Spontaneous Symmetry Breaking of Achiral Molecules Leading to the Formation of Homochiral Superstructures that Exhibit Mechanoluminescence

Abstract

Chirality, with its intrinsic symmetry‐breaking feature, is frequently utilized in the creation of acentric crystalline functional materials that exhibit intriguing optoelectronic properties. On the other hand, the development of chiral crystals from achiral molecules offers a solution that bypasses the need for enantiopure motifs, presenting a promising alternative and thereby expanding the possibilities of the self‐assembly toolkit. Nevertheless, the rational design of achiral molecules that prefer spontaneous symmetry breaking during crystallization has so far been obscure. In this study, we present a series of six achiral molecules, demonstrating that when these conformationally flexible molecules adopt a cis‐conformation and engage in multiple non‐covalent interactions along a helical path, they collectively self‐assemble into chiral superstructures consisting of single‐handed supramolecular columns. When these homochiral supramolecular columns align in parallel, they form polar crystals that exhibit intense luminescence upon grinding or scraping. We therefore demonstrate our molecular design strategy could significantly increase the likelihood of symmetry breaking in achiral molecular synthons during self‐assembly, offering a facile access to novel chiral crystalline materials with unique optoelectronic properties.

On Sense and Deform: Molecular Luminescence for Mechanoscience

“…A large number of hexafluoroacetylacetonate (hfa), dibenzoylmethide (dbm), thenoyl-trifluoroacetonate (tta), and other chelates with capping/bridging ligands have been demonstrated to be ML active. 29 ML and PL spectral profiles are generally reported to be "similar"; thus, ML is expected to 106 (Z)-BFBM-TPE, 170 CDpP-G/B, 171 BCPC, 172 TXENE-Y, 154 1a, 173 ImBr, 174 FCO-CzS, 141 XPy, 148 TPA-o-3COOMe, 175 [Sm(dbm) 4 ]TEA, 176 [Eu(tta) 3 (phen)], 177 [Ru(bpy) 3 ]Cl 2 , 178 PL intensity contrast in a homometallic complex, 130 which was explained by the characteristic direct-lanthanide(III) excitation process in ML (denoted as the "f-f priority rule" in contrast to ligand-excitation in PL). 51 The number of studies reporting the chromic shifts in ML of organic molecules relative to their PL is also increasing, similar to those observed in MRL (PL in ground forms), as depicted by the blue and red shades in Figure 4.…”

“…132 Indeed, Yang et al recently demonstrated the critical role of polarity in molecular ML activity by comparing spontaneously polarized and nonpolarized crystalline phases (see section 3.5). 148 Lanthanide(III) coordination complexes are considered as promising candidates for ML-active materials, unlike MRL (Figure 4). A large number of hexafluoroacetylacetonate (hfa), dibenzoylmethide (dbm), thenoyl-trifluoroacetonate (tta), and other chelates with capping/bridging ligands have been demonstrated to be ML active.…”

“…149 Yang et al also induced bright ML of achiral Py-derived molecules by clamping them in a helical column through H-bonds, which spontaneously polarized by adopting a parallel orientation (FPy, Figures 3 and 7d-iii), while an analogue with antiparallel orientation was ML-inert despite the similar PLQY (Φ PL = 54 and 45%, respectively). 148 Therefore, isolating the brightness under photoirradiation and that under mechanical-stimulation is reasonable.…”

“…Examples of molecular ML activation by (a-i) ML-inactive host crystal, (a-ii) ML-active host crystals, (b) host polymers including mechanochemiluminescence, (c) photochromic reactions, (d-i) mechanically ductile and polar crystal packing in (d-ii) constrained CP chains and (d-iii) spontaneously oriented columns (ML photos, chemical structures, and schematics, reproduced with permissions from ref /ref /ref /ref /ref /ref , copyright 2023/2018/2023/2023/2017/2024 John Wiley & Sons, Inc., ref , copyright 2018, The Royal Society of Chemistry, ref , copyright 2022 American Chemical Society, ref /ref , copyright 2012/2024 Springer Nature BV).…”

“…Inset shows a group of ML-active Eu(III) CPs (Z = d(Ph)t/dpedot, 149 d(p-Tol)f, 150 tppb, 160 dpa, 161 bipyo, 162 acbz 163 ) and lanthanide(III) complexes ([Ln(ppi) 3 (dpepo)] 164 and [Ln-(tmh) 3 (dmap)] 165 ). (Others: Scotch tape, 166 BP-TPY, 167 CAC-n (n = 4, 6, 8), 168 naphthalene/Py/ANT/(E)-stilbene/RiTB/p-terphenyl/ coronene/[Ir(ppy) 3 ]/[Cu(dmp)(xantphos)]/[Eu(acac) 3 (phen)], 169 m-/p-NH 2 -TPE,106 (Z)-BFBM-TPE,170 CDpP-G/B, 171 BCPC,172 TXENE-Y, 154 1a,173 ImBr,174 FCO-CzS,141 XPy,148 TPA-o-3COOMe,175 [Sm(dbm) 4 ]TEA,176 [Eu(tta) 3 (phen)],177 [Ru(bpy) 3 ]Cl 2 ,178 and [Mn-(tppo) 2 Br 2 ] 179 ).T h i s c o n t e n t i s o n l y l i c e n s e d f o r c o n s u m p t i o n b y A u t h o r i z e d U s e r s a f f i l i a t e d w i t h s c i t e .…”