Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials

TLDR: The possibility to change the molecular assembled structures of organic and organometallic materials through mechanical stimulation is emerging as a general and powerful concept for the design of functional materials, enabling the development of molecular materials with mechanoresponsive luminescence characteristics.

Abstract: The possibility to change the molecular assembled structures of organic and organometallic materials through mechanical stimulation is emerging as a general and powerful concept for the design of functional materials. In particular, the photophysical properties such as photoluminescence color, quantum yield, and emission lifetime of organic and organometallic fluorophores can significantly depend on the molecular packing, enabling the development of molecular materials with mechanoresponsive luminescence characteristics. Indeed, an increasing number of studies have shown in recent years that mechanical force can be utilized to change the molecular arrangement, and thereby the optical response, of luminescent molecular assemblies of π-conjugated organic or organometallic molecules. Here, the development of such mechanoresponsive luminescent (MRL) molecular assemblies consisting of organic or organometallic molecules is reviewed and emerging trends in this research field are summarized. After a brief introduction of mechanoresponsive luminescence observed in molecular assemblies, the concept of "luminescent molecular domino" is introduced, before molecular materials that show turn-on/off of photoluminescence in response to mechanical stimulation are reviewed. Mechanically stimulated multicolor changes and water-soluble MRL materials are also highlighted and approaches that combine the concept of MRL molecular assemblies with other materials types are presented in the last part of this progress report.

Figures

Figure 10. Covalent attachment of MRL materials to other materials of interest. a) Molecular structures of amphiphilic pyrene derivative 58 and water-soluble linker 59. b) Schematic illustration of the attachment of MRL micelles onto glass or polymer beads. c,d) Pictures documenting the change of the photoluminescence color of the glass or polymer beads. The MRL micelles were covalently attached to the surface of the beads. Adapted with permission. [91] Copyright 2014, American Chemical Society.

Figure 4. Single-component MRL materials that display mechanically induced on-off switching. a) Molecular structures of compounds 28 and 29 and emission spectra of 28 before and after mechanical grinding. b) Molecular structures of 30 and 31 and photographs of the pristine and ground powders of 30. c) Molecular structure of 32 and pictures showing mechano- and acid-sensing behavior. d) Molecular structures 33 and 34 and pictures illustrating the mechanoresponsive properties. e) Molecular structures of 35 and 36 and images illustrating their photoluminescence color changes in response to external stimuli. Part a is adapted with permission. [94] Copyright 2009, American Chemical Society. Part b is adapted with permission. [95] Copyright 2012, Wiley-VCH. Part c is adapted with permission. [96] Copyright 2014, The Royal Society of Chemistry. Part d is adapted with permission. [97] Copyright 2014, The Royal Society of Chemistry. Part e is adapted with permission. [98] Copyright 2011, Wiley-VCH.

Figure 5. Molecular structures of 37, 38 and 39 and photographs demonstrating the reversible MRL behavior of the mixture under UV irradiation. Adapted with permission. [104] Copyright 2011, Wiley-VCH.

Figure 6. Multi-color MRL materials based on a single component. a) Molecular structures of compounds 40 and 41, procedures to obtain three different luminescent colors, and schematic illustration of the assembled structures of the LC mixture that from the cubic, columnar and mesomorphic phases. b) Molecular structures of AIE-active compounds 42–44 and pictures illustrating the photoluminescence color changes of 43. c) Molecular structure of compound 45, schematic representation of the multilamellar structure of 45Y and pictures revealing the

Figure 8. Multi-color MRL materials based on two luminophores. a) Molecular structures of 53 and 54 and pictures showing the mechano-responsive behavior of the mixture. b) Molecular structure of 55 and emission spectra that document changes of the material’s

Figure 7. Organometallic multi-color MRL materials. a) Molecular structure of compound 50 and the phase transition diagram for 50. b) Molecular structure of compound 51 and pictures documenting the photoluminescence color changes in response to external stimuli. c) Molecular structure of compound 52, photographs of the powder forms of 52 showing different photoluminescence colors and schematic representation of the molecular arrangements of each emitting state. Part a is adapted with permission. [111] Copyright 2011, Wiley-VCH. Part b is adapted with permission. [112] Copyright 2012, The Chemical Society of Japan. Part c is adapted with permission. [113] Copyright 2015, The Royal Society of Chemistry.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Progress report mechanoresponsive luminescent molecular assemblies: an emerging class of materials" ?

Mechanochromic luminescence is defined as the phenomenon in which a material displays a major and reversible change in photoluminescence color in response to mechanical stimulation this paper. 

Q2. What is the effect of the imperfect H-type aggregation on the luminescent?

The imperfect H-type aggregation allows the materials to show a radiative decay from the lower exciton state, leading to yellow photoluminescence. 

Q3. Why does the light of 37 appear after mechanical grinding?

Due to the phase separation, the electron transfer from 37 to 38 is disturbed and the greenish-blue luminescence of 37 appears after mechanical grinding. 

Q4. What is the effect of the Y-micelles on the surface of glass beads?

When sufficient mechanical stress is applied to the surface of the materials bearing Y-micelles, Y-micelles transform into green-emitting micelles (G-micelles) in water. 

Q5. What is the important requirement to achieve MRL behavior?

In other words, the presence of at least twothermodynamically (meta)stable states is the most significant requirement to achieve MRL behavior. 

Q6. What is the effect of mechanical stimulation on the photoluminescence properties of the amphiphil?

If an amphiphile having a fluorophore forms supramolecular assemblies in water and this supramolecular architecture is altered in response to mechanical stimulation in water, the photoluminescence properties of the system can be also expected to be responsive to mechanical force. 

Q7. What is the effect of the pressure on the photoluminescence color of the powder?

As the applied pressure increases, the photoluminescence color of the powder gradually changes from green (at 0 GPa) to red (at 7.92 GPa). 

Q8. What is the common approach to induce metastable states?

One typical approach to induce such kinetically-trapped metastable states is the introduction of relatively bulky and flexible substituents. 

Q9. How can kinetically trapped molecules be converted to thermodynamically stable states?

The kinetically trapped molecular assemblies can be converted to thermodynamically stable states by mechanical stimulation (and typically also other mechanisms, including heating, exposure to solvents, etc.). 

Q10. What is the reason for the formation of one-dimensional cylindrical micelles?

The formation of one-dimensional cylindrical micelles is ascribed to the weak hydrophobic interaction between the anisotropic micelles. 

Q11. What is the effect of the introduction of the methyl groups on the emission band?

The authors concluded that the introduction of the methyl groups stabilized structures which display no aurophilic interactions and, consequently, the emission band shows a blue shift upon transformation from the green emissive, metastable crystals. 

Q12. What are the properties of the MRL materials?

The rapidly growing international efforts to develop MRL materials have yielded alarge number of organic or organometallic molecules that exhibit mechanoresponsive properties. 

Q13. What is the way to achieve a thermodynamically stable assembly?

Another way to achieve two different stable states is to interfere with the formation ofthermodynamically stable assemblies, for example by kinetically trapping molecular assemblies in thermodynamically metastable forms. 

Q14. Why do cylindrical micelles not appear in the TEM images?

no cylindrical molecular assemblies appear in the TEM images, because the green-emissive micelles are not able to form one-dimensional aggregates. 

Q15. What is the ring-opening reaction of the dipeptide derivative 54?

The appearance of red photoluminescence is based on the ring-opening reaction of the rhodamine B moiety of 55 as observed for dipeptide derivative 54 described above. 

Q16. What is the effect of hydrostatic pressure on the photoluminescence color of anthrac?

In the hydrostatic pressure experiment, the photoluminescence color also shows a significant red sift from green to red as the pressure increases and subsequent releasing pressure recovers the initial green photoluminescence (Figure 6f). 

Q17. How long did it take to achieve complete color change?

When the same vortex experiment was conducted for smaller glass beads (Φ < 106 μm) bearing Y-micelles, it required longer time (1 h) to achieve complete color change. 

Q18. Why have so many people been interested in the construction of ordered assemblies?

The construction of ordered assemblies of organic and/or organometallic compoundshas attracted much attention in the past decades, because such architectures possess great potential as sophisticated functional materials. [1] 

Q19. What is the average hydrodynamic diameter of the micelles obtained from dynamic light scattering measurements?

The average hydrodynamic diameter of the micelles obtained from dynamic light scattering measurements is approximately 7 nm, which is consistent with diameters of the micellar structures observed in the TEM images.