Photochemical and photophysical properties of a poly(propylene amine) dendrimer functionalised with E-stilbene units

Article abstract of DOI:10.1039/b404463k

A second generation poly(propylene amine) dendrimer (2) functionalised at the periphery with eight E-stilbene and eight 4-tert-butylbenzenesulfonyl units has been prepared. The absorption spectrum, fluorescence spectrum and decay, E ? Z photoisomerization, and photocyclization of the Z-isomer of the stilbene units have been investigated in air equilibrated acetonitrile solutions. For comparison purposes, a reference compound of the peripheral dendrimer units, namely 4-tert-butyl-N-propyl-N-(4-styryl-benzyl)-benzenesulfonamide (1), has also been studied. The quantum yield of the E→Z photoisomerization reaction (0.30) and the fluorescence quantum yield of the E isomer (0.014) are substantially smaller for the units appended to the dendrimer compared to those of the reference compound 1 (0.50 and 0.046, respectively). The presence of a red tail and the biexponential decay of the emission band of the dendrimer indicate formation of excimers between the stilbene units appended at the polypropylene amine) dendritic structure. Under the experimental conditions used (λ_exc = 313 nm), a Z/E photostationary state (around 9:1 for both reference compound 1 and dendrimer 2) is reached in the time scale of minutes. On continuing irradiation, other photoreactions take place in the time scale of hours: the stilbene moiety of compound 1 undergoes photocyclization to phenanthrene (quantum yield 0.015), whereas in dendrimer 2 photocyclization to phenanthrene is accompanied by other processes, including a photoreaction involving the internal amine groups.

Full text of DOI:10.1039/b404463k

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A R T I C L E
Photochemical and photophysical properties of a poly(propylene
amine) dendrimer functionalised with E-stilbene units†
Veronica Vicinelli,a Paola Ceroni,a Mauro Maestri,a Mariachiara Lazzari,a
Vincenzo Balzani,*a Sang-Kyu Lee,b Jeroen van Heystb and Fritz Vögtle*b
a
Dipartimento di Chimica “G. Ciamician”, Università di Bologna, via Selmi 2, I-40126 Bologna,
Italy
b
Kekulé-Institut für Organische Chemie und Biochemie der Universität Bonn,
Gerhard-Domagk Strasse 1, D-53121 Bonn, Germany
Received 26th March 2004, Accepted 27th May 2004
First published as an Advance Article on the web 12th July 2004
A second generation poly(propylene amine) dendrimer (2) functionalised at the periphery with eight E-stilbene and eight
4-tert-butylbenzenesulfonyl units has been prepared. The absorption spectrum, fluorescence spectrum and decay, E  Z
photoisomerization, and photocyclization of the Z-isomer of the stilbene units have been investigated in air equilibrated
acetonitrile solutions. For comparison purposes, a reference compound of the peripheral dendrimer units, namely 4-tert-
butyl-N-propyl-N-(4-styryl-benzyl)-benzenesulfonamide (1), has also been studied. The quantum yield of the E→Z
photoisomerization reaction (0.30) and the fluorescence quantum yield of the E isomer (0.014) are substantially smaller for
the units appended to the dendrimer compared to those of the reference compound 1 (0.50 and 0.046, respectively). The
presence of a red tail and the biexponential decay of the emission band of the dendrimer indicate formation of excimers
between the stilbene units appended at the poly(propylene amine) dendritic structure. Under the experimental conditions used
(exc = 313 nm), a Z/E photostationary state (around 9:1 for both reference compound 1 and dendrimer 2) is reached in the
time scale of minutes. On continuing irradiation, other photoreactions take place in the time scale of hours: the stilbene moiety
of compound 1 undergoes photocyclization to phenanthrene (quantum yield 0.015), whereas in dendrimer 2 photocyclization
to phenanthrene is accompanied by other processes, including a photoreaction involving the internal amine groups.
-cyclodextrin8 and cucurbit[8]uril,9 or when two stilbene-like
units are self-assembled in pseudorotaxane-type structures10 or
in hydrogen-bonded tape architectures.11 Z-stilbene undergoes
either photoisomerization to the E-isomer (a, Scheme 1) or
photocyclization to dihydrophenanthrene12 which can revert to the
Z-isomer but, in the presence of oxygen, is irreversibly transformed
into phenanthrene13 (c, Scheme 1). Photocyclization of Z-stilbenes is
indeed a valuable synthetic route for generation of phenanthrenes.
Introduction
Stilbene, which can exist as an E (lower energy) and Z (higher
energy) isomers (a, Scheme 1), is one of the most interesting
molecules from a photochemical and photophysical viewpoint.1
As early as 1931, Olson2 discussed the mechanism of the Z/E (at
that time, called cis/trans) photoisomerization and in 1968 Wald3
discussed the molecular basis of visual excitation demonstrating
the involvement of geometrical isomerization. A vast amount of
experimental results has since been gathered concerning the photo-
chemical and photophysical behavior of stilbene and stilbene-like
molecules under a variety of experimental conditions.1 Among
the other points of interest of stilbene photochemistry, it should
be recalled that it provides a classic example of the fact that light
excitation can allow the selective conversion of a lower energy (E)
to a higher energy (Z) isomer.
The photophysical and photochemical events that can take
place upon light excitation of stilbene can be summarized as
follows. In solution at ambient temperature E-stilbene and some
of its derivatives show a relatively strong fluorescence in the range
340–400 nm, whereas the fluorescence of the Z isomer is extremely
weak.1d,f The singlet excited state of E-stilbene can interact with
amines to give exciplexes.4 Formation of exciplexes causes a
quenching of the E-stilbene fluorescence and the appearance of
a broad emission in the 400–500 nm region.4a Excimers are also
formed as precursors of the photocyclodimerization of E-stilbene
and excimer emission has also been reported.5
Besides photochemical E  Z photoisomerization (a, Scheme 1),
other photochemical reactions can be obtained upon irradiation of
stilbene. E-stilbene can undergo photodimerization (b, Scheme 1),
a reaction first observed by Ciamician and Silber.6 This photo-
reaction, which originates from the singlet excited state, requires
very high concentration of E-stilbene, but in water, where stilbene
molecules are partly associated, photodimerization can be observed
also in diluted solution (1 M).7 The extent of dimerization is
considerably high when the reaction takes place in the cavity of
Scheme 1 (a) Photoisomerization of E- and Z-stilbene; (b) photocyclo-
dimerization of E-stilbene; (c) photocyclization of Z-stilbene to dihydro-
phenanthrene followed by oxidation to phenanthrene.
Because of their very interesting photochemical and photo-
physical properties, stilbenoid compounds have found several
applications in material science,1d e.g., for optical brighteners,14
light-emitting diodes,15 functionalization of surfaces,16 photo-
polymerizable materials,17 nonlinear optics,18 and photoresponsive
hybrid materials.19 Recently, stilbene has also been used in the field
of molecular machines20 and to investigate antibody-based photo-
chemical sensors for diagnostic and clinical applications.21
† Dedicated to Jean-Pierre Sauvage on the occasion of his 60th birthday.
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 2 0 7 – 2 2 1 3
2 2 0 7

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Table 1 Spectroscopic, photophysical and photochemical data (acetonitrile solution, 298 K)
Emissiona
Absorption
max/nm
Photoreactionb

/M−1 cm−1

max/nm


E→Z
%Zc
E-stilbene
E-1
E-2
294, 306
298, 310
300, 312
27100, 26200
31400, 31900
202400, 198100
352
0.031
0.046
0.014
<200 psd
<200 ps
<200ps, 3.5 ns
0.51
0.50
0.30
92
90
90
353
361, 410e
a ex= 291 nm. b irr= 313 nm. cAt the pseudo-photostationary state. d 70 ps in n-pentane or hexane.35 e See text and Fig. 2.
In the last few years, great attention has been devoted to
dendrimers, tree-like macromolecules with a well defined
Results and discussion
We have investigated the absorption and emission spectra, and
photochemical reactivity of E-stilbene (Scheme 1), E-1 reference
compound, and E-2 dendrimer (Scheme 2) in acetonitrile solution
at 298 K. The results are illustrated and discussed in the following
sections. The most important data have been gathered in Table 1.
composition, a high degree of order, and the possibility to contain
selected chemical units at predetermined sites of their structure.22
Particularly interesting dendrimers are those containing photoactive
units.23 A great number of photoluminescent dendrimers have been
investigated, in an attempt to construct antenna systems that might
be useful for solar energy conversion as well as for other purposes,
such as signal amplification in luminescence sensors,24–26 and
spectral energy concentrator (‘molecular lens’).27 Less attention
has been devoted to photoreactive dendrimers, with the exception
of species containing photoisomerizable azobenzene unit(s) in the
core,28 branches,29 or appended at the periphery.30 Since stilbene
is at the same time a luminescent and photoreactive unit, its
incorporation in dendrimer structures should result in interesting
functional compounds. Recently, several dendrimers with a stilbene
core have been investigated31 and a few other studies on stilbene
containing dendrimers have appeared.32–34
Absorption and emission spectra
The UV absorption spectrum of E-stilbene in n-pentane is known to
exhibit two absorption bands, one with maximum near 230 nm, and
the other one with two peaks at 292 and 306 nm.1f The absorption
spectra of E-stilbene, E-1 reference compound, and dendrimer
E-2 in acetonitrile solution are shown in Fig. 1. As one can see,
the spectra of E-1 and E-2 are slightly red-shifted compared to E-
stilbene, presumably because of the presence of a substituent in the
para position.1b Compared with the spectrum of eight E-1 units, E-2
shows somewhat smaller  values, a result that could be attributed
to incomplete functionalization. The spectrum of E-2, however,
differs from that of E-1 also because of the presence of a weak
tail at low energy, suggesting that the observed spectral differences
can be due to the fact that in E-2 the stilbene units ‘feel’ a different
environment since they can interact with other components of the
dendritic structure.
In this paper we have investigated the properties of a second
generation poly(propylene amine) dendrimer (POPAM) function-
alized at the periphery with eight E-stilbene units (2, Scheme 2).
For comparison purposes, a model compound of the peripheral
dendrimer units, namely 4-tert-butyl-N-propyl-N-(4-styryl-
benzyl)benzenesulfonamide (1, Scheme 2), has also been studied.
Scheme 2 Structure formulas of the E-form of 4-tert-butyl-N-propyl-N-(4-stiryl-benzyl)-benzenesulfonamide (E-1), and of the second generation
poly(propylene amine) dendrimer functionalized at the periphery with eight E-stilbene and eight 4-tert-butylbenzenesulfonyl units (E-2).
2 2 0 8
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 2 0 7 – 2 2 1 3

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to the interaction between naphthalene units. In such compounds
we also found that, by addition of trifluoroacetic acid, protonation
of the amine groups caused, as expected, the disappearance of the
charge transfer/exciplex emission and a strong increase in the
monomer emission.36,37 In the case of E-2, which contains 6 amine
units, we have found that addition of an excess (30 equivalents per
dendrimer) of trifluoroacetic acid has no effect on the emission
spectrum. Therefore, we conclude that the broad and weak band
with maximum around 410 nm (Fig. 2, inset) is not related to the
presence of charge-transfer/exciplex species involving amines and
can therefore be assigned to dimer/excimer stilbene-based species.
We can also conclude that the protonation of the amine groups of
the dendrimer does not cause structural rearrangements capable of
affecting the interactions between stilbene units.
Fig. 1 Absorption spectra of E-stilbene (solid line), E-1 reference
compound (dotted line) and E-2 dendrimer (dashed line) in acetonitrile
solutions at 298 K.
Photoisomerization reactions
E-stilbene is known to exhibit a fluorescence band with

max = 352 nm,  = 0.035, and  = 70 ps in n-pentane or hexane.35
As mentioned in the introduction, stilbene and stilbenoid
compounds undergo a variety of photochemical processes. We
have investigated the photochemical behavior of stilbene, reference
compound E-1, and dendrimer E-2 in acetonitrile solution at 298 K.
Some relevant data are gathered in Table 1.
The emission spectra in acetonitrile solution of the examined
compounds are shown in Fig. 2 and some relevant data are gathered
in Table 1. E-stilbene shows an emission band with maximum at
352 nm,  = 0.031, and lifetime too short (<200 ps) to be measured
with our equipment. The fluorescence band of the E-1 reference
compound (max = 353 nm) is similar to that of E-stilbene, but
somewhat more intense ( = 0.046); the excited state lifetime is
again too short (<200 ps) to be measured. The emission spectrum
of dendrimer E-2 is slightly red-shifted (max = 361 nm), much
less intense ( = 0.014), and exhibits a long tail at low energy.
Subtraction of the spectrum of E-1 from that of E-2 (after normal-
ization at 353 nm) yields a broad and weak band with max around
410 nm (Fig. 2, inset) that can be assigned to the emission of
stilbene units perturbed by intradendritic interactions. The relatively
long emission decay (3.5 ns) that can be observed in the case of E-2
is indeed consistent with the presence of another emitting species
besides unperturbed stilbene units, whose lifetime is shorter than
200 ps. It has been reported that interaction of stilbene with amines
gives rise to a broad and weak emission band in the 450–550 nm
region with lifetime in the nanosecond time scale.4a Since several
amine groups are present in the E-2 dendritic structure, the broad
and weak band observed at 410 nm might thus be due to exciplexes
or ground state charge-transfer complexes involving amines. An
alternative assignment could involve excimers or stilbene dimers.
As we have seen above, the lower intensity of the absorption band
around 300 nm and the slight red shift for E-2 compared with E-1
is indeed consistent with the presence of ground state interactions.
Excitation spectrum of E-2, collected at 500 nm, matches its absorp-
tion spectrum, thus ruling out the presence of impurity quenching of
the stilbene emission and definitively assigning the long emission
decay to intradendritic interactions of peripheral stilbene units.
It is well known1 that E-stilbene undergoes an E→Z photo-
isomerization upon excitation with UV light. The spectral changes
observed on irradiating with 313 nm light in acetonitrile solutions
of E-stilbene, reference compound E-1, and dendrimer E-2 are
shown in Fig. 3. Clearly, the E→Z photoisomerization reaction is a
clean process for each one of the three compounds until a pseudo-
photostationary state is reached (see also below). From the known
spectrum of Z-stilbene (Fig. 3a), it can be calculated that in the case
of stilbene such a pseudo-photostationary state is reached when
92% of the E-isomer has been converted into the Z one. Since the
pure forms of Z-1, and Z-2 were not available, we have used the
spectrum of Z-stilbene to estimate the composition of the pseudo-
photostationary state for compounds 1 and 2, obtaining a Z/E ratio
about 9:1 in both cases. For each compound, the quantum yield of
the E→Z photoisomerization reaction was measured at very low
conversion ratios (see Experimental). The value obtained for the
reference compound E-1 (0.50) is substantially the same as that
obtained for E-stilbene (0.51), whereas that of dendrimer E-2 (0.30)
is considerably smaller. The lower quantum yield of isomerization
found for dendrimer E-2 could be due to steric crowding, but it
seems more likely related to the occurence of excited state deacti-
vation via excimer formation, as shown by the fluorescence results.
In stilbene-containing monolayers and vesicles5a and DNA stilbene
systems5b it has been found that excimer formation is accompanied
by [2 + 2] dimerization (Scheme 1b). However no evidence for such
a reaction has been found for E-2. Apparently, in our dendrimer, the
excimer concentration is very low and/or the excimer cannot reach
a structure useful for dimerization.
Photocyclization reaction of the Z-isomers
As we have seen above, irradiation with 313 nm light of E-stil-
bene, E-1, and E-2 led, in less than 10 minutes, to a pseudo-photo-
stationary state in which the two isomers are present in a 9:1 Z/E
ratio (Table 1). On a much longer time scale, however, continuous
irradiation with 313 nm light caused other spectral changes, as
illustrated in Fig. 4.
In the case of stilbene, the absorption (Fig. 4a) and emission
spectra (Fig. 4a, inset) obtained after 10 hours of irradiation showed
the characteristic bands of phenanthrene. This result indicates the
occurrence of the well known photocyclization of the Z-isomer
to dihydrophenanthrene,12 followed by spontaneous oxidation
to phenanthrene (Scheme 1c).13 The photocyclization reaction is
very clean and takes place with a low quantum yield (0.015). The
absorbance values indicate that under the experimental conditions
used around 90% of stilbene was converted to phenanthrene; com-
plete photocyclization would probably occur on a much longer time
scale because of the very low quantum yield.
Fig. 2 Emission spectra (exc =291 nm) of E-stilbene (solid line), E-1
reference compound (dotted line), and E-2 dendrimer (dashed line) in
acetonitrile solutions at 298 K. The inset shows the spectrum obtained after
subtraction of the normalized spectrum of E-1 from that of E-2.
In previous investigations we have found that dendrimers
containing propylene amine36 or cyclam37 units functionalized
at the periphery with naphthalene show three different types of
emission bands, namely ‘monomer’ emission, charge transfer/
exciplex emission deriving from interaction of the naphthalene units
with the internal amine groups, and dimer/excimer emission due
Fig. 4b shows that, under the experimental conditions used, a
clean photocyclization reaction occurs also in the case of reference
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 2 0 7 – 2 2 1 3
2 2 0 9

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Conclusions
We have investigated the spectroscopic, photophysical, and
photochemical properties of stilbene, reference compound 1, and
dendrimer 2 in acetonitrile solution at ambient temperature. The
results obtained show that 1 behaves like stilbene, whereas the
behaviour of the stilbene units of dendrimer 2 substantially differs
from that of their model compound 1. Such a ‘dendrimer effect’38
shows up as follows: (i) small differences in the absorption spectrum
of the E-stilbene units, caused by interactions among contiguous
unitsinthedendriticstructure;(ii)strongdecreaseinthefluorescence
quantum yield of the E-stilbene units and appearance of a broad and
weak emission at lower energies, assigned to the formation of E-
stilbene excimers; (iii) strong decrease in the quantum yield of the
E→Z photoisomerization reaction, presumably because of excited
state deactivation via excimer formation; (iv) lower quantum yield
of the photocyclization process of the Z-isomer to phenanthrene; (v)
occurrence of secondary reactions, one of which involves the inter-
nal amine groups of the dendrimer, during the slow photocyclization
process. Finally, there is no evidence that the dendritic architecture
favors a [2 + 2] dimerization between E-stilbene units.
Fig. 3 Spectral changes observed on a short time scale (min) upon
irradiation of acetonitrile solutions of E-stilbene (a), E-1 (b) and E-2 (c)
with 313 nm light. The concentration of stilbene units was the same
in all cases (3.05 × 10−5 M). The solid and dotted curves represent the
spectra of the solutions before and after (photostationary state) irradiation,
respectively. The dashed line in (a) represents the absorption spectrum of
pure Z-stilbene.
Experimental
Synthesis of N-propyl-(4-tert-butylbenzenesulfonyl)amide
To the mixture of 1-propylamine (400 mg, 6.8 mmol) and triethyl-
amine (830 mg, 8.2 mmol) in chloroform 4-tert-butylbenzene-
sulfonyl chloride (1.6 g, 6.8 mmol) in chloroform was added
dropwise. After stirring for 24 h at RT the mixture was washed with
water, aq. NaHCO3 and again with water. The organic phase was
dried with Na2SO4 and the solvent was removed in vacuo. Further
purification was achieved by column chromatography (SiO2,
40–60 nm, dichloromethane/methanol: 40/1) yielding 1.5 g (87%)
of a colourless solid. 1H NMR: (400 MHz, CDCl3, 25 °C),  [ppm] =
3
0.87 (t, 3H, JHH = 7.45 Hz, CH2CH2CH3), 1.34 (s, 9H, C(CH3)3),
3
3
1.50 (sext, 2H, JHH = 7.33 Hz, JHH = 7.20 Hz, CH2CH2CH3),
2.91(q, 2H, 3JHH = 7.07 Hz, 3JHH 6.32 Hz, NHCH2CH2CH3), 4.59 (t,
1H, 3JHH = 6.19 Hz, NHCH2), 7.51 (AA′BB′, 2H, 3JHH = 8.72 Hz),
7.79 (AA′BB′, 2H, 3JHH = 8.84 Hz). 13C NMR: (100.6 MHz, CDCl3,
25 °C),  [ppm] = 11.10 (CH2CH2CH3), 23.00 (CH2CH2CH3), 31.10
(C(CH3)3), 35.13 (C(CH3)3), 44.99 (CH2CH2CH3), 126.04, 126.92

(CAr), 137.03, 156.31 (CqAr). EI-MS: m/z (%): 255.1 (M + H , 20).
C13H21NO2S: (255.38).
Synthesis of the E form of 4-tert-Butyl-N-propyl-N-(4-styryl-
benzyl)benzenesulfonamide (E-1)
Fig. 4 Spectral changes observed on a long time scale (10 h) upon
irradiation of acetonitrile solutions of E-stilbene (a), E-1 (b) and
E-2 (c) with 313 nm light. The solid and dotted curves represent the
initial (photostationary state of the E→Z isomerization) and final (end of
irradiation) spectra, respectively. The insets show the emission spectra of
the solutions at the end of irradiation.
N-Propyl-(4-tert-butylbenzenesulfonyl)amide (200 mg, 0.78 mmol)
and potassium carbonate (330 mg, 2.34 mmol) was dissolved in
100 ml acetonitrile. Under reflux 4-bromomethyl-stilbene (260 mg,
0.94 mmol) was added dropwise and the mixture was stirred for
3 d. After filtering the undissolved potassium carbonate, the solvent
was removed in vacuo and the residue was collected in chloroform.
After washing with water, aq. NaHCO3 and again with water the
organic phase was dried with Na2SO4 and the solvent was removed
again in vacuo. Further purification was achieved by column
chromatography (SiO2, 40–60 nm, dichloromethane/n-hexane: 2/1,
compound 1, with the same quantum yield as for stilbene. The
behaviour of dendrimer 2, however, is substantially different
(Fig. 4c). Photocyclization to phenanthrene does take place, as
shown by the increase in absorption at 250 nm and by the appear-
ance of the characteristic phenanthrene emission band (Fig. 4c,
inset), but the lack of isosbestic points shows that other processes
take place besides photocyclization. In order to understand whether
the dendrimer amine groups play some role in determining the
observed spectral changes, we have protonated E-2 by addition of
30 eqs of trifluoroacetic acid before irradiation. Under such condi-
tions, the formation of the absorption tail above 330 nm was no
longer observed upon prolonged irradiation, showing that, in the
unprotonated form, there is indeed a reaction involving the amine
groups. It is not clear, however, whether such a reaction takes place
from a Z-stilbene or a phenanthrene excited state. The complex-
ity of the reactions taking place upon prolonged irradiation of 2 is
also shown by the lack of isosbestic points even in the cases of the
acidified solutions, and the presence of a red tail around 450 nm in
the emission spectrum of the photoproducts (Fig. 4c, inset).
1
1% Et3N) yielding 300 mg (86%) of a colourless solid. H NMR:
3
(400 MHz, CDCl3, 25 °C),  [ppm] = 0.74 (t, 3H, JHH = 7.33 Hz,
CH2CH2CH3), 1.36 (s, 9H, C(CH3)3), 1.39 (sext, 2H, 3JHH = 7.33 Hz,
3
3JHH = 7.58 Hz, CH2CH2CH3), 3.09(t, 2H, JHH = 7.58 Hz,
NHCH2CH2CH3), 4.34 (s, 2H, Ar–CH2N), 7.09 (s, 2H, CHCH),
7.24–7.27 (m, 4H, Ar–H), 7.36 (t, 2H, 3JHH = 7.71 Hz, Ar–H), 7.44
3
(d, 2H, JHH = 8.31 Hz, Ar–H), 7.50–7.54 (m, 4H, Ar–H), 7.77
3
(AA′BB′, 2H, JHH = 8.71 Hz). 13C NMR: (100.6 MHz, CDCl3,
25 °C),  [ppm] = 11.16 (CH2CH2CH3), 21.61 (CH2CH2CH3),
31.14 (C(CH3)3), 35.14 (C(CH3)3), 50.09 (CH2CH2CH3), 51.80 (Ar–
CH2N), 126.03, 126.53, 126.61, 127.03, 127.70, 128.19 (CHCH),
128.63, 128.71, 128.92 (CHCH), 136.14, 136.84, 137.17, 137.28,

156.17 (CAr). FAB-MS: m/z (%): 447.2 (M + H , 30). GC-MS:

Rt = 20.99 min; m/z (%) = 447 (M ). C28H33NO2S: (447.63).
2 2 1 0
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 2 0 7 – 2 2 1 3

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Synthesis of the sulfonamide-dendrimer 8-Cascade: 1,2-
diaminoethane[4-N,N,N′,N′]:(1-azabutylidene)2:N-[1-
thiodioxo-4-tert-butylbenzene] (4, Scheme 3)
42.7, 52.6, 126.2, 127.1, 137.0, 156.1. FAB-MS: m/z (%): 2316.1
(M + H , 20). C118H188N14O16S8: (2315.3).

Synthesis of the E form of 8-Cascade: 1,2-diaminoethane[4-
N,N,N′,N′]:(1-azabutylidene)2:N-(1-benzyl-2-phenylethylene)-
N-[1-thiodioxo-4-tert-butylbenzene] (E-2, Scheme 3)
The starting oligopropyleneamine(POPAM)dendrimer-(NH2)8 (3,
Scheme 3) and 10 equivalents of triethylamine were dissolved in
50 ml of dry dichloromethane. To a refluxing mixture, a solution
of 8 equivalents of 4-tert-butylbenzenesulfonyl chloride in 50 ml
of dichloromethane was added dropwise. After stirring for four
days under reflux the solvent was removed in vacuo and the
residue was collected in dichloromethane. After washing with
water, aq. Na2CO3 and again with water the organic phase was
dried with Na2SO4. The solvent was removed in vacuo yield-
The monosubstituted POPAM-dendrimer (4) and 1.5-fold excess
of potassium carbonate were dissolved in 50 ml of acetonitrile. To
this suspension 4-bromomethyl-stilbene (20% excess) in 50 ml
of acetonitrile was added dropwise. The mixture was stirred for
three days under reflux. After filtering the undissolved potassium
carbonate, the solvent was removed in vacuo and the residue
was collected in dichloromethane. After washing with water, aq.
Na2CO3 and again with water, the organic phase was dried with
Na2SO4. Further purification was achieved by column chromato-
graphy (SiO2, 40–63 m, CH2Cl2/ MeOH/ Et3N: 100/1/1) yield-
1
ing a yellow solid (90%). H NMR: (250 MHz, CDCl3, 25 °C),
 [ppm] = 1.30 (s, 72 H, CH3), 1.36 (br, 24 H, CH2CH2CH2), 2.44
(br, 36 H, NCH2), 2.92 (br, 16 H, SO2NHCH2), 7.46 (d, 16 H,
3
3JHH = 8.5 Hz, Ar–H), 7.76 (d, 16 H, JHH = 8.3 Hz, Ar–H). 13C
1
ing a yellow solid (29%). H NMR: (400 MHz, CDCl3, 25 °C),
NMR: (60.9 MHz, CDCl3, 25 °C),  [ppm] = 25.9, 31.3, 35.2,
Scheme 3 Schematic representation of the synthesis of E-2.
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 2 0 7 – 2 2 1 3
2 2 1 1

View Article Online
10 D. G. Amirsakis, A. M. Elizarov, M. A. Garcia-Garibay, P. T. Glink,
 [ppm] = 1.25 (s, 72 H, CH3), 1.96 (s, br, 24 H, CH2CH2CH2), 2.64
(br, 36 H, NCH2), 3.15 (br, 16 H, SO2NHCH2), 4.12 (br, 16H, Ar-
CH2), 6.80–7.52 (m, 104 H, Ar–H, CH2CH2), 7.70 (m, 16 H, Ar–
H). 13C NMR: (100.6 MHz, CDCl3, 25 °C),  [ppm] = 31.1, 35.2,
47.0, 53.1, 56.5, 62.4, 126.5, 126.7, 126.8, 127.3, 127.8, 128.8,
129.6, 127.3, 135.6, 136.6, 137.0, 156.1. MALDI-TOP-MS: m/z
J. F. Stoddart, A. J. P. White and D. J. Williams, Angew. Chem., Int. Ed.,
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
(%): 3294.7 ([C195H249N14O16S8] , 10). C238H284N14O16S8: (3853.4)
Photophysical and photochemical experiments
The photophysical properties (absorption and emission spectra,
emission quantum yields, and excited state lifetimes) have been
investigated in acetonitrile. The equipment used was peviously
described.24 Luminescence quantum yields were measured
following the method described by Demas and Crosby39 (standard
used: naphthalene in cyclohexane,  = 0.23 40). The estimated
experimental errors are: 2 nm on the band maximum, 5% on the
molar extinction coefficient, 10% on the fluorescence quantum
yield, 5% on the fluorescence lifetime.
Photochemical reactions were performed on stirred solutions,
by using a Hanau Q 400 medium pressure Hg lamp (150 W).
Selection of the irradiation wavelength (313 nm) was accomplished
by the use of an interference filter. The number of incident
photons was measured by ferrioxalate actinometry.41 The E→Z
photoisomerization quantum yield (E→Z) and the composition
of the photostationary state of the isomerization reaction were
determined from the change in absorbance at 310 nm, assuming
that the absorption spectrum of the Z isomer of the stilbene units of
1 and 2 is the same as that of Z-stilbene. The estimated error on the
quantum yield and on the composition of the photostationary state
is 10%. The quantum yield of the photocyclization reaction that
converts the Z-stilbene unit of 1 into a phenanthrene unit was es-
timated from the increase in absorbance at 254 nm (phenanthrene
band). In both cases, the quantum yield values observed at low
conversion percentages (<10%) were extrapolated to t = 0 in order
to avoid interference from light absorption by the photoproducts.
24 F. Vögtle, S. Gestermann, C. Kauffmann, P. Ceroni, V. Vicinelli and
V. Balzani, J. Am. Chem. Soc., 2000, 122, 10398.
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Photochem. Photobiol. Sci., 2002, 1, 45; (b) A. Dirksen, E. Zuidema,
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T. Mizutani, J. Hayakawa, A. Momotake, M. Ikegami, R. Nagahata
and T. Arai, Photochem. Photobiol., 2002, 76, 13; (c) J. Hayakawa,
A. Momotake and T. Arai, Chem. Commun., 2003, 94; (d) M. Imai,
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Acknowledgements
This work has been supported in Italy by MIUR (Supramolecular
Devices Project), University of Bologna (Funds for Selected
Topics). We would also like to acknowledge the generous support
from the European LIMM (Light Induced Molecular Motion, IST-
2001-35503) and SUSANA (SUpramolecular Self-Assembly of
Nano Structures, HPRN-CT-2002-00185) projects. We are also
grateful to COST D11/0007 for support.
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