. Mater. Chem.,
2003, 13, 1900.
29 S. H. Mashraqui, R. S. Kenny, S. G. Ghadigaonkar, A. Krishnan,
M. Bhattacharya and P. K. Das, Opt. Mater., 2004, 27, 257.
30 P. V. James, P. K. Sudeep, C. H. Suresh and K. G. Thomas,
J. Phys. Chem. A, 2006, 110, 4329.
1988 | J. Mater. Chem., 2007, 17, 1981–1988
This journal is ß The Royal Society of Chemistry 2007