each other (Scheme 4). Figs. 4 and 5 are typical of normal
excimer ECL (of 3b) and ICT ECL (of 1e) respectively depicted
compared with the corresponding fluorescence curves. A survey
of the literature on the excimers of common fluorophores
revealed that only alkyl spacer linked bisarenes of a specific
type (e.g., naphthalene, phenanthrene etc.) exhibit excimer pho-
toluminescence while the free arene counterparts do not.13,14
Free quinoline is not known to exhibit excimer emission.
Further, the reduction in intensity of ECL accompanied by the
blue-shift (as compared to photoluminescence) is indicative of
aggregation.13 In the case of 3d–3f the only possibility by which
they can form a blue-shifted excimer is by trans excimer
formation.
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)3
and is shown in Scheme 2 where A represents the acceptor
(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 (eqns 1
and 2). Then they collide to form an excimer (eqns 3 and 4). For
‘1d–1f ’ the ECL mechanism (Scheme 3) is quite different
from that for ‘na–nc’. The radical ions collide neck-to-neck to
generate the ICT state directly (eqn. 5). The third blue-shifted
ECL for 3d–3f is a sequel to H-type excimer or trans excimer
as shown in Scheme 4. This is a unique property of the
4-quinolinyl system due to the symmetrical nature of the
excimer.
Conclusion
In summary, we have disclosed a new family of compounds
showing ECL based on quinoline and isoquinoline acceptors
and aryl donors linked by a triple bond. Various donor substi-
tuted phenylquinolinylethynes and phenylisoquinolinylethynes
were prepared in good yields and their ECL properties were
studied. In all of the ECL active systems no co-reactant was
used. The ECL for weak donor substituted compounds
(1a–1c,2a,2b,3a–3c,4a,4b,5a–5c) is believed to be from the
normal excimer formed by annihilation of radical ions gener-
ated electrochemically. Compounds with strong donor groups
(1d–1f,5d,5e), show ECL from their ICT states. A strange
aggregation of H-type excimer formation in 3d–3f is believed
to be responsible for the observed blue-shift of ECL in
comparison with their solution fluorescence maxima. It can be
seen that only the 4-quinolinyl systems show H-type excimers
and this may be due to the favorable geometrical arrangement
(with a center of symmetry) of the excimers of 4-quinolinyl
derivatives. From the annihilation enthalpy changes of
reaction, we understand that ECL for compounds 1a–1f are
derived from singlet states and the rest (2–5) from a triplet–
triplet annihilation mechanism. Thus the present study has
thrown light on the fundamental aspects governing the ECL
phenomenon. The compounds reported here may have possible
application as sensors in molecular recognition oriented
towards immunoassay due to the presence of a hydrogen
bonding site in the fluorophore moiety. Introduction of many
binding sites at the fluorophore moiety would be more helpful
for the studies. Efforts in this direction will be pursued by us in
the future.
Scheme 4 Proposed structure of the H-type dimer of 3e.
Fig.
4
Typical comparative fluorescence (solid line) and ECL
(squares) spectra of 3b showing red shifted (excimer) ECL.
Acknowledgements
Financial support by the National Science Council of Taiwan is
gratefully acknowledged.
Fig. 5 Typical comparative fluorescence (solid line) and ICT ECL
(squares) spectra of 1e showing close overlap of both solution
fluorescence and ECL spectra.
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Direct formation of excimers by radical ion comproportion-
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1601