Photophysical, electrochemical, and crystallographic investigation of conjugated fluoreno azomethines and their precursors

Article abstract of DOI:10.1039/b618098a

The photophysical investigation of amino- and aldehyde-substituted fluorenes revealed that these compounds are not only highly fluorescent, but dissipation of their singlet excited energy occurs by a combination of nonradiative means involving intersystem crossing (ISC) and internal conversion (IC). Quantification of the triplet state formed by ISC was possible by laser-flash photolysis (LFP). The efficiency by which this manifold was populated varied between 10 and 40% depending on the fluorene substitution. Condensation of these aldehyde and amine precursors yielded conjugated thiopheno azomethines with robust covalent bonds. Fluorescence of the azomethine fluorene derivatives was reduced relative to their precursors while the degree of IC remained unchanged. Deactivation of the singlet excited state occurred predominately by ISC and the resulting triplet state was rapidly and efficiently quenched by energy transfer by the azomethine linkage. Cyclic voltammetry of the fluoreno azomethines showed both oxidation and reduction processes, and the measured redox potentials and the band-gaps are lower than a bisfluorene analogue. The fluoreno azomethine LUMO energy levels are sufficiently low, making them compatible with common cathodes, therefore eliminating the use of an electron-injection layer. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b618098a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Photophysical, electrochemical, and crystallographic investigation of
conjugated fluoreno azomethines and their precursors{
Sergio Andre´s Pe´rez Guar`ın, Ste´phane Dufresne, Derek Tsang,{ Assa Sylla§ and W. G. Skene*
Received 12th December 2006, Accepted 30th March 2007
First published as an Advance Article on the web 23rd April 2007
DOI: 10.1039/b618098a
The photophysical investigation of amino- and aldehyde-substituted fluorenes revealed that these
compounds are not only highly fluorescent, but dissipation of their singlet excited energy occurs
by a combination of nonradiative means involving intersystem crossing (ISC) and internal
conversion (IC). Quantification of the triplet state formed by ISC was possible by laser-flash
photolysis (LFP). The efficiency by which this manifold was populated varied between 10 and
40% depending on the fluorene substitution. Condensation of these aldehyde and amine
precursors yielded conjugated thiopheno azomethines with robust covalent bonds. Fluorescence
of the azomethine fluorene derivatives was reduced relative to their precursors while the degree of IC
remained unchanged. Deactivation of the singlet excited state occurred predominately by ISC and
the resulting triplet state was rapidly and efficiently quenched by energy transfer by the azomethine
linkage. Cyclic voltammetry of the fluoreno azomethines showed both oxidation and reduction
processes, and the measured redox potentials and the band-gaps are lower than a bisfluorene
analogue. The fluoreno azomethine LUMO energy levels are sufficiently low, making them
compatible with common cathodes, therefore eliminating the use of an electron-injection layer.
further complicated by electrochemical excitation selection
Introduction
rules that limit the desired fluorescent state to 25%,
Fluorene and its derivatives have received much attention
subsequently affording low emission efficiencies in functioning
because of their unique optical properties that are suitable for
devices. The remaining 75% results in triplets that can lead to
emitting devices.1 The most important application of such
unwanted delayed emission and/or red-shifted contaminated
compounds involves organic light-emitting diodes, in which
emission. Even though fluorene exhibits high fluorescence
the fluorenes play a pivotal role in the emitting layer in these
quantum yields, it also undergoes ISC to populate its triplet
functional devices. These are subsequently used in flexible light
state. This manifold shift can potentially promote undesired
displays and/or low power-consumption emitting devices.2
phosphorescence that is bathochromically-shifted relative to
Although fluorene and its various derivatives have been
the fluorescence. Long-lived species additionally result from
extensively investigated because of the many interesting
this process, which contaminate the pristine emission. Ideal
properties for commercial applications,3 many synthetic and
fluorenes for the emitting layer therefore possess high
property challenges must still be overcome before a viable,
fluorescence quantum yields with little ISC, or if the triplet is
practical, and ideal device can be produced. Current challenges
formed, it is deactivated by nonemissive means.
of fluorene-derived compounds include difficult synthesis
Azomethines (-NLC-) are highly attractive alternatives to
requiring stringent reaction conditions, inconsistent reprodu-
current coupling methods to afford conjugated compounds
cibility, problematical purification from residual metal cata-
because of their isoelectronic properties relative to their carbon
lysts, and poor color quality.4,5 For newer generations of
analogues.11 They have the advantage of ease of synthesis
devices, tunable color with pristine emission is desired, in
without the use of stringent reaction conditions or metal
addition to overcoming the synthetic challenges.6,7
catalysts. Azomethines can also be easily purified, unlike their
Even though many fluorene derivatives have been synthe-
carbon analogues, and they exhibit good oxidative and
reductive resistance.12,13 Previous examination of azomethines
sized and investigated in the hope of achieving pristine blue
emission, their emission is usually contaminated with
undesired green color arising from keto defects5,7,8 or inter-
ensuing compounds were unsuitable for working devices.14
as functional materials, however, showed the properties of the
and intrachain quenching,9 and excimer formation.10 This is
This primarily was a result of a limited number of diamino
monomers possessing inadequate emissive and electrical/
Department of Chemistry, University of Montreal, Pavillon J. A.
Bombardier, C. P. 6128, succ. Centre-ville, Montreal, QC, H3C 3J7,
Canada. E-mail: w.skene@umontreal.ca; Fax: +1 (514) 340-5290;
Tel: +1 (514) 340-5174
{ Electronic supplementary information (ESI) available: Normalized
absorption, fluorescence, phosphorescence and 1H-NMR spectra. See
DOI: 10.1039/b618098a
{ Present address: University of Toronto, Toronto, Canada.
§ Present address: Universite´ Pierre et Marie Curie, Paris, France.
conductive properties, poor solubility, and oxidative decom-
position. Consequently, research in this area was abandoned
until the recent introduction of new monomers such as stable
aminothiophenes15 and the fluorene derivatives reported in
Chart 1. Such compounds have renewed the interest in
azomethines because of their promising properties that are
compatible with functional materials.12,16,17 Of particular
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Chart 1 Structures of fluorene derivatives and azomethines investigated.
interest are fluorene monomers which have gained a wide
importance because of their spectroscopic properties that are
ideal for emitting devices.3,18 Owing to our previous success
with thiopheno azomethines displaying little ISC,19 we
investigated the photophysics of fluorene-derived azomethines.
The triplet state was particularly studied by steady-state and
time-resolved means to see whether this manifold was
populated and if its deactivation occurred by unwanted
phosphorescence. The redox properties of the synthesized
conjugated fluoreno azomethines were also examined to assess
their compatibility with conventional oligofluorenes and to
ultimately gauge their suitability as alternative materials for
functional devices
the desired products in suitable yields. The simple method was
suitable only for the formation of 8, which easily precipitated
upon its formation. However, this product proved only
marginally soluble in all organic solvents suitable for analyses,
suggesting that the compound is rigid and adopts a planar and
linear conformation. Alternatively, high yields of the fluoreno
azomethines were possible by using dehydration protocols
with TiCl4 with short reaction times. The selective formation
of 9 vs. 10 was achieved by controlling the stoichiometry of 2,
while the two hexyl side-chains conferred good solubility to
these compounds in most organic solvents. The resulting
azomethine bonds were found to resist reduction with common
reducing agents such as NaBH4 and undergo reduction only
with refluxing with DIBAL. Azomethine hydrolysis occurs
only with prolonged heating with concentrated acid in water-
saturated organic solvents. No product decomposition was
observed even after eighteen months in solution under ambient
conditions.
Results & discussion
Synthesis
The required dialdehyde precursor 6 was obtained via a one-
step Grignard reaction from the dibromo derivative 4 by
trapping the electrophile with DMF. This route was preferred
over halogen–metal exchange with n-BuLi owing to the low
yields and the plethora of side products obtained with the low
temperature n-BuLi reaction. Given our previous experience
with azomethine formation,19 it was anticipated that the
synthesis of 8–10 would be straightforward. However,
prolonged reaction times in excess of 8 days were required
for the condensation of 2 with 6 to proceed. Mild dehydration
conditions employing isopropanol that are normally sufficient
to induce azomethine formation unfortunately did not afford
Photophysics
We initially sought out to examine the spectroscopic and
electrochemical properties of the fluoro azomethines (8–10)
given the limited number of extensive photophysical studies
of azomethines and owing to their potential use in emitting
devices.20 Unfortunately, assessment of the fluoreno azo-
methines properties proved challenging given the limited
number of coherent studies that examined the spectroscopy
or electrochemistry of the various fluorene monomers reported
in Chart 1. For this reason, we also examined the photophysics
2802 | J. Mater. Chem., 2007, 17, 2801–2811
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number of fluorenes and to the increase in degree of
conjugation. This is in contrast to 721, whose absorption and
fluorescence spectra are hypsochromically-shifted relative to
its azomethine analogue 8 confirming its limited degree of
conjugation. The molar absorption coefficients also increase
with the increased degree of conjugation found with 9 and 10
owing to highly permitted p–p* transitions.
The lack of structured absorption and emission for the
substituted fluorenes is evident from Fig. 1. This is in contrast
to the unsubstituted fluorene (1), and suggests there is no
defined order in the singlet excited and ground states for the
substituted fluorenes. This implies the structures are not
consistently rigid and they undergo a certain degree of rotation
and twisting. This is supported by the moderate absorption
and emission bathochromic shifts observed instead of pro-
nounced shifts, associated with highly-conjugated compounds
such as thiopheno azomethines.19 This suggests the fluorenes
are not highly coplanar and rigid, supported by the crystal-
lographic data" (vide infra).
Fig. 1 Normalized absorption of 6 (circles), 9 (squares), and 10
(triangles) measured in anhydrous and deaerated acetonitrile. Inset:
Normalized emission spectra of 6 (circles), 9 (squares), and 10
(triangles) measured in anhydrous and deaerated acetonitrile excited
at their respective maximum.
The relative energy difference between the ground and the
excited states can be calculated from the intercept of the
combined normalized absorption and emission spectra.22 Such
calculations provide the relative HOMO–LUMO energy
difference (DE) from the spectral overlap. Additionally, the
absorption method further provides an estimate for the band-
gap obtained from the absorption onset in the red region of
the spectrum. From the combined spectroscopic results for
the various fluoreno azomethines reported in Table 1, the
wide range of properties is apparent, including the similarity
between the azomethines and the oligofluorene analogue 7.
1 is known to be highly fluorescent and its derivatives
examined exhibit the same behavior with Wfl y 0.7.23,24 The
amino derivatives (2 and 3) are the exception with reduced
fluorescence quantum yields. This is most likely a result of the
free amine group undergoing known deactivation by bond
rotation and/or charge transfer.23,25 This notwithstanding, the
and electrochemistry of the fluoreno azomethines along with
their monomers and an oligofluorene carbon analogue, 7.
Bathochromic shifts in the absorption spectra are observed
along the series going from 6 to 9 and to 10, illustrated in
Fig. 1. The observed absorption shifts of 29 nm going from 8
to 9 and the 41 nm shift for the formation of 10 from 9 confirm
the conjugated nature of the azomethine derivatives. These
pronounced absorption changes are indicative of the conjuga-
tion degree arising from azomethine bond formation. The
observed transitions are dominated by lowering of the excited
electronic p–p* levels owing to the stabilization from increas-
ing the degree of conjugation. This is a result of the energy
levels undergoing a pronounced stabilization from the hetero-
conjugated bond. The same trend holds true for the
fluorescence (inset Fig. 1) in which bathochromic shifts of 48
and 39 nm occur with the addition of each fluorene unit for 9
and 10, respectively. The fluorescence bathochromic shift
correlates with the stabilization resulting from the increased
" CCDC reference number 630800. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b618098a
Table 1 Photophysical properties of fluorene derivatives
lem
/
DE/ Eg/
kr/
knr
/
eT2T
/
lphos
/
tphos
/
Compound labs e/cm21 M21 nm Wfl (77 K) eVb eVc tfl/nsd 108 s21 e 108s21 f
l
T2T/nmg WT M21 cm21 i tT/ms WPhos nmk msl
a
h
j
1
2
3
4
5
6
7
8
9
10
a
261 17 500
290 16 000
297 18 700
280 18 100
316 18 600
326 18 800
327 31 650
361 30 578
355 30 494
396 34 800
302 0.72 (0.86) 4.4 3.9 10m
0.7
5.0
2.9
6.8
6.8
5.9
6.1
3.6
2.1
1.8
0.3
4.1
5.4
2.3
1.5
2.4
3.0
5.5
6.3
6.5
386n
450
437
440
405
470
525
0.1n 12 000o
0.11 13 000
n/ap n/ap
0.06 11 500
0.3 13 500
0.2 36 000
0.4 6 300
153n 0.36 482
5.6
6.3
7.2
8.3
10
359 0.55
387 0.35
340 0.75
365 0.82
3.7 3.6 1.1
3.4 3.4 1.2
3.9 3.8 1.1
3.6 3.5 1.2
8.1
2.4
6.3
2.2
5.9
14
—
—
—
0.15 478
0.23 482
0.09 484
0.38 495
0.25 506
0.42 450
0.51 495
0.46 525
0.13 555
383 0.71 (0.82) 3.6 3.4 1.2
380 0.67
445 0.40
42
2.9 3.4 1.1
2.9 2.9 1.1
4.5
6.3
5.9
7.6
No signal
No signal
No signal
—
—
—
—
—
—
433 0.25 (0.28) 3.1 2.9 1.2
472 0.22 (0.31) 2.9 2.8 1.2
Fluorescence quantum yield relative to 2-aminofluorene (Wfl = 0.52)45 at room temperature. Values in parentheses were measured at 77 K in
b
4 : 1 methanol–ethanol matrix. HOMO–LUMO energy level difference measured from the intercept of the normalized absorption and
c
d
emission spectra. Spectroscopically determined band-gap taken from the absorption onset. Fluorescence lifetime measured at maximum lem
.
Radiative decay rate constant kr = wfl/tfl. Nonradiative decay rate constant knr = kr(1 2 wfl)/wfl. Triplet-triplet absorption spectrum
e
f
g
h
maximum. Triplet quantum yield of transient. Molar absorption coefficient of measured triplet transients according to eqn (1) relative to
i
benzophenone (eT2T = 8 400 M21 cm21).29,49 Phosphorescence quantum yield. Phosphorescence maximum. Phosphorescence lifetime
j
k
l
measured at phosphorescence maximum. Literature value.50 Literature value.51 Literature value.29,49 Compound 3 decomposes with
prolonged laser irradiation, therefore WT and eT2T cannot be measured.
m
n
o
p
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fluorescence quantum yields of the fluoreno azomethine are
systemically lower than their precursors. This is not surprizing
since thiopheno azomethine derivatives are known to deacti-
vate their excited state predominately by internal conversion.
To confirm the reason for this reduced emission, the
excited-state properties of the azomethines were examined by
steady-state and time-resolved spectroscopic means. Mono-
exponential fluorescence lifetimes were measured and they
confirm a unimolecular deactivation process of the excited
singlet. Given the comparable radiative (kf) and non-radiative
(knr) rate constants for all the compounds, including 8–10, and
their similarity to 1, the excited-state deactivation must
therefore occur by similar nonradiative channels such as either
internal conversion (IC) or intersystem crossing (ISC) to the
triplet state. Deactivation by photoionization and photo-
isomerization can be eliminated because no photoisomers or
ionic intermediates were observed. Confirmation of IC
deactivation is possible by fluorescence measurements at
77 K. At this temperature, all modes of energy dissipation
by bond rotation are suppressed. Surprizingly, the fluorescence
measurements at this low temperature confirm that IC
accounts for roughly only 9% of the excited-state deactivation
according to: Wfl (77 K) 2 Wfl (RT) # WIC. The unaccounted
energy dissipation of the singlet excited state must therefore
occur by ISC according to the energy-conversation equation:
Wfl + WISC + WIC # 1.
The singlet to triplet manifold shift is most likely a result of
the heavy atoms that induce spin-orbit coupling by the heavy
atom effect.26 The manifold shift is further enhanced by the
azomethine nitrogen that additionally contributes to the heavy
atom effect. Furthermore, the high degree of conjugation is
understood to narrow the singlet–triplet energy gap, resulting
in increased ISC. Laser-flash photolysis (LFP) studies of the
fluorene derivatives were undertaken to confirm formation of
the triplet state by ISC. Direct visualization of the fluorene
monomer triplets were observed between 386 and 470 nm
following excitation at 266 nm. The absorption differences
observed for the triplets of 1–6 are due to stabilization effects
induced by the electronic groups, while the bathochromic shift
observed at 525 nm for 7 (Fig. 2) is from the increased degree
of conjugation for this oligofluorene. The triplet nature of
the produced transients is confirmed by their unimolecular
decays in addition to being quenched with standard triplet
quenchers such as oxygen, 1,3-cyclohexadiene, naphthalene,
and b-carotene.27 All compounds from Chart 1 yielded a
noticeable triplet with the exception of azomethines 8-10 that
surprizingly yielded no signal by LFP.
Fig. 2 Transient absorption spectrum of 7 recorded in acetonitrile.
Time windows recorded at: 8 ms (open squares), 12 ms (closed circles),
18 ms (open circles), 25 ms (closed triangles), 50 ms (open triangles), and
100 ms (closed squares) after the excitation pulse at 266 nm. Inset: first-
order decay of triplet transient of 7 monitored at 525 nm.
viable information from this equation. Observing the growth
of a common transient for both benzophenone and the
fluorene derivatives simplifies eqn (1) such that eTT becomes
unity. This is possible by using a triplet quencher in sufficient
quantity such that it quenches 95% of the produced fluorene
and benzophenone triplets. b-Carotene is an ideal quencher
because its triplet energy is lower than that of the fluorene
derivatives. This results in rapid exothermic triplet quenching
by energy transfer (ET), requiring only a small concentration
of the quencher. Direct excitation of fluorene generates the
singlet excited state eqn (2) that undergoes ISC to form the
triplet state eqn (3). This manifold is then quenched
quantitatively by b-carotene by ET resulting in its triplet
(eqn (4)), which is visible at 550 nm (Inset Fig. 3). Comparing
Quantification of the produced triplets (WT = WISC) and
their corresponding molar absorption coefficients (eTT) are
possible by LFP in addition to visualizing the triplets. These
values are of interest because not only are they unknown, but
they also provide important information required to assess the
suitability of these compounds for emitting applications. Of
particular interest is the triplet state that can potentially
phosphoresce leading to emission contamination thereby
limiting their use in functional devices. Quantification of the
triplet state relative to an actinometer (benzophenone) is
possible according to eqn (1). However, either WT or eTT of the
compound of interest must be known in order to extract any
Fig. 3 Maximum triplet b-carotene formed by energy transfer as a
function of laser power with different triplet donors for determining
the triplet quantum yield of benzophenone (squares), 2 (circles), and 7
(triangles) Inset: b-carotene triplet monitored at 550 nm in acetonitrile.
2804 | J. Mater. Chem., 2007, 17, 2801–2811
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the maximum signal of triplet b-carotene produced under
quenching processes are reduced. Emission from the low-lying
triplet state is seen as a bathochromic emission relative to the
fluorescence emission for all the compounds, including
the azomethines. The relative phosphorescence shifts as a
function of fluorene-substitution follows the same trend as the
fluorescence emission. This confirms the increased stability,
hence lowering of the energy states, arising from the increased
conjugation with 7–10. Interestingly, phosphorescence is
observed for all the azomethine derivatives and confirms these
conjugated compounds form their triplet state in significant
amounts. Furthermore, this corroborates that deactivation of
the azomethine triplet states occurs by ET. The triplet state
can undergo energy dissipation either by radiative phos-
phorescence or by nonradiative intersystem crossing. The
similarity of WT determined by LFP and the phosphorescence
quantum yield (WPhos) measured by steady-state emission for
all the fluorene compounds, except 8–10, imply their triplets
are deactivated uniquely by emissive modes and not by
nonradiative means. Furthermore, the similar values validate
the LFP measurements and they additionally confirm the
predominate deactivation mode for the triplet state is by
phosphorescence. This implies that the electrochemically-
generated triplets will most likely result in delayed emission
causing unwanted red-shifted emission contamination, while
the fluoreno azomethines triplet self-quenching will provide
pristine emission.
similar conditions for both benzophenone and the fluorene
derivatives as
a function of laser power provides the
representative plots shown in Fig. 3. The triplet quantum
yields are derived from the ratio of the slopes of benzophenone
and the fluorene derivatives according to eqn (1). Owing to the
strong ground-state bleaching of certain compounds con-
comitant with the screening effect of b-carotene at longer
wavelengths, the singlet recovery method of Lament was also
used to measure the triplet quantum yields.28 Both methods
gave mutually the same results for 1, 2, and 7, and serve to
validate the two methods for WT determination. The eTT for
the various fluorene derivatives are determined by the same
power-dependence study against triplet benzophenone (WT = 1
and eTT = 8 400 M21 cm21) measured at 525 nm in the
absence of the b-carotene quencher.29,30 The resulting calcu-
lated values in Table 1 show the eTT for the fluorene precursors
are . 12 000 M21 cm21 and they are influenced by the
electronic group on the fluorenes. Only in the case of 7, was a
small eTT observed, yet it showed the highest triplet quantum
yield. This is surprizing since it serves as a model compound
for materials currently used in emitting devices and therefore
it was expected to exhibit the lowest WT. The high WT
measured is supported by the overall deactivation processes
(Wfl + WIC + WT) that are equal to unity within experimental
error. Interestingly, the azomethine derivatives do not produce
any visible triplet.
Electrochemistry
DAbsUnknownWRef eRef
WUnknown
~
(1)
DAbsRef eUnknown
The fluoreno azomethines were found to undergo a one-
electron oxidative processes observable by cyclic voltammetry.
Not only are the observed oxidation potentials for all the
azomethines studied comparable to those of 7, their radical-
cation formation is also quasi-reversible, seen in Fig. 4. This is
in stark contrast to previous examples of azomethines that
undergo irreversible oxidation regardless of the experimental
conditions. The irreversible anodic process also occurs with
Fluorene DDhDuA FluoreneÃ
(2)
(3)
ISC
Fluoreneà DDDA 3Fluorene
ET
3Fluorenezb-Carotene DDDA Fluorenez3b-Carotene (4)
From the fluorescence data in Table 1, WT for the
azomethines 8-10 must be y 0.7, according to the energy-
conversation equation above. Their triplets should therefore be
detectable by LFP. Since no LFP signal was observed, the
triplet state must be efficiently self-quenched by the azo-
methine bond. Given the fastest lifetime that can be resolved
by the LFP system is y 100 ns and the effective quenching
concentration of intramolecular quenching is 1 M ([Q]), the
intramolecular rate constant (kq) for fluorene-triplet quench-
ing by the azomethine bond must be ca. 1 6 107 M21 s21
according to kobs = k0 + kq[Q], where k0 is the triplet kinetic in
the absence of quencher. The calculated rate constant is slower
than diffusion controlled (1010 M21 s21) implying the energy-
transfer quenching process is endothermic, however the large
effective concentration of the azomethine quencher assures
efficient quenching. The fluorene azomethines are therefore
suitable candidates for emitting devices since the unwanted
triplet state is efficiently deactivated by nonradiative means,
leading to pristine emission from the singlet state only.
Phosphorescence measurements at 77 K were done to
validate the LFP results. At this low temperature, all
diffusion-deactivation processes are suppressed and any
Fig. 4 Cyclic voltammograms of 7 (open squares) and 8 (closed
squares) measured at 100 mV s21 in anhydrous dichloromethane.
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most fluorene derivatives.31 Since earlier azomethine electro-
chemical studies involved only homologous aryl units, they
not only exhibited irreversible oxidation,32,33 but they also
exhibited extremely high oxidation potentials. These undesired
properties limit the usefulness of other azomethines as
functional materials.12,32,34,35 In fact, earlier azomethine
examples were so reactive they sustained oxidative decom-
position of both products and reagents.
barrier height for the electron-injection process.37,38 This is
expected to lead to simpler device construction concomitant
with increased device efficiency.
ox
onset
IP~HOMO~E
(SCE)z4:4
(SCE)z4:4
(5)
(6)
red
onset
EA~LUMO~E
The oxidation potentials combined with the reduction
potentials provide an accurate representation of the band-
gaps, tabulated in Table 2. The measured electrochemical
band-gaps (Eg) of the fluoreno azomethines are lower than
their fluorenyl polymer analogues whose Eg = 3.04 eV.39 These
low values support the overall high stability of these
compounds owing to their degree of conjugation, determined
spectroscopically. Further corroborating evidence of the data
obtained between the two methods is derived by the similar
electrochemically measured Eg that are similar to the spectro-
scopically-derived values.
Not only do 8–10 undergo an oxidation process demon-
strating their p-doping-type behavior, they also undergo a
reversible electrochemical reduction that is observed at
relatively low potentials. Given the similar reduction potentials
of the fluorene derivatives to their fluoreno azomethines, the
observed cathodic process is not from reduction of the
azomethine bond. This is further supported by the lack of
azomethine reduction with standard reducing agents such as
NaBH4, and further confirms the robustness this bond.
Rather, the process is ascribed to reduction of the fluorenyl
moiety, confirming their n-doping-type character. This is
opposite to conventional oligofluorenes that are reduced only
at high cathodic potentials. The fluoreno azomethine low
reduction potentials equate to low work-function barriers
making them compatible with currently used cathodes,
consequently eliminating the use of an electron-hole injection
(EIL) layer for these compounds when incorporated into
emitting devices.
Crystal structure
Similar to their carbon analogues, two geometric isomers are
possible with azomethines, E and Z. Absolute assignment to
either of the two azomethine isomers is unfortunately difficult
1
by H-NMR. Confirmation of the exclusive formation of one
isomer, which is assumed to be the most thermodynamically
stable E isomer is derived from the NMR data that show only
one imine peak. The crystal structure obtained for 10 confirms
that both azomethine bonds adopt the E isomer (Fig. 5).
Formation of the thermodynamically stable E isomer is
assumed for all the azomethines given the presence of only
The ionization potential (IP) and electron affinities (EA) are
required to assess the HOMO–LUMO properties of the
azomethines and the fluorene derivatives. These values are
calculated from the oxidation and reduction potentials mea-
sured by cyclic voltammetry according to the well established
eqn (5) and (6), respectively.36 The HOMO and LUMO levels
of 8–10 are reported in Table 2. Even though the calculated
HOMO levels are comparable to 7, the low reduction potential
results in LUMO levels that are lower than conventional
oligofluorenes. The fluoreno azomethine LUMO energy
levels are reduced such that they are comparable in energy
to commonly used cathodes such as calcium (2.9 eV) and
magnesium (3.7 eV).37 The energy compatibility implies a low
work-function for electron injection from the cathode to the
fluorenyl LUMO. Significant enhancement can be expected for
the injection of electrons from the cathodes to 9 and 10 such
that an electron-injection layer is not required to overcome the
1
one imine peak in all the H-NMR spectra. Two independent
molecules of 10 were found in the asymmetric unit-cell and one
of them exhibited disorder while both consisted of the E
isomer. Owing to the extreme disorder of one of the molecules
within the lattice, some restraints were imposed in order to
achieve a good correlation with the final structural refinement.
Some optimized parameters within standard deviation from
the ordered molecule were consequently applied to the
disordered molecule in order to achieve the refined structure.
Identical anisotropic displacement parameters were restrained
for every atom of the disordered molecule. Separate aniso-
tropic displacement parameters were applied while some bond
lengths such as the azomethine were restrained in the
disordered molecule as to achieve a good correlation factor.
For clarity reasons, only the compound consisting of no
disorder is presented in Fig. 5.
Table 2 Electrochemical properties of fluorene derivatives
Compound Epaa/V Epcb/V HOMOc/eV LUMOc/eV Egd/eV
The isolated crystal structure shows the fluorene units are
orientated in an anti-parallel arrangement in addition to
allowing assignment of the geometric isomer. The anti-parallel
configuration is dissimilar to 7 whose fluorenyl moieties are
parallel and coplanar.40 The advantage of this configuration is
a high degree of conjugation, supported by the spectroscopic
results. Unlike their thiophene analogues,41 the fluorene units
do not adopt entirely planar and linear configurations with
respect to one another. Rather, the average mean-planes of the
terminal fluorenes are twisted 26.7u and 65.3u from the mean
plane of the central fluorene and its two azomethine bonds,
illustrated in the bottom of Fig. 5 and tabulated in Table 3.
1
2
3
4
5
6
7
8
9
10
a
0.91
0.90
0.53
2.25
1.28
1.38
1.54
1.53
1.42
1.41
21.31
21.02
21.47
—
5.2
5.0
4.9
6.2
5.4
5.6
5.6
5.6
5.7
5.6
3.3
3.5
3.0
2.8
3.2
3.3
2.1
3.6
3.6
3.4
c
1.9
1.5
1.9
3.4e
2.2
2.3
3.5
2.0
2.1
2.2
—
21.29
f
22.3
21.12
21.18
21.52
b
Oxidation potential.
Reduction potential.
Relative to the
d
vacuum level. Electrochemical band-gap. Spectroscopic band-gap.
e
f
Literature value.52
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Fig. 5 Crystal structure for 10" showing the atom labels (top) with ellipsoids drawn at 40% probability, seen along the a crystal lattice axis
(middle), and parallel to the plane of the central fluorene unit (bottom).
The measured mean-plane angles are similar to homoaryl
azomethines that have twisted mean planes varying between
55u and 65u from the central aryl unit.17,34,42,43 Deviation from
planarity is required for the homoaryl azomethines in order to
minimize the energy strain resulting from steric hindrance
between the ortho hydrogen on the homoaryl ring and the
The p-stacking and C-H–p interactions of the fluoreno
azomethines are also in part responsible for the apparent high
degree of crystallinity. The resulting combined strong interac-
tions result in a densely-packed network, in which the fluorene
azomethines are stacked in an anti-parallel array leading to
a zig-zag-like packing arrangement represented in Fig. 6.44
azomethine hydrogen, H112…H117 and H131…H141, that
43
˚
are separated only by 2.19 A.
Table 3 Selected parameters from the crystal structure of 10
Side A
Side B
Plane anglea/u
-CLN-/s
Aryl-C/s
LN-Aryl/s
a
26.44 (3)
1.287 (3)
1.465 (3)
1.418 (3)
65.50 (4)
1.290 (3)
1.461 (3)
1.427 (3)
Mean-plane angle of the terminal group relative to the mean plane
of the central aryl unit.
Fig. 6 Crystal packing of 10.
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comparing the amount of b-carotene triplet produced from
both the sample of study and with benzophenone (WT = 1).27,47
The amount of triplet b-carotene produced by direct excitation
at 266 nm was accounted for when determining the triplet
quantum yields according to Fig. 3. Owing to the strong
overlap between the ground-state and excited-state absor-
bances of same compounds, the triplet quantum yield for these
compounds was determined using the power-dependence
singlet recovery method by Lament.28 The triplet-extinction
coefficients were determined from the measured triplet
quantum yields according to eqn (1) by comparing the laser-
power dependence triplet signal of the compounds of study
relative to benzophenone (eTT = 8 400 M21 cm21) excited at
266 nm.29,47
Table 4 Details of crystal structure determination of 10.
Formula
C57H60N2
Mw/g mol21; F(000)
Crystal color and form
Crystal size/mm
T/K; dcalcd/g cm23
Crystal system
773.07; 1664
Yellow plate
0.20 6 0.10 6 0.06
100 (2); 1.138
Triclinic
¯
Space group
˚
Unit cell a/A
P1
13.568 (9)
˚
b/A
14.907 (10)
24.077 (18)
˚
c/A
a/u
b/u
c/u
76.898 (3)
74.505 (2)
77.736 (2)
4510.4 (5); 4
1.94–66.17 ; 0.873
13 780/17 354; 0.038
0.49; Semi-empirical
0.0533; 0.1427
0.0634; 0.1507
1.049
3
˚
V/A ; Z
h range/u; completeness
Reflections, collected/independent; Rint
m/mm21; Abs. corr.
R1(F); wR(F2) [I . 2s(I)]
R1(F); wR(F2) (all data)
GoF(F2)
Electrochemical measurements
Cyclic voltammetric measurements were performed on a Bio
Analytical Systems EC Epsilon potentiostat at scan rates of
100 mV s21. Compounds were dissolved in anhydrous and
deaerated dichloromethane at 1024 M with 0.1 M Bu4NPF6. A
platinum electrode and a saturated Ag/AgCl electrode were
employed as auxiliary and reference electrodes, respectively.
Ferrocene was added at the end of the electrochemical
measurements to serve as an internal reference.48
Max. residual e2 density/e2
0.70
23
˚
A
Specific crystallographic data pertaining to 10 are reported
in Table 4.
Experimental
Materials and general experimental procedures
Synthetic procedures
All reagents were commercially available from Aldrich, and
were used as received unless otherwise stated. Anhydrous and
deaerated solvents were obtained via a Glass Contour solvent
purification system. 1H-NMR and 13C-NMR spectra were
recorded on a Bruker 400 MHz spectrometer with the appro-
priate deuterated solvents. 13C-NMR proved challenging for
compounds 8–10 owing to their limited solubility in deuterated
DMSO.
2-Bromo-9,9-dihexyl-9H-fluorene. 2-Bromofluorene (1.6 g,
6.5 mmol) was dissolved in DMSO (35 mL), 1-bromohexane
(2 mL, 14 mmol) was added, along with benzyltriethylammo-
nium chloride (5 mol%, 73 mg, 0.02 mmol). This was poured
into 10 mL 50% NaOH solution (7.58 g NaOH), with stirring
under N2. The solution turned orange, red, then purple over
ten minutes. After stirring for 20 hours at room temperature,
the solution was poured onto water (80 mL), and extracted
with four portions of ether (65 mL). The extracts were
combined and washed with three portions of distilled water
(100 mL). The organic layer was dried with MgSO4 and
reduced in vacuo. The bright yellow oil was chromatographed
(SiO2, hexanes) to recover a pale yellow oil (2.50 g, 93%).
1H-NMR (400 MHz, CDCl3): d = 7.68 (m, 1H), 7.57 (dd, 1H),
7.46 (m, 2H), 7.34 (m, 3H), 1.95 (m, 4H), 1.1 (m, 12H), 0.79 (t,
6H), 0.61 (m, 4H). HR-MS calculated for [C25H23Br + H]+:
413.1834, found 413.1832.
Spectroscopic measurements
Absorption measurements were performed on a Cary-500
spectrometer, and fluorescence studies were done on an
Edinburgh Instruments FLS-920 fluorimeter after deaerating
the samples thoroughly with nitrogen for 20 minutes.
Fluorescence quantum yields were measured at 1025 M by
exciting the corresponding compounds compared to 2-amino-
fluorene (W = 0.52)45 excited at the same wavelength. The
actinometer absorbances, and those of the compounds,
were matched to within 5% at the excitation wavelength.
Phosphorescence measurements were done on a Cary Eclipse
using 4 : 1 methanol–ethanol glass matrix at 77 K and
by exciting at the compound’s absorption maximum.
Phosphorescence quantum yields were determined by com-
paring optically-matched samples relative to fluorenone
(Wphos = 0.06 in ethanol) at 77 K.46 The triplet–triplet
absorption spectra were measured in anhydrous acetonitrile
using a Luzchem mini-LFP system excited at 266 nm with the
fourth harmonic from a Continuum YAG:Nd Sure-lite laser.
The triplet quantum yields were measured by optically
matching samples within 5% at 266 nm and by quenching
with b-carotene (10 mM). The b-carotene triplet growth was
monitored at 550 nm as a function of laser power and the
triplet quantum yields were calculated according to eqn (1) by
9,99-Dihexyl-2-fluorenylboronic acid. 9,99-Dihexyl-2-bromo-
fluorene (300 mg, 0.73 mmol) was added to anhydrous THF
under N2 (5 mL). The solution was cooled to 278 uC and
n-BuLi (1 mL of 1 M solution, 1 mmol) was added dropwise.
The solution turned green and was allowed to stir for 1 hour.
B(OCH3)3 (0.2 mL, 1.2 mol) was added and the solution
turned clear yellow. The reaction was allowed to stir overnight
before being slowly warmed to room temperature. Distilled
water (5 mL) was added, and stirring was continued for 1 hour
at room temperature. HCl (2 M, 4 mL) was then added, and
the THF was removed under vacuum. The remaining aqueous
solution was extracted three times with ether. The organic
extracts were combined and dried with MgSO4 and the crude
product was recovered as a solid, then it was chromatographed
2808 | J. Mater. Chem., 2007, 17, 2801–2811
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with ethyl acetate–hexanes (1 : 4 v/v) to afford the product as a
1
colorless oil (81 mg, 29%). H-NMR (400 MHz, CDCl3): d =
was isolated as a yellow solid (183.7 mg, 100%). The product
was insoluble and subsequent characterization was not
possible.
8.37 (d, 1H), 8.29 (s, 1H), 7.94 (d, 1H), 7.87 (m, 1H), 7.43–7.48
(m, 3H), 2.1 (m, 4H), 0.5–1.1 (m, 8 H).
7-[(9H-Fluoren-2-ylimino)methyl]-9,9-dioctyl-9H-fluorene-2-
carbaldehyde (9). To 6 (50 mg, 0.11 mmol) was added 2
(16.1 mg, 0.09 mmol) in anhydrous ethanol, under N2 and a
catalytic amount of TFA. No heating was required and the
reaction was allowed to stir at room temperature for 12 hours.
The crude solid was chromatographed on activated basic
alumina using ethyl acetate–hexanes (1 : 4 v/v) to yield
the product as a yellow powder (41 mg, 60%). 1H-NMR
(400 MHz,[D] CDCl3): d = 10.09 (s, 1H), 8.65 (s, 1H), 8.00 (s,
1H), 7.90 (m, 4H), 7.82 (m, 3H), 7.55 (d, 1H, J = 7.3 Hz), 7.49
(s, 1H), 7.39 (t, 1H, J = 7.3 Hz), 7.31 (m, 2H), 3.25 (s, 2H), 2.08
(m, 3H), 1.55 (s, 4H), 1.06 (m, 18 H), 0.8 (t, 6H, J = 7.0 Hz),
0.61 (m, 3H). HR-MS(+) calculated for [C44H51NO + H]+:
610.4026, found: 610.4021.
9,9-Dihexyl-2-(9,9-dihexyl-9H-fluoren-7-yl)-9H-fluorene (7).
9,99-Dihexyl-2-fluorenylboronic acid (256 mg, 0.68 mmol) was
dissolved in toluene (7 mL), in which was added 9,99-dihexyl-2-
bromofluorene (600 mg, 1.45 mmol) dissolved in toluene
(2 mL). Aqueous Na2CO3 (10 mL, 2 M) was added in one
portion, and the solution was stirred under N2. Pd(PPh3)4
(23 mg, 0.03 mmol) was added after 2 hours of stirring while
the flask was protected from light and refluxed for 30 hours.
The solution was poured into hexanes (125 mL) and
washed with saturated aqueous NaCl. The organic layer was
separated, dried with MgSO4, and evaporated in vacuo to
recover a golden-yellow oil that was subsequently purified by
SiO2 chromatography with hexanes. The title compound was
isolated as a colorless oil that crystallized as white crystals
1
overnight (168 mg, 74%). H-NMR (400 MHz, CDCl3): d =
(13E)-N-((2-((E)-(9H-Fluoren-7-ylimino)methyl)-9,9-dioctyl-
9H-fluoren-7-yl)methylene)-9H-fluoren-2-amine (10). To
7.77 (dd, 2H), 7.65 (m, 2H), 7.35 (m, 3H), 2.05 (m, 4H),
1.09 (m, 12H), 0.78 (m, 10H). 13C-NMR (300 MHz, CDCl3):
d = 152.8, 151.2, 141.0, 140.7, 140.5, 127.2, 127.0, 126.3,
123.2, 121.6, 120.0, 119.9, 55.4, 40.6, 31.8, 29.9, 24.0, 22.8,
14.3. HR-MS: calculated for [C50H66 + H]+: 667.52372, found:
667.5224.
6
(100 mg, 0.22 mmol) was added 1,4-diazabicyclo[2.2.2]octane
(DABCO) (300 mg, 2.68 mmol) and dissolved in anhydrous
toluene under nitrogen at 0 uC. To this mixture was slowly
added titanium(IV) chloride (TiCl4) in toluene [1 M] (0.67 mL,
067 mmol) over 30 minutes. The reaction was warmed to room
temperature followed by the addition of 2 (121 mg, 0.67 mmol)
in anhydrous toluene and then refluxed for 1 hour. Upon
cooling to room temperature, the mixture was filtered through
a plug of basic activated alumina (Al2O3). The title compound
was isolated by flash chromatography on basic activated
alumina with ethyl acetate–hexanes (1 : 4 v/v) to yield the title
compound as a bright yellow powder (112 mg, 65%). 1H-NMR
(400 MHz, [D] acetone-d6): d = 8.82 (s, 2H), 8.18 (s, 2H), 8.05
(s, 4H), 7.93 (d, 2H, J = 8.2 Hz), 7.88 (d, 2H, J = 7.5 Hz), 7.59
(t, 4H, J = 7.5 Hz), 7.40 (t, 4H, J = 7.5 Hz), 7.32 (t, 2H,
J = 7.5 Hz), 3.99 (s, 4H), 1.29 (m, 12H), 1.10 (m, 16H), 0.79 (m,
6H). HR-MS(+) calculated for [C57H60N2 + H]+: 773.4815,
found: 773.4829.
9,9-Dioctylfluorene-2,7-dicarboxaldehyde (6). Fine magne-
sium mesh (1.24 g, 51 mmol) was added to newly flame-dried
glassware under nitrogen. To this was added 2,7-dibromo-9,9-
dioctylfluorene (2.80 g, 5.1 mmol) dissolved in anhydrous
THF. The remaining dibromofluorene solution was added
dropwise at room temperature once the Grignard reaction
began. The reaction was then refluxed for 24 hours. To the
resulting brown-colored solution was added a solution of
DMF (0.56 mL, 7.3 mmol) diluted in anhydrous THF (2 mL)
after cooling to 0 uC. Aqueous HCl (1.5 M, 30 mL) was
added to the brown solution after 2 days of stirring at room
temperature, and then poured into water (150 mL). The
aqueous phase was extracted and then washed with ethyl
acetate. The combined organic layers were dried over MgSO4
then concentrated under vacuum. The crude product was
purified by silica-column chromatography with hexanes–ethyl
acetate (20 : 1 v/v) to afford the title compound as a yellow
solid (2.01 g, 88%). 1H-NMR (400 MHz, acetone-d6): d =
10.15 (s, 2H), 8.18 (d, 2H, J = 7.84 Hz), 8.10 (s, 2H), 8.02 (d,
2H, J = 7.76 Hz), 2.23 (t, 4H, J = 8.2 Hz), 1.05–1.23 (m, 24H),
0.8 (t, 6H J = 7.08 Hz). 13C-NMR (300 MHz, acetone-d6):
d = 193.19, 153.98, 146.86, 138.24, 130.97, 124.95, 122.81,
56.81, 40.81, 32.8, 30.8, 29.7, 29.4, 24.9, 23.6, 14.66 ppm.
HR-MS(+) calculated for [C31H42OS + H]+: 447.3269, found:
447.3257.
Conclusion
Steady-state and time-resolved investigation of fluorene and its
derivatives showed these compounds are highly fluorescent.
The predominate nonradiative deactivation mode of the singlet
excited state occurs by ISC with only a minor contribution
from IC. Fluoreno azomethine self-quenching of the triplet
state leads to non-emissive triplet states. Conversely, triplet
deactivation of fluorene and its derivatives occur by radiative
phosphorescence. The fluoreno azomethines’ oxidation and
reduction potentials and their band-gaps can be modulated
such that they are similar to their carbon analogues and they
are lower than their aryl azomethine homologues, making
them compatible for use in functional devices. Moreover, the
LUMO levels can be tailored according to their degree of
conjugation and substitution such that the energy levels
are compatible with commonly used cathodes in OLEDs.
The self-quenched triplet state concomitant with the n- and
p-doping properties thus make fluoreno azomethines suitable
(9H-Fluoren-2-yl)-(9H-fluoren-2-ylmethylene)amine (8). To 2
(93.3 mg, 0.51 mmol) was added 2-fluorene carboxaldehyde
(100 mg, 0.51 mmol) in anhydrous ethanol, under N2 and
a catalytic amount of TFA. The solution was stirred at
room temperature for 12 hours until a yellow precipitate
was formed. It was subsequently filtered and the product
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 2801–2811 | 2809

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Acknowledgements
The authors acknowledge financial support from the Natural
Sciences and Engineering Research Council Canada, the
University of Montreal, Centre for Self-Assembled Chemical
Structures, and additional equipment funding from the
Canada Foundation for Innovation. DT thanks NSERC for
an UGA scholarship, and SD thanks the Universite´ de
Montre´al for a graduate scholarship. AS also acknowledges
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´
La Ministe`re des Affaires Etrange`res (France) for a travel
award. Gratitude is extended to Dr M. Simard for assistance
with the crystallographic measurements and to Prof. D.
Zargarian for helpful suggestions. Prof. J. C. Scaiano is also
acknowledged for kindly providing access to 308 nm LFP for
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