A multichromophoric dendrimer: From synthesis to energy up-conversion in a rigid matrix

Article abstract of DOI:10.1039/c1cc16000a

A dendrimer with a [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) complex as a core and four diphenylanthracene units at the periphery was prepared from a scaffold based on a bipyridyl ligand bearing four terminal alkyne groups. Upon green lig

Full text of DOI:10.1039/c1cc16000a

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COMMUNICATION
A multichromophoric dendrimer: from synthesis to energy up-conversion
in a rigid matrixw
Giacomo Bergamini,a Paola Ceroni,*a Pierangelo Fabbrizib and Stefano Cicchi*b
Received 27th September 2011, Accepted 20th October 2011
DOI: 10.1039/c1cc16000a
A dendrimer with a [Ru(bpy)3]2+ (bpy = 2,20-bipyridine) complex
as a core and four diphenylanthracene units at the periphery was
prepared from a scaffold based on a bipyridyl ligand bearing four
terminal alkyne groups. Upon green light excitation, the dendrimer
shows blue luminescence even in a rigid matrix at 77 K thanks to the
dendritic multichromophoric structure.
that can be reacted in a Copper Catalyzed Azide–Alkyne
Cycloaddition (CuAAC).7 This scaffold is highly versatile and
can be decorated at the periphery with four long chains, com-
pound 5, or four diphenylanthracene units, compound 6
(Scheme 1). The crucial step of the synthesis is the alkylation
of the dianion of 4,40-dimethyl-[2,20]bipyridine (2) with benzyl
bromide 1 to afford 3 (see ESIw). The use of LDA/CsOAc8
(unprecedented in the literature) was mandatory to obtain
acceptable yields in the alkylation process since the use of simple
LDA9 or LDA/tBuOK10 (both at ꢀ60 1C and ꢀ80 1C) gave
complex reaction mixtures which afforded lower yields of 3. The
CuAAC reaction of 4, after desilylation of 3, with n-dodecyl azide
gave compound 5 while the same reaction with 9-(4-azidomethyl-
phenyl)-10-phenyl-anthracene gave 6. Dendrimer 7 was prepared
by reacting compound 6 with [Ru(bpy)2Cl2].
Dendrimers are repeatedly branched macromolecules with a
well-defined and tree-like structure.1 They are ideal scaffolds to
arrange multiple and different functional units within a nano-
object according to a predetermined pattern. Several examples of
dendrimers containing photoactive units have been investigated
in the last decade2 and photoinduced electron and energy transfer
processes have been investigated for sensing with signal amplifi-
cation and light-harvesting purposes.
The absorption spectrum of dendrimer 7 (Fig. 1) compared to
that of model compounds shows that the lowest energy band at
459 nm is the characteristic metal-to-ligand charge transfer band
(MLCT) of Ru(II) polypyridine complexes.11 A slight red shift is
observed compared to pristine [Ru(bpy)3]2+ (lmax = 451 nm) and
it is likely due to the substituents on one of the bpy ligands which
lower the energy of the corresponding MLCT transition. The
vibrationally structured band in the range between 330–410 nm is
mainly due to the DPA chromophores (S0 - S1(p,p*)) with
almost no shift compared to pristine 9,10-diphenylanthracene.12
The band at 290 nm is centered on the bpy ligand, as evident by
In the present communication, a dendritic structure (7 in
Scheme 1) consisting of a [Ru(bpy)3]2+ (bpy = 2,20-bipyridine)
complex as a core and four diphenylanthracene (hereafter called
DPA) units at the periphery has been synthesized. These chromo-
phoric units have been selected because in fluid solution they are
known3 to exhibit energy up-conversion by low-power excitation,
the process in which incident photons of a given energy are
converted into photons of higher energy.4 The novelty of the
present approach is the use of a multichromophoric system in
which energy up-conversion works even in a rigid matrix at 77 K
since it does not rely on bimolecular processes. To the best of our
knowledge, dendrimers have never been used for this application,
apart from one example reported by us,5 based on chromophores
absorbing and emitting in the UV region.
the comparison with the well-known spectrum of [Ru(bpy)3]2+
.
Upon excitation at 373 nm, where most of the light (ca. 85%)is
absorbed by DPA, two emission bands are observed (Fig. 1, inset)
due to the fluorescence of DPA12 at 410 nm and the phospho-
rescence of the Ru(II) complex at 617 nm with a slight red shift
We have recently developed a ‘‘click’’ approach to triazole
disubstituted bipyridyl compounds.6 In this work we expand the
number of triazole substituents per bpy moiety by the synthesis
of a scaffold (compound 4) bearing four terminal alkyne groups
compared to [Ru(bpy)3]2+ 11
The luminescence results show that
.
the S1(p,p*) excited state of peripheral DPA units of dendrimer 7 is
quenched 40 times compared to the model compound, and that of
the Ru(II) complex is quenched 10 times (Zq in Table 1). For more
details on the quenching efficiency, see the ESI.w As reported in
Fig. 2, both energy and electron transfer to the [Ru(bpy)3]2+ core
are thermodynamically allowed. In the case of electron transfer,
DG1 (Bꢀ0.5 eV) can be estimated by the energy of S1(p,p*) and
the redox potentials of 9,10-diphenyl anthracene and [Ru(bpy)3]2+
model compounds.13 The efficiency of the energy transfer process is
very low, as demonstrated by the excitation spectrum performed
with lem = 620 nm, in which the characteristic bands of DPA
chromophores in the range 330–410 nm are barely visible.
a Dipartimento di Chimica ‘‘G. Ciamician’’, Centro di ricerca
interuniversitario per la conversione chimica dell’energia solare,
`
Universita di Bologna, via Selmi 2, I-40126 Bologna, Italy.
E-mail: paola.ceroni@unibo.it; Fax: +39 051 2099456;
Tel: +39 051 2099535
b Dipartimento di Chimica ‘‘Ugo Schiff’’, Universita` di Firenze,
via della lastruccia 3-13, I-50019 Sesto Fiorentino, Firenze, Italy.
E-mail: stefano.cicchi@unifi.it; Fax: +39 055 4573531;
Tel: +39 055 4573496
w Electronic supplementary information (ESI) available: Details on
the synthesis, the quenching efficiency, up-converted emission, and
experimental setups. See DOI: 10.1039/c1cc16000a
c
12780 Chem. Commun., 2011, 47, 12780–12782
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Scheme 1 Synthetic procedure for compounds 5 and 6 (eqn (1)) and 7 (eqn (2)).
Fig. 2 Schematic energy level diagram showing the absorption
(dashed grey lines), emission (solid black lines), and non-radiative
processes (wavy lines) occurring in dendrimer 7 at 298 K. CT is a
charge transfer state corresponding to photoinduced oxidation of
DPA and reduction of [Ru(bpy)3]2+ moieties.
Fig. 1 Absorption spectra of a 1.6 ꢁ 10ꢀ5 M solution of 7 (red line)
in CH3CN at 298 K. For comparison purposes, normalized absorption
spectra of 9,10-diphenylanthracene (green line) and [Ru(bpy)3](ClO4)2
(blue line) are reported. Inset shows the emission spectrum upon
excitation at 373 nm. For the sake of clarity, in the range between
530–780 nm the emission spectra have been normalized.
At 77 K in C2H5OH/CH2Cl2 1 : 1 (v/v) rigid matrix, fluores-
cence of DPA and phosphorescence of Ru(II) complex can be
observed upon excitation at 373 nm. At variance with the
experimental results in fluid solution, quenching of an S1(p,p*)
excited state of peripheral DPA units is less efficient and takes
place by energy transfer, as estimated by the excitation spec-
trum. Indeed, electron transfer is precluded as a result of the
destabilization of the CT level at 77 K due to the lack of
solvent repolarization.14
Table 1 Emission data for dendrimer 7 compared to model compounds
in CH3CN at 298 K in air-equilibrated solution
lem/nm
t/ns
Fem
Zq
Upon laser excitation of a 4.2 ꢁ 10ꢀ5 M deaerated solution
of dendrimer 7 at 532 nm, fluorescence of DPA at 410 nm is
observed both at 298 K in CH3CN and at 77 K in C2H5OH/
CH2Cl2 1 : 1 (v/v) rigid matrix (Fig. 3 and S1, ESIw). The
corresponding decay time is much longer (ms-time scale in both
cases) than that reported in Table 1. This is due to delayed
fluorescence of DPA obtained by an energy up-conversion
mechanism. This process is based on sensitized triplet–triplet
annihilation of DPA chromophores, as described by the
following equations:
7
410
617
610
410
o0.5
15.2
157
0.023a
0.003b
0.028c
0.90c
0.99
0.90
[Ru(bpy)3]2+
DPA
5.4
a
lex = 373 nm. Emission quantum yield is estimated by taking into
account the percentage of light absorbed by the core chromophore at
b
c
the excitation wavelength. lex = 460 nm. From ref. 13.
The quenching of [Ru(bpy)3]2+ phosphorescence is due to
energy transfer from 3MLCT to populate the T1(p,p*) excited
state of the DPA units with 90% efficiency (Table 1).
Ru(S0) + hn - *Ru(S1)
(1)
c
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In conclusion, we have synthesized a dendrimer constituted
by a [Ru(bpy)3]2+ core and four DPA units at the periphery.
The photophysical processes show a quite complex pattern of
energy and electron transfer processes. The most interesting
result is the observation of the blue emission of DPA chromo-
phores upon green-laser light excitation at 532 nm of the
Ru(II) core both in fluid CH3CN solution and in a C2H5OH/
CH2Cl2 1 : 1 (v/v) rigid matrix at 77 K. This result is very
important in view of a potential application of energy
up-conversion in a solid state device for wavelength shifting
in spectroscopy. By proper choice of the chromophoric units
and based on previously reported examples in fluid solution,16
these dendritic systems may be applied to the sensitization of
photovoltaics by harvesting the red and near infrared region of
the solar spectrum.
Fig. 3 Emission intensity decay at 425 nm as a function of excitation
power of a 4.2 ꢁ 10ꢀ5 M solution of 7 in a C2H5OH/CH2Cl2 1 : 1 (v/v)
rigid matrix at 77 K. lex = 532 nm. Inset shows the emission intensity
at 3 ms as a function of the normalized laser power.
This work has been supported by Fondazione Carisbo
(‘‘Dispositivi nanometrici basati su dendrimeri e nanoparticelle’’),
bilateral project Italy–China 2011 (MAE DGPCC), MIUR
(PRIN 20085ZXFEE).
*Ru(S1) - *Ru(T1)
(2)
(3)
*Ru(T1) + DPA(S0) - Ru(S0) + *DPA(T1)
*DPA(T1) + *DPA(T1) - *DPA(S1) + DPA(S0) (4)
*DPA(S1) - DPA(S0) + hn0
(5)
Notes and references
z In deaerated solutions at 298 K, the lifetimes of the lowest triplet
excited states are: 0.89 ms and 8000 ms for [Ru(bpy)3]2+ and 9,10-
diphenylanthracene.13
where Ru stands for [Ru(bpy)3]2+. This mechanism is consistent
with the energy level diagram shown in Fig. 2.
1 (a) Designing Dendrimers, ed. S. Campagna, P. Ceroni and
F. Puntoriero, John Wiley
It takes advantage of the unitary efficiency of process (2) for
Ru(II) polypyridine complexes,11 and of the presence of
multiple DPA units in close proximity at the periphery of
the dendrimer.
& Sons, Hoboken, 2012;
(b) F. Vogtle, G. Richardt and N. Werner, Dendrimer Chemistry,
¨
Wiley-VCH, Chichester, 2009; (c) G. R. Newkome and F. Vogtle,
¨
Dendrimers and Dendrons, Wiley-VCH, Weinheim, 2001;
(d) Dendrimers and Other Dendritic Polymers, ed. J. M. J. Fre
and D. A. Tomalia, John Wiley & Sons, Chichester, 2001.
´
chet
For an equimolar solution (4.2 ꢁ 10ꢀ5 M) of the two model
compounds under the same experimental conditions, the
emission intensity of DPA delayed fluorescence is higher at
298 K compared to dendrimer 7, while it is not observed at all
at 77 K. The higher intensity observed for the separated
components at 298 K is due to the fact that quenching of
the S1 state of 9,10-diphenylanthracene by energy and electron
transfer processes to the Ru(II) complex core is precluded.
Dynamic quenching processes cannot compete with the
intrinsic deactivation of the S1 state of 9,10-diphenylanthracene
since its lifetime is too short and the concentration of
[Ru(bpy)3]2+ is too low.14 Processes (3) and (4) can occur
intermolecularly in fluid solution thanks to the longer lifetime
of the triplet excited states of both chromophores,z but are
precluded in rigid matrix. Therefore, at 77 K up-converted
emission is observed only for the multichromophoric dendrimer
7 in which processes (3) and (4) occur intramolecularly. A further
proof of the intramolecular nature of process (4) is given by the
plot of the delayed luminescence intensity as a function of the
excitation power (Fig. 3, inset). This plot is not quadratic, but
linear at low power and reaches a plateau at higher value due to
excitation of all the [Ru(bpy)3]2+ chromophores of the solution
within the laser pulse.
2 For some recent reviews, see: (a) G. Bergamini, E. Marchi and
P. Ceroni, Coord. Chem. Rev., 2011, 255, 2458–2468; (b) D. Astruc,
E. Boisselier and C. Orneals, Chem. Rev., 2010, 110, 1857;
(c) R. Deloncle and A.-M. Caminade, J. Photochem. Photobiol.,
C, 2010, 11, 25; (d) W.-S. Li and T. Aida, Chem. Rev., 2009,
109, 6047.
3 R. R. Islangulov, D. V. Kozlov and F. N. Castellano, Chem.
Commun., 2005, 3776–3778.
4 For recent reviews, see: (a) P. Ceroni, Chem.–Eur. J., 2011, 17,
9560–9564; (b) T. N. Singh-Rachford and F. N. Castellano, Coord.
Chem. Rev., 2010, 254, 2560–2573.
5 G. Bergamini, P. Ceroni, M. Maestri, V. Balzani, S.-K. Lee and
F. Vogtle, Photochem. Photobiol. Sci., 2004, 3, 898–905.
¨
6 (a) P. Fabbrizzi, B. Cecconi and S. Cicchi, Synlett, 2011, 223;
(b) S. Paek, C. Baik, M.-S. Kang and J. Ko, J. Organomet. Chem.,
2010, 695, 821.
7 (a) V. A. Rostovstev, L. G. Green, V. V. Fokin and
K. B. Sharpless, Angew. Chem., Int. Ed., 2002, 41, 2596;
(b) P. Fabbrizzi, S. Cicchi, A. Brandi, E. Sperotto and G. van
Koten, Eur. J. Org. Chem., 2009, 5423.
8 W. Bauer and L. Lochmann, J. Am. Chem. Soc., 1992, 114, 7482.
9 Y.-R. Hong and C. B. Gorman, J. Org. Chem., 2003, 68, 9019.
10 M. Schlosser, Pure Appl. Chem., 1988, 60, 1627.
11 S. Campagna, F. Puntoriero, F. Nastasi, G. Bergamini and
V. Balzani, Top. Curr. Chem., 2007, 280, 117.
12 I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic
Molecules, Academic Press, London, 1965.
13 M. Montalti, A. Credi, L. Prodi and M. T. Gandolfi, Handbook of
Photochemistry, CRC Press, Boca Raton, FL, USA, 3rd edn, 2006,
ch. 7.
14 V. Balzani and F. Scandola, Supramolecular Photochemistry, Ellis
Horwood, New York, 1991.
15 S. Ji, W. Wu, W. Wu, H. Guo and J. Zhao, Angew. Chem., Int. Ed.,
2011, 50, 1626.
The advantage of the present multichromophoric dendritic
structure, compared to the previously reported examples based
on the two separated chromophores,3,4,15 is that energy
up-conversion can be observed also in rigid media, a very
important step toward solid-state devices. Moreover, at 77 K
quenching of DPA fluorescence is less efficient than at 298 K
since the photoinduced electron transfer is precluded.
16 See e.g.: S. Baluschev, V. Yakutkin, G. Wegner, T. Miteva,
G. Nelles, A. Yasuda, S. Chernov, S. Aleshchenkov and
A. Cheprakov, Appl. Phys. Lett., 2007, 90, 181103.
c
12782 Chem. Commun., 2011, 47, 12780–12782
This journal is The Royal Society of Chemistry 2011