Arylethynylacridines: Electrochemiluminescence and photophysical properties

Article abstract of DOI:10.1039/b410829a

Electrogenerated chemiluminescence (ECL) of six new ethyne-based acridine derivatives (1-6) has been studied. The new acridine derivatives were synthesized by cross-coupling of 9-chloroacridine and corresponding donor-substituted phenylethynes under modified Sonogashira conditions. The donor groups were varied in the order of increasing steric hindrance and donor strength at the donor site. The solution phase photophysical properties and ECL of these compounds were studied comparatively in acetonitrile solvent. The UV-Visible spectra of compounds 1-5 exhibit closely the same maxima. Density functional theory (DFT) has been invoked to analyze and understand the unexpected UV-Visible absorption behavior. Compounds with weak electron donors produce excimer ECL irrespective of steric hindrance at the donor site, while the compound with a stronger donor gives rise to ECL that is blue-shifted with respect to its photoluminescence spectrum. All except one of these compounds also exhibit solid state fluorescence which may be useful for solid state devices such as organic light emitting diodes (OLEDs) and as laser dyes. The observed properties are discussed with reference to the structure of the compounds synthesized.

Full text of DOI:10.1039/b410829a

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A R T I C L E
Arylethynylacridines: electrochemiluminescence and photophysical
properties†‡
Arumugasamy Elangovan, Hsing-Hua Chiu, Shu-Wen Yang and Tong-Ing Ho*
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan.
E-mail: hall@ntu.edu.tw; Fax: +886-2-2363-6359; Tel: +886-2-2369-1801
Received 16th July 2004, Accepted 8th September 2004
First published as an Advance Article on the web 30th September 2004
Electrogenerated chemiluminescence (ECL) of six new ethyne-based acridine derivatives (1–6) has been studied.
The new acridine derivatives were synthesized by cross-coupling of 9-chloroacridine and corresponding donor-
substituted phenylethynes under modified Sonogashira conditions. The donor groups were varied in the order of
increasing steric hindrance and donor strength at the donor site. The solution phase photophysical properties and
ECL of these compounds were studied comparatively in acetonitrile solvent. The UV-Visible spectra of compounds
1–5 exhibit closely the same maxima. Density functional theory (DFT) has been invoked to analyze and understand
the unexpected UV-Visible absorption behavior. Compounds with weak electron donors produce excimer ECL
irrespective of steric hindrance at the donor site, while the compound with a stronger donor gives rise to ECL that
is blue-shifted with respect to its photoluminescence spectrum. All except one of these compounds also exhibit solid
state fluorescence which may be useful for solid state devices such as organic light emitting diodes (OLEDs) and as
laser dyes. The observed properties are discussed with reference to the structure of the compounds synthesized.
its homocyclic analog derivatives such as rubrene, 9-phenyl-
Introduction
anthracene and 9,10-diphenylanthracene, have been well studied
Emission of light by recombination of charged radical ions
and used in ECL,3 there are no reports of acridine based bipolar
which are generated by electrolysis of organic molecules having
molecules for luminescent, especially ECL studies and applica-
redox centers is called electrogenerated chemiluminescence (or
tions though there is a report on the use of acridine in mixed
electrochemiluminescence, ECL).1 By sweeping of negative and
systems with polynuclear hydrocarbons.4 This poor attention
positive potentials in sequential cycles with short time intervals,
may be due to the high reduction potential of acridine itself
the excited states of electroactive species are made to populate
the emitting state which subsequently attains the ground state
[E1/2(1) = −1.62 V and E1/2(2) = −2.38 V].5 The photophysics
of acridine and its derivatives have recently been well studied
emitting light of appropriate wavelength. As for the mecha-
theoretically and experimentally.6 Exploitation of the electron
nism, either a singlet or a triplet state of the electroactive species
transfer process in long distances in biological systems such
can be formed depending on the annihilation enthalpy change
as proteins and DNA has received much attention.7 Several
(DH0) during the electron transfer reaction. The ion annihila-
fluorophores including pyrene, 9-aminoacridine and their alkyl
tion can generate the singlet state (S-route)2 if the magnitude of
chain-tethered derivatives have been among the well known
DH0 is sufficiently larger than the energy needed for the excited
intercalators.8 Further, detection of anions by acridine based
singlet state (S1). On the other hand, if the DH0 is lower than
bis-tren [tren = tris(2-aminoethyl)amine], the macrobicyclic
S1, but it is sufficient to generate the triplet state, then the elec-
tris-acridine cryptand, was studied in water through enhance-
tron transfer reaction will lead to the formation of the triplet
ment of fluorescence upon complexation.9 In addition, acridine
state.2b However, it is yet possible to achieve singlet state by
has been incorporated as an integral part of certain polyimdes
triplet–triplet annihilation, and this is known as a T-route.2b As
which were applied as emissive layers and/or hole transport
a result, the light emitted from the electrochemically generated
layers in the fabrication of light emitting devices (LED) due to
excited state of the electroactive species normally is fluores-
their higher thermal stability caused by acridine units.10 Acridine
cence. ECL has been widely applied as a sensitive and selective
is of interest to us because of its ability to intercalate into/bind
detecting technique for many analytical applications.2f It has
with many biologically important molecular fragments and
also been used as an excellent model for investigating the elec-
report information about the host.
tron transfer mechanism.2d Unlike fluorescence, which requires
a light source in order to excite molecules, ECL does not require
a light source for excitation and hence is considered superior
Structural modification in the acridine ring, especially exten-
sion of p-conjugation and introduction of electron push–pull
action, would bring about changes in the properties of acridine,
to fluorescence techniques in certain analytical studies.1c While
such as redox behavior and visible light photophysics, conducive
many molecules have been known to exhibit ECL activity, most
for electro- and photoanalyses.
requiring amines as co-reactants, simple integral molecules
We have been involved in the design, synthesis and study of the
emitting ECL, especially without the use of any co-reactant,
ECL properties of novel ethyne based donor–acceptor systems.11
are of interest to us.
Acridine is structurally similar to anthracene but possesses
Pursuant to our persistent endeavor of finding new molecules
for ECL and LED, we have now chosen acridine as an electron
a nitrogen heteroatom in its central ring. However, although
acceptor unit in the new systems and varied the electron donors
linked by a C–C triple bond in the order of increasing steric
† Electronic supplementary information (ESI) available: 1H- and
hindrance and donor strength. This attempt is more appropriate
13C NMR and ECL spectra, and HOMO and LUMO surfaces of all
in the light of our recent observation that weaker donor cause
compounds, and crystal structure and crystallographic details, includ-
excimer ECL and stronger donors cause ICT ECL in the ethyne
ing bond lengths and angles, for compound 6. See http://www.rsc.org/
based quinoline systems.11a We would like to probe the validity
suppdata/ob/b4/b410829a/
of the theory by the introduction of more steric hindrance at the
donor site. The structures of the molecules synthesized for the
present study are shown in Scheme 1.
‡ CCDC reference number 244931. See http://www.rsc.org/suppdata/
ob/b4/b410829a/ for crystallographic data for compound 6 in .cif or
other electronic format.
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 1 1 3 – 3 1 1 8
3 1 1 3

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26 mg (10%) and a stirring bar were charged in a two neck
round-bottomed flask equipped with a condenser and the setup
was degassed and back-filled three times with a gaseous mixture
of Ar + H2. THF (8–10 mL) was introduced via a syringe into
the reaction flask followed by 6 equivalent of TEA and finally by
a solution of the terminal acetylene (1.1 mmol) [4-tolylacetylene
(Aldrich) 127 mg] in 5 mL THF via a syringe under the mild
reducing atmosphere at about 60 °C. After the addition of the
alkyne, the reaction mixture was refluxed for 8–10 h. When
the TLC indicated disappearance of starting compounds, the
volatiles were evaporated and the residue was recrystallized
from toluene to get pure 2 as yellow crystals 222 mg (76%); mp:
156–158 °C; 1H NMR (400 MHz, CDCl3): d 8.55 (d, J 8.4 Hz,
2H), 8.23 (d, J 8.8 Hz, 2H), 7.79 (t, J 7.6 Hz, 2H), 7.67 (d, J 8.4
Hz, 2H), 7.61 (t, J 7.5 Hz, 2H), 7.27 (d, J 7.6 Hz, 2H), 2.43 (s,
3H); 13C NMR (100 MHz, CDCl3): d 148.76, 140.35, 132.29,
130.73, 130.11, 129.80, 128.66, 127.10, 126.84, 126.81, 119.70,
106.14, 84.09, 22.32; High Mass (M+) m/z: 293.1205 calculated
for C22H15N: 293.1204.
Scheme 1 Structures of phenylethynylacridines prepared and used in
the present study.
9-Phenylethynyl-acridine (1). Mp: 164–165 °C; 1H NMR
(400 MHz, CDCl3): d 8.55 (d, J 8.4 Hz, 2H), 8.24 (d, J 8.8 Hz,
2H), 7.79 (m, 4H), 7.62 (t, J 7.5 Hz, 2H), 7.46 (m, 3H); 13C
NMR (100 MHz, CDCl3): d 148.75, 132.32, 130.66, 130.16,
129.86, 128.99, 128.21, 126.97, 126.86, 126.82, 122.73, 105.56,
84.53; High Mass (M+) m/z: 279.1028 calculated for C21H13N:
279.1048.
Experimental
All the chemicals and reagents were purchased from Acros
Organics, unless specified otherwise, and were used as received.
Dichlorobis(triphenylphosphine)palladium(II)
was
either
prepared in-house or from commercial source (Acros).
9-Chloroacridine was prepared from N-phenylanthranilic acid
and phosphoryl chloride according to the reported method.12
Solvents were distilled as per the standard methods and purged
with argon before use. Triethylamine (TEA) and tetrahydrofuran
(THF) were distilled and purged with a mixture of approxi-
mately 1:1 argon and hydrogen before use. 1H-NMR spectra of
the samples were recorded with a 400 MHz Varian instrument
and 13C-NMR spectra were recorded with the same instrument
at 100.1 MHz operator frequency in CDCl3 solvent (Merck) with
CHCl3 internal standard (d 7.24 ppm for 1H and 77 ppm, middle
of the three peaks, for 13C spectra. Mass spectra were recorded
with a JEOL SX 102A instrument on a nitrobenzyl alcohol
matrix. TLC was run on Merck precoated aluminium plates
(Si 60 F254). Column chromatography was run on Merck silica
gel (60–120 mesh) or neutral alumina (Merck) 70–230 mesh.
All UV-Visible spectra were recorded on a HITACHI U-2000
spectrophotometer with 10 lM solution of the compounds in
CH3CN and all fluorescence spectra on a HITACHI F-3010
fluorescence spectrophotometer using similar solution con-
centrations. Fluorescence quantum yields (U) were determined
in CH2Cl2 with reference to coumarin 1 standard (U = 0.5 in
MeOH).13 CV measurements were done on a CH Instruments
Electrochemical Analyzer. The cell used is a three-electrode
cell consisting of a carbon disc (2.0 mm) working electrode,
a platinum wire counter electrode and a Ag/AgCl reference
electrode. The scan rate was 100 mV s−1. ECL spectra were
recorded at room temperatures using a setup consisting of an
F-3010 Fluorescence spectrophotometer, a CV-27 Voltammo-
graph with a computer interface. The electrode surfaces were
prepared freshly before CV and ECL experiments. The carbon
disc electrode was rubbed against alumina paste and the Pt
electrode was cleaned by rinsing with dilute nitric acid followed
by water and then fired with a naked flame to ensure maximum
cleanliness of the electrode. Typically a 1 mM concentration of
the compound solution in degassed acetonitrile2b with 0.05 M
tetrabutylammonium perchlorate (TBAP) was used. To gener-
ate the annihilation reaction, the platinum electrode was pulsed
between the first reduction and first oxidation potentials and the
pulse interval was controlled on a computer. All measurements
were done at room temperature (22–23 °C).
9-(4-Methoxy-phenylethynyl)-acridine (3). Mp: 167–169 °C;
1H NMR (400 MHz, CDCl3): d 8.56 (d, J 8.8 Hz, 2H), 8.24 (d, J
8.8 Hz, 2H), 7.80 (t, J 8.6 Hz, 2H), 7.73 (d, J 8.8 Hz, 2H), 7.62
(t, J 8.6 Hz, 2H), 6.99 (d, J 8.8 Hz, 2H), 3.88 (s, 3H); 13C NMR
(100 MHz, CDCl3): d 160.89, 148.85, 133.95, 130.58, 130.19,
128.69, 127.08, 126.72, 126.64, 114.83, 114.69, 106.10, 83.68,
55.92; High Mass (M+) m/z: 309.1148 calculated for C22H15NO:
309.1154.
9-(4-Isopropoxy-phenylethynyl)-acridine (4). Mp: 145–147 °C;
1H NMR (400 MHz, CDCl3): d 8.54 (d, J 8.4 Hz, 2H), 8.21 (d, J
8.8 Hz, 2H), 7.78 (t, J 7.8 Hz, 2H), 7.69 (d, J 8.8 Hz, 2H), 7.60 (t,
J 7.4 Hz, 2H), 6.95 (d, J 8.8 Hz, 2H), 4.62 (septet, J 6.0 Hz, 1 H),
1.37 (d, J 6.4 Hz, 2H); 13C NMR (100 MHz, CDCl3): d 159.38,
148.85, 134.02, 130.65, 130.18, 128.88, 127.14, 126.76, 126.67,
116.31, 114.40, 106.36, 83.58, 70.59, 22.57; High Mass (M+) m/z:
337.1471 calculated for C24H19NO: 337.1467.
9-[4-(2-Ethyl-hexyloxy)-phenylethynyl]-acridine (5). Pale
1
liquid; H NMR (400 MHz, CDCl3): d 8.53 (d, J 8.4 Hz, 2H),
8.21 (d, J 8.8 Hz, 2H), 7.77 (t, J 7.8 Hz, 2H), 7.69 (d, J 10.4 Hz,
2H), 7.59 (t, J 7.5 Hz, 2H), 6.96 (d, J 8.4 Hz, 2H), 3.89 (d, J
6.0 Hz, 2H), 1.75 (sextet, J 6.0 Hz, 1H), 1.46 (m, 4H), 1.32 (m,
4H), 0.91 (m, 6H); 13C NMR (100 MHz, CDCl3): d 160.84,
148.87, 133.94, 130.60, 130.19, 128.86, 127.13, 126.75, 126.65,
115.28, 114.50, 106.38, 83.63, 71.22, 39.91, 31.09, 29.68, 24.46,
23.64, 14.71, 11.76; High Mass (M+) m/z: 407.2240 calculated for
C29H29NO: 407.2249.
9-[4-(N,N-Dimethylamino)-phenyl]ethynyl-acridine (6). Mp:
230–232 °C; 1H NMR (400 MHz, CDCl3): d 8.56 (d, J 8.0 Hz,
2H), 8.20 (d, J 8.8 Hz, 2H), 7.77 (t, J 7.7 Hz, 2H), 7.64 (d, J
9.2 Hz, 2H), 7.58 (t, J 7.5 Hz, 2H), 6.72 (d, J 9.2 Hz, 2H), 3.04
(s, 6H); 13C NMR (100 MHz, CDCl3) d 151.19, 148.94, 133.76,
130.52, 130.11, 129.60, 127.33, 126.5, 126.33, 112.16, 109.17,
108.4, 83.59, 40.68; High Mass (M+) m/z: 322.1470 calculated
for C23H18N2: 322.1470.
Theoretical calculations
General procedure for the synthesis of 9-p-tolylethynylacridine (2)
Theoretical calculations were performed using Spartan’04™
for Windows, Wavefunction Inc., Irvine, CA 92612, USA.
Molecules were built and optimized at the B3LYP level of
9-Chloroacridine (prepared according to ref. 12) 213.7 mg
(1 mmol), (PPh3)2PdCl2 14 mg (2 mol%), CuI 4 mg (2%), PPh3
3 1 1 4
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 1 1 3 – 3 1 1 8

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Table 1 Photophysical data of compounds 1–6
a
Compound
kmaxAbs/nm, eV [emax
]
kmaxFlu/nm, eV
Ub
Stokes shift/nm
Solid state emission kmax/nm
1
2
3
4
5
6
417, 2.97 [1.62]
419, 2.96 [1.38]
422, 2.94 [1.46]
424, 2.92 [2.03]
424, 2.92 [2.50]
446, 2.78 [2.15]
431, 2.88
434, 2.86
464, 2.67
469, 2.64
466, 2.66
569, 2.18
0.16
0.27
0.59
0.53
0.62
0.043
14
15
42
44
42
487
520
540
494
—
123
554
a (×104 M−1 cm−1). b Fluorescence quantum yield (U) measured in CH2Cl2 with reference to coumarin 1 (U = 0.50 in MeOH).13
density functional theory (DFT) using 6-31G* basis set at
ground state, and orbital energies and HOMO–LUMO energy
gaps were calculated subsequently. HOMO and LUMO surfaces
are provided in the Supplementary Information.†
and the donor group remains cut off. This indicates that the
electronic transition is vastly borne by the acridinyl moiety and
hence it is observed that compounds 2–5 absorb at closely simi-
lar wavelengths showing structured bands. However the scene
takes a dramatic twist in the case of 6 for which the HOMO is
localised on the donor bearing phenyl moiety and the LUMO
entirely on the acridinyl moiety (see Supplementary Information
for figures†). Therefore the electronic transition in the case of 6
is from the HOMO of the donor to the LUMO of the acridine
suggesting ICT. This can be seen from its structureless absorp-
tion band at the longest wavelength.
Fig. 2 shows the calculated energy level diagram for the
compounds 1–6 and acridine. It can be seen that the LUMO
energy is reduced relative to that of acridine whereas the HOMO
is raised slightly for 1–5 and highest for 6. The HOMO–LUMO
energy gaps remain in the range of 3.14–3.36 eV for 1–6
while the gap for free acridine is 3.7 eV. The theoretical gaps
are expectedly higher by 0.15–0.25 eV than the experimental
values (Table 1) as the calculations were done for molecules in
the gas phase. However reduction in the energy gap for 1–6 can
be attributed to the extension of p-conjugation.
Result and discussion
Compounds 1–6 were prepared in good yields by palladium-
catalyzed Sonogashira coupling of 9-chloroacridine with donor-
bearing terminal arylalkynes under a mild reducing atmosphere
of hydrogen and argon according to our reported method.14 All
1
compounds were characterized by H-NMR, 13C-NMR and
high resolution mass spectra (see Supplementary Information
for spectra†).
Photophysical properties
The UV-Visible absorption spectra of all the compounds were
recorded in acetonitrile solvent at room temperatures (22–23 °C).
All compounds absorb moderately (cf. emax Table 1) and exhibit
absorption maxima characteristic of a p–p* transition as the
longest wavelength absorption along with a b-band absorption
around 260 nm. Compounds 1–5 show closely similar maxima
in their absorption spectra. Although Table 1 carries the maxi-
mum absorption wavelengths (and energy in eV), the onset of
the maxima occur at about 440 nm (2.8 eV) (Fig. 1). Compound
6 stands out from the series by showing the longest wavelength
of absorption starting at about 500 nm (2.5 eV) with a maximum
at 446 nm. This is due to the efficient charge transfer caused by
the introduction of a strong electron donor NMe2 group which
may increase the energy level of highest occupied molecular
orbital (HOMO).
Fig. 2 Calculated HOMO–LUMO energy levels for 1–6 and free
acridine.
All the compounds are visibly fluorescent green in methylene
chloride–chloroform solutions with 6 being yellow–green
fluorescent. The solution fluorescence spectra of compounds
1–5 were recorded in acetonitrile while that of 6 in methylene
chloride as it did not show fluorescence in acetonitrile (Fig. 3).
Compounds 1 and 2 show closely similar emission maxima
around 430 nm with vibrational fine structure, while 3–5 show
slightly red-shifted emission around 465 nm as a structureless
band with enhanced emission quantum yields (cf. U) and the
trend is also observed in the Stokes shift. For 1 and 2 there is
only a marginal Stokes shift while for 3–5 the shift is consider-
ably larger. Compound 6 is outstanding in its emission property
as well as in that it shows a distinct broad emission maximum
at 569 nm albeit with a lower quantum yield and with a Stokes
shift of 123 nm. This behavior is attributed to the presence
of a strong electron donating NMe2 group which makes the
molecule’s energy difference between excited state geometry and
ground state geometry larger. Probably this molecule undergoes
torsional motion to achieve maximum twist between the donor
Fig. 1 UV-visible absorption spectra of 1–6 recorded in CH3CN
(10 lM).
The anomolous UV-Visible absorption behavior could be
understood by analyzing the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
surfaces of these compounds using density functional theory.
The orbital surfaces and energy levels can be compared with
those of free acridine. The HOMO and LUMO for compound
1 are well delocalized throughout the molecule. Whereas for
2–5 both HOMO and LUMO are both localised mainly on the
acridinyl moiety with extension only up to the C–C triple bond
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 1 1 3 – 3 1 1 8
3 1 1 5

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and acceptor moieties with respect to the C–C triple bond. The
lower fluorescence quantum yield may be due to the strong
intramolecular charge transfer (polar compound) which either
quenches fluorescence or leads to a low lying excited state. This
state of affairs is more prominent especially in highly polar
acetonitrile in which no fluorescence was observed.
Fig. 5 Cyclic voltammogram of 6 in acetonitrile (1 mM with 50 mM
TBAP. Scan rate: 100 mV s−1).
In general, extension of p-conjugation has reduced the reduc-
tion potential and favored easier reduction as compared to the
parent acridine.5
ECL spectra were recorded11d with a sample concentration of
1.0 mM in the presence of TBAP (50 mM) in CH3CN. Voltage
was pulsed between the first positive and first negative peak
potentials with varying stepping pulse rates and intervals in order
to get a optimum ECL spectra (see ESI for spectra and voltage
pulse rates†). All compounds exhibited ECL with appreciable
intensities relative to Ru(bpy)32+. Compounds 1–5 show ECL
with maxima red shifted in comparison with their fluorescence
maxima by 20–40 nm. It may be safely ascribed to the emission
from the excimer of the annihilated product responsible for ECL
emission (E-route). Compound 6 can be considered as emitting
ECL from an aggregate in which the acridine moiety of two
molecules face each other inversely with the NMe2 substituted
phenyl donor groups projecting away from the centre (Fig. 6).
This kind of structure is most likely formed at the concentration
used for ECL studies which is 100 folds more concentrated than
that used for fluorescence studies.
Fig. 3 Fluorescence emission spectra of 1–6 [1–5 were recorded in
CH3CN (10 lM) and 6 in CH2Cl2].
Compounds 1–4 and 6, which are solids, were found to emit
fluorescence in the solid state upon irradiation with black light
(364 nm). Hence, qualitative solid state emission spectra were
recorded and are shown in Fig. 4. Compounds 1–3 appear to
show excimer emission while 4 and 6 appear to show mono-
meric emission which can be compared with solution photo-
luminescence.
Fig. 6 Structure of aggregate formation proposed for ECL of 6.
X-Ray crystal structure analysis of compound 6 revealed
features supporting the foregoing argument.16 The compound
crystallized as a triclinic prism with two acridinyl heads invert-
edly facing each other with the N,N-dimethylaminophenyl
groups projecting nearly perpendicularly away in opposite
directions (Fig. 7) consistent with the structure depicted in
Fig. 6. The dihedral angle between the plane of the acridine and
that of the 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 Å. At higher
concentrations under ECL measurement conditions the ECL
emissive species may attain a structure as shown in Fig. 6 and
hence the blue shifted ECL spectra for 6. For comparison, the
fluorescence spectrum of 6 was recorded in CH2Cl2 as well as at
the same concentration used for ECL measurements (1 mM).
We found that the photoluminescence maximum is only red
shifted by about 10 nm (573 nm) as compared to that at 10 lM
solution and with drastically reduced intensity (one-seventh).
Therefore the ECL emissive species is believed to be unique
and quite different from that of the photoluminescent species.
The ECL spectra could be reproduced for 8–10 electrochemical
cycles, however the intensity showed reduction upon cycling
any further. A similar observation has been made in the case
of N,N-dimethylaminophenyl-4-quinolinylethyne (compound 6
minus one phenyl ring in the acridine).11c
Fig. 4 Solid state fluorescence spectra of 1–4 and 6.
Electrochemical analysis and ECL
Cyclic voltammetry (CV) was performed on all the com-
pounds to determine their electroactivity and to arrive at
their redox potential values before electrochemiluminescence
studies. A 1 mM concentration solution of the compound in
degassed acetonitrile with 50 mM TBAP was employed for
the CV measurements with a 100 mV s−1 scan rate. All the
compounds had a reversible reduction peak and an irrevers-
ible or a quasi-reversible oxidation peak. The reduction peak
potential for compounds 1–5 occurs at about −0.85 V and the
oxidation potential at about 1.5–1.6 V (Table 2). Compound 6
follows a trend observed in its photophysical properties. The
CV trace of compound 6 is shown in Fig. 5; the reduction peak
potential at −0.84 V can be ascribed to the reduction at the
acridine moiety5 and the oxidation peak at 0.98 V is due to the
oxidation occurring at the N,N-dimethylaminophenyl group.11c
Introduction of the strong electron donating NMe2 group has
brought down the oxidation potential. This is indicative of the
reduction in the energy gap between the LUMO and HOMO in
this system. This reduction in the HOMO–LUMO gap is also
discernible from the photophysical properties of 6 (vide supra).
3 1 1 6
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 1 1 3 – 3 1 1 8

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Table 2 Electrochemical and ECL data of 1–6
Compound
kmaxECL/nm, eV
Relative intensitya
Ep,OX/V
Ep,RED/V
−DH0/eVb
1
2
3
4
5
6
473, 2.62
482, 2.57
480, 2.58
487, 2.55
479, 2.59
534, 2.32
0.044
0.022
0.09
0.012
0.021
0.01
1.668
1.672
1.643
1.536
1.540
0.984
−0.867
−0.853
−0.805
−0.859
−0.864
−0.842
2.375
2.365
2.288
2.235
2.244
1.666
a Relative to Ru(bpy)32+ (1.0). b Calculated using the equation: −DH0 = Ep,OX − Ep,RED − 0.16 V.15
properties to the resultant molecules which are not commonly
anticipated for the parent compounds. The observance of solid
state fluorescence emission promotes more interest in these
molecules for further application in solid state devices such as
organic light emitting diodes and lasers. Though the ECL was
studied in acetonitrile solvent, introduction of functional groups
Fig. 7 X-Ray crystal packing diagram showing p-stacking interac-
tion between the two adjacent acridinyl units. The inter-planar distance
between the two planes of acridine was measured to be 3.59 Å.
−
such as SO3 could make them water soluble and the ECL
immunoassay could well be carried out in aqueous solutions. In
view of the potential binding nature of the acridines to a variety
of guest species, the compounds reported here and their water-
soluble derivatives may find application in immunoassay and
other sensing applications.
Based on the foregoing arguments a probable mechanism
may be proposed for the formation of an aggregate under ECL
conditions as shown in Scheme 2.
Acknowledgements
Financial support for this work was provided by the National
Science Council, Taiwan. We thank Professor S. M. Peng and
Mr G. S. Lee for providing X-ray data for compound 6.
References
1 (a) L. R. Faulkner, and A. J. Bard, Electrogenerated Chemilumines-
cence, in Electrochemical Methods, John Wiley & Sons, New York,
1980, pp. 621–627; (b) A. W. Knight and G. M. Greenway, Analyst,
1994, 119, 879; (c) A. W. Knight, Trends Anal. Chem., 1999, 18, 47;
(d) R. Y. Lai, E. F. Fabrizio, S. A. Jenekhe and A. J. Bard, J. Am.
Chem. Soc., 2001, 123, 9112; (e) I. Prieto, J. Teetsov, M. A. Fox,
D. A. Vanden Bout and A. J. Bard, J. Phys. Chem., 2001, A105,
520; (f) M. Oyama and S. Okazaki, Anal. Chem., 1998, 70, 5079;
(g) A. Kapturkievicz, J. Electroanal. Chem., 1990, 290, 135;
(h) A. Kapturkievicz, J. Electroanal. Chem., 1991, 302, 13.
2 (a) T. Kuwana, in Electroanalytical Chemistry, A. J. Bard, ed.,
Marcel Dekker, New York, 1966, vol. 1, ch. 3; (b) A. J. Bard and
L. R. Faulkner, in Electroanalytical Chemistry, A. J. Bard, ed.,
Marcel Dekker, New York, 1977, vol. 10, p. 1; (c) D. M. Hercules,
Acc. Chem. Res., 1969, 2, 301; (d) R. Y. Lai, E. F. Fabrizio, L. Lu,
S. A. Jenekhe and A. J. Bard, J. Am. Chem. Soc., 2001, 123, 9112;
(e) L. R. Faulkner and A. J. Bard, Electrogenerated Chemilumines-
cence, in Electrochemical methods, John Wiley & Sons, New York,
1980, pp. 621–627; (f) M. M. Richter, Chem. Rev., 2004, 104, 3003;
(g) N. R. Armstrong, R. M. Whightman and E. M. Gross, Annu.
Rev. Phys. Chem., 2001, 52, 391.
Scheme 2 Probable mechanism for the formation of an aggregate
during ECL.
In order to ascertain the mechanism of ECL emission,
whether it is a S-route or a T-route, the enthalpy change of
radical ion annihilation reaction (−DH0) was calculated from
the equation:15
−DH0 = Ep,OX − Ep,RED − 0.16 eV
The enthalpy change values for radical ion-annihilation reac-
tions, along with the ECL maxima are given in Table 2. The
enthalpy changes of the annihilation reaction in all the cases
studied here have lower values (1.7–2.4 eV) as compared to their
ECL maxima (2.3–2.6 eV) and hence the annihilation reac-
tion did not produce sufficient energy to populate the singlet
exited state and therefore triplet–triplet annihilation must have
occurred to provide the energy required for the formation of
excited state which in turn emits ECL.
3 (a) D. M. Hercules, J. Am. Chem. Soc., 1969, 91, 301; (b) R. E. Visco
and E. A. Chandross, J. Am. Chem. Soc., 1964, 86, 5350;
(c) K. S. V. Santhanam and A. J. Bard, J. Am. Chem. Soc., 1965, 87,
139; (d) D. M. Hercules, Science, 1964, 145, 808.
4 T. D. Santa Cruz, D. L. Akins and R. L. Birke, J. Am. Chem. Soc.,
1976, 98, 1677.
5 Ref. 4 and B. J. Tabner and J. R. Yandle, J. Chem. Soc. A, 1968,
381.
6 (a) Ò. Rubio-Pons, L. Serrano-Andrés and M. Merchán, J. Phys.
Chem., 2001, 105, 9664; (b) J. Prochorow, I. Deperasieska and
O. Morawski, J. Mol. Struct., 2000, 555, 97; (c) P. Nikolov and
H. Görner, J. Photochem. Photobiol., A, 1996, 101, 137.
7 (a) S. A. Wallin, E. D. A. Stemp, A. M. Everest, J. M. Nocek,
T. L. Netzel and B. M. Hoffman, J. Am. Chem. Soc., 1991, 113,
1842; (b) A. W. Axup, M. Albin, S. L. Mayo, R. J. Crutchley and
H. B. Gray, J. Am. Chem. Soc., 1988, 110, 435; (c) D. N. Beratan
and J. N. Onuchic, Photosynth. Res., 1989, 22, 173; (d) J. K.
Barton, V. K. Challa and N. J. Turro, J. Am. Chem. Soc., 1986, 108,
6391; (e) N. J. Turro and J. K. Barton, Science, 1988, 241, 5361;
(f) E. D. A. Stemp, M. R. Arkin and J. K. Barton, J. Am. Chem.
Soc., 1995, 117, 2375; (g) C. J. Murphy, M. M. Arkin, N. D. Ghatlia,
S. Bossman, N. J. Turro and J. K. Barton, Proc. Natl. Acad. Sci.
USA, 1991, 91, 5315.
Conclusion
A new series of donor–acceptor ethynes with acridine as
acceptor and several donor substituted phenyl groups have
been synthesized and their ECL properties were studied. All
the compounds exhibit ECL from the annihilation of the
radical anions and radical cations via T-route. Compounds
with weak donors, irrespective of whether sterically capable
of causing hindrance or not, emit excimer ECL. Compounds
with strong donors such as the NMe2 group gives rise to blue
shifted ECL. Further it is hereby confirmed that the excimer
ECL and monomeric ECL are determined by strength of the
electron donating group rather than steric hindrance at the
donor site. While neither the parent acridine nor the isolated
donor-substituted benzene is capable of producing ECL, it
may be inferred that the extension of conjugation imparts new
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 1 1 3 – 3 1 1 8
3 1 1 7

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8 (a) A. R. Peacocke and N. J. H. Sherrett, Trans. Faraday Soc., 1956,
52, 261; (b) L. S. Lerman, J. Mol. Biol., 1961, 3, 18; (c) D. M. Bassini,
J. Wirz, R. Hochstrasser and W. Leupin, J. Photochem. Photobiol., A,
1996, 100, 65; (d) K. Fukui and K. Tanaka, Nucleic Acids Res., 1996,
24, 3962; (e) K. Fukui, M. Morimoto, H. Segawa, K. Tanaka and
T. Shimidzu, Bioconjugate Chem., 1996, 7, 349; (f) A. I. Kononov,
E. B. Moroshkina, N. V. Tkachenko and H. Lemmetyinen, J. Phys.
Chem. B, 2001, 105, 535; (g) S. Hess, W. B. Davis, A. A. Voityk,
N. Rösch, M. E. Michel-Beyerle, N. P. Ernsting, S. A. Kovalenko
and J. L. Pérez Lustres, ChemPhysChem, 2002, 1439; (h) J. Joseph,
N. V. Eldho and D. Ramaiah, Chem Eur. J., 2003, 9, 5926.
9 M.-P. Teulade-Fichou, J.-P. Vigneron and J.-M. Lehn, J. Chem. Soc.,
Perkin Trans. 2, 1996, 2169.
10 (a) H. M. Gajiwala and R. Zand, Macromolecules, 1995, 28, 481;
(b) R. D. Patil and H. M. Gajiwala, Polymer, 1995, 38, 4557;
(c) S. Xu, M. Yang, J. Wang, H. Ye and X. Liu, Synth. Metals, 2003,
132, 145.
11 (a) A. Elangovan, T.-Y. Chen, C.-Y. Chen and T.-I. Ho, Chem.
Commun., 2003, 2146; (b) C.-Y. Chen, J.-H. Ho, S.-L. Wang and T.-I.
Ho, Photochem. Photobiol. Sci., 2003, 2, 1232; (c) A. Elangovan,
S.-W. Yang, J.-H. Lin, K.-M. Kao and T.-I. Ho, Org. Biomol. Chem.,
2004, 2, 1597; (d) F.-C. Chen, J.-H. Ho, C.-Y. Chen, Y. O. Su and
T.-I. Ho, J. Electroanal. Chem., 2001, 499, 17.
12 G. D. Jaycox, G. W. Gribble and M. P. Hacker, J. Heterocycl. Chem.,
1987, 24, 1405.
13 G. A. Reynolds and K. H. Drexhage, Opt. Commun., 1975, 13, 222.
14 (a) A. Elangovan, Y.-H. Wang and T.-I. Ho, Org. Lett., 2003, 5,
1841; (b) K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron
Lett., 1975, 16, 4467.
15 L. R. Faulkner, H. Tachikawa and A. J. Bard, J. Am. Chem. Soc.,
1972, 94, 691.
16 Crystal data for 6: formula C23H18N2; formula weight: 322.39; unit
cell parameters with standard deviations, cell length a: 9.2827(2) Å;
cell length b: 9.3930(2) Å; cell length c: 10.7357(3) Å; cell angle
a: 81.7260 (10)°; cell angle b: 72.9830 (10)°; cell angle c: 71.801 (2)°;
cell volume: 849.25(3) Å3; symmetry space group: P1; cell formula
units Z: 2; temperature of study: 295(2) K; absorption coefficient:
0.02(2) mm−1; reflections collected: 5997; independent reflections:
3827 (Rint = 0.0192); final R indices [I > 2r(I)]: R1 = 0.0483,
wR2 = 0.1278; R indices (all data): R1 = 0.0671, wR2 = 0.1449.
3 1 1 8
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 1 1 3 – 3 1 1 8