ICYS Annual Report 2022

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].u.a[ ytisnetnI6420]%Energy [kJ/mol][ saera palrevO10BeforeAfter(a)(c)(b)8 x 104(d)Crystal dataGrindingBefore grindingAfter grinding1618555045ExcimerMonomer40353.30Average pyrene distance [Å]30 Before press After press Crystal2024222? [deg]262830Crystal data141210864203.353.403.453.50Figure 1. The molecular structure of 1 (a) and crystal structure of 1 and hydrogen atoms was omitted for clarification. (b) Side view of 1. (c) Top view of 1. The overlap area is highlighted in yellow.The molecular and packing structure of 1 was evaluated using a single-crystal X-ray structural analysis of 1 which formed a spring structure (Figure 1b, c). This spring structure was in good agreement with the spring structure obtained from the DFT calculations. The molecular spring of 1 has helicity; P and M helicity are arranged alternately and their space groups were belonging to the nonpolar space group (P-1) (Figure 1c). The dihedral angle, average distance, and overlap area between the planes of terminal pyrene units of 1 are 9.885 ̊, 3.2375 Å, and only 33%, respectively (Figure 1b). Its average distance was as long as that of the previous expanded molecular spring (3.232–3.285 Å).1 Furthermore, inter-molecular interactions between the pyrene moieties in 1 were examined by Hirshfeld surface analysis, which revealed that the C-C distances corresponding to the π−π stacking interactions are far apart both intra- and inter-molecularly; thus pyrene of 1 is isolated both intra- and inter-molecularly, even though it forms a spring structure in the solid.Interestingly, the fluorescent color of 1 in the solid state can be converted by mechanical force; in other words, 1 is the mechanically responsive luminescence material (Figure 2a). The evaporation of CHCl3 solution of 1 results in the ivory powder (1before), which shows the blue emission and low quantum yield of 0.07. This blue-emitting ivory powder was changed to blue-green-emitting ivory solid by mechanical force with an agate mortar (1after). The quantum yield of 1after was 0.38 which was Research Digest intra- and the Figure 2. (a) Photographs of 1before and 1after under UV irradiation at 365 nm. (b) PXRD patterns of 1before and 1after. (c) The contracting motion of 1, was estimated by DFT calculation. (d) A plot of relative energies of 1 against the average distance of terminal pyrenes of 1.References1) Ohta, E.; Sato, H.; Ushiyama, H., et al., Nat Chem 2011, 3, 68-73.2) Ge, Y.; Wen, Y.; Yang, B., et al., J. Mater. Chem. C 2020, 8, 11830-11838.five times higher than that of 1before but lower than that of 1 in CHCl3. To clarify the different crystal structures of these two solids, a powder X-ray diffraction (PXRD) pattern of 1before showed clear refraction peaks, indicating that 1 formed a well-defined structure before the application of force (Figure 2b). Meanwhile, the PXRD pattern of 1after did not show any clear diffraction peaks. Thus, 1 formed an amorphous state by grinding their crystalline samples.To investigate the possibility of intramolecular pyrenes showing excimer emission even in the solid state, the change in energy and overlap area when the terminal pyrenes were moved towards each other was estimated by DTF calculations (Figure 2c, d). When the terminal pyrenes were moved towards each other, the relative energy of 1 decreased and the overlap area of intramolecular pyrenes increased accordingly from 33% to about 50%. Not only the distance between pyrenes but also their overlap area has a significant effect on pyrene excimer emission. For example, it has been reported that pyrenes show excimer emission when the overlap area exceeds 40%.2 In addition, temperature-dependent fluorescence spectra have shown that pyrene within a molecular spring can be even closer to each other. Therefore, the change in emission color from blue to blue-green of 1 is dominated by intermolecular excimer emission as the highly crystalline structure changes to amorphous, while isolated intramolecular pyrenes can also show excimer emission by moving towards each other and increasing their overlap area. 1. Outline of ResearchThe detection and visualization of deformation in the organic crystal that arises upon applying mechanical forces are useful for both fundamental studies and practical applications. In this study, we synthesized a novel o-phenylene derivative bearing two pyrenes (1) (Figure 1a), whose luminescent color of powder 1 changes from blue to blue-green by grinding it with an agate mortar. XRD, fluorescence spectra, and DFT calculation of 1 revealed the expanded spring structure with a highly crystalline nature changed to a contracted spring structure with amorphous nature after the application of mechanical forces. Therefore, the structural change of the crystal and the molecular spring was successfully visualized.2. Research ActivitiesMechanically Responsible Fluorescence of Molecular SpringHayato ANETAI