Unusual fluorescene emission from ethynyltriphenylene-substituted diacetylenic molecular hinge. Formation of intramolecular excimer

Article abstract of DOI:10.1039/b923441a

A diacetylenic molecular hinge bearing two ethynyltriphenylene units (1) has been synthesized. Evidence from ¹H NMR and variable temperature NMR (VT-NMR) of 1 in comparison to model compounds bearing only one triphenylene unit suggests that there is an equilibrium between the open conformer and the intramolecularly π-π interacting closed conformer in solution (equilibrium constant K = 6.5 at 298 K in CDCl₃) arising from the rotation of the diacetylenic hinge. Unusual fluorescence emission observed from 1 has been assigned to excimer formation arising from intramolecularly π-π interacting triphenylene units in the excited state. Steady state and picosecond time resolved fluorescence spectra of 1 were nearly identical and corresponded to intramolecular excimer emission.

Full text of DOI:10.1039/b923441a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Unusual fluorescene emission from ethynyltriphenylene-substituted
diacetylenic molecular hinge. Formation of intramolecular excimer†
Ritesh Nandy and Sethuraman Sankararaman*
Received 10th November 2009, Accepted 8th March 2010
First published as an Advance Article on the web 18th March 2010
DOI: 10.1039/b923441a
A diacetylenic molecular hinge bearing two ethynyltriphenylene units (1) has been synthesized.
Evidence from 1H NMR and variable temperature NMR (VT-NMR) of 1 in comparison to model
compounds bearing only one triphenylene unit suggests that there is an equilibrium between the open
conformer and the intramolecularly p–p interacting closed conformer in solution (equilibrium constant
K = 6.5 at 298 K in CDCl3) arising from the rotation of the diacetylenic hinge. Unusual fluorescence
emission observed from 1 has been assigned to excimer formation arising from intramolecularly p–p
interacting triphenylene units in the excited state. Steady state and picosecond time resolved
fluorescence spectra of 1 were nearly identical and corresponded to intramolecular excimer emission.
through an ethynyl bridge to a rigid acetylenic hinge (Scheme 2).6
Introduction
We address the question of whether or not p-stacking interactions
would lead to the stabilization of certain conformers, such as the
closed conformer in Scheme 2, in solution phase. Earlier we had
used a pyrene chromophore to study p-stacking interactions.6,7 We
have reported the crystal structures of both the closed p-stacked
conformer and the open conformer arising out of rotation of
the diacetylenic hinge. (Scheme 2, p = pyrene, n = 2). Ours is
the first example of structural characterization of conformational
isomers arising out of the rotation of a diacetylenic hinge.7 We have
attributed the stability of the closed conformer to the p-stacking
interaction of the two pyrene units.
It is well known that many aromatic molecules form excimers
following electronic excitation and then emit from the excimer
state at a longer wavelength compared to the monomer emission.1
Excimer emission is generally observed at high concentrations.
Pyrene is a typical example of such a molecule that readily emits
from the excimer state at concentrations >10-3 M.1,2 However,
there are few polycyclic aromatic molecules that do not form
an excimer in the excited state and hence emission from the
excimer state is very rare.3 Triphenylene is a good example of
such a molecule. Triphenylene and its derivatives do not emit
from the excimer state even at high concentrations,3,4 except when
they are present in a constrained environment. Recently, excimer
emission of a triphenylene derivative has been reported from a gel
state.5 Depending upon the type of overlap in the excited state,
two types of excimer emissions have been suggested, one from
the eclipsed overlap and the other from the staggered overlap
(Scheme 1).5 Emission from the solid state of triphenylene can
also be considered as excimer emission due to extensive ground
state intermolecular interactions in the crystalline state.
Scheme 2 Conformational equilibrium of an acetylenic molecular hinge
in open and closed forms.
In the present study we have made use of the same rigid
diacetylenic molecular hinge8 with triphenylene instead of pyrene
(Scheme 3). The present study is aimed at studying the fluorescence
emission of 1, in particular emission from the intramolecular
excimer state of triphenylene as it is very uncommon to observe
excimer emission from triphenylene. In the present context the
intramolecular excimer state corresponds to the two tripheny-
lene units coming closer in the excited state as a result of rotation
of the diacetylene hinge. As an extension of our earlier work on
the pyrene derivatives we also address the question of whether the
closed conformer is present in the ground state in equilibrium with
the open form (Scheme 2). We probe the excited state using steady
Scheme 1 Two types of overlap of triphenylene in the excimer state.
We have been interested in studying p-stacking interactions
in molecular systems where large aromatic units are connected
Department of Chemistry, Indian Institute of Technology Madras, Chennai,
600036, India. E-mail: sanka@iitm.ac.in; Fax: +91 44 22570545
† Electronic supplementary information (ESI) available: Copies of 1H and
13C NMR of all the synthesized compounds, 1H–1H COSY NMR of 1
and 5, time resolved decay curves, temperature dependent fluorescence.
See DOI: 10.1039/b923441a
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Scheme 3 Synthesis of target 1.
state and time resolved fluorescence spectroscopy and the ground
state using NMR spectroscopy.
Results and discussion
Synthesis
Synthesis of target 1 was accomplished in 4 steps. Sono-
gashira coupling of 2-iodotriphenylene (2) and 4-t-butyl-2-
ethynylbenzaldehdye (3) yielded 4 in 58% isolated yield. Con-
version of 4 to terminal acetylene 5 was accomplished using
the Corey–Fuchs method9 in an overall yield of 59%. Oxidative
coupling10 of terminal acetylene 5 yielded the desired product (1)
in 72% yield (Scheme 3). Target 1 was characterized by various
spectroscopic data. Compound 6 with only one triphenylene unit
was used as a model compound. It was synthesized by the
Cadiot–Chadkiewicz coupling11 of bromoethynylbenzene (7) with
5 (Scheme 4). For the sake of convenience numbering of the
triphenylene ring protons is shown in Scheme 4.
Comparison of 1H NMR spectra of 1 and model compounds
In triphenylene the bay region protons (H1, H4, H5, H8, H9,
H12) appear at 8.5–8.6 ppm and the non-bay region protons (H2,
H3, H6, H7, H10, H11) at d 7.6–7.7 ppm. In 5 one of the bay
region protons (H1) appeared at 8.87 ppm as a doublet (J 1.4 Hz),
more deshielded than the rest due to the anisotropic effect of the
adjacent acetylenic bond (Fig. 1). Similarly H3, a non-bay region
proton adjacent to the acetylenic bond, appeared at 7.83 ppm
(dd, J 8.6 and 1.4 Hz), more deshielded than the rest of the non-
bay region protons. In case of 6, protons H1 and H3 appeared
at slightly higher chemical shift values than that of 5 at 8.95
and 7.89 ppm, respectively, due to an increase in conjugation
Scheme 4 Synthesis of model compound 6 with one triphenylene unit.
evident that 1 should possess a symmetrical structure (presumably
a C2 axis) resulting in the chemical equivalence of a given proton
in one triphenylene ring to that of the same proton on the other
ring. Furthermore, the proton resonances in 1 are more resolved
and spread out than those of 5 and 6. For example, two of the
bay region protons, namely H4 and H5, are well resolved and
appeared at 8.13 and 8.0 ppm as doublets, respectively. One of
the non-bay region protons, H6, is the most shielded one and
appeared at 7.22 ppm as a triplet. H1 appeared at 8.5 ppm as
a doublet. These assignments are based on a careful analysis
1
(Fig. 1). The aromatic region of the H NMR spectra of 1, 5
and 6 is compared in Fig. 1. It is evident that protons of 1 are
more shielded by about 0.4 ppm than those of 5 and 6. It is also
1
of the H–1H COSY spectra of 1 and 5. In the absence of any
p–p interaction between the triphenylene units in 1 the chemical
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Fig. 1 Comparison of the aromatic region of the 1H NMR spectra of 5
(top), 6 (middle) and 1 (bottom) in CDCl3 at 25 ◦C.
shift of the triphenylene ring protons would have been either same
as that of 6 or slightly more deshielded than 6 due to further
increase in conjugation. However, a reverse trend is observed
for 1. The shielding of the protons of 1 compared to those of
5 and 6 is due to intramolecular p-stacking interaction of the two
triphenylene units in 1. p-Stacking usually results in the shielding
1
the protons in H NMR spectrum due to ring current effect of
the aromatic rings.12,13 The spreading out of resonances is due
to shielding of different protons to different extents depending
on their placement in the ring current effect.14 In the absence of
any heteroatom substituents exerting strong electronic effects the
shielding of protons of 1 compared to 5 and 6 as well as enhanced
resolution of the ring protons in 1 compared to 5 and 6 could only
be explained on the basis of p-stacking and consequent ring current
effect of one triphenylene ring on the proton chemical shifts of the
other.
Fig. 2 VT-1H NMR spectra of 1 in CDCl2CDCl2. Only the aromatic
region is shown.
Variable temperature 1H NMR (VT-1H NMR) studies of 1
The question of whether or not 1 exists as an equilibrium mixture
of the open and closed forms as shown in Scheme 2 in the ground
electronic state with p–p interactions between the triphenylene
Fig. 3 Effect of temperature on the chemical shift of H4 and H5 protons
in 1.
1
moieties in the closed form is conveniently studied by H NMR
spectroscopy. The chemical shifts of aromatic protons are sensitive
to p–p interactions.12–14 Typically, aggregation due to p-stacking
results in the shielding of aromatic proton resonances resulting
in lower chemical shift values due to an aromatic ring current
effect. At any given temperature NMR would give the average
chemical shift arising out of the equilibrium in Scheme 2. It
is raised, the intramolecular p–p interaction is disturbed and as
a consequence the chemical shift of various protons increased
(Fig. 2). The observed chemical shifts are only an average of
the open and closed forms of 1. With increasing temperature the
population of open is increased in the equilibrium (Scheme 2) and
hence the chemical shifts of various hydrogens are increased. Thus,
the 1H NMR data clearly supports the existence of the equilibrium
between the open and closed forms of 1 in the solution phase. The
chemical shift of triphenylene protons in the model compounds 4
and 5 could be taken as equal to the open form of 1 (devoid of p–p
interactions). The poor solubility of 1 hampered measurement of
NMR below 298 K in CDCl2CDCl2. Therefore, low temperature
measurements were made in CDCl3 in the temperature range of
233–298 K (ESI, Fig. S24†). As the temperature is lowered the
chemical shift of various triphenylene protons decreased until
253 K. Below this temperature the chemical shift of various
protons were invariant. The chemical shift of H1 proton remained
at 8.43 ppm below 253 K.
1
should be emphasized here that the H NMR spectra of the model
compounds 5 and 6 with only one triphenylene unit were independent
of temperature of measurement. Being a weakmolecular interaction
(interaction energy is about 2–10 kJ mol-1)15 p–p interaction is
perturbed by temperature. An increase in temperature generally
results in breaking of the p–p interactions. A systematic inves-
1
tigation of the effect of temperature on the H NMR spectrum
of 1 was undertaken in the temperature range of 298 to 408 K
in CDCl2CDCl2. Upon increasing the temperature the chemical
shift of various triphenylene protons increased gradually (Fig. 2).
For example the chemical shift of H1, H4, H5 and H6, increased
from d 8.45, 8.09, 7.99, 7.14 ppm, respectively, at 298 K to d 8.58,
8.21, 8.13, 7.25 ppm, respectively, at 408 K. The effect of increasing
temperature on the chemical shift of H4 and H5 is shown in Fig. 3,
and a similar trend was observed for H1 and H6. It should be
noted that in this temperature range the chemical shifts of the
phenyl protons remained unchanged (Fig. 2). As the temperature
As an approximation, the chemical shift of H1 in 1 at 233 K
was taken as that of the closed form. The chemical shift of
H1 in the model compound 6 was taken as that of the open
form. Using these values the equilibrium constant K as given in
Scheme 2 was estimated to be 6.5 at 298 K in CDCl3 (ESI†). This
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value is comparable to that reported earlier for the corresponding
pyrene derivative from our laboratory.7 Using VT-NMR data the
thermodynamic parameters for the equilibrium were estimated
(DG = -4.6 kJ mol-1, DH = -14.2 kJ mol-1 and DS = -32.2 J
mol-1 K-1 in CDCl3) (ESI†).
Absorption and fluorescence emission studies
A comparison of the absorption spectra of 1, 5 and 6 in
cyclohexane is shown in Fig. 4. Compared to the spectrum of
5 which has an absorption cut-off at 360 nm the bands of 1
and 6 are broader and more red shifted as expected due to
extended conjugation. The additional broad band in the region
380–400 nm for 1 might be due to intramolecular p–p interaction
arising from the closed form of 1. This additional broad band
in 1 in the longer wavelength region in comparison with the
model compounds is reminiscent of the “cyclophane band” arising
due to intramolecular p–p interaction in cyclophanes wherein the
Fig. 5 Fluorescence emission spectrum of 1 (a) (ff = 0.10), 6 (b) (ff =
0.14) and 5 (c) (ff = 0.19) in cyclohexane (1 ¥ 10-5 M) and 1 (d) in the solid
state. l(ex) = 310 nm.
bands in the region 400–450 nm. Emission from eclipsed overlap
of the two triphenylene units gives red-shifted emission around
550 nm.5 Our results are consistent with staggered overlap of
the two triphenylene units in the closed conformer (Scheme 2).
Furthermore, the fluorescence emission was measured within the
narrow temperature range of 283–343 K (ESI, Fig. S3†). Systems
that undergo ground state p-stacking usually show a small blue
shift in the emission wavelength at higher temperatures due to
the disruption of the p-stacking interactions. Our observations
are consistent with the observations in the literature18 in that
the fluorescence emission was shifted from 438 nm at 283 K to
433 nm at 343 K (ESI, Fig. S3†). These observations based on
the absorption and fluorescence emission data further support
the presence of equilibrium shown in Scheme 2 in solution.
Considering that the barrier for rotation along the diacetylenic
bond in 1 is very low, emission from the excimer state of 1 where
the two triphenylene units come close (as in the closed form in
Scheme 2) is reasonable. The excimer emission of 1 arises from
the closed form that exists in the ground state of 1 in addition
to the excimer formed in the excited state by the rotation of the
diacetylenic axis. The fluorescence quantum efficiencies (ff) of 1,
5 and 6, respectively, are 0.10, 0.19 and 0.14 in cyclohexane. They
were measured using 9.10-diphenylanthracene as the standard.1
˚
aromatic units exist within the distance of 3.2–3.5 A when the
bridge length is small.16
Fig. 4 Absorption spectrum of 1 (a), 6 (b) and 5 (c) (---) in cyclohexane
(1 ¥ 10-5 M).
In cyclohexane, triphenylene emits from the monomer state at
370 nm.4 It does not exhibit any excimer emission even in more
concentrated solutions. However, emission in the solid state at
408 nm could be attributed to excimer emission in view of the
ground state p–p interactions in the solid state as revealed from
the crystal structure of triphenylene.17 Similarly for the model
compound 5, fluorescence emission occurred at 375 and 383 (sh)
nm in CH2Cl2, and 432 nm in the solid state. Compared to
triphenylene, both the absorption and emission bands of 5 are
red-shifted due to extended conjugation of the phenylethynyl unit.
The large bathochromic shifts of approximately 38–55 nm between
the emission bands in solution state and in solid state, respectively,
of triphenylene and 5 are due to emission from the monomer state
in solution and from the excimer state in solid. In the case of 1,
fluorescence emission in cyclohexane showed two bands at 411
(sh) and 438 nm (Fig. 5). Emission in the solid state was only
slightly red-shifted by 5 nm and appeared at 443 nm as a broad
featureless band. Compared to 5 and 6, emission of 1 in solution
phase is red-shifted by 50 nm. The fluorescence emission of 1 was
independent of concentration as well as the excitation wavelength
(ESI, Fig. S1 and S2†). The emission spectrum of 1 in solution
phase is very similar to the excimer emission of a triphenylene
derivative reported in the gel state with two bands.5 Based on
spectral comparison we conclude that the two triphenylene units
are p-stacked in a staggered manner (Scheme 1). p-Stacking of the
two triphenylene units in a staggered manner gives two emission
Time resolved fluorescence studies
The time resolved fluorescence spectrum of 1 was recorded, and
the fluorescence lifetimes of 1 and 5 were also measured using
the picosecond time correlated single photon counting (TCSPC)
technique. The time resolved fluorescence spectrum of 1 measured
incyclohexane is showninFig. 6anditclosely resembled the steady
spectrum (Fig. 5) with two maxima at 400 and 431 nm. The time
resolved fluorescence spectrum was also reminiscent of the excimer
spectrum of triphenylene derivative reported in the literature.5
The spectrum corresponds to excimer emission arising from the
staggered p-stacked triphenylene units in the closed form. It should
be noted that under the picosecond time regime of measurement
the monomer emission spectrum of 1 was not observed, rather the
excimer emission spectrum was directly observed.
The fluorescence lifetime measurements for 1 and 5 were made
in cyclohexane using a diode laser as the excitation source at 266
and 280 nm with a pulse width of 50 and 150 ps, respectively. The
monomer fluorescence decay of 5 observed at 375 nm followed a
clean first order decay and the fluorescence life time was 10.3
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measurements. Steady state fluorescence spectra were measured
on a Hitachi F4500 spectrometer.
2-Iodotriphenylene (2)
To a suspension of triphenylene (300 mg, 1.31 mmol) in acetic
acid (50 mL), water (5 mL) and sulfuric acid (0.5 mL) was added
iodine (200 mg, 0.79 mmol) and potassium iodate (68 mg, 0.32
mmol). The reaction mixture was heated at 55 ◦C for 1 day. It
was cooled to room temperature and poured into water (100 mL).
The mixture was extracted with CH2Cl2 (100 mL) and the organic
layer was washed with sodium thiosulfate solution (2 ¥ 50 mL) and
finally with water (100 mL). It was dried over anhydrous sodium
sulfate and solvent was removed under reduced pressure to give
a pale brown solid. It was purified by column chromatography
over silica gel using hexane to give a colorless solid (360 mg)
which contained an inseparable mixture of 2-iodotriphenylene
and triphenylene (45 : 55). The mixture was used as such for
further reaction. A small amount (100 mg) of the mixture was
purified by column chromatography and pure 2-iodotriphenylene
Fig. 6 Time resolved fluorescence emission spectra of 1 measured in
cyclohexane after 100 ps, 490 ps and 1.03 ns delay after the laser excitation
at l (ex) = 266 nm, pulse width = 50 ps.
0.1 ns, as expected (ESI, Fig. S22†).5 The excimer fluorescence
decay of 1 was followed at 406 and 436 nm (ESI, Fig S23†). The
decay profile was nearly identical at both the wavelengths. The
decay of excimer emission of 1 at these two wavelengths followed
a more complex kinetics presumably due to many conformational
isomers (open and closed forms shown in Scheme 2 are the two
extreme cases) and could be fitted to only biexponential decay. The
decay was too fast compared to that of 5. An approximate estimate
of the lifetime of the excimer emission of 1 is about 0.39 ns.
1
was characterized by H and 13C NMR spectroscopy. (Note: use
of excess iodinating agent resulted in the formation of a mixture
of mono and polyiodotriphenylenes which were inseparable). dH
(400 MHz, CDCl3) 8.93 (d, J 1.6 Hz, 1 H, 1-H), 8.51-8.63 (m, 4
H), 8.31 (d, J 8.4 Hz, 1 H, 4-H), 7.89 (dd, J 1.6, 8.4 Hz, 1 H, 3-H),
7.62-7.69 (m, 4H) ppm. dC (100 MHz, CDCl3) 135.79, 132.48,
131.75, 129.99, 129.79, 129.16, 129.06, 128.37, 127.81, 127.69,
127.42, 127.39, 125.03, 123.37, 123,33, 123.30, 123.07, 93.31 ppm;
m/z [%] MALDI-TOF MS calcd for C18H11I, 354; found 354 [M+,
100], 355 [M++1, 40]; 356 [M++2, 10].
Conclusions
We have synthesized a molecular hinge (1) with ethynyltriph-
1
enylene units as pendant groups. Evidence from H NMR and
VT-NMR of 1 suggest that there is an equilibrium between
the open conformer and the intramolecularly p–p interacting
closed conformer in solution. The extra band observed in the
electronic spectrum of 1 at longer wavelengths in comparison to
the model compounds 5 and 6 is indicative of p–p interaction
between the triphenylene units in the ground state. Observation
of a longer wavelength broad band is reminiscent of such
bands in cyclophanes where the aromatic units are held face-
to-face16 within p-stacking distance. Fluorescence spectroscopy
also revealed unusual excimer emission from 1, unusual be-
cause triphenylene moiety is known to emit from the excimer
state only under constrained environment. The observed spec-
trum is reminiscent of the excimer emission spectrum reported
for another triphenylene derivative in the literature.5 In the
closed conformer, the two triphenylene units are intramolecularly
p-stacked in a staggered manner as evident from the comparison
of fluorescence spectrum with the literature.5 The steady state
and the time resolved picosecond fluorescence spectra of 1 were
nearly identical. Based on the NMR (electronic ground state) and
UV-Vis and fluorescence (electronic excited state) spectroscopic
data we conclude that in molecular hinge 1, the closed conformer
with the two triphenylene units with p-stacking interactions is
present in equilibrium with the open form.
2-(5-tert-Butyl-2-formylphenylethynyl)triphenylene (4)
A Schlenk flask was charged with a mixture of 2-iodotriphenylene
and triphenylene (1 : 1 mole ratio) (4.66 g), Pd(PPh3)2Cl2 (208 mg,
0.30 mmol), PPh3 (155 mg, 0.60 mmol), CuI (113 mg, 0.60 mmol),
degassed THF (60 mL) and Et3N (60 mL). The reaction mixture
was stirred at room temperature for 15 min and 4-tert-butyl-2-
ethynylbenzaldehyde (1.0 g, 5.38 mmol) was added. Stirring was
continued for 3 h at room temperature. The solvent was removed
and the residue was dissolved in CH2Cl2 (200 mL). The solution
was washed with 5% aqueous HCl solution (2 ¥ 100 mL) and
finally with water (100 mL). It was dried over anhydrous sodium
sulfate and solvent was removed under reduced pressure. The
crude product was purified by column chromatography over silica
gel. Elution with hexane yielded triphenylene followed by elution
using a mixture of hexane and CH2Cl2 (90 : 10, v/v) yielded 4 as a
◦
≡
colorless solid (1.28 g, 58%). Mp 75–77 C. IR (neat) 2206 (C C),
-1
=
1689 (C O) cm . dH (400 MHz, CDCl3) 10.72 (s, 1 H, CHO), 8.80
(d, J 1.6 Hz, 1 H, 1-H), 8.56-8.64 (m, 5 H), 7.94 (d, J 8.4 Hz, 1 H,
4-H), 7.76 (dd, J 1.6 and 8.4 Hz, 1 H, 3-H), 7.74 (d, J 1.6 Hz, 1 H,
3-H phenyl), 7.62-7.68 (m, 4 H), 7.52 (dd, J 1.6 and 8.0 Hz, 1 H,
4-H phenyl), 1.42 (s, 9 H, C(Me)3) ppm. dC (100 MHz, CDCl3)
191.40 (CHO), 157.82, 133.69, 130.22, 130.17, 130.07, 129.99,
129.76, 129.16, 128.94, 127.76, 127.69, 127.41, 127.37, 127.34,
126.97, 126.64, 126.21, 123.55, 123.38, 123.36, 123.31, 121.02,
Experimental
General experimental methods19 and instrumentation for time re-
solved fluorescence spectrum and decay measurements20 have been
published elsewhere. The solutions were thoroughly degassed by
purging either dry nitrogen or dry argon prior to the fluorescence
≡
≡
96.04 (C C), 86.33 (C C), 35.34, 30.97 ppm; m/z [%] ESI Q-
TOF MS 413 [M++1] (100). HRMS calcd. for C31H25O [M + H+]
413.1905; found 413.1904.
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◦
2-(5-tert-Butyl-2-ethynylphenylethynyl)triphenylene (5)
The reaction mixture was stirred at 60 C for 6 h. It was cooled
to room temperature and neutralized with 5% aq HCl solution
(20 mL). The mixture was extracted with CH2Cl2 (2 ¥ 20 mL) and
the organic layer was washed with water (2 ¥ 15 mL). The organic
layer was dried over sodium sulfate and evaporated to dryness. The
crude product was purified by column chromatography over silica
gel using a mixture of hexane and CH2Cl2 (90 : 10, v/v) to yield 1
as a colorless solid (0.180 g, 72%). Mp 217 ◦C (decomp). IR (neat)
2957, 2866, 2190 cm-1; UV-Vis (CH2Cl2): l (log e) 257 (5.044), 267
(5.048), 312 (4.795), 332 (4.771), 361 (4.494). dH NMR (400 MHz,
CDCl3) 8.50 (s, 1 H, 1-H), 8.33-8.40 (m, 3 H), 8.13 (d, J 8.0 Hz, 1
H, 4-H), 8.00 (d, J 8.6 Hz, 1 H, 5-H), 7.62 (d, J 1.6 Hz, 1 H), 7.59
(d, J 8.2 Hz, 1 H) 7.37-7.51 (m, 5 H), 7.20 (dd, J 7.3, 7.9 Hz, 1 H,
6-H), 1.40 (s, 9 H, C(Me)3) ppm; dC NMR (100 MHz, CDCl3)
152.51, 132.49, 129.80, 129.64, 129.53, 129.29, 129.22, 129.00,
128.88, 128.64, 127.33, 127,12, 126.99, 126.91, 126.83, 126.77,
125.35, 123.48, 123.20, 123.20, 122.88, 122.72, 121.96, 121.32,
94.70, 89.13, 81.63, 77.82, 34.98, 31.08 ppm; m/z MALDI-TOF
MS calcd for C64H46, 814; found 813 [M+-1].
To a solution of PPh3 (2.033 g, 7.76 mmol) in dry CH2Cl2
(50 mL) zinc powder (578 mg, 7.76 mmol) was added and the
reaction mixture was cooled to 0 C. CBr4 (1.288 g, 3.88 mmol)
◦
was added portionwise over 15 min. The reaction mixture was
brought to room temperature and stirring was continued for
3 h. Aldehyde 4 (400 mg, 0.97 mmol) was added and stirring
was continued for 8 h. The reaction mixture was filtered and
the residue was washed with CH2Cl2. The washings along with
the filtrate were evaporated under reduced pressure. The crude
product was purified by column chromatography over silica gel
using a mixture of hexane and CH2Cl2 (95 : 5, v/v) to yield
a colorless solid which contained triphenylphosphine and the
desired dibromovinyl derivative. The white solid was dissolved
in CH2Cl2 (50 mL) and excess methyl iodide (5 mL) was added
to form methyltriphenylphosphonium iodide. The mixture was
stirred overnight. The solvent was removed and crude product was
purified by column chromatography over silica gel using a mixture
of hexane and CH2Cl2 (95 : 5 v/v) to yield the dibromovinyl
derivative as a colorless solid (469 mg, 85%). 2-(5-tert-Butyl-2-
Synthesis of model compound 6
(1,1-dibromovinyl)phenylethynyl)-triphenylene. Mp: 80–82 ◦C. IR
-1
≡
(neat) 2958, 2191 (C C) cm . dH (400 MHz, CDCl3) 8.86 (d, J
To a stirred solution of a mixture containing 5 (15 mg, 0.03 mmol),
piperidine (3.74 mg, 0.044 mmol), CuBr (0.28 mg, 0.002 mmol),
and hydroxylamine hydrochloride (2.08 mg, 0.03 mmol) in
methanol (10 mL) under nitrogen atmosphere at room
temperature was added a degassed solution of phenylbromoethyne
(7, 8.0 mg, 0.04 mmol) in methanol (3 mL) over a period of
15 min. The mixture was stirred for 3 h at room temperature
and then heated for 15 h at 60 ◦C. Solvent was removed under
reduced pressure in the rotary evaporator, and the crude product
was poured into ice cold water (10 mL) and extracted with
dichloromethane (2 ¥ 5 mL). The organic extract was washed
with saturated brine solution (5 mL). The organic layer was dried
over Na2SO4 and solvent was removed under reduced pressure.
The crude product was purified by column chromatography over
silica gel using a mixture of hexane and CH2Cl2 (95: 5, v/v)
to yield 6 as a white solid (8 mg, 45%). In this◦reaction the
1.6 Hz, 1 H, 1-H), 8.62-8.68 (m, 5 H), 7.97 (s, 1 H), 7.78-7.82 (m,
2 H), 7.66-7.70 (m, 4 H), 7.42 (dd, J 2.4, 8.0 Hz, 1 H, 4-H phenyl),
1.38 (s, 9 H, C(Me)3) ppm; dC NMR (100 MHz, CDCl3) 151.63,
135.81, 134.53, 130.12, 130.01, 129.81, 129.78, 129.30, 129.14,
129.07, 127.75, 127.69, 127.64, 127.43, 127.40, 126.88, 125.56,
123.54, 123.42,123.39, 123.33, 122.46, 121.64, 94.99, 90.68, 88.64,
34.81, 31.10 ppm; m/z MALDI-TOF MS calcd for C32H24Br2,
568; found 567 [M+-1]. An oven dried Schlenk flask was charged
with the dibromovinyl derivative (300 mg, 0.53 mmol) and dry
THF (30 mL). The solution was cooled to -78 ◦C and LDA (1.59
mmol) [freshly prepared from n-BuLi (1.59 mmol, 1.3 mL of 1.2 M
solution in hexane) and diisopropylamine (161 mg, 0.22 mL)] was
added and stirring was continued for 1 h. Upon completion, the
reaction mixture was quenched with saturated NH4Cl solution
(20 mL) at -78 ◦C. The reaction mixture was extracted with
CH2Cl2 (30 mL). The organic layer was washed with water (2 ¥
30 mL) and dried over anhydrous sodium sulfate. The solvent
was evaporated to dryness and crude product was purified by
column chromatography over silica gel using a mixture of hexane
and CH2Cl2 (95 : 5, v/v) to yield 5 as a colorless solid (0.149 g,
dimer (1, 7 mg) was also isolated. 6: Mp: 169–171 C. IR (neat)
-1
≡
2961, 2867, 2216 (C C) cm . dH NMR (500 MHz, CDCl3) 8.95
(d, J 1.0 Hz, 1 H, 1-H), 8.63-8.70 (m, 5 H), 7.89 (dd, J 2, 9.0 Hz,
1 H, 3-H), 7.67-7.69 (m, 3 H), 7.55-7.61 (m, 4 H), 7.34-7.41(m,
5 H) 1.35 (s, 9 H, C(Me)3) ppm; dC NMR (100 MHz, CDCl3)
152.52, 132.68, 132.51, 130.12, 130.02, 129.92, 129.77, 129.74,
129.34, 129.22, 129.12, 128.76, 128.46, 127.62, 127.49, 127.43,
127.35, 126.77, 125.57, 123.55, 123.45, 123.42, 123.36, 123.21,
121.93, 121.83, 121.73, 94.10, 89.22, 82.39, 80.62, 74.34, 34.98,
31.04 ppm; m/z MALDI-TOF MS calcd for C40H28 508; found
508 [M+, 100%], 509 [M++1, 30%], 510 [M++2, 5%], 493 [M+-CH3,
10%].
◦
-1
≡
69%). Mp 73–76 C. IR (neat) 3283, 2958, 2222 (C C) cm .
UV-Vis (CH2Cl2) l (log e) 259 (4.726), 285 (4.503), 310 (4.348),
324 (4.442), 343 (4.340); dH NMR (400 MHz, CDCl3) 8.87 (d, J
1.4 Hz, 1 H, 1-H), 8.61-8.68 (m, 5 H), 7.83 (dd, J 1.5, 8.6 Hz, 1
H, 3-H), 7.65-7.70 (m, 5 H), 7.51 (d, J 8.2 Hz, 1 H, 3-H phenyl),
7.35 (dd, J 2.0, 8.2 Hz, 1 H, 4-H phenyl) 3.40 (s, 1 H, acetylenic
H), 1.36 (s, 9 H, C(Me)3) ppm; dC NMR (100 MHz, CDCl3)
152.04, 132.45, 130.10, 129.99, 129.72, 129.69, 129.35, 129.14,
128.97, 127.61, 127.57, 127.36, 126.98, 125.89, 125.49, 123.52,
123.40, 123.36, 123.31, 121.95, 121.81, 93.26, 89.27, 82,44, 80.50,
34,85, 31.07 ppm; m/z [%] MALDI-TOF MS calcd for C32H24,
408; found 408 [M+, 100], 409 [M++1, 23], 410 [M++2, 5].
Acknowledgements
We thank IIT Madras for a fellowship to RN; SAIF, IIT Madras
for spectroscopic data and DST, New Delhi for financial support;
Prof. P. Ramamurthy, Dr C. Selvaraju and K. Duraimurugan
NCUFP, University of Madras for time resolved measurements
and Prof. A. K. Mishra, Department of Chemistry, IIT Madras
for useful discussions.
Synthesis of 1
Cu(OAc)2·H2O (486 mg, 2.44 mmol) was dissolved in acetonitrile
(28 mL) and pyridine (7 mL) and 5 (250 mg, 0.61 mmol) was added.
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