Understanding the reversible anodic behaviour and fluorescence properties of fluorenylazomethines A structure-property study

Article abstract of DOI:10.1139/V10-080

A series of fluorenylazomethine dyads and triads were prepared by simple condensation between the corresponding amine and aldehyde fluorene derivatives. These compounds were prepared as model compounds for investigating the effects of substitution and electronic groups on both the electrochemical properties and fluorescence quantum yields. It was found that the oxidation potential could be decreased by both incorporating electron donating groups and increasing the degree of conjugation. It was further found that alkylation in the fluorene's 9-position increased the azomethine degree of conjugation by forcing all the fluorene moieties to be coplanar with the azomethine bonds to which they are attached. Meanwhile, reversible radical cation behaviour was possible by substituting the terminal 2,2′-positions with atoms other than hydrogen. The radical cation was theoretically found to be distributed evenly across the fluorene, corroborating the reversible anodic behaviour with 2,2′-substitution. The fluorescence quantum yields of the azomethines were not found to be dependent on substitution. This was because the azomethine fluorescence was found to be quenched relative to their precursors regardless of substitution. The fluorescence could be restored at both low temperature and by acid protonation.

Full text of DOI:10.1139/V10-080

9
45
Understanding the reversible anodic behaviour and
fluorescence properties of fluorenylazomethines —
A structure–property study
Satyananda Barik, Sayuri Friedland, and W.G. Skene
Abstract: A series of fluorenylazomethine dyads and triads were prepared by simple condensation between the corre-
sponding amine and aldehyde fluorene derivatives. These compounds were prepared as model compounds for investigating
the effects of substitution and electronic groups on both the electrochemical properties and fluorescence quantum yields. It
was found that the oxidation potential could be decreased by both incorporating electron donating groups and increasing
the degree of conjugation. It was further found that alkylation in the fluorene’s 9-position increased the azomethine degree
of conjugation by forcing all the fluorene moieties to be coplanar with the azomethine bonds to which they are attached.
Meanwhile, reversible radical cation behaviour was possible by substituting the terminal 2,2’-positions with atoms other
than hydrogen. The radical cation was theoretically found to be distributed evenly across the fluorene, corroborating the re-
versible anodic behaviour with 2,2’-substitution. The fluorescence quantum yields of the azomethines were not found to be
dependent on substitution. This was because the azomethine fluorescence was found to be quenched relative to their pre-
cursors regardless of substitution. The fluorescence could be restored at both low temperature and by acid protonation.
Key words: fluorenylimines, reversible oxidation, cyclic voltammetry, azomethines, Schiff base, radical ions.
R e´ sum e´ : On a pr e´ par e´ une s e´ rie de dyades et de triades de la fluor e´ nylazom e´ thine par simple condensation entre des d e´ ri-
v e´ s de l’amine et de l’ald e´ hyde du fluor e` ne. On a pr e´ par e´ ces compos e´ s comme mod e` les pour des e´ tudes sur les effets de
substitution et des groupes e´ lectroniques tant sur les propri e´ t e´ s e´ lectrochimiques que sur les rendements quantiques de
fluorescence. On a trouv e´ que le potentiel d’oxydation peut eˆ tre r e´ duit tant par l’incorporation de groupes e´ lectrodonneurs
que par une augmentation du degr e´ de conjugaison. On a aussi trouv e´ que l’alkylation en position 9 des fluor e` nes aug-
mente le degr e´ de conjugaison des azom e´ thines en for c¸ ant toutes les entit e´ s fluor e` nes a` eˆ tre coplanaires avec les liaisons
azom e´ thines auxquelles sont attach e´ es. Par ailleurs, un comportement de cation radical r e´ versible est possible si l’on sub-
stitue les positions terminales en 2,2’ par des atomes autres que l’hydrog e` ne. On a trouv e´ sur une base th e´ orique que le ca-
tion radical peut eˆ tre distribu e´ d’une fa c¸ on uniforme sur l’ensemble de la portion fluor e` ne, ce qui confirme le
comportement anodique r e´ versible avec une substitution 2,2’. Les r e´ sultats obtenus sugg e` rent que les rendements quanti-
ques de fluorescence des azom e´ thines ne d e´ pendent pas de la substitution. Cette conclusion repose sur le fait que la fluo-
rescence de l’azom e´ thine est d e´ sactiv e´ e par rapport a` celles de leurs pr e´ curseurs quelle que soit la substitution. La
fluorescence peut eˆ tre restaur e´ e aussi bien par une basse temp e´ rature que par une protonation acide.
Mots-cl e´ s : fluor e´ nylimines, oxydation r e´ versible, voltamp e´ rom e´ trie cyclique, azom e´ thines, base de Schiff, ions radicaux.
with vinylene linkages.7–9 Their preparation uses Gilch and
Introduction
Horner–Emmons protocols, requiring rigorous reaction condi-
tions such as anhydrous solvents and inert atmospheres. These
coupling methods, unfortunately, produce significant byprod-
ucts requiring product purification in order not to compromise
the material’s colour emission and device performance.
Azomethines (–N=C–) are highly attractive alternatives to
vinylene linkages owing to their easy preparation not requir-
Conjugated polymers are of great interest in part due to
their electrochemical and optoelectronic properties that are
well-suited for use in plastic devices such as organic field
effect transistors (OFET), light emitting diodes (OLED),
and organic photovoltaics (OPVD), to name but a few.1–4
Polyfluorenes are particularly interesting because of their in-
herent fluorescence making them appropriate emitting materi-
5
10
als for OLED usage. Many polyfluorene derivatives have
ing stringent reaction conditions. They are further advanta-
been prepared and investigated to afford materials with im-
proved device efficiency and color purity for addressing the
performance requirements for the next generation of plastic
electronics and consumer demands. Desired property im-
provements are possible by connecting the fluorene segments
geous because their preparation is environmentally friendly
with water being the unique byproduct produced. The prepa-
ration of azomethines, therefore, requires little or no purifi-
cation. Moreover, the azomethine bond is isoelectronic to its
all-carbon counterpart, making it ideally suited for func-
5–7
Received 5 May 2010. Accepted 6 June 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on 18 August 2010.
1
2
S. Barik, S. Friedland, and W. Skene. Centre of Self-Assembled Chemical Structures, D e´ partement de Chimie, Universit e´ de
Montr e´ al, CP 6128, succ. Centre-ville, Montr e´ al, QC H3C 3J7, Canada.
1
Present address: Department of Chemistry, McGill University, Montreal, QC, Canada.
Corresponding author (e-mail: w.skene@umontreal.ca).
2
Can. J. Chem. 88: 945–953 (2010)
doi:10.1139/V10-080
Published by NRC Research Press

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Can. J. Chem. Vol. 88, 2010
Chart 1. Monomers and oligomers prepared and investigated and some representative analogues.
tional materials usage, but with the advantage of simple
preparation and little purification.
Despite these advantages, azomethine fluorenes have not
been fully exploited as functional materials in emitting devi-
ces. This is a result of previously investigated fluorenylazo-
tions of fluorenyl compounds with tailored properties for
specific applications. Herein, we present the preparation and
electrochemical studies of a series of fluorenylazomethines,
represented in Chart 1, for understanding the structure–rever-
sible oxidation relationship.
1
1–13
methine derivatives that were irreversibly oxidized and were
nonfluorescent.1
4,15
Such properties, unfortunately, preclude
Experimental
their use in functioning devices, and as a result, have at-
tracted little attention for property improvement. It is there-
fore important to understand the reasons for irreversible
oxidation to design and prepare new azomethine derivatives
with functional materials properties. For this reason, we in-
vestigated the effect of fluorenylazomethine structure on the
electrochemical properties to achieve reversible oxidation.
We were further enticed to undertake such structure–property
studies to bring to light the potential uses of fluorenylazome-
thines as functional materials. This is in part because azome-
thines are dismissed as useful functional materials owing to
the limited properties of previously examined compounds.
We therefore examined the electrochemical properties of a
series of model fluorenylazomethines complemented with
theoretical studies to better understand the oxidation process.
The knowledge gained from such studies is not only pivotal
for demonstrating the suitability of azomethines as functional
materials, but also for designing and preparing future genera-
Materials and general experimental procedures
All reagents were commercially available from Sigma-Al-
drich and were used as received unless otherwise stated. An-
hydrous and deaerated solvents were obtained via a Glass
1
13
Contour Solvent Purification System. H NMR and
C
NMR spectra were recorded on a Bruker 400 MHz spec-
trometer with the appropriate deuterated solvents.
Spectroscopic measurements
Absorption measurements were performed on a Cary-500
spectrometer and fluorescence studies were done on an Ed-
inburgh Instruments FLS-920 fluorimeter after deaerating
the samples thoroughly with nitrogen for 20 min. Relative
fluorescence quantum yields were measured at 10–5 mol/L
by exciting the corresponding compounds at its maximum
absorption in spectroscopic grade dichloromethane relative
to itself at 77 K under the same conditions.
Published by NRC Research Press

Barik et al.
947
Electrochemical measurements
the crude product was purified by column chromatography
with hexanes/ethyl acetate (7:3) to give a mixture of 2-ni-
tro-9,9-dihexylfluorene and 2,7-dinitro-9,9-dihexylfluorene.
The products were further recrystalized from EtOH to sepa-
rate the two products, which were isolated as yellow solids.
Cyclic voltammetric measurements were performed on a
Bio Analytical Systems EC Epsilon potentiostat at scan rates
of 100 mV/s. Compounds were dissolved in anhydrous and
–
4
deaerated dichloromethane at 10 mol/L with 0.5 mol/L
Bu NPF . A platinum electrode and a saturated Ag/AgCl
4
6
2
,7-Dinitro-9,9-dihexylfluorene
Isolated yield of 66% (1.94 g). H NMR (400 MHz,
CDCl ) d: 8.33 (dd, 2H), 8.28 (d, 2H), 7.93 (d, 2H), 2.09
electrode were employed as auxiliary and reference electro-
des, respectively, and ferrocene was added at the end of the
electrochemical measurements to serve as an internal refer-
ence.
The highest occupied molecular orbital (HOMO) and low-
est unoccupied molecular orbital (LUMO) energy levels and
spin densities were calculated using density functional
1
3
13
(
dd, 4H), 1.02–1.13 (m, 12H), 0.76 (t, 4H), 0.55 (t, 6H).
C
NMR (100 MHz, CDCl ) d: 152.7, 147.5, 127.7, 123.6,
3
1
21.6, 120.1, 56.0, 40.4, 31.8, 29.9, 24.1, 22.9, 14.3.
2
-Nitro-9,9-dihexylfluorene
theory (DFT) calculation methods available in Spartan 06
1
H NMR (400 MHz, CDCl ) d: 8.27 (d, 1H), 8.23 (d, 1H),
Wavefunction, Inc.) with the 6-31g* basis set.1 The bond
6
3
(
7
(
.82 (dd, 2H), 7.41–7.44 (m, 3H), 2.03 (dd, 4H), 1.05–1.13
angles, distances, torsions, and other parameters were exper-
imentally derived from the X-ray data for an analogous
compound. The crystallographic data were used as the opti-
mized geometry from which the single point energy was cal-
culated to semiempirically calculate the molecular orbitals
and spin densities. The rotational barriers were semiempiri-
m, 12H), 0.76 (t, 4H), 0.60 (t, 6H). 13C NMR (100 MHz,
CDCl ) d: 152.7, 152.3, 148.0, 147.5, 129.6, 127.8, 123.6,
1
2
3
23.5, 121.6, 120.1, 118.6, 56.0, 40.5, 31.8, 29.9, 24.1,
2.9, 14.3.
2
-Amino-9,9-dihexylfluorene (1)
1
7
cally calculated using the AM1 method. The desired dihe-
2
-Nitro-9,9-dihexylfluorene (0.84 g, 2.21 mmol) was dis-
dral angel was varied from 08 to 3608 and constrained at a
solved in 10 mL of EtOH and stirred at room temperature
under N . A catalytic amount of 10% Pd/C (0.2 g) was
added followed by the addition of hydrazine monohydrate
given angle. The heat of formation (DH ) for the given an-
f
2
gle was subsequently calculated without additional structural
optimization.
(
1.6 g, 11.05 mmol) and the mixture was refluxed for 2 h.
The reaction mixture was filtered to remove the Pd/C and
the product was extracted with dichloromethane. The or-
ganic layer was washed with water (3 Â 50 mL) and brine
Synthetic procedures
The syntheses of 3, 4, 7, and 8 were prepared according
to reported procedures.
1
8
solution and then dried over MgSO . The solvent was
4
evaporated and the crude product was purified by column
chromatography with hexanes/ethyl acetate (1:1) as eluent
to give the product as a brown sticky liquid with an isolated
yield of 78%. The product was unstable and was kept under
9
,9-Dihexylfluorene
Fluorene (5 g, 30.8 mmol) was dissolved in DMSO
50 mL) and triethylbenzyl ammonium bromide (0.1 g,
(
1
9
1
.5 mol) was added under N2. A 50% aqueous NaOH
N atmosphere and used immediately for subsequent reac-
2
(
15 mL) solution was added while stirring at room temper-
1
tions. H NMR (400 MHz, CDCl ) d: 7.57 (d, 1H), 7.48 (d,
3
ature. After 0.5 h, 1-bromohexane (12.41 g, 75.2 mmol)
was added dropwise for 15 min. The reaction mixture was
then stirred at room temperature for 4 d. The reaction mix-
ture was diluted with an excess of diethyl ether and the
aqueous layer was removed. The organic layer was washed
with water, 2 mol/L HCl, and brine and then dried over
1
4
H), 7.26 (m, 3H), 7.19 (dd, 2H), 6.68 (d, 2H), 1.90 (dd,
H), 1.04 (m, 12H), 0.78 (t, 6H), 0.64 (d, 4H). C NMR
13
(
1
2
100 MHz, CDCl ) d: 153.0, 150.1, 148.2, 128.9, 125.7,
22.9, 120.9, 118.7, 114.3, 110.2, 55.1, 41.0, 31.9, 30.2,
4.1, 23.0, 14.4. MS m/z: 350.54 (M ).
3
+
MgSO . The solvent was removed under vacuum and the
2,7-Diamino-9,9-dihexylfluorene (2)
4
residue was purified by column chromatography over silica
gel with hexanes as eluent to give the product as a white
The solution of 2,7-dinitro-9,9-dihexylfluorene (1.2 g,
2.8 mmol) in 20 mL of EtOH was stirred at room tempera-
1
solid at –40 8C (7.8 g, 90%). H NMR (400 MHz, CDCl )
ture under N . To the above solution, a catalytic amount of
3
2
d: 7.73 (d, 2H), 7.31–7.38 (m, 6H), 1.99 (dd, 4H), 1.07–
10% Pd/C (0.1 g) was added followed by hydrazine mono-
hydrate (1.02 g, 20.18 mmol) and the mixture was refluxed
for 2 h. The reaction mixture was filtered and the product
was extracted with dichloromethane. The organic layer was
washed with water (3 Â 50 mL) and brine solution and then
1
3
1.33(m, 12H), 0.77 (t, 4H), 0.63(t, 6H).
C NMR
(
1
100 MHz, CDCl ) d: 151.1, 141.5, 127.4, 121.1, 123.2,
20.0, 55.4, 40.8, 31.9, 30.1, 24.1, 23.0, 14.2.
3
2,7-Dinitro-9,9-dihexylfluorene
dried over MgSO . The solvent was evaporated and the
4
The nitration was done similarly to published proce-
crude product was purified by column chromatography with
hexanes/ethyl acetate (1:1) to afford the title compound as a
brown liquid with an isolated yield of 72%. The product was
unstable and was stored under N2 until used. 1H NMR
dures.20
A
solution of 9,9-dihexylfluorene (2.12 g,
6
.33 mmol) in 20 mL of HNO in a 100 mL roundbottom
3
flask was heated to reflux for 2 h under N atmosphere.
2
The reaction mixture was then cooled to room temperature
and poured onto ice and the product was extracted with di-
chloromethane. The organic layer was washed with water
(400 MHz, CDCl ) d: 7.35 (d, 2H), 6.66 (d, 4H), 3.94 (br,
3
NH ), 1.83 (dd, 4H), 1.06 (m, 12H), 0.78 (t, 6H), 0.66 (d,
2
13
4H). C NMR (100 MHz, CDCl ) d: 152.1, 144.3, 133.8,
3
three times, saturated NaHCO , and brine solution and then
119.5, 114.6, 110.7, 55.0, 41.3, 32.0, 30.2, 24.1, 23.1, 14.4.
MS m/z: 356.3 (M ).
3
+
finally dried over MgSO . The solvent was evaporated and
4
Published by NRC Research Press

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48
Can. J. Chem. Vol. 88, 2010
9
,9-Dioctylfluorene-2,7-dicarboxaldehyde (3)
A 250 mL two-necked flask containing 9,9-dioctyl-2,7-di-
until the solution was acidic. The solution was stirred for
30 min then the product was extracted in dichloromethane.
The organic layer was isolated while the aqueous layer was
washed with dichloromethane once more and the combined
organic extracts were dried over Na SO . The crude product
bromofluorene (2.2 g, 4.01 mmol) in anhydrous THF
40 mL) was stirred under N at –78 8C in an acetone–dry
(
2
21
ice bath. n-Butyl lithium (7.6 mL, 16.0 mmol) was added
under vigorous stirring and stirred for 1.5 h. A deaerated
mixture of DMF (5 mL) and anhydrous THF (3 mL) was
added dropwise to the reaction mixture at –78 8C and al-
lowed to stir for 1 h at room temperature. The reaction mix-
ture was cooled to 0 8C and 10% HCl was added until the
solution was acidic (tested by litmus paper). After stirring
for 30 min, the mixture was partitioned between dichloro-
methane and water and the organic layer was isolated. The
aqueous layer was washed with dichloromethane and the
combined organic extracts were dried over Na SO . The
2
4
was purified by column chromatography with hexanes/di-
chloromethane (1:1) to afford the title product as colorless
1
crystals (0.9 g, 58%). H NMR (400 MHz, CDCl ) d: 10.08
3
(
2
s, 1H, CHO), 7.83–7.88 (m, 3H), 7.64 (d, 1H), 7.51 (d, 2H),
13
.01 (dd, 4H), 1.05 (m, 20H), 0.81 (t, 6H), 0.57 (d, 4H).
C
NMR (100 MHz, CDCl ) d: 192.5, 154.6, 151.5, 146.7,
3
1
4
38.9, 136.0, 130.9, 130.8, 126.8, 123.5, 122.6, 120.5, 56.0,
0.4, 32.1, 32.0, 30.2, 29.5, 24.1, 22.9, 14.4, MS m/z: 497.0
+
(
M ).
2
4
crude product was purified by column chromatography using
hexanes/dichloromethane (1:1) as eluent. The product was
obtained as colourless crystals (58%, 1.05 g). 1H NMR
7
-[(9’,9’-Dihexylfluoren-2’-ylimino)-methyl]-9,9-
dioctylfluorene-2-carboxaldehyde (6)
Both 3 (100 mg, 0.22 mmol) and 1 (180.1 mg,
(
400 MHz, CDCl ) d: 10.1 (s, 2H, CHO), 7.91–7.96 (m,
3
0
.514 mmol) were dissolved in absolute ethanol under N2.
6
H), 2.0 (dd, 4H), 1.02 (m, 20H), 0.78 (t, 6H), 0.56 (d, 4H).
A catalytic amount of TFA (60 mL of 1% in absolute etha-
nol) was added and the reaction mixture was stirred over-
night at room temperature. The solvent was evaporated to
afford a dark oil that was extracted with dichloromethane
1
3
C NMR (100 MHz, CDCl ) d: 192.5, 153.2, 146.0, 136.8,
3
1
1
30.7, 123.8, 121.7, 56.0, 40.4, 32.1, 30.2, 29.5, 24.2, 22.9,
4.4. MS m/z: 446.4 (M ).
+
(
25 mL). The organic layer was washed with water and
9
,9-Dihexylfluorene-2-carboxaldehyde (4)
The synthesis was followed according to known meth-
brine solution and then dried over MgSO . The crude prod-
uct was chromatographed on activated basic alumina with
ethyl acetate/hexanes (1:1 v/v) to yield the product as a yel-
low oil (80 mg, 60%). H NMR (400 MHz, CDCl3) d: 10.08
(s, 1H), 8.65 (s, 1H), 8.00 (s, 1H), 7.89 (m, 4H), 7.86 (m,
3H), 7.74 (d, 1H, J = 7.3 Hz), 7.41 (s, 1H), 7.4 (t, 1H, J =
7.3 Hz), 7.31 (m, 2H), 2.01 (m, 8H), 1.08 (m, 30 H), 0.79 (t,
12H, J = 7.0 Hz), 0.61 (m, 8H). 13C NMR (100 MHz,
CDCl ) d: 192.8, 160.7, 152.5, 152.4, 151.9, 151.2, 147.9,
139.9, 135.7, 130.9, 129.2, 127.5, 127.3, 127.1, 123.5,
4
ods.17 The solution of 2-bromo-9,9-dihexylfluorene (4.0 g,
9
.67 mmol) in anhydrous THF (50 mL) was purged with N2
1
at –78 8C in an acetone–dry ice bath. To the above solution,
n-butyl lithium (9.21 mL, 19.35 mmol) was added dropwise
and stirred for 1.5 h. A degassed solution of DMF (2 mL)
and anhydrous THF (3 mL) was added dropwise while
maintaining the reaction mixture at –78 8C, after which the
temperature was raised to room temperature and stirred for 1
h. The reaction mixture was cooled to 0 8C and 10% HCl
was added until the solution was acidic as verified by litmus
paper. After stirring for 30 min, the mixture was partitioned
between dichloromethane and water. The organic layer was
extracted and the aqueous layer was washed again with di-
chloromethane. The combined organic fractions were dried
3
1
3
23.4, 121.3, 120.3, 119.8, 116.5, 55.6, 40.6, 32.0, 31.9,
1.8, 30.1, 30.0, 24.2, 24.1, 23.1, 23.0, 22.9, 14.4, 14.3. MS
m/z: 778.49.
2’,7’-[(9,9,9@,9@-Tetraoctylfluorene-7,7@-diylimino)-methyl]-
9’,9’-dihexyl-2,2@-dibromotrifluorene (9)
over Na SO . After solvent removal, the crude product was
purified by column chromatography with hexanes/dichloro-
methane (1:1). The product was obtained as a colourless
liquid (2.15 g, 48%). H NMR (400 MHz, CDCl ) d: 10.09
2
4
Both 6 (0.511 g, 1.02 mmol) and 4 (0.150 g, 0.411 mmol)
were dissolved in absolute ethanol (5 mL) under N . TFA
2
1
3
(
60 mL of 1% in ethanol) was added to the reaction mixture
(
s, 1H, CHO), 7.91 (s, 1H), 7.86 (dd, 2H), 7.97 (dd, 1H),
and then it was stirred for 1 h at room temperature. The sol-
vent was evaporated to afford a brown oil that was extracted
with dichloromethane (25 mL). The organic layer was
washed with water (2 Â 50 mL) and brine solution (2 Â
7
0
1
1
2
.41 (m, 3H), 2.02 (dd, 4H), 1.04 (m, 12H), 0.75 (t, 6H),
.569 (d, 4H). 1 C NMR (100 MHz, CDCl ) d: 192.7,
52.5, 147.9, 139.9, 135.7, 130.9, 129.2, 127.5, 123.5,
23.4, 121.3, 120.3, 55.8, 40.64, 31.8, 30.0, 29.5, 24.1,
2.9, 14.4. MS m/z: 363.24 (M ).
3
3
5
0 mL) and then dried over MgSO . The product was iso-
4
+
lated as a yellow solid (44%) after flash column chromatog-
raphy with basic activated alumina with hexanes/ethyl
acetate (7:3). H NMR (400 MHz, CDCl ) d: 8.64 (d, 2H),
7
(
(
2
-Bromo-9,9-dioctylfluorene-7-carboxaldehyde (5)
To the solution containing 9,9-dioctyl-2,7-dibromofluor-
1
3
.9 (d, 4H), 7.62 (dd, 4H), 7.52 (d, 4H), 7.31 (d, 2H), 6.65
ene (3.0 g, 5.47 mmol) in anhydrous THF (60 mL) purged
with N at –78 8C was added n-butyl lithium (2.84 mL,
7
1
4H), 2.06 (dd, 12H), 1.08 (m, 50H), 0.81 (m, 18H), 0.63
2
m, 12H). 13C NMR (100 MHz, CDCl ) d: 160.3, 154.1,
.11 mmol) and the reaction was allowed to stir for another
.5 h. Afterwards, deaerated DMF (1 mL) diluted in anhy-
3
1
53.2, 152.6, 151.4, 146.2, 143.8, 140.5, 139.8, 139.7,
drous THF (1 mL) was added dropwise to the reaction mix-
ture at –78 8C, after which the temperature was allowed to
warm to room temperature and then stirred for 1 h. The re-
action mixture was cooled to 0 8C and 10% HCl was added
136.2, 136.0, 132.5, 130.5, 129.3, 126.7, 122.8, 122.0,
120.7, 120.3, 119.6, 119.2, 116.4, 114.4, 110.2, 56.0, 55.3,
41.2, 40.6, 32.0, 30.4, 30.3, 30.2, 29.6, 24.1, 24.0, 23.0,
+
22.9, 14.4. MS m/z: 1323.6 (M ).
Published by NRC Research Press

Barik et al.
949
Fig. 1. Cyclic voltammogram of 6 (*) and 9 (*) measured at
00 mV/s.
Results and discussion
1
Synthesis
Compounds 6–9 were prepared and investigated to pro-
vide insight into the fluorescence and electrochemical be-
haviour of fluorenylazomethines. The compounds were
chosen as model compounds for such studies given the com-
plexity of absolute characterization of their polymeric coun-
terparts. The advantage of investigating these model
compounds is that precise structure–property relationships
can be accurately obtained. The compounds were judiciously
selected because the effect of the imine bond, 9-fluorenyl al-
kylation, degree of conjugation, and substitution of the fluo-
rene’s 2,7-positions on both the electrochemical and
photophysical properties could be investigated. The required
fluorenyl aldehyde precursors were prepared from 9,9-dioc-
tyl 2,7-dibromofluorene by standard metal–halogen ex-
change with n-butyl lithium and quenching with DMF.22
The various aldehyde derivatives were obtained by varying
the n-BuLi/dibromofluorene ratio. The complementary ami-
nofluorene derivatives were obtained by nitrating 9,9-di-
hexyl-fluorene, prepared from fluorene by known methods,
followed by reduction with activated Pd/C (10%).23 The re-
sulting amino fluorenes were extremely sensitive to ambient
conditions and underwent spontaneous decomposition. Con-
sequently, they were used immediately once synthesized for
preparing the targeted azomethines. The desired azomethines
were prepared using mild coupling conditions by refluxing
the complementary fluorenes in absolute ethanol in the pres-
ence of a catalytic amount of TFA. The desired products
were readily purified by column chromatography without
any decomposition and were obtained in sufficient purity
for electrochemical and spectroscopic characterization. The
extended degree of conjugation of 6–9 make these com-
pounds stable and less sensitive to acid hydrolysis to their
aliphatic counter parts.
increased degree of conjugation. A significantly larger de-
crease in the oxidation is expected for 8 as a result of the
increased degree of conjugation; however, previous crystal-
lographic studies showed that the terminal fluorenes are
24
highly twisted with respect to the central axis. Therefore,
it has a limited degree of conjugation, located predominantly
on the =N–fluorene–N= central moiety. Conversely, 9 is
highly conjugated according to the extremely low Epa meas-
ured. The difference in conjugation between 8 and 9 is most
likely a result of the alkyl substitution that forces coplanari-
zation of the fluorene and azomethine units to minimize in-
terchain interactions. Although unequivocal confirmation of
the extended conjugation can be had from the crystal struc-
ture, repeated attempts to crystallize 9 were, unfortunately,
unsuccessful. The optimized geometry was subsequently cal-
culated via DFT for examining the structure of 9. We first
tested this method for accurately calculating the optimized
azomethine geometries by comparing calculated structures
with those measured experimentally from known crystal
structure data. The calculated optimized geometries were
consistent with those obtained by X-ray diffraction (XRD).
The resulting structure calculated for 9 showed the terminal
fluorenes to be twisted by 128 from the plane described by
the central fluorene and the azomethines. This is in contrast
Electrochemical studies
The electrochemical behaviour is an important property
for functional materials. Given that the exciton in an emit-
ting device is produced by the recombination of an electron
and hole, it is desired that the emitting layer be able to re-
peatedly sustain the harsh oxidation and reduction environ-
ment found in the device environment for ensuring device
longevity. The redox properties of the fluorenylazomethines
were therefore investigated and the structure-redox poten-
tials examined. The effects of substitution in the 2, 2’-posi-
tions, degree of conjugation, and alkylation on the
electrochemical properties are possible with the compounds
in Chart 1.
The effect of the single heteroatomic bond on the Epa is
evident by comparing 6 with 4. The oxidation potential of
the fluorenylazomethine is increased by 130 mV. Although
the Epa of 6 is expected to be reduced significantly com-
pared to 4 as a result of the increased degree conjugation
arising from the azomethine bond, the heteroconjugated
bond is a good electron-withdrawing group and counterbal-
ances some of the energetic gain from the increased conju-
gation. The Epa of 7 is increased relative to 6 owing to the
absence of the electron-withdrawing aldehyde group. Mean-
while, the Epa of 8 is 120 mV lower than 7 because of its
to 8, whose terminal fluorenes are twisted by 26.78 and
65.38 from the central fluorene plane.1
8,25
The low Epa of 9
relative to the other azomethines is therefore a result of both
its extended degree of conjugation and the weak electron-
donating terminal bromines.
As seen in Fig. 1, the first oxidation process of 9 is rever-
sible. This is in contrast to the other fluorenylazomethines,
such as 6, that undergo irreversible oxidation. The electro-
chemical sample of 9 was thoroughly deaerated to ensure
that the observed reversible process was not from the reduc-
tion of residual oxygen. The peak persisted despite persistent
nitrogen purging. The anodic process corresponds to a one-
Published by NRC Research Press

9
50
Can. J. Chem. Vol. 88, 2010
Fig. 2. Calculated spin densities shown in red for the radical cation 9. The alkyl groups in the 9-positions are omitted for clarity.
Fig. 3. Normalized fluorescence spectra of 2 (~), 3 (^), 6 ($), and 9 (*) excited at their respective maximum. Inset: normalized emis-
sion spectra of 9 at room temperature (~) and 77 K (~).
electron transfer process while oxygen reduction is a two-
electron process. The radical cation produced from the one-
electron oxidation of 9 is therefore stable, based on the ob-
served reversible oxidation, while similar intermediates for
the other azomethines are unstable, evidenced by the irrever-
sible anodic processes.
processes can thus be tuned courtesy of substitution in the
2,2’-positions. It is therefore expected that polymers derived
from 2 and 3 should sustain reversible oxidation.
Fluorescence studies
Fluorescence investigations involving previously studied
azomethines have exclusively used relative actinometry.
The challenge with this approach is the lack of universal
reference whose emission yield is accurately known and is
both wavelength and solvent independent. Also, there is no
general actinometer that consistently absorbs across the en-
tire visible spectrum for measuring the emission yields of
highly conjugated compounds regardless of the degree of
conjugation. As a result of these limitations, large variations
of fluorescence quantum yields are reported and cannot be
accurately known. The absolute quantum yields of the fluo-
renylazomethine model compounds and their precursors
were therefore measured using an integrating sphere because
of their variable absorbance and fluorescence, evident in
Fig. 3. As seen in Table 1, the fluorescence yields of the
precursors 1–5 are all quenched. This is not surprising given
The spin densities of the radical cation of 9 were investi-
gated to understand the reversible anodic behaviour. As seen
in Fig. 2, the calculated spin densities is evenly distributed
over most of the carbons of 9. The 2,2’-positions are substi-
tuted with bromine for 9 while for the other azomethines,
such as 6–8, these positions are unsubstituted. Therefore, the
resulting radical cation undergoes homocross-coupling accord-
ing to known means when it is located in the terminal posi-
26
tion. This leads to the irreversible oxidation, as observed for
27,28
6–8.
The terminal halogens of 9 effectively prevent radical
cross-coupling and make the one-electron transfer process re-
versible. Alkylation in the 9-position is also important for re-
versible radical cation formation, according to Fig. 2.
Azomethine structure and substitution play important roles in
determining both the oxidation potentials at which the radical
cation is produced and its reversible formation. The fluoreny-
lazomethine oxidation potentials and the reversibility of these
that the fluorescence of fluorene is highly dependent on sub-
stitution.2
9,30
The addition of the azomethine bond to the flu-
Published by NRC Research Press

Barik et al.
951
Table 1. Photophysical and electrochemical properties of fluorenylazomethine derivatives and their precursors.
labs
(nm)
lem
(nm)
Ffl
Eg
Epa
(V)
1.36
—
—
1.00
1.15
0.86
1.30
1.53
1.41
0.56
Epc
(V)
–1.31
—
—
–1.02
–1.06
–1.90
–1.64
–1.12
–1.52
—
HOMO LUMO
Eg
(eV)
1.9
—
—
2.0
2.5
2.76
2.9
2.1
2.2
2.6
(77 K)a
(eV)b
c
d
(eV)
5.2
—
e
(eV)
3.3
—
e
f
Compound
Fluoreneg
261
292
398
339
326
330
384
361
396
410
302
389
494
383
366
425
509
445
472
470
0.72
0
0.01
0
0.01
0.02
0.02 (0.32)
0.01 (0.05)
0.01 (0.31)
0.02 (0.38)
3.9
3.5
2.5
3.4
3.5
3.4
2.6
2.9
2.8
2.6
h
1
2
h
—
—
3
4
5
6
7
8
5.4
5.8
5.26
5.7
5.6
5.6
4.9
3.4
3.3
2.5
2.8
3.5
3.4
2.3
i
i
9
a
Fluorescence quantum yield at room temperature. Values in parentheses are the emission yields measured at 77 K and
calculated relative to the absolute room temperature fluorescence yields.
b
Spectroscopically determined energy gap taken from the absorption onset.
c
+
Oxidation potential relative to saturated Ag/Ag electrode.
Reduction potential relative to Ag/Ag .
d
+
e
Relative to the vacuum level.
Electrochemical energy gap.
f
g
h
Reference 25.
Reliable electrochemical data for 1 and 2 could not be obtained as a result of their instability under our experimental
conditions.
i
Reference 18.
orene moiety does not increase the fluorescence. In all
cases, the fluorenylazomethine fluorescence is quenched re-
gardless of degree of oligomerization.
Fig. 4. Rotational bond energies of the fluorene–N= (black) and
CH–fluorene (blue) bonds for 6 and 2,2’-bifluorene (red) semiem-
pirically calculated from the heats of formation (DHf) using AM1.
=
With increasing oligomerization, the number of degrees of
freedom increases, representing additional modes of fluorescence
deactivation. Fluorescence deactivation by aryl–azomethine bond
rotation was subsequently investigated to determine whether
this deactivation mode was responsible for the quenched
azomethine fluorescence. This was done by examining the
fluorescence emission at 77 K, at which temperature all non-
radiative fluorescence deactivation modes by bond rotation
are suppressed. Therefore, fluorescence increase is expected
at 77 K if deactivation by nonradiative means occurs. The
oligofluorenes 6–9 all exhibited significant increases in fluo-
rescence yields at this reduced temperature as seen in the
inset of Fig. 3. Unfortunately, accurate quantitative measure-
ments of the emission yields at 77 K cannot be obtained be-
cause they are reliant on the weak room temperature
fluorescence signals. Nonetheless, the fluorescence quantum
yields of the oligofluorenylazomethines at low 77 K surpass
those of their precursors at room temperature and reach the
same order of magnitude as native fluorene.
To provide further insight into the origins of the temper-
ature dependent fluorescence of the azomethines, the rota-
tional barrier around the =N–fluorenyl and =CH–fluorenyl
bonds were semiempirically calculated. A simple alkylated
dyad analogue of 7 was used as a model compound because
its crystal structure data is known, which were used for
inputting the correct optimized ground state geometry.
since its fluorescence yields are reduced relative to native
fluorene as a result of nonradiative deactivation around the
fluorene–fluorene bond.24 As seen in Fig. 4, the bond rota-
tion energies for both the azomethine and fluorene dyads
are similar. This suggests that fluorescence quenching by
nonradiative bond rotation is a possible deactivation mode
for azomethines. The absolute fluorescence yield of 9 was
subsequently examined in thin films, where bond rotation
deactivation processes cannot occur. A fluorescence quan-
tum yield of 0.08 was measured in the solid state, confirm-
ing that deactivation by means other than simple fluorene–
N= and =CH–fluorene bond rotation as responsible for azo-
methine fluorescence quenching. Meanwhile, the fluores-
The heats of formation (DH ) of each torsion around the
f
fluorene–N= and =CH–fluorene bonds were calculated.
Although absolute DH values are not possible as a result of
f
scaling errors, the relative values can, however, be accu-
rately calculated. The semiempirically calculated bond rota-
tion energies for an analogous fluorene–fluorene dyad were
also calculated. 2,2’-Bifluorene was used as a benchmark
Published by NRC Research Press

9
52
Can. J. Chem. Vol. 88, 2010
cence of both 6 and 9 could, however, be significantly re-
stored by protonating the azomethine bond with TFA. The
measured absolute fluorescence yield for these two com-
pounds was 0.4 and 0.11, respectively. The fluorescence on/
off switching by acid protonation corroborates previous
(3) Kamtekar, K. T.; Monkman, A. P.; Bryce, M. R. Adv. Mater.
2010, 22 (5), 572. doi:10.1002/adma.200902148. PMID:
20217752.
(
4) Lee, K.; Nair, P. R.; Scott, A.; Alam, M. A.; Janes, D. B. J.
Appl. Phys. 2009, 105 (10), 102046. doi:10.1063/1.3116630.
5) Simas, E. R.; Gehlen, M. H.; Glogauer, A.; Akcelrud, L. J.
(
studies that the major deactivation mode is by intramolecu-
lar photoinduced electron transfer.31 Fluorenylazomethines
Phys. Chem.
A 2008, 112 (23), 5054. doi:10.1021/
jp711934d. PMID:18481838.
are therefore interesting because their fluorescence can be
tuned by both temperature and acid doping.
(
6) Zhang, K.; Chen, Z.; Yang, C.; Zou, Y.; Gong, S.; Qin, J.;
Cao, Y. J. Phys. Chem. C 2008, 112 (10), 3907. doi:10.
1
021/jp077433+.
Conclusion
(7) Mikroyannidis, J. A.; Gibbons, K. M.; Kulkarni, A. P.; Je-
nekhe, S. A. Macromolecules 2008, 41 (3), 663. doi:10.
A series of dyad and triad fluorenylazomethines were pre-
pared with different electronic groups and substitution. The
oxidation potential was contingent on both substitution and
degree of conjugation. Although homoaryl groups are nor-
mally twisted by up to 658 from the azomethines to which
they are linked, increased planarity and hence increased de-
gree of conjugation was possible by alkylating the fluorene’s
1
021/ma071504l.
8) Sun, M.; Zhong, C.; Li, F.; Cao, Y.; Pei, Q. Macromolecules
010, 43 (4), 1714. doi:10.1021/ma9024762. PMID:17343421.
9) Bezgin, B.; Yagan, A.; Onal, A. M. J. Electroanal. Chem.
009, 632 (1–2), 143. doi:10.1016/j.jelechem.2009.04.011.
(
(
2
2
(
10) Guar `ı n, S. A.; Bourgeaux, M.; Dufresne, S.; Skene, W. G. J.
Org. Chem. 2007, 72 (7), 2631. doi:10.1021/jo070100o.
PMID:17343421.
9-position. Substitution in the 2,2’-position was also found to
play an important role in determining the reversibility of the
electrochemically generated radical cation. Meanwhile, the
fluorescence of the azomethines remained quenched similar
to their precursors. The fluorescence could be restored at
low temperature and by acid protonation. The fluorenylazo-
methines therefore have sensor properties. The collective
knowledge gained from the combined electrochemical and
fluorimetric studies is pivotal for the design and preparation
for the future generation of azomethines for emitting appli-
cations and will lead to highly fluorescent materials with de-
sired reversible oxidation. The results further suggest that
polymers derived from 2 and 3 will exhibit desired reversi-
ble oxidation, making them suitable materials for emitting
applications.
(
11) Wang, C.; Shieh, S.; LeGoff, E.; Kanatzidis, M. G. Macro-
molecules 1996, 29 (9), 3147. doi:10.1021/ma9514131.
(12) Yang, C.-J.; Jenekhe, S. A. Chem. Mater. 1991, 3 (5), 878.
doi:10.1021/cm00017a025.
(13) Kuder, J. E.; Gibson, H. W.; Wychick, D. J. Org. Chem.
1975, 40 (7), 875. doi:10.1021/jo00895a013.
(
14) Tsai, F.-C.; Chang, C.-C.; Liu, C.-L.; Chen, W.-C.; Jenekhe,
S. A. Macromolecules 2005, 38 (5), 1958. doi:10.1021/
ma048112o.
(
(
(
15) Liu, C.-L.; Chen, W.-C. Macromol. Chem. Phys. 2005, 206
(
21), 2212. doi:10.1002/macp.200500236.
16) Sherrill, C. D. J. Chem. Phys. 2010, 132 (11), 110902.
doi:10.1063/1.3369628.
17) Thiel, W. In Theory and Applications of Computational
Chemistry: The First Forty Years; Dykstra, C. E., Kim, K.
S., Frenking, G., Scuseria, G. E., Eds.; Elsevier: Amserdam,
2005; pp 559–580.
Supplementary data
(
(
(
18) Guar `ı n, S. A. P.; Dufresne, S.; Tsang, D.; Sylla, A.; Skene, W.
G. J. Mater. Chem. 2007, 17 (27), 2801. doi:10.1039/b618098a.
19) Chen, Q.-Q.; Liu, F.; Ma, Z.; Peng, B.; Wei, W.; Huang, W.
Chem. Lett. 2008, 37 (2), 178. doi:10.1246/cl.2008.178.
20) Dudek, S. P.; Pouderoijen, M.; Abbel, R.; Schenning, A. P.
H. J.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127 (33),
11763. doi:10.1021/ja052054k. PMID:16104754.
Supplementary data for this article are available on the
journal Web site (canjchem.nrc.ca).
Acknowledgements
The Natural Sciences and Engineering Reserach Council
of Canada (NSERC) is thanked for a Discovery Grant (DG)
and a Research Tools and Instruments (RTI) Grant, and the
Centre for Self-Assembled Chemical Structures (CSACS)
for a Strategic Research Grant (SRG), which allowed this
work to be performed. In addition, the Canada Foundation
for Innovation (CFI) is thanked for additional equipment
funding. WGS also thanks both the Alexander von Hum-
boldt Foundation and the Royal Society of Chemistry
(21) Giuseppone, N.; Fuks, G.; Lehn, J.-M. Chem. Eur. J. 2006,
12 (6), 1723. doi:10.1002/chem.200501037.
(
(
(
22) Belfield, K. D.; Morales, A. R.; Kang, B.-S.; Hales, J. M.; Ha-
gan, D. J.; Van Stryland, E. W.; Chapela, V. M.; Percino, J.
Chem. Mater. 2004, 16 (23), 4634. doi:10.1021/cm049872g.
23) Dudek, S. P.; Pouderoijen, M.; Abbel, R.; Schenning, A. P.
H. J.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127 (33),
1
1763. doi:10.1021/ja052054k. PMID:16104754.
(
RSC) for a J. W. T. Jones Travelling Fellowship, allowing
24) Guarin, S. A. P.; Dufresne, S.; Tsang, D.; Sylla, A.; Skene,
this manuscript to be completed. SF thanks NSERC and the
Reactive Intermediate Student Exchange Program (RISE)
program for an undergraduate scholarship.
W. G. Polymeric Materials: Science and Engineering
(
PMSE) Preprints; American Chemical Society: Washing-
ton, DC, 2007; Vol. 96, p 244.
(
25) Dufresne, S.; P e´ rez Guar `ı n, S. A.; Bolduc, A.; Bourque, A.
N.; Skene, W. G. Photochem. Photobiol. Sci. 2009, 8 (6),
796. doi:10.1039/b819735k. PMID:19492107.
(26) Roncali, J. Chem. Rev. 1992, 92 (4), 711. doi:10.1021/
cr00012a009.
References
(
1) Br e´ das, J.-L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Acc.
Chem. Res. 2009, 42 (11), 1691. doi:10.1021/ar900099h.
PMID:19653630.
(
2) Bredas, J.-L.; Durrant, J. R. Acc. Chem. Res. 2009, 42 (11),
(27) Roncali, J.; Blanchard, P.; Fr e` re, P. J. Mater. Chem. 2005,
1
689. doi:10.1021/ar900238j. PMID:19916562.
15 (16), 1589. doi:10.1039/b415481a.
Published by NRC Research Press

Barik et al.
953
(
28) Hapiot, P.; Lagrost, C.; LeFloch, F.; Raoult, E.; Rault-
Berthelot, J. Chem. Mater. 2005, 17 (8), 2003. doi:10.1021/
cm048331o.
(30) Murphy, R. S.; Moorlag, C. P.; Green, W. H.; Bohne, C. J.
Photochem. Photobiol. A 1997, 110 (2), 123. doi:10.1016/
S1010-6030(97)00191-3.
(
29) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of
Molecular Photochemistry: An Introduction; University
Science Books: Sausalito, CA, 2009.
(31) Dufresne, S.; Callaghan, L.; Skene, W. G. J. Phys. Chem. B
2009, 113 (47), 15541. doi:10.1021/jp907391y.
Published by NRC Research Press