Strategy for Stimuli-Induced Spin Control Using a Liquescent Radical

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Strategy for Stimuli-Induced Spin Control Using a Liquescent Radical Cation Shuichi Suzuki,* Ryochi Maya, Yoshiaki Uchida, and Takeshi Naota* Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan

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S Supporting Information *

ABSTRACT: A liquescent salt based on an N-pentylphenothiazine radical cation (1•+·NTf2−) exhibited a unique crystal−crystal phase transition from a paramagnetic orange solid to a diamagnetic green solid induced by brief, weak, and pinpoint mechanostress. Electron spin resonance and electronic spectroscopies revealed that this unprecedented solid-state spin controllability was attributable to mechanostresstriggered sequential association of the highly mobile radical species occurring under neat conditions.

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the molecular fluidity.13,14 Consequently, we found that the spin and color properties of the liquescent radical cation salt 1•+·NTf2−, which bears high molecular mobility in the condensed state, were controllable via the introduction of indirect, instantaneous, weak, and pinpoint mechanostress as a stimuli-triggered initiation of domino rearrangement of its molecular alignment.

INTRODUCTION Recent advancements in organic radicals have enabled their utilization as promising components for molecule-based magnetic and electronic materials.1−4 The association characteristics of these molecules have a significant influence on their electronic, electric, and magnetic properties.1−4 In solution state, some of the charged radicals with a π-conjugated system have displayed dynamic changes in spin properties that were attributed to their association/dissociation.5,6 These fascinating species could potentially be used as molecule-based materials with simultaneous modulation of spin-related properties if their association structures could be dynamically changed through external stimuli. Controls of radical spin properties in the solid state have previously been achieved under harsh conditions, such as high temperatures,7 high pressures,8 and grinding.9 All of these conditions require energy-consuming processes to ensure stoichiometric conversion of the entire system. Therefore, an urgent topic in the development of future functional radical materials is the creation of molecules with spin properties that can be significantly controlled upon brief and weak external stimuli based on domino-type sequential changes10 in molecular arrangements. However, such a sophisticated spin control and the necessary rational molecular designs have long remained undiscovered, likely due to difficulties in achieving π-conjugated radicals with sufficient molecular mobility,11 as they definitively require molecular rigidity in their structures. This dilemma has inspired the development of new methodologies employing molecular fluidity in the radical associates under neat conditions. We designed a phenothiazine radical cation bearing a bis(trifluoromethanesulfonyl)imide anion, 1•+·NTf2−, in which (1) the stable open-shell system on the π-conjugated platform provides variable electronic structures depending on its association12 and (2) flexibility and charge delocalization of the anion increase the phase transition controllability within a narrow range of ambient temperatures upon enhancement of © 2019 American Chemical Society

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RESULTS AND DISCUSSION Radical cation salt 1•+·NTf2− displayed unique color changes in conjunction with phase transitions induced by thermal and mechanical stimuli (Figures 1 and S1). A deep green crystal (GC) of 1•+·NTf2−, which was obtained by recrystallization from dichloromethane/cyclohexane, transformed to an orange liquid (OL) state upon heating to 100 °C.15 Thermogravimetric analysis (TGA) of the compound revealed no weight loss until 150 °C under aerated conditions (Figure S2), demonstrating that the phase transition occurred without any decomposition. Upon lowering the temperature to 50 °C, the OL changed to an orange solid (OS) in a kinetically locked state, which became a thermodynamically stable green solid (GS) state very slowly upon standing at 30 °C (Figures 1b and S1 and Movie S1). Notably, the kinetic OS state transformed to the thermodynamic GS state upon brief, weak, and pinpoint mechanostress, whereas the OS state remained unchanged without mechanostress at 50 °C (Figures 1c and S1 and Movies S2 and S3). Such a dynamic thermo- and mechanochromism has never been observed in analogues such as 1•+·BF4− [mp. 165 °C (decomp)], 1•+·PF6− [mp. 159 °C (decomp)], and 1•+·SbF6− [mp. 161 °C (decomp)] under the similar conditions, suggesting that the flexibility of the Received: April 6, 2019 Accepted: May 28, 2019 Published: June 10, 2019 10031

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Figure 1. (a) Chemical structure of 1•+·NTf2−. (b) Photographs of spin-state and color changes (Movie S1); DM = diamagnetic, PM = paramagnetic; GC = green crystal, OL = orange liquid, OS = orange solid, GS = green solid. (c) Photographs of changes from PM-OS to DM-GS induced by a pinpoint mechanostress made with a steel needle over a cover glass (Movie S2).

Figure 2. Variations in (a,b) ESR and (c) electronic spectra of 1•+·NTf2− in various states upon (a,c) thermal and (b) mechanical stimuli. Transformations (a,c) between DM-GC, PM-OL, PM-OS, and DM-GS states (top to bottom) upon temperature changes at 30, 100, 30, and 30 °C, respectively, and (b) PM-OS to DM-GS states (Figure 1c and Movie S3) upon pinpoint mechanostress at 50 °C.

NTf2− anion assists in lowering the melting point less than the decomposed point (see Figure S3). Figure 1c and Movie S2 clearly demonstrate that the green spot caused by pinpoint stress on the OS spread to all areas spontaneously after standing at 50 °C. The magnetic properties of 1•+·NTf2− changed drastically with its phase transitions. Figures 2a and S4 illustrate the changes in the electron spin resonance (ESR) spectra that occurred during the transitions between the GC, OL, OS, and GS states. Although the GC state was almost ESR silent, the signal intensity increased significantly during the transition to the OL state upon heating. The kinetic OS state remained ESR active, whereas the thermodynamic GS state prepared by cooling was entirely ESR silent. The hysteretic changes in the ESR intensities were observed repeatedly (Figure S5). These spectral changes demonstrated that (1) the radical cation 1•+ in the GC and GS states formed dimer species with strong antiferromagnetic interactions, thereby exhibiting diamagnetic (DM) properties, and (2) the radical cations in the OL and OS states interacted weakly and thus showed paramagnetic (PM) properties. The strong antiferromagnetic interaction in the DM-GC state was also clarified by the temperature-dependent χp value in terms of a superconducting quantum interference device, which was estimated with the singlet−triplet model as 2J/kB = −2.6 × 103 K (Figure S6). Most importantly, the paramagnetic property of the kinetic solid (PM-OS) could be drastically changed to a diamagnetic solid (DM-GS) by pinpoint mechanostress (Figure 2b). This is the first demonstration of dynamic spin control of the radical solids

by instantaneous, weak, and pinpoint addition of mechanostress. To gain further insights into the mechanism of stimuliinduced spin-state control, we examined the electronic structures of 1•+·NTf2− in various states. The changes in electronic spectra of various 1•+·NTf2− aggregates are shown in Figures 2c (400−900 nm) and S7 (300−1500 nm). The spectra of the DM-GC state exhibited characteristic absorption bands at approximately 480 and 790 nm, which suggested the formation of associated phenothiazine radical cations.12b The PM-OL state displayed a distinct absorption pattern with a strong band at approximately 513 nm and a weak band in the near-IR region. These bands were attributed to the monomer of the phenothiazine radical cation based on its spectrum in dichloromethane (Figure S7). The absorption bands of the PM-OS state were similar to those of the PM-OL state, although the PM-OS state exhibited a significantly greater intensity in the near-IR band, which suggested that the electronic structure of the PM-OS state was intermediary between the DM-GC and PM-OL states. The spectrum of the DM-GS state was identical to that of the DM-GC state. These spectra indicated that the present phase transitions originated from the association and dissociation of the dimer structure of the phenothiazine radical cation. The dissociation and association of the radical dimer during the phase transitions, accompanied by drastic changes in spin and color properties, were established by X-ray diffraction (XRD) analysis. The single-crystal XRD pattern of the DMGC state revealed that crystallographically independent units 1 10032

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Figure 3. (a) Packing of π-dimer structure along the a (top) and c (bottom) axes. (b) Powder XRD patterns of the DM-GC, PM-OL, PM-OS, and DM-GS (top to bottom) states, measured at 30, 120, 30, and 30 °C, respectively.

and 2 formed a π-dimer, where the planar phenothiazine unit experienced face-to-face contact in an eclipsed and head-tohead manner (Figures 3a and S8). The intermolecular distance of the S−S contact in the π-dimer was estimated to be 2.98 Å, which is significantly shorter than the sum of the van der Waals radii.16,17 The crystal packing structure revealed that no significant dimer−dimer interactions were present in any of the axis projections (Figures 3a and S9). Powder XRD measurements (Figures 3b, S10, and S11) revealed that the sharp diffraction peaks in the DM-GC state nearly disappeared in the PM-OL state, changed to a distinctly sharp pattern in the PMOS state, and finally returned back to the original crystal pattern in the DM-GS state. A schematic diagram of the stimuli-induced spin control of 1•+·NTf2− is shown in Figure S12. The diamagnetic property of the GC state was rationalized as reflecting a co-facial association in a π-dimer (Figure S12a), where the π-orbitals of each radical unit effectively overlapped at their positive spin densities (Figure S8). This state switched to the paramagnetic OL state upon the cleavage of the inter-radical interactions in the π-dimer (Figure S12b), which readily arises thermally within the range of ambient temperatures owing to the high fluxional behavior of the ion pair. The liberated ion pair in the PM-OL state underwent acute rearrangement of species upon cooling to 50 °C (Figure S12c), at which point the ion pair lost its fluidity by offsetting the π-stacking interactions with least molecular motions to afford metastable, kinetic crystals that still exhibited paramagnetic properties. The proposed offset stacking in the paramagnetic kinetic crystal OS seems to be evidenced by an N-octyl analogue crystal bearing an offset stacking arrangement (Figure S13) with paramagnetic properties (Figure S14) and UV/vis/near-IR patterns close to that of the PM-OS of 1•+·NTf2− (Figure 2c). Introduction of weak, brief mechanostress to a pinpoint region of the kinetically formed, offset stacking alignment of the PM-OS state gave rise to a regional phase transition to a thermodynamically stable, πdimer structure (Figure S12d).18 The subsequent π-dimer domain region underwent anisotropic growth via the consecutive domino-type molecular movement to convert the PM-OS state to the DM-GC state spontaneously across the entirety of the crystal region. Additional results including an unprecedented single-crystal-to-single-crystal transformation

also suggested changes in the structure spin and color properties (Tables S1 and S2 and Figures S15 and 16).

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CONCLUSIONS We have prepared a liquescent radical ion salt, 1•+·NTf2−, that displayed drastic spin-state changes induced by heating and cooling processes or a brief, weak, and pinpoint mechanostress accompanied by domino transfer of molecular alignment. The ESR/electronic spectra and XRD patterns indicated that the unprecedented changes are very likely attributable to dynamic changes in the association structures of 1•+ during the crystal− liquid and the crystal−crystal transformations. The observed phenomena indicate that flexible ion pairs with charged radical ions that are capable of associations are one of a rational strategy for molecule-based materials with extreme sensitivity to external stimuli.

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EXPERIMENTAL SECTION General. Melting points were measured in glass plates on a Yanagimoto melting point apparatus. UV/vis/near-IR absorption spectra in solution state and KBr pellets were obtained on a JASCO V-650 spectrophotometer and a Shimadzu UV/vis/ near-IR scanning spectrometer UV-3100 PC, respectively. UV/ vis/near-IR spectra of neat samples were recorded using an Ocean Optics HR4000 spectrometer. Differential scanning calorimetry (DSC) was performed with a SHIMADZU DSC60. TGA and differential thermal analysis were measured using a SHIMADZU DTG-60. ESR spectra were recorded with a JEOL JES-FE1XG. The magnetic susceptibility measurements were performed using a Quantum Design SQUID magnetometer, MPMS-XL. Single-crystal X-ray data were collected by a Rigaku XtaLAB P200 diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71075 Å). Powder XRD was performed on a Philips X’Pert Pro MPD diffractometer using C u Kα r a d i a t i o n (λ = 1 .5 4 18 Å ) . Si l v e r b is (trifluoromethanesulfonyl)imide, dehydrated dichloromethane, and cyclohexane were commercially available and used without further purification. N-n-Pentyl-10H-phenothiazine (1) and Nn-octyl-10H-phenothiazine (N-octyl analogue) were prepared according to the literature.19 Compounds 1•+·BF4−, 1•+·PF6−,

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and 1•+·SbF6− were prepared using corresponding silver salts by the similar method of 1•+·NTf2−. Synthesis of N-n-Pentyl-10H-phenothiazin-10-ium10-yl Bis(trifluoromethanesulfonyl)imide (1•+·NTf2−). A suspension of N-n-pentyl-10H-phenothiazine (1) (300 mg, 1.12 mmol) and silver bis(trifluoromethanesulfonyl)imide (442 mg, 1.14 mmol) in dehydrated dichloromethane (5 mL) was stirred for 1 h at room temperature. After the resulting insoluble solid was removed by filtration through a pad of Celite, the filtrate was concentrated under reduced pressure. The residue was purified by recrystallization from dichloromethane/cyclohexane to give 1•+·NTf2− as deep green crystals of DM-GC (553 mg, 93%). 1 • + ·NTf 2 − : C19H19O4N2F6S3; MW 549.54; mp: 100−103 °C; MS (FAB+-MS) m/z: 269.12 ([C17H19NS]+); MS (FAB−-MS) m/z: 279.92 ([C2O4NF6S2]−); IR (KBr): 3127 (w), 3070 (w), 2962 (w), 2938 (w), 2881 (w), 2867 (w), 1564 (w), 1539 (w), 1513 (w), 1445 (w), 1350 (s), 1332 (m), 1193 (s), 1137 (m), 1054 (m), 755 (m), 740 (w), 712 (w), 614 (m), 570 (m), 514 (w) cm−1; Anal. Calcd for C19H19O4N2F6S3: C, 41.53; H, 3.49; N, 5.10. Found: C, 41.72; H, 3.27; N, 5.13. Synthesis of N-n-Octyl-10H-phenothiazin-10-ium-10yl Bis(trifluoromethanesulfonyl)imide (N-Octyl Analogue). The N-octyl analogue was prepared from N-n-octyl10H-phenothiazine (216 mg, 0.694 mmol) by the similar method of 1•+·NTf2−. The crude product was purified by recrystallization from dichloromethane/cyclohexane to give the N-octyl analogue as deep orange crystals (382 mg, 93%). MW 591.63; mp: 99−100 °C; MS (FAB+-MS) m/z: 311.17 ([C20H25NS]+); MS (FAB−-MS) m/z: 279.92 ([C2O4NF6S2]−); IR (KBr): 3127 (w), 3078 (w), 3004 (w), 2930 (m), 2869 (w), 2363 (w), 1684 (w), 1653 (w), 1559 (m), 1539 (m), 1437 (m), 1349 (s), 1199 (s), 1135 (m), 1050 (m), 764 (m), 738 (m), 618 (s), 568 (s), 515 (m) cm−1; Anal. Calcd for C22H25O4N2F6S3: C, 44.66; H, 4.26; N, 4.74. Found: C, 44.50; H, 4.23; N, 4.72.

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Phase transition behavior by a pinpoint and weak mechanostress at 50 °C with a preheated steel needle over a cover glass (MOV) Behaviors with and without mechanostress at 50 °C using a preheated steel needle over a cover glass (MOV) Crystallographic data of 1•+·NTf2− (CIF) Crystallographic data of the N-octyl analogue (CIF) Crystallographic data of the polymorph of 1•+·NTf2− (CIF) Crystallographic data of the polymorph of 1•+·NTf2− after heating at 80 °C (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.S.). *E-mail: [email protected] (T.N.). ORCID

Shuichi Suzuki: 0000-0001-6294-9084 Yoshiaki Uchida: 0000-0001-5043-9239 Takeshi Naota: 0000-0001-5348-3390 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research [JSPS KAKENHI #grant numbers JP26102005 (S.S.), JP17K05783 (S.S.), JP16H06516 (T.N.), and JP17H04896 (Y.U.)]. We also thank Prof. Dr. Yasushi Morita (Aichi Institute of Technology) and Prof. Dr. Takumi Konno (Osaka University) for UV/vis/near-IR measurements and Dr. Hiroyasu Sato (Rigaku) for helpful discussions about X-ray crystal structure analysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00982. Photographs of 1•+·NTf2− in the different states; TG and DSC curves; calculated structures of NTf2−; temperature dependency of ESR intensity; ESR spectra changes at variable temperature; temperature dependence of χp for 1•+·NTf2−; UV/vis/near-IR spectra of 1•+· NTf2−; dimer structure in the DM-GC state; packing structure in the DM-GC state; powder XRD patterns at the phase transition; powder XRD patterns in the DMGC and DM-GS states; proposed mechanism of the changes in radical associates; packing structure of the Noctyl analogue; ESR and UV/vis/near-IR spectra of the N-octyl analogue; calculated exchange interactions of the intermolecular contacts; packing diagram and dimer structures in a polymorph crystal of 1•+·NTf2−; results of time-dependent density functional theory calculations; and crystallographic data (PDF) Phase transition behaviors during temperature changes (MOV) 10034

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