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A Mechanofl uorochromic Push–Pull Small Molecule with Aggregation-Controlled Linear and Nonlinear Optical
Properties
Yue Jiang , Denis Gindre , Magali Allain , Ping Liu , Clément Cabanetos , and Jean Roncali *
Y. Jiang, Dr. D. Gindre, M. Allain, Dr. C. Cabanetos, Prof. J. Roncali
Moltech-Anjou
CNRS UMR 6200, University of Angers 2 Bd Lavoisier 49045 Angers , France
E-mail: jeanroncali@gmail.com Y. Jiang, Prof. P. Liu
Research Institute of Materials Science South China University of Technology Guangzhou 510640 , China
DOI: 10.1002/adma.201501697
We report here on the optical properties of a small push–pull molecule involving a diphenylamine donor block N -substituted by an oligo-oxyethylene chain connected to a dicyanovinyl acceptor group through a thienyl π-conjugating spacer ( 2 ).
The parent compound containing a hexyl chain ( 1 ) was also synthesized as reference ( Scheme 1 ). Although these struc- tures were selected on the basis of our recent work on small molecular donors for organic photovoltaics, [ 26 ] we note that tri- phenylamine blocks have already been used for the design of stimulable chromophores. [ 25,27,28 ]
Compared to the already known compounds that exhibit AIE and/or MCF behavior our new molecule differs by the extreme simplicity of the structure and by the fact that, to the best of our knowledge, it is the fi rst example of material showing non- linear optical mechanochromic properties. The introduction of a hydrophilic oligo–oxyethylene chain at the opposed side of the hydrophobic dicyanovinyl acceptor end-group was expected to confer amphiphilic properties on the molecule. The resulting molecular assemblies are thus expected to be subjected to two counter-acting forces, namely, dipole interactions which favor head-to-tail molecular packing and hydrophilic/lipophilic inter- actions which should in contrast promote cofacial arrangement.
Such a confl icting situation is expected to generate a metastable state which has been suggested to play an important role in the generation of mechano-stimulable properties. [ 15 ]
The synthesis and characterization of the two molecules are described in the Supporting Information. The UV–vis absorp- tion spectrum of compounds 1 and 2 in dichloromethane (DCM) solution shows a fi rst transition around 350 nm and a broad absorption band in the 400–600 nm region attributed to an internal charge transfer (ICT). [ 29 ] Both spectra present very close absorption maxima ( λ max ) at 511 and 504 nm and molec- ular absorption coeffi cients of 47 000 and 44 000 M −1 cm −1 for 1 and 2 , respectively (Figure S1, Supporting Information). Thin fi lms spun-cast on glass exhibit broadened and slightly red- shifted absorption bands ( λ max = 513 nm for both materials) due to intermolecular interactions in the solid state. A band gap of E g ≈ 2.00 eV was estimated from the long-wavelength absorp- tion edge of both compounds.
After 20–30 min storage in ambient conditions the fi lms of compound 1 remain unchanged, while the fi lms of compound 2 undergo discoloration ( Figure 1 ). The absorption spectra recorded at various time intervals reveal a progressive bleaching of the main absorption band. The spectrum of the fi nal state presents a shoulder at ≈350 nm, a main absorption band with λ max at 420 nm, and a very weak transition at ≈620 nm resulting in a pale beige color. Redissolution of the fi nal fi lm in DCM gives a deep-red solution with an absorption spectrum identical The control of the optical properties of organic solids has
become a topic of considerable fundamental interest and a key of many advanced applications of materials based on π-conjugated chromophores. In the past decades, electrochromic [ 1,2 ] and light-emitting devices, [ 3,4 ] lasers, [ 5,6 ] or solar cells [ 7,8 ] based on organic materials have been proposed and investigated. All these applications resort to active materials with specifi c optical and electrical properties, which implies an intensifi cation of research on the structural control of parameters such as the energy level of the frontiers orbitals, light-harvesting properties, photoluminescence effi ciency, and charge transport. Although these variables are essentially controlled at the molecular level through the composition topology and rigidity of the chromo- phoric system, intermolecular interactions exert a determining infl uence on the optical and electrical properties of the mate- rial. Thus, charge transport, absorption, and emission of light or exciton diffusion are for a large part controlled by intermo- lecular interactions and molecular packing. Although optical properties of organic solids implicitly refer to a static molecular organization, in recent years, some fascinating new dynamic phenomena have been disclosed. Thus, in 2001, Tang and co- workers reported that some molecular systems that are none- missive in diluted solutions become highly photo-luminescent in the aggregated phase and they introduced the term of aggre- gation-induced emission (AIE). [ 9–11 ] More recently, mechano- chromic (MC) and/or mechanofl uorochromic (MFC) organic solids have been described. [ 12–19 ] In these materials, the absorp- tion and/or emission of light can be modulated by mechanical forces such as hydrostatic pressure, smearing, or grinding. [ 12 ] In recent years, MC and MFC materials based on various struc- tures have been synthesized including modifi ed phenylenevi- nylene oligomers, [ 12–17 ] pyrene derivatives, [ 18 ] dendridic liquid crystals, [ 13,19 ] tetrathiazolylthiophene, [ 20 ] boron diketonates and β-diketones, [ 21 ] and metal complexes. [ 22–24 ] It is noteworthy that examples of MFC materials based on AIE active molecules have also been described. [ 25 ]
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to the initial one. This indicates that the molecular structure remains intact during the optical changes in the solid as con- fi rmed by mass spectrometry.
The modifi cation of the optical spectrum suggests that in as-cast fi lms, the material is in a metastable state that spontane- ously evolves toward a more stable state. This process can be attributed to a modifi cation of the molecular packing eventu- ally accompanied with geometrical changes in the molecular structure. However, in view of the previously noted relative insensitivity of the λ max of the ICT band on
steric effects in the donor block, this contri- bution is thought to play a limited role. [ 30 ] On the other hand, the bleaching of the main absorption band and emergence of two new transitions at higher and lower energies is consistent with a progressive transition from J to H aggregates. [ 31 ]
The photograph of a stabilized fi lm of 2 partially scratched with a spatula (Figure 1 c) shows that this mechanical stimulation restores the red color of the as-cast fi lm.
Furthermore, the picture taken under illu- mination with 365 nm light shows that this part of the fi lm emits a deep-red photolumi- nescence (Figure 1 d), thus demonstrating the MCF properties of the material. A further illustration is given in Figure 1 e. Writing on a stabilized fi lm (1) with the tip of a spatula produces red scars (2). This writing sponta- neously self-heals to restore the pale beige state (1) after ≈30 min in ambient conditions.
On the other hand, a 5 min thermal treat- ment at 110 °C also erases the writing while the fi lm turns reddish (3).
In order to better understand these results, fi lms of 2 have been analyzed by powder X-ray diffraction at the different stages of the MC processes. Due to the rapid discoloration of the as-cast fi lm, it has not been possible
to record the spectrum corresponding to the very initial state ( t = 0). The spectrum recorded 20 min after fi lm deposition ( Figure 2 a) shows two peaks at 2 θ = 5 and 10° and a weak one at ≈28°. After 5 min at 110 °C these peaks disappear (Figure 2 b) while the color turns to red. However, after ≈30 min at room temperature the peaks reappear while the reddish color van- ishes (Figure 2 c). These results indicate that the metastable deep-red form of the freshly cast fi lm is essentially amor- phous and spontaneously crystallizes when stored in ambient conditions.
The crystallographic structure of a single crystal of 2 (Figure 2 ) shows that the unit cell contains two conforma- tions that differ by the dihedral angle between the inner and outer phenyl rings (62.23° and 75.73°). Moreover, one con- formation is less planar than the other with dihedral angles between the dicyanovinyl group and the thiophene, and the thiophene and inner phenyl rings of 6.81° and 18.96° in one case and 2.23° and 11.96° for the other (see the Supporting Information). Unlike the head-to-tail arrangement often observed for strongly dipolar molecules, [ 33 ] the crystal of 2 contains both head-to-tail and face-to-face stacking alternately distributed.
The photoluminescence of compound 2 has been analyzed in solvents of increasing polarity namely hexane, toluene, DCM, THF, and acetonitrile. Although only small variations were observed on the absorption spectrum (Figure S2, Supporting Information), the maximum of the fl uorescence emission spec- trum presents a considerable red shift from ≈520 nm in hexane to ≈650 nm in acetonitrile ( Figure 3 a,b). The linear correlation www.MaterialsViews.com
Scheme 1. Chemical structure of compounds 1 and 2 .
Figure 1. a) Photographs of fi lms of compounds 1 and 2 on glass; top as-cast fi lms, bottom after 20 min in ambient conditions. b) Absorption spectra of a fi lm of 2 on glass at various time intervals. c) Photograph of a stabilized fi lm of 2 on glass partially scratched with a spatula.
d) Same fi lm irradiated with 375 nm light. e).Writing/erasing cycle of a fi lm of 2 . 1) Stabilized fi lm, 2) after writing, 3) after 5 min at 110 °C.
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with the E t parameter of solvent polarity [ 34 ] (Figure S3, Supporting Information) is consistent with an excited state with intramolecular charge transfer and a large increase of the transition dipole moment in the excited state.
The molecular aggregation of 2 has been investigated on THF solutions containing increasing amounts of water. As could be expected, the addition of water produces the bleaching of the main absorption band, while the spectrum obtained after addition of 80% of water becomes very similar to that of the stabilized fi lm with transitions at ≈350 and 420 nm plus some remaining absorption at ≈500 nm (Figure 3 c).
The fl uorescence emission spectrum of 2 in THF with an excitation wavelength of 420 nm shows two maxima at 500 and 610 nm. Addition of water leads to the aggre- gation-caused quenching (ACQ) of fl uores- cence which is complete for 80% of water (Figure S4, Supporting Information). How- ever, the spectra recorded with an excitation wavelength of 365 nm show that this ACQ of the long wavelength emission is accom- panied with the simultaneous emergence of an emission band at 440–460 nm (Figure 3 d).
These results show that the aggregate phase, corresponding to the crystalline state of the stabilized fi lm, is also emissive but at a shorter wavelength than the amorphous state. Thus, addition of water causes the simultaneous ACQ of the long-wavelength emission and the AIE of the short-wavelength fl uorescence. This dual behavior which leads to highly contrasted red and blue photolu- minescence emissions (Figure 3 e) is consistent with a MFC behavior involving the interconversion between two states.
The highly dipolar push–pull structure of compound 2 and the noncentrosymmetric arrangement of dipoles indicated by the presence of face-to-face dimers in the crystallographic struc- ture suggest that this material could exhibit bulk second-order nonlinear (NLO) properties. Figure 4 a shows an image of a stabilized fi lm of 2 at high magnifi cation. The fi lm contains, in fact, circular objects with a darker periphery. The mechanism of formation of these objects is not fully understood, but the amphiphilic nature of the molecule probably plays a major role.
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Figure 2. Left: powder X-ray diffraction spectra of a fi lm of 2 . a) 20 min after spin-casting;
b) same fi lm after 5 min at 110 °C; c) same fi lm after 30 min at room temperature. Right:
crystallographic structure of compound 2 . [ 32 ]
Figure 3. a) Photographs of solutions of 2 under 365 nm light in solvents of increasing polarity from left to right: hexane, toluene, DCM, THF, acetonitrile; b) fl uorescence emission spectra in these solvents ( λ exc = 500 nm); c) UV–vis absorption spectra of compound 2 in THF and 80:20 H 2 O/THF. d) Fluorescence emission spectra of 2 in THF/H 2 O mixtures (excitation 365 nm); e) photographs of solutions in THF and 80:20 H 2 O/THF under 365 nm light.
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Under laser irradiation at 800 nm, these peripheral zones pre- sumed to be the most crystalline, produce an intense second harmonic generation (SHG) response at 400 nm (4b) (see the Supporting Information for experimental detail). Smearing part of the fi lm causes the extinction of the SHG signal which remains clearly visible in the spared zone (Figure 4 c,d). Con- sidered in the light of the above-discussed absorption, emis- sion, and X-ray diffraction data, these results suggest that SHG is produced by crystalline domains, and that the mechanical destruction of the local noncentrosymmetric order leads to the extinction of the SHG signal. A further support to this hypoth- esis is given by the fact that thermal annealing causes the tem- porary disappearance of the SHG signal which is again detected after ≈30 min in ambient conditions.
In order to gain further information on the variations of the energy levels associated with the interconversion processes, the cyclic voltammetry (CV) of compound 2 has been investigated in solution and the solid state. To the best of our knowledge, no investigation of the changes in electrochemical properties associated with MC processes has been reported so far. The CV recorded in DCM shows a reversible oxidation wave corre- sponding to the formation of the cation-radical with an anodic peak potential ( E pa ) of 1.00 V. This CV is very similar to that of the reference compound 1 . A fi lm of compound 2 was drop-cast on platinum electrode from a DCM solution. This electrode was then immersed in a 0.20 M solution of KNO 3 in water . This aqueous medium was selected in order to avoid the probable dissolution of the fi lm in organic solvents. The CV of this fi lm shows a broad anodic wave with an E pa of ≈1.60 V ( Figure 5 ).
Due to the rapid crystallization of the as-cast fi lm, this CV corresponds in fact to the stable crystalline form. In order to prevent this fast crystallization, a fi lm of 2 was drop-cast on a Pt electrode from a DCM solution containing 30 g L −1 of low molecular weight PVC. After solvent evaporation, compound 2 is embedded in a polymer matrix which freezes the system in
its initial deep-red form. The CV of this fi lm shows an anodic shoulder at ≈1.20 V. Taking into account the overvoltage associated with charge and mass transport in a polymer matrix, this value is in satisfying agreement with the 1.00 V measured in solution. The onset of the oxidation waves leads to esti- mated values of the HOMO level of −5.84 and −6.20 eV for the red and the crystalline form, respectively. Interestingly, the poten- tial difference of ≈0.40 V found between the E pa values of the red and the crystalline form agrees well with the 0.47 eV energy difference between the absorption maxima of the two states (Figure 1 ). This result is consistent with a transition between the two forms involving essentially a variation of the HOMO level.
To summarize, a push–pull molecule containing a diphenylamine donor block N -substituted by an oligooxyethylene chain has been synthesized. Film casting from solutions produces a metastable deep-red amorphous material which rapidly evolves toward an almost colorless stable crystalline form. Mechanical stimulation by writing, rubbing, or smearing restores the ini- tial deep-red form which spontaneously returns to the crystal- line state. The analysis of aggregation in solution reveals a dual
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Figure 4. a) Optical microscopy of a stabilized fi lm of 2 on glass. b) SHG response of the same fi lm under 800 nm laser irradiation. c) Optical microscopy of the fi lm after smearing. d) SHG response after smearing. Bars: 50 µm.
Figure 5. CV of compound 2 . Black line: in 0.10 M Bu 4 NPF 6 /CH 2 Cl 2 , plat- inum electrodes, scan rate 100 mV cm −1 . Red line: fi lm cast on platinum electrode in 0.20 M KNO 3 /H 2 O, scan rate 20 mV s −1 . Blue line: fi lm cast on platinum from a DCM solution containing PVC in 0.20 M KNO 3 /H 2 O, scan rate 20 mV s −1 .
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behavior with ACQ of the long-wavelength luminescence and AIE at short wavelengths. The noncentrosymmetric crystalline form exhibits an intense bulk SHG response which is tempo- rary suppressed by mechanical or thermal stimulation.
These various results are consistent with the idea that the introduction of a polyether chain in the structure leads to a metastable material interconvertible into two states with dif- ferent absorption, photoluminescence, and NLO properties by mechanical stimulation or controlled aggregation.
Although it is clear that these very preliminary results pose many questions which will require much further experimental and theoretical work, the original linear and nonlinear optical properties of this type of molecules can contribute to extend the advanced applications of organic materials in optical storage, optoelectronic devices, sensors, [ 12 ] or transient electronics. [ 35,36 ]
Supporting Information
Supporting information is available from the Wiley Online Library or from the author.
Acknowledgements
The authors thank the Chinese CSC Scholarship Program for the grant of Y.J.
Received: April 10, 2015 Revised: May 19, 2015 Published online: June 18, 2015
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