Phenylene alkylene dendrons with site-specific incorporated fluorescent pyrene probes

Article abstract of DOI:10.1021/jo050302p

This report deals with the synthesis and the spectroscopic properties of two second generation (G2) dendrons with site-specific incorporated phenyl pyrene derivatives as solvatochromic fluorescent probes. The generations that do not carry the probe are eq

Full text of DOI:10.1021/jo050302p

Phenylene Alkylene Dendrons with Site-Specific Incorporated
Fluorescent Pyrene Probes
Matthias Beinhoff,†,§ Wilfried Weigel,*,‡ Wolfgang Rettig,‡ Irene Bruedgam,† Hans Hartl,† and
A. Dieter Schlueter†,|
Institute of Chemistry, Free University Berlin, Takustr. 3, 14195 Berlin, Germany, Institute of Chemistry,
Humboldt University Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany, Max-Planck-Institute of
Colloids and Interfaces, 14424 Potsdam, Germany, and Institute of Polymers, Wolfgang-Pauli-Str. 10,
ETH Hoenggerberg, HCI J 541, CH-8093 Zuerich, Switzerland
weigel@chemie.hu-berlin.de
Received February 17, 2005
This report deals with the synthesis and the spectroscopic properties of two second generation
(G2) dendrons with site-specific incorporated phenyl pyrene derivatives as solvatochromic fluorescent
probes. The generations that do not carry the probe are equipped with volume dummies, pyrene
moieties that do not show a solvatochromic effect. Two complementary G2 phenylene alkylene
dendrons were synthesized using Suzuki-Miyaura cross coupling. Most of the reactions used in
the 10-step sequence generating the target compounds proceeded in good yields. The incorporated
probes can be selectively photoexcited and show solvatochromic shifts that are of the same
magnitude as for the free probes in a homogeneous solvent environment. In addition to the charge-
transfer fluorescence, a broad emission band is observed that is assigned to an intramolecular
exciplex formation between the aryl pyrene chromophores.
Introduction
details of the influence of the structure of the dendrimers
in solution on these processes remain an open question.
Because of their flexibility, many dendrimers can exist
in numerous low-energy conformations. Solvation is a
crucial parameter that determines if the dendrimers exist
in more extended or collapsed structures.5,6 Furthermore
The investigation of the photophysical properties of
dendrimers with incorporated chromophores has been the
subject of considerable interest in recent years.1 Because
of the unique molecular architecture of dendrimers,
especially the encapsulation effects of active core func-
tionalities and their potential as light-harvesting anten-
nas and as framework for redox-active functionalities,
they have received great attention.2-4 However, many
(3) (a) Adronov, A.; Fre´chet, J. M. J. Chem. Commun. 2000, 1701-
1710. (b) Brousmiche, D. W.; Serin, J. M.; Fre´chet, J. M. J.; He, G. S.;
Lin, T.-C.; Chung, S.-J.; Prasad, P. N.; Kannan, R.; Tan, L.-S. J. Phys.
Chem. B 2004, 108, 8592-8600. (c) Hahn, U.; Gorka, M.; Voegtle, F.;
Vicinelli, V.; Ceroni, P.; Maestri, M.; Balzani, V. Angew. Chem. 2002,
114, 3747-3750; Angew. Chem., Int. Ed. 2002, 41, 3595-3598.
(4) (a) Ceroni, P.; Vicinelli, V.; Maestri, M.; Balzani, V.; Mueller,
W. M.; Mueller, U.; Hahn, U.; Osswald, F.; Voegtle, F. New. J. Chem.
2002, 25, 989-993. (b) Ghaddar, T. H.; Wishart, J. F.; Thompson, D.
W.; Ehitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2002, 124, 8285-
8289. (c) Lor, M.; De Belder, G.; Schweitzer, G.; Fron, E.; Viaene, L.;
Cotlet, M.; Weil, T.; Muellen, K.; Verhoeven, J. W.; Van der Auweraer,
M.; De Schryver, F. C. Photochem. Photobiol. Sci. 2003, 2, 501-510.
(5) (a) Hecht, S.; Vladimirov, N.; Fre´chet, J. M. J. J. Am. Chem.
Soc. 2001, 123, 18-25. (b) Riley, J. M.; Alkan, S.; Chen, A.; Shapiro,
M.; Khan, W. A.; Murphy, W. R., Jr.; Hanson, J. E. Macromolecules
2001, 34, 1797-1809.
* To whom correspondence should be addressed. Phone: +49-30-
20935583. Fax: +49-30-20935574.
† Free University Berlin.
‡ Humboldt University Berlin.
§ Max-Planck-Institute of Colloids and Interfaces.
| ETH Zuerich.
(1) (a) Newkome, G. R.; Moorefield, C. N.; Voegtle, F. Dendrimers
and Dendrons-Concepts, Syntheses, Applications; Wiley-VCH: Wein-
heim, 2001. (b) Dendrimers and other Dendritic Polymers; Fre´chet, J.
M. J., Tomalia, D. A., Eds.; Wiley: New York, 2001.
(2) Hecht, S.; Fre´chet, J. M. J. Angew. Chem. 2001, 113, 76-94;
Angew. Chem., Int. Ed. 2001, 40, 74-91.
10.1021/jo050302p CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/27/2005
J. Org. Chem. 2005, 70, 6583-6591
6583

Beinhoff et al.
smaller dendrimers can be expected to form more open
structures in contrast to larger molecules that should
have more spherical shapes with a denser packing. As a
consequence, site isolation by encapsulation, for example,
strongly depends on the solvent properties.7 Structural
and solvent effects can also be expected to have a strong
influence on the efficiency of excimer-like interactions as
well as energy and electron transfer, processes that
strongly rely on the relative positions and energies of the
photoexcited states of the involved chromophores and
redox-active groups, respectively.8
of dendrimers. However, the possible variation of the
solvent density inside a dendrimer that could strongly
affect the microenvironment and especially the local
polarity at each site has not received much attention. If
the density of a dendrimer gradually decreases from its
interior to its exterior as, e.g., SANS measurements6,14
and computational studies6,15 indicate, the local polarity
created by generation-specific penetration of polar solvent
molecules would be expected to increase in the same
direction resulting in a polarity gradient, especially for
spherical dendrimers with a high degree of symmetry. A
similar polarity gradient has been supposed to have an
effect on the charge-transfer efficiency of photoexcited
polyelectrolyte multilayer systems.16 Such an effect could
be applied for dendritic systems that are equipped with
electron acceptors at defined generations to realize a
stepwise electron transfer through the dendritic frame-
work. A different stabilization of the formed radical
anions17 after charge separation due to different degrees
of solvation in each generation could provide the free-
energy gradient as the necessary driving force.
In our concept, we will apply fluorescent polarity
probes to study the possible polarity gradient in dendritic
structures with the probes incorporated at specific gen-
erations.18 Because of the bulkiness of such probes in
comparison to the dendrimer skeleton, the concept in-
volves so-called volume dummies. They will be attached
to those generations that do not carry the probes in order
to maintain the same sterical and solvation situation in
comparable generations of a set of dendrimers, Figure
1a.
In this project, the cyano-substituted phenyl pyrene
derivative 1 (Figure 1b) will be applied as fluorescence
probe. 1 was found to be a suitable probe due to its strong
solvatochromic effect and sufficient fluorescence quantum
yield.19 For mixtures of 1 and 2, it was already
shown that 1 fulfills the requirement to be exclusively
photoexcitable in the presence of the volume dummy
2.19
The tuning of the properties of dendrimers is commonly
achieved by derivatization of the peripheral end groups
or the core moiety. Contrary to this approach, the
internal functionalization of the volume between is
pursued with less activity.20 Especially the site-specific
incorporation of functional groups by premodified branch-
ing units seems to be a valuable goal, since there are only
a handful of examples known at present.21
The attachment of functional probes has facilitated the
detailed investigation of these properties.
A generation-dependent accessibility was shown for
dendrons by incorporating anthracene as a single fluo-
rescent probe unit in a specific location and studying the
accessibility of these locations using an intermolecular
photoinduced electron-transfer-based fluorescence quench-
ing process.9 Especially pyrene has been applied as a
probe due to its well-known fluorescence characteristics.
The relative structural permeabilities of a single pyrenyl
residue attached to an amine within a series of symmetric
poly(amido) dendrimers was investigated by quenching
experiments. The increasing blocking of a pyrenyl residue
with the growing dendrimer network could be shown.10
Pyrene photoactive cores were used as probes to inves-
tigate the isolation of the core functionality by fluores-
cence quenching, energy transfer, and solvatochromic
methods. With increasing chain length as well as solvent
polarity enhanced site isolation was found.5 Pyrene
moieties in poly(propylene imine) dendrimers were used
to probe the extent of steric crowding. More excimer
emission was observed for higher generations with little
evidence of interdendrimer interactions.11 Pyrenes were
also used for studies of the energy transfer in dendrimers.
Energy transfer from pyrene units at the periphery to a
diphenylanthracene core was observed for dendrons in
solution and particles of dendrimer aggregates.12 Excimer
emission was also observed as the predominant fluores-
cence in a series of pyrene-capped dendrons that can be
effectively quenched when a suitable donor group is
attached at the focal point. These results indicate sub-
stantial electronic coupling between the appended chro-
mophores and a suitable donor as the core unit.13
A more detailed knowledge of generation-specific sol-
vent effects should contribute to the basic understanding
(6) Ballauff, M.; Likos, C. N. Angew. Chem. 2004, 116, 3060-3082;
Angew. Chem., Int. Ed. 2004, 43, 2998-3020.
(14) (a) Poetschke, D.; Ballauff, M.; Lindner, P.; Fischer, M.; Voegtle,
F. Macromolecules 1999, 32, 4079-4087. (b) Poetschke, D.; Ballauff,
M.; Lindner, P.; Fischer, M.; Voegtle, F. Macromol. Chem. Phys. 2000,
201, 330-339. (c) Ballauff, M. Top. Curr. Chem. 2001, 212, 176-194.
(15) (a) Mansfield, M. L.; Klushin, L. I. Macromolecules 1993, 26,
4262-4268. (b) Boris, D.; Rubinstein, M. Macromolecules 1996, 29,
7251-7260. (c) Murat, M.; Grest, G. S. Macromolecules 1996, 29, 1278-
1285. (d) Wallace, E. J.; Buzza, D. M. A.; Read, D. J. Macromolecules
2001, 34, 7140-7146. (e) Maiti, P. K.; Cagin, T.; Wang, G.; Goddard,
W. A., III. Macromolecules 2004, 37, 6236-6254.
(7) Chasse, T. L.; Sachdeva, R.; Li, Q.; Li, Z.; Petrie, R. J.; Gorman,
C. B. J. Am. Chem. Soc. 2003, 125, 8250-8254.
(8) (a) Hofkens, J.; Zatterini, L.; De Belder, G.; Gensch, T.; Maus,
M.; Vosch, T.; Karni, Y.; Schweitzer, G.; De Schryver, F. C.; Hermann,
A.; Muellen, K. Chem. Phys. Lett. 1999, 304, 1-9. (b) Gorman, C. B.;
Smith, J. C.; Hager, M. W.; Parkhurst, B. L.; Gracz, H. S.; Haney, C.
A. J. Am. Chem. Soc. 1999, 121, 9958-9966. (c) Thomas, K. R. J.;
Thompson, A. L.; Sivakumar, A. V.; Bardeen, C. J.; Thayumanavan,
S. J. Am. Chem. Soc. 2005, 127, 373-383.
(9) Sivanadan, K.; Aathimanikandan, S. V.; Arges, C. G.; Bardeen,
C. J.; Thayumanavan, S. J. Am. Chem. Soc. 2005, 127, 2020-2021.
(10) Cardona, C. M.; Wilkes, T.; Ong, W.; Kaifer, A. E.; McCarley,
T. D.; Pandey, S.; Baker, G. A.; Kane, M. N.; Baker, S. N.; Bright, F.
V. J. Phys. Chem. B 2002, 106, 8649-8656.
(16) Tedeschi, C.; Li, L.; Moehwald, H.; Spitz, C.; von Seggern, D.;
Menzel, R.; Kirstein, S. J. Am. Chem. Soc. 2004, 126, 3218-3227.
(17) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry,
2nd ed.; VCH: Weinheim, 1988.
(18) Beinhoff, M.; Modrakowski, C.; Weigel, W.; Rettig, W.; Schluet-
er, A. D. Am. Chem. Soc., Polym. Mater. Sci. Eng. 2001, 84, 751-752.
(19) Beinhoff, M.; Weigel, W.; Jurczok, M.; Rettig, W.; Bruedgam,
I.; Hartl, H.; Modrakowski, C.; Schlueter, A. D. Eur. J. Org. Chem.
2001, 3819-3829.
(20) Hecht, S. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1047-
1058.
(11) Baker, L.-A.; Crooks, R. M. Macromolecules 2000, 33, 9034-
9039.
(12) Kobayashi, M.; Machida, S.; Takahashi, S.; Horie, K.; Yoshika-
wa, H.; Masuhara, H. Chem. Lett. 2002, 394-395.
(13) Stewart, G. N.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 4354-
4360.
6584 J. Org. Chem., Vol. 70, No. 17, 2005

Phenylene Alkylene Dendrons
FIGURE 1. (a) Dendrimers with generation-specific incorporated polarity probes and volume dummies. (b) Structures of the
parent molecules of polarity probe 1 and the corresponding volume dummy 2.
SCHEME 1
Here we report on the initial convergent synthesis22
of a set of two G2 dendrons with generation-specific
attached solvation probes and volume dummy molecules
based on AB2-type building blocks obtained by derivati-
zation of the phenyl pyrenes 1 and 2. The dendrimer
backbone consists of phenylene and alkylene units and
is generated by applying Suzuki-Miyaura cross-coupling
conditions, i.e., coupling of in situ prepared alkylboranes
and aromatic halides.23,24 This strategy allows for ac-
celerated convergent construction of all-hydrocarbon
dendrons because formation of one generation can be
achieved in a single preparative step. The UV absorption
and photoemission properties of the two G2 dendrons are
investigated and discussed in regard to solvatochromic
effects and intramolecular interaction of the incorporated
chromophores.
3 and 4 in yields of 62 and 90%, respectively (Scheme 1),
as the mixture of their isomers. The ratio of the 3-, 6-,
and 8-regioisomers was about 1:2:2, revealed by 1H NMR
spectroscopy. Repeated recrystallization instead of col-
umn chromatography was the most efficient way to
remove the 3-isomer from the mixture of the isomers. The
remaining mixture of the 6- and 8-isomers 3a/3b was
used in the following steps.
Results and Discussion
Syntheses. To attach the probe 1 and the volume
dummy 2 covalently to a dendritic skeleton, they have
to be equipped with functional groups. Their incorpora-
tion can be realized either on the benzonitrile part or the
pyrene part. The latter approach is subject of this report.
Since Ti(IV)-catalyzed formylation of pyrene selectively
gives the monoformyl product in high yields, this reaction
was chosen for the second substitution step.19,25,26 Reac-
tion of pyrenes 1 and 2 with dichloromethoxymethane
in the presence of TiCl4 gave the corresponding aldehydes
Small amounts of the pure samples of 3a and 3b were
obtained for their characterization by repeated fraction-
ated recrystallization. In 2D heteronuclear multiple
quantum coherence (HMQC) NMR experiments, all
signals of the 1H and 13C NMR spectra could be assigned.
The large downfield shifts of H-5 (3a) and H-9 (3b) are
caused by the strong deshielding of the neighboring
carbonyl group. This was confirmed by single-crystal
X-ray diffraction of 3b which represents one of the few
examples of a pyrene derivative single-crystal X-ray
structure with two acyclic carbon substituents in the
ortho (1-, 3-, 6-, and 8-) positions known to present.27 The
carbonyl group lies in the pyrene plane with a deviation
of 2°. The carbonyl oxygen points toward the proton H-9.
The O-H-9 distance of 2.32 Å is significantly shorter
than the sum of the van der Waals radii (2.60 Å). Since
the distance O-C-9 is 2.96 Å and the angle C-9-H-9-O
amounts to 125°, one can interpret such an arrangement
as an attractive intramolecular C-H‚‚‚O bonding.28
These values match very well with other hydrogen bonds
of the same kind. The dihedral angle of the phenyl group
(21) See for example: (a) Newkome, G. R.; Moorefield, C. N.; Keith,
J. M.; Baker, G. R.; Escamilla, G. H. Angew. Chem. 1994, 106, 701-
703; Angew. Chem., Int. Ed. Engl. 1994, 33, 666. (b) Galliot, C.; Larre´,
C.; Caminade, A.-M.; Majoral, J.-P. Science 1997, 277, 1981-1984. (c)
Freeman, A. W. L.; Christoffels, A. J.; Fre´chet, J. M. J. J. Org. Chem.
2000, 65, 7612-7617. (d) Bo, Z.; Schaefer, A.; Franke, P.; Schlueter,
A. D. Org. Lett. 2000, 2, 1645-1648. (e) Shultz, L. G.; Zhao, Y.;
Zimmerman, S. C. Angew. Chem. 2001, 113, 2016-2020; Angew.
Chem., Int. Ed. 2001, 40, 1962-1966.
(22) (a) Grayson, S. M.; Fre´chet, J. M. J. Chem. Rev. 2001, 101,
3819-3868. (b) Freeman, A. W.; Fre´chet, J. M. J. In ref 1b, pp 91-
110.
(23) (a) Miyaura, N.; Ishiyama, T.; Sasaki, H.; Ishikawa, M.; Satoh,
M. J. Am. Chem. Soc. 1989, 111, 314-321. (b) Chemler, S. R.; Trauner,
D.; Danishefsky, S. Angew. Chem. 2001, 113, 4676-4701; Angew.
Chem., Int. Ed. 2001, 40, 4544-4568.
(24) Beinhoff, M.; Karakaya, B.; Schlueter, A. D. Synthesis 2003,
79-90.
(27) (a) Schmitt, P.; Mandon, D.; Fischer, J.; Weiss, R. New J. Chem.
1992, 16, 763-765. (b) Sugiura, K.; Mikami, S.; Iwasaki, K.; Hino, S.;
Asato, E.; Sakata, Y. J. Mater. Chem. 2000, 10, 315-319.
(28) (a) Desiraju, G. R. Acc. Chem. Res. 1991, 24, 290-296. (b)
Desiraju, G. R. Acc. Chem. Res. 1996, 29, 441-449. (c) Desiraju, G. R.
Acc. Chem. Res. 2002, 35, 565-573.
(25) For other examples see: Rieche, A.; Gross, H.; Hoeft, E. Chem.
Ber. 1963, 96, 88-94.
(26) Yamazo, T.; Miyazawa, A.; Tashiro, M. J. Chem. Soc., Perkin
Trans 1 1993, 3127-3135.
J. Org. Chem, Vol. 70, No. 17, 2005 6585

Beinhoff et al.
SCHEME 2
SCHEME 5
SCHEME 3
SCHEME 4
tested. The best result was obtained when an excess of
the alcohol was deprotonated with sodium hydride in
THF and benzyl chloride was added along with catalytic
amounts of crown ether 15[K]5.32 The yields of the
isolated ethers 15-17 ranged between 82 and 89%.
The monomers 15 and 16 carry the fluorescent probe,
compound 17, the volume dummy. To convergently
construct dendrons with the probe unit at a specific site,
peripheral units were synthesized by converting the
halogens of the monomers 15-17 into propyl TBDMS
ethers. Incorporation of a peripheral functionality allows
a flexible subsequent derivatization of the dendrimers.
The TBDMS group as a hydroxy-protecting group was
chosen because of its mild cleavage conditions. In the
present system, the pyrene moieties are attached to the
hydrocarbon skeleton of the dendron by two hydrolyti-
cally sensitive benzylic functions. Allyl TBDMS ether 18
was therefore in situ hydroborated with 9-BBN-H and
reacted with monomers 15 and 17 in the presence of Pd-
[(PPh)3]4 in a homogeneous THF/aqueous base system to
give the peripheral building blocks 19 and 20 in an
isolated yield of 52 and 58%, respectively (Scheme 5). The
comparatively low yields were due to a loss of product in
the column chromatography purification step because the
monocoupling products and the desired compounds 19
and 20 exhibit very similar Rf values (TLC). All precursor
molecules with an ethylene bridge between the pyrene
and the benzonitrile unit showed a higher propensity to
solidify than the phenyl pyrene analogue. This trend
culminated in the observation that the volume dummy
molecule 20 is an amorphous solid, whereas the probe
molecule 19 is an oil.
and the pyrene plane is 62°, which corresponds to the
calculated value of 61° for phenyl pyrene.27,29
Aldehydes 3 and 4 were quantitatively (thin-layer
chromatography, TLC) reduced to benzyl alcohols 5 and
6 with NaBH4, and the hydroxy goups converted into the
corresponding benzyl chlorides 7 and 8 with thionyl
chloride on a multigram scale (Scheme 2). Because of
their low stability on silica gel, the benzyl chlorides were
purified by recrystallization from the reaction mixture.
3,5-Dibromo and diiodo homoallyl benzenes were shown
to act as AB2 monomers in the convergent synthesis of
dendrons by Suzuki-Miyaura cross coupling.23 To apply
this strategy to the present probe and dummy concept,
the related compounds 13 and 14 were synthesized,
which carry a benzylic hydroxy function for subsequent
attachments (Scheme 3). 1,3,5-Tribromobenzene (9) was
monolithiated with butyllithium in diethyl ether at -78
°C, and the resulting lithium organyl intermediate was
trapped with dimethylformamide (DMF) to give aldehyde
11.30 The 1,3,5-triiodobenzene (10) was lithiated in
toluene at room temperature and subsequently reacted
with DMF to prevent multiple lithiation and formation
of butylated byproducts. Both aldehydes 11 and 12 were
allylated under mild conditions with allyl trimethoxy
silane in the presence of catechol and triethylamine to
give the butenols 13 and 14 in yields of about 85%.31
Butenols 13 and 14 were coupled to the benzyl chlo-
rides 7 and 8, respectively, by modified Williamson
conditions to afford the AB2 monomers 15-17 (Scheme
4). Various conditions for the ether bond formation of a
secondary alkoxide with chloro methyl pyrene were
Repetition of the procedure, i.e., hydroboration of the
terminal olefin at the focal point of the peripheral units
19 and 20 and cross coupling with the opposing mono-
mers 17 or 15/16, respectively, resulted in both G2
dendrons 21 and 22 (Schemes 6 and 7). They carry the
solvatochromic probe molecule either in the second
(outer) or in the first (inner) generation. The side
products, i.e., the monocoupling product and the excess
(29) Reynders, P.; Kuehnle, W.; Zachariasse, K. A. J. Am. Chem.
Soc. 1990, 112, 3929-3939.
(30) Chen, L. S.; Chen, G. J.; Tamborski, C. J. Organomet. Chem.
1981, 215, 281-291.
(31) Hosomi, A.; Kohra, S.; Ogata, K.; Yunagi, T.; Tominaga, Y. J.
Org. Chem. 1990, 55, 2415-2420.
(32) Yamana, K.; Takei, M.; Nakano, H. Tetrahedron Lett. 1997, 38,
6051-6054.
6586 J. Org. Chem., Vol. 70, No. 17, 2005

Phenylene Alkylene Dendrons
SCHEME 6
SCHEME 7
of 25% of the hydroborated G1 dendron, could not be
removed quantitatively by column chromatography. Pu-
rification was finally achieved by preparative gel perme-
ation chromatography (GPC) separation. Because of the
multistep workup procedure, the isolated yields of the
G2 dendrons 21 and 22 were as small as 11 and 50%,
respectively.
The target compounds were obtained as analytically
pure materials, and their structures have unequivocally
been characterized. All 1H NMR signals besides the
pyrenyl protons could be assigned (Figure 2). Formation
of the product is indicated by the new resonances for the
protons H-7 to H-9 compared to the spectra of the starting
material. Interestingly, the absorptions for the aromatic
J. Org. Chem, Vol. 70, No. 17, 2005 6587

Beinhoff et al.
FIGURE 2. 1H NMR spectrum of G2 dendron 21 (§: CHCl3).
protons of the branching benzene units are all separated
with a maximum difference of 0.33 ppm from H-6 to H-11.
The signals of H-3, H-9, H-18, and H-23 all show a large
diastereotopic splitting, although partly superimposed
with other signals. The matrix-assisted laser desorption-
ionization time-of-flight (MALDI TOF) mass spectro-
metric analyses displayed intense peaks for the molecular
ion of sodium and potassium adducts. Their isotope
distribution fits the calculated data.
Spectroscopic Properties. The UV absorption and
fluorescence properties of the dendrons 21 and 22 with
incorporated probe units in the first and second genera-
tion, respectively, were investigated in comparison to the
free cyano-substituted phenyl pyrene derivative 119 as
model probe and the corresponding dummy 2 with an
ethyl group as a tether between the pyrene and the
benzonitrile moiety. Besides the change of the microen-
vironment of the incorporated probes, the influence of the
dendritic backbone, especially in regard to possible
intramolecular interactions between the chromophores,
was of special interest.
The normalized UV absorption and fluorescence spec-
tra of the dendrons 21 and 22 are given in Figure 3. As
it was shown for mixtures of the free probe 1 and the
dummy 2,19 a selective excitation of the incorporated
probes in 21 and 22 can be realized for excitation
wavelengths >360 nm.
The UV absorption spectra of 21 and 22 show the main
maximum at about 352 nm, Table 1. In contrast to the
free probe, the maxima are red shifted by about 10 nm
and a fine structure with multiple maxima is observed
independent of the solvent polarity. The fluorescence
spectra in the nonpolar solvent methylcyclohexane (MCH)
have a maximum at about 410 nm for both compounds.
In acetonitrile (ACN), the spectra are considerably red
shifted with maxima at 432 nm for 21 and 437 nm for
22. This solvatochromic effect indicates that a certain
charge-transfer (CT) character is present in the excited
FIGURE 3. UV absorption in MCH and fluorescence spectra
of the dendrons 21 and 22 in MCH and ACN. The thin lines
represent the decomposed fluorescence spectra.33
states, with the pyrene moiety as the donor and the
benzonitrile group as the acceptor unit.19
Interestingly, in addition to the CT band, a red-shifted
shoulder between 470 and 505 nm appears in the
emission spectra of the dendrons and the intensity of the
shoulder with respect to the main peak is higher for 22
than for 21, Figure 3. In the case of 22, the relative
intensity of the shoulder is higher in ACN than in MCH.
The Stokes shifts were calculated as the difference of
the energies of the absorption and fluorescence maxima,
determined after deconvolution of the superimposed
spectra,33 with values of about 4000 cm-1 in MCH and
(33) Using the program: Peakfit 4.06 ed.; SPSS Inc.: 1995.
6588 J. Org. Chem., Vol. 70, No. 17, 2005

Phenylene Alkylene Dendrons
TABLE 1. UV Absorption and Fluorescence Maxima,
Stokes Shifts, and Fluorescence Quantum Yields of the
Free Probe 1, the Dummy Molecule 2, and the Dendrons
with Incorporated Probes in the First or Second
Generation
compd solvent λabs, nm λflu, nma νST, cm-1 b Φfl
τfl, nsc
b
1
MCH
ACN
MCH
ACN
MCH
343
344
344
344
352
391, 406
427
4050
5650
3000
2950
4950
7350
5250
8500
4100
7950
5550
8600
0.69
5.8
2.7
7.3
7.2
0.81
2
378, 389
378, 388
409
0.042
0.063
0.38
0.043
0.28
21
475 shd
432
ACN
MCH
ACN
352
352
352
503 shd
411
0.015
0.36
22
2.4
489 shd
437
0.096 31.3e
0.24
0.10
1.7
505 shd
13.6e
a Fluorescence maxima of 21 and 22 from the deconvoluted
spectra.33 b Error about 10%. c Error about 5%, excitation at 357
nm and detection at the fluorescence maxima. d Shoulder. e De-
tection at 550 nm to minimize contribution of the CT emission.
FIGURE 4. Decrease of the shoulder at 490 nm of 22 in MCH
relative to the main fluorescence peak with decreasing tem-
perature. The appearance of a new small peak at 375 nm at
low temperature can be assumed to indicate the formation of
a pure locally excited state of the probe chromophore.37
5400 cm-1 in ACN for the CT band and values between
7000 and 8600 cm-1 for the red-shifted emission, Table
1. The fluorescence quantum yields were calculated for
both emission bands from the decomposed fluorescence
spectra with higher values for the CT than for the red-
shifted emission. The values for the CT bands are slightly
lower in comparison to the values of the free probe 1,
Table 1. The solvatochromic shifts and the quantum
yields for the CT bands of 21 and 22 are of the same
magnitude as the values reported for 1. These two
measures have been the key parameters in the selection
of 1 as suitable probe molecule.19 This indicates that the
attachment of the pyrene moiety of the probe to the
dendritic backbone does not affect the formation of the
photoexcited CT state that is combined with a geo-
metrical relaxation of the pyrene and benzonitrile sub-
units of the probe. As expected, no significant differences
have been found in the spectroscopic parameters of the
dendrones 21 and 22 probably due to the small size and
the flexibility of these dendritic subunits that levels out
any differences in the microenvironment of the incorpo-
rated probes.
The fluorescence decay times have been measured for
1, 2, and 22 in MCH and ACN with the shorter lifetimes
in the highly polar solvent, Table 1. In comparison to the
main CT emission, fluorescence lifetimes of more than
one magnitude higher have been determined for the
species that is assigned to the red-shifted shoulder.
The observed shoulder in the range of 470-505 nm
indicates the existence of a second emissive state. Pyrenes
are well known for the ability to form excimers in
solutions of concentrations higher than 10-5 M with
typically broad and red-shifted emission bands, where
the interacting chromophores are arranged in a sandwich-
like structure.34 In principle, such an intermolecular
interaction could also be the reason for the formation of
the second emission band observed for 21 and 22. In this
case, a concentration dependence of the relative intensi-
ties of the CT band vs the shoulder in the fluorescence
spectra would be expected. However, such dependence
was not found for the spectra of 22 in the range of the
measured sample concentrations (10-7-10-6 M).
Interactions between different chromophores could also
occur intramolecularly for 21 and 22. In this case, it can
be assumed that a geometrical reorientation between the
subunits of the molecules must take place. It is well
known that such processes are affected by a change of
the solvent viscosity.35,36 A significant decrease of the
intensity of the shoulder relative to the main peak by
lowering the temperature was observed for 22 in MCH
and butyronitrile. At 77 K, the red-shifted emission band
disappears completely, i.e., the reorientation process that
leads to the formation of this excited state is completely
frozen out, Figure 4. This observed temperature effect
strongly supports the assumption of an intramolecular
exciplex formation.
After excitation of the probe chromophore in 1, 21, or
22 and initial population of the Franck-Condon state a
geometrically relaxed fluorescent CT state is formed as
it was described for other phenyl pyrene derivatives.19,37
In the cases of 21 and 22, a subsequent interaction
between the chromophores of the photoexcited probe and
the dummy moiety that requires a change of the confor-
mation of several single bonds in the dendron leads to
the formation of the exciplex as illustrated in Scheme 8.
This is consistent with the results of the lifetime
measurements for 22 where the relatively long fluores-
cence lifetimes (31 and 14 ns) of the red-shifted exciplex
emission in contrast to the values for the CT emission of
about 2 ns can be explained by the stronger electronic
coupling between the donor and acceptor moiety in the
latter case due to the shorter distance between both
units.38 However, the short fluorescence lifetime of the
(35) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. Rev. 2003,
103, 3899-4031.
(36) Zhang, B.-W.; Cao, Y.; Bai, J.-W.; Chen, J.-R. J. Photochem.
Photobiol. A 1997, 106, 169-175. (b) Kawakami, J.; Furuta, T.;
Nakamuru, J.; Uchida, A.; Iwamura, M. Bull. Chem. Soc. Jpn. 1999,
72, 47-54.
(37) Weigel, W.; Rettig, W.; Dekhtyar, M.; Modrakowski, C.; Bein-
hoff, M.; Schlueter, A. D. J. Phys. Chem. A 2003, 107, 5941-5947.
(38) Kawakami, J.; Nakamura, J.; Iwamura, M. J. Photochem.
Photobiol., A 2001, 140, 199-206.
(34) Turro, N. J. Modern Molecular Photochemistry; The Benjamin/
Cummings Publishing Company: Menlo Park, CA, 1978.
J. Org. Chem, Vol. 70, No. 17, 2005 6589

Beinhoff et al.
SCHEME 8. Charge Transfer within the Probe
Unit after Photoexcitation of 22 and Subsequent
Exciplex Formation with the Dummy Moiety as
the Donor and the Photoexcited Probe as
Acceptor
to the solvent density in the interior of larger dendrimers
those synthesis is under way. In contrast to the free probe
molecules, the G2 dendrons show an additional broad
emission band in the fluorescence spectra that is assigned
to the formation of an exciplex between the aryl pyrene
chromophores. The exciplex formation requires a geo-
metrical reorientation between the subunits of the G2
dendrons that are highly flexible due to their relatively
small size and can be frozen out at low temperatures.
Experimental Section
X-(4-Benzonitrile)-pyrene-1-carbaldehyde, Three Iso-
mers (X ) 3, 6, 8) (3). To a solution of 4-pyren-1-yl-
benzonitrile 1 (11.51 g, 37.91 mmol) and dichloro methyl
methyl ether (4.30 mL, 47.54 mmol, 1.25 equiv) in CH2Cl2 (250
mL), titanium(IV)chloride (11.5 mL, 104.9 mmol, 2.77 equiv)
was added within 30 min at 0 °C. The dark mixture was stirred
at room temperature overnight. Water (100 mL) was added,
the phases separated, and the aqueous one was extracted with
CH2Cl2. The combined organic layers were dried over MgSO4,
and the solvent was evaporated. The residue was purified by
column chromatography on silica gel (CH2Cl2). Yield: 62%
(7.79 g) of 3 as a mixture of regioisomers (3-, 6-, and 8-formyl
isomer). By recrystallization of suitable fractions pure (1H
NMR), samples of the 6- and the 8-isomers 3a and 3b were
obtained; their structures were elucidated by 2D HMQC NMR
experiments. The structure of 8-isomer 3b was solved by X-ray
analysis.
[X-(4-Benzonitrile)-pyren-1-yl]-methanol, Two Isomers
(X ) 6 and 8) (5). Aldehyde 3 (11.95 g, 36.06 mmol) was solved
in THF (500 mL), and 2-propanol (1000 mL) was added. After
addition of sodium borohydride (7.25 g, 191.65 mmol, 5.32
equiv), the reaction mixture was stirred for 2 h at ambient
temperature. The unreacted sodium borohydride was hydro-
lyzed by dropwise addition of diluted hydrochloric acid to the
ice-cooled mixture until no more hydrogen evolution occurred.
The organic solvents were distilled off, and the residue was
extracted with dichloromethane. The combined organic layers
were dried and filtered, and the solvent evaporated. Column
chromatography (CH2Cl2, then CH2Cl2:methanol ) 20:1) gave
11.80 g (98%) of a mixture of two regioisomers of [X-(4-
benzonitrile)-pyren-1-yl]-methanol 5.
1-Chloromethyl-X-(4-benzonitrile)-pyrene, One Isomer
(X ) 6 or 8) (7). To a suspension of alcohol 5 (9.87 g, 29.61
mmol) in CH2Cl2 (350 mL) a solution of thionyl chloride in CH2-
Cl2 (50 mL) was added dropwise at room temperature (40 min).
The clear solution was stirred until gas evolution stopped (3
h). To the ice-cooled solution, aqueous NaOH (1 M, 150 mL)
was added carefully, and the phases were separated. The
aqueous one was extracted with CH2Cl2, the combined organic
layers were dried over MgSO4 and filtered, and the solvent
evaporated. The resulting brownish residue was recrystallized
from CH2Cl2:methanol to give 8.60 g (83%) of 7 as an
amorphous solid. The reaction proceeded quantitatively as the
TLC showed no alcohol. The reduced yield may be due to the
loss of product in the recrystallization step. It was found that
the benzyl chloride was not stable on silica gel (2D TLC).
3,5-Diiodobenzaldehyde (12). To a suspension of 1,3,5-
triiodobenzene 10 (17.39 g, 38.15 mmol) in toluene (650 mL),
a 1.6 M solution of n-butyllithium in hexane (28.00 mL, 44.80
mmol, 1.17 equiv) was added within 60 min at room temper-
ature. After 3 days, N,N-dimethyl formamide (10.0 mL, 119.0
mmol, 3.12 equiv) was added all at once and the mixture was
stirred for an additional 1 day with a mechanical stirrer. Water
(200 mL) was added, the phases were separated, and the
aqueous one was extracted with toluene twice. The combined
organic layers were dried over MgSO4, and the filtrate
evaporated. Chromatographic separation through silica gel
(hexanes:EtOAc ) 10:1) gave 9.02 g (66%) of 12 as colorless
CT state in the nanosecond range should be long enough
to allow reorientation of the chromophores involved in
the exciplex formation at least from those ground-state
conformations that are close to the proposed exciplex
structure. The intramolecular exciplex formation also
explains the observed higher intensity of the shoulder
in the fluorescence spectra of 22 compared to 21 since
the probability for exciplex formation between the
photoexcited probe and the pyrene chromophore of the
dummy is higher in 22 due to the better probe/dummy
ratio of 1:2 vs 2:1 in 21.
Conclusions
In summary, we have presented the initial convergent
synthesis of a set of two G2 dendrons with generation-
specific attached solvatochromic probes and volume dum-
mies. The site-specific incorporation of functional groups
was achieved by derivatization of phenyl pyrenes to
obtain premodified branching units. The phenylene alky-
lene dendrimer backbone was constructed in an acceler-
ated manner by applying Suzuki-Miyaura cross-coupling
conditions where one generation can be synthesized in a
single preparative step. The incorporated probes can be
selectively photoexcited and the strong solvatochromic
shifts observed in the same magnitude as for the parent
probe molecules in a homogeneous solvent environment
can be explained by the high charge transfer character
of the excited states. This solvatochromic effect will be
used as a tool to probe the microenvironment in regard
6590 J. Org. Chem., Vol. 70, No. 17, 2005

Phenylene Alkylene Dendrons
crystals. When Et2O or THF at -78 °C was used instead of
toluene, the product was accompanied by butylated side-
products.
solution of 9-BBN-H (12.55 mL of a 0.5 M solution in THF,
6.28 mmol, 3.00 equiv) at 0 °C, and the solution was stirred
for 18 h at ambient temperature. After addition of aqueous
sodium hydroxide (1 M, 20 mL), 15 (1.30 g, 2.09 mmol), the
catalyst precursor Pd(PPh3)4 (0.15 g, 1.3‚10-4 mol, 3.1 mol-%
per coupling) and additional THF (20 mL), the suspension was
stirred for 17 h at 60 °C. Extractive workup with diethyl ether
and column chromatography on silica gel (CH2Cl2) gave 0.88
g (52%) of 19 as a highly viscous oil.
1-(3,5-Dibromo-phenyl)-but-3-en-1-ol(13).Allyl-trimethoxy-
silane (18.00 mL, 106.83 mmol, 1.53 equiv) was added drop-
wise to a suspension of 18.50 g (70.10 mmol) 3,5-dibromo-
benzaldehyde 9, 23.01 g (208.97 mmol, 2.98 equiv) catechol,
and triethylamine (50 mL, 358.73 mmol, 5.12 equiv). The
mixture was stirred at 70 °C for 24 h. The acidified (25%
hydrochloric acid) suspension was extracted with dichlo-
romethane (5×), the combined organic phases were washed
with brine and dried over MgSO4, and the solvent was
evaporated. The residue was purified by column chromatog-
raphy on silica gel (hexanes/EtOAc ) 10:1), and the product
was distilled in vacuo to give 17.96 g (84%) of 84 as a colorless
liquid.
1-(4-Benzonitrile)-X-[1-(3,5-dibromo-phenyl)-but-3-en-
yloxymethyl]-pyrene, Two Isomers (X ) 6, 8) (15). To a
suspension of sodium hydride (0.332 g of a 60% suspension in
mineral oil, 8.65 mmol, 2.03 equiv) in THF (5 mL) was added
carefully a solution of 1-(3,5-dibromo-phenyl)-but-3-en-1-ol 13
(2.57 g, 8.40 mmol, 1.97 equiv) in THF (5 mL). After 1 h
1-chloromethyl-X-(4-benzonitrile)-pyrene 7 (1.50 g, 4.26 mmol),
[15]-crown-5 (0.094 g, 0.43 mmol, 0.1 equiv), and THF (50 mL)
were added, and the solution was stirred overnight. A small
amount of silica gel was added, and the solvent was removed.
The residue was purified by column chromatography on silica
gel (CH2Cl2) to yield 2.21 g (83%) of 15 as greenish glassy
material that could not be recrystallized.
Acknowledgment. Financial support by the
Deutsche Forschungsgemeinschaft (DFG, Sonderfors-
chungsbereich 448, TP A1 and B3) and the Fonds der
Chemischen Industrie is gratefully acknowledged. We
thank Dr. A. Schaefer for recording the 2D NMR
spectra, Dr. S. Weidner and Dr. P. Franke for the
MALDI TOF mass spectra, C. Zimmermann for the GPC
separations, and A. Rothe for recording some of the UV
absorption and fluorescence spectra.
Supporting Information Available: General experimen-
tal methods, synthesis of compounds 4, 6, 8, 14, 16, 17, 20,
21, and 22, analytical data of all compounds, 1H NMR spectra
of 3a and 3b, crystallographic data, including an ORTEP plot,
tables with crystal data, structure refinement, figures of
crystal lattice and unit cell, the CIF file of compound 3b, and
the concentration dependent fluorescence spectra of 22. This
material is available free of charge via the Internet at
http://pubs.acs.org.
1-(1-{3,5-Bis-[3-(tert-butyl-dimethyl-silanyloxy)-propyl]-
phenyl}-but-3-enyl oxymethyl)-X-(4-benzonitrile)-pyrene,
Two Isomers (X ) 6, 8) (19). Allyloxy-dimethyl-tert-butyl-
silane 18 (1.33 mL, 6.26 mmol, 2.99 equiv) was added to a
JO050302P
J. Org. Chem, Vol. 70, No. 17, 2005 6591