Brightly Tricolored Mechanochromic Luminescence from a Single‐Luminophore Liquid Crystal: Reversible Writing and Erasing of Images

Abstract

Luminescent materials that switch their photoluminescent properties in response to various external stimuli have attracted much attention for a decade because of their potential application for memory devices, sensors, security materials, and informational displays. [1][2][3][4][5] To induce change in the luminescent colors of organic and organometallic materials, one could switch the molecularly assembled structures. [4][5][6][7] Crystals, [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] liquid crystals, [25][26][27][28] and polymers [29][30][31][32] have been reported to change their luminescent colors by mechanical and thermal stimuli. The observed phenomena are referred to as piezo(mechano)chromic or thermochromic luminescence. Such materials contain just one type of luminophore and they form only two luminescent states. In the condensed states, to date, multiemission colors that are generated from a single luminophore and switch between their colors using external stimuli have not been achieved for organic and organometallic materials; the one exception is a crystalline compound. [16] If such materials are prepared using liquid crystals, it leads to new applications of stimuli-responsive luminescent materials that are flexible, sophisticated, and highly functional.Liquid crystals are functional soft materials that exhibit mobile and ordered states. [33] Because of their dynamic properties, liquid crystals are good candidates for stimuliresponsive luminescent materials. [25][26][27][28] In our previous studies, we have prepared pyrene-, anthracene-, and naphthalenebased liquid crystals that show piezo(mechano)chromic luminescence and thermochromic luminescence in liquidcrystalline (LC) states. [25,26] However, multiluminescent colors have not been achieved for these LC materials.

Bio‐Inspired Phosphole‐Lipids: From Highly Fluorescent Organogels to Mechanically Responsive FRET

“…[10] But more importantly, the FRET efficiency is highly dependent on the donoracceptor distance, which makes such intermolecular process very useful for the sensing of external stimuli, particularly in the application toward mechanical responses. [5,6,12] It is therefore inherently plausible that both efficient exciton migration (donor-donor) and the enhanced energy transfer (donor-acceptor) work together to enhance the orange emission of the acceptor upon mechanical grinding as a result of created trap sites. In the drop-cast film, the electrostatic repulsion prevents strong intermolecular interactions between the bulky phospholium donor (4 b) and the acceptor (2 b) species, leading to less efficient energy transfer.…”

Bioinspirierte Phosphol‐Lipide: von stark fluoreszierenden Organogelen zu mechanisch induziertem FRET

“…[11] Auf unseren Studien basierend, kann angenommen werden, dass dieser mechanisch induzierte FRET hauptsächlich vom Donor-Akzeptor-Abstand abhängt. [5,6,12] Es ist daher sehr wahrscheinlich, dass die verstärkte, orangefarbene Emission des Akzeptors bei mechanischer Beanspruchung durch das Zusammenspiel von effizienter Exziton-Migration (Donor-Donor) und gesteigertem Energietransfer (Donor-Akzeptor) durch Bildung von Fallstellen zustande kommt. Daher wird im unbehandelten Film hauptsächlich die blaue Emission des Donors (4 b) beobachtet.…”

“…Außerdem wurde bereits gezeigt, dass effiziente Exziton-Migration in Organogelen, Nanopartikeln und sogar amorphen Festkçrpern mçglich ist. [5,6,12] Es ist daher sehr wahrscheinlich, dass die verstärkte, orangefarbene Emission des Akzeptors bei mechanischer Beanspruchung durch das Zusammenspiel von effizienter Exziton-Migration (Donor-Donor) und gesteigertem Energietransfer (Donor-Akzeptor) durch Bildung von Fallstellen zustande kommt. ¾nderungen der Dipol-Ausrichtung von Donor und Akzeptor, die den Energietransfer ebenfalls beeinflussen kçnnen, kçnnen jedoch nicht vollständig ausgeschlossen werden.…”

High‐Contrast Red–Green–Blue Tricolor Fluorescence Switching in Bicomponent Molecular Film

“…Sagara and Kato reported a single chromophore-based tricolor switching system, which is implemented by supramolecular structural changes of the liquid crystalline chromophore by mechanical forces to give luminescence color switching in green, yellow, and red. [4] The Tian group has also demonstrated a multicolor mechanochromic system, which is triggered by modulating the degree of molecular p-p overlap through varying external pressure. [3] However, it was noted in these works that achieving color change in the full RGB range with such single chromophoric systems is inherently limited, because the supramolecular stacking control of the molecular units to give excitonic and/or excimeric coupling are rather limited to the narrow-range color modulation.…”

“…An ideal multicolor fluorescence switching system requires high sensitivity, contrast, and reversibility in switching, high luminescence quantum yield, and independent switching of RGB emitting elements without crosstalk between them. As for the reversible luminescence switching, [1] one of the most successful strategies is to control the molecular stacking and/or supramolecular structure of emitting elements by physical means, which can be distinguished from the irreversible chemical alternation of their molecular structures.[2] Although there have already been a few examples of external stimuli-responsive multicolor reversible fluorescence switches, [3][4][5][6][7] a practical RGB switching system for the full color reproduction with high luminescence quantum yield and high switching contrast in the solid state has not been accomplished yet. Sagara and Kato reported a single chromophore-based tricolor switching system, which is implemented by supramolecular structural changes of the liquid crystalline chromophore by mechanical forces to give luminescence color switching in green, yellow, and red.…”