First synthesis and electrogenerated chemiluminescence of novel p-substituted phenyl-2-quinolinylethynes

Article abstract of DOI:10.1039/b305943j

A series of p-substituted phenyl-2-quinolinylethynes as blue-green emitters were synthesized using a modified Sonogashira coupling reaction and their electrogenerated chemiluminescence properties are reported.

Full text of DOI:10.1039/b305943j

First synthesis and electrogenerated chemiluminescence of novel
p-substituted phenyl-2-quinolinylethynes†‡
Arumugasamy Elangovan, Ting-Yu Chen, Chih-Yuan Chen and Tong-Ing Ho*
Department of Chemistry, National Taiwan University, Taipei – 106, Taiwan, R O C.
E-mail: hall@ntu.edu.tw; Fax: +886-2-2363-6359; Tel: +886-2-2369-1801
Received (in Cambridge, UK) 28th May 2003, Accepted 30th June 2003
First published as an Advance Article on the web 15th July 2003
A series of p-substituted phenyl-2-quinolinylethynes as blue-
green emitters were synthesized using a modified Sonoga-
shira coupling reaction and their electrogenerated chem-
iluminescence properties are reported.
Electrogenerated chemiluminescence (ECL) is an important
property of organic molecules and coordination compounds of
Ru ions. ECL can be realized by the annihilation of radical
Scheme 1 Synthesis of arylquinolinylethynes.
cations and radical anions.1 ECL has been found to be better
than conventional chemiluminescence, in that it is characterized
absorption maxima consistent with ICT emission behavior.
by high sensitivity, selectivity and it is convenient to generate
Minimized ground state molecular modelling calculations
the required reagents in solution in the vicinity of an electrode.2
carried out using Spartan™ 4.0 indicated increasing twisting
between the donor and acceptor moieties as the electron
donating ability is increased (Table 1, q, the twist angle between
the quinoline–triple bond plane and the donor bearing phenyl).
Development of new analytical systems has been a topic of
current interest.3–5 Recently ECL from intramolecular charge
transfer (ICT) systems has drawn much attention.3,6–7
We have studied the photophysical properties of ICT systems
1–3 show small q while 4–8 show larger q.
consisting of an N,N-dialkylaminophenyl donor and a 2-quino-
The CV curve for 3 recorded¶ between +2.7 V and 23 V is
linyl acceptor connected by a double bond.8–11 We found that
shown in Fig. 1. The reduction potentials (21.94 V and 22.3 V)
the quinoline moiety can act as a good electron acceptor,
correspond to reduction at the quinoline moiety and acetylene
especially if it is connected through a double bond to a good
electron donor. These systems show larger bathochromic shifts
in the absorption and emission spectra.11 However ECL
properties of alkynes are rare and, to the best of our knowledge,
no ECL properties have been reported for arylquinolinylethynes
so far. We became interested in studying donor–acceptor
systems with a triple bond as connector since the acetylenic
bond has been shown to be a good connector for these donors
and acceptors and in photoactive molecular conductors.12,13
We have shown14 that diarylethynes can be conveniently
prepared by Sonogashira crosscoupling of alkynes and aryl
halides with diminished homocoupling by using a dilute
hydrogen atmosphere. This, coupled with our quest for new
simple compounds exhibiting ECL, prompted us to attempt the
synthesis of arylquinolinylethynes with donor substituents and
study their electrochemiluminescence properties. A series of
novel ICT compounds from N,N-dialkylanilinyl donors and
2-quinolinyl acceptor linked by a triple bond were synthesized
using our modified Sonogashira reaction conditions (Scheme
1).§
function respectively.16,17 The positive potential at +1.71 V is
ascribed to oxidation at the methoxyphenyl site (for 4–8 the
donor is the N,N-dialkylaminophenyl moiety and they show
similar oxidation potentials 0.85–1.05 V). The first reduction
and oxidation potentials which correspond to the reduction of
the quinoline and oxidation of the donor moiety respectively are
recorded in Table 1.
ECL for 1–8 was measured according to our previously
published method.18 The ECL spectrum for 5 between 21.8 V
(0.2 s) and +0.9 V (0.2 s) with 2 s intermission is shown in Fig.
2, and the ECL maxima for 1–8 are summarized in Table 1.
No excimer peaks were observed in the ECL spectra, as seen
from Fig. 2. The ECL spectra of 1 and 2 showed very low
intensity. Therefore a strong donor group on the phenyl moiety
is found to be essential for the detection of ECL.
The absorption and emission maxima for 1–8 are given in
Table 1. 4–8 show ICT behavior11 through solvatochromic
studies. 4–8 show larger Stokes’ shifts in their fluorescence and
† Electronic supplementary information (ESI) available: experimental. See
http://www.rsc.org/suppdata/cc/b3/b305943j/
‡ Dedicated to Professor F. D. Lewis on the eve of his 60th birthday.
Fig. 1 Cyclic voltammogram of 3.
Table 1 Photophysical and electrochemical data of arylquinolinylethynesa
Product
lmaxAbs/nm
lmaxFlu/nm, eV
Shift/nm
lmaxECL/nm, eV
Ep,RED/V
Ep,OX/V
2DH0/eVb
q/°
0.0
0.0
0.5
53.8
7.6
8.5
1
2
3
4
5
6
7
8
326
316
346
373
379
380
366
359
361, 3.44
364, 3.41
405, 3.06
527, 2.35
528, 2.35
533, 2.33
531, 2.34
523, 2.37
35
48
59
154
149
153
165
164
463, 2.66
472, 2.62
479, 2.59
540, 2.30
539, 2.30
547, 2.27
527, 2.35
531, 2.33
21.88
22.16
21.94
21.97
21.89
21.91
21.84
21.94
2.16
2.11
1.71
0.92
0.96
1.11
0.98
1.01
3.88
4.11
3.49
2.73
2.69
2.86
2.66
2.79
18.9
33.3
a Conditions of optical and CV measurements: CH3CN, 25 °C. b Calculated from 2DHo = Ep,OX 2 Ep,RED 2 0.16 eV.15
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CHEM. COMMUN., 2003, 2146–2147
This journal is © The Royal Society of Chemistry 2003

with diminished homocoupling and their ECL properties were
studied. The ECL for 1–3 is believed to be from the excimer
formed by annihilation of radical ions generated electro-
chemically. For 4–8, due to their larger twisting donor–acceptor
angle, the ECL can be generated by the annihilation of their
radical ions to populate their ICT states. Preliminary studies on
the usefulness of these compounds indicate possible application
as anion sensors. For example, 4 was found to detect F2 ion by
quenching of emission. Research in this direction will form a
future study.
Financial support by the National Science Council, Taiwan,
Republic of China is gratefully acknowledged.
Fig. 2 ECL spectrum of 5.
Notes and references
The annihilation enthalpy change (2DHo) calculated from
the oxidation and reduction potentials15 (Table 1) is sufficient to
populate the singlet state of 1–8 (S-route). The singlet excited
state of 1–8 can be reached in the electron transfer reaction of
the radical ions.
§ Coupling without the use of hydrogen-degassing resulted in very low
yields of products together with major diphenylbutadiynes. Typical
experimental procedure:14 4-iodo-N,N-diethylaniline (1 mmol),
(PPh3)2PdCl2 (1 mol%), CuI (1 mol%) were taken in a flask and degassed
and back-filled with a mixture of N2 + H2 3 times and triethylamine (8 mL,
degassed with N2 + H2) followed by trimethylsilylethyne (1.1 mmol) were
added. The resulting mixture was stirred for 2 h at rt. The solvent was
evaporated and worked up with ether and saturated NaHCO3 solution. The
ether solution after drying was passed through a short alumina column and
then evaporated to get the pure TMS derivative in 99.6% yield. This was
hydrolyzed by stirring in MeOH with 0.4 g K2CO3 for 2 h followed by
aqueous work-up to get fine low melting needles of pure 4-N,N-
diethylaminophenylethyne in 94% yield. This acetylene was coupled with
2-chloroquinoline by refluxing in THF (degassed with N2 + H2) with 2
mol% Pd catalyst, 2 mol% CuI and 4 equiv. TEA. After 24 h, the volatiles
were evaporated and the crude product was extracted with ether and
evaporated. Column chromatography of the residue on silica gel afforded
pure 5 in 95% yields for 70% conversion (details in ESI†).
¶ See ESI for details.† Typically a 1 mmol solution of the compound in dry
degassed CH3CN with 0.05 M tetrabutylammonium hexafluorophosphate
(TBAP) was electrolysed under argon using a carbon disc (2.0 mm) working
electrode, a Pt wire counter electrode and a Ag/AgCl reference electrode
with a scan rate of 100 mV s21. The ECL spectra were recorded using a
setup consisting of a F-3010 Fluorescence spectrophotometer, a CV-27
voltammeter with a PC interface under CV conditions. The ECL intensities
were measured with 40 nm slit width relative to tris(2,2A-bipyridyl)rutheniu-
m(II) chloride.
1–3 show larger shifts in the ECL maxima as compared with
their fluorescence maxima, while ECL maxima for compounds
4–8 are pretty much the same as their fluorescence maxima. The
ECL for 1–3 can be explained on the basis of excimer emission
generated from the annihilation of the radical anions and radical
cations of 1–319 (E-route). No excimer emission is observed
when higher concentrations of 1–3 were employed for photo-
excitation. Even though there are reports4,19 that ECL excimer
emissions can be observed, no excimer emission was recorded
by direct photoexcitation. Direct formation of excimers by
radical ion comproportionation reactions is more probable
under the ECL experimental conditions since the radical ions,
when annihilated, should be in close proximity with the
appropriate geometry. The ECL for 4–8 derived from the
annihilation arising from collisions between the radical cations
and radical anions of these compounds to generate ICT states.7
The requirement for the generation of the ICT state is provided
by the large twist angle between the plane of the quinoline
moiety and the donor bearing phenyl moiety as seen from the
larger calculated twist angle.
The mechanism of ECL emission can be divided into two
categories. For 1–3 with no or weak electron donating
substituents and small twist angle, it is less favorable to
populate the ICT state. Due to the planar geometry, they tend to
show excimer type ECL emission albeit with less efficiency.
The mechanism is similar to that already reported for poly(9,9-
dioctylfluorene)4 and is shown in Scheme 2. A represents the
acceptor (2-quinoline) moiety and D the donor (substituted
phenyl) moiety of the same molecule. During electrochemical
redox reaction the radical anion and radical cation are formed
(eqn. 1 and 2). Then they collide to form an excimer (eqn. 3 and
4). For 4–8 the ECL mechanism (Scheme 3) is quite different
from that for 1–3. The radical ions collide neck-to-neck to
generate the ICT state directly (eqn. 5).
1 L. R. Faulkner and A. J. Bard, Electrogenerated Chemiluminescence, in
Electrochemical methods, John Wiley & Sons, New York, 1980, pp.
621–627.
2 (a) A. W. Knight and G. M. Greenway, Analyst, 1994, 119, 879; (b) A.
W. Knight, Trends Anal. Chem., 1999, 18(1), 47.
3 R. Y. Lai, E. F. Fabrizio, L. Lu, S. A. Jenekhe and A. J. Bard, J. Am.
Chem. Soc., 2001, 123, 9112.
4 I. Prieto, J. Teetsov, M. A. Fox, D. A. Vanden Bout and A. J. Bard, J.
Phys. Chem. A, 2001, 105, 520.
5 M. Oyama and S. Okazaki, Anal. Chem., 1998, 70, 5079.
6 A. Kapturkievicz, J. Electroanal. Chem., 1990, 290, 135.
7 A. Kapturkievicz, J. Electroanal. Chem., 1991, 302, 13.
8 S.-L. Wang, T.-C. Lee and T.-I. Ho, J. Photochem. Photobiol. A: Chem.,
2002, 151, 21.
9 S.-L. Wang and T.-I. Ho, Spectrochim. Acta Part A, 2001, 57, 361.
10 S.-L. Wang and T.-I. Ho, J. Photochem. Photobiol. A: Chem., 2000,
135, 119.
11 S.-L. Wang and T.-I. Ho, Chem. Phys. Lett., 1997, 268, 434.
12 A. Harriman and R. Zeissel, Coord. Chem. Rev., 1998, 171, 331.
13 (a) J. G. Kushmerick, D. B. Holt, S. K. Pollack, M. A. Ratner, J. C.
Yang, T. L. Schull, J. Naciri, M. H. Moore and R. Shashidhar, J. Am.
Chem. Soc., 2002, 124, 10654; (b) E. Galopini, W. Guo, W. Zhang, P.
G. Hoertz, P. Qu and G. J. Meyer, J. Am. Chem. Soc., 2002, 124, 7801;
(c) C. Hortholary and C. Coudret, J. Org. Chem., 2003, 68, 2167; (d) S.
Huang and J. M. Tour, J. Am. Chem. Soc., 1999, 121, 4098.
14 A. Elangovan, Y.-H. Wang and T.-I. Ho, Org. Lett., 2003, 5(11),
1841.
15 L. R. Faulkner, H. Tachikawa and A. J. Bard, J. Am. Chem. Soc., 1972,
94, 691.
16 W. H. Smith and A. J. Bard, J. Am. Chem. Soc., 1975, 97(22), 6491.
17 W. F.-X. Ding, J. R.-Y. Xie, G.-Z. Ji and X.-K. Jiang, J. Chem. Res. (M),
1998, 1482.
Scheme 2 Plausible mechanism of ECL emission for 1–3.
Scheme 3 Plausible mechanism of ECL emission for 4–8.
In summary we have uncovered a new family of compounds
showing ECL based on quinoline acceptors and aryl donors
linked by a triple bond. Donor substituted phenyl-2-quinoliny-
lethynes (3–8) were prepared for the first time in good yields
18 F.-C. Chen, J.-H. Ho, C.-Y. Chen, Y. O. Su and T.-I. Ho, J. Electroanal.
Chem., 2001, 499, 17.
19 E. A. Chandross, J. W. Longworth and R. E. Visco, J. Am. Chem. Soc.,
1965, 87, 3259.
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