A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE19 May 2022A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission Cheng Yi, Yin-Shan Meng, Liang Zhao, Nian-Tao Yao, Qiang Liu, Wen Wen, Rui-Xia Li, Yuan-Yuan Zhu, Hiroki Oshio and Tao Liu Cheng Yi State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning , Yin-Shan Meng State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning , Liang Zhao State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning , Nian-Tao Yao State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning , Qiang Liu State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning , Wen Wen State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning , Rui-Xia Li State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning , Yuan-Yuan Zhu State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning Anhui Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, Anhui , Hiroki Oshio State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning and Tao Liu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning https://doi.org/10.31635/ccschem.022.202201950 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The utilization of spin crossover (SCO) to modulate the luminescence properties in smart multifunctional materials and multichannel sensors is promising. However, it is challenging to build a strong coupling between SCO and luminescence in one system. Herein, we present a mononuclear compound [Fe(tpe-abpt)2(SeCN)2]·4DMF ( 1·4DMF, tpe-abpt: (4-(1,1,2,2-tetraphenylethene))-N-(3,5-bis(pyridin-2-yl)-4H-1,2,4-triazol-4yl)methanimine) showing aggregation-induced emission (AIE) and thermally induced SCO properties. Variable-temperature single-crystal structural analysis reveals that SCO changes the number of pathways and strength of intermolecular interactions, resulting in deactivation of nonradiative decay and significant enhancement of luminescence. The photoluminescence (PL) intensity of 1·4DMF exhibited a fivefold increase upon the spin transition from the low-spin to the high-spin states. In contrast with the current strategy of controlling the Förster resonance energy transfer (FRET) process by utilizing SCO to tune the overlap degree between the emission band of the luminophore and UV–vis absorption band of high-spin and low-spin states, we developed a new approach to tune the intermolecular interactions between AIE luminogens (AIEgens) by utilizing a subtle SCO-induced structural transformation, therefore leading to effective coupling between SCO and luminescence and a significant change in luminescence upon SCO. Our results provide a rational strategy to build smart multifunctionalized materials with remarkably synergetic SCO and luminescence. Download figure Download PowerPoint Introduction The pursuit of smart multifunctional materials that possess external stimuli-responsive magnetic and optical properties has attracted increasing interest both in fundamental science and for potential applications to switches, sensors, intelligent devices, and so on.1–8 In particular, the combination of spin crossover (SCO) and luminescence is very promising since it provides a new route to modulate the luminescent signal of an organic luminophore via high-spin (HS) and low-spin (LS) conversion of a metal ion. More importantly, the luminescence shows superior sensitivities upon physicochemical changes induced by subtle environmental modulation of the SCO center. As a consequence, the luminescent characteristic with a high spatial and temporal resolution is superior in studies of the SCO molecule at the single molecular level. Such synergetic property also renders SCO-based luminescence materials as promising candidates for nano- and single-molecule devices.9–13 However, it is nonetheless challenging in two respects to build a strong coupling between luminescence and SCO in one system. First, it is difficult to preserve both the SCO and luminescence properties because both SCO and luminescence are sensitive to subtle structural variations in the process of incorporating each other.14 Moreover, the luminophore often suffers from the luminescent self-quenching problem in the solid state.15–17 The second is in building an effective coupling pathway between SCO and luminescence. Currently the main strategy is to tune the overlap between the emission band of the luminophore and the UV–vis absorption band of the FeII center via SCO so as to control the Förster resonance energy transfer (FRET) process.18 The drawback is that the changes in the UV–vis absorption band during the HS and LS conversion is usually small, which makes it difficult to modulate the luminescent intensity over a wide range.10,19 Despite the fact that several typical luminophores such as pyrene, rhodamines, and anthracene have been successfully implanted into SCO systems, the coupling between them is weak, resulting in only small changes in luminescent intensity.19–26 Although challenging, it is therefore important to make a strong and effective coupling to ultimately realize the switching of luminescence. Our attention moves to a family of special luminophores, the aggregation-induced emission luminogens (AIEgens). In comparison to classic luminophores that suffer from aggregation-caused quenching (ACQ), the AIEgens can exhibit much stronger emission in aggregation or even in the solid state, which is due to the restricted intramolecular motion and suppressed nonradiative decay. Owing to this unique property, AIEgens have been extensively studied in the fields of optics, electronics, energy, and bioscience27–34 and would be an ideal candidate for the construction of an SCO-luminescence coupled system. In addition, the very recent study by Wu et al.35 illustrates that the luminescence of AIEgens is more sensitive to the intramolecular conformational isomerization and intermolecular interaction changes than the ACQ-type luminophors. Taking this into consideration, the bond length variation on the SCO center would cause structural changes of AIEgens as well as the distances and intermolecular interactions between them and therefore may affect the intermolecular energy transfer pathways and efficiency. As a consequence, the AIEgens are expected to show drastic changes in luminescence intensity upon the spin-state conversion. However, related studies addressing this issue have not yet been explored. Motivated by this, we decided to design a suitable ligand that can show the AIE effect and provide a suitable ligand field for the SCO. Herein, we report a new AIE ligand of (4-(1,1,2,2-tetraphenylethene))-N-(3,5-bis(pyridin-2-yl)-4H-1,2,4-triazol-4yl)methanimine and its ferrous complex of [Fe(tpe-abpt)2(SeCN)2]·4DMF ( 1·4DMF) (Scheme 1). In 1·4DMF, the abpt unit (4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) coordinates to the FeII ion to realize the SCO,36–40 and the tpe (tetraphenylethylene) group plays a role in the AIE property.41–46 1·4DMF showed enhanced and suppressed luminescence emission upon LS→HS and HS→LS conversions, respectively. A significant luminescent intensity change over five-fold was observed, which was due to the SCO-tuned intermolecular interactions between tpe units. Scheme 1 | Schematic illustration of SCO responsive AIE by SCO-tuned intermolecular interactions between tpe units. The grey dashed lines represented the intermolecular contact interactions. Download figure Download PowerPoint Experimental Methods Materials and syntheses All reagents were commercially available and used without further purification. tpe-CHO and abpt were synthesized according to a procedure in the literature.47,48 Synthesis of (4-(1,1,2,2-tetraphenylethene))-N-(3,5-bis(pyridin-2-yl)-4H-1,2,4-triazol-4yl)methanimine (tpe-abpt) (4-(1,1,2,2-tetraphenylethene))-N-(3,5-bis(pyridin-2-yl)-1,2,4-triazol-4yl)methanimine (abpt, 1.00 mmol) was added to a 100 mL three-neck round bottom flask containing 40 mL ethanol. Under magnetic stirring, the solution was slowly heated to 85 °C. When all the raw materials were completely dissolved, 20 drops of glacial acetic acid were slowly added. The reaction was stopped after stirring for 12 h at 85 °C. The solution was cooled to room temperature, and yellow-green powder was precipitated, filtered, recrystallized in ethanol, and dried in vacuum. Tpe-abpt was dissolved in dichloromethane, and crystals that could be used for analysis were obtained by volatilization method. Single-crystal X-ray analyses revealed that tpe-abpt crystallizes in the triclinic space group P 1 ¯ ( Supporting Information Figure S1 and Table S2). The yield was 73% based on abpt. 1H NMR (500 MHz, DMSO-d6) δ 8.80 (s, 1H), 8.59–8.55 (m, 2H), 8.09 (d, J = 7.8 Hz, 2H), 8.01 (t, J = 7.8 Hz, 2H), 7.55 (d, J = 8.1 Hz, 2H), 7.52–7.49 (m, 2H), 7.22–7.08 (m, 11H), 7.03–6.97 (m, 6H). Synthesis of Fe(tpe-abpt)2(SeCN)2·4DMF (1·4DMF) and Fe(tpe-abpt)2(SeCN)2·2DMF (1·2DMF) A methanol/water (v/v = 2∶1) mixture solution of Fe(NCSe)2 (0.05 mmol) in the presence of a small quantity of ascorbic acid was added to a dimethylformamide (DMF; 20 mL) solution of tpe-abpt (0.1 mmol). The mixture was stirred for 1 h, and then the solution was concentrated to 2.5 mL. Different crystals were obtained by slowly diffusing ether into the DMF solution. The colors and shapes of 1·4DMF and 1·2DMF were different. 1·4DMF was a pink flake-like crystal while 1·4DMF was a red rod-like crystal. They were easily separated by hand with the help of the optical microscope. Anal. Calcd (%) for 1·4DMF (C92H84FeN18O4Se2): C, 64.26; H, 4.92; N, 14.66. Found: C, 64.54; H, 5.16; N, 14.37. Anal. Calcd (%) for 1·2DMF (C86H70FeN16O2Se2): C, 65.65; H, 4.48; N, 14.25. Found: C, 65.94; H, 4.63; N, 14.07. Physical measurements Structure determination and refinement The single-crystal X-ray diffraction data for tpe-abpt, 1·4DMF and 1·2DMF were collected on a Bruker D8 VENTURE CMOS-based diffractometer (Bruker AXS Company, Karlsruhe, Germany) (Mo-Kα radiation, λ = 0.71073 Å) using the SMART and SAINT programs. Final unit cell parameters were based on all observed reflections from integration of all frame data. The structures were solved with the ShelXT structure solution program using intrinsic phasing and refined with the ShelXL refinement package, using least-squares minimization that was implanted in Olex2. For all compounds, all nonhydrogen atoms were refined anisotropically, and the hydrogen atoms of organic ligands were located geometrically and fixed with isotropic thermal parameters. 57Fe Mössbauer spectra measurement Zero-field 57Fe Mössbauer spectra were recorded on a Topo-logic 500A spectrometer (Topologic Systems Company, Japan) at different temperatures with a proportional counter. The temperature of the sample was controlled by Model 9700 digital temperature controller from the same scientific instruments company (Topologic Systems Company, Japan). The Doppler velocity of the spectrometer was calibrated with respect to α-Fe. Magnetic measurements Magnetic measurements were carried out with a Quantum Design physical property measurement system using the polycrystalline sample. All dc susceptibilities were corrected for diamagnetic contribution from the sample holder and the molecule using Pascal's constants. UV–vis–NIR absorption spectroscopy The solid state absorption spectra presented in this work were recorded as UV–vis reflectance spectra by a HITACHI HU4150 spectrophotometer (HITACHI Company, Tokyo, Japan). Luminescence spectroscopy Luminescence spectra of the solid sample were collected using an Edinburgh FLS1000 fluorescence spectrophotometer (Edinburgh Company, Edinburgh, United Kingdom) coupled with a liquid helium-type refrigerator with a solid sample holder. Luminescence decay of compound 1·4DMF in DMF/water mixtures with different water fractions (WFs) were measured by an Edinburgh FLS1000 fluorescence spectrophotometer (Edinburgh Company, Edinburgh, United Kingdom). The quantum yield of 1·4DMF was determined by the integrating sphere on an Edinburgh FLS1000 fluorescence spectrophotometer (Edinburgh Company, Edinburgh, United Kingdom) (150 mm in diameter with the inner surface coated with polytetrafluoroethylene (PTFE) to enable efficient scattering of light). Elemental analysis The elemental analysis was performed with an Elementar Vario EL III (Elementar Company, Hanau, Germany). Results and Discussion Crystal structure The synthesis of the AIE ligand tpe-abpt is described in Supporting Information Scheme S1. Pink flake-like crystals of 1·4DMF were obtained by slowly diffusing ether into a DMF solution. Single-crystal X-ray analyses revealed that 1·4DMF crystallizes in the triclinic space group P 1 ¯ ( Supporting Information Table S1). The FeII ion is coordinated by two nitrogen atoms from two isoselencyanate anions and four nitrogen atoms from the two tpe-abpt ligands, providing a N6 octahedral coordination environment (Figure 1 and Supporting Information Figure S2). Four DMF molecules are located in the lattice voids of the unit cell. Thermogravimetric analysis (TGA) confirmed negligible release of the DMF molecules below 100 °C ( Supporting Information Figure S3). Significant changes of coordination bond lengths were observed upon the temperature variation. At 110 K, the Fe–Ntriazole bond (1.969(1) Å) is slightly shorter than the Fe–Npyridyl bond (2.015(1) Å) but is longer than the Fe–NSeCN bond (1.943(1) Å). These bond lengths are characteristic of FeIILS species ( Supporting Information Tables S3 and S4). At 300 K, the Fe–Ntriazole, Fe–Npyridyl, and Fe–NSeCN bond lengths increase to 2.133(4), 2.225(4), and 2.108(6) Å, respectively, indicating the occurrence of the spin transition from LS to HS.36,39 It is also worth noting that intermolecular short-contact interactions function between phenyl and other groups with the average distance of 2.845 Å at 300 K, which will interfere with their rotation and movement ( Supporting Information Table S5). Figure 1 | The crystal structure of complex 1·4DMF at 300 K. Hydrogen atoms are omitted for clarity. Download figure Download PowerPoint AIE property Since the tpe moiety is a well-known emitter that exhibits the AIE property,49 the luminescent properties of the ligand tpe-abpt and 1·4DMF were investigated in a DMF/water mixture solution. The 365 nm light was used for excitation throughout the emission measurements. The ligand tpe-abpt showed an emission band centered at 484 nm in pure DMF solution, which is similar to those for tpe derivatives ( Supporting Information Figure S5).41–46 Supporting Information Figure S5 shows photoluminescence (PL) spectra and WF dependence of the maximum peak intensity at 484 nm for tpe-abpt. The PL intensity of tpe-abpt was very weak in pure DMF solution, and the intensities were almost constant in value up to the WF of 60%. However, a remarkable enhancement of the PL intensity was observed above the WF of 65%, reaching its maximum value at 75%. Upon further increasing the WF to 99%, the PL intensity slightly decreased, which might be caused by the further aggregation of molecules, resulting in nonradiative decay. The maximum PL intensity centered at 484 nm is approximately 44 times larger than that in the pure DMF solution. This behavior can be explained by the restriction of intramolecular motion.50,51 1·4DMF also exhibited the AIE property (Figures 2a and 2b and Supporting Information Figure S6). The PL spectrum and intensity variations upon changing the WF for 1·4DMF showed similar behavior to that of tpe-abpt. The PL intensity of 1·4DMF at 484 nm in pure DMF solution was very weak and showed the constant value, which was followed by the rapid increase to the maximum value at the WF of 80%. The intensity ratio at the WF of 0% to 80% was about 82, which is about two times larger than that for tpe-abpt (44). To better demonstrate the AIE property, the quantum yield of 1·4DMF in the pure DMF solution was determined. In the pure DMF solution, the quantum yield of 1·4DMF showed a rather small value of 0.29%. When the WF was increased to 80%, the quantum yield was 30.91%, which is 106 times higher than that in the pure DMF solution. These results clearly demonstrate that 1·4DMF satisfactorily inherits the AIE property. At the same time, the luminescence decay experiments before and after aggregating were conducted. The lifetime data of 1·4DMF in the pure DMF solution and DMF/water mixture (WF = 80%) are 6.0 and 1.7 ns, respectively ( Supporting Information Figure S7). The luminescent lifetimes are typical for fluorophore-decorated iron-based complexes. Figure 2 | (a) PL spectra of 1·4DMF in DMF/water mixtures (10 μM) with different WFs. (b) PL intensity at 484 nm as a function of WF for 1·4DMF at room temperature. Inset in (b) showed the photographs of 1·4DMF in DMF/water mixtures with different WFs under 365 nm UV illumination. Download figure Download PowerPoint Magnetic property Temperature-dependent magnetic susceptibility data revealed that 1·4DMF showed SCO behavior both in solids and DMF solution (Figure 3). In the crystalline solid, a gradual spin-state transition was observed in the temperature range from 200 to 400 K. The χMT value at 400 K was 3.98 cm3 mol−1 K, which was slightly higher than the spin-only value for an FeIIHS ion but still within the typical range for mononuclear FeII-based SCO complexes.52,53 Upon cooling, the χMT values began to drop to χMT = 0.78 cm3 mol−1 K at 200 K, indicative of the occurrence of SCO behavior. The transition temperature T1/2 was 295 K. Upon further cooling, the χMT values slowly decreased to 0.63 cm3 mol−1 K at 110 K, suggesting that the spin transition was incomplete. 57Fe Mössbauer spectra recorded at 110 K revealed that 15.4% of FeII ions remained in the HS state ( Supporting Information Figure S8). Further measurement in the heating mode revealed no thermal hysteresis in this system. Magnetic susceptibility measurements of 1·4DMF in DMF solution were also carried out in the temperature range of 233–353 K. The spin states in the solution were monitored by 1H NMR measurements, where the chemical shift of methyl groups (2.75 ppm) for DMF-d7 was changed upon the spin state changing when compared with that in the absence of paramagnetic 1·4DMF ( Supporting Information Figure S9). The Evans method was applied to calculate the χMT values (details were provided in the Supporting Information).54 1·4DMF in the deuterated DMF solution underwent a gradual SCO from 233 to 353 K with the χMT values varying from 0.68 to 3.90 cm3 mol−1 K ( Supporting Information Table S7). The χMT value at 353 K was close to the value expected for the FeIIHS species, indicative of the complete spin transition to the HS state. Further measurements in the lower temperature range were not conducted due to the freezing of DMF at 212 K. the χMT were by the solution with gradual and complete SCO (Figure where the χMT is to be cm3 mol−1 K and is the ideal ( = ( 1 ( Figure | dependence of the magnetic susceptibility for 1·4DMF in solid state under applied dc field of and temperature of 2 K The data of χMT in DMF were collected between 233 K and 353 K of temperature dependence of χMT for solution 1·4DMF Download figure Download PowerPoint The obtained values of change = and change = J in the typical range of FeII = mol−1 and = J mol−1 The T1/2 was to be K by T1/2 = which K to the one = 295 in This is because can the interactions between SCO of SCO and luminescence The luminescent and magnetic studies for 1·4DMF in solution demonstrate the of AIE and SCO properties. This to the synergetic property of SCO and luminescence in the crystalline state. the temperature dependence of emission spectra of crystalline 1·4DMF was measured with the excitation of = 365 nm (Figure The emission maximum of 1·4DMF was observed nm at K ( Supporting Information Figure When the sample at the PL intensity decreased and the value at K, which is due to the thermal quenching the PL intensity began to an increase was observed as the temperature was from to K (Figure and Supporting Information Figure The PL intensity the maximum value at K where the spin transition was almost It is worth noting that the significant enhancement of the PL intensity within the SCO temperature range and that the maximum of PL intensity at K is times higher than that at K. with such significant variation of the luminescence property is ( Supporting Information Table S8). These results the remarkable of the SCO and luminescent properties in Figure | (a) Temperature-dependent luminescence emission spectra for 1·4DMF in solid state = 365 (b) of the HS of FeII ion and PL intensity at nm as a function of temperature for Download figure Download PowerPoint To the variations in the PL intensity were caused by the temperature luminescent spectra of tpe-abpt in solids were recorded ( Supporting Information Figure For tpe-abpt at K, a emission band was observed at nm under the excitation of 365 nm light ( Supporting Information Figure The emission spectrum of tpe-abpt was similar to the typical emission spectra of tpe Upon its PL intensity was decreased during the temperature measured as a of the thermal quenching effect ( Supporting Information Figure enhancement of the PL intensity was in the same temperature range from to K ( Supporting Information Figure To further the between luminescence and we also studied its complex ( which is similar to 1·4DMF ( Supporting Information and confirmed negligible release of the DMF molecules below 100 °C ( Supporting Information Figure S4). Magnetic susceptibility measurement revealed an SCO in the solid state from to 200 K. The of 1·2DMF was K, lower than that of 1·4DMF ( Supporting Information Figure As the PL intensity of 1·2DMF in a solid was with the increase of temperature from to 400 K ( Supporting Figure The enhancement of PL intensity from to K, which is the same temperature range for its SCO process and UV–vis spectra variation ( Supporting Information and All results that synergetic between luminescence and SCO has successfully been The intensity of luminescent emission can be by changing the spin state of the FeII ion. It is that the coupling between SCO and luminescence on the degree of overlap between the emission band for the luminophore and the UV–vis absorption band for the SCO To this, the UV–vis absorption spectra of 1·4DMF were measured in the temperature range of K (Figure At K, a absorption band from to nm was observed, and this band can be to the transition of the FeIILS ion. When 1·4DMF was heated from to K, the absorption intensity at nm not show a significant Upon heating to K, a of absorption intensity was observed (Figure and we that the transition band for FeIILS ion with the emission band for The energy transfer from the ligand luminophore to the FeII center was as the HS was resulting in the increase of the PL no interaction between tpe and FeII the energy transfer via the However, the small variation in UV–vis absorption intensity at nm at K to at 400 such a remarkable change in PL This that the luminescent change may not from the between the SCO unit and the To this we compared the structures of 1·4DMF at different that the tpe unit not show rotation or a small of the molecular was observed due to the of the However, the intermolecular contact interactions tpe To at the of the changes in intermolecular contact interactions, the single crystal data of 1·4DMF were further collected from 110 to 300 K at different temperatures (Figures At 110 K, of intermolecular short-contact interactions with an average distance of Å were in 1·4DMF (Figure and Supporting Information Table S5). When the temperature was increased to K, of short-contact interactions still remained (Figure However, when further heated to 300 K, the number of interactions was to with the average distance increased to 2.845 Å (Figure Such a of short-contact interactions from to 300 K is caused by the SCO-induced structural can further that increasing the temperature to 400 K may cause the of this of the fact that the luminescence property of AIEgens is sensitive to the conformational and intermolecular interaction changes compared to organic luminophores, it is that the in interaction number and increase in distances between tpe will the energy transfer between therefore nonradiative decay and leading to a remarkable increase in PL This was also by the study on It showed change in intermolecular interactions ( Supporting Information Table due to the SCO. It in a small change of PL we can that SCO-tuned intermolecular interactions more to the increase of PL intensity for Figure | (a) Temperature-dependent UV–vis absorption spectroscopy of 1·4DMF in solid state. (b) The PL intensity of the maximum emission = and absorption intensity at nm as a function of temperature for solid Download figure Download PowerPoint To we a new ligand tpe-abpt and its ferrous complex 1·4DMF, which showed both SCO and AIE properties. The of ligand tpe-abpt not only the strong solid luminescence emission in the crystalline state but also provide a sensitive luminescence unit that is responsive to subtle SCO-induced changes in intermolecular interactions. Variable-temperature luminescent measurement

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