Optodynamers: Expression of color and fluorescence at the interface between two films of different dynamic polymers

Article abstract of DOI:10.1039/b712454f

Color and fluorescence changes were expressed at the interface between two different polyhydrazone polymer films, induced by hydrazone bond exchange and component recombination through the interface, resulting in an extension of conjugation. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712454f

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Optodynamers: expression of color and fluorescence at the interface
between two films of different dynamic polymers{
Takashi Ono,ab Shunsuke Fujii,ab Tadahito Noboriab and Jean-Marie Lehn*a
Received (in Cambridge, UK) 14th August 2007, Accepted 18th September 2007
First published as an Advance Article on the web 4th October 2007
DOI: 10.1039/b712454f
obtained by condensation of the corresponding hydrazine and
aldehyde parent molecules.
Color and fluorescence changes were expressed at the interface
between two different polyhydrazone polymer films, induced
by hydrazone bond exchange and component recombination
through the interface, resulting in an extension of conjugation.
The absorption area of the bis-hydrazone compound 1 showed
lmax = 384 nm and extended to around 430 nm, conferring a light
yellow color in CHCl3 solution (Fig. 2). Compound 1 is expected
to present a certain degree of conjugation through two hydrazone
bonds and a thiophene moiety (Fig. 1).
Dynamic polymers, dynamers,1 are supramolecular2,3 or molecu-
lar1,4,5 polymers based on monomeric components linked respec-
tively by non-covalent interactions and by reversible covalent
bonds. They possess in principle the ability to exchange,
incorporate, eliminate or recombine components and thus give
access to a range of novel properties.
Introduction of a phenyl group instead of benzyl on the
hydrazone moieties was effective in expanding the degree of
conjugation in 2 with respect to 1. Indeed the absorption band of
the bis-hydrazone compound 2 shifted to lmax = 421 nm and
extended to around 480 nm, yielding a vivid yellow color in CHCl3
solution. Furthermore, compound 2 exhibited notable fluorescence
around 450–550 nm under excitation at 421 nm. The distinct
yellow–green fluorescence was also observed visually for the neat
powder under 365 nm excitation. Furthermore, compound 4
containing a single phenyl hydrazone unit on one thiophene
moiety displayed much less effective conjugation and compound 5
derived from an aliphatic aldehyde presented a much more blue
shifted absorption (Fig. 2).
In particular, covalent dynamers, based for instance on
reversible hydrazone bonds,4,5 are unique materials displaying
unusual features due to their ability to undergo bond rearrange-
ment and constitutional change.5–9 Indeed, they have been shown
to be amenable to marked modifications in mechanical properties,8
as well as to respond to chemical and physical effectors.9 The
induction of optical properties would be of special interest, as it
could both demonstrate the occurrence of constitutional
dynamics10 in a system and implement dynamics to confer novel
physical features.
In view of these results, two polyhydrazone polymers P1 and P2
(Fig. 3) were selected as initial materials to demonstrate color and
fluorescence changes induced by bond exchanges and component
migration through the interface of these two polymer films.
Polycondensation between the corresponding bis-hydrazine and
dialdehyde monomers in CHCl3 or in CHCl3–THF mixture in the
presence of acid catalyst{ gave respectively an almost colorless,
slightly turbid film for the polymer P1 and a light yellow film for
the polymer P2. A tert-butyl group was introduced in the polymer
P2 to increase solubility and facilitate the polycondensation
reaction. The polymer P3 was prepared as a reference material
in the same manner, giving a dark orange film. These films were
soft, slightly stretchy and easily cut with scissors. Gel permeation
chromatography gave a molecular weight Mn = 38400 and a
distribution Mw/Mn = 2.9 for the polymer P1, Mn = 13900 and
We here describe some results of our studies on the modulation
of optical properties in a system of two films of hydrazone-based
dynamic polymers, demonstrating i) the expression of novel optical
features, ii) that reveals the occurrence of bond reorganization and
component diffusion through the interface of the two films, and iii)
allows the generation of optical patterns.
The process relies on the modification of conjugation on bond
recombination, leading to changes in absorption and emission
properties due to the generation of moieties presenting a more
extended conjugation pathway.
Polymeric or supramolecularly assembled materials possessing
conjugated moieties have attracted considerable attention for
implementation in systems such as sensors or molecular electronic
devices, because they have an inherent potential to change their
conjugation and optical properties by recombination or reconfi-
guration in response to external stimuli.11
In order to obtain information on the color and fluorescence of
bis-hydrazone containing moieties presenting different extents of
conjugation, UV–visible and emission spectroscopic measurements
were first performed on the model compounds 1–3 (Fig. 1),
aISIS, Universite´ Louis Pasteur, 8 alle´e Gaspard Monge, F-67083
Strasbourg cedex, France. E-mail: lehn@isis.u-strasbg.fr
bR&D Center, Mitsui Chemicals, Inc., 580-32 Nagaura, Sodegaura,
Chiba, 299-0265, Japan
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b712454f
Fig. 1 Structures of hydrazone compounds. The conjugation domain is
represented in red.
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Fig. 4 UV–visible absorption spectra in CHCl3 of the hydrazone
polymers (a) P1, (b) P2 and (c) P3, and (d) heating time dependence of
polymer blend P1 and P2; fluorescence emission spectra in CHCl3 of (e)
the hydrazone polymer P3 and (f) heating time dependence of the
fluorescence of polymer blend P1 and P2 under excitation at 425 nm.
Fig. 2 UV–visible absorption spectra of the hydrazone compounds 1–5,
(a) N,N-dibenzylhydrazine and (b) thiophene-2,5-dicarboxaldehyde in
CHCl3 (0.05 mM on the basis of hydrazone, hydrazine or aldehyde) and
fluorescence emission spectrum of bis-hydrazone compound 2 in CHCl3
(0.02 mM) under excitation at 412 nm.
dynamic polymers P1 and P2, generating the functional fragment
characteristic of polymer P3. The fourth combination that results
from combination of the monomer of P1 and P2 yields the bis-
hydrazone moieties shown in polymer P4. It would not be
expected to present notable absorption or emission features in view
of the presence of the isolating trimethylene chain between the two
hydrazone groups.12
Color and fluorescence expressions may be expected to emerge
even at the interface of two different dynamer films. Indeed,
superimposing films of the dynamers P1 and P2 and applying
bond exchange conditions should lead to the appearance of the
absorption and emission features characteristic of dynamer P3 in
the overlapping region of the two films (Fig. 5, top). A simple
demonstration was performed (Fig. 5, bottom) with two polymer
films P1 and P2 (54 mm and 64 mm thickness, respectively). Two
films cut by scissors were superimposed on a glass plate and heated
at around 160–170 uC with a heat gun for 5 min. Only the domain
where the two films were superimposed changed to vivid yellow
and produced a yellow–green fluorescence observable visually
under 365 nm excitation, thus generating a distinctive pattern. On
the other hand, the colors of the rest of the films remained almost
the same as the original ones and no fluorescence was observed.
Fig. 3 Structures of polyhydrazone dynamers P1–P4. The conjugation
pathway is shown in red.
Mw/Mn = 3.1 for the polymer P2, Mn = 34500 and Mw/Mn = 2.7
for the polymer P3 (with polystyrene calibration).
Changes in UV–visible absorption and fluorescence induced by
bond exchange and crossover component recombination were
demonstrated firstly by means of a polymer blend film, prepared
by just mixing solutions of the two homopolymers P1 and P2 in
presence of added acid. The visual color of the blend polymer film
obtained, initially light yellow, changed progressively to vivid
yellow on heating at 120 uC. The UV–visible absorption around
400–450 nm in CHCl3 evolved toward that of polymer P3 as a
function of heating time (Fig. 4). In a similar way, a fluorescence
band appeared around 450–600 nm and its intensity increased
toward that of polymer P3. These results indicate that the change
of color and the emergence of fluorescence resulted from bond
exchange and crossover component recombination between the
Fig. 5 Visual color change and fluorescence emergence by heating of
superimposed polymer films P1 and P2. (Top) Schematic representation;
(bottom) an illustrative example: (a) color before heating under sunlight,
(b) color after heating, (c) fluorescence after heating under excitation at
365 nm.
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responding to external stimuli, and may extend over the
modulation of other properties such as electronic (redox
modifications, conductivity), magnetic, and mechanical. The
dynamic polymers are illustrative of the potential offered by the
application of the principles of constitutional dynamic chemis-
try1,4,10 to materials science.
Notes and references
{ Equimolar amounts of the corresponding bis-hydrazine (compounds L
and P, ESI{) and the dialdehyde monomers at concentrations around
0.04 M each were dissolved in the mixture of CHCl3 and THF (8 : 2 in
volume ratio; for the polymer P1) or CHCl3 (for the polymers P2, P3),
followed by addition of pentadecafluorooctanoic acid in 0.1 molar ratio
with respect to the resulting total hydrazone bonds. The solution was
heated at 60 uC for 12 h, then poured into a petri dish of 50 mm diameter
made of fluoroplastic, followed by evaporation at 60 uC at normal pressure
until most of the solvent had disappeared and then kept at 60 uC in vacuo
for 12 h. About 200 mg of the total amounts of monomers were used to
obtain polymer film of around 0.04–0.06 mm thickness.
Fig. 6 Cross-sectional schematic representation of the superimposition
of two hydrazone polymer layers.
§ Polymer thin layers were prepared using a 2 wt% THF solution of the
polymer by spin-coating onto 5 cm square quartz plates, which were rinsed
with acetone prior to use. The spin-speed and the period were 500 rpm and
60 s, respectively. These layers were subsequently dried in vacuo at 60 uC for
12 h.
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Francis, Boca Raton, 2nd edn, 2005, ch. 1, p. 3.
Fig. 7 Fluorescence emission spectra of the overlapping domain of neat
hydrazone polymer films P1 and P2 under excitation at 425 nm as a
function of heating time.
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12 In addition to the four ‘‘symmetrical’’ domains present in P1–P4, the
blend may also contain ‘‘unsymmetrical’’ domains having different
components on each side.
13 J.-M. Lehn, in Supramolecular Science: Where It Is and Where It Is
Going, ed. R. Ungaro and E. Dalcanale, Kluwer, Dordrecht, The
Netherlands, 1999, p. 287.
A detailed study on the emergence of fluorescence at the
interface was performed with two thin layers (average thickness of
about 0.15–0.2 mm, determined by a stylus method) of dynamers
P1 and P2 coated on a quartz plate§, which were superimposed
face-to-face (Fig. 6) and heated to 120 uC in an oven under a
stream of argon. The fluorescence intensity of the overlapping part
around 450–600 nm, measured on the layers, increased with
heating time in a manner similar to the case of the polymer blend
(Fig. 7). The result indicates that hydrazone bond exchange and
component migration occurred by heating through the interface of
the polymer layers P1 and P2, leading to expression of
fluorescence along with conjugation extension between the
hydrazone moieties.
In conclusion, the present results show that color and
fluorescence of polyhydrazone dynamers can be modified by
hydrazone bond exchange and component recombination. They
also demonstrate that such processes can occur even at the
interface of two different polymer films, resulting in chemical
interconnection of the films. This feature has promising potential
for implementation in optical materials for systems such as
molecular sensing or photoactive devices. It gives access to
smart and adaptive dynamic materials1,10,13 controlled by and
4362 | Chem. Commun., 2007, 4360–4362
This journal is ß The Royal Society of Chemistry 2007