Photophysical, photochemical, and electrochemical properties of dendrimers with a dimethoxybenzil core

Article abstract of DOI:10.1039/b615196e

Three dendrimers consisting of a dimethoxybenzil core and branches that contain two (G0), four (G1), and eight (G2) naphthalene units at the periphery and zero (G0), two (G1), and six (G2) dimethoxybenzene units in the branches have been synthesized and t

Full text of DOI:10.1039/b615196e

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Photophysical, photochemical, and electrochemical properties of
dendrimers with a dimethoxybenzil corew
a
a
a
a
Carlo Giansante, Paola Ceroni,* Vincenzo Balzani, Mauro Maestri,
b
b
Sang-Kyu Lee and Fritz Vogtle*
¨
Received (in Montpellier, France) 18th October 2006, Accepted 9th January 2007
First published as an Advance Article on the web 30th January 2007
DOI: 10.1039/b615196e
Three dendrimers consisting of a dimethoxybenzil core and branches that contain two (G0), four
G1), and eight (G2) naphthalene units at the periphery and zero (G0), two (G1), and six (G2)
(
dimethoxybenzene units in the branches have been synthesized and their photophysical,
photochemical, and electrochemical properties have been investigated. For comparison purposes,
the properties of dimethoxybenzil (MB) and of a dendron containing four naphthalene and three
dimethoxybenzene units (D2) have also been studied. The properties of the dendrimers in the
ground state (absorption spectra and electrochemical behavior) are those expected for their
noninteracting component units. The excited state properties, however, are substantially
controlled by electronic interactions between the dimethoxybenzil core and the naphthalene units
contained in the branches. In dichloromethane–chloroform 1 : 1 (v/v) solution at 298 K, energy
1 1
transfer from the lowest excited state (S ) of the naphthalene units to the lower lying S excited
state of the dimethoxybenzil core takes place with high efficiency. In a rigid matrix at 77 K,
selective excitation of the dimethoxybenzil chromophore yields an emission band that exhibits a
spectral evolution: in the millisecond time scale it shows a spectral profile very similar to the
dimethoxybenzil phosphorescence, whereas in the second time scale it is very similar to the
naphthalene-type phosphorescence. Energy transfer from the T
dimethoxybenzil core to the T excited state of the naphthalene units takes place at 77 K, but not
at 298 K, because the T excited state of the dimethoxybenzil core moves to energy lower than
1
excited state of the
1
1
that of the naphthalene chromophore. The photochemical results show that the dimethoxybenzil
core maintains its intrinsic photoreactivity toward dioxygen, and that on increasing dendrimer
generation a photoreaction between core and branches predominates.
the spectra of the component units. When a single chromo-
Introduction
phoric group is in an excited state, however, the interaction
with the nearby units is often strong enough to change its
luminescence behavior because of formation of excimers or
1
,2
Dendrimers are repeatedly branched tree-like compounds,
that can be synthesized with well defined composition, a high
degree of order, and the possibility to contain selected chemi-
cal units in predetermined sites of their structure. These
macromolecules are currently attracting the interest of a great
number of scientists because of their unusual chemical, phy-
sical and biological properties and the wide range of potential
5
6
7
exciplexes and the occurrence of energy or electron transfer
processes. In suitably designed dendrimers energy transfer
processes can be spatially and energetically controlled, thereby
opening the way towards the construction of antenna systems
8
for solar energy conversion, luminescence sensors with signal
3
applications.
9
amplification, and systems capable of changing the wave-
8c,10
In the last few years, several families of dendrimers contain-
ing different chromophoric and luminophoric units have been
4
synthesized. In the dendritic structure, the electronic interac-
length of light.
4a
Continuing our studies on dendrimers, we have investi-
gated the photophysical, photochemical, and electrochemical
properties of three dendrimers consisting of a dimethoxybenzil
core and branches that contain two (G0), four (G1), and eight
tion between nearby units is usually small in the ground state,
so that each unit can be independently excited and the
dendrimer absorption spectrum is close to the summation of
(G2) naphthalene units at the periphery and zero (G0), two
(G1), and six (G2) dimethoxybenzene units in the branches
(Scheme 1). Benzil, naphthalene, and dimethoxybenzene are
a
Dipartimento di Chimica ‘‘G. Ciamician’’, Universita` di Bologna, Via
Selmi 2, I-40126 Bologna, Italy. E-mail: paola.ceroni@unibo.it; Fax:
+39-051-2099456
Kekule´-Institut fu¨r Organische Chemie und Biochemie der
Universita¨t Bonn, Gerhard-Domagk Strasse 1, D-53121 Bonn,
Germany. E-mail: voegtle@uni-bonn.de; Fax: +49-228-735662
well known luminescent species. In particular, both fluores-
11
cence and phosphorescence are exhibited by benzil and
11
naphthalene in suitable experimental conditions, opening
b
the possibility to investigate singlet–singlet and triplet–triplet
energy transfer. For comparison purposes, the properties of
w This paper was published as part of the special issue on Dendrimers
and Dendritic Polymers: Design, Properties and Applications.
1
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Fig. 1 Absorption spectra of MB and D2 (a) and dendrimers G0–G2
(
2 2 3
b) in CH Cl –CHCl 1 : 1 (v/v) at 298 K.
chromophores increases on increasing dendrimer generation
and, as a consequence, the absorbance at 275 nm increases
while the shoulder at 300 nm is practically constant. Quanti-
tative comparison with the spectra of the model compounds
shows that there is no significant interaction in the ground
state. In particular, the spectrum of G2 is almost coincident
with that obtained considering the sum of the spectra of two
dendrons D2 and a MB unit.
Fluorescence spectra and lifetimes. Upon excitation at
Scheme 1
2
77 nm, dendrimers G0–G2 show two emission bands: the
first one, with maximum at 335 nm (Fig. 2), is due to the
fluorescence of the naphthalene group; the second one, with
dimethoxybenzil (MB), 2-methylnaphthalene, and of a den-
dron containing four naphthalene units and three dimethox-
ybenzene units (D2) have also been studied.
Results and discussion
Photophysical properties
Absorption spectra. Dendron D2 absorbs at higher energy
(
lmax = 275 nm) compared to MB (lmax = 300 nm) (Fig.
(a)). In particular, the absorption spectrum of D2 is domi-
1
nated by the four naphthalene groups, since the three di-
methoxybenzene moieties, that absorb in the same spectral
region, have a molar absorption coefficient (e275 nm = 2200
ꢁ
ꢁ
1
ꢁ1 12
ꢁ1 13
M
cm
)
smaller than that of naphthalene (e275 nm = 4300
cm ). The absorption spectra of the dendrimers (Fig.
1
M
1
(b)) show the features of both the dimethoxybenzil core, with
Fig. 2 Fluorescence spectra of dendrimers G0–G2 and dendron D2 in
a shoulder at 300 nm, and the dimethoxybenzene and
naphthalene units contained in the branches, with a maximum
at 275 nm. The number of naphthalene and dimethoxybenzene
2 2 3
CH Cl –CHCl 1 : 1 (v/v) at 298 K with the same absorbance at the
excitation wavelength. lex = 277 nm. Inset shows the normalized
emission spectra of G2 and a model compound, 2-methylnaphthalene.
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Therefore, an energy transfer process from the lowest excited
state (S
1 1
) of the naphthalene units to the lower lying S excited
state of the dimethoxybenzil core takes place with high
1
7
efficiency (F 4 0.9). A further proof of a high energy
transfer efficiency comes from the fact that the excitation
spectra of G0–G2 (recorded with lem = 500 nm) are practi-
cally coincident with the corresponding absorption spectra.
The quantum yield of the naphthalene type emission slightly
increases on increasing dendrimer generation, most likely
because of the increasing distance between the donor and
acceptor partners of the energy transfer process. The photo-
induced energy transfer process from the S
lene to the S state of the dimethoxybenzil core is active also at
7 K (Scheme 2), since a significant quenching of the naphtha-
1
state of naphtha-
1
7
Fig. 3 Fluorescence spectra of G2 (solid line) and MB (dashed-dotted
line) in CH Cl –CHCl 1 : 1 (v/v) at 298 K with the same absorbance
lene emission intensity and lifetime is observed when dendri-
mers G0–G2 are compared to dendron D2. However,
sensitization of the dimethoxybenzil has not been evidenced
because its fluorescence quantum yield is so low that the
corresponding band is completely covered by the much more
intense phosphorescence band.
2
2
3
at the excitation wavelength. lex = 277 nm.
maximum at 495 nm and much weaker in intensity (see, for
example, Fig. 3), is assigned to the dimethoxybenzil fluores-
cence, because of the close similarity to that recorded for MB.
The small quantum yield of the dimethoxybenzil fluorescence
No fluorescence of the dimethoxybenzene group has been
observed in dendrimers G1 and G2, as expected on the basis of
the behaviour of the dendron D2 where the potentially fluor-
is due to the close proximity of the S
1
(np*) and T (np*) excited
1
1
states that favors intersystem crossing. The emission bands
4
1
5
centred at 335 nm of G1, G2, and D2 show a weak tail at
lower energy, compared with that exhibited by 2-methyl-
naphthalene (inset of Fig. 2), suggesting formation of
escent S excited state of dimethoxybenzene is quenched by
1
energy and electron transfer processes involving the naphtha-
1
6
lene units. Furthermore, in the case of dendrimers G1 and
G2 another lower-lying excited state located on the core is
present, so that an additional quenching channel for the
fluorescent excited state of dimethoxybenzene is available.
The results obtained for G0–G2 from fluorescence lifetime
measurements are not easily interpreted because multi-expo-
nential decays were observed due to spectral overlaps. The
reference compound MB has a very short lifetime (o0.5 ns),
while D2 shows a biexponential decay: the shorter component
is the usual naphthalene-type emission, while the longer one is
1
6
naphthalene excimers.
It should be noticed that the naphthalene-type emission in
the dendrimers is strongly quenched compared to D2, as
evidenced by the spectra displayed in Fig. 2 and the quantum
yield data reported in Table 1. On the other hand, emission of
the benzil core at 495 nm is sensitised. Indeed, practically
coincident emission intensities at 495 nm were obtained upon
excitation at 277 nm of isoabsorbing solutions of G2 and MB
(
Fig. 3), although in the case of G2 about 80% of the light is
1
6
absorbed by naphthalene units at the excitation wavelength.
assigned to an excimer emission.
In the case of the
Table 1 Photophysical data in dichloromethane–chloroform 1 : 1 (v/v) solution, unless otherwise noted
Absorption
Fluorescence
298 K
Phosphorescence
298 K
a
298 K
77 K
ꢁ
1
ꢁ1
lmax/nm
e
max/M cm
lmax/nm
F
em
t/ns
lmax/nm
t/ms
lmax/nm
t/s
4
b
ꢁ3
MB
D2
G0
G1
300
276
288
280
278
25 000
27 200
34 400
47 500
80 100
495
335
335
3 ꢂ 10ꢁ
o0.5
550
25
494
(
2.7 ꢂ 10
(2.7 ꢂ 10
ꢁ3
492)
)
2.8 ꢂ 10ꢁ
2
5.4, 66
c
—
—
c
480
480)
1.2
(1.5)
(
ꢁ
ꢁ
4
4
d
d
d
ꢁ3
6 ꢂ 10
3 ꢂ 10
o0.5, 3.2
550
550
550
30
30
30
502
(502)
2.5 ꢂ 10 , 0.9
b
4
95
o0.5
(1.4)
ꢁ
ꢁ
3
4
ꢁ3
335
95
1 ꢂ 10
3 ꢂ 10
o0.5, 3.2
502
(502)
1.6 ꢂ 10 , 1.0
b
4
o0.5
(1.5)
ꢁ
ꢁ
3
4
ꢁ3
G2
335
95
3 ꢂ 10
o0.5, 3.1
492
(518)
b
1.1 ꢂ 10 , 1.0
b
4
3 ꢂ 10
o0.5
(1.2)
a
Data reported in parenthesis have been obtained in dichloromethane–methanol 1 : 1 (v/v) rigid matrix at 77 K. Prompt fluorescence to be
c
d
distinguished from delayed fluorescence displayed in Fig. 4. No emission has been observed. The two lifetimes reported correspond to the
naphthalene monomer emission with maximum at 335 nm and the excimer emission tail at lower energy. The value of the shorter lifetime cannot be
determined with the equipment used (see Experimental section).
1
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to the dimethoxybenzil phosphorescence. The shoulder is in
the same position of the fluorescence band observed in aerated
solution while it is not present in the phosphorescence spectra
recorded at 77 K (vide infra) and is thus attributed to delayed
1
4b
fluorescence, i.e. to the thermal population at 298 K of the
S excited state from the T one. Indeed, the ratio of emission
1
1
intensities at 495 and 548 nm is not dependent on the delay
time used to register the emission spectrum. This result
indicates that the two emission bands originate from two
1 1
states with the same lifetime, i.e. S and T are in thermal
equilibrium. Similar spectra have been obtained for dendri-
mers G0–G2 upon selective excitation of the dimethoxybenzil
chromophore at 330 nm (see e.g. G1 in Fig. 4). The phosphor-
escence intensity of isoabsorbing solutions at the excitation
wavelength are slightly higher and the corresponding lifetimes
are slightly longer in the case of the dendrimers compared to
MB reference compound (Table 1). These results demonstrate
1
that no quenching of the T excited state of the dimethoxy-
benzil core is brought about by the naphthalene moieties and
suggest that either (i) the dendritic wedges protect the di-
methoxybenzil excited state by quenching from residual dioxy-
gen or (ii) a more rigid structure of the dimethoxybenzil core in
the dendrimers leads to lower non-radiative decay rate con-
stant. On the other hand, no emission has been recorded,
under the same experimental conditions, in the case of D2.
Indeed, D2 phosphorescence can be observed only at 77 K
(vide infra) because the T excited state of naphthalene has a
1
Scheme 2
very long lifetime (Table 1) that allows bimolecular quenching
by impurities present even at very low concentration.
dendrimers, the shorter component (o0.5 ns) can be assigned
to the naphthalene-type emission, that is highly quenched
compared to D2, and the longer one to the naphthalene
excimer emission (see inset of Fig. 2). The small emission
quantum yield of the dimethoxybenzil chromophore and the
partial spectral overlap with the naphthalene excimer emission
prevented a reliable estimation of the emission lifetime of the core.
Laser-flash photolysis experiments performed on MB and
dendrimers G0–G2 in deaerated dichloromethane–chloroform
1
: 1 (v/v) solutions showed, upon excitation of the dimethox-
ybenzil moiety (lex = 355 nm), transient absorption spectra
characteristic of T - T transitions of the dimethoxybenzil
1
n
1
8
chromophore. Time evolution of this spectral changes, e.g.
at lmax = 480 nm, was not monoexponential and depended on
laser intensity and chromophore concentration. The decays
can be fitted by a combination of a first- and second-order
1 n
Phosphorescence spectra, T –T transient absorption spectra
and lifetimes. The emission spectrum recorded with a 10 ms
delay for a deaerated dichloromethane–chloroform 1 : 1 (v/v)
solution of MB shows a maximum at 548 nm (t = 25 ms) and a
shoulder at 495 nm (Fig. 4). The maximum can be attributed
process (see experimental section), i.e. intrinsic deactivation of
19
1
T and bimolecular triplet–triplet annihilation. The triplet
excited state lifetimes (first-order component) are in very good
agreement with data found by analysing phosphorescence
2
0
intensity decays. Triplet–triplet annihilation rate constants
(
(
(
kT–T), measured under the same experimental conditions
l
k
ex = 355 nm, labs = 480 nm), decreased in going from MB
6
ꢁ1 ꢁ1
s ) to dendrimers (e.g., in the case of
T–T = 1.2 ꢂ 10 M
6
ꢁ1 ꢁ1
G1 k
= 0.5 ꢂ 10 M
s ). This experimental result shows
T–T
that, as expected, the dimethoxybenzil core in the dendrimers is
protected against self-quenching by the dendritic branches.
In dichloromethane–chloroform 1 : 1 (v/v) rigid matrix at 77
K, MB and D2 show two phosphorescence bands very close in
energy (Fig. 5(a)). The main differences are: (i) the vibrational
structure present only in the phosphorescence of D2, (ii) the
lifetime values: 2.7 ms for MB and 1.2 s for D2 (Table 1) and
(
iii) the phosphorescence quantum yield much lower for
1
naphthalene than that of benzil. The phosphorescence of
1
MB is assigned to the np* triplet excited state (T ) of the
dimethoxybenzil chromophore, and that of D2 to the pp* T1
excited state of the naphthalene unit, as evidenced by
1
Fig.
CH
4
Phosphorescence spectra of MB and G1 in deaerated
–CHCl 1 : 1 (v/v) solution at 298 K, under the same experi-
2
Cl
2
3
mental conditions. lex = 330 nm.
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is slightly higher in the case of dendrimers compared to MB
(
Table 1) and no transient absorption features typical of
naphthalene (2-methylnaphthalene shows a transient absorp-
tion spectrum with lmax at ca. 400 nm) has been observed
upon selective excitation of the dimethoxybenzil core (lex
=
3
55 nm). The lack of energy transfer at room temperature can
be explained considering that the dimethoxybenzil phosphor-
escence shows a quite strong blue shift in going from 298 to
77 K, as expected for a np* transition where lack of solvent
repolarization in rigid matrix leads to an increase of the
corresponding excited state energy. A much less pronounced
shift is expected for the naphthalene T
of its pp* character. As a consequence, the lowest T
state in the dendrimers is located on the dimethoxybenzil at
1
excited state, because
1
excited
2
98 K and on the naphthalene at 77 K.
Another interesting aspect of the phosphorescence bands
observed for these dendrimers is that only in the case of G2
there is a substantial solvent effect. The G2 phosphorescence is
red-shifted compared to MB, G0 and G1 in dichloromethane–
methanol rigid matrix and blue-shifted in dichlormethane–
chloroform (Table 1). The peculiar behaviour of G2 can be
due to a different degree of back-folding of the dendrons in the
two solvent mixtures, thus creating an environment of differ-
ent polarity from the bulk of the solution.
In aerated solution, all the examined compounds are able to
transfer energy to dioxygen with high efficiency as shown by the
observation of the singlet dioxygen emission band at 1260 nm.
Fig. 5 Phosphorescence spectra of MB and D2 (a) and of dendrimers
G1 (b) in CH Cl –CHCl 1 : 1 (v/v) at 77 K. lex = 330 nm.
2
2
3
comparison of the phosphorescence bands of 2-methyl-
Photochemical properties
1
6
naphthalene. Under the same experimental conditions, den-
drimers G0–G2 show a complex behaviour: upon selective
excitation of the dimethoxybenzil chromophore at 330 nm, the
phosphorescence spectrum recorded with a relatively short
delay (30 ms) shows a spectral profile very similar to that
observed for MB, while recorded with a longer delay (100 ms)
is very similar to the naphthalene-type phosphorescence (see
e.g., Fig. 5(b)). Correspondingly, two lifetimes have been
observed (Table 1): a shorter one (ms time scale) assigned to
the dimethoxybenzil chromophore, and a longer one (second
Upon irradiation at 313 nm of an air-equilibrated
2 2 3
CH Cl –CHCl 1 : 1 (v/v) solution of MB, strong changes in
the absorption spectra are observed (Fig. 6(a)): the
1
time scale) due to the naphthalene moiety. The T excited
states of naphthalene and dimethoxybenzil units are very close
in energy (Fig. 5(a)), but spectral evolution of the phosphor-
escence band for dendrimers G0–G2 and observation of
naphthalene phosphorescence upon selective excitation of
dimethoxybenzil core are indicative of an energy transfer
process from the dimethoxybenzil to the naphthalene chro-
mophore (Scheme 2). The efficiency of this process is low, as
demonstrated by the slight decrease of dimethoxybenzil triplet
excited state lifetime (Table 1), and an estimation of the
naphthalene sensitization efficiency and rate constant is diffi-
cult. Indeed, quantitative measurements of emission intensities
at 77 K are difficult, especially in the present case since the
naphthalene phosphorescence quantum yield is much lower
1
1
than that of benzil.
Contrary to what happens at 77 K, at 298 K there is no
evidence for energy transfer from the T1 excited state of
dimethoxybenzil to that of naphthalene in dendrimers
G0–G2. The emission quantum yield and lifetime of the T1
Fig. 6 Absorption changes of MB (a) and G2 (b) in air-equilibrated
excited state of the dimethoxybenzil unit at room temperature
CH Cl –CHCl 1 : 1 (v/v) solution during irradiation at 313 nm.
2
2
3
1
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characteristic dimethoxybenzil band at 300 nm disappears and
a new one with maximum at 255 nm increases with an
isosbestic point at 273 nm. The reaction quantum yield,
measured from the disappearance of the band at 300 nm, is
the first one (E1/2 = ꢁ1.22 V vs. SCE) is completely reversible,
while the second one (E1/2 = ꢁ1.47 V vs. SCE) shows some
degree of chemical irreversibility. The spectroelectrochemical
ꢁ
characterization of the monoanionic species MB has been
0
.09 in air-equilibrated solution, while it is negligible (o0.005)
performed in an OTTLE cell and evidences the appearance of
new absorption bands at 370 and 600 nm.
in deaerated solution where no absorption change was ob-
served for irradiation times comparable to those of the air-
equilibrated experiments. Clearly, dioxygen is involved in the
photochemical reaction, likely due to the formation of benzoyl
In the cathodic region G2 undergoes two one-electron
reduction processes likely centred on the dimethoxybenzil core
(E1/2(I) = ꢁ1.19 V E1/2(II) = ꢁ1.66 V vs. SCE), but the
second peak is cathodically shifted and presents an higher
degree of chemical irreversibility. The spectroelectrochemical
investigation of the monoanionic species of G2 confirms that
the first reduction process involves the dimethoxybenzil core
since the absorption changes are similar to those observed for
the reduction of MB. The spectroelectrochemistry in corre-
spondence of the second reduction process is prevented by the
narrower potential window available in the spectroelectro-
chemical experimental conditions.
2
1
peroxide, as already reported.
Fig. 6(b) shows the absorption changes recorded upon
irradiation at 313 nm of an air-equilibrated CH Cl –CHCl
: 1 (v/v) solution of G2. Although the photoreaction
2
2
3
1
quantum yield is similar (0.07) to that obtained for MB,
substantial qualitative differences from the MB photoreactiv-
ity can be observed: (i) two new absorption bands appear upon
irradiation, one at higher energy (lmax = 250 nm) compared
to that of dimethoxybenzil, and one at lower energy (lmax
E350 nm); (ii) no effect of oxygen has been observed (F =
The electrochemical reversibility (high rate of heterogeneous
electron transfer) of the first reduction process of the benzyl
core in G2 shows that there is no significant site isolation effect
in the dendritic structure. On the other hand, the shift of the
second reduction process towards more negative potentials
can be attributed to a lower stabilization of the dianionic
species in the environment created by the dendrons compared
0
.07 both in air equilibrated and deaerated solutions).
In the case of air-equilibrated solutions of G0 and G1
irradiation at 313 nm caused spectral changes that, at the
beginning, were similar to those observed under the same
experimental conditions for MB, i.e. disappearance of the
band at 300 nm and increase of a new one at higher energy.
On going on with irradiation, however, a new band at 250 nm
and a red tail (lmax E350 nm) were formed, reminiscent of the
photochemical reactivity of G2. Two different families of
spectra, characterized by different isosbestic points, were in-
deed obtained, indicating the occurrence of two successive
photochemical reactions. In deaerated solution, the photoche-
mical reactivity of G0 and G1 is quite similar to that exhibited
by G2, but with a much lower quantum yield (see Table 2).
Irradiation at 313 nm of a deaerated solution containing
MB (4 ꢂ 10ꢁ M) and D2 (4 ꢂ 10 M) leads to spectral
changes similar to those observed in the case of G2: the
dimethoxybenzil band disappears and a new band at lower
energy (lmax E350 nm) increases.
In conclusion, the results obtained show that the dimethoxy-
benzil core maintains its photoreactivity toward dioxygen, and
that on increasing dendrimer generation the branches perhaps
protect the core towards reaction with dioxygen, but introduce
a new reaction channel involving the dendron itself.
2ꢁ
to the MB species in the bulk of the solution.
In the anodic region the cyclic voltammogram of G2 pre-
sents a multielectronic chemically irreversible process, that can
be assigned to the oxidation of the dimethoxybenzene units of
2
2
ꢁ1
the dendrons (Epa = +1.93 V vs. SCE at 0.2 V s ).
Conclusions
5
ꢁ4
We have investigated the photophysical, photochemical, and
electrochemical properties of G0, G1, and G2 dendrimers and
of their core (MB) and branches (D2) reference compounds.
The properties of the dendrimers in the ground state (absorp-
tion spectra and electrochemical behavior) are those expected
for their dimethoxybenzil, naphthalene, and dimethoxyben-
zene noninteracting units. The excited state properties, how-
ever, are substantially controlled by electronic interactions
between the dimethoxybenzil core and the naphthalene units
contained in the branches, whereas the dimethoxybenzene
units play a less important role. In this family of dendrimers
multiple photoinduced energy transfer processes take place:
Electrochemical characterization
The cyclic voltammogram recorded for MB in dichloro-
methane/TBAPF
6
solution shows two reduction processes:
from the lowest singlet excited state (S ) of the naphthalene
1
a
Table 2 Photoreaction quantum yield upon irradiation at 313 nm in
dichloromethane solution at 298 K
units to that of the dimethoxybenzil core both at 298 and 77 K
and from the T
the T excited state of the naphthalene units at 77 K, but not at
98 K. The latter is a quite unusual result in dendritic species.
At 298 K, the T excited state the dimethoxybenzil core moves
1
excited state of the dimethoxybenzil core to
1
F
react
2
Air-equilibrated solution
Deaerated solution
1
MB
G0
G1
G2
a
0.09
0.11
0.10
0.07
o0.005
0.007
0.01
to lower energy, thus preventing the photoinduced energy
transfer. As far as the photochemical behavior is concerned,
the dimethoxybenzil unit maintains its intrinsic photoreactiv-
ity toward dioxygen even when it belongs to the dendrimer
core, and on increasing dendrimer generation a photoreaction
between core and branches also takes place.
0.07
Reported quantum yields were obtained by extrapolation to time
zero of the initial part of the photoreaction.
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Experimental
under reduced pressure. Purification by column chromatogra-
2
phy (SiO ; dichloromethane) yielded a yellowish solid (180 mg).
Synthetic procedure
TLC (SiO ): R = 0.6 (dichloromethane).
2
f
1
0
H NMR (300 MHz, CDCl , 25 1C): d [ppm] 5.31 (s, 4H,
4
,4 -Dimethoxybenzil (MB) is commercially available (Fluka).
3
3
CH O), 7.08 (AA‘BB’, 4H, J
= 9.04 Hz), 7.47–7.53 (m,
2
HH
0
4
,4 -Dihydroxybenzil
6
H, naph-H), 7.82–7.89 (m, 8H, naph-H), 7.95 (AA‘BB’, 4H,
3
J
HH = 9.04 Hz).
1
3
C NMR (100.6 MHz, CDCl
CH O), 115.20, 125.01, 126.34, 126.44, 126.51, 126.57,
27.78, 127.95, 128.64, 132.40, 133.21, 133.26, 133.31, 163.00
ar), 193.38 (ketone-C).
3
, 25 1C): d [ppm] 70.48
(
2
1
(C
0
"
Route A. 4,4 -Dimethoxybenzil MB (2.0 g; 7.4 mmol) and
FAB-MS: m-NBA, m/z (%) = 523.2 (M + H , 40).
hexadecyltributylphosphonium bromide (760 mg; 1.5 mmol)
as catalyst in aqueous HBr (48%; 40 ml), were heated to reflux
for 6 h. The reaction mixture was then poured into water and
the mixture extracted several times with ethyl acetate. The
C H O : 522.59.
22
2
7
3
0 0
,4 -Bis[3,5-bis(2 -oxymethylnaphthyl)benzyloxy]benzil (G1)
4
organic phase was dried with MgSO
under reduced pressure. The residue was purified by column
chromatography (SiO ; dichloromethane–acetone 5 : 1) yield-
4
and the solvent removed
2
ing 100 mg of a yellowish solid.
0
Route B. 4,4 -Dimethoxybenzil MB (2.0 g; 7.4 mmol) was
heated to reflux for 3 h in a mixture of aqueous HBr (48%)
and glacial acetic acid (50 ml; 1 : 1 v/v). The reaction mixture
was poured into water and then extracted several times with
ethyl acetate. The organic phase was washed two times with
The experimental procedure is analogous to the synthesis of G0.
0
Scale: 4,4 -Dihydroxybenzil 50 (400 mg; 1.48 mmol), 3,5-
0
bis(2 -oxymethylnaphthyl)benzyl bromide (1.5 g; 3.11 mmol),
4
water, dried with MgSO and the solvent removed under
potassium carbonate (620 mg; 4.44 mmol) and [18]crown-6
reduced pressure. The residue was purified by column chro-
(
120 mg; 0.45 mmol) in dry acetone (150 ml). Purification by
column chromatography (SiO ; dichloromethane) yielded a
yellowish solid (820 mg; 53%).
TLC (SiO ): R = 0.6 (dichloromethane).
matography (SiO ; dichloromethane–acetone 5 : 1) yielding a
2
2
yellowish solid.
TLC (SiO
2
): R
f
= 0.5 (dichloromethane–acetone 5 : 1 v/v).
, 25 1C): d [ppm] 6.78 (AA‘BB’,
H, JHH = 8.9 Hz), 7.73 (AA‘BB‘, 4H, JHH = 8.9 Hz).
2
f
1
H NMR (400 MHz, CDCl
3
3
1
H NMR (400 MHz, CDCl , 25 1C): d [ppm] 5.07 (s, 4H, ar-
3
3
4
4
CH O), 5.20 (s, 8H, naph-CH O), 6.68 (t, 2H, J = 2.2 Hz,
2
2
HH
1
3
C NMR (100.6 MHz, CDCl
3
, 25 1C): d [ppm] 115.82,
25.04, 132.58, 163.65, 194.31 (Car), 194.31 (ketone-C).
4
ar-H), 6.70 (d, 4H, JHH = 2.2 Hz, ar-H), 6.98 (AA‘BB’, 4H,
1
4
J
HH = 9.0 Hz), 7.46–7.52 (m, 12H, naph-H), 7.81–7.86
"
GC-MS: R
: 242.23.
For the syntheses of the naphthalene-decorated Fre´chet-
t
= 10.51 min; m/z (%) = 242 (M , 10).
4
m, 16H, naph-H) 7.90 (AA‘BB’, 4H, JHH = 9.0 Hz).
13
(
C
14
H
10
O
4
C NMR (100.6 MHz, CDCl
3
, 25 1C): d [ppm] 70.17, 70.36
CH O), 102.03, 106.50, 115.19, 125.25, 126.15, 126.30,
(
2
dendrons, and for general procedures for the preparation of
dendritic benzyl alcohols/bromides with naphthalene units,
including compound D2, see ref. 23.
1
1
26.40, 126.58, 127.77, 127.96, 128.46, 132.36, 133.13,
33.32, 133.18, 138.40, 160.35, 163.84 (Car), 194.38 (ketone-C).
"
FAB-MS: m-NBA, m/z (%) = 1047.5 (M + H , 10).
0
0
C H O : 1047.19.
72 54 8
4
,4 -Bis(2 -oxymethylnaphthyl)benzil (G0)
0
0
0
00
4
,4 -Bis[3,5-bis[3 ,5 -bis(2 -oxymethylnaphthyl)-
benzyloxy]benzyloxy]benzil (G2)
0
4
,4 -Dihydroxybenzil (210 mg; 0.86 mmol), 2-bromomethyl-
naphthalene (440 mg; (2.00 mmol), potassium carbonate (360
mg; 2.60 mmol) and [18]crown-6 (100 mg; 0.35 mmol) were
dissolved in dry acetone (50 ml) and heated to reflux for 48 h.
The precipitate was filtered off, washed with dichloromethane,
and the solvent removed under reduced pressure. The residue
was dissolved in dichloromethane, washed with water, sodium
hydrogen carbonate-solution, and again with water. The
organic phase was dried with Na SO and the solvent removed
The experimental procedure is analogous to the synthesis
of G0.
2
4
1
256 | New J. Chem., 2007, 31, 1250–1258
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0
Scale: 4,4 -Dihydroxybenzil (91.5 mg; 0.38 mmol), 3,5-
are the rate constants of the first- and second-order processes,
respectively.
0
0
00
bis[3 ,5 -bis(2 -oxymethylnaphthyl)benzyloxy]benzyl bromide
800 mg; 0.79 mmol), potassium carbonate (260 mg; 1.90
mmol) and [18]crown-6 (40 mg; 0.15 mmol) in dry acetone
80 ml). Purification by column chromatography (SiO ; di-
(
Continuous irradiation experiments were performed by a
medium pressure mercury lamp. An interference filter (Oriel)
(
was used to select a narrow spectral range with l
=
2
max
ꢁ
5
chloromethane) yielded a yellowish solid (260 mg; 33%).
TLC (SiO ): R = 0.6 (dichloromethane).
H NMR (400 MHz, CDCl , 25 1C): d [ppm] 4.96 (s, 12H,
Ar-CH O-Ar), 5.17 (s, 16H, naph-CH
2.3 Hz, Ar-H), 6.59 (d, 4H, JHH = 2.3 Hz, ar-H), 6.65 (t,
313 nm. The irradiated solution (3 ml, 2 ꢂ 10 M) was
2
f
contained in a spectrophotometric cell. The intensity of the
ꢁ8
1
ꢁ1
3
incident light (8.3 ꢂ 10 einstein min at 313 nm), measured
26
4
2
2
O-ar), 6.55 (t, 2H, JHH
by the ferrioxalate actinometer,
experiments.
was the same in all
4
=
4
4
4
H, JHH = 2.2 Hz, Ar-H), 6.71 (d, 8H, JHH = 2.2 Hz, Ar-
The estimated experimental errors are: ꢅ2 nm on the band
maximum, 5% on the molar absorption coefficient, 10% on
the fluorescence and photoreaction quantum yield, 5% on the
lifetime.
3
H), 6.96 (AA‘BB’, 4H, JHH = 9.0 Hz), 7.44–7.50 (m, 24H,
3
naph-H), 7.78–7.84 (m, 32H, naph-H) 7.90 (AA‘B’, 4H, JHH
= 9.0 Hz).
13
C NMR (100.6 MHz, CDCl , 25 1C): d [ppm] 70.05, 70.12,
3
7
1
1
0.29 (CH O), 101.80, 101.85, 106.43, 106.53, 115.16, 125.29,
2
26.09, 126.25, 126.38, 126.56, 127.75, 127.97, 128.41, 132.37,
33.11, 133.32, 134.28, 138.29, 139.24, 160.17, 160.27, 163.85
Electrochemical and spectroelectrochemical measurements.
(
Car), 193.37 (ketone-C).
Electrochemical experiments were carried out in argon-purged
MALDI-TOF-MS: Matrix: DHB; m/z (%) = 2119.6
"
[M + Na] , 100)
2 2
CH Cl (Romil Hi-Dryt) solutions at room temperature with
(
an EcoChemie Autolab 30 multipurpose instrument interfaced
to a personal computer. The working electrode was a glassy
2
carbon electrode (0.08 cm , Amel); the counter electrode was a
C
143 110
H O16: 2096.40.
Photophysical and photochemical experiments
Pt spiral and a silver wire was employed as a quasi-reference
electrode (QRE). The potentials reported are referred to SCE
by measuring the AgQRE potential with respect to ferrocene
The experiments were performed in dichloromethane–chloro-
form 1 : 1 (v/v) mixture, unless otherwise noted. Deaerated
solutions were obtained by three freeze–pump–thaw cycles.
UV-Vis absorption spectra were recorded with a Perkin
Elmer l40 spectrophotometer, using quartz cells with path-
length of 1.0 cm. Emission spectra and phosphorescence life-
(
+0.46 V vs. SCE). The concentration of the compounds
ꢁ
4
examined was of the order of 5 ꢂ 10 M; 0.1 M tetrabutyl-
ammonium hexafluorophosphate (TBAPF ) was added as
supporting electrolyte. Cyclic voltammograms were obtained
6
ꢁ
1
with scan rates in the range 0.05–5 V s . The number of
electrons exchanged in each process was estimated by compar-
ing the current intensity of the corresponding CV wave with
that observed for the monoelectronic oxidation of ferrocene,
after correction for differences in the diffusion coefficients.
The experimental error on the potential values was estimated
to be ꢅ10 mV.
times were obtained with
a
Perkin Elmer LS-50
spectrofluorimeter, equipped with a Hamamatsu R928 photo-
tube. Fluorescence quantum yields were measured following
2
the method of Demas and Crosby (standard used: naphtha-
4
2
5
lene, F = 0.23 in deaerated cyclohexane). Prompt fluores-
cence lifetimes were measured by time-correlated single-
photon counting (0.5 ns time resolution) by an Edinburgh
Instrument equipped with a TCC900 card and a D2 lamp.
Nanosecond transient absorption experiments were performed
in argon purged acetonitrile solutions. A Continuum Surelite
SLI-10 Nd:YAG laser was used to excite the sample with 10-ns
pulses at 355 nm. The monitoring beam was supplied by a Xe
arc lamp, and the signal was detected by a red sensitive
photodiode after passing through a high radiance monochro-
mator. Differential absorption spectra were recorded point-by-
point, while kinetic measurements were made at fixed wave-
length. 32 individual laser shots were averaged to improve the
reliability of each acquisition. Transient absorption decays
were fitted by considering both a first- (intrinsic deactivation)
and a second-order (triplet-triplet annihilation) component,
according to the following equations:
Spectroelectrochemical experiments were performed in an
optically transparent thin layer electrode (OTTLE) cell con-
stituted by two quartz windows, a three-electrode set melt-
sealed in a polyethylene spacer. The three electrodes are: two
ꢁ
1
Pt minigrids (32 wires cm ) as working and counter electro-
des and a silver wire as quasi-reference electrode, positioned
very close to the working electrode to minimize Ohmic resis-
tance. The cell has a sandwich type arrangement. The light is
passed through the central part of the working electrode
minigrid (ca. 80% transmittance) and the light path is of the
order of 100–200 mm. Diffusion of electrogenerated species
between the electrodes is negligible on the experimental time
scale (10–50 s). The solution used for spectroelectrochemical
experiments was prepared as previously described for cyclic
voltammetry. In order to localize the right potential window
corresponding to the redox process of interest, a thin layer
ꢃ
d½ Aꢄ
ꢃ
0 ꢃ
2
ꢃ
ꢁ
¼ k½ Aꢄ þ k ½ Aꢄ ) ½ Aꢄ
ꢁ1
cyclic voltammetry (TLCV) was recorded at 5 mV s before
dt
ꢃ
½
Aꢄ k
the spectroelectrochemistry. After refilling the OTTLE cell
with new solution, the electrolysis potential was varied with
0
kt
¼
ꢃ
ꢃ
0
0
0
ð½ Aꢄ k þ kÞe ꢁ ½ Aꢄ k
0
2
0–50 mV steps and UV-VIS-NIR absorption spectra were
where [*A] is the concentration of excited state (*A) at zero
0
recorded with an Agilent Technologies 8543 diode array
spectrophotometer.
0
time, when the laser pulse of excitation is switched on, k and k
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Acknowledgements
8 (a) S. Jordens, G. De Belder, M. Lor, G. Schweitzer, M. Van der
Auweraer, T. Weil, E. Reuther, K. Mu
Photochem. Photobiol. Sci., 2003, 2, 177; (b) U. Hahn, M. Gorka,
F. Vogtle, V. Vicinelli, P. Ceroni, M. Maestri and V. Balzani,
Angew. Chem., Int. Ed., 2002, 41, 3595; (c) V. Vicinelli, P. Ceroni,
M. Maestri, V. Balzani, M. Gorka and F. Vogtle, J. Am. Chem.
¨
llen and F. C. De Schryver,
We would like to thank Susanna Bazzani, Giacomo Bergami-
ni, and Dr Stephan Bitter for useful discussions. This work has
been supported by FIRB (Manipolazione molecolare per
macchine nanometriche) and by the ‘‘Fonds der Chemischen
Industrie’’, for which we are very grateful.
¨
¨
Soc., 2002, 124, 6461; (d) M.-S. Choi, T. Aida, T. Yamazaki and I.
Yamazaki, Chem. Eur. J., 2002, 8, 2668; (e) J. M. Serin,
D. W. Brousmiche and J. M. J. Fre
605.
(a) M.-H. Xu, J. Lin, Q.-S. Hu and L. Pu, J. Am. Chem. Soc., 2002,
24, 14239; (b) V. J. Pugh, Q. S. Hu, X. Zuo, F. D. Lewis and L.
Pu, J. Org. Chem., 2001, 66, 6136; (c) V. Balzani, P. Ceroni, S.
´
chet, Chem. Commun., 2002,
2
9
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6
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1
258 | New J. Chem., 2007, 31, 1250–1258
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