Synthesis and electrogenerated chemiluminescence of donor-substituted phenylethynylcoumarins

Article abstract of DOI:10.1021/jo0493424

Two series of donor-bearing phenylethynylcoumarins have been synthesized, and their photophysical properties have been evaluated. Chemiluminescence was observed through the annihilation of their electrogenerated radical ions and was found to be only sligh

Full text of DOI:10.1021/jo0493424

Synthesis and Electrogenerated Chemiluminescence of
Donor-Substituted Phenylethynylcoumarins
Arumugasamy Elangovan, Jui-Hsien Lin, Shu-Wen Yang, Hsien-Yi Hsu, and Tong-Ing Ho*
Department of Chemistry, National Taiwan University, Taipei-106, Taiwan, Republic of China
hall@ntu.edu.tw
Received April 20, 2004
Two series of donor-bearing phenylethynylcoumarins have been synthesized, and their photophysical
properties have been evaluated. Chemiluminescence was observed through the annihilation of their
electrogenerated radical ions and was found to be only slightly affected by the presence of various
donor groups on the phenyl moiety linked through the C-C triple bond. The overall properties of
the two series of compounds are discussed with respect to their structures. The observed electronic
absorption properties are explained with the help of computational studies.
Introduction
interested in studying the effect of the extension of
conjugation in coumarin compounds via introduction of
a C-C triple bond with an electron push directed toward
the coumarinyl moiety on ECL and other photophysical
properties. Coumarin was chosen because of its excellent
Electrogenerated chemiluminescence (ECL) is becom-
ing increasingly important due to its potential application
in immunoassay. ECL is considered advantageous be-
1
cause it does not require a light source for excitation.
Although a wide variety of organic molecules have been
found to exhibit ECL,1 most of them exhibit emission
from the annihilation of their radical ions produced at
the vicinity of electrodes in the presence of a co-reactant,
especially tertiary amines. To study the ECL behavior
of highly fluorescent laser dyes is of interest in view of
their excellent fluorescence properties. The major re-
quirements for compounds to be suitable for ECL include
high fluorescence quantum yield, the electrochemical
stability - that of radical anions and radical cations, and
the efficiency of radical ion-annihilation and energy
4
photochemical and photophysical nature especially in
4e
biological activity as well as in organic light-emitting
b-f
4f,g
devices (OLED).
Despite remarkable properties and
uses, there are no reports on the ECL properties of the
ethynyl compounds of coumarins, although two reports
are available on the syntheses and photophysical proper-
5
ties and one is available on the incorporation in the
6
polymer backbone. For the present study, we chose two
different positions of linkage, 3-coumarinyl and 7-cou-
marinyl derivatives. The donor groups were varied by
substituting the p-hydrogen of the phenylethynylcou-
i
2
marins with Me, OMe, O Pr, and NMe groups. These
2
transfer. Park and Bard have studied for the first time
groups have been chosen to study the effect of increasing
donor strength in that order as well as of the introduction
of steric hindrance at the donor site on the optical and
ECL properties of the resultant molecules. We have
observed that the OMe-substituted phenylquinolinyl-
ethynes showed excimer ECL due to the low twist angle,
the electrochemical behavior and ECL of selected laser
dyes, coumarin 2, coumarin 30, oxazine 1, Nile Blue,
Rhodamin B, and Rhodamine 6G, which are well-known
fluorescent molecules.
In our continued endeavor of finding new molecules
for the ECL and for light-emitting devices, we became
3
2
while the NMe -substituted counterpart exhibited mono-
3
a
meric intramolecular charge-transfer (ICT) ECL. Our
expectation in the present study is that the introduction
(1) References are only comprehensive and not extensive. (a) Richter,
i
M. M. Chem. Rev. 2004, 104, 3003. (b) Kuwana, T. In Electroanalytical
Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1966; Vol. 1,
Chapter 3. (c) Bard, A. J.; Faulkner, L. R. In Electroanalytical
Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol. 10,
p 1. (d) Hercules, D. M. Acc. Chem. Res. 1969, 2, 301. (e) Lai, R. Y.;
Fabrizio, E. F.; Lu, L.; Jenekhe, S. A.; Bard, A. J. J. Am. Chem. Soc.
of the O Pr group, a bulkier group than OMe, would
(4) (a) Reynolds, G. A.; Drexhage, K. H. Opt. Commun. 1975, 13,
222. (b) Maeda, M. Lasers and Dyes; OHM: Tokyo, 1984. (c) Elderfield,
R. C., Ed. Heterocyclic Compounds; John Wiley: New York; 1951; Vol.
2, p 173. (d) Katritzky, A. R., Rees, C. W., Eds. Comprehensive
Heterocyclic Chemistry; Pergamon Press: Oxford, 1984; Vol. 3, Part
2B, p 641. (e) See, for example: Coumarins-Biology, Applications and
Mode of Action; O’Kennedy, R., Thornes, R. D., Eds.; Wiley: Chichester,
1997. (f) Kido, J.; Lizumi, Y. Appl. Phys. Lett. 1998, 73, 2721. (g) Tasch,
S.; Brandst a¨ tter, C.; Meghdadi, F.; Leising, G.; Froyer, G.; Athouel, L.
Adv. Mater. 1997, 9, 33. (h) Muthuramu, K.; Ramamurthy, V. J. Org.
Chem. 1982, 47, 3976.
(5) (a) Schiedel, M.-S.; Briehn, C. A.; B a¨ uerle, P. Angew. Chem., Int.
Ed. 2001, 40, 4677. (b) Mori, A.; Ahmed, M. S. M.; Sekiguchi, A.; Masui,
K.; Koike, T. Chem. Lett. 2002, 756.
(6) Fomine, S.; Fomina, L.; S a` nchez, C.; Ortiz, A.; Ogawa, T. Polym.
J. 1997, 29, 49.
2
001, 123, 9112. (f) Faulkner L. R.; Bard, A. J. Electrogenerated
Chemiluminescence. In Electrochemical Methods; John Wiley & Sons:
New York; 1980; pp 621-627. (g) Oyama M.; Okazaki, S. Anal. Chem.
1
998, 70, 5079. (h) Kapturkievicz, A. J. Electroanal. Chem. 1990, 290,
1
35. (i) Kapturkievicz, A. J. Electroanal. Chem. 1991, 302, 13. (j)
Knight, A. W.; Greenway, G. M. Analyst 1994, 119, 879 and references
therein. (k) Miao, W.; Bard, A. J. Anal. Chem. 2003, 75, 5825.
(
(
2) Park, S. M.; Bard, A. J. J. Electroanal. Chem. 1977, 77, 137.
3) (a) Elangovan, A.; Chen, T.-Y.; Chen, C.-Y.; Ho, T.-I. Chem.
Commun. 2003, 2146. (b) Chen, C.-Y.; Ho, J.-H.; Wang, S.-L.; Ho, T.-
I. Photochem. Photobiol. Sci. 2003, 2, 1232. (c) Elangovan, A.; Yang,
S.-W.; Lin, J.-H.; Kao, K.-M.; Ho, T.-I. Org. Biomol. Chem. 2004, 2,
1
597.
10.1021/jo0493424 CCC: $27.50 © 2004 American Chemical Society
8086
J. Org. Chem. 2004, 69, 8086-8092
Published on Web 10/21/2004

Synthesis and ECL of New Phenylethynylcoumarins
SCHEME 1. Structures of Donor-Substituted
Phenylethynylcoumarins 1-10
Solution-Phase Photophysical Properties. The
UV-visible absorption spectra of all of the compounds
were recorded in acetonitrile solvent (except 5 which was
recorded in CH
long wavelength absorption maxima are given in Table
. From the spectra of 1-5 (Figure 1), it can be seen that
2 2
Cl ) at a concentration of 10 µM, and the
1
there is a minimal and gradual increase in the λmax values
upon increasing the donor strength in both of the series.
The maximum red shift can be observed for 5. Figure 2
shows the UV spectra of 6-10, for which the longest
wavelength absorption appears as a very weak tail
around 330-340 nm. This may be due to the fact that
the donors in compounds 6-10 are not in effective
communication with the coumarinyl acceptor unit as
compared to the 3-coumarinyl systems (1-5), and hence
poor charge-transfer transition was observed for 6-10.
Free coumarin has been known to show an absorption
8
maximum at 310 nm. This intriguing absorption behav-
ior of 7-coumarin systems can be interpreted with the
help of semiempirical calculations. Two molecules from
each family of 3- and 7-isomers, 1, 5, 6, and 10, were
chosen for the study. Structures were built and mini-
mized at the PM3 level, and the various properties and
orbital surfaces were obtained for singlet molecules at
the ground state. Molecule 1 has a planar structure in
s
C symmetry (dihedral angle 0°) with a heat of formation
SCHEME 2
of 38.10 kcal/mol and a dipole moment of 4.06 D. The
HOMO and LUMO levels were found spread all over the
molecule, indicating uniform distribution of π-electrons
throughout the molecule (see Supporting Information for
figures).
s
On the contrary, molecule 6, which also has a C point
group with a heat of formation of 37.35 kcal/mol and a
dipole moment of 4.56 D, is twisted at a dihedral angle
of 90°. The HOMO and LOMO levels are not spread
throughout the molecule but localized on the coumarin
subunit, indicating that there is no contribution from the
phenyl moiety. Upon imposing a dihedral angle con-
straint of 0°, the molecule attains planarity, although
with an infinitesimal change in energy and dipole mo-
ment, and shows orbital distributions similar to those of
1
.
The molecule 5 also has a planar structure in C
1
symmetry with an energy of formation of 34.32 kcal/mol
and a dipole moment of 4.07 D. The HOMO level
indicates partial π-electron distribution at the acceptor
coumarin end with major distribution at the donor-
bearing phenyl group. Molecule 10 too has a twisted
impart a higher twist between the donor and coumarin
acceptor moieties and thus alter the ECL maxima. The
structures of the compounds studied in the present work
are listed in Scheme 1.
1
structure at the ground state in C symmetry with a heat
of formation of 33.54 kcal/mol and a dipole moment of
4.36 D. The dihedral angle between the planes of the
donor-substituted phenyl and coumarinyl acceptor units
was measured to be 90.01° with HOMO localized on the
donor part and LUMO localized on the acceptor coumarin
part. As a result, the donor and acceptor moieties do not
couple effectively and remain electronically detached
from each other. Imposing a dihedral angle constraint
of 0° enhanced the energy of the molecule to 37.77 kcal/
mol and the dipole moment to 6.05 D. The planarized
Result and Discussion
Compounds 1-5 were readily prepared by the modified
Sonogashira coupling of 3-iodocoumarin and donor-
substituted phenylacetylenes, while 6-10 were prepared
5
from 7-iodocoumarin and the same donor-phenylacety-
lenes under a mild reducing atmosphere of hydrogen and
7
argon according to Scheme 2. The synthetic yields and
melting point are summarized along with the photo-
physical data in Table 1.
s
molecule has a C symmetry and shows the HOMO and
LUMO levels similar to the 3-coumarin derivative, with
moderate π-electron distribution over acceptor and donor
moieties, respectively.
(
7) (a) Elangovan, A.; Wang Y.-H.; Ho, T.-I. Org. Lett. 2003, 5, 1841.
b) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
6, 4467.
(
1
J. Org. Chem, Vol. 69, No. 23, 2004 8087

Elangovan et al.
TABLE 1. Synthetic Yield, Melting Point, and Photophysical Data of Compounds 1-10
yield
(%)
λmaxabs (bnm),a
λmaxflu (nm),a
Stokes shift
(nm)
compound
mp (°C)
[ꢀ]
eV
Φc
1
2
3
4
5
6
7
8
9
89
88
85
83
88
86
90
85
85
92
176-177
127-128
162-165
147-148
205-206
74-76
183-186
102-107
112-116
125-129
342 [2.36]
345 [2.81]
353 [2.41]
356 [2.87]
406
416
456
464
0.88
64
71
103
108
144
64
136
157
160
105
0.73
0.55
0.43
d
d
406 [2.44]
550
0.009
0.011
0.016
0.029
0.034
0.009
300 [3.26]
304 [3.03]
308 [3.16]
310 [2.59]
333 [3.63]
364
440
465
470
438
10
a
c ) 10 µM in CH3CN. b (×10 M cm-1). c Measured in CH2Cl2 with reference to coumarin 1 (Φ ) 0.99 in CH3COOEt). d Spectra of
4
-1
compound 5 recorded in CH2Cl2.
FIGURE 3. Fluorescence spectra of 3-coumarin derivatives
-4 in CH CN and 5 in CH Cl (10 µM).
FIGURE 1. UV-visible spectra of 3-coumarin derivatives 1-4
in CH CN and 5 in CH Cl (10 µM).
3 2 2
1
3
2
2
FIGURE 4. Fluorescence spectra of 7-coumarin derivatives
FIGURE 2. UV-visible spectra of 3-coumarin derivatives
-10 in CH CN (10 µM).
6-10 in CH CN (10 µM).
3
6
3
lecular charge transfer from the HOMO of the donor-
bearing phenyl moiety to the LUMO of the coumarin
subunit during electronic absorption.
From these observations, it can be inferred that the
electronic coupling between the donor and the acceptor
is more pronounced in the 3-coumarin derivatives than
in the 7-coumarin derivatives, although the latter ap-
parently has a longer resonance conjugation than the
former. This could also be a reason for observing better
overall properties in the former system of compounds
than in the latter (vide infra). The reason for the
appearance of the strong structureless long-wavelength
absorption band at 333 nm for 10 is due to the intramo-
The fluorescence spectra of 1-4 and 6-10 were
recorded in acetonitrile, while that of 5 was recorded in
CH Cl as it is not fluorescent in highly polar CH CN.
2
2
3
As in the case of absorption spectra, fluorescence spectra
(Figures 3 and 4) also take the similar course of shifting
the maxima of emission to the red upon increasing the
donor strength with 5 showing the longest wavelength
of emission in the series. Comparing the Stokes shift, 1
8088 J. Org. Chem., Vol. 69, No. 23, 2004

Synthesis and ECL of New Phenylethynylcoumarins
-
0.85 to -0.93 V are attributable to the reduction of the
coumarin moiety. The reduction potential values are
closely the same as the acceptor is the same in each
family, with very minor variations caused by the presence
of substituents, but these substituents seem to have no
regular impact on them. The first oxidation peak poten-
tials can be regarded as due to the oxidation at the donor-
bearing phenyl group. The oxidation peak potentials
show a gradual downward trend upon increasing the
donor strength. The unsubstituted phenylethynylcou-
marins have the highest oxidation potentials [1.93 V (for
6
) to 1.95 V (for 1)], while the strongest donor-bearing
compounds show the lowest oxidation peak potentials
0.89 V (for 10) to 0.90 V (for 5)]. The minor reduction in
[
the reduction potential of -0.07 to -0.17 V is indicative
of mild influence from the donor groups.
Electrochemiluminescence. ECL spectra were re-
corded with a sample concentration of 1.0 mM in the
3
presence of TBAP (50 mM) in CH CN with varying pulse
3
FIGURE 5. Cyclic voltammogram of 7 in CH CN (1.0 mM
rates and intervals to obtain an optimum ECL spectra.
The enthalpy change of the radical ion annihilation
with 50 mM TBAP) at 50 mV/s scan rate.
9
and 2 (3-coumarinyl family) bearing H and Me groups
do not show a great shift, whereas their counterparts in
the 7-coumarinyl family, 6 and 7, show a large Stokes
shift. In general, the Stokes shift of the 7-coumarinyl
family is larger than that of the 3-coumarinyl family
reaction (-∆H°) calculated from the equation
-∆H° ) Ep,OX - Ep,RED - 0.16 eV
for radical ion-annihilation reactions, along with the ECL
maxima, are given in Table 2. The unsubstituted cou-
(except 10). In the case of 3-coumarin derivatives, a
stronger donor is required to induce larger Stokes shift.
This indicates that the charge-transfer interaction is
more facilitated with the introduction of stronger donors
than weaker donors in these systems (cf., 2 and 4). In
the case of 7-coumarin derivatives (6-10), there seems
to be a periodical increment in the Stokes shift values
upon increasing the donor strength. Here, the shift is
sensitive to even milder push-pull enhancement (cf., 7
and 9).
10
marin shows a photoluminescence maximum at 363 nm,
whereas ECL shows a maximum at 428 nm under similar
conditions of measurements. Extending conjugation along
the molecular axis with the triple bond coupled with
various donors is found to shift the maxima to the red.
The ECL maxima of 6 seem little affected by the
extension of conjugation. It can be seen from Table 2 that
the position of ECL maxima for 1-4 and 6-9 appear at
4
40-456 nm. Their relative positions of ECL are not
The Φ values for 1-4 are much higher than those of
-10. The emission quantum yield decreases upon
much altered by the presence of substituents unlike the
solution photoluminescence maxima wherein there is a
gradual increase in the shift that ranges from 364 to 470
nm. In this situation, there seems to be no pronounced
role for the donors in these systems. Only for strong donor
group-substituted molecules 5 and 10 are the ECL
maxima shifted considerably. It appears from these
observations that the ECL emitting species emits light
from a state that is different from that of the photolu-
minescent state. It may be a special type of excimer state/
aggregation state. An H-type excimer or a trans excimer
formation (Figure 6) in high concentration can be con-
sidered responsible for this behavior under ECL experi-
mental conditions. It may be noted for comparison that
the excimer emission maximum of the R,ω-biscoumarin
6
increasing the donor strength and is the highest for 1
and lowest for 5 (and for 10). Possible quenching by
intermolecular electron transfer from the N,N-dimethyl-
anilino group to the coumarin of the neighboring molecule
or even the strong excited state ICT itself could be the
reason for this reduction in quantum yield. Surprisingly,
while the fluorescence quantum yields decrease upon
increasing the donor strength in 1-5, it increases in 6-9.
Again, as revealed by the π-electron distribution from
PM3 level calculations, the twisted geometry of the
7
-coumarin family of compounds disturbs the electronic
communication between the donor and acceptor groups,
and this could be one of the reasons for the observed
photophysical properties. This is also supported by the
11
long-chain compound has been observed at 450 nm.
observation that compound 10, which contains an NMe
2
Considering the energy of the ECL emission maxima in
eV and the enthalpy change for the annihilation reaction
group as donor, did in fact show photoluminescence from
MeCN, whereas 5, which also contains the same donor
group, did not.
(
-∆H° in eV), it is clear that the emission is generated
via triplet-triplet annihilation because the electrogen-
erated radical ions do not have sufficient energy to
populate the singlet state. Hence, triplet-triplet an-
Electrochemistry. Cyclic voltammetry was employed
to study the electrochemical behavior and to determine
the reduction and oxidation peak potentials of compounds
1
-10, and a typical CV trace of 7 is shown in Figure 5.
(
8) (a) Lewis, F. D.; Barancyk, S. V. J. Am. Chem. Soc. 1989, 111,
All compounds exhibited reversible first reduction peak
and an irreversible oxidation peak. The peak oxidation
8653. (b) Kim, H.-C.; Kreiling, S.; Greiner, A.; Hampp, N. Chem. Phys.
Lett. 2003, 372, 899.
(
9) Faulkner, L. R.; Tachikawa, H.; Bard, A. J. J. Am. Chem. Soc.
(E
p,OX) and peak reduction (Ep,RED) potentials are given
1
972, 94, 691.
in Table 2. The reduction peak potentials in the range of
(10) Wood, P. D.; Johnston, L. J. J. Phys. Chem. A 1998, 102, 5585.
J. Org. Chem, Vol. 69, No. 23, 2004 8089

Elangovan et al.
TABLE 2. ECL Maxima and Electrochemical Data of 1-10
λmaxECL (nm),
relative
intensity
Ep,OX
(V)
Ep,RED
(V)
-∆H°
(eV)
a
compound
eV
1
2
3
4
5
6
7
8
9
439, 2.82
456, 2.72
438, 2.83
451, 2.75
505, 2.45
456, 2.72
448, 2.77
454, 2.73
455, 2.73
478, 2.59
428, 2.89
0.60
1.95
1.88
1.59
1.55
0.90
1.93
1.74
1.55
1.49
0.89
2.29
-0.93
-0.83
-0.89
-0.92
-0.88
-0.91
-0.89
-0.92
-0.85
-0.91
-1.00
2.72
2.55
2.32
2.31
1.62
2.68
2.47
2.31
2.18
1.64
3.13
0.03
0.20
0.50
0.06
0.008
0.005
0.003
0.004
0.0001
0.096
10
b
coumarin
a
Relative to Ru(bpy)32+ ) 1.0. b Unsubstituted free coumarin.
rescence measurements). In view of the fact that the
configurational twist can occur in solution, which is not
possible in the solid crystalline state, it may be assumed
that the donor tail groups can lie twisted away from the
two π-stacked coumarin acceptor headgroups. Scheme 3
provides a probable pathway by which an H-excimer can
be formed during the annihilation reaction of the radical
ions under ECL conditions. The tendency of the molecules
to undergo π-stacking should naturally favor alignment
of the chromophore to fit in the most comfortable motif.
The motif shown in Figure 6 would be plausible. Similar
FIGURE 6. Proposed structure of the H-type excimer or the
trans excimer.
observations were made with donor-substituted phenyl-
c
4
-quinolinylethynes.3 In addition, it may be noted that
there are a few reports of reduction in the emission
intensity with a blue shift in the maxima of excimer
fluorescence of 9-anthronitrile.14 While there are some
reports of excimer emission of certain alkyl-tethered
bisarenes,15 there are no reports of excimer emission for
free coumarin so as to have substantial correlation
between the observed ECL and photoexcimer emissions
of coumarin. Hence, we prefer to arrive at a modest
inference that the only way by which the chromophores
studied here can exhibit excimer (in ECL) would be by
formation of an H-type or a trans excimer, as shown in
Figure 6, which would also amount to the reduction in
dipole moment and hence the ECL blue shift in compari-
son with photoluminescence. Theoretical treatment of the
FIGURE 7. X-ray crystal structure of 4 with thermal el-
lipsoids drawn at the 50% probability level.
nihilation must have occurred to provide sufficient energy
to generate the ECL emitting state.1a,e,9 It is of interest
to note that the intensities of ECL emission for 1-5 are
very much higher than those for 6-10. Compound 5 does
emit electrochemiluminescence, while it does not emit
photoluminescence in the same solvent. This observation
also supports our argument that the ECL is from a
completely different state as compared to the photolu-
minescent state for this and other systems as well.
Comparing the photoluminescence quantum yield and
relative intensities of ECL, there appears to be close
consistency between the two series in that the 3-coumarin
derivatives are better luminescent materials than 7-cou-
marin derivatives. However, the lesser relative ECL
intensity for some compounds as compared to their
photoluminescence quantum yield could be due to the
formation of (trans) excimer and self-quenching.
(
11) Liu, T.; Wu, S. Wuli Huaxue Xuebao 1996, 12, 677; Chem. Abstr.
1
25: 300343.
(12) Sharma, J.; Singh, P. K.; Singh, K. P.; Khanna, R. N. Org. Prep.
Proced. Int. 1995, 27, 84.
(
13) Iodosalicylaldehyde was prepared according to: Klein, S. I.;
Guertin, K. R.; Spada, A. P.; Pauls, H. W.; Gong, Y.; McGarry, D. G.
U.S. patent, 2001, 6323227. 4-Iodosalicylaldehyde was cyclized with
acetic anhydride to obtain 7-iodocoumarin in 40% yield according to:
Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s
Textbook of Practical Organic Chemistry; Wiley: New York, 1989; p
1040.
(
14) (a) Morsi, S. E.; Carr, D.; El-Bayoumi, M. A. Chem. Phys. Lett.
From the X-ray crystal structure of compound 4 (Figure
), it can be seen that the molecule has a planar
1
978, 58, 571. (b) Ebeid, E. M.; El-Bayoumi, M. A.; Williams, J. O. J.
7
Chem. Soc., Faraday Trans. 1 1978, 74, 1457. (c) Kasha, M.; Rawls,
H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371.
structure, indicating effective conjugation through the
molecular axis. Further, an intermolecular distance of
(
15) (a) Lewis, F. D.; Zhang, Y.; Liu, X.; Xu, N.; Letsinger, R. L. J.
Phys. Chem. B 1999, 103, 2570. (b) Chandross, E. A.; Dempster, C. J.
J. Am. Chem. Soc. 1970, 92, 3586. (c) Lim, E. C. Acc. Chem. Res. 1987,
3
.6 Å was measured between two molecules in the crystal
2
0, 8. (d) DeSchryver, F. C.; Collart, P.; Vandendriessche, J.;
Goedeweeck, R.; Swinnen, A. -M.; Van der Auveraer, M. Acc. Chem.
Res. 1987, 20, 159 and references therein.
unit cell, suggesting pronounced interaction between
π-faces of adjacent molecules16 in a head-on-head tails-
away fashion (Figure 8).
(16) (a) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885. (b)
Coates, G. W.; Dunn, A. R.; Henling, L. M.; Ziller, J. W.; Lobkovsky,
E. B.; Grubbs, R. H. J. Am. Chem. Soc. 1998, 120, 3641. (c) Hunter, C.
A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J. Chem. Soc., Perkin Trans.
2 2001, 651.
This kind of π-stacking in the solid state may be
extended to high-concentration solutions under ECL
-
3
measurements (10 M, 100-folds higher than for fluo-
8090 J. Org. Chem., Vol. 69, No. 23, 2004

Synthesis and ECL of New Phenylethynylcoumarins
FIGURE 8. Head-on-head tails-away mode of crystal packing with the π-stacking interaction viewed along the “b” axis.
SCHEME 3. Probable Mechanism for the
Formation of the H-Excimer
behavior seems to be affected only by the presence of
strong donors such as the NMe group. All compounds
2
are believed to show ECL emission via the triplet-triplet
annihilation pathway as deduced from their lower an-
nihilation reaction enthalpy. No co-reactant was used
throughout to generate ECL. The effect of the twist angle
between the donor and acceptor could not be ascertained
due to the formation of the probable trans excimer whose
ECL emission maxima are vastly independent of the
donor groups in these systems under ECL experimental
conditions. The present study, however, supports our
view that the photoluminescent state and electrochemi-
1
7
luminescent state may be not the same, but different.
Studies on the possible application of these compounds
in light-emitting devices are underway.
ECL state would require expensive calculations as the
species may be a singlet or triplet and an open shell.
Experimental Section
Conclusion
All of the chemicals and reagents were purchased from a
commercial supplier and were used as received. Dicholobis-
Donor-substituted phenylethynylcoumarins have been
synthesized in good yields by the modified Sonogashira
coupling reaction, and the ECL and other photophysical
properties of the ethynes were studied. Compounds 1-5
showed expected UV-vis absorption spectra, whereas
those of 6-10 are quite intriguing as they exhibit the
highest absorbance maxima reminiscent of free coumarin
with very weak long-wavelength absorption regardless
of their apparent longer conjugation. This character has
been analyzed with the aid of the PM3 level of theory,
which predicted planar geometry for 3-isomers (1-5) but
twisted geometry for 7-isomers (6-10). As a result of this
twist, orbital contributions to the electronic absorption
of these molecules were very weak, and the absorption
spectra appeared close to that of free coumarin. While
the photophysical properties showed some trend consis-
tent with the structure-property relationship, the ECL
(
triphenylphosphine)palladium(II) was either prepared in-
1
2
house or from a commercial source. 3-Iodocoumarin and
13
7
-iodocoumarin were prepared according to the reported
methods. Solvents were distilled as per the standard methods
and were purged with argon before use. Triethylamine (TEA)
and tetrahydrofuran (THF) were distilled and purged with a
mixture of approximately 1:1 argon and hydrogen before use.
1
H NMR spectra of the samples were recorded with a 400 MHz
13
instrument, and C NMR spectra were recorded with the same
instrument at 100.1 MHz operator frequency in CDCl
3
solvent
1
with CHCl
3
internal standard (δ 7.24 ppm for H and 77 ppm,
(
17) Supporting evidence: When this paper was under review, a
paper appeared that postulates the possibility of the annihilation
reaction producing excited states other than the photoluminescence
state. See: Ketter, J. B.; Wightman, R. M. J. Am. Chem. Soc. 2004,
1
26, 10183.
18) Spartan’04 Windows; Wavefunction, Inc., 18401 Von Karman
Ave., Suite 370, Irvine, CA 92612.
(
J. Org. Chem, Vol. 69, No. 23, 2004 8091

Elangovan et al.
middle of the three peaks, for 13C spectra). Mass spectra were
recorded on nitrobenzyl alcohol matrix. TLC was run on
precoated aluminum plates (Si 60 F254). Column chromatog-
raphy was run on silica gel (60-120 mesh) or neutral alumina
144.2, 139.3, 131.8, 131.7, 129.0, 127.5, 124.7, 118.9, 118.8,
+
116.6, 113.1, 96.1, 82.7, 21.7. MS (M ) m/z, 260.0838; calcu-
12 2
lated for C18H O , 260.0837. The characterization data for the
rest of the compounds are provided in the Supporting Informa-
tion.
7
0-230 mesh. All UV-visible spectra were recorded for a 10
µM solution of the compounds in CH CN, and all fluorescence
3
Theoretical Calculations. Semiempirical calculations
18
spectra were recorded using similar solution concentrations
unless otherwise specified. CV measurements were done using
a cell consisting of a carbon disk (2.0 mm) working electrode,
a platinum wire counter electrode, and a Ag/AgCl reference
electrode. The scan rates were 50 and 100 mV/s. ECL spectra
were recorded at room temperature using a setup consisting
of a fluorescence spectrophotometer and a cyclic voltammo-
graph with a computer interface. Typically, 1 mM concentra-
tion of the compound solution in acetonitrile with 0.05 M
tetrabutylammonium perchlorate (TBAP) were used. To gen-
erate the annihilation reaction, the platinum electrode was
pulsed between first reduction and first oxidation potentials,
and the pulse interval was controlled on a computer. All
measurements were done at room temperatures (22-24 °C).
General Procedure for the Synthesis of 2. 3-Iodocou-
were performed using Spartan’04(W). Structures were drawn
at the entry-level of input and minimized. Equilibrium geom-
etry was obtained at the PM3 level of theory for each molecule
at the ground state from its initial geometry subject to
symmetry. The total charge was kept neutral, and the mul-
tiplicity was kept at singlet. Orbitals and energies, atomic
charges, vibrational modes, and thermodynamic properties
were chosen as output parameters. HOMO and LUMO orbital
surfaces were then obtained from the output. Constraints were
imposed for 6 and 10 by constraining the dihedral angle
between the plane of the phenyl ring and that of the coumari-
nyl ring to “0°” and then were minimized. The calculation was
resubmitted for the planarized structure, and the orbitals and
properties were recalculated.
marin (prepared according to ref 12), 272 mg (1 mmol), (PPh
3
)
2
-
Acknowledgment. Financial support from the Na-
tional Science Council, Taiwan, is gratefully acknowl-
edged. We would like to thank Professor S. M. Peng and
Mr. G. S. Lee for providing X-ray data for compound 4.
We also thank Prof. K. T. Liu for providing access to
the Spartan’04(W) program.
PdCl , 14 mg (2 mol %), CuI, 4 mg (2%), PPh , 26 mg (10%),
2
3
and a stirrer were charge in a two-neck round-bottomed flask
equipped with a condenser, and the setup was degassed and
2
back-filled three times with a gaseous mixture of Ar + H .
THF, 8 mL, was introduced from a still into the reaction flask
followed by 6 equiv of triethylamine, and finally a solution of
the terminal acetylene (1.1 mmol) (4-tolylacetylene 127 mg)
in 5 mL of THF was added via a syringe under the mild
reducing atmosphere at about 60 °C. After the addition of the
alkyne, the reaction mixture was refluxed for 6 h. When the
TLC indicated the disappearance of the starting iodo com-
pound, the volatiles were evaporated and the residue was
Supporting Information Available: 1H and 13C NMR
spectra of compounds 1-10; X-ray crystal data for 4; CV trace
for 1-10; raw ECL spectra for the 3-coumarin family (1-5);
selected ECL spectra for the 7-coumarin family (6, 8, and 10);
details of theoretical calculations; HOMO-LUMO orbital
surfaces for 1, 5, 6, and 10, (PM3) and their Cartesian
coordinates for these molecules. This material is available free
of charge via the Internet at http://pubs.acs.org.
chromatographed to obtain 230 mg (88%) of analytically pure
1
2
1
2
. mp: 127-128 °C. H NMR (400 MHz, CDCl
3
): δ 7.91 (s,
H), 7.53-7.46 (m, 4H), 7.33-7.26 (m, 2H), 7.15 (d, J ) 8 Hz,
H), 2.36 (s, 3H). C NMR (100 MHz, CDCl
1
3
3
): δ 159.2, 153.0,
JO0493424
8092 J. Org. Chem., Vol. 69, No. 23, 2004