Synthesis and photoresponsive behavior of optically active methacrylic homopolymers containing side-chain spiropyran chromophores

Article abstract of DOI:10.1016/j.reactfunctpolym.2012.04.011

Novel optically-active methacrylic homopolymers bearing in the side chain one or more chiral groups of one single configuration (based on the l-lactic acid residue) linked to the spiropyran chromophore, have been successfully synthesized and fully characterized. These intrinsically chiral polymers exhibit remarkable thermal stability, with glass transition temperatures in the range 100-130 °C and decomposition temperatures around 270°C. The chiroptical characterization indicates the occurrence of asymmetric induction on the electronic transitions of the side-chain chromophore related to the number of l-lactic acid residues interposed between the main chain and the spiropyran chromophore. In the presence of acid, these systems can be used to modulate the protonation of polymeric azopyridine moieties upon photoisomerization of the spiropyran group. In addition to UV-Vis spectroscopy, the proton transfer process occurring between the macromolecular components can be also followed by CD spectroscopy, the system thus behaving as a chiroptical switch.

Full text of DOI:10.1016/j.reactfunctpolym.2012.04.011

Reactive & Functional Polymers 72 (2012) 469–477
Contents lists available at SciVerse ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Synthesis and photoresponsive behavior of optically active methacrylic
homopolymers containing side-chain spiropyran chromophores
b
a
c
Luigi Angiolini a, Tiziana Benelli a, , Erika Bicciocchi , Loris Giorgini , Françisco M. Raymo
⇑
a Dip. di Chimica Industriale e dei Materiali and INSTM UdR-Bologna, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
b CSIRO Molecular and Health Technologies, Bag 10, Clayton South, Victoria 3169, Australia
c Laboratory for Molecular Photonics, Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, FL 33146-0431, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel optically-active methacrylic homopolymers bearing in the side chain one or more chiral groups of
one single configuration (based on the -lactic acid residue) linked to the spiropyran chromophore, have
been successfully synthesized and fully characterized.
These intrinsically chiral polymers exhibit remarkable thermal stability, with glass transition
temperatures in the range 100–130 °C and decomposition temperatures around 270 °C. The chiroptical
characterization indicates the occurrence of asymmetric induction on the electronic transitions of the
Received 27 October 2011
Received in revised form 13 March 2012
Accepted 23 April 2012
L
Available online 27 April 2012
Keywords:
side-chain chromophore related to the number of
chain and the spiropyran chromophore.
L-lactic acid residues interposed between the main
Photochromic chiral polymers
Circular dichroism
Spiropyrans
Photochromism
Polymeric chiroptical switches
In the presence of acid, these systems can be used to modulate the protonation of polymeric azopyri-
dine moieties upon photoisomerization of the spiropyran group. In addition to UV–Vis spectroscopy, the
proton transfer process occurring between the macromolecular components can be also followed by CD
spectroscopy, the system thus behaving as a chiroptical switch.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
An important class of photochromic compounds includes deriv-
atives of spiropyran (SP) and spirooxazine, which are characterized
An intense interest has currently arisen toward investigations
dealing with the amplification of chirality of polymeric materials,
in solution as well as in the solid state [1–4]. To this regard, in pre-
vious studies we reported on optically active photochromic meth-
acrylic polymers bearing in the side chain both a chiral group of
by a marked photochromism [14–17]. It is well known that the col-
orless SP, upon irradiation with ultraviolet (UV) light, isomerizes to
the purple merocyanine open form (ME) through heterolytic cleav-
age of the spiro carbon–oxygen bond. The process is thermally and
photochemically reversible. Furthermore, the photogenerated
open form, after addition of acid, switches completely to the
protonated species (MEH) which, if irradiated with visible light,
releases a proton and switches back to the SP form. The absorption
and emission properties of these three species (SP, ME and MEH)
significantly differ and allow to use these compounds for repeated
data recording and optical data storage [14,18–23] (Fig. 1).
Fascinating strategies to implement memory devices based on
these spiropyran photochromic compounds have indeed been
proposed, such as the intermolecular communication between
different molecular switches based on photon and/or proton trans-
fer [19,20,24].
Recently [25], we also reported the possibility to couple an
homopolymeric methacrylic spirobenzopyran system, poly[10-(2-
methacryloxyethyl)-30,30-dimethyl-6-nitrospiro(2H-1-benzopy-
ran-2,20-indoline)] {poly[M-SP]}, with a polymer functionalized
with chiral azopyridine moieties, poly[(S)-3-methacryloyloxy-1-
[4-(2-pyridilazo)phenyl] pyrrolidine] {poly[(S)-AZ]}, which can
exist in the unprotonated {poly[(S)-AZ]} and protonated form
{poly[(S)-AZH]} (Fig. 2).
one single configuration (i.e. the L-lactic acid residue) and trans-
azoaromatic [5,6] or porphyrin chromophores [7] that provide
the macromolecules with the further possibility of assuming a con-
formational dissymmetry of one prevailing screw sense, which can
be revealed by the presence, in the circular dichroism (CD) spectra,
of dichroic bands related to the electronic transitions of the chro-
mophore [8]. This functional combination allows the polymers to
display additional properties typical of dissymmetric systems
(optical activity, exciton splitting of chiroptical absorptions) which
are of interest in the field of chiral nanotechnology [9,10].
On the other hand, photochromic materials are well known can-
didates for several technological applications, such as devices for
the optical storage of information, optical switches and, in general,
as materials exhibiting photoresponsive properties when irradi-
ated with light of suitable frequency and intensity [11–13].
⇑
Corresponding author. Tel./fax: +39 051 2093720.
E-mail address: tiziana.benelli@unibo.it (T. Benelli).
1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2012.04.011

470
L. Angiolini et al. / Reactive & Functional Polymers 72 (2012) 469–477
In this context, we considered the opportunity of preparing and
CH3
investigating a new group of optically-active methacrylic polymers
bearing in the side chain one or two chiral groups of one single
O
NO2
N
configuration based on the L-lactic acid residue linked to the
CH3
CH3
VIS light
photochromic spiropyran chromophore, in order to use this mate-
rial in combination with poly[(S)-AZ], so as to observe the hydro-
gen transfer also when these distinct macromolecular systems
are both optically active.
SP
UV light
To this purpose, in the present article we describe the prepa-
ration of the chiral polymeric derivatives poly[10-{2-[(S)-2-meth-
acryloyloxypropanoyloxy]ethyl}-30,30-dimethyl-6-nitrospiro(2H-
1-benzopyran-2,20-indoline)] {poly[(S)-ML-SP]} and poly[10-(2-
{(S)-2-[(S)-2-methacryloyloxypropanoyloxy]propanoyloxy}ethyl)-
30,30-dimethyl-6-nitrospiro(2H-1-benzopyran-2,20-indoline)]
{poly[(S,S)-MLL-SP]} (Fig. 2). These new polymeric materials have
been characterized by common spectroscopic techniques with par-
ticular attention to their thermal, chiroptical and photochromic
properties.
-
HO
O
CH3
CH3
+
+
N
N
NO2
NO2
CH3
CH3
CH3
CH3
MEH
ME
The photoisomerization with UV and/or Vis light of the spiropy-
ran moiety, in the presence of trifluoroacetic acid (TFA), has been
investigated in the presence of poly[(S)-AZ] and the photorespon-
sive properties, as well as the resulting signal communication be-
tween these macromolecular switches, observed by UV–Vis and
CD spectroscopy.
TFA
Fig. 1. The switching cycle associated with the three species SP, ME and MEH.
It was observed that the proton release by the protonated azo-
pyridine compound to the ME species can be modulated by con-
trolling the interconversion between SP and ME through UV and
Vis light irradiation [20,24,25] and that the obtained signal com-
munication between these polymeric switches can be monitored
not only by UV–Vis spectroscopy, but also by CD [25], owing to
the presence in poly[(S)-AZ] of a chiral group of one prevailing con-
figuration (i.e. the pyrrolidine residue) interposed between the
polymeric backbone and the trans-azopyridine chromophore.
Although numerous investigations have been devoted to photo-
sensitive polymers containing spiropyran side groups [14,17,22,
26], to our knowledge, polymeric systems bearing two distinct
functional groups (i.e. spiropyran and chiral groups of one single
absolute configuration) directly linked to the side chain have not
yet been reported and could be of potential interest for several ad-
vanced applications and to investigate the influence of the macro-
molecular chirality on the peculiar properties of this particular
group of photoresponsive polymeric derivatives.
2. Experimental
2.1. Physico-chemical characterization
1H and 13C NMR spectra were obtained at room temperature, on
5–10% w/v CDCl3 solutions, using a Varian NMR Gemini 300 spec-
trometer. Chemical shifts are given in ppm from tetramethylsilane
(TMS) as the internal reference. 1H NMR spectra were run at
300 MHz by using the following experimental conditions: 24,000
data points, 4.5-kHz spectral width, 2.6-s acquisition time, 128
transients. 13C NMR spectra were recorded at 75.5 MHz under full
proton decoupling by using the following experimental conditions:
24,000 data points, 20-kHz spectral width, 0.6-s acquisition time,
64,000 transients. FT-IR spectra were carried out on a Perkin–El-
mer 1750 spectrophotometer equipped with an Epson Endeavor
II data station, on samples prepared as KBr pellets.
Number average molecular weights of the polymers ðMnÞ and
their polydispersity index ðMw=MnÞ were determined in THF solu-
tion by SEC using an HPLC Lab Flow 2000 apparatus equipped with
an injector Rheodyne 7725i, a Phenomenex Phenogel 5-micron
MXL column and a UV–Vis detector Linear Instrument model
UVIS-200, working at 254 nm. Calibration curves were obtained
by using monodisperse polystyrene standards in the range 800–
35,000.
The glass transition temperature values (Tg) of polymers were
determined by differential scanning calorimetry (DSC) on a TA
Instrument DSC 2920 Modulated apparatus adopting a tempera-
ture program consisting of three heating and two cooling ramps
starting from room temperature (heating/cooling rate 10 °C/min
under nitrogen atmosphere) on samples of 5–9 mg.
CH3
CH3
(
)
n
CH2
C
C
R
(
)
n
CH2
H
C
C
O
O
O
*
N
N
poly[(S)-AZ]
O
NO2
N
N
The initial thermal decomposition temperature (Td) was deter-
mined on the polymeric samples with a Perkin–Elmer TGA-7 ther-
mogravimetric analyzer by heating the samples in air at a rate of
20 °C/min.
Checking of liquid crystalline behavior was carried out with a
Zeiss Axioscope2 polarizing microscope through crossed polarizers
fitted with a Linkam THMS 600 hot stage.
Optical activity measurements were accomplished at 25 °C on
dichloroethane solutions (c ꢀ 0.250 g dLꢁ1) with a Perkin Elmer
341 digital polarimeter, equipped with a Toshiba sodium bulb,
poly[M-SP]
N
R=
O
CH3
O
C
poly[(S)-ML-SP]
R=
R=
O
O
CH
*
O
O
CH3
O
C
CH3
CH
O
C
poly[(S,S)-MLL-SP]
CH
*
O
*
Fig. 2. Chemical structures of the investigated polymers.

L. Angiolini et al. / Reactive & Functional Polymers 72 (2012) 469–477
471
using a cell path length of 1 dm. Specific optical rotation values at
the sodium D line are expressed as deg dmꢁ1 gꢁ1 cm3.
(d, 2H, CH2@), 5.90 (d, 1H, CH@CHACO), 4.30 (m, 2H, CH2AO),
3.60–3.35 (m, 2H, CH2AN), 1.90 (s, 3H, methacrylic CH3), 1.30
and 1.15 (2s, 6H, indoline CH3) ppm.
UV–Vis absorption spectra of the samples in solution were re-
corded at 25 °C in dichloroethane with a Varian Cary 100 Bio spec-
trophotometer. The spectral region 800–220 nm was investigated
by using cells path length of 0.1 cm at chromophore concentration
FT-IR: 3081 (mCH arom.), 2946 (mCH aliph.), 1713 (
mC@C methacrylic), 1611 and 1509 (mC@C arom.), 1162 (
and 743 (dCH arom. rings) cmꢁ1
m
C@O), 1637
(
mCAO), 809
.
of about 5.0 ꢂ 10ꢁ4 mol Lꢁ1
.
CD spectra were carried out on a Jasco 810 A dichrograph, under
2.3.2. 10-{2-[(S)-2-Methacryloyloxypropanoyloxy]ethyl}-30,30-
dimethyl-6-nitrospiro(2H-1-benzopyran-2,20-indoline) [(S)-ML-SP].
Yield 24%
the same conditions as the UV–Vis spectra. The
pressed as L molꢁ1 cmꢁ1, were calculated from the following
expression: = [ ]/3300, where the molar ellipticity [ ] in
deg cm2 dmolꢁ1 refers to one spiropyran chromophore.
De values, ex-
1H NMR: 8.05 and 7.95 (d and s, 2H, arom. ortho to nitro group),
7.30–6.70 (m, 5H, arom.), 6.60 (d, 1H, CH@CHACO), 6.20 and 5.65
(d, 2H, CH2@), 5.90 (d, 1H, CH@CHACO), 5.15 (q, 1H, CHACH3), 4.30
(m, 2H, CH2AO), 3.60–3.35 (m, 2H, CH2AN), 1.95 (s, 3H, metha-
crylic CH3), 1.45 (d, 3H, CH3ACH), 1.30 and 1.15 (2s, 6H, indoline
CH3) ppm.
D
e
H
H
Irradiations experiments at 365 nm were carried out at 25 °C on
dichloroethane solutions (2 mL) using the same cells path length
and solution concentrations as the UV–Vis and CD measurements,
with a Mineralight UVGL-25 lamp. Irradiations at 436 nm were
carried out in the same conditions using the emission from a
150 W medium pressure Hg lamp (Hanau), filtered by a 436 nm
interference filter (Balzer) with a 5 nm bandwidth.
13C NMR: 170.9 (C@O lactic ester), 167.3 (C@O methacrylic es-
ter), 159.8, 147.8, 143.6, 142.0, 136.2, 129.1, 128.5, 126.4, 123.2,
122.7, 120.6, 119.1, 116.2, 107.3 and 107.1 (CH@CH pyran), 136.0
(CH2@CACH3), 126.2 (CH2@), 69.5 (CH3ACH), 63.8 (C(CH3)2), 53.3
(CH2AO), 42.6 (CH2AN), 26.5 and 20.3 (indoline CH3), 18.2 (meth-
acrylic CH3), 17.4 (CH3ACH) ppm.
2.2. Materials
Methacryloyl chloride (Sigma–Aldrich, Milan, Italy) was dis-
tilled (bp 95 °C) under inert atmosphere in the presence of traces
of 2,6-di-tertꢃbutyl-p-cresol as polymerization inhibitor just before
FT-IR: 3080 (mCH arom.), 2961 (mCH aliph.), 1750 (mC@O lactic es-
ter), 1723 (mC@O methacrylic ester), 1640 (mC@C methacrylic), 1611
and 1509 (mC@C arom.), 1385 (d CH3), 1160 (
mCAO), 807 and 744
(dCH arom. rings) cmꢁ1
.
use. (+)-L-Lactic acid (Aldrich), 1,3-diisopropylcarbodiimide (DIPC,
Aldrich) and 4-dimethylaminopyridine (Aldrich) were used as re-
ceived. 4-Dimethylaminopyridinium 4-toluensulfonate (DPTS)
was prepared from 4-dimethylaminopyridine and 4-toluensulfonic
acid as described [27].
2.3.3. 10-(2-{(S)-2-[(S)-2-Methacryloyloxypropanoyloxy]propanoy-
loxy}ethyl)-30,30-dimethyl-6-nitrospiro(2H-1-benzopyran-2,
20-indoline) [(S,S)-MLL-SP]. Yield 12%
2,20-Azobisisobutyronitrile (AIBN) (Aldrich) was crystallized
from abs. ethanol before use.
1H NMR: 8.05 and 7.95 (d and s, 2H, arom. ortho to nitro group),
7.30–6.70 (m, 5H, arom.), 6.60 (d, 1H, CH@CHACO), 6.20 and 5.65
(d, 2H, CH2@), 5.90 (d, 1H, CH@CHACO), 5.20–5.05 (m, 2H,
CHACH3), 4.30 (m, 2H, CH2AO), 3.60–3.35 (m, 2H, CH2AN), 1.95
(s, 3H, methacrylic CH3), 1.45 (d, 3H, CH3ACHACOOACH), 1.30
and 1.15 (2s, 6H, indoline CH3) ppm.
Methylene dichloride (CH2Cl2), dichoroethane (DCE) and tetra-
hydrofuran (THF) were purified and dried according to reported
procedures [28] and stored over molecular sieves (4 Å) under
nitrogen.
All other reagents and solvents (Aldrich) were used as received
without further purification.
13C NMR: 170.9, 170.7 (C@O lactic ester), 167.3 (C@O metha-
crylic ester), 159.9, 147.8, 143.4, 141.8, 136.3, 129.2, 128.4, 126.5,
123.4, 122.5, 120.7, 119.1, 116.2, 107.4 and 107.1 (CH@CH pyran),
136.1 (CH2@CACH3), 126.1 (CH2@), 69.6 and 69.2 (CH3ACH), 63.9
(C(CH3)2), 53.4 (CH2AO), 42.8 (CH2AN), 26.4 and 20.4 (indoline
CH3), 18.3 (methacrylic CH3), 17.4 (CH3ACH) ppm.
2-(30,30-Dimethyl-6-nitro-30H-spiro[chromene-2,20-indol]-10-
yl)-ethanol (SP-OH) [29], (S)-(ꢁ)-methacryloyl-
L-lactic acid [(S)-
ML] [30] and poly[(S)-3-methacryloyloxy-1-[4-(2-pyridy-
lazo)phenyl] pyrrolidine] {poly[(S)-AZ]} [25] were prepared as
reported.
FT-IR: 3076 (mCH arom.), 2956 (mCH aliph.), 1748 (mC@O lactic es-
ter), 1723 (mC@O methacrylic ester), 1639 (mC@C methacrylic), 1610
2.3. Synthesis of the spiropyran monomers
and 1509 (mC@C arom.), 1386 (d CH3), 1159 (
mCAO), 807 and 744
(dCH arom. rings) cmꢁ1
.
The spiropyran alcohol SP-OH (13.5 mmol) was added at room
temperature under nitrogen flow to a solution of the chiral acid (S)-
ML (13.5 mmol) in dry CH2Cl2 (20 mL), in the presence of 2,6-di-
tert-butyl-p-cresol (0.10 g) as polymerization inhibitor. DIPC
(2.7 mL, 17.6 mmol), and DPTS (13.5 mmol) were added to the stir-
red solution as coupling agent and as condensation activator,
respectively [27]. The mixture was kept at room temperature for
48 h, the solid N,N0-diisopropylurea formed filtered off and the or-
ganic solution washed repeatedly with aq. 0.1 M HCl, 5% Na2CO3
and H2O, in that order. After drying the organic layer on anhydrous
Na2SO4 and evaporation of the solvent under vacuum, the crude
mixture of products was purified by column chromatography in
the dark (SiO2, CH2Cl2 as eluent) obtaining three products, i.e. M-
SP, (S)-ML-SP and (S,S)-MLL-SP, in the pure form, which were
characterized as follows.
2.4. Synthesis of polymeric derivatives
The homopolymerization reactions of (S)-ML-SP and (S,S)-MLL-
SP were carried out in glass vials using 2,20-azobisisobutyronitrile
(AIBN) as thermal initiator (2 wt% with respect to the monomers)
and dry THF as solvent (0.5 g of monomer in 5 mL of THF). The
reaction mixture were introduced into the vial under nitrogen
atmosphere, submitted to several freeze–thaw cycles and heated
at 60 °C for 72 h. The reactions were then stopped by pouring the
mixtures into a large excess (100 mL) of methanol, and the coagu-
lated polymer filtered off. The solid polymeric products were puri-
fied by repeated precipitations in methanol and finally dried under
vacuum to constant weight.
The dried polymers were soluble in THF and CHCl3 as well as in
strong polar solvents such as nitrobenzene, DMF, DMA or DMSO.
Relevant data for the synthesized products are reported in Table 1.
Conversions were determined gravimetrically and all the prod-
ucts characterized by FT-IR, 1H and 13C NMR.
2.3.1. 10-(2-Methacryloyloxyethyl)-30,30-dimethyl-6-nitrospiro(2H-1-
benzopyran-2,20-indoline) [M-SP]. Yield 40%
1H NMR: 8.06 and 8.02 (d and s, 2H, arom. ortho to nitro group),
7.22–6.80 (m, 5H, arom.), 6.70 (d, 1H, CH@CHACO), 6.10 and 5.60

472
L. Angiolini et al. / Reactive & Functional Polymers 72 (2012) 469–477
2.4.1. Poly[10-{2-[(S)-2-methacryloyloxypropanoyloxy]ethyl}-30,30-
dimethyl-6-nitrospiro(2H-1-benzopyran-2,20-indoline)] {poly[(S)-ML-
SP]}
Taking into account the mild conditions adopted, similar to
those reported for the synthesis of other optically active polyesters
starting from chiral hydroxy acid reagents reported in the litera-
ture [31,32], it is reasonable to presume that no racemization at
the asymmetric center occurs during the reaction and therefore
1H NMR: 8.05–7.85 (m, 2H, arom. ortho to nitro group), 7.20–
6.50 (m, 5H, arom. and 1H, CH@CHACO), 5.95–5.75 (m, 1H,
CH@CHACO), 5.00–4.75 (m, 1H, CHACH3), 4.40–4.05 (m, 2H,
CH2AO), 3.60–3.20 (m, 2H, CH2AN), 2.10–0.40 (m, 5H, backbone
CH3 and CH2, 6H, indoline CH3 and 3H, CH3ACH) ppm.
13C NMR: 177.4 (C@O methacrylic ester), 170.9 (C@O lactic es-
ter), 159.9, 147.2, 141.8, 136.4, 129.2, 128.5, 126.6, 123.5, 122.5,
120.7, 119.2, 116.2, 107.4 and 107.1 (CH@CH pyran), 69.9
(CH3ACH), 63.6 (C(CH3)2), 55.2 (backbone CH2AC), 53.5 (CH2AO),
45.7 (CH2AC backbone), 42.9 (CH2AN), 26.5 and 20.5 (indoline
CH3), 19.6 and 17.8 (backbone CH3), 17.4 (CH3ACH) ppm.
that the obtained products are as optically pure as the
precursor.
L-lactic acid
The structures of the above mentioned products were con-
firmed by 1H NMR analysis, which displays, besides the resonances
of the vinylic protons of monomeric methacrylate (around 5.70 and
6.20 ppm) and the protons of the spiropyran moiety, an increasing
number of quartets (substantially overlapped) and doublets due to
the lactic acid CH and CH3 groups, respectively.
Differently from (S)-ML-SP and (S,S)-MLL-SP, the achiral mono-
mer M-SP, previously synthesized by a different way and already
investigated in its homopolymeric form [25], was not submitted
to polymerization.
FT-IR: 3042(
methacrylic ester), 1611 and 1521 (
CAO), 806 and 746 (dCH arom. rings) cmꢁ1
.
m
CH arom.), 2952 (mCH aliph.), 1732 (mC@O lactic and
mC@C arom.), 1385 (d CH3), 1126
(m
The chiral monomers were then radically homopolymerized in
THF solution by adopting the usual method employing AIBN (2%
w/w of monomer) as thermal initiator. The obtained polymeric
products, purified by repeated dissolution and reprecipitation in
methanol, displayed comparable average molecular weight and
polydispersity values (Table 1), as determined by SEC, and the ex-
pected IR spectra, showing the disappearance of the absorption
band at 1636 cmꢁ1, related to the methacrylic double bond.
Accordingly, in the 1H NMR spectra, the resonances of the vinylide-
nic protons of the monomer were absent. The purified polymeric
products were completely soluble in several solvents such as
CHCl3, THF, N-methylpyrrolidone, dimethylacetamide, dimethyl-
sulfoxide, this behavior favoring the possibility of their application
also as thin films.
2.4.2. Poly[10-(2-{(S)-2-[(S)-2-methacryloyloxypropanoyloxy]-
propanoyloxy}ethyl)-30,30-dimethyl-6-nitrospiro(2H-1-benzopyran-
2,20-indoline)] {poly[(S,S)-MLL-SP]}
1H NMR: 8.05–7.90 (m, 2H, arom. ortho to nitro group), 7.20–
6.60 (m, 5H, arom. and 1H, CH@CHACO), 5.95–5.80 (m, 1H,
CH@CHACO), 5.20–4.85 (m, 2H, CHACH3), 4.40–4.05 (m, 2H,
CH2AO), 3.60–3.30 (m, 2H, CH2AN), 2.10–0.80 (m, 5H, backbone
CH3 and CH2, 6H, indoline CH3 and 6H, CH3ACH) ppm.
13C NMR: 177.5 (C@O methacrylic ester), 170.8 (C@O lactic es-
ter), 160.0, 147.3, 141.8, 136.4, 129.3, 128.5, 126.6, 123.5, 122.5,
120.7, 119.2, 116.2, 107.4 and 107.1 (CH@CH pyran), 69.4
(CH3ACH), 63.9 (C(CH3)2), 55.2 (backbone CH2AC), 53.5 (CH2AO),
45.7 (CH2AC backbone), 42.9 (CH2AN), 26.5 and 20.5 (indoline
CH3), 19.6 and 17.8 (backbone CH3), 17.4 (CH3ACH) ppm.
The thermal stability of the polymeric derivatives, as deter-
mined by thermogravimetric analysis (TGA), resulted good under
the adopted conditions, with decomposition temperature values
around 271–273 °C (Table 1). Such a behavior is similar to that re-
FT-IR: 3042(
methacrylic ester), 1610 and 1520 (
CAO), 807 and 744 (dCH arom. rings) cmꢁ1
.
m
CH arom.), 2969 (mCH aliph.), 1738 (mC@O lactic and
m
C@C arom.), 1385 (d CH3), 1126
(m
ported for the analog poly[M-SP] [25], lacking of the L-lactic acid
residue, and is indicative of a remarkable presence of strong dipo-
lar interactions in the solid state between the chromophores lo-
cated in the macromolecular side chains, thus making these
materials, also, interesting for potential applications.
3. Results and discussion
3.1. Synthesis and structural characterization
The observation with a polarizing microscope did not reveal any
liquid–crystalline behavior and DSC thermograms showed only
second-order transitions, originated by glass transitions (Tg), with
no melting peaks, thus suggesting that these materials are substan-
tially amorphous in the solid state (Table 1 and Fig. 3).
As previously reported, poly[M-SP] is characterized by a high Tg
value (166 °C) which was ascribed to the stiffness and steric hin-
drance of the spiropyran group located in the side chain [25].
The insertion of an increasing number of lactic acid residues be-
tween the backbone and the chromophore, makes the side chain
of poly[(S)-ML-SP] and poly[(S,S)-MLL-SP] increasingly flexible,
however does not prevent the presence in the solid state of strong
dipolar interactions between the spiropyran moieties, as demon-
strated by their still high Tg values (123 and 99 °C, respectively)
(Fig. 3).
The key intermediate 2-(30,30-dimethyl-6-nitro-30H-spiro[chro-
mene-2,20-indol]-10-yl)-ethanol (SP-OH) [29] was used to prepare
the new optically-active monomers (S)-ML-SP and (S,S)-MLL-SP
by reaction with the chiral acid (S)-ML [30] in the presence of DIPC
and DPTS as coupling agent and condensation activator, respec-
tively [27] (Scheme 1).
Under these conditions, SP-OH and (S)-ML may give esterifica-
tion and transesterification of the methacrylic ester. As a conse-
quence, spirobenzopyran methacrylic monomers with different
numbers of lactic acid residues linked together were actually ob-
tained and separated by column chromatography. The structures
of the different products achieved along with their relative
amounts are reported in Scheme 1.
Table 1
3.2. UV–Vis and photochromic properties
Characterization data of polymeric derivatives.
b
c
a
a
Sample
Yield %
Tg (°C) Td (°C)
As previously reported for other spiropyran derivatives
[20,24,25,33–35], the absorption spectra (Table 2) of colorless solu-
tions of poly[(S)-ML-SP], poly[(S,S)-MLL-SP] and of the correspond-
ing monomers show three typical absorption bands: the first one
located at 340 nm is related to the internal charge-transfer transi-
tion of the spiropyran system; the other two, at 270 and 243 nm,
more intense, to the electronic transitions of the individual aro-
matic rings.
Mn;SEC g/mol
Mw=Mn
Poly[M-SP]d
Poly[(S)-ML-SP]
Poly[(S,S)-MLL-SP] 39
73
47
11,800
5500
5300
1.8
1.8
1.5
166
123
99
275
273
271
a
b
c
Determined by SEC in THF at 25 °C.
Determined by DSC, heating rate of 10 °C/min under nitrogen atmosphere.
Determined by TGA, heating rate of 20 °C/min in air.
Ref. [25].
d

L. Angiolini et al. / Reactive & Functional Polymers 72 (2012) 469–477
473
CH3
CH2
C
C
O
O
CH3
C
OH
O
CH2
HC CH3
*
C
O
C
O
n=0-2
+
N
O
O
HC CH3
*
NO2
C
O
N
OH
O
NO2
(S)-ML
SP-OH
(n=0, 40% yield)
(n=1, 24% yield)
(n=2, 12% yield)
M-SP
(S)-ML-SP
(S,S)-MLL-SP
Scheme 1. Synthetic route to the spiropyran monomers.
Table 2
UV–Vis spectral data of polymeric compounds and their corresponding monomers in
dichloroethane solution at 25 °C.
Samples
1st Band
2nd Band
3rd Band
a
b
a
b
a
b
Poly[M-SP]
kmax
emax
kmax
emax
kmax
emax
M-SPc
341
341
342
340
341
340
8700
8100
8600
8700
8700
8600
268
269
268
270
268
269
16,400
16,200
16,300
16,100
16,300
15,900
243
243
243
243
244
243
20,200
20,100
20,200
20,700
20,100
20,700
Poly[M-SP]c
(S)-ML-SP
Poly[(S)-ML-SP]
(S,S)-MLL-SP
Poly[(S,S)-MLL-SP]
Poly[(S)-ML-SP]
a
b
c
Wavelength of maximum absorbance, expressed in nm.
Expressed in L molꢁ1 cmꢁ1 and calculated for one single chromophore.
Ref. [25].
Poly[(S,S)-MLL-SP]
1.2
1.0
0.8
0.6
0.4
0.2
0.0
50
100
150
200
T (°C)
Fig. 3. DSC traces (second heating cycle) of poly[M-SP], poly[(S)-ML-SP] and
poly[(S,S)-MLL-SP] at heating rate of 10 °C/min under nitrogen atmosphere.
As an example, the UV–Vis absorption spectra of poly[(S)-ML-
SP] in dichloroethane before and after irradiation at 365 nm and
subsequent treatment with one equivalent of TFA are reported in
Fig. 4.
By irradiating the solution poly[(S)-ML-SP] with UV light, the
neutral compound switches to the purple zwitterionic merocya-
nine isomer (ME) because of the cleavage of a CAO bond. Thus,
the conjugation between the aromatic rings increases, and an
300
400
500
λ (nm)
600
700
absorption band at 575 nm, related to the merocyanine
p–
pꢄ
electronic transition, appears. Poly[(S,S)-MLL-SP] and poly[M-SP]
show similar behavior, except for the intensity of the merocyanine
band.
Fig. 4. UV–Vis spectra of poly[(S,S)-MLL-SP] in dichloroethane before (—) and
after (- - -) irradiation at 365 nm and subsequent treatment with one equivalent
of TFA (ꢃꢃꢃ).

474
L. Angiolini et al. / Reactive & Functional Polymers 72 (2012) 469–477
By using the molar extinction coefficient of the open form of the
optical data-storage applications, similarly to the achiral polymeric
derivative poly[M-SP].
parent compound 10,30-dihydro-10,30,30-trimethyl-6-nitrospiro[2H-
1-benzopyran-2,20(2H)-indole] [36], it is possible to evaluate the
amount of ME% at the photostationary state. The obtained data
show that, after the same irradiation time, poly[(S,S)-MLL-SP]
and poly[(S)-ML-SP] display lower amounts of open form than
the related achiral homopolymer poly[M-SP] (21%, 27% and 30%,
respectively). By contrast, monomers (S)-ML-SP and (S,S)-MLL-
SP, which do not posses any structural restriction and are ran-
domly dispersed in dilute solution, show amounts of merocyanine
form around 9%, similarly to M-SP [25,35].
3.3. Chiroptical properties of spiropyran derivatives
To investigate the optical activity in dilute solution, the specific
and molar optical rotation values at the sodium D line {½a D25
ꢅ
and
½
U D
25} (Table 3) were determined for the polymeric derivatives
ꢅ
and the CD spectra (Table 4) of all the chiral samples were
recorded.
The specific optical rotation values shown by poly[(S)-ML-SP]
and poly[(S,S)-MLL-SP] (ꢁ22 and ꢁ41 deg dmꢁ1 gꢁ1 cm3, respec-
tively) (Table 3) are higher than those of the corresponding mono-
mers (S)-ML-SP and (S,S)-MLL-SP (ꢁ4 and ꢁ31 deg dmꢁ1 gꢁ1 cm3,
respectively) and can be ascribed to the establishment of increased
overall dissymmetry in the macromolecules. The remarkable opti-
cal activity enhancement shown by the (S,S)-MLL derivatives with
respect to the (S)-ML ones, could be attributed to the increasing
number of chiral centers of a single absolute configuration present
in each monomeric unit.
As reported in Table 4, the CD spectra of monomeric and poly-
meric chiral spiropyran derivatives show, in correspondence of the
UV–Vis absorption bands (Table 2), dichroic signals of low inten-
sity. The intensities of these bands are significantly higher in the
polymer with respect to the corresponding monomer and increase
with the number of chiral centers present in the side chain: indeed,
the CD spectrum of poly[(S,S)-MLL-SP] is enhanced in amplitude if
compared to that one of poly[(S)-ML-SP] (Table 4 and Fig. 6), in
agreement with the observed optical rotation values. A similar
trend, although of lower magnitude, is shown by the related mono-
mers (S)-ML-TPP and (S,S)-MLL-TPP.
Such a behavior can be ascribed to the occurrence of dipolar
interactions between spiropyran chromophores in the side chains,
stabilizing the merocyanine form at the equilibrium, which pro-
gressively fade when the flexibility of the side chains increases
due to the presence of a longer chiral spacer located between the
chromophore and the macromolecular backbone.
This hypothesis is confirmed by the different spiropyran back-
isomerization rates of solutions stored in the dark. Under these
conditions, the merocyanine residue thermally isomerizes back
to the initial neutral closed spiropyran form, which is thermody-
namically more stable, and the process can be followed by
monitoring the absorbance change at 570 nm (Fig. 5). The back-
isomerization rates appear almost similar for poly[M-SP] [25,35]
and poly[(S)-ML-SP], but relatively faster for poly[(S,S)-MLL-SP]
as a direct consequence of the above mentioned reduction of
dipolar interchromophoric interactions stabilizing the ME form.
Upon addition of 1 equivalent of TFA to the solution at the
photostationary state after UV irradiation, the purple ME form
switches to the yellow-green protonated form MEH. Accordingly,
the absorption at 575 nm fades and a new band appears at
435 nm (Fig. 4). By irradiating with Vis light, or by adding a base
(triethylamine), MEH can be converted again to the initial neutral
spiropyran form, thus restoring the original absorption spectrum.
In conclusion, the new chiral polymeric compounds, as well as
the corresponding monomers, can switch between three different
states, each of them characterized by its own absorption proper-
ties. These features, together with their significant thermal stabil-
ity, suggest the possibility of use for repeated recording of data and
The dichroic signals are clearly due to the intrinsic chirality of
the L-lactic acid residue, which is able to induce an asymmetric
perturbation on the electronic transitions of the spiropyran chro-
mophore giving rise to differential circularly polarized light
absorption in solution. As frequently observed in the literature
[1–4,8], the optical activity of polymeric derivatives is remarkably
higher with respect to the related monomers, in consequence of
the restricted conformational mobility due to the macromolecular
structure.
Differently from our previous studies [5–7] concerning optically
active polymeric derivatives bearing side-chain azoaromatic or
porphyrin chromophores, in the present case the spiropyran chro-
mophore does not appear to originate in their CD spectra dichroic
excitonic couplets which are usually correlated to the presence of
conformational dissymmetry of the macromolecules with a pre-
vailing helix sense. In this case the absence of particularly struc-
tured dichroic signals (excitonic couplets) in the CD spectra of
the homopolymers suggests that no conformational dissymmetry
is present or that the chirally perturbed electronic transitions of
the spiropyran chromophores are insufficient to disclose it.
However, the presence in these materials of the spiropyran
chromophore possessing induced optical activity, may allow to
1.0
0.8
0.6
0.4
0.2
0.0
Table 3
Specific and molar optical rotation of spiropyran derivatives.
a
b
Sample
½
a 2D5
ꢅ
½ ꢅ
U 2D5
ML-SP
Poly[(S)-ML-SP]
MLL-SP
ꢁ4
ꢁ19.7
ꢁ22
ꢁ31
ꢁ41
ꢁ108.4
ꢁ175.1
ꢁ231.5
0
10
20
30
40
Poly[(S,S)-MLL-SP]
time (min)
a
Specific optical rotation, expressed as deg dmꢁ1 gꢁ1 cm3, in dichloroethane
solution.
Fig. 5. UV–Vis absorbance values expressed as At/A0 (at 570 nm) of poly[(S)-ML-SP]
(d) and poly[(S,S)-MLL-SP] (s) at 25 °C in dichloroethane solution during thermal
back isomerization after irradiation at 365 nm.
b
Molar optical rotation, expressed as deg dmꢁ1 molꢁ1 dL and calculated as
(½a 2D5
ꢅ ꢃM/100), where M represents the molecular weight of the monomeric unit.

L. Angiolini et al. / Reactive & Functional Polymers 72 (2012) 469–477
475
Table 4
CD spectral data of chiral spiropyran derivatives in dichloroethane solution at 25 °C.
12
Sample
1st Band
2nd Band
3rd Band
10
8
a
b
a
b
a
b
k1
D
e1
k2
D
e2
k3
De3
(S)-ML-SP
Poly[(S)-ML-SP]
(S,S)-MLL-SP
–
342
–
–
265
268
266
266
ꢁ0.32
ꢁ0.45
ꢁ0.41
ꢁ0.60
246
247
244
246
ꢁ0.60
ꢁ0.66
ꢁ0.74
ꢁ1.07
+0.07
–
+0.12
6
Poly[(S,S)-MLL-SP]
343
4
a
Wavelength (in nm) of maximum dichroic absorption.
b
De
expressed in L molꢁ1 cmꢁ1 and calculated for one repeating unit in the
2
polymers.
0
-2
-4
-6
-8
2
1
0
-10
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-1
-2
250
300
350
λ (nm)
400
450
500
Fig. 6. CD spectra in dichloroethane solution of poly[(S)-ML-SP] (—) and poly[(S,S)-
MLL-SP] (- - -).
investigate their photochromic properties by chiroptical methods,
also, in addition to be of interest in the field of the chiral nanotech-
nology, and to investigate the amplification of chirality in poly-
meric materials.
300
400
500
700
600
λ (nm)
3.4. Photoresponsive properties and photomodulation of proton
transfer between azopyridine and spiropyran dyes
Fig. 7. UV–Vis (bottom) and CD spectra (top) of a solution of poly[(S,S)-MLL-SP]
and poly[(S)-AZ] 1:1 in dichloroethane (5 ꢂ 10ꢁ4 M) (ꢃ-ꢃ-ꢃ), in the presence of one
equivalent of TFA (- - -), upon irradiation at 365 nm (ꢃꢃꢃ) and at 436 nm (—).
The literature reports the possibility to photo-modulate the
proton transfer between low molecular weight spiropyran and
azopyridine systems in solution [20,24]. Recently, it was demon-
strated that the same process can also take place when these func-
tionalities are bonded to distinct macromolecular chains [25] or to
the same macromolecular chain in copolymeric derivatives [35].
Moreover, the insertion of a pyrrolidine group of one prevailing
configuration between the polymeric backbone and the trans-azo-
pyridine dye allows to monitor the reversible and reproducible sig-
nal communication by CD, as well as by UV–Vis spectroscopy.
In this context, solutions of the investigated chiral spiropyran
polymeric materials containing an equimolar amount of azopyri-
dine homopolymer {poly[(S)-AZ]} have been irradiated alterna-
tively by UV and Visible light in the presence of one equivalent
of TFA, affording the protonated form {poly[(S)-AZH]}. All the
derivatives displayed a UV–Vis behavior similar to that previously
reported for the mixture of the corresponding homopolymers
poly[M-SP] and poly[(S)-AZ] [25].
isomerizes to the open form [(S,S)-ME], which deprotonates (S)-
AZH to give the (S,S)-MLL-MEH and (S)-AZ species. Such behavior
is consistent with the absorption spectra changes: the absorption
bands around 270 and 540 nm, related to (S,S)-MLL-SP and (S)-
AZH species, respectively, decrease, while that one at 425 nm,
due to (S,S)-MLL-MEH and (S)-AZ, increases in intensity. The back
isomerization of (S,S)-MLL-MEH to (S,S)-MLL-SP is obtained by
irradiation at 436 nm, thus inducing again the proton transfer to
(S)-AZ. As a result, the absorption band related to merocyanine de-
creases, while those around 270 and 540 nm increase again.
This process is reproducible and reversible: by submitting the
materials to several cycles of UV and Visible photoirradiation, in
fact, it is possible to tune the absorbance at a specific wavelength
(e.g. at 460 nm, as shown in the inset of Fig. 8 (bottom) for
poly[(S,S)-MLL-SP]), with a good fatigue resistance. Indeed, after
several irradiations steps with UV and Vis light alternatively acting
on the same solution, the values of absorbance remain quite
similar. This indicates that no degradation photoreactions or
relevant variation of interchromophoric interactions take place.
As shown in Fig. 7 (bottom) for poly[(S,S)-MLL-SP] and poly[(S)-
AZH], in fact, upon irradiation at 365 nm, the spiropyran moiety

476
L. Angiolini et al. / Reactive & Functional Polymers 72 (2012) 469–477
Thus, several photoinduced cycles could be performed without any
significant change in the proton-transfer behavior.
optically-active (S)-3-hydroxy pyrrolidinyl group of one single con-
figuration [37] and suggests the presence of cooperative interac-
tions between side-chain azopyridine chromophores disposed in
a mutual chiral geometry of one prevailing helicity.
The above described photoinduced changes in the absorption
spectra are accompanied by changes of dichroic signals. As re-
ported in Fig. 7 (top), the 1:1 mixture of poly[(S,S)-MLL-SP] and
poly[(S)-AZ] displays, upon addition of one equivalent of TFA, a
CD spectrum very similar to that one of the homopolymer
poly[(S)-AZH] alone [25]. In fact, in addition to a weak positive
By irradiating this solution at 365 nm, the formation of the mer-
ocyanine state is promoted, with proton transfer from the (S)-AZH to
the (S,S)-MLL-ME form, with a consequent noticeable change in the
position and shape of the CD bands related to the (S)-AZ/(S)-AZH
ratio. However, the shape and intensities of the CD signals in the
spectral range 220–300 nm are practically unaffected by proton-
transfer phenomena during the photoisomerization, and hence the
process can be followed by observing the CD variations connected
to the more intense transitions of the azopyridine chromophore.
The back isomerization is promoted by irradiation with Visible
light and, as a result, the CD bands related to the protonated
(S)-AZH form increase again. This phenomenon is reversible and
the system does not show any degradation after several irradiation
cycles (see inset in Fig. 8, top).
band around 270 nm related to the
p–
pꢄ electronic transition,
two intense dichroic signals of opposite sign and similar intensity,
with a cross-over point in correspondence of the first UV maxi-
mum absorbance (at about 490 nm), related to the chiral (S)-AZH
moieties, are displayed. Low intensity signals in the spectral range
220–270 nm, related to the chiral (S,S)-MLL-SP moieties, are also
present.
As exhaustively discussed in the literature [25], the excitonic
splitting of the azoaromatic electronic transitions is typical of
methacrylic azobenzene polymers bearing in the side chain the
As expected, the homopolymer with only one chiral residue of
12
lactic acid {poly[(S)-ML-SP]} shows a similar behavior as
poly[(S,S)-MLL-SP] when irradiated in the presence of poly[(S)-
AZ] and TFA.
6
9
6
In conclusion, the present investigation shows the possibility to
tune the protonation of the azopyridinic group by controlling the
photoisomerization of intrinsically chiral spiropyran moieties when
these groups are bonded to a distinct macromolecular chain, also.
-6
3
4. Conclusions
Novel homopolymeric methacrylates bearing in the side chains
the optically-active L-lactic acid residue linked to the spiropyran
0
chromophore were prepared by radical polymerization of the cor-
responding monomers.
Thermal analysis shows the presence of remarkable interaction
between neighboring spiropyran moieties related to the flexibility
of the side-chain chiral spacer.
-3
-6
1,5
The CD spectra of these macromolecular materials in solution
are characterized by enhanced dichroic signals related to the elec-
tronic transitions of the spiropyran chromophores with respect to
the monomers. The polymers display remarkable reversible
changes of their photochromic properties evidenced by UV–Vis
measurements in response to environmental stimuli. In particular,
when these systems are mixed in solution with an optically active
methacrylate homopolymer bearing in the side chain the azopyri-
dine chromophore, in the presence of acid, a modulation of azo-
pyridine protonation by photo-isomerization of the spiropyran
component can be achieved. The signal communication between
these polymeric switches can be monitored by UV–Vis as well as
CD spectroscopy, and is completely reversible and reproducible.
The chemical anchorage of spiropyran photochromic molecules
to polymeric matrices thus allows the combination of their intrin-
sic photochemical properties with those typical of polymers, such
as processability and stability.
1.5
0,6
1.0
0.5
0.0
Acknowledgements
Financial support from Consortium INSTM and University of
Bologna (Marco Polo Program) are gratefully acknowledged.
300
400
500
600
700
λ (nm)
References
Fig. 8. Photochromic modulation of absorbance intensity (bottom) and ellipticity
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365 nm (—) and at 436 nm (ꢃꢃꢃ). Inset: intensity variation at 460 nm of the UV and
CD spectra measured upon alternating irradiation at 365 (UV) and 436 (Vis) nm.
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