Substituent effects on the fluorescent spin-crossover Fe(II) complexes of rhodamine 6G hydrazones

Juan Yuan*ab, Mei-Jiao Liub, Shu-Qi Wuc, Xin Zhua, Nan Zhangb, Osamu Sato*c and Hui-Zhong Kou*b
aSchool of Pharmacy, Henan University of Chinese Medicine, Zhengzhou 450046, P. R. China. E-mail: hnzz_yuan@163.com
bDepartment of Chemistry, Tsinghua University, Beijing 100084, P. R. China. E-mail: kouhz@mail.tsinghua.edu.cn
cInstitute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka Nishi-ku, 819-0395 Fukuoka, Japan. E-mail: sato@cm.kyushu-u.ac.jp

Received 26th January 2019 , Accepted 14th March 2019

First published on 14th March 2019


Two new complexes were constructed via the coordination of fluorophores with Fe(II) ions to study the effects of ligand substituents on fluorescent-spin crossover (SCO) materials. Single-crystal structural determination suggests that the central iron(II) of complexes 1–2 adopted an N4O2 coordination configuration, chelated by two ligands L1 or L2. The π⋯π stacking interaction between the π-conjugated xanthene groups of the adjacent ligands for complex 1 is apparently stronger than that for complex 2. Magnetic susceptibility measurements show that complex 1 is high spin, while complex 2 is diamagnetic at room temperature. Interestingly, the desolvated form 1-d of complex 1 exhibits SCO behavior (Tc = 175 K) without hysteresis. Significantly, complex 1-d displays the correlation between the spin crossover and the fluorescence.


Introduction

Spin-crossover complexes, exhibiting bistability between the high-spin (HS) and low-spin (LS) states1–3 of metallic ions, have potential applications in molecular switches, information storage and display devices.4–6 Recently, the research on incorporating functional moieties into spin-crossover systems has emerged with the aim of constructing multifunctional molecular devices.7–9 Fluorescent-SCO bifunctional materials can find their potential application in fluorescence thermometers, biomarkers, drug delivery, etc.10–12 From the synthesis viewpoint, the reaction of SCO active metal ions with fluorescent ligands is the most effective way of obtaining fluorescent–SCO materials. The facile crystallization of such materials favors the investigations on the relationship between the SCO center and the fluorophore. However, it is challenging to obtain synergetic SCO–fluorescent complexes,13–20 especially structurally elucidated ones.17–20 The first crystallographically characterized SCO–fluorescent FeII complex, reported by Garcia and co-workers,17 showed fluorescence with variable emission maxima at different temperatures (λmax = 394 nm at 80 K, 414 nm and 510 nm at 259 K) due to the cis-keto–enol transformation of the Schiff base ligand, whereas the fluorescence intensity was not well synchronized with the SCO curve.

Recently, we prepared a three-step SCO FeII complex [Fe(L′)2](ClO4)2·xsolv based on the rhodamine 6G hydrazone-labeled L′ ligand (Scheme 1),19 and the solvent-free form of the complex exhibits synergetic fluorescence and SCO around room temperature (Tc↑ = 343 K and Tc↓ = 303 K). The fluorescence comes from the ring-opened rhodamine group. This result indicates that rhodamine 6G pyridylhydrazone derivatives are excellent tridentate ligands to construct SCO-fluorescent complexes with the existence of Npyridyl, Nimine, and Ocarbonyl coordination atoms and a xanthene fluorophore. It has been documented that the change in the substituent of ligands can affect the SCO behavior of the complexes by modifying the field strength and the steric hindrance of ligands.21–26 In order to investigate the influence of the substituent on the SCO-fluorescent FeII complexes of rhodamine 6G pyridylhydrazone ligands, we report two new FeII complexes 1 and 2 based on L1 and L2 (Scheme 1, L1 = 6-bromopyridine-2-carbaldehyde rhodamine 6G hydrazone, L2 = pyridine-2-carbaldehyde rhodamine 6G hydrazone). Complexes 1 and 2 display usual magnetic properties of high spin or diamagnetic (low spin), respectively; however, the desolvated form (1-d) of complex 1 displays coupled SCO-fluorescent behavior. Herein, the synthesis and characterization of complexes 1 and 2 are described.


image file: c9qi00111e-s1.tif
Scheme 1 Structure of the ring-closed (left) and ring-opened (right) ligands L1 (R = Br), L2 (R = H) and L′ (R = CH3).

Experimental

Synthesis

All the starting materials and solvents were commercially available and were used without further purification. The ligands L1 and L2 were synthesized according to a procedure described earlier.19 Caution! The complexes between metal ions and organic ligands with perchlorate anions are potentially explosive, great care should be taken when using and handling these compounds.
[FeII(L1)2](ClO4)2·3CH3OH·CH2Cl2·H2O (1). The ligand L1 (0.2 mmol) was dissolved in dichloromethane (5 mL) to afford a colorless solution. A methanol solution (5 mL) of Fe(ClO4)2·6H2O (0.1 mmol) was added to the above solution under stirring. The solution turned reddish brown immediately. After stirring for a few minutes, the resulting solution was filtered and slow diffusion of ether into the filtrate in a monotube at room temperature gave dark green cube crystals suitable for X-ray diffraction measurements. Yield: about 65% (based on the Fe(II) salt). The single crystals of complex 1 tend to lose solvents slowly. The solvent-free complex 1-d of complex 1 is obtained by heating the crystals of 1 at 127 °C in a vacuum oven for 2 h. Calculated for C64H60Br2Cl2N10O12Fe (1-d): C 53.09, H 4.18, N 9.67; found: C 53.1, H 3.8, N 9.8.
[FeII(L2)2](ClO4)2·2CH3OH·1.75H2O (2). Using L2 instead of L1, dark green lamellate crystals of complex 2 were obtained in a way similar to that for complex 1. The crystals are stable in air. Yield: about 80% (based on the Fe(II) salt). Calculated for C66H73.5N10O15.75Cl2Fe: C 57.21, H 5.35, N 10.11; found: C 57.2, H 4.9, N 10.0.

Materials and methods

Variable-temperature magnetic susceptibility measurements were carried out on a Quantum Design MPMS-5S SQUID magnetometer working under a magnetic field of 1000 Oe. The corrections were made to the magnetic data for the background by deducting an empty holder. Solid-state emission spectra were recorded on an Edinburgh FLS 920 fluorescence spectrophotometer by using solid samples. IR absorption spectra were recorded in the range of 4000–400 cm−1 on KBr pellets using a WQF-510A FT–IR spectrophotometer. UV–vis spectra were obtained on a TU-1901 spectrophotometer. Powder X-ray diffraction (XRD) measurements were carried out at room temperature on a Bruker D8 ADVANCE X-ray diffractometer. Thermogravimetric analyses (TGA) were performed on a DTU-3A simultaneous thermal analyzer under a nitrogen atmosphere at a heating rate of 10 °C min−1. Elemental analyses for C, H, and N were performed on an Elementar Vario Cario Erballo analyzer.

DFT calculations

Density functional theory (DFT) calculations were employed with the Gaussian 09 program. Geometric optimizations were performed at the B3LYP/6-311G* level of theory with the molecular geometries where the rhodamine groups were replaced by hydrogen atoms (the ligands are abbreviated as HL1S, HL’S and HL2S, respectively). Electronic structural calculations were conducted at the TPSSh/6-311G* level of theory.27

Crystallography

Single crystal X-ray diffraction data were collected on a Rigaku SuperNova, Dual, Cu at zero, AtlasS2 diffractometer. The structures were solved by direct methods Olex2 1.2 and refined by full-matrix least-squares (SHELXL-2013 or Olex2 1.2) on F2. Hydrogen atoms were added geometrically and refined using a riding model.

Results and discussion

Syntheses and general characterization

The well-shaped dark green crystals of complexes 1 and 2 were obtained by diffusion of ether into a MeOH–CH2Cl2 reddish brown solution of 1 and 2, respectively. The color change of the solution is due to the formation of ring-opened ligands upon coordination.28 The solubility of complex 2 is higher than that of complex 1 in MeOH–CH2Cl2 solution, and the crystallization of complex 2 is also faster than that of complex 1. The crystals of complex 1 are efflorescent and become partially desolvated upon exposure to air, while the crystals of complex 2 are stable. It is probably because the noncoordinating CH2Cl2 solvent molecules in the crystals of complex 1 are volatile. Thermogravimetric analyses (TGA) on 1 (Fig. S1, ESI) indicate that a continuous loss of mass before 100 °C and a plateau in the range of 100–200 °C are observed, consistent with the complete removal of the solvent molecules to yield stable solvent free compound 1-d when heated at 127 °C. TGA analysis reveals a large mass loss of nearly 69.90% for complexes 1 and 2 at about 215 °C, indicating the decomposition of the perchlorate complexes. The TGA of complex 2 shows the loss of mass by 2.3% at 130 °C, corresponding to 1.75 lattice water molecules per molecule. The powder XRD pattern of 1 does not coincide with that calculated from the single-crystal X-ray diffraction data, revealing that 1 has lost some solvent molecules during the XRD measurement (Fig. S2). The powder XRD pattern of 2 is in good agreement with the calculated results, which indicates the structural stability of complex 2 at room temperature.

The IR spectra of ligands L1, L2 and complexes 1–2 were recorded (Fig. S3). The characteristic absorption peaks of complexes 1 and 2 are very similar, different from those of ligands L1 and L2 because the ligands are ring-closed in the solid state. The IR spectra show the characteristic absorption peaks at 1090 cm−1 for perchlorate in complexes 1 and 2. The peaks, located at 1720 cm−1 (L1) and 1742 cm−1 (L2), indicate the existence of C[double bond, length as m-dash]O in ligands, while the absence of these two peaks in the spectrum of complexes 1 and 2 is consistent with the coordination of an iron(II) metal center. The peaks, located at 1606 cm−1 (complexes 1 and 2) and 1620 cm−1 (ligands L1 and L2), are due to the C[double bond, length as m-dash]N stretching. The UV-vis absorption spectra of ligands L1, L2 and complexes 1–2 in ethanol solution were obtained (Fig. 1). The absorption spectra of complexes 1 and 2 are very similar, and show strong absorptions centred at about 305 nm and 525 nm. The band at 525 nm should be responsible for the π–π* transition of the xanthene groups of the ring-opened ligand, while the strong bands at 305 nm are probably due to the π–π* transition of the Schiff base part. The ligands L1 and L2 show an absorption peak at ca. 305 nm, and no absorption at about 525 nm is observed, which also indicates that the free ligands L1 and L2 are ring-closed in ethanol.


image file: c9qi00111e-f1.tif
Fig. 1 The UV-vis spectra for L1, L2 and complexes 1–2 in ethanol (5 × 10−5 mol L−1).

Crystal structures

The structures of complex 1 at 110 K and 173 K and complex 2 at 110 K were determined, respectively. Crystal data and structural details of complexes 1 and 2 are listed in Table 1, and selected bond lengths and bond angles are given in Table 2. Complexes 1 and 2 are not isomorphous although they have a similar [Fe(Lx)2]2+ cationic structure, while complex 1 is isomorphous to the analogous complex [Fe(L′)2](ClO4)2·xsolv.19
Table 1 Crystal data for complexes 1 and 2
Complexes 1 2
T (K) 110 173 173
Formula C68H76Br2Cl4N10O16Fe C66H73.5N10O15.75Cl2Fe
Formula weight 1646.85 1385.59
Crystal system Monoclinic Monoclinic
Space group P21/c P21/c
a (Å) 20.2940(6) 20.3424(6) 11.1308(6)
b (Å) 28.6311(9) 28.6600(9) 27.8856(14)
c (Å) 12.6951(3) 12.6888(3) 22.265(2)
β (°) 94.094(2) 94.166(2) 103.407(8)
V3) 7357.5(4) 7378.2(4) 6722.6(8)
Z 4 4
ρcalc(g cm−3) 1.487 1.483 1.369
μ (mm−1) 4.856 4.842 3.152
F(000) 3384 3384 2902
Data/restraints/parameters 12[thin space (1/6-em)]487/101/958 12[thin space (1/6-em)]539/102/959 11[thin space (1/6-em)]415/54/875
GOF on F2 1.045 1.019 1.048
R1 [I > 2σ(I)] 0.0925 0.0932 0.1048
wR2 (all data) 0.2662 0.2802 0.3135
CCDC 1891017 1891018 1891019


Table 2 Selected bond distances (Å) and bond angles (°) for complexes 1 and 2
Complexes 1 (110 K) 1 (173 K) 2 (173 K)
Fe(1)–O(1) 2.052(4) 2.053(4) 1.945(4)
Fe(1)–O(2) 2.070(4) 2.069(4) 1.936(4)
Fe(1)–N(1) 2.242(5) 2.245(5) 1.937(6)
Fe(1)–N(2) 2.104(4) 2.104(5) 1.852(7)
Fe(1)–N(4) 2.268(4) 2.270(5) 1.956(5)
Fe(1)–N(5) 2.118(5) 2.122(5) 1.858(7)
Fe(1)–Oav 2.062(4) 2.061(4) 1.941(4)
Fe(1)–Nimine,av 2.111(5) 2.113(5) 1.855(7)
Fe(1)–Npy,av 2.255(5) 2.258(5) 1.946(7)
O(1)–Fe(1)–N(1) 147.09(16) 147.33(18) 161.8(3)
O(2)–Fe(1)–N(4) 146.94(17) 146.33(18) 161.2(3)
N(2)–Fe(1)–N(5) 166.71(18) 166.8(2) 174.2(2)


The central iron(II) of complexes 1 and 2 is surrounded by two tridentate pyridyl aroylhydrazone ligands L1 or L2, and adopts an N4O2 octahedral coordination configuration (Fig. 2). The average Fe–O/N bond distances for complex 2 at 173 K are 1.941(4) Å (Fe–Oav) and 1.901(7) Å (Fe–Nav), typical of low spin diamagnetic iron(II) with the N4O2 donor set (Table S1, ESI).19,29–34 The average Fe–N bond length of 2.185(5) Å at 173 K for complex 1 is typical of HS iron(II) ions,19,29–34 which is in agreement with the magnetic data. Complex 1 showed a larger deformation with the trans O–Fe–N or N–Fe–N bond angles in the range of 146.43°–166.91°, indicating that the spin state of iron(II) for complex 1 is different from that of complex 2 at 173 K with the trans-N–Fe–N bond angle of 174.2(2)°.29–34 The crystal data of complex 1 at 110 K do not show much difference in the bond distances and bond angles from the 173 K data, showing that complex 1 is high spin at 173 K and 110 K, and there is no SCO between 110 K and 173 K for the complex. On the basis of available structural data for analogous FeIIN4O2 complexes (Table S1, ESI), we can find that the Fe–Npy bond distances are generally larger than the Fe–Nimine irrespective of HS or LS Fe(II) species, and the Fe–Nimine bond distances for LS complexes are always less than 2.0 Å (1.833(8)–1.968(6) Å) whereas the Fe–Nimine bond distances for HS complexes are always larger than 2.0 Å (2.024(3)–2.136(7) Å). Therefore, the Fe–Nimine bond distances may be used as an indicator for spin state determination. For HS FeIIN4O2 complexes, the Fe–Npy and Fe–O bond distances are in the range of 2.083(4)–2.264(2) Å and 2.053(4)–2.145(5) Å, respectively, and are in the range of 1.915(9)–2.048(6) Å and 1.936(4)–2.047(5) Å for LS FeIIN4O2 complexes.


image file: c9qi00111e-f2.tif
Fig. 2 Crystal structure of the [Fe(L1)2]2+ cation for complex 1 (top) and [Fe(L2)2]2+ cation for complex 2 (bottom). Hydrogen atoms have been omitted for clarity.

For both complexes 1 and 2, adjacent molecules were connected by π⋯π stacking interactions between the large π-conjugated xanthene groups of the adjacent ligands (L1 or L2) to form a one-dimensional (1D) supramolecular chain along the crystallographic c axis. As shown in Fig. 3, the π⋯π stacking interactions for complexes 1 and 2 are obviously different, though the stacking modes are both offset face-to-face stacking. For complex 1, the xanthene groups of the adjacent ligands are parallel with the centroid-to-centroid distances of 3.649 Å and 3.964 Å, and face-to-face distances of 3.648 Å and 3.837 Å at 173 K, while for complex 2, the xanthene groups of the adjacent ligands are not parallel with a dihedral angle of 6.36°, and the adjacent xanthene groups have little overlap. The centroid-to-centroid distance is 4.140 Å.


image file: c9qi00111e-f3.tif
Fig. 3 1D supramolecular chain structure of complexes 1 (top) and 2 (bottom).

Overall, the molecular structure of 1 is apparently different from that of complex 2 due to the ligand substituent effect. The former is high spin with a strong intermolecular π–π stacking interaction, and the latter is low spin with a weak π–π interaction. From the structural viewpoint, complex 1 is very similar to the analogous complex of L′.19

Magnetic properties

The magnetic properties of complexes 1, 2 and 1-d were investigated. Complex 2 is diamagnetic at room temperature, consistent with the crystal data mentioned above. The magnetic susceptibilities for complexes 1 and 1-d were obtained by first heating the solvated sample of 1 from 5 K to 400 K, and then varying the temperature in the range of 400–5–400 K, as shown in Fig. 4.
image file: c9qi00111e-f4.tif
Fig. 4 Temperature dependence of χmT for complexes 1 and 1-d.

For complex 1, the χmT value is about 3.24 cm3 K mol−1 at 300 K, which falls within the normal range for isolated HS Fe(II) ions with S = 2. χmT is almost constant in the temperature range of 230–300 K, and then begins to decrease slightly reaching the plateau value of 3.1 cm3 K mol−1 at 70 K, with the temperature decreasing (Fig. 4). Below 20 K, χmT decreases rapidly to 2.56 cm3 K mol−1 at 5 K. The decrease of χmT below 20 K is most likely due to the zero-field splitting of HS Fe(II) and/or the intermolecular antiferromagnetic interaction. However, the decrease of χmT in the temperature range of 230–70 K should be due to the SCO of some desolvated Fe(II) molecules when compared with the χmT vs. T plots for 1-d. As previously stated, complex 1 tends to slowly lose solvent molecules, and the existence of some desolvated solid 1-d in 1 is inevitable.

The magnetic behavior of the desolvated complex 1-d (Fig. 4) is different from that of complex 1. Complex 1-d that was in situ produced by heating 1 at 400 K in the SQUID exhibits incomplete SCO behavior in the temperature range of 230–70 K (Fig. 4). The χmT value is 2.40 cm3 K mol−1 at 70 K (a plateau of χmT), corresponding to the HS fraction γHS of 0.74 calculated from the equation γHS = [(χmT)m − (χmT)LS]/[(χmT)HS − (χmT)LS], where (χmT)m is the experimental value at the temperature, and (χmT)LS or (χmT)HS is the value for the 100% LS (S = 0, (χmT)LS = 0 cm3 K mol−1) or HS (S = 2, (χmT)HS = 3.30 cm3 K mol−1) situation, respectively. Below 30 K, χmT decreases rapidly reaching the minimum value of 2.05 cm3 K mol−1 at 10 K owing to the ZFS of the remaining HS Fe(II) molecules and/or the intermolecular antiferromagnetic interaction. The spin transition temperature is 175 K and no apparent hysteresis can be observed (Fig. 4). It has been documented that the lattice solvent is one of the important factors affecting magnetism because the removal of solvents changes the intermolecular interactions.22,29,35,36 Complex 1-d displays gradual SCO and a similar phenomenon has been observed in the desolvated analogous complex [Fe(L′)2](ClO4)2 that shows abrupt SCO around room temperature with a hysteresis loop of 40 K.19

It is worth noting that isostructural complexes 1, 2 and [Fe(L′)2](ClO4)2·xsolv19 have the same perchlorate counter-anion, but their magnetic behavior is different. Complex 1 is HS paramagnetic, and complex 2 displays LS diamagnetic, while [Fe(L′)2](ClO4)2·xsolv exhibits three-step SCO. This indicates that the substituent (electron donating groups of Br, H, or Me) on ligands L1, L2 and L′ plays a main role in the magnetic behavior of these complexes, which has been frequently observed.22,26,28–33,37–44 The substituents can affect not only the strength of the ligand field, but also the steric hindrance of ligands because the substituents are next to the coordinating nitrogen atom. The strength of the ligand field is expected to be in the order of L′ (Me) > L2 (H) > L1 (Br) according to the electron donating ability of the substituents, while the steric hindrance of ligands is L′ > L1 > L2. Generally speaking, the stronger the strength of the ligand field is, the more likely the complex is in low-spin state; the larger steric hindrance favors the high-spin state. Apparently, the magnetic data suggest that these two factors concurrently affect the magnetic behavior of these analogous complexes, and the steric effect plays a dominant role in the magnetism of these species.

To gain more insights into the electronic structures of these compounds, density functional theory (DFT) calculations have been performed at the TPSSh/6-311G* level of theory on the optimized molecular models where the rhodamine groups are replaced by hydrogen atoms for simplicity (Fig. S4). All optimized structures didn't exhibit the imaginary frequencies, confirming the validity of these geometries. The Fe–L bond lengths of the optimized geometries are listed in Table S2, and the calculated energies of the five 3d-centered orbitals in Table S3. Though the absolute values of the d-orbitals are quite different, the orbital splitting patterns in [Fe(L′)2]2+ and [Fe(L′S)2] motifs are almost the same,19 certificating the reliability of our simplified models.44 The Mülliken charge on the Npyridine atoms of the deprotonated ligands (Fig. S5) is −0.292, −0.322 and −0.295, respectively, revealing that the methyl group acts as an electron-donating group while the bromide atom acts as a weak electron-withdrawing group. The calculated crystal field splitting energies, Δo, in Table S3, indicate that the strength of the ligand field is in the order [Fe(L1S)2] < [Fe(L′S)2] < [Fe(L2S)2]. Moreover, the [Fe(L2S)2] motif exhibits the shortest Fe–Npyridine bond lengths (ca. 1.978 Å) compared with those of the [Fe(L′S)2] (ca. 2.048 Å) and [Fe(L1S)2] motifs (ca. 2.071 Å) due to the steric hindrance. Thus, the overall ligand field strength is L1 (Br) < L′ (Me) < L2 (H), consistent with the observed spin states of the three complexes. The present results indicate that the incorporation of steric ortho electron donating groups in the ligands is an effective way of modulating SCO in this family of complexes.

Variable-temperature fluorescence

To study the correlation between the fluorescence and the spin state of the Fe(II) ions, the temperature dependence of the emission spectra for complexes 1-d and 2 in the range of 80 K–300 K was measured with the same excitation wavelength λex of 355 nm, as shown in Fig. 5 and 6. For complex 1-d, the measurements were performed in the warming mode because of the absence of hysteresis in the complex. For reference, the room-temperature emission spectra for ligands L1 and L2 in the solid state were also obtained with a λex of 355 nm (Fig. S4).
image file: c9qi00111e-f5.tif
Fig. 5 (a) Temperature-dependent emission spectra of complex 1-d (λex = 355 nm); (b) χmT and the fluorescence intensity of the maximum emission (λem = 570 nm) for 1-d.

image file: c9qi00111e-f6.tif
Fig. 6 (a) Temperature-dependent emission spectra of complex 2 (λex = 355 nm); (b) the fluorescence intensity of the maximum emission (λem = 555 nm) for complex 2.

The free ligands L1 and L2 exhibit a strong emission band at 475 nm and 460 nm, respectively, due to the pyridylaldehyde Schiff base part. The slight difference in emission peaks (475 nm for L1 and 460 nm for L2) may be attributed to the influence of substituents on L1 and L2. There is no obvious emission peak between 500 and 600 nm for L1 and L2, corresponding to the ring-closed rhodamine amide moiety in free ligands.19,28 However, obvious emission bands at 570 nm for complex 1-d (Fig. 5a) and at 555 nm for complex 2 (Fig. 6a) can be observed, indicating that the coordination of L1 and L2 to FeII leads to the formation of the ring-opened rhodamine amide moiety.45–47 Compared with complex 2 and [Fe(L′)2](ClO4)2,19 the fluorescence spectra for complex 1-d show a broad emission band with a slight red shift, which is because the emission band (475 nm) for the Schiff base part merged with those of the xanthene moiety of ligand L1.

As mentioned above, complex 2 is diamagnetic in the room temperature range, which indicates that the fluorescence intensities of complex 2 are not affected by spin transition in the temperature range of 80–300 K, and should be temperature-dependent. As expected, upon heating, the emission intensity of complex 2 decreased continuously with increasing temperature owing to the suppression of thermal fluorescence quenching (Fig. 6b).19 However, complex 1-d exhibited different temperature-dependent fluorescence emissions (Fig. 5b) in the range of 80–300 K. Upon heating, the emission intensity of 1-d initially decreased with increasing temperature, reaching the lowest value at 120 K. Upon further heating, the emission intensity started to increase and finally reached the highest value at 200 K (Fig. 5b) before decreasing again until 300 K with the continuous rise of the temperature. A comparison of the magnetic and fluorescence properties shows that the abnormal region for fluorescence intensity is in good agreement with the spin transition process (Fig. 5b). Moreover, the emission intensity of the HS state was higher than that of the LS state in the spin transition temperature range, demonstrating that fluorescence is indeed affected by the spin state of Fe(II) ions. The results provide direct evidence for the correlation between fluorescence and thermal-induced SCO. The mechanism of this correlation should be related to the energy transfer between the ferrous ions and the xanthene fluorophores.19

It is worth noting that the fluorescence intensities in the LS region for complex 1-d are different from that for the analogous [Fe(L′)2](ClO4)2,19 weak for the former and strong for the latter (Fig. 5b). This can be understood by considering the temperature dependence of fluorescence intensity for complex 2 where the intensities change very slightly below 150 K (Fig. 6b). Therefore, we assume that the fluorescence intensity for LS 1-d responds similarly to temperature, whereas [Fe(L′)2](ClO4)2 and [Zn(L′)2](ClO4)2 show obvious variations in fluorescence intensity at low temperatures.19

Conclusions

We have studied the influence of substituents on the magnetic and fluorescence properties of FeII complexes based on rhodamine 6G derivative ligands. The results show that ortho substituents with the ligand field effect and the steric effect influence the magnetic properties of the complexes. Complex 1 (R = Br) is high spin and complex 2 (R = H) is diamagnetic, while [Fe(L′)2](ClO4)2 (R = Me)19 exhibits three-step SCO. The desolvated species (1-d) shows incomplete SCO behavior. Moreover, complex 1-d displays the correlation between the spin crossover and the fluorescence. These results reveal that ligand substituents can modulate both the magnetism and fluorescence, which opens up a new way for the preparation of novel multifunctional materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the NSFC (Project No. 21571113 and 21771115), the project of the Scientific and Technological in Henan Province (Project No. 172102310433), and the Key Scientific Research Project of Colleges and Universities in Henan Province (16A350006). The solid fluorescence spectra were obtained at the Analytical Instrumentation Center of Peking University. We thank Ming-Xing Chen for the help with the fluorescence measurements.

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Footnote

Electronic supplementary information (ESI) available: TGA, PXRD, IR and fluorescence spectra. CCDC 1891017–1891019. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qi00111e

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