Synthesis and chiroptical properties of chiral azoaromatic dendrimers with a C3-symmetrical core

Article abstract of DOI:10.1002/chir.20712

New chiral azoaromatic dendrimeric systems have been synthesized starting from 1,3,5-benzenetricarbonyl trichloride as the core molecule. The simultaneous presence of the (S)-3-hydroxy pyrrolidinyl ring as the optically active moiety and the azobenzene donor-acceptor conjugated system as the photochromic group with permanent dipole moment, makes these systems potentially interesting as materials for advanced applications in nanotechnologies. All the compounds obtained have been characterized with particular attention to the effects induced by changing the electron-with-drawing group in the chromophoric moiety and to their optical activity. A strong nonlinear enhancement of chiroptical properties related to the number of chiral units linked to the symmetrical core is observed in these derivatives, which indicates the presence of conformationally chiral substructures.

Full text of DOI:10.1002/chir.20712

CHIRALITY 22:99–109 (2010)
Synthesis and Chiroptical Properties of Chiral Azoaromatic
Dendrimers with a C3-Symmetrical Core
*
LUIGI ANGIOLINI, TIZIANA BENELLI, AND LORIS GIORGINI
Dipartimento di Chimica Industriale e dei Materiali and INSTM UdR-Bologna, University of Bologna,
40136 Bologna, Italy
ABSTRACT
New chiral azoaromatic dendrimeric systems have been synthesized
starting from 1,3,5-benzenetricarbonyl trichloride as the core molecule. The simultane-
ous presence of the (S)-3-hydroxy pyrrolidinyl ring as the optically active moiety and
the azobenzene donor-acceptor conjugated system as the photochromic group with per-
manent dipole moment, makes these systems potentially interesting as materials for
advanced applications in nanotechnologies. All the compounds obtained have been char-
acterized with particular attention to the effects induced by changing the electron-with-
drawing group in the chromophoric moiety and to their optical activity. A strong nonlin-
ear enhancement of chiroptical properties related to the number of chiral units linked to
the symmetrical core is observed in these derivatives, which indicates the presence of
C
VV
conformationally chiral substructures. Chirality 22:99–109, 2010.
2009 Wiley-Liss, Inc.
KEY WORDS: chiral dendrimers; chiral nanotechnology; chiroptical properties;
photochromic materials; azobenzene; multifunctional materials
INTRODUCTION
seems to open new possibilities for the use of azobenzene-
containing materials as chiroptical switches, besides the
usual applications in optics.
Intense attention has currently arisen in investigations
dealing with chiral nanotechnology1,2 and the amplification
of chirality of polymeric materials.3–5
The need to seek new optical materials and devices has
led in recent years to the study of photoresponsive dendri-
tic macromolecular systems.28–30 This class of compounds
is very attractive because of their unique physical proper-
ties31,32 related to their three-dimensional branched struc-
ture and globular symmetrical conformation. Furthermore,
the incorporation of specific functional groups within their
structural interior or their periphery, makes them available
as potential advanced materials.28,33
Several studies are reported in the literature in this
direction28,34; for instance, a dendritic polyisophthalate
endcapped with 12 naphthyl groups35 is promising as a de-
vice for optical storage of information and holographic re-
cording, because of its compact shape and low molecular
weight which provides low viscosity and good compatibil-
ity with monomers and corresponding polymers.
In this context, chiral dendrimers,6–10 supramolecular
helical polymers,5,11,12 and optically active low molecular
weight compounds with C3 symmetry have found increas-
ing interest, because they are expected to be useful in
chemical operations including asymmetric catalysis, chiral
recognition, and resolution.13,14 This field has been investi-
gated by Moberg15 and reviewed recently, with particular
attention on C3-symmetrical nanoarchitecture (supramo-
lecular and macromolecular), by Gibson.16
It is well known that polymers containing azoaromatic
moieties are of remarkable potential interest for several
advanced technological applications, such as optical data
storage,17 nonlinear optical (NLO) switches,18 holographic
memories19,20 and in general as materials exhibiting pho-
toresponsive properties when irradiated with light of suita-
ble frequency and intensity.21–23
However, relatively few studies have been conducted on
the preparation of photochromic chiral dendrimers and
their structural–chiroptical properties. By selecting appro-
priate chiral elements of one single configuration and plac-
ing them in predefined positions, the influence of the
three-dimensional disposition of these chiral elements on
Furthermore, the presence of a chiral group of one pre-
vailing configuration interposed between the polymeric
backbone and the trans-azoaromatic chromophore allows
the polymers to display both the properties typical of dis-
symmetric systems (optical activity, exciton splitting of
dichroic absorptions), as well as features typical of photo-
chromic materials (photorefractivity, photoresponsiveness,
NLO properties).24,25
Contract grant sponsor: MIUR ; Contract grant number: PRIN2007.
Contract grant sponsor: Consortium INSTM.
In addition, we have recently observed26,27 that it is pos-
sible to photomodulate the chiroptical properties of thin
films of chiral photochromic polymers by irradiation with
circularly polarized (CP) light of one single left (L) or right
*Correspondence to: L. Angiolini, Dipartimento di Chimica Industriale e
dei, Materiali and INSTM UdR-Bologna, University of Bologna, Viale
Risorgimento 4, 40136 Bologna, Italy. E-mail: luigi.angiolini@unibo.it
Received for publication 25 July 2008; Accepted 6 February 2009
DOI: 10.1002/chir.20712
Published online 22 April 2009 in Wiley InterScience
(R) rotation sense. This unexpected new phenomenon (www.interscience.wiley.com).
C
VV
2009 Wiley-Liss, Inc.

100
ANGIOLINI ET AL.
the chiroptical properties of photochromic optically active
materials containing the trans-azoaromatic moiety can
then be evaluated.
In this context, we have focused our attention on the
synthesis of a couple of novel derivatives with a C3-sym-
metric core functionalized with three and six optically
active azoic alcohols (see Fig. 1). With the aim of investi-
gating the interactions which can be established between
chromophores, systems with different electron-withdraw-
ing groups in 40 position of the azoaromatic residue have
also been prepared and studied.
All the derivatives obtained have been fully character-
ized and their spectroscopic properties compared with
those of a model compound, bis-[(S)-HAP] (see Fig. 1), in
order to better understand the structure–property relation-
ships of these compounds, with particular attention to opti-
cal activity and chiroptical properties.
MATERIALS AND METHODS
General
1H and 13C NMR spectra were obtained at room tempera-
ture, on 5–10% CDCl3 solutions, unless otherwise stated,
using a Varian NMR Gemini 300 spectrometer. Chemical
shifts are given in ppm from tetramethylsilane (TMS) as
1
the internal reference. H NMR spectra were run at 300
MHz by using the following experimental conditions:
24,000 data points, 4.5-kHz spectral width, 2.6-sec acquisi-
tion time, 128 transients. 13C NMR spectra were recorded
at 75.5 MHz, under full proton decoupling, by using the fol-
lowing experimental conditions: 24,000 data points, 20-kHz
spectral width, 0.6-sec acquisition time, 64,000 transients.
FTIR spectra were recorded with a Perkin-Elmer 1750
spectrophotometer, equipped with an Epson Endeavor II
data station, on samples prepared as KBr pellets. Melting
points (uncorrected) were determined in glass capillaries
on a Bu¨chi 510 apparatus with a heating rate of 18C/min.
UV–Vis absorption spectra were recorded at 258C in the
700–250 nm spectral region with a Perkin-Elmer Lambda
19 spectrophotometer on DMA solutions by using cell
path lengths of 0.1 cm.
Concentrations in azobenzene chromophore of about
3 ꢀ 1024 mol L21 were used.
Optical activity measurements were made at 258C with
DMA solutions with a Perkin Elmer 341 digital polarime-
ter, equipped with a Toshiba sodium bulb, using a cell
path length of 1 dm. Specific and molar rotation values at
the sodium D line are expressed as deg dm21 g21 cm3
and deg dm21 mol21 dL, respectively.
Circular dichroism (CD) spectra were recorded at 258C
on DMA solutions on a Jasco 810 A dichrograph, using
the same path lengths and solution concentrations as for
the UV–Vis measurements. De values, expressed as L
mol21 cm21 were calculated from the following expres-
sion: De 5 [Y]/3300, where the molar ellipticity [Y] in
deg cm2 dmol21 refers to one azobenzene chromophore.
Fig. 1. Chemical structures of bis-[(S)-HAP], ter-[(S)-HAP-R] and
hexakis-[(S)-HAP-R].
Chemicals
The azoic alcohols (S)-(2)-3-hydroxy-1-(4-azobenzene)
pyrrolidine [(S)-HAP], (S)-(2)-3-hydroxy-1-(40-cyano-4-azo-
Chirality DOI 10.1002/chir

CHIRAL AZOAROMATIC DENDRIMERS
101
Ter-{(S)-3-[1-(40-cyano-4-azobenzene)-pyrrolidine]}
trimesate [ter-[(S)-HAP-C]]
benzene) pyrrolidine [(S)-HAP-C] and (S)-(2)-3-hydroxy-
1-(40-nitro-4-azobenzene) pyrrolidine [(S)-HAP-N] were
synthesized as previously reported.36,37
Chloroform, CH2Cl2, THF, toluene, and DMF were puri-
fied and dried according to the reported procedures38 and
stored under nitrogen. All other reagents and solvents
(Aldrich) were used as received.
Ter-[(S)-HAP-C] was purified by column chromatogra-
phy (SiO2 70–230, CH2Cl2/Ethyl Acetate 4:1 v/v as eluent)
and subsequent crystallization from CHCl3 (M.p. 221–2258C).
Elemental analysis: found C 69.6%, H 4.7%, N 16.3%; cal-
culated for C60H48N12O6 (1033.1): C 69.76%, H 4.68%, N
16.27%.
Method A: Yield 28%.
Method B: Yield 48%.
General Procedure for the Synthesis of
Chiral Trimesic Acid Esters
1H NMR (DMF-d7): 7.40-7.20 (m, 12H, arom ortho and
meta to CN; 6H, arom metha to amino group; 3H, core),
6.05 (dd, 6H, arom ortho to amino group), 5.10 (s, 3H,
CHꢁꢁO), 3.20-2.90 (m, 12H, 2- and 5-CH2), 1.80 (m, 6H, 4-
CH2) ppm.
Method A. A solution of 1,3,5-benzenetricarbonyl tri-
chloride (1) (0.00075 mol) in dry CH2Cl2 (8 ml) was added
dropwise, under nitrogen, to an ice-cooled solution of the
opportune azoic alcohol (0.005 mol) in dry CH2Cl2 (30 ml),
in the presence of triethylamine (TEA) (0.022 mol) and a
catalytic amount of 4-dimethylamino pyridine (DMAP).
The mixture was kept ice-cooled for 2 h, then left at room
temperature for one night, filtered and washed with 0.1 M
HCl, 5% Na2CO3 and finally with water. The organic layer
was dried (Na2SO4) and the solvent evaporated under
reduced pressure to give crude ter-[(S)-HAP-R].
13C NMR (DMF-d7): 164.7 (C¼¼O core), 156.0, 151.6,
144.3 (arom 4-, 1- and 10-C), 134.5 (1-C core), 134.2 (arom
30-C), 132.2 (arom 40-C), 126.5 (arom 3-C), 123.2 (arom 20-
C), 119.4 (CN), 112.7 (arom 2-C), 112.3 (CH core), 76.2
(CH-O), 54.2 (NꢁꢁCH2ꢁꢁCH), 46.7 (NꢁꢁCH2ꢁꢁCH2), 31.5
(CHꢁꢁCH2ꢁꢁCH2) ppm.
FTIR (KBr): 3063 (mCꢁꢁH arom), 2854 (mCꢁꢁH aliph),
2223 (mCN), 1720 (mC¼¼O), 1597 and 1517 (mC¼¼ arom),
C
1381 and 1242 (mCꢁꢁO), 846 and 821 (dCH 1,4-disubst.
Method B. A solution of the opportune azoic alcohol
(0.0041 mol), 18-crown-6 ether (0.0037 mol) and DMAP
(0.008 mol) in dry toluene (160 ml) was kept 2 h at reflux
under nitrogen in a Dean-Stark trap. A solution of 1
(0.00124 mol) in dry toluene (8 ml) was then added drop-
wise and the solution kept at reflux over night. The reaction
mixture was filtered and the crude product purified by col-
umn chromatography (SiO2 70–230, CH2Cl2/Ethyl Acetate
4:1 v/v as eluent) followed by crystallization from CHCl3.
arom ring), 742 (dCH 1,3,5 trisubst. arom ring) cm21
.
Ter-{(S)-3-[1-(40-nitro-4-azobenzene)-pyrrolidine]} trimes-
ate [ter-[(S)-HAP-N]].
Ter-[(S)-HAP-N] was crystallized from CHCl3 (M.p.
232–2368C).
Elemental analysis: found C 62.5%, H 4.4%, N 17.4%; cal-
culated for C57H48N12O12 (1093.1): C 62.63%, H 4.43%, N
15.38%.
Method B: Yield 38%
1H NMR (nitrobenzene-d6): 8.98 (s, 3H, core), 8.30 (dd,
6H, arom ortho to NO2), 7.97 (dd, 6H, arom meta to amino
group and arom 20-H), 6.75 (dd, 4H, arom ortho to amino
group), 5.90 (s, 3H, CH-O), 4.0-3.70 (m, 12H, 20- e 50-CH2),
2.50 (m, 6H, 40-CH2) ppm.
Ter-{(S )-3-[1-(4-Azobenzene)-Pyrrolidine] Trimesate
[ter-[(S )-HAP]]
Ter-[(S)-HAP] was purified by column chromatography
(SiO2 70–230, CH2Cl2/Ethyl Acetate 4:1 v/v as eluent) and
subsequent crystallization from THF/Abs. Ethanol (M.p.
201–2068C).
13C NMR (nitrobenzene-d6): 165.0 (C¼¼O core), 157.8
(arom 40), 151.7, 148.2, 145.1 (arom 4-, 1- and 10-C), 132.3
(1-C core), 127.0 (arom 30-C), 125.2, 123.8 (arom 3-C and
20-C), 112.7 (arom 2-C), 112.3 (CH core), 76.0 (CHꢁꢁO),
54.1 (NꢁꢁCH2ꢁꢁCH), 46.8 (NꢁꢁCH2ꢁꢁCH2), 31.9
(CHꢁꢁCH2ꢁꢁCH2) ppm.
Elemental analysis: found C 71.4%, H 5.4%, N 13.2%; calcu-
lated for C57H51N9O6 (958,1): C 71.46%, H 5.37%, N 13.16%.
Method A: Yield 25%
1H NMR (DMF-d7): 7.32 (s, 3H, core) 7.22-7.12 (m, 12H,
arom meta to amino group and arom 20-H), 6.88-6.70 (m,
9H, arom 30- and 40-H), 6.10 (dd, 6H, arom ortho to amino
group), 5.08 (s, 3H, CHꢁꢁO), 3.23-2.85 (m, 12H, 2- and 5-
CH2), 1.78 (m, 6H, 4-CH2) ppm.
FTIR (KBr): 3093 (mCꢁꢁH arom), 2919 and 2856 (mCꢁꢁH
aliph), 1723 (mC¼¼O), 1603 and 1515 (mC¼¼ arom), 1381
C
and 1243 (mCꢁꢁO), 1338 (mN¼¼O,s), 858 and 825 (dCH 1,4-dis-
ubst. arom ring), 743 (dCH 1,3,5 trisubst arom ring) cm21
.
13C NMR (DMF-d7): 166.1 (C¼¼O core), 152.1, 149.8,
144.5 (arom 4-, 1- e 10-C), 135.9 (CH core), 133.5 (1-C
core), 131.6 (arom 40-C), 131.2 (arom 30-C), 127.0 (arom 3-
C), 124.0 (arom 20-C), 114.0 (arom 2-C), 77.7 (CHꢁꢁO),
55.6 (NꢁꢁCH2ꢁꢁCH), 47.9 (NꢁꢁCH2ꢁꢁCH2), 32.9
(CHꢁꢁCH2ꢁꢁCH2) ppm.
Synthesis of Chiral First Generation Dendrimers: ter-
(di-{( S)-3-[1-(4-azobenzene)pyrrolidine]}-isophthalate)
Trimesate [hexakis-[( S)-HAP]] and ter-(di-{( S)-3-[1-
(40-cyano-4-azobenzene)pyrrolidine]}-isophthalate)
Trimesate [hexakis-[( S)-HAP-C]]
FTIR (KBr): 3065 (mCꢁꢁH arom), 2850 and 2983 (mCꢁꢁH
Ter-(dimethyl isophthalate) trimesate (3). This
aliph), 1720 (mC¼¼O), 1597 and 1515 (mC¼¼ arom), 1385 product was prepared with a synthetic approach similar to
C
and 1245 (mCꢁꢁO), 819 (dCH 1,4-disubst. arom ring), 768 that one previously reported by Shi and coworkers,35 but
and 689 (dCH monosubst. arom ring), 742 (dCH 1,3,5 tri- with a different purification procedure. A solution of 1
subst. arom ring) cm21
.
(0.001 mol) in dry toluene (20 ml) was added dropwise,
Chirality DOI 10.1002/chir

102
ANGIOLINI ET AL.
under nitrogen to a mixture of 5-hydroxy dimethyl (arom 3-C), 122.4 (arom 20-C), 121.3 (2-C isophthalate),
isophthalate (2) (0.03 mol) and TEA (catalytic amount). 121.0 (4- and 6-C isophthalate), 112.3 (arom 2-C), 75.5
The stirred mixture was refluxed and monitored by FTIR (CHꢁꢁO), 54.1 (NꢁꢁCH2ꢁꢁCH), 46.4 (NꢁꢁCH2ꢁꢁCH2),
until the signal at 1761 cm21 related to the acyl chloride 31.5 (CHꢁꢁCH2ꢁꢁCH2) ppm.
disappeared. The solvent was evaporated under reduced
FTIR (KBr): 3061 (mC-H arom), 2919 and 2850 (mCꢁꢁH
pressure and the crude product 3 purified by column aliph), 1718 (mC¼¼O), 1602 and 1515 (mC¼¼ arom), 1385
C
chromatography (SiO2 70–230, CH2Cl2/Ethyl acetate 15:1 and 1221 (mCꢁꢁO), 821 (dCH 1,4-disubst. arom ring), 759
as eluent) (Yield 66%).
and 689 (dCH monosubst. arom ring), 723 (dCH 1,3,5 tri-
1H NMR (CDCl3): 9.25 (s, 3H core), 8.65 (s, 3H arom subst. arom ring) cm21
isophthalate 20-H), 8.20 (s, 6H arom isophthalate 40 and 60-
H), 3.99 (s, 18H, CH3) ppm.
.
Hexakis-[(S)-HAP-C]. Following the procedure des-
cribed for hexakis-[(S)-HAP], this product was purified by
column chromatography (SiO2 70–230, CH2Cl2/ethyl ace-
tate 15:1 v/v as eluent) and crystallized in abs. ethanol
(Yield 24%).
FTIR (KBr): 3089 (mCꢁꢁH arom), 2957 and 2846 (mCꢁꢁH
aliph), 1733 (mC¼¼O), 1592 (mC¼¼ arom), 1461 (das CH3),
C
1433 (ds CH3), 1329 and 1256 (mCꢁꢁO), 758 (dCH isophtha-
late arom ring), 743 (dCH 1,3,5 trisubst. arom ring) cm21
.
Elemental analysis: found C 68.9%, H 4.4%, N 14.2%;
calculated for C135H102N24O18 (2348.4): C 69.05%, H 4.38%,
N 14.31%.
Ter-(5-hydroxyisophthalic acid) trimesate ester
(4). To a solution of 3 (0.00115 mol) in CHCl3 (10 ml)
10 ml a aqueous solution of 40% KOH and methanol were
added until the two layers were totally mixed. The solution
was left under stirring for 1 h and the obtained white solid
filtered and dissolved in water previously acidified with aq.
HCl (pH 5 1). The product was finally extracted with ethyl
acetate, dried over Na2SO4 and the organic phase evaporated
under reduced pressure to give a white material (yield 97%).
1H NMR (DMSO-d6): 10.20 (s, 6H, ꢁꢁOH), 8.73 (s, 3H,
core), 7.99 (s, 3H, arom isophthalate 20-H), 7.50 (s, 6H,
arom isophthalate 40 and 60-H) ppm.
1H-NMR (DMF-d7): 8.12-7.84 (m, 24H, arom ortho and
meta to CN; 12H, arom meta to amino group; 3H, core; 3H,
isophthalate 2-H), 7.65 (s, 6H, isophthalate 4 and 6-H), 6.83
(dd, 6H, arom ortho to amino group), 5.75 (s,3H, CH-O), 3.95-
2.64 (m, 24H, 2- and 5-CH2), 2.36-2.60 (m,12H, 4-CH2) ppm.
13C NMR (DMF-d7): 165.3 (C¼¼O isophthalate), 162.7
(C¼¼O core), 158.9 (5-C isophthalate), 155.8 (arom 4-C),
151.4 (arom 10-C), 143.9 (1-C arom) 133.0 (C core), 132.3
(CH core), 134.0 (arom 30-C), 126.4 (arom 3-C), 123.0
(arom 20-C), 121.2 (2-C isophthalate), 121.0 (4- and 6-C
isophthalate), 119.3 (CN), 112.6 (arom 2-C), 112.0 (arom
40-C), 75.4 (CHꢁꢁO), 54.1 (NꢁꢁCH2ꢁꢁCH), 46.5 (Nꢁꢁ
CH2ꢁꢁCH2), 31.4 (CHꢁꢁCH2ꢁꢁCH2) ppm.
FTIR (KBr): 3420 (mOꢁꢁH), 3087 (mCꢁꢁH arom), 2960 and
2890 (mCꢁꢁH aliph), 1702 (mC¼¼O), 1600 (mC¼¼ arom), 1412
C
and 1278 (mCꢁꢁO), 744 (dCH 1,3,5 trisubst. arom ring) cm21
.
FTIR (KBr): (mCꢁꢁH arom), 2854 (mCꢁꢁH aliph), 2223
Hexakis-[(S)-HAP]. A solution of 4 (0.142 mmol) in
15 ml SOCl2 (14.6 mmol) was refluxed for 3 h under nitro-
gen. Excess SOCl2 was then removed under reduced pres-
sure and the residue, dissolved in 5 ml of dry toluene, was
added to a solution of (S)-HAP (0.0013 mol) in 50 ml of
dry toluene in presence of 18-crown-6 ether (0.17 g) and
DMAP (0.16 g). The reaction mixture was refluxed for 20
h under nitrogen and the solvent then evaporated. The
remaining residue was dissolved in CH2Cl2, washed with
(mCN), 1720 (mC¼¼O), 1596 and 1516 (mC¼¼ arom), 1378
C
and 1224 (mCꢁꢁO), 846 and 821 (dCH 1,4-disubst. arom
ring), 742 (dCH 1,3,5 trisubst. arom ring) cm21
.
Synthesis of Model Compound bis-{( S)-3-
[1-(4-azobenzene)-pyrrolidine]}-5-Hydroxy
Isophthalate [Bis-[( S)-HAP]]
bis-{(S)-3-[1-(4-azobenzene)-pyrrolidine]}-5-(tert-butyl-
0.1 M HCI, 5% Na2CO3 and finally with water. The organic dimethylsiloxy) isophthalate (7). A solution of 5-(tert-
layer was dried over Na2SO4 and the solvent evaporated butyldimethylsiloxy) isophthalic acid (5) (0.00266 mol),
under reduced pressure to give the crude product. Hexa- synthesized in two steps as reported in literature by Miller
kis-[(S)-HAP] was purified by column chromatography starting from commercial 5-hydroxyisophthalic acid,39 was
(SiO2 70–230, CH2Cl2/ethyl acetate 4:1 v/v as eluent) and refluxed in 8 ml of SOCl2 overnight under nitrogen.
crystallized in abs. ethanol (Yield 29%).
Excess of SOCl2 was evaporated under reduced pressure
Elemental analysis: found C 70.3%, H 4.9%, N 11.4%; cal- and the intermediate 5-(tert-butyldimethylsiloxy)isophtha-
culated for C129H108N18O18 (2198.4): C 70.48%, H 4.95%, N loyl dichloride (6),39 dissolved in 10 ml of dry toluene, was
11.47%.
added to a solution of (S)-HAP (7.98 mmol) in 130 ml of
1H NMR (DMF-d7): 8.05 (s, 3H isophthalate 2-H), 7.95 dry toluene in the presence of 18-crown-6 ether (1.40 g)
(s, 3H core), 7.90-7.80 (m, 24H, arom meta to amino group and DMAP (1.70 g). The reaction mixture was kept at
and arom 20-H), 7.68 (s, 6H isophthalate 4 and 6-H), 7.58- reflux for 20 h under nitrogen atmosphere, filtered, and
7.40 (m, 18H, arom 30- and 40-H), 6.83-6.65 (dd, 12H, arom the solvent evaporated. The remaining residue was dis-
ortho to amino group), 5.78 (s, 3H, CHꢁꢁO), 3.85-3.53 (m, solved in CH2Cl2 and washed with HCl 0.1 M, 5% Na2CO3
24H, 20- and 50-CH2), 2.50-2.30 (m, 12H, 40-CH2) ppm.
and finally with water.
The organic layer was dried (Na2SO4) and the solvent
13C NMR (DMF-d7): 165.3 (C¼¼O isophthalate), 162.7
(C¼¼O core), 158.9 (5-C isophthalate), 153.5 (arom 4-C), evaporated under reduced pressure to give the crude
150.6 (arom 10-C), 143.9 (arom 1-C), 133.5 (C core), 132.3 product 7 which was purified by column chromatography
(CH core), 130.0 (arom 40-C), 129.6 (arom 30-C), 125.5 (SiO2 70–230, CH2Cl2/ehtyl Acetate 15:1 v/v). Yield 61%.
Chirality DOI 10.1002/chir

CHIRAL AZOAROMATIC DENDRIMERS
103
Scheme 1. Synthesis of chiral derivatives ter-[(S)-HAP-R] and hexakis-[(S)-HAP-R].
1H NMR (CDCl3): 8.18 (s, 1H, arom isophthalate 2-H), arom ortho to amino group), 5.68 (s, 2H, CH-O), 3.85-3.55
7.96-7.80 (m, 8H, arom metha to amino group and 20-H), (m, 8H, 20-, and 50-CH2), 2.35 (m, 4H, 40-CH2) ppm.
7.65 (s, 2H, arom isophthalate 4- and 6-H), 7.52-7.40 (m,
13C NMR (CDCl3): 165.8 (C¼¼O), 157.3 (arom isophtha-
6H, arom 30- and 40-H), 6.65 (dd, 4H, arom ortho to amino late 5-C), 153.7 (arom 4-C), 150.1 (arom 10-C), 144.6 (arom 1-
group), 5.68 (s, 2H, CHꢁꢁO), 3.85-3.59 (m, 8H, 2- and 5- C), 132.3 (arom isophthalate 1- and 3-C), 130.2 (arom 40-C),
CH2), 2.40 (m, 4H, 4-CH2) ppm.
129.6 (arom 30-C), 125.9 (arom 3-C), 123.3 (arom isophtha-
late 2-C), 122.9 (arom 20-C), 121.7 (arom isophthalate 4- and
6-C), 112.3 (arom 2-C), 75.4 (CHꢁꢁO), 54.4 (NꢁꢁCH2ꢁꢁCH),
46.5 (NꢁꢁCH2ꢁꢁCH2), 32.0 (CHꢁꢁCH2ꢁꢁCH2) ppm.
Bis-[(S)-HAP]. A solution of 7 (1.64 mmol) in 150 ml
of dry THF and 1.6 ml of tetrabutylammonium fluoride
(TFBA) 1 M in THF was stirred for 1 h. The solvent was
evaporated and the obtained solid dissolved in CH2Cl2,
washed with HCl 0.1 M, 5% Na2CO3 and finally with water.
The organic layer was dried (Na2SO4) and the solvent
evaporated under reduced pressure to give the crude
product which was purified by chromatography (SiO2 70–
230, CH2Cl2/ethyl acetate 15:1 v/v) (Yield 46%, M.p. 75–
808C).
FTIR (KBr): 3412 (mOꢁꢁH), 3056 (mCꢁꢁH arom), 2926
and 2852 (mCꢁꢁH aliph), 1722 (mC¼¼O), 1602 and 1515
(mC¼¼ arom), 1382 and 1240 (mCꢁꢁO), 821 (dCH 1,4-dis-
C
ubst. arom ring), 759 and 689 (dCH monosubst. arom
ring), 723 (dCH 1,3,5 trisubst. arom ring) cm21
.
RESULTS AND DISCUSSION
Synthesis and Characterization
Elemental analysis: found C 70.6%, H 5.3%, N 12.2%;
The trimesic acid ester derivatives ter-[(S)-HAP], ter-
calculated for C40H36N6O5 (680.8): C 70.57%, H 5.33%, [(S)-HAP-C] and ter-[(S)-HAP-N] (see Fig. 1), have been
N 12.35%.
obtained by direct esterification of 1,3,5-tricarbonyl ben-
1H NMR (CDCl3): 8.18 (s, 1H, arom isophthalate 2-H), zene trichloride (1) with the opportune azoic alcohol by
7.94-7.80 (m, 8H, arom meta to amino group and 20-H), two different synthetic pathways in order to optimize the
7.65 (s, 2H, arom isophthalate 4- and 6-H), 6.65 (dd, 4H, reaction yields (Scheme 1).
Chirality DOI 10.1002/chir

104
ANGIOLINI ET AL.
Fig. 2. H1 NMR spectra in DMF-d7 solution of (a) ter-[(S)-HAP] and (b) hexakis-[(S)-HAP]. Starred signals refer to solvent resonances.
Performing the reaction in dry CH2Cl2 in the presence d7 showing, among the other resonances, a singlet at 8.60
of TEA and DMAP, the desired compounds are obtained ppm related to the aromatic protons of the aromatic core,
with yields around 25–28%; better results (yields around is reported in Figure 2.
45–48%) are however achieved by a synthetic pathway sim-
The synthesis of hexakis-[(S)-HAP-R] derivatives has
ilar to that one previously reported by McGrath,28–30 using been carried out, starting from the functionalization of 1
18-crown-6 ether as complexing agent and DMAP as cata- with 5-hydroxydimethyl isophthalate (2) by a procedure
lyst and neutralizing agent of the HCI produced.
similar to that one reported in the literature.35 The
The structures of these products have been confirmed obtained intermediate 3 has been then hydrolyzed in
1
by H- and 13C NMR, FTIR, and elemental analysis. As an excess of KOH by monitoring the reaction progress until
1
example, the H NMR spectrum of ter-[(S)-HAP] in DMF- the disappearance, in the FTIR spectrum, of the signal at
Chirality DOI 10.1002/chir

CHIRAL AZOAROMATIC DENDRIMERS
105
Scheme 2. Synthesis of the model compound bis-[(S)-HAP].
1733 cm21 related to the methyl ester, the structure of the kis-[(S)-HAP]. This new product has been obtained in two
acid product 4 being finally confirmed by 1H and 13C steps by reaction of 5-(tert-butyldimethylsyloxy)isophtha-
NMR. This dendritic acid was previously obtained by hy- loyl dichloride (6), synthesized as reported in the litera-
drolysis of 3 in the presence of AlCl3 and NaI in water/ ture,39 with the azoic alcohol (S)-HAP to give intermediate
acetonitrile after 15 h reaction.35 By using selective alka- 7, subsequently desilylated with TFBA40 (Scheme 2).
line hydrolysis in CHCl3/water/methanol 1:1:1 v/v mix-
ture, instead, we have been able to obtain product 4 in atives decrease from ter-[(S)-HAP-R] to the first genera-
quantitative yield, after about 1 h reaction. tion product, hexakis-[(S)-HAP-R] because of the
As reported in Table 1, the yields of dendrimeric deriv-
Compound 4 has been treated with SOCl2 and finally reduced reactivity of the acid substrate, essentially
functionalized with the azoic alcohols (S)-HAP or (S)- because of steric hindrance of the azoic ester produced
HAP-C, in the presence of DMAP and 18-crown-6 ether, that prevents further functionalization. Repeated attempts
to give the hexakis systems which have been characterized to obtain the second generation product by direct esterifi-
by FTIR, 1H and 13C NMR.
cation of the hexa-acyl chloride of 4 with bis-[(S)-HAP]
The 1H NMR spectrum of hexakis-[(S)-HAP] in DMF-d7, in the presence of DMAP and 18-crown-6 ether, however,
reported as an example in Figure 2, displays, besides the were unsuccessful, affording incomplete functionalization
signals also shown by the ter-derivative, three singlets of 4 with formation of complex mixtures of several
related to the aromatic protons of the isophthalic rings products constituted by mono- and bis-azo derivatives.
(3H at 8.05 and 6H at 7.65 ppm, protons b and c, respec- Such behavior, which is consistent with the findings
tively, in Fig. 2b) and the aromatic core (3H at 10.45 ppm, from other convergent dendrimer syntheses,6,41,42 rules
proton a in Fig. 2b).
out the possibility of obtaining higher generation den-
We have also synthesized bis-[(S)-HAP] (see Fig. 1) as drimers capped with azoaromatic groups by this syn-
a model compound for the three constituent units of hexa- thetic pathway.
TABLE 1. Characterization data of dendrimeric derivatives
Samples
Yield (%)
[a]2D5 a
c (g/dL)
[F]2D5 b
M (g/mol)
nc
[Fn]2D5/nd
bis-[(S)-HAP]
ter-[(S)-HAP]
92
25e
1561
1553
n.d.
0.165
0.235
13819
15298
n.d.
680.76
958.09
2
3
3
3
6
6
1
11912
11755
n.d.
ter-[(S)-HAP-C]
ter-[(S)-HAP-N]
hexakis-[(S)-HAP]
hexakis-[(S)-HAP-C]
(S)-PAPg
28e, 48f
38f
1033.12
1093.08
2195.36
2270.21
351.45
n.d.
n.d.
n.d.
29
24
1854
n.d.
0.134
118774
n.d.
13128
n.d.
14.0g
0.501g
114.0g
114.0g
aSpecific optical rotation in DMA solution, expressed as deg dm21 g21 cm3.
bMolar optical rotation in DMA solution, expressed as deg dm21 mol21 dL and calculated as ([a]2D5ꢀM/100), where M represents the molecular weight
of the product.
cn represents the number of chiral residues present in each molecule.
dMolar optical rotation per one chiral residue in DMA solution, expressed as deg dm21 mol21 dL and calculated as ([F]2D5/n).
eSynthetic method A.
fSynthetic method B.
gIn CHCl3 solution,. Ref. 36.
Chirality DOI 10.1002/chir

106
ANGIOLINI ET AL.
TABLE 2. UV-vis spectra of dendrimeric derivatives in DMA solution at 258C
1st band
2nd band
3rd band
emaxꢀ1023b
kmax
emaxꢀ1023b
kmax
emaxꢀ1023b
a
a
a
Samples
kmax
bis-[(S)-HAP]
ter-[(S)-HAP]
ter-[(S)-HAP-C]
ter-[(S)-HAP-N]
hexakis-[(S)-HAP]
hexakis-[(S)-HAP-C]
417
416
457
489
418
460
33.5
34.5
36.3
33.3
33.0
35.1
318
8.6
270
266
275
286
268
276
14.2
12.8
14.5
11.9
14.6
13.7
317
316
9.0
7.1
aWavelength of maximum absorbance, expressed in nm.
bExpressed in L mol21 cm21 and calculated for one mole of azoaromatic chromophore.
Optical Activity
UV–Vis Analysis
The optical activity data of the investigated products are
The UV–Vis spectra of all dendrimeric derivatives in
DMA solution (Table 2) exhibit, in the 250–700 nm spectral
reported in Table 1.
1
Previous H NMR data36 concerning the optical purity region, two absorption bands; one, more intense, is cen-
of 40-unsubstituted pivaloyl derivative bearing one azoaro- tered between 410 and 500 nm and is attributed to elec-
matic group in the side-chain, (S)-(1)-3-pivaloyloxy-1-(4- tronic transitions such as n-p*, p-p* and internal charge
azobenzene) pyrrolidine [(S)-PAP], prepared through a transfer of the azobenzene chromophore. The other, cen-
similar synthetic pathway, indicated that this compound tered at around 260–290 nm, is attributed to the p-p* elec-
had an enantiomeric excess higher than 90%, thus exclud- tronic transition of the aromatic ring.45 In addition, the
ing the possibility of racemization at the pyrrolidine asym- spectra of bis-[(S)-HAP] and hexakis-[(S)-HAP-R] show a
metric center in the course of the synthesis. Thus, a simi- further UV band close to 318 nm, related to the electronic
lar optical purity has reasonably been assumed also for the transitions of the 5-hydroxyisophthalate residue (Table 2).
derivatives reported in the present paper.
The specific {[a]D25 (optical rotation values of bis- and ter- R], no absorbance is displayed in that spectral region.
[(S)-HAP] in DMA solution appear similar, while a re- As expected, a remarkable bathochromic shift is noted
When this dendritic fragment is absent, as in ter-[(S)-HAP-
markable increase of activity is observed on passing to the in both the UV–Vis bands on passing from ter-[(S)-HAP]
hexakis derivative (Table 1). The optical rotation values of to ter-[(S)-HAP-C] and to ter-[(S)-HAP-N] (see Fig. 3).
40-substituted azoaromatic derivatives could not be meas- This effect is clearly related, as previously reported,46–49 to
ured at the sodium D-line due to their high absorbance at the increasing electron-withdrawing capability of the 40-
that wavelength.
substituent at the azoaromatic chromophore and hence to
Of course, the related molar optical rotation values
([F]D25 (of bis-, ter- and hexakis-[(S)-HAP] derivatives
increase noticeably with the molecular weight, however,
each compound bears a different number n of chiral
groups, the molar optical rotation per chiral unit, calcu-
lated as [F]D25/n, allows a better comparison among the
samples. Even so, the values in the last column of Table 1
confirm that hexakis-[(S)-HAP] is characterized by a
greater optical activity than bis- and ter-[(S)-HAP], thus
indicating that the chirality of these derivatives depends
strongly on the increased branching of the system.
Indeed, the related model compound (S)-PAP, containing
only one (S)-HAP unit, previously investigated,36 displays
negligible optical activity, as reported in Table 1 ([Fn]D25
114 deg dm21 mol21 dL, in CHCl3), and the molar rota-
tion [Fn]D25/n of bis-[(S)-HAP], the constituent dendron of
hexakis-[(S)-HAP] is remarkably lower (11912 deg dm21
mol21 dL against 13128 deg dm21 mol21 dL).
These findings appear to be related, as previously
reported,6,43,44 to the presence of a dendritic chiral sub-
structure, induced, in this case, by an optically active
group of one prevailing absolute configuration (S), favor-
ing the assumption of a preferred chiral arrangement of
the dendritic structure of the samples.
Fig. 3. UV–Vis spectra in DMA solution of ter-[(S)-HAP] (—), ter-[(S)-
HAP-C] (- - -), and ter-[(S)-HAP-N] (ꢀꢀꢀꢀ).
Chirality DOI 10.1002/chir

CHIRAL AZOAROMATIC DENDRIMERS
107
TABLE 3. CD spectra of dendrimeric derivatives in DMA solution at 258C
1st band
2nd band
3rd band
a
b
c
a
b
a
b
a
b
Samples
k1
De1
k0
k2
De2
k3
De3
k4
De4
bis-[(S)-HAP]
ter-[(S)-HAP]
430
448
492
524
430
491
410
13.75
14.92
16.73
15.61
13.30
14.39
20.51
320
20.82
267
n.d.
273
289
268
283
258
10.26
n.d.
395
446
471
386
422
446
20.38
21.78
21.37
ter-[(S)-HAP-C]
ter-[(S)-HAP-N]
hexakis-[(S)-HAP]
hexakis-[(S)-HAP-C]
(S)-PAPd
20.23
20.28
10.28
10.25
10.22
320
321
20.77
20.49
aWavelength (in nm) of maximum dichroic absorption.
bDe expressed in L mol21 cm21 and calculated for one mole of azoaromatic chromophore.
cWavelength (in nm) of the cross-over point of dichroic bands.
dIn CHCl3 solution,. Ref. 36.
the extent of conjugation, giving rise to a progressive
As reported in Figure 4, the CD spectra of ter-derivatives
reduction of the electronic transition energy in the system. show two dichroic bands of opposite sign and different in-
As reported in Table 2, no substantial difference in kmax tensity in the 350–600 nm region. Being connected to the
and extinction coefficient values was found on passing
from bis- to ter- and hexakis-[(S)-HAP]. Such behavior,
noted in other dendrimeric systems,40 suggests that each
chromophore unit is actually isolated in solution without
any effect originated in the dendrimeric structure, or that
dipolar interchromophoric interactions between azoaro-
matic moieties bonded to the same aromatic ring are of
the same extent in all the samples. This has, of course, im-
portant consequences in the CD spectra, where the elec-
tronic transitions related to the azoaromatic chromophore
are used as probes of conformational chirality.
electronic transitions of the first UV band, the dichroic sig-
nals show the expected bathochromism related to the sub-
stituent effect mentioned above. In contract with previ-
ously reported linear methacrylic polymers36,37,50 bearing
the same optically active pyrrolidine group linked in the
side chain through the nitrogen atom to the azoaromatic
chromophore, the cross-over points appear slightly shifted
with respect to the UV maxima absorptions.
As the first UV band is due to several electronic transi-
tions which can be affected differently by the overall sys-
tem chirality, the resulting CD spectra could originate in
the overlap of a very intense positive band likely con-
nected to the shoulder present in the UV–Vis spectra of all
the ter-[(S)-HAP-R] at about 445, 490, and 520 nm, respec-
tively, and a less intense positive excitonic couplet
connected to the maximum of the first UV band. The
Chiroptical Properties
To investigate further optical activity in dilute solution, the
CD spectra of all samples have been recorded and the molar
ellipticity values (De), normalized to the number n of chiral
residues present in each structure, measured (Table 3).
Fig. 4. CD spectra in DMA solution of ter-[(S)-HAP] (—), ter-[(S)-
HAP-C] (- - -), and ter-[(S)-HAP-N] (ꢀꢀꢀꢀ).
Fig. 5. CD spectra in DMA solution of bis-[(S)-HAP] (—), hexakis-[(S)-
HAP] (- - -), and hexakis-[(S)-HAP-C] (ꢀꢀꢀꢀ).
Chirality DOI 10.1002/chir

108
ANGIOLINI ET AL.
7. Chow HF, Fok LF, Mak CC. Synthesis and characterization of optically
active, homochiral dendrimers. Tetrahedron Lett 1994;35:3547–3550.
presence of this last signal may suggest the existence to a
small extent of cooperative interactions between azoaro-
matic chromophores disposed in a mutual chiral geome-
try.51 Indeed, the CD spectrum of model (S)-PAP36 dis-
plays only two weak bands at 410 and 258 nm (Table 3),
related to the UV–Vis absorptions, indicating that the ab-
sence of the above interactions is a consequence of the
lack of any structural restriction.
By contrast, very different CD spectra are shown by bis-
and hexakis-[(S)-HAP] with respect to ter-[(S)-HAP-R]
(see Fig. 5). These compounds display positive and struc-
tured dichroic bands due to the overlap of several elec-
tronic transitions related to the first UV band. Instead,
around 320 nm a weak negative signal is present, attribut-
able to the isophthalic ring transitions which, therefore,
are also affected by the chirality of the system. As
expected, hexakis-[(S)-HAP-C] shows a similar behavior,
but the dichroic bands are shifted to higher wavelengths
because of the substituent effect (see Fig. 5).
8. Seebach DG, Herrmann F, Lengweiler UD, Bachmann BM, Amrein
W. Synthesis and enzymatic degradation of dendrimers from (R)-3-
hydroxybutanoic acid and trimesic acid. Angew Chem Int Ed Engl
1996;35:2795–2797.
9. Chang HT, Chen CT, Kondo T, Siuzdak G, Sharpless KB. Asymmetric
dihydroxylation enables rapid construction of chiral dendrimers based
on 1,2-diols. Angew Chem Int Ed Engl 1996;35:182–186.
10. Thomas CW, Tor Y. Dendrimers and chirality, Chirality 1998;10:53–59.
11. Angiolini L, Benelli T, Giorgini L, Salatelli E. Optically active photo-
chromic polymers with three-arm star structure by atom transfer radi-
cal polymerization. Macromolecules 2006;39:3731–3737.
12. Wulff G, Zweering U. Optically active, isotactic homopolymers of a-
olefins with main-chain chirality and the first preparation of optically
active c3-symmetrical polymers. Chem Eur J 1999;5:1898–1904.
13. Lehn JM. Perspectives in supramolecular chemistry—from molecular
recognition towards molecular information processing and self-organi-
zation. Angew Chem Int Ed 1990;29:1304–1319.
14. Lorenzo MO, Baddeley CJ, Muryn C, Raval R. Extended surface chir-
ality from supramolecular assemblies of adsorbed chiral molecules.
Nature 2000;404:376–379.
The similarity between the CD spectra of bis- and hexa-
kis-[(S)-HAP] indicates that the electronic transitions of
the azoaromatic chromophore and the isophthalic residue
are unaffected by the extension of the dendrimeric struc-
ture of the samples. Actually, each pair of azoaromatic moi-
eties bonded to the isophthalic ring appears to behave in-
dependently from the others. This behavior could be
ascribed to easier rotation around the ester bond with
respect to the ter-derivatives, which display a more
crowded central aromatic ring with a higher local density
of chromophores and hence a reduced rotational freedom.
These findings indicate that the CD spectra are in
agreement with the results of UV–Vis spectra, but do not
confirm the much higher optical rotation values given by
the hexakis derivative with respect to bis-[(S)-HAP].
Further research aimed to clarify the structural features
and the chirality of the above-described systems, particu-
larly in the solid state, are currently in progress, along
with the investigation of their photoresponsive properties,
carried out by irradiation with linearly polarized (pro-
ducing birefringence modulation) and circularly polarized
light (possibility of chiroptical switching).
15. Moberg C. C3 Symmetry in asymmetric catalysis and chiral recogni-
tion. Angew Chem Int Ed 1998;37:248–268.
16. Gibson SE, Castaldi MP. Applications of chiral C3-symmetric mole-
cules. Chem Commun 2006;3045–3062.
17. Gibbons WM, Shannon PJ, Sun ST, Swetlin BJ. Surface-mediated
alignment of nematic liquid crystals with polarized laser light. Nature
1991;351:49–50.
18. Delaire JA, Nakatani K. Linear and nonlinear optical properties of pho-
tochromic molecules and materials. Chem Rev 2000;100:1817–1846.
19. Todorov T, Nikolova L, Tomova N. Polarization holography. II. Polar-
ization holographic gratings in photoanisotropic materials with and
without intrinsic birefringence. Appl Opt 1984;23:4588–4591.
20. Andruzzi L, Altomare A, Ciardelli F, Solaro R, Hvilsted S, Ramanujam
PS. Holographic gratings in azobenzene side-chain polymethacrylates.
Macromolecules 1999;32:448–454.
21. Kim N. Recent development of the nonlinear optical active material
system. Macromol Symp 1996;101:235–242.
22. Prasad PN, Williams DJ. Introduction to nonlinear optical effects in
molecules and polymers. New York: Wiley; 1991.
23. Burland DM, Miller RD, Walsh CA. Second-order nonlinearity in
poled-polymer systems. Chem Rev 1994;94:31–75.
24. Carlini C, Angiolini L, Caretti D. Photochromic optically active poly-
mers, in: Salamone JC, editor. Polymeric materials encyclopedia,
Vol. 7. Boca Raton: CRC Press; 1996. p 5116–5123.
25. Verbiest T, Kauranen M, Persoon A. Second-order nonlinear optical
properties of chiral thin films. J. Mater Chem 1999;9:2005–2012.
LITERATURE CITED
26. Angiolini L, Bozio R, Giorgini L, Pedron D, Turco G, Dauru` A. Photo-
modulation of the chiroptical properties of new chiral methacrylic poly-
mers with side chain azobenzene moieties. Chem Eur J 2002;8:4241–4247.
1. Zhang J, Albelda Mt, Liu Y, Canary JW. Chiral Nanotechnology. Chir-
ality 2005;17:404–420.
27. Angiolini L, Benelli T, Bozio R, Dauru` A, Giorgini L, Pedron D. Photo-
induced chiroptical bistability in new chiral methacrylic azobenzene-
containing polymers. Synthetic Metals 2003;139:743–746.
2. Green MM, Nolte RJM, Meijer EW, Denmark SE, Siegel J. Topics in ster-
eochemistry. Materials-chirality, Vol. 24. New York: Wiley-VCH; 2003.
3. Kajitani T, Okoshi K, Sakurai S, Kumaki J, Yashima E. Helix-sense
controlled polymerization of a single phenyl isocyanide enantiomer
leading to diastereomeric helical polyisocyanides with opposite helix-
sense and cholesteric liquid crystals with opposite twist-sense. J Am
Chem Soc 2006;128:708–709.
28. Junge DM, McGrath DV. Photoresponsive azobenzene-containing
dendrimers with multiple discrete states. J Am Chem Soc 1999;121:
4912–4913.
29. Ghosh S, Banthia AK, Chen Z. Synthesis and photoresponsive study
of azobenzene centered polyamidoamine dendrimers. Tetrahedron
2005;61:2889–2896.
4. Yashima E, Maeda K, Nishimura T. Detection and amplification of
chirality by helical polymers. Chem Eur J 2004;10:42–51.
30. Momotake A, Arai T. Photochemistry and photophysics of stilbene
dendrimers and related compounds. J Photochem Photobiol C: Photo-
chem Rev 2004;5:1–25.
5. Wilson AJ, Masuda M, Sijbesma RP, Meijer EW. Chiral amplification
in the transcription of supramolecular helicity into a polymer back-
bone. Angew Chem Int Ed 2005;44:2275–2279.
31. Zeng F, Zimmerman SC. Dendrimers in supramolecular chemistry: from
molecular recognition to self-assembly. Chem Rev 1997;97:1681–1712.
6. Romagnoli B, van Baal I, Price DW, Harwood LM, Hayes W. Chiral
poly(aromatic amide ester) dendrimers bearing an amino acid derived
32. Bosman AW, Jansenn HM, Mejer EW. About dendrimers: structure,
physical properties, and applications. Chem Rev 1999;99:1665–1688.
C3-symmetric core—synthesis and properties. Eur
2004;2004:4148–4157.
J Org Chem
Chirality DOI 10.1002/chir

CHIRAL AZOAROMATIC DENDRIMERS
109
33. Adronov A, Gilat SL, Free´chet JMJ, Ohta K, Neuwahl FVR, Fleming
GR. Light harvesting and energy transfer in laser-dye-labeled poly(aryl
ether) dendrimers. J Am Chem Soc 2000;122:1175–1185.
43. Seebach D, Lapierre JM, Greiveldinger G, Skobridis K. Synthesis of
chiral starburst dendrimers from phb-derived triols as central cores.
Helv Chim Acta 1994;77:1673–1688.
34. Archut A, Azzellini GC, Balzani V, De Cola L, Vo¨gtle F. Toward photo-
switchable dendritic hosts. Interaction between azobenzene-function-
alized dendrimers and eosin. J Am Chem Soc 1998;120:12187–12191.
44. McElhanon JR, McGrath DV. Constitution, configuration, and the op-
tical activity of chiral dendrimers. J Am Chem Soc 1998;120:1647–
1656.
35. Kou H, Shi W, Lougnot DJ, Lu Y, Ming H. Dendritic polyisophthalate
endcapped with naphthyl groups for holographic recording. Polym
Adv Technol 2004;15:508–513.
45. Altomare A, Ciardelli F, Ghiloni MS, Solaro R, Tirelli N. Chiral metha-
crylic polymers containing permanent dipole azobenzene chromo-
phores. 13C NMR spectra and photochromic properties. Macromol
Chem Phys 1997;198:1739–1752.
36. Angiolini L, Caretti D, Giorgini L, Salatelli E. Synthesis and chiroptical
properties of optically active photochromic methacrylic polymers bear-
ing in the side chain the (S)-3-hydroxypyrrolidinyl group conjugated
with the trans-azoaromatic chromophore. J Polym Sci Part A: Polym
Chem 1999;37:3257–3268.
46. Angiolini L, Carlini C, Caretti D, Salatelli E. Optically active polymers
bearing side-chain photochromic moieties: synthesis and chiroptical
properties of methacrylic and acrylic homopolymers with pendant L-
lactic acid or L-alanine residues connected to trans-4-aminoazoben-
zene. Macromol Chem Phys 1995;196:2737–2750.
37. Angiolini L, Caretti D, Giorgini L, Salatelli E. Methacrylic polymers
bearing side-chain permanent dipole azobenzene chromophores
spaced from the main chain by chiral moieties: synthesis and charac-
terization. Polymer 2001;42:4005–4016.
47. Chiellini E, Solaro R, Ledwith A, Galli G. Optically active vinyl poly-
mers containing fluorescent groups, part 8. Synthesis and properties
of copolymers of N-vinylcarbazole and (-)-menthyl acrylate and
(-)-menthyl methacrylate. Macromolecules 1980;13:1654–1660.
38. Perrin DD, Amarego WLF, Perrin DR. Purification of laboratory chem-
icals. Oxford: Pergamon Press; 1966.
48. Majumdar RN, Carlini C. Spectroscopic characterization and chirop-
tical properties of the copolymers of (-)3-menthyl acrylate and (-)3-
menthyl methacrylate with styrene. Makromol Chem 1980;181:201–
214.
39. Miller TM, Kwock EW, Neenan TX. Synthesis of four generations of
monodisperse aryl ester dendrimers based on 1,3,5-benzenetricarbox-
ylic acid. Macromolecules 1992;25:3143–3148.
49. Carlini C, Gurzoni F. Optically active polymers containing side-chain
benzophenone chromophores. Polymer 1983;24:101–106.
40. Godbert N, Bryce MR. Highly-charged organic nanoparticles: redox-
active dendrimers incorporating 9,10-bis(1,3-dithiol-2-ylidene)-9,10-
dihydroanthracene units. J Mater Chem 2002;12:27–36.
50. Angiolini L, Benelli T, Giorgini L, Salatelli E. Optically active photo-
chromic methacrylic polymers with controlled average molecular
weight and defined end-groups by atom transfer radical polymeriza-
tion. Polymer 2005;46:2424–2432.
41. Hawker CJ, Fre´chet JMJ. A new convergent approach to monodis-
perse dendritic macromolecules.
1990;15:1010–1013.
J Chem Soc Chem Commun
51. Painelli A, Terenziani F, Angiolini L, Benelli T, Giorgini L. Chiral
interactions in azobenzene dimers: a combined experimental and
theoretical study. Chem Eur J 2005;11:6053–6063.
42. Hawker CJ, Fre´chet JMJ. Preparation of polymers with controlled mo-
lecular architecture. A new convergent approach to dendritic macro-
molecules. J Am Chem Soc 1990;112:7638–7647.
Chirality DOI 10.1002/chir