Synthesis, electronic properties, and electrochemiluminescence of donor-substituted phenylethynylanthronitriles

Article abstract of DOI:10.1021/jo0477953

A new series of π-conjugated donor-acceptor compounds (1-6) with inherent redox centers have been prepared and studied with respect to their electronic properties. The photophysical characteristics of these compounds have been studied in relation to their

Full text of DOI:10.1021/jo0477953

Synthesis, Electronic Properties, and Electrochemiluminescence
of Donor-Substituted Phenylethynylanthronitriles
†
†
†
†
Arumugasamy Elangovan, Kuo-Ming Kao, Shu-Wen Yang, Yu-Ling Chen,
,†
,‡
Tong-Ing Ho,* and Yuhlong Oliver Su*
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, and Department of
Chemistry, National Chi-Nan University, Nantou 545, Taiwan
hall@ntu.edu.tw
Received December 16, 2004
A new series of π-conjugated donor-acceptor compounds (1-6) with inherent redox centers have
been prepared and studied with respect to their electronic properties. The photophysical
characteristics of these compounds have been studied in relation to their structures. Cyclic
voltammetry and UV-vis spectroelectrochemistry were used to probe the ground-state electronic
properties of the neutral and charged species. The observed electronic absorption properties of the
neutral and charged molecules are explained with the help of frontier orbital structures and
electrostatic potential maps obtained from density functional theory (DFT, B3LYP/6-31G*)
calculations. Electrochemiluminescence (ECL) of this series of donor-substituted phenylethynyl-
anthronitriles with different donors was also studied. The structure-property relationship of all
of the compounds is discussed.
Introduction
monly anticipated luminescence applications, there is a
special and unique application which involves production
of light by chemiluminescence via an encounter reaction
of radical ions formed by electrolysis of redox active
molecules. Annihilation of radical ions, which are gener-
ated sequentially with a time interval at the surface of
an electrode, can lead to the formation of electronically
excited states of molecules by energetic electron-transfer
reactions at the electrified interface. These excited species
emit energy in the form of fluorescence at a certain
wavelength. This technique is called as electrogenerated
The use of π-conjugated organic materials has gained
much attention because of their potential application in
electronic devices such as light-emitting diodes and field-
effect transistors. The structural flexibility and facile
processing of these materials could potentially help to
reduce the cost and enhance the applicability of these
materials in various fields. Ethyne-based donor-acceptor
molecules are emerging as new organic materials for
luminescent applications.3 Apart from the most com-
1
2
,4
†
National Taiwan University.
(4) (a) Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org. Lett. 2003, 5,
4245. (b) Pohl, R.; Anzenbacher, P., Jr. Org. Lett. 2003, 5, 2769. (c)
Metivier, R.; Amengual, R.; Leray, I.; Michelet, V.; Genet, J.-P. Org.
Lett. 2004, 6, 739. (d) Anderson, J. D.; McDonald, E. M.; Lee, P. A.;
Anderson, M. L.; Ritchie, E. L.; Hall, H. K.; Hopkins, T.; Mash, E. A.;
Wang, J.; Padias, A.; Thayumanavan, S.; Barlow, S.; Marder, S. R.;
Jabbour, G. E.; Shaheen, S.; Kippelen, B.; Peyghambarian, N.; Wight-
man, R. M.; Armstrong, N. R. J. Am. Chem. Soc. 1998, 120, 9646. (e)
Armstrong, N. R.; Anderson, J. D.; Lee, P. A.; McDonald, E. M.;
Wightman, R. M.; Hall, H. K.; Hopkins, T.; Padias, A.; Thayumanavan,
S.; Barlow, S.; Marder, S. R. SPIE 1999, 3476, 178. (f) Armstrong, N.
R.; Whightman, R. M.; Gross, E. M. Annu. Rev. Phys. Chem. 2001, 52,
391.
‡
National Chi-Nan University.
(
1) (a) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J.
H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.;
Br e´ das, J.-L.; L o¨ gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121.
b) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed.
(
1
998, 37, 402.
2) Brown, A. R.; Pomp, A.; Hart, C. M.; de Leeuw, D. M. Science
995, 270, 972.
3) (a) Yamaguchi, Y.; Kobayashi, S.; Miyamura, S.; Okamoto, Y.;
(
1
(
Wakamiya, T.; Matsubura, Y.; Yoshida, Z.-I. Angew. Chem., Int. Ed.
004, 43, 366. (b) Boydston, A. J.; Yin, Y.; Pagenkopf, B. L. J. Am.
Chem. Soc. 2004, 126, 3724.
2
10.1021/jo0477953 CCC: $30.25 © 2005 American Chemical Society
4460
J. Org. Chem. 2005, 70, 4460-4469
Published on Web 04/23/2005

ECL of Donor-Substituted Phenylethynylanthronitriles
chemiluminescence or, often, electrochemiluminescence
SCHEME 1. General Synthetic Route for the
Preparation of PEANs 1-6
5
-15
(ECL).
ECL emission can occur directly from the
singlet excited state (S route), via triplet-triplet an-
nihilation (T route), or via excimer formation (E route).5b
Excimers are homodimers of molecules in solution
which exist in the photoexcited state.16 Many excimers
are formed between molecules whose π-systems interact
effectively leading to the formation of appropriate states,
and they possess structures prominently governed by
their monomer chemical structures. The formation of
excited states in the solid state by application of an
electric field (as in OLED) has been shown to follow a
mechanism similar to that followed in solution (ECL).4
An understanding of the process occurring at the molec-
ular level (even at the plasma level) is important for
enhancing our ability to control and manipulate the
emissive state. Design, synthesis, and study of new
organic molecules for ECL are, therefore, a live research
area. Recently, the radical ions of some water-soluble
porphyrins and zinc porphyrins were produced, and we
recorded the ECL emissions.10 A series of intramolecular
charge-transfer (ICT) donor-acceptor stilbenoid systems
with the N,N-dialkylamino group as the donor and with
a pyridyl, thiophenyl, or aryl group as the acceptor were
prepared, and their ECL emissions were studied.11 It was
found that most of the stilbenoids showed ICT-ECL
through direct annihilation of radical ions. Only poor ICT
compounds with weak electron-demanding thiophene as
the acceptor showed excimer ECL. The coplanarity of the
donor and the quinolinyl and isoquinolinyl acceptor
moieties with respect to the conjugated triple bond plays
a role in determining the type and emission wavelength
d-f
trile acceptor moieties was synthesized, and the ECL and
other photophysical properties were studied.
The anthronitrile moiety was chosen because of the
following: (i) It is a fluorophore and a good electron
acceptor, and when connected appropriately with an
electron donor through conjugation, it will portray pho-
tophysical characteristics, such as absorption and emis-
sion in the visible region; in other words, the HOMO-
LUMO gap can be reduced advantageously. (ii) 9-Cyano-
1
0-haloanthracenes have been found to produce, upon
electroreduction, radical anion intermediates stable enough
for the electrochemical studies, and all the related
activation parameters could be determined with sufficient
accuracy even at room temperatures;13 substitution of the
halogen with donor groups via conjugation would help
realize the tunable properties. (iii) Anthracene and
diphenylanthracene have been among the very early
aromatic compounds to be studied14 for ECL, and now
our attention has been drawn to the extension of π-con-
jugation through the molecule seeking for new aspects.
1
2a,b
of ECL.
Formation of the H-type excimer has been
proposed to be responsible for the observed blue-shifted
ECL emission with respect to photoluminescence in
p-N,N-diethylaminophenyl-4-quinolinylethyne1 and p-
N,N-dimethylaminophenylethynylacridine (see text and
ref 27).
2b
In the present work, a series of new diarylethyne
molecules with donor-substituted phenyls and anthroni-
(
iv) The anthronitrile moiety can impart intermolecular
(
5) (a) Faulkner, L. R.; Bard, A. J. Electrogenerated Chemilumi-
π-π interactions, and hence, the chromophore activity
can be controlled and the emission tuned by the ap-
propriate conditions.
nescence. In Electrochemical Methods; John Wiley & Sons: New York,
1
980; pp 621-627. (b) Faulkner, L. R.; Bard, A. J. In Electroanalytical
Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol. 10,
pp 1-95.
(
6) (a) Knight, A. W.; Greenway, G. M. Analyst 1994, 119, 879. (b)
Knight, A. W. Trends Anal. Chem. 1999, 18, 47.
Results and Discussion
(
7) (a) Lai, R. Y.; Fabrizio, E. F.; Jenekhe, S. A.; Bard, A. J. J. Am.
Chem. Soc. 2001, 123, 9112. (b) Knorr, A.; Daub, J. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 2664.
Compounds 1-6 were synthesized by coupling the
(
8) Prieto, I.; Teetsov, J.; Fox, M. A.; Vanden Bout, D. A.; Bard, A.
corresponding terminal aryl acetylenes with 9-bromo-10-
anthronitrile under modified Sonogashira conditions as
J. J. Phys. Chem. A 2001, 105, 520.
17
(
9) (a) Oyama, M.; Okazaki, S. Anal. Chem. 1998, 70, 5079. (b)
Kapturkievicz, A. J. Electroanal. Chem. 1990, 290, 135. (c) Kapturk-
ievicz, A. J. Electroanal. Chem. 1991, 302, 13.
shown in Scheme 1. A summary of photophysical data is
presented in Table 1, and the electrochemical character-
istics of these compounds are furnished in Table 2.
Photophysical Properties. UV-vis absorption spec-
tra of all of the compounds (1-6) exhibited â-band
absorption at about 275 nm. As can be seen from Figure
1, for those phenylethynylanthronitriles (PEANs) with
(
10) Chen, F.-C.; Ho, J.-H.; Chen, C.-Y.; Su, Y. O.; Ho, T.-I. J.
Electroanal. Chem. 2001, 499, 17.
11) Chen, C.-Y.; Ho, J.-H.; Wang, S.-L.; Ho, T.-I. Photochem.
Photobiol. Sci. 2003, 2, 1232.
(
(
12) (a) Elangovan, A.; Chen, T.-Y.; Chen, C.-Y.; Ho, T.-I. Chem.
Commun. 2003, 2146. (b) Elangovan, A.; Yang, S.-W.; Lin, J.-H.; Kao,
K.-M.; Ho, T.-I. Org. Biomol. Chem. 2004, 2, 1597.
(
(
13) Heinze, J.; Schwart, J. J. Electroanal. Chem. 1981, 126, 283.
14) (a) Faulkner, L. R.; Bard, A. J. J. Am. Chem. Soc. 1968, 90,
6
284. (b) Hercules, D. M. Acc. Chem. Res. 1969, 2, 301. (c) Visco, R.
(16) Turro, N. J. Modern Molecular Photochemistry; Benjamin-
Cummings: Redwood City, CA, 1978.
E.; Chandross, E. A. J. Am. Chem. Soc. 1964, 86, 5350. (d) Santhanam,
K. S. V.; Bard, A. J. J. Am. Chem. Soc. 1965, 87, 139. (e) Hercules, D.
M. Science 1964, 145, 808. (f) Richter, M. M. Chem. Rev. 2004, 104,
(17) (a) Elangovan, A.; Wang, Y.-H.; Ho, T.-I. Org. Lett. 2003, 5,
1841. (b) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 4467.
3
003.
(
972, 94, 691.
15) Faulkner, L. R.; Tachikawa, H.; Bard, A. J. J. Am. Chem. Soc.
(18) Reynolds, G. A.; Drexhage, K. H. Optics Commun. 1975, 13,
222.
1
J. Org. Chem, Vol. 70, No. 11, 2005 4461

Elangovan et al.
TABLE 1. Photophysical Data of 1-6
a
Maxima at the longest-absorption wavelength. b ∆EH-L is the
HOMO-LUMO gap calculated from the onset of the visible
c
3
-1
-1 d
absorption maxima. In units of ×10
M
cm
.
Emission
spectra for 4-6 were recorded in CH2Cl2 because no emission was
noticed in CH3CN; emission spectra for 1-3 in CH2Cl2 showed
e
FIGURE 2. Fluorescence emission spectra of PEANs 1-6.
little shift in the maxima. Using coumarin 334 as the standard
Φ ) 0.69 in MeOH).
1
8
Spectra of 1-3 were recorded in ACN and those of 4-6 in CH -
2
(
-
5
Cl
2
(10 M); inset is a magnified view of the spectra of 4-6.
TABLE 2. ECL and Electrochemical Data for PEANs
-6 Recorded in Acetonitrile with 50 mM TBAP
The preceding argument is supplemented by the emis-
1
sion characteristics as well. Compounds 1 and 2 emit
closely at the same maximum with vibrational fine
structure while 3 attains a moderate charge transfer
given its structureless emission band in ACN again
caused by the increase in donor strength (Figure 2).
Although the amine substituted compounds 4-6 benefi-
cially exhibited longer red-shifts and higher intensities
in the absorption spectra, their emission intensities were
diminished in polar ACN when compared to those of 1-3
with no or weaker donors. In polar solvents such as ACN,
for the stronger ICT compounds, the energy level of the
ICT state becomes so low that back-electron transfer
occurs to populate the ground state, and therefore 4-6
did not show fluorescence in ACN. In less polar methyl-
ene chloride, 4-6 show emission with a Stokes shift of
PEAN
λmaxECL (nm, eV)
Ep,ox (V)
Ep,red (V)
-∆H° (eV)a
1
2
3
4
5
6
542, 2.28
541, 2.28
536, 2.30
556, 2.22
550, 2.24
540, 2.28
1.62
1.62
1.49
0.90
0.87
0.91
-0.92
-0.91
-0.89
-0.90
-0.90
-0.90
2.38
2.37
2.22
1.64
1.61
1.65
a
Calculated using the equation ∆H° ) Ep,ox - Ep,red - 0.16.15
>
100 nm. In general, the increased Stokes shift is
indicative of better stabilization of the emitting state of
the acceptor moiety that is in communication with the
donor moiety at remote.
While the absorption maxima of 1-6 were found to be
>
40 nm red-shifted in comparison with that of the
Abs
Flu
parent, 9-cyanoanthracene (λmax
) 400 nm, λmax
)
4
4
40), the emission maxima were found to be shifted from
0 to 140 nm. Compounds 1-3 showed intense green
emission, while 4-6 showed beautiful orange-red fluo-
rescence in solution (CH Cl /CHCl ). Since molecules
showing a preferably narrow emission typically close to
2
2
3
FIGURE 1. UV-vis absorption spectra of PEANs 1-6 in ACN
4,19
-
5
600-650 nm would be suitable for red-emitting LEDs
(
10 M).
and those emitting at about 500 nm would be suitable
for green-emitting LEDs, the reported compounds may
be of use as dopants in an EL device. The photolumines-
cence quantum yields (Φ measured in CH Cl ) are
poor donors (R ) H, Me, OMe), the longest wavelength
of the absorption maximum is about 440 nm as a
structured band (3 broadens slightly because of the
enhancement of donor strength). This implies that the
vertically excited lowest singlet state (the Franck-
Condon state) has little or no charge transfer. From the
2
2
moderate to quite high (from 0.05 to 0.73, those of 1-3
were the highest). The reduction in the Φ values of 4-6
may be the result of the lower energy of the emitting
states which may facilitate nonradiative decay or produce
low-energy states that decay to the ground state by back-
electron donation.
-
1
approximate spacing of about 1250 cm , the transition
is likely the result of the stretching contribution ν(CC)
from the polyaromatic anthronitrile unit. For those
2 2
PEANs which have stronger donors (R ) NMe , NEt ,
The electronic absorption spectral properties can be
better understood by analyzing the calculated highest-
piperidinyl), the maximum shifts to >480 nm with a
structureless feature. This vast red-shift can be at-
tributed to the absorption from the charge-transfer
transition at the lowest Franck-Condon excited state.
(
19) Duong, H. M.; Bendikov, M.; Steiger, D.; Zhang, Q.; Sonmez,
G.; Yamada, J.; Wudl, F. Org. Lett. 2003, 5, 4433.
4462 J. Org. Chem., Vol. 70, No. 11, 2005

ECL of Donor-Substituted Phenylethynylanthronitriles
FIGURE 3. HOMO and LUMO levels of PEANs 1-6 calculated at the B3LYP/6-31G* level of density functional theory with a
6-31G* basis set using Spartan’04 for Windows.
occupied molecular orbital (HOMO) and the lowest-
unoccupied molecular orbital (LUMO) surfaces of the
molecules (Figure 3). Molecule 1 (B3LYP/6-31G*) has a
planar structure with frontier orbital distributed through-
out the backbone. Thus, this compound has an absorption
spectrum that shows a weak structured transition as the
longest-wavelength absorption. Molecules 2 and 3, though
they have moderate electron-donating groups, such as Me
and OMe, their B3LYP/6-31G*-optimized structures at
the ground state are not planar but twisted with a
dihedral angle of 90°. Their HOMO and LUMO are
mainly localized on the acceptor anthronitrile subunit,
and there is no contribution from the donor-bearing
phenyl moiety. Thus, their electronic absorption implies
a vertical excitation of the electron from the HOMO of
the anthronitrile to the LUMO of the same unit.
FIGURE 4. Energy level diagram showing the influence of
The entire scenario takes a turn in the case of
molecules 4-6. It is interesting that these molecules,
although they are twisted in a manner similar to 2 and
the donor substituents on the HOMO and LUMO energies.
By comparing the B3LYP/6-31G* HOMO and LUMO
energy levels of compounds 1-6 (Figure 4), one can see
that both HOMO and LUMO energies are higher by 0.2-
0.3 eV for the strong donor-substituted PEANs 4-6 than
those for 1-3 which indicates the moderate influence of
the strong donor moieties; they are also consistent with
the experimental results observed in the absorption and
emission spectra.
3
, have their HOMO and LUMO at different locations;
namely, the HOMO is localized on the donor-bearing
phenyl moiety, and the LUMO is localized on the acceptor
anthronitrile subunit. Thus, these molecules experience
intramolecular charge transfer during electronic absorp-
tion, and hence, their absorption spectra show a strong
structureless band at the longest wavelength. These
molecules make good examples of systems with remotely
communicating groups that can be applied in molecular
electronics.
Electrochemistry. Cyclic voltammograms (CV) were
recorded to determine the electrochemical activity and
the reduction and oxidation potential values of these
J. Org. Chem, Vol. 70, No. 11, 2005 4463

Elangovan et al.
FIGURE 5. CV traces for PEANs 1 (a) and 4 (b) recorded in ACN containing 50 mM TBAP at a scan rate of 50 mV/s.
FIGURE 6. UV-vis spectra of the radical anion (a) and radical cation (b) of PEAN 1.
-
3
compounds. CV of 1-6 (1.0 × 10 M) were recorded in
acetonitrile with 50 mM tetrabutylammonium perchlo-
rate (TBAP) as the supporting electrolyte at a scan rate
of 0.05 V/s, and the reduction and oxidation peak
potentials are recorded in Table 2.
buting to both HOMO and LUMO which indicates the
involvement of the bridging unit in the redox processes.
All compounds are found to show stability for about 8-10
electrochemical cycles.
UV-Vis Spectroelectrochemistry. UV-vis spectro-
electrochemical experiments were carried out on all of
the compounds to obtain more information about the
redox process and the eventual products of the reaction.
The UV-vis spectra were recorded for the radical ions
generated by electrolysis. All compounds, when reduced,
showed the formation of a new band at about 320 nm
and another set of long-wavelength bands centered at
about 650 nm with the tail extending up to 750-800 nm.
The intensity of the longest-wavelength band also in-
creases with an increase in the voltage (-0.3 V every 30
s for 4-5 min). This increase in intensity is accompanied
by a decrease in the intensity of bands at 260 and 450
nm. Figures 6 and 7 are typical UV-vis absorption
spectra of PEANs 1 and 4 (reduced and oxidized),
respectively, as representative cases of weak and strong
donor-substituted PEANs (the spectra for the rest are
provided in the Supporting Information). The appearance
of two isosbestic points in the reduction spectra indicates
the existence of two species, namely, the neutral and
radical anions of the respective compounds, in equilib-
rium.
The reversible reduction peak potentials occur in the
vicinity of -0.9 V which corresponds to the reduction at
the 9-cyanoanthracene moiety. The extension of conjuga-
tion through a triple bond has altered the reduction
potential significantly in comparison to the reported
reduction potential of -1.27 V for the unsubstituted
parent, 9-cyanoanthracene.20 As seen in Figure 3, PEANs
2-3 have the HOMO and LUMO localized mainly on the
acceptor anthronitrile unit with delocalization extending
up to the CtC triple bond. Hence, the redox process
occurring in these compounds can be assumed to take
place mainly on the part of the π-extended acceptor
anthronitrile. The oxidation curve is irreversible for 1-3;
however, a well-defined peak corresponding to the peak
potential could be observed, and the reduction curves are
fully reversible (Figure 5).
PEANs 4-6 showed quasi-reversible oxidation peaks
and fully reversible reduction peaks. The HOMOs for
PEANs 4-6 are localized on the donor-nearing phenyl
moiety, and the LUMOs are localized on the acceptor
anthronitrile subunit with the CtC triple bond contri-
The UV-vis spectra of the radical cations of PEANs
1-3, with no or weak donors, produce long-wavelength
(
20) Cheng, E.; Sun, T.-C.; Su, O. Y. J. Chin. Chem. Soc. 1993, 40,
5
51.
4464 J. Org. Chem., Vol. 70, No. 11, 2005

ECL of Donor-Substituted Phenylethynylanthronitriles
FIGURE 7. UV-vis spectra of the radical anion (a) and radical cation (b) of PEAN 4.
maxima that are too weak or do not show (e.g., Figure
7
b), whereas PEANs 4-6, which have oxidizable amine
groups, exhibit the formation of new long-wavelength
bands centered at about 630 nm. They have features
which signify the existence of mixtures of free and radical
cations of the reactant molecules in equilibrium as
revealed by the noticeable formation of two isosbestic
points at about 335 and 430 nm. The trend observed in
the UV-vis spectroelectrochemistry is rationalized with
the help of the HOMO and LUMO frontier orbital
diagrams in Figure 3. As discussed earlier, since the
HOMO and LUMO are localized at the acceptor part for
PEANs 2 and 3, the radical ions in these systems are
generated only within this subunit of the molecule.
However, the formation of long-wavelength maxima
beyond the photoabsorption maxima can be attributed
to the formation of the charge resonance state wherein
resonance occurs between the reducible center (the
anthronitrile moiety) and the other end (the phenyl
moiety) facilitated by conjugation through the triple bond.
The formation of new long-wavelength bands in PEANs
FIGURE 8. Electrostatic potential maps for the PEAN (from
•
+
•-
•+
•-
top to bottom): 1, 1 , and 1 (left) and 6, 6 , and 6 (right).
The maps for the radical ions were obtained using density
functional theory (DFT-B3LYP/6-31G*). Red is more negative.
Blue is less electronegative and more electropositive.
4-6 is plausible given the occurrence of the HOMO on
the donor part of these molecules; this is not the case
with PEANs 2 and 3 in which there are no such
oxidizable groups and the HOMOs are found on the
acceptor part. Thus, the appearance of long-wavelength
bands in PEANs 4-6 indicates that radical ions are
indeed formed and involved in resonance with the ac-
ceptor moiety, and hence, these groups are electrochemi-
cally active. More prominent absorption bands appear for
the reduced states as well, and the charge resonance in
such cases leads to stronger absorption which results in
the appearance of bands more intense than those in the
oxidized states. The electrochemical reversibility, as
confirmed by the reproducibility of the absorption spectra
of the neutral compound solution from the electrochemi-
cal cycling for all of the compounds, was satisfactory.
The preceding discussion is supported by computa-
tional DFT calculations for the charged molecules. Figure
at the center of the molecule. Both radical ions have very
different charge densities in comparison to that of the
neutral molecule. PEAN 6 attains planarity in the
charged state; the radical anion carries more negative
charge at the acceptor anthronitrile unit (red), and the
radical cation carries more positive charge at the donor
site (less green and more blue, more positive). The
planarization caused by charging, together with the
electrostatic difference, can induce charge resonance
between the communicating groups, namely, the donor
and the acceptor, and thus produce new absorption
bands.21
Figure 9 shows the HOMO and LUMO surfaces for the
charged species of 1 and 6. As seen from this figure, the
radical anion of 1 carries the HOMO coefficient at the
acceptor unit and the LUMO at the phenyl moiety. Thus,
the electronic absorption involving charge resonance can
be expected and has been observed (Figure 7a). The
radical cation has an orbital coefficient almost uniformly
distributed in both the HOMO and LUMO, and therefore,
no charge resonance can be expected for electronic
absorption; hence, the spectrum is flat at the region
8
depicts the electrostatic potential density maps for the
representative molecules 1 (no donor) and 6 (strong
donor). Charged molecule 1 has a planar geometry
similar to that of the neutral state; the radical anion
carries more negative charge at the center of the acceptor
anthronitrile moiety (red, more negative), and the radical
cation carries less electron density (blue, more positive)
J. Org. Chem, Vol. 70, No. 11, 2005 4465

Elangovan et al.
FIGURE 9. HOMO (bottom) and LUMO (top) surfaces for radical ions of 1 and 6 (DFT-B3LYP/6-31G*).
shown in Figure 7b. The situation changes when a strong
donor is substituted for the weak donor. With a piperidi-
nyl unit at the phenyl moiety (PEAN 6), the radical anion
carries the HOMO and LUMO at two different locations;
the radical cation seems to have an almost uniform
distribution throughout the charged molecule. Thus, this
molecule (as well as the other strong donor-substituted
molecules 4 and 5) experiences a charge difference in
their charged state, and therefore, they undergo charge
resonance during absorption.
Electrochemiluminescence. ECL spectra were re-
corded in acetonitrile at 1.0 and 0.10 mM concentrations
with 50 mM tetrabutylammonium perchlorate as the
supporting electrolyte in a cell setup similar to one that
was previously published.10 The electrodes were pulsed
between the first oxidation and first reduction peak
potentials at various pulse intervals to generate radical
FIGURE 10. ECL spectrum of 3 in ACN (10- M).
4
ions and the subsequent annihilation reaction. Satisfac-
-4
tory ECL spectra were obtained at a 10 M concentration
of the sample, while no ECL was observed at the lower
concentrations used for photoluminescence measure-
ments. Moreover, the ECL emission intensity diminished
SCHEME 2
-3
greatly at higher concentrations (10 M) when compared
-5
with those of photoluminescence (10 M). A typical ECL
spectrum of compound 3 is depicted in Figure 10, and
the maxima of all compounds are recorded in Table 2.
As can be seen from Tables 1 and 2, the ECL maxima
for 1-3 are red-shifted by 40-80 nm, and for compounds
(21) (a) Lewis, I. C.; Singer, L. S. J. Chem. Phys. 1965, 43, 2712. (b)
Howarth, O. W.; Fraenkel, G. K. J. Am. Chem. Soc. 1966, 88, 4514. (c)
Howarth, O. W.; Fraenkel, G. K. J. Chem. Phys. 1970, 52, 6258. (d)
Badger, B.; Brocklehurst, B.; Russell, R. D. Chem. Phys. Lett. 1967, 1,
1
22. (e) Badger, B.; Brocklehurst, B. Nature 1968, 219, 263. (f) Badger,
B.; Brocklehurst, B. J. Chem. Soc., Faraday Trans. 1969, 65, 2582. (g)
Badger, B.; Brocklehurst, B. J. Chem. Soc., Faraday Trans. 1969, 65,
2
588. (h) Badger, B.; Brocklehurst, B. J. Chem. Soc., Faraday Trans.
970, 66, 2939. (i) Kawai, K.; Miyamoto, K.; Tojo, S.; Majima, T. J.
1
4
-6, the ECL maxima are blue-shifted by 40-68 nm
Am. Chem. Soc. 2003, 125, 912. (j) Kawai, K.; Yoshida, H.; Takada,
T.; Tojo, S.; Majima, T. J. Phys. Chem. 2004, 108, 13547. (k) Kochi, J.
K.; Rathore, R.; Magu e` res, P. L. J. Org. Chem. 2000, 65, 6826. (l)
Tsuchida, A.; Ikawa, T.; Yamamoto, M.; Ishida, A.; Takamuku, S. J.
Phys. Chem. 1995, 99, 14793 and references therein. (m) B u¨ schel, M.;
Stadler, C.; Lambert, C.; Beck, M.; Daub, J. J. Electroanal. Chem. 2000,
when compared with their fluorescence maxima. The
enthalpy change (-∆H°) values for the radical ion an-
nihilation, calculated from the peak potentials, are
greater than the ECL maximal energies for 1 and 2, and
hence, they can follow the S route. The annihilation
enthalpy changes for 3-6 are not sufficient to populate
the excited emission state. The direct annihilation of
radical ions can generate a triplet state instead, and
4
84, 24. (n) Beer, G.; Niederalt, C.; Grimme, S.; Daub, J. Angew. Chem.,
Int. Ed. 2000, 39, 3252. (o) Sakabara, P. J.; Berridge, R.; McInnes, E.
J. L.; West, D. P.; Coles, S. J.; Hursthouse, M. B.; M u¨ llen, K. J. Mater.
Chem. 2004, 14, 1964. (p) Dunsch, L.; Rapta, P.; Schulte, N.; Dieter
Schl u¨ ter, A. Angew. Chem., Int. Ed. 2002, 41, 2082.
4466 J. Org. Chem., Vol. 70, No. 11, 2005

ECL of Donor-Substituted Phenylethynylanthronitriles
compounds to crystallize large enough for the X-ray
diffraction analysis. Hence, a model compound, namely,
N,N-dimethylaminophenylethynylanthracene (7),
FIGURE 11. H excimer or trans excimer formation proposed
for the ECL-emitting state of PEAN.
was prepared so that a single-crystal X-ray analysis could
be obtained. Figure 12 shows the packing diagram of
compound 7 which depicts the existence of two adjacent
molecules in the proximity of the moieties of the molecule
which are aligned in an anti H-H fashion with an
interplanar distance of 3.48 Å.
hence the emission state can be generated by the triplet-
triplet annihilation process (T route, Scheme 2).
24
The possibility of the excimer formation in step 5 of
the above mechanism can be thought to be promoted by
the π-π interaction between the two acceptor subunits
as shown in eq 6 of Scheme 2. The excited state may have
a structure in which the two acceptor headgroups align
head to head with the donor tailgroups projecting away
from one another in an anti H-H fashion as shown in
Figure 11.
The donor-bearing phenyl group is projecting in a near-
perpendicular fashion relative to the anthracene moiety.
The dihedral angle between the plane of the donor-
bearing phenyl group and that of the anthracene group
was determined to be 73.7°, and the interplanar distance
between the two offset-parallel anthracene planes of
adjacent molecules was measured to be 3.48 Å which is
The parent 9-cyanoanthracene is known to form an
excimer in solution, glass, and crystalline matrixes, and
the energy of excimer-type ECL emission, in general, is
in agreement with the published photoexcimer energy of
parent 9-cyanoanthracene.22 Further, it is interesting to
note that the ECL maxima of 1-6 occur very closely at
about 540-550 nm regardless of the nature of the donor
groups. In view of the preceding discussions, we infer that
the ECL emissions in 1-6 are all from a similar type of
species, and we presume it to be from a special type of
excimer as shown in Figure 11. A remarkable recent
publication by Ketter and Wightman23 has indicated the
possibility of an annihilation reaction producing as yet
unknown excited states other than the photolumines-
cence state. These authors have studied the tuning of the
emission states of the benzophenone/phenoxathiin or
benzophenone/4-methoxythioanisole system among oth-
ers. They have discovered the formation of an excimer of
benzophenone in the ECL; it was a species that was not
found in its photoluminescence upon increasing the
concentration of benzophenone 100 fold.
25
within the range of a typical π-π interaction. The ECL
of compound 7 has also been found to be blue-shifted by
about 30 nm with respect to its own photoluminescence
maxima (to be reported elsewhere along with many other
26
related compounds).
A similar situation has also recently been encountered
27
by us in the family of phenylethynylacridines. The N,N-
dimethylaminophenylethynylacridine (8),
which showed a blue-shift of about 35 nm compared to
its photoluminescence maximum, crystallized in a tri-
clinic prism with two acridinyl heads facing each other
invertedly with N,N-dimethylaminophenyl groups pro-
jecting nearly perpendicularly in opposite directions
In the present study, we have noticed the formation of
an ECL emission maximum that is either red-shifted
(
1
Figure 13), similar to the structure depicted in Figure
1. The dihedral angle between the plane of acridine and
(
PEANs 1-3) or blue-shifted (PEANs 4-6) with respect
to the photoluminescence maxima. The structure, pro-
posed as a H-type or trans excimer with donor-bearing
phenyl tails projecting perpendicularly away from the two
inversely face-to-face aligned acceptor anthronitrile head-
groups (resembling the letter H), also amounts to a
decrease in the dipole moment of the species. Given the
steric nature of the bimolecular complex, only the pro-
posed supramolecular assembly is more plausible because
they cannot form the normal cis excimer in which case
the ECL maxima would be shifted more to the red
that of donor-bearing phenyl is measured to be 73.3°, and
the interplanar distance between the two offset-parallel
planes of the acridines at 91.83° was measured to be 3.59
(
24) Crystal data for 7: Formula C24H19N; formula weight 321.40.
Unit cell parameters with standard deviations (unit cell lengths in
angstroms): cell length a 9.2040(2) Å, cell length b 9.3250(2) Å, cell
length c 11.0960(3) Å; cell angle R 72.871(2)°, cell angle â 80.4240-
3
(
10)°, cell angle γ 71.4130(10)°; cell volume 859.86(4) Å ; symmetry
space group H-M, P 1h ; cell formula units Z 2; temperature of study
-1
295(2) K; absorption coefficient 0.072 mm ; reflections collected 5163;
independent reflections 2982 (Rint ) 0.0226); final R indices [I > 2σ-
(
especially for 4-6) than to the blue, compared to the
(
0
I)] R ) 0.0451, R
.1361.
(25) (a) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885. (b)
w w
) 0.1232; R indices (all data) R ) 0.0582, R )
photoluminescence maxima (see ref 12a). Attempts to
draw evidence from the X-ray crystal structure analysis
were unsuccessful because of the inability of these
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.
(26) Ho, T.-I.; Elangovan, A.; Hsu, H.-Y.; Yang, S.-W. J. Phys. Chem.
B 2005, 109, XXXX.
(27) Elangovan, A.; Chiu, H.-H.; Yang, S.-W.; Ho, T.-I. Org. Biomol.
Chem. 2004, 2, 3113.
(
22) (a) Morsi, S. E.; Carr, D.; El-Bayoumi, M. A. Chem. Phys. Lett.
978, 58, 571. (b) Ebeid, E. M.; Morsi, S. E.; El-Bayoumi, M. A.;
Williams, J. O. J. Chem. Soc., Faraday Trans. 1978, 74, 1457.
23) Ketter, J. B.; Wightman, R. M. J. Am. Chem. Soc. 2004, 126,
0183.
1
(
1
J. Org. Chem, Vol. 70, No. 11, 2005 4467

Elangovan et al.
FIGURE 12. Crystal packing diagram of 7 showing the π-π interactions between the two adjacent anthracene moieties of the
molecule. It is aligned in an anti H-H fashion with an interplanar distance of 3.48 Å.
FIGURE 13. X-ray crystal packing diagram showing π-stacking interactions between the two adjacent acridinyl units. The
interplanar distance between the two planes of acridine was measured to be 3.59 Å (from ref 27).
SCHEME 3. Schematic of Routes of Formation of
the Various Energy States for the PL and ECL of
PEANs 1-6
and respective states. As discussed earlier, the ECL
states for all of the compounds (represented by red bars
in the scheme) appear at states with close to the same
energy, whereas the PL states (represented by gray bars)
occur at different energy levels consistent with the
structure of free compounds (cf. Tables 1 and 2). The
radical ion-annihilation reaction can thus produce new
excited states other than the PL states.
Conclusion
We have synthesized a new series of donor-substituted
phenylethynylanthronitriles (PEANs 1-6) and studied
their photophysical and ECL properties. Compounds
1
-3, bearing weak donor groups, exhibit no charge
Å.28 A similar trend was also noticed in the case of donor-
transfer, and their electronic absorption spectra are of
Franck-Condon-type. Compounds 4-6 show strong ICT
interactions in the ground and excited states. The ener-
gies of excited ICT states for 4-6 become so low that they
are not fluorescent in high-polarity acetonitrile solvent.
Density functional theory provided evidence for the
observed electronic properties, in that the properties of
29
substituted phanyethynylcoumarins. The extent of the
ECL blue-shift from the PL maximum in the present
family of compounds is the largest (up to 68 nm) in our
studies so far.
In light of the preceding discussion and in view of the
energy states of both photoluminescence (PL) and elec-
trochemiluminescence (ECL), we can draw a schematic
of the whole picture (Scheme 3) which summarizes the
processes taking place during the electronic transitions
2
and 3, bearing weak donors, are controlled by the
acceptor anthronitrile moiety and those of 4-6 are of the
ICT-type involving both the donor and acceptor moieties.
The electronic absorption, electro reduction and oxida-
tion, and UV-vis spectroelectrochemistry can be ex-
plained by the orbital distribution and electrostatic
charge densities of these neutral compounds and their
radical ions.
(
18 2
28) Crystal data for 8: Formula C23H N ; formula weight 322.39.
Unit cell parameters with standard deviations (unit cell lengths in
Angstroms): cell length a 9.2827(2) Å, cell length b 9.3930(2) Å, cell
length c 10.7357(3) Å; cell angle R 81.7260(10)°, cell angle â 72.9830-
3
(
10)°, cell angle γ 71.801(2)°; cell volume 849.25(3) Å ; symmetry space
group P 1h ; cell formula units Z 2; temperature of study 295(2) K;
-1
absorption coefficient 0.02(2) mm ; reflections collected 5997; inde-
All compounds reported here exhibit electrogenerated
chemiluminescence (ECL) without the use of a co-
reactant. Surprisingly, all of the compounds emit ECL
at almost the same maxima centered at 545 nm regard-
pendent reflections 3827 (Rint ) 0.0192); final R indices [I > 2σ(I)] R
)
w w
0.0483, R ) 0.1278; R indices (all data) R ) 0.0671, R ) 0.1449.
(
29) Elangovan, A.; Lin, J.-H.; Yang, S.-W.; Hsu, H.-Y.; Ho, T.-I. J.
Org. Chem. 2004, 69, 8086.
4468 J. Org. Chem., Vol. 70, No. 11, 2005

ECL of Donor-Substituted Phenylethynylanthronitriles
less of the nature of the donor subunits. The observation
of ECL in high-polarity acetonitrile while photolumines-
cence was not observed in the same solvent at even
higher concentrations of the fluorophore supports the
notion that the ECL-emitting state is different from the
photoluminescent state. The possibility of the formation
of a new kind of arrangement in high-concentration
solutions is rationalized by the ability of such structurally
related compounds to undergo π-π interaction. Thus, it
is possible to generate a new excited state by the
appropriate structural modification of fluorophores and
the choice of conditions. These compounds appear to have
qualities suitable for further application of ECL, and
some of them could be good candidates for light-emitting
diodes. Preliminary EL studies involving PEAN 4 have
indicated that this compound can be applied as a dopant
in a device to emit red light. Fabrication of sophisticated
devices with better multicolor tuning and extensive
device property studies will be pursued in the future.
spectroelectrochemistry was performed with an optically trans-
parent thin-layer electrode cell coupled with an UV-vis
2
0
spectrophotometer. ECL spectra were recorded at room
temperature using a setup consisting of a fluorescence spec-
trophotometer and a voltammograph with a computer inter-
face. The electrode surfaces were prepared freshly before the
CV and ECL experiments. The carbon disk electrode was
rubbed against alumina paste, and the Pt wire and Pt mesh
electrodes were cleaned by rinsing with dilute nitric acid
followed by water; then they were finally fired with a naked
flame to ensure maximum cleanliness of the electrode. Typi-
cally 0.1 and 1.0 mM concentrations of the compound in
acetonitrile with 0.05 M tetrabutylammonium perchlorate
(
TBAP) were loaded into the quartz cell, degassed with pure
argon, and electrolyzed between platinum wire and platinum
mesh electrodes. The platinum electrode was pulsed between
first reduction and first oxidation potentials to generate
reaction, and the pulse interval was controlled on a computer.
The general synthetic method and the characterization data
of compounds 1-6 are provided in the Supporting Information.
Theoretical Calculations. Density functional theoretical
and semiempirical calculations were performed using Spar-
3
0
tan’04 (for Windows). The structures were drawn at the
entry-level of input and minimized. Equilibrium geometry was
obtained at the B3LYP level of DFT for each molecule at the
ground state from its initial geometry subject to symmetry
Experimental Section
Materials and Methods. Phenyl acetylene and tolyl
acetylene were obtained from commercial sources and were
used as received. The rest of the terminal alkynes were
31
with a 6-31G* basis set. The total charge was kept neutral,
anionic, or cationic as required, and the multiplicity was set
at singlet or doublet as required. Orbitals and energies, atomic
charges, vibrational modes, and thermodynamic properties
were chosen as output parameters. HOMO and LUMO orbital
surfaces and electrostatic potential density maps were then
obtained from the output.
1
2,17a
reported earlier from our lab.
Dichlorobis(triphenylphos-
phine)palladium(II) was either prepared in-house or obtained
from a commercial source. Solvents were distilled as per the
standard methods and purged with argon before use. Triethyl-
amine (TEA) and tetrahydrofuran (THF) were distilled and
purged with a mixture of approximately 1:1 argon/hydrogen
1
before use. H NMR spectra of the samples were recorded with
1
3
Acknowledgment. Financial support for this work
was provided by the National Science Council of Taiwan.
a 300/400 MHz instrument, and C NMR spectra were
recorded at 100.1 MHz operator frequency in CDCl
3
solvent
1
with CHCl
3
as the internal standard (δ 7.24 for H and 77
1
3
Supporting Information Available: Characterization
middle of the three peaks for C spectra). Mass spectra were
recorded on a nitrobenzyl alcohol matrix. TLC was run on
precoated aluminum plates (Si 60 F254). Column chromatog-
raphy was run on silica gel (60-120 mesh) and neutral
alumina (70-230 mesh). All UV-vis spectra were recorded
on a spectrophotometer with a 10 µM solution of the com-
2 2
data, UV-vis and fluorescence spectra in CH Cl , raw ECL,
1
13
H NMR, and C NMR spectra of all compounds, Cartesian
coordinates for all neutral molecules and radical ions of 1 and
6
, and the UV-vis spectra of the radical ions of all compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
3
pounds in CH CN, and all fluorescence spectra were recored
on a fluorescence spectrophotometer using similar solution
concentrations. Quantum yields were determined using cou-
marin 334 as the standard (Φ ) 0.69 in MeOH). CV
measurements were done on a voltammograph 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. UV-vis
JO0477953
1
8
(
30) Spartan’04 for Windows; Wavefunction, Inc.: Irvine, CA, 2003.
31) (a) Hehre, W. J. A Guide to Molecular Mechanics and Quantum
(
Chemical Calculations; Wavefunction, Inc.: Irvine, CA, 2003. (b) Lee,
C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (c) Beck, A. D. J.
Chem. Phys. 1993, 98, 5648.
J. Org. Chem, Vol. 70, No. 11, 2005 4469