Synthesis, absorption, and fluorescence-emission properties of 1,3,6,8-tetraethynylpyrene and its derivatives

Article abstract of DOI:10.1002/ejoc.200500222

The synthesis of several 1,3,6,8-tetraethynylpyrene derivatives is reported. The effect of extended acetylenic conjugation on their absorption and fluorescence-emission properties is studied. Significant bathochromic shifts of both the absorption and fluorescence emission bands are observed. These derivatives emit fluorescence in the visible (400-550 nm) region. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejoc.200500222

FULL PAPER
Synthesis, Absorption, and Fluorescence-Emission Properties of 1,3,6,8-
Tetraethynylpyrene and Its Derivatives
Gandikota Venkataramana[a] and Sethuraman Sankararaman*[a]
Keywords: Alkynes / Conjugation / Fluorescence / Quantum yield / Pyrene
The synthesis of several 1,3,6,8-tetraethynylpyrene deriva-
tives is reported. The effect of extended acetylenic conjuga-
tion on their absorption and fluorescence-emission properties
is studied. Significant bathochromic shifts of both the absorp-
tion and fluorescence emission bands are observed. These
derivatives emit fluorescence in the visible (400–550 nm) re-
gion.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
jugation by introducing unsaturated functional groups to
the core of the fluorophore. One such group is the acetyle-
nic group. In a recent paper the absorption and fluorescence
emission properties of the dimer of 1-ethynylpyrene, namely
1,4-bis(1-pyrenyl)butadiyne, have been reported,[5] and
polymers of 1-ethynylpyrene and 1-trimethylsilylethynylpyr-
ene have also been reported.[6] These polymers exhibit high
thermal stability and absorb and emit in the visible region.
Crystalline oligopyrene nanowires exhibiting multicolored
emission have also been reported.[7] In the present study we
have used acetylenic groups to extend the conjugation of
the pyrene chromophore. We describe the synthesis of
1,3,6,8-tetraethynylpyrene and several of its derivatives
bearing both hydrophilic and hydrophobic substituents and
studies on the electronic absorption and fluorescence emis-
sion properties of these molecules.
Introduction
Pyrene (1) is a prototypical fluorescent molecule with de-
sirable photophysical properties that make it useful as a
fluorescence probe. Pyrene and its derivatives have been
widely used as fluorescence probes in many applications.
For example, pyrene-labeled oligonucleotides have been
used as probes to study DNA hybridization,[1] and pyrene-
labeled lipids have been used to study the depth-dependent
quenching of fluorescence in lipid bilayers.[2] Recently, the
synthesis of a pyrene-based fluorescent dendrimer has been
reported wherein the core unit is a 1,3,6,8-tetrasubstituted
pyrene and the peripheral units contain monosubstituted
pyrene units.[3] In spite of its wide use there are two major
drawbacks in using pyrene as a fluorescence probe. The ab-
sorption and emission wavelengths of the pyrene monomer
are confined to the UV region and pyrene forms an excimer
above concentrations of 0.1 m. In order to probe bio-
logical membranes using fluorescence techniques it is desir-
able to have a fluorophore probe that absorbs and emits in
the longer wavelength region, preferably in the visible re-
gion of the electromagnetic spectrum in order to minimize
the spectral overlap of the intrinsic fluorescence of the bi-
omolecules such as tryptophan, tyrosine, and NADH that
occur in the UV region. Furthermore, molecular systems
that are light emitters in the visible region are potentially
useful in the fabrication of organic light emitting devices
(OLED).[4] Therefore it is desirable to design molecules that
have emission in the visible region. The most common
method to bathochromically shift the absorption and emis-
sion characteristics of a fluorophore is to extend the π-con-
Results and Discussion
1,3,6,8-Tetrabromopyrene (2)[8] is readily obtained by the
exhaustive bromination of pyrene (Scheme 1), and it served
as the starting material for the synthesis of 1,3,6,8-tetraeth-
ynylpyrene derivatives.
Scheme 1. Synthesis of 1,3,6,8-tetrabromopyrene. Reagents and
conditions: (a) Br2, nitrobenzene, 120 °C, 94% (2).
[a] Department of Chemistry, Indian Institute of Technology Ma-
dras,
Chennai 600036, India
Fax: +91-44-2257-0545
E-mail: sanka@iitm.ac.in
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
The Sonogashira coupling of tetrabromide 2 with various
terminal acetylenes yielded the corresponding tetraethynyl
derivatives 3a–f in moderate to good yields (Scheme 2).
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200500222
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1,3,6,8-Tetraethynylpyrene and Its Derivatives
FULL PAPER
Figure 1. MALDI-TOF mass spectra showing the molecular ion
region of 3e (A) and 3c (B) and the isotope peaks.
In order to utilize tetraethynylpyrene (3g) as an acetyle-
nic building block to extend the conjugation of the pyrene
chromophore we investigated the Sonogashira coupling of
3g. The coupling of 3g and 1-bromo-2-(trimethylsilylethyn-
yl)benzene (4) gave the expected tetrasubstituted derivative
5, albeit in poor yield (10%), along with an insoluble red
brown solid. Iodoarenes are generally more reactive in the
Sonogashira coupling reaction.[10] However, in the present
case when 1-iodo-2-(trimethylsilylethynyl)benzene was used
instead of 4 no significant improvement in the yield of 5
was observed (Scheme 4).
Scheme 2. Synthesis of 1,3,6,8-tetraethynylpyrene derivatives (3a–
f). Reagents and conditions: (a) [PdCl2(PPh3)2], diisopropylamine,
PPh3, CuI, 60–70 °C.
The tetraethynyl derivatives 3a–f possess D2h point-group
1
symmetry; therefore their H NMR spectra are simple and
display a pair of singlets in a 2:1 ratio for the pyrene ring
protons. The four protons on carbon atoms 4, 5, 9, and 10
appear as a singlet in the region of δ = 8.4–8.6 ppm and the
two protons on carbon atoms 2 and 7 appear as a singlet
in the region of δ = 8.2–8.3 ppm. The products arising from
partial substitution, namely di- and trisubstituted deriva-
tives, give a complex multiplet pattern for the pyrene ring
1
protons in the H NMR spectrum and are readily discerni-
ble from the tetrasubstituted derivatives.[9] The tetrakis(tri-
methylsilylethynyl) derivative 3a was obtained as an orange
solid in 33% yield. Removal of the TMS groups by treat-
ment with tetra-n-butylammonium fluoride in THF yielded
tetraethynylpyrene 3g as a pale-yellow solid in 93% yield
(Scheme 3).
Scheme 4. Extention of conjugation of pyrene using 3g as the build-
ing block. Reagents and conditions: (a) [PdCl2(PPh3)2], diisopro-
pylamine, PPh3, CuI, 60–70 °C.
The electronic absorption and fluorescence-emission
data along with the quantum yield of fluorescence for com-
pounds 3a–g and 5 are listed in Table 1 and are compared
with that of pyrene (1).[11] The spectra were recorded in
THF, typically in the concentration range of 10–5–10–6 .
Therefore, the reported fluorescence data correspond to
only the monomer emission. The solutions were degassed
Scheme 3. Synthesis of 1,3,6,8-tetraethynylpyrene (3g). Reagents
and conditions: (a) nBu4NF, THF, room temp., 1 h, 93% (3g).
Tetraethynylpyrene (3g) is a stable solid that can be by purging with dry N2 prior to quantum yield measure-
stored at room temperature in air for a prolonged period of ments. The fluorescence intensity of compounds 3a–g was
time. While 3a is soluble in all the common organic sol- unaffected by the presence of atmospheric oxygen as the
vents, including hexane, compound 3g is rather insoluble. fluorescence intensities of 3a–g are the same before and af-
The NMR spectra of 3g were recorded in [D8]THF. Deriva- ter degassing.
tives 3b–f are also highly insoluble in common organic sol-
Typically, three absorption bands were observed in the
vents. Nevertheless, the compounds were thoroughly char- region 350–450 nm for derivatives 3a–d. The corresponding
acterized by various spectroscopic techniques. In particular, bands for 3e and 3f are bathochromically shifted to the re-
the MALDI-TOF mass spectra of these derivatives display gion 375–475 nm (Figure 2). While derivatives 3a–d show
the molecular ion peak along with the isotope peaks in ex- well-resolved sharp absorption bands, those of 3e and 3f
pected intensity ratios (Figure 1).
are broad and less resolved. The longest wavelength π–π*
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G. Venkataramana, S. Sankararaman
Table 1. Electronic absorption, fluorescence emission, and quantum yield of fluorescence of 3a–g and 5.[a]
FULL PAPER
Compound
Absorption
Fluorescence emission
λmax [nm] (λex)
Φf[c]
[b]
λmax [nm] (log ε [–1 cm–1])
1
3a
3b
3c[d]
3d
3e
3f
335 (5.10), 319 (5.05), 306 (4.98), 288 (5.05), 272 (5.13), 261 (5.08), 239 (5.34)
435 (5.19), 409 (5.05), 387 (4.85), 314 (5.20), 302 (5.01), 289 (4.94), 256 (5.02), 237 (5.08)
420 (3.71), 358 (4.17), 341 (4.22), 303 (4.34), 290 (4.37), 243 (4.70)
420 (4.72), 395 (4.62), 374 (4.45), 307 (4.70), 296 (4.57), 257 (4.59)
424 (5.13), 398 (5.02), 377 (4.85), 308 (5.17), 295 (5.01), 254 (5.07)
462 (4.77), 437 (4.70), 338 (4.97), 240 (4.96)
393, 389 (sh), 384, 379, 373, (336)
498, 465, 439, (389)
0.60
0.42
0.49
0.68[d]
0.36
0.18
0.16
0.59
nd
438 (sh), 416, (358)
481 (sh), 450, 425, (375)
485, 455, 427, (410)
552 (sh), 512, 480, (485)
552 (sh), 514, 482, (375)
472 (sh), 442, 418, (374)
558 (sh), 521, 492, (349)
465 (4.42), 438 (4.36), 339 (4.71), 249 (4.65)
414 (4.60), 390 (4.47), 370 (4.21), 305 (4.55), 289 (4.53), 252 (4.33), 238 (4.53)
476 (4.74), 446 (4.59), 420 (sh), 351 (4.87)
3g
5
[a] Spectra were measured for 10–5–10–6  solutions in THF. [b] wavelength of excitation. [c] Using anthracene (Φf = 0.31) as the fluores-
cence standard. [d] In DMSO. sh = shoulder, nd = not determined.
bands of pyrene derivatives 3a–g are bathochromically the excited singlet state. Furthermore, both the absorption
shifted by 85–130 nm in comparison to pyrene[11] due to the and emission bands of the phenyl-substituted derivatives 3e
extended conjugation of the pyrene chromophore with the and 3f are much broader than those of 3a–d (Figure 2 and
acetylenic units. The phenylethynyl derivatives 3e and 3f Figure 3).
show the largest bathochromic shift of the absorption
bands due to the extension of conjugation with the pendent
phenyl groups. Similarly, the fluorescence emission bands
of 3a–g are also bathochromically shifted in comparison to
that of pyrene.[11] For example, compared to the S0,0 emis-
sion band of pyrene at 373 nm, the S0,0 emission band of
3e and 3f occur at 480 and 482 nm, respectively, a shift of
nearly 90 nm. The lowest energy emission bands of these
derivatives occur at 552 nm, well into the visible region, and
are about 150 nm bathochromically shifted compared to
that of pyrene. The fluorescence spectra are independent of
the excitation wavelength. The quantum efficiency of fluo-
rescence was determined using anthracene as the standard
fluorophore in THF. The quantum efficiency of fluores-
cence emission for 3a–d and 3g was in the range of 0.4–0.7;
Figure 3. Fluorescence emission spectra: (A) 3a (----) and 3e (___);
(B) 3d (----) and 3f (___) in THF.
these values are comparable to that of pyrene. However, the
quantum efficiencies of fluorescence for 3e and 3f are 0.18
and 0.16, respectively, much lower than that of the other
derivatives. In the case of 3a–d the chromophore, namely
Conclusions
The extension of conjugation of the pyrene chromophore
by acetylenic substituents has effectively served to shift the
wavelength of absorption and fluorescence emission into
the visible region of the electromagnetic spectrum. The
quantum efficiencies of fluorescence of the tetraethynyl de-
rivatives are comparable to that of pyrene, except in the
case of the phenyl-substituted derivatives 3e and 3f, whose
fluorescence quantum efficiencies are low due to the deacti-
vation of the excited state resulting from the free rotation
of the phenyl groups. These derivatives emit fluorescence in
the visible region and hence they are potentially useful as
organic light emitting materials in the fabrication of light
emitting devices.
tetraethynylpyrene, is highly rigid and hence the competing
nonradiative pathways do not occur efficiently. In the case
of 3e and 3f the free rotation of the phenyl substituents that
are in conjugation with the pyrene chromophore is possible,
allowing competing nonradiative pathways for the decay of
Experimental Section
General Remarks: 1H and 13C NMR spectroscopy: Bruker AM-400
or Jeol GSX 400 NB at 400 MHz and 100 MHz, respectively; TMS
as internal standard. Mass spectrometry: MALDI-TOF MS with
an Applied Biosystems Voyager 6316 spectrometer using either α-
cyano-4-hydroxycinnamic acid or tetracyanoethylene as the matrix;
Figure 2. Electronic absorption spectra: (A) 3a (-----) and 3e (___);
(B) 3d (----) and 3f (___) in THF.
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1,3,6,8-Tetraethynylpyrene and Its Derivatives
FULL PAPER
ESI-MS with a Micromass Micro Q-TOF spectrometer; EI-MS
with a Finnigan MAT 90X spectrometer. IR spectra were recorded
with a Perkin–Elmer Spectrum One spectrometer. UV/Vis spectra
were recorded with a Cary 5E Varian UV/Vis/NIR spectrometer.
All the fluorescence measurements were made with a Jobin Yvon
Horiba Fluorolog-3 spectrometer. The fluorescence quantum yields
were measured using anthracene as the standard according to a
literature procedure.[12] Although atmospheric oxygen did not af-
fect the fluorescence intensity of compounds 3a–g, the solutions
were degassed prior to fluorescence measurements. The quantum
yields were corrected for the refractive indices of the solvents used.
All reactions were carried out under dry nitrogen in oven-dried
Schlenk flasks, unless indicated otherwise. Column chromatog-
raphy was performed on silica gel (60–120 or 240–400 mesh). TLCs
were run on Macherey–Nagel polygram sil G/UV254 plates. All the
compounds reported herein did not melt up to 200 °C and the tem-
perature for the onset of decomposition, as determined by thermo-
gravimetric analysis, is given in the individual cases. THF was dis-
tilled from sodium.
(ESI-MS, MeOH/H2O): calcd. for C36H34NaO4 553.2355; found
553.2393 [M + Na+]; calcd. for C36H35O5 513.2430; found 513.2451
[M + OH].
1,3,6,8-Tetrakis(3-hydroxy-1-propynyl)pyrene
(3c):
Propargyl
alcohol (0.65 g, 11.6 mmol) was used and the reaction was carried
out at 70 °C for 42 h. The crude product was triturated with CHCl3
(25 mL) and the yellow solid obtained was filtered and washed with
water and dried to give 3c as a pale-yellow solid (0.52 g, 65%).
Temperature for onset of decomposition: 200 °C. IR (KBr): ν =
˜
1
3272 cm–1 (OH), 2218 (CϵC). H NMR ([D6]DMSO): δ = 8.61 (s,
4 H), 8.19 (s, 2 H), 5.60 (t, J = 5.7 Hz, 4 H, OH) 4.56 (d, J =
5.7 Hz, 8 H) ppm. 13C NMR ([D6]DMSO): δ = 130.9, 126.5, 118.4,
115.0, 109.4, 97.3, 81.1, 49.7 ppm. MALDI-TOF MS: m/z (%) =
418 (100) [M+], 419 (65) [M+ + 1], 420 (20) [M+ + 2].
1,3,6,8-Tetrakis(3,3-diethoxy-1-propynyl)pyrene
(3d):
Toluene
(20 mL) was used instead of THF. Propiolaldehyde diethyl acetal
(1.23 g, 9.65 mmol) was used and the reaction was stirred at 70 °C
for 24 h. The crude product was dissolved in CHCl3 and washed
with water (75 mL) followed by saturated brine solution (75 mL).
After removal of the solvent, the product was further purified by
column chromatography on silica gel using hexane/ethyl acetate
(9:1, v/v) to give 3d as a yellow solid (0.62 g, 45%). Further purifi-
cation was done by recrystallization from CHCl3. Temperature for
1,3,6,8-Tetrabromopyrene (2):[8] Bromine (35.0 g, 0.22 mol) was
added dropwise, with vigorous stirring, to a solution of pyrene (1;
10.0 g, 0.049 mol) in nitrobenzene (200 mL) at 120 °C. The mixture
was kept at 120 °C for 4 h and then cooled to room temperature
to yield a pale-green precipitate. This was filtered, washed with
ethanol (150 mL), and dried under vacuum. The solid product
(24.1 g, 94%) was insoluble in all the common organic solvents. It
was identified as 2 by EI-MS data, which display the isotope peaks
in the expected ratio. The product was used as such for the Sonoga-
shira coupling reactions. MS (EI, 70 eV): m/z (%) = 522 (12), 520
(50), 518 (70), 516 (48), 514 (12) [M+] (isotope peaks in the ratio
1:4:6:4:1), 441, 439, 438, 437, 435 [M+ – Br] (isotope peaks), 360
(19), 358 (38), 356 (19) [M+ – 2 Br], 198 (100) [M+ – 4 Br].
onset of decomposition: 238 °C. IR (KBr): ν = 2222 cm–1 (CϵC).
˜
1H NMR (CDCl3): δ = 8.53 (s, 4 H), 8.26 (s, 2 H), 5.63 (s, 4 H)
3.88 (m, 4 H), 3.74 (m, 4 H), 1.28 (t, J = 7.0 Hz, 24 H) ppm. 13C
NMR (CDCl3): δ = 134.5, 132.2, 127.1, 123.5, 117.6, 92.0, 91.2,
82.9, 61.2, 15.1 ppm. MALDI-TOF MS: m/z (%) = 706 (100) [M+],
707 (65) [M+ + 1], 708 (25) [M+ + 2]. ESI-MS: m/z (%) = 729.5
(82) [M + Na+], 661.4 (58) [M+ – OC2H5].
1,3,6,8-Tetrakis(phenylethynyl)pyrene (3e): Phenylacetylene (0.89 g,
8.68 mmol) was used and the reaction mixture was heated at 70 °C
for 16 h. The yellow precipitate was filtered and washed with
CHCl3 (75 mL) and benzene (100 mL) and dried to yield 3e as a
yellow powder (1.08 g, 93%). Temperature for onset of decomposi-
General Procedure for the Sonogashira Coupling of 2: Compound 2
(1.0 g, 1.93 mmol), [PdCl2(PPh3)2] (67 mg, 0.096 mmol), CuI
(18 mg, 0.096 mmol), PPh3 (50 mg, 0.193 mmol), and the terminal
alkyne (11.6 mmol) were added to a degassed solution of diisopro-
pylamine (20 mL) and THF (20 mL) under N2. The resulting mix-
ture was stirred at 60–70 °C for the time mentioned in the individ-
ual cases. The reaction mixture was then cooled to room tempera-
ture and solvent was removed to give the crude reaction mixture,
which was further worked up as indicated in the individual cases.
tion: 375 °C. IR (KBr): ν = 2200 cm–1 (CϵC). MALDI-TOF MS:
˜
m/z (%) = 602 (100) [M+], 603 (70) [M+ + 1], 604 (20) [M+ + 2].
HRMS (EI, 70 eV): calcd. for C48H26 602.20345; found 602.20505.
1,3,6,8-Tetrakis{[4-(trifluoromethyl)phenyl]ethynyl}pyrene (3f): Tri-
ethylamine (10 mL) was used instead of diisopropylamine. 4-(Tri-
fluoromethylethynyl)benzene (0.33 g, 1.93 mmol) was used and the
reaction was carried out at 70 °C for 10 h. The orange precipitate
was filtered and washed with CHCl3 (50 mL) followed by water
(50 mL) and dried to yield 3f (0.31 g, 92%). Temperature for onset
1,3,6,8-Tetrakis(trimethylsilylethynyl)pyrene
(3a):
Trimethyl-
silylacetylene (1.14 g, 11.57 mmol) was used and the reaction was
carried out at 65 °C for 12 h. Column chromatographic purification
of the crude product on silica gel with hexane as the eluent yielded
of decomposition: 273 °C. IR (KBr): ν = 2206 cm–1 (CϵC). 1H
˜
3a (0.38 g, 33%) as a red orange solid. Temperature for onset of
1
decomposition: 325 °C. IR (KBr): ν = 2958, 2152 cm–1 (CϵC). H
NMR (CD3COCD3): δ = 8.83 (s, 4 H), 8.54 (s, 2 H), 7.99 (d, J =
8.2 Hz, 8 H), 7.84 (d, J = 8.2 Hz, 8 H). MALDI-TOF MS: m/z (%)
= 874 (100) [M+], 875 (80) [M+ + 1]. HRMS (EI, 70 eV): calcd. for
C52H22F12 874.15298; found 874.15250.
˜
NMR (CDCl3): δ = 8.53 (s, 4 H), 8.28 (s, 2 H), 0.39 (s, 36 H) ppm.
13C NMR (CDCl3): δ = 134.5, 131.9, 126.8, 123.4, 118.5, 102.8,
101.3, 0.1 ppm. MS (EI, 70 eV): m/z (%) = 586 (20) [M+], 490 (100),
179 (70), 73 (55). HRMS: calcd. for C36H42Si4 586.23636; found
586.23571.
1,3,6,8-Tetraethynylpyrene (3g): A red orange solution of 3a (0.5 g,
0.85 mmol) in degassed THF (30 mL) was treated with nBu4NF
2- (26 mg, 0.085 mmol) and the resulting mixture was stirred at 30–
35 °C for 1 h. The reaction mixture was then poured into ice cold
water (100 mL) and the solid obtained was filtered, washed with
water (2×100 mL), and dried to yield 3g (0.24 g, 93%) as a pale-
1,3,6,8-Tetrakis(3-hydroxy-3-methyl-1-butynyl)pyrene
(3b):
Methyl-3-butyn-2-ol (1.62 g, 19.3 mmol) was used and the reaction
was carried out at 60 °C for 16 h. The crude product was purified
by column chromatography on silica gel using a mixture of hexane/
acetone (4:1, v/v) as the eluent to yield 3b (0.92 g, 89%) as a yellow
solid. Temperature for onset of decomposition: 175 °C. IR (KBr):
yellow solid. Temperature for onset of decomposition: 350 °C. IR
1
(KBr): ν = 3280, 2099 cm–1 (CϵC). H NMR ([D8]THF): δ = 8.68
˜
ν = 3291 cm–1 (OH), 2224 (CϵC). 1H NMR ([D6]DMSO): δ = 8.43
(s, 4 H), 8.34 (s, 2 H), 4.28 (s, 4 H) ppm. 13C NMR ([D8]THF): δ
˜
(s, 4 H), 8.08 (s, 2 H), 5.76 (s, 4 H, OH) 1.70 (s, 24 H) ppm. 13C = 135.5, 132.6, 127.6, 123.5, 119.2, 86.1, 81.7 ppm. MS (EI, 70 eV):
NMR ([D6]DMSO): δ = 132.6, 130.4, 126.1, 122.6, 118.2, 103.0,
m/z (%) = 298 (45) [M+], 274 (100). HRMS: calcd. for C24H10
78.1, 64.0, 31.6 ppm. MALDI-TOF MS: m/z = 530 [M+]. HRMS 298.07825; found 298.07687.
Eur. J. Org. Chem. 2005, 4162–4166
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G. Venkataramana, S. Sankararaman
FULL PAPER
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1,3,6,8-Tetrakis{[2-(trimethylsilylethynyl)phenyl]ethynyl}pyrene (5):
1-Bromo-2-(trimethylsilylethynyl)benzene
(4)[13]
(4.58 g,
18.1 mmol) and tetraethynylpyrene 3g (0.9 g, 3.0 mmol) were
stirred at 70 °C for 15 h. The crude product was purified by column
chromatography on silica gel using 10% ethyl acetate/hexane (v/v)
as the eluent. The product 5 was obtained as a dark-red solid (0.3 g,
10%). IR (KBr): ν = 2156 cm–1 (CϵC). 1H NMR (CDCl3): δ =
˜
8.91 (s, 4 H), 8.53 (s, 2 H), 7.71–7.59 (m, 8 H), 7.40–7.32 (m, 8 H),
0.29 (s, 18 H), 0.05 (s, 18 H) ppm. 13C NMR (CDCl3): δ = 137.8,
132.7, 131.9, 129.7, 129.0, 128.6, 128.3, 127.4, 125.8, 125.2, 119.0,
103.6, 99.2, 94.8, 91.5, 1.0, 0.1 ppm. MALDI-TOF MS: m/z = 986
[M+], 971 [M+ – Me].
Acknowledgments
We gratefully acknowledge financial support from the Volkswagen
Stiftung, Germany, and CSIR, New Delhi. We thank Prof. T. Pra-
deep and Mr. D. M. David Jaba Singh, IIT Madras, for recording
the MALDI-TOF MS data, Dr. U. Papke, TU Braunschweig, Ger-
many, for recording the HRMS data, and Ms. T. Shyamala, IIT
Madras, for fluorescence measurements. We thank Prof. H. Hopf,
TU Braunschweig, Germany, and Prof. A. K. Mishra, IIT Madras,
for useful discussions.
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Published Online: August 8, 2005
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