Blue-emitting copolymers of isoquinoline and fluorene

Article abstract of DOI:10.1016/j.reactfunctpolym.2011.05.006

5,8-Linked copolymers of isoquinoline with fluorene were synthesized by Pd(0)-catalyzed Suzuki polycondensation reaction. The polymers showed good solubility in common organic solvents and the number average molecular weights determined from GPC analysis fall in the range of 7.03-8.08 × 10 ³ g/mol. The photoluminescence spectra of these neutral materials in solution exhibit emission maxima in the range 408-426 nm. An X-ray diffraction study on a single crystal of a 5,8-di(9,9-dimethylfluoren-2-yl) isoquinoline model compound shows it to have a non-planar conformation and sheds light on the factors that drive the solid-state packing. The alkylation of the imine nitrogen was achieved by treatment with methyl iodide and the alkylated products were characterized by ¹H NMR, ¹³C NMR and UV-vis spectroscopy. The absorption maxima of model compounds and polymers were red shifted by 46-70 nm upon N-alkylation.

Full text of DOI:10.1016/j.reactfunctpolym.2011.05.006

Reactive & Functional Polymers 71 (2011) 849–856
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Blue-emitting copolymers of isoquinoline and fluorene
Renchu Scaria a, Nigel T. Lucas b, Klaus Müllen c, Josemon Jacob a,
⇑
a Centre for Polymer Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India
b Department of Chemistry, University of Otago, Dunedin 9054, New Zealand
c Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 March 2011
Received in revised form 10 May 2011
Accepted 12 May 2011
Available online 18 May 2011
5,8-Linked copolymers of isoquinoline with fluorene were synthesized by Pd(0)-catalyzed Suzuki poly-
condensation reaction. The polymers showed good solubility in common organic solvents and the num-
ber average molecular weights determined from GPC analysis fall in the range of 7.03–8.08 Â 103 g/mol.
The photoluminescence spectra of these neutral materials in solution exhibit emission maxima in the
range 408–426 nm. An X-ray diffraction study on a single crystal of a 5,8-di(9,9-dimethylfluoren-2-yl)
isoquinoline model compound shows it to have a non-planar conformation and sheds light on the factors
that drive the solid-state packing. The alkylation of the imine nitrogen was achieved by treatment with
methyl iodide and the alkylated products were characterized by 1H NMR, 13C NMR and UV–vis spectros-
copy. The absorption maxima of model compounds and polymers were red shifted by 46–70 nm upon
N-alkylation.
Keywords:
Conjugated polymers
Polycations
Isoquinoline
N-alkylation
Ó 2011 Elsevier Ltd. All rights reserved.
Solid-state packing
1. Introduction
synthesis and opto-electronic properties of oligomers or polymers
bearing isoquinoline moieties with a 5,8-linkage in the conjugated
The optical and electronic properties of polymers containing
N-heterocyclic moieties have been studied extensively for their
potential use in organic light emitting diodes (OLEDs), organic
solar cells and chemical sensors [1–4]. Quinoline, quinoxaline,
oxadiazole and pyridine derivatives have been widely used as elec-
tron transporting/hole blocking materials in LED devices and LED
blends because these types of units have excellent properties such
as high electron affinity, high thermal and oxidation stability, and
good charge injection [5–8]. Within the class of conjugated poly-
mers, materials containing N-heterocyclic moieties have attracted
considerable attention not only because of their electron-accepting
abilities but also for the possibility of metal complexation, N-oxi-
dation, N-protonation, and N-alkylation, leading to polymers with
unique properties that are different from neutral polymers [4,9–
14]. Recently, Swager and Izuhara have reported on a new class
of nitrogen-containing polyheterocycles with high electron affinity
[15,16]. Polyquinolines have been widely studied as n-type
semiconductors and emissive materials in electronic devices
[17–22]. Compared to quinoline based materials, there have been
limited reports on materials derived on isoquinoline for electronic
applications [23–26]. Recently, isoquinoline based metal
complexes have been studied for their potential application in
phosphorescence-based organic and polymeric light-emitting
devices [27–29]. Apart from a few reports on polyisoquinolines,
backbone have not yet been explored. In this contribution, the syn-
thesis, characterization and optical studies of model compounds
and copolymers derived from isoquinoline and dialkylfluorene
have been investigated. Oligo- and polyfluorenes have emerged
as the most attractive blue-emitting materials due to their high
efficiency and good thermal stability of polymers [30–37]. Combin-
ing the electron affinity of isoquinoline and blue light-emitting
nature of highly soluble polyfluorenes, the copolymers are ex-
pected to possess good solubility, thermal stability and blue
light-emitting properties. The absorption maxima of neutral poly-
mers can be extended by introduction of cations into the polymer
backbone. In a previous study, we reported on the synthesis of
conjugated polyfluorenyl cations and the control of cation density
and solubility by copolymerization [38]. In the case of isoquinoline
based materials, the presence of an electron deficient nitrogen
atom offers the possibility to tune the optical properties by
N-alkylation. N-alkylation is widely investigated as a means to
tailor the optical properties of polyquinoline and polypyridine
derivatives. The N-alkylated polymers bearing cations in the back-
bone have exhibited a red shift in absorption maxima as compared
to their neutral polymers [13,34,39]. Isoquinoline with different
linkage positions offers the possibility to synthesize novel organic
materials with tunable optical properties. In this study, copolymers
and model compounds of 5,8-linked isoquinoline with fluorene
have been prepared, their optical and thermal properties
investigated, and the solid-state structure of a model compound
⇑
Corresponding author. Tel.: +91 9911061890; fax: +91 11 2659 1421.
E-mail address: jacob@polymers.iitd.ac.in (J. Jacob).
1381-5148/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2011.05.006

850
R. Scaria et al. / Reactive & Functional Polymers 71 (2011) 849–856
determined. In addition, conjugated polycations formed by N-
alkylation of the isoquinoline unit are discussed.
140.78, 139.06, 138.91, 138.66, 138.01, 137.64, 134.73, 130.50,
129.10, 128.89, 127.87, 127.53, 127.10, 124.46, 124.25, 122.66,
120.19, 120.03, 118.71, 47.01, 27.19.
2. Experimental
2.3.2. Synthesis of 2,7-di(isoquinol-5-yl)-9,9-dimethylfluorene (7)
In a Schlenk flask, 5-bromoisoquinoline 3 (0.28 g, 0.63 mmol),
2,7-bis(4,4,5,5-tetramethyl-[1,3,2]dioxaborolane)-9,9-dimethylflu-
orene 5 (0.46 g, 2.20 mmol), 2 M K2CO3 (4.6 mL, 9.19 mmol) and
6 mL of 1,4-dioxane were taken and purged with N2 for 15 min.
Then, Pd(PPh3)4 (0.06 g, 2 mol%) was added and the reaction mix-
ture was heated with stirring at 100 °C for 20 h. The cooled reac-
tion mixture was extracted into chloroform and the organic layer
was washed with brine and then dried over anhydrous Na2SO4.
The solvent was removed under vacuum and the residue was puri-
fied by column chromatography over silica gel with 0–20% ethyl
acetate in hexane as the eluent, to give 7 as a yellow solid
(0.15 g, 53%). 1H NMR (300 MHz, CDCl3): d = 9.34 (s, 2H), 8.54 (d,
2H, J = 5.7 Hz), 8.04 (d, 2H, J = 8.1 Hz), 7.94 (d, 2H, J = 7.8 Hz),
7.85 (d, 2H, J = 5.7 Hz), 7.79 (d, 2H, J = 6.9 Hz), 7.73 (t, 2H,
J = 7.35 Hz), 7.59 (s, 2H), 7.53 (d, 2H, J = 7.5 Hz). 13C NMR
(75 MHz): d = 154.24, 152.92, 143.42, 139.49, 138.35, 138.26,
134.18, 130.96, 129.02, 127.11, 126.82, 124.26, 120.19, 118.54,
47.16, 27.21.
2.1. Materials
Dry solvents, stored in a glove box, were used for specified reac-
tions. 5,8-dibromoisoquinoline (2) and 5-bromoisoquinoline (3)
were synthesized according to literature procedures [40]. 9,9-
Dimethylfluorene-2-(4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane) (4),
2,7-bis(4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane)-9,9-dimethylflu-
orene (5), 2,7-bis(4,4,5,5-tetramethyl-[1,3,2]-dioxaboralan-2-yl)-
9,9-dioctylfluorene (8a) and 2,7-bis(4,4,5,5-tetramethy-[1,3,2]-
dioxaboralan-2-yl)-9,9-bis(2-ethylhexyl)fluorene (8b) were also
synthesized in accordance with literature procedures [41–44].
2.2. Characterization
1H and 13C NMR spectra were recorded on a Bruker 300 MHz
spectrometer operating respectively at 300 MHz for 1H and
75 MHz for 13C NMR using deuterated solvents with tetramethyl-
silane (TMS) as a reference. UV–vis absorption spectra were
recorded in THF solution on a Perkin-Elmer Lambda 25 Spectro-
photometer. Photoluminescence measurements were carried out
with a Fluorolog HORIBAJOBIN YVON spectrophotometer using a
xenon-arc lamp as a source, in THF solution. Thermal degradation
was studied by TGA on a Perkin Elmer Pyris 7 thermal analysis sys-
tem under a dynamic atmosphere of nitrogen at a heating rate of
20 °C/min. Differential scanning calorimetry (DSC) data were
recorded using a TA DSC Q200 calibrated with indium at heating/
cooling rates of 10 °C minÀ1 under nitrogen. The molecular weights
of the polymers were determined by Gel Permeation Chromatogra-
phy against polystyrene standard in THF at 30 °C. The cyclic
voltammetric studies were conducted on an Autolab 30, Potentio-
stat/Galvanostat at a constant scan rate of 20 mV sÀ1. Platinum
wires were used as both the counter and working electrodes, and
Ag/AgCl electrode was used as the reference electrode. Thin films
of copolymers 9a and 9b on platinum electrodes were prepared
by dipping the electrode into a 0.5–1.0 wt.% copolymer solution
in toluene. The resulting films were dried in a vacuum oven at
80 °C. Cyclic voltammogram of 12a and 12b were taken from a
solution of corresponding compounds in 0.1 M tetrabutylammo-
nium tetrafluoroborate (TBABF4) in acetonitrile, which was used
as the electrolyte.
2.4. Synthesis of polymers
2.4.1. Synthesis of 9a
A mixture of 5,8-dibromoisoquinoline 2 (0.25 g, 0.87 mmol),
2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaboralan-2-yl)-9,9-dioctyl-
fluorene 8a (0.56 g, 0.87 mmol) and Pd(PPh3)4 (0.020 mg,
2 mol%) were added to a degassed mixture of toluene (3 mL), Ali-
quat 336 (3 drops) and 1.7 mL of 2 M Na2CO3. The mixture was vig-
orously stirred at 85 °C for 48 h under a nitrogen atmosphere.
Then, bromobenzene (45 lL, 0.43 mmol) was added as an end cap-
ping agent to the mixture and heated for an additional 12 h. After
cooling to room temperature, the viscous solution was poured into
300 mL of methanol. The precipitated polymer was isolated by fil-
tration. It was further purified by Soxhlet extraction with acetone
followed by reprecipitation from methanol. The polymer was dried
under reduced pressure to yield 9a as a pale yellow solid (0.36 g,
80%). 1H NMR (300 MHz, CDCl3): d = 9.51 (s, 1H), 8.57 (d, 1H),
7.99–7.89 (4H, br), 7.74 (s, 1H), 7.62–7.59 (5H, br), 2.11 (4H, br),
1.17–0.82 (m, 30H, br). UV–vis (THF) kmax
: 358 nm, GPC:
Mn = 8.08 Â 103 g/mol, PDI = 2.85.
2.3. Synthesis of model compounds
2.3.1. Synthesis of 5,8-di(9,9-dimethylfluoren-2-yl) isoquinoline (6)
5,8-dibromoisoquinoline 2 (0.25 g, 0.87 mmol), 9,9-dimethyl
2.4.2. Synthesis of 9b
A mixture of 5,8-dibromoisoqunoline 2 (0.26 g, 0.90 mmol), 2,7-
bis(4,4,5,5-tetramethy-1,3,2-dioxaboralan-2-yl)-9,9-bis(2-ethylhexyl)
fluorene 8b (0.58 g, 0.90 mmol) and Pd(PPh3)4 (0.021 mg, 2 mol%)
were added to a degassed solution of toluene (3 mL), Aliquat 336
(three drops) and 1.8 mL of 2 M Na2CO3. The mixture was stirred
fluorene-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)
4
(0.84 g,
2.63 mmol), 2 M K2CO3 (2.6 mL, 5.22 mmol) were added to 5 mL
of 1,4-dioxane in a Schlenk flask. The solution was purged with
N2 for 15 min, and then Pd(PPh3)4 (0.06 g, 2 mol%) was added.
The reaction mixture was heated with stirring at 100 °C. The reac-
tion was followed by TLC and after 20 h was worked up. The cooled
reaction mixture was extracted into chloroform and the organic
layer was washed with brine and then dried over anhydrous
Na2SO4. The solvent was removed under vacuum and the residue
was purified by column chromatography over silica gel with 0–
10% ethyl acetate in hexane as the eluent, to give 6 as a white solid
(0.32 g, 71%). 1H NMR (300 MHz, CDCl3): d = 9.47 (s, 1H), 8.45 (d,
1H, J = 6 Hz), 7.88 (m, 3H), 7.83–7.79 (m, 3H), 7.70 (d, 1H,
J = 7.2 Hz), 7.64 (s, 1H), 7.59–7.49 (m, 5H), 7.39–7.35 (m, 4H). 13C
NMR (75 MHz): d = 154.03, 153.96, 153.85, 151.61, 143.04,
vigorously at 85 °C for 48 h. Bromobenzene (47 lL, 0.45 mmol)
was then added as end capping agent to the mixture and further
heated for 12 h with stirring. The reaction mixture was cooled to
room temperature and the viscous solution poured into 300 mL
of methanol. The precipitated polymer was isolated by filtration
and purified by Soxhlet extraction with acetone followed by repre-
cipitation from methanol. The polymer was dried under reduced
pressure to give 9b in 78% yield (0.36 g). 1H NMR (300 MHz,
CDCl3): d = 9.53 (s, 1H), 8.55 (d, 1H), 7.97–7.60 (m, 9H), 2.13 (t
br, 4H), 1.00–0.66 (m br, 30H). UV–vis (THF) kmax: 354 nm, GPC:
Mn = 7.03 Â 103 g/mol, PDI = 3.04.

R. Scaria et al. / Reactive & Functional Polymers 71 (2011) 849–856
851
2.5. Synthesis and characterization of isoquinolinium salts
3. Results and discussion
2.5.1. Synthesis of 10
3.1. Synthesis and characterization of model compounds and polymers
Iodomethane (0.049 mL, 0.77 mmol) was added dropwise to
5,8-di(9,9-dimethylfluoren-2-yl)isoquinoline
6
(0.050 g, 9.74 Â
Scheme 1 outlines the synthetic scheme for model compounds
and polymers made by the Suzuki coupling reaction. Coupling of 2
and 4 in a mixture of dioxane and aqueous K2CO3 solution gave
5,8-di(9,9-dimethylfluoren-2-yl)isoquinoline (6) in 71% yield. In a
similar manner, the coupling of 3 and 5 gave 2,7-di(isoquinol-5-
yl)-9,9-dimethylfluorene (7) in 53% yield. The reaction of 2 with
2,7-bis(4,4,5,5-tetramethy-1,3,2 dioxaboralan-2-yl)-9,9-dioctylflu-
orene (8a) under Suzuki polymerization conditions furnished 9a
in 80% yield with a degree of polymerization (Pn) of 15 and a poly-
dispersity (Mw/Mn, PDI) of 2.85. 9b was obtained by reaction of 2
with 2,7-bis(4,4,5,5-tetramethy-[1,3,2]-dioxaboralan-2-yl)-9,9-bis
(2-ethylhexyl)fluorene (8b) under identical conditions in 82% yield
with a Pn of 13 and a PDI of 3.05. Although the measured PDI is
slightly higher than normal, repeated Soxhelt extraction and repre-
cipitation did not improve the PDI of the copolymers. We believe
the molecular weights can be further improved by use of alkyl
chains with longer solubilising groups and will be reported sepa-
rately. The compounds 6 and 7 and the polymers 9a and 9b show
good solubility in common organic solvents.
The molecular structure of the polymers and model compounds
were confirmed by 1H NMR spectroscopy. In 9b the orientation of
isoquinoline units in the polymer chain are random with respect to
each other, as evidenced from their 1H NMR spectra. In the case of
model compound 6, a single crystal was subjected to X-ray diffrac-
tion analysis enabling insight into the molecular confirmation and
solid state organization of the molecule. The thermal properties of
the polymers were determined by DSC and TGA measurements.
The degradation temperature of polymers at 5% weight loss (Td)
was in the range of 408–438 °C. The glass transition temperatures
(Tg) of 9a and 9b are 91 °C and 95 °C, respectively. The physical
properties of polymers are summarized in Table 1.
10À2 mmol) dissolved in 7 mL of chloroform. The reaction mixture
was stirred at room temperature for 24 h and the yellow solution
was extracted with dichloromethane to yield 10 as an yellow solid
(0.062 g, 96%). 1H NMR (300 MHz, CDCl3): d = 9.32 (s, 1H), 9.02 (d,
1H, 6.3 Hz), 8.42 (d, 1H, 6.3 Hz), 8.12 (d, 1H, 7.5 Hz), 7.96 (t, 2H,
8.1 Hz), 7.85 (d, 1H, 7.8 Hz), 7.76–7.67 (m, 3H), 7.62 (d, 1H,
J = 7.5 Hz), 7.5 (s, 1H), 7.41–7.34 (m, 7H), 4.71 (s, 3H), 1.73 (s,
6H). 13C NMR (75Mz, CDCl3) d = 154.85, 154.59, 153.93, 153.77,
148.42, 143.15, 140.44, 139.99, 139.66, 138.03, 136.40, 135.94,
135.15, 134.41, 131.46, 129.66, 128.82, 127.97, 127.13, 126.73,
124.75, 124.51, 124.01, 122.64, 120.81, 120.48, 49.89, 47.16, 27.07.
2.5.2. Synthesis of 11
To 2,7-di(isoquinol-5-yl)-9,9-dimethylfluorene 7 (0.050 g, 0.011
mmol) dissolved in 7 mL of chloroform, iodomethane (0.055 mL,
0.88 mmol) was added. The reaction mixture was stirred at room
temperature for 24 h to yield an orange precipitate which is
washed with chloroform to yield 11 as an orange solid (0.065 g,
98%). 1H NMR (300 MHz, DMSO-d6): d = 10.09 (s, 2H), 8.67 (d,
2H, J = 6.3 Hz), 8.54 (d, 2H, J = 7.8 Hz), 8.35–8.29 (m, 6 H), 8.18–
8.14 (m, 4H), 7.61(d, 2H, J = 7.5 Hz), 4.50 (s, 6H), 1.59 (s, 6H). 13C
NMR (75 MHz, DMSO-d6): d = 154.59, 151.02, 139.36, 138.32,
137.06, 136.31, 131.01, 129.83, 129.16, 127.74, 124.50, 123.39,
121.17, 47.84, 47.13, 26.72.
2.5.3. Synthesis of 12a
Iodomethane (0.12 mL, 1.93 mmol) was added dropwise to 9a
(0.125 g, 2.414 Â 10–4 mol of monomer) in 6 mL of chloroform.
The reaction mixture was stirred at room temperature for 24 h to
yield a red precipitate which is washed with chloroform to yield
12a as a red solid (0.120 g, 75%). 1H NMR (300 MHz, DMSO-d6):
d = 9.68 (s br, 1H), 8.79 (d br, 1H), 8.40–7.71 (m br, 9H), 4.54 (s,
3H), 2.21 (t br, 4H), 1.13 – 0.78 (m br, 30H).
3.2. Optical properties of model compounds and polymers
2.5.4. Synthesis of 12b
In order to study the optical properties of polymers and model
compounds, UV–vis spectra were recorded in THF solution, which
are depicted in Fig. 1. When isoquinoline is end-capped at 5,8 posi-
tions with fluorene units (6), the absorbance maximum is observed
at 340 nm. On the other hand, when fluorene is end-capped with
an isoquinoline unit at both ends (7), the absorbance maximum
is observed at 327 nm. The observed trend is in accordance with
what is expected as 7 has lower conjugation length compared to
6. Also, the extent of conjugation between neighbouring fluorene
and isoquinoline units will largely depend on the twist angle.
The absorption maxima of 9a and 9b are at 358 nm and 354 nm,
respectively.
The photoluminescence spectra of the model compounds and
polymers were recorded in THF and are shown in Fig. 2. When flu-
orene is end capped with isoquinoline units on either side, the
emission maximum is observed at 408 nm. However, when iso-
quinoline is end caped at 5,8-positions with fluorene units, the
emission maximum is at 424 nm. The observed Stokes shifts for
6 and 7 are 84 nm and 81 nm, respectively. Polymers 9a and 9b
show emission maxima at 417 nm and 426 nm giving Stokes shifts
of 59 nm and 72 nm respectively. The magnitude of Stokes shift in
conjugated systems describes the structure reorganization be-
tween the ground and excited states and/or energy migration to
minority segments having greater conjugation lengths [47]. The
smaller Stokes shift of the polymers compared to model com-
pounds indicates the larger structure reorganization (bond twist-
ing, solution relaxation or change in electronic structure)
Iodomethane (0.12 mL, 1.93 mmol) was added dropwise to 9b
(0.125 g, 2.41 Â 10À4 mol of monomer) in 6 mL of chloroform.
The reaction mixture was stirred at room temperature for 24 h to
yield a red precipitate which is washed with chloroform to yield
12b as a red solid (0.120 g, 75%). 1H NMR (300 MHz, DMSO-d6):
d = 9.59 (s, 1H, br), 8.86 (d, 1H, br), 8.32–7.70 (m, 9H, br), 4.54 (s,
3H), 2.24 (t, 4H, br), 0.91–0.62 (m, 30H, br).
2.6. X-ray crystallography
Data were collected at 91 K with
a Bruker Kappa X8 diffractometer with APEX II detector, and
employing graphite monochromated Mo K radiation generated
x and u scans to 50.78° 2h on
a
from a fine-focus sealed tube. The data integration and reduction
were undertaken with SAINT and XPREP (Bruker Analytical X-ray
Instruments Inc, Madison, Wisconsin, USA, 1995) and subsequent
computations were carried out with the Xseed graphical interface
to SHELX97 [45]. A Gaussian absorption correction was applied to
the data [46]. The structure was solved in the monoclinic space
group P21/c (#14) by direct methods with SHELXS-97 [45], then
extended and refined with SHELXL-97. The non-hydrogen atoms
in the asymmetric unit were modelled with anisotropic displace-
ment parameters. A riding atom model with group displacement
parameters was used for the hydrogen atoms, with the exception
of the chloroform hydrogen atom, which was fully refined to inves-
tigate its interaction with the isoquinoline nitrogen atom.

852
R. Scaria et al. / Reactive & Functional Polymers 71 (2011) 849–856
2.2 eq NBS
Con.H2SO4
1.1 eq NBS
Con.H2SO4
Br
Br
Br
N
N
N
3
2
1
O
O
2M K2CO3, Pd(PPh3)4
Dioxane, 100 0
2M K2CO3, Pd(PPh3)4
Dioxane, 100 0
O
B
O
O
B
B
C
C
O
4
5
N
N
N
6
7
Br
Br
O
O
O
O
2M K2CO3, Pd(PPh3)4
toluene, 85 0C, 48h
B
B
+
*
*
n
N
R
R
R
R
N
2
8 a R = n-octyl
9 a
9 b
R = n-octyl
8 b R = 2-ethylhexyl
R = 2-ethylhexyl
Scheme 1. Synthetic scheme for model compounds and polymers.
involved in the latter case. The photophysical properties of poly-
mers and model compounds are summarized in Table 2.
Table 1
Physical properties of polymers.
Polymer
Yield (%)
Mn  103(g/mol)a
Mw/Mn
Td (°C)b
Tg (°C)c
a
3.3. Solid state structure of the neutral model compound 6
9a
9b
80
78
8.08
7.03
2.85
3.04
424
403
91
95
Slow diffusion of methanol into a chloroform solution of
5,8-di(9,9-dimethylfluoren-2-yl)isoquinoline (6) yielded needle-
like crystals, one of which was subjected to X-ray diffraction
structure analysis (Fig. 3). The compound crystallized as a 1:1
chloroform adduct with four molecules of each per unit cell in
the centrosymmetric P21/c space group. Attempts to obtain single
a
Number–average molecular weight (Mn) and polydispersity index (Mw/Mn)
determined by means of GPC in THF on the basis of polystyrene calibration.
b
Degradation temperature at 5% weight loss (Td) measured by TGA at a heating
rate of 20 °C/min under N2 atmosphere.
c
Glass transition temperature measured by DSC at a heating rate of 10 °C/min
under N2 atmosphere.
Fig. 2. Photoluminescence spectra of model compounds and polymers in THF
Fig. 1. UV–vis spectra of model compounds and polymers in THF solution.
solution.

R. Scaria et al. / Reactive & Functional Polymers 71 (2011) 849–856
853
Table 2
Photophysical properties of polymers and model compounds.
Polymer/model compound
kmax (nm)a
kem (nm)b
9a
9b
6
358
354
340
327
417
426
424
408
7
a
UV–vis absorption maxima recorded in THF solution.
Photoluminescence emission maxima recorded in THF solution.
b
crystals of 6 were only successful using chloroform and the study
confirms that the solvent plays an active role in the crystallization.
The unsaturated core of 6 adopts a non-planar conformation, with
dihedral angles of 52.6° (C5–C12) and 67.3° (C8–C32) between the
isoquinoline and respective fluorene units. All bond lengths and
angles are within the usual ranges expected. Few other 5- or 8-
substituted isoquinolines have been structurally characterized
[48–51]; this is the first example of a structurally characterized
5,8-diaryl substituted isoquinoline.
The crystal packing provides insight into how this type of mol-
ecule may organize within the thin films typically needed in opto-
electronic devices. Tightly-packed sheets of 6, stacked along the a-
axis, display face-to-face
p-stacking between pairs of isoquinoline
units (separation 3.52 Å) and several C–HÁ Á ÁC(
p)
interactions
involving fluorene moiety hydrogens (C14–H14Á Á ÁC43 2.83 Å,
C24–H24aÁ Á ÁC34 2.86 Å, C36–H36Á Á ÁC3 2.86 Å, C44–H44aÁ Á ÁC6
2.89 Å) (Fig. 4). Further stabilization of the structure arises from
C11–H11Á Á ÁN interactions (2.71 Å) within the inversion-related
pairs. The layers of 6 are separated by chloroform molecules which
appear to interact strongly with the isoquinoline nitrogen through
a Cl3C–HÁ Á ÁN hydrogen bond (2.27 Å; C–H–N 158.8°), but isolate
the layers of 6 from one another. Nevertheless, the aforementioned
combination of close intermolecular contacts results in good inter-
action between the molecules of 6 within the b–c planes of the
crystal.
Fig. 4. Crystal structure packing diagrams of 6ÁCHCl3, (a) showing inversion-related
pairs with short C–HÁ Á ÁN contacts (red) involving a fluorene and chloroform, and
C–HÁ Á ÁC(
p) intermolecular close contacts between fluorenes (blue). Note the
p
-stacking overlap of the isoquinoline fragments, as viewed from above; (b)
packing diagram, as viewed along the c-axis, showing two layers of 6 separated by
chloroform molecules. Interlayer -stacking of the isoquinoline fragments is
p
highlighted with orange dotted lines. (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of this article.)
3.4. Synthesis and characterization of isoquinolininum salts
3.5. Cyclic Voltammetric Studies
Reaction of the neutral model compounds and polymers with
iodomethane generated the corresponding isoquinolininum salts.
Alkylation induced a change in solution color from colorless to yel-
low. The formation of cations is evidenced by significant shift in
signals in 1H NMR spectra, along with the presence of a new peak
at around 4.5 ppm due to N-CH3 protons. As examples, the 1H NMR
spectra of 9b and 12b are given in Figs. 5 and 6, respectively.
The degree of methylation can be determined by peak ratios of
N-methyl protons and aromatic protons, which indicates complete
conversion to the cationic form. The resulting structures of cations
are given in Chart 1. Compound 10 is soluble in chloroform, but the
other materials show good solubility only in polar solvents such as
dimethyl sulphoxide (DMSO) and N,N-dimethylformamide (DMF).
Cyclic voltammetry (CV) was performed to estimate the highest
occupied molecular orbital (HOMO) and lowest unoccupied molec-
ular orbital (LUMO) energy levels of the synthesized polymers. The
HOMO levels of the polymers were calculated from the onset oxi-
dation potential according to the equation HOMO = Àe(Eox + 4.4)
eV [52]. The LUMO levels of the polymers were determined by sub-
traction of the band gap energy (Eg) from the HOMO energy level
following LUMO = HOMO – Eg. The optical band gaps (Eg) were cal-
culated from the absorption edge of the copolymer films. The re-
sults of the electrochemical measurements are summarized in
Table 3.
Fig. 3. ORTEP representation of a crystal structure of 6ÁCHCl3 showing the numbering scheme. The chloroform solvate molecule has been omitted for clarity. Displacement
ellipsoids are depicted at 50% probability.

854
R. Scaria et al. / Reactive & Functional Polymers 71 (2011) 849–856
Fig. 5. 1H NMR spectrum of 9b in CDCl3 solution.
Fig. 6. 1H NMR spectrum of 12b in d6-DMSO.
On the basis of roughly evaluated oxidation potentials, the
UV–vis spectra of the cationic model compounds and polymers
in DMF solution are depicted in Fig. 7.
HOMO levels of 9a and 9b was estimated as À5.94 and
À5.95 eV respectively. The LUMO energy levels of 9a and 9b
are À2.89 and À2.96 eV respectively. The HOMO energy levels
of 12a and 12b are À5.98 and À5.99 eV, and their LUMO energy
levels are À3.52 and À3.55 eV respectively. Upon N-methylation,
the HOMO energy levels of polymers 9a and 9b are lowered by
0.04 eV, while the LUMO energy levels are lowered by 0.63 and
0.59 eV respectively. Thus, the formation of positive charge on
the ring nitrogen by methylation lowered the energy levels of
both HOMO and LUMO.
The absorption spectra of the polycations reveal a new wave-
length absorption band approximately around 400 nm. The new
absorption band probably originates from optical transition involv-
ing charge transfer from dialkyl fluorene moieties to positively
charged nitrogen centre of isoquinoline residue. The polycations
12a and 12b have kmax of 404 nm and 405 nm respectively. The
cations 10 and 11 generated by N-alkylation of the precursor mod-
el compounds show kmax values of 392 nm and 397 nm, respec-
tively. Thus, upon cation formation the absorption maxima are
significantly red shifted compared to their neutral compounds.
The formation of a positive charge on the nitrogen atom by
N-methylation lowered the energy of both HOMO and LUMO orbi-
tals. However, presence of the greater amount of electron density
on the nitrogen in the LUMO lowered its energy more than that
of the HOMO; therefore, the optical band gap decreases, resulting
3.6. Optical properties of isoquinolininum salts
In order to study the effect of N-alkylation on the optical prop-
erties, the conjugated cations were generated by N-alkylation from
the precursor materials by treatment with methyl iodide. The

R. Scaria et al. / Reactive & Functional Polymers 71 (2011) 849–856
855
_
+
N
_
+
N
+
_
I
I
N
I
CH3
CH3
CH3
10
11
*
*
*
n
*
n
_
+
+
I
N
_
N
I
CH3
CH3
12 a
Chart 1. Isoquinolinium salts of the model compounds and polymers.
12 b
the model compounds, the significantly larger red shift in the
absorption maximum in the case of 11 compared to 10 is due to
the presence two N-alkylated isoquinoline moieties in the former.
The structural changes induced by N-alkylation could also affect
the extent of conjugation between neighbouring fluorene units
and isoquinoline units causing a large variation in absorption
maxima.
The photoluminescence (PL) quantum yields of the polymers in
solution were investigated by using anthracene (UPL = 0.27) as a
standard [54]. The PL quantum yields of 9a and 9b in THF solution
are 0.84 and 0.62 respectively and that of 12a and 12b in ethanol
solution are 0.07 and 0.10 respectively. In general, the charge
transfer process is non emissive or weakly fluorescent and com-
petes with fluorescence [55]. Thus, N-alkylation results in lower
quantum yields compared to neutral polymers.
Table 3
Electrochemical properties of polymers.
Polymers
Eox (V)
HOMO (eV)b
LUMO (eV)c
Eogpt (eV)a
9a
9b
12a
12b
1.54
1.55
1.58
1.59
3.05
2.99
2.46
2.44
À5.94
À5.95
À5.98
À5.99
À2.89
À2.96
À3.52
À3.55
a
Optical band gap, calculated from the absorption edge of the copolymer films,
g = 1240/kedge
E
.
b
Determined from the onset oxidation potential. HOMO = Àe(Eox + 4.4) eV;
c
LUMO = (HOMO – Eg) eV.
4. Conclusions
In conclusion, a new series of isoquinoline based oligomers and
polymers have been synthesized and characterized by a number of
methods. The absorption maxima of the neutral materials in solu-
tion fall in the range of 327–358 nm. An X-ray diffraction study of
the di(fluorenyl)isoquinoline model compound shows significant
twisting of the fluorene cores relative to the isoquinoline linker
in the solid state. Conversion of the model compounds and poly-
mers to a cationic form by N-alkylation generated the correspond-
ing isoquinolininum salts with red shifted absorption maxima. The
absorption maxima of the conjugated polycations fall in the range
of 392–405 nm. Thus, the optical properties of isoquinoline based
model compounds and polymers can be tuned by N-alkylation,
an aspect that increases the attractiveness of N-methylated poly-
mers for use as electron transport layer in OLEDs.
Acknowledgements
Fig. 7. UV–vis spectra of isoquinolininum salts in DMF solution.
The authors acknowledge generous financial support from the
Department of Science and Technology, New Delhi, India and the
Max Planck Society, Germany. We thank Dr. S. K. Dhawan and
Dr. Ritu Srivastava, National Physical Laboratory, New Delhi for
extending their laboratory facilities.
in the longer wavelength absorption [53]. It should be noted that a
red shifted absorbance along with appearance of new absorption
band was observed when poly(para-pyridylvinylene)s were
N-alkylated with methyl triflate [39]. In the case of 9a and 9b,
the red shift upon conversion to cationic forms 12a and 12b is
46 nm and 51 nm, respectively, while in the case of model com-
pounds 6 and 7, conversion to cationic forms 10 and 11 red shifted
the absorption maxima by 52 nm and 70 nm, respectively. Among
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