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.Herein we report a new type of stimuli-responsive luminescent liquid crystal that exhibits three luminescent colors, which can be switched by mechanical and thermal stimuli (Figure 1). The liquid crystal is composed of equimolar amounts of the dumbbell-shaped compound 1 and compound 2 (Scheme 1). The three luminescent colors observed are reddish-orange, yellow, and green, which are easily distinguished by the naked eye. Moreover, the LC mixture of 1 and 2 contains only one type of luminophore, 9,10-bis(phenylethynyl)anthracene, [34,35] and no additives are required to induce the luminescent color changes. Unlike crystalline materials, the thin-film states are easily prepared (see below). The change in the luminescent colors and the phase-transition behavior of the LC mixture are summarized in Figure 1.The mixture forms a thermotropic micellar cubic phase upon heating from room temperature to 146 8C. Recently thermotropic cubic phases classified as micellar cubic phases were reported. [25,36] The characterization of the LC phases is discussed in the Supporting Information. Under UV irradiation (365 nm), reddish-orange photoluminescence is observed for the micellar cubic phase (Figure 1, top left). We have found that me...

Design amphiphilic dipolar π-systems for stimuli-responsive luminescent materials using metastable states

“…Recently, luminescent materials that are responsive to mechanical stimuli have attracted considerable attention because they have the unique ability to tune their molecular, physical or chemical properties via macroscopic stimulation [5][6][7][8] . Several mechanistic explanations for the mechanically stimulated changes in the colour of the luminescence have been proposed: a phase transformation of the solid structure [9][10][11][12][13][14][15][16] , excimer formation or dissolution [17][18][19] and mechanical stress-induced chemical changes 20,21 . However, the development of new materials with luminescent mechanochromism still depends heavily on serendipitous discovery or random screening; studies based on rationally designed luminophoric compounds and sensitivity to mechanical stimuli remains rare.…”

Electrofluorochromism in π-conjugated ionic liquid crystals

“…M aterials able to switch their photoluminescence in response to various external stimuli can be potentially used for memory devices, sensors, security materials and informational displays [1][2][3][4][5][6] . Specifically, materials in which photoluminescence is modulated by redox processes are known as electrofluorochromic 7 .…”

Functional liquid-crystalline polymers and supramolecular liquid crystals

“…[131][132][133] Sagara and Kato reported mechanochromic liquid crystals consisting of π-conjugated organic molecules and dendritic side chains. 134 Pyrene, 134 naphthalene, 135 oligothiophene 136,137 and anthracene [138][139][140][141] derivatives have been used as the central luminescent cores of the dendritic molecules. For example, fan-shaped dendrons have been employed to prepare luminescent compounds 25-28.…”

“…142 The photoluminescent color change is observed for these molecules upon shear-induced phase transition. [133][134][135][136][137][138][139]142 Some π-conjugated LC molecules with branched or dendritic side chains show unique mechanochromic properties such as reversible luminescent color change 137 and tricolored luminescence. 139 The type of LC phase transition depends on the molecular structures of dendritic moieties.…”

“…[133][134][135][136][137][138][139]142 Some π-conjugated LC molecules with branched or dendritic side chains show unique mechanochromic properties such as reversible luminescent color change 137 and tricolored luminescence. 139 The type of LC phase transition depends on the molecular structures of dendritic moieties. Cubic-columnar phase transition is observed for the luminescent molecules with fan-shaped dendrons, 133-136,138,139 while smectic-smectic phase transition is induced for those with fork-like mesogens 143 composed of rigid-rod mesogenic moieties.…”