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Mechanofluorochromism: from molecular engineering to the elaboration of smart materials

Periodic Reporting for period 3 - MECHANO-FLUO (Mechanofluorochromism: from molecular engineering to the elaboration of smart materials)

Reporting period: 2020-03-01 to 2021-08-31

Fluorescent materials, designed through appropriate molecular engineering, are able to signal different stimuli with a high sensitivity, in a non-destructive manner. In particular, mechanofluorochromism relates to fluorescent compounds for which emission spectrum in the solid state changes upon application of a mechanical stimulus. The interest for this phenomenon has enormously increased for the last 5 to 6 years but many examples remain purely qualitative: neither the nature of the mechanical stress (pressure, shearing force…) that triggers the fluorescence changes nor its intensity are known.
The overall objective of the MECHANO-FLUO project is to prepare mechanofluorochromic molecules and materials, understand and tune their photophysical and mechanofluorochromic properties, and implement these new materials as quantitative mechanical force sensors. To achieve this overall objective, several workpackages are defined:
- The first one comprises the synthesis of a small library of mechanofluorochromic molecules, in our group or in external collaborations, if possible displaying different mechanical stimuli (pressure, shearing), with various sensitivities. We aim at relating the molecular structure to the sensitivity to different mechanical stimuli and to obtain the appropriate molecular materials for the others parts of the project.
- In the second workpackage, we aim at studying mechanofluorochromism at the micro- to nanoscale. Going from the macroscale to the nanoscale could potentially yield an amplification of the mechanofluorochromic response and allow the development ultra-sensitive mechanofluorochromic probes. Understanding and controlling the mechanofluorochromic response at the nanoscale is also necessary in order to develop nanostructured materials allowing local force measurements.
- The third work package covers the quantification of the mechanofluorochromic response at the macroscopic scale. This quantification will be performed on the pure molecular materials (powder or thin films) and on polymer doped by our mechanofluorochromic materials. This could find an application in stress metrology: fluorescent materials with a quantitative
mechanofluorochromic response could give access to the stress level in various materials and thus constitute an entirely new method for in situ control of the damaging of materials, such as composite materials classically used in aeronautics.
- In the fourth workpackage, we will select biocompatible mechanofluorochromic materials, in the form of nanoparticles or embedded into suitable polymer matrices to study mechanobiology at the cellular scale. We hope to provide a tool for direct force measurement at the cellular scale, which would have tremendous implications for the
comprehension of biological phenomena where mechanotransduction is implied, especially embryogenesis and tumor proliferation.
While the first two workpackages mainly have implications in the field of materials chemistry (beyond mechanofluorochromism, a better understanding of the sensitivity of fluorescent materials to forces could be useful for different applications, such as for example the development of flexible organic electronics devices), the third and fourth work packages aim providing innovative tools in other fields of science, namely solid state mechanics and mechanobiology. We also hope that practical applications will come from the development of mechanofluorochromic polymers coatings.
Workpackage 1 has progressed according to plan and several new mechanofluorochromic molecules have been prepared and studied, in our group and through national or international collaborations. In particular, we have developed a series of boron diketonates complexes for we which we have been able to identify the emissive species before and after mechanical stimulation, in collaboration with computational chemists (Dr. Ilaria Ciofini, Paris, PI of the ERC project STRIGES). These results have been published in Advanced Materials (2018). In collaboration with the group of Prof Tsuyoshi Kawai (NAIST, Nara, Japan), we have also been able to develop the first compounds that are mechanofluorochromic and at the same time are CPL (Circularly Polarized Luminescence) emitters, and display a change in CPL emission upon mechanical stimulation. These results have been published in Chemical Science (2019).
Concerning the WP2 of the project, the proof of principle has been established: mechanofluorochromism has been evidenced at the nanoscale for two families of compounds. The results showing nanoscale mechanofluorochromism on the first compound have been published in J. Phys. Chem. Letters (2019). The second compound we have prepared display an off to on fluorescence emission switch upon shearing. We have studied its behavior at the nanoscale and we have been able to show that the fluorescence intensity observed after mechanical stimulation increases with the force applied, in the 20-200 nN range. The corresponding article is in press for Chem. Commun (ocotober 2019).
Concerning the quantification of mechanofluorochromism at the macroscale (WP3), we have established a collaboration with Dr. Laurence Bodelot (Laboratoire de Mécaniques des Solides, Ecole Polytechnique, Palaiseau, France). A device has been built to quantify the forces that can trigger the fluorescence changes on a molecular solid. Two compounds have been studied that revealed sensitive to shearing forces only, and shearing thresholds have been determined. Preliminary studies on the preparation mechanofluorochromic polymers have also been started in 2018.
The understanding of the molecular origins of mechanofluorochromism on boron diketonate complexes clearly represents a progress beyond the state of the art in the field, and provides us solid bases to develop performant new compounds in this family. In particular, we intent to develop further the collaboration with Prof. Kawai(NAIST, Japan) in order to optimize the “mechano-CPL” effect.
Concerning the study of mechanofluorochromism at the micro- to nanoscale, we have been able to show, on one compound displaying an off to on fluorescence switch, that the fluorescence intensity observed after mechanical stimulation increased with the force applied. It is the first time that such force to fluorescence intensity relationship is clearly established. We have also studied other compounds, with various chemical structures and we have shown that in some cases the mechanofluorochromic responses at the macroscale and nanoscale were qualitatively similar while in other cases they were different. During the second half of the project, we hope to better rationalize this observation, especially through two improvements of our AFM coupled to fluorescence microscope setup: i) we plan to record the fluorescence emission during the AFM mechanical stimulation (so far, fluorescence and AFM images have been acquired sequentially) and ii) we plan to add a Fluorescence Lifetime Imaging Microscopy modality to our setup.
Concerning the quantification of mechanofluorochromism at the macroscale, we are working in order to obtain a quantification of the shearing threshold necessary to observe a change in fluorescence for at least two different compounds, in order to describe a method to quantify the mechanofluorochromic response for different molecular materials, whereas for the moment this mechanofluorochromic response is most of the time only qualitatively observed by manually grinding a powder in a mortar. Until the end of the project, we will also continue our efforts to obtain mechanofluorochromic polymers, either by non-covalent or covalent incorporation of mechanofluorochromic dyes in a polymer matrix, and we will work to quantify the fluorescence response to a mechanical stimulation at the macroscale on polymer samples.