Toward photoswitchable dendritic hosts. Interaction between azobenzene- functionalized dendrimers and eosin

Article abstract of DOI:10.1021/ja9822409

Two poly(propyleneimine) dendrimers bearing up to 32 photoisomerizable azobenzene groups in the periphery have been used as potential hosts for eosin Y (2',4',5',7'-tetrabromofluorescein dianion). The all-E azobenzene dendrimers can be reversibly switched

Full text of DOI:10.1021/ja9822409

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J. Am. Chem. Soc. 1998, 120, 12187-12191
12187
Toward Photoswitchable Dendritic Hosts. Interaction between
Azobenzene-Functionalized Dendrimers and Eosin
Andreas Archut,1a Gianluca Camillo Azzellini,1b,c Vincenzo Balzani,*,1b
Luisa De Cola,1b and Fritz Vo1gtle*,1a
Contribution from the Kekule´-Institut fu¨r Organische Chemie und Biochemie der UniVersita¨t Bonn,
Gerhard-Domagk Strasse 1, D-53121 Bonn, Germany, and Dipartimento di Chimica “G. Ciamician”,
UniVersita` di Bologna, Via Selmi 2, I-40126 Bologna, Italy
ReceiVed June 26, 1998
Abstract: Two poly(propyleneimine) dendrimers bearing up to 32 photoisomerizable azobenzene groups in
the periphery have been used as potential hosts for eosin Y (2′,4′,5′,7′-tetrabromofluorescein dianion). The
all-E azobenzene dendrimers can be reversibly switched to their Z form by light excitation. Both the E and
Z forms of the dendrimers quench the eosin fluorescence by a static mechanism. The quenching is most
likely due to an electron-transfer reaction between the singlet excited state of eosin and the tertiary amine
units present along the branches of the dendrimers. Quenching by the Z form of the dendrimers is more
efficient than quenching by the E form. The E f Z and Z f E photoisomerization reactions of the azobenzene
units of the dendrimers are sensitized by eosin via a triplet-triplet energy transfer mechanism. The results
obtained indicate that eosin is hosted by the dendrimers and suggest that the Z forms are more efficient hosts
than the E forms.
Introduction
characterization of dendrimers containing light switchable
units.9
Cascade molecules,2 nowadays commonly called dendrimers,3
are well-defined, highly branched macromolecules constructed
from an initiator core upon which radially branched layers,
termed generations, are covalently attached. Potentially im-
portant practical applications of dendrimers are related to the
possibility of encapsulating guest molecules.4 Examples of
dynamic5 and static4 guest encapsulation have already been
reported. In particular, Meijer et al.4,6 have shown that when
poly(propyleneimine) dendrimers bearing a bulky shell of 64
amino acids in the periphery (dendritic boxes) are constructed
in the presence of guest molecules, such molecules can be
irreversibly imprisoned into internal cavities of the dendrimer
and then site-selectively liberated by suitable chemical reac-
tions.7
For practical applications (e.g., drug delivery), a dendritic
box should be opened and closed reversibly by means of a
simple, external stimulus. Light is a particularly useful and
efficient stimulus to cause reversible structural changes in
molecular and supramolecular systems.8 We are therefore
engaged in a research program aimed at the synthesis and
It is well-known that azobenzene-type compounds undergo
an efficient and fully reversible photoisomerization reaction.10
For this reason, they have been extensively used to construct
photoswitchable devices.8,11 We have found that the thermo-
dynamically stable E isomers of the azobenzene groups con-
tained in the periphery of poly(propyleneimine) dendrimers
(para, P, and meta, M, carboxamide substituted; first, G1, and
fourth, G4, generations; Figure 1) are reversibly switched to
the Z form by 313 nm light and can then be converted back to
the E form by irradiation with 254 nm light or by heating.9
Other research groups have investigated the isomerization of
dendrimers containing an azobenzene as the central linker.12
Isomerization of azobenzene units involves a large structural
rearrangement (Figure 2a). In going from the E to the Z isomer,
the distance between the para carbon atoms of azobenzene
decreases from 9 to 5.5 Å and the dipole moment increases
from 0 (since the E form is planar and symmetric) to 3.0 D.10b
Structural changes in the peripheral units of a dendrimer (Figure
2b) can modify the surface properties and, in large architectures,
can also cause rearrangements in the internal cavities. For all
these reasons, we thought that dendrimers bearings azobenzene
(1) (a) Bonn University. (b) University of Bologna. (c) On leave from
the Instituto de Qu´ımica, Universidade de Sa˜o Paulo, C.P. 26077, Sa˜o Paulo,
Brazil.
(2) Buhleier, E.; Wehner, W.; Vo¨gtle, F. Synthesis 1978, 155.
(3) (a) Newkome, G. R.; Moorefield, C.; Vo¨gtle, F. Dendritic Macro-
molecules: Concepts, Syntheses, PerspectiVes; VCH: Weinheim, 1996. (b)
Tomalia, D. A.; Durst, H. D. Top. Curr. Chem. 1993, 165, 193. (c) Gorman,
C. AdV. Mater. 1998, 10, 295.
(8) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Hor-
wood: Chichester, 1991. Balzani, V.; Scandola, F. In ComprehensiVe
Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D.
D., Vo¨gtle, F., Eds.; Pergamon: Oxford, 1996; Vol. 10, p 687.
(9) Archut, A.; Vo¨gtle, F.; De Cola, L.; Azzellini, G. C.; Balzani, V.;
Ramanujam, P. S.; Berg, R. H. Chem. Eur. J. 1998, 4, 669.
(10) (a) Rau, H. In Photochromism, Molecules and Systems; Du¨rr, H.,
Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; Chapter 4. (b) Kumar,
G. S.; Neckers, D. C. Chem. ReV. 1989, 89, 1915.
(11) (a) Shinkai, S.; Manabe, O. Top. Curr. Chem. 1984, 121, 67. (b)
Shinkai, S. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L.,
Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Eds.; Pergamon: Oxford,
1996; Vol. 1, p 671.
(4) Jansen, J. F. G. A.; de Brabander-van der Berg, E. M. M.; Meijer, E.
W. Science 1994, 266, 1226.
(5) (a) Fre´chet, J. M. J. Science 1994, 263, 1710. (b) Turro, C.; Niu, S.;
Bossmann, S. H.; Tomalia, D. A.; Turro, N. J. J. Phys. Chem. 1995, 99,
5512. (c) Naylor, A. M.; Goddard, W. A., III; Kiefer, G. E.; Tomalia, D.
A. J. Am. Chem. Soc. 1989, 111, 2339.
(6) Jansen, J. F. G. A.; Meijer, E. W. J. Am. Chem. Soc. 1995, 117,
4417.
(7) See also: Miklis, P.; Cagin, T.; Goddard, W. A., III. J. Am. Chem.
Soc. 1997, 119, 7458.
(12) (a) Junge, D. M.; McGrath, D. V. Chem. Commun. 1997, 857. (b)
Jiang, D.-L.; Aida, T. Nature 1997, 388, 454.
10.1021/ja9822409 CCC: $15.00 © 1998 American Chemical Society
Published on Web 12/02/1998

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12188 J. Am. Chem. Soc., Vol. 120, No. 47, 1998
Archut et al.
Figure 1. First (G1) and fourth (G4) generation dendrimers. P and M indicate para and meta carboxamide substitution. The structure of eosin is
also shown.
Figure 2. Photoisomerization of azobenzene and of the fourth generation dendrimers bearing 32 photoisomerizable azobenzene groups in the
periphery.
groups in the periphery could play the role of photoswitchable
hosts.
To investigate this possibility, we have examined the
photochemical and photophysical properties of solutions con-
taining dendrimers and the potential host eosin Y (2′,4′,5′,7′-
tetrabromofluorescein dianion; hereafter simply called eosin,
Figure 1). The reason for choosing eosin was 2-fold: (i) its
strong fluorescence,13 which could be affected in case of

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Toward Photoswitchable Dendritic Hosts
J. Am. Chem. Soc., Vol. 120, No. 47, 1998 12189
inclusion in the dendrimer, and (ii) the energy of its lowest triplet
state,13 which is higher than that of the lowest azobenzene triplet
state14 and could therefore sensitize the photoisomerization
reactions of the peripheral units of the dendrimer.
Experimental Section
All the experiments were carried out in dimethylformamide (DMF)
solution at 293 K, unless otherwise noted. The preparation and
characterization of the dendrimers and the equipment used were
described in a previous paper.9,15
Procedure. To perform quenching and sensitization experiments
and to compare the results obtained with molecules as different as
azobenzene and the fourth generation dendrimers which contain up to
32 azobenzene units, we chose very carefully the experimental
conditions. Limitations were imposed by (i) the need to use the same
irradiation wavelength to minimize experimental errors on the rate of
the photochemical reactions, (ii) a partial overlap between the spectra
of eosin and azobenzene chromophoric groups at the irradiation
wavelength (vide infra), (iii) the need to use spectrophotometric analysis
to follow the photoisomerization reaction,10 and (iv) self-association
of eosin molecules at high concentrations.16 The conditions in which
the experimental errors could be minimized were as follows: eosin
concentration 5.0 × 10-6 M and azobenzene concentration 1.6 × 10-3
M, corresponding to an absorbance of 0.95 at 450 nm. To keep constant
the number of azobenzene units in the experiments involving the G1
and G4 dendrimers, the concentration of the dendrimers was adjusted
so as to have the same absorbance of the 1.6 × 10-3 M azobenzene
solutions. This means that the theoretical15 concentration of the G1
and G4 dendrimers was 4.0 × 10-4 and 5.0 × 10-5 M, respectively.
Correction of eosin fluorescence intensity for inner filter effects was
performed as indicated in the literature.17,18 Irradiation was performed
at 365 nm for the conversion of the E to the Z form and at 545 nm for
the study of the kinetics of the E f Z and Z f E photoisomerization
processes. The reactions were followed by the absorbance changes in
the maximum of the n-π* absorption bands (450 and 435 nm for the
E and Z form, respectively). In the direct photoisomerization experi-
ments, the initial absorbance of the solution was <0.1, so that the
photochemical reactions followed a first-order rate law. The photo-
sensitization of the isomerization reaction by excited eosin was a
pseudo-first-order process since its rate depended on the concentration
of the azobenzene species.
Figure 3. Absorption spectra of DMF solutions containing (a) 5.0 ×
10-5 M E-PG4, (b) 5.0 × 10-6 M eosin, (c) 5.0 × 10-5 M E-PG4 and
5.0 × 10-6 M eosin, and (d) 5.0 × 10-5 M Z-PG4 and 5.0 × 10-6
M
eosin. The inset shows the fluorescence band of solutions containing
eosin alone (curve b) and eosin in the presence of E-PG4 and Z-PG4
(curves c and d, respectively).
Table 1. Quenching of the Eosin Fluorescencea
compd
E form
Z form
azobenzeneb
PG1c
97
92
67
90
100
90
50
PG4d
MG4d
66
a Corrected fluorescence intensities, compared to the fluorescence
intensity of eosin alone taken as 100%. Eosin concentration, 5.0 ×
10-6 M. DMF solution, 293 K. The experimental error is estimated to
be 5%. b 1.6 × 10-3 M. c 4.0 × 10-4 M. d 5.0 × 10-5 M.
carboxamine-substituted azobenzene dendrimer bearing up to
32 azobenzene units in the E form (E-PG4) is shown in Figure
3 (curve a). The band with λmax ) 450 nm (ꢀ ) 1.9 × 104
M-1 cm-1) is due to the nfπ* transition of the azobenzene
units.10 The spectrum of 5.0 × 10-6 M eosin in DMF solution
is shown in Figure 3 (curve b). The intense band in the visible
region (λmax ) 535 nm; ꢀ ) 1.0 × 105 M-1 cm-1) is slightly
affected by concentration because of formation of molecular
aggregates.16 The spectrum of a solution containing both E-PG4
and eosin (Figure 3, curve c) is identical, within experimental
error, to the sum of the spectra of the two separated compounds.
Upon irradiation of the E-PG4 solution with 365 nm light, a
photostationary state is reached in which more than 95% of the
azobenzene groups are in their Z form. The spectrum of a
Experiments on solutions containing concentrations of the various
species different from those stated above were not performed because
of the large experimental errors expected and the impossibility of
making direct comparisons with experiments carried out in the selected
conditions.
solution containing 5.0 × 10-5 M Z-PG4 and 5.0 × 10-6
M
Results and Discussion
eosin (Figure 3, curve d) is again identical, within experimental
error, to the sum of the spectra of the two separated compounds.
Similar results have been obtained in the case of E-MG4 and
Z-MG4. These results show that the interaction between the
two species, if any, is clearly too weak to affect the absorption
spectra.
Absorption Spectra. The absorption spectrum in the visible
region of a 5.0 × 10-5 M solution of the fourth generation para-
(13) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry; Dekker: New Yok, 1993.
(14) Ronayette, J.; Arnaud, R.; Lemaire, J. Can. J. Chem. 1974, 52, 1858.
(15) A certain polydispersity, particularly of the higher generations
originating from imperfections of the polyamine dendrimer skeletons, cannot
be eliminated with the current workup procedures. MALDI-TOF mass
spectroscopy does not show peaks that could be attributed to impurities
originating from structural defects of the polyamine precursor or from
incomplete conversion with the activated esters up to generation 3. In the
case of generation 4 there are additional peaks below the molecule ion peak
that originate from imperfect polyamine starting material, that is dendritic
polyamines with less than 32 propylene amine arms in the periphery. For
the sake of reliability of the photochemical investigations we have thus not
extended our study to generation 5 azobenzene dendrimers. Structural defects
in poly(propyleneimine) dendrimers have been investigated and categorized
recently: Ashton, P. R.; Boyd, S. E.; Brown, C. L.; Nepogodiev, S. A.;
Meijer, E. W.; Peerlings, H. W. I.; Stoddart, J. F. Chem. Eur. J. 1997, 3,
974. Hummelen, J. C.; van Dogen, J. L. J.; Meijer, E. W. Chem. Eur. J.
1997, 3, 1493.
Quenching of Eosin Fluorescence. Eosin displays a strong
fluorescence band in the red region of the spectrum (in water:
λmax ) 570 nm, Φ ) 0.69, τ ) 3.6 ns;10 in DMF: λmax ) 560
nm, τ ) 3.8 ns). The inset to Figure 3 shows the fluorescence
band of solutions containing eosin alone (curve b) and eosin in
the presence of E-PG4 and Z-PG4 (curves c and d, respectively).
Clearly, the intensity of the fluorescence band is much smaller
when the dendrimers are present. When the E-PG4 and Z-PG4
dendrimers were replaced by 32 times more concentrated
azobenzene (to keep the number of azobenzene units ap-
proximately constant), no quenching was observed within the
experimental error. The corrected fluorescence intensity data
obtained for the examined compounds are shown in Table 1.
As one can see, the quenching is smaller for the first generation
dendrimer, whose concentration was 8 times higher than that
(16) Valdes-Aguilera, O.; Neckers, D. C. Acc. Chem. Res. 1989, 22, 171.
(17) Lakowicz, J. H. Principles of Fluorescence Spectroscopy; Plenum
Press: New York, 1986; Chapter 2.
(18) Credi, A.; Prodi, L. Spectrochim. Acta A 1998, 54, 159.

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12190 J. Am. Chem. Soc., Vol. 120, No. 47, 1998
Archut et al.
of the fourth generation compounds. In the assumption of a
dynamic quenching mechanism, we can use the Stern-Volmer
equation19 to evaluate the quenching constants. With use of
the data of Table 1, the measured eosin lifetime, and the
concentration of the fourth generation dendrimers, values in the
range of 5 × 1011 to 5 × 1012 M-1 s-1 are obtained, which are
much higher than the diffusion controlled limit (about 7 × 109
M-1 s-1 for DMF solution at 298 K).13 This, of course, shows
that a dynamic quenching mechanism is implausible. Further-
more, in the case of a dynamic mechanism, the fluorescence
lifetime should be affected in parallel with the fluorescence
intensity,19 but we found that the quenching of the fluorescence
intensity by the fourth generation dendrimers is not accompanied
by any change in the fluorescence lifetime. These results clearly
show that the observed fluorescence quenching does not occur
by diffusion, but by a static mechanism implying association
between the eosin and dendrimer.
The static quenching of the eosin fluorescence intensity in
the presence of the dendrimers could be due either to a change
in the rate constants of the intrinsic radiative and/or nonradiative
deactivation processes of the dye caused by the different
“solvation” enviroment in the dendrimer pockets or to a true
quenching process by energy or electron transfer. In the first
case, however, the fluorescence lifetime would most likely
change, contrary to what we observed. Therefore, we consider
the possibility of a static energy or electron-transfer quenching.
The fluorescent excited state of eosin (whose energy is 2.17
eV)13 lies below the lowest singlet excited state of the azoben-
zene units, as it clearly appears from the absorption spectra
shown in Figure 3. Therefore, quenching of the eosin fluores-
cent singlet excited state by energy transfer to the azobenzene
chromophoric units would imply formation of triplet azoben-
zene, a process which, being spin forbidden for both Coulombic
and exchange energy transfer, is likely too slow to compete
with the short lifetime (3.8 ns) of the excited eosin singlet. On
the other hand, energy transfer to the amine units present in
the branches of the dendrimer is not possible because such units
do not have energy levels which lie below the excited eosin
singlet.
Figure 4. Curve a is the spectrum of a solution containing 5.0 × 10-5
M E-PG4 and 5.0 × 10-6 M eosin. Curve b is the spectrum recorded
after prolonged irradiation of the E form at 365 nm, when the
photostationary state containing >90% Z form is obtained. Curve c is
the spectrum obtained upon excitation of the Z form at 545 nm up to
a new photostationary state. Leaving the solution in the dark, curve c
goes back to curve a because the fraction of Z form present in the
photostationary state under 545 nm excitation is transformed to the
stable E form.
amine units of the dendrimers, undergoes an irreversible
oxidation process with Ep ) +0.65 V vs Ag-AgNO3.21 On
the basis of the excited-state energy and electrochemical data,
the reductive quenching of the excited state of eosin by the
amine units is considerably exergonic22 and it can therefore
account for the observed quenching process.23
In any case, the important point is that fluorescence quenching
occurs by a static mechanism, so that we can conclude that the
quenchable eosin molecules are hosted inside the dendrimer.
The results obtained (Table 1) show that the minimum value
for the fraction of eosin molecules hosted in Z-PG4 is 50%. It
is interesting to note that for both the PG4 and MG4 dendrimers
the quenching ability (which is related to the hosting ability)
of the Z form is higher than that of the E form.
Direct and Eosin-Sensitized Photoisomerization of the
Azobenzene Units. Irradiation of E-PG4 in DMF solution with
545 nm light caused the E f Z isomerization reaction of the
azobenzene units. For short irradiation times (i.e., until there
is no appreciable absorption by the Z isomer product), the
reaction was first order, with a rate constant of 6.3 × 10-3
min-1. Irradiation of a solution containing 5.0 × 10-5 M Z-PG4
and 5.0 × 10-6 M eosin, where the 545 nm excitation light is
absorbed by both components (Figure 3, curve c), caused a
pseudo-first-order isomerization reaction with a rate constant
of 9.7 × 10-3 min-1. The increase in the rate constant compared
to the direct photoreaction indicates that the light absorbed by
eosin is effective to cause isomerization. Similar experiments
were carried out for other compounds, for the back Z f E
photoisomerization (λirradiation ) 545 nm, Figure 4), and for both
aerated and deaerated solutions. The results obtained, gathered
in Table 2, show that (i) all the compounds undergo direct E f
Z and Z f E photoisomerization, (ii) the light absorbed by eosin
is effective to promote the photoisomerization reactions (sen-
sitized photoisomerization), (iii) the sensitized reaction is more
efficient in the case of the Z f E isomerization, as it usually
happens for azobenzene-type compounds,10,24 (iv) the efficiency
of the sensitized reaction is different for the various compounds
As far as electron-transfer quenching is concerned, it should
be pointed out that both eosin and azobenzene are relatively
difficult to oxidize and to reduce (for eosin in water, E°(Eos/
Eos-) ) -0.85 V, E°(Eos+/Eos) ) +1.1 V;20 for azobenzene,
E°(Az/Az-) ) -1.36 V vs SCE in DMF,21 whereas no oxidation
process is reported in the literature). Since the energy of the
fluorescent excited state of eosin is only 2.17 eV, a fast electron-
transfer quenching reaction involving the azobenzene units does
not seem plausible since, judging from the above-mentioned
electrochemical data, both the reductive and the oxidative
quenching processes do not appear to be exergonic.22 The amine
units present in the branches of the dendrimer, however, are
easily oxidized. Triethylamine, which is a good model for the
(19) (a) Turro, N. J. Modern Molecular Photochemistry; Benjamin:
Menlo Park, 1978. (b) Gilbert, A.; Baggot, J. Essentials of Molecular
Photochemistry; Blackwell: Oxford, 1991.
(20) Moser, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1984, 106, 6567.
(21) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Non-
aqueous Systems; Dekker: New York, 1970.
(22) For an excited-state electron-transfer reaction
A* + B f A- + B+ (reductive quenching)
the thermodynamic driving force can be estimated from the approximate
equation:
∆G° ) -∆E°° - E(A/A-) + E(B+/B)
where E(A/A-) and E(B+/B) are the energies (in eV) of the one-electron-
reduction processes, and ∆E°° is the excited-state spectroscopic energy.19
An equivalent equation can be written for the oxidative quenching.
(23) Photosensitized oxidation of tertiary aliphatic amines by eosin has
been observed: Bartholomeu, R. F.; Davidson, R. S. J. Chem. Soc. (C)
1971, 2347.