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. 2023 Mar 30;8(14):12922–12927. doi: 10.1021/acsomega.3c00099

15N CP/MAS NMR as a Tool for the Mechanistic Study of Mechanical Stimuli-Responsive Materials: Evidence for the Conformational Change of an Emissive Dimethylacridane Derivative

Toyotaka Nakae †, Mineyuki Hattori ‡,*, Yoshinori Yamanoi §,*
PMCID: PMC10099412  PMID: 37065051

Abstract

graphic file with name ao3c00099_0005.jpg

Mechanochromic luminescent molecules are currently attracting considerable attention because of their promising technological applications, but understanding their mechanism of action is challenging and is thus hindering our deeper understanding of these materials. The conformational change of 9,9′-dimethyl-9,10-dihydroacridane derivative 1 was examined using solid-state 15N nuclear magnetic resonance (NMR) spectroscopic techniques without using a specifically 15N-labeled compound. A difference between the two conformers was clearly observed in the measurements and was assigned to the ⟨pl⟩ and ⟨bf⟩ spatial structures. The results were supported by quantum chemical calculations on 15N NMR chemical shifts of each isomer. The technique presented here can clearly identify the structural changes caused by crushing a powder sample. Such structural changes are difficult to determine using X-ray diffraction (XRD) measurements.

1. Introduction

Stimuli-responsive crystalline molecules have attracted much interest over the past several years.1 Among them, mechanofluorochromic materials have made considerable progress in various intelligent responsive devices and security technologies.2 There are many research examples of mechanochromic emission, but direct observation of structural variation before and after grinding a crystalline sample remains challenging. Most structural information on crystalline samples comes from single-crystal and/or powder X-ray diffraction studies, but these methods are insufficient to obtain structural information on amorphous samples, as such samples tend to show complex conformers. Thus, the investigation of structure–property relationships is key for rational molecular design to improve the performance of mechanochromic materials.

Solid-state NMR spectroscopy is increasingly employed to characterize solid-state functional materials,3 as it is a nondestructive characterization method suitable for complex materials and insoluble compounds. Many studies to date have focused on 13C and 29Si nuclei, whereas there have been limited measurements of 15N nuclei.415N NMR spectroscopy is hampered by low sensitivity due to the very low natural abundance of 15N (0.37%). The low nitrogen concentration in many molecules led researchers to believe that 15N NMR investigations were impossible without 15N enrichment. However, enrichment with 15N is often not synthetically feasible. Thus, there are few reports of 15N NMR measurements of mechanochromic materials.

We recently reported emissive organosilane crystalline compounds with nitrogen atoms in heterocyclic components.5 These compounds exhibit mechanochromic luminescence owing to a combination of the flexibility of the Si–Si bond and the conformational isomers of the cyclic N-arylamine.6,7 Among them, 4,7-bis(2-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-1,1,2,2-tetramethyldisilanyl)benzo[c][1,2,5]thiadiazole adopts various conformational isomers 1α, 1β, and 1γ, which contribute to changes in luminescence properties (Figure 1). The conformational structure in the crystalline state was found as 1α studied by X-ray diffraction experiments. Grinding a crystalline sample with a mortar and pestle results in red-shifted emission. The morphology of the ground sample was amorphous, and no clear difference was observed in the solid-state 13C NMR data between the crystalline and amorphous states in a preliminary study. Few investigations have focused on the isomeric conformational structure in the amorphous state of mechanochromic materials.8 In this study, we used solid-state 15N NMR spectroscopy to identify structural modifications induced by the mechanochemical effect of grinding nitrogen-containing heterocycle 1 without (partial) 15N enrichment, and we present experimental and computational investigations of the relationship between the mechanochromic behavior and molecular structural changes of 1.

Figure 1.

Figure 1

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Possible conformational isomers 1α, 1β and 1γ related to mechanochromic emission.

2. Results and Discussion

2.1. 15N CP/MAS NMR Measurements

Compound 1 containing the 9,9′-dimethyl-9,10-dihydroacridane moiety possibly has quasi-equatorial (⟨eq⟩), quasi-axial (⟨ax⟩), butterfly (⟨bf⟩), and planar (⟨pl⟩) conformers. The ⟨bf-eq⟩ isomer is often observed in the crystalline state of the 9,9′-dimethyl-9,10-dihydroacridane structure, which shows a dihedral angle between the two phenyl groups in the 9,9′-dimethyl-9,10-dihydroacridane ring of about 30°.9 The structures of only a few ⟨pl⟩ isomers have been reported based on single-crystal X-ray structural analysis.10 Despite being far from common, 15N NMR spectroscopy can provide useful information about the environment around N atoms, a key element in our material. The 15N NMR spectrum of 1 in the crystalline state was measured to distinguish different classes and the connectivity of nitrogen in the molecule. The spectrum required the accumulation of 8,000 to 16,000 scans over a period of 24–48 h to obtain an adequate signal-to-noise ratio. The 15N CP/MAS spectra of 15N-unlabeled 1 are shown in Figure 2. A singlet signal was observed in the crystalline sample at −269.8 ppm relative to nitromethane. This signal can be assigned to the ⟨bf-eq⟩ conformer of 9,9′-dimethyl-9,10-dihydroacridane (Figure 2a) and is supported by single-crystal X-ray diffraction. The 15N chemical shift of the benzothiadiazole group was observed at around −45 ppm (Figure S1a).11 These observations were consistent with the presence of only one type of 9,9′-dimethyl-9,10-dihydroacridane group.

Figure 2.

Figure 2

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Solid-state 15N NMR of 1 with reference to 15N nitromethane as an external reference at rt. (a) Crystalline state. (b) Amorphous state with a grinding crystalline state. (c) After thermal annealing of the amorphous sample at 110 °C. LB = 0.

The grinding process induces a crystalline-to-amorphous phase transition, causing a change in the structure of the conformational isomers at the molecular level. Figure 2b shows the 15N NMR spectrum for an amorphous sample of 1 after grinding and suggests the presence of multiple chemically distinct species. The peaks are located at −269.7 and −273.9 ppm, which indicates a different nitrogen environment of 9,9′-dimethyl-9,10-dihydroacridane in the ground sample compared with the unground sample.12 We assigned the peak at lower magnetic field (−269.7 ppm) and at higher magnetic field (−273.9 ppm) to nitrogen atoms of ⟨bf-eq⟩ and another conformer, respectively, because the remaining peak at −269.7 ppm must be ⟨bf-eq⟩. These results indicate that there is a chemical shift difference between the conformational isomers because the chemical shifts were affected by nitrogen nuclei in the molecule. The broadening of the peak in the amorphous state is presumably a result of random dipolar interaction in the amorphous state due to the deconstruction of CH−π intermolecular interactions with a lack of long-range periodicity. The 15N chemical shift of the benzothiadiazole group did not change during the grinding process (Figure S1b).

Because of the flexible framework of 1, the molecule can easily alter its conformational structure to effectively optimize intermolecular interactions. The two broad peaks disappear and the original single sharp peak at −269.9 ppm reappears after annealing at 100 °C (Figure 2c), indicating that the crystalline phase was recovered. This observation is consistent with the amorphous state returning to the crystalline state upon thermal stimulation. The 15N CP/MAS NMR spectra indicate that the mechanochromic luminescence of 1 is relevant to molecular conformation and packing mode.

2.2. Theoretical Calculations

We calculated the energies of each conformational isomer of phenyl 9,9′-dimethyl-9,10-dihydroacridane to clarify its utility as a model compound. Possible conformational isomers and a proposed energy diagram are shown in Figure 3. The frequency analysis was performed on all optimized structures to ensure that the energy of the structure is the local minimum. Although ⟨bf-eq⟩ is the most stable isomer which is consistent with only one isomer observed in the crystal structure, studies performed on phenyl 9,9′-dimethyl-9,10-dihydroacridane revealed that the ⟨bf⟩ and ⟨pl⟩ conformations interconverted freely in the amorphous state with a low kinetic barrier (ΔG≠ ≈ 0.4 kcal/mol) and no conformational energy difference (ΔE = 0.0 kcal/mol). This result supported the theoretical possibility of coexisting populations of these conformations at rt. Therefore, when the crystalline sample is crushed under mechanical stress, ⟨bf⟩ and ⟨pl⟩ are present in equal amounts in the amorphous state at rt. The ⟨bf-ax⟩ conformer is also potentially present in the amorphous sample at a lower population due to the high internal energy (ΔG≠ ≈ 5.6 kcal/mol). Therefore, the ⟨bf-ax⟩ conformer cannot be detected by NMR owing to the small amount present in the sample. The above results imply that 1 in the amorphous sample is a mixture of ⟨bf-ax⟩ and ⟨pl-eq⟩ at a ratio of ∼1:1 and that grinding overcomes the transition energy for the internal conversion from ⟨bf-eq⟩ to ⟨pl-eq⟩ or ⟨bf-ax⟩. The calculation showed significant conformational flexibility and adoption of several conformers in the amorphous state.

Figure 3.

Figure 3

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Structure–property relation. For simplicity, energy calculations were performed on phenyl 9,9′-dimethyl-9,10-dihydroacridane. The unit for numbers in the energy diagram is kcal/mol.

Computational aspects of 15N NMR shielding constants (chemical shifts) provide a powerful tool in structural studies of organic compounds.13 To verify the nitrogen assignments, 15N NMR chemical shift calculations of phenyl 9,9′-dimethyl-9,10-dihydroacridane were performed based on the gauge including atomic orbital (GIAO) calculations using the Gaussian 16 program at the B3LYP/6-311+G** level of theory.14 The calculated results for each isomer are shown in the Supporting Information. The calculated 15N NMR chemical shifts of ⟨bf-eq⟩, ⟨pl-eq⟩, and ⟨ax-bf⟩ were −259.5, −264.1, and −263.8 ppm relative to nitromethane, respectively.15 The comparison of ⟨bf-eq⟩ with ⟨pl-eq⟩ and ⟨ax-bf⟩ using 15N NMR showed a high field shift of ∼4 ppm, consistent with the results of 15N NMR experimental measurements. The finding allows for a rationale for the mechanism of stimuli-responsive emission.

3. Conclusions

The present work investigated crystalline and amorphous samples of 1 using solid-state 15N NMR and theoretical calculations. Because no spectroscopic evidence on conformational changes in the amorphous sample was presented, solid-state 15N NMR complements X-ray diffraction studies on the conformations of nitrogen-containing heterocycles. The CP/MAS 15N NMR data presented here provide more detailed structural information than previously reported 13C CP/MAS NMR studies. The 15N NMR measurements indicate that a donor–acceptor–donor molecule of 1 after grinding exhibits at least two distinguishable environments around the N atoms between the ⟨bf⟩ and ⟨pl⟩ isomers with high conformational mobility. 15N NMR chemical shifts were key to assigning the structures of the conformational forms and were sensitive to small changes in the local environment of the N atoms. The two different 15N NMR peaks observed were derived from different nitrogen substitution patterns. The assignment was performed based on computational 15N NMR studies for each isomer. 15N NMR supported the fact that disruption and restoration of intermolecular CH−π interaction triggered by grinding and heating causes the conformational change of 9,9′-dimethyl-9,10-dihydroacridane, which are consistent with a previously proposed mechanism based on thermodynamic arguments. The energy levels of 1α, 1β, and 1γ are within 5.6 kcal/mol, and calculations suggest that 1 possesses significant conformational flexibility. Our work provides further evidence of a conformational change in 1 upon mechanical stimuli. Because 15N NMR chemical shifts have a high regularity correlated to structure, they can be used as diagnostic indicators for identifying the structure of compounds in the solid state.

4. Experimental Section

4.1. Materials

All chemicals and reagents for CP/MAS 15N NMR were obtained from commercial sources and used without additional purification. Compound 1 was prepared according to procedures previously reported by our group.6b

4.2. Solid-State NMR Spectroscopy

The sample was packed into a 4 mm zirconia rotor and measured with 15N cross-polarization/magic angle spinning (CP/MAS) NMR using a spectrometer (Bruker AVANCE III HD 600WB) at a Larmor frequency of 60.86 MHz. A Bruker MAS probe head (MAS4DR) was used with a HR-MAS rotor with 4 mm diameter (HZ05538) and a Teflon insert (50 μL), and the sample spin rate was 8 kHz. The amounts of crystalline and amorphous samples for 15N CP/MAS NMR measurements were 38.37 and 29.92 mg, respectively. The 15N chemical shifts were referenced to nitromethane at 0 ppm.16 NH4Cl (10 atom % 15N) was used as a second reference material, the NH4 signal of which was set at −341.15 ppm. The samples were measured at ambient probe temperature.

4.3. Computational Details

The Gaussian 16 rev. C program was used to predict the 15N NMR shielding constants of 1. The geometries of all isomers of 1 were optimized at the B3LYP/6-31+G** level of theory, and the 15N chemical shifts were calculated using the optimized structures by the gauge-independent atomic orbital method. The GIAO magnetic shielding tensor was −119.9 ppm for 15N in nitromethane, and calculated 15N NMR shifts were referenced to nitromethane.

Acknowledgments

This work was financially supported in part by the Nagase Science and Technology Foundation, the Tonen General Sekiyu Research/Development Encouragement & Scholarship Foundation, a Grant-in-Aid for Scientific Research (C) (No. JP22K05250), and Scientific Research on the Innovative Area “Soft Crystal: Science and Photofunctions of Flexible Response Systems with High Order” (Area 2903, No. JP17H06369) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Solid-state NMR measurements were supported by the AIST Nanocharacterization Facility (ANCF) in the Advanced Research Infrastructure for Materials and Nanotechnology (No. JPMXP1222AT5021) sponsored by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c00099.

  • 15N NMR of 1, and calculated data of nitromethane, ⟨bf-eq⟩, ⟨pl-eq⟩, and ⟨ax-bf⟩ of phenyl 9,9′-dimethyl-9,10-dihydroacridane (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c00099_si_001.pdf (407.2KB, pdf)

References

  1. a Kato M.; Ito H.; Hasegawa M.; Ishii K. Soft Crystals: Flexible Response Systems with High Structural Order. Chem. – Eur. J. 2019, 25, 5105–5112. 10.1002/chem.201805641. [DOI] [PMC free article] [PubMed] [Google Scholar]; b Kato M. Luminescent Platinum Complexes Having Sensing Functionalities. Bull. Chem. Soc. Jpn. 2007, 80, 287–294. 10.1246/bcsj.80.287. [DOI] [Google Scholar]; c Sagara Y.; Kato T. Mechanically induced luminescence changes in molecular assemblies. Nat. Chem. 2009, 1, 605–610. 10.1038/nchem.411. [DOI] [PubMed] [Google Scholar]; d Mellerup S. K.; Vang S. Boron-based stimuli responsive materials. Chem. Soc. Rev. 2019, 48, 3537–3549. 10.1039/C9CS00153K. [DOI] [PubMed] [Google Scholar]; e Jobbágy C.; Deák A. Stimuli-Responsive Dynamic Gold Complexes. Eur. J. Inorg. Chem. 2014, 2014, 4434–4449. 10.1002/ejic.201402249. [DOI] [Google Scholar]; f Varughese S. Non-covalent routes to tune the optical properties of molecular materials. J. Mater. Chem. C 2014, 2, 3499–3516. 10.1039/c3tc32414a. [DOI] [Google Scholar]; g Wang X.-d.; Wolfbeis O. S.; Meier R. J. Luminescent probes and sensors for temperature. Chem. Soc. Rev. 2013, 42, 7834–7869. 10.1039/c3cs60102a. [DOI] [PubMed] [Google Scholar]; h Li B.; Fan H.-T.; Zang S.-Q.; Li H.-Y.; Wang L.-Y. Metal-containing crystalline luminescent thermochromic materials. Coord. Chem. Rev. 2018, 377, 307–329. 10.1016/j.ccr.2018.09.004. [DOI] [Google Scholar]
  2. a Zhao J.; Chi Z.; Zhang Y.; Mao Z.; Yang Z.; Ubba E.; Chi Z. Recent progress in the mechanofluorochromism of cyanoethylene derivatives with aggregation-induced emission. J. Mater. Chem. C 2018, 6, 6327–6353. 10.1039/C8TC01648H. [DOI] [Google Scholar]; b Chi Z.; Zhang X.; Xu B.; Zhou X.; Ma C.; Zhang Y.; Liu S.; Xu J. Recent advances in organic mechanofluorochromic materials. Chem. Soc. Rev. 2012, 41, 3878–3896. 10.1039/c2cs35016e. [DOI] [PubMed] [Google Scholar]; c Mechanochromic Fluorescent Materials: Phenomena, Materials and Applications; Xu J.; Chi Z., Eds.; Royal Society of Chemistry: Cambridge, UK, 2014; Vol. 8. [Google Scholar]; d Sagara Y.; Yamane S.; Mitani M.; Weder C.; Kato T. Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials. Adv. Mater. 2016, 28, 1073–1095. 10.1002/adma.201502589. [DOI] [PubMed] [Google Scholar]; e Ito S. Recent Advances in Mechanochromic Luminescence of Organic Crystalline Compounds. Chem. Lett. 2021, 50, 649–660. 10.1246/cl.200874. [DOI] [Google Scholar]
  3. Molugu T. R.; Lee S.; Brown M. Concepts and methods of solid-state NMR spectroscopy applied to biomembranes. Chem. Rev. 2017, 117, 12087–12132. 10.1021/acs.chemrev.6b00619. [DOI] [PubMed] [Google Scholar]
  4. For the representative examples, see:; a Hayashi S.; Hayamizu K. Chemical Shift Standards in High-Resolution Solid-State NMR (2) 15N Nuclei. Bull. Chem. Soc. Jpn. 1991, 64, 688–690. 10.1246/bcsj.64.688. [DOI] [Google Scholar]; b Munowitz M.; Bachovchin W. W.; Herzfeld J.; Dobson C. M.; Griffin R. G. Acid-Base and Tautomeric Equilibria in the Solid State: 15N NMR Spectroscopy of Histidine and Imidazole. J. Am. Chem. Soc. 1982, 104, 1192–1196. 10.1021/ja00369a007. [DOI] [Google Scholar]; c Mason J. Nitrogen Nuclear Magnetic Resonance Spectroscopy in Inorganic, Organometallic, and Bioinorganic Chemistry. Chem. Rev. 1981, 81, 205–227. 10.1021/cr00043a001. [DOI] [Google Scholar]
  5. For representative examples, see:; a Nakae M.; Nishio M.; Yamanoi Y. Photofunctional organosilicon compounds. Bull. Jpn. Soc. Coord. Chem. 2020, 76, 31–39. 10.4019/bjscc.76.31. [DOI] [Google Scholar]; b Usuki T.; Omoto K.; Shimada M.; Yamanoi Y.; Kasai H.; Nishibori E.; Nishihara H. Effects of substituents on the blue luminescence of disilane-linked donor-acceptor-donor triads. Molecules 2019, 24, 521. 10.3390/molecules24030521. [DOI] [PMC free article] [PubMed] [Google Scholar]; c Usuki T.; Shimada M.; Yamanoi Y.; Ohto T.; Tada H.; Kasai H.; Nishibori E.; Nishihara H. Aggregation-Induced Emission Enhancement from Disilane-Bridged Donor-Acceptor-Donor Luminogens Based on the Triarylamine Functionality. ACS Appl. Mater. Interfaces 2018, 10, 12164–12172. 10.1021/acsami.7b14802. [DOI] [PubMed] [Google Scholar]; d Shimada M.; Tsuchiya M.; Sakamoto R.; Yamanoi Y.; Nishibori E.; Sugimoto K.; Nishihara H. Bright Solid-State Emission of Disilane-Bridged Donor–Acceptor–Donor and Acceptor–Donor–Acceptor Chromophores. Angew. Chem., Int. Ed. 2016, 55, 3022–3026. 10.1002/anie.201509380. [DOI] [PubMed] [Google Scholar]; e Shimada M.; Yamanoi Y.; Matsushita T.; Kondo T.; Nishibori E.; Hatakeyama A.; Sugimoto K.; Nishihara H. Optical Properties of Disilane-Bridged Donor-Acceptor Architectures: Strong Effect of Substituents on Fluorescence and Nonlinear Optical Properties. J. Am. Chem. Soc. 2015, 137, 1024–1027. 10.1021/ja511177e. [DOI] [PubMed] [Google Scholar]; f Nakae T.; Miyabe H.; Nishio M.; Yamada T.; Yamanoi Y. Synthesis, structure and photophysical properties of yellow-green and blue photoluminescent dinuclear and octanuclear copper(I) iodide complexes with a disilanylene-bridged bispyridine ligand. Molecules 2021, 26, 6852. 10.3390/molecules26226852. [DOI] [PMC free article] [PubMed] [Google Scholar]; g Shimada M.; Yamanoi Y.; Ohto T.; Pham S.-T.; Yamada R.; Tada H.; Omoto K.; Tashiro S.; Shionoya M.; Hattori M.; Jimura K.; Hayashi S.; Koike H.; Iwamura M.; Nozaki K.; Nishihara H. Multifunctional Octamethyltetrasila[2.2]cyclophanes: Conformational Variations, Circularly Polarized Luminescence, and Organic Electroluminescence. J. Am. Chem. Soc. 2017, 139, 11214–11221. 10.1021/jacs.7b05671. [DOI] [PubMed] [Google Scholar]
  6. a Nakae T.; Nishio M.; Usuki T.; Ikeya M.; Nishimoto C.; Ito S.; Nishihara H.; Hattori M.; Hayashi S.; Yamada T.; Yamanoi Y. Luminescent behavior elucidation of disilane-bridged D–A–D triad composed of phenothiazine and thienopyrazine. Angew. Chem., Int. Ed. 2021, 60, 22871–22878. 10.1002/anie.202108089. [DOI] [PubMed] [Google Scholar]; b Miyabe H.; Ujita M.; Nishio M.; Nakae T.; Usuki T.; Ikeya M.; Nishimoto C.; Ito S.; Hattori M.; Takeya S.; Hayashi S.; Saito D.; Kato M.; Nishihara H.; Yamada T.; Yamanoi Y. A Series of D–A–D Structured Disilane-Bridged Triads: Structure and Stimuli-Responsive Luminescence Studies. J. Org. Chem. 2022, 87, 8928–8938. 10.1021/acs.joc.2c00641. [DOI] [PubMed] [Google Scholar]
  7. For other examples on conformational isomers of N-aryl cyclic amines, see:; a Okazaki M.; Takeda Y.; Data P.; Pander P.; Higginbotham H.; Monkmanb A. P.; Minakata S. Thermally activated delayed fluorescent phenothiazine–dibenzo[a,j]phenazine–phenothiazine triads exhibiting tricolor-changing mechanochromic luminescence. Chem. Sci. 2017, 8, 2677–2686. 10.1039/C6SC04863C. [DOI] [PMC free article] [PubMed] [Google Scholar]; b Takeda Y.; Data P.; Minakata S. Alchemy of donor–acceptor–donor multi-photofunctional organic materials: from construction of electron-deficient azaaromatics to exploration of functions. Chem. Commun. 2020, 56, 8884–8894. 10.1039/D0CC03322G. [DOI] [PubMed] [Google Scholar]; c Tanaka H.; Shizu K.; Nakanotani H.; Adachi C. Dual Intramolecular Charge-Transfer Fluorescence Derived from a Phenothiazine-Triphenyltriazine Derivative. J. Phys. Chem. C 2014, 118, 15985–15994. 10.1021/jp501017f. [DOI] [Google Scholar]; d Etherington M. K.; Franchello F.; Gibson J.; Northey T.; Santos J.; Ward J. S.; Higginbotham H. F.; Data P.; Kurowska A.; Santos P. L. D.; Graves D. R.; Batsanov A. S.; Dias F. B.; Bryce M. R.; Penfold T. J.; Monkman A. P. Regio- and conformational isomerization critical to design of efficient thermally-activated delayed fluorescence emitters. Nat. Commun. 2017, 8, 14987 10.1038/ncomms14987. [DOI] [PMC free article] [PubMed] [Google Scholar]; e Data P.; Okazaki M.; Minakata S.; Takeda Y. Thermally activated delayed fluorescence vs. room temperature phosphorescence by conformation control of organic single molecules. J. Mater. Chem. C 2019, 7, 6616–6621. 10.1039/C9TC00909D. [DOI] [Google Scholar]; f Takeda Y.; Mizuno H.; Okada Y.; Okazaki M.; Minakata S.; Penford T.; Fukuhara G. Hydrostatic Pressure – Controlled Ratiometric Luminescence Responses of Dibenzo[a,j]phenazine – Cored Mechanoluminophore. ChemPhotoChem 2019, 3, 1203–1211. 10.1002/cptc.201900190. [DOI] [Google Scholar]
  8. For representative examples on mechanochromism investigated by 13C or 31P CP/MAS NMR, see:; a Sheth A. R.; Lubach J. W.; Munson E. J.; Muller F. X.; Grant D. J. W. Mechanochromism of Piroxicam Accompanied by Intermolecular Proton Transfer Probed by Spectroscopic Methods and Solid-Phase Changes. J. Am. Chem. Soc. 2005, 127, 6641–6651. 10.1021/ja045823t. [DOI] [PubMed] [Google Scholar]; b Utrera-Melero R.; Huitorel B.; Cordier M.; Mevellec J.-Y.; Massuyeau F.; Latouche C.; Martineau-Corcos C.; Perruchas S. Combining Theory and Experiment to Get Insight into the Amorphous Phase of Luminescent Mechanochromic Copper Iodide Clusters. Inorg. Chem. 2020, 59, 13607–13620. 10.1021/acs.inorgchem.0c01967. [DOI] [PubMed] [Google Scholar]
  9. For example, see:; a Lv X.; Cao X.; Wu H.; Lin H.; Ni F.; Huang H.; Zou Y.; Yang C. Realize efficient organic after glow from simple halogenated acridan derivatives. Chem. Eng. J. 2021, 419, 129598 10.1016/j.cej.2021.129598. [DOI] [Google Scholar]; b Han M.; Xu Z.; Lu J.; Xie Y.; Li Q.; Li Z. Intramolecular-locked triphenylamine derivatives with adjustable room temperature phosphorescence properties by the substituent effect. Mater. Chem. Front. 2021, 6, 33–39. 10.1039/D1QM01184G. [DOI] [Google Scholar]
  10. a Hladka I.; Volyniuk D.; Bezvikonnyi O.; Kinzhybalo V.; Bednarchuk T. J.; Danyliv Y.; Lytvyn R.; Lazauskas A.; Grazulevicius J. V. Polymorphism of derivatives of tert-butyl substituted acridan and perfluorobiphenyl as sky-blue OLED emitters exhibiting aggregation induced thermally activated delayed fluorescence. J. Mater. Chem. C 2018, 6, 13179–13189. 10.1039/C8TC04867C. [DOI] [Google Scholar]; b Zheng K.; Ni F.; Chen Z.; Zhong C.; Yang C. Polymorph-Dependent Thermally Activated Delayed Fluorescence Emitters: Understanding TADF from a Perspective of Aggregation State. Angew. Chem., Int. Ed. 2020, 59, 9972–9976. 10.1002/anie.201913210. [DOI] [PubMed] [Google Scholar]; c Lin T. A.; Chatterjee T.; Tsai W.-L.; Lee W.-K.; Wu M.-J.; Jiao M.; Pan K.-C.; Yi C.-L.; Chung C.-L.; Wong K.-T.; Wu C.-C. Sky-Blue Organic Light Emitting Diode with 37% External Quantum Efficiency Using Thermally Activated Delayed Fluorescence from Spiroacridine-Triazine Hybrid. Adv. Mater. 2016, 28, 6976–6983. 10.1002/adma.201601675. [DOI] [PubMed] [Google Scholar]; d Liang D.; Chen X.-L.; Liao J.-Z.; Hu J.-Y.; Jia J.-H.; Lu C.-Z. Highly Efficient Cuprous Complexes with Thermally Activated Delayed Fluorescence for Solution-Processed Organic Light-Emitting Devices. Inorg. Chem. 2016, 55, 7467–7475. 10.1021/acs.inorgchem.6b00763. [DOI] [PubMed] [Google Scholar]; e Keruckiene R.; Guzauskas M.; Narbutaitis E.; Tsiko U.; Volyniuk D.; Lee P.-H.; Chen C.-H.; Chiu T.-L.; Lin C.-F.; Lee J.-H.; Grazulevicius J. V. Exciplex-forming derivatives of 2,7-di-tert-butyl-9,9-dimethylacridan and benzotrifluoride for efficient OLEDs. Org. Electron. 2020, 78, 105576 10.1016/j.orgel.2019.105576. [DOI] [Google Scholar]; f Fecková M.; Kalis I. K.; Roisnel T.; Poul P.; Pytela O.; Klikar M.; Guen F. R.; Bureš F.; Fakis M.; Achelle S. Photophysics of 9,9-Dimethylacridan-Substituted Phenylstyrylpyrimidines Exhibiting Long-Lived Intramolecular Charge-Transfer Fluorescence and Aggregation-Induced Emission Characteristics. Chem. – Eur. J. 2021, 27, 1145–1159. 10.1002/chem.202004328. [DOI] [PubMed] [Google Scholar]
  11. Yavari I.; Botto R. E.; Roberts J. D. Nitrogen-15 nuclear resonance spectroscopy. Furazans and related systems. J. Org. Chem. 1978, 43, 2542–2544. 10.1021/jo00406a056. [DOI] [Google Scholar]
  12. Levy G. C.; Lichter R. L.. Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy; John Wiley & Sons: New York, 1979. [Google Scholar]
  13. Encyclopedia of NMR; Grant D. M.; Harris R. K., Eds.; Wiley: Chichester, 1996. [Google Scholar]
  14. For example, see:; a Wolinski K.; James V.; Hinton F.; Pulay P. Efficient Implementation of the Gauge-Independent Atomic Orbital Method for NMR Chemical Shift Calculations. J. Am. Chem. Soc. 1990, 112, 8251–8260. 10.1021/ja00179a005. [DOI] [Google Scholar]; b Ditchfield R. Self-consistent perturbation theory of diamagnetism I. A gauge-invariant LCAO method for N.M.R. chemical shifts. Mol. Phys. 1974, 27, 789–807. 10.1080/00268977400100711. [DOI] [Google Scholar]
  15. Calculated 15N absolute shielding tensors (σ) were converted into 15N NMR chemical shifts (δ) using the equation: δ = (σCH3NO2 – σ)/(1 – 10–6σCH3NO2), where σCH3NO2 and σ are the 15N NMR isotopic absolute shielding tensors of nitromethane and samples, respectively. Both values must be calculated at the same level of theory and with the same basis sets. See:; a Samultsev D. O.; Semenov V. A.; Krivdin L. B. On the accuracy factors and computational cost of the GIAO–DFT calculation of 15N NMR chemical shifts of amides. Magn. Reason. Chem. 2017, 55, 1015–1021. 10.1002/mrc.4625. [DOI] [PubMed] [Google Scholar]; b Samultsev D. O.; Semenov V. A.; Krivdin L. B. On the accuracy of the GIAO-DFT calculation of 15N NMR chemical shifts of the nitrogen-containing heterocycles–a gateway to better agreement with experiment at lower computational cost. Magn. Reson. Chem. 2014, 52, 222–230. 10.1002/mrc.4055. [DOI] [PubMed] [Google Scholar]
  16. a Ueda T.; Nagatomo S.; Masui H.; Nakamura N.; Hayashi S. Hydrogen Bonds in Crystalline Imidazoles Studied by 15N NMR and ab initio MO Calculations. Z. Naturforsch. A 1999, 54, 437–442. 10.1515/zna-1999-6-715. [DOI] [Google Scholar]; b Bertani P.; Raya J.; Bechinger B. 15N chemical shift referencing in solid state NMR. Solid State Nucl. Magn. Reson. 2014, 61–62, 15–18. 10.1016/j.ssnmr.2014.03.003. [DOI] [PubMed] [Google Scholar]

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