Material Design for Piezochromic Luminescence:  Hydrogen-Bond-Directed Assemblies of a Pyrene Derivative

Abstract

For more than a decade, tunable solid-state organic luminescent materials have been attracting considerable interest in various fields of application. 1 The tuning and switching of the luminescence of organic solids by controlling the mode of molecular packing instead of alteration of the chemical structure are attractive targets for both fundamental research and practical applications, but successful examples are quite limited and have been reported only recently. 2,3 In this letter, we will present a novel material design for piezochromic luminescence based on a pressure-dependent mode of molecular packing. Piezochromism 4,5 is the phenomenon of color change caused by mechanical grinding and reversion to the original color by heating or recrystallization, in which the presence of two different pressure-dependent stable or metastable states is essential. 4 Up to now, organic piezochromic substances that show a color change by their luminescence are quite limited. 6 Since luminescence can be detected with high sensitivity, materials that show piezochromic luminescence can find a wide variety of applications such as optical recording and strain-or pressure-sensing systems.For a better understanding and prediction of molecular packing in organic crystals, Kitaigorodskii's close-packing principle 7 and Etter's first hydrogen-bond rule 8 are regarded as useful concepts and have been discussed extensively. 9 Using these concepts as competing factors, we adopted a novel design principle to incorporate two strongly demanding factors for molecular packing, a planar aromatic core and multiple hydrogen-bonding sites, into a single molecular structure. The crystals of disk-shaped aromatics tend to be closely packed with an interplane distance of 0.34-0.36 nm, 4 while hydrogen-bond-directed columnar assemblies have a wider interplane distance of 0.47-0.48 nm. 10 The designed molecule, an amide-substituted tetraphenylpyrene derivative C6TPPy, successfully showed a piezochromic luminescence by pressureinduced alteration of the molecular packing and subsequent heatinginduced restoration. This design principle is simple and applicable to developing a broad range of materials showing piezochromic luminescence.1,3,6,8-Tetraphenylpyrene (TPPy) has a planar aromatic core and is a highly efficient fluorophore showing strong blue luminescence in solution (quantum yield Φ ) 0.9 in cyclohexane). 11 To the para position of the phenyl groups of this parent molecule, four hexyl amide units were introduced as the multiple hydrogen-bonding sites (C6TPPy, Figure 1a). The addition of methanol to a chloroform solution of C6TPPy resulted in precipitation of a white powder (B-form), which exhibited strong blue luminescence upon irradiation of UV-light ( Figure 1b). Once the precipitate was formed, this B-form solid did not dissolve again in chloroform or other common organic solvents but dissolved only slowly in warm 1-methyl-2-pyrrolidinone. Interestingly, this blue-emitting white solid (B-form) was converted to a yellowish solid showing a str...

Quantitative evaluation of thermodynamic parameters for thermal back-reaction after mechanically induced fluorescence change

“…Thus, we studied 1,3-bis-(4-methoxyphenyl)-1,3-propanedione (2a) and 1,3-bis-(4-tertbutylphenyl)-1,3-propanedione (2b) as ligands next. Both 2aBF 2 and 2bBF 2 also showed a crystal-to-amorphous phase change 5 subsequent to the mechanical perturbation, as confirmed by XRD spectra, and the substituent dependence of the rate constants and the activation parameters were determined in the same manner. These experimental data are included in the electronic supplementary information (ESI).…”

Synthesis and Mechanofluorochromism of 4-[Bis(4-methylphenyl)amino]acetophenone

“…Among them, materials that exhibit reversible change in fluorescent color induced by mechanical stress such as grinding, namely "mechanofluorochromism", have recently been received attention. Several examples of both organometallic and pure organic materials have been reported to exhibit mechanofluorochromism [1][2][3][4][5][6][7][8][9][10][11][12][13] and these phenomena were generally believed to be due to change in intermolecular interactions caused by altering the molecular arrangements in the crystalline state by mechanical stress.…”

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.…”