We, a group of Dr. Toshiki Mutai (Univ. Tokyo) and Prof. Dr. Satoshi Takamizawa (Yokohama City Univ.) have developed 'superelastochromic crystal', which upon controlling the strength of the mechanical load to a luminescent organic crystal, the luminescence color changes reversibly in accordance to force-induced crystal-to-crystal phase transition.

Chromism is a color change of molecular solid materials induce by external stimuli such as photons, heat, chemicals, etc. From a practical viewpoint, chromic materials have been intensively studied to develop stimulus-responsive smart materials in various fields. Simply saying, the color changes are easily detectable by naked eyes or common spectroscopy. A typical chromism proceeds in a one-way, two-state cycle as illustrated in Figure 1(a); that is, the first stimulus ST1 promptly switches the initial state (A) to another state (B), and the return process, (B) to (A), need different stimulus ST2. On the other hand, a chromic system that can be controlled by variation of the strength of a stimulus, not by two different stimuli (Figure 1(b)), will have wider practical applicability.

A chromism induced by mechanical forces such as pressing, shearing, cutting is called 'mechanochromism', which is appealing because mechanical forces are the most fundamental and common stimuli in nature that can be generated without any specific apparatus. What has attracted more in recent years is mechanochromic luminescence because of its high sensitivity. However, most of reported to date show typical chromic behavior as indicated in Figure 1(a), except for those based on elastic deformation such as amorphous gel materials and molecular crystals under very high pressure (3–10 GPa).

The term 'superelasticity' is diffusion-less plastic deformation with spontaneous shape recoverability accompanying reversible phase transition, that is, the shape of a solid changes upon mechanical loading and recovers its initial shape just by unloading. Superelasticity has been regarded as a characteristic property of a limited number of metal alloys for more than a half century, until Takamaizawa’s report on terephthalamide in 2014 [1]. A group of Mutai has been studying the solid-state luminescence of 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine derivatives (HPIPs, Figure 2a), which is based on the excited-state intramolecular proton-transfer (ESIPT) resulting in a large Stokes shift. It is to be emphasized that some of HPIPs show remarkable polymorph-dependent luminescence color, because they form an environment-sensitive zwitterionic keto form in the excited state [2,3]. Compound 7Cl forms two polymorphic crystals showing yellow-green (YG) and orange (O) luminescence under UV light (365 nm) (Figure 2b).

The mechanical loading on a crystal YG generates orange luminescent phase (O) in the middle, which is gradually enlarged by further mechanical loading (Figure 3(a)). Then, subsequent unloading the mechanical force results in complete recovery of YG crystalline phase.

Such type of mechanochromic luminescence can be called “superelastochromism”, which enables real-time, reversible, and stepless control of the abundance ratio of biphasic color emissions by only controlling a single stimulus, force stress. The unique chromic system based on spontaneous reversibility holds potential for realizing molecule-based informative mechanical sensing.

1. S. Takamizawa, Y. Miyamoto, Superelastic organic crystals, Angew. Chem. Int. Ed., 53, 6970–6973 (2014).

2. T. Mutai, H. Tomoda, T. Ohkawa, Y. Yabe, K. Araki, Switching of polymorph-dependent ESIPT luminescence of an imidazo[1,2-a]pyridine derivative. Angew. Chem. Int. Ed. 47, 9522–9524 (2008).

3. T. Mutai, H. Shoni, Y. Shigemitsu, K. Araki, Three-color polymorph-dependent luminescence: crystallographic analysis and theoretical study on excited-state intramolecular proton transfer (ESIPT) luminescence of cyano-substituted imidazo[1,2-a]pyridine. CrystEngComm 16, 3890–3895 (2014).