Electrochemistry and ECL of DSBFNPC
A R T I C L E S
Ru(bpy)32+ f Ru(bpy)33+ + e-
(bpy)2Ru2+(bpy) + e- f (bpy)2Ru2+(bpy•-)
(1)
alternating pulsing or sweeping the potential to produce the
reactants. The electron-transfer reaction (ion annihilation) can
produce excited singlet or triplet states, depending on the energy
of the reaction and the energy of the excited state. If the
magnitude of the enthalpy (∆Hann) of an ion annihilation reaction
is larger than the energy (Es) needed for the excited singlet state,
the direct generation of an excited singlet state is possible; this
(2)
Ru(bpy)33+ + (bpy)2Ru2+(bpy•-) f (bpy)2Ru3+(bpy•-)* +
2+
Ru(bpy)3 (3)
(bpy)2Ru3+(bpy•-)* f Ru(bpy)32+ + hν
(4)
is called the S-route or an energy-sufficient system.14 If |∆Hann
|
of the ion annihilation reaction is lower than Es but higher than
the triplet state energy (Et), the generation of an excited singlet
state is still possible by a triplet-triplet annihilation reaction
(a T-route or energy-deficient system).14 If |∆Hann| is close to
Es, the T-route can contribute to the formation of singlet state
species in addition to the S-route, and this is called the ST-
route.15,16 In some cases, excimer or exciplex emission has also
been observed (the E-route).14 Many polyaromatic hydrocarbons,
the first compounds studied for ECL applications, produced
emission by these reaction schemes.14,17
In this case, the oxidized form contains a Ru(III) center and
the reduced form contains a bipyridine radical anion. The excited
state can be written as the charge-transfer state shown in eqs 3
and 4. The fluorene ring provides a good emitting center but
has poor cation radical stability so it is not a good candidate
for ECL. However, by functionalization with the appropriate
redox center, it is possible to improve the stability of the
oxidized fluorene compound for ECL. As discussed earlier,
because of their attractive properties, 9,9′-spirobifluorene de-
rivatives have been proposed as suitable materials in the field
of photoelectronic devices, including ECL devices and photo-
voltaic cells.12,13,21 Molecules incorporating carbazole moieties
have been investigated extensively as light-emitting materials
and as hole-transporting materials in OLEDs because of their
high reversibility upon electrochemical oxidation.22 For example,
poly(N-vinyl carbazole) is often used as a hole transport layer,23
and highly efficient devices with this polymer as a hole transport
polymer have been reported.24 Recently, luminescent thin films
of conjugated polymers containing both of the fluorene and
carbazole moieties as the emitting layer for OLEDs have been
reported.25,26 The fluorene moieties with strong blue fluorescence
play the key role of luminescing sites, and the carbazole units
with coupling activity under anodic oxidation play the role of
the linking sites in electropolymerization. In addition, a non-
conjugated hybrid of carbazole and fluorene has been reported
to produce efficient green and red electrophosphorescent
devices.27 This new material combines characteristics of both
carbazole and fluorene.
Useful ECL compounds must be able to generate stable
radical cations and anions with sufficient energy in the electron-
transfer reaction to generate an excited state that emits with
good quantum efficiency. However, many molecules that emit
strongly do not produce strong ECL, because decomposition
of the radical ion causes one of the redox processes to be
chemically irreversible. In others, the radical cation or anion
cannot be generated before the background oxidation or
reduction of the solvent-supporting electrolyte. In these cases,
ECL coreactants can be added to generate a stable radical
counterion required for annihilation.14 ECL coreactants are
species that, upon electrochemical oxidation or reduction,
undergo a bond cleavage reaction to produce intermediates that
are strong reductants or oxidants, capable of reacting with one
of the electrogenerated species to produce excited states capable
of emitting light.18
In recent years, compounds that contain a coreactant directly
attached to the emitting dye with one reversible electrochemical
process, or compounds that have donor and acceptor moieties
covalently bonded with a capability to generate ECL through
charge-transfer states, have been studied19,20 with the hope of
improving the radical cation and anion stability and obtaining
structures that contain charge-transfer characteristics analogous
to those of Ru(bpy)32+. In this case, the oxidized form is a Ru-
(III) center and the reduced form is a bipyridine radical anion.
The excited state can be written as the well-known charge-
transfer state shown in the following reactions
Experimental Section
Materials. N-Phenylcarbazole (Aldrich) and anhydrous acetonitrile
(MeCN, Aldrich) were used without further purification, and anhydrous
benzene (Aldrich) was used after distillation. Tetra-n-butylammonium
hexafluorophosphate (TBAPF6) (Aldrich) was dried in a vacuum oven
at 125 °C before being transferred directly into an inert atmosphere
glovebox (Vacuum Atmospheres Corp.). All solutions were prepared
in the glovebox with fresh anhydrous solvents and sealed in airtight
vessels for measurements completed outside the box.
Synthesis. A mixture of 3,6-dipinacolbronic ester-9-phenylcarba-
zole36 (495 mg, 1.0 mmol), 2-bromo-9,9′-spirobifluorene1 (827 mg, 2.1
mmol), Pd(PPh3)4 (213 mg, 0.2 mmol), P(t-Bu)3 (2 mL, 0.05 M in
(15) Bezman, R.; Faulkner, L. R. J. Am. Chem. Soc. 1972, 94, 6317.
(16) Tachikawa, H.; Bard, A. J. Chem. Phys. Lett. 1974, 26, 246.
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T.; Bard, A. J. J. Am. Chem. Soc. 1971, 93, 5968. (c) Bezman, R.; Faulkner,
L. R. J. Am. Chem. Soc. 1972, 94, 6324. (d) Debad, J. D.; Morris, J. C.;
Lynch, V.; Magnus, P.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 2374. (e)
Debad, J. D.; Morris, J. C.; Magnus, P.; Bard, A. J. J. Org. Chem. 1997,
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F.; Salbeck, J.; Spreitzer, H.; Gratzel, M. Nature 1998, 395, 583.
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