Pyrene-excimers-based antenna systems

Article abstract of DOI:10.1002/chem.200801379

A series of dendrimeric compounds bearing pyrene units were synthesized to afford light-harvesting antennae based on the formation of intramolecular excimers. The synthetic plan profited from the efficiency of the Huisgen reaction, the 1,3-dipolar cycload

Full text of DOI:10.1002/chem.200801379

DOI: 10.1002/chem.200801379
Pyrene-Excimers-Based Antenna Systems
Stefano Cicchi,*[a] Pierangelo Fabbrizzi,[a] Giacomo Ghini,[a] Alberto Brandi,[a]
Paolo Foggi,*[b, c, e] Agnese Marcelli,[b] Roberto Righini,[b] and Chiara Botta*[d]
Abstract: A series of dendrimeric compounds bearing pyrene units were synthe-
sized to afford light-harvesting antennae based on the formation of intramolecular
excimers. The synthetic plan profited from the efficiency of the Huisgen reaction,
the 1,3-dipolar cycloaddition of azides and terminal alkynes, which allowed ready
assembly of the different building blocks. The three molecular antennae obtained,
of increasing generation, revealed efficient energy transfer both in solution and in
the solid state.
Keywords: azides · catalysis · cyclo-
addition
transfer
· dendrimers · energy
Introduction
makes the formation of the excimers concentration inde-
pendent.[3] The new fluorescent properties of the excimeric
species are at the basis of analytical applications[4] because
of the strong dependence of fluorescence on the environ-
ment.[5] The high sensitivity of fluorescence detection makes
such an approach very fruitful.[6] Much less addressed by in-
vestigators has been the possibility of decorating the periph-
ery of the dendrimers with pyrene units. Nevertheless, a
dendrimer might be an excellent scaffold to anchor many
chromophores together and give rise to the formation of a
number of intramolecular excimers. This possibility was
rather used as a probe to study the structural characteristics
of dendrimers than for the production of devices.[7,8,9]
As a result of our ongoing research into the synthesis of
light-harvesting antennae,[10,11] we were interested in the syn-
thesis of new dendrimers with their outer parts decorated
with pyrene units to build up light-harvesting systems based
on the excimeric fluorescence of pyrene, thus profiting from
the supramolecular process (i.e., the excimer formation)
within a molecular assembly. Such an approach has, to the
best of our knowledge, only one precedent concerning the
quenching of the pyrene excimers through a FRET pro-
cess.[12] Furthermore, the possibility of inducing electrolumi-
nescence makes it possible to anticipate applications in the
field of organic light-emitting devices (OLEDs).[13] For a
simple and efficient build up of the saturated scaffold, the
Huisgen reaction was chosen as, after the recent develop-
ment of the catalytic form,[14] it has been demonstrated to
be a reliable and efficient synthetic tool.[15,16] Using a simple
variation of a procedure described by Fokin and co-work-
ers,[15] which proved to be efficient for the construction of
complex dendrimeric structures, the preparation of three
Among the fluorescent chromophores used for the produc-
tion of devices, pyrene draws the highest attention. Such
wide interest is due to the peculiar properties of this mole-
cule, which can form excimers[1] even at relatively low con-
centration. The photophysical properties of pyrene excimers
have been, in fact, studied over quite a few years.[2] The
grafting of more than a pyrene unit on a suitable scaffold
[a] Dr. S. Cicchi, Dr. P. Fabbrizzi, Dr. G. Ghini, Prof. A. Brandi
Dipartimento Chimica Organica Universitꢀ di Firenze Italian Inter-
university Consortium on Material Science and Technology (INSTM)
Via della Lastruccia 13
50019 Sesto Fiorentino (FI) (Italy)
Fax (+39)0553531
E-mail: stefano.cicchi@unifi.it
[b] Prof. P. Foggi, Dr. A. Marcelli, Prof. R. Righini
European Laboratory for Non-linear Spectroscopy (LENS)
via Nello Carrara 1, Sesto F. no. (FI) (Italy)
E-mail: foggi@lens.unifi.it
[c] Prof. P. Foggi
Dipartimento di Chimica, Universitꢀ di Perugia
via Elce di sotto 8, 06123 Perugia (Italy)
[d] Dr. C. Botta
Istituto per lo Studio delle Macromolecole (ISMAC), CNR
via Bassini 15, 20133 Milano (Italy)
E-mail: chiara.botta@ismac.cnr.it
[e] Prof. P. Foggi
INOA-CNR
Largo E. Fermi 6, 50125 Firenze (Italy)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801379.
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FULL PAPER
dendrimeric compounds substituted with pyrene units at the
periphery was successfully carried out.
monomeric pyrene fluorescence and a broad, featureless
emission centered at approximately 475 nm. The last over-
riding contribution was observed even at very low concen-
tration (10ꢀ6 m); therefore, the signal can be attributed to in-
tramolecular excimer formation. Although the UV absorp-
tion was not significantly sensitive to the nature of the sol-
vent, the photoluminescence spectra varied markedly with
Results and Discussion
The first generation of the pyrene-substituted dendron was
readily obtained starting from ester 1,[15] which was convert-
ed into the diacetylenic compound 3.[15] The CuI-catalyzed
Huisgen reaction afforded finally 5 (Scheme 1). The rela-
tively long arm that characterizes the pyrene units was
chosen to guarantee full flexibility in the final assembly and
to avoid any steric obstacle in the formation of the excimer-
ic species. The reaction was very efficient and afforded 5 in
high yield.
that. It is well known that in bisACTHUNTGRNEUNG(pyrenil) compounds the
extent of excimer emission was limited by the probability of
a molecule reaching, within the lifetime of the excited state,
a conformation suitable for excimer formation and by the
stabilization of the excimer.[17] The observed ratio between
the excimer and monomeric fluorescence quantum yield
MON
FFEXC/FF
was plotted in Figure 1 as a function of the vis-
cosity of the solvent. Despite the different molecular nature,
the ratio decreased sharply as the h value increased[18] and
became zero at high viscosity.
MON
Figure 1. Plot of the FFEXC/FF
variation with the viscosity h of the sol-
Scheme 1. CuI-catalyzed Huisgen reaction to yield compound 5.
vent. The line is just a guide for the eye.
The structure of 5 was readily
assigned on the basis of its very
symmetrical H NMR spectrum,
Table 1. The wavelengths of maximum absorption lMAX, emission wavelengths lEM, and fluorescence quantum
yields FF for 5.[a]
1
the signal at d=7.43 ppm (s,
2H) was assigned to the pro-
tons on the triazole rings (see
the Experimental Section). The
spectroscopic properties of this
first model compound were in-
vestigated in several different
deoxygenated solvents. The ab-
sorption and emission data are
summarized in Table 1.
The UV absorption spectra
were characterized by two well-
structured bands at roughly 275
and 345 nm, typical of the
pyrene units, whereas the emis-
MAX
MAX
MON
EXC
MON
Solvent
l1
l2
lEM
FF
FF
FFEXC/FF
[nm]
277
[nm]
344
[nm]
dichloromethane
methanol
377/397
475
377/396
477
377/397
475
376/397
476
378/398
481
377/397
476
0.028
0.025
0.08
0.05
0.11
0.13
0.23
5.0
4.0
2.1
1.8
1.4
0.85
0
0.14
0.10
0.17
0.09
0.16
275
277
275
278
277
277
342
345
342
346
344
344
DMF
ethanol
dimethyl sulfoxide
1-octanol
0.11
0
ethylene glycol
377/397
[a] The lMAX, lEM, and FF values were collected in solutions (10ꢀ6 m); MON and EXC indicate the monomeric
pyrene and the intramolecular excimer fluorescence, respectively; the excitation wavelength was 275 nm in
sion spectra exhibited typical CH2Cl2 and 345 nm in the other solvents.
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S. Cicchi, P. Foggi, C. Botta et al.
Encouraged by the interesting properties of the first gen-
eration dendron, the synthesis of the two subsequent gener-
ations of the donor scaffold, that is, compounds 9 and 13,
was also undertaken and achieved by following the same ap-
proach, with small variations, in excellent yield (Scheme 2
and Scheme 3). The branching units 6 and 7 were used to in-
crease the number of acetylenic groups at the periphery.
The UV absorption and fluorescence spectra of 5, 9, and
13 were recorded in DMF (Figure 2). The study was restrict-
ed to DMF as the solvent because of the limited solubility
of 9 and 13 in other solvents. The UV absorption spectra
that resulted are almost superimposable for the three differ-
ent compounds. In contrast, in the fluorescence spectra, the
ratio of excimer/monomer emission was dependent on the
generation of the three compounds, with the maximum
emission observed for 13.
For the production of light-harvesting antenna systems, a
suitable acceptor chromophore that has to fulfill the main
requisite of the Fçrster theory,[19] which is generally applied
for non-conjugated light-harvesting antenna systems, was
needed. The requisite was that the fluorescence spectra of
the donor chromophore should overlap with the absorption
spectrum of the acceptor one. For this reason, a fluorophore
with a maximum absorption at around 500 nm should be
linked to the dendrimer. Two suitable candidates, 14 and 15,
were accordingly identified.
Nitrobenzofurazane 14 is a
suitable fluorophore that was
previously used by us in other
antenna systems.[11] Commer-
cially available as a chloride
salt, 14 becomes fluorescent
after conjugation with an amino
group. Styrylpyridinium deriva-
tives, such as 15, which are
readily available from the alky-
lation of the pyridine nitrogen
atom of N,N-diethyl-N-[4-{(E)-2-(4-pyridinyl)ethenyl}phe-
nyl]amine (19),[20,21] are versatile fluorophores that are well-
known red emitters used mainly as fluorescent probes. In
both cases, the absorption of the potential acceptor chromo-
phores overlaps efficiently with the excimeric emission of
the donor (see Figure 1 in the Supporting Information).
Compounds 14 and 15 appear to be suited for the construc-
tion of a light-harvesting system in which 5 will act as the
donor in an intramolecular energy-transfer process.
For the synthesis of the antenna system, it was necessary
to modify the structure of the dendron to anchor the accept-
or chromophore to the structure. To this end, dendron 16
was synthesized. Substitution of the terminal bromine atom
with a secondary amine and the final SNAr process with
chloride 17 afforded 18, a simple model of an antenna (over-
all yield: 92%). The same inter-
mediate 16 was treated with 4-
styryl-pyridine derivative 19 to
afford pyridinium salt 20 in
76% yield (Scheme 4).
The absorption spectra of 18
and 20 were compared to the
absorption spectra of the isolat-
ed donor and acceptor chromo-
phores (Figure 3). The donor
and acceptor portions could in-
teract appreciably in the anten-
na systems 18 and 20, in fact
the bands associated with the
acceptor underwent an ipo-
chromism. The effect was
strongly apparent in the UV
spectrum of 18, in which the
molar extinction coefficient
showed
a
30% decrease
(Figure 3). On the other hand,
a red shift was evident only in
the UV spectrum of 20. These
data suggested that the optical
properties of the acceptor
moiety in the antenna system
could not be considered as an
equivalent of the chromophore
in solution.
Compound 18, the first to be
analyzed, showed some disap-
Scheme 2. Synthesis of dendron 9.
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Formation of Intramolecular Excimers
FULL PAPER
Scheme 3. Synthesis of dendron 13.
spectrum showed a very poorly resolved band caused by the
overlapping of the excimeric emission with the emission of
the acceptor. Much more rewarding results were obtained
with 20, whose emission spectrum (i.e., lecx =345 nm) is re-
ported in Figure 4. The fluorescence emission was normal-
ized at 377 nm, which corresponds to the value of maximum
intensity, and we emphasized the fact that we obtained the
emission in the whole visible spectrum. To prevent mislead-
ing information from the figure, we noticed that the fluores-
cence quantum yield (see Table 2) both of the monomer and
of the excimer decreased going from donor to antenna mol-
ecule.
The concentration was kept very low (3.5ꢂ10ꢀ7 m) to
avoid any intermolecular effects. In the emission profile, it
was easy to recognize three different contributions from the
monomeric pyrene, the excimer emission and the acceptor
emission. The hypothesis of a direct excitation of the last
chromophore was excluded by the absence of an emission
from a solution of 15 excited at the same wavelength and
concentration reported above. This evidence guaranteed the
intramolecular nature of the process.
Figure 2. UV absorption and fluorescence (lexc =345 nm) spectra of 5, 9,
and 13 in DMF. The emission profiles were normalized at 377 nm.
pointing features that suggested that the study should be
stopped. In fact, because of a blue shift of more than 10 nm
(data not showed) of the acceptor emission, the resulting
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S. Cicchi, P. Foggi, C. Botta et al.
Scheme 4. Synthesis of compound 18 and pyridinium salt 20. DIPEA=diisopropylethylamine.
Figure 4. Normalized emission spectra of 20 (dashed line) and 5 (continu-
ous line) in DMF.
Figure 3. The absorption spectra (dotted line) of 18 (right) in CH2Cl2 and
20 (left) in DMF were compared to their correspondent donor (solid
line) and acceptor (dashed line) moieties.
possible to estimate the quantum efficiency f of the energy
transfer by the ratio reported in Equation (1).[22]
The total (monomer+excimer) fluorescence quantum
yield (FFD) of the donor in the antenna system (i.e., 20) was
compared with that of the dendron (i.e., 5) (FFD)0, both ex-
cited at 345 nm. For a simple donor/acceptor system, it is
FFD
¼ 1ꢀf
ð1Þ
ðFFDÞ0
From the data reported in Table 2, a value of f=0.9 was cal-
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Formation of Intramolecular Excimers
FULL PAPER
Table 2. Emission data of 5, 9, 13, 20, 21, and 23.[a]
R
R0
ð3Þ
f ¼ 1ꢀ
Dendrons
Antennae
Entry
5
9
13
20
21
23
MON
EXC
By applying Equation (3) to the
data collected for 5 and 20
(Table 2, entry 5), the energy-
transfer efficiency was 0.5.
On the basis of the results
obtained with the first-genera-
tion compounds, the second-
1
2
3
FF
FF
0.08ꢁ0.01 0.029ꢁ0.004 0.031ꢁ0.005 0.014ꢁ0.002 0.0053ꢁ0.0007 0.007ꢁ0.001
0.17ꢁ0.04 0.20ꢁ0.03
0.32ꢁ0.06
0.016ꢁ0.003 0.013ꢁ0.002
0.006ꢁ0.002 0.009 ꢁ0.002
0.0048ꢁ0.0009
0.018 ꢁ0.002
(FFA
)
–
–
–
D
(l=345 nm)
qFA
4
–
–
–
0.007ꢁ0.001 0.011 ꢁ0.002
0.018 ꢁ0.002
(l=488 nm)
MON
5
6
FFEXC/FF
2.1
–
6.9
–
10.3
–
1.1
0.5
2.4
0.65
0.7
0.93
f[b]
[a] The fluorescence quantum yield of the monomer and of the excimer are given in entries 1 and 2, respec-
tively; the fluorescence quantum yields of the acceptor in the antenna system indirectly excited at 345 nm and
directly excited by absorption at 488 nm and are given in entries 3 and 4. [b] f is the estimated energy-transfer
efficiency.
and
third-generation
com-
pounds 21 and 23, respectively,
were synthesized. The synthesis
was again performed by modi-
fying the precedent synthesis of
culated. However, such a high value surely overrated the
the
donor
dendrons
real efficiency because, in this case, the energy transfer was
complicated by the presence of another process, that is, the
excimer formation. Regardless of how complex the involved
photophysical processes are, it is evident that the fluores-
cence quantum yield of the donor is decreased in the pres-
ence of the acceptor. The present study is limited to static
measurements and does not allow full unveiling of the
mechanism of the excimer pathway. Time-resolved measure-
ments will be necessary to detail the process. Nevertheless,
from the kinetic pathway shown in Figure 5, it was possible
to determinate the relative quantum yield for the monomer
and the excimer emission.
(Scheme 5). In the last step of the synthesis of 21, the alkyla-
tion of pyridine 19, some evidence of decomposition was no-
ticed, probably caused by prolonged heating in CHCl3. For
this reason, the 1,3-dipolar cycloaddition reaction, which
took place at room temperature and overnight, was carried
out as the final step in the synthesis of 23. The purification
of these molecular antennae was performed by chromatog-
raphy on alumina, and all the compounds where fully identi-
1
fied and characterized by ESI mass-spectrometric, H and
13C NMR spectroscopic, and elemental analysis. The absorp-
tion and fluorescence spectra of 21 and 23 are reported in
the Figure 6. The calculated fluorescence quantum yields FF
of the monomer and excimer and the efficiency of the den-
drons and antennae are reported in Table 2.
The complete picture of the three antenna systems and
their dendrons allowed an estimation of the energy-transfer
efficiency, and a net increment passing from 20 to 23 was
obtained. For a wider characterization of the spectroscopic
properties of these compounds and to explore the potential
application in OLEDs, their UV absorption and fluores-
cence spectra were obtained in the solid state. The three
donors 5, 9, and 13 were studied as cast-films formed from
diluted solutions in CH2Cl2.
The absorption spectra of the films (Figure 7) are slightly
broadened and red shifted (5 nm) with respect to those of
the solutions (see Figure 2). These results are in agreement
with those reported for pyrene-based materials in the pres-
ence of pyrene preassociation[25] in the ground state. On the
contrary, the emission spectra (Figure 7) exclusively showed
the emission of the excimer. The different behavior ob-
served in the solid state and in solution can be explained by
the different nature of the excitations. In solution, excimer
formation, at the low concentrations used, is an intramolecu-
lar process and dynamic excimers are formed after excita-
tion thanks to the mobility of the pyrene units (influenced
by the viscosity of the solvent). On the other hand, in the
solid films, the tendency of the aromatic units toward
p stacking induces partial pyrene aggregation or preassocia-
tion in the ground state and pyrene excimer formation
occurs both intra- and intermolecularly. Because of the ri-
gidity of the environment, static excimers are responsible
Figure 5. Schematic processes for excimer formation/deactivation. E=ex-
cimer, M=monomer, knr =non-radiative rate constant.
It is well known[23] that the accepted expression of a quan-
tum yield emission is given by Equation (2):
kF
ki
P
*
FF ¼ F
ð2Þ
where F* is the formation efficiency of the emitting state,
kF is the rate constant of the emission, and ꢀki is the sum of
all the rates involved in depopulation of the emission state.
By defining R0 and R, the ratios between the fluorescence
quantum yields of the excimer and monomer relative to the
dendron molecule and the antenna system, respectively, it
was possible to demonstrate (see note i in the Supporting
Information) that the ratio R/R0 was strictly connected to
the efficiency f of the energy-transfer process given in Equa-
tion (3).
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759

S. Cicchi, P. Foggi, C. Botta et al.
Scheme 5. Synthesis of the second- and third-generation compounds 21 and 23.
for the emission in contrast to the dynamic excimers typical
of the solutions.[2] The small red shift in the energy levels of
the aggregated pyrenes observed in the films however
makes them efficient energy traps for the excitation, which
because of the fast intermolecular migration (Fçrster and
Dexter energy transfers) typical of solid-state samples[25] re-
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Formation of Intramolecular Excimers
FULL PAPER
Figure 6. Absorption spectra (left) and fluorescence spectra (right) of 20–23. The intensity of the emission profile was divided to the absorbance of the
sample at lexc =345 nm.
Figure 8. Absorption and emission spectra (cast-film) of 20–23.
Figure 7. Solid-state (cast-film) absorption and fluorescence spectra of 5,
9, and 13.
Table 3. Fluorescence quantum yields of 5, 9, 13, 15, 20, 21, and 23 (cast-
films).
combine only from static excimers. The same measurements
made with the isolated acceptor 15 showed that this mole-
cule is almost non-fluorescent in the solid state (FF <0.01 at
l=500 nm), even if its fluorescence spectrum is not appreci-
ably different from the spectrum obtained in solution (see
Figure 2 in the Supporting Information).
This situation was radically altered in the molecular an-
tennae. Again the absorption spectra did not show any re-
markable variation (Figure 8), as it was the sum of the ab-
sorption spectra of the two chromophores, whereas the
emission spectra showed exclusively the band associated
with the emission of the acceptor (Figure 8, l=350 nm).
The higher efficiency of the energy transfer in the solid
state, with respect to the solution, might be because of the
presence of both long-range Fçrster-type and short-range
Dexter-type energy transfers.[25] All the data collected in the
solid state are reported in Table 3.
Compound
U
)
qFA
(l=500 nm)
D
ACHTUNGTRENNUNG
5
9
–
–
–
13
15
20
21
23
<0.01
0.02
0.07
0.11
show that 21 and 23 can be used as red emitters in OLED
applications.
The efficiency of the acceptor emission increases with the
generation and reaches a very promising value of 0.13 for
the third-generation antenna in the solid state. These results
The synthesis of the third-generation compounds 20, 21, and
23 demonstrated the possibility of assembling molecular an-
tenna systems based on the formation of excimers. The
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S. Cicchi, P. Foggi, C. Botta et al.
nol. The products appeared to be pure as shown by NMR spectroscopic
measurements and TLC (CHCl3/methanol, 20:1)
static spectroscopic study, although not able to give precise
details of the complete energy-transfer process, suggests a
high efficiency for all the systems assembled. From the anal-
ysis of the photoluminescence in the solid state, we have
shown that because of energy-transfer and hopping process-
es, a pure red emission is attained. The quite high efficiency
of this emission for the antenna systems of the second and
third generation demonstrates that they are very promising
red emitters for OLED applications.[26]
Pyr2-G1-COOCH3 (5): The product was synthesized according to general
procedure A from azide 4 (0.34 g, 1.13 mmol) and acetylenic core 3
(0.138 g, 0.57 mmol) and obtained as a brown waxy solid (0.45 g, 94%).
1H NMR (400 MHz, CDCl3, 208C, TMS): d=8.21–8.12 (m, 6H), 8.08 (d,
J
N
ACHTUNGTREN(NUGN H,H)=7.8 Hz, 2H),
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
2.09–1.99 (m, 4H), 1.93–1.82 ppm (m, 4H); 13C NMR (50 MHz, CDCl3,
208C, TMS): d=166.3, 165.6, 135.4, 132.1, 131.3, 130.7, 129.9, 128.4,
127.4, 127.1, 126.7, 125.8, 124.9, 124.7, 123.0, 122.6, 62.2, 52.4, 50.3, 32.8,
30.1, 28.4 ppm; IR (KBr): n˜ =3036 (w), 2937 (w), 2861 (w), 1728 (m),
1594 (m), 1457 (m), 1436 (m) 1316 (d), 1234 (m), 1170 (s), 1041 (s), 840
(s), 768 cmꢀ1 (m); MS (ESI): m/z (%): 881.27 (41) [M+K]+, 865.36 (100)
[M+Na+], 842.36 (13) [M]+; elemental analysis (%) calcd for
C54H46N6O4: C 76.94, H 5.50, N 9.97; found: C 77.17, H 5.52, N 10.
Experimental Section
Safety warning: Azides are potentially explosive. Maximum caution must
be used when manipulating these compounds, especially in large-scale re-
actions.
Pyr4-G2-COOCH3 (9): The product was synthesized according to general
procedure A from azide 4 (0.2 g, 0.67 mmol) and acetylenic core 8
(0.095 g, 0.17 mmol) and obtained as a brown waxy solid (0.27 g, 91%).
1H NMR (400 MHz, CDCl3, 208C, TMS): d=8.15–8.09 (m, 16H), 8.06–
Methyl 3,5-bisACHTUNGTRENNUNG{[3,5-bis(2-propynyloxy)benzyl]oxy}benzoate (Acet4-G2-
COOCH3) (8): A suspension of methyl 3,5-dihidroxybenzoate (1; 0.228 g,
0.93 mmol), 1-bromomethyl-3,5-bis(prop-2-ynyloxy)benzene (7; 0.530 g,
1.86 mmol, 2 equiv), K2CO3 (1.03 g, 7.46 mmol), and [18]crown-6 (5 mg)
in acetone (25 mL) was heated to reflux under nitrogen for 48 h. The re-
action mixture was then evaporated to dryness and partitioned between
water and dichloromethane. The organic layer was dried over Na2SO4,
evaporated, and the solid crystallized in methanol to give the product as
white crystals (0.39 g, 60%). 1H NMR (400 MHz, CDCl3, 208C, TMS):
8.00 (m 8H), 7.98–7.89 (m, 8H), 7.73 (d, J
4H), 7.22 (d, J(H,H)=2.3 Hz, 2H), 6.72–6.79 (m, 1H), 6.60 (d, J
2 Hz, 4H), 6.54–6.49 (m, 2H), 5.04 (s, 8H), 4.92 (s, 4H), 4.24 (t, J
8 Hz, 8H), 3.85 (s, 3H), 3.26 (t, J(H,H)=8 Hz, 8H), 2.02–1.88 (m, 8H),
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
1.86–1.72 ppm (m, 8H); 13C NMR (50 MHz, CDCl3, 208C, TMS): d=
166.6, 159.4, 143.5, 139, 135.5, 131.9, 131.2, 130.7, 129.8, 128.4, 127.4,
127.3, 127, 126.6, 125.8, 124.9, 124.8, 122.9, 122.8, 108.4, 107, 106.5, 101.4,
76.6, 69.9, 61.9, 52.4, 50.6, 50.2, 32.7, 30, 28.3 ppm; IR (KBr): n˜ =3036
(w), 2935 (w), 1717 (m), 1594 (s), 1436 (m), 1299 (m), 1231 (m), 1157 (s),
1043 (s), 844 (s), 768 (m), 708 (m), 679 (m), 587 cmꢀ1 (m); MS (ESI): m/z
(%): 1800.6 (10%) [M+K]+, 1784.7 (27) [M+Na]+, 1762.6 (10), 1447.5
(37), 1003.4 (39), 987.4 (100), 855.4 (40), 685.6 (64), 569.5 (46); elemental
analysis (%) calcd for C114H96N12O8: C 77.71, H 5.49, N 9.54; found: C
77.32, H 5.45, N 9.46.
d=7.27 (d, J
(H,H)=2.3 Hz, 4H), 6.57 (t, J
(H,H)=2.4 Hz, 8H), 3.90 (s, 3H), 2.52 ppm (t, JCAHTUNGTRENNNUG
A
ACHTUNGTNER(NUNG H,H)=2.3 Hz, 1H), 6.68 (t, J-
A
ACHTUNGTRENNUNG
J
A
13C NMR (50 MHz, CDCl3, 208C, TMS): d=166.3, 159.2, 158.5, 138.7,
131.7, 108.1, 107, 106.5, 101.5, 78, 75.5, 69.6, 55.6, 52 ppm; IR (KBr): n˜ =
3285 (m), 2909 (w), 2113 (w), 1713 (m), 1600 (s), 1465 (m), 1437 (s), 1386
(m), 1330 (s), 1268 (m), 1243 (m), 1185 (s), 1160 (m), 1060 (m), 1050 (m),
1013 (m), 830 (m), 763 (m), 678 cmꢀ1 (m); MS (EI, 70 eV): m/z (%):
564.2 (8) [M]+, 365.2 (15.51), 199 (100), 128 (64.4); elemental analysis
(%) calcd for C34H28O8: C 72.33, H 5.00; found: C 72.21, H 4.89.
Pyr8-G3-COOCH3 (13): The product was synthesized according to gener-
al procedure A from azide 4 (0.1 g, 0.33 mmol) and acetylenic core 12
(0.050 g, 0.04 mmol) and was obtained as a brown waxy solid (0.130 g,
87%). 1H NMR (400 MHz, CDCl3, 208C, TMS): d=8.1–8.03 (m, 24H),
Methyl
benzoate (Acet8-G3-COOCH3) (12): A solution of methyl 3,5-dihidroxy-
benzoate (1; 0.025 g, 0.15 mmol), 1,3-bis{[3,5-bis(2-propynyloxy)benzyl]-
3,5-bisACHTUNGTRENNUNG[(3,5-bisACTHUNGTREN{NUGN [3,5-bis(2-propynyloxy)benzyl]oxy}benzyl)oxy]-
8.0–7.97 (m, 16H), 7.93–7.86 (m, 24H), 7.67 (d, J
(s, 8H), 7.20 (d, J(H,H)=2 Hz, 2H), 6.67 (s, 1H), 6.56–6.55 (m, 12), 6.47
(s, 6H), 4.98 (s, 16H), 4.88 (s, 4H), 4.85 (s, 8H), 4.15 (t, J(H,H)=8 Hz,
16H), 3.79 (s, 3H), 3.20 (t, J(H,H)=8 Hz, 16H), 1.91–1.84 (m, 16H),
ACHTUNGTREN(NUNG H,H)=8 Hz, 8H), 7.27
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
oxy}-5-(bromoethyl)benzene (8; 0.184 g, 0.3 mmol), K2CO3 (0.165 g,
1.2 mmol), and [18]crown-6 (10 mg) in acetone (8 mL) was heated to
reflux under nitrogen for 48 h. The reaction mixture was then evaporated
to dryness and partitioned between water and dichloromethane. The or-
ganic layer was dried over Na2SO4, evaporated, and the solid crystallized
in methanol to give the product as white crystals (0.165 g, 90%).
1H NMR (400 MHz, CDCl3, 208C, TMS): d=7.27 (s, 2H), 6.76 (t, J-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
1.76–1.68 ppm (m, 16H); 13C NMR (50 MHz, CDCl3, 208C): d=159.5,
159.1, 143.3, 139, 138.6, 135.2, 130.9, 130.4, 129.5, 128.1, 127.1, 126.8,
126.3, 125.5, 124.6, 124.4, 122.7, 122.4, 106, 101, 69.4, 61.6, 50.6, 49.8, 32.3,
29.6, 29.5, 27.9 ppm; IR (KBr): n˜ =3025 (w), 2920 (w), 2868 (w), 2366
(w), 1720 (w), 1593 (s), 1452 (m), 1369 (w), 1296 (w), 1155 (s), 1040 (m),
837 (m), 764 (w), 706 (w), 675 cmꢀ1 (w); MS (ESI): m/z (%): 3664.6 (10),
2464 (20), 1935.7 (26), 1863.5 (100), 1843.5 (17), 1823.1 (13); elemental
analysis (%) calcd for C234H196N24O16: C 78.06, H 5.49, N 9.34; found: C
77.82, H 5.47, N 9.29.
A
R
ACHTUNGTRNE(NUNG H,H)=
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
159.8, 158.9, 139.6, 139.3, 116.5, 107.1, 101.6, 79.6, 79.1, 79, 78.3, 77.8,
69.6, 56, 49, 48.6, 48.2, 47.7, 47.3 ppm; IR (KBr): n˜ =3287 (m), 2905 (w),
2115 (w), 1709 (m), 1601 (s), 1465 (m), 1427 (s), 1383 (m), 1310 (s), 1265
(m), 1231 (m), 1187 (s), 1161 (m), 1056 (m), 1014 (m), 818 (m), 762 cmꢀ1
(m); MS (ESI): m/z (%): 1222 (100), 1204.9 (6) [M]+; elemental analysis
(%) calcd for C74H60O16: C 73.74, H 5.02; found: C 73.62, 5.01.
Pyr2-G1-COOCH3 (18): The product was synthesized according to gener-
al procedure A from azide 4 (0.25 g, 0.84 mmol) and acetylenic core 7
(0.37 g, 0.42 mmol) and obtained as a brown waxy solid (0.58 g, 93%).
1H NMR (400 MHz, CDCl3, 208C, TMS): d=8.22–8.06 (m, 10H), 8.03–
7.91 (m, 6H), 7.80 (d, J
ACHTUNGTRENNUNG
6.53 (s, 1H), 5.09 (s, 4H), 4.46 (s, 2H), 4.35 (t, JAHCTUNGTRENNUNG
General procedure (A): copper(I)-catalyzed Huisgen reaction (azyde–
alkyne cycloaddition; CuAAC): The corresponding acetylenic core (50,
25, and 12.5 mol% for the first-, second-, and third-generation com-
pounds, respectively), CuSO4·5H2O (5 mol%), and sodium ascorbate
(10 mol%) were added to a solution of the azide (100 mol%) in the min-
imum amount of THF/H2O (1:1). The reaction mixture was stirred for
3 h, the THF was evaporated under vacuum, and the residue was parti-
tioned between water and CHCl3. The organic layer was dried over
Na2SO4 and evaporated to dryness. The crude products (usually brown
oils) were washed with methanol under reflux, cooled to room tempera-
ture to form brown waxy solids, filtered, and washed again with metha-
(t, J=7.5 Hz, 4H), 2.1–2.0 (m, 4H), 1.90–1.82 ppm (m, 4H); 13C NMR
(50 MHz, CDCl3, 208C, TMS): d=159.1, 139.5, 139.3, 135.2, 131, 130.4,
129.6, 129.5, 129.3, 128.1, 127, 126.4, 125.6, 124.6, 124.5, 122.3, 122.7,
107.6, 101.5, 61.7, 49.8, 45.9, 32.3, 29.7, 28 ppm; IR (KBr): n˜ =3045 (m),
2954 (s), 1600 (s), 1461 (s), 1166 (s), 1050 cmꢀ1 (m); MS (ESI): m/z (%):
899.4 (90) [M+Na]+, 876.5 (22) [M]+, 855.45 (100), 833.45 (26),
797.5(96); elemental analysis (%) calcd for C53H45N6O2Br: C 72.51, H
5.17, N 9.57; found: C 72.51, H 5.07, N 9.33.
Pyr2-G1-(styrylpyrydinium) bromide (20):
A solution of 19 (0.083 g,
0.33 mmol) and 18 (0.29 g, 0.33 mmol) in CHCl3 (20 mL) was heated to
762
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ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 754 – 764

Formation of Intramolecular Excimers
FULL PAPER
reflux for 24 h. The solvent was evaporated to dryness and the crude
product purified by column chromatography on silica gel with dichloro-
methane/methanol of increasing polarity (20:1, 10:1, 5:1) as the eluent
(0.25 g, 67%). Rf =0.22 (dichloromethane/methanol, 20:1, final product is
the red spot); 1H NMR (400 MHz, CDCl3, 208C, TMS): d=8.78 (d, J-
Pyr4-G2-(styrylpyridinium) bromide (21): Compound 11a (0.44 g,
0,24 mmol) and 19 (0.061 g, 0.24 mmol) were dissolved in CHCl3 (10 mL)
and heated to reflux for 24 h. The solvent was evaporated, and the crude
product was purified by column chromatography on neutral aluminum
oxide, starting the elution with CHCl3 then increasing the polarity
(CHCl3/methanol 30:1, 20:1, 10:1) to yield a red solid (0.38 g, 76%). Rf =
0.34 (CHCl3/MeOH, 20:1, red spot on an aluminum oxide TLC plate);
1H NMR (400 MHz, CDCl3, 208C, TMS): d=8.72–8.66 (bs, 2H), 8.12–
A
ACHTUNGTREN(NUGN H,H)=7.8 Hz, 2H),
AHCTUNGTRENNUNG
7.83 (m, 32H), 7.67 (d, J
5H), 6.78–6.36 (m, 10H), 5.57–5.46 (m, 2H), 4.99 (s, 8H), 4.93 (m, 4H),
4.19 (t, J(H,H)=8 Hz, 8H), 3.33–3.24 (m, 4), 3.18 (t, J(H,H)=8 Hz, 8H),
1.95–1.83 (m, 8H), 1.78–1.66 (m, 8H), 1.09 ppm (t, J(H,H)=7 Hz, 6H);
ACHTUNGTRENUN(NG H,H)=7.8, 4H), 7.47 (s, 4H), 7.30–7.21 (m,
8 Hz, 4H), 2.0–1.90 (m, 4H), 1.78–1.66 (m, 4H), 1.17–1.08 ppm (m, 6H);
13C NMR (50 MHz, CDCl3, 208C, TMS): d=159.6, 153.6, 149.8, 142.7,
135.7, 131.2, 130.8, 130.6, 129.6, 128.3, 127.4, 127.2, 127, 126.5, 125.8,
124.7, 123.1, 121.9, 121.5, 115.4, 111.3, 108.2, 61.8, 50.5, 50.1, 44.6, 32.6,
30.2, 28.4, 12.8 ppm; IR (KBr): n˜ =3031 (w), 2929 (w), 2852 (w), 1634
(m), 1571 (s), 1518 (m), 1449 (m), 1401 (w), 1346 (m), 1321 (m), 1267
(m), 1149 (s), 1043 (m), 842 cmꢀ1 (s); MS (ESI): m/z (%): 1049.5 (100)
[M]+, 876.3 (8), 751.4 (38), 750.4 (67), 712.4 (12), 534.3 (55); elemental
analysis (%) calcd for C70H65BrN8O2: C 74.39, H 5.80, N 9.91; found: C
73.97, H 5.73, N 9.87.
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
13C NMR (50 MHz, [D6]DMSO, 208C, TMS): d=160.2, 159.6, 143.6,
142.8, 139.5, 136.7, 131.2, 130.7, 129.6, 128.9, 128.4, 127.8, 127.6, 126.9,
126.5, 125.3, 124.5, 123.8, 111.8, 107.1, 79.7, 69.8, 61.81, 49.8, 44.3, 32.4,
30.3, 28.8, 13.1 ppm; IR (KBr): n˜ =3036 (w), 2929 (w), 2853 (w), 1639
(m), 1568 s), 1520 m), 1446 (m), 1407w), 1342(m), 1321(m), 1268(m),
1149 (s), 1041(m), 835 cmꢀ1(s); MS (ESI): m/z (%): 1967.9 (90), 1967.91
(63) [M+], 1669.8 (100), 1370.6 (62), 1070.6 (26), 1049.6 (19), 996.1 (25);
elemental analysis (%) calcd for C130H115BrN14O6: C 76.19, H 9.57, N
9.57; found: C 75.96, H 9.46, N 9.61.
Pyr2-G1-N-propylamine (18a): Compound 18 (0.09 g, 0.1 mmol), was dis-
solved in propylamine (2 mL) and CHCl3 (2 mL) and stirred for 4 h at
room temperature. The reaction mixture was evaporated to dryness and
the residue was dissolved in CHCl3 (30 mL) and washed with NaHCO3
(sat. solution, 4ꢂ100 mL). The organic layer was dried over Na2SO4 and
evaporated. The product was used for the next step without further pu-
rification. 1H NMR (200 MHz, CDCl3, 208C, TMS): d=8.29–7.89 (m,
Acet8-G3-(styrylpyridinium) bromide (22b): Compound 22a (0.15 g.
0.118 mmol) and 19 (0.03 g, 0.118 mmol) were dissolved in toluene
(10 mL) and heated at 808C overnight. The toluene was evaporated and
the residue was purified by column chromatography on neutral aluminum
oxide, starting the elution with CHCl3 increasing the polarity (CHCl3/
methanol 20:1) to yield a red waxy solid (0.15 g, 83%). Rf =0.1 (CHCl3/
MeOH, 20:1, red spot on an aluminum oxide TLC plate); 1H NMR
16H), 7.74 (d, J
1H), 5.09 (s, 4H), 4.25 (t, J
(H,H)=8 Hz, 4H), 2.52 (t, J(H,H)=6 Hz, 2H), 2.06–1.85 (m, 4H), 1.86–
165 (m, 4H), 1.56–1.40 (m, 2H), 0.89 ppm (t, J(H,H)=6 Hz, 3H).
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
(400 MHz, CDCl3, 208C, TMS): d=9.0 (d, J
(H,H)=6.4 Hz, 2H), 7.41–7.36 (m, 3H), 6.83 (d, J
6.64–6.55 (m, 15H), 6.5–6.42 (m, 7H), 5.77 (s, 2H), 4.93 (s, 4H), 4.91 (s,
8H), 4.60 (d, J(H,H)=2.8 Hz, 16H), 3.37 (q, J(H,H)=6.8 Hz, 4H), 2.51
(t, J(H,H)=2.4 Hz, 8H), 1.16 ppm (t, J
(H,H)=2.4 Hz, 6H); 13C NMR
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
J
N
ACHTUNGTRENNUNG
Pyr2-G1-N-propylamine-N-nitrobenzofurazan (20): Compound 18a
(0.083 g, 0.097 mmol) and 4-chloro-7-nitrobenzofurazan (17; 0.039 g,
0.194 mmol) were dissolved in CHCl3 and stirred overnight at room tem-
perature. DIPEA (2 drops) was added to the solution, and the reaction
mixture was stirred for 1 h. The reaction mixture was washed with
NaHCO3 and brine. The organic layer was dried over Na2SO4 and evapo-
rated under vacuum. The crude product was purified by column chroma-
tography on neutral aluminum oxide eluting with CHCl3 to yield a dark
orange gum (0.092 g, 92%). 1H NMR (400 MHz, CDCl3, 208C, TMS):
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
(50 MHz, CDCl3, 208C, TMS): d=160, 159.6, 158.6, 149.9, 143.3, 139.4,
139.1, 136, 130.9, 122.2, 121.6, 115.8, 111.4, 108.1, 106.8, 101.7, 78.4, 77.8,
77.2, 76.6, 76, 69.7, 56, 44.7, 12.8 ppm; IR (KBr): n˜ =3277 (s), 2963 (w),
2919 (w), 2863 (w), 2112 (w), 1591 (s), 1518 (m), 1451 (m), 1323 (w),
1150 (s), 1043 (m), 953 (w), 831 (w), 752 (w), 674 (w), 640 cmꢀ1 (w); MS
(ESI): m/z (%): 1610.3 (5), 1450.36 (4), 1412.4 (100) [M]+, 1321.8 (5); el-
emental analysis (%) calcd for C90H79BrN2O14: C 72.43, H 5.34, N 1.88;
found : C 72.14, H 5.32, N 1.91.
d=8.09–7.84 (m, 17H), 7.70 (d, J
(d, J(H,H)=7.8 Hz, 1H), 6.49 (s, 1H), 6.31 (s, 2H), 5.02 (s, 4H), 4.30 (t,
(H,H)=7.1 Hz, 4H), 3.74–3.67 (m, 2H), 3.25 (t, J(H,H)=7.6, 4H),
2.04–1.93 (m, 4H), 1.85–1.71 (m, 4H), 1.69–1.60 (m, 2H), 1.30–1.18 (m,
2H), 0.92 ppm (t, J
(H,H)=7.4 Hz, 3H); 13C NMR (50 MHz, CDCl3,
ACHTUNGTRNE(NUNG H,H)=7.8 Hz, 2H), 7.45 (s, 2H), 7.21
ACHTUNGTRENNUNG
Pyr8-G3-(styrylpyridinium) bromide (23): The product was synthesized
according to general procedure A from azide 4 (0.079 g, 0.026 mmol) and
acetylenic core 22b (0.05 g, 0.033 mmol). The crude product was purified
by column chromatography on neutral aluminum oxide, eluting with
CHCl3/methanol (20:1) to yield a red waxy solid (0.112 g, 87%). Rf =0.32
(CHCl3/MeOH, 20:1, red spot on an aluminum oxide TLC plate);
J
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
208C, TMS): d=159.8, 144.7, 143.3, 142.3, 137.2, 135.5, 135, 131.9, 131.1,
130.6, 130, 129.6, 128.3, 128.1, 127.3, 127.2, 127.1, 126.6, 125.9, 124.9,
124.7, 122.9, 122.7, 122.3, 105.6, 101.8, 100.9, 62, 56.5, 56.1, 50.3, 32.7,
30.1, 29.9, 28.4, 11.2 ppm; IR (KBr): n˜ =3030 (w), 2925 (w), 2852 (w),
1636 (m), 1570 (s), 1518 (m), 1446 (m), 1401 (w), 1380 (m) 1345 (m),
1328 (m), 1267 (m), 1149 (s), 1043 (m), 842 (s), 636 (m), 530 cmꢀ1 (m);
MS (ESI): m/z (%): 1041.5 (15) [MꢀNa+], 1018.4 (100) [M]+, 976.3 (34),
714.4 (70), 614.7 (37), 302.8 (60); elemental analysis (%) calcd for
C62H54N10O5: C 73.07, H 5.34, N 13.74; found: C 72.87, H 5.29, N 13.69.
1H NMR (400 MHz, CDCl3, 208C, TMS): d=8.51 (d, J
2H), 8.0–7.92 (m, 24H), 7.90–7.88 (m, 16H), 7.87–7.81 (m, 24H), 7.56 (d,
(H,H)=8 Hz, 8H), 7.3–7.23 (m, 10H), 7.91 (d, J(H,H)=8.4 Hz, 2H),
7.0–6.96 (m, 3H), 6.63 (s, 1H), 6.55 (d, J(H,H)=10.4 Hz, 12H), 4.43 (d,
(H,H)=6.4 Hz, 6H), 6.37 (d, J(H,H)=12.8 Hz, 2H), 6.15 (d, J(H,H)=
15.6 Hz, 1H), 5.40 (bs, 2H), 4.90–4.80 (m, 28H), 4.04 (t, J(H,H)=6.8 Hz,
16H), 3.20–3.13 (m, 4H), 3.05 (t, J(H,H)=6.8 Hz, 16H), 1.79–1.71 (m,
16H), 1.63–1.56 (m, 16H), 0.99 ppm (t, J
(H,H)=7.2 Hz, 6H); 13C NMR
ACHTUNGTRENUN(NG H,H)=6 Hz,
J
N
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
J
N
N
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Pyr4-G2-Br (11a): The product was synthesized according to general pro-
AHCTUNGTRENNUNG
cedure
A from azide 4 (0.29 g, 0.97 mmol) and acetylenic core 11
(50 MHz, CDCl3, 208C, TMS): d=159.7, 159.4, 143.3, 139.4, 135.6, 131.2,
130.6, 129.6, 128.3, 127.3, 127, 126.5, 125.8, 124.7, 123, 106.4, 101.2, 61.8,
50, 32.6, 30, 28.3 ppm; IR (KBr): n˜ =3020 (w), 2919 (w), 2851 (w), 1597,
(m), 1451 (m), 1323 (w), 1149 (s), 1037 (m), 841 (m), 746 cmꢀ1 (w); MS
(ESI): m/z (%): 1903.4 (6) [M2+], 1734.8 (5), 1610.2 (6), 1505.3 (5),
1444.3 (16), 1411.2 (100), 1269.6 (5) [M3+], 1221.8 (21), 1045.8 (14); ele-
mental analysis (%) calcd for C250H215N26O14Br: C 77.24, H 5.57, N 9.37;
found C 76.99, H 5.49, N 9.41.
(0.148 g, 0.24 mmol) and obtained as a brown waxy solid (0.416 g, 95%).
1H NMR (400 MHz, CDCl3, 208C, TMS): d=8.13–7.80 (m, 32H), 7.70 (d,
JACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
7.45 Hz, 8H), 1.97–1.85 (m, 8H), 1.81–1.69 ppm (m, 8H); 13C NMR
(50 MHz, CDCl3, 208C, TMS): d=159.4, 159.2, 143.2, 138.9, 135.2, 131,
130.4, 129.5, 128.1, 127.1, 126.8, 126.4, 125.5, 124.6, 124.4, 122.7, 122.4,
106.1, 101, 69.4, 67.7, 61.6, 49.7, 32.2, 30.1, 29.6, 27.9, 25.4 ppm; IR (KBr):
n˜ =3043 (m), 2960 (s), 1605 (s), 1461 (s), 1162 (s), 1049 cmꢀ1 (m); MS
(ESI): m/z (%): 1716.6 (8), 1567.2 (24), 1417.6 (47), 1267.5 (39), 1118
(20), 685.6 (28), 569.5 (20), 554.45 (74), 552.4 (100); elemental analysis
(%) calcd for C113H95BrN12O6: C 75.53, H 5.33, N, 9.35; found C 75.42, H
5.30, N 9.28.
Chem. Eur. J. 2009, 15, 754 – 764
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
763

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Acknowledgements
Funding from the Italian FIRB 2004 “Molecular compounds and hybrid
nanostructured materials with resonant and non-resonant optical proper-
ties for photonic devices” (contract no. RBNE033KMA) and from the
European Community under the contract RIII-CT-2003-506350 is ac-
knowledged. Mrs Brunella Innocenti and Mr Maurizio Passaponti are ac-
knowledged for technical assistance.
[11] P. L. Gentili, M. Mugnai, L. Bussotti, R. Righini, P. Foggi, S. Cicchi,
G. Ghini, S. Viviani, A. Brandi, J. Photochem. Photobiol. A 2007,
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Received: July 7, 2008
Revised: October 1, 2008
Published online: December 3, 2008
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