A re-evaluation of the photophysical properties of 1,4-bis(phenylethynyl)benzene: A model for poly(phenyleneethynylene)

Article abstract of DOI:10.1021/ja025568h

Photophysical measurements, recorded in aerated cyclohexane at 283 K, indicate that 1,4-bis-(phenylethynyl)benzene behaves in a conventional manner, undergoing emission from the lowest vibrational level of the first excited singlet state; there is no evid

Full text of DOI:10.1021/ja025568h

Published on Web 06/13/2002
A Re-evaluation of the Photophysical Properties of
1,4-Bis(phenylethynyl)benzene: A Model for
Poly(phenyleneethynylene)
Andrew Beeby,* Karen Findlay, Paul J. Low,* and Todd B. Marder
Contribution from the Department of Chemistry, UniVersity of Durham,
Durham DH1 3LE, United Kingdom
Received January 11, 2002
Abstract: Photophysical measurements, recorded in aerated cyclohexane at 283 K, indicate that 1,4-bis-
(phenylethynyl)benzene behaves in a conventional manner, undergoing emission from the lowest vibrational
level of the first excited singlet state; there is no evidence for aggregation of this material in cyclohexane
solution in the concentration range (1-250) × 10-6 mol dm-3. However, in highly viscous, low-temperature
glasses, the material does exhibit inhomogeneous fluorescence behavior, and wavelength-dependent
excitation and emission spectra, indicative of a slow rate of relaxation of conformers of the excited states
compared to the rate of fluorescence.
1. Introduction
is dependent upon the relative orientation of the planar aromatic
moieties. Indeed, it has recently been claimed that conforma-
tional changes may be used to modify the conductive properties
of a 1,4-bis(phenylethynyl)benzene derivative, giving rise to a
molecular switch.5 The HOMO-LUMO gap in these materials,
and hence their emissive properties, will vary with the effective
conjugation length, which in turn will alter their photo- and
electroluminescence characteristics. To engineer molecular
devices based upon conjugated frameworks, a firm grasp of the
factors that control the geometry of the compound, and hence
the π-conjugation pathway, in both the ground and excited states
is required.
We,1c,6 and others,1-5,7 have had an interest in the chemistry
and properties of compounds featuring phenylethynyl motifs
for some time. The barrier to rotation about the alkynyl-aryl
single bond is very low, estimated at less than 1 kcal/mol,8 and
engineering control over the molecular conformation in materials
derived from the elementary framework 1 (Chart 1) is a
significant challenge. Nevertheless, a number of groups have
recently reported success in achieving a degree of such control
after constraining molecular and polymeric materials in Lang-
muir films,9 or self-assembled monolayers,10 as well as through
use of intramolecular tethers.11
Many ethynylated aromatic systems, such as 1,4-bis(phenyl-
ethynyl)benzenes, 9,10-bis(phenylethynyl)anthracenes, 2,5-bis-
(phenylethynyl)thiophenes, and 2,5-bis(phenylethynyl) metal-
lacylopentadienes, display interesting structural, electronic,
nonlinear optical, and luminescent properties. The stiff, linear
nature of these compounds often results in liquid-crystalline
behavior,1 and it is interesting to note that many of the models
used to rationalize the phase behavior assume cylindrical
symmetry along the ethynyl axis. The fluorescent and elec-
troluminescent properties of both molecular and polymeric
systems based upon these motifs have prompted speculation
about the suitability of these compounds for alternatives to poly-
(phenylene)vinylene (PPV) as emitting layers in electrolumi-
nescent devices.2 The conjugated π-system has led to the
development of molecular wire-like architechtures,3 for which
remarkably low resistivities have been measured.4
The origin of many of these fascinating properties may be
directly attributed to the extended, linear π-conjugation that runs
along the principal molecular axis which, at any point in time,
* Corresponding author. Andrew.Beeby@durham.ac.uk; P.J.Low@
durham.ac.uk
(1) (a) Miteva, T.; Palmer, L.; Kloppenburg, L.; Neher, D.; Bunz, U. H. F.
Macromolecules 2000, 33, 652-654. (b) Kloppenburg, L.; Jones, D.;
Claridge, J. B.; zur Loye, H.-C.; Bunz, U. H. F. Macromolecules 1999,
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10.1021/ja025568h CCC: $22.00 © 2002 American Chemical Society

Photophysical Properties of 1,4-Bis(phenylethynyl)benzene
A R T I C L E S
Chart 1. Compounds 1 and 2
isomers of 1, i.e., a planar structure and a twisted form. If true,
this observation would have profound implications for the
behavior of these materials in a range of applications, as well
as being an example of very unusual photophysical behavior.
We had not previously observed such anomalous behavior
in several closely related systems,6 and Grummt and colleagues
have claimed the fluorescence spectrum of 1 is wavelength
independent.7j Furthermore, neither we nor Grummt’s group had
seen the unusual low-energy shoulder (λmax ) 360 nm) on the
UV-vis profile of 1 reported by Levitus et al.14 This prompted
us to reexamine carefully the spectroscopic properties of 1 in
solution.
In terms of measurements concerned with the dynamic
processes of materials based upon the core framework of 1,
Sluch et al. have undertaken picosecond time-resolved emission
studies of a substituted nonamer 2 (Chart 1).12 Their results are
consistent with a thermally populated distribution of ground-
state conformations and a first excited state with a considerably
larger barrier to rotational distortion from the lowest energy
planar orientation. The viscosity of the solvent medium was
also shown to play a significant role in the dynamic process,
with high-viscosity solvents hindering the relative torsional
motion of the aryl groups. Similar models have been proposed13
to account for the photophysical properties of 1,4-bis(anthra-
cenyl-9-ethynyl)benzene6c and 1,4-bis(phenylethynyl)anthra-
cene.6c,7a-c
We were greatly intrigued by a recent report of a series of
photophysical measurements of the parent species 1,4-bis-
(phenylethynyl)benzene (1).14 This report described the observa-
tion of significant and systematic shifts in the fluorescence
emission spectrum of 1 with excitation wavelength. The
observation of two discrete spectral profiles, rather than a smooth
transition from one spectral profile, clearly indicated the
presence of two, spectroscopically distinct species in solution.
These species were attributed to two discrete configurational
2. Experimental Section
A sample of 1 was prepared by Sonogashira coupling of 1,4-
diiodobenzene with phenylacetylene in diisopropylamine with a mixed
Pd(PPh3)4 (3%)/CuI (3%) catalyst.15 The product was purified by
recrystallization from toluene, yielding analytically pure material as
determined by elemental analysis (C22H14 requires C 94.97%, H 5.03%;
1
found C 94.59% H 4.99%), H and 13C NMR, and GC-MS. UV-vis
spectra were recorded with an ATI Unicam UV-2 spectrophotometer.
Samples were held in silica cells with path lengths of 1, 2, 10, and 25
mm and the spectra recorded over the range 200-400 nm with a 1 nm
data interval and a 2 nm band-pass. Fluorescence spectra were recorded
with a Jobin-Yvon Horiba Fluorolog 3-22 Tau-3 spectrofluorimeter.
The spectra of dilute solutions, with an absorbance of <0.1 in the range
250-400 nm, were recorded by using conventional 90° geometry, and
a front-face geometry was used to study highly absorbing solutions.
Both the excitation and emission spectra were fully corrected by using
the manufacturers correction curves for the spectral response of the
excitation and emission optical components. A spectral band-pass of
2.5 nm was used for both the excitation and emission monochromators.
Excitation and emission matrices were acquired by recording the
emission spectra over the range 300-500 nm and stepping the excitation
wavelength from 230 to 400 nm in 2.5 nm increments. Fluorescence
lifetimes were recorded with the same spectrometer operating in the
phase-modulation mode. The phase shift and modulation were recorded
over the frequency range 50-300 MHz, and the data fitted using the
Jobin-Yvon software package. Low-temperature spectra were recorded
in an Oxford Instruments DN1704 cryostat and temperature controller
(ITC 6). A mixture of diethyl ether, 2-methylbutane, and ethanol in
the ratio 5:5:2 (v/v), EPA, was used for the low-temperature studies
since this forms an optically transparent glass at low temperatures.
(7) For a selection of recent references see: (a) Piskunov, A. V.; Moroz, A.
A.; Shvartsberg, M. S. IzV. Akad. Nauk SSSR, Ser. Khim. 1990, 1441-
1443. (b) Nakatsuji, S.; Akiyama, S.; Nakatsuji, S.; Nomura, K.; Matsuda,
K.; Nakashima, K. J. Chem. Soc., Chem. Commun. 1991, 948. (c) Matsuda,
K.; Uesugi, Y.; Nakashima, K. J. Chem. Soc., Perkin Trans. 1992, 755.
(d) Jones, L., II; Schumm, J. S.; Tour, J. M. J. Org. Chem. 1997, 62, 1388.
(e) Li, H.; Powell, D. R.; Firman, T. K.; West, R. Macromolecules 1998,
31, 1093-1098. (f) Lahiri, S.; Thompson, J. L.; Moore, J. S. J. Am. Chem.
Soc. 2000, 122, 11315-11319. (g) Dirk, S. M.; Price, D. W.; Chanteau,
S.; Kosynkin, D. V.; Tour, J. M. Tetrahedron 2001, 57, 5109-5121. (h)
Tour, J. M.; Rawlett, A. M.; Kozaki, M.; Yao, Y. X.; Jagessar, R. C.; Dirk,
S. M.; Price, D. W.; Reed, M. A.; Zhou, C. W.; Chen, J.; Wang, W. Y.;
Campbell, I. Chem. Eur. J. 2001, 7, 5118-5134. (i) Kawai, T.; Saski, T.;
Irie, M. Chem. Commun. 2001, 711. (j) Birckner, E.; Grummt, U.-W.;
Go¨ller, A. H.; Pautszch, T.; Egbe, D. A. M.; Al-Higari, M.; Klemm, E. J.
Phys. Chem. A 2001, 105, 10307. (k) Tan, C.; Pinto, M. R.; Schanze, K.
S. Chem. Commun. 2002, 446-447.
(8) (a) Abramenkov, A. V.; Almenningen, A.; Cyvin, B. N.; Cyvin, S. J.;
Jonvik, T.; Khaikin, L. S.; Romming, C.; Vilkonv, L. V. Acta Chim. Scand.
1988, A42, 674-84. (b) Inoue, K.; Takeuchi, H.; Konaka, S. J. Chem. Phys.
A 2001, 105, 6711-16. (c) Okuyama, K.; Hasegawa, T.; Ito, M.; Mikami,
N. J. Phys. Chem. 1984, 88, 1711-1716.
(9) Kim, J.; Swager, T. M. Nature 2001, 411, 1030-34.
(10) Reinerth, W. A.; Jones, L., II; Burgin, T. P.; Zhou, C.; Muller, C. J.;
Deshpande, M. R.; Reed, M. A.; Tour, J. M. Nanotechnology 1998, 9, 246-
250.
(11) (a) McFarland, S. A.; Finney, N. S. J. Am. Chem. Soc. 2002, 124, 1178.
(b) Crisp, G. T.; Bubner, T. P. Tetrahedron 1997, 53, 11881-11898.
(12) Sluch, M. I.; Godt, G.; Bunz, U. H. F.; Berg, M. A. J. Am. Chem. Soc.
2001, 123, 6447.
(13) (a) Cherkasov, A. S.; Veselova, B. M.; Krasovitskii, B. M.; Shershukov,
V. M. Opt. Spectrosc. (USSR) 1985, 59, 59-63. (b) Levitus, M.; Garica-
Garibay, M. A. J. Phys. Chem. A 2000, 104, 8632-8637. (c) Schmieder,
K.; Levitus, M.; Dang, H.; Garcia-Garibay, M. A. J. Phys. Chem. A 2002,
106, 1551-1556.
(14) Levitus, M.; Schmieder, K.; Ricks, H.; Shimizu, K. D.; Bunz, U. H. F.;
Garcia-Gariby, M. A. J. Am. Chem. Soc. 2001, 123, 4259-4265.
3. Results and Discussion
In cyclohexane solution, the UV-vis spectrum of 1 prepared
in this manner is consistent with that published earlier by one
of us,6e featuring a series of partially resolved absorption bands
between 250 and 350 nm, with a sharp band-edge at the red
end of the absorption profile (Figure 1). Similar spectra were
observed in toluene, acetonitrile, and dioxane solutions.6e,7j,16
A maximum extinction coefficient of 58 000 ( 1 000 mol-1
dm3 cm-1 was determined at the 320 nm peak. The same
spectral profile was obtained at concentrations from 10-6 to 2.5
× 10-4 mol dm-3 in cyclohexane, with a linear dependence of
the absorption upon concentration (Figure 1). The linear nature
of the Beer-Lambert plot suggests that concentration-dependent
aggregation phenomena are not significant in this solvent.
(15) A minor modification of the procedure described in ref 6a was employed,
as detailed in the Supporting Information.
(16) Beeby, A.; Low, P. J.; Marder, T. B.; Nyugen, P.; Dai, C. Unpublished
work.
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A R T I C L E S
Beeby et al.
Figure 1. UV absorption spectra of 1 × 10-6 (lower trace) and 250 ×
10-6 mol dm-3 (upper trace) solutions of 1 in cyclohexane at 293 K. The
upper trace has been offset by 10 000 mol-1 dm3 cm-1 for clarity. The
inset shows the absorbance at the peak, 320 nm, as a function of
concentration. The value obtained for 250 × 10-6 mol dm-3 falls on this
line but has been omitted for clarity.
Figure 3. Fluorescence excitation and emission matrix obtained from a 1
× 10-6 mol dm-3 solution of 1 in cyclohexane. The emission spectra have
been projected onto the right-hand-side panel. The ridge of small peaks
that appear at excitation wavelengths longer than 350 nm are the result of
Rayleigh scatter by the sample.
Figure 2. Normalized fluorescence excitation and emission spectra obtained
from a 1 × 10-6 mol dm-3 solution of 1 in cyclohexane. The excitation
spectra a and c were obtained by using emission wavelengths of 450 and
350 nm, respectively, and the emission spectra b and d were obtained by
using excitation wavelengths of 270 and 340 nm, respectively. The small
peak at 397 nm in spectrum b is due to Raman scatter from the solvent.
Both the excitation and emission monochromators were set to 2.5 nm fwhm.
Figure 4. Data obtained by the phase-modulation technique for the
determination of the fluorescence lifetime of 1 in aerated cyclohexane, using
excitation at 320 nm and emission at 360 nm. The curves show the fit of
the phase (squares) and modulation (triangles) for a single-exponential decay
with a lifetime of 0.53 ns. The normalized residuals are shown below.
ø2 ) 0.80.
Crucially, the spectra do not exhibit the shoulder at 360 nm
described by Levitus et al.14
explained by the effects of reabsorption in these optically dense
solutions17,18 rather than the effects of co-facial aggregation
proposed elsewhere.7e
Fluorescence emission spectra were collected by using
excitation wavelengths from 230 to 400 nm (Figure 2). We
observed constant emission band profiles across this range of
excitation wavelengths. The emission spectrum is similar to that
observed by Levitus et al. using short excitation wavelengths,
and shows some vibrational fine structure with peaks at 346,
362, and 375 nm, tailing to the red.14 Excitation spectra of the
emission were independent of the selected emission wavelength
and closely resemble the UV-vis absorption spectrum.
As the homogeneity of the fluorescence spectrum is of vital
importance to the question of whether this material has any dual
emission characteristics, we have also recorded the excitation-
emission matrix, EEM, of a dilute solution of 1 in cyclohexane,
illustrated in Figure 3. These data further confirm the homo-
geneity of the excitation and emission spectra. The emission
spectrum of a more concentrated solution of 1, 250 × 10-6 mol
dm-3, was recorded by using a front-face geometry that reduces,
but does not entirely eliminate, inner-filter effects. The observed
spectrum shows a similar spectral profile to that of very dilute
solutions, but the blue-shifted band at 350 nm has a reduced
intensity. This is not an unusual phenomenon, and it can be
Fluorescence lifetimes, determined by the phase-modulation
technique from cyclohexane solutions, gave a value for 1 of
0.63 ( 0.05 ns, which was found to be independent of the
excitation and emission wavelengths used, consistent with our
data reported earlier.6e A typical fit is illustrated in Figure 4.
Similarly, Birckner at al. found the fluorescence lifetime of 1
to vary between 0.59 (toluene) and 0.69 ns (methylcyclohexane).7j
This is much shorter than reported by Levitus et al., where
values ranged between 0.74 and 0.97 ns depending upon the
excitation and emission wavelengths used.14 Their data were
obtained by using the method of time-correlated single-photon
counting, and close examination of the fluorescence decay plot
presented in ref 14 shows that the instrument response profile
of the equipment was rather long, >1 ns, and that a relatively
small number of counts were acquired.19 A system containing
(17) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience:
London, UK, 1970.
(18) Dhami, S.; deMello, A. J.; Rumbles, G.; Bishop, S. M.; Phillips, D.; Beeby,
A. Photochem. Photobiol. 1995, 61, 341-346.
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Photophysical Properties of 1,4-Bis(phenylethynyl)benzene
A R T I C L E S
propose that in the low-temperature glass there is a more limited
range of conformations, with a greater bias toward the planar
form. However, since the rotational barrier in the ground state
is low, cooling to 77 K does not adequately reduce kT to limit
the material to only the planar form.
Several models have been proposed to account for the
dynamic behavior of 1 and structurally related systems on the
basis of photophysical studies. Cherkasov et al. proposed a two-
state model in which the aromatic rings adopt coplanar or
orthogonal conformations to account for the low-temperature
fluorescence behavior of 9,10-bis(phenylethynyl)anthracene.13a
A similar description was proposed by Levitus et al. to account
for their observations of 1.14
Figure 5. Absorption spectra of 1 in EPA at 298 (lower trace) and 77 K
(upper trace). The spectra have been corrected for changes in solvent density
and the low-temperature spectrum is shown offset by 20 000 mol-1 dm3
cm-1 for clarity.
However, in contrast to these hypotheses, we suggest that
there is a continuum of rotational conformers of 1 in solution
and that the distribution of these conformers in the ground
electronic state changes with temperature.
Our observations and interpretations are therefore entirely in
agreement with the conclusions of Sluch et al., who investigated
the dynamics of the fluorescence of a substituted oligophen-
yleneethynylene, 2.12 Their work indicated that rotational
relaxation occurred very rapidly in the excited state, with a
recorded time constant of 60 ps, and they interpreted their data
in terms of a quadratic coupling model. Using fast time-resolved
emission spectroscopy, they demonstrated that the blue edge
of the emission shifted in a subtle fashion at early times after
excitation, evidence for the rapid planarization of the excited
state. In their model, Sluch et al. suggest that the system behaves
as though the potential well is very small in the ground electronic
state but that in the first excited state the planar form is
considerably more stable than the nonplanar conformers. Indeed,
these interpretations are further supported by subsequent work
on anthrylethynylenes reported by Garcia-Garibay’s group.13b,c
Figure 6. Normalized, corrected fluorescence excitation and emission
spectra of 1 in EPA at (a, b) 77 and (c, d) 298 K determined with excitation
at 320 nm and emission at 360 nm.
two distinct emitting species would be expected to show a
biexponential decay, although the low signal-to-noise ratio and
the unacceptably low reduced ø2 values obtained for the single
exponential decays may have prevented the authors from
considering such fits.
4. Conclusion
Photophysical measurements, recorded in aerated cyclohexane
at 283 K, indicate that 1 behaves in a conventional manner,
undergoing emission from the lowest vibrational level of the
first excited singlet state. We see no evidence whatsoever for
any anomalous excited state behavior that supports the presence
of two distinct rotamers or aggregates in cyclohexane solution
in the concentration range (1-250) × 10-6 mol dm-3, and
suggest that the data reported in the previous study involving
excitation at wavelengths greater than 350 nm arise not from
1, but rather from a second chemically distinct species, despite
the careful efforts made by the authors to ensure sample
purity.14,20 However, there is evidence that in highly viscous,
low-temperature glasses, the material does exhibit inhomoge-
neous fluorescence behavior, and that the excitation and
emission spectra are wavelength dependent, indicative of a slow
rate of relaxation of rotamers of the excited states compared to
the rate of fluorescence.
At 77 K, the absorption spectrum of 1 in EPA shows changes
in the absorption profile, with a significant increase in absorption
at the red edge (Figure 5). Qualitatively, the excitation spectrum
collected under these conditions is similar to the absorption
spectrum and more like the mirror image of the emission
spectrum, showing a strong (0,0) band and a small Stokes shift
(Figure 6). Closer examination reveals a more complex behavior
in the low-temperature glass, with some evidence of changes
in the emission spectrum with excitation wavelength. For
example, upon excitation at 350 nm, the red edge of the
absorption band gives an emission spectrum with a sharp blue
edge. However, for progressively shorter excitation wavelengths,
this rising edge becomes less well-defined and the vibrational
fine structure in the emission spectra show subtle but distinct
shifts. This can only be attributed to the fact that in the low-
temperature glasses we have a continuum of conformers, each
with its own absorption and emission spectrum, and that on the
fluorescence time scale these conformers are now long-lived,
i.e., the rate of rotation of the phenyl rings with respect to one
another is comparable to the fluorescence lifetime, 0.53 ns. We
Acknowledgment. We thank the EPSRC and the University
of Durham for financial support of this work. The assistance of
Drs. Alan Kenwright (NMR) and Mike Jones (GC-MS) in
determining the purity of 1 is gratefully acknowledged. Prof.
(19) O’Connor, D. V.; Phillips, D. Time Correlated Single Photon Counting;
Academic Press: New York, 1984.
(20) Garcia-Garibay, M. A. Personal communication.
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A R T I C L E S
Beeby et al.
M. A. Garcia-Garibay is thanked for helpful discussions, and a
preprint of ref 13c.
and GC-MS (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Supporting Information Available: Details of the synthesis
1
and characterization of 1, including H and 13C NMR spectra
JA025568H
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8284 J. AM. CHEM. SOC. VOL. 124, NO. 28, 2002