Design, synthesis and optical response of pyridine-cored V-shaped stilbenoid dendrimers

Article abstract of DOI:10.2174/157017810791112522

Design, synthesis and characterization of new series of first-generation V-shaped dendrimers bearing phenylenevinylene dendritic branches in periphery and pyridine as a core is described. A preliminary study of the optical properties of the resulting compounds was conducted by UV/vis and fluorescence spectroscopy.

Full text of DOI:10.2174/157017810791112522

Letters in Organic Chemistry, 2010, 7, 203-207
203
Design, Synthesis and Optical Response of Pyridine-Cored V-Shaped
Stilbenoid Dendrimers
Debabrata Jana and Binay K. Ghorai*
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, India
Received October 28, 2009: Revised January 19, 2010: Accepted January 28, 2010
Abstract: Design, synthesis and characterization of new series of first-generation V-shaped dendrimers bearing
phenylenevinylene dendritic branches in periphery and pyridine as a core is described. A preliminary study of the optical
properties of the resulting compounds was conducted by UV/vis and fluorescence spectroscopy.
Keywords: Stilbenoid dendrimer, two-fold Heck reaction, fluorosolvatochromism.
INTRODUCTION
While the photophysical properties of several dendrimers
with electron deficient cores have been investigated and well
characterized [12], stilbenoid dendrimers with pyridine as a
core is not well known. Our interest is in the synthesis of
novel conjugated V-shaped 1st generation dendrimer with
pyridine core for optoelectronic applications. In this paper,
we wish to present a convergent synthesis of stilbenoid
dendrimers via the strategic use of multifold Heck reactions
and the influence of the length and orientation of linkage in
phenylenevinylene skeleton on the optical absorption and
emission properties with the fluorosolvatochromism and pH
sensitivity.
The interest in dendrimers was mainly focused on
discovering synthetic routes toward novel members of
unique family of macromolecule. Today, however, scientists
working in this field have increasingly shifted their attention
to the modification of existing dendrimers in order to explore
the material properties of these regular highly branched
molecules. Such modifications of the core molecule of
dendrimers can tune the photophysical properties. Recently,
conjugated oligomers are subjected to important
investigations from both academic and industrial laboratories
[1] due to their promising applications, such as organic light
emitting diodes (OLEDs) [2], solar cell [3], field-effect
transistors (FETs) [4], and models [5] to understand the
fundamental properties of their analogous polydiverse
polymers. Among these oligomers, molecules with a D-ꢀ-A
or D-ꢀ-A-ꢀ-D structure (where D is an electron-donating
group, A an electron accepting group, and ꢀ a conjugating
moiety) are of high interest due to their fluorescence
properties with internal charge transfer (ICT) and as
chromophores for second and third-order nonlinear optics
(NLO) [6].
Wittig or Horner-Emmons reactions have been mostly
used for the synthesis of stilbenoid dendrimers [13]. For such
strategies, special bifunctional synthons are usually required,
both for dendron preparation as well as for the core-coupling
reactions. Syntheses of stilbenoid dendrimers via
a
sequential Heck reaction (for dendron propagation) and
Horner–Emmons condensation (for core-coupling) have also
been reported. However, stilbene synthesis via Wittig or
Horner–Emmons reactions usually gives rise to a cis-trans
mixture of products, necessitating an extra isomerization step
to produce the desired trans-stilbene linkages. It must be
mentioned that lack of stereochemical homogeneity in the
dendrimer structure can severely affect its optoelectronic
property. We decided to use the Heck arylation reaction to
construct all the stilbenoid linkages (both at the core and in
the periphery).
The pyridine ring is an excellent candidate to be
incorporated in such structures. Indeed, this heterocyclic
moiety has a moderate electron-withdrawing character,
significant aromaticity that can lead to highly conjugated
molecules, as well as basic and potential ligand properties
that can also be used to modulate the optoelectronic
properties of molecules. On the other hand, dendritic
poly(phenylenevinylenes) [7], also called stilbenoid
dendrimers, represent an important group within this class of
material. Several studies have been published to date
concerning the synthesis and properties of these materials
[8]. For example, such compounds have been used
successfully as charge transporting [9], light-emitting [10],
and electron transfer materials [11]. It has also been
demonstrated that phenylenevinylene dendrite arms can
function as light-harvesting antennae [10b].
RESULTS AND DISCUSSION
Requisite 1-vinyl(4-tert-butylstyrene)benzene (1) [14]
required for our study was synthesized from the readily
available methyl 4-bromobenzoate (2) in good yield as
outlined in Scheme 1. At first Heck reaction on 2 with 4-t-
butylstyrene under Jeffery’s conditions [15] provided 3
(76%), and subsequent treatment of 3 with lithium
aluminium hydride in THF gave the alcohol 4 (79%) and the
later was oxidized (PDC, CH2Cl2, rt) to produce the styryl-
benzaldehyde 5 (86%). Wittig reaction of 5 (Ph3P+MeI-, n-
BuLi, THF) at -20 °C afforded compound 1 in 72% yield.
*Address correspondence to this author at the Department of Chemistry,
Bengal Engineering and Science University, Shibpur, Howrah 711 103,
India; Tel: +913326684561; Fax: +913326682916;
E-mail: bkghorai@yahoo.co.in
1570-1786/10 $55.00+.00
© 2010 Bentham Science Publishers Ltd.

204 Letters in Organic Chemistry, 2010, Vol. 7, No. 3
Jana and Ghorai
X
CO2Me
CO2Me
b
d
a
Br
2
4, X = CH2OH ( 79%)
5, X = CHO (86% )
3 (76%)
c
1 (72%)
Scheme 1. Reagents and conditions: (a) 4-t-Butylstyrene, Pd(OAc)2, n-Bu4NBr, KOAc, DMF, 90 °C, 24 h; (b) LiAlH4, THF, rt; (c) PDC,
CH2Cl2, rt; (d) Ph3P+MeI-, n-BuLi, THF, -20 °C.
Br
Br
NH2
CO2Me
a
d
b
CO2Me
X
7
8 (65%)
6 (78%)
9, X = CO2Me (75%)
10, X = CHO (60% )
c
t
Scheme 2. Reagents and conditions: (a) (i) Br2, HOAc; (ii) BuONO, DMF, 60 °C; (b) 4-t-Butylstyrene (2.5 equiv.), Pd(OAc)2, n-Bu4NBr,
KOAc, DMF, 90 °C, 36 h; (c) LiAlH4, THF, rt; (d) (i) PDC, CH2Cl2, rt; (ii) Ph3P+MeI-, n-BuLi, THF, -20 °C.
Dendron 6 was synthesized from methyl anthranilate (7)
according to the literature method depicted in Scheme 2 [8c].
Dibromination of methyl anthranilate (7) followed by
deamination (tBuONO, DMF) gave methyl 3,5-
dibromobenzoate (8) in 65% yield [16]. Two-fold Heck
atom of pyridine ring enhances the solubility of the mono-
and bis-coupled products, which lead to the complete
conversions with
purification process.
a considerable easy isolation and
Thus, two-fold Heck reaction of 2,6-dibromopyridine
(11) with 4-tert-butylvinyl styrene derivative under Jeffery’s
phase transfer conditions [10% Pd(OAc)2, n-Bu4NBr, KOAc,
DMF, 90 °C] give rise to the first generation dendrimer 12
[17] in 45% yield (Scheme 3). Under the same conditions,
two-fold Heck reaction of 11 with excess of styrene
derivative 6 led to the fluorescent meta-branched V-shaped
pyridine-cored dendrimer 13 [18] was formed in 30% yield.
An additional example using the same two-fold Heck
reaction strategy with dendron 1 led to the formation of a
highly fluorescent compound 14 [19] in low yield (15%).
reaction on
8
with 4-t-butylstyrene under Jeffery’s
conditions gave 9, which on treatment with lithium
aluminium hydride in THF afforded the alcohol 10. PDC
oxidation of 10 followed by Wittig reaction yielded the
desired dendron 6.
Generally, multifold Heck coupling reaction for
stilbenoid dendrimer preparation afforded low yields due to
the insolubility of partially coupled intermediates, which
were observed to precipitate from solution during early
reaction times [14]. We have chosen 2,6-dibromopyridine as
a core molecule to prepare stilbenoid dendrimer because it
has a ꢀ-acceptor property, which control the absorption and
emission spectra in different solvents and also the nitrogen
We investigated the absorption and emission behaviors of
12, 13 and 14 in different solvents. The results of these
investigations are summarized in Table 1. The absorption

Design, Synthesis and Optical Response of Pyridine-Cored
Letters in Organic Chemistry, 2010, Vol. 7, No. 3
205
N
N
12
13
b
a
Br
N
Br
c
N
11
14
Scheme 3. Synthesis of stilbenoid dendritic molecules. Reagents and conditions: (a) 4-t-Butylstyrene, Pd(OAc)2, n-Bu4NBr, KOAc, DMF, 90
°C, 24 h, 45%; (b) 6 (4.0 equiv), Pd(OAc)2, n-Bu4NBr, KOAc, DMF, 90 °C, 48 h, 30%; (c) 1 (4.0 equiv), Pd(OAc)2, n-Bu4NBr, KOAc, DMF,
90 °C, 48 h, 15%.
Table 1. Spectroscopic Data of 12-14
UV/Vis
ꢀmax [nm]
Excitation
ꢀmax [nm]
Fluorescence
ꢀmax [nm]
Compd
Solvent
Toluene
Chloroform
Tetrahydrofuran
Methanol
313
316,339
318,340
-
338
341
342
342
338
344
342
342
372
375
375
375
399
415
409
465
392
400
397
409
421
445
437
492
12
Toluene
313,335
318
Chloroform
Tetrahydrofuran
Methanol
13
14
317
317
Toluene
348
Chloroform
Tetrahydrofuran
Methanol
351
350
350
All spectra were recorded at room temperature at C = (1.0 x 10-6 M to 1.2 x 10-6 M).
spectra are nearly independent of solvent polarity, except for
a slight, insignificant red shift indicates a negligible
intramolecular interaction between solvent molecule and
pyridine system in the ground state. In contrast, the emission

206 Letters in Organic Chemistry, 2010, Vol. 7, No. 3
Jana and Ghorai
spectra exhibit distinct solvent dependence behavior. Broad
structure less emission and large maximum fluorescence
wave length shifts were observed on increasing the solvent
polarity along with a successive decrease in the fluorescence
intensity. This solvatochromic behavior, which results from
the stabilization of the highly polar emitting state by polar
solvents, is typical for compounds that undergo an internal
charge transfer upon excitation and has been fully
documented for numerous fluorophores or cores containing
donor-acceptor units [6a]. As an example, the
photoluminescence (PL) spectra of compound 12 (Fig. 1)
shows broad and strong red shift in more polar solvent (e.g.
methanol) with a ꢀmax of 465 nm, compare to the ꢀmax of 399
nm in less polar solvent (e.g. toluene).
Fig. (2). UV/vis absorption changes of 12 (1.05 x 10-5 M) in CHCl3
with TFA (10-2 M) addition.
N
H
CF3COO
15
In conclusion, we have successfully synthesized and
characterized a series of new 2,6-bis(arylvinyl)pyridines in a
straightforward manner by two-fold Heck coupling reaction
between 2,6-dibromopyridine and appropriate vinylphenyl-
ene moiety. This well known protocol permits highly
stereoselective trans coupling. The material exhibits
moderate emission solvatochromism and red shifted broad
structureless bands are obtained in polar solvents, an
observation characteristic of intramolecular charge transfer
at excited state. The pH sensing properties of absorption
spectra were also studied particularly. Further work in this
area is currently underway in this laboratory.
Fig. (1). Emission spectra of compound 12 (1.2 x 10-6 M) in various
solvents.
It is well known that meta-substituted systems are weaker
light emitters than the corresponding para isomers; exactly
same phenomenon is observed for our V-shaped pyridine-
cored dendrimer. The emission spectra of compound 13
shows lower ꢀmax than that of compound 14 which implies
that extended conjugation in para position is more efficient
for red shift than in meta position. The PL (ꢀmax) of
compound 13 in chloroform appears in 400 nm where as for
compound 14 in same solvent appears in 445 nm.
ACKNOWLEDGEMENTS
The PL spectrum of compound 12 in toluene (excitation
at 338 nm) showed an emission band at 399 nm whereas for
compound 14 in same solvent (excited at 372 nm) showed at
421 nm, this change probably attributed to the length of the
peripheral vinylphenylene arms.
Financial support from UGC [No.37-93/2009(SR)] and
CSIR (research fellowship to D.J.), New Delhi is gratefully
acknowledged. We thank IICB, Kolkata for providing us
MALDI-MS facility.
The nitrogen atoms of all prepared pyridines are basic
centers that can be protonated. Thus, the effect of
protonation on the UV-vis absorption spectra of CHCl3
solutions of compound 12 is illustrated in Fig. (2). The
spectra show a new similar intensity red-shifted band at 402
nm corresponding to the protonated species 15. It may be
noted that the absorption band for the neutral compound on
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1
[9]
solids. Data for 12: Mp 172 °C; H NMR (500 MHz, CDCl3): ꢀ
7.67 (d, 2H, J = 16.0 Hz), 7.64 (t, 1H, J = 7.5 Hz), 7.56 (d, 4H, J =
8.3 Hz), 7.41 (d, 4H, J = 8.3 Hz), 7.27 (d, 2H, J = 7.5 Hz), 7.19 (d,
2H, J = 16.0 Hz), 1.35 (s, 18H); 13C NMR (125 MHz, CDCl3): ꢀ
155.6, 151.4, 136.7, 134.0, 132.6, 127.6, 126.9, 125.6, 120.0, 34.7,
31.2; MALDI-TOF MS: m/e : 396.0 (MH+).
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Data for 13: 1H NMR (500 MHz, CDCl3): ꢀ 7.88 (t, 1H, J = 7.5
Hz), 7.85 (d, 2H, J = 16.0 Hz), 7.68 (s, 4H), 7.59 (br s, 2H), 7.52
(d, 8H, J = 8.3 Hz), 7.48 (d, 2H, J = 7.5 Hz), 7.43 (d, 8H, J = 8.3
Hz), 7.37 (d, 2H, J = 16.0 Hz), 7.23 (d, 4H, J = 16.0 Hz), 7.15 (d,
4H, J = 16.0 Hz), 1.35 (s, 36H); 13C NMR (75 MHz, CDCl3): ꢀ
155.4, 150.9, 138.3, 137.4, 134.4, 132.5, 129.1, 128.8, 127.5,
126.3, 125.6, 124.6, 124.2, 122.1, 120.4, 34.6, 31.3; MALDI-TOF
MS: m/e : 916.7 (MH+).
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Data for 14: 1H NMR (300 MHz, CDCl3): ꢀ 7.86 (d, 4H, J = 8.1
Hz), 7.65 (d, 4H, J = 8.1 Hz), 7.49 (m, 5H), 7.41 (m, 6H), 7.26 (d,
4H, J = 16.2 Hz), 7.10 (d, 4H, J = 16.2 Hz), 1.34 (s, 18H);
MALDI-TOF MS: m/e : 600.0 (MH+).