Bridged tolanes: A twisted tale

Article abstract of DOI:10.1021/jo5010235

The rotational motion of tolanes along their acetylene axis is not fully understood. What happens to the optical and electronic properties if the tolane backbone is forced into a twisted conformation? Several tethers were investigated to obtain tolanophan

Full text of DOI:10.1021/jo5010235

Article
pubs.acs.org/joc
Bridged Tolanes: A Twisted Tale
Sebastian Menning,† Maximilian Kramer,‡ Andrew Duckworth,§ Frank Rominger,† Andrew Beeby,§,*
̈
Andreas Dreuw,‡,∥,* and Uwe H. F. Bunz†,∥,*
†Organisch-Chemisches Institut, Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
̈
‡Interdisziplinares Zentrum fur Wissenschaftliches Rechnen (IWR), Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld
̈
̈
̈
368, 69120 Heidelberg, Germany
§Department of Chemistry, Durham University, South Road, Durham DH 3LE, U.K.
∥Centre for Advanced Materials (CAM), Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
S
* Supporting Information
ABSTRACT: The rotational motion of tolanes along their
acetylene axis is not fully understood. What happens to the
optical and electronic properties if the tolane backbone is
forced into a twisted conformation? Several tethers were
investigated to obtain tolanophanes, fixing the torsion angle of
the two phenyl rings. X-ray crystal structures revealed tether-
specific torsion angles in the solid state. The absorption,
emission, and excitation spectra were recorded. Twisted tethered tolane conformers showed blue-shifted absorption; emission
spectra were all torsionally independent and identical. The tethered tolanes were embedded in a rigid matrix by freezing to 77 K;
well-resolved emission spectra were recorded for planar tolanes, but for twisted systems unexpectedly long-lived
phosphorescence was observed. How is this triplet emission explained? Quantum chemical calculations (TDDFT/cam-
B3LYP/6-31G*) of the unsubstituted tolane showed that intersystem crossing (ISC) is favored with large spin−orbit coupling,
which occurs when the molecular orbitals are orthogonal to each other; this is the case at the crossing of S1/T7. Also, a small
energy difference between singlet and triplet states is required; we found that ISC can favorably take place at four crossings: S1/
T6, S1/T7, S1/T8,9, S1/T10.
INTRODUCTION
■
Tolane (diphenylacetylene) is a fundamental building block for
conjugated organic oligomers, polymers and dendrimers.1−4
The tolane framework is conformationally rigid, except for its
single degree of freedom, the rotation around the acetylene axis,
featuring a barrier of 2.4 kJ/mol.5 Influencing the rotational
motion of these linear chromophores is a significant tool to
influence their photophysical properties.6−13
If the torsion angle α between the two aromatic rings
switches from a planar to an orthogonal geometry, conjugation
of the system will reduce. This decrease of electronic
communication is illustrated by the position of the HOMO
(highest occupied molecular orbital) and LUMO (lowest
unoccupied molecular orbital) energies. The higher HOMO−
LUMO gap in a fully twisted tolane leads to the observed
hypsochromic shift in absorption.9,14,15 Standard quantum
chemical calculations paint this simplified picture (see
Figure 1. Simplified illustration of low-lying excited states of tolane.
Supporting Information).
The photophysical behavior of tolane is peculiar. In its
ground state, tolane has D2h symmetry. Excitation to the linear
11B1u (ππ*) state (lifetime τ = 8 ps) is followed by internal
conversion (IC) to the trans-bent 11Au (πσ*) “dark” state (τ =
200 ps) with C2h symmetry (Figure 1).16−18 The two π-orbitals
of the C−C triple bond enhance the spin−orbit coupling,
which is responsible for an intersystem crossing (ISC),
resulting in the triplet state T1.19 The main fluorescence arises
from 11B1u → 11Ag. Emission from the “dark” state (11Au →
11Ag) is weak due to its bent, stilbene-like structure.20 The
triplet state (13B1u) is linear with a lifetime of τ = 1 μs.21−23
Received: May 11, 2014
© XXXX American Chemical Society
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Tolanes twist either through sterically demanding
groups15,24−32 or by choosing a suitable tether, connecting
the two phenyl rings. Yang et al. used pentiptycene groups to
impart steric bulk into the tolane system and prepared a series
of tolane oligomers. Because of their twisted conformation both
a blue-shift in absorption and emission was observed at 80 K.
For tolanophanes, different tethers were investigated, but up
to now the highest torsion angle in the solid state was
34°.14,33,34 In 2013 we reported a vertically twisted tolane,
phosphorescing brightly at 77 K. In this contribution we expand
synthetic access to further tethered tolanes and explore their
photophysics. All of the twisted systems show a fascinating
long-lived triplet emission while planar tolanes fluoresce.35
Table 1. Reaction Conditions for Syntheses of Bridged
Tolanes
RESULTS AND DISCUSSION
■
Syntheses. The tolanophanes 2, 4−10 were prepared in a
two-step reaction. Diol 1,36,37 made by Sonogashira reaction of
2-iodobenzyl alcohol with acetylene gas, was transformed into
the tolanophanes 2 and 4−10 by reaction with diacid chlorides
in the presence of an auxiliary base under high dilution
conditions (Scheme 1). To influence the torsion angle of 2, the
Scheme 1. Syntheses of Bridged Tolanes (X = Tether)
malonyl-tether in 2 was dimethylated in the α-position
furnishing 3 in 56% yield. As a structurally flexible tether,
succinyl was inserted, resulting in the flexible ring system 4.
Stiffening the tether of this system was achieved by the
syntheses of the tolanophanes 5−10 (Table 1). The tethered
tolanes were obtained after aqueous workup followed by
chromatographic purification. Yields vary from 6 to 35% in this
ring closure reaction.
Ground State Conformations. We obtained single
crystalline specimen from compounds 1−9 using different
solvent mixtures (see Supporting Information),38 and colorless
needles of 2−9 were analyzed by X-ray crystallography. For
tolane 10 repeated attempts of crystallization failed due to its
low solubility. Because of ring strain, tolane axes have little
deviation from linearity (<9°). The strapped systems show
different torsion angles depending on the tether we used
(Figure 2).39
For tolanes 2, 4, 6 and 9 torsions angles around 80° resulted.
2 was methylated into 3 to see if the torsion angle would
change. We found two molecules in the unit cell of 3 with α1 =
69° and α2 = 80°. Surprisingly, compound 4 with the softer
succinyl bridge shows maximum twist, at least in the solid state.
Tolanophanes 6 and 9, where the tether is stiffer, twist by 75
and 72°. An almost planar conformation is observed for 5, 7
and 8.
Figure 2. Crystal structures and torsion angles of 2−9. *Two
independent molecules were found in the unit cell. Both had planar
conformation with torsion angles of α1 = 4°; α2 = 6°. Here we show
the conformer with the torsion angle α2 due to similarity.
B3LYP/cc-pVTZ (Table 2).40−42 Most tolanes have several
ground state conformers, which span a range of torsion angles.
For example tolanes 2 and 3 possess a twisted global minimum
with a twist angle of 72° that is about 10 kJ/mol lower in
energy than two almost planar isomers. However, dependent
on the tether of the different tolanes, the relative energies of
planar and twisted isomers change and, as a consequence, their
Boltzmann distribution at thermal equilibrium. While the
tolanes 2, 3, 6, 9 and 10 prefer twisted conformations with
Boltzmann populations of the corresponding minima with
We computed all possible ground state conformations of all
investigated tolanes 2−10 at the theoretical level of DFT-D3/
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n-hexane at room temperature. Figure 3a shows the absorption
spectra of the planar systems (5, 7 and 8) with their main
maxima at 310 nm. The twisted derivatives 2−4, 6, 9 (and 10)
experience a hypsochromic shift in their UV−vis spectra
(Figure 3b); decreased conjugation results in a higher
HOMO−LUMO gap. As an exception, tolanes 4 and 10
should be focused on. The shape of the absorbance shows a
similarity to the planar derivatives, despite the almost
perpendicular geometry of 4 in the crystal structure. This
could be explained by the flexible (4) and long (10) tethers
allowing facile planarization in solution. The excitation spectra
show the relative emission intensity at 360 nm and reflect the
absorption spectra (see Supporting Information). While the
UV−vis spectra represent the torsion angle quite adequately,
the shape of the emission spectra is identical, regardless of
twisting (Figure 3c). In solution planarization of the excited
state is apparently fast, so that emission does not occur from
the twisted species.43
Neither extinction coefficients nor fluorescence quantum
yields are dependent on the twist angle (Table 3). The “open”
tolane 1 has an absorption coefficient of 2 × 104 L mol−1 cm−1,
while planar 8, with the stiffest tether of all the planar
derivatives, shows an absorptivity of 3 × 104 L mol−1 cm−1. For
twisted tolanes 2−4, 6, 9 (and 10) the coefficients range from
1−1.5 × 104 L mol−1 cm−1. The quantum yields were measured
in two solvents, n-hexane and dichloromethane, and are lower
in the more polar solvent, as expected. Torsion angle and
quantum yield are apparently independent, and for all tolanes
the fluorescence lifetimes were short (∼400 ps). From the
steady state measurements we can glean that some tethered
tolanes show a twisted ground state conformation in solution as
suggested by X-ray crystallography and quantum chemical
calculations. Considering the excited states, however, they all
behave identically and emit from a planar conformation.
Low Temperature Spectroscopy. At room temperature
all bridged tolanes show the same fluorescence spectra; they are
all in a planar excited state conformation (Figure 3c).43 To trap
the ground state conformations and preserve them in the
excited state, we dissolved the tolanophanes in EPA (mixture of
diethyl ether/isopentane/ethanol 5:5:2), a transparent organic
glass at 77 K.
Table 2. Overview of the Tolanes 2−10 with the According
Torsion Angle and Their Boltzmann Population at the
Theoretical Level of DFT-D3/B3LYP/cc-pVTZ
Boltzman
ensemble
[%]
relative
energy
[kJ/mol]
torsion angle α,
crystal [deg]
torsion angle α,
calculated [deg]
tolane
2
79
72
18
86
72
20
85
79
6
94.4
4.1
0
7.8
10.2
0.0
9.5
11.9
0.0
1.9
2.6
5.3
5.4
0.0
3.7
11.0
15.6
18.3
0.0
13.7
18.6
0.0
2.8
3.0
7.6
0.0
7.5
0.0
0.0
3.2
8.3
1.5
3
4
69, 80
82
97.1
2.1
0.8
48.5
22.9
17.2
5.8
77
73
29
6
5.6
5
11
80.7
18.0
1.0
88
4
77
83
80
83
66
11
80
87
12
9
0.2
0.1
6
7
75
6
99.5
0.4
0.1
60.1
19.3
17.8
2.8
8
4, 6
95.4
4.6
80
67
50
56
60
9
72
99.9
76.6
20.8
2.7
10
−
more than 80%, the tolanes 5, 7 and 8 exhibit preferentially
planar ground state structures. The only exception is tolane 4,
which, because of its flexible tether, can adopt planar as well as
twisted conformations at room temperature.
Considering all the tethered systems, a simple prediction of
the twist angle depending on the molecular structure is not
possible. Synthesis followed by X-ray analysis or a priori
quantum chemical calculations are necessary. But how does the
variation of the twist angle α influences their photophysical
behavior, or does it not at all?
Freezing the compounds to 77 K, we “lock in” the ground
state conformation and assume that the structure in the glass is
similar to that observed in the crystalline state. Figure 4 shows
the spectra at room temperature (top, red) and at 77 K
(bottom, blue) of the two twisted systems 3, 4 and the planar
derivative 5. Spectra of other tethered tolanes are not shown
but similar (see Supporting Information). Both absorption and
Excited State Conformations. For all the targets
absorption, excitation and emission spectra were recorded in
Figure 3. Absorption spectra of (a) planar and (b) twisted conformers. (c) Emission spectra of compounds 1−10. All spectra in n-hexane at room
temperature.
C
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Table 3. Photophysical Properties of Tolanes 1−10
b
c
b
b
c
b
b
d
tolane torsion angle α [deg] λmax [nm] absorption ε [104 L mol−1 cm−1
]
λmax [nm] emission
Φ
[%] Φ [%] τ1
[ps] τ2
[ps]
τ
[s]
298K
298K
77K
1
2
3
4
5
6
7
8
9
10
71
285
265
265
286
306
285
308
307
269
286
2.0
1.3
1.3
1.3
1.9
0.6
1.3
3.0
1.6
1.5
310
311
311
309
309
311
311
310
311
310
5.1
1.0
7.1
420
424
430
437
460
115
108
175
54
−
1.4
79
22.9
15.5
21.9
27.8
4.1
69, 80
82
4.7
1.4
1.1
−
10.8
33.9
0.2
11
3
a
e
e
75
−
446
−
0.8
6
2.2
0.2
31
−
−
1.6
4, 6
72
25.5
0.4
4.8
424
78
e
e
0.2
−
−
−
4.5
e
0.1
438
67
1.1
a
b
c
d
Shoulder. In n-hexane. In dichloromethane. In diethyl ether/isopentane/ethanol 5:5:2 (EPA). Lack of appreciable fluorescence to detect signal.
Figure 4. Absorption, excitation and emission spectra in EPA of the twisted conformers 3 (a) and 4 (b) and the planar species 5 (c). Black:
Absorption at rt (top) and 77 K (bottom). Red: Excitation (λem = 360 nm) and emission at rt. Blue: Excitation (λem = 360 nm) and emission at 77 K.
Blue dotted: Excitation (λem = 460 nm) at 77 K. Torsion angles from the X-ray crystal structure.
emission spectra of the planar phanes (5, 7, 8) are well
resolved.
The anticipated fluorescence blue-shift, when going from
room temperature to 77 K, was observed for the twisted
derivatives 2, 3, 9 (and 10). For 4, 6 and the planar tolanes 5, 7,
8 (and 1) no hypsochromic shift was recorded. Additionally, for
all twisted conformers strong phosphorescence around 450 nm
was measured, and for 10 as well, which is why we reason it to
be twisted.
It is noteworthy that excitation at different wavelengths (λex =
260−300 nm, 10 nm-steps, see Supporting Information) gave
varying emission spectra with respect to the ratio of
fluorescence/phosphorescence intensities. This occurs from
different existing conformers. Exciting at 260 nm the twisted
specimen emits and gives a strong phosphorescence signal,
while the planar isomer gets excited at 300 nm recording only
fluorescence. This could as well be seen in the excitation
spectra, which detected the phosphorescence (λem = 460 nm),
showing the maximum at 260−270 nm (Figure 4, blue dotted
Figure 5. 3D illustration of luminescent decay of tolane 2.
line). These effects are observed in analogy to literature
observations.9
absorption profile.9,14,15 In addition (vide supra) the torsion
angle also influences the occurrence of phosphorescence at low
temperatures. To gain insight into the photophysics of tolanes
in general, a first important step is a detailed understanding of
the photophysics of the unsubstituted parent tolane. Toward
that objective, the vertical excited singlet and triplet states of
tolane were computed along the twisting coordinate at the
theoretical level of TDDFT/CAM B3LYP/6-31G*.44−47
At the planar geometry, the two lowest lying and thus
relevant transitions S1 and S2 exhibit excitation energies of 4.7
and 5.2 eV and oscillator strengths of 1.1 and 0.0 respectively.
In the molecular orbital picture the S1 state is represented by a
HOMO to LUMO transition. The S2 state corresponds mainly
In rigid systems triplets have a chance to phosphoresce as
radiationless decay is less prevalent. The lifetime of the triplet
state for unsubstituted tolane is 1 μs.21 Measuring the
phosphorescence lifetime of the twisted representatives 2−4,
6, 9 (and 10), however, gives values around 1 s (Table 3).
Figure 5 illustrates the fast decay of the fluorescence (280−380
nm) and the very slow disappearance of the phosphorescence
signal (400−700 nm) of compound 2. Such long-lived
phosphorescence is new for tolanes and remarkable.
Quantum Chemical Calculations. Our and other
researchers’ experiments show that the torsion angle between
the phenyl rings of tolanes determines the shape of their
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to an electronic transition from the HOMO−4 (!) to the
LUMO. HOMO and HOMO−4 reflect the orthogonal π-
bonds of the central triple bond. At the fully twisted
configuration, i.e., a torsion angle of 90°, the energy of the S1
and S2 states are interchanged possessing excitation energies of
5.2 and 5.1 eV, respectively (Figure 6). The HOMO to LUMO
of 5.9 eV. This explains the strongly blue-shifted absorption
spectra of twisted tolanes in contrast to planar ones.
Inspection of the potential energy curves in Figure 6 reveals
that the allowed HOMO−LUMO excitation always leads to a
planarization of the tolanes. The global planar minimum on the
S1 surface of tolane lies at 0°, and from all other angles a
barrierless transition into this minimum is possible in S1. This
nicely explains why planar and twisted tolanes exhibit different
UV−vis absorption spectra, but why the fluorescence spectra
are all equivalent and correspond to those of planar tolanes.
To further understand why twisted tolanes exhibit
phosphorescence at low temperatures, while planar ones do
not show this feature, the excited singlet and triplet states are
considered. For phosphorescence to occur, intersystem crossing
(ISC) must happen, which is facilitated by two requirements:
large spin−orbit couplings and a small S-T energy difference.
Upon initial excitation, the S1 state is populated, and we assume
that ISC occurs from this state. Following the S1 curve of tolane
along the twisting coordinate from 0° to 90° (Figure 6), four
curve crossings with triplet states are identified: S1/T6 at 25°,
S1/T7 at 65°, S1/T8/9 at 70° and S1/T10 at 75°, where ISC can
favorably occur. However, large spin−orbit coupling is also
required for efficient ISC, and to estimate its magnitude, El
Sayed rules can be invoked.48 According to these rules, spin−
orbit coupling is particularly large when the molecular orbitals
involved in the crossing of electronic singlet and triplet states
are perpendicular to each other. Inspection of the molecular
orbitals involved in these states reveals that the most likely
place for ISC to occur is at the crossing between S1 and T7 at a
torsion angle of 65°, since only for this pair of states, the
orbitals are perpendicular to each other. Hence, at the
theoretical level of TDDFT/CAM B3LYP/6-31G*, the most
likely crossing for ISC to take place is between S1 and T7
(Figure 7).
Figure 6. Potential energy curves of the lowest lying singlet (blue) and
triplet (red) excited states of tolane along the twisting coordinate of
the two phenyl rings. The curves were computed at the theoretical
level of TDDFT/CAM B3LYP/6-31G*. The circles highlight relevant
crossings between the lowest singlet and triplet states.
This finding explains why only twisted tolanes exhibit
phosphorescence at low temperatures. A tolane with small
torsion angle is initially excited into the S1 state, and ISC is very
slow, because tolanes tend to planarize in the S1 state, and the
initial excitation is not high enough to reach the state crossing
with T7 at 65° torsion. The situation is different when a twisted
tolane is excited in a frozen matrix. Then the initial excitation
energy lies higher than the state crossing, and moreover, the
induced planarization of the tolane leads barrierless to the S1/
T7 crossing where most likely ISC can take place. Eventually,
transition loses its oscillator strength continuously along the
twisting coordinate until both states have no oscillator strength
any more at 90°. At this geometry, the lowest excited state with
substantial oscillator strength of 1.5 is S6 at an excitation energy
Figure 7. Molecular orbitals involved in lowest singlet state S1 and in the energetically accessible triplet states T1 to T7 computed at the level of
TDDFT/CAM B3LYP/6-31G*.
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1
colorless solid: Rf 0.43 (PE/EA 4:1); mp 232 °C; H NMR (300
MHz) δ 3.40 (s, 2H), 5.16 (s, 4H), 7.35−7.43 (m, 6H), 7.56−7.59 (m,
2H); 13C NMR (76 MHz) δ 42.0, 67.7, 90.8, 124.4, 128.9, 129.3,
130.7, 132.8, 136.4, 166.3; IR (ν) 2954, 1751, 1724, 1495, 1462, 1405,
1377, 1297, 1276, 1210, 1151, 990, 760, 739; HRMS (EI) calcd for M+
306.0892, found 306.0884. Anal. Calcd for (C19H14O4): C 74.50, H
4.61. Found: C 74.31, H 4.63.
the molecules decay nonradiatively into the T1 minimum at 0°
and phosphoresce strongly red-shifted. In agreement with the
experimental findings, only tolanes with large arrested torsion
angles can exhibit efficient phosphorescence.
Tethered Tolanes. The experimental findings agree
perfectly to the calculated results. Compounds 5, 7 and 8 are
planar and nonphosphorescent and exhibit exclusively fluo-
rescence at low temperature. In contrast, the twisted tolanes 2,
3, 6, 9 and 10 show predominantly phosphorescence. Tolane 4
is an intermediate case as it shows both strong fluorescence and
phosphorescence at low temperature in agreement with the
calculated population of planar as well as twisted ground state
minima.
Tolane 3. To a solution of tolane 2 (250 mg, 816 μmol, 1.0 equiv)
in THF (10 mL) at 0 °C was added sodium hydride (58.8 mg, 2.45
mmol, 3.0 equiv). After stirring for 30 min methyl iodide (0.15 mL,
2.45 mmol, 3.0 equiv) was added, and the reaction mixture was stirred
at room temperature for 48 h. The reaction mixture was quenched
with ice−water (50 mL) and extracted with DCM (3 × 60 mL). The
organic extract was washed with water (3 × 200 mL) and brine (150
mL), dried over MgSO4 and concentrated in vacuo. After chromato-
graphic purification tolane 3 (154 mg, 461 μmol, 56%) was obtained as
CONCLUSION
1
■
a colorless solid: Rf 0.64 (PE/EA 4:1); mp 182 °C; H NMR (300
MHz) δ 1.43 (s, 6H), 5.09 (s, 4H), 7.34−7.42 (m, 6H), 7.54−7.57 (m,
2H); 13C NMR (76 MHz) δ 22.8, 48.9, 67.7, 90.7, 124.5, 128.8, 129.2,
130.8, 132.5, 136.7, 173.1; IR (ν) 2987, 1749, 1723, 1496, 1455, 1390,
1376, 1271, 1246, 1170, 1126, 929, 762, 751; HRMS (EI) calcd for M+
334.1205, found 334.1227.
We have prepared a series of tolanophanes with torsion angles
that can be “dialed in”, depending upon the chemical nature of
the bridging group. Twisting of the tolanes is documented by
crystal structures, DFT-calculations and optical (absorption)
spectroscopy. As the absorption spectra blue-shift in depend-
ence of the torsion angle, i.e., twisting diminishes the
conjugation, the emission is torsion angle independent, when
measured in solution. Emission spectra of twisted tolanes
recorded in organic glasses at cryogenic temperatures show
efficient phosphorescence with emissive lifetimes of ca. 1 s.
Quantum chemical calculations show that the S1 intersects with
the T7-state and ISC is facile. Relaxation to T1 is followed by
phosphorescence. Over all, twisting tolanes leads to surprising
and exciting changes in their optical and electronic properties.
The most unusual feature is their long-lived strong phosphor-
escence observed in cryogenic matrices.
Tolane 4. GP. To NaHCO3 (1.06 g, 12.6 mmol, 5.0 equiv) in
DCM (200 mL) were added 1 (600 mg, 2.52 mmol, 1.0 equiv) in
DCM (75 mL) and succinyl chloride (0.3 mL, 2.77 mmol, 1.1 equiv)
in DCM (75 mL) using two dropping funnels. Purification gave 4 (162
mg, 0.506 μmol, 20%) as a colorless solid: Rf 0.39 (PE/EA 4:1); mp
195 °C; 1H NMR (300 MHz) δ 2.63 (s, 4H), 5.22 (s, 4H), 7.33−7.41
(m, 4H), 7.44−7.47 (m, 2H), 7.59−7.62 (m, 2H); 13C NMR (76
MHz) δ 30.1, 66.4, 91.2, 124.4, 128.8, 129.1, 131.2, 133.0, 136.9,
171.7; IR (ν) 2956, 1722, 1495, 1462, 1417, 1377, 1336, 1229, 1165,
990, 976, 956, 879, 769; HRMS (EI) calcd for M+ 320.1049, found
320.1050. Anal. Calcd for (C20H16O4): C 74.99, H 5.03. Found: C
74.82, H 5.00.
Tolane 5. GP. Diol 1 (600 mg, 2.52 mmol, 1.0 equiv) in DCM (75
mL) and trans-1,2-cyclohexanedicarbonyl chloride50,51 (579 mg, 2.77
mmol, 1.1 equiv) in DCM (75 mL) were added to NaHCO3 (1.06 g,
12.6 mmol, 5.0 equiv) in DCM (150 mL) using two dropping funnels.
After purification 5 (305 mg, 815 μmol, 32%) was isolated as a
EXPERIMENTAL SECTION
■
Materials and Methods. Reactions were carried out in oven-dried
glassware under an atmosphere of nitrogen using the common
Schlenk-technique. For thin layer chromatography (TLC) silica gel
plates were used and examined under ultraviolet light irradiation (254
and 365 nm). Column chromatography was performed using silica gel
(particle size: 0.04−0.063 mm). All NMR spectra (1H,13C) were
recorded at room temperature at 300 or 500 MHz in deuterated
chloroform (CDCl3). The chemical shifts (δ) were reported in parts
per million (ppm) and referenced internally to the solvent signal.49
The following abbreviations were used in the listings of the NMR
resonances: s (singlet), d (doublet), t (triplet), m (multiplet). For high
resolution mass spectra (HRMS) the samples were ionized by electron
impact (EI) in the positive mode and analyzed by magnetic sector.
Solvents were abbreviated as PE (petroleum ether), EA (ethyl acetate),
EtOH (ethanol).
1
colorless solid: Rf 0.60 (PE/EA 4:1); mp 255 °C; H NMR (300
MHz) δ 1.17−1.39 (m, 4H), 1.75 (d, 2H, J 8.4 Hz), 2.06 (d, 2H, J 9.7
Hz), 2.55−2.65 (m, 2H), 5.03 (d, 2H, J 11.3 Hz), 5.35 (d, 2H, J 11.2
Hz), 7.24−7.34 (m, 4H), 7.44−7.47 (m, 2H), 7.60−7.63 (m, 2H); 13C
NMR (76 MHz) δ 25.2, 29.1, 45.2, 66.4, 91.5, 124.5, 128.8, 129.1,
131.3, 133.1, 136.9, 174.7; IR (ν) 2937, 2861, 1722, 1495, 1446, 1389,
1372, 1316, 1256, 1220, 1170, 1112, 1002, 982, 757; HRMS (EI) calcd
for M+ 374.1518, found 374.1514. Anal. Calcd for (C24H22O4): C
76.99, H 5.92. Found: C 76.85, H 5.89.
Tolane 6. According to reference:52 To a solution of 1 (500 mg,
2.10 mmol, 1.0 equiv) in dry DCM (400 mL) were given NEt3 (0.8
mL, 5.67 mmol, 2.7 equiv) and DMAP (0.128 g, 1.05 mmol, 0.5
equiv). Then fumaryl chloride (0.3 mL, 2.31 mmol, 1.1 equiv) was
added slowly to the mixture, and the whole solution was stirred for 20
min. After washing with water (3 × 300 mL) and brine (300 mL) the
organic layer was dried over MgSO4 and concentrated in vacuo. After
chromatographic purification and final recrystallization from EtOH 6
(67.7 mg, 0.213 mmol, 10%) was obtained as a colorless solid: Rf 0.27
General Procedure (GP). Simultaneous, dropwise addition of a
solution of diol 1 and a second solution of the corresponding
dichloride to base (NaHCO3 or NEt3, DMAP) in solvent (DCM or
THF) was followed by stirring for 10 h at room temperature. In case
of using DCM as solvent the organic phase was washed with water,
brine, dried over MgSO4 and evaporated under reduced pressure. If
THF was used as solvent the mixture was concentrated in vacuo, and
the residue was dissolved in DCM followed by aqueous workup as
described above. After chromatographic purification (PE/EA 4:1) the
desired tolanophane was obtained.
1
(PE/EA 4:1); mp 167 °C; H NMR (300 MHz) δ 5.39 (s, 4H), 7.17
(s, 2H), 3.35−7.44 (m, 4H), 7.49−7.51 (m, 2H), 7.57−7.59 (m, 2H);
13C NMR (126 MHz) δ 67.7, 91.2, 122.2, 129.1, 129.5, 130.3, 133.4,
133.5, 137.0, 165.2; IR (ν) 2916, 1713, 1636, 1496, 1478, 1446, 1324,
1261, 1159, 1023, 987, 761; HRMS (EI) calcd for M+ 318.0892, found
318.0877.
Tolane 1. 2,2′-(1,2-Ethynediyl)bis-benzenemethanol was synthe-
Tolane 7. Norborna-2,5-diene-2,3-dicarboxylic acid53 (401 mg,
1.85 mmol, 1.1 equiv) was added slowly to a solution of 1 (400 mg,
1.68 mmol, 1.0 equiv), NEt3 (0.63 mL, 4.53 mmol, 2.7 equiv) and
DMAP (103 mg, 839 μmol, 0.5 equiv) in DCM (320 mL). After
stirring with water the solution was washed with water (3 × 300 mL),
brine (300 mL) and dried over MgSO4. Concentration in vacuo and
final chromatographic purification provided 7 (45.7 mg, 120 μmol,
sized by known procedure, see reference.36,37
Tolane 2. GP. The DCM-solutions (75 mL each) of 1 (600 mg,
2.52 mmol, 1.0 equiv) and malonyl chloride (0.3 mL, 2.77 mmol, 1.1
equiv) were added to a suspension of NaHCO3 (635 mg, 7.55 mmol,
3.0 equiv) in DCM (200 mL) over a period of 4 h. After the described
workup the crude product was subjected to chromatographic
purification to provide tolane 2 (273 mg, 891 μmol, 35%) as a
F
dx.doi.org/10.1021/jo5010235 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
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1
7%) as a colorless solid: Rf 0.50 (PE/EA 4:1); mp 183 °C; H NMR
(300 MHz) δ 2.01 (dt, 1H, J 6.8, 1.4 Hz), 2.22 (dt, 1H, J 6.8, 1.4 Hz),
3.90 (dt, 2H, J 3.8, 1.6 Hz), 5.28 (s, 4H), 6.89 (t, 2H, J 1.9 Hz), 7.33−
7.42 (m, 4H), 7.45−7.48 (m, 2H), 7.58−7.61 (m, 2H); 13C NMR (76
MHz) δ 53.8, 67.2, 72.4. 91.8, 124.6, 128.8, 129.2, 131.4, 132.7, 136.7,
142.5, 151.0, 166.1; IR (ν) 2981, 2942, 1718, 1704, 1557, 1489, 1380,
1258, 1084, 1049, 932, 753, 727; HRMS (EI) calcd for M+ 382.1205,
found 382.1191.
Tolane 8. According to GP the THF-solutions (25 mL each) of 1
(500 mg, 2.10 mmol, 1.0 equiv) and phthaloyl chloride (0.3 mL, 2.31
mmol, 1.1 equiv) were dropped into a mixture of NEt3 (2.9 mL, 8.39
mmol, 10.0 equiv) and THF (300 mL). Following the workup as
described above and chromatographic purification (PE/EA 4:1) tolane
8 (136 mg, 369 μmol, 18%) was obtained as a colorless solid: Rf 0.43
(PE/EA 4:1); mp 243 °C; 1H NMR (300 MHz) δ 5.43 (s, 4H), 7.37−
7.40 (m, 4H), 7.46 (td, 2H, J 3.3, 2.9 Hz), 7.55−7.58 (m, 2H), 7.62−
7.68 (m, 4H). 13C NMR (76 MHz) δ 67.9, 92.2, 124.6, 128.7, 128.7,
129.2, 131.0, 131.5, 131.9, 133.4, 136.2, 167.8; IR (ν) 3061, 2952,
1716, 1602, 1496, 1450, 1373, 1276, 1249, 1119, 938, 757, 703, 693;
HRMS (EI) calcd for M+ 368.1049, found 368.1029. Anal. Calcd for
(C24H16O4): C 78.25, H 4.38. Found: C 77.87, H 4.56.
Tolane 9. GP. The THF-solutions (25 mL each) of 1 (500 mg,
2.10 mmol, 1.0 equiv) and isophthaloyl chloride (469 mg, 2.31 mmol,
1.1 equiv) were given to NEt3 (2.9 mL, 8.39 mmol, 10.0 equiv) in
THF (300 mL). Purification as described and column chromatography
led to 9 (255 mg, 692 μmol, 33%) as a colorless solid: Rf 0.46 (PE/EA
4:1); mp 208 °C; 1H NMR (300 MHz) δ 5.53 (s, 4H), 7.30−7.37 (m,
4H), 7.39−7.45 (m, 3H), 7.67−7.70 (m, 2H), 8.01 (dd, 2H, J 7.7, 1.8
Hz), 9.77 (t, 1H, J 1.7 Hz); 13C NMR (76 MHz) δ 66.2, 92.2, 123.8,
128.7, 128.8, 130.5, 131.5, 132.4, 135.2, 136.7, 137.0, 165.3; IR (ν)
3137, 2952, 1719, 1491, 1448, 1428, 1368, 1287, 1126, 1060, 756, 719,
708; HRMS (EI) calcd for M+ 368.1049, found 368.1078. Anal. Calcd
for (C24H16O4): C 78.25, H 4.38. Found: C 77.95, H 4.45.
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1
0.40 (PE/EA 4:1); mp 250 °C; H NMR (300 MHz) δ 5.45 (s, 4H),
7.35−7.42 (m, 6H), 7.62−7.65 (m, 2H), 7.80 (s, 4H); 13C NMR (76
MHz) δ 65.9, 91.5, 123.1, 128.7, 129.0, 129.3, 129.6, 132.7, 133.6,
137.2, 165.3; IR (ν) 3055, 2920, 1703, 1495, 1453, 1408, 1376, 1363,
1246, 1083, 1015, 927, 880, 748, 729; HRMS (EI) calcd for M+
368.1049, found 368.1027.
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ASSOCIATED CONTENT
* Supporting Information
NMR spectra, X-ray crystal structures (CIF), photophysical
data and quantum chemical calculation details. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
(35) Menning, S.; Kramer, M.; Coombs, B. A.; Rominger, F.; Beeby,
̈
A.; Dreuw, A.; Bunz, U. H. F. J. Am. Chem. Soc. 2013, 135, 2160.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: andrew.beeby@durham.ac.uk.
*E-mail: dreuw@uni-heidelberg.de.
*E-mail: uwe.bunz@oci.uni-heidelberg.de.
̌
■
(36) Stara,
Fiedler, P. Tetrahedron 1998, 54, 11209.
́
I. G.; Stary,
́
I.; Kollar
́
ovic,
̌
A.; Teply,
́
F.; Saman, D.;
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1178.
(38) CCDC 918376 (1), 918377 (2), 970881 (3), 970882 (4),
970883 (5), 970884 (6), 970885 (7), 918378 (8), 918379 (9) contain
the supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
(39) Torsion angles were determined by fitting a plane through each
phenyl ring and measuring the angle of the planes.
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Notes
The authors declare no competing financial interest.
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