Synthesis, photophysics and molecular structures of luminescent 2,5-bis(phenylethynyl)thiophenes (BPETs)

Article abstract of DOI:10.1039/b701172e

The Sonogashira cross-coupling of two equivalents of para-substituted ethynylbenzenes with 2,5-diiodothiophene provides a simple synthetic route for the preparation of 2,5-bis(para-R-phenylethynyl)thiophenes (R = H, Me, OMe, CF₃, NMe₂

Full text of DOI:10.1039/b701172e

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Synthesis, photophysics and molecular structures of luminescent
2,5-bis(phenylethynyl)thiophenes (BPETs)
Jamie S. Siddle,a Richard M. Ward,a Jonathan C. Collings,a Simon R. Rutter,a
Laurent Porres,a Lucas Applegarth,a Andrew Beeby,a Andrei S. Batsanov,a
Amber L. Thompson,a Judith A. K. Howard,a Abdou Boucekkine,b Karine Costuas,b
Jean-Franc¸
ois Haletb and Todd B. Marder*a
Received (in Montpellier, France) 25th January 2007, Accepted 21st March 2007
First published as an Advance Article on the web 12th April 2007
DOI: 10.1039/b701172e
The Sonogashira cross-coupling of two equivalents of para-substituted ethynylbenzenes with
2,5-diiodothiophene provides a simple synthetic route for the preparation of 2,5-bis(para-R-
phenylethynyl)thiophenes (R = H, Me, OMe, CF3, NMe2, NO2, CN and CO2Me) (1a–h).
Likewise, 2,5-bis(pentafluorophenylethynyl)thiophene (2) was prepared by the coupling of 2,5-
diiodothiophene with pentafluorophenylacetylene. All compounds were characterised by NMR,
IR, Raman and mass spectroscopy, elemental analysis, and their absorption and emission spectra,
quantum yields and lifetimes were also measured. The spectroscopic studies of 1a–h and 2 show
that both electron donating and electron withdrawing para-subsituents on the phenyl rings shift
the absorption and emission maxima to lower energies, but that acceptors are more efficient in
this regard. The short singlet lifetimes and modest fluorescence quantum yields (ca. 0.2–0.3)
observed are characteristic of rapid intersystem crossing. The single-crystal structures of 2,5-
bis(phenylethynyl)thiophene, 2,5-bis(para-carbomethoxyphenylethynyl)thiophene, 2,5-bis(para-
methylphenylethynyl)thiophene and 2,5-bis(pentafluorophenylethynyl)thiophene were determined
by X-ray diffraction at 120 K. DFT calculations show that the all-planar form of the compounds
is the lowest in energy, although rotation of the phenyl groups about the CRC bond is facile
and TD-DFT calculations suggest that, similar to 1,4-bis(phenylethynyl)benzene analogues, the
absorption spectra in solution arise from a variety of rotational conformations. Frequency
calculations confirm the assignments of the compounds’ IR and Raman spectra.
coupling of terminal alkynes19–23 or alkynyl Grignard re-
Introduction
agents34 with 2,5-halothiophenes, giving BPETs in good
yields. Pd-catalysed cross-coupling has also been used to
produce BPETs in good yields from triorganoindium com-
pounds and 2,5-dibromothiophene.35 Other methods include
longer reaction sequences involving elimination reactions. A
one-pot synthesis using benzyl sulfone derivatives and 2-for-
myl-5-phenylethynylthiophene, incorporating two sequential
eliminations from the reaction intermediates, has been devel-
oped, forming symmetrical or unsymmetrical BPETs in good
yields.36 The compound 1,8-diphenyl-1,7-octadiyne-3,6-dione,
synthesised in nine steps from 1,4-butanediol, can also be
converted to a BPET by a reaction with bis(triphenyl)tin
sulfide in the presence of BCl3.37
Conjugated systems, such as 1,4-bis(phenylethynyl)benzenes
(BPEBs)1–15
and
9,10-bis(phenylethynyl)anthracenes
(BPEAs),16–18 show interesting structural, electronic and lu-
minescent properties, and have been well studied; however,
there are far fewer examples of 2,5-bis(phenylethynyl)thio-
phenes (BPETs). The few BPETs that have been reported
are of interest due to their luminescence19–24 and non-linear
optical25–27 properties, as well as their liquid crystalline phase
behaviour, which they owe to the bent nature of the thiophene
moiety.28–31 An optically active (axially chiral) bis-BPET
derivative has been described,32 as well as transition metal
p-complexes containing BPETs as ligands.33a Closely related
to the BPETs are 2,5-bis(pyridylethynyl)thiophenes,33b–d
which have the capacity to coordinate to transition metal
atoms (via nitrogen), and thus can be used to create photo-
conducting solids or molecular wires.
To complement our current work on luminescent mer,cis-
[tris(trimethylphosphine)-trimethylsilylethynyl-2,5-bis(para-R-
phenylethynyl)-3,4-(para-R-phenyl)rhodacyclopenta-2,4-dienes],38
we have synthesised
a number of structurally related
There are a number of routes for the synthesis of BPETs.
The most straightforward methods utilise the catalytic cross-
2,5-bis(para-R-phenylethynyl)thiophenes with the aim of eval-
uating the role of the heteroatom on the optical properties of
heterocyclopentadienes. Herein, we report the synthesis and
optical properties of 2,5-bis(para-R-phenylethynyl)thio-
phenes, R = H (1a), Me (1b), OMe (1c), CF3 (1d), NMe2
(1e), NO2 (1f), CN (1g) and CO2Me (1h), 2,5-bis(pentafluoro-
phenylethynyl)-thiophene (2) and the crystal and molecular
a Department of Chemistry, Durham University, South Road, Durham,
UK DH1 3LE. E-mail: todd.marder@durham.ac.uk; Fax: +44 191-
384-4737; Tel: +44 191-334-2037
b
´
UMR CNRS 6226 Sciences Chimiques de Rennes, Universite de
Rennes 1, Campus de Beaulieu, 35042 Rennes, France
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generate no more than 50% yield based on 2,5-diiodothio-
phene, and (b) generate a significant amount (10%) of the 4-
Me2N–C6H4–CRC–CRC–C6H4-4-NMe2 diyne, further
consuming 20% of the starting alkyne during the catalyst
activation step.1,42 Both of these factors should significantly
reduce the overall yield of 1e obtained. We have prepared
compound 1e in moderate yield using 2 mol% Pd catalyst and
did not observe a significant amount of diyne formation (i.e.
beyond that required for catalyst precursor reduction). We
also note that analytically pure samples of several of the
compounds showed melting ranges that were slightly broader
than expected, possibly an indication of transient liquid crystal
phase behaviour,30b but this aspect was not investigated
further.
Scheme 1
structures of selected examples. A more detailed study of the
photophysical behaviour of the parent compound 1a is given
in a companion paper.39
Optical properties
The optical properties (absorption and fluorescence maxima,
fluorescence quantum yields and lifetimes) for compounds
1a–h and 2 are presented in Table 1. All compounds show
an intense absorption band in the UV region (Fig. 1 and Fig.
2). With the exception of the nitro derivative, 1f, these
chromophores are also fluorescent in the UV-vis region with
moderate quantum yields of 0.19–0.33 (Fig. 3 and Fig. 4). It is
apparent that the presence of both electron donating and
electron withdrawing groups at the para-position of the phenyl
rings shifts the absorption as well as the emission maxima
bathochromically from 350 nm for 1a to 386 and 387 nm for 1e
and 1f, respectively, in absorption, and from 382 nm for 1a to
434 and 402 nm for 1e and 1h, respectively, in emission.
Compound 1f is very weakly emissive in toluene, resulting
from the presence of the NO2 groups, which lead to non-
radiative deactivation of the excited state, meaning its quan-
tum yield could not be determined accurately. This presum-
ably results from the fact that the p - p* transition is higher
in energy than the nitro n - p* transition, as is often the case
for such compounds. However, a quantum yield of 0.17 was
measured in the more polar CH2Cl2 solvent. The excited state
lifetimes of these compounds are short compared to those of
related BPEA and BPEB systems,3b,15a,b,16a and this is mir-
rored in the lower quantum yields. This observation can be
attributed to the excited singlet state, S1, undergoing more
Results and discussion
Syntheses
Compounds 1a–h and 2 were synthesised via the Sonogashira
cross-coupling reaction of 2,5-diiodothiophene with two
equivalents of the appropriate substituted ethynylbenzene at
room temperature for 24–96 h, except for 1h, which was stirred
at room temperature for 24 h and then heated to 80 1C for 2 h,
catalysed by 2 mol% Pd(PPh3)2Cl2 and CuI in triethylamine
or diethylamine,40 as shown in Scheme 1. Reactions were
monitored by GC-MS or TLC, and the compounds were
isolated after passage through a short silica gel column eluting
with hexane, hexane/CH2Cl2 or hot toluene. Compounds
1a,22,34–37 1c33,41 and 1e19,20 have previously been reported,
although no synthetic or characterisation details were given
for 1c. Analogues of 1c, containing longer alkoxy substituents,
have been reported very recently, along with their liquid
crystal phase behaviour and optical properties.30b Compound
1e is reported to have been synthesized in almost quantitative
yield; however, the procedure, surprisingly, used a 1 : 1 ratio of
4-N,N-dimethylaminophenylacetylene to 2,5-diiodothiophene
with 10 mol% of a Pd(II) catalyst precursor, which should: (a)
Table 1 Spectroscopic data for compounds 1a–h and 2 in toluene, unless otherwise noted.
lmax
abs/nm
e/molÀ1
lmax
em/nm
Observed S1–S0
energy/cmÀ1b
Calculated S1–S0
energy/cmÀ1a
Stokes
shift/cmÀ1
Lifetime
t/ps
cmÀ1 dm3 fcalc
F
a
Compound
R
1a
1b
1c
H
Me
OMe
350
353
345
359
354
386
387
394c
365
365
351
33 000
40 000
39 000
40 000
40 000
52 000
44 000
42 000
49 000
48 000
39 000
1.56
1.72
1.76
382
386
394
26 200
25 900
25 400
25 600
25 200
24 800
0.20
0.23
0.22
2400
2400
2500
240
240
270
1d
1e
1f
CF3
NMe2
NO2
1.72
1.99
1.69
387
434
435
547c
401
402
383
b
25 800
23 000
23 500
25 000
23 300
22 100
0.22
0.33
—
2400
2900
2900
7100c
2500
2500
2400
d
230
400
—
0.17c
0.31
0.29
0.19
930c
320
310
210
1g
1h
2
CN
CO2Me
Fd
1.93
1.89
1.60
24 900
24 900
26 100
23 800
23 700
25 300
c
a
From gas phase TD-DFT calculations; f = oscillator strength. Estimated from lmax em (see text). Measurements in CH2Cl2. F at the 2, 3, 4,
5 and 6 positions of the phenyl ring.
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Fig. 1 Absorption spectra of compounds 1a–c and 1e.
Fig. 3 Emission spectra of compounds 1a–c and 1e.
Fig. 2 Absorption spectra of compounds 1d and 1f–h.
rapid intersystem crossing (ISC) to the triplet state, T1, for the
BPETs compared to their BPEB and BPEA analogues.39 ISC
is faster due to the introduction of the sulfur, a moderately
heavy atom which enhances spin–orbit coupling in the
molecule.
Fig. 4 Emission spectra of compounds 1d, 1f and 1h.
identical to that reported very recently for the EtO derivati-
ve,30b confirming the accuracy of our data.
A plot of the absorbance maxima against the Hammett
constant43 for the para-substituent (sp), shown in Fig. 5,
displays a similar trend to those that we have observed for
the analogous BPEBs1 and BPEAs.16 The absorption maxima
for the BPETs are intermediate between the values for BPEBs
and BPEAs, with the BPEAs being the most red-shifted of
these series. The graph shows a shift of absorbance maxima to
lower energy with both stronger electron withdrawing and
electron donating para-substituents on the phenyl ring, sig-
nifying a reduced energy gap between the S0 ground state and
S1 excited state in each case. This is the result of the fact that
electron donors raise the HOMO more than the LUMO while
electron acceptors lower the LUMO more than the HOMO
(vide infra). However, in each of the series, the effect of
acceptor substituents is greater than that of donor substitu-
ents, as can be seen from the gradients of each plot. The
difference in gradient between the acceptor series and the
donor series increases from BPEBs to BPETs to BPEAs,
demonstrating that the photophysical properties for these
related compounds, as well as being influenced by the para-
substituents, are also dependent on the nature of the central
core. Finally, we note that the optical data for the MeO
derivative, 1c, reported herein is, as expected, essentially
Crystal structures
The structures of 1a, 1b, 1h and 2 each contain one indepen-
dent molecule (Fig. 6) possessing neither crystallographic
symmetry nor local symmetry, except for 1b, which has an
approximate C2 axis. Selected geometric parameters are listed
in Table 2. The geometry of the planar thiophene ring is very
similar in all four structures, but differs significantly from that
of unsubstituted thiophene. The latter proved undeterminable
in the solid state due to rotational disorder, complicated
polymorphism and an incommensurate structure,44 but was
recently established (with much difficulty) in the gas phase by a
combination of electron diffraction, NMR, rotational and
microwave spectroscopy, and ab initio and DFT calcula-
tions,45 giving equilibrium (re) bond lengths C–S 1.704(2),
CQC 1.372(3) and C–C 1.421(4) A. Longer C–S bonds in 1
and 2 probably indicate increased conjugation with the CRC
bonds. Indeed, the intervening C(2)–C(6) and C(5)–C(8) bonds
are marginally shorter than the corresponding Csp–Csp2 bonds
in tolan46 (mean 1.433(3) A) or 1,4-bis(phenylethynyl)benze-
ne4b,12 (mean 1.430(3) A). Comparison of the CRC bond
lengths is inconclusive, as these distances are not very sensitive
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Table 2 Mean bond distances (A) and angles (1) between ring planesa
and/or vectors. Calculated distances (DFT, B3LYP, 6-31G*) are
italicised
1a
1b
1h
2
C–S
1.733(3) 1.727(2) 1.729(2) 1.723(2)
1.758 1.758 1.757 1.758
1.380(4) 1.378(2) 1.380(3) 1.374(3)
1.386 1.386 1.387 1.387
1.410(4) 1.405(2) 1.402(3) 1.401(3)
1.411 1.411 1.409 1.409
1.422(3) 1.422(2) 1.418(3) 1.418(3)
1.403 1.403 1.402 1.402
1.204(4) 1.183(2) 1.207(3) 1.192(3)
1.219 1.219 1.219 1.217
1.443(3) 1.444(2) 1.426(3) 1.425(3)
CQC
C(3)–C(4)
C(2,5)–CR
CRC
Fig. 5 Plots of absorption maxima vs. Hammett Constant (sp+/spÀ
)
RC–C(Ph)
for the BPETs, 1a–h, and the analogous 1,4-bis(para-R-phenylethy-
nyl)benzenes (BPEBs) and 9,10-bis(para-R-phenylethynyl)anthracenes
(BPEAs).
1.422
33.6
9.7
39.2
9.1
1.422
5.8
5.7
10.2
0.9
0.6
1.420
2.8
3.4
3.4
6.6
1.404
4.5
23.5
21.9
0.7
0.4
i/ii
i/iii
ii/iii
C(6)RC(7)/ii
C(8)RC(9)/iii
C(2)–C(6)/C(5)–C(8)
to changes in electronic structure, whilst being rather sensitive
to the experimental procedure used to determine them, such as
the availability of high-angle reflections.46a DFT calculations
(see Table 2) predict much stronger conjugation.
3.8
0.8
148.6(2) 147.9(2) 153.9(3) 151.6(3)
C(11)–C(14)/C(21)–C(24) 141.1(2) 147.0(2) 175.5(2) 158.7(2)
a
For the definition of planes i–iii, see Fig. 6.
Molecules 1b and 1h are nearly planar, as found for the
DFT optimised geometries, vide infra; the carboxylate groups
in 1h have a mutually anti-orientation and are nearly coplanar
with the adjacent benzene rings, ii and iii, the dihedral angles
being 3.3 and 5.71, respectively (Fig. 7).
The central thiophene ring inevitably makes the molecular
rod non-linear. Indeed, the exocyclic bonds C(2)–C(6) and
C(5)–C(8) are not continuations of the bisector lines of the
angles S(1)–C(2)–C(3) and S(1)–C(5)–C(4), but deviate from
the latter towards the sulfur atom by several degrees. Thus, the
angles between the C(2)–C(6) and C(5)–C(8) bonds (148–1541,
see Table 2) are considerably smaller than between the bisector
lines (ca. 1571) mentioned above. Note that this effect is also
observed in unsubstituted thiophene, where the S(1)–C(2)–H
and C(3)–C(2)–H angles are very different (119.9(3) and
128.5(3)1) and the C(2)–H and C(5)–H bonds form an angle
of 147.4(3)1.
In 1b, the acetylenic (–CRC–) moieties are practically
linear, whereas in 1h they are substantially bent, which results,
paradoxically, in the overall straightening of the molecule,
making the C(11)Á Á ÁC(14) and C(21)Á Á ÁC(24) vectors more
co-linear (Table 2). In 1a, the acetylenic groups are bent to a
similar extent but, in contrast with 1h, out of the thiophene
plane, so that no overall straightening occurs. In 1a and 2, the
phenyl rings adopt considerably unsymmetrical orientations.
The crystal packing motif of 1b is a longitudinally-slanted
infinite stack, in which the adjacent molecules are related by
the b translation, with a mean interplanar separation of d =
3.57 A. The so-called ‘aromatic slip angle’, y, between the
translation and the normal to the molecular plane, equals
45.51, so that the CRC bonds of one molecule overlap with
Fig. 6 Molecular structures of 1a, 1b, 1h and 2 (50% thermal
ellipsoids). The arrow indicates the approximate 2-fold axis (see
Fig. 7).
Fig. 7 Molecular structures of 1a, 1b, 1h and 2 viewed down the
approximately 2-fold axis of the thiophene ring (- in Fig. 6).
Hydrogen atoms are omitted for clarity.
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set, as defined in Gaussian03,47 for all atoms. Geometries were
optimised for all compounds, and in all cases, a coplanar
arrangement of the aryl and thiophene rings was found to be
the lowest in energy, as has been observed previously for
BPEBs.13,15b Details of the computed bond distances for
compounds 1a,b,h and 2 are given in Table 2, for comparison
with the experimental values determined from the X-ray
diffraction experiments. All of the computed distances are
within 1.5% of their experimental counterparts, and most
are within 1%. We also performed vibration frequency calcu-
lations on all of the optimised geometries, indicating that they
are indeed minima. The frequency calculations also allowed us
to assign the compounds’ absorptions in their IR and Raman
spectra, especially the symmetric stretching mode of the two
CRC units, which gives rise to a strong band in each of the
Raman spectra, and its asymmetric counterpart, which leads
to a strong band in the IR spectra. We note that the correction
factor for the calculated frequencies in both the IR and
Raman spectra is 0.96 in all cases, in excellent agreement with
that normally observed for DFT calculations with this combi-
nation of functional and basis set.48 Even though the com-
pounds all have C2v geometries, and thus lack inversion
centres, due to the nature of the angles that the CRC vectors
make with the thienyl rings, there is clearly little change in
dipole moment for the symmetric CRC stretching mode and
little change in polarisability for the asymmetric CRC
stretching mode, as these bands are predicted to be very weak
in the IR and Raman spectra, respectively, and are not
observed experimentally. Also of interest is the fact that the
two bands are calculated to be ca. 10 cmÀ1 apart, with the IR-
active asymmetric stretching mode always being predicted to
be at a slightly higher energy. However, in the experimental
spectra, these bands are almost coincident in energy, within
experimental error. Although we have not computed barriers
to rotation about the Caryl–Calkynyl bonds, these are expected
to be quite small in both solution and the gas phase, again, in
line with what has been found previously for BPEBs,13 and
consistent with the fact that various torsion angles (up to
33.61, see Table 2) are observed in the solid-state structures,
vide supra. The frequency calculations show very low energy
modes corresponding to this rotational motion, as well as to a
bending mode which changes the angle between the two CRC
vectors.
Fig. 8 Crystal packing of 1b (hydrogen atoms are omitted).
the phenyl and thiophene rings of the next. The stacks are
arranged into a layer parallel to the (1 0 À2) plane (Fig. 8). The
adjacent layers, related by the screw axis, are obviously
parallel, and the directions of the stacks in these layers,
[1 0 0], are also parallel. However, both the mean planes and
the long axes of the molecules belonging to adjacent layers are
practically perpendicular (angles 89 and 881, respectively). The
packing of 2 is similar, albeit that in this case, the stacked
molecules are related by the a translation, with d = 3.49 A and
y = 45.01. The planes of molecules belonging to adjacent
layers form an angle of 751, and their long axes are staggered
by 851. In 1a, the corresponding angles are 70 and 851, but the
intra-stack slip is larger (y = 54.91) and more laterally
directed. Thus, the overlap of molecules within a ‘stack’
(generated, as in 1b, by the b translation) is actually slight,
even though d is as short as 3.30 A. In contrast, the more linear
molecules of 1h pack in a slanted herringbone manner
into a layer, in which the planes of contacting molecules meet
at a 451 angle (Fig. 9). The long axes of all molecules within a
layer are parallel to one another and inclined by ca. 241 to the
crystallographic axis a, which is normal to the mean plane of
the layer.
Gas phase TD-DFT calculations permitted assignment of
the lowest energy allowed S1 ’ S0 electronic absorptions in
the UV-vis spectra to the HOMO–LUMO transition in all
cases, and these are predicted to have large oscillator strengths
(f = ca. 1.5–1.9), corresponding to the observed large extinc-
tion coefficients, which are in the range ca. 33 000–52 000
molÀ1 cmÀ1 dm3 (Table 1). The calculated S1 ’ S0 absorp-
tions are 41–45 nm red-shifted from the observed values of
DFT calculations on 1a–h and 2
We have performed DFT and TD-DFT calculations on all of
the BPETs using the B3LYP functional and the 6-31G* basis
lmax for 1a–e and 2, and 65, 55 and 56 nm red-shifted for 1f–h,
respectively, which is a result of the increased p-conjugation in
the planar structures found in the calculations. In solution,
nearly free rotation around the CRC bonds leads to
Frank–Condon absorption arising from a weighted average
of all rotamers.13,15b Thus, it is known13,15b that at ambient
temperature, the observed absorption profile is blue-shifted
due to the presence of rotational isomers, in which the S1 ’ S0
Fig. 9 Crystal packing of 1h, viewed down the long axis of the
molecule.
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transition energies increase with the torsion angle between
adjacent rings. Note that p-conjugation falls off with cos2y.
However, the positions of the 0,0 transitions can be estimated
from the absorption and emission spectra. Low temperature
spectra of 1a reveal the presence of an intense 0,0 transition of
the planar conformation, which is virtually coincident with the
emission maximum; in non-polar solvents, the 0,0 bands show
a negligible Stokes shift.39 Thus, in Table 1, we collect the
observed transition energies in solution (using the emission
maxima), along with those calculated from the TD-DFT
studies in the gas phase. The observed and calculated values
are generally in good agreement, with the observed values
typically being 600–900 cmÀ1 (ca. 3–4%) higher in energy than
the calculated ones. The notable exceptions are the NMe2
compound, 1e, for which the observed value is actually 300
cmÀ1 lower in energy than the calculated one, and the NO2
compound, 1f, for which the observed value is 1400 cmÀ1
higher in energy than the observed value. These larger dis-
crepancies could well be a result of solvation effects, which are
more important for the cases wherein there is a larger degree of
charge transfer, vide infra. We note that a TD-DFT calcula-
tion on 1f, in which both nitrophenyl groups have been rotated
by 451 (in the same direction), leads to a 10 nm blue shift in the
absorption energy, along with a reduction in the oscillator
strength. In contrast, rotating one of the nitrophenyl groups
by 901 leads to a 44 nm blue shift. Interestingly, when both
nitrophenyl groups are rotated by 901, effectively decoupling
their p-systems from the diethynylthiophene core, the
HOMO–LUMO transition has an oscillator strength of 0
and the lowest energy absorption is shifted by 133 nm. The
conjugation effect is smaller for the donor-substituted and
weakly acceptor-substituted systems than it is for the strong
acceptor compounds, increasing with acceptor strength. This
is also in keeping with the steeper slopes of the Hammett plots
for the acceptors compared to the donors shown in Fig. 6.
There is no significant charge transfer evident in the S1 ’ S0
transition of the parent compound 1a (see HOMO and
LUMO in Fig. 10). In contrast, some degree of charge transfer
is evident for the donor cases, wherein the HOMO has a
significant contribution from the para-substituent, whereas the
LUMO is more localised in the centre of the molecule (e.g.
NMe2 compound 1e in Fig. 10). Likewise, for the acceptors
(e.g. NO2 compound 1f in Fig. 10), we see the inverse charge
transfer, with the HOMO being more localised at the centre of
the molecule and the LUMO having significant NO2 charac-
ter. We confirmed the charge transfer by computing the
contributions of each moiety (thienylene, ethynyl, phenylene
and para-substituent) to the HOMO and LUMO. Consistent
with this is the small red shift (7 nm) in the absorption
maximum for 1f when the solvent is changed from toluene
to CH2Cl2, although the emission maximum red-shifts by
112 nm, giving rise to a large Stokes shift of 7100 cmÀ1 in
CH2Cl2, indicating a substantial electronic/structural reorga-
nisation in the singlet excited state prior to emission, especially
in more polar solvents, which are capable of better stabilising
the accompanying charge redistribution. Further details on the
structure of the excited state of 1a and its photophysics (in
both singlet and triplet states) can be found in a companion
paper.39
Fig. 10 Plots of the HOMO and LUMO for compounds 1a (BPET),
1e (NMe2) and 1f (NO2), and their respective energies.
Conclusions
We have shown that the Pd/Cu-catalysed Sonogashira cross-
coupling methodology provides a convenient route to a series
of para-substituted 2,5-bis(phenylethynyl)thiophenes. These
chromophores show interesting photophysical properties,
which have been studied in solution at room temperature.
The presence of strong donors or acceptors is found to red-
shift both the absorption and emission bands. The compounds
have the moderate quantum yields and short singlet lifetimes
characteristic of rapid intersystem crossing to the triplet
state.36
Experimental
General
All reactions were performed under dry nitrogen using stan-
dard Schlenk techniques. The amine solvents used in syntheses
were dried over CaH2 and distilled under dry nitrogen.
Reagents purchased from commercial suppliers were tested
for purity by GC-MS before use. Ethynylbenzenes were
synthesised by literature methods,49 except for phenylacety-
lene, which was purchased from Aldrich. Crude yields of ca.
80 Æ 15% were typical. As we required high purities for
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spectroscopic measurements, the yields reported here are of
analytically pure material, obtained after one or more recrys-
tallisations.
sured using a dilute LUDOXs suspension as the scattering
sample, setting the monochromator at the emission wave-
length of the laser, giving an instrument response function
(IRF) of 200 or 100 ps at 396 or 300 nm, respectively. The
resulting intensity decay was a convolution of the fluorescence
decay with the IRF, and iterative reconvolution of the IRF
with a decay function and non-linear least-squares analysis
were used to analyse the convoluted data.50,52,53
Reactions were monitored in situ by GC-MS or by TLC to
assure the complete disappearance of 2,5-diiodothiophene
prior to workup. GC-MS analyses were performed on an
Agilent Technologies 6890 N gas chromatograph equipped
with a 5973 inert mass selective detector and a 10 m fused silica
capillary column (5% cross-linked phenylmethylsilicone) un-
der the following operating conditions: injector temperature
250 1C, detector temperature 300 1C, the oven temperature
was ramped from 70 to 280 1C at 20 1C minÀ1. UHP helium
was used as the carrier gas. NMR spectra were recorded in
CDCl3 on Varian Mercury-200, Unity-300 and Inova-500, and
Bruker Avance-400 spectrometers at the following frequencies:
1H: 200, 300 and 400 MHz, 13C{1H}: 100 and 126 MHz,
19F{1H}: 188 MHz. 13C NMR assignments were made with the
assistance of HSQC and HMBC experiments. Mass spectra for
compounds 1a–d, 1g and 2 were obtained on a Thermo
Finnigan Trace MS spectrometer operating in EI mode.
Spectra for 1e, 1f and 1i were obtained on a Thermo Finnigan
DSQ spectrometer operating in EI mode. Elemental analyses
were performed using an Exeter Analytical E440 machine by
departmental services at Durham University. Melting point
values were measured on a Stuart Scientific SM3 apparatus.
IR spectra were recorded as KBr disks using a Perkin-Elmer
Spectrum 100 series FT-IR spectrometer. Raman spectra were
recorded on solid samples using a Horiba Jobin Yvon Lab-
RamHR Raman microscope with the laser set at 785 nm.
UV-vis, fluorescence spectra, lifetime and quantum yield
measurements were recorded in toluene if not indicated other-
wise. UV-vis absorption spectra and extinction coefficients
were obtained on a Hewlett-Packard 8453 diode array spectro-
photometer using standard 1 cm width quartz cells. Fluores-
cence spectra and quantum yield measurements were recorded
on a Horiba Jobin Yvon Fluorolog FL 3-22 Tau spectro-
photometer. The spectra of dilute solutions, with absorbance
maxima of less than 0.1, were recorded using a conventional
901 geometry. The emission spectra were fully corrected using
the manufacturer’s correction curves for the spectral response
of the emission optical components. The quantum yield of
each compound was estimated by comparing it with standards
of known quantum yield. The absorbance of the sample was
kept below 0.12 to avoid inner filter effects, and all measure-
ments were carried out at room temperature. The fluorescence
quantum yields of compounds 1a–d, 1g, 1i and 2 were mea-
sured against 1,4-di-(5-phenyl-1,3-oxazole-2-yl)-benzene (PO-
POP) in cyclohexane (F = 0.97)50 and quinine sulfate in 0.1 M
H2SO4 (F = 0.54),50 and that of 1e and 1f were measured
against norharmane in 0.1 M H2SO4 (F = 0.58)51 and 9,10-
diphenylanthracene in C6H12 (F = 0.9).50 The fluorescence
lifetimes of 1a–e, 1g, 1i and 2 were measured by time-corre-
lated single photon counting (TCSPC) using either a 396 nm
pulsed laser diode or the 3rd harmonic of a cavity-dumped,
mode-locked Ti-sapphire laser (Coherent MIRA, 300 nm).
The fluorescence emission was collected at right angles to the
excitation source, with the emission wavelength selected using
a monochromator and detected by a single photon avalanche
diode (SPAD). The instrument response function was mea-
Syntheses
2,5-Bis(phenylethynyl)thiophene (1a). The compounds 2,5-
diiodothiophene (0.67 g, 2.00 mmol), Pd(PPh3)2Cl2 (0.03 g,
0.04 mmol) and CuI (0.008 g, 0.04 mmol) were added to a
Schlenk flask that had been evacuated and refilled with nitro-
gen three times. Dry, de-gassed NEt3 (50 mL) was added via a
cannula. Phenylacetylene (0.43 g, 4.20 mmol) was then added
under a positive pressure of nitrogen and the reaction mixture
was stirred at room temperature for 72 h. The NEt3 solvent
was then removed in vacuo. The residual solid was applied to
the top of a 5 cm silica gel column eluted with hot toluene. The
toluene was removed in vacuo and the product was recrystal-
lised from a toluene/hexane solution. Yield of analytically
pure material 0.15 g (26%), m.p. 83.9–84.5 1C. IR (/cmÀ1
)
1595 (arene ring), 2199 (CRC, asym). Raman (/cmÀ1) 1445,
1595 (arene ring), 2198 (CRC, sym). 1H NMR (400 MHz): d
7.54 (m, 4 H, CHarom), 7.36 (m, 6 H, CHarom), 7.17 (s, 2 H,
CHthiophene). 13C{1H} NMR (100 MHz): d 131.82 (Cthio),
131.50 (Carom), 128.67 (Carom), 128.42 (Carom), 124.68 (Cthio),
122.63 (Carom), 94.07 (CRC), 82.28 (CRC). MS (EI): m/z
284. Anal. calc. for C20H12S: C, 84.47; H, 4.25. Found: C,
84.09; H, 4.14%.
2,5-Bis(para-methylphenylethynyl)thiophene (1b). The com-
pounds 2,5-diiodothiophene (0.67 g, 2.00 mmol),
Pd(PPh3)2Cl2 (0.03 g, 0.04 mmol) and CuI (0.008 g, 0.04
mmol) were added under nitrogen to a Schlenk flask. Dry,
de-gassed NEt3 (50 mL) was added via a cannula. 4-Methyl-
phenylacetylene (0.49 g, 4.20 mmol) was then added under a
positive pressure of nitrogen. The reaction mixture was stirred
at room temperature for 24 h and the NEt3 was then removed
in vacuo. The residual solid was applied to the top of a 5 cm
silica gel column eluted with 1 : 1 CH2Cl2/hexane, and the
solvent was removed in vacuo. Analytically pure product was
obtained as a red crystalline solid after recrystallisation from a
hot hexane/toluene solution. Yield 0.17 g (27%), m.p.
167.9–168.2 1C. IR (/cmÀ1) 1530 (arene ring), 2192 (CRC,
asym). Raman (/cmÀ1) 1438, 1604, (arene ring), 2195 (CRC,
sym). 1H NMR (200 MHz): d 7.43 (d, J = 8 Hz, 4 H, CHarom),
7.17 (d, J = 8 Hz, 4 H, CHarom), 7.14 (s, 2 H, CHthiophene), 2.38
(s, 6 H, CH3). 13C{1H} NMR (100 MHz): d 138.89 (Carom),
131.56 (Cthio), 131.39 (Carom), 129.18 (Carom), 124.69 (Cthio),
119.57 (Carom), 94.19 (CRC), 81.71 (CRC), 21.54 (CH3).
MS (EI): m/z 312. Anal. calc. for C22H16S: C, 84.57; H, 5.16.
Found: C, 84.68; H, 5.16%.
2,5-Bis(para-methoxyphenylethynyl)thiophene (1c). The
compounds 2,5-diiodothiophene (0.67 g, 2.00 mmol),
Pd(PPh3)2Cl2 (0.03 g, 0.04 mmol) and CuI (0.008 g, 0.04
mmol) were added under nitrogen to a Schlenk flask. Dry,
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de-gassed NEt3 (50 mL) was added via a cannula and 4-
methoxyphenylacetylene (0.55 g, 4.20 mmol) was then added
under a positive pressure of nitrogen. The reaction mixture
was stirred at room temperature for 72 h and the NEt3 was
then removed in vacuo. The residual solid was added to the top
of a 5 cm silica gel column eluted with 1 : 1 CH2Cl2/hexane.
The solvents were removed in vacuo. Recrystallisation of the
crude product from hot toluene/CH2Cl2 gave a bright yellow,
analytically pure solid. Yield: 0.23 g, (34%), m.p. 119.6–122.4 1C.
IR (/cmÀ1) 1602 (arene ring), 2198 (CRC, asym). Raman (/
1452, 1609 (arene ring), 2192 (CRC, sym). 1H NMR (300
MHz): d 7.39 (d, J = 9 Hz, 4 H, CHarom), 7.05 (s, 2 H,
CHthiophene), 6.65 (d, J = 9 Hz, 4 H, CHarom), 3.00 (s, 12 H,
N(CH3)2). 13C{1H} NMR (100 MHz): d 150.46 (Carom), 132.89
(Carom), 130.98 (Cthio), 124.90 (Cthio), 111.98 (Carom), 109.52
(Carom), 95.29 (CRC), 80.78 (CRC), 40.41 (N(CH3)2). MS
(EI): m/z 370. Anal. calc. for C24H22N2S: C, 77.80; H, 5.98; N,
7.56. Found: C, 77.29; H, 5.91; N, 7.57%.
2,5-Bis(para-nitrophenylethynyl)thiophene (1f). The com-
pounds 2,5-diiodothiophene (1.34 g, 4.00 mmol),
Pd(PPh3)2Cl2 (0.056 g, 0.08 mmol) and CuI (0.015 g, 0.08
mmol) were added under nitrogen to a Schlenk flask. Dry, de-
gassed NEt3 (50 mL) was added via a cannula and 4-nitro-
phenylacetylene (1.24 g, 8.40 mmol) was then added under a
positive pressure of nitrogen. The reaction mixture was stirred
at room temperature for 72 h and the NEt3 was then removed
in vacuo. The residual solid was added to the top of a 5 cm
silica gel column eluted with 1 : 1 CH2Cl2/hexane and the
solvents were removed in vacuo. Analytically pure product was
obtained as a gold coloured solid after two recrystallisations
from hot toluene. Yield: 0.84 g (56%), m.p. 193.9–195.6 1C. IR
(/cmÀ1) 1333 (NO2, asym), 1511, 1591 (arene ring), 2200
(CRC, asym). Raman (/cmÀ1) 1329 (NO2, sym), 1589 (arene
ring), 2195 (CRC, sym). 1H NMR (200 MHz): d 8.25 (d, J =
9 Hz, 4 H, CHarom), 7.68 (d, J = 9 Hz, 4 H, CHarom), 7.28 (s, 2
H, CHthiophene). 13C{1H} NMR (100 MHz): d 147.30 (Carom),
133.23 (Cthio), 132.14 (Carom), 129.25 (Carom), 124.72 (Cthio),
123.75 (Carom), 92.83 (CRC), 87.10 (CRC). MS (EI): m/z
374. Anal. calc. for C20H10N2O4S: C, 64.16; H, 2.69; N, 7.48.
Found: C, 64.19; H, 2.63; N, 7.35%.
1
cmÀ1) 1441, 1607 (arene ring), 2197 (CRC, sym). H NMR
(200 MHz): d 7.46 (d, J = 9 Hz, 4 H, CHarom), 7.11 (s, 2 H,
CHthiophene), 6.88 (d, J = 9 Hz, 4 H, CHarom), 3.84 (s, 6 H,
OCH3). 13C{1H} NMR (100 MHz): d 159.95 (Carom), 133.03
(Carom), 131.33 (Cthio), 124.64 (Cthio), 114.75 (Carom), 114.10
(Carom), 93.95 (CRC), 81.11 (CRC), 55.33 (OCH3). MS
(EI): m/z 344. Anal. calc. for C22H16O2S: C, 76.72; H, 4.68.
Found: C, 76.52; H, 4.65%.
2,5-Bis(para-trifluoromethylphenylethynyl)thiophene
(1d).
The compounds 2,5-diiodothiophene (0.67 g, 2.00 mmol),
Pd(PPh3)2Cl2 (0.03 g, 0.04 mmol) and CuI (0.008 g, 0.04
mmol) were added under nitrogen to a Schlenk flask. Dry,
de-gassed NEt3 (50 mL) was added via a cannula and 4-
trifluoromethylphenylacetylene (0.72 g, 4.20 mmol) was then
added under a positive pressure of nitrogen. The reaction
mixture was stirred at room temperature for 94 h and the
NEt3 was then removed in vacuo. The residual solid was added
to the top of a 5 cm silica gel column eluted with hexane and
the solvent was removed in vacuo. The solid was recrystallised
from a small amount of hot hexane to give a yellow, analyti-
cally pure, crystalline solid. Yield: 0.50 g (60%), m.p.
107.8–112.2 1C. IR (/cmÀ1) 1609 (arene ring), 2197 (CRC,
asym). Raman (/cmÀ1) 1442, 1617 (arene ring), 2202 (CRC,
sym). 1H NMR (200 MHz): d 7.62 (s, 8 H, CHarom), 7.23 (s, 2
H, CHthiophene). 19F{1H} NMR (188 Hz): d À63.28 (s, 6 F,
CF3). 13C{1H} NMR (126 MHz): d 132.88 (Cthio), 131.91
(Carom), 130.57 (q, JCF = 33 Hz, Carom), 126.53 (Carom),
125.62 (q, JCF = 3 Hz, Carom), 124.83 (Cthio), 124.10 (q, JCF
= 272 Hz, CF3), 92.95 (CRC), 84.41 (CRC). MS (EI): m/z
420. Anal. calc. for C22H10F6S: C, 62.86; H, 2.40. Found: C,
62.65; H, 2.39%.
2,5-Bis(para-cyanophenylethynyl)thiophene (1g). The com-
pounds 2,5-diiodothiophene (0.67 g, 2.00 mmol),
Pd(PPh3)2Cl2 (0.03 g, 0.04 mmol) and CuI (0.008 g, 0.04
mmol) were added under nitrogen to a Schlenk flask. Dry,
de-gassed NHEt2 (50 mL) was added via a cannula and 4-
cyanophenylacetylene (0.53 g, 4.20 mmol) was then added
under a positive pressure of nitrogen. The reaction mixture
was stirred at room temperature for 63 h and the NHEt2 was
removed in vacuo. The residual solid was added to the top of a
5 cm silica gel column eluted with 1 : 1 CH2Cl2/hexane and the
solvents were removed in vacuo. Analytically pure product was
obtained as a tan solid after two recrystallisations from hot
toluene. Yield: 0.23 g (34%), m.p. 250.6–252.6 1C. IR (/cmÀ1
)
2,5-Bis(para-N,N-dimethylaminophenylethynyl)thiophene (1e).
The compounds 2,5-diiodothiophene (0.58 g, 1.72 mmol),
Pd(PPh3)2Cl2 (0.024 g, 0.034 mmol) and CuI (0.006 g, 0.034
mmol) were added under nitrogen to a Schlenk flask. Dry, de-
gassed NEt3 (50 mL) was added via a cannula and 4-N,N-
dimethylaminophenylacetylene (0.53 g, 3.62 mmol) was then
added under a positive pressure of nitrogen. The reaction
mixture was stirred at room temperature for 96 h and the
NEt3 was then removed in vacuo. The residual solid was added
to the top of a 5 cm alumina pad, which was first eluted with
hexane (150 ml) to remove any unreacted starting materials.
The column was eluted with 1 : 4 CH2Cl2/hexane and the
solvents were then removed in vacuo. The product was recrys-
tallised from CH2Cl2/hexanes to yield a bright yellow solid.
Yield 0.25 g (39%), m.p. 194.4–198.0 1C. IR (/cmÀ1) 1531,
1605 (arene ring), 2194 (CRC, asym). Raman (/cmÀ1) 1257,
1597 (arene ring), 2202 (CRC, sym), 2226 (CRN, sym).
Raman (/cmÀ1) 1436, 1602 (arene ring), 2197 (CRC, asym).
1H NMR (200 MHz): d 7.63 (m, 8 H, CHarom), 7.24 (s, 2 H,
CHthiophene). 13C{1H} NMR (100 MHz): d 132.99 (Cthio),
132.13 (Carom), 131.90 (Carom), 127.31 (Carom), 124.63 (Cthio),
118.33 (CRN), 112.03 (Carom), 92.89 (CRC), 86.24 (CRC).
MS (EI): m/z 334. Anal. calc. for C22H10N2S: C, 79.02; H,
3.01; N, 8.38. Found: C, 78.77; H, 2.98; N, 8.16%.
2,5-Bis(para-carbomethoxyphenylethynyl)thiophene
(1h).
The compounds 2,5-diiodothiophene (0.67 g, 2.00 mmol),
Pd(PPh3)2Cl2 (0.03 g, 0.04 mmol) and CuI (0.008 g, 0.04
mmol) were added under nitrogen to a Schlenk flask. Dry,
de-gassed NEt3 (50 mL) was added via a cannula and 4-
carbomethoxyphenylacetylene (0.67 g, 4.20 mmol) was then
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Table 3 Crystal dataa
Compound
1a
1b
1h
2
CCDC no.
Formula
Formula weight
T/K
641166
C20H12
284.38
120
641167
C22H16
312.41
120
641168
C24H16O4S
400.43
641169
S
S
C20H2F10
464.28
120
S
120
Symmetry
Space group (no.)
a/A
b/A
Monoclinic
P21/n (#14)
14.752(1)
5.7461(2)
17.339(1)
90.87(1)
1469.6(1)
4
Monoclinic
P21/n (#14)
16.072(2)
5.0902(5)
21.234(2)
112.22(1)
1608.1(3)
4
Orthorhombic
Pca21 (#29)
7.1798(8)
45.453(4)
5.8879(4)
90
1921.5(3)
4
0.20
Monoclinic
P21/n (#14)
4.9212(7)
31.274(4)
11.3413(15)
101.83(1)
1708.4(4)
4
c/A
b/1
V/A3
Z
m/mmÀ1
Reflections collected
Unique reflections
0.21
9135
0.20
17 948
0.30
16 756
19 478
3365, 2098b
0.067
0.050
4255, 3243b
0.048
0.046
4881, 4505b
0.033
0.046
3921, 3129b
0.039
0.047
Rint
R(F)b
wR(F2)
a
0.127
0.109
0.116
0.110
For crystallographic data in CIF or other electronic format see DOI: 10.1039/b701172e Reflections with F2 4 2s(F2).
b
added under a positive pressure of nitrogen. The reaction
mixture was stirred at room temperature for 66 h and then
heated at 80 1C for 2 h. The NEt3 was then removed in vacuo.
The residual solid was added to the top of a 5 cm silica gel
column eluted with 1 : 1 CH2Cl2/hexane and the solvents were
removed in vacuo. Analytically pure product was isolated as a
pale yellow solid after recrystallisation from hot toluene/
hexane solution. Yield: 0.40 g (50%), m.p. 201.5–202.5 1C.
IR (/cmÀ1) 1602 (arene ring), 1717 (CQO, asym), 2191
(CRC, asym). Raman (/cmÀ1) 1448, 1607 (arene ring), 1717
(C=O, sym), 2197 (CRC, sym). 1H NMR (200 MHz): d 8.03
(d, J = 8 Hz, 4 H, CHarom), 7.58 (d, J = 8 Hz, 4 H, CHarom),
7.21 (s, 2 H, CHthiophene), 3.94 (s, 6 H, CO2Me). 13C{1H}
NMR (100 MHz): d 166.42 (CO2), 132.54 (Cthio), 131.35
(Carom), 129.91 (Carom), 129.59 (Carom), 127.16 (Carom),
124.73 (Cthio), 93.66 (CRC), 85.02 (CRC), 52.26 (CH3).
MS (EI): m/z 401. Anal. calc. for C24H16O4S: C, 71.98; H,
4.03. Found: C, 71.77; H, 3.96%.
(d, JCF = 242 Hz, Carom), 142.08 (d, JCF = 258 Hz, Carom),
140.90 (d, JCF = 252 Hz, Carom), 133.85 (Cthio), 124.51 (Cthio),
99.87 (t, J = 13 Hz, Carom), 93.67 (CRC), 78.70 (CRC). MS
(EI): m/z 464. Anal. calc. for C20H2F10S: C, 51.74; H, 0.43.
Found: C, 51.55; H, 0.48%.
X-Ray crystallography
Single-crystal diffraction experiments (Table 3) were carried
out on Bruker three-circle diffractometers with CCD area
detectors SMART 1 K or, for 1h, SMART 6 K, using
graphite-monochromated Mo-Ka radiation (l = 0.71073 A)
and Cryostream (Oxford Cryosystems) open-flow N2 cryo-
stats. The structures were solved by direct methods and refined
by full-matrix least-squares against F2 on all data using
SHELXTL software.54 Non-hydrogen atoms were refined in
anisotropic and hydrogen atoms in isotropic approximations.
Acknowledgements
T. B. M., A. B. and J. A. K. H. thank One NorthEast for
support through the Nanotechnology UIC programme. T. B.
M. and J.-F. H. thank the Royal Society and the CNRS for
their support of this collaboration via an International Joint
Project—Europe Grant. R. M. W., A. L. T., S. R. R. and L. A.
thank the EPSRC for postgraduate studentships. We thank
Ms K. Wongkhan for a preliminary optical polarised light
microscopy examination of compound 1g. We are grateful to
Durham University for the provision of high-performance
computer facilities.
2,5-Bis(pentafluorophenylethynyl)thiophene (2). The com-
pounds 2,5-diiodothiophene (0.67 g, 2.00 mmol),
Pd(PPh3)2Cl2 (0.03 g, 0.04 mmol) and CuI (0.008 g, 0.04
mmol) were added under nitrogen to a Schlenk flask. Dry,
de-gassed NEt3 (50 mL) was added via a cannula and penta-
fluorophenylacetylene (0.81 g, 4.20 mmol) was then added
under a positive pressure of nitrogen. The reaction mixture
was stirred at room temperature for 24 h and the NEt3 was
then removed in vacuo. The residual solid was added to the top
of a 5 cm silica gel column eluted with 1 : 1 CH2Cl2/hexane
and the solvents were removed in vacuo. The product was
recrystallised from a hot hexane/toluene solution to give the
analytically pure material as large yellow crystals. Yield 0.39 g,
(42%), m.p. 153.7–156.1 1C. IR (/cmÀ1) 1532 (arene ring),
2218 (CRC, asym). Raman (/cmÀ1) 1424 (arene ring),
2217 (CRC, sym). 1H NMR (200 MHz): d 7.32 (s, 2 H,
CHthiophene). 19F{1H} NMR (188 MHz): d À135.70 (m, 4 F,
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New J. Chem., 2007, 31, 841–851 | 849

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