Upon the intrinsic optical properties of oligo(p-phenyleneethynylene)s (OPEs). Synthesis of OPE3 for experimental gas-phase absorption studies

Article abstract of DOI:10.1016/j.tet.2008.08.068

A new oligo(p-phenyleneethynylene) (OPE3) was synthesized by step-wise Pd-catalyzed cross-coupling reactions. The molecule incorporates an amine functionality, which allows for transfer of the protonated molecule to the gas phase by electrospray ionization leaving the OPE entity neutral. This method has allowed for the first experimental gas-phase absorption spectrum of an OPE3 by action spectroscopy, employing an electrostatic ion storage ring in combination with a laser system. The studies reveal the effect of having a positive charge in proximity to the conjugated backbone of OPE3 in the absence of any interfering solvent molecules. In addition, ionization energies and electron affinities of OPE2-OPE17 were calculated at the density functional theory (DFT) level.

Full text of DOI:10.1016/j.tet.2008.08.068

Tetrahedron 64 (2008) 11475–11479
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Upon the intrinsic optical properties of oligo(p-phenyleneethynylene)s (OPEs).
Synthesis of OPE3 for experimental gas-phase absorption studies
Torben Ryhding a, Maj-Britt Suhr Kirketerp b, Umesh Kadhane b, Morten Køcks Lykkegaard b, Subhasis
,
a,
*
Panja b, Steen Brøndsted Nielsen b , Mogens Brøndsted Nielsen
*
a Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark
b Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
A new oligo(p-phenyleneethynylene) (OPE3) was synthesized by step-wise Pd-catalyzed cross-coupling
reactions. The molecule incorporates an amine functionality, which allows for transfer of the protonated
molecule to the gas phase by electrospray ionization leaving the OPE entity neutral. This method has
allowed for the first experimental gas-phase absorption spectrum of an OPE3 by action spectroscopy,
employing an electrostatic ion storage ring in combination with a laser system. The studies reveal the
effect of having a positive charge in proximity to the conjugated backbone of OPE3 in the absence of any
interfering solvent molecules. In addition, ionization energies and electron affinities of OPE2–OPE17
were calculated at the density functional theory (DFT) level.
Received 5 June 2008
Accepted 20 August 2008
Available online 27 August 2008
Keywords:
Cross-coupling
Electrostatic interactions
Ionization energy
OPE
Ó 2008 Elsevier Ltd. All rights reserved.
Spectroscopy
1. Introduction
be evaporated for gas-phase absorption spectroscopy. Instead
charged chromophores can often be brought in the gas phase by
Conjugated polymers have found wide applications in materials
chemistry as laser dyes, light-emitting diodes, organic conductors,
photoconductors and nonlinear optical (NLO) materials.1 Their
materials’ properties are to a large extent determined by the optical
band gap. The poly(p-phenyleneethynylene)s (PPEs) represent one
type of conjugated polymers that by suitable functionalization can
be employed, for example, for explosive detection or polarizers for
LC displays.2 In the conjugated oligomer approach,3 the optical
properties of a series of monodisperse oligomers are investigated
and then extrapolated to ideal infinite polymers. These conjugated
oligomers are interesting in their own right, for example, as mo-
lecular wires for molecular electronics.4
One challenge is to make meaningful comparisons between
experimental excitation energies obtained in solution or in the solid
state and those calculated by quantum chemical means on isolated
molecules. Intermolecular or molecule–solvent interactions as well
as solid-state packing effects may perturb the electronic structure
and cause a shift in the transition energy. This problem calls for
studies on isolated molecules in the gas phase, free of a disturbing
environment. However, often the macromolecules are too fragile to
electrospray ionization (ESI) and their absorption can then be
measured by sophisticated action spectroscopy techniques, that is,
absorption is monitored by a detection of ionic fragmentation. In
Aarhus, we use the electrostatic ion storage ring, ELISA,5 as a gas-
phase optical cell for such measurements. As an example, several
biochromophores have been investigated using this approach,
shedding light on absorption tuning by protein interactions.6
For investigating neutral chromophores by this method, we
need to attach a charged group, a so-called spectator charge, to
allow for electrospray ionization. Otherwise the chromophore itself
would become charged under the conditions. This spectator charge
should be placed at a remote position if the goal is to reveal the
intrinsic properties of the neutral chromophore; i.e., the spectator
charge should for this purpose be placed at a position at which it
does not interfere with the chromophore. On the other hand, the
method also allows us to investigate systematically the influence of
approaching a positive or negative charge to the chromophore. We
became interested to explore the properties of oligo(p-phenyl-
eneethynylene)s (OPEs, Fig. 1) and present here the synthesis of an
OPE3 incorporating an amino group as protonation site. The first
experimental gas-phase absorption spectrum was recorded of this
OPE under the influence of a positive ammonium group as spec-
tator charge. The result of this nontrivial experiment can be com-
pared directly with quantum chemical calculations. In addition, the
* Corresponding authors.
E-mail addresses: sbn@phys.au.dk (S.B. Nielsen), mbn@kiku.dk (M.B. Nielsen).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.08.068

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T. Ryhding et al. / Tetrahedron 64 (2008) 11475–11479
2.2. Gas-phase action spectroscopy
H
Gas-phase experiments on the OPE3 5 were carried out at the
ELectrostatic Ion Storage ring in Aarhus (ELISA). The experimental
setup is shown in Figure 2 and has been described in detail
elsewhere.5,6 Electrospray ionization of 5 dissolved in water and
methanol (1:1) with 5% acetic acid added produced the bare 5$Hþ
ions in the gas phase. Ions were accumulated for 0.1 s in a 22-pole
ion trap in which they were thermalized from collisions with
helium buffer gas. The ion bunch was accelerated to 22-keV ki-
netic energies, and selection of ions of interest according to their
mass-to-charge ratio was then done by a bending magnet (5$Hþ:
m/z¼378). These ions were injected into the ring and stored. The
storage time in the ring was about a second, limited by collisions
with residual gas at a pressure of a few times 10ꢁ11 mbar. Ionic
dissociation in the same side as the injection side was monitored
from counting the neutral particles that hit the microchannel plate
detector at the end of the section. After about 35.1 ms, ions were
irradiated with a 4-ns pulse of light from an EKSPLA laser that
produced tunable light between 210 and 390 nm. Light absorption
was monitored by an increased yield of neutrals from delayed
dissociation of the photoexcited ions. A measurement of the
neutrals yield divided by the number of photons in the pulse and
normalized to the yield from collision induced dissociation as
a function of wavelength provided the absorption spectrum.
The decay spectrum of isolated 5$Hþ ions in the ring is
shown in Figure 3. The revolution time of the ions in the ring is
n
OPE3
OPE(n+1)
"spectator charge"
- allows for ESI
H
n
Figure 1. Oligo(p-phenyleneethynylene)s (OPEs). To allow for electrospray ionization
(ESI), a charged group is attached, a so-called spectator charge.
intrinsic properties of OPEs were investigated in a computational
study of ionization energies and electron affinities.
2. Results and discussion
2.1. Synthesis
The synthesis of the OPE3 model compound is shown in
Scheme 1. We chose to attach an amine functionality, being
precursor for an ammonium spectator ion, by an ethyl linker to the
chromophore. First,
a
Sonogashira cross-coupling reaction7
between phenylacetylene 1 and aryl iodide 28 gave compound 3.
This compound was desilylated using K2CO3 in MeOH/THF, and the
resulting terminal alkyne was then subjected to a cross-coupling
reaction with the aryl bromide 4 to furnish the OPE3 5. Here the
catalyst conditions for coupling of aryl bromides developed by
Hundertmark et al.9 were employed. These conditions allowed
coupling of the aryl bromide 4 at 40 ꢀC. The product 5 exhibited
limited solubility, but it was soluble enough to allow for the gas-
phase spectroscopic studies aimed at.
Laser pulse
Ion bunch
Laser power meter
Laser
Magnet
Injection
Micro-channel plate
detector
1 m
+
I
SiMe3
Accelerator with
electrospray ion source
1
2
Figure 2. The electrostatic ion storage ring in Aarhus, ELISA, in combination with
a laser system. The photofragment yield was measured as a function of wavelength
from the signal of neutrals hitting the detector. See text for details.
[Pd(PPh3)2Cl2], CuI
Et3N, THF
80%
105
105
SiMe3
104
0.0
0.5
Time after
1.0
3
photoexcitation (ms)
1) K2CO3, MeOH, THF
2)
Br
104
NH2
25%
4
[Pd(PhCN)2Cl2], CuI
(t-Bu)3P, Et3N
THF, toluene, 40 °C
103
0
10
20
30
40
50
Time (ms)
NH2
Figure 3. Decay spectrum of 5$Hþ ions. After 35.1 ms of storage, the ions were pho-
toexcited with 320-nm light. The inset shows the signal as a function of time after
photoexcitation.
5
Scheme 1.

T. Ryhding et al. / Tetrahedron 64 (2008) 11475–11479
11477
0.3
0.2
0.1
0.0
6
7
James et al.14 have calculated the excitation energy of the
parent OPE3 and obtained a value of 360 nm using time-de-
pendent density functional theory (TD-DFT) and a value of 365 nm
at the ZINDO/S level. We also performed TD-DFT calculations
(B3LYP/6-311þþG
*
)
on B3LYP/6-31G(d) geometry-optimized
200
240
280 320
Wavelength (nm)
360
400
structures using the Gaussian 03 program package.15 The calcu-
lated value for the neutral, parent OPE3 is 367 nm, while the ethyl-
substituted OPE3s 6 and 7 exhibit slightly redshifted values of 370
and 374 nm, respectively. Despite the fact that the calculated
values are close to the experimental one, they still seem to over-
estimate the intrinsic absorption maximum as the experimental
value is influenced by the positive charge in proximity to the
conjugated system.
Figure 4. Absorption spectrum of 5$Hþ ions isolated in vacuo. The grey box indicates
the region in which lmax is usually found in solution for OPE3s (see text for details).
78 ms. The high number of neutrals detected immediately after
injection is due to metastable ions that have been excited during
extraction from the ion trap. After a few milliseconds, the signal
is dominated by collisional decay in the ring. After 35.1 ms, the
ions were photoexcited with 320-nm light, which resulted in
a large number of neutrals from delayed ionic dissociation. The
decay is due to one-photon absorption according to a power
dependence study (data not shown) and is well-described by an
2.3. Computational study: ionization energies and electron
affinities
A selection of OPE(n)s (n¼2–17) was geometry-optimized at
the semi-empirical PM3 level. The structure of fully planar
OPE17 is shown in Figure 5. Then, single-point energies were
calculated at the B3LYP/6-311þþG(2d,p) level on the neutral,
radical cation and radical anion structures (using the geometries
of the neutral molecules). From these data, the vertical ioniza-
tion energies (IE) and vertical electron affinities (EA) were cal-
culated and collected in Table 1. These energies are plotted as
exponential with a time constant of 46 ms. From a separate ex-
periment in which ring voltages were switched to store daughter
ions,10 the dominant reaction channel was found to be loss of
ammonia.
In Figure 4 we present the absorption spectrum of bare 5$Hþ
ions. The gas-phase spectrum reveals maximum absorption at
lmax¼357 nm, a shoulder at ca. 317 nm and an absorption band
with maximum below 210 nm, the lower limit of the laser. Previous
solution studies (in hexane) on the parent unsubstituted OPE3
revealed an absorption maximum at 319 nm and a shoulder at
338 nm.11 In chloroform, absorption at 322 nm was reported and in
dioxane at 320 nm.12 For compound 6 incorporating two ethyl
substituents at the central ring, a slightly redshifted absorption
maximum at 327 nm in dichloromethane was reported.13 It is
noticed that the absorption band of the neutral chromophore in
solution phase is in between the two bands observed for the cation
in gas phase. Since a small redshift of the absorption is expected if
the cation is dissolved in an apolar solvent, the difference between
the absorption of the cation and neutral chromophore must be
ascribed to the presence of the charged ammonium group. In other
words, the ammonium group is not an innocent spectator when
placed at the current position. A simple explanation of the two
bands of the cation is an asymmetric charge distribution in the
excited state: either the active electron is closer to the positive
charge in the excited state than in the ground state or it has moved
in the opposite direction, away from the charge. In the first case, we
have an excited state that is stabilized more than the ground state
and in the second a ground state that is stabilized more than the
excited state by the presence of the charge. This model would result
in one band to the blue and one to the red relative to the neutral
chromophore. To test this idea, we plan in the future to place an
ammonium group symmetrically at both sides of the chromophore,
in which case only a single band is expected. In another approach,
the spectator charge is to be placed at a more remote position in
order not to interfere with the absorption properties.
a function of the inverse total number (N) of
p-electrons in
Figure 6. Linear extrapolation to an infinite PPE provides an IE of
5.80 eV. Hovewer, it should be noted that the points do not
follow a straight line perfectly, and this IE of an infinite polymer
seems to be slightly overestimated. For comparison, an experi-
mental IE of PPE was determined to be 6.3 eV with ultraviolet
photoelectron spectroscopy and 5.8 eV with cyclic voltamme-
try.16 By extrapolation of the calculated data, an EA of 2.34 eV
for PPE is obtained. Deviation from a perfect straight line is
Figure 5. PM3-optimized structure of planar OPE17.
Table 1
Vertical ionization energies (IE) and electron affinities (EA) calculated at the B3LYP/
6-311þþG(2d,p) level on PM3 geometry-optimized neutral structures
OPE2
OPE3
OPE4
OPE5
OPE6
OPE7
OPE8
OPE9
IE/eV
EA/eV
7.60
0.17
7.02
0.84
6.72
1.19
6.54
1.40
6.41
1.57
6.31
1.70
6.27
1.79
6.19
1.88
OPE10 OPE11 OPE12 OPE13 OPE14 OPE15 OPE16 OPE17
IE/eV
6.15
6.14
1.97
6.05
2.02
6.03
2.06
6.01
2.09
5.94
2.12
5.93
2.15
6.01
2.16
EA/eV 1.94

11478
8.0
T. Ryhding et al. / Tetrahedron 64 (2008) 11475–11479
4. Experimental
IE = 5.80 eV + 26.1 eV / N
4.1. General experimental procedures
7.5
7.0
6.5
6.0
Dry solvents were provided by an Innovative Technology Pure-
solv apparatus, or dried over 4 Å molecular sieves. Merck silica gel
(0.040–0.063 mm) was used for flash column chromatography. TLC
was performed on silica gel coated aluminium foils (Merck alumina
foil 60 F254). NMR spectra were obtained using a Varian 300 MHz
NMR spectrometer at room temperature. Fast Atom Bombardment
(FAB) mass spectra were obtained on a Jeol JMS-HX 110 Tandem
Mass Spectrometer in the positive ion mode. Electrospray ioniza-
tion spectra were recorded on a ZAB-EQ (VG-Analytical) in-
strument. Melting points were measured on a Reichert melting
point apparatus equipped with a microscope and are uncorrected.
Elemental analyses were performed at Department of Chemistry,
University of Copenhagen.
5.5
2.5
4.2. 1,4-Diethyl-2-(phenylethynyl)-5-(trimethylsilylethynyl)-
benzene (3)
EA = 2.34 eV - 32.0 eV / N
2.0
1.5
1.0
0.5
0.0
To a solution of the iodide 2 (0.51 g, 1.43 mmol), Pd(PPh3)2Cl2
(50 mg, 0.07 mmol) and CuI (24 mg, 0.13 mmol) in THF (10 mL)
were added phenylacetylene 1 (0.20 mL, 1.7 mmol) and Et3N
(2 mL). The reaction mixture was stirred for 24 h under Ar. Then, it
was diluted with CH2Cl2 (20 mL) and washed with saturated
aqueous NH4Cl (20 mL). The organic phase was dried with MgSO4,
filtered and concentrated in vacuo. Purification by flash chroma-
tography (SiO2, c-hexane) gave the product as a colourless oil
(0.38 g, 80%). 1H NMR (CDCl3, 300 MHz)
d
¼0.28 (s, 9H, CH3), 1.27 (t,
J¼7.5 Hz, 3H, CH3), 1.30 (t, J¼7.4 Hz, 3H, CH3), 2.79 (q, J¼7.5 Hz, 2H,
CH2), 2.84 (q, J¼7.4 Hz, 2H, CH2), 7.36 (m, 5H, Ar), 7.54 (m, 2H, Ar).
0.00
0.02
0.04
1 / N
0.06
0.08
13C NMR (CDCl3, 75 MHz)
94.30, 99.36, 104.07, 122.54, 122.87, 123.66, 128.53, 128.63, 131.72,
131.74, 132.07, 143.56, 144.19. MS (FAB): m/z¼330 (Mþ). Anal. Calcd
for C23H26Si: C, 83.57; H, 7.93. Found: C, 83.51; H, 8.17.
d
¼0.23, 14.77, 14.95, 27.37, 27.40, 88.38,
Figure 6. Ionization energies (IE) and electron affinities (EA) for OPEs as a function of
the inverse number of -electrons (N).
p
4.3. 1-[4-(2-Aminoethyl)phenylethynyl]-2,5-diethyl-4-
(phenylethynyl)benzene (5)
more significant here and may underestimate the EA of PPE by
ca. 0.1 eV. Subtracting the linearly extrapolated IE and EA values
gives a band gap of 3.46 eV for PPE. This value is the estimated
band gap of an isolated, completely planar PPE. For comparison,
alkyl-substituted PPEs typically exhibit a broad unresolved ab-
sorption in solution at 384/388 nm (ca. 3.2 eV), and an absorp-
tion in films at 435 nm (2.9 eV).2,17
Compound 3 (0.121 g, 0.367 mmol) was treated with K2CO3
(0.20 g, 1.4 mmol) in a MeOH/CH2Cl2 mixture (1:1, 10 mL). After
complete desilylation (judged by TLC, approx. 45 min), the mixture
was diluted with Et2O (20 mL) and washed with water (2ꢂ20 mL).
The organic phase was dried with MgSO4, filtered and concentrated
in vacuo to furnish the deprotected alkyne. Pd(PhCN)2Cl2 (25 mg,
0.065 mmol) and CuI (6 mg, 0.03 mmol) were dissolved in THF
(1 mL) and toluene (1 mL). Then, i-Pr2NH (0.34 mL) and t-Bu3P
(0.25 mL, 10% in hexanes) were added to this catalyst mixture. The
catalyst solution was then added to the deprotected alkyne and aryl
bromide 4 (0.109 g, 0.55 mmol) under argon, and the reaction
mixture was left overnight at 40 ꢀC. The mixture was concentrated
in vacuo, whereupon the residue was dissolved in warm toluene.
Pentane was added to precipitate the product as a slightly yellow
solid (37 mg, 25%) of limited solubility. Mp 180 ꢀC (decomp.). 1H
3. Conclusions
An OPE3 incorporating an amino functionality was prepared by
step-wise cross-coupling reactions. The amino group allows for
transfer of the chromophore as the protonated species to the gas
phase by electrospray ionization. Electrospray ionization in com-
bination with the storage ring technique prevents the necessity for
evaporating the molecules in order to perform gas-phase spec-
troscopy and we have measured the first experimental gas-phase
absorption spectrum of an OPE3 ammonium cation. The major
absorption maximum is found to be significantly redshifted in
the gas phase as compared to solution phase, which is ascribed to
NMR (DMSO-d6, 300 MHz):
d
¼1.21 (m, 6H, CH3), 2.75–3.03 (m, 8H,
4ꢂCH2), 7.21 (d, J¼7.2 Hz, 2H, Ar), 7.34 (d, J¼7.3 Hz, 2H, Ar), 7.44 (m,
2H, Ar), 7.50 (m, 3H, Ar). HRMS (ESI): m/z¼378.2211 (MHþ); calcd
for C28H28N: 378.2216.
the stabilizing effect of the excess charge on the diffuse pp state.
*
The method is by no way trivial and the optical properties of other
OPE model compounds await to be investigated in future work.
In addition, we have calculated vertical ionization energies and
electron affinities for a large selection of OPEs and predict by ex-
trapolation values of 5.80 and 2.34 eV, respectively, for an isolated,
completely planar PPE.
Acknowledgements
This work was supported by Lundbeckfonden and Direktør Ib
Henriksens Fond.

T. Ryhding et al. / Tetrahedron 64 (2008) 11475–11479
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