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A Mechanofluorochromic 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* 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 specific optical and electrical properties, which implies an intensification of research on the structural control of parameters such as the energy level of the frontiers orbitals, light-harvesting properties, photoluminescence efficiency, and charge transport. Although these variables are essentially controlled at the molecular level through the composition topology and rigidity of the chromophoric system, intermolecular interactions exert a determining influence on the optical and electrical properties of the material. Thus, charge transport, absorption, and emission of light or exciton diffusion are for a large part controlled by intermolecular 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 coworkers reported that some molecular systems that are nonemissive in diluted solutions become highly photo-luminescent in the aggregated phase and they introduced the term of aggregation-induced emission (AIE).[9–11] More recently, mechanochromic (MC) and/or mechanofluorochromic (MFC) organic solids have been described.[12–19] In these materials, the absorption 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 structures have been synthesized including modified phenylenevinylene 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]

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: [email protected] Y. Jiang, Prof. P. Liu Research Institute of Materials Science South China University of Technology Guangzhou 510640, China

DOI: 10.1002/adma.201501697

Adv. Mater. 2015, 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 structures were selected on the basis of our recent work on small molecular donors for organic photovoltaics,[26] we note that triphenylamine 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 first example of material showing nonlinear 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 interactions which should in contrast promote cofacial arrangement. Such a conflicting 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 absorption spectrum of compounds 1 and 2 in dichloromethane (DCM) solution shows a first 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 (λmax) at 511 and 504 nm and molecular absorption coefficients of 47 000 and 44 000 M−1 cm−1 for 1 and 2, respectively (Figure S1, Supporting Information). Thin films spun-cast on glass exhibit broadened and slightly redshifted absorption bands (λmax = 513 nm for both materials) due to intermolecular interactions in the solid state. A band gap of Eg ≈ 2.00 eV was estimated from the long-wavelength absorption edge of both compounds. After 20–30 min storage in ambient conditions the films of compound 1 remain unchanged, while the films 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 final state presents a shoulder at ≈350 nm, a main absorption band with λmax at 420 nm, and a very weak transition at ≈620 nm resulting in a pale beige color. Redissolution of the final film in DCM gives a deep-red solution with an absorption spectrum identical

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Scheme 1. Chemical structure of compounds 1 and 2.

to record the spectrum corresponding to the very initial state (t = 0). The spectrum recorded 20 min after film deposition (Figure 2a) shows two peaks at 2θ = 5 and 10° and a weak one at ≈28°. After 5 min at 110 °C these peaks disappear (Figure 2b) while the color turns to red. However, after ≈30 min at room temperature the peaks reappear while the reddish color vanishes (Figure 2c). These results indicate that the metastable deep-red form of the freshly cast film is essentially amorphous 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 conformations that differ by the dihedral angle between the inner and outer phenyl rings (62.23° and 75.73°). Moreover, one conformation 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 fluorescence emission spectrum presents a considerable red shift from ≈520 nm in hexane to ≈650 nm in acetonitrile (Figure 3a,b). The linear correlation

to the initial one. This indicates that the molecular structure remains intact during the optical changes in the solid as confirmed by mass spectrometry. The modification of the optical spectrum suggests that in as-cast films, the material is in a metastable state that spontaneously evolves toward a more stable state. This process can be attributed to a modification of the molecular packing eventually accompanied with geometrical changes in the molecular structure. However, in view of the previously noted relative insensitivity of the λmax of the ICT band on steric effects in the donor block, this contribution 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 film of 2 partially scratched with a spatula (Figure 1c) shows that this mechanical stimulation restores the red color of the as-cast film. Furthermore, the picture taken under illumination with 365 nm light shows that this part of the film emits a deep-red photoluminescence (Figure 1d), thus demonstrating the MCF properties of the material. A further illustration is given in Figure 1e. Writing on a stabilized film (1) with the tip of a spatula produces red scars (2). This writing spontaneously self-heals to restore the pale beige state (1) after ≈30 min in ambient conditions. On the other hand, a 5 min thermal treatment at 110 °C also erases the writing while the film turns reddish (3). In order to better understand these results, Figure 1. a) Photographs of films of compounds 1 and 2 on glass; top as-cast films, bottom films of 2 have been analyzed by powder after 20 min in ambient conditions. b) Absorption spectra of a film of 2 on glass at various X-ray diffraction at the different stages of the time intervals. c) Photograph of a stabilized film of 2 on glass partially scratched with a spatula. MC processes. Due to the rapid discoloration d) Same film irradiated with 375 nm light. e).Writing/erasing cycle of a film of 2. 1) Stabilized of the as-cast film, it has not been possible film, 2) after writing, 3) after 5 min at 110 °C. 2





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The fluorescence 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 aggregation-caused quenching (ACQ) of fluorescence which is complete for 80% of water (Figure S4, Supporting Information). However, the spectra recorded with an excitation wavelength of 365 nm show that this ACQ of the long wavelength emission is accompanied with the simultaneous emergence of an emission band at 440–460 nm (Figure 3d). These results show that the aggregate phase, corresponding to the crystalline state of the stabilized film, is also emissive but at a shorter wavelength than the amorphous state. Thus, addition of water causes the Figure 2. Left: powder X-ray diffraction spectra of a film of 2. a) 20 min after spin-casting; simultaneous ACQ of the long-wavelength b) same film after 5 min at 110 °C; c) same film after 30 min at room temperature. Right: emission and the AIE of the short-wavelength crystallographic structure of compound 2.[32] fluorescence. This dual behavior which leads to highly contrasted red and blue photoluminescence emissions (Figure 3e) is consistent with a MFC with the Et parameter of solvent polarity[34] (Figure S3, behavior involving the interconversion between two states. Supporting Information) is consistent with an excited state The highly dipolar push–pull structure of compound 2 and with intramolecular charge transfer and a large increase of the the noncentrosymmetric arrangement of dipoles indicated by transition dipole moment in the excited state. the presence of face-to-face dimers in the crystallographic strucThe molecular aggregation of 2 has been investigated on ture suggest that this material could exhibit bulk second-order THF solutions containing increasing amounts of water. As nonlinear (NLO) properties. Figure 4a shows an image of a could be expected, the addition of water produces the bleaching stabilized film of 2 at high magnification. The film contains, in of the main absorption band, while the spectrum obtained after fact, circular objects with a darker periphery. The mechanism addition of 80% of water becomes very similar to that of the of formation of these objects is not fully understood, but the stabilized film with transitions at ≈350 and 420 nm plus some amphiphilic nature of the molecule probably plays a major role. remaining absorption at ≈500 nm (Figure 3c).

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) fluorescence emission spectra in these solvents (λexc = 500 nm); c) UV–vis absorption spectra of compound 2 in THF and 80:20 H2O/THF. d) Fluorescence emission spectra of 2 in THF/H2O mixtures (excitation 365 nm); e) photographs of solutions in THF and 80:20 H2O/THF under 365 nm light.




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its initial deep-red form. The CV of this film 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 estimated values of the HOMO level of −5.84 and −6.20 eV for the red and the crystalline form, respectively. Interestingly, the potential difference of ≈0.40 V found between the Epa 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 Figure 4. a) Optical microscopy of a stabilized film of 2 on glass. b) SHG response of the same N-substituted by an oligooxyethylene chain film under 800 nm laser irradiation. c) Optical microscopy of the film after smearing. d) SHG has been synthesized. Film casting from response after smearing. Bars: 50 µm. solutions produces a metastable deep-red amorphous material which rapidly evolves toward an almost colorless stable crystalline form. Mechanical Under laser irradiation at 800 nm, these peripheral zones prestimulation by writing, rubbing, or smearing restores the inisumed to be the most crystalline, produce an intense second tial deep-red form which spontaneously returns to the crystalharmonic generation (SHG) response at 400 nm (4b) (see the line state. The analysis of aggregation in solution reveals a dual Supporting Information for experimental detail). Smearing part of the film causes the extinction of the SHG signal which remains clearly visible in the spared zone (Figure 4c,d). Considered in the light of the above-discussed absorption, emission, 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 hypothesis is given by the fact that thermal annealing causes the temporary 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 corresponding to the formation of the cation-radical with an anodic peak potential (Epa) of 1.00 V. This CV is very similar to that of the reference compound 1. A film 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 KNO3 in water. This aqueous medium was selected in order to avoid the probable dissolution of the film in organic solvents. The CV of this film shows a broad anodic wave with an Epa of ≈1.60 V (Figure 5). Due to the rapid crystallization of the as-cast film, this CV corresponds in fact to the stable crystalline form. In order to Figure 5. CV of compound 2. Black line: in 0.10 M Bu4NPF6/CH2Cl2, platprevent this fast crystallization, a film of 2 was drop-cast on a inum electrodes, scan rate 100 mV cm−1. Red line: film cast on platinum −1 Pt electrode from a DCM solution containing 30 g L of low electrode in 0.20 M KNO3/H2O, scan rate 20 mV s−1. Blue line: film cast molecular weight PVC. After solvent evaporation, compound 2 on platinum from a DCM solution containing PVC in 0.20 M KNO3/H2O, is embedded in a polymer matrix which freezes the system in scan rate 20 mV s−1.

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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:

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