Synthesis and photophysical properties of poly-(phenylenevinylene) dendrimers with a ruthenium tris-bipyridine core

Article abstract of DOI:10.1016/j.cplett.2008.06.036

The photophysical processes have been investigated in first, second and third generation dendrimers with poly-(phenylenevinylene) branches and a ruthenium tris-bipyridine core, RuDn (n = 1-3). These dendrimers show very efficient forward singlet-singlet energy transfer from the branches to the ruthenium core upon UV irradiation, with efficiencies of 0.99 for RuD1 and 0.88 for RuD2 and RuD3 in CH₂Cl₂. The RuDn dendrimers show a bi-exponential emission decay in CH₂Cl₂, when excited with a 460 nm light with short lifetimes, however, the emission decay lifetimes become mono-exponential in 10% Triton X-100 aqueous solution (τ = 840 ns for RuD1, 890 ns for RuD2 and 1120 ns for RuD3).

Full text of DOI:10.1016/j.cplett.2008.06.036

Chemical Physics Letters 460 (2008) 543–547
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Synthesis and photophysical properties of poly-(phenylenevinylene)
dendrimers with a ruthenium tris-bipyridine core
*
Tania I. Houjeiry, Tarek H. Ghaddar
Department of Chemistry, American University of Beirut, P.O. Box 11-0236, Beirut, Lebanon
a r t i c l e i n f o
a b s t r a c t
Article history:
The photophysical processes have been investigated in first, second and third generation dendrimers
with poly-(phenylenevinylene) branches and a ruthenium tris-bipyridine core, RuDn (n = 1–3). These
dendrimers show very efficient forward singlet–singlet energy transfer from the branches to the ruthe-
nium core upon UV irradiation, with efficiencies of 0.99 for RuD1 and 0.88 for RuD2 and RuD3 in CH2Cl2.
The RuDn dendrimers show a bi-exponential emission decay in CH2Cl2, when excited with a 460 nm light
with short lifetimes, however, the emission decay lifetimes become mono-exponential in 10% Triton X-
Received 29 February 2008
In final form 11 June 2008
Available online 15 June 2008
100 aqueous solution (
s
= 840 ns for RuD1, 890 ns for RuD2 and 1120 ns for RuD3).
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
luminescence properties due to the photophysical properties of the
active components (core and branching units).
The key reactions in natural photosynthetic systems involve en-
ergy and electron transfer reactions. These reactions are usually
driven by an absorbed photon by which sunlight is converted into
chemical energy by successive electronic energy transfer (EET)
reactions and charge transfer (CT) events to a chemical redox site
[1–3]. The first critical electron transfer reaction is initiated by
light harvested in a complex array of antenna pigments. The exci-
tation energy then migrates by energy transfer reactions to a spe-
cific electron donor with about unit efficiency [3]. Dendrimers are
regarded as excellent candidates for light harvesting antenna sys-
tems due to their three-dimensional hyper-branched structure
[4,5]. This unique architecture enables the synthesis of dendrimers
with high number of light absorbing or photoactive sites within an
energy or redox gradient framework [6–8]. As such, dendrimers
can transfer the harvested-light energy, through the energy gradi-
ent framework, by multi-step electron [9–12] or energy transfer
reactions to a redox active site. Dendrimers containing high con-
centrations of redox and photoactive chromophores have been
extensively studied for their energy transfer properties [13–25].
Poly-(phenylenevinylene) dendrimers derivatives have been
widely investigated for almost a decade due to their excellent ther-
mal and optoelectronic properties [26,27]. Here we report the syn-
thesis and photophysical properties of a novel dendritic system
that is capable of a very efficient forward singlet–singlet energy
transfer from poly-(phenylenevinylene) branching units to a ruthe-
nium tris-bipyridine core upon UV light irradiation. As such, these
composite dendrimers may reveal new advantages in their electro-
2. Experimental
2.1. Materials and instrumentation
All chemicals were purchased from Aldrich and used as sup-
plied. The dendritic poly-(phenylenevinylene) aldehydes [28]
(Gn-CHO, n = 1–3) and 4,40-bis(diethylmethylphosphonate)-2,20-
bipyridine [29] were synthesized using reported procedures in
the literature. Methylene chloride (CH2Cl2) and tetrahydrofuran
(THF) were distilled from calcium hydride (CaH2) and sodium/ben-
zophenone prior to use, respectively. The NMR spectra were re-
corded on
a Bruker AM 300 MHz spectrometer. Absorption
spectra were recorded on a Jasco V-570 UV/VIS/NIR spectropho-
tometer. Emission spectra were measured on a JobinYvon Horiba
Fluorolog-3 spectrofluorometer. Time-correlated single photon
counting (TCSPC) emission decay measurements were done using
an IBH NanoLED-460 diode laser for the excitation source and an
IBH TBX-04 and R928 detectors, and the lifetimes were obtained
by curve-fitting with IBH DAS 6.0 program. For steady-state and
lifetime emission measurements the respective dendrimer solution
was prepared in CH2Cl2 with an optical density of 0.1 at the excita-
tion wavelength, followed by 3 cycles of freeze–thaw in order to
get rid of the dissolved oxygen.
2.2. Preparation of G1-bpy, G2-bpy and G3-bpy
Potassium t-butoxide (3.0 equiv) was added to a solution
of the corresponding Gn-CHO (2 equiv) and 4,40-bis(diet-
hylmethylphosphonate)-2,20-bipyridine (1 equiv) in dry THF.
The mixture was stirred under nitrogen for 12 h, after which the
* Corresponding author. Fax: +961 1 365217.
E-mail address: tg02@aub.edu.lb (T.H. Ghaddar).
0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.06.036

544
T.I. Houjeiry, T.H. Ghaddar / Chemical Physics Letters 460 (2008) 543–547
reaction was quenched with water (4ꢀ) and the milky solution was
extracted with CH2Cl2 (3ꢀ). The organic layer was separated, dried
over anhydrous Na2SO4 then the solvent was removed under vac-
uum. The off-white powder was collected and dried in a vacuum
oven overnight.
2.3.3. RuD3 (58% yield)
1H NMR (300 MHz, CDCl3) d 1.27 (s, 288H), 7.05–7.77 (m,
160H), 8.24 (d, J = 8.2 Hz, 4H), 8.54 (s, 2H). 13C NMR unresolved.
MALDI-TOF (DHB) m/z calcd
5116.9, found 5116.7.
C
378H460F6N2PRu (M-2bpyꢁPF6)+
3. Results and discussion
2.2.1. G1-bpy (88% yield)
1H NMR (300 MHz, CDCl3) d 1.32 (s, 72H; CH3), 7.02–7.07 (d,
J = 16.2 Hz, 4H), 7.20–7.40 (m, 16 H), 7.24 (d, J = 5.0 Hz, 2H) 7.45–
7.50 (d, J = 16.2 Hz, 4H), 7.59 (m, 6H), 8.56 (s, 2H), 8.65 (d,
J = 5.0 Hz, 2H). 13C NMR (75 MHz, CDCl3) d 31.49, 34.77, 118.62,
120.99, 121.18, 122.37, 124.08, 125.06, 126.55, 127.37, 130.61,
133.51, 136.30, 136.90, 138.50, 146.03, 149.44, 151.13, 156.08.
MALDI-TOF (CHCA) m/z calcd C90H108N2 (M+H)+ 1217.9, found
1218.9.
The dendritic poly-(phenylenevinylene) aldehydes (Gn-CHO,
n = 1–3) were synthesized using reported procedure in the
literature [28]. The bipyridine-based poly-(phenylenevinylene)
dendrons (Gn-bpy, n = 1–3) were synthesized starting from the
above mentioned dendritic aldehydes and 4,40-bis(diet-
hylmethylphosphonate)-2,20-bipyridine [29] utilizing the Horner–
Wadsworth–Emmons reaction, (Scheme 1). The reaction of the
different Gn-bpy dendrons with (bpy)2RuCl2 (where bpy is 2,20-
bipyridine) in a 1:1 ethanol/chloroform mixture afforded the three
different ruthenium-based poly-(phenylenevinylene) dendrimers
(RuDn, n = 1–3), (Scheme 1). All of the dendrimers were character-
ized using 1H NMR, 13C NMR and MALDI-TOF mass spectrometry.
The 1H NMR spectra of the dendrimers become very complicated
when the generations become higher, however, all of the dendri-
mers possess the correct ratio of aliphatic to aromatic protons.
2.2.2. G2-bpy (87% yield)
1H NMR (300 MHz in CDCl3) d 1.25 (s, 144 H), 7.09–7.14 (d,
J = 16.2 Hz, 8H), 7.21–7.67 (m, 64 H), 8.61 (s, 2H), 8.68 (d,
J = 5.0 Hz, 2H). 13C NMR (75 MHz, CDCl3) d 31.52, 34.93, 118.45,
120.98, 121.23, 122.31, 123.78, 124.29, 124.52, 125.04, 126.97,
127.67, 128.48, 129.54, 130.36, 133.09, 136.43, 137.19, 137.74,
138.33, 138.39, 145.66, 149.70, 151.12, 156.59. MALDI-TOF (CHCA)
m/z calcd C186H220N2 (M+H)+ 2483.7, found 2484.7.
2.2.3. G3- bpy (93% yield)
1H NMR (300 MHz in CDCl3) d 1.22 (s, 288 H), 6.97–7.03 (d,
J = 16.2 Hz, 16 H) 7.09–7.57 (138 H), 8.53 (s, 2H), 8.55 (d,
J = 5.0 Hz, 2H, bpy H). 13C NMR (75 MHz, CDCl3) d 31.48, 34.88,
118.45, 120.93, 121.220, 122.28, 123.78, 124.13, 124.38, 124.62,
127.12, 127.65, 128.74, 128.98, 129.37, 129.7, 130.33, 133.02,
136.38, 137.26, 137.83,138.07, 138.23, 138.30, 138.37, 138.42,
145.63, 149.70, 151.01, 151.09, 156.57. MALDI-TOF (CHCA) m/z
calcd C378H444N2 (M+H)+ 5015.6, found 5016.5.
O
O
P OEt
OEt
EtO P
EtO
+
N
N
n
H
O
G
n
n
-CHO
= 1 to 3
i
2.3. Preparation of RuD1, RuD2 and RuD3
The same procedure was followed in preparing the three differ-
ent RuDn dendrimers. The corresponding Gn-bpy (1 equiv) was
added to a solution of (bpy)2RuCl2 (1 equiv) in 1:1 ethanol/CHCl3
and was refluxed in the dark for two days under an Ar atmosphere.
The solvent was then taken off under vacuum and the product was
re-dissolved in acetone and precipitated down with aqueous
NH4PF6. The solid was filtered and then purified on a silica gel col-
umn by gradient elution 0–10% acetone in CH2Cl2. The main band
was collected and the solvent was taken off under vacuum.
n
n
G
n
-bpy
N
N
n
= 1 to 3
ii
2.3.1. RuD1 (68% yield)
1H NMR (300 MHz, CDCl3) d 1.25 (s, 72H), 7.05–7.10 (d,
J = 16.2 Hz, 2H), 7.30–7.83 (m, 44H), 8.21 (d, J = 8.2 Hz, 4H), 8.45
(s, 2H). 13C NMR (75 MHz, CDCl3) d 31.45, 34.85, 121.04, 121.21,
122.38, 123.98, 124.28, 124.71, 125.55, 127.00, 128.03, 128.21,
130.96, 136.19, 137.17, 137.88, 138.66, 147.08, 151.11, 151.28,
156.36, 156.59, 156.77. MALDI-TOF (CHCA) m/z calcd C110H124F6-
N6PRu (MꢁPF6)+ 1776.2, found 1776.0.
n
n
RuD
n
2.3.2. RuD2 (60% yield)
N
N
+2
1H NMR (300 MHz, CDCl3) d 1.27 (s, 144H), 7.05–7.77 (m, 88H),
8.21 (d, J = 8.2 Hz, 4H), 8.54 (s, 2H). 13C NMR (75 MHz, CDCl3) d
156.70, 156.56, 156.34, 151.14, 151.06, 147.13, 138.46, 138.30,
137.85, 137.59, 136.39, 136.23, 130.32, 130.23, 129.82, 128.10,
128.01, 127.56, 127.31, 125.24, 124.90, 124.30, 124.23, 123.82,
122.20, 120.92, 34.88, 31.47. MALDI-TOF (CHCA) m/z calcd
n
= 1 to 3
N
Ru
N
N
N
-
.2PF6
Scheme 1. Reagents and conditions: (i) t-BuOK, THF, RT, 12 h. (ii) (bpy)2RuCl2, 1:1
EtOH:CHCl3, reflux for 48 h, NH4PF6.
C
110H124F6N6PRu (MꢁPF6)+ 3042.1, found 3042.1.

T.I. Houjeiry, T.H. Ghaddar / Chemical Physics Letters 460 (2008) 543–547
545
All of the olefinic protons that can be distinguished in the aromatic
region of the NMR spectra appeared as doublets with a coupling
constant of 16.2 Hz, suggesting an all E configuration as expected.
Fig. 1 shows the absorption spectra of the three different den-
drons Gn-bpy (n = 1–3) in CH2Cl2. The Gn-bpy dendrons show
absorption maxima between 318 and 322 nm (1pp* electronic
transitions), (Table 1), with no detectable absorption in the visible
region, which implies that the absorbing units are stilbene-like
moieties (kmax = 308 nm and 320 nm for stilbene). Owing to the
meta linking (cross conjugation), it is not surprising that the
absorption maximum of the Gn-bpy dendron does not change with
generation number. However, the high increase in the extinction
coefficient as we go higher in generation number is due to the
exponential increase in the light absorbing phenylenevinylene
units.
A strong emission is observed in CH2Cl2 at room temperature
for the first, second and third generation dendrons (kem = 420 nm
for G1-bpy, 400 and 414 nm for G2-bpy and 394 and 412 nm for
G3-bpy), (Fig. 1 and Table 1). The high fluorescence intensity for
these dendrons decreases and the emission maximum blue shifts
as we go from generation 1 to 3. Similar results have been previ-
ously reported with rigid dendrons and were accounted for by
the enhancement of the non-radiative decay pathways in larger
dendrimers by which the excitation energy gets dissipated [6].
Moreover, the G2-bpy and G3-bpy emission spectra show addi-
tional vibronic side bands that are not resolved in the G1-bpy case.
This can be attributed to the fact that as the dendrimer generation
increases, the dendrimer’s ‘arms’ may start playing a significant
role in defining the solvent shell around the dendrimer which in
turn would result in such differences in the emission spectra, i.e.,
blue shifting of the emission maxima and clearer appearance of
the vibronic side bands.
Fig. 2 shows the absorption spectra of the three different RuDn
(n = 1–3) dendrimers in CH2Cl2. The two distinctive peaks are cen-
tered at 320 and 470 nm. The former is due to the intraligand elec-
tronic transitions 1IL of the stilbenoid dendrons, and the latter is
the normal MLCT transition of the ruthenium tris-bipyridine core.
The bipyridine (bpy) intraligand pp* transition is centered at
292 nm as can be seen from the RuD1 and RuD2 absorption spec-
tra. In spite the fact that in the RuD3 case the bpy pp* transition is
not resolved, the molar absorptivity at 320 nm for RuD3 compared
with G3-bpy is higher by 60000 Mꢁ1cmꢁ1. This indicates that the
band between 290 and 320 nm in the RuD3 case arises from a
bpy-localized pp* and a stilbenoid centered (1IL) transitions. Thus,
this explains the presence of two maxima in the absorption spectra
between 290 and 320 nm for RuD1 and RuD2 and only one maxi-
mum for RuD3.
The emission spectra of the three RuDn dendrimers upon excit-
ing the ruthenium tris-bipyridine MLCT band (kex = 470 nm) in
methylene chloride is shown in Fig. 2. The emission maxima of
the RuDn dendrimers are centered at kem = 652 nm, (Table 1). Upon
exciting the RuDn dendrimers at 320 nm, where the poly-(phenyl-
enevinylene) units absorb most of the light, an enhancement of the
ruthenium tris-bipyridine core emission in the three RuDn dendri-
mers is observed. In addition, the emission of the poly-(phenylene-
vinylene) arms is quenched by more than 99% for the three RuDn
dendrimers. These results are attributed to a very efficient sin-
glet–singlet energy transfer reaction from the poly-(phenylenevin-
ylene) arms to the ruthenium core. To further investigate this
forward energy transfer reaction, we measured the photolumines-
cence excitation spectra of the three RuDn dendrimers in methy-
lene chloride. The energy transfer quantum yield can be
estimated quantitatively by comparing the absorption spectrum
and excitation spectrum of the RuDn dendrimers by monitoring
the ruthenium core emission. Fig. 3 shows the corrected photolu-
minescence excitation spectra of the three RuDn dendrimers and
their absorption (1–transmittance) spectra. The excitation spectra
are normalized to the respective absorption band between 470
and 550 nm, where light is absorbed mainly by the ruthenium core.
The fact that the absorption and the respective excitation spectra
of the RuD1 dendrimer are coincident across the entire excitation
spectrum, this indicates that the forward singlet–singlet energy
transfer from the poly-(phenylenevinylene) arms to the ruthenium
core is close to unity for RuD1 (>99%). For RuD2 and RuD3, the sin-
glet–singlet energy transfer efficiency is calculated to be ꢂ88%.
Measuring the rate of energy transfer in the three RuDn
dendrimers upon 320 nm excitation was beyond our TCSPC time-
resolution (>100 ps), indicating that kET P 1010sꢁ1. The emission
lifetimes for the Gn-bpy and RuDn upon 320 nm and 460 nm
excitation, respectively, are summarized in Table 1. The emis-
sion decay lifetimes of the RuDn dendrimers deviate from the
5
4
3
2
1
0
300
350
400
450
500
Wavelength (nm)
Fig. 1. Steady state fluorescence (right, kex = 320 nm) and UV–vis (left) spectra of:
(dashed) G1-bpy, (dotted) G2-bpy and (solid) G3-bpy in CH2Cl2 at 298 K.
Table 1
Photophysical Data of the Gn-bpy and the RuDn Dendrimers
Ufa,b
Lifetime (
s
/ns) (I)a,b,c
Lifetime (s/l
s) (I)c,d
a
Entry
Absorption (k/nm) (
e
ꢀ 105/Mꢁ1 cmꢁ1
)
Emission (k/nm)a,b
G1-bpy
G2-bpy
G3-bpy
RuD1
RuD2
RuD3
322 (1.15)
320 (2.53)
318 (5.60)
294 (1.12), 320 (1.03), 470 (0.19)
294 (2.12), 320 (2.68), 470 (0.19)
320 (6.20), 470 (0.19)
420
400, 414
394, 412
652
0.61
0.34
0.22
0.005
0.003
0.002
9.3
8.0
7.2
–
–
–
93 (0.83), 344 (0.17)
75 (0.78), 298 (0.22)
108 (0.49), 297 (0.51)
0.84 (1.0)
0.89 (1.0)
1.12 (1.0)
652
652
a
Measured in CH2Cl2 at 298 K.
b
c
kex = 320 nm and 460 nm for Gn-bpy and RuDn, respectively.
kem = 420 nm and 650 nm for Gn-bpy and RuDn, respectively.
Measured in 10% aqueous Triton X-100 at 298 K.
d

546
T.I. Houjeiry, T.H. Ghaddar / Chemical Physics Letters 460 (2008) 543–547
104
6
4
2
0
103
b
102
a
0
1
2
3
4
5
6
Time (μs)
300
400
500
600
700
Wavelength (nm)
Fig. 4. Lifetime emission spectra of RuD2 in (a) CH2Cl2 and (b) 10% aqueous Triton
X-100, kex = 460 nm and kem = 650 nm at 298 K.
Fig. 2. Steady state fluorescence (right, kex = 470 nm) and UV–vis (left) spectra of:
(dashed) RuD1, (dotted) RuD2 and (solid) RuD3 in CH2Cl2 at 298 K.
examined the RuDn lifetime emission measurements in 10% aque-
ous Triton X-100, a non-ionic micellar solution, in order to get rid
of any possible aggregation. Interestingly, in the three RuDn cases
the emission decays became mono-exponential with emission life-
a
0.6
times
s = 0.84, 0.89 and 1.12 ls for RuD1, RuD2 and RuD3, respec-
tively, Table 1 and Fig. 4. These findings suggested that in low
polarity solvent, such as CH2Cl2, self-quenching of the ruthenium
emission would take place since the highly hydrophobic branching
units on different dendrimer molecule may aggregate in a way
where they are exposed more to the solvent shell than the posi-
tively charged ruthenium core. Such self-quenching would be pres-
ent to a lesser extent in the above used micellar solution, since the
ruthenium core would be in this case exposed more to the solvent
shell than the branching units. The probability of finding a dendri-
mer molecule present in a single micelle unit can be obtained using
Poisson’s equation
0.4
0.2
0.0
350
400
450
500
550
600
Wavelength (nm)
b
0.6
0.4
0.2
0.0
PðnÞ ¼ ðxneꢁxÞ=n!
where P(n) is the probability of finding a micelle unit containing ‘n’
dendrimer molecules, and ‘x’ is the ratio of the dendrimer mole-
cules to that of the micelles. For the given Triton X-100 micellar
concentration (154 mM, 10% by weight) and a dendrimer concen-
tration of 5.3 ꢀ 10ꢁ6 M, the probability of finding one dendrimer
molecule present in one micelle is 3.4 ꢀ 10ꢁ5, and two dendrimer
molecules in one micelle is 5.9 ꢀ 10ꢁ10. The latter number indi-
cates that the chances of finding two or more dendrimer molecules
in the same micelle are negligible. Therefore, bimolecular non-
radiative energy transfer and/or collisional quenching would be
negligible in such micellar medium. It is worth mentioning here
that a Triton X-100 micelle’s effective hydrodynamic diameter is
P10 nm [31], which is large enough to accommodate the different
dendrimers in study.
350
400
450
500
550
600
Wavelength (nm)
c
0.6
0.4
0.2
0.0
4. Conclusions
350
400
450
500
550
600
In conclusion, we have synthesized novel dendrimers bearing
phenylenevinylene-branching units at the periphery and a ruthe-
nium tris-bipyridine moiety at the core. These dendrimers show
very efficient and fast energy transfer rates from the periphery to
the core upon UV light irradiation. The RuDn show a bi-exponential
emission decay in CH2Cl2, when excited with a 460 nm light, with
shorter lifetimes than typical ruthenium polypyridine complexes,
however, the emission decay lifetimes become mono-exponential
in 10% Triton X-100 aqueous solution. This is attributed to aggrega-
tion of the three RuDn dendrimers in CH2Cl2 due to the highly
Wavelength (nm)
Fig. 3. Photoluminescence excitation (dashed, kem = 650 nm) and 1ꢁtransmittance
(solid) spectra of: (a) RuD1, (b) RuD2, (c) RuD3 in CH2Cl2 at 298 K.
mono-exponential decay as we go higher in dendrimer generation
in CH2Cl2, and are shorter than the reported values in the literature
for analogous ruthenium polypyridyl complexes such as RuðbpyÞ3þ2
(s
= 0.8ꢁ1.1
ls) [30]. In order to investigate the origin of the short
and bi-exponential emission decay of the RuDn dendrimers, we re-

T.I. Houjeiry, T.H. Ghaddar / Chemical Physics Letters 460 (2008) 543–547
547
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electroluminescence properties of these dendrimers.
Acknowledgments
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This work was supported by the University Research Board
(URB) at the American University of Beirut (AUB), and the Lebanese
National Council for Scientific research (LNCSR).
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