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...

Organic luminescent solids are attracting increasing interest in various fields of application. Modification or alteration of the chemical structures of their component molecules is the most common approach for tuning their luminescence properties. However, for dynamic tuning or switching of solid-state luminescence with high efficiency and reproducibility successful examples are limited as chemical reactions in the solid state frequently encounter insufficient conversion, one-way reactions or loss of their luminescence properties. One promising approach is to control the luminescence properties by altering the mode of solid-state molecular packing without chemical reactions. Here, we show that 2,2':6',2''-terpyridine, practically non-luminescent in the form of amorphous solid or needle crystal, shows strong blue luminescence upon formation of a plate crystal. Efficient and reproducible on-off switching of solid-state luminescence is demonstrated by heat-mode interconversion between the plate and needle crystals. Because alteration of the mode of molecular packing does not require chemical reactions, the present findings would open the way for the development of novel organic luminescent solids that can be switched on and off by external thermal stimuli.

Reproducible switching of the polymorph‐dependent excited‐state intramolecular proton‐transfer (ESIPT) luminescence of an imidazo[1,2‐a]pyridine between blue‐green and yellow (see picture) is achieved by thermal control of its solid‐state molecular packing. Thus, ESIPT is a promising mechanism for packing‐to‐luminescence transduction and amplification that offers a novel design concept for tunable organic luminescent solids.