Efficient and stable blue electrogenerated chemiluminescence of fluorene-substituted aromatic hydrocarbons

Article abstract of DOI:10.1002/anie.200904156

(Figure Presented) Properly shielded: The fluorene groups used as capping agents for new diphenylanthracene (DPA), pyrene, and anthracene derivatives impart steric hindrance which prevents interchromophore interactions, giving these molecules high photolu

Full text of DOI:10.1002/anie.200904156

Communications
DOI: 10.1002/anie.200904156
Optical Materials
Efficient and Stable Blue Electrogenerated Chemiluminescence of
Fluorene-Substituted Aromatic Hydrocarbons**
Khalid M. Omer, Sung-Yu Ku, Ken-Tsung Wong,* and Allen J. Bard*
Tailored molecular structure is the key to tune the electronic
and spectroscopic properties of a molecule. This is always a
goal in electrogenerated chemiluminescence (ECL), that is, to
have a material with high photoluminescence (PL) quantum
yield and stable, long-lived radical ions in solution upon
oxidation and reduction. Among various ECL active materi-
als, polyaromatic hydrocarbons (PAHs) were the earliest
subjects of ECL studies,[1] and substituted PAHs have been
widely investigated.[2] Indeed, the PAHs 9,10-diphenylanthra-
cene (DPA) and rubrene (R) have usually been used as ECL
standards.[1a] These molecules are intense ECL emitters
because of their high fluorescence quantum yields and
stable radical cations and anions in aprotic media. However,
the ECL emission of DPA and R eventually decays. This
might be attributed to the irreversibility of the second
oxidation because of the instability of the dication, for
example, DPA2+. Although for generation of ECL spectra,
the radical ions are typically obtained by pulsing the electrode
potentials, slightly beyond ( ꢀ 100 mV) the first reduction and
oxidation peak potentials, there is always some disproportio-
nation of the radical cation leading to dication production.
Thus, instability of doubly charged ions leads to slow
decomposition. If the second oxidation wave is made more
electrochemically reversible, the stability of ECL should be
significantly improved. A similar argument has been made in
considering the stability of electrochromic devices involving
the generation of radical ions.[3]
the same as the fluorescence spectrum because both methods
produce the same excited state. However, the ECL spectra of
some PAHs, like pyrene, show another extra broad and
structureless peak at longer wavelengths, which is attributed
to the formation of excimer.[4] Long wavelength emission can
also be attributed to the formation of a byproduct during the
electrolysis, for example, by the decomposition of the radical
cation, as seen in ECL of anthracene.[1e]
Fluorene-based molecules, like oligofluorenes[2a] and ter-
fluorenes[5] are of interest in ECL and organic light emitting
devices (OLED) because of their good electrochemical and
thermal stabilities and high quantum yields. The present work
reports that capping a DPA derivative, pyrene and anthra-
cene, with two fluorene derivatives produces new aromatic
hydrocarbons (FDF, FPF, FAF) (Figure 1) with very interest-
ing ECL behavior, with enhanced ECL efficiency and stability
as compared to their parent PAHs.
Figure 2 depicts a comparison of the cyclic voltammo-
grams (CV) of FAF, FDF, and FPF and their parent counter-
parts. The electrochemical data are summarized in Table 1.
FAF exhibits two oxidation peaks (Figure 2a). The first
oxidation at Eꢁ1;ox = 1.13 V vs. SCE is reversible, at a less
In aprotic media, ECL involves light emission due to the
electron transfer process in the vicinity of the electrode with
the intensity largely governed by the stability of the electro-
generated radical ions over the time scale of the potential
sweeps (or steps) and the quantum efficiency of the generated
excited state.[1a] The ECL spectrum of a compound is usually
[*] K. M. Omer, A. J. Bard
Center for Electrochemistry, Department of Chemistry and Bio-
chemistry and Center for Nano- and Molecular Science and
Technology, University of Texas at Austin
Austin, TX 78712 (USA)
E-mail: ajbard@mail.utexas.edu
S.-Y. Ku, K.-T. Wong
Department of Chemistry, National Taiwan University
106 Taipei (Taiwan)
E-mail: kenwong@ntu.edu.tw
[**] We thank Roche (BioVeris), The Robert A. Welch Foundation
(F0021), the National Science Foundation NSF (CHE-0808927), and
the National Science Council of Taiwan for supporting this work. We
thank Shu-Hua Chou for her assistance with theoretical calcula-
tions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904156.
Figure 1. Chemical structures of the new ECL active aromatic com-
pounds and model compounds.
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is unstable, which we believe is due to the lack of
additional substitutions on the C2 and C6 positions of
anthracene. In addition, FAF shows two sequential
reduction peaks at Eꢁ1;red = ꢂ1.92 V and Eꢁ2;red = ꢂ2.29 V
vs. SCE, whereas anthracene exhibits only one higher
reduction peak at Eꢁ1;red = ꢂ2.09 V. Although the electro-
chemical behavior of FAF is close to that of DPA, the
second reduction of FAF is clearly seen while that of DPA
could not be detected within the potential window of the
solvent used in this experiment.
FDF, with C2, C6, C9, and C10 aryl-blocked anthra-
cene core, reversibly oxidizes to give its first oxidation at
Eꢁ1;ox = 1.05 V (for DPA, E1ꢁ;ox = 1.15 V)[8] and the second
reversible oxidation peak at Eꢁ2;ox = 1.48 V (compared to
DPA with an irreversible wave, even at higher scan rates,
E
p2,ox = 1.58 V)(Figure 2b)). The observed peak separa-
tion for the reversible waves was ca. 80 mV, larger than
expected peak splitting for ideal nernstian behavior,
where a one-electron redox wave is expected to have a
peak separation of ca. 59 mV. However, the internal
standard, ferrocene, which is known to show nernstian
behavior showed a similar peak separation under the
same conditions. Thus, the observed peak separation can
be attributed to ohmic drop ( ꢀ 1200 ohm) that is often
observed with aprotic solvents. Scan rate studies (Sup-
porting Information, Figure S1, S2) showed that the
anodic and cathodic peak currents (ipa, ipc) of the first
oxidation wave were proportional to the square root of
scan rate (n1/2) while the corresponding peak potentials
(Epa, Epc) were independent of n. Additionally, the peak
current ratio (ipa/ipc) was approximately unity down to a
scan rate of 100 mVsꢂ1, indicating the absence of a
subsequent chemical reaction upon oxidation. FDF shows
two reversible reduction peaks with each reduction peak
height and separation ( ꢀ 80 mV) equal to those of the
oxidation waves, indicating that each reduction is a one-
electron transfer process. The potentials of the reduction
waves were E1ꢁ;red = ꢂ1.78 V and E2ꢁ;red = ꢂ2.27 V, while for
DPA, only one reduction at Eꢁ1;red = ꢂ2.06 V was detected.
The lower first oxidation potential (by 100 mV) and reduction
potential (by 280 mV) and the reversibility of the second
oxidation and reduction processes indicate that radical ions
and doubly charged moieties are more stable in FDF than in
DPA. This can be attributed to the enhanced p-conjugation,
rendering the delocalization of charges throughout the parent
DPA core and fluorene substitutions, although the DFT
calculation (B3LYP/STO-3G) on the energy-optimized
molecular geometry of FDF (Figure S3) reveals the dihedral
angles between the DPA core and two C9-biphenyl substi-
tuted fluorene rings are 29.48 and 31.58, respectively. Thus, the
introduction of fluorene peripherals on C2 and C6 of
anthracene not only stabilizes the radical ions, but also
blocks the active positions subject to decomposition, giving
rise to extra stabilization on the highly charged species
(dication and dianion).
Figure 2. Cyclic voltammograms of a) FAF and anthracene, b) FDF and
DPA, and c) FPF and pyrene. Conditions: 1 mm compound, 0.1m Bu4NPF6
as supporting electrolyte in benzene/MeCN (1:1), scan rate 100 mVsꢂ1
;
working electrode: Pt disk (1 mm diameter), counter electrode: Pt wire,
reference electrode: Ag wire (calibrated vs Fc/Fc+). Normalized absorption
(solid line) and emission spectra (dotted line, excited at absorption
maxima) of d) FAF and anthracene, e) FDF and DPA, and f) FPF and pyrene
in MeCN/benzene (1:1).
Table 1: Electrochemical data of FAF, FDF, and FPF as compared to their
parent counterparts.[a]
Oxid. [Vꢁ]
E1;ox
Red. [Vꢁ]
Cmpd.
E1ꢁ;ox
E1ꢁ;red
E2;red
D [cm2 sꢂ1
]
FAF
anthracene
FDF
DPA
FPF
1.13
1.24(Ep)
1.05
1.15
1.10
1.57(Ep)
–
1.48
1.58(Ep)
1.43
–
ꢂ1.92
ꢂ2.09
ꢂ1.87
ꢂ2.06
ꢂ2.00
ꢂ2.19
ꢂ2.29
9.5ꢀ10ꢂ6
–
ꢂ2.27
9.0ꢀ10ꢂ6
9.0ꢀ10ꢂ6
–
ꢂ2.30
pyrene
1.16
–
[a] All potentials are versus SCE,* where Eo(Fc/Fc+)=0.424 V vs. SCE. D:
diffusion coefficient.
positive potential than the parent anthracene by 130 mV
(anthracene, Ep = 1.24 V; Ep is peak potential).[6] The second
oxidation of FAF at Ep2,ox = 1.57 V is irreversible even with a
scan rate of 2000 mVsꢂ1. As shown in Figure 2a, the oxidation
of anthracene is totally irreversible. Aikens et al. suggested
that the irreversible oxidation of anthracene can be attributed
to the nucleophilic attack by solvent on the meso positions of
anthracene leading to the decomposition of the radical
cations.[7] Thus, C9 and C10 aryl-substituted anthracenes,
such as DPA and FAF, can efficiently suppress the decom-
position process, and allow reversible oxidations. The irre-
versible second oxidation of FAF indicates the dication FAF2+
As compared to pyrene (Figure 2c), FPF shows two
oxidation waves E1ꢁ;ox = 1.10, E2ꢁ;ox = 1.43 V vs. SCE. The first
oxidation is reversible at all scan rates ranging from 50 mV to
10 Vsꢂ1, while the second oxidation shows reversibility only
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with the scan rates beyond 250 mVsꢂ1. In addition, two
sequential reversible reduction peaks (Eꢁ1;red = ꢂ2.00 V,
Eꢁ2;red = ꢂ2.30 V vs. SCE), even at low scan rates, were
detected. The lower potentials and reversibility of electron
transfer processes as compared to those of pyrene agree
with the stabilization effects of fluorene substitutions. The
perturbation of the redox potentials, after capping with
fluorenes, is more pronounced in FDF than in FPF,
implying better electronic interactions between fluorenyl
peripherals and DPA than in the case of pyrene. However,
it is reasonable to anticipate that these new molecules
should provide efficient ECL with higher stability due to
the increase in lifetimes of the oppositely charged radicals,
which subsequently annihilate to form the excited states.
Electronic absorption and photoluminescence (PL)
spectra of FAF, FDF, and FPF are shown in Figure 2 d–f.
The photophysical data are summarized in Table 2. All
spectra of the new compounds show evident shifts to lower
energy as compared to their parent compounds because of
the increased p-conjugation in the new chromophores,
agreeing with our observations in electrochemical studies.
Figure 3. Normalized ECL spectra of a) FAF, b) DPA and FDF, c) pyrene
and FPF taken in the same solution used for electrochemistry experiments
shown in Figure 1. Pulsing is from Ep1,ox =+80 mV to Ep1,red =ꢂ80 mV with
integration time of 10 s for each sample and a slit width of 0.5 mm.
d) Transient ECL experiment, electrochemical current (blue line), ECL
These new compounds generally show small Stokes shifts, intensity (red line) for FDF. Pulse width: 0.1 s, sampling time: 1 ms,
pulsing pattern: 0 V to negative (Ep1,red =ꢂ80 mV) to anodic
which can be attributed to the structural rigidity and low
(Ep1,ox =+80 mV).
Table 2: Photophysical properties of FDF, FPF, and FAF as compared to
their parent compounds.[a]
Table 3: Photophysical and ECL parameters of FAF, FDF, and FPF.
[b]
[c]
Cmpd.
lmax,Abs [nm]
lmax,PL [nm]
FPL Stokes
Cmpd. lmax,PL [nm]
lmax,ECL [nm]
Es[a] DH8an FPL Fr,ECL
shift
FAF
FDF
437
450
2.92 2.95
2.81 2.82
0.62 0.65
0.90 0.90
FAF
341, 360, 380, 400
437
0.62 37
24
451, 482,
516(sh)
430
460, 485,
513(sh)
445
anthracene 310, 327, 340, 357,
376
400, 423, 450, 482
FPF
3.05 3.00
0.81 0.85
FDF
DPA
FPF
385, 407, 430
355, 373, 393
373
452, 485, 516(sh)
408, 429, 450(sh)
430
392, 413, 440,
470(ex)
0.90 22
15
0.81 57
52
[a] Es
calculated ꢁ based
on
1= (Eabsorption
+
Eemission).
2
[b] DHan ¼ E1ꢁ;ox ꢂ E1;red ꢂ 0:1 eV. [c] Fr,ECL is the relative ECL compared
ECL
DPA
to DPA, taking F =1. sh: shoulder.
pyrene
311, 324, 340
[a] Solvent is MeCN/benzene (1:1) for all the compounds. sh: shoulder,
ex: excimer.
Anthracene shows poor ECL through direct annihilation
reaction in aprotic solvents because of the irreversibility of
oxidation (Figure 2a).[12] Since the radical cation of anthra-
cene has a very short life-time, a pulse width of 0.05 s and
rigorous purification of DMFare required for the detection of
anthracene ECL. In contrast, FAF, with fluorene protection
on the C9 and C10 positions of anthracene, exhibits good
electrochemical reversibility for both the oxidation and
reduction processes, leading to very strong and stable bright
blue ECL (450 nm) (Figure 3a). The ECL spectrum of FDF
compared to DPA is shown in Figure 3b. FDF gives a
maximum ECL emission peak at lECL = 485 nm, which is red-
shifted ca. 50 nm as compared to that of DPA. The ECL
emission observed for FDF was almost as intense as that of
DPA under the same conditions. They have similar fluores-
polarity of the excited states.[10] The rigid structural features
also contribute to the high efficiencies of PL, leading to high
quantum yields of 0.62 for FAF, 0.90 for FDF, and 0.81 for
FPF. As indicated in Figure 2 f, a broad tailing can be detected
in the pyrene emission spectrum, which is ascribed to the
excimer of pyrene formed in MeCN/Benzene (1:1). The red-
shifted PL without excimer emission of FPF indicates the
introduction of fluorene substitutions and not only increases
the p-conjugation, but also imparts steric hindrance that
prevents excimer formation.
Figure 3 depicts the ECL spectra of the highly efficient
emitters FAF, FDF, and FPF compared with their parent
compounds. The ECL results are summarized in Table 3. The
high intensity allowed the ECL spectra of these new emitters
to be taken with only a 10 s integration time; such strong
emitters are very rare in ECL research. Typically, slight red
shifts in the ECL spectra were observed as compared to their
PL. This is attributed to the inner filter effect due to the
difference in concentrations used for PL and ECL.
PL
PL
cence and ECL quantum yields (F = 0.90, F
= 0.91,
FDF
DPA
ECL
F
ꢀ FEDCPLA). The question of long-term stability upon pulse
FDF
cycling has always been of interest in ECL because of possible
device applications.[13] Although the radical ions are appa-
rently stable on the CV time scale, the ECL of DPA tends to
decrease with extended pulsing. As shown in Figure 3d, the
transient ECL of FDF retains a constant intensity for each
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pulse (pulsing from Ep1,ox + 80 mV to Ep1,red ꢂ80 mV),
indicating the ECL emission with FDF is more stable than
DPA. The ECL of FDF was stable for at least 20000 pulses
(0.1 s/pulse). These results are consistent with our observa-
tions that the second oxidation of DPA is not stable even with
scan rates up to 1 Vsꢂ1, while the second oxidation of FDF is
stable even at a scan rate of 100 mVsꢂ1.
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The ECL of pyrene from radical ion annihilation shows
only excimer emission at lECL = 470 nm.[4] Instead, FPF shows
bright blue ECL (lECL = 445 nm) without the indication of
excimer formation during the electron transfer processes
(Figure 3c). The steric hindrance of fluorene substitution in
FPF prevents p-stacking for excimer formation during the
electron transfer reaction, leading a high ECL quantum yield
of 0.85 for FPF. By comparing the enthalpy of annihilation
reaction, DHan ¼ Eoꢁx ꢂ Erꢁed ꢂ 0:1 (in eV), to the energy
required for excited state formation, Es, as shown in Table 3,
ECL for all three compounds follows the S-route, that is,
availability of sufficient energy in the electron transfer
reaction to populate to the singlet excited state directly.
In conclusion, fluorene was used as a capping agent to
produce DPA, pyrene, and anthracene derivatives as new
aromatic hydrocarbons (FDF, FPF, FAF). The introduction of
fluorene groups imparts steric hindrance that prevents
interchromophore interactions, giving these molecules high
PL quantum yields. The fluorene substitutions also block the
active positions of the PAH cores subject to electrochemical
decomposition permitting the formation of stable radical ions
and enhancing p-conjugation facilitating the charge delocal-
ization. More importantly, the C2 and C6 substitutions on the
DPA core provide extra stabilization for the highly charged
species (dication and dianion), allowing FDF to have highly
efficient (FEFDCLF = 0.90) and stable (up to 20000 pulses) blue
ECL. The calculated enthalpies of these annihilation reac-
tions indicate that the electron transfer reactions for ECL
follow an S-route.
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Received: July 27, 2009
Revised: September 25, 2009
Published online: November 4, 2009
Keywords: electrochemistry ·
.
electrogenerated chemiluminescence · oligofluorene ·
polyaromatic hydrocarbon
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